Effects of induced systemic hypothyroidism upon the retina: regulation of thyroid hormone receptor alpha and photoreceptor production.
OBJECTIVE
Investigate the effects of systemic hypothyroidism upon the differentiated, growing, and regenerating retina of postmetamorphic winter flounder, a vertebrate that experiences a thyroid hormone (TH) induced metamorphosis during development.


METHODS
A loss-of-signal strategy was utilized in which TH signaling was disrupted by inhibiting TH synthesis. Induced hypothyroidism was confirmed by radioimmunoassay. Reverse transcriptase PCR (RT-PCR), real-time quantitative PCR (qPCR), molecular cloning, non-isotopic in situ hybridization, western blot analysis, and indirect immunohistochemistry techniques were performed to analyze retinal thyroid hormone receptors (TR), photoreceptor production, and the phenotypic repertoire of differentiated retinal cells as a function of TH signaling status.


RESULTS
Molecular bases for TH signaling were supported by retinal expression of TH receptors alpha and beta. TH-dependent transcriptional regulation of TRalpha but not TRbeta was indicated, with induced hypothyroidism producing an increase in TRalpha expression. Evidence for post-transcriptional regulation of retinal TRalpha was observed. The repertoire of inner retinal cell types in premetamorphic fish (a naturally low TH condition) matched that observed in the central retinas of both normal postmetamorphic fish (a naturally elevated TH condition) and postmetamorphic fish rendered hypothyroidic. In differentiated postmetamorphic retina there was no evidence for significant differences in opsin expression between normal and hypothyroidic animals. Induced hypothyroidism did, however, significantly affect the types of photoreceptors that were produced in postmetamorphic retina: as a hypothyroidic postmetamorphic retina grew or regenerated following injury, the phenotypic repertoire of newly-produced photoreceptors matched that observed for premetamorphic retina, in which rods, SWS2-expressing "blue" cones, and LWS-expressing "red" cones are absent, and only the RH2-expressing "green" cone type is present. The effects of induced hypothyroidism upon photoreceptor specification (manifestation of the rod lineage) and differentiation (expression of a particular opsin by specified cones) were apparently reversible.


CONCLUSIONS
The results suggest a TH-dependent regulation of retinal TRalpha, a lack of TH-dependent regulation of the phenotypic identity of differentiated retinal cells, and the operation of similar cytogenic mechanisms during retinal growth and regeneration. The principal conclusion is that TH signaling significantly affects, in a targeted manner, the production of both rod and cone photoreceptors during retinal growth and regeneration.
Mader
Michelle M
MM
Department of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, NY 13210, USA.
Cameron
David A
DA
D000818:Animals; D002454:Cell Differentiation; D002455:Cell Division; D005432:Flounder; D007037:Hypothyroidism / Q000139:chemically induced / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology*; D008675:Metamorphosis, Biological; D010641:Phenotype; D020419:Photoreceptor Cells, Vertebrate / Q000473:pathology*; D012038:Regeneration; D012160:Retina / Q000378:metabolism* / Q000473:pathology* / Q000503:physiopathology; D015398:Signal Transduction; D013890:Thiourea; D037021:Thyroid Hormone Receptors alpha / Q000096:biosynthesis*; D014018:Tissue Distribution; D014284:Triiodothyronine / Q000378:metabolism
D037021:Thyroid Hormone Receptors alpha; D014284:Triiodothyronine; D013890:Thiourea