Vascular smooth muscle cell durotaxis depends on substrate stiffness gradient strength

Substrate elasticity regulates the behavior of human monocyte-derived macrophages

Macrophages play a key role in atherosclerosis, cancer, and in the response to implanted medical devices. In each of these situations, the mechanical environment of a macrophage can vary from soft to stiff. However, how stiffness affects macrophage behavior remains uncertain. Using substrates of varying stiffness, we show macrophage phenotype and function depends on substrate stiffness. Notably, the cell area increases slightly from a sphere after 18 h on substrates mimicking healthy arterial stiffness (1-5 kPa), whereas macrophages on stiffer substrates (280 kPa-70 GPa) increased in area by nearly eight-fold. Macrophage migration is random regardless of substrate stiffness. The total average track speed was 7.8 ± 0.5 μm/h, with macrophages traveling fastest on the 280-kPa substrate (12.0 ± 0.5 μm/h) and slowest on the 3-kPa substrate (5.0 ± 0.4 μm/h). In addition F-actin organization in macrophages depends on substrate stiffness. On soft substrates, F-actin is spread uniformly throughout the cytoplasm, whereas on stiff substrates F-actin is functionalized into stress fibers. The proliferation rate of macrophages was faster on stiff substrates. Cells plated on the 280-kPa gel had a significantly shorter doubling time than those plated on the softer substrate. However, the ability of macrophages to phagocytose 1-μm particles did not depend on substrate stiffness. In conclusion, the results herein show macrophages are mechanosensitive; they respond to changes in stiffness by modifying their area, migration speed, actin organization, and proliferation rate. These results are important to understanding how macrophages respond in complex mechanical environments such as an atherosclerotic plaque.

A genetic variant of cortactin linked to acute lung injury impairs lamellipodia dynamics and endothelial wound healing

Inflammatory mediators released in acute lung injury (ALI) trigger the disruption of interendothelial junctions, leading to loss of vascular barrier function, protein-rich pulmonary edema, and severe hypoxemia. Genetic signatures that predict patient recovery or disease progression are poorly defined, but recent genetic screening of ALI patients has identified an association between lung inflammatory disease and a single nucleotide polymorphism (SNP) in the gene for the actin-binding and barrier-regulatory protein cortactin. This study investigated the impact of this disease-linked cortactin variant on wound healing processes that may contribute to endothelial barrier restoration. A microfabricated platform was used to quantify wound healing in terms of gap closure speed, lamellipodia dynamics, and cell velocity. Overexpression of wild-type cortactin in endothelial cells (ECs) improved directional cell motility and enhanced lamellipodial protrusion length, resulting in enhanced gap closure rates. By contrast, the cortactin SNP impaired wound closure and cell locomotion, consistent with the observed reduction in lamellipodial protrusion length and persistence. Overexpression of the cortactin SNP in lung ECs mitigated the barrier-enhancing activity of sphingosine 1-phosphate. These findings suggest that this common cortactin variant may functionally contribute to ALI predisposition by impeding endothelial wound healing.

Actin flow and talin dynamics govern rigidity sensing in actin-integrin linkage through talin extension

At cell-substrate adhesion sites, the linkage between actin filaments and integrin is regulated by mechanical stiffness of the substrate. Of potential molecular regulators, the linker proteins talin and vinculin are of particular interest because mechanical extension of talin induces vinculin binding with talin, which reinforces the actin-integrin linkage. For understanding the molecular and biophysical mechanism of rigidity sensing at cell-substrate adhesion sites, we constructed a simple physical model to examine a role of talin extension in the stiffness-dependent regulation of actin-integrin linkage. We show that talin molecules linking between retrograding actin filaments and substrate-bound integrin are extended in a manner dependent on substrate stiffness. The model predicts that, in adhesion complexes containing ≈30 talin links, talin is extended enough for vinculin binding when the substrate is stiffer than 1 kPa. The lifetime of talin links needs to be 2-5 s to achieve an appropriate response of talin extension against substrate stiffness. Furthermore, changes in actin velocity drastically shift the range of substrate stiffness that induces talin-vinculin binding. Our results suggest that talin extension is a key step in sensing and responding to substrate stiffness at cell adhesion sites.

Cooperative effects of matrix stiffness and fluid shear stress on endothelial cell behavior

Arterial hemodynamic shear stress and blood vessel stiffening both significantly influence the arterial endothelial cell (EC) phenotype and atherosclerosis progression, and both have been shown to signal through cell-matrix adhesions. However, the cooperative effects of fluid shear stress and matrix stiffness on ECs remain unknown. To investigate these cooperative effects, we cultured bovine aortic ECs on hydrogels matching the elasticity of the intima of compliant, young, or stiff, aging arteries. The cells were then exposed to laminar fluid shear stress of 12 dyn/cm(2). Cells grown on more compliant matrices displayed increased elongation and tighter EC-cell junctions. Notably, cells cultured on more compliant substrates also showed decreased RhoA activation under laminar shear stress. Additionally, endothelial nitric oxide synthase and extracellular signal-regulated kinase phosphorylation in response to fluid shear stress occurred more rapidly in ECs cultured on more compliant substrates, and nitric oxide production was enhanced. Together, our results demonstrate that a signaling cross talk between stiffness and fluid shear stress exists within the vascular microenvironment, and, importantly, matrices mimicking young and healthy blood vessels can promote and augment the atheroprotective signals induced by fluid shear stress. These data suggest that targeting intimal stiffening and/or the EC response to intima stiffening clinically may improve vascular health.

Microglia mechanics: immune activation alters traction forces and durotaxis

Microglial cells are key players in the primary immune response of the central nervous system. They are highly active and motile cells that chemically and mechanically interact with their environment. While the impact of chemical signaling on microglia function has been studied in much detail, the current understanding of mechanical signaling is very limited. When cultured on compliant substrates, primary microglial cells adapted their spread area, morphology, and actin cytoskeleton to the stiffness of their environment. Traction force microscopy revealed that forces exerted by microglia increase with substrate stiffness until reaching a plateau at a shear modulus of ~5 kPa. When cultured on substrates incorporating stiffness gradients, microglia preferentially migrated toward stiffer regions, a process termed durotaxis. Lipopolysaccharide-induced immune-activation of microglia led to changes in traction forces, increased migration velocities and an amplification of durotaxis. We finally developed a mathematical model connecting traction forces with the durotactic behavior of migrating microglial cells. Our results demonstrate that microglia are susceptible to mechanical signals, which could be important during central nervous system development and pathologies. Stiffness gradients in tissue surrounding neural implants such as electrodes, for example, could mechanically attract microglial cells, thus facilitating foreign body reactions detrimental to electrode functioning.

Stem cell mechanobiology: diverse lessons from bone marrow

A stem cell niche is defined by various chemical and physical features that influence whether a stem cell remains quiescent, divides, or differentiates. We review mechanical determinants that affect cell fate through actomyosin forces, nucleoskeleton remodeling, and mechanosensitive translocation of transcription factors. Current methods for physical characterization of tissue microenvironments are summarized together with efforts to recapitulate niche mechanics in culture. We focus on mesenchymal stem cells, particularly in osteogenesis and adipogenesis, and on blood stem cells - both of which reside in mechanically diverse marrow microenvironments. Given the explosion of efforts with pluripotent stem cells, the evident mechanosensitivity of clinically relevant, multipotent marrow cells underscores an increasing need to examine and understand in vivo and in vitro physical properties on length scales that cells sense.

Vascular smooth muscle cell functional contractility depends on extracellular mechanical properties

Vascular smooth muscle cells' primary function is to maintain vascular homeostasis through active contraction and relaxation. In diseases such as hypertension and atherosclerosis, this function is inhibited concurrent to changes in the mechanical environment surrounding vascular smooth muscle cells. It is well established that cell function and extracellular mechanics are interconnected; variations in substrate modulus affect cell migration, proliferation, and differentiation. To date, it is unknown how the evolving extracellular mechanical environment of vascular smooth muscle cells affects their contractile function. Here, we have built upon previous vascular muscular thin film technology to develop a variable-modulus vascular muscular thin film that measures vascular tissue functional contractility on substrates with a range of pathological and physiological moduli. Using this modified vascular muscular thin film, we found that vascular smooth muscle cells generated greater stress on substrates with higher moduli compared to substrates with lower moduli. We then measured protein markers typically thought to indicate a contractile phenotype in vascular smooth muscle cells and found that phenotype is unaffected by substrate modulus. These data suggest that mechanical properties of vascular smooth muscle cells' extracellular environment directly influence their functional behavior and do so without inducing phenotype switching.

Review of cellular mechanotransduction on micropost substrates

As physical entities, living cells can sense and respond to various stimulations within and outside the body through cellular mechanotransduction. Any deviation in cellular mechanotransduction will not only undermine the orchestrated regulation of mechanical responses, but also lead to the breakdown of their physiological function. Therefore, a quantitative study of cellular mechanotransduction needs to be conducted both in experiments and in computational simulations to investigate the underlying mechanisms of cellular mechanotransduction. In this review, we present an overview of the current knowledge and significant progress in cellular mechanotransduction via micropost substrates. In the aspect of experimental studies, we summarize significant experimental progress and place an emphasis on the coupled relationship among cellular spreading, focal adhesion and contractility as well as the influence of substrate properties on force-involved cellular behaviors. In the other aspect of computational investigations, we outline a coupled framework including the biochemically motivated stress fiber model and thermodynamically motivated adhesion model and present their predicted biomechanical responses and then compare predicted simulation results with experimental observations to further explore the mechanisms of cellular mechanotransduction. At last, we discuss the future perspectives both in experimental technologies and in computational models, as well as facing challenges in the area of cellular mechanotransduction.

Cell-Substrate Interactions Feedback to Direct Cell Migration along or against Morphological Polarization

In response to external stimuli, cells polarize morphologically into teardrop shapes prior to moving in the direction of their blunt leading edge through lamellipodia extension and retraction of the rear tip. This textbook description of cell migration implies that the initial polarization sets the direction of cell migration. Using microfabrication techniques to control cell morphologies and the direction of migration without gradients, we demonstrate that after polarization, lamelipodia extension and attachment can feedback to change and even reverse the initial morphological polarization. Cells do indeed migrate faster in the direction of their morphologically polarization. However, feedback from subsequent lamellipodia extension and attachment can be so powerful as to induce cells to reverse and migrate against their initial polarization, albeit at a slower speed. Constitutively active mutants of RhoA show that RhoA stimulates cell motility when cells are guided either along or against their initial polarization. Cdc42 activation and inhibition, which results in loss of directional motility during chemotaxis, only reduces the speed of migration without altering the directionality of migration on the micropatterns. These results reveal significant differences between substrate directed cell migration and that induced by chemotactic gradients.

Age-related vascular stiffening: causes and consequences

Arterial stiffening occurs with age and is closely associated with the progression of cardiovascular disease. Stiffening is most often studied at the level of the whole vessel because increased stiffness of the large arteries can impose increased strain on the heart leading to heart failure. Interestingly, however, recent evidence suggests that the impact of increased vessel stiffening extends beyond the tissue scale and can also have deleterious microscale effects on cellular function. Altered extracellular matrix (ECM) architecture has been recognized as a key component of the pre-atherogenic state. Here, the underlying causes of age-related vessel stiffening are discussed, focusing on age-related crosslinking of the ECM proteins as well as through increased matrix deposition. Methods to measure vessel stiffening at both the macro- and microscale are described, spanning from the pulse wave velocity measurements performed clinically to microscale measurements performed largely in research laboratories. Additionally, recent work investigating how arterial stiffness and the changes in the ECM associated with stiffening contributed to endothelial dysfunction will be reviewed. We will highlight how changes in ECM protein composition contribute to atherosclerosis in the vessel wall. Lastly, we will discuss very recent work that demonstrates endothelial cells (ECs) are mechano-sensitive to arterial stiffening, where changes in stiffness can directly impact EC health. Overall, recent studies suggest that stiffening is an important clinical target not only because of potential deleterious effects on the heart but also because it promotes cellular level dysfunction in the vessel wall, contributing to a pathological atherosclerotic state.

Hydrogel-based methods for engineering cellular microenvironment with spatiotemporal gradients

Natural cellular microenvironment consists of spatiotemporal gradients of multiple physical (e.g. extracellular matrix stiffness, porosity and stress/strain) and chemical cues (e.g. morphogens), which play important roles in regulating cell behaviors including spreading, proliferation, migration, differentiation and apoptosis, especially for pathological processes such as tumor formation and progression. Therefore, it is essential to engineer cellular gradient microenvironment incorporating various gradients for the fabrication of normal and pathological tissue models in vitro. In this article, we firstly review the development of engineering cellular physical and chemical gradients with cytocompatible hydrogels in both two-dimension and three-dimension formats. We then present current advances in the application of engineered gradient microenvironments for the fabrication of disease models in vitro. Finally, concluding remarks and future perspectives for engineering cellular gradients are given.

Measurement systems for cell adhesive forces

Cell adhesion to the extracellular matrix (ECM) involves integrin receptor-ligand binding and clustering to form focal adhesion (FA) complexes, which mechanically link the cell's cytoskeleton to the ECM and regulate fundamental cell signaling pathways. Although elucidation of the biochemical events in cell-matrix adhesive interactions is rapidly advancing, recent studies show that the forces underlying cell-matrix adhesive interactions are also critical to cell responses. Therefore, multiple measurement systems have been developed to quantify the spatial and temporal dynamics of cell adhesive forces, and these systems have identified how mechanical events influence cell phenotype and FA structure-function relationships under physiological and pathological settings. This review focuses on the development, methodology, and applications of measurement systems for probing (a) cell adhesion strength and (b) 2D and 3D cell traction forces.

A 3D tension bioreactor platform to study the interplay between ECM stiffness and tumor phenotype

Extracellular matrix (ECM) structure, composition, and stiffness have profound effects on tissue development and pathologies such as cardiovascular disease and cancer. Accordingly, a variety of synthetic hydrogel systems have been designed to study the impact of ECM composition, density, mechanics, and topography on cell and tissue phenotype. However, these synthetic systems fail to accurately recapitulate the biological properties and structure of the native tissue ECM. Natural three dimensional (3D) ECM hydrogels, such as collagen or hyaluronic acid, feature many of the chemical and physical properties of tissue, yet, these systems have limitations including the inability to independently control biophysical properties such as stiffness and pore size. Here, we present a 3D tension bioreactor system that permits precise mechanical tuning of collagen hydrogel stiffness, while maintaining consistent composition and pore size. We achieve this by mechanically loading collagen hydrogels covalently-conjugated to a polydimethylsiloxane (PDMS) membrane to induce hydrogel stiffening. We validated the biological application of this system with oncogenically transformed mammary epithelial cell organoids embedded in a 3D collagen I hydrogel, either uniformly stiffened or calibrated to create a gradient of ECM stiffening, to visually demonstrate the impact of ECM stiffening on transformation and tumor cell invasion. As such, this bioreactor presents the first tunable 3D natural hydrogel system that is capable of independently assessing the role of ECM stiffness on tissue phenotype.

Impact of dimensionality and network disruption on microrheology of cancer cells in 3D environments

Dimensionality is a fundamental component that can have profound implications on the characteristics of physical systems. In cell biology, however, the majority of studies on cell physical properties, from rheology to force generation to migration, have been performed on 2D substrates, and it is not clear how a more realistic 3D environment influences cell properties. Here, we develop an integrated approach and demonstrate the combination of mitochondria-tracking microrheology, microfluidics, and Brownian dynamics simulations to explore the impact of dimensionality on intracellular mechanics and on the effects of intracellular disruption. Additionally, we consider both passive thermal and active motor-driven processes within the cell and demonstrate through modeling how active internal fluctuations are modulated via dimensionality. Our results demonstrate that metastatic breast cancer cells (MDA-MB-231) exhibit more solid-like internal motions in 3D compared to 2D, and actin network disruption via Cytochalasin D has a more pronounced effect on internal cell fluctuations in 2D. Our computational results and modeling show that motor-induced active stress fluctuations are enhanced in 2D, leading to increased local intracellular particle fluctuations and apparent fluid-like behavior.

Biomechanical properties at the cellular and subcellular levels are important in providing proper biological functions, from cell migratory capabilities to intracellular transport. Deregulation in these properties can lead to disease states such as cancer metastasis. We develop and demonstrate an integrated experimental and computational approach to study intracellular mechanics. We demonstrate that a key environmental factor, dimensionality, plays a significant role in modulating intracellular mechanical behavior. This is important as typical cell biology and mechanics experiments are performed on 2D substrates, which do not capture the physiological features of 3D matrices and may not induce physiologically accurate cell properties. We further develop an effective temperature model to describe how dimensionality changes intracellular particle motion by altering the activity of molecular motors.

Extracellular matrix presentation modulates vascular smooth muscle cell mechanotransduction

The development of atherosclerosis involves phenotypic changes among vascular smooth muscle cells (VSMCs) that correlate with stiffening and remodeling of the extracellular matrix (ECM). VSMCs are highly sensitive to the composition and mechanical state of the surrounding ECM, and ECM remodeling during atherosclerosis likely contributes to pathology. We hypothesized that ECM mechanics and biochemistry are interdependent in their regulation of VSMC behavior and investigated the effect of ligand presentation on certain stiffness-mediated processes. Our findings demonstrate that substrate stiffening is not a unidirectional stimulus-instead, the influence of mechanics on cell behavior is highly conditioned on ligand biochemistry. This "stiffness-by-ligand" effect was evident for VSMC adhesion, spreading, cytoskeletal polymerization, and focal adhesion assembly, where VSMCs cultured on fibronectin (Fn)-modified substrates showed an augmented response to increasing stiffness, whereas cells on laminin (Ln) substrates showed a dampened response. By contrast, cells on Fn substrates showed a decrease in myosin light chain (MLC) phosphorylation and elongation with increasing stiffness, whereas Ln supported an increase in MLC phosphorylation and no change in cell shape with increasing stiffness. Taken together, these findings show that identical cell populations exhibit opposing responses to substrate stiffening depending on ECM presentation. Our results also suggest that the shift in VSMC phenotype in a developing atherosclerotic lesion is jointly regulated by stromal mechanics and biochemistry. This study highlights the complex influence of the blood vessel wall microenvironment on VSMC phenotype and provides insight into how cells may integrate ECM biochemistry and mechanics during normal and pathological tissue function.

A novel 2.5D culture platform to investigate the role of stiffness gradients on adhesion-independent cell migration

Current studies investigating the role of biophysical cues on cell migration focus on the use of culture platforms with static material parameters. However, migrating cells invivo often encounter spatial variations in extracellular matrix stiffness. To better understand the effects of stiffness gradients on cell migration, we developed a 2.5D cell culture platform where cells are sandwiched between stiff tissue culture plastic and soft alginate hydrogel. Under these conditions, we observed migration of cells from the underlying stiff substrate into the alginate matrix. Observation of migration into alginate in the presence of integrin inhibition as well as qualitative microscopic analyses suggested an adhesion-independent cell migration mode. Observed migration was dependent on alginate matrix stiffness and the RhoA-ROCK-myosin-II pathway; inhibitors specifically targeting ROCK and myosin-II arrested cell migration. Collectively, these results demonstrate the utility of the 2.5D culture platform to advance our understanding of the effects of stiffness gradients and mechanotransductive signaling on adhesion-independent cell migration.

Static axial stretching enhances the mechanical properties and cellular responses of fibrin microthreads

Fibrin microthreads are a platform technology that can be used for a variety of applications, and therefore the mechanical requirements of these microthreads differ for each tissue or device application. To develop biopolymer microthreads with tunable mechanical properties, we analyzed fibrin microthread processing conditions to strengthen the scaffold materials without the use of exogenous crosslinking agents. Fibrin microthreads were extruded, dried, rehydrated and static axially stretched 0-200% of their original lengths; then the mechanical and structural properties of the microthreads were assessed. Stretching significantly increased the tensile strength of microthreads 3-fold, yielding scaffolds with tensile strengths and stiffnesses that equaled or exceeded values reported previously for carbodiimide crosslinked threads without affecting intrinsic material properties such as strain hardening or Poisson's ratio. Interestingly, these stretching conditions did not affect the rate of proteolytic degradation of the threads. The swelling ratios of stretched microthreads decreased, and scanning electron micrographs showed increases in grooved topography with increased stretch, suggesting that stretching may increase the fibrillar alignment of fibrin fibrils. The average cell alignment with respect to the longitudinal axis of the microthreads increased 2-fold with increased stretch, further supporting the hypothesis that stretching microthreads increases the alignment of fibrin fibrils on the surfaces of the scaffolds. Together, these data suggest that stretching fibrin microthreads generates stronger materials without affecting their proteolytic stability, making stretched microthreads ideal for implantable scaffolds that require short degradation times and large initial loading properties. Further modifications to stretched microthreads, such as carbodiimide crosslinking, could generate microthreads to direct cell orientation and align tissue deposition, with additional resistance to degradation for use as a long-term scaffold for tissue regeneration.

Lipid rafts direct macrophage motility in the tissue microenvironment

Infiltrating leukocytes are exposed to a wide range of tissue elasticities. While we know the effects of substrate elasticity on acute inflammation via the study of neutrophil migration, we do not know its effects on leukocytes that direct chronic inflammatory events. Here, we studied morphology and motility of macrophages, the innate immune cells that orchestrate acute and chronic inflammation, on polyacrylamide hydrogels that mimicked a wide range of tissue elasticities. As expected, we found that macrophage spreading area increased as substrate elasticity increased. Unexpectedly, we found that morphology did not inversely correlate with motility. In fact, velocity of steady-state macrophages remained unaffected by substrate elasticity, while velocity of biologically stimulated macrophages was limited on stiff substrates. We also found that the lack of motility on stiff substrates was due to a lack of lipid rafts on the leading edge of the macrophages. This study implicates lipid rafts in the mechanosensory mechanism of innate immune cell infiltration.

The contractile strength of vascular smooth muscle myocytes is shape dependent

Vascular smooth muscle cells in muscular arteries are more elongated than those in elastic arteries. Previously, we reported changes in the contractility of engineered vascular smooth muscle tissue that appeared to be correlated with the shape of the constituent cells, supporting the commonly held belief that elongated muscle geometry may allow for the better contractile tone modulation required in response to changes in blood flow and pressure. To test this hypothesis more rigorously, we developed an in vitro model by engineering human vascular smooth muscle cells to take on the same shapes as those seen in elastic and muscular arteries and measured their contraction during stimulation with endothelin-1. We found that in the engineered cells, actin alignment and nuclear eccentricity increased as the shape of the cell elongated. Smooth muscle cells with elongated shapes exhibited lower contractile strength but greater percentage increase in contraction after endothelin-1 stimulation. We analysed the relationship between smooth muscle contractility and subcellular architecture and found that changes in contractility were correlated with actin alignment and nuclear shape. These results suggest that elongated smooth muscle cells facilitate muscular artery tone modulation by increasing its dynamic contractile range.

Design and utilization of macrophage and vascular smooth muscle cell co-culture systems in atherosclerotic cardiovascular disease investigation

Atherosclerotic cardiovascular disease has been acknowledged as a chronic inflammatory condition. Monocytes and macrophages lead the inflammatory pathology of atherosclerosis whereas changes in atheromatous plaque thickness and matrix composition are attributed to vascular smooth muscle cells. Because these cell types are key players in atherosclerosis progression, it is crucial to utilize a reliable system to investigate their interaction. In vitro co-culture systems are useful platforms to study specific molecular mechanisms between cells. This review aims to summarize the various co-culture models that have been developed to investigate vascular smooth muscle cell and monocyte/macrophage interactions, focusing on the monocyte/macrophage effects on vascular smooth muscle cell function.

Microscale patterning of hydrogel stiffness through light-triggered uncaging of thiols

Mammalian cell behavior is strongly influenced by physical and chemical cues originating from the extracellular matrix (ECM). In vivo, ECM signals are displayed in a spatiotemporally complex fashion, often composed as gradients and in concentration profiles that change in time. Most in vitro models to study the role of ECM signals in regulating cell behavior are limited in capturing this microenvironmental complexity, as they are static and homogeneous. In order to achieve a dynamic control of the physical properties of a hydrogel network, we here designed a chemical scheme to control poly(ethylene glycol) (PEG) hydrogel stiffness in space, time and intensity. Specifically, we combined caging chemistry and Michael-type addition to enable the light-triggered local control of hydrogel crosslinking density. Thiol moieties of one of the reactive PEG macromers undergoing crosslinking were equipped with caging groups to prevent their susceptibility to the counter-reactive vinyl sulfone groups on the termini of the complementary PEG macromers. Thus, the crosslinking density of the hydrogel network could be tuned by uncaging with light which directly translated into differential patterns of hydrogel stiffness. Using this approach, user-defined stiffness patterns in a range of soft tissue microenvironments (i.e. between 3-8 kPa) were obtained and shown to influence the migratory behavior of primary human mesenchymal stem cells (hMSC). Stiffness gradients in the higher range (5.5-8 kPa) were able to elicit durotaxis towards the more densely crosslinked regions, whereas those in the lower range (3-5.5 kPa) showed no significant directional preference in hMSC migration. Our patterning tool should be useful for the manipulation of cell fate in various other contexts.

Digital Plasmonic Patterning for Localized Tuning of Hydrogel Stiffness

The mechanical properties of the extracellular matrix (ECM) can dictate cell fate in biological systems. In tissue engineering, varying the stiffness of hydrogels-water-swollen polymeric networks that act as ECM substrates-has previously been demonstrated to control cell migration, proliferation, and differentiation. Here, "digital plasmonic patterning" (DPP) is developed to mechanically alter a hydrogel encapsulated with gold nanorods using a near-infrared laser, according to a digital (computer-generated) pattern. DPP can provide orders of magnitude changes in stiffness, and can be tuned by laser intensity and speed of writing. In vitro cellular experiments using A7R5 smooth muscle cells confirm cell migration and alignment according to these patterns, making DPP a useful technique for mechanically patterning hydrogels for various biomedical applications.

Cell migration on planar and three-dimensional matrices: a hydrogel-based perspective

The migration of cells is a complex process that is dependent on the properties of the surrounding environment. In vivo, the extracellular environment is complex with a wide range of physical features, topographies, and protein compositions. There have been a number of approaches to design substrates that can recapitulate the complex architecture in vivo. Two-dimensional (2D) substrates have been widely used to study the effect of material properties on cell migration. However, such substrates do not capture the intricate structure of the extracellular environment. Recent advances in hydrogel assembly and patterning techniques have enabled the design of new three-dimensional (3D) scaffolds and microenvironments. Investigations conducted on these matrices provide growing evidence that several established migratory trends obtained from studies on 2D substrates could be significantly different when conducted in a 3D environment. Since cell migration is closely linked to a wide range of physiological functions, there is a critical need to examine migratory trends on 3D matrices. In this review, our goal is to highlight recent experimental studies on cell migration within engineered 3D hydrogel environments and how they differ from planar substrates. We provide a detailed examination of the changes in cellular characteristics such as morphology, speed, directionality, and protein expression in 3D hydrogel environments. This growing field of research will have a significant impact on tissue engineering, regenerative medicine, and in the design of biomaterials.

Directed migration of mesenchymal cells: where signaling and the cytoskeleton meet

Cell migration directed by spatial cues, or taxis, is a primary mechanism for orchestrating concerted and collective cell movements during development, wound repair, and immune responses. Compared with the classic example of amoeboid chemotaxis, in which fast-moving cells such as neutrophils are directed by gradients of soluble factors, directed migration of slow-moving mesenchymal cells such as fibroblasts is poorly understood. Mesenchymal cells possess a distinctive organization of the actin cytoskeleton and associated adhesion complexes as its primary mechanical system, generating the asymmetric forces required for locomotion without strong polarization. The emerging hypothesis is that the molecular underpinnings of mesenchymal taxis involve distinct signaling pathways and diverse requirements for regulation.

Vascular smooth muscle cell culture in microfluidic devices

This paper presents a microfluidic device enabling culture of vascular smooth muscle cells (VSMCs) where extracellular matrix coating, VSMC seeding, culture, and immunostaining are demonstrated in a tubing-free manner. By optimizing droplet volume differences between inlets and outlets of micro channels, VSMCs were evenly seeded into microfluidic devices. Furthermore, the effects of extracellular matrix (e.g., collagen, poly-l-Lysine (PLL), and fibronectin) on VSMC proliferation and phenotype expression were explored. As a platform technology, this microfluidic device may function as a new VSMC culture model enabling VSMC studies.

Time-dependent migratory behaviors in the long-term studies of fibroblast durotaxis on a hydrogel substrate fabricated with a soft band

Durotaxis, biased cell movement up a stiffness gradient on culture substrates, is one of the useful taxis behaviors for manipulating cell migration on engineered biomaterial surfaces. In this study, long-term durotaxis was investigated on gelatinous substrates containing a soft band of 20, 50, and 150 μm in width fabricated using photolithographic elasticity patterning; sharp elasticity boundaries with a gradient strength of 300 kPa/50 μm were achieved. Time-dependent migratory behaviors of 3T3 fibroblast cells were observed during a time period of 3 days. During the first day, most of the cells were strongly repelled by the soft band independent of bandwidth, exhibiting the typical durotaxis behavior. However, the repellency by the soft band diminished, and more cells crossed the soft band or exhibited other mixed migratory behaviors during the course of the observation. It was found that durotaxis strength is weakened on the substrate with the narrowest soft band and that adherent affinity-induced entrapment becomes apparent on the widest soft band with time. Factors, such as changes in surface topography, elasticity, and/or chemistry, likely contributing to the apparent diminishing durotaxis during the extended culture were examined. Immunofluorescence analysis indicated preferential collagen deposition onto the soft band, which is derived from secretion by fibroblast cells, resulting in the increasing contribution of haptotaxis toward the soft band over time. The deposited collagen did not affect surface topography or surface elasticity but did change surface chemistry, especially on the soft band. The observed time-dependent durotaxis behaviors are the result of the mixed mechanical and chemical cues. In the studies and applications of cell migratory behavior under a controlled stimulus, it is important to thoroughly examine other (hidden) compounding stimuli in order to be able to accurately interpret data and to design suitable biomaterials to manipulate cell migration.

Fibroblasts and the ground they walk on

Fibroblast migration is essential to normal wound healing and pathological matrix deposition in fibrosis. This review summarizes our understanding of how fibroblasts navigate 2D and 3D extracellular matrices, how this behavior is influenced by the architecture and mechanical properties of the matrix, and how migration is integrated with the other principle functions of fibroblasts, including matrix deposition, contraction, and degradation.

Synergistic effects of matrix nanotopography and stiffness on vascular smooth muscle cell function

Vascular smooth muscle cells (vSMCs) retain the ability to undergo modulation in their phenotypic continuum, ranging from a mature contractile state to a proliferative, secretory state. vSMC differentiation is modulated by a complex array of microenvironmental cues, which include the biochemical milieu of the cells and the architecture and stiffness of the extracellular matrix. In this study, we demonstrate that by using UV-assisted capillary force lithography (CFL) to engineer a polyurethane substratum of defined nanotopography and stiffness, we can facilitate the differentiation of cultured vSMCs, reduce their inflammatory signature, and potentially promote the optimal functioning of the vSMC contractile and cytoskeletal machinery. Specifically, we found that the combination of medial tissue-like stiffness (11 MPa) and anisotropic nanotopography (ridge width_groove width_ridge height of 800_800_600 nm) resulted in significant upregulation of calponin, desmin, and smoothelin, in addition to the downregulation of intercellular adhesion molecule-1, tissue factor, interleukin-6, and monocyte chemoattractant protein-1. Further, our results allude to the mechanistic role of the RhoA/ROCK pathway and caveolin-1 in altered cellular mechanotransduction pathways via differential matrix nanotopography and stiffness. Notably, the nanopatterning of the stiffer substrata (1.1 GPa) resulted in the significant upregulation of RhoA, ROCK1, and ROCK2. This indicates that nanopatterning an 800_800_600 nm pattern on a stiff substratum may trigger the mechanical plasticity of vSMCs resulting in a hypercontractile vSMC phenotype, as observed in diabetes or hypertension. Given that matrix stiffness is an independent risk factor for cardiovascular disease and that CFL can create different matrix nanotopographic patterns with high pattern fidelity, we are poised to create a combinatorial library of arterial test beds, whether they are healthy, diseased, injured, or aged. Such high-throughput testing environments will pave the way for the evolution of the next generation of vascular scaffolds that can effectively crosstalk with the scaffold microenvironment and result in improved clinical outcomes.

Multiscale relationships between fibronectin structure and functional properties

Cell behavior is tightly coupled to the properties of the extracellular matrix (ECM) to which they attach. Fibronectin (Fn) forms a supermolecular, fibrillar component of the ECM that is prominent during development, wound healing and the progression of numerous diseases. This indicates that Fn has an important function in controlling cell behavior during dynamic events in vivo. The multiscale architecture of Fn molecules assembled into these fibers determines the ligand density of cell adhesion sites on the surface of the Fn fiber, Fn fiber porosity for cell signaling molecules such as growth factors, the mechanical stiffness of the Fn matrix and the adhesivity of Fn for its numerous soluble ligands. These parameters are altered by mechanical strain applied to the ECM. Recent efforts have attempted to link the molecular properties of Fn with bulk properties of Fn matrix fibers. Studies of isolated Fn fibers have helped to characterize the fiber's material properties and, in combination with models of Fn molecular behavior in the fibers, have begun to provide insights into the Fn molecular arrangement and intermolecular adhesions within the fibers. A review of these studies allows the development of an understanding of the mechanobiological functions of Fn.

ONE-DIMENSIONAL MODELING AND SIMULATIONS OF MIGRATION OF CULTURED FIBROBLASTS

Cell migration is crucial for many physiological functions such as wound healing, immuno-response and carcinogenesis. In this study an one-dimensional model of migration of fibroblasts was developed by modeling and integrating five subcellular processes, namely, actin protrusion, focal adhesion formation, stress fiber formation, polarization and retraction. The direction of migration was determined by polarization, which was related to direction of the stiffness gradient of the substrate. By controlling intensity of ultraviolet exposure on type-I collagen, a substrate with a stiffness gradient could be fabricated. Kinematic analyses of positions of the cell front, the nucleus and the cell rear, were utilized as inputs to the model. Simulation results of five live NIH 3T3 fibroblasts showed that the model was capable of simulating fast moving, slow moving and back-and-forth moving of the cells on the substrate.

Phenotypic modulation of smooth muscle cells by chemical and mechanical cues of electrospun tecophilic/gelatin nanofibers

The ability of mature smooth muscle cells (SMCs) to modulate their phenotype in response to environmental cues is a critical issue related to vascular diseases. A tissue engineered vascular graft shall promote the contractile phenotype of vascular SMCs. To this aim, Tecophilic/gelatin (TP/gel) was electrospun at different weight ratios of TP/gelatin (100:0, 70:30, 50:50, 30:70), leading to differences in biochemical and mechanical properties of the nanofibers which in turn influenced the phenotype of SMCs. Results indicated that both the substrate with higher ligand density and lower stiffness could enhance SMC contractility and reduce cell proliferation. However, observing the highest SMCs contractility on electrospun TP(70)/gel(30) among the composite scaffolds demonstrated stiffness as the most critical parameter. Due to conflicting effects of softness versus minor fraction of gelatin (reduced ligand density) within TP(70)/gel(30) fibers, a relatively high proliferation of SMCs was still observed on TP(70)/gel(30) scaffold. The surface of TP(70)/gel(30) scaffold was further modified through physical adsorption of gelatin molecules so as to increase the ligand density on its surface, whereby a functional vascular construct that promotes the contractile behavior of SMCs with low cell proliferation was developed.

Determinants of maximal force transmission in a motor-clutch model of cell traction in a compliant microenvironment

The mechanical stiffness of a cell's environment exerts a strong, but variable, influence on cell behavior and fate. For example, different cell types cultured on compliant substrates have opposite trends of cell migration and traction as a function of substrate stiffness. Here, we describe how a motor-clutch model of cell traction, which exhibits a maximum in traction force with respect to substrate stiffness, may provide a mechanistic basis for understanding how cells are tuned to sense the stiffness of specific microenvironments. We find that the optimal stiffness is generally more sensitive to clutch parameters than to motor parameters, but that single parameter changes are generally only effective over a small range of values. By contrast, dual parameter changes, such as coordinately increasing the numbers of both motors and clutches offer a larger dynamic range for tuning the optimum. The model exhibits distinct regimes: at high substrate stiffness, clutches quickly build force and fail (so-called frictional slippage), whereas at low substrate stiffness, clutches fail spontaneously before the motors can load the substrate appreciably (a second regime of frictional slippage). Between the two extremes, we find the maximum traction force, which occurs when the substrate load-and-fail cycle time equals the expected time for all clutches to bind. At this stiffness, clutches are used to their fullest extent, and motors are therefore resisted to their fullest extent. The analysis suggests that coordinate parameter shifts, such as increasing the numbers of motors and clutches, could underlie tumor progression and collective cell migration.

Substrates with patterned extracellular matrix and subcellular stiffness gradients reveal local biomechanical responses

A substrate fabrication process is developed to pattern both the extracellular matrix (ECM) and rigidity at sub-cellular spatial resolution. When growing cells on these substrates, it is found that cells respond locally in their cytoskeleton assembly. The presented method allows unique insight into the biological interpretation of mechanical signals, whereas photolithography-based fabrication is amenable to integration with complex microfabricated substructures.

Engineering physical microenvironment for stem cell based regenerative medicine

Regenerative medicine has rapidly evolved over the past decade owing to its potential applications to improve human health. Targeted differentiations of stem cells promise to regenerate a variety of tissues and/or organs despite significant challenges. Recent studies have demonstrated the vital role of the physical microenvironment in regulating stem cell fate and improving differentiation efficiency. In this review, we summarize the main physical cues that are crucial for controlling stem cell differentiation. Recent advances in the technologies for the construction of physical microenvironment and their implications in controlling stem cell fate are also highlighted.

Optical measurement of arterial mechanical properties: from atherosclerotic plaque initiation to rupture

During the pathogenesis of coronary atherosclerosis, from lesion initiation to rupture, arterial mechanical properties are altered by a number of cellular, molecular, and hemodynamic processes. There is growing recognition that mechanical factors may actively drive vascular cell signaling and regulate atherosclerosis disease progression. In advanced plaques, the mechanical properties of the atheroma influence stress distributions in the fibrous cap and mediate plaque rupture resulting in acute coronary events. This review paper explores current optical technologies that provide information on the mechanical properties of arterial tissue to advance our understanding of the mechanical factors involved in atherosclerosis development leading to plaque rupture. The optical approaches discussed include optical microrheology and traction force microscopy that probe the mechanical behavior of single cell and extracellular matrix components, and intravascular imaging modalities including laser speckle rheology, optical coherence elastography, and polarization-sensitive optical coherence tomography to measure the mechanical properties of advanced coronary lesions. Given the wealth of information that these techniques can provide, optical imaging modalities are poised to play an increasingly significant role in elucidating the mechanical aspects of coronary atherosclerosis in the future.

Microfluidic device with dual mechanical cues for cell migration investigation

Cell migration plays an important role in numerous physiological and pathological conditions, such as angiogenesis, wound healing and cancer metastasis. Understanding the fundamental mechanisms of cell migration is crucial to develop strategies for disease treatment and regenerative medicine. Several biomechanical cues have been well studied about their effects on guiding cell migration. However, the effects of dual or multiple cues on cell migration are barely addressed. In this work, we developed a microfluidic-based device to study the combinatory effects of osmotic and stiffness gradient on cell migration. Computer simulation and experimental validation showed that the device was capable of providing stable osmotic and stiffness gradient to cultured cells at the same time. Preliminary results suggest that our device has a valuable potential in studying cell migration in complex conditions which better recapitulate the complex environmental conditions in vivo.

Substrates with engineered step changes in rigidity induce traction force polarity and durotaxis

Rigidity sensing plays a fundamental role in multiple cell functions ranging from migration, to proliferation and differentiation^1-5. During migration, single cells have been reported to preferentially move toward more rigid regions of a substrate in a process termed durotaxis. Durotaxis could contribute to cell migration in wound healing and gastrulation, where local gradients in tissue rigidity have been described. Despite the potential importance of this phenomenon to physiology and disease, it remains unclear how rigidity guides these behaviors and the underlying cellular and molecular mechanisms. To investigate the functional role of subcellular distribution and dynamics of cellular traction forces during durotaxis, we developed a unique microfabrication strategy to generate elastomeric micropost arrays patterned with regions exhibiting two different rigidities juxtaposed next to each other. After initial cell attachment on the rigidity boundary of the micropost array, NIH 3T3 fibroblasts were observed to preferentially migrate toward the rigid region of the micropost array, indicative of durotaxis. Additionally, cells bridging two rigidities across the rigidity boundary on the micropost array developed stronger traction forces on the more rigid side of the substrate indistinguishable from forces generated by cells exclusively seeded on rigid regions of the micropost array. Together, our results highlighted the utility of step-rigidity micropost arrays to investigate the functional role of traction forces in rigidity sensing and durotaxis, suggesting that cells could sense substrate rigidity locally to induce an asymmetrical intracellular traction force distribution to contribute to durotaxis.

Microfabricated polyacrylamide devices for the controlled culture of growing cells and developing organisms

The ability to spatially confine living cells or small organisms while dynamically controlling their aqueous environment is important for a host of microscopy applications. Here, we show how polyacrylamide layers can be patterned to construct simple microfluidic devices for this purpose. We find that polyacrylamide gels can be molded like PDMS into micron-scale structures that can enclose organisms, while being permeable to liquids, and transparent to allow for microscopic observation. We present a range of chemostat-like devices to observe bacterial and yeast growth, and C. elegans nematode development. The devices can integrate PDMS layers and allow for temporal control of nutrient conditions and the presence of drugs on a minute timescale. We show how spatial confinement of motile C. elegans enables for time-lapse microscopy in a parallel fashion.

Simulation of the Mechanical Response of Cells on Micropost Substrates

Experimental studies where cells are seeded on micropost arrays in order to quantify their contractile behavior are becoming increasingly common. Interpretation of the data generated by this experimental technique is difficult, due to the complexity of the processes underlying cellular contractility and mechanotransduction. In the current study, a coupled framework that considers strain rate dependent contractility and remodeling of the cytoskeleton is used in tandem with a thermodynamic model of tension dependent focal adhesion formation to investigate the biomechanical response of cells adhered to micropost arrays. Computational investigations of the following experimental studies are presented: cell behavior on different sized arrays with a range of post stiffness; stress fiber and focal adhesion formation in irregularly shaped cells; the response of cells to deformations applied locally to individual posts; and the response of cells to equibiaxial stretching of micropost arrays. The predicted stress fiber and focal adhesion distributions; in addition to the predicted post tractions are quantitatively and qualitatively supported by previously published experimental data. The computational models presented in this study thus provide a framework for the design and interpretation of experimental micropost studies.

Designing degradable hydrogels for orthogonal control of cell microenvironments

Degradable and cell-compatible hydrogels can be designed to mimic the physical and biochemical characteristics of native extracellular matrices and provide tunability of degradation rates and related properties under physiological conditions. Hence, such hydrogels are finding widespread application in many bioengineering fields, including controlled bioactive molecule delivery, cell encapsulation for controlled three-dimensional culture, and tissue engineering. Cellular processes, such as adhesion, proliferation, spreading, migration, and differentiation, can be controlled within degradable, cell-compatible hydrogels with temporal tuning of biochemical or biophysical cues, such as growth factor presentation or hydrogel stiffness. However, thoughtful selection of hydrogel base materials, formation chemistries, and degradable moieties is necessary to achieve the appropriate level of property control and desired cellular response. In this review, hydrogel design considerations and materials for hydrogel preparation, ranging from natural polymers to synthetic polymers, are overviewed. Recent advances in chemical and physical methods to crosslink hydrogels are highlighted, as well as recent developments in controlling hydrogel degradation rates and modes of degradation. Special attention is given to spatial or temporal presentation of various biochemical and biophysical cues to modulate cell response in static (i.e., non-degradable) or dynamic (i.e., degradable) microenvironments. This review provides insight into the design of new cell-compatible, degradable hydrogels to understand and modulate cellular processes for various biomedical applications.

Master equation-based analysis of a motor-clutch model for cell traction force

Microenvironmental mechanics play an important role in determining the morphology, traction, migration, proliferation, and differentiation of cells. A stochastic motor-clutch model has been proposed to describe this stiffness sensitivity. In this work, we present a master equation-based ordinary differential equation (ODE) description of the motor-clutch model, from which we derive an analytical expression to for a cell's optimum stiffness (i.e. the stiffness at which the traction force is maximal). This analytical expression provides insight into the requirements for stiffness sensing by establishing fundamental relationships between the key controlling cell-specific parameters. We find that the fundamental controlling parameters are the numbers of motors and clutches (constrained to be nearly equal), and the time scale of the on-off kinetics of the clutches (constrained to favor clutch binding over clutch unbinding). Both the ODE solution and the analytical expression show good agreement with Monte Carlo motor-clutch output, and reduce computation time by several orders of magnitude, which potentially enables long time scale behaviors (hours-days) to be studied computationally in an efficient manner. The ODE solution and the analytical expression may be incorporated into larger scale models of cellular behavior to bridge the gap from molecular time scales to cellular and tissue time scales.

Rigidity-patterned polyelectrolyte films to control myoblast cell adhesion and spatial organization

In vivo, cells are sensitive to the stiffness of their micro-environment and especially to the spatial organization of the stiffness. In vitro studies of this phenomenon can help to better understand the mechanisms of the cell response to spatial variations of the matrix stiffness. In this work, we design polelyelectrolyte multilayer films made of poly(L-lysine) and a photo-reactive hyaluronan derivative. These films can be photo-crosslinked through a photomask to create spatial patterns of rigidity. Quartz substrates incorporating a chromium mask are prepared to expose selectively the film to UV light (in a physiological buffer), without any direct contact between the photomask and the soft film. We show that these micropatterns are chemically homogeneous and flat, without any preferential adsorption of adhesive proteins. Three groups of pattern geometries differing by their shape (circles or lines), size (form 2 to 100 μm) or interspacing distance between the motifs are used to study the adhesion and spatial organization of myoblast cells. On large circular micropatterns, the cells form large assemblies that are confined to the stiffest parts. Conversely, when the size of the rigidity patterns is subcellular, the cells respond by forming protrusions. Finally, on linear micropatterns of rigidity, myoblasts align and their nuclei drastically elongate in specific conditions. These results pave the way for the study of the different steps of myoblast fusion in response to matrix rigidity in well-defined geometrical conditions.