ENDOCYTIC DEPLETION OF L-MAG FROM CNS MYELIN IN QUAKING MICE

Quaking is an autosomal recessive hypo/dysmyelinating mutant mouse whi ch has a 1-Mbp deletion on chromosome 17. The mutation exhibits pleiot rophy and does not include genes encoding characterized myelin protein s, The levels of the 67-kD isoform of the myelin-associated glycoprote in (S-MAG) relative to those of the 72-kD isoform (L-MAG) are increase d in the quaking CNS, but not in other dysmyelinating mutants. Abnorma l expression of MAG isoforms in quaking may result from altered transc ription of the MAG gene or from abnormal sorting, transport, or target ing of L-MAG or S-MAG. To test these hypotheses, we have determined th e distribution of L-MAG and S-MAG in cervical spinal cord of 7-, 14-, 21-, 28-, and 35-d-old quaking mice. In 7-d-old quaking and control sp inal cord, L- and S-MAG was detectable in periaxonal regions of myelin ated fibers and in the perinuclear cytoplasm of oligodendrocytes. Betw een 7 and 35 d, L-MAG was removed from the periaxonal membrane of quak ing but not control mice. Compared to control mice, a significant incr ease in MAG labeling of endosomes occurred within oligodendrocyte cyto plasm of 35-d-old quaking mice. S-MAG remained in periaxonal membranes of both quaking and control mice. Analysis of the cytoplasmic domain of L-MAG identifies amino acid moths at tyrosine 35 and tyrosine 65 wh ich meet the criteria for ''tyrosine internalization signals'' that di rect transmembrane glycoproteins into the endocytic pathway. These res ults establish that L-MAG is selectively removed from the periaxonal m embrane of CNS-myelinated fibers by receptor-mediated endocytosis. The loss of L-MAG from quaking periaxonal membranes results from increase d endocytosis of L-MAG and possibly a decrease in L-MAG production.