Alzheimer's Disease Frontal Cortex Mitochondria Show a Loss of Individual Respiratory Proteins but Preservation of Respiratory Supercomplexes

Alzheimer's disease (AD), the most common cause of sporadic dementia of in adults, shows increased risk of occurrence with aging and is destined to become a major sociomedical tragedy over the next few decades. Although likely complex in origin, sporadic AD is characterized by a progressive and stereotyped neuropathology with aggregated protein deposition (esp beta amyloid (BA) and hyperphosphorylated tau (P-tau)) and neuronal degeneration. To date, prevention of BA synthesis or immune-mediated removal of BA has failed to alter AD progression. Development and testing of P-tau therapeutics are a work in progress. AD brain tissues show multiple system deficits, including loss of respiratory capacity. In the present study there were no differences in mitochondrial mass between AD and CTL samples. We examined mitochondrial preparations of postmortem AD and CTL frontal cortex for relative levels of individual respiratory protein complexes by Western immunoblotting. ANOVA revealed deficiencies of all respiratory complex subunits in AD; post-ANOVA t-testing revealed significant differences in levels of subunits for complexes II, III, and V, borderline significance for subunit of complex IV, and no difference for subunit of complex I. We also examined mitochondrial extracts with blue-native gel electrophoresis combined with immunoblotting for subunits of complexes I and III to search for “respiratory supercomplexes” (RSC's). We found that levels of RSC's did not differ between AD and CTL samples. Mitochondrial preparations from end-stage AD brain tissue showed loss of individual ATP-producing respiration subunits but preservation of levels of assembled respiratory subunits into RSC's. Possible explanations include insufficient sensitivity of our method of RSC detection to find loss of individual subunits, or normal levels of RSC's in AD brain mitochondria coupled with decreased levels of nonassembled respiratory complex subunits. Disease-altering therapies of early AD could include stimulation of mitochondrial biogenesis to overcome loss of respiratory subunits.