NAD + replacement therapy with nicotinamide riboside does not improve cardiac function in a model of mitochondrial heart disease. (2017)

Rationale Protein lysine acetylation is an important post‐translational modification that regulates cardiac metabolism. Sirtuin 3 (SIRT3) is an NAD + ‐dependent deacetylase that controls mitochondrial deacetylation. Loss of SIRT3 expression causes increased mitochondrial protein acetylation along with cardiac hypertrophy, fibrosis and increased susceptibility to mechanical and ischemic stress. The childhood heart disease of Friedreich's Ataxia (FRDA) is characterized by cardiac hypertrophy and failure. There is no effective therapy. FRDA is caused by loss of frataxin (FXN), a mitochondrial protein involved in energy homeostasis. FRDA model hearts have increased mitochondrial protein acetylation and impaired SIRT3 activity, which may contribute to the pathophysiology of this heart disease and represent a therapeutic target. Objective There is evidence that NAD + repletion by supplementation with its precursor, nicotinamide riboside (NR), can increase SIRT3 activity and improve mitochondrial function. We tested the hypothesis that NR therapy would normalize lysine acetylation and improve cardiac function in a model of FRDA heart disease. Methods and Results A conditional mouse model of FRDA with ablation of FXN (FXN KO) in the heart (n=6) and healthy controls (n=6) were treated with oral administration of NR (10 ml/kg/day) from wean at day 30 for 5 weeks. Results were compared to untreated age‐ and sex‐matched animals. We evaluated hearts using echocardiography, cardiac catheterization, histology, protein expression and lysine acetylation. We used the acetylation of a known target of SIRT3, superoxide dismutase 2 (SOD2), as readout for SIRT3 activity. The acetylation state of SOD2 was increased in FXN KO mice compared to controls, and did not change after NR therapy. Surprisingly, FXN KO hearts demonstrated increased global mitochondrial protein lysine acetylation in response to treatment with NR. SIRT3 protein expression did not differ between groups. Cardiac function declined steadily with age in FXN KO animals and there was no improvement after NR supplementation in FXN KO animals. Such outcome parameters included left ventricle thickness (LVPWd) (FXN KO 0.85 ±02.3 vs FXN KO +NR 0.78 ±0.12 mm, p =0.53), ejection fraction (FXN KO 26.6 ±9.7 vs. FXN KO +NR 30.6±13.5%, p =0.53), fractional shortening (FXN KO 12.4 ±4.7 vs. FXN KO +NR 14.5 ±6.7 %, p =0.51) and rates of contraction (FXN KO 5165 ±1019 vs FXN KO +NR 4683 ±760 mmHg/sec, p =0.42) and relaxation (FXN KO −4099 ±1022 vs. FXN KO +NR −3613 ±807 mmHg/sec, p =0.42). Diffuse cardiac fibrosis was evident in FXN KO hearts at 5 weeks post‐wean and there was no observable difference after NR therapy. Discussion In FXN KO hearts, SIRT3 may be irreversibly inactivated, possibly via lipid peroxidation from an increase in reactive oxygen species known to arise secondary to the loss of frataxin. Combining NR therapy with SIRT3 replacement, or trialing longer or larger doses of NR, may be more effective in modifying SIRT3 acetylation targets in the FXN KO heart and warrants further investigation. Conclusion In conclusion, NAD + supplementation with NR in the FRDA model of mitochondrial heart disease does not alter SIRT3 activity or improve cardiac function. Support or Funding Information This work was supported by an NHLBI NRSA fellowship grant #F31HL126489 to ARS, grants from the Friedreich's Ataxia Research Alliance (FARA) and Muscular Dystrophy Association (MDA) to RMP.