Two olfactory receptors-OR2A4/7 and OR51B5-differentially affect epidermal proliferation and differentiation.

Olfactory receptors (ORs), which belong to the G-protein coupled receptor family, are expressed in various human tissues, including skin. Cells in non-olfactory tissues tend to express more than one individual OR gene, but function and interaction of two or more ORs in the same cell type has only been marginally analysed. Here, we revealed OR2A4/7 and OR51B5 as two new ORs in human skin cells and identified cyclohexyl salicylate and isononyl alcohol as agonists of these receptors. In cultured human keratinocytes, both odorants induce strong Ca2+ signals that are mediated by OR2A4/7 and OR51B5, as demonstrated by the receptor knockdown experiments. Activation of corresponding receptors induces a cAMP-dependent pathway. Localization studies and functional characterization of both receptors revealed several differences. OR2A4/7 is expressed in suprabasal keratinocytes and basal melanocytes of the epidermis and influences cytokinesis, cell proliferation, phosphorylation of AKT and Chk-2 and secretion of IL-1. In contrast, OR51B5 is exclusively expressed in suprabasal keratinocytes, supports cell migration and regeneration of keratinocyte monolayers, influences Hsp27, AMPK1 and p38MAPK phosphorylation and interestingly, IL-6 secretion. These findings underline that different ORs perform diverse functions in cutaneous cells, and thus offering an approach for the modulated treatment of skin diseases and wound repair.

Chemosensory information processing between keratinocytes and trigeminal neurons.

Trigeminal fibers terminate within the facial mucosa and skin and transmit tactile, proprioceptive, chemical, and nociceptive sensations. Trigeminal sensations can arise from the direct stimulation of intraepithelial free nerve endings or indirectly through information transmission from adjacent cells at the peripheral innervation area. For mechanical and thermal cues, communication processes between skin cells and somatosensory neurons have already been suggested. High concentrations of most odors typically provoke trigeminal sensations in vivo but surprisingly fail to activate trigeminal neuron monocultures. This fact favors the hypothesis that epithelial cells may participate in chemodetection and subsequently transmit signals to neighboring trigeminal fibers. Keratinocytes, the major cell type of the epidermis, express various receptors that enable reactions to multiple environmental stimuli. Here, using a co-culture approach, we show for the first time that exposure to the odorant chemicals induces a chemical communication between human HaCaT keratinocytes and mouse trigeminal neurons. Moreover, a supernatant analysis of stimulated keratinocytes and subsequent blocking experiments with pyrodoxalphosphate-6-azophenyl-2',4'-disulfonate revealed that ATP serves as the mediating transmitter molecule released from skin cells after odor stimulation. We show that the ATP release resulting from Javanol® stimulation of keratinocytes was mediated by pannexins. Consequently, keratinocytes act as chemosensors linking the environment and the trigeminal system via ATP signaling.

A synthetic sandalwood odorant induces wound-healing processes in human keratinocytes via the olfactory receptor OR2AT4.

As the outermost barrier of the body, the skin is exposed to multiple environmental factors, including temperature, humidity, mechanical stress, and chemical stimuli such as odorants that are often used in cosmetic articles. Keratinocytes, the major cell type of the epidermal layer, express a variety of different sensory receptors that enable them to react to various environmental stimuli and process information in the skin. Here we report the identification of a novel type of chemoreceptors in human keratinocytes, the olfactory receptors (ORs). We cloned and functionally expressed the cutaneous OR, OR2AT4, and identified Sandalore, a synthetic sandalwood odorant, as an agonist of this receptor. Sandalore induces strong Ca(2+) signals in cultured human keratinocytes, which are mediated by OR2AT4, as demonstrated by receptor knockdown experiments using RNA interference. The activation of OR2AT4 induces a cAMP-dependent pathway and phosphorylation of extracellular signal-regulated kinases (Erk1/2) and p38 mitogen-activated protein kinases (p38 MAPK). Moreover, the long-term stimulation of keratinocytes with Sandalore positively affected cell proliferation and migration, and regeneration of keratinocyte monolayers in an in vitro wound scratch assay. These findings combined with our studies on human skin organ cultures strongly indicate that the OR 2AT4 is involved in human keratinocyte re-epithelialization during wound-healing processes.