Mammalian mtDNA gene expression: Concepts learned from in vivo models

Mammalian mitochondrial DNA (mtDNA) expression is essential for oxidative phosphorylation (OXPHOS) and its in vivo regulation requires significant refinement. Here, we review key insights from mouse models carrying genetic modifications to the mtDNA expression machinery. While in vitro studies defined the basic machinery, mouse models reveal that mitochondrial transcription often exceeds immediate needs and may not be the primary rate-limiting step for OXPHOS biogenesis. Instead, mitochondria produce a transcript surplus regulated by nucleoid compaction and post-transcriptional stabilization. This apparent excess capacity is uncoupled from protein output under basal conditions but becomes critical during physiological stress or pathology. Using current and emerging genetic tools, researchers are now deciphering how regulatory layers coordinate to sustain systemic energy demands. These lessons highlight the importance of in vivo systems for identifying regulatory control points of mtDNA expression and developing targeted therapies for mitochondrial disorders.