Studying rod photoreceptor development in zebrafish

Abstract

The zebrafish has rapidly become a favored model vertebrate organism, well suited for studies of developmental processes using large-scale genetic screens. In particular, zebrafish morphological and behavioral genetic screens have led to the identification of genes important for development of the retinal photoreceptors. This may help clarify the genetic mechanisms underlying human photoreceptor development and dysfunction in retinal diseases. In this review, we present the advantages of zebrafish as a vertebrate model organism, summarize retinal and photoreceptor cell development in zebrafish, with emphasis on the rod photoreceptors, and describe zebrafish visual behaviors that can be used for genetic screens. We then describe some of the photoreceptor cell mutants that have been isolated in morphological and behavioral screens and discuss the limitations of current screening methods for uncovering mutations that specifically affect rod function. Finally, we present some alternative strategies to target the rod developmental pathway in zebrafish.

Fig. 1

Traditional screen to uncover recessive mutations in F3 larvae. Male fish are mutagenized with ENU and mated to females to produce an F1 population. The F1 fish are mated to generate F2 families. Sibling mating of F2 families uncovers recessive mutations in 1/4 of the progeny in 1/4 of the crosses.

Fig. 2

Rapid development of the zebrafish eye. (A) 36 hpf. (B) 54 hpf. (C) 120 hpf. Note the progressive lamination of the retina.

Fig. 3

Zebrafish visual behavior assays. (A) Optokinetic response (OKR). (B) Population screening for larval optomotor response (OMR). (C) Adult OMR. (D) Escape response. This figure was graciously provided by Stephan Neuhauss.

Fig. 4

Immunofluorescence labeling identifies alterations in rod photoreceptor patterning and morphology. (A, B) Two views of rod labeling in wild-type 5 dpf larvae. (C) The rods in this mutant display an altered, “stringy” morphology. (D) In this mutant, the pattern and spacing of the rods is not normal. (E, F) In this mutant, the rods look normal in the outer nuclear layer, but ectopic rods are also visible in the inner retina, and sometimes the choroid fissure fails to close (F).