Abstract
Purpose :
Color vision in vertebrates requires differential expression of multiple specific cone opsins in distinct cone populations. One model for the regulation of the human long and medium wavelength sensitive (LWS/MWS) opsin tandem array suggests an upstream regulatory region randomly interacts with replicated opsin genes, resulting in the mutually exclusive expression of a specific opsin. However, our prior studies of the long wavelength sensitive (lws1/lws2) array in zebrafish (sharing a common ancestral LWS gene with the human array) demonstrate that thyroid hormone (TH) serves as a regulator of this gene array in larvae/juveniles (Mackin et al., 2019). In the current study we investigate potential roles for the TH receptor isoform thrb2 in regulating lws1 vs. lws2 expression and in mediating the effects of exogenous TH using a crx: thrb2-YFP transgenic line, in which thrb2 is over- and ectopically expressed in all photoreceptors and in a subset of bipolar neurons (Suzuki et al., 2013).
Methods :
Larval and adult carriers of this transgene were examined for expression of selected opsin transcripts using fluorescent confocal microscopy following multiplex hybridization chain reaction (HCR) in situs, and larvae were also subjected to exogenous TH treatments (hyperthyroid). Lws1 and lws2 expressing cells of transgenic larvae were counted and compared to WT clutch mates (controls) for both euthyroid and hyperthyroid groups.
Results :
Larval transgenics (4dpf) displayed reduced (though not significantly) numbers of lws1+ cones in comparison to wildtype clutch mates (n=8, p=0.0746), and treatment of transgenics with TH resulted in less robust increases in lws1+ cones and decreases in lws2+ cones (n=5, p=0.003) in comparison to treated wildtype clutch mates. Interestingly, untreated adult transgenics showed expanded expression domains of lws1 into central retina, surrounding the optic nerve head, an expression pattern not seen in wildtype retinas.
Conclusions :
These findings support potential roles for thrb2 in regulating differential expression of lws1 vs. lws2, but also suggest its roles are complex, and likely TH- and age-dependent.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.