The gene encoding rat phosphoglycerate mutase subunit M: cloning and promoter analysis in skeletal muscle cells

The Ankrd2, Cdkn1c and Calcyclin Genes are Under the Control of MyoD During Myogenic Differentiation

Skeletal muscle development requires the coordinated expression of numerous transcription factors to control the specification of the muscle fate in mesodermal cells and the differentiation of the committed myoblasts into functional contractile fibers. The bHLH transcription factor MyoD plays a key role in these processes, since its forced expression is sufficient to induce the myogenesis in a variety of non-muscle cells in culture. Consistent with this observation, the majority of skeletal muscle genes require MyoD to activate their own transcription. In order to identify novel MyoD-target genes we generated C2C12 MyoD-silenced clones, and used a muscle-specific cDNA microarray to study the induced modifications of the transcriptional profile. Gene expression was analyzed at three different stages in differentiating MyoD(-)C2C12 myoblasts. These microarray data sets identified many additional uncharacterized downstream MyoD transcripts that may play important functions in muscle cell differentiation. Among these genes, we concentrated our study on the cell cycle regulators Cdkn1c and calcyclin and on the muscle-specific putative myogenic regulator Ankrd2. Bioinformatic and functional studies on the promoters of these genes clarified their dependence on MyoD activity. Clues of other regulatory mechanisms that might interact with the principal bHLH transcription factor have been revealed by the unexpected up-regulation in MyoD(-) cells of these novel (and other) target transcripts, at the differentiation stage in which MyoD became normally down-regulated.

Effects of hypoxia and thyroid hormone on mRNA levels and activity of phosphoglycerate mutase in rabbit tissues

AIM

In the present work, we studied the effects of hypoxia and triiodothyronine (T(3)) on phosphoglycerate mutase (PGAM) activity and expression in rabbit liver, brain, and skeletal muscle under in vivo conditions.

METHODS

Hypoxia was induced in a methacrylate cage with a mixture of 90% nitrogen and 10% oxygen. Hyperthyroidism was induced daily by T(3) injection (250 microg/kg).

RESULTS

Hypoxia increases the PGAM activity in liver and brain, tissues which possess type PGAM-BB isozyme, but does not affect the PGAM activity in muscle which possesses type PGAM-MM isozyme. T(3) administration increases the PGAM activity in muscle and liver, but does not affect the enzyme activity in the brain. In all cases, the activity changes in parallel with those of PGAM mRNA levels.

CONCLUSION

The tissue-specific effects of hypoxia and T(3) could be explained by the tissue-specific distribution of both PGAM isozyme and T(3) receptors.

Effects of thyroid hormone on mRNAs of phosphoglycerate mutase subunits in rat muscle during development

BACKGROUND

We previously showed that triiodothyronine (T3) stimulates muscle phosphoglycerate mutase (PGAM) activity and isozyme transition in rat skeletal and cardiac muscles.

METHODS

The effects of T3 on PGAM types B and M subunit expression in rat muscle during development are reported.

RESULTS

T3 administration during the first 21 days of rat life more than doubles type M PGAM mRNA levels, but produces minor effects on type B PGAM mRNA levels. The antihormone propylthiouracil (PTU) slightly decreases both type B and M mRNA levels, but this decrease is not statistically significant.

CONCLUSION

Thyroid hormone influences PGAM mRNA isozyme levels differently and increases type M mRNA.

A 1.3-kb upstream 5' region of the rat phosphoglycerate mutase m gene confers testis and skeletal muscle-specific expression in transgenic mice

Spermatogenesis is a complex process that occurs in successive mitotic, meiotic and post-meiotic phases and involves a highly regulated selective gene-expression pattern. However, this process has not been well characterised at the gene expression level due to the absence of germinal cell lines. We previously demonstrated that the rat skeletal muscle-specific gene for the glycolytic enzyme phosphoglycerate mutase is also specifically expressed in meiotic and haploid male germ cells from testis (12). To analyse the promoter elements that regulate the transcription of the phosphoglycerate mutase m gene (pgam-m)during spermatogenesis, we developed transgenic mice for a construct containing 1.3 kb from the pgam-m promoter linked to the Escherichia coli LacZ gene. RNA analysis by retrotranscription and PCR amplification of transgene expression showed transcriptional activity in the testis with a pattern during testis development that was identical to the endogenous gene. The transgene was also active in skeletal muscle but not in the adult heart in all the transgenic lines analysed. Collectively, these studies demonstrate that the 1.3 kb pgam-m promoter contains sufficient sequences to specify temporally regulated testis-specific expression as well as skeletal-muscle expression.

Segregated regulatory elements direct beta-myosin heavy chain expression in response to altered muscle activity

Our previous transgenic analyses revealed that a 600-base pair beta-myosin heavy chain (betaMyHC) promoter conferred mechanical overload (MOV) and non-weight-bearing (NWB) responsiveness to a chloramphenicol acetyltransferase reporter gene. Whether the same DNA regulatory element(s) direct betaMyHC expression following MOV or NWB activity in vivo remains unknown. We now show that a 293-base pair betaMyHC promoter fused to chloramphenicol acetyltransferase (beta293) responds to MOV, but not NWB activity, indicating a segregation of these two diverse elements. Inclusion of the betaMyHC negative regulatory element (-332 to -300; betaNRE) within transgene beta350 repressed expression in all transgenic lines. Electrophoretic mobility shift assays showed highly enriched binding activity only in NWB soleus nuclear extracts that was specific to the distal region of the betaNRE sense strand (dbetaNRE-S; -332 to -311). Supershift electrophoretic mobility shift assay revealed that the binding at the distal region of the betaNRE sense strand was antigenically distinct from cellular nucleic acid-binding protein and Y-box-binding factor 1, two proteins shown to bind this element. Two-dimensional UV cross-linking and shift Southwestern blotting analyses detected two proteins (50 and 52 kDa) that bind to this element. These in vivo results demonstrate that segregated betaMyHC promoter elements transcriptionally regulate betaMyHC transgene expression in response to two diverse modes of neuromuscular activity.

PGAM-M expression is regulated pretranslationally in hindlimb muscles and under altered loading conditions

Enzymatic activity from the muscle-specific isoform of phosphoglycerate mutase (PGAM-M) is higher within glycolytic skeletal muscles than in oxidative muscles. The hypothesis that PGAM-M is regulated pretranslationally among muscles of the hindlimb was tested using enzymatic assays, Western blots, and Northern blots. We further investigated the regulatory level(s) at which PGAM-M gene expression is controlled during hindlimb unweighting. PGAM-M mRNA and immunoreactive protein levels were fourfold lower in the rat soleus muscle than in the tibialis anterior (TA), plantaris, and extensor digitorum longus muscles. Four weeks of unweighting induced a 2.5-fold increase in PGAM enzymatic activity within the soleus muscle, a 1.8-fold increase in PGAM-M immunoreactivity, and a 3. 5-fold increase in PGAM-M mRNA. To examine potential transcriptional regulatory mechanisms, the proximal 400 bp of the rat PGAM-M promoter were linked to a firefly luciferase and injected into normal and unweighted TA and soleus muscles. Firefly luciferase activity was elevated two- to threefold in the TA and the unweighted soleus over the normal soleus muscle. These data suggest that PGAM-M expression is pretranslationally regulated among muscle types and within unweighted slow-twitch muscle. Furthermore, the proximal 400 bp of the PGAM-M promoter contains cis-acting sequences to allow muscle-type-specific expression of a reporter gene and responsiveness to soleus muscle unweighting.

CARP, a cardiac ankyrin repeat protein, is downstream in the Nkx2-5 homeobox gene pathway

To identify the molecular pathways that guide cardiac ventricular chamber specification, maturation and morphogenesis, we have sought to characterize factors that regulate the expression of the ventricular myosin light chain-2 gene, one of the earliest markers of ventricular regionalization during mammalian cardiogenesis. Previously, our laboratory identified a 28 bp HF-la/MEF-2 element in the MLC-2v promoter region, which confers cardiac ventricular chamber-specific gene expression during murine cardiogenesis, and showed that the ubiquitous transcription factor YB-1 binds to the HF-la site in conjunction with a co-factor. In a search for interacting co-factors, a nuclear ankyrin-like repeat protein CARP (cardiac ankyrin repeat protein) was isolated from a rat neonatal heart cDNA library by yeast two-hybrid screening, using YB-1 as the bait. Co-immunoprecipitation and GST-CARP pulldown studies reveal that CARP forms a physical complex with YB-1 in cardiac myocytes and immunostaining shows that endogenous CARP is localized in the cardiac myocyte nucleus. Co-transfection assays indicate that CARP can negatively regulate an HF-1-TK minimal promoter in an HF-1 sequence-dependent manner in cardiac myocytes, and CARP displays a transcriptional inhibitory activity when fused to a GAL4 DNA-binding domain in both cardiac and noncardiac cell context. Northern analysis revealed that carp mRNA is highly enriched in the adult heart, with only trace levels in skeletal muscle. During murine embryogenesis, endogenous carp expression was first clearly detected as early as E8.5 specifically in heart and is regulated temporally and spatially in the myocardium. Nkx2-5, the murine homologue of Drosophila gene tinman was previously shown to be required for heart tube looping morphogenesis and ventricular chamber-specific myosin light chain-2 expression during mammalian heart development. In Nkx2-5(−/−)embryos, carp expression was found to be significantly and selectively reduced as assessed by both whole-mount in situ hybridizations and RNase protection assays, suggesting that carp is downstream of the homeobox gene Nkx2-5 in the cardiac regulatory network. Co-transfection assays using a dominant negative mutant Nkx2-5 construct with CARP promoter-luciferase reporter constructs in cardiac myocytes confirms that Nkx2-5 either directly or indirectly regulates carp at the transcriptional level. Finally, a carp promoter-lacZ transgene, which displays cardiac-specific expression in wild-type and Nkx2-5(+/−) background, was also significantly reduced in Nkx2-5(−/−) embryos, indicating that Nkx2-5 either directly or indirectly regulates carp promoter activity during in vivo cardiogenesis as well as in cultured cardiac myocytes. Thus, CARP is a YB-1 associated factor and represents the first identified cardiac-restricted downstream regulatory gene in the homeobox gene Nkx2-5 pathway and may serve as a negative regulator of HF-1-dependent pathways for ventricular muscle gene expression.

Distribution of phosphoglycerate mutase isozymes in rat, rabbit and human tissues

The distribution of phosphoglycerate mutase isozymes (types MM, MB and BB) in rat, rabbit and human tissues has been studied by electrophoresis on cellulose acetate and by highly-resolutive ion exchange chromatography. In the three species, muscle is the tissue with higher phosphoglycerate mutase activity. Heart is the only tissue with the three phosphoglycerate mutase isozymes in substantial amounts. Skeletal muscle contains mostly type MM isozyme and the other tissues possess almost exclusively type BB isozyme. Even in the presence of inhibitors, adenylate kinase can interfere with the staining reactions when large samples are analyzed and a long period of incubation is required.

Quantitative discrimination of MEF2 sites

Myocyte-specific enhancer factor 2 (MEF2) is a family of closely related transcription factors that play a key role in the differentiation of muscle tissues and are important in the muscle-specific expression of a number of genes. Given the centrality of MEF2 in muscle differentiation, regulatory regions newly determined to be muscle specific are often studied for potential MEF2 binding sites. Possible sites are often located by comparison to a homologous gene or by matching to the consensus MEF2 sequence. Enough data have accumulated that a richer description of the MEF2 binding site, a position weight matrix, can be reliably constructed and its usefulness can be assessed. It was shown that scores from such a matrix approximate MEF2 binding energy and enable recognition of naturally occurring MEF2 sites with high sensitivity and specificity. Regulation of genes via MEF2-like sites is complicated by the fact that a number of transcription factors are involved. Not only is MEF2 itself a family of proteins, but several other, nonhomologous, transcription factors overlap MEF2 in DNA-binding specificity. Thus, more quantitative methods for recognizing potential sites may help with the lengthy process of disentangling the complex regulatory circuits of muscle-specific expression.

EFIA/YB-1 is a component of cardiac HF-1A binding activity and positively regulates transcription of the myosin light-chain 2v gene

Transient assays in cultured ventricular muscle cells and studies in transgenic mice have identified two adjacent regulatory elements (HF-1a and HF-1b/MEF-2) as required to maintain ventricular chamber-specific expression of the myosin light-chain 2v (MLC-2v) gene. A rat neonatal heart cDNA library was screened with an HF-1a binding site, resulting in the isolation of EFIA, the rat homolog of human YB-1. Purified recombinant EFIA/YB-1 protein binds to the HF-1a site in a sequence-specific manner and contacts a subset of the HF-1a contact points made by the cardiac nuclear factor(s). The HF-1a sequence contains AGTGG, which is highly homologous to the inverted CCAAT core of the EFIA/YB-1 binding sites and is found to be essential for binding of the recombinant EFIA/YB-1. Antiserum against Xenopus YB-3 (100% identical in the DNA binding domain and 89% identical in overall amino acid sequence to rat EFIA) can specifically abolish a component of the endogenous HF-1a complex in the rat cardiac myocyte nuclear extracts. In cotransfection assays, EFIA/YB-1 increased 250-bp MLC-2v promoter activity by 3.4-fold specifically in the cardiac cell context and in an HF-1a site-dependent manner. EFIA/YB-1 complexes with an unknown protein in cardiac myocyte nuclear extracts to form the endogenous HF-1a binding activity. Immunocoprecipitation revealed that EFIA/YB-1 has a major associated protein of approximately 30 kDa (p30) in cardiac muscle cells. This study suggests that EFIA/YB-1, together with the partner p30, binds to the HF-1a site and, in conjunction with HF-1b/MEF-2, mediates ventricular chamber-specific expression of the MLC-2v gene.

The muscle-specific phosphoglycerate mutase gene is specifically expressed in testis during spermatogenesis

Spermatogenesis is a dramatic differentiation process which involves very selective but poorly characterized gene-expression patterns. To gain insight into this process, we have investigated the expression during spermatogenesis of the genes that encode phosphoglycerate mutase, an essential glycolytic enzyme for the spermatozoa energy supply. By using cDNA and genomic probes we demonstrate the presence in testis of a mRNA corresponding to the muscle-specific phosphoglycerate mutase which shows a longer poly(A) tail. This muscle-specific gene is submitted to developmental regulation during testis maturation and begins to be expressed at postnatal day 22, when germ cells start to enter into meiosis. Northern blot and in situ hybridization experiments show that in contrast to what happens during skeletal-muscle differentiation, PGAM-M gene expression during spermatogenesis is not coupled to constitutive phosphoglycerate mutase (PGAM-B) gene repression. Thus, the muscle-specific PGAM-M gene constitutes a meiotic gene and therefore represents a very interesting model to study differential tissue-specific gene expression.