Oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) are responsible for producing the multilayered myelin sheath that surrounds the majority of nerve axons. The myelin sheath promotes rapid, saltatory nerve conduction and is essential for proper nervous system function in higher vertebrates. The process by which axons are ensheathed with the multilayered myelin membrane is exceptionally complex. A number of molecules have been shown to be essential for the formation of the myelin structure. For example, there are myelin-specific proteins and lipids that appear to play critical roles in myelin formation and maintenance. Additionally, the interactions of myelinating cells with the extracellular environment and with their target axons are critical for proper myelin sheath function. The underlying hypothesis of this proposal is that glycosphingolipids represent an important class of molecules in myelinating cell function. We believe that glycosylated lipids facilitate many key steps in the formation and maintenance of the myelin sheath, including mediating the interactions of myelinating cells with axons as well as with their extracellular environment. Until recently, the glycosphingolids have largely been characterized using biochemical techniques. The focus of the proposed studies is to use genetic approaches to better understand the role that these molecules specifically play in the myelination process. We will examine the myelinating cell function of enzymes that transfer either glucose (UDP-glucose:ceramide glucosyltransferase) or galactose (UDP-galactose: ceremide galactosyltransferase) to ceramide. Glucosylceramide serves as the backbone for the majority of the gangliosides (sulfated glycolipids), and galactocerebroside, and its sulfated derivative sulfatide, represent the most abundant glycolipid components of the myelin sheath. In myelinating cells gangliosides and the galactolipids have been shown to be residents of lipid rafts, along with a class of cell adhesion proteins that are attached to the plasma membrane through a glycan linkage, (glycophosphatidylinositol [GPI]-linked proteins). We will examine whether the function of the glycolipids, at least in part, is mediated through the transport of the GPI-linked proteins by these raft domains. Together the studies described in this proposal should provide us with a better appreciation of the importance of glycosphingolipids in the complex biological processes of myelinating cells
