Abstract
Purpose :
Vertebrate color vision relies on distinct cone opsins expressed in specific populations. In the retina, the presence of thyroid hormone receptor β (THRB) is crucial for the expression of long-wavelength-sensitive opsin (LWS) in red cone photoreceptors. The regulation of the tandemly duplicated lws array remains responsive to thyroid hormone (TH) during zebrafish development (Mackin et al., 2019) and in adults (Farre et al., 2023). Our study investigates the impact of experimentally altered TH signaling on the zebrafish lws array by overexpressing the thyroid hormone receptor beta 2 isoform (thrb2) in all photoreceptors, using the transgenic line Tg(crx: mYFP-2a-trβ2) and allowing thyroid gland ablation via metronidazole treatment, using Tg(tg: nVenus-2a-nfsB) wp.rt8.
Methods :
Hemizygous Tg(crx: mYFP-2a-trβ2) were crossed with homozygous Tg(tg:nVenus-2a-nfsB) wp.rt8lines, ensuring universal Venus and nfsB expression in thyroglobulin+ cells in embryos, with or without the thrb2 gain-of-function (GOF) transgene. To induce athyroidy, 2.5dpf embryos underwent a 48 or 72-hour treatment with 10mM metronidazole (MTZ) in 0.1% DMSO (0.1% DMSO was the control), with sampling at 4 days post fertilization (dpf) or 6dpf. Athyroidy was confirmed by the absence of Venus reporter-expressing thyroglobulin cells post-MTZ treatment. Manual quantification of lws1+ and lws2+ cones was conducted using fluorescence confocal microscopy acquired images following hybridization chain reaction (HCR) in situ.
Results :
At 4dpf, lws1+ve cone numbers were significantly reduced in the athyroid larvae harboring the thrb2 GOF transgene compared to the euthyroid and normal thrb2 control group (N=7, F (3, 24) = 5.085, P=0.0072) whereas there were no statistical differences between any other conditions. However, at 6dpf, a significant reduction in lws1+ve cell numbers was observed for all groups compared to the euthyroid, normal thrb2 control group (N=10, F (3, 36) = 22.53, P<0.0001).
Conclusions :
These results support potential roles for Thrb2 in differential regulation of the lws array. Reduced TH signaling appears to repress lws1 and overexpression of thrb2 may exacerbate this effect during this developmental timepoint.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.