Adult neurogenesis is a highly conserved phenomenon that neural stem cells (NSCs) produce new neurons, astrocytes or oligodendrocytes in the adult brain. In the adult mammalian brain, neurogenesis is restricted to the subventircular zone (SVZ) and the subgranular zone (SGZ). In contrast, zebrafish have 16 NSCs niches and can continue to produce new neurons through life. In the optic tectum where optic nerves project, radial glial cells (RGCs) located in the deeper layer express several stem cell markers as Sox2 and msi1, and are quiescent, while RGCs in the telencephalon are proliferative and work as NSCs. We confirmed that the number of proliferative RGCs in the optic tectum was fewer than that in telencephalon.
Zebrafish have also great ability to regenerate brain after the traumatic brain injury compared to mammalian. In this study, we established the stab injury of the optic tectum and analyzed the proliferation of RGCs after stab injury. We confirmed that the induction of RGC proliferation after stab injury. Then we analyze the differentiation of RGCs by BrdU labeling for 24 hours and confirmed that the number of HuC, neuronal marker, and BrdU-double positive cells was increased in the injured optic tectum at 1 week post injury compared with those in the intact side.
To analyze molecular mechanisms that control the proliferation of RGCs in response to stab injury, we injected zymsoan A into cerebrospinal fluid using cerebroventircular microinjection and confirmed that the proliferation of RGCs was also induced at 1day post injection. We treated zebrafish with dexamethasone (Dex), anti-inflammatory drug, 2 days before and after stab injury. The proliferation of RGCs in the Dex-treated groups was decreased at 2dpi. These results suggest that inflammatory responses have a key role in the induction of RGC proliferation after the tissue damage in the PGZ. Wnt signaling is well known to regulate stem cell proliferation and differentiation during development and also contributes to the regenerative response after traumatic brain injury. To elucidate contribution of Wnt signaling to the proliferation of RGCs, we analyzed the Wnt reporter zebrafish after the stab injury. We confirmed that the expression of GFP was detected in the BLBP-positive cells after the stab injury in optic tectum. Then we treated injured-zebrafish with the Wnt inhibitor and analyzed the proliferation of RGCs. The number of proliferative RGCs was significantly decreased compared with that of DMSO-treated zebrafish. These results suggested that the up-regulation of Wnt signaling may induce the proliferation of RGCs in response to stab injury.