The free radical gas, nitric oxide (NO) is a pivotal signaling messenger. NO was named “Molecule of the Year” in 1992 by the journal Science and the discovery that NO plays a central role in cardiovascular system function led to a Nobel Prize for Drs Furchgott, Ignarro and Murad in 1998. Since then, NO has proven to be an even more versatile messenger, performing important roles in many systems including the nervous, immune and motor systems.
In the Percival lab, we study NO signaling in normal and diseased skeletal and cardiac muscle. We depend on strong healthy skeletal muscle for movement and breathing and the heart for pumping blood. Our primary interest is the function of NO synthesized by the enzyme neuronal nitric oxide synthase (nNOS). The functions of nNOS in muscle, particularly the less common splice variants, remain to be fully understood. We focus on the canonical NO-cGMP mode of signaling, where NO acts to stimulate cGMP synthesis by soluble guanylyl cyclase (sGC), a major receptor for NO. cGMP then in turn binds and activates downstream targets such as protein kinase G and ion channels (Figure 1).
There are at least four splice variants of nNOS (nNOSα, nNOSβ, nNOSγ and nNOSμ). In skeletal muscle, nNOSμ was thought to be the only nNOS expressed; however we recently found that nNOSβ and sGC were expressed at the Golgi complex in muscle cells (Figure 2). Both nNOSμ and nNOSβ isoforms are necessary for optimal exercise performance. The mechanisms governing exercise capacity are not only important for understanding the limits of athletic performance, they are important because they are inextricably linked with human longevity. Furthermore, nNOSμ may not only regulate exercise capacity, it may also act as an “activity sensor” that initiates different signaling programs in an adaptive response to inactivity or endurance type exercise. For a more detailed description of current knowledge about nNOS function in skeletal muscle please read our 2011 review in Biophysical Reviews listed in the Publications section.
Abnormalities in nNOS signaling are a common pathogenic feature of many neuromuscular diseases including limb girdle and Duchenne/Becker muscular dystrophies. We use the mdx mouse model of Duchenne/Becker muscular dystrophy to understand the changes in nNOS function and to develop potential NO-based treatments for muscular dystrophy. We have found that pharmacologically enhancing NO-cGMP signaling, using drugs such as Viagra®, is powerful for reducing dystrophic muscle disease.
To better understand nNOS-cGMP signaling, the Percival lab is focused on addressing the following questions:
1. What happens when muscle runs out of gas?
Or more specifically, what are the functions of nNOS isoforms in muscle? Using knockout mice, we study the impact of the loss of nNOSβ and nNOSμ on muscle. We have found that both nNOSβ and nNOSμ are necessary to maintain normal skeletal muscle size, strength and fatigue resistance. Our long term goal is to understand the underlying mechanisms responsible. This has led to studies of nNOS isoform regulation of mitochondria, the “batteries” providing energy for the cell. We believe this work will lead to a better understanding of the molecular mechanisms governing exercise performance and may eventually provide mechanistic insights into why exercise is so good for you.
2. How can you use a potentially highly diffusible free radical gas as a messenger?
We and others have identified three spatially and functionally distinct nNOS signaling compartments in skeletal muscle created by the differential targeting of nNOS splice forms (Figure 2). nNOSμ is localized to the sarcolemma and cytosol and nNOSβ is localized the Golgi complex. Thus, we believe that tight control of nNOS localization and its targets facilitates the specific use of NO as a gaseous messenger. We are interested in understanding the mechanisms governing the localization of nNOS splice variants and their targets and how localization relates to function.
3. Can we just say “NO” to muscular dystrophy?
As stated above, we depend on strong healthy skeletal muscles for movement and breathing. This is exemplified by the disease Duchenne Muscular Dystrophy (DMD), where the loss of skeletal muscle mass and function leads to a loss of ambulation by the end of the first decade of life and respiratory failure in second or third decades of life. DMD is caused by the loss of dystrophin which disrupts the tight spatial and regulatory controls on nitric oxide-cGMP signaling. We use the mdx mouse model of DMD to study the perturbation of nitric oxide-cGMP signaling and to test novel drug-based approaches to alleviate disease pathology. We have found that the use of sildenafil (Viagra®, Revatio®) to amplify NO-cGMP signals can prevent or even reverse skeletal and cardiac muscle dysfunction in the mdx mouse model of DMD. These findings led to Phase 2 clinical trials (NCT01168908) to test its efficacy in humans. We hope to build on our results with sildenafil and will be testing different approaches to amplify NO-cGMP signaling in mouse models of muscular dystrophy in the future.
A postdoctoral associate position is available to study nitric oxide signaling in the Percival lab.
The position involves the study of the metabolic functions of skeletal muscle neuronal nitric oxide synthase with special emphasis on the regulation of mitochondrial energetics in the context of obesity in novel mouse models. Highly motivated individuals with a Ph.D or M.D. with a strong interest in these areas and background in one or more of: molecular and cellular biology, muscle physiology, mitochondrial biology, animal models of disease or mouse transgenesis are encouraged to apply.
Applicants should email a statement of research interests, curriculum vitae, and the contact details of three references to: Dr Justin Percival, email@example.com.