Inflammasomes integrate cytosolic evidence of infection or damage to mount inflammatory responses. The inflammasome sensor NLRP1 is expressed in human keratinocytes and coordinates inflammation in the skin. We found that diverse stress signals induce human NLRP1 inflammasome assembly by activating MAP kinase p38: While the ribotoxic stress response to UV and microbial molecules exclusively activates p38 through MAP3K ZAKα, infection with arthropod-borne alphaviruses, including Semliki Forest and Chikungunya virus, activates p38 through ZAKα and potentially other MAP3K. We demonstrate that p38 directly phosphorylates NLRP1 and that serine 107 in the linker region is critical for activation. NLRP1 phosphorylation is followed by ubiquitination of NLRP1PYD, N-terminal degradation of NLRP1, and nucleation of inflammasomes by NLRP1UPA-CARD. In contrast, activation of NLRP1 by nanobody-mediated ubiquitination, viral proteases, or inhibition of DPP9 was independent of p38 activity. Taken together, we define p38 activation as a unifying signaling hub that controls NLRP1 inflammasome activation by integrating a variety of cellular stress signals relevant to the skin.

Highlights d Mass spectrometry resource for inflammasome-activated protein release in macrophages d Inhibition of inflammasome assembly and protein release by ER-Golgi disruption d Dissection of protein release by gasdermin, organelle damage, and extracellular vesicles d Identification of released constitutive and induced alarmins

Inflammation and infection can trigger local tissue Na + accumulation. This Na +-rich environment boosts proinflammatory activation of monocyte/macrophage-like cells (MΦs) and their antimicrobial activity. Enhanced Na +-driven MΦ function requires the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5), which augments nitric oxide (NO) production and contributes to increased autophagy. However, the mechanism of Na + sensing in MΦs remained unclear. High extracellular Na + levels (high salt [HS]) trigger a substantial Na + influx and Ca 2+ loss. Here, we show that the Na + /Ca 2+ exchanger 1 (NCX1, also known as solute carrier family 8 member A1 [SLC8A1]) plays a critical role in HS-triggered Na + influx, concomitant Ca 2+ efflux, and subsequent augmented NFAT5 accumulation. Moreover, interfering with NCX1 activity impairs HS-boosted inflammatory signaling, infection-triggered autolysosome formation, and subsequent antibacterial activity. Taken together, this demonstrates that NCX1 is able to sense Na + and is required for amplifying inflammatory and antimicrobial MΦ responses upon HS exposure. Manipulating NCX1 offers a new strategy to regulate MΦ function.