Surface chemistry and cell biological tools for the analysis of cell adhesion and migration

Programming and monitoring surface-confined DNA computing

DNA-based molecular computing has evolved to encompass a diverse range of functions, demonstrating substantial promise for both highly parallel computing and various biomedical applications. Recent advances in DNA computing systems based on surface reactions have demonstrated improved levels of specificity and computational speed compared to their solution-based counterparts that depend on three-dimensional molecular collisions. Herein, computational biomolecular interactions confined by various surfaces such as DNA origamis, nanoparticles, lipid membranes and chips are systematically reviewed, along with their manipulation methodologies. Monitoring techniques and applications for these surface-based computing systems are also described. The advantages and challenges of surface-confined DNA computing are discussed.

Sequential Photoactivation of Self-Assembled Monolayers to Direct Cell Adhesion and Migration

Dynamic substrates for cell culture control the spatial and temporal presentation of extracellular matrix ligands that interact with adherent cells. This paper reports a photoactive surface chemistry that can repeatedly activate regions of the substrate for cell adhesion, spreading, and migration. The approach uses self-assembled monolayers presenting the integrin ligand RGD that is caged with a nitrophenyl-based photoprotecting group. The group is also modified with a maltoheptaose oligosaccharide to prevent nonspecific protein adsorption and cell attachment. The peptide is uncaged when irradiated with a laser source at 405 nm on a microscope to reveal micron-size regions for single cell attachment. This method is applied to studies of gap junction-mediated communication between two neighboring cells and requires the patterning of an initial receiver cell population and then the patterning of a second sender population to give a culture wherein each pair of cells are separated by 30 μm. Finally, activation of the region between the cells permits cell-cell contact and gap junction assembly between the sender and receiver cells. This example demonstrates the broad relevance of this method to studying complex phenotypes in cell culture.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Engineering nanoscale stem cell niche: direct stem cell behavior at cell-matrix interface

Biophysical cues on the extracellular matrix (ECM) have proven to be significant regulators of stem cell behavior and evolution. Understanding the interplay of these cells and their extracellular microenvironment is critical to future tissue engineering and regenerative medicine, both of which require a means of controlled differentiation. Research suggests that nanotopography, which mimics the local, nanoscale, topographic cues within the stem cell niche, could be a way to achieve large-scale proliferation and control of stem cells in vitro. This Progress Report reviews the history and contemporary advancements of this technology, and pays special attention to nanotopographic fabrication methods and the effect of different nanoscale patterns on stem cell response. Finally, it outlines potential intracellular mechanisms behind this response.

Haptotaxis is cell type specific and limited by substrate adhesiveness

Motile cells navigate through tissue by relying on tactile cues from gradients provided by extracellular matrix (ECM) such as ligand density or stiffness. Mesenchymal stem cells (MSCs) and fibroblasts encounter adhesive or 'haptotactic' gradients at the interface between healthy and fibrotic tissue as they migrate towards an injury site. Mimicking this phenomenon, we developed tunable RGD and collagen gradients in polyacrylamide hydrogels of physiologically relevant stiffness using density gradient multilayer polymerization (DGMP) to better understand how such ligand gradients regulate migratory behaviors. Independent of ligand composition and fiber deformation, haptotaxis was observed in mouse 3T3 fibroblasts. Human MSCs however, haptotaxed only when cell-substrate adhesion was indirectly reduced via addition of free soluble matrix ligand mimetic peptides. Under basal conditions, MSCs were more contractile than fibroblasts. However, the presence of soluble adhesive peptides reduced MSC-induced substrate deformations; increased contractility may contribute to limited migration, but modulating cytoskeletal assembly was ineffective at promoting MSC haptotaxis. When introduced to gradients of increased absolute ligand concentrations, 3T3s displayed increased contractility and no longer haptotaxed. These data suggest that haptotactic behaviors are limited by adhesion and that although both cell types may home to tissue to aid in repair, fibroblasts may be more responsive to ligand gradients than MSCs.

Effect of surface chemistry on the integrin induced pathway in regulating vascular endothelial cells migration

The migration of vascular endothelial cells (ECs) is essential for reendothelialization after implantation of cardiovascular biomaterials. Reendothelialization is largely determined by surface properties of implants. In this study, surfaces modified with various chemical functional groups (CH3, NH2, COOH, OH) prepared by self-assembled monolayers (SAMs) were used as model system. Expressions and distributions of critical proteins in the integrin-induced signaling pathway were examined to explore the mechanisms of surface chemistry regulating EC migration. The results showed that SAMs modulated cell migration were in the order CH3>NH2>OH>COOH, determined by differences in the expressions of focal adhesion components and Rho GTPases. Multiple integrin subunits showed difference in a surface chemistry-dependent manner, which induced a stepwise activation of signaling cascades associated with EC migration. This work provides a broad overview of surface chemistry regulated endothelial cell migration and establishes association among the surface chemistry, cell migration behavior and associated integrin signaling events. Understanding the relationship between these factors will help us to understand the surface/interface behavior between biomaterials and cells, reveal molecular mechanism of cells sensing surface characterization, and guide surface modification of cardiovascular implanted materials.

Chelation gradients for investigation of metal ion binding at silica surfaces

Centimeter-long surface gradients in bi- and tridentate chelating agents have been formed via controlled rate infusion, and the coordination of Cu(2+) and Zn(2+) to these surfaces has been examined as a function of distance by X-ray photoelectron spectroscopy (XPS). 3-(Trimethoxysilylpropyl)ethylenediamine and 3-(trimethoxysilylpropyl)diethylenetriamine were used as precursor silanes to form the chelation gradients. When the gradients were exposed to a metal ion solution, a series of coordination complexes formed along the length of the substrate. For both chelating agents at the three different concentrations studied, the amine content gradually increased from top to bottom as expected for a surface chemical gradient. While the Cu 2p peak area had nearly the same profile as nitrogen, the Zn 2p peak area did not and exhibited a plateau along much of the gradient. The normalized nitrogen-to-metal peak area ratio (N/M) was found to be highly dependent on the type of ligand, its surface concentration, and the type of metal ion. For Cu(2+), the N/M ratio ranged from 8 to 11 on the diamine gradient and was ∼4 on the triamine gradient, while for Zn(2+), the N/M ratio was 4-8 on diamine and 5-7 on triamine gradients. The extent of protonation of amine groups was higher for the diamine gradients, which could lead to an increased N/M ratio. Both 1:1 and 1:2 ligand/metal complexes along with dinuclear complexes are proposed to form, with their relative amounts dependent on the ligand, ligand density, and metal ion. Collectively, the methods and results described herein represent a new approach to study metal ion binding and coordination on surfaces, which is especially important to the extraction, preconcentration, and separation of metal ions.

Switchable substrates for analyzing and engineering cellular functions

Cellular activity is highly dependent on the extracellular environment, which is composed of surrounding cells and extracellular matrices. This focus review summarizes recent advances in chemically and physically engineered switchable substrates designed to control such cellular microenvironments by application of an external stimulus. Special attention is given to their molecular design, switching strategies, and representative examples for bioanalytical and biomedical applications.

Creating adhesive and soluble gradients for imaging cell migration with fluorescence microscopy

Cells can sense and migrate towards higher concentrations of adhesive cues such as the glycoproteins of the extracellular matrix and soluble cues such as growth factors. Here, we outline a method to create opposing gradients of adhesive and soluble cues in a microfluidic chamber, which is compatible with live cell imaging. A copolymer of poly-L-lysine and polyethylene glycol (PLL-PEG) is employed to passivate glass coverslips and prevent non-specific adsorption of biomolecules and cells. Next, microcontact printing or dip pen lithography are used to create tracks of streptavidin on the passivated surfaces to serve as anchoring points for the biotinylated peptide arginine-glycine-aspartic acid (RGD) as the adhesive cue. A microfluidic device is placed onto the modified surface and used to create the gradient of adhesive cues (100% RGD to 0% RGD) on the streptavidin tracks. Finally, the same microfluidic device is used to create a gradient of a chemoattractant such as fetal bovine serum (FBS), as the soluble cue in the opposite direction of the gradient of adhesive cues.

Aminoalkoxysilane reactivity in surface amine gradients prepared by controlled-rate infusion

The reactivity of a series of substituted aminoalkoxysilanes for surface amine gradient formation has been studied using a newly developed time-based exposure method termed controlled-rate infusion (CRI). The aminoalkoxysilanes used include those that contain primary, secondary, and tertiary monoamines as well as more than one amine group (diamine and triamine). X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of a gradient in each case and to acquire detailed information on gradient composition from which kinetic data were obtained. The total area under the N 1s XPS spectra allows for the extent of amine modification to be quantitatively assessed along each gradient. The N 1s peaks actually appear as doublets, providing additional data on the level of protonation and, hence, amine basicity on the dry surface. The degree of protonation showed an interesting trend toward smaller values running from top to bottom along gradients incorporating the most basic amines. The gradient profiles, including initial steepness and extent of saturation, were shown to be highly dependent on the aminoalkoxysilane precursor employed. The highest levels of modification were achieved for the diamine and primary monoamine precursors while the more hindered amines produced lower levels of surface modification and took longer for saturation to be achieved. By fitting the gradient data to a simple first-order kinetic model, rate constants for the condensation reaction between each aminosilane and accessible surface silanol groups were obtained. The rate constants follow the trend: triamine ~ diamine > monoamine and primary > secondary > tertiary, indicating kinetic factors also play an important role in controlling surface modification. The presence of more than one amine group on the silane is concluded to enhance the rate of condensation to the surface silanol groups, while the more hindered secondary and tertiary amines slow condensation. Collectively, the results provide valuable new data on how the number of amine groups, degree of substitution, and steric hindrance influence silane reactivity with silica surfaces, amine surface coverage, and basicity along the gradient profile.

Reversible and oriented immobilization of ferrocene-modified proteins

Adopting supramolecular chemistry for immobilization of proteins is an attractive strategy that entails reversibility and responsiveness to stimuli. The reversible and oriented immobilization and micropatterning of ferrocene-tagged yellow fluorescent proteins (Fc-YFPs) onto β-cyclodextrin (βCD) molecular printboards was characterized using surface plasmon resonance (SPR) spectroscopy and fluorescence microscopy in combination with electrochemistry. The proteins were assembled on the surface through the specific supramolecular host-guest interaction between βCD and ferrocene. Application of a dynamic covalent disulfide lock between two YFP proteins resulted in a switch from monovalent to divalent ferrocene interactions with the βCD surface, yielding a more stable protein immobilization. The SPR titration data for the protein immobilization were fitted to a 1:1 Langmuir-type model, yielding K(LM) = 2.5 × 10(5) M(-1) and K(i,s) = 1.2 × 10(3) M(-1), which compares favorably to the intrinsic binding constant presented in the literature for the monovalent interaction of ferrocene with βCD self-assembled monolayers. In addition, the SPR binding experiments were qualitatively simulated, confirming the binding of Fc-YFP in both divalent and monovalent fashion to the βCD monolayers. The Fc-YFPs could be patterned on βCD surfaces in uniform monolayers, as revealed using fluorescence microscopy and atomic force microscopy measurements. Both fluorescence microscopy imaging and SPR measurements were carried out with the in situ capability to perform cyclic voltammetry and chronoamperometry. These studies emphasize the repetitive desorption and adsorption of the ferrocene-tagged proteins from the βCD surface upon electrochemical oxidation and reduction, respectively.

Cell fluidics: producing cellular streams on micropatterned synthetic surfaces

Patterning cell-adhesive molecules on material surfaces provides a powerful tool for controlling and guiding cell locomotion and migration. Here we report fast, reliable, easy to implement methods to fabricate large patterns of proteins on synthetic substrates to control the direction and speed of cells. Two common materials exhibiting very different protein adsorption capacities, namely, polystyrene and Teflon, were functionalized with micrometric stripes of laminin. The protein was noncovalently immobilized onto the surface by following either lithographically controlled wetting (LCW) or micromolding in capillaries (MIMIC). These techniques proved to be sufficiently mild so as not to interfere with the protein adhesion capability. Cells adhered onto the functionalized stripes and remained viable for more than 20 h. During this time frame, cells migrated along the lanes and the dynamics of motion was strongly affected by the substrate surface chemistry and culturing conditions. In particular, enhanced mismatches of cell adhesive properties obtained by the juxtaposition of bare and laminin-functionalized Teflon caused cells to move slowly and their movement to be highly confined. The patterning procedure was also effective at guiding migration on conventional cell culture dishes that were functionalized with laminin patterns, even in the presence of serum proteins, although to a lesser extent compared to that for Teflon. This work demonstrates the possibility of creating well-defined, long-range cellular streams on synthetic substrates by pursuing straightforward functionalizing techniques that can be implemented for a broad class of materials under conventional, long-time cell-culturing conditions. The procedure effectively confines cells to migrate along predefined patterns and can be implemented in different biomedical and biotechnological applications.

Bio-inspired materials for parsing matrix physicochemical control of cell migration: a review

Cell motility is ubiquitous in both normal and pathophysiological processes. It is a complex biophysical response elicited via the integration of diverse extracellular physicochemical cues. The extracellular matrix directs cell motility via gradients in morphogens (a.k.a. chemotaxis), adhesive proteins (haptotaxis), and stiffness (durotaxis). Three-dimensional geometrical and proteolytic cues also constitute key regulators of motility. Therefore, cells process a variety of physicochemical signals simultaneously, while making informed decisions about migration via intracellular processing. Over the last few decades, bioengineers have created and refined natural and synthetic in vitro platforms in an attempt to isolate these extracellular cues and tease out how cells are able to translate this complex array of dynamic biochemical and biophysical features into functional motility. Here, we review how biomaterials have played a key role in the development of these types of model systems, and how recent advances in engineered materials have significantly contributed to our current understanding of the mechanisms of cell migration.

Tissue morphing control on dynamic gradient surfaces

In this report, we develop smart surfaces for the spatial and temporal control of mammalian cell behavior. We integrate a bioactive surface strategy with a photo-electroactive surface strategy to generate dynamic ligand surface gradients for controlling cell adhesion, tissue shape morphing, and cell tissue migration.

Microfluidics-based devices: New tools for studying cancer and cancer stem cell migration

Cell movement is highly sensitive to stimuli from the extracellular matrix and media. Receptors on the plasma membrane in cells can activate signal transduction pathways that change the mechanical behavior of a cell by reorganizing motion-related organelles. Cancer cells change their migration mechanisms in response to different environments more robustly than noncancer cells. Therefore, therapeutic approaches to immobilize cancer cells via inhibition of the related signal transduction pathways rely on a better understanding of cell migration mechanisms. In recent years, engineers have been working with biologists to apply microfluidics technology to study cell migration. As opposed to conventional cultures on dishes, microfluidics deals with the manipulation of fluids that are geometrically constrained to a submillimeter scale. Such small scales offer a number of advantages including cost effectiveness, low consumption of reagents, high sensitivity, high spatiotemporal resolution, and laminar flow. Therefore, microfluidics has a potential as a new platform to study cell migration. In this review, we summarized recent progress on the application of microfluidics in cancer and other cell migration researches. These studies have enhanced our understanding of cell migration and cancer invasion as well as their responses to subtle variations in their microenvironment. We hope that this review will serve as an interdisciplinary guidance for both biologists and engineers as they further develop the microfluidic toolbox toward applications in cancer research.