Project 3

A novel role of beta actin in the organization of the mitochondrial genome

The mitochondrial genome is normally organized in the form of nucleoids, primordial nuclei that are segregated at the inner mitochondrial membrane. We recently discovered that mitochondria have a pool of β-actin which forms a “mitoskeleton” in the mitochondrial matrix. We showed that this actin-based mitoskeleton is required for segregation of the mitochondrial genome. In the absence of β-actin, mitochondrial DNA transcription levels drop as a consequence of mtDNA aggregation, leading to impaired mitochondrial membrane potential (MMP).

Figure 4. A mitochondrial pool of β-actin is required for proper mitochondrial DNA segregation. (A-B) Super-resolution microscopy shows that TFAM-containing nucleoids tend to form clustered aggregates in bulbous mitochondria [MitoTracker Deep Red (red) and anti-TFAM antibody (green) staining in WT and KO cells; scale bar, 5 μm]. Arrows point to examples of enlarged mitochondria. Insets show magnifications of wt and KO mitochondria [scale bar, 1 μm]. Arrowheads in indicate TFAM-based nucleoid aggregates (see Xie, Venit et al., 2018). (C) A model showing β-actin-dependent mitochondrial DNA segregation. In the β-actin KO cells this mechanism is impaired and it leads to a drop in mitochondrial DNA transcription, in membrane potential and ATP synthesis.

Based on these novel findings, we are now very interested to nail down how actin promotes mitochondrial genome segregation. To address this question, we are currently using a combination of advanced biochemistry and super-resolution microscopy to explore how actin is imported into mitochondria, how its polymerization state is regulated within mitochondria and the potential role of specific myosin isoforms in mitochondrial genome segregation.