Aging is characterized by a gradual loss of cellular homeostasis and tissue function. This is a major cause of frailty in the elderly, which reduces their quality of life and increases the susceptibility to morbidity and mortality. Even though exercise is one of the most potent ways to reduce frailty, our capacity to exercise and achieve benefits from it declines with age.
In our recent study titled “Impairment of an Endothelial NAD+-H2S Signaling Network Is a Reversible Cause of Vascular Aging,” published in the journal Cell, we may have identified one of the main causes of frailty and discovered a simple way to reverse it.
Vascular aging, frailty and SIRT1 connection
Blood vessels play an important role in the proper maintenance of tissue function. They supply oxygen and nutrients to different tissues and remove waste products. Aging significantly impairs this function as blood vessel density and blood flow to different tissues decrease with age. Aging of the vasculature is an often-overlooked driver of several age-related diseases due to decreased perfusion of major tissues, resulting in the loss of exercise capacity, impaired wound healing, and frailty, and diseases including cardiovascular diseases, cerebrovascular diseases, ischemia, vascular dementia, sarcopenia, and others.
Endothelial cells form the inner lining of blood vessels and capillaries. A decline in endothelial function is one of the main causes of vascular aging, manifested as increased aortic stiffness and loss of vascular density. While the first process has been studied extensively, the second is poorly understood.
Given the importance of endothelial cells to age-related diseases and overall functional decline, we investigated the key pathways within endothelial cells that contribute to a loss of capillary formation and blood flow during aging. We asked if any canonical pathways that regulate lifespan could also regulate vascular health. We tested the involvement of the sirtuin family of deacylases that is a key mediator of the response to exercise and can increase lifespan in different model organisms, including mice.
SIRT1, a mammalian sirtuin, uses nicotinamide adenine dinucleotide (NAD+) as a substrate to modulate the activity of most major metabolic and defense pathways by removing acetyl groups from lysine residues. Interestingly when we deleted SIRT1 specifically in endothelial cells, blood vessel density in the muscle of these animals decreased by 40% and they ran only half as far, while overexpression of SIRT1 increased number of blood vessels in muscle and the mice ran almost two times further before getting exhausted.
Regular exercise is highly potent in improving blood flow in our body. However, it is not clear why we become desensitized to exercise as we age. Using an ‘exercise mimetic’ mouse in which the transcriptional co-activator PGC-1α was overexpressed, our results indicate that endothelial SIRT1 is a major mediator of the benefits of exercise on fitness. Together, these results indicated that a reduction in the activity of endothelial SIRT1 during aging is the reason why blood flow in muscle decreases and we get frail with age.
Getting stronger with NAD+
NAD+ is a key cellular metabolite that is used as a cofactor in a multitude of enzymatic reactions inside our body. Its importance was appreciated since being discovered in the early 1900’s, however, it has recently gained heightened attention for its newly discovered role in aging. The activity of SIRT1 relies primarily on the availability of NAD+ inside the cells.
Several labs, including ours, have shown previously that NAD+ levels decrease with age in different tissues, thereby reducing SIRT1 activity. In humans, this decline starts as early as 30 years of age, and by 50, levels of NAD+ can be half. Restoration of NAD+ levels by treating with NAD+ boosters – such as nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR) – is becoming an effective strategy to activate SIRT1 in aged animals and ameliorate age-related diseases.
Consistent with this, providing NMN in drinking water of 20-month-old mice (equivalent to 60-year-old humans) for two months improved their running capacity by 56%-80% in different endurance tests. This improvement was attributed to an increased number of capillaries and blood flow in the muscle of these animals. We showed that at the molecular level, NMN restored NAD+ levels in the endothelial cells and stimulated SIRT1 activity, thereby improving the ability of these cells to form new blood vessels and regulate proper blood flow. Interestingly, when combined with exercise training, the effect of NMN treatment was also augmented. We concluded that NMN acts as an “exercise mimetic” that stimulates capillary formation as effectively as endurance training.
Hydrogen sulfide improves NAD+’s action
Just like NAD+, hydrogen sulfide (H2S) is another molecule that is made inside our cells and has gained attention for its anti-aging properties. Given the similarity of the effects of NMN and H2S in sensing nutrient intake, we tested if a hydrogen sulfide precursor, sodium hydrosulfide (NaHS), could also reduce frailty in mice. We saw that it could increase endurance slightly, and to our surprise, giving hydrogen sulfide precursors with NMN had a synergistic effect on running capacity.
In one experiment, the treatment of 32-month old mice (equivalent to a 90-year-old human) with both molecules doubled the endurance of these animals. H2S also helped restore NAD+ levels in the cells and improved the capacity of NAD+ to increase blood vessel density in the muscle via SIRT1, indicating that H2S signaling is upstream of SIRT1. Single or combination treatments, therefore, hold promise for the treatment and prevention of age-related disease, especially those accelerated by a decline in blood flow.
Are there any side effects?
Given the ability of NMN to promote blood vessel formation, in other words, angiogenesis, the question arises: Does it promote tumor growth? In our initial assessment of the animals above and in a mouse model of hepatocarcinoma, we reported that we did not find any evidence of increased tumor burden in the NMN-treated animals. Although more work is warranted to address this issue, some recent results generated in our lab indicate that NMN might be actually effective in reducing tumor burden in some rodent cancer models.
In addition to aged mice, NMN was equally effective in improving muscle capillary density and blood flow in animals with hind-limb ischemia. Thus, NMN and other NAD+ boosters could be useful for treating peripheral arterial disease and other post-ischemic events such as a stroke or myocardial infarction, and we are keen to explore the effects of NMN in different age-related diseases caused by reduced blood flow. This work also sets the stage for the testing of different H2S precursors in vascular diseases, either alone or in combination with NAD+ precursors. And finally, it will also be interesting to test if endothelial NAD+-H2S pathway extends mouse lifespan.
In an accompanying article published in the same issue of Cell titled “Amino Acid Restriction Triggers Angiogenesis via GCN2/ATF4 Regulation of VEGF and H2S Production,” we collaborated with James Mitchell’s team to show that dietary restriction of sulfur amino acids improved muscle capillary density in mice via production of H2S. Taken together, these two studies uncover new pathways that mediate the health benefits of diet and exercise, and highlight novel approaches to mimic them that could one day reduce frailty, treat a variety of age-related diseases, and even increase human lifespan.
These findings are described in the article entitled Impairment of an Endothelial NAD+-H2S Signaling Network Is a Reversible Cause of Vascular Aging, recently published in the journal Cell. This work was conducted by a team including Abhirup Das and David A. Sinclair from Harvard Medical School and the University of New South Wales.