Viscoelastic voyages - Biophysical perspectives on cell intercalation during Drosophila gastrulation.

Developmental processes are driven by a combination of cytoplasmic, cortical, and surface-associated forces. However, teasing apart the contributions of these forces and how a viscoelastic cell responds has long been a key question in developmental biology. Recent advances in applying biophysical approaches to these questions is leading to a fundamentally new understanding of morphogenesis. In this review, we discuss how computational analysis of experimental findings and in silico modeling of Drosophila gastrulation processes has led to a deeper comprehension of the physical principles at work in the early embryo. We also summarize many of the emerging methodologies that permit biophysical analysis as well as those that provide direct and indirect measurements of force directions and magnitudes. Finally, we examine the multiple frameworks that have been used to model tissue and cellular behaviors.

Phosphatidylinositol 3,4-bisphosphate synthesis and turnover are spatially segregated in the endocytic pathway.

Phosphoinositides play crucial roles in intracellular membrane dynamics and cell signaling, with phosphatidylinositol (PI) 3-phosphates being the predominant phosphoinositide lipids at endosomes and lysosomes, whereas PI 4-phosphates, such as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), are enriched at the cell surface including sites of endocytosis. How PI 4-phosphates and PI 3-phosphates are dynamically interconverted within the endocytic pathway and how this is controlled in space and time remains poorly understood. Here, combining live imaging, genome engineering, and acute chemical and genetic manipulations, we found that local synthesis of PI(3,4)P2 by phosphatidylinositol 3-kinase C2α at plasma membrane clathrin-coated pits is spatially segregated from its hydrolysis by the PI(3,4)P2-specific inositol polyphosphate 4-phosphatase 4A (INPP4A). We observed that INPP4A is dispensable for clathrin-mediated endocytosis and is undetectable in endocytic clathrin-coated pits. Instead, we found that INPP4A partially localizes to endosomes and that loss of INPP4A in HAP1 cancer cells perturbs signaling via AKT kinase and mTOR complex 1. These results reveal a function for INPP4-mediated PI(3,4)P2 hydrolysis in local regulation of growth factor and nutrient signals at endosomes in cancer cells. They further suggest a model whereby synthesis and turnover of PI(3,4)P2 are spatially segregated within the endocytic pathway to couple endocytic membrane traffic to growth factor and nutrient signaling.

Rab8 directs furrow ingression and membrane addition during epithelial formation in Drosophila melanogaster.

One of the most fundamental changes in cell morphology is the ingression of a plasma membrane furrow. The Drosophila embryo undergoes several cycles of rapid furrow ingression during early development that culminate in the formation of an epithelial sheet. Previous studies have demonstrated the requirement for intracellular trafficking pathways in furrow ingression; however, the pathways that link compartmental behaviors with cortical furrow ingression events are unclear. Here, we show that Rab8 has striking dynamic behaviors in vivo. As furrows ingress, cytoplasmic Rab8 puncta are depleted and Rab8 accumulates at the plasma membrane in a location that coincides with known regions of directed membrane addition. We additionally use CRISPR/Cas9 technology to N-terminally tag Rab8, which is then used to address endogenous localization and function. Endogenous Rab8 displays partial coincidence with Rab11 and the Golgi, and this colocalization is enriched during the fast phase of cellularization. When Rab8 function is disrupted, furrow formation in the early embryo is completely abolished. We also demonstrate that Rab8 behaviors require the function of the exocyst complex subunit Sec5 as well as the recycling endosome protein Rab11. Active, GTP-locked Rab8 is primarily associated with dynamic membrane compartments and the plasma membrane, whereas GDP-locked Rab8 forms large cytoplasmic aggregates. These studies suggest a model in which active Rab8 populations direct furrow ingression by guiding the targeted delivery of cytoplasmic membrane stores to the cell surface through interactions with the exocyst tethering complex.

Exocyst-Dependent Membrane Addition Is Required for Anaphase Cell Elongation and Cytokinesis in Drosophila.

Mitotic and cytokinetic processes harness cell machinery to drive chromosomal segregation and the physical separation of dividing cells. Here, we investigate the functional requirements for exocyst complex function during cell division in vivo, and demonstrate a common mechanism that directs anaphase cell elongation and cleavage furrow progression during cell division. We show that onion rings (onr) and funnel cakes (fun) encode the Drosophila homologs of the Exo84 and Sec8 exocyst subunits, respectively. In onr and fun mutant cells, contractile ring proteins are recruited to the equatorial region of dividing spermatocytes. However, cytokinesis is disrupted early in furrow ingression, leading to cytokinesis failure. We use high temporal and spatial resolution confocal imaging with automated computational analysis to quantitatively compare wild-type versus onr and fun mutant cells. These results demonstrate that anaphase cell elongation is grossly disrupted in cells that are compromised in exocyst complex function. Additionally, we observe that the increase in cell surface area in wild type peaks a few minutes into cytokinesis, and that onr and fun mutant cells have a greatly reduced rate of surface area growth specifically during cell division. Analysis by transmission electron microscopy reveals a massive build-up of cytoplasmic astral membrane and loss of normal Golgi architecture in onr and fun spermatocytes, suggesting that exocyst complex is required for proper vesicular trafficking through these compartments. Moreover, recruitment of the small GTPase Rab11 and the PITP Giotto to the cleavage site depends on wild-type function of the exocyst subunits Exo84 and Sec8. Finally, we show that the exocyst subunit Sec5 coimmunoprecipitates with Rab11. Our results are consistent with the exocyst complex mediating an essential, coordinated increase in cell surface area that potentiates anaphase cell elongation and cleavage furrow ingression.

Lattice light sheet microscopy reveals 4D force propagation dynamics and leading-edge behaviors in an embryonic epithelium in Drosophila.

How pulsed contractile dynamics drive the remodeling of cell and tissue topologies in epithelial sheets has been a key question in development and disease. Due to constraints in imaging and analysis technologies, studies that have described the in vivo mechanisms underlying changes in cell and neighbor relationships have largely been confined to analyses of planar apical regions. Thus, how the volumetric nature of epithelial cells affects force propagation and remodeling of the cell surface in three dimensions, including especially the apical-basal axis, is unclear. Here, we perform lattice light sheet microscopy (LLSM)-based analysis to determine how far and fast forces propagate across different apical-basal layers, as well as where topological changes initiate from in a columnar epithelium. These datasets are highly time- and depth-resolved and reveal that topology-changing forces are spatially entangled, with contractile force generation occurring across the observed apical-basal axis in a pulsed fashion, while the conservation of cell volumes constrains instantaneous cell deformations. Leading layer behaviors occur opportunistically in response to favorable phasic conditions, with lagging layers "zippering" to catch up as new contractile pulses propel further changes in cell topologies. These results argue against specific zones of topological initiation and demonstrate the importance of systematic 4D-based analysis in understanding how forces and deformations in cell dimensions propagate in a three-dimensional environment.

Measuring the hierarchy of molecular events during clathrin-mediated endocytosis.

A well-orchestrated hierarchy of molecular events is required for successful initiation and maturation of clathrin-coated pits (CCPs). Nevertheless, CCPs display a broad range of lifetimes. This dynamic heterogeneity could either reflect differences in the temporal hierarchy of molecular events, or similar CCP maturation processes with variable kinetics. To address this question, we have used multi-channel image acquisition and automated analysis of CCP dynamics in combination with a new method to quantify the time courses of recruitment of endocytic factors to CCPs of different lifetimes. Using this approach we have extracted the kinetics of recruitment and disassembly of fluorescently labeled clathrin and/or AP-2 throughout the entire lifetime of temporally defined CCP cohorts. On the basis of these analyses, we can (i) directly correlate recruitment profiles of these two proteins; (ii) define five distinct CCP maturation phases, i.e. initiation, growth, maturation, separation and departure; (iii) distinguish events with absolute versus fractional timing and (iv) provide information on the spatial distribution of fluorophores during CCP maturation. Emerging from these analyses is a more clearly defined role for AP-2 in determining the temporal hierarchy for clathrin recruitment and CCP maturation. This method provides a new means to identify other such hierarchies during CCP maturation.

Hotspots organize clathrin-mediated endocytosis by efficient recruitment and retention of nucleating resources.

The formation of clathrin-coated pits (CCPs) at the plasma membrane has been reported to sometimes occur repeatedly at predefined sites. However, defining such CCP 'hotspots' structurally and mechanistically has been difficult due to the dynamic and heterogeneous nature of CCPs. Here, we explore the molecular requirements for hotspots using a global assay of CCP dynamics. Our data confirmed that a subset of CCPs is nucleated at spatially distinct sites. The degree of clustering of nucleation events at these sites is dependent on the integrity of cortical actin, and the availability of certain resources, including the adaptor protein AP-2 and the phospholipid PI(4,5)P(2) . We observe that modulation in the expression level of FCHo1 and 2, which have been reported to initiate CCPs, affects only the number of nucleations. Modulation in the expression levels of other accessory proteins, such as SNX9, affects the spatial clustering of CCPs but not the number of nucleations. On the basis of these findings, we distinguish two classes of accessory proteins in clathrin-mediated endocytosis (CME): nucleation factors and nucleation organizers. Finally, we observe that clustering of transferrin receptors spatially randomizes pit nucleation and thus reduces the role of hotspots. On the basis of these data, we propose that hotspots are specialized cortical actin patches that organize CCP nucleations from within the cell by more efficient recruitment and/or retention of the resources required for CCP nucleation partially due to the action of nucleation organizers.

Replisome loading reduces chromatin motion independent of DNA synthesis.

Chromatin has been shown to undergo diffusional motion, which is affected during gene transcription by RNA polymerase activity. However, the relationship between chromatin mobility and other genomic processes remains unclear. Hence, we set out to label the DNA directly in a sequence unbiased manner and followed labeled chromatin dynamics in interphase human cells expressing GFP-tagged proliferating cell nuclear antigen (PCNA), a cell cycle marker and core component of the DNA replication machinery. We detected decreased chromatin mobility during the S-phase compared to G1 and G2 phases in tumor as well as normal diploid cells using automated particle tracking. To gain insight into the dynamical organization of the genome during DNA replication, we determined labeled chromatin domain sizes and analyzed their motion in replicating cells. By correlating chromatin mobility proximal to the active sites of DNA synthesis, we showed that chromatin motion was locally constrained at the sites of DNA replication. Furthermore, inhibiting DNA synthesis led to increased loading of DNA polymerases. This was accompanied by accumulation of the single-stranded DNA binding protein on the chromatin and activation of DNA helicases further restricting local chromatin motion. We, therefore, propose that it is the loading of replisomes but not their catalytic activity that reduces the dynamics of replicating chromatin segments in the S-phase as well as their accessibility and probability of interactions with other genomic regions.

Cargo and dynamin regulate clathrin-coated pit maturation.

Total internal reflection fluorescence microscopy (TIR-FM) has become a powerful tool for studying clathrin-mediated endocytosis. However, due to difficulties in tracking and quantifying their heterogeneous dynamic behavior, detailed analyses have been restricted to a limited number of selected clathrin-coated pits (CCPs). To identify intermediates in the formation of clathrin-coated vesicles and factors that regulate progression through these stages, we used particle-tracking software and statistical methods to establish an unbiased and complete inventory of all visible CCP trajectories. We identified three dynamically distinct CCP subpopulations: two short-lived subpopulations corresponding to aborted intermediates, and one longer-lived productive subpopulation. In a manner dependent on AP2 adaptor complexes, increasing cargo concentration significantly enhances the maturation efficiency of productive CCPs, but has only minor effects on their lifetimes. In contrast, small interfering RNA (siRNA) depletion of dynamin-2 GTPase and reintroduction of wild-type or mutant dynamin-1 revealed dynamin's role in controlling the turnover of abortive intermediates and the rate of CCP maturation. From these data, we infer the existence of an endocytic restriction or checkpoint, responsive to cargo and regulated by dynamin.

Interface extension is a continuum property suggesting a linkage between AP contractile and DV lengthening processes.

In the early Drosophila embryo, the elongation of the anterior-posterior (AP) body axis is driven by cell intercalation in the germband epithelium. Neighboring cells intercalate through the contraction of AP interfaces (between AP neighbors) into higher-order vertices, which then resolve through the extension of new dorsal-ventral (DV) interfaces (between DV neighbors). Although interface contraction has been extensively studied, less is known about how new interfaces are established. Here we show that DV interface elongation behaviors initiate at the same time as AP contractions, and that DV interfaces which are newly created from resolution of higher-order vertices do not appear to possess a unique 'identity;' instead, all horizontal interfaces undergo lengthening, elongating through ratchetlike sliding behaviors analogous to those found in AP interfaces. Cortical F-actin networks are essential for high area oscillation amplitudes required for effective ratcheting. Our results suggest that, contrary to canonical models, the elongation of new DV interfaces is not produced by a mechanistically separate process. Instead, medial myosin populations drive oscillating radial forces in the cells to generate transient force asymmetries at all tricellular vertices, which-combined with planar polarized stabilization-produce directional ratcheted sliding to generate both AP interface contraction and DV interface elongation.

Robust single-particle tracking in live-cell time-lapse sequences.

Single-particle tracking (SPT) is often the rate-limiting step in live-cell imaging studies of subcellular dynamics. Here we present a tracking algorithm that addresses the principal challenges of SPT, namely high particle density, particle motion heterogeneity, temporary particle disappearance, and particle merging and splitting. The algorithm first links particles between consecutive frames and then links the resulting track segments into complete trajectories. Both steps are formulated as global combinatorial optimization problems whose solution identifies the overall most likely set of particle trajectories throughout a movie. Using this approach, we show that the GTPase dynamin differentially affects the kinetics of long- and short-lived endocytic structures and that the motion of CD36 receptors along cytoskeleton-mediated linear tracks increases their aggregation probability. Both applications indicate the requirement for robust and complete tracking of dense particle fields to dissect the mechanisms of receptor organization at the level of the plasma membrane.

A PtdIns(3,4,5)P3 dispersal switch engages cell ratcheting at specific cell surfaces.

Force generation in epithelial tissues is often pulsatile, with actomyosin networks generating contractile forces before cyclically disassembling. This pulsed nature of cytoskeletal forces implies that there must be ratcheting mechanisms that drive processive transformations in cell shape. Previous work has shown that force generation is coordinated with endocytic remodeling; however, how ratcheting becomes engaged at specific cell surfaces remains unclear. Here, we report that PtdIns(3,4,5)P3 is a critical lipid-based cue for ratcheting engagement. The Sbf RabGEF binds to PIP3, and disruption of PIP3 reveals a dramatic switching behavior in which medial ratcheting is activated and epithelial cells begin globally constricting apical surfaces. PIP3 enrichments are developmentally regulated, with mesodermal cells having high apical PIP3 while germband cells have higher interfacial PIP3. Finally, we show that JAK/STAT signaling constitutes a second pathway that combinatorially regulates Sbf/Rab35 recruitment. Our results elucidate a complex lipid-dependent regulatory machinery that directs ratcheting engagement in epithelial tissues.

Vascular Aging in the Invertebrate Chordate, Botryllus schlosseri.

Vascular diseases affect over 1 billion people worldwide and are highly prevalent among the elderly, due to a progressive deterioration of the structure of vascular cells. Most of our understanding of these age-related cellular changes comes from in vitro studies on human cell lines. Further studies of the mechanisms underlying vascular aging in vivo are needed to provide insight into the pathobiology of age-associated vascular diseases, but are difficult to carry out on vertebrate model organisms. We are studying the effects of aging on the vasculature of the invertebrate chordate, Botryllus schlosseri. This extracorporeal vascular network of Botryllus is transparent and particularly amenable to imaging and manipulation. Here we use a combination of transcriptomics, immunostaining and live-imaging, as well as in vivo pharmacological treatments and regeneration assays to show that morphological, transcriptional, and functional age-associated changes within vascular cells are key hallmarks of aging in B. schlosseri, and occur independent of genotype. We show that age-associated changes in the cytoskeleton and the extracellular matrix reshape vascular cells into a flattened and elongated form and there are major changes in the structure of the basement membrane over time. The vessels narrow, reducing blood flow, and become less responsive to stimuli inducing vascular regression. The extracorporeal vasculature is highly regenerative following injury, and while age does not affect the regeneration potential, newly regenerated vascular cells maintain the same aged phenotype, suggesting that aging of the vasculature is a result of heritable epigenetic changes.

Characterizing the cellular architecture of dynamically remodeling vascular tissue using 3-D image analysis and virtual reconstruction.

Epithelial tubules form critical structures in lung, kidney, and vascular tissues. However, the processes that control their morphogenesis and physiological expansion and contraction are not well understood. Here we examine the dynamic remodeling of epithelial tubes in vivo using a novel model system: the extracorporeal vasculature of Botryllus schlosseri, in which the disruption of the basement membrane triggers rapid, massive vascular retraction without loss of barrier function. We developed and implemented 3-D image analysis and virtual reconstruction tools to characterize the cellular morphology of the vascular wall in unmanipulated vessels and during retraction. In both control and regressed conditions, cells within the vascular wall were planar polarized, with an integrin- and curvature-dependent axial elongation of cells and a robust circumferential alignment of actin bundles. Surprisingly, we found no measurable differences in morphology between normal and retracting vessels under extracellular matrix (ECM) disruption. However, inhibition of integrin signaling through focal adhesion kinase inhibition caused disruption of cellular actin organization. Our results demonstrate that epithelial tubes can maintain tissue organization even during extreme remodeling events, but that the robust response to mechanical signals-such as the response to loss of vascular tension after ECM disruption-requires functional force sensing machinery via integrin signaling.

Global and local regulation of clathrin-coated pit dynamics detected on patterned substrates.

Live-cell imaging of individual clathrin-coated pit (CCP) dynamics has revealed a broad variation in their internalization kinetics, but the functional significance and mechanistic underpinnings of this heterogeneity remain unknown. One contributing factor may be the spatial variations in the underlying actin cortex. To test this, we cultured cells on fibronectin (Fn) micropatterned substrates to vary the cortical actin mechanics in a defined manner. Under these conditions, stress fibers became organized to bridge adhesive islands, creating spatial heterogeneity in the cortical actin architecture. CCP lifetimes within the Fn-coated islands were selectively prolonged. This differential effect was not due to adherence to Fn-coated surfaces, and was not observed in cells grown on patterned surfaces that did not induce organized stress fiber assembly. Pharmacological agents that lower cortical tension selectively lowered CCP lifetimes within Fn islands, thus abolishing the spatial heterogeneity in the CCP dynamics. Although we cannot rule out the possibility that other factors might locally affect CCP dynamics at Fn islands, our data suggest that localized modulation in cortical tension may spatially regulate clathrin-mediated endocytosis.

Cell ratcheting through the Sbf RabGEF directs force balancing and stepped apical constriction.

During Drosophila melanogaster gastrulation, the invagination of the prospective mesoderm is driven by the pulsed constriction of apical surfaces. Here, we address the mechanisms by which the irreversibility of pulsed events is achieved while also permitting uniform epithelial behaviors to emerge. We use MSD-based analyses to identify contractile steps and find that when a trafficking pathway initiated by Sbf is disrupted, contractile steps become reversible. Sbf localizes to tubular, apical surfaces and associates with Rab35, where it promotes Rab GTP exchange. Interestingly, when Sbf/Rab35 function is compromised, the apical plasma membrane becomes deeply convoluted, and nonuniform cell behaviors begin to emerge. Consistent with this, Sbf/Rab35 appears to prefigure and organize the apical surface for efficient Myosin function. Finally, we show that Sbf/Rab35/CME directs the plasma membrane to Rab11 endosomes through a dynamic interaction with Rab5 endosomes. These results suggest that periodic ratcheting events shift excess membrane from cell apices into endosomal pathways to permit reshaping of actomyosin networks and the apical surface.

Nanoscale coupling of endocytic pit growth and stability.

Clathrin-mediated endocytosis, an essential process for plasma membrane homeostasis and cell signaling, is characterized by stunning heterogeneity in the size and lifetime of clathrin-coated endocytic pits (CCPs). If and how CCP growth and lifetime are coupled and how this relates to their physiological function are unknown. We combine computational modeling, automated tracking of CCP dynamics, electron microscopy, and functional rescue experiments to demonstrate that CCP growth and lifetime are closely correlated and mechanistically linked by the early-acting endocytic F-BAR protein FCHo2. FCHo2 assembles at the rim of CCPs to control CCP growth and lifetime by coupling the invagination of early endocytic intermediates to clathrin lattice assembly. Our data suggest a mechanism for the nanoscale control of CCP growth and stability that may similarly apply to other metastable structures in cells.

Vertex sliding drives intercalation by radial coupling of adhesion and actomyosin networks during Drosophila germband extension.

Oriented cell intercalation is an essential developmental process that shapes tissue morphologies through the directional insertion of cells between their neighbors. Previous research has focused on properties of cell-cell interfaces, while the function of tricellular vertices has remained unaddressed. Here, we identify a highly novel mechanism in which vertices demonstrate independent sliding behaviors along cell peripheries to produce the topological deformations responsible for intercalation. Through systematic analysis, we find that the motion of vertices connected by contracting interfaces is not physically coupled, but instead possess strong radial coupling. E-cadherin and Myosin II exist in previously unstudied populations at cell vertices and undergo oscillatory cycles of accumulation and dispersion that are coordinated with changes in cell area. Additionally, peak enrichment of vertex E-cadherin/Myosin II coincides with interface length stabilization. Our results suggest a model in which asymmetric radial force balance directs the progressive, ratcheted motion of individual vertices to drive intercalation.

Planar polarized Rab35 functions as an oscillatory ratchet during cell intercalation in the Drosophila epithelium.

The coordination between membrane trafficking and actomyosin networks is essential to the regulation of cell and tissue shape. Here, we examine Rab protein distributions during Drosophila epithelial tissue remodeling and show that Rab35 is dynamically planar polarized. Rab35 compartments are enriched at contractile interfaces of intercalating cells and provide the first evidence of interfacial monopolarity. When Rab35 function is disrupted, apical area oscillations still occur and contractile steps are observed. However, contractions are followed by reversals and interfaces fail to shorten, demonstrating that Rab35 functions as a ratchet ensuring unidirectional movement. Although actomyosin forces have been thought to drive interface contraction, initiation of Rab35 compartments does not require Myosin II function. However, Rab35 compartments do not terminate and continue to grow into large elongated structures following actomyosin disruption. Finally, Rab35 represents a common contractile cell-shaping mechanism, as mesoderm invagination fails in Rab35 compromised embryos and Rab35 localizes to constricting surfaces.Various stages of tissue morphogenesis involve the contraction of epithelial surfaces. Here, the authors identify the Rab GTPase Rab35 as an essential component of this contractile process, which functions as a membrane ratchet to ensure unidirectional movement of intercalating cells.

Regulation of clathrin-mediated endocytosis by hierarchical allosteric activation of AP2.

The critical initiation phase of clathrin-mediated endocytosis (CME) determines where and when endocytosis occurs. Heterotetrameric adaptor protein 2 (AP2) complexes, which initiate clathrin-coated pit (CCP) assembly, are activated by conformational changes in response to phosphatidylinositol-4,5-bisphosphate (PIP2) and cargo binding at multiple sites. However, the functional hierarchy of interactions and how these conformational changes relate to distinct steps in CCP formation in living cells remains unknown. We used quantitative live-cell analyses to measure discrete early stages of CME and show how sequential, allosterically regulated conformational changes activate AP2 to drive both nucleation and subsequent stabilization of nascent CCPs. Our data establish that cargoes containing Yxxφ motif, but not dileucine motif, play a critical role in the earliest stages of AP2 activation and CCP nucleation. Interestingly, these cargo and PIP2 interactions are not conserved in yeast. Thus, we speculate that AP2 has evolved as a key regulatory node to coordinate CCP formation and cargo sorting and ensure high spatial and temporal regulation of CME.