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.

Signs of normal proliferation in the telencephalon of adult male songbirds (serinus serinus), as shown by PCNA-positivity.

The immunocytochemical expression of Proliferating Cell Nuclear Antigen (PCNA) (a cycline that coadjuvates DNA polymerase delta) becomes appreciable in the cell cycle when DNA synthesis occurs; hence cells in the S phase can be revealed by means of monoclonal antibodies. Therefore, PCNA can be considered a marker of proliferation, and numerous literature reports have demonstrated the reliability of the PCNA test. Since normal neurogenic events can still occur in the brain tissue of adult homeothermic vertebrates (especially songbirds), we evaluated if the persistence of spontaneous proliferation could be revealed in adult male songbirds (Serinus serinus) using the PCNA marker, the same test we used previously to study the persistence of natural proliferation in the encephalon of adult heterothermic vertebrates. The patterns of PCNA positivity showed normal proliferation in the telencephalon of the adult male Serinus serinus. This activity was shown by cells interposed among the epithelial cells lining the lateral side of each ventricular cavity, both in correspondence to the apical tracts and declivities of the ependyma and arranged, here and there, either in groups or slightly separated. As in our previous studies on PCNA expression and persistence of spontaneous encephalic proliferation in adult poikilothermal vertebrates (in the telencephalon of Podarcis, Triturus and Rana, and in the telencephalon, mesencephalon and cerebellum of Carassius), the results of the present research largely agree with the findings of previous Authors, usually obtained with different methods. This agreement confirms the reliability of the PCNA test used for this type of investigation.

PCNA positivity in the mesencephalic matrix areas in the adult of a Teleost, Carassius carassius L.

Adult Teleosts still display karyokinetic activity involving also the central nervous system so that physiological neurogenesis does not cease when adult body size is reached but later. In the brain this is due to the persistence in several districts of neural type elements in the ependymal epithelium or in the area surrounding it, which combine to form matrix zones or matrix areas. These elements may undergo proliferative and/or maturational events, including also post-traumatic neurogenic events, either natural or experimentally induced, as a result of which the degree of brain plasticity is inferior only to that observed in the Amphibians. As part of our project of re-examining the matrix areas of normal adult heterothermic Vertebrates based on the use of a recent proliferative marker displaying the expression of the Proliferating Cellular Nuclear Antigen (PCNA). This is a reliable immunocytochemical test owing to its role in the cell cycle and to the ubiquity of PCNA due to the theoretical basis of the method, has now been applied to the mesencephalon of the Teleosts, which is the location of matrix areas that are absent in the Amphibians and Reptiles. The immunocytochemical patterns we observed in Carassius carassius displayed a much strong DNA synthesis activity than that we found in the telencephalon of the same specimens. These spontaneous neurogenic manifestations are not limited to the optic tectum, and are not restricted to the already well-known caudal, ventral and dorsal matrix zones. This evidence is discussed and compared with literature data describing the results of classical histologic, autoradiographic or immunocytochemical techniques under light and electron microscopy in various species of normal adult Teleosts or in specimens that had undergone reparative or regenerative processes after experimental intervention.