A team of Harvard Medical School scientists led by Dr. David Sinclair has successfully reversed the aging of blood vessels in a new study, and human trials are currently underway. The therapy restores blood vessel growth and muscle vitality and boosts exercise endurance in aging animals.
Age-related muscle wastage is linked to vascular aging
The new study published in Cell identified the cellular process behind aging of the vascular system and its effect on the muscular system [1]. The research team has also reversed the vascular aging process, at least in mice.
The study results highlight the disruption to the crosstalk that normally happens between the muscles and blood vessels that allow them both to remain healthy. They used the precursors of two molecules found naturally in the body to reverse muscle loss and blood vessel aging in mice.
During the aging process, the smallest blood vessels begin to shrink and die, leading to reduced blood flow and less oxygen reaching organs and tissues. Vascular aging leads to a myriad of conditions, including vascular diseases, neurological conditions, muscle loss, impaired wound healing, and frailty.
The blood vessels are lined with endothelial cells, which are vital for the growth and health o
f blood vessels that supply oxygen and nutrients to the surrounding tissues and connected organs. As the endothelial cells age, the blood vessels begin to waste away, new blood vessels fail to form, and the blood supply to tissues steadily begins to fall. The loss of blood supply to organs and tissues causes the build-up of toxins, low oxygen levels, and the loss of critical nutrients.
This is particularly apparent in muscle tissue, as it is heavily vascularized and relies on a plentiful blood supply to work. This study strongly suggests that this is why muscles waste away and weaken with age; this condition, known as sarcopenia, leads to the characteristic frailty typical of old age. Exercise can offset this loss of muscle somewhat, but even that becomes increasingly ineffective at stopping the progression of sarcopenia.
Dr. Sinclair and his team wanted to know what causes blood flow to decline and why exercise loses its effectiveness against sarcopenia. Perhaps most importantly, they wanted to know if this process could be reversed.
Unraveling vascular aging and muscle wastage
They conducted a number of experiments and discovered that blood flow declines as the result of endothelial cells losing a protein known as SIRT1. Some readers may recall that the SIRT1 protein has been shown to extend lifespan in yeast and mice, and it plays an important role in metabolism.
The research team showed that this loss of SIRT1 is caused by the decline of NAD+, a key regulator of protein interactions in the cell that is also instrumental in DNA repair. NAD+ was discovered in 1906 by Harden and Young, and you can learn more about this molecule and its history here. Previous research by Dr. Sinclair and others shows that NAD+ levels also decline with age and that NAD+ also increases the activity of SIRT1.
The study results show that NAD+ and SIRT1 facilitate crosstalk between the endothelial cells in the blood vessel walls and the muscle cells. The experiment shows that in young mice, SIRT1 signaling activates, which leads to the creation of new blood capillaries, the smallest blood vessels that supply oxygen and nutrients to the tissues. However, the study strongly suggests that as NAD+ and SIRT1 levels decline during aging, blood flow decreases, which leaves muscle tissue starved of oxygen and nutrients, thus leading to sarcopenia.
The researchers tested this by knocking out the SIRT1 gene in the endothelial cells of young mice; they found that these mice had reduced capillary density and less capillaries when compared to healthy control mice with functioning SIRT1 genes. In exercise tests, the SIRT1 knockout mice had poor exercise endurance and were only able to run half the distance of the control mice.
In order to determine the role of SIRT1 in exercise-induced blood vessel growth, the research team studied how SIRT1 knockout mice responded to exercise. They placed the mice on a month-long exercise program and then looked at their hind leg muscles. Compared to the control mice, the SIRT1 knockout mice showed a significantly lower ability to form new blood vessels after exercise.
It has long been known that exercise causes the formation of blood vessels in response to growth factor signals released by muscles put under a heavy exercise workload. However, these results showed that SIRT1 appears to be the primary messenger that relays growth factor signaling from muscles to blood vessels, which signals the formation of new blood capillaries. The data showed that endothelial cells lacking SIRT1 were desensitized to growth factor signaling from muscles and did not respond to it. This explains why age-related loss of SIRT1 leads to muscle wastage and the decline of blood vessels.
Increasing SIRT1 levels to prevent age-related muscle wasting
Given these results, the researchers wondered if boosting levels of SIRT1 could prevent sarcopenia by stimulating blood vessel growth. Given that a decline of NAD+ levels reduced SIRT1 activity, thus disrupting the ability of mice to grow new blood vessels, they focused on increasing NAD+ levels to possibly reverse this.
In order to do this, they used the NAD+ precursor NMN, a compound that the body converts into NAD+ as part of the NAD salvaging pathway. You can see below how NMN and other precursors relate to the creation of NAD+; as you may note, NMN is only one step away from conversion into NAD+, making it more efficient compared to nicotinamide riboside (NR), which is also featured in the diagram.
They first tested endothelial cells from humans and mice and exposed them to NMN; the results were increased growth capacity and a reduced rate of cell death.
The next step was to give NMN treatment to a group of 20-month-old mice (about the same as human 70-year-olds) for a period of two months. They found that NMN restored the number and density of blood capillaries to levels seen in young mice. The blood flow to muscles was increased and was significantly higher than that observed in control mice of the same age.
The most obvious change was in the ability of the aged mice to exercise. There was an increase in exercise capacity of between 56 and 80 percent compared to the untreated control mice.
A second compound to boost SIRT1 activity
The researchers were keen to see if the effects of NMN could be increased even more, so they added a second compound to the therapy. They added sodium hydrosulfide (NaHS), which is also known to increase SIRT1 activity.
They tested the combo in 32-month-old mice (similar to 90-year-old humans) for four weeks and found that the mice were, on average, able to run around for double the amount of time as untreated mice. The mice just given NMN ran 1.6 times farther than untreated mice. This shows that the combination has a significant effect on exercise endurance.
Finally, the NMN treatment did not increase the density of blood vessels or the exercise capacity in young sedentary mice, but it did so in young mice that had regular exercise for at least a month.
Together, these results show the vital role that NAD+ and SIRT1 play in the crosstalk between blood vessels and muscle tissue and also why exercise loses its effectiveness as we age.
The next step is to replicate these findings and then move towards the development of NMN-based drugs that can mimic the effects of exercise by increasing blood flow and oxygenation to muscle and other tissues by encouraging new blood vessel formation. Such drugs have potential for combating sarcopenia and might even help organs that have experienced damage due to loss of blood supply, which is common in heart attacks and strokes.
Despite all this positive news, a word of caution should be offered here. Encouraging the formation of new blood vessels should be done with caution, as it may also fuel the growth of tumors. If you already have a tumor, using this therapy could provide that tumor with additional blood supply and nutrients to fuel its growth. That said, Dr. Sinclair was keen to point out that the experiments in this study provided no evidence that use of NMN encouraged tumor development in the treated animals.
Conclusion
While there has been considerable hype surrounding NAD+ precursors in the last few years, the results presented by the Sinclair lab make for compelling reading. If we can reverse the aging of the vascular system with compounds that increase NAD+ and stimulate SIRT1, then we can potentially develop related therapies in the near future to combat age-related decline.
With human trials currently underway for NMN, we should hopefully have some data soon, and we can then see if the results in mice translate to humans; there is plenty of reason to be optimistic that they will, because our species share NAD+ biology.
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Literature
[1] Sinclair D. Bonkowski, M. Impairment of an Endothelial NAD+-H2S Signaling Network Is a Reversible Cause of Vascular Aging (2018) doi.org/10.1016/j.cell.2018.02.008
