Plasma membrane of cultured oligodendrocytes: III. Relatedness to myelin

Abstract

We have compared highly purified fractions of oligodendrocyte plasma membrane to myelin by one- and two dimensional gel electrophoresis and found them to be distinct. The major myelin proteins--proteolipid protein (PLP), DM-20, and myelin basic protein (MBP), which dominate the sodium dodecyl sulfate polyacrylamide gel electrophoresis pattern of myelin--were minor components of the plasmalemma. However, 2',3', cyclic nucleotide phosphodiesterase (CNPase) and myelin-associated glycoprotein (MAG) were represented equally in both membranes. Labeling the cells with various precursors followed by isolation of plasmalemma revealed that newly synthesized PLP, DM-20, CNPase, and MAG were incorporated into the plasma membrane of "floating" oligodendrocytes (i.e., nonattached to substratum). This was not so with MBP. Nevertheless, scattered patches of MBP were localized on the plasma membrane of intact cells using the immunogold method at the electron microscopic level. The data are consistent with the notion that MBP is not a constituent of the plasma membrane of mature oligodendrocytes (the MBP patches on intact cells are likely remnants from past association with myelin) but is rapidly associated with the plasmalemma of myelinating oligodendrocytes (i.e., attached cells). It is suggested that phosphorylation of MBP provides the triggering signal for plasma membrane association. In order to analyze the minor proteins in myelin and compare them to the plasma membrane by two-dimensional gel electrophoresis, myelin was extracted with chloroform:methanol to remove PLP, DM-20, and MBP. Even in the absence of PLP, DM-20, and MBP the pattern of extracted myelin still differed from that of plasmalemma indicating that their minor protein compositions were not the same. Myelin was characterized by a group of proteins that clustered at pI 5.5-6.5 and Mr 40,000-60,000 of which alpha-tubulins, beta-tubulins, and actin are part: the plasmalemma had tubulins and actin but in different proportions. Our findings indicate that in addition to PLP, DM-20, and MPB, myelin is also enriched relative to the plasmalemma in another group of proteins.

Functional implications for the microtubule-associated protein tau: localization in oligodendrocytes.

“…A colocalization between MBP and the microtubule protein tubulin has also been reported (48). In a comparative study of the protein composition of highly purified fractions of OLG plasmalemma and myelin by two-dimensional gel electrophoresis, Szuchet et al (49) showed that both membranes had actin and a-and f3-tubulins. However, there were significant differences in their contents: OLG plasmalemma was enriched in actin relative to the tubulins; the opposite was the case for myelin.…”

CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process

“…Given that OLG plasma membrane and myelin are compositionally distinct beyond the major proteins (Jahn et al, 2009;Szuchet et al, 1988;Vanrobaeys et al, 2005), questions concerning myelin synthesis, transport and deposition are highly pertinent and, yet, remain largely unanswered. We have addressed these topics using transmission electron microscopy (TEM), immuno-EM and threedimensional electron tomography (3D-ET) on an in vitro model consisting of pure cultures of OLGs and on embryonic (E) avian optic nerve at stages E12, E15, E17 and E21.…”

Biosynthesis and expression of the myelin‐associated glycoprotein in cultured oligodendrocytes from adult bovine brain

“…Immunocytochemical examination of sections of CNS tissue demonstrated that in vivo it is present in the periaxonal membranes and cytoplasm of oligodendrocytes (Sternberger et al, 1979;Trapp et al, 1989). It has also been detected immunocytochemically in cultured rat (Dubois-Dalcq et al, 1986) and mouse (Poltorak et al, 1987) oligodendrocytes and by Western blotting in cultured porcine (Gebicke-Harter et al, 1984) and ovine (Yim et al, 1986;Szuchet et al, 1988) oligodendrocytes. However, biochemical investigation of MAG in cultured oligodendrocytes is limited so far to cells from developing rat brain where the presence of mRNA for both isoforms (Tropak et al, 1988) and its phosphorylation (Bambrick and Braun, 1991) have been demonstrated.…”