Le neurotrofine e i loro specifici recettori nel cervello di Danio rerio

Neurotrophins include a family of growth factors acting on proliferation, differentiation and survival of developing neuronal poulations belonging to both central and peripheral nervous system. Particularly in the postnatal brain, they regulate plasticity and stability of synapses in the CNS, and the complex brain circuitry that regulates behavior. Under- or over-expression of neurotrophins cause neurodegenerative and psychiatric disorders. Neurotrophin signals are transduced by a class of receptors (Trks) with intracytoplasmic tyrosine-kinase domain. All neurotrophins and Trk receptors are well conserved during vertebrate phylogenesis. The aim of this study is to investigate neurotrophins and their Trk receptors in zebrafish or Danio rerio, a teleost fish used in laboratories throughout the world as animal model. It was originally employed for genetic developmental studies and, more recently, for human diseases purposes, included neurodegenerative pathologies. The brain of zebrafish, as in other fish, is extremely interesting for studies regarding adult neurogenesis and reparative processes, due to its high levels of neuronal precursor proliferation and differentiation, which are many times greater than in mammals. Immunochemical and immunocitochemical analysis have been used to investigate the presence and localization of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4) and their specific receptors (TrkA, TrkB, TrkC). Western blottings analysis, performed on zebrafish brain homogenates, showed bands corresponding to expected molecular weights for zebrafish neurotrophins and Trk receptors. Single and double immunocitochemical labeling were carried out on microtomic paraffinic or vibratome agarose sections. Slides were observed at light or confocal microscope. Immunoreactivity to all tested neurotrophins and Trk receptors was widely diffused throughout the zebrafish brain. Particularly, it was localized in neurons and their dendrites/axons, in vessel walls and in different cell populations with glial charcteristics. Generally, cell identification was made on morphological criteria and by means of specific markers. The pattern of distribution of immunoreactivity, such as close proximity or colocalization of neurotrophin/ receptor, suggested an autocrine or paracrine mode of action and the presence of immunolabeling in numerous fibers is likely due to active retrograde and/or anterograde transport. Furthermore, their localization in cell populations with glial properties has been occurred in several neurogenic niches, leading to the hypothesis that these neurotrophic factors might play roles in regulating adult neurogenesis of zebrafish brain. Particularly for BDNF, often it was colocalized with S100β (a glial marker) and PCNA (proliferative marker) in cells of proliferative areas. These results were further confirmed by using two zebrafish transgenic lines: gfap-gfp and nestin-gfp zebrafish, kindly provided by Prof. U. Strahle of Tossicology and Genetic Institute, Forschungzentrum, Karlsruhe, Germany. In conclusion, the research gives ample morphological evidence regarding the involvement of neurotrophins in many biological functions, mostly cell proliferation, of zebrafish brain and represents a useful and promising starting point for future studies devoted to adult neurogenesis and brain repair