Identification of HB-EGF positive and HB-EGF responsive cell populations in zebrafish olfactory epithelium

Abstract

The peripheral olfactory epithelium (OE) is an exception to the limited capacity of the adult nervous system to undergo neurogenesis, which supports structural repair in response to traumatic injury. It continuously and lifelong generates new olfactory sensory neurons (OSNs) at high rate to replace dying cells. Additionally, the OE is capable of regenerating efficiently following severe structural damage. Similar to the mammalian OE, maintenance and regenerative neurogenesis in zebrafish are associated with the selective activity of globose (GBC) and horizontal basal cells (HBC), respectively, which have distinctive structural and functional characteristics. Transcriptome analysis of the regenerating OE suggested that the diffusible signaling factor heparin-binding epidermal growth factor-like growth factor (HB-EGF) may play a critical role at the onset of regenerative neurogenesis. In response to tissue damage, expression levels of HB-EGF are transiently and rapidly upregulated as an early damage response. HB-EGF stimulates mitotic activity in HBCs through the epidermal growth factor receptor (EGFR) signaling pathway. However, the tissue expression of both HB-EGF and EGFR in the zebrafish OE were not known. The studies described in this thesis identified HB-EGF-expressing and HB-EGF responsive cells by selective in situ-hybridization against hbegfa and egfra transcripts. Hbegfa is expressed largely by Sox2-positive basal cells, including tp63-positive HBCs, in addition to HuC/D- positive OSNs and additional cells that may comprise sustentacular glial cells. In response to injury, the number of hbegfa-positive cells increases across all cell populations. In contrast, egfra is expressed by two cell types with distinct tissue distribution, which include HBCs occupying basal OE layers and a subpopulation of OSNs that are restricted to the apical surface of the zebrafish OE. The expression analysis complements previous findings on the proposed role of HB-EGF during OE regeneration and suggests a model of signaling that includes paracrine stimulation between basal cells and other resident OE cells onto injury-responsive HBCs.