The dynamics of human mitochondrial nucleoids within the mitochondrial network

Doctoral theses defended at the University of Tartu, summary in Estonian.

In eukaryotic cells, mitochondria are organelles of endosymbiotic origin with a well-known function in the synthesis of ATP via oxidative phosphorylation. Mitochondria also play an important role in calcium homeostasis, programmed cell death and β-oxidation of free fatty acids. Mitochondria possess a small and compact genome, which encoded in the mtDNA, which encodes for essential subunits of the electron transport chain, mitochondrial tRNAs and rRNAs. Human mtDNA is packaged exclusively by the TFAM protein into units called nucleoids. Nucleoids are associated with the inner mitochondrial membrane and based on their composition and function at least two pools of nucleoids exist. Nucleoids actively engaged in replication and characterized by the presence of helicase Twinkle are associated with a cholesterol rich membrane structure (replication microdomain). This structure is in turn associated with ER-mitochondria junctions termed mitochondria associated ER membranes (MAM). MAM contacts are essential for mtDNA segregation, as mitochondrial division and nucleoid allocation occur in the vicinity of a subset of MAM. However, it remains to be elucidated how nucleoids impact the formation of MAM at sites of mtDNA synthesis. The results in this thesis provide further knowledge on the interplay between nucleoids and organelle tethers. It was found that a downregulation of nucleoid packaging protein TFAM results in nucleoid aggregation, significant decrease in mtDNA copy number, increased proportion of replicating nucleoids and reorganization of MAM contacts. These results show that the nucleoid integrity is prerequisite for the formation of ER-mitochondria connections. Additionally, this study helps to shed light onto the functions of the proteins involved in mtDNA and mtRNA metabolism, thereby contributing to the understanding of defects associated with mtDNA maintenance.