Left-right asymmetry in Drosophila

The sex of organ geometry

Organs have a distinctive yet often overlooked spatial arrangement in the body1-5. We propose that there is a logic to the shape of an organ and its proximity to its neighbours. Here, by using volumetric scans of many Drosophila melanogaster flies, we develop methods to quantify three-dimensional features of organ shape, position and interindividual variability. We find that both the shapes of organs and their relative arrangement are consistent yet differ between the sexes, and identify unexpected interorgan adjacencies and left-right organ asymmetries. Focusing on the intestine, which traverses the entire body, we investigate how sex differences in three-dimensional organ geometry arise. The configuration of the adult intestine is only partially determined by physical constraints imposed by adjacent organs; its sex-specific shape is actively maintained by mechanochemical crosstalk between gut muscles and vascular-like trachea. Indeed, sex-biased expression of a muscle-derived fibroblast growth factor-like ligand renders trachea sexually dimorphic. In turn, tracheal branches hold gut loops together into a male or female shape, with physiological consequences. Interorgan geometry represents a previously unrecognized level of biological complexity which might enable or confine communication across organs and could help explain sex or species differences in organ function.

A Fluorescent Sex-Sorting Technique for Insects with the Demonstration in Drosophila melanogaster

Recent advances in insect genetic engineering offer alternative genetic biocontrol solutions to control populations of pests and disease vectors. While success has been achieved, sex-sorting remains problematic for scaling many genetic biocontrol interventions. Here, we describe the development of a genetically stable sex-sorting technique for female and male selection with a proof of concept in Drosophila melanogaster termed SEPARATOR (Sexing Element Produced by Alternative RNA-splicing of A Transgenic Observable Reporter). This elegant approach utilizes dominantly expressed fluorescent proteins and differentially spliced introns to ensure sex-specific expression. The system has the potential for adaptability to various insect species and application for high-throughput insect sex-sorting.

Identification and functional analysis of CG3526 in spermatogenesis of Drosophila melanogaster

Spermatogenesis is a critical part of reproduction in insects; however, its molecular mechanism is still largely unknown. In this study, we identified a testis-specific gene CG3526 in Drosophila melanogaster. Bioinformatics analysis showed that CG3526 contains a zinc binding domain and 2 C2 H2 type zinc fingers, and it is clustered to the vertebrate really interesting new gene (RING) family E3 ubiquitin-protein ligases. When CG3526 was knocked down by RNA interference (RNAi), the testis became much smaller in size, and the apical tip exhibited a sharp and thin end instead of the blunt and round shape in the control testis. More importantly, compared to the control flies, only a few mature sperm were present in the seminal vesicle of C587-Gal4 > CG3526 RNAi flies. Immunofluorescence staining of the testis from CG3526 RNAi flies showed that the homeostasis of testis stem cell niche was disrupted, cell distribution in the apical tip was scattered, and the process of spermatogenesis was not completed. Furthermore, we found that the phenotype of CG3526 RNAi flies' testis was similar to that of testis of Stat92E RNAi flies, the expression level of CG3526 was significantly downregulated in the Stat92EF06346 mutant flies, and the promoter activity of CG3526 was upregulated by STAT92E. Taken together, our results indicated that CG3526 is a downstream effector gene in the JAK-STAT signaling pathway that plays a key role in the spermatogenesis of Drosophila.

A fluorescent sex-sorting technique for insects with the demonstration in Drosophila melanogaster

Recent advances in insect genetic engineering offer alternative genetic biocontrol solutions to control populations of pests and disease vectors. While success has been achieved, sex-sorting remains problematic for scaling many genetic biocontrol interventions. Here we describe the development of a sex-sorting technique for female and male selection with a proof-of-concept in D. melanogaster termed SEPARATOR (Sexing Element Produced by Alternative RNA-splicing of A Transgenic Observable Reporter). This approach utilizes dominant fluorescent proteins and differentially spliced introns to ensure sex-specific expression. The system has the potential for adaptability to various insect species and application for high-throughput insect sex-sorting.

Drosophila class-I myosins that can impact left-right asymmetry have distinct ATPase kinetics

Myosin-1D (myo1D) is important for Drosophila left-right asymmetry, and its effects are modulated by myosin-1C (myo1C). De novo expression of these myosins in nonchiral Drosophila tissues promotes cell and tissue chirality, with handedness depending on the paralog expressed. Remarkably, the identity of the motor domain determines the direction of organ chirality, rather than the regulatory or tail domains. Myo1D, but not myo1C, propels actin filaments in leftward circles in in vitro experiments, but it is not known if this property contributes to establishing cell and organ chirality. To further explore if there are differences in the mechanochemistry of these motors, we determined the ATPase mechanisms of myo1C and myo1D. We found that myo1D has a 12.5-fold higher actin-activated steady-state ATPase rate, and transient kinetic experiments revealed myo1D has an 8-fold higher MgADP release rate compared to myo1C. Actin-activated phosphate release is rate limiting for myo1C, whereas MgADP release is the rate-limiting step for myo1D. Notably, both myosins have among the tightest MgADP affinities measured for any myosin. Consistent with ATPase kinetics, myo1D propels actin filaments at higher speeds compared to myo1C in in vitro gliding assays. Finally, we tested the ability of both paralogs to transport 50 nm unilamellar vesicles along immobilized actin filaments and found robust transport by myo1D and actin binding but no transport by myo1C. Our findings support a model where myo1C is a slow transporter with long-lived actin attachments, whereas myo1D has kinetic properties associated with a transport motor.

Asymmetric activity of NetrinB controls laterality of the Drosophila brain

Left-Right (LR) asymmetry of the nervous system is widespread across animals and is thought to be important for cognition and behaviour. But in contrast to visceral organ asymmetry, the genetic basis and function of brain laterality remain only poorly characterized. In this study, we performed RNAi screening to identify genes controlling brain asymmetry in Drosophila. We found that the conserved NetrinB (NetB) pathway is required for a small group of bilateral neurons to project asymmetrically into a pair of neuropils (Asymmetrical Bodies, AB) in the central brain in both sexes. While neurons project unilaterally into the right AB in wild-type flies, netB mutants show a bilateral projection phenotype and hence lose asymmetry. Developmental time course analysis reveals an initially bilateral connectivity, eventually resolving into a right asymmetrical circuit during metamorphosis, with the NetB pathway being required just prior symmetry breaking. We show using unilateral clonal analysis that netB activity is required specifically on the right side for neurons to innervate the right AB. We finally show that loss of NetB pathway activity leads to specific alteration of long-term memory, providing a functional link between asymmetrical circuitry determined by NetB and animal cognitive functions.

Meiosis initiation: a story of two sexes in all creatures great and small

Meiosis facilitates diversity across individuals and serves as a major driver of evolution. However, understanding how meiosis begins is complicated by fundamental differences that exist between sexes and species. Fundamental meiotic research is further hampered by a current lack of human meiotic cells lines. Consequently, much of what we know relies on data from model organisms. However, contextualising findings from yeast, worms, flies and mice can be challenging, due to marked differences in both nomenclature and the relative timing of meiosis. In this review, we set out to combine current knowledge of signalling and transcriptional pathways that control meiosis initiation across the sexes in a variety of organisms. Furthermore, we highlight the emerging links between meiosis initiation and oncogenesis, which might explain the frequent re-expression of normally silent meiotic genes in a variety of human cancers.

The nuclear activity of the actin-binding Moesin protein is necessary for gene expression in Drosophila

Ezrin-Radixin-Moesin (ERM) proteins play an essential role in the cytoplasm by cross-linking actin filaments with plasma membrane proteins. Research has identified the nuclear localization of ERMs, as well as the involvement of a single Drosophila ERM protein, Moesin, in nuclear mRNA exports. However, the question of how important the nuclear activity of ERM proteins are for the life of an organism has so far not been explored. Here, we present the first attempt to reveal the in vivo relevance of nuclear localization of Moesin in Drosophila. With the help of a nuclear export signal, we decreased the amount of Moesin in the nuclei of the animals. Furthermore, we observed various developmental defects, demonstrating the importance of ERM function in the nucleus for the first time. Transcriptome analysis of the mutant flies revealed that the lack of nuclear Moesin function leads to expression changes in nearly 700 genes, among them heat-shock genes. This result together with additional findings revealed that in Drosophila the expression of protein chaperones requires the nuclear functions of Moesin. DATABASE: GEO accession number: GSE155778.

Characterization of the Drosophila suzukii β2-tubulin gene and the utilization of its promoter to monitor sex separation and insemination

The Drosophila melanogaster β2-tubulin gene (Dm-β2t) controls the function of microtubules in the testis and sperm, and has been evaluated for use in biocontrol strategies based on the sterile insect technique, including sexing and the induction of male sterility. The spotted-wing Drosophila (Drosophila suzukii) is native to eastern Asia but has spread globally as an invasive pest of fruit crops, so biocontrol strategies are urgently required for this species. We therefore isolated the β2tubulin ortholog Ds-β2t from the USA laboratory strain of D. suzukii and confirmed the presence of functional motifs by aligning orthologs from multiple insects. The developmental expression profile of Ds-β2t was determined by RT-PCR using gene-specific primers and was similar to that of Dm-β2t. We then isolated the Ds-β2t promoter and used it to generate transgenic strains expressing a testis-specific fluorescent protein starting from the thirdinstar larvae. Efficient sexing was achieved based on fluorescence detection, and the transgenic males showed a similar survival rate to wild-type males. Fluorescence imaging and PCR were also used to confirm the insemination of wild-type females by transgenic males. We therefore confirm that D. suzukii strains expressing fluorescent markers under the control of the Ds-β2t promoter can be used for sexing and the confirmation of mating, and we discuss the wider potential of the Ds-β2t promoter in the context of genetic control strategies for D. suzukii.

Gene expression during genital morphogenesis in the ground beetle Carabus maiyasanus

To investigate the developmental genetics of genital formation in the carabid beetle Carabus maiyasanus, we compared gene expression patterns among five stages using transcriptomic RNA sequencing data from abdominal segments and genitalia in the third (last) larval instar (including prepupa) and pupal stages. We identified 18 839 genes, of which 10 796 were differentially expressed among stages or between sexes. There were relatively few differentially expressed genes (DEGs) between the sexes (3%). The DEGs were clustered into six groups, mainly according to stage-specific expression patterns. Genes in clusters 1-3 showed high expression levels before pupation and low expression levels during the pupal period, whereas genes in clusters 4-6 showed high expression levels from the prepupal to the pupal stages. Genes related to the initial pupation process and differentiation of genital discs in Drosophila were involved in clusters 4 and 6 and showed low expression levels at early third instar and elevated expression levels from the early prepupal stage, suggesting that the pupation process and genital differentiation started in the prepupal stage. Clusters 4 and 5 included developmental genes related to organ size control, which may be important in the formation of internal genital structures during the pupal stage.

The Drosophila actin nucleator DAAM is essential for left-right asymmetry

Left-Right (LR) asymmetry is essential for organ positioning, shape and function. Myosin 1D (Myo1D) has emerged as an evolutionary conserved chirality determinant in both Drosophila and vertebrates. However, the molecular interplay between Myo1D and the actin cytoskeleton underlying symmetry breaking remains poorly understood. To address this question, we performed a dual genetic screen to identify new cytoskeletal factors involved in LR asymmetry. We identified the conserved actin nucleator DAAM as an essential factor required for both dextral and sinistral development. In the absence of DAAM, organs lose their LR asymmetry, while its overexpression enhances Myo1D-induced de novo LR asymmetry. These results show that DAAM is a limiting, LR-specific actin nucleator connecting up Myo1D with a dedicated F-actin network important for symmetry breaking.

Although our body looks symmetrical when viewed from the outside, it is in fact highly asymmetrical when we consider the shape and implantation of organs. For example, our heart is on the left side of the thorax, while the liver is on the right. In addition, our heart is made up of two distinct parts, the right heart and the left heart, which play different roles for blood circulation. These asymmetries, called left-right asymmetries, play a fundamental role in the morphogenesis and function of visceral organs and the brain. Aberrant LR asymmetry in human results in severe anatomical defects leading to embryonic lethality, spontaneous abortion and a number of congenital disorders. Our recent work has identified a particular myosin (Myo1D) as a major player in asymmetry in Drosophila and vertebrates. Myosins are proteins that can interact with the skeleton of cells (called the cytoskeleton) to transport other proteins, contract the cells, allow them to move, etc. In this work, we were able to identify all the genes of the cytoskeleton involved with myosin in left-right asymmetry, in particular a so-called 'nucleator' gene because it is capable of forming new parts of the cytoskeleton necessary for setting up asymmetries.

Individual, but not population asymmetries, are modulated by social environment and genotype in Drosophila melanogaster

Theory predicts that social interactions can induce an alignment of behavioral asymmetries between individuals (i.e., population-level lateralization), but evidence for this effect is mixed. To understand how interaction with other individuals affects behavioral asymmetries, we systematically manipulated the social environment of Drosophila melanogaster, testing individual flies and dyads (female-male, female-female and male-male pairs). In these social contexts we measured individual and population asymmetries in individual behaviors (circling asymmetry, wing use) and dyadic behaviors (relative position and orientation between two flies) in five different genotypes. We reasoned that if coordination between individuals drives alignment of behavioral asymmetries, greater alignment at the population-level should be observed in social contexts compared to solitary individuals. We observed that the presence of other individuals influenced the behavior and position of flies but had unexpected effects on individual and population asymmetries: individual-level asymmetries were strong and modulated by the social context but population-level asymmetries were mild or absent. Moreover, the strength of individual-level asymmetries differed between strains, but this was not the case for population-level asymmetries. These findings suggest that the degree of social interaction found in Drosophila is insufficient to drive population-level behavioral asymmetries.

Sertraline induces DNA damage and cellular toxicity in Drosophila that can be ameliorated by antioxidants

Sertraline hydrochloride is a commonly prescribed antidepressant medication that acts by amplifying serotonin signaling. Numerous studies have suggested that children of women  taking sertraline during pregnancy have an increased risk of developmental defects. Resolving the degree of risk for human fetuses requires comprehensive knowledge of the pathways affected by this drug. We utilized a Drosophila melanogaster model system to assess the effects of sertraline throughout development. Ingestion of sertraline by females did not affect their fecundity or embryogenesis in their progeny. However, larvae that consumed sertraline experienced delayed developmental progression and reduced survival at all stages of development. Genetic experiments showed that these effects were mostly independent of aberrant extracellular serotonin levels. Using an ex vivo imaginal disc culture system, we showed that mitotically active sertraline-treated tissues accumulate DNA double-strand breaks and undergo apoptosis at increased frequencies. Remarkably, the sertraline-induced genotoxicity was partially rescued by co-incubation with ascorbic acid, suggesting that sertraline induces oxidative DNA damage. These findings may have implications for the biomedicine of sertraline-induced birth defects.

Molecular to organismal chirality is induced by the conserved myosin 1D

The emergence of asymmetry from an initially symmetrical state is a universal transition in nature. Living organisms show asymmetries at the molecular, cellular, tissular, and organismal level. However, whether and how multilevel asymmetries are related remains unclear. In this study, we show that Drosophila myosin 1D (Myo1D) and myosin 1C (Myo1C) are sufficient to generate de novo directional twisting of cells, single organs, or the whole body in opposite directions. Directionality lies in the myosins' motor domain and is swappable between Myo1D and Myo1C. In addition, Myo1D drives gliding of actin filaments in circular, counterclockwise paths in vitro. Altogether, our results reveal the molecular motor Myo1D as a chiral determinant that is sufficient to break symmetry at all biological scales through chiral interaction with the actin cytoskeleton.

From cytoskeletal dynamics to organ asymmetry: a nonlinear, regulative pathway underlies left-right patterning

Consistent left-right (LR) asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single-cell behaviour and patterning of the LR axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct molecular and anatomical errors over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key 'determinant' genes. We provide novel functional data, in Xenopus laevis, on conserved elements of the cytoskeleton that drive asymmetry, and comparatively analyse it together with previously published results in the field. Our new observations and meta-analysis demonstrate that despite aberrant expression of upstream regulatory genes, embryos can progressively normalize transcriptional cascades and anatomical outcomes. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

HCN4 ion channel function is required for early events that regulate anatomical left-right patterning in a nodal and lefty asymmetric gene expression-independent manner

Laterality is a basic characteristic of all life forms, from single cell organisms to complex plants and animals. For many metazoans, consistent left-right asymmetric patterning is essential for the correct anatomy of internal organs, such as the heart, gut, and brain; disruption of left-right asymmetry patterning leads to an important class of birth defects in human patients. Laterality functions across multiple scales, where early embryonic, subcellular and chiral cytoskeletal events are coupled with asymmetric amplification mechanisms and gene regulatory networks leading to asymmetric physical forces that ultimately result in distinct left and right anatomical organ patterning. Recent studies have suggested the existence of multiple parallel pathways regulating organ asymmetry. Here, we show that an isoform of the hyperpolarization-activated cyclic nucleotide-gated (HCN) family of ion channels (hyperpolarization-activated cyclic nucleotide-gated channel 4, HCN4) is important for correct left-right patterning. HCN4 channels are present very early in Xenopus embryos. Blocking HCN channels (Ih currents) with pharmacological inhibitors leads to errors in organ situs. This effect is only seen when HCN4 channels are blocked early (pre-stage 10) and not by a later block (post-stage 10). Injections of HCN4-DN (dominant-negative) mRNA induce left-right defects only when injected in both blastomeres no later than the 2-cell stage. Analysis of key asymmetric genes' expression showed that the sidedness of Nodal, Lefty, and Pitx2 expression is largely unchanged by HCN4 blockade, despite the randomization of subsequent organ situs, although the area of Pitx2 expression was significantly reduced. Together these data identify a novel, developmental role for HCN4 channels and reveal a new Nodal-Lefty-Pitx2 asymmetric gene expression-independent mechanism upstream of organ positioning during embryonic left-right patterning.

Conserved roles for cytoskeletal components in determining laterality

Consistently-biased left-right (LR) patterning is required for the proper placement of organs including the heart and viscera. The LR axis is especially fascinating as an example of multi-scale pattern formation, since here chiral events at the subcellular level are integrated and amplified into asymmetric transcriptional cascades and ultimately into the anatomical patterning of the entire body. In contrast to the other two body axes, there is considerable controversy about the earliest mechanisms of embryonic laterality. Many molecular components of asymmetry have not been widely tested among phyla with diverse bodyplans, and it is unknown whether parallel (redundant) pathways may exist that could reverse abnormal asymmetry states at specific checkpoints in development. To address conservation of the early steps of LR patterning, we used the Xenopus laevis (frog) embryo to functionally test a number of protein targets known to direct asymmetry in plants, fruit fly, and rodent. Using the same reagents that randomize asymmetry in Arabidopsis, Drosophila, and mouse embryos, we show that manipulation of the microtubule and actin cytoskeleton immediately post-fertilization, but not later, results in laterality defects in Xenopus embryos. Moreover, we observed organ-specific randomization effects and a striking dissociation of organ situs from effects on the expression of left side control genes, which parallel data from Drosophila and mouse. Remarkably, some early manipulations that disrupt laterality of transcriptional asymmetry determinants can be subsequently "rescued" by the embryo, resulting in normal organ situs. These data reveal the existence of novel corrective mechanisms, demonstrate that asymmetric expression of Nodal is not a definitive marker of laterality, and suggest the existence of amplification pathways that connect early cytoskeletal processes to control of organ situs bypassing Nodal. Counter to alternative models of symmetry breaking during neurulation (via ciliary structures absent in many phyla), our data suggest a widely-conserved role for the cytoskeleton in regulating left-right axis formation immediately after fertilization of the egg. The novel mechanisms that rescue organ situs, even after incorrect expression of genes previously considered to be left-side master regulators, suggest LR patterning as a new context in which to explore multi-scale redundancy and integration of patterning from the subcellular structure to the entire bodyplan.

Left-right asymmetric cell intercalation drives directional collective cell movement in epithelial morphogenesis

Morphogenetic epithelial movement occurs during embryogenesis and drives complex tissue formation. However, how epithelial cells coordinate their unidirectional movement while maintaining epithelial integrity is unclear. Here we propose a novel mechanism for collective epithelial cell movement based on Drosophila genitalia rotation, in which epithelial tissue rotates clockwise around the genitalia. We found that this cell movement occurs autonomously and requires myosin II. The moving cells exhibit repeated left–right-biased junction remodelling, while maintaining adhesion with their neighbours, in association with a polarized myosin II distribution. Reducing myosinID, known to cause counter-clockwise epithelial-tissue movement, reverses the myosin II distribution. Numerical simulations revealed that a left–right asymmetry in cell intercalation is sufficient to induce unidirectional cellular movement. The cellular movement direction is also associated with planar cell-shape chirality. These findings support a model in which left–right asymmetric cell intercalation within an epithelial sheet drives collective cellular movement in the same direction.

Coordinated epithelial movement during embryogenesis drives complex tissue formation, but how this movement is coordinated to maintain epithelial integrity is not clear. Here the authors show that left-right asymmetry in cell intercalation drives clockwise rotation of epithelia in Drosophila genital development.

Hox control of Drosophila larval anatomy; The Alary and Thoracic Alary-Related Muscles

The body plan of arthropods and vertebrates involves the formation of repetitive segments, which subsequently diversify to give rise to different body parts along the antero-posterior/rostro-caudal body axis. Anatomical variations between body segments are crucial for organ function and organismal fitness. Pioneering work in Drosophila has established that Hox transcription factors play key roles both in endowing initially identical segments with distinct identities and organogenesis. The focus of this review is on Alary Muscles (AMs) and the newly discovered Thoracic Alary-Related Muscles (TARMs). AMs and TARMs are thin muscles which together connect the circulatory system and different midgut regions to the exoskeleton, while intertwining with the respiratory tubular network. They were hypothesized to represent a new type of muscles with spring-like properties, maintaining internal organs in proper anatomical positions during larval locomotion. Both the morphology of TARMs relative to AMs, and morphogenesis of connected tissues is under Hox control, emphasizing the key role of Hox proteins in coordinating the anatomical development of the larva.

The Atypical Cadherin Dachsous Controls Left-Right Asymmetry in Drosophila

Left-right (LR) asymmetry is essential for organ development and function in metazoans, but how initial LR cue is relayed to tissues still remains unclear. Here, we propose a mechanism by which the Drosophila LR determinant Myosin ID (MyoID) transfers LR information to neighboring cells through the planar cell polarity (PCP) atypical cadherin Dachsous (Ds). Molecular interaction between MyoID and Ds in a specific LR organizer controls dextral cell polarity of adjoining hindgut progenitors and is required for organ looping in adults. Loss of Ds blocks hindgut tissue polarization and looping, indicating that Ds is a crucial factor for both LR cue transmission and asymmetric morphogenesis. We further show that the Ds/Fat and Frizzled PCP pathways are required for the spreading of LR asymmetry throughout the hindgut progenitor tissue. These results identify a direct functional coupling between the LR determinant MyoID and PCP, essential for non-autonomous propagation of early LR asymmetry.

Left-right asymmetry in the light of TOR: An update on what we know so far

The internal left‐right (LR) asymmetry is a characteristic that exists throughout the animal kingdom from roundworms over flies and fish to mammals. Cilia, which are antenna‐like structures protruding into the extracellular space, are involved in establishing LR asymmetry during early development. Humans who suffer from dysfunctional cilia often develop conditions such as heterotaxy, where internal organs appear to be placed randomly. As a consequence to this failure in asymmetry development, serious complications such as congenital heart defects (CHD) occur. The mammalian (or mechanistic) target of rapamycin (mTOR) pathway has recently emerged as an important regulator regarding symmetry breaking. The mTOR pathway governs fundamental processes such as protein translation or metabolism. Its activity can be transduced by two complexes, which are called TORC1 and TORC2, respectively. So far, only TORC1 has been implicated with asymmetry development and appears to require very precise regulation. A number of recent papers provided evidence that dysregulated TORC1 results in alterations of motile cilia and asymmetry defects. In here, we give an update on what we know so far of mTORC1 in LR asymmetry development.

Diversity and convergence in the mechanisms establishing L/R asymmetry in metazoa

Differentiating left and right hand sides during embryogenesis represents a major event in body patterning. Left-Right (L/R) asymmetry in bilateria is essential for handed positioning, morphogenesis and ultimately the function of organs (including the brain), with defective L/R asymmetry leading to severe pathologies in human. How and when symmetry is initially broken during embryogenesis remains debated and is a major focus in the field. Work done over the past 20 years, in both vertebrate and invertebrate models, has revealed a number of distinct pathways and mechanisms important for establishing L/R asymmetry and for spreading it to tissues and organs. In this review, we summarize our current knowledge and discuss the diversity of L/R patterning from cells to organs during evolution.

The myosin ID pathway and left-right asymmetry in Drosophila

Drosophila is a classical model to study body patterning, however left-right (L/R) asymmetry had remained unexplored, until recently. The discovery of the conserved myosin ID gene as a major determinant of L/R asymmetry has revealed a novel L/R pathway involving the actin cytoskeleton and the adherens junction. In this process, the HOX gene Abdominal-B plays a major role through the control of myosin ID expression and therefore symmetry breaking. In this review, we present organs and markers showing L/R asymmetry in Drosophila and discuss our current understanding of the underlying molecular genetic mechanisms. Drosophila represents a valuable model system revealing novel strategies to establish L/R asymmetry in invertebrates and providing an evolutionary perspective to the problem of laterality in bilateria.

Reciprocal signaling between the ectoderm and a mesendodermal left-right organizer directs left-right determination in the sea urchin embryo

During echinoderm development, expression of nodal on the right side plays a crucial role in positioning of the rudiment on the left side, but the mechanisms that restrict nodal expression to the right side are not known. Here we show that establishment of left-right asymmetry in the sea urchin embryo relies on reciprocal signaling between the ectoderm and a left-right organizer located in the endomesoderm. FGF/ERK and BMP2/4 signaling are required to initiate nodal expression in this organizer, while Delta/Notch signaling is required to suppress formation of this organizer on the left side of the archenteron. Furthermore, we report that the H(+)/K(+)-ATPase is critically required in the Notch signaling pathway upstream of the S3 cleavage of Notch. Our results identify several novel players and key early steps responsible for initiation, restriction, and propagation of left-right asymmetry during embryogenesis of a non-chordate deuterostome and uncover a functional link between the H(+)/K(+)-ATPase and the Notch signaling pathway.

Paraxial left-sided nodal expression and the start of left-right patterning in the early chick embryo

A common element during early left-right patterning of the vertebrate body is left-sided nodal expression in the early-somite stage lateral plate mesoderm. Leftward cell movements near the node of the gastrulating chick embryo recently offered a plausible mechanism for breaking the presomite-stage molecular symmetry in those vertebrates which lack rotating cilia on the notochord or equivalent tissues. However, the temporal and functional relationships between generation of the known morphological node asymmetry, onset of leftward cell movements and establishment of stable molecular asymmetry in the chick remain unresolved. This study uses high-resolution light microscopy and in situ gene expression analysis to show that intranodal cell rearrangement during the phase of counter-clockwise node torsion at stage 4+ is immediately followed by symmetry loss and rearrangement of shh and fgf8 expression in node epiblast between stages 5- and 5+. Surprisingly, left-sided nodal expression starts at stage 5-, too, but lies in the paraxial mesoderm next to the forming notochordal plate, and can be rendered symmetrical by minimal mechanical disturbance of distant tissue integrity at stage 4. The "premature" paraxial nodal expression together with morphological and molecular asymmetries in, and near, midline compartments occurring at defined substages of early gastrulation help to identify a new narrow time window for early steps in left-right patterning in the chick and support the concept of a causal relationship between a-still enigmatic-chiral (motor) protein, cell movements and incipient left-right asymmetry in the amniote embryo.

DE-Cadherin regulates unconventional Myosin ID and Myosin IC in Drosophila left-right asymmetry establishment

In bilateria, positioning and looping of visceral organs requires proper left-right (L/R) asymmetry establishment. Recent work in Drosophila has identified a novel situs inversus gene encoding the unconventional type ID myosin (MyoID). In myoID mutant flies, the L/R axis is inverted, causing reversed looping of organs, such as the gut, spermiduct and genitalia. We have previously shown that MyoID interacts physically with β-Catenin, suggesting a role of the adherens junction in Drosophila L/R asymmetry. Here, we show that DE-Cadherin co-immunoprecipitates with MyoID and is required for MyoID L/R activity. We further demonstrate that MyoIC, a closely related unconventional type I myosin, can antagonize MyoID L/R activity by preventing its binding to adherens junction components, both in vitro and in vivo. Interestingly, DE-Cadherin inhibits MyoIC, providing a protective mechanism to MyoID function. Conditional genetic experiments indicate that DE-Cadherin, MyoIC and MyoID show temporal synchronicity for their function in L/R asymmetry. These data suggest that following MyoID recruitment by β-Catenin at the adherens junction, DE-Cadherin has a twofold effect on Drosophila L/R asymmetry by promoting MyoID activity and repressing that of MyoIC. Interestingly, the product of the vertebrate situs inversus gene inversin also physically interacts with β-Catenin, suggesting that the adherens junction might serve as a conserved platform for determinants to establish L/R asymmetry both in vertebrates and invertebrates.

Functional correlates of positional and gender-specific renal asymmetry in Drosophila

BACKGROUND

In humans and other animals, the internal organs are positioned asymmetrically in the body cavity, and disruption of this body plan can be fatal in humans. The mechanisms by which internal asymmetry are established are presently the subject of intense study; however, the functional significance of internal asymmetry (outside the brain) is largely unexplored. Is internal asymmetry functionally significant, or merely an expedient way of packing organs into a cavity?

METHODOLOGY/PRINCIPAL FINDINGS

Like humans, Drosophila shows internal asymmetry, with the gut thrown into stereotyped folds. There is also renal asymmetry, with the rightmost pair of renal (Malpighian) tubules always ramifying anteriorly, and the leftmost pair always sitting posteriorly in the body cavity. Accordingly, transcriptomes of anterior-directed (right-side) and posterior-directed (left-side) Malpighian (renal) tubules were compared in both adult male and female Drosophila. Although genes encoding the basic functions of the tubules (transport, signalling) were uniformly expressed, some functions (like innate immunity) showed positional or gender differences in emphasis; others, like calcium handling or the generation of potentially toxic ammonia, were reserved for just the right-side or left-side tubules, respectively. These findings correlated with the distinct locations of each tubule pair within the body cavity. Well known developmental genes (like dorsocross, dachshund and doublesex) showed continuing, patterned expression in adult tubules, implying that somatic tissues maintain both left-right and gender identities throughout life. Gender asymmetry was also noted, both in defence and in male-specific expression of receptors for neuropeptide F and sex-peptide: NPF elevated calcium only in male tubules.

CONCLUSIONS/SIGNIFICANCE

Accordingly, the physical asymmetry of the tubules in the body cavity is directly adaptive. Now that the detailed machinery underlying internal asymmetry is starting to be delineated, our work invites the investigation, not just of tissues in isolation, but in the context of their unique physical locations and milieux.

Canonical Wnt signaling in the visceral muscle is required for left-right asymmetric development of the Drosophila midgut

Many animals develop left-right (LR) asymmetry in their internal organs. The mechanisms of LR asymmetric development are evolutionarily divergent, and are poorly understood in invertebrates. Therefore, we studied the genetic pathway of LR asymmetric development in Drosophila. Drosophila has several organs that show directional and stereotypic LR asymmetry, including the embryonic gut, which is the first organ to develop LR asymmetry during Drosophila development. In this study, we found that genes encoding components of the Wnt-signaling pathway are required for LR asymmetric development of the anterior part of the embryonic midgut (AMG). frizzled 2 (fz2) and Wnt4, which encode a receptor and ligand of Wnt signaling, respectively, were required for the LR asymmetric development of the AMG. arrow (arr), an ortholog of the mammalian gene encoding low-density lipoprotein receptor-related protein 5/6, which is a co-receptor of the Wnt-signaling pathway, was also essential for LR asymmetric development of the AMG. These results are the first demonstration that Wnt signaling contributes to LR asymmetric development in invertebrates, as it does in vertebrates. The AMG consists of visceral muscle and an epithelial tube. Our genetic analyses revealed that Wnt signaling in the visceral muscle but not the epithelium of the midgut is required for the AMG to develop its normal laterality. Furthermore, fz2 and Wnt4 were expressed in the visceral muscles of the midgut. Consistent with these results, we observed that the LR asymmetric rearrangement of the visceral muscle cells, the first visible asymmetry of the developing AMG, did not occur in embryos lacking Wnt4 expression. Our results also suggest that canonical Wnt/β-catenin signaling, but not non-canonical Wnt signaling, is responsible for the LR asymmetric development of the AMG. Canonical Wnt/β-catenin signaling is reported to have important roles in LR asymmetric development in zebrafish. Thus, the contribution of canonical Wnt/β-catenin signaling to LR asymmetric development may be an evolutionarily conserved feature between vertebrates and invertebrates.

Population stochasticity, random determination of handedness, and the genetic basis of antisymmetry

Conspicuous lateral asymmetries of organisms are classified into two major categories: antisymmetry (AS), characterized by almost equal frequencies of dextral and sinistral morphs, and directional asymmetry (DA), in which one morph dominates. I compared and characterized two types of genes, both with existing examples, in their roles in the evolutionary transitions between AS and DA for the first time. Handedness genes (HGs) determine the chirality in a strict sense, while randomization genes (RGs) randomize the chirality. A theory predicts that, in an AS population maintained by HGs under negative frequency-dependent selection, RGs harness fluctuation of the morph frequencies as their driving force and thus increase their frequency until half of the population flips the phenotype. These predictions were confirmed by simulations. Consequently, RGs mask the genetic effects of HGs, which provides a possible explanation for the apparent lack of a genetic basis for AS in empirical AS studies.

Low frequency vibrations disrupt left-right patterning in the Xenopus embryo

The development of consistent left-right (LR) asymmetry across phyla is a fascinating question in biology. While many pharmacological and molecular approaches have been used to explore molecular mechanisms, it has proven difficult to exert precise temporal control over functional perturbations. Here, we took advantage of acoustical vibration to disrupt LR patterning in Xenopus embryos during tightly-circumscribed periods of development. Exposure to several low frequencies induced specific randomization of three internal organs (heterotaxia). Investigating one frequency (7 Hz), we found two discrete periods of sensitivity to vibration; during the first period, vibration affected the same LR pathway as nocodazole, while during the second period, vibration affected the integrity of the epithelial barrier; both are required for normal LR patterning. Our results indicate that low frequency vibrations disrupt two steps in the early LR pathway: the orientation of the LR axis with the other two axes, and the amplification/restriction of downstream LR signals to asymmetric organs.

UVRAG is required for organ rotation by regulating Notch endocytosis in Drosophila

Heterotaxy characterized by abnormal left-right body asymmetry causes diverse congenital anomalies. Organ rotation is a crucial developmental process to establish the left-right patterning during animal development. However, the molecular basis of how organ rotation is regulated is poorly understood. Here we report that Drosophila UV-resistance associated gene (UVRAG), a tumor suppressor that regulates autophagy and endocytosis, plays unexpected roles in controlling organ rotation. Loss-of-function mutants of UVRAG show seriously impaired organ rotation phenotypes, which are associated with defects in endocytic trafficking rather than autophagy. Blunted endocytic degradation by UVRAG deficiency causes endosomal accumulation of Notch, resulting in abnormally enhanced Notch activity. Knockdown of Notch itself or expression of a dominant negative form of Notch transcriptional co-activator Mastermind is sufficient to rescue the rotation defect in UVRAG mutants. Consistently, UVRAG-mutated heterotaxy patient cells also display highly increased Notch protein levels. These results suggest evolutionarily conserved roles of UVRAG in organ rotation by regulating Notch endocytic degradation.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Everything you always wanted to know about sex ... in flies

'Everything you always wanted to know about sex' is a workshop organized as part of the annual Drosophila Research Conference of the Genetics Society of America. This workshop provides an intellectual venue for interaction among research groups that study sexual dimorphism from the molecular, evolutionary, genomic, and behavioral perspectives. The speakers summarize the key ideas behind their research for people working in other fields, outline unsolved questions, and offer their opinions about future directions. The 2010 workshop highlighted the power of the Drosophila model for understanding sexual dimorphism at levels ranging from cell biology and gene regulation to population genetics and genome evolution, and demonstrated the importance of cross-disciplinary interactions in the study of sex. In this respect, Drosophila sets a good example for research in other organisms, including humans and their mammalian relatives.

Drosophila provides rapid modeling of renal development, function, and disease

The evolution of specialized excretory cells is a cornerstone of the metazoan radiation, and the basic tasks performed by Drosophila and human renal systems are similar. The development of the Drosophila renal (Malpighian) tubule is a classic example of branched tubular morphogenesis, allowing study of mesenchymal-to-epithelial transitions, stem cell-mediated regeneration, and the evolution of a glomerular kidney. Tubule function employs conserved transport proteins, such as the Na(+), K(+)-ATPase and V-ATPase, aquaporins, inward rectifier K(+) channels, and organic solute transporters, regulated by cAMP, cGMP, nitric oxide, and calcium. In addition to generation and selective reabsorption of primary urine, the tubule plays roles in metabolism and excretion of xenobiotics, and in innate immunity. The gene expression resource FlyAtlas.org shows that the tubule is an ideal tissue for the modeling of renal diseases, such as nephrolithiasis and Bartter syndrome, or for inborn errors of metabolism. Studies are assisted by uniquely powerful genetic and transgenic resources, the widespread availability of mutant stocks, and low-cost, rapid deployment of new transgenics to allow manipulation of renal function in an organotypic context.

Coupling of apoptosis and L/R patterning controls stepwise organ looping

Handed asymmetry in organ shape and positioning is a common feature among bilateria, yet little is known about the morphogenetic mechanisms underlying left-right (LR) organogenesis. We utilize the directional 360° clockwise rotation of genitalia in Drosophila to study LR-dependent organ looping. Using time-lapse imaging, we show that rotation of genitalia by 360° results from an additive process involving two ring-shaped domains, each undergoing 180° rotation. Our results show that the direction of rotation for each ring is autonomous and strictly depends on the LR determinant myosin ID (MyoID). Specific inactivation of MyoID in one domain causes rings to rotate in opposite directions and thereby cancels out the overall movement. We further reveal a specific pattern of apoptosis at the ring boundaries and show that local cell death is required for the movement of each domain, acting as a brake-releaser. These data indicate that organ looping can proceed through an incremental mechanism coupling LR determination and apoptosis. Furthermore, they suggest a model for the stepwise evolution of genitalia posture in Diptera, through the emergence and duplication of a 180° LR module.

Left-right asymmetric morphogenesis of the anterior midgut depends on the activation of a non-muscle myosin II in Drosophila

Many animals exhibit stereotypical left-right (LR) asymmetry in their internal organs. The mechanisms of LR axis formation required for the subsequent LR asymmetric development are well understood, especially in some vertebrates. However, the molecular mechanisms underlying LR asymmetric morphogenesis, particularly how mechanical force is integrated into the LR asymmetric morphogenesis of organs, are poorly understood. Here, we identified zipper (zip), encoding a Drosophila non-muscle myosin II (myosin II) heavy chain, as a gene required for LR asymmetric development of the embryonic anterior midgut (AMG). Myosin II is known to directly generate mechanical force in various types of cells during morphogenesis and cell migration. We found that myosin II was involved in two events in the LR asymmetric development of the AMG. First, it introduced an LR bias to the directional position of circular visceral muscle (CVMU) cells, which externally cover the midgut epithelium. Second, it was required for the LR-biased rotation of the AMG. Our results suggest that myosin II in CVMU cells plays a crucial role in generating the force leading to LR asymmetric morphogenesis. Taken together with previous studies in vertebrates, the involvement of myosin II in LR asymmetric morphogenesis might be conserved evolutionarily.

Tenectin is a novel alphaPS2betaPS integrin ligand required for wing morphogenesis and male genital looping in Drosophila

Morphogenesis of the adult structures of holometabolous insects is regulated by ecdysteroids and juvenile hormones and involves cell-cell interactions mediated in part by the cell surface integrin receptors and their extracellular matrix (ECM) ligands. These adhesion molecules and their regulation by hormones are not well characterized. We describe the gene structure of a newly described ECM molecule, tenectin, and demonstrate that it is a hormonally regulated ECM protein required for proper morphogenesis of the adult wing and male genitalia. Tenectin's function as a new ligand of the PS2 integrins is demonstrated by both genetic interactions in the fly and by cell spreading and cell adhesion assays in cultured cells. Its interaction with the PS2 integrins is dependent on RGD and RGD-like motifs. Tenectin's function in looping morphogenesis in the development of the male genitalia led to experiments that demonstrate a role for PS integrins in the execution of left-right asymmetry.

Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

Left-right asymmetry in the chick embryo requires core planar cell polarity protein Vangl2

Consistent left-right patterning is a fascinating and biomedically important problem. In the chick embryo, it is not known how cells determine their position (left or right) relative to the primitive streak, which is required for subsequent asymmetric gene expression cascades. We show that the subcellular localization of Vangl2, a core planar cell polarity (PCP) protein, is consistently polarized, giving cells in the blastoderm a vector pointing toward the primitive streak. Moreover, morpholino-mediated loss-of-function of Vangl2 by electroporation into chicks at very early stages randomizes the normally left-sided expression of Sonic hedgehog. Strikingly, Vangl2 morpholinos also induce a desynchronization of asymmetric gene expression within the left and right domains of Hensen's node. These data reveal the existence of polarized planar cell polarity protein localization in gastrulating chick and demonstrate that the PCP pathway is functionally required for normal asymmetry in the chick upstream of Sonic hedgehog. These data suggest a new and widely applicable class of models for the spread and coordination of left-right patterning information in the embryonic blastoderm.

Is left-right asymmetry a form of planar cell polarity?

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

The development and evolution of left-right asymmetry in invertebrates: lessons from Drosophila and snails

The unique nature of body handedness, which is distinct from the anteroposterior and dorsoventral polarities, has been attracting growing interest in diverse biological disciplines. Recent research progress on the left-right asymmetry of animal development has focused new attention on the mechanisms underlying the development and evolution of invertebrate handedness. This exploratory review of currently available information illuminates the prospective value of Drosophila and pulmonate snails for innovative new research aimed at elucidating these mechanisms. For example, findings in Drosophila and snails suggest that an actin filament-dependent mechanism may be evolutionarily conserved in protostomes. The polarity conservation of primary asymmetry across most metazoan phyla, which visceral handedness represents, indicates developmental constraint and purifying selection as possible but unexplored mechanisms. Comparative studies using Drosophila and snails, which have the great advantages of using genetic and evolutionary approaches, will accelerate our understanding of the mechanisms governing the conservation and diversity of animal handedness.