Mitochondrial Targeting of the Enteropathogenic Escherichia coli Map Triggers Calcium Mobilization, ADAM10-MAP Kinase Signaling, and Host Cell Apoptosis

Abstract

The ability of diarrheagenic bacterial pathogens, such as enteropathogenic Escherichia coli (EPEC), to modulate the activity of mitogen-activated protein kinases (MAPKs) and cell survival has been suggested to benefit bacterial colonization and infection. However, our understanding of the mechanisms by which EPEC modulate these functions is incomplete. In this study, we show that the EPEC type III secreted effector Map stimulates the sheddase activity of the disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) and the ERK and p38 MAPK signaling cascades. Remarkably, all these activities were dependent upon the ability of Map to target host mitochondria, mainly via its mitochondrial toxicity region (MTR). Map targeting of mitochondria disrupted the mitochondrial membrane potential, causing extrusion of mitochondrial Ca2+ into the host cell cytoplasm. We also found that Map targeting of mitochondria is essential for triggering host cell apoptosis. Based on these findings, we propose a model whereby Map imported into mitochondria causes mitochondrial dysfunction and Ca2+ efflux into the host cytoplasm. Since Ca2+ has been reported to promote ADAM10 activation, the acute elevation of Ca2+ in the cytoplasm may stimulate the ADAM10 sheddase activity, resulting in the release of epidermal growth factors that stimulate the ERK signaling cascade. As p38 activity is also Ca2+ sensitive, elevation in cytoplasmic Ca2+ may independently also activate p38. We hypothesize that Map-dependent MAPK activation, combined with Map-mediated mitochondrial dysfunction, evokes mitochondrial host cell apoptosis, potentially contributing to EPEC colonization and infection of the gut.IMPORTANCE Enteropathogenic E. coli (EPEC) is an important human diarrhea-causing bacterium. The pathogenic effects of EPEC largely depend upon its ability to inject a series of proteins, termed effectors, into the host cells. One such effector is the mitochondrion-associated protein (Map). Map has been shown to induce actin-rich projections (i.e., filopodia) on the infected cell surface and activate a Rho GTPase enzyme termed Cdc42. Nonetheless, although most injected Map localizes to host mitochondria, its functions in the mitochondria remain unknown. Here, we show that Map targeting of mitochondria stimulates the disruption of mitochondrial membrane potential to induce Ca2+ efflux into the host cytoplasm. The efflux stimulates the activity of a protein termed ADAM10, which induces activation of a mitogen-activated protein kinase cascade leading to host cell apoptosis. As apoptosis plays a central role in host-pathogen interactions, our findings provide novel insights into the functions of mitochondrial Map in promoting the EPEC disease.