Changes in thyroid hormone reception precede SWS1 opsin downregulation in trout retina

Abstract

SUMMARYRainbow trout undergo natural cone degeneration and thus are interesting models for examining mechanisms of neural degeneration. They have ultraviolet-sensitive (UVS) cones that are lost over most of the retina during development; only a small functional population remains in the dorsal retina. How this spatial distribution of UVS cones is maintained is unclear. Thyroxine (T4) induces UVS cone loss, and local thyroid hormone regulation was hypothesized to control UVS cone distribution. Thyroid hormone receptor alpha (TRα), thyroid hormone receptor beta (TRβ) and Type 2 deiodinase (D2) regulate thyroid hormone exposure to target cells. Regional retinal expression of these genes was investigated during exogenous T4 treatment and natural smoltification of rainbow trout. Each retina from dark-adapted parr, T4-treated parr and natural smolts was divided into four quadrants, and total RNA was isolated. Quantitative real-time RT-PCR analysis demonstrated that all retinal quadrants had increased accumulation of TRβ transcripts 2 days post-T4 treatment, corresponding to initiation of SWS1 opsin downregulation. Smolts exhibited decreased accumulation of TRα and TRβ transcripts in all quadrants, but this effect was most pronounced in the dorso-temporal (DT) retinal quadrant where UVS cones persist. By contrast, in 2 day T4-treated parr, the DT quadrant showed increased expression of TRα and TRβ. Furthermore, D2 transcripts decreased in the DT quadrant of T4-treated parr but increased in the DT quadrant of smolts. These results suggest that T4 upregulates TRβ expression to initiate SWS1 opsin downregulation, while TRα and TRβ downregulation occurs to prevent natural loss of UVS cones from the DT retina.

In ovo thyroxine exposure alters later UVS cone loss in juvenile rainbow trout

“…This process also occurs in land-locked salmonid species such as the popular fish model, rainbow trout (Oncorhynchus mykiss), which spend their entire lifespan in freshwater systems. In rainbow trout, loss of UVS cones is preceded by the downregulation of the SWS1 opsin gene (Allison et al, 2003;Veldhoen et al, 2006;Raine and Hawryshyn,al., 2006;Raine and Hawryshyn, 2009;Raine et al, 2010). Additionally, consistent upregulation of TR gene expression prior to T4-induced down-regulation of SWS1 suggests that this receptor isoform plays a key role in this developmental process, similar to that found in mice (Raine and Hawryhshyn, 2009;Ng et al, 2010;Raine et al, 2010).…”

“…Total RNA was isolated from each juvenile retina. RNA isolation and quantification procedures followed protocols developed in previous studies (Raine et al, 2004;Veldhoen et al, 2006;Raine and Hawryshyn, 2009;Raine et al, 2010). Briefly, samples were homogenized using a Retsch MM301 mixer mill (Retsch GmBH & Co. Ltd KG, Hann, Germany) and total RNA isolated with an Absolutely RNA Miniprep kit (Stratagene, LaJolla, CA, USA).…”

“…The O. mykiss gene-specific primers used in this study, shortwavelength-sensitive 1 and 2 (SWS1, SWS2), long-wavelengthsensitive (LWS) and rhodopsin 1 and 2 (RH1 and RH2), were developed and validated in previous studies (Table1) (Raine et al, 2004;Veldhoen et al, 2006;Raine and Hawryshyn, 2009;Raine et al, 2010). Although the present study focuses primarily on SWS1, other opsins were also examined here, as previous studies showed that TH-induced downregulation of one opsin can lead to an increase in expression of other opsin(s) (e.g.…”

Thyroid Hormone Controls Cone Opsin Expression in the Retina of Adult Rodents

“…Opsin switches in postmitotic photoreceptors are well known in the developing mammalian retina, with ultraviolet/blue (SWS1) opsin expression preceding green (MWS/LWS) opsin expression (Szél et al, 1994;Cornish et al, 2004) and have been observed, during metamorphosis, in the larval eye of the fruit fly (Sprecher and Desplan, 2008) and in young fish (Gan and Novales Flamarique, 2010; references therein). In the retina of juvenile salmonids (alevin and parr stages), for example, "single cones" undergo a TH-induced switch in opsin expression from ultraviolet (SWS1) to blue (SWS2) opsin (Cheng and Flamarique, 2007;Cheng et al, 2009), with the expression levels of thyroid receptor isoforms TR␣ and TR␤ raising after developmental or experimentally induced surges in thyroid hormone (Raine and Hawryshyn, 2009;Gan and Novales Flamarique, 2010;Raine et al, 2010). In adult mammals, comparable opsin shifts have not been described, except minor circadian oscillations of opsin transcript levels (von Schantz et al, 1999), which are thought not to alter the spectral phenotype of the photoreceptor.…”

Thyroid hormone accelerates opsin expression during early photoreceptor differentiation and induces opsin switching in differentiated TRα‐expressing cones of the salmonid retina

“…In the retina of rainbow trout parr, quantitative RT-PCR analyses have shown that TRa and TRb transcription is up-regulated following 3 days of exogenous thyroid hormone exposure (Raine and Hawryshyn, 2009). This up-regulation is most pronounced in the dorso-temporal retina, the only area with a remaining population of UV opsin-expressing cones at this stage in the animal's life cycle (Cheng and Novales Flamarique, 2007;Cheng et al, , 2009.…”