Accumulated mtDNA mutations specifically impair complex I-linked respiration

Oxidative phosphorylation (OxPhos) within mitochondria relies on the coordinated synthesis of both nuclear- and mitochondrial-encoded protein subunits comprising the mitochondrial respiratory complexes (Complexes I–V). Mitochondrial DNA (mtDNA) encodes 13 proteins vital for Complex I, III, IV, and V function. Accumulated mutations in mtDNA are causally linked to aging and several age-related diseases, presumably as a consequence of impaired respiration. However, how high mtDNA mutation burden impinges on cellular bioenergetics across the major organ systems remains only partially resolved. Due to the growing links connecting mtDNA mutations to human pathophysiology, here we leveraged a comprehensive mitochondrial phenotyping platform to assess the phenotypic consequences of increased mtDNA mutation burden across tissues (brown adipose, brain, colon, heart, kidney, liver, lung, and bone marrow-derived mononuclear cells) of the mouse. Remarkably, despite widespread reductions in OxPhos protein expression, mitochondrial respiratory capacity under mixed substrate conditions was largely preserved across tissues. More detailed analysis revealed that NAD-linked respiration exhibited partial functional deficits in most tissues, consistent with functional deficiencies in complex I. In contrast, respiration routed from CII-CIII-CIV-CV remained intact. Together, these findings highlight Complex I as the primary functional consequence of mtDNA mutational load.