Cellular and Molecular Mechanisms of Border Cell Migration Analyzed Using Time-Lapse Live-Cell Imaging

The pattern of the follicle cell diversification in ovarian follicles of the true fruit flies, Tephritidae

In flies (Diptera), the ovary displays several distinct patterns of the follicular epithelium formation and diversification. Two main patterns have been identified in the true flies or Brachycera, namely the Rhagio type and the Drosophila type. These patterns align with the traditional division of Brachycera into Orthorrhapha and Cyclorrhapha. However, studies of the follicular epithelium morphogenesis in cyclorrhaphans other than Drosophila are scarce. We characterise the developmental changes associated with the emergence of follicle cell (FC) diversity in two cyclorrhaphans belonging to the family Tephritidae (Brachycera, Cyclorrhapha). Our analysis revealed that the diversification of FCs in these species shows characteristics of both the Rhagio and Drosophila types. First, a distinct cluster of FCs, consisting of polar cells and border-like cells, differentiates at the posterior pole of the ovarian follicle. This feature is unique to the Rhagio type and has only been reported in species representing the Orthorrhapha group. Second, morphological criteria have identified a significantly smaller number of subpopulations of FCs than in Drosophila. Furthermore, while the general pattern of FC migration is similar to that of Drosophila, the distinctive migration of the anterior-dorsal FCs is absent. In the studied tephritids, the migration of the anterior polar cell/border cell cluster towards the anterior pole of the oocyte is followed by the posterior migration of the main body cuboidal FCs to cover the expanding oocyte. Finally, during the onset of vitellogenesis, a distinct subset of FCs migrates towards the centre of the ovarian follicle to cover the oocyte's anterior pole. Our study also highlights specific actions of some FCs that accompany the migration process, which has not been previously documented in cyclorrhaphans. These results support the hypothesis that the posterior and centripetal migrations of morphologically unique FC subsets arose in the common ancestor of Cyclorrhapha. These events appear to have occurred fairly recently in the evolutionary timeline of Diptera.

Mechanistic insights into mesenchymal-amoeboid transition as an intelligent cellular adaptation in cancer metastasis and resistance

Malignant cell plasticity is an important hallmark of tumor biology and crucial for metastasis and resistance. Cell plasticity lets cancer cells adapt to and escape the therapeutic strategies, which is the leading cause of cancer patient mortality. Epithelial cells acquire mobility via epithelial-mesenchymal transition (EMT), whereas mesenchymal cells enhance their migratory ability and clonogenic potential by acquiring amoeboid characteristics through mesenchymal-amoeboid transition (MAT). Tumor formation, progression, and metastasis depend on the tumor microenvironment (TME), a complex ecosystem within and around a tumor. Through increased migration and metastasis of cancer cells, the TME also contributes to malignancy. This review underscores the distinction between invasion pattern morphological manifestations and the diverse structures found within the TME. Furthermore, the mechanisms by which amoeboid-associated characteristics promote resistance and metastasis and how these mechanisms may represent therapeutic opportunities are discussed.

Drosophila Clueless ribonucleoprotein particles display novel dynamics that rely on the availability of functional protein and polysome equilibrium

The cytoplasm is populated with many ribonucleoprotein (RNP) particles that post-transcriptionally regulate mRNAs. These membraneless organelles assemble and disassemble in response to stress, performing functions such as sequestering stalled translation pre-initiation complexes or mRNA storage, repression and decay. Drosophila Clueless (Clu) is a conserved multi-domain ribonucleoprotein essential for mitochondrial function that forms dynamic particles within the cytoplasm. Unlike well-known RNP particles, stress granules and Processing bodies, Clu particles completely disassemble under nutritional or oxidative stress. However, it is poorly understood how disrupting protein synthesis affects Clu particle dynamics, especially since Clu binds mRNA and ribosomes. Here, we capitalize on ex vivo and in vivo imaging of Drosophila female germ cells to determine what domains of Clu are necessary for Clu particle assembly, how manipulating translation using translation inhibitors affects particle dynamics, and how Clu particle movement relates to mitochondrial association. Using Clu deletion analysis and live and fixed imaging, we identified three protein domains in Clu, which are essential for particle assembly. In addition, we demonstrated that overexpressing functional Clu disassembled particles, while overexpression of deletion constructs did not. To examine how decreasing translation affects particle dynamics, we inhibited translation in Drosophila germ cells using cycloheximide and puromycin. In contrast to stress granules and Processing bodies, cycloheximide treatment did not disassemble Clu particles yet puromycin treatment did. Surprisingly, cycloheximide stabilized particles in the presence of oxidative and nutritional stress. These findings demonstrate that Clu particles have novel dynamics in response to altered ribosome activity compared to stress granules and Processing bodies and support a model where they function as hubs of translation whose assembly heavily depends on the dynamic availability of polysomes.

The Enigmas of Tissue Closure: Inspiration from Drosophila

Hollow structures are essential for development and physiological activity. The construction and maintenance of hollow structures never cease throughout the lives of multicellular animals. Epithelial tissue closure is the main strategy used by living organisms to build hollow structures. The high diversity of hollow structures and the simplicity of their development in Drosophila make it an excellent model for the study of hollow structure morphogenesis. In this review, we summarize the tissue closure processes in Drosophila that give rise to or maintain hollow structures and highlight the molecular mechanisms and distinct cell biology involved in these processes.

Collective cell migration relies on PPP1R15-mediated regulation of the endoplasmic reticulum stress response

Collective cell migration is integral to many developmental and disease processes. Previously, we discovered that protein phosphatase 1 (Pp1) promotes border cell collective migration in the Drosophila ovary. We now report that the Pp1 phosphatase regulatory subunit dPPP1R15 is a critical regulator of border cell migration. dPPP1R15 is an ortholog of mammalian PPP1R15 proteins that attenuate the endoplasmic reticulum (ER) stress response. We show that, in collectively migrating border cells, dPPP1R15 phosphatase restrains an active physiological protein kinase R-like ER kinase- (PERK)-eIF2α-activating transcription factor 4 (ATF4) stress pathway. RNAi knockdown of dPPP1R15 blocks border cell delamination from the epithelium and subsequent migration, increases eIF2α phosphorylation, reduces translation, and drives expression of the stress response transcription factor ATF4. We observe similar defects upon overexpression of ATF4 or the eIF2α kinase PERK. Furthermore, we show that normal border cells express markers of the PERK-dependent ER stress response and require PERK and ATF4 for efficient migration. In many other cell types, unresolved ER stress induces initiation of apoptosis. In contrast, border cells with chronic RNAi knockdown of dPPP1R15 survive. Together, our results demonstrate that the PERK-eIF2α-ATF4 pathway, regulated by dPPP1R15 activity, counteracts the physiological ER stress that occurs during collective border cell migration. We propose that in vivo collective cell migration is intrinsically "stressful," requiring tight homeostatic control of the ER stress response for collective cell cohesion, dynamics, and movement.

Steroid hormone signaling synchronizes cell migration machinery, adhesion and polarity to direct collective movement

Migratory cells - either individually or in cohesive groups - are critical for spatiotemporally regulated processes such as embryonic development and wound healing. Their dysregulation is the underlying cause of formidable health problems such as congenital abnormalities and metastatic cancers. Border cell behavior during Drosophila oogenesis provides an effective model to study temporally regulated, collective cell migration in vivo. Developmental timing in flies is primarily controlled by the steroid hormone ecdysone, which acts through a well-conserved, nuclear hormone receptor complex. Ecdysone signaling determines the timing of border cell migration, but the molecular mechanisms governing this remain obscure. We found that border cell clusters expressing a dominant-negative form of ecdysone receptor extended ineffective protrusions. Additionally, these clusters had aberrant spatial distributions of E-cadherin (E-cad), apical domain markers and activated myosin that did not overlap. Remediating their expression or activity individually in clusters mutant for ecdysone signaling did not restore proper migration. We propose that ecdysone signaling synchronizes the functional distribution of E-cadherin, atypical protein kinase C (aPKC), Discs large (Dlg1) and activated myosin post-transcriptionally to coordinate adhesion, polarity and contractility and temporally control collective cell migration.

Specific prostaglandins are produced in the migratory cells and the surrounding substrate to promote Drosophila border cell migration

Introduction: A key regulator of collective cell migration is prostaglandin (PG) signaling. However, it remains largely unclear whether PGs act within the migratory cells or their microenvironment to promote migration. Here we use Drosophila border cell migration as a model to uncover the cell-specific roles of two PGs in collective migration. The border cells undergo a collective and invasive migration between the nurse cells; thus, the nurse cells are the substrate and microenvironment for the border cells. Prior work found PG signaling is required for on-time border cell migration and cluster cohesion. Methods: Confocal microscopy and quantitative image analyses of available mutant alleles and RNAi lines were used to define the roles of the PGE2 and PGF2α synthases in border cell migration. Results: We find that the PGE2 synthase cPGES is required in the substrate, while the PGF2α synthase Akr1B is required in the border cells for on-time migration. Akr1B acts in both the border cells and their substrate to regulate cluster cohesion. One means by which Akr1B may regulate border cell migration and/or cluster cohesion is by promoting integrin-based adhesions. Additionally, Akr1B limits myosin activity, and thereby cellular stiffness, in the border cells, whereas cPGES limits myosin activity in both the border cells and their substrate. Decreasing myosin activity overcomes the migration delays in both akr1B and cPGES mutants, indicating the changes in cellular stiffness contribute to the migration defects. Discussion: Together these data reveal that two PGs, PGE2 and PGF2α, produced in different locations, play key roles in promoting border cell migration. These PGs likely have similar migratory versus microenvironment roles in other collective cell migrations.

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.

Transcriptome analysis reveals temporally regulated genetic networks during Drosophila border cell collective migration

BACKGROUND

Collective cell migration underlies many essential processes, including sculpting organs during embryogenesis, wound healing in the adult, and metastasis of cancer cells. At mid-oogenesis, Drosophila border cells undergo collective migration. Border cells round up into a small group at the pre-migration stage, detach from the epithelium and undergo a dynamic and highly regulated migration at the mid-migration stage, and stop at the oocyte, their final destination, at the post-migration stage. While specific genes that promote cell signaling, polarization of the cluster, formation of protrusions, and cell-cell adhesion are known to regulate border cell migration, there may be additional genes that promote these distinct active phases of border cell migration. Therefore, we sought to identify genes whose expression patterns changed during border cell migration.

RESULTS

We performed RNA-sequencing on border cells isolated at pre-, mid-, and post-migration stages. We report that 1,729 transcripts, in nine co-expression gene clusters, are temporally and differentially expressed across the three migration stages. Gene ontology analyses and constructed protein-protein interaction networks identified genes expected to function in collective migration, such as regulators of the cytoskeleton, adhesion, and tissue morphogenesis, but also uncovered a notable enrichment of genes involved in immune signaling, ribosome biogenesis, and stress responses. Finally, we validated the in vivo expression and function of a subset of identified genes in border cells.

CONCLUSIONS

Overall, our results identified differentially and temporally expressed genetic networks that may facilitate the efficient development and migration of border cells. The genes identified here represent a wealth of new candidates to investigate the molecular nature of dynamic collective cell migrations in developing tissues.

Differential cell motion: A mathematical model of anterior posterior sorting

Here, we investigate how a subpopulation of cells can move through an aggregate of cells. Using a stochastic force-based model of Dictyostelium discoideum when the population is forming a slug, we simulate different strategies for prestalk cells to reliably move to the front of the slug while omitting interaction with the substrate thus ignoring the overall motion of the slug. Of the mechanisms that we simulated, prestalk cells being more directed is the best strategy followed by increased asymmetric motive forces for prestalk cells. The lifetime of the cell adhesion molecules, while not enough to produce differential motion, did modulate the results of the strategies employed. Finally, understanding and simulating the appropriate boundary conditions are essential to correctly predict the motion.

Exploring Cell Migration Mechanisms in Cancer: From Wound Healing Assays to Cellular Automata Models

PURPOSE

Cell migration is a critical driver of metastatic tumor spread, contributing significantly to cancer-related mortality. Yet, our understanding of the underlying mechanisms remains incomplete.

METHODS

In this study, a wound healing assay was employed to investigate cancer cell migratory behavior, with the aim of utilizing migration as a biomarker for invasiveness. To gain a comprehensive understanding of this complex system, we developed a computational model based on cellular automata (CA) and rigorously calibrated and validated it using in vitro data, including both tumoral and non-tumoral cell lines. Harnessing this CA-based framework, extensive numerical experiments were conducted and supported by local and global sensitivity analyses in order to identify the key biological parameters governing this process.

RESULTS

Our analyses led to the formulation of a power law equation derived from just a few input parameters that accurately describes the governing mechanism of wound healing. This groundbreaking research provides a powerful tool for the pharmaceutical industry. In fact, this approach proves invaluable for the discovery of novel compounds aimed at disrupting cell migration, assessing the efficacy of prospective drugs designed to impede cancer invasion, and evaluating the immune system's responses.

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.

ArfGAP1 regulates the endosomal sorting of guidance receptors to promote directed collective cell migration in vivo

Chemotaxis drives diverse migrations important for development and involved in diseases, including cancer progression. Using border cells in the Drosophila egg chamber as a model for collective cell migration, we characterized the role of ArfGAP1 in regulating chemotaxis during this process. We found that ArfGAP1 is required for the maintenance of receptor tyrosine kinases, the guidance receptors, at the plasma membrane. In the absence of ArfGAP1, the level of active receptors is reduced at the plasma membrane and increased in late endosomes. Consequently, clusters with impaired ArfGAP1 activity lose directionality. Furthermore, we found that the number and size of late endosomes and lysosomes are increased in the absence of ArfGAP1. Finally, genetic interactions suggest that ArfGAP1 acts on the kinase and GTPase Lrrk to regulate receptor sorting. Overall, our data indicate that ArfGAP1 is required to maintain guidance receptors at the plasma membrane and promote chemotaxis.

Septins regulate border cell surface geometry, shape, and motility downstream of Rho in Drosophila

Septins self-assemble into polymers that bind and deform membranes in vitro and regulate diverse cell behaviors in vivo. How their in vitro properties relate to their in vivo functions is under active investigation. Here, we uncover requirements for septins in detachment and motility of border cell clusters in the Drosophila ovary. Septins and myosin colocalize dynamically at the cluster periphery and share phenotypes but, surprisingly, do not impact each other. Instead, Rho independently regulates myosin activity and septin localization. Active Rho recruits septins to membranes, whereas inactive Rho sequesters septins in the cytoplasm. Mathematical analyses identify how manipulating septin expression levels alters cluster surface texture and shape. This study shows that the level of septin expression differentially regulates surface properties at different scales. This work suggests that downstream of Rho, septins tune surface deformability while myosin controls contractility, the combination of which governs cluster shape and movement.

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.

Who's really in charge: Diverse follower cell behaviors in collective cell migration

Collective cell migrations drive morphogenesis, wound healing, and cancer dissemination. Cells located at the front are considered leaders while those behind them are defined topologically as followers. Leader cell behaviors, including chemotaxis and their coupling to followers, have been well-studied and reviewed. However, the contributions of follower cells to collective cell migration represent an emerging area of interest. In this perspective, we highlight recent research into the broadening array of follower cell behaviors found in moving collectives. We describe examples of follower cells that possess cryptic leadership potential and followers that lack that potential but contribute in diverse and sometimes surprising ways to collective movement, even steering from behind. We highlight collectives in which all cells both lead and follow, and a few passive passengers. The molecular mechanisms controlling follower cell function and behavior are just emerging and represent an exciting frontier in collective cell migration research.

The Vast Utility of Drosophila Oogenesis

In this chapter, we highlight examples of the diverse array of developmental, cellular, and biochemical insights that can be gained by using Drosophila melanogaster oogenesis as a model tissue. We begin with an overview of ovary development and adult oogenesis. Then we summarize how the adult Drosophila ovary continues to advance our understanding of stem cells, cell cycle, cell migration, cytoplasmic streaming, nurse cell dumping, and cell death. We also review emerging areas of study, including the roles of lipid droplets, ribosomes, and nuclear actin in egg development. Finally, we conclude by discussing the growing conservation of processes and signaling pathways that regulate oogenesis and female reproduction from flies to humans.

Quantitative Image Analysis of Dynamic Cell Behaviors During Border Cell Migration

Drosophila border cells have emerged as a genetically tractable model to investigate dynamic collective cell migration within the context of a developing organ. Studies of live border cell cluster migration have revealed similarities with other migrating collectives, including formation and restriction of cellular protrusions to the front of the cluster, supracellular actomyosin contractility of the entire collective, and intra-collective cell motility. Here, we describe protocols to prepare ex vivo cultures of stage 9 egg chambers followed by live time-lapse imaging of fluorescently labeled border cells to image dynamic cell behaviors. We provide options to perform live imaging using either a widefield epifluorescent microscope or a confocal microscope. We further outline steps to quantify various cellular behaviors and protein dynamics of live migrating border cells using the Fiji image processing package of ImageJ. These methods can be adapted to other migrating cell collectives in cultured tissues and organs.

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.

Live Imaging of the Drosophila Testis Stem Cell Niche

Live imaging of adult tissue stem cell niches provides key insights into the dynamic behavior of stem cells, their differentiating progeny, and their neighboring support cells, but few niches are amenable to this approach. Here, we discuss a technique for long-term live imaging of the Drosophila testis stem cell niche. Culturing whole testes ex vivo for up to 18 h allows for tracking of cell-type-specific behaviors under normal and various chemically or genetically modified conditions. Fixing and staining tissues after live imaging allows for the molecular confirmation of cell identity and behavior. By using live imaging in intact niches, we can better uncover the cellular and molecular mechanisms that regulate stem cell function in vivo.

A Scribble/Cdep/Rac pathway controls follower-cell crawling and cluster cohesion during collective border-cell migration

Collective cell movements drive normal development and metastasis. Drosophila border cells move as a cluster of 6-10 cells, where the role of the Rac GTPase in migration was first established. In border cells, as in most migratory cells, Rac stimulates leading-edge protrusion. Upstream Rac regulators in leaders have been identified; however, the regulation and function of Rac in follower border cells is unknown. Here, we show that all border cells require Rac, which promotes follower-cell motility and is important for cluster compactness and movement. We identify a Rac guanine nucleotide exchange factor, Cdep, which also regulates follower-cell movement and cluster cohesion. Scribble, Discs large, and Lethal giant larvae localize Cdep basolaterally and share phenotypes with Cdep. Relocalization of Cdep::GFP partially rescues Scribble knockdown, suggesting that Cdep is a major downstream effector of basolateral proteins. Thus, a Scrib/Cdep/Rac pathway promotes cell crawling and coordinated, collective migration in vivo.

Two Rac1 pools integrate the direction and coordination of collective cell migration

Integration of collective cell direction and coordination is believed to ensure collective guidance for efficient movement. Previous studies demonstrated that chemokine receptors PVR and EGFR govern a gradient of Rac1 activity essential for collective guidance of Drosophila border cells, whose mechanistic insight is unknown. By monitoring and manipulating subcellular Rac1 activity, here we reveal two switchable Rac1 pools at border cell protrusions and supracellular cables, two important structures responsible for direction and coordination. Rac1 and Rho1 form a positive feedback loop that guides mechanical coupling at cables to achieve migration coordination. Rac1 cooperates with Cdc42 to control protrusion growth for migration direction, as well as to regulate the protrusion-cable exchange, linking direction and coordination. PVR and EGFR guide correct Rac1 activity distribution at protrusions and cables. Therefore, our studies emphasize the existence of a balance between two Rac1 pools, rather than a Rac1 activity gradient, as an integrator for the direction and coordination of collective cell migration.

Previous studies suggested a chemokine receptor governed gradient of Rac1 activity is essential for collective guidance of Drosophila border cells. Here, Zhou et al. report that two distinct Rac1 pools at protrusions and cables, not Rac1 activity gradient, integrate the direction and coordination for collective guidance.

Hypoxia controls plasma membrane targeting of polarity proteins by dynamic turnover of PI4P and PI(4,5)P2

Phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-biphosphate (PIP2) are key phosphoinositides that determine the identity of the plasma membrane (PM) and regulate numerous key biological events there. To date, mechanisms regulating the homeostasis and dynamic turnover of PM PI4P and PIP2 in response to various physiological conditions and stresses remain to be fully elucidated. Here, we report that hypoxia in Drosophila induces acute and reversible depletion of PM PI4P and PIP2 that severely disrupts the electrostatic PM targeting of multiple polybasic polarity proteins. Genetically encoded ATP sensors confirmed that hypoxia induces acute and reversible reduction of cellular ATP levels which showed a strong real-time correlation with the levels of PM PI4P and PIP2 in cultured cells. By combining genetic manipulations with quantitative imaging assays we showed that PI4KIIIα, as well as Rbo/EFR3 and TTC7 that are essential for targeting PI4KIIIα to PM, are required for maintaining the homeostasis and dynamic turnover of PM PI4P and PIP2 under normoxia and hypoxia. Our results revealed that in cells challenged by energetic stresses triggered by hypoxia, ATP inhibition and possibly ischemia, dramatic turnover of PM PI4P and PIP2 could have profound impact on many cellular processes including electrostatic PM targeting of numerous polybasic proteins.

Polarized interfacial tension induces collective migration of cells, as a cluster, in a 3D tissue

In embryogenesis and cancer invasion, cells collectively migrate as a cluster in 3D tissues. Many studies have elucidated mechanisms of either individual or collective cell migration on 2D substrates; however, it remains unclear how cells collectively migrate as a cluster through 3D tissues. To address this issue, we considered the interfacial tension at cell-cell boundaries expressing cortical actomyosin contractions and cell-cell adhesive interactions. The strength of this tension is polarized; i.e., spatially biased within each cell according to a chemoattractant gradient. Using a 3D vertex model, we performed numerical simulations of multicellular dynamics in 3D space. The simulations revealed that the polarized interfacial tension enables cells to migrate collectively as a cluster through a 3D tissue. In this mechanism, interfacial tension induces unidirectional flow of each cell surface from the front to the rear along the cluster surface. Importantly, this mechanism does not necessarily require convection of cells, i.e., cell rearrangement, within the cluster. Moreover, several migratory modes were induced, depending on the strengths of polarity, adhesion, and noise; i.e., cells migrate either as single cells, as a cluster, or aligned like beads on a string, as occurs in embryogenesis and cancer invasion. These results indicate that the simple expansion and contraction of cell-cell boundaries enables cells to move directionally forward and to produce the variety of collective migratory movements observed in living systems.

A coarse-grained approach to model the dynamics of the actomyosin cortex

Background

The dynamics of the actomyosin machinery is at the core of many important biological processes. Several relevant cellular responses such as the rhythmic compression of the cell cortex are governed, at a mesoscopic level, by the nonlinear interaction between actin monomers, actin crosslinkers, and myosin motors. Coarse-grained models are an optimal tool to study actomyosin systems, since they can include processes that occur at long time and space scales, while maintaining the most relevant features of the molecular interactions.

Results

Here, we present a coarse-grained model of a two-dimensional actomyosin cortex, adjacent to a three-dimensional cytoplasm. Our simplified model incorporates only well-characterized interactions between actin monomers, actin crosslinkers and myosin, and it is able to reproduce many of the most important aspects of actin filament and actomyosin network formation, such as dynamics of polymerization and depolymerization, treadmilling, network formation, and the autonomous oscillatory dynamics of actomyosin.

Conclusions

We believe that the present model can be used to study the in vivo response of actomyosin networks to changes in key parameters of the system, such as alterations in the attachment of actin filaments to the cell cortex.

Supplementary Information

The online version contains supplementary material available at (10.1186/s12915-022-01279-2).

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.

The Drosophila spectraplakin Short stop regulates focal adhesion dynamics by crosslinking microtubules and actin

The spectraplakin family of proteins includes ACF7/MACF1 and BPAG1/dystonin in mammals, VAB-10 in Caenorhabditis elegans, Magellan in zebrafish, and Short stop (Shot), the sole Drosophila member. Spectraplakins are giant cytoskeletal proteins that cross-link actin, microtubules, and intermediate filaments, coordinating the activity of the entire cytoskeleton. We examined the role of Shot during cell migration using two systems: the in vitro migration of Drosophila tissue culture cells and in vivo through border cell migration. RNA interference (RNAi) depletion of Shot increases the rate of random cell migration in Drosophila tissue culture cells as well as the rate of wound closure during scratch-wound assays. This increase in cell migration prompted us to analyze focal adhesion dynamics. We found that the rates of focal adhesion assembly and disassembly were faster in Shot-depleted cells, leading to faster adhesion turnover that could underlie the increased migration speeds. This regulation of focal adhesion dynamics may be dependent on Shot being in an open confirmation. Using Drosophila border cells as an in vivo model for cell migration, we found that RNAi depletion led to precocious border cell migration. Collectively, these results suggest that spectraplakins not only function to cross-link the cytoskeleton but may regulate cell–matrix adhesion.

theLiTE™: A Screening Platform to Identify Compounds that Reinforce Tight Junctions

Tight junctions (TJ) are formed by transmembrane and intracellular proteins that seal the intercellular space and control selective permeability of epithelia. Integrity of the epithelial barrier is central to tissue homeostasis and barrier dysfunction has been linked to many pathological conditions. TJ support the maintenance of cell polarity through interactions with the Par complex (Cdc42-Par-6-Par-3-aPKC) in which Par-6 is an adaptor and links the proteins of the complex together. Studies have shown that Par-6 overexpression delays the assembly of TJ proteins suggesting that Par-6 negatively regulates TJ assembly. Because restoring barrier integrity is of key therapeutic and prophylactic value, we focus on finding compounds that have epithelial barrier reinforcement properties; we developed a screening platform (theLiTE™) to identify compounds that modulate Par-6 expression in follicular epithelial cells from Par-6-GFP Drosophila melanogaster egg chambers. Hits identified were then tested whether they improve epithelial barrier function, using measurements of transepithelial electrical resistance (TEER) or dye efflux to evaluate paracellular permeability. We tested 2,400 compounds, found in total 10 hits. Here we present data on six of them: the first four hits allowed us to sequentially build confidence in theLiTE™ and two compounds that were shortlisted for further development (myricetin and quercetin). We selected quercetin due to its clinical and scientific validation as a compound that regulates TJ; food supplement formulated on the basis of this discovery is currently undergoing clinical evaluation in gastroesophageal reflux disease (GERD) sufferers.

Mapping Asymmetry in Collective Cell Migration: Lessons from Border Cells in Drosophila Oogenesis

Asymmetry in the migrating group of cells is critical for efficient directed movement observed in normal development and in pathological conditions like tumor cell metastasis. This is conspicuously detected at the level of polarized protrusions and differential localization of various polarity proteins in collectively moving clusters. Over the years, border cell migration in Drosophila oogenesis has emerged as an excellent model system for studying polarity in the migrating group of cells. Here we report two protocols employing live cell imaging and tissue immunohistochemistry to evaluate the polarity in migrating border cell clusters.

Peroxidasin Enhances Basal Phenotype and Inhibits Branching Morphogenesis in Breast Epithelial Progenitor Cell Line D492

The human breast is composed of terminal duct lobular units (TDLUs) that are surrounded by stroma. In the TDLUs, basement membrane separates the stroma from the epithelial compartment, which is divided into an inner layer of luminal epithelial cells and an outer layer of myoepithelial cells. Stem cells and progenitor cells also reside within the epithelium and drive a continuous cycle of gland remodelling that occurs throughout the reproductive period. D492 is an epithelial cell line originally isolated from the stem cell population of the breast and generates both luminal and myoepithelial cells in culture. When D492 cells are embedded into 3D reconstituted basement membrane matrix (3D-rBM) they form branching colonies mimicking the TDLUs of the breast, thereby providing a well-suited in vitro model for studies on branching morphogenesis and breast development. Peroxidasin (PXDN) is a heme-containing peroxidase that crosslinks collagen IV with the formation of sulfilimine bonds. Previous studies indicate that PXDN plays an integral role in basement membrane stabilisation by crosslinking collagen IV and as such contributes to epithelial integrity. Although PXDN has been linked to fibrosis and cancer in some organs there is limited information on its role in development, including in the breast. In this study, we demonstrate expression of PXDN in breast epithelium and stroma and apply the D492 cell line to investigate the role of PXDN in cell differentiation and branching morphogenesis in the human breast. Overexpression of PXDN induced basal phenotype in D492 cells, loss of plasticity and inhibition of epithelial-to-mesenchymal transition as is displayed by complete inhibition of branching morphogenesis in 3D culture. This is supported by results from RNA-sequencing which show significant enrichment in genes involved in epithelial differentiation along with significant negative enrichment of EMT factors. Taken together, we provide evidence for a novel role of PXDN in breast epithelial differentiation and mammary gland development.

Fascin limits Myosin activity within Drosophila border cells to control substrate stiffness and promote migration

A key regulator of collective cell migrations, which drive development and cancer metastasis, is substrate stiffness. Increased substrate stiffness promotes migration and is controlled by Myosin. Using Drosophila border cell migration as a model of collective cell migration, we identify, for the first time, that the actin bundling protein Fascin limits Myosin activity in vivo. Loss of Fascin results in: increased activated Myosin on the border cells and their substrate, the nurse cells; decreased border cell Myosin dynamics; and increased nurse cell stiffness as measured by atomic force microscopy. Reducing Myosin restores on-time border cell migration in fascin mutant follicles. Further, Fascin’s actin bundling activity is required to limit Myosin activation. Surprisingly, we find that Fascin regulates Myosin activity in the border cells to control nurse cell stiffness to promote migration. Thus, these data shift the paradigm from a substrate stiffness-centric model of regulating migration, to uncover that collectively migrating cells play a critical role in controlling the mechanical properties of their substrate in order to promote their own migration. This understudied means of mechanical regulation of migration is likely conserved across contexts and organisms, as Fascin and Myosin are common regulators of cell migration.

NaNuTrap: a technique for in vivo cell nucleus labelling using nanobodies

In vivo cell labelling is challenging in fast developmental processes because many cell types differentiate more quickly than the maturation time of fluorescent proteins, making visualization of these tissues impossible with standard techniques. Here, we present a nanobody-based method, Nanobody Nuclear Trap (NaNuTrap), which works with the existing Gal4/UAS system in Drosophila and allows for early in vivo cell nuclei labelling independently of the maturation time of the fluorescent protein. This restores the utility of fluorescent proteins that have longer maturation times, such as those used in two-photon imaging, for live imaging of fast or very early developmental processes. We also present a more general application of this system, whereby NaNuTrap can convert cytoplasmic GFP expressed in any existing transgenic fly line into a nuclear label. This nuclear re-localization of the fluorescent signal can improve the utility of the GFP label, e.g. in cell counting, as well as resulting in a general increase in intensity of the live fluorescent signal. We demonstrate these capabilities of NaNuTrap by effectively tracking subsets of cells during the fast movements associated with gastrulation.

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.

Drosophila models of metastasis

An important goal in the fight against cancer is to understand how tumors become invasive and metastatic. A crucial early step in metastasis is thought to be the epithelial mesenchymal transition (EMT), the process in which epithelial cells transition into a more migratory and invasive, mesenchymal state. Since the genetic regulatory networks driving EMT in tumors derive from those used in development, analysis of EMTs in genetic model organisms such as the vinegar fly, Drosophila melanogaster, can provide great insight into cancer. In this review I highlight the many ways in which studies in the fly are shedding light on cancer metastasis. The review covers both normal developmental events in which epithelial cells become migratory, as well as induced events, whereby normal epithelial cells become metastatic due to genetic manipulations. The ability to make such precise genetic perturbations in the context of a normal, in vivo environment, complete with a working innate immune system, is making the fly increasingly important in understanding metastasis.

Septate junction proteins are required for egg elongation and border cell migration during oogenesis in Drosophila

Protein components of the invertebrate occluding junction—known as the septate junction (SJ)—are required for morphogenetic developmental events during embryogenesis in Drosophila melanogaster. In order to determine whether SJ proteins are similarly required for morphogenesis during other developmental stages, we investigated the localization and requirement of four representative SJ proteins during oogenesis: Contactin, Macroglobulin complement-related, Neurexin IV, and Coracle. A number of morphogenetic processes occur during oogenesis, including egg elongation, formation of dorsal appendages, and border cell (BC) migration. We found that all four SJ proteins are expressed in egg chambers throughout oogenesis, with the highest and the most sustained levels in the follicular epithelium (FE). In the FE, SJ proteins localize along the lateral membrane during early and mid-oogenesis, but become enriched in an apical-lateral domain (the presumptive SJ) by stage 11. SJ protein relocalization requires the expression of other SJ proteins, as well as Rab5 and Rab11 like SJ biogenesis in the embryo. Knocking down the expression of these SJ proteins in follicle cells throughout oogenesis results in egg elongation defects and abnormal dorsal appendages. Similarly, reducing the expression of SJ genes in the BC cluster results in BC migration defects. Together, these results demonstrate an essential requirement for SJ genes in morphogenesis during oogenesis, and suggest that SJ proteins may have conserved functions in epithelial morphogenesis across developmental stages.

Visualizing the Effects of Oxidative Damage on Drosophila Egg Chambers using Live Imaging

Live imaging of Drosophila melanogaster ovaries has been instrumental in understanding a variety of basic cellular processes during development, including ribonucleoprotein particle movement, mRNA localization, organelle movement, and cytoskeletal dynamics. There are several methods for live imaging that have been developed. Due to the fact that each method involves dissecting individual ovarioles placed in media or halocarbon oil, cellular damage due to hypoxia and/or physical manipulation will inevitably occur over time. One downstream effect of hypoxia is to increase oxidative damage in the cells. The purpose of this protocol is to use live imaging to visualize the effects of oxidative damage on the localization and dynamics of subcellular structures in Drosophila ovaries after induction of controlled cellular damage. Here, we use hydrogen peroxide to induce cellular oxidative damage and give examples of the effects of such damage on two subcellular structures, mitochondria and Clu bliss particles. However, this method is applicable to any subcellular structure. The limitations are that hydrogen peroxide can only be added to aqueous media and would not work for imaging that uses halocarbon oil. The advantages are that hydrogen peroxide is readily available and inexpensive, acts quickly, its concentrations can be modulated, and oxidative damage is a good approximation of damage caused by hypoxia as well as general tissue damage due to manipulation.

Coupled oscillators coordinate collective germline growth

Developing oocytes need large supplies of macromolecules and organelles. A conserved strategy for accumulating these products is to pool resources of oocyte-associated germline nurse cells. In Drosophila, these cells grow more than 100-fold to boost their biosynthetic capacity. No previously known mechanism explains how nurse cells coordinate growth collectively. Here, we report a cell cycle-regulating mechanism that depends on bidirectional communication between the oocyte and nurse cells, revealing the oocyte as a critical regulator of germline cyst growth. Transcripts encoding the cyclin-dependent kinase inhibitor, Dacapo, are synthesized by the nurse cells and actively localized to the oocyte. Retrograde movement of the oocyte-synthesized Dacapo protein to the nurse cells generates a network of coupled oscillators that controls the cell cycle of the nurse cells to regulate cyst growth. We propose that bidirectional nurse cell-oocyte communication establishes a growth-sensing feedback mechanism that regulates the quantity of maternal resources loaded into the oocyte.

The ArfGAP Drongo Promotes Actomyosin Contractility during Collective Cell Migration by Releasing Myosin Phosphatase from the Trailing Edge

Collective cell migration is involved in various developmental and pathological processes, including the dissemination of various cancer cells. During Drosophila melanogaster oogenesis, a group of cells called border cells migrate collectively toward the oocyte. Herein, we show that members of the Arf family of small GTPases and some of their regulators are required for normal border cell migration. Notably, we found that the ArfGAP Drongo and its GTPase-activating function are essential for the initial detachment of the border cell cluster from the basal lamina. We demonstrate through protein localization and genetic interactions that Drongo controls the localization of the myosin phosphatase in order to regulate myosin II activity at the back of the cluster. Moreover, we show that toward the class III Arf, Drongo acts antagonistically to the guanine exchange factor Steppke. Overall, our work describes a mechanistic pathway that promotes the local actomyosin contractility necessary for border cell detachment.

Cadherin-Mediated Cell Coupling Coordinates Chemokine Sensing across Collectively Migrating Cells

The directed migration of cells sculpts the embryo, contributes to homeostasis in the adult, and, when dysregulated, underlies many diseases [1, 2]. During these processes, cells move singly or as a collective. In both cases, they follow guidance cues, which direct them to their destination [3-6]. In contrast to single cells, collectively migrating cells need to coordinate with their neighbors to move together in the same direction. Recent studies suggest that leader cells in the front sense the guidance cue, relay the directional information to the follower cells in the back, and can pull the follower cells along [7-19]. In this manner, leader cells steer the collective and set the collective's overall speed. However, whether follower cells also participate in steering and speed setting of the collective is largely unclear. Using chimeras, we analyzed the role of leader and follower cells in the collectively migrating zebrafish posterior lateral line primordium. This tissue expresses the chemokine receptor Cxcr4 and is guided by the chemokine Cxcl12a [20-23]. We find that leader and follower cells need to sense the attractant Cxcl12a for efficient migration, are coupled to each other through cadherins, and require coupling to pull Cxcl12a-insensitive cells along. Analysis of cell dynamics in chimeric and protein-depleted primordia shows that Cxcl12a-sensing and cadherin-mediated adhesion contribute jointly to direct migration at both single-cell and tissue levels. These results suggest that all cells in the primordium need to sense the attractant and adhere to each other to coordinate their movements and migrate with robust directionality.

Prostaglandins regulate invasive, collective border cell migration

While prostaglandins (PGs), short-range lipid signals, regulate single cell migration, their roles in collective migration remain unclear. To address this, we use Drosophila border cell migration, an invasive, collective migration that occurs during Stage 9 of oogenesis. Pxt is the Drosophila cyclooxygenase-like enzyme responsible for PG synthesis. Loss of Pxt results in both delayed border cell migration and elongated clusters, whereas somatic Pxt knockdown causes delayed migration and compacted clusters. These findings suggest PGs act in both the border cells and nurse cells, the substrate on which the border cells migrate. As PGs regulate the actin bundler Fascin, and Fascin is required for on-time migration, we assessed whether PGs regulate Fascin to promote border cell migration. Coreduction of Pxt and Fascin results in delayed migration and elongated clusters. The latter may be due to altered cell adhesion, as loss of Pxt or Fascin, or coreduction of both, decreases integrin levels on the border cell membranes. Conversely, integrin localization is unaffected by somatic knockdown of Pxt. Together these data lead to the model that PG signaling controls Fascin in the border cells to promote migration and in the nurse cells to maintain cluster cohesion.

What makes leader cells arise: Intrinsic properties and support from neighboring cells

Cancer cells collectively invading as a cohesive and polarized group is termed collective invasion, which is a fundamental property of many types of cancers. In this multicellular unit, cancer cells are heterogeneous, consisting of two morphologically and functionally distinct subpopulations, leader cells and follower cells. Leader cells at the invasive front are responsible for exploring the microenvironment, paving the way, and transmitting information to follower cells. Here, in this review, we will describe the important role of leader cells in collective invasion and the emerging underlying mechanisms of leader cell formation including intrinsic properties and the support from neighboring cells. It will help us to elucidate the essence of collective invasion and provide new anticancer therapeutic clues.

Fascin regulates protrusions and delamination to mediate invasive, collective cell migration in vivo

BACKGROUND

The actin bundling protein Fascin is essential for developmental cell migrations and promotes cancer metastasis. In addition to bundling actin, Fascin has several actin-independent roles; how these other functions contribute to cell migration remains unclear. Border cell migration during Drosophila oogenesis provides an excellent model to study Fascin's various roles during invasive, collective cell migration.

RESULTS

On-time border cell migration during Stage 9 requires Fascin (Drosophila Singed). Fascin functions not only within the migrating border cells, but also within the nurse cells, the substrate for this migration. Fascin genetically interacts with the actin elongation factor Enabled to promote on-time Stage 9 migration and overexpression of Enabled suppresses the defects seen with loss of Fascin. Loss of Fascin results in increased, shorter and mislocalized protrusions during migration. Additionally, loss of Fascin inhibits border cell delamination and increases E-Cadherin (Drosophila Shotgun) adhesions on both the border cells and nurse cells.

CONCLUSIONS

Overall, Fascin promotes on-time border cell migration during Stage 9 and contributes to multiple aspects of this invasive, collective cell migration, including both protrusion dynamics and delamination. These findings have implications beyond Drosophila, as border cell migration has emerged as a model to study mechanisms mediating cancer metastasis.

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.

Protein phosphatase 1 activity controls a balance between collective and single cell modes of migration

Collective cell migration is central to many developmental and pathological processes. However, the mechanisms that keep cell collectives together and coordinate movement of multiple cells are poorly understood. Using the Drosophila border cell migration model, we find that Protein phosphatase 1 (Pp1) activity controls collective cell cohesion and migration. Inhibition of Pp1 causes border cells to round up, dissociate, and move as single cells with altered motility. We present evidence that Pp1 promotes proper levels of cadherin-catenin complex proteins at cell-cell junctions within the cluster to keep border cells together. Pp1 further restricts actomyosin contractility to the cluster periphery rather than at individual internal border cell contacts. We show that the myosin phosphatase Pp1 complex, which inhibits non-muscle myosin-II (Myo-II) activity, coordinates border cell shape and cluster cohesion. Given the high conservation of Pp1 complexes, this study identifies Pp1 as a major regulator of collective versus single cell migration.

In Vivo Quantitative Imaging Provides Insights into Trunk Neural Crest Migration

Neural crest (NC) cells undergo extensive migrations during development. Here, we couple in vivo live imaging at high resolution with custom software tools to reveal dynamic migratory behavior in chick embryos. Trunk NC cells migrate as individuals with both stochastic and biased features as they move dorsoventrally to form peripheral ganglia. Their leading edge displays a prominent fan-shaped lamellipodium that reorients upon cell-cell contact. Computational analysis reveals that when the lamellipodium of one cell touches the body of another, the two cells undergo "contact attraction," often moving together and then separating via a pulling force exerted by lamellipodium. Targeted optical manipulation shows that cell interactions coupled with cell density generate a long-range biased random walk behavior, such that cells move from high to low density. In contrast to chain migration noted at other axial levels, the results show that individual trunk NC cells navigate the complex environment without tight coordination between neighbors.

Identifying conserved molecular targets required for cell migration of glioblastoma cancer stem cells

Glioblastoma (GBM) is the most prevalent primary malignant brain tumor and is associated with extensive tumor cell infiltration into the adjacent brain parenchyma. However, there are limited targeted therapies that address this disease hallmark. While the invasive capacity of self-renewing cancer stem cells (CSCs) and their non-CSC progeny has been investigated, the mode(s) of migration used by CSCs during invasion is currently unknown. Here we used time-lapse microscopy to evaluate the migratory behavior of CSCs, with a focus on identifying key regulators of migration. A head-to-head migration assay demonstrated that CSCs are more invasive than non-CSCs. Time-lapse live cell imaging further revealed that GBM patient-derived CSC models either migrate in a collective manner or in a single cell fashion. To uncover conserved molecular regulators responsible for collective cell invasion, we utilized the genetically tractable Drosophila border cell collective migration model. Candidates for functional studies were generated using results from a targeted Drosophila genetic screen followed by gene expression analysis of the human homologs in GBM tumors and associated GBM patient prognosis. This strategy identified the highly conserved small GTPase, Rap1a, as a potential regulator of cell invasion. Alteration of Rap1a activity impaired the forward progress of Drosophila border cells during development. Rap1a expression was elevated in GBM and associated with higher tumor grade. Functionally, the levels of activated Rap1a impacted CSC migration speed out of spheres onto extracellular matrix. The data presented here demonstrate that CSCs are more invasive than non-CSCs, are capable of both collective and single cell migration, and express conserved genes that are required for migration and invasion. Using this integrated approach, we identified a new role for Rap1a in the migration of GBM CSCs.

Clustered cell migration: Modeling the model system of Drosophila border cells

In diverse developmental contexts, certain cells must migrate to fulfill their roles. Many questions remain unanswered about the genetic and physical properties that govern cell migration. While the simplest case of a single cell moving alone has been well-studied, additional complexities arise in considering how cohorts of cells move together. Significant differences exist between models of collectively migrating cells. We explore the experimental model of migratory border cell clusters in Drosophila melanogaster egg chambers, which are amenable to direct observation and precise genetic manipulations. This system involves two special characteristics that are worthy of attention: border cell clusters contain a limited number of both migratory and non-migratory cells that require coordination, and they navigate through a heterogeneous three-dimensional microenvironment. First, we review how clusters of motile border cells are specified and guided in their migration by chemical signals and the physical impact of adjacent tissue interactions. In the second part, we examine questions around the 3D structure of the motile cluster and surrounding microenvironment in understanding the limits to cluster size and speed of movement through the egg chamber. Mathematical models have identified sufficient gene regulatory networks for specification, the key forces that capture emergent behaviors observed in vivo, the minimal regulatory topologies for signaling, and the distribution of key signaling cues that direct cell behaviors. This interdisciplinary approach to studying border cells is likely to reveal governing principles that apply to different types of cell migration events.

Minimal Network Topologies for Signal Processing during Collective Cell Chemotaxis

Cell-cell communication plays an important role in collective cell migration. However, it remains unclear how cells in a group cooperatively process external signals to determine the group's direction of motion. Although the topology of signaling pathways is vitally important in single-cell chemotaxis, the signaling topology for collective chemotaxis has not been systematically studied. Here, we combine mathematical analysis and simulations to find minimal network topologies for multicellular signal processing in collective chemotaxis. We focus on border cell cluster chemotaxis in the Drosophila egg chamber, in which responses to several experimental perturbations of the signaling network are known. Our minimal signaling network includes only four elements: a chemoattractant, the protein Rac (indicating cell activation), cell protrusion, and a hypothesized global factor responsible for cell-cell interaction. Experimental data on cell protrusion statistics allows us to systematically narrow the number of possible topologies from more than 40,000,000 to only six minimal topologies with six interactions between the four elements. This analysis does not require a specific functional form of the interactions, and only qualitative features are needed; it is thus robust to many modeling choices. Simulations of a stochastic biochemical model of border cell chemotaxis show that the qualitative selection procedure accurately determines which topologies are consistent with the experiment. We fit our model for all six proposed topologies; each produces results that are consistent with all experimentally available data. Finally, we suggest experiments to further discriminate possible pathway topologies.

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.

Coordination of protrusion dynamics within and between collectively migrating border cells by myosin II

Collective cell migration is emerging as a major driver of embryonic development, organogenesis, tissue homeostasis, and tumor dissemination. In contrast to individually migrating cells, collectively migrating cells maintain cell–cell adhesions and coordinate direction-sensing as they move. While nonmuscle myosin II has been studied extensively in the context of cells migrating individually in vitro, its roles in cells migrating collectively in three-dimensional, native environments are not fully understood. Here we use genetics, Airyscan microscopy, live imaging, optogenetics, and Förster resonance energy transfer to probe the localization, dynamics, and functions of myosin II in migrating border cells of the Drosophila ovary. We find that myosin accumulates transiently at the base of protrusions, where it functions to retract them. E-cadherin and myosin colocalize at border cell-border cell contacts and cooperate to transmit directional information. A phosphomimetic form of myosin is sufficient to convert border cells to a round morphology and blebbing migration mode. Together these studies demonstrate that distinct and dynamic pools of myosin II regulate protrusion dynamics within and between collectively migrating cells and suggest a new model for the role of protrusions in collective direction sensing in vivo.