NAD+ : The healthy aging molecule

 Highlights

• NAD+ is a critical coenzyme required for many biological reactions and without it we would be dead in 30 seconds.
• NAD+ is essential to convert the food we eat into energy our cells can use to function.
• NAD+ switches on other important cellular pathways such as sirtuins, which are needed to support healthy aging.  

Introduction

NAD+ is a coenzyme found in every cell of the body, and without it we would be dead in 30 seconds. As a coenzyme its presence is required to support more than 500 biological reactions within the cell. Therefore, NAD+ is involved in many different cellular processes such as cellular energy production, immune cell signaling, mitochondrial function, and DNA repair. This article will look at some of the key roles of NAD+ within our cells and why NAD+ is seen as a key molecule to support healthy aging.

NAD+ was first discovered in 1906 by Sir Arthur Harden and William John Young, while studying the fermentation process in yeast. While we have known about NAD+ for over 100 years it is only recently that the potential longevity benefits of NAD+ have been appreciated.

 NAD+ Structure

NAD stands for nicotinamide adenine dinucleotide. It is composed of two nucleotides (di meaning two in Greek) one containing an adenine group and the other containing a nicotinamide group. NAD is often seen written as NAD+, NADH, NADP+, NADPH and these are all different forms of NAD with a slight change in the number of hydrogens or phosphate groups attached. (1)

Because NAD can take various forms it is involved in a wide array of cellular processes. It links changes in the cells environment to the responses required inside the cell by altering cellular metabolism, cell signaling and transcription. It is via all these mechanisms that NAD+ elicits the positive benefits of many healthy lifestyle factors such as calorie restriction and exercise. (2)

The wide array of functions NAD+ has can be grouped into 2 main categories:

1. Cellular metabolism – the group of reactions involved in converting the food we eat into ATP which can be used by our cells for energy. During these reactions NAD+ functions as an electron acceptor and donator. When NAD+ accepts electrons, it becomes NADH (known as the reduced form) and when NADH donates electrons it goes back to NAD+ (the oxidized form). These reactions do not consume NAD+, it simply alternates between several structures (e.g. NAD+ / NADH).

2. NAD+ dependent signaling – these are all the biochemical reactions which depend on NAD+ to occur. It is this role of NAD+ which has been of great interest to scientists as we continue to learn about the ways in which NAD+ supports our cellular health. These reactions include DNA repair and activation of sirtuins (longevity genes). They also consume NAD+ so ensuring NAD+ levels remain high is essential to keep these processes working efficiently.

The NAD+ : NADH Ratio

Another important factor is the ratio of NAD+ to NADH. In young cells the proportion of NAD+ is much higher than NADH, however in older cells as NAD+ production declines the ratio drifts towards higher levels of NADH which is unfavorable. This is because it is the NAD+ form which is needed for cellular signaling. (3) However, it is possible to push the ratio back towards NAD+ by activating enzymes such as NQO1 – which power the enzymatic reaction that converts NADH to NAD+. The compound alpha lipoic acid (ALA) has been shown to activate NQO1 and is one of the ingredients included in Nuchido TIME+ for this reason .

The main benefit of high NAD levels is the other downstream pathways that it activates. NAD+ is the fuel powering these health promoting pathways. (4)

Sirtuins

Sirtuins are a family of proteins (of which there are seven SIRT1-7), that are found in various locations within our cells. SIRT1 and SIRT6 are found in the nucleus, SIRT7 in the nucleolus, SIRT3, SIRT4, and SIRT5 are in the mitochondria, and SIRT1, SIRT2 and SIRT5 are also located in the cytosol. (5)

They are essential to cellular health, energy metabolism and the regulation of gene expression. Their job is to remove acetyl markers from specific sites on the DNA, which changes the DNA structure and, consequently, which genes are activated. They are constantly present within our cells but to be active they require the presence of NAD+. (6)

Sirtuins are often referred to as longevity genes because of their role in eliciting stress responses and regulating aging pathways within the cell. Unfortunately, activation of sirtuins declines with age as our NAD+ levels decline. So, it is by keeping NAD+ levels high that we can keep our sirtuins active as we age and support our cellular heath. (7)

PARPs

PARPs are another family of proteins activated by NAD+, there are 17 human PARP proteins, many of which are central to DNA repair. Our DNA is constantly facing damage from UV light from the sun, free radicals from our mitochondria (even more so when they become damaged), toxins, and chemicals. It is estimated that each cell suffers tens of thousands of DNA damage events every day, so having well-functioning DNA repair mechanisms is vital.

When we are young and have high levels of NAD+ our DNA is very efficient at repairing damage because PARP proteins use NAD+ as a fuel, breaking it down to power the DNA repair action. However, as we age that process becomes dysfunctional in part due to declining NAD+ levels allowing DNA damage to accumulate. DNA damage is so key to the aging process that it is one of the hallmarks of aging. (8-12)

CD38

CD38 is an enzyme expressed on the surface of immune cells and is one of the largest consumers of NAD+. CD38 regulates calcium signaling to active immune cells but it is very inefficient at using NAD+. It can waste around 100 molecules of NAD+ for each reaction. Additionally, the levels of CD38 expressed on our cells increases with age. This is due to ‘inflammaging’ the age-related increase in baseline inflammation levels. (13-15)

When cells accumulate too much damage, and they cannot be repaired the cells become senescent. These cells stop replicating, to prevent the spread of damage but they are still metabolically active. They are often described as zombie cells. A proportion of these senescent cells will secrete chemicals called the SASP (senescence associated secretory phenotype) which will negatively impact the cells around it and also cause them to become senescent. The inflammatory factors in the SASP can also cause cells to increase expression or upregulate CD38 further wasting NAD+ levels.  (16, 17)

Therefore, inhibiting CD38 even a little bit can have a very positive impact on NAD+ levels. Apigenin, a polyphenol found in parsley has been found to inhibit CD38 and that is why it is included in the Nuchido TIME+ formulation. (18)

NAD+ Recycling

Of particular importance is that all the biochemical reactions which breakdown NAD+ to function leave behind nicotinamide (NAM) as a byproduct. The cell has a recycling process called the ‘Salvage Pathway’ that can convert this waste NAM back into fresh NAD+ for the cell to use. Due to the high consumption of NAD+ by these various processes the Salvage Pathway is responsible for producing the majority of NAD+ within our cells. (19)

However, the enzyme which powers the Salvage Pathway called NAMPT also declines with age meaning the cell struggles to recycle waste NAM. This is one of the main reasons NAD+ levels decline with age, so restoring the activity of the Salvage Pathway is paramount to boosting NAD+ levels. (20)

It is therefore clear, that NAD+ is so important to our health because it acts as a regulator of so many important cellular pathways. When we are young our cells are very good at making and recycling NAD+ so our cells continuously have sufficient supply to power all the NAD+ dependent reactions. However, as we age our NAD+ levels naturally decline, and it is this decline which is thought to be a main reason why we develop age-related diseases. That is why research is focusing on ways to boost cellular NAD+ levels as we age and keep us feeling our best.

References

(1)    Covarrubias, A. J., Perrone, R., Grozio, A., & Verdin, E. (2021). NAD+ metabolism and its roles in cellular processes during ageing. Nature Reviews Molecular Cell Biology, 22(2), 119-141.

(2)    Poljsak, B., Kovač, V., & Milisav, I. (2020). Healthy lifestyle recommendations: do the beneficial effects originate from NAD+ amount at the cellular level?. Oxidative medicine and cellular longevity, 2020.

(3)    Clement, J., Wong, M., Poljak, A., Sachdev, P., & Braidy, N. (2019). The plasma NAD+ metabolome is dysregulated in “normal” aging. Rejuvenation research, 22(2), 121-130

(4)    Aman, Y., Qiu, Y., Tao, J., & Fang, E. F. (2018). Therapeutic potential of boosting NAD+ in aging and age-related diseases. Translational Medicine of Aging, 2, 30-37.

(5)    Haigis, M. C., & Sinclair, D. A. (2010). Mammalian sirtuins: biological insights and disease relevance. Annual Review of Pathology: Mechanisms of Disease, 5, 253-295.

(6)    Imai, S. I., & Guarente, L. (2014). NAD+ and sirtuins in aging and disease. Trends in cell biology, 24(8), 464-471.

(7)    Grabowska, W., Sikora, E., & Bielak-Zmijewska, A. (2017). Sirtuins, a promising target in slowing down the ageing process. Biogerontology, 18(4), 447-476.

(8)    Schumacher, B., Pothof, J., Vijg, J., & Hoeijmakers, J. H. (2021). The central role of DNA damage in the ageing process. Nature, 592(7856), 695-703.

(9)    Petr, M. A., Tulika, T., Carmona-Marin, L. M., & Scheibye-Knudsen, M. (2020). Protecting the aging genome. Trends in cell biology, 30(2), 117-132

(10)Garinis, G. A., Van der Horst, G. T., Vijg, J., & Hoeijmakers, J. H. (2008). DNA damage and ageing: new-age ideas for an age-old problem. Nature cell biology, 10(11), 1241-1247.

(11) Wilk, A., Hayat, F., Cunningham, R., Li, J., Garavaglia, S., Zamani, L., ... & Sobol, R. W. (2020). Extracellular NAD+ enhances PARP-dependent DNA repair capacity independently of CD73 activity. Scientific reports, 10(1), 1-21.

(12)López-Otín, C., Blasco, M. A., Partridge, L., Serrano, M., & Kroemer, G. (2013). The hallmarks of aging. Cell, 153(6), 1194-1217.

(13)Schultz, M. B., & Sinclair, D. A. (2016). Why NAD+ declines during aging: It’s destroyed. Cell metabolism, 23(6), 965-966.

(14)Aksoy, P., White, T. A., Thompson, M., & Chini, E. N. (2006). Regulation of intracellular levels of NAD: a novel role for CD38. Biochemical and biophysical research communications, 345(4), 1386-1392.

(15)Camacho-Pereira, J., Tarragó, M. G., Chini, C. C., Nin, V., Escande, C., Warner, G. M., ... & Chini, E. N. (2016). CD38 dictates age-related NAD decline and mitochondrial dysfunction through an SIRT3-dependent mechanism. Cell metabolism, 23(6), 1127-1139.

(16)Childs, B. G., Durik, M., Baker, D. J., & Van Deursen, J. M. (2015). Cellular senescence in aging and age-related disease: from mechanisms to therapy. Nature medicine, 21(12), 1424-1435.

(17)Di Micco, R., Krizhanovsky, V., Baker, D., & di Fagagna, F. D. A. (2021). Cellular senescence in ageing: from mechanisms to therapeutic opportunities. Nature Reviews Molecular Cell Biology, 22(2), 75-95.

(18)Escande, C., Nin, V., Price, N. L., Capellini, V., Gomes, A. P., Barbosa, M. T., ... & Chini, E. N. (2013). Flavonoid apigenin is an inhibitor of the NAD+ ase CD38: implications for cellular NAD+ metabolism, protein acetylation, and treatment of metabolic syndrome. Diabetes, 62(4), 1084-1093.

(19)Fang, E. F., Lautrup, S., Hou, Y., Demarest, T. G., Croteau, D. L., Mattson, M. P., & Bohr, V. A. (2017). NAD+ in aging: molecular mechanisms and translational implications. Trends in molecular medicine, 23(10), 899-916.

(20)Liu, X., & Huang, T. The Role of NAD+ in Anti-Aging Therapies. life, 15, 21-24.