Michel Sémériva

Genetic control of cell morphogenesis during Drosophila melanogaster cardiac tube formation

Tubulogenesis is an essential component of organ development, yet the underlying cellular mechanisms are poorly understood. We analyze here the formation of the Drosophila melanogaster cardiac lumen that arises from the migration and subsequent coalescence of bilateral rows of cardioblasts. Our study of cell behavior using three-dimensional and time-lapse imaging and the distribution of cell polarity markers reveals a new mechanism of tubulogenesis in which repulsion of prepatterned luminal domains with basal membrane properties and cell shape remodeling constitute the main driving forces. Furthermore, we identify a genetic pathway in which roundabout, slit, held out wings, and dystroglycan control cardiac lumen formation by establishing nonadherent luminal membranes and regulating cell shape changes. From these data we propose a model for D. melanogaster cardiac lumen formation, which differs, both at a cellular and molecular level, from current models of epithelial tubulogenesis. We suggest that this new example of tube formation may be helpful in studying vertebrate heart tube formation and primary vasculogenesis.

Steroid-dependent modification of Hox function drives myocyte reprogramming in the Drosophila heart

In the Drosophila larval cardiac tube, aorta and heart differentiation are controlled by the Hox genes Ultrabithorax (Ubx) and abdominal A (abdA), respectively. There is evidence that the cardiac tube undergoes extensive morphological and functional changes during metamorphosis to form the adult organ, but both the origin of adult cardiac tube myocytes and the underlying genetic control have not been established. Using in vivo time-lapse analysis, we show that the adult fruit fly cardiac tube is formed during metamorphosis by the reprogramming of differentiated and already functional larval cardiomyocytes, without cell proliferation. We characterise the genetic control of the process, which is cell autonomously ensured by the modulation of Ubx expression and AbdA activity. Larval aorta myocytes are remodelled to differentiate into the functional adult heart, in a process that requires the regulation of Ubx expression. Conversely, the shape, polarity, function and molecular characteristics of the surviving larval contractile heart myocytes are profoundly transformed as these cells are reprogrammed to form the adult terminal chamber. This process is mediated by the regulation of AbdA protein function, which is successively required within these persisting myocytes for the acquisition of both larval and adult differentiated states. Importantly, AbdA specificity is switched at metamorphosis to induce a novel genetic program that leads to differentiation of the terminal chamber. Finally, the steroid hormone ecdysone controls cardiac tube remodelling by impinging on both the regulation of Ubx expression and the modification of AbdA function. Our results shed light on the genetic control of one in vivo occurring remodelling process, which involves a steroid-dependent modification of Hox expression and function.

Control of Cardiac Rhythm by ORK1, a Drosophila Two-Pore Domain Potassium Channel

Unravelling the mechanisms controlling cardiac automatism is critical to our comprehension of heart development and cardiac physiopathology. Despite the extensive characterization of the ionic currents at work in cardiac pacemakers, the precise mechanisms initiating spontaneous rhythmic activity and, particularly, those responsible for the specific control of the pacemaker frequency are still matters of debate and have not been entirely elucidated. By using Drosophila as a model animal to analyze automatic cardiac activity, we have investigated the function of a K+ channel, ORK1 (outwardly rectifying K+ channel-1) in cardiac automatic activity. ORK1 is a two-pore domain K+ (K2P) channel, which belongs to a diverse and highly regulated superfamily of potassium-selective leak channels thought to provide baseline regulation of membrane excitability. Cardiac-specific inactivation of Ork1 led to an increase in heart rhythm. By contrast, when overexpressed, ORK1 completely prevented heart beating. In addition, by recording action potentials, we showed that the level of Ork1 activity sets the cardiac rhythm by controlling the duration of the slow diastolic depolarization phase. Our observations identify a new mechanism for cardiac rhythm control and provide the first demonstration that K2P channels regulate the automatic cardiac activity.

Signalling Pathways Involved in Adult Heart Formation Revealed by Gene Expression Profiling in Drosophila

Drosophila provides a powerful system for defining the complex genetic programs that drive organogenesis. Under control of the steroid hormone ecdysone, the adult heart in Drosophila forms during metamorphosis by a remodelling of the larval cardiac organ. Here, we evaluated the extent to which transcriptional signatures revealed by genomic approaches can provide new insights into the molecular pathways that underlie heart organogenesis. Whole-genome expression profiling at eight successive time-points covering adult heart formation revealed a highly dynamic temporal map of gene expression through 13 transcript clusters with distinct expression kinetics. A functional atlas of the transcriptome profile strikingly points to the genomic transcriptional response of the ecdysone cascade, and a sharp regulation of key components belonging to a few evolutionarily conserved signalling pathways. A reverse genetic analysis provided evidence that these specific signalling pathways are involved in discrete steps of adult heart formation. In particular, the Wnt signalling pathway is shown to participate in inflow tract and cardiomyocyte differentiation, while activation of the PDGF-VEGF pathway is required for cardiac valve formation. Thus, a detailed temporal map of gene expression can reveal signalling pathways responsible for specific developmental programs and provides here substantial grasp into heart formation.

The multienzyme complex containing nine aminoacyl-tRNA synthetases is ubiquitous from Drosophila to mammals.

In all mammalian cells studied so far, a multienzyme complex containing the nine aminoacyl-tRNA synthetases specific for the amino acids Glu, Pro, Ile, Leu, Met, Gln, Lys, Arg and Asp was characterized. The complexes purified from various sources display very similar polypeptide compositions; they are composed of 11 polypeptides with molecular masses ranging from 18 to 150 kDa. By contrast, the corresponding enzymes from prokaryotes and lower eukaryotes behave as free enzymes. In order to test for the ubiquity of the multisynthetase complex in all metazoan species, we have searched for a similar complex in Drosophila. We have purified to homogeneity, from Schneider cells, a high molecular weight complex comprising the same nine synthetase activities. Its polypeptide composition resembles that of the complexes isolated from mammalian sources. By using the Western blotting procedure, some of the constituent polypeptides of the Drosophila complex were assigned to specific aminoacyl-tRNA synthetases. These findings support the proposal according to which the multisynthetase complex is an idiosyncratic feature of all higher eukaryotic cells.

The fabulous destiny of the Drosophila heart

For the last 15 years the fly cardiovascular system has attracted developmental geneticists for its potential as a model system of organogenesis. Heart development in Drosophila indeed provides a remarkable system for elucidating the basic molecular and cellular mechanisms of morphogenesis and, more recently, for understanding the genetic control of cardiac physiology. The success of these studies can in part be attributed to multidisciplinary approaches, the multiplicity of existing genetic tools, and a detailed knowledge of the system. Striking similarities with vertebrate cardiogenesis have long been stressed, in particular concerning the conservation of key molecular regulators of cardiogenesis and the new data presented here confirm Drosophila cardiogenesis as a model not only for organogenesis but also for the study of molecular mechanisms of human cardiac disease.

Plasma membranes from rat intestinal epithelial cells at different stages of maturation. I. Preparation and characterization of plasma membrane subfractions originating from crypt cells and from villous cells

To determine the mechanism of the maturation of the brush border membrane in intestinal epithelial cells, purification of the plasma membrane from undifferentiated rat crypt cells and of the basal-lateral membrane from villous cells has been performed. The method is based on density perturbation of the mitochondria to selectively disrupt their association with the membrane. With both cell populations, two membrane subfractions displaying the same respective density on sucrose gradient have been obtained with an overall yield of 15--20% and a 10-fold enrichment of the plasma membrane markers 5-nucleotidase and (Na+ + K+)-dependent, ouabain-sensitive ATPase chosen to follow their purification. The four fractions were constituted by sheets and apparently closed vesicles of various sizes. Each fraction was characterized by a distinct protein composition and different levels of enzyme activities. The cells, used for the preparation of the membranes, were isolated as a villus to crypt gradient. This separation and that of the membranes, led to the conclusion that the (Na+ + K+)-dependent ATPase is localized principally in the plasma membrane of all cells whatever their state of maturation, while 5-nucleotidase is predominantly located in the basal-lateral membrane of the villous cells and may serve as a specific marker for the purification of this membrane. Finally it has been shown that aminopeptidase, dissacharidases and alkaline phosphatase do not appear simultaneously in the maturation process of the cells, alkaline phosphatase being absent from the crypt cells and aminopeptidase being the first to be synthesized. This enzyme seems to appear in the crypt cells membrane before being integrated into the mature brush border membrane.

Cellular interactions during heart morphogenesis in the Drosophila embryo

Summry— The formation of the dorsal vessel or heart in a Drosophila melanogaster embryo can be divided into three main steps: i) the determination step allows individualization of heart precursor cells from the dorsal mesoderm. They are arranged in clusters of seven to nine cells, located in each of the eleven segments of the trunk. Preliminary observations suggest that the gene Notch could participate in the choice of fate that the cardioblasts and the pericardial cells will adopt within the cardiogenic region. In the same line, a new gene, whose expression, as revealed by a P‐lacZ insertion, is initiated at gastrulation in the developing mesoderm and becomes restricted within the mesoderm to the myogenic lineages, could participate in the determination of the cardioblasts identity; ii) once the cardioblasts have separated from the dorsal mesoderm, they reorganize to from an epithelial monolayer. The gene coding for the α‐subunit of the transduction protein Go, which is expressed in the cardioblasts shortly before this step, could be involved in this process. Indeed, mutants in the Go α gene are affected in the formation of the cardiac endothelium; and iii) the last step consists of the migration of the cardiac epithelium towards the dorsal midline of the embryo to form the dorsal vessel by apposition of the two layers of cardioblasts. We show that an extracellular matrix component is specifically expressed at the surface of the dorsal vessel and could participate in the interaction between the dorsalmost ectodermal cells and the heart during this migration step.

Response to Mechanical Stress Is Mediated by the TRPA Channel Painless in the Drosophila Heart

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

Downstream of homeotic genes: in the heart of Hox function.

A functional organ is constituted of diverse cell types. Each one occupies a distinct position and is associated to specific morphological and physiological functions. The identification of the genetic programs controlling these elaborated and highly precise features of organogenesis is crucial to understand how a mature organ works under normal conditions, and how pathologies can develop. Recently, a number of studies have reported a critical role for Hox genes in one example of organogenesis: cardiogenesis in Drosophila. Beyond the interest in understanding the molecular basis of functional cardiogenesis, this system might provide a model for proposing new paradigms of how Hox genes achieve their action throughout development.