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

Preparation, Properties, and Bioapplications of Block Copolymer Nanopatterns

Block copolymer (BCP) self-assembly has emerged as a feasible method for large-scale fabrication with remarkable precision - features that are not common for most of the nanofabrication techniques. In this review, recent advancements in the molecular design of BCP along with state-of-the-art processing methodologies based on microphase separation alone or its combination with different lithography methods are presented. Furthermore, the bioapplications of the generated nanopatterns in the development of protein arrays, cell-selective surfaces, and antibacterial coatings are explored. Finally, the current challenges in the field are outlined and the potential breakthroughs that can be achieved by adopting BCP approaches already applied in the fabrication of electronic devices are discussed.

Block copolymer nanopatterns affect cell spreading: Stem versus cancer bone cells

Bone healing after a tumor removal can be promoted by biomaterials that enhance the bone regeneration and prevent the tumor relapse. Herein, we obtained several nanopatterns by self-assembly of polystyrene-block-poly-(2-vinylpyridine) (PS-b-P2VP) with different molecular weights and investigated the adhesion and morphology of human bone marrow mesenchymal stem cells (BMMSC) and osteosarcoma cell line (SaOS-2) on these patterns aiming to identify topography and chemistry that promote bone healing. We analyzed > 2000 cells per experimental condition using imaging software and different morphometric descriptors, namely area, perimeter, aspect ratio, circularity, surface/area, and fractal dimension of cellular contour (FDC). The obtained data were used as inputs for principal component analysis, which showed distinct response of BMMSC and SaOS-2 to the surface topography and chemistry. Among the studied substrates, micellar nanopatterns assembled from the copolymer with high molecular weight promote the adhesion and spreading of BMMSC and have an opposite effect on SaOS-2. This nanopattern is thus beneficial for bone regeneration after injury or pathology, e.g. bone fracture or tumor removal.

DNA Origami Nanoarrays for Multivalent Investigations of Cancer Cell Spreading with Nanoscale Spatial Resolution and Single-Molecule Control

We present a strategy for the fabrication of biomimetic nanoarrays, based on the use of DNA origami, that permits the multivalent investigation of ligand-receptor molecule interactions in cancer cell spreading, with nanoscale spatial resolution and single-molecule control. We employed DNA origami to control the nanoscale spatial organization of integrin- and epidermal growth factor (EGF)-binding ligands that modulate epidermal cancer cell behavior. By organizing these multivalent DNA nanostructures in nanoarray configurations on nanopatterned surfaces, we demonstrated the cooperative behavior of integrin and EGF ligands in the spreading of human cutaneous melanoma cells: this cooperation was shown to depend on both the number and ratio of the selective ligands employed. Notably, the multivalent biochips we have developed allowed for this cooperative effect to be demonstrated with single-molecule control and nanoscale spatial resolution. By and large, the platform presented here is of general applicability for the study, with molecular control, of different multivalent interactions governing biological processes from the function of cell-surface receptors to protein-ligand binding and pathogen inhibition.

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.

Specific Neuron Placement on Gold and Silicon Nitride-Patterned Substrates through a Two-Step Functionalization Method

The control of neuron-substrate adhesion has been always a challenge for fabricating neuron-based cell chips and in particular for multielectrode array (MEA) devices, which warrants the investigation of the electrophysiological activity of neuronal networks. The recent introduction of high-density chips based on the complementary metal oxide semiconductor (CMOS) technology, integrating thousands of electrodes, improved the possibility to sense large networks and raised the challenge to develop newly adapted functionalization techniques to further increase neuron electrode localization to avoid the positioning of cells out of the recording area. Here, we present a simple and straightforward chemical functionalization method that leads to the precise and exclusive positioning of the neural cell bodies onto modified electrodes and inhibits, at the same time, cellular adhesion in the surrounding insulator areas. Different from other approaches, this technique does not require any adhesion molecule as well as complex patterning technique such as μ-contact printing. The functionalization was first optimized on gold (Au) and silicon nitride (Si3N4)-patterned surfaces. The procedure consisted of the introduction of a passivating layer of hydrophobic silane molecules (propyltriethoxysilane [PTES]) followed by a treatment of the Au surface using 11-amino-1-undecanethiol hydrochloride (AT). On model substrates, well-ordered neural networks and an optimal coupling between a single neuron and single micrometric functionalized Au surface were achieved. In addition, we presented the preliminary results of this functionalization method directly applied on a CMOS-MEA: the electrical spontaneous spiking and bursting activities of the network recorded for up to 4 weeks demonstrate an excellent and stable neural adhesion and functional behavior comparable with what expected using a standard adhesion factor, such as polylysine or laminin, thus demonstrating that this procedure can be considered a good starting point to develop alternatives to the traditional chip coatings to provide selective and specific neuron-substrate adhesion.

Molecular Occupancy of Nanodot Arrays

Single-molecule nanodot arrays, in which a biomolecule of choice (protein, nucleic acid, etc.) is bound to a metallic nanoparticle on a solid substrate, are becoming an increasingly important tool in the study of biomolecular and cellular interactions. We have developed an on-chip measurement protocol to monitor and control the molecular occupancy of nanodots. Arrays of widely spaced nanodots and nanodot clusters were fabricated on glass surfaces by nanolithography and functionalized with fluorescently labeled proteins. The molecular occupancy was determined by monitoring individual fluorophore bleaching events, while accounting for fluorescence quenching effects. We found that the occupancy can be interpreted as a packing problem, and depends on nanodot size and binding ligand concentration, where the latter is easily adjusted to compensate the flexibility of dimension control in nanofabrication. The results are scalable with nanodot cluster size, extending to large area close packed arrays. As an example, the nanoarray platform was used to probe the geometric requirement of T-cell activation at the single-molecule level.

Modification of surface/neuron interfaces for neural cell-type specific responses: a review

Surface/neuron interfaces have played an important role in neural repair including neural prostheses and tissue engineered scaffolds. This comprehensive literature review covers recent studies on the modification of surface/neuron interfaces. These interfaces are identified in cases both where the surfaces of substrates or scaffolds were in direct contact with cells and where the surfaces were modified to facilitate cell adhesion and controlling cell-type specific responses. Different sources of cells for neural repair are described, such as pheochromocytoma neuronal-like cell, neural stem cell (NSC), embryonic stem cell (ESC), mesenchymal stem cell (MSC) and induced pluripotent stem cell (iPS). Commonly modified methods are discussed including patterned surfaces at micro- or nano-scale, surface modification with conducting coatings, and functionalized surfaces with immobilized bioactive molecules. These approaches to control cell-type specific responses have enormous potential implications in neural repair.

Fabrication of RGD micro/nanopattern and corresponding study of stem cell differentiation

Micropatterns of gold (Au) nanoarrays on inorganic and polymeric substrates were fabricated by combining block copolymer micelle nanolithography to obtain gold nanoarrays on glass, photolithography plus hydrofluoric acid (HF) etching to generate microislands, and transfer lithography to shift the gold micro/nanopatterns from glass to a bioinert poly(ethylene glycol) (PEG) hydrogel surface. Further the modification of the gold nanodots via cell-adhesive arginine-glycine-aspartate (RGD) ligands was carried out to achieve peptide micro/nanopatterns. Whereas the micro/nanopatterns of noble metals could be useful in various applications, the peptide micro/nanopatterns especially enable persistent cell localization on adhesive micropatterns of RGD nanoarrays on the background of potently nonfouling PEG hydrogels, and thus offer a powerful tool to investigate cell-material interactions on both molecular and cellular levels. As a demonstration, we cultured human mesenchymal stem cells (hMSCs) on micro/nanopatterns with RGD nanoarrays of nanospacings 46 and 95 nm, and with micropans of side lengths 35 and 65 μm (four groups in total). The osteogenic and adipogenic differentiation of hMSCs was conducted, and the potential effect of RGD nanospacing and the effect of cell spreading size on cell differentiation were decoupled for the first time. The results reveal that RGD nanospacing, independent of cell spreading size, acts as a strong regulator of cell tension and stem cell differentiation, which cannot be concluded unambiguously based on either merely micropatterns or nanopatterns.

The height of cell-adhesive nanoposts generated by block copolymer/surfactant complex systems influences the preosteoblast phenotype

In tissue engineering, the nanoscale topography of the substrate is important, because transplanted cells can recognize and respond to topographical patterns, allowing control of gene expression and tissue formation. In this study, we hypothesized that the height of cell-adhesive nanoposts could regulate cell phenotype. Nano-patterned surfaces were generated via self-assembly of polystyrene-b-poly(ethylene oxide)/dodecylbenzenesulfonic acid (PS-b-PEO/DBSA) complex systems. The height of PS nanoposts, which are considered to be cell-adhesion domains, was varied from 11 to 43 nm, while nanopost size and the center-to-center distance between nanoposts were kept constant. Adhesion, growth, and differentiation of mouse preosteoblasts (MC3T3-E1) cultured on the nano-patterned surfaces were significantly influenced by nanopost height. This approach therefore holds great promise for the design of biomedical devices, as well as tissue engineering scaffolds.

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.

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Magnetically attachable stencils and the non-destructive analysis of the contribution made by the underlying matrix to cell migration

Cell migration is controlled by the integration of numerous distinct components. Consequently, the analysis of cell migration is advancing towards comprehensive, multifaceted in vitro models. To accurately evaluate the contribution of an underlying substrate to cell motility in complex cellular environments we developed a migration assay using magnetically attachable stencils (MAts). When attached to a culture surface, MAts create a defined void in the cell monolayer without disrupting the cells or damaging the underlying substrate. Quantitative analysis of migration into this void reveals the substrate's contribution to migration. The magnetically-guided placement of a microfabricated stencil allows for full experimental control of the substrate on which migration is analyzed. MAts enable the evaluation of intact, defined matrix, and make it possible to analyze migration on unique surfaces such as micropatterned proteins, nano-textured surfaces, and pliable hydrogels. These studies also revealed that mechanical disruption, including the damage that occurs during scratch assays, diminishes migration and confounds the analysis of individual cell behavior. Analysis of migration on increasingly complex biomaterials reveals that the contribution of the underlying matrix depends not only on its molecular composition but also its organization and the context in which it is presented.

Translation of the cell adhesion molecule ALCAM in axonal growth cones – regulation and functional importance

ALCAM is a cell adhesion molecule that is present on extending axons and has been shown to be crucial for elongation and navigation of retinal ganglion cell (RGC) axons. In the present study, we show that ALCAM mRNA is present in axonal growth cones of RGCs in vivo and in vitro, and that translation of ALCAM occurs in RGC growth cones separated from their soma. This growth cone translation is regulated by the 3′-untranslated region (3′-UTR) of ALCAM and depends on the activity of the kinases ERK and TOR (target of rapamycin). We also investigated the impact of the growth cone translation of ALCAM on axonal functions. Growth cone translation of ALCAM is crucial for the enhanced elongation of axons extending in contact with ALCAM protein. The local translation of ALCAM in the growth cone is able to rapidly counterbalance experimentally induced ALCAM internalization, thereby contributing to the maintenance of constant ALCAM levels in the plasma membrane. Assays where RGC axons have the choice to grow on laminin or both ALCAM and laminin – as is the case in the developing retina – reveal that the axonal preference for ALCAM-containing lanes depends on translation of ALCAM in growth cones. Taken together, these results show for the first time that translation of a cell adhesion molecule in growth cones, as well as the impact of this local translation on the behavior of axon and growth cone.

Effect of adsorbed fibronectin on the differential adhesion of osteoblast-like cells and Staphylococcus aureus with and without fibronectin-binding proteins

The influence of fibronectin (Fn) coated surfaces patterned with poly(ethylene glycol) microgels having inter-gel spacings between 0.5 and 3.0 μm on the adhesion of Staphylococcus aureus strains with and without Fn-binding proteins and cellular adhesion/spreading was investigated. Quantitative force measurements between a S. aureus cell and a patterned surface showed that the adhesion force between the bacterium and the patterned surface increased substantially after Fn adsorption, regardless of the strain used, but decreased with decreasing inter-gel spacing. In flow-chamber experiments, the Fn-binding strain adhered at a higher rate after Fn adsorption than the strain lacking Fn-binding proteins. In both cases, the adhesion rates decreased with decreasing inter-gel spacing. Osteoblast-like cells could bind to patterned surfaces despite the microgels, and adsorbed Fn substantially amplified this effect. Even under highly non-adhesive conditions associated with closely spaced microgels, adsorbed Fn preserves a window of inter-gel spacing around 1 μm where the adhesion of staphylococcal cells is hindered while cells can still adhere and spread.

Nanolithographic control of the spatial organization of cellular adhesion receptors at the single-molecule level

The ability to control the placement of individual molecules promises to enable a wide range of applications and is a key challenge in nanoscience and nanotechnology. Many biological interactions, in particular, are sensitive to the precise geometric arrangement of proteins. We have developed a technique which combines molecular-scale nanolithography with site-selective biochemistry to create biomimetic arrays of individual protein binding sites. The binding sites can be arranged in heterogeneous patterns of virtually any possible geometry with a nearly unlimited number of degrees of freedom. We have used these arrays to explore how the geometric organization of the extracellular matrix (ECM) binding ligand RGD (Arg-Gly-Asp) affects cell adhesion and spreading. Systematic variation of spacing, density, and cluster size of individual integrin binding sites was used to elicit different cell behavior. Cell spreading assays on arrays of different geometric arrangements revealed a dramatic increase in spreading efficiency when at least four liganded sites were spaced within 60 nm or less, with no dependence on global density. This points to the existence of a minimal matrix adhesion unit for fibronectin defined in space and stoichiometry. Developing an understanding of the ECM geometries that activate specific cellular functional complexes is a critical step toward controlling cell behavior. Potential practical applications range from new therapeutic treatments to the rational design of tissue scaffolds that can optimize healing without scarring. More broadly, spatial control at the single-molecule level can elucidate factors controlling individual molecular interactions and can enable synthesis of new systems based on molecular-scale architectures.