Mitochondrial biogenesis
Mitochondrial biogenesis is the process by which new mitochondria are formed in the cell. Mitochondrial biogenesis is activated by numerous different signals during times of cellular stress or in response to environmental stimuli. The mitochondrion is a key regulator of the metabolic activity of the cell, and is also an important organelle in both production and degradation of free radicals. It is reckoned that higher mitochodrial copy number (or higher mitochondrial mass) is protective for the cell.
Mitochondria are produced from the transcription and translation of genes both in the nuclear genome and in the mitochondrial genome. The majority of mitochondrial protein comes from the nuclear genome, while the mitochondrial genome encodes most parts of the electron transport chain along with mitochondrial rRNA and tRNA. A major adaptation to mitochondrial biogenesis results in more mitochondrial tissues which increases metabolic enzymes for glycolysis, oxidative phosphorylation and ultimately a greater mitochondrial metabolic capacity.[Peter D. Wagner Summer 2011 Lecture]
The master regulators of mitochondrial biogenesis appear to be the peroxisome proliferator-activated receptor gamma (PGC) family of transcriptional coactivators, including PGC-1α, PGC-1β, and the PGC-related coactivator, PRC. PGC-1α, in particular, is thought to be a master regulator. It is known to co-activate nuclear respiratory factor 2 (NRF2/GABPA), and together with NRF-2 coactivates nuclear respiratory factor 1 (NRF1). The NRFs, in turn, activate the mitochondrial transcription factor A (tfam), which is directly responsible for transcribing nuclear-encoded mitochondrial proteins. This includes both structural mitochondrial proteins as well as those involved in mtDNA transcription, translation, and repair.