Abstract
Purpose:
Replacing photoreceptors lost in degenerative disease remains an elusive goal of stem cell therapy, particularly generating cone photoreceptors, which are responsible for daytime vision, colour detection, and visual acuity. We use the zebrafish, a diurnal animal with cone-rich retinas, to elucidate the intrinsic signaling that orchestrates cone differentiation from photoreceptor progenitors in the contexts of embryonic development and adult neural regeneration. Here we test the hypothesis that disrupting thyroid hormone receptor β (THRβ) affects cone subtype specification, and that such effects are differentially dependent on the developmental stage.
Methods:
We engineered a transgenic zebrafish permitting conditional disruption of THRβ activity via expression of a dominant negative THRβ (dnTHRβ), along with a GFP marker, under the heat shock inducible promoter hsp70. This dominant negative receptor is expected to dimerize with native THRβ and inhibit the transcription regulation activity of THRβ. Transgenic larval zebrafish were heat shocked to induce transgene expression at different stages of retinal development, and spatiotemporal aspects of GFP were documented. Cone subtype abundances in heated transgenic fish were compared to wild type fish.
Results:
When heat shocked, transgenic larvae express GFP in developing retina, eye lens, brain, and muscle tissue for several hours. Disruption of THRβ activity via expression of dnTHRβ early in retinal development increased red cone abundance (7% increase, p=0.009) relative to UV and blue cones, apparently at the expense of UV cones (20% decrease compared to red and blue cones, p=0.03). But disruption later in development resulted in a decrease in red cones (33% decrease, p<0.001) relative to UV and blue cones.
Conclusions:
We are elaborating on the emerging network of genes known to control cone photoreceptor specification in zebrafish (DuVal et al. 2014 PMID 24681822), towards promoting cone regeneration. Red cone photoreceptor abundance changed differentially depending on the timing of THRβ disruption in our model, elaborating on results of THRβ knockdown in a previous study (Suzuki et al. 2013 PMID 23980162). Because altered timing of THRβ disruption appears to differentially affect photoreceptor specification, our data implicates differential roles for THRβ during the various states of retinal progenitor cell competency.