Epidermal TRPV4 ion channels regulate UVB induced sunburn by triggering inflammmasome activation and ERK signaling

Abstract

Skin inflammation is an evolutionary-honed protective mechanism that serves to clear noxious cues and irritants and initiate regeneration. Calcium-permeable transient-receptor-potential (TRP) ion channels have critical functions in sensory transduction which is sensitized in skin inflammation. Skin sensory transduction relies on skin-innervating sensory neurons in the dorsal root ganglion (DRG), but also on innervated keratinocytes (KC). The multimodally-activated TRPV4 is robustly expressed in KC, where it can readily be activated by Ultraviolet-B (UVB). Our goal was to deconstruct keratinocyte TRPV4-mediated signaling, specifically how TRPV4 can facilitate inflammatory injury, thus lowering pain thresholds and rendering KC into pain-generator cells. We wanted to uncover the effect of TRPV4-mediated signaling on UVB-induced inflammasome activation in KC given the powerful impact of the activated inflammasome on pro-inflammatory/pro-algesic secretory signaling from KC to innervating DRG neurons, using mouse models and cultured human KC. In mice, our evidence suggests that TRPV4 functions as calcium-permeable channel upstream of the KC inflammasome. Furthermore, we found that UVB induced activation of TRPV4 caused rapid - within minutes - ERK phosphorylation, caspase-1 activation and IL1ß secretion. In human primary KC we demonstrated that UVB induced secretion of IL1ß was dependent on the NLRP1 inflammasome. Direct chemical TRPV4 activation could also activate NLRP1 and to lesser extent NLPR3. Building on our previous work, we now define at increased resolution TPRV4-dependent forefront signaling mechanisms in KC in response to UVB, showing TRPV4 upstream of the NLRP1 inflammasome in KC, subsequent rapid MAPK ERK activation and pro-inflammatory/pro-algesic secretory function.