COUP-TFI and -TFII nuclear receptors are expressed in amacrine cells and play roles in regulating the differentiation of retinal progenitor cells

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are members of the steroid/thyroid hormone receptor superfamily. We have shown that two homologous COUP-TF genes, COUP-TFI and COUP-TFII, are expressed in developing mouse retina with a unique gradient along the dorsal-ventral axis. In this work, we aimed to characterize the detailed expression patterns of COUP-TFs in mature retina. Their functions in retinal progenitor cell differentiation into subtypes of mature retinal cells were also examined. Immunostaining of frozen mouse retinal sections with antibodies against COUP-TFs and markers for retinal subtypes revealed that COUP-TFI and -TFII are expressed in amacrine cells, especially in a glycinergic subtype in mature mouse retina. Forced expression of COUP-TFI and -TFII in mouse retinal explant culture by retrovirus-mediated gene transfer promoted amacrine and cone photoreceptor cell differentiation, whereas that of rod photoreceptors decreased. Cell proliferation and apoptosis were not affected by the perturbation of COUP-TFI and -TFII expression levels. Using the Y79 retinoblastoma cell line, we observed that COUP-TFI and -TFII suppressed the transcriptional activation of the Nrl gene. We then analyzed one another member of COUP-TF transcription factors, COUP-TFγ, whose structure is relatively distant from those of COUP-TFI and -TFII. It is expressed mainly in horizontal cells and has weak activity in inducing amacrine cells when COUP-TFγ was ectopically expressed in retinal explants. In summary, we found that COUP-TFI and -TFII play roles in amacrine cell differentiation, and COUP-TFγ has distinct expression pattern and roles during retinal development.