DeltaC and DeltaD Signaling Contributes to the Proper Development of Photoreceptor Subtypes in Zebrafish

Abstract

Purpose: Rod and cone photoreceptors are derived from the same common retinal progenitor cells. The signaling pathways that regulate the production of photoreceptors are not well characterized. Classical studies have shown that Delta-Notch signaling is essential for the maintenance of retinal progenitor cells and regulate the proper timing of retinogenesis. However, the specificity of different Delta ligands on photoreceptor development is not well understood. The purpose of this study was to investigate the specific functions of two Delta ligands, deltaC (dlc) and deltaD (dld), in zebrafish photoreceptor development.

Methods: To determine the roles of these Delta ligands on photoreceptor development, the eyes of the dlc^b663 and dld^tg249 mutants were characterized by immunostaining and in situ hybridization using photoreceptor markers. Thyroid hormone pathway is known to affect cone development in vertebrates. To determine the interaction between the Delta-Notch and thyroid hormone pathways, dlc^b663, dld^tg249 and wild-type (WT) embryos were treated with exogenous thyroid hormone (T3) followed by an analysis of photoreceptor gene expression using in situ hybridization. To demonstrate the extent to which the change in photoreceptor development would lead to a change in the visual behavior of the mutant embryos, visual motor response (VMR) and optokinetic reflex (OKR) were measured on these embryos.

Results: First, the differentiation of blue and red cones was promoted in the dlc^b663 and dld^tg249 mutants respectively. Second, the differentiation of rods was suppressed in both mutants. Third, uv, blue and rod photoreceptors were suppressed after T3 treatment in the WT embryos, while these effects of T3 treatment on the dlc^b663 and dld^tg249 mutants was not as severe. Finally, the VMR of both mutants was altered compared with the WT embryos.

Conclusions: Photoreceptor differentiation was affected in unique manners in both dlc^b663 and dld^tg249 mutants. In addition, we observed that the DeltaC and DeltaD pathways have a functional interaction with the thyroid hormone pathway. Also, the changes in photoreceptor differentiation might cause the observed alteration in visual behavior. Overall, this study has revealed novel roles of Delta ligands in photoreceptor development and visual function.

Keywords: 648 photoreceptors • 698 retinal development • 497 development