The Role of Caspase-7 during Infection with Listeria monocytogenes and during Intoxication with Listeriolysin O.

Abstract

The mammalian plasma membrane is a critical barrier that guards the integrity and function of the cell. Damage to this lipid bilayer commonly occurs during infection by microbial pathogens, many of which encode pore-forming toxins (PFT). Host cells are resilient to damage by physiological levels of PFT, but the molecular mechanisms that govern adaptation to membrane damage are poorly characterized. We found that a host cysteine protease, caspase-7, is activated during infection with the bacterial pathogen Listeria monocytogenes, and that activation was attributable to bacterial expression of the pore-forming toxin listeriolysin O (LLO). Exogenous treatment of cells with purified LLO induced membrane-damage-dependent transient activation of caspase-7. Transient activation correlated with rapid resolution of membrane damage and improved survival comparing wild type and caspase-7 deficient cells. Caspase-7 deficiency resulted in decreased membrane blebbing in response to toxin treatment, implicating blebbing as one possible mechanism by which host cells maintain membrane integrity and survival upon intoxication. Caspase-7 deficient cells also displayed less aggregation of Annexin A1, a calcium sensor that acts as a plug to sequester the cell cytosol from the membrane lesion, which may contribute to the inability of these cells to heal. The process of cellular blebbing is observed in both apoptotic and non-apoptotic contexts and relies on complex regulation of the actinomyosin network. We found that inhibition of Rho kinases or myosin II, key regulators of the cortical cytoskeleton, also inhibited blebbing and membrane repair in response to toxin treatment, largely mimicking the phenotype of caspase-7 deficient macrophages. Thus, we have uncovered a novel mechanism of membrane repair that relies on proteins originally implicated in the execution of programmed cell death, caspase-7, ROCK, and myosin II. We propose that the dualistic functions of these proteins are due in part to signal strength; low-level damage or activation promotes survival whereas high-level activation leads to apoptosis. In this way, contextual signals allow cells to repurpose existing machinery that would otherwise stimulate apoptosis, to promote blebbing and survival after damage by pore-forming toxins.