The role of photoreceptors in human skin physiology : potential targets for light-based wound healing treatments : identification of opsins and cryptochromes and the effect of photobiomodulation on human skin and in cultured primary epidermal keratinocytes and dermal fibroblasts

The positive effect of photobiomodulation in wound healing has previously been reported, however there is a considerable lack of knowledge regarding the molecular mechanisms involved, and no consensus on light parameters. Cytochrome c oxidase (CCO) is established as the main photoreceptor in cells, but light also induces nitric oxide (NO), production of reactive oxygen species (ROS) and activation of ion channels. Emerging new molecular targets include the GPCRs opsins (OPNs) and the circadian clock transcription factors, cryptochromes (CRYs). Localisation of OPN1-SW, OPN3, OPN5, CRY1 and CRY2 was seen in female facial and abdominal human skin. Furthermore, expression of these photoreceptors was retained in primary epidermal keratinocytes and dermal fibroblasts in culture; both cell types expressed OPN1-SW, OPN3, CRY1 and CRY2, at the mRNA and protein level. OPN2 was only expressed in cultured dermal fibroblasts, while in line with in situ expression, OPN5 was only expressed in cultured keratinocytes. The photoreceptor-expressing cultured epidermal keratinocytes demonstrated a dose- and wavelength- dependent response in both metabolic activity and cell migration in a scratch-wound assay. Specifically, low dose (2 J/cm2) blue light (447 nm) increased metabolic activity, but it did not impact keratinocyte migration. In contrast, high dose (30 J/cm2) blue light had no effect on metabolism, but inhibited migration of epidermal keratinocytes. Red light (655 nm) at 30 J/cm2 stimulated metabolic activity but did not modulate migration, while a higher dose of 60 J/cm2 had no effect on keratinocyte metabolic activity. In order to study OPN3 and CRY1 function, they were silenced in keratinocytes using siRNA; additionally 8 μM KL001 was used to stabilize CRY1. KL001 inhibited migration, and induced KRT1 and KRT10, an effect which was abrogated by knockdown of OPN3. Interestingly, knockdown of OPN3 upregulated CRY1 expression, while KL001 upregulated OPN3 expression, indicating a regulation by OPN3 of the molecular epidermal clock. Low levels of blue light increased early differentiation of epidermal keratinocytes, which was mediated by OPN3 and circadian clock mechanisms. However, low levels of blue light decreased keratinocyte DNA synthesis, which was mediated by circadian clock independently of OPN3. Translation of parameters ex vivo showed increasing re-epithelialisation and induction of OPN3 and CRY1 expression following exposure to 2 J/cm2 of blue light; however high doses of blue light inhibited re-epithelialisation. Red light, also increased re-epithelialisation, but had no effect on OPN3 or CRY1 expression. In conclusion, photoreceptors are expressed in human skin and they mediate DNA synthesis, migration and differentiation of epidermal keratinocytes. Furthermore, low dose of blue light interacts with OPN3 to induce epidermal differentiation, through the regulation of the circadian clock. A better understanding of the molecular mechanisms behind the photobiomodulation response in vitro will help to develop light based therapies for human wound healing. Interestingly, selected light parameters translated to human ex vivo skin showed a beneficial effect of low doses of blue (2 J/cm2) and red (30 J/cm2) light in re-epithelialisation.