Differential regulation of microtubule-associated protein 1B (MAP1B) in rat CNS and PNS during development.

MAP1B is a major cytoskeletal protein in growing axons and is strongly regulated during brain development. The present studies compare the expression of MAP1B mRNA, the protein, and its phosphorylated isoform in spinal cord and dorsal root ganglia (DRGs) with brain. In spinal cord and brain, MAP1B mRNA levels were highest in early stages of development, decreased several fold during postnatal development, and remained low in adults. In contrast, there were no significant changes of MAP1B mRNA levels during development of DRG and they remained high in adults. The levels of MAP1B protein decreased in brain and spinal cord in parallel with the changes of their mRNA. The protein levels in DRG remained relatively high but declined in the sciatic nerve. Phosphorylated MAP1B was expressed in high levels during the early stages of development in brain, spinal cord, and sciatic nerve and decreased rapidly to undetectable levels postnatally except for sciatic nerve where it remained detectable. Immunohistochemical analysis showed that phosphorylated MAP1B was absent from DRG cell bodies at all stages but was present in axons of DRG and motor neurons in both spinal cord and sciatic nerve. Immunostaining also confirmed Western blot analysis indicating that MAP1B was initially abundant within the spinal cord but was at later stages present only in motor neurons and the central processes of DRG neurons. These results reflect differential distribution of MAP1B isoforms at different stages of development and in different regions of the nervous system.