Fat2 acts through the WAVE regulatory complex to drive collective cell migration during tissue rotation

CYRI controls epidermal wound closure and cohesion of invasive border cell cluster in Drosophila

Cell motility is crucial for many biological processes including morphogenesis, wound healing, and cancer invasion. The WAVE regulatory complex (WRC) is a central Arp2/3 regulator driving cell motility downstream of activation by Rac GTPase. CYFIP-related Rac1 interactor (CYRI) proteins are thought to compete with WRC for interaction with Rac1 in a feedback loop regulating lamellipodia dynamics. However, the physiological role of CYRI proteins in vivo in healthy tissues is unclear. Here, we used Drosophila as a model system to study CYRI function at the cellular and organismal levels. We found that CYRI is not only a potent WRC regulator in single macrophages that controls lamellipodial spreading but also identified CYRI as a molecular brake on the Rac-WRC-Arp2/3 pathway to slow down epidermal wound healing. In addition, we found that CYRI limits invasive border cell migration by controlling cluster cohesion and migration. Thus, our data highlight CYRI as an important regulator of cellular and epithelial tissue dynamics conserved across species.

Atypical cadherin, Fat2, regulates axon terminal organization in the developing Drosophila olfactory receptor neurons

The process of how neuronal identity confers circuit organization is intricately related to the mechanisms underlying neurodegeneration and neuropathologies. Modeling this process, the olfactory circuit builds a functionally organized topographic map, which requires widely dispersed neurons with the same identity to converge their axons into one a class-specific neuropil, a glomerulus. In this article, we identified Fat2 (also known as Kugelei) as a regulator of class-specific axon organization. In fat2 mutants, axons belonging to the highest fat2-expressing classes present with a more severe phenotype compared to axons belonging to low fat2-expressing classes. In extreme cases, mutations lead to neural degeneration. Lastly, we found that Fat2 intracellular domain interactors, APC1/2 (Adenomatous polyposis coli) and dop (Drop out), likely orchestrate the cytoskeletal remodeling required for axon condensation. Altogether, we provide a potential mechanism for how cell surface proteins' regulation of cytoskeletal remodeling necessitates identity specific circuit organization.

Fat2 polarizes Lar and Sema5c to coordinate the motility of collectively migrating epithelial cells

Migrating epithelial cells globally align their migration machinery to achieve tissue-level movement. Biochemical signaling across leading-trailing cell-cell interfaces can promote this alignment by partitioning migratory behaviors like protrusion and retraction to opposite sides of the interface. However, how signaling proteins become organized at interfaces to accomplish this is poorly understood. The follicular epithelial cells of Drosophila melanogaster have two signaling modules at their leading-trailing interfaces - one composed of the atypical cadherin Fat2 (also known as Kugelei) and the receptor tyrosine phosphatase Lar, and one composed of Semaphorin5c and its receptor Plexin A. Here, we show that these modules form one interface signaling system with Fat2 at its core. Trailing edge-enriched Fat2 concentrates both Lar and Semaphorin5c at leading edges of cells, but Lar and Semaphorin5c play little role in the localization of Fat2. Fat2 is also more stable at interfaces than Lar or Semaphorin5c. Once localized, Lar and Semaphorin5c act in parallel to promote collective migration. We propose that Fat2 serves as the organizer of this interface signaling system by coupling and polarizing the distributions of multiple effectors that work together to align the migration machinery of neighboring cells.

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Expect the unexpected: conventional and unconventional roles for cadherins in collective cell migration

Migrating cell collectives navigate complex tissue environments both during normal development and in pathological contexts such as tumor invasion and metastasis. To do this, cells in collectives must stay together but also communicate information across the group. The cadherin superfamily of proteins mediates junctional adhesions between cells, but also serve many essential functions in collective cell migration. Besides keeping migrating cell collectives cohesive, cadherins help follower cells maintain their attachment to leader cells, transfer information about front-rear polarity among the cohort, sense and respond to changes in the tissue environment, and promote intracellular signaling, in addition to other cellular behaviors. In this review, we highlight recent studies that reveal diverse but critical roles for both classical and atypical cadherins in collective cell migration, specifically focusing on four in vivo model systems in development: the Drosophila border cells, zebrafish mesendodermal cells, Drosophila follicle rotation, and Xenopus neural crest cells.

Constriction imposed by basement membrane regulates developmental cell migration

The basement membrane (BM) is a specialized extracellular matrix (ECM), which underlies or encases developing tissues. Mechanical properties of encasing BMs have been shown to profoundly influence the shaping of associated tissues. Here, we use the migration of the border cells (BCs) of the Drosophila egg chamber to unravel a new role of encasing BMs in cell migration. BCs move between a group of cells, the nurse cells (NCs), that are enclosed by a monolayer of follicle cells (FCs), which is, in turn, surrounded by a BM, the follicle BM. We show that increasing or reducing the stiffness of the follicle BM, by altering laminins or type IV collagen levels, conversely affects BC migration speed and alters migration mode and dynamics. Follicle BM stiffness also controls pairwise NC and FC cortical tension. We propose that constraints imposed by the follicle BM influence NC and FC cortical tension, which, in turn, regulate BC migration. Encasing BMs emerge as key players in the regulation of collective cell migration during morphogenesis.

Fat2 polarizes Lar and Sema5c to coordinate the motility of collectively migrating epithelial cells

Migrating epithelial cells globally align their migration machinery to achieve tissue-level movement. Biochemical signaling across leading-trailing cell-cell interfaces can promote this alignment by partitioning migratory behaviors like protrusion and retraction to opposite sides of the interface. However, how the necessary signaling proteins become organized at this site is poorly understood. The follicular epithelial cells of Drosophila melanogaster have two signaling modules at their leading-trailing interfaces-one composed of the atypical cadherin Fat2 and the receptor tyrosine phosphatase Lar, and one composed of Semaphorin 5c and its receptor Plexin A. Here we show that these modules form one interface signaling system with Fat2 at its core. Trailing edge-enriched Fat2 concentrates both Lar and Sema5c at cells' leading edges, likely by slowing their turnover at this site. Once localized, Lar and Sema5c act in parallel to promote collective migration. Our data suggest a model in which Fat2 couples and polarizes the distributions of multiple effectors that work together to align the migration machinery of neighboring cells.

A Low-Tech Flow Chamber for Live Imaging of Drosophila Egg Chambers During Drug Treatments

The Drosophila egg chamber is a powerful system to study epithelial cell collective migration and polarized basement membrane secretion. A strength of this system is the ability to capture these dynamic processes in ex vivo organ culture using high-resolution live imaging. Ex vivo culture also allows acute pharmacological or labeling treatments, extending the versatility of the system. However, many current ex vivo egg chamber culture setups do not permit easy medium exchange, preventing researchers from following individual egg chambers through multiple treatments. Here we present a method to immobilize egg chambers in an easy-to-construct flow chamber that permits imaging of the same egg chamber through repeated solution exchanges. This will allow researchers to take greater advantage of the wide variety of available pharmacological perturbations and other treatments like dyes to study dynamic processes in the egg chamber.

Fat2 polarizes the WAVE complex in trans to align cell protrusions for collective migration

For a group of cells to migrate together, each cell must couple the polarity of its migratory machinery with that of the other cells in the cohort. Although collective cell migrations are common in animal development, little is known about how protrusions are coherently polarized among groups of migrating epithelial cells. We address this problem in the collective migration of the follicular epithelial cells in Drosophila melanogaster. In this epithelium, the cadherin Fat2 localizes to the trailing edge of each cell and promotes the formation of F-actin-rich protrusions at the leading edge of the cell behind. We show that Fat2 performs this function by acting in trans to concentrate the activity of the WASP family verprolin homolog regulatory complex (WAVE complex) at one long-lived region along each cell's leading edge. Without Fat2, the WAVE complex distribution expands around the cell perimeter and fluctuates over time, and protrusive activity is reduced and unpolarized. We further show that Fat2's influence is very local, with sub-micron-scale puncta of Fat2 enriching the WAVE complex in corresponding puncta just across the leading-trailing cell-cell interface. These findings demonstrate that a trans interaction between Fat2 and the WAVE complex creates stable regions of protrusive activity in each cell and aligns the cells' protrusions across the epithelium for directionally persistent collective migration.

Somatic Mutation of FAT Family Genes Implicated Superior Prognosis in Patients With Stomach Adenocarcinoma

FAT family genes encode protocadherin, which regulates tumor cell proliferation and migration. Although transcriptional levels of FAT family members had been reported in multiple malignant tumors, the association between mutation and prognosis of the FAT family in stomach adenocarcinoma (STAD) has not been investigated. Herein, we performed a multi-omics integrative bioinformatics analysis using genomic and mRNA expression data to explore the role of gene mutations across the FAT family on clinical outcomes of STAD. The results showed that FAT mutations occurred in 174 of 435 (40%) of the samples. Patients with FAT mutations possessed significantly better progression-free survival (P = 0.019) and overall survival (P = 0.034) than those with non-FAT mutations, and FAT mutations exhibited significantly higher tumor mutational burden (TMB) and microsatellite instability. Notably, FAT mutations had a greater effect on somatic single-nucleotide variation than copy number variation and resulted in more abundant DNA damage repair (DDR) mutations. Further investigation demonstrated that FAT mutations contributed to an inflammatory tumor microenvironment (TME), as indicated by significantly increased numbers of activated CD4 and CD8 T cells, and significantly decreased numbers of mast cell, plasmacytoid dendritic cell, type 2 T helper cell, and high expression of immune-promoting genes. Moreover, biological process antigen processing and presentation, DNA replication, and DDR-related pathways were significantly upregulated in patients with FAT mutations. Collectively, FAT mutations significantly improved the survival of patients with STAD by enhancing tumor immunogenicity (e.g., TMB and DDR mutations) and an inflamed TME, indicating that the FAT family might be a potential prognostic and therapeutic biomarker for STAD.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Directing with restraint: Mechanisms of protrusion restriction in collective cell migrations

Cell migration is necessary for morphogenesis, tissue homeostasis, wound healing and immune response. It is also involved in diseases. In particular, cell migration is inherent in metastasis. Cells can migrate individually or in groups. To migrate efficiently, cells need to be able to organize into a leading front that protrudes by forming membrane extensions and a trailing edge that contracts. This organization is scaled up at the group level during collective cell movements. If a cell or a group of cells is unable to limit its leading edge and hence to restrict the formation of protrusions to the front, directional movements are impaired or abrogated. Here we summarize our current understanding of the mechanisms restricting protrusion formation in collective cell migration. We focus on three in vivo examples: the neural crest cell migration, the rotatory migration of follicle cells around the Drosophila egg chamber and the border cell migration during oogenesis.

Fat3 acts through independent cytoskeletal effectors to coordinate asymmetric cell behaviors during polarized circuit assembly

The polarized flow of information through neural circuits depends on the orderly arrangement of neurons, their processes, and their synapses. This polarity emerges sequentially in development, starting with the directed migration of neuronal precursors, which subsequently elaborate neurites that form synapses in specific locations. In other organs, Fat cadherins sense the position and then polarize individual cells by inducing localized changes in the cytoskeleton that are coordinated across the tissue. Here, we show that the Fat-related protein Fat3 plays an analogous role during the assembly of polarized circuits in the murine retina. We find that the Fat3 intracellular domain (ICD) binds to cytoskeletal regulators and synaptic proteins, with discrete motifs required for amacrine cell migration and neurite retraction. Moreover, upon ICD deletion, extra neurites form but do not make ectopic synapses, suggesting that Fat3 independently regulates synapse localization. Thus, Fat3 serves as a molecular node to coordinate asymmetric cell behaviors across development.

Collective cell migration driven by filopodia-New insights from the social behavior of myotubes

Collective migration is a key process that is critical during development, as well as in physiological and pathophysiological processes including tissue repair, wound healing and cancer. Studies in genetic model organisms have made important contributions to our current understanding of the mechanisms that shape cells into different tissues during morphogenesis. Recent advances in high-resolution and live-cell-imaging techniques provided new insights into the social behavior of cells based on careful visual observations within the context of a living tissue. In this review, we will compare Drosophila testis nascent myotube migration with established in vivo model systems, elucidate similarities, new features and principles in collective cell migration.

Adherens junction regulates cryptic lamellipodia formation for epithelial cell migration

Collective migration of epithelial cells plays crucial roles in various biological processes such as cancer invasion. In migrating epithelial sheets, leader cells form lamellipodia to advance, and follower cells also form similar motile apparatus at cell-cell boundaries, which are called cryptic lamellipodia (c-lamellipodia). Using adenocarcinoma-derived epithelial cells, we investigated how c-lamellipodia form and found that they sporadically grew from around E-cadherin-based adherens junctions (AJs). WAVE and Arp2/3 complexes were localized along the AJs, and silencing them not only interfered with c-lamellipodia formation but also prevented follower cells from trailing the leaders. Disruption of AJs by removing αE-catenin resulted in uncontrolled c-lamellipodia growth, and this was brought about by myosin II activation and the resultant contraction of AJ-associated actomyosin cables. Additional observations indicated that c-lamellipodia tended to grow at mechanically weak sites of the junction. We conclude that AJs not only tie cells together but also support c-lamellipodia formation by recruiting actin regulators, enabling epithelial cells to undergo ordered collective migration.

Filopodia-based contact stimulation of cell migration drives tissue morphogenesis

Cells migrate collectively to form tissues and organs during morphogenesis. Contact inhibition of locomotion (CIL) drives collective migration by inhibiting lamellipodial protrusions at cell-cell contacts and promoting polarization at the leading edge. Here, we report a CIL-related collective cell behavior of myotubes that lack lamellipodial protrusions, but instead use filopodia to move as a cohesive cluster in a formin-dependent manner. We perform genetic, pharmacological and mechanical perturbation analyses to reveal the essential roles of Rac2, Cdc42 and Rho1 in myotube migration. These factors differentially control protrusion dynamics and cell-matrix adhesion formation. We also show that active Rho1 GTPase localizes at retracting free edge filopodia and that Rok-dependent actomyosin contractility does not mediate a contraction of protrusions at cell-cell contacts, but likely plays an important role in the constriction of supracellular actin cables. Based on these findings, we propose that contact-dependent asymmetry of cell-matrix adhesion drives directional movement, whereas contractile actin cables contribute to the integrity of the migrating cell cluster.

Ancestral roles of atypical cadherins in planar cell polarity

Planar cell polarization, PCP, describes a form of organization where every cell within a group acquires the same planar characteristics, whether it is orientation of cell division, direction of migration, or localization of a cellular structure. PCP is essential for correct organization of cells into tissues and building a proper body plan. Here we use Hydra, an organism with a single axis of symmetry and a very simple body plan to investigate the function of the cell adhesion molecules Fat-like and Dachsous. We show that Hydra Fat-like and Dachsous are planar polarized, providing a demonstration of planar polarization of proteins in a nonbilaterian organism. We also discover roles for Hydra Fat-like in cell adhesion, spindle orientation, and tissue organization.

Fat, Fat-like, and Dachsous family cadherins are giant proteins that regulate planar cell polarity (PCP) and cell adhesion in bilaterians. Their evolutionary origin can be traced back to prebilaterian species, but their ancestral function(s) are unknown. We identified Fat-like and Dachsous cadherins in Hydra, a member of phylum Cnidaria a sister group of bilaterian. We found Hydra does not possess a true Fat homolog, but has homologs of Fat-like (HyFatl) and Dachsous (HyDs) that localize at the apical membrane of ectodermal epithelial cells and are planar polarized perpendicular to the oral–aboral axis of the animal. Using a knockdown approach we found that HyFatl is involved in local cell alignment and cell–cell adhesion, and that reduction of HyFatl leads to defects in tissue organization in the body column. Overexpression and knockdown experiments indicate that the intracellular domain (ICD) of HyFatl affects actin organization through proline-rich repeats. Thus, planar polarization of Fat-like and Dachsous cadherins has ancient, prebilaterian origins, and Fat-like cadherins have ancient roles in cell adhesion, spindle orientation, and tissue organization.

The cytoskeletal regulator HEM1 governs B cell development and prevents autoimmunity

The WAVE regulatory complex (WRC) is crucial for assembly of the peripheral branched actin network constituting one of the main drivers of eukaryotic cell migration. Here, we uncover an essential role of the hematopoietic-specific WRC component HEM1 for immune cell development. Germline-encoded HEM1 deficiency underlies an inborn error of immunity with systemic autoimmunity, at cellular level marked by WRC destabilization, reduced filamentous actin, and failure to assemble lamellipodia. Hem1^-/- mice display systemic autoimmunity, phenocopying the human disease. In the absence of Hem1, B cells become deprived of extracellular stimuli necessary to maintain the strength of B cell receptor signaling at a level permissive for survival of non-autoreactive B cells. This shifts the balance of B cell fate choices toward autoreactive B cells and thus autoimmunity.

Integrins regulate epithelial cell shape by controlling the architecture and mechanical properties of basal actomyosin networks

Forces generated by the actomyosin cytoskeleton are key contributors to many morphogenetic processes. The actomyosin cytoskeleton organises in different types of networks depending on intracellular signals and on cell-cell and cell-extracellular matrix (ECM) interactions. However, actomyosin networks are not static and transitions between them have been proposed to drive morphogenesis. Still, little is known about the mechanisms that regulate the dynamics of actomyosin networks during morphogenesis. This work uses the Drosophila follicular epithelium, real-time imaging, laser ablation and quantitative analysis to study the role of integrins on the regulation of basal actomyosin networks organisation and dynamics and the potential contribution of this role to cell shape. We find that elimination of integrins from follicle cells impairs F-actin recruitment to basal medial actomyosin stress fibers. The available F-actin redistributes to the so-called whip-like structures, present at tricellular junctions, and into a new type of actin-rich protrusions that emanate from the basal cortex and project towards the medial region. These F-actin protrusions are dynamic and changes in total protrusion area correlate with periodic cycles of basal myosin accumulation and constriction pulses of the cell membrane. Finally, we find that follicle cells lacking integrin function show increased membrane tension and reduced basal surface. Furthermore, the actin-rich protrusions are responsible for these phenotypes as their elimination in integrin mutant follicle cells rescues both tension and basal surface defects. We thus propose that the role of integrins as regulators of stress fibers plays a key role on controlling epithelial cell shape, as integrin disruption promotes reorganisation into other types of actomyosin networks, in a manner that interferes with proper expansion of epithelial basal surfaces.

Morphogenesis involves global changes in tissue architecture driven by cell shape changes. Mechanical forces generated by actomyosin networks and force transmission through adhesive complexes power these changes. The actomyosin cytoskeleton organises in different types of networks, which localise to precise regions and perform distinct roles. However, they are rarely independent and, often, reorganisation of a given structure can promote the formation of another, conversions proposed to underlie many morphogenetic processes. Nonetheless, the mechanisms controlling actomyosin network dynamics during morphogenesis remain poorly characterised. Here, using the Drosophila follicular epithelium, we show that cell-ECM interactions mediated by integrins are required for the correct distribution of actin in the different actin networks. Elimination of integrins results in redistribution of actin from stress fibers into a new type of protrusions that dynamically emanate from the cortex and extend into the stress fibers. Changes in area protrusions correlate with bursts of myosin accumulated in stress fibers and constriction pulses of the cell membrane. We also found that integrin mutant cells show increased membrane tension and reduced basal cell surface. As these defects are rescued by eliminating the F-actin protrusions, we believe these structures prevent proper basal surface growth. Thus, we propose that integrin function as regulators of stress fibers assembly and maintenance controls epithelial cell shape, as its disruption promotes reorganisation into other actomyosin networks, conversions that interfere with proper epithelial basal surface expansion.

Genetic dissection of active forgetting in labile and consolidated memories in Drosophila

Different memory components are forgotten through distinct molecular mechanisms. In Drosophila, the activation of 2 Rho GTPases (Rac1 and Cdc42), respectively, underlies the forgetting of an early labile memory (anesthesia-sensitive memory, ASM) and a form of consolidated memory (anesthesia-resistant memory, ARM). Here, we dissected the molecular mechanisms that tie Rac1 and Cdc42 to the different types of memory forgetting. We found that 2 WASP family proteins, SCAR/WAVE and WASp, act downstream of Rac1 and Cdc42 separately to regulate ASM and ARM forgetting in mushroom body neurons. Arp2/3 complex, which organizes branched actin polymerization, is a canonical downstream effector of WASP family proteins. However, we found that Arp2/3 complex is required in Cdc42/WASp-mediated ARM forgetting but not in Rac1/SCAR-mediated ASM forgetting. Instead, we identified that Rac1/SCAR may function with formin Diaphanous (Dia), a nucleator that facilitates linear actin polymerization, in ASM forgetting. The present study, complementing the previously identified Rac1/cofilin pathway that regulates actin depolymerization, suggests that Rho GTPases regulate forgetting by recruiting both actin polymerization and depolymerization pathways. Moreover, Rac1 and Cdc42 may regulate different types of memory forgetting by tapping into different actin polymerization mechanisms.

Differentiation of follicular epithelium in polytrophic ovaries of Pieris napi (Lepidoptera: Pieridae)-how far to Drosophila model

Lepidoptera together with its sister group Trichoptera belongs to the superorder Amphiesmenoptera, which is closely related to the Antliophora, comprising Diptera, Siphonaptera, and Mecoptera. In the lepidopteran Pieris napi, a representative of the family Pieridae, the ovaries typical of butterflies are polytrophic and consist of structural ovarian units termed ovarioles. Each ovariole is composed of a terminal filament, germarium, vitellarium, and ovariole stalk. The germarium houses developing germ cell clusters and somatic prefollicular and follicular cells. The significantly elongated vitellarium contains linearly arranged ovarian follicles in successive stages of oogenesis (previtellogenesis, vitellogenesis, and choriogenesis). Each follicle consists of an oocyte and seven nurse cells surrounded by follicular epithelium. During oogenesis, follicular cells diversify into five morphologically and functionally distinct subpopulations: (1) main body follicular cells (mbFC), (2) stretched cells (stFC), (3) posterior terminal cells (pFC), (4) centripetal cells (cpFC), and (5) interfollicular stalk cells (IFS). Centripetal cells are migratorily active and finally form the micropyle. Interfollicular stalk cells derive from mbFC as a result of mbFC intercalation. Differentiation and diversification of follicular cells in Pieris significantly differ from those described in Drosophila in the number of subpopulations and their origin and function during oogenesis.

Division orientation: disentangling shape and mechanical forces

Oriented cell divisions are essential for the generation of cell diversity and for tissue shaping during morphogenesis. Cells in tissues are mechanically linked to their neighbors, upon which they impose, and from which they experience, physical force. Recent work in multiple systems has revealed that tissue-level physical forces can influence the orientation of cell division. A long-standing question is whether forces are communicated to the spindle orienting machinery via cell shape or directly via mechanosensing intracellular machinery. In this article, we review the current evidence from diverse model systems that show spindles are oriented by tissue-level physical forces and evaluate current models and molecular mechanisms proposed to explain how the spindle orientation machinery responds to extrinsic force.

The migration and fusion events related to ROCK activity strongly influence the morphology of chicken embryo intestinal organoids

The method of organoid culture has become a tool widely used in gastrointestinal research, but so far, the migration of organoids derived from gut epithelium and formed in 3D Matrigel matrix has not been reported and studied. The intestinal epithelial tissue derived from 19-day-old chicken embryo was cultured in Matrigel and the dynamic properties of the forming organoids were analyzed by time-lapse image analysis. It was observed that about one in ten organoids actively moved through the matrix, at a speed of 10-20 μm/h. Moreover, rotation was observed in the majority of organoids that did not migrate long distances. The fusion events took place between organoids, which collided during the movement or growth. In our previous paper, we showed that the presence of Toll-like receptor 4 ligand, Escherichia coli lipopolysaccharide (LPS, 1 μg/ml), increased the mean organoid diameter. Here, we confirm this result and demonstrate that the Rho-associated protein kinase (ROCK) inhibitor Y-27632 (10 μM) did not completely abolish organoid migration, but prevented the fusion events, in both LPS-treated and untreated cultures. In consequence, in the presence of Y-27632, the differences between cultures incubated with and without LPS were not visible. We conclude that migration and fusion of organoids may influence their morphology and suggest that these phenomena should be taken into account during the design of experimental settings.

Distinct Interaction Sites of Rac GTPase with WAVE Regulatory Complex Have Non-redundant Functions in Vivo

Cell migration often involves the formation of sheet-like lamellipodia generated by branched actin filaments. The branches are initiated when Arp2/3 complex [1] is activated by WAVE regulatory complex (WRC) downstream of small GTPases of the Rac family [2]. Recent structural studies defined two independent Rac binding sites on WRC within the Sra-1/PIR121 subunit of the pentameric WRC [3, 4], but the functions of these sites in vivo have remained unknown. Here we dissect the mechanism of WRC activation and the in vivo relevance of distinct Rac binding sites on Sra-1, using CRISPR/Cas9-mediated gene disruption of Sra-1 and its paralog PIR121 in murine B16-F1 cells combined with Sra-1 mutant rescue. We show that the A site, positioned adjacent to the binding region of WAVE-WCA mediating actin and Arp2/3 complex binding, is the main site for allosteric activation of WRC. In contrast, the D site toward the C terminus is dispensable for WRC activation but required for optimal lamellipodium morphology and function. These results were confirmed in evolutionarily distant Dictyostelium cells. Moreover, the phenotype seen in D site mutants was recapitulated in Rac1 E31 and F37 mutants; we conclude these residues are important for Rac-D site interaction. Finally, constitutively activated WRC was able to induce lamellipodia even after both Rac interaction sites were lost, showing that Rac interaction is not essential for membrane recruitment. Our data establish that physical interaction with Rac is required for WRC activation, in particular through the A site, but is not mandatory for WRC accumulation in the lamellipodium.

Epithelial Cell Chirality Revealed by Three-Dimensional Spontaneous Rotation

Our understanding of the left-right (LR) asymmetry of embryonic development, in particular the contribution of intrinsic handedness of the cell or cell chirality, is limited due to the confounding systematic and environmental factors during morphogenesis and a ack of physiologically relevant in vitro 3D platforms. Here we report an efficient two-layered biomaterial platform for determining the chirality of individual cells, cell aggregates, and self-organized hollow epithelial spheroids. This bioengineered niche provides a uniform defined axis allowing for cells to rotate spontaneously with a directional bias toward either clockwise or counterclockwise directions. Mechanistic studies reveal an actin-dependent, cell-intrinsic property of 3D chirality that can be mediated by actin cross-linking via α-actinin-1. Our findings suggest that the gradient of extracellular matrix is an important biophysicochemical cue influencing cell polarity and chirality. Engineered biomaterial systems can serve as an effective platform for studying developmental asymmetry and screening for environmental factors causing birth defects.

A Cell Migration Tracking Tool Supports Coupling of Tissue Rotation to Elongation

Cell migration is indispensable to morphogenesis and homeostasis. Live imaging allows mechanistic insights, but long-term observation can alter normal biology, and tools to track movements in vivo without perturbation are lacking. We develop here a tool called M-TRAIL (matrix-labeling technique for real-time and inferred location), which reveals migration histories in fixed tissues. Using clones that overexpress GFP-tagged extracellular matrix (ECM) components, motility trajectories are mapped based on durable traces deposited onto basement membrane. We applied M-TRAIL to Drosophila follicle rotation, comparing in vivo and ex vivo migratory dynamics. The rate, trajectory, and cessation of rotation in wild-type (WT) follicles measured in vivo and ex vivo were identical, as was rotation failure in fat2 mutants. However, follicles carrying intracellularly truncated Fat2, previously reported to lack rotation ex vivo, in fact rotate in vivo at a reduced speed, thus revalidating the hypothesis that rotation is required for tissue elongation. The M-TRAIL approach could be applied to track and quantitate in vivo cell motility in other tissues and organisms.

Evolution of mechanotransduction via YAP/TAZ in animal epithelia

Mechanical stretch forces can control the growth of epithelial tissues such as mammalian skin, whose surface area is precisely coordinated with body size. In skin keratinocytes cultured in vitro, mechanical forces acting via Integrin adhesions and the actin cytoskeleton have been shown to induce nuclear translocation of YAP/TAZ co-activators to induce cell proliferation. Furthermore, conditional knockouts of both YAP (also called YAP1) and TAZ (also called WWTR1) in mouse skin resemble the phenotype of skin-specific loss of Integrin beta1 (ITGB1), indicating that this signalling mechanism is important in vivo. Curiously, Integrins are dispensable in Drosophila to activate the sole YAP/TAZ homolog Yorkie (Yki), which has lost the C-terminal PDZ-binding motif needed to promote nuclear localization of YAP/TAZ in mammalian cells. Differences in the structure of the epidermis between deuterostomes (e.g.: stratified squamous skin of mammals) and protostomes (e.g.: monolayered columnar epidermis of Drosophila) may explain this evolutionary divergence. Monolayered columnar epithelia feature a well-differentiated apical membrane domain, where proteins such as Crumbs, Expanded, Merlin and Kibra activate the Hippo pathway to repress Drosophila Yki. Stratified squamous epithelia lack an apical domain and thus depend primarily on basal Integrin adhesions to activate YAP/TAZ in basal layer stem cells via multiple postulated signalling mechanisms. Finally, YAP and TAZ retain the ability to sense the apical domain in the columnar epithelial cells lining internal organs such as the lung bronchus, where YAP/TAZ localize to the nucleus in proliferating basal layer stem cells but translocate to the cytoplasm in differentiated columnar cells.

The WAVE Regulatory Complex and Branched F-Actin Counterbalance Contractile Force to Control Cell Shape and Packing in the Drosophila Eye

Contractile forces eliminate cell contacts in many morphogenetic processes. However, mechanisms that balance contractile forces to promote subtler remodeling remain unknown. To address this gap, we investigated remodeling of Drosophila eye lattice cells (LCs), which preserve cell contacts as they narrow to form the edges of a multicellular hexagonal lattice. We found that during narrowing, LC-LC contacts dynamically constrict and expand. Similar to other systems, actomyosin-based contractile forces promote pulses of constriction. Conversely, we found that WAVE-dependent branched F-actin accumulates at LC-LC contacts during expansion and functions to expand the cell apical area, promote shape changes, and prevent elimination of LC-LC contacts. Finally, we found that small Rho GTPases regulate the balance of contractile and protrusive dynamics. These data suggest a mechanism by which WAVE regulatory complex-based F-actin dynamics antagonize contractile forces to regulate cell shape and tissue topology during remodeling and thus contribute to the robustness and precision of the process.

FAT2 is a novel independent prognostic factor for the poor prognosis of gastric carcinoma

BACKGROUND

This study investigated the clinical implication of FAT2 in the progression, metastasis, and prognosis of gastric cancer.

METHODS

The expression of FAT2 in 436 clinicopathologically characterized gastric cancer cases and 92 control human non-tumor mucosa were analyzed by immunohistochemistry. Consequently, survival analysis was conducted to investigate the association of FAT2 expression and the development of gastric cancers.

RESULTS

FAT2 protein was found highly expressed in 90 of 92 (97.83%) control human non-tumor mucosa, while was highly expressed in 126 of 436 (28.90%) tumors samples and low in 310 of 436 (72.10%). The expression of FAT2 was associated with age, tumor size, depth of invasion, Lauren's classification, lymph node and distant metastases, regional lymph node stage, TNM stage, and prognosis. In particular, for stage I, II, and III tumors patients the 5-year survival rate was lower in those with high expression of FAT2 than those with low expression. In stage IV tumors, the expression of FAT2 was not associated with the 5-year survival rate. Lauren's classification and distant metastases, TNM stage, and expression of FAT2 were independent prognostic factors in the patients with gastric cancer, as revealed by Cox regression analysis.

CONCLUSION

The expression of FAT2 in gastric cancer was significantly associated with lymph node and distant metastases, and poor prognosis. FAT2 was also associated with the collective invasion and influenced the prognosis of those patients.

Epithelial rotation is preceded by planar symmetry breaking of actomyosin and protects epithelial tissue from cell deformations

Symmetry breaking is involved in many developmental processes that form bodies and organs. One of them is the epithelial rotation of developing tubular and acinar organs. However, how epithelial cells move, how they break symmetry to define their common direction, and what function rotational epithelial motions have remains elusive. Here, we identify a dynamic actomyosin network that breaks symmetry at the basal surface of the Drosophila follicle epithelium of acinar-like primitive organs, called egg chambers, and may represent a candidate force-generation mechanism that underlies the unidirectional motion of this epithelial tissue. We provide evidence that the atypical cadherin Fat2, a key planar cell polarity regulator in Drosophila oogenesis, directs and orchestrates transmission of the intracellular actomyosin asymmetry cue onto a tissue plane in order to break planar actomyosin symmetry, facilitate epithelial rotation in the opposite direction, and direct the elongation of follicle cells. In contrast, loss of this rotational motion results in anisotropic non-muscle Myosin II pulses that are disorganized in plane and causes cell deformations in the epithelial tissue of Drosophila eggs. Our work demonstrates that atypical cadherins play an important role in the control of symmetry breaking of cellular mechanics in order to facilitate tissue motion and model epithelial tissue. We propose that their functions may be evolutionarily conserved in tubular/acinar vertebrate organs.

Movement of epithelial tissues is essential for organ and body formation as well as function. To facilitate epithelial movements, cells need an internal or external source of mechanical force and a collective decision in which direction to move. However, little is known about the underlying mechanism of collective cell movement in living and moving epithelial tissues. Using high-speed confocal imaging of rotating follicle epithelia in acinar-like Drosophila egg chambers, we find that individual cells polarize their actomyosin network, a potent force-generating source, at their basal surface. We show that the atypical cadherin Fat2, a key regulator of planar cell polarity in Drosophila oogenesis, unifies and amplifies the polarized non-muscle Myosin II of individual follicle cells to break the symmetry of actomyosin contractility at the epithelial level. We propose that this is essential to facilitate epithelial rotation, and thereby directed cell elongation, at the basal surface of follicle cells. In contrast, a lack of unidirectional actomyosin contractility results in disrupted non-muscle Myosin II polarity within follicle cells and causes asynchronous Myosin II pulses that deform follicle cells. This demonstrates the critical function of Fat2, in the planar symmetry breaking of actomyosin, in epithelial motility, and potentially in organ development.

Variations in basement membrane mechanics are linked to epithelial morphogenesis

The regulation of morphogenesis by the basement membrane (BM) may rely on changes in its mechanical properties. To test this, we developed an atomic force microscopy-based method to measure BM mechanical stiffness during two key processes in Drosophila ovarian follicle development. First, follicle elongation depends on epithelial cells that collectively migrate, secreting BM fibrils perpendicularly to the anteroposterior axis. Our data show that BM stiffness increases during this migration and that fibril incorporation enhances BM stiffness. In addition, stiffness heterogeneity, due to oriented fibrils, is important for egg elongation. Second, epithelial cells change their shape from cuboidal to either squamous or columnar. We prove that BM softens around the squamous cells and that this softening depends on the TGFβ pathway. We also demonstrate that interactions between BM constituents are necessary for cell flattening. Altogether, these results show that BM mechanical properties are modified during development and that, in turn, such mechanical modifications influence both cell and tissue shapes.

Fat2 and Lar Dance a Pas de Deux during Collective Cell Migration

What coordinates the internal leading and trailing edges in collectively migrating cells is largely unknown. In this issue of Developmental Cell, Barlan et al. (2017) delineate a Fat2/Lar planar signaling pathway at the basal, motile cell-cell contacts of Drosophila egg chamber follicle cells.

Cellular functions of WASP family proteins at a glance

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family function as nucleation-promoting factors for the ubiquitously expressed Arp2/3 complex, which drives the generation of branched actin filaments. Arp2/3-generated actin regulates diverse cellular processes, including the formation of lamellipodia and filopodia, endocytosis and/or phagocytosis at the plasma membrane, and the generation of cargo-laden vesicles from organelles including the Golgi, endoplasmic reticulum (ER) and the endo-lysosomal network. Recent studies have also identified roles for WASP family members in promoting actin dynamics at the centrosome, influencing nuclear shape and membrane remodeling events leading to the generation of autophagosomes. Interestingly, several WASP family members have also been observed in the nucleus where they directly influence gene expression by serving as molecular platforms for the assembly of epigenetic and transcriptional machinery. In this Cell Science at a Glance article and accompanying poster, we provide an update on the subcellular roles of WHAMM, JMY and WASH (also known as WASHC1), as well as their mechanisms of regulation and emerging functions within the cell.

Fat2 and Lar Define a Basally Localized Planar Signaling System Controlling Collective Cell Migration

Collective migration of epithelial cells underlies diverse tissue-remodeling events, but the mechanisms that coordinate individual cell migratory behaviors for collective movement are largely unknown. Studying the Drosophila follicular epithelium, we show that the cadherin Fat2 and the receptor tyrosine phosphatase Lar function in a planar signaling system that coordinates leading and trailing edge dynamics between neighboring cells. Fat2 signals from each cell's trailing edge to induce leading edge protrusions in the cell behind, in part by stabilizing Lar's localization in these cells. Conversely, Lar signals from each cell's leading edge to stimulate trailing edge retraction in the cell ahead. Fat2/Lar signaling is similar to planar cell polarity signaling in terms of sub-cellular protein localization; however, Fat2/Lar signaling mediates short-range communication between neighboring cells instead of transmitting long-range information across a tissue. This work defines a key mechanism promoting epithelial migration and establishes a different paradigm for planar cell-cell signaling.

Functional Coordination of WAVE and WASP in C. elegans Neuroblast Migration

Directional cell migration is critical for metazoan development. We define two molecular pathways that activate the Arp2/3 complex during neuroblast migration in Caenorhabditis elegans. The transmembrane protein MIG-13/Lrp12 is linked to the Arp2/3 nucleation-promoting factors WAVE or WASP through direct interactions with ABL-1 or SEM-5/Grb2, respectively. WAVE mutations partially impaired F-actin organization and decelerated cell migration, and WASP mutations did not inhibit cell migration but enhanced migration defects in WAVE-deficient cells. Purified SEM-5 and MIG-2 synergistically stimulated the F-actin branching activity of WASP-Arp2/3 in vitro. In GFP knockin animals, WAVE and WASP were largely organized into separate clusters at the leading edge, and the amount of WASP was less than WAVE but could be elevated by WAVE mutations. Our results indicate that the MIG-13-WAVE pathway provides the major force for directional cell motility, whereas MIG-13-WASP partially compensates for its loss, underscoring their coordinated activities in facilitating robust cell migration.

Configuring a robust nervous system with Fat cadherins

Atypical Fat cadherins represent a small but versatile group of signaling molecules that influence proliferation and tissue polarity. With huge extracellular domains and intracellular domains harboring many independent protein interaction sites, Fat cadherins are poised to translate local cell adhesion events into a variety of cell behaviors. The need for such global coordination is particularly prominent in the nervous system, where millions of morphologically diverse neurons are organized into functional networks. As we learn more about their biological functions and molecular properties, increasing evidence suggests that Fat cadherins mediate contact-induced changes that ultimately impose a structure to developing neuronal circuits.

Fat-like cadherins in cell migration-leading from both the front and the back

When cells migrate through the body, their motility is continually influenced by interactions with other cells. The Fat-like cadherins are cell-cell signaling proteins that promote migration in multiple cell types. Recent studies suggest, however, that Fat-like cadherins influence motility differently in mammals versus Drosophila, with the cadherin acting at the leading edge of mammalian cells and the trailing edge of Drosophila cells. As opposed to this being a difference between organisms, it is more likely that the Fat-like cadherins are highly versatile proteins that can interact with the migration machinery in multiple ways. Here, I review what is known about how Fat-like cadherins promote migration, and then explore where conserved features may be found between the mammalian and Drosophila models.

The repertoire of epithelial morphogenesis on display: Progressive elaboration of Drosophila egg structure

Epithelial structures are foundational for tissue organization in all metazoans. Sheets of epithelial cells form lateral adhesive junctions and acquire apico-basal polarity perpendicular to the surface of the sheet. Genetic analyses in the insect model, Drosophila melanogaster, have greatly advanced our understanding of how epithelial organization is established, and how it is modulated during tissue morphogenesis. Major insights into collective cell migrations have come from analyses of morphogenetic movements within the adult follicular epithelium that cooperates with female germ cells to build a mature egg. Epithelial follicle cells progress through tightly choreographed phases of proliferation, patterning, reorganization and migrations, before they differentiate to form the elaborate structures of the eggshell. Distinct structural domains are organized by differential adhesion, within which lateral junctions are remodeled to further shape the organized epithelia. During collective cell migrations, adhesive interactions mediate supracellular organization of planar polarized macromolecules, and facilitate crawling over the basement membrane or traction against adjacent cell surfaces. Comparative studies with other insects are revealing the diversification of morphogenetic movements for elaboration of epithelial structures. This review surveys the repertoire of follicle cell morphogenesis, to highlight the coordination of epithelial plasticity with progressive differentiation of a secretory epithelium. Technological advances will keep this tissue at the leading edge for interrogating the precise spatiotemporal regulation of normal epithelial reorganization events, and provide a framework for understanding pathological tissue dysplasia.

Symmetry Breaking in an Edgeless Epithelium by Fat2-Regulated Microtubule Polarity

Planar cell polarity (PCP) information is a critical determinant of organ morphogenesis. While PCP in bounded epithelial sheets is increasingly well understood, how PCP is organized in tubular and acinar tissues is not. Drosophila egg chambers (follicles) are an acinus-like "edgeless epithelium" and exhibit a continuous, circumferential PCP that does not depend on pathways active in bounded epithelia; this follicle PCP directs formation of an ellipsoid rather than a spherical egg. Here, we apply an imaging algorithm to "unroll" the entire 3D tissue surface and comprehensively analyze PCP onset. This approach traces chiral symmetry breaking to plus-end polarity of microtubules in the germarium, well before follicles form and rotate. PCP germarial microtubules provide chiral information that predicts the direction of whole-tissue rotation as soon as independent follicles form. Concordant microtubule polarity, but not microtubule alignment, requires the atypical cadherin Fat2, which acts at an early stage to translate plus-end bias into coordinated actin-mediated collective cell migration. Because microtubules are not required for PCP or migration after follicle rotation initiates, while dynamic actin and extracellular matrix are, polarized microtubules lie at the beginning of a handoff mechanism that passes early chiral PCP of the cytoskeleton to a supracellular planar polarized extracellular matrix and elongates the organ.

Rotating for elongation: Fat2 whips for the race

Dynamic rearrangements of the actin cytoskeleton are crucial for cell shape and migration. In this issue, Squarr et al. (2016. J. Cell Biol.http://dx.doi.org/10.1083/jcb.201508081) show that the cadherin superfamily protein Fat2 regulates actin-rich protrusions driving collective cell migration during Drosophila melanogaster egg morphogenesis through its interaction with the WAVE regulatory complex.

Molecular Control of Actin Dynamics In Vivo: Insights from Drosophila

The actin cytoskeleton provides mechanical support for cells and generates forces to drive cell shape changes and cell migration in morphogenesis. Molecular understanding of actin dynamics requires a genetically traceable model system that allows interdisciplinary experimental approaches to elucidate the regulatory network of cytoskeletal proteins in vivo. Here, we will discuss some examples of how advances in Drosophila genetics and high-resolution imaging techniques contribute to the discovery of new actin functions, signaling pathways, and mechanisms of actin regulation in vivo.