Tbx2b is required for ultraviolet photoreceptor cell specification during zebrafish retinal development

Abstract

The vertebrate rod and cone photoreceptors are highly specialized sensory neurons that transduce light into the chemical and electrical signals of the nervous system. Although the physiological properties of cones and rods are well known, only a handful of genes have been identified that regulate the specification of photoreceptor subtypes. Taking advantage of the mosaic organization of photoreceptors in zebrafish, we report the isolation of a mutation resulting in a unique change in photoreceptor cell fate. Mutation of the lots-of-rods (lor) locus results in a near one-for-one transformation of UV-cone precursors into rods. The transformed cells exhibit morphological characteristics and a gene-expression pattern typical of rods, but differentiate in a temporal and spatial pattern consistent with UV-cone development. In mutant larvae and adults, the highly ordered photoreceptor mosaic is maintained and degeneration is not observed, suggesting that lor functions after the specification of the other photoreceptor subtypes. In genetic chimeras, lor functions cell-autonomously in the specification of photoreceptor cell fate. Linkage analysis and geneticcomplementation testing indicate that lor is an allele of tbx2b/fby (from beyond). fby was identified by a pineal complex phenotype, and carries a nonsense mutation in the T-box domain of the tbx2b transcription factor. Homozygous fby mutant larvae and lor/fby transheterozygotes also display the lots-of-rods phenotype. Based upon these data, we propose a previously undescribed function for tbx2b in photoreceptor cell precursors, to promote the UV cone fate by repressing the rod differentiation pathway.cone ͉ Danio rerio ͉ T-box ͉ mosaic V ertebrates have evolved 2 major classes of retinal photoreceptors: rods, which mediate dim light vision, and cones, which detect light of greater intensity, have a faster temporal resolution, and mediate color vision. Largely from the analysis of mutations in mice and humans, a transcriptional network regulating photoreceptor cell development has been proposed. The presumptive photoreceptor progenitors sequentially express the homeobox transcription factors (TFs) Otx2 and Crx (1-3), and in their absence, photoreceptor precursors are not specified or fail to differentiate. Rod specification requires the additional expression of the Maf-family TF Nrl and its target Nr2e3 (4, 5). Nrl acts as a molecular switch; in its absence, precursors adopt the short-wavelength (S) opsin cone fate (5, 6), and mis-and over-expression of Nrl transforms most if not all cone precursors into functional rods (7,8). NR2E3 expression, which is disrupted in enhanced S-cone syndrome in humans and the rd7 mouse, is required for the repression of cone-specific genes in rod precursors (4, 9, 10). However, it remains to be determined if a reciprocal system exists in cone precursors for repressing rodspecific genes.The zebrafish retina, in addition to rods, possesses 4 cone subtypes, each with a distinct morphology and expressing a unique opsin (11-17). The spatial...

Diminished foraging performance of a mutant zebrafish with reduced population of ultraviolet cones

“…Because these results could have arisen from a difference in overall photon catch by cones between the two fish groups (as may be expected from the replacement of UV cones by rods in the lor retina) [32], additional experiments were carried out under a 450 long pass (450 LP) background (figure 1a). The intensity of the 450 LP background was 1.15 times that of the full spectrum background, resulting in an estimated total photon catch of lor fish that was 1.02 times that of control fish under the full spectrum background.…”

“…Because lor zebrafish photoreceptors have only been characterized by molecular and immunohistochemical means [32], and there is no knowledge of the absorbance of the visual pigments or the functional output of the retina, I first characterized the photoreceptor visual pigments and spectral sensitivity of this mutant. This knowledge was essential to assess the visual capabilities of the animal.…”

“…Mutation screens carried out in the Fadool laboratory identified the lor (lots of rods) mutant characterized by greatly diminished numbers of UV cones, their place in the photoreceptor mosaic occupied, instead, by rods [32]. Thus, the lor mutant provides a unique opportunity to test whether UV cones improve the foraging performance of zebrafish.…”

Cone photoreceptor types in zebrafish are generated by symmetric terminal divisions of dedicated precursors

“…Past studies have provided insight into the transcriptional regulation of cone versus rod photoreceptor fate (37,42,43), but, other than for L cones, it is not yet known what factors dictate cones to express UV-, S-, or M-opsin. For example, perturbation of tbx2b causes a fate switch of UV cones to rods but does not alter the number of other cone types (42). We showed here, however, that, although not yet identified, these factors are likely to act at the cone-precursor stage rather than after cone differentiation.…”

The bHLH Transcription Factor NeuroD Governs Photoreceptor Genesis and Regeneration Through Delta-Notch Signaling

“…[41][42][43] Reciprocal transplants were performed between wild-type (WT) and NeuroD knockdown embryos and larvae were incubated in EdU as described above. To label donor cells, embryos were injected with 5% tetramethyl rhodamine-dextran (D-1816, 10,000 MW; Life Technologies, Carlsbad, CA, USA) at the 1-to 2-cell stage.…”