Myocardin is required for cardiomyocyte survival and maintenance of heart function.

Despite intense investigation over the past century, the molecular mechanisms that regulate maintenance and adaptation of the heart during postnatal development are poorly understood. Myocardin is a remarkably potent transcriptional coactivator expressed exclusively in cardiac myocytes and smooth muscle cells during postnatal development. Here we show that myocardin is required for maintenance of cardiomyocyte structure and sarcomeric organization and that cell-autonomous loss of myocardin in cardiac myocytes triggers programmed cell death. Mice harboring a cardiomyocyte-restricted null mutation in the myocardin gene (Myocd) develop dilated cardiomyopathy and succumb from heart failure within a year. Remarkably, ablation of the Myocd gene in the adult heart leads to the rapid-onset of heart failure, dilated cardiomyopathy, and death within a week. Myocd gene ablation is accompanied by dissolution of sarcomeric organization, disruption of the intercalated disc, and cell-autonomous loss of cardiomyocytes via apoptosis. Expression of myocardin/serum response factor-regulated myofibrillar genes is extinguished, or profoundly attenuated, in myocardin-deficient hearts. Conversely, proapoptotic factors are induced and activated in myocardin-deficient hearts. We conclude that the transcriptional coactivator myocardin is required for maintenance of heart function and ultimately cardiomyocyte survival.

Identification of a hypaxial somite enhancer element regulating Pax3 expression in migrating myoblasts and characterization of hypaxial muscle Cre transgenic mice.

Pax3 encodes a transcription factor that functions in the embryonic central nervous system, neural crest, and somitic mesoderm. Prior studies suggest that distinct regulatory elements regulate temporal and spatial expression of Pax3 in neural crest and mesoderm. Here, we describe a discrete enhancer element, conserved between mouse and human genomes, that directs Pax3 expression in the ventral-lateral lip of interlimb somites. These regions give rise to hypaxial musculature including limb, ventral body wall, diaphragm, and tongue muscles. Transgenic mice harboring the hypaxial muscle enhancer driving lacZ expression initiate beta-galactosidase expression at E10.0, significantly later than endogenous Pax3 expression in presomitic and segmented mesoderm. Initiation of transgene expression is not dependent on Pax3 itself, since expression is detectable in homozygous Splotch embryos. Transgenic mice expressing Cre recombinase in hypaxial myoblasts were generated and characterized. These results suggest that Pax3 is differentially regulated within the somite in both spatial and temporal domains. Hypaxial muscle Cre mice will allow for specific manipulation of gene expression in this subset of developing skeletal muscle.

High-efficiency somatic mutagenesis in smooth muscle cells and cardiac myocytes in SM22alpha-Cre transgenic mice.

The cytoskeletal protein SM22alpha is expressed in visceral and vascular smooth muscle cells (SMCs), in cardiac myocytes, and in the myotomal components of the somites during murine embryonic development. In this report, we describe the generation and characterization of transgenic mice expressing Cre-recombinase under the transcriptional control of the -2.8-kb SM22alpha promoter. Following interbreeding with the R26R reporter strain, Cre-dependent beta-galactosidase expression was observed as early as embryonic day 9.5 in SMCs of the developing vasculature, in cardiac myocytes, but not in the somites. In adult mice, Cre-mediated recombination was observed in vascular SMCs throughout the venous and arterial systems, in visceral SMCs in multiple organs, and in cardiac, but not skeletal muscle. Importantly, Cre-mediated recombination was present in nearly 100% of arterial SMCs, including in the aorta. These mice are thus an important new tool for performing in vivo loss-of-function studies of genes expressed in vascular SMCs.