R3HDML is a small, signal-peptide-bearing CAP-superfamily precursor that, despite its inherited name, does not contain an R3H domain. The conserved precursor is cleaved before secretion to yield an extracellular CAP-domain protein. Mouse R3hdml is produced by activated and differentiating skeletal- muscle satellite cells and acts extracellularly to promote their proliferation and myogenic differentiation, supporting postnatal skeletal-muscle development and regeneration after injury. The immediate receptor, extracellular-matrix target, or enzymatic activity is unknown; peptidase inhibition remains a structural prediction rather than a demonstrated biochemical function. Direct causal evidence is from mouse, and conservation of this myogenic role in human cells has not yet been tested.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT places R3HDML in the extracellular region, consistent with the precursor topology and direct secretion of the mouse ortholog. Reason: Mouse satellite cells secrete a processed R3hdml protein, and extracellular recombinant protein rescues R3hdml-silenced myogenic cells. The human protein has the same signal-peptide/propeptide/CAP-domain architecture, so extracellular action is a well-supported core localization. Supporting Evidence: PMID:31524320 R3hdml protein was secreted upon satellite cell differentiation. |
| GO:0060090 molecular adaptor activity | IBA GO_REF:0000033 | REMOVE | Summary: PAINT propagated molecular adaptor activity from mouse Glipr1l1 across a broad and functionally diverse CAP-family ancestral node to R3HDML. Reason: The sole seed, Glipr1l1, is a specialized outer-acrosomal-membrane protein modeled in protein localization during the acrosome reaction. R3HDML is a different CAP-family paralog that is processed and secreted by satellite cells; neither the mouse R3hdml study nor human protein evidence shows that it bridges a macromolecular complex. Shared CAP architecture is inadequate evidence for this source-specific adaptor activity. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG CONTEXT OR TISSUE MISMATCH ROLE CONFLATION Sources checked: MGI:MGI:1916536 · mouse Glipr1l1 SUPPORTS SOURCE BUT NOT TARGET The seed activity belongs to a sperm outer-acrosomal-membrane paralog functioning during the acrosome reaction, not to R3HDML. PANTHER:PTN000036124 · broad PTHR10334 ancestral node UNRESOLVED This node spans numerous divergent CAP-family subfamilies and is too broad to justify transfer of the Glipr1l1-specific adaptor role. Supporting Evidence: file:human/R3HDML/R3HDML-deep-research-manual.md CAP-domain membership does not establish a shared adaptor activity across this highly functionally diversified family, so the transfer to R3HDML should be removed. |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping predicts secretion of the human signal-peptide-bearing precursor. Reason: Direct conditioned-medium and rescue evidence for mouse R3hdml confirms the sequence-based secretion prediction, and the mature human protein lacks any membrane anchor. Supporting Evidence: PMID:31524320 adding the R3hdml protein to the culture media rescued the phenotypes seen in R3hdml‐silenced C2C12 cells |
| GO:2000288 positive regulation of myoblast proliferation | ISS PMID:31524320 R3hdml regulates satellite cell proliferation and differenti... | NEW | Summary: Proposed orthology-supported annotation. Mouse knockout and cell-culture experiments show that R3hdml promotes satellite-cell/myoblast proliferation. Reason: R3hdml loss reduces satellite-cell proliferation and cell-cycle markers, while the one-to-one human ortholog retains the processed CAP architecture. ISS conveys the mouse-to-human evidence boundary. Supporting Evidence: PMID:31524320 Inactivation of R3hdml led to a reduced amount of skeletal muscle and decreased proliferation of myogenic satellite cells. |
| GO:0051149 positive regulation of muscle cell differentiation | ISS PMID:31524320 R3hdml regulates satellite cell proliferation and differenti... | NEW | Summary: Proposed orthology-supported annotation. R3hdml knockdown attenuates myotube formation, and extracellular R3hdml protein rescues the defect. Reason: The loss-and-rescue cell experiments support a positive extracellular role in myogenic differentiation. The annotation is transferred to human with ISS rather than presented as a direct human assay. Supporting Evidence: PMID:31524320 Silencing of the R3hdml gene in C2C12 cells attenuated myotube formation, and those phenotypes were rescued in the presence of R3hdml protein. |
| GO:0043415 positive regulation of skeletal muscle tissue regeneration | ISS PMID:31524320 R3hdml regulates satellite cell proliferation and differenti... | NEW | Summary: Proposed orthology-supported annotation. Mouse R3hdml loss delays recovery after cardiotoxin injury, and local re-expression restores recovery. Reason: The knockout and in vivo rescue supply causal evidence for a positive role in skeletal-muscle regeneration; ISS appropriately transfers the conserved protein's role to human. Supporting Evidence: PMID:31524320 Handgrip strength recovery was delayed after CTX injection in R3hdml KO mice compared with littermate controls. Furthermore, overexpression of R3hdml at forearm rescued the wild‐type phenotype. |
| GO:0048643 positive regulation of skeletal muscle tissue development | ISS PMID:31524320 R3hdml regulates satellite cell proliferation and differenti... | NEW | Summary: Proposed orthology-supported annotation. R3hdml-null mice have reduced skeletal-muscle mass and impaired satellite-cell proliferation during the developmental period in which R3hdml is normally expressed. Reason: Genetic loss supports a positive contribution to postnatal skeletal-muscle development. The human annotation is explicitly an orthology transfer. Supporting Evidence: PMID:31524320 Body weight and skeletal muscle mass of R3hdml knockout (KO) mice are lower compared to control mice. |
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Download this section (compressed HTML)Q: Which cell-surface receptor, extracellular-matrix component, or protease is the immediate target of mature R3HDML in satellite cells?
Suggested experts: skeletal-muscle stem-cell biologists, CAP-superfamily biochemists
Q: Does human R3HDML retain the satellite-cell myogenic role demonstrated in mouse, despite intestine-enriched RNA in adult human tissue atlases?
Suggested experts: human muscle-regeneration specialists
Q: Is furin-like propeptide cleavage required for R3HDML secretion, stability, or biological activity?
Suggested experts: secretory-pathway biochemists
Experiment: Knock out R3HDML in primary human satellite cells or iPSC-derived myogenic progenitors, then perform mature-protein and cleavage-resistant rescue while measuring proliferation, differentiation, fusion, and Akt/IGF-1 responses.
Hypothesis: Secreted, proteolytically matured human R3HDML is required cell-nonautonomously for normal satellite-cell proliferation and differentiation.
Type: CRISPR genetics, human myogenesis, and rescue analysis
Experiment: Screen purified mature human R3HDML against extracellular serine proteases and quantitative peptide-substrate panels, with matched CAP-domain mutants and stoichiometric binding controls.
Hypothesis: R3HDML directly inhibits a restricted extracellular serine protease rather than acting only as a nonenzymatic ligand.
Type: biochemical activity and inhibition profiling
Experiment: Use affinity labeling, cell-surface proteomics, and genome-wide CRISPR resistance screening during recombinant-R3HDML stimulation of human myogenic progenitors.
Hypothesis: Mature R3HDML engages a specific surface receptor upstream of IGF-1/Akt and myogenic cell-cycle responses.
Type: receptor identification and genetic epistasis
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The direct molecular activity, receptor, substrate, and extracellular binding partner of mature R3HDML are unknown.
OPEN BIOLOGY MF_DARK
What is known: R3hdml is demonstrably secreted, extracellular protein rescues loss in mouse myogenic cells, and loss affects proliferation and differentiation, but direct Akt binding was not detected and receptor activation versus extracellular- matrix/protease regulation remain untested alternatives.
Significance: Resolving this mechanism is necessary for a defensible molecular-function term and for understanding how R3HDML controls satellite cells.
What would resolve it: Purified-protein protease panels, receptor capture, extracellular interactomics, and separation-of-function rescue mutants should distinguish ligand, adaptor, matrix-regulator, and peptidase-inhibitor models.
Provenance (the field's own admissions):
Gap: Conservation of the mouse skeletal-muscle developmental and regenerative role in human R3HDML has not been demonstrated.
OPEN BIOLOGY BP_DARK
What is known: The human protein retains the signal peptide, propeptide, and CAP domain, while the only causal functional study uses mouse cells and mice and adult human expression resources emphasize intestinal tissue.
Significance: Direct human evidence is needed before treating the myogenic process terms as more than well-motivated orthology transfers.
What would resolve it: Endogenous expression, secretion, knockout, and mature-protein rescue should be tested in primary human satellite cells and regenerating human muscle models.
Provenance (the field's own admissions):
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