NAD+ and Longevity: The Role of Mitochondrial Peptides

Mitochondria, the energy producing organelles in virtually every human cell, have long been central to theories of aging. The original "mitochondrial free radical theory" proposed that accumulated oxidative damage to mitochondrial DNA drives aging. While that model has been refined (the relationship between reactive oxygen species and aging is more nuanced than initially thought), the fundamental importance of mitochondrial function in aging remains well established.

Two recent developments have transformed our understanding of mitochondria's role in longevity. The first is the discovery that mitochondria produce bioactive peptides, called mitochondrial derived peptides or MDPs (Merry et al., 2020, Am J Physiol Endocrinol Metab; PMID: 32776825). The second is the recognition that NAD+ (nicotinamide adenine dinucleotide), a critical mitochondrial coenzyme, declines gradually with age in tissues of both rodents and humans (Covarrubias et al., 2021, Nat Rev Mol Cell Biol; PMID: 33353981). These two fields are now converging, revealing an integrated mitochondrial signaling network that influences aging at its most fundamental level.

NAD+ is a coenzyme present in every living cell, essential for hundreds of metabolic reactions including oxidative phosphorylation (energy production), DNA repair, epigenetic regulation, and cell signaling. NAD+ levels decline with age in several human and animal tissues, a decline linked to many hallmarks of aging (Yoshino et al., 2018, Cell Metabolism; PMID: 29249689). The decline is driven by multiple factors. CD38, an NAD+ consuming enzyme, increases with age related inflammation (Covarrubias et al., 2020, Nat Metab; PMID: 33199924). PARP enzymes, which repair DNA damage, also consume NAD+, and DNA damage accumulates with age. The NAMPT enzyme is rate limiting for NAD+ biosynthesis via the salvage pathway, the dominant NAD+ production route in most tissues (Khaidizar et al., 2021, Int J Mol Sci; PMID: 33918226). And senescent cells and inflammatory signaling drive CD38 expression, creating a vicious cycle of NAD+ depletion and further cellular dysfunction (Covarrubias et al., 2020, Nat Metab; PMID: 33199924). NAD+ restoration strategies, including precursors like NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside), as well as direct NAD+ IV infusions, have become a cornerstone of longevity medicine. Small human trials have reported that NMN supplementation raises blood NAD+ levels, but whether that improves long term health outcomes is not yet known (Yi et al., 2023, Geroscience; PMID: 36482258).

The discovery of mitochondrial derived peptides has opened an entirely new chapter in mitochondrial biology. These small peptides, encoded within the mitochondrial genome, function as retrograde signals from mitochondria to the nucleus and peripheral tissues, communicating mitochondrial status and activating protective pathways (Merry et al., 2020, Am J Physiol Endocrinol Metab; PMID: 32776825).

Humanin was the first MDP discovered, identified in 2001 in a screen for factors that protect neurons from Alzheimer's disease related cell death (Hashimoto et al., 2001, Proc Natl Acad Sci U S A; PMID: 11371646). It is a 24 amino acid peptide encoded within the mitochondrial 16S rRNA gene (Karachaliou et al., 2023, Biology (Basel); PMID: 38132360). It protects neurons from amyloid beta toxicity, oxidative stress, and excitotoxicity, which are mechanisms relevant to Alzheimer's and other neurodegenerative diseases (Hashimoto et al., 2001, PNAS; PMID: 11371646). In rodent studies, humanin improved insulin sensitivity, acting through the hypothalamus to reduce hepatic glucose production, and a potent analogue lowered blood glucose in diabetic rats (Muzumdar et al., 2009, PLoS One; PMID: 19623253). Lower circulating levels of humanin and other MDPs have been associated with metabolic conditions such as obesity and diabetes (Merry et al., 2020, Am J Physiol Endocrinol Metab; PMID: 32776825). Circulating humanin levels generally decline with age across species, and children of centenarians have higher humanin levels than age matched controls (Yen et al., 2020, Aging (Albany NY); PMID: 32575074). Humanin also activates STAT3 signaling and has protective effects in stressed cells (Muzumdar et al., 2009, PLoS One; PMID: 19623253).

MOTS c (mitochondrial open reading frame of the 12S rRNA type c) is a 16 amino acid peptide first described in 2015 (Lee et al., 2015, Cell Metab; PMID: 25738459). It has been called the "exercise mimetic peptide" for its remarkable metabolic effects. It activates AMP activated protein kinase (AMPK), the master metabolic sensor that is also activated by exercise, caloric restriction, and metformin (Lee et al., 2015, Cell Metab; PMID: 25738459). This positions MOTS c at the nexus of the most well validated longevity pathways (Lee et al., 2015, Cell Metabolism; PMID: 25738459). In mouse studies, MOTS c prevented age dependent and high fat diet induced insulin resistance as well as diet induced obesity (Lee et al., 2015, Cell Metab; PMID: 25738459). Exercise increases MOTS c levels in human skeletal muscle and in circulation, and MOTS c can regulate nuclear genes involved in metabolism and stress adaptation (Reynolds et al., 2021, Nat Commun; PMID: 33473109). It may be a key molecular mediator of exercise's metabolic benefits. Under metabolic stress, it also translocates to the nucleus, regulates antioxidant response genes, and interacts with the stress responsive transcription factor Nrf2 (Kim et al., 2018, Cell Metab; PMID: 29983246).

NAD+ and mitochondrial derived peptides are connected through several converging pathways. NAD+ is essential for the electron transport chain; as NAD+ declines, mitochondrial function deteriorates, which may alter MDP production and secretion. NAD+ is also the required substrate for sirtuin enzymes (SIRT1 through SIRT7), which are NAD+ dependent deacetylases central to longevity pathways. MOTS c activates AMPK, and AMPK in turn raises cellular NAD+ levels and enhances SIRT1 activity (Cantó et al., 2009, Nature; PMID: 19262508). NAD+, through sirtuins and other NAD+ dependent enzymes, influences chromatin remodelling and gene expression, processes that change with age (Covarrubias et al., 2021, Nat Rev Mol Cell Biol; PMID: 33353981).

Translating this biology into clinical practice is still in its early stages. NAD+ restoration is the most accessible intervention: oral NAD+ precursors such as NMN, tested at 300 to 900 mg per day in clinical trials, raise blood NAD+ levels (Yi et al., 2023, Geroscience; PMID: 36482258)). Clinical trials show NMN is well tolerated and raises blood NAD+ levels, with emerging evidence for improvements in physical performance (Yi et al., 2023, Geroscience; PMID: 36482258). MOTS c administration is being explored clinically in some longevity medicine practices, though human clinical trial data is limited. Humanin analogues are in preclinical and early clinical development for neurodegenerative and metabolic conditions. HNG (S14G humanin), a more potent synthetic analogue, improved cognitive deficits and reduced amyloid pathology in a mouse model of Alzheimer's disease (Zhang et al., 2012, Pharmacol Biochem Behav; PMID: 21993310).

This multi modal approach reflects the reality that aging is not a single pathway process. It is the result of interconnected declines across multiple systems, and effective longevity medicine will likely require interventions that address this complexity.