We have used cDNA probes specific for three of the major brain microtubule-associated proteins (MAPS), MAP1, MAP2, and MAPS, to study the timing of appearance, relative abundance, and intracellular compartmentalization of MAP gene transcripts in developing rat brain. The MAPI probe hybridizes throughout the brain, in both grey and white matter. MAN mRNA is detected only in grey matter and appears in cerebral neurons only after they have ceased dividing and have migrated to the cortical plate. The MAPS cDNA hybridizes throughout the embryonic brain, but by P12, MAP5 mRNA distribution is restricted to relatively immature areas. MAN mRNA, found in dendrites in the developing brain, persists in some adult dendrites. MAP5 mRNA, like β-tubulin mRNA, is found only in the cell bodies of developing neurons, indicating that the protein must be transported from the soma into processes. MAP1 mRNA is found only in the proximal regions of cortical pyramidal cell dendrites in both developing and adult brain. The diverse distributions of MAP gene transcripts emphasize the importance of these proteins in generating heterogeneity of microtubule function and indicate that MAP compartmentalization within neurons is regulated in part by differential mRNA transport.
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