Abstract
Purpose :
Zebrafish photoreceptors comprise one rod type and four cone types maximally sensitive to ultraviolet (UV), blue, green, and red light. The transcriptional networks that govern the developmental diversification of cone types remain incompletely understood. Here, we define the transcriptional corepressor Sterile Alpha Motif Domain Containing 7 (samd7) as a key cell fate determinant of rods and long-wavelength sensitive (i.e., green and red) cones in zebrafish.
Methods :
Using single cell RNA-sequencing data obtained from fluorescently sorted crx:GFP+ photoreceptors and fluorescent in situ hybridization, we identified photoreceptor-subtype-specific patterns of samd7 expression in the larval zebrafish retina. We then used CRISPR-Cas9 to engineer a samd7 mutant zebrafish and identified changes in the larval samd7 mutant retina using RNA-sequencing, antibody staining, and transgenic zebrafish imaging. We additionally performed RNA-sequencing using fluorescently sorted thrb:tdTomato+ red cones from samd7-/- and WT backgrounds. To determine if samd7 was sufficient to repress blue and UV opsin expression, either crx:GFP or crx:samd7-2A-GFP constructs were injected into samd7-/- embryos at the single-cell stage and antibody staining for blue and UV opsin was performed. We further characterized changes in the adult samd7 mutant retina by RNA-sequencing, flatmount imaging of the cone mosaic, and ex vivo transretinal electrical recordings.
Results :
We show that samd7 is expressed exclusively in developing photoreceptors and mature red cones, green cones, and rods. In samd7 mutant larvae, green cones are transfated into supernumerary blue cones, red cones are transformed into hybrid red/UV-cones, and the number of rods is severely reduced. Overexpression of samd7 is sufficient to repress blue opsin in native and supernumerary blue cones. In the adult samd7-/- retina, changes in gene expression resemble those of the larval mutant; in addition, the photoreceptor mosaic is disrupted, and the electrical responses of rods and cones are reduced.
Conclusions :
Taken together, these results indicate that samd7 acts in red cones, green cones, and rods to sustain normal programs of gene expression, acting primarily in cones to prevent ectopic expression of blue- and UV-cone-specific genes.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.