Human nonvisual opsin 3 regulates pigmentation of epidermal melanocytes through functional interaction with melanocortin 1 receptor

Abstract

Opsins form a family of light-activated, retinal-dependent, G protein-coupled receptors (GPCRs) that serve a multitude of visual and nonvisual functions. Opsin 3 (OPN3 or encephalopsin), initially identified in the brain, remains one of the few members of the mammalian opsin family with unknown function and ambiguous light absorption properties. We recently discovered that OPN3 is highly expressed in human epidermal melanocytes (HEMs)-the skin cells that produce melanin. The melanin pigment is a critical defense against ultraviolet radiation (UVR), and its production is mediated by the Gαs-coupled melanocortin 1 receptor (MC1R). The physiological function and light sensitivity of OPN3 in melanocytes are yet to be determined. Here, we show that in HEMs, OPN3 acts as a negative regulator of melanin production by modulating the signaling of MC1R. OPN3 negatively regulates the cyclic adenosine monophosphate (cAMP) response evoked by MC1R via activation of the Gαi subunit of G proteins, thus decreasing cellular melanin levels. In addition to their functional relationship, OPN3 and MC1R colocalize at both the plasma membrane and in intracellular structures, and can form a physical complex. Remarkably, OPN3 can bind retinal, but does not mediate light-induced signaling in melanocytes. Our results identify a function for OPN3 in the regulation of the melanogenic pathway in epidermal melanocytes; we have revealed a light-independent function for the poorly characterized OPN3 and a pathway that greatly expands our understanding of melanocyte and skin physiology. opsin3 | encephalopsin | melanocytes | melanocortin 1 receptor | pigmentation U nlike most mammals, which have melanin-producing melanocytes predominantly in the hair follicle bulb (1), humans are uniquely equipped with melanocytes in the outermost layer of the skin, the epidermis (2, 3). These neural crest-derived melanocytes are the only source of the photoprotective pigment melanin in human skin, and thus are critical for the defense against solar ultraviolet radiation (UVR)-induced genotoxic damage (4-6).Solar UVR at the surface of the earth is composed of ∼5% short-wavelength UVB rays and ∼95% long-wavelength UVA rays. Much of our current knowledge about melanogenesis in epidermal melanocytes stems from the well-characterized UVBinduced melanin pathway (7). UVB elicits DNA damage in epidermal keratinocytes, triggering facultative skin darkening through increased melanin production in neighboring melanocytes (8). UVB-irradiated keratinocytes and melanocytes locally secrete α-melanocyte stimulating hormone (α-MSH), an agonist of the Gαs-coupled melanocortin 1 receptor (MC1R) that is primarily expressed on melanocytes (8,9). MC1R has a pivotal role in determining pigmentation, as several naturally occurring loss-offunction MC1R variants are associated with the redhead phenotype (10, 11) characterized by a pale complexion and increased sensitivity to UVR (12). Downstream, α-MSH-induced MC1R activation leads to stimulation of adenylyl cyclase (AC) and production of...

The expression of opsins in the human skin and its implications for photobiomodulation: A Systematic Review

“…In 2018, Regazzetti et al proposed that OPN3 might be a key receptor responsible for visible light‐induced hyperpigmentation by demonstrating that violet light (415 nm) irradiation on cultured NHMs activates calcium‐dependent microphthalmia‐associated transcription factor (MITF) pathway and upregulates melanogenesis‐associated enzymes, whereas the violet light – induced effect was abrogated by silencing OPN3 57 . In 2019, Ozdeslik et al also suggested that OPN3 might play a key role in the regulation of melanogenesis, but proposed a different mechanism of action 58 . Ozdeslik et al found that OPN3 acts as a negative modulator of melanin production via coupling to Gαi pathway, which inhibits melanocortin 1 receptor (MC1R) – mediated cAMP response leading to melanin production.…”

“…Advances in molecular biology techniques eventually led to a discovery of opsins as potential photosensors in the skin and inspired great scientific interest concerning the role of opsins in the skin. A number of studies demonstrated that opsins modulate various physiological processes of the skin, including wound healing, melanogenesis, photoaging, and hair growth 54,58 …”

“…Several studies have already demonstrated the great potentials of PBM for treating dermatological conditions by targeting these opsins 21 . Recent studies, which demonstrated the stimulatory effect on hair follicle stem cells and melanocytes with specific wavelengths of light irradiation, suggest that PBM could be a new promising treatment for hair loss and skin pigmentation disorders in near future 58,60 …”

Cell-autonomous light sensitivity via Opsin3 regulates fuel utilization in brown adipocytes

“…A decade later, Opn3 was found to be also highly expressed in adipose tissue [13]. Interestingly, distant homologs of Opn3 are capable of mediating direct photoreception in various nonvisual tissues by activating G protein-coupled receptor (GPCR) signaling in a light-dependent manner [14][15][16]. These lines of evidence prompted us to ask whether adipose tissue can indeed sense light directly via Opn3-GPCR signaling and, if so, what the metabolic consequences of these signaling pathways are.…”

Adaptive thermogenesis in mice requires adipocyte light-sensing via Opsin 3

“…Coupled with studies showing light-dependent signaling by these opsins (Kato et al, 2016;Kojima et al, 2011;Koyanagi et al, 2013a;Yamashita et al, 2010), this has raised the possibility of extraocular photoreception in mammals. Evidence for the function of these signals is limited to date but includes a possible role for OPN4 in acutely-regulated dilation of blood vessels (Sikka et al, 2014(Sikka et al, , 2016 and in the modulation of melanocyte pigmentation (de Assis et al, 2018;Ozdeslik et al, 2019). It has also been suggested that adipocyte function might be modulated by light stimulation of OPN4 (Ondrusova et al, 2017).…”