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James M Fadool, Karen Alvarez-Delfin, Orleiquis Guerra, Mailin Sotolongo-Lopez; Regulation of Spatial Pattering of Rods and Cones in the Larval Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4033.
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© ARVO (1962-2015); The Authors (2016-present)
Humans are largely dependent upon cone vision. The highest cone density is found in the fovea positioned at the center of gaze, with comparatively few present in the rod-dominated retinal periphery. Surprisingly, few mechanisms are known that regulate the spatial patterning of rod and cone photoreceptors. The larval zebrafish retina is anatomically and functionally cone-dominated with conspicuously few rods in the central retina and greater numbers in the ventral retina and periphery. Our strategy takes advantage of these regional differences in the numbers of rods to identify fundamental mechanisms regulate their spatial patterning.
Immunolabeling and expression of cell-specific reporter genes were analyzed by confocal microscopy. Zebrafish embryos and larvae harboring mutations of lor/tbx2b, lak/ath7, lep/pct2, ljr, or morpholino knockdown of six7 or trb2 were used throughout the study. Transcriptional activity was measured using luciferase reporter assays of HEK293 cells co-transfected with transcription factor constructs and opsin promoter-luciferase reporter plasmids.
In the central retina of larval zebrafish, cones outnumber rods 20:1. Our published data demonstrate that genetic mutation of tbx2b/lor leads to an increased number and the uniform distribution of rods due to a cell fate switch of SWS1 cones into rods. However, the increase in rod number following knockdown of six7 is associated with continued cell proliferation in the central retina. Furthermore, our data show that the effects are additive. Larvae double mutant for tbx2b/lor and six7/ljr demonstrate twice the number of rods as either mutation alone. Surprisingly, no changes in rod patterning were associated with the increased photoreceptor numbers observed in lak/ath7 or lep/pct2 mutant larvae. In luciferase reporter assays, tbx2b alone had no effect upon transcription from opsin promoter constructs but represses the Crx/Nrl-induced transcriptional activation of the rho-promoter, and Crx activation of the SWS1 promoter. No effects were observed using six7.
Our data show that spatial patterning of photoreceptors can be regulated independently by factors controlling cell fate or cellular proliferation; that tbx2b and six7 act through distinct molecular mechanisms. However the absence of patterning defects in lak/ath7 or lep/pct2 mutant larvae suggests that additional factors are also involved.
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