Mitochondrial Translation Occurs Preferentially in the Peri-Nuclear Mitochondrial Network of Cultured Human Cells

SIMPLE SUMMARY: All the cells in our bodies need energy to survive. This chemical energy is generated by a subcompartment of the cell called the mitochondrial network. The complexes that generate this cellular fuel, require 13 protein components to be synthesised inside this mitochondrial network. Our project looked to see whether the manufacture of these protein components was evenly distributed within the network. We identified that there is a gradient of activity that is highest near the nucleus and diminishes towards the edges of the cell and that this did not seem to be caused by a lack of the machinery responsible for the protein synthesis. This work is important because it now allows us to use this newly refined technology to investigate whether this pattern changes in patients with mitochondrial disorders. These are a group of diseases that cause malfunctions in muscle, brain, nerve and other tissues that get progressively worse with time and which are currently untreatable. A deeper understanding of what has gone wrong may help us understand the disease process better, with a longer-term view of treating the disorders. ABSTRACT: Human mitochondria are highly dynamic organelles, fusing and budding to maintain reticular networks throughout many cell types. Although extending to the extremities of the cell, the majority of the network is concentrated around the nucleus in most of the commonly cultured cell lines. This organelle harbours its own genome, mtDNA, with a different gene content to the nucleus, but the expression of which is critical for maintaining oxidative phosphorylation. Recent advances in click chemistry have allowed us to visualise sites of mitochondrial protein synthesis in intact cultured cells. We show that the majority of translation occurs in the peri-nuclear region of the network. Further analysis reveals that whilst there is a slight peri-nuclear enrichment in the levels of mitoribosomal protein and mitochondrial rRNA, it is not sufficient to explain this substantial heterogeneity in the distribution of translation. Finally, we also show that in contrast, a mitochondrial mRNA does not show such a distinct gradient in distribution. These data suggest that the relative lack of translation in the peripheral mitochondrial network is not due to an absence of mitoribosomes or an insufficient supply of the mt-mRNA transcripts.