RNA-Seq data of mouse retina, retinal pigment epithelium and cones
The differentiation of cone photoreceptors, which mediate daylight vision and color vision, depends critically upon thyroid hormone.However, the route of access of blood-borne thyroid hormone to cones is undefined because cones reside behind the blood-retina barrier. This study uses genetic manipulation of a membrane transporter for thyroid hormone (Mct8) in mouse models to show a role for the retinal pigment epithelium (RPE), which forms the outer blood-retina barrier, in the control of cone differentiation.The results suggest a paracrine-like mechanism promotes thyroid hormone-mediated cone differentiation. The findings suggest that in addition to the transport of essential solutes and support of photoreceptor homeostasis, the RPE controls  hormonal signaling required for cone differentiation. Overall design: RNA-seq analysis was performed on purified RPE cells, bulk retinal tissue, and also on isolated single cones from mouse genetic  models. To distinguish between M-dominant and S-dominant cone types, samples were taken from superior and inferior areas of the retina, respectively. RPE samples were taken from superior and inferior areas of the eye to investigate the possibility of spatial distinctions in the transcriptome of RPE cells. An Mct8-knockout allele (Mct8-) was used to determine the total requirement for Mct8. The retinal transcriptome of a hypothyroid strain, Tshr-/-, was also analyzed.  The role of the RPE was established by RPE-specific conditional deletion of a floxed Mct8 allele using an RPE-specific Cre driver transgene (Best1cre). As an X-linked gene, for females, the Mct8 flox allele was combined with a germline Mct8 null allele (Mct8 fl/null).  The results of the study suggest that Mct8 thyroid hormone transporter in the RPE promotes differentiation of cones in the retina.