Integrin-dependent actomyosin contraction regulates epithelial cell scattering

The effects of BMP2 and the mechanisms involved in the invasion and angiogenesis of IDH1 mutant glioma cells

PURPOSE

This study investigated the effect of an isocitrate dehydrogenase 1 (IDH1) mutation (mutIDH1) on the invasion and angiogenesis of human glioma cells.

METHODS

Doxycycline was used to induce the expression of mutIDH1 in glioma cells. Transwell and wound healing assays were conducted to assess glioma cell migration and invasion. Western blotting and cell immunofluorescence were used to measure the expression levels of various proteins. The influence of bone morphogenetic protein 2 (BMP2) on invasion, angiogenesis-related factors, BMP2-related receptor expression, and changes in Smad signaling pathway-related proteins were evaluated after treatment with BMP2. Differential gene expression and reference transcription analysis were performed.

RESULTS

Successful infection with recombinant lentivirus expressing mutIDH1 was demonstrated. The IDH1 mutation promoted glioma cell migration and invasion while positively regulating the expression of vascularization-related factors and BMP2-related receptors. BMP2 exhibited a positive regulatory effect on the migration, invasion, and angiogenesis of mutIDH1-glioma cells, possibly mediated by BMP2-induced alterations in Smad signaling pathway-related factors.After BMP2 treatment, the differential genes of MutIDH1-glioma cells are closely related to the regulation of cell migration and cell adhesion, especially the regulation of Smad-related proteins. KEGG analysis confirmed that it was related to BMP signaling pathway and TGF-β signaling pathway and cell adhesion. Enrichment analysis of gene ontology and genome encyclopedia further confirmed the correlation of these pathways.

CONCLUSION

Mutation of isocitrate dehydrogenase 1 promotes the migration, invasion, and angiogenesis of glioma cells, through its effects on the BMP2-driven Smad signaling pathway. In addition, BMP2 altered the transcriptional patterns of mutIDH1 glioma cells, enriching different gene loci in pathways associated with invasion, migration, and angiogenesis.

The highly metastatic 4T1 breast carcinoma model possesses features of a hybrid epithelial/mesenchymal phenotype

Epithelial-mesenchymal transitions (EMTs) are thought to promote metastasis via downregulation of E-cadherin (also known as Cdh1) and upregulation of mesenchymal markers such as N-cadherin (Cdh2) and vimentin (Vim). Contrary to this, E-cadherin is retained in many invasive carcinomas and promotes collective cell invasion. To investigate how E-cadherin regulates metastasis, we examined the highly metastatic, E-cadherin-positive murine 4T1 breast cancer model, together with the less metastatic, 4T1-related cell lines 4T07, 168FARN and 67NR. We found that 4T1 cells display a hybrid epithelial/mesenchymal phenotype with co-expression of epithelial and mesenchymal markers, whereas 4T07, 168FARN, and 67NR cells display progressively more mesenchymal phenotypes in vitro that relate inversely to their metastatic capacity in vivo. Using RNA interference and constitutive expression, we demonstrate that the expression level of E-cadherin does not determine 4T1 or 4T07 cell metastatic capacity in mice. Mechanistically, 4T1 cells possess highly dynamic, unstable cell-cell junctions and can undergo collective invasion without E-cadherin downregulation. However, 4T1 orthotopic tumors in vivo also contain subregions of EMT-like loss of E-cadherin. Thus, 4T1 cells function as a model for carcinomas with a hybrid epithelial/mesenchymal phenotype that promotes invasion and metastasis.

Photoactivatable substrates show diverse phenotypes of leader cells in collective migration when moving along different extracellular matrix proteins

In cancer metastasis, collectively migrating clusters are discriminated into leader and follower cells that move through extracellular matrices (ECMs) with different characteristics. The impact of changes in ECM protein types on leader cells and migrating clusters is unknown. To address this, we investigated the response of leader cells and migrating clusters upon moving from one ECM protein to another using a photoactivatable substrate bearing photocleavable PEG (PCP), whose surface changes from protein-repellent to protein-adhesive in response to light. We chose laminin and collagen I for our study since they are abundant in two distinct regions in living tissues, namely basement membrane and connective tissue. Using the photoactivatable substrates, the precise deposition of the first ECM protein in the irradiated areas was achieved, followed by creating well-defined cellular confinements. Secondary irradiation enabled the deposition of the second ECM protein in the new irradiated regions, resulting in region-selective heterogeneous and homogenous ECM protein-coated surfaces. Different tendencies in leader cell formation from laminin into laminin compared to those migrating from laminin into collagen were observed. The formation of focal adhesion and actin structures for cells within the same cluster in the ECM proteins responded according to the underlying ECM protein type. Finally, integrin β1 was crucial for the appearance of leader cells for clusters migrating from laminin into collagen. However, when it came to laminin into laminin, integrin β1 was not responsible. This highlights the correlation between leader cells in collective migration and the biochemical signals that arise from underlying extracellular matrix proteins.

Enhancing PKA-dependent mesothelial barrier integrity reduces ovarian cancer transmesothelial migration via inhibition of contractility

Cancer-mesothelial cell interactions are critical for multiple solid tumors to colonize the surface of peritoneal organs. Understanding mechanisms of mesothelial barrier dysfunction that impair its protective function is critical for discovering mesothelial-targeted therapies to combat metastatic spread. Here, we utilized a live cell imaging-based assay to elucidate the dynamics of ovarian cancer spheroid transmesothelial migration and mesothelial-generated mechanical forces. Treatment of mesothelial cells with the adenylyl cyclase agonist forskolin strengthens cell-cell junctions, reduces actomyosin fibers, contractility-driven matrix displacements, and cancer spheroid transmigration in a protein kinase A (PKA)-dependent mechanism. We also show that inhibition of the cytoskeletal regulator Rho-associated kinase in mesothelial cells phenocopies the anti-metastatic effects of forskolin. Conversely, upregulation of contractility in mesothelial cells disrupts cell-cell junctions and increases the clearance rates of ovarian cancer spheroids. Our findings demonstrate the critical role of mesothelial cell contractility and mesothelial barrier integrity in regulating metastatic dissemination within the peritoneal microenvironment.

Myosin light chain phosphatase is a downstream target of Rho-kinase in endothelin-1-induced transactivation of the TGF-β receptor

BACKGROUND

Rho-kinase (ROCK) regulates actomyosin contraction, coronary vasospasm, and cytoskeleton dynamics. ROCK and of NADPH oxidase (NOX) play an essential role in cardiovascular disease and proteoglycan synthesis, which promotes atherosclerosis by trapping low density lipoprotein. ROCK is activated by endothelin-1 (ET1) and transactivates the transforming growth factor beta receptor (TGFβR1), intensifying Smad signaling and proteoglycan production. This study aimed to identify the role of myosin light chain phosphatase (MLCP) as a downstream target of ROCK in TβR1 transactivation.

METHODS

Vascular smooth muscle cells were treated with ET1 and inhibitors of ROCK and MLCP were added. The phosphorylation levels of Smad2C, myosin light chain (MLC), and MLCP were monitored by western blot, and the mRNA expression of chondroitin 4-O-sulfotransferase 1 (C4ST1) was assessed by quantitative real-time PCR.

RESULTS

We examined ROCK's role in ET1-induced TGFβR1 activation. ROCK phosphorylated MLCP at the MYPT1 T853 residue, blocked by the ROCK inhibitor Y27632. ROCK also increased MLC phosphorylation and actomyosin contraction in response to ET1, enhanced by the phosphatase inhibitor Calyculin A. Calyculin A also increased C4ST1 expression, GAG-chain synthesizing enzymes.

CONCLUSIONS

This work suggests that ROCK is involved in ET1-mediated TβR1 activation through increased MLCP phosphorylation, which leads to Smad2C phosphorylation and stimulates C4ST1 expression.

Actin Cytoskeleton Remodeling Accompanied by Redistribution of Adhesion Proteins Drives Migration of Cells in Different EMT States

Epithelial-mesenchymal transition (EMT) is a process during which epithelial cells lose epithelial characteristics and gain mesenchymal features. Here, we used several cell models to study migratory activity and redistribution of cell-cell adhesion proteins in cells in different EMT states: EGF-induced EMT of epithelial IAR-20 cells; IAR-6-1 cells with a hybrid epithelial-mesenchymal phenotype; and their more mesenchymal derivatives, IAR-6-1-DNE cells lacking adherens junctions. In migrating cells, the cell-cell adhesion protein α-catenin accumulated at the leading edges along with ArpC2/p34 and α-actinin. Suppression of α-catenin shifted cell morphology from fibroblast-like to discoid and attenuated cell migration. Expression of exogenous α-catenin in MDA-MB-468 cells devoid of α-catenin drastically increased their migratory capabilities. The Y654 phosphorylated form of β-catenin was detected at integrin adhesion complexes (IACs). Co-immunoprecipitation studies indicated that α-catenin and pY654-β-catenin were associated with IAC proteins: vinculin, zyxin, and α-actinin. Taken together, these data suggest that in cells undergoing EMT, catenins not participating in assembly of adherens junctions may affect cell migration.

Bioengineered Hydrogels Recapitulate Fibroblast Heterogeneity in Cancer

Recently mapped transcriptomic landscapes reveal the extent of heterogeneity in cancer-associated fibroblasts (CAFs) beyond previously established single-gene markers. Functional analyses of individual CAF subsets within the tumor microenvironment are critical to develop more accurate CAF-targeting therapeutic strategies. However, there is a lack of robust preclinical models that reflect this heterogeneity in vitro. In this study, single-cell RNA sequencing datasets acquired from head and neck squamous cell carcinoma tissues to predict microenvironmental and cellular features governing individual CAF subsets are leveraged. Some of these features are then incorporated into a tunable hyaluronan-based hydrogel system to culture patient-derived CAFs. Control over hydrogel degradability and integrin adhesiveness enabled derivation of the predominant myofibroblastic and inflammatory CAF subsets, as shown through changes in cell morphology and transcriptomic profiles. Last, using these hydrogel-cultured CAFs, microtubule dynamics are identified, but not actomyosin contractility, as a key mediator of CAF plasticity. The recapitulation of CAF heterogeneity in vitro using defined hydrogels presents unique opportunities for advancing the understanding of CAF biology and evaluation of CAF-targeting therapeutics.

N-cadherin and β1 integrin coordinately regulate growth plate cartilage architecture

Spatial and temporal regulation of chondrocyte maturation in the growth plate drives growth of many bones. One essential event to generate the ordered cell array characterizing growth plate cartilage is the formation of chondrocyte columns in the proliferative zone via 90-degree rotation of daughter cells to align with the long axis of the bone. Previous studies have suggested crucial roles for cadherins and integrin β1 in column formation. The purpose of this study was to determine the relative contributions of cadherin- and integrin-mediated cell adhesion in column formation. Here we present new mechanistic insights generated by application of live time-lapse confocal microscopy of cranial base explant cultures, robust genetic mouse models, and new quantitative methods to analyze cell behavior. We show that conditional deletion of either the cell-cell adhesion molecule Cdh2 or the cell-matrix adhesion molecule Itgb1 disrupts column formation. Compound mutants were used to determine a potential reciprocal regulatory interaction between the two adhesion surfaces and identified that defective chondrocyte rotation in a N-cadherin mutant was restored by a heterozygous loss of integrin β1. Our results support a model for which integrin β1, and not N-cadherin, drives chondrocyte rotation and for which N-cadherin is a potential negative regulator of integrin β1 function.

Force-dependent focal adhesion assembly and disassembly: A computational study

Cells interact with the extracellular matrix (ECM) via cell-ECM adhesions. These physical interactions are transduced into biochemical signals inside the cell which influence cell behaviour. Although cell-ECM interactions have been studied extensively, it is not completely understood how immature (nascent) adhesions develop into mature (focal) adhesions and how mechanical forces influence this process. Given the small size, dynamic nature and short lifetimes of nascent adhesions, studying them using conventional microscopic and experimental techniques is challenging. Computational modelling provides a valuable resource for simulating and exploring various "what if?" scenarios in silico and identifying key molecular components and mechanisms for further investigation. Here, we present a simplified mechano-chemical model based on ordinary differential equations with three major proteins involved in adhesions: integrins, talin and vinculin. Additionally, we incorporate a hypothetical signal molecule that influences adhesion (dis)assembly rates. We find that assembly and disassembly rates need to vary dynamically to limit maturation of nascent adhesions. The model predicts biphasic variation of actin retrograde velocity and maturation fraction with substrate stiffness, with maturation fractions between 18-35%, optimal stiffness of ∼1 pN/nm, and a mechanosensitive range of 1-100 pN/nm, all corresponding to key experimental findings. Sensitivity analyses show robustness of outcomes to small changes in parameter values, allowing model tuning to reflect specific cell types and signaling cascades. The model proposes that signal-dependent disassembly rate variations play an underappreciated role in maturation fraction regulation, which should be investigated further. We also provide predictions on the changes in traction force generation under increased/decreased vinculin concentrations, complementing previous vinculin overexpression/knockout experiments in different cell types. In summary, this work proposes a model framework to robustly simulate the mechanochemical processes underlying adhesion maturation and maintenance, thereby enhancing our fundamental knowledge of cell-ECM interactions.

MAP4K4 regulates forces at cell-cell and cell-matrix adhesions to promote collective cell migration

Collective cell migration is not only important for development and tissue homeostasis but can also promote cancer metastasis. To migrate collectively, cells need to coordinate cellular extensions and retractions, adhesion sites dynamics, and forces generation and transmission. Nevertheless, the regulatory mechanisms coordinating these processes remain elusive. Using A431 carcinoma cells, we identify the kinase MAP4K4 as a central regulator of collective migration. We show that MAP4K4 inactivation blocks the migration of clusters, whereas its overexpression decreases cluster cohesion. MAP4K4 regulates protrusion and retraction dynamics, remodels the actomyosin cytoskeleton, and controls the stability of both cell-cell and cell-substrate adhesion. MAP4K4 promotes focal adhesion disassembly through the phosphorylation of the actin and plasma membrane crosslinker moesin but disassembles adherens junctions through a moesin-independent mechanism. By analyzing traction and intercellular forces, we found that MAP4K4 loss of function leads to a tensional disequilibrium throughout the cell cluster, increasing the traction forces and the tension loading at the cell-cell adhesions. Together, our results indicate that MAP4K4 activity is a key regulator of biomechanical forces at adhesion sites, promoting collective migration.

Cell Migration in Three Dimensions

Cell migration plays an essential role in many pathophysiological processes, including embryonic development, wound healing, immunity, and cancer invasion, and is therefore a widely studied phenomenon in many different fields from basic cell biology to regenerative medicine. During the past decades, a multitude of increasingly complex methods have been developed to study cell migration. Here we compile a series of current state-of-the-art methods and protocols to investigate cell migration in a variety of model systems ranging from cells, organoids, tissue explants, and microfluidic systems to Drosophila, zebrafish, and mice. Together they cover processes as diverse as nuclear deformation, energy consumption, endocytic trafficking, and matrix degradation, as well as tumor vascularization and cancer cell invasion, sprouting angiogenesis, and leukocyte extravasation. Furthermore, methods to study developmental processes such as neural tube closure, germ layer specification, and branching morphogenesis are included, as well as scripts for the automated analysis of several aspects of cell migration. Together, this book constitutes a unique collection of methods of prime importance to those interested in the analysis of cell migration.

Actomyosin-mediated cellular tension promotes Yap nuclear translocation and myocardial proliferation through α5 integrin signaling

The cardiomyocyte phenotypic switch from a proliferative to terminally differentiated state results in the loss of regenerative potential of the mammalian heart shortly after birth. Nonmuscle myosin IIB (NM IIB)-mediated actomyosin contractility regulates cardiomyocyte cytokinesis in the embryonic heart, and NM IIB levels decline after birth, suggesting a role for cellular tension in the regulation of cardiomyocyte cell cycle activity in the postnatal heart. To investigate the role of actomyosin contractility in cardiomyocyte cell cycle arrest, we conditionally activated ROCK2 kinase domain (ROCK2:ER) in the murine postnatal heart. Here, we show that α5/β1 integrin and fibronectin matrix increase in response to actomyosin-mediated tension. Moreover, activation of ROCK2:ER promotes nuclear translocation of Yap, a mechanosensitive transcriptional co-activator, and enhances cardiomyocyte proliferation. Finally, we show that reduction of myocardial α5 integrin rescues the myocardial proliferation phenotype in ROCK2:ER hearts. These data demonstrate that cardiomyocytes respond to increased intracellular tension by altering their intercellular contacts in favor of cell-matrix interactions, leading to Yap nuclear translocation, thus uncovering a function for nonmuscle myosin contractility in promoting cardiomyocyte proliferation in the postnatal heart.

Mechanical stress driven by rigidity sensing governs epithelial stability

Epithelia act as a barrier against environmental stress and abrasion and in vivo they are continuously exposed to environments of various mechanical properties. The impact of this environment on epithelial integrity remains elusive. By culturing epithelial cells on 2D hydrogels, we observe a loss of epithelial monolayer integrity through spontaneous hole formation when grown on soft substrates. Substrate stiffness triggers an unanticipated mechanical switch of epithelial monolayers from tensile on soft to compressive on stiff substrates. Through active nematic modelling, we find that spontaneous half-integer defect formation underpinning large isotropic stress fluctuations initiate hole opening events. Our data show that monolayer rupture due to high tensile stress is promoted by the weakening of cell-cell junctions that could be induced by cell division events or local cellular stretching. Our results show that substrate stiffness provides feedback on monolayer mechanical state and that topological defects can trigger stochastic mechanical failure, with potential application towards a mechanistic understanding of compromised epithelial integrity during immune response and morphogenesis.

Photoactivatable surfaces resolve the impact of gravity vector on collective cell migratory characteristics

Despite considerable interest in the impact of space travel on human health, the influence of the gravity vector on collective cell migration remains unclear. This is primarily because of the difficulty in inducing collective migration, where cell clusters appear in an inverted position against gravity, without cellular damage. In this study, photoactivatable surfaces were used to overcome this challenge. Photoactivatable surfaces enable the formation of geometry-controlled cellular clusters and the remote induction of cellular migration via photoirradiation, thereby maintaining the cells in the inverted position. Substrate inversion preserved the circularity of cellular clusters compared to cells in the normal upright position, with less leader cell appearance. Furthermore, the inversion of cells against the gravity vector resulted in the remodeling of the cytoskeletal system via the strengthening of external actin bundles. Within the 3D cluster architecture, enhanced accumulation of active myosin was observed in the upper cell-cell junction, with a flattened apical surface. Depending on the gravity vector, attenuating actomyosin activity correlates with an increase in the number of leader cells, indicating the importance of cell contractility in collective migration phenotypes and cytoskeletal remodeling.

Mechanotransduction in the pathogenesis of non-alcoholic fatty liver disease

Mechanobiology is a domain of interdisciplinary research that aims to explore the impact of physical force, applied externally or internally, on cell and tissue function, including development, growth, and differentiation. Mechanotransduction is a term that describes how cells sense physical forces (such as compression, stretch, and shear stress), convert them into biochemical signals, and mount adaptive responses integrated by the nucleus. There is accumulating evidence that mechanical forces extensively inform the biological behaviour of liver cells in health and disease. Recent research has elucidated many cellular and molecular mechanisms involved in this process including the pleiotropic control and diverse effects of the paralogous transcription co-activators YAP/TAZ, which play a prominent role in mechanotransduction. The liver sinusoids represent a unique microenvironment in which cells are exposed to mechanical cues originating in the cytoskeleton and at interfaces with adjacent cells, the extracellular matrix, and vascular or interstitial fluids. In non-alcoholic fatty liver disease (NAFLD), hepatocellular lipid accumulation and ballooning, activation of inflammatory responses, dysfunction of liver sinusoidal endothelial cells, and transdifferentiation of hepatic stellate cells into a pro-contractile and pro-fibrotic phenotype have been associated with aberrant cycles of mechanosensing and mechanoresponses. The downstream consequences of disrupted mechanical homeostasis likely contribute to the progression of NAFLD and promote the development of portal hypertension, cirrhosis, and hepatocellular carcinoma. Identification of molecular targets involved in pathogenic mechanotransduction will allow for the development of novel strategies to prevent the progression of liver disease in NAFLD.

The influence of entropic crowding in cell monolayers

Cell-cell interaction dictates cell morphology and organization, which play a crucial role in the micro-architecture of tissues that guides their biological and mechanical functioning. Here, we investigate the effect of cell density on the responses of cells seeded on flat substrates using a novel statistical thermodynamics framework. The framework recognizes the existence of nonthermal fluctuations in cellular response and thereby naturally captures entropic interactions between cells in monolayers. In line with observations, the model predicts that cell area and elongation decrease with increasing cell seeding density-both are a direct outcome of the fluctuating nature of the cellular response that gives rise to enhanced cell-cell interactions with increasing cell crowding. The modeling framework also predicts the increase in cell alignment with increasing cell density: this cellular ordering is also due to enhanced entropic interactions and is akin to nematic ordering in liquid crystals. Our simulations provide physical insights that suggest that entropic cell-cell interactions play a crucial role in governing the responses of cell monolayers.

Adherens junctions stimulate and spatially guide integrin activation and extracellular matrix deposition

Cadherins and integrins are intrinsically linked through the actin cytoskeleton and are largely responsible for the mechanical integrity and organization of tissues. We show that cadherin clustering stimulates and spatially guides integrin activation. Adherens junction (AJ)-associated integrin activation depends on locally generated tension and does not require extracellular matrix ligands. It leads to the creation of primed integrin clusters, which spatially determine where focal adhesions will form if ligands are present and where ligands will be deposited. AJs that display integrin activation are targeted by microtubules facilitating their disassembly via caveolin-based endocytosis, showing that integrin activation impacts the stability of the core cadherin complex. Thus, the interplay between cadherins and integrins is more intimate than what was once believed and is rooted in the capacity of active integrins to be stabilized via AJ-generated tension. Altogether, our data establish a mechanism of cross-regulation between cadherins and integrins.

Force Estimation during Cell Migration Using Mathematical Modelling

Cell migration is essential for physiological, pathological and biomedical processes such as, in embryogenesis, wound healing, immune response, cancer metastasis, tumour invasion and inflammation. In light of this, quantifying mechanical properties during the process of cell migration is of great interest in experimental sciences, yet few theoretical approaches in this direction have been studied. In this work, we propose a theoretical and computational approach based on the optimal control of geometric partial differential equations to estimate cell membrane forces associated with cell polarisation during migration. Specifically, cell membrane forces are inferred or estimated by fitting a mathematical model to a sequence of images, allowing us to capture dynamics of the cell migration. Our approach offers a robust and accurate framework to compute geometric mechanical membrane forces associated with cell polarisation during migration and also yields geometric information of independent interest, we illustrate one such example that involves quantifying cell proliferation levels which are associated with cell division, cell fusion or cell death.

Precise Tuning and Characterization of Viscoelastic Interfaces for the Study of Early Epithelial-Mesenchymal Transition Behaviors

There is growing evidence that cellular functions are regulated by the viscoelastic nature of surrounding matrices. This study aimed to investigate the impact of interfacial viscoelasticity on adhesion and epithelial-mesenchymal transition (EMT) behaviors of epithelial cells. The interfacial viscoelasticity was manipulated using spin-coated thin films composed of copolymers of ε-caprolactone and d,l-lactide photo-cross-linked with benzophenone, whose mechanical properties were characterized using atomic force microscopy and a rheometer. The critical range for the morphological transition of epithelial Madin-Darby canine kidney (MDCK) cells was of the order of 10² ms relaxation time, which was 1-2 orders of magnitude smaller than the relaxation times reported (10-10² s). An analysis of strain rate-dependent viscoelastic properties revealed that the difference was caused by the different strain rate/frequency used for the mechanical characterization of the interface and bulk. Furthermore, decoupling of the interfacial viscous and elastic terms demonstrated that E/N-cadherin expression levels were regulated differently by interfacial relaxation and elasticity. These results confirm the significance of precise manipulation and characterization of interfacial viscoelasticity in mechanobiology studies on EMT progression.

Deconstructing the role of myosin contractility in force fluctuations within focal adhesions

Force fluctuations exhibited in focal adhesions that connect a cell to its extracellular environment point to the complex role of the underlying machinery that controls cell migration. To elucidate the explicit role of myosin motors in the temporal traction force oscillations, we vary the contractility of these motors in a dynamical model based on the molecular clutch hypothesis. As the contractility is lowered, effected both by changing the motor velocity and the rate of attachment/detachment, we show analytically in an experimentally relevant parameter space, that the system goes from decaying oscillations to stable limit cycle oscillations through a supercritical Hopf bifurcation. As a function of the motor activity and the number of clutches, the system exhibits a rich array of dynamical states. We corroborate our analytical results with stochastic simulations of the motor-clutch system. We obtain limit cycle oscillations in the parameter regime as predicted by our model. The frequency range of oscillations in the average clutch and motor deformation compares well with experimental results.

In-Depth Molecular Characterization of Neovascular Membranes Suggests a Role for Hyalocyte-to-Myofibroblast Transdifferentiation in Proliferative Diabetic Retinopathy

Background

Retinal neovascularization (RNV) membranes can lead to a tractional retinal detachment, the primary reason for severe vision loss in end-stage disease proliferative diabetic retinopathy (PDR). The aim of this study was to characterize the molecular, cellular and immunological features of RNV in order to unravel potential novel drug treatments for PDR.

Methods

A total of 43 patients undergoing vitrectomy for PDR, macular pucker or macular hole (control patients) were included in this study. The surgically removed RNV and epiretinal membranes were analyzed by RNA sequencing, single-cell based Imaging Mass Cytometry and conventional immunohistochemistry. Immune cells of the vitreous body, also known as hyalocytes, were isolated from patients with PDR by flow cytometry, cultivated and characterized by immunohistochemistry. A bioinformatical drug repurposing approach was applied in order to identify novel potential drug options for end-stage diabetic retinopathy disease.

Results

The in-depth transcriptional and single-cell protein analysis of diabetic RNV tissue samples revealed an accumulation of endothelial cells, macrophages and myofibroblasts as well as an abundance of secreted ECM proteins such as SPARC, FN1 and several types of collagen in RNV tissue. The immunohistochemical staining of cultivated vitreal hyalocytes from patients with PDR showed that hyalocytes express α-SMA (alpha-smooth muscle actin), a classic myofibroblast marker. According to our drug repurposing analysis, imatinib emerged as a potential immunomodulatory drug option for future treatment of PDR.

Conclusion

This study delivers the first in-depth transcriptional and single-cell proteomic characterization of RNV tissue samples. Our data suggest an important role of hyalocyte-to-myofibroblast transdifferentiation in the pathogenesis of diabetic vitreoretinal disease and their modulation as a novel possible clinical approach.

Cytoplasmic pressure maintains epithelial integrity and inhibits cell motility

Cytoplasmic pressure, a function of actomyosin contractility and water flow, can regulate cellular morphology and dynamics. In mesenchymal cells, cytoplasmic pressure powers cell protrusion through physiological three-dimensional extracellular matrices. However, the role of intracellular pressure in epithelial cells is relatively unclear. Here we find that high cytoplasmic pressure is necessary to maintain barrier function, one of the hallmarks of epithelial homeostasis. Further, our data show that decreased cytoplasmic pressure facilitates lamellipodia formation during the epithelial to mesenchymal transition (EMT). Critically, activation of the actin nucleating protein Arp2/3 is required for the reduction in cytoplasmic pressure and lamellipodia formation in response to treatment with hepatocyte growth factor (HGF) to induce EMT. Thus, elevated cytoplasmic pressure functions to maintain epithelial tissue integrity, while reduced cytoplasmic pressure triggers lamellipodia formation and motility during HGF-dependent EMT.

Cell-in-cell phenomenon: leukocyte engulfment by non-tumorigenic cells and cancer cell lines

BACKGROUND

Research on cell-in-cell (CIC) phenomena, including entosis, emperipolesis and cannibalism, and their biological implications has increased in recent years. Homotypic and heterotypic engulfment of various target cells by numerous types of host cells has been studied in vitro and in tissue sections. This work has identified proteins involved in the mechanism and uncovered evidence for CIC as a potential histopathologic predictive and prognostic marker in cancer. Our experimental study focused on non-professional phagocytosis of leukocytes.

RESULTS

We studied the engulfment of peripheral blood mononuclear cells isolated from healthy donors by counting CIC structures. Two non-tumorigenic cell lines (BEAS-2B, SBLF-9) and two tumour cell lines (BxPC3, ICNI) served as host cells. Immune cells were live-stained and either directly co-incubated or treated with irradiation or with conventional or microwave hyperthermia. Prior to co-incubation, we determined leukocyte viability for each batch via Annexin V-FITC/propidium iodide staining. All host cells engulfed their targets, with uptake rates ranging from 1.0% ± 0.5% in BxPC3 to 8.1% ± 5.0% in BEAS-2B. Engulfment rates of the cancer cell lines BxPC3 and ICNI (1.6% ± 0.2%) were similar to those of the primary fibroblasts SBLF-9 (1.4% ± 0.2%). We found a significant negative correlation between leukocyte viability and cell-in-cell formation rates. The engulfment rate rose when we increased the dose of radiotherapy and prolonged the impact time. Further, microwave hyperthermia induced higher leukocyte uptake than conventional hyperthermia. Using fluorescent immunocytochemistry to descriptively study the proteins involved, we detected ring-like formations of diverse proteins around the leukocytes, consisting, among others, of α-tubulin, integrin, myosin, F-actin, and vinculin. These results suggest the involvement of actomyosin contraction, cell-cell adhesion, and the α-tubulin cytoskeleton in the engulfment process.

CONCLUSIONS

Both non-tumorigenic and cancer cells can form heterotypic CIC structures by engulfing leukocytes. Decreased viability and changes caused by microwave and X-ray irradiation trigger non-professional phagocytosis.

Topological data analysis of collective and individual epithelial cells using persistent homology of loops

Interacting, self-propelled particles such as epithelial cells can dynamically self-organize into complex multicellular patterns, which are challenging to classify without a priori information. Classically, different phases and phase transitions have been described based on local ordering, which may not capture structural features at larger length scales. Instead, topological data analysis (TDA) determines the stability of spatial connectivity at varying length scales (i.e. persistent homology), and can compare different particle configurations based on the "cost" of reorganizing one configuration into another. Here, we demonstrate a topology-based machine learning approach for unsupervised profiling of individual and collective phases based on large-scale loops. We show that these topological loops (i.e. dimension 1 homology) are robust to variations in particle number and density, particularly in comparison to connected components (i.e. dimension 0 homology). We use TDA to map out phase diagrams for simulated particles with varying adhesion and propulsion, at constant population size as well as when proliferation is permitted. Next, we use this approach to profile our recent experiments on the clustering of epithelial cells in varying growth factor conditions, which are compared to our simulations. Finally, we characterize the robustness of this approach at varying length scales, with sparse sampling, and over time. Overall, we envision TDA will be broadly applicable as a model-agnostic approach to analyze active systems with varying population size, from cytoskeletal motors to motile cells to flocking or swarming animals.

High-Throughput Methods in the Discovery and Study of Biomaterials and Materiobiology

The complex interaction of cells with biomaterials (i.e., materiobiology) plays an increasingly pivotal role in the development of novel implants, biomedical devices, and tissue engineering scaffolds to treat diseases, aid in the restoration of bodily functions, construct healthy tissues, or regenerate diseased ones. However, the conventional approaches are incapable of screening the huge amount of potential material parameter combinations to identify the optimal cell responses and involve a combination of serendipity and many series of trial-and-error experiments. For advanced tissue engineering and regenerative medicine, highly efficient and complex bioanalysis platforms are expected to explore the complex interaction of cells with biomaterials using combinatorial approaches that offer desired complex microenvironments during healing, development, and homeostasis. In this review, we first introduce materiobiology and its high-throughput screening (HTS). Then we present an in-depth of the recent progress of 2D/3D HTS platforms (i.e., gradient and microarray) in the principle, preparation, screening for materiobiology, and combination with other advanced technologies. The Compendium for Biomaterial Transcriptomics and high content imaging, computational simulations, and their translation toward commercial and clinical uses are highlighted. In the final section, current challenges and future perspectives are discussed. High-throughput experimentation within the field of materiobiology enables the elucidation of the relationships between biomaterial properties and biological behavior and thereby serves as a potential tool for accelerating the development of high-performance biomaterials.

Nano-structure of vitronectin/heparin on cell membrane for stimulating single cell in iPSC-derived embryoid body

Individual cell environment stimulating single cell is a suitable strategy for the generation of sophisticated multicellular aggregates with localized biochemical signaling. However, such strategy for induced pluripotent stem cell (iPSC)-derived embryoid bodies (EBs) is limited because the presence of external stimulation can inhibit spontaneous cellular communication, resulting in misdirection in the maturation and differentiation of EBs. In this study, a facile method of engineering the iPSC membrane to stimulate the inner cell of EBs while maintaining cellular activities is reported. We coated the iPSC surface with nanoscale extracellular matrix fabricated by self-assembly between vitronectin and heparin. This nano-coating allowed iPSC to retain its in vitro properties including adhesion capability, proliferation, and pluripotency during its aggregation. More importantly, the nano-coating did not induce lineage-specific differentiation but increased E-cadherin expression, resulting in promotion of development of EB. This study provides a foundation for future production of sophisticated patient-specific multicellular aggregates by modification of living cell membranes.

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VTN/HEP nano-coating acts as a flexible individual cellular environment

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VTN/HEP nano-coating stimulates embryoid body to promote its development

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VTN/HEP nano-coating preserves spontaneous cell aggregation

The epithelial-mesenchymal transition and the cytoskeleton in bioengineered systems

The epithelial-mesenchymal transition (EMT) is intrinsically linked to alterations of the intracellular cytoskeleton and the extracellular matrix. After EMT, cells acquire an elongated morphology with front/back polarity, which can be attributed to actin-driven protrusion formation as well as the gain of vimentin expression. Consequently, cells can deform and remodel the surrounding matrix in order to facilitate local invasion. In this review, we highlight recent bioengineering approaches to elucidate EMT and functional changes in the cytoskeleton. First, we review transitions between multicellular clusters and dispersed individuals on planar surfaces, which often exhibit coordinated behaviors driven by leader cells and EMT. Second, we consider the functional role of vimentin, which can be probed at subcellular length scales and within confined spaces. Third, we discuss the role of topographical patterning and EMT via a contact guidance like mechanism. Finally, we address how multicellular clusters disorganize and disseminate in 3D matrix. These new technologies enable controlled physical microenvironments and higher-resolution spatiotemporal measurements of EMT at the single cell level. In closing, we consider future directions for the field and outstanding questions regarding EMT and the cytoskeleton for human cancer progression. Video Abstract.

You Talking to Me? Cadherin and Integrin Crosstalk in Biomaterial Design

While much work has been done in the design of biomaterials to control integrin-mediated adhesion, less emphasis has been put on functionalization of materials with cadherin ligands. Yet, cell-cell contacts in combination with cell-matrix interactions are key in driving embryonic development, collective cell migration, epithelial to mesenchymal transition, and cancer metastatic processes, among others. This review focuses on the incorporation of both cadherin and integrin ligands in biomaterial design, to promote what is called the "adhesive crosstalk." First, the structure and function of cadherins and their role in eliciting mechanotransductive processes, by themselves or in combination with integrin mechanosensing, are introduced. Then, biomaterials that mimic cell-cell interactions, and recent applications to get insights in fundamental biology and tissue engineering, are critically discussed.

VE-cadherin endocytosis controls vascular integrity and patterning during development

Tissue morphogenesis requires dynamic intercellular contacts that are subsequently stabilized as tissues mature. The mechanisms governing these competing adhesive properties are not fully understood. Using gain- and loss-of-function approaches, we tested the role of p120-catenin (p120) and VE-cadherin (VE-cad) endocytosis in vascular development using mouse mutants that exhibit increased (VE-cad^GGG/GGG) or decreased (VE-cad^DEE/DEE) internalization. VE-cad^GGG/GGG mutant mice exhibited reduced VE-cad-p120 binding, reduced VE-cad levels, microvascular hemorrhaging, and decreased survival. By contrast, VE-cad^DEE/DEE mutants exhibited normal vascular permeability but displayed microvascular patterning defects. Interestingly, VE-cad^DEE/DEE mutant mice did not require endothelial p120, demonstrating that p120 is dispensable in the context of a stabilized cadherin. In vitro, VE-cadDEE mutant cells displayed defects in polarization and cell migration that were rescued by uncoupling VE-cadDEE from actin. These results indicate that cadherin endocytosis coordinates cell polarity and migration cues through actin remodeling. Collectively, our results indicate that regulated cadherin endocytosis is essential for both dynamic cell movements and establishment of stable tissue architecture.

Phenotypic Plasticity of Cancer Cells Based on Remodeling of the Actin Cytoskeleton and Adhesive Structures

There is ample evidence that, instead of a binary switch, epithelial-mesenchymal transition (EMT) in cancer results in a flexible array of phenotypes, each one uniquely suited to a stage in the invasion-metastasis cascade. The phenotypic plasticity of epithelium-derived cancer cells gives them an edge in surviving and thriving in alien environments. This review describes in detail the actin cytoskeleton and E-cadherin-based adherens junction rearrangements that cancer cells need to implement in order to achieve the advantageous epithelial/mesenchymal phenotype and plasticity of migratory phenotypes that can arise from partial EMT.

Mapping transmembrane binding partners for E-cadherin ectodomains

We combine proximity labeling and single molecule binding assays to discover transmembrane protein interactions in cells. We first screen for candidate binding partners by tagging the extracellular and cytoplasmic regions of a "bait" protein with BioID biotin ligase and identify proximal proteins that are biotin tagged on both their extracellular and intracellular regions. We then test direct binding interactions between proximal proteins and the bait, using single molecule atomic force microscope binding assays. Using this approach, we identify binding partners for the extracellular region of E-cadherin, an essential cell-cell adhesion protein. We show that the desmosomal proteins desmoglein-2 and desmocollin-3, the focal adhesion protein integrin-α2β1, the receptor tyrosine kinase ligand ephrin-B1, and the classical cadherin P-cadherin, all directly interact with E-cadherin ectodomains. Our data shows that combining extracellular and cytoplasmic proximal tagging with a biophysical binding assay increases the precision with which transmembrane ectodomain interactors can be identified.

Collectively stabilizing and orienting posterior migratory forces disperses cell clusters in vivo

Individual cells detach from cohesive ensembles during development and can inappropriately separate in disease. Although much is known about how cells separate from epithelia, it remains unclear how cells disperse from clusters lacking apical-basal polarity, a hallmark of advanced epithelial cancers. Here, using live imaging of the developmental migration program of Drosophila primordial germ cells (PGCs), we show that cluster dispersal is accomplished by stabilizing and orienting migratory forces. PGCs utilize a G protein coupled receptor (GPCR), Tre1, to guide front-back migratory polarity radially from the cluster toward the endoderm. Posteriorly positioned myosin-dependent contractile forces pull on cell-cell contacts until cells release. Tre1 mutant cells migrate randomly with transient enrichment of the force machinery but fail to separate, indicating a temporal contractile force threshold for detachment. E-cadherin is retained on the cell surface during cell separation and augmenting cell-cell adhesion does not impede detachment. Notably, coordinated migration improves cluster dispersal efficiency by stabilizing cell-cell interfaces and facilitating symmetric pulling. We demonstrate that guidance of inherent migratory forces is sufficient to disperse cell clusters under physiological settings and present a paradigm for how such events could occur across development and disease.

Crosstalk between Cell Adhesion Complexes in Regulation of Mechanotransduction

Physical forces regulate numerous biological processes during development, physiology, and pathology. Forces between the external environment and intracellular actin cytoskeleton are primarily transmitted through integrin-containing focal adhesions and cadherin-containing adherens junctions. Crosstalk between these complexes is well established and modulates the mechanical landscape of the cell. However, integrins and cadherins constitute large families of adhesion receptors and form multiple complexes by interacting with different ligands, adaptor proteins, and cytoskeletal filaments. Recent findings indicate that integrin-containing hemidesmosomes oppose force transduction and traction force generation by focal adhesions. The cytolinker plectin mediates this crosstalk by coupling intermediate filaments to the actin cytoskeleton. Similarly, cadherins in desmosomes might modulate force generation by adherens junctions. Moreover, mechanotransduction can be influenced by podosomes, clathrin lattices, and tetraspanin-enriched microdomains. This review discusses mechanotransduction by multiple integrin- and cadherin-based cell adhesion complexes, which together with the associated cytoskeleton form an integrated network that allows cells to sense, process, and respond to their physical environment.

Gold nanoparticles disrupt actin organization and pulmonary endothelial barriers

This study explored the impact of gold nanoparticles on the metabolic activity and morphology of human pulmonary endothelial cell monolayers. We developed a gold nanoparticle library of three different sizes and two surface chemistries that include anionic citrate and the cationic polyelectrolyte poly(allylamine hydrochloride). The nanoparticles were characterized in cell culture medium to assess how their physical properties are altered after exposure to biological fluids. A bovine serum albumin pretreatment protocol was developed to stabilize the nanoparticles in cell culture medium. Results of this study show that an 18 h exposure of human pulmonary artery endothelial cells to the different nanoparticles modestly affects cellular metabolic activity. However, nanoparticle exposure perturbs the cortical actin networks and induces the formation of intercellular gaps. In particular, exposure to the poly(allylamine hydrochloride)-coated particles reduces the area of cell-cell junctions-a change that correlates with increased leakiness of endothelial barriers. The presence of excess polyelectrolyte capping agents in the supernatant of poly(allylamine hydrochloride)-coated nanoparticles significantly impacts endothelial morphology. Pretreatment of the particle supernatant with bovine serum albumin mitigates the negative effects of free or bound polyelectrolytes on endothelial cell monolayers.

Investigating Optimal Time Step Intervals of Imaging for Data Quality through a Novel Fully-Automated Cell Tracking Approach

Computer-based fully-automated cell tracking is becoming increasingly important in cell biology, since it provides unrivalled capacity and efficiency for the analysis of large datasets. However, automatic cell tracking’s lack of superior pattern recognition and error-handling capability compared to its human manual tracking counterpart inspired decades-long research. Enormous efforts have been made in developing advanced cell tracking packages and software algorithms. Typical research in this field focuses on dealing with existing data and finding a best solution. Here, we investigate a novel approach where the quality of data acquisition could help improve the accuracy of cell tracking algorithms and vice-versa. Generally speaking, when tracking cell movement, the more frequent the images are taken, the more accurate cells are tracked and, yet, issues such as damage to cells due to light intensity, overheating in equipment, as well as the size of the data prevent a constant data streaming. Hence, a trade-off between the frequency at which data images are collected and the accuracy of the cell tracking algorithms needs to be studied. In this paper, we look at the effects of different choices of the time step interval (i.e., the frequency of data acquisition) within the microscope to our existing cell tracking algorithms. We generate several experimental data sets where the true outcomes are known (i.e., the direction of cell migration) by either using an effective chemoattractant or employing no-chemoattractant. We specify a relatively short time step interval (i.e., 30 s) between pictures that are taken at the data generational stage, so that, later on, we may choose some portion of the images to produce datasets with different time step intervals, such as 1 min, 2 min, and so on. We evaluate the accuracy of our cell tracking algorithms to illustrate the effects of these different time step intervals. We establish that there exist certain relationships between the tracking accuracy and the time step interval associated with experimental microscope data acquisition. We perform fully-automatic adaptive cell tracking on multiple datasets, to identify optimal time step intervals for data acquisition, while at the same time demonstrating the performance of the computer cell tracking algorithms.

The role of single cell mechanical behavior and polarity in driving collective cell migration

The directed migration of cell collectives is essential in various physiological processes, such as epiboly, intestinal epithelial turnover, and convergent extension during morphogenesis as well as during pathological events like wound healing and cancer metastasis. Collective cell migration leads to the emergence of coordinated movements over multiple cells. Our current understanding emphasizes that these movements are mainly driven by large-scale transmission of signals through adherens junctions. In this study, we show that collective movements of epithelial cells can be triggered by polarity signals at the single cell level through the establishment of coordinated lamellipodial protrusions. We designed a minimalistic model system to generate one-dimensional epithelial trains confined in ring shaped patterns that recapitulate rotational movements observed in vitro in cellular monolayers and in vivo in genitalia or follicular cell rotation. Using our system, we demonstrated that cells follow coordinated rotational movements after the establishment of directed Rac1-dependent polarity over the entire monolayer. Our experimental and numerical approaches show that the maintenance of coordinated migration requires the acquisition of a front-back polarity within each single cell but does not require the maintenance of cell-cell junctions. Taken together, these unexpected findings demonstrate that collective cell dynamics in closed environments as observed in multiple in vitro and in vivo situations can arise from single cell behavior through a sustained memory of cell polarity.

Reciprocal integrin/integrin antagonism through kindlin-2 and Rho GTPases regulates cell cohesion and collective migration

Collective cell behaviour during embryogenesis and tissue repair requires the coordination of intercellular junctions, cytoskeleton-dependent shape changes controlled by Rho GTPases, and integrin-dependent cell-matrix adhesion. Many different integrins are simultaneously expressed during wound healing, embryonic development, and sprouting angiogenesis, suggesting that there is extensive integrin/integrin cross-talk to regulate cell behaviour. Here, we show that fibronectin-binding β1 and β3 integrins do not act synergistically, but rather antagonize each other during collective cell processes in neuro-epithelial cells, placental trophoblasts, and endothelial cells. Reciprocal β1/β3 antagonism controls RhoA activity in a kindlin-2-dependent manner, balancing cell spreading, contractility, and intercellular adhesion. In this way, reciprocal β1/β3 antagonism controls cell cohesion and cellular plasticity to switch between extreme and opposing states, including epithelial versus mesenchymal-like phenotypes and collective versus individual cell migration. We propose that integrin/integrin antagonism is a universal mechanism to effectuate social cellular interactions, important for tissue morphogenesis, endothelial barrier function, trophoblast invasion, and sprouting angiogenesis.

Early Events in Actin Cytoskeleton Dynamics and E-Cadherin-Mediated Cell-Cell Adhesion during Epithelial-Mesenchymal Transition

Epithelial-mesenchymal transition (EMT) plays an important role in development and also in initiation of metastasis during cancer. Disruption of cell-cell contacts during EMT allowing cells to detach from and migrate away from their neighbors remains poorly understood. Using immunofluorescent staining and live-cell imaging, we analyzed early events during EMT induced by epidermal growth factor (EGF) in IAR-20 normal epithelial cells. Control cells demonstrated stable adherens junctions (AJs) and robust contact paralysis, whereas addition of EGF caused rapid dynamic changes at the cell-cell boundaries: fragmentation of the circumferential actin bundle, assembly of actin network in lamellipodia, and retrograde flow. Simultaneously, an actin-binding protein EPLIN was phosphorylated, which may have decreased the stability of the circumferential actin bundle. Addition of EGF caused gradual replacement of linear E-cadherin–based AJs with dynamic and unstable punctate AJs, which, unlike linear AJs, colocalized with the mechanosensitive protein zyxin, confirming generation of centripetal force at the sites of cell-cell contacts during EMT. Our data show that early EMT promotes heightened dynamics at the cell-cell boundaries—replacement of stable AJs and actin structures with dynamic ones—which results in overall weakening of cell-cell adhesion, thus priming the cells for front-rear polarization and eventual migration.

Stiffness Sensing by Cells

Physical stimuli are essential for the function of eukaryotic cells, and changes in physical signals are important elements in normal tissue development as well as in disease initiation and progression. The complexity of physical stimuli and the cellular signals they initiate are as complex as those triggered by chemical signals. One of the most important, and the focus of this review, is the effect of substrate mechanical properties on cell structure and function. The past decade has produced a nearly exponentially increasing number of mechanobiological studies to define how substrate stiffness alters cell biology using both purified systems and intact tissues. Here we attempt to identify common features of mechanosensing in different systems while also highlighting the numerous informative exceptions to what in early studies appeared to be simple rules by which cells respond to mechanical stresses.

Emerging themes and unifying concepts underlying cell behavior regulation by the pericellular space

Cells reside in a complex three-dimensional (3D) microenvironment where physical, chemical, and architectural features of the pericellular space regulate important cellular functions like migration, differentiation, and morphogenesis. A major goal of tissue engineering is to identify which properties of the pericellular space orchestrate these emergent cell behaviors and how. In this review, we highlight recent studies at the interface of biomaterials and single cell biophysics that are lending deeper insight towards this goal. Advanced methods have enabled the decoupling of architectural and mechanical features of the microenvironment, revealing multiple mechanisms of adhesion and mechanosensing modulation by biomaterials. Such studies are revealing important roles for pericellular space degradability, hydration, and adhesion competition in cell shape, volume, and differentiation regulation. STATEMENT OF SIGNIFICANCE: Cell fate and function are closely regulated by the local extracellular microenvironment. Advanced methods at the interface of single cell biophysics and biomaterials have shed new light on regulators of cell-pericellular space interactions by decoupling more features of the complex pericellular milieu than ever before. These findings lend deeper mechanistic insight into how biomaterials can be designed to fine-tune outcomes like differentiation, migration, and collective morphogenesis.

Motility-limited aggregation of mammary epithelial cells into fractal-like clusters

Migratory cells transition between dispersed individuals and multicellular collectives during development, wound healing, and cancer. These transitions are associated with coordinated behaviors as well as arrested motility at high cell densities, but remain poorly understood at lower cell densities. Here, we show that dispersed mammary epithelial cells organize into arrested, fractal-like clusters at low density in reduced epidermal growth factor (EGF). These clusters exhibit a branched architecture with a fractal dimension of [Formula: see text], reminiscent of diffusion-limited aggregation of nonliving colloidal particles. First, cells display diminished motility in reduced EGF, which permits irreversible adhesion upon cell-cell contact. Subsequently, leader cells emerge that guide collectively migrating strands and connect clusters into space-filling networks. Thus, this living system exhibits gelation-like arrest at low cell densities, analogous to the glass-like arrest of epithelial monolayers at high cell densities. We quantitatively capture these behaviors with a jamming-like phase diagram based on local cell density and EGF. These individual to collective transitions represent an intriguing link between living and nonliving systems, with potential relevance for epithelial morphogenesis into branched architectures.

p90 ribosomal S6 kinase (RSK) phosphorylates myosin phosphatase and thereby controls edge dynamics during cell migration

Cell migration is essential to embryonic development, wound healing, and cancer cell dissemination. Cells move via leading-edge protrusion, substrate adhesion, and retraction of the cell's rear. The molecular mechanisms by which extracellular cues signal to the actomyosin cytoskeleton to control these motility mechanics are poorly understood. The growth factor-responsive and oncogenically activated protein extracellular signal-regulated kinase (ERK) promotes motility by signaling in actin polymerization-mediated edge protrusion. Using a combination of immunoblotting, co-immunoprecipitation, and myosin-binding experiments and cell migration assays, we show here that ERK also signals to the contractile machinery through its substrate, p90 ribosomal S6 kinase (RSK). We probed the signaling and migration dynamics of multiple mammalian cell lines and found that RSK phosphorylates myosin phosphatase–targeting subunit 1 (MYPT1) at Ser-507, which promotes an interaction of Rho kinase (ROCK) with MYPT1 and inhibits myosin targeting. We find that by inhibiting the myosin phosphatase, ERK and RSK promote myosin II–mediated tension for lamella expansion and optimal edge dynamics for cell migration. These findings suggest that ERK activity can coordinately amplify both protrusive and contractile forces for optimal cell motility.

Traction microscopy with integrated microfluidics: responses of the multi-cellular island to gradients of HGF

Collective cellular migration plays a central role in development, regeneration, and metastasis. In these processes, mechanical interactions between cells are fundamental but measurement of these interactions is often hampered by technical limitations. To overcome some of these limitations, here we describe a system that integrates microfluidics with traction microscopy (TM). Using this system we can measure simultaneously, and in real time, migration speeds, tractions, and intercellular tension throughout an island of confluent Madin-Darby canine kidney (MDCK) cells. The cell island is exposed to hepatocyte growth factor (HGF) at a controlled gradient of concentrations; HGF is known to elicit epithelial-to-mesenchymal transition (EMT) and cell scattering. As expected, the rate of expansion of the cell island was dependent on the concentration of HGF. Higher concentrations of HGF reduced intercellular tensions, as expected during EMT. A novel finding, however, is that the effects of HGF concentration and its gradient were seen within an island. This integrated experimental system thus provides an integrated tool to better understand cellular forces during collective cellular migration under chemical gradients.

Physical defects in basement membrane-mimicking collagen-IV matrices trigger cellular EMT and invasion

In fibrosis and cancer, degradation of basement membrane (BM) and cell invasion are considered as key outcomes of a cellular transformation called epithelial-mesenchymal transition (EMT). Here, we pose a converse question - can preexisting physical defects in the BM matrix cause EMT in normal epithelial cells? On a BM-mimicking matrix of collagen-IV-coated polyacrylamide (PA) gel, we have discovered a reverse phenomenon in which preexisting defects trigger EMT in normal epithelial cells. Through spatiotemporal measurements and simulations in silico, we demonstrate that the EMT precedes cellular mechanoactivation on defective matrices, but they occur concurrently on stiff matrices. The defect-dependent EMT caused cell invasion though a stroma-mimicking collagen-I layer, which could be disabled through MMP9 inhibition. Our findings reveal that the known BM degradation caused by cellular EMT and invasion is not a one-way process. Instead, normal epithelial cells can exploit physical defects in the BM matrix to undergo disease-like cellular transformations.

Correlative 3D Structured Illumination Microscopy and Single-Molecule Localization Microscopy for Imaging Cancer Invasion

Super-resolution microscopy methods enable resolution of biological molecules in their cellular or tissue context at the nanoscale. Different methods have their strengths and weaknesses. Here we present a method that enables correlative confocal, structured illumination microscopy (SIM) and single-molecule localization microscopy (SMLM) imaging of structures involved in formation of invadopodia on the same sample. This enables up to four colors to be visualized in three dimensions at a resolution of between 120 and 10 nm for SIM and SMLM, respectively.

Regulation of epithelial migration by epithelial cell adhesion molecule requires its Claudin-7 interaction domain

Epithelial cell adhesion molecule (EpCAM) is a glycoprotein on the surface of epithelial cells that is essential for intestinal epithelial integrity and expressed at high levels in many epithelial derived cancers and circulating tumor cells. Here we show the effect of EpCAM levels on migration of Madin-Darby-Canine Kidney (MDCK) epithelial cells. MDCK cells depleted of EpCAM show increased activation of extracellular signal-regulated kinase (ERK) and of myosin, and increased cell spreading and epithelial sheet migration into a gap. In contrast, over-expression of EpCAM inhibits ERK and myosin activation, and slows epithelial sheet migration. Loss of EpCAM is rescued by EpCAM-YFP mutated in the extracellular domain required for cis-dimerization whereas EpCAM-YFP with a mutation that inhibits Claudin-7 interaction cannot rescue increased ERK, myosin activation, and increased migration in EpCAM-depleted cells. In summary, these results indicate that interaction of EpCAM and Claudin-7 at the cell surface negatively regulates epithelial migration by inhibiting ERK and actomyosin contractility.

Force localization modes in dynamic epithelial colonies

Collective cell behaviors, including tissue remodeling, morphogenesis, and cancer metastasis, rely on dynamics among cells, their neighbors, and the extracellular matrix. The lack of quantitative models precludes understanding of how cell-cell and cell-matrix interactions regulate tissue-scale force transmission to guide morphogenic processes. We integrate biophysical measurements on model epithelial tissues and computational modeling to explore how cell-level dynamics alter mechanical stress organization at multicellular scales. We show that traction stress distribution in epithelial colonies can vary widely for identical geometries. For colonies with peripheral localization of traction stresses, we recapitulate previously described mechanical behavior of cohesive tissues with a continuum model. By contrast, highly motile cells within colonies produce traction stresses that fluctuate in space and time. To predict the traction force dynamics, we introduce an active adherent vertex model (AAVM) for epithelial monolayers. AAVM predicts that increased cellular motility and reduced intercellular mechanical coupling localize traction stresses in the colony interior, in agreement with our experimental data. Furthermore, the model captures a wide spectrum of localized stress production modes that arise from individual cell activities including cell division, rotation, and polarized migration. This approach provides a robust quantitative framework to study how cell-scale dynamics influence force transmission in epithelial tissues.

Cellular sheddases are induced by Merkel cell polyomavirus small tumour antigen to mediate cell dissociation and invasiveness

Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high propensity for recurrence and metastasis. Merkel cell polyomavirus (MCPyV) is recognised as the causative factor in the majority of MCC cases. The MCPyV small tumour antigen (ST) is considered to be the main viral transforming factor, however potential mechanisms linking ST expression to the highly metastatic nature of MCC are yet to be fully elucidated. Metastasis is a complex process, with several discrete steps required for the formation of secondary tumour sites. One essential trait that underpins the ability of cancer cells to metastasise is how they interact with adjoining tumour cells and the surrounding extracellular matrix. Here we demonstrate that MCPyV ST expression disrupts the integrity of cell-cell junctions, thereby enhancing cell dissociation and implicate the cellular sheddases, A disintegrin and metalloproteinase (ADAM) 10 and 17 proteins in this process. Inhibition of ADAM 10 and 17 activity reduced MCPyV ST-induced cell dissociation and motility, attributing their function as critical to the MCPyV-induced metastatic processes. Consistent with these data, we confirm that ADAM 10 and 17 are upregulated in MCPyV-positive primary MCC tumours. These novel findings implicate cellular sheddases as key host cell factors contributing to virus-mediated cellular transformation and metastasis. Notably, ADAM protein expression may be a novel biomarker of MCC prognosis and given the current interest in cellular sheddase inhibitors for cancer therapeutics, it highlights ADAM 10 and 17 activity as a novel opportunity for targeted interventions for disseminated MCC.

Cell-Cell Contact and Receptor Tyrosine Kinase Signaling

The behavior of cells within tissues is governed by the activities of adhesion receptors that provide spatial cues and transmit forces through intercellular junctions, and by growth-factor receptors, particularly receptor tyrosine kinases (RTKs), that respond to biochemical signals from the environment. Coordination of these two activities is essential for the patterning and polarized migration of cells during morphogenesis and for homeostasis in mature tissues; loss of this coordination is a hallmark of developing cancer and driver of metastatic progression. Although much is known about the individual functions of adhesion and growth factor receptors, we have a surprisingly superficial understanding of the mechanisms by which their activities are coordinated.