Developmental Aspects of Myelin Membrane Protein Synthesis: Spatial Segregation of Polyribosomes
The rapid and precise construction of the myelin sheath is one of the most remarkable aspects of nervous system development and maturation. In the CNS, the myelinating cell is the oligodendrocyte, a cell with a small soma that supports via slender cytoplasmic “arms” thirty or more individual, flattened expanses of spirally-wrapped, tightly compacted myelin membrane. Thin section electron microscopy of the mature myelin sheath reveals an alternating pattern of thick (major dense, MDL) and thin (intraperiod, IDL) lines that represent the apposition of the cytoplasmic and extracellular aspects, respectively, of the oligodendrocyte plasma membrane bilayer (1). During rapid myelination, this cell generates vast quantities of plasma membrane, and inserts into this lipid matrix the myelin-specific polypeptides that presumably participate in the initial organization and subsequent maintenance of the mature sheath. How the oligodendrocyte precisely targets these proteins to the developing myelin domain of the plasma membrane is a challenging problem, especially since it is difficult to imagine that complete control of the local assembly process along individual myelin internodes resides solely in the cell soma, which comprises only a small fraction of the cell volume, and is for the most part located a considerable distance from the myelinating regions. In this short paper, we summarize the evidence that the oligodendrocyte controls the myelination process in part locally at each developing internode, by selectively segregating to the myelinating region those free polysomes engaged in synthesizing a specific set of major myelin proteins, the myelin basic proteins.
KeywordsMyelin Basic Protein Myelin Sheath Myelin Membrane Guanidinium Hydrochloride Free Polysome
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