An assay was developed for rapid and sensitive analysis of myosin heavy chain (MHC) mRNA expression in rodent skeletal muscle. Only 2 microg of total RNA were necessary for the simultaneous analysis of relative mRNA expression of six different MHC genes. We designed synthetic DNA fragments as internal standards, which contained the relevant primer sequences for the adult MHC mRNAs type I, IIa, IIx, IIb as well as the embryonic and neonatal MHC mRNAs. A known amount of the synthetic fragment was added to each polymerase chain reaction (PCR) and yielded a product of different size than the amplified MHC mRNA fragment. The ratio of amplified MHC fragment to synthetic fragment allowed us to calculate percentages of the gene expression of the different MHC genes in a given muscle sample. Comparison with the traditional Northern blot analysis demonstrated that our reverse transcriptase-PCR-based assay was reliable, fast, and quantitative over a wide range of relative MHC mRNA expression in a spectrum of adult and neonatal rat skeletal muscles. Furthermore, the high sensitivity of the assay made it very useful when only small quantities of tissue were available. Statistical analysis of the signals for each MHC isoform across the analyzed samples showed a highly significant correlation between the PCR and the Northern signals as Pearson correlation coefficients ranged between 0.77 and 0.96 (P < 0.005). This assay has potential use in analyzing small muscle samples such as biopsies and samples from pre- and/or neonatal stages of development.

Recent advances in chromatin biology have enhanced our understanding of gene regulation. It is now widely appreciated that gene regulation is dependent upon post-translational modifications to the histones which package genes in the nucleus of cells. Active genes are known to be associated with acetylation of histones (H3ac) and trimethylation of lysine 4 in histone H3 (H3K4me3). Using chromatin immunoprecipitation (ChIP), we examined histone modifications at the myosin heavy chain (MHC) genes expressed in fast vs. slow fiber-type skeletal muscle, and in a model of muscle unloading, which results in a shift to fast MHC gene expression in slow muscles. Both H3ac and H3K4me3 varied directly with the transcriptional activity of the MHC genes in fast fiber-type plantaris and slow fiber-type soleus. During MHC transitions with muscle unloading, histone H3 at the type I MHC becomes de-acetylated in correspondence with down-regulation of that gene, while upregulation of the fast type IIx and IIb MHCs occurs in conjunction with enhanced H3ac in those MHCs. Enrichment of H3K4me3 is also increased at the type IIx and IIb MHCs when these genes are induced with muscle unloading. Downregulation of IIa MHC, however, was not associated with corresponding loss of H3ac or H3K4me3. These observations demonstrate the feasibility of using the ChIP assay to understand the native chromatin environment in adult skeletal muscle, and also suggest that the transcriptional state of types I, IIx and IIb MHC genes are sensitive to histone modifications both in different muscle fiber-types and in response to altered loading states.

Cardiac β-myosin heavy chain (β-MHC) gene expression is mainly regulated through transcriptional processes. Although these results are based primarily on in vitro cell culture models, relatively little information is available concerning the interaction of key regulatory factors thought to modulate MHC expression in the intact rodent heart. Using a direct gene transfer approach, we studied the in vivo transcriptional activity of different-length β-MHC promoter fragments in normal control and in altered thyroid states. The test β-MHC promoter was fused to a firefly luciferase reporter gene, whereas the control α-MHC promoter was fused to the Renilla luciferase reporter gene and was used to account for variations in transfection efficiency. Absolute reporter gene activities showed that β- and α-MHC genes were individually and reciprocally regulated by thyroid hormone. The β-to-α ratios of reporter gene expression demonstrated an almost threefold larger β-MHC gene expression in the longest than in the shorter promoter fragments in normal control animals, implying the existence of an upstream enhancer. A mutation in the putative thyroid response element of the −408-bp β-MHC promoter construct caused transcriptional activity to drop to null. When studied in the −3,500-bp β-MHC promoter, construct activity was reduced (∼100-fold) while thyroid hormone responsiveness was retained. These findings suggest that, even though the bulk of the thyroid hormone responsiveness of the gene is contained within the first 215 bp of the β-MHC promoter sequence, the exact mechanism of triiodothyronine (T3) action remains to be elucidated.

The main goal of this study was to examine the transcriptional activity of different-length beta-myosin heavy chain (beta-MHC) promoters in the hypertensive rodent heart using the direct gene transfer approach. A hypertensive state was induced by abdominal aortic constriction (AbCon) sufficient to elevate mean arterial pressure by approximately 45% relative to control. Results show that beta-MHC promoter activity of all tested wild-type constructs, i.e., -3500, -408, -299, -215, -171, and -71 bp, was significantly increased in AbCon hearts. In the normal control hearts, expression of the -71-bp construct was comparable to that of the promoterless vector, but its induction by AbCon was comparable to that of the other constructs. Additional results, based on mutation analysis and DNA gel mobility shift assays targeting betae1, betae2, GATA, and betae3 elements, show that these previously defined cis-elements in the proximal promoter are indeed involved in maintaining basal promoter activity; however, none of these elements, either individually or collectively, appear to be major players in mediating the hypertension response of the beta-MHC gene. Collectively, these results indicate that three separate regions on the beta-MHC promoter are involved in the induction of the gene in response to hypertension: 1) a distal region between -408 and -3500 bp, 2) a proximal region between -299 and -215 bp, and 3) a basal region within -71 bp of the transcription start site. Future research needs to further characterize these responsive regions to more fully delineate beta-MHC transcriptional regulation in response to pressure overload.

No abstract