Skeletal muscle injury, fibrosis and transforming growth factor-beta.

Several animal models were assessed for their ability to produce pathology with a potential for developing fibrosis. Pathology was determined using standard histochemical and immunohistochemical techniques. In a model of ethanol-induced myopathy, cytochrome p450 1A1/2 was induced in endothelial cells (i.e. capillaries) but there was insufficient pathology to produce fibrosis. Two models of overload, tenotomy of the gastrocnemius muscle and a tetanus toxin overload model showed morphological changes including: degenerating fibers, central nuclei, variable myofiber sizes, and an expansion of the extracellular matrix, but changes were non-uniformly distributed making this a poor fibrosis model. A tetanus-toxin model of repetitive strain was studied. After 6 weeks of repeated strain, extensive pathology, including central nuclei, ringed fibers, and an uniform expansion of the extracellular matrix, was apparent. Recovery of skeletal muscle from 6 weeks of repeated-strain injury was evaluated using morphometric analysis, scanning electron microscopy, and high performance liquid chromatography. After 3 months, a significant deficit in muscle mass and myofiber area remained, an expanded extracellular matrix and collagen content was elevated with a 75% increase in the collagen crosslinks to collagen, indicating recovery from chronic strain is slow and incomplete with residual fibrosis. The presence of transforming growth factor-β, a prominent cytokine identified in other fibrotic organs, was documented after acute strain injury using immunohistochemistry, reverse transcriptase polymerase chain reaction, and western blotting. Transforming growth factor-β2 was observed in areas of myofiber injury and necrosis 24 and 72 hours post-strain injury. Transcript levels for transforming growth factor-β2 did not appear to change in response to injury. Transcript levels for transforming growth factor-β1, collagen I, and collagen III increased at 24 hours post-strain injury. Protein levels for transforming growth factor-β 1 and -β2 increased 48 hours post-strain injury. The ability of skeletal muscle to recover from injury is dependent on injury type, the frequency of injury, and the duration of injury. With repeated strain injury, skeletal muscle fibrosis can occur. Although the role of transforming growth factor-β in skeletal muscle fibrosis has yet to be established, it has been localized to areas of myofiber injury, suggesting transforming growth factor-β may play a role in skeletal muscle injury and recovery