Left-right asymmetry: class I myosins show the direction

Myosin superfamily members during myelin formation and regeneration

Myelin is an insulator that forms around axons that enhance the conduction velocity of nerve fibers. Oligodendrocytes dramatically change cell morphology to produce myelin throughout the central nervous system (CNS). Cytoskeletal alterations are critical for the morphogenesis of oligodendrocytes, and actin is involved in cell differentiation and myelin wrapping via polymerization and depolymerization, respectively. Various protein members of the myosin superfamily are known to be major binding partners of actin filaments and have been intensively researched because of their involvement in various cellular functions, including differentiation, cell movement, membrane trafficking, organelle transport, signal transduction, and morphogenesis. Some members of the myosin superfamily have been found to play important roles in the differentiation of oligodendrocytes and in CNS myelination. Interestingly, each member of the myosin superfamily expressed in oligodendrocyte lineage cells also shows specific spatial and temporal expression patterns and different distributions. In this review, we summarize previous findings related to the myosin superfamily and discuss how these molecules contribute to myelin formation and regeneration by oligodendrocytes.

Actin-membrane linkers: Insights from synthetic reconstituted systems

At the cell surface, the actin cytoskeleton and the plasma membrane interact reciprocally in a variety of processes related to the remodeling of the cell surface. The actin cytoskeleton has been known to modulate membrane organization and reshape the membrane. To this end, actin-membrane linking molecules play a major role in regulating actin assembly and spatially direct the interaction between the actin cytoskeleton and the membrane. While studies in cells have provided a wealth of knowledge on the molecular composition and interactions of the actin-membrane interface, the complex molecular interactions make it challenging to elucidate the precise actions of the actin-membrane linkers at the interface. Synthetic reconstituted systems, consisting of model membranes and purified proteins, have been a powerful approach to elucidate how actin-membrane linkers direct actin assembly to drive membrane shape changes. In this review, we will focus only on several actin-membrane linkers that have been studied by using reconstitution systems. We will discuss the design principles of these reconstitution systems and how they have contributed to the understanding of the cellular functions of actin-membrane linkers. Finally, we will provide a perspective on future research directions in understanding the intricate actin-membrane interaction.

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.

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.

E and ID proteins regulate cell chirality and left-right asymmetric development in Drosophila

How left-right (LR) asymmetric forms in the animal body is a fundamental problem in Developmental Biology. Although the mechanisms for LR asymmetry are well studied in some species, they are still poorly understood in invertebrates. We previously showed that the intrinsic LR asymmetry of cells (designated as cell chirality) drives LR asymmetric development in the Drosophila embryonic hindgut, although the machinery of the cell chirality formation remains elusive. Here, we found that the Drosophila homologue of the Id gene, extra macrochaetae (emc), is required for the normal LR asymmetric morphogenesis of this organ. Id proteins, including Emc, are known to interact with and inhibit E-box-binding proteins (E proteins), such as Drosophila Daughterless (Da). We found that the suppression of da by wild-type emc was essential for cell chirality formation and for normal LR asymmetric development of the embryonic hindgut. Myosin ID (MyoID), which encodes the Drosophila Myosin ID protein, is known to regulate cell chirality. We further showed that Emc-Da regulates cell chirality formation, in which Emc functions upstream of or parallel to MyoID. Abnormal Id-E protein regulation is involved in various human diseases. Our results suggest that defects in cell shape may contribute to the pathogenesis of such diseases.

Cell chirality: emergence of asymmetry from cell culture

Increasing evidence suggests that intrinsic cell chirality significantly contributes to the left-right (LR) asymmetry in embryonic development, which is a well-conserved characteristic of living organisms. With animal embryos, several theories have been established, but there are still controversies regarding mechanisms associated with embryonic LR symmetry breaking and the formation of asymmetric internal organs. Recently, in vitro systems have been developed to determine cell chirality and to recapitulate multicellular chiral morphogenesis on a chip. These studies demonstrate that chirality is indeed a universal property of the cell that can be observed with well-controlled experiments such as micropatterning. In this paper, we discuss the possible benefits of these in vitro systems to research in LR asymmetry, categorize available platforms for single-cell chirality and multicellular chiral morphogenesis, and review mathematical models used for in vitro cell chirality and its applications in in vivo embryonic development. These recent developments enable the interrogation of the intracellular machinery in LR axis establishment and accelerate research in birth defects in laterality.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

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.

Unconventional myosin ID is expressed in myelinating oligodendrocytes

Myelin is a dynamic multilamellar structure that ensheathes axons and is crucial for normal neuronal function. In the central nervous system (CNS), myelin is produced by oligodendrocytes that wrap many layers of plasma membrane around axons. The dynamic membrane trafficking system, which relies on motor proteins, is required for myelin formation and maintenance. Previously, we found that myosin ID (Myo1d), a class I myosin, is enriched in the rat CNS myelin fraction. Myo1d is an unconventional myosin and has been shown to be involved in membrane trafficking in the recycling pathway in an epithelial cell line. Western blotting revealed that Myo1d expression begins early in myelinogenesis and continues to increase into adulthood. The localization of Myo1d in CNS myelin has not been reported, and the function of Myo1d in vivo remains unknown. To demonstrate the expression of Myo1d in CNS myelin and to begin to explore the function of Myo1d in myelination, we produced a new antibody against Myo1d that has a high titer and specificity for rat Myo1d. By using this antibody, we demonstrated that Myo1d is expressed in rat CNS myelin and is especially abundant in abaxonal and adaxonal regions (the outer and inner cytoplasm-containing loops, respectively), but that expression is low in peripheral nervous system myelin. In culture, Myo1d was expressed in mature rat oligodendrocytes. Furthermore, an increase in expression of Myo1d during maturation of CNS white matter (cerebellum and corpus callosum) was demonstrated by histological analysis. These results suggest that Myo1d may be involved in the formation and/or maintenance of CNS myelin.

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.

Micropatterning chiral morphogenesis

A recently developed technique enables quantitative study of the initiation of left-right asymmetry using cells grown on micropatterns with close appositional boundaries. It was found that mammalian cells exhibit either a left or right bias in their migratory behavior, which was determined by cell phenotype, different for certain cancer and normal cells, and dependent on functionality of the actin cytoskeleton. We discuss here the relevance of this simple technique to study of development and birth defects in laterality.

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.

Mechanical force alters morphogenetic movements and segmental gene expression patterns during Drosophila embryogenesis

The development of an organism is accompanied by various cellular morphogenetic movements, changes in cellular as well as nuclear morphology and transcription programs. Recent evidence suggests that intra and inter-cellular connections mediated by various adhesion proteins contribute to defining nuclear morphology. In addition, three dimensional organization of the cell nucleus regulate the transcription programs. However the link between cellular morphogenetic movements and its coupling to nuclear function in a developmental context is poorly understood. In this paper we use a point perturbation by tissue level laser ablation and sheet perturbation by application of force using magnetic tweezers to alter cellular morphogenetic movements and probe its impact on nuclear morphology and segmental gene expression patterns. Mechanical perturbations during blastoderm stage in a developing Drosophila embryo resulted in localized alterations in nuclear morphology and cellular movement. In addition, global defects in germ-band (GB) extension and retraction are observed when external force is applied during morphogenetic movements, suggesting a long-range physical coupling within the GB layer of cells. Further local application of force resulted in redistribution of non muscle myosin-II in the GB layer. Finally these perturbations lead to altered segmental gene (engrailed) expression patterns later during the development. Our observations suggest that there exists a tight regulation between nuclear morphology and cellular adhesive connections during morphogenetic movement of cells in the embryo. The observed spatial changes in patterning genes, with perturbation, highlight the importance of nuclear integrity to cellular movement in establishing gene expression program in a developmental system.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Micropatterned mammalian cells exhibit phenotype-specific left-right asymmetry

Left-right (LR) asymmetry (handedness, chirality) is a well-conserved biological property of critical importance to normal development. Changes in orientation of the LR axis due to genetic or environmental factors can lead to malformations and disease. While the LR asymmetry of organs and whole organisms has been extensively studied, little is known about the LR asymmetry at cellular and multicellular levels. Here we show that the cultivation of cell populations on micropatterns with defined boundaries reveals intrinsic cell chirality that can be readily determined by image analysis of cell alignment and directional motion. By patterning 11 different types of cells on ring-shaped micropatterns of various sizes, we found that each cell type exhibited definite LR asymmetry (p value down to 10(-185)) that was different between normal and cancer cells of the same type, and not dependent on surface chemistry, protein coating, or the orientation of the gravitational field. Interestingly, drugs interfering with actin but not microtubule function reversed the LR asymmetry in some cell types. Our results show that micropatterned cell populations exhibit phenotype-specific LR asymmetry that is dependent on the functionality of the actin cytoskeleton. We propose that micropatterning could potentially be used as an effective in vitro tool to study the initiation of LR asymmetry in cell populations, to diagnose disease, and to study factors involved with birth defects in laterality.

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.

The Drosophila serine protease homologue Scarface regulates JNK signalling in a negative-feedback loop during epithelial morphogenesis

In Drosophila melanogaster, dorsal closure is a model of tissue morphogenesis leading to the dorsal migration and sealing of the embryonic ectoderm. The activation of the JNK signal transduction pathway, specifically in the leading edge cells, is essential to this process. In a genome-wide microarray screen, we identified new JNK target genes during dorsal closure. One of them is the gene scarface (scaf), which belongs to the large family of trypsin-like serine proteases. Some proteins of this family, like Scaf, bear an inactive catalytic site, representing a subgroup of serine protease homologues (SPH) whose functions are poorly understood. Here, we show that scaf is a general transcriptional target of the JNK pathway coding for a secreted SPH. scaf loss-of-function induces defects in JNK-controlled morphogenetic events such as embryonic dorsal closure and adult male terminalia rotation. Live imaging of the latter process reveals that, like for dorsal closure, JNK directs the dorsal fusion of two epithelial layers in the pupal genital disc. Genetic data show that scaf loss-of-function mimics JNK over-activity. Moreover, scaf ectopic expression aggravates the effect of the JNK negative regulator puc on male genitalia rotation. We finally demonstrate that scaf acts as an antagonist by negatively regulating JNK activity. Overall, our results identify the SPH-encoding gene scaf as a new transcriptional target of JNK signalling and reveal the first secreted regulator of the JNK pathway acting in a negative-feedback loop during epithelial morphogenesis.

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.

H,K-ATPase protein localization and Kir4.1 function reveal concordance of three axes during early determination of left-right asymmetry

Consistent laterality is a fascinating problem, and study of the Xenopus embryo has led to molecular characterization of extremely early steps in left-right patterning: bioelectrical signals produced by ion pumps functioning upstream of asymmetric gene expression. Here, we reveal a number of novel aspects of the H+/K+-ATPase module in chick and frog embryos. Maternal H+/K+-ATPase subunits are asymmetrically localized along the left-right, dorso-ventral, and animal-vegetal axes during the first cleavage stages, in a process dependent on cytoskeletal organization. Using a reporter domain fused to molecular motors, we show that the cytoskeleton of the early frog embryo can provide asymmetric, directional information for subcellular transport along all three axes. Moreover, we show that the Kir4.1 potassium channel, while symmetrically expressed in a dynamic fashion during early cleavages, is required for normal LR asymmetry of frog embryos. Thus, Kir4.1 is an ideal candidate for the K+ ion exit path needed to allow the electroneutral H+/K+-ATPase to generate voltage gradients. In the chick embryo, we show that H+/K+-ATPase and Kir4.1 are expressed in the primitive streak, and that the known requirement for H+/K+-ATPase function in chick asymmetry does not function through effects on the circumferential expression pattern of Connexin43. These data provide details crucial for the mechanistic modeling of the physiological events linking subcellular processes to large-scale patterning and suggest a model where the early cytoskeleton sets up asymmetric ion flux along the left-right axis as a system of planar polarity functioning orthogonal to the apical-basal polarity of the early blastomeres.

Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

Roles for Drosophila melanogaster myosin IB in maintenance of enterocyte brush-border structure and resistance to the bacterial pathogen Pseudomonas entomophila

Drosophila myosin IB (Myo1B) is one of two class I myosins in the Drosophila genome. In the larval and adult midgut enterocyte, Myo1B is present within the microvillus (MV) of the apical brush border (BB) where it forms lateral tethers between the MV membrane and underlying actin filament core. Expression of green fluorescent protein-Myo1B tail domain in the larval gut showed that the tail domain is sufficient for localization of Myo1B to the BB. A Myo1B deletion mutation exhibited normal larval gut physiology with respect to food uptake, clearance, and pH regulation. However, there is a threefold increase in terminal deoxynucleotidyl transferase dUTP nick-end labeling-positive enterocyte nuclei in the Myo1B mutant. Ultrastructural analysis of mutant midgut revealed many perturbations in the BB, including membrane tethering defects, MV vesiculation, and membrane shedding. The apical localization of both singed (fascin) and Dmoesin is impaired. BBs isolated from mutant and control midgut revealed that the loss of Myo1B causes the BB membrane and underlying cytoskeleton to become destabilized. Myo1B mutant larvae also exhibit enhanced sensitivity to oral infection by the bacterial pathogen Pseudomonas entomophila, and severe cytoskeletal defects are observed in the BB of proximal midgut epithelial cells soon after infection. Resistance to P. entomophila infection is restored in Myo1B mutant larvae expressing a Myo1B transgene. These results indicate that Myo1B may play a role in the local midgut response pathway of the Imd innate immune response to Gram-negative bacterial infection.

Strategies to establish left/right asymmetry in vertebrates and invertebrates

Left/right (L/R) asymmetry is essential during embryonic development for organ positioning, looping and handed morphogenesis. A major goal in the field is to understand how embryos initially determine their left and right hand sides, a process known as symmetry breaking. A number of recent studies on several vertebrate and invertebrate model organisms have provided a more complex view on how L/R asymmetry is established, revealing an apparent partition between deuterostomes and protostomes. In deuterostomes, nodal cilia represent a conserved symmetry-breaking process; nevertheless, growing evidence shows the existence of pre-cilia L/R asymmetries involving active ion flows. In protostomes like snails and Drosophila, symmetry breaking relies on different mechanisms, involving, in particular, the actin cytoskeleton and associated molecular motors.