Short-term administration of Nicotinamide Mononucleotide preserves cardiac mitochondrial homeostasis and prevents heart failure

NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

Targeting mitochondria for cardiovascular disorders: therapeutic potential and obstacles

A large body of evidence indicates that mitochondrial dysfunction has a major role in the pathogenesis of multiple cardiovascular disorders. Over the past 2 decades, extraordinary efforts have been focused on the development of agents that specifically target mitochondria for the treatment of cardiovascular disease. Despite such an intensive wave of investigation, no drugs specifically conceived to modulate mitochondrial functions are currently available for the clinical management of cardiovascular disease. In this Review, we discuss the therapeutic potential of targeting mitochondria in patients with cardiovascular disease, examine the obstacles that have restrained the development of mitochondria-targeting agents thus far, and identify strategies that might empower the full clinical potential of this approach.

Both gain and loss of Nampt function promote pressure overload-induced heart failure

The heart requires high-energy production, but metabolic ability declines in the failing heart. Nicotinamide phosphoribosyl-transferase (Nampt) is a rate-limiting enzyme in the salvage pathway of nicotinamide adenine dinucleotide (NAD) synthesis. NAD is directly involved in various metabolic processes and may indirectly regulate metabolic gene expression through sirtuin 1 (Sirt1), an NAD-dependent protein deacetylase. However, how Nampt regulates cardiac function and metabolism in the failing heart is poorly understood. Here we show that pressure-overload (PO)-induced heart failure is exacerbated in both systemic Nampt heterozygous knockout (Nampt+/-) mice and mice with cardiac-specific Nampt overexpression (Tg-Nampt). The NAD level declined in Nampt+/- mice under PO (wild: 377 pmol/mg tissue; Nampt+/-: 119 pmol/mg tissue; P = 0.028). In cultured cardiomyocytes, Nampt knockdown diminished mitochondrial NAD content and ATP production (relative ATP production: wild: 1; Nampt knockdown: 0.56; P = 0.0068), suggesting that downregulation of Nampt induces mitochondrial dysfunction. On the other hand, the NAD level was increased in Tg-Nampt mice at baseline but not during PO, possibly due to increased consumption of NAD by Sirt1. The expression of Sirt1 was increased in Tg-Nampt mice, in association with reduced overall protein acetylation. PO-induced downregulation of metabolic genes was exacerbated in Tg-Nampt mice. In cultured cardiomyocytes, Nampt and Sirt1 cooperatively suppressed mitochondrial proteins and ATP production, thereby promoting mitochondrial dysfunction. In addition, Nampt overexpression upregulated inflammatory cytokines, including TNF-α and monocyte chemoattractant protein-1. Thus endogenous Nampt maintains cardiac function and metabolism in the failing heart, whereas Nampt overexpression is detrimental during PO, possibly due to excessive activation of Sirt1, suppression of mitochondrial function, and upregulation of proinflammatory mechanisms.NEW & NOTEWORTHY Nicotinamide phosphoribosyl-transferase (Nampt) is a rate-limiting enzyme in the salvage pathway of nicotinamide adenine dinucleotide synthesis. We demonstrate that pressure overload-induced heart failure is exacerbated in both systemic Nampt heterozygous knockout mice and mice with cardiac-specific Nampt overexpression. Both loss- and gain-of-function models exhibited reduced protein acetylation, suppression of metabolic genes, and mitochondrial energetic dysfunction. Thus endogenous Nampt maintains cardiac function and metabolism in the failing heart, but cardiac-specific Nampt overexpression is detrimental rather than therapeutic.

Calcium Signaling and Reactive Oxygen Species in Mitochondria

In heart failure, alterations of Na⁺ and Ca²⁺ handling, energetic deficit, and oxidative stress in cardiac myocytes are important pathophysiological hallmarks. Mitochondria are central to these processes because they are the main source for ATP, but also reactive oxygen species (ROS), and their function is critically controlled by Ca²⁺ During physiological variations of workload, mitochondrial Ca²⁺ uptake is required to match energy supply to demand but also to keep the antioxidative capacity in a reduced state to prevent excessive emission of ROS. Mitochondria take up Ca²⁺ via the mitochondrial Ca²⁺ uniporter, which exists in a multiprotein complex whose molecular components were identified only recently. In heart failure, deterioration of cytosolic Ca²⁺ and Na⁺ handling hampers mitochondrial Ca²⁺ uptake and the ensuing Krebs cycle-induced regeneration of the reduced forms of NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate), giving rise to energetic deficit and oxidative stress. ROS emission from mitochondria can trigger further ROS release from neighboring mitochondria termed ROS-induced ROS release, and cross talk between different ROS sources provides a spatially confined cellular network of redox signaling. Although low levels of ROS may serve physiological roles, higher levels interfere with excitation-contraction coupling, induce maladaptive cardiac remodeling through redox-sensitive kinases, and cell death through mitochondrial permeability transition. Targeting the dysregulated interplay between excitation-contraction coupling and mitochondrial energetics may ameliorate the progression of heart failure.

Impaired SIRT3 activity mediates cardiac dysfunction in endotoxemia by calpain-dependent disruption of ATP synthesis

BACKGROUND

Sepsis-induced cardiomyopathy contributes to the high mortality of septic shock in critically ill patients. Since the underlying mechanisms are incompletely understood, we hypothesized that sepsis-induced impairment of sirtuin 3 (SIRT3) activity contributes to the development of septic cardiomyopathy.

METHODS AND RESULTS

Treatment of mice with lipopolysaccharide (LPS) for 6 h resulted in myocardial NAD⁺ depletion and increased mitochondrial protein acetylation, indicating impaired myocardial SIRT3 activity due to NAD⁺ depletion. LPS treatment also resulted in impaired cardiac output in isolated working hearts, indicating endotoxemia-induced cardiomyopathy. Maintaining normal myocardial NAD⁺ levels in LPS-treated mice by Poly(ADP-ribose)polymerase 1 (PARP1) deletion prevented cardiac dysfunction, whereas additional SIRT3 deficiency blunted this beneficial effect, indicating that impaired SIRT3 activity contributes to cardiac dysfunction in endotoxemia. Measurements of mitochondrial ATP synthesis suggest that LPS-induced contractile dysfunction may result from cardiac energy depletion due to impaired SIRT3 activity. Pharmacological inhibition of mitochondrial calpains using MDL28170 normalized LPS-induced cleavage of the ATP5A1 subunit of ATP synthase and normalized contractile dysfunction, suggesting that cardiac energy depletion may result from calpain-mediated cleavage of ATP5A1. These beneficial effects were completely blunted by SIRT3 deficiency. Finally, a gene set enrichment analysis of hearts of patients with septic, ischemic or dilated cardiomyopathy revealed a sepsis-specific suppression of SIRT3 deacetylation targets, including ATP5A1, indicating a functional relevance of SIRT3-dependent pathways in human sepsis.

CONCLUSIONS

Impaired SIRT3 activity may mediate cardiac dysfunction in endotoxemia by facilitating calpain-mediated disruption of ATP synthesis, suggesting SIRT3 activation as a potential therapeutic strategy to treat septic cardiomyopathy.

Emerging potential benefits of modulating NAD+ metabolism in cardiovascular disease

Nicotinamide adenine dinucleotide (NAD⁺) and related metabolites are central mediators of fuel oxidation and bioenergetics within cardiomyocytes. Additionally, NAD⁺ is required for the activity of multifunctional enzymes, including sirtuins and poly(ADP-ribose) polymerases that regulate posttranslational modifications, DNA damage responses, and Ca²⁺ signaling. Recent research has indicated that NAD⁺ participates in a multitude of processes dysregulated in cardiovascular diseases. Therefore, supplementation of NAD⁺ precursors, including nicotinamide riboside that boosts or repletes the NAD⁺ metabolome, may be cardioprotective. This review examines the molecular physiology and preclinical data with respect to NAD⁺ precursors in heart failure-related cardiac remodeling, ischemic-reperfusion injury, and arrhythmias. In addition, alternative NAD⁺-boosting strategies and potential systemic effects of NAD⁺ supplementation with implications on cardiovascular health and disease are surveyed.