BACKGROUND: Thyroid hormone (TH) signaling is involved in complex processes regulating development and energy balance, especially in organs with high-rate metabolism such as the retina. Despite evidence of TH involvement in regulation of retinal functions and disease susceptibility, a comprehensive analysis of the tissue distribution of the components of the retinal TH regulatory system is lacking. The aims of the present study were (i) to provide a detailed description of the retinal expression sites and localization patterns of deiodinases (DIO2 and DIO3), TH receptors (THRα and THRβ), and TH transporters (monocarboxylate transporter 8, MCT8, and organic anion-transporting polypeptide 1C1, OATP1C1); (ii) to recapitulate the retinal alterations following genetic deletions of specific TH regulatory elements in specific knockout (KO) mice.

METHODS: We first predicted the cell-specific expression of TH regulatory elements in mouse and human retina via bioinformatic analysis of single-cell RNAseq databases. The results were subsequently validated in mouse retinas with RNAscope and protein localization was assessed with immunofluorescence. Cone photoreceptor morphology and retinal layer thicknesses were evaluated in the retinas of KO mice.

RESULTS: Dio2 mRNA was expressed by Müller glial cells and neuronal populations of the inner retina, while prominent DIO2 immunoreactivity was in cone photoreceptors, with an evident mismatch between mRNA and protein localization. DIO3 was expressed by virtually all neural cell populations of the retina. THRα and THRβ were predominantly localized to the inner and outer retina, respectively, with prominent expression of THRα in putative amacrine cells and ganglion cells, and of THRβ in cone photoreceptors. Both MCT8 and OATP1C1 were detected in the retinal pigment epithelium, in Müller glia, and in blood vessels. MCT8 was also expressed in some retinal ganglion cells. In KO mice, drastic alterations were observed in cone density and morphology and in retinal layer thickness following either Dio2 or Dio3 genetic deletion. In contrast, deletion of Thrs or TH transporters resulted in only minor changes, indicating potential compensatory mechanisms.

CONCLUSIONS: The present mapping of the TH regulatory system in the mouse retina provides a framework for understanding molecular and cellular mechanisms regulating local TH availability and signaling in health and disease.