Subcellular spatial segregation of integrin subtypes by patterned multicomponent surfaces

Development of an integrin αv-based universal marker, capable of both prediction and direction of stem cell fate

Integrin-mediated focal adhesion (FA) and subsequent cytoskeletal reorganization influence cell morphology, migration, and ultimately cell fate. Previous studies have used various patterned surfaces with defined macroscopic cell shapes or nanoscopic FA distributions to explore how different substrates affect the fate of human bone marrow mesenchymal stem cells (BMSCs). However, there is currently no straightforward relationship between BMSC cell fates induced by patterned surfaces and FA distribution substrates. In this study, we conducted single-cell image analysis of integrin αv-mediated FA and cell morphological features of BMSCs during biochemically induced differentiation. This enabled the identification of distinct FA features that can discriminate between osteogenic and adipogenic differentiation, demonstrating that integrin αv-mediated focal adhesion (FA) can be used as a non-invasive biomarker for real time observation. Based on these results, we developed an organized microscale fibronectin (FN) patterned surface where the fate of BMSC could be precisely manipulated by these FA features. Notably, even in the absence of any biochemical inducers, such as those contained in the differentiation medium, BMSCs cultured on these FN patterned surfaces exhibited upregulation of differentiation markers comparable to BMSCs cultured using conventional differentiation methods. Therefore, the present study reveals the application of these FA features as universal markers not only for predicting differentiation status, but also for regulating cell fate by precisely controlling the FA features with a new cell culture platform. STATEMENT OF SIGNIFICANCE: Although the effects of material physiochemical properties on cell morphology and subsequent cell fate decisions have been extensively studied, a simple yet intuitive correlation between cellular features and differentiation remains unavailable. We present a single cell image-based strategy for predicting and directing stem cell fate. By using a specific integrin isoform, integrin αv, we identified distinct geometric features that can be used as a marker for discriminating between osteogenic and adipogenic differentiation in real-time. From these data, new cell culture platforms capable of regulating cell fate by precisely controlling FA features and cell area can be developed.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Integrating Soft Hydrogel with Nanostructures Reinforces Stem Cell Adhesion and Differentiation

Biophysical cues can regulate stem cell behaviours and have been considered as critical parameters of synthetic biomaterials for tissue engineering. In particular, hydrogels have been utilized as promising biomimetic and biocompatible materials to emulate the microenvironment. Therefore, well-defined mechanical properties of a hydrogel are important to direct desirable phenotypes of cells. Yet, limited research pays attention to engineering soft hydrogel with improved cell adhesive property, which is crucial for stem cell differentiation. Herein, we introduce silica nanoparticles (SiO2 NPs) onto the surface of methacrylated hyaluronic (MeHA) hydrogel to manipulate the presentation of cell adhesive ligands (RGD) clusters, while remaining similar bulk mechanical properties (2.79 ± 0.31 kPa) to that of MeHA hydrogel (3.08 ± 0.68 kPa). RGD peptides are either randomly decorated in the MeHA hydrogel network or on the immobilized SiO2 NPs (forming MeHA–SiO2). Our results showed that human mesenchymal stem cells exhibited a ~1.3-fold increase in the percentage of initial cell attachment, a ~2-fold increase in cell spreading area, and enhanced expressions of early-stage osteogenic markers (RUNX2 and alkaline phosphatase) for cells undergoing osteogenic differentiation with the osteogenic medium on MeHA–SiO2 hydrogel, compared to those cultured on MeHA hydrogel. Importantly, the cells cultivated on MeHA–SiO2 expressed a ~5-fold increase in nuclear localization ratio of the yes-associated protein, which is known to be mechanosensory in stem cells, compared to the cells cultured on MeHA hydrogel, thereby promoting osteogenic differentiation of stem cells. These findings demonstrate the potential use of nanomaterials into a soft polymeric matrix for enhanced cell adhesion and provide valuable guidance for the rational design of biomaterials for implantation.

On the adhesion-velocity relation and length adaptation of motile cells on stepped fibronectin lanes

The biphasic adhesion-velocity relation is a universal observation in mesenchymal cell motility. It has been explained by adhesion-promoted forces pushing the front and resisting motion at the rear. Yet, there is little quantitative understanding of how these forces control cell velocity. We study motion of MDA-MB-231 cells on microlanes with fields of alternating Fibronectin densities to address this topic and derive a mathematical model from the leading-edge force balance and the force-dependent polymerization rate. It reproduces quantitatively our measured adhesion-velocity relation and results with keratocytes, PtK1 cells, and CHO cells. Our results confirm that the force pushing the leading-edge membrane drives lamellipodial retrograde flow. Forces resisting motion originate along the whole cell length. All motion-related forces are controlled by adhesion and velocity, which allows motion, even with higher Fibronectin density at the rear than at the front. We find the pathway from Fibronectin density to adhesion structures to involve strong positive feedbacks. Suppressing myosin activity reduces the positive feedback. At transitions between different Fibronectin densities, steady motion is perturbed and leads to changes of cell length and front and rear velocity. Cells exhibit an intrinsic length set by adhesion strength, which, together with the length dynamics, suggests a spring-like front-rear interaction force. We provide a quantitative mechanistic picture of the adhesion-velocity relation and cell response to adhesion changes integrating force-dependent polymerization, retrograde flow, positive feedback from integrin to adhesion structures, and spring-like front-rear interaction.

Antibody Printing Technologies

Antibody microarrays are routinely employed in the lab and in the clinic for studying protein expression, protein-protein, and protein-drug interactions. The microarray format reduces the size scale at which biological and biochemical interactions occur, leading to large reductions in reagent consumption and handling times while increasing overall experimental throughput. Specifically, antibody microarrays, as a platform, offer a number of different advantages over traditional techniques in the areas of drug discovery and diagnostics. While a number of different techniques and approaches have been developed for creating micro and nanoscale antibody arrays, issues relating to sensitivity, cost, and reproducibility persist. The aim of this review is to highlight current state-of the-art techniques and approaches for creating antibody arrays by providing latest accounts of the field while discussing potential future directions.

Fibronectin-based nanomechanical biosensors to map 3D surface strains in live cells and tissue

Mechanical forces are integral to cellular migration, differentiation and tissue morphogenesis; however, it has proved challenging to directly measure strain at high spatial resolution with minimal perturbation in living sytems. Here, we fabricate, calibrate, and test a fibronectin (FN)-based nanomechanical biosensor (NMBS) that can be applied to the surface of cells and tissues to measure the magnitude, direction, and strain dynamics from subcellular to tissue length-scales. The NMBS is a fluorescently-labeled, ultra-thin FN lattice-mesh with spatial resolution tailored by adjusting the width and spacing of the lattice from 2-100 µm. Time-lapse 3D confocal imaging of the NMBS demonstrates 2D and 3D surface strain tracking during mechanical deformation of known materials and is validated with finite element modeling. Analysis of the NMBS applied to single cells, cell monolayers, and Drosophila ovarioles highlights the NMBS's ability to dynamically track microscopic tensile and compressive strains across diverse biological systems where forces guide structure and function.

Extracellular matrix alignment dictates the organization of focal adhesions and directs uniaxial cell migration

Physical features of the extracellular matrix (ECM) heavily influence cell migration strategies and efficiency. Migration in and on fibrous ECMs is of significant physiologic importance, but limitations in the ability to experimentally define the diameter, density, and alignment of native ECMs in vitro have hampered our understanding of how these properties affect this basic cell function. Here, we designed a high-throughput in vitro platform that models fibrous ECM as collections of lines of cell-adhesive fibronectin on a flat surface to eliminate effects of dimensionality and topography. Using a microcontact printing approach to orthogonally vary line alignment, density, and size, we determined each factor's individual influence on NIH3T3 fibroblast migration. High content imaging and statistical analyses revealed that ECM alignment is the most critical parameter in influencing cell morphology, polarization, and migratory behavior. Specifically, increasing ECM alignment led cells to adopt an elongated uniaxial morphology and migrate with enhanced speed and persistence. Intriguingly, migration speeds were tightly correlated with the organization of focal adhesions, where cells with the most aligned adhesions migrated fastest. Highly organized focal adhesions and associated actin stress fibers appeared to define the number and location of protrusive fronts, suggesting that ECM alignment influences active Rac1 localization. Utilizing a novel microcontact-printing approach that lacks confounding influences of substrate dimensionality, mechanics, or differences in the adhesive area, this work highlights the effect of ECM alignment on orchestrating the cytoskeletal machinery that governs directed uniaxial cell migration.

Recent Advances in Engineering the Stem Cell Microniche in 3D

Conventional 2D cell culture techniques have provided fundamental insights into key biochemical and biophysical mechanisms responsible for various cellular behaviors, such as cell adhesion, spreading, division, proliferation, and differentiation. However, 2D culture in vitro does not fully capture the physical and chemical properties of the native microenvironment. There is a growing body of research that suggests that cells cultured on 2D substrates differ greatly from those grown in vivo. This article focuses on recent progress in using bioinspired 3D matrices that recapitulate as many aspects of the natural extracellular matrix as possible. A range of techniques for the engineering of 3D microenvironment with precisely controlled biophysical and chemical properties, and the impact of these environments on cellular behavior, is reviewed. Finally, an outlook on future challenges for engineering the 3D microenvironment and how such approaches would further our understanding of the influence of the microenvironment on cell function is provided.

3D Microcontact Printing for Combined Chemical and Topographical Patterning on Porous Cell Culture Membrane

Micrometer-scale biochemical or topographical patterning is commonly used to guide the cell attachment and growth, but the ability to combine these patterns into an integrated surface with defined chemical and geometrical characteristics still remains a technical challenge. Here, we present a technical solution for simultaneous construction of 3D morphologies, in the form of channels, on porous membranes along with precise transfer of extracellular matrix proteins into the channels to create patterns with geometrically restricting features. By combining the advantages of microthermoforming and microcontact printing, this technique offers a unique patterning process that provides spatiotemporal control over morphological and chemical feature in a single step. By use of our 3D-microcontact printing (3DμCP), determined microstructures like channels with different depths and widths even with more complex patterns can be fabricated. Collagen, fibronectin, and laminin were successfully transferred inside the predesigned geometries, and the validity of the process was confirmed by antibody staining. Cells cultivated on 3DμCP patterned polycarbonate membrane have shown selective adhesion and growth. This technique offers a novel tool for creating freeform combinatorial patterning on the thermoformable surface.

Motile Dendritic Cells Sense and Respond to Substrate Geometry

Dendritic cell (DC) migration is required for efficient presentation of antigen to T cells and the initiation of an adaptive immune response. In spite of its importance, many aspects of DC migration have not been characterized. DCs encounter a variety of environments with different stiffness and geometry, but the effect of these parameters on DC migration has not yet been determined. We addressed this question by comparing DC motility on standard migration surfaces (polydimethylsiloxane (PDMS)-coated coverslips) and micropost array detectors (mPADs). These two surfaces differ in both stiffness and geometry. We found that DC migration was affected by substrate type, with significant increases in speed and significant decreases in persistence time on mPADs made of PDMS as compared to spin-coated PDMS coverslips. To determine whether the geometry or compliance of the post arrays was responsible for these changes in DC migration, we quantified DC motility on mPADs of identical geometry but different stiffness. Migration was indistinguishable on these mPADs, suggesting that DCs are responsive to geometry of ligand presentation and not stiffness. Further, by micropatterning ligands on flat PDMS surfaces in similar geometries to the mPAD arrays, we determined that DCs respond to the geometry of printed ligand. Finally, we used a variety of small molecule inhibitors to identify pathways involved in geometry sensing. We saw a significant role for myosin contractility and α₅β₁ integrin engagement. We also noted significant reorganization of the actin cytoskeleton into dynamic actin rings when DCs were motile on posts. From these experiments, we conclude that DCs are insensitive to substrate compliance in the range tested but respond to changes in geometry via a mechanism that involves integrin function, myosin contractility, and remodeling of the actin cytoskeleton. As a possible explanation, we postulate a consistent role for filopodial extension and contraction as the driver of DC motility.

The Arp2/3 complex binding protein HS1 is required for efficient dendritic cell random migration and force generation

Dendritic cell migration to the T-cell-rich areas of the lymph node is essential for their ability to initiate the adaptive immune response. While it has been shown that the actin cytoskeleton is required for normal DC migration, the role of many of the individual cytoskeletal molecules is poorly understood. In this study, we investigated the contribution of the Arp2/3 complex binding protein, haematopoietic lineage cell-specific protein 1 (HS1), to DC migration and force generation. We quantified the random migration of HS1^-/- DCs on 2D micro-contact printed surfaces and found that in the absence of HS1, DCs have greatly reduced motility and speed. This same reduction in motility was recapitulated when adding Arp2/3 complex inhibitor to WT DCs or using DCs deficient in WASP, an activator of Arp2/3 complex-dependent actin polymerization. We further investigated the importance of HS1 by measuring the traction forces of HS1^-/- DCs on micropost array detectors (mPADs). In HS1 deficient DCs, there was a significant reduction in force generation (3.96 ± 0.40 nN per cell) compared to WT DCs (13.76 ± 0.84 nN per cell). Interestingly, the forces generated in DCs lacking WASP were only slightly reduced compared to WT DCs. Taken together, these findings show that HS1 and Arp2/3 complex-mediated actin polymerization are essential for the most efficient DC random migration and force generation.

Protein-Substrate Adhesion in Microcontact Printing Regulates Cell Behavior

Microcontact printing (μCP) is widely used to create patterns of biomolecules essential for studies of cell mechanics, migration, and tissue engineering. However, different types of μCPs may create micropatterns with varied protein-substrate adhesion, which may change cell behaviors and pose uncertainty in result interpretation. Here, we characterize two μCP methods for coating extracellular matrix (ECM) proteins (stamp-off and covalent bond) and demonstrate for the first time the important role of protein-substrate adhesion in determining cell behavior. We found that, as compared to cells with weaker traction force (e.g., endothelial cells), cells with strong traction force (e.g., vascular smooth muscle cells) may delaminate the ECM patterns, which reduced cell viability as a result. Importantly, such ECM delamination was observed on patterns by stamp-off but not on the patterns by covalent bonds. Further comparisons of the displacement of the ECM patterns between the normal VSMCs and the force-reduced VSMCs suggested that the cell traction force plays an essential role in this ECM delamination. Together, our results indicated that μCPs with insufficient adhesion may lead to ECM delamination and cause cell death, providing new insight for micropatterning in cell-biomaterial interaction on biointerfaces.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

New advances in probing cell-extracellular matrix interactions

The extracellular matrix (ECM) provides structural and biochemical support to cells within tissues. An emerging body of evidence has established that the ECM plays a key role in cell mechanotransduction - the study of coupling between mechanical inputs and cellular phenotype - through either mediating transmission of forces to the cells, or presenting mechanical cues that guide cellular behaviors. Recent progress in cell mechanotransduction research has been facilitated by advances of experimental tools, particularly microtechnologies, engineered biomaterials, and imaging and analytical methods. Microtechnologies have enabled the design and fabrication of controlled physical microenvironments for the study and measurement of cell-ECM interactions. Advances in engineered biomaterials have allowed researchers to develop synthetic ECMs that mimic tissue microenvironments and investigate the impact of altered physicochemical properties on various cellular processes. Finally, advanced imaging and spectroscopy techniques have facilitated the visualization of the complex interaction between cells and ECM in vitro and in living tissues. This review will highlight the application of recent innovations in these areas to probing cell-ECM interactions. We believe cross-disciplinary approaches, combining aspects of the different technologies reviewed here, will inspire innovative ideas to further elucidate the secrets of ECM-mediated cell control.

Revealing the Role of Microscale Architecture in Immune Synapse Function Through Surface Micropatterning

The immune synapse has emerged as a compelling example of structural complexity within cell-cell interfaces. This chapter focuses on the use of microcontact printing to isolate and investigate how spatial organization of signaling molecules drives the function of immune cells. In the process detailed here, multiple rounds of microcontact printing are combined to create patterned surfaces that control the relative spatial localization of CD3 and CD28 signaling in T cells, effectively replacing an antigen presenting cell with an engineered surface. A set of approaches used to address key issues of T cell activation are described and discussed.

Multiplex cell microarrays for high-throughput screening

Microarray technology was developed in the early 1990s to measure the transcription levels of thousands of genes in parallel. The basic premise of high-density arraying has since been expanded to create cell microarrays. Cells on chip are powerful experimental tools for high-throughput and multiplex screening of samples or cellular functions. Miniaturization increases assay throughput while reducing both reagent consumption and cell population heterogeneity effect, making these systems attractive for a wide range of assays, from drug discovery to toxicology, stem cell research and therapy. It is usual to functionalize the surface of a substrate to design cell microarrays. One form of cell microarrays, the transfected cell microarray, wherein plasmid DNA or siRNA spotted on the surface of a substrate is reverse-transfected locally into adherent cells, has become a standard tool for parallel cell-based analysis. With the advent of technology, cells can also be directly spotted onto functionalized surfaces using robotic fluid-dispensing devices or printed directly on bio-ink material. We are providing herein an overview of the latest developments in optical cell microarrays allowing high-throughput and high-content analysis.

Cell-cell contact and matrix adhesion promote αSMA expression during TGFβ1-induced epithelial-myofibroblast transition via Notch and MRTF-A

During epithelial-mesenchymal transition (EMT) epithelial cells lose cell-cell adhesion, exhibit morphological changes, and upregulate the expression of cytoskeletal proteins. Previous studies have demonstrated that complete disruption of cell-cell contact can promote transforming growth factor (TGF)-β1-induced EMT and the expression of the myofibroblast marker alpha smooth muscle actin (αSMA). Furthermore, increased cell spreading mediates TGFβ1-induced αSMA expression during EMT. Here, we sought to examine how the presence of partial cell-cell contacts impacts EMT. A microfabrication approach was employed to decouple the effects of cell-cell contact and cell-matrix adhesion in TGFβ1-induced EMT. When cell spreading is controlled, the presence of partial cell-cell contacts enhances expression of αSMA. Moreover, cell spreading and intercellular contacts together control the subcellular localization of activated Notch1 and myocardin related transcription factor (MRTF)-A. Knockdown of Notch1 or MRTF-A as well as pharmacological inhibition of these pathways abates the cell-cell contact mediated expression of αSMA. These data suggest that the interplay between cell-matrix adhesion and intercellular adhesion is an important determinant for some aspects of TGFβ1-induced EMT.

Protrusive and Contractile Forces of Spreading Human Neutrophils

Human neutrophils are mediators of innate immunity and undergo dramatic shape changes at all stages of their functional life cycle. In this work, we quantified the forces associated with a neutrophil's morphological transition from a nonadherent, quiescent sphere to its adherent and spread state. We did this by tracking, with high spatial and temporal resolution, the cell's mechanical behavior during spreading on microfabricated post-array detectors printed with the extracellular matrix protein fibronectin. Two dominant mechanical regimes were observed: transient protrusion and steady-state contraction. During spreading, a wave of protrusive force (75 ± 8 pN/post) propagates radially outward from the cell center at a speed of 206 ± 28 nm/s. Once completed, the cells enter a sustained contractile state. Although post engagement during contraction was continuously varying, posts within the core of the contact zone were less contractile (-20 ± 10 pN/post) than those residing at the geometric perimeter (-106 ± 10 pN/post). The magnitude of the protrusive force was found to be unchanged in response to cytoskeletal inhibitors of lamellipodium formation and myosin II-mediated contractility. However, cytochalasin B, known to reduce cortical tension in neutrophils, slowed spreading velocity (61 ± 37 nm/s) without significantly reducing protrusive force. Relaxation of the actin cortical shell was a prerequisite for spreading on post arrays as demonstrated by stiffening in response to jasplakinolide and the abrogation of spreading. ROCK and myosin II inhibition reduced long-term contractility. Function blocking antibody studies revealed haptokinetic spreading was induced by β2 integrin ligation. Neutrophils were found to moderately invaginate the post arrays to a depth of ∼1 μm as measured from spinning disk confocal microscopy. Our work suggests a competition of adhesion energy, cortical tension, and the relaxation of cortical tension is at play at the onset of neutrophil spreading.

Substrate engagement of integrins α5β1 and αvβ3 is necessary, but not sufficient, for high directional persistence in migration on fibronectin

The interplay between specific integrin-mediated matrix adhesion and directional persistence in cell migration is not well understood. Here, we characterized fibroblast adhesion and migration on the extracellular matrix glycoproteins fibronectin and vitronectin, focusing on the role of α₅β₁ and α_vβ₃ integrins. Fibroblasts manifested high directional persistence in migration on fibronectin-, but not vitronectin-coated substrates, in a ligand density-dependent manner. Fibronectin stimulated α₅β₁-dependent organization of the actin cytoskeleton into oriented, ventral stress fibers, and assembly of dynamic, polarized protrusions, characterized as regions free of stress fibers and rich in nascent adhesions at their edge. Such protrusions correlated with persistent, local leading edge advancement, but were not sufficient, nor necessary for directional migration over longer times. Selective blocking of α_vβ₃ or α₅β₁ integrins using small molecule integrin antagonists reduced directional persistence on fibronectin, indicating integrin cooperativity in maintaining directionality. On the other hand, patterned substrates, designed to selectively engage either integrin, or their combination, were not sufficient to establish directional migration. Overall, our study demonstrates adhesive coating-dependent regulation of directional persistence in fibroblast migration and challenges the generality of the previously suggested role of β₁ and β₃ integrins in directional migration.

Motile Human Neutrophils Sense Ligand Density Over Their Entire Contact Area

Neutrophils are key components of the immune system and motility is central their function during the inflammatory response. We have previously demonstrated that neutrophils are capable of switching their motile phenotype between amoeboid-like and keratocyte-like in response to the ligand density of adhesion molecules (Henry et al. in Int Biol 6:348-356, 2014). In this study, we engineered planar micropatterned surfaces that presented adhesion molecules in local islands of high density, separated by regions largely devoid of ligands. By controlling the geometry of islands we made arrays in which the local (on island) adhesion density was high but the global (multi-island) adhesion density over the entire cell-substrate interface was low. Neutrophils in contact with these island arrays assumed a well-spread and directionally-persistent motile phenotype (keratocyte-like) in contrast to the classical amoeboid morphology they display on uniform fields of high adhesion density. By virtue of our rationally designed substrates, we were able to conclude that neutrophils were integrating the stimulation received across their entire contact interface; furthermore, they were able to mount this whole cell response on the timescale of seconds. This work demonstrates the capacity of adhesive microenvironments to direct the phenotype of cell motility, which has broader implications in physiologic processes such as inflammation and cancer metastasis.

Segregation versus colocalization: orthogonally functionalized binary micropatterned substrates regulate the molecular distribution in focal adhesions

Orthogonally functionalized binary micropatterned substrates are produced using a novel protocol. The use of adequate peptido-mimetics enables an unprecedented segregation of purified αvβ3 and α5β1 integrins in adjacent microislands and evidences the preference of U2OS cells to colocalize such receptors. Moreover, this tendency can be altered by varying the geometry and composition of the micropatterns.

Extracellular matrix biomimicry for the creation of investigational and therapeutic devices

The extracellular matrix (ECM) is a web of fibrous proteins that serves as a scaffold for tissues and organs, and is important for maintaining homeostasis and facilitating cellular adhesion. Integrin transmembrane receptors are the primary adhesion molecules that anchor cells to the ECM, thus integrating cells with their microenvironments. Integrins play a critical role in facilitating cell-matrix interactions and promoting signal transduction, both from the cell to the ECM and vice versa, ultimately mediating cell behavior. For this reason, many advanced biomaterials employ biomimicry by replicating the form and function of fibrous ECM proteins. The ECM also acts as a reservoir for small molecules and growth factors, wherein fibrous proteins directly bind and present these bioactive moieties that facilitate cell activity. Therefore biomimicry can be enhanced by incorporating small molecules into ECM-like substrates. Biomimetic ECM materials have served as invaluable research tools for studying interactions between cells and the surrounding ECM, revealing that cell-matrix signaling is driven by mechanical forces, integrin engagement, and small molecules. Mimicking pathological ECMs has also elucidated disease specific cell behaviors. For example, biomimetic tumor microenvironments have been used to induce metastatic cell behaviors, and have thereby shown promise for in vitro cancer drug testing and targeting. Further, ECM-like substrates have been successfully employed for autologous cell recolonization for tissue engineering and wound healing. As we continue to learn more about the mechanical and biochemical characteristics of the ECM, these properties can be harnessed to develop new biomaterials, biomedical devices, and therapeutics.

Astrocytes alignment and reactivity on collagen hydrogels patterned with ECM proteins

To modulate the surface properties of collagen and subsequent cell-surface interactions, a method was developed to transfer protein patterns from glass coverslips to collagen type I hydrogel surfaces. Two proteins and one proteoglycan found in central nervous system extracellular matrix as well as fibrinogen were patterned in stripes onto collagen hydrogel and astrocytes were cultured on these surfaces. The addition of the stripe protein patterns to hydrogels created astrocyte layers in which cells were aligned with underlying patterns and had reduced chondroitin sulfate expression compared to the cells grown on collagen alone. Protein patterns were covalently cross-linked to the collagen and stable over four days in culture with no visible cellular modifications. The present method can be adapted to transfer other types of protein patterns from glass coverslips to collagen hydrogels.

Two-dimensional motility of a macrophage cell line on microcontact-printed fibronectin

The ability of macrophages to migrate to sites of infection and inflammation is critical for their role in the innate immune response. Macrophage cell lines have made it possible to study the roles of individual proteins responsible for migration using molecular biology, but it has not been possible to reliably elicit the motility of macrophage cell lines in two dimensions. In the past, measurements of the motility of macrophage cell lines have been largely limited to transwell assays which provide limited quantitative information on motility and limited ability to visualize cell morphology. We used microcontact printing to create polydimethylsiloxane (PDMS) surfaces functionalized with fibronectin that otherwise support little macrophage adhesion. We used these surfaces to measure macrophage migration in two dimensions and found that these cells migrate efficiently in a uniform field of colony-stimulating factor-1, CSF-1. Knockdown of Cdc42 led to a nonstatistically significant reduction in motility, whereas chemical inhibition of PI3K activity led to a complete loss of motility. Inhibition of the RhoA kinase, ROCK, did not abolish the motility of these cells but caused a quantitative change in motility, reducing motility significantly on high concentrations of fibronectin but not on low concentrations. This study illustrates the importance of studying cell motility on well controlled materials to better understand the exact roles of specific proteins on cell migration. © 2014 Wiley Periodicals, Inc.

Microcontact peeling as a new method for cell micropatterning

Micropatterning is becoming a powerful tool for studying morphogenetic and differentiation processes of cells. Here we describe a new micropatterning technique, which we refer to as microcontact peeling. Polydimethylsiloxane (PDMS) substrates were treated with oxygen plasma, and the resulting hydrophilic layer of the surface was locally peeled off through direct contact with a peeling stamp made of aluminum, copper, or silicon. A hydrophobic layer of PDMS could be selectively exposed only at the places of the physical contact as revealed by water contact angle measurements and angle-resolved X-ray photoelectron spectroscopy, which thus enabled successful micropatterning of cells with micro-featured peeling stamps. This new micropatterning technique needs no procedure for directly adsorbing proteins to bare PDMS in contrast to conventional techniques using a microcontact printing stamp. Given the several unique characteristics, the present technique based on the peel-off of inorganic materials may become a useful option for performing cell micropatterning.

Integrated micro/nanoengineered functional biomaterials for cell mechanics and mechanobiology: a materials perspective

The rapid development of micro/nanoengineered functional biomaterials in the last two decades has empowered materials scientists and bioengineers to precisely control different aspects of the in vitro cell microenvironment. Following a philosophy of reductionism, many studies using synthetic functional biomaterials have revealed instructive roles of individual extracellular biophysical and biochemical cues in regulating cellular behaviors. Development of integrated micro/nanoengineered functional biomaterials to study complex and emergent biological phenomena has also thrived rapidly in recent years, revealing adaptive and integrated cellular behaviors closely relevant to human physiological and pathological conditions. Working at the interface between materials science and engineering, biology, and medicine, we are now at the beginning of a great exploration using micro/nanoengineered functional biomaterials for both fundamental biology study and clinical and biomedical applications such as regenerative medicine and drug screening. In this review, an overview of state of the art micro/nanoengineered functional biomaterials that can control precisely individual aspects of cell-microenvironment interactions is presented and they are highlighted them as well-controlled platforms for mechanistic studies of mechano-sensitive and -responsive cellular behaviors and integrative biology research. The recent exciting trend where micro/nanoengineered biomaterials are integrated into miniaturized biological and biomimetic systems for dynamic multiparametric microenvironmental control of emergent and integrated cellular behaviors is also discussed. The impact of integrated micro/nanoengineered functional biomaterials for future in vitro studies of regenerative medicine, cell biology, as well as human development and disease models are discussed.

Cross talk between CD3 and CD28 is spatially modulated by protein lateral mobility

Functional convergence of CD28 costimulation and TCR signaling is critical to T-cell activation and adaptive immunity. These receptors form complex microscale patterns within the immune synapse, although the impact of this spatial organization on cell signaling remains unclear. We investigate this cross talk using micropatterned surfaces that present ligands to these membrane proteins in order to control the organization of signaling molecules within the cell-substrate interface. While primary human CD4(+) T cells were activated by features containing ligands to both CD3 and CD28, this functional convergence was curtailed on surfaces in which engagement of these two systems was separated by micrometer-scale distances. Moreover, phosphorylated Lck was concentrated to regions of CD3 engagement and exhibited a low diffusion rate, suggesting that costimulation is controlled by a balance between the transport of active Lck to CD28 and its deactivation. In support of this model, disruption of the actin cytoskeleton increased Lck mobility and allowed functional T-cell costimulation by spatially separated CD3 and CD28. In primary mouse CD4(+) T cells, a complementary system, reducing the membrane mobility increased the sensitivity to CD3-CD28 separation. These results demonstrate a subcellular reaction-diffusion system that allows cells to sense the microscale organization of the extracellular environment.

Ligand density elicits a phenotypic switch in human neutrophils

Neutrophils are mediators of innate immunity and motility is critical to their function. We used microcontact printing to investigate the relationship between density of adhesive ligands and the dynamics of neutrophil motility. We show that neutrophils adopt a well-spread morphology without a uropod on moderate densities of adhesion ligand. As density is increased, the morphology switches to a classic amoeboid shape. In addition to the morphological differences, the dynamics of motility were quantitatively distinct. Well-spread cells without uropods glide slowly with high persistence, while amoeboid cells made frequent directional changes migrating quickly with low persistence. Using an antibody panel against various integrin chains, we show that adhesion and motility on fibronectin are mediated by MAC-1 (αMβ2). The phenotypic switch could be generalized to other surface ligands, such as bovine serum albumin, to which the promiscuous MAC-1 also binds. These results suggest that neutrophils are capable of displaying multiple modes of motility as dictated by their adhesive environment.

Micropatterned multicolor dynamically adhesive substrates to control cell adhesion and multicellular organization

We present a novel technique to examine cell-cell interactions and directed cell migration using micropatterned substrates of three distinct regions: an adhesive region, a nonadhesive region, and a dynamically adhesive region switched by addition of a soluble factor to the medium. Combining microcontact printing with avidin-biotin capture chemistry, we pattern nonadhesive regions of avidin that become adhesive through the capture of biotinylated fibronectin. Our strategy overcomes several limitations of current two-color dynamically adhesive substrates by incorporating a third, permanently nonadhesive region. Having three spatially and functionally distinct regions allows for the realization of more complex configurations of cellular cocultures as well as intricate interface geometries between two cell populations for diverse heterotypic cell-cell interaction studies. We can now achieve spatial control over the path and direction of migration in addition to temporal control of the onset of migration, enabling studies that better recapitulate coordinated multicellular migration and organization in vitro. We confirm that cellular behavior is unaltered on captured biotinylated fibronectin as compared to printed fibronectin by examining the cells' ability to spread, form adhesions, and migrate. We demonstrate the versatility of this approach in studies of migration and cellular cocultures, and further highlight its utility by probing Notch-Delta juxtacrine signaling at a patterned interface.

Topographical control of multiple cell adhesion molecules for traction force microscopy

Cellular traction forces are important quantitative measures in cell biology as they have provided much insight into cell behavior in contexts such as cellular migration, differentiation, and disease progression. However, the complex environment in vivo permits application of cell traction forces through multiple types of cell adhesion molecules. Currently available approaches to differentiate traction forces among multiple cell adhesion molecules are limited to specialized approaches to decouple cell-cell from cell-extracellular matrix (ECM) tractions. Here, we present a technique which uses indirect micropatterning onto a polyacrylamide gel to pattern multiple, spatially distinct fluorescently labeled ECM proteins, specifically gelatin and fibronectin (Fn), and confine the area to which cells can adhere. We found that cells interacting with both gelatin and Fn altered their traction forces significantly in comparison to cells on Fn-only substrates. This crosstalk interaction resulted in a decrease in overall traction forces on dual-patterned substrates as compared to cells on Fn-only substrates. This illustrates the unique need to study such interactions and demonstrates great potential in future studies in multi-ligand environments. Current micropatterning techniques on glass can easily be adapted to present other protein classes, such as cadherins, while maintaining control of adhesion spacing, cell spread area, and stiffness, each of which are important regulators of cell mechanobiology.

"Stamp-off" to micropattern sparse, multicomponent features

Spatially patterned subtractive de-inking, a process we term "stamp-off," provides a simple method to generate sparse, multicomponent protein micropatterns. It has been applied to control cell adhesion, study adhesion biology, as well as to micropattern fragile surfaces. This technique can also readily be applied to study nanoscale interactions between cell membrane receptors and surface-immobilized ligands. It is based on conventional microcontact printing and as such requires the same reagents, including photolithographically defined masters, a spin-coater, poly(dimethyl siloxane) (PDMS), and conventional cell culture reagents such as glass coverslips and adhesive proteins. Stamp-off is conceptually simplified into three steps: (1) generation of an appropriate cell culture substrate, PDMS-coated glass, (2) micropatterning with stamp-off, and (3) cell deposition. After elaborating each of these three methods, we discuss limitations of the technique and its applications.

Patterned hydrogels for simplified measurement of cell traction forces

To understand mechanobiology, a quantitative understanding of how cells interact mechanically with their environment is needed. Cell mechanics is important to study as they play a role in cell behaviors ranging from cell signaling to epithelial to mesenchymal transition in physiological processes such as development and cancer. To study changes in cell contractile behavior, numerous quantitative measurement techniques have been developed based on the measurement of deformations of a substrate from an initial state. Herein, we present details on a technique we have developed for the measurements of 2D cellular traction forces with the goal of facilitating adaptation of this technique by other investigators. This technique is flexible in that it utilizes well-studied methods for microcontact printing and fabrication of polyacrylamide hydrogels to generate regular arrays of patterns that can be transferred onto the hydrogels. From the deformation of the arrays, an automated algorithm can be used to quantitatively determine the traction forces exerted by the cells onto the adhesion points. The simplicity and flexibility of this technique make it a useful contribution to our toolbox for measurement of cell traction forces.

Cell adhesion and shape regulate TGF-beta1-induced epithelial-myofibroblast transition via MRTF-A signaling

Myofibroblasts, specialized cells that play important roles in wound healing and fibrosis, can develop from epithelial cells through an epithelial-mesenchymal transition (EMT). During EMT, epithelial cells detach from neighboring cells and acquire an elongated, mesenchymal-like morphology. These phenotypic changes are accompanied by changes in gene expression patterns including upregulation of a variety of cytoskeletal associated proteins which contribute to the ability of myofibroblasts to exert large contractile forces. Here, the relationship between cell shape and cytoskeletal tension and the expression of cytoskeletal proteins in transforming growth factor (TGF)-β1-induced EMT is determined. We find that culturing cells in conditions which permit cell spreading and increased contractility promotes the increased expression of myofibroblast markers and cytoskeletal associated proteins. In contrast, blocking cell spreading prevents transdifferentiation to the myofibroblast phenotype. Furthermore, we find that cell shape regulates the expression of cytoskeletal proteins by controlling the subcellular localization of myocardin related transcription factor (MRTF)-A. Pharmacological inhibition of cytoskeletal tension or MRTF-A signaling blocks the acquisition of a myofibroblast phenotype in spread cells while overexpression of MRTF-A promotes the expression of cytoskeletal proteins for all cell shapes. These data suggest that cell shape is a critical determinant of myofibroblast development from epithelial cells.

Contact inhibition of locomotion probabilities drive solitary versus collective cell migration

Contact inhibition of locomotion (CIL) is the process whereby cells collide, cease migrating in the direction of the collision, and repolarize their migration machinery away from the collision. Quantitative analysis of CIL has remained elusive because cell-to-cell collisions are infrequent in traditional cell culture. Moreover, whereas CIL predicts mutual cell repulsion and 'scattering' of cells, the same cells in vivo are observed to undergo CIL at some developmental times and collective cell migration at others. It remains unclear whether CIL is simply absent during collective cell migration, or if the two processes coexist and are perhaps even related. Here, we used micropatterned stripes of extracellular matrix to restrict cell migration to linear paths such that cells polarized in one of two directions and collisions between cells occurred frequently and consistently, permitting quantitative and unbiased analysis of CIL. Observing repolarization events in different contexts, including head-to-head collision, head-to-tail collision, collision with an inert barrier, or no collision, and describing polarization as a two-state transition indicated that CIL occurs probabilistically, and most strongly upon head-to-head collisions. In addition to strong CIL, we also observed 'trains' of cells moving collectively with high persistence that appeared to emerge from single cells. To reconcile these seemingly conflicting observations of CIL and collective cell migration, we constructed an agent-based model to simulate our experiments. Our model quantitatively predicted the emergence of collective migration, and demonstrated the sensitivity of such emergence to the probability of CIL. Thus CIL and collective migration can coexist, and in fact a shift in CIL probabilities may underlie transitions between solitary cell migration and collective cell migration. Taken together, our data demonstrate the emergence of persistently polarized, collective cell movement arising from CIL between colliding cells.

Single-vesicle patterning of uniform, giant polymersomes into microarrays

Giant, cell-sized polymersomes are functionalized and patterned at the single vesicle level. Microfluidic methods are employed to generate uniform diameter vesicles with high loading efficiencies and microcontact printing is used to generate patterns of adhesive ligand. A simple sensory capability is demonstrated with the immobilized array of vesicles.

Mesenchymal stem cell mechanobiology and emerging experimental platforms

Experimental control over progenitor cell lineage specification can be achieved by modulating properties of the cell's microenvironment. These include physical properties of the cell adhesion substrate, such as rigidity, topography and deformation owing to dynamic mechanical forces. Multipotent mesenchymal stem cells (MSCs) generate contractile forces to sense and remodel their extracellular microenvironments and thereby obtain information that directs broad aspects of MSC function, including lineage specification. Various physical factors are important regulators of MSC function, but improved understanding of MSC mechanobiology requires novel experimental platforms. Engineers are bridging this gap by developing tools to control mechanical factors with improved precision and throughput, thereby enabling biological investigation of mechanics-driven MSC function. In this review, we introduce MSC mechanobiology and review emerging cell culture platforms that enable new insights into mechanobiological control of MSCs. Our main goals are to provide engineers and microtechnology developers with an up-to-date description of MSC mechanobiology that is relevant to the design of experimental platforms and to introduce biologists to these emerging platforms.

Decoupling substrate stiffness, spread area, and micropost density: a close spatial relationship between traction forces and focal adhesions

Mechanical cues can influence the manner in which cells generate traction forces and form focal adhesions. The stiffness of a cell's substrate and the available area on which it can spread can influence its generation of traction forces, but to what extent these factors are intertwined is unclear. In this study, we used microcontact printing and micropost arrays to control cell spreading, substrate stiffness, and post density to assess their effect on traction forces and focal adhesions. We find that both the spread area and the substrate stiffness influence traction forces in an independent manner, but these factors have opposite effects: cells on stiffer substrates produce higher average forces, whereas cells with larger spread areas generate lower average forces. We show that post density influences the generation of traction forces in a manner that is more dominant than the effect of spread area. Additionally, we observe that focal adhesions respond to spread area, substrate stiffness, and post density in a manner that closely matches the trends seen for traction forces. This work supports the notion that traction forces and focal adhesions have a close relationship in their response to mechanical cues.

Micro- and nanoscale engineering of cell signaling

It is increasingly recognized that cell signaling, as a chemical process, must be considered at the local, micrometer scale. Micro- and nanofabrication techniques provide access to these dimensions, with the potential to capture and manipulate the spatial complexity of intracellular signaling in experimental models. This review focuses on recent advances in adapting surface engineering for use with biomolecular systems that interface with cell signaling, particularly with respect to surfaces that interact with multiple receptor systems on individual cells. The utility of this conceptual and experimental approach is demonstrated in the context of epithelial cells and T lymphocytes, two systems whose ability to perform their physiological function is dramatically impacted by the convergence and balance of multiple signaling pathways.

A Novel Technique for Micro-patterning Proteins and Cells on Polyacrylamide Gels

Spatial patterning of proteins (extracellular matrix, ECM) for living cells on polyacrylamide (PA) hydrogels has been technically challenging due to the compliant nature of the hydrogels and their aqueous environment. Traditional micro-fabrication process is not applicable. Here we report a simple, novel and general method to pattern a variety of commonly used cell adhesion molecules, i.e. Fibronectin (FN), Laminin (LN) and Collagen I (CN), etc. on PA gels. The pattern is first printed on a hydrophilic glass using polydimethylsiloxane (PDMS) stamp and micro-contact printing (μCP). Pre-polymerization solution is applied on the patterned glass and is then sandwiched by a functionalized glass slide, which covalently binds to the gel. The hydrophilic glass slide is then peeled off from the gel when the protein patterns detach from the glass, but remain intact with the gel. The pattern is thus transferred to the gel. The mechanism of pattern transfer is studied in light of interfacial mechanics. It is found that hydrophilic glass offers strong enough adhesion with ECM proteins such that a pattern can be printed, but weak enough adhesion such that they can be completely peeled off by the polymerized gel. This balance is essential for successful pattern transfer. As a demonstration, lines of FN, LN and CN with widths varying from 5-400 μm are patterned on PA gels. Normal fibroblasts (MKF) are cultured on the gel surfaces. The cell attachment and proliferation are confined within these patterns. The method avoids the use of any toxic chemistry often used to pattern different proteins on gel surfaces.

Microengineered synthetic cellular microenvironment for stem cells

Stem cells possess the ability of self-renewal and differentiation into specific cell types. Therefore, stem cells have great potentials in fundamental biology studies and clinical applications. The most urgent desire for stem cell research is to generate appropriate artificial stem cell culture system, which can mimic the dynamic complexity and precise regulation of the in vivo biochemical and biomechanical signals, to regulate and direct stem cell behaviors. Precise control and regulation of the biochemical and biomechanical stimuli to stem cells have been successfully achieved using emerging micro/nanoengineering techniques. This review provides insights into how these micro/nanoengineering approaches, particularly microcontact printing and elastomeric micropost array, are applied to create dynamic and complex environment for stem cells culture.

A micropatterning and image processing approach to simplify measurement of cellular traction forces

Quantification of the traction forces that cells apply to their surroundings has been critical to the advancement of our understanding of cancer, development and basic cell biology. This field was made possible through the development of engineered cell culture systems that permit optical measurement of cell-mediated displacements and computational algorithms that allow conversion of these displacements into stresses and forces. Here, we present a novel advancement of traction force microscopy on polyacrylamide (PAA) gels that addresses limitations of existing technologies. Through an indirect patterning technique, we generated PAA gels with fluorescent 1 μm dot markers in a regularized array. This improves existing traction measurements since (i) multiple fields of view can be measured in one experiment without the need for cell removal; (ii) traction vectors are modeled as discrete point forces, and not as a continuous field, using an extremely simple computational algorithm that we have made available online; and (iii) the pattern transfer technique is amenable to any of the published techniques for producing patterns on glass. In the future, this technique will be used for measuring traction forces on complex patterns with multiple, spatially distinct ligands in systems for applying strain to the substrate, and in sandwich cultures that generate quasi-three-dimensional environments for cells.

Synergistic regulation of cell function by matrix rigidity and adhesive pattern

Cell-extracellular matrix (ECM) interactions play a critical role in regulating cellular behaviors. Recent studies of cell-ECM interactions have mainly focused on the actomyosin based and adhesion mediated mechanosensing pathways to understand how individual mechanical signals in the cell microenvironment, such as matrix rigidity and adhesive ECM pattern, are sensed by the cell and further trigger downstream intracellular signaling cascades and cellular responses. However, synergistic and collective regulation of cellular behaviors by matrix rigidity and adhesive ECM pattern are still elusive and largely uncharacterized. Here, we generated a library of microfabricated polydimethylsiloxane (PDMS) micropost arrays to study the synergistic and independent effects of matrix rigidity and adhesive ECM pattern on mechanoresponsive behaviors of both NIH/3T3 fibroblasts and human umbilical vein endothelial cells (HUVECs). We showed that both cell types were mechanosensitive and their cell spreading, FA formation, cytoskeletal contractility, and proliferation were all strongly dependent on both substrate rigidity and adhesive ECM pattern. We further showed that under the same substrate rigidity condition, smaller and closer adhesive ECM islands would cause both cells to spread out more, form more adhesion structures, and have a higher proliferation rate. The influence of adhesive ECM pattern on rigidity-mediated cytoskeletal contractility was cell type specific and was only significant for NIH/3T3. Morphometric analysis of cell populations revealed a strong correlation between focal adhesion and cell spreading, regardless of substrate rigidity and adhesive ECM pattern. We also observed a strong correlation between cellular traction force and cell spreading, with a substantially smaller independent effect of substrate rigidity on traction force. Our study here had determined key aspects of the biomechanical responses of adherent cells to independent and collective changes of substrate rigidity and adhesive ECM pattern.