MEF2: a transcriptional target for signaling pathways controlling skeletal muscle growth and differentiation

Down-regulation of an ankyrin repeat-containing protein, V-1, during skeletal muscle differentiation and its re-expression in the regenerative process of muscular dystrophy

Using Western blot analysis and immunohistochemical methods, we examined the expression of V-1, a member of the ankyrin repeat-containing protein family, during differentiation and regeneration of skeletal muscle. The expression of V-1 was high in cultured myoblasts and decreased during their differentiation into myotubes, while high expression was maintained when muscle differentiation was inhibited by treatment with basic fibroblast growth factor. Down-regulation of V-1 also occurred during in vivo muscle differentiation from embryonic to postnatal stages, reaching an undetectable level in mature skeletal muscle. In contrast, strong V-1 immunoreactivity was detected again in myoblasts and regenerating muscle fibers with a small diameter, which were observed in Duchenne muscular dystrophy and its animal model, mdx mouse. Thus, it seems that V-1 is a good marker for early stage of muscle regeneration and changes of its expression suggest that V-1 plays a role in prenatal muscle differentiation and postnatal muscle regeneration.

Gastrin stimulates cyclooxygenase-2 expression in intestinal epithelial cells through multiple signaling pathways. Evidence for involvement of ERK5 kinase and transactivation of the epidermal growth factor receptor

Gastrin is a hormone produced by G-cells in the normal gastric antrum. However, colorectal carcinoma cells may aberrantly produce gastrin and exhibit increased expression of cholecystokinin B (CCK-B)/gastrin receptors. Gastrin is trophic for the normal gastric oxyntic mucosa and exerts a growth-promoting action on gastrointestinal malignancy. Thus, gastrin may act as an autocrine/paracrine or endocrine factor in the initiation and progression of colorectal carcinoma. The molecular mechanisms involved have not been elucidated. Hypergastrinemia induced by Helicobacter pylori infection is associated with increased cyclooxygenase-2 (COX-2) expression in gastric and colorectal tissues, suggesting the possibility that gastrin up-regulates COX-2 expression in these tissues; this has not been confirmed. We report here that gastrin significantly increases the expression of COX-2 mRNA and protein, the activity of the COX-2 promoter, and the release of prostaglandin E(2) from a rat intestinal epithelial cell line transfected with the CCK-B receptor. These actions were dependent upon the activation of multiple MAPK signal pathways, including ERK5 kinase; transactivation of the epidermal growth factor receptor; and the increased expression and activities of transcription factors ELK-1, activating transcription factor-2, c-Fos, c-Jun, activator protein-1, and myocyte enhancer factor-2. Thus, our findings identify the signaling pathways coupling the CCK-B receptor with up-regulation of COX-2 expression. This effect may contribute to this hormone-dependent gastrointestinal carcinogenesis, especially in the colon.

MyoD induces apoptosis in the absence of RB function through a p21(WAF1)-dependent re-localization of cyclin/cdk complexes to the nucleus

During differentiation of skeletal myoblasts, MyoD promotes growth arrest through the induction of the cdk inhibitor p21 and the accumulation of hypophosphorylated RB protein. Myoblasts lacking RB function fail to accomplish full differentiation and undergo apoptosis. Here we show that exogenous MyoD induces apoptosis in several cell backgrounds sharing RB inactivation. This process is associated with increased levels of cell cycle-driving proteins and aberrant cell cycle progression. The inability of MyoD to induce apoptosis in a p21-null background, highlights a requirement of p21 in RB-regulated apoptosis during myogenesis. This pro-apoptotic function of p21 cannot be exerted by simple p21 over-expression, but requires the co-operation of MyoD. We also suggest that the essential aspect of p21 activity involved in such a process is related to its ability to induce the nuclear accumulation and aberrant activity of cyclin/cdk complexes. These results establish a novel link between MyoD, p21 and RB during myogenesis, providing new insights into the antagonism between muscle differentiation and loss of RB function.

Transgenic and gene targeting studies of hair cell function in mouse inner ear

Despite the rapid discovery of a large number of genes in sensory hair cells of the inner ear, the functional roles of these genes in hair cells remain largely undetermined. Recent advances in transgenic and gene targeting technologies in mice have offered unprecedented opportunities to genetically manipulate the expression of these genes and to study their functional roles in hair cells in vivo. Transgenic analyses have revealed the presence of hair-cell-specific promoters in the genes encoding Math1, myosin VIIa, Pou4f3, and the alpha9 subunit of the acetylcholine receptor (alpha9 AChR). Targeted inactivation using embryonic stem cell technology and transgenic expression studies have revealed the roles of several genes involved in hair cell lineage (Math1), differentiation (Pou4f3), mechanotransduction (Myo1c, and Myo7a), electromotility (Prestin), and efferent modulation (Chrna9, encoding alpha9 AChR). Although many of these genes also play roles in other tissues, inactivation of these genes in hair cells alone will soon be possible by using the Cre-loxP system. Also imminent is the development of genetic methods to inactivate genes specifically in mouse hair cells at a desired time, by using inducible systems established in other types of neurons. Combining these types of manipulation of gene expression will enable hearing researchers to elucidate some of the fundamental and unique features of hair cell function such as mechanotransduction, frequency tuning, active mechanical amplification, and efferent modulation.

Patterns of gene expression in atrophying skeletal muscles: response to food deprivation

During fasting and many systemic diseases, muscle undergoes rapid loss of protein and functional capacity. To define the transcriptional changes triggering muscle atrophy and energy conservation in fasting, we used cDNA microarrays to compare mRNAs from muscles of control and food-deprived mice. Expression of >94% of genes did not change, but interesting patterns emerged among genes that were differentially expressed: 1) mRNAs encoding polyubiquitin, ubiquitin extension proteins, and many (but not all) proteasome subunits increased, which presumably contributes to accelerated protein breakdown; 2) a dramatic increase in mRNA for the ubiquitin ligase, atrogin-1, but not most E3s; 3) a significant suppression of mRNA for myosin binding protein H (but not other myofibrillar proteins) and IGF binding protein 5, which may favor cell protein loss; 4) decreases in mRNAs for several glycolytic enzymes and phosphorylase kinase subunits, and dramatic increases in mRNAs for pyruvate dehydrogenase kinase 4 and glutamine synthase, which should promote glucose sparing and gluconeogenesis. During fasting, metallothionein mRNA increased dramatically, mRNAs for extracellular matrix components fell, and mRNAs that may favor cap-independent mRNA translation rose. Significant changes occurred in mRNAs for many growth-related proteins and transcriptional regulators. These transcriptional changes indicate a complex adaptive program that should favor protein degradation and suppress glucose oxidation in muscle. Similar analysis of muscles atrophying for other causes is allowing us to identify a set of atrophy-specific changes in gene expression.

Identification of novel MyoD gene targets in proliferating myogenic stem cells

A major control point for skeletal myogenesis revolves around the muscle basic helix-loop-helix gene family that includes MyoD, Myf-5, myogenin, and MRF4. Myogenin and MRF4 are thought to be essential to terminal differentiation events, whereas MyoD and Myf-5 are critical to establishing the myogenic cell lineage and producing committed, undifferentiated myogenic stem cells (myoblasts). Although mouse genetic studies have revealed the importance of MyoD and Myf-5 for myoblast development, the genetic targets of MyoD and Myf-5 activity in undifferentiated myoblasts remain unknown. In this study, we investigated the function of MyoD as a transcriptional activator in undifferentiated myoblasts. By using conditional expression of MyoD, in conjunction with suppression subtractive hybridizations, we show that the Id3 and NP1 (neuronal pentraxin 1) genes become transcriptionally active following MyoD induction in undifferentiated myoblasts. Activation of Id3 and NP1 represents a stable, heritable event that does not rely on continued MyoD activity and is not subject to negative regulation by an activated H-Ras G12V protein. These results are the first to demonstrate that MyoD functions as a transcriptional activator in myogenic stem cells and that this key myogenic regulatory factor exhibits different gene target specificities, depending upon the cellular environment.

m-Bop, a repressor protein essential for cardiogenesis, interacts with skNAC, a heart- and muscle-specific transcription factor

The m-Bop protein encoded by the mouse Bop gene is strongly expressed in heart and skeletal muscle, and recent studies with Bop knockout mice have demonstrated that m-Bop is essential for cardiogenesis in vivo and can act as a HDAC-dependent repressor in vitro. In the present studies, m-Bop was observed to interact with skNAC, a reported transcriptional activator specific to heart and skeletal muscle. The amino-terminal S region of the split S-ET domain of m-Bop as well as the MYND domain were required for interaction with skNAC in both the two-hybrid system and in coimmunoprecipitation experiments from cultured mammalian cells. As shown previously for interaction of the MYND domain-containing transcriptional corepressor, BS69, with several viral and cellular oncoproteins, a PXLXP motif in skNAC was required for interaction with m-Bop. Similar kinetics of induction and localization of m-Bop and skNAC during the induction of myogenesis in cultured C2C12 cells suggests a possible associated role for these proteins during this process.

Cyclin D-cdk4 activity modulates the subnuclear localization and interaction of MEF2 with SRC-family coactivators during skeletal muscle differentiation

Prior work has indicated that D-type cyclin-cdk4 complexes, which are only active in proliferating cells, can suppress the skeletal muscle differentiation program in proliferating myoblasts. In this study, we show that cyclin D-cdk activity can block the activity of the MEF2 family of transcriptional regulators, which are crucial regulators of skeletal muscle gene expression. We have found that cyclin D-cdk activity blocks the association of MEF2C with the coactivator protein GRIP-1 and thereby inhibits the activity of MEF2. During skeletal muscle differentiation, GRIP-1 is localized to punctate nuclear structures and can apparently tether MEF2 to such structures. Cotransfection of GRIP-1 can both potentiate the transcriptional activity of a Gal4-MEF2C construct and induce MEF2C localization to punctate nuclear structures. Consistent with the absence of punctate nuclear GRIP-1 in proliferating myoblasts, we have found that ectopic cyclin D-cdk4 expression disrupts the localization of both GRIP-1 and MEF2C to these punctate subnuclear structures. Our findings indicate that cyclin D-cdk4 activity represses skeletal muscle differentiation in proliferating cells by blocking the association of MEF2 with the coactivator GRIP-1 and concomitantly disrupts the association of these factors with punctate nuclear subdomains within the cell.

Androgen receptor enhances myogenin expression and accelerates differentiation

Animal and clinical studies indicated that the androgen-AR signaling pathway is required for appropriate development of sexually dimorphic skeletal muscles and increases lean muscle mass, muscle strength, and muscle protein synthesis. However, the detailed mechanisms by which the androgen-AR signaling pathway regulates skeletal muscle development need further study at the molecular level. C2C12 myoblast cells stably transfected with the Flag-tagged AR were used to analyze the role of androgen-AR signaling pathway in skeletal muscle development. The results indicate that the androgen-AR signaling pathway may suppress skeletal myoblast cell growth and accelerate myoblast cell differentiation via enhanced myogenin expression. This is a first report showing the role of androgen-AR signaling pathway in regulation of myoblast cell growth and myogenic regulatory factors.

Muscle development genes: their relevance in neuromuscular disorders

Myogenesis is a complex cascade of events that involves the specification and differentiation of muscle precursor cells or myoblasts, their fusion to form primary and secondary myotubes and subsequent maturation into muscle fibres. In addition, the development of axial muscle requires the migration of muscle precursor cells. These events are under strict genetic control. The contribution of individual genes to this process has been highlighted both by the phenotype of mice with targeted inactivation of individual myogenic regulatory factors and by rare human disorders in which the involvement of these genes has been demonstrated. The inactivation of known myogenic regulatory genes is associated with abnormal regulation of skeletal muscle differentiation and has an effect on regeneration but does not cause progressive muscle weakness or wasting. This review summarises recent developments in this field and will be of particular relevance to those interested in neuromuscular disorders. We also examine the possibility that some rare human conditions associated with abnormal muscle formation may be due to genetic defects in one of the myogenic regulatory genes.

Role of SHP-2 in fibroblast growth factor receptor-mediated suppression of myogenesis in C2C12 myoblasts

Ligand activation of the fibroblast growth factor receptor (FGFR) represses myogenesis and promotes activation of extracellular signal-regulated kinases 1 and 2 (Erks). The precise mechanism through which the FGFR transmits both of these signals in myoblasts remains unclear. The SH2 domain-containing protein tyrosine phosphatase, SHP-2, has been shown to participate in the regulation of FGFR signaling. However, no role for SHP-2 in FGFR myogenic signaling is known. In this study, we show that stimulation of C2C12 myoblasts with FGF-2 induces SHP-2 complex formation with tyrosyl-phosphorylated FGFR substrate 2 alpha (FRS-2 alpha). Both the catalytic activity and, to a much lesser extent, the Grb2 binding-tyrosyl phosphorylation sites of SHP-2 are required for maximal FGF-2-induced Erk activity and Elk-1 transactivation. When overexpressed in C2C12 myoblasts, wild-type SHP-2, but not a catalytically inactive SHP-2 mutant, potentiates the suppressive effects of FGF-2 on muscle-specific gene expression. In addition, expression of a constitutively active mutant of SHP-2 is sufficient to prevent myogenesis. The constitutively active mutant of SHP-2 induces hyper-tyrosyl phosphorylation of FRS-2 alpha but fails to stimulate or potentiate either FGF-2-induced Erk activation or Elk-1 transactivation. These data suggest that in myoblasts, SHP-2 represses myogenesis via a pathway that is independent of the Erks. We propose that SHP-2 plays a pivotal role in FGFR signaling in myoblasts via both Erk-dependent and Erk-independent pathways.

AVP induces myogenesis through the transcriptional activation of the myocyte enhancer factor 2

The neurohypophyseal nonapeptide Arg8 vasopressin (AVP) promotes differentiation of cultured L6 and L5 myogenic cell lines and mouse primary satellite cells. Here, we investigated the molecular mechanism involved in the induction of the myogenic program by AVP. In L6 cells, AVP treatment rapidly induces Myf-5, myogenin, and myocyte enhancer factor 2 (MEF2) mRNAs, without affecting the expression of known myogenic growth factors such as IGF-I, IGF-II, or their receptors. In the presence of cycloheximide, AVP up-regulates the expression of MEF2, but not of myogenin, indicating that the synthesis of a protein intermediate is not necessary for MEF2 induction. Notably, AVP treatment activates a calcium/calmodulin kinase signaling pathway that induces cytosolic compartmentalization of the histone deacetylase 4, a mechanism related to the transcriptional activation of MEF2. The activity of chloramphenicol acetyltransferase reporter constructs carrying the Myo184 and Myo84 fragments of the myogenin promoter is also induced by AVP. Mutation of the MEF2 site completely abolishes the response to AVP, whereas deletion of the E1 site present in pMyo84 does not impair this response. Together, these results show that AVP induces myogenic differentiation through the transcriptional activation of MEF2, a mechanism that is critical for myogenesis.

Identification and cloning of a new protein that binds the 3(') untranslated region of alpha-striated tropomyosin

The 3' untranslated region of muscle tropomyosin (TM UTR) induces muscle differentiation when transcribed in primary fibroblasts. This sequence binds protein in extracts from cell types that differentiate upon TM UTR transcription. To identify the protein(s) bound by the TM UTR, an avian embryo fibroblast library was induced to express protein in solution and extracts from these pools were screened with electromobility shift assays using a TM UTR RNA probe. Positive pools were progressively fractionated until a pool containing a single positive clone was obtained. The TM UTR-binding protein (UBP) clone thus isolated contains 751 nt, 618 of which represent a single open reading frame. UBP is related to a human autoantigen, Sjogren's syndrome antigen B (SSB) beginning with the start of the UBP open reading frame. This homology is to the 5' end of SSB in a region containing an RNA-binding motif of 70 amino acids. The deduced amino acid sequence of UBP predicts phosphorylation sites for protein kinase C, casein kinase 2, and cAMP-dependent protein kinase and asparginine glycosylation sites. The observed size of UBP by UV cross-linking with a TM UTR probe is of the same size as the protein bound in fibroblast extract. UBP is expressed in primary fibroblasts, but not in fibroblast or myogenic cell lines, suggesting that its expression is restricted. The full-length UBP mRNA is approximately 3 kB, suggesting a long 5' untranslated region. Transient transfection of cultured cells with UBP directs production of a protein that binds the TM UTR, confirming that these sequences interact in vivo. These observations suggest that we have identified a novel protein that binds to the TM UTR in vitro and in vivo. Determining the function of this protein will facilitate determining the mechanism by which the TM UTR induces differentiation.

Activation of the MEF2 transcription factor in skeletal muscles from myotonic mice

Becker syndrome, a recessive nondystrophic myotonia caused by mutations in the chloride channel 1 gene (CLCN1), is characterized by delayed muscle relaxation after contraction. The ADR (arrested development of righting response) mouse is an animal model for Becker syndrome. Skeletal muscles from ADR myotonic animals show an increased number of oxidative fibers with a lack of glycolytic fibers as well as signs of muscle hypertrophy. Through breeding ADR myotonic mice with mice harboring a MEF2-dependent reporter gene, we found that the transcriptional activity of MEF2 was dramatically enhanced in myotonic muscles. Post-translational induction of MEF2 transcriptional activity correlated with the activation of p38 MAPK and did not affect MEF2 DNA-binding affinity. Expression of class II histone deacetylases (HDACs), which repress MEF2-dependent gene expression, was significantly reduced in skeletal muscles from myotonic mice. These findings suggest that the combined effects of class II HDAC deficiency and p38 MAPK activation lead to potent upregulation of MEF2 transcriptional activity, which contributes to the long-term changes in gene expression and fiber-type transformation observed in myotonic skeletal muscles. These findings provide new molecular targets for potential treatment of congenital myotonia.

Phosphorylation of pRb is required for HGF-induced muscle cell proliferation and is p27kip1-dependent

Hepatocyte growth factor (HGF) plays a crucial role in the differentiation of skeletal muscle cells, where a process in which the retinoblastoma protein (pRb) has been implicated. We addressed the role of pRb in HGF-mediated effects on the proliferation and differentiation of adult skeletal muscle myoblasts. HGF shifted pRb to its hyperphosphorylation forms and increased the transactivation of E2F1, a transcription factor required for S phase entry. A constitutively active pRb mutant blocked HGF-dependent pRb phosphorylation and transactivation of E2F1 and increased cell proliferation. Accordingly, this mutant reversed the inhibitory effects of HGF on the expression of the cyclin-dependent kinase (CDK) inhibitor p27 and myogenic differentiation markers. HGF-mediated pRb phosphorylation was reversed by ectopic expression of p27, but neither the myogenic regulatory factor, MEF2, nor the myogenic inhibitory protein Twist had that effect. These results suggest that in response to HGF signaling, there is a decrease in p27 expression that results in an accumulation of hyperphosphorylated Rb protein, and subsequent progression of myoblasts into the G1 phase of the cell cycle.

A Twist in fate: evolutionary comparison of Twist structure and function

The general requirement to induce mesoderm and allocate cells into different mesodermal tissues such as body muscle or heart is common in many animal embryos. Since the discovery of the twist gene, there has been great progress toward unraveling the molecular mechanisms that control mesoderm specification and differentiation. Twist was first identified in Drosophila as a gene crucial for proper gastrulation and mesoderm formation. In the fly embryo, Twist continues to play additional roles, allocating mesodermal cells into the body wall muscle fate and patterning a subset of these muscles. Twist is also required for proper differentiation of the adult musculature. Twist homologues have been identified in a great variety of organisms, which span the phylogenetic tree. These organisms include other invertebrates such as jellyfish, nematode, leech and lancelet as well as vertebrates such as frog, chick, fish, mouse and human. The Twist family shares both homology in structure across the basic helix-loop-helix domain and in expression during mesoderm and muscle development in most species. Here we review the current state of knowledge of the Twist family and consider how Twist functions during development. Moreover, we highlight experimental evidence that shows common themes that Twist employs during specification and patterning of the mesoderm among evolutionarily distant organisms. Conserved principles and the molecular mechanisms underlying them are discussed.

CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle

BACKGROUND

A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

RESULTS

We examined the consequences of C/EBPalpha expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPalpha. Ectopic expression of C/EBPalpha in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

CONCLUSION

We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

Dominant-interfering forms of MEF2 generated by caspase cleavage contribute to NMDA-induced neuronal apoptosis

Myocyte enhancer factor-2 (MEF2) transcription factors are activated by p38 mitogen-activated protein kinase during neuronal and myogenic differentiation. Recent work has shown that stimulation of this pathway is antiapoptotic during development but proapoptotic in mature neurons exposed to excitotoxic or other stress. We now report that excitotoxic (N-methyl-D-aspartate) insults to mature cerebrocortical neurons activate caspase-3, -7, in turn cleaving MEF2A, C, and D isoforms. MEF2 cleavage fragments containing a truncated transactivation domain but preserved DNA-binding domain block MEF2 transcriptional activity via dominant interference. Transfection of constitutively active MEF2 (MEF2C-CA) rescues MEF2 transcriptional activity after N-methyl-D-aspartate insult and prevents neuronal apoptosis. Conversely, dominant-interfering MEF2 abrogates neuroprotection by MEF2C-CA. These results define a pathway to excitotoxic neuronal stress/apoptosis via caspase-catalyzed cleavage of MEF2. Additionally, we show that similar MEF2 cleavage fragments are generated in vivo during focal stroke damage. Hence, this pathway appears to have pathophysiological relevance in vivo.

Signal transduction and the control of gene expression

More than 2000 transcription factors are encoded in the human genome. Such proteins have often been classified according to common structural elements. But because transcription factors evolved in the service of biologic function, we propose an alternative grouping of eukaryotic transcription factors on the basis of characteristics that describe their roles within cellular regulatory circuits.

ECM is required for skeletal muscle differentiation independently of muscle regulatory factor expression

Transcription of specific skeletal muscle genes requires the expression of the muscle regulatory factor myogenin. To assess the role of the extracellular matrix (ECM) in skeletal muscle differentiation, the specific inhibitors of proteoglycan synthesis, sodium chlorate and beta-D-xyloside, were used. Treatment of cultured skeletal muscle cells with each inhibitor substantially abolished the expression of creatine kinase and alpha-dystroglycan. This inhibition was totally reversed by the addition of exogenous ECM. Myoblast treatment with each inhibitor affected the deposition and assembly of the ECM constituents glypican, fibronectin, and laminin. These treatments did not affect MyoD, MEF2A, and myogenin expression and nuclear localization. Differentiated myoblast treatment with RGDS peptides completely inhibited myogenesis without affecting the expression or nuclear localization of myogenin. Integrin-mediated signaling of focal adhesion kinase was partially inhibited by chlorate and beta-D-xyloside, an effect reversed by the addition of exogenous ECM gel. These results suggested that the expression of myogenin is not sufficient to successfully drive skeletal muscle formation and that ECM is required to complete the skeletal muscle differentiation process.

Cytokine stimulation of energy expenditure through p38 MAP kinase activation of PPARgamma coactivator-1

Cachexia is a chronic state of negative energy balance and muscle wasting that is a severe complication of cancer and chronic infection. While cytokines such as IL-1alpha, IL-1beta, and TNFalpha can mediate cachectic states, how these molecules affect energy expenditure is unknown. We show here that many cytokines activate the transcriptional PPAR gamma coactivator-1 (PGC-1) through phosphorylation by p38 kinase, resulting in stabilization and activation of PGC-1 protein. Cytokine or lipopolysaccharide (LPS)-induced activation of PGC-1 in cultured muscle cells or muscle in vivo causes increased respiration and expression of genes linked to mitochondrial uncoupling and energy expenditure. These data illustrate a direct thermogenic action of cytokines and p38 MAP kinase through the transcriptional coactivator PGC-1.

Isolation and characterization of chicken TaxREB107, a putative DNA binding protein abundantly expressed in muscle

Myogenic regulatory factors (MRFs) are vital transcription factors that act at multiple points during development to establish the skeletal muscle phenotype. This class of muscle-restricted, basic helix-loop-helix (bHLH) proteins acts in concert with additional transcriptional modulators to precisely control muscle gene expression. We have isolated the chicken homologue of Tax responsive element binding protein 107 (TaxREB107). The cDNA is 83% homologous at the amino acid level to human and mouse TaxREB107 and contains a centrally located leucine zipper motif. Northern analysis demonstrated that the gene is expressed in multiple tissues including skeletal muscle. Immunofluorescent staining revealed that the cTaxREB107 protein is located in both the nuclear and cytoplasmic compartments. Distinct localization to the nucleoli supports the evidence that TaxREB107 is a ribosomal protein. Because TaxREB proteins also are implicated in transcriptional regulation, we overexpressed cTaxREB107 in embryonic myoblasts. cTaxREB107 increased troponin I reporter gene activity as well as MRF-directed transcription from a multimerized skeletal muscle E-box reporter gene (4Rtk-luc). However, cotransfection of expression plasmids coding for MyoD and cTaxREB107 did not produce an increase in 4Rtk-luc suggesting that cTaxREB107 enhances myogenic gene transcription through a means independent of a physical association with MyoD. In conclusion, our results define a role for cTaxREB107 during avian myogenesis as a positive modulator of skeletal muscle gene expression.

Insulin-like growth factor-mediated muscle differentiation: collaboration between phosphatidylinositol 3-kinase-Akt-signaling pathways and myogenin

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development in the embryo and in regeneration in the adult. To study mechanisms of IGF action in muscle, we developed a myogenic cell line that overexpresses IGF-binding protein-5. C2BP5 cells remain quiescent in low serum differentiation medium until the addition of IGF-I. Here we use this cell line to identify signaling pathways controlling IGF-mediated differentiation. Induction of myogenin by IGF-I and myotube formation were prevented by the phosphatidylinositol (PI) 3-kinase inhibitor, LY294002, even when included 2 days after growth factor addition, whereas expression of active PI 3-kinase could promote differentiation in the absence of IGF-I. Differentiation also was induced by myogenin but was blocked by LY294002. The differentiation-promoting effects of IGF-I were mimicked by a modified membrane-targeted inducible Akt-1 (iAkt), and iAkt was able to stimulate differentiation of C2 myoblasts and primary mouse myoblasts incubated with otherwise inhibitory concentrations of LY294002. These results show that an IGF-regulated PI 3-kinase-Akt pathway controls muscle differentiation by mechanisms acting both upstream and downstream of myogenin.

Signaling to the nucleus by an L-type calcium channel-calmodulin complex through the MAP kinase pathway

Increases in the intracellular concentration of calcium ([Ca2+]i) activate various signaling pathways that lead to the expression of genes that are essential for dendritic development, neuronal survival, and synaptic plasticity. The mode of Ca2+ entry into a neuron plays a key role in determining which signaling pathways are activated and thus specifies the cellular response to Ca2+. Ca2+ influx through L-type voltage-activated channels (LTCs) is particularly effective at activating transcription factors such as CREB and MEF-2. We developed a functional knock-in technique to investigate the features of LTCs that specifically couple them to the signaling pathways that regulate gene expression. We found that an isoleucine-glutamine ("IQ") motif in the carboxyl terminus of the LTC that binds Ca2+-calmodulin (CaM) is critical for conveying the Ca2+ signal to the nucleus. Ca2+-CaM binding to the LTC was necessary for activation of the Ras/mitogen-activated protein kinase (MAPK) pathway, which conveys local Ca2+ signals from the mouth of the LTC to the nucleus. CaM functions as a local Ca2+ sensor at the mouth of the LTC that activates the MAPK pathway and leads to the stimulation of genes that are essential for neuronal survival and plasticity.

Myocyte enhancer factor 2A and 2D undergo phosphorylation and caspase-mediated degradation during apoptosis of rat cerebellar granule neurons

Myocyte enhancer factor 2 (MEF2) proteins are important regulators of gene expression during the development of skeletal, cardiac, and smooth muscle. MEF2 proteins are also present in brain and recently have been implicated in neuronal survival and differentiation. In this study we examined the cellular mechanisms regulating the activity of MEF2s during apoptosis of cultured cerebellar granule neurons, an established in vitro model for studying depolarization-dependent neuronal survival. All four MEF2 isoforms (A, B, C, and D) were detected by immunoblot analysis in cerebellar granule neurons. Endogenous MEF2A and MEF2D, but not MEF2B or MEF2C, were phosphorylated with the induction of apoptosis. The putative sites that were phosphorylated during apoptosis are functionally distinct from those previously reported to enhance MEF2 transcription. The increased phosphorylation of MEF2A and MEF2D was followed by decreased DNA binding, reduced transcriptional activity, and caspase-dependent cleavage to fragments containing N-terminal DNA binding domains and C-terminal transactivation domains. Expression of the highly homologous N terminus of MEF2A (1-131 amino acids) antagonized the transcriptional activity and prosurvival effects of a constitutively active mutant of MEF2D (MEF2D-VP16). We conclude that MEF2A and MEF2D are prosurvival factors with high transcriptional activity in postmitotic cerebellar granule neurons. When these neurons are induced to undergo apoptosis by lowering extracellular potassium, MEF2A and MEF2D are phosphorylated, followed by decreased DNA binding and cleavage by a caspase-sensitive pathway to N-terminal fragments lacking the transactivation domains. The degradation of MEF2D and MEF2A and the generation of MEF2 fragments that have the potential to act as dominant-inactive transcription factors lead to apoptotic cell death.

Differential localization of HDAC4 orchestrates muscle differentiation

The class II histone deacetylases HDAC4 and HDAC5 interact specifically with the myogenic MEF2 transcription factor and repress its activity. Here we show that HDAC4 is cytoplasmic during myoblast differentiation, but relocates to the nucleus once fusion has occurred. Inappropriate nuclear entry of HDAC4 following overexpression suppresses the myogenic programme as well as MEF2-dependent transcription. Activation of the Ca(2+)/calmodulin signalling pathway via constitutively active CaMKIV prevents nuclear entry of HDAC4 and HDAC4-mediated inhibition of differentiation. Consistent with a role of phosphorylation in HDAC4 cytoplasmic localisation, HDAC4 binds to 14-3-3 proteins in a phosphorylation-dependent manner. Together these data establish a role for HDAC4 in muscle differentiation. Recently, HDAC5 has also been implicated in muscle differentiation. However, despite the functional similarities of HDAC4 and HDAC5, their intracellular localisations are opposed, suggesting a distinct role for these enzymes during muscle differentiation.

Genomic cloning of the Hsc71 gene in the hermaphroditic teleost Rivulus marmoratus and analysis of its expression in skeletal muscle: identification of a novel muscle-preferred regulatory element

To further our understanding of the role of stress proteins in development as well as in adaptation of fish to adverse environmental conditions, we undertook molecular analyses of stress protein encoding genes from the hermaphroditic teleost Rivulus marmoratus. We isolated a genomic clone containing the Hsc71 gene (rm-hsc71m) and its upstream sequences. rm-Hsc71m is not induced by external stress, but is enriched in a tissue-specific manner during early development. In adult, the strongest expression appeared in skeletal muscle, whereas lower expression was seen in the gill, eye and brain. To understand the regulatory basis of high muscle expression of rm-hsc71m, transfection of R.marmoratus muscle tissue was performed using 5' deletion fragments containing the rm-hsc71m promoter driving EGFP expression. An upstream region from -2.7 to -1.9 kb was identified as a muscle-specific regulatory region. Within this region, we identified at least three sites with the novel sequence TGTnACA interacting with a fish muscle factor having an M(r) of 32 000. Our data indicate that rm-hsc71m expression in skeletal muscle is controlled by a muscle-specific regulatory element containing this novel motif.

Early-age heat exposure affects skeletal muscle satellite cell proliferation and differentiation in chicks

Exposure of young chicks to thermal conditioning (TC; i.e., 37 degrees C for 24 h) resulted in significantly improved body and muscle growth at a later age. We hypothesized that TC causes an increase in satellite cell proliferation, necessary for further muscle hypertrophy. An immediate increase was observed in satellite cell DNA synthesis in culture and in vivo in response to TC of 3-day-old chicks to levels that were significantly higher than those of control chicks. This was accompanied by a marked induction of insulin-like growth factor-I (IFG-I), but not hepatocyte growth factor in the breast muscle. No significant difference between treatments in plasma IGF-I levels was observed. A marked elevation in muscle regulatory factors on day 5, followed by a decline in cell proliferation on day 6 together with continuous high levels of IGF-I in the TC chick muscle may indicate accelerated cell differentiation. These data suggest a central role for IGF-I in the immediate stimulation of satellite cell myogenic processes in response to heat exposure.

CHAMP, a novel cardiac-specific helicase regulated by MEF2C

MEF2C is a MADS-box transcription factor required for cardiac myogenesis and morphogenesis. In MEF2C mutant mouse embryos, heart development arrests at the looping stage (embryonic day 9.0), the future right ventricular chamber fails to form, and cardiomyocyte differentiation is disrupted. To identify genes regulated by MEF2C in the developing heart, we performed differential array analysis coupled with subtractive cloning using RNA from heart tubes of wild-type and MEF2C-null embryos. Here, we describe a novel MEF2C-dependent gene that encodes a cardiac-restricted protein, called CHAMP (cardiac helicase activated by MEF2 protein), that contains seven conserved motifs characteristic of helicases involved in RNA processing, DNA replication, and transcription. During mouse embryogenesis, CHAMP expression commences in the linear heart tube at embryonic day 8.0, shortly after initiation of MEF2C expression in the cardiogenic region. Thereafter, CHAMP is expressed specifically in embryonic and postnatal cardiomyocytes. At the trabeculation stage of heart development, CHAMP expression is highest in the trabecular region in which cardiomyocytes have exited the cell cycle and is lowest in the proliferative compact zone. These findings suggest that CHAMP acts downstream of MEF2C in a cardiac-specific regulatory pathway for RNA processing and/or transcriptional control.

Life and death of a cardiac myocyte: principles of cellular biology

If the future of extracorporeal circulation is to include approaches to enhance localized or widespread distribution of cells, and/or gene transfer for augmentation of cardiac function, it is imperative that we gain an increased understanding of the mechanisms that define the cardiac myocyte phenotype. The purpose of this paper is to review the natural history of the cardiac myocyte. A variety of signals determine the cellular processes that characterize birth, growth, differentiation and death of cardiomyocytes. Examined here are primary aspects of the molecular genetics of growth and development, including signal transduction, protein phosphorylation, the cell division cycle, and transcriptional activation. This review is intended to be an update on insights into molecular aspects of the cell, with emphasis on gene expression during cardiac myogenesis and a discussion of its relevance to the field of extracorporeal circulation. In addition, the current status of research in myogenesis is presented.

A role for histone deacetylase HDAC1 in modulating the transcriptional activity of MyoD: inhibition of the myogenic program

The molecular mechanism(s) that are responsible for suppressing MyoD's transcriptional activities in undifferentiated skeletal muscle cells have not yet been determined. We now show that MyoD associates with a histone deacetylase-1 (HDAC1) in these cells and that this interaction is responsible for silencing MyoD-dependent transcription of endogenous p21 as well as muscle-specific genes. Specifically, we present evidence that HDAC1 can bind directly to MyoD and use an acetylated MyoD as a substrate in vitro, whereas a mutant version of HDAC1 (H141A) can not. Further more, this mutant also fails to repress MyoD-mediated transcription in vivo, and unlike wild-type HDAC1 it can not inhibit myogenic conversion, as judged by confocal microscopy. Finally, we show that an endogenous MyoD can be acetylated upon its conversion to a hypophosphorylated state and only when the cells have been induced to differentiate. These results provide for a model which postulates that MyoD may be co-dependent on HDAC1 and P/CAF for temporally controlling its transcriptional activity before and after the differentiation of muscle cells.

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).

MEKK2 associates with the adapter protein Lad/RIBP and regulates the MEK5-BMK1/ERK5 pathway

MEKK2 and MEKK3 are two closely related mitogen-activated protein kinase (MAPK) kinase kinases. The kinase domains of MEKK2 and MEKK3 are nearly identical, although their N-terminal regulatory domains are significantly divergent. By yeast two-hybrid library screening, we have identified MEK5, the MAPK kinase in the big mitogen-activated protein kinase 1 (BMK1)/ERK5 pathway, as a binding partner for MEKK2. MEKK2 expression stimulates BMK1/ERK5 activity, the downstream substrate for MEK5. Compared with MEKK3, MEKK2 activated BMK1/ERK5 to a greater extent, which might correlate with a higher affinity MEKK2-MEK5 interaction. A dominant negative form of MEK5 blocked the activation of BMK1/ERK5 by MEKK2, whereas activation of c-Jun N-terminal kinase (JNK) was unaffected, showing that MEK5 is a specific downstream effector of MEKK2 in the BMK1/ERK5 pathway. Activation of BMK1/ERK5 by epidermal growth factor and H2O2 in Cos7 and HEK293 cells was completely blocked by a kinase-inactive MEKK3 (MEKK3kin(-)), whereas MEKK2kin(-) had no effect. However, in D10 T cells, expression of MEKK2kin(-) but not MEKK3kin(-) inhibited BMK1/ERK5 activity. Two-hybrid screening also identified Lck-associated adapter/Rlk- and Itk-binding protein (Lad/RIBP), a T cell adapter protein, as a binding partner for MEKK2. MEKK2 and Lad/RIBP colocalize at the T cell contact site with antigen-loaded presenting cells, demonstrating cotranslocation of MEKK2 and Lad/RIBP during T cell activation. MEKK3 neither binds Lad/RIBP nor is recruited to the T cell contact with antigen presenting cell. MEKK2 and MEKK3 are differentially associated with signaling from specific upstream receptor systems, whereas both activate the MEK5-BMK1/ERK5 pathway.

Myocyte enhancer factors-2B and -2C are required for adhesion related kinase repression of neuronal gonadotropin releasing hormone gene expression

Synthesis of the hypothalamic peptide, gonadotropin releasing hormone (GnRH), is paramount for reproductive function. GnRH neurons originate in the olfactory region and migrate into the forebrain during development. We recently implicated adhesion related kinase (Ark) in GnRH neuron development based on its differential expression in two GnRH producing cell lines, GT1-7 and Gn10. The Ark membrane receptor encodes an extracellular domain resembling cell adhesion molecules and an intracellular tyrosine kinase. Ark is expressed in Gn10 cells derived from migrating GnRH neurons but not GT1-7 cells of the post-migratory phenotype. Here, we show that Ark and GnRH transcripts are colocalized in the cribriform plate at midgestation, suggesting that Ark is expressed in migrating GnRH neurons in vivo. Furthermore, we have identified the GnRH gene as a downstream target of Ark signaling. Ark inhibits GnRH gene expression in GnRH neuronal cells via the coordinated binding of myocyte enhancer factor-2B and -2C (MEF-2B and -2C) and a putative homeoprotein within the proximal rat GnRH promoter. Given that MEF-2 proteins are widely expressed in the brain, these studies provide further evidence for MEF-2 action during neuronal development. Moreover, our studies elucidate a potential role for Ark in regulating GnRH gene expression during GnRH neuronal migration.

CaM kinase signaling induces cardiac hypertrophy and activates the MEF2 transcription factor in vivo

Hypertrophic growth is an adaptive response of the heart to diverse pathological stimuli and is characterized by cardiomyocyte enlargement, sarcomere assembly, and activation of a fetal program of cardiac gene expression. A variety of Ca(2+)-dependent signal transduction pathways have been implicated in cardiac hypertrophy, but whether these pathways are independent or interdependent and whether there is specificity among them are unclear. Previously, we showed that activation of the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin or its target transcription factor NFAT3 was sufficient to evoke myocardial hypertrophy in vivo. Here, we show that activated Ca(2+)/calmodulin-dependent protein kinases-I and -IV (CaMKI and CaMKIV) also induce hypertrophic responses in cardiomyocytes in vitro and that CaMKIV overexpressing mice develop cardiac hypertrophy with increased left ventricular end-diastolic diameter and decreased fractional shortening. Crossing this transgenic line with mice expressing a constitutively activated form of NFAT3 revealed synergy between these signaling pathways. We further show that CaMKIV activates the transcription factor MEF2 through a posttranslational mechanism in the hypertrophic heart in vivo. Activated calcineurin is a less efficient activator of MEF2-dependent transcription, suggesting that the calcineurin/NFAT and CaMK/MEF2 pathways act in parallel. These findings identify MEF2 as a downstream target for CaMK signaling in the hypertrophic heart and suggest that the CaMK and calcineurin pathways preferentially target different transcription factors to induce cardiac hypertrophy.