The contribution of purinergic signaling to olfactory neurogenesis in zebrafish

Abstract

Compared to other sensory epithelia, the olfactory epithelium (OE) has a remarkably high regenerative capacity: continuous neurogenesis compensates for regular turnover of olfactory sensory neurons (OSNs) while reactive neurogenesis fully recovers lost cells and OE function after damage. It is known from the rodent OE that two populations of multipotent basal progenitor cells, the horizontal (HBCs) and globose basal cells (GBCs) contribute to the repopulation of the OE, under injury and maintenance conditions, respectively. However, progenitor cell populations with equivalent function and how they are regulated in the zebrafish OE are less well characterized. A recent and interesting hypothesis is that purinergic signaling may play a critical role in the regulation of neurogenesis. According to this scenario, purine compounds, such as ATP, would be released from dying cells and stimulate proliferation of progenitors either directly, or indirectly through sustentacular glia cells (SCs). In order to examine the role of purinergic signaling, calcium-imaging experiments were conducted on ex vivo vibratome sections through the adult zebrafish OE. Morphometric measurements of Ca2+ responses upon extracellular ATP stimulation were compared to cell measurements obtained via immunohistochemistry using antibodies against cell type-specific markers of non-neuronal OE cells, such as Krt5, Sox2 and Tp63. ATP stimulation activates two distinct Sox2+ cell populations with different response kinetics, one of which shows HBC-like characteristics, whereas the other resembles SCs. To more directly investigate the physiological effect of purinergic stimulation onto neurogenesis, fish were injected intraperitoneally with ATP or ATP and the P2 receptor antagonist suramin and changes in proliferation were examined using BrdU incorporation assays. ATP stimulation resulted in a significant increase in cell proliferation in the OE and the observed increase could be blocked by co-application of suramin, suggesting a direct purinergic effect. It was found that the majority of newborn cells commit to a neural fate and that krt5+ HBC-like cells were not the main contributors to the response.