The Drosophila wing hearts originate from pericardial cells and are essential for wing maturation

The opportunities and challenges of using Drosophila to model human cardiac diseases

The Drosophila heart tube seems simple, yet it has notable anatomic complexity and contains highly specialized structures. In fact, the development of the fly heart tube much resembles that of the earliest stages of mammalian heart development, and the molecular-genetic mechanisms driving these processes are highly conserved between flies and humans. Combined with the fly's unmatched genetic tools and a wide variety of techniques to assay both structure and function in the living fly heart, these attributes have made Drosophila a valuable model system for studying human heart development and disease. This perspective focuses on the functional and physiological similarities between fly and human hearts. Further, it discusses current limitations in using the fly, as well as promising prospects to expand the capabilities of Drosophila as a research model for studying human cardiac diseases.

Reduction of Drosophila Mitochondrial RNase P in Skeletal and Heart Muscle Causes Muscle Degeneration, Cardiomyopathy, and Heart Arrhythmia

In this study, we examine the cause and progression of mitochondrial diseases linked to the loss of mtRNase P, a three-protein complex responsible for processing and cleaving mitochondrial transfer RNAs (tRNA) from their nascent transcripts. When mtRNase P function is missing, mature mitochondrial tRNA levels are decreased, resulting in mitochondrial dysfunction. mtRNase P is composed of Mitochondrial RNase P Protein (MRPP) 1, 2, and 3. MRPP1 and 2 have their own enzymatic activity separate from MRPP3, which is the endonuclease responsible for cleaving tRNA. Human mutations in all subunits cause mitochondrial disease. The loss of mitochondrial function can cause devastating, often multisystemic failures. When mitochondria do not provide enough energy and metabolites, the result can be skeletal muscle weakness, cardiomyopathy, and heart arrhythmias. These symptoms are complex and often difficult to interpret, making disease models useful for diagnosing disease onset and progression. Previously, we identified Drosophila orthologs of each mtRNase P subunit (Roswell/MRPP1, Scully/MRPP2, Mulder/MRPP3) and found that the loss of each subunit causes lethality and decreased mitochondrial tRNA processing in vivo. Here, we use Drosophila to model mtRNase P mitochondrial diseases by reducing the level of each subunit in skeletal and heart muscle using tissue-specific RNAi knockdown. We find that mtRNase P reduction in skeletal muscle decreases adult eclosion and causes reduced muscle mass and function. Adult flies exhibit significant age-progressive locomotor defects. Cardiac-specific mtRNase P knockdowns reduce fly lifespan for Roswell and Scully, but not Mulder. Using intravital imaging, we find that adult hearts have impaired contractility and exhibit substantial arrhythmia. This occurs for roswell and mulder knockdowns, but with little effect for scully. The phenotypes shown here are similar to those exhibited by patients with mitochondrial disease, including disease caused by mutations in MRPP1 and 2. These findings also suggest that skeletal and cardiac deficiencies induced by mtRNase P loss are differentially affected by the three subunits. These differences could have implications for disease progression in skeletal and heart muscle and shed light on how the enzyme complex functions in different tissues.

Identification of Bipotential Blood Cell/Nephrocyte Progenitors in Drosophila: Another Route for Generating Blood Progenitors

The Drosophila lymph gland is the larval hematopoietic organ and is aligned along the anterior part of the cardiovascular system, composed of cardiac cells, that form the cardiac tube and its associated pericardial cells or nephrocytes. By the end of embryogenesis the lymph gland is composed of a single pair of lobes. Two additional pairs of posterior lobes develop during larval development to contribute to the mature lymph gland. In this study we describe the ontogeny of lymph gland posterior lobes during larval development and identify the genetic basis of the process. By lineage tracing we show here that each posterior lobe originates from three embryonic pericardial cells, thus establishing a bivalent blood cell/nephrocyte potential for a subset of embryonic pericardial cells. The posterior lobes of L3 larvae posterior lobes are composed of heterogeneous blood progenitors and their diversity is progressively built during larval development. We further establish that in larvae, homeotic genes and the transcription factor Klf15 regulate the choice between blood cell and nephrocyte fates. Our data underline the sequential production of blood cell progenitors during larval development.

Wing hearts in four-winged Ultrabithorax-mutant flies-the role of Hox genes in wing heart specification

Wings are probably the most advanced evolutionary novelty in insects. In the fruit fly Drosophila melanogaster, proper development of wings requires the activity of so-called wing hearts located in the scutellum of the thorax. Immediately after the imaginal ecdysis, these accessory circulatory organs remove hemolymph and apoptotic epidermal cells from the premature wings through their pumping action. This clearing process is essential for the formation of functional wing blades. Mutant flies that lack intact wing hearts are flightless and display malformed wings. The embryonic wing heart progenitors originate from two adjacent parasegments corresponding to the later second and third thoracic segments. However, adult dipterian flies harbor only one pair of wings and only one pair of associated wing hearts in the second thoracic segment. Here we show that the specification of WHPs depends on the regulatory activity of the Hox gene Ultrabithorax. Furthermore, we analyzed the development of wing hearts in the famous four-winged Ultrabithorax (Ubx) mutant, which was first discovered by Ed Lewis in the 1970s. In these flies, the third thoracic segment is homeotically transformed into a second thoracic segment resulting in a second pair of wings instead of the club-shaped halteres. We show that a second pair of functional wing hearts is formed in the transformed third thoracic segment and that all wing hearts originate from the wild-type population of wing heart progenitor cells.

Suppression of store-operated calcium entry causes dilated cardiomyopathy of the Drosophila heart

Store-operated Ca2+ entry (SOCE) is an essential Ca2+ signaling mechanism present in most animal cells. SOCE refers to Ca2+ influx that is activated by depletion of sarco/endoplasmic reticulum (S/ER) Ca2+ stores. The main components of SOCE are STIM and Orai. STIM proteins function as S/ER Ca2+ sensors, and upon S/ER Ca2+ depletion STIM rearranges to S/ER-plasma membrane junctions and activates Orai Ca2+ influx channels. Studies have implicated SOCE in cardiac hypertrophy pathogenesis, but SOCE's role in normal heart physiology remains poorly understood. We therefore analyzed heart-specific SOCE function in Drosophila, a powerful animal model of cardiac physiology. We show that heart-specific suppression of Stim and Orai in larvae and adults resulted in reduced contractility consistent with dilated cardiomyopathy. Myofibers were also highly disorganized in Stim and Orai RNAi hearts, reflecting possible decompensation or upregulated stress signaling. Furthermore, we show that reduced heart function due to SOCE suppression adversely affected animal viability, as heart specific Stim and Orai RNAi animals exhibited significant delays in post-embryonic development and adults died earlier than controls. Collectively, our results demonstrate that SOCE is essential for physiological heart function, and establish Drosophila as an important model for understanding the role of SOCE in cardiac pathophysiology.

The Insect Circulatory System: Structure, Function, and Evolution

Although the insect circulatory system is involved in a multitude of vital physiological processes, it has gone grossly understudied. This review highlights this critical physiological system by detailing the structure and function of the circulatory organs, including the dorsal heart and the accessory pulsatile organs that supply hemolymph to the appendages. It also emphasizes how the circulatory system develops and ages and how, by means of reflex bleeding and functional integration with the immune system, it supports mechanisms for defense against predators and microbial invaders, respectively. Beyond that, this review details evolutionary trends and novelties associated with this system, as well as the ways in which this system also plays critical roles in thermoregulation and tracheal ventilation in high-performance fliers. Finally, this review highlights how novel discoveries could be harnessed for the control of vector-borne diseases and for translational medicine, and it details principal knowledge gaps that necessitate further investigation.

Developmental dynamics of butterfly wings: real-time in vivo whole-wing imaging of twelve butterfly species

Colour pattern development of butterfly wings has been studied from several different approaches. However, developmental changes in the pupal wing tissues have rarely been documented visually. In this study, we recorded real-time developmental changes of the pupal whole wings of 9 nymphalid, 2 lycaenid, and 1 pierid species in vivo, from immediately after pupation to eclosion, using the forewing-lift method. The developmental period was roughly divided into four sequential stages. At the very early stage, the wing tissue was transparent, but at the second stage, it became semi-transparent and showed dynamic peripheral adjustment and slow low-frequency contractions. At this stage, the wing peripheral portion diminished in size, but simultaneously, the ventral epithelium expanded in size. Likely because of scale growth, the wing tissue became deeply whitish at the second and third stages, followed by pigment deposition and structural colour expression at the fourth stage. Some red or yellow (light-colour) areas that emerged early were “overpainted” by expanding black areas, suggesting the coexistence of two morphogenic signals in some scale cells. The discal spot emerged first in some nymphalid species, as though it organised the entire development of colour patterns. These results indicated the dynamic wing developmental processes common in butterflies.

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.

Beyond aerodynamics: The critical roles of the circulatory and tracheal systems in maintaining insect wing functionality

Insect wings consist almost entirely of lifeless cuticle; yet their veins host a complex multimodal sensory apparatus and other tissues that require a continuous supply of water, nutrients and oxygen. This review provides a survey of the various living components in insect wings, as well as the specific contribution of the circulatory and tracheal systems to provide all essential substances. In most insects, hemolymph circulates through the veinal network in a loop flow caused by the contraction of accessory pulsatile organs in the thorax. In other insects, hemolymph oscillates into and out of the wings due to the complex interaction of several factors, such as heartbeat reversal, intermittent pumping of the accessory pulsatile organs in the thorax, and the elasticity of the wall of a special type of tracheae. A practically unexplored subject is the need for continuous hydration of the wing cuticle to retain its flexibility and toughness, including the associated problem of water loss due to evaporation. Also, widely neglected is the influence of the hemolymph mass and the circulating flow in the veins on the aerodynamic properties of insect wings during flight. Ventilation of the extraordinarily long wing tracheae is probably accomplished by intricate interactions with the circulatory system, and by the exchange of oxygen via cutaneous respiration.

Drosophila immune cells extravasate from vessels to wounds using Tre1 GPCR and Rho signaling

In contrast to vertebrates, adult Drosophila melanogaster have an open cardiovascular system. However, Thuma et al. find that in late pupation, hemolymph flows through Drosophila wing veins, providing a unique genetic and live-imaging opportunity to investigate the mechanisms driving immune cell extravasation from vessels to wounds and reveal new roles for Tre1 and Rho signaling in this process.

Inflammation is pivotal to fight infection, clear debris, and orchestrate repair of injured tissues. Although Drosophila melanogaster have proven invaluable for studying extravascular recruitment of innate immune cells (hemocytes) to wounds, they have been somewhat neglected as viable models to investigate a key rate-limiting component of inflammation—that of immune cell extravasation across vessel walls—due to their open circulation. We have now identified a period during pupal development when wing hearts pulse hemolymph, including circulating hemocytes, through developing wing veins. Wounding near these vessels triggers local immune cell extravasation, enabling live imaging and correlative light-electron microscopy of these events in vivo. We show that RNAi knockdown of immune cell integrin blocks diapedesis, just as in vertebrates, and we uncover a novel role for Rho-like signaling through the GPCR Tre1, a gene previously implicated in the trans-epithelial migration of germ cells. We believe this new Drosophila model complements current murine models and provides new mechanistic insight into immune cell extravasation.

Distinct domains in the matricellular protein Lonely heart are crucial for cardiac extracellular matrix formation and heart function in Drosophila

The biomechanical properties of extracellular matrices (ECMs) are critical to many biological processes, including cell-cell communication and cell migration and function. The correct balance between stiffness and elasticity is essential to the function of numerous tissues, including blood vessels and the lymphatic system, and depends on ECM constituents (the "matrisome") and on their level of interconnection. However, despite its physiological relevance, the matrisome composition and organization remain poorly understood. Previously, we reported that the ADAMTS-like protein Lonely heart (Loh) is critical for recruiting the type IV collagen-like protein Pericardin to the cardiac ECM. Here, we utilized Drosophila as a simple and genetically amenable invertebrate model for studying Loh-mediated recruitment of tissue-specific ECM components such as Pericardin to the ECM. We focused on the functional relevance of distinct Loh domains to protein localization and Pericardin recruitment. Analysis of Loh deletion constructs revealed that one thrombospondin type 1 repeat (TSR1-1), which has an embedded WXXW motif, is critical for anchoring Loh to the ECM. Two other thrombospondin repeats, TSR1-2 and TSR1-4, the latter containing a CXXTCXXG motif, appeared to be dispensable for tethering Loh to the ECM but were crucial for proper interaction with and recruitment of Pericardin. Moreover, our results also suggested that Pericardin in the cardiac ECM primarily ensures the structural integrity of the heart, rather than increasing tissue flexibility. In conclusion, our work provides new insights into the roles of thrombospondin type 1 repeats and advances our understanding of cardiac ECM assembly and function.

Distinct subsets of Eve-positive pericardial cells stabilise cardiac outflow and contribute to Hox gene-triggered heart morphogenesis in Drosophila

The Drosophila heart, composed of discrete subsets of cardioblasts and pericardial cells, undergoes Hox-triggered anterior-posterior morphogenesis, leading to a functional subdivision into heart proper and aorta, with its most anterior part forming a funnel-shaped cardiac outflow. Cardioblasts differentiate into Tin-positive 'working myocytes' and Svp-expressing ostial cells. However, developmental fates and functions of heart-associated pericardial cells remain elusive. Here, we show that the pericardial cells that express the transcription factor Even Skipped adopt distinct fates along the anterior-posterior axis. Among them, the most anterior Antp-Ubx-AbdA-negative cells form a novel cardiac outflow component we call the outflow hanging structure, whereas the Antp-expressing cells differentiate into wing heart precursors. Interestingly, Hox gene expression in the Even Skipped-positive cells not only underlies their antero-posterior diversification, but also influences heart morphogenesis in a non-cell-autonomous way. In brief, we identify a new cardiac outflow component derived from a subset of Even Skipped-expressing cells that stabilises the anterior heart tip, and demonstrate non-cell-autonomous effects of Hox gene expression in the Even Skipped-positive cells on heart morphogenesis.

Macroevolutionary shifts of WntA function potentiate butterfly wing-pattern diversity

Butterfly wing patterns provide a rich comparative framework to study how morphological complexity develops and evolves. Here we used CRISPR/Cas9 somatic mutagenesis to test a patterning role for WntA, a signaling ligand gene previously identified as a hotspot of shape-tuning alleles involved in wing mimicry. We show that WntA loss-of-function causes multiple modifications of pattern elements in seven nymphalid butterfly species. In three butterflies with a conserved wing-pattern arrangement, WntA is necessary for the induction of stripe-like patterns known as symmetry systems and acquired a novel eyespot activator role specific to Vanessa forewings. In two Heliconius species, WntA specifies the boundaries between melanic fields and the light-color patterns that they contour. In the passionvine butterfly Agraulis, WntA removal shows opposite effects on adjacent pattern elements, revealing a dual role across the wing field. Finally, WntA acquired a divergent role in the patterning of interveinous patterns in the monarch, a basal nymphalid butterfly that lacks stripe-like symmetry systems. These results identify WntA as an instructive signal for the prepatterning of a biological system of exuberant diversity and illustrate how shifts in the deployment and effects of a single developmental gene underlie morphological change.

Pupal development and pigmentation process of a polka-dotted fruit fly, Drosophila guttifera (Insecta, Diptera)

Various organisms have color patterns on their body surfaces, and these color patterns are thought to contribute to physiological regulation, communication with conspecifics, and signaling with the environment. An adult fly of Drosophila guttifera (Insecta: Diptera: Drosophilidae) has melanin pigmentation patterns on its body and wings. Though D. guttifera has been used for research into color pattern formation, how its pupal development proceeds and when the pigmentation starts have not been well studied. In this study, we defined the pupal stages of D. guttifera and measured the pigment content of wing spots from the pupal period to the period after eclosion. Using a transgenic line which carries eGFP connected with an enhancer of yellow, a gene necessary for melanin synthesis, we analyzed the timing at which the yellow enhancer starts to drive eGFP. We also analyzed the distribution of Yellow-producing cells, as indicated by the expression of eGFP during pupal and young adult periods. The results suggested that Yellow-producing cells were removed from wings within 3 h after eclosion, and wing pigmentation continued without epithelial cells. Furthermore, the results of vein cutting experiments showed that the transport of melanin precursors through veins was necessary for wing pigmentation. These results showed the importance of melanin precursors transported through veins and of extracellular factors which were secreted from epithelial cells and left in the cuticle.

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.

Bioassay and Scanning Electron Microscopic Observations Reveal High Virulence of Entomopathogenic Fungus, Beauveria bassiana, on the Onion Maggot (Diptera: Anthomyiidae) Adults

When flies were dipped in 1 × 10⁸ conidia/ml conidia suspensions and then kept in the incubator (22 ± 1 °C, 70 ± 5% RH), scanning electron microscope observations revealed that, at 2 h, the majority of adhering Beauveria bassiana conidia were attached to either the wing surface or the interstitial area between the macrochaetae on the thorax and abdomen of the onion maggot adults. Germ tubes were being produced and had oriented toward the cuticle by 18 h. Penetration of the insect cuticle had occurred by 36 h, and by 48 h, germ tubes had completely penetrated the cuticle. Fungal mycelia had emerged from the insect body and were proliferating after 72 h. The superficial area and structure of the wings and macrochaetae may facilitate the attachment of conidia and enable effective penetration. The susceptibility of adults to 12 isolates, at a concentration of 1 × 10⁷ conidia/ml, was tested in laboratory experiments. Eight of the more potent strains caused in excess of 85% adult mortality 8 d post inoculation, while the median lethal time (LT₅₀) of these strains was <6 d. The virulence of the more effective strains was further tested, and the median lethal concentrations (LC₅₀) were calculated by exposing adults to doses ranging from 10³-10⁷ conidia/ml. The lowest LC₅₀ value, found in the isolate XJWLMQ-32, for the adults was 3.87 × 10³ conidia/ml. These results demonstrate that some B. bassiana strains are highly virulent to onion maggot adults and should be considered as potential biocontrol agents against the adult flies.

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.

Precise long-range migration results from short-range stepwise migration during ring gland organogenesis

Many organs are specified far from the location they occupy when functional, having to migrate long distances through the heterogeneous and dynamic environment of the early embryo. We study the formation of the main Drosophila endocrine organ, the ring gland, as a new model to investigate in vivo the genetic regulation of collective cell migration. The ring gland results from the fusion of three independent gland primordia that migrate from the head towards the anterior aorta as the embryo is experiencing major morphogenetic movements. To complete their long-range migration, the glands extend filopodia moving sequentially towards a nearby intermediate target and from there to more distal ones. Thus, the apparent long-range migration is composed of several short-range migratory steps that facilitate reaching the final destination. We find that the target tissues react to the gland's proximity by sending filopodia towards it. Our finding of a succession of independent migration stages is consistent with the stepwise evolution of ring gland assembly and fits with the observed gland locations found in extant crustaceans, basal insects and flies.

Influence of ovarian muscle contraction and oocyte growth on egg chamber elongation in Drosophila

Organs are formed from multiple cell types that make distinct contributions to their shape. The Drosophila egg chamber provides a tractable model to dissect such contributions during morphogenesis. Egg chambers consist of 16 germ cells (GCs) surrounded by a somatic epithelium. Initially spherical, these structures elongate as they mature. This morphogenesis is thought to occur through a 'molecular corset' mechanism, whereby structural elements within the epithelium become circumferentially organized perpendicular to the elongation axis and resist the expansive growth of the GCs to promote elongation. Whether this epithelial organization provides the hypothesized constraining force has been difficult to discern, however, and a role for GC growth has not been demonstrated. Here, we provide evidence for this mechanism by altering the contractile activity of the tubular muscle sheath that surrounds developing egg chambers. Muscle hypo-contraction indirectly reduces GC growth and shortens the egg, which demonstrates the necessity of GC growth for elongation. Conversely, muscle hyper-contraction enhances the elongation program. Although this is an abnormal function for this muscle, this observation suggests that a corset-like force from the egg chamber's exterior could promote its lengthening. These findings highlight how physical contributions from several cell types are integrated to shape an organ.

Hemolymph circulation in insect flight appendages: physiology of the wing heart and circulatory flow in the wings of the mosquito, Anopheles gambiae

The wings of insects are composed of membranes supported by interconnected veins. Within these veins are epithelial cells, nerves and tracheae, and their maintenance requires the flow of hemolymph. For this purpose, insects employ accessory pulsatile organs (auxiliary hearts) that circulate hemolymph throughout the wings. Here, we used correlative approaches to determine the functional mechanics of hemolymph circulation in the wings of the malaria mosquito, Anopheles gambiae. Examination of sectioned tissues and intravital videos showed that the wing heart is located underneath the scutellum and is separate from the dorsal vessel. It is composed of a single pulsatile diaphragm (indicating that it is unpaired) that contracts at 3 Hz and circulates hemolymph throughout both wings. The wing heart contracts significantly faster than the dorsal vessel, and there is no correlation between the contractions of these two pulsatile organs. The wing heart functions by aspirating hemolymph out of the posterior wing veins, which forces hemolymph into the wings via anterior veins. By tracking the movement of fluorescent microspheres, we show that the flow diameter of the wing circulatory circuit is less than 1 µm, and we present a spatial map detailing the flow of hemolymph across all the wing veins, including the costa, sub-costa, ambient costa, radius, media, cubitus anterior, anal vein, and crossveins. We also quantified the movement of hemolymph within the radius and within the ambient costa, and show that hemolymph velocity and maximum acceleration are higher when hemolymph is exiting the wing.

Nano-architecture of gustatory chemosensory bristles and trachea in Drosophila wings

In the Drosophila wing anterior margin, the dendrites of gustatory neurons occupy the interior of thin and long bristles that present tiny pores at their extremities. Many attempts to measure ligand-evoked currents in insect wing gustatory neurons have been unsuccessful for technical reasons. The functions of this gustatory activity therefore remain elusive and controversial. To advance our knowledge on this understudied tissue, we investigated the architecture of the wing chemosensory bristles and wing trachea using Raman spectroscopy and fluorescence microscopy. We hypothesized that the wing gustatory hair, an open-ended capillary tube, and the wing trachea constitute biological systems similar to nano-porous materials. We present evidence that argues in favour of the existence of a layer or a bubble of air beneath the pore inside the gustatory hair. We demonstrate that these hollow hairs and wing tracheal tubes fulfil conditions for which the physics of fluids applied to open-ended capillaries and porous materials are relevant. We also document that the wing gustatory hair and tracheal architectures are capable of trapping volatile molecules from the environment, which might increase the efficiency of their spatial detection by way of wing vibrations or during flight.

Klf15 Is Critical for the Development and Differentiation of Drosophila Nephrocytes

Insect nephrocytes are highly endocytic scavenger cells that represent the only invertebrate model for the study of human kidney podocytes. Despite their importance, nephrocyte development is largely uncharacterised. This work tested whether the insect ortholog of mammalian Kidney Krüppel-Like Factor (Klf15), a transcription factor required for mammalian podocyte differentiation, was required for insect nephrocyte development. It was found that expression of Drosophila Klf15 (dKlf15, previously known as Bteb2) was restricted to the only two nephrocyte populations in Drosophila, the garland cells and pericardial nephrocytes. Loss of dKlf15 function led to attrition of both nephrocyte populations and sensitised larvae to the xenotoxin silver nitrate. Although pericardial nephrocytes in dKlf15 loss of function mutants were specified during embryogenesis, they failed to express the slit diaphragm gene sticks and stones and did not form slit diaphragms. Conditional silencing of dKlf15 in adults led to reduced surface expression of the endocytic receptor Amnionless and loss of in vivo scavenger function. Over-expression of dKlf15 increased nephrocyte numbers and rescued age-dependent decline in nephrocyte function. The data place dKlf15 upstream of sns and Amnionless in a nephrocyte-restricted differentiation pathway and suggest dKlf15 expression is both necessary and sufficient to sustain nephrocyte differentiation. These findings explain the physiological relevance of dKlf15 in Drosophila and imply that the role of KLF15 in human podocytes is evolutionarily conserved.

A Drosophila Model of Epidermolysis Bullosa Simplex

The blistering skin disorder Epidermolysis bullosa simplex (EBS) results from dominant mutations in K5 or K14 genes, encoding the intermediate filament network of basal epidermal keratinocytes. The mechanisms governing keratin network formation and collapse due to EBS mutations remain incompletely understood. Drosophila lacks cytoplasmic intermediate filaments, providing a ‚null’ environment to examine the formation of keratin networks and determine mechanisms by which mutant keratins cause pathology. Here, we report that ubiquitous co-expression of transgenes encoding wild-type human K14 and K5 resulted in the formation of extensive keratin networks in Drosophila epithelial and non-epithelial tissues, causing no overt phenotype. Similar to mammalian cells, treatment of transgenic fly tissues with phosphatase inhibitors caused keratin network collapse, validating Drosophila as a genetic model system to investigate keratin dynamics. Co-expression of K5 and a K14^R125C mutant that causes the most severe form of EBS resulted in widespread formation of EBS-like cytoplasmic keratin aggregates in epithelial and non-epithelial fly tissues. Expression of K14^R125C/K5 caused semi-lethality; adult survivors developed wing blisters and were flightless due to lack of intercellular adhesion during wing heart development. This Drosophila model of EBS is valuable for the identification of pathways altered by mutant keratins and for development of EBS therapies.

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.

Genetic background and GxE interactions modulate the penetrance of a naturally occurring wing mutation in Drosophila melanogaster

Many genes involved in producing complex traits are incompletely penetrant. One such example is vesiculated, an X-linked gene in Drosophila melanogaster that results in wing defects. To examine the genetic architecture of a complex trait (wings containing vesicles), we placed a naturally occurring variant into multiple autosomal backgrounds and quantified penetrance and expressivity at a range of developmental temperatures. We found significant epistasis, genotype-by-environment interactions, and maternal effects. Sex and temperature effects were modulated by genetic background. The severity of wing phenotypes also varied across different genetic backgrounds, and expressivity was positively correlated with penetrance. We also found evidence of naturally segregating suppressors of vesiculated. These suppressors were present on both the second and third chromosomes, and complex interactions were observed. Taken together, these findings indicate that multiple genetic and environmental factors modulate the phenotypic effects of a naturally occurring vesiculated allele.

The Iroquois complex is required in the dorsal mesoderm to ensure normal heart development in Drosophila

Drosophila heart development is an invaluable system to study the orchestrated action of numerous factors that govern cardiogenesis. Cardiac progenitors arise within specific dorsal mesodermal regions that are under the influence of temporally coordinated actions of multiple signaling pathways. The Drosophila Iroquois complex (Iro-C) consists of the three homeobox transcription factors araucan (ara), caupolican (caup) and mirror (mirr). The Iro-C has been shown to be involved in tissue patterning leading to the differentiation of specific structures, such as the lateral notum and dorsal head structures and in establishing the dorsal-ventral border of the eye. A function for Iro-C in cardiogenesis has not been investigated yet. Our data demonstrate that loss of the whole Iro complex, as well as loss of either ara/caup or mirr only, affect heart development in Drosophila. Furthermore, the data indicate that the GATA factor Pannier requires the presence of Iro-C to function in cardiogenesis. Furthermore, a detailed expression pattern analysis of the members of the Iro-C revealed the presence of a possibly novel subpopulation of Even-skipped expressing pericardial cells and seven pairs of heart-associated cells that have not been described before. Taken together, this work introduces Iro-C as a new set of transcription factors that are required for normal development of the heart. As the members of the Iro-C may function, at least partly, as competence factors in the dorsal mesoderm, our results are fundamental for future studies aiming to decipher the regulatory interactions between factors that determine different cell fates in the dorsal mesoderm.

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 ultrastructure of Drosophila heart cells

The functionality of the Drosophila heart or dorsal vessel is achieved by contributions from several tissues. The heart tube itself is composed of different types of cardiomyocytes that form an anterior aorta and a posterior heart chamber, inflow tracts and intracardiac valves. Herein we present an in-depth ultrastructural analysis of all cell types present in the Drosophila heart at different developmental stages. We demonstrate that the lumen-forming cardiomyocytes reveal a complex subcellular architecture that changes during development. We show that ostial cells, for which it was previously shown that they are specified during embryogenesis, start to differentiate at the end of embryogenesis displaying opening structures that allow inflow of hemolymph. Furthermore we found, that intracardiac valve cells differentiate during larval development and become enlarged during the 3. instar larval stages by the formation of cellular cytoplasmic free cavities. Moreover we were able to demonstrate, that the alary muscles are not directly connected to the heart tube but by extracellular matrix fibers at any stage of development. Our present work will provide a reference for future investigations on normal heart development and for analyses of mutant phenotypes that are caused by defects on the subcellular level.

Phospholipid homeostasis regulates lipid metabolism and cardiac function through SREBP signaling in Drosophila

The epidemic of obesity and diabetes is causing an increased incidence of dyslipidemia-related heart failure. While the primary etiology of lipotoxic cardiomyopathy is an elevation of lipid levels resulting from an imbalance in energy availability and expenditure, increasing evidence suggests a relationship between dysregulation of membrane phospholipid homeostasis and lipid-induced cardiomyopathy. In the present study, we report that the Drosophila easily shocked (eas) mutants that harbor a disturbance in phosphatidylethanolamine (PE) synthesis display tachycardia and defects in cardiac relaxation and are prone to developing cardiac arrest and fibrillation under stress. The eas mutant hearts exhibit elevated concentrations of triglycerides, suggestive of a metabolic, diabetic-like heart phenotype. Moreover, the low PE levels in eas flies mimic the effects of cholesterol deficiency in vertebrates by stimulating the Drosophila sterol regulatory element-binding protein (dSREBP) pathway. Significantly, cardiac-specific elevation of dSREBP signaling adversely affects heart function, reflecting the cardiac eas phenotype, whereas suppressing dSREBP or lipogenic target gene function in eas hearts rescues the cardiac hyperlipidemia and heart function disorders. These findings suggest that dysregulated phospholipid signaling that alters SREBP activity contributes to the progression of impaired heart function in flies and identifies a potential link to lipotoxic cardiac diseases in humans.

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.

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.

Bringing together components of the fly renal system

The function of all animal excretory systems is to rid the body of toxins and to maintain homeostatic balance. Although excretory organs in diverse animal species appear superficially different they are often built on two common principals: filtration and tubular secretion/reabsorbtion. The Drosophila excretory system is composed of filtration nephrocytes and Malpighian (renal) tubules. Here we review recent molecular genetic data on the development and differentiation of nephrocytes and renal tubules. We focus in particular on the molecular mechanisms that underpin key cell and tissue behaviours during morphogenesis, drawing parallels with other species where they exist. Finally we assess the implications of patterned tissue differentiation for the subsequent regulation of renal function. These studies highlight the continuing usefulness of the fly to provide fundamental insights into the complexities of organ formation.

The Drosophila wing hearts consist of syncytial muscle cells that resemble adult somatic muscles

In Drosophila, hemolymph circulation in the wings is accomplished by a pair of wing hearts located in the thorax. The embryonic progenitors of these organs were only recently discovered and found to belong to the cardiac mesoderm. In this study, the functional morphology and the structure of mature organs were studied by light and electron microscopy to characterize the tissues arising from this new set of progenitors. Each wing heart consists of 7-8 muscle cells providing the pumping force, a thin layer of non-contractile mononucleated cells separating the muscle cells from the body cavity, and acellular suspending strands opposing the muscle contractions. The muscle cells are multinucleated syncytia attached to the cuticle via epidermal tendon cells. They have central nuclei and sarcomeres with discontinuous Z-discs, A-bands, and I-bands, whereas H-bands and M-bands are indiscernible. From 9 to 11 actin filaments surround each myosin filament. Mitochondria are abundantly interspersed between myofibrils and accumulated in characteristic outpockets of the plasma membrane. The analysis revealed that the wing heart muscles resemble in their ultrastructure and their mode of attachment adult somatic muscles. This suggests that, despite their origin in the cardiac mesoderm, wing heart progenitors are functionally related to somatic adult muscle precursors.

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.

Bithorax complex genes control alary muscle patterning along the cardiac tube of Drosophila

Cardiac specification models are widely utilized to provide insight into the expression and function of homologous genes and structures in humans. In Drosophila, contractions of the alary muscles control hemolymph inflow and support the cardiac tube, however embryonic development of these muscles remain largely understudied. We found that alary muscles in Drosophila embryos appear as segmental pairs, attaching dorsally at the seven-up (svp) expressing pericardial cells along the cardiac dorsal vessel, and laterally to the body wall. Normal patterning of alary muscles along the dorsal vessel was found to be a function of the Bithorax Complex genes abdominal-A (abd-A) and Ultrabithorax (Ubx) but not of the orphan nuclear receptor gene svp. Ectopic expression of either abd-A or Ubx resulted in an increase in the number of alary muscle pairs from seven to 10, and also produced a general elongation of the dorsal vessel. A single knockout of Ubx resulted in a reduced number of alary muscles. Double knockouts of both Ubx and abd-A prevented alary muscles from developing normally and from attaching to the dorsal vessel. These studies demonstrate an additional facet of muscle development that depends upon the Hox genes, and define for the first time mechanisms that impact development of this important subset of muscles.

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

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

Cardiac gene regulatory networks in Drosophila

The Drosophila system has proven a powerful tool to help unlock the regulatory processes that occur during specification and differentiation of the embryonic heart. In this review, we focus upon a temporal analysis of the molecular events that result in heart formation in Drosophila, with a particular emphasis upon how genomic and other cutting-edge approaches are being brought to bear upon the subject. We anticipate that systems-level approaches will contribute greatly to our comprehension of heart development and disease in the animal kingdom.