Myelin basic protein deficit in the PNS of mld mutant mice recovers during development.

Myelination was studied between 15 and 135 days postnatally in peripheral nerves of myelin deficient (mld) mice and in unaffected littermates. The nerve weights were not affected by the mutation and showed a 4-fold increase during the developmental period studied. The amounts of myelin present in peripheral nerves, as shown by biochemical and morphological techniques, were slightly reduced in mld in comparison to control mice. In controls, the concentration of myelin doubled during the investigation period. The increase of myelin basic protein (MBP) in total nerve homogenate paralleled the deposition of myelin, but the MBP concentration remained constant in normal myelin. In contrast, in mld myelin MBP concentrations were extremely low until 60 days of age and increased thereafter to reach almost normal values at 135 days. Similarly, the amounts of myelin isolated at 85 and 135 days were normal. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP; EC 3.1.4.37), the myelin-specific enzyme, showed normal specific activities in mld nerves. In mld and control myelin, CNP-specific activities decreased during development suggesting a preferential localization of CNP in Schwann cell plasma membranes. In contrast to the central nervous system, other myelin proteins were not altered in mld peripheral nervous system (PNS) and the very low MBP content had no severe repercussions on the composition and structure of the myelin sheath. Furthermore, Schwann cells appeared normal in mld PNS. Nevertheless, more subtle alterations could be detected. Slightly decreased amounts of myelin were observed in young mld mice and preliminary results indicate discrete alterations of the myelin periodicity.(ABSTRACT TRUNCATED AT 250 WORDS)