Specific muscle identities are regulated by Kruppel during Drosophila embryogenesis

KLF10: a point of convergence in cancer cachexia

PURPOSE OF THE REVIEW

Cancer-associated cachexia is a wasting syndrome entailing loss in body mass and a shortened life expectancy. There is currently no effective treatment to abrogate this syndrome, which leads to 20-30% of deaths in patients with cancer. While there have been advancements in defining signaling factors/pathways in cancer-induced muscle wasting, targeting the same in the clinic has not been as successful. Krüppel-like factor 10 (KLF10), a transcription factor implicated in muscle regulation, is regulated by the transforming growth factor-beta signaling pathway. This review proposes KLF10 as a potential convergence point of diverse signaling pathways involved in muscle wasting.

RECENT FINDINGS

KLF10 was discovered as a target of transforming growth factor-beta decades ago but more recently it has been shown that deletion of KLF10 rescues cancer-induced muscle wasting. Moreover, KLF10 has also been shown to bind key atrophy genes associated with muscle atrophy in vitro .

SUMMARY

There is an elevated need to explore targets in cachexia, which will successfully translate into the clinic. Investigating a convergence point downstream of multiple signaling pathways might hold promise in developing effective therapies for cachexia.

Drosophila Myoblast Fusion: Invasion and Resistance for the Ultimate Union

Cell-cell fusion is indispensable for creating life and building syncytial tissues and organs. Ever since the discovery of cell-cell fusion, how cells join together to form zygotes and multinucleated syncytia has remained a fundamental question in cell and developmental biology. In the past two decades, Drosophila myoblast fusion has been used as a powerful genetic model to unravel mechanisms underlying cell-cell fusion in vivo. Many evolutionarily conserved fusion-promoting factors have been identified and so has a surprising and conserved cellular mechanism. In this review, we revisit key findings in Drosophila myoblast fusion and highlight the critical roles of cellular invasion and resistance in driving cell membrane fusion.

Integrin signaling downregulates filopodia during muscle-tendon attachment

Cells need to sense their environment to ensure accurate targeting to specific destinations. This occurs in developing muscles, which need to attach to tendon cells before muscle contractions can begin. Elongating myotube tips form filopodia, which are presumed to have sensory roles, and are later suppressed upon building the attachment site. Here, we use live imaging and quantitative image analysis of lateral transverse (LT) myotubes in Drosophila to show that filopodia suppression occurs as a result of integrin signaling. Loss of the integrin subunits αPS2 and βPS (also known as If and Mys, respectively, in flies) increased filopodia number and length at stages when they are normally suppressed. Conversely, inducing integrin signaling, achieved by the expression of constitutively dimerised βPS cytoplasmic domain (diβ), prematurely suppressed filopodia. We discovered that the integrin signal is transmitted through the protein G protein-coupled receptor kinase interacting ArfGAP (Git) and its downstream kinase p21-activated kinase (Pak). Absence of these proteins causes profuse filopodia and prevents the filopodial inhibition mediated by diβ. Thus, integrin signaling terminates the exploratory behavior of myotubes seeking tendons, enabling the actin machinery to focus on forming a strong attachment and assembling the contractile apparatus.

Summary: Integrins signal through Git and Pak to downregulate filopodia when muscles reach their target attachment site in Drosophila.

Diverse integrin adhesion stoichiometries caused by varied actomyosin activity

Cells in an organism are subjected to numerous sources of external and internal forces, and are able to sense and respond to these forces. Integrin-mediated adhesion links the extracellular matrix outside cells to the cytoskeleton inside, and participates in sensing, transmitting and responding to forces. While integrin adhesion rapidly adapts to changes in forces in isolated migrating cells, it is not known whether similar or more complex responses occur within intact, developing tissues. Here, we studied changes in integrin adhesion composition upon different contractility conditions in Drosophila embryonic muscles. We discovered that all integrin adhesion components tested were still present at muscle attachment sites (MASs) when either cytoplasmic or muscle myosin II was genetically removed, suggesting a primary role of a developmental programme in the initial assembly of integrin adhesions. Contractility does, however, increase the levels of integrin adhesion components, suggesting a mechanism to balance the strength of muscle attachment to the force of muscle contraction. Perturbing contractility in distinct ways, by genetic removal of either cytoplasmic or muscle myosin II or eliminating muscle innervation, each caused unique alterations to the stoichiometry at MASs. This suggests that different integrin-associated proteins are added to counteract different kinds of force increase.

APC/CFzr regulates cardiac and myoblast cell numbers and plays a crucial role during myoblast fusion

Somatic muscles are formed by the iterative fusion of myoblasts into muscle fibres. This process is driven by the recurrent recruitment of proteins to the cell membrane to induce F-actin nucleation at the fusion site. Although various proteins involved in myoblast fusion have been identified, knowledge about their sub-cellular regulation is rather elusive. We identified the anaphase-promoting complex (APC/C) adaptor Fizzy related (Fzr) as an essential regulator of heart and muscle development. We show that APC/CFzr regulates the fusion of myoblasts as well as mitotic exit of pericardial cells, cardioblasts and myoblasts. Surprisingly, over-proliferation is not causative for the observed fusion defects. Instead, fzr mutants exhibit smaller F-actin foci at the fusion site, and display reduced membrane breakdown between adjacent myoblasts. We show that lack of APC/CFzr causes the accumulation and mislocalisation of Rols and Duf, two proteins involved in the fusion process. Duf seems to serve as direct substrate of the APC/CFzr, and its destruction depends on the presence of distinct degron sequences. These novel findings indicate that protein destruction and turnover constitute major events during myoblast fusion.

Dynamics of transcriptional (re)-programming of syncytial nuclei in developing muscles

Background

A stereotyped array of body wall muscles enables precision and stereotypy of animal movements. In Drosophila, each syncytial muscle forms via fusion of one founder cell (FC) with multiple fusion competent myoblasts (FCMs). The specific morphology of each muscle, i.e. distinctive shape, orientation, size and skeletal attachment sites, reflects the specific combination of identity transcription factors (iTFs) expressed by its FC. Here, we addressed three questions: Are FCM nuclei naive? What is the selectivity and temporal sequence of transcriptional reprogramming of FCMs recruited into growing syncytium? Is transcription of generic myogenic and identity realisation genes coordinated during muscle differentiation?

Results

The tracking of nuclei in developing muscles shows that FCM nuclei are competent to be transcriptionally reprogrammed to a given muscle identity, post fusion. In situ hybridisation to nascent transcripts for FCM, FC-generic and iTF genes shows that this reprogramming is progressive, beginning by repression of FCM-specific genes in fused nuclei, with some evidence that FC nuclei retain specific characteristics. Transcription of identity realisation genes is linked to iTF activation and regulated at levels of both transcription initiation rate and period of transcription. The generic muscle differentiation programme is activated independently.

Conclusions

Transcription reprogramming of fused myoblast nuclei is progressive, such that nuclei within a syncytial fibre at a given time point during muscle development are heterogeneous with regards to specific gene transcription. This comprehensive view of the dynamics of transcriptional (re)programming of post-mitotic nuclei within syncytial cells provides a new framework for understanding the transcriptional control of the lineage diversity of multinucleated cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0386-2) contains supplementary material, which is available to authorized users.

The Hunchback temporal transcription factor establishes, but is not required to maintain, early-born neuronal identity

Background

Drosophila and mammalian neural progenitors typically generate a diverse family of neurons in a stereotyped order. Neuronal diversity can be generated by the sequential expression of temporal transcription factors. In Drosophila, neural progenitors (neuroblasts) sequentially express the temporal transcription factors Hunchback (Hb), Kruppel, Pdm, and Castor. Hb is necessary and sufficient to specify early-born neuronal identity in multiple lineages, and is maintained in the post-mitotic neurons produced during each neuroblast expression window. Surprisingly, nothing is currently known about whether Hb acts in neuroblasts or post-mitotic neurons (or both) to specify first-born neuronal identity.

Methods

Here we selectively remove Hb from post-mitotic neurons, and assay the well-characterized NB7-1 and NB1-1 lineages for defects in neuronal identity and function.

Results

We find that loss of Hb from embryonic and larval post-mitotic neurons does not affect neuronal identity. Furthermore, removing Hb from post-mitotic neurons throughout the entire CNS has no effect on larval locomotor velocity, a sensitive assay for motor neuron and pre-motor neuron function.

Conclusions

We conclude that Hb functions in progenitors (neuroblasts/GMCs) to establish heritable neuronal identity that is maintained by a Hb-independent mechanism.

We suggest that Hb acts in neuroblasts to establish an epigenetic state that is permanently maintained in early-born neurons.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0078-1) contains supplementary material, which is available to authorized users.

Genetic dissection of the Transcription Factor code controlling serial specification of muscle identities in Drosophila

Each Drosophila muscle is seeded by one Founder Cell issued from terminal division of a Progenitor Cell (PC). Muscle identity reflects the expression by each PC of a specific combination of identity Transcription Factors (iTFs). Sequential emergence of several PCs at the same position raised the question of how developmental time controlled muscle identity. Here, we identified roles of Anterior Open and ETS domain lacking in controlling PC birth time and Eyes absent, No Ocelli, and Sine oculis in specifying PC identity. The windows of transcription of these and other TFs in wild type and mutant embryos, revealed a cascade of regulation integrating time and space, feed-forward loops and use of alternative transcription start sites. These data provide a dynamic view of the transcriptional control of muscle identity in Drosophila and an extended framework for studying interactions between general myogenic factors and iTFs in evolutionary diversification of muscle shapes.

DOI:http://dx.doi.org/10.7554/eLife.14979.001

Animals have many different muscles of various shapes and sizes that are suited to specific tasks and behaviors. The fruit fly known as Drosophila has a fairly simple musculature, which makes it an ideal model animal to investigate how different muscles form.

In fruit fly embryos, cells called progenitor cells divide to produce the cells that will go on to form the different muscles. Proteins called identity Transcription Factors are present in progenitor cells. Different combinations of identity Transcription Factors can switch certain genes on or off to control the muscle shapes in specific areas of an embryo. However, progenitor cells born in the same area but at different times display different patterns of identity Transcription Factors; this suggests that timing also influences the orientation, shape and size of a developing muscle, also known as muscle identity.

Dubois et al. used a genetic screen to look for identity Transcription Factors and the roles these proteins play in muscle formation in fruit flies. Tracking the activity of these proteins revealed a precise timeline for specifying muscle identity. This timeline involves cascades of different identity Transcription Factors accumulating in the cells, which act to make sure that distinct muscle shapes are made. In flies with specific mutations, the timing of these events is disrupted, which results in muscles forming with different shapes to those seen in normal flies. The findings of Dubois et al. suggest that the timing of when particular progenitor cells form, as well as their location in the embryo, contribute to determine the shapes of muscles.

The next step following on from this work is to use video-microscopy to track identity Transcription Factors when the final muscle shapes emerge. Further experiments will investigate how identity Transcription Factors work together with proteins that are directly involved in muscle development.

DOI:http://dx.doi.org/10.7554/eLife.14979.002

KLF7 Regulates Satellite Cell Quiescence in Response to Extracellular Signaling

Retaining muscle stem satellite cell (SC) quiescence is important for the maintenance of stem cell population and tissue regeneration. Accumulating evidence supports the model where key extracellular signals play crucial roles in maintaining SC quiescence or activation, however, the intracellular mechanisms that mediate niche signals to control SC behavior are not fully understood. Here, we reported that KLF7 functioned as a key mediator involved in low-level TGF-β signaling and canonical Notch signaling-induced SC quiescence and myoblast arrest. The data obtained showed that KLF7 was upregulated in quiescent SCs and nonproliferating myoblasts. Silence of KLF7 promoted SCs activation and myoblasts proliferation, but overexpression of KLF7 induced myogenic cell arrest. Notably, the expression of KLF7 was regulated by TGF-β and Notch3 signaling. Knockdown of KLF7 diminished low-level TGF-β and canonical Notch signaling-induced SC quiescence. Investigation into the mechanism revealed that KLF7 regulation of SC function was dependent on p21 and acetylation of Lys227 and/or 231 in the DNA binding domain of KLF7. Our study provides new insights into the regulatory network of muscle stem cell quiescence. Stem Cells 2016;34:1310-1320.

Fear-of-intimacy mediated zinc transport controls the function of Zn-finger transcription factors involved in myogenesis

Zinc is a component of one tenth of all human proteins. Its cellular concentration is tightly regulated because its dyshomeostasis has catastrophic health consequences. Two families of zinc transporters control zinc homeostasis in organisms, but there is little information about their specific developmental roles. We show that the ZIP transporter fear-of-intimacy (foi) is necessary for the formation of Drosophila muscles. In foi mutants, myoblasts segregate normally, but their specification is affected, leading to the formation of a misshapen muscle pattern and distorted midgut. The observed phenotypes could be ascribed to the inactivation of specific zing-finger transcription factors (ZFTFs), supporting the hypothesis that they a consequence of a zinc intracellular depletion. Accordingly, foi phenotypes can be rescued by mesodermal expression of other ZIP members with similar subcellular localization. We propose that Foi acts mostly as a transporter to regulate zinc intracellular homeostasis, thereby impacting on the activity of ZFTFs that control specific developmental processes. Our results additionally suggest a possible explanation for the presence of large numbers of zinc transporters in organisms based on differences in ion transport specificity and/or degrees of activity among transporters.

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.

Muscle cell fate choice requires the T-box transcription factor midline in Drosophila

Drosophila Midline (Mid) is an ortholog of vertebrate Tbx20, which plays roles in the developing heart, migrating cranial motor neurons, and endothelial cells. Mid functions in cell-fate specification and differentiation of tissues that include the ectoderm, cardioblasts, neuroblasts, and egg chambers; however, a role in the somatic musculature has not been described. We identified mid in genetic and molecular screens for factors contributing to somatic muscle morphogenesis. Mid is expressed in founder cells (FCs) for several muscle fibers, and functions cooperatively with the T-box protein H15 in lateral oblique muscle 1 and the segment border muscle. Mid is particularly important for the specification and development of the lateral transverse (LT) muscles LT3 and LT4, which arise by asymmetric division of a single muscle progenitor. Mid is expressed in this progenitor and its two sibling FCs, but is maintained only in the LT4 FC. Both muscles were frequently missing in mid mutant embryos, and LT4-associated expression of the transcription factor Krüppel (Kr) was lost. When present, LT4 adopted an LT3-like morphology. Coordinately, mid misexpression caused LT3 to adopt an LT4-like morphology and was associated with ectopic Kr expression. From these data, we concluded that mid functions first in the progenitor to direct development of LT3 and LT4, and later in the FCs to influence whichever of these differentiation profiles is selected. Mid is the first T-box factor shown to influence LT3 and LT4 muscle identity and, along with the T-box protein Optomotor-blind-related-gene 1 (Org-1), is representative of a new class of transcription factors in muscle specification.

Kruppel-like factors in muscle health and disease

Kruppel-like factors (KLF) are zinc-finger DNA-binding transcription factors that are critical regulators of tissue homeostasis. Emerging evidence suggests that KLFs are critical regulators of muscle biology in the context of cardiovascular health and disease. The focus of this review is to provide an overview of the current state of knowledge regarding the physiologic and pathologic roles of KLFs in the three lineages of muscle: cardiac, smooth, and skeletal.

Whole-genome analysis of muscle founder cells implicates the chromatin regulator Sin3A in muscle identity

Skeletal muscles are formed in numerous shapes and sizes, and this diversity impacts function and disease susceptibility. To understand how muscle diversity is generated, we performed gene expression profiling of two muscle subsets from Drosophila embryos. By comparing the transcriptional profiles of these subsets, we identified a core group of founder cell-enriched genes. We screened mutants for muscle defects and identified functions for Sin3A and 10 other transcription and chromatin regulators in the Drosophila embryonic somatic musculature. Sin3A is required for the morphogenesis of a muscle subset, and Sin3A mutants display muscle loss and misattachment. Additionally, misexpression of identity gene transcription factors in Sin3A heterozygous embryos leads to direct transformations of one muscle into another, whereas overexpression of Sin3A results in the reverse transformation. Our data implicate Sin3A as a key buffer controlling muscle responsiveness to transcription factors in the formation of muscle identity, thereby generating tissue diversity.

A guide to study Drosophila muscle biology

The development and molecular composition of muscle tissue is evolutionarily conserved. Drosophila is a powerful in vivo model system to investigate muscle morphogenesis and function. Here, we provide a short and comprehensive overview of the important developmental steps to build Drosophila body muscle in embryos, larvae and pupae. We describe key methods, including muscle histology, live imaging and genetics, to study these steps at various developmental stages and include simple behavioural assays to assess muscle function in larvae and adults. We list valuable antibodies and fly strains that can be used for these different methods. This overview should guide the reader to choose the best marker or the appropriate method to obtain high quality muscle morphogenesis data in Drosophila.

Tup/Islet1 integrates time and position to specify muscle identity in Drosophila

The LIM-homeodomain transcription factor Tailup/Islet1 (Tup) is a key component of cardiogenesis in Drosophila and vertebrates. We report here an additional major role for Drosophila Tup in specifying dorsal muscles. Tup is expressed in the four dorsal muscle progenitors (PCs) and tup-null embryos display a severely disorganized dorsal musculature, including a transformation of the dorsal DA2 into dorsolateral DA3 muscle. This transformation is reciprocal to the DA3 to DA2 transformation observed in collier (col) mutants. The DA2 PC, which gives rise to the DA2 muscle and to an adult muscle precursor, is selected from a cluster of myoblasts transiently expressing both Tinman (Tin) and Col. The activation of tup by Tin in the DA2 PC is required to repress col transcription and establish DA2 identity. The transient, partial overlap between Tin and Col expression provides a window of opportunity to distinguish between DA2 and DA3 muscle identities. The function of Tup in the DA2 PC illustrates how single cell precision can be reached in cell specification when temporal dynamics are combined with positional information. The contributions of Tin, Tup and Col to patterning Drosophila dorsal muscles bring novel parallels with chordate pharyngeal muscle development.

Receptor-type guanylyl cyclase Gyc76C is required for development of the Drosophila embryonic somatic muscle

Guanylyl cyclases mediate a number of physiological processes, including smooth muscle function and axonal guidance. Here, we report a novel role for Drosophila receptor-type guanylyl cyclase at 76C, Gyc76C, in development of the embryonic somatic muscle. In embryos lacking function of Gyc76C or the downstream cGMP-dependent protein kinase (cGK), DG1, patterning of the somatic body wall muscles was abnormal with ventral and lateral muscle groups showing the most severe defects. In contrast, specification and elongation of the dorsal oblique and dorsal acute muscles of gyc76C mutant embryos was normal, and instead, these muscles showed defects in proper formation of the myotendinous junctions (MTJs). During MTJ formation in gyc76C and pkg21D mutant embryos, the βPS integrin subunit failed to localize to the MTJs and instead was found in discrete puncta within the myotubes. Tissue-specific rescue experiments showed that gyc76C function is required in the muscle for proper patterning and βPS integrin localization at the MTJ. These studies provide the first evidence for a requirement for Gyc76C and DG1 in Drosophila somatic muscle development, and suggest a role in transport and/or retention of integrin receptor subunits at the developing MTJs.

Diversification of muscle types in Drosophila: upstream and downstream of identity genes

Understanding gene regulatory pathways underlying diversification of cell types during development is one of the major challenges in developmental biology. Progressive specification of mesodermal lineages that are at the origin of body wall muscles in Drosophila embryos has been extensively studied during past years, providing an attractive framework for dissecting cell type diversification processes. In particular, it has been found that muscle founder cells that are at the origin of individual muscles display specific expression of transcription factors that control diversification of muscle types. These factors, encoded by genes collectively called muscle identity genes, are activated in discrete subsets of muscle founders. As a result, each founder cell is thought to carry a unique combinatorial code of identity gene expression. Considering this, to define temporally and spatially restricted expression of identity genes, a set of coordinated upstream regulatory inputs is required. But also, to realize the identity program and to form specific muscle types with distinct properties, an efficient battery of downstream identity gene targets needs to be activated. Here we review how the specificity of expression and action of muscle identity genes is acquired.

Combinatorial coding of Drosophila muscle shape by Collier and Nautilus

The diversity of Drosophila muscles correlates with the expression of combinations of identity transcription factors (iTFs) in muscle progenitors. Here, we address the question of when and how a combinatorial code is translated into muscle specific properties, by studying the roles of the Collier and Nautilus iTFs that are expressed in partly overlapping subsets of muscle progenitors. We show that the three dorso-lateral (DL) progenitors which express Nautilus and Collier are specified in a fixed temporal sequence and that each expresses additionally other, distinct iTFs. Removal of Collier leads to changes in expression of some of these iTFs and mis-orientation of several DL muscles, including the dorsal acute DA3 muscle which adopts a DA2 morphology. Detailed analysis of this transformation revealed the existence of two steps in the attachment of elongating muscles to specific tendon cells: transient attachment to alternate tendon cells, followed by a resolution step selecting the final sites. The multiple cases of triangular-shaped muscles observed in col mutant embryos indicate that transient binding of elongating muscle to exploratory sites could be a general feature of the developing musculature. In nau mutants, the DA3 muscle randomly adopts the attachment sites of the DA3 or DO5 muscles that derive from the same progenitor, resulting in a DA3, DO5-like or bifid DA3-DO5 orientation. In addition, nau mutant embryos display thinner muscle fibres. Together, our data show that the sequence of expression and combinatorial activities of Col and Nau control the pattern and morphology of DL muscles.

Drosophila araucan and caupolican integrate intrinsic and signalling inputs for the acquisition by muscle progenitors of the lateral transverse fate

A central issue of myogenesis is the acquisition of identity by individual muscles. In Drosophila, at the time muscle progenitors are singled out, they already express unique combinations of muscle identity genes. This muscle code results from the integration of positional and temporal signalling inputs. Here we identify, by means of loss-of-function and ectopic expression approaches, the Iroquois Complex homeobox genes araucan and caupolican as novel muscle identity genes that confer lateral transverse muscle identity. The acquisition of this fate requires that Araucan/Caupolican repress other muscle identity genes such as slouch and vestigial. In addition, we show that Caupolican-dependent slouch expression depends on the activation state of the Ras/Mitogen Activated Protein Kinase cascade. This provides a comprehensive insight into the way Iroquois genes integrate in muscle progenitors, signalling inputs that modulate gene expression and protein activity.

In Drosophila, as in vertebrates, the muscular system consists of different types of muscles that must act in coordination with the nervous system to control the adequate release of contraction power required for the proper functioning of the organism. Therefore, the acquisition of specific identities by individual muscles is a key step in the generation of the muscular system. In Drosophila, muscle progenitors (specific myoblasts that seed the formation of mature muscles) integrate positional and temporal signalling inputs, resulting in the expression of unique combinations of muscle identity genes, which confer on them specific fates. Up to now, very little was known of how this integration takes place at a molecular level and how a particular code is translated into a specific muscle fate. Here we show that the acquisition of the lateral transverse muscle fate requires the repression mediated by Araucan and Caupolican, two homeoproteins of the Iroquois Complex, of other muscle identity genes, like slouch and vestigial. The repressor or activator function of the Iroquois proteins depends on the activity of the Ras signalling pathway. Therefore, our work places Iroquois genes at a nodal point that integrates signalling inputs and regulates protein activity and cell fate determination.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

Regulation and functions of the lms homeobox gene during development of embryonic lateral transverse muscles and direct flight muscles in Drosophila

BACKGROUND

Patterning and differentiation of developing musculatures require elaborate networks of transcriptional regulation. In Drosophila, significant progress has been made into identifying the regulators of muscle development and defining their interactive networks. One major family of transcription factors involved in these processes consists of homeodomain proteins. In flies, several members of this family serve as muscle identity genes to specify the fates of individual muscles, or groups thereof, during embryonic and/or adult muscle development. Herein, we report on the expression and function of a new Drosophila homeobox gene during both embryonic and adult muscle development.

METHODOLOGY/PRINCIPAL FINDINGS

The newly described homeobox gene, termed lateral muscles scarcer (lms), which has yet uncharacterized orthologs in other invertebrates and primitive chordates but not in vertebrates, is expressed exclusively in subsets of developing muscle tissues. In embryos, lms is expressed specifically in the four lateral transverse (LT) muscles and their founder cells in each hemisegment, whereas in larval wing imaginal discs, it is expressed in myoblasts that develop into direct flight muscles (DFMs), which are important for proper wing positioning. We have analyzed the regulatory inputs of various other muscle identity genes with overlapping or complementary expression patterns towards the cell type specific regulation of lms expression. Further we demonstrate that lms null mutants exhibit reduced numbers of embryonic LT muscles, and null mutant adults feature held-out-wing phenotypes. We provide a detailed description of the pattern and morphology of the direct flight muscles in the wild type and lms mutant flies by using the recently-developed ultramicroscopy and show that, in the mutants, all DFMs are present and present normal morphologies.

CONCLUSIONS/SIGNIFICANCE

We have identified the homeobox gene lms as a new muscle identity gene and show that it interacts with various previously-characterized muscle identity genes to regulate normal formation of embryonic lateral transverse muscles. In addition, the direct flight muscles in the adults require lms for reliably exerting their functions in controlling wing postures.

Downstream of identity genes: muscle-type-specific regulation of the fusion process

In all metazoan organisms, the diversification of cell types involves determination of cell fates and subsequent execution of specific differentiation programs. During Drosophila myogenesis, identity genes specify the fates of founder myoblasts, from which derive all individual larval muscles. Here, to understand how cell fate information residing within founders is translated during differentiation, we focus on three identity genes, eve, lb, and slou, and how they control the size of individual muscles by regulating the number of fusion events. They achieve this by setting expression levels of Mp20, Pax, and mspo, three genes that regulate actin dynamics and cell adhesion and, as we show here, modulate the fusion process in a muscle-specific manner. Thus, these data show how the identity information implemented by transcription factors is translated via target genes into cell-type-specific programs of differentiation.

Diversification of muscle types: recent insights from Drosophila

Myogenesis is a highly conserved process ending up by the formation of contracting muscles. In Drosophila embryos, myogenesis gives rise to a segmentally repeated array of thirty distinct fibres, each of which represents an individual muscle. Since Drosophila offers a large range of genetic tools for easily testing gene functions, it has become one of the most studied and consequently best-described model organisms for muscle development. Over the last two decades, the Drosophila model system has enabled major advances in our understanding of how the initially equivalent mesodermal cells become competent for entering myogenic differentiation and how each distinct type of muscle is specified. Here we present an overview of Drosophila muscle development with a special focus on the diversification of muscle types and the genes that control acquisition of distinct muscle properties.

Drosophila Tey represses transcription of the repulsive cue Toll and generates neuromuscular target specificity

Little is known about the genetic program that generates synaptic specificity. Here we show that a putative transcription factor, Teyrha-Meyhra (Tey), controls target specificity, in part by repressing the expression of a repulsive cue, Toll. We focused on two neighboring muscles, M12 and M13, which are innervated by distinct motoneurons in Drosophila. We found that Toll, which encodes a transmembrane protein with leucine-rich repeats, was preferentially expressed in M13. In Toll mutants, motoneurons that normally innervate M12 (MN12s) formed smaller synapses on M12 and instead appeared to form ectopic nerve endings on M13. Conversely, ectopic expression of Toll in M12 inhibited synapse formation by MN12s. These results suggest that Toll functions in M13 to prevent synapse formation by MN12s. We identified Tey as a negative regulator of Toll expression in M12. In tey mutants, Toll was strongly upregulated in M12. Accordingly, synapse formation on M12 was inhibited. Conversely, ectopic expression of tey in M13 decreased the amount of Toll expression in M13 and changed the pattern of motor innervation to the one seen in Toll mutants. These results suggest that Tey determines target specificity by repressing the expression of Toll. These results reveal a mechanism for generating synaptic specificity that relies on the negative regulation of a repulsive target cue.

Cytoskeletal remodeling during myotube assembly and guidance: coordinating the actin and microtubule networks

The formation of a multinucleated muscle fiber from individual myoblasts is a complex morphological event that requires dramatic cytoskeletal rearrangements. This multistep process includes myoblast fusion, myotube migration and elongation, myotube target recognition, and finally attachment to form a stable adhesion complex. Many of the studies directed towards understanding the developmental process of muscle morphogenesis at the cellular level have relied on forward genetic screens in model systems such as Drosophila melanogaster for mutations affecting individual stages in myogenesis. Through the analyses of these gene products, proteins that regulate the actin or microtubule cytoskeleton have emerged as important players in each of these steps. We recently demonstrated that RacGAP50C, an essential protein that functions as a cytoskeletal regulator during cell division, also plays an important role in organizing the polarized microtubule network in the elongating myotube. Here we review the current literature regarding Drosophila myogenesis and illustrate several steps of muscle development with respect to the diverse roles that the cytoskeleton plays during this process. Furthermore, we discuss the significance of cytoskeletal coordination during these multiple steps.

Multi-step control of muscle diversity by Hox proteins in the Drosophila embryo

Hox transcription factors control many aspects of animal morphogenetic diversity. The segmental pattern of Drosophila larval muscles shows stereotyped variations along the anteroposterior body axis. Each muscle is seeded by a founder cell and the properties specific to each muscle reflect the expression by each founder cell of a specific combination of 'identity' transcription factors. Founder cells originate from asymmetric division of progenitor cells specified at fixed positions. Using the dorsal DA3 muscle lineage as a paradigm, we show here that Hox proteins play a decisive role in establishing the pattern of Drosophila muscles by controlling the expression of identity transcription factors, such as Nautilus and Collier (Col), at the progenitor stage. High-resolution analysis, using newly designed intron-containing reporter genes to detect primary transcripts, shows that the progenitor stage is the key step at which segment-specific information carried by Hox proteins is superimposed on intrasegmental positional information. Differential control of col transcription by the Antennapedia and Ultrabithorax/Abdominal-A paralogs is mediated by separate cis-regulatory modules (CRMs). Hox proteins also control the segment-specific number of myoblasts allocated to the DA3 muscle. We conclude that Hox proteins both regulate and contribute to the combinatorial code of transcription factors that specify muscle identity and act at several steps during the muscle-specification process to generate muscle diversity.

A misexpression screen to identify regulators of Drosophila larval hemocyte development

In Drosophila, defense against foreign pathogens is mediated by an effective innate immune system, the cellular arm of which is composed of circulating hemocytes that engulf bacteria and encapsulate larger foreign particles. Three hemocyte types occur: plasmatocytes, crystal cells, and lamellocytes. The most abundant larval hemocyte type is the plasmatocyte, which is responsible for phagocytosis and is present either in circulation or in adherent sessile domains under the larval cuticle. The mechanisms controlling differentiation of plasmatocytes and their migration toward these sessile compartments are unclear. To address these questions we have conducted a misexpression screen using the plasmatocyte-expressed GAL4 driver Peroxidasin-GAL4 (Pxn-GAL4) and existing enhancer-promoter (EP) and EP yellow (EY) transposon libraries to systematically misexpress approximately 20% of Drosophila genes in larval hemocytes. The Pxn-GAL4 strain also contains a UAS-GFP reporter enabling hemocyte phenotypes to be visualized in the semitransparent larvae. Among 3412 insertions screened we uncovered 101 candidate hemocyte regulators. Some of these are known to control hemocyte development, but the majority either have no characterized function or are proteins of known function not previously implicated in hemocyte development. We have further analyzed three candidate genes for changes in hemocyte morphology, cell-cell adhesion properties, phagocytosis activity, and melanotic tumor formation.

Visualizing new dimensions in Drosophila myoblast fusion

Over several years, genetic studies in the model system, Drosophila melanogastor, have uncovered genes that when mutated, lead to a block in myoblast fusion. Analyses of these gene products have suggested that Arp2/3-mediated regulation of the actin cytoskeleton is crucial to myoblast fusion in the fly. Recent advances in imaging in Drosophila embryos, both in fixed and live preparations, have led to a new appreciation of both the three-dimensional organization of the somatic mesoderm and the cell biology underlying myoblast fusion.

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.

Daughterless dictates Twist activity in a context-dependent manner during somatic myogenesis

Somatic myogenesis in Drosophila relies on the reiterative activity of the basic helix-loop-helix transcriptional regulator, Twist (Twi). How Twi directs multiple cell fate decisions over the course of mesoderm and muscle development is unclear. Previous work has shown that Twi is regulated by its dimerization partner: Twi homodimers activate genes necessary for somatic myogenesis, whereas Twi/Daughterless (Da) heterodimers lead to the repression of these genes. Here, we examine the nature of Twi/Da heterodimer repressive activity. Analysis of the Da protein structure revealed a Da repression (REP) domain, which is required for Twi/Da-mediated repression of myogenic genes, such as Dmef2, both in tissue culture and in vivo. This domain is crucial for the allocation of mesodermal cells to distinct fates, such as heart, gut and body wall muscle. By contrast, the REP domain is not required in vivo during later stages of myogenesis, even though Twi activity is required for muscles to achieve their final pattern and morphology. Taken together, we present evidence that the repressive activity of the Twi/Da dimer is dependent on the Da REP domain and that the activity of the REP domain is sensitive to tissue context and developmental timing.

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.

The Him gene reveals a balance of inputs controlling muscle differentiation in Drosophila

Tissue development requires the controlled regulation of cell-differentiation programs. In muscle, the Mef2 transcription factor binds to and activates the expression of many genes and has a major positive role in the orchestration of differentiation [1–4]. However, little is known about how Mef2 activity is regulated in vivo during development. Here, we characterize a gene, Holes in muscle (Him), which our results indicate is part of this control in Drosophila. Him expression rapidly declines as embryonic muscle differentiates, and consistent with this, Him overexpression inhibits muscle differentiation. This inhibitory effect is suppressed by mef2, implicating Him in the mef2 pathway. We then found that Him downregulates the transcriptional activity of Mef2 in both cell culture and in vivo. Furthermore, Him protein binds Groucho, a conserved, transcriptional corepressor, through a WRPW motif and requires this motif and groucho function to inhibit both muscle differentiation and Mef2 activity during development. Together, our results identify a mechanism that can inhibit muscle differentiation in vivo. We conclude that a balance of positive and negative inputs, including Mef2, Him, and Groucho, controls muscle differentiation during Drosophila development and suggest that one outcome is to hold developing muscle cells in a state with differentiation genes poised to be expressed.

Expression and functional analysis of a novel Fusion Competent Myoblast specific GAL4 driver

In the Drosophila embryo, body wall muscles are formed by the fusion of two cell types, Founder Cells (FCs) and Fusion Competent Myoblasts (FCMs). Using an enhancer derived from the Dmef2 gene ([C/D]( *)), we report the first GAL4 driver specifically expressed in FCMs. We have determined that this GAL4 driver causes expression in a subset of FCMs and, upon fusion, in developing myotubes from stage 14 onwards. In addition, we have shown that using this Dmef2-5x[C/D]( *)-GAL4 driver to express dominant negative Rac in only FCMs causes a partial fusion block. This novel GAL4 driver will provide a useful reagent to study Drosophila myoblast fusion and muscle differentiation.

3D analysis of founder cell and fusion competent myoblast arrangements outlines a new model of myoblast fusion

Formation of the Drosophila larval body wall muscles requires the specification, coordinated cellular behaviors and fusion of two cell types: Founder Cells (FCs) that control the identity of the individual muscle and Fusion Competent Myoblasts (FCMs) that provide mass. These two cell types come together to control the final size, shape and attachment of individual muscles. However, the spatial arrangement of these cells over time, the sequence of fusion events and the contribution of these cellular relationships to the fusion process have not been addressed. We analyzed the three-dimensional arrangements of FCs and FCMs over the course of myoblast fusion and assayed whether these issues impact the process of myoblast fusion. We examined the timing of the fusion process by analyzing the fusion profile of individual muscles in wild type and fusion mutants. We showed that there are two temporal phases of myoblast fusion in wild type embryos. Limited fusion events occur during the first 3 h of fusion, while the majority of fusion events occur in the remaining 2.5 h. Altogether, our data have led us to propose a new model of myoblast fusion where the frequency of myoblast fusion events may be influenced by the spatial arrangements of FCs and FCMs.

Defective decapentaplegic signaling results in heart overgrowth and reduced cardiac output in Drosophila

During germ-band extension, Decapentaplegic (Dpp) signals from the dorsal ectoderm to maintain Tinman (Tin) expression in the underlying mesoderm. This signal specifies the cardiac field, and homologous genes (BMP2/4 and Nkx2.5) perform this function in mammals. We showed previously that a second Dpp signal from the dorsal ectoderm restricts the number of pericardial cells expressing the transcription factor Zfh1. Here we report that, via Zfh1, the second Dpp signal restricts the number of Odd-skipped-expressing and the number of Tin-expressing pericardial cells. Dpp also represses Tin expression independently of Zfh1, implicating a feed-forward mechanism in the regulation of Tin pericardial cell number. In the adjacent dorsal muscles, Dpp has the opposite effect. Dpp maintains Krüppel and Even-skipped expression required for muscle development. Our data show that Dpp refines the cardiac field by limiting the number of pericardial cells. This maintains the boundary between pericardial and dorsal muscle cells and defines the size of the heart. In the absence of the second Dpp signal, pericardial cells overgrow and this significantly reduces larval cardiac output. Our study suggests the existence of a second round of BMP signaling in mammalian heart development and that perhaps defects in this signal play a role in congenital heart defects.

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.

Kruppel-like Factors (KLFs) in muscle biology

The Kruppel-like Factor (KLF) family of zinc-finger transcription factors are critical regulators of cell differentiation, phenotypic modulation and physiologic function. An emerging body of evidence implicates an important role for these factors in cardiovascular biology, however, the role of KLFs in muscle biology is only beginning to be understood. This article reviews the published data describing the role of KLFs in the heart, smooth muscle, and skeletal muscle and highlights the importance of these factors in cardiovascular development, physiology and disease pathobiology.

The Wiskott-Aldrich syndrome protein (WASP) is essential for myoblast fusion in Drosophila

In higher organisms, mononucleated myoblasts fuse to form multinucleated myotubes. During this process, myoblasts undergo specific changes in cell morphology and cytoarchitecture. Previously, we have shown that the actin regulator Kette (Hem-2/Nap-1) is essential for myoblast fusion. In this study, we describe the role of the evolutionary conserved Wiskott-Aldrich syndrome protein that serves as a regulator for the Arp2/3 complex for myoblast fusion. By screening an EMS mutagenesis collection, we discovered a new wasp allele that does not complete fusion during myogenesis. Interestingly, this new wasp3D3-035 allele is characterized by a disruption of fusion after precursor formation. The molecular lesion in this wasp allele leads to a stop codon preventing translation of the CA domain. Usually, the WASP protein exerts its function through the Arp2/3-interacting CA domain. Accordingly, a waspDeltaCA that is expressed in a wild-type background acts as dominant-negative during the fusion process. Furthermore, we show that the myoblast fusion phenotype of kette mutant embryos can be suppressed by reducing the gene dose of wasp3D3-035. Thus, Kette antagonizes WASP function during myoblast fusion.

Parcas, a regulator of non-receptor tyrosine kinase signaling, acts during anterior-posterior patterning and somatic muscle development in Drosophila melanogaster

We have isolated parcas (pcs) in a screen to identify novel regulators of muscle morphogenesis. Pcs is expressed in the ovary and oocyte during oogenesis and again in the embryo, specifically in the developing mesoderm, throughout muscle development. pcs is first required in the ovary during oogenesis for patterning and segmentation of the early Drosophila embryo due primarily to its role in the regulation of Oskar (Osk) levels. In addition to the general patterning defects observed in embryos lacking maternal contribution of pcs, these embryos show defects in Wingless (Wg) expression, causing losses of Wg-dependent cell types within the affected segment. pcs activity is required again later during embryogenesis in the developing mesoderm for muscle development. Loss and gain of function studies demonstrate that pcs is necessary at distinct times for muscle specification and morphogenesis. Pcs is predicted to be a novel regulator of non-receptor tyrosine kinase (NRTK) signaling. We have identified one target of Pcs regulation, the Drosophila Tec kinase Btk29A. While Btk29A appears to be regulated by Pcs during its early role in patterning and segmentation, it does not appear to be a major target of Pcs regulation during muscle development. We propose that Pcs fulfils its distinct roles during development by the regulation of multiple NRTKs.

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.

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.

Drosophila Heartless acts with Heartbroken/Dof in muscle founder differentiation

The formation of a multi-nucleate myofibre is directed, in Drosophila, by a founder cell. In the embryo, founders are selected by Notch-mediated lateral inhibition, while during adult myogenesis this mechanism of selection does not appear to operate. We show, in the muscles of the adult abdomen, that the Fibroblast growth factor pathway mediates founder cell choice in a novel manner. We suggest that the developmental patterns of Heartbroken/Dof and Sprouty result in defining the domain and timing of activation of the Fibroblast growth factor receptor Heartless in specific myoblasts, thereby converting them into founder cells. Our results point to a way in which muscle differentiation could be initiated and define a critical developmental function for Heartbroken/Dof in myogenesis.

In the fly embryo, the founder cells that direct myofibre formation are selected through Notch-mediated signaling. The authors show that in adult animals, founder cells are specified by signaling through the FGF pathway.

Muscle building; mechanisms of myotube guidance and attachment site selection

The complex muscle patterns of higher organisms arise as migrating myoblasts are guided toward and connect with specific attachment sites. We review here the current understanding of myotube migration, focusing on its dynamic nature and the few molecular cues that have been identified to date. Much of this knowledge comes from studies in Drosophila, where powerful methods for in vivo imaging and genetic manipulation can be used to tackle this important but largely unsolved problem in developmental biology.

Transcription of Drosophila troponin I gene is regulated by two conserved, functionally identical, synergistic elements

The Drosophila wings-up A gene encodes Troponin I. Two regions, located upstream of the transcription initiation site (upstream regulatory element) and in the first intron (intron regulatory element), regulate gene expression in specific developmental and muscle type domains. Based on LacZ reporter expression in transgenic lines, upstream regulatory element and intron regulatory element yield identical expression patterns. Both elements are required for full expression levels in vivo as indicated by quantitative reverse transcription-polymerase chain reaction assays. Three myocyte enhancer factor-2 binding sites have been functionally characterized in each regulatory element. Using exon specific probes, we show that transvection is based on transcriptional changes in the homologous chromosome and that Zeste and Suppressor of Zeste 3 gene products act as repressors for wings-up A. Critical regions for transvection and for Zeste effects are defined near the transcription initiation site. After in silico analysis in insects (Anopheles and Drosophila pseudoobscura) and vertebrates (Ratus and Coturnix), the regulatory organization of Drosophila seems to be conserved. Troponin I (TnI) is expressed before muscle progenitors begin to fuse, and sarcomere morphogenesis is affected by TnI depletion as Z discs fail to form, revealing a novel developmental role for the protein or its transcripts. Also, abnormal stoichiometry among TnI isoforms, rather than their absolute levels, seems to cause the functional muscle defects.