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Two isoforms of myelin-associated glycoprotein accumulate in quaking mice: only the large polypeptide is
P E Braun1, E Horvàth, A M Edwards
1Department of Biochemistry, McGill University, Montreal, Canada.
Abstract:
We have shown that the developmentally regulated appearance of the two myelin-associated glycoprotein (MAG) polypeptides in normal mouse brain myelin does not reflect the developmental pattern of differential splicing of primary gene transcripts as determined earlier by RNase protection assays. Contrary to expectation, the large (L-MAG) and small (S-MAG) polypeptides are present in about equal amounts at a relatively early stage of myelination, day 24 or earlier. In quaking (qk) mutant mouse brain myelin, both MAG polypeptides are evident at all ages examined; the relatively greater abundance of S-MAG compared to L-MAG at early ages (days 18 and 22) confirms our earlier observation on in vitro translations of mRNA. At later ages (day 27 and beyond) both isoform are present in approximately equal amounts. The L-MAG but not the S-MAG polypeptide can be phosphorylated by kinases that are endogenous to isolated qk myelin, analogous to the phosphorylation we have observed in vivo in normal mice and in their isolated myelin.
Insights
The developmental appearance of myelin-associated glycoprotein (MAG) isoforms in mouse brain myelin is not solely driven by differential splicing. Both large (L-MAG) and small (S-MAG) forms are present early, with phosphorylation differences observed in quaking mutants.
Area of Science:
- Neuroscience
- Molecular Biology
- Developmental Biology
Background:
- Myelin-associated glycoprotein (MAG) is crucial for myelin development and function.
- Two MAG isoforms, large (L-MAG) and small (S-MAG), arise from differential splicing.
- Previous studies suggested developmental regulation of MAG isoforms via splicing.
Purpose of the Study:
- To investigate the developmental expression patterns of L-MAG and S-MAG in normal and quaking (qk) mutant mouse brains.
- To determine if differential splicing of MAG gene transcripts explains the developmental appearance of MAG isoforms.
- To examine the phosphorylation status of MAG isoforms in normal and qk mutant myelin.
Main Methods:
- Analysis of MAG polypeptide abundance in mouse brain myelin at various developmental stages.
- Comparison of MAG isoform expression in normal mice versus quaking (qk) mutant mice.
- Assessment of in vitro kinase activity on MAG isoforms in isolated myelin.
Main Results:
- The appearance of L-MAG and S-MAG in normal mouse brain myelin does not correlate with differential splicing patterns.
- Both L-MAG and S-MAG are present in roughly equal amounts by day 24 of development.
- In qk mutant myelin, S-MAG is more abundant than L-MAG at early ages (18-22 days), with equal amounts later (27+ days).
- Endogenous myelin kinases phosphorylate L-MAG but not S-MAG in qk mutants, similar to normal mice.
Conclusions:
- Developmental regulation of MAG isoforms is not solely explained by differential splicing of primary gene transcripts.
- Both MAG isoforms are present early in development, challenging previous assumptions.
- Phosphorylation of L-MAG by endogenous myelin kinases occurs in both normal and qk mutant mice.