Myogenic regulatory factors such as MyoD and myogenin are differentially expressed in fast and slow fibers ( 3), and a MyoD-binding E-box is necessary for the expression of MyHC-2B ( 4). Fast and slow motor unit firing patterns are transduced into different gene programs within the myofibers, but the specific effector signaling pathways have been only partially elucidated. The slow and the various fast fiber phenotypes result from the concerted activation of complex gene programs comprising various contractile proteins and metabolic enzymes. Slow fibers express MyHC-slow, are oxidative and fatigue-resistant, whereas fast fibers display a graded range of functional properties according to the scheme 2A↔2X↔2B, with MyHC-2A having the slowest and 2B the fastest shortening velocity, and 2A being oxidative and fatigue-resistant and 2B glycolytic and easily fatigable. Four muscle fiber types have been distinguished based on the kinetic properties of their myosin heavy chain (MyHC) ATPase: 1 type1/slow and 3 type2/fast. Fiber type composition depends on developmental cues during embryogenesis, but activity is the major controller of fiber plasticity in the adult ( 1, 2). This characteristic, in turn, determines the specific performances of each muscle. Slow and fast motor neuron activity triggers both muscle contraction and the transcription of activity-dependent genes, ensuring muscle growth and an adequate composition in fast and slow fibers.
Our data contribute to the elucidation of the mechanisms whereby activity can modulate the phenotype and performance of skeletal muscle. We provide evidence that the transcription of slow and fast myosin heavy chain (MyHC) genes uses different combinations of NFAT family members, ranging from MyHC-slow, which uses all 4 NFAT isoforms, to MyHC-2B, which only uses NFATc4. Our results show that, depending on the applied activity pattern, different combinations of NFAT family members translocate to the nucleus contributing to the transcription of fiber type specific genes. We analyzed the role of NFAT family members in vivo by transient transfection in skeletal muscle using a loss-of-function approach by RNAi. Here, we show that 4 nuclear factor of activated T cell (NFAT) family members act coordinately downstream of Cn in the specification of muscle fiber types. The calcium/calmodulin-dependent phosphatase calcineurin (Cn) has been shown to mediate the transcriptional effects of motor neuron activity, but precisely how 4 distinct muscle fiber types are composed and maintained in response to activity is largely unknown. The intracellular signals that convert fast and slow motor neuron activity into muscle fiber type specific transcriptional programs have only been partially defined.