TMEM263

TMEM263 is a small multi-pass membrane protein required for postnatal longitudinal growth through its support of the growth hormone (GH)/IGF-1 axis (PMID:29930570, PMID:38241182, PMID:37577461). In mice, deletion of Tmem263 causes severe proportional dwarfism driven by disruption of GH signaling: null animals have low circulating IGF-1, reduced hepatic GH receptor expression, and impaired GH-induced JAK2/STAT5 signaling, with consequent feminization of the male liver transcriptome resembling Stat5b-null or hypophysectomized males (PMID:38241182, PMID:37577461). The growth requirement is conserved across species, as loss-of-function mutations cause autosomal dwarfism in chickens (PMID:29930570) and lethal rhizomelic skeletal dysplasia in a human fetus (PMID:34238371). At the cellular level, TMEM263 is an ER-resident protein whose two transmembrane domains fold into a hairpin that targets it to the ER and to lipid droplets, where it is both necessary and sufficient for lipid droplet biogenesis. How the protein's role in lipid droplet biology connects mechanistically to hepatic GH receptor expression and JAK2/STAT5 signaling has not been resolved in the available corpus.

1. 2018 Medium

   Establishing that TMEM263 loss-of-function is causal for a growth phenotype, rather than merely correlated, anchored the gene's biological role in body growth.

   Evidence Whole-genome sequencing, fine mapping, and co-segregation of a p.Trp59* nonsense variant with autosomal dwarfism in chickens

   PMID:29930570

   Open questions at the time

   • No in vitro functional reconstitution of the mutation's effect
   • The noted GH1 interaction was not validated biochemically
   • Cellular mechanism linking the protein to growth was undefined
2. 2021 Low

   Identification of a human case extended the growth phenotype across species, suggesting TMEM263 acts within the GH signaling pathway in humans.

   Evidence Whole exome sequencing of a fetus with severe lethal rhizomelic skeletal dysplasia identifying a homozygous frameshift mutation

   PMID:34238371

   Open questions at the time

   • Single case, WES-only, no functional validation
   • Pathway involvement inferred rather than experimentally demonstrated
   • No mechanistic link to GH signaling established in human cells
3. 2024 High

   A knockout mouse pinpointed the mechanism of the growth defect to disruption of the GH/IGF-1 axis at the level of hepatic GH receptor expression and JAK2/STAT5 signaling.

   Evidence Constitutive Tmem263 knockout mice with serum IGF-1 measurement, hepatic GHR expression, GH-induced JAK2/STAT5 signaling assays, and liver transcriptomics

   PMID:37577461 PMID:38241182

   Open questions at the time

   • Molecular mechanism by which TMEM263 supports GHR expression is undefined
   • Whether the effect is liver-cell-autonomous or pituitary/systemic is unresolved
   • Direct physical role of TMEM263 in the GHR/JAK2 complex not demonstrated
4. 2025 Medium

   Defining TMEM263 as an ER/lipid-droplet protein necessary and sufficient for lipid droplet biogenesis provided a candidate cellular function underlying the growth phenotypes.

   Evidence ER-resident protein screen, transmembrane-hairpin domain mutagenesis, live-cell lipid droplet imaging, and loss-of-function in cultured cells and zebrafish (preprint)

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Proposed neutral-lipid condensation mechanism not biochemically reconstituted
   • Connection between lipid droplet function and hepatic GH receptor signaling not established

• How TMEM263's lipid droplet / ER membrane function mechanistically controls hepatic GH receptor expression and JAK2/STAT5 signaling remains the central open question.
• No molecular intermediary linking lipid droplet biology to GHR expression identified
• No direct binding partner within the GH signaling machinery validated
• Tissue-specific conditional models not reported