Olfactory receptors are expressed by different cell types throughout the body and regulate physiological cell functions beyond olfaction. In particular, the olfactory receptor OR2AT4 has been shown to stimulate keratinocyte proliferation in the skin. Here, we show that the epithelium of human hair follicles, particularly the outer root sheath, expresses OR2AT4, and that specific stimulation of OR2AT4 by a synthetic sandalwood odorant (Sandalore®) prolongs human hair growth ex vivo by decreasing apoptosis and increasing production of the anagen-prolonging growth factor IGF-1. In contrast, co-administration of the specific OR2AT4 antagonist Phenirat® and silencing of OR2AT4 inhibit hair growth. Together, our study identifies that human hair follicles can engage in olfactory receptor-dependent chemosensation and require OR2AT4-mediated signaling to sustain their growth, suggesting that olfactory receptors may serve as a target in hair loss therapy.
Olfactory receptors (ORs) are part of an evolutionarily ancient chemosensory signaling system that long predates the development of smell sensation (olfaction)
Interestingly, several ORs are also expressed in human epidermis19,20, including OR2AT4, whose selective activation by the synthetic sandalwood odorant (Sandalore®) promotes human epidermal keratinocyte migration and proliferation in vitro and wound re-epithelialization ex vivo20. Sandalore®-induced Ca2+ signaling could be blocked in OR2AT4-transfected Hana3A cells when this was co-applied at equimolar concentrations with the potent competitive OR2AT4 antagonist in presence of Sandalore®, Phenirat®20. Given the intimate connections between hair growth and wound healing21,22,23,24, we hypothesized that this OR might also impact on human hair growth. This hypothesis was investigated by immunohistology, qRT-PCR, western blot, microarray, phospho-kinase assay, and gene silencing in healthy, organ-cultured human scalp hair follicles (HFs)25.
The present study shows that human HFs express a specific OR, namely, OR2AT4. The activation of this OR by its specific agonist, Sandalore®, prolongs anagen maintenance ex vivo by decreasing hair matrix keratinocytes apoptosis and increasing the production of IGF-1 in the outer root sheath (ORS). The anagen-prolonging effect mediated by Sandalore® is OR2AT4 dependent, as confirmed by co-administration of Sandalore® with the OR2AT4 competitive antagonist, Phenirat®, as well as the specific knock-down of OR2AT4 in human HFs. Taken together, we show that human HFs can engage in chemosensation and that the specific activation of OR2AT4 is required to sustain HF growth.
Human HFs express OR2AT4
Immunofluorescence microscopy, qRT-PCR, and western blot analysis revealed that human scalp HFs in the anagen VI stage of the hair cycle26,27 express OR2AT4 at the transcript and protein level. Interestingly, OR2AT4 protein was predominantly expressed by suprabulbar keratinocytes of the proximal ORS , while hair matrix keratinocytes also expressed low-level OR2AT4 protein , both in healthy scalp skin in situ27 and in amputated microdissected anagen HFs . Of note, OR2AT4 expression was downregulated during spontaneous, apoptosis-driven HF regression . Thus, using the primary antibody employed here20, intrafollicular OR2AT4 expression is strikingly restricted to defined epithelial HF compartments and is hair cycle dependent.