Domains outside of the DNA-binding domain impart target gene specificity to myogenin and MRF4

Expression and function of scleraxis in the developing auditory system

A study of genes expressed in the developing inner ear identified the bHLH transcription factor Scleraxis (Scx) in the developing cochlea. Previous work has demonstrated an essential role for Scx in the differentiation and development of tendons, ligaments and cells of chondrogenic lineage. Expression in the cochlea has been shown previously, however the functional role for Scx in the cochlea is unknown. Using a Scx-GFP reporter mouse line we examined the spatial and temporal patterns of Scx expression in the developing cochlea between embryonic day 13.5 and postnatal day 25. Embryonically, Scx is expressed broadly throughout the cochlear duct and surrounding mesenchyme and at postnatal ages becomes restricted to the inner hair cells and the interdental cells of the spiral limbus. Deletion of Scx results in hearing impairment indicated by elevated auditory brainstem response (ABR) thresholds and diminished distortion product otoacoustic emission (DPOAE) amplitudes, across a range of frequencies. No changes in either gross cochlear morphology or expression of the Scx target genes Col2A, Bmp4 or Sox9 were observed in Scx(-/-) mutants, suggesting that the auditory defects observed in these animals may be a result of unidentified Scx-dependent processes within the cochlea.

Effects of oral creatine and resistance training on myogenic regulatory factor expression

PURPOSE

This study examined 12 wk of creatine (Cr) supplementation and heavy resistance training on skeletal muscle creatine kinase (M-CK) mRNA expression and the mRNA and protein expression of the myogenic regulatory factors Myo-D, myogenin, MFR-4, and Myf5.

METHODS

Twenty-two untrained males were randomly assigned to either a control (CON), placebo (PLC), or Cr (CRT) group in a double-blind fashion. Muscle biopsies were obtained before and after training. PLC and CRT trained thrice weekly using 3 sets of 6-8 repetitions at 85-90% 1-RM on the leg press, knee extension, and knee curl exercises. CRT ingested 6 g.d-1 of Cr for 12 wk while PLC consumed the equal amount of placebo.

RESULTS

After training, M-CK mRNA expression, as well as myogenin and MRF-4 mRNA and protein expression, were found to be significantly greater for CRT compared with PLC and CON, whereas PLC was also significantly different from CON (P < 0.05). For Myo-D mRNA and protein, both CRT and PLC were significantly different from CON (P < 0.05), but CRT and PLC were not different from one another. No significant differences were located for Myf5 mRNA or protein (P > 0.05). M-CK mRNA was correlated with myogenin (r = 0.916) and MRF-4 (r = 0.883) protein (P < 0.05).

CONCLUSION

When combined with heavy resistance training, Cr supplementation increases M-CK mRNA expression, likely due to concomitant increases in the expression of myogenin and MRF-4. Therefore, increases in myogenin and MRF-4 mRNA and protein may play a role in increasing myosin heavy chain expression, already shown to occur with Cr supplementation.

The C-terminus of myogenin, but not MyoD, targets upregulation of MEF2C expression

The myogenic regulatory family of basic helix-loop-helix transcription factors, including MyoD and myogenin, functions cooperatively with the myocyte-specific enhancer binding factor 2 (MEF2) family during skeletal myogenesis. Previously, using aggregated P19 cells, we have shown that myogenin upregulates MEF2C expression while MyoD does not [Ridgeway et al., J. Biol. Chem. 275 (2000) 41-46]. In order to identify the domain of myogenin responsible for activating MEF2C expression, a series of chimeras of MyoD and myogenin were generated. Only chimeras containing the C-terminal region of myogenin were able to activate MEF2C in aggregated P19 cells, suggesting that the C-terminus of myogenin is responsible for the regulation of specific target genes.

Thoracic skeletal defects in myogenin- and MRF4-deficient mice correlate with early defects in myotome and intercostal musculature

Myogenin and MRF4 are skeletal muscle-specific bHLH transcription factors critical for muscle development. In addition to a variety of skeletal muscle defects, embryos homozygous for mutations in myogenin or MRF4 display phenotypes in the thoracic skeleton, including rib fusions and sternal defects. These skeletal defects are likely to be secondary because myogenin and MRF4 are not expressed in the rib cartilage or sternum. In this study, the requirement for myogenin and MRF4 in thoracic skeletal development was further examined. When a hypomorphic allele of myogenin and an MRF4-null mutation were placed together, the severity of the thoracic skeletal defects was greatly increased and included extensive rib cartilage fusion and fused sternebrae. Additionally, new rib defects were observed in myogenin/MRF4 compound mutants, including a failure of the rib cartilage to contact the sternum. These results suggested that myogenin and MRF4 share overlapping functions in thoracic skeletal formation. Spatial expression patterns of skeletal muscle-specific markers in myogenin- and MRF4-mutant embryos revealed early skeletal muscle defects not previously reported. MRF4-/- mice displayed abnormal intercostal muscle morphology, including bifurcation and fusion of adjacent intercostals. myogenin/MRF4-mutant combinations displayed ventral myotome defects, including a failure to express normal levels of myf5. The results suggested that the early muscle defects observed in myogenin and MRF4 mutants may cause subsequent thoracic skeletal defects, and that myogenin and MRF4 have overlapping functions in ventral myotome differentiation and intercostal muscle morphogenesis.

Mutation of MyoD-Ser237 abolishes its up-regulation by c-Mos

Recently we have shown that Mos could activate myogenic differentiation by promoting heterodimerisation of MyoD and E12 proteins. Here, we demonstrate that MyoD can be efficiently phosphorylated by in vitro kinase assay with purified Mos immunoprecipitated from transfected cells. Comparative two-dimensional tryptic phosphopeptide mapping combined with site-directed mutagenesis revealed that Mos phosphorylates MyoD on serine 237. Mutation of serine 237 to a non-phosphorylable alanine (MyoD-Ala237) abolished the positive regulation of MyoD by Mos following overexpression in proliferating 10T1/2 cells. Taken together, our data show that direct phosphorylation of MyoD-Ser237 by Mos plays a positive role in increasing MyoD activity during myoblast proliferation.

Transcription factor BETA2 acts cooperatively with E2A and PDX1 to activate the insulin gene promoter

The insulin gene is efficiently expressed only in pancreatic beta cells. Using reverse transcriptase-polymerase chain reaction analysis, we show that insulin mRNA levels are at least 10(5)-fold higher in beta cells than non-beta cells. To examine the underlying mechanisms, we expressed beta cell transcription factors by transfection of non-beta cells. Separate expression of BETA2, E2A, or PDX1 led to modest (<10-fold) activation of the insulin promoter, whereas co-expression of the three proteins produced synergistic, high level activation (160-fold). This level of activity is approximately 25% that observed in transfected beta cell lines. Of the three factors studied, BETA2 appears to play a dominant role. Efficient transcription required a C-terminal activation domain of BETA2 and an N-terminal region, which does not function as an independent activation domain. The myogenic basic helix-loop-helix (bHLH) protein MyoD was unable to bind and activate the promoter, even when its DNA binding region was replaced with that of BETA2. Our results demonstrate the central importance of BETA2 in insulin gene transcription and the importance of sequences outside the canonical DNA binding domain in permitting efficient DNA binding and cell-specific activity of the insulin gene promoter.

Myocyte enhancer factor 2C and myogenin up-regulate each other's expression and induce the development of skeletal muscle in P19 cells

Two families of transcription factors, myogenic regulatory factors (MRFs) and myocyte enhancer factor 2 (MEF2), function synergistically to regulate myogenesis. In addition to activating structural muscle-specific genes, MRFs and MEF2 activate each other's expression. The MRF, myogenin, can activate MEF2 DNA binding activity when transfected into fibroblasts and, in turn, the myogenin promoter contains essential MEF2 DNA binding elements. To determine which MEF2 is involved in this regulation, P19 cells stably expressing MyoD and myogenin were compared for their ability to activate the expression of MEF2 family members. There was very little cross-activation of MyoD expression by myogenin and vice versa. Myogenin expression, and not MyoD, was found to up-regulate MEF2C expression. MEF2A, -B, and -D expression levels were not up-regulated by overexpression of either MyoD or myogenin. To examine whether MEF2C can differentially regulate MyoD or myogenin expression, P19 cell lines overexpressing MEF2C were analyzed. MEF2C induced myogenesis in P19 cells and up-regulated the expression of myogenin with 25-fold greater efficiency than that of MyoD. Therefore, myogenin and MEF2C participate in a regulatory loop in differentiating stem cells. This positive regulation does not extend to MyoD or the other MEF2 family members. Consequently, MEF2C appears to play a specific role in early events of myogenesis.

Human bHLH transcription factor gene myogenin (MYOG): genomic sequence and negative mutation analysis in patients with severe congenital myopathies

The myogenin gene encodes an evolutionarily conserved basic helix-loop-helix transcription (bHLH) factor that is required for differentiation of skeletal muscle, and its homozygous deletion in mice results in perinatal death from respiratory failure due to the lack of muscle fibers. Since the histology of skeletal muscle in myogenin null mice is reminiscent of that found in severe congenital myopathy patients, many of whom also die of respiratory complications, we sought to test the hypothesis that an aberrant human myogenin (myf4) coding region could be associated with some congenital myopathy conditions. With PCR amplification, we found similarly sized PCR products for the three exons of the myogenin gene in DNA from 37 patient and 40 control individuals. In contrast to previously reported sequencing of human myogenin (myf4), we describe with automated sequencing several base differences in flanking and coding regions plus an additional 659 and 498 bp in the first and second introns, respectively, in all 37 patient and 40 control samples. We also find a variable length (CA)-dinucleotide repeat in the second intron, which may have utility as a marker for future linkage studies. In summary, no causative mutations were detected in the myogenin coding locus of genomic DNA from 37 patients with severe congenital myopathy.

Overlapping functions of the myogenic bHLH genes MRF4 and MyoD revealed in double mutant mice

The myogenic basic helix-loop-helix (bHLH) genes - MyoD, Myf5, myogenin and MRF4 - exhibit distinct, but overlapping expression patterns during development of the skeletal muscle lineage and loss-of-function mutations in these genes result in different effects on muscle development. MyoD and Myf5 have been shown to act early in the myogenic lineage to establish myoblast identity, whereas myogenin acts later to control myoblast differentiation. In mice lacking myogenin, there is a severe deficiency of skeletal muscle, but some residual muscle fibers are present in mutant mice at birth. Mice lacking MRF4 are viable and have skeletal muscle, but they upregulate myogenin expression, which could potentially compensate for the absence of MRF4. Previous studies in which Myf5 and MRF4 null mutations were combined suggested that these genes do not share overlapping myogenic functions in vivo. To determine whether the functions of MRF4 might overlap with those of myogenin or MyoD, we generated double mutant mice lacking MRF4 and either myogenin or MyoD. MRF4/myogenin double mutant mice contained a comparable number of residual muscle fibers to mice lacking myogenin alone and myoblasts from those double mutant mice formed differentiated multinucleated myotubes in vitro as efficiently as wild-type myoblasts, indicating that neither myogenin nor MRF4 is absolutely essential for myoblast differentiation. Whereas mice lacking either MRF4 or MyoD were viable and did not show defects in muscle development, MRF4/MyoD double mutants displayed a severe muscle deficiency similar to that in myogenin mutants. Myogenin was expressed in MRF4/MyoD double mutants, indicating that myogenin is insufficient to support normal myogenesis in vivo. These results reveal unanticipated compensatory roles for MRF4 and MyoD in the muscle differentiation pathway and suggest that a threshold level of myogenic bHLH factors is required to activate muscle structural genes, with this level normally being achieved by combinations of multiple myogenic bHLH factors.

MRF4 can substitute for myogenin during early stages of myogenesis

MRF4, myogenin, MyoD, and Myf-5 are the four members of the basic helix-loop-helix family of muscle-specific regulatory factors (MRFs). We examined whether MRF4 could substitute for myogenin in vivo by determining if the myofiber- and MRF4-deficient phenotype of myogenin (-/-) mice could be rescued by a myogenin promoter-MRF4 transgene. When the transgene was expressed at a physiological level in myogenin-deficient fetuses, we found that expression of the endogenous MRF4 gene was restored to normal levels, whereas MyoD levels were unchanged. Thus, MRF4 can participate in a positive autoregulatory loop and can substitute for myogenin to activate its own promoter. Myogenin-deficient fetuses that expressed the transgene also had more myosin, more and larger myofibers, and a more normal ribcage morphology than myogenin-deficient littermates without the transgene. The transgene failed, however, to restore normal numbers of myofibers or viability to myogenin-deficient mice, because the approximately 1.6 kb myogenin promoter fragment was not expressed in most late-forming myofibers. These results demonstrate that MRF4 is able to substitute for myogenin to activate MRF4 expression and promote myofiber formation during the early stages of myogenesis.

Effect of atrophy and contractions on myogenin mRNA concentration in chick and rat myoblast omega muscle cells

The skeletal rat myoblast omega (RMo) cell line forms myotubes that exhibit spontaneous contractions under appropriate conditions in culture. We examined if the RMo cells would provide a model for studying atrophy and muscle contraction. To better understand how to obtain contractile cultures, we examined levels of contraction under different growing conditions. The proliferation medium and density of plating affected the subsequent proportion of spontaneously contracting myotubes. Using a ribonuclease protection assay, we found that exponentially growing RMo myoblasts contained no detectable myogenin or herculin mRNA, while differentiating myoblasts contained high levels of myogenin mRNA but no herculin mRNA. There was no increase in myogenin mRNA concentration in either primary chick or RMo myotubes whose contractions were inhibited by depolarizing concentrations of potassium (K+). Thus, altered myogenin mRNA concentrations are not involved in atrophy of chick myotubes. Depolarizing concentrations of potassium inhibited spontaneous contractions in both RMo cultures and primary chick myotube cultures. However, we found that the myosin concentration of 6-d-old contracting RMo cells fed medium plus AraC was 11 +/- 3 micrograms myosin/microgram DNA, not significantly different from 12 +/- 4 micrograms myosin/microgram DNA (n = 3), the myosin concentration of noncontracting RMo cells (treated with 12 mM K+ for 6 d). Resolving how RMo cells maintained their myosin content when contraction is inhibited may be important for understanding atrophy.

The myogenic regulatory factor MRF4 represses the cardiac alpha-actin promoter through a negative-acting N-terminal protein domain

Cardiac alpha-actin is activated early during the development of embryonic skeletal muscle and cardiac myocytes. The gene product remains highly expressed in adult striated cardiac muscle yet is dramatically reduced in skeletal muscle. Activation and repression of cardiac alpha-actin gene activity in developing skeletal muscle correlates with changes in the relative content of the four myogenic regulatory factors. Cardiac alpha-actin promoter activity, assessed in primary chick myogenic cultures, was activated by endogenous myogenic regulatory factors but was inhibited in the presence of co-expressed MRF4. By exchanging N- and C-terminal domains of MRF4 and MyoD, the N terminus of MRF4 was identified as the mediator of repressive activity, revealing a novel negative regulatory role for MRF4. The relative ratios of myogenic regulatory factors may have fundamental roles in selecting specific muscle genes for activation and/or repression.

Myogenic regulatory factors can activate TATA-containing promoter elements via an E-box independent mechanism

We have studied the effect of several myogenic regulatory factors on the activity of the promoter for a mouse gene encoding a skeletal myosin heavy chain (MyHC) expressed in adult (type IIB) muscle fibers. Co-transfection of myogenic factors is necessary for activity of the IIB promoter in mouse C2 myotubes in culture but not in quail myotubes in culture. Although this promoter contains one E-box within the first 192 base pairs upstream of the transcriptional start site, mutations in this motif demonstrate that it is not required for the transactivation effect of the myogenic factors. Analysis of other mutants suggests that the MEF2 and MHox DNA-binding factor binds to an evolutionarily conserved AT-rich motif. In addition, the IIB promoter appears to require the conserved TATA motif (CTATAAAAG) in order to be activated by the AT-rich sequences. The IIB promoter constructs produce RNA transcripts which begin at the natural site of transcriptional initiation in quail myotubes and in mouse C2 myotubes after co-transfection with myogenic factors; a second, minor, start site is also used in the co-transfected C2 myotubes. Results obtained after transfection of the mouse IIB promoter constructs in quail myotube cultures suggest that the overexpression of myogenic factors in C2 cultures does not result in an environment in which the control of IIB MyHC promoter activity is aberrant. Therefore, either the myogenic factors themselves, or other proteins induced by them, seem to interact directly with the basal transcription seem to interact directly with the basal transcription machinery to allow muscle-specific gene expression.

Myogenin binds to and represses c-fos promoter

Myogenin (a member of the myogenic basic helix-loop-helix transcription factor family) seems to be the main effector of proliferation repression, a crucial step which precedes muscle cell terminal differentiation during muscle development. Proliferation repression most likely occurs through inhibition of proliferation-associated genes such as the proto-oncogene, c-fos. Here, we demonstrate that myogenin binds to an E-box located in the main element of the c-fos promoter, the serum response element (SRE). Results from co-transfection experiments indicate that myogenin acts as a repressor for the SRE. Our data suggest that myogenin could play a role in c-fos inhibition at the onset of muscle cell terminal differentiation.

HLH forced dimers: tethering MyoD to E47 generates a dominant positive myogenic factor insulated from negative regulation by Id

Basic-helix-loop-helix (bHLH) class transcription factors bind DNA as hetero- and homodimers. In murine myogenic cells the HLH network includes multiple members of the E protein, MyoD, and Id families; changes in the network characterize muscle determination and differentiation and have been proposed as causal for these developmental transitions. To test the importance of HLH partner choice in these cellular decisions, we have designed a strategy in which the identity of a bHLH dimer is specified by joining two monomers via a flexible polypeptide linker. A MyoD-E47 polyprotein avidly bound the same DNA targets as its unlinked counterpart, but, unlike intermolecular dimers that are very sensitive to inhibition by Id, MyoD-E47 was resistant to Id challenge. In cells MyoD-E47 acted as a dominant positive myogenic factor, capable of initiating myogenic determination and also substantially bypassing negative regulation of differentiation by serum growth factors.

Muscle deficiency and neonatal death in mice with a targeted mutation in the myogenin gene

Myogenin is a muscle-specific transcription factor that can induce myogenesis in a variety of cell types in tissue culture. To test myogenin's role in vivo, mice homozygous for a targeted mutation in the myogenin gene were generated. These mice survive fetal development but die immediately after birth and show a severe reduction of all skeletal muscle. Myogenin-mutant mice differ from mice carrying mutations in genes for the related myogenic factors Myf5 and MyoD, which have no muscle defects. Myogenin is therefore essential for the development of functional skeletal muscle.

Selective accumulation of MyoD and myogenin mRNAs in fast and slow adult skeletal muscle is controlled by innervation and hormones

Each of the myogenic helix-loop-helix transcription factors (MyoD, Myogenin, Myf-5, and MRF4) is capable of activating muscle-specific gene expression, yet distinct functions have not been ascribed to the individual proteins. We report here that MyoD and Myogenin mRNAs selectively accumulate in hindlimb muscles of the adult rat that differ in contractile properties: MyoD is prevalent in fast twitch and Myogenin in slow twitch muscles. The distribution of MyoD and Myogenin transcripts also differ within a single muscle and correlate with the proportions of fast glycolytic and slow oxidative muscle fibres, respectively. Furthermore, the expression of a transgene consisting of a muscle-specific cis-regulatory region from the myoD gene controlling lacZ was primarily associated with the fast glycolytic fibres. Alteration of the fast/slow fibre type distribution by thyroid hormone treatment or by cross-reinnervation resulted in a corresponding alteration in the MyoD/Myogenin mRNA expression pattern. These findings show that the expression of specific myogenic helix-loop-helix regulators is under the control of innervation and humoral factors and may mediate differential control of contractile protein gene expression in adult muscle.

XASH1, a Xenopus homolog of achaete-scute: a proneural gene in anterior regions of the vertebrate CNS

The pro-neural achaete-scute complex (ASC) of Drosophila encodes four homologous proteins, each containing a basic helix-loop-helix (bHLH) domain, characteristic of a large family of transcription factors. We have isolated XASH1, a Xenopus homolog of achaete-scute. The XASH1 protein is very similar to the ASC proteins of Drosophila and the rat homolog, MASH1. XASH1 is expressed in the embryonic anterior central nervous system in a dynamic sequence, first in the midbrain, then in the forebrain, and then in the eye and hindbrain. In the larva, XASH1 expression correlates with regions of continued neurogenesis in the CNS, revealing the pattern of rhombomeres in the hindbrain, and other proliferative zones in the eye and midbrain. As a heterodimer with the bHLH protein E12, XASH1 binds specifically to an enhancer sequence derived from the promoter of the proneural achaete gene of Drosophila. This binding is inhibited by the extramacrochaete protein, a negative regulator of ASC gene function and neurogenesis in Drosophila. The combined evidence described in this paper strongly suggests that XASH1 plays a role in Xenopus neurogenesis similar to that played by the ASC genes in Drosophila.