The myosin alkali light chain proteins and their genes

Myosin isoforms and their light chains from the ventricular muscle of the urodelan amphibian Pleurodeles waltlii: comparison with myosin from skeletal muscles

Myosin extracted from ventricular muscle of the urodelan amphibian Pleurodeles waltlii was analyzed in comparison with myosin extracted from skeletal muscles by native, one-dimensional SDS gel electrophoresis and two-dimensional gel electrophoresis. Two myosin isoforms were detected in ventricular muscle using pyrophosphate gel electrophoresis. These isomyosins contained two types of light chain subunits, LC1v and LC2v. Two-dimensional gel electrophoresis showed that LC1v comigrated with the slow light chain LC1s, whereas LC2v was characterized by a specific mobility, distinct from LC2s and LC2f. Diaphragm muscle was characterized by the coexistence of larval and adult myosin isoforms.

Morderate diabetes alters myosin isoenzyme distribution in cardiac but not skeletal muscle of male rats

Diabetes is known to alter the myosin phenotype of striated muscle, but the impact of the same diabetic state on different types of striated muscles remains unknown. Therefore, this study determined the myosin isoenzyme profile in the left ventricle, soleus, plantaris, and extensor digitorium longus (EDL) of young male rats made moderately diabetic with streptozotocin, (45 mg/kg, ip). Eight weeks after the single streptozotocin injection, tissues were collected and subsequently electrophoretically analyzed for native myosin isoenzyme distribution. Skeletal muscles were additionally analyzed for myosin heavy chain distribution. Neither the native myosin isoform nor the myosin heavy chain (MHC) distribution profiles of the skeletal muscles were altered by the diabetic state. In contrast, the high ATPase cardiac isoform, VI, was significantly replaced by the low ATPase isoform, V3 (p < 0.05). These results demonstrate that striated muscle responds to a moderate diabetic state in a limited and muscle specific fashion. Significantly, the change in the cardiac myosin isoform profile is comparable to that which occurs in a more severe diabetic state.

A skeletal muscle-specific enhancer regulated by factors binding to E and CArG boxes is present in the promoter of the mouse myosin light-chain 1A gene

The mouse myosin light-chain 1A (MLC1A) gene, expressed in the atria of the adult heart, is one of the first muscle genes to be activated when skeletal as well as cardiac muscles form in the embryo. It is also transcribed in skeletal muscle cell lines at the onset of differentiation. Transient transfection assays of mouse skeletal muscle cell lines with DNA constructs containing MLC1A promoter fragments fused to the chloramphenicol acetyltransferase (CAT) gene show that the first 630 bp of the promoter is sufficient to direct expression of the reporter gene during myotube formation. Two E boxes located at bp -76 and -519 are necessary for this regulation. MyoD and myogenin proteins bind to them as heterodimers with E12 protein and, moreover, transactivate them in cotransfection experiments with the MLC1A promoter in nonmuscle cells. Interestingly, the effect of mutating each E box is less striking in primary cultures than in the C2 or Sol8 muscle cell line. A DNA fragment from bp -36 to -597 confers tissue- and stage-specific activity to the herpes simplex virus thymidine kinase promoter in both orientations, showing that the skeletal muscle-specific regulation of the MLC1A gene is under the control of a muscle-specific enhancer which extends into the proximal promoter region. At bp -89 is a diverged CArG box, CC(A/T)6AG, which binds the serum response factor (SRF) in myotube nuclear extracts, as does the wild-type sequence, CC(A/T)6GG. Both types of CArG box also bind a novel myotube-enriched complex which has contact points with the AT-rich part of the CArG box and adjacent 3' nucleotides. Mutations within the CArG box distinguish between the binding of this complex and binding of SRF; only SRF binding is directly involved in the specific regulation of the MLC1A gene in skeletal muscle cell lines.

Inhibition of transcription factor GATA-4 expression blocks in vitro cardiac muscle differentiation

Commitment of mesodermal cells to the cardiac lineage is a very early event that occurs during gastrulation, and differentiation of cardiac muscle cells begins in the presomite stage prior to formation of the beating heart tube. However, the molecular events, including gene products that are required for differentiation of cardiac muscle cells, remain essentially unknown. GATA-4 is a recently characterized cardiac muscle-restricted transcription factor whose properties suggest an important regulatory role in heart development. We tested the role of GATA-4 in cardiac differentiation, using the pluripotent P19 embryonal carcinoma cells, which can be differentiated into beating cardiac muscle cells. In this system, GATA-4 transcripts and protein are restricted to cells committed to the cardiac lineage, and induction of GATA-4 precedes expression of cardiac marker genes and appearance of beating cells. Inhibition of GATA-4 expression by antisense transcripts blocks development of beating cardiac muscle cells and interferes with expression of cardiac muscle markers. These data indicate that GATA-4 is necessary for development of cardiac muscle cells and identify for the first time a tissue-specific transcription factor that may be crucial for early steps of mammalian cardiogenesis.

Chamber-specific expression of human myocardial proteins detected by two-dimensional gel electrophoresis

High resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), followed by computer-assisted image analysis (PDQUEST) was used to screen atrial and ventricular protein patterns for quantitative and qualitative differences in protein expression. Myocardial proteins from left ventricular (LV) and right atrial (RA) samples from end-stage, failing explanted hearts and from a healthy donor heart (control) were separated by 2-D large gel electrophoresis. Ten RA versus ten LV gels from explanted dilated cardiomyopathic (DCM) hearts were analyzed for quantitative differences in their spot patterns. Of the 197 spots matched to every gel, 40 spots differed significantly in intensity between RA and LV for DCM patients. A larger number of atrial and ventricular gels (20 RA, 20 LV) from DCM patients and from a healthy donor heart (4 RA, 4 LV gels) were analyzed for qualitative differences in protein expression. Three protein spots (SSP 1120: M(r)/pI:20.5 kDa/4.6; SSP 1119: M(r)/pI:20.6 kDa/4.5; SSP 0117:M(r)/pI:20.7/ < 4.5) that are present in all RA gels for DCM patients are absent in all LV gels. Two protein spots (SSP 0112: M(r)/pI:17.2 kDa,/ < 4.4; SSP 0114:M(r)/pI:17.6 kDa/ < 4.4) occur only in all LV gels but not in the RA gels. These five qualitatively differing spots are identical in DCM patients and in the healthy donor heart. Some of the differing spots were internally sequenced and identified as myosin light chain isoforms (myosin light chain 2, atrial; myosin light chain 2, ventricular; myosin light chain 1, atrial) with the Protein Identification Resource (PIR) accession numbers A44451, S03708, A30881, respectively. Additionally, phosphoglycerate mutase (PIR: JQ0750) and ATP synthase alpha chain (PIR: S17193) were identified. Thus, quantitative and qualitative differences between atrial and ventricular protein patterns were identified by 2-D PAGE. A characteristic distribution of myosin light chains between atrial and ventricular human myocardium was found using our approach.

Myosin light chain 3F regulatory sequences confer regionalized cardiac and skeletal muscle expression in transgenic mice

The myosin light chain IF/3F locus contains two independent promoters, MLC1F and MLC3F, which are differentially activated during skeletal muscle development. Transcription at this locus is regulated by a 3' skeletal muscle enhancer element, which directs correct temporal and tissue-specific expression from the MLC1F promoter in transgenic mice. To investigate the role of this enhancer in regulation of the MLC3F promoter in vivo, we have analyzed reporter gene expression in transgenic mice containing lacZ under transcriptional control of the mouse MLC3F promoter and 3' enhancer element. Our results show that these regulatory elements direct strong expression of lacZ in skeletal muscle; the transgene, however, is activated 4-5 d before the endogenous MLC3F promoter, at the time of initiation of MLC1F transcription. In adult mice, transgene activity is downregulated in muscles that have reduced contributions of type IIB fibers (soleus and diaphragm). The rostrocaudal positional gradient of transgene expression documented for MLC1F transgenic mice (Donoghue, M., J. P. Merlie, N. Rosenthal, and J. R. Sanes. 1991. Proc. Natl. Acad. Sci. USA. 88:5847-5851) is not seen in MLC3F transgenic mice. Although MLC3F was previously thought to be restricted to skeletal striated muscle, the MLC3F-lacZ transgene is expressed in cardiac muscle from 7.5 d of development in a spatially restricted manner in the atria and left ventricular compartments, suggesting that transcriptional differences exist between cardiomyocytes in left and right compartments of the heart. We show here that transgene-directed expression of the MLC3F promoter reflects low level expression of endogenous MLC3F transcripts in the mouse heart.

Muscle-specific gene expression during myogenesis in the mouse

Over the past decade, significant advances in molecular biological techniques have substantially increased our understanding of in vivo myogenesis, supplementing the information that previously had been obtained from classical embryological and morphological studies of muscle development. In this review, we have attempted to correlate morphogenetic events in developing murine muscle with the expression of genes encoding the MyoD family of myogenic regulatory factors and the contractile proteins. Differences in the pattern of expression of these genes in murine myotomal and limb muscle are discussed in the context of muscle cell lineage and environmental factors. The differences in gene expression in these two types of muscle suggest that no single coordinated pattern of gene activation is required during the initial formation of the muscles of the mouse.

A flagellar calmodulin gene of Naegleria, coexpressed during differentiation with flagellar tubulin genes, shares DNA, RNA, and encoded protein sequence elements

Two calmodulins are synthesized during differentiation of Naegleria gruberi from amoebae to flagellates; one remains in the cell body and the other becomes localized in the flagella. The single, intronless, expressed gene for flagellar calmodulin has been cloned and sequenced. The encoded protein is a typical calmodulin with four putative calcium-binding domains, but it has an amino-terminal extension of 10 divergent amino acids preceding conserved calmodulin residue 4. The transcripts encoding flagellar calmodulin and flagellate cell body calmodulin are clearly divergent. Expression of the flagellar calmodulin gene is differentiation-specific; its mRNA appears and then disappears concurrently with those encoding flagellar alpha- and beta-tubulin. Three provocative sequence elements are shared among these unrelated coexpressed genes: (i) a palindromic DNA sequence element is found in duplicate or triplicate upstream to each transcribed region; (ii) a perfect 12-nucleotide match is found near the AUG start codon of flagellar calmodulin and alpha-tubulin; and (iii) the novel amino-terminal extension of flagellar calmodulin contains a 5-amino-acid element similar to the amino terminus of flagellar alpha-tubulin. These shared sequence elements are proposed to have roles in differentiation, possibly in regulation of transcription, mRNA stability, and localization of these proteins to flagella.

A hormone-encoding gene identifies a pathway for cardiac but not skeletal muscle gene transcription

In contrast to skeletal muscle, the mechanisms responsible for activation and maintenance of tissue-specific transcription in cardiac muscle remain poorly understood. A family of hormone-encoding genes is expressed in a highly specific manner in cardiac but not skeletal myocytes. This includes the A- and B-type natriuretic peptide (ANP and BNP) genes, which encode peptide hormones with crucial roles in the regulation of blood volume and pressure. Since these genes are markers of cardiac cells, we have used them to probe the mechanisms for cardiac muscle-specific transcription. Cloning and functional analysis of the rat BNP upstream sequences revealed unexpected structural resemblance to erythroid but not to muscle-specific promoters and enhancers, including a requirement for regulatory elements containing GATA motifs. A cDNA clone corresponding to a member of the GATA family of transcription factors was isolated from a cardiomyocyte cDNA library. Transcription of this GATA gene is restricted mostly to the heart and is undetectable in skeletal muscle. Within the heart, GATA transcripts are localized in ANP- and BNP-expressing myocytes, and forced expression of the GATA protein in heterologous cells markedly activates transcription from the natural cardiac muscle-specific ANP and BNP promoters. This GATA-dependent pathway defines the first mechanism for cardiac muscle-specific transcription. Moreover, the present findings reveal striking similarities between the mechanisms controlling gene expression in hematopoietic and cardiac cells and may have important implications for studies of cardiogenesis.

A hormone-encoding gene identifies a pathway for cardiac but not skeletal muscle gene transcription

In contrast to skeletal muscle, the mechanisms responsible for activation and maintenance of tissue-specific transcription in cardiac muscle remain poorly understood. A family of hormone-encoding genes is expressed in a highly specific manner in cardiac but not skeletal myocytes. This includes the A- and B-type natriuretic peptide (ANP and BNP) genes, which encode peptide hormones with crucial roles in the regulation of blood volume and pressure. Since these genes are markers of cardiac cells, we have used them to probe the mechanisms for cardiac muscle-specific transcription. Cloning and functional analysis of the rat BNP upstream sequences revealed unexpected structural resemblance to erythroid but not to muscle-specific promoters and enhancers, including a requirement for regulatory elements containing GATA motifs. A cDNA clone corresponding to a member of the GATA family of transcription factors was isolated from a cardiomyocyte cDNA library. Transcription of this GATA gene is restricted mostly to the heart and is undetectable in skeletal muscle. Within the heart, GATA transcripts are localized in ANP- and BNP-expressing myocytes, and forced expression of the GATA protein in heterologous cells markedly activates transcription from the natural cardiac muscle-specific ANP and BNP promoters. This GATA-dependent pathway defines the first mechanism for cardiac muscle-specific transcription. Moreover, the present findings reveal striking similarities between the mechanisms controlling gene expression in hematopoietic and cardiac cells and may have important implications for studies of cardiogenesis.

Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells

Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.

Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells

Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.

Transcriptional control of the chicken cardiac myosin light-chain gene is mediated by two AT-rich cis-acting DNA elements and binding of serum response factor

Transcriptional control of the cardiac/slow skeletal alkali myosin light-chain (MLC1c/1s) gene is mediated, in part, by two highly conserved AT-rich cis-acting elements present in the immediate 5' flanking region. These elements cooperate to form an enhancer that can impart tissue specificity to heterologous promoters that are themselves not tissue specific in their pattern of expression. In the chicken, one of these elements matches the binding site for myocyte-specific enhancer-binding factor 2, while the other is a cis-acting element present in the transcriptional control regions of all striated alkali MLC genes (except MLC3f) and is referred to as the MLC box. The central decanucleotide core region of the MLC box closely resembles the CArG (CC[A/T]6GG) box of the serum response element, and the binding of muscle nuclear protein complexes to this element can be competed for with a synthetic serum response element. On the basis of their competition profiles and requirements for nonspecific competitor, two nuclear protein complexes, which compete for binding to the CArG-like region of the MLC box, have been identified. One of the complexes binds to a mutation of the CArG-like region that inactivates transcription of a linked reporter gene, while binding of the other complex is inhibited by this mutation. This latter complex reacts with an antibody to serum response factor (SRF) and exhibits the same binding characteristics as purified SRF. These results demonstrate that transcriptional control of the chicken MLC1c/1s gene resides in an upstream enhancer that is composed of two separate AT-rich elements, both of which are required to drive expression of a linked reporter gene. The binding of a nuclear protein complex containing SRF to one of these elements, the MLC box, is required for gene activation and apparently inhibited by other nuclear factors whose binding overlaps that of the SRF complex.

Transcriptional control of the chicken cardiac myosin light-chain gene is mediated by two AT-rich cis-acting DNA elements and binding of serum response factor

Transcriptional control of the cardiac/slow skeletal alkali myosin light-chain (MLC1c/1s) gene is mediated, in part, by two highly conserved AT-rich cis-acting elements present in the immediate 5' flanking region. These elements cooperate to form an enhancer that can impart tissue specificity to heterologous promoters that are themselves not tissue specific in their pattern of expression. In the chicken, one of these elements matches the binding site for myocyte-specific enhancer-binding factor 2, while the other is a cis-acting element present in the transcriptional control regions of all striated alkali MLC genes (except MLC3f) and is referred to as the MLC box. The central decanucleotide core region of the MLC box closely resembles the CArG (CC[A/T]6GG) box of the serum response element, and the binding of muscle nuclear protein complexes to this element can be competed for with a synthetic serum response element. On the basis of their competition profiles and requirements for nonspecific competitor, two nuclear protein complexes, which compete for binding to the CArG-like region of the MLC box, have been identified. One of the complexes binds to a mutation of the CArG-like region that inactivates transcription of a linked reporter gene, while binding of the other complex is inhibited by this mutation. This latter complex reacts with an antibody to serum response factor (SRF) and exhibits the same binding characteristics as purified SRF. These results demonstrate that transcriptional control of the chicken MLC1c/1s gene resides in an upstream enhancer that is composed of two separate AT-rich elements, both of which are required to drive expression of a linked reporter gene. The binding of a nuclear protein complex containing SRF to one of these elements, the MLC box, is required for gene activation and apparently inhibited by other nuclear factors whose binding overlaps that of the SRF complex.

Myosin polymorphism and differential expression in adult human skeletal muscle

1. Myosin heavy chain (HC) and light chain (LC) isoforms are expressed in a tissue-specific and developmentally-regulated manner in human skeletal muscle. 2. At least seven myosin HC isoforms are expressed in skeletal muscle of the adult. 3. Histochemically-delineated fibre types (based on the stability of myofibrillar actomyosin adenosine triphosphatase activity) in limb muscles correlate with the myosin HC content. 4. Alterations in the phenotypic expression of myosin provides a mechanism of adaptation to stresses placed upon the muscle (e.g. increased and decreased usage).

Identification of an allelic variant of isoform MLC1-V/sB (human myosin light chain)

A study of 250 specimens of human myocardium by two-dimensional gel electrophoresis revealed an allelic variant of isoform MLC1-V/sB, which was identified by immunoblotting with monoclonal antibody against MLC1-V/sB and peptide mapping after in situ tryptic digestion of electroblotted proteins. The substitution Asn-144 for His-144 was found in this new allelic variant of MLC1-V/sB.

Contractile protein gene expression in primary myotubes of embryonic mouse hindlimb muscles

The time course of contractile protein [actin, myosin heavy chain (MHC) and myosin light chain (MLC)] gene expression in the hindlimb muscles of the embryonic mouse (&lt; 15 days gestation) has been correlated with the expression of genes for the myogenic regulatory factors, myogenin and MyoD, and with morphogenetic events. At 14 days gestation, secondary myotubes are not yet present in crural muscles (M. Ontell and K. Kozeka (1984) Am. J. Anat. 171, 133–148; M. Ontell, D. Bourke and D. Hughes (1988) Am. J. Anat. 181, 267–278); therefore, all transcripts for contractile proteins found in these muscles must be produced in primary myotubes. In situ hybridization, with 35S-labeled antisense cRNAs, demonstrates the versatility of primary myotubes in that transcripts for (1) alpha-cardiac and alpha-skeletal actin, (2) MHCembryonic, MHCperinatal and MHC beta/slow, and (3) MLC1A, MLC1F and MLC3F are detectable at 14 days gestation. While the general patterns of early activation of the cardiac genes and early activation of the genes for the developmental isoforms are preserved in both myotomal and limb muscles (D. Sassoon, I. Garner and M. Buckingham (1988) Development 104, 155–164 and G. E. Lyons, M. Ontell, R. Cox, D. Sassoon and M. Buckingham (1990) J. Cell Biol. 111, 1465–1476 for myotomal muscle), there are a number of differences in contractile protein gene expression. For example, in the myotome, when myosin light chain genes are initially transcribed, hybridization signal with probe for MLC1A mRNA is greater than that with probe for MLC1F transcripts, whereas the relative intensity of signal with these same probes is reversed in the hindlimb. The order in which myosin heavy chain genes are activated is also different, with MHCembryonic and MHCperinatal preceding the appearance of MHC beta/slow transcripts in limb muscles, while MHCembryonic and MHC beta/slow appear simultaneously in the myotomes prior to MHCperinatal. In the myotome, an intense hybridization signal for alpha-cardiac and a weak signal for alpha-skeletal actin transcripts are detectable prior to myosin mRNAs, whereas in the limb alpha-cardiac actin transcripts accumulate with myosin transcripts before alpha-skeletal actin mRNA is detectable. These differences indicate that there is no single coordinate pattern of expression of contractile protein genes during initial formation of the muscles of the mouse.(ABSTRACT TRUNCATED AT 400 WORDS)

fos/jun repression of cardiac-specific transcription in quiescent and growth-stimulated myocytes is targeted at a tissue-specific cis element

Unlike that of skeletal muscle cells in which growth and differentiation appear mutually exclusive, growth stimulation of cardiac cells is characterized by transient expression of early response nuclear proto-oncogenes as well as induction of several cardiac-specific markers. This observation led to the speculation that these proto-oncogenes, particularly c-fos and c-jun, might act as positive regulators of cardiac transcription. We have examined the role of c-jun and c-fos in basal and growth-stimulated cardiac transcription, using the cardiac-specific atrial natriuretic factor (ANF) gene as a marker. The results indicate that c-jun and c-fos are negative regulators of ANF transcription. Inducers of jun and fos activity, such as mitogens and growth factors, inhibited endogenous ANF transcripts. In transient cotransfection assays, jun and fos were able to trans-repress the ANF promoter in both quiescent and alpha 1-adrenergic stimulated myocytes. This repression was specific to myocyte cultures and was not observed in nonmuscle cells. Deletion analysis indicated that repression does not require typical AP-1-binding sites (tetradecanoyl phorbol acetate response elements) or serum response elements but is targeted at a cardiac-specific element within the ANF promoter. Various Fos-related proteins, including Fra-1, Fos B, and v-Fos, were able to trans-repress ANF transcription. In addition, C-terminal c-fos mutants which no longer repress transcription of such early growth response genes as c-fos and EGR-1 retained the ability to repress ANF transcription. Repression by c-jun occurs via the N-terminal activation domain and does not require the DNA-binding domain, suggesting that proto-oncogene repression involves interaction with one or more limiting cardiac-specific coactivators.

fos/jun repression of cardiac-specific transcription in quiescent and growth-stimulated myocytes is targeted at a tissue-specific cis element

Unlike that of skeletal muscle cells in which growth and differentiation appear mutually exclusive, growth stimulation of cardiac cells is characterized by transient expression of early response nuclear proto-oncogenes as well as induction of several cardiac-specific markers. This observation led to the speculation that these proto-oncogenes, particularly c-fos and c-jun, might act as positive regulators of cardiac transcription. We have examined the role of c-jun and c-fos in basal and growth-stimulated cardiac transcription, using the cardiac-specific atrial natriuretic factor (ANF) gene as a marker. The results indicate that c-jun and c-fos are negative regulators of ANF transcription. Inducers of jun and fos activity, such as mitogens and growth factors, inhibited endogenous ANF transcripts. In transient cotransfection assays, jun and fos were able to trans-repress the ANF promoter in both quiescent and alpha 1-adrenergic stimulated myocytes. This repression was specific to myocyte cultures and was not observed in nonmuscle cells. Deletion analysis indicated that repression does not require typical AP-1-binding sites (tetradecanoyl phorbol acetate response elements) or serum response elements but is targeted at a cardiac-specific element within the ANF promoter. Various Fos-related proteins, including Fra-1, Fos B, and v-Fos, were able to trans-repress ANF transcription. In addition, C-terminal c-fos mutants which no longer repress transcription of such early growth response genes as c-fos and EGR-1 retained the ability to repress ANF transcription. Repression by c-jun occurs via the N-terminal activation domain and does not require the DNA-binding domain, suggesting that proto-oncogene repression involves interaction with one or more limiting cardiac-specific coactivators.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.

Localization of the gene coding for ventricular myosin regulatory light chain (MYL2) to human chromosome 12q23-q24.3

Human myosin light chain-2 (MYL2) is an important protein involved in the regulation of myosin ATPase activity in smooth muscle. In cardiac muscle, the precise role of MYL2 is not well understood; however, an increase in ventricular MYL2 is observed during myocardial hypertrophy in cardiac patients with valve stenosis. The chromosomal location of the gene coding for MYL2 was identified using a cloned cDNA for human MYL2. Southern blot analysis of DNA from a human/rodent somatic cell hybrid mapping panel showed that the BamHI fragment that hybridized with this cDNA probe was concordant with chromosome 12. The 768-bp cDNA was hybridized to human metaphase chromosomes. The results revealed a significant clustering of silver grains over chromosome 12 bands q23-q24.3, indicating that the gene coding for MYL2 is located in this region.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Probing myosin light chain 1 structure with monoclonal antibodies

Five monoclonal antibodies that react with different regions of myosin light chain 1 from human ventricular myocardial muscle were used to obtain information on interactions between the light chain 1 and heavy chains and generally on the tertiary structure of the light chain 1 within the myosin head. We performed Western blot assays of the five antibodies with myosins from different cardiac and skeletal muscles, with different proteolytic fragments of bovine ventricular myosin light chain 1 (LC1) and to different recombinant fragments of human ventricular LC1 and rat fast skeletal light chain LC1/LC3. The five antibodies were mapped in three different regions of the light chain 1: two antibodies mapped within the first eight amino-terminal residues, two between residues 71 and 74, and one between residues 129 and 134. The apparent dissociation constants of the last three antibodies, determined by antibody-antigen equilibria in solution, were lower than when isolated light chains were used as antigens. It is probable that the corresponding amino acids involved in the antibody epitopes were either involved in interactions between the light and heavy myosin subunits, or somehow hindered by the myosin heavy chain bulk. In contrast, the apparent dissociation constants measured for both other antibodies were higher when myosin, rather than isolated light chains, was used as antigen. Thus LC1 fixation to heavy chains within the myosin molecule induced conformation changes at the amino-terminal end of the light chain 1. No difference in the accessibility of this mobile LC1 segment was detected in the presence of actin. Finally, observed differences in epitope accessibility on the light chain LC1 in myosin, as compared with chymotryptic subfragment 1 (SF1), indicated conformational differences between native myosin and extensively studied SF1 molecules.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Regulation of acetylcholinesterase in avian heart. Studies on ontogeny and the influence of vagotomy

This article examines the role of innervation in regulating expression of acetylcholinesterase (AchE), butyrylcholinesterase (BuchE), and the muscarinic acetylcholine receptor (mAchR) in avian heart. Two distinct approaches are taken. The first approach examines the relation between the onsets of parasympathetic and sympathetic innervation and the appearance of AchE and BuchE. All molecular forms of AchE and BuchE are present in early embryonic chick heart well before the onset of parasympathetic and sympathetic innervation. These molecular forms are characterized by sedimentation coefficients of 4.5S, 11S, 15S, and 19S. With further development, the amounts of AchE fall; the reductions in AchE parallel the onset of functional parasympathetic innervation. The amounts of BuchE increase progressively throughout embryonic development, independent of autonomic innervation, and in mature chick heart predominate over the much less abundant amounts of AchE. The 15S and 19S forms of AchE in heart are lost during early embryogenesis but reappear in skeletal muscle during later embryogenesis. The second approach examines the influence of vagotomy and sympathetic denervation of 8-day-old chick myocardium on expression of the molecular forms of AchE, BuchE, mAchR, and beta-adrenergic receptors. The amounts of AchE and BuchE molecular forms in avian heart are not measurably influenced by bilateral vagotomy for a duration of 4 days, unilateral vagotomy for a duration of 25 days, or sympathetic denervation. A measurable upregulation is observed in muscarinic receptors (35-46%) after vagotomy but not sympathectomy and in beta-adrenergic receptors (29%) after sympathectomy but not vagotomy. In all cases, results in atria and ventricles are nearly identical. The present results indicate that expression of AchE in the myocardium is unique and different from that in skeletal muscle and not directly linked with autonomic innervation.

Transcription of the embryonic myosin light chain gene is restricted to type II muscle fibers in human adult masseter

We have previously demonstrated that the embryonic myosin light chain (MLC1emb) isoform whose expression is restricted to the early fetal stages in most mammalian skeletal muscles, persists throughout development in human masseter muscle. In order to go further in this study, we have compared the developmental profile of MLC1emb gene transcription in human masseter and quadriceps muscles using both Northern blotting and in situ hybridization techniques. Interestingly, whereas expression of this gene was observed in all fibers during fetal stages in both muscles, transcription in adult masseter was found to be restricted to type II fibers. Existence of a masseter-specific pathway of muscle gene regulation is discussed.

Fish myosin alkali light chains originate from two different genes

We have determined the amino acid sequences of mullet white skeletal muscle myosin alkali light chains, LC1 and LC3. These are the first available fish myosin sequences. There are 29 differences between the mullet LC1 and LC3 sequences, spread throughout all regions of their polypeptide chains, leading us to conclude that fish myosin alkali light chains originate from two different genes. This finding is in sharp contrast to all previous studies on vertebrate fast skeletal muscle myosins, which showed that in birds and mammals LC1 and LC3 are produced from a single gene by alternative RNA transcription and splicing, yielding proteins which differ only in their N-terminal segments.

Intracompartmental sorting of essential myosin light chains: molecular dissection and in vivo monitoring by epitope tagging

The isoprotein-specific intracompartmental sorting of the three essential myosin light chains (LCs), the skeletal muscle LC-1f and LC-3f and the nonmuscle LC-3nm, was investigated. Epitope tagging was used to monitor the intracellular localization to different cytoskeletal structures of the exogenously introduced constructs in adult rat cardiomyocytes (ARCs), which exhibit both stress fibers and regenerating myofibrils. LC-1f and LC-3f bind almost exclusively to the sarcomeric myosin heavy chain (MHC) with high affinity, while the LC-3nm interacts with stress fibers and sarcomeres equally well. Sorting appears to be directed by a hierarchical order of different affinities. Domain mapping by deletion and by construction of a LC-1f/3nm chimera suggests that the LCs are composed of three functionally distinct domains: a basal MHC binding site in the C-terminus; the central part, modulating the preferential interaction with MHC isoforms; and the isoprotein-specific N-terminus of the essential LC, which is probably not involved in the sorting process.

Modification in the expression and localization of contractile and cytoskeletal proteins in Schwartz-Jampel syndrome

Muscle biopsies taken from 4 patients with clinical diagnosis of Schwartz-Jampel syndrome were analyzed by enzyme-histochemical immunocytochemical and biochemical techniques. In situ distribution of the different myosin heavy chain (MHC) isoforms together with that of the cytoskeletal proteins vimentin, desmin and titin was determined in type I, type IIA, type IIB and type IIC fibers. The same muscle biopsies were analyzed for their content in myosin light chains (MLC) by two-dimensional gel electrophoresis and native myosin isoforms by pyrophosphate gel electrophoresis. The opportunity to study 4 patients of different ages, all members of the same family, permitted us to reveal several interesting features in this rare and so far poorly understood muscle pathology. (i) We observed a predominance of slow (type I) fibers in the oldest patient. (ii) Two classes of small clusters of atrophic type IIC fibers were observed. The first class corresponded to fibers which coexpressed embryonic, fetal and fast, but not slow, MHC isoforms. The fibers also displayed an abnormal distribution of desmin, vimentin and titin. The second class was composed by fibers coexpressing embryonic, fetal, fast and slow, MHC isoforms. In contrast to that observed for the first class, fibers in the second class displayed a normal pattern of expression of desmin, vimentin and titin. (iii) A familial heterogeneity was observed between the 4 patients. The pathological processes involved in the evolution of this syndrome are discussed.

Expression of myosin regulatory light chains in rat brain: characterization of a novel isoform

We have characterized cDNA clones of mRNAs encoding two distinct isoforms of myosin regulatory light chain expressed in rat brain. One clone, isolated from a cultured astrocyte cDNA library, is derived from a 1200-base mRNA that is expressed at high levels in cultured astrocytes, and at higher levels in the embryonic brain than in the adult brain. The nucleotide sequence of this cDNA is essentially identical to a previously reported cDNA encoding a smooth muscle isoform from rat aorta cells (Taubman et al., J. Cell Biol., 104 (1987) 1505-1515). The second clone hybridized to a 1300-base mRNA that is expressed abundantly in the adult brain and is the predominant species in cultured neuroblasts. Both mRNAs are expressed, to varying extents, in other muscle and nonmuscle tissues. The deduced amino acid sequences of the two isoforms differ in 4 residues out of 171. On the basis of the tissue distribution of their mRNAs and a comparison of identities among the known amino acid sequences of myosin regulatory light chains we suggest that both proteins should be considered as non-muscle isoforms. We conclude that there are at least two isoforms of the myosin regulatory light chain expressed in rat brain and that their expression is under both cell-specific and developmental regulation.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

Developmental regulation of myosin gene expression in mouse cardiac muscle

Expression of the two isoforms of cardiac myosin heavy chain (MHC), MHC alpha and MHC beta, in mammals is regulated postnatally by a variety of stimuli, including serum hormone levels. Less is known about the factors that regulate myosin gene expression in rapidly growing cardiac muscle in embryos. Using isoform-specific 35S-labeled cRNA probes corresponding to the two MHC genes and the two myosin alkali light chain (MLC) genes expressed in cardiac muscle, we have investigated the temporal and spatial pattern of expression of these different genes in the developing mouse heart by in situ hybridization. Between 7.5 and 8 d post coitum (p.c.), the newly formed cardiac tube begins to express MHC alpha, MHC beta, MLC1 atrial (MLC1A), and MLC1 ventricular (MLC1V) gene transcripts at high levels throughout the myocardium. As a distinct ventricular chamber forms between 8 and 9 d p.c., MHC beta mRNAs begin to be restricted to ventricular myocytes. This process is complete by 10.5 d p.c. During this time, MHC alpha mRNA levels decrease in ventricular muscle cells but continue to be expressed at high levels in atrial muscle cells. MHC alpha transcripts continue to decrease in ventricular myocytes until 16 d p.c., when they are detectable at low levels, but then increase, and finally replace MHC beta mRNAs in ventricular muscle by 7 d after birth. Like MHC beta, MLC1V transcripts become restricted to ventricular myocytes, but at a slower rate. MLC1V mRNAs continue to be detected at low levels in atrial cells until 15.5 d p.c. MLC1A mRNA levels gradually decrease but are still detectable in ventricular cells until a few days after birth. This dynamic pattern of changes in the myosin phenotype in the prenatal mouse heart suggests that there are different regulatory mechanisms for cell-specific expression of myosin isoforms during cardiac development.

The expression of myosin genes in developing skeletal muscle in the mouse embryo

Using in situ hybridization, we have investigated the temporal sequence of myosin gene expression in the developing skeletal muscle masses of mouse embryos. The probes used were isoform-specific, 35S-labeled antisense cRNAs to the known sarcomeric myosin heavy chain and myosin alkali light chain gene transcripts. Results showed that both cardiac and skeletal myosin heavy chain and myosin light chain mRNAs were first detected between 9 and 10 d post coitum (p.c.) in the myotomes of the most rostral somites. Myosin transcripts appeared in more caudal somites at later stages in a developmental gradient. The earliest myosin heavy chain transcripts detected code for the embryonic skeletal (MHCemb) and beta-cardiac (MHC beta) isoforms. Perinatal myosin heavy chain (MHCpn) transcripts begin to accumulate at 10.5 d p.c., which is much earlier than previously reported. At this stage, MHCemb is the major MHC transcript. By 12.5 d p.c., MHCpn and MHCemb mRNAs are present to an equal extent, and by 15.5 d p.c. the MHCpn transcript is the major MHC mRNA detected. Cardiac MHC beta transcripts are always present as a minor component. In contrast, the cardiac MLC1A mRNA is initially more abundant than that encoding the skeletal MLC1F isoform. By 12.5 d p.c. the two MLC mRNAs are present at similar levels, and by 15.5 d p.c., MLC1F is the predominant MLC transcript detected. Transcripts for the ventricular/slow (MLC1V) and another fast skeletal myosin light chain (MLC3F) are not detected in skeletal muscle before 15 d p.c., which marks the beginning of the fetal stage of muscle development. This is the first stage at which we can detect differences in expression of myosin genes between developing muscle fibers. We conclude that, during the development of the myotome and body wall muscles, different myosin genes follow independent patterns of activation and accumulation. The data presented are the first detailed study of myosin gene expression at these early stages of skeletal muscle development.

Comparative structural and enzymatic properties of skeletal muscle myosin from neonatal and adult rabbits

Several structural and enzymatic properties of myosin from skeletal muscles of neonatal and adult rabbits were compared. Electrophoretic analyses and proteolysis experiments indicated that differences between the two myosin types could be attributed to their heavy subunits. Circular dichroism measurements of subfragment-1 species, and trypsin-digested derivatives showed that the neonatal protein contained less alpha-helices than the adult form. The Mg2(+)-ATPase activity of neonatal myosin was lower than that of adult myosin, especially in the presence of actin. In comparison with adult subfragment-1, it was found that the binding of ATP analogues such as adenosine 5'-[beta, gamma-imino]triphosphate and PPi, or that of ATP (as deduced from the apparent KmATP) to neonatal subfragment-1 in the presence of actin was enhanced, while that of ADP was decreased. On the other hand, the association of actin with the ADP - neonatal-subfragment-1 complex was weaker. These features must be expressed in the cyclical actin-myosin association/dissociation steps occurring in ATP hydrolysis, and more particularly in the reassociation of actin with the ATP-hydrolysis-products - myosin complex.

Actin and myosin expression during development of cardiac muscle from cultured embryonal carcinoma cells

P19 embryonal carcinoma cells are multipotential stem cells that differentiate into striated muscle as well as some other cell types when aggregated and exposed to dimethyl sulfoxide (DMSO). Immunofluorescence experiments using monospecific antibodies indicated that the majority of muscle cells were mononucleate and contained four myosin isoforms normally found in cardiac muscle; atrial and ventricular myosin heavy chains, ventricular myosin light chain 1, and atrial myosin light chain 2. Northern blot analysis of RNA isolated from differentiating cultures indicated that cardiac actin and skeletal actin mRNAs were expressed at similar levels and with identical kinetics during the differentiation of P19-derived myocytes. These results demonstrate that most of the P19-derived myocytes are of the cardiac type and suggest that they closely resemble the cells of the early embryonic myocardium.

Human skeletal muscle myosin light chains analyzed by immobilized pH gradients during ontogenesis: identification of new phosphorylatable isoforms of light chain 2

Previous studies using two-dimensional gel electrophoresis have described adult and fetal isoforms of skeletal muscle myosin light chains (MLC). They have also revealed an embryo-specific light chain (LC1emb), apparently absent in most adult skeletal muscles. In order to characterize more thoroughly the MLC family, we have analyzed the MLCs from human skeletal muscle at different developmental stages using a two-dimensional electrophoresis technique with an immobilized pH gradient in the first dimension. The high resolution of this novel technique, resolving components which in isoelectric points are less than or equal to 0.01 pH, combined with sensitive silver staining, has allowed us to identify four phosphorylatable isoforms of MLC2: two slow-myosin light chains (MLC2Sa and b), two fast myosin light chains (MLC2Fa and b), and their phosphorylated counterparts: MLC2SaP and bP, MLC2FaP and bP. The following major modifications during development were observed: (i) The embryonic LC (LC1emb) persists up to at least 26 weeks of fetal life. (ii) The polymorphism of LC2 is already evident at 10 weeks of development but only the nonphosphorylated forms of LC2S and LC2F seem to be present. The LC2Fa form is predominant. As early as 26 weeks of fetal life, the 4 phosphorylated forms are detected. In the adult, LC2Fb is a minor component. (iii) LC3F (fast) is already expressed at an early embryonic stage (10 weeks).

Characterization of human myosin light chains 1sa and 3nm: implications for isoform evolution and function

We have isolated a cDNA clone for the human slow-twitch muscle isoform myosin light-chain 1slow-a (MLC1sa) from a skeletal muscle library and for the human nonmuscle isoform myosin light-chain 3nonmuscle (MLC3nm) from a fibroblast library. The nucleotide sequence of both isoforms was determined, and isoform-specific probes were constructed. In addition, MLC1sa was subsequently isolated from the fibroblast library. MLC1sa and MLC3nm were found to be very closely related to each other and distant from all other myosin light-chain isoforms so far described. We concluded that MLC1sa arose by duplication of MLC3nm rather than from any other isoform. A comparison was made between all human myosin light chains described to date and a model proposed for the evolution of this multigene family. A comparison between human and chicken myosin light-chain isoforms showed that human isoforms are more similar to their chicken counterparts than to human MLC1sa. The expression of MLC1sa and MLC3nm was studied in humans, rabbits, mice, and rats. MLC1sa was detected at the onset of both human and murine myogenesis in vitro. With development, MLC1sa may be replaced by the other slow-twitch muscle isoform, 1sb, in slow-twitch skeletal muscle, but the proportion of MLC1sa to 1sb expression varies between different species. MLC1sa was detected in nonmuscle cells in humans, mice, and rats. MLC3nm was the major nonmuscle alkaline myosin light chain in all species tested, but its pattern of expression in nonmuscle tissues was not identical to that of beta- or gamma-actin. We have shown that in the human, as in the chicken, one exon is spliced out of the MLC3nm transcript in smooth muscle to give an alternative product. We concluded that all alkali myosin light-chain isoforms may be functionally different.

Characterization of human myosin light chains 1sa and 3nm: implications for isoform evolution and function

We have isolated a cDNA clone for the human slow-twitch muscle isoform myosin light-chain 1slow-a (MLC1sa) from a skeletal muscle library and for the human nonmuscle isoform myosin light-chain 3nonmuscle (MLC3nm) from a fibroblast library. The nucleotide sequence of both isoforms was determined, and isoform-specific probes were constructed. In addition, MLC1sa was subsequently isolated from the fibroblast library. MLC1sa and MLC3nm were found to be very closely related to each other and distant from all other myosin light-chain isoforms so far described. We concluded that MLC1sa arose by duplication of MLC3nm rather than from any other isoform. A comparison was made between all human myosin light chains described to date and a model proposed for the evolution of this multigene family. A comparison between human and chicken myosin light-chain isoforms showed that human isoforms are more similar to their chicken counterparts than to human MLC1sa. The expression of MLC1sa and MLC3nm was studied in humans, rabbits, mice, and rats. MLC1sa was detected at the onset of both human and murine myogenesis in vitro. With development, MLC1sa may be replaced by the other slow-twitch muscle isoform, 1sb, in slow-twitch skeletal muscle, but the proportion of MLC1sa to 1sb expression varies between different species. MLC1sa was detected in nonmuscle cells in humans, mice, and rats. MLC3nm was the major nonmuscle alkaline myosin light chain in all species tested, but its pattern of expression in nonmuscle tissues was not identical to that of beta- or gamma-actin. We have shown that in the human, as in the chicken, one exon is spliced out of the MLC3nm transcript in smooth muscle to give an alternative product. We concluded that all alkali myosin light-chain isoforms may be functionally different.

Heterogenic mRNAs with an identical protein-coding region of the human embryonic myosin alkali light chain in skeletal muscle cells

The formation of human myotubes in culture is accompanied by the induction of developmentally regulated, muscle-specific genes. We have studied the expression of human myosin light chain proteins and mRNAs during myogenesis in culture, in particular the skeletal embryonic myosin light chain 1 (MC1emb), which is indistinguishable from MLC1 of adult atrial cardiac muscle (MLC1A) as has been shown for rodent and bovine MLC1emb. We have identified distinct MLC1emb/MLC1A mRNAs in cultured human skeletal muscle cells that differ in their 5' and 3' untranslated regions but contain identical protein-coding regions. The alternative 3' untranslated region is detectable also in RNA of human atria. The different MLC1emb RNAs are likely to be encoded by one gene. It appears that the two MLC1emb 5' untranslated regions of the human gene are specific for man. In the mouse, only one 5' untranslated region of the MLC1emb gene has been detected.

A possible role for myosin light chain 1 slow of bovine muscle

Myosin light chain 1 slow (LC1slow) was purified from the bovine slow-twitch muscle masseter and used as an antigen in chicken. The antibody fraction from egg yolk was employed to show the distribution of LC1slow in samples of diverse bovine muscles. Adjacent cryosections were cut. One section was stained for myofibrillar adenosine triphosphatase (ATPase) activity and the other was challenged with the antibody fraction, followed by a fluorescently-labelled secondary antibody. The ATPase section (viewed with conventional optical microscopy) displayed fast and slow fibres, whereas the antibody-labelled section (viewed with epifluorescent optics) showed which of the slow fibres contained LC1slow. In some of the muscles examined, the slow fibres contained variable amounts of LC1slow. In other muscles, such as those involved in chewing, the slow fibres were all rich in LC1slow. Slow fibres in yet other muscles, for example deep hip muscles and an extraocular muscle, were devoid of LC1slow. Based on the probable functions of the muscles examined, the hypothesis is advanced that LC1slow is associated with movement, its absence from slow fibres signifying a postural role for those slow fibres. The present work also documents the LC1slow content and muscle fibre composition of 14 hind leg muscles of a single ox. The results suggest postural and locomotory roles for certain muscles.

Myosin light chain 3 synthesis during chick pectoralis muscle development in ovo

The level of myosin light chain 3 (LC3) in vertebrate skeletal muscle is developmentally regulated in a tissue-specific manner. We have used the RNA-cDNA hybridization assay to quantitate LC3 mRNA levels at various stages of chick pectoralis muscle development in ovo. The LC3 mRNA was found significantly in breast muscle only on Day 16 in ovo and later, the level of mRNA ranging from about 30 to 32% of that present in adult tissue. These values are in good agreement with the corresponding levels of LC3 in embryonic muscle. These results do not support the earlier reports that the protein and mRNA for LC3 accumulate in a noncoordinate manner in embryonic pectoralis muscle and they suggest that LC3 synthesis in ovo is regulated primarily at the transcriptional level.

Characterization of a rat myosin alkali light chain gene expressed in ventricular and slow twitch skeletal muscles

Mammalian cardiac muscle contains two myosin alkali light chains: 1) the atrial light chain (MLC1A), and 2) the ventricular light chain (MLC1V) predominantly expressed either in the atrium or in the ventricle. In this report we describe the isolation and characterization of the complete gene for rat MLC1V. The rat MLC1V gene is approximately 6.5 kb long and the mRNA coding sequences are organized in 7 different exons. Comparison of this gene sequence with other known MLC1 gene sequences revealed that the exon-intron organization is highly conserved within the MLC1 gene family. The derived protein sequence of rat MLC1V showed a higher sequence homology with human ventricular (96%) MLC1V than with rat fast skeletal MLC1f (74%), suggesting functional similarities between different MLC1V proteins. S1 nuclease mapping and primer extension analysis demonstrated that this gene is expressed only in ventricular and slow twitch skeletal muscle tissues and is transcribed from the same promoter and transcription initiation site.