Opalin, or oligodendrocytic myelin paranodal and inner loop protein, is a transmembrane protein detected specifically in mammalian oligodendrocytes, and may play significant role in oligodendrocyte differentiation and myelination.Opalin has binding sites for Myt1 and cAMP-response element binding protein (CREB). Over-expression of Myt1, treatment of the cell with leukemia inhibitory factor (LIF), and cAMP analog (CREB activator) enhanced the expression of endogenous Opalin in Oli-neu cells and activated the oligodendrocyte enhancer. Thus LIF, cAMP signaling cascades and Myt1 may play significant roles in the differentiation of oligodendrocytes through their action on the Opalin oligodendrocyte enhancer. Enzymatic deglycosylation showed that myelin Opalin contained N- and O-glycans, and that the O-glycans, at least, had negatively charged sialic acids. Site-directed mutations at the glycan sites impaired the cell surface localization of Opalin. In addition to the somata and processes of oligodendrocytes, Opalin immunoreactivity was observed in myelinated axons in a spiral fashion, and was concentrated in the paranodal loop region. Immunogold electron microscopy demonstrated that Opalin was localized at particular sites in the paranodal loop membrane. These results suggest a role for highly sialylglycosylated Opalin in an intermembranous function of the myelin paranodal loops in the central nervous system.
