Expression of MAP1B protein and its phosphorylated form MAP1B-P in the CNS of a continuously growing fish, the rainbow trout

Microtubule-associated protein-1B (MAP1B), and particularly its phosphorylated isoform MAP1B-P, play an important role in axonal outgrowth during development of the mammalian nervous system and have also been shown to be associated with axonal plasticity in the adult. Here, we used antibodies and mRNA probes directed against mammalian MAP1B to extend our analysis to fish species, trout (Oncorhynchus mykiss), at different stages of development. The specificity of the cross-reaction of our anti-total-MAP1B/MAP1B-P antibodies was confirmed by Western blotting. Trout MAP1B-like proteins exhibited about the same apparent molecular weight (320 kDa) as rat-MAP1B. Immunohistochemistry and in situ hybridization analysis performed on hindbrain and spinal cord revealed the presence of MAP1B in neurons and some glial subpopulations. Primary sensory neurons and motoneurons maintain high levels of MAP1B expression from early stages throughout adulthood, as has been shown for mammals. Unlike mammals, however, MAP1B and axon-specific MAP1B-P continue to be strongly expressed by hindbrain neurons projecting into spinal cord, with the important exception of Mauthner cells. MAP1B/MAP1B-P immunostaining were also detected elsewhere within the brain, including axons of the retino-tectal projection. This obvious difference between adult fish and mammals is likely to reflect the capacity of fish for continued growth and regeneration. Our results suggest that MAP1B/MAP1B-P expression is generally maintained in neurons known to regenerate after axotomy. The regenerative potential of the adult nervous system may in fact depend on continued expression of neuron-intrinsic growth related proteins, a feature of MAP1B that appears phylogenetically conserved.