Simultaneous Single-Cell Measurements Demonstrate a Positive Correlation between RNA Copy Number for Mitochondrial Division and Fusion Genes and Mitochondrial Fragmentation

Mitochondria are some of the most highly dynamic organelles in eukaryotic cells. The spatial distribution and morphology of mitochondria undergo constant changes via division and fusion to adapt to cellular conditions and to maintain various biological processes. However, how mitochondrial division and fusion are coordinated within a cell is currently unclear. Here, we performed microscopic observation using single molecule fluorescence in situ hybridization (smFISH) combined with immunostaining to simultaneously measure RNA molecules transcribed from mitochondrial division or fusion genes and the number of mitochondria in an individual cell. The RNA copy number in a single cell varied widely; with up to a 6-10 fold difference among cells. Notably, the expression levels of both types of genes seemed to be positively correlated with the number of fragmented mitochondria. Moreover, multiplexed smFISH imaging confirmed that the expression levels of eight species of mitochondrial division and fusion genes were significantly correlated in individual cells. These findings suggest that the increased expression of mitochondria! division/fusion genes is likely to trigger the homogenization of mitochondrial component molecules such as DNA, RNA, proteins, and lipids among mitochondria through structural remodeling via frequent division and fusion.