Guardian of the Cell


Scientists unravel the structure, key features of a human immune-surveillance protein, setting the stage for more-precise immune therapies

protein structure
Scientists have identified the key structural and functional features of a critical immune protein in humans that guards against cancer, viral and bacterial infections.

 

The human body is built for survival. Each one of its cells is closely guarded by a set of immune proteins armed with nearly foolproof radars that detect foreign or damaged DNA.

One of the cells’ most critical sentinels is a “first responder” protein known as cGAS, which senses the presence of foreign and cancerous DNA and initiates a signaling cascade that triggers the body’s defenses.

The 2012 discovery of cGAS ignited a firestorm of scientific inquiry, resulting in more than 500 research publications, but the structure and key features of the human form of the protein continued to elude scientists.

Now, scientists at Harvard Medical School and Dana-Farber Cancer Institute have, for the first time, identified the structural and functional differences in human cGAS that set it apart from cGAS in other mammals and underlie its unique function in people.

A report on the team’s work, published July 12 in Cell, outlines the protein’s structural features that explain why and how human cGAS senses certain types of DNA, while ignoring others.

“The structure and mechanism of action of human cGAS have been critical missing pieces in immunology and cancer biology,” said senior investigator Philip Kranzusch, assistant professor of microbiology and immunobiology at Harvard Medical School and Dana-Farber Cancer Institute. “Our findings detailing the molecular makeup and function of human cGAS close this critical gap in our knowledge.” Importantly, the findings can inform the design of small-molecule drugs tailored to the unique structural features of the human protein—an advance that promises to boost the precision of cGAS-modulating drugs that are currently in development as cancer therapies. “Several promising experimental immune therapies currently in development are derived from the structure of mouse cGAS, which harbors key structural differences with human cGAS,” Kranzusch said. “Our discovery should help refine these experimental therapies and spark the design of new ones. It will pave the way toward structure-guided design of drugs that modulate the activity of this fundamental protein.”

The team’s findings explain a unique feature of the human protein—its capacity to be highly selective in detecting certain types of DNA and its propensity to get activated far more sparingly, compared with the cGAS protein in other animals.

Specifically, the research shows that human cGAS harbors mutations that make it exquisitely sensitive to long lengths of DNA but render it “blind” or “insensitive” to short DNA fragments.

“Human cGAS is a highly discriminating protein that has evolved enhanced specificity toward DNA,” said co-first author Aaron Whiteley, a postdoctoral researcher in the Department of Microbiology and Immunobiology at Harvard Medical School. “Our experiments reveal what underlies this capability.”

Location, location, location

In all mammals, cGAS works by detecting DNA that’s in the wrong place. Under normal conditions, DNA is tightly packed and protected in the cell’s nucleus—the cellular “safe”—where genetic information is stored. DNA has no business roaming freely around the cell. When DNA fragments do end up outside the nucleus and in the cell’s cytosol, the liquid that encases the cell’s organelles, it’s usually a sign that something ominous is afoot, such as damage coming from within the cell or foreign DNA from viruses or bacteria that has made its way into the cell.

The cGAS protein works by recognizing such misplaced DNA. Normally, it lies dormant in cells. But as soon as it senses the presence of DNA outside the nucleus, cGAS springs into action. It makes another chemical—a second messenger—called cGAMP, thus setting in motion a molecular chain reaction that alerts the cell to the abnormal presence of DNA. At the end of this signaling reaction, the cell either gets repaired or, if damaged beyond repair, it self-destructs.

But the health and integrity of the cell are predicated on cGAS’ ability to distinguish harmless DNA from foreign DNA or self-DNA released during cell damage and stress. “It’s a fine balancing act that keeps the immune system in equilibrium. An overactive cGAS can spark autoimmunity, or self-attack, while cGAS that fails to detect foreign DNA can lead to tumor growth and cancer development,” said co-first author Wen Zhou, a postdoctoral researcher at Harvard Medical School and Dana-Farber Cancer Institute.

The current study reveals the evolutionary changes to the protein’s structure that allow human cGAS to ignore some DNA encounters while responding to others.

A foe, an accomplice

For their work, the team turned to an unlikely collaborator—Vibrio cholerae, the bacterium that causes cholera, one of humankind’s oldest scourges.

Taking advantage of a cholera enzyme that shares similarities with cGAS, the scientists were able to recreate the function of both human and mouse cGAS in the bacterium.

Teaming up with colleagues from the lab of Harvard Medical School bacteriologist John Mekalanos, the scientists designed a chimeric, or hybrid, form of cGAS that included genetic material from both the human and mouse forms of the protein. Then they compared the ability of the hybrid cGAS to recognize DNA against both the intact mouse and intact human versions of the protein.

In a series of experiments, the scientists observed activation patterns between the different types of cGAS, progressively narrowing down the key differences that accounted for differential DNA activation among the three.

The experiments revealed that out of the 116 amino acids that differ in human and mouse cGAS, only two accounted for the altered function of human cGAS. Indeed, human cGAS was capable of recognizing long DNA with great precision but it ignored short DNA fragments. The mouse version of the protein, by contrast, did not differentiate between long and short DNA fragments

“These two tiny amino acids make a world of difference,” Whiteley said. “They allow the human protein to be highly selective and respond only to long DNA, while ignoring short DNA, essentially rendering the human protein more tolerant of DNA presence in the cytosol of the cell.”

Plotting the genetic divergence on an evolutionary timescale, the scientists determined that the human and mouse cGAS genes parted ways sometime between 10 million and 15 million years ago.

The two amino acids responsible for sensing long DNA and tolerating short DNA are found solely in humans and nonhuman primates, such as gorillas, chimps and bonobos. The scientists hypothesize that the ability to ignore short DNA but recognize long DNA must have conferred some evolutionary benefits. “It could be a way to guard against an overactive immune system and chronic inflammation,” Kranzusch said. “Or it could be that the risk of certain human diseases is lowered by not recognizing short DNA.”

In a final set of experiments, the team determined the atomic structure of the human cGAS in its active form as it binds to DNA. To do so, they used a visualization technique known as X-ray crystallography, which reveals the molecular architecture of protein crystals based on a pattern of scattered X-ray beams.

Profiling the structure of cGAS “in action” revealed the precise molecular variations that allowed it to selectively bind to long DNA, while ignoring short DNA.

“Understanding what makes the structure and function of human cGAS different from those in other species was the missing piece,” Kranzusch said. “Now that we have it, we can really start designing drugs that work in humans, rather than mice.”

Other investigators included Carina de Oliveira Mann, Benjamin Morehouse, Radosław Nowak, Eric Fischer, and Nathanael Gray. The work was supported by the Claudia Adams Barr Program for Innovative Cancer Research, by the Richard and Susan Smith Family Foundation, by the Charles H. Hood Foundation, by a Cancer Research Institute CLIP Grant, by the National Institute of Allergy and Infectious Diseases grant AI-01845, by National Cancer Institute grant R01CA214608, by the Jane Coffin Childs Memorial Fund for Medical Research, by a Cancer Research Institute Eugene V. Weissman Fellow award, and by a National Institutes of Health T32 grant 5T32CA207021-02.

Relevant Disclosures: The Dana-Farber Cancer Institute and Harvard Medical School have patents pending for human cGAS technologies, on which the authors are inventors.

Harvard Medical School Harvard Medical School (http://hms.harvard.edu) has more than 11,000 faculty working in 10 academic departments located at the School’s Boston campus or in hospital-based clinical departments at 15 Harvard-affiliated teaching hospitals and research institutes: Beth Israel Deaconess Medical Center, Boston Children’s Hospital, Brigham and Women’s Hospital, Cambridge Health Alliance, Dana-Farber Cancer Institute, Harvard Pilgrim Health Care Institute, Hebrew SeniorLife, Joslin Diabetes Center, Judge Baker Children’s Center, Massachusetts Eye and Ear/Schepens Eye Research Institute, Massachusetts General Hospital, McLean Hospital, Mount Auburn Hospital, Spaulding Rehabilitation Network and VA Boston Healthcare System.

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Exercise is Statistically as Good as Pharmaceuticals to Treat Diseases.



A recent study 
published in BMJ found that physical activity is as effective as drug interventions for patients with existing cardiovascular diseases and other chronic conditions such as diabetes.

In the few conditions where the life-saving benefits of exercise have been studied, physical activity was often found to be as effective as drugs at reducing the risk of death, according to the first study to aggregate and assess the comparative benefits of drugs and exercise for reducing mortality in a wide range of illnesses.

 

“We were surprised to find that exercise seems to have such powerful life-saving effects for people with serious chronic conditions,” said Huseyin Naci, an HMS visiting fellow in population medicine at the Harvard Pilgrim Health Care Institute, and a graduate student at the London School of Economics. “It was also surprising to find that so little is known about the potential benefits of physical activity for health in so many other illnesses.”

Regular physical activity has been shown to reduce the morbidity and mortality from many chronic diseases. Millions suffer from chronic illnesses that can be prevented or improved through regular physical activity. This include heart disease, heart attack, type 2 diabetes, obesity, colon cancer, hip fractures, stroke and high blood pressure. On average, people who are physically active outlive those who are inactive.

Despite the well-known benefits of physical activity, most adults and many children lead a relatively sedentary lifestyle and are not active enough to achieve these health benefits. A sedentary lifestyle is defined as engaging in no leisure-time physical activity (exercises, sports, physically active hobbies) in a two-week period.

Study Details

In addition to providing guidance for patients and clinicians about the importance of discussing the potential benefits of exercise, the researchers highlighted the importance of continuing to research the value of exercise for health.

The researchers argue that more trials comparing the effectiveness of exercise and drugs are urgently needed to help doctors and patients make the best treatment decisions. In the meantime, they say exercise “should be considered as a viable alternative to, or alongside, drug therapy.”

“We’re not saying people who have had a stroke should go off their medication and head to the gym,” Naci said, “but having a conversation with their physician about incorporating exercise into their treatment might be beneficial in many cases.”

Preventable illness makes up approximately 80% percent of the burden of illness and 90% of all healthcare costs. Preventable illness accounts for eight of the nine leading categories of death.

In the United States, 80 percent of people 18 and older failed to meet the recommended levels of aerobic and muscle-strengthening physical activity in 2011, according to the CDC. What’s more, the average number of retail prescriptions per capita for calendar year 2011 was 12.1, according to the Kaiser Family Foundation.

For people with chronic ailments, exercise used to be viewed as asking for trouble. However, current evidence suggests that in both health and disease, the overall prognosis is better for the exerciser than for the sedentary. For example, a recent study showed that intensive workouts can not only slow the progress of coronary disease, but actually restore lost coronary function when the disease is still stable.

“We’ve yet to find a disease state where exercise isn’t helpful.” said Miriam Nelson, Ph.D, from Tufts University.

For the current study, the researchers analyzed the results of 305 randomized controlled trials involving 339,274 individuals and found no statistically detectable differences between exercise and drug interventions for secondary prevention of heart disease and prevention of diabetes.

Exercise Often More Effective Than Drugs

Among stroke patients, exercise was more effective than drug treatment.

The authors point out that the amount of trial evidence on the mortality benefits of exercise is considerably smaller than that on the benefits of drugs, and this may have had an impact on their results. Of the nearly 340,000 cases analyzed, only 15,000 patients had had exercise-based interventions.

For chronically ill individuals, the psychological as well as physical benefits of exercise can be profound. Even ten minutes of light exercise a day, can help most chronically ill patients feel more vibrant, energetic and alert.

“Exercise is empowering and energizing, and it increases your sense of control over the situation. You’re never too sick or too old to get started exercising,” stated Bess Marcus, Ph.D, of Brown’s University.

The researchers argue in the paper that this “blind spot” in available scientific evidence “prevents prescribers and their patients from understanding the clinical circumstances where drugs might provide only modest improvement but exercise could yield more profound or sustainable gains in health.”

Participation in regular physical activity– at least 30 minutes of moderate activity on at least five days per week, or 20 minutes of vigorous physical activity at least three times per week–is critical to sustaining good health. Youth should strive for at least one hour of exercise a day. Regular physical activity has beneficial effects on most (if not all) organ systems, and consequently it helps to prevent a broad range of health problems and diseases. People of all ages, both male and female, derive substantial health benefits from physical activity.

Regular physical activity reduces the risk of developing or dying from some of the leading causes of illness in the United States. Regular physical activity improves health in the following ways:

·         Reduces the risk of dying prematurely from heart disease and other conditions;

·         Reduces the risk of developing diabetes;

·         Reduces the risk of developing high blood pressure;

·         Reduces blood pressure in people who already have high blood pressure;

·         Reduces the risk of developing colon and breast cancer5;

·         Helps to maintain a healthy weight;

·         Helps build and maintain healthy bones, muscles, and joints;

·         Helps older adults to become stronger and better able to move about without falling;

·         Reduces feelings of depression and anxiety; and

·         Promotes psychological well-being. 


Exercise v.s. Diet v.s. Drugs

Exercise v.s. diet v.s. drugs is often the debate that many health professionals evaluate. By examining each disease through clinical trials, we can better determine the efficacy of both exercise and diet in the treatment of many common ailments. Diet, for example, is the cornerstone of diabetes care, but if diet is combined with exercise, diabetics dramatically improve their condition by more than 45% than with diet alone.

CONDITION

TYPE OF EXERCISE

MAXIMUM IMPROVEMENT WITH EXERCISE

MAXIMUM IMPROVEMENT WITH DRUGS

MAXIMUM IMPROVEMENT WITH DIET

High Blood Pressure

Aerobic

15%

9%

11%

Diabetes

Strength training, flexibility, low-impact aerobic

52%

5%

38%

Stroke

Strength training, flexibility, low-impact aerobic

28%

7%

Heart Disease

Aerobic

33%

11%

26%

High LDL cholesterol

13%

20%

Low HDL cholesterol

Aerobic

15%

High Blood Sugar

Aerobic

15%

11%

30%

Arthritis Pain

Strength training, flexibility, low-impact aerobic

40%

12%

Low Bone Density

Weight bearing

3%

2%


Regular physical activity is associated with lower mortality rates for both older and younger adults. Even those who are moderately active on a regular basis have lower mortality rates than those who are least active. Regular physical activity leads to cardiovascular fitness, which decreases the risk of cardiovascular disease mortality in general and coronary artery disease mortality in particular. High blood pressure is a major underlying cause of cardiovascular complications and mortality. Regular physical activity can prevent or delay the development of high blood pressure, and reduces blood pressure in persons with hypertension.

Despite this uncertainty, the authors claim that based on the available data physical activity is potentially as effective as many drug interventions and more trials to address the disparity between exercise and drug-based treatment evidence are needed.

 

“What we don’t know about the benefits of exercise may be hurting us,” Naci said.

Sources:
bmj.com
nlm.nih.gov
preventdisease.com
medicalnewstoday.com