Among the most abundant components of myelin are the galactolipids galactocerebroside (Ga1C) and sulfatide. In spite of this abundance, the roles that these molecules play in the myelin sheath are not well understood. Until recently, our concept of Ga1C and sulfatide functions had been principally defined by immunological and chemical perturbation studies that implicate these lipids in oligodendrocyte differentiation, myelin formation, and myelin stability. Recently, however, genetic studies have allowed us to re-analyze the functions of these lipids. We have cloned the gene encoding the enzyme UDP- galactose: ceramide galactosyltransferase (CGT), which is required for myelin galactolipid synthesis, and we have generated mice that are incapable of synthesizing either Ga1C or sulfatide by inactivating the CGT gene in the mouse germline. These galactolipid-deficient animals exhibit a severe tremor, hindlimb paralysis, and electrophysiological deficits. In addition, ultrastructural studies have revealed hypomyelinated white matter tracts with unstable myelin sheaths and a variety of myelin irregularities. Longitudinal EM analysis has exposed a number of profound abnormalities at the node of Ranvier, including increased nodal length, reversed lateral loops, and compromised axo-oligodendrocytic junctions. Collectively, these observations indicate that cell-cell interactions, which are essential in the formation and maintenance of a properly functioning myelin sheath, are compromised in the galactolipid-deficient mice. The studies described in this proposal are designed to further explore the function of the myelin galactolipids, using the CGT mutant animals as a key resource. We will determine the degree to which glucocerebroside, a lipid that atypically accumulates in the CGT mutant's myelin, compensates for the loss of the galactolipids. We will further explore the role that the galactolipids play in myelinating cell development and in the early stages of the myelination process. The function of the galactolipids in node of Ranvier formation will also be examined in detail. Furthermore, the contribution that these molecules make to the stability of the myelin sheath will be studied. Together, this work will significantly increase our understanding of the function of the galactolipids in myelinating cell development, the myelination process, and in the stability of the myelin sheath.
