New publication: Mitochondrial protease PARL mediates the processing of the cell death regulator Smac
Mitochondria are central metabolic organelles that regulate cell death pathways through releasing death-promoting proteins such as cytochrome c and Smac. The group of Thomas Langer has now found that the mitochondrial protease PARL cleaves Smac, which is a prerequisite for its release and the initiation of cell death.
Mitochondria are commonly known as the major site of cellular energy conversion and often designated as the ´powerhouses` of the living cell. Besides their metabolic functions that are essential for cell survival, however, mitochondria can also initiate a detrimental cascade that culminates in the death of a cell. This pathway is initiated by the release of death-promoting proteins from mitochondria, such as cytochrome c and Smac. Structural changes of mitochondria facilitate the release of cytochrome c from mitochondria in dying cells: normally present in internal membrane compartments, termed cristae, rearrangement of these compartments upon initiation of apoptotic cell death allows the release of cytochrome c from mitochondria to the cytosol. These membrane rearrangements are regulated by the dynamin-like GTPase OPA1, whose function is controlled by proteases. The rhomboid protease PARL has been linked to apoptosis and, consistently, the loss of PARL leads to multisystemic atrophy and cachectic death in mice. It was initially proposed that PARL regulates the remodeling of cristae during apoptosis, but other proteases were later demonstrated to be responsible for OPA1 cleavage in dying cells. Therefore, the role of PARL in apoptosis remained a mystery.
In order to understand how PARL affects cell survival and cell death, the group of Thomas Langer initially asked which mitochondrial proteins are cleaved by the protease PARL by comparing normal cells to cells lacking functional PARL. Using various proteomic approaches and quantitative mass spectroscopy, they found that PARL mediates proteolytic processing of the mitochondrial protein Smac. Smac is released from mitochondria and initiates a cell death cascade by binding to and inhibiting XIAP, which inhibits apoptosis of healthy cells. “We examined cells lacking PARL and found that they were much less sensitive to cell death than normal cells. When we introduced mature Smac into the cells or used synthetic molecules mimicking Smac function, cells again behaved normally”, explains Shotaro Saita, the first author of the paper. Smac cleavage by PARL was found to be required for the release of Smac from mitochondria. It came as a surprise that, while loss of PARL led to impaired maturation of Smac and prevented cell death, cytochrome c could still be released from mitochondria by cell death stimuli. “We could show that Smac cleavage and release from mitochondria constitute a second, critical process in the mitochondrial cell death pathway, which is largely independent of the release of cytochrome c” comments Thomas Langer. “We made significant progress, but many questions remain. It will be in particular important to understand the role of Smac cleavage by PARL in different cells and tissues, as mice lacking Smac do not show significant abnormalities. This might be due to differences in the level of XIAP, the protein targeted by Smac”. The researchers are now working to further define the role of PARL in cell death pathways and how it regulates Smac function. These studies will likely be of direct medical relevance, as small molecules mimicking the inhibitory effect of Smac are in clinical trials for new cancer therapies.
Shotaro Saita, Hendrik Nolte, Kai Uwe Fiedler, Hamid Kashkar, A. Saskia Venne, René P. Zahedi, Marcus Krüger and Thomas Langer. PARL mediates Smac proteolytic maturation in mitochondria to promote apoptosis. Nature Cell Biology, doi:10.1038/ncb3488
Prof. Dr. Thomas Langer
Institute for Genetics and Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD)
Tel. +49 221 478 84263