ABSTRACT
During retinal development, the cell-fate of photoreceptors is committed long before maturation, which entails the expression of opsins and functional transduction of light. The mechanisms that delay the maturation of photoreceptors remain unknown. We have recently reported that immature photoreceptors express the LIM domain transcription factors Islet2 and Lim3, as well as the cell-surface glycoprotein axonin1 [Fischer et al., (2008a) J Comp Neurol 506:584-603]. As the photoreceptors mature to form outer segments and express photopigments, the expression of the Islet2, Lim3 and axonin1 is diminished. The purpose of this study was to investigate whether thyroid hormone (TH) influences the maturation of photoreceptors. We studied the maturation of photoreceptors across the gradient of maturity that exists in far peripheral regions of the post-natal chicken retina [Ghai et al., (2008) Brain Res 1192:76-89]. We found that intraocular injections of TH down-regulated Islet2, Lim3 and axonin1 in photoreceptors in far peripheral regions of the retina. By contrast, TH stimulated the up-regulation of red-green opsin, violet opsin, rhodopsin and calbindin in photoreceptors. We found a correlation between the onset of RLIM (RING finger LIM-domain binding protein) and down-regulation of Islet2 and Lim3 in maturing photoreceptors; RLIM is known to interfere with the transcriptional activity of LIM-domain transcription factors. We conclude that TH stimulates the maturation of photoreceptors in the avian retina. We propose that TH inhibits the expression of Islet2 and Lim3, which thereby permits photoreceptor maturation and the onset of photopigment-expression.
