Caspase-1 autoproteolysis is differentially required for NLRP1b and NLRP3 inflammasome function

Abstract

Inflammasomes are caspase-1-activating multiprotein complexes. The mouse nucleotide-binding domain and leucine rich repeat pyrin containing 1b (NLRP1b) inflammasome was identified as the sensor of Bacillus anthracis lethal toxin (LT) in mouse macrophages from sensitive strains such as BALB/c. Upon exposure to LT, the NLRP1b inflammasome activates caspase-1 to produce mature IL-1β and induce pyroptosis. Both processes are believed to depend on autoproteolysed caspase-1. In contrast to human NLRP1, mouse NLRP1b lacks an N-terminal pyrin domain (PYD), indicating that the assembly of the NLRP1b inflammasome does not require the adaptor apoptosis-associated speck-like protein containing a CARD (ASC). LT-induced NLRP1b inflammasome activation was shown to be impaired upon inhibition of potassium efflux, which is known to play a major role in NLRP3 inflammasome formation and ASC dimerization. We investigated whether NLRP3 and/or ASC were required for caspase-1 activation upon LT stimulation in the BALB/c background. The NLRP1b inflammasome activation was assessed in both macrophages and dendritic cells lacking either ASC or NLRP3. Upon LT treatment, the absence of NLRP3 did not alter the NLRP1b inflammasome activity. Surprisingly, the absence of ASC resulted in IL-1β cleavage and pyroptosis, despite the absence of caspase-1 autoprocessing activity. By reconstituting caspase-1/caspase-11 −/− cells with a noncleavable or catalytically inactive mutant version of caspase-1, we directly demonstrated that noncleavable caspase-1 is fully active in response to the NLRP1b activator LT, whereas it is nonfunctional in response to the NLRP3 activator nigericin. Taken together, these results establish variable requirements for caspase-1 cleavage depending on the pathogen and the responding NLR.interleukin-1beta | pyroptosis | lethal toxin | macrophage | dendritic cell

Caspase-1: an integral regulator of innate immunity

“…Moreover, autoproteolysis of procaspase-1 was dependent on ASC-mediated inflammasome assembly in response to stimulation of NLRP1b, NLRC3, NLRC4, and AIM2. ASC-and autoproteolysis-independent activation of caspase-1 was sufficient for the induction of pyroptosis following NLRP1b or NLRC4 stimulation and IL-1β secretion upon NLRP1b stimulation [76,[78][79][80]. Together, these data document differential requirements for procaspase-1 expression, its enzymatic activity and autoproteolysis for inflammasome functions, including the autoprocessing-independent enzymatic activity of procaspase-1 and scaffolding functions of procaspase-1, independent of its enzymatic activity.…”

“…As revealed by uncleavable caspase-1 variants, the procaspase-1 zymogen induced pyroptosis upon NLRP1b, NLRC4, or AIM2 stimulation [78,79]. Secretion of IL-1β depended on procaspase-1 autoproteolysis after NLRC4, NLRP3, or AIM2 activation, but was induced by uncleavable procaspase-1 upon NLRP1b stimulation using Bacillus anthracis lethal toxin [78,79]. Therefore, procaspase-1 can be activated independent of its autoproteolysis.…”

“…The model of inflammasome-mediated activation of caspase-1 involves the binding of procaspase-1 to the adaptor protein ASC, connecting procaspase-1 to an intracellular sensor molecule, belonging to the NLR or PYHIN family [2]. Recently, several studies elucidated differential requirements for the expression, autoprocessing and enzymatic activity of procaspase-1 contributing to cytokine secretion and pyroptosis (Table 1) [33,[76][77][78][79][80]. Expression of caspase-1 was shown to be essential for pyroptosis and the processing and secretion of IL-1β.…”

“…Interestingly, the enzymatic activity of caspase-1 was dispensable for the secretion of proIL-1β following activation of NLRP3 with MSU, Alum, ATP, or Nigericin and the secretion of IL-1α in response to NLRP3 stimulation with ATP, Nigericin, or Candida, suggesting a protease-independent function of caspase-1 [77]. As revealed by uncleavable caspase-1 variants, the procaspase-1 zymogen induced pyroptosis upon NLRP1b, NLRC4, or AIM2 stimulation [78,79]. Secretion of IL-1β depended on procaspase-1 autoproteolysis after NLRC4, NLRP3, or AIM2 activation, but was induced by uncleavable procaspase-1 upon NLRP1b stimulation using Bacillus anthracis lethal toxin [78,79].…”

Current Knowledge on Procaspase-1 Variants with Reduced or Abrogated Enzymatic Activity in Autoinflammatory Disease

“…However, NLRP1b inflammasome formation in response to Bacillus anthracis lethal toxin does not result in autoprocessing of procaspase-1, nevertheless leading to IL-1β processing and release and pyroptosis [46,47]. Hence, procaspase-1 autoprocessing is a variable requirement for enzymatic activity of caspase-1 and depends on the respective PAMPs and the corresponding nodlike receptor (NLR).…”