Syndapin promotes pseudocleavage furrow formation by actin organization in the syncytial Drosophila embryo

Reshaping the Syncytial Drosophila Embryo with Cortical Actin Networks: Four Main Steps of Early Development

Drosophila development begins as a syncytium. The large size of the one-cell embryo makes it ideal for studying the structure, regulation, and effects of the cortical actin cytoskeleton. We review four main steps of early development that depend on the actin cortex. At each step, dynamic remodelling of the cortex has specific effects on nuclei within the syncytium. During axial expansion, a cortical actomyosin network assembles and disassembles with the cell cycle, generating cytoplasmic flows that evenly distribute nuclei along the ovoid cell. When nuclei move to the cell periphery, they seed Arp2/3-based actin caps which grow into an array of dome-like compartments that house the nuclei as they divide at the cell cortex. To separate germline nuclei from the soma, posterior germ plasm induces full cleavage of mono-nucleated primordial germ cells from the syncytium. Finally, zygotic gene expression triggers formation of the blastoderm epithelium via cellularization and simultaneous division of ~6000 mono-nucleated cells from a single internal yolk cell. During these steps, the cortex is regulated in space and time, gains domain and sub-domain structure, and undergoes mesoscale interactions that lay a structural foundation of animal development.

Adhesion and Polarity protein distribution-regulates hexagon dominated plasma membrane organization in Drosophila blastoderm embryos

Epithelial cells contain polarity complexes on the lateral membrane and are organized in a hexagon-dominated polygonal array. The mechanisms regulating the organization of polygonal architecture in metazoan embryogenesis are not completely understood. Drosophila embryogenesis enables mechanistic analysis of epithelial polarity formation and its impact on polygonal organization. The plasma membrane (PM) of syncytial Drosophila blastoderm embryos is organized as a polygonal array with pseudocleavage furrow formation during the almost synchronous cortical division cycles. We find that polygonal (PM) organization arises in the metaphase (MP) of division cycle 11, and hexagon dominance occurs with an increase in furrow length in the metaphase of cycle 12. There is a decrease in cell shape index in metaphase from cycles 11 to 13. This coincides with Drosophila E-cad (DE-cadherin) and Bazooka enrichment at the edges and the septin, Peanut at the vertices of the furrow. We further assess the role of polarity and adhesion proteins in pseudocleavage furrow formation and its organization as a polygonal array. We find that DE-cadherin depletion leads to decreased furrow length, loss of hexagon dominance, and increased cell shape index. Bazooka and Peanut depletion lead to decreased furrow length, delay in onset of hexagon dominance from cycle 12 to 13, and increased cell shape index. Hexagon dominance occurs with an increase in furrow length in cycle 13 and increased DE-cadherin, possibly due to the inhibition of endocytosis. We conclude that polarity protein recruitment and regulation of endocytic pathways enable pseudocleavage furrow stability and the formation of a hexagon-dominated polygon array.

Dysregulation of the endoplasmic reticulum blocks recruitment of centrosome-associated proteins resulting in mitotic failure

The endoplasmic reticulum (ER) undergoes a remarkable transition in morphology during cell division to aid in the proper portioning of the ER. However, whether changes in ER behaviors modulate mitotic events is less clear. Like many animal embryos, the early Drosophila embryo undergoes rapid cleavage cycles in a lipid-rich environment. Here, we show that mitotic spindle formation, centrosomal maturation, and ER condensation occur with similar time frames in the early syncytium. In a screen for Rab family GTPases that display dynamic function at these stages, we identified Rab1. Rab1 disruption led to an enhanced buildup of ER at the spindle poles and produced an intriguing 'mini-spindle' phenotype. ER accumulation around the mitotic space negatively correlates with spindle length/intensity. Importantly, centrosomal maturation is defective in these embryos, as mitotic recruitment of key centrosomal proteins is weakened after Rab1 disruption. Finally, division failures and ER overaccumulation is rescued by Dynein inhibition, demonstrating that Dynein is essential for ER spindle recruitment. These results reveal that ER levels must be carefully tuned during mitotic processes to ensure proper assembly of the division machinery.

Lipid Polarization during Cytokinesis

The plasma membrane of eukaryotic cells is composed of a large number of lipid species that are laterally segregated into functional domains as well as asymmetrically distributed between the outer and inner leaflets. Additionally, the spatial distribution and organization of these lipids dramatically change in response to various cellular states, such as cell division, differentiation, and apoptosis. Division of one cell into two daughter cells is one of the most fundamental requirements for the sustenance of growth in all living organisms. The successful completion of cytokinesis, the final stage of cell division, is critically dependent on the spatial distribution and organization of specific lipids. In this review, we discuss the properties of various lipid species associated with cytokinesis and the mechanisms involved in their polarization, including forward trafficking, endocytic recycling, local synthesis, and cortical flow models. The differences in lipid species requirements and distribution in mitotic vs. male meiotic cells will be discussed. We will concentrate on sphingolipids and phosphatidylinositols because their transbilayer organization and movement may be linked via the cytoskeleton and thus critically regulate various steps of cytokinesis.

Pseudocleavage furrows restrict plasma membrane-associated PH domain in syncytial Drosophila embryos

Syncytial cells contain multiple nuclei and have local distribution and function of cellular components despite being synthesized in a common cytoplasm. The syncytial Drosophila blastoderm embryo shows reduced spread of organelle and plasma membrane-associated proteins between adjacent nucleo-cytoplasmic domains. Anchoring to the cytoarchitecture within a nucleo-cytoplasmic domain is likely to decrease the spread of molecules; however, its role in restricting this spread has not been assessed. In order to analyze the cellular mechanisms that regulate the rate of spread of plasma membrane-associated molecules in the syncytial Drosophila embryos, we express a pleckstrin homology (PH) domain in a localized manner at the anterior of the embryo by tagging it with the bicoid mRNA localization signal. Anteriorly expressed PH-domain forms an exponential gradient in the anteroposterior axis with a longer length scale as compared to Bicoid. Using a combination of experiments and theoretical modeling, we find that the characteristic distribution and length scale emerge due to plasma membrane sequestration and restriction within an energid. Loss of plasma membrane remodeling to form pseudocleavage furrows shows an enhanced spread of PH-domain but not Bicoid. Modeling analysis suggests that the enhanced spread of the PH-domain occurs due to the increased spread of the cytoplasmic population of the PH-domain in pseudocleavage furrow mutants. Our analysis of cytoarchitecture interaction in regulating plasma membrane protein distribution and constraining its spread has implications on the mechanisms of spread of various molecules such as morphogens in syncytial cells.

Restriction of subapical proteins during cellularization depends on the onset of zygotic transcription and the formin Dia

Cortical domains are characterized by spatially restricted polarity proteins. The pattern of cortical domains is dynamic and changes during cell differentiation and development. Although there is a good understanding for how the cortical pattern is maintained, e. g. by mutual antagonism, less is known about how the initial pattern is established, and its dynamics coordinated with developmental progression. Here we investigate the initial restriction of subapical marker proteins during the syncytial-cellular transition in Drosophila embryos. The subapical markers Canoe/Afadin, the complex ELMO-Sponge, Baz and Arm become initially restricted between apical and lateral domains during cellularization. We define the role of zygotic genome activation as a timer for subapical domain formation. Subapical markers remained widely spread in embryos treated with α-amanitin and became precociously restricted in mutant embryos with premature zygotic transcription. In contrast, remodeling of the nuclear division cycle without cytokinesis to a full cell cycle is not a prerequisite for subapical domain formation, since we observed timely subapical restriction in embryos undergoing an extra nuclear cycle. We provide evidence that earliest subapical markers ELMO-Sponge and Canoe are required for subapical accumulation of Baz. Supporting an important role of cortical F-actin in subapical restriction, we found that the formin Dia was required for Baz restriction, and its distribution depended on the onset of zygotic gene expression. In summary, we define zygotic transcription as a timer, to which subapical markers respond in a dia-dependent mechanism.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization

Actomyosin contractility is regulated by Rho-GTP in cell migration, cytokinesis and morphogenesis in embryo development. Whereas Rho activation by Rho-GTP exchange factor (GEF), RhoGEF2, is well known in actomyosin contractility during cytokinesis at the base of invaginating membranes in Drosophila cellularization, Rho inhibition by RhoGTPase-activating proteins (GAPs) remains to be studied. We have found that the RhoGAP, GRAF, inhibits actomyosin contractility during cellularization. GRAF is enriched at the cleavage furrow tip during actomyosin assembly and initiation of ring constriction. Graf depletion shows increased Rho-GTP, increased Myosin II and ring hyper constriction dependent upon the loss of the RhoGTPase domain. GRAF and RhoGEF2 are present in a balance for appropriate activation of actomyosin ring constriction. RhoGEF2 depletion and abrogation of Myosin II activation in Rho kinase mutants suppress the Graf hyper constriction defect. Therefore, GRAF recruitment restricts Rho-GTP levels in a spatiotemporal manner for inhibiting actomyosin contractility during cellularization.

F-BAR domain protein Syndapin regulates actomyosin dynamics during apical cap remodeling in syncytial Drosophila embryos

Branched actin networks driven by Arp2/3 interact with actomyosin filaments in processes such as cell migration. Similar interactions occur in the syncytial Drosophila blastoderm embryo where expansion of apical caps by Arp2/3-driven actin polymerization occurs in interphase, and cap buckling at contact edges by Myosin II to form furrows takes place in metaphase. Here, we study the role of Syndapin (Synd), an F-BAR domain-containing protein, in apical cap remodeling prior to furrow extension. We found that depletion of synd resulted in larger apical caps. Super-resolution and TIRF microscopy showed that control embryos had long apical actin protrusions in caps during interphase and short protrusions during metaphase, whereas synd depletion led to formation of sustained long protrusions, even during metaphase. Loss of Arp2/3 function in synd mutants partly reverted defects in apical cap expansion and protrusion remodeling. Myosin II levels were decreased in synd mutants, an observation consistent with the expanded cap phenotype previously reported for Myosin II mutant embryos. We propose that Synd function limits branching activity during cap expansion and affects Myosin II distribution in order to bring about a transition in actin remodeling activity from apical cap expansion to lateral furrow extension.

DE-cadherin and Myosin II balance regulates furrow length for onset of polygon shape in syncytial Drosophila embryos

Cell shape morphogenesis, from spherical to polygonal, occurs in epithelial cell formation in metazoan embryogenesis. In syncytial Drosophila embryos, the plasma membrane incompletely surrounds each nucleus and is organized as a polygonal epithelial-like array. Each cortical syncytial division cycle shows a circular to polygonal plasma membrane transition along with furrow extension between adjacent nuclei from interphase to metaphase. In this study, we assess the relative contribution of DE-cadherin (also known as Shotgun) and Myosin II (comprising Zipper and Spaghetti squash in flies) at the furrow to polygonal shape transition. We show that polygonality initiates during each cortical syncytial division cycle when the furrow extends from 4.75 to 5.75 μm. Polygon plasma membrane organization correlates with increased junctional tension, increased DE-cadherin and decreased Myosin II mobility. DE-cadherin regulates furrow length and polygonality. Decreased Myosin II activity allows for polygonality to occur at a lower length than controls. Increased Myosin II activity leads to loss of lateral furrow formation and complete disruption of the polygonal shape transition. Our studies show that DE-cadherin-Myosin II balance regulates an optimal lateral membrane length during each syncytial cycle for polygonal shape transition.This article has an associated First Person interview with the first author of the paper.

Syndapin constricts microvillar necks to form a united rhabdomere in Drosophila photoreceptors

Drosophila photoreceptors develop from polarized epithelial cells that have apical and basolateral membranes. During morphogenesis, the apical membranes subdivide into a united bundle of photosensory microvilli (rhabdomeres) and a surrounding supporting membrane (stalk). By EMS-induced mutagenesis screening, we found that the F-Bin/Amphiphysin/Rvs (F-BAR) protein syndapin is essential for apical membrane segregation. The analysis of the super-resolution microscopy, STORM and the electron microscopy suggest that syndapin localizes to the neck of the microvilli at the base of the rhabdomere. Syndapin and moesin are required to constrict the neck of the microvilli to organize the membrane architecture at the base of the rhabdomere, to exclude the stalk membrane. Simultaneous loss of syndapin along with the microvilli adhesion molecule chaoptin significantly enhanced the disruption of stalk-rhabdomere segregation. However, loss of the factors involving endocytosis do not interfere. These results indicated syndapin is most likely functioning through its membrane curvature properties, and not through endocytic processes for stalk-rhabdomere segregation. Elucidation of the mechanism of this unconventional domain formation will provide novel insights into the field of cell biology.

RNAi Screen in Tribolium Reveals Involvement of F-BAR Proteins in Myoblast Fusion and Visceral Muscle Morphogenesis in Insects

In a large-scale RNAi screen in Tribolium castaneum for genes with knock-down phenotypes in the larval somatic musculature, one recurring phenotype was the appearance of larval muscle fibers that were significantly thinner than those in control animals. Several of the genes producing this knock-down phenotype corresponded to orthologs of Drosophila genes that are known to participate in myoblast fusion, particularly via their effects on actin polymerization. A new gene previously not implicated in myoblast fusion but displaying a similar thin-muscle knock-down phenotype was the Tribolium ortholog of Nostrin, which encodes an F-BAR and SH3 domain protein. Our genetic studies of Nostrin and Cip4, a gene encoding a structurally related protein, in Drosophila show that the encoded F-BAR proteins jointly contribute to efficient myoblast fusion during larval muscle development. Together with the F-Bar protein Syndapin they are also required for normal embryonic midgut morphogenesis. In addition, Cip4 is required together with Nostrin during the profound remodeling of the midgut visceral musculature during metamorphosis. We propose that these F-Bar proteins help govern proper morphogenesis particularly of the longitudinal midgut muscles during metamorphosis.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

The ABD on the nascent polypeptide and PH domain are required for the precise Anillin localization in Drosophila syncytial blastoderm

Targeting proteins to regions where they are required is essential for proper development of organisms. For achievement of this, subcellular mRNA localization is one of the critical mechanisms. Subcellular mRNA localization is an evolutionarily conserved phenomenon from E. coli to human and contributes to limiting the regions at which its products function and efficiently supplies substrates for protein translation. During early Drosophila embryogenesis, while 71% of the 3370 mRNAs analyzed have shown prominent subcellular localization, the underlying molecular mechanisms have not been elucidated. Here, we reveal that anillin mRNA, one of the localized mRNAs in early Drosophila embryo, localizes to the tip of the pseudo-cleavage furrow in the Drosophila syncytial blastoderm using in situ hybridization combined with immunohistochemistry. Localization analyses with transgenic fly lines carrying a series of deletion mRNAs indicate that this localization is dependent on its own nascent polypeptides including the actin binding domain (ABD). In addition to the mRNA localization, it is revealed that the pleckstrin homology (PH) domain of Anillin protein is also required for its proper localization. Thus, we indicate that the precise localization of Anillin protein is tightly regulated by the ABD on the nascent polypeptide and PH domain in the Drosophila syncytial blastoderm.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

Differentially-dimensioned furrow formation by zygotic gene expression and the MBT

Despite extensive work on the mechanisms that generate plasma membrane furrows, understanding how cells are able to dynamically regulate furrow dimensions is an unresolved question. Here, we present an in-depth characterization of furrow behaviors and their regulation in vivo during early Drosophila morphogenesis. We show that the deepening in furrow dimensions with successive nuclear cycles is largely due to the introduction of a new, rapid ingression phase (Ingression II). Blocking the midblastula transition (MBT) by suppressing zygotic transcription through pharmacological or genetic means causes the absence of Ingression II, and consequently reduces furrow dimensions. The analysis of compound chromosomes that produce chromosomal aneuploidies suggests that multiple loci on the X, II, and III chromosomes contribute to the production of differentially-dimensioned furrows, and we track the X-chromosomal contribution to furrow lengthening to the nullo gene product. We further show that checkpoint proteins are required for furrow lengthening; however, mitotic phases of the cell cycle are not strictly deterministic for furrow dimensions, as a decoupling of mitotic phases with periods of active ingression occurs as syncytial furrow cycles progress. Finally, we examined the turnover of maternal gene products and find that this is a minor contributor to the developmental regulation of furrow morphologies. Our results suggest that cellularization dynamics during cycle 14 are a continuation of dynamics established during the syncytial cycles and provide a more nuanced view of developmental- and MBT-driven morphogenesis.

One of the primary events that must occur repeatedly throughout a complex animal’s lifetime is the ingression of a plasma membrane furrow. Furrow formation and ingression are requisite elements of cell division, and drive the physical separation of one cell into two cells. However, the mechanisms that permit an embryo to change the length and size of a furrow are unclear. Here, we show that a combination of higher ingression rates and longer duration phases drive changes in furrow dimensions through the introduction of a new ingression phase. These changes are driven by the embryo’s own genome, and suggest that zygotic transcription controls organismal form at an earlier time point than previously appreciated. Additionally, the failure to properly lengthen furrows as development proceeds causes defects in chromosome segregation during cell division and results in massive genomic instability. Our data demonstrate the importance of the dynamic regulation of furrow dimensions to organismal form and viability.

Syndapin bridges the membrane-cytoskeleton divide during furrow extension

BAR domain proteins can regulate ‘membrane reservoirs’ that provide surface area and buffer membrane tension. Syndapin is an F-BAR and SH3 domain containing protein involved in cytoskeletal remodelling and endocytosis. The Syndapin F-BAR domain is uniquely versatile compared to others in the family and can bend phospholipid membranes into tubules of various diameters and directly bind actin. The Syndapin SH3 domain can also interact with actin remodelling proteins and modulate cytoskeletal contractility. Pseudocleavage furrow extension in the syncytial division cycles of Drosophila embryos requires the homeostatic control of conserved processes that control plasma membrane tension and actin contractility. We find that Syndapin plays an important role in promoting pseudocleavage furrow extension. We propose a model involving roles for Syndapin in membrane dynamics and direct or indirect effect on the cytoskeleton to explain how it affects pseudocleavage furrow growth, independent of its role in endocytosis.

Syndapin/SDPN-1 is required for endocytic recycling and endosomal actin association in the C. elegans intestine

Syndapin/Pascin family F-BAR domain proteins bind directly to membrane lipids and are associated with actin dynamics at the plasma membrane. Previous reports have also implicated mammalian syndapin 2 in endosome function during receptor recycling, but precise analysis of a putative recycling function for syndapin in mammalian systems is difficult because of syndapin effects on the earlier step of endocytic uptake, and potential redundancy among the three separate genes that encode mammalian syndapin isoforms. Here we analyze the endocytic transport function of the only C. elegans syndapin, SDPN-1. We find that SDPN-1 is a resident protein of the early and basolateral recycling endosomes in the C. elegans intestinal epithelium, and sdpn-1 deletion mutants display phenotypes indicating a block in basolateral recycling transport. sdpn-1 mutants accumulate abnormal endosomes positive for early endosome and recycling endosome markers that are normally separate, and such endosomes accumulate high levels of basolateral recycling cargo. Furthermore, we observed strong colocalization of endosomal SDPN-1 with the F-actin biosensor Lifeact, and found that loss of SDPN-1 greatly reduced Lifeact accumulation on early endosomes. Taken together our results provide strong evidence for an in vivo function of syndapin in endocytic recycling, and suggest that syndapin promotes transport via endosomal fission.