Site-specific presentation of single recombinant proteins in defined nanoarrays

TiO2 Nano-Biopatterning Reveals Optimal Ligand Presentation for Cell-Matrix Adhesion Formation

Nanoscale organization of transmembrane receptors is critical for cellular functions, enabled by the nanoscale engineering of bioligand presentation. Previously, a spatial threshold of ≤60 nm for integrin binding ligands in cell-matrix adhesion is demonstrated using monoliganded gold nanoparticles. However, the ligand geometric arrangement is limited to hexagonal arrays of monoligands, while plasmonic quenching limits further investigation by fluorescence-based high-resolution imaging. Here, these limitations are overcome with dielectric TiO2 nanopatterns, eliminating fluorescence quenching, thus enabling super-resolution fluorescence microscopy on nanopatterns. By dual-color super-resolution imaging, high precision and consistency among nanopatterns, bioligands, and integrin nanoclusters are observed, validating the high quality and integrity of both nanopattern functionalization and passivation. By screening TiO2 nanodiscs with various diameters, an increase in fibroblast cell adhesion, spreading area, and Yes-associated protein (YAP) nuclear localization on 100 nm diameter compared with smaller diameters was observed. Focal adhesion kinase is identified as the regulatory signal. These findings explore the optimal ligand presentation when the minimal requirements are sufficiently fulfilled in the heterogenous extracellular matrix network of isolated binding regions with abundant ligands. Integration of high-fidelity nano-biopatterning with super-resolution imaging allows precise quantitative studies to address early signaling events in response to receptor clustering and their nanoscale organization.

Proteins on Supported Lipid Bilayers Diffusing around Proteins Fixed on Acrylate Anchors

Mobility of proteins and lipids plays a major role in physiological processes. Platforms which were developed to study protein interaction between immobilized and mobile proteins suffer from shortcomings such as fluorescence quenching or complicated fabrication methods. Here we report a versatile platform comprising immobilized histidine-tagged proteins and biotinylated proteins in a mobile phase. Importantly, multiphoton photolithography was used for easy and fast fabrication of the platform and allows, in principle, extension of its application to three dimensions. The platform, which is made up of functionalized polymer structures embedded in a mobile lipid bilayer, shows low background fluorescence and allows for mobility of arbitrary proteins.

Morphological Plasticity of Human Melanoma Cells Is Determined by Nanoscopic Patterns of E- and N-Cadherin Interactions

Loss of E-cadherin and concomitant upregulation of N-cadherin is known as the cadherin switch, and has been implicated in melanoma progression. Mechanistically, homophilic ligation of N-cadherin-expressing melanoma cells with N-cadherin presented within the microenvironment is thought to facilitate invasion. However, the biophysical aspects governing molecular specificity and function of such interactions remain unclear. By using precisely defined nano-patterns of N- or E-cadherin (with densities tunable by more than one order of magnitude from 78 to 1,128 ligands/μm²), we analyzed adhesion and spreading of six different human melanoma cell lines with distinct constitutive cadherin expression patterns. Cadherin-mediated homophilic cell interactions (N/N and E/E) with cadherin-functionalized nano-matrices revealed an unexpected functional dichotomy inasmuch as melanoma cell adhesion was cadherin density-dependent, while spreading and lamellipodia formation were independent of cadherin density. Surprisingly, E-cadherin-expressing melanoma cells also interacted with N-cadherin-presenting nano-matrices, suggesting heterophilic (N/E) interactions. However, cellular spreading in these cases occurred only at high densities of N-cadherin (i.e., >285 ligands/μm²). Overall, our approach using nano-patterned biomimetic surfaces provides a platform to further refine the roles of cadherins in tumor cell behavior and it revealed an intriguing flexibility of mutually compensating N- and E-cadherin interactions relevant for melanoma progression.

Spatial Control of Biological Ligands on Surfaces Applied to T Cell Activation

In this chapter, we present techniques, based on molecular-scale nanofabrication and selective self-assembly, for the presentation of biomolecules of interest (ligands, receptors, etc.) on a surface with precise spatial control and arbitrary geometry at the single-molecule level. Metallic nanodot arrays are created on glass coverslips and are then used as anchors for the immobilization of biological ligands via thiol linking chemistry. The nanodot size is controlled by both lithography and metallization. The reagent concentration in self-assembly can be adjusted to ensure single-molecule occupancy for a given dot size. The surrounding glass is backfilled by a protein-repellent layer to prevent nonspecific adsorption. Moreover, bifunctional surfaces are created, whereby a second ligand is presented on the background, which is frequently a requirement for simulating complex cellular functions involving more than one key ligand. This platform serves as a novel and powerful tool for molecular and cellular biology, e.g., to study the fundamental mechanisms of receptor-mediated signaling.

Nanoscale Tuning of VCAM-1 Determines VLA-4-Dependent Melanoma Cell Plasticity on RGD Motifs

The biophysical fine-tuning of cancer cell plasticity is crucial for tumor progression but remains largely enigmatic. Although vascular cell adhesion molecule-1 (VCAM-1/CD106) has been implicated in melanoma progression, here its presentation on endothelial cells was associated with diminished melanoma cell spreading. Using a specific nanoscale modulation of VCAM-1 (tunable from 70 to 670 ligands/μm²) next to integrin ligands (RGD motifs) in a bifunctional system, reciprocal regulation of integrin α4 (ITGA4/VLA-4/CD49d)-dependent adhesion and spreading of melanoma cells was found. As the VCAM-1/VLA-4 receptor pair facilitated adhesion, while at the same time antagonizing RGD-mediated spreading, melanoma cell morphogenesis on these bifunctional matrices was directly regulated by VCAM-1 in a dichotomic and density-dependent fashion. This was accompanied by concordant regulation of F-actin cytoskeleton remodeling, Rac1-expression, and paxillin-related adhesion formation. The novel function of VCAM-1 was corroborated in vivo using two murine models of pulmonary metastasis. The regulation of melanoma cell plasticity by VCAM-1 highlights the complex regulation of tumor-matrix interactions.Implications: Nanotechnology has revealed a novel dichotomic function of the VCAM-1/VLA-4 interaction on melanoma cell plasticity, as nanoscale tuning of this interaction reciprocally determines adhesion and spreading in a ligand density-dependent manner. Mol Cancer Res; 16(3); 528-42. ©2017 AACR.

Nanopatterned Adhesive, Stretchable Hydrogel to Control Ligand Spacing and Regulate Cell Spreading and Migration

Spatial molecular patterning enables the regulation of adhesion receptor clustering and can thus play a pivotal role in multiple biological activities such as cell adhesion, viability, proliferation, and differentiation. A wide range of nanopatterned, adhesive interfaces have been designed to decipher the essence of molecular-scale interactions between cells and the adhesive interface. Although an interligand spacing of less than 70 nm is a proven prerequisite for the formation of stable focal adhesions, there is a paucity of data concerning how cells behave on substrates featuring heterogeneous adhesiveness. In this study, a stretchable hydrogel functionalized with a quasi-hexagonally arranged nanoarray was stretched along one direction, resulting in ligands periodically arranged in a pattern resembling a centered rectangular lattice with an interligand spacing smaller than 70 nm in one direction and greater than 70 nm in the orthogonal direction. This substrate was utilized to modulate interligand spacing and investigate cell adhesion and migration. An interligand spacing larger than 70 nm-even in just one direction-prevented the establishment of stable focal adhesions. The stretched interface promoted dynamic remodeling at cell contacts, resulting in higher cellular mobility. Our nanopatterned stretchable hydrogel permits reversible control over cell adhesion and migration on nanopatterned ligand interfaces.

Controlled Self-Assembly of Hexagonal Nanoparticle Patterns on Nanotopographies

Diblock copolymer micelle nanolithography (BCML) is a versatile and efficient method to cover large surface areas with hexagonally ordered arrays of metal nanoparticles, in which the nanoparticles are equally spaced. However, this method falls short of providing a controlled allocation of such regular nanoparticle arrays with specific spacing into micropatterns. We present here a quick and high-throughput method to generate quasi-hexagonal nanoparticle structures with well-defined interparticle spacing on segments of nanotopographic Si substrates. The topographic height of these segments plays a dominant role in dictating the spacing between the gold nanoparticles, as the nanoparticle arrangement is controlled by immersion forces and by their self-assembly within the segments. Our novel strategy of employing a single-step BCML routine is a highly promising method for the fabrication of regular gold nanopatterns in micropatterns for a wide range of applications.

Tuning neuron adhesion and neurite guiding using functionalized AuNPs and backfill chemistry

Gold nanoparticles are used to investigate the dependence of neuron adhesion on the density of cell binding sites and particle backfill. Neurons viability and neurite development depend differently on cell attractive and cell repellant surface cues.

Nanoscale integrin ligand patterns determine melanoma cell behavior

Cells use integrin receptors to adhere onto surfaces by binding to ligands such as the arginine-glycine-aspartic acid (RGD) motif. Cancer cells make use of this adhesion process, which has motivated the development of integrin-directed drugs. However, those drugs may exert paradoxical effects on tumor progression, which raises the question of how integrin function is governed in tumor cells on the nanoscale. We have utilized precisely defined and tunable RGD ligand site densities spanning 1 order of magnitude, i.e., 103 to 1145 ligand sites/μm(2), by using RGD-functionalized gold nanoparticle patterns immobilized on glass by block copolymer (micellar) nanolithography. In an αVβ3 integrin-dependent fashion, human melanoma cells spread, formed focal contacts, and reorganized cytoskeletal fibers on a physiologically relevant RGD density of 349 sites/μm(2). Intriguingly, low doses of solute RGD "shifted" the optimal densities of immobilized ligand along with corresponding melanoma cell integrin clusters and cytoskeletal changes toward those typical for "intermediate" ligand presentation. Consequently, melanoma cells were forced into a "permissive" state, optimizing interactions with suboptimal nanostructured biomimetic surfaces, thus providing an explanation for the seemingly paradoxical effects on tumor progression and a potential clue for individualized antitumoral therapies.

Synthetic control of isolated, single functional groups on silica surfaces

We report control of the density of isolated, single functional groups in homogeneously mixed trichloroalkyl silanes on various silica surfaces. The functional groups are covalently bound to a silane derived from the Rink resin. This Rink-silane is reactive to any nucleophile. Control over the density of functional groups is achieved by diluting the immersion solution containing the Rink-silane with an inert silane, octadecyltrichlorsilane. The isolated nature of the functional groups is confirmed by the stochastic blinking of fluorescent single boron-dipyrromethane dyes imaged in total internal reflection geometry. The robust character of silane monolayers allows facile covalent binding and cleavage of molecular species from silica surfaces as well as general synthetic transformations to be conducted. This is shown by the covalent attachment and then cleavage of a naphthalene chromophore. This low-cost and scalable platform has great potential for use in sensing, molecular electronics, semiconductor processing, and the investigation of fundamental processes in catalysis and the kinetics of molecular association.

Adhesion maturation of neutrophils on nanoscopically presented platelet glycoprotein Ibα

Neutrophilic granulocytes play a fundamental role in cardiovascular disease. They interact with platelet aggregates via the integrin Mac-1 and the platelet receptor glycoprotein Ibα (GPIbα). In vivo, GPIbα presentation is highly variable under different physiological and pathophysiological conditions. Here, we quantitatively determined the conditions for neutrophil adhesion in a biomimetic in vitro system, which allowed precise adjustment of the spacings between human GPIbα presented on the nanoscale from 60 to 200 nm. Unlike most conventional nanopatterning approaches, this method provided control over the local receptor density (spacing) rather than just the global receptor density. Under physiological flow conditions, neutrophils required a minimum spacing of GPIbα molecules to successfully adhere. In contrast, under low-flow conditions, neutrophils adhered on all tested spacings with subtle but nonlinear differences in cell response, including spreading area, spreading kinetics, adhesion maturation, and mobility. Surprisingly, Mac-1-dependent neutrophil adhesion was very robust to GPIbα density variations up to 1 order of magnitude. This complex response map indicates that neutrophil adhesion under flow and adhesion maturation are differentially regulated by GPIbα density. Our study reveals how Mac-1/GPIbα interactions govern cell adhesion and how neutrophils process the number of available surface receptors on the nanoscale. In the future, such in vitro studies can be useful to determine optimum therapeutic ranges for targeting this interaction.

Fine tuning and efficient T cell activation with stimulatory aCD3 nanoarrays

Anti-CD3 (aCD3) nanoarrays fabricated by self-assembled nanopatterning combined with site-directed protein immobilization techniques represent a novel T cell stimulatory platform that allows tight control over ligand orientation and surface density. Here, we show that activation of primary human CD4+ T cells, defined by CD69 upregulation, IL-2 production and cell proliferation, correlates with aCD3 density on nanoarrays. Immobilization of aCD3 through nanopatterning had two effects: cell activation was significantly higher on these surfaces than on aCD3-coated plastics and allowed unprecedented fine-tuning of T cell response.

Regulation of hematopoietic stem cell behavior by the nanostructured presentation of extracellular matrix components

Hematopoietic stem cells (HSCs) are maintained in stem cell niches, which regulate stem cell fate. Extracellular matrix (ECM) molecules, which are an essential part of these niches, can actively modulate cell functions. However, only little is known on the impact of ECM ligands on HSCs in a biomimetic environment defined on the nanometer-scale level. Here, we show that human hematopoietic stem and progenitor cell (HSPC) adhesion depends on the type of ligand, i.e., the type of ECM molecule, and the lateral, nanometer-scaled distance between the ligands (while the ligand type influenced the dependency on the latter). For small fibronectin (FN)-derived peptide ligands such as RGD and LDV the critical adhesive interligand distance for HSPCs was below 45 nm. FN-derived (FN type III 7-10) and osteopontin-derived protein domains also supported cell adhesion at greater distances. We found that the expression of the ECM protein thrombospondin-2 (THBS2) in HSPCs depends on the presence of the ligand type and its nanostructured presentation. Functionally, THBS2 proved to mediate adhesion of HSPCs. In conclusion, the present study shows that HSPCs are sensitive to the nanostructure of their microenvironment and that they are able to actively modulate their environment by secreting ECM factors.

Nanoscale ligand spacing influences receptor triggering in T cells and NK cells

Bioactive nanoscale arrays were constructed to ligate activating cell surface receptors on T cells (the CD3 component of the TCR complex) and natural killer (NK) cells (CD16). These arrays are formed from biofunctionalized gold nanospheres with controlled interparticle spacing in the range 25-104 nm. Responses to these nanoarrays were assessed using the extent of membrane-localized phosphotyrosine in T cells stimulated with CD3-binding nanoarrays and the size of cell contact area for NK cells stimulated with CD16-binding nanoarrays. In both cases, the strength of response decreased with increasing spacing, falling to background levels by 69 nm in the T cell/anti-CD3 system and 104 nm for the NK cell/anti-CD16 system. These results demonstrate that immune receptor triggering can be influenced by the nanoscale spatial organization of receptor/ligand interactions.

Depending on its nano-spacing, ALCAM promotes cell attachment and axon growth

ALCAM is a member of the cell adhesion molecule (CAM) family which plays an important role during nervous system formation. We here show that the two neuron populations of developing dorsal root ganglia (DRG) display ALCAM transiently on centrally and peripherally projecting axons during the two phases of axon outgrowth. To analyze the impact of ALCAM on cell adhesion and axon growth, DRG single cells were cultured on ALCAM-coated coverslips or on nanopatterns where ALCAM is presented in physiological amino-carboxyl terminal orientation at highly defined distances (29, 54, 70, 86, and 137 nm) and where the interspaces are passivated to prevent unspecific protein deposition. Some axonal features (branching, lateral deviation) showed density dependence whereas others (number of axons per neuron, various axon growth parameters) turned out to be an all-or-nothing reaction. Time-lapse analyses revealed that ALCAM density has an impact on axon velocity and advance efficiency. The behavior of the sensory axon tip, the growth cone, partially depended on ALCAM density in a dose-response fashion (shape, dynamics, detachment) while other features did not (size, complexity). Whereas axon growth was equally promoted whether ALCAM was presented at high (29 nm) or low densities (86 nm), the attachment of non-neuronal cells depended on high ALCAM densities. The attachment of non-neuronal cells to the rather unspecific standard proteins presented by conventional implants designed to enhance axonal regeneration is a severe problem. Our findings point to ALCAM, presented as 86 nm pattern, for a promising candidate for the improvement of such implants since this pattern drives axon growth to its full extent while at the same time non-neuronal cell attachment is clearly reduced.

Control of cell adhesion and neurite outgrowth by patterned gold nanoparticles with tunable attractive or repulsive surface properties

Guiding of neuronal cells on surfaces is required for the investigation of fundamental aspects of neurobiology, for tissue engineering, and for numerous bioelectronic applications. A modular method to establish nanostructured chemical templates for local deposition of gold nanoparticles is presented. A process comprising nanoimprint lithography, silanization, lift-off, and gold nanoparticle immobilization is used to fabricate the particle patterns. The chemical composition of the surface can be modified by in situ adsorption of cell-binding ligands to locally addressed particles. The versatility of this approach is demonstrated by inverting the binding affinity between rat cortical neurons and nanopatterned surfaces via wet-chemical means and thereby reversing the pattern of guided neurons.

Nanostructured material surfaces--preparation, effect on cellular behavior, and potential biomedical applications: a review

Nanostructures play important roles in vivo, where nanoscaled features of extracellular matrix (ECM) components influence cell behavior and resultant tissue formation. This review summarizes some of the recent developments in fostering new concepts and approaches to nanofabrication, such as top-down and bottom-up and combinations of the two. As in vitro investigations demonstrate that man-made nanotopography can be used to control cell reactions to a material surface, its potential application in implant design and tissue engineering becomes increasingly evident. Therefore, we present recent progress in directing cell fate in the field of cell mechanics, which has grown rapidly over the last few years, and in various tissue-engineering applications. The main focus is on the initial responses of cells to nanostructured surfaces and subsequent influences on cellular functions. Specific examples are also given to illustrate the potential nanostructures may have for biomedical applications and regenerative medicine.

Sensitivity of protein adsorption to architectural variations in a protein-resistant polymer brush containing engineered nanoscale adhesive sites

Patchy polymer brushes contain nanoscale (5-15 nm) adhesive elements, such as polymer coils or nanoparticles, embedded at their base at random positions on the surface. The competition between the brush's steric (protein resistant) repulsions and the attractions from the discrete adhesive elements provides a precise means to control bioadhesion. This differs from the classical approach, where functionality is placed on the brush's periphery. The current study demonstrates the impact of poly(etheylene glycol) (PEG) brush architecture and ionic strength on fibrinogen adsorption on brushes containing embedded poly-l-lysine (PLL, 20K MW) coils or "patches". The consistent appearance of a fibrinogen adsorption threshold, a minimum loading of patches on the surface, below which protein adsorption does not occur, suggests multivalent protein capture: Adsorbing proteins simultaneously engage several patches. The surface composition (patch loading) at the threshold is extremely sensitive to the brush height and ionic strength, varying up to a factor of 5 in the surface loading of the PLL patches (~50% of the range of possible surfaces). Variations in ionic strength have a similar effect, with the smallest thresholds seen for the largest Debye lengths. While trends with brush height were the clearest and most dominant, consideration of the PEG loading within the brush or its persistence length did not reveal a critical brush parameter for the onset of adsorption. The lack of straightforward correlation on brush physics was likely a result of multivalent binding, (producing an additional dependence on patch loading), and might be resolved for univalent adsorption onto more strongly binding patches. While studies with similar brushes placed uniformly on a surface revealed that the PEG loading within the brush is the best indicator of protein resistance, the current results suggest that brush height is more important for patchy brushes. Likely the interactions producing brush extension normal to the interface act similarly to drive lateral tether extension to obstruct patches.

Single-molecule protein arrays enabled by scanning probe block copolymer lithography

The ability to control the placement of individual protein molecules on surfaces could enable advances in a wide range of areas, from the development of nanoscale biomolecular devices to fundamental studies in cell biology. Such control, however, remains a challenge in nanobiotechnology due to the limitations of current lithographic techniques. Herein we report an approach that combines scanning probe block copolymer lithography with site-selective immobilization strategies to create arrays of proteins down to the single-molecule level with arbitrary pattern control. Scanning probe block copolymer lithography was used to synthesize individual sub-10-nm single crystal gold nanoparticles that can act as scaffolds for the adsorption of functionalized alkylthiol monolayers, which facilitate the immobilization of specific proteins. The number of protein molecules that adsorb onto the nanoparticles is dependent upon particle size; when the particle size approaches the dimensions of a protein molecule, each particle can support a single protein. This was demonstrated with both gold nanoparticle and quantum dot labeling coupled with transmission electron microscopy imaging experiments. The immobilized proteins remain bioactive, as evidenced by enzymatic assays and antigen-antibody binding experiments. Importantly, this approach to generate single-biomolecule arrays is, in principle, applicable to many parallelized cantilever and cantilever-free scanning probe molecular printing methods.

Supported membranes embedded with fixed arrays of gold nanoparticles

We present a supported membrane platform consisting of a fluid lipid bilayer membrane embedded with a fixed array of gold nanoparticles. The system is realized by preforming a hexagonal array of gold nanoparticles (∼5-7 nm) with controlled spacing (∼50-150 nm) fixed to a silica or glass substrate by block copolymer lithography. Subsequently, a supported membrane is assembled over the intervening bare substrate. Proteins or other ligands can be associated with the fluid lipid component, the fixed nanoparticle component, or both, providing a hybrid interface consisting of mobile and immobile components with controlled geometry. We test different biochemical coupling strategies to bind individual proteins to the particles surrounded by a fluid lipid membrane. The coupling efficiency to nanoparticles and the influence of nanoparticle arrays on the surrounding membrane integrity are characterized by fluorescence imaging, correlation spectroscopy, and super-resolution fluorescence microscopy. Finally, the functionality of this system for live cell experiments is tested using the ephrin-A1-EphA2 juxtacrine signaling interaction in human breast epithelial cells.

Single component and selective competitive protein adsorption in a patchy polymer brush: opposition between steric repulsions and electrostatic attractions

This work explores the use of "patchy" polymer brushes to control protein adsorption rates on engineered surfaces and to bind targeted species from protein mixtures with high selectivity but without invoking molecular recognition. The brushes of interest contain embedded cationic "patches" composed of isolated adsorbed poly(l-lysine) coils (PLL) that are about 10 nm in diameter and are randomly arranged on a silica substrate. Around these patches is a protein-resistant poly(ethylene glycol) (PEG) brush that is formed from the adsorption of a PLL-g-PEG graft copolymer on the remaining silica surface. Electrostatic attractions between individual cationic patches and the negative regions of approaching proteins may be energetically insufficient to bind proteins. Furthermore, protein-patch attractions are reduced by steric repulsions between proteins and the PEG brush. We show that protein adsorption, gauged by ultimate short-term coverages and by the initial protein adsorption rate, exhibits an adhesion threshold: pure PEG brushes of the architectures employed here and brushes containing sparse loadings of PLL patches do not adsorb protein. Above a critical PLL patch loading or threshold, protein adsorption proceeds, often dramatically. The PLL patch thresholds are specific to the protein of interest, allowing surfaces to be engineered to adhesively discriminate different proteins within a mixture. The separation achieved is remarkably sharp: one protein adsorbs, but the second is completely rejected from the interface. The surfaces in this study, by virtue of their well-controlled and well-characterized patchy nature, distinguish themselves from multicomponent brushes or brushes used to end-tether peptide sequences and nucleotides.

Selective surface chemistry using alumina nanoparticles generated from block copolymers

Developing orthogonal surface chemistry techniques that perform at the nanoscale is key to achieving precise control over molecular patterning on surfaces. We report the formation and selective functionalization of alumina nanoparticle arrays generated from block copolymer templates. This new material provides an alternative to gold for orthogonal surface chemistry at the nanometer scale. Atomic force microscopy and X-ray photoelectron spectroscopy confirm these particles show excellent selectivity over silica for phosphonic and carboxylic acid adsorption. As this is the first reported synthesis of alumina nanoparticles from block copolymer templates, characterizations via Fourier transform infrared spectroscopy, Auger electron spectroscopy, and transmission electron microscopy are presented. Reproducible formation of alumina nanoparticles was dependent on a counterintuitive synthetic step wherein a small amount of water is added to an anhydrous toluene solution of block copolymer and aluminum chloride. The oxidation environment of the aluminum in these particles, as measured by Auger electron spectroscopy, is similar to that of native aluminum oxide and alumina grown by atomic layer deposition. This discovery expands the library of available surface chemistries for nanoscale molecular patterning.

Measuring cell adhesion forces: theory and principles

Cell adhesion is an essential prerequisite for survival, communication, and navigation of cells in organisms. It is maintained by the organized binding of molecules from the cell membrane to the extracellular space. This chapter focuses on direct measurements of cellular binding strength at the level of single adhesion molecules. Using atomic force microscopy-based force measurements, adhesion strength can be monitored as a function of adhesion time and environmental conditions. In this way, cellular adhesion strategies like changes in affinity and avidity of adhesion molecules (e.g., integrins) are characterized as well as the molecular arrangement of adhesion molecules in the cell membrane (e.g., molecular clusters, focal adhesion spots, and linkage to the cytoskeleton or tether). Some prominent values for the data evaluation are presented as well as constraints and preparative techniques for successful cell adhesion force experiments.

Polymeric substrates with tunable elasticity and nanoscopically controlled biomolecule presentation

Despite tremendous progress in recent years, nanopatterning of hydrated polymeric systems such as hydrogels still represents a major challenge. Here, we employ block copolymer nanolithography to arrange gold nanoparticles on a solid template, followed by the transfer of the pattern to a polymeric hydrogel. In the next step, these nanoparticles serve as specific anchor points for active biomolecules. We demonstrate the engineering of poly(ethylene glycol) hydrogel surfaces with respect to elasticity, nanopatterning, and functionalization with biomolecules. For the first time, biomolecule arrangement on the nanometer scale and substrate stiffness can be varied independently from each other. Young's moduli, a measure of the compliance of the substrates, can be tuned over 4 orders of magnitude, including the values for all of the different tissues found in the human body. Structured hydrogels can be used to pattern any histidine-tagged protein as exemplified for his-protein A as an acceptor for immunoglobulin. When cell-adhesion-promoting peptide cRGDfK is selectively coupled to gold nanoparticles, the surfaces provide cues for cell-surface interaction and allow for the study of the modulation of cellular adhesion by the mechanical properties of the environment. Therefore, these substrates represent a unique multipurpose platform for studying receptor/ligand interactions with adhering cells, mechanotransduction, and cell-adhesion-dependent signaling.

Enzyme mediated site-specific surface modification

Stable tethering of bioactive peptides like RGD to surfaces can be achieved via chemical bonding, biotin streptavidin interaction, or photocross-linking. More challenging is the immobilization of proteins, since methods applied to immobilize peptides are either not specific or versatile enough or might even compromise the protein's bioactivity. To overcome this limitation, we have employed a scheme that by enzymatic (transglutaminase) reaction allows the site-directed and site-specific coupling of growth factors and other molecules to nonfouling poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) coated surfaces under physiological conditions. By our modular and flexible design principle, we are able to functionalize these surfaces directly with peptides and growth factors or precisely position poly(ethylene glycol) (PEG)-like hydrogels for the presentation of growth factors as exemplified with vascular endothelial growth factor (VEGF).

Surface-specific interaction of the extracellular domain of protein L1 with nitrilotriacetic acid-terminated self-assembled monolayers

We report a study on the interaction of the extracellular domain of trans-membrane proteins N-cadherin and L1 with nitrilotriacetic acid (NTA)-terminated self-assembled monolayers (SAMs) grown on silver and gold surfaces. Quartz crystal microbalance (QCM) and reflection absorption infrared spectroscopy (RAIRS) measurements reveal that upon addition of protein to an NTA-SAM there is a subsequent change in the mass and average chemical structure inside the films formed on the metal substrates. By using vibrational sum frequency generation (VSFG) spectroscopy and making a comparison to SAMs prepared with n-alkanethiols, we find that the formed NTA-SAMs are terminated by ethanol molecules from solution. The ethanol signature vanishes after the addition of L1, which indicates that the L1 proteins can interact specifically with the NTA complex. Although the RAIRS spectra display signatures in the amide and fingerprint regions, the VSFG spectra display only a weak feature at 866 cm(-1), which possibly indicates that some of the abundant phenyl rings in the complex are ordered. Although cell biology experiments suggest the directional complexation of L1, the VSFG experiments suggest that the alpha-helices and beta-sheets of L1 lack any preferential ordering.

Nanopatterns biofunctionalized with cell adhesion molecule DM-GRASP offered as cell substrate: spacing determines attachment and differentiation of neurons

The density/spacing of plasma membrane proteins is thought to be crucial for their function; clear-cut experimental evidence, however, is still rare. We examined nanopatterns biofunctionalized with cell adhesion molecule DM-GRASP with respect to their impact on neuron attachment and neurite growth. Data analysis/modeling revealed that these cellular responses improve with increasing DM-GRASP density, with the exception of one spacing which does not allow for the anchorage of a cytoskeletal protein (spectrin) to three DM-GRASP molecules.

Patterned and switchable surfaces for biomolecular manipulation

The interactions of biomolecules and cells with solid interfaces play a pivotal role in a range of biomedical applications and have therefore been studied in great detail. An improved understanding of these interactions results in the ability to manipulate DNA, proteins and other biomolecules, as well as cells, spatially and temporally at surfaces with high precision. This in turn engenders the development of advanced devices, such as biosensors, bioelectronic components, smart biomaterials and microarrays. Spatial control can be achieved by the production of patterned surface chemistries using modern high-resolution patterning technologies based on lithography, microprinting or microfluidics, whilst temporal control is accessible through the application of switchable surface architectures. The combination of these two surface properties offers unprecedented control over the behaviour of biomolecules and cells at the solid-liquid interface. This review discusses the behaviour of biomolecules and cells at solid interfaces and highlights fundamental and applied research exploring patterned and switchable surfaces.

Cell interactions with hierarchically structured nano-patterned adhesive surfaces

The activation of well-defined numbers of integrin molecules in predefined areas by adhesion of tissue cells to biofunctionalized micro-nanopatterned surfaces was used to determine the minimum number of activated integrins necessary to stimulate focal adhesion formation. This was realized by combining micellar and conventional e-beam lithography, which enabled deposition of 6 nm large gold nanoparticles on predefined geometries. Patterns with a lateral spacing of 58 nm and a number of gold nanoparticles, ranging from 6 to 3000 per adhesive patch, were used. For α(v) β(3)-integrin activation, gold nanoparticles were coated with c(-RGDfK-)-thiol peptides, and the remaining glass surface was passivated to prevent non-specific protein adsorption and cell adhesion. Results show that focal adhesion formation is dictated by the underlying hierarchical nanopattern. Adhesive patches with side lengths of 3000 nm and separated by 3000 nm, or with side lengths of 1000 nm and separated by 1000 nm, containing approximately 3007 ± 193 or 335 ± 65 adhesive gold nanoparticles, respectively, induced the formation of actin-associated, paxillin-rich focal adhesions, comparable in size and shape to classical focal adhesions. In contrast, adhesive patches with side lengths of 500, 250 or 100 nm, and separated from adjacent adhesive patches by their respective side lengths, containing 83 ± 11, 30 ± 4, or 6 ± 1 adhesive gold nanoparticles, respectively, showed a significant increase in paxillin domain length, caused by bridging the pattern gap through an actin bundle in order to mechanically, synergistically strengthen each single adhesion site. Neither paxillin accumulation nor adhesion formation was induced if less than 6 c(-RGDfK-)-thiol functionalised gold nanoparticles per adhesion site were presented to cells.

Cell adhesion to agrin presented as a nanopatterned substrate is consistent with an interaction with the extracellular matrix and not transmembrane adhesion molecules

BACKGROUND

Molecular spacing is important for cell adhesion in a number of ways, ranging from the ordered arrangement of matrix polymers extracellularly, to steric hindrance of adhesion/signaling complexes intracellularly. This has been demonstrated using nanopatterned RGD peptides, a canonical extracellular matrix ligand for integrin interactions. Cell adhesion was greatly reduced when the RGD-coated nanoparticles were separated by more than 60 nm, indicating a sharp spacing-dependent threshold for this form of cell adhesion.

RESULTS

Here we show a similar dependence of cell adhesion on the spacing of agrin, a protein that exists as both a secreted, matrix-bound form and a type-2 transmembrane form in vivo. Agrin was presented as a substrate for cell adhesion assays by anchoring recombinant protein to gold nanoparticles that were arrayed at tunable distances onto glass coverslips. Cells adhered well to nanopatterned agrin, and when presented as uniformly coated substrates, adhesion to agrin was comparable to other well-studied adhesion molecules, including N-Cadherin. Adhesion of both mouse primary cortical neurons and rat B35 neuroblastoma cells showed a spacing-dependent threshold, with a sharp drop in adhesion when the space between agrin-coated nanoparticles increased from 60 to 90 nm. In contrast, adhesion to N-Cadherin decreased gradually over the entire range of distances tested (uniform, 30, 60, 90, and 160 nm). The spacing of the agrin nanopattern also influenced cell motility, and peptide competition suggested adhesion was partially integrin dependent. Finally, differences in cell adhesion to C-terminal agrin fragments of different lengths were detected using nanopatterned substrates, and these differences were not evident using uniformly coated substrates.

CONCLUSION

These results suggest nanopatterned substrates may provide a physiological presentation of adhesive substrates, and are consistent with cells adhering to agrin through a mechanism that more closely resembles an interaction with the extracellular matrix than a transmembrane adhesion molecule.

Cell adhesion and response to synthetic nanopatterned environments by steering receptor clustering and spatial location

During adhesion and spreading, cells form micrometer-sized structures comprising transmembrane and intracellular protein clusters, giving rise to the formation of what is known as focal adhesions. Over the past two decades these structures have been extensively studied to elucidate their organization, assembly, and molecular composition, as well as to determine their functional role. Synthetic materials decorated with biological molecules, such as adhesive peptides, are widely used to induce specific cellular responses dependent on cell adhesion. Here, we focus on how surface patterning of such bioactive materials and organization at the nanoscale level has proven to be a useful strategy for mimicking both physical and chemical cues present in the extracellular space controlling cell adhesion and fate. This strategy for designing synthetic cellular environments makes use of the observation that most cell signaling events are initiated through recruitment and clustering of transmembrane receptors by extracellular-presented signaling molecules. These systems allow for studying protein clustering in cells and characterizing the signaling response induced by, e.g., integrin activation. We review the findings about the regulation of cell adhesion and focal adhesion assembly by micro- and nanopatterns and discuss the possible use of substrate stiffness and patterning in mimicking both physical and chemical cues of the extracellular space.

Cooperativity in adhesion cluster formation during initial cell adhesion

We have studied the initial phase of cell adhesion as a function of the lateral organization of individual integrin molecules with single-cell force microscopy. Nanostructures, consisting of hexagonally ordered gold dots, were prepared with diblock-copolymer micelle lithography and functionalized with arginine- glycine-aspartate peptides, thus defining integrin position with nanometer resolution. Adhesion strength was characterized with an atomic force microscope and both cell detachment forces and work of detachment showed a reinforcement of adhesion if the distance between integrin molecules was <70 nm. This reinforcement had already occurred at cell-substrate contact times <5 min. We believe our results show quantitatively the relevance of the distance between adjacent integrin binding sites rather than their density. Furthermore, we propose a model describing the cooperative stabilization of early integrin clusters as a function of receptor patterning at the nanoscale.

Induction of cell polarization and migration by a gradient of nanoscale variations in adhesive ligand spacing

Cell interactions with adhesive surfaces play a vital role in the regulation of cell proliferation, viability, and differentiation, and affect multiple biological processes. Since cell adhesion depends mainly on the nature and density of the adhesive ligand molecules, spatial molecular patterning, which enables the modulation of adhesion receptor clustering, might affect both the structural and the signaling activities of the adhesive interaction. We herein show that cells plated on surfaces that present a molecularly defined spacing gradient of an integrin RGD ligand can sense small but consistent differences in adhesive ligand spacing of about 1 nm across the cell diameter, which is approximately 61 mum when the spacing includes 70 nm. Consequently, these positional cues induce cell polarization and initiate cell migration and signaling. We propose that differential positional clustering of the integrin transmembrane receptors is used by cells for exploring and interpreting their environment, at high spatial sensitivity.

Cell adhesion and polarisation on molecularly defined spacing gradient surfaces of cyclic RGDfK peptide patches

In vivo cell migration and location are orchestrally guided by soluble and bound chemical gradients. Here, gradients of extracellular matrix molecules are formed synthetically by the combination of a surface nanopatterning technique called block copolymer nanolithography (BCN) and a biofunctionalisation technique. A modified substrate dip-coating process of BCN allows for the formation of precise molecular gradients of cyclic RGDfK peptide patches at interfaces, which are presented to cells for testing cell adhesion and polarisation. Surfaces formed by BCN consist of hexagonally ordered gold dot patterns with a gradient in particle spacing. Each dot serves as a chemical anchor for the binding of cyclic RGDfK peptides, which are specifically recognised by alpha(v)beta(3) integrins. Due to steric hindrance only up to one integrin binds to one functionalised gold dot which forms a peptide patch spacing. We demonstrate how cell morphology, adhesion area, actin and vinculin distribution as well as cell body polarisation are influenced by the peptide patch spacing gradient. As a consequence, these gradients of adhesive ligands induce cell orientation towards smaller particle spacing when the gradient strength is 15nm/mm at least. This implicates that an adherent cell's sensitivity to differentiate between ligand patch spacing is approximately 1nm across the cell body.

Cell adhesion molecule DM-GRASP presented as nanopatterns to neurons regulates attachment and neurite growth

Adhesion and neurite formation of neurons and neuroblastoma cells critically depends on the lateral spacing of the cell adhesion molecule DM-GRASP offered as nanostructured substrate.