Mobile and three-dimensional presentation of adhesion proteins within microwells.

On traditional cell culture substrates cells adhere to a planar 2D surface where ligands are presented immobile. A more realistic presentation of cell adhesion ligands which can account for lateral mobility and a more tissue-like 3D presentation would allow studies addressing fundamental questions of significant importance for applications such as tissue engineering and implant intregration. To study the effect of lateral mobility of cell membrane interaction cues in three dimensions, we have developed and characterized a platform which generically enables patterning of single cells into microwells presenting a cell membrane mimetic interface pre-patterned to its walls. Here, we describe its application in presenting a soluble cell adhesive ligand coupled through streptavidin-antibody linkage to lipids in a supported lipid bilayer (SLB) coated microwell. The lateral mobility of the presented ligands was controlled through a small change in temperature. The SLB phospholipid composition was choosen such that below its melting transition at 30 °C the ligands are immobile, while above 30 °C they are laterally mobile. The platform thus enables the investigation of cell adhesion to either laterally immobile or mobile E-cadherin ligand presented on the same cell membrane mimetic surface.

In vitro expressed GPCR inserted in polymersome membranes for ligand-binding studies.

The dopamine receptor D2 (DRD2), a G-protein coupled receptor is expressed into PBd(22)-PEO(13) and PMOXA(20)-PDMS(54)-PMOXA(20) block copolymer vesicles. The conformational integrity of the receptor is confirmed by antibody- and ligand-binding assays. Replacement of bound dopamine is demonstrated on surface-immobilized polymersomes, thus making this a promising platform for drug screening.

A novel microfluidics-based method for probing weak protein-protein interactions.

We report the use of a novel microfluidics-based method to detect weak protein-protein interactions between membrane proteins. The tight junction protein, claudin-2, synthesised in vitro using a cell-free expression system in the presence of polymer vesicles as membrane scaffolds, was used as a model membrane protein. Individual claudin-2 molecules interact weakly, although the cumulative effect of these interactions is significant. This effect results in a transient decrease of average vesicle dispersivity and reduction in transport speed of claudin-2-functionalised vesicles. Polymer vesicles functionalised with claudin-2 were perfused through a microfluidic channel and the time taken to traverse a defined distance within the channel was measured. Functionalised vesicles took 1.19 to 1.69 times longer to traverse this distance than unfunctionalised ones. Coating the channel walls with protein A and incubating the vesicles with anti-claudin-2 antibodies prior to perfusion resulted in the functionalised vesicles taking 1.75 to 2.5 times longer to traverse this distance compared to the controls. The data show that our system is able to detect weak as well as strong protein-protein interactions. This system offers researchers a portable, easily operated and customizable platform for the study of weak protein-protein interactions, particularly between membrane proteins.

Selective deposition and self-assembly of triblock copolymers into matrix arrays for membrane protein production.

To improve the stability of cell membrane mimics, there has been growing interest in the use of block copolymers. Here, we present an easy approach to create an array of planar polymeric matrices capable of hosting membrane proteins. The array of polymeric matrices was formed by the selective deposition of triblock copolymers onto an array of hydrophilic islands situated within a hydrophobic background. The thickness of these matrices corresponds to the length of a single polymer chain. These polymeric matrices were used to host cell-free expressed membrane proteins, and offers a prototype from which a membrane protein array can be created for diagnostics or drug discovery purposes.

Single cell 3-D platform to study ligand mobility in cell-cell contact.

Lateral mobility and dimensionality have both been shown to influence cellular behavior, but have yet to be combined and applied in a single in vitro platform to address, e.g., cell adhesion in a setting mimicking the three-dimensional environment of neighboring cells in a reductionist way. To study the effect of the lateral mobility of cell adhesive ligands in three dimensions we present and characterize a platform, which enables patterning of single cells into microwells presenting a cell membrane mimetic interface pre-patterned to its walls. Soluble E-cadherin extracellular domains coupled through an optimized streptavidin-antibody linkage to lipids in a supported lipid bilayer (SPB) were presented on the microwell walls as either laterally mobile or immobile ligands. The fluidity was controlled through a small change in temperature by choosing phospholipids for the SPB with a lipid phase transition temperature around 30 °C. The platform thus enabled the investigation of cell adhesion to either laterally immobile or mobile E-cadherin ligands presented on the same cell membrane mimetic surface. Chinese hamster ovary (CHO) cells engineered to express E-cadherin that were cultured on the platform demonstrated that enhanced cadherin lateral mobility significantly decreased the formation of actin bundles and resulted in more diffuse actin organization, while constraining the cell shape to that of the microwell. This example highlights the potential to use in vitro cell culture platforms to mimic direct cell-cell interaction in a controlled environment that nevertheless captures the dynamic nature of the native cell environment.

Multicompartmentalized polymersomes for selective encapsulation of biomacromolecules.

Multicompartmentalized polymersomes are formed using block co-polymers PMOXA-PDMS-PMOXA and PS-PIAT, and are subsequently proven to be capable of selective encapsulation of biomacromolecules. This architecture mimics the compartmentalization found in cells and may serve as a simple, albeit robust, model system.

Proteopolymersomes: in vitro production of a membrane protein in polymersome membranes.

Polymersomes are stable self-assembled architectures which mimic cell membranes. For characterization, membrane proteins can be incorporated into such bio-mimetic membranes by reconstitution methods, leading to so-called proteopolymersomes. In this work, we demonstrate the direct incorporation of a membrane protein into polymersome membranes by a cell-free expression system. Firstly, we demonstrate pore formation in the preformed polymersome membrane using α-hemolysin. Secondly, we use claudin-2, a protein involved in cell-cell interactions, to demonstrate the in vitro expression of a membrane protein into these polymersomes. Surface plasmon resonance (Biacore) binding studies with the claudin-2 proteopolymersomes and claudin-2 specific antibodies are performed to show the presence of the in vitro expressed protein in polymersome membranes.

Dimensionality controls cytoskeleton assembly and metabolism of fibroblast cells in response to rigidity and shape.

BACKGROUND: Various physical parameters, including substrate rigidity, size of adhesive islands and micro-and nano-topographies, have been shown to differentially regulate cell fate in two-dimensional (2-D) cell cultures. Cells anchored in a three-dimensional (3-D) microenvironment show significantly altered phenotypes, from altered cell adhesions, to cell migration and differentiation. Yet, no systematic analysis has been performed that studied how the integrated cellular responses to the physical characteristics of the environment are regulated by dimensionality (2-D versus 3-D). METHODOLOGY/PRINCIPAL FINDINGS: Arrays of 5 or 10 microm deep microwells were fabricated in polydimethylsiloxane (PDMS). The actin cytoskeleton was compared for single primary fibroblasts adhering either to microfabricated adhesive islands (2-D) or trapped in microwells (3-D) of controlled size, shape, and wall rigidity. On rigid substrates (Young's Modulus = 1 MPa), cytoskeleton assembly within single fibroblast cells occurred in 3-D microwells of circular, rectangular, square, and triangular shapes with 2-D projected surface areas (microwell bottom surface area) and total surface areas of adhesion (microwell bottom plus wall surface area) that inhibited stress fiber assembly in 2-D. In contrast, cells did not assemble a detectable actin cytoskeleton in soft 3-D microwells (20 kPa), regardless of their shapes, but did so on flat, 2-D substrates. The dependency on environmental dimensionality was also reflected by cell viability and metabolism as probed by mitochondrial activities. Both were upregulated in 3-D cultured cells versus cells on 2-D patterns when surface area of adhesion and rigidity were held constant. CONCLUSION/SIGNIFICANCE: These data indicate that cell shape and rigidity are not orthogonal parameters directing cell fate. The sensory toolbox of cells integrates mechanical (rigidity) and topographical (shape and dimensionality) information differently when cell adhesions are confined to 2-D or occur in a 3-D space.

Beyond the lotus effect: roughness influences on wetting over a wide surface-energy range.

To enhance our understanding of liquids in contact with rough surfaces, a systematic study has been carried out in which water contact angle measurements were performed on a wide variety of rough surfaces with precisely controlled surface chemistry. Surface morphologies consisted of sandblasted glass slides as well as replicas of acid-etched, sandblasted titanium, lotus leaves, and photolithographically manufactured golf-tee shaped micropillars (GTMs). The GTMs display an extraordinarily stable, Cassie-type hydrophobicity, even in the presence of hydrophilic surface chemistry. Due to pinning effects, contact angles on hydrophilic rough surfaces are shifted to more hydrophobic values, unless roughness or surface energy are such that capillary forces become significant, leading to complete wetting. The observed hydrophobicity is thus not consistent with the well-known Wenzel equation. We have shown that the pinning strength of a surface is independent of the surface chemistry, provided that neither capillary forces nor air enclosure are involved. In addition, pinning strength can be described by the axis intercept of the cosine-cosine plot of contact angles for rough versus flat surfaces with the same surface chemistries.

Micro-well arrays for 3D shape control and high resolution analysis of single cells.

In addition to rigidity, matrix composition, and cell shape, dimensionality is now considered an important property of the cell microenvironment which directs cell behavior. However, available tools for cell culture in two-dimensional (2D) versus three-dimensional (3D) environments are difficult to compare, and no tools exist which provide 3D shape control of single cells. We developed polydimethylsiloxane (PDMS) substrates for the culture of single cells in 3D arrays which are compatible with high-resolution microscopy. Cell adhesion was limited to within microwells by passivation of the flat upper surface through 'wet-printing' of a non-fouling polymer and backfilling of the wells with specific adhesive proteins or lipid bilayers. Endothelial cells constrained within microwells were viable, and intracellular features could be imaged with high resolution objectives. Finally, phalloidin staining of actin stress fibers showed that the cytoskeleton of cells in microwells was 3D and not limited to the cell-substrate interface. Thus, microwells can be used to produce microenvironments for large numbers of single cells with 3D shape control and can be added to a repertoire of tools which are ever more sought after for both fundamental biological studies as well as high throughput cell screening assays.

Nanopatterns with biological functions.

Both curiosity and a desire for efficiency have advanced our ability to manipulate materials with great precision on the micrometer and, more recently, on the nanometer scale. Certainly, the semiconductor and integrated circuit industry has put the pressure on scientist and engineers to develop better and faster nanofabrication techniques. Furthermore, our curiosity as to how life works, and how it can be improved from a medical perspective, stands to gain a great deal from advances in nanotechnology. Novel nanofabrication techniques are opening up the possibilities for mimicking the inherently nano-world of the cell, i.e., the nanotopographies of the extracellular matrix (ECM) and the nanochemistry presented on both the cell membrane and the ECM. In addition, biosensing applications that rely on fabrication of high-density, precision arrays, e.g., DNA or gene chips and protein arrays, will gain significantly in efficiency and, thus, in usefulness once it becomes possible to fabricate heterogeneous nanoarrays. Clearly, continued advances in nanotechnology are desired and required for advances in biotechnology. In this review, we describe the leading techniques for generating nanopatterns with biological function including parallel techniques such as extreme ultraviolet interference lithography (EUV-IL), soft-lithographic techniques (e.g., replica molding (RM) and microcontact printing (muCP)), nanoimprint lithography (NIL), nanosphere lithography (NSL) (e.g., colloid lithography or colloidal block-copolymer micelle lithography) and the nanostencil technique, in addition to direct-writing techniques including e-beam lithography (EBL), focused ion-beam lithography (FIBL) and dip-pen nanolithography (DPN). Details on how the patterns are generated, how biological function is imparted to the nanopatterns, and examples of how these surfaces can and are being used for biological applications will be presented. This review further illustrates the rapid pace by which advances are being made in the field of nanobiotechnology, owing to an increasing number of research endeavors, for an ever increasing number of applications.