Salmonella Promotes ASC Oligomerization-dependent Caspase-1 Activation

Innate immune cells sense and respond to the cytoplasmic infection of bacterial pathogens through NLRP3, NLRC4 or AIM2 inflammasome depending on the unique molecular pattern of invading pathogens. The infection of flagellin- or type III secretion system (T3SS)-containing Gram-negative bacteria such as Salmonella enterica serovar Typhimurium (S. typhimurium) or Pseudomonas aeruginosa (P. aeruginosa) triggers NLRC4-dependent caspase-1 activation leading to the secretion of proinflammatory cytokines such as interleukin-1-beta (IL-1beta) and IL-18. Previous studies have shown that apoptosis-associated speck-like protein containing a CARD (ASC) is also required for Salmonella-induced caspase-1 activation, but it is still unclear how ASC contributes to the activation of NLRC4 inflammasome in response to S. typhimurium infection. In this study, we demonstrate that S. typhimurium triggers the formation of ASC oligomer in a potassium depletion-independent manner as determined by in vitro crosslinking and in situ fluorescence imaging. Remarkably, inhibition of potassium efflux failed to block Salmonella-promoted caspase-1 activation and macrophage cell death. These results collectively suggest that ASC is substantially oligomerized to facilitate the activation of caspase-1 in response to S. typhimurium infection. Contrary to NLRP3 inflammasome, intracellular potassium depletion is not critical for NLRC4 inflammasome signaling by S. typhimurium.

Pyrin Domain (PYD)-containing Inflammasome in Innate Immunity

Inflammasome is a cytosolic multiprotein complex to activate caspase-1 leading to the subsequent processing of inactive pro-interleukin-1-beta (Pro-IL-1beta) into its active interleukin-1 beta (IL-1beta) in response to pathogen- or danger-associated molecular pattern. In recent years, a huge progress has been made to identify inflammasome component as a molecular platform to recruit and activate caspase-1. Nucleotide-binding oligomerization domain-like receptor (NLR) family proteins such as NLRP1, NLRP3 or interleukin-1beta-converting enzyme (ICE)-protease activating factor (IPAF) have been first characterized to form inflammasome complex to induce caspase-1 activation. More recently, non-NLR type, pyrin-domain (PYD)-containing proteins such as pyrin or absent in melanoma2 (AIM2) were also proposed to form caspase-1-activating inflammasome machinery with apoptosis-associated speck-like protein containing a CARD (ASC), an essential adaptor molecule. Inflammasome pathways were shown to be crucial for protecting host organisms against diverse pathogen infections, but accumulating evidences also suggest that excessive activation of inflammasome/caspase-1 might be related to the pathogenesis of inflammation-related diseases. Indeed, mutations in NLRP3 or pyrin are closely associated with autoinflammatory diseases such as familial Mediterranean fever (FMF) syndrome or Muckle-Wells syndrome (MWS), indicating that the regulation of caspase-1 activity by inflammasome is a central process in these hereditary inflammatory disorders. Here, recent advances on the molecular mechanism of caspase-1 activation by PYD-containing inflammasomes are summarized and discussed.