Surely we have all experienced muscle soreness following exercise. This is mainly caused by membrane damage of the muscle fibres. Skeletal muscle fibres are organised in myofibrils, which in turn are composed of repeating structural units arranged in series, the sarcomeres. Desmin is an intermediate filament protein that is located on the periphery of the sarcomeres, linking individual myofibrils laterally. Its strategic location suggests that it plays a major protective role in muscle structural integrity, as well as force transmission. It is known that when desmin is not expressed, the myofibrils have greater mobility and are more prone to misalignment. Nevertheless, the protective role of desmin during exercise-induced mechanical stress has not yet been ascertained. In a paper recently published in J. Appl. Physiol., Kurt Haubold and collaborators examined the athletic skills of mice lacking desmin, in order to investigate the role of this protein in maintenance of muscle fibre integrity.
The authors of this study placed normal mice, and animals that lacked desmin, in cages with access to running wheels attached to bicycle computers. After recording the times and distances that the desmin-null animals ran over a three-week period, the team compared their performances with their wild-type cousins.
Mice lacking desmin were significantly more indolent than wild-type mice,since they spent less time running and ran shorter distances. Moreover, the average speed of the lazy mice decreased 20% throughout the experiment, as they were unable to adapt to voluntary running. The animals also lacked endurance and performed less well under stress during treadmill tests at increased speeds. For example, the mice that did not express desmin only maintained a speed of 20 m min-1 for less than 12 min, while most of their wild-type counterparts sustained that speed for at least 25 min. In addition, they could not run as fast as the wild-type mice in the stress test:less than half of desmin-null mice managed to run at 25 m min-1,whilst scampering along at high speed was no sweat for the wild-type mice. The relatively poor exercising ability of mice lacking desmin shows that this protein is essential for force generation and transmission through muscle fibres.
Knowing that exercise-induced muscle membrane damage can result in increased levels of serum creatine kinase activity, the team tested desmin's protective properties by looking for evidence of damage-induced creatine kinase efflux while the animals exercised. However, the mice didn't show elevated levels of serum creatine kinase following exercise on the wheel. So it seems that impaired voluntary exercise performance in desmin-null mice is not caused by muscle membrane damage.
The team also investigated the protective role of desmin against muscle damage by measuring creatine kinase levels following downhill running on a treadmill at a 16° incline. Downhill running induces eccentric muscle contractions and is a potential cause of muscle membrane damage. Surprisingly,mice lacking desmin enjoyed downhill running as much as the wild-type ones and had significantly lower serum creatine kinase activity. Again, it appears that the loss of desmin does not make muscles more susceptible to damage. The authors hypothesise that desmin loss impairs mitochondrial function and negatively influences aerobic exercise performance.
These results indicate that desmin's most important function associated with exercise is not the protection of muscle fibres from mechanical insults,but lateral force transmission and maintenance of myofibrillar organisation. And there is always the possibility that mutations in the desmin-encoding gene may have a similar effect in humans and adversely affect our exercise performance (well, that's my excuse, anyway)!