Spatial, temporal and mechanistic characterization of apoptotic death in the developing subventricular zone

The neonatal subventricular zone (SVZ) is a site of continued postnatal neurogenesis, and is the source of cortical glial cells. Apoptosis is an endogenous process of cell destruction, and is a key event in the proper development of the SVZ. Despite its importance, there is still a lack of knowledge regarding the temporal and spatial occurrence of neonatal SVZ apoptosis, cell types affected and the underlying intrinsic and extrinsic mechanisms that guide the process. This thesis addresses these issues, and in addition, finds a nontraditional mode of neurotrophic action for cell survival in the neonatal SVZ. We assessed SVZ apoptosis by subregion, employing the cell death markers, pH2ax and cleaved caspase 3. The medial SVZ contained the highest density of dying cells at p0, while at p7 there was no significant difference in the apoptotic cell density distribution in the SVZ subregions. Combining cell type specific markers with the death markers used, revealed immature postmitotic neurons were the primary cell type cleared in the p0 medial SVZ. The majority of dying cells in the p7 dorsolateral SVZ (SVZdl) were unable to be identified. Using stereotactic injection of a GFP expressing lentivirus, we determined the p0 medial SVZ cell population to be migratory cells bound for the olfactory bulb. An investigation into the intrinsic and extrinsic mechanisms mediating cell death in the neonatal SVZ, showed BH3-only protein Bim expression in the p0 and p7 SVZ, as well as significantly decreased p0 medial SVZ apoptosis in Bim knockout mice. Bim knockout mice did not show a significant change in apoptosis in the p7 SVZdl. TrkB knockout mice have shown a survival role for the receptor in the lateral ganglionic eminence of the neonatal SVZ. To test this in the p0 medial SVZ using a more specific method, a TrkB blocking antibody was injected into the p0 medial SVZ. This resulted in a significantly higher number of apoptotic cells in the p1 medial SVZ versus controls. These studies demonstrate the dynamic nature of the SVZ with its changing density and identity of apoptotic cells within the subregions. It has also shown the influence of Bim and TrkB signaling in neonatal SVZ apoptosis and survival. Finally, it has identified a premigratory cell population in the p0 medial SVZ, whose survival is mediated by neurotrophin signaling at their site of origin.