MEGF8 controls osteogenic differentiation through post-transcriptional regulation of BMP-SMAD signaling in craniosynostosis.

Carpenter syndrome, caused by biallelic mutations in MEGF8 or RAB23, manifests with craniosynostosis through incompletely defined mechanisms. While both genes encode negative regulators of Hedgehog (Hh) signaling, we demonstrate that MEGF8 maintains cranial suture patency through a distinct, Hh-independent pathway. Loss of MEGF8 disrupts ubiquitination and lysosomal degradation of BMPR1A, leading to BMPR1A accumulation and hyperactivation of canonical BMP-SMAD1/5/9 signaling, which accelerates osteogenic differentiation of cranial mesenchyme. Using Megf8 mutant mice, we show tissue-specific specialization: limb defects are Hh-dependent and rescued by SMO inhibition, whereas craniosynostosis is BMP-driven and refractory to Hh blockade, with BMP type I receptor inhibition selectively rescuing the cranial phenotype. Comparative analyses reveal that MEGF8 and RAB23 promote osteogenic differentiation through distinct mechanisms-MEGF8 via ubiquitin-mediated BMPR1A turnover and BMP-SMAD activation, RAB23 through FGF-ERK signaling-despite both affecting GLI1-mediated transcription. Reintroduction of human MEGF8 in MEGF8-knockdown cells restores BMPR1A protein levels, validating the specificity of MEGF8-mediated BMPR1A regulation. These findings suggest that MEGF8 modulates BMP signaling post-transcriptionally, establishes tissue-specific regulatory mechanisms in syndromic disorders, and demonstrates how divergent pathways converge on shared phenotypes, with implications for pathway-specific therapeutic strategies.