Defects of mitochondrial RNA turnover lead to the accumulation of double-stranded RNA in vivo

The RNA helicase SUV3 and the polynucleotide phosphorylase PNPase are involved in the degradation of mitochondrial mRNAs but their roles in vivo are not fully understood. Additionally, upstream processes, such as transcript maturation, have been linked to some of these factors, suggesting either dual roles or tightly interconnected mechanisms of mitochondrial RNA metabolism. To get a better understanding of the turn-over of mitochondrial RNAs in vivo, we manipulated the mitochondrial mRNA degrading complex in Drosophila melanogaster models and studied the molecular consequences. Additionally, we investigated if and how these factors interact with the mitochondrial poly(A) polymerase, MTPAP, as well as with the mitochondrial mRNA stabilising factor, LRPPRC. Our results demonstrate a tight interdependency of mitochondrial mRNA stability, polyadenylation and the removal of antisense RNA. Furthermore, disruption of degradation, as well as polyadenylation, leads to the accumulation of double-stranded RNAs, and their escape out into the cytoplasm is associated with an altered immune-response in flies. Together our results suggest a highly organised and inter-dependable regulation of mitochondrial RNA metabolism with far reaching consequences on cellular physiology.

Although a number of factors have been implemented in the turnover of mitochondrial (mt) DNA-derived transcripts, their exact functions and interplay with one another is not entirely clear. Several of these factors have been proposed to co-ordinately regulate both transcript maturation, as well as degradation, but the order of events during mitochondrial RNA turnover is less well understood. Using a range of different genetically modified Drosophila melanogaster models, we studied the involvement of the RNA helicase SUV3, the polynucleotide phosphorylase PNPase, the leucine-rich pentatricopeptide repeat motif-containing protein LRPPRC, and the mitochondrial RNA poly(A) polymerase MTPAP, in stabilisation, polyadenylation, and degradation of mitochondrial transcripts. Our results show a tight collaborative activity of these factors in vivo and reveal a clear hierarchical order of events leading to mitochondrial mRNA maturation. Furthermore, we demonstrate that the loss of SUV3, PNPase, or MTPAP leads to the accumulation of mitochondrial-derived antisense RNA in the cytoplasm of cells, which is associated with an altered immune-response in flies.