Intracellular Toll-like receptor of birds that senses unmethylated CpG DNA and is the functional counterpart of mammalian TLR9, which is absent from avian genomes. It has a leucine-rich-repeat ectodomain, a transmembrane segment and a cytoplasmic TIR domain, resides in the endoplasmic reticulum in resting cells and signals from endolysosomal compartments, its activation being blocked by agents that prevent endosomal maturation. Expression of the receptor in cells that otherwise lack a CpG sensor confers NF-kappa-B activation in response to synthetic CpG oligodeoxynucleotides and to bacterial genomic DNA, with sequence preferences of its own, and reducing receptor expression in avian macrophages reduces their response to the same ligands, leading to induction of inflammatory mediators. Signalling from the receptor has two arms in chicken macrophages, an NF-kappa-B arm that induces inflammatory cytokines and chemokines and an interferon arm that induces interferon beta; interferon-beta transcription downstream of the receptor requires both NF-kappa-B and IRF7 and, unlike mammalian TLR9 signalling, does not require TBK1. Two of its leucine-rich repeats resemble a repeat implicated in ligand binding in mammalian TLR9, but the receptor is not a TLR9 orthologue: it belongs to the TLR21/TLR22 branch that also contains mouse TLR13 and has homologues in fish and amphibians, so its similarity to TLR9 is convergent in ligand rather than inherited. Transcripts are found across tissues.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002224 toll-like receptor signaling pathway | IBA GO_REF:0000033 | MODIFY | Summary: Correct, and the receptor-specific child is available and experimentally supported. Reason: Two independent studies show NF-kappa-B-dependent signalling initiated at this receptor by CpG DNA, in heterologous cells and, by knockdown, in avian macrophages, which is what GO:0035682 toll-like receptor 21 signaling pathway describes. The receptor identity is unambiguous here, unlike the mouse TLR11/TLR12 pair, so the numbered term can be used safely. Note that the term to avoid would be a TLR9 pathway term: the functional resemblance to TLR9 is convergence on a ligand, not orthology, and GOA has correctly not made that transfer. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN002808083 SUPPORTS TRANSFER Family-level pathway assertion is sound; direct evidence on the chicken receptor licenses the receptor-specific child. Proposed replacements: toll-like receptor 21 signaling pathway Supporting Evidence: PMID:20498358 NF-kappaB in response to unmethylated CpG DNA PMID:19573927 expression of TLR21 but not TLR7 or 15 resulted in marked NF-kappaB activation upon stimulation with exogenous ODN |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | MODIFY | Summary: The receptor is ER-resident when resting and signals from endolysosomes, not from the plasma membrane. Reason: The ancestral node is seeded by cell-surface receptors, and the location did not transfer. Confocal microscopy and endoglycosidase H sensitivity place the resting receptor in the endoplasmic reticulum, activation is blocked by chloroquine and by bafilomycin A, and the receptor was found in the same intracellular compartments as human TLR9. Endosome is proposed as the compartment in which the receptor acts; the resting ER pool is described in the description rather than annotated, because the evidence describes trafficking rather than steady-state residence in one place. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN002808045 SUPPORTS SOURCE BUT NOT TARGET Donors are surface TLRs; this nucleic-acid-sensing branch is intracellular and its activation requires endosomal maturation. Proposed replacements: endosome Supporting Evidence: PMID:20498358 functional in endolysosomes PMID:19573927 to the endoplasmic reticulum (ER) of resting cells file:CHICK/TLR21/TLR21-deep-research-falcon.md TLR21 is localized in the endosome, consistent with its role in distinguishing foreign nucleic acids from self-DNA |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: True but generic. Reason: Sound TIR-domain inference, superseded by the proposed GO:0035682. |
| GO:0016020 membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Correct for a single-pass membrane protein. Reason: True but uninformative; the informative locations are the endoplasmic reticulum and the endolysosome. |
| GO:0038023 signaling receptor activity | IBA GO_REF:0000033 | MODIFY | Summary: The receptor recognises unmethylated CpG DNA, a pathogen-associated pattern, so the specific molecular function applies. Reason: GO:0038023 is correct but generic. Expressing this receptor in cells lacking a CpG sensor confers responsiveness to CpG oligodeoxynucleotides and to bacterial genomic DNA with defined sequence preferences, and knocking it down in avian macrophages removes that responsiveness, which is functional evidence for recognition by this receptor. GO:0038187 is the conventional molecular function for characterised TLRs. GO:0045322 unmethylated CpG binding was considered and rejected as a proposal: no direct binding assay exists for the chicken receptor, and even human TLR9 holds that term only by ISS and IEA, so asserting it here would exceed the evidence. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN002808045 SUPPORTS TRANSFER The node assertion is correct; functional data on the chicken receptor support the specific child. Proposed replacements: pattern recognition receptor activity Supporting Evidence: PMID:20498358 NF-kappaB in response to unmethylated CpG DNA PMID:19573927 expression of TLR21 but not TLR7 or 15 resulted in marked NF-kappaB activation upon stimulation with exogenous ODN |
| GO:0032728 positive regulation of interferon-beta production | IDA PMID:37951324 Mechanisms of type I interferon production by chicken TLR21. | NEW | Summary: Activation of this receptor by CpG DNA in chicken macrophages induces interferon-beta transcription, through an NF-kappa-B- and IRF7-dependent but TBK1-independent route. Reason: The receptor is the entity that performs the sensing step which initiates this output, not a molecule consumed by it, so the participation test is met: CpG recognition by the receptor is what turns on the interferon-beta response. The cached record (abstract only) shows interferon-beta transcription induced by chTLR21 activation in chicken macrophages and its dependence on NF-kappa-B and IRF7; it does not report manipulation of the receptor itself, so the evidence code is IDA rather than IMP. The comparator check supports the proposal rather than undermining it. The functional counterparts in mammals carry exactly this term from equivalent experiments: human TLR9 and mouse Tlr9 both hold GO:0032728 (human by IDA from PMID:16286015, mouse by IDA from PMID:15356140), and human TLR3 holds it by IDA and IMP. The nucleic-acid-sensing TLRs are conventionally annotated to the interferon-production terms for the interferon they induce, and this receptor is the avian sensor in that role. The specific interferon-beta child is used rather than the type I parent because interferon-beta transcription is what was measured. This is not redundant with the proposed GO:0035682 pathway term, which is neither an ancestor nor a descendant of it. Supporting Evidence: PMID:37951324 triggers the activation of NF-ΞΊB and induces a potent type-I interferon response PMID:37951324 the transcription of interferon beta (IFNB) by chTLR21 was found to be dependent on both NF-ΞΊB and IRF7 signalling, but independent of the TBK1 kinase |
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Download this section (compressed HTML)Q: Does chicken TLR21 bind CpG DNA directly? All the evidence is functional (heterologous expression, knockdown, pharmacological blockade of endosomal maturation). A direct binding assay would license GO:0045322 unmethylated CpG binding, which is currently held even by human TLR9 only by ISS.
Q: Should GO record the relationship between TLR21 and TLR9 as functional analogy? Both receptors sense the same ligand class from different branches of the family, and annotation transfer between them would be wrong while a note of the analogy would be informative.
Q: Which UniProt entry should carry chicken TLR21 annotations? Several Ensembl models of the same gene exist in TrEMBL, this one has no RefSeq or GeneID cross-reference, and none is reviewed, so annotations risk being split across accessions.
Q: Does the avian receptor depend on UNC93B1 for trafficking, as the zebrafish and mammalian nucleic-acid-sensing receptors do? That dependence is established for zebrafish TLR21 and for mouse TLR13 but has not been tested in chicken.
Experiment: Measure direct binding of defined CpG and GpC oligodeoxynucleotides to the purified chicken TLR21 ectodomain at endolysosomal and neutral pH, by fluorescence polarisation or surface plasmon resonance, with methylated ligands as controls.
Hypothesis: The receptor binds unmethylated CpG DNA directly and in a pH-dependent manner, which would convert the current functional annotations into a molecular binding annotation.
Type: direct binding assay
Experiment: Test the trafficking requirement by co-expressing chicken TLR21 with and without chicken UNC93B1 in avian cells, following localisation and CpG responsiveness, and by disrupting UNC93B1 in a chicken macrophage line.
Hypothesis: Like its zebrafish counterpart and mouse TLR13, the avian receptor needs UNC93B1 to leave the endoplasmic reticulum and become responsive.
Type: trafficking dependence assay
Experiment: Map the sequence preference of the chicken receptor against a matched panel of oligodeoxynucleotides also tested on zebrafish TLR21 and on mammalian TLR9, using the same reporter system for all three.
Hypothesis: The CpG motif preferences differ systematically between receptors, which matters both for adjuvant design in poultry and for deciding how far ligand-level annotations may be shared across these receptors.
Type: ligand specificity assay
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