DR JUSTIN PERCIVAL
Dr NO
The exact reasons behind why exercise conveys health benefits remain elusive. Dr Justin Percival and his collea gues are investigating the role of nitric oxide in skeletal muscles, and their findings could lead to novel therapies for a range of diseases school, the 1998 Nobel Prize was given in recognition of the fact that NO represented a ‘new principle for signalling in biological systems’. So when I had the opportunity to join Dr Stanley Froehner’s lab and study NO in the context of human muscle disease, I jumped at the chance. Also, my early interest and efforts on translating our findings into a potential therapy were strengthened by meeting parents of children with Duchenne muscular dystrophy (DMD) at Parent Project Muscular Dystrophy conferences. For me, it highlighted an urgent need for new and safe therapies, because after nearly a quarter of a century since the landmark discovery that loss of dystrophin caused DMD, steroids remain the primary and very limited drug treatment option for DMD. Could you outline the long- and short-term objectives of your lab’s research? Decades of research are unequivocal; moderate exercise is good for us and reduces the chances of developing disease. Our long-term goal is to understand why. We know that skeletal muscle nitric oxide (NO) controls exercise capacity, so we want to determine if NO contributes to the systemic benefits of exercise by focusing on NO’s metabolic and endocrine functions in muscle. In the short term, we are building more precise and powerful genetic tools to explore these functions of NO in muscle. We are also developing new clinical applications for PDE5 inhibitors based on our recent understanding of their function in muscle. What sparked your interest in the area of skeletal muscle NO? As an undergraduate student at Victoria University in New Zealand, I found free radical chemistry incredibly interesting. I was lucky to briefly study free radical photochemistry in human plasma with Drs Rod Tilbury and Bill Jordan. Just as I started graduate 82 INTERNATIONAL INNOVATION
Your lab is studying NO signalling in normal and diseased skeletal and cardiac muscle. What exciting discoveries have you made so far? For me, the most exciting discovery is that skeletal muscle expresses a second neuronal NO synthase (nNOS) called nNOSβ at the Golgi. Our findings show that peak muscle performance requires two nNOS enzymes, nNOSµ and nNOSβ, and that these enzymes promote muscle fatigue resistance. This data argues against an old and increasingly challenged paradigm about how NO signalling occurs. This old idea is that NO signalling is primarily diffusion limited; that NO gas diffuses randomly to targets down a diffusion gradient. But we found nNOSµ and nNOSβ enzymes are localised very close to each other in muscle and have quite separate functions, which is not possible if NO freely diffuses. Of course, we were also very excited by our findings that the PDE5 inhibitor sildenafil can reduce respiratory and cardiac muscle dysfunction in an mdx mouse model of DMD. We hope that these preclinical studies can be
translated into clinical practice one day and provide a new palliative therapy for DMD until dystrophin replacement treatments can be developed. Clinical studies are underway, but it may take some time to work out the ideal treatment regime for patients. Which key players are you collaborating with to conduct your research? We are fortunate to have several excellent collaborators whose expertise allows us to address the multifaceted roles of NO in muscle. Dr David Marcinek at the University of Washington is a key collaborator. His group is developing sophisticated in vivo spectroscopy techniques that they use to understand the impact of free radicals on skeletal muscle mitochondria function. Together, we uncovered new aspects of mitochondrial dysfunction in muscular dystrophy. We are currently working with Drs Emmanuel Buys and Peter Brouckaert to define the downstream targets of NO in skeletal muscle. Also, we are very fortunate to have as colleagues at the University of Miami, Drs Glen Kerrick and Joshua Hare, with whom we are exploring the regulation of calcium handling by nNOS and cGMP-independent NO signalling, respectively. Where are you hoping to focus your research efforts over the coming months? In the short term, we are focused on identifying effectors and target pathways of NO in skeletal muscle. We are also generating new nNOS knockout models that will allow us to precisely define the roles of nNOS splice variants in myokine secretion and skeletal muscle metabolism. Since the functions of skeletal muscle PDE5 are poorly defined, we will also investigate the roles of muscle PDE5 and strongly believe that this knowledge will enable us to develop new therapeutic applications for PDE5 inhibitors.
NO to muscle disease In the quest to better understand why exercise is so good for us, researchers at the University of Miami have revealed some novel insights into muscle nitric oxide signalling, which could lead to new therapeutic uses for PDE5 inhibitors NITRIC OXIDE (NO) is a biomarker of cardiovascular health, and the importance of NO signalling in the cardiovascular, immune and central nervous systems has been known for well over a decade. NO is also a biomarker of healthy skeletal muscle, where it is largely synthesised by neuronal NO synthase (nNOS) from arginine and oxygen. Healthy skeletal muscles with higher endurance exercise capacity express more nNOS, which is known to promote muscle fatigue resistance. Conversely, unhealthy muscles have less nNOS or defective NO signalling. Thus, it is becoming clear that skeletal muscle nNOS may play a role in the ‘use it or lose it’ phenomenon; the loss of muscle size and strength that accompanies a cessation in exercise.
EXERCISE PILLS Although it is well known that exercise helps to promote health, the pathways responsible for the health benefits it affords are not well understood. A research group based at the University of Miami is aiming to elucidate this by studying the role of NO signalling in muscles. Led by Dr Justin Percival, the group’s previous discovery of an entirely new NO pathway in skeletal muscle has led them to investigate the possibility that NO may link exercise capacity and myokine secretion – protein messengers that oppose harmful inflammation, promote healthier metabolism and which are thought to be partly responsible for the benefits of exercise. Percival hopes that his team’s studies will provide evidence that NO is an important regulator of the secretory function of muscle and in doing so gain answers to the question of why exercise has such positive whole body benefits. “This is an exciting new area of research which has great potential for identifying new targets, or myokine ‘exercise pills’ that in theory could be used to treat conditions like neuromuscular disease, diabetes, obesity and mental illness that may benefit from exercise,” he adds.
REPURPOSING VIAGRA Much of Percival’s research has involved studies of mouse models for Duchenne muscular dystrophy (DMD), a genetic disease caused by the loss of dystrophin that incites skeletal and cardiac muscle weakness and wasting, leading to respiratory or cardiac failure before patients reach 30 years of
age. There is an urgent need for new, safe therapies for the treatment of DMD, for which steroids are currently the only available treatment option. In two exciting recent studies, Percival, while working in Dr Stanley Froehner’s lab, repurposed the PDE5 inhibitor sildenafil (otherwise known as Viagra) to amplify NO signalling and treat DMD; work which has received a lot of attention from the scientific community and media. They treated mice with Viagra and found that it reduced respiratory muscle weakness and fibrosis. “This was a very exciting result for us because Viagra may be able to reduce the fibrosis that contributes to respiratory dysfunction which is the major cause of premature mortality in DMD patients,” Percival postulates. In collaboration with Dr Joseph Beavo’s lab, they also found that Viagra rapidly restored cardiac muscle strength in aged mice with established cardiac muscle weakness. Several clinical trials are now underway to test the efficacy of PDE5 inhibitors in improving muscle function in DMD patients.
NEW FRONTIER Percival’s studies in NO represent an exciting and emerging new frontier in skeletal muscle biology that may allow researchers to better understand the mystery of exactly why exercise conveys health benefits. His early research on the biology of NO was supported by Parent Project Muscular Dystrophy (PPMD), a non-profit organisation founded by parents of children with DMD. Although eventually his work did lead to the consideration of PDE5 inhibitors as a novel therapy for the disease, it was not originally focused on therapy development. “For me, this highlights the foresight of PPMD, since the parents didn’t only fund therapy-focused translational science. It also highlights both the importance of funding discovery science and the role serendipity plays in science. We cannot be sure where the next advance in treatment will come from,” he muses. Furthermore, understanding how to prevent the loss of functional skeletal muscle has many clinical applications beyond the treatment of DMD. These range from therapies for patients confined to their beds for long periods, senior citizens where loss of mobility is a significant cause of morbidity and indirect mortality, muscle wasting in cancer patients, people with heart disease and obesity, and even to astronauts whose muscles waste away in the absence of gravity.
INTELLIGENCE NOVEL INSIGHTS INTO MUSCLE NITRIC OXIDE SIGNALLING LEAD TO NEW THERAPEUTIC USES FOR PDE5 INHIBITORS OBJECTIVES We seek to understand the skeletal muscle functions of the multifaceted signalling molecule nitric oxide. Exploring NO function will help us unlock the mystery of how exercise promotes whole body health and provide new therapeutic targets for the treatment of neuromuscular disease and other diseases that may benefit from exercise.
KEY COLLABORATORS Dr Emmanuel Buys, Massachusetts General Hospital Dr Joshua Hare, University of Miami Dr Glenn Kerrick, University of Miami Dr David Marcinek, University of Washington
FUNDING Institutional start up funds
CONTACT Dr Justin Percival Project Coordinator Department of Molecular and Cellular Pharmacology University of Miami Miller School of Medicine Pharmacology, R-189 University of Miami Miami, FL 33101 USA T +1 305 243 7303 E
[email protected] JUSTIN PERCIVAL received his BSc from Victoria University (New Zealand) and his PhD from the University of Sydney (Australia). He then worked at the University of Washington prior to establishing his lab as Assistant Professor at the University of Miami. Percival has contributed to the understanding of nitric oxide signalling in muscle and to the development of phosphodiesterase 5 inhibitors as treatments for Duchenne muscular dystrophy.
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