Self-assembling peptide hydrogel fosters chondrocyte extracellular matrix production and cell division: implications for cartilage tissue repair

Biomimetic self-assembling peptides as injectable scaffolds for hard tissue engineering

The production of bone-, dentine- and enamel-like biomaterials for the engineering of mineralized (hard) tissues is a high-priority in regenerative medicine and dentistry. An emerging treatment approach involves the use of short biomimetic peptides that self-assemble to form micrometer-long nanofibrils with well defined surface chemistry and periodicity that display specific arrays of functional groups capable of mineral nucleation. The fibrils also give rise to dynamically stable 3D scaffold gels for the potential control of crystal disposition and growth. Peptides can also be injected in their monomeric fluid state, with subsequent self-assembly and gelation in situ triggered by physiological conditions. In this way, they can infiltrate and self-assemble within irregular or microscopic cavities, for restorative treatment of bone defects, dentinal hypersensitivity or dental decay. Cell adhesion and proliferation is also supported by these scaffolds, offering further advantages for applications in hard tissue engineering. These self-assembling matrices also provide well defined model systems that can contribute greatly to the elucidation of the biological mechanisms of protein-mediated biomineralization.

Slow release of molecules in self-assembling peptide nanofiber scaffold

Biological hydrogels consisting of self-assembling peptide nanofibers are potentially excellent materials for various controlled molecular release applications. The individual nanofiber consists of ionic self-complementary peptides with 16 amino acids (RADA16, Ac-RADARADARADARADA-CONH(2)) that are characterized by a stable beta-sheet structure and undergo self-assembly into hydrogels containing approximately 99.5% w/v water. We report here on the diffusion properties of phenol red, bromophenol blue, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine, 3-PSA), 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (4-PSA), and Coomassie Brilliant Blue G-250 (CBBG) through RADA16 hydrogels. The apparent diffusivity (D ) of phenol red (1.05+/-0.08 x 10(-10) m(2) s(-1)) is higher than that of 3-PSA (0.050+/-0.004 x 10(-10) m(2) s(-1)) and 4-PSA (0.007+/-0.002 x 10(-10) m(2) s(-1)). The difference in 3-PSA and 4-PSA diffusivities suggests that the sulfonic acid groups directly facilitate electrostatic interactions with the RADA16 fiber surface. Bromophenol blue and CBBG were not released from the hydrogel, suggesting that they interact strongly with the peptide hydrogel scaffold. The diffusivities (D ) of the dyes decreased with increasing hydrogel peptide concentration, providing an alternate route of controlling release kinetics. These results indicate that release profiles can be tailored through controlling nanofiber-diffusant molecular level interactions.

Peptide-based biomaterials for protease-enhanced drug delivery

Controlled delivery of drugs in response to environments has the potential of targeting therapies and personalized treatments. Here, we described self-assembled peptide sequences that release therapeutic payloads upon specific interaction with disease-associated proteases. The core peptide sequence consists of a protease cleavable region flanked by two self-assembly motifs. In aqueous solution, the peptides self-assemble as a gel scaffold. With treatment of the model preparations with the appropriate protease, the matrix can be degraded in a controlled fashion, where the degradation rate is fine-tuned by varying the peptide compositions. Protease-mediated drug release was demonstrated by enzymatic treatment of a model therapeutic peptide incorporated into the optimized matrix. Our results suggest that this type of material may have far-reaching applications for functionally targeted drug delivery.

Capturing complex 3D tissue physiology in vitro

The emergence of tissue engineering raises new possibilities for the study of complex physiological and pathophysiological processes in vitro. Many tools are now available to create 3D tissue models in vitro, but the blueprints for what to make have been slower to arrive. We discuss here some of the 'design principles' for recreating the interwoven set of biochemical and mechanical cues in the cellular microenvironment, and the methods for implementing them. We emphasize applications that involve epithelial tissues for which 3D models could explain mechanisms of disease or aid in drug development.

A material decoy of biological media based on chitosan physical hydrogels: application to cartilage tissue engineering

The cartilage tissue has a limited self-regenerative capacity. Tissue-engineering represents a promising trend for cartilage repair. The present study was aimed to develop a biomaterial formulation by combining fragments of chitosan hydrogel with isolated rabbit or human chondrocytes. We first reported the properties of the constructs elaborated with rabbit chondrocytes and pure chitosan physical hydrogels with defined molecular weight, acetylation degree and polymer concentration. Morphological data showed that chondrocytes were not penetrating the hydrogels but tightly bound to the surface of the fragments and spontaneously formed aggregates of combined cell/chitosan. A significant amount of neo-formed cartilage-like extracellular matrix (ECM) was first accumulated in-between cells and hydrogel fragments and furthermore was widely distributed within the neo-construct. The optimal biological response was obtained with hydrogel fragments concentrated at 1.5% (w/w) of polymer made from a chitosan with a degree of acetylation between 30 and 40%. Such hydrogels were then mixed with human chondrocytes. The phenotype of the cells was analyzed by using chondrocytic (mRNA expression of mature type II collagen and aggrecan as well as secretion of proteoglycans of high molecular weight) and non chondrocytic (mRNA expression of immature type II collagen and type I collagen) molecular markers. As compared with human chondrocytes cultured without chitosan hydrogel which rapidly dedifferentiated in primary culture, cells mixed with chitosan rapidly loose the expression of type I and immature type II collagen while they expressed mature type II collagen and aggrecan. In these conditions, chondrocytes maintained their phenotype for as long as 45 days, thus forming cartilage-like nodules. Taken together, these data suggest that a chitosan hydrogel does not work as a scaffold, but could be considered as a decoy of cartilage ECM components, thus favoring the binding of chondrocytes to chitosan. Such a biological response could be described by the concept of reverse encapsulation.

Peptide−TiO2 Surface Interaction in Solution by Ab Initio and Molecular Dynamics Simulations

Ab initio periodic calculations and classical molecular dynamics (MD) simulations were performed to investigate the adsorption mode of alanine and a number of short peptides, in particular two peptides, alanine-glutamic acid and alanine-lysine, taken as model systems for the ionic self-complementary oligopeptide EAK16-II, onto TiO(2) (110) rutile surface, and their conformational characteristics upon adsorption. The atomistic description of the rutile surface and its interactions with water and peptide molecules were based on ab initio calculations, the TIP3P water model, the AMBER force field, and available parameters. By comparison with ab initio calculations, it is shown that MD simulations of reasonable duration can describe the main characteristics of the peptide-TiO(2) surface interaction in solution, at least on a short time scale. Atom-atom radial distribution functions, atom-surface distances, backbone and side chain dihedral angle distributions, and peptide-surface interaction energies have been analyzed. Once adsorbed onto the TiO(2) rutile surface by a bidentate interaction of both carboxyl oxygens with two adjacent Ti atoms, the small peptide studied showed a clear propensity to remain there and undergo relatively limited hinge-bending motions.

Biomimetic self-assembled nanofibers

Peptide-amphiphiles, peptides to which a non-peptidic hydrophobic moiety has been added to the N or C terminal end, have been demonstrated to be a versatile method for simultaneously controlling nanostructure and chemical functionality. These amphiphiles are able to self-assemble, in a controlled fashion, into nanofibers with diameter between 6-10 nm and with length in excess of 1000 nm. At proper concentration these nanofibers form a viscoelastic gel capable of entrapping living cells and eliciting specific responses from them. Because of the flexibility of the display of chemical functionality on a controlled nanofibrous scaffold, applications for peptide-amphiphiles have been proposed including heterogeneous catalysis, nanoelectronics, drug delivery, and tissue engineering.

Chondrogenic differentiation and functional maturation of bovine mesenchymal stem cells in long-term agarose culture

BACKGROUND

The developmental history of the chondrocyte results in a cell whose biosynthetic activities are optimized to maintain the concentration and organization of a mechanically functional cartilaginous extracellular matrix. While useful for cartilage tissue engineering studies, the limited supply of healthy autologous chondrocytes may preclude their clinical use. Consequently, multipotential mesenchymal stem cells (MSCs) have been proposed as an alternative cell source.

OBJECTIVE

While MSCs undergo chondrogenesis, few studies have assessed the mechanical integrity of their forming matrix. Furthermore, efficiency of matrix formation must be determined in comparison to healthy chondrocytes from the same donor. Given the scarcity of healthy human tissue, this study determined the feasibility of isolating bovine chondrocytes and MSCs, and examined their long-term maturation in three-dimensional agarose culture.

EXPERIMENTAL DESIGN

Bovine MSCs were seeded in agarose and induced to undergo chondrogenesis. Mechanical and biochemical properties of MSC-laden constructs were monitored over a 10-week period and compared to those of chondrocytes derived from the same group of animals maintained similarly.

RESULTS

Our results show that while chondrogenesis does occur in MSC-laden hydrogels, the amount of the forming matrix and measures of its mechanical properties are lower than that produced by chondrocytes under the same conditions. Furthermore, some important properties, particularly glycosaminoglycan content and equilibrium modulus, plateau with time in MSC-laden constructs, suggesting that diminished capacity is not the result of delayed differentiation.

CONCLUSIONS

These findings suggest that while MSCs do generate constructs with substantial cartilaginous properties, further optimization must be done to achieve levels similar to those produced by chondrocytes.

Atomistic simulation approach to a continuum description of self-assembled beta-sheet filaments

We investigated the supramolecular structure and continuum mechanical properties of a beta-sheet nanofiber comprised of a self-assembling peptide ac-[RARADADA]2-am using computer simulations. The supramolecular structure was determined by constructing candidate filaments with dimensions compatible with those observed in atomic force microscopy and selecting the most stable ones after running molecular dynamics simulations on each of them. Four structures with different backbone hydrogen-bonding patterns were identified to be similarly stable. We then quantified the continuum mechanical properties of these identified structures by running three independent simulations: thermal motion analysis, normal mode analysis, and steered molecular dynamics. Within the range of deformations investigated, the filament showed linear elasticity in transverse directions with an estimated persistence length of 1.2-4.8 microm. Although side-chain interactions govern the propensity and energetics of filament self-assembly, we found that backbone hydrogen-bonding interactions are the primary determinant of filament elasticity, as demonstrated by its effective thickness, which is smaller than that estimated by atomic force microscopy or from the molecular geometry, as well as by the similar bending stiffness of a model filament without charged side chains. The generality of our approach suggests that it should be applicable to developing continuum elastic ribbon models of other beta-sheet filaments and amyloid fibrils.

Self-assembling peptides as injectable lubricants for osteoarthritis

The self-assembly of peptides is explored as an alternative route towards the development of new injectable joint lubricants for osteoarthritis (OA). The versatility of the peptide chemistry allows the incorporation of behavior reminiscent of hyaluronic acid (HA), while the triggered in situ self-assembly provides easy delivery of the samples by injection due to the low viscosity of the peptide solutions (that are initially monomeric). Using design criteria based on the chemical properties of HA, a range of de novo peptides were prepared with systematic alterations of charge and hydrophilicity that self-assembled into nematic fluids and gels in physiological solution conditions. The frictional characteristics of the peptides were evaluated using cartilage on cartilage sliding contacts along with their rheological characteristics. Peptide P(11)-9, whose molecular, mesoscopic, and rheological properties most closely resembled HA was found to be the most effective lubricant amongst the peptides. In healthy static and dynamic friction testing (corresponding to healthy joints) P(11)-9 at 20-40 mg/mL performed similar to HA at 10 mg/mL. In friction tests with damaged cartilage (corresponding to early stage OA) P(11)-9 was a less efficient lubricant than HA, but still the best among all the peptides tested. The results indicate that de novo self-assembling peptides could be developed as an alternate therapeutic lubricant for early stage OA.

Cartilage tissue engineering: its potential and uses

PURPOSE OF REVIEW

The prevalent nature of osteoarthritis, a cartilage degenerative disease that results in the erosion of joint surfaces and loss of mobility, underscores the importance of developing functional articular cartilage replacement. Recent research efforts have focused on tissue engineering as a promising approach for cartilage regeneration and repair. Tissue engineering is a multidisciplinary research area that incorporates both biological and engineering principles for the purpose of generating new, living tissues to replace the diseased/damaged tissue and restore tissue/organ function. This review surveys and highlights the current concepts and recent progress in cartilage tissue engineering, and discusses the challenges and potential of this rapidly advancing field of biomedical research.

RECENT FINDINGS

Cartilage tissue engineering is critically dependent on selection of appropriate cells (differentiated or progenitor cells); fabrication and utilization of biocompatible and mechanically suitable scaffolds for cell delivery; stimulation with chondrogenically bioactive molecules introduced in the form of recombinant proteins or via gene transfer; and application of dynamic, mechanical loading regimens for conditioning of the engineered tissue constructs, including the design of specialized biomechanically active bioreactors.

SUMMARY

Cell selection, scaffold design and biological stimulation remain the challenges of function tissue engineering. Successful regeneration or replacement of damaged or diseased cartilage will depend on future advances in our understanding of the biology of cartilage and stem cells and technological development in engineering.

PuraMatrix facilitates bone regeneration in bone defects of calvaria in mice

Artificial bones have often used for bone regeneration due to their strength, but they cannot provide an adequate environment for cell penetration and settlement. We therefore attempted to explore various materials that may allow the cells to penetrate and engraft in bone defects. PuraMatrix is a self-assembling peptide scaffold that produces a nanoscale environment allowing both cellular penetration and engraftment. The objective of this study was to investigate the effect of PuraMatrix on bone regeneration in a mouse bone defect model of the calvaria. Matrigel was used as a control. The expression of bone-related genes (alkaline phosphatase, Runx2, and Osterix) in the PuraMatrix-injected bone defects was stronger than that in the Matrigel-injected defects. Soft X-ray radiographs revealed that bony bridges were clearly observed in the defects treated with PuraMatrix, but not in the Matrigel-treated defects. Notably, PuraMatrix treatment induced mature bone tissue while showing cortical bone medullary cavities. The area of newly formed bones at the site of the bone defects was 1.38-fold larger for PuraMatrix than Matrigel. The strength of the regenerated bone was 1.72-fold higher for PuraMatrix (146.0 g) than for Matrigel (84.7 g). The present study demonstrated that PuraMatrix injection favorably induced functional bone regeneration.

Peptide-Based Viscoelastic Matrices for Drug Delivery and Tissue Repair

Molecular self-assembly has paved the way to create novel, supramolecular, functional biomaterials. Peptide-based biomaterials are gaining interest as a result of their programmability, biodegradability, and bioresorbability. Further, unlike polymeric materials, peptides can be made monodisperse with precise control over sequence, chain length, and stereochemistry. Peptide-based viscoelastic matrices have been designed and characterized for various biomedical applications, such as tissue engineering scaffolds or drug delivery vehicles. The 'holy grail' in designing an ideal tissue engineering scaffold lies in mimicking the cues of the tissue's natural extracellular matrix (ECM). Some of the key elements of ECM that are incorporated into these peptide scaffolds include cell-adhesive and protease-sensitive sequences for enhanced cell-cell and cell-biomaterial interactions. Peptide-based viscoelastic matrices can also be engineered with drug carrying protease-sensitive sequences for controlled and site-specific drug delivery. Molecular-level engineering of simple oligopeptide modules can be used to control the position and density of the bio-mimetic functionalities in the supramolecular structures, which demonstrates the power of the 'bottom-up' approach in self-assembly.

Chitosan-alginate as scaffolding material for cartilage tissue engineering

Tissue compatibility of chitosan-alginate scaffolds was studied in vitro in terms of cell morphology, proliferation, and functionality using HTB-94 cells. The scaffold has an interconnected 3D porous structure, and was fabricated by thermally induced phase separation followed by freeze drying. Cell proliferation on the chitosan-alginate scaffold was found to be faster than on a pure chitosan scaffold. After cell culture for 2 weeks in vitro, the cells on the chitosan scaffold gradually assumed a fibroblast-like morphology while the cells on the chitosan-alginate scaffold retained their spherical morphology throughout the period of study. SDS-PAGE electrophoresis and Western blot assays for proteins extracted from cells grown on scaffolds indicated that production of cartilage-specific collagen type II, a marker for chondrocytic phenotype, increased from week 2 to week 3 on the chitosan-alginate scaffold but decreased on the chitosan scaffold. This study suggested that chitosan-alginate scaffolds promote cell proliferation, enhance phenotype expression of HTB-94 chondrocytes, and may potentially serve as an improved alternative to chitosan scaffolds for cartilage tissue engineering.

Three Distinct Read-Out Modes for Enzyme Activity Can Operate in a Semi-Wet Supramolecular Hydrogel

Assays of hydrolytic enzyme activity, such as of glycosidases and phosphatase, as well as several proteases, using a semi-wet supramolecular hydrogel array composed of a glycosylated amino acetate are described. It has been demonstrated that the microcavity formed by gel fibrils is suitable to immobilize native enzymes without denaturation under semi-wet conditions, and thus the nanofiber has been rationally used as a sensing domain to monitor enzymatic reactions. By using a fluorogenic substrate, reducing the size of the hydrogel can significantly improve the problem of suppressed diffusion within the gel matrix thus making the hydrogel a promising semi-wet matrix for evaluating enzyme activity. Confocal laser scanning microscopy observations have shown that an environmentally sensitive fluorescent probe accumulates in the hydrophobic domain of the gel fiber and emits fluorescence more strongly upon hydrolytic cleavage of the substrate peptides. Not only a simple environmentally sensitive probe but also a FRET (fluorescence resonance energy transfer)-type read-out mode can be devised to analyze the enzymatic hydrolysis-triggered redistribution of the probe between the nanospace and the nanofiber to accomplish a more clearly distinguished enzyme assay. Thus, it is clear that three distinct read-out modes, that is, 1) fluorogenic substrates, 2) substrates bearing an environmentally sensitive probe, or 3) a substrate exhibiting FRET, can operate under the semi-wet hydrogel conditions used in these investigations. In addition, owing to the unique properties of the present supramolecular hydrogel in semi-wet conditions, that is, its phase-segregation properties and dynamics, the supramolecular substrate/enzyme array has successfully been used for high-throughput screening of single and multiple enzymes based on their activity, lysate analysis, and quantitative evaluation of inhibitor potency and selectivity.

Delivery of TGF-β1 and chondrocytes via injectable, biodegradable hydrogels for cartilage tissue engineering applications

In this work, novel hydrogel composites, based on the biodegradable polymer, oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs) were utilized as injectable cell and growth factor carriers for cartilage tissue engineering applications. Specifically, bovine chondrocytes were embedded in composite hydrogels co-encapsulating gelatin MPs loaded with transforming growth factor-beta1 (TGF-beta1). Hydrogels with embedded cells co-encapsulating unloaded MPs and those with no MPs served as controls in order to assess the effects of MPs and TGF-beta1 on chondrocyte function. Samples were cultured up to 28 days in vitro. By 14 days, cell attachment to embedded gelatin MPs within the constructs was observed via light microscopy. Bioassay results showed that, over the 21 day period, there was a statistically significant increase in cellular proliferation for samples containing gelatin MPs, but no increase was exhibited in samples without MPs over the culture period. The release of TGF-beta1 further increased cell construct cellularity. Over the same time period, glycosaminoglycan content per cell remained constant for all formulations, suggesting that the dramatic increase in cell number for samples with TGF-beta1-loaded MPs was accompanied by maintenance of the cell phenotype. Overall, these data indicate the potential of OPF hydrogel composites containing embedded chondrocytes and TGF-beta1-loaded gelatin MPs as a novel strategy for cartilage tissue engineering.

Exploring and engineering the cell surface interface

Cells are inherently sensitive to local mesoscale, microscale, and nanoscale patterns of chemistry and topography. We review current approaches to control cell behavior through the nanoscale engineering of materials surfaces. Far-reaching implications are emerging for applications including medical implants, cell supports, and materials that can be used as instructive three-dimensional environments for tissue regeneration.

Development of a tissue-engineered composite implant for treating traumatic paraplegia in rats

This study was designed to assess a new composite implant to induce regeneration of injured spinal cord in paraplegic rats following complete cord transection. Neuronal xenogeneic cells from biopsies of adult nasal olfactory mucosa (NOM) of human origin, or spinal cords of human embryos, were cultured in two consecutive stages: stationary cultures in a viscous semi-solid gel (NVR-N-Gel) and in suspension on positively charged microcarriers (MCs). A tissue-engineered tubular scaffold, containing bundles of parallel nanofibers, was developed. Both the tube and the nanofibers were made of a biodegradable dextran sulphate-gelatin co-precipitate. The suturable scaffold anchored the implant at the site of injury and provided guidance for the regenerating axons. Implants of adult human NOM cells were implanted into eight rats, from which a 4 mm segment of the spinal cord had been completely removed. Another four rats whose spinal cords had also been transected were implanted with a composite implant of cultured human embryonic spinal cord cells. Eight other cord-transected rats served as a control group. Physiological and behavioral analysis, performed 3 months after implantation, revealed partial recovery of function in one or two limbs in three out of eight animals of the NOM implanted group and in all the four rats that were implanted with cultured human embryonic spinal cord cells. Animals of the control group remained completely paralyzed and did not show transmission of stimuli to the brain. The utilization of an innovative composite implant to bridge a gap resulting from the transection and removal of a 4 mm spinal cord segment shows promise, suggesting the feasibility of this approach for partial reconstruction of spinal cord lesions. Such an implant may serve as a vital bridging station in acute and chronic cases of paraplegia.

Custom design of the cardiac microenvironment with biomaterials

Many strategies for repairing injured myocardium are under active investigation, with some early encouraging results. These strategies include cell therapies, despite little evidence of long-term survival of exogenous cells, and gene or protein therapies, often with incomplete control of locally-delivered dose of the factor. We propose that, ultimately, successful repair and regeneration strategies will require quantitative control of the myocardial microenvironment. This precision control can be engineered through designed biomaterials that provide quantitative adhesion, growth, or migration signals. Quantitative timed release of factors can be regulated by chemical design to direct cellular differentiation pathways such as angiogenesis and vascular maturation. Smart biomaterials respond to the local environment, such as protease activity or mechanical forces, with controlled release or activation. Most of these new biomaterials provide much greater flexibility for regenerating tissues ex vivo, but emerging technologies like self-assembling nanofibers can now establish intramyocardial cellular microenvironments by injection. This may allow percutaneous cardiac regeneration and repair approaches, or injectable-tissue engineering. Finally, materials can be made to multifunction by providing sequential signals with custom design of differential release kinetics for individual factors. Thus, new rationally-designed biomaterials no longer simply coexist with tissues, but can provide precision bioactive control of the microenvironment that may be required for cardiac regeneration and repair.

Designer self-assembling peptide nanofiber scaffolds for 3D tissue cell cultures

Biomedical researchers have become increasingly aware of the limitations of time-honored conventional 2D tissue cell cultures where most tissue cell studies have been carried out. They are now searching for 3D cell culture systems, something between a petri dish and a mouse. It has become apparent that 3D cell culture offers a more realistic micro- and local-environment where the functional properties of cells can be observed and manipulated that is not possible in animals. A newly designer self-assembling peptide scaffolds may provide an ideally alternative system. The important implications of 3D tissue cell cultures for basic cell biology, tumor biology, high-content drug screening, and regenerative medicine and beyond could be profound.

New properties from PLA-PEO-PLA hydrogels

Polymeric materials are important in many medical applications. Regenerative medicine offers the potential to repair or replace damaged tissue and polymers are an essential component of many tissue engineering approaches. Hydrogels have many advantageous properties but, generally, lack robust mechanical properties. At the same time, mounting evidence points to the importance of the matrix modulus when constructing devices. In this context, triblock copolymers made from poly(-lactide)-poly(ethylene glycol)-poly(-lactide) have been prepared and formulated into hydrogels. Investigations into their mechanical properties found the elastic modulus to be greater than 10 kPa which is at least one order of magnitude stiffer than previously reported from macromolecules composed of similar monomers. Part of the reason is the presence of crystalline lactide domains. Creating hydrogels with tailored modulus across the kPa range will likely have important ramifications in regenerative medicine.

Self-complementary peptides for the formation of collagen-like triple helical supramolecules

Collagen is acknowledged as one of the most prominent biomaterials on account of its high biocompatibility and biostability. The development of artificial collagens to replace the animal-derived collagens presents a challenge in the formation of safer and highly functionalized biomaterials. Here, a novel peptide-based system for obtaining collagen-like supramolecules via a spontaneous self-assembling process is described. The designed collagen-like peptides are self-complementary trimers in which each of the 24-mer peptide strands is tethered by two cystine knots forming a staggered arrangement. Their self-assembling ability in aqueous solution was analyzed by circular dichroism, ultrafiltration, and laser diffraction particle size estimation. The obtained results indicate that the staggered trimers form large supramolecular architectures through intermolecular triple helix-formation.

Production and purification of self-assembling peptides in Ralstonia eutropha

Self-assembling peptides have emerged as an attractive scaffold material for tissue engineering, yet the expense associated with solid phase chemical synthesis has limited their broad use. In addition, the fidelity of chemical synthesis constrains the length of polypeptides that can be produced homogeneously by this method. Template-derived biosynthesis by recombinant DNA technology may overcome both of these problems. However, recovery of polypeptides from recombinant protein expression systems typically involves multi-step purification schemes. In this study, we report an integrated approach to recombinantly produce and purify self-assembling peptides from the recently developed expression host Ralstonia eutropha. The purification is based on the specific affinity of carbohydrate binding modules (CBMs) to cellulose. In a first step, we identified CBMs that express well in R. eutropha by assembling a fusion library of green fluorescent protein (GFP) and CBMs and determining the fluorescence of cell-free extracts. Three GFP::CBM fusions were found to express at levels similar to GFP alone, of which two CBMs were able to mediate cellulose binding of the GFP::CBM fusion. These two CBMs were then fused to multiple repeats of the self-assembling peptide RAD16-I::E (N-RADARADARADARADAE-C). The fusion protein CBM::E::(RAD16-I::E)4 was expressed in R. eutropha and purified using the CBM's affinity for cellulose. Subsequent proteolytic cleavage with endoproteinase GluC liberated RAD16-I::E peptide monomers with similar properties to the chemically synthesized counterpart RAD16-I.

The enhancement of osteoblast growth and differentiation in vitro on a peptide hydrogel-polyHIPE polymer hybrid material

The objective of this study was to investigate the effect of combining two biomaterials on osteoblast proliferation, differentiation and mineralised matrix formation in vitro. The first biomaterial has a well-defined architecture and is known as PolyHIPE polymer (PHP). The second biomaterial is a biologically inspired self-assembling peptide hydrogel (RAD16-I, also called PuraMatrix) that produces a nanoscale environment similar to native extracellular matrix (ECM). Our work investigates the effect of combining RAD16-I with two types of PHP (HA (Hydroxyapatite)-PHP and H (Hydrophobic)-PHP) and evaluates effects on osteoblast growth and differentiation. Results demonstrated successful incorporation of RAD16-I into both types of PHP. Osteoblasts were observed to form multicellular layers on the combined biomaterial surface and also within the scaffold. Dynamic cell seeding and culturing techniques were compared to static seeding methods and produced a more even distribution of cells throughout the constructs. Cells were found to penetrate the scaffold to a maximum depth of 3 mm after 35 days in culture. There was a significant increase in cell number in H-PHP constructs coated with RAD16-I compared to H-PHP alone. Our results show that RAD16-I enhances osteoblast differentiation and indicates that the incorporation of this peptide provides a more permissive environment for osteoblast growth. We have developed a microcellular polymer containing a nanoscale environment to enhance cell: biomaterial interactions and promote osteoblast growth in vitro.

Cytocompatibility of self-assembled beta-hairpin peptide hydrogel surfaces

MAX1 is a 20 amino acid peptide that undergoes triggered self-assembly to form a rigid hydrogel. When dissolved in aqueous solutions, this peptide exists in an ensemble of random coil conformations rendering it fully soluble. The addition of an exogenous stimulus results in peptide folding into beta-hairpin conformation. This folded structure undergoes rapid assembly into a highly crosslinked hydrogel network. DMEM cell culture media is one stimulus able to initiate folding and consequent self-assembly of MAX1. The cytocompatibility of this gel towards NIH 3T3 murine fibroblasts is demonstrated. Gels were shown to be non-toxic to the fibroblast cells. MAX1 hydrogels also foster the ability of the cells to attach to the hydrogel scaffold in the absence or presence of serum proteins. Additionally MAX1 hydrogels were able to support fibroblast proliferation to confluency with little effect on the rheological properties of the scaffold. MAX1 hydrogels meet the preliminary mechanical and cytocompatibiltiy requirements of a tissue engineering scaffold.

Cooperation between Artificial Receptors and Supramolecular Hydrogels for Sensing and Discriminating Phosphate Derivatives

This study has successfully demonstrated that the cooperative action of artificial receptors with semi-wet supramolecular hydrogels may produce a unique and efficient molecular recognition device not only for the simple sensing of phosphate derivatives, but also for discriminating among phosphate derivatives. We directly observed by confocal laser scanning microscopy that fluorescent artificial receptors can dynamically change the location between the aqueous cavity and the hydrophobic fibers upon guest-binding under semi-wet conditions provided by the supramolecular hydrogel. On the basis of such a guest-dependent dynamic redistribution of the receptor molecules, a sophisticated means for molecular recognition of phosphate derivatives can be rationally designed in the hydrogel matrix. That is, the elaborate utilization of the hydrophobic fibrous domains, as well as the water-rich hydrophilic cavities, enables us to establish three distinct signal transduction modes for phosphate sensing: the use of (i) a photoinduced electron transfer type of chemosensor, (ii) an environmentally sensitive probe, and (iii) an artificial receptor displaying a fluorescence resonance energy transfer type of fluorescent signal change. Thus, one can selectively sense and discriminate the various phosphate derivatives, such as phosphate, phospho-tyrosine, phenyl phosphate, and adenosine triphosphate, using a fluorescence wavelength shift and a seesaw type of ratiometric fluorescence change, as well as a simple fluorescence intensity change. It is also shown that an array of the miniaturized hydrogel is promising for the rapid and high-throughput sensing of these phosphate derivatives.

Self-assembling short oligopeptides and the promotion of angiogenesis

Because an adequate blood supply to and within tissues is an essential factor for successful tissue regeneration, promoting a functional microvasculature is a crucial factor for biomaterials. In this study, we demonstrate that short self-assembling peptides form scaffolds that provide an angiogenic environment promoting long-term cell survival and capillary-like network formation in three-dimensional cultures of human microvascular endothelial cells. Our data show that, in contrast to collagen type I, the peptide scaffold inhibits endothelial cell apoptosis in the absence of added angiogenic factors, accompanied by enhanced gene expression of the angiogenic factor VEGF. In addition, our results suggest that the process of capillary-like network formation and the size and spatial organization of cell networks may be controlled through manipulation of the scaffold properties, with a more rigid scaffold promoting extended structures with a larger inter-structure distance, as compared with more dense structures of smaller size observed in a more compliant scaffold. These findings indicate that self-assembling peptide scaffolds have potential for engineering vascularized tissues with control over angiogenic processes. Since these peptides can be modified in many ways, they may be uniquely valuable in regeneration of vascularized tissues.

Cell adhesion ability of artificial extracellular matrix proteins containing a long repetitive Arg-Gly-Asp sequence

We generated recombinant artificial extracellular matrix (ECM) proteins containing repetitive Arg-Gly-Asp (RGD) sequences: double (RGD2), 21 (RGD21) and 43 (RGD43) repeats of RGD. RGD43-coated glass slides promoted fibroblast NIH3T3 cell adhesion and spreading on the surface. Since actin stress fibers and focal contacts were also observed in cells adhering on RGD43-coated glass slides, it was suggested that the RGD peptides in RGD43 transmitted an adhesion signal via integrins and promoted cell adhesion. We coated recombinant ECM proteins, each containing repetitive RGD domains, on polystyrene plates and investigated the effects of RGD length on the cell adhesion ability using three different cell lines, namely, fibroblast NIH3T3, HeLa cancer and neuronal PC12 cell lines. The results indicated that RGD43 had a cell adhesion ability superior to those of natural extracellular matrix proteins, fibronectin and laminin, although the effects of RGD repeat length on the cell adhesion ability depended on the cell line. As an artificial three-dimensional scaffold for cell cultivation, we also prepared an RGD43 hydrogel by a cross-linking reaction using glutaraldehyde. On the RGD43 hydrogel scaffold, fibroblast cells also successfully adhered under serum-free conditions.

Self-assembling peptide amphiphile nanofiber matrices for cell entrapment

We have developed a class of peptide amphiphile (PA) molecules that self-assemble into three-dimensional nanofiber networks under physiological conditions in the presence of polyvalent metal ions. The assembly can be triggered by adding PA solutions to cell culture media or other synthetic physiological fluids containing polyvalent metal ions. When the fluids contain suspended cells, PA self-assembly entraps cells in the nanofibrillar matrix, and the cells survive in culture for at least three weeks. We also show that entrapment does not arrest cell proliferation and motility. Biochemical and ultrastructural analysis by electron microscopy indicate that entrapped cells internalize the nanofibers and possibly utilize PA molecules in their metabolic pathways. These results demonstrate that PA nanofibrillar matrices have the potential to be used for cell transplantation or other tissue engineering applications.

Evaluation of medium supplemented with insulin-transferrin-selenium for culture of primary bovine calf chondrocytes in three-dimensional hydrogel scaffolds

Insulin-transferrin-selenium (ITS) was investigated as a complete or partial replacement for fetal bovine serum (FBS) during in vitro culture of bovine calf chondrocytes in hydrogel scaffolds. Chondrocyte-seeded agarose and self-assembling peptide hydrogels were maintained in Dulbecco's modified Eagle's medium plus 10% FBS, 1% ITS plus 0.2% FBS, or 1% ITS and evaluated for biosynthesis, cell division, and surface outgrowth of fibroblastic-like cells and fibrous capsule formation over several weeks of culture. In peptide hydrogels, cells cultured in ITS plus 0.2% FBS medium exhibited high rates of biosynthesis and showed similar cell division trends as seen in 10% FBS cultures. ITS medium alone did not support glycosaminoglycan accumulation beyond 5 days of culture, and cell division was less than that in both serum-containing cultures. Extensive cellular outgrowth and fibrous capsule formation were observed in 10% FBS medium, whereas little outgrowth was observed in ITS plus 0.2% FBS and none was seen in ITS medium alone. In agarose hydrogels, chondrocyte biosynthesis and cell division in ITS medium were similar to that in 10% serum culture over 5 weeks, and cellular outgrowth was eliminated. Taken together, ITS was suitable as a partial (peptide) or complete (agarose) substitute for serum, and also provided the benefit of reducing or eliminating cell outgrowth and fibrous capsule formation on the hydrogel surface.

Dynamic reassembly of peptide RADA16 nanofiber scaffold

Nanofiber structures of some peptides and proteins as biological materials have been studied extensively, but their molecular mechanism of self-assembly and reassembly still remains unclear. We report here the reassembly of an ionic self-complementary peptide RADARADARADARADA (RADA16-I) that forms a well defined nanofiber scaffold. The 16-residue peptide forms stable beta-sheet structure and undergoes molecular self-assembly into nanofibers and eventually a scaffold hydrogel consisting of >99.5% water. In this study, the nanofiber scaffold was sonicated into smaller fragments. Circular dichroism, atomic force microscopy, and rheology were used to follow the kinetics of the reassembly. These sonicated fragments not only quickly reassemble into nanofibers that were indistinguishable from the original material, but their reassembly also correlated with the rheological analyses showing an increase of scaffold rigidity as a function of nanofiber length. The disassembly and reassembly processes were repeated four times and, each time, the reassembly reached the original length. We proposed a plausible sliding diffusion model to interpret the reassembly involving complementary nanofiber cohesive ends. This reassembly process is important for fabrication of new scaffolds for 3D cell culture, tissue repair, and regenerative medicine.

The effect of functionalized self-assembling peptide scaffolds on human aortic endothelial cell function

A class of designed self-assembling peptide nanofiber scaffolds with more than 99% water content has been shown to be a good biological material for cell culture. Here, we report the functionalization of one of these peptide scaffolds, RAD16-I (AcN-RADARADARADARADA-CONH2), by direct solid phase synthesis extension at the amino terminal with three short-sequence motifs. These motifs are present in two major protein components of the basement membrane, laminin 1 (YIGSR, RYVVLPR) and collagen IV (TAGSCLRKFSTM). These motifs have been previously shown to promote specific biological activities including endothelial cell adhesion, spreading, and tubular formation. Therefore, the generic functionalized peptide developed was AcN-X-GG-RADARADARADARADA-CONH2 with each motif represented by "X". We show in this work that these tailor-made peptide scaffolds enhance the formation of confluent cell monolayers of human aortic endothelial cells (HAEC) in culture. Moreover, additional assays designed to evaluate endothelial cell function showed that HAEC monolayers obtained on these scaffolds not only maintained LDL uptake activity but also enhanced nitric oxide release and elevated laminin 1 and collagen IV deposition. These results suggest that this new scaffold provide a better physiological substrate for endothelial cell culture and suggest its further application for biomedical research, cancer biology and regenerative biology.

Repeated rapid shear-responsiveness of peptide hydrogels with tunable shear modulus

A pair of mutually attractive but self-repulsive decapeptides, with alternating charged/neutral amino acid sequence patterns, was found to co-assemble into a viscoelastic material upon mixing at a low total peptide concentration of 0.25 wt %. Circular dichroism spectroscopy of individual decapeptide solutions revealed their random coil conformation. Transmission electron microscopy images showed the nanofibrillar network structure of the hydrogel. Dynamic rheological characterization revealed its high elasticity and shear-thinning nature. Furthermore, the co-assembled hydrogel was capable of rapid recoveries from repeated shear-induced breakdowns, a property desirable for designing injectable biomaterials for controlled drug delivery and tissue engineering applications. A systematic variation of the neutral amino acids in the sequence revealed some of the design principles for this class of biomaterials. First, viscoelastic properties of the hydrogels can be tuned through adjusting the hydrophobicity of the neutral amino acids. Second, the beta-sheet propensity of the neutral amino acid residue in the peptides is critical for hydrogelation.

A cultural renaissance: in vitro cell biology embraces three-dimensional context

Increasingly, researchers are recognizing the limitations of two-dimensional (2-D), monolayer cell culture and embracing more realistic three-dimensional (3-D) cell culture systems. Currently, 3-D culture techniques are being employed by neuroscientists to grow cells from the central nervous system. From this work, it has become clear that 3-D cell culture offers a more realistic milieu in which the functional properties of neurons can be observed and manipulated in a manner that is not possible in vivo. The implications of this technical renaissance in cell culture for both clinical and basic neuroscience are significant and far-reaching.

Tissue engineering of cartilage using an injectable and adhesive chitosan-based cell-delivery vehicle

OBJECTIVE

Adult articular cartilage shows a limited intrinsic repair response to traumatic injury. To regenerate damaged cartilage, cell-assisted repair is thus viewed as a promising therapy, despite being limited by the lack of a suitable technique to deliver and retain chondrogenic cells at the defect site.

DESIGN

We have developed a cytocompatible chitosan solution that is space-filling, gels within minutes, and adheres to cartilage and bone in situ. This unique combination of properties suggested significant potential for its use as an arthroscopically injectable vehicle for cell-assisted cartilage repair. The primary goal of this study was to assess the ability of this polymer system, when loaded with primary articular chondrocytes, to support cartilage formation in vitro and in vivo. The chitosan gel was cultured in vitro, with and without chondrocytes, as well as injected subcutaneously in nude mice to form subcutaneous dorsal implants. In vitro and in vivo constructs were collectively analyzed histologically, for chondrocyte mRNA and protein expression, for biochemical levels of glycosaminoglycan, collagen, and DNA, and for mechanical properties.

RESULTS

Resulting tissue constructs revealed histochemical, biochemical and mechanical properties comparable to those observed in vitro for primary chondrocytes cultured in 2% agarose. Moreover, the gel was retained after injection into a surgically prepared, rabbit full-thickness chondral defect after 1 day in vivo, and in rabbit osteochondral defects, up to 1 week.

CONCLUSIONS

The in situ-gelling chitosan solution described here can support in vitro and in vivo accumulation of cartilage matrix by primary chondrocytes, while persisting in osteochondral defects at least 1 week in vivo.

Fibrous crystalline hydrogels formed from polymers possessing a linear poly(ethyleneimine) backbone

Novel thermoreversible physical hydrogels formed from polymers with linear and star architectures possessing a linear poly(ethyleneimine) (PEI) backbone have been investigated. The hydrogelation occurred simply upon natural cooling of hot aqueous solutions of PEIs to room temperature. The X-ray diffraction and differential scanning calorimetry measurements for the resultant hydrogels unambiguously indicated that the hydrogelation originated from the formation of dihydrate crystalline structures of PEI. These crystalline hydrogels are structurally unique and hierarchical. Microscopic images revealed that the morphologies of the crystalline hydrogels depend on their molecular architectures. The linear PEI resulted in branched fibrous bundles organized by unit crystalline nanofibers with a width of ca. 5-7 nm. The six-armed star with benzene ring core produced fanlike fibrous bundles while the four-armed star with porphyrin core assembled into asterlike aggregates. The critical concentration of gelation (C(G)) was low (about 0.2 approximately 0.3%) and the thermoreversible gel-sol transition temperatures (T(G)) were controllable from approximately 43 to approximately 79 degrees C. The hydrogels formed in the presence of the various aqueous additives including organic solvents, hydrophilic polymers, physical cross-linker, chemical cross-linker, and base enabling modification and functionalization during synthesis. The mechanical properties of the hydrogels could be improved by chemical cross-linking of preformed hydrogels by glutaraldehyde. Physically and physical/chemical cross-linked hydrogels served as excellent template roles in biomimetic silicification, which produced silica-PEI hybrid powder or monolith constructed by nanofibers.

Entrapment of migrating hippocampal neural cells in three-dimensional peptide nanofiber scaffold

Isolation and expansion of self-renewing neural cells ex vivo are required for neural tissue repair in regenerative medicine. Neurogenesis occurs in restricted areas of postnatal mammalian brain including dentate gyrus and subventricular zone. We developed a simple method to entrap migrating neural cells (potential neuroprogenitors) from postnatal hippocampal organotypic cultures in three-dimensional (3-D) peptide nanofiber scaffolds. A few hours after placing the hippocampal slices in culture, cell proliferation activity at the "interface zone" between the tissue slice and the membrane culture surface was observed. Pulse-chase experiments using 5-bromodeoxyuridine (BrdU), which measures mitotic activity, showed that a number of cells incorporated BrdU at the interface zone. The number of BrdU(+) cells increased exponentially during the first 3 days of exposure to the label. The BrdU(+) cells also stained positive for glial fibrillary acidic protein (2.2 +/- 0.5%), a marker for astroglia; and for betaIII tubulin (7.3 +/- 2.8%) and nestin (2.7 +/- 0.9%), markers for neural progenitors. When hippocampal slices were cultured on a peptide nanofiber scaffold layer (~500 microm thick), a more extended interface zone between each tissue slice and the scaffold was formed. Moreover, the migrating BrdU(+) cell population entrapped in the 3-D peptide scaffold was readily isolated by mechanically disrupting the scaffold and then used for conventional 2-D culture systems for further studies. This simple method may be useful not only in developing technology for neural progenitor cell isolation and enrichment in vitro, but also for expanding cells for cell-based therapies of regenerative medicine.

Synthetic biomaterials as instructive extracellular microenvironments for morphogenesis in tissue engineering

New generations of synthetic biomaterials are being developed at a rapid pace for use as three-dimensional extracellular microenvironments to mimic the regulatory characteristics of natural extracellular matrices (ECMs) and ECM-bound growth factors, both for therapeutic applications and basic biological studies. Recent advances include nanofibrillar networks formed by self-assembly of small building blocks, artificial ECM networks from protein polymers or peptide-conjugated synthetic polymers that present bioactive ligands and respond to cell-secreted signals to enable proteolytic remodeling. These materials have already found application in differentiating stem cells into neurons, repairing bone and inducing angiogenesis. Although modern synthetic biomaterials represent oversimplified mimics of natural ECMs lacking the essential natural temporal and spatial complexity, a growing symbiosis of materials engineering and cell biology may ultimately result in synthetic materials that contain the necessary signals to recapitulate developmental processes in tissue- and organ-specific differentiation and morphogenesis.

Effects of dynamic compressive loading on chondrocyte biosynthesis in self-assembling peptide scaffolds

Dynamic mechanical loading has been reported to affect chondrocyte biosynthesis in both cartilage explant and chondrocyte-seeded constructs. In this study, the effects of dynamic compression on chondrocyte-seeded peptide hydrogels were analyzed for extracellular matrix synthesis and retention over long-term culture. Initial studies were conducted with chondrocyte-seeded agarose hydrogels to explore the effects of various non-continuous loading protocols on chondrocyte biosynthesis. An optimized alternate day loading protocol was identified that increased proteoglycan (PG) synthesis over control cultures maintained in free-swelling conditions. When applied to chondrocyte-seeded peptide hydrogels, alternate day loading stimulated PG synthesis up to two-fold higher than that in free-swelling cultures. While dynamic compression also increased PG loss to the medium throughout the 39-day time course, total PG accumulation in the scaffold was significantly higher than in controls after 16 and 39 days of loading, resulting in an increase in the equilibrium and dynamic compressive stiffness of the constructs. Viable cell densities of dynamically compressed cultures differed from free-swelling controls by less than 20%, demonstrating that changes in PG synthesis were due to an increase in the average biosynthesis per viable cell. Protein synthesis was not greatly affected by loading, demonstrating that dynamic compression differentially regulated the synthesis of PGs. Taken together, these results demonstrate the potential of dynamic compression for stimulating PG synthesis and accumulation for applications to in vitro culture of tissue engineered constructs prior to implantation.

Biomaterials: where we have been and where we are going

Since its inception just over a half century ago, the field of biomaterials has seen a consistent growth with a steady introduction of new ideas and productive branches. This review describes where we have been, the state of the art today, and where we might be in 10 or 20 years. Herein, we highlight some of the latest advancements in biomaterials that aim to control biological responses and ultimately heal. This new generation of biomaterials includes surface modification of materials to overcome nonspecific protein adsorption in vivo, precision immobilization of signaling groups on surfaces, development of synthetic materials with controlled properties for drug and cell carriers, biologically inspired materials that mimic natural processes, and design of sophisticated three-dimensional (3-D) architectures to produce well-defined patterns for diagnostics, e.g., biological microelectromechanical systems (bioMEMs), and tissue engineering.

Fabrication of molecular materials using peptide construction motifs

Biotechnology has generally been associated with gene cloning and expression, genomics, high throughput drug discovery, biomedical advancement and agricultural development. That is about to change. Biotechnology will expand to encompass discovery and fabrication of biological and molecular materials with diverse structures, functionalities and utilities. The advent of nanobiotechnology and nanotechnology have accelerated this trend. Analogous to the construction of an intricate architectural structure, diverse and numerous structural motifs are used to assemble a sophisticated complex. Nature has selected, produced and evolved numerous molecular architectural motifs over billions of years for particular functions. These molecular motifs can now be used to build materials from the bottom up. Biotechnology will continue to harness nature's enormous power to benefit other disciplines and society as a whole.

Kinetic control of dimer structure formation in amyloid fibrillogenesis

Amyloid fibril formation involves nonfibrillar oligomeric intermediates, which are important as possible cytotoxic species in neurodegenerative diseases. However, their transient nature and polydispersity have made it difficult to identify their formation mechanism or structure. We have investigated the dimerization process, the first step in aggregate formation, by multiple molecular dynamics simulations of five beta-sheet-forming peptides. Contrary to the regular beta-sheet structure of the amyloid fibril, the dimers exhibit all possible combinations of beta-sheets, with an overall preference for antiparallel arrangements. Through statistical analysis of 1,000 dimerization trajectories, each 1 ns in length, we have demonstrated that the observed distribution of dimer configurations is kinetically determined; hydrophobic interactions orient the peptides so as to minimize the solvent accessible surface area, and the dimer structures become trapped in energetically unfavorable conformations. Once the hydrophobic contacts are present, the backbone hydrogen bonds form rapidly by a zipper-like mechanism. The initial nonequilibrium structures formed are stable during the 1-ns simulation time for all five peptides at room temperature. In contrast, at higher temperatures, where rapid equilibration among different configurations occurs, the distribution follows the global energies. The relaxation time of dimers at room temperature was estimated to be longer than the time for diffusional encounters with other oligomers at typical concentrations. These results suggest that kinetic trapping could play a role in the structural evolution of early aggregates in amyloid fibrillogenesis.