The fabulous destiny of the Drosophila heart

Cardiac Development and Animal Models of Congenital Heart Defects

The major events of cardiac development, including early heart formation, chamber morphogenesis and septation, and conduction system and coronary artery development, are briefly reviewed together with a short introduction to the animal species commonly used to study heart development and model congenital heart defects (CHDs).

Regulation of gene expression during ontogeny of physiological function in the brackishwater amphipod Gammarus chevreuxi

Embryonic development is a complex process involving the co-ordinated onset and integration of multiple morphological features and physiological functions. While the molecular basis of morphological development in embryos is relatively well known for traditional model species, the molecular underpinning of the development of physiological functions is not. Here, we used global gene expression profiling to investigate the transcriptional changes associated with the development of morphological and physiological function in the amphipod crustacean Gammarus chevreuxi. We compared the transcriptomes at three timepoints during the latter half of development, characterised by different stages of the development of heart form and function: 10 days post fertilisation (dpf, Early: no heart structure visible), 15 dpf (Middle: heart present but not fully functional), and 18 dpf (Late: regular heartbeat). Gene expression profiles differed markedly between developmental stages, likely representing a change in the activity of different processes throughout the latter period of G. chevreuxi embryonic development. Differentially expressed genes belonged to one of three distinct clusters based on their expression patterns across development. One of these clusters, which included key genes relating to cardiac contractile machinery and calcium handling, displayed a pattern of sequential up-regulation throughout the developmental period studied. Further analyses of these transcripts could reveal genes that may influence the onset of a regular heartbeat. We also identified morphological and physiological processes that may occur alongside heart development, such as development of digestive caeca and the cuticle. Elucidating the mechanisms underpinning morphological and physiological development of non-model organisms will support improved understanding of conserved mechanisms, addressing the current phylogenetic gap between relatively well known model species.

Drosophila Accessory Gland: A Complementary In Vivo Model to Bring New Insight to Prostate Cancer

Prostate cancer is the most common cancer in aging men. Despite recent progress, there are still few effective treatments to cure its aggressive and metastatic stages. A better understanding of the molecular mechanisms driving disease initiation and progression appears essential to support the development of more efficient therapies and improve patient care. To do so, multiple research models, such as cell culture and mouse models, have been developed over the years and have improved our comprehension of the biology of the disease. Recently, a new model has been added with the use of the Drosophila accessory gland. With a high level of conservation of major signaling pathways implicated in human disease, this functional equivalent of the prostate represents a powerful, inexpensive, and rapid in vivo model to study epithelial carcinogenesis. The purpose of this review is to quickly overview the existing prostate cancer models, including their strengths and limitations. In particular, we discuss how the Drosophila accessory gland can be integrated as a convenient complementary model by bringing new understanding in the mechanisms driving prostate epithelial tumorigenesis, from initiation to metastatic formation.

Heat shock proteins and small nucleolar RNAs are dysregulated in a Drosophila model for feline hypertrophic cardiomyopathy

In cats, mutations in myosin binding protein C (encoded by the MYBPC3 gene) have been associated with hypertrophic cardiomyopathy (HCM). However, the molecular mechanisms linking these mutations to HCM remain unknown. Here, we establish Drosophila melanogaster as a model to understand this connection by generating flies harboring MYBPC3 missense mutations (A31P and R820W) associated with feline HCM. The A31P and R820W flies displayed cardiovascular defects in their heart rates and exercise endurance. We used RNA-seq to determine which processes are misregulated in the presence of mutant MYBPC3 alleles. Transcriptome analysis revealed significant downregulation of genes encoding small nucleolar RNA (snoRNAs) in exercised female flies harboring the mutant alleles compared to flies that harbor the wild-type allele. Other processes that were affected included the unfolded protein response and immune/defense responses. These data show that mutant MYBPC3 proteins have widespread effects on the transcriptome of co-regulated genes. Transcriptionally differentially expressed genes are also candidate genes for future evaluation as genetic modifiers of HCM as well as candidate genes for genotype by exercise environment interaction effects on the manifestation of HCM; in cats as well as humans.

Heat shock proteins and small nucleolar RNAs are dysregulated in a Drosophila model for feline hypertrophic cardiomyopathy

In cats, mutations in myosin binding protein C (encoded by the MYBPC3 gene) have been associated with hypertrophic cardiomyopathy (HCM). However, the molecular mechanisms linking these mutations to HCM remain unknown. Here, we establish Drosophila melanogaster as a model to understand this connection by generating flies harboring MYBPC3 missense mutations (A31P and R820W) associated with feline HCM. The A31P and R820W flies displayed cardiovascular defects in their heart rates and exercise endurance. We used RNA-seq to determine which processes are misregulated in the presence of mutant MYBPC3 alleles. Transcriptome analysis revealed significant downregulation of genes encoding small nucleolar RNA (snoRNAs) in exercised female flies harboring the mutant alleles compared to flies that harbor the wild-type allele. Other processes that were affected included the unfolded protein response and immune/defense responses. These data show that mutant MYBPC3 proteins have widespread effects on the transcriptome of co-regulated genes. Transcriptionally differentially expressed genes are also candidate genes for future evaluation as genetic modifiers of HCM as well as candidate genes for genotype by exercise environment interaction effects on the manifestation of HCM; in cats as well as humans.

Cbl-Associated Protein CAP contributes to correct formation and robust function of the Drosophila heart tube

The formation of a tube-like structure is a basic step in the making of functional hearts in vertebrates and invertebrates and therefore, its understanding provides important information on heart development and function. In Drosophila, the cardiac tube originates from two bilateral rows of dorsally migrating cells. On meeting at the dorsal midline, coordinated changes in cell shape and adhesive properties transform the two sheets of cells into a linear tube. ECM and transmembrane proteins linked to the cytoskeleton play an important role during these dynamic processes. Here we characterize the requirement of Cbl-Associated Protein (CAP) in Drosophila heart formation. In embryos, CAP is expressed in late migrating cardioblasts and is located preferentially at their luminal and abluminal periphery. CAP mutations result in irregular cardioblast alignment and imprecisely controlled cardioblast numbers. Furthermore, CAP mutant embryos show a strongly reduced heart lumen and an aberrant shape of lumen forming cardioblasts. Analysis of double heterozygous animals reveals a genetic interaction of CAP with Integrin- and Talin-encoding genes. In post-embryonic stages, CAP closely colocalizes with Integrin near Z-bands and at cell-cell contact sites. CAP mutants exhibit a reduced contractility in larval hearts and show a locally disrupted morphology, which correlates with a reduced pumping efficiency. Our observations imply a function of CAP in linking Integrin signaling with the actin cytoskeleton. As a modulator of the cytoskeleton, CAP is involved in the establishment of proper cell shapes during cardioblast alignment and cardiac lumen formation in the Drosophila embryo. Furthermore, CAP is required for correct heart function throughout development.

Matrix metalloproteinases regulate ECM accumulation but not larval heart growth in Drosophila melanogaster

The Drosophila heart provides a simple model to examine the remodelling of muscle insertions with growth, extracellular matrix (ECM) turnover, and fibrosis. Between hatching and pupation, the Drosophila heart increases in length five-fold. If major cardiac ECM components are secreted remotely, how is ECM "self assembly" regulated? We explored whether ECM proteases were required to maintain the morphology of a growing heart while the cardiac ECM expanded. An increase in expression of Drosophila's single tissue inhibitor of metalloproteinase (TIMP), or reduced function of metalloproteinase MMP2, resulted in fibrosis and ectopic deposition of two ECM Collagens; type-IV and fibrillar Pericardin. Significant accumulations of Collagen-IV (Viking) developed on the pericardium and in the lumen of the heart. Congenital defects in Pericardin deposition misdirected further assembly in the larva. Reduced metalloproteinase activity during growth also increased Pericardin fibre accumulation in ECM suspending the heart. Although MMP2 expression was required to remodel and position cardiomyocyte cell junctions, reduced MMP function did not impair expansion of the heart. A previous study revealed that MMP2 negatively regulates the size of the luminal cell surface in the embryonic heart. Cardiomyocytes align at the midline, but do not adhere to enclose a heart lumen in MMP2 mutant embryos. Nevertheless, these embryos hatch and produce viable larvae with bifurcated hearts, indicating a secondary pathway to lumen formation between ipsilateral cardiomyocytes. MMP-mediated remodelling of the ECM is required for organogenesis, and to prevent assembly of excess or ectopic ECM protein during growth. MMPs are not essential for normal growth of the Drosophila heart.

Pericardin, a Drosophila collagen, facilitates accumulation of hemocytes at the heart

Hematopoietic cell lineages support organismal needs by responding to positional and systemic signals that balance proliferative and differentiation events. Drosophila provides an excellent genetic model to dissect these signals, where the activity of cues in the hemolymph or substrate can be traced to determination and differentiation events of well characterized hemocyte types. Plasmatocytes in third instar larvae increase in number in response to infection and in anticipation of metamorphosis. Here we characterize hemocyte clustering, proliferation and transdifferentiation on the heart or dorsal vessel. Hemocytes accumulate on the inner foldings of the heart basement membrane, where they move with heart contraction, and are in proximity to the heart ostia and pericardial nephrocytes. The numbers of hemocytes vary, but increase transiently before pupariation, and decrease by 4 h before pupa formation. During their accumulation at the heart, plasmatocytes can proliferate and can transdifferentiate into crystal cells. Serrate expressing cells as well as lamellocyte-like, Atilla expressing ensheathing cells are associated with some, but not all hemocyte clusters. Hemocyte aggregation is enhanced by the presence of a heart specific Collagen, Pericardin, but not the associated pericardial cells. The varied and transient number of hemocytes in the pericardial compartment suggests that this is not a hematopoietic hub, but a niche supporting differentiation and rapid dispersal in response to systemic signals.

Drosophila melanogaster: A Model Organism to Study Cancer

Cancer is a multistep disease driven by the activation of specific oncogenic pathways concomitantly with the loss of function of tumor suppressor genes that act as sentinels to control physiological growth. The conservation of most of these signaling pathways in Drosophila, and the ability to easily manipulate them genetically, has made the fruit fly a useful model organism to study cancer biology. In this review we outline the basic mechanisms and signaling pathways conserved between humans and flies responsible of inducing uncontrolled growth and cancer development. Second, we describe classic and novel Drosophila models used to study different cancers, with the objective to discuss their strengths and limitations on their use to identify signals driving growth cell autonomously and within organs, drug discovery and for therapeutic approaches.

Selective Filopodia Adhesion Ensures Robust Cell Matching in the Drosophila Heart

The ability to form specific cell-cell connections within complex cellular environments is critical for multicellular organisms. However, the underlying mechanisms of cell matching that instruct these connections remain elusive. Here, we quantitatively explored the dynamics and regulation of cell matching processes utilizing Drosophila cardiogenesis. We found that cell matching is highly robust at boundaries between cardioblast (CB) subtypes, and filopodia of different CB subtypes have distinct binding affinities. Cdc42 is involved in regulating this selective filopodia binding adhesion and influences CB matching. Further, we identified adhesion molecules Fasciclin III (Fas3) and Ten-m, both of which also regulate synaptic targeting, as having complementary differential expression in CBs. Altering Fas3 expression changes differential filopodia adhesion and leads to CB mismatch. Furthermore, only when both Fas3 and Ten-m are lost is CB alignment severely impaired. Our results show that differential adhesion mediated by selective filopodia binding efficiently regulates precise and robust cell matching.

Whole genome analysis of a schistosomiasis-transmitting freshwater snail

Biomphalaria snails are instrumental in transmission of the human blood fluke Schistosoma mansoni. With the World Health Organization's goal to eliminate schistosomiasis as a global health problem by 2025, there is now renewed emphasis on snail control. Here, we characterize the genome of Biomphalaria glabrata, a lophotrochozoan protostome, and provide timely and important information on snail biology. We describe aspects of phero-perception, stress responses, immune function and regulation of gene expression that support the persistence of B. glabrata in the field and may define this species as a suitable snail host for S. mansoni. We identify several potential targets for developing novel control measures aimed at reducing snail-mediated transmission of schistosomiasis.

On the Evolution of the Cardiac Pacemaker

The rhythmic contraction of the heart is initiated and controlled by an intrinsic pacemaker system. Cardiac contractions commence at very early embryonic stages and coordination remains crucial for survival. The underlying molecular mechanisms of pacemaker cell development and function are still not fully understood. Heart form and function show high evolutionary conservation. Even in simple contractile cardiac tubes in primitive invertebrates, cardiac function is controlled by intrinsic, autonomous pacemaker cells. Understanding the evolutionary origin and development of cardiac pacemaker cells will help us outline the important pathways and factors involved. Key patterning factors, such as the homeodomain transcription factors Nkx2.5 and Shox2, and the LIM-homeodomain transcription factor Islet-1, components of the T-box (Tbx), and bone morphogenic protein (Bmp) families are well conserved. Here we compare the dominant pacemaking systems in various organisms with respect to the underlying molecular regulation. Comparative analysis of the pathways involved in patterning the pacemaker domain in an evolutionary context might help us outline a common fundamental pacemaker cell gene programme. Special focus is given to pacemaker development in zebrafish, an extensively used model for vertebrate development. Finally, we conclude with a summary of highly conserved key factors in pacemaker cell development and function.

Dissecting the Role of the Extracellular Matrix in Heart Disease: Lessons from the Drosophila Genetic Model

The extracellular matrix (ECM) is a dynamic scaffold within organs and tissues that enables cell morphogenesis and provides structural support. Changes in the composition and organisation of the cardiac ECM are required for normal development. Congenital and age-related cardiac diseases can arise from mis-regulation of structural ECM proteins (Collagen, Laminin) or their receptors (Integrin). Key regulators of ECM turnover include matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of matrix metalloproteinases (TIMPs). MMP expression is increased in mice, pigs, and dogs with cardiomyopathy. The complexity and longevity of vertebrate animals makes a short-lived, genetically tractable model organism, such as Drosophila melanogaster, an attractive candidate for study. We survey ECM macromolecules and their role in heart development and growth, which are conserved between Drosophila and vertebrates, with focus upon the consequences of altered expression or distribution. The Drosophila heart resembles that of vertebrates during early development, and is amenable to in vivo analysis. Experimental manipulation of gene function in a tissue- or temporally-regulated manner can reveal the function of adhesion or ECM genes in the heart. Perturbation of the function of ECM proteins, or of the MMPs that facilitate ECM remodelling, induces cardiomyopathies in Drosophila, including cardiodilation, arrhythmia, and cardia bifida, that provide mechanistic insight into cardiac disease in mammals.

Mapping Heart Development in Flies: Src42A Acts Non-Autonomously to Promote Heart Tube Formation in Drosophila

Congenital heart defects, clinically identified in both small and large animals, are multifactorial and complex. Although heritable factors are known to have a role in cardiovascular disease, the full genetic aetiology remains unclear. Model organism research has proven valuable in providing a deeper understanding of the essential factors in heart development. For example, mouse knock-out studies reveal a role for the Integrin adhesion receptor in cardiac tissue. Recent research in Drosophila melanogaster (the fruit fly), a powerful experimental model, has demonstrated that the link between the extracellular matrix and the cell, mediated by Integrins, is required for multiple aspects of cardiogenesis. Here we test the hypothesis that Integrins signal to the heart cells through Src42A kinase. Using the powerful genetics and cell biology analysis possible in Drosophila, we demonstrate that Src42A acts in early events of heart tube development. Careful examination of mutant heart tissue and genetic interaction data suggests that Src42A’s role is independent of Integrin and the Integrin-related Focal Adhesion Kinase. Rather, Src42A acts non-autonomously by promoting programmed cell death of the amnioserosa, a transient tissue that neighbors the developing heart.

Lab-on-chips for manipulation of small-scale organisms to facilitate imaging of neurons and organs

Caenorhabditis elegans (C. elegans) and Drosophila melanogaster (D. melanogaster) are widely-used model organisms for neurological and cardiac studies due to their simple neuronal (302 neurons in C. elegans) and cardiac (simple tubular organ in D. melanogaster) systems. However, their small sizes and continuous mobility impede their precise and timely manipulation, hence, limiting the assays that can be done using conventional manual methods. This has resulted in a need for technologies that allow multidirectional manipulation of model organisms to enable studies on target neurons and organs throughout the body. By integration of rotatable glass capillaries with pneumatic suction into microfluidic devices, we propose novel Lab-on-Chips for multi-directional manipulation and imaging of small organisms. These hybrid Lab-on-Chips can facilitate the processes of animal handling and stimuli control, using modules for single-organism selection, orientation, imaging and chemical stimulation. We show the applications of these hybrid microdevices in manipulating C. elegans for neuronal imaging (neuron-level assay) or D. melanogaster for heart screening (organ level assay). These devices can enhance the throughput of biological assays on whole-organisms and find their applications in drug discovery and toxicology.

Formation and function of intracardiac valve cells in the Drosophila heart

Drosophila harbors a simple tubular heart that ensures hemolymph circulation within the body. The heart is built by a few different cell types, including cardiomyocytes that define the luminal heart channel and ostia cells that constitute openings in the heart wall allowing hemolymph to enter the heart chamber. Regulation of flow directionality within a tube, such as blood flow in arteries or insect hemolymph within the heart lumen, requires a dedicated gate, valve, or flap-like structure that prevents backflow of fluids. In the Drosophila heart, intracardiac valves provide this directionality of hemolymph streaming, with one valve being present in larvae and three valves in the adult fly. Each valve is built by two specialized cardiomyocytes that exhibit a unique histology. We found that the capacity to open and close the heart lumen relies on a unique myofibrillar setting as well as on the presence of large membranous vesicles. These vesicles are of endocytic origin and probably represent unique organelles of valve cells. Moreover, we characterised the working mode of the cells in real time. Valve cells exhibit a highly flexible shape and during each heartbeat, oscillating shape changes result in closing and opening of the heart channel. Finally, we identified a set of novel valve cell markers useful for future in-depth analyses of cell differentiation in wildtype and mutant animals.

Methylene Blue Partially Rescues Heart Defects in a Drosophila Model of Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a Polyglutamine disease caused by the presence of CAG repeats in the first exon of Huntingtin (Htt), a large protein with multiple functions. In addition to neurodegeneration of specific brain regions, notably the striatum, HD also shows alterations in peripheral tissues, such as the heart, skeletal muscles or peripheral endocrine glands. Mutant Huntingtin (mHtt)-driven mitochondrial impairment may underlie some of the CNS and peripheral tissues dysfunctions, especially in tissues with high energy demand such as the heart.

OBJECTIVE

The aim of this study is to characterize two new inducible Drosophila HD heart models and to assay the therapeutic potential of methylene blue in these HD models.

METHODS

We report the construction of inducible Drosophila HD heart models, expressing two Nter fragments of the protein encompassing either exon 1 or the first 171 amino acids and the characterization of heart phenotypes in vivo.

RESULTS

We show that both mHtt fragments are able to impair fly cardiac function with different characteristics. Additionally, expression of mHtt, which was limited to adulthood only, leads to mild heart impairment, as opposed to a strong and age-dependent phenotype observed when mHtt expression was driven during both developmental and adult stages. We report that treatment with methylene blue (MB), a protective compound in mitochondria-related diseases, partially protects the fly's heart against mHtt-induced toxicity, but does not rescue neuronal or glial phenotypes in other fly models of HD. This may be linked to its low penetration through the fly's blood-brain barrier.

CONCLUSIONS

Our data suggest that improvement of mitochondrial function by MB, or related compounds, could be an efficient therapeutic strategy to prevent cardiac failure in HD patients.

On the Morphology of the Drosophila Heart

The circulatory system of Drosophilamelanogaster represents an easily amenable genetic model whose analysis at different levels, i.e., from single molecules up to functional anatomy, has provided new insights into general aspects of cardiogenesis, heart physiology and cardiac aging, to name a few examples. In recent years, the Drosophila heart has also attracted the attention of researchers in the field of biomedicine. This development is mainly due to the fact that several genes causing human heart disease are also present in Drosophila, where they play the same or similar roles in heart development, maintenance or physiology as their respective counterparts in humans. This review will attempt to briefly introduce the anatomy of the Drosophila circulatory system and then focus on the different cell types and non-cellular tissue that constitute the heart.

Probing the Electrophysiology of the Developing Heart

Many diseases that result in dysfunction and dysmorphology of the heart originate in the embryo. However, the embryonic heart presents a challenging subject for study: especially challenging is its electrophysiology. Electrophysiological maturation of the embryonic heart without disturbing its physiological function requires the creation and deployment of novel technologies along with the use of classical techniques on a range of animal models. Each tool has its strengths and limitations and has contributed to making key discoveries to expand our understanding of cardiac development. Further progress in understanding the mechanisms that regulate the normal and abnormal development of the electrophysiology of the heart requires integration of this functional information with the more extensively elucidated structural and molecular changes.

Regulation of Heart Rate in Drosophila via Fragile X Mental Retardation Protein

RNA binding proteins play a pivotal role in post-transcriptional gene expression regulation, however little is understood about their role in cardiac function. The Fragile X (FraX) family of RNA binding proteins is most commonly studied in the context of neurological disorders, as mutations in Fragile X Mental Retardation 1 (FMR1) are the leading cause of inherited mental retardation. More recently, alterations in the levels of Fragile X Related 1 protein, FXR1, the predominant FraX member expressed in vertebrate striated muscle, have been linked to structural and functional defects in mice and zebrafish models. FraX proteins are established regulators of translation and are known to regulate specific targets in different tissues. To decipher the direct role of FraX proteins in the heart in vivo, we turned to Drosophila, which harbors a sole, functionally conserved and ubiquitously expressed FraX protein, dFmr1. Using classical loss of function alleles as well as muscle specific RNAi knockdown, we show that Drosophila FMRP, dFmr1, is required for proper heart rate during development. Functional analyses in the context of cardiac-specific dFmr1 knockdown by RNAi demonstrate that dFmr1 is required cell autonomously in cardiac cells for regulating heart rate. Interestingly, these functional defects are not accompanied by any obvious structural abnormalities, suggesting that dFmr1 may regulate a different repertoire of targets in Drosophila than in vertebrates. Taken together, our findings support the hypothesis that dFmr1 protein is essential for proper cardiac function and establish the fly as a new model for studying the role(s) of FraX proteins in the heart.

Fine-Tuning of the Actin Cytoskeleton and Cell Adhesion During Drosophila Development by the Unconventional Guanine Nucleotide Exchange Factors Myoblast City and Sponge

The evolutionarily conserved Dock proteins function as unconventional guanine nucleotide exchange factors (GEFs). Upon binding to engulfment and cell motility (ELMO) proteins, Dock-ELMO complexes activate the Rho family of small GTPases to mediate a diverse array of biological processes, including cell motility, apoptotic cell clearance, and axon guidance. Overlapping expression patterns and functional redundancy among the 11 vertebrate Dock family members, which are subdivided into four families (Dock A, B, C, and D), complicate genetic analysis. In both vertebrate and invertebrate systems, the actin dynamics regulator, Rac, is the target GTPase of the Dock-A subfamily. However, it remains unclear whether Rac or Rap1 are the in vivo downstream GTPases of the Dock-B subfamily. Drosophila melanogaster is an excellent genetic model organism for understanding Dock protein function as its genome encodes one ortholog per subfamily: Myoblast city (Mbc; Dock A) and Sponge (Spg; Dock B). Here we show that the roles of Spg and Mbc are not redundant in the Drosophila somatic muscle or the dorsal vessel. Moreover, we confirm the in vivo role of Mbc upstream of Rac and provide evidence that Spg functions in concert with Rap1, possibly to regulate aspects of cell adhesion. Together these data show that Mbc and Spg can have different downstream GTPase targets. Our findings predict that the ability to regulate downstream GTPases is dependent on cellular context and allows for the fine-tuning of actin cytoskeletal or cell adhesion events in biological processes that undergo cell morphogenesis.

Cellular Mechanisms of Drosophila Heart Morphogenesis

Many of the major discoveries in the fields of genetics and developmental biology have been made using the fruit fly, Drosophila melanogaster. With regard to heart development, the conserved network of core cardiac transcription factors that underlies cardiogenesis has been studied in great detail in the fly, and the importance of several signaling pathways that regulate heart morphogenesis, such as Slit/Robo, was first shown in the fly model. Recent technological advances have led to a large increase in the genomic data available from patients with congenital heart disease (CHD). This has highlighted a number of candidate genes and gene networks that are potentially involved in CHD. To validate genes and genetic interactions among candidate CHD-causing alleles and to better understand heart formation in general are major tasks. The specific limitations of the various cardiac model systems currently employed (mammalian and fish models) provide a niche for the fly model, despite its evolutionary distance to vertebrates and humans. Here, we review recent advances made using the Drosophila embryo that identify factors relevant for heart formation. These underline how this model organism still is invaluable for a better understanding of CHD.

Cdc42 and formin activity control non-muscle myosin dynamics during Drosophila heart morphogenesis

Cdc42 and the formins dDAAM and Diaphanous play pivotal roles in heart lumen formation through the spatiotemporal regulation of the actomyosin network.

During heart formation, a network of transcription factors and signaling pathways guide cardiac cell fate and differentiation, but the genetic mechanisms orchestrating heart assembly and lumen formation remain unclear. Here, we show that the small GTPase Cdc42 is essential for Drosophila melanogaster heart morphogenesis and lumen formation. Cdc42 genetically interacts with the cardiogenic transcription factor tinman; with dDAAM which belongs to the family of actin organizing formins; and with zipper, which encodes nonmuscle myosin II. Zipper is required for heart lumen formation, and its spatiotemporal activity at the prospective luminal surface is controlled by Cdc42. Heart-specific expression of activated Cdc42, or the regulatory formins dDAAM and Diaphanous caused mislocalization of Zipper and induced ectopic heart lumina, as characterized by luminal markers such as the extracellular matrix protein Slit. Placement of Slit at the lumen surface depends on Cdc42 and formin function. Thus, Cdc42 and formins play pivotal roles in heart lumen formation through the spatiotemporal regulation of the actomyosin network.

Cdc42 is required in a genetically distinct subset of cardiac cells during Drosophila dorsal vessel closure

The embryonic heart tube is formed by the migration and subsequent midline convergence of two bilateral heart fields. In Drosophila the heart fields are organized into two rows of cardioblasts (CBs). While morphogenesis of the dorsal ectoderm, which lies directly above the Drosophila dorsal vessel (DV), has been extensively characterized, the migration and concomitant fundamental factors facilitating DV formation remain poorly understood. Here we provide evidence that DV closure occurs at multiple independent points along the A-P axis of the embryo in a "buttoning" pattern, divergent from the zippering mechanism observed in the overlying epidermis during dorsal closure. Moreover, we demonstrate that a genetically distinct subset of CBs is programmed to make initial contact with the opposing row. To elucidate the cellular mechanisms underlying this process, we examined the role of Rho GTPases during cardiac migration using inhibitory and overexpression approaches. We found that Cdc42 shows striking cell-type specificity during DV formation. Disruption of Cdc42 function specifically prevents CBs that express the homeobox gene tinman from completing their dorsal migration, resulting in a failure to make connections with their partnering CBs. Conversely, neighboring CBs that express the orphan nuclear receptor, seven-up, are not sensitive to Cdc42 inhibition. Furthermore, this phenotype was specific to Cdc42 and was not observed upon perturbation of Rac or Rho function. Together with the observation that DV closure occurs through the initial contralateral pairing of tinman-expressing CBs, our studies suggest that the distinct buttoning mechanism we propose for DV closure is elaborated through signaling pathways regulating Cdc42 activity in this cell type.

Distinct functions of the laminin β LN domain and collagen IV during cardiac extracellular matrix formation and stabilization of alary muscle attachments revealed by EMS mutagenesis in Drosophila

BACKGROUND

The Drosophila heart (dorsal vessel) is a relatively simple tubular organ that serves as a model for several aspects of cardiogenesis. Cardiac morphogenesis, proper heart function and stability require structural components whose identity and ways of assembly are only partially understood. Structural components are also needed to connect the myocardial tube with neighboring cells such as pericardial cells and specialized muscle fibers, the so-called alary muscles.

RESULTS

Using an EMS mutagenesis screen for cardiac and muscular abnormalities in Drosophila embryos we obtained multiple mutants for two genetically interacting complementation groups that showed similar alary muscle and pericardial cell detachment phenotypes. The molecular lesions underlying these defects were identified as domain-specific point mutations in LamininB1 and Cg25C, encoding the extracellular matrix (ECM) components laminin β and collagen IV α1, respectively. Of particular interest within the LamininB1 group are certain hypomorphic mutants that feature prominent defects in cardiac morphogenesis and cardiac ECM layer formation, but in contrast to amorphic mutants, only mild defects in other tissues. All of these alleles carry clustered missense mutations in the laminin LN domain. The identified Cg25C mutants display weaker and largely temperature-sensitive phenotypes that result from glycine substitutions in different Gly-X-Y repeats of the triple helix-forming domain. While initial basement membrane assembly is not abolished in Cg25C mutants, incorporation of perlecan is impaired and intracellular accumulation of perlecan as well as the collagen IV α2 chain is detected during late embryogenesis.

CONCLUSIONS

Assembly of the cardiac ECM depends primarily on laminin, whereas collagen IV is needed for stabilization. Our data underscore the importance of a correctly assembled ECM particularly for the development of cardiac tissues and their lateral connections. The mutational analysis suggests that the β6/β3/β8 interface of the laminin β LN domain is highly critical for formation of contiguous cardiac ECM layers. Certain mutations in the collagen IV triple helix-forming domain may exert a semi-dominant effect leading to an overall weakening of ECM structures as well as intracellular accumulation of collagen and other molecules, thus paralleling observations made in other organisms and in connection with collagen-related diseases.

The evolution and conservation of left-right patterning mechanisms

Morphological asymmetry is a common feature of animal body plans, from shell coiling in snails to organ placement in humans. The signaling protein Nodal is key for determining this laterality. Many vertebrates, including humans, use cilia for breaking symmetry during embryonic development: rotating cilia produce a leftward flow of extracellular fluids that induces the asymmetric expression of Nodal. By contrast, Nodal asymmetry can be induced flow-independently in invertebrates. Here, we ask when and why flow evolved. We propose that flow was present at the base of the deuterostomes and that it is required to maintain organ asymmetry in otherwise perfectly bilaterally symmetrical vertebrates.

Matricellular proteins in development: perspectives from the Drosophila heart

The Drosophila model represents an attractive system in which to study the functional contribution of specific genes to organ development. Within the embryo, the heart tube serves as an informative developmental paradigm to analyze functional aspects of matricellular proteins. Here, we describe two essential extracellular matricellular proteins, Multiplexin (Mp) and Lonely heart (Loh). Each of these proteins contributes to the development and morphogenesis of the heart tube by regulating the activity/localization of essential extracellular proteins. Mp, which is secreted by heart cardioblasts and is specifically distributed in the lumen of the heart tube, binds to the signaling protein Slit, and facilitates its local signaling at the heart's luminal domain. Loh is an ADAMTS-like protein, which serves as an adapter protein to Pericardin (a collagen-like protein), promoting its specific localization at the abluminal domain of the heart tube. We also introduce the Drosophila orthologues of matricellular proteins present in mammals, including Thrombospondin, and SPARC, and discuss a possible role for Teneurins (Ten-A and Ten-M) in the heart. Understanding the role of these proteins provides a novel developmental perspective into the functional contribution of matricellular proteins to organ development.

Methylene blue rescues heart defects in a Drosophila model of Friedreich's ataxia

Friedreich's ataxia (FRDA), the most common hereditary ataxia, is characterized by progressive degeneration of the central and peripheral nervous system, hypertrophic cardiomyopathy and a high risk of diabetes. FRDA is caused by abnormally low levels of frataxin, a highly conserved mitochondrial protein. Drosophila has been previously successfully used to model FRDA in various cell types, including neurons and glial cells. Here, we report the development of a Drosophila cardiac model of FRDA. In vivo heart imaging revealed profound impairments in heart function in frataxin-depleted Drosophila, including a strong increase in end-systolic and end-diastolic diameters and a decrease in fractional shortening (FS). These features, reminiscent of pathological phenotypes in humans, are fully rescued by complementation with human frataxin, suggesting conserved cardiac functions of frataxin between the two organisms. Oxidative stress is not a major factor of heart impairment in frataxin-depleted flies, suggesting the involvement of other pathological mechanisms notably mitochondrial respiratory chain (MRC) dysfunction. Accordingly, we report that methylene blue (MB), a compound known to act as an alternative electron carrier that bypasses mitochondrial complexes I-III, was able to prevent heart dysfunction. MB also partially rescued the phenotype when administered post-symptomatically. Analysis of MB derivatives demonstrates that only compounds with electron carrier properties are able to prevent the heart phenotype. Thus MB, a compound already used for several clinical applications, appears promising for the treatment of the heart dysfunctions that are a major cause of death of FRDA patients. This work provides the grounds for further evaluation of MB action in mammals.

The ultrastructure of the Aedes aegypti heart

Comparative structural analyses of the heart and associated tissues in 4th instar larvae (L4), pupae and adults of Aedes aegypti were undertaken using a combination of microscopy techniques. The Ae. aegypti heart consists of cardiomyocytes arranged in a helical fashion, and it is physically associated with intersegmental groups of pericardial cells (PCs) and the alary muscles (AMs). Ramifications commonly present in AMs are more developed in adults than in the immature stages. Pericardial cells absorb and store extracellular components as shown by the uptake of carmine dye fed in larval diet. We also observed that carmine stained inclusions corresponding to electron-dense structures resembling lysosomes that were more abundant and prominent in pupae, suggestive of increase of waste accumulation during pupation. The results presented here expand on previously known aspects of the mosquito heart and describe for the first time comparative aspects of the morphology of the heart in different developmental stages.

Getting to the heart of the matter: long non-coding RNAs in cardiac development and disease

Cardiogenesis in mammals requires exquisite control of gene expression and faulty regulation of transcriptional programs underpins congenital heart disease (CHD), the most common defect among live births. Similarly, many adult cardiac diseases involve transcriptional changes and sometimes have a developmental basis. Long non-coding RNAs (lncRNAs) are a novel class of transcripts that regulate cellular processes by controlling gene expression; however, detailed insights into their biological and mechanistic functions are only beginning to emerge. Here, we discuss recent findings suggesting that lncRNAs are important factors in regulation of mammalian cardiogenesis and in the pathogenesis of CHD as well as adult cardiac disease. We also outline potential methodological and conceptual considerations for future studies of lncRNAs in the heart and other contexts.

Evolutionary origins of the blood vascular system and endothelium

Every biological trait requires both a proximate and evolutionary explanation. The field of vascular biology is focused primarily on proximate mechanisms in health and disease. Comparatively little attention has been given to the evolutionary basis of the cardiovascular system. Here, we employ a comparative approach to review the phylogenetic history of the blood vascular system and endothelium. In addition to drawing on the published literature, we provide primary ultrastructural data related to the lobster, earthworm, amphioxus, and hagfish. Existing evidence suggests that the blood vascular system first appeared in an ancestor of the triploblasts over 600 million years ago, as a means to overcome the time-distance constraints of diffusion. The endothelium evolved in an ancestral vertebrate some 540-510 million years ago to optimize flow dynamics and barrier function, and/or to localize immune and coagulation functions. Finally, we emphasize that endothelial heterogeneity evolved as a core feature of the endothelium from the outset, reflecting its role in meeting the diverse needs of body tissues.

Fermitins, the orthologs of mammalian Kindlins, regulate the development of a functional cardiac syncytium in Drosophila melanogaster

The vertebrate Kindlins are an evolutionarily conserved family of proteins critical for integrin signalling and cell adhesion. Kindlin-2 (KIND2) is associated with intercalated discs in mice, suggesting a role in cardiac syncytium development; however, deficiency of Kind2 leads to embryonic lethality. Morpholino knock-down of Kind2 in zebrafish has a pleiotropic effect on development that includes the heart. It therefore remains unclear whether cardiomyocyte Kind2 expression is required for cardiomyocyte junction formation and the development of normal cardiac function. To address this question, the expression of Fermitin 1 and Fermitin 2 (Fit1, Fit2), the two Drosophila orthologs of Kind2, was silenced in Drosophila cardiomyocytes. Heart development was assessed in adult flies by immunological methods and videomicroscopy. Silencing both Fit1 and Fit2 led to a severe cardiomyopathy characterised by the failure of cardiomyocytes to develop as a functional syncytium and loss of synchrony between cardiomyocytes. A null allele of Fit1 was generated but this had no impact on the heart. Similarly, the silencing of Fit2 failed to affect heart function. In contrast, the silencing of Fit2 in the cardiomyocytes of Fit1 null flies disrupted syncytium development, leading to severe cardiomyopathy. The data definitively demonstrate a role for Fermitins in the development of a functional cardiac syncytium in Drosophila. The findings also show that the Fermitins can functionally compensate for each other in order to control syncytium development. These findings support the concept that abnormalities in cardiomyocyte KIND2 expression or function may contribute to cardiomyopathies in humans.

The evolutionary origins of chordate hematopoiesis and vertebrate endothelia

The vertebrate circulatory system is the most complex vascular system among those of metazoans, with key innovations including a multi-chambered heart and highly specialized blood cells. Invertebrate vessels, on the other hand, consist of hemal spaces between the basal laminae of epithelia. How the evolutionary transition from an invertebrate-type system to the complex vertebrate one occurred is, however, poorly understood. We investigate here the development of the cardiovascular system of the cephalochordate amphioxus Branchiostoma lanceolatum in order to gain insight into the origin of the vertebrate cardiovascular system. The cardiac markers Hand, Csx (Nkx2-5) and Tbx4/5 reveal a broad cardiac-like domain in amphioxus; such a decentralized organization during development parallels that seen in the adult anatomy. Our data therefore support the hypothesis that amphioxus never possessed a proper heart, even transiently during development. We also define a putative hematopoietic domain, supported by the expression of the hematopoietic markers Scl and Pdvegfr. We show that this area is closed to the dorsal aorta anlages, partially linked to excretory tissues, and that its development is regulated by retinoic acid, thus recalling the aorta-gonads-mesonephros (AGM) area of vertebrates. This region probably produces Pdvegfr+ hemal cells, with an important role in amphioxus vessel formation, since treatments with an inhibitor of PDGFR/VEGFR lead to a decrease of Laminin in the basal laminae of developing vessels. Our results point to a chordate origin of hematopoiesis in an AGM-like area from where hemal Pdvegfr+ cells are produced. These Pdvegfr+ cells probably resemble the ancestral chordate blood cells from which the vertebrate endothelium later originated.

Genes and networks regulating cardiac development and function in flies: genetic and functional genomic approaches

The Drosophila heart has emerged as a powerful model system for cardiovascular research. This simple organ, composed of only 104 cardiomyocytes and associated pericardiac cells, has been the focus of numerous candidate gene approaches in the last 2 decades, which have unraveled a number of transcription factors and signaling pathways involved in the regulation of early cardiac development. Importantly, these regulators seem to have largely conserved functions in mammals. Recent studies also demonstrated the usefulness of the fly circulatory system to investigate molecular mechanisms involved in the control of the establishment and maintenance of the cardiac function. In this review, we have focused on how new technological and conceptual advances in the field of functional genomics have impacted research on the cardiovascular system in Drosophila. Genome-scale genetic screens were conducted taking advantage of recently developed ribonucleic acid interference transgenic lines and molecularly defined genetic deficiencies, which have provided new insights into the genetics of both the developmental control of heart formation and cardiac function. In addition, a comprehensive picture of the transcriptional network controlling heart formation is emerging, thanks to newly developed genomic approaches which allow global and unbiased identification of the underlying components of gene regulatory circuits.

Growth factors, nutrient signaling, and cardiovascular aging

Growth factors regulated by specific macronutrients have been shown to promote aging and accelerate mortality in the majority of the organisms studied. In particular, the enzymes activated by growth hormone, insulin, and insulin-like growth factor-1 in mammals and their orthologs in simple model organisms represent perhaps the best-understood proteins involved in the aging process. Dietary restriction, which reduces the level of insulin-like growth factor-1 and of other growth factors, has been associated with protection from diabetes, cancer, and cardiovascular diseases, and deficiencies in growth hormone signaling and insulin-like growth factor-1 are strongly associated with protection from cancer and diabetes in both mice and humans; however, their role in cardiac function and cardiovascular diseases is controversial. Here, we review the link between growth factors, cardiac function, and heart disease with focus on the cardioprotective and sensitizing effect of growth factors in both model organisms and humans.

Integrins are required for cardioblast polarisation in Drosophila

BACKGROUND

The formation of a tubular organ, such as the heart, requires the communication of positional and polarity signals between migratory cells. Key to this process is the establishment of a new luminal domain on the cell surface, generally from the apical domain of a migratory cell. This domain will also acquire basal properties, as it will produce a luminal extracellular matrix. Integrin receptors are the primary means of cell adhesion and adhesion signaling with the extracellular matrix. Here we characterise the requirement of Integrins in a genetic model of vasculogenesis, the formation of the heart in Drosophila.

RESULTS

As with vertebrates, the Drosophila heart arises from lateral mesoderm that migrates medially to meet their contralateral partners, to then assemble a midline vessel. During migration, Integrins are among the first proteins restricted to the presumptive luminal domain of cardioblasts. Integrins are required for normal levels of leading edge membrane motility. Apical accumulation of Integrins is enhanced by Robo, and reciprocally, apicalisation of luminal factors like Slit and Robo requires Integrin function. Integrins may provide a template for the formation of a lumen by stabilising lumen factors like Robo. Subsequent to migration, Integrin is required for normal cardioblast alignment and lumen formation. This phenotype is most readily modified by other mutations that affect adhesion, such as Talin and extracellular matrix ligands.

CONCLUSION

Our findings reveal an instructive role for Integrins in communicating polarising information to cells during migration, and during transition to an epithelial tube structure.

Spire, an actin nucleation factor, regulates cell division during Drosophila heart development

The Drosophila dorsal vessel is a beneficial model system for studying the regulation of early heart development. Spire (Spir), an actin-nucleation factor, regulates actin dynamics in many developmental processes, such as cell shape determination, intracellular transport, and locomotion. Through protein expression pattern analysis, we demonstrate that the absence of spir function affects cell division in Myocyte enhancer factor 2-, Tinman (Tin)-, Even-skipped- and Seven up (Svp)-positive heart cells. In addition, genetic interaction analysis shows that spir functionally interacts with Dorsocross, tin, and pannier to properly specify the cardiac fate. Furthermore, through visualization of double heterozygous embryos, we determines that spir cooperates with CycA for heart cell specification and division. Finally, when comparing the spir mutant phenotype with that of a CycA mutant, the results suggest that most Svp-positive progenitors in spir mutant embryos cannot undergo full cell division at cell cycle 15, and that Tin-positive progenitors are arrested at cell cycle 16 as double-nucleated cells. We conclude that Spir plays a crucial role in controlling dorsal vessel formation and has a function in cell division during heart tube morphogenesis.

A novel method for obtaining semi-thin cross sections of the Drosophila heart and their labeling with multiple antibodies

The Drosophila heart has become an exciting model for elucidating the molecular basis for cardiac function in higher organisms. To complement the genetic approaches that have recently identified an array of genes essential for cardiac function, we developed a method to obtain optimal semi-thin cross sections of embryonic, larval, and adult fly hearts in a desired orientation. A procedure for fluorescent labeling of these sections with multiple markers has also been developed, allowing the detection of proteins at high subcellular resolution. Sections obtained by our method reveal changes in cell shape between embryonic heart and aorta cardioblasts and elucidate the morphology of the adult heart. Analysis of the adult heart reveals the precise cardiac tube morphology, differential distribution of the extracellular matrix protein Laminin within the cardiac tube, as well as individual hand-positive, and Held Out Wings (HOW)-positive luminal cells that might represent blood cells. In summary, our method enables visualization of cross sections of the embryonic and adult hearts at high resolution while maintaining the ability to co-label the sections with multiple markers, thereby facilitating the analysis of cardiac tube formation and maintenance at different developmental stages.

Syndecan contributes to heart cell specification and lumen formation during Drosophila cardiogenesis

The transmembrane proteoglycan Syndecan contributes to cell surface signaling of diverse ligands in mammals, yet in Drosophila, genetic evidence links Syndecan only to the Slit receptor Roundabout and to the receptor tyrosine phosphatase LAR. Here we characterize the requirement for syndecan in the determination and morphogenesis of the Drosophila heart, and reveal two phases of activity, indicating that Syndecan is a co-factor in at least two signaling events in this tissue. There is a stochastic failure to determine heart cell progenitors in a subset of abdominal hemisegments in embryos mutant for syndecan, and subsequent to Syndecan depletion by RNA interference. This phenotype is sensitive to gene dosage in the FGF receptor (Heartless), its ligand, Pyramus, as well as BMP-ligand Decapentaplegic (Dpp) and co-factor Sara. Syndecan is also required for lumen formation during assembly of the heart vessel, a phenotype shared with mutations in the Slit and Integrin signaling pathways. Phenotypic interactions of syndecan with slit and Integrin mutants suggest intersecting function, consistent with Syndecan acting as a co-receptor for Slit in the Drosophila heart.

Cardiac remodeling in Drosophila arises from changes in actin gene expression and from a contribution of lymph gland-like cells to the heart musculature

Understanding the basis of normal heart remodeling can provide insight into the plasticity of the cardiac state, and into the potential for treating diseased tissue. In Drosophila, the adult heart arises during metamorphosis from a series of events, that include the remodeling of an existing cardiac tube, the elaboration of new inflow tracts, and the addition of a layer of longitudinal muscle fibers. We have identified genes active in all these three processes, and studied their expression in order to characterize in greater detail normal cardiac remodeling. Using a Transglutaminase-lacZ transgenic line, that is expressed in the inflow tracts of the larval and adult heart, we confirm the existence of five inflow tracts in the adult structure. In addition, expression of the Actin87E actin gene is initiated in the remodeling cardiac tube, but not in the longitudinal fibers, and we have identified an Act87E promoter fragment that recapitulates this switch in expression. We also establish that the longitudinal fibers are multinucleated, characterizing these cells as specialized skeletal muscles. Furthermore, we have defined the origin of the longitudinal fibers, as a subset of lymph gland cells associated with the larval dorsal vessel. These studies underline the myriad contributors to the formation of the adult Drosophila heart, and provide new molecular insights into the development of this complex organ.

Expression of Slit and Robo genes in the developing mouse heart

Development of the mammalian heart is mediated by complex interactions between myocardial, endocardial, and neural crest-derived cells. Studies in Drosophila have shown that the Slit-Robo signaling pathway controls cardiac cell shape changes and lumen formation of the heart tube. Here, we demonstrate by in situ hybridization that multiple Slit ligands and Robo receptors are expressed in the developing mouse heart. Slit3 is the predominant ligand transcribed in the early mouse heart and is expressed in the ventral wall of the linear heart tube and subsequently in chamber but not in atrioventricular canal myocardium. Furthermore, we identify that the homeobox gene Nkx2-5 is required for early ventral restriction of Slit3 and that the T-box transcription factor Tbx2 mediates repression of Slit3 in nonchamber myocardium. Our results suggest that patterned Slit-Robo signaling may contribute to the control of oriented cell growth during chamber morphogenesis of the mammalian heart.

The transmembrane receptor Uncoordinated5 (Unc5) is essential for heart lumen formation in Drosophila melanogaster

Transport of liquids or gases in biological tubes is fundamental for many physiological processes. Our knowledge on how tubular organs are formed during organogenesis and tissue remodeling has increased dramatically during the last decade. Studies on different animal systems have helped to unravel some of the molecular mechanisms underlying tubulogenesis. Tube architecture varies dramatically in different organs and different species, ranging from tubes formed by several cells constituting the cross section, tubes formed by single cells wrapping an internal luminal space or tubes that are formed within a cell. Some tubes display branching whereas others remain linear without intersections. The modes of shaping, growing and pre-patterning a tube are also different and it is still not known whether these diverse architectures and modes of differentiation are realized by sharing common signaling pathways or regulatory networks. However, several recent investigations provide evidence for the attractive hypothesis that the Drosophila cardiogenesis and heart tube formation shares many similarities with primary angiogenesis in vertebrates. Additionally, another important step to unravel the complex system of lumen formation has been the outcome of recent studies that junctional proteins, matrix components as well as proteins acting as attractant and repellent cues play a role in the formation of the Drosophila heart lumen. In this study we show the requirement for the repulsively active Unc5 transmembrane receptor to facilitate tubulogenesis in the dorsal vessel of Drosophila. Unc5 is localized in the luminal membrane compartment of cardiomyocytes and animals lacking Unc5 fail to form a heart lumen. Our findings support the idea that Unc5 is crucial for lumen formation and thereby represents a repulsive cue acting during Drosophila heart tube formation.

Response to mechanical stress is mediated by the TRPA channel painless in the Drosophila heart

Mechanotransduction modulates cellular functions as diverse as migration, proliferation, differentiation, and apoptosis. It is crucial for organ development and homeostasis and leads to pathologies when defective. However, despite considerable efforts made in the past, the molecular basis of mechanotransduction remains poorly understood. Here, we have investigated the genetic basis of mechanotransduction in Drosophila. We show that the fly heart senses and responds to mechanical forces by regulating cardiac activity. In particular, pauses in heart activity are observed under acute mechanical constraints in vivo. We further confirm by a variety of in situ tests that these cardiac arrests constitute the biological force-induced response. In order to identify molecular components of the mechanotransduction pathway, we carried out a genetic screen based on the dependence of cardiac activity upon mechanical constraints and identified Painless, a TRPA channel. We observe a clear absence of in vivo cardiac arrest following inactivation of painless and further demonstrate that painless is autonomously required in the heart to mediate the response to mechanical stress. Furthermore, direct activation of Painless is sufficient to produce pauses in heartbeat, mimicking the pressure-induced response. Painless thus constitutes part of a mechanosensitive pathway that adjusts cardiac muscle activity to mechanical constraints. This constitutes the first in vivo demonstration that a TRPA channel can mediate cardiac mechanotransduction. Furthermore, by establishing a high-throughput system to identify the molecular players involved in mechanotransduction in the cardiovascular system, our study paves the way for understanding the mechanisms underlying a mechanotransduction pathway.

Cells sense mechanical forces and design an appropriate response crucial for cell and organ shape and differentiation during development, as well as for physiological adaptation. In particular, cardiac muscle continuously adapts to the mechanical constraints generated by its own rhythmic contractile activity. Consequently, defects in mechanosensation lead to severe pathologies, including cardiomyopathies and atherosclerosis. However, despite their well recognized functional importance, the molecular mechanisms of mechanotransduction are poorly understood. Here we study the Drosophila heart to investigate the genetic basis of mechanotransduction. We show that the heart responds to mechanical constraints by diastolic heart arrests, and we demonstrate that this phenotype can be used to identify genes controlling this particular mechanotransduction pathway. We show that the cation channel, Painless, first identified in the pain response pathway, also plays an essential function in the mechanotransduction pathway. The model system we have developed allows, for the first time, analysis of gene function in a mechanotransduction process in vivo, in the presence of endogenous mechanical constraints. These results establish the basis for an in-depth characterization of mechanotransduction pathways.