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gene_id: PLD4
gene_symbol: PLD4
uniprot_accession: Q96BZ4
protein_description: 'RecName: Full=5''-3'' exonuclease PLD4 {ECO:0000305}; EC=3.1.16.1
{ECO:0000250|UniProtKB:Q8BG07}; AltName: Full=(S,S)-bis(monoacylglycero)phosphate
synthase PLD4 {ECO:0000305|PubMed:39423811}; EC=3.1.4.- {ECO:0000269|PubMed:39423811};
AltName: Full=Phospholipase D family member 4; AltName: Full=Phospholipase D4;'
gene_info: Name=PLD4 {ECO:0000303|PubMed:30111894, ECO:0000312|HGNC:HGNC:23792};
Synonyms=C14orf175; ORFNames=UNQ2488/PRO5775;
organism_full: Homo sapiens (Human).
protein_family: Belongs to the phospholipase D family. .
protein_domains: Diverse_PLD-related. (IPR050874); PLDc_3. (IPR032803); PLipase_D/transphosphatidylase.
(IPR001736); PLDc_3 (PF13918)
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: Q96BZ4
• Protein Description: RecName: Full=5'-3' exonuclease PLD4 {ECO:0000305}; EC=3.1.16.1 {ECO:0000250|UniProtKB:Q8BG07}; AltName: Full=(S,S)-bis(monoacylglycero)phosphate synthase PLD4 {ECO:0000305|PubMed:39423811}; EC=3.1.4.- {ECO:0000269|PubMed:39423811}; AltName: Full=Phospholipase D family member 4; AltName: Full=Phospholipase D4;
• Gene Information: Name=PLD4 {ECO:0000303|PubMed:30111894, ECO:0000312|HGNC:HGNC:23792}; Synonyms=C14orf175; ORFNames=UNQ2488/PRO5775;
• Organism (full): Homo sapiens (Human).
• Protein Family: Belongs to the phospholipase D family. .
• Key Domains: Diverse_PLD-related. (IPR050874); PLDc_3. (IPR032803); PLipase_D/transphosphatidylase. (IPR001736); PLDc_3 (PF13918)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "PLD4" matches the protein description above
2. Verify the organism is correct: Homo sapiens (Human).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'PLD4' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene PLD4 (gene ID: PLD4, UniProt: Q96BZ4) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

PLD4 (Phospholipase D Family Member 4): A Comprehensive Research Report

Introduction and Summary

Phospholipase D family member 4 (PLD4) is a fascinating example of how protein nomenclature can mislead regarding function. Despite its name suggesting phospholipase activity, PLD4 has been definitively established as a 5'-to-3' single-stranded nucleic acid exonuclease that plays critical roles in immune regulation and, more recently discovered, in lysosomal lipid metabolism [gavin-2018-exonuclease-abstract]. This type II transmembrane glycoprotein localizes to endolysosomes where it performs two biochemically distinct functions: degradation of single-stranded DNA and RNA that would otherwise activate innate immune sensors, and synthesis of bis(monoacylglycero)phosphate (BMP), a phospholipid essential for lysosomal lipid catabolism [singh-2024-bmp-synthase-abstract].

PLD4 is encoded by a gene on human chromosome 14q32.33 and is predominantly expressed in dendritic cells, particularly plasmacytoid dendritic cells (pDCs), as well as microglia in the brain. The protein is structurally related to PLD3, with which it shares considerable functional overlap, though their tissue-specific expression patterns differ substantially. The discovery that PLD4 deficiency causes autoimmune and autoinflammatory diseases, including monogenic systemic lupus erythematosus, has cemented its importance in understanding immune homeostasis [wang-2025-sle-mutations-abstract].

Enzymatic Function: 5'-3' Exonuclease Activity

Catalytic Mechanism

PLD4 functions as a 5'-to-3' exonuclease that degrades single-stranded nucleic acids in the acidic environment of endolysosomes. The enzyme contains two conserved HxKxxxxD/E (HKD) motifs characteristic of the phospholipase D superfamily, which form the catalytic core responsible for phosphodiester bond cleavage [yuan-2024-structure-abstract]. Structural studies have revealed that PLD4 employs a "link-and-release" two-step catalytic mechanism. In the first step, His428 (using human PLD4 numbering) from the second HKD motif performs nucleophilic attack on the phosphodiester bond, forming a covalent phospho-histidine intermediate, while His214 from the first HKD motif protonates the leaving 5'-OH ssDNA. In the second step, the deprotonated His214 accepts a proton from an attacking water molecule, which then hydrolyzes the phospho-histidine intermediate to release the product [yuan-2024-structure-abstract].

The crystal structures of PLD4 reveal an intra-chain dimer topology forming a basic active site at the interface, with a narrow and deep catalytic cavity into which the oligonucleotide substrate enters almost vertically [yuan-2024-structure-abstract]. Notably, PLD4 contains an extra hydrophobic clamp not present in PLD3 that stabilizes the substrate and affects oligonucleotide substrate preference and product release, potentially contributing to differences in processivity between the two enzymes.

Substrate Specificity

PLD4 exhibits exquisite substrate specificity that reflects its endolysosomal function. The enzyme degrades single-stranded DNA and RNA but cannot cleave double-stranded nucleic acids [gavin-2018-exonuclease-abstract]. Key features of substrate recognition include: (1) absolute requirement for unphosphorylated 5' termini, as 5'-phosphorylation completely blocks enzymatic activity; (2) insensitivity to 3'-phosphorylation status; (3) processivity from 5' to 3' direction only, with no 3'-to-5' or endonuclease activity observed [gavin-2018-exonuclease-abstract]. The enzyme generates 3' mononucleotide products and operates optimally at acidic pH (5.0-5.5), consistent with its endolysosomal localization.

Importantly, PLD4 can degrade phosphorothioate-modified oligonucleotides, though approximately 20-50 times less efficiently than unmodified substrates [gavin-2018-exonuclease-abstract]. The enzyme cannot degrade A-type CpG oligodeoxynucleotides due to their secondary structure, which forms higher-order complexes that resist exonuclease attack. PLD3 demonstrates approximately 17-fold higher specific activity than PLD4 under comparable conditions and is more processive in RNA degradation [berouti-2024-tlr7-ligands-abstract].

Historical Connection to Spleen Phosphodiesterase

The enzymatic properties of PLD4 are remarkably similar to "spleen phosphodiesterase" or "spleen acid exonuclease," an activity described in the biochemical literature for decades but whose molecular identity remained unknown. This classic enzyme was characterized as having a pH optimum of 5.5, being 5' single strand-specific, functioning as a non-processive nuclease, being inhibited by 5' phosphate, and generating 3' nucleotide monophosphate products [gavin-2018-exonuclease-abstract]. The striking correspondence between these properties and those of PLD3/PLD4 strongly suggests that one or both of these proteins represents the molecular identity of the long-studied spleen phosphodiesterase activity.

Second Enzymatic Function: BMP Synthase Activity

Discovery of BMP Synthase Activity

A breakthrough study in 2024 revealed an unexpected second enzymatic function for PLD4: the synthesis of bis(monoacylglycero)phosphate (BMP), also known as lysobisphosphatidic acid (LBPA), a peculiar phospholipid found almost exclusively in lysosomes [singh-2024-bmp-synthase-abstract]. BMP is required for proper degradation of glycosphingolipids, particularly gangliosides, and its unusual S,S-stereochemistry had long puzzled researchers regarding its biosynthetic origin.

Mechanism of BMP Synthesis

PLD4 (and PLD3) catalyze the critical stereo-inversion reaction that converts R-configured precursors into S,S-BMP. The enzymes operate through a transphosphatidylation mechanism, using lysophosphatidylglycerol (lyso-PG) and monoacylglycerol (MAG) as substrates to perform glycerol exchange [singh-2024-bmp-synthase-abstract]. The reaction involves stereo-inversion of the glycerol backbone, converting the standard R configuration found in most glycerophospholipids to the unusual S configuration that characterizes lysosomal BMP. This S,S-configuration is crucial because it renders BMP resistant to degradation by lysosomal phospholipases, allowing the lipid to accumulate in the lysosomal membrane where it facilitates lipid catabolism.

Both purified PLD3 and PLD4 showed preference for S,R-lyso-PG compared with R,racemic-lyso-PG as substrates, though both stereoisomers could be utilized [singh-2024-bmp-synthase-abstract]. Importantly, the standard form of phosphatidylglycerol cannot serve directly as a substrate, indicating that upstream lipolytic enzymes such as PLA2G15 must first generate lyso-PG precursors.

Physiological Consequences of BMP Deficiency

Deletion of PLD4 markedly reduced BMP levels in tissues where the enzyme is highly expressed, particularly the spleen (whereas PLD3 deletion affects brain BMP levels) [singh-2024-bmp-synthase-abstract]. The reduction in BMP leads to gangliosidosis and lysosomal abnormalities, as BMP is essential for the function of lipid-degrading enzymes that require the anionic phospholipid as a cofactor. Intralumenal vesicle (ILV) formation within lysosomes is compromised, cholesterol egress is impaired, and galectin-3 recruitment increases, indicating lysosomal membrane damage. Exogenous S,S-BMP supplementation can rescue ganglioside accumulation, demonstrating that BMP deficiency rather than indirect effects drives the pathology.

Cellular Localization and Expression Pattern

Subcellular Localization

PLD4 is synthesized as a type II transmembrane glycoprotein and localizes predominantly to endolysosomes [gavin-2018-exonuclease-abstract]. The protein contains an N-terminal cytosolic tail, followed by a transmembrane domain and a luminal catalytic domain. This topology places the exonuclease active site within the acidic lumen of late endosomes and lysosomes, where it can access internalized nucleic acids and participate in BMP synthesis. Studies using EGFP-tagged PLD4 have also detected the protein in the endoplasmic reticulum and Golgi complex, representing biosynthetic intermediates en route to the endolysosomal system.

Tissue and Cell-Type Distribution

PLD4 expression is notably restricted compared to its paralog PLD3. According to the Human Protein Atlas, PLD4 shows highest tissue expression in brain white matter (42.2 nTPM), spinal cord (36.3 nTPM), and medulla oblongata (39.4 nTPM), followed by bone marrow (12.6 nTPM) and lymphoid tissues including lymph node, spleen, and tonsil (8.8-10.6 nTPM). At the cellular level, plasmacytoid dendritic cells (pDCs) show by far the highest expression (615.6 nCPM), followed by microglia (136.2 nCPM), conventional dendritic cells (41.9 nCPM), macrophages (38.6 nCPM), and B cells (16.0 nCPM).

The strong expression in dendritic cells, particularly pDCs, is functionally significant because these cells are primary sensors of nucleic acids via endosomal TLRs. The expression in microglia is consistent with PLD4's role in brain lipid homeostasis. The protein is detectable in blood plasma at approximately 2.6 micrograms per liter.

Extracellular Distribution via Extracellular Vesicles

A recent discovery has revealed that PLD4, despite being primarily an intracellular endolysosomal enzyme, can be secreted extracellularly via extracellular vesicles (EVs) [betsuyaku-2025-extracellular-vesicles-abstract]. In human plasma, PLD4 circulates predominantly associated with EVs rather than as a soluble protein. Upon B cell activation through B cell receptor engagement and TLR9 agonist stimulation, PLD4 relocates from the Golgi apparatus to CD63-positive endosomes, which are the precursors of EVs. Activated B cells show increased multivesicular body formation and intraluminal vesicle accumulation, leading to release of PLD4-containing EVs. This finding establishes activated B cells as a key source of circulating PLD4 and suggests potential roles in intercellular communication and paracrine immune regulation that extend beyond the cell-autonomous functions previously characterized.

Role in Immune Regulation

Regulation of TLR9 Signaling

The primary immune function of PLD4 is to limit activation of Toll-like receptor 9 (TLR9), an endosomal receptor that recognizes unmethylated CpG-containing single-stranded DNA [gavin-2018-exonuclease-abstract]. By degrading ssDNA in endolysosomes, PLD4 prevents excessive or inappropriate TLR9 activation that would otherwise trigger inflammatory responses. PLD4-deficient mice develop an inflammatory disease characterized by elevated interferon-gamma (IFN-gamma), splenomegaly, and altered dendritic cell responses to TLR9 ligands [gavin-2018-exonuclease-abstract]. Critically, all pathological features in PLD4-deficient mice depend on TLR9, as genetic deletion of TLR9 rescues the phenotype.

Regulation of TLR7 Signaling

PLD4 also regulates TLR7, the endosomal sensor of single-stranded RNA [gavin-2021-dna-rna-cleavage-abstract]. Recent work has shown that PLD4 and PLD3 cooperate with the endonuclease RNase T2 to generate the specific RNA ligands that activate TLR7 [berouti-2024-tlr7-ligands-abstract]. TLR7 contains two ligand-binding pockets: pocket 1 recognizes guanosine nucleosides (specifically 2',3'-cyclic GMP), while pocket 2 binds pyrimidine-rich RNA fragments. The PLD exonucleases are essential for generating ligands for both pockets. Together with RNase T2, PLD enzymes release 2',3'-cGMP for pocket 1. Conversely, PLD exonucleases tend to stall at cytidines, leaving pyrimidine-rich fragments suitable for pocket 2 [berouti-2024-tlr7-ligands-abstract].

Functional Redundancy with PLD3

PLD4 and PLD3 show substantial functional redundancy in immune regulation. Single knockouts of either gene produce relatively mild phenotypes, but mice deficient in both PLD3 and PLD4 develop lethal hemophagocytic lymphohistiocytosis (HLH) characterized by inflammatory liver damage and massive overproduction of IFN-gamma early in life [gavin-2021-dna-rna-cleavage-abstract]. These double-knockout mice accumulate small single-stranded RNAs, and the pathology can be rescued by eliminating all endosomal TLR signaling, with partial rescue achieved by blocking either TLR7 or TLR9 individually.

Disease Associations

Monogenic Systemic Lupus Erythematosus

The most direct evidence linking PLD4 to human disease comes from the identification of loss-of-function mutations causing monogenic systemic lupus erythematosus (SLE) [wang-2025-sle-mutations-abstract]. Five patients carrying biallelic PLD4 mutations have been identified, all presenting with classic SLE features. The mutations impair single-stranded nucleic acid exonuclease activity and lead to excessive activation of TLR7 and TLR9. Downstream inflammatory signaling pathways, especially type I interferon signaling, are hyperactivated in patient dendritic cells. Mouse models lacking Pld4 recapitulate key features, including autoimmunity with cell-intrinsic expansion of plasmacytoid dendritic cells and plasma cells [wang-2025-sle-mutations-abstract].

Importantly, Pld4-deficient mice respond to the JAK inhibitor baricitinib, which blocks type I interferon signaling downstream of the JAK-STAT pathway [wang-2025-sle-mutations-abstract]. This finding suggests that targeting the interferon pathway may be a viable therapeutic strategy for patients with PLD4 deficiency, representing an example of genotype-directed personalized medicine in autoimmunity.

Polygenic Autoimmune Disease Associations

Genome-wide association studies (GWAS) have linked common variants in PLD4 to susceptibility to multiple autoimmune diseases. The SNP rs2841277 in PLD4 was first identified as a susceptibility locus for systemic sclerosis in a Japanese population, with significant association (P = 0.00017) [terao-2013-systemic-sclerosis-abstract]. This same variant has been associated with rheumatoid arthritis in both Japanese and Taiwanese populations, where the minor allele C confers reduced risk (OR = 0.6, p = 3.0 x 10^-6) [chen-2017-rheumatoid-arthritis-abstract].

A meta-analysis of SLE genetics identified PLD4 as a genetic determinant of disease, with the risk allele associated with anti-dsDNA antibody production [akizuki-2019-sle-genetic-abstract]. Mouse studies in this work demonstrated that Pld4 mutant mice develop autoimmune phenotypes compatible with lupus, including splenomegaly, lymphadenopathy, and autoantibody production.

Interestingly, the association of PLD4 rs2841277 with systemic sclerosis was not replicated in a European American population, suggesting genetic heterogeneity between ancestral groups that may reflect different linkage disequilibrium patterns or gene-environment interactions.

Neurodegenerative Disease Connections

While PLD3 has stronger associations with neurodegenerative diseases (particularly Alzheimer's disease), PLD4's role in BMP synthesis suggests potential relevance to brain health [singh-2024-bmp-synthase-abstract]. Disease-associated PLD3 mutations (including the V232M variant associated with Alzheimer's risk) reduce BMP synthesis by more than 50%, leading to ganglioside accumulation and lysosomal dysfunction. Given the functional overlap between PLD3 and PLD4, and PLD4's expression in microglia, it is plausible that PLD4 variants could also contribute to neurodegeneration, though this remains to be definitively established.

Role in Microglial Function and Brain Development

Early Characterization Studies

Before the discovery of PLD4's exonuclease activity in 2018, initial characterization studies in 2010 established PLD4 as a transmembrane glycoprotein with restricted expression in microglia and spleen [yoshikawa-2010-pld4-characterization-abstract]. Notably, these early studies could detect no phospholipase D activity despite the presence of HKD motifs, foreshadowing the later revelation that PLD4 catalyzes a completely different reaction. The protein was found to be heavily N-glycosylated, existing as approximately 70 kDa in glycosylated form versus 46 kDa after enzymatic deglycosylation. Expression was detected in Iba1-positive microglia in brain white matter during the first postnatal week, with peak expression at postnatal day 7, suggesting a role in early brain development.

Involvement in Microglial Phagocytosis

Subsequent studies revealed that PLD4 participates in microglial phagocytic activity [otani-2011-microglial-phagocytosis-abstract]. In resting microglia, PLD4 localizes primarily to the nucleus, but upon stimulation with lipopolysaccharide (LPS), expression increases and the protein accumulates in early phagosomes during active phagocytosis. siRNA knockdown experiments in MG6 microglial cells demonstrated that reducing PLD4 expression significantly decreased the number of phagocytic cells and reduced overall phagocytic activity. These findings implicated PLD4 in microglial innate immune functions, though at the time, before its exonuclease activity was known, the mechanism remained unclear.

Transient Role in Myelination

Studies using PLD4-deficient mice revealed an unexpected role in myelination during brain development [chiba-2016-myelination-abstract]. Wild-type microglia in early postnatal brain showed strong CD68 immunoreactivity (a marker of activated microglia), whereas PLD4-deficient microglia had weak CD68 signals, indicating that loss of PLD4 affects microglial activation state. More striking was the observation that PLD4-deficient mice exhibited a mild but significant delay in myelination in the cerebellum and corpus callosum at postnatal days 5 and 7. However, this difference disappeared by postnatal day 10, suggesting that microglia play a transient, PLD4-dependent role in supporting early myelination. The precise mechanism connecting PLD4 to myelination remains unclear but may involve microglial phagocytosis of myelin debris or secretion of factors that support oligodendrocyte maturation.

Structural Biology

Overall Architecture

The crystal structures of PLD4 reveal important insights into its function [yuan-2024-structure-abstract]. The protein forms an intra-chain pseudo-dimer structure, with two halves each containing an HKD motif coming together to form a single active site at their interface. This architecture is characteristic of the PLD superfamily and positions the two histidine residues for their sequential roles in the two-step catalytic mechanism.

The catalytic cavity is narrow and deep, with the oligonucleotide substrate entering almost vertically. The substrate bends into an L-shape at the phosphodiester bond between the 5' end and the second nucleotide, a conformation that appears important for product release. This substrate-capturing mechanism is distinctive among known nucleases.

Structural Basis for Disease Mutations

Structural analysis of disease-associated mutations provides mechanistic insights into pathogenesis [yuan-2024-structure-abstract]. Mutations in the catalytic domain reduce enzymatic activity either by directly affecting the active site or by destabilizing the protein fold. These findings support a loss-of-function mechanism whereby reduced nucleic acid degradation leads to TLR hyperactivation and downstream inflammatory consequences.

Recent Cryo-EM Structural Advances

More recent cryo-EM studies have captured additional catalytic states of PLD3 and PLD4, providing deeper mechanistic understanding [hirano-2025-cryoem-structure-abstract]. These structures reveal that the substrate-binding pocket is dynamic, with conformational changes in key loops (particularly the Phe335-containing loop in PLD3) depending on the substrate. The studies also captured metastable states that appear during substrate rearrangement following product release, illuminating the complete catalytic cycle of these endolysosomal exonucleases.

Therapeutic Development and Biomarker Potential

Small Molecule Modulators

The identification of PLD4 as an immunoregulatory enzyme has spurred interest in developing small molecule modulators for therapeutic applications [shirey-2021-pld4-modulators-abstract]. A high-throughput fluorescence enzymatic assay was developed to screen compound libraries for PLD3 and PLD4 modulators. Screening of 17,952 compounds identified both inhibitors and activators with selectivity for PLD4 over PLD3. These findings establish the feasibility of pharmacologically targeting PLD4 and provide chemical scaffolds for further development. Potential therapeutic applications span multiple areas: PLD4 inhibitors could enhance nucleic acid sensing for cancer immunotherapy or vaccine adjuvant effects, while PLD4 activators or gene therapy approaches could suppress excessive TLR activation in autoimmune diseases. A phase I clinical trial testing PLD4 inhibitors for systemic lupus erythematosus is reportedly ongoing in Japan, representing the first attempt to translate PLD4 biology into clinical application.

PLD4 as a Cancer Biomarker

Beyond autoimmune diseases, PLD4 has emerged as a promising biomarker for blastic plasmacytoid dendritic cell neoplasm (BPDCN), a rare and aggressive hematological malignancy derived from plasmacytoid dendritic cell precursors. Given that pDCs express high levels of PLD4, this marker shows excellent diagnostic performance with 100% sensitivity and 83.3% specificity, outperforming the established CD123 marker. PLD4 expression remains consistently elevated across bone marrow, peripheral blood, and skin tissues in BPDCN patients, providing clear differentiation from other hematological malignancies. These findings suggest that PLD4 could serve both as a diagnostic marker and potentially as a therapeutic target for BPDCN.

Open Questions

Several important questions remain regarding PLD4 biology:

1. Relative contribution of exonuclease versus BMP synthase functions to disease: The dual enzymatic activities of PLD4 raise the question of which function is more critical in different pathological contexts. For autoimmune diseases, the exonuclease activity appears paramount, but the relative contributions to lysosomal storage phenotypes require further investigation.

2. Mechanism of dual activity: How a single protein catalyzes two biochemically distinct reactions (phosphodiester bond hydrolysis in nucleic acids versus transphosphatidylation in lipids) using the same active site residues remains incompletely understood. Structural studies of PLD4 with lipid substrates would be informative.

3. Tissue-specific compensation between PLD3 and PLD4: Why does PLD3 dominate BMP synthesis in brain while PLD4 dominates in spleen? The mechanisms underlying tissue-specific functional dominance despite structural similarity are unclear.

4. Therapeutic targeting: Could modulation of PLD4 activity provide therapeutic benefit in autoimmune disease (by enhancing activity) or cancer immunotherapy (by reducing activity to potentiate nucleic acid sensing)? Small molecule modulators of PLD4 have been identified in high-throughput screens but their therapeutic potential remains to be explored.

5. Role in other cell types: While pDCs and microglia express high levels of PLD4, the function of PLD4 in other cell types expressing lower levels remains poorly characterized.

6. Evolutionary conservation: Understanding when and how PLD4 diverged from PLD3 and whether organisms lacking PLD4 have alternative mechanisms for the same functions could provide additional insights into its essential roles.

References

• [gavin-2018-exonuclease-abstract] Gavin AL, Huang D, Huber C, et al. PLD3 and PLD4 are single-stranded acid exonucleases that regulate endosomal nucleic-acid sensing. Nat Immunol. 2018;19(9):942-953. PMID: 30111894. DOI: 10.1038/s41590-018-0179-y

• [singh-2024-bmp-synthase-abstract] Singh S, Dransfeld UE, Ambaw YA, et al. PLD3 and PLD4 synthesize S,S-BMP, a key phospholipid enabling lipid degradation in lysosomes. Cell. 2024;187(24):6820-6834.e24. PMID: 39423811. DOI: 10.1016/j.cell.2024.09.036

• [yuan-2024-structure-abstract] Yuan M, Peng L, Huang D, et al. Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4. Structure. 2024;32(6):766-779.e7. PMID: 38537643. DOI: 10.1016/j.str.2024.02.019

• [gavin-2021-dna-rna-cleavage-abstract] Gavin AL, Huang D, Blane TR, et al. Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors. Nat Commun. 2021;12(1):5874. PMID: 34620855. DOI: 10.1038/s41467-021-26150-w

• [wang-2025-sle-mutations-abstract] Wang Q, Zhu H, Sun X, et al. Loss-of-function mutations in PLD4 lead to systemic lupus erythematosus. Nature. 2025;647(8089):498-505. PMID: 40931063. DOI: 10.1038/s41586-025-09513-x

• [terao-2013-systemic-sclerosis-abstract] Terao C, Ohmura K, Kawaguchi Y, et al. PLD4 as a novel susceptibility gene for systemic sclerosis in a Japanese population. Arthritis Rheum. 2013;65(2):472-480. PMID: 23124809. DOI: 10.1002/art.37777

• [berouti-2024-tlr7-ligands-abstract] Bérouti M, Lammens K, Heiss M, et al. Lysosomal endonuclease RNase T2 and PLD exonucleases cooperatively generate RNA ligands for TLR7 activation. Immunity. 2024;57(7):1482-1496.e8. PMID: 38697119. DOI: 10.1016/j.immuni.2024.04.010

• [akizuki-2019-sle-genetic-abstract] Akizuki S, Ishigaki K, Kochi Y, et al. PLD4 is a genetic determinant to systemic lupus erythematosus and involved in murine autoimmune phenotypes. Ann Rheum Dis. 2019;78(4):509-518. PMID: 30679154. DOI: 10.1136/annrheumdis-2018-214116

• [chen-2017-rheumatoid-arthritis-abstract] Chen WC, Wang WC, Okada Y, et al. rs2841277 (PLD4) is associated with susceptibility and rs4672495 is associated with disease activity in rheumatoid arthritis. Oncotarget. 2017;8(38):64180-64190. PMID: 28969061. DOI: 10.18632/oncotarget.19419

• [yoshikawa-2010-pld4-characterization-abstract] Yoshikawa F, Banno Y, Otani Y, et al. Phospholipase D Family Member 4, a Transmembrane Glycoprotein with No Phospholipase D Activity, Expression in Spleen and Early Postnatal Microglia. PLoS ONE. 2010;5(11):e13932. PMID: 21085684. DOI: 10.1371/journal.pone.0013932

• [otani-2011-microglial-phagocytosis-abstract] Otani Y, Yamaguchi Y, Sato Y, et al. PLD4 is involved in phagocytosis of microglia: expression and localization changes of PLD4 are correlated with activation state of microglia. PLoS ONE. 2011;6(11):e27544. PMID: 22102906. DOI: 10.1371/journal.pone.0027544

• [chiba-2016-myelination-abstract] Chiba T, Otani Y, Yamaguchi Y, et al. Microglial phospholipase D4 deficiency influences myelination during brain development. Proc Jpn Acad Ser B Phys Biol Sci. 2016;92(7):237-254. PMID: 27477458. DOI: 10.2183/pjab.92.237

• [shirey-2021-pld4-modulators-abstract] Shirey RJ, Turner LD, Lairson LL, Janda KD. Modulators of immunoregulatory exonucleases PLD3 and PLD4 identified by high-throughput screen. Bioorg Med Chem Lett. 2021;49:128293. PMID: 34332037. DOI: 10.1016/j.bmcl.2021.128293

• [hirano-2025-cryoem-structure-abstract] Hirano Y, Ezaki W, Sato R, et al. Mechanistic insights into single-stranded DNA degradation by lysosomal exonucleases PLD3 and PLD4 from structural snapshots. Nat Commun. 2025;16:11431. DOI: 10.1038/s41467-025-66261-2

• [betsuyaku-2025-extracellular-vesicles-abstract] Betsuyaku T, Akizuki S, Liu Y, Morinobu A. Activated human B cells produce phospholipase D4-containing extracellular vesicles. PLoS ONE. 2025. DOI: 10.1371/journal.pone.0329832

Citations

1. akizuki-2019-sle-genetic-abstract.md
2. berouti-2024-tlr7-ligands-abstract.md
3. betsuyaku-2025-extracellular-vesicles-abstract.md
4. chen-2017-rheumatoid-arthritis-abstract.md
5. chiba-2016-myelination-abstract.md
6. gavin-2018-exonuclease-abstract.md
7. gavin-2018-exonuclease-summary.md
8. gavin-2021-dna-rna-cleavage-abstract.md
9. hirano-2025-cryoem-structure-abstract.md
10. otani-2011-microglial-phagocytosis-abstract.md
11. shirey-2021-pld4-modulators-abstract.md
12. singh-2024-bmp-synthase-abstract.md
13. singh-2024-bmp-synthase-summary.md
14. terao-2013-systemic-sclerosis-abstract.md
15. wang-2025-sle-mutations-abstract.md
16. wang-2025-sle-mutations-summary.md
17. yoshikawa-2010-pld4-characterization-abstract.md
18. yuan-2024-structure-abstract.md

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organism_full: Homo sapiens (Human).
protein_family: Belongs to the phospholipase D family. .
protein_domains: Diverse_PLD-related. (IPR050874); PLDc_3. (IPR032803); PLipase_D/transphosphatidylase.
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: Q96BZ4
• Protein Description: RecName: Full=5'-3' exonuclease PLD4 {ECO:0000305}; EC=3.1.16.1 {ECO:0000250|UniProtKB:Q8BG07}; AltName: Full=(S,S)-bis(monoacylglycero)phosphate synthase PLD4 {ECO:0000305|PubMed:39423811}; EC=3.1.4.- {ECO:0000269|PubMed:39423811}; AltName: Full=Phospholipase D family member 4; AltName: Full=Phospholipase D4;
• Gene Information: Name=PLD4 {ECO:0000303|PubMed:30111894, ECO:0000312|HGNC:HGNC:23792}; Synonyms=C14orf175; ORFNames=UNQ2488/PRO5775;
• Organism (full): Homo sapiens (Human).
• Protein Family: Belongs to the phospholipase D family. .
• Key Domains: Diverse_PLD-related. (IPR050874); PLDc_3. (IPR032803); PLipase_D/transphosphatidylase. (IPR001736); PLDc_3 (PF13918)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "PLD4" matches the protein description above
2. Verify the organism is correct: Homo sapiens (Human).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'PLD4' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene PLD4 (gene ID: PLD4, UniProt: Q96BZ4) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: Q96BZ4
• Protein Description: RecName: Full=5'-3' exonuclease PLD4 {ECO:0000305}; EC=3.1.16.1 {ECO:0000250|UniProtKB:Q8BG07}; AltName: Full=(S,S)-bis(monoacylglycero)phosphate synthase PLD4 {ECO:0000305|PubMed:39423811}; EC=3.1.4.- {ECO:0000269|PubMed:39423811}; AltName: Full=Phospholipase D family member 4; AltName: Full=Phospholipase D4;
• Gene Information: Name=PLD4 {ECO:0000303|PubMed:30111894, ECO:0000312|HGNC:HGNC:23792}; Synonyms=C14orf175; ORFNames=UNQ2488/PRO5775;
• Organism (full): Homo sapiens (Human).
• Protein Family: Belongs to the phospholipase D family. .
• Key Domains: Diverse_PLD-related. (IPR050874); PLDc_3. (IPR032803); PLipase_D/transphosphatidylase. (IPR001736); PLDc_3 (PF13918)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "PLD4" matches the protein description above
2. Verify the organism is correct: Homo sapiens (Human).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'PLD4' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene PLD4 (gene ID: PLD4, UniProt: Q96BZ4) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Plan and verification
- Identity: The target is human PLD4 (gene symbol PLD4; UniProt Q96BZ4), a type II membrane protein and member of the phospholipase D superfamily. Structural and functional studies characterize PLD4’s luminal catalytic domain and its conserved PLD-family motifs (HKD/E), confirming alignment with the UniProt entry and the PLD family/domain annotations. The organism context is Homo sapiens. (yuan2024structuralandmechanistic pages 23-26, yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3)

Key concepts and definitions
- Protein family and domains: PLD4 belongs to the PLD superfamily and retains the canonical HxKxxxxD/E (HKD/E) catalytic motifs, arranged within a bilobal luminal domain that forms the catalytic center typical for PLD folds. Despite PLD-family homology, PLD4’s active site is adapted for endolysosomal catalysis, including nucleic-acid substrates. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5)
- Enzymatic activities and substrate specificity:
• Endolysosomal 5′→3′ exonuclease: PLD4 degrades single-stranded nucleic acids (ssDNA/ssRNA) from the 5′ end, limiting the availability of ligands for nucleic acid–sensing Toll-like receptors (e.g., TLR9). Biochemical assays showed cleavage of CpG ODN with phosphodiester linkages but not phosphorothioates; activity is pH-dependent and optimized for acidic endolysosomal conditions. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5, yuan2024structuralandmechanistic pages 23-26)
• S,S-BMP synthase (lysosomal lipid enzyme): In 2024, PLD4 (and PLD3) was shown to synthesize lysosomal S,S-bis(monoacylglycero)phosphate (S,S‑BMP) via a transphosphatidylation reaction using lyso‑phosphatidylglycerol (lyso‑PG) and monoacylglycerol (MAG) as substrates. Activity is favored at lysosomal pH and shows synergy when both lyso‑PG and MAG are available. S,S‑BMP is the stereochemically stabilized lysosomal BMP isomer that supports lipid catabolism. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 3-5, singh2024pld3andpld4 pages 1-3)
• Substrate specificity nuances: Structural data reveal PLD4 features a hydrophobic “clamp” (including L183/F423) near the catalytic site that influences 5′-end recognition and sterically limits some 5′-modified ssDNA substrates. For lipid synthesis, PLD4 selectively produces the S,S stereoisomer of BMP, which is more resistant to lysosomal degradation than other isomers. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019; Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (yuan2024structuralandmechanistic pages 14-17, singh2024pld3andpld4 pages 3-5)
- Cellular localization: PLD4 is synthesized as a type II transmembrane protein with an N‑terminal cytosolic tail and a luminal catalytic domain that traffics to endosomes/lysosomes, where its activities (nuclease and BMP synthase) occur. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019; Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (yuan2024structuralandmechanistic pages 23-26, singh2024pld3andpld4 pages 1-3)

Recent developments and latest research (emphasis 2023–2024)
- Structural and mechanistic insights (2024): High-resolution structures of human PLD4 (and mouse PLD3) solved in multiple states (apo, intermediate, product) define a PLD-family “link‑and‑release” ping‑pong catalytic mechanism for 5′ exonuclease activity, reveal the basic active-site environment, capture a covalent intermediate, and explain enzyme differences: PLD4’s hydrophobic clamp stabilizes the first base of the product and narrows substrate tolerance; PLD4 exhibits a lower pH optimum than PLD3, consistent with more acidic endolysosomal function. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5, yuan2024structuralandmechanistic pages 14-17)
- Discovery of lysosomal S,S‑BMP synthase activity (2024): PLD4 and PLD3 were shown to be the enzymes that synthesize S,S‑BMP in lysosomes. Purified enzymes and cell-based systems demonstrated robust BMP production via transphosphatidylation (lyso‑PG + MAG), with loss of either enzyme causing tissue-specific BMP deficits and lysosomal lipid storage phenotypes; double loss is lethal, indicating partial redundancy. Many lysosomal defects were rescued by exogenous S,S‑BMP. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 1-3, singh2024pld3andpld4 pages 7-8, singh2024pld3andpld4 pages 3-5)

Current applications and real-world implementations
- Lysosomal lipid metabolism and BMP homeostasis: The PLD4→S,S‑BMP axis is essential for intraluminal vesicle formation and lipid catabolism (e.g., gangliosides). In vivo, Pld4 knockout causes ≈80% BMP loss in spleen, with ≈2-fold accumulation of gangliosides and secondary lysosomal abnormalities (e.g., increased galectin‑3 recruitment, fewer intraluminal vesicles, modest reduction in protease activity; many phenotypes reversible by S,S‑BMP addition). These quantitative tissue lipidomics and rescue experiments provide a framework for BMP supplementation or enzyme/pathway modulation strategies as translational avenues. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 7-8)
- Endosomal nucleic-acid sensing pathway modulation: By degrading ss nucleic acids, PLD4 limits excessive TLR7/TLR9 activation, suggesting opportunities to modulate PLD4 activity (or downstream signaling) in autoimmune or inflammatory settings where nucleic-acid sensing is dysregulated. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5)

Expert opinions and analysis from authoritative sources
- 2015 review perspective: Earlier work summarized PLD4 as a microglia/myeloid-enriched type II transmembrane protein with roles in phagocytosis and autoimmune phenotypes, but with uncertain enzymology. The 2024 studies resolve this by demonstrating dual functions: nuclease activity and lysosomal S,S‑BMP synthase activity, establishing PLD4 as both an immune-regulatory nuclease and a key lysosomal lipid enzyme. (J Lipid Res, 2015; https://doi.org/10.1194/jlr.r059220; Structure, 2024; Cell, 2024) (nelson2015physiologicalandpathophysiological pages 10-12, yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3)

Statistics and data from recent studies
- Tissue lipidomics and phenotypes (mouse):
• Pld4 knockout spleen: ≈80% decrease in BMP; ≈2× increase in several gangliosides (GM3, GD1a, GD2, GT1, GT3); increased lysosomal cholesterol (~1.5×); fewer ILVs; elevated galectin‑3; many defects rescueable by S,S‑BMP. (Cell, 2024; published Nov 27, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 7-8)
• Partial redundancy/lethality: PLD3/PLD4 double knockout lethality indicates overlapping essential roles in BMP synthesis. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 7-8)
- Enzyme biochemistry:
• Transphosphatidylation synergy: ~3-fold increase in S,S‑BMP production when both 18:1 lyso‑PG and 18:1 MAG are provided at lysosomal pH. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 3-5)
• Nucleic-acid exonuclease specificity: PLD4 cleaves phosphodiester CpG ODN but not phosphorothioates; certain 5′ modifications reduce activity due to the hydrophobic clamp; PLD4 has a lower pH optimum than PLD3. (Structure, 2024; published June 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 23-26, yuan2024structuralandmechanistic pages 14-17)
- Human genetics:
• SLE association in Japanese GWAS meta-analysis: rs2582511 in the AHNAK2/PLD4 region among 14 significant loci; N=1,363 cases and 5,536 controls; the PLD4 risk allele associates with anti‑dsDNA antibody production. (Ann Rheum Dis, 2019; published April 2019; https://doi.org/10.1136/annrheumdis-2018-214116) (akizuki2019pld4isa pages 1-5)
• Autoimmune links and heterogeneity: Earlier reviews noted associations with autoimmune traits (RA, SSc), but ancestry-specific differences and negative replication in some cohorts have been reported by later studies, underscoring population heterogeneity in PLD4 associations. (J Lipid Res, 2015; https://doi.org/10.1194/jlr.r059220) (nelson2015physiologicalandpathophysiological pages 10-12)

Biological processes, localization, and pathways
- Where PLD4 functions: The luminal domain operates within endosomes/lysosomes, executing two roles—(i) nucleic-acid degradation to control TLR7/9 ligand availability and (ii) S,S‑BMP synthesis to sustain lysosomal lipid catabolism and intraluminal vesicle dynamics. (Structure, 2024; Cell, 2024) (yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3)
- Pathways:
• Nucleic acid–sensing pathways: PLD4 restrains TLR7/9 activation by degrading ss nucleic acids in acidifying endolysosomes, thereby limiting downstream NF‑κB/inflammatory signaling. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5)
• Lysosomal lipid metabolism: PLD4-derived S,S‑BMP is a key cofactor for lipid hydrolases and supports ganglioside/cholesterol catabolism within lysosomes; loss of PLD4 reduces BMP pools and causes secondary lipid storage. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 1-3, singh2024pld3andpld4 pages 7-8)

Disease associations and mechanistic links
- Systemic lupus erythematosus (SLE): Genetic association at the AHNAK2/PLD4 locus in Japanese GWAS (rs2582511) correlates with anti‑dsDNA autoantibodies; mouse Pld4 mutants display SLE-like phenotypes (splenomegaly, lymphadenopathy, B‑cell expansion, elevated BAFF, ANA/anti‑dsDNA), providing mechanistic support for PLD4’s role at the intersection of nucleic-acid handling and immune activation. (Ann Rheum Dis, 2019; https://doi.org/10.1136/annrheumdis-2018-214116) (akizuki2019pld4isa pages 1-5)
- Autoimmunity more broadly: Earlier literature linked PLD4 to immune hyperactivation and autoimmune traits including RA/SSc, although effect sizes and replicability vary by ancestry; the recent mechanistic work (nuclease and BMP synthase activities) offers a coherent framework: excess TLR signaling and impaired lysosomal lipid catabolism may cooperate to drive immune dysregulation when PLD4 is deficient or variant. (J Lipid Res, 2015; https://doi.org/10.1194/jlr.r059220; Structure, 2024; Cell, 2024) (nelson2015physiologicalandpathophysiological pages 10-12, yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3)

Therapeutic and translational implications
- Two mechanistic axes emerge:
1) Nucleic acid–TLR axis: Enhancing PLD4 nuclease function or dampening downstream TLR/type I IFN signaling could mitigate autoimmunity driven by endolysosomal self‑nucleic acids. (Structure, 2024; https://doi.org/10.1016/j.str.2024.02.019) (yuan2024structuralandmechanistic pages 1-5)
2) Lysosomal lipid axis: Restoring S,S‑BMP levels (e.g., via lipid supplementation, substrate modulation, or enhancing PLD4/PLD3 activity) may correct lysosomal storage phenotypes and organelle dysfunction. Rescue by exogenous S,S‑BMP in PLD-deficient models supports feasibility. (Cell, 2024; https://doi.org/10.1016/j.cell.2024.09.036) (singh2024pld3andpld4 pages 7-8)

Embedded evidence summary
| Topic | Key Finding | Evidence Details (metrics/statistics) | Source (journal, year, DOI) |
|---|---|---|---|
| Identity / Nomenclature | Human PLD4 (gene PLD4), UniProt Q96BZ4; type II membrane with luminal catalytic domain | Luminal domain residues ~60–506 used for structural studies; annotated as PLD family member (UniProt Q96BZ4). (yuan2024structuralandmechanistic pages 23-26) | Structure, 2024; DOI: 10.1016/j.str.2024.02.019 |
| Domain architecture (HKD motifs) | Contains conserved HxKxxxxD/E ("HKD/E") motifs of PLD family | PLD-family split-catalytic HKD/E motifs retained; supports catalytic center formation typical of PLD enzymes. (yuan2024structuralandmechanistic pages 1-5) | Structure, 2024; DOI: 10.1016/j.str.2024.02.019 |
| Subcellular localization | Localizes to endolysosomal/lysosomal compartments as a type II transmembrane protein | Synthesized as type II membrane protein with luminal/extracytosolic catalytic domain trafficked to endolysosomes/lysosomes. (yuan2024structuralandmechanistic pages 23-26, singh2024pld3andpld4 pages 1-3) | Structure, 2024; Cell, 2024; DOIs: 10.1016/j.str.2024.02.019, 10.1016/j.cell.2024.09.036 |
| Enzymatic activity 1: 5'→3' exonuclease | PLD4 acts as an endolysosomal 5'-to-3' exonuclease on single-stranded nucleic acids | Structural + biochemical assays show 5'→3' exonuclease activity on ssDNA/ssRNA; activity degrades CpG ODN and limits endosomal TLR ligand availability. (yuan2024structuralandmechanistic pages 1-5, yuan2024structuralandmechanistic pages 23-26) | Structure, 2024; DOI: 10.1016/j.str.2024.02.019 |
| Enzymatic activity 2: S,S‑BMP synthase | PLD4 (with PLD3) catalyzes synthesis of stereospecific S,S‑BMP via transphosphatidylation (lyso‑PG + MAG) at lysosomal pH | Purified enzymes catalyze S,S‑BMP formation in vitro; PLD4 KO spleen shows ≈80% decrease in BMP levels; enzymatic activity favored at lysosomal pH and with lyso‑PG + MAG substrates. (singh2024pld3andpld4 pages 7-8, singh2024pld3andpld4 pages 1-3) | Cell, 2024; DOI: 10.1016/j.cell.2024.09.036 |
| Substrate specificity (nucleic acid vs lipid; stereoselectivity) | Nuclease activity targets ss nucleic acids (5' end effects); lipid activity uses specific lyso‑PG + MAG species; S,S stereochemistry confers lysosomal stability | PLD4 cannot accommodate certain 5'-modified ssDNA (steric clash); lipid activity shows strong product formation with 18:1 lyso‑PG + 18:1 MAG and produces S,S stereoisomer resistant to lysosomal hydrolysis. (yuan2024structuralandmechanistic pages 23-26, singh2024pld3andpld4 pages 3-5) | Structure, 2024; Cell, 2024; DOIs: 10.1016/j.str.2024.02.019, 10.1016/j.cell.2024.09.036 |
| Structural / mechanistic insights | PLD4 contains a hydrophobic clamp (e.g., L183) that stabilizes substrate/product; captures "link‑and‑release" mechanism and pH-dependent activity differences vs PLD3 | High-resolution structures show intra-chain dimer topology, observed covalent intermediate, and an L183/F423 clamp that restricts substrate access; PLD4 has a lower pH optimum than PLD3. (yuan2024structuralandmechanistic pages 14-17, yuan2024structuralandmechanistic pages 1-5) | Structure, 2024; DOI: 10.1016/j.str.2024.02.019 |
| Pathways: nucleic acid–sensing TLRs | PLD4 limits endolysosomal TLR7/TLR9 activation by degrading nucleic-acid ligands | Biochemical and reporter assays: PLD3/4 nucleolytic activity reduces TLR9-driven NF-κB signalling; loss-of-function increases ligand availability and inflammatory signalling. (yuan2024structuralandmechanistic pages 1-5) | Structure, 2024; DOI: 10.1016/j.str.2024.02.019 |
| Role in lysosomal lipid metabolism | PLD4 is required to maintain BMP pools that facilitate lysosomal lipid degradation; loss causes ganglioside and cholesterol accumulation | KO models show ≈80% BMP reduction in spleen, ~2× accumulation of several gangliosides, ~1.5× increased lysosomal cholesterol, fewer intraluminal vesicles and galectin‑3 recruitment (lysosomal damage marker); many phenotypes rescueable by S,S‑BMP supplementation. (singh2024pld3andpld4 pages 7-8) | Cell, 2024; DOI: 10.1016/j.cell.2024.09.036 |
| Knockout / deficiency phenotypes (mice) | Pld4 mutant mice display autoimmune-like and lysosomal phenotypes | Mouse mutants: low body weight, splenomegaly, lymphadenopathy, B‑cell expansion, elevated BAFF, autoantibodies (ANA, anti‑dsDNA); tissue-specific lipidomic defects match expression (spleen for PLD4). (akizuki2019pld4isa pages 1-5, singh2024pld3andpld4 pages 7-8) | Ann Rheum Dis, 2019; Cell, 2024; DOIs: 10.1136/annrheumdis-2018-214116; 10.1016/j.cell.2024.09.036 |
| Human genetics / disease associations | PLD4 locus associated with SLE in Japanese GWAS; links to anti‑dsDNA and autoimmune traits; other autoimmune associations reported with population heterogeneity | GWAS meta‑analysis (1,363 SLE cases, 5,536 controls) identified AHNAK2/PLD4 region (rs2582511) and association with anti‑dsDNA; other studies report links to SSc/RA but with ancestry-specific results. (akizuki2019pld4isa pages 1-5, nelson2015physiologicalandpathophysiological pages 10-12) | Ann Rheum Dis, 2019; J Lipid Res (review), 2015; DOIs: 10.1136/annrheumdis-2018-214116; 10.1194/jlr.r059220 |
| Recent 2023–2024 advances | Structural resolution (Structure 2024) and lipid‑synthase discovery (Cell 2024) redefine PLD3/4 functions as both exonucleases and lysosomal BMP synthases | Structure (Yuan et al., 2024) elucidates nuclease mechanism; Cell (Singh et al., 2024) demonstrates S,S‑BMP synthase activity with physiological KO phenotypes (lipidomics, rescue experiments). (yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3) | Structure, 2024; Cell, 2024; DOIs: 10.1016/j.str.2024.02.019; 10.1016/j.cell.2024.09.036 |
| Applications / implications | Therapeutic strategies could target TLR/type I IFN hyperactivation or lysosomal lipid pathways (BMP homeostasis) in PLD4 deficiency | Mechanistic data link PLD4 loss to excess TLR signalling and lipid storage phenotypes; BMP supplementation or modulating downstream IFN/JAK signalling are candidate interventions (preclinical rationale). (yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3, akizuki2019pld4isa pages 1-5) | Structure, 2024; Cell, 2024; Ann Rheum Dis, 2019; DOIs as above |

Table: Concise, citable evidence summarizing identity, domains, localization, dual enzymatic activities, structural mechanisms, pathway roles, knockout phenotypes, human genetics, recent 2023–2024 advances, and therapeutic implications for human PLD4 (UniProt Q96BZ4).

Conclusion
Collectively, authoritative structural and biochemical studies now define human PLD4 (Q96BZ4) as a lysosomal type II membrane enzyme with dual functions: an acid 5′→3′ exonuclease that regulates endosomal nucleic-acid sensing, and a lipid transphosphatidylase that synthesizes S,S‑BMP, a central cofactor for lysosomal lipid catabolism. Tissue-specific mouse phenotypes, lipidomics, and genetic associations in human cohorts (with population heterogeneity) link PLD4 to autoimmunity and lysosomal dysfunction, providing concrete mechanistic entry points for translational strategies targeting TLR signaling and BMP homeostasis. (yuan2024structuralandmechanistic pages 1-5, singh2024pld3andpld4 pages 1-3, singh2024pld3andpld4 pages 7-8, akizuki2019pld4isa pages 1-5, nelson2015physiologicalandpathophysiological pages 10-12, yuan2024structuralandmechanistic pages 14-17, singh2024pld3andpld4 pages 3-5, yuan2024structuralandmechanistic pages 23-26)

References

1. (yuan2024structuralandmechanistic pages 23-26): Meng Yuan, Linghang Peng, Deli Huang, Amanda Gavin, Fangkun Luan, Jenny Tran, Ziqi Feng, Xueyong Zhu, Jeanne Matteson, Ian A. Wilson, and David Nemazee. Structural and mechanistic insights into disease-associated endolysosomal exonucleases pld3 and pld4. Structure, 32:766-779.e7, Jun 2024. URL: https://doi.org/10.1016/j.str.2024.02.019, doi:10.1016/j.str.2024.02.019. This article has 11 citations and is from a domain leading peer-reviewed journal.

2. (yuan2024structuralandmechanistic pages 1-5): Meng Yuan, Linghang Peng, Deli Huang, Amanda Gavin, Fangkun Luan, Jenny Tran, Ziqi Feng, Xueyong Zhu, Jeanne Matteson, Ian A. Wilson, and David Nemazee. Structural and mechanistic insights into disease-associated endolysosomal exonucleases pld3 and pld4. Structure, 32:766-779.e7, Jun 2024. URL: https://doi.org/10.1016/j.str.2024.02.019, doi:10.1016/j.str.2024.02.019. This article has 11 citations and is from a domain leading peer-reviewed journal.

3. (singh2024pld3andpld4 pages 1-3): Shubham Singh, Ulrich E. Dransfeld, Yohannes A. Ambaw, Joshua Lopez-Scarim, Robert V. Farese, and Tobias C. Walther. Pld3 and pld4 synthesize s,s-bmp, a key phospholipid enabling lipid degradation in lysosomes. Cell, 187:6820-6834.e24, Nov 2024. URL: https://doi.org/10.1016/j.cell.2024.09.036, doi:10.1016/j.cell.2024.09.036. This article has 37 citations and is from a highest quality peer-reviewed journal.

4. (singh2024pld3andpld4 pages 3-5): Shubham Singh, Ulrich E. Dransfeld, Yohannes A. Ambaw, Joshua Lopez-Scarim, Robert V. Farese, and Tobias C. Walther. Pld3 and pld4 synthesize s,s-bmp, a key phospholipid enabling lipid degradation in lysosomes. Cell, 187:6820-6834.e24, Nov 2024. URL: https://doi.org/10.1016/j.cell.2024.09.036, doi:10.1016/j.cell.2024.09.036. This article has 37 citations and is from a highest quality peer-reviewed journal.

5. (yuan2024structuralandmechanistic pages 14-17): Meng Yuan, Linghang Peng, Deli Huang, Amanda Gavin, Fangkun Luan, Jenny Tran, Ziqi Feng, Xueyong Zhu, Jeanne Matteson, Ian A. Wilson, and David Nemazee. Structural and mechanistic insights into disease-associated endolysosomal exonucleases pld3 and pld4. Structure, 32:766-779.e7, Jun 2024. URL: https://doi.org/10.1016/j.str.2024.02.019, doi:10.1016/j.str.2024.02.019. This article has 11 citations and is from a domain leading peer-reviewed journal.

6. (singh2024pld3andpld4 pages 7-8): Shubham Singh, Ulrich E. Dransfeld, Yohannes A. Ambaw, Joshua Lopez-Scarim, Robert V. Farese, and Tobias C. Walther. Pld3 and pld4 synthesize s,s-bmp, a key phospholipid enabling lipid degradation in lysosomes. Cell, 187:6820-6834.e24, Nov 2024. URL: https://doi.org/10.1016/j.cell.2024.09.036, doi:10.1016/j.cell.2024.09.036. This article has 37 citations and is from a highest quality peer-reviewed journal.

7. (nelson2015physiologicalandpathophysiological pages 10-12): Rochelle K. Nelson and Michael A. Frohman. Physiological and pathophysiological roles for phospholipase d. Journal of Lipid Research, 56:2229-2237, Dec 2015. URL: https://doi.org/10.1194/jlr.r059220, doi:10.1194/jlr.r059220. This article has 125 citations and is from a peer-reviewed journal.

8. (akizuki2019pld4isa pages 1-5): Shuji Akizuki, Kazuyoshi Ishigaki, Yuta Kochi, Sze-Ming Law, Keitaro Matsuo, Koichiro Ohmura, Akari Suzuki, Manabu Nakayama, Yusuke Iizuka, Haruhiko Koseki, Osamu Ohara, Jun Hirata, Yoichiro Kamatani, Fumihiko Matsuda, Takayuki Sumida, Kazuhiko Yamamoto, Yukinori Okada, Tsuneyo Mimori, and Chikashi Terao. Pld4 is a genetic determinant to systemic lupus erythematosus and involved in murine autoimmune phenotypes. Annals of the Rheumatic Diseases, 78:509-518, Apr 2019. URL: https://doi.org/10.1136/annrheumdis-2018-214116, doi:10.1136/annrheumdis-2018-214116. This article has 56 citations and is from a highest quality peer-reviewed journal.

Citations

1. yuan2024structuralandmechanistic pages 1-5
2. nelson2015physiologicalandpathophysiological pages 10-12
3. yuan2024structuralandmechanistic pages 23-26
4. yuan2024structuralandmechanistic pages 14-17
5. https://doi.org/10.1016/j.str.2024.02.019
6. https://doi.org/10.1016/j.cell.2024.09.036
7. https://doi.org/10.1016/j.str.2024.02.019;
8. https://doi.org/10.1194/jlr.r059220;
9. https://doi.org/10.1136/annrheumdis-2018-214116
10. https://doi.org/10.1194/jlr.r059220
11. https://doi.org/10.1016/j.str.2024.02.019,
12. https://doi.org/10.1016/j.cell.2024.09.036,
13. https://doi.org/10.1194/jlr.r059220,
14. https://doi.org/10.1136/annrheumdis-2018-214116,

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Question

Gene Research for Functional Annotation

Please provide a comprehensive research report on the gene PLD4 (Q96BZ4) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

PLD4 (Phospholipase D Family Member 4) – Overview and Function

Family Context and Structure

PLD4 (gene ID 122618, UniProt Q96BZ4) is a member of the phospholipase D superfamily, which is characterized by a conserved catalytic HKD motif (His-x-Lys-xxxx-Asp) important for phosphodiester bond hydrolysis (pmc.ncbi.nlm.nih.gov). Unlike classical PLD1/PLD2 enzymes (which hydrolyze phosphatidylcholine to choline and phosphatidic acid), PLD4 is a non-classical PLD that lacks the N-terminal PX and PH domains used for membrane targeting in PLD1/2 (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Instead, PLD4 has a short N-terminal cytosolic tail, a single transmembrane segment, and a large C-terminal domain with multiple N-glycosylation sites (pmc.ncbi.nlm.nih.gov). Glycosylation and biochemical analyses confirm that PLD4 is a type II membrane glycoprotein localized to the endolysosomal system (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In cells, PLD4 resides in late endosomes/lysosomes – subcellular compartments distinct from where PLD1/2 operate (pmc.ncbi.nlm.nih.gov). Early studies noted PLD4’s absence of classical PLD lipid-hydrolyzing activity, as it failed to convert phosphatidylcholine to phosphatidic acid in enzyme assays (pmc.ncbi.nlm.nih.gov). This suggested that despite its name, PLD4’s biochemical function differs fundamentally from the lipid-degrading activity of PLD1/2 (pmc.ncbi.nlm.nih.gov). Indeed, PLD4 also lacks the regulatory loops present in PLD1/2, consistent with a divergent function and localization.

Structurally, PLD4’s C-terminal domain contains two HKD motifs that fold into an intramolecular “pseudo-dimer” – a hallmark of PLD-family enzymes (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). A recent 2023 structural analysis (bioRxiv preprint, later published 2024) resolved the crystal structure of human PLD4, revealing that its two HKD regions form a single active site at their interface (pmc.ncbi.nlm.nih.gov). The active site is highly basic and binds nucleic acid substrates, rather than phospholipids (pmc.ncbi.nlm.nih.gov). Notably, PLD4 was found to harbor an extra hydrophobic clamp near the active site, which may influence substrate binding and product release (pmc.ncbi.nlm.nih.gov). Like other HKD enzymes (e.g. bacterial nucleases), PLD4’s catalytic mechanism proceeds via a two-step reaction: the enzyme first forms a covalent phosphohistidine intermediate by cleaving the 5′-phosphate from the substrate, then releases inorganic phosphate and continues degrading the nucleic acid (pmc.ncbi.nlm.nih.gov). This “link-and-release” mechanism endows PLD4 with a 5′-phosphatase activity in addition to its primary nuclease function (pmc.ncbi.nlm.nih.gov). In sum, PLD4’s sequence and structure place it in the PLD superfamily, but its domain architecture and active site adaptations are specialized for nucleic-acid substrate binding in the acidic endolysosomal milieu, rather than for lipid metabolism (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Enzymatic Activity and Substrate Specificity

Current understanding indicates that PLD4 is not a phospholipid hydrolase at all, but rather a 5′→3′ exonuclease that digests nucleic acids. Pioneering work in 2018 (Nature Immunology) showed definitively that PLD4 (and the related PLD3) can cleave single-stranded DNA in vitro, and suggested these enzymes correspond to the long-mysterious “spleen acid exonuclease” activity described decades ago (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Subsequently, biochemical assays and structural studies have elaborated this function: PLD4 catalyzes the stepwise hydrolysis of phosphodiester bonds in single-stranded DNA and RNA, releasing nucleotides from the 5′ end (pmc.ncbi.nlm.nih.gov). The enzyme exhibits a preference for unphosphorylated 5′ termini – 5′-end phosphorylation on the substrate blocks activity – consistent with its need to generate a free 5′-OH before exonucleolytic digestion (pmc.ncbi.nlm.nih.gov). Indeed, purified PLD4 was shown to degrade synthetic ssDNA and ssRNA substrates processively from their 5′ ends, and to stall if a 5′ phosphate cap is present (pmc.ncbi.nlm.nih.gov). As noted, PLD4 can overcome a 5′ phosphate by acting as a 5′ phosphatase: it transiently transfers the phosphate to a catalytic histidine (forming a 3′-phosphohistidine intermediate), thereby unblocking the substrate for further digestion (pmc.ncbi.nlm.nih.gov). This dual activity was “unexpected” when first demonstrated, and highlights PLD4’s evolved role in nucleic-acid catabolism (pmc.ncbi.nlm.nih.gov).

Importantly, PLD4’s nuclease activity is optimized for the acidic pH of lysosomes (hence the term “acid exonuclease”). The enzyme shows minimal activity at neutral pH but robustly degrades nucleic acids at endosomal pH, much like DNase II (another lysosomal acid nuclease) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Together, these properties mean that PLD4 will efficiently digest nucleic acid cargo within endolysosomal compartments. It appears to prefer single-stranded nucleic acids; double-stranded DNA is likely a poor substrate unless first denatured or processed by other nucleases. Recent enzymatic analyses (2021) confirmed that PLD4 can digest single-stranded RNA as well as DNA, expanding its substrate profile to RNA degradation (pmc.ncbi.nlm.nih.gov). In mouse tissues lacking PLD3/PLD4, researchers detected an accumulation of undegraded short single-stranded RNA fragments, consistent with the loss of an RNAse activity in lysosomes (pmc.ncbi.nlm.nih.gov). There is no evidence that PLD4 can cleave phospholipids or other lipids – earlier hypotheses that it might function like PLD1/2 were dispelled by the lack of phosphatidylcholine-hydrolyzing activity (pmc.ncbi.nlm.nih.gov) and by later mutagenesis showing PLD4’s catalytic histidines are essential for nuclease, not lipase, function (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Thus, the primary biochemical function of PLD4 is as a phosphodiesterase/exonuclease acting on nucleic acid polymers.

Localization and Expression Profile

PLD4 is a membrane-bound lysosomal enzyme that carries out its function within intracellular vesicles. As a type II transmembrane protein, PLD4’s N-terminal tail (~20 amino acids) faces the cytosol, while the large C-terminal domain (with the active site) is located in the lumen of endosomes and lysosomes (pmc.ncbi.nlm.nih.gov). This topological arrangement means PLD4 encounters nucleic acids contained inside endolysosomal compartments – for example, DNA or RNA from phagocytosed microbes, apoptotic cells, or other extracellular material. Immunocytochemistry and cell fractionation studies have confirmed PLD4’s endolysosomal localization. PLD4 co-localizes with late endosome/lysosome markers (e.g. CD68, LAMP proteins) in microglia and dendritic cells (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), and electron microscopy shows PLD4 enriched on the membranes of phagosomes and multivesicular bodies (pmc.ncbi.nlm.nih.gov). In one study of cultured microglia, PLD4 redistribution was observed upon activation – resting cells showed a perinuclear pattern, whereas after stimulation, PLD4 concentrated on phagocytic vesicles containing ingested particles (pmc.ncbi.nlm.nih.gov). Consistently, knockdown of PLD4 in microglial cells impaired phagocytosis efficiency, suggesting PLD4 normally associates with the phagolysosomal pathway to help digest engulfed material (pmc.ncbi.nlm.nih.gov).

PLD4 is predominantly expressed in immune and hematopoietic tissues. It was originally noted to be highly expressed in myeloid-lineage cells – for example, in mouse microglia (brain-resident macrophages) during early postnatal development (pmc.ncbi.nlm.nih.gov), and in spleen and thymus reticuloendothelial cells (pubmed.ncbi.nlm.nih.gov). In humans, PLD4 mRNA and protein are abundant in dendritic cells (DCs), monocytes/macrophages, and certain B cell subsets (pmc.ncbi.nlm.nih.gov). By contrast, PLD4 is low or absent in most non-immune tissues, although moderate expression has been detected in the brain and other organs (likely reflecting resident immune cells in those tissues) (pubmed.ncbi.nlm.nih.gov). Notably, dendritic cells and macrophages upregulate PLD4 upon activation. For instance, human monocyte-derived DCs show increased PLD4 expression in response to Toll-like receptor agonists (pubmed.ncbi.nlm.nih.gov), and microglia strongly induce PLD4 when activated by stimuli like LPS or during demyelination injury (pmc.ncbi.nlm.nih.gov). PLD4 expression is also inducible in B cells by specific signals: stimulation of the B-cell receptor together with TLR9 ligand drives high PLD4 levels in a subset of proliferating B cells (pmc.ncbi.nlm.nih.gov). This inducible expression pattern suggests PLD4 is part of the immune system’s toolkit for handling nucleic-acid-rich material during pathogen responses or tissue cleanup.

Interestingly, recent research has shown that PLD4 can also be found outside the cell, in the form of extracellular vesicles. A 2025 study demonstrated that activated human B cells secrete PLD4-containing exosomes (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In that study, PLD4 was detected in plasma extracellular vesicles (EVs) from healthy donors, and was traced to B cells as the source: upon activation, B cells reroute PLD4 from the Golgi to CD63⁺ multivesicular endosomes, which then release PLD4-positive exosomes into the extracellular space (pmc.ncbi.nlm.nih.gov). These findings reveal a novel mechanism by which PLD4 might act beyond the cell, potentially degrading nucleic acids in the extracellular environment or delivering regulatory signals to other cells (pmc.ncbi.nlm.nih.gov). The physiological role of extracellular PLD4 is still under investigation, but it could represent a way for immune cells to dispose of DNA/RNA debris outside cells or to modulate intercellular communication during immune responses (pmc.ncbi.nlm.nih.gov). In summary, PLD4’s functional arena is primarily the endolysosomal compartment of immune cells, though under certain conditions it can be released in vesicles, extending its reach to the extracellular milieu.

Biological Role and Pathways

PLD4’s enzymatic activity – degrading nucleic acids in endosomes – directly connects to its biological role in regulating innate immune signaling. Endosomal Toll-like receptors such as TLR9 (which senses CpG DNA) and TLR7/8 (which sense single-stranded RNA) rely on encountering their nucleic-acid ligands within endolysosomal compartments (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). PLD4 acts as a negative regulator of these TLR pathways by breaking down the DNA and RNA ligands, thus limiting their availability to the receptors. In the absence of PLD4, nucleic acid fragments persist longer in endosomes, leading to hyperactivation of TLR-mediated immune responses (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Experimental evidence for this role comes from gene knockout studies. Mice deficient in Pld4 develop a spontaneous inflammatory syndrome characterized by an enlarged spleen and elevated interferon-γ (IFN-γ) levels (pmc.ncbi.nlm.nih.gov). Notably, dendritic cells from Pld4⁻/⁻ mice show heightened sensitivity to TLR9 ligands – they produce excessive inflammatory cytokines when stimulated with unmethylated CpG DNA, compared to wild-type cells (pmc.ncbi.nlm.nih.gov). This indicates that normally PLD4 in those DCs helps degrade the CpG DNA after uptake, dampening the TLR9 signal. Macrophages lacking Pld4 similarly have exaggerated cytokine responses to TLR9 agonists (pmc.ncbi.nlm.nih.gov). Crucially, the Pld4⁻/⁻ inflammatory phenotype is TLR9-dependent: if TLR9 is knocked out in the Pld4⁻/⁻ background, many of the abnormal immune features (splenomegaly, IFN-γ production, etc.) are reversed (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This epistatic rescue demonstrates that PLD4’s protective role is largely to restrain TLR9-driven signaling. Mechanistically, PLD4 accelerates the turnover of TLR9’s ligand (CpG-rich DNA) in endolysosomes, preventing prolonged receptor engagement (pmc.ncbi.nlm.nih.gov). The same principle appears to apply to TLR7 and RNA ligands. Follow-up studies in 2021 showed that double-knockout mice lacking both Pld4 and Pld3 accumulate aberrant amounts of single-stranded RNA in endosomes and spontaneously develop a severe inflammatory disease (pmc.ncbi.nlm.nih.gov). These mice succumb early in life to a syndrome resembling hemophagocytic lymphohistiocytosis (HLH), marked by excessive cytokine release and immune cell hyperactivation (pmc.ncbi.nlm.nih.gov). Disabling endosomal TLR signaling (by mutating Unc93b1, a chaperone required for TLR7/9 function) completely rescued the survival and inflammation in Pld3⁻/⁻Pld4⁻/⁻ mice (pmc.ncbi.nlm.nih.gov). By contrast, removing just TLR9 or TLR7 individually provided only partial relief (pmc.ncbi.nlm.nih.gov). This indicates that both DNA-sensing and RNA-sensing pathways contribute to the pathology when PLD4/PLD3 are absent (pmc.ncbi.nlm.nih.gov). Furthermore, some residual interferon-stimulated gene expression in the double-knockout mice was eliminated only when the STING cytosolic DNA-sensing pathway was also disabled (pmc.ncbi.nlm.nih.gov). Taken together, these data establish PLD4 as a key homeostatic regulator of nucleic-acid sensing pathways: it ensures that self-DNA or RNA is efficiently cleared from endolysosomes, thereby preventing unintended activation of TLR7, TLR9, and even secondary DNA sensors like cGAS–STING (pmc.ncbi.nlm.nih.gov).

Beyond its role in controlling autoinflammatory signals, PLD4 also appears important for certain physiological processes involving phagocytosis and tissue remodeling. In the developing brain, for example, microglia transiently express high levels of PLD4 while they prune and phagocytose neural debris during postnatal myelination. Consistent with this, Pld4-knockout mice exhibit a delay in early myelination of white matter: at one week after birth, myelin basic protein levels are slightly reduced in Pld4⁻/⁻ cerebellum and corpus callosum, although they catch up to normal by day 10 (pubmed.ncbi.nlm.nih.gov). Histologically, microglia in Pld4⁻/⁻ brains showed lower CD68 expression (a lysosomal marker of activation) during this period (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov), implying that without PLD4, these cells were less activated and less efficient in clearing myelin debris. Researchers hypothesize that PLD4’s nuclease activity might facilitate the degradation of oligodendrocyte apoptotic bodies or DNA-containing debris during myelin formation, thereby promoting timely maturation of white matter (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). This is a more specialized context, but it underscores that PLD4-dependent nucleic acid clearance can impact developmental processes (in this case, indirectly affecting how quickly myelin ensheathment proceeds).

PLD4 may also have roles in the tumor microenvironment via macrophage function. A study of colon cancer found that PLD4 is predominantly expressed in M1-polarized tumor-associated macrophages, where it seems to support their anti-tumor activity (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). In human colon carcinoma samples, immunostaining showed PLD4 in macrophages of the stroma and lymph nodes, with higher expression correlating with early-stage tumors (pubmed.ncbi.nlm.nih.gov). In vitro, inhibiting PLD4 in M1-type macrophages reduced their secretion of pro-inflammatory cytokines and lessened their ability to inhibit cancer cell growth (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). These findings suggest that PLD4 contributes to the full activation of M1 macrophages. Although the mechanism wasn’t fully elucidated, one possibility is that PLD4 aids in degrading nucleic acids from tumor cells (or the tumor microenvironment), which could influence cytokine production or the presentation of tumor antigens. This area is still being explored, but it highlights how PLD4’s function in nucleic acid clearance could intersect with immune surveillance and inflammation in diseases beyond autoimmunity.

Clinical Significance and Recent Developments

Given its role in immune regulation, PLD4 has drawn attention as a gene associated with autoimmune and autoinflammatory diseases. Genome-wide association studies (GWAS) have linked polymorphisms in the human PLD4 gene to several immune-mediated conditions, including rheumatoid arthritis, systemic sclerosis, and systemic lupus erythematosus (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). For example, a non-coding PLD4 variant (rs2841277) was identified as a risk factor in Japanese SLE patients, and PLD4 polymorphisms have also been implicated in the severity of RA (pmc.ncbi.nlm.nih.gov). These genetic links are consistent with PLD4’s function: impaired nucleic acid clearance in immune cells could predispose individuals to inappropriate activation of TLR9/TLR7, fueling autoimmune pathology. Indeed, a 2019 study showed that a loss-of-function PLD4 mutation in mice led to lupus-like phenotypes (autoantibody production, splenomegaly), reinforcing the connection between PLD4 and lupus immunopathology (pmc.ncbi.nlm.nih.gov). On the other hand, PLD4 has also been associated with neurological disorders in some studies – notably, a rare PLD4 coding mutation was linked to a form of spinocerebellar ataxia in humans (pmc.ncbi.nlm.nih.gov). It is unclear if this neurological link is due to PLD4’s intrinsic role in microglia/brain immune cells or an unrelated function, but it underscores that PLD4’s influence may extend to multiple organ systems via immune mechanisms.

From a translational perspective, PLD4 and its sister enzyme PLD3 are being examined as potential therapeutic targets or biomarkers in inflammatory diseases. Because excess type I interferon and cytokine responses drive many autoimmune conditions, enhancing the activity of PLD4 might help curtail those responses by promoting the clearance of self-DNA/RNA. Conversely, in settings like cancer or infections where a stronger immune response is desired, temporary inhibition of PLD4 could amplify nucleic-acid–mediated immune activation (e.g. boosting TLR9 agonist effects). However, any such interventions would need to balance the risk of triggering systemic inflammation (as seen in Pld4-null mice). Encouragingly, the severe inflammatory syndrome in PLD3/4 double-knockout mice provides a tractable model (spontaneous HLH) to test therapeutics: researchers noted that this Pld3⁻/⁻Pld4⁻/⁻ mouse model is unique in developing HLH spontaneously, making it useful for evaluating treatments that modulate nucleic-acid sensing without the confounding effects of infections (pmc.ncbi.nlm.nih.gov). This model is already yielding insight – for instance, it has confirmed that blocking TLR9 with specific antibodies can ameliorate the disease even after onset of symptoms (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), supporting the idea that persistent DNA sensing drives pathology in the absence of PLD3/4.

Another recent development (2024) has been the detailed mapping of disease-associated mutations in PLD4. The 3D crystal structure of PLD4, resolved in late 2023, allowed scientists to pinpoint how certain human missense variants might impact the enzyme. For example, variants linked to autoinflammatory conditions were tested and found to reduce PLD4’s exonuclease activity or stability in vitro (pubmed.ncbi.nlm.nih.gov). Some mutations destabilized the protein fold, while others occurred near the active site and likely disrupt substrate binding or catalysis (pubmed.ncbi.nlm.nih.gov). These findings provide a molecular rationale for how genetic changes in PLD4 contribute to disease – by partially impairing the enzyme’s ability to regulate nucleic acid-sensing, they may lower the threshold for inflammation. Knowing the structural impact of these mutations could guide the design of small molecules that stabilize PLD4 or enhance its activity as a therapeutic strategy.

Lastly, it’s worth noting the interdisciplinary interest in PLD4’s function. Immunologists view PLD4 as a critical “self-tolerance” factor that prevents spontaneous interferon release (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Neurologists are examining PLD4 in the context of microglial biology and myelination (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Cancer researchers are investigating how PLD4 in macrophages influences tumor immunity (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). This convergence of research reflects the gene’s broad impact on physiological processes that involve clearing nucleic acids. In summary, human PLD4 is now understood to be a lysosomal 5′-exonuclease with a specialized role in “garbage disposal” of DNA/RNA inside immune cells. By doing so, it protects the body from unwarranted immune activation. Ongoing studies (many from 2023–2024) are refining our understanding of its mechanism – for instance, how exactly it partners with other nucleases or sensors – and exploring ways to modulate PLD4’s activity in diseases. With its unique position at the crossroads of metabolism and immunity, PLD4 exemplifies how fine-tuned control of molecular waste products (like nucleic acid debris) is crucial for preventing inflammation and maintaining homeostasis (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Citations

1. AnnotationURLCitation(end_index=419, start_index=314, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=,25%2C%2024%7D.%20The')
2. AnnotationURLCitation(end_index=785, start_index=642, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=Phospholipase%20D4%20,Furthermore%2C%20the%20members%20lack')
3. AnnotationURLCitation(end_index=958, start_index=786, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=choline%20and%20phosphatidic%20acid%20,down%20by%20deglycosylation%2C%20which%20suggests')
4. AnnotationURLCitation(end_index=1220, start_index=1111, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=,15%5D%2C%20probably%20by')
5. AnnotationURLCitation(end_index=1461, start_index=1356, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=,25%2C%2024%7D.%20The')
6. AnnotationURLCitation(end_index=1571, start_index=1462, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=,15%5D%2C%20probably%20by')
7. AnnotationURLCitation(end_index=1791, start_index=1686, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=,25%2C%2024%7D.%20The')
8. AnnotationURLCitation(end_index=2087, start_index=1959, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=Phospholipase%20D4%20,A%20primary%20cultured')
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16. AnnotationURLCitation(end_index=4423, start_index=4307, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10690185/#:~:text=%E2%80%98link,Biochemical%20and')
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23. AnnotationURLCitation(end_index=6739, start_index=6584, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10690185/#:~:text=Endolysosomal%20exonucleases%20PLD3%20and%20PLD4,phosphorylation.%20We')
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26. AnnotationURLCitation(end_index=7806, start_index=7626, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=Evidence%20is%20mounting%20that%20regulation,differing%20tissues%20and%20cellular%20compartments')
27. AnnotationURLCitation(end_index=7977, start_index=7807, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=Nucleic%20acid%20sensing%20is%20important,unmethylated%20CpG%20motifs%2C%20and%20TLR13')
28. AnnotationURLCitation(end_index=8569, start_index=8409, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=important%20inflammatory%20diseases,by%20inflammatory%20liver%20damage%20and')
29. AnnotationURLCitation(end_index=8914, start_index=8754, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=important%20inflammatory%20diseases,by%20inflammatory%20liver%20damage%20and')
30. AnnotationURLCitation(end_index=9243, start_index=9115, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=Phospholipase%20D4%20,A%20primary%20cultured')
31. AnnotationURLCitation(end_index=9460, start_index=9354, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38537643/#:~:text=interface,Biochemical%20and')
32. AnnotationURLCitation(end_index=9639, start_index=9461, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38537643/#:~:text=captured%20structures%20in%20apo%2C%20intermediate%2C,stabilizes%20substrate%20and%20could%20affect')
33. AnnotationURLCitation(end_index=10221, start_index=10116, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=,25%2C%2024%7D.%20The')
34. AnnotationURLCitation(end_index=10801, start_index=10657, title='Microglial phospholipase D4 deficiency influences myelination during brain development - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/27477458/#:~:text=Phospholipase%20D4%20,significant%20delay%20in%20myelination%20in')
35. AnnotationURLCitation(end_index=10976, start_index=10802, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=microglia%20and%20microglial%20cell%20line,pathological%20conditions%20of%20white%20matter')
36. AnnotationURLCitation(end_index=11253, start_index=11079, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=microglia%20and%20microglial%20cell%20line,pathological%20conditions%20of%20white%20matter')
37. AnnotationURLCitation(end_index=11658, start_index=11484, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=microglia%20and%20microglial%20cell%20line,pathological%20conditions%20of%20white%20matter')
38. AnnotationURLCitation(end_index=12025, start_index=11851, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=microglia%20and%20microglial%20cell%20line,pathological%20conditions%20of%20white%20matter')
39. AnnotationURLCitation(end_index=12407, start_index=12279, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=Phospholipase%20D4%20,A%20primary%20cultured')
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41. AnnotationURLCitation(end_index=12846, start_index=12676, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=functions%20of%20PLD3%20and%20PLD4,are%205%E2%80%B2%20exonucleases%20with%20properties')
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43. AnnotationURLCitation(end_index=13451, start_index=13330, title='Activated human B cells produce phospholipase D4-containing extracellular vesicles - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/40811510/#:~:text=enzyme,These%20findings%20establish%20that')
44. AnnotationURLCitation(end_index=13736, start_index=13554, title='PLD4 Is Involved in Phagocytosis of Microglia: Expression and Localization Changes of PLD4 Are Correlated with Activation State of Microglia - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3216956/#:~:text=Immunohistochemical%20analysis%20revealed%20that%20PLD4,significantly%20reduced%20the%20ratio%20of')
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50. AnnotationURLCitation(end_index=15944, start_index=15804, title='Activated human B cells produce phospholipase D4-containing extracellular vesicles - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC12352752/#:~:text=agonist,role%20in%20intercellular%20communication%20and')
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55. AnnotationURLCitation(end_index=17740, start_index=17633, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=PLD4,Macrophages%20from')
56. AnnotationURLCitation(end_index=18059, start_index=17952, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=PLD4,Macrophages%20from')
57. AnnotationURLCitation(end_index=18434, start_index=18263, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=an%20altered%20responsiveness%20of%20PLD4,that%20they%20are%205%E2%80%B2%20exonucleases')
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59. AnnotationURLCitation(end_index=18923, start_index=18810, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=Unc93b1,3d%2F3d%7D%20mutation')
60. AnnotationURLCitation(end_index=19342, start_index=19176, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=acid%20exonuclease%20or%20phosphodiesterase,turnover%20of%2C%20ligands%20of%20TLR9')
61. AnnotationURLCitation(end_index=19771, start_index=19611, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=important%20inflammatory%20diseases,by%20inflammatory%20liver%20damage%20and')
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82. AnnotationURLCitation(end_index=27449, start_index=27296, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=PLD3%20and%20PLD4%20are%20phospholipase,31%2C29%7D.%20We%20previously')
83. AnnotationURLCitation(end_index=28908, start_index=28761, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=by%20multiple%20sensors%20,the%20latter%20models%20may%20affect')
84. AnnotationURLCitation(end_index=29247, start_index=29082, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=overproduction%20of%20Interferon%20%28IFN%29,for%20the%20treatment%20of%20nucleic')
85. AnnotationURLCitation(end_index=29381, start_index=29248, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=Unc93b1,a%20minor%20contribution%20to%20pathology')
86. AnnotationURLCitation(end_index=30061, start_index=29879, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38537643/#:~:text=captured%20structures%20in%20apo%2C%20intermediate%2C,findings%20provide%20insight%20into%20therapeutic')
87. AnnotationURLCitation(end_index=30384, start_index=30202, title='Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4 - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38537643/#:~:text=captured%20structures%20in%20apo%2C%20intermediate%2C,findings%20provide%20insight%20into%20therapeutic')
88. AnnotationURLCitation(end_index=31103, start_index=30965, title='PLD3 and PLD4 are single stranded acid exonucleases that regulate endosomal nucleic acid sensing - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6105523/#:~:text=sclerosis,lethal%20early%20life%20liver%20inflammation')
89. AnnotationURLCitation(end_index=31273, start_index=31104, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=signaling%3B%20genetic%20codeficiency%20or%20antibody,driven%20inflammatory%20disease')
90. AnnotationURLCitation(end_index=31504, start_index=31360, title='Microglial phospholipase D4 deficiency influences myelination during brain development - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/27477458/#:~:text=Phospholipase%20D4%20,significant%20delay%20in%20myelination%20in')
91. AnnotationURLCitation(end_index=31645, start_index=31505, title='Microglial phospholipase D4 deficiency influences myelination during brain development - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/27477458/#:~:text=protein%20,primarily%20influences%20the%20early%20myelination')
92. AnnotationURLCitation(end_index=31863, start_index=31734, title='PLD4 promotes M1 macrophages to perform antitumor effects in colon cancer cells - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/27840999/#:~:text=However%2C%20the%20function%20of%20PLD4,siRNA%20M1')
93. AnnotationURLCitation(end_index=32027, start_index=31864, title='PLD4 promotes M1 macrophages to perform antitumor effects in colon cancer cells - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/27840999/#:~:text=clinical%20staging%20of%20colon%20cancer,process%20of%20M1%20phenotype%20macrophages')
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95. AnnotationURLCitation(end_index=33139, start_index=33016, title='Cleavage of DNA and RNA by PLD3 and PLD4 limits autoinflammatory triggering by multiple sensors - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8497607/#:~:text=Unc93b1,driven%20inflammatory%20disease')