NRROS (LRRC33) is a single-pass type I transmembrane leucine-rich repeat protein, a paralog of GARP (LRRC32), expressed mainly in myeloid cells and highly in brain microglia. Its extracellular LRR domain forms disulfide bonds with the latency-associated peptide (LAP) of proTGF-beta1, tethering latent TGF-beta1 to the cell surface as a "milieu molecule" from which mature TGF-beta1 is released by integrin alphaVbeta8 in a highly localized manner. A substantial pool of NRROS resides in the endoplasmic reticulum, where complexes with proTGF-beta1 form, and only a mature glycoform reaches the plasma membrane. NRROS-dependent TGF-beta1 activation maintains homeostatic microglia; loss-of-function variants cause infantile-onset seizures with neurodegeneration and brain calcification. Earlier proposals that it negatively regulates reactive oxygen species or directly inhibits Toll-like receptor signaling are not supported by knockout studies.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000107 | MODIFY | Summary: ER localization transferred from mouse Nrros. Reason: Correct but general; NRROS is a single-pass membrane protein in the ER membrane (term already carried with EXP evidence). Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:32197075 Both wild-type and mutant NRROS were found to co-localize with the ER marker calnexin in a similar manner PMID:32197075 NRROS is a membrane protein and has been found to localize on the endoplasmic reticulum (ER) and cell surface. |
| GO:0005783 endoplasmic reticulum | ISS GO_REF:0000024 | MODIFY | Summary: ER localization by sequence similarity to mouse. Reason: As above; refine to ER membrane. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:32197075 Both wild-type and mutant NRROS were found to co-localize with the ER marker calnexin in a similar manner |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:32197075 Bi-allelic LoF NRROS Variants Impairing Active TGF-Ξ²1 Delive... | ACCEPT | Summary: Wild-type and variant NRROS colocalize with calnexin in transfected HEK293T cells. Reason: A large fraction of NRROS resides in the ER membrane, where it forms disulfide-linked complexes with proTGF-beta1 before a minor glycoform reaches the surface; the ER is a genuine, though biosynthetic, location. Supporting Evidence: PMID:32197075 Both wild-type and mutant NRROS were found to co-localize with the ER marker calnexin in a similar manner PMID:32197075 NRROS is a membrane protein and has been found to localize on the endoplasmic reticulum (ER) and cell surface. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Automated ER membrane mapping. Reason: Consistent with experimental colocalization. Supporting Evidence: PMID:32197075 Both wild-type and mutant NRROS were found to co-localize with the ER marker calnexin in a similar manner file:human/NRROS/NRROS-uniprot.txt SUBCELLULAR LOCATION: Cell membrane {ECO:0000269|PubMed:23545260, ECO:0000269|PubMed:29909984}; Single-pass type I membrane protein {ECO:0000255}. Endoplasmic reticulum membrane {ECO:0000269|PubMed:32197075} |
| GO:0005886 plasma membrane | EXP PMID:23545260 Identification and characterization of a unique leucine-rich... | ACCEPT | Summary: Plasma membrane localization in the TLR desensitization study. Reason: Consistent with cell-surface detection of the mature glycoform complexed with proTGF-beta1. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt SUBCELLULAR LOCATION: Cell membrane {ECO:0000269|PubMed:23545260, ECO:0000269|PubMed:29909984}; Single-pass type I membrane protein {ECO:0000255}. Endoplasmic reticulum membrane {ECO:0000269|PubMed:32197075} PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA |
| GO:0005886 plasma membrane | EXP PMID:29909984 A Milieu Molecule for TGF-Ξ² Required for Microglia Function ... | ACCEPT | Summary: Cell-surface LRRC33-proTGF-beta1 complexes on microglia/macrophages. Reason: The functional pool presenting latent TGF-beta1 to integrin alphaVbeta8 is at the plasma membrane. Supporting Evidence: PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Automated plasma membrane mapping. Reason: Correct. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt SUBCELLULAR LOCATION: Cell membrane {ECO:0000269|PubMed:23545260, ECO:0000269|PubMed:29909984}; Single-pass type I membrane protein {ECO:0000255}. Endoplasmic reticulum membrane {ECO:0000269|PubMed:32197075} |
| GO:0006801 superoxide metabolic process | ISS NOT GO_REF:0000024 | ACCEPT | Summary: NOT superoxide metabolic process, by similarity to mouse. Reason: Accepting the negation: Lrrc33-/- macrophages generate ROS normally and loss of Cybb does not rescue the phenotype, refuting the original NRROS (negative regulator of ROS) hypothesis. Supporting Evidence: PMID:29909984 no difference was seen between Lrrc33β/β and WT macrophages in ROS generation either with or without LPS+ IFN-Ξ³ treatment PMID:29909984 Strikingly, the neurological phenotype was not reversed in Lrrc33β/β;Cybbβ/β mice that lacked the ability to generate reactive oxygen species. |
| GO:0006954 inflammatory response | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Inflammatory response inherited. Reason: Loss of NRROS causes reactive microglia and inflammation-associated neurodegeneration, and NRROS dampens TLR-induced cytokine output, probably as a consequence of TGF-beta1 activation; an indirect role. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling PMID:29909984 Lrrc33-/- mice lack CNS vascular abnormalities associated with deficiency in TGF-Ξ²-activating integrins but have microglia with a reactive phenotype |
| GO:0006954 inflammatory response | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Inflammatory response transferred from mouse. Reason: As above. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling |
| GO:0006954 inflammatory response | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Inflammatory response by similarity. Reason: As above. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling PMID:23545260 The LRRs of LRRC33 is required for the interaction with TLR and its inhibitory effect on NF-ΞΊB and AP-1 activation as well as cytokine production. |
| GO:0006955 immune response | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Immune response inherited. Reason: Indirect role via TGF-beta1 activation in myeloid cells; peripheral. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling |
| GO:0006955 immune response | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Immune response transferred from mouse. Reason: As above. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling |
| GO:0006955 immune response | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Immune response by similarity. Reason: As above. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt ability to inhibit TLR-mediated NF-kappa-B activation and cytokine production is probably a consequence of its role in TGF-beta-1 signaling |
| GO:0009986 cell surface | IBA GO_REF:0000033 | ACCEPT | Summary: Cell surface inherited at the GARP/LRRC33 node. Reason: Mature NRROS presents proTGF-beta1 at the cell surface. Supporting Evidence: PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA |
| GO:0009986 cell surface | IDA PMID:29909984 A Milieu Molecule for TGF-Ξ² Required for Microglia Function ... | ACCEPT | Summary: Cell-surface detection of LRRC33 and proTGF-beta1 complexes. Reason: Direct evidence. Supporting Evidence: PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA PMID:29909984 Only the 120 kDa LRRC33 band was present on the cell surface |
| GO:0009986 cell surface | IEA GO_REF:0000120 | ACCEPT | Summary: Automated cell surface. Reason: Correct. Supporting Evidence: PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA |
| GO:0014005 microglia development | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Microglia development transferred from mouse. Reason: Microglial homeostasis and identity depend on TGF-beta1 activated via NRROS; NRROS acts upstream by presenting TGF-beta, not as a developmental effector. Supporting Evidence: PMID:29909984 Lrrc33-/- mice lack CNS vascular abnormalities associated with deficiency in TGF-Ξ²-activating integrins but have microglia with a reactive phenotype |
| GO:0014005 microglia development | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Microglia development by similarity. Reason: As above. Supporting Evidence: PMID:29909984 Lrrc33-/- mice lack CNS vascular abnormalities associated with deficiency in TGF-Ξ²-activating integrins but have microglia with a reactive phenotype |
| GO:0030511 positive regulation of transforming growth factor beta receptor signaling pathway | IEA GO_REF:0000107 | ACCEPT | Summary: Positive regulation of TGF-beta receptor signaling transferred from mouse. Reason: Core role: NRROS is required for integrin-dependent activation of latent TGF-beta1 in microglia and macrophages, enabling TGF-beta receptor signaling. Supporting Evidence: PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. file:human/NRROS/NRROS-uniprot.txt Required for activation of latent TGF-beta-1 in macrophages and microglia: associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGFB1, and regulates integrin-dependent activation of TGF-beta-1 |
| GO:0030511 positive regulation of transforming growth factor beta receptor signaling pathway | ISS GO_REF:0000024 | ACCEPT | Summary: Positive regulation of TGF-beta receptor signaling by similarity. Reason: Core role. Supporting Evidence: PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. file:human/NRROS/NRROS-uniprot.txt Required for activation of latent TGF-beta-1 in macrophages and microglia: associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGFB1, and regulates integrin-dependent activation of TGF-beta-1 |
| GO:0045087 innate immune response | ISS NOT GO_REF:0000024 | ACCEPT | Summary: NOT innate immune response, by similarity to mouse. Reason: Accepting the negation as curated: the TLR- and ROS-centred innate immune roles originally proposed are not supported by knockout data, which point to TGF-beta1 presentation instead. Supporting Evidence: PMID:29909984 no difference was seen between Lrrc33β/β and WT macrophages in ROS generation either with or without LPS+ IFN-Ξ³ treatment PMID:29909984 sequence identity between LRRC33 and TLRs is too low at 15β18% to suggest a functional relationship |
| GO:0050431 transforming growth factor beta binding | IBA GO_REF:0000033 | ACCEPT | Summary: TGF-beta binding inherited at the GARP/LRRC33 node. Reason: NRROS covalently binds the LAP prodomain of proTGF-beta1, like its paralog GARP. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt Required for activation of latent TGF-beta-1 in macrophages and microglia: associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGFB1, and regulates integrin-dependent activation of TGF-beta-1 PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. |
| GO:0050431 transforming growth factor beta binding | IEA GO_REF:0000120 | ACCEPT | Summary: Automated TGF-beta binding. Reason: Core binding activity. Supporting Evidence: file:human/NRROS/NRROS-uniprot.txt Required for activation of latent TGF-beta-1 in macrophages and microglia: associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGFB1, and regulates integrin-dependent activation of TGF-beta-1 |
| GO:0050431 transforming growth factor beta binding | IPI PMID:29909984 A Milieu Molecule for TGF-Ξ² Required for Microglia Function ... | ACCEPT | Summary: LRRC33 forms disulfide-linked complexes with proTGF-beta1. Reason: Direct experimental evidence of the core binding activity. Supporting Evidence: PMID:29909984 Importantly, LRRC33 and proTGF-Ξ²1 were present on the cell surface and their expression was increased by PMA PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. |
| GO:0141069 receptor ligand inhibitor activity | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Receptor ligand inhibitor activity transferred from mouse. Reason: NRROS tethers TGF-beta1 in its latent, LAP-bound form until integrin-mediated release; while bound, the ligand cannot engage its receptor. The net biological role is to enable localized activation, so this term captures only the latency-maintaining aspect. Supporting Evidence: PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. file:human/NRROS/NRROS-uniprot.txt Required for activation of latent TGF-beta-1 in macrophages and microglia: associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGFB1, and regulates integrin-dependent activation of TGF-beta-1 |
| GO:0141069 receptor ligand inhibitor activity | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Receptor ligand inhibitor activity by similarity. Reason: As above. Supporting Evidence: PMID:29909984 LRRC33 on the surface of microglia functions as a milieu molecule and enables highly localized, integrin-Ξ±VΞ²8-dependent TGF-Ξ² activation. |
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Q: Does NRROS bind TGF-beta2 or TGF-beta3 prodomains in vivo, or is it specific to TGF-beta1?
Experiment: Surface biotinylation and glycosidase sensitivity time courses of endogenous NRROS in human iPSC-derived microglia, with and without TGFB1 co-expression, to quantify ER versus surface pools and their dependence on complex formation.
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