Mitochondrial Ca²⁺ uniporter haploinsufficiency leads to sexually dimorphic redox imbalance and metabolic remodelling in the mouse brain

The mitochondrial Ca²⁺ uniporter (MCU) links energy metabolism to cell excitability and signalling throughout the lifespan. However, whether neural metabolism responds to MCU impairments in a sex-specific manner has remained unknown, especially in models with partial MCU downregulation. Using hippocampal slices from adult heterozygous Mcu knock-out (hKO) mice, we observed sexually dimorphic changes in NAD(P)H autofluorescence dynamics following neuronal stimulation. In male mice, these signals were preserved despite decreased mitochondrial Ca²⁺ uptake, likely due to increased MDH2 levels and potentially other enzymes from the tricarboxylic acid cycle, the malate aspartate shuttle, and glycolysis. In contrast to males, neural tissue from female hKO mice showed delayed NAD(P)H production and limited NAD⁺ availability when compared to sex-matched controls, despite intact mitochondrial Ca²⁺ uptake. In addition, both male and female hKO mice exhibit decreased NADP⁺ levels and GSH/GSSG ratios (along with increased protein S-glutathionylation), indicating a weakened antioxidant capacity. Strikingly, markers of oxidative damage were also decreased (albeit more prominently in male mice), suggesting attenuated generation of reactive oxygen species. In addition, sex-specific changes in the hippocampal metabolome were manifested in hKO mice, along with a common decrease in spermidine levels. However, spermidine-dependent hypusination of eIF5A remained unaltered, suggesting further compensatory mechanisms at this age. In summary, our findings indicate that brain tissue can adapt to partial MCU deficits by salvaging most mitochondrial NADH production in active states, while compromising redox signalling and the polyamine pathway. The interplay between these molecular phenotypes likely impacts neurological conditions and potentially cognitive impairment with age. KEY POINTS: The inactivation of one Mcu allele (which encodes the mitochondrial Ca²⁺ uniporter) leads to altered neuronal excitability and attenuated mitochondrial Ca²⁺ elevations in active neurons from 6- to 12-months-old female and male mice, respectively. Tissue autofluorescence imaging reveals delayed mitochondrial NAD(P)H production in stimulated hippocampal tissue from female but not male heterozygous Mcu knockout mice. Mitochondrial Ca²⁺ uniporter haploinsufficiency is characterized by a sex-specific decrease in oxidative stress markers in the brain, despite a decline in NADP⁺ levels and the GSH/GSSG ratio in both male and female mice. Changes in the abundance of enzymes and polar metabolites in brain tissue reveal sexually dimorphic metabolic remodelling in the context of Mcu haploinsufficiency. Life-long downregulation of the mitochondrial Ca²⁺ uniporter results in decreased hippocampal spermidine levels in adult male and female mice.