Integrin-cytoskeletal interactions in migrating fibroblasts are dynamic, asymmetric, and regulated

Single Nanovesicles Tracking Reveals Their Heterogeneous Extracellular Adsorptions

The vigorous nanomedicine offers significant possibilities for effective therapeutics of various diseases, and nanovesicles (NVs) represented by artificial liposomes and natural exosomes and cytomembranes especially show great potential. However, their complex interactions with cells, particularly the heterogeneous extracellular adsorptions, are difficult to analyze spatiotemporally due to the transient dynamics. In this study, by single NVs tracking, the extracellular NVs adsorptions are directly observed and their heterogeneous characteristics are revealed. Briefly, plenty of NVs adsorbed on HCT116 cells are tracked and classified, and it is discovered that they exhibit various diffusion properties from different extracellular regions: stable adsorptions on the rear surface and restricted adsorptions on the front protrusion. After the hydrolysis of hyaluronic acid in the extracellular matrix by hyaluronidase, the restricted adsorptions are further weakened and manifested as dissociative adsorptions, which demonstrated reduced total NVs adsorptions from a single-cell and single-particle perspective. Compared with traditional static analysis, the spatiotemporal tracking and heterogeneous results not only reveal the extracellular NVs-cell interactions but also inspire a wide variety of nanomedicine and their nano-investigations.

Crawling, waving, inch worming, dilating, and pivoting mechanics of migrating cells: Lessons from Ken Jacobson

Research on the locomotion of single cells on hard, flat surfaces brought insight into the mechanisms of leading-edge protrusion, spatially graded adhesion, front-rear coordination, and how intracellular and traction forces are harnessed to execute various maneuvers. Here, we highlight how, by studying a variety of cell types, shapes, and movements, Ken Jacobson and his collaborators made several discoveries that triggered the mechanistic understanding of cell motility. We then review the recent advancements and current perspectives in this field.

Controlling in and out - the future of interfering with immune cell trafficking in inflammatory bowel disease

INTRODUCTION

Immune cell trafficking is a key requirement in the pathogenesis of inflammatory bowel diseases. Consistently, therapeutic strategies to target immune cell trafficking have been established and continue to be developed for the treatment of ulcerative colitis and Crohn's disease.

AREAS COVERED

In this review, we briefly summarize the most important checkpoints of intestinal immune cell trafficking and their importance during IBD. Moreover, we provide an overview of associated therapeutic targets and previous as well as current efforts on treatment strategies related to these targets.

EXPERT OPINION

Finally, we comment on potential future developments that might shape the field of immune cell trafficking in the context of IBD.

Cytocompatibility of Graphene Monolayer and Its Impact on Focal Cell Adhesion, Mitochondrial Morphology and Activity in BALB/3T3 Fibroblasts

This study investigates the effect of graphene scaffold on morphology, viability, cytoskeleton, focal contacts, mitochondrial network morphology and activity in BALB/3T3 fibroblasts and provides new data on biocompatibility of the "graphene-family nanomaterials". We used graphene monolayer applied onto glass cover slide by electrochemical delamination method and regular glass cover slide, as a reference. The morphology of fibroblasts growing on graphene was unaltered, and the cell viability was 95% compared to control cells on non-coated glass slide. There was no significant difference in the cell size (spreading) between both groups studied. Graphene platform significantly increased BALB/3T3 cell mitochondrial activity (WST-8 test) compared to glass substrate. To demonstrate the variability in focal contacts pattern, the effect of graphene on vinculin was examined, which revealed a significant increase in focal contact size comparing to control-glass slide. There was no disruption in mitochondrial network morphology, which was branched and well connected in relation to the control group. Evaluation of the JC-1 red/green fluorescence intensity ratio revealed similar levels of mitochondrial membrane potential in cells growing on graphene-coated and uncoated slides. These results indicate that graphene monolayer scaffold is cytocompatible with connective tissue cells examined and could be beneficial for tissue engineering therapy.

Loss of the Essential Autophagy Regulators FIP200 or Atg5 Leads to Distinct Effects on Focal Adhesion Composition and Organization

Autophagy is an essential catabolic intracellular pathway that maintains homeostasis by degrading long-lived proteins, damaged organelles, and provides an energy source during nutrient starvation. It is now understood that autophagy has discrete functions as a selective lysosomal degradation pathway targeting large cytosolic structural and signaling complexes to influence cell motility and adhesion. We provide evidence suggesting the primary autophagy regulators Atg5 and FIP200 both play a role in cell motility and extracellular matrix adhesion. However, their loss of function has a differential impact on focal adhesion composition and organization, as well as signaling in response to fibronectin induced cell spreading. This differential impact on focal adhesions is illustrated by smaller focal adhesion complexes and a decrease in FAK, paxillin, and vinculin expression associated with FIP200 loss of function. In contrast, Atg5 loss of function results in production of large and stable focal adhesions, characterized by their retention of phosphorylated FAK and Src, which correlates with increased vinculin and FAK protein expression. Importantly, autophagy is upregulated during processes associated with focal adhesion reorganization and their exhibits colocalization of autophagosomes with focal adhesion cargo. Interestingly, FIP200 localizes to vinculin-rich focal adhesions and its loss negatively regulates FAK phosphorylation. These data collectively suggest FIP200 and Atg5 may have both autophagy-dependent and -independent functions that provide distinct mechanisms and impacts on focal adhesion dynamics associated with cell motility.

Interferometric fluorescence cross correlation spectroscopy

Measuring transport properties like diffusion and directional flow is essential for understanding dynamics within heterogeneous systems including living cells and novel materials. Fluorescent molecules traveling within these inhomogeneous environments under the forces of Brownian motion and flow exhibit fluctuations in their concentration, which are directly linked to the transport properties. We present a method utilizing single photon interference and fluorescence correlation spectroscopy (FCS) to simultaneously measure transport of fluorescent molecules within aqueous samples. Our method, within seconds, measures transport in thousands of homogenous voxels (100 nm)³ and under certain conditions, eliminates photo-physical artifacts associated with blinking of fluorescent molecules. A comprehensive theoretical framework is presented and validated by measuring transport of quantum dots, associated with VSV-G receptor along cellular membranes as well as within viscous gels.

7-O-methylpunctatin, a Novel Homoisoflavonoid, Inhibits Phenotypic Switch of Human Arteriolar Smooth Muscle Cells

Remodeling of arterioles is a pivotal event in the manifestation of many inflammation-based cardio-vasculopathologies, such as hypertension. During these remodeling events, vascular smooth muscle cells (VSMCs) switch from a contractile to a synthetic phenotype. The latter is characterized by increased proliferation, migration, and invasion. Compounds with anti-inflammatory actions have been successful in attenuating this phenotypic switch. While the vast majority of studies investigating phenotypic modulation were undertaken in VSMCs isolated from large vessels, little is known about the effect of such compounds on phenotypic switch in VSMCs of microvessels (microVSMCs). We have recently characterized a novel homoisoflavonoid that we called 7-O-methylpunctatin (MP). In this study, we show that MP decreased FBS-induced cell proliferation, migration, invasion, and adhesion. MP also attenuated adhesion of THP-1 monocytes to microVSMCs, abolished FBS-induced expression of MMP-2, MMP-9, and NF-κB, as well as reduced activation of ERK1/2 and FAK. Furthermore, MP-treated VSMCs showed an increase in early (myocardin, SM-22α, SM-α) and mid-term (calponin and caldesmon) differentiation markers and a decrease in osteopontin, a protein highly expressed in synthetic VSMCs. MP also reduced transcription of cyclin D1, CDK4 but increased protein levels of p21 and p27. Taken together, these results corroborate an anti-inflammatory action of MP on human microVSMCs. Therefore, by inhibiting the synthetic phenotype of microVSMCs, MP may be a promising modulator for inflammation-induced arteriolar pathophysiology.

Extent of Cell Confinement in Microtracks Affects Speed and Results in Differential Matrix Strains

During metastasis, cancer cells navigate through a spatially heterogeneous extracellular matrix (ECM). Physical properties of ECM, including the degree of confinement, influence cell migration behavior. Here, utilizing in vitro three-dimensional collagen microtracks, we demonstrate that cell-ECM interactions, specifically the degree of spatial confinement, regulate migratory behavior. We found that cells migrate faster when they are fully confined, contacting all four walls (top, bottom, and two sides) of a collagen microtrack, compared with cells that are partially confined, contacting less than four walls. When fully confined, cells exhibit fewer but larger vinculin-containing adhesions and create greater strains in the surrounding matrix directed toward the cell body. In contrast, partially confined cells develop a more elongated morphology with smaller but significantly more vinculin-containing adhesions and displace the surrounding matrix less than fully confined cells. The resulting effect of increasing cell contractility via Rho activation is dependent on the number of walls with which the cell is in contact. Although matrix strains increase in both fully and partially confined cells, cells that are partially confined increase speed, whereas those in full confinement decrease speed. Together, these results suggest that the degree of cell-ECM contact during confined migration is a key determinant of speed, morphology, and cell-generated substrate strains during motility, and these factors may work in tandem to facilitate metastatic cell migration.

Release of cholesterol-rich particles from the macrophage plasma membrane during movement of filopodia and lamellipodia

Cultured mouse peritoneal macrophages release large numbers of ~30-nm cholesterol-rich particles. Here, we show that those particles represent fragments of the plasma membrane that are pulled away and left behind during the projection and retraction of filopodia and lamellipodia. Consistent with this finding, the particles are enriched in proteins found in focal adhesions, which attach macrophages to the substrate. The release of particles is abolished by blocking cell movement (either by depolymerizing actin with latrunculin A or by inhibiting myosin II with blebbistatin). Confocal microscopy and NanoSIMS imaging studies revealed that the plasma membrane-derived particles are enriched in 'accessible cholesterol' (a mobile pool of cholesterol detectable with the modified cytolysin ALO-D4) but not in sphingolipid-sequestered cholesterol [a pool detectable with ostreolysin A (OlyA)]. The discovery that macrophages release cholesterol-rich particles during cellular locomotion is likely relevant to cholesterol efflux and could contribute to extracellular cholesterol deposition in atherosclerotic plaques.

Engineering approaches to studying cancer cell migration in three-dimensional environments

Cancer is one of the most devastating diseases of our time, with 17 million new cancer cases and 9.5 million cancer deaths in 2018 worldwide. The mortality associated with cancer results primarily from metastasis, i.e. the spreading of cancer cells from the primary tumour to other organs. The invasion and migration of cells through basement membranes, tight interstitial spaces and endothelial cell layers are key steps in the metastatic cascade. Recent studies demonstrated that cell migration through three-dimensional environments that mimic the in vivo conditions significantly differs from their migration on two-dimensional surfaces. Here, we review recent technological advances made in the field of cancer research that provide more 'true to the source' experimental platforms and measurements for the study of cancer cell invasion and migration in three-dimensional environments. These include microfabrication, three-dimensional bioprinting and intravital imaging tools, along with force and stiffness measurements of cells and their environments. These techniques will enable new studies that better reflect the physiological environment found in vivo, thereby producing more robust results. The knowledge achieved through these studies will aid in the development of new treatment options with the potential to ultimately lighten the devastating cost cancer inflicts on patients and their families. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Dissecting single-cell molecular spatiotemporal mobility and clustering at focal adhesions in polarised cells by fluorescence fluctuation spectroscopy methods

Quantitative fluorescence fluctuation spectroscopy from optical microscopy datasets is a very powerful tool to resolve multiple spatiotemporal cellular and subcellular processes at the molecular level. In particular, raster image correlation spectroscopy (RICS) and number and brightness analyses (N&B) yield molecular mobility and clustering dynamic information extracted from real-time cellular processes. This quantitative information can be inferred in a highly flexible and detailed manner, i.e. 1) at the localisation level: from full-frame datasets and multiple regions of interest within; and 2) at the temporal level: not only from full-frame and multiple regions, but also intermediate temporal events. Here we build on previous research in deciphering the molecular dynamics of paxillin, a main component of focal adhesions. Cells use focal adhesions to attach to the extracellular matrix and interact with their local environment. Through focal adhesions and other adhesion structures, cells sense their local environment and respond accordingly; due to this continuous communication, these structures can be highly dynamic depending on the extracellular characteristics. By using a previously well-characterised model like paxillin, we examine the powerful sensitivity and some limitations of RICS and N&B analyses. We show that cells upon contact to different surfaces show differential self-assembly dynamics in terms of molecular diffusion and oligomerisation. In addition, single-cell studies show that these dynamics change gradually following an antero-posterior gradient.

Temporal Variation in Single-Cell Power-Law Rheology Spans the Ensemble Variation of Cell Population

Changes in the cytoskeletal organization within cells can be characterized by large spatial and temporal variations in rheological properties of the cell (e.g., the complex shear modulus G^∗). Although the ensemble variation in G^∗ of single cells has been elucidated, the detailed temporal variation of G^∗ remains unknown. In this study, we investigated how the rheological properties of individual fibroblast cells change under a spatially confined environment in which the cell translational motion is highly restricted and the whole cell shape remains unchanged. The temporal evolution of single-cell rheology was probed at the same measurement location within the cell, using atomic force microscopy-based oscillatory deformation. The measurements reveal that the temporal variation in the power-law rheology of cells is quantitatively consistent with the ensemble variation, indicating that the cell system satisfies an ergodic hypothesis in which the temporal statistics are identical to the ensemble statistics. The autocorrelation of G^∗ implies that the cell mechanical state evolves in the ensemble of possible states with a characteristic timescale.

Biological responses to immobilized microscale and nanoscale surface topographies

Cellular responses are highly influenced by biochemical and biomechanical interactions with the extracellular matrix (ECM). Due to the impact of ECM architecture on cellular responses, significant research has been dedicated towards developing biomaterials that mimic the physiological environment for design of improved medical devices and tissue engineering scaffolds. Surface topographies with microscale and nanoscale features have demonstrated an effect on numerous cellular responses, including cell adhesion, migration, proliferation, gene expression, protein production, and differentiation; however, relationships between biological responses and surface topographies are difficult to establish due to differences in cell types and biomaterial surface properties. Therefore, it is important to optimize implant surface feature characteristics to elicit desirable biological responses for specific applications. The goal of this work was to review studies investigating the effects of microstructured and nanostructured biomaterials on in vitro biological responses through fabrication of microscale and nanoscale surface topographies, physico-chemical characterization of material surface properties, investigation of protein adsorption dynamics, and evaluation of cellular responses in specific biomedical applications.

Integrin and cadherin clusters: A robust way to organize adhesions for cell mechanics

Recent studies at the nanometer scale have revealed that relatively uniform clusters of adhesion proteins (50-100 nm) constitute the modular units of cell adhesion sites in both cell-matrix and cell-cell adhesions. Super resolution microscopy and membrane protein diffusion studies both suggest that even large focal adhesions are formed of 100 nm clusters that are loosely aggregated. Clusters of 20-50 adhesion molecules (integrins or cadherins) can support large forces through avidity binding interactions but can also be disassembled or endocytosed rapidly. Assembly of the clusters of integrins is force-independent and involves gathering integrins at ligand binding sites where they are stabilized by cytoplasmic adhesion proteins that crosslink the integrin cytoplasmic tails plus connect the clusters to the cell cytoskeleton. Cooperative-signaling events can occur in a single cluster without cascading to other clusters. Thus, the clusters appear to be very important elements in many cellular processes and can be considered as a critical functional module.

Advanced fluorescence microscopy techniques for the life sciences

The development of super-resolved fluorescence microscopy, for which the Nobel Prize was awarded in 2014, has been a topic of interest to physicists and biologists alike. It is inevitable that numerous questions in biomedical research cannot be answered by means other than direct observation. In this review, advances to fluorescence microscopy are covered in a widely accessible fashion to facilitate its use in decisions related to its acquisition and utilization in biomedical research.

Three-Dimensional Reflectance Traction Microscopy

Cells in three-dimensional (3D) environments exhibit very different biochemical and biophysical phenotypes compared to the behavior of cells in two-dimensional (2D) environments. As an important biomechanical measurement, 2D traction force microscopy can not be directly extended into 3D cases. In order to quantitatively characterize the contraction field, we have developed 3D reflectance traction microscopy which combines confocal reflection imaging and partial volume correlation postprocessing. We have measured the deformation field of collagen gel under controlled mechanical stress. We have also characterized the deformation field generated by invasive breast cancer cells of different morphologies in 3D collagen matrix. In contrast to employ dispersed tracing particles or fluorescently-tagged matrix proteins, our methods provide a label-free, computationally effective strategy to study the cell mechanics in native 3D extracellular matrix.

Actomyosin pulls to advance the nucleus in a migrating tissue cell

The cytoskeletal forces involved in translocating the nucleus in a migrating tissue cell remain unresolved. Previous studies have variously implicated actomyosin-generated pushing or pulling forces on the nucleus, as well as pulling by nucleus-bound microtubule motors. We found that the nucleus in an isolated migrating cell can move forward without any trailing-edge detachment. When a new lamellipodium was triggered with photoactivation of Rac1, the nucleus moved toward the new lamellipodium. This forward motion required both nuclear-cytoskeletal linkages and myosin activity. Apical or basal actomyosin bundles were found not to translate with the nucleus. Although microtubules dampen fluctuations in nuclear position, they are not required for forward translocation of the nucleus during cell migration. Trailing-edge detachment and pulling with a microneedle produced motion and deformation of the nucleus suggestive of a mechanical coupling between the nucleus and the trailing edge. Significantly, decoupling the nucleus from the cytoskeleton with KASH overexpression greatly decreased the frequency of trailing-edge detachment. Collectively, these results explain how the nucleus is moved in a crawling fibroblast and raise the possibility that forces could be transmitted from the front to the back of the cell through the nucleus.

Cell mechanics: principles, practices, and prospects

Cells generate and sustain mechanical forces within their environment as part of their normal physiology. They are active materials that can detect mechanical stimulation by the activation of mechanosensitive signaling pathways, and respond to physical cues through cytoskeletal re-organization and force generation. Genetic mutations and pathogens that disrupt the cytoskeletal architecture can result in changes to cell mechanical properties such as elasticity, adhesiveness, and viscosity. On the other hand, perturbations to the mechanical environment can affect cell behavior. These transformations are often a hallmark and symptom of a variety of pathologies. Consequently, there are now a myriad of experimental techniques and theoretical models adapted from soft matter physics and mechanical engineering to characterize cell mechanical properties. Interdisciplinary research combining modern molecular biology with advanced cell mechanical characterization techniques now paves the way for furthering our fundamental understanding of cell mechanics and its role in development, physiology, and disease. We describe a generalized outline for measuring cell mechanical properties including loading protocols, tools, and data interpretation.We summarize recent advances in the field and explain how cell biomechanics research can be adopted by physicists, engineers, biologists, and clinicians alike.

Quantifying cell-to-cell variation in power-law rheology

Among individual cells of the same source and type, the complex shear modulus G(∗) exhibits a large log-normal distribution that is the result of spatial, temporal, and intrinsic variations. Such large distributions complicate the statistical evaluation of pharmacological treatments and the comparison of different cell states. However, little is known about the characteristic features of cell-to-cell variation. In this study, we investigated how this variation depends on the spatial location within the cell and on the actin filament cytoskeleton, the organization of which strongly influences cell mechanics. By mechanically probing fibroblasts arranged on a microarray, via atomic force microscopy, we observed that the standard deviation σ of G(∗) was significantly reduced among cells in which actin filaments were depolymerized. The parameter σ also exhibited a subcellular spatial dependence. Based on our findings regarding the frequency dependence of σ of the storage modulus G('), we proposed two types of cell-to-cell variation in G(') that arise from the purely elastic and the frequency-dependent components in terms of the soft glassy rheology model of cell deformability. We concluded that the latter inherent cell-to-cell variation can be reduced greatly by disrupting actin networks, by probing at locations within the cell nucleus boundaries distant from the cell center, and by measuring at high loading frequencies.

Coating extracellular matrix proteins on a (3-aminopropyl)triethoxysilane-treated glass substrate for improved cell culture

We demonstrate that a (3-aminopropyl)triethoxysilane-treated glass surface is superior to an untreated glass surface for coating with extracellular matrix (ECM) proteins when used as a cell culture substrate to observe cell physiology and behavior. We found that MDCK cells cultured on untreated glass coated with ECM removed the coated ECM protein and secreted different ECM proteins. In contrast, the cells did not remove the coated ECM protein when seeded on (3-aminopropyl)triethoxysilane-treated (i.e., silanized) glass coated with ECM. Furthermore, the morphology and motility of cells grown on silanized glass differed from those grown on non-treated glass, even when both types of glass were initially coated with laminin. We also found that cells on silanized glass coated with laminin had higher motility than those on silanized glass coated with fibronectin. Based on our results, we suggest that silanized glass is a more suitable cell culture substrate than conventional non-treated glass when coated by ECM for observations of ECM effects on cell physiology.

Aggregation state determines the localization and function of M1- and M23-aquaporin-4 in astrocytes

An aggregation state–dependent mechanism for segregation of plasma membrane protein complexes confers specific functional roles to the M1 and M23 isoforms of the water channel AQP4.

The astrocyte water channel aquaporin-4 (AQP4) is expressed as heterotetramers of M1 and M23 isoforms in which the presence of M23–AQP4 promotes formation of large macromolecular aggregates termed orthogonal arrays. Here, we demonstrate that the AQP4 aggregation state determines its subcellular localization and cellular functions. Individually expressed M1–AQP4 was freely mobile in the plasma membrane and could diffuse into rapidly extending lamellipodial regions to support cell migration. In contrast, M23–AQP4 formed large arrays that did not diffuse rapidly enough to enter lamellipodia and instead stably bound adhesion complexes and polarized to astrocyte end-feet in vivo. Co-expressed M1– and M23–AQP4 formed aggregates of variable size that segregated due to diffusional sieving of small, mobile M1–AQP4-enriched arrays into lamellipodia and preferential interaction of large, M23–AQP4-enriched arrays with the extracellular matrix. Our results therefore demonstrate an aggregation state–dependent mechanism for segregation of plasma membrane protein complexes that confers specific functional roles to M1– and M23–AQP4.

Ibuprofen inhibits migration and proliferation of human coronary artery smooth muscle cells by inducing a differentiated phenotype: role of peroxisome proliferator-activated receptor γ

OBJECTIVES

The search for agents that are capable of preventing restenosis and reduce the risk of late thrombosis is of utmost importance. In this study we aim to evaluate the in vitro effects of ibuprofen on proliferation and migration of human coronary artery smooth muscle cells and on endothelial cells.

METHODS

Cell proliferation was evaluated by trypan blue exclusion. Cell migration was assessed by wound-healing 'scratch' assay and time-lapse video microscopy. Protein expression was assessed by immunoblotting, and morphology by immunocytochemistry. The involvement of the PPARγ pathway was studied with the agonist troglitazone, and the use of selective antagonists such as PGF2α and GW9662.

KEY FINDINGS

We demonstrate that ibuprofen inhibits proliferation and migration of HCASMCs and induces a switch in HCASMCs towards a differentiated and contractile phenotype, and that these effects are mediated through the PPARγ pathway. Importantly we also show that the effects of ibuprofen are cell type-specific as it does not affect migration and proliferation of endothelial cells.

CONCLUSIONS

Taken together, our results suggest that ibuprofen could be an effective drug for the development of novel drug-eluting stents that could lead to reduced rates of restenosis and potentially other complications of DES implantation.

Myosin 1E localizes to actin polymerization sites in lamellipodia, affecting actin dynamics and adhesion formation

Because the actin network in active lamellipodia is continuously assembling at the edge, moving inward and disassembling, there is a question as to how actin-binding proteins and other components are transported to the leading edge and how nascent adhesions are stabilized. Active transport could play a significant role in these functions but the components involved are unknown. We show here that Myosin 1E (a long tailed Myosin 1 isoform) rapidly moves to the tips of active lamellipodia and to actin-rich early adhesions, unlike Myosin 1G, 1B or 1C (short tailed isoforms). Myosin 1E co-localizes with CARMIL, FHOD1, Arp3 and β3-integrin in those early adhesions. But these structures precede stable paxillin-rich adhesions. Myosin 1E movement depends upon actin-binding domains and the presence of an SH3 oligomerization domain. Overexpression of a Myosin 1E deletion mutant without the extreme C-terminal interacting (SH3) domain (Myosin 1EΔSH3) increases edge fluctuations and decreases stable adhesion lifetimes. In contrast, overexpression of Myosin 1E full tail domain (TH1+TH2+TH3/SH3) decreases edge fluctuation. In Myosin 1E knockdown cells, and more prominently in cells treated with Myosin 1 inhibitor, cell-matrix adhesions are also short-lived and fail to mature. We suggest that, by moving to actin polymerization sites and early adhesion sites in active lamellipodia, Myosin 1E might play important roles in transporting not only important polymerizing proteins but also proteins involved in adhesion stabilization.

The cellular mechanics of an invasive lifestyle

Invasive behaviour is the hallmark of a variety of cell types of animal, plant, and fungal origin. Here we review the purpose and mechanism of invasive growth and migration. The focus is on the physical principles governing the process, the source of invasive force, and the cellular mechanism by which the cell penetrates the substrate. The current experimental methods for measuring invasive force and the modelling approaches for studying invasive behaviour are explained, and future experimental strategies are proposed.

Geometric control of cell migration

Morphological polarization involving changes in cell shape and redistribution of cellular signaling machinery, initiate the migration of mammalian cells. Golgi complex typically localizes in front of the nucleus, and this frontwards polarization has been proposed to be involved in directional migration. However, the sequence of events remains unresolved. Does Golgi polarization precede directional migration or vice-versa? We address this question by constraining cells to specific areas and shapes then tracking their motile behavior and the spatio-temporal distribution of Golgi apparatus upon release. Results show that while the position of the Golgi complex depends on the cell geometry, the subcellular localization of the Golgi complex does not define the cell's leading edge. Cells constrained within elongated geometries exhibit polarized extension of lamellipodia and upon release, migrate preferentially along the long axis of the cell. Minimally constrained cells released from larger areas however, exhibit retarded migration regardless of lamellipodia protrusion activity.

Deformation gradients imprint the direction and speed of en masse fibroblast migration for fast healing

En masse cell migration is more relevant than single cell migration in physiological processes of tissue formation, such as embryogenesis, morphogenesis and wound healing. In these situations, cells are influenced by the proximity of other cells including interactions facilitated by substrate mechanics. Here we found that when fibroblasts migrated en masse over a hydrogel, they established a well-defined deformation field by traction forces and migrated along a trajectory defined by field gradients. The mechanics of the hydrogel determined the magnitude of the gradient. For materials stiff enough to withstand deformation related to cellular traction forces, such patterns did not form. Furthermore, migration patterns functioned poorly on very soft matrices where only a minimal traction gradient could be established. The largest degree of alignment and migration velocity occurred on the gels with the largest gradients. Granulation tissue formation in punch wounds of juvenile pigs was correlated strongly with the modulus of the implanted gel in agreement with in vitro en masse cell migration studies. These findings provide basic insight into the biomechanical influences on fibroblast movement in early wounds and relevant design criteria for development of tissue-engineered constructs that aim to stimulate en masse cell recruitment for rapid wound healing.

Force mapping during the formation and maturation of cell adhesion sites with multiple optical tweezers

Focal contacts act as mechanosensors allowing cells to respond to their biomechanical environment. Force transmission through newly formed contact sites is a highly dynamic process requiring a stable link between the intracellular cytoskeleton and the extracellular environment. To simultaneously investigate cellular traction forces in several individual maturing adhesion sites within the same cell, we established a custom-built multiple trap optical tweezers setup. Beads functionalized with fibronectin or RGD-peptides were placed onto the apical surface of a cell and trapped with a maximum force of 160 pN. Cells form adhesion contacts around the beads as demonstrated by vinculin accumulation and start to apply traction forces after 30 seconds. Force transmission was found to strongly depend on bead size, surface density of integrin ligands and bead location on the cell surface. Highest traction forces were measured for beads positioned on the leading edge. For mouse embryonic fibroblasts, traction forces acting on single beads are in the range of 80 pN after 5 minutes. If two beads were positioned parallel to the leading edge and with a center-to-center distance less than 10 µm, traction forces acting on single beads were reduced by 40%. This indicates a spatial and temporal coordination of force development in closely related adhesion sites. We also used our setup to compare traction forces, retrograde transport velocities, and migration velocities between two cell lines (mouse melanoma and fibroblasts) and primary chick fibroblasts. We find that maximal force development differs considerably between the three cell types with the primary cells being the strongest. In addition, we observe a linear relation between force and retrograde transport velocity: a high retrograde transport velocity is associated with strong cellular traction forces. In contrast, migration velocity is inversely related to traction forces and retrograde transport velocity.

Analytic study of three-dimensional single cell migration with and without proteolytic enzymes

Cell motility is a fundamental physiological process that regulates cellular fate in healthy and diseased systems. Cells cultured in 3D environments often exhibit biphasic dependence of migration speed with cell adhesion. Much is not understood about this very common behavior. A phenomenological model for 3D single-cell migration that exhibits biphasic behavior and highlights the important role of steric hindrance is developed and studied analytically. Changes in the biphasic behavior in the presence of proteolytic enzymes are investigated. Our methods produce a framework to determine analytic formulae for the mean cell speed, allowing general statements in terms of parameters to be explored, which will be useful when interpreting future experimental results. Our formula for mean cell speed as a function of ligand concentration generalizes and extends previous computational models that have shown good agreement with in vitro experiments.

Cell-substratum adhesion strength as a determinant of hepatocyte aggregate morphology

Cultured hepatocytes typically form multicellular aggregates which are either monolayered or spheroidal in morphology. We propose that the aggregate morphology resulting from a particular cell-substratum interaction has a biophysical basis: when cell contractile forces are greater than cell-substratum adhesion forces, spheroidal aggregates form; when cell contractile forces are weaker than cell-substratum adhesion forces, cells remain essentially spread and form monolayered aggregates. We tested this hypothesis by systematically varying the morphology of hepatocellular aggregates formed on substrata coated with a series of different concentrations of Matrigel, and correlating aggregate morphology with the cell-substratum adhesion strength measured in a shear flow detachment assay. Aggregate morphology was binary-spheroidal aggregates formed at low Matrigel concentrations and monolayered aggregates formed at high Matrigel concentrations. Cell-substratum adhesion strength was similarly binary, with low adhesion strengths correlated with spheroidal aggregates and high adhesion strengths correlated with formation of monolayered aggregates. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 415-426, 1997.

Mechanotransduction in cells

Cell-matrix and cell-cell adhesions critically influence cell metabolism, protein synthesis, cell survival, cytoskeletal architecture and consequently cell mechanical properties such as migration, spreading and contraction. An important group of adhesive transmembrane receptors that mechanically link the ECM (extracellular matrix) with the internal cytoskeleton are integrins which are intimately connected with the FAs (focal adhesions) which consists of many proteins. The transient formation of FAs is greatly augmented either through externally applied tension to the cell or internally through myosin II-driven cell contractility. Exactly which protein(s) within FAs sense, transmit and respond to mechanical stress is currently debated and numerous candidates have been proposed.

3D Traction forces in cancer cell invasion

Cell invasion through a dense three-dimensional (3D) matrix is believed to depend on the ability of cells to generate traction forces. To quantify the role of cell tractions during invasion in 3D, we present a technique to measure the elastic strain energy stored in the matrix due to traction-induced deformations. The matrix deformations around a cell were measured by tracking the 3D positions of fluorescent beads tightly embedded in the matrix. The bead positions served as nodes for a finite element tessellation. From the strain in each element and the known matrix elasticity, we computed the local strain energy in the matrix surrounding the cell. We applied the technique to measure the strain energy of highly invasive MDA-MB-231 breast carcinoma and A-125 lung carcinoma cells in collagen gels. The results were compared to the strain energy generated by non-invasive MCF-7 breast and A-549 lung carcinoma cells. In all cases, cells locally contracted the matrix. Invasive breast and lung carcinoma cells showed a significantly higher contractility compared to non-invasive cells. Higher contractility, however, was not universally associated with higher invasiveness. For instance, non-invasive A-431 vulva carcinoma cells were the most contractile cells among all cell lines tested. As a universal feature, however, we found that invasive cells assumed an elongated spindle-like morphology as opposed to a more spherical shape of non-invasive cells. Accordingly, the distribution of strain energy density around invasive cells followed patterns of increased complexity and anisotropy. These results suggest that not so much the magnitude of traction generation but their directionality is important for cancer cell invasion.

A silicone-based stretchable micropost array membrane for monitoring live-cell subcellular cytoskeletal response

External forces are increasingly recognized as major regulators of cellular structure and function, yet the underlying mechanism by which cells sense forces and transduce them into intracellular biochemical signals and behavioral responses ('mechanotransduction') is largely undetermined. To aid in the mechanistic study of mechanotransduction, herein we devised a cell stretching device that allowed for quantitative control and real-time measurement of mechanical stimuli and cellular biomechanical responses. Our strategy involved a microfabricated array of silicone elastomeric microposts integrated onto a stretchable elastomeric membrane. Using a computer-controlled vacuum, this micropost array membrane (mPAM) was activated to apply equibiaxial cell stretching forces to adherent cells attached to the microposts. Using the mPAM, we studied the live-cell subcellular dynamic responses of contractile forces in vascular smooth muscle cells (VSMCs) to a sustained static equibiaxial cell stretch. Our data showed that in response to a sustained cell stretch, VSMCs regulated their cytoskeletal (CSK) contractility in a biphasic manner: they first acutely enhanced their contraction to resist rapid cell deformation ('stiffening') before they allowed slow adaptive inelastic CSK reorganization to release their contractility ('softening'). The contractile response across entire single VSMCs was spatially inhomogeneous and force-dependent. Our mPAM device and live-cell subcellular contractile measurements will help elucidate the mechanotransductive system in VSMCs and thus contribute to our understanding of pressure-induced vascular disease processes.

Force- and Ca2+-dependent internalization of integrins in cultured endothelial cells

The effects of mechanical force applied to the integrin clusters at focal contacts were examined in cultured human umbilical vein endothelial cells. When a fibronectin-coated glass bead was attached to the apical cell surface, focal contacts formed beneath the bead that became linked to focal contacts at the basal cell membrane by actin stress fibers in 5 minutes. Integrin dynamics at the basal focal contacts were monitored in live cells in response to a localized mechanical stimulus generated by displacing the glass bead. Traction force transmitted to the basal focal contacts through the stress fibers was monitored by measuring the deformation of the polyacrylamide gel substratum. The force declined in a few seconds, probably owing to decreases in the elastic modulus of the stress fibers. This transient mechanical stimulus caused the dephosphorylation of paxillin and disassembly of integrin clusters at the basal cell membrane in 20 minutes. The disassembly was mediated mainly by clathrin-dependent endocytosis of integrins. The integrin internalization was inhibited in Ca2+- and K+-free solution, and by phenylarsine oxide, a phosphatase inhibitor. These results suggest that a transient mechanical stimulus applied to focal contacts induces Ca2+-dependent dephosphorylation of some proteins, including paxillin, and facilitates clathrin-dependent endocytosis of integrins.

Effective drift and diffusion of a particle jumping between mobile and immobile states

We study propagation of a particle that jumps between two states, in which it moves with different velocities and diffusion coefficients. To simplify analysis, in the main part of the paper we derive formulas assuming that in one of the states the particle is immobile. A generalization to the case when the particle is mobile in both states is given at the end of the paper. The formulas show how the effective drift velocity and effective diffusion coefficient depend on jump rates between the two states as well as on the particle velocities and diffusion coefficients in these states. Specifically, we find that the effective diffusion coefficient can exhibit a non-monotonic behavior as a function of the ratio of the jump rates.

Integrins in cell migration

Integrin-based adhesion has served as a model for studying the central role of adhesion in migration. In this article, we outline modes of migration, both integrin-dependent and -independent in vitro and in vivo. We next discuss the roles of adhesion contacts as signaling centers and linkages between the ECM and actin that allows adhesions to serve as traction sites. This includes signaling complexes that regulate migration and the interplay among adhesion, signaling, and pliability of the substratum. Finally, we address mechanisms of adhesion assembly and disassembly and the role of adhesion in cellular polarity.

Dynamic molecular processes mediate cellular mechanotransduction

Cellular responses to mechanical forces are crucial in embryonic development and adult physiology, and are involved in numerous diseases, including atherosclerosis, hypertension, osteoporosis, muscular dystrophy, myopathies and cancer. These responses are mediated by load-bearing subcellular structures, such as the plasma membrane, cell-adhesion complexes and the cytoskeleton. Recent work has demonstrated that these structures are dynamic, undergoing assembly, disassembly and movement, even when ostensibly stable. An emerging insight is that transduction of forces into biochemical signals occurs within the context of these processes. This framework helps to explain how forces of varying strengths or dynamic characteristics regulate distinct signalling pathways.

Quantification and regulation of cell migration

Cell migration is essential for many physiological and pathological processes that include embryonic development, the immune response, wound healing, angiogenesis, and cancer metastasis. It is also important for emerging tissue engineering applications such as tissue reconstitution and the colonization of biomedical implants. By summarizing results from recent experimental and theoretical studies, this review outlines the role played by growth factors or substrate-adhesion molecules in modulating cell motility and shows that cell motility can be an important factor in determining the rates of tissue formation. The application of cell motility assays and the use of theoretical models for analyzing cell migration and proliferation are also discussed.

Forcing a connection: impacts of single-molecule force spectroscopy on in vivo tension sensing

Mechanical tension plays a large role in cell development ranging from morphology to gene expression. On the molecular level, the effects of tension can be seen in the dynamic arrangement of membrane proteins as well as the recruitment and activation of intracellular proteins. Forces applied to biopolymers during in vitro force measurements offer greater understanding of the effects of tension on molecules in live cells, and experimental techniques involving test tubes and live cells can often overlap. Indeed, when forces exerted on cellular components can be calibrated ex vivo with force spectroscopy, a powerful tool is available for researchers in probing cellular mechanotransduction on the molecular scale. This review will discuss the techniques used in measuring both cellular traction forces and single-molecule force spectroscopy. Emphasis will be placed on the use of fluorescence reporter systems for the development of in vivo tension sensors that can be used for calibration with single molecule force methods.

Stretchy proteins on stretchy substrates: the important elements of integrin-mediated rigidity sensing

Matrix and tissue rigidity guides many cellular processes, including the differentiation of stem cells and the migration of cells in health and disease. Cells actively and transiently test rigidity using mechanisms limited by inherent physical parameters that include the strength of extracellular attachments, the pulling capacity on these attachments, and the sensitivity of the mechanotransduction system. Here, we focus on rigidity sensing mediated through the integrin family of extracellular matrix receptors and linked proteins and discuss the evidence supporting these proteins as mechanosensors.

When Cre-mediated recombination in mice does not result in protein loss

Cre/loxP recombination enables cellular specificity and, in the case of inducible systems, temporal control of genomic deletions. Here we used a SM22α tamoxifen-inducible Cre line to inactivate β1 integrin in adult smooth muscle. Interestingly, analysis of two distinct β1 loxP transgenic mice revealed vastly different outcomes after β1 integrin deletion. Lethality occurred 4 weeks postinduction in one Cre/loxP line, while no apparent phenotype was seen in the other line. Genetic analysis revealed appropriate DNA excision in both cases; however, differences were found in the degree of protein loss with absolutely no change in protein levels in the model that lacked a phenotype. Seeking to understand protein persistence despite appropriate recombination, we first validated the flox allele using a constitutive Cre line and demonstrated its ability to mediate effective protein inactivation. We then examined the possibility of heterozygous cell selection, protein turnover, and deletion efficiency with no success for explaining the phenotype. Finally, we documented the presence of the Cre-recombination episomal product, which persisted in tissue samples with no protein loss. The product was only noted in cells with low proliferative capacity. These findings highlight the potential for protein expression from the products of Cre-recombinase excised genes, particularly when deletion occurs in low turnover populations.

Cell migration at the interface of a dual chemical-mechanical gradient

Cell migration plays a critical role in numerous physiological processes, such as wound healing, response to inflammation, and cancer metastasis. In recent years, accumulating evidence indicates that cell movement is regulated not only by chemical signals but also by mechanical stimuli. In this study, the primary goal is to identify whether a chemical or mechanical stimulus plays the decisive role in directing cell migration. Measuring the motility of cells when they are presented with a combination of chemical and mechanical cues will provide insight into the complex physiological phenomena that guide and direct migration. A novel polyacrylamide hydrogel was designed with an interfacial region where the chemical and mechanical properties varied in opposing directions. One side of the interface was stiff (high Young's modulus) with a low protein concentration, whereas the other side of the interface was compliant (low Young's modulus) with a high protein concentration. The chemical gradient was created by varying the collagen (type I) concentration and the mechanical gradient was introduced by changing the extent of cross-linking in the polymer. The length of the interface with opposing chemical-mechanical profiles was found to be approximately 100 mum. Our results demonstrate that when Balb/c 3T3 fibroblasts were presented with a choice, they either migrated preferentially toward the high-collagen-compliant (low Young's modulus) side of the interfacial region or remained on the high-collagen region, suggesting a more dominant role for chemical stimuli in directing fibroblast locomotion.

Mechanics rules cell biology

Cells in the musculoskeletal system are subjected to various mechanical forces in vivo. Years of research have shown that these mechanical forces, including tension and compression, greatly influence various cellular functions such as gene expression, cell proliferation and differentiation, and secretion of matrix proteins. Cells also use mechanotransduction mechanisms to convert mechanical signals into a cascade of cellular and molecular events. This mini-review provides an overview of cell mechanobiology to highlight the notion that mechanics, mainly in the form of mechanical forces, dictates cell behaviors in terms of both cellular mechanobiological responses and mechanotransduction.

Fluctuations of cytoskeleton-bound microbeads--the effect of bead-receptor binding dynamics

The cytoskeleton (CSK) of living cells is a crosslinked fiber network, subject to ongoing biochemical remodeling processes that can be visualized by tracking the spontaneous motion of CSK-bound microbeads. The bead motion is characterized by anomalous diffusion with a power-law time evolution of the mean square displacement (MSD), and can be described as a stochastic transport process with apparent diffusivity D and power-law exponent β: MSD ∼ D (t/t(0))(β). Here we studied whether D and β change with the time that has passed after the initial bead-cell contact, and whether they are sensitive to bead coating (fibronectin, integrin antibodies, poly-L-lysine, albumin) and bead size (0.5-4.5 µm). The measurements are interpreted in the framework of a simple model that describes the bead as an overdamped particle coupled to the fluctuating CSK network by an elastic spring. The viscous damping coefficient characterizes the degree of bead internalization into the cell, and the spring constant characterizes the strength of the binding of the bead to the CSK. The model predicts distinctive signatures of the MSD that change with time as the bead couples more tightly to the CSK and becomes internalized. Experimental data show that the transition from the unbound to the tightly bound state occurs in an all-or-nothing manner. The time point of this transition shows considerable variability between individual cells (2-30 min) and depends on the bead size and bead coating. On average, this transition occurs later for smaller beads and beads coated with ligands that trigger the formation of adhesion complexes (fibronectin, integrin antibodies). Once the bead is linked to the CSK, however, the ligand type and bead size have little effect on the MSD. On longer timescales of several hours after bead addition, smaller beads are internalized into the cell more readily, leading to characteristic changes in the MSD that are consistent with increased viscous damping by the cytoplasm and reduced binding strength.

A new lock-step mechanism of matrix remodelling based on subcellular contractile events

Myofibroblasts promote tissue contractures during fibrotic diseases. To understand how spontaneous changes in the intracellular calcium concentration, [Ca(2+)](i), contribute to myofibroblast contraction, we analysed both [Ca(2+)](i) and subcellular contractions. Contractile events were assessed by tracking stress-fibre-linked microbeads and measured by atomic force microscopy. Myofibroblasts exhibit periodic (approximately 100 seconds) [Ca(2+)](i) oscillations that control small (approximately 400 nm) and weak (approximately 100 pN) contractions. Whereas depletion of [Ca(2+)](i) reduces these microcontractions, cell isometric tension is unaffected, as shown by growing cells on deformable substrates. Inhibition of Rho- and ROCK-mediated Ca(2+)-independent contraction has no effect on microcontractions, but abolishes cell tension. On the basis of this two-level regulation of myofibroblast contraction, we propose a single-cell lock-step model. Rho- and ROCK-dependent isometric tension generates slack in extracellular matrix fibrils, which are then accessible for the low-amplitude and high-frequency contractions mediated by [Ca(2+)](i). The joint action of both contraction modes can result in macroscopic tissue contractures of approximately 1 cm per month.

Computational study of proteolysis-driven single cell migration in a three-dimensional matrix

Cell migration is a fundamental process that is crucial to a variety of physiological events. While traditional approaches have focused on two-dimensional (2D) systems, recent efforts have shifted to studying migration in three-dimensional (3D) matrices. A major distinction that has emerged is the increased importance of cell-matrix interactions in 3D environments. In particular, cell motility in 3D matrices is more dependent on matrix metalloproteinases (MMPs) to degrade steric obstacles than in 2D systems. In this study, we implement the effects of MMP-mediated proteolysis in a force-based computational model of 3D migration, testing two matrix ligand-MMP relationships that have been observed experimentally: linear and log-linear. The model for both scenarios predicts maximal motility at intermediate matrix ligand and MMP levels, with the linear case providing more physiologically compelling results. Recent experimental results suggesting MMP influence on integrin expression are also integrated into the model. While the biphasic behavior is retained, with MMP-integrin feedback peak cell speed is observed in a low ligand, high MMP regime instead of at intermediate ligand and MMP levels for both ligand-MMP relationships. The simulation provides insight into the expanding role of cell-matrix interactions in cell migration in 3D environments and has implications for cancer research.