Caspase-11 and caspase-1 differentially modulate actin polymerization via RhoA and Slingshot proteins to promote bacterial clearance

Abstract

Inflammasomes are multiprotein complexes that include members of the NOD-like receptor family and caspase-1. Caspase-1 is required for the fusion of the Legionella vacuole with lysosomes. Caspase-11, independently of the inflammasome, also promotes phagolysosomal fusion. However, it is unclear how these proteases alter intracellular trafficking. Here, we show that caspase-11 and caspase-1 function in opposing manners to phosphorylate and dephosphorylate cofilin, respectively upon infection with Legionella. Caspase-11 targets cofilin via the RhoA GTPase, whereas caspase-1 engages the Slingshot phosphatase. The absence of either caspase-11 or caspase-1 maintains actin in the polymerized or depolymerized form, respectively and averts the fusion of pathogen-containing vacuoles with lysosomes. Therefore, caspase-11 and caspase-1 converge on the actin machinery with opposing effects to promote vesicular trafficking.

CASP4/caspase-11 promotes autophagosome formation in response to bacterial infection

“…CASP4 restricts the growth of L. pneumophila , an intracellular pathogen that resides in autophagosomes in WT macrophages [3,16,25]. Notably, B. cenocepacia is also cleared by autophagosomes in healthy macrophages [5], but the role of CASP4 in intracellular trafficking of this bacterium is not known.…”

“…Microtubules and actin filaments provide a scaffold to allow vesicular trafficking, and remodeling of the actin cytoskeleton involves CASP4 [16,25,36]. Depolymerization of actin filaments specifically inhibits starvation-induced autophagosome formation, indicating a critical role for actin during the early stages of autophagy [37].…”

“…However, this is the first report of CASP4 deficiency negatively affecting autophagy in macrophages, which may explain the ability of other autophagy-interacting bacteria to be cleared in the presence of CASP4 [2,8,16]. In contrast, clearance of nonpathogenic E. coli does not depend on CASP1 or CASP4 [16,25]. Notably, B. cenocepacia lacking a functional T6SS fails to induce autophagy in macrophages.…”

“…Likewise, we found reduced amounts of CFL1 adjacent to the B. cenocepacia -containing vacuole in CASP4-deficient cells. casp4 −/- macrophages also exhibit significantly lower amounts of active RHOA and inactive CFL1, even under basal conditions [25]. In addition, the B. cenocepacia T6SS effector TecA mediates the deamidation of RHOA in murine cells [32].…”

“…However, during L. pneumophila infection, inflammasome activation promotes autophagy [24]. Specifically, CASP4 activity modulates actin polymerization in L. pneumophila -infected macrophages, and hence facilitates the trafficking of phagosomes to lysosomal compartments for bacterial clearance [16,25]. Interestingly, transportation of phagosomes containing nonpathogenic Escherichia coli is not influenced by CASP4, suggesting differential regulation of endocytic pathways depending on the captured cargo [16].…”