Lupus muscle atrophy is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass. Muscular atrophy decreases quality of life as the sufferer becomes unable to perform certain tasks or worsen the risks of accidents while performing those (like walking). Muscular atrophy increases the risks of falling in conditions such as IBM (inclusion body myositis). Muscular atrophy affects a major number of elderly.
During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass. This condition is called “sarcopenia”. The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the “satellite cells” which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival.
In addition to the simple loss of muscle mass (atrophy), or the age-related decrease in muscle function (sarcopenia), there are other diseases which may be caused by structural defects in the muscle (muscular dystrophy), or by inflammatory reactions in the body directed against muscle (the myopathies).
Lupus muscle atrophy occurs by a change in the normal balance between protein synthesis and protein degradation. During atrophy, there is a down-regulation of protein synthesis pathways, and an activation of protein breakdown pathways. The particular protein degradation pathway which seems to be responsible for much of the muscle loss seen in a muscle undergoing atrophy is the ATP-dependent ubiquitin/proteasome pathway. In this system, particular proteins are targeted for destruction by the ligation of at least four copies of a small peptide called ubiquitin onto a substrate protein. When a substrate is thus “poly-ubiquitinated”, it is targeted for destruction by the proteasome. Particular enzymes in the ubiquitin/proteasome pathway allow ubiquitination to be directed to some proteins but not others – specificity is gained by coupling targeted proteins to an “E3 ubiquitin ligase”. Each E3 ubiquitin ligase binds to a particular set of substrates, causing their ubiquitination.
(Source: Sandri M. 2008. Signaling in Muscle Atrophy and Hypertrophy. Physiology 23: 160-170.)
How to treat lupus muscle atrophy?
Muscle atrophy can be opposed by the signaling pathways which induce muscle hypertrophy, or an increase in muscle size. Therefore one way in which exercise induces an increase in muscle mass is to down regulate the pathways which have the opposite effect.
One important rehabilitation tool for muscle atrophy includes the use of functional electrical stimulation to stimulate the muscles. This has seen a large amount of success in the rehabilitation of paraplegic patients.
Since the absence of muscle-building amino acids can contribute to muscle wasting (that which is torn down must be rebuilt with like material), amino acid therapy may be helpful for regenerating damaged or atrophied muscle tissue. The branched-chain amino acids or BCAAs (leucine, isoleucine, and valine) are critical to this process, in addition to lysine and other amino acids.
(Source: D.Zhang et al., Functional Electrical Stimulation in Rehabilitation Engineering: A survey, Nenyang technological University, Singapore)