NNT deficiency alters cardiac structure and function without impairing mitochondrial bioenergetics in aged mice

Proton-translocating NAD(P)⁺ transhydrogenase (NNT) is highly expressed in cardiac tissue, where it physiologically supports mitochondrial NADPH production. However, under certain pathological conditions, NNT may shift toward consuming the mitochondrial NADPH pool. Although NNT has been implicated in redox homeostasis, its contribution to cardiac function during aging remains uncertain. In this study, we assessed cardiac morphology and function, as well as mitochondrial bioenergetics and Ca²⁺ handling, in NNT-deficient (Nnt^-/-) mice and congenic wild-type controls (Nnt^+/+) at adult (5 months), middle (12 months), and older (23 months) ages. NNT-deficient mice developed age-related cardiac hypertrophy, along with a moderately reduced ejection fraction and fractional shortening at older ages, suggesting left ventricular dysfunction. These changes were associated with increased mitochondrial H₂O₂ release under specific conditions, whereas mitochondrial bioenergetic parameters and Ca²⁺ retention capacity remained largely unaffected by the Nnt genotype at all ages. Our findings indicate that NNT plays a protective role in the aging heart by maintaining redox balance and that NNT deficiency may contribute to late-onset cardiac dysfunction without causing overt mitochondrial bioenergetic failure. These results provide insight into the potential cardiac consequences of pathogenic NNT variants in humans.