Oligodendrocytes in aging mice lacking myelin-associated glycoprotein are dystrophic but not apoptotic

Although MAG-null mice myelinate relatively normally except for subtle structural abnormalities in the periaxonal region of myelin sheaths, they develop more severe pathological changes as they age. The purpose of this study was to further define the biochemical aspects of CNS pathology caused by an absence of MAG. Proteins associated with myelin and oligodendrocytes were quantified by densitometry of western blots in whole brain homogenates, as well as in isolated myelinated axons and myelin. Neither myelin yields, nor levels of myelin basic protein and proteolipid protein, were decreased in comparison to control levels in 14-month-old MAG-null mice. On the other hand, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and the 120 kD neural cell adhesion molecule (N-CAM) were substantially reduced in whole brain, myelinated axons, and myelin. Tubulin, Na(+)K(+)ATPase and Fyn tyrosine kinase were also reduced significantly in myelin-related fractions, but not in whole brain homogenate. The decreased levels of these proteins suggest pathological abnormalities in oligodendrocytes. Furthermore, significant reductions of CNPase and 120 kD NCAM were also present at 2 months, indicating that the oligodendroglial abnormalities begin at a relatively early age. Neither TUNEL assays nor multiplex RT-PCR for mRNAs of apoptosis-related proteins in the aging MAG-null mice provided evidence for apoptotic oligodendrocytes. These biochemical findings suggest oligodendroglial damage in MAG-null mice and support the morphological observations pointing to a progressive "dying-back oligodendrogliopathy" as a consequence of MAG deficiency.