The adult heart lacks reserve cardiocytes and cannot regenerate. Therefore, a large acute myocardial infarction often develops into congestive heart failure. To attempt to prevent this progression, we transplanted skeletal myoblasts into cryoinfarcted myocardium of the same rabbits (autologous transfer), monitored cardiac function in vivo for two to six weeks and examined serial sections of the hearts by light and electron microscopy. Islands of different sizes comprising elongated, striated cells that retained characteristics of both skeletal and cardiac cells were found in the cryoinfarct. In rabbits in which myoblasts were incorporated, myocardial performance was improved. The ability to regenerate functioning muscle after autologous myoblast transplantation could have a important effect on patients after acute myocardial infarction.

Gene expression in skeletal muscles of adult vertebrates is altered profoundly by changing patterns of contractile work. Here we observed that the functional activity of MEF2 transcription factors is stimulated by sustained periods of endurance exercise or motor nerve pacing, as assessed by expression in transgenic mice of a MEF2-dependent reporter gene (desMEF2-lacZ). This response is accompanied by transformation of specialized myo®ber subtypes, and is blocked either by cyclosporin A, a speci®c chemical inhibitor of calcineurin, or by forced expression of the endogenous calcineurin inhibitory protein, myocyte-enriched calcineurin interacting protein 1. Calcineurin removes phosphate groups from MEF2, and augments the potency of the transcriptional activation domain of MEF2 fused to a heterologous DNA binding domain. Across a broad range, the enzymatic activity of calcineurin correlates directly with expression of endogenous genes that are transcriptionally activated by muscle contractions. These results delineate a molecular pathway in which calcineurin and MEF2 participate in the adaptive mechanisms by which skeletal myo®bers acquire specialized contractile and metabolic properties as a function of changing patterns of muscle contraction.

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Skeletal muscle adapts to different patterns of motor nerve activity by alterations in gene expression that match specialized properties of contraction, metabolism, and muscle mass to changing work demands (muscle plasticity). Calcineurin, a calcium͞calmodulin-dependent, serine-threonine protein phosphatase, has been shown to control programs of gene expression in skeletal muscles, as in other cell types, through the transcription factor nuclear factor of activated T cells (NFAT). This study provides evidence that the function of NFAT as a transcriptional activator is regulated by neuromuscular stimulation in muscles of intact animals and that calcium influx from the transient receptor potential (TRPC3) channel is an important determinant of NFAT activity. Expression of TRPC3 channels in skeletal myocytes is up-regulated by neuromuscular activity in a calcineurin-dependent manner. These data suggest a mechanism for cellular memory in skeletal muscles whereby repeated bouts of contractile activity drive progressively greater remodeling events. Skeletal muscles consist of a mosaic of multinucleated myofibers differing in size, metabolic capacity, mitochondrial content, and contractile properties. Determinants of specialized myofiber phenotypes are programmed during fetal development but are modulated in response to changing motor nerve input (1-3). This plasticity allows myofibers to match cellular capacities to different physiologic demands, as demonstrated in classic cross-innervation and chronic lowfrequency stimulation experiments performed on adult muscles (4, 5). These adaptive responses are minimal after a single period of neuromuscular activity but become fully manifest only after repeated bouts of activity pursued over a period of days to weeks, as exemplified by the results of training in human athletes. Thus, the physiologically important phenomenon of muscle plasticity, familiar to muscle physiologists for decades (6, 7), includes a form of cellular memory, the molecular basis of which has been poorly understood.We have reported that calcineurin, a serine-threonine phosphatase, functions as an effector for calcium-dependent signaling pathways in skeletal myofibers by inducing a program of gene expression associated with slow oxidative myofibers (8, 9). Calcineurin activates the transcription factors nuclear factor of activated T cells (NFAT) and myocyte enhancer factor 2, which in turn alter the expression of target genes (5, 9, 10). Constitutively active calcineurin expressed in myofibers of intact animals activates genes that encode contractile proteins, ion channels, and myoglobin that establish the slow oxidative myofiber phenotype (5).How are changes in intracellular calcium associated with differing patterns of excitation-contraction coupling interpreted by calcineurin͞NFAT to alter gene transcription? The present study used a transgenic mouse model to assess the functional state of NFAT proteins in the skeletal muscle of intact animals. In addition, we assessed the relative contributions of diff...