Expression and Localization of Myelin Basic Protein in Oligodendrocytes and Transfected Fibroblasts

Abstract

Myelin basic protein (MBP) is a major structural component of myelin. It is expressed exclusively in myelinating glia (oligodendrocytes in the CNS and Schwann cells in the PNS) and is localized to the cytoplasmic surface of the plasma membrane and myelin membrane produced by these cells. The work described here concerns the mechanism of plasma membrane localization of MBP in myelinating glial cells and whether it involves differentiated functions specific to these cells or general functions of plasma membrane assembly common to all cells. To this end, the subcellular localization of endogenous MBP in mouse oligodendrocytes was compared with that of transiently expressed MBP in monkey fibroblasts (Cos-1 cells) transfected with an MBP expression vector containing cDNA for rat 14K MBP. The steady-state levels of MBP-specific RNA and of MBP polypeptide expressed in the transfected fibroblasts were comparable to the levels expressed in oligodendrocytes in primary culture. MBP localization was analyzed in whole cells by immunofluorescence and in specific intracellular compartments by subcellular fractionation. The results show that MBP expressed in wild-type oligodendrocytes is localized to the plasma membrane. In contrast, MBP expressed in transfected fibroblasts appears dispersed in the cytoplasm and is distributed uniformly among the various subcellular fractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential and cell development-dependent localization of myelin mRNAs in oligodendrocytes

“…As the cell body has a much larger volume than the processes, especially the tiny secondary ones, the prominent signal present in the processes indicates a much higher local concentration of MBP mRNA in the processes than in the cell body. These observations are consistent with earlier work (Colman et al, 1982;Barbarese et al, 1988;Verity and Campagnoni, 1988), demonstrating that MBP mRNAs are transported to or near the site where its protein product is synthesized, followed by instantaneous insertion of MBP into the myelin membrane. Note that the distribution of total MBP mRNA is not significantly affected by the culturing time of the cells (Fig.…”

“…The MBP gene contains seven exons, from which the MBP mRNA isoforms are formed by alternative splicing (Campagnoni, 1988). In myelinating OLG the majority of mRNAs coding for MBP are present in the processes (Colman et al, 1982;Barbarese et al, 1988;Verity and Campagnoni, 1988;Amur-Umarjee et al, 1990). Therefore, mechanisms for specific transport and, presumably, for sorting of MBP mRNAs must exist in OLG.…”

Regulation of Cytoskeleton by Myelin Components: Studies on Shiverer Oligodendrocytes Carrying an <i>Mbp</i> Transgene

“…One possibility is that the amount of MBP (any isoform) is critical for domain formation. However, this does not appear to be likely since MBP domains are not observed in shiverer and other cell types that have been transfected with individual MBP isoforms, and which express these isoforms MBP at high levels [36][37][38]Dyer,unpubl. observations].…”

Expression of Myelin Protein Genes and Other Myelin Components in an Oligodendrocytic Cell Line Conditionally Immortalized with a Temperature‐Sensitive Retrovirus

“…In normal oligodendrocytes, several posttranscriptional regulatory steps exist that may modulate MBP polypeptide levels. These steps include translocation of MBP mRNAs (Trapp et al, 1987;Verity and Campagnoni, 1988a;Amur-Umarjee et al, 1990b), translational modulation by steroids (Verdi et al, 1989), posttranslational modification (reviewed by Campagnoni and Macklin, 1988), and potentially unidentified membrane assembly steps (Barbarese et al, 1988;Verity and Campagnoni, 1988b;Allinquant et al, 1991). The absence of any one of these potential regulatory steps, e.g., the localization of the MBP mRNAs to the cell bodies of N20.1 cells, could result in rapid degradation of MBP polypeptides.…”