A role for the Drosophila neurogenic genes in mesoderm differentiation

Genetic Control of Muscle Diversification and Homeostasis: Insights from Drosophila

In the fruit fly, Drosophila melanogaster, the larval somatic muscles or the adult thoracic flight and leg muscles are the major voluntary locomotory organs. They share several developmental and structural similarities with vertebrate skeletal muscles. To ensure appropriate activity levels for their functions such as hatching in the embryo, crawling in the larva, and jumping and flying in adult flies all muscle components need to be maintained in a functionally stable or homeostatic state despite constant strain. This requires that the muscles develop in a coordinated manner with appropriate connections to other cell types they communicate with. Various signaling pathways as well as extrinsic and intrinsic factors are known to play a role during Drosophila muscle development, diversification, and homeostasis. In this review, we discuss genetic control mechanisms of muscle contraction, development, and homeostasis with particular emphasis on the contractile unit of the muscle, the sarcomere.

Notch signaling at the crossroads of innate and adaptive immunity

Notch signaling is an evolutionarily conserved cell-to-cell signaling pathway that regulates cellular differentiation and function across multiple tissue types and developmental stages. In this review, we discuss our current understanding of Notch signaling in mammalian innate and adaptive immunity. The importance of Notch signaling is pervasive throughout the immune system, as it elicits lineage and context-dependent effects in a wide repertoire of cells. Although regulation of binary cell fate decisions encompasses many of the functions first ascribed to Notch in the immune system, recent advances in the field have refined and expanded our view of the Notch pathway beyond this initial concept. From establishing T cell identity in the thymus to regulating mature T cell function in the periphery, the Notch pathway is an essential, recurring signal for the T cell lineage. Among B cells, Notch signaling is required for the development and maintenance of marginal zone B cells in the spleen. Emerging roles for Notch signaling in innate and innate-like lineages such as classical dendritic cells and innate lymphoid cells are likewise coming into view. Lastly, we speculate on the molecular underpinnings that shape the activity and versatility of the Notch pathway.

The Notch pathway regulates both the proliferation and differentiation of follicular cells in the panoistic ovary of Blattella germanica

The Notch pathway is an essential regulator of cell proliferation and differentiation during development. Its involvement in insect oogenesis has been examined in insect species with meroistic ovaries, and it is known to play a fundamental role in cell fate decisions and the induction of the mitosis-to-endocycle switch in follicular cells (FCs). This work reports the functions of the main components of the Notch pathway (Notch and its ligands Delta and Serrate) during oogenesis in Blattella germanica, a phylogenetically basal species with panoistic ovary. As is revealed by RNAi-based analyses, Notch and Delta were found to contribute towards maintaining the FCs in an immature, non-apoptotic state. This ancestral function of Notch appears in opposition to the induction of transition from mitosis to endocycle that Notch exerts in Drosophila melanogaster, a change in the Notch function that might be in agreement with the evolution of the insect ovary types. Notch was also shown to play an active role in inducing ovarian follicle elongation via the regulation of the cytoskeleton. In addition, Delta and Notch interactions were seen to determine the differentiation of the posterior population of FCs. Serrate levels were found to be Notch-dependent and are involved in the control of the FC programme, although they would appear to play no crucial role in panoistic ovary oogenesis.

A new level of plasticity: Drosophila smooth-like testes muscles compensate failure of myoblast fusion

The testis of Drosophila resembles an individual testis tubule of mammals. Both are surrounded by a sheath of smooth muscles, which in Drosophila are multinuclear and originate from a pool of myoblasts that are set aside in the embryo and accumulate on the genital disc later in development. These muscle stem cells start to differentiate early during metamorphosis and give rise to all muscles of the inner male reproductive system. Shortly before the genital disc and the developing testes connect, multinuclear nascent myotubes appear on the anterior tips of the seminal vesicles. Here, we show that adhesion molecules are distinctly localized on the seminal vesicles; founder cell (FC)-like myoblasts express Dumbfounded (Duf) and Roughest (Rst), and fusion-competent myoblast (FCM)-like cells mainly express Sticks and stones (Sns). The smooth but multinuclear myotubes of the testes arose by myoblast fusion. RNAi-mediated attenuation of Sns or both Duf and Rst severely reduced the number of nuclei in the testes muscles. Duf and Rst probably act independently in this context. Despite reduced fusion in all of these RNAi-treated animals, myotubes migrated onto the testes, testes were shaped and coiled, muscle filaments were arranged as in the wild type and spermatogenesis proceeded normally. Hence, the testes muscles compensate for fusion defects so that the myofibres encircling the adult testes are indistinguishable from those of the wild type and male fertility is guaranteed.

Summary:Drosophila testes muscles arise from stem cells and can compensate for fusion defects to safeguard fertility; this plasticity may compensate for the observed lack of satellite cells in Drosophila.

Specification of the somatic musculature in Drosophila

The somatic muscle system formed during Drosophila embryogenesis is required for larvae to hatch, feed, and crawl. This system is replaced in the pupa by a new adult muscle set, responsible for activities such as feeding, walking, and flight. Both the larval and adult muscle systems are comprised of distinct muscle fibers to serve these specific motor functions. In this way, the Drosophila musculature is a valuable model for patterning within a single tissue: while all muscle cells share properties such as the contractile apparatus, properties such as size, position, and number of nuclei are unique for a particular muscle. In the embryo, diversification of muscle fibers relies first on signaling cascades that pattern the mesoderm. Subsequently, the combinatorial expression of specific transcription factors leads muscle fibers to adopt particular sizes, shapes, and orientations. Adult muscle precursors (AMPs), set aside during embryonic development, proliferate during the larval phases and seed the formation of the abdominal, leg, and flight muscles in the adult fly. Adult muscle fibers may either be formed de novo from the fusion of the AMPs, or are created by the binding of AMPs to an existing larval muscle. While less is known about adult muscle specification compared to the larva, expression of specific transcription factors is also important for its diversification. Increasingly, the mechanisms required for the diversification of fly muscle have found parallels in vertebrate systems and mark Drosophila as a robust model system to examine questions about how diverse cell types are generated within an organism.

The role of TORC1 in muscle development in Drosophila

Myogenesis is an important process during both development and muscle repair. Previous studies suggest that mTORC1 plays a role in the formation of mature muscle from immature muscle precursor cells. Here we show that gene expression for several myogenic transcription factors including Myf5, Myog and Mef2c but not MyoD and myosin heavy chain isoforms decrease when C2C12 cells are treated with rapamycin, supporting a role for mTORC1 pathway during muscle development. To investigate the possibility that mTORC1 can regulate muscle in vivo we ablated the essential dTORC1 subunit Raptor in Drosophila melanogaster and found that muscle-specific knockdown of Raptor causes flies to be too weak to emerge from their pupal cases during eclosion. Using a series of GAL4 drivers we also show that muscle-specific Raptor knockdown also causes shortened lifespan, even when eclosure is unaffected. Together these results highlight an important role for TORC1 in muscle development, integrity and function in both Drosophila and mammalian cells.

Drosophila polypyrimidine tract-binding protein (DmPTB) regulates dorso-ventral patterning genes in embryos

The Drosophila polypyrimidine tract-binding protein (dmPTB or hephaestus) plays an important role during embryogenesis. A loss of function mutation, heph⁰³⁴²⁹, results in varied defects in embryonic developmental processes, leading to embryonic lethality. However, the suite of molecular functions that are disrupted in the mutant remains unknown. We have used an unbiased high throughput sequencing approach to identify transcripts that are misregulated in this mutant. Misregulated transcripts show evidence of significantly altered patterns of splicing (exon skipping, 5′ and 3′ splice site switching), alternative 5′ ends, and mRNA level changes (up and down regulation). These findings are independently supported by reverse-transcription-polymerase chain reaction (RT-PCR) analysis and in situ hybridization. We show that a group of genes, such as Zerknüllt, z600 and screw are among the most upregulated in the mutant and have been functionally linked to dorso-ventral patterning and/or dorsal closure processes. Thus, loss of dmPTB function results in specific misregulated transcripts, including those that provide the missing link between the loss of dmPTB function and observed developmental defects in embryogenesis. This study provides the first comprehensive repertoire of genes affected in vivo in the heph mutant in Drosophila and offers insight into the role of dmPTB during embryonic development.

Collier/OLF/EBF-dependent transcriptional dynamics control pharyngeal muscle specification from primed cardiopharyngeal progenitors

In vertebrates, pluripotent pharyngeal mesoderm progenitors produce the cardiac precursors of the second heart field as well as the branchiomeric head muscles and associated stem cells. However, the mechanisms underlying the transition from multipotent progenitors to distinct muscle precursors remain obscured by the complexity of vertebrate embryos. Using Ciona intestinalis as a simple chordate model, we show that bipotent cardiopharyngeal progenitors are primed to activate both heart and pharyngeal muscle transcriptional programs, which progressively become restricted to corresponding precursors. The transcription factor COE (Collier/OLF/EBF) orchestrates the transition to pharyngeal muscle fate both by promoting an MRF-associated myogenic program in myoblasts and by maintaining an undifferentiated state in their sister cells through Notch-mediated lateral inhibition. The latter are stem cell-like muscle precursors that form most of the juvenile pharyngeal muscles. We discuss the implications of our findings for the development and evolution of the chordate cardiopharyngeal mesoderm.

The Notch target E(spl)mδ is a muscle-specific gene involved in methylmercury toxicity in motor neuron development

Methylmercury (MeHg) is a ubiquitous environmental toxin that has a selective and potent impact on the nervous system, particularly during neural development yet, the mechanisms for its apparent neurodevelopmental specificity are unknown. The Notch receptor pathway has been implicated as a MeHg target in several studies. Notch signaling mediates cell-cell signals in a number of developmental contexts including neurogenesis and myogenesis, where it fundamentally acts to repress differentiation. Previous work in our lab has shown that MeHg causes preferential upregulation of a canonical Notch response gene, E(spl)mδ, in Drosophila embryos. In parallel, MeHg is seen to disrupt outgrowth of embryonic intersegmental motor nerves (ISN), which can be mimicked by expression of activated Notch in embryonic neurons. However, overexpression of E(spl)mδ in developing neurons fails to elicit motor neuron outgrowth defects, pointing to a non-autonomous role for E(spl)mδ in motor axon development. In this study we investigate a role for E(spl)mδ in conveying the toxicity of MeHg in the embryo. We find that endogenous expression of the E(spl)mδ gene localizes to developing somatic muscles in embryos. Notably, E(spl)mδ expression is seen in several muscles that are known synaptic targets for both the ISN and the segmental motor nerve (SN). We also demonstrate that the SN, similar to the ISN, exhibits disrupted axon outgrowth in response to MeHg. E(spl)mδ can induce a SN motor neuron phenotype, similar to MeHg treatment; but, only when E(spl)mδ expression is targeted to developing muscles. E(spl)mδ overexpression in developing muscles also results in aberrant muscle morphology, which is not apparent with expression of the closely related E(spl)mγ in developing muscles. Our data point to a role for the Notch target E(spl)mδ in mediating MeHg toxicity in embryonic development by disrupting the coordinated targeting of motor neurons to their muscle targets.

Myosin heavy chain-like localizes at cell contact sites during Drosophila myoblast fusion and interacts in vitro with Rolling pebbles 7

Besides representing the sarcomeric thick filaments, myosins are involved in many cellular transport and motility processes. Myosin heavy chains are grouped into 18 classes. Here we show that in Drosophila, the unconventional group XVIII myosin heavy chain-like (Mhcl) is transcribed in the mesoderm of embryos, most prominently in founder cells (FCs). An ectopically expressed GFP-tagged Mhcl localizes in the growing muscle at cell-cell contacts towards the attached fusion competent myoblast (FCM). We further show that Mhcl interacts in vitro with the essential fusion protein Rolling pebbles 7 (Rols7), which is part of a protein complex established at cell contact sites (Fusion-restricted Myogenic-Adhesive Structure or FuRMAS). Here, branched F-actin is likely needed to widen the fusion pore and to integrate the myoblast into the growing muscle. We show that the localization of Mhcl is dependent on the presence of Rols7, and we postulate that Mhcl acts at the FuRMAS as an actin motor protein. We further show that Mhcl deficient embryos develop a wild-type musculature. We thus propose that Mhcl functions redundantly to other myosin heavy chains in myoblasts. Lastly, we found that the protein is detectable adjacent to the sarcomeric Z-discs, suggesting an additional function in mature muscles.

Functional analysis of the NHR2 domain indicates that oligomerization of Neuralized regulates ubiquitination and endocytosis of Delta during Notch signaling

The Notch pathway plays an integral role in development by regulating cell fate in a wide variety of multicellular organisms. A critical step in the activation of Notch signaling is the endocytosis of the Notch ligands Delta and Serrate. Ligand endocytosis is regulated by one of two E3 ubiquitin ligases, Neuralized (Neur) or Mind bomb. Neur is comprised of a C-terminal RING domain, which is required for Delta ubiquitination, and two Neur homology repeat (NHR) domains. We have previously shown that the NHR1 domain is required for Delta trafficking. Here we show that the NHR1 domain also affects the binding and internalization of Serrate. Furthermore, we show that the NHR2 domain is required for Neur function and that a point mutation in the NHR2 domain (Gly430) abolishes Neur ubiquitination activity and affects ligand internalization. Finally, we provide evidence that Neur can form oligomers in both cultured cells and fly tissues, which regulate Neur activity and, by extension, ligand internalization.

Dlk1 is necessary for proper skeletal muscle development and regeneration

Delta-like 1homolog (Dlk1) is an imprinted gene encoding a transmembrane protein whose increased expression has been associated with muscle hypertrophy in animal models. However, the mechanisms by which Dlk1 regulates skeletal muscle plasticity remain unknown. Here we combine conditional gene knockout and over-expression analyses to investigate the role of Dlk1 in mouse muscle development, regeneration and myogenic stem cells (satellite cells). Genetic ablation of Dlk1 in the myogenic lineage resulted in reduced body weight and skeletal muscle mass due to reductions in myofiber numbers and myosin heavy chain IIB gene expression. In addition, muscle-specific Dlk1 ablation led to postnatal growth retardation and impaired muscle regeneration, associated with augmented myogenic inhibitory signaling mediated by NF-κB and inflammatory cytokines. To examine the role of Dlk1 in satellite cells, we analyzed the proliferation, self-renewal and differentiation of satellite cells cultured on their native host myofibers. We showed that ablation of Dlk1 inhibits the expression of the myogenic regulatory transcription factor MyoD, and facilitated the self-renewal of activated satellite cells. Conversely, Dlk1 over-expression inhibited the proliferation and enhanced differentiation of cultured myoblasts. As Dlk1 is expressed at low levels in satellite cells but its expression rapidly increases upon myogenic differentiation in vitro and in regenerating muscles in vivo, our results suggest a model in which Dlk1 expressed by nascent or regenerating myofibers non-cell autonomously promotes the differentiation of their neighbor satellite cells and therefore leads to muscle hypertrophy.

Gene deletion screen for cardiomyopathy in adult Drosophila identifies a new notch ligand

RATIONALE

Drosophila has been recognized as a model to study human cardiac diseases.

OBJECTIVE

Despite these findings, and the wealth of tools that are available to the fly community, forward genetic screens for adult heart phenotypes have been rarely performed because of the difficulty in accurately measuring cardiac function in adult Drosophila.

METHODS AND RESULTS

Using optical coherence tomography to obtain real-time analysis of cardiac function in awake Drosophila, we performed a genomic deficiency screen in adult flies. Based on multiple complementary approaches, we identified CG31665 as a novel gene causing dilated cardiomyopathy. CG31665, which we name weary (wry), has structural similarities to members of the Notch family. Using cell aggregation assays and gamma-secretase inhibitors we show that Wry is a novel Notch ligand that can mediate cellular adhesion with Notch expressing cells and transactivates Notch to promote signaling and nuclear transcription. Importantly, Wry lacks a DSL (Delta-Serrate-Lag) domain that is common feature to the other Drosophila Notch ligands. We further show that Notch signaling is critically important for the maintenance of normal heart function of the adult fly.

CONCLUSIONS

In conclusion, we identify a previously unknown Notch ligand in Drosophila that when deleted causes cardiomyopathy. Our study suggests that Notch signaling components may be a therapeutic target for dilated cardiomyopathy.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

Mind bomb 2, a founder myoblast-specific protein, regulates myoblast fusion and muscle stability

A fundamental step during Drosophila myogenesis is the specification of founder myoblasts (FMs). Founders possess the information required for the acquisition of muscle identity and for the execution of the myogenic programme, whereas fusion-competent myoblasts (FCMs) acquire this information after fusing to founders. Very little is known about genes that implement the execution of the myogenic programme. Here we characterise Mind bomb 2 (Mib2), a protein with putative E3 ubiquitin ligase activity that is exclusive of FMs and necessary for at least two distinct steps of the founder/myotube differentiation programme. Thus, in mib2 mutants, the early process of myoblast fusion is compromised, as FMs undergo a reduced number of rounds of fusion with FCMs. At later stages, with the onset of muscle contraction, many muscles degenerate, display aberrant sarcomeric structure and detach from tendons. The fusion process requires intact E3-RING-finger domains of Mib2 (the putative catalytic sites), probably to eliminate the FCM-specific activator Lmd from nascent myotubes. However, these sites appear dispensable for muscle integrity. This, and the subcellular accumulation of Mib2 in Z and M bands of sarcomeres, plus its physical interaction with nonmuscle myosin (a Z-band-localised protein necessary for the formation of myofibrils), suggest a structural role for Mib2 in maintaining sarcomeric stability. We suggest that Mib2 acts sequentially in myoblast fusion and sarcomeric stability by two separable processes involving distinct functions of Mib2.

Myoblast fusion in Drosophila

Myogenic differentiation in Drosophila melanogaster, as in many other organisms, involves the generation of multinucleate muscle fibers through the fusion of myoblasts. Prior to fusion, the myoblasts become specified as one of two distinct cell types. They then become competent to fuse and express genes associated with cell recognition and adhesion. Initially, cell-type- specific adhesion molecules mediate recognition and fusion between these two distinct populations of myoblasts. Intracellular proteins that are essential for the fusion process are then recruited to points of cell-cell contact at the membrane, where the cell surface molecules have become localized. Many of these cytosolic proteins contribute to reorganization of the cytoskeleton through activation of small guanosine triphosphatases and recruitment of actin nucleating proteins. Following the initial fusion event, the ultimate size of the syncytia is achieved through multiple rounds of fusion between the developing syncytia and mononucleate myoblasts. Ultrastructural changes associated with cell fusion include recruitment of electron-dense vesicles to points of cell-cell contact, resolution of these vesicles into fusion plaques, fusion pore formation, and membrane vesiculation. This chapter reviews our current understanding of the genes, pathways, and ultrastructural events associated with fusion in the Drosophila embryo, giving rise to multinucleate syncytia that will be used throughout larval life.

A key role of Pox meso in somatic myogenesis of Drosophila

The Pax gene Pox meso (Poxm) was the first and so far only gene whose initial expression was shown to occur specifically in the anlage of the somatic mesoderm, yet its role in somatic myogenesis remained unknown. Here we show that it is one of the crucial genes regulating the development of the larval body wall muscles in Drosophila. It has two distinct functions expressed during different phases of myogenesis. The early function, partially redundant with the function of lethal of scute [l(1)sc], demarcates the ;Poxm competence domain', a domain of competence for ventral and lateral muscle development and for the determination of at least some adult muscle precursor cells. The late function is a muscle identity function, required for the specification of muscles DT1, VA1, VA2 and VA3. Our results led us to reinterpret the roles of l(1)sc and twist in myogenesis and to propose a solution of the 'l(1)sc conundrum'.

The MARVEL domain protein, Singles Bar, is required for progression past the pre-fusion complex stage of myoblast fusion

Multinucleated myotubes develop by the sequential fusion of individual myoblasts. Using a convergence of genomic and classical genetic approaches, we have discovered a novel gene, singles bar (sing), that is essential for myoblast fusion. sing encodes a small multipass transmembrane protein containing a MARVEL domain, which is found in vertebrate proteins involved in processes such as tight junction formation and vesicle trafficking where--as in myoblast fusion--membrane apposition occurs. sing is expressed in both founder cells and fusion competent myoblasts preceding and during myoblast fusion. Examination of embryos injected with double-stranded sing RNA or embryos homozygous for ethane methyl sulfonate-induced sing alleles revealed an identical phenotype: replacement of multinucleated myofibers by groups of single, myosin-expressing myoblasts at a stage when formation of the mature muscle pattern is complete in wild-type embryos. Unfused sing mutant myoblasts form clusters, suggesting that early recognition and adhesion of these cells are unimpaired. To further investigate this phenotype, we undertook electron microscopic ultrastructural studies of fusing myoblasts in both sing and wild-type embryos. These experiments revealed that more sing mutant myoblasts than wild-type contain pre-fusion complexes, which are characterized by electron-dense vesicles paired on either side of the fusing plasma membranes. In contrast, embryos mutant for another muscle fusion gene, blown fuse (blow), have a normal number of such complexes. Together, these results lead to the hypothesis that sing acts at a step distinct from that of blow, and that sing is required on both founder cell and fusion-competent myoblast membranes to allow progression past the pre-fusion complex stage of myoblast fusion, possibly by mediating fusion of the electron-dense vesicles to the plasma membrane.

Shaping leg muscles in Drosophila: role of ladybird, a conserved regulator of appendicular myogenesis

Legs are locomotor appendages used by a variety of evolutionarily distant vertebrates and invertebrates. The primary biological leg function, locomotion, requires the formation of a specialised appendicular musculature. Here we report evidence that ladybird, an orthologue of the Lbx1 gene recognised as a hallmark of appendicular myogenesis in vertebrates, is expressed in leg myoblasts, and regulates the shape, ultrastructure and functional properties of leg muscles in Drosophila. ladybird expression is progressively activated in myoblasts associated with the imaginal leg disc and precedes that of the founder cell marker dumbfounded. The RNAi-mediated attenuation of ladybird expression alters properties of developing myotubes, impairing their ability to grow and interact with the internal tendons and epithelial attachment sites. It also affects sarcomeric ultrastructure, resulting in reduced leg muscle performance and impaired mobility in surviving flies. The over-expression of ladybird also results in an abnormal pattern of dorsally located leg muscles, indicating different requirements for ladybird in dorsal versus ventral muscles. This differential effect is consistent with the higher level of Ladybird in ventrally located myoblasts and with positive ladybird regulation by extrinsic Wingless signalling from the ventral epithelium. In addition, ladybird expression correlates with that of FGF receptor Heartless and the read-out of FGF signalling downstream of FGF. FGF signals regulate the number of leg disc associated myoblasts and are able to accelerate myogenic differentiation by activating ladybird, leading to ectopic muscle fibre formation. A key role for ladybird in leg myogenesis is further supported by its capacity to repress vestigial and to down-regulate the vestigial-governed flight muscle developmental programme. Thus in Drosophila like in vertebrates, appendicular muscles develop from a specialised pool of myoblasts expressing ladybird/Lbx1. The ladybird/Lbx1 gene family appears as a part of an ancient genetic circuitry determining leg-specific properties of myoblasts and making an appendage adapted for locomotion.

The NHR1 domain of Neuralized binds Delta and mediates Delta trafficking and Notch signaling

Notch signaling, which is crucial to metazoan development, requires endocytosis of Notch ligands, such as Delta and Serrate. Neuralized is a plasma membrane-associated ubiquitin ligase that is required for neural development and Delta internalization. Neuralized is comprised of three domains that include a C-terminal RING domain and two neuralized homology repeat (NHR) domains. All three domains are conserved between organisms, suggesting that these regions of Neuralized are functionally important. Although the Neuralized RING domain has been shown to be required for Delta ubiquitination, the function of the NHR domains remains elusive. Here we show that neuralized, a well-characterized neurogenic allele, exhibits a mutation in a conserved residue of the NHR1 domain that results in mislocalization of Neuralized and defects in Delta binding and internalization. Furthermore, we describe a novel isoform of Neuralized and show that it is recruited to the plasma membrane by Delta and that this is mediated by the NHR1 domain. Finally, we show that the NHR1 domain of Neuralized is both necessary and sufficient to bind Delta. Altogether, our data demonstrate that NHR domains can function in facilitating protein-protein interactions and in the case of Neuralized, mediate binding to its ubiquitination target, Delta.

Evidence for the involvement of dominant-negative Notch molecules in the normal course of Drosophila development

Notch signaling is used to specify cell types during animal development. A high level specifies one cell type, whereas a low level specifies the alternate type. The effector of Notch signaling is the Notch intracellular domain. Upon its release from the plasma membrane in response to Delta binding the Notch extracellular domain, the Notch intracellular domain combines with the transcription factor Suppressor of Hairless and promotes the expression of target genes. Using a panel of antibodies made against different extracellular and intracellular regions of Notch, we show that cell types and tissues with low levels of Notch signaling are enriched for Notch molecules detected only by the extracellular domain antibodies. This enrichment often follows enrichment for Notch molecules detected only by antibodies made against the Suppressor of Hairless binding region. Notch molecules lacking most of the intracellular domain or containing only the Suppressor of Hairless binding region are produced during development. Such molecules are known to suppress Notch signaling, possibly by taking away Delta or Suppressor of Hairless from the full-length Notch. Thus, it is possible that dominant-negative Notch molecules are produced in the normal course of tissue differentiation in Drosophila as part of an auto-down-regulation mechanism.

An integrated strategy for analyzing the unique developmental programs of different myoblast subtypes

An important but largely unmet challenge in understanding the mechanisms that govern the formation of specific organs is to decipher the complex and dynamic genetic programs exhibited by the diversity of cell types within the tissue of interest. Here, we use an integrated genetic, genomic, and computational strategy to comprehensively determine the molecular identities of distinct myoblast subpopulations within the Drosophila embryonic mesoderm at the time that cell fates are initially specified. A compendium of gene expression profiles was generated for primary mesodermal cells purified by flow cytometry from appropriately staged wild-type embryos and from 12 genotypes in which myogenesis was selectively and predictably perturbed. A statistical meta-analysis of these pooled datasets--based on expected trends in gene expression and on the relative contribution of each genotype to the detection of known muscle genes--provisionally assigned hundreds of differentially expressed genes to particular myoblast subtypes. Whole embryo in situ hybridizations were then used to validate the majority of these predictions, thereby enabling true-positive detection rates to be estimated for the microarray data. This combined analysis reveals that myoblasts exhibit much greater gene expression heterogeneity and overall complexity than was previously appreciated. Moreover, it implicates the involvement of large numbers of uncharacterized, differentially expressed genes in myogenic specification and subsequent morphogenesis. These findings also underscore a requirement for considerable regulatory specificity for generating diverse myoblast identities. Finally, to illustrate how the developmental functions of newly identified myoblast genes can be efficiently surveyed, a rapid RNA interference assay that can be scored in living embryos was developed and applied to selected genes. This integrated strategy for examining embryonic gene expression and function provides a substantially expanded framework for further studies of this model developmental system.

MicroRNA1 influences cardiac differentiation in Drosophila and regulates Notch signaling

MicroRNAs (miRNAs) are genomically encoded small RNAs that hybridize with messenger RNAs, resulting in degradation or translational inhibition of targeted transcripts. The potential for miRNAs to regulate cell-lineage determination or differentiation from pluripotent progenitor or stem cells is unknown. Here, we show that microRNA1 (miR-1) is an ancient muscle-specific gene conserved in sequence and expression in Drosophila. Drosophila miR-1 (dmiR-1) is regulated through a serum response factor-like binding site in cardiac progenitor cells. Loss- and gain-of-function studies demonstrated a role for dmiR-1 in modulating cardiogenesis and in maintenance of muscle-gene expression. We provide in vivo evidence that dmiR-1 targets transcripts encoding the Notch ligand Delta, providing a potential mechanism for the expansion of cardiac and muscle progenitor cells and failure of progenitor cell differentiation in some dmiR-1 mutants. These findings demonstrate that dmiR-1 may "fine-tune" critical steps involved in differentiation of cardiac and somatic muscle progenitors and targets a pathway required for progenitor cell specification and asymmetric cell division.

Subdividing the embryo: a role for Notch signaling during germ layer patterning in Xenopus laevis

The development of all vertebrate embryos requires the establishment of a three-dimensional coordinate system in order to pattern embryonic structures and create the complex shape of the adult organism. During the process of gastrulation, the three primary germ layers are created under the guidance of numerous signaling pathways, allowing cells to communicate during development. Cell-cell communication, mediated by receptors of the Notch family, has been shown to be involved in mediating diverse cellular behaviors during development and has been implicated in the regulation of cell fate decisions in both vertebrate and invertebrate organisms. In order to investigate a role for Notch signaling during boundary formation between the mesoderm and endoderm during gastrulation, we manipulated Notch signaling in gastrula stage embryos and examined gene expression in resultant tissues and organs. Our findings demonstrate a much broader role for Notch signaling during germ layer determination than previously reported in a vertebrate organism. Activation of the Notch pathway, specifically in gastrula stage embryos, results in a dramatic decrease in the expression of genes necessary to create many different types of mesodermal tissues while causing a dramatic expansion of endodermal tissue markers. Conversely, temporally controlled suppression of this pathway results in a loss of endodermal cell types and an expansion of molecular markers of mesoderm. Thus, our data are consistent with and significantly extend the implications of prior observations suggesting roles for Notch signaling during germ layer formation and establish an evolutionarily conserved role for Notch signaling in mediating mesoderm-endoderm boundaries during early vertebrate development.

Embryonic even skipped-dependent muscle and heart cell fates are required for normal adult activity, heart function, and lifespan

The Drosophila pair-rule gene even skipped (eve) is required for embryonic segmentation and later in specific cell lineages in both the nervous system and the mesoderm. We previously generated eve mesoderm-specific mutants by combining an eve null mutant with a rescuing transgene that includes the entire locus, but with the mesodermal enhancer removed. This allowed us to analyze in detail the defects that result from a precisely targeted elimination of mesodermal eve expression in the context of an otherwise normal embryo. Absence of mesodermal eve causes a highly selective loss of the entire eve-expressing lineage in this germ layer, including those progeny that do not continue to express eve, suggesting that mesodermal eve precursor specification is not implemented. Despite the resulting absence of a subset of muscles and pericardial cells, mesoderm-specific eve mutants survive to fertile adulthood, providing an opportunity to examine the effects of these developmental abnormalities on adult fitness and heart function. We find that in these mutants, flying ability, myocardial performance under normal and stressed conditions, and lifespan are severely reduced. These data imply a nonautonomous role of the affected pericardial cells and body wall muscles in developing and/or maintaining cardiac performance and possibly other functions contributing to normal lifespan. Given the similarities of molecular-genetic control between Drosophila and vertebrates, these findings suggest that peri/epicardial influences may well be important for proper myocardial function.

Distinct roles for Mind bomb, Neuralized and Epsin in mediating DSL endocytosis and signaling in Drosophila

Ligands of the Delta/Serrate/Lag2 (DSL) family must normally be endocytosed in signal-sending cells to activate Notch in signal-receiving cells. DSL internalization and signaling are promoted in zebrafish and Drosophila, respectively, by the ubiquitin ligases Mind bomb (Mib) and Neuralized (Neur). DSL signaling activity also depends on Epsin, a conserved endocytic adaptor thought to target mono-ubiquitinated membrane proteins for internalization. Here, we present evidence that the Drosophila ortholog of Mib (Dmib) is required for ubiquitination and signaling activity of DSL ligands in cells that normally do not express Neur, and can be functionally replaced by ectopically expressed Neur. Furthermore, we show that both Dmib and Epsin are required in these cells for some of the endocytic events that internalize DSL ligands, and that the two Drosophila DSL ligands Delta and Serrate differ in their utilization of these Dmib- and Epsin-dependent pathways: most Serrate is endocytosed via the actions of Dmib and Epsin, whereas most Delta enters by other pathways. Nevertheless, only those Serrate and Delta proteins that are internalized via the action of Dmib and Epsin can signal. These results support and extend our previous proposal that mono-ubiquitination of DSL ligands allows them to gain access to a select, Epsin-dependent, endocytic pathway that they must normally enter to activate Notch.

Lateral inhibition in proneural clusters: cis-regulatory logic and default repression by Suppressor of Hairless

Lateral inhibition, wherein a single cell signals to its neighbors to prevent them from adopting its own fate, is the best-known setting for cell-cell communication via the Notch (N) pathway. During peripheral neurogenesis in Drosophila, sensory organ precursor (SOP) cells arise within proneural clusters (PNCs), small groups of cells endowed with SOP fate potential by their expression of proneural transcriptional activators. SOPs use N signaling to activate in neighboring PNC cells the expression of multiple genes that inhibit the SOP fate. These genes respond transcriptionally to direct regulation by both the proneural proteins and the N pathway transcription factor Suppressor of Hairless [Su(H)], and their activation is generally highly asymmetric; i.e. only in the inhibited(non-SOP) cells of the PNC, and not in SOPs. We show that the substantially higher proneural protein levels in the SOP put this cell at risk of inappropriately activating the SOP-inhibitory genes, even without input from N-activated Su(H). We demonstrate that this is prevented by direct `default'repression of these genes by Su(H), acting through the same binding sites it uses for activation in non-SOPs. We show that de-repression of even a single N pathway target gene in the SOP can extinguish the SOP cell fate. Finally, we define crucial roles for the adaptor protein Hairless and the co-repressors Groucho and CtBP in conferring repressive activity on Su(H) in the SOP. Our work elucidates the regulatory logic by which N signaling and the proneural proteins cooperate to create the neural precursor/epidermal cell fate distinction during lateral inhibition.

The ubiquitin ligase Drosophila Mind bomb promotes Notch signaling by regulating the localization and activity of Serrate and Delta

The receptor Notch and its ligands of the Delta/Serrate/LAG2 (DSL) family are the central components in the Notch pathway, a fundamental cell signaling system that regulates pattern formation during animal development. Delta is directly ubiquitinated by Drosophila and Xenopus Neuralized, and by zebrafish Mind bomb, two unrelated RING-type E3 ubiquitin ligases with common abilities to promote Delta endocytosis and signaling activity. Although orthologs of both Neuralized and Mind bomb are found in most metazoan organisms, their relative contributions to Notch signaling in any single organism have not yet been assessed. We show here that a Drosophila ortholog of Mind bomb (D-mib) is a positive component of Notch signaling that is required for multiple Neuralized-independent, Notch-dependent developmental processes. Furthermore, we show that D-mib associates physically and functionally with both Serrate and Delta. We find that D-mib uses its ubiquitin ligase activity to promote DSL ligand activity, an activity that is correlated with its ability to induce the endocytosis and degradation of both Delta and Serrate (see also Le Borgne et al., 2005). We further demonstrate that D-mib can functionally replace Neuralized in multiple cell fate decisions that absolutely require endogenous Neuralized, a testament to the highly similar activities of these two unrelated ubiquitin ligases in regulating Notch signaling. We conclude that ubiquitination of Delta and Serrate by Neuralized and D-mib is an obligate feature of DSL ligand activation throughout Drosophila development.

The quantitative genetic basis of male mating behavior in Drosophila melanogaster

Male mating behavior is an important component of fitness in Drosophila and displays segregating variation in natural populations. However, we know very little about the genes affecting naturally occurring variation in mating behavior, their effects, or their interactions. Here, we have mapped quantitative trait loci (QTL) affecting courtship occurrence, courtship latency, copulation occurrence, and copulation latency that segregate between a D. melanogaster strain selected for reduced male mating propensity (2b) and a standard wild-type strain (Oregon-R). Mating behavior was assessed in a population of 98 recombinant inbred lines derived from these two strains and QTL affecting mating behavior were mapped using composite interval mapping. We found four QTL affecting male mating behavior at cytological locations 1A;3E, 57C;57F, 72A;85F, and 96F;99A. We used deficiency complementation mapping to map the autosomal QTL with much higher resolution to five QTL at 56F5;56F8, 56F9;57A3, 70E1;71F4, 78C5;79A1, and 96F1;97B1. Quantitative complementation tests performed for 45 positional candidate genes within these intervals revealed 7 genes that failed to complement the QTL: eagle, 18 wheeler, Enhancer of split, Polycomb, spermatocyte arrest, l(2)05510, and l(2)k02206. None of these genes have been previously implicated in mating behavior, demonstrating that quantitative analysis of subtle variants can reveal novel pleiotropic effects of key developmental loci on behavior.

Epsin potentiates Notch pathway activity in Drosophila and C. elegans

Endocytosis and trafficking within the endocytosis pathway are known to modulate the activity of different signaling pathways. Epsins promote endocytosis and are postulated to target specific proteins for regulated endocytosis. Here, we present a functional link between the Notch pathway and epsins. We identify the Drosophila ortholog of epsin, liquid facets (lqf), as an inhibitor of cardioblast development in a genetic screen for mutants that affect heart development. We find that lqf inhibits cardioblast development and promotes the development of fusion-competent myoblasts, suggesting a model in which lqf acts on or in fusion-competent myoblasts to prevent their acquisition of the cardioblast fate. lqf and Notch exhibit essentially identical heart phenotypes, and lqf genetically interacts with the Notch pathway during multiple Notch-dependent events in Drosophila. We extended the link between the Notch pathway and epsin function to C. elegans, where the C. elegans lqf ortholog acts in the signaling cell to promote the glp-1/Notch pathway activity during germline development. Our results suggest that epsins play a specific, evolutionarily conserved role to promote Notch signaling during animal development and support the idea that they do so by targeting ligands of the Notch pathway for endocytosis.

Founder myoblasts and fibre number during adult myogenesis in Drosophila

We have examined the mechanisms underlying the setting of myotubes and choice of myotube number in adult Drosophila. We find that the pattern of adult myotubes is prefigured by a pattern of duf-lacZ-expressing myoblasts at appropriate locations. Selective expression of duf-lacZ in single myoblasts emerges from generalized, low-level expression in all adult myoblasts during the third larval instar. The number of founders, thus chosen, corresponds to the number of fibres in a muscle. In contrast to the embryo, the selection of individual adult founder cells during myogenesis does not depend on Notch-mediated lateral inhibition. Our results suggest a general mechanism by which multi-fibre muscles can be patterned.

Notch signaling patterns Drosophila mesodermal segments by regulating the bHLH transcription factor twist

One of the first steps in embryonic mesodermal differentiation is allocation of cells to particular tissue fates. In Drosophila, this process of mesodermal subdivision requires regulation of the bHLH transcription factor Twist. During subdivision, Twist expression is modulated into stripes of low and high levels within each mesodermal segment. High Twist levels direct cells to the body wall muscle fate, whereas low levels are permissive for gut muscle and fat body fate. We show that Su(H)-mediated Notch signaling represses Twist expression during subdivision and thus plays a critical role in patterning mesodermal segments. Our work demonstrates that Notch acts as a transcriptional switch on mesodermal target genes, and it suggests that Notch/Su(H) directly regulates twist, as well as indirectly regulating twist by activating proteins that repress Twist. We propose that Notch signaling targets two distinct 'Repressors of twist' - the proteins encoded by the Enhancer of split complex [E(spl)C] and the HLH gene extra machrochaetae (emc). Hence, the patterning of Drosophila mesodermal segments relies on Notch signaling changing the activities of a network of bHLH transcriptional regulators, which, in turn, control mesodermal cell fate. Since this same cassette of Notch, Su(H) and bHLH regulators is active during vertebrate mesodermal segmentation and/or subdivision, our work suggests a conserved mechanism for Notch in early mesodermal patterning.

Functional Notch signaling is required for BMP4-induced inhibition of myogenic differentiation

The bone morphogenetic protein (BMP) and Notch signaling pathways are crucial for cellular differentiation. In many cases, the two pathways act similarly; for example, to inhibit myogenic differentiation. It is not known whether this inhibition is caused by distinct mechanisms or by an interplay between Notch and BMP signaling. Here we demonstrate that functional Notch signaling is required for BMP4-mediated block of differentiation of muscle stem cells, i.e. satellite cells and the myogenic cell line C2C12. Addition of BMP4 during induction of differentiation dramatically reduced the number of differentiated satellite and C2C12 cells. Differentiation was substantially restored in BMP4-treated cultures by blocking Notch signaling using either the gamma-secretase inhibitor L-685,458 or by introduction of a dominant-negative version of the Notch signal mediator CSL. BMP4 addition to C2C12 cells increased transcription of two immediate Notch responsive genes, Hes1 and Hey1, an effect that was abrogated by L-685,458. A 3 kb Hey1-promoter reporter construct was synergistically activated by the Notch 1 intracellular domain (Notch 1 ICD) and BMP4. The BMP4 mediator SMAD1 mimicked BMP activation of the Hey1 promoter. A synthetic Notch-responsive promoter containing no SMAD1 binding sites responded to SMAD1, indicating that DNA-binding activity of SMAD1 is not required for activation. Accordingly, Notch 1 ICD and SMAD1 interacted in binding experiments in vitro. Thus, the data presented here provide evidence for a direct interaction between the Notch and BMP signaling pathways, and indicate that Notch has a crucial role in the execution of certain aspects of BMP-mediated differentiation control.

Notch and Ras signaling pathway effector genes expressed in fusion competent and founder cells during Drosophila myogenesis

Drosophila muscles originate from the fusion of two types of myoblasts, founder cells (FCs) and fusion-competent myoblasts (FCMs). To better understand muscle diversity and morphogenesis, we performed a large-scale gene expression analysis to identify genes differentially expressed in FCs and FCMs. We employed embryos derived from Toll10b mutants to obtain primarily muscle-forming mesoderm, and expressed activated forms of Ras or Notch to induce FC or FCM fate, respectively. The transcripts present in embryos of each genotype were compared by hybridization to cDNA microarrays. Among the 83 genes differentially expressed, we found genes known to be enriched in FCs or FCMs, such as heartless or hibris, previously characterized genes with unknown roles in muscle development, and predicted genes of unknown function. Our studies of newly identified genes revealed new patterns of gene expression restricted to one of the two types of myoblasts, and also striking muscle phenotypes. Whereas genes such as phyllopod play a crucial role during specification of particular muscles, others such as tartan are necessary for normal muscle morphogenesis.

Modifiers of muscle and heart cell fate specification identified by gain-of-function screen in Drosophila

The homeobox genes ladybird in Drosophila and their vertebrate counterparts Lbx1 genes display restricted expression patterns in a subset of muscle precursors and are both implicated in diversification of muscle cell fates. In order to gain new insights into mechanisms controlling conserved aspects of cell fate specification, we have performed a gain-of-function (GOF) screen for modifiers of the mesodermal expression of ladybird genes using a collection of EP element carrying Drosophila lines. Amongst the identified genes, several have been previously implicated in cell fate specification processes, thus validating the strategy of our screen. Observed GOF phenotypes have led us to identification of an important number of candidate genes, whose myogenic and/or cardiogenic functions remain to be investigated. Amongst them, the EP insertions close to rhomboid, yan and rac2 suggest new roles for these genes in diversification of muscle and/or heart cell lineages. The analysis of loss and GOF of rhomboid and yan reveals their new roles in specification of ladybird-expressing precursors of adult muscles (LaPs) and ladybird/tinman-positive pericardial cells. Observed phenotypes strongly suggest that rhomboid and yan act at the level of progenitor and founder cells and contribute to the diversification of mesodermal fates. Our analysis of rac2 phenotypes clearly demonstrates that the altered mesodermal level of Rho-GTPase Rac2 can influence specification of a number of cardiac and muscular cell types including those expressing ladybird. Finding that in rac2 mutants ladybird and even skipped-positive muscle founders are overproduced, indicate a new early function for this gene during segregation of muscle progenitors and/or specification of founder cells. Intriguingly, rhomboid, yan and rac2 act as conserved components of Receptor Tyrosine Kinases (RTKs) signalling pathways, suggesting that RTK signalling constitutes a part of a conserved regulatory network governing diversification of muscle and heart cell types.

Control of Myoblast Fusion by a Guanine Nucleotide Exchange Factor, Loner, and Its Effector ARF6

Myoblast fusion is essential for the formation and regeneration of skeletal muscle. In a genetic screen for regulators of muscle development in Drosophila, we discovered a gene encoding a guanine nucleotide exchange factor, called loner, which is required for myoblast fusion. Loner localizes to subcellular sites of fusion and acts downstream of cell surface fusion receptors by recruiting the small GTPase ARF6 and stimulating guanine nucleotide exchange. Accordingly, a dominant-negative ARF6 disrupts myoblast fusion in Drosophila embryos and in mammalian myoblasts in culture, mimicking the fusion defects caused by loss of Loner. Loner and ARF6, which also control the proper membrane localization of another small GTPase, Rac, are key components of a cellular apparatus required for myoblast fusion and muscle development. In muscle cells, this fusigenic mechanism is coupled to fusion receptors; in other fusion-competent cell types it may be triggered by different upstream signals.

General outlines of the molecular genetics of the Notch signalling pathway in Drosophila melanogaster: a review

The Notch signalling pathway appears to be ubiquitous in virtually all cell-cell contacts in all metazoan animals, and is best known and most throughout studied in Drosophila melanogaster. In this species the Notch signalling pathway regulates, with both positive and negative signals, the differentiation of at least central and peripheral nervous system and eye, wing disc, oogenesis, segmental appendages such as antennae and legs, and muscles, through lateral inhibition or induction. In general, the pathway works as follows: Notch is most likely a dimeric transmembrane receptor at the cell surface, where it is activated by its ligands Serrate and or Delta from the neighbouring cell Fringe, discriminating between the two ligands. Then, the receptor is cleaved by a proteolytic mechanism in which Presenilin plays an important role, and the intracellular domain is transferred to the nucleus, where it, together with the Suppressor of Hairless protein, constitutes a transcription factor which activates the Notch target genes, mainly located in the Enhancer of split complex. These target genes then encode repressor proteins.

The regulation of Notch signaling controls satellite cell activation and cell fate determination in postnatal myogenesis

We have studied the role of Notch-1 and its antagonist Numb in the activation of satellite cells during postnatal myogenesis. Activation of Notch-1 promoted the proliferation of myogenic precursor cells expressing the premyoblast marker Pax3. Attenuation of Notch signaling by increases in Numb expression led to the commitment of progenitor cells to the myoblast cell fate and the expression of myogenic regulatory factors, desmin, and Pax7. In many intermediate progenitor cells, Numb was localized asymmetrically in actively dividing cells, suggesting an asymmetric cell division and divergent cell fates of daughter cells. The results indicate that satellite cell activation results in a heterogeneous population of precursor cells with respect to Notch-1 activity and that the balance between Notch-1 and Numb controls cellular homeostasis and cell fate determination.

Hes6 regulates myogenic differentiation

Hes6 is a basic helix-loop-helix transcription factor homologous to Drosophila Enhancer of Split (EoS) proteins. It is known to promote neural differentiation and to bind to Hes1, a related protein that is part of the Notch signalling pathway, affecting Hes1-regulated transcription. We show that Hes6 is expressed in the murine embryonic myotome and is induced on C2C12 myoblast differentiation in vitro. Hes6 binds DNA containing the Enhancer of Split E box (ESE) motif, the preferred binding site of Drosophila EoS proteins, and represses transcription of an ESE box reporter. When overexpressed in C2C12 cells, Hes6 impairs normal differentiation, causing a decrease in the induction of the cyclin-dependent kinase inhibitor, p21Cip1, and an increase in the number of cells that can be recruited back into the cell cycle after differentiation in culture. In Xenopus embryos, Hes6 is co-expressed with MyoD in early myogenic development. Microinjection of Hes6 RNA in vivo in Xenopus embryos results in an expansion of the myotome, but suppression of terminal muscle differentiation and disruption of somite formation at the tailbud stage. Analysis of Hes6 mutants indicates that the DNA-binding activity of Hes6 is not essential for its myogenic phenotype, but that protein-protein interactions are. Thus, we demonstrate a novel role for Hes6 in multiple stages of muscle formation.

Reciprocal regulatory interactions between the Notch and Ras signaling pathways in the Drosophila embryonic mesoderm

Convergent intercellular signals must be precisely integrated in order to elicit specific biological responses. During specification of muscle and cardiac progenitors from clusters of equivalent cells in the Drosophila embryonic mesoderm, the Ras/MAPK pathway--activated by both epidermal and fibroblast growth factor receptors--functions as an inductive cellular determination signal, while lateral inhibition mediated by Notch antagonizes this activity. A critical balance between these signals must be achieved to enable one cell of an equivalence group to segregate as a progenitor while its neighbors assume a nonprogenitor identity. We have investigated whether these opposing signals directly interact with each other, and we have examined how they are integrated by the responding cells to specify their unique fates. Our findings reveal that Ras and Notch do not function independently; rather, we have uncovered several modes of cross-talk between these pathways. Ras induces Notch, its ligand Delta, and the epidermal growth factor receptor antagonist, Argos. We show that Delta and Argos then synergize to nonautonomously block a positive autoregulatory feedback loop that amplifies a fate-inducing Ras signal. This feedback loop is characterized by Ras-mediated upregulation of proximal components of both the epidermal and fibroblast growth factor receptor pathways. In turn, Notch activation in nonprogenitors induces its own expression and simultaneously suppresses both Delta and Argos levels, thereby reinforcing a unidirectional inhibitory response. These reciprocal interactions combine to generate the signal thresholds that are essential for proper specification of progenitors and nonprogenitors from groups of initially equivalent cells.

Cross-repressive interactions of identity genes are essential for proper specification of cardiac and muscular fates in Drosophila

In Drosophila embryos, founder cells that give rise to cardiac precursors and dorsal somatic muscles derive from dorsally located progenitors. Individual fates of founder cells are thought to be specified by combinatorial code of transcription factors encoded by identity genes. To date, a large number of identity genes have been identified; however, the mechanisms by which these genes contribute to cell fate specification remain largely unknown. We have analysed regulatory interactions of ladybird (lb), msh and even skipped (eve), the three identity genes specifying a subset of heart and/or dorsal muscle precursors. We show that deregulation of each of them alters the number of cells that express two other genes, thus changing the ratio between cardiac and muscular cells, and the ratio between different cell subsets within the heart and within the dorsal muscles. Specifically, we demonstrate that mutation of the muscle identity gene msh and misexpression of the heart identity gene lb lead to heart hyperplasia with similar cell fate modifications. In msh mutant embryos, the presumptive msh-muscle cells switch on lb or eve expression and are recruited to form supernumerary heart or dorsal muscle cells, thus indicating that msh functions as a repressor of lb and eve. Similarly, overexpression of lb represses endogenous msh and eve activity, hence leading to the respecification of msh and eve positive progenitors, resulting in the overproduction of a subset of heart cells. As deduced from heart and muscle phenotypes of numb mutant embryos, the cell fate modifications induced by gain-of-function of identity genes are not lineage restricted. Consistent with all these observations, we propose that the major role of identity genes is to maintain their restricted expression by repressing other identity genes competent to respond positively to extrinsic signals. The cross-repressive interactions of identity genes are likely to ensure their localised expression over time, thus providing an essential element in establishing cell identity.

myoblasts incompetent encodes a zinc finger transcription factor required to specify fusion-competent myoblasts in Drosophila

We report a new gene, myoblasts incompetent, essential for normal myogenesis and myoblast fusion in Drosophila. myoblasts incompetent encodes a putative zinc finger transcription factor related to vertebrate Gli proteins and to Drosophila Cubitus interruptus. myoblasts incompetent is expressed in immature somatic and visceral myoblasts. Expression is predominantly in fusion-competent myoblasts and a loss-of-function mutation in myoblasts incompetent leads to a failure in the normal differentiation of these cells and a complete lack of myoblast fusion. In the mutant embryos, founder myoblasts differentiate normally and form mononucleate muscles, but genes that are specifically expressed in fusion-competent cells are not activated and the normal downregulation of twist expression in these cells fails to occur. In addition, fusion-competent myoblasts fail to express proteins characteristic of the general pathway of myogenesis such as myosin and Dmef2. Thus myoblasts incompetent appears to function specifically in the general pathway of myogenesis to control the differentiation of fusion-competent myoblasts.

Drosophila neuralized is a ubiquitin ligase that promotes the internalization and degradation of delta

The Drosophila gene neuralized (neur) has long been recognized to be essential for the proper execution of a wide variety of processes mediated by the Notch (N) pathway, but its role in the pathway has been elusive. In this report, we present genetic and biochemical evidence that Neur is a RING-type, E3 ubiquitin ligase. Next, we show that neur is required for proper internalization of Dl in the developing eye. Finally, we demonstrate that ectopic Neur targets Dl for internalization and degradation in a RING finger-dependent manner, and that the two exist in a physical complex. Collectively, our data indicate that Neur is a ubiquitin ligase that positively regulates the N pathway by promoting the endocytosis and degradation of Dl.

Drosophila Lame duck, a novel member of the Gli superfamily, acts as a key regulator of myogenesis by controlling fusion-competent myoblast development

A hallmark of mature skeletal muscles is the presence of multinucleate muscle fibers. In Drosophila, the formation of muscle syncytia requires the cooperative participation of two types of myoblasts, founder cells and fusion-competent myoblasts. We show that a newly identified gene, lame duck (lmd), has an essential regulatory role in the specification and function of fusion-competent myoblasts. Embryos that lack lmd function show a loss of expression of two key differentiation and fusion genes, Mef2 and sticks-and-stones, in fusion-competent myoblasts and are completely devoid of multinucleate muscle fibers. By contrast, founder cells are specified and retain their capability to differentiate into mononucleate muscle cells. lmd encodes a novel member of the Gli superfamily of transcription factors and is expressed in fusion-competent myoblasts and their precursors in a Wingless- and Notch-dependent manner. The activity of the Lmd protein appears to be additionally controlled by its differential cytoplasmic versus nuclear localization. Results from an independent molecular screen for binding factors to a myoblast-specific Mef2 enhancer further demonstrate that Lmd is a direct transcriptional regulator of Mef2 in fusion-competent myoblasts.

Isolation of a murine homologue of the Drosophila neuralized gene, a gene required for axonemal integrity in spermatozoa and terminal maturation of the mammary gland

The Drosophila neuralized gene shows genetic interactions with Notch, Enhancer of split, and other neurogenic genes and is thought to be involved in cell fate specification in the central nervous system and the mesoderm. In addition, a human homologue of the Drosophila neuralized gene has been described as a potential tumor suppressor gene in malignant astrocytomas. We have isolated a murine homologue of the Drosophila and human Neuralized genes and, in an effort to understand its physiological function, derived mice with a targeted deletion of this gene. Surprisingly, mice homozygous for the introduced mutation do not show aberrant cell fate specifications in the central nervous system or in the developing mesoderm. This is in contrast to mice with targeted deletions in other vertebrate homologues of neurogenic genes such as Notch, Delta, and Cbf-1. Male Neuralized null mice, however, are sterile due to a defect in axoneme organization in the spermatozoa that leads to highly compromised tail movement and sperm immotility. In addition, female Neuralized null animals are defective in the final stages of mammary gland maturation during pregnancy.

Drosophila rolling pebbles: a multidomain protein required for myoblast fusion that recruits D-Titin in response to the myoblast attractant Dumbfounded

The fusion of myoblasts leading to the formation of myotubes is an integral part of skeletal myogenesis in many organisms. In Drosophila, specialized founder myoblasts initiate fusion through expression of the receptor-like attractant Dumbfounded (Duf), which brings them into close contact with other myoblasts. Here, we identify Rols7, a gene expressed in founders, as an essential component for fusion during myotube formation. During fusion, Rols7 localizes in a Duf-dependent manner at membrane sites that contact other myoblasts. These sites are also enriched with D-Titin, which functions to maintain myotube structure and morphology. When Rols7 is absent or its localization is perturbed, the enrichment of D-Titin fails to occur. Rols7 integrates the initial event of myoblast attraction with the downstream event of myotube structural organization by linking Duf to D-Titin.