The mitochondrial intermembrane space protein Smac/DIABLO in signal transduction and immune response
Medical Microbiology and Mycology, Hygiene, Molecular Infection Biology
Final Report Abstract
Apoptotic cell death is a form of suicide where the cell kills itself for the benefit of the organism. A typical signalling pathway of apoptosis involves the mitochondria: the mitochondrial outer membrane is permeabilized (the process is termed MOMP), and mitochondrial proteins activate proteases in the cytosol that orchestrate the killing. An intriguing new concept has been introduced a number of years ago. Mitochondrial permeabilization can be partial, and all the events initiated remain at a ‘sub-lethal’ level. We had observed that sub-lethal signalling occurs during microbial infection, contributing to the inflammatory process. This project had the goals of advancing our understanding of the events of sub-lethal signalling at mitochondria, and to test some of the effects in tissue organoids and in mice. We identified new molecular steps at mitochondria. We found that sub-lethal signals activate the pore-forming protein BAX in a novel fashion. During apoptosis, BAX forms large ‘macropores’ that permit not only the rupture of the outer membrane but also herniation of the inner membrane. This is linked to opening and reorganisation of the mitochondrial cristae. Because most cytochrome c is stored in cristae, macropores permit the release of most cellular cytochrome c, and this is required for apoptosis. Sub-lethal signals, on the other hand, generate only small pores that very likely only release the intermembrane portion of cytochrome c, which is insufficient to kill the cell. Importantly, mitochondria seem to be able to repair the damage and to recover completely. The release of the intermembrane space protein SMAC during sub-lethal signalling was further shown to have a pro-inflammatory function during infection. We showed most of these effects with the gastric pathogen Helicobacter pylori (Hp) and confirmed the appearance of these signals in biopsies from Hp-positive patients. We had initially proposed to study the effects also in organoids from SMAC-deficient mice and to infect the mice with Helicobacter. We were unable to do the experiments due to animal breeding difficulties during the pandemic and are completing the infection experiments only now. We have however established the experimental system of gastric organoids from human induced pluripotent stem cells (iPSC). We have deleted SMAC in the iPSC and have established the experimental system of infecting the epithelial cells from these organoids to test for the inflammatory effects of SMAC. In summary, we believe that two findings from this project were particularly important. First, we identified sub-lethal signals during Hp-infection in patients; this is the first demonstration of this event in humans in vivo. Secondly, we identified a mechanism of non-lethal MOMP at the outer mitochondrial membrane: mitochondria can partially permeabilized but repair the damage, and the implications for signalling may be immense. The iPSC-model will be of substantial use for follow-up projects.
Publications
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Mitochondria supply sub-lethal signals for cytokine secretion and DNA-damage in H. pylori infection. Cell Death & Differentiation, 29(11), 2218-2232.
Dörflinger, Benedikt; Badr, Mohamed Tarek; Haimovici, Aladin; Fischer, Lena; Vier, Juliane; Metz, Arlena; Eisele, Bianca; Bronsert, Peter; Aumann, Konrad; Höppner, Jens; Waguia, Kontchou Collins; Parui, Ishita; Weber, Arnim; Kirschnek, Susanne & Häcker, Georg
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Sub-lethal signals in the mitochondrial apoptosis apparatus: pernicious by-product or physiological event?. Cell Death & Differentiation, 30(2), 250-257.
Häcker, Georg & Haimovici, Aladin
