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Regulation of Myelination: Schwann Cell Transition from a Myelin‐Maintaining State to a Quiescent State After Permanent Nerve Transection
Author(s) -
Poduslo Joseph F.,
Dyck Peter J.,
Berg Carole T.
Publication year - 1985
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1985.tb05428.x
Subject(s) - myelin , sciatic nerve , schwann cell , anatomy , myelin sheath , chemistry , myelin associated glycoprotein , axoplasmic transport , pronase , biology , central nervous system , endocrinology , biochemistry , enzyme , trypsin
Permanent nerve transection of the adult rat sciatic nerve forces Schwann cells in the distal nerve segment from a myelin‐maintaining to a quiescent state. This transition was followed by serial morphometric evaluation of the percentage fascicular area having myelin (myelin percent of area) in transverse sections of the distal nerve segment and revealed a rapid decline from a normal value of 36.6% to 3.2% by 14 days for the sciatic nerve to < 1.0% throughout the remaining time course (up to 105 days). No evidence of axonal reentry into the distal nerve segment or new myelin formation was observed at times under 70 days. In some of the distal nerve segments at 70, 90, and 105 days, new myelinated fibers were observed that usually consisted of only a few myelinated fibers at the periphery and in the worst case amounted to 1.6% (myelin percent of area). Radioactive precursor incorporation of [ 3 H]mannose into endoneurial slices at 4 and 7 days after transection revealed two species of the major myelin glycoprotein, P 0 , with M r of 28,500 and 27,700. By 14 days after nerve transection, only the 27,700 M r species remained. Incorporation of [ 3 H]mannose into the 27,700 M r species increased progressively to 35 days after transection and then began to decline at 70 and 105 days. Alterations in the oligosaccharide structure of this down‐regulated myelin glycoprotein accounted for the progressive increase in mannose incorporation. Lectin affinity chromatography of pronase‐digested P 0 glycopeptides on concanavalin A‐Sepharose revealed that the 28,500 M r species of P 0 had the complex‐type oligosaccharide as the predominant oligosaccharide structure (92%). In contrast, the high mannose‐type oligosaccharide was the predominate structure for the 27,700 M r form, which increased to 70% of the total radioactivity by 35 days after nerve transection. Since the biosynthesis of the complex‐type oligosaccharide chains on glycoproteins involves high mannose‐type intermediates, the mechanism of down‐regulation in the biosynthesis of this major myelin glycoprotein, therefore, results in a biosynthetic switch from the complex‐type oligosaccharide structure as an end product to the predominantly high mannose‐type oligosaccharide structure as a biosynthetic intermediate. This biosynthetic switch occurs gradually between 7 and 14 days after nerve transection and likely reflects a decreased rate of processing through the Golgi apparatus. It remains to be determined if the high mannose‐type oligosaccharide chain on P 0 can undergo additional processing steps in this permanent nerve transection model.