Human Skin Can "See" Light with Opsins: A Bright Future for Light-based Therapies?

Denise Falcone1, Irene Castellano Pellicena2, and Natallia E. Uzunbajakava3
1Radboud University Medical Center, Department of Dermatology, Nijmegen, The Netherlands
2Centre for Skin Sciences, University of Bradford, Bradford, West Yorkshire, United Kingdom
3Philips Research, Eindhoven, The Netherlands

March 15, 2018

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Photoreceptors in Vision
Phototransduction is the mechanism whereby photons are ‘received’ by a photoreceptor and converted into a cellular response.1,2 In humans, the most well-known example of phototransduction is the image forming process of human vision, in which photons interact with light-sensitive opsin family proteins: rhodopsin (OPN2), and the three types of OPN1 with short-, medium- and long-wavelength absorption maxima (OPN1-SW, OPN1-MW, and OPN1-LW). OPN2 is expressed in rod cells of the retina and mediates dim-light vision, whereas OPN1 is expressed in cone cells and mediates color vision.1,2

Opsin proteins belong to the family of G-protein coupled receptors (GPCRs), the largest family of integral membrane proteins accounting for 3-4% of the human genome.3 GPCRs can be activated by a variety of stimuli and mediate intracellular signal transduction cascades controlling gene transcription.1

Photoreceptors Beyond Vision
Until recently, phototransduction in humans was exclusively attributed to the cells of the vision system, mediated by OPN1 and OPN2. New evidence described the existence of melanopsin (OPN4) acting as a non-image forming photoreceptor in the retina, and considered to have light-regulated tasks.4 Likewise, another opsin (peropsin) was identified in human ocular tissues and hypothesized to play a role in the retinal pigment epithelium.5

As for tissues outside the visual system, in humans expression of several types of opsins was reported in the skin, hair follicles, and resident cells among other tissues.1,6-16

Photobiomodulation and Potential New Targets of Light
Photobiomodulation (PBM) is the process of addressing cellular targets by photons in the UV-visible-near infrared spectrum, at low irradiance and radiant exposure, to initiate photochemical and photobiological interactions and subsequent tissues responses that are physiologically relevant.17-19 PBM has been attracting an increasing attention, as its application for the treatment of a range of cutaneous and general health conditions demonstrated encouraging results.17-19

However, one cannot fully dispel the existing skepticism about this field, due to inconsistencies in experimental outcomes, study designs, and applied optical parameters, as well as a lack of complete understanding on how photons are ’sensed’ by cells and which molecular cascades are triggered.20 Several endogenous photoreceptors and chromophores in human skin are suggested to mediate the therapeutic action of PBM, such as cytochrome c oxidase as well as nitrosated- and flavo-proteins.21 Lamentably, no complete and scientifically-objectified picture of the role of these potential 'receptors' of light photons has been formed yet. This impedes the progress of PBM towards truly effective and evidence-based therapies.

The recently discovered opsin photoreceptors represent a potentially new molecular target in PBM. Understanding their possible implication in signal transduction could pave the path towards new and more effective PBM-based therapies, addressing a wide range of health conditions. Therefore, in this short communication we will focus on summarizing new findings about opsin expression outside of visual tissues, with a special emphasis on human skin and hair follicles. In addition, we will discuss possible implications of this knowledge for future therapies based on PBM with UV-visible light.

Opsins in Human Skin and Hair Follicles
Epidermal Keratinocytes
Tutsumi et al. reported on the expression of the visual opsins, OPN1-SW, OPN1-M/LW, and OPN2, in human facial skin by RT-PCR and immunohistochemistry.11 The results on the epidermal presence of opsins were recently confirmed by Buscone et al., who showed the expression of OPN2 and OPN3 in human beard and scalp skin by immunofluorescence, and Castellano-Pellicena et al., who demonstrated that OPN1-SW, OPN3, and OPN5 are expressed in facial and abdominal human skin.6,12 Another opsin, peropsin, was detected by immunostaining, localizing primarily in the suprabasal epidermis.8

The functional role of opsins in photo-sensing was investigated as well. Castellano-Pellicena et al. found that low levels of blue light (~ 2J/cm2) at 450 nm induced early differentiation in epidermal keratinocyte cultures, where the effect was abrogated by silencing OPN3, and concluded that OPN3 plays a role in modulating light-dependent epidermal keratinocyte differentiation.12,13 Kim et al. observed that violet light (380–420 nm) significantly increased the expression of OPN2 mRNA, which in turn decreased the mRNA expression of keratinocyte differentiation markers.14 Violet light (380 and 400 nm) also increased the intracellular calcium flux response in N/TERT-1 keratinocytes, an effect abrogated by peropsin knock-down, thereby suggesting that peropsin contributes to phototransduction of violet light in human keratinocytes.8 A single irradiation by blue light delayed the recovery of the skin barrier after acute disruption in in vivo studies on mice and humans,15,22 whereas repeated irradiations alleviated symptoms of psoriasis vulgaris and atopic dermatitis in clinical studies.23,24 These findings suggest that future visible light-based therapies could address skin conditions characterized by abnormal epidermal differentiation and barrier homeostasis.

Dermal Fibroblasts
Using immunofluorescence, Buscone et al. showed the expression of OPN2 and OPN3 in dermal fibroblasts of human skin obtained from either beard or scalp regions.6 In addition, Castellano-Pellicena et al. demonstrated that OPN1-SW and OPN3 are expressed in cells isolated from human abdominal, breast, and facial skin.12,13 Presence of opsins in human dermal fibroblasts might explain the findings of Mignon et al., who reported that low-levels of blue light at 450 nm (2 J/cm2) had a stimulatory effect on their metabolism, leading to increased production of collagen, whereas higher levels (30 J/cm2 and higher) had an inhibitory effect on cell metabolism and protein synthesis.25 Further investigations of the role of opsins in mediating the impact of light on dermal fibroblasts could potentially pave the way to therapies addressing wound healing and skin rejuvenation.

Epidermal Melanocytes
Wicks et al. showed that OPN2 is expressed in human epidermal melanocytes in in vitro cell cultures and contributes to UV phototransduction.7 Upon UV exposure (90% in the range 320-400 nm and 10% in the range 280-320 nm), significant melanin production was measured within one hour and up to 24 hours thereafter. Likewise, Passeron and co-workers demonstrated the expression of OPN3 in normal human melanocytes and its key role in mediating melanogenesis induced by blue light at 415 and 465 nm.9 Together with the work of Bigliardi and colleagues on peropsin expression in keratinocytes and its sensitivity to violet light, these findings identify OPN2, OPN3, and peropsin as new potential targets for regulating melanogenesis, for instance in pigmentary disorders.8

Cutaneous Mast Cells
Siiskonen et al. used PCR to demonstrate that freshly isolated mast cells (MCs) from breast skin, cultured MCs from breast skin and foreskin, and LAD2-MCs express OPN1-MW, OPN2, and OPN3.16 Expression of OPN1-MW and OPN3 was also detected in freshly isolated MCs from eyelid skin. Freshly isolated MCs from abdominal skin and peripheral CD34+ stem cell-derived mast cells were negative for the investigated photoreceptors, suggesting that their expression might be connected to MCs differentiation and functionality. It is further proven that light-based therapies addressing IgE/MC-mediated allergic reactions might be envisaged in the future.

Hair Follicles
Buscone and co-workers detected the expression of OPN2 and OPN3 in the outer- and inner-root sheath cells of human hair follicles, respectively.6 Treatment with low-levels of blue light at 453 nm significantly prolonged the anagen phase in hair follicles cultured ex vivo, which was correlated with sustained proliferation in the light-treated samples. This light-induced cell proliferation was abrogated by silencing OPN3, thereby implying its role in mediating the hair growth ex vivo. This hypothesis was further supported by microarray data, which showed that OPN3 expression in hair follicle outer root sheath cells is related to genes controlling proliferation and apoptosis.6 Future therapies targeting opsins might prove the beneficial role of light for hair growth or hair removal.

Opsins in Other 'Non-Visual' Mammalian Tissues
Systemic and Pulmonary Vessels
Berkowitz and colleagues demonstrated that OPN4 is expressed in mouse aorta and that OPN3 and OPN4 are both expressed in rat pulmonary arteries and pulmonary arterial smooth muscle cells.26,27 Intriguingly, the authors showed that these opsins were responsible for mediating photorelaxation, defined as the reversible relaxation of blood vessels to 'cold' light, and that the maximum response was achieved in the blue spectral range (430-460 nm). These results, which still need to be confirmed to be valid for human tissue, suggest that phototherapy might provide an alternative treatment strategy for pulmonary vasoconstrictive disorders and for diseases in which altered vasoreactivity is a significant pathological contributor.26,27

The endogenous expression of OPN3, OPN4, and OPN5 has been observed in murine brain, albeit their functional role remains to be unraveled.28,29 In the last decade, opsin expression in the brain has also been genetically targeted in mouse models.30,31 In this technique, called optogenetics, the exchange of light with the nervous system is achieved by implanting a small fiber-optic probe into the brain, which is typically interfaced with a light source for input and to a fast camera or photomultiplier for readout. This robust hardware allows for the activation of specific cell populations, genetically expressing opsins in a time-controlled manner. For instance, by using blue laser light to target channelrhodopsin-2(ChR2) in astrocytes (brain cells), forming the interface between the brain vasculature and the central nervous system, the local cerebral blood flow could be manipulated and non-invasively monitored.32 This opens up the possibility to study, in vivo, neurovascular interactions in both healthy and neurovascular dysfunction models.

Future Outlook
We witness growing evidence that in human non-visual tissue opsins serve as an endogenous 'optogenetic system', orchestrating light-based regulation of several physiological functions including skin pigmentation, epidermal and dermal cell differentiation, and hair cycle control. Additionally, light-induced and opsin-dependent vasorelaxation of blood vessels was shown in a murine model. More studies are one the horizon seeking to unravel potential of opsins in mast cell-mediated allergic reactions and neurovascular interactions. And so, optical radiation across the UV and visible spectrum at low levels of irradiance and radiant exposure (with or without the addition of pharmaceutical components) could become a new treatment strategy for a range of skin ailments as well as systemic health disorders.23,26,33 Achieving this urges us, however, to further unveil functional roles of opsins in photo-sensing and to couple it to clinical applications.

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