Glycoproteins of Myelin and Myelin-Forming Cells

Abstract

Myelin is formed as an extension and modification of the surface membrane of the oligodendrocyte in the central nervous system (CNS) and the Schwann cell in the peripheral nervous system (PNS). The extended plasma membranes are wrapped around the axons in a spiral fashion, leading to a structure that is seen in the electron microscope to consist of alternating major dense and intraperiod lines. The relationship of the myelin sheath to the myelin-forming cells and the axon, as well as the morphological and biochemical differences between CNS and PNS myelin, are described in detail elsewhere (Morell et al., 1989). However, the basic structures of CNS and PNS myelin sheaths are similar, with the major dense lines consisting of the structure formed by the junction of the cytoplasmic surfaces of the plasma membrane of myelin-forming cells and the intraperiod lines corresponding to the point at which the extra-cellular surfaces come together. The membranes composing the compacted myelin sheaths are characterized by a high lipid content and only a few major proteins. CNS myelin has two major proteins: the hydrophobic 30-kDa proteolipid protein (PLP) accounting for about half of the total, and the highly positively charged, 14- to 21-kDa myelin basic proteins (MBPs) accounting for over one-third of the total. More than half of the total protein in PNS myelin is the integral, 30-kDa P0 protein, while about one-fourth to one-third is accounted for by lower-molecular-weight positively charged proteins including the same MBPs found in CNS myelin and another 14-kDa component called the P2 protein. Current concepts about how these major myelin proteins are incorporated into the myelin sheath are summarized in Figure 1.