Myopathy literally means “muscle disease”. Any condition that negatively affects muscle performance can be seen as a myopathy. In humans and animals, they can arise from a wide variety of hereditary and/or acquired causes. These include abnormalities in i) structural proteins (e.g. muscular dystrophies), ii) impaired muscle metabolism (e.g. disorders of carbohydrate or fat metabolism) or iii) immune-mediated inflammation (e.g. polymyositis, dermatomyositis). The primary feature of all these different abnormalities is that of muscle weakness that affects predominantly large muscle groups like the hip, thigh or shoulder muscles.
How strong a muscle can contract, or how much force it can produce, is directly related to the number of contracting muscle fibres and the contractile force generated by each of the individual fibres. As was discussed under Muscle 101, a muscle consists of various fibre types that, when activated, contributed to the total amount of force produced. Therefore, weakness may result from either a quantitative (decrease in the number of functional fibres) or qualitative (impaired muscle fibre contractility) factors.
How does a myopathy bring about muscle weakness?
Many myopathies that were passed down from the parents (hereditary), are caused by mutations in a particular gene coding for structural muscle proteins or enzymes that are essential for adequate energy production within the muscle fibres. Abnormal or dysfunctional structural proteins could theoretically lead to impaired contractility of muscle fibres, and was shown in some forms of muscular dystrophies. As the disease progresses, loss of muscle fibres most likely contributes to the weakness, and is probably the reason for the progressive nature of these disorders. The metabolic myopathies also cause weakness, with the primary factor a lack in ATP production. For example, the muscle from patients with McArdle’s disease (glycogen storage disease V), are unable to utilise its stored muscle glycogen due to a mutation in the enzyme (i.e. phosphorylase) that breaks it down to glucose units. These patients struggle to exercise and when they do, can lead to severe muscle cramps, pain, discomfort, and in some extreme cases, hospitalisation.
However, the mechanism of weakness in acquired muscle disorders is less obvious. The most frequent group of acquired muscle disorders is that of the immune and inflammatory myopathies (IIMs), consisting of polymyositis, dermatomyositis, inclusion body myositis, non-specific inflammatory myopathy and necrotising autoimmune myopathy. Weakness in this group of disorders is usually attributed to loss of muscle fibres due to inflammatory necrosis. However, it is unlikely that a decrease in the number of muscle fibres due to inflammatory necrosis is the sole mechanism responsible for weakness in IIMs, and it is probable that qualitative changes in muscle fibre contractility play an important role.
For example, the images shown above is of an athlete that was complaining of muscle cramps. By using the ATPase staining method at three pHs, it was possible to identify type II fibres that have abnormal centres (arrows). It could indicate a loss in contractile apparatus (i.e. myosin and actin) from the centre of the fibre.
Clarifying muscle weakness on a single fibre level
In vitro single muscle fibre contractility studies make it possible to directly assess the function of the cellular contractile apparatus in both healthy and diseased muscle, and are ideally suited to study muscle function at a cellular level. This technique, together with other molecular techniques, will be employed to study factors involved in muscle weakness in IIMs and McArdle’s disease. The advantage of an in vitro technique to assess contractility are two fold: i) any neurological abnormality is evaded and ii) any metabolic abnormality is side stepped.
An in-depth understanding of the mechanisms involved in the development of these disorders is of critical importance. By doing this research, we hope to gain a better understanding of the effects these disorders have on the way muscle fibres contract. Such information could potentially lead to different approaches to the treatment of these disorders.
Click on any project to read more about the topic, what we are doing and our findings (if available).
Understanding inflammatory myopathies and weakness
Why are inflamed muscles weak?
Idiopathic inflammatory myopathies (IIMs) are a group of muscle disorders caused by an immune-mediated attack on skeletal muscle tissue, and consist of polymyositis (PM), dermatomyositis (DM), inclusion body myositis (IBM), non-specific inflammatory myopathy (NIM) and necrotising autoimmune myopathy (NAM). All these myopathies present with some level of weakness. The exact mechanism of weakness in IIMs is still unknown, but may theoretically be from a decrease in the actual number of muscle fibres, decreased contractility (performance) of the individual muscle fibres, or both. To date, it has been assumed this weakness result from a decrease in the number of muscle fibres as a result of necrosis. It is due to this assumption that the fibre contractility has been poorly studied. Only one study has looked at the muscle fibres’ function in untreated DM and IBM. However, this study had a number of flaws. Also, a number of observations argue against necrosis as the only factor contributing to the weakness. These include:
☆ a lack of correlation between weakness and the degree of inflammation in muscle,
☆ the relatively small amount of necrotic fibres on muscle histology as compared to the degree of weakness, and
☆ how quickly patients respond to the treatment with corticosteroids.
Other non-immune effects on muscle may also contribute significantly to weakness in IIMs by affecting the contractile apparatus. These include an acquired deficiency of AMP-deaminase 1 (possibly interleukin-1 mediated) and depression of muscle fibre contractility by tumour necrosis factor α (TNF-α), as suggested by animal studies. Furthermore, the mechanism by which corticosteroids improve muscle function is also poorly studied. Possible explanations include the inhibition of secretion of TNF-α and decreased levels of TNF-α receptors, as well as increased AMP-deaminase 1 enzyme via decreased expression of interleukin-1.
What are we doing?
We are currently comparing the contractile properties of single muscle fibres from patients with untreated IIMs to those of healthy volunteers. Apart from the functional single fibre tests, we are also assessing a number of proteins (as mentioned above) in the muscle.
What we have done…
We have recently performed the first training study on a participant with advanced stage inclusion body myositis (IBM) and the data has made us very excited. Click on the video below to watch a narrated presentation of this extraordinary achievement.
Establishing the single fibre in vitro contracture test in Africa
The dangers of poor calcium regulation…
Malignant hyperthermia (MH) is a fatal condition triggered by volatile anaesthetic agents. Once a patient is subjected to the anaesthetic (e.g. halothane), symptoms of MH appear within minutes. The most prominent symptoms are involuntary muscle contractions and elevated body temperatures (>40˚C), the latter due to increased skeletal muscle metabolism. The cause of MH is due to mutations in the ryanodine receptors located in the sarcoplasmic reticulum.
These receptors form part of the channels that release calcium into the cytosol to initiate contraction. The mutation causes the ryanodine receptors to be highly sensitive to caffeine and certain anaesthetics (e.g. halothane). The increased sensitivity triggers the release of calcium, leading to involuntary muscle contraction. Treatment is usually a large dose of dantrolene, which blocks the release of calcium from the sarcoplasmic reticulum.
Genetic vs. functional diagnosis
A number of ryanodine receptor mutations have already been identified. Patients who has a reaction during anaesthesia would be genetically screened from a mere blood sample. However, an unknown mutation can easily go undetected. The only sure way to establish whether an individual is susceptible to MH, is to perform a muscle biopsy and subject it to the In Vitro Contracture Test (IVCT). This test requires a large muscle sample (approximately 30 mm in length and 3 – 5 mm thick), rendering the process very invasive. Most patients complain about severe pain during the healing process, and are usually left with significant scaring.
The IVCT test itself involves the muscle to be bathed in a solution containing various amounts of caffeine and halothane. If muscle contraction starts before the recommended threshold, the patient is usually classified as MH susceptible and confirmed with a genetic screening test. Muscle testing on the rest of the family will also be required.
What are we doing?
Technology has since evolved so that contracture tests may be performed on single isolated muscle fibres. The upside of this technology is that a very small muscle biopsy may be sufficient (5 mm in length) and could be obtained using a muscle biopsy needle. To confirm the validity of using the single fibre method to assist in diagnosing MH, and because there is no IVCT facility available in Africa, the MyoLab has embarked on developing, establishing and validating a single fibre technique to assist in diagnosing MH. This research heavily relies on small muscle biopsy samples from individuals that have previously been diagnosed with MH. For more information, please contact the MyoLab.
Muscle function in metabolic myopathies
The muscle from patients with McArdle’s disease are unable to utilise its stored muscle glycogen due to a mutation in their myophosphorylase enzyme. Patients with this mutation are unable to utilise their intramuscular glycogen, and struggle to exercise. and when they do, lead to severe muscle cramps, pain, discomfort, and in some extreme cases, hospitalisation. The category of metabolic myopathies consist of mainly inherited genetic defects that affect the supply of ATP through the respective pathways. These pathways mainly include that of fat oxidation, mitochondria and glycolysis. Studies on the functional components of affected muscle are limited and very scarce, especially on how contractility on a molecular level is affected.
The MyoLab, in partnership with local and international institutions, are currently investigating how contractility is affected and how exercise training improves these parameters.
The muscle from patients with McArdle’s disease are unable to utilise their stored muscle glycogen due to a mutation in their myophosphorylase enzyme. Patients with this mutation are unable to utilise their intramuscular glycogen, and thus struggle to exercise.
However, a previous study found that, during exercise, McArdle patients had much greater electrical activity within their muscles compared to healthy individuals. The authors of that study argued that more muscle fibres are being activated to compensate for the weakness associated with this disease. Besides the fact that McArdle patients lack phosphorylase, very little is actually known about the functional components of their muscle fibres, such as muscle fibre type, muscle fibre size and the contractility of the fibres. In order to answer whether McArdle patients have an altered muscle fibre type (which could lead to less force production and hence, weakness), we performed a relative muscle fibre type analyses (Kohn et al., 2014).
To our surprise, there was no difference in the fibre type composition of McArdle patients’ muscle and that of healthy controls (see graph below). We were able to show this in both the Biceps brachii and Vastus lateralis muscles. To further explain the weakness that McArdle patients experience, we analysed the cross-sectional areas of the various fibre types (Henning et al., 2016). Again, there was no difference between fibre size of healthy individuals and that of McArdle patients in both the arm and leg muscles.
We therefore concluded that fibre type and fibre size is not compromised or altered as a result of this disorder.
What we are doing next?
We are in the process of measuring the contractility of various metabolic myopathies on a single fibre level.