Tracking the fate of her4 expressing cells in the regenerating retina using her4:Kaede zebrafish.

The Basic-Helix-Loop-Helix-Orange (bHLH-O) transcription factor Hairy-related 4 (her4) is a downstream effector of Notch-Delta signaling that represses expression of typically pro-neural genes in proliferative domains of the central nervous system. Notch-Delta signaling in the retina has been shown to increase in response to injury and influences neuroprotective properties of Müller glia. In contrast to mammals, teleost fish are able to regenerate retinal neurons in response to injury. In zebrafish, her4 is upregulated in the regenerating neural retina in response to both acute and chronic photoreceptor damage, but the contribution of her4 expressing cells to neurogenesis following acute or chronic retinal damage has remained unexplored. Here we investigate the role of her4 in the regenerating retina in a background of chronic, rod-specific degeneration as well as following acute light damage. We demonstrate that her4 is expressed in the persistently neurogenic ciliary marginal zone (CMZ), as well as in small subsets of slowly proliferating Müller glia in the inner nuclear layer (INL) of the central retina. We generated a transgenic line of zebrafish that expresses the photoconvertible Kaede reporter driven by a her4 promoter and validated that expression of the transgene faithfully recapitulates endogenous her4 expression. Lineage tracing analysis revealed that her4-expressing cells in the INL contribute to the rod lineage, and her4 expressing cells in the CMZ are capable of generating any retinal cell type except rod photoreceptors. Our results indicate that her4 is involved in a replenishing pathway that maintains populations of stem cells in the central retina, and that the magnitude of the her4-associated proliferative response mirrors the extent of retinal damage.