The role of WNT signaling during regenerative neurogenesis in the zebrafish olfactory epithelium

Abstract

The peripheral olfactory epithelium (OE) is a potentially insightful exception to the limited capacity of neurogenesis in the adult nervous system. The OE undergoes continuous generation of olfactory sensory neurons (OSNs) to replace senescent and dying neurons by maintenance neurogenesis. In addition, the OE is capable of rapid regeneration upon traumatic injury by repair neurogenesis. Yet, the molecular signals that contribute to these two modes of neurogenesis are not well established. Transcriptome profiling of gene expression during OE regeneration revealed that components and downstream target genes of the Wnt/β-catenin signaling pathway are strongly upregulated following OE injury. This suggests that Wnt/βcatenin signaling may critically contribute to the regulation of OE neurogenesis by triggering mitotic activity in relevant stem/progenitor cells. To examine the role of Wnt/β-catenin signaling for both maintenance and repair neurogenesis, Wnt activity was manipulated in the intact and damaged OE by using pharmacological agonists and antagonists. Pharmacological activation of the Wnt pathway induced massive proliferation responses across the entire OE, including the sensory OE, which resembled the neurogenesis pattern during OE regeneration. Inhibition of the pathway, on the other hand, had a minor effect on maintenance neurogenesis in the intact OE, however, resulted in the partial suppression of damage-induced proliferation. Importantly, anti-β-catenin immunostaining revealed the presence of Wnt-responsive cells in regions of active maintenance neurogenesis in the intact OE, while in agonist-treated OE the βcatenin expression pattern resembled expression observed under damage conditions. The specificity and efficiency of the agonists were validated by examining the changes in the expression levels of components and downstream genes of the Wnt pathway by a semiquantitative RT-PCR approach. These results suggest that Wnt/β-catenin signaling is necessary and sufficient for maintenance and repair neurogenesis and that Wnt pathway activity induces cell proliferation from two types of progenitor cell populations in the zebrafish OE.