Enzymatic proteolysis of a surface-bound alpha-helical polypeptide

In this work, we studied the interactions of enzymes with model substrate surfaces using label-free techniques. Our model system was based on serine proteases (a class of enzymes that digests proteins) and surface-bound polypeptide substrates. While previous studies have focused on bulk media factors such as pH, ionic strength, and surfactants, this study focuses on the role of the surface-bound substrate itself. In particular, we assess how the substrate density of a polypeptide with an alpha-helical secondary structure influences surface reactivity. An alpha-helical secondary structure was chosen based on literature indicating that stable alpha-helices can resist enzymatic digestion. To investigate the protease resistance of a surface-bound a-helix, we designed an a-helical polypeptide (SS-polypeptide, where SS = disulfide), used it to form films of varying surface coverage and then measured responses of the films to enzymatic exposure. Using quartz-crystal microbalance with dissipation (QCM-D), angle-resolved X-ray photoelectron spectroscopy (AR-XPS), grazing-angle infrared spectroscopy (GAIRS), and other techniques, we characterized the degradation of films to determine how the lateral packing density of the surface-bound SS-polypeptide substrate affected surface proteolysis. Characterization of pure SS-polypeptide films indicated dense packing of helices that maintained their helical structure and were generally oriented normal to the surface. We found that films of pure SS-polypeptide significantly resisted enzymatic digestion, while incorporation of very minor amounts of a diluent in such films resulted in rapid digestion. In part, this may be due to the need for the enzyme to bind several peptides along the peptide substrate within the cleft for digestion to occur. Only SS-polypeptide films that were densely packed and did not permit catalytic access to multiple peptides (e.g., terminal peptides only) were resistant to enzymatic proteolysis.

Fluorescence quantification for surface plasmon excitation

Surface plasmon resonance and surface plasmon fluorescence spectroscopy in combination have the potential to distinguish multicomponent surface processes. However, surface intensity variations from resonance angle shifts lead to a nonlinear response in the fluorescence intensity. We report a method to account for surface intensity variations using the experimentally measured relationship between fluorescence and reflectivity. We apply this method to monitor protease adsorption and proteolytic substrate degradation simultaneously. Multilayer protein substrates are prepared for these degradation studies using a layer-by-layer technique.

Viral infection of human progenitor and liver-derived cells encapsulated in three-dimensional PEG-based hydrogel

We have studied the encapsulation of human progenitor cells into 3D PEG hydrogels. Replication-incompetent lentivirus promoter reporter vectors were found to efficiently detect the in vivo expression of human hepatic genes in hydrogel-encapsulated liver progenitor cells. Similarly, hydrogel-encapsulated cells could be efficiently infected with hepatitis C virus, and progeny infectious virus could be recovered from the media supernatants of the hydrogels. Provocatively, the diameters of these virus particles range from approximately 50 to 100 nm, while the calculated mesh size of the 8 k hydrogel is 44.6 +/- 1.7 A. To reconcile how viral particles can penetrate the hydrogels to infect the encapsulated cells, we propose that microfractures/defects of the hydrogel result in a functional pore size of up to 20 fold greater than predicted by theoretical mesh calculations. These results suggest a new model of hydrogel structure, and have exciting implications for tissue engineering and hepatitis virus studies.

Development of hydrogel-based keratoprostheses: a materials perspective

Research and development of artificial corneas (keratoprostheses) in recent years have evolved from the use of rigid hydrophobic materials such as plastics and rubbers to hydrophilic, water-swollen hydrogels engineered to support not only peripheral tissue integration but also glucose diffusion and surface epithelialization. The advent of the AlphaCor core-and-skirt hydrogel keratoprosthesis has paved the way for a host of new approaches based on hydrogels and other soft materials that encompass a variety of materials preparation strategies, from synthetic homopolymers and copolymers to collagen-based bio-copolymers and, finally, interpenetrating polymer networks. Each approach represents a unique strategy toward the same goal: to develop a new hydrogel that mimics the important properties of natural donor corneas. We provide a critical review of these approaches from a materials perspective and discuss recent experimental results. While formidable technical hurdles still need to be overcome, the rapid progress that has been made by investigators with these approaches is indicative that a synthetic donor cornea capable of surface epithelialization is now closer to becoming a clinical reality.

Glucose-permeable interpenetrating polymer network hydrogels for corneal implant applications: a pilot study

Epithelialization of a keratoprosthesis requires that the implant material be sufficiently permeable to glucose. We have developed a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN) hydrogel that can provide adequate passage of glucose from the aqueous humor to the epithelium in vivo. A series of PEG/PAA IPNs with varying PEG macromonomer molecular weights were synthesized and evaluated through swelling studies to determine their water content and diffusion experiments to assess their permeability to glucose. One of the PEG/PAA hydrogels prepared in this study had a glucose diffusion coefficient nearly identical to that of the human cornea (approximately 2.5 x 10(-6) cm(2)/sec). When implanted intrastromally in rabbit corneas, this hydrogel was retained and well-tolerated in 9 out of 10 cases for a period of 14 days. The retained hydrogels stayed optically clear and the epithelium remained intact and multilayered, indicating that the material facilitated glucose transport from the aqueous humor to the anterior part of the eye. The results from these experiments indicate that PEG/PAA hydrogels are promising candidates for corneal implant applications such as keratoprostheses and intracorneal lenses, and that the PEG/PAA IPN system in general is useful for creating permeable substrates for ophthalmic and other biomedical applications.

Design and fabrication of an artificial cornea based on a photolithographically patterned hydrogel construct

We describe the design and fabrication of an artificial cornea based on a photolithographically patterned hydrogel construct, and demonstrate the adhesion of corneal epithelial and fibroblast cells to its central and peripheral components, respectively. The design consists of a central "core" optical component and a peripheral tissue-integrable "skirt." The core is composed of a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) double-network with high strength, high water content, and collagen type I tethered to its surface. Interpenetrating the periphery of the core is a microperforated, but resilient poly(hydroxyethyl acrylate) (PHEA) hydrogel skirt that is also surface-modified with collagen type I. The well-defined microperforations in the peripheral component were created by photolithography using a mask with radially arranged chrome discs. Surface modification of both the core and skirt elements was accomplished through the use of a photoreactive, heterobifunctional crosslinker. Primary corneal epithelial cells were cultured onto modified and unmodified PEG/PAA hydrogels to evaluate whether the central optic material could support epithelialization. Primary corneal fibroblasts were seeded onto the PHEA hydrogels to evaluate whether the peripheral skirt material could support the adhesion of corneal stromal cells. Cell growth in both cases was shown to be contingent on the covalent tethering of collagen. Successful demonstration of cell growth on the two engineered components was followed by fabrication of core-skirt constructs in which the central optic and peripheral skirt were synthesized in sequence and joined by an interpenetrating diffusion zone.

Progress in the development of interpenetrating polymer network hydrogels

Interpenetrating polymer networks (IPNs) have been the subject of extensive study since their advent in the 1960s. Hydrogel IPN systems have garnered significant attention in the last two decades due to their usefulness in biomedical applications. Of particular interest are the mechanical enhancements observed in "double network" IPN systems which exhibit nonlinear increases in fracture properties despite being composed of otherwise weak polymers. We have built upon pioneering work in this field as well as in responsive IPN systems to develop an IPN system based on end-linked poly-(ethylene glycol) (PEG) and loosely crosslinked poly(acrylic acid) (PAA) with hydrogen bond-reinforced strain-hardening behavior in water and high initial Young's moduli under physiologic buffer conditions through osmotically induced pre-stress. Uniaxial tensile tests and equilibrium swelling measurements were used to study PEG/PAA IPN hydrogels having second networks prepared with varying crosslinking and photoinitiator content, pH, solids content, and comonomers. Studies involving the addition of non-ionic comonomers and neutralization of the second network showed that template polymerization appears to be important in the formation of mechanically enhanced IPNs.

Enrichment of deuterium oxide at hydrophilic interfaces in aqueous solutions

The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. We use neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulk liquid solutions of H2O/D2O mixtures. We find that near the interface the neutron scattering length density is larger than in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a region adjacent to the quartz that is enriched in D2O and extends 5-10 nm into the solution. This suggests caution when interpreting results where D2O is substituted for H2O in aqueous interfacial chemistry.

Quartz resonator signatures under Newtonian liquid loading for initial instrument check

The quartz crystal microbalance (QCM) has been increasingly utilized in the monitoring of the deposition of thin macromolecular films. Studies in the deposition of polymers, biomaterials, and interfacial reactions under electrochemical environment are some of the conditions for the study of these material and deposition properties at a lipid interface. Numerous studies have shown the difficulties in configuring an experimental setup for the QCM such that the recorded data reflect only the behavior of the quartz crystal and its load, and not some artifact. Such artifacts for use in liquids include mounting stress, surface properties such as hydrophobicity, surface roughness coupling to loading liquids, influence of compressional waves, and even problems with the electronic circuitry including the neglect of the quartz capacitance and the hysteretic effects of electronic components. It is thought useful to obtain a simple test by which the user could make a quick initial assessment of the instrument's performance. When a smooth quartz crystal resonator is immersed from air into a Newtonian liquid, the resonance and loss characteristics of the QCM are changed. A minimum of two experimental parameters is needed to characterize these changes. One of the changes is that of the resonant frequency. The second is characterized by either a change in the equivalent circuit resistance (DeltaR) or a change in the resonance dissipation (DeltaD). Two combinations of these observables, in terms of either Deltaf and DeltaR or Deltaf and DeltaD, which we define as Newtonian signatures of S(1) and S(2), are calculated to have fixed values and to be independent of the harmonic and of the physical values of the Newtonian liquid. We have experimentally determined the values of S(1) and S(2) using three different QCM systems. These are the standard oscillator, the network analyzer, and the QCM dissipation instrument. To test the sensitivity of these signatures to surface roughness, which is potential experimental artifact, we determined the values of S(1) and S(2) for roughened crystals and found that these signatures do reflect that experimental condition. Moreover, these results were qualitatively in accord with the roughness scaling factor described by Martin.

Creation of lipid partitions by deposition of amphipathic viral peptides

Phospholipid vesicles exhibit a natural characteristic to fuse and reform into a continuous single bilayer membrane on hydrophilic solid substrates such as glass, mica, and silica. The resulting solid-supported bilayer mimics physiological tendencies such as lipid flip-flop and lateral mobility. The lateral mobility of fluorescently labeled lipids fused into solid-supported bilayers is found to change upon deposition on the membrane surface of an amphipathic alpha-helical peptide (AH) derived from the hepatitis C virus (HCV) NS5A protein. The binding of the AH peptide to a phospholipid bilayer, with the helical axis parallel to the bilayer, leads to immobilization of the bilayer. We used AFM to better understand the mechanistic details of this specific interaction, and determined that the diminished fluidity of the bilayer is due to membrane thinning. Utilizing this specific interaction between AH peptides and lipid molecules, we demonstrate a novel process for the creation of lipid partition by employing AH peptides as agents to immobilize lipid molecules, thus creating a patterned solid support with partition-defined areas of freely mobile lipid bilayers. This architecture could have a wide range of applications in novel sensing, biotechnology, high-throughput screening, and biomimetic strategies.

Employing two different quartz crystal microbalance models to study changes in viscoelastic behavior upon transformation of lipid vesicles to a bilayer on a gold surface

By analyzing the viscoelastic properties of two distinct layers, a layer of "soft" vesicles and a "rigid" bilayer, we have created a model system to permit the study of film behavior in the region of nonlinear mass and frequency change (non-Sauerbrey). The structural transformation of lipid vesicles to a bilayer is shown to be accompanied by significant changes in their physical properties. After the adsorption and saturation of intact vesicles on gold surfaces, the adsorbed vesicle layer exhibits a soft, water-rich, viscoelastic state. The AH peptide, a vesicle-destabilizing agent, is then added to trigger the formation of a much thinner (approximately 5 nm), compact, and rigid bilayer. In this study, we used the quartz crystal microbalance with dissipation technique. Large non-Sauerbrey frequency and energy dissipation changes characterize the viscoelastic nature of adsorbed intact vesicle films thicker than approximately 10 nm. Once the transformation is complete, the frequency changes along with zero energy dissipation for sufficiently thin films (t approximately 5 nm) were effectively modeled with the Sauerbrey equation. Furthermore, we checked the validity of the Voigt-Voinova model in which the quartz substrate is treated as a Voigt element, which is beyond the Sauerbrey description. The calculations treating the film as having a constant viscosity agreed well with the Voigt-Voinova model. These results were compared to calculations done using the electromechanical (EM) model, which does not require a series expansion. The Voigt-Voinova results were in excellent agreement with the EM model, providing evidence that the expansion used in their study is quite accurate.

Kinetics of oligonucleotide hybridization to DNA probe arrays on high-capacity porous silica substrates

We have investigated the kinetics of DNA hybridization to oligonucleotide arrays on high-capacity porous silica films that were deposited by two techniques. Films created by spin coating pure colloidal silica suspensions onto a substrate had pores of approximately 23 nm, relatively low porosity (35%), and a surface area of 17 times flat glass (for a 0.3-microm film). In the second method, latex particles were codeposited with the silica by spin coating and then pyrolyzed, which resulted in larger pores (36 nm), higher porosity (65%), and higher surface area (26 times flat glass for a 0.3-microm film). As a result of these favorable properties, the templated silica hybridized more quickly and reached a higher adsorbed target density (11 vs. 8 times flat glass at 22 degrees C) than the pure silica. Adsorption of DNA onto the high-capacity films is controlled by traditional adsorption and desorption coefficients, as well as by morphology factors and transient binding interactions between the target and the probes. To describe these effects, we have developed a model based on the analogy to diffusion of a reactant in a porous catalyst. Adsorption values (k(a), k(d), and K) measured on planar arrays for the same probe/target system provide the parameters for the model and also provide an internally consistent comparison for the stability of the transient complexes. The interpretation of the model takes into account factors not previously considered for hybridization in three-dimensional films, including the potential effects of heterogeneous probe populations, partial probe/target complexes during diffusion, and non-1:1 binding structures. The transient complexes are much less stable than full duplexes (binding constants for full duplexes higher by three orders of magnitude or more), which may be a result of the unique probe density and distribution that is characteristic of the photolithographically patterned arrays. The behavior at 22 degrees C is described well by the predictive equations for morphology, whereas the behavior at 45 degrees C deviates from expectations and suggests that more complex phenomena may be occurring in that temperature regime.

Binding dynamics of hepatitis C virus' NS5A amphipathic peptide to cell and model membranes

Membrane association of the hepatitis C virus NS5A protein is required for viral replication. This association is dependent on an N-terminal amphipathic helix (AH) within NS5A and is restricted to a subset of host cell intracellular membranes. The mechanism underlying this specificity is not known, but it may suggest a novel strategy for developing specific antiviral therapy. Here we have probed the mechanistic details of NS5A AH-mediated binding to both cell-derived and model membranes by use of biochemical membrane flotation and quartz crystal microbalance (QCM) with dissipation. With both assays, we observed AH-mediated binding to model lipid bilayers. When cell-derived membranes were coated on the quartz nanosensor, however, significantly more binding was detected, and the QCM-derived kinetic measurements suggested the existence of an interacting receptor in the target membranes. Biochemical flotation assays performed with trypsin-treated cell-derived membranes exhibited reduced AH-mediated membrane binding, while membrane binding of control cytochrome b5 remained unaffected. Similarly, trypsin treatment of the nanosensor coated with cellular membranes abolished AH peptide binding to the cellular membranes but did not affect the binding of a control lipid-binding peptide. These results therefore suggest that a protein plays a critical role in mediating and stabilizing the binding of NS5A's AH to its target membrane. These results also demonstrate the successful development of a new nanosensor technology ideal both for studying the interaction between a protein and its target membrane and for developing inhibitors of that interaction.

Glyco-acrylate copolymers for bilayer tethering on benzophenone-modified substrates

Model biological membranes are becoming increasingly important for studying fundamental biophysical phenomena and developing membrane-based devices. To address the anticipated problem of non-physiological interactions between membrane proteins and substrates seen in "solid-supported lipid bilayers" that are formed directly on hydrophilic substrates, we have developed a polymer-tethered lipid bilayer system based on a random copolymer with multiple lipid analogue anchors and a glyco-acrylate backbone. This system is targeted at applications that, most importantly, require stability and robustness since each copolymer has multiple lipid analogues that insert into the bilayer. We have combined this copolymer with a flexible photochemical coupling scheme that covalently attaches the copolymer to the substrate. The Langmuir isotherms of mixed copolymer/free lipid monolayers measured at the air-water interface indicate that the alkyl chains of the copolymer lipid analogues and the free lipids dominate the film behavior. In addition, no significant phase transitions are seen in the isotherms, while hysteresis experiments confirm that no irreversible states are formed during the monolayer compression. Isobaric creep experiments at the air-water interface and AFM experiments of the transferred monolayer are used to guide processing parameters for creating a fluid, homogeneous bilayer. Bilayer homogeneity and fluidity are monitored using fluorescence microscopy. Continuous bilayers with lateral diffusion coefficients of 0.6 microm(2)/s for both leaflets of the bilayer are observed for a 5% copolymer system.

Quantitative analysis of tethered vesicle assemblies by quartz crystal microbalance with dissipation monitoring: Binding dynamics and bound water content

To implement the molecular recognition properties of membrane proteins for applications including biosensors and diagnostic arrays, the construction of a biomimetic platform capable of maintaining protein structure and function is required. In this paper, we describe a tethered phospholipid vesicle assembly that overcomes the major limitations of planar supported lipid bilayers and alternative biomimetic membrane platforms and characterize it using quartz crystal microbalance with dissipation monitoring (QCM-D) and fluorescence microscopy. We provide evidence of a one-step mechanism for bilayer formation and monitor the subsequent adsorption and binding of streptavidin, vesicles, and streptavidin-coated microspheres. For all three species, we identify a critical surface density above which a significant amount of coupled interstitial water contributes to the response of the quartz resonator in a phenomenon similar to dynamic coupling due to surface roughness. A Sauerbrey-type analysis is sufficient to accurately interpret the QCM-D results for streptavidin binding if water is treated as an additional inertial mass, but viscoelastic models must be invoked for vesicle and microsphere binding. Additionally, we present evidence of vesicle flattening, possibly enhanced by a biotin-mediated membrane-membrane interaction.

Fluid supported lipid bilayers containing monosialoganglioside GM1: A QCM-D and FRAP study

In an effort to use model fluid membranes for immunological studies, we compared the formation of planar phospholipid bilayers supported on silicon dioxide surfaces with and without incorporation of glycolipids as the antigen for in situ antibody binding. Dynamic light scattering measurements did not differentiate the hydrodynamic volumes of extruded small unilamellar vesicles (E-SUVs) containing physiologically relevant concentrations (0.5-5 mol%) of monosialoganglioside GM1 (GM1) from exclusive egg yolk L-alpha-phosphatidylcholine (egg PC) E-SUVs. However, quantifiable differences in deposition mass and dissipative energy loss emerged in the transformation of 5 mol% GM1/95 mol% egg PC E-SUVs to planar supported lipid bilayers (PSLBs) by vesicle fusion on thermally evaporated SiO2, as monitored by the quartz crystal microbalance with dissipation (QCM-D) technique. Compared to the 100 mol% egg PC bilayers on the same surface, E-SUVs containing 5 mol% GM1 reached a approximately 12% higher mass and a lower dissipative energy loss during bilayer transformation. PSLBs with 5 mol% GM1 are approximately 18% heavier than 100 mol% egg PC and approximately 11% smaller in projected area per lipid, indicating an increased rigidity and a tighter packing. Subsequent binding of polyclonal immunoglobulin G anti-GM1 to the PSLBs was performed in situ and showed specificity. The anti-GM1 to GM1 ratios at equilibrium were roughly proportional to the concentrations of anti-GM1 administered in the solution. Fluorescence recovery after photobleaching was utilized to verify the retained, albeit reduced lateral fluidity of the supported membranes. Five moles percentage of GM1 membranes (GM1 to PC ratio approximately 1:19) decorated with 1 mol% N-(Texas Red sulfonyl)-1,2-dihexadecanoyl-sn-glycerol-3-phosphoethanolamine (Texas Red DHPE) exhibited an approximately 16% lower diffusion coefficient of 1.32+/-0.06 microm2/s, compared to 1.58+/-0.04 microm2/s for egg PC membranes without GM1 (p<0.01). The changes in vesicle properties and membrane lateral fluidity are attributed to the interactions of GM1 with itself and GM1 with other membrane lipids. This system allows for molecules of interest such as GM1 to exist on a more biologically relevant surface than those used in conventional methods such as ELISA. Our analysis of rabbit serum antibodies binding to GM1 demonstrates this platform can be used to test for the presence of anti-lipid antibodies in serum.