Research progress on the structure and function of endomucin

Abstract Endomucin is a type I integral membrane glycoprotein, which is expressed in venous and capillary endothelial cells. It consists of 261 amino acids with an extracellular domain that is highly O‐glycosylated at serine and threonine residues and has several potential N‐glycosylation sites. Endomucin plays an important role in biological processes such as cell interaction, molecular cell signaling, angiogenesis and cell migration, and in recent years it has also been identified as an anti‐adhesion molecule and a marker of endothelial cells. While it has been shown to be involved in a number of physiological and pathological mechanisms, many of its functions remain unknown, and further study is needed. This article reviews research progress on the function of endomucin to date, in order to provide guidance for future studies.


| INTRODUC TI ON
Endothelial cells rely on mechanical and humoral factors to maintain the endothelial barrier function and to avoid inappropriate loss of humor and solutes. Under physiological conditions, endothelial cells activate extracellular signals that lead to spatio-temporal regulation of gene expression. Under pathological conditions such as inflammation, fluid permeability in the infected area is greatly increased, thus promoting host defense mechanisms. 1 Therefore, studying the molecular mechanisms of endothelial cell-specific regulation may reveal the cellular processes that lead to the occurrence and development of many diseases.
Endomucin, a component of the glycocalyx, is a highly O-glycosylated single transmembrane sialomucin expressed by endothelial cells, specifically by capillaries and venous endothelium, but not by most arterial endothelium. 2,3 Endomucin also exists in the lymphatic sinus endothelium, but not in the lymphatic endothelial cells of the subcapsular sinus. 4 Mai Nguyen and others have established that the protein sequence of the long splicing variant of endomucin in humans and mice contains 261 amino acid residues and consists of a long extracellular part (aa 1-C190), a transmembrane part (aa 191-C214) and cytoplasmic sequence (aa 215-C261) ( Figures 1 and 2). In addition, 30% of its amino acids are serine or threonine residues, allowing it to covalently bind to O-glycans through its extracellular domain. 5-7

| MAIN FUN C TI ON OF ENDOMUCIN
Endomucin is mainly expressed by endothelial cells around the postcapillary venules and a large number of highly vascularized tissues such as the heart, kidney and lung. 2,5,8 Post-capillary venules are the main site for leukocyte recruitment during internal perfusion. Endomucin inhibits leukocyte-endothelial cell adhesion via lymphocyte function-associated antigen 1 (LFA-1)-mediated binding to intercellular adhesion molecule 1 (ICAM-1) in non-inflamed tissues, and downregulation of endomucin by tumor necrosis factor (TNF)-α critically promotes neutrophil infiltration into inflamed tissues. 9 Treatment of endothelium with TNF-α or the strong oxidant pervanadate leads to loss of cell-surface endomucin and increases the levels of the C-terminal fragment (EMCN-CTF). Chemical inhibition of the cell surface protein ADAM10 alone or in combination with ADAM17 can block the release of EMCN-CTF induced by TNFα. Thus endomucin represents a potential therapeutic target in the management of vascular inflammation. 10 Endomucin is also expressed in retinal endothelial cells, and the expression level is reduced under high blood glucose in vitro and in vivo. Its overexpression can restore the glycocalyx of retinal endothelial cells induced by streptozotocin in diabetic rats. 11 In addition to reducing the adhesion of leukocytes to endothelial cells, overexpression of endomucin can also reduce inflammation, stabilize the blood-retinal barrier, and inhibit vascular leakage. 12 Endomucin is the target of endothelial glycocalyx degradation; it protects diabetic patients from retinal vascular degeneration by restoring the glycocalyx of endothelial cells. Therefore, it represents a new treatment strategy for diabetic retinopathy.
Another mechanism by which endomucin enhances glycocalyx is through its interaction with galectin. Previous studies have found that O-glycans on transmembrane mucins interact with galactose lectins to form multivalent protective crystal complexes, creating organized transmembrane mucins and aggregating on the glycocalyx on the cell surface. 13,14 Endomucin, also called endothelial sialomucin, interferes with the assembly of focal adhesion complexes and inhibits interaction between cells and the extracellular matrix, and maintains normal physiological functions.

| Effect of endomucin on biological functions of endothelial cells
Endothelial cell migration is the central link in the process of vascular morphogenesis. 19 As is well known, the cytoskeleton is an important part of cell morphology and motor regulation. In endothelial cells with reduced endomucin expression, a decrease in F-actin expression can be observed, which suggests that the presence of endomucin is a necessary condition for the formation of F-actin in endothelial cells. 20 Studies have shown that expression of endomucin is not affected by treatment with IL-1 injection, but the level of fucosylation is significantly increased after IL-1 treatment. Furthermore, endomucin glycosylation in endothelial cells is directly related to monocyte-endothelial cell adhesion, which is achieved by blocking anti-endomucin or sLex (sialyl-Lewis X, sLex) antibodies in endothelial cells overexpressing FUT7 (α1,3-karst-based transfer Enzyme VII gene). 6 In recent years, evidence has accumulated showing that endomucin is an anti-adhesion molecule. Activation of factors stimulating endomucin can down-regulate the expression of endomucin on the surface of endothelial cells and at the same time increase the adhesion between white blood cells and endothelial cells, while the absence of endomucin leads to an increase in the interaction between white blood cells and endothelial cells. 6,9

| OTHER FUN C TI ON S OF ENDOMUCIN
Endomucin was initially considered to be an endothelial cell-specific protein, but it has also been shown to be involved in the development of embryonic stem cells. 21 Endomucin has been shown to be a better marker for hematopoietic stem cells than CD34 protein.

Human adult hematopoietic stem cells (HSCs) can be identified by
hematopoietic progenitors associated with the expression of endomucin. 22 Beside HSCs, endomucin can be also used as a marker in the study of skin endothelium. 3 In terms of bone formation and development, the VEGF pathway controls the coupling of angiogenesis and osteogenesis in orthopedic implant-bone integration by affecting the formation of CD31hi-EMCNhi endothelial cells. 23 Endomucin was found to be associated with rheumatoid arthritis in a Japanese population, and the endomucin allele associated with rheumatoid arthritis susceptibility may also be involved in the pathogenesis of rheumatoid arthritis. 24  At present, there are few studies on the adhesion or anti-adhesion activity of endomucin. It has been reported that endomucin membrane protein is highly sensitive to L-selectin oligosaccharides, and can be used as a ligand of L-selectin to play a similar biological role. 32 As a typical sialic acid mucin, endomucin is highly accessible on the cell surface, enabling some part of it to support or prevent cell adhesion. At the same time, endomucin is also a type of glycoprotein on the membrane. Its structural and expression level changes may affect the biological functions of cells such as invasion, metastasis, and adhesion. Whether endomucin affects the adhesion between tumor cells and endothelial cells has not been reported. The study of the adhesion between tumor cells and endothelial cells will provide a new theoretical basis for tumor metastasis.

CO N FLI C T O F I NTE R E S T
None.