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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.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.
The tlr5b gene (UniProt: A0ACM8R384) encodes Toll-like receptor 5b precursor in zebrafish (Danio rerio), a member of the Toll-like receptor family of pattern recognition receptors (PRRs) (vaart2012pathogenrecognitionand pages 1-2, meijer2004expressionanalysisof pages 1-2). The protein contains characteristic structural features including leucine-rich repeats (LRRs) in the extracellular domain, a single transmembrane segment, and a cytoplasmic Toll/interleukin-1 receptor (TIR) signaling domain, consistent with the InterPro domains reported in the UniProt entry (meijer2004expressionanalysisof pages 1-2). Zebrafish possess two TLR5 paralogs, TLR5a and TLR5b, which arose through gene duplication during teleost evolution (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 6-7, meijer2004expressionanalysisof pages 9-10).
TLR5b functions as a non-enzymatic pattern recognition receptor whose primary substrate specificity is bacterial flagellin, the major structural protein of bacterial flagella (voogdt2018duplicatedtlr5of pages 1-1, yang2002tolllikereceptorsignaling pages 12-14, voogdt2018duplicatedtlr5of pages 1-2). Rather than catalyzing a biochemical reaction, TLR5b acts as a molecular sensor that recognizes this conserved pathogen-associated molecular pattern (PAMP). The receptor has been experimentally demonstrated to respond to flagellins from multiple bacterial species, including Salmonella enterica serovar Enteritidis and Pseudomonas aeruginosa (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 1-2).
The C-terminal region of flagellin directly contacts TLR5, and sequence variations in this region can influence the strength of receptor activation (yang2002tolllikereceptorsignaling pages 12-14). Different flagellar serotypes activate zebrafish TLR5 to varying degrees, indicating that ligand recognition involves specific structural requirements within the flagellin molecule (yang2002tolllikereceptorsignaling pages 12-14).
A defining characteristic that distinguishes zebrafish TLR5b from mammalian TLR5 is its requirement for heterodimerization with TLR5a to achieve functional flagellin signaling (voogdt2018duplicatedtlr5of pages 6-7, voogdt2018duplicatedtlr5of pages 5-5, voogdt2018duplicatedtlr5of pages 1-1). This represents a fundamental departure from the canonical TLR5 paradigm observed in mammals, birds, and reptiles, where TLR5 functions as a homodimer (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 6-7).
Extensive experimental evidence demonstrates that full-length TLR5b alone cannot activate NF-κB signaling in response to flagellin stimulation, even when expressed with the trafficking chaperone UNC93B1 (voogdt2018duplicatedtlr5of pages 1-2, voogdt2018duplicatedtlr5of pages 5-5). Instead, productive receptor activation requires the complementary pairing of TLR5a and TLR5b ectodomains and intracellular domains (voogdt2018duplicatedtlr5of pages 5-6, voogdt2018duplicatedtlr5of pages 6-7). Domain-swapping experiments revealed that while the N-terminal ectodomain of TLR5b can bind flagellin in a homodimeric configuration, the cytoplasmic domains must be heterodimeric for signal transduction (voogdt2018duplicatedtlr5of pages 6-7).
This obligate heterodimerization reflects subfunctionalization following gene duplication, where the two paralogs have undergone concerted coevolution to create complementary, non-interchangeable roles in a unique flagellin-sensing receptor complex (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 6-7). Morpholino-mediated knockdown of either tlr5a or tlr5b individually blocks flagellin-induced inflammatory gene activation, confirming their functional interdependence in vivo (yang2015commonandspecific pages 2-4, yang2015commonandspecific pages 1-2).
Recent studies in related cyprinid fish have revealed unexpected ligand versatility in TLR5 paralogs. In grass carp (Ctenopharyngodon idella), CiTLR5b homodimers were shown to recognize double-stranded RNA (dsRNA), including the synthetic analog poly(I:C), and to activate interferon-responsive elements during viral infection (weber2022fishtlr5develops pages 1-2, weber2022fishtlr5develops pages 2-3). This dsRNA recognition occurred independently of the flagellin-sensing TLR5a/TLR5b heterodimeric configuration and involved distinct binding sites from those used for flagellin recognition (weber2022fishtlr5develops pages 1-2).
However, this dual-ligand capability has not been directly demonstrated for zebrafish TLR5b, and functional differences between fish species necessitate caution in extrapolating these findings (weber2022fishtlr5develops pages 1-2, davis2026tolllikereceptorsin pages 12-13, voogdt2018duplicatedtlr5of pages 1-1). The canonical and experimentally validated function of zebrafish TLR5b remains bacterial flagellin recognition through heterodimerization with TLR5a (voogdt2018duplicatedtlr5of pages 1-1).
Subcellular localization studies using heterologous expression systems revealed that zebrafish TLR5b predominantly accumulates in LAMP-1-positive vesicular compartments, corresponding to late endosomes or lysosomes (voogdt2018duplicatedtlr5of pages 3-4). This intracellular localization contrasts with mammalian TLR5, which is typically found at the plasma membrane with tissue-specific polarization—apical in respiratory epithelium and basolateral in intestinal epithelium (yang2017tlr5beyondthe pages 1-2).
The intracellular localization of TLR5b was not altered by flagellin stimulation, suggesting a constitutive vesicular residence rather than ligand-induced trafficking (voogdt2018duplicatedtlr5of pages 3-4). This localization pattern is consistent with TLR5b being a transmembrane protein that requires proper cellular trafficking for function but raises questions about how extracellular bacterial flagellin accesses the receptor in these compartments.
The trafficking chaperone UNC93B1 plays a critical role in TLR5b function by facilitating receptor redistribution and enabling productive heterodimerization with TLR5a (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 3-4). UNC93B1 expression enhances flagellin-induced NF-κB activation in cell-based assays, indicating that proper receptor trafficking is essential for signal transduction (voogdt2018duplicatedtlr5of pages 1-1). This chaperone-dependent mechanism suggests that TLR5b subcellular localization is actively regulated and contributes to the specificity of immune responses.
TLR5b activates innate immune signaling primarily through the myeloid differentiation primary response protein MyD88 (li2017patternrecognitionreceptors pages 2-3, li2017patternrecognitionreceptors pages 1-2, stockhammer2009transcriptomeprofilingand pages 2-3). Following ligand-induced conformational changes in the TLR5a–TLR5b heterodimer, the cytoplasmic TIR domains recruit MyD88, which in turn engages IRAK family kinases (li2017patternrecognitionreceptors pages 2-3, yang2002tolllikereceptorsignaling pages 1-6). This initiates a signaling cascade involving TRAF6, the TAK1/TAB complex, and ultimately the IKK complex (li2017patternrecognitionreceptors pages 2-3, yang2002tolllikereceptorsignaling pages 1-6).
The signaling pathway bifurcates into two major branches:
NF-κB Activation: The IKK complex phosphorylates IκB proteins, leading to their degradation and allowing NF-κB family members (Rel, RelA, RelB, NF-κB2) to translocate to the nucleus (stockhammer2009transcriptomeprofilingand pages 11-12, voogdt2018duplicatedtlr5of pages 5-5). Reconstitution experiments with TLR5a and TLR5b in reporter cell lines directly demonstrated NF-κB luciferase activity in response to flagellin (voogdt2018duplicatedtlr5of pages 5-5, voogdt2018duplicatedtlr5of pages 6-7).
MAPK/AP-1 Activation: The TAK1 complex also activates mitogen-activated protein kinase (MAPK) cascades, including ERK, JNK, and p38, which converge on AP-1 transcription factors (Fos, Jun, ATF family members) (stockhammer2009transcriptomeprofilingand pages 11-12, yang2002tolllikereceptorsignaling pages 1-6). Infection and flagellin stimulation induce expression of fos, jun, atf3, atf4, and atf5 in zebrafish embryos (stockhammer2009transcriptomeprofilingand pages 11-12).
Functional genomic studies using morpholino-mediated TLR5a/b knockdown combined with flagellin challenge have identified specific downstream target genes:
Pro-inflammatory Cytokines: il1b (interleukin-1β) and il8 (CXCL8/interleukin-8) are rapidly induced by flagellin in a TLR5-dependent manner (stockhammer2009transcriptomeprofilingand pages 11-12, yang2015commonandspecific pages 2-4). TLR5a/b depletion significantly reduced but did not completely abolish these responses, indicating both TLR5-dependent and TLR5-independent recognition of flagellated bacteria (stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 11-11).
Chemokines: cxcl-C1c is strongly induced by flagellin, and its activation is almost completely dependent on TLR5a/b, as demonstrated by morpholino knockdown experiments (stockhammer2009transcriptomeprofilingand pages 11-12).
Matrix Metalloproteinases: mmp9 (matrix metalloproteinase 9) is among the most consistently induced genes across multiple infection models and flagellin stimulations (stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 12-13). TLR5a/b knockdown nearly abolished mmp9 induction, establishing it as a bona fide TLR5 target gene (stockhammer2009transcriptomeprofilingand pages 11-12). Mmp9 likely contributes to inflammatory cell recruitment and tissue remodeling during bacterial infection.
Negative Feedback Regulators: irak3, encoding a putative negative regulator of TLR signaling, is induced in a TLR5a/b-dependent manner, suggesting an intrinsic feedback mechanism to limit inflammatory responses (stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 10-11).
Additional genes induced by flagellin or bacterial infection include tnfa (tumor necrosis factor α), NF-κB pathway components (nfkbiaa, nfkbiab), and signaling intermediates (traf1, pik3cg, socs family members) (stockhammer2009transcriptomeprofilingand pages 11-12, yang2015commonandspecific pages 2-4).
Morpholino experiments targeting MyD88 revealed pathway-specific gene regulation. Genes such as il1b, mmp9, and irak3 require MyD88 for induction, confirming their position in the canonical TLR5–MyD88 pathway (stockhammer2009transcriptomeprofilingand pages 2-3). However, ifn1 (type I interferon) and, to some extent, il8 can be induced during Salmonella infection independently of MyD88, suggesting the existence of MyD88-independent signaling branches or compensatory pathways during whole-bacterium infection (stockhammer2009transcriptomeprofilingand pages 2-3).
Both tlr5a and tlr5b are upregulated during Salmonella typhimurium infection in zebrafish embryos (stockhammer2009transcriptomeprofilingand pages 12-13, stockhammer2009transcriptomeprofilingand pages 11-12, yang2015commonandspecific pages 2-4). Interestingly, this upregulation occurs with both wild-type flagellated strains and nonflagellated LPS mutants, indicating that TLR5 gene expression during infection is induced by broader infection-associated signals beyond direct flagellin recognition (stockhammer2009transcriptomeprofilingand pages 12-13).
Functional studies using morpholino-mediated TLR5a/b knockdown followed by bacterial challenge or purified flagellin injection demonstrated that these receptors are required for robust inflammatory responses to flagellated bacteria (stockhammer2009transcriptomeprofilingand pages 10-11, stockhammer2009transcriptomeprofilingand pages 11-12). However, during intact bacterial infection, TLR5a/b knockdown did not completely prevent inflammatory gene induction, likely because multiple PRRs (TLR2, TLR4, and others) can recognize different PAMPs from Gram-negative bacteria (stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 11-11).
TLR5 expression has been documented in zebrafish during mycobacterial infection and is induced in response to Vibrio anguillarum challenge, suggesting a broad role in recognition of diverse flagellated bacterial pathogens (tanekhy2016theroleof pages 8-10, tanekhy2016theroleof pages 13-14, meijer2004expressionanalysisof pages 1-2). The receptor likely contributes to early antibacterial innate immunity, particularly in mucosal tissues where flagellated bacteria are commonly encountered (yang2002tolllikereceptorsignaling pages 12-14).
Zebrafish TLR5a and TLR5b arose through gene duplication during teleost fish evolution (voogdt2018duplicatedtlr5of pages 1-1, meijer2004expressionanalysisof pages 9-10). Phylogenetic and synteny analyses support a tandem duplication model, where an ancestral TLR5 gene was duplicated to produce two closely linked paralogs (voogdt2018duplicatedtlr5of pages 6-7, zhang2023genomewideidentificationand pages 5-8). This duplication is distinct from the broader teleost-specific whole-genome duplication (3R) but may have been facilitated by the genomic plasticity characteristic of teleost genomes (zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 1-2).
Non-teleost vertebrates, including mammals, birds, reptiles, and the spotted gar (a basal ray-finned fish), retain only a single TLR5 ortholog that functions as a homodimeric receptor (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 6-7). The retention of both TLR5 duplicates in zebrafish, rather than loss of one copy, resulted from subfunctionalization through the duplication-degeneration-complementation (DDC) process (voogdt2018duplicatedtlr5of pages 6-7). Each paralog acquired complementary, non-redundant functions, such that both are now required for effective flagellin sensing.
Genome-wide surveys of fish TLR5 genes reveal extensive diversity across teleost lineages, reflecting different evolutionary trajectories following whole-genome duplications and lineage-specific gene losses (zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 1-2, zhang2023genomewideidentificationand pages 8-10). Some fish species possess membrane-bound TLR5 (TLR5M) and soluble TLR5 (TLR5S) forms, while others retain multiple membrane-bound paralogs designated TLR5a, TLR5b, and additional copies (TLR5Ma, TLR5Mb1, TLR5Mb2) (zhang2023genomewideidentificationand pages 5-8).
Differences at flagellin-interaction sites and potential viral-binding sites among TLR5 isoforms suggest lineage-specific functional divergence and adaptation to distinct pathogen pressures (zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 2-3). The discovery that grass carp TLR5b can recognize both flagellin (as a heterodimer) and dsRNA (as a homodimer) exemplifies this functional versatility, though whether such dual ligand recognition represents an ancestral feature or a derived innovation remains unclear (weber2022fishtlr5develops pages 1-2, weber2022fishtlr5develops pages 2-3).
The primary experimental approach for investigating TLR5b function in zebrafish has been transient morpholino-mediated knockdown (stockhammer2009transcriptomeprofilingand pages 3-4, yang2015commonandspecific pages 2-4, stockhammer2009transcriptomeprofilingand pages 4-4). Translation-blocking morpholinos targeting the 5' untranslated region of tlr5a and tlr5b mRNA have been injected into zebrafish embryos, either separately or in combination, followed by assessment of inflammatory gene expression after flagellin challenge or bacterial infection (stockhammer2009transcriptomeprofilingand pages 3-4, stockhammer2009transcriptomeprofilingand pages 11-12, yang2015commonandspecific pages 1-2).
These studies consistently demonstrate that knockdown of either tlr5a or tlr5b prevents flagellin-induced activation of target genes such as il1b, mmp9, and cxcl-C1c, supporting the functional interdependence of the two paralogs (yang2015commonandspecific pages 2-4, stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 10-11). Standard control morpholinos have been used to control for nonspecific effects (stockhammer2009transcriptomeprofilingand pages 3-4, stockhammer2009transcriptomeprofilingand pages 4-4).
However, morpholino approaches have inherent limitations, including potential off-target effects, variable knockdown efficiency, and inability to assess long-term developmental compensation. To date, stable CRISPR/Cas9-mediated tlr5b knockout lines have not been reported in the literature examined, representing an important gap in the functional characterization (stockhammer2009transcriptomeprofilingand pages 4-4, voogdt2018duplicatedtlr5of pages 6-7).
Complementary gain-of-function approaches have employed overexpression of wild-type, chimeric, and mutant TLR5 constructs in heterologous cell lines (typically HeLa or HEK293 cells) coupled with NF-κB luciferase reporter assays (voogdt2018duplicatedtlr5of pages 5-6, voogdt2018duplicatedtlr5of pages 5-5, voogdt2018duplicatedtlr5of pages 6-7). These studies have been instrumental in defining the heterodimeric architecture of zebrafish TLR5, revealing that productive signaling requires specific combinations of TLR5a and TLR5b extracellular and intracellular regions (voogdt2018duplicatedtlr5of pages 5-6, voogdt2018duplicatedtlr5of pages 6-7).
Domain-swapping experiments demonstrated that the N-terminal ectodomain of TLR5b can function in homodimeric configurations within chimeric receptors, whereas full-length native TLR5b requires heterodimerization (voogdt2018duplicatedtlr5of pages 3-4, voogdt2018duplicatedtlr5of pages 6-7). These structure–function studies have provided mechanistic insights complementary to in vivo knockdown experiments.
A comprehensive genome-wide analysis by Zhang et al. (2023) characterized TLR5 diversity across fish species, identifying three TLR5M isotypes (TLR5Ma, TLR5Mb1, TLR5Mb2) and TLR5S forms resulting from various combinations of whole-genome duplications and gene losses (zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 1-2). This work established that TLR5 copy-number variation in fishes reflects both WGD events and lineage-specific adaptations, with functional divergence at pathogen-recognition sites suggesting subfunctionalization or neofunctionalization (zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 8-10).
Weber (2022) highlighted the discovery that grass carp TLR5b can sense viral dsRNA in addition to bacterial flagellin, representing a significant expansion of TLR5 function beyond its traditional role (weber2022fishtlr5develops pages 1-2, weber2022fishtlr5develops pages 2-3). This finding challenges the established paradigm that TLR5 exclusively recognizes flagellin and raises questions about evolutionary constraints on ligand specificity. Whether zebrafish TLR5b possesses similar dual recognition capacity awaits experimental validation (weber2022fishtlr5develops pages 1-2, davis2026tolllikereceptorsin pages 12-13).
Davis et al. (2026) reviewed the translational potential of fish TLRs, including TLR5, for adjuvant-based vaccine design in aquaculture (davis2026tolllikereceptorsin pages 12-13). Flagellin's potent immunostimulatory properties through TLR5 activation make it an attractive vaccine adjuvant candidate for enhancing protective immunity against bacterial and viral pathogens in farmed fish (davis2026tolllikereceptorsin pages 12-13). The heterodimeric requirement for TLR5a/TLR5b in zebrafish suggests that both receptor components must be considered in designing TLR5-targeted immunotherapies.
| Feature | Description |
|---|---|
| Protein name and gene symbol | Toll-like receptor 5b precursor; gene symbol tlr5b (synonym tlr5); target accession UniProt A0ACM8R384. |
| Organism | Zebrafish, Danio rerio (formerly Brachydanio rerio). The literature identifies tlr5a and tlr5b as the two zebrafish counterparts of vertebrate TLR5 (li2017patternrecognitionreceptors pages 2-3, yang2015commonandspecific pages 2-4). |
| Protein family | Toll-like receptor family; an innate-immune pattern-recognition receptor and type-I single-pass transmembrane glycoprotein (vaart2012pathogenrecognitionand pages 1-2, voogdt2018duplicatedtlr5of pages 1-1, meijer2004expressionanalysisof pages 1-2). |
| Primary molecular function | Serves as a non-catalytic receptor subunit that detects bacterial flagellin in cooperation with TLR5a and converts ligand-induced receptor rearrangement into intracellular inflammatory signaling, notably NF-κB activation (voogdt2018duplicatedtlr5of pages 6-7, voogdt2018duplicatedtlr5of pages 1-1). |
| Ligand specificity | The experimentally established zebrafish ligand is bacterial flagellin, including flagellins from Salmonella enterica and Pseudomonas aeruginosa; TLR5b therefore has a recognition ligand rather than an enzymatic substrate (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 1-2). A potential second ligand class—dsRNA/poly(I:C)—has been demonstrated for grass-carp TLR5b homodimers, but direct recognition by zebrafish TLR5b has not been established and should not be transferred across species without testing (weber2022fishtlr5develops pages 1-2, davis2026tolllikereceptorsin pages 12-13, voogdt2018duplicatedtlr5of pages 1-1). |
| Receptor configuration | Native full-length zebrafish TLR5b is not an independently signaling flagellin-responsive homodimer. Productive flagellin recognition and signaling require a complementary TLR5a–TLR5b heterodimer; isolated TLR5b N-terminal ectodomain regions can support a homodimeric arrangement in engineered constructs, but the complete intracellular signaling configuration must include contributions from both paralogs (voogdt2018duplicatedtlr5of pages 1-1, voogdt2018duplicatedtlr5of pages 5-6, voogdt2018duplicatedtlr5of pages 6-7). |
| Cellular localization | An integral membrane protein. In heterologous-cell imaging experiments, zebrafish TLR5b accumulated principally in LAMP-1-positive late-endosomal/lysosomal vesicles rather than predominantly at the plasma membrane. The trafficking chaperone UNC93B1 promoted receptor redistribution and functional association with TLR5a; this localization should be interpreted as direct cell-model evidence, not yet a complete in-vivo zebrafish tissue map (voogdt2018duplicatedtlr5of pages 3-4, voogdt2018duplicatedtlr5of pages 1-1). |
| Key structural domains | N-terminal signal peptide/precursor organization; extracellular leucine-rich repeats and LRR C-terminal cysteine-rich flanking region for ligand recognition; one transmembrane helix; and a cytosolic Toll/interleukin-1 receptor (TIR) domain for adaptor recruitment and signaling. This architecture is characteristic of TLR proteins and agrees with the supplied InterPro assignments IPR001611, IPR003591, IPR032675, IPR000483, and IPR000157 (vaart2012pathogenrecognitionand pages 1-2, meijer2004expressionanalysisof pages 1-2). |
| Evolutionary origin | TLR5a and TLR5b are distinct paralogs rather than splice isoforms. Recent comparative phylogenetic and synteny analysis favors a tandem duplication of an ancestral membrane-form TLR5 gene for the zebrafish M/M pair, followed by subfunctionalization and concerted coevolution into a complementary heterodimer; broader teleost TLR5 diversity also reflects whole-genome duplication and gene loss (voogdt2018duplicatedtlr5of pages 6-7, zhang2023genomewideidentificationand pages 5-8, zhang2023genomewideidentificationand pages 8-10). |
Table: This table consolidates the identity, architecture, ligand specificity, receptor configuration, localization, and evolutionary interpretation of zebrafish TLR5b. It distinguishes direct zebrafish evidence from dsRNA sensing reported only for another cyprinid species.
| Component/Process | Details with References |
|---|---|
| Ligand recognition and receptor activation | Bacterial flagellin engages a functional TLR5a–TLR5b heterodimer. Ligand-induced ectodomain rearrangement is transmitted across the membrane to juxtapose the cytosolic TIR domains; full-length TLR5b alone does not produce effective flagellin-induced NF-κB signaling in the tested system (voogdt2018duplicatedtlr5of pages 6-7, voogdt2018duplicatedtlr5of pages 1-2, voogdt2018duplicatedtlr5of pages 5-5). |
| Trafficking regulator | Zebrafish UNC93B1 promotes redistribution and productive association of TLR5a and TLR5b. TLR5b was observed in LAMP-1-positive late-endosomal/lysosomal vesicles in heterologous cells (voogdt2018duplicatedtlr5of pages 3-4). |
| Adaptor proteins | MyD88 is the best-supported adaptor for zebrafish flagellin–TLR5 signaling. General zebrafish TLR pathway inventories also include TIRAP/Mal and TRIF/TICAM1, but their direct recruitment by TLR5b has not been established with the same specificity (li2017patternrecognitionreceptors pages 2-3, li2017patternrecognitionreceptors pages 1-2, stockhammer2009transcriptomeprofilingand pages 2-3). |
| Proximal kinases and signaling intermediates | The canonical inferred cascade is TIR-domain signaling → MyD88 → IRAK-family kinases → TRAF6 → TAK1/TAB complex. Zebrafish pathway studies identify Irak and Traf6 as conserved signaling intermediates, while irak3 is induced as a likely negative-feedback regulator (li2017patternrecognitionreceptors pages 2-3, yang2002tolllikereceptorsignaling pages 1-6, stockhammer2009transcriptomeprofilingand pages 10-11). |
| NF-κB branch | TAK1 activates the IKK complex, promoting IκB degradation and NF-κB nuclear activity. Functional TLR5a/TLR5b reconstitution directly activates an NF-κB reporter; infection-associated targets include rel, rela, relb, nfkb2, nfkbiaa, and nfkbiab (stockhammer2009transcriptomeprofilingand pages 11-12, voogdt2018duplicatedtlr5of pages 5-5, yang2002tolllikereceptorsignaling pages 1-6). |
| MAPK/AP-1 branch | The conserved pathway predicts activation of ERK, JNK, and p38 MAPKs and AP-1-family factors. Zebrafish flagellin/infection responses include fos, jun, atf3, atf4, and atf5, although their dependence on TLR5b alone has not been genetically isolated (stockhammer2009transcriptomeprofilingand pages 11-12, yang2002tolllikereceptorsignaling pages 1-6). |
| Key downstream genes | Flagellin induces inflammatory and defense-associated genes including il1b, il8/cxcl8, cxcl-C1c, mmp9, and irak3. TLR5a/b depletion nearly abolished mmp9, cxcl-C1c, and irak3 regulation and reduced il1b and il8 responses (stockhammer2009transcriptomeprofilingand pages 11-12). |
| Biological processes | The pathway mediates recognition of flagellated bacteria, pro-inflammatory cytokine and chemokine production, leukocyte-recruiting signals, extracellular-matrix remodeling through Mmp9, and feedback limitation of TLR signaling through Irak3 (stockhammer2009transcriptomeprofilingand pages 11-12, stockhammer2009transcriptomeprofilingand pages 12-13). |
| Response during bacterial infection | Both tlr5a and tlr5b are upregulated during Salmonella Typhimurium infection. Their induction also occurs with nonflagellated bacteria, indicating that infection-associated expression is broader than direct flagellin sensing; redundant pattern-recognition pathways can preserve inflammation during whole-bacterium infection (stockhammer2009transcriptomeprofilingand pages 12-13, stockhammer2009transcriptomeprofilingand pages 11-11). |
| In-vivo loss-of-function evidence | Embryonic morpholino knockdown—either simultaneous or separate targeting of tlr5a and tlr5b—followed by bloodstream injection of purified flagellin reduced or prevented inflammatory gene induction. Separate depletion of either paralog blocked flagellin-induced il1b, supporting their functional interdependence (stockhammer2009transcriptomeprofilingand pages 3-4, yang2015commonandspecific pages 2-4, yang2015commonandspecific pages 1-2). |
| Cell-based functional and structure–function evidence | Overexpression of wild-type, chimeric, and mutant receptors in reporter cells, followed by Salmonella flagellin stimulation and NF-κB luciferase measurement, showed that productive signaling requires complementary TLR5a/TLR5b extracellular and intracellular regions (voogdt2018duplicatedtlr5of pages 5-6, voogdt2018duplicatedtlr5of pages 5-5, voogdt2018duplicatedtlr5of pages 6-7). |
| Evidence limitation | Most zebrafish loss-of-function evidence is based on transient morpholinos rather than stable tlr5b CRISPR mutants. Consequently, developmental compensation, paralog-specific in-vivo functions, tissue distribution, and effects on pathogen burden or survival remain incompletely resolved (stockhammer2009transcriptomeprofilingand pages 3-4, stockhammer2009transcriptomeprofilingand pages 4-4, voogdt2018duplicatedtlr5of pages 6-7). |
Table: This table traces zebrafish TLR5b signaling from flagellin recognition and TLR5a heterodimerization to MyD88-associated NF-κB/AP-1 responses. It also distinguishes directly demonstrated findings from pathway inference and summarizes the principal experimental limitations.
TLR5b represents a specialized innate immune receptor in zebrafish that has evolved a unique heterodimeric configuration with TLR5a for bacterial flagellin recognition. This obligate heterodimerization, intracellular vesicular localization, and dependence on UNC93B1 trafficking distinguish zebrafish TLR5b from its mammalian TLR5 counterpart. The receptor activates MyD88-dependent signaling cascades culminating in NF-κB and AP-1 transcription factors, which drive expression of pro-inflammatory cytokines, chemokines, and antimicrobial effectors critical for host defense against flagellated bacteria.
Recent discoveries of potential dual-ligand recognition capabilities in related fish species, combined with comprehensive evolutionary analyses revealing extensive TLR5 diversification across teleosts, suggest that fish TLR5 paralogs may possess broader functional versatility than previously appreciated. Future studies employing stable genetic knockouts, tissue-specific expression mapping, and direct ligand-binding assays will be essential for fully elucidating TLR5b's role in zebrafish immunity and its potential applications in aquaculture disease management.
References
(vaart2012pathogenrecognitionand pages 1-2): Michiel van der Vaart, Herman P. Spaink, and Annemarie H. Meijer. Pathogen recognition and activation of the innate immune response in zebrafish. Advances in Hematology, 2012:1-19, Jul 2012. URL: https://doi.org/10.1155/2012/159807, doi:10.1155/2012/159807. This article has 237 citations.
(meijer2004expressionanalysisof pages 1-2): Annemarie H Meijer, S.F Gabby Krens, Indira A Medina Rodriguez, Shuning He, Wilbert Bitter, B Ewa Snaar-Jagalska, and Herman P Spaink. Expression analysis of the toll-like receptor and tir domain adaptor families of zebrafish. Molecular immunology, 40 11:773-83, Jan 2004. URL: https://doi.org/10.1016/j.molimm.2003.10.003, doi:10.1016/j.molimm.2003.10.003. This article has 650 citations and is from a peer-reviewed journal.
(voogdt2018duplicatedtlr5of pages 1-1): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(voogdt2018duplicatedtlr5of pages 6-7): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(meijer2004expressionanalysisof pages 9-10): Annemarie H Meijer, S.F Gabby Krens, Indira A Medina Rodriguez, Shuning He, Wilbert Bitter, B Ewa Snaar-Jagalska, and Herman P Spaink. Expression analysis of the toll-like receptor and tir domain adaptor families of zebrafish. Molecular immunology, 40 11:773-83, Jan 2004. URL: https://doi.org/10.1016/j.molimm.2003.10.003, doi:10.1016/j.molimm.2003.10.003. This article has 650 citations and is from a peer-reviewed journal.
(yang2002tolllikereceptorsignaling pages 12-14): S Yang. Toll-like receptor signaling in the innate immune system of zebrafish larvae. Unknown journal, 2002.
(voogdt2018duplicatedtlr5of pages 1-2): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(voogdt2018duplicatedtlr5of pages 5-5): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(voogdt2018duplicatedtlr5of pages 5-6): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(yang2015commonandspecific pages 2-4): Shuxin Yang, Rubén Marín-Juez, Annemarie H. Meijer, and Herman P. Spaink. Common and specific downstream signaling targets controlled by tlr2 and tlr5 innate immune signaling in zebrafish. BMC Genomics, Jul 2015. URL: https://doi.org/10.1186/s12864-015-1740-9, doi:10.1186/s12864-015-1740-9. This article has 50 citations and is from a peer-reviewed journal.
(yang2015commonandspecific pages 1-2): Shuxin Yang, Rubén Marín-Juez, Annemarie H. Meijer, and Herman P. Spaink. Common and specific downstream signaling targets controlled by tlr2 and tlr5 innate immune signaling in zebrafish. BMC Genomics, Jul 2015. URL: https://doi.org/10.1186/s12864-015-1740-9, doi:10.1186/s12864-015-1740-9. This article has 50 citations and is from a peer-reviewed journal.
(weber2022fishtlr5develops pages 1-2): Alexander N R Weber. Fish tlr5 develops a taste for viral rna. EMBO Reports, Jun 2022. URL: https://doi.org/10.15252/embr.202255443, doi:10.15252/embr.202255443. This article has 10 citations and is from a highest quality peer-reviewed journal.
(weber2022fishtlr5develops pages 2-3): Alexander N R Weber. Fish tlr5 develops a taste for viral rna. EMBO Reports, Jun 2022. URL: https://doi.org/10.15252/embr.202255443, doi:10.15252/embr.202255443. This article has 10 citations and is from a highest quality peer-reviewed journal.
(davis2026tolllikereceptorsin pages 12-13): Isabella A. Davis, Kavi R. Miryala, Roy Curtiss, and Banikalyan Swain. Toll-like receptors in fish: evolution, structure, immune signaling, and translational prospects for adjuvant-based vaccine design in aquaculture. Jul 2026. URL: https://doi.org/10.3389/fimmu.2026.1880904, doi:10.3389/fimmu.2026.1880904. This article has 0 citations and is from a peer-reviewed journal.
(voogdt2018duplicatedtlr5of pages 3-4): Carlos G. P. Voogdt, Jaap A. Wagenaar, and Jos P. M. van Putten. Duplicated tlr5 of zebrafish functions as a heterodimeric receptor. Proceedings of the National Academy of Sciences, 115:E3221-E3229, Mar 2018. URL: https://doi.org/10.1073/pnas.1719245115, doi:10.1073/pnas.1719245115. This article has 57 citations and is from a highest quality peer-reviewed journal.
(yang2017tlr5beyondthe pages 1-2): Jingyi Yang and Huimin Yan. Tlr5: beyond the recognition of flagellin. Cellular & Molecular Immunology, 14:1017-1019, Nov 2017. URL: https://doi.org/10.1038/cmi.2017.122, doi:10.1038/cmi.2017.122. This article has 115 citations and is from a peer-reviewed journal.
(li2017patternrecognitionreceptors pages 2-3): Yajuan Li, Yuelong Li, Xiaocong Cao, Xiangyu Jin, and Tengchuan Jin. Pattern recognition receptors in zebrafish provide functional and evolutionary insight into innate immune signaling pathways. Cellular and Molecular Immunology, 14:80-89, Oct 2017. URL: https://doi.org/10.1038/cmi.2016.50, doi:10.1038/cmi.2016.50. This article has 227 citations and is from a peer-reviewed journal.
(li2017patternrecognitionreceptors pages 1-2): Yajuan Li, Yuelong Li, Xiaocong Cao, Xiangyu Jin, and Tengchuan Jin. Pattern recognition receptors in zebrafish provide functional and evolutionary insight into innate immune signaling pathways. Cellular and Molecular Immunology, 14:80-89, Oct 2017. URL: https://doi.org/10.1038/cmi.2016.50, doi:10.1038/cmi.2016.50. This article has 227 citations and is from a peer-reviewed journal.
(stockhammer2009transcriptomeprofilingand pages 2-3): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(yang2002tolllikereceptorsignaling pages 1-6): S Yang. Toll-like receptor signaling in the innate immune system of zebrafish larvae. Unknown journal, 2002.
(stockhammer2009transcriptomeprofilingand pages 11-12): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(stockhammer2009transcriptomeprofilingand pages 11-11): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(stockhammer2009transcriptomeprofilingand pages 12-13): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(stockhammer2009transcriptomeprofilingand pages 10-11): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(tanekhy2016theroleof pages 8-10): Mahmoud Tanekhy. The role of toll-like receptors in innate immunity and infectious diseases of teleost. Aquaculture Research, 47:1369-1391, May 2016. URL: https://doi.org/10.1111/are.12607, doi:10.1111/are.12607. This article has 43 citations and is from a peer-reviewed journal.
(tanekhy2016theroleof pages 13-14): Mahmoud Tanekhy. The role of toll-like receptors in innate immunity and infectious diseases of teleost. Aquaculture Research, 47:1369-1391, May 2016. URL: https://doi.org/10.1111/are.12607, doi:10.1111/are.12607. This article has 43 citations and is from a peer-reviewed journal.
(zhang2023genomewideidentificationand pages 5-8): Kai Zhang, Ming Chen, Haobin He, Hongyan Kou, Li Lin, and Rishen Liang. Genome-wide identification and characterization of toll-like receptor 5 (tlr5) in fishes. Frontiers in Genetics, Jan 2023. URL: https://doi.org/10.3389/fgene.2022.1083578, doi:10.3389/fgene.2022.1083578. This article has 9 citations and is from a peer-reviewed journal.
(zhang2023genomewideidentificationand pages 1-2): Kai Zhang, Ming Chen, Haobin He, Hongyan Kou, Li Lin, and Rishen Liang. Genome-wide identification and characterization of toll-like receptor 5 (tlr5) in fishes. Frontiers in Genetics, Jan 2023. URL: https://doi.org/10.3389/fgene.2022.1083578, doi:10.3389/fgene.2022.1083578. This article has 9 citations and is from a peer-reviewed journal.
(zhang2023genomewideidentificationand pages 8-10): Kai Zhang, Ming Chen, Haobin He, Hongyan Kou, Li Lin, and Rishen Liang. Genome-wide identification and characterization of toll-like receptor 5 (tlr5) in fishes. Frontiers in Genetics, Jan 2023. URL: https://doi.org/10.3389/fgene.2022.1083578, doi:10.3389/fgene.2022.1083578. This article has 9 citations and is from a peer-reviewed journal.
(zhang2023genomewideidentificationand pages 2-3): Kai Zhang, Ming Chen, Haobin He, Hongyan Kou, Li Lin, and Rishen Liang. Genome-wide identification and characterization of toll-like receptor 5 (tlr5) in fishes. Frontiers in Genetics, Jan 2023. URL: https://doi.org/10.3389/fgene.2022.1083578, doi:10.3389/fgene.2022.1083578. This article has 9 citations and is from a peer-reviewed journal.
(stockhammer2009transcriptomeprofilingand pages 3-4): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.
(stockhammer2009transcriptomeprofilingand pages 4-4): Oliver W Stockhammer, Anna Zakrzewska, Zoltán Hegedûs, Herman P Spaink, and Annemarie H Meijer. Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to salmonella infection. The Journal of Immunology, 182:5641-5653, May 2009. URL: https://doi.org/10.4049/jimmunol.0900082, doi:10.4049/jimmunol.0900082. This article has 303 citations.