Neuromancer Tbx20-related genes (H15/midline) promote cell fate specification and morphogenesis of the Drosophila heart

Posttranscriptional regulation of the T-box gene midline via the 3'UTR in Drosophila is complex and cell- and tissue-dependent

The T-box (Tbx) proteins have a 180-230 amino acid DNA-binding domain, first reported in the Brachyury (T) protein. They are highly conserved among metazoans. They regulate a multitude of cellular functions in development and disease. Here, we report posttranscriptional and translational regulation of midline (mid), a Tbx member in Drosophila. We found that the 3'UTR of mid has mRNA degradation elements and AT-rich sequences. In Schneider S2 cells, mid-mRNA could be detected only when the transgene was without the 3'UTR. Similarly, the 3'UTR linked to the Renilla luciferase reporter significantly reduced the activity of the Luciferase, whereas deleting only the degradation elements from the 3'UTR resulted in reduced activity, but not as much. Overexpression of mid in MP2, an embryonic neuroblast, showed no significant difference in the levels of mid-mRNA between the 2 transgenes, with and without the 3'UTR, indicating the absence of posttranscriptional regulation of mid in MP2. Moreover, while elevated mid-RNA was detected in MP2 in nearly all hemisegments, only a fifth of those hemisegments had elevated levels of the protein. Overexpression of the 2 transgenes resulted in MP2-lineage defects at about the same frequency. These results indicate a translational/posttranslational regulation of mid in MP2. The regulation of ectopically expressed mid in the wing imaginal disc was complex. In the wing disc, where mid is not expressed, the ectopic expression of the transgene lacking the 3'UTR had a higher level of mid-RNA and the protein had a stronger phenotypic effect. These results indicate that the 3'UTR can subject mid-mRNA to degradation in a cell- and tissue-specific manner. We further report a balancer-mediated transgenerational modifier effect on the expression and gain of function effects of the 2 transgenes.

Single-cell profiling of the developing embryonic heart in Drosophila

Drosophila is an important model for studying heart development and disease. Yet, single-cell transcriptomic data of its developing heart have not been performed. Here, we report single-cell profiling of the entire fly heart using ∼3000 Hand-GFP embryos collected at five consecutive developmental stages, ranging from bilateral migrating rows of cardiac progenitors to a fused heart tube. The data revealed six distinct cardiac cell types in the embryonic fly heart: cardioblasts, both Svp+ and Tin+ subtypes; and five types of pericardial cell (PC) that can be distinguished by four key transcription factors (Eve, Odd, Ct and Tin) and include the newly described end of the line PC. Notably, the embryonic fly heart combines transcriptional signatures of the mammalian first and second heart fields. Using unique markers for each heart cell type, we defined their number and location during heart development to build a comprehensive 3D cell map. These data provide a resource to track the expression of any gene in the developing fly heart, which can serve as a reference to study genetic perturbations and cardiac diseases.

Shared cis-regulatory modules control expression of the tandem paralogs midline and H15 in the follicular epithelium

The posterior end of the follicular epithelium is patterned by midline (MID) and its paralog H15, the Drosophila homologs of the mammalian Tbx20 transcription factor. We have previously identified two cis-regulatory modules (CRMs) that recapitulate the endogenous pattern of mid in the follicular epithelium. Here, using CRISPR/Cas9 genome editing, we demonstrate redundant activity of these mid CRMs. Although the deletion of either CRM alone generated marginal change in mid expression, the deletion of both CRMs reduced expression by 60%. Unexpectedly, the deletion of the 5' proximal CRM of mid eliminated H15 expression. Interestingly, expression of these paralogs in other tissues remained unaffected in the CRM deletion backgrounds. These results suggest that the paralogs are regulated by a shared CRM that coordinates gene expression during posterior fate determination. The consistent overlapping expression of mid and H15 in various tissues may indicate that the paralogs could also be under shared regulation by other CRMs in these tissues.

The Role of Tbx20 in Cardiovascular Development and Function

Tbx20 is a member of the Tbx1 subfamily of T-box-containing genes and is known to play a variety of fundamental roles in cardiovascular development and homeostasis as well as cardiac remodeling in response to pathophysiological stresses. Mutations in TBX20 are widely associated with the complex spectrum of congenital heart defects (CHDs) in humans, which includes defects in chamber septation, chamber growth, and valvulogenesis. In addition, genetic variants of TBX20 have been found to be associated with dilated cardiomyopathy and heart arrhythmia. This broad spectrum of cardiac morphogenetic and functional defects is likely due to its broad expression pattern in multiple cardiogenic cell lineages and its critical regulation of transcriptional networks during cardiac development. In this review, we summarize recent findings in our general understanding of the role of Tbx20 in regulating several important aspects of cardiac development and homeostasis and heart function.

Heart Development and Regeneration in Non-mammalian Model Organisms

Cardiovascular disease is a serious threat to human health and a leading cause of mortality worldwide. Recent years have witnessed exciting progress in the understanding of heart formation and development, enabling cardiac biologists to make significant advance in the field of therapeutic heart regeneration. Most of our understanding of heart development and regeneration, including the genes and signaling pathways, are driven by pioneering works in non-mammalian model organisms, such as fruit fly, fish, frog, and chicken. Compared to mammalian animal models, non-mammalian model organisms have special advantages in high-throughput applications such as disease modeling, drug discovery, and cardiotoxicity screening. Genetically engineered animals of cardiovascular diseases provide valuable tools to investigate the molecular and cellular mechanisms of pathogenesis and to evaluate therapeutic strategies. A large number of congenital heart diseases (CHDs) non-mammalian models have been established and tested for the genes and signaling pathways involved in the diseases. Here, we reviewed the mechanisms of heart development and regeneration revealed by these models, highlighting the advantages of non-mammalian models as tools for cardiac research. The knowledge from these animal models will facilitate therapeutic discoveries and ultimately serve to accelerate translational medicine.

Restriction on self-renewing asymmetric division is coupled to terminal asymmetric division in the Drosophila CNS

Neuronal precursor cells undergo self-renewing and non-self-renewing asymmetric divisions to generate a large number of neurons of distinct identities. In Drosophila, primary precursor neuroblasts undergo a varying number of self-renewing asymmetric divisions, with one known exception, the MP2 lineage, which undergoes just one terminal asymmetric division similar to the secondary precursor cells. The mechanism and the genes that regulate the transition from self-renewing to non-self-renewing asymmetric division or the number of times a precursor divides is unknown. Here, we show that the T-box transcription factor, Midline (Mid), couples these events. We find that in mid loss of function mutants, MP2 undergoes additional self-renewing asymmetric divisions, the identity of progeny neurons generated dependent upon Numb localization in the parent MP2. MP2 expresses Mid transiently and an over-expression of mid in MP2 can block its division. The mechanism which directs the self-renewing asymmetric division of MP2 in mid involves an upregulation of Cyclin E. Our results indicate that Mid inhibits cyclin E gene expression by binding to a variant Mid-binding site in the cyclin E promoter and represses its expression without entirely abolishing it. Consistent with this, over-expression of cyclin E in MP2 causes its multiple self-renewing asymmetric division. These results reveal a Mid-regulated pathway that restricts the self-renewing asymmetric division potential of cells via inhibiting cyclin E and facilitating their exit from cell cycle.

Nerve cells in the brain, spinal cord, gut and so on in all organisms are generated from stem cells. These primary cells divide to self-renew and at the same time generate a secondary precursor cell that terminally divides to produce two cells that differentiate into neurons of different identities, or glial cells or a neuron and a glia. The secondary cells never self-renew, the reason for which is not known. We found that in embryos that lack the activity of a gene called midline, precursors such as MP2 that normally divides into two neurons, self-renews and generates a neuron at the same time. The identity of the differentiating progeny is tied to how the asymmetrically localized determinant Numb is distributed in the precursor cell. When this gene, midline, is over expressed, it blocks MP2 division. The way Midline protein works is that it represses the cyclin E gene via binding to sites in its promoter, preventing the over-expression of Cyclin E and thus blocking cells from entering the cell cycle. A deregulation of cyclin E as in loss of function midline mutants allows one of the daughter cells of MP2 to re-enter cell cycle as MP2, just as an over-expression of the cyclin E gene also does. These results show a mechanism by which restriction on self-renewing asymmetric division is coupled to terminal asymmetric division and works through Midline and Cyclin E. This work addresses one of the fundamental problems is biology.

Diversification of heart progenitor cells by EGF signaling and differential modulation of ETS protein activity

For coordinated circulation, vertebrate and invertebrate hearts require stereotyped arrangements of diverse cell populations. This study explores the process of cardiac cell diversification in the Drosophila heart, focusing on the two major cardioblast subpopulations: generic working myocardial cells and inflow valve-forming ostial cardioblasts. By screening a large collection of randomly induced mutants, we identified several genes involved in cardiac patterning. Further analysis revealed an unexpected, specific requirement of EGF signaling for the specification of generic cardioblasts and a subset of pericardial cells. We demonstrate that the Tbx20 ortholog Midline acts as a direct target of the EGFR effector Pointed to repress ostial fates. Furthermore, we identified Edl/Mae, an antagonist of the ETS factor Pointed, as a novel cardiac regulator crucial for ostial cardioblast specification. Combining these findings, we propose a regulatory model in which the balance between activation of Pointed and its inhibition by Edl controls cardioblast subtype-specific gene expression.

Organs contain many different kinds of cells, each specialised to perform a particular role. The fruit fly heart, for example, has two types of muscle cells: generic heart muscle cells and ostial heart muscle cells. The generic cells contract to force blood around the body, whilst the ostial cells form openings that allow blood to enter the heart. Though both types of cells carry the same genetic information, each uses a different combination of active genes to perform their role.

During development, the cells must decide whether to become generic or ostial. They obtain signals from other cells in and near the developing heart, and respond by turning genes on or off. The response uses proteins called transcription factors, which bind to regulatory portions of specific genes.

The sequence of signals and transcription factors that control the fate of developing heart muscle cells was not known. So Schwarz et al. examined the process using a technique called a mutagenesis screen. This involved triggering random genetic mutations and looking for flies with defects in their heart muscle cells. Matching the defects to the mutations revealed genes responsible for heart development.

Schwarz et al. found that for cells to develop into generic heart muscle cells, a signal called epidermal growth factor (EGF) switches on a transcription factor called Pointed in the cells. Pointed then turns on another transcription factor that switches off the genes for ostial cells. Conversely, ostial heart muscle cells develop when a protein called ‘ETS-domain lacking’ (Edl) interferes with Pointed, allowing the ostial genes to remain on. The balance between Pointed and Edl controls which type of heart cell each cell will become.

Many cells in other tissues in fruit flies also produce the Pointed and Edl proteins and respond to EGF signals. This means that this system may help to decide the fate of cells in other organs. The EGF signaling system is also present in other animals, including humans. Future work could reveal whether the same molecular decision making happens in our own hearts.

Tbx20 drives cardiac progenitor formation and cardiomyocyte proliferation in zebrafish

Tbx20 is a T-box transcription factor that plays essential roles in the development and maintenance of the heart. Although it is expressed by cardiac progenitors in all species examined, an involvement of Tbx20 in cardiac progenitor formation in vertebrates has not been previously described. Here we report the identification of a zebrafish tbx20 mutation that results in an inactive, truncated protein lacking any functional domains. The cardiac progenitor population is strongly diminished in this mutant, leading to the formation of a small, stretched-out heart. We found that overexpression of Tbx20 results in an enlarged heart with significantly more cardiomyocytes. Interestingly, this increase in cell number is caused by both enhanced cardiac progenitor cell formation and the proliferation of differentiated cardiomyocytes, and is dependent upon the activity of Tbx20's T-box and transcription activation domains. Together, our findings highlight a previously unappreciated role for Tbx20 in promoting cardiac progenitor formation in vertebrates and reveal a novel function for its activation domain in cardiac cell proliferation during embryogenesis.

Tbx20 Is an Essential Regulator of Embryonic Heart Growth in Zebrafish

The molecular mechanisms that regulate cardiomyocyte proliferation during embryonic heart growth are not completely deciphered yet. In a forward genetic N-ethyl-N-nitrosourea (ENU) mutagenesis screen, we identified the recessive embryonic-lethal zebrafish mutant line weiches herz (whz). Homozygous mutant whz embryos display impaired heart growth due to diminished embryonic cardiomyocyte proliferation resulting in cardiac hypoplasia and weak cardiac contraction. By positional cloning, we found in whz mutant zebrafish a missense mutation within the T-box 20 (Tbx20) transcription factor gene leading to destabilization of Tbx20 protein. Morpholino-mediated knock-down of Tbx20 in wild-type zebrafish embryos phenocopies whz, indicating that the whz phenotype is due to loss of Tbx20 function, thereby leading to significantly reduced cardiomyocyte numbers by impaired proliferation of heart muscle cells. Ectopic overexpression of wild-type Tbx20 in whz mutant embryos restored cardiomyocyte proliferation and heart growth. Interestingly, ectopic overexpression of Tbx20 in wild-type zebrafish embryos resulted, similar to the situation in the embryonic mouse heart, in significantly reduced proliferation rates of ventricular cardiomyocytes, suggesting that Tbx20 activity needs to be tightly fine-tuned to guarantee regular cardiomyocyte proliferation and embryonic heart growth in vivo.

The T-box transcription factor Midline regulates wing development by repressing wingless and hedgehog in Drosophila

Wingless (Wg) and Hedgehog (Hh) signaling pathways are key players in animal development. However, regulation of the expression of wg and hh are not well understood. Here, we show that Midline (Mid), an evolutionarily conserved transcription factor, expresses in the wing disc of Drosophila and plays a vital role in wing development. Loss or knock down of mid in the wing disc induced hyper-expression of wingless (wg) and yielded cocked and non-flat wings. Over-expression of mid in the wing disc markedly repressed the expression of wg, DE-Cadherin (DE-Cad) and armadillo (arm), and resulted in a small and blistered wing. In addition, a reduction in the dose of mid enhanced phenotypes of a gain-of-function mutant of hedgehog (hh). We also observed repression of hh upon overexpression of mid in the wing disc. Taken together, we propose that Mid regulates wing development by repressing wg and hh in Drosophila.

Transcriptional Regulation of Heart Development in Zebrafish

Cardiac transcription factors orchestrate the complex cellular and molecular events required to produce a functioning heart. Misregulation of the cardiac transcription program leads to embryonic developmental defects and is associated with human congenital heart diseases. Recent studies have expanded our understanding of the regulation of cardiac gene expression at an additional layer, involving the coordination of epigenetic and transcriptional regulators. In this review, we highlight and discuss discoveries made possible by the genetic and embryological tools available in the zebrafish model organism, with a focus on the novel functions of cardiac transcription factors and epigenetic and transcriptional regulatory proteins during cardiogenesis.

Adaptation and detoxification mechanisms of Vetiver grass (Chrysopogon zizanioides) growing on gold mine tailings

Vetiver grass (Chrysopogon zizanioides) was investigated for its potential use in the rehabilitation of gold mine tailings, its ability to extract and accumulate toxic metals from the tailings and its metal tolerant strategies. Vetiver grass was grown on gold mine tailings soil, in a hothouse, and monitored for sixteen weeks. The mine tailings were highly acidic and had high electrical conductivity. Vetiver grass was able to grow and adapt well on gold mine tailings. The results showed that Vetiver grass accumulated large amounts of metals in the roots and restricted their translocation to the shoots. This was confirmed by the bioconcentration factor of Zn, Cu, and Ni of >1 and the translocation factor of <1 for all the metals. This study revealed the defense mechanisms employed by Vetiver grass against metal stress that include: chelation of toxic metals by phenolics, glutathione S-tranferase, and low molecular weight thiols; sequestration and accumulation of metals within the cell wall that was revealed by the scanning electron microscopy that showed closure of stomata and thickened cell wall and was confirmed by high content of cell wall bound phenolics. Metal induced reactive oxygen species are reduced or eliminated by catalase, superoxide dismutase and peroxidase dismutase.

The drosophila T-box transcription factor midline functions within Insulin/Akt and c-Jun-N terminal kinase stress-reactive signaling pathways to regulate interommatial bristle formation and cell survival

We recently reported that the T-box transcription factor midline (mid) functions within the Notch-Delta signaling pathway to specify sensory organ precursor (SOP) cell fates in early-staged pupal eye imaginal discs and to suppress apoptosis (Das et al.). From genetic and allelic modifier screens, we now report that mid interacts with genes downstream of the insulin receptor(InR)/Akt, c-Jun-N-terminal kinase (JNK) and Notch signaling pathways to regulate interommatidial bristle (IOB) formation and cell survival. One of the most significant mid-interacting genes identified from the modifier screen is dFOXO, a transcription factor exhibiting a nucleocytoplasmic subcellular distribution pattern. In common with dFOXO, we show that Mid exhibits a nucleocytoplasmic distribution pattern within WT third-instar larval (3(o)L) tissue homogenates. Because dFOXO is a stress-responsive factor, we assayed the effects of either oxidative or metabolic stress responses on modifying the mid mutant phenotype which is characterized by a 50% loss of IOBs within the adult compound eye. While metabolic starvation stress does not affect the mid mutant phenotype, either 1 mM paraquat or 20% coconut oil, oxidative stress inducers, partially suppresses the mid mutant phenotype resulting in a significant recovery of IOBs. Another significant mid-interacting gene we identified is groucho (gro). Mid and Gro are predicted to act as corepressors of the enhancer-of-split gene complex downstream of Notch. Immunolabeling WT and dFOXO null 3(o)L eye-antennal imaginal discs with anti-Mid and anti-Engrailed (En) antibodies indicate that dFOXO is required to activate Mid and En expression within photoreceptor neurons of the eye disc. Taken together, these studies show that Mid and dFOXO serve as critical effectors of cell fate specification and survival within integrated Notch, InR/dAkt, and JNK signaling pathways during 3(o)L and pupal eye imaginal disc development.

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.

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.

Visualizing late insect embryogenesis: extraembryonic and mesodermal enhancer trap expression in the beetle Tribolium castaneum

The beetle Tribolium castaneum has increasingly become a powerful model for comparative research on insect development. One recent resource is a collection of piggyBac transposon-based enhancer trap lines. Here, we provide a detailed analysis of three selected lines and demonstrate their value for investigations in the second half of embryogenesis, which has thus far lagged behind research on early stages. Two lines, G12424 and KT650, show enhanced green fluorescent protein (EGFP) expression throughout the extraembryonic serosal tissue and in a few discrete embryonic domains. Intriguingly, both lines show for the first time a degree of regionalization within the mature serosa. However, their expression profiles illuminate distinct aspects of serosal biology: G12424 tracks the tissue's rapid maturation while KT650 expression likely reflects ongoing physiological processes. The third line, G04609, is stably expressed in mesodermal domains, including segmental muscles and the heart. Genomic mapping followed by in situ hybridization for genes near to the G04609 insertion site suggests that the transposon has trapped enhancer information for the Tribolium orthologue of midline (Tc-mid). Altogether, our analyses provide the first live imaging, long-term characterizations of enhancer traps from this collection. We show that EGFP expression is readily detected, including in heterozygote crosses that permit the simultaneous visualization of multiple tissue types. The tissue specificity provides live, endogenous marker gene expression at key developmental stages that are inaccessible for whole mount staining. Furthermore, the nonlocalized EGFP in these lines illuminates both the nucleus and cytoplasm, providing cellular resolution for morphogenesis research on processes such as dorsal closure and heart formation. In future work, identification of regulatory regions driving these enhancer traps will deepen our understanding of late developmental control, including in the extraembryonic domain, which is a hallmark of insect development but which is not yet well understood.

Transcriptional regulation of Drosophila gonad formation

The formation of the Drosophila embryonic gonad, involving the fusion of clusters of somatic gonadal precursor cells (SGPs) and their ensheathment of germ cells, provides a simple and genetically tractable model for the interplay between cells during organ formation. In a screen for mutants affecting gonad formation we identified a SGP cell autonomous role for Midline (Mid) and Longitudinals lacking (Lola). These transcriptional factors are required for multiple aspects of SGP behaviour including SGP cluster fusion, germ cell ensheathment and gonad compaction. The lola locus encodes more than 25 differentially spliced isoforms and we have identified an isoform specific requirement for lola in the gonad which is distinct from that in nervous system development. Mid and Lola work in parallel in gonad formation and surprisingly Mid overexpression in a lola background leads to additional SGPs at the expense of fat body cells. Our findings support the idea that although the transcription factors required by SGPs can ostensibly be assigned to those being required for either SGP specification or behaviour, they can also interact to impinge on both processes.

Methods to assess Drosophila heart development, function and aging

In recent years the Drosophila heart has become an established model for many different aspects of human cardiac disease. This model has allowed identification of disease-causing mechanisms underlying congenital heart disease and cardiomyopathies and has permitted the study of underlying genetic, metabolic and age-related contributions to heart function. In this review we discuss methods currently employed in the analysis of the Drosophila heart structure and function, such as optical methods to infer heart function and performance, electrophysiological and mechanical approaches to characterize cardiac tissue properties, and conclude with histological techniques used in the study of heart development and adult structure.

The midline protein regulates axon guidance by blocking the reiteration of neuroblast rows within the Drosophila ventral nerve cord

Guiding axon growth cones towards their targets is a fundamental process that occurs in a developing nervous system. Several major signaling systems are involved in axon-guidance, and disruption of these systems causes axon-guidance defects. However, the specific role of the environment in which axons navigate in regulating axon-guidance has not been examined in detail. In Drosophila, the ventral nerve cord is divided into segments, and half-segments and the precursor neuroblasts are formed in rows and columns in individual half-segments. The row-wise expression of segment-polarity genes within the neuroectoderm provides the initial row-wise identity to neuroblasts. Here, we show that in embryos mutant for the gene midline, which encodes a T-box DNA binding protein, row-2 neuroblasts and their neuroectoderm adopt a row-5 identity. This reiteration of row-5 ultimately creates a non-permissive zone or a barrier, which prevents the extension of interneuronal longitudinal tracts along their normal anterior-posterior path. While we do not know the nature of the barrier, the axon tracts either stall when they reach this region or project across the midline or towards the periphery along this zone. Previously, we had shown that midline ensures ancestry-dependent fate specification in a neuronal lineage. These results provide the molecular basis for the axon guidance defects in midline mutants and the significance of proper specification of the environment to axon-guidance. These results also reveal the importance of segmental polarity in guiding axons from one segment to the next, and a link between establishment of broad segmental identity and axon guidance.

During nervous system development, once formed from neuroblasts, neurons grow axons in the direction of their synaptic partners. Genetic forces guide these axon growth cones towards the target. This is known as axon guidance or pathfinding. There are a number of proteins that regulate axon-pathfinding. The well-known examples are the Slit and its receptor Roundabout, and Netrin and its receptor Frazzled. The Drosophila embryo and the nervous system are divided into segments by segmentation genes. Mutations in segmentation genes affect axon guidance, although how they do so is not well understood. In our work described here, we show that the T-box protein Midline prevents mis-specification of neuroblast rows, in particular, it prevents row 2 from becoming row 5. Thus, in midline mutants, row 2 changes into row 5, ultimately creating a non-permissive barrier that prevents axons from following their defined path. Thus, axons stop and diverge when they reach this barrier. Our results show how mutations in segmentation genes can affect axon guidance and how significant the environment is for axon-pathfinding. Our work is also a cautionary reminder that guidance defects need to be interpreted with care and can arise due to a variety of other defects.

A comparative study of Pointed and Yan expression reveals new complexity to the transcriptional networks downstream of receptor tyrosine kinase signaling

The biochemical regulatory network downstream of receptor tyrosine kinase (RTK) signaling is controlled by two opposing ETS family members: the transcriptional activator Pointed (Pnt) and the transcriptional repressor Yan. A bistable switch model has been invoked to explain how pathway activation can drive differentiation by shifting the system from a high-Yan/low-Pnt activity state to a low-Yan/high-Pnt activity state. Although the model explains yan and pnt loss-of-function phenotypes in several different cell types, how Yan and Pointed protein expression dynamics contribute to these and other developmental transitions remains poorly understood. Toward this goal we have used a functional GFP-tagged Pnt transgene (Pnt-GFP) to perform a comparative study of Yan and Pnt protein expression throughout Drosophila development. Consistent with the prevailing model of the Pnt-Yan network, we found numerous instances where Pnt-GFP and Yan adopt a mutually exclusive pattern of expression. However we also observed many examples of co-expression. While some co-expression occurred in cells where RTK signaling is presumed low, other co-expression occurred in cells with high RTK signaling. The instances of co-expressed Yan and Pnt-GFP in tissues with high RTK signaling cannot be explained by the current model, and thus they provide important contexts for future investigation of how context-specific differences in RTK signaling, network topology, or responsiveness to other signaling inputs, affect the transcriptional response.

Response to the dorsal anterior gradient of EGFR signaling in Drosophila oogenesis is prepatterned by earlier posterior EGFR activation

Spatially restricted epidermal growth factor receptor (EGFR) activity plays a central role in patterning the follicular epithelium of the Drosophila ovary. In midoogenesis, localized EGFR activation is achieved by the graded dorsal anterior localization of its ligand, Gurken. Graded EGFR activity determines multiple dorsal anterior fates along the dorsal-ventral axis but cannot explain the sharp posterior limit of this domain. Here, we show that posterior follicle cells express the T-box transcription factors Midline and H15, which render cells unable to adopt a dorsal anterior fate in response to EGFR activation. The posterior expression of Midline and H15 is itself induced in early oogenesis by posteriorly localized EGFR signaling, defining a feedback loop in which early induction of Mid and H15 confers a molecular memory that fundamentally alters the outcome of later EGFR signaling. Spatial regulation of the EGFR pathway thus occurs both through localization of the ligand and through localized regulation of the cellular response.

The Drosophila T-box transcription factor Midline functions within the Notch-Delta signaling pathway to specify sensory organ precursor cell fates and regulates cell survival within the eye imaginal disc

We report that the T-box transcription factor Midline (Mid), an evolutionary conserved homolog of the vertebrate Tbx20 protein, functions within the Notch-Delta signaling pathway essential for specifying the fates of sensory organ precursor (SOP) cells. These findings complement an established history of research showing that Mid regulates the cell-fate specification of diverse cell types within the developing heart, epidermis and central nervous system. Tbx20 has been detected in unique neuronal and epithelial cells of embryonic eye tissues in both mice and humans. However, the mechanisms by which either Mid or Tbx20 function to regulate cell-fate specification or other critical aspects of eye development including cell survival have not yet been elucidated. We have also gathered preliminary evidence suggesting that Mid may play an indirect, but vital role in selecting SOP cells within the third-instar larval eye disc by regulating the expression of the proneural gene atonal. During subsequent pupal stages, Mid specifies SOP cell fates as a member of the Notch-Delta signaling hierarchy and is essential for maintaining cell viability by inhibiting apoptotic pathways. We present several new hypotheses that seek to understand the role of Mid in regulating developmental processes downstream of the Notch receptor that are critical for specifying unique cell fates, patterning the adult eye and maintaining cellular homeostasis during eye disc morphogenesis.

In vitro site selection of a consensus binding site for the Drosophila melanogaster Tbx20 homolog midline

We employed in vitro site selection to identify a consensus binding sequence for the Drosophila melanogaster Tbx20 T-box transcription factor homolog Midline. We purified a bacterially expressed T-box DNA binding domain of Midline, and used it in four rounds of precipitation and polymerase-chain-reaction based amplification. We cloned and sequenced 54 random oligonucleotides selected by Midline. Electromobility shift-assays confirmed that 27 of these could bind the Midline T-box. Sequence alignment of these 27 clones suggests that Midline binds as a monomer to a consensus sequence that contains an AGGTGT core. Thus, the Midline consensus binding site we define in this study is similar to that defined for vertebrate Tbx20, but differs from a previously reported Midline binding sequence derived through site selection.

Transactivation in Drosophila of human enhancers by human transcription factors involved in congenital heart diseases

BACKGROUND

The human transcription factors (TFs) GATA4, NKX2.5 and TBX5 form part of the core network necessary to build a human heart and are involved in Congenital Heart Diseases (CHDs). The human natriuretic peptide precursor A (NPPA) and α-myosin heavy chain 6 (MYH6) genes are downstream effectors involved in cardiogenesis that have been demonstrated to be in vitro targets of such TFs.

RESULTS

To study the interactions between these human TFs and their target enhancers in vivo, we overexpressed them in the whole Drosophila cardiac tube using the UAS/GAL4 system. We observed that all three TFs up-regulate their natural target enhancers in Drosophila and cause developmental defects when overexpressed in eyes and wings.

CONCLUSIONS

A strong potential of the present model might be the development of combinatorial and mutational assays to study the interactions between human TFs and their natural target promoters, which are not easily undertaken in tissue culture cells because of the variability in transfection efficiency, especially when multiple constructs are used. Thus, this novel system could be used to determine in vivo the genetic nature of the human mutant forms of these TFs, setting up a powerful tool to unravel the molecular genetic mechanisms that lead to CHDs.

JAK/Stat signaling regulates heart precursor diversification in Drosophila

Intercellular signal transduction pathways regulate the NK-2 family of transcription factors in a conserved gene regulatory network that directs cardiogenesis in both flies and mammals. The Drosophila NK-2 protein Tinman (Tin) was recently shown to regulate Stat92E, the Janus kinase (JAK) and Signal transducer and activator of transcription (Stat) pathway effector, in the developing mesoderm. To understand whether the JAK/Stat pathway also regulates cardiogenesis, we performed a systematic characterization of JAK/Stat signaling during mesoderm development. Drosophila embryos with mutations in the JAK/Stat ligand upd or in Stat92E have non-functional hearts with luminal defects and inappropriate cell aggregations. Using strong Stat92E loss-of-function alleles, we show that the JAK/Stat pathway regulates tin expression prior to heart precursor cell diversification. tin expression can be subdivided into four phases and, in Stat92E mutant embryos, the broad phase 2 expression pattern in the dorsal mesoderm does not restrict to the constrained phase 3 pattern. These embryos also have an expanded pericardial cell domain. We show the E(spl)-C gene HLHm5 is expressed in a pattern complementary to tin during phase 3 and that this expression is JAK/Stat dependent. In addition, E(spl)-C mutant embryos phenocopy the cardiac defects of Stat92E embryos. Mechanistically, JAK/Stat signals activate E(spl)-C genes to restrict Tin expression and the subsequent expression of the T-box transcription factor H15 to direct heart precursor diversification. This study is the first to characterize a role for the JAK/Stat pathway during cardiogenesis and identifies an autoregulatory circuit in which tin limits its own expression domain.

Tinman/Nkx2-5 acts via miR-1 and upstream of Cdc42 to regulate heart function across species

Cdc42 regulates cardiac function in mice and flies downstream of a conserved Tinman/Nkx2-5–miR-1 signaling network.

Unraveling the gene regulatory networks that govern development and function of the mammalian heart is critical for the rational design of therapeutic interventions in human heart disease. Using the Drosophila heart as a platform for identifying novel gene interactions leading to heart disease, we found that the Rho-GTPase Cdc42 cooperates with the cardiac transcription factor Tinman/Nkx2-5. Compound Cdc42, tinman heterozygous mutant flies exhibited impaired cardiac output and altered myofibrillar architecture, and adult heart–specific interference with Cdc42 function is sufficient to cause these same defects. We also identified K⁺ channels, encoded by dSUR and slowpoke, as potential effectors of the Cdc42–Tinman interaction. To determine whether a Cdc42–Nkx2-5 interaction is conserved in the mammalian heart, we examined compound heterozygous mutant mice and found conduction system and cardiac output defects. In exploring the mechanism of Nkx2-5 interaction with Cdc42, we demonstrated that mouse Cdc42 was a target of, and negatively regulated by miR-1, which itself was negatively regulated by Nkx2-5 in the mouse heart and by Tinman in the fly heart. We conclude that Cdc42 plays a conserved role in regulating heart function and is an indirect target of Tinman/Nkx2-5 via miR-1.

A method to measure myocardial calcium handling in adult Drosophila

RATIONALE

Normal cardiac physiology requires highly regulated cytosolic Ca(2+) concentrations and abnormalities in Ca(2+) handling are associated with heart failure. The majority of approaches to identifying the components that regulate intracellular Ca(2+) dynamics rely on cells in culture, mouse models, and human samples. However, a genetically robust system for unbiased screens of mutations that affect Ca(2+) handling remains a challenge.

OBJECTIVE

We sought to develop a new method to measure myocardial Ca(2+) cycling in adult Drosophila and determine whether cardiomyopathic fly hearts recapitulate aspects of diseased mammalian myocardium.

METHODS AND RESULTS

Using engineered transgenic Drosophila that have cardiac-specific expression of Ca(2+)-sensing fluorescent protein, GCaMP2, we developed methods to measure parameters associated with myocardial Ca(2+) handling. The following key observations were identified: (1) Control w(1118) Drosophila hearts have readily measureable Ca(2+)-dependent fluorescent signals that are dependent on L-type Ca(2+) channels and SR Ca(2+) stores and originate from rostral and caudal pacemakers. (2) A fly mutant, held-up(2) (hdp(2)), that has a point mutation in troponin I and has a dilated cardiomyopathic phenotype demonstrates abnormalities in myocardial Ca(2+) handling that include increases in the duration of the 50% rise in intensity to peak intensity, the half-time of fluorescence decline from peak, the full duration at half-maximal intensity, and decreases in the linear slope of decay from 80% to 20% intensity decay. (3) Hearts from hdp(2) mutants had reductions in caffeine-induced Ca(2+) increases and reductions in ryanodine receptor (RyR) without changes in L-type Ca(2+) channel transcripts in comparison with w(1118).

CONCLUSIONS

Our results show that the cardiac-specific expression of GCaMP2 provides a means of characterizing propagating Ca(2+) transients in adult fly hearts. Moreover, the adult fruit fly heart recapitulates several aspects of Ca(2+) regulation observed in mammalian myocardium. A mutation in Drosophila that causes an enlarged cardiac chamber and impaired contractile function is associated with abnormalities in the cytosolic Ca(2+) transient as well as changes in transcript levels of proteins associated with Ca(2+) handling. This new methodology has the potential to permit an examination of evolutionarily conserved myocardial Ca(2+)-handing mechanisms by applying the vast resources available in the fly genomics community to conduct genetic screens to identify new genes involved in generated Ca(2+) transients and arrhythmias.

Drosophila models of cardiac disease

The fruit fly Drosophila melanogaster has emerged as a useful model for cardiac diseases, both developmental abnormalities and adult functional impairment. Using the tools of both classical and molecular genetics, the study of the developing fly heart has been instrumental in identifying the major signaling events of cardiac field formation, cardiomyocyte specification, and the formation of the functioning heart tube. The larval stage of fly cardiac development has become an important model system for testing isolated preparations of living hearts for the effects of biological and pharmacological compounds on cardiac activity. Meanwhile, the recent development of effective techniques to study adult cardiac performance in the fly has opened new uses for the Drosophila model system. The fly system is now being used to study long-term alterations in adult performance caused by factors such as diet, exercise, and normal aging. The fly is a unique and valuable system for the study of such complex, long-term interactions, as it is the only invertebrate genetic model system with a working heart developmentally homologous to the vertebrate heart. Thus, the fly model combines the advantages of invertebrate genetics (such as large populations, facile molecular genetic techniques, and short lifespan) with physiological measurement techniques that allow meaningful comparisons with data from vertebrate model systems. As such, the fly model is well situated to make important contributions to the understanding of complicated interactions between environmental factors and genetics in the long-term regulation of cardiac performance.

Genetic and genomic dissection of cardiogenesis in the Drosophila model

The linear heart tube of the fruit fly Drosophila has served as a very valuable model for studying the regulation of early heart development. In the past, regulatory genes of Drosophila cardiogenesis have been identified largely through candidate approaches. The vast genetic toolkit available in this organism has made it possible to determine their functions and build regulatory networks of transcription factors and signaling inputs that control heart development. In this review, we summarize the major findings from this study and present current approaches aiming to identify additional players in the specification, morphogenesis, and differentiation of the heart by forward genetic screens. We also discuss various genomic and bioinformatic approaches that are currently being developed to extend the known transcriptional networks more globally which, in combination with the genetic approaches, will provide a comprehensive picture of the regulatory circuits during cardiogenesis.

The Tbx20 homologs midline and H15 specify ventral fate in the Drosophila melanogaster leg

Regional fates in the developing limbs of Drosophila melanogasterare controlled by selector gene transcription factors. Ventral fate in the fly leg is specified by the expression of the ligand Wingless. We present evidence that midline and H15, members of the Tbx20 class of T-box transcription factors, are key mediators of the Wingless signal in the formation of the ventral region of the fly leg. midline and H15 are restricted to identical ventral domains of expression through activation by Wingless and repression by the dorsal signal Decapentaplegic. midline and H15 function redundantly and cell autonomously in the formation of ventral-specific structures. Conversely, midlineis sufficient to induce ventral fate. Finally, the induction of ectopic ventral fate by mid is compromised when Wingless signaling is attenuated, suggesting that Wingless acts both upstream and in parallel with midline/H15 to specify ventral fate. Based on these results,we propose that midline and H15 may be considered as the selector genes for ventral leg fate.

Partial loss of GATA factor Pannier impairs adult heart function in Drosophila

The GATA transcription factor encoded by pannier (pnr) is a critical regulator of heart progenitor formation in Drosophila. Mutations in GATA4, the mammalian homolog of pnr, have also been implicated in causing human cardiac disease in a haploinsufficient manner. Mouse models of Gata4 loss-of-function and gain-of-function studies underscored the importance of Gata4 in regulating cardiac progenitor cells specification and differentiation. However, it is not known whether pnr/Gata4 is directly involved in establishing and maintaining adult heart physiology because of the lethality associated with defective heart function and redundancy among various GATA factors in vertebrates. Here, we took advantage of the Drosophila heart model to examine the function of pnr in adult heart physiology. We found that pnr heterozygous mutants result in defective cardiac performance in response to electrical pacing of the heart as well as in elevated arrhythmias. Adult-specific disruption of pnr function using a dominant-negative form pnrEnR revealed a cardiac autonomous requirement of pnr in regulating heart physiology. Moreover, we identified Tbx20/neuromancer (nmr) as a potential downstream mediator of pnr in regulating cardiac performance and rhythm regularity, based on the observation that overexpression of nmr genes, but not of tinman, partially rescues the adult defects in pnr mutants. We conclude that pnr is not only essential for early cardiac progenitor formation, along with tinman and T-box factors, but also plays an important role in establishing and/or maintaining proper heart function, which is partially through another key regulator Tbx20/nmr.

Non-autonomous modulation of heart rhythm, contractility and morphology in adult fruit flies

The outermost layer of the vertebrate heart originates from migratory mesothelial cells (epicardium) that give rise to coronary vascular smooth muscles and fibroblasts. The role of the epicardium in myocardial morphogenesis and establishment of normal heart function is still largely unknown. Here, we use Drosophila to investigate non-autonomous influences of epicardial-like tissue surrounding the heart tube on the structural and functional integrity of the myocardium. It has previously been shown that during Drosophila heart formation, mesodermal expression of the homeobox transcription factor even-skipped (eve) is required for specification of a subset of non-myocardial progenitors in the precardiac mesoderm. These progenitors may share some similarities with the vertebrate epicardium. To investigate a non-autonomous epicardial-like influence on myocardial physiology, we studied the consequences of reduced mesodermal Eve expression and epi/pericardial cell numbers on the maturation of the myocardial heart tube, its contractility, and acquisition of a normal heart rhythm in the Drosophila model. Targeting the eve repressor ladybird early (lbe) with the minimal eve mesodermal enhancer efficiently eliminates the mesodermal Eve lineages. These flies exhibit defects in heart structure, including a reduction in systolic and diastolic diameter (akin to 'restrictive cardiomyopathy'). They also exhibit an elevated incidence of arrhythmias and intermittent asystoles, as well as compromised performance under stress. These abnormalities are restored by eve reexpression or by lbe-RNAi co-overexpression. The data suggest that adult heart function in Drosophila is likely to be modulated non-autonomously, possibly by paracrine influences from neighboring cells, such as the epi/pericardium. Thus, Drosophila may serve as a model for finding genetic effectors of epicardial-myocardial interactions relevant to higher organisms.

Drug discovery through functional screening in the Drosophila heart

Although advancements in the preventive and therapeutic strategies of cardiac diseases have successfully improved the prognosis of many types of cardiac diseases, they are still challengeable targets because of their high mortality and large medical expenses. Moreover, because heart function is tightly associated with quality of life, it is important to elucidate the genetic and molecular basis of disease progression. One of the recent advances for assessing protein function is reverse chemical genetics, which has the advantages that complement classical reverse genetics and should advance efforts at drug discovery for many diseases. Toward that end an appropriate biological assay system is required to describe specific heart phenotypes. Recent studies have shown that many aspects of Drosophila heart development and function are similar to those observed in the human heart, making Drosophila a useful model system with the advantage of a simpler genetic organization and shorter life span. Here we describe several assay systems that can be used to characterize Drosophila heart function. The first method is an external electrical pacing assay that is used to assess the response to stress in the adult fly. The incidence of pacing-induced heart dysfunction measured by this method strongly correlates with natural aging and mutation in genes known to be involved in human cardiac dysfunction. Consequently, this method can be used to identify unapparent heart failure phenotypes. This procedure is applicable for both genetic and pharmacological screening. The second method is an image-based heart performance assay. This method provides details of the dynamics of heart contraction in real time similar to clinical echocardiography. This method may be used for secondary drug screening as well as for more detailed analysis of the genetic and pharmacological phenotypes of Drosophila hearts.

The Drosophila homolog of vertebrate Islet1 is a key component in early cardiogenesis

In mouse, the LIM-homeodomain transcription factor Islet1 (Isl1) has been shown to demarcate a separate cardiac cell population that is essential for the formation of the right ventricle and the outflow tract of the heart. Whether Isl1 plays a crucial role in the early regulatory network of transcription factors that establishes a cardiac fate in mesodermal cells has not been fully resolved. We have analyzed the role of the Drosophila homolog of Isl1, tailup (tup), in cardiac specification and formation of the dorsal vessel. The early expression of Tup in the cardiac mesoderm suggests that Tup functions in cardiac specification. Indeed, tup mutants are characterized by a reduction of the essential early cardiac transcription factors Tin, Pnr and Dorsocross1-3 (Doc). Conversely, Tup expression depends on each of these cardiac factors, as well as on the early inductive signals Dpp and Wg. Genetic interactions show that tup cooperates with tin, pnr and Doc in heart cell specification. Germ layer-specific loss-of-function and rescue experiments reveal that Tup also functions in the ectoderm to regulate cardiogenesis and implicate the involvement of different LIM-domain-interacting proteins in the mesoderm and ectoderm. Gain-of-function analyses for tup and pnr suggest that a proper balance of these factors is also required for the specification of Eve-expressing pericardial cells. Since tup is required for proper cardiogenesis in an invertebrate organism, we believe it is appropriate to include tup/Isl1 in the core set of ancestral cardiac transcription factors that govern a cardiac fate.

Transcription factor neuromancer/TBX20 is required for cardiac function in Drosophila with implications for human heart disease

neuromancer/Tbx20 (nmr) genes are cardiac T-box transcription factors that are evolutionarily conserved from flies to humans. Along with other known congenital heart disease genes, including tinman/Nkx2-5, dorsocross/Tbx5/6, and pannier/Gata4/6, they are important for specification and morphogenesis of the embryonic heart. The Drosophila heart has proven to be an excellent model to study genes involved in establishing and maintaining the structural integrity of the adult heart, as well as genes involved in maintaining physiological function. Using this model, we have identified nmr as a gene required in adult fly hearts for the maintenance of both normal myofibrillar architecture and cardiac physiology. Moreover, we have discovered synergistic interactions between nmr and other cardiac transcription factors, including tinman/Nkx2-5, in regulating cardiac performance, rhythmicity, and cardiomyocyte structure, reminiscent of similar interactions in mice. This suggests a remarkably conserved role for this network of cardiac transcription factors in the genetic control of the adult heart. In addition, nmr-tinman interactions also influence the expression of potential downstream effectors, such as ion channels. Interestingly, genetic screening of patients with dilated cardiomyopathy and congenital heart disease has revealed TBX20 variants in three sporadic and two familial cases that were not found in controls. These findings suggest that the fly heart might serve as an identifier of candidate genes involved in human heart disease.

Neuromancer1 and Neuromancer2 regulate cell fate specification in the developing embryonic CNS of Drosophila melanogaster

T-box genes encode a large family of transcription factors that regulate many developmental processes in vertebrates and invertebrates. In addition to their roles in regulating embryonic heart and epidermal development in Drosophila, we provide evidence that the T-box transcription factors neuromancer1 (nmr1) and neuromancer2 (nmr2) play key roles in embryonic CNS development. We verify that nmr1 and nmr2 function in a partially redundant manner to regulate neuronal cell fate by inhibiting even-skipped (eve) expression in specific cells in the CNS. Consistent with their redundant function, nmr1 and nmr2 exhibit overlapping yet distinct protein expression profiles within the CNS. Of note, nmr2 transcript and protein are expressed in identical patterns of segment polarity stripes, defined sets of neuroblasts, many ganglion mother cells and discrete populations of neurons. However, while we observe nmr1 transcripts in segment polarity stripes and specific neural precursors in early stages of CNS development, we first detect Nmr1 protein in later stages of CNS development where it is restricted to discrete subsets of Nmr2-positive neurons. Expression studies identify nearly all Nmr1/2 co-expressing neurons as interneurons, while a single Eve-positive U/CQ motor neuron weakly co-expresses Nmr2. Lineage studies map a subset of Nmr1/2-positive neurons to neuroblast lineages 2-2, 6-1, and 6-2 while genetic studies reveal that nmr2 collaborates with nkx6 to regulate eve expression in the CNS. Thus, nmr1 and nmr2 appear to act together as members of the combinatorial code of transcription factors that govern neuronal subtype identity in the CNS.

Diverged and conserved aspects of heart formation in a spider

Heart development exhibits some striking similarities between vertebrates and arthropods, for example in both cases the heart develops as a linear tube from mesodermal cells. Furthermore, the underlying molecular pathways exhibit a significant number of similarities between vertebrates and the fruit fly Drosophila, suggesting a common origin of heart development in the last common ancestor of flies and vertebrates. However, there is hardly any molecular data from other animals. Here we show that many of the key genes are also active in heart development in the spider Cupiennius salei. Spiders belong to the chelicerates and are distantly related to insects with respect to the other arthropods. The tinman/Nkx2.5 ortholog is the first gene to be specifically expressed in the presumptive spider heart, like in flies and vertebrates. We also show that tinman is expressed in a similar way in the beetle Tribolium castaneum. Taken together this demonstrates that tinman has a conserved role in the specification of the arthropod heart. In addition, we analyzed the expression of other heart genes (decapentaplegic, Wnt5, H15, even-skipped, and Mef2 ) in Cupiennius. The expression of these genes suggests that the genetic pathway of heart development may be largely conserved among arthropods. However, a major difference is seen in the earlier expression of the even-skipped gene in the developing spider heart compared with Drosophila, implying that the role of even-skipped in heart formation might have changed during arthropod evolution. The most striking finding, however, is that in addition to the dorsal tissue of the fourth walking leg segment and the opisthosomal segments, we discovered tinman-expressing cells that arise from a position dorsal to the cephalic lobe and that contribute to the anterior dorsal vessel. In contrast to the posterior heart tissue, these cells do not express the other heart genes. The spider heart thus is composed of two distinct populations of cells.

The T-box genes H15 and optomotor-blind in the spiders Cupiennius salei, Tegenaria atrica and Achaearanea tepidariorum and the dorsoventral axis of arthropod appendages

Dorsoventral axis formation in the legs of the fly Drosophila melanogaster requires the T-box genes optomotor-blind (omb) and H15. Evolutionary conservation of the patterning functions of these genes is unclear, because data on H15 expression in the spider Cupiennius salei did not support a general role of H15 in ventral fate specification. However, H15 has a paralogous gene, midline (mid) in Drosophila and H15 duplicates are also present in Cupiennius and the millipede Glomeris marginata. H15 therefore seems to have been subject to gene duplication opening the possibility that the previous account on Cupiennius has overlooked one or several paralogs. We have studied omb- and H15-related genes in two additional spider species, Tegenaria atrica and Achearanea tepidariorum and show that in both species one of the H15 genes belongs to a third group of spider H15 genes that has an expression pattern very similar to the H15 pattern in Drosophila. The expression patterns of all omb-related genes are also very similar to the omb expression pattern in Drosophila. These data suggest that the dorsoventral patterning functions of omb and H15 are conserved in the arthropods and that the previous conclusions were based on an incomplete data set in Cupiennius. Our results emphasize the importance of a broad taxon sampling in comparative studies.

Dystrophin deficiency in Drosophila reduces lifespan and causes a dilated cardiomyopathy phenotype

A number of studies have been conducted recently on the model organism Drosophila to determine the function of genes involved in human disease, including those implicated in neurological disorders, cancer and metabolic and cardiovascular diseases. The simple structure and physiology of the Drosophila heart tube together with the available genetics provide a suitable in vivo assay system for studying cardiac gene functions. In our study, we focus on analysis of the role of dystrophin (Dys) in heart physiology. As in humans, the Drosophila dys gene encodes multiple isoforms, of which the large isoforms (DLPs) and a truncated form (Dp117) are expressed in the adult heart. Here, we show that the loss of dys function in the heart leads to an age-dependent disruption of the myofibrillar organization within the myocardium as well as to alterations in cardiac performance. dys RNAi-mediated knockdown in the mesoderm also shortens lifespan. Knockdown of all or deletion of the large isoforms increases the heart rate by shortening the diastolic intervals (relaxation phase) of the cardiac cycle. Morphologically, loss of the large DLPs isoforms causes a widening of the cardiac tube and a lower fractional shortening, a phenotype reminiscent of dilated cardiomyopathy. The dilated dys mutant phenotype was reversed by expressing a truncated mammalian form of dys (Dp116). Our results illustrate the utility of Drosophila as a model system to study dilated cardiomyopathy and other muscular-dystrophy-associated phenotypes.

Spatial specificity of mesodermal even-skipped expression relies on multiple repressor sites

Individual cardiac progenitors emerge at defined positions within each segment in the trunk mesoderm. Their specification depends on segmental information from the pre-patterned ectoderm, which provides positional information to the underlying cardiac mesoderm via inductive signals. This pattern is further reinforced by repressive interactions between transcription factors that are expressed in neighboring sets of cardiac progenitors. For example, even-skipped (eve) and ladybird early (lbe) gene products mark adjacent cardiac cell clusters within a segment, and their antagonistic interaction results in mutually exclusive expression domains. Lbe acts directly on the eve mesodermal enhancer (eme) to participate in restricting its expression anteriorly. We hypothesized that additional repressive activities must regulate the precise pattern of eve expression in the cardiac mesoderm via this enhancer. In this study, we identified two additional repressor motifs: 4 copies of an "AT"-rich motif (M1a-d) and 2 copies of an "GC"-rich motif (M2 a,b), which when mutated cause expansion of eme-dependent reporter gene expression. We have also examined potential negative regulators of eve and found that their overexpression is sufficient to repress eve as well as the eme enhancer via these sites. Our data suggest that a combination of factors is likely to interact with multiple essential repressor sites to confer precise spatial specificity of eve expression in the cardiac mesoderm.

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.

Heart development in Drosophila

The Drosophila heart, also called the dorsal vessel, is an organ for hemolymph circulation that resembles the vertebrate heart at its transient linear tube stage. Dorsal vessel morphogenesis shares several similarities with early events of vertebrate heart development and has proven to be an insightful system for the study of cardiogenesis due to its relatively simple structure and the productive use of Drosophila genetic approaches. In this review, we summarize published findings on Drosophila heart development in terms of the regulators and genetic pathways required for cardiac cell specification and differentiation, and organ formation and function. Emerging genome-based strategies should further facilitate the use of Drosophila as an advantageous system in which to identify previously unknown genes and regulatory networks essential for normal cardiac development and function.

Exploring in vitro roles of siRNA in cardiovascular disease

RNA interference (RNAi) is an adaptive defense mechanism through which double stranded RNAs silence cognate genes in a sequence-specific manner. It has been employed widely as a powerful tool in functional genomics studies, target validation and therapeutic product development. Similarly, the application of small interfering RNA (siRNA) to the silencing of the disease-causing genes involved in cardiovascular diseases has made great progress. In this overview, we attempt to provide a brief outline of the current understanding of the mechanism of RNAi and its potential application to the cardiovascular system, with particular emphasis on its ability to identify the pathophysiological function of genes related to several important cardiovascular disorders. The prospects of RNAi-based therapeutics, as well as the advantages and potential problems, are also discussed.