MAINTENANCE OF NAD+ HOMEOSTASIS IN SKELETAL MUSCLE DURING AGING AND EXERCISE

What is NAD+?

Nicotinamide adenine dinucleotide (NAD+) is made up of two covalently bonded mononucleotides, one nicotinamide mononucleotide (NMN) and AMP. NAD+ plays an important role in a variety of cellular functions related to metabolism, signal transduction and redox balance. Discovered more than a century ago by Sir Arthur Harden as a chemical catalyst for fermentation and winemaking, this classical coenzyme participates in many chemical reactions in cells, but its biological role is limited. Its comprehensive biology was not revealed until the past few decades. 

Nicotinamide adenine dinucleotide (NAD) is a versatile chemical compound that serves as a coenzyme in metabolism and as a substrate to support the enzymatic functions of: sirtuins (SIRT), poly (ADP- ribose) polymerase-1 (PARP-1) and cyclic ADP ribose hydrolase (CD38). 

Under normal physiological conditions, NAD+ consumption is regulated based on the synthesis of NAD+, which is mainly through the salvage pathway, which is catalyzed by nicotinamide photribosyltransferase (NAMPT). 

However, aging and muscle contraction enhance NAD+ utilization, whereas NAD+ replenishment is limited by the cellular source of NAD+ precursors and/or enzyme expression. 

Maintains NAD+ homeostasis in skeletal muscle

Initially, NAD+ was primarily considered a reducing agent, accepting hydrogen ions from other substrates and being reduced to NADH. This reaction is reversible and NADH is converted back to NAD+, so the sum of NAD+ and NADH will remain constant. However, when NAD+ is used in non-redox reactions, such as in signal transduction, its role is expanded and its levels are often depleted.

Maintains NAD+ homeostasis in skeletal muscle

The recognition of NAD+ as a multifunctional signaling molecule was primarily driven by research in aging and nutrition. In yeast and invertebrates, NAD+ supplementation increases lifespan and counteracts age-related functional decline. 

Michael N. Sack and Toren Finkel's studies of sirtuins (SIRTs), a family of NAD+-dependent protein deacetylases, have provided a mechanistic link between NAD+ homeostasis and cellular function. Increased protein acetylation is responsible for many age-related and aging musculoskeletal disorders, such as mitochondrial impairment (due to increased PGC-1α and FoxO3a acetylation), decreased antioxidant capacity metabolism (mainly due to acetylation of MnSOD) and increased inflammation (due to acetylation of p65).

According to Claudia CS Chini 2017 Research, deacetylation (as well as removal of succinyl, malonyl, glutamyl and palmitoyl residues) of proteins requires higher SIRT activity, but ironically, SIRT1 activity in skeletal muscle does not. decreased with aging despite increased SIRT1 protein expression. As a required substrate for SIRT, reduced NAD+ limits the deacetylation capacity of SIRT1, whereas maintaining NAD+ helps ensure SIRT function and promote longevity in yeast, some invertebrates and mice, while enhancing disease resistance in mammals and humans.

The hallmark of aging is reduced mitochondrial function, evidenced by reduced metabolic fuel utilization, ATP production, and oxygen consumption. However, the mechanism of age-related decline in the quality and quantity of mitochondria has not previously been completely clear. The discovery that NAD+ is depleted in the bodies of older organisms, including skeletal muscle, provides new insight into decades of aging research.

On the other hand, in addition to the natural aging process, exercise increases metabolic and oxidative stress on skeletal muscle and requires higher NAD+ turnover. Exercise increases SIRT's consumption of NAD+ for the degradation of key enzymes and transcription factors, as well as PARP-1 for DNA polynucleotide chain repair. This need appears to be met primarily by upregulation of NAMPT to recycle NAM, but the efficiency of NAD+ recovery may be limited by cellular sources of NAD+ precursors.

Aging gradually tilts the balance between NAD+ consumption and biosynthesis in favor of NAD+ consumption. Increased acetylation of proteins, enzymes, and key transcription factors in the cell requires higher SIRT activity, while PARR-1-catalyzed pADPr cleavage and CD38-catalyzed NAD+ hydrolysis increase increases with aging, leading to a NAD+ deficit. 

How to increase NAD+ levels?

Theoretically, increased NAD+ consumption could be supplemented by increased intracellular NAD+ synthesis. The Preiss–Handler pathway converts dietary NA to nicotinic acid mononucleotide (NAMN), then nicotinic acid adenine dinucleotide (NAAD), and finally NAD+. Meanwhile, De novo NAD+ biosynthesis from dietary tryptophan requires more steps with limited usefulness. 

A more important pathway that organisms rely on is the NAD+ salvage pathway controlled by nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme that converts NAM cleaved by SIRT, CD38, and PARP-1 to NMN, and finally NAD+. This pathway not only recycles NAM to maintain intracellular NAD+ concentrations but also reduces NAM inhibition on SIRT. 

There is evidence from the David W. Frederick et al-2016 Study that a decline in NAMPT activity in old age reduces NAM recycling and NAD+ recovery, leading to muscle degeneration. On the other hand, dietary supplementation of NAM or NR can increase intracellular and mitochondrial NAD+ levels, improve metabolic function, and even increase lifespan in aged mice.

Indeed according to research by Eric Verdin, 2015, in mammals and humans, dietary supplementation of NAD+ precursors, such as nicotinic acid (NA), nicotinamide mononucleotide (NMN) and nicotinamide ribose ( NR) helps significantly improve age-related mitochondrial degradation, metabolic dysfunction, insulin resistance, neurological disorders and exercise tolerance. 

According to research by Mario Mehmel et al. – 2020, under normal conditions, dietary sources of niacin (NA, NAM) at a daily dose of 20mg will maintain healthy NAD+ levels. However, higher NAD+ levels have been shown to improve a number of cardiovascular, metabolic, neurodegenerative and aging disorders. 

Therefore, there is increasing interest in both research and clinical practice to enhance cellular NAD+ synthesis. To date, Carlo Custodero – 2020 research shows that dietary supplementation of NAD+ precursors has been shown to be significantly effective. In addition to tryptophan, four NAD+ precursors are commonly accepted as potential dietary supplements: NA, NAM, NMN, and NR. 

NMN has anti-aging effects at the cellular level, reducing aging of the skin, muscles, bones, liver, vision, improving insulin sensitivity, immune function and physical activity levels. Nowadays, in addition to eating scientifically and living a healthy lifestyle, you can supplement functional foods containing Nicotinamide Mononucleotide , which is a precursor enzyme of Coenzyme NAD.