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Annika Balraj, Takeshi Yoshimatsu, Ralph F Nelson; Thyroxin b2 receptor (trb2) overexpression alters cone spectra in zebrafish.. Invest. Ophthalmol. Vis. Sci. 2016;57(12):586. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The trb2 nuclear receptor triggers progenitors to become L-cones. When expressed in all cone progenitors (Patterson et al, 2015), the tetrachromatic (L, red; M, green; S, blue; UV) zebrafish becomes an L- cone monochromat. In gnat2:MYFP-2A-trβ2 (gnat2:trb2), trb2 is expressed in differentiated cones, causing co-expression of L-opsin in M- and UV-cones at 5dpf (Suzuki et al, 2013). We hypothesized that mixed opsin cones might sensitize ERG responses at long wavelengths (wl), and desensitize UV- and M-cones on red backgrounds.
Both gnat2:trb2 and wild type (WT) siblings were studied at 5, 6, 7, and 12 dpf. gnat2:trb2 larvae showed MYFP fluorescence in the pupil and pineal gland. Dissected eyes were perfused with oxygenated MEM containing 20mM Na Aspartate to block post-synaptic glutamatergic receptors and isolate cone PIII responses. Using a microelectrode, ERG responses were recorded at 10 wavelengths (330-650nm). Backgrounds were either infrared (IR) or red (627nm). Spectra were fit by a ‘sum of Hill functions’ model (Nelson & Singla, 2009). The wl dependence of red to IR background sensitivity provided an index of L-opsin distribution in cone types. For 650nm stimuli, red background sensitivity was 26% of IR background sensitivity and increased towards 100% with shorter wl stimuli. A red background half-desensitized-wavelength (wl1/2) was measured at 63% of IR background sensitivity.
5-12dpf WT: The primary peak was 370nm, and the secondary peak was 550nm. The median wl1/2 was 490nm. 5dpf: gnat2:trβ2 and WT spectra were similar on both IR and red backgrounds. The gnat2:trβ2 wl1/2 was 490nm. 6dpf: on IR background, gnat2:trβ2 sensitivity was greater than wild type for wavelengths longer than 400nm. wl1/2 was 400nm, suggesting L-, M- and S- cones were desensitized by red backgrounds. 7dpf: gnat2:trβ2 IR background sensitivity in the UV was less than WT, but similar at longer wl. wl1/2 was 390nm, suggesting L-, M-, S- and UV- cones were desensitized by the red background. 12dpf: gnat2:trβ2 IR spectra showed normal (WT) UV sensitivity, but an enhanced 550nm peak. wl1/2 was 490nm.
The results support the hypothesis of mixed opsin cones in gnat2:trb2, but only for 6 and 7dpf. At 12dpf, a red spectral enhancement persists, but red desensitization of non-L cones is lost. Mixed L-opsin expression in non L-cones appears ultimately rejected by 12dpf.
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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