Deciphering Sarcopenia

As a young hepatologist in the mid-1990s caring for patients with end-stage liver disease, Srinivasan Dasarathy, MD, observed firsthand the malnutrition and loss of skeletal muscle mass that typified these patients and the complications that resulted.

At the time, cirrhosis was thought to cause malnutrition. Surgeons were reluctant to perform liver transplants on patients they believed were too malnourished to survive. And in patients who did undergo transplantation, although a new liver resolved the clinical and metabolic abnormalities of cirrhosis, it did not appear to improve patients’ body composition and restore muscle mass. Most patients’ muscle loss actually worsened after transplantation, and mortality was higher in patients with ongoing muscle loss compared with patients who showed an increase in muscle mass after transplant.

What was the connection between the liver and skeletal muscles?, Dr. Dasarathy wondered. Why would chronic liver disease (and other chronic diseases, too) decrease muscle mass, and why wouldn’t transplantation reverse the condition? What were the underlying metabolic processes that caused the decline? If identified, could they be treated or prevented?

There were no definitive answers to be found in the medical literature. Adult malnutrition was a broad, catchall term without a consistent definition, making it difficult to compare results in studies examining nutritional status in chronic liver disease and post-transplant outcomes. Sarcopenia — a word coined in 1988 by Tufts University clinical nutritionist Irwin Rosenberg, MD, to describe loss of skeletal muscle mass and function — seemed to be applicable to what Dr. Dasarathy and others observed in cirrhotic patients, but Dr. Rosenberg insisted it was a phenomenon exclusive to aging, not chronic disease. The tools to measure muscle mass were imprecise, with no agreed-upon reference standard for diagnosing malnutrition in cirrhosis. And finally, muscle physiologists and biologists who might have insights on the mechanisms of cirrhotic muscle wasting were focused instead on muscle growth and development, or on dystrophic diseases and aging.

In sum, then, despite muscle loss being the most frequent complication of cirrhosis, affecting patients even after transplant, it was the least studied. There was no basic understanding of its etiology, and no effective treatment options.

Dr. Dasarathy realized that only a systematic, long-term investigative effort involving both basic and translational research could attempt to answer the fundamental questions he was raising. He was trained as a clinician, not a researcher. But he decided to take up the challenge.

“It was a long, painful and lonely journey,” he admits. Progress at first was incremental. Skeptics abounded. Funding was hard to obtain. Colleagues had to teach him how to culture muscle cells and write persuasive grant proposals. He still keeps a pillow in his office, a reminder of the many nights he slept on the floor there so he could monitor ongoing experiments. His wife brought meals and changes of clothes so he could keep working.

More than two decades later, Dr. Dasarathy’s efforts have borne considerable fruit. He is an internationally recognized authority on sarcopenia in cirrhosis and other chronic diseases. His work is supported by five National Institutes of Health R01 grants. Using advanced multiomics analyses and genome association studies, cellular and animal models and human subjects, Dr. Dasarathy and his team of researchers have made groundbreaking discoveries illuminating the metabolic and molecular perturbations that occur in cirrhotic sarcopenia, with particular insights on skeletal muscle responses to various cellular stresses.

Those findings are laying the foundation for novel, targeted therapies to treat sarcopenia in chronic disease, potentially utilizing exercise, nutritional supplementation, drugs to stimulate protein synthesis and other promising methods.

“Dr. Dasarathy has been a pioneer in the field of sarcopenia research,” says Miguel Regueiro, MD, Chair of Cleveland Clinic’s Digestive Disease and Surgery Institute. “His tireless efforts and persistence to discover the mechanistic reasons for muscle-wasting in patients with cirrhosis has defined him as the preeminent scientist in the field. His humble persona, sharp focus on finding the answers and dedication to improving patient outcomes have made him the brilliant physician-researcher he is today.”

Starting from scratch

At the outset of Dr. Dasarathy’s sarcopenia research, investigators had established that loss of muscle mass, the major component of malnutrition, was a common element of cirrhosis, occurring in as many as 70% of patients. It also was documented that cirrhotic muscle depletion was a major predictor of poor clinical outcomes, including reduced survival, decreased quality of life and development of other complications, even after liver transplantation.

Despite this recognition of the clinical relevance of muscle wasting to cirrhosis (even if the condition wasn’t overtly called sarcopenia at the time), there was no understanding of the mechanisms involved, and no evidence-based explanation for why less muscle mass should lead to higher morbidity and mortality. It was obvious to Dr. Dasarathy that, without a clear picture of the molecular and metabolic underpinnings of sarcopenia, there was no logical basis upon which to develop successful targeted therapies.

But how to proceed? Progress necessitated developing a working knowledge of diverse and complex fields such as aging, molecular signaling, nutraceuticals, hepatology, transplant immunology and clinical nutrition, among others. It also required that Dr. Dasarathy make a shift in mindset, from clinician to researcher, in how he dealt with unknowns and unforeseen outcomes in the lab.

In a clinical setting, a hypothesis that produces a positive result — finding a medication that resolves a patient’s symptoms, for example — is a desirable conclusion to a problem. In research, a validated hypothesis can be a dead end, whereas a negative result is a catalyst for further inquiry and new hypotheses that may lead to greater insights.

With Dr. Dasarathy’s clinical background, negative or unexpected results in his early sarcopenia research left him feeling defeated. A colleague, Cleveland Clinic molecular cardiology researcher Sathyamangla Prasad, PhD, helped him change his attitude. The advice was a major turning point in Dr. Dasarathy’s scientific career.

“He would say ‘This is great!’” Dr. Dasarathy recalls. “I would say, ‘How is this great? The results are the opposite of what I want.’ He would say, ‘This is what you write your grant on. You find out why your rationale was incorrect. You figure it out.’ He taught me how to not worry about what the observations are, but to spend my time on thinking how to interpret them.”

Dr. Dasarathy began to methodically investigate the metabolic and molecular bases of cirrhotic sarcopenia. “We worked systematically,” he says. “We used very complex bioinformatics approaches. Then we did cellular studies. Then we did organelles studies and metabolic studies, then animal studies. And we used those data to perform human studies.”

He credits Arthur McCullough, MD, the Digestive Disease & Surgery Institute’s former Director of Research and, more recently, Dr. Regueiro, the institute’s Chair, for championing his research and career path, especially when external funding was scarce. “I was writing grants and the grants would consistently not be discussed” by reviewers. “I was pretty demoralized. And Art McCullough kept saying, ‘I believe in you.’ Dr. Regueiro also is an amazing man. I’m fortunate that I have had incredible institutional support.”

Probing sarcopenia’s underpinnings

Dr. Dasarathy came up with the term liver-muscle axis to describe the metabolic partnership and feedback loop between the liver and skeletal muscles that he helped elucidate.

Skeletal muscle is the body’s major protein storehouse. Muscle mass is maintained by a careful balance between protein synthesis, protein breakdown and regeneration, the latter regulated by muscle-committed stem cells. Cirrhosis is a state of accelerated starvation and hypermetabolism. Cirrhotic patients’ reduced dietary intake, decreased protein consumption and impaired digestion and nutrient absorption contribute to depleted muscle mass through metabolic changes and alterations in protein turnover and energy disposal.

Dr. Dasarathy and his team made fundamental discoveries involving how skeletal muscles respond to various cellular stresses, mediated by hyperammonemia, hypoxia and ethanol exposure.

“We discovered that the rate of protein synthesis and specific type of protein breakdowns are important in contributing to muscle loss,” he says. “We discovered that the reason cirrhotic patients get fatigued and are not able to function is because their muscle proteins are modified. They don’t make energy or ATP. They don’t slide as well and therefore they’re not able to generate the same amount of contractile force. We then used a complementary approach to examine what happens to the chromosomes in the muscle cell, how they are translated into messenger RNA, how proteins are made from the messenger RNA, how those proteins are modified and how they change the metabolic functions of the cell. This multi-layered approach has resulted in our identifying not only mitochondrial dysfunction, but also that many chronic diseases are really accelerated aging. The muscle responses of a 40-year-old cirrhotic and a 60- or 70-year-old noncirrhotic are functionally similar. These are potentially modifiable senescence factors.”

Clinical translational studies involving human patients led Dr. Dasarathy and his team to develop a number of strategies to potentially reverse sarcopenia using supplemental L-leucine, hyaluronan35, hydroxymethylbutyrate and ammonia-lowering regimens.

Exploring exercise as a therapy

Exercise to combat sarcopenic muscle-wasting is another promising area that Dr. Dasarathy and his colleagues are investigating. Cirrhosis is a state of anabolic resistance. Although exercise is an anabolic stimulant that can increase muscle mass and improve contractility in healthy people, there is little research assessing its impact in cirrhosis and whether it can overcome anabolic resistance.

“Our current work focuses on the mechanistic basis of responses to exercise in patients with chronic disease, including studies on actomyosin complex responses and the causes of anabolic resistance in these patients,” Dr. Dasarathy says. “We are also using systems biology and machine-learning approaches to examine the differences between responders and nonresponders to exercise. If we know what the difference within a responder or non-responder is — which organelle, which specific function is defective — we can target those functions. So even if exercise itself is not effective, any dysfunctions or defects we identify may be beneficial therapeutic targets. Are there novel signatures that would indicate a cirrhotic patient is not likely to respond to exercise? Or that exercise alone is not enough? Then we would know we need to do something else for them. And this is the most interesting part: Are there metabolic pathways or molecules that can be targeted to improve exercise responses?”

Dr. Dasarathy is developing a novel clinical research program focused on exercise biology in chronic disease to address those questions.

Wider impacts

Although his research has largely focused on sarcopenia in cirrhosis, Dr. Dasarathy believes his findings have broader relevance.

“What we are doing is not applicable to just liver disease,” he says. “It’s applicable to diseases in general that have sarcopenia as a component and almost all chronic diseases have sarcopenia. Using this extraordinarily powerful combination of approaches, we can translate basic understanding of biology into novel means to improve health and outcomes.”

A recent insight is that older patients with chronic diseases have both primary (aging-associated) sarcopenia and secondary (chronic disease-associated) sarcopenia, which Dr. Dasarathy and his team dubbed compound sarcopenia. Based on an analysis of hospitalized general medical population patients and patients with cirrhosis, the researchers found that compound sarcopenia in older cirrhotic patients is associated with higher inpatient mortality, increased lengths of stay and higher hospitalization costs. The results support the concept of an accelerated aging phenotype in patients with cirrhosis, and indicate that sarcopenia of aging exacerbates the disease-related sarcopenia of cirrhosis.

Looking back at his quarter-century of sarcopenia research, Dr. Dasarathy attributes his success to persistence. “Everybody has to start somewhere,” he says, “and where nothing exists, you have to start from nothing. I’m like a cork. You can push the cork down, but you can’t keep it down.”