Ca²⁺ waves in keratinocytes are transmitted to sensory neurons: The involvement of extracellular ATP and P2Y₂ receptor activation

ATP acts as an intercellular messenger in a variety of cells. In the present study, we have characterized the propagation of Ca²⁺ waves mediated by extracellular ATP in cultured NHEKs (normal human epidermal keratinocytes) that were co-cultured with mouse DRG (dorsal root ganglion) neurons. Pharmacological characterization showed that NHEKs express functional metabotropic P2Y₂ receptors. When a cell was gently stimulated with a glass pipette, an increase in [Ca²⁺]_i (intracellular Ca²⁺ concentration) was observed, followed by the induction of propagating Ca²⁺ waves in neighbouring cells in an extracellular ATP-dependent manner. Using an ATP-imaging technique, the release and diffusion of ATP in NHEKs were confirmed. DRG neurons are known to terminate in the basal layer of keratinocytes. In a coculture of NHEKs and DRG neurons, mechanical-stimulation-evoked Ca²⁺ waves in NHEKs caused an increase in [Ca²⁺]_i in the adjacent DRG neurons, which was also dependent on extracellular ATP and the activation of P2Y₂ receptors. Taken together, extracellular ATP is a dominant messenger that forms intercellular Ca²⁺ waves in NHEKs. In addition, Ca²⁺ waves in NHEKs could cause an increase in [Ca²⁺]_i in DRG neurons, suggesting a dynamic cross-talk between skin and sensory neurons mediated by extracellular ATP.