Now, research led by scientists at Harvard Medical School reveals a critical step in a molecular chain of events that allows cells to mend their broken DNA.
The findings, published March 24 in Science, offer a critical insight into how and why the body’s ability to fix DNA dwindles over time and point to a previously unknown role for the signaling molecule NAD as a key regulator of protein-to-protein interactions in DNA repair. NAD, identified a century ago, is already known for its role as a controller of cell-damaging oxidation.
Additionally, experiments conducted in mice show that treatment with the NAD precursor NMN mitigates age-related DNA damage and wards off DNA damage from radiation exposure.
The scientists caution that the effects of many therapeutic substances are often profoundly different in mice and humans owing to critical differences in biology. However, if affirmed in further animal studies and in humans, the findings can help pave the way to therapies that prevent DNA damage associated with aging and with cancer treatments that involve radiation exposure and some types of chemotherapy, which along with killing tumors can cause considerable DNA damage in healthy cells. Human trials with NMN are expected to begin within six months, the researchers said.
“Our results unveil a key mechanism in cellular degeneration and aging but beyond that they point to a therapeutic avenue to halt and reverse age-related and radiation-induced DNA damage,” said senior author David Sinclair, professor in the Department of Genetics at HMS and professor at the University of New South Wales School of Medicine in Sydney, Australia.
A previous study led by Sinclair showed that NMN reversed muscle aging in mice.
A plot with many characters
The investigators started by looking at a cast of proteins and molecules suspected to play a part in the cellular aging process. Some of them were well-known characters, others more enigmatic figures.
The researchers already knew that NAD, which declines steadily with age, boosts the activity of the SIRT1 protein, which delays aging and extends life in yeast, flies and mice. Both SIRT1 and PARP1, a protein known to control DNA repair, consume NAD in their work.
Another protein DBC1, one of the most abundant proteins in humans and found across life forms from bacteria to plants and animals, was a far murkier presence. Because DBC1 was previously shown to inhibit vitality-boosting SIRT1, the researchers suspected DBC1 may also somehow interact with PARP1, given the similar roles PARP1 and SIRT1 play.
“We thought if there is a connection between SIRT1 and DBC1, on one hand, and between SIRT1 and PARP1 on the other, then maybe PARP1 and DBC1 were also engaged in some sort of intracellular game,” said Jun Li, first author on the study and a research fellow in the Department of Genetics at HMS.
To get a better sense of the chemical relationship among the three proteins, the scientists measured the molecular markers of protein-to-protein interaction inside human kidney cells. DBC1 and PARP1 bound powerfully to each other. However, when NAD levels increased, that bond was disrupted. The more NAD present inside cells, the fewer molecular bonds PARP1 and DBC1 could form. When researchers inhibited NAD, the number of PARP1-DBC1 bonds went up. In other words, when NAD is plentiful, it prevents DBC1 from binding to PARP1 and meddling with its ability to mend damaged DNA.
What this suggests, the researchers said, is that as NAD declines with age, fewer and fewer NAD molecules are around to stop the harmful interaction between DBC1 and PARP1. The result: DNA breaks go unrepaired and, as these breaks accumulate over time, precipitate cell damage, cell mutations, cell death and loss of organ function.
Next, to understand how exactly NAD prevents DBC1 from binding to PARP1, the team homed in on a region of DBC1 known as NHD, a pocket-like structure found in some 80,000 proteins across life forms and species whose function has eluded scientists. The team’s experiments showed that NHD is an NAD binding site and that in DBC1, NAD blocks this specific region to prevent DBC1 from locking in with PARP1 and interfering with DNA repair.
And, Sinclair added, since NHD is so common across species, the finding suggests that by binding to it, NAD may play a similar role averting harmful protein interactions across many species to control DNA repair and other cell survival processes.
To determine how the proteins interacted beyond the lab dish and in living organisms, the researchers treated young and old mice with the NAD precursor NMN, which makes up half of an NAD molecule. NAD is too large to cross the cell membrane, but NMN can easily slip across it. Once inside the cell, NMN binds to another NMN molecule to form NAD.
As expected, old mice had lower levels of NAD in their livers, lower levels of PARP1 and a greater number of PARP1 with DBC1 stuck to their backs.
However, after receiving NMN with their drinking water for a week, old mice showed marked differences both in NAD levels and PARP1 activity. NAD levels in the livers of old mice shot up to levels similar to those seen in younger mice. The cells of mice treated with NMN also showed increased PARP1 activity and fewer PARP1 and DBC1 molecules binding together. The animals also showed a decline in molecular markers that signal DNA damage.
In a final step, scientists exposed mice to DNA-damaging radiation. Cells of animals pre-treated with NMN showed lower levels of DNA damage. Such mice also didn’t exhibit the typical radiation-induced aberrations in blood counts, such as altered white cell counts and changes in lymphocyte and hemoglobin levels. The protective effect was seen even in mice treated with NMN after radiation exposure.
Taken together, the results shed light on the mechanism behind cellular demise induced by DNA damage. They also suggest that restoring NAD levels by NMN treatment should be explored further as a possible therapy to avert the unwanted side effects of environmental radiation, as well as radiation exposure from cancer treatments.
In December 2016, a collaborative project between the Sinclair Lab and Liberty Biosecurity became a national winner in NASA’s iTech competition for their concept of using NAD-boosting molecules as a potential treatment in cosmic radiation exposure during space missions.
Co-authors on the research included Michael Bonkowski, Basil Hubbard, Alvin Ling, Luis Rajman, Sebastian Moniot, Clemens Steegborn, Dapeng Zhang, L. Aravind, Bo Qin, Zhenkun Lou, and Vera Gorbunova.
The work was funded by the Glenn Foundation for Medical Research, the American Federation for Aging Research, Edward Schulak, grants from the National Institute on Aging and the National Institutes of Health, by the National Library of Medicine/NIH intramural program, the National Cancer Institute, and by Deutsche Forschungsgemeinschaft.
Aging increases cell-to-cell transcriptional variability upon immune stimulation
Scientists have resolved a key question in aging research by showing how mouse immune cells of different ages respond to stimulation. Study demonstrates weaker response of older cells is due to their coordination breaking down, making their response to immune stimulation more variable. Single-cell sequencing technology allows scientists to profile individual cells independently to view cellular activity in high resolution.
Aging is characterized by progressive loss of physiological and cellular functions, but the molecular basis of this decline remains unclear. We explored how aging affects transcriptional dynamics using single-cell RNA sequencing of unstimulated and stimulated naïve and effector memory CD4+ T cells from young and old mice from two divergent species. In young animals, immunological activation drives a conserved transcriptomic switch, resulting in tightly controlled gene expression characterized by a strong upregulation of a core activation program, coupled with a decrease in cell-to-cell variability. Aging perturbed the activation of this core program and increased expression heterogeneity across populations of cells in both species. These discoveries suggest that increased cell-to-cell transcriptional variability will be a hallmark feature of aging across most, if not all, mammalian tissues.
The research, published August 6 in the journal Cell Reports, focused on an organ called the thymus, which produces T lymphocytes, critical immune cells that must be continuously replenished to respond to new infections.
“The thymus begins to atrophy rapidly in very early adulthood, simultaneously losing its function,” said TSRI Professor Howard Petrie. “This new study shows for the first time a mechanism for the long-suspected connection between normal immune function and antioxidants.”
Scientists have been hampered in their efforts to develop specific immune therapies for the elderly by a lack of knowledge of the underlying mechanisms of this process.
To explore these mechanisms, Dr. Petrie and his team developed a computational approach for analyzing the activity of genes in two major thymic cell types — stromal cells and lymphoid cells — in mouse tissues, which are similar to human tissues in terms of function and age-related atrophy. The team found that stromal cells were specifically deficient in an antioxidant enzyme called catalase, which resulted in elevated levels of the reactive oxygen by-products of metabolism and, subsequently, accelerated metabolic damage.
To confirm the central role of catalase, the scientists increased levels of this enzyme in genetically altered animal models, resulting in preservation of thymus size for a much longer period. In addition, animals that were given two common dietary antioxidants, including vitamin C, were also protected from the effects of aging on the thymus.
Taken together, the findings provide support for the “free-radical theory” of aging, which proposes that reactive oxygen species such as hydrogen peroxide, produced during normal metabolism, cause cellular damage that contributes to aging and age-related diseases.
While other studies have suggested that sex hormones, particularly androgens such as testosterone, play a major role in the aging process, it fails to answer the key question — why does the thymus atrophy so much more rapidly than other body tissues?
“There’s no question that the thymus is remarkably responsive to androgens,” Dr. Petrie noted, “but our study shows that the fundamental mechanism of aging in the thymus, namely accumulated metabolic damage, is the same as in other body tissues. However, the process is accelerated in the thymus by a deficiency in the essential protective effects of catalase, which is found at higher levels in almost all other body tissues.”
A less strenuous form of exercise known as whole-body vibration (WBV) can mimic the muscle and bone health benefits of regular exercise in mice, according to a new study. WBV consists of a person sitting, standing or lying on a machine with a vibrating platform. When the machine vibrates, it transmits energy to the body, and muscles contract and relax multiple times during each second.
A less strenuous form of exercise known as whole-body vibration (WBV) can mimic the muscle and bone health benefits of regular exercise in mice, according to a new study published in the Endocrine Society’s journal Endocrinology.
WBV consists of a person sitting, standing or lying on a machine with a vibrating platform. When the machine vibrates, it transmits energy to the body, and muscles contract and relax multiple times during each second.
Many people find it challenging to exercise regularly and that is contributing to the obesity and diabetes epidemics. These disorders can increase the risk of bone fractures. Physical activity can help to decrease this risk and reduce the negative metabolic effects of each condition.
“Our study is the first to show that whole-body vibration may be just as effective as exercise at combatting some of the negative consequences of obesity and diabetes,” said the study’s first author, Meghan E. McGee-Lawrence, Ph.D., of Augusta University in Augusta, Ga. “While WBV did not fully address the defects in bone mass of the obese mice in our study, it did increase global bone formation, suggesting longer-term treatments could hold promise for preventing bone loss as well.”
To conduct the study, researchers examined two groups of 5-week-old male mice. One group consisted of normal mice, while the other group was genetically unresponsive to the hormone leptin, which promotes feelings of fullness after eating. Mice from each group were assigned to sedentary, WBV or treadmill exercise conditions.
After a week-long period to grow used to the exercise equipment, the groups of mice began a 12-week exercise program. The mice in the WBV group underwent 20 minutes of WBV at a frequency of 32 Hz with 0.5g acceleration each day. Mice in the treadmill group walked for 45 minutes daily at a slight incline. For comparison, the third group did not exercise. Mice were weighed weekly during the study.
The genetically obese and diabetic mice showed similar metabolic benefits from both WBV and exercising on the treadmill. Obese mice gained less weight after exercise or WBV than obese mice in the sedentary group, although they remained heavier than normal mice. Exercise and WBV also enhanced muscle mass and insulin sensitivity in the genetically obese mice. Although there were no significant effects in the young healthy mice, the low-intensity exercise and WBV protocols were designed for successful completion by obese mice. These findings suggest that WBV may be a useful supplemental therapy to combat metabolic dysfunction in individuals with morbid obesity.