| Endomucin (EMCN, MUC14) is a type I transmembrane, mucin-like O‑glycosylated glycoprotein that resides in the endothelial glycocalyx of venous and microvascular endothelium and also labels long‑term, multi‑lineage repopulating hematopoietic stem cells in both mouse and human systems. The luminal extracellular region extends into the glycocalyx and is rich in glycosylation, generating an anti‑adhesive barrier that reduces direct leukocyte and platelet contact with the endothelial surface and contributes to organization of signaling receptors at the luminal membrane. A defined N‑terminal segment of the EMCN extracellular domain, encompassing amino acids 21–121, is sufficient to support vascular endothelial growth factor (VEGF)‑induced VEGF receptor 2 (VEGFR2) internalization and angiogenic function, and mutation of specific N‑glycosylation sites within this region abolishes VEGFR2 binding and internalization, highlighting a direct structural requirement for EMCN‑dependent control of VEGFR2 endocytosis and downstream signaling. EMCN thereby modulates VEGF‑driven endothelial migration, proliferation, and tube formation by selectively regulating VEGFR2 trafficking rather than serving only as a passive glycocalyx component, integrating receptor endocytosis with maintenance of an anti‑adhesive luminal interface that shapes permeability, leukocyte access, and angiogenic output in microvascular beds. In the hematopoietic compartment, EMCN expression marks a subset of long‑term hematopoietic stem cells that exhibit higher long‑term multi‑lineage repopulating capacity, increased quiescence, and distinctive transcriptional programs compared with EMCN‑negative counterparts, linking surface EMCN to stem‑cell maintenance states within the niche. EMCN is essential for efficient transendothelial migration and mobilization of long‑term HSCs during hematopoietic stress and transplantation, where endothelial EMCN contributes to maintenance and self‑renewal of the HSC pool and sustained blood cell production after engraftment. Across tissues, EMCN enrichment in specialized bone vasculature and vascular beds involved in inflammatory and malignant processes associates its expression pattern with vascular inflammatory diseases, bone remodeling defects, and cancer, in which changes in EMCN levels accompany altered angiogenesis, endothelial activation, and stem‑cell trafficking and provide a context‑dependent biomarker of vascular and hematopoietic states. |
