The myoD gene family: nodal point during specification of the muscle cell lineage

Muscle fiber pattern is independent of cell lineage in postnatal rodent development

Muscle fibers specialized for fast or slow contraction are arrayed in characteristic patterns within developing limbs. Clones of myoblasts analyzed in vitro express fast and slow myosin isoforms typical of the muscle from which they derive. As a result, it has been suggested that distinct myoblast lineages generate and maintain muscle fiber pattern. We tested this hypothesis in vivo by using a retrovirus to label myoblasts genetically so that the fate of individual clones could be monitored. Both myoblast clones labeled in muscle in situ and clones labeled in tissue culture and then injected into various muscles contribute progeny to all fiber types encountered. Thus, extrinsic signals override the intrinsic commitment of myoblast nuclei to particular programs of gene expression. We conclude that in postnatal development, pattern is not dictated by myoblast lineage.

The MyoD family of myogenic factors is regulated by electrical activity: isolation and characterization of a mouse Myf-5 cDNA

A full-length cDNA coding for a homolog of the human Myf-5 was isolated from a BC3H-1 mouse library and characterized. The clone codes for a protein of 255 amino acids that is 89%, 88% and 68% identical to the human, bovine and Xenopus myf-5, respectively. The mouse Myf-5 cDNA (mmyf-5), as well as sequences coding for MyoD, myogenin and Mrf-4, were used to probe Northern blots to analyze the effects of innervation on the expression of the MyoD family of myogenic factors. Mouse myf-5, MyoD and myogenin mRNAs levels were found to decline in hind limb muscles of mice between embryonic day 15 (E15) and the first postnatal week, a period that coincides with innervation. In contrast, Mrf-4 transcripts increase during this period and reach steady-state levels by 1-week after birth. To distinguish if the changes in myogenic factor expression are due to a developmental program or to innervation, mRNA levels were analyzed at different times after muscle denervation. Mmyf-5 transcripts begin to accumulate 2 days postdenervation; after 1 week levels are 7-fold higher than in innervated muscle. Mrf-4, MyoD and myogenin transcripts begin to accumulate as soon as 8h after denervation, and attain levels that are 8-, 15- and 40-fold higher than found in innervated skeletal muscle, respectively. The accumulation of these three mRNAs precedes the increase of nicotinic acetylcholine receptor alpha subunit transcripts, a gene that is transcriptionally regulated by MyoD-related factors in vitro. Using extracellular electrodes to directly stimulate in situ the soleus muscle of rats, we found that 'electrical activity' per se, in absence of the nerve, represses the increases of myogenic factor mRNAs associated with denervation.

Functional antagonism between c-Jun and MyoD proteins: a direct physical association

The product of the proto-oncogene Jun inhibits myogenesis. Constitutive expression of Jun in myoblasts interferes with the expression and the function of MyoD protein. In transient transfection assays Jun inhibits transactivation of the MyoD promoter, the muscle creatine kinase enhancer, and a reporter gene linked to MyoD DNA-binding sites. Conversely, MyoD suppresses the transactivation by Jun of genes linked to an AP-1 site. We demonstrate that both in vivo and in vitro MyoD and Jun proteins physically interact. Mutational analysis suggests that this interaction occurs via the leucine zipper domain of Jun and the helix-loop-helix region of MyoD.

The transcription of MyoD1 and myogenin genes in thymic cells in vivo

The skeletal muscle specific genes MyoD1 and myogenin are closely associated with commitment of cells to the myogenic lineage and differentiation of skeletal muscle precursor cells. The transcription of these genes was studied in the thymus where mononuclear cells termed myoid cells appear to closely resemble skeletal muscle precursors. In thymus from adult SJL/J and BALB/c mice, in situ hybridization with either MyoD1 or myogenin riboprobes showed probe-positive cells concentrated in the medullary region. In neonatal thymus, mRNA for these genes was not detected. These data are the first demonstration in a higher vertebrate of MyoD1 and myogenin expression in a tissue other than skeletal muscle. The sustained expression of MyoD1 and myogenin genes in thymi of adult mice shows that myoid cells are not equivalent to quiescent stem cells of mature skeletal muscle. In addition, studies with antistriational antibodies indicate that myoid cells do not continue to differentiate within the normal murine thymic environment. This arrested differentiation process presents an unusual model for investigating conditions regulating myogenesis in vivo.

Twin of I-POU: A two amino acid difference in the I-POU homeodomain distinguishes an activator from an inhibitor of transcription

I-POU, a POU domain nuclear protein that lacks two conserved basic amino acids of the POU homeodomain is coexpressed in the developing Drosophila nervous system with a second POU domain transcription factor, Cf1-a. I-POU does not bind to DNA but forms a POU domain-mediated, high affinity heterodimer with Cf1-a, inhibiting its ability to bind and activate the dopa decarboxylase gene. The I-POU/Cf1-a dimerization interface encompasses only the N-terminal basic region and helices 1 and 2 of the POU homeodomains with precise amino acid and alpha-helical requirements. twin of I-POU, an alternatively spliced transcript of the I-POU gene, encodes a protein containing the two basic amino acid residues absent in I-POU. Twin of I-POU is incapable of dimerizing with Cf1-a, but can act as a positive transcription factor on targets distinct from those regulated by Cf1-a. These findings suggest that the I-POU genomic locus simultaneously generates both a specific activator and inhibitor of gene transcription, capable of modulating two distinct regulatory programs during neural development.

Purification, cloning, and RXR identity of the HeLa cell factor with which RAR or TR heterodimerizes to bind target sequences efficiently

We have purified and cloned a HeLa cell nuclear protein that strongly stimulates binding of retinoic acid and thyroid hormone receptors (RARs and TRs) to response elements. The purified protein is a human retinoid X receptor beta (hRXR beta). Three murine members of the RXR family (mRXR alpha, beta, and gamma) have also been cloned, and their interactions with RARs and TRs have been investigated. Under conditions where RAR, RXR, and TR bound poorly as homodimers to various response elements, strongly cooperative RAR-RXR and TR-RXR binding was observed. The binding efficiency was dependent on the sequence, relative orientation, and spacing of the repeated motifs of response elements. We show also that unstable RAR-RXR heterodimers were formed in solution, and that C-terminal sequences and the DNA-binding domains of both receptors were required for efficient formation of stable heterodimers on response elements. These findings suggest a convergence of the signaling pathways of some members of the nuclear receptor superfamily.

Human calcitonin gene regulation by helix-loop-helix recognition sequences

Human calcitonin (CT) gene transcription is regulated by proximal 5' flanking sequences which mediate cAMP-induced expression, and by a distal basal enhancer region. Using transient expression of CT-CAT constructs, we showed that the basal enhancer is active in a CT-producing small cell lung cancer cell line (DMS53) and the thyroid C cell derived tumor line, TT, but is inactive in non-CT-producing cell lines. In deletional and direct mutational analyses of the distal enhancer region, disruption of two elements resembling recognition sequences for the helix-loop-helix (HLH) family of transcriptional regulatory proteins resulted in a significant loss of basal transcriptional enhancer action. These results suggest that HLH recognition motifs may mediate a significant portion of constitutive CT gene transcriptional activity in these cells. Nuclear protein extracts from DMS53 cells formed specific binding complexes with oligonucleotides containing two of these candidate enhancer sequences. However, proteins capable of binding to these CT gene HLH consensus recognition sites were not restricted to CT-producing cells. We conclude that members of the HLH protein family, some expressed ubiquitously and some expressed or activated in a tissue-restricted fashion, may combine to enhance CT gene transcription in tumor cells of neuroendocrine derivation.

Growth factors, growth factor response elements, and the cardiac phenotype

Fibroblast growth factors (FGF) and type beta-1 transforming growth factor (TGF beta 1) are pleiotropic regulatory peptides which are expressed in myocardium in a precise developmental and spatial program and are up-regulated, in the adult heart, by ischemia or a hemodynamic burden. The accumulation of trophic factors after aortic banding supports the hypothesis that autocrine or paracrine pathways might function to mediate, in part, the consequences of mechanical load. Our laboratory has demonstrated that cardiac muscle cells are targets for the action of peptide growth factors and, more specifically, that modulation of the cardiac phenotype by basic FGF (bFGF) and TGF beta 1 strongly resembles the induction of fetal cardiac genes--including skeletal alpha-actin (SkA), beta-myosin heavy chain, and atrial natriuretic factor--which are characteristic of pressure-overload hypertrophy. Unexpectedly, and despite effects like those of bFGF on five other cardiac genes, acidic FGF (aFGF) was found to repress, rather than stimulate, SkA transcription in neonatal cardiac muscle cells. The proximal 200 nucleotides of a heterologous SkA promoter were sufficient for basal tissue-specific transcription, for induction by bFGF, and for inhibition by aFGF. Thus, both positive and negative regulation by peptide growth factors can be localized to the proximal SkA promoter. Full promoter activity required each of three CC[A/T]6GG motifs similar to the serum response element (SRE) for activation of the c-fos proto-oncogene, as previously shown for SkA transcription in a skeletal muscle background. The most proximal SRE, SRE1, was sufficient in the absence of other SkA promoter sequences for efficient tissue-specific expression in cardiac myocytes (versus cardiac fibroblasts), and was stimulated by bFGF to the same extent as the full-length promoter and endogenous gene. Despite its ability to repress the SkA promoter, aFGF had no significant effect on SRE1. Both FGFs up-regulated the canonical fos SRE, to a comparable degree. Thus, SRE1 can discriminate between signals generated in cardiac myocytes by bFGF and aFGF. In cardiac myocyte extracts, two predominant proteins contact SRE1: serum response factor (SRF) and a second protein, F-ACT-1. Thus, serum response factor and F-ACT-1 are candidate trans-acting factors for basal transcription of the SkA gene in cardiac muscle cells and for induction of SkA by bFGF and, potentially, other trophic signals.

Molecular characterization of NSCL, a gene encoding a helix-loop-helix protein expressed in the developing nervous system

We report here the molecular cloning and chromosomal localization of an additional member of the helix-loop-helix (HLH) family of transcription factors, NSCL. The NSCL gene was identified based on its hybridization to the previously described hemopoietic HLH gene, SCL. Murine NSCL cDNA clones were obtained from a day 11.5 mouse embryo cDNA library. The coding region is 399 base pairs and encodes a predicted protein of 14.8 kDa. The nucleotide sequence shows 71% identity and the amino acid sequence shows 61% identity to murine SCL in the HLH domain. The NSCL protein-coding region terminates six amino acids beyond the second amphipathic helix of the HLH domain. Expression of NSCL was detected in RNA from mouse embryos between 9.5 and 14.5 days postcoitus, with maximum levels of expression at 10.5-12 days. Examination of 12- and 13-day mouse embryos by in situ hybridization revealed expression of NSCL in the developing nervous system. The NSCL gene was mapped to murine chromosome 1. The very restricted pattern of NSCL expression suggests an important role for this HLH protein in neurological development.

Coordinate change in phenotype in a mouse cell line selected for CD8 expression

A CD4+, CD8+ derivative of the CD4+, CD8- cell line SAKRTLS 12.1 was isolated by fluorescence activated cell sorting for CD8+ cells. This derivative showed a co-ordinate change in a number of independent characters: The parental cell line was CD4+, CD8-, CD3+, CD5hi, HSA+, DEXR, CD44hi, while the derivative was CD4+, CD8+, CD3-, CD5(10), HSA+, DEXS, CD44(10). The derivative expressed the Thy-1.1, Ly-2.1, and Ly-3.1 surface antigens, consistent with origin from the SAKRTLS 12.1 parental cell line, and showed a drug resistance profile identical to that of the parent. It was not possible to isolate revertants with a phenotype identical to that of the parental cell line. Activation of the structural gene coding for CD8 alpha chain was correlated with demethylation at several sites. We interpret these results to mean that this CD8+ derivative of SAKRTLS 12.1 arose as a result of an alteration of a gene that coordinately regulates multiple genes whose expression changes during thymocyte differentiation. Gene methylation may contribute, directly or indirectly, to some or all of the changes in gene expression observed.

Identification of skeletal muscle precursor cells in vivo by use of MyoD1 and myogenin probes

The activation of mononuclear muscle precursor cells after crush injury to mouse tibialis anterior muscles was monitored in vivo by in situ hybridization with MyoD1 and myogenin probes. These genes are early markers of skeletal muscle differentiation and have been extensively studied in vitro. The role in vivo of these regulatory proteins during myogenesis of mature muscle has not been studied previously. MyoD1 and myogenin mRNA were present in occasional mononuclear cells of uninjured muscle. Increased MyoD1 and myogenin mRNA sequences in mononuclear cells were detected as early as 6 h after injury, peaked between 24 and 48 h, and thereafter declined to pre-injury levels at about 8 days. The mRNAs were detected in mononuclear cells throughout the muscle, with the majority of cells located some distance from the site of crush injury. The presence of MyoD1 and myogenin mRNA at 6 to 48 h indicates that transcription of these genes is occurring at the same time as replication of muscle precursor cells in vivo. At no time were significant levels of mRNA for these genes detected in myotubes. MyoD1 and myogenin provide precise markers for the very early identification and study of mononuclear skeletal muscle precursor cells in muscle regenerating in vivo.

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.

MyoD and myogenin are coexpressed in regenerating skeletal muscle of the mouse

The differential expression of genes triggering myogenesis might cause or reflect differences among myoblasts. Little is known about the presence of MyoD1 and myogenin during the process of regeneration. We therefore examined the expression of MyoD1 and myogenin in muscle regeneration after grafting. Immunostaining of regenerating skeletal muscle of the mouse revealed the presence of both MyoD1 and myogenin. In mononucleated cells the proteins were not detected until shortly before fusion into myotubes. They persisted in the nuclei of regenerated muscle fibers for at least 2 weeks. MyoD1 and myogenin were not detected in nonregenerating control muscle.

Cloning and characterization of the gene encoding rabbit cardiac calsequestrin

A cDNA encoding rabbit cardiac calsequestrin was isolated and characterized. The deduced nascent cardiac calsequestrin contains 409 amino acids of which 26% are acidic residues, and had 93% and 67% aa identity with canine cardiac calsequestrin and rabbit fast-twitch skeletal muscle calsequestrin, respectively. RNA blot analyses indicate that this mRNA is expressed in atrium, ventricle and to a lesser amount in slow-twitch skeletal muscle. This mRNA transcript is not expressed in adult fast-twitch skeletal muscle, smooth muscle, or nonmuscle tissues. Analysis of in vitro skeletal muscle myogenesis using a mouse myoblast cell line C2C12, demonstrates that both cardiac and skeletal calsequestrin isoforms are coproduced during muscle differentiation.

The Drosophila single-minded gene encodes a helix-loop-helix protein that acts as a master regulator of CNS midline development

Development of the Drosophila CNS midline cells is dependent upon the function of the single-minded (sim) gene. Sequence analysis shows that sim is a member of the basic-helix-loop-helix class of transcription factors. Cell fate experiments establish that sim is required for early events in midline cell development, including a synchronized cell division, proper formation of nerve cell precursors, and positive auto-regulation of its midline expression. Induction of ectopic sim protein under the control of the hsp70 promoter shows that sim can direct cells of the lateral CNS to exhibit midline cell morphology and patterns of gene expression. We propose that sim functions as a master developmental regulator of the CNS midline lineage.

TAL2, a helix-loop-helix gene activated by the (7;9)(q34;q32) translocation in human T-cell leukemia

Tumor-specific alteration of the TAL1 gene occurs in almost 25% of patients with T-cell acute lymphoblastic leukemia (T-ALL). We now report the identification of TAL2, a distinct gene that was isolated on the basis of its sequence homology with TAL1. The TAL2 gene is located 33 kilobase pairs from the chromosome 9 breakpoint of t(7;9)(q34;q32), a recurring translocation specifically associated with T-ALL. As a consequence of t(7;9)(q34;q32), TAL2 is juxtaposed with sequences from the T-cell receptor beta-chain gene on chromosome 7. TAL2 sequences are actively transcribed in SUP-T3, a T-ALL cell line that harbors the t(7;9)(q34;q32). The TAL2 gene product includes a helix-loop-helix protein dimerization and DNA binding domain that is especially homologous to those encoded by the TAL1 and LYL1 protooncogenes. Hence, TAL2, TAL1, and LYL1 constitute a discrete subgroup of helix-loop-helix proteins, each of which can potentially contribute to the development of T-ALL.

The E-cadherin promoter: functional analysis of a G.C-rich region and an epithelial cell-specific palindromic regulatory element

The cell-cell adhesion molecule E-cadherin is specifically expressed in epithelia and is involved in the maintenance of the epithelial phenotype. Expression of E-cadherin is downregulated in many poorly differentiated carcinomas, which leads to higher motility and invasiveness of the cells. To examine the mechanisms that regulate tissue-specific expression, we have characterized the promoter of the E-cadherin gene. We found that an upstream fragment (positions -178 to +92) mediates strong expression of a chloramphenicol acetyltransferase reporter gene in epithelial cells (i.e., 60% of the level obtained with simian virus 40 promoter/enhancer constructs), whereas in nonepithelial cells this promoter was either inactive or much less active. By DNase I footprinting and gel retardation analysis as well as through functional dissection of the regulatory sequences, we identified two regions that contribute to tissue-specific activity of the promoter: (i) a G-C-rich region between -25 and -58 that generates basic epithelial promoter activity, most likely in combination with an "initiator" element present at the single transcription start site of the gene, and (ii) a palindromic sequence between -75 and -86 (named E-pal) that potentiates the activity of the proximal E-cadherin promoter and confers epithelial cell-specific activity on a simian virus 40 promoter. The E-pal sequence is homologous to cis regulatory elements active in keratin gene promoters and competes with these elements for nuclear factor binding. Interestingly, the activity of the E-cadherin promoter was reduced in dedifferentiated breast carcinoma cells, indicating that the identified elements are subject to negative regulation during tumor progression.

Characterisation of a genomic clone covering the structural mouse MyoD1 gene and its promoter region

We have isolated the mouse MyoD1 gene flanked by its promoter region by screening a genomic library with synthetic oligonucleotides. The structural gene is interrupted by two G + C rich introns. Transfection of the cloned gene inserted into an expression vector converts fibroblasts to myoblasts. Sequence analysis of about 650 bp of the 5' upstream region revealed the presence of several potential regulatory elements such as a TATA-box, an AP2-box, two SP1-boxes and a CAAT-box. In addition, there are three half palindromic estrogen response elements, a potential cAMP response element and various muscle specific elements such as a muscle-specific CAAT-box (MCAT) and four potential binding sites for MyoD1. Using S1 protection analysis the major start site of transcription in muscle and myoblast cells was mapped 3 bp upstream of the published cDNA 5' end. Promoter activity of the 650 bp upstream fragment was tested by in vitro transcription and by transfection analysis of myoblasts and fibroblasts. In all promoter test systems used, MyoD1 promoter activity was detected in myoblasts as well as in fibroblasts. Furthermore, DNA methylation was found to turn off MyoD1 promoter activity both in myoblasts and in fibroblasts.

Characterization of two Drosophila POU domain genes, related to oct-1 and oct-2, and the regulation of their expression patterns

We have characterized two genes from Drosophila melanogaster that encode proteins with POU domains showing a high degree of identity with the human Oct-1 and Oct-2 transcription factors. These POU domain genes, pdm-1 and pdm-2, are expressed at high levels during early embryogenesis and at lower levels throughout the rest of development. Both genes are expressed as two stripes in the presumptive abdominal region during the blastoderm stage, followed by thirteen stripes in the germ band extended stage. This pattern of expression is altered in mutants for a gap gene (hunchback) and a pair-rule gene (fushi tarazu). In later stage embryos, both pdm-1 and pdm-2 are expressed in selected neuroblasts in the ventral nervous system, with higher levels in the three thoracic segments and lower levels in the abdominal segments. The low level of expression in the abdominal segments is maintained by the genes within the bithorax complex (BX-C). We have also identified the cells in the dorsal and lateral clusters of the peripheral nervous system that express pdm-1 and pdm-2, and show that some of these cells derive from lineages that require BX-C functions. Together, these results suggest that previously characterized members of the embryonic regulatory hierarchy specify the patterns of the POU domain gene expression, which, in turn, function during neurogenesis and perhaps in earlier stages.

Evidence that enteric neurons may derive from the sympathoadrenal lineage

The first neurons that differentiate in the embryonic foregut of mammals transiently express catecholamine biosynthetic enzymes and accumulate catecholamine. Since this transmitter is found predominantly in cells of the sympatho-adrenal (SA) lineage, it has been suggested that enteric and sympathetic neurons may derive from the same progenitor. Enteric neurons would then lose the catecholamine phenotype during further development, as the two lineages diverge. We have further investigated this possibility using the SA1 monoclonal antibody that binds selectively to SA progenitor cells in the embryonic rat. We find that SA1 binds to the tyrosine hydroxylase+, neurofilament+, and SCG10+ cells of the Embryonic Day 14.5 (E14.5) rat foregut. We also find that a marker for later neuronal differentiation in the SA lineage, B2, also appears in the myenteric plexus concomitant with the loss of SA1 staining. Thus, at least some enteric neuronal precursors may exhibit the SA1----B2 antigenic switch previously observed in developing sympathetic neurons at E14.5. SA1 staining in the foregut partially overlaps with staining for neuropeptide Y, vasoactive intestinal polypeptide, and serotonin. These results support the hypothesis that enteric and sympathetic neurons derive from a common progenitor and that as the markers for the SA lineage are down-regulated, the many types of enteric neurons begin to differentiate.

Identification and characterization of a T-cell-specific enhancer adjacent to the murine CD4 gene

Expression of the CD4 and CD8 glycoproteins is a tightly regulated process tied to the maturation of functionally distinct classes of thymocytes. Therefore, understanding of the mechanism of expression of the genes encoding CD4 and CD8 is likely to yield important insight into regulation of the differentiated functions of T cells. Here, we report the identification of a T-cell-specific enhancer in a DNase I-hypersensitive region about 13 kb 5' of the transcription initiation site of the murine CD4 gene. Within the minimal enhancer element, at least three nuclear protein binding sites were identified by DNase I footprint analysis. One site contains the consensus motif for TCF-1 alpha/LEF-1, a recently identified HMG box transcription factor primarily expressed in pre-B and T cells. By Southwestern (DNA-protein) blotting and binding competition analyses, the protein binding to this site was found to be indistinguishable from TCF-1 alpha/LEF-1. Mutagenesis of this site resulted in loss of factor binding but had a relatively minor effect on enhancer activity. In contrast, mutations in another site, containing two consensus binding motifs for basic helix-loop-helix proteins, abolished factor binding and dramatically reduced enhancer activity. None of the protein binding sites had activity on its own, suggesting that the CD4 enhancer requires the interaction of multiple regulatory sites.

Expression of Id, a negative regulator of helix-loop-helix DNA binding proteins, is down-regulated at confluence and enhanced by dexamethasone in a mouse osteoblastic cell line, MC3T3E1

The message of Id (for "inhibitor of DNA binding") was expressed in an osteoblastic cell line, MC3T3E1, when the cells were in early cultures, while undetectable after the cells became confluent. The abundance of Id message in MC3T3E1 cells in early cultures was increased when the cells were treated with dexamethasone. This effect was time and dose dependent in a range between 10(-11)M and 10(-7)M. Id message expression was not enhanced by TGF-beta, 1,25-dihydroxyvitamin D3, retinoic acid, interleukin 1-alpha, interleukin 6, tumor necrosis factor-alpha or parathyroid hormone. These observations indicate for the first time the presence of HLH protein family member, Id, in osteoblast-like cells and its regulation by dexamethasone.

Apolipoprotein A-1 expression is resistant to dimethyl sulfoxide inhibition of myogenic differentiation

Primary cultures of chick embryonic muscle (CEM) were analyzed for the differential expression of a 26-kDa protein during myogenesis. We have identified this 26-kDa protein as apolipoprotein A-1 (Apo A-1), the major protein of serum high density lipoprotein particles. Apo A-1 was expressed in a pattern temporally similar to those of muscle-specific proteins, by myoblasts at very low levels, and by myotubes at high levels. The half-life of Apo A-1 in CEM cell homogenates was 23 min. This fast turnover rate appeared to be due to the secretion of Apo A-1 into the culture medium. To further characterize the relationship of Apo A-1 expression and myogenic differentiation, CEM cultures were treated with dimethyl sulfoxide (DMSO). In the presence of 2% DMSO, myotubes exhibited an atrophied morphology and an inhibition of the synthesis and accumulation of muscle-specific sarcomeric proteins. During recovery from DMSO treatment, the expression and accumulation of muscle-specific proteins returned to high levels. In contrast, the rates of synthesis and secretion of Apo A-1 in control, DMSO-treated, and DMSO-recovered CEM cells were nearly equivalent. These results indicate that the expression of Apo A-1 is not strictly linked to the expression of muscle-specific sarcomeric proteins in skeletal muscle and suggest that independent, or additional regulatory mechanisms exist which modulate Apo A-1 expression during myogenesis.

Clonal cells from embryonic retinal cell lines express qualitative electrophysiological differences

Cells from the embryonic quail retina were immortalized with the v-mil oncogene and cloned by limiting dilution. Their phenotype was examined using the whole-cell patch clamp method. Three membrane currents, IK(IR), INa and IK, were found at different frequencies within a sample of 170 cells drawn from a large clone. Nearly all combinations of these three markers were found and the frequency of combinations showed that the markers assorted independently. Examination of clones of less than 10 cells showed that heterogeneity originates with a high probability within clones, arguing that chromosomal mutation, for example, is unlikely to account for phenotypic diversity. A possible explanation is that phenotypic differences between cells might reflect the local exchange of instructive signals. If so, then the genes for the three phenotypic markers are controlled independently.

Two genetically and molecularly distinct functions involved in early neurogenesis reside within the Enhancer of split locus of Drosophila melanogaster

Molecular correlation of the genetic aspects of the function of the neurogenic gene Enhancer of split [E(spl)] has previously been hampered by the densely transcribed nature of the chromosomal region within which it resides. We present data indicating that two distinct molecular species contribute to E(spl) function. Analysis of new E(spl) alleles has allowed us to define two complementing functions within the locus. Subsequent phenotypic analysis of different E(spl) deficiencies combined with P element-transformed constructs has demonstrated that these two functions correspond to: (1) a family of helix-loop-helix (HLH) protein-encoding genes and (2) the single copy gene E(spl) m9/10, whose product shares homology with G-protein beta subunits. The zygotically active E(spl) HLH genes can, at least partially, substitute for one another's functions and their total copy number determines the activity of the locus. E(spl) m9/10 acts synergistically with the E(spl) HLH genes and other neurogenic genes in the process of neurogenesis. The maternal component of E(spl) m9/10 has the most pronounced effect in neurogenesis, while its zygotic component is predominantly required during postembryonic development. The lethality of trans-heterozygotes of null E(spl) deficiency alleles with a strong Delta point mutation is a result of the concomitant reduction in activity of both E(spl) HLH and m9/10 functions. Immunocytochemical localization of the E(spl) m9/10 protein has revealed that it is a ubiquitously distributed nuclear component in embryonic, larval and imaginal tissues.

Identification and characterization of a T-cell-specific enhancer adjacent to the murine CD4 gene

Expression of the CD4 and CD8 glycoproteins is a tightly regulated process tied to the maturation of functionally distinct classes of thymocytes. Therefore, understanding of the mechanism of expression of the genes encoding CD4 and CD8 is likely to yield important insight into regulation of the differentiated functions of T cells. Here, we report the identification of a T-cell-specific enhancer in a DNase I-hypersensitive region about 13 kb 5' of the transcription initiation site of the murine CD4 gene. Within the minimal enhancer element, at least three nuclear protein binding sites were identified by DNase I footprint analysis. One site contains the consensus motif for TCF-1 alpha/LEF-1, a recently identified HMG box transcription factor primarily expressed in pre-B and T cells. By Southwestern (DNA-protein) blotting and binding competition analyses, the protein binding to this site was found to be indistinguishable from TCF-1 alpha/LEF-1. Mutagenesis of this site resulted in loss of factor binding but had a relatively minor effect on enhancer activity. In contrast, mutations in another site, containing two consensus binding motifs for basic helix-loop-helix proteins, abolished factor binding and dramatically reduced enhancer activity. None of the protein binding sites had activity on its own, suggesting that the CD4 enhancer requires the interaction of multiple regulatory sites.

Widespread expression of MyoD genes in Xenopus embryos is amplified in presumptive muscle as a delayed response to mesoderm induction

The MyoD gene codes for an important regulatory factor in skeletal myogenesis. To explore the relationship between mesoderm induction in Xenopus embryos and expression of MyoD, I have monitored MyoD mRNA levels in normal embryos and cultured explants by RNase protection. Transcription from the two Xenopus MyoD gene copies is activated weakly across the whole embryo at the midblastula transition, and this activation occurs in the absence of mesoderm induction. In response to induction this basal expression is amplified 50- to 100-fold, but in animal-pole explants 6-10 hr elapse before induced mRNAs appear, and this induction requires prior protein synthesis. The promiscuous transcripts disappear from animal explants at a time when induction "competence" is lost, suggesting a link between these events. The data highlight a broad, but transient, permissiveness for MyoD expression in embryos, which is propagated and amplified only in presumptive muscle in response to induction. Moreover, muscle-specific MyoD expression is a relatively late (postgastrulation) event in the mesoderm-induction cascade.

Fiber type- and position-dependent expression of a myosin light chain-CAT transgene detected with a novel histochemical stain for CAT

We recently generated and characterized transgenic mice in which regulatory sequences from a myosin light chain gene (MLC1f/3f) are linked to the chloramphenicol acetyltransferase (CAT) gene. Transgene expression in these mice is specific to skeletal muscle and graded along the rostrocaudal axis: adult muscles derived from successively more caudal somites express successively higher levels of CAT. To investigate the cellular basis of these patterns of expression, we developed and used a histochemical stain that allows detection of CAT in individual cells. Our main results are as follows: (a) Within muscles, CAT is detected only in muscle fibers and not in associated connective tissue, blood vessels, or nerves. Thus, the tissue specificity of transgene expression observed by biochemical assay reflects a cell-type specificity demonstrable histochemically. (b) Within individual muscles, CAT levels vary with fiber type. Like the endogenous MLC1f/3f gene, the transgene is expressed at higher levels in fast-twitch (type II) than in slow-twitch (type I) muscle fibers. In addition, CAT levels vary among type II fiber subtypes, in the order IIB greater than IIX greater than IIA. (c) Among muscles that are similar in fiber type composition, the average level of CAT per fiber varies with rostrocaudal position. This position-dependent variation in CAT level is apparent even when fibers of a single type are compared. From these results, we conclude that fiber type and position affect CAT expression independently. We therefore infer the existence of separate fiber type-specific and positionally graded transcriptional regulators that act together to determine levels of transgene expression.

Mammalian achaete-scute homolog 1 is transiently expressed by spatially restricted subsets of early neuroepithelial and neural crest cells

Using monoclonal antibodies, we have examined the expression pattern of MASH1, a basic helix-loop-helix protein that is a mammalian homolog of the Drosophila achaete-scute proteins. In Drosophila, achaete-scute genes are required for the determination of a subset of neurons. In the rat embryo, MASH1 expression is confined to subpopulations of neural precursor cells. The induction of MASH1 precedes, but is extinguished upon, overt neuronal differentiation. MASH1 is expressed in the forebrain by spatially restricted domains of neuroepithelium and in the peripheral nervous system exclusively by precursors of sympathetic and enteric neurons. The features of early and transient expression, in spatially restricted subpopulations of neural precursors, are similar to those observed for achaete-scute. Thus, the amino acid sequence conservation between MASH1 and achaete-scute is reflected in a parallel conservation of cell type specificity of expression, similar to the case of mammalian MyoD and Drosophila nautilus. These data support the idea that helix-loop-helix proteins may represent an evolutionarily conserved family of cell-type determination genes, of which MASH1 is the first neural-specific member identified in vertebrates.

A family of C/EBP-related proteins capable of forming covalently linked leucine zipper dimers in vitro

Mouse and rat genomic DNA libraries were screened by reduced stringency hybridization with the DNA-binding domain of the c/ebp gene as a probe. Three genes were isolated that encode bZIP DNA-binding proteins (designated CRP1, CRP2, and CRP3) with strong amino acid sequence similarities to the C/EBP-binding domain. CRP2 is identical to the protein described recently by other groups as NF-IL6, LAP, IL-6DBP, and AGP/EBP, whereas CRP1 and CRP3 represent novel proteins. Several lines of evidence indicate that these three proteins, along with C/EBP, comprise a functional family. Each bacterially expressed polypeptide binds to DNA as a dimer with recognition properties that are virtually identical to those of C/EBP. Every member also bears a conserved cysteine residue at or near the carboxyl terminus, immediately following the leucine zipper, that at least in vitro allows efficient disulfide cross-linking between paired zipper helices. We developed a gel assay for covalent dimers to assess leucine zipper specificities among the family members. The results demonstrate that all pairwise combinations of dimer interactions are possible. To the extent that we have examined them, the same heterodimeric complexes can be detected intracellularly following cotransfection of the appropriate pair of genes into recipient cells. All members are also capable of activating in vivo transcription from promoters that contain a C/EBP-binding site. Our findings indicate that a set of potentially interacting C/EBP-like proteins exists, whose complexity is comparable to that of other bZIP protein subfamilies such as Jun, Fos, and ATF/CREB.

Helix-loop-helix genes translocated in lymphoid leukemia

A new class of DNA-binding proteins with a helix-loop-helix (HLH) structure has recently been described. Many of these transcriptional regulators are known to play a central role in cell-specification and differentiation processes. Four members of the HLH family are now implicated in the development of human lymphoid malignancies as a result of aberrant expression following chromosomal translocation events. This review focuses on two of these family members: SCL and LYL-1.

Muscle-specific transcriptional activation by MyoD

We focus on the mechanism by which MyoD activates transcription. Previous experiments showed that when the 13-amino-acid basic region of E12 replaced the corresponding basic region of MyoD, the resulting MyoD-E12Basic chimeric protein could bind specifically to muscle-specific enhancers in vitro and form dimers with E12, but could not activate a cotransfected reporter gene or convert 10T1/2 cells to muscle. Here we show that back mutation of this chimeric protein (with the corresponding residues in MyoD) re-establishes activation, and we identify a specific alanine involved in increasing DNA binding and a specific threonine required for activation. Using a reporter gene containing MyoD-binding sites located downstream from the transcription start site, we show that MyoD-E12Basic can bind in vivo and thereby inhibit expression of the reporter. In vivo binding is also supported by the fact that the addition of the "constitutive" VP16 activation domain to MyoD-E12Basic restores full trans-activation potential. The normal MyoD-activation domain maps within the amino-terminal 53 residues and can be functionally replaced by the activation domain of VP16. The activity of the MyoD-activation domain is dramatically elevated when deletions are made almost anywhere in the rest of the MyoD molecule, suggesting that the activation domain in MyoD is usually masked. Surprisingly, MyoD-E12Basic can activate transcription in CV1 and B78 cells (but not in 10T1/2 or 3T3 cells), suggesting that the activation function of the basic domain requires a specific factor present in CV1 and B78 cells. We propose that to function, the masked MyoD-activation domain requires the participation of a second factor that recognizes the basic region. We refer to such a factor as a recognition factor.

Helix-loop-helix transcription factor E47 activates germ-line immunoglobulin heavy-chain gene transcription and rearrangement in a pre-T-cell line

E47 is a helix-loop-helix transcription factor that binds to sites in the immunoglobulin heavy-chain and kappa light-chain gene enhancers. Other proteins of this type are involved in cell-type determination. A possible role for E47 in B-cell development was tested by overexpressing a cDNA encoding E47 in the pre-T-cell line 2017. We found a dramatic activation of a germ-line heavy-chain gene transcript in these stable transfectants and an equally large induction of immunoglobulin D-to-J rearrangement, the first recognized step in B-cell development. Germ-line kappa light-chain gene transcription and rearrangement were unaffected, but transcription of the recombination-activating genes RAG-1 and RAG-2 and the lymphoid-specific transcription factor Oct-2 was increased. These T cells did not transcribe their rearranged DJ alleles, however, and failed to progress to the next stage of heavy-chain gene assembly, V-to-DJ rearrangement. Because transcription factor E47 can induce pre-T cells to carry out events of B-cell differentiation, it may be a crucial determinant of the earliest stages of B-cell development.

Mechanisms of complex transcriptional regulation: implications for brain development

The large number of transcription factors, their diverse sequence-specific interactions with DNA sites and with other transcription factors, and their ability to be modified in response to a variety of environmental cues and intracellular signals provide combinatorial codes for highly complex and yet highly organized patterns of gene expression likely to underlie the determination of diversity of neuronal phenotypes. Subtle differences in the combinations of transcription factors are likely to have profound consequences for cell phenotype, similar to the mechanism involved in the specification of cell types in yeast (reviewed in Herskowitz, 1989). Although our current understanding of transcriptional regulation in the brain comes largely from phenomenological studies, recent technical progress on two fronts promises a bright future. Homologous recombination technology in embryonic stem cells (reviewed in Capecchi, 1989; Rossant, 1990) allows the disruption of particular genes in transgenic mice and definition of the roles of identified transcription factors in mammalian neurogenesis. A second technological advance, targeted tumorigenesis, has provided neuronal model cell lines (Mellon et al., 1990; reviewed in Cepko, 1988; McKay et al., 1988) that mimic certain neuronal differentiation pathways. These combined genetic, cell biological, and biochemical approaches will greatly facilitate the study of neural development and function.

The Id gene is activated by serum but is not required for de-differentiation in rat vascular smooth muscle cells

Primary rat vascular smooth muscle cells cultured on fibronectin in the absence of serum lost smooth-muscle-specific myosin heavy chain but did not enter the cell cycle and proliferate until they were stimulated by serum. Under these conditions accumulation of Id mRNA occurred only in response to serum and was maximal during the G1 phase of the cycle.

Mutagenesis of the myogenin basic region identifies an ancient protein motif critical for activation of myogenesis

Myogenin is a muscle-specific nuclear factor that acts as a genetic switch to activate myogenesis. Myogenin, MyoD, and a growing number of proteins implicated in transcriptional control share sequence homology within a basic region and an adjacent helix-loop-helix motif. Here we identify by site-directed mutagenesis a 12-amino acid subdomain of the myogenin basic region essential for binding of DNA and activation of myogenesis. The basic region of the widely expressed helix-loop-helix protein E12 is conserved at 8 of these 12 residues and can mediate DNA binding when placed in myogenin, but it cannot activate myogenesis. Replacement of each of the four nonconserved residues of the myogenin basic region with the corresponding residues of E12 reveals two adjacent amino acids (Ala86-Thr) that can impart muscle specificity to the basic region. These residues are specific to, and conserved in, the basic regions of all known myogenic helix-loop-helix proteins from Drosophila to man, suggesting that they constitute part of an ancient protein motif required for activation of the myogenic program.

Ubiquitous MyoD transcription at the midblastula transition precedes induction-dependent MyoD expression in presumptive mesoderm of X. laevis

We have used a quantitative reverse transcription-polymerase chain reaction assay to detect MyoD mRNA during early embryonic development of Xenopus laevis. We find that during a short period of time following the midblastula transition MyoD becomes transcriptionally activated at a low level ubiquitously throughout the embryo. Restriction of MyoD expression to muscle precursor cells appears as a subsequent event, in which the process of mesoderm induction stabilizes transcription only in the marginal zone of the embryo, the presumptive mesoderm.

The oncofetal gene Pem encodes a homeodomain and is regulated in primordial and pre-muscle stem cells

The oncofetal gene, Pem, is expressed in a stage specific manner during murine ontogeny. The carboxy terminal portion of the predicted Pem protein has significant similarity to homeodomains of the Drosophila prd family. The Pem gene is expressed in undifferentiated embryonal stem (ES) and embryonal carcinoma (EC) cell lines. Pem mRNA is induced 35-fold in ES cells differentiated in the absence of retinoic acid. Pem mRNA is increased in EC cells differentiated towards parietal or visceral endoderm, consistent with the abundant Pem expression in embryonic yolk sac. In 10T mesenchymal stem cells committed to muscle cell differentiation, Pem mRNA expression is dramatically increased. The elevation in Pem expression preceded the induction of the muscle master regulatory gene, myoD. We conclude that the Pem gene encodes a candidate transcription factor which is developmentally regulated.

Cloning of a factor required for activity of the Ah (dioxin) receptor

The aryl hydrocarbon (Ah) receptor binds various environmental pollutants, such as polycyclic aromatic hydrocarbons, heterocyclic amines, and polychlorinated aromatic compounds (dioxins, dibenzofurans, and biphenyls), and mediates the carcinogenic effects of these agents. The complementary DNA and part of the gene for an 87-kilodalton human protein that is necessary for Ah receptor function have been cloned. The protein is not the ligand-binding subunit of the receptor but is a factor that is required for the ligand-binding subunit to translocate from the cytosol to the nucleus after binding ligand. The requirement for this factor distinguishes the Ah receptor from the glucocorticoid receptor, to which the Ah receptor has been presumed to be similar. Two portions of the 87-kilodalton protein share sequence similarities with two Drosophila proteins, Per and Sim. Another segment of the protein shows conformity to the consensus sequence for the basic helix-loop-helix motif found in proteins that bind DNA as homodimers or heterodimers.

Cardiac growth factors

The role of polypeptide growth factors in cardiovascular ontogeny, function, and pathologic states is poorly understood. Recent investigations demonstrate that the myocardium produces both known and novel growth factors, which are highly regulated during development and disease, and have suggested that peptide growth factors may direct cardiac organogenesis and adaptation. Aspects of growth factor production, transduction, and action in myocardium are distinct to the cardiac muscle lineage and were not foreseen from results in simpler systems. Transforming growth factor beta 1 and fibroblast growth factors (FGFs) selectively up-regulate an ensemble of tissue-specific genes associated with the fetal myocardium. One of these, encoding the skeletal muscle isoform of alpha-actin, is activated by basic FGF yet is inhibited by acidic FGF. A serum response element of this gene is selectively induced, in cardiac myocytes, by basic FGF but not acidic FGF. Thus, cardiac muscle is an especially intriguing model for the analysis of growth factor signalling pathways that control differentiated gene transcription.

Role of environmental signals and transcriptional regulators in neural crest development

The processes by which undifferentiated cells are assigned to particular fates are far from clear. We review recent work that has examined this problem in the neural crest, a multipotential cell population that gives rise to peripheral neurons in vertebrates. Peripheral neuronal differentiation appears to occur in a series of developmental steps that can be regulated independently by signals in the environment. Furthermore, such steps are reflected by corresponding changes in the pattern of regulatory transcription factor expression in differentiating neural crest cells. The determination of neuronal identity may proceed by a series of parallel regulatory pathways involving transcription factors acting both in cascades and in combinatorial arrays.