Molecular basis of skeletal muscle plasticity--from gene to form and function.

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.