New insights into the development of cerebellar nuclei neurons in mice

Field: Neuroscience (Life Sciences)

Development of the central nervous system begins with the specification of the neuroectoderm, delineation of the neural sheet into segments that will give rise to different brain compartment, and follows with the massive neurogenesis and neuronal migration. Cerebellum is a part of this complicated system with largest number of neurons. During cerebellar development, cerebellar nuclei neurons and Purkinje cells are the earliest born among the different neuronal subtypes. Purkinje cells are the sole output of the cerebellar cortex and they project to the cerebellar nuclei, which in turn represent the main output of the cerebellum. The cerebellum is generated from the rhombic lip and the ventricular zone. We used whole mount/section immunohistochemistry, mouse cerebellar and embryonic cultures, Dye Tracers and In Situ Hybridization to examine the origin of a new subset of cerebellar nuclei neurons from the mesencephalon during early cerebellar development. The isthmic organizer plays an active role in differentiating the mesencephalon (midbrain) from the rhombencephalon (hindbrain). Our results show that a subset of cerebellar nuclei neurons, which are immunopositive for α-synuclein (SNCA) and orthodenticle homeobox 2 (Otx2), originate from the mesencephalon and cross the isthmus toward the rostral end of the nuclear transitory zone. Double immunoflourscence of the SNCA with Otx2 or p75 neurotrophin receptor (p75ntr) indicates that these cells are derived from neural crest cells. We also showed that this population of neurons with nerve fibers terminates at the subpial surface of putative lobules VI/VII. The SNCA+/Otx2+/p75+ cells, which divide the cerebellar primordium into rosterodorsal and caudoventral compartments, show increased cleaved caspase-3 activation to activate axonal guidance. These results strongly suggest that early cerebellar nuclei neurons originate from the mesencephalic neural crest population. Their presence in the nuclear transitory zone suggests that maybe these neurons/fibers have a regulatory role as a signaling center, such as for axonal guidance and neuronal migration during early cerebellar development. To question the importance of the expression of SNCA during cerebellar development we find a mouse with ablation of Snca gene and study it for any abnormalities during embryonic and adult cerebellum. Our findings indicate that the mesencephalic derived cells are still there while expressing P75NTR and not SNCA and this ablation does not interrupt the cerebellar architecture development and zone and stripe pattern formation and shown normal cerebellar morphology in postnatal mouse cerebellum. To explore and trace the distribution and pattern of p75NTR expression we used postnatal cerebellum in normal and Acp2 mutant mice with cerebellar defect. It was shown that P75NTR expressed in the same pattern as HSP25 zone and stripe phenotype (strongly present in Purkinje cells of the central and nodular zones) in adult cerebellum in both control and Acp2 mutant mice. In this thesis, I provide data about existence of the third germinal zone, the mesencephalon, in the developing mouse after that the role of SNCA expression was questioned during mouse cerebellar development. Beside spatiotemporal expression of SNCA, p75NTR expression was studied.