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Sara Patterson, Takeshi Yoshimatsu, Tara Suresh, Ralph F Nelson; The Role of Thyroid Hormone Receptor B2 (trβ2) in Photoreceptor Opsin Development. Invest. Ophthalmol. Vis. Sci. 2016;57(12):587.
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© ARVO (1962-2015); The Authors (2016-present)
In the transgenic zebrafish line crx:MYFP-2A-trβ2, the highly conserved cone-rod homeobox (crx) gene promoter drives thyroid hormone β2 (trβ2) expression (Suzuki et al., 2013). Crx is expressed in photoreceptor progenitors and trβ2 is a necessary for subsequent red cone development. This transgenic provides the opportunity to study thyroid hormone in photoreceptor development.
Both crx:trβ2 and wildtype (WT) cousins were studied as larvae at 5-12 days post-fertilization (dpf) and adults at 6-9 months old. Eyes were removed and perfused with oxygenated MEM containing 20mM L-Aspartate or 10mM CNQX to isolate the photoreceptor and ON-bipolar responses, respectively. Microelectrode ERG responses were recorded to 9 wavelengths (330-650nm), each at 7 irradiances. A 627nm background was used for red adaptation trials. Spectral sensitivity was calculated with an ERG model summing 4 Hill functions, one for each zebrafish cone type (Nelson & Singla, 2009). To examine Vitamin A2 L-opsin expression, a 5th Hill function was added using a 600nm λmax.
In crx:trβ2, photoreceptor ERG responses showed increased long-wavelength sensitivity and diminished, but not entirely absent, short-wavelength sensitivity. By adulthood, crx:trβ2 photoreceptor spectra resembled WT spectra with only a small increase in long-wavelength sensitivity UV- and S-opsin immunostaining found expression at 5 and 12dpf. While S-opsin was widespread, UV-opsin appeared primarily in the peripheral retina. Red adaptation altered only long-wavelength photoreceptor sensitivity but decreased ON-bipolar sensitivity at all wavelengths in both larvae and adults. A2 sensitivity was stronger in crx:trβ2 than WTs and decreased from 5 and 12dpf in both genotypes.
Zebrafish photoreceptor differentiation is normally complete by 4dpf. However, photoreceptor ERG responses in crx:trβ2 developed towards WT sensitivity between 5dpf and adulthood. The large decrease in both short- and long-wavelength ON-bipolar sensitivity in red-adaptation trials suggests crx:trβ2 may also impact photoreceptor-bipolar cell connectivity. The decrease in Vitamin A2 600nm sensitivity from 5 to 12dpf suggests thyroid hormone may be involved in the early retinal development of zebrafish. Understanding how short-wavelength opsins and spectral sensitivity returns in crx:trβ2 could have important implications for photoreceptor development and regeneration research.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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