Transient receptor potential V3 (TRPV3) and TRPV4 are heat-activated cation channels expressed in keratinocytes. It has been proposed that heat-activation of TRPV3 and/or TRPV4 in the skin may release diffusible molecules which would then activate termini of neighboring dorsal root ganglion (DRG) neurons. Here we show that adenosine triphosphate (ATP) is such a candidate molecule released from keratinocytes upon heating in the co-culture systems. Using TRPV1-deficient DRG neurons, we found that increase in cytosolic Ca 2+ -concentration in DRG neurons upon heating was observed only when neurons were co-cultured with keratinocytes, and this increase was blocked by P2 purinoreceptor antagonists, PPADS and suramin. In a co-culture of keratinocytes with HEK293 cells (transfected with P2X 2 cDNA to serve as a biosensor), we observed that heat-activated keratinocytes secretes ATP, and that ATP release is compromised in keratinocytes from TRPV3-deficient mice. This study provides evidence that ATP is a messenger molecule for mainly TRPV3-mediated thermotransduction in skin.

Transient receptor potential vanilloid 4 (TRPV4) channel is a physiological sensor for hypo-osmolarity, mechanical deformation, and warm temperature. The channel activation leads to various cellular effects involving Ca 2؉ dynamics. We found that TRPV4 interacts with ␤-catenin, a crucial component linking adherens junctions and the actin cytoskeleton, thereby enhancing cell-cell junction development and formation of the tight barrier between skin keratinocytes. TRPV4-deficient mice displayed impairment of the intercellular junction-dependent barrier function in the skin. In TRPV4-deficient keratinocytes, extracellular Ca 2؉ -induced actin rearrangement and stratification were delayed following significant reduction in cytosolic Ca 2؉ increase and small GTPase Rho activation. TRPV4 protein located where the cell-cell junctions are formed, and the channel deficiency caused abnormal cell-cell junction structures, resulting in higher intercellular permeability in vitro. Our results suggest a novel role for TRPV4 in the development and maturation of cell-cell junctions in epithelia of the skin. Transient receptor potential vanilloid 4 (TRPV4),3 a member of the TRP superfamily of cation channels, is a Ca 2ϩ -permeable channel expressed in both neuronal and non-neuronal cells. Channel activation allows cation influx into cells, leading to various Ca 2ϩ -dependent processes (1, 2). TRPV4 can be activated by a variety of chemical and physical stimuli such as synthetic phorbol ester 4␣-phorbol 12,13-didecanoate (4␣-PDD) (3), a botanical agent (bisandrographolide A) (4), anandamide metabolites including arachidonic acid and epoxyeicosatrienoic acids (5), hypo-osmotic stimulation (6, 7), shear stress (8), mechanical stretch (9), and moderate warmth (27-35°C) (7, 10). Therefore, the TRPV4 channel functions as a multimodal transducer in various tissues and cells.Keratinocytes in the skin epidermis express TRPV4 (10, 11), and it has been proposed that the channel is involved in the detection of warm temperature (12). Skin keratinocytes express another warm temperature-sensitive TRP channel, TRPV3 (activated by temperature above 32°C), which is also implicated in temperature sensation in mice (13). Because both TRPV3 and TRPV4 are expressed in keratinocytes and are activated by a similar range of temperatures, these channels likely have distinct functions in the skin. Consistent with this idea, a recent report provided evidence that TRPV3, rather than TRPV4, mainly participates in transmission of warm temperature information from keratinocytes to adjacent nerve endings through ATP release (14). It has also been reported that mutation of TRPV3 is linked to defective hair growth and dermatitis in rodents (15, 16), although the involvement of TRPV4 has not been confirmed.The skin constitutes an interface between the external environment and the body, serving as a hydrophobic barrier essential for protection against infection from the outside and dehydration from the inside. The skin barrier function is achieved by keratinocytes in...