Sub-lethal mitochondrial outer membrane permeabilization: mechanisms and regulation in bacterial infection

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Mitochondria regulate energy levels and metabolism and can, through the release of the intermembrane space proteins cytochrome c and Smac, trigger caspase-activation and apoptosis. An intriguing, currently evolving field is the generation and role of ‘sub-lethal’ signals in the mitochondrial apoptosis apparatus. In addition to the established complete and quick release of cytochrome c/Smac during apoptosis, mitochondria can also release only small amounts of these proteins, activating small amounts of apoptotic caspases. Current evidence suggests that these sub-lethal signals are pro-inflammatory and may be part of an innate immune response; sub-lethal signals have been observed in all cellular infections tested to date. Sub-lethal signals use the main apoptosis machinery, including Bcl-2-family proteins, and they appear to be linked to the structure of the mitochondrial network. The goals of this project are to understand key events and players at mitochondria during sub-lethal signaling, and to map these events during bacterial infection of non-professional immune cells. During apoptosis, active BH3-only proteins, such as Bim, Noxa and Bad, activate the effectors of intermembrane space protein release, Bax and Bak. During sub-lethal signaling, as far as is known, the players appear to be the same but the actual events are unclear. We will assess the events at mitochondria upon experimental induction of Bim, focusing on the activation of Bax, Bak, the question of localized vs. widespread activation of the mitochondrial network and the reciprocal interaction of sub-lethal signals and fission-fusion activity of the mitochondrial network. We will test the hypothesis that the BH3-only proteins Noxa and Bad, known to be far less active in the induction of apoptosis than Bim, may in fact be more relevant to the induction of sub-lethal signals. These experiments will identify some of the cell biological events of sub-lethal signaling at mitochondria. In the second part of the project, we will use these results and test for mitochondrial events during infection of human epithelial cells with the two pathogens, Helicobacter pylori (Hp) and Chlamydia trachomatis (Ctr). Both pathogens generate sub-lethal signals and inflammatory activity through Bax/Bak; while Ctr has been described to inhibit mitochondrial fission, Hp can increase fission. Through studying the infection with these two pathogens, we expect to be able to understand some of the sub-lethal events at mitochondria during infection, and better to appreciate the link between apoptosis-system and fission-fusion events at the mitochondrial network. In our view, sub-lethal signals at mitochondria are important events during infection, and they may determine the initiation of inflammation and an immune response during infection. The experiments in this project will help clarify this role.