Differential Requirement of P2X7 Receptor and Intracellular K+ for Caspase-1 Activation Induced by Intracellular and Extracellular Bacteria

Interleukin-1β (IL-1β) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases. The maturation and secretion of IL-1β are mediated by caspase-1, a protease that processes pro-IL-1β into biologically active IL-1β. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration of K⁺ potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and intracellular K⁺ in IL-1β secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists such as lipopolysaccharide or infection with extracellular bacteria <i>Staphylococcus aureus</i> and <i>Escherichia coli</i>, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through depletion of intracellular K⁺ caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K⁺. Consistently, caspase-1 activation induced by intracellular <i>Salmonella</i> or <i>Listeria</i> was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally in the absence of P2X7 receptor-mediated cytoplasmic K⁺ perturbations.