Thyroid hormone receptor beta mutations alter photoreceptor development and function in Danio rerio (zebrafish)

Abstract

We investigate mutations in trβ2, a splice variant of thrb, identifying changes in function, structure, and behavior in larval and adult zebrafish retinas. Two N-terminus CRISPR mutants were identified. The first is a 6BP+1 insertion deletion frameshift resulting in a truncated protein. The second is a 3BP in frame deletion with intact binding domains. ERG recordings of isolated cone signals showed that the 6BP+1 mutants did not respond to red wavelengths of light while the 3BP mutants did respond. 6BP+1 mutants lacked optomotor and optokinetic responses to red/black and green/black contrasts. Both larval and adult 6BP +1 mutants exhibit a loss of red-cone contribution to the ERG and an increase in UV-cone contribution. Transgenic reporters show loss of cone trβ2 activation in the 6BP+1 mutant but increase in the density of cones with active blue, green, and UV opsin genes. Antibody reactivity for red-cone LWS1 and LWS2 opsin was absent in the 6BP+1 mutant, as was reactivity for arrestin3a. Our results confirm a critical role for trβ2 in long-wavelength cone development.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

“…trβ2 expression is specifically required for expression of the red cone opsin ( Suzuki et al, 2013 ) lws1 ( Suzuki et al, 2013 ; Mackin et al, 2019 ; Volkov et al, 2020 ). Consequently, knockdown of trβ2 reduces the number of red cones ( Suzuki et al, 2013 ); zebrafish thrb mutants, as larvae or adults, have an anatomical loss of red cones ( Volkov et al, 2020 ), which is associated with reduced response to red light and a loss of red cone inputs to the ERG ( Deveau et al, 2020 ). Exposing zebrafish larvae to T3 from 2 to 4 dpf increased expression and distribution of lws1 ( Mackin et al, 2019 ), consistent with effects in TRβ2 mutants.…”

“…For example, reductions in trβ2 leads to an increase in the number of cones expressing UV opsin in zebrafish ( Suzuki et al, 2013 ; Volkov et al, 2020 ), similar to the increase in S-cone number observed in TRβ2-null mice ( Roberts et al, 2006 ), suggesting trβ2 , and TH signaling, determines L-cone vs. UV-cone fate ( Suzuki et al, 2013 ). Exposing trβ2 mutant zebrafish larvae (2–4 dpf) to T3 caused a dose dependent increase in rh2-2 and rh2-3 expression ( Mackin et al, 2019 ) in green cones, which was observed physiologically as reduced green sensitivity and a shift to a longer peak wavelength in photopic ERG recordings ( Deveau et al, 2020 ), another example of a TH-induced shift to longer wavelength sensitivity. These results in zebrafish agree with those from human retinal organoid cultures, which show that TH binding to TRβ2 is required for L/M cone development ( Eldred et al, 2018 ).…”

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

“…Each of these layers has a specific function in the visual system (Bilotta & Saszik, 2001; Stenkamp, 2015). For color vision, photoreceptors with different wavelength sensitivities develop from retinal progenitor cells (Deveau et al, 2020). The rods mediate vision at low light intensity, and the cones mediate daylight and color vision (Takechi & Kawamura, 2005).…”

“…Among others, Harpavat and Cepko (2003) concluded that binding of THs to TH receptor β (TRβ) determines the correct expression of cone opsins in both zebrafish (Suzuki et al, 2013; Volkov et al, 2020) and rodents (Roberts et al, 2006). The role of THs in eye development is further supported by the fact that TRs, iodothyronine deiodinase (DIO) enzymes, and thyroperoxidase (TPO) are expressed in vertebrate eyes (Bagci et al, 2015; Deveau et al, 2020; Heijlen et al, 2014; Houbrechts et al, 2016; Marelli et al, 2016; Suzuki et al, 2013). In particular, TRβ2 and DIOs appear to be key regulators of cone photoreceptor development in the retina (Deveau et al, 2020; Ng et al, 2010; Suzuki et al, 2013) and are needed, for example, for the expression of cone opsins (Suzuki et al, 2013).…”

Biochemistry and physiology of zebrafish photoreceptors

“…Both the mechanisms red-shift the zebrafish photoreceptor spectral sensitivity. Moreover, in TH receptor-defective fish, retinal progenitors designed to become red cones are transfated into UV cones, providing another mechanism for TH to regulate long-wavelength vision [ 180 , 195 , 37 ].…”