Receptor-Type Protein-Tyrosine Phosphatase μ Is Expressed in Specific Vascular Endothelial Bedsin Vivo

We investigated the localization of receptor-type protein-tyrosine phosphatase μ (RPTPμ) in tissues by immunofluorescence. RPTPμ immunoreactivity was found almost exclusively within vascular endothelial cells. RPTPμ was more abundant in the arterial tree than in the venous circulation. This pattern of expression was opposite to that of the von Willebrand factor and demonstrated a lack of difference in expression of VE-cadherin. RPTPμ was undetectable in the endocardium. In agreement with previous work on nonendothelial cell lines, RPTPμ was exclusively at the lateral aspects of endothelial cellsin vivoand at cell–cell contacts as well asex vivoin two- or three-dimensional endothelial cell cultures, and expression levels were upregulated by cell density. RPTPμ was detected in few other cells: bronchial and biliary epithelia and cardiocytes (intercalated discs). Our results identify RPTPμ as a new marker of endothelial cell heterogeneity and suggest a possible role in endothelial-specific functions, involving cell–cell contact.