Opsin 3, encoding a non-visual photoreceptor, is a pseudogene in cattle

Abstract

The non-visual photoreceptor opsin 3 (OPN3) performs various functions by directly accepting light and activating G protein-coupled receptor signalling in non-visual tissues. OPN3, which is expressed in brown adipocytes, induces the expression of uncoupling protein 1 to generate heat upon exposure to blue light. There have been few reports on OPN3 expression in ruminants, and the mRNA sequence registered in a database as bovine OPN3 is the only predicted sequence. Therefore, we examined OPN3 expression in cattle. In the bovine retina, amplified fragments of the visual opsins, rhodopsin and opsin 1 and the non-visual opsins, opsin 4 and opsin 5, were detected using reverse transcription PCR, whereas no amplified PCR products of OPN3 were detected. Eleven tissues were used to analyse OPN3 expression in the entire body of bovines, including the testis and placenta, where OPN3 is highly expressed in humans and mice. No amplified PCR products were detected in any of the tissues. In a comparison among animal species, OPN3 was found to be expressed in mouse and horse testes, whereas no expression was observed in cattle and goat tissues. RNA sequencing revealed no OPN3 transcripts in the bovine placenta. Direct genomic DNA sequencing revealed a stop codon in the middle of the bovine OPN3 sequence. These results suggest that bovine OPN3 is pseudogenic as it does not encode a complete receptor protein. They also indicated that OPN3 is pseudogenic not only in cattle but also in other Cetartiodactyla, including goats.