High tide or low tide: the transport and metabolism of mitochondrial nucleotides

Mitochondria are multifaceted organelles that support numerous cellular metabolic pathways, including the biosynthesis of nucleotides required for cell growth and proliferation. Owing to an ancient endosymbiotic origin, mitochondria contain multiple copies of their own genome and therefore demand sufficient (deoxy)nucleotides in the mitochondrial matrix for DNA replication and transcription into RNA. Disturbed mitochondrial deoxynucleotide homeostasis can lead to a decline in mitochondrial DNA abundance and integrity, causing mitochondrial diseases with diverse and severe symptoms. Mitochondrial nucleotides are not only required for nucleic acid synthesis but also for bioenergetics and mitochondrial enzymatic activity. This review first explores how mitochondria supply energy and anabolic precursors for nucleotide synthesis and how the mitochondrial network influences the spatial control of cellular nucleotide metabolism. Then follows an in-depth discussion of the mechanisms that supply mitochondria with sufficient and balanced nucleotides and why these mechanisms are relevant to human mitochondrial disease. Lastly, the review highlights the emergence of regulated mitochondrial nucleotide supply in physiological processes including innate immunity and discusses the implications of dysregulated mitochondrial and cytosolic nucleotide homeostasis in pathophysiology.