Exercise-induced alterations and loss of sarcomeric M-line organization in the diaphragm muscle of obscurin knockout mice

We recently reported that skeletal muscle fibers of obscurin knockout (KO) mice present altered distribution of ankB, disorganization of the sub-sarcolemmal microtubules and reduced localization of dystrophin at costameres. In addition, these mice have impaired running endurance and increased exercise-induced sarcolemmal damage compared to wild-type animals. Here, we report results from a combined approach of physiological, morphological, and structural studies in which we further characterize the skeletal muscles of obscurin KO mice. A detailed examination of exercise performance, using different running protocols, revealed that the reduced endurance of obscurin KO animals on the treadmill depends on exercise intensity and age. Indeed, a mild running protocol did not evidence significant differences between control and obscurin KO mice whereas comparison of running abilities of 2, 6 and 11 month-old mice exercised at exhaustion revealed a progressive, age-dependent reduction of the exercise tolerance in KO mice. Histological analysis indicated that a heavy exercise induced leucocyte infiltration, fibrotic connective tissue deposition and hyper-contractures in the diaphragm of KO mice. On the same line, electron microscopy revealed that in the diaphragm of exercised obscurin KO mice, but not in the hind limb muscles, both M-line and H-zone of sarcomeres appeared wavy and less defined. Altogether, these results suggest that obscurin is required for the maintenance of morphological and ultrastructural integrity of skeletal muscle fibers against damage induced by intense mechanical stress and point to the diaphragm as the skeletal muscle most severely affected in obscurin-deficient mice.