beta-MHC Gene Research Articles

A MyoD1-independent muscle-specific enhancer controls the expression of the beta-myosin heavy chain gene in skeletal and cardiac muscle cells.

The beta-cardiac myosin heavy chain is the major contractile protein expressed in two sarcomeric muscles of distinct embryologic origins, the ventricular myocardium and slow twitch skeletal muscle. Characterization of the cis-acting regulatory sequences of the human and the rat beta-MHC genes established that their expression in these two muscle types is controlled, at least in part, by common mechanisms involving a muscle-specific enhancer. This enhancer consists of distinct but cooperative subelements that interact with muscle-specific nuclear proteins. In contrast to other muscle-specific enhancers, the beta-MHC gene enhancer is unresponsive, directly or indirectly, to the muscle lineage-determining and muscle gene-transactivating helix-loop-helix factors MyoD and myogenin. A MyoD-binding site in the rat beta-MHC promoter is not required for transcriptional activity in skeletal and cardiac cells, but is necessary for activation in 10T1/2 and CV1 cells transfected with MyoD. In addition, this element is absent from the human beta-MHC promoter. Thus, MyoD and MyoD-related processes are neither required nor sufficient for the expression of the beta-MHC gene either in cardiac or skeletal muscle cells. These observations provide evidence for the existence of myogenic regulatory programs that precede and/or differ from those governed by known myogenic helix-loop-helix transactivators.

The presence of a DNA binding complex correlates with E beta class II MHC gene expression.

The class II major histocompatibility complex (MHC, Ia) antigens are a family of membrane proteins whose expression is strictly regulated. They have a limited tissue distribution and their expression is regulated both developmentally and in response to external stimuli. Here we report the identification of a DNA binding protein complex (termed complex A) within the murine E beta MHC gene, which is restricted to cells that express Ia antigens. Complex A binding activity is developmentally regulated in cells of the B lineage in accordance with class II expression and is responsive to two different Ia-inducing lymphokines, interferon-gamma in macrophages and interleukin-4 in pre-B cells. The DNA target sequence in complex A includes three previously defined transcriptional motifs W, X and Y, and acts as a cis-acting transcription element. Complex A is present both in cells that are constitutive for class II MHC expression and in cells that have been induced for class II MHC expression. These results suggest that complex A may play a critical role in the regulation of class II MHC gene expression.

The myosin gene switching in human cardiac hypertrophy.

Cardiac hypertrophy is associated with qualitative as well as quantitative changes in myocardial cells. To analyze the molecular basis of isozymic transitions of cardiac myosins in response to pressure overload, we have constructed and characterized two types of myosin heavy chain (MHC) cDNA clones, specifying alpha- and beta-MHCs, and two types of myosin alkali light chain cDNA clones, complementary to atrial type (ALC1) and ventricular type (VLC1) mRNAs from a human fetal heart cDNA library. Using the S1 nuclease mapping procedure, we showed that the MCH isozymic transitions from alpha- to beta-MHC in the pressure overloaded atria are produced by changes in the relative level of alpha- and beta-MHC gene expression. In addition, we observed that the expression of VLC1 gene is also induced in the atria subjected to severe pressure overload. Thus, it appears that the increased expression of VLC1 gene, together with the isogene switch from alpha- to beta-MHC gene, may participate in the adaptation of myocardium to new functional requirement. Then, to get a better understanding of the genetic mechanisms involved in the regulation of isogene expression, we have isolated and sequenced genomic clone for VLC1 isoform. Sequence analysis has identified multiple potential cis regulatory elements within a 686-bp upstream region. This region includes 28-bp alternating purine/pyrimidine sequences and two segment exhibiting homology to consensus sequence proposed for viral and cellular enhancer elements. In particular, a comparison of the VLC1 upstream gene sequence with those available for several muscle-specific genes revealed that CC(A + T-rich)6GG elements and CATTCCT sequence are conserved. These results suggested that CArG box (-96 to -87) has an important role in the positive regulation of the VLC1 gene and this element may be involved in the co-regulation of VLC1 and cardiac alpha-actin genes.

Increased class I and class II MHC products and mRNA in kidneys of MRL-lpr/lpr mice during autoimmune nephritis and inhibition by cyclosporine.

The expression of MHC products in the kidneys of MRL-1pr/1pr mice was investigated. As previously described, these mice develop lupus-like nephritis with intraglomerular and peritubular Ig deposition, vasculitis, and interstitial mononuclear cell infiltration at about 12 wk of age. As the nephritis appeared, the expression of MHC class I and II products rose, as demonstrated by absorption and by specific binding of radiolabeled antibodies. Hybridization of kidney RNA with specific probes revealed an increase in specific mRNA for MHC class I and II genes and for beta2 microglobulin. Using rat monoclonals against mouse class I and II MHC products, and goat anti-rat Ig as second antibody, we showed that the increase in renal class I and II expression was localized to the basolateral membranes of tubular cells, and, in the case of class I, in arteries and glomeruli. The sites of tubular MHC expression corresponded closely to the sites of extensive peritubular Ig deposition. High doses of cyclosporine given for 6 to 8 wk reduced the peritubular Ig deposits, renal Ia and H-2K expression, and specific mRNA for beta 2-microglobulin and MHC genes, but did not reduce anti-DNA antibody levels in serum. Thus the peritubular Ig deposits and tubular MHC induction coincided in timing and location, and in their resolution with cyclosporine. The results raise the possibility that the increase in renal MHC expression not only accompanies the renal lesions, but may play a role in their pathogenesis.

The baboon beta-myosin heavy-chain gene: construction and characterization of cDNA clones and gene expression in cardiac tissues.

We have constructed a cDNA library from baboon ventricle and have used a rabbit beta-myosin heavy chain (beta-MHC) cDNA probe to isolate cross-hybridizing clones. The nucleotide sequence of one such clone, lambda BMHC beta 14, contains a portion of the coding region of the light meromyosin (LMM) region and the 3'-untranslated region of the baboon ventricular MHC. This cDNA clone is identified as containing beta-MHC sequences on the basis of similarity with the 3'-untranslated regions of beta-MHC genes from man (96% homologous) and rat (71% homologous), and dissimilarity with the 3'-untranslated region of the rat alpha-MHC gene (25% homologous). Alignment and comparison of the baboon cDNA nucleotide sequence with a human cDNA sequence reveal two amino acid substitutions in the LMM region that cause differences in their hydrophilicity profiles. These differences may alter MHC functions such as filament assembly. We have used baboon beta-MHC cDNA clones to construct probes for S1 nuclease protection studies to detect and to distinguish cardiac MHC gene transcripts in baboon ventricle, atrium, and diaphragm. As in human tissues, beta-MHC gene transcripts are detected in RNA from baboon ventricle and diaphragm. In baboon atrium, we detect beta-MHC gene transcripts as well as transcripts that may represent expression of the alpha-MHC gene. This study represents the first examination of cardiac MHC genes and gene expression in tissues from a large mammal that is closely related to man.

Expression of the cardiac ventricular alpha- and beta-myosin heavy chain genes is developmentally and hormonally regulated.

The cardiac ventricular myosin phenotype is developmentally and hormonally regulated. The genes coding for the two myosin heavy chains ( MHCs ), alpha and beta, have been recently isolated and characterized. In this study, we establish the precise temporal expression of these MHC genes in correlation with the myosin phenotype both during cardiac development and in response to different thyroid hormone levels and also document their expression in other muscle tissues. The close correlation observed between the relative abundance of the alpha- and beta-MHC mRNAs and corresponding isozymes demonstrates that the MHC phenotype is produced by the expression of the alpha- and beta-MHC genes and is regulated by changes in the level of their respective mRNAs. The opposite effect of thyroid hormone on the expression of the alpha- and beta-MHC genes in the ventricular myocardium indicates that these genes are regulated in an antithetic fashion. Finally, the MHC mRNAs encoded by the alpha- and beta-MHC genes are also present in the atrial myocardium and in the soleus, respectively.

Cardiac alpha- and beta-myosin heavy chain genes are organized in tandem.

Two ventricular myosin heavy chains (MHCs), alpha and beta, which exhibit different levels of ATPase activity, are differentially expressed during development, in response to thyroid hormone and in several pathological conditions. We have isolated and analyzed the structure of the genes coding for alpha- and beta-MHC mRNAs in the rat. Detailed analysis of eight overlapping MHC genomic clones shows that the alpha- and beta-MHC genes are organized in tandem and span 50 kilobases of the chromosome. The beta-MHC gene, predominantly expressed in late fetal life, is located 4 kilobases upstream from the alpha-MHC gene, predominantly expressed in the adult. These two genes are very closely related at the nucleotide sequence level, suggesting that they have arisen by duplication of a common ancestor, yet their expression in the ventricular myocardium has been shown to be regulated in an antithetic fashion by thyroid hormone.