Cardiolipin Provides a Platform for Caspase-1 Activation and NLRP3 Inflammasome Assembly

Rationale: The NLRP3 inflammasome promotes innate immunity through Caspase-1 activation and IL-1β and IL-18 maturation. Two-step NLRP3 activation requires an initial NFκB-activating priming stimulus and secondary agonist which induces cation flux and mitochondrial stress. However, the dynamics of inflammasome assembly and extent of mitochondrial disruption required for NLRP3 activation remain uncertain. NLRP3 interacts with mitochondrial cardiolipin, and we hypothesized cardiolipin could activate NLRP3 in vitro.

Methods: Utilizing a novel in vitro broken cell system we analyzed the interaction of cardiolipin with the components of the NLRP3 inflammasome

 

Results: Cardiolipin specifically induced caspase-1 autocatalysis, and NLRP3 and Caspase-1 formed high molecular weight complexes in the presence of cardiolipin. However, NLRP3 and ASC were dispensable for the molecular weight shift of Caspase-1. Finding that Caspase-1 directly binds cardiolipin, we examined whether Caspase-1 associates with mitochondria endogenously. NLRP3 and Caspase-1 localize to mitochondria upon priming, and mitochondrial association of Caspase-1 is NLRP3- and ASC- independent. In contrast, ASC associates upon secondary stimulus in a NLRP3-dependent manner. Furthermore, mitochondrial membrane remodeling mediated by fission-inducing DRP1 appears to positively regulate NLRP3 inflammasome activation.  

Conclusions: Our findings suggest that Caspase-1 binds cardiolipin on stressed mitochondria, support the hypothesized pattern-recognition activity of inflammatory caspases, and solidify mitochondria as critical regulators of immunity by serving as supramolecular organizing centers.