Immune system-related CD9 is expressed in mouse central nervous system myelin at a very late stage of myelination

CD9 is a tetra-membrane-spanning glycoprotein involved in cell adhesion, migration, and proliferation in both the immune and the immature nervous system. In this study, CD9 expression was detected in myelin of mouse brain, starting at postnatal day 16. The amount of CD9 protein continuously increased with age and persisted in the adult brain. It appeared later than myelin proteolipid protein (PLP), a typical late myelin marker. Mature oligodendrocytes were abundant in CD9, although it was also detected in astrocytes and microglial cells in vitro. CD9 appeared at the end of the myelination process and was localized along the outermost membrane of compact myelin. CD9 is known to associate with integrins, which are candidate receptors for extracellular matrix and transmit extracellular signals into the cells. Taken together, CD9 at the surface of central nervous system (CNS) mature myelin may have a unique function to facilitate signal transduction and enhance myelin membrane adhesion to extracellular matrices at very late stages of development.