Transcriptional activity of MEF2 during mouse embryogenesis monitored with a MEF2-dependent transgene

Class II histone deacetylases act as signal-responsive repressors of cardiac hypertrophy

The heart responds to stress signals by hypertrophic growth, which is accompanied by activation of the MEF2 transcription factor and reprogramming of cardiac gene expression. We show here that class II histone deacetylases (HDACs), which repress MEF2 activity, are substrates for a stress-responsive kinase specific for conserved serines that regulate MEF2-HDAC interactions. Signal-resistant HDAC mutants lacking these phosphorylation sites are refractory to hypertrophic signaling and inhibit cardiomyocyte hypertrophy. Conversely, mutant mice lacking the class II HDAC, HDAC9, are sensitized to hypertrophic signals and exhibit stress-dependent cardiomegaly. Thus, class II HDACs act as signal-responsive suppressors of the transcriptional program governing cardiac hypertrophy and heart failure.

Angiotensin II induces myocyte enhancer factor 2- and calcineurin/nuclear factor of activated T cell-dependent transcriptional activation in vascular myocytes

It is well known that angiotensin II (Ang II) is implicated in the phenotypic modulation and hypertrophy of vascular smooth muscle cells (VSMCs). To study the mechanisms by which Ang II contributes to the pathological changes of VSMCs, we examined whether Ang II stimulated myocyte enhancer factor 2 (MEF2)- and calcineurin/nuclear factor of activated T cell (NFAT)-dependent transcriptional activation of genes in VSMCs. Ang II increased the DNA binding activity of MEF2A and its expression at the protein level. Ang II induced c-jun promoter activity, and this increase was inhibited by dominant-negative mutants of MEF2A and mitogen-activated protein kinase kinase 6 but not by calcineurin inhibitors. Ang II stimulated NFAT DNA binding activity and NFAT-dependent gene transcription, and these effects of Ang II were inhibited by calcineurin inhibitors. Furthermore, Ang II induced the promoter activity of the nonmuscle-type myosin heavy chain B gene, which we used as a marker of the dedifferentiated state of VSMCs, and this increase was inhibited by calcineurin inhibitors but not by the dominant-negative mutants of MEF2A or mitogen-activated protein kinase kinase 6. Finally, Ang II increased protein synthesis, and this increase was inhibited by infection with an adenovirus construct that expresses the dominant-negative mutant of MEF2A but not by calcineurin inhibitors. These results suggest that Ang II stimulates the MEF2- and calcineurin/NFAT-dependent pathways and that these pathways have distinct roles in VSMCs.

Transcriptional regulation of vertebrate cardiac morphogenesis

Transcription factors can regulate the expression of other genes in a tissue-specific and quantitative manner and are thus major regulators of embryonic developmental processes. Several transcription factors that regulate cardiac genes specifically have been described, and the recent discovery that dominant inherited transcription factor mutations cause congenital heart defects in humans has brought direct medical relevance to the study of cardiac transcription factors in heart development. Although this field of study is extensive, several major gaps in our knowledge of the transcriptional control of heart development still exist. This review will concentrate on recent developments in the field of cardiac transcription factors and their roles in heart formation.

MEF2: a calcium-dependent regulator of cell division, differentiation and death

The decision of a cell to divide, differentiate or die is dependent on the coupling of cytoplasmic signals to the activation and repression of specific sets of genes in the nucleus. Many of the signal transduction pathways that control these cellular decisions are activated by elevation of intracellular calcium. Recent studies have revealed a central role for the myocyte enhancer factor-2 (MEF2) family of transcription factors in linking calcium-dependent signaling pathways to the genes responsible for cell division, differentiation and death. This article describes the post-translational mechanisms that confer calcium-sensitivity to MEF2 and its downstream target genes, and considers how this transcription factor can control diverse and mutually exclusive cellular decisions.

Nerve activity-dependent modulation of calcineurin signaling in adult fast and slow skeletal muscle fibers

This study tested the hypothesis that calcineurin signaling is modulated in skeletal muscle cells by fluctuations in nerve-mediated activity. We show that dephosphorylation of NFATc1, MEF2A, and MEF2D transcription factors by calcineurin in all muscle types is dependent on nerve activity and positively correlated with muscle usage under normal weightbearing conditions. With increased nerve-mediated activity, calcineurin dephosphorylation of these targets was found to be potentiated in a way that paralleled the higher muscle activation profiles associated with functional overload or nerve electrical stimulation conditions. We also establish that muscle activity must be sustained above native levels for calcineurin-dependent dephosphorylation of MEF2A and MEF2D to be transduced into an increase in MEF2 transcriptional function, suggesting that calcineurin cooperates with other activity-linked events to signal via these proteins. Finally, examination of individual fiber responses to overload and nerve electrical stimulation revealed that calcineurin-MEF2 signaling occurs in all fiber types but most readily in fibers that are normally least active (i.e. those expressing IIx and IIb myosin heavy chain (MHC)), suggesting that signaling via this phosphatase is also dependent upon the activation history of the muscle cell.

Different pathways regulate expression of the skeletal myosin heavy chain genes

Mammalian skeletal muscles are a mosaic of different fiber types largely defined by differential myosin heavy chain (MyHC) expression. Little is known about the molecular mechanisms regulating expression of the MyHC gene family members in different fiber types. In this work, we identified several cis- and trans-elements that regulate expression of the three adult fast MyHC genes. Despite multiple DNA-binding motifs for well characterized muscle transcription factors upstream of all three fast MyHC genes, expression of MyoD/Myf-5, calcineurin, or NFAT3 had different effects on the three promoters. MyoD or Myf-5 overexpression preferentially activated the IIb promoter, whereas NFAT or activated calcineurin overexpression preferentially activated the IIa promoter. Calcineurin had a 50-100-fold stimulatory effect on the IIa promoter, and the known downstream effectors of calcineurin (myocyte enhancer factor-2 and NFAT) cannot completely account for this activation. Finally, we identified two elements critical for regulating MyHC-IId/x expression: a 130-base pair enhancer element and a CArG-like element that inhibited IId/x promoter activity in vitro. Thus, we have found specific regulatory pathways that are distinct for the three adult fast MyHC genes. These elements are logical candidates for fiber-specific control of skeletal muscle gene expression in vivo.

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.

Muscle specificity encoded by specific serum response factor-binding sites

Serum response factor (SRF) is a MADS box transcription factor that regulates muscle-specific and growth factor-inducible genes by binding the consensus sequence CC(A/T)6GG, known as a CArG box. Because SRF expression is not restricted solely to muscle, its expression alone cannot account for the muscle specificity of some of its target genes. To understand further the role of SRF in muscle-specific transcription, we created transgenic mice harboring lacZ transgenes linked to tandem copies of different CArG boxes with flanking sequences. CArG boxes from the SM22 and skeletal alpha-actin promoters directed highly restricted expression in developing smooth, cardiac, and skeletal muscle cells during early embryogenesis. In contrast, the CArG box and flanking sequences from the c-fos promoter directed expression throughout the embryo, with no preference for muscle cells. Systematic swapping of the core and flanking sequences of the SM22 and c-fos CArG boxes revealed that cell type specificity was dictated in large part by sequences immediately flanking the CArG box core. Sequences that directed widespread embryonic expression bound SRF more strongly than those that directed muscle-restricted expression. We conclude that sequence variations among CArG boxes influence cell type specificity of expression and account, at least in part, for the ability of SRF to distinguish between growth factor-inducible and muscle-specific genes in vivo.

Control of cardiac-specific transcription by p300 through myocyte enhancer factor-2D

The transcriptional integrator p300 regulates gene expression by interaction with sequence-specific DNA-binding proteins and local remodeling of chromatin. p300 is required for cardiac-specific gene transcription, but the molecular basis of this requirement is unknown. Here we report that the MADS (MCM-1, agamous, deficiens, serum response factor) box transcription factor myocyte enhancer factor-2D (MEF-2D) acts as the principal conduit for cardiac transcriptional activation by p300. p300 activation of the native 2130-base pair human skeletal alpha-actin promoter required a single hybrid MEF-2/GATA-4 DNA motif centered at -1256 base pairs. Maximal expression of the promoter in cultured myocytes and in vivo correlated with binding of both MEF-2 and p300, but not GATA-4, to this AT-rich motif. p300 and MEF-2 were coprecipitated from cardiac nuclear extracts by an oligomer containing this element. p300 was found exclusively in a complex with MEF-2D at this and related sites in other cardiac-restricted promoters. MEF-2D, but not other MEFs, significantly potentiated cardiac-specific transcription by p300. No physical or functional interaction was observed between p300 and other factors implicated in skeletal actin transcription, including GATA-4, TEF-1, or SRF. These results show that, in the intact cell, p300 interactions with its protein targets are highly selective and that MEF-2D is the preferred channel for p300-mediated transcriptional control in the heart.

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.

An overlapping CArG/octamer element is required for regulation of desmin gene transcription in arterial smooth muscle cells

The desmin gene encodes an intermediate filament protein that is present in skeletal, cardiac, and smooth muscle cells. This study shows that the 4-kb upstream region of the murine desmin promoter directs expression of a lacZ reporter gene throughout the heart from E7.5 and in skeletal muscle and vascular smooth muscle cells from E9. 5. The distal fragment (-4005/-2495) is active in arterial smooth muscle cells but not in venous smooth muscle cells or in the heart in vivo. It contains a CArG/octamer overlapping element (designated CArG4) that can bind the serum response factor (SRF) and an Oct-like factor. The desmin distal fragment can replace a SM22alpha regulatory region (-445/-126) that contains two CArG boxes, to cis-activate a minimal (-125/+65) SM22alpha promoter fragment in arterial smooth muscle cells of transgenic embryos. lacZ expression was abolished when mutations were introduced into the desmin CArG4 element that abolished the binding of SRF and/or Oct-like factor. These data suggest that a new type of combined CArG/octamer element plays a prominent role in the regulation of the desmin gene in arterial smooth muscle cells, and SRF and Oct-like factor could cooperate to drive specific expression in these cells.

The MADS-Box factor CeMEF2 is not essential for Caenorhabditis elegans myogenesis and development

MEF2 is an evolutionarily conserved MADS (MCM1, Agamous, Deficiens, and serum response factor) box-type transcription factor that plays a critical role in vertebrate and Drosophila melanogaster myogenesis. We have addressed the developmental role of the single MEF2-like factor, CeMEF2, in Caenorhabditis elegans. Using expression assays and two mef-2 deletion alleles, we show that CeMEF2 is not required for proper myogenesis or development. Moreover, a putative null mef-2 allele fails to enhance or suppress the phenotypes of mutants in CeMyoD or CeTwist. Our results suggest that despite its evolutionary conservation of sequence and DNA binding properties, CeMEF2 has adopted a divergent role in development in the nematode compared with Drosophila and vertebrates.

Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha-actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] Dev. Biol. 159:706-719; Schoenwolf and Garcia-Martinez [1995] Cell Mol. Biol. Res. 41:233-240; Garcia-Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so-called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent-shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight-heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post-transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events.

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.

A protein encoded within the Down syndrome critical region is enriched in striated muscles and inhibits calcineurin signaling

Here we describe a small family of proteins, termed MCIP1 and MCIP2 (for myocyte-enriched calcineurin interacting protein), that are expressed most abundantly in striated muscles and that form a physical complex with calcineurin A. MCIP1 is encoded by DSCR1, a gene located in the Down syndrome critical region. Expression of the MCIP family of proteins is up-regulated during muscle differentiation, and their forced overexpression inhibits calcineurin signaling to a muscle-specific target gene in a myocyte cell background. Binding of MCIP1 to calcineurin A requires sequence motifs that resemble calcineurin interacting domains found in NFAT proteins. The inhibitory action of MCIP1 involves a direct association with the catalytic domain of calcineurin, rather than interference with the function of downstream components of the calcineurin signaling pathway. The interaction between MCIP proteins and calcineurin may modulate calcineurin-dependent pathways that control hypertrophic growth and selective programs of gene expression in striated muscles.

Autoregulation and multiple enhancers control Math1 expression in the developing nervous system

Development of the vertebrate nervous system requires the actions of transcription factors that establish regional domains of gene expression, which results in the generation of diverse neuronal cell types. MATH1, a transcription factor of the bHLH class, is expressed during development of the nervous system in multiple neuronal domains, including the dorsal neural tube, the EGL of the cerebellum and the hair cells of the vestibular and auditory systems. MATH1 is essential for proper development of the granular layer of the cerebellum and the hair cells of the cochlear and vestibular systems, as shown in mice carrying a targeted disruption of Math1. Previously, we showed that 21 kb of sequence flanking the Math1-coding region is sufficient for Math1 expression in transgenic mice. Here we identify two discrete sequences within the 21 kb region that are conserved between mouse and human, and are sufficient for driving a lacZ reporter gene in these domains of Math1 expression in transgenic mice. The two identified enhancers, while dissimilar in sequence, appear to have redundant activities in the different Math1 expression domains except the spinal neural tube. The regulatory mechanisms for each of the diverse Math1 expression domains are tightly linked, as separable regulatory elements for any given domain of Math1 expression were not found, suggesting that a common regulatory mechanism controls these apparently unrelated domains of expression. In addition, we demonstrate a role for autoregulation in controlling the activity of the Math1 enhancer, through an essential E-box consensus binding site.

An Nkx-dependent enhancer regulates cGATA-6 gene expression during early stages of heart development

The evolutionarily conserved GATA-6 transcription factor is an early and persistent marker of heart development in diverse vertebrate species. We previously found evidence for a functionally conserved heart-specific enhancer upstream of the chicken GATA-6 (cGATA-6) gene and in the present study we used transgenic mouse assays to further characterize this regulatory module. We show that this enhancer is activated in committed precursor cells within the cardiac crescent and that it remains active in essentially all cardiogenic cells through the linear heart stage. Although this enhancer can account for cGATA-6 gene expression early in the cardiogenic program, it is not able to maintain expression throughout the heart later in development. In particular, the enhancer is sequentially downregulated along the posterior to anterior axis, with activity becoming confined to outflow tract myocardium. Enhancers with similar properties have been shown to regulate the early heart-restricted expression of the mouse Nkx2.5 transcription factor gene. Whereas these Nkx2.5 enhancers are GATA-dependent, we show that the cGATA-6 enhancer is Nkx-dependent. We speculate that these enhancers are silenced to allow GATA-6 and Nkx2.5 gene expression to be governed by region-specific enhancers in the multichambered heart.