LRIT3 is a type-I single-pass membrane glycoprotein with an extracellular leucine-rich-repeat region, immunoglobulin-like domain, and fibronectin type-III domain. In the retina it acts from rod and cone photoreceptor terminals as a trans-synaptic organizer of the depolarizing-bipolar-cell glutamate signalplex, enabling postsynaptic localization of nyctalopin and the TRPM1 channel and normal ON-pathway visual transmission. Biallelic human LRIT3 variants cause complete congenital stationary night blindness, and human retinal staining places LRIT3 puncta in the outer plexiform layer; endogenous mouse tagging and photoreceptor- specific rescue resolve the functional protein predominantly to the presynaptic side. Overexpressed human LRIT3 can alter FGFR1 maturation and PLC-gamma signaling in cultured cells, but this context is distinct from its established retinal role. Two splice isoforms are described; isoform 2 lacks the canonical five-LRR segment, without demonstrated isoform-specific localization or function.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: UniProt's controlled-vocabulary mapping assigns the type-I membrane glycoprotein LRIT3 to the endoplasmic reticulum membrane. Reason: Human LRIT3 expressed in HEK293 cells was predominantly ER-modified, and its single transmembrane helix makes ER-membrane residence during biosynthesis credible. This is retained as a non-core trafficking location because the physiological retinal protein acts principally at presynaptic photoreceptor terminals, while the ER observation comes from an overexpression system. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB-SubCell:SL-0097 · endoplasmic reticulum membrane controlled vocabulary SUPPORTS TRANSFER Supporting Evidence: PMID:22673519 Surprisingly, LRIT3 existed predominantly as ER-modified forms. |
| GO:0030425 dendrite | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Combined automated methods transfer dendritic localization from the exact mouse Lrit3 ortholog and the UniProt dendrite vocabulary mapping. Reason: Direct human antibody staining placed LRIT3 puncta in a pattern resembling ON-bipolar dendritic tips, so this curator-supported localization is retained. Later photoreceptor-specific rescue and endogenous mouse tagging resolve the functional protein presynaptically, making dendrite a non-core, cell-side- ambiguous observation rather than the principal site of action. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SUPPORTS TRANSFER ensembl:ENSMUSP00000140184 · mouse Lrit3 orthology source SUPPORTS TRANSFER UniProtKB-SubCell:SL-0283 · dendrite controlled vocabulary SUPPORTS TRANSFER Supporting Evidence: PMID:23246293 Human LRIT3 antibody staining revealed in the outer plexiform layer of the human retina a punctate-labeling pattern resembling the dendritic tips of bipolar cells; similar patterns have been observed for other proteins implicated in cCSNB. PMID:40263339 These observations suggest that LRIT3 is confined to pre-synaptic compartment and is expressed solely by photoreceptors. |
| GO:0043204 perikaryon | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: UniProt's controlled-vocabulary mapping places LRIT3 in the neuronal perikaryon. Reason: This controlled-vocabulary assignment reflects an older retinal localization model but lacks direct positive evidence for a somatic LRIT3 pool. Current endogenous tagging instead confines mouse LRIT3 to presynaptic photoreceptor terminals, so perikaryon is marked over-annotated. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK Sources checked: UniProtKB-SubCell:SL-0197 · perikaryon controlled vocabulary SOURCE WEAK OR INFERRED Supporting Evidence: PMID:40263339 These observations suggest that LRIT3 is confined to pre-synaptic compartment and is expressed solely by photoreceptors. |
| GO:0051286 cell tip | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Ensembl Compara transfers cell-tip localization from the exact mouse Lrit3 ortholog. Reason: Mouse puncta were historically described at ON-bipolar dendritic tips, whereas photoreceptor-targeted rescue and endogenous tagging support a presynaptic rod- and cone-terminal origin. A photoreceptor terminal remains a specialized cell tip, so the generic term is retained outside the core while the more informative presynapse term identifies the principal site of action. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SUPPORTS TRANSFER ensembl:ENSMUSP00000140184 · mouse Lrit3 orthology source SUPPORTS TRANSFER Supporting Evidence: PMID:31189098 In contrast, we demonstrate that LRIT3 is expressed presynaptically, in rod photoreceptors (rods), and when we restore LRIT3 expression in Lrit3-/- rods, we restore expression of the postsynaptic glutamate signalplex and rod-driven vision. |
| GO:0043204 perikaryon | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Manual sequence-similarity transfer assigns perikaryon localization from the exact mouse Lrit3 ortholog. Reason: The exact mouse ortholog makes the sequence-similarity transfer mechanically traceable, but there is no positive evidence for a somatic LRIT3 pool. Newer endogenous tagging establishes mouse LRIT3 at presynaptic photoreceptor terminals, so perikaryon is marked over-annotated. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SOURCE WEAK OR INFERRED Supporting Evidence: PMID:40263339 These observations suggest that LRIT3 is confined to pre-synaptic compartment and is expressed solely by photoreceptors. |
| GO:0040036 regulation of fibroblast growth factor receptor signaling pathway | IDA PMID:22673519 Leucine-rich repeat, immunoglobulin-like and transmembrane d... | KEEP AS NON CORE | Summary: In transfected human HEK293 cells, LRIT3 increased the Golgi-modified pool of FGFR1 and altered the PLC-gamma branch of FGFR1 signaling. Reason: The direction-neutral regulatory term is supported by direct experiments with human LRIT3 and FGFR1. However, the phenotype was measured under heterologous overexpression, did not enhance FGF-stimulated ERK or AKT signaling, and the PLC-gamma effect was ligand-independent. It therefore remains a supported but context-limited non-core activity rather than the established retinal function. Supporting Evidence: PMID:22673519 These results suggest that LRIT3 facilitates exit of FGFR1 from the ER. PMID:22673519 However, phosphorylation of PLC-γ was not dependent on bFGF1. These results suggest that LRIT3 can aberrantly activate the PLC-γ branch of the FGFR1-signaling pathway. |
| GO:0099084 postsynaptic specialization organization | ISO PMID:31189098 Presynaptic Expression of LRIT3 Transsynaptically Organizes ... | NEW | Summary: Proposed human annotation by orthology for presynaptic LRIT3 organizing the postsynaptic depolarizing-bipolar-cell signalplex. Reason: Photoreceptor-specific expression in Lrit3-null mice restores postsynaptic signalplex assembly and rod-driven vision, while nyctalopin/TRPM1 dependence and domain-rescue experiments define the organized postsynaptic specialization. GO:0099084 captures assembly and arrangement of that postsynaptic structure without claiming that human LRIT3 is itself postsynaptic. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SUPPORTS TRANSFER Supporting Evidence: PMID:31189098 Our results demonstrate that, in the retina, the LRR-containing protein LRIT3 acts as a transsynaptic organizer of the postsynaptic complex required for normal synaptic function. |
| GO:0050804 modulation of chemical synaptic transmission | ISO PMID:28334377 LRIT3 Differentially Affects Connectivity and Synaptic Trans... | NEW | Summary: Proposed human annotation by orthology for LRIT3-dependent modulation of retinal photoreceptor-to-bipolar-cell transmission. Reason: Mouse loss-of-function and rescue establish that LRIT3 is required for normal ON-bipolar signaling and also affects downstream OFF-pathway responses. The direction-neutral term is appropriate because LRIT3 organizes the signalplex rather than uniformly increasing or decreasing all retinal transmission. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SUPPORTS TRANSFER Supporting Evidence: PMID:28334377 CONCLUSIONS: These results suggest that LRIT3 is likely involved in coordination of the transsynaptic communication between cones and ON-BCs during synapse formation and function. |
| GO:0098793 presynapse | ISO PMID:40263339 Efficient in vivo labeling of endogenous proteins with SMART... | NEW | Summary: Proposed human annotation by orthology for LRIT3 localization to the presynaptic photoreceptor side of rod and cone synapses. Reason: Endogenous mouse tagging and photoreceptor-specific rescue independently place LRIT3 in presynaptic rods and cone pedicles. GO:0098793 captures the functional side of the synapse more accurately than the historical dendrite/cell-tip descriptions, while the human annotation remains explicitly orthology-based. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:W8DXL4 · mouse Lrit3 SUPPORTS TRANSFER Supporting Evidence: PMID:40263339 These observations suggest that LRIT3 is confined to pre-synaptic compartment and is expressed solely by photoreceptors. |
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Download this section (compressed HTML)Q: Does human LRIT3 bind nyctalopin directly, and which LRIT3 domain or residues form the trans-synaptic interface?
Suggested experts: Retinal synapse experts, Structural biologists
Q: Is endogenous human LRIT3 exclusively presynaptic in rods and cones, or is there a distinct postsynaptic pool at bipolar-cell dendritic tips?
Suggested experts: Human retinal cell biologists, Advanced imaging experts
Q: Which LRIT3-dependent step separately recruits nyctalopin and TRPM1 to the depolarizing-bipolar-cell signalplex?
Suggested experts: Ion-channel trafficking experts, Synaptic organizer experts
Q: Is LRIT3 isoform 2 expressed as a stable retinal protein, and can it reach the synapse or organize the signalplex without the canonical LRR region?
Suggested experts: Isoform biology experts, Retinal proteomics experts
Q: Does endogenous LRIT3 regulate FGFR1 trafficking in any normal human cell type, and is that activity connected to retinal biology?
Suggested experts: FGF-signaling experts, Retinal developmental biologists
Experiment: Measure purified ectodomain binding by biolayer interferometry and crosslinking mass spectrometry, determine a complex structure if stable, and test interface mutants in photoreceptor-specific rescue of Lrit3-null mouse retina.
Hypothesis: LRIT3 directly binds nyctalopin through its LRR region to organize a trans-synaptic complex.
Type: quantitative biochemistry and structural rescue
Experiment: Introduce small endogenous tags into LRIT3 in human retinal organoids, image with expansion microscopy against pre- and postsynaptic landmarks, and validate the tagged protein's trafficking and signaling competence.
Hypothesis: Human LRIT3 is predominantly presynaptic at rod and cone terminals.
Type: endogenous tagging and spatial proteomics
Experiment: Perform time-resolved photoreceptor-specific rescue with LRR, Ig-like, and cytoplasmic-tail mutants and quantify nyctalopin and TRPM1 localization, signalplex composition, and ON-bipolar electrophysiology independently.
Hypothesis: LRIT3 recruits nyctalopin and TRPM1 through separable domain-dependent steps.
Type: domain-resolved in vivo rescue
Experiment: Combine retinal long-read sequencing and targeted proteomics with matched isoform- specific trafficking, surface-expression, partner-binding, and functional rescue assays.
Hypothesis: LRIT3 isoform 2 is unstable or functionally deficient because it lacks the canonical LRR segment.
Type: isoform-resolved structure-function analysis
Experiment: Perturb endogenous LRIT3 in cells that naturally coexpress FGFR1, quantify ER-to- Golgi trafficking and ligand-dependent PLC-gamma/ERK/AKT outputs, and compare with the original HEK293 overexpression design.
Hypothesis: LRIT3 modulates FGFR1 only under overexpression and is dispensable for endogenous FGFR1 trafficking in retinal cells.
Type: endogenous signaling perturbation
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The direct extracellular binding interface by which LRIT3 organizes nyctalopin, TRPM1, and the depolarizing-bipolar-cell signalplex is unresolved.
OPEN BIOLOGY MF_DARK
Significance: Co-immunoprecipitation and domain-deletion rescue identify functional dependencies but do not distinguish direct binary contacts, multimerization, or an intervening synaptic partner.
What would resolve it: Reconstitute purified LRIT3 ectodomains with nyctalopin and TRPM1 extracellular regions, combine quantitative binding with crosslinking mass spectrometry and structural analysis, and validate interface mutants by retinal rescue.
Provenance (the field's own admissions):
Gap: The exact cell-side distribution and molecular partners of endogenous human LRIT3 at rod and cone synapses remain unresolved.
OPEN BIOLOGY CC_DARK
Significance: Human antibody staining resembled bipolar dendritic tips, whereas mouse endogenous tagging and rescue support a presynaptic photoreceptor origin; synaptic apposition can make these patterns difficult to distinguish.
What would resolve it: Endogenously tag LRIT3 in human retinal organoids or donor tissue and combine expansion microscopy with cell-type-specific proteomics to resolve pre- versus postsynaptic pools and their partners.
Provenance (the field's own admissions):
Gap: The expression, trafficking, and function of human LRIT3 isoform 2 are unknown.
OPEN BIOLOGY RESIDUAL_SUBGAP
Significance: Isoform 2 removes the canonical five-LRR segment implicated by mouse rescue studies in trafficking and signalplex assembly, but no endogenous isoform-specific protein evidence exists.
What would resolve it: Use long-read retinal transcriptomics and isoform-specific proteomics, then compare the two isoforms in surface-trafficking, partner-binding, and Lrit3-null retinal rescue assays.
Gap: The endogenous physiological relevance of LRIT3-dependent FGFR1 maturation and signaling outside the overexpression assay is unknown.
OPEN BIOLOGY BP_DARK
Significance: The direct human-cell assay is reproducible evidence for FGFR-pathway modulation, but no retinal or other native tissue phenotype currently connects it to the core visual function.
What would resolve it: Test endogenous FGFR1 trafficking and signaling after LRIT3 perturbation in human retinal and non-retinal cells that naturally coexpress both proteins, with ligand- controlled measurements and rescue at physiological expression levels.
Provenance (the field's own admissions):
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