Freeze-Fracture Localization of Myelin Proteins

Cullen, Michael

Abstract

Myelin is a morphologically and functionlly specialized membrane formed and maintained by glial cells. Both the peripheral nervous system (PNS) and central nervous system (CNS) myelin sheaths in the mouse contain large basic protein (LBP) and small basic protein (SBP). PNS and CNS myelin each contain an additional major protein (PO or PLP, respectively); these are the most abundant kinds of protein in each type of myelin. Biochemical extraction of LBP and SBP from the PNS myelin sheath results in splitting and unraveling of the sheath along the major dense line. Mice with either the """"""""shiverer"""""""" or the """"""""myelin deficient"""""""" mutation lack PLP and/or LBP and SBP. The CNS myelin sheath of these mutant animals lacks a major dense line. In freeze-fracture studies of the CNS and PNS myelin sheaths of normal animals, various sizes and shapes of intramembranous particles are evident on membrane leaflets associated with either the major dense line (P-face) or the intraperiod line (E-face). The objective of the proposed research is to use a morphometric approach to correlate the freeze-fracture observations with the biochemical data. Mutant mice lacking PLP and/or basic proteins will be compared with each other and with normal mice. Species which differ from mice in the amounts of myelin proteins (rabbits, guinea pigs, and chickens) will also be analyzed. Morphological identification of specific myelin proteins provides a sensitive and reliable tool to assay individual sheaths for the presence, density, and spatial distribution of these proteins. Thus, using the freeze-fracture technique, it will be possible to detail the sequence of membrane changes during myelinogenesis. Future studies will include the study of membrane changes during myelin breakdown and remyelination following nerve lesions.