Regulation of the twist target gene tinman by modular cis-regulatory elements during early mesoderm development

Establishment of distinct MyoD, E2A, and twist DNA binding specificities by different basic region-DNA conformations

Basic helix-loop-helix (bHLH) proteins perform a wide variety of biological functions. Most bHLH proteins recognize the consensus DNA sequence CAN NTG (the E-box consensus sequence is underlined) but acquire further functional specificity by preferring distinct internal and flanking bases. In addition, induction of myogenesis by MyoD-related bHLH proteins depends on myogenic basic region (BR) and BR-HLH junction residues that are not essential for binding to a muscle-specific site, implying that their BRs may be involved in other critical interactions. We have investigated whether the myogenic residues influence DNA sequence recognition and how MyoD, Twist, and their E2A partner proteins prefer distinct CAN NTG sites. In MyoD, the myogenic BR residues establish specificity for particular CAN NTG sites indirectly, by influencing the conformation through which the BR helix binds DNA. An analysis of DNA binding by BR and junction mutants suggests that an appropriate BR-DNA conformation is necessary but not sufficient for myogenesis, supporting the model that additional interactions with this region are important. The sequence specificities of E2A and Twist proteins require the corresponding BR residues. In addition, mechanisms that position the BR allow E2A to prefer distinct half-sites as a heterodimer with MyoD or Twist, indicating that the E2A BR can be directed toward different targets by dimerization with different partners. Our findings indicate that E2A and its partner bHLH proteins bind to CAN NTG sites by adopting particular preferred BR-DNA conformations, from which they derive differences in sequence recognition that can be important for functional specificity.

Transcription of the myogenic regulatory gene Mef2 in cardiac, somatic, and visceral muscle cell lineages is regulated by a Tinman-dependent core enhancer

The MADS-box transcription factor MEF2 is expressed specifically in developing cardiac, somatic, and visceral muscle cell lineages during Drosophila embryogenesis and is required for myoblast differentiation and muscle morphogenesis. To define the mechanisms that regulate Mef2 transcription, we have analyzed the Mef2 upstream region for sequences sufficient to recapitulate the expression pattern of the gene in Drosophila embryos. Here we describe a complex enhancer located 5.8 kb upstream of the Drosophila Mef2 gene that controls transcription in cardial cells of the dorsal vessel, a subset of somatic muscle founder cells, and the visceral muscle cells. The core of this enhancer contains two evolutionarily conserved binding sites for the homeodomain protein Tinman (Tin), expressed in developing cardiac, somatic, and visceral muscle lineages. Both Tin binding sites are required for enhancer activity in all three muscle cell lineages. Whereas the 285-bp enhancer core alone is sufficient for expression in cardiac cells, expression in somatic founder cells and visceral muscle is dependent on the core enhancer plus unique flanking sequences that include an evolutionarily conserved E box. These results reveal an essential role for Tin in activation of Mef2 transcription in multiple myogenic lineages and demonstrate that transcriptional activity of Tin is dependent on combinatorial interactions with other factors unique to different muscle cell types.

The homeodomain transcription factor NK-4 acts as either a transcriptional activator or repressor and interacts with the p300 coactivator and the Groucho corepressor

NK-4 (tinman) encodes an NK-2 class homeodomain transcription factor that is required for development of the Drosophila dorsal mesoderm, including heart. Genetic evidence suggests its important role in mesoderm subdivision, yet the properties of NK-4 as a transcriptional regulator and the mechanism of gene transcription by NK-4 are not completely understood. Here, we describe its properties as a transcription factor and its interaction with the p300 coactivator and the Groucho corepressor. We demonstrate that NK-4 can activate or repress target genes in cultured cells, depending on functional domains that are conserved between Drosophila melanogaster and Drosophila virilis NK-4 genes. Using GAL4-NK-4 fusion constructs, we have mapped a transcriptional activation domain (amino acids 1-110) and repression domains (amino acids 111-188 and the homeodomain) and found an inhibitory function for the homeodomain in transactivation by NK-4. Furthermore, we demonstrate that NK-4-dependent transactivation is augmented by the p300 coactivator and show that NK-4 physically interacts with p300 via the activation domain. In addition, cotransfection experiments indicate that the repressor activity of NK-4 is strongly enhanced by the Groucho corepressor. Using immunoprecipitation and in vitro pull-down assays, we show that NK-4 directly interacts with the Groucho corepressor, for which the homeodomain is required. Together, our results indicate that NK-4 can act as either a transcriptional activator or repressor and provide the first evidence of NK-4 interactions with the p300 coactivator and the Groucho corepressor.

Building the heart piece by piece: modularity of cis-elements regulating Nkx2-5 transcription

Heart formation in Drosophila is dependent on the homeobox gene tinman. The homeobox gene Nkx2-5 is closely related to tinman and is the earliest known marker for cardiogenesis in vertebrate embryos. Recent studies of cis-regulatory elements required for Nkx2-5 expression in the developing mouse heart have revealed an extraordinary array of independent cardiac enhancers, and associated negative regulatory elements, that direct transcription in distinct regions of the embryonic heart. These studies demonstrate the modularity in cardiac transcription, in which different regulatory elements respond to distinct sets of transcription factors to control gene expression in different compartments of the developing heart. We consider the potential mechanisms underlying such transcriptional complexity, its possible significance for cardiac function, and the implications for evolution of the multichambered heart.

TGF-beta family signal transduction in Drosophila development: from Mad to Smads

The transforming growth factor-beta (TGF-beta) superfamily encompasses a large group of soluble extracellular proteins that are potent regulators of development in both vertebrates and invertebrates. Drosophila TGF-beta family members include three proteins with homology to vertebrate bone morphogenetic proteins (BMPs): Decapentaplegic (Dpp), Screw, and Glass bottom boat-60A. Genetic studies of Dpp signaling led to the identification of Smad proteins as central mediators of signal transduction by TGF-beta family members. Work in mammalian tissue culture has elucidated a biochemical model for signal transduction, in which activation of receptor serine-threonine kinase activity leads to phosphorylation of specific Smad proteins and translocation of heteromeric Smad protein complexes to the nucleus. Once in the nucleus Smad proteins interact with other DNA binding proteins to regulate transcription of specific target genes. Dissection of Dpp-response elements from genes expressed during embryonic mesoderm patterning and midgut morphogenesis provides important insights into the contributions of Smad proteins and tissue-specific transcription factors to spatial regulation of gene expression. Genetic studies in Drosophila are now expanding to include multiple BMP ligands and receptors and have uncovered activities not explained by the current signal transduction model. Identification of more ligand sequences and demonstration of a functional Drosophila activin-like signal transduction pathway suggest that all TGF-beta signal transduction pathways are present in flies.

Lack of regulation in the heart forming region of avian embryos

The ability to regenerate a heart after ablation of cardiogenic mesoderm has been demonstrated in early stage fish and amphibian embryos but this type of regulation of the heart field has not been seen in avians or mammals. The regulative potential of the cardiogenic mesoderm was examined in avian embryos and related to the spatial expression of genes implicated in early cardiogenesis. With the identification of early cardiac regulators such as bmp-2 and nkx-2.5, it is now possible to reconcile classical embryological studies with molecular mechanisms of cardiac lineage determination in vivo. The most anterior lateral embryonic cells were identified as the region that becomes the heart and removal of all or any subset of these cells resulted in the loss of corresponding cardiac structures. In addition, removal of the lateral heart forming mesoderm while leaving the lateral endoderm intact also results in loss of cardiac structures. Thus the medial anterior mesoderm cannot be recruited into the heart lineage in vivo even in the presence of potentially cardiac inducing endoderm. In situ analysis demonstrated that genes involved in early events of cardiogenesis such as bone morphogenetic protein 2 (bmp-2) and nkx-2.5 are expressed coincidentally with the mapped far lateral heart forming region. The activin type IIa receptor (actR-IIa) is a potential mediator of BMP signaling since it is expressed throughout the anterior mesoderm with the highest level of expression occurring in the lateral prospective heart cells. The posterior boundary of actR-IIa is consistent with the posterior boundary of nkx-2.5 expression, supporting a model whereby ActR-IIa is involved in restricting the heart forming region to an anterior subset of lateral cells exposed to BMP-2. Analysis of the cardiogenic potential of the lateral plate mesoderm posterior to nkx-2.5 and actR-IIa expression demonstrated that these cells are not cardiogenic in vitro and that removal of these cells from the embryo does not result in loss of heart tissue in vivo. Thus, the region of the avian embryo that will become the heart is defined medially, laterally, and posteriorly by nkx-2.5 gene expression. Removal of all or part of the nkx-2.5 expressing region results in the loss of corresponding heart structures, demonstrating the inability of the chick embryo to regenerate cardiac tissue in vivo at stages after nkx-2.5 expression is initiated.

Identification of upstream regulatory regions in the heart-expressed homeobox gene Nkx2-5

Nkx2-5 marks the earliest recognizable cardiac progenitor cells, and is activated in response to inductive signals involved in lineage specification. Nkx2-5 is also expressed in the developing foregut, thyroid, spleen, stomach and tongue. One approach to elucidate the signals involved in cardiogenesis was to examine the transcriptional regulation of early lineage markers such as Nkx2-5. We generated F0 transgenic mice, which carry Nkx2-5 flanking sequences linked to a lacZ reporter gene. We identified multiple regulatory regions located within the proximal 10.7 kb of the Nkx2-5 gene. In addition to a proximal promoter, we identified a second promoter and a novel upstream exon that could participate in the regulation of Nkx2-5 transcription. Although used rarely in normal development, this novel exon could be spliced into the Nkx2-5 coding region in several ways, thereby potentially creating novel Nkx2-5 protein isoforms, whose transcriptional activity is greatly diminished as compared to wild-type Nkx2-5. An enhancer that directs expression in pharynx, spleen, thyroid and stomach was identified within 3.5 kb of exon 1 between the coding exon 1 and the novel upstream exon 1a. Two or more enhancers upstream of exon 1a were capable of driving expression in the cardiac crescent, throughout the myocardium of the early heart tube, then in the outflow tract and right ventricle of the looped heart tube. A negative element was also located upstream of exon1a, which interacted in complex ways with enhancers to direct correct spatial expression. In addition, potential autoregulatory elements can be cooperatively stimulated by Nkx2-5 and GATA-4. Our results demonstrate that a complex suite of interacting regulatory domains regulate Nkx2-5 transcription. Dissection of these elements should reveal essential features of cardiac induction and positive and negative signaling within the cardiac field.

Control of early cardiac-specific transcription of Nkx2-5 by a GATA-dependent enhancer

The homeobox gene Nkx2-5 is the earliest known marker of the cardiac lineage in vertebrate embryos. Nkx2-5 expression is first detected in mesodermal cells specified to form heart at embryonic day 7.5 in the mouse and expression is maintained throughout the developing and adult heart. In addition to the heart, Nkx2-5 is transiently expressed in the developing pharynx, thyroid and stomach. To investigate the mechanisms that initiate cardiac transcription during embryogenesis, we analyzed the Nkx2-5 upstream region for regulatory elements sufficient to direct expression of a lacZ transgene in the developing heart of transgenic mice. We describe a cardiac enhancer, located about 9 kilobases upstream of the Nkx2-5 gene, that fully recapitulates the expression pattern of the endogenous gene in cardiogenic precursor cells from the onset of cardiac lineage specification and throughout the linear and looping heart tube. Thereafter, as the atrial and ventricular chambers become demarcated, enhancer activity becomes restricted to the developing right ventricle. Transcription of Nkx2-5 in pharynx, thyroid and stomach is controlled by regulatory elements separable from the cardiac enhancer. This distal cardiac enhancer contains a high-affinity binding site for the cardiac-restricted zinc finger transcription factor GATA4 that is essential for transcriptional activity. These results reveal a novel GATA-dependent mechanism for activation of Nkx2-5 transcription in the developing heart and indicate that regulation of Nkx2-5 is controlled in a modular manner, with multiple regulatory regions responding to distinct transcriptional networks in different compartments of the developing heart.

Drosophila mef2 expression during mesoderm development is controlled by a complex array of cis-acting regulatory modules

The function of the Drosophila mef2 gene, a member of the MADS box supergene family of transcription factors, is critical for terminal differentiation of the three major muscle cell types, namely somatic, visceral, and cardiac. During embryogenesis, mef2 undergoes multiple phases of expression, which are characterized by initial broad mesodermal expression, followed by restricted expression in the dorsal mesoderm, specific expression in muscle progenitors, and sustained expression in the differentiated musculatures. In this study, evidence is presented that temporally and spatially specific mef2 expression is controlled by a complex array of cis-acting regulatory modules that are responsive to different genetic signals. Functional testing of approximately 12 kb of 5' flanking region of the mef2 gene showed that the initial widespread mesodermal expression is achieved through a 280-bp twist-dependent enhancer. The subsequent dorsal mesoderm-restricted mef2 expression is mediated through a 460-bp dpp-responsive regulatory module, which involves the function of the Smad4 homolog Medea and contains several binding sites for Medea and Mad. The analysis also showed that regulated mef2 expression in the caudal and trunk visceral mesoderm, which give rise to longitudinal and circular gut musculatures, respectively, is under the control of distinct enhancer elements. In addition, mef2 expression in the cardioblasts of the heart is dependent upon at least two distinct enhancers, which are active at different periods during embryogenesis. Moreover, multiple regulatory elements are differentially activated for specific expression in presumptive muscle founders, prefusion myoblasts, and differentiated muscle fibers. Taken together, the presented data suggest that specific expression of the mef2 gene in myogenic lineages in the Drosophila embryo is the result of multiple genetic inputs that act in an additive manner upon distinct enhancers in the 5' flanking region.

The taxonomy of developmental control in Caenorhabditis elegans

The Caenorhabditis elegans genome sequence was surveyed for transcription factor and signaling gene families that have been shown to regulate development in a variety of species. About 10 to 25 percent of the genes in most of the gene families already have been genetically analyzed in C. elegans, about half of the genes detect probable orthologs in other species, and about 10 to 25 percent of the genes are, at present, unique to C. elegans. Caenorhabditis elegans is also missing genes that are found in vertebrates and other invertebrates. Thus the genome sequence reveals universals in developmental control that are the legacy of metazoan complexity before the Cambrian explosion, as well as genes that have been more recently invented or lost in particular phylogenetic lineages.

A GATA-dependent nkx-2.5 regulatory element activates early cardiac gene expression in transgenic mice

nkx-2.5 is one of the first genes expressed in the developing heart of early stage vertebrate embryos. Cardiac expression of nkx-2.5 is maintained throughout development and nkx-2.5 also is expressed in the developing pharyngeal arches, spleen, thyroid and tongue. Genomic sequences flanking the mouse nkx-2.5 gene were analyzed for early developmental regulatory activity in transgenic mice. Approximately 3 kb of 5′ flanking sequence is sufficient to activate gene expression in the cardiac crescent as early as E7.25 and in limited regions of the developing heart at later stages. Expression also was detected in the developing spleen anlage at least 24 hours before the earliest reported spleen marker and in the pharyngeal pouches and their derivatives including the thyroid. The observed expression pattern from the −3 kb construct represents a subset of the endogenous nkx-2.5 expression pattern which is evidence for compartment-specific nkx-2.5 regulatory modules. A 505 bp regulatory element was identified that contains multiple GATA, NKE, bHLH, HMG and HOX consensus binding sites. This element is sufficient for gene activation in the cardiac crescent and in the heart outflow tract, pharynx and spleen when linked directly to lacZ or when positioned adjacent to the hsp68 promoter. Mutation of paired GATA sites within this element eliminates gene activation in the heart, pharynx and spleen primordia of transgenic embryos. The dependence of this nkx-2. 5 regulatory element on GATA sites for gene activity is evidence for a GATA-dependent regulatory mechanism controlling nkx-2.5 gene expression. The presence of consensus binding sites for other developmentally important regulatory factors within the 505 bp distal element suggests that combinatorial interactions between multiple regulatory factors are responsible for the initial activation of nkx-2.5 in the cardiac, thyroid and spleen primordia.

Analysis of a Caenorhabditis elegans Twist homolog identifies conserved and divergent aspects of mesodermal patterning

Mesodermal development is a multistep process in which cells become increasingly specialized to form specific tissue types. In Drosophila and mammals, proper segregation and patterning of the mesoderm involves the bHLH factor Twist. We investigated the activity of a Twist-related factor, CeTwist, during Caenorhabditis elegans mesoderm development. Embryonic mesoderm in C. elegans derives from a number of distinct founder cells that are specified during the early lineages; in contrast, a single blast cell (M) is responsible for all nongonadal mesoderm formation during postembryonic development. Using immunofluorescence and reporter fusions, we determined the activity pattern of the gene encoding CeTwist. No activity was observed during specification of mesodermal lineages in the early embryo; instead, the gene was active within the M lineage and in a number of mesodermal cells with nonstriated muscle fates. A role for CeTwist in postembryonic mesodermal cell fate specification was indicated by ectopic expression and genetic interference assays. These experiments showed that CeTwist was responsible for activating two target genes normally expressed in specific subsets of nonstriated muscles derived from the M lineage. In vitro and in vivo assays suggested that CeTwist cooperates with the C. elegans E/Daughterless homolog in directly activating these targets. The two target genes that we have studied, ceh-24 and egl-15, encode an NK-2 class homeodomain and an FGF receptor (FGFR) homolog, respectively. Twist activates FGFR and NK-homeodomain target genes during mesodermal patterning of Drosophila and similar target interactions have been proposed to modulate mesenchymal growth during closure of the vertebrate skull. These results suggest the possibility that a conserved pathway may be used for diverse functions in mesodermal specification.

Smad proteins act in combination with synergistic and antagonistic regulators to target Dpp responses to the Drosophila mesoderm

Dorsal mesoderm induction in arthropods and ventral mesoderm induction in vertebrates are closely related processes that involve signals of the BMP family. In Drosophila, induction of visceral mesoderm, dorsal muscles, and the heart by Dpp is, at least in part, effected through the transcriptional activation and function of the homeobox gene tinman in dorsal mesodermal cells during early embryogenesis. Here we present a functional dissection of a tinman enhancer that mediates the Dpp response. We provide evidence that mesoderm-specific induction of tinman requires the binding of both activators and repressors. Screens for binding factors yielded Tinman itself and the Smad4 homolog Medea. We show that the binding and synergistic activities of Smad and Tinman proteins are critical for mesodermal tinman induction, whereas repressor binding sites prevent induction in the dorsal ectoderm and amnioserosa. Thus, integration of positive and negative regulators on enhancers of target genes appears to be an important mechanism in tissue-specific induction by TGF-beta molecules.

Signalling by the Drosophila epidermal growth factor receptor is required for the specification and diversification of embryonic muscle progenitors

Muscle development initiates in the Drosophila embryo with the segregation of single progenitor cells, from which a complete set of myofibres arises. Each progenitor is assigned a unique fate, characterized by the expression of particular identity genes. We now demonstrate that the Drosophila epidermal growth factor receptor provides an inductive signal for the specification of a large subset of muscle progenitors. In the absence of the receptor or its ligand, SPITZ, specific progenitors fail to segregate. The resulting unspecified mesodermal cells undergo programmed cell death. In contrast, receptor hyperactivation generates supernumerary progenitors, as well as the duplication of at least one SPITZ-dependent myofibre. The development of individual muscles is differentially sensitive to variations in the level of signalling by the epidermal growth factor receptor. Such graded myogenic effects can be influenced by alterations in the functions of Star and rhomboid. In addition, muscle patterning is dependent on the generation of a spatially restricted, activating SPITZ signal, a process that may rely on the localized mesodermal expression of RHOMBOID. Thus, the epidermal growth factor receptor contributes both to muscle progenitor specification and to the diversification of muscle identities.

Muscle development: a transcriptional pathway in myogenesis

Recent studies have substantially advanced our understanding of the transcriptional program regulating development of the different muscle types in Drosophila. For body wall muscle, a pathway can now be drawn that links the transcription factor Dorsal, inherited from the egg, with the differentiated-muscle protein tropomyosin.

bagpipe-Dependent expression of vimar, a novel Armadillo-repeats gene, in Drosophila visceral mesoderm

Two homeobox-containing genes, tinman and bagpipe, play important roles during the specification of the midgut visceral musculature from the mesoderm during Drosophila embryogenesis. Expression of tinman in the dorsal mesoderm activates the expression of the bagpipe gene in segmental subsets of those cells, which then become determined to form the midgut visceral mesoderm. Understanding how the bagpipe gene affects this specification requires the isolation and characterization of its downstream target genes. Using an enhancer trap line that expresses its marker in the midgut visceral mesoderm, we have cloned and characterized a novel gene (vimar) that is expressed embryonically in the mid and hindgut visceral mesoderm, as well as in the CNS and PNS. The expression of this gene in the midgut visceral mesoderm initiates shortly after bagpipe expression and depends on bagpipe function. Maternal and zygotic transcripts are produced from this gene by alternative polyadenylation, and encode the same 634-amino acid protein. The vimar protein contains 15 tandem copies of the Armadillo repeat, a protein interaction domain, and is similar to mammalian Smg guanine dissociation stimulator protein, which stimulates the activity of a number of different p21 small G-proteins. These results, together with the observed lethality of vimar mutations, indicate that vimar is one of the bagpipe target genes that are required for normal development and differentiation of the midgut visceral mesoderm.