Cellular recognition of synthetic peptide amphiphiles in self-assembled monolayer films

Issues of ligand accessibility and mobility in initial cell attachment

The influence of lateral ligand mobility on cell attachment and receptor clustering has previously been explored for membrane-anchored molecules involved in cell-cell adhesion. In this study, we considered instead a cell binding motif from the extracellular matrix. Even though the lateral mobility of extracellular matrix ligands in membranes does not occur in vivo, we believe it is of interest for cell engineering in vitro. As is the case for cell-cell adhesion molecules, lateral mobility of extracellular matrix ligands could influence cell attachment and, subsequently, cell behavior in cell culture. In this paper, the accessibility and functionality of extracellular matrix ligands presented at surfaces were evaluated for the conditions of laterally mobile versus non-mobile ligands by studying ligand-antibody binding events and early cell attachment as a function of ligand concentration. We compare the initial attachment of rat-derived adult hippocampal progenitor (AHP) cells on laterally mobile, supported phospholipid bilayer membranes to non-mobile, poly-L-lysine-grafted-poly(ethylene glycol) (PLL-g-PEG) polymer films functionalized with a range of laminin-derived IKVAV-containing peptide densities. To this end, synthesis of a new PLL-g-PEG/PEG-IKVAV polymer is described. The characterization of available IKVAV peptides on both surface presentations schemes was explored by studying the mass uptake of anti-IKVAV antibodies using a combination of the surface-sensitive techniques quartz crystal microbalance with dissipation monitoring, surface plasmon resonance spectroscopy, and optical waveguide lightmode spectroscopy. IKVAV-containing peptides presented on laterally mobile, supported phospholipid bilayers and non-mobile PLL-g-PEG were recognized by the anti-IKVAV antibody in a dose-dependent manner, indicating that the amount of available IKVAV ligands increases proportionally with ligand density over the concentrations tested. Attachment of AHP cells to IKVAV-functionalized PLL-g-PEG and supported phospholipid bilayers followed a sigmoidal dependence on peptide concentration, with a critical concentration of approximately 3 pmol/cm2 IKVAV ligands required to support initial AHP cell attachment for both surface modifications. There appeared to be little influence of IKVAV peptide mobility on the initial attachment of AHP cells. Although the spread in the cell attachment data was larger for the PLL-g-PEG surface modification, this was reduced when observed after 24 h, indicating that the cells might need longer times to establish attachment strengths equivalent to those observed on peptide-functionalized supported lipid bilayers. The present study is a step toward understanding the influence of extracellular-matrix-derived ligand mobility on cell fate. Further analysis should focus on the systematic tuning of lateral ligand diffusion, as well as a comparison between the response of non-spreading cells (i.e., AHPs), versus spreading cells (i.e., fibroblasts).

Targeted drug delivery utilizing protein-like molecular architecture

Nanotechnology-based drug delivery systems (nanoDDSs) have seen recent popularity due to their favorable physical, chemical, and biological properties, and great efforts have been made to target nanoDDSs to specific cellular receptors. CD44/chondroitin sulfate proteoglycan (CSPG) is among the receptors overexpressed in metastatic melanoma, and the sequence to which it binds within the type IV collagen triple-helix has been identified. A triple-helical "peptide-amphiphile" (alpha1(IV)1263-1277 PA), which binds CD44/CSPG, has been constructed and incorporated into liposomes of differing lipid compositions. Liposomes containing distearoyl phosphatidylcholine (DSPC) as the major bilayer component, in combination with distearoyl phosphatidylglycerol (DSPG) and cholesterol, were more stable than analogous liposomes containing dipalmitoyl phosphatidylcholine (DPPC) instead of DSPC. When dilauroyl phosphatidylcholine (DLPC):DSPG:cholesterol liposomes were prepared, monotectic behavior was observed. The presence of the alpha1(IV)1263-1277 PA conferred greater stability to the DPPC liposomal systems and did not affect the stability of the DSPC liposomes. A positive correlation was observed for cellular fluorophore delivery by the alpha1(IV)1263-1277 PA liposomes and CD44/CSPG receptor content in metastatic melanoma and fibroblast cell lines. Conversely, nontargeted liposomes delivered minimal fluorophore to these cells regardless of the CD44/CSPG receptor content. When metastatic melanoma cells and fibroblasts were treated with exogeneous alpha1(IV)1263-1277, prior to incubation with alpha1(IV)1263-1277 PA liposomes, to potentially disrupt receptor/liposome interactions, a dose-dependent decrease in the amount of fluorophore delivered was observed. Overall, our results suggest that PA-targeted liposomes can be constructed and rationally fine-tuned for drug delivery applications based on lipid composition. The selectivity of alpha1(IV)1263-1277 PA liposomes for CD44/CSPG-containing cells represents a targeted-nanoDDS with potential for further development and application.

Cell adhesion and growth to Peptide-patterned supported lipid membranes

Lipid vesicles displaying RGD peptide amphiphiles were fused with glass coverslips to control the ability of these surfaces to support cell adhesion and growth. Cell adhesion was prevented on phosphatidylcholine bilayers in the absence of RGD, whereas cells adhered and grew in the presence of accessible RGD amphiphiles. This specific interaction between cells and RGD peptides was further explored in a concentration-dependent fashion by creating surface composition arrays using microfluidics. For the range of concentrations studied adhesion and growth were favored by increased peptide concentration, but this concentration dependence was found to diminish in the higher concentration regions of the array. Developing peptide composition gradients in a membrane environment is demonstrated as an effective method to screen biological probes for cell adhesion and growth.

Stabilization of peptide fibrils by hydrophobic interaction

Hydrophobic interactions play an important role in assembly processes in aqueous environments. In case of peptide amphiphiles, hydrophobicity is combined with hydrogen bonding to yield well-defined peptide-based aggregates. Here, we report a systematic study after the role of hydrophobic interactions on both stabilization and morphology of a peptide fibrillar assembly. For this purpose, alkyl tails were connected to a known beta-sheet forming peptide with the sequence KTVIIE. The introduction of n-alkyl groups induced thermal stability to the assemblies without affecting the morphology of the peptide aggregates.

Self-assembly and applications of biomimetic and bioactive peptide-amphiphiles

Peptide-amphiphiles are amphiphilic structures with a hydrophilic peptide headgroup that incorporates a bioactive sequence and has the potential to form distinct structures, and a hydrophobic tail that serves to align the headgroup, drive self-assembly, and induce secondary and tertiary conformations. In this paper we review the different self-assembled structures of peptide-amphiphiles that range from micelles and nanofibers, to patterned membranes. We also describe several examples where peptide-amphiphiles have found applications as soft bioactive materials for model studies of bioadhesion and characterization of different cellular phenomena, as well as scaffolds for tissue engineering, regenerative medicine, and targeted drug delivery.

Fibronectin terminated multilayer films: protein adsorption and cell attachment studies

Electrostatically driven layer-by-layer (LbL) assembly is a simple and robust method for producing structurally tailored thin film biomaterials, of thickness ca. 10nm, containing biofunctional ligands. We investigate the LbL formation of multilayer films composed of polymers of biological origin (poly(L-lysine) (PLL) and dextran sulfate (DS)), the adsorption of fibronectin (Fn)--a matrix protein known to promote cell adhesion--onto these films, and the subsequent spreading behavior of human umbilical vein endothelial cells (HUVEC). We employ optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microgravimetry with dissipation (QCMD) to characterize multilayer assembly in situ, and find adsorbed Fn mass on PLL-terminated films to exceed that on DS terminated films by 40%, correlating with the positive charge and lower degree of hydration of PLL terminated films. The extent and initial rate of Fn adsorption to both PLL and DS-terminated films exceed those onto the bare substrate, indicating the important role of electrostatic complexation between negatively charged protein and positively charged PLL at or near the film surface. We use phase-contrast optical microscopy to investigate the time-dependent morphological changes of HUVEC as a function of layer number, charge of terminal layer, and the presence of Fn. We observe HUVEC to attach, spread, and lose circularity on all surfaces. Positively charged PLL-terminated films exhibit a greater extent of cell spreading than do (negatively charged) DS-terminated films, and spreading is enhanced while circularity loss is suppressed by the presence of adsorbed Fn. The number of layers plays a significant role only for DS-terminated films with Fn, where spreading on a bilayer greatly exceeds that on a multilayer, and PLL-terminated films without Fn, where initial spreading is significantly higher on a monolayer. We observe initial cell spreading to be followed by retraction (i.e. decreased cell area and circularity with time) for films without Fn, and for DS-terminated films with Fn. Overall, the Fn-coated PLL monolayer and the Fn-coated PLL-terminated multilayer are the best performing films in promoting cell spreading. We conclude the presence of Fn to be an important factor (more so than film charge or layer number) in controlling the interaction between multilayer films and living cells, and thus to represent a promising strategy toward in vivo applications such as tissue engineering.

Self-assembly of model DNA-binding peptide amphiphiles

Peptide amphiphiles combine the specific functionality of proteins with the engineering convenience of synthetic amphiphiles. These molecules covalently link a peptide headgroup, typically from an active fragment of a larger protein, to a hydrophobic alkyl tail. Our research is aimed at forming and characterizing covalently stabilized, self-assembled, peptide-amphiphile aggregates that can be used as a platform for the examination and modular design and construction of systems with engineering biological activity. We have studied the self-assembly properties of a model DNA-binding amphiphile, having a GCN4 peptide as the headgroup and containing a polymerizable methacrylic group in the tail region, using a combination of small-angle X-ray scattering, small-angle neutron scattering, and cryo- transmission electron microscopy. Our results reveal a variety of morphologies in this system. The peptide amphiphiles assembled in aqueous solution to helical ribbons and tubules. These structures transformed into lamella upon DNA binding. In contrast with common surfactants, the specific interaction between the headgroups seems to play an important role in determining the microstructure. The geometry of the self-assembled aggregate can be controlled by means of adding a cosurfactant. For example, the addition of SDS induced the formation of spherical micelles.

Learning the chemical language of cell-surface interactions

Harvesting, training, and domesticating living organisms to apply their intrinsic capabilities to desired applications is one of the most fundamental of human accomplishments. Extending this concept to the cellular level requires precise control of the chemical interface between live cells and synthetic materials. One great challenge is to progress from empirically surveying cellular behaviors to the predictive design of synthetic substrates that communicate specific signals to cells. This requires a detailed understanding of cell signaling mechanisms as well as the ability to synthesize hybrid materials that incorporate biological molecules in precisely defined ways. A number of promising advances in the development of synthetic interfaces with cells suggest that the move to predictive design is under way.

Tailoring surface properties of biomedical polymers by implantation of Ar and He ions

Ion implantation at 25 and 100 keV has been used as a tool for the modification of the surface properties of two biomedical polymers. The modulation induced by the different energy dispersion mechanisms of Ar and He have allowed satisfactory modifications for both the activation of the surfaces of chemically functional polycaprolactone (PCL) and the stabilization of anti-fouling poly(ethylene glycol) (PEG). In both cases the implantations have been performed at doses of 10(14) cm(-2) by taking into account the effect of different current densities, which are shown to distinctly influence the fragmentation-crosslinking of the target polymers. The resultant films were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, time of flight secondary ion mass spectroscopy and atomic force microscopy. Both shifts in zeta potential versus pH curves and the alteration of the polar components of the surface free energy (contact angle measurements) were correlated with the composition analysis. The response of the modified surfaces towards biomolecular interaction is demonstrated by the induction of preferential adsorption on irradiated PCL and the inhibited adsorption onto implanted PEG regions for selected oligopeptides and proteins.

Synthesis and biological activity of N-terminal lipidated and/or fluorescently labeled conjugates of astressin as corticotropin releasing factor antagonists

This report describes the synthesis of eight N-terminally modified astressin analogs and their biochemical evaluation as corticotropin releasing factor (CRF) antagonists. The lipidated astressin derivatives were tested on rat CRF receptor type 1 and 2alpha and were found to be active as CRF antagonists (rCRFR1: pA(2)=7.5-8.3; rCRFR2alpha: pA(2)=7.5-9.0) with nearly equal activities as compared to unmodified astressin (rCRFR1: pA(2)=8.3+/-0.09; rCRFR2alpha: pA(2)=8.7+/-0.08).

Solid-state NMR structural studies of peptides immobilized on gold nanoparticles

In this paper we describe solid-state NMR experiments that provide information on the structures of surface-immobilized peptides. The peptides are covalently bound to alkanethiolates that are self-assembled as monolayers on colloidal gold nanoparticles. The secondary structure of the immobilized peptides was characterized by quantifying the Ramachandran angles phi and psi. These angles were determined in turn from distances between backbone carbonyl 13C spins, measured with the double-quantum filtered dipolar recoupling with a windowless sequence experiment, and by determination of the mutual orientation of chemical shift anisotropy tensors of 13C carbonyl spins on adjacent peptide planes, obtained from the double-quantum cross-polarization magic-angle spinning spectrum. It was found that peptides composed of periodic sequences of leucines and lysines were bound along the length of the peptide sequence and displayed a tight alpha-helical secondary structure on the gold nanoparticles. These results are compared to similar studies of peptides immobilized on hydrophobic surfaces.

Lipopeptides incorporated into supported phospholipid monolayers have high specific activity at low incorporation levels

The ability to present cell adhesion molecule (CAM) ligands in controlled amounts on a culture surface would greatly facilitate the control of cell growth and differentiation. Supported lipid monolayer/bilayer systems have previously been developed that allow for presentation of CAM ligands for cell interaction; however, these systems have employed peptide loadings much higher than those used in poly(ethylene glycol) (PEG)-based immobilization systems. We report the development of synthetic methods that can be used for the efficient and versatile creation of many linear and cyclic lipid-linked peptide moieties. Using RGD-based peptides for the alpha5beta1 integrin as a model system, we have demonstrated that these lipopeptides support efficient cell binding and spreading at CAM ligand loadings as low as 0.1 mol %, which is well below that previously reported for supported lipid systems. Engineered lipopeptide-based surfaces offer unique presentation options not possible with other immobilization systems, and the high activity at low loadings we have shown here may be extremely useful in presenting multiple CAM ligands for studying cell growth, differentiation, and signaling.

Adsorbed layers of oriented fibronectin: A strategy to control cell-surface interactions

Fibronectin (Fn) is a matrix protein known to induce cell attachment and spreading through its cell binding site and related synergy sites. Fn-coated surfaces are therefore useful in tissue engineering and other cell contacting applications, but a problem with many immobilization strategies is a random distribution of molecular orientations. We sought to control Fn orientation, and thus enhance the availability of its cell binding site, by immobilizing Fn via a carboxymethyl dextran layer onto which are chemically attached monoclonal antibodies specific to a region near to Fn's C terminus (and thus away from the cell binding site). Using optical waveguide lightmode spectroscopy, we show the presence of chemically coupled antibodies to yield a considerably denser and thicker Fn layer, consistent with a more vertically aligned protein. Human umbilical vein endothelial cells spread significantly faster, and in a more spherically symmetric way, on an oriented Fn layer (i.e., in the presence of immobilized monoclonal antibodies) as compared with a control Fn layer (i.e., in the absence of bound antibodies). However, we observe human umbilical vein endothelial cell spreading on the oriented Fn layer to be similar to that on a Fn layer in the absence of a carboxymethyl dextran layer, suggesting that although orienting Fn is a promising strategy, coupling strategies using linkers other than dextran may be needed.

Model surfaces engineered with nanoscale roughness and RGD tripeptides promote osteoblast activity

Cell adhesion to biomaterials is a prerequisite for tissue integration with the implant surface. Herein, we show that we can generate a model silica surface that contains a minimal-length arginine-glycine-aspartic acid (RGD) peptide that maintains its biological activity. In the first part of this study, attachment of MC3T3-E1 osteoblast-like cells was investigated on silicon oxide, amine terminated substrates [i.e., 3-aminopropyl triethoxysilane (APTS)], grafted RGD, and physisorbed RGD control. The APTS layer exhibited nanoscale roughness and presented amine functional groups for grafting a minimal RGD tripeptide devoid of any flanking groups or spacers. Contact angle measurements indicated that the hydrophobicity of the APTS surface was significantly lower than that of the surface with grafted RGD (RGD-APTS). Atomic force microscopy showed that surfaces covered with RGD-APTS were smoother (Ra = 0.71 nm) than those covered with APTS alone (Ra = 1.59 nm). Focusing mainly on cell morphology, experiments showed that the RGD-APTS hybrid provided an optimum surface for cell adhesion, spreading, and cytoskeletal organization. Discrete focal adhesion plaques were also observed consistent with successful cell signaling events. In a second set of experiments, smooth, monolayers of APTS (Ra = 0.1 nm) were used to prepare arginine-glycine-aspartic acid-serine (RGDS)-APTS and arginine-glycine-glutamic acid-serine (RGES)-APTS (control) substrates. Focusing mainly on cell function, integrin and gene expression were all enhanced for rate osteosarcoma cells on surfaces containing grafted RGDS. Both sets of studies demonstrated that grafted molecules of RGD(S) enhance both osteoblast-like cell adhesion and function.

Differential modulation of human melanoma cell metalloproteinase expression by alpha2beta1 integrin and CD44 triple-helical ligands derived from type IV collagen

Tumor cell binding to components of the basement membrane is well known to trigger intracellular signaling pathways. Signaling ultimately results in the modulation of gene expression, facilitating metastasis. Type IV collagen is the major structural component of the basement membrane and is known to be a polyvalent ligand, possessing sequences bound by the alpha1beta1, alpha2beta1, and alpha3beta1 integrins, as well as cell surface proteoglycan receptors, such as CD44/chondroitin sulfate proteoglycan (CSPG). The role of alpha2beta1 integrin and CD44/CSPG receptor binding on human melanoma cell activation has been evaluated herein using triple-helical peptide ligands incorporating the alpha1(IV)382-393 and alpha1(IV)1263-1277 sequences, respectively. Gene expression and protein production of matrix metalloproteinases-1 (MMP-1), -2, -3, -13, and -14 were modulated with the alpha2beta1-specific sequence, whereas the CD44-specific sequence yielded significant stimulation of MMP-8 and lower levels of modulation of MMP-1, -2, -13, and -14. Analysis of enzyme activity confirmed different melanoma cell proteolytic potentials based on engagement of either the alpha2beta1 integrin or CD44/CSPG. These results are indicative of specific activation events that tumor cells undergo upon binding to select regions of basement membrane collagen. Based on the present study, triple-helical peptide ligands provide a general approach for monitoring the regulation of proteolysis in cellular systems.

Biomimetic Peptide−Amphiphiles for Functional Biomaterials: The Role of GRGDSP and PHSRN

The study we present involves the use of a biomimetic system that allows us to study specific interactions in the alpha(5)beta(1) receptor-GRGDSP ligand system with an atomic force microscope (AFM). Bioartificial membranes that mimic the adhesion domain of the extracellular matrix protein fibronectin are constructed from peptide-amphiphiles. A novel peptide-amphiphile is designed that contains both GRGDSP (Gly-Arg-Gly-Asp-Ser-Pro, the primary recognition site for alpha(5)beta(1)) and PHSRN (Pro-His-Ser-Arg-Asn, the synergy binding site for alpha(5)beta(1)) sequences in a single peptide formulation, separated by a spacer. Two different antibodies are used to immobilize and activate isolated alpha(5)beta(1) integrins on the AFM tip. The interaction measured between immobilized alpha(5)beta(1) integrins and peptide-amphiphiles is specific for integrin-peptide binding and is affected by divalent cations in a way that accurately mimics the adhesion function of the alpha(5)beta(1) receptor. The strength of the PHSRN synergistic effect depends on the accessibility of this sequence to alpha(5)beta(1) integrins. An increase in adhesion is observed compared to surfaces displaying only GRGDSP peptides when the new biomimetic peptide-amphiphiles are diluted with lipidated poly(ethylene glycol), which provides more space for the peptide headgroups to bend and expose more of the PHSRN at the interface.

Induction of Endothelial Cell Activation by a Triple Helical α2β1 Integrin Ligand, Derived from Type I Collagen α1(I)496–507

Endothelial cell activation involves the elevated expression of cell adhesion molecules, chemoattractants, chemokines, and cytokines. These expression profiles may be regulated by integrin-mediated cell signaling pathways. In the current study, an alpha2beta1 integrin triple helical peptide ligand derived from type I collagen residues alpha1(I)496-507 was examined for induction of human aortic endothelial cell (HAEC) activation. In addition, a "miniextracellular matrix" composed of a mixture of the alpha1(I)496-507 ligand and a second, alpha-helical ligand incorporating the endothelial cell proliferating region of SPARC (secreted protein acidic and rich in cysteine) was studied for induction of HAEC activation. Following HAEC adhesion to alpha1(I)496-507, mRNA expression of E-selectin-1, vascular and intercellular cell adhesion molecules-1, and monocytic chemoattractant protein-1 was stimulated, whereas that of endothelin-1 was inhibited. Enzyme-linked immunosorbent assay analysis demonstrated that E-selectin-1 and monocytic chemoattractant protein-1 expression was also stimulated, whereas endothelin-1 protein expression diminished. Engagement of the alpha2beta1 integrin initiated a HAEC response similar to that of tumor necrosis factor-alpha-induced HAECs but was not sufficient to induce an inflammatory response. Addition of the SPARC119-122 region had only a slight effect on HAEC activation. Other cell-extracellular matrix interactions appear to be required to elicit an inflammatory response. The alpha2beta1 integrin specific triple helical peptide ligand described herein represents a more general in vitro model system by which gene expression and protein production profiles induced by binding to a single cellular receptor type can be quantified.

Structural studies of biomaterials using double-quantum solid-state NMR spectroscopy

Proteins directly control the nucleation and growth of biominerals, but the details of molecular recognition at the protein-biomineral interface remain poorly understood. The elucidation of recognition mechanisms at this interface may provide design principles for advanced materials development in medical and ceramic composites technologies. Here, we describe both the theory and practice of double-quantum solid-state NMR (ssNMR) structure-determination techniques, as they are used to determine the secondary structures of surface-adsorbed peptides and proteins. In particular, we have used ssNMR dipolar techniques to provide the first high-resolution structural and dynamic characterization of a hydrated biomineralization protein, salivary statherin, adsorbed to its biologically relevant hydroxyapatite (HAP) surface. Here, we also review NMR data on peptides designed to adsorb from aqueous solutions onto highly porous hydrophobic surfaces with specific helical secondary structures. The adsorption or covalent attachment of biological macromolecules onto polymer materials to improve their biocompatibility has been pursued using a variety of approaches, but key to understanding their efficacy is the verification of the structure and dynamics of the immobilized biomolecules using double-quantum ssNMR spectroscopy.

Structure and function of integral membrane protein domains resolved by peptide-amphiphiles: application to phospholamban

We have used synthetic lipidated peptides ("peptide-amphiphiles") to study the structure and function of isolated domains of integral transmembrane proteins. We used 9-fluorenylmethyloxycarbonyl (Fmoc) solid-phase peptide synthesis to prepare full-length phospholamban (PLB(1-52)) and its cytoplasmic (PLB(1-25)K: phospholamban residues 1-25 plus a C-terminal lysine), and transmembrane (PLB(26-52)) domains, and a 38-residue model alpha-helical sequence as a control. We created peptide-amphiphiles by linking the C-terminus of either the isolated cytoplasmic domain or the model peptide to a membrane-anchoring, lipid-like hydrocarbon tail. Circular dichroism measurements showed that the model peptide-amphiphile, either in aqueous suspension or in lipid bilayers, had a higher degree of alpha-helical secondary structure than the unlipidated model peptide. We hypothesized that the peptide-amphiphile system would allow us to study the function and structure of the PLB(1-25)K cytoplasmic domain in a native-like configuration. We compared the function (inhibition of the Ca-ATPase in reconstituted membranes) and structure (via CD) of the PLB(1-25) amphiphile to that of PLB and its isolated transmembrane and cytoplasmic domains. Our results indicate that the cytoplasmic domain PLB(1-25)K has no effect on Ca-ATPase (calcium pump) activity, even when tethered to the membrane in a manner mimicking its native configuration, and that the transmembrane domain of PLB is sufficient for inhibition of the Ca-ATPase.

Production of interferon-beta by fibroblast cells on membranes prepared with RGD-containing peptides

The production of interferon-beta by NB1-RGB fibroblast cells cultured on protein and peptide membranes prepared from silk fibroin, motif peptides of silk fibroin [(AG)(n)] containing arginine-glycine-aspartic acid (RGD) peptide, and Pronectin was investigated. The cell density on various protein and peptide membranes was approximately the same, although the production of interferon-beta depended significantly on the membranes where the cells were cultured. The highest production of interferon-beta was observed when the cells were cultured on (AG)(6)RGD(AG)(7) membranes prepared with hexafluoroacetone (HFA) as the casting solvent. On RGD-containing peptide membranes more centrally located in the peptides, the cells produced more interferon-beta when the peptide membranes were prepared with HFA as the casting solvent. However, there was no enhanced production of interferon-beta by cells on (AG)(6)RGD(AG)(7) membranes prepared with 9 mol/L LiBr or 4.5 mol/L LiClO(4) solution as the casting solvent. Therefore, both the chemical composition and the secondary and higher order structure of the peptide membranes are important for enhanced production of interferon-beta. The blocking of integrin beta(1) on the cells by anti-integrin beta(1) antibody prevented the enhanced production of interferon-beta on (AG)(6)RGD(AG)(7) membranes prepared with HFA. We suggest that the cells must bind to the RGD sequence having the appropriate conformation through their integrin beta(1) for enhanced production of interferon-beta.

Characterization of peptide-amphiphiles possessing cellular activation sequences

Numerous approaches have been described for modifying biomaterials to incorporate extracellular matrix components. "Peptide-amphiphiles", whereby monoalkyl hydrocarbon chains are covalently linked to peptide sequences, have been shown previously to (a) form specific molecular architecture with enhanced stability and (b) promote cell adhesion, spreading, and signaling. The present study has examined the use of chimeric peptide-amphiphiles for inducing protein-like structures and peptide-amphiphile mixtures for enhancing surface bioactivity. The alpha-helical propensity of a 21 residue peptide, incorporating the SPARC(119-122) angiogenesis-inducing sequence and either unmodified or acylated with a C(6), C(10), C(14), C(16), C(18), C(18:1), or C(18:1-OH) monoalkyl hydrocarbon chain, has been examined. Peptide and peptide-amphiphile structures were characterized by circular dichroism and one- and two-dimensional NMR spectroscopic techniques. The 21 residue peptide alone does not form a distinct structure in solution, whereas N-terminal acylation by monoalkyl hydrocarbon chains results in the 21 residue peptide-amphiphile adopting a predominantly alpha-helical structure in solution. The thermal stability of the alpha-helix increases with increasing hydrocarbon chain length. The SPARC(119-122) peptide-amphiphiles were then screened for promotion of endothelial cell adhesion and spreading. The greatest activity was achieved by using a mixture of the alpha-helical SPARC(119-122) peptide-amphiphile, a triple-helical peptide-amphiphile incorporating the alpha2beta1 integrin binding site from type I collagen, and a pseudolipid. The pseudolipid is most likely required for a spatial distribution of the peptide-amphiphiles that allows for optimal cellular interactions. Overall, we have found that incorporation of bioactive sequences within peptide-amphiphiles results in the induction of an ordered structure of the bioactive sequence and that mixtures of peptide-amphiphiles can be used to promote endothelial cell behaviors comparable to extracellular matrix components.

Comparison of reagents for shape analysis of fixed cells by automated fluorescence microscopy

BACKGROUND

Cell size and shape have been implicated as potentiators of intracellular signaling events and as indicators of abnormal cell behavior. Automated microscopy and image analysis can provide quantitative information about the size and shape of cultured cells, but it requires that the edge of a cell be clearly identified. Generating adequate contrast at the edge of thin well-spread cells can be challenging.

METHODS

We compared six (five chemically reactive and one lipophilic) fluorescent molecules--5-chloromethyl fluorescein diacetate (CMFDA, CellTracker green), fluorescein-5-maleimide, fluorescein-5-isothiocyanate (FITC), 5-iodoacetamidofluorescein, 5(6)-carboxy fluorescein-N-hydroxysuccinimidyl ester, and N-fluorescein-1,2-dihexadecanoyl-sn-glycerol-3-phosphoethanolamine--for their effectiveness as stains for automated morphology analysis of fixed cells.

RESULTS

Formaldehyde-fixed rat aortic smooth muscle cells stained with fluorescein-5-maleimide or FITC exhibited an average intensity that was at least twofold greater than cells stained with CMFDA even when subjected to a 25-fold shorter exposure time. Cell area determined with the higher intensity stains was less sensitive to threshold settings during automated cell morphology analysis.

CONCLUSION

A procedure that includes the use of fluorescein-5-maleimide or FITC for staining fixed cell provides sensitivity sufficient to permit rapid, automated, morphologic analysis of well-spread fixed cells.

Micromechanical coupling between cell surface receptors and RGD peptides

Contact between an adherent cell and the extracellular matrix (ECM) promotes the recruitment of structural and signaling molecules to the cytoplasmic domain of integrins, which mediate cell adhesion, cell migration, and cell growth. It is unclear whether the intracellular recruitment of these cytoplasmic molecules enhances the affinity between the ECM and the extracellular domain of the cell surface receptors (integrins). Using soft microneedles coated with Arg-Gly-Asp (RGD) peptides, a sequence commonly shared by ECM proteins, we apply a localized ramp shear stress to the surface of a HeLa cell and measure the cell stiffness and the collective (or apparent) unbinding lifetime of its surface receptors to RGD. These measurements demonstrate that both cell stiffness and the collective cell surface receptor-RGD unbinding lifetime increase with the duration of the pre-shear cell-microneedle contact and with the rate of shear applied to the cell membrane. These parameters are also crucially dependent on the integrity of the actin filament network. Our results are consistent with a model of positive feedback signaling where RGD-mediated initial recruitment of cytoskeletal proteins to the cytoplasmic domain of integrins directly enhances the interaction between the extracellular domain of integrins and the RGD sequence of ECM molecules.

Influence of hydrophobic mismatch and palmitoylation on the association of transmembrane alpha-helical peptides with detergent-resistant membranes

The aim of this study was to gain insight into the mechanism through which transmembrane proteins are targeted to liquid ordered (L(o)) phase domains or rafts. This was investigated by analyzing the Triton X-100 resistance of designed transmembrane peptides in model membranes of 1,2-dioleoyl-sn-glycero-3-phosphocholine, sphingomyelin and cholesterol (1/1/1, molar ratio), which contain both L(o) phase domains and fluid bilayers. By using peptides with one or two palmitate chains covalently linked to their N-terminus or with variable hydrophobic lengths, the roles of protein palmitoylation and of mismatch between the transmembrane segment of the protein and the bilayer thickness, respectively, were investigated. The results show that neither hydrophobic matching nor palmitoylation is sufficient for partitioning of peptides into L(o) phase domains. It is concluded that the L(o) phase itself, due to the tight packing of the lipids, constitutes an unfavorable environment for accommodation of protein transmembrane segments.

Assembly of alpha-helical peptide coatings on hydrophobic surfaces

The adsorption or covalent attachment of biological macromolecules onto polymer materials to improve their biocompatibility has been pursued using a variety of approaches, but key to understanding their efficacy is the verification of the structure and dynamics of the immobilized biomolecules. Here we present data on peptides designed to adsorb from aqueous solutions onto highly porous hydrophobic surfaces with specific helical secondary structures. Small linear peptides composed of alternating leucine and lysine residues were synthesized, and their adsorption onto porous polystyrene surfaces was studied using a combination of solid-state NMR techniques. Using conventional solid-state NMR experiments and newly developed double-quantum techniques, their helical structure was verified. Large-amplitude dynamics on the NMR time scale were not observed, suggesting irreversible adsorption of the peptides. Their association, adsorption, and structure were examined as a function of helix length and sequence periodicity, and it was found that, at higher solution concentrations, peptides as short as seven amino acids adsorb with defined secondary structures. Two-dimensional double-quantum experiments using (13)C-enriched peptide sequences allow high-resolution determination of secondary structure in heterogeneous environments where the peptides are a minor component of the material. These results shed light on how polymeric surfaces may be surface-modified by structured peptides and demonstrate the level of molecular structural and dynamic information solid-state NMR can provide.

Peptide-amphiphile nanofibers: a versatile scaffold for the preparation of self-assembling materials

Twelve derivatives of peptide-amphiphile molecules, designed to self-assemble into nanofibers, are described. The scope of amino acid selection and alkyl tail modification in the peptide-amphiphile molecules are investigated, yielding nanofibers varying in morphology, surface chemistry, and potential bioactivity. The results demonstrate the chemically versatile nature of this supramolecular system and its high potential for manufacturing nanomaterials. In addition, three different modes of self-assembly resulting in nanofibers are described, including pH control, divalent ion induction, and concentration.

Surface modification of neural recording electrodes with conducting polymer/biomolecule blends

The interface between micromachined neural microelectrodes and neural tissue plays an important role in chronic in vivo recording. Electrochemical polymerization was used to optimize the surface of the metal electrode sites. Electrically conductive polymers (polypyrrole) combined with biomolecules having cell adhesion functionality were deposited with great precision onto microelectrode sites of neural probes. The biomolecules used were a silk-like polymer having fibronectin fragments (SLPF) and nonapeptide CDPGYIGSR. The existence of protein polymers and peptides in the coatings was confirmed by reflective microfocusing Fourier transform infrared spectroscopy (FTIR). The morphology of the coating was rough and fuzzy, providing a high density of bioactive sites for interaction with neural cells. This high interfacial area also helped to lower the impedance of the electrode site and, consequently, to improve the signal transport. Impedance spectroscopy showed a lowered magnitude and phase of impedance around the biologically relevant frequency of 1 kHz. Cyclic voltammetry demonstrated the intrinsic redox reaction of the doped polypyrrole and the increased charge capacity of the coated electrodes. Rat glial cells and human neuroblastoma cells were seeded and cultured on neural probes with coated and uncoated electrodes. Glial cells appeared to attach better to polypyrrole/SLPF-coated electrodes than to uncoated gold electrodes. Neuroblastoma cells grew preferentially on and around the polypyrrole/CDPGYIGSR-coated electrode sites while the polypyrrole/CH(3)COO(-)-coated sites on the same probe did not show a preferential attraction to the cells. These results indicate that we can adjust the chemical composition, morphology, electronic transport, and bioactivity of polymer coatings on electrode surfaces on a multichannel micromachined neural probe by controlling electrochemical deposition conditions.

Adhesion of α5β1 receptors to biomimetic substrates constructed from peptide amphiphiles

Biomimetic membrane surfaces functionalized with fragments of the extracellular matrix protein, fibronectin, are constructed from mixtures of peptide and polyethylene glycol (PEG) amphiphiles. Peptides from the primary binding loop, GRGDSP, were used in conjunction with the synergy site peptide, PHSRN, in the III(9-10) sites of human fibronectin. These peptides were attached to dialkyl lipid tails to form peptide amphiphiles. PEG amphiphiles were mixed in the layer to minimize non-specific adhesion in the background. GRGDSP and PEG amphiphiles or GRGDSP, PHSRN, and PEG amphiphiles were mixed in various ratios and deposited on solid substrates from the air-water interface using Langmuir-Blodgett techniques. In this method, peptide composition, density, and presentation could be controlled accurately. The effectiveness of these substrates to mimic native fibronectin is evaluated by their ability to generate adhesive forces when they are in contact with purified activated alpha5beta1 integrin receptors that are immobilized on an opposing surface. Adhesion is measured using a contact mechanical approach (JKR experiment). The effects of membrane composition, density, temperature, and peptide conformation on adhesion to activated integrins in this simulated cell adhesion setup were determined. Addition of the synergy site, PHSRN, was found to increase adhesion of alpha5beta1, to biomimetic substrates markedly. Increased peptide mobility (due to increased experimental temperature) increased integrin adhesion markedly at low peptide concentrations. A balance between peptide density and steric accessibility of the receptor binding face to alpha5beta1 integrin was required for highest adhesion.

The microenvironment of immobilized Arg-Gly-Asp peptides is an important determinant of cell adhesion

This paper uses self-assembled monolayers on gold as a model system to demonstrate that the attachment and spreading of Swiss 3T3 fibroblasts depends strongly on the microenvironment of immobilized RGD peptides. This work utilized monolayers that present mixtures of Arg-Gly-Asp peptides, which are ligands for cellular integrin receptors, and oligo(ethylene glycol) groups, which resist the nonspecific adsorption of protein. The microenvironment of the peptide ligands was controlled by altering the length of the surrounding oligo(ethylene glycol) groups on the monolayer. By using thiols that present either tri-, tetra-, penta-, or hexa(ethylene glycol) units, the average distance separating the glycol groups and the peptide ligand is altered while the structure and properties of the background remain unchanged. Cell attachment to monolayers presenting a fixed density of peptide decreased as the length of the oligo(ethylene glycol) group increased. The average projected area of attached cells showed a similar trend. At lower densities of immobilized peptide, decreases in both cell attachment and projected cell area were more pronounced. Attachment and spreading did not depend on density of peptide on monolayers presenting tri(ethylene glycol) groups, but showed a high sensitivity to density of ligand on monolayers presenting longer glycol oligomers. Experiments that used a soluble peptide to inhibit the attachment of cells to monolayers demonstrated that the strength of the cell-substrate interaction decreased on monolayers presenting longer glycol groups. Together, these results suggest that the microenvironment of the peptide ligand influences the affinity of the integrin-peptide interaction and that weaker interactions display a density-dependent enhancement of binding during cell attachment and spreading. This finding is an important consideration in studies that correlate biological function with the composition of ligands on a substrate. This finding also represents an important principle for the design of biologically active materials because it illustrates the degree to which the presentation of adhesion motifs can modify the response of mammalian cells.

Fabrication of nanometer-sized protein patterns using atomic force microscopy and selective immobilization

A new methodology is introduced to produce nanometer-sized protein patterns. The approach includes two main steps, nanopatterning of self-assembled monolayers using atomic force microscopy (AFM)-based nanolithography and subsequent selective immobilization of proteins on the patterned monolayers. The resulting templates and protein patterns are characterized in situ using AFM. Compared with conventional protein fabrication methods, this approach is able to produce smaller patterns with higher spatial precision. In addition, fabrication and characterization are completed in near physiological conditions. The adsorption configuration and bioreactivity of the proteins within the nanopatterns are also studied in situ.

Induction of protein-like molecular architecture by monoalkyl hydrocarbon chains

Numerous approaches have been described for creating relatively small folded biomolecular structures. "Peptide-amphiphiles," whereby monoalkyl or dialkyl hydrocarbon chains are covalently linked to peptide sequences, have been shown previously to form specific molecular architecture of enhanced stability. The present study has examined the use of monoalkyl hydrocarbon chains as a more general method for inducing protein-like structures. Peptide and peptide-amphiphiles have been characterized by CD and one- and two-dimensional nmr spectroscopic techniques. We have examined two structural elements: alpha-helices and collagen-like triple helices. The alpha-helical propensity of a 16-residue peptide either unmodified or acylated with a C(6) or C(16) monoalkyl hydrocarbon chain has been examined initially. The 16-residue peptide alone does not form a distinct structure in solution, whereas the 16-residue peptide adopts predominantly an alpha-helical structure in solution when a C(6) or C(16) monoalkyl hydrocarbon chain is N-terminally acylated. The thermal stability of the alpha-helix is greater upon addition of the C(16) compared with the C(6) chain, which correlates to the extent of aggregation induced by the respective hydrocarbon chains. Very similar results are seen using a 39-residue triple-helical model peptide, in that structural thermal stability (a) is increasingly enhanced as alkyl chain length is increased and (b) correlates to the extent of peptide-amphiphile aggregation. Overall, structures as diverse as alpha-helices, triple helices, and turns/loops have been shown to be induced and/or stabilized by alkyl chains. Increasing alkyl chain length enhances stability of the structural element and induces aggregates of defined sizes. Hydrocarbon chains may be useful as general tools for protein-like structure initiation and stabilization as well as biomaterial modification.