The regulation of skin pigmentation in response to environmental light by pineal Type II opsins and skin melanophore melatonin receptors

Coupling skin colour with the light/dark cycle helps regulate body temperature in ectotherms. In X. laevis, nocturnal release of melatonin from the pineal complex induces pigment aggregation and skin lightening. This nocturnal blanching is initiated by a sensor (type II opsin) that triggers melatonin release when light intensity falls below a minimum threshold, and an effector (melatonin receptor) in the skin which induces pigment aggregation. The sensor/s and effector/s belong to two families of G-protein coupled receptors that originated from a common ancestor, but diverged with subsequent evolution. The aim of this work was to identify candidate sensor/s and effector/s that regulate melatonin-mediated skin colour variation. In X. laevis, we identified a developmental time (stage 43/44) when skin lightening depends on pineal complex photosensitivity alone. At this stage, the pineal complex comprises the frontal organ and pineal gland. A total of 37 type II opsin (14 duplicated) and 6 melatonin receptor (3 duplicated) genes were identified through a full genome analysis of the allotetraploid, X. laevis. These genes were grouped into subfamilies based on their predicted amino acid sequences and the presence of specific amino acids essential for their function. The pineal complex expresses mainly blue light sensitive opsins [pinopsin, parietopsin, opn3, and melanopsins (opn4 and opn4b)] and UV-light sensitive opsins (opn5 and parapinopsin), while visual opsins and va-ancient opsin are absent, as determined by RT-PCR and in situ hybridization. The photoisomerase retinal G-protein coupled receptor, and an uncharacterized opn6b opsin, are also expressed. The spectral sensitivity that triggers melatonin secretion, and therefore melanophore aggregation, falls in the visible spectrum (470–650 ηm) and peaks in the blue/green range, pointing to the involvement of opsins with sensitivities therein. The effector-melatonin receptors expressed in skin melanophores are mtnr1a and mtnr1c. Our data point to candidate proteins required in the neuroendocrine circuit that underlies the circadian regulation of skin pigmentation, and suggest that multiple initiators and effectors likely participate.