Return of the first inflammasome

Abstract

The assembly of inflammasomes is linked to the detection of pathogens and other danger signals by intracellular pattern-recognition receptors of the mammalian innate immune system. The human inflammasome sensor NLRP1 is activated by N-terminal proteolytic cleavage and subsequent degradation, causing the release of the C-terminal NLRP1^UPA-CARD fragment and the recruitment of the adaptor protein ASC and caspase-1, resulting in the processing of IL-1&#946;/IL-18 and pyroptotic cell death.<br />To study NLRP1 inflammasomes, I characterized HEK 293T and N/TERT-1 keratinocyte inflammasome reporter cell lines, and I identified two NLRP1^PYD-specific nanobodies which, combined with the E3 ligase receptor VHL, allowed the precise stimulation of endogenous NLRP1 by targeted NLRP1^PYD ubiquitination and subsequent N-terminal degradation.<br />Using the reporter cell lines, I found that various stimuli of the ribotoxic stress response activate human NLRP1 in a p38-dependent manner. In addition, infection with alphaviruses, including Semliki Forrest virus and Chikungunya virus, caused p38-dependent NLRP1 activation. p38 kinases directly phosphorylate the N-terminal linker region of the inflammasome sensor, in which serine 107 represents a critical phosphorylation site. I propose that phosphorylation of the N-terminal linker generates a phospho-degron which is recognized by cullin RING E3 ligases, causing the ubiquitination of NLRP1^PYD, N-terminal degradation, and inflammasome assembly.<br />In addition to delineating p38-mediated NLRP1 activation, I identified novel viral NLRP1 stimuli and established lymphocytes as NLRP1-competent cell types.