Mechanisms of membrane biogenesis in oligodendrocytes: Roles for Sec8 and Rab3a

Rapid nerve impulse transmission is essential for integration of information in the nervous system. In vertebrates this has been ensured by the acquisition of the myelin sheath, synthesized as a membrane extension by oligodendrocytes (OL) in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). Substantial information has been acquired during the last decades, first on the biophysical and biochemical characteristics of the myelin sheath, and more recently on the molecular components and developmental formation of the axo-glial apparatus. However, little is known about the molecular basis of myelin membrane extension, polarization and maintenance. The goal of this study was to identify and characterize components of the trafficking machinery involved in OL differentiation and myelin membrane biogenesis. For this, we used fluorescence microscopy, co-immunoprecipitation, RNA interference and overexpression analyses and we took advantage of the well-characterized OL lineage. We found that OLs express several key molecules necessary for the targeting of transport carriers to areas of rapid membrane growth. One of these molecules, Sec8, promotes OL process branching and myelin-like membrane formation in vitro. In mature OLs, Sec8, together with CASK, a multidomain scaffolding protein, associates with oligodendrocyte specific protein (OSP), suggesting that Sec8 plays a role in OSP recruitment to the OL membrane. These results suggest that Sec8 plays a central role in oligodendrocyte process growth and membrane formation, and it may regulate the recruitment of carriers that transport myelin proteins such as OSP to sites of membrane growth. Rab3a, another regulator of exocytosis, is important for mature OL differentiation and it may play a role in myelin membrane biogenesis. In OL progenitors, CASK associates with syndecan-3, a co-receptor in fibroblast growth factor-2 (FGF-2) signaling, indicating that CASK may be part of a cytoplasmic scaffolding complex that links transmembrane signaling molecules with the cytoskeleton. In conclusion, we have characterized a series of molecules involved in OL membrane biogenesis, findings that contribute to a better understanding of the mechanisms that regulate myelin membrane formation and homeostasis, and to developing future therapies aimed at promoting remyelination.