Research Group Leader - Institute for Genetics, Faculty of Math. Nat. Sciences
Mitochondria are mobile organelles that constantly fuse and divide. This plasticity is crucial for neuronal survival and is associated with age-related diseases. Mitochondrial fusion depends on the mitofusin proteins: MFN1 and MFN2 in humans and Fzo1 in yeast. Dysfunction of MFN2 causes the neuropathy Charcot-Marie-Tooth type 2A (CMT2A). In addition, it is associated with Parkinson’s disease, with cardiovascular pathologies and with type 2 diabetes and obesity.
We aim at understanding the molecular mechanism on how mitofusins drive mitochondrial membrane merging and also unravel disease-underlying functions of MFN2.
Our research: We are interested in several aspects of mitochondrial dynamics and quality-control processes, which depend on mitofusins and on ubiquitin, a conserved and essential post-translational modifier. First, we study at a molecular level the mechanisms allowing mitochondria to fuse or divide, via both positive and negative regulation of mitofusins by ubiquitin, using yeast and cell culture models. Under healthy conditions ubiquitin protects mitofusin from degradation and induces mitochondrial fusion, thus maintaining mitochondrial interconnectivity. Instead, upon stress, additional ubiquitylation of mitofusins triggers their degradation by the proteasome, thus repressing mitochondrial fusion and leading to mitochondrial fragmentation by ongoing fission events. Second, we are interested in understanding how mitochondria, E3 ligases and deubiquitylases coordinately manage ubiquitin pools.
Our successes: We contributed to the field of mitochondrial dynamics by deciphering a multistep process requiring an intimate coupling of oligomerisation, GTPase activity and post-translational modifications of Fzo1. Moreover, we uncovered two different pathways of ubiquitylation/deubiquitylation of mitofusins. We found that these two pathways integrate a deubiquitylation cascade governed by the ubiquitin-dependent chaperone Cdc48. In addition, our results pointed to opposing functions of deubiquitylases also for the general regulation of ubiquitin homeostasis. This demonstrated a fine tuned cascade regulating ubiquitin pools and also the ubiquitylation status of mitofusins, which is a prerequisite for the modulation of mitochondrial fusion. These results provide novel cues for the understanding of diseases caused by defective mitofusins. Finally, our lab provided novel insights in the field of mitochondrial quality control, focusing on the clearance of defective mitochondria, a process termed as mitophagy. In this regard, our group focuses on the modulation of mitofusins by ubiquitin during mitophagy, characterized in yeast and in cultured cells.
Our goals: We aim at deciphering the molecular mechanisms regulating the multi-step process of mitofusin-mediated mitochondrial fusion. Our long-term purpose is transferring the basic molecular mechanisms of mitochondrial fusion into therapeutic targets of diseases associated with mitofusins.
Our methods/techniques: We combine genetic, biochemical and cell biological methods, using the single cellular model organism S. cerevisiae and also cell culture models, in order to understand molecular mechanisms regulating mitochondrial dynamics. Mostly, we focus on the characterization of different post-translational modifications, targeting mitofusins and impacting on mitochondrial fate. Mechanistic characterization of the mitochondrial dynamics process in yeast are further transferred to cell culture, allowing novel insight to the regulation of human mitofusins and the identification of defects causing CMT2A.
Figure 1: Mitochondrial dynamics. Mitochondria are very dynamic organelles that constantly fuse and divide. This plasticity is crucial for embryonic development, for neuronal survival, and is associated with mitophagy and apoptosis (EMBO Rep 2014 15(3) 231-243).
Figure 2:Mitofusins control mitochondrial morphology, from yeast to mammals. (A and B) Immunofluorescence and (B) transmission electron microscopy (TEM) of cells deficient for mitofusins, in yeast (A) and in cell culture models (B).