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Robert Mackin, Diana Mitchell, Deborah L Stenkamp; Differential Regulation of Tandemly-Duplicated Cone Opsins by Thyroid Signaling in the Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2017;58(8):124.
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© ARVO (1962-2015); The Authors (2016-present)
Human trichromatic color vision requires that the tandemly-duplicated LWS/MWS (long- and medium-wavelength sensitive) cone opsin genes are differentially expressed in subsets of cones. We previously showed that retinoic acid is involved in the differential regulation of tandemly-duplicated LWS cone opsins in zebrafish. Here we test the hypothesis that thyroid hormone (T3) is also involved.
Embryos were treated with DMSO or 100nM T3 from 2-4 days post fertilization (dpf) for gain-of-function. For loss-of-function, Tg(tg:nVenus-2a-nfnB)wp.rt8 embryos were treated with 10mM metronidazole from 3-4dpf, which destroys the thyroid gland. To visualize the cellular presence of T3, a thyroid receptor β Ligand Trap (T3LT) transgenics were used, in which cells express GFP upon binding of T3 to a fusion protein. LWS (“red-sensitive”) cones were visualized using trβ2:tdTomato transgenics. Expression of LWS1 and LWS2 was measured by qPCR, and visualized using LWS:PAC(H) transgenics, in which LWS1 is reported by GFP and LWS2 by RFP. Fluorescence was imaged in whole mounted eyes and cryosections using confocal microscopy.
In controls, LWS2 showed high levels of expression, while LWS1 showed little or no expression at 4dpf. In embryos treated with 100nM T3, LWS1 increased by 10 fold (p=2.84E-07), while LWS2 decreased by 5 fold (p=2.16E-07). Thyroid ablation resulted in an increase in LWS2 at 6dpf (p=.008) and no significant change in LWS1 when measured by qPCR. Thyroid-ablated LWS:PAC(H) embryos had fewer GFP+ (LWS1+) cones compared to controls, suggesting a reduction of LWS1 in individual cones. T3-treated T3LT embryos showed GFP (reporting T3) colocalized with the tdTomato reporter for trb2 at 3dpf, indicating T3 presence in LWS cones. At 4dpf there were fewer co-labeled cones.
These results significantly extend findings that nuclear signaling molecules influence differential expression of tandemly-duplicated cone opsin genes. Exogenous T3 robustly increased LWS1 expression and decreased LWS2. Thyroid ablation caused the opposite outcome, consistent with endogenous roles for T3 in regulating differential expression of LWS1 and LWS2. The presence of T3 in LWS cones suggests the potential for a cell-autonomous mechanism. The shift in T3 localization from LWS cones to a different subset of cones suggests complex regulation of T3 metabolism and/or localization.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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