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

Tissue engineering: state of the art in oral rehabilitation

More than 85% of the global population requires repair or replacement of a craniofacial structure. These defects range from simple tooth decay to radical oncologic craniofacial resection. Regeneration of oral and craniofacial tissues presents a formidable challenge that requires synthesis of basic science, clinical science and engineering technology. Identification of appropriate scaffolds, cell sources and spatial and temporal signals (the tissue engineering triad) is necessary to optimize development of a single tissue, hybrid organ or interface. Furthermore, combining the understanding of the interactions between molecules of the extracellular matrix and attached cells with an understanding of the gene expression needed to induce differentiation and tissue growth will provide the design basis for translating basic science into rationally developed components of this tissue engineering triad. Dental tissue engineers are interested in regeneration of teeth, oral mucosa, salivary glands, bone and periodontium. Many of these oral structures are hybrid tissues. For example, engineering the periodontium requires growth of alveolar bone, cementum and the periodontal ligament. Recapitulation of biological development of hybrid tissues and interfaces presents a challenge that exceeds that of engineering just a single tissue. Advances made in dental interface engineering will allow these tissues to serve as model systems for engineering other tissues or organs of the body. This review will begin by covering basic tissue engineering principles and strategic design of functional biomaterials. We will then explore the impact of biomaterials design on the status of craniofacial tissue engineering and current challenges and opportunities in dental tissue engineering.

A cell-free protein-producing gel

Proteins are important biomaterials and are generally produced in living cells. Here, we show a novel DNA hydrogel that is capable of producing functional proteins without any living cells. This protein-producing gel (termed 'the P-gel system' or 'P-gel') consists of genes as part of the gel scaffolding. This is the first time that a hydrogel has been used to produce proteins. The efficiency was about 300 times higher than current, solution-based systems. In terms of volumetric yield, the P-gel produced up to 5 mg ml(-1) of functional proteins. The mechanisms behind the high efficiency and yield include improved gene stability, higher local concentration and a faster enzyme turnover rate due to a closer proximity of genes. We have tested a total of 16 different P-gels and have successfully produced all 16 proteins including membrane and toxic proteins, demonstrating that the P-gel system can serve as a general protein production technology.

Differential maturation and structure-function relationships in mesenchymal stem cell- and chondrocyte-seeded hydrogels

Degenerative disease and damage to articular cartilage represents a growing concern in the aging population. New strategies for engineering cartilage have employed mesenchymal stem cells (MSCs) as a cell source. However, recent work has suggested that chondrocytes (CHs) produce extracellular matrix (ECM) with superior mechanical properties than MSCs do. Because MSC-biomaterial interactions are important for both initial cell viability and subsequent chondrogenesis, we compared the growth of MSC- and CH-based constructs in three distinct hydrogels-agarose (AG), photocrosslinkable hyaluronic acid (HA), and self-assembling peptide (Puramatrix, Pu). Bovine CHs and MSCs were isolated from the same group of donors and seeded in AG, Pu, and HA at 20 million cells/mL. Constructs were cultured for 8 weeks with biweekly analysis of construct physical properties, viability, ECM content, and mechanical properties. Correlation analysis was performed to determine quantitative relationships between formed matrix and mechanical properties for each cell type in each hydrogel. Results demonstrate that functional chondrogenesis, as evidenced by increasing mechanical properties, occurred in each MSC-seeded hydrogel. Interestingly, while CH-seeded constructs were strongly dependent on the 3D environment in which they were encapsulated, similar growth profiles were observed in each MSC-laden hydrogel. In every case, MSC-laden constructs possessed mechanical properties significantly lower than those of CH-seeded AG constructs. This finding suggests that methods for inducing MSC chondrogenesis have yet to be optimized to produce cells whose functional matrix-forming potential matches that of native CHs.

Early tissue patterning recreated by mouse embryonic fibroblasts in a three-dimensional environment

Cellular self-organization studies have been mainly focused on models such as Volvox, the slime mold Dictyostelium discoideum, and animal (metazoan) embryos. Moreover, animal tissues undergoing regeneration also exhibit properties of embryonic systems such as the self-organization process that rebuilds tissue complexity and function. We speculated that the recreation in vitro of the biological, biophysical, and biomechanical conditions similar to those of a regenerative milieu could elicit the intrinsic capacity of differentiated cells to proceed to the development of a tissue-like structure. Here we show that, when primary mouse embryonic fibroblasts are cultured in a soft nanofiber scaffold, they establish a cellular network that causes an organized cell contraction,proliferation, and migration that ends in the formation of a symmetrically bilateral structure with a distinct central axis. A subset of mesodermal genes (brachyury, Sox9, Runx2) is upregulated during this morphogenetic process. The expression of brachyury was localized first at the central axis, extending then to both sides of the structure. The spontaneous formation of cartilage-like tissue mainly at the paraxial zone followed expression ofSox9 and Runx2. Because cellular self-organization is an intrinsic property of the tissues undergoing development,this model could lead to new ways to consider tissue engineering and regenerative medicine.

Co-assembling peptides as defined matrices for endothelial cells

Self-assembling peptides and peptide derivatives bearing cell-binding ligands are increasingly being investigated as defined cell culture matrices and as scaffolds for regenerative medicine. In order to systematically refine such scaffolds to elicit specific desired cell behaviors, ligand display should ideally be achieved without inadvertently altering other physicochemical properties such as viscoelasticity. Moreover, for in vivo applications, self-assembled biomaterials must exhibit low immunogenicity. In the present study, multi-peptide co-assembling hydrogels based on the beta-sheet fibrillizing peptide Q11 (QQKFQFQFEQQ) were designed such that they presented RGDS or IKVAV ligands on their fibril surfaces. In co-assemblies of the ligand-bearing peptides with Q11, ligand incorporation levels capable of influencing the attachment, spreading, morphology, and growth of human umbilical vein endothelial cells (HUVECs) did not significantly alter the materials' fibrillization, beta-turn secondary structure, or stiffness. RGDS-Q11 specifically increased HUVEC attachment, spreading, and growth when co-assembled into Q11 gels, whereas IKVAV-Q11 exerted a more subtle influence on attachment and morphology. Additionally, Q11 and RGDS-Q11 were minimally immunogenic in mice, making Q11-based biomaterials attractive candidates for further investigation as defined, modular extracellular matrices for applications in vitro and in vivo.

Controlled release of functional proteins through designer self-assembling peptide nanofiber hydrogel scaffold

The release kinetics for a variety of proteins of a wide range of molecular mass, hydrodynamic radii, and isoelectric points through a nanofiber hydrogel scaffold consisting of designer self-assembling peptides were studied by using single-molecule fluorescence correlation spectroscopy (FCS). In contrast to classical diffusion experiments, the single-molecule approach allowed for the direct determination of diffusion coefficients for lysozyme, trypsin inhibitor, BSA, and IgG both inside the hydrogel and after being released into the solution. The results of the FCS analyses and the calculated pristine in-gel diffusion coefficients were compared with the values obtained from the Stokes-Einstein equation, Fickian diffusion models, and the literature. The release kinetics suggested that protein diffusion through nanofiber hydrogels depended primarily on the size of the protein. Protein diffusivities decreased, with increasing hydrogel nanofiber density providing a means of controlling the release kinetics. Secondary and tertiary structure analyses and biological assays of the released proteins showed that encapsulation and release did not affect the protein conformation and functionality. Our results show that this biocompatible and injectable designer self-assembling peptide hydrogel system may be useful as a carrier for therapeutic proteins for sustained release applications.

Effect of swelling ratio of injectable hydrogel composites on chondrogenic differentiation of encapsulated rabbit marrow mesenchymal stem cells in vitro

An injectable, biodegradable hydrogel composite of oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs) has been investigated as a cell and growth factor carrier for cartilage tissue engineering applications. In this study, hydrogel composites with different swelling ratios were prepared by cross-linking OPF macromers with poly(ethylene glycol) (PEG) repeating units of varying molecular weights from 1000 approximately 35000. Rabbit marrow mesenchymal stem cells (MSCs) and MPs loaded with transforming growth factor-beta1 (TGF-beta1) were encapsulated in the hydrogel composites to examine the effect of the swelling ratio of the hydrogel composites on the chondrogenic differentiation of encapsulated rabbit marrow MSCs both in the presence and in the absence of TGF-beta1. The swelling ratio of the hydrogel composites increased as the PEG molecular weight in the OPF macromers increased. Chondrocyte-specific genes were expressed at higher levels in groups containing TGF-beta1-loaded MPs and varied with the swelling ratio of the hydrogel composites. OPF hydrogel composites with PEG repeating units of molecular weight 35000 and 10000 with TGF-beta1-loaded MPs exhibited a 159 +/- 95- and a 89 +/- 31-fold increase in type II collagen gene expression at day 28, respectively, while OPF hydrogel composites with PEG repeating units of molecular weight 3000 and 1000 with TGF-beta1-loaded MPs showed a 27 +/- 10- and a 17 +/- 7-fold increase in type II collagen gene expression, respectively, as compared to the composites with blank MPs at day 0. The results indicate that chondrogenic differentiation of encapsulated rabbit marrow MSCs within OPF hydrogel composites could be affected by their swelling ratio, thus suggesting the potential of OPF composite hydrogels as part of a novel strategy for controlling the differentiation of stem cells.

Introducing chemical functionality in Fmoc-peptide gels for cell culture

Aromatic short peptide derivatives, i.e. peptides modified with aromatic groups such as 9-fluorenylmethoxycarbonyl (Fmoc), can self-assemble into self-supporting hydrogels. These hydrogels have some similarities to extracellular matrices due to their high hydration, relative stiffness and nanofibrous architecture. We previously demonstrated that Fmoc-diphenylalanine (Fmoc-F(2)) provides a suitable matrix for two-dimensional (2D) or three-dimensional (3D) culture of primary bovine chondrocytes. In this paper we investigate whether the introduction of chemical functionality, such as NH(2), COOH or OH, enhances compatibility with different cell types. A series of hydrogel compositions consisting of combinations of Fmoc-F(2) and n-protected Fmoc amino acids, lysine (K, with side chain R=(CH(2))(4)NH(2)), glutamic acid (D, with side chain R=CH(2)COOH), and serine (S, with side chain R=CH(2)OH) were studied. All compositions produced fibrous scaffolds with fibre diameters in the range of 32-65 nm as assessed by cryo-scanning electron microscopy and atomic force microscopy. Fourier transform infrared spectroscopy analysis suggested that peptide segments adopt a predominantly antiparallel beta-sheet conformation. Oscillatory rheology results show that all four hydrogels have mechanical profiles of soft viscoelastic materials with elastic moduli dependent on the chemical composition, ranging from 502 Pa (Fmoc-F(2)/D) to 21.2 KPa (Fmoc-F(2)). All gels supported the viability of bovine chondrocytes as assessed by a live-dead staining assay. Fmoc-F(2)/S and Fmoc-F(2)/D hydrogels in addition supported viability for human dermal fibroblasts (HDF) while Fmoc-F(2)/S hydrogel was the only gel type that supported viability for all three cell types tested. Fmoc-F(2)/S was therefore investigated further by studying cell proliferation, cytoskeletal organization and histological analysis in 2D culture. In addition, the Fmoc-F(2)/S gel was shown to support retention of cell morphology in 3D culture of bovine chondrocytes. These results demonstrate that introduction of chemical functionality into Fmoc-peptide scaffolds may provide gels with tunable chemical and mechanical properties for in vitro cell culture.

Self-assembling materials for therapeutic delivery

A growing number of medications must be administered through parenteral delivery, i.e., intravenous, intramuscular, or subcutaneous injection, to ensure effectiveness of the therapeutic. For some therapeutics, the use of delivery vehicles in conjunction with this delivery mechanism can improve drug efficacy and patient compliance. Macromolecular self-assembly has been exploited recently to engineer materials for the encapsulation and controlled delivery of therapeutics. Self-assembled materials offer the advantages of conventional crosslinked materials normally used for release, but also provide the ability to tailor specific bulk material properties, such as release profiles, at the molecular level via monomer design. As a result, the design of materials from the "bottom up" approach has generated a variety of supramolecular devices for biomedical applications. This review provides an overview of self-assembling molecules, their resultant structures, and their use in therapeutic delivery. It highlights the current progress in the design of polymer- and peptide-based self-assembled materials.

Cells and biomaterials in cartilage tissue engineering

Cartilage defects are notoriously difficult to repair and owing to the long-term prognosis of osteoarthritis, and a rapidly aging population, a need for new therapies is pressing. Cell-based therapies for cartilage regeneration were introduced into patients in the early 1990s. Since that time the technology has developed from a simple cell suspension to more complex 3D structures. Cells, both chondrocytes and stem cells, have been incorporated into scaffold material with the aim to better recreate the natural environment of the cell, while providing more structural support to withstand the large forces applied on the de novo tissue. This review aims to provide an overview of potential cell sources and different scaffold materials, which are in development for cartilage tissue engineering.

Transplantation of human mesenchymal stems cells into intervertebral discs in a xenogeneic porcine model

STUDY DESIGN

Experimental and descriptive study of a xenotransplantation model in minipigs.

OBJECTIVE

To study survival and function of human mesenchymal stem cells (hMSCs) after transplantation into injured porcine spinal discs, as a model for cell therapy.

SUMMARY OF BACKGROUND DATA

Biologic treatment options of the intervertebral disc are suggested for patients with chronic low back pain caused by disc degeneration.

METHODS

Three lumbar discs in each of 9 minipigs were injured by aspiration of the nucleus pulposus (NP), 2 weeks later hMSCs were injected in F12 media suspension (cell/med) or with a hydrogel carrier (Puramatrix) (cell/gel). The animals were sacrificed after 1, 3, or 6 months. Disc appearance was visualized by magnetic resonance imaging. Immunohistochemistry methods were used to detect hMSCs by antihuman nuclear antibody staining, and further performed for Collagen II, Aggrecan, and Collagen I. SOX 9, Aggrecan, Versican, Collagen IA, and Collagen IIA and Collagen IIB human mRNA expression was analyzed by real-time PCR.

RESULTS

At magnetic resonance imaging all injured discs demonstrated degenerative signs. Cell/gel discs showed fewer changes compared with cell/med discs and only injured discs at later time points. hMSCs were detected in 9 of 10 of the cell/gel discs and in 8 of 9 of the cell/med discs. Immunostaining for Aggrecan and Collagen type II expression were observed in NP after 3 and 6 months in gel/cell discs and colocalized with the antihuman nuclear antibody. mRNA expression of Collagen IIA, Collagen IIB, Versican, Collagen 1A, Aggrecan, and SOX9 were detected in both cell/med and cell/gel discs at the time points 3 and 6 months by real-time PCR.

CONCLUSION

hMSCs survive in the porcine disc for at least 6 months and express typical chondrocyte markers suggesting differentiation toward disc-like cells. As in autologous animal models the combination with a three-dimensional-hydrogel carrier seems to facilitate differentiation and survival of MSCs in the disc. Xenotransplantation seems to be valuable in evaluating the possibility for human cell therapy treatment for intervertebral discs.

Cultura de condrócitos em arcabouço tridimensional: hidrogel de alginato

OBJETIVOS: O presente estudo teve como objetivo cultivar condrócitos retirados da articulação do joelho de coelhos encapsulados em hidrogel de alginato (HA) e caracterizar a produção de matriz extracelular (ECM). MÉTODOS: A cartilagem articular foi removida do joelho de coelhos, com três a seis meses, fragmentada em pedaços de 1mm e submetida à digestão enzimática. Uma concentração de 1x106 céls/mL foram ressuspensas em uma solução de alginato de sódio a 1,5% (w/v), em seguida fez-se o processo de gelatinização em CaCl2 (102 mM), permitindo a formação do HA e cultivo em meio DMEM-F12 durante quatro semanas. A distribuição das células e a ECM foram acessadas através das secções histológicas coradas com e azul de toluidina hematoxilina e eosina (HE). RESULTADOS: Houve um aumento no número e na viabilidade dos condrócitos durante as quatro semanas de cultura. Através das análises histológicas dos HAs corados com azul de toluidina e HE foi possível observar a distribuição definida dos condrócitos no hidrogel, assemelhando-se a grupos isógenos e formação de matriz territorial. CONCLUSÃO: Este estudo demonstrou a eficiência do HA como arcabouço para ser usado na cultura de condrócitos, constituindo uma alternativa no reparo de lesões na cartilagem articular.

Gel de plaquetas: arcabouço 3D para cultura celular

INTRODUÇÃO: O reparo tissular é o objetivo final da cirurgia. A cultura celular requer arcabouço mecânico que dê suporte ao crescimento celular e difusão dos nutrientes. O uso do plasma rico em plaquetas (PRP) como um arcabouço 3D possui diversas vantagens: é material biológico, de fácil absorção pós-transplante, rico em fatores de crescimento, em especial PDGF- ββ e TGF-β que estimula síntese de matriz extracelular na cartilagem. OBJETIVO: Desenvolver arcabouço 3D à base de PRP. MATERIAIS E MÉTODOS: Duas formas foram idealizadas: Sphere e Carpet. Condições estéreis foram utilizadas. O gel de plaquetas permaneceu em cultura celular, observado diariamente em microscópio invertido. RESULTADOS: Ambos arcabouços obtiveram sucesso, com aspectos positivos e negativos. DISCUSSÃO: A forma Sphere não aderiu ao plástico. Observou-se retração do gel e investigação ao microscópio dificultada devido às áreas opacas no campo visual. A forma Carpet não aderiu ao plástico e apresentou-se translúcida. O tempo de estudo foi de 20 dias. CONCLUSÕES: A produção de um arcabouço 3D PRP foi um sucesso, e trata-se de uma alternativa que necessita ser mais utilizado e investigado para que se consolide em uma rota eficiente e confiável na tecnologia de engenharia tissular, particularmente em cultura de tecido cartilaginoso.

High-tech applications of self-assembling supramolecular nanostructured gel-phase materials: from regenerative medicine to electronic devices

It is likely that nanofabrication will underpin many technologies in the 21st century. Synthetic chemistry is a powerful approach to generate molecular structures that are capable of assembling into functional nanoscale architectures. There has been intense interest in self-assembling low-molecular-weight gelators, which has led to a general understanding of gelation based on the self-assembly of molecular-scale building blocks in terms of non-covalent interactions and packing parameters. The gelator molecules generate hierarchical, supramolecular structures that are macroscopically expressed in gel formation. Molecular modification can therefore control nanoscale assembly, a process that ultimately endows specific material function. The combination of supramolecular chemistry, materials science, and biomedicine allows application-based materials to be developed. Regenerative medicine and tissue engineering using molecular gels as nanostructured scaffolds for the regrowth of nerve cells has been demonstrated in vivo, and the prospect of using self-assembled fibers as one-dimensional conductors in gel materials has captured much interest in the field of nanoelectronics.

The effect of self-assembling peptide nanofiber scaffolds on mouse embryonic fibroblast implantation and proliferation

Development of new materials for tissue engineering can be facilitated by the capacity to efficiently monitor in vivo the survival, proliferation and differentiation behaviour of cells implanted in different target tissues. We present here the application of a previously developed platform that allows to monitor in real time the survival and proliferative behaviour of implanted cells in two anatomical sites: subcutaneous and intramuscular. Basically, the system is based on the use of a non-invasive bioluminescence imaging (BLI) technique to detect luciferase expressing C57BL/6 cells, mouse embryonic fibroblasts, seeded in two sets of scaffolds: 1, a RAD16-I self-assembling peptide nanofiber matrix and 2, a composite consisted of the same RAD16-I nanofibers contained into a microporous biorubber scaffold. Interestingly, our results indicated considerable differences in the behaviour of implanted cells in each scaffold type. We observed that the self-assembling peptide scaffold alone foster cell survival and promotes cell proliferation where the composite scaffold not. Since self-assembling peptide scaffolds presents value stiffness proximal to the implanted tissues it is suggestive to think that harder materials will provide a physical constriction for cells to proliferate as well as mechanical discontinuity. We therefore propose that it is important to close match the implantation environment with the cell/material constructs in order to obtain the best response of the cells, illustrating the convenience of this strategy for the development of new tissue engineering platforms.

Transglutaminase 2 cross-linking of matrix proteins: biological significance and medical applications

This review summarises the functions of the enzyme tissue transglutaminase (TG2) in the extracellular matrix (ECM) both as a matrix stabiliser through its protein cross-linking activity and as an important cell adhesion protein involved in cell survival. The contribution of extracellular TG2 to the pathology of important diseases such as cancer and fibrosis are discussed with a view to the potential importance of TG2 as a therapeutic target. The medical applications of TG2 are further expanded by detailing the use of transglutaminase cross-linking in the development of novel biocompatible biomaterials for use in soft and hard tissue repair.

A thixotropic nanocomposite gel for three-dimensional cell culture

Thixotropic materials, which become less viscous under stress and return to their original state when stress is removed, have been used to deliver gel-cell constructs and therapeutic agents. Here we show that a polymer-silica nanocomposite thixotropic gel can be used as a three-dimensional cell culture material. The gel liquefies when vortexed--allowing cells and biological components to be added--and resolidifies to trap the components when the shear force from spinning is removed. Good permeability of nutrients and gases through the gel allows various cell types to proliferate and be viable for up to three weeks. Human mesenchymal stem cells cultured in stiffer gels developed bone-like behaviour, showing that the rheological properties of the gel can control cell differentiation. No enzymatic, chemical, or photo-crosslinking, changes in ionic strength or temperature are required to form or liquefy the gel, offering a way to sub-culture cells without using trypsin-a protease commonly used in traditional cell culture techniques.

Transient exposure to transforming growth factor beta 3 under serum-free conditions enhances the biomechanical and biochemical maturation of tissue-engineered cartilage

A goal of cartilage tissue engineering is the production of cell-laden constructs possessing sufficient mechanical and biochemical features to enable native tissue function. This study details a systematic characterization of a serum-free (SF) culture methodology employing transient growth factor supplementation to promote robust maturation of tissue-engineered cartilage. Bovine chondrocyte agarose hydrogel constructs were cultured under free-swelling conditions in serum-containing or SF medium supplemented continuously or transiently with varying doses of transforming growth factor beta 3 (TGF-beta3). Constructs were harvested weekly or bi-weekly and assessed for mechanical and biochemical properties. Transient exposure (2 weeks) to low concentrations (2.5-5 ng/mL) of TGF-beta3 in chemically defined medium facilitated robust and highly reproducible construct maturation. Constructs receiving transient TGF-beta3 exposure achieved native tissue levels of compressive modulus (0.8 MPa) and proteoglycan content (6-7% of wet weight) after less than 2 months of in vitro culture. This maturation response was far superior to that observed after continuous growth factor supplementation or transient TGF-beta3 treatment in the presence of serum. These findings represent a significant advance in developing an ex vivo culture methodology to promote production of clinically relevant and mechanically competent tissue-engineered cartilage constructs for implantation to repair damaged articular surfaces.

Self-organization of a chiral D-EAK16 designer peptide into a 3D nanofiber scaffold

Self-assembling peptide nanofiber scaffolds are an excellent material for applications such as tissue repair, tissue regeneration, instant stopping of bleeding, and slow drug release. We report a new self-assembling peptide D-EAK16 consisting purely of D-amino acids. D-EAK16 and L-EAK16 display mirror-image CD spectra at 20 degrees C. Like L-EAK16, D-EAK16 self-assembles into nanofibers, thus demonstrating that chiral self-assembling peptide nanofiber scaffolds can be made from both L- and D-amino acids. We also show that D-peptide nanofibers are resistant to natural proteases and may thus be useful in biotechnology, nanobiotechnology, tissue repair and tissue regeneration as well as other medical applications.

Reversible transitions between peptide nanotubes and vesicle-like structures including theoretical modeling studies

Peptide-based self-assembling systems are increasingly attractive because of their wide range of applications in different fields. Peptide nanostructures are flexible with changes in the ambient conditions. Herein, a reversible shape transition between self-assembled dipeptide nanotubes (DPNTs) and vesicle-like structures is observed upon a change in the peptide concentration. SEM, TEM, AFM, and CD spectroscopy were used to follow this transition process. We show that dilution of a peptide-nanotube dispersion solution results in the formation of vesicle-like structures, which can then be reassembled into the nanotubes by concentrating the solution. A theoretical model describing this shape-transition phenomenon is presented to propose ways to engineer assembling molecules in order to devise other systems in which the morphology can be tuned on demand.

Tensile properties of engineered cartilage formed from chondrocyte- and MSC-laden hydrogels

OBJECTIVE

The objective of this study was to determine the capacity of chondrocyte- and mesenchymal stem cell (MSC)-laden hydrogel constructs to achieve native tissue tensile properties when cultured in a chemically defined medium supplemented with transforming growth factor-beta3 (TGF-beta3).

DESIGN

Cell-laden agarose hydrogel constructs (seeded with bovine chondrocytes or MSCs) were formed as prismatic strips and cultured in a chemically defined serum-free medium in the presence or absence of TGF-beta3. The effects of seeding density (10 vs 30 million cells/mL) and cell type (chondrocyte vs MSC) were evaluated over a 56-day period. Biochemical content, collagenous matrix deposition and localization, and tensile properties (ramp modulus, ultimate strain, and toughness) were assessed biweekly.

RESULTS

Results show that the tensile properties of cell-seeded agarose constructs increase with time in culture. However, tensile properties (modulus, ultimate strain, and toughness) achieved on day 56 were not dependent on either the initial seeding density or the cell type employed. When cultured in medium supplemented with TGF-beta3, tensile modulus increased and plateaued at a level of 300-400 kPa for each cell type and starting cell concentration. Ultimate strain and toughness also increased relative to starting values. Collagen deposition increased in constructs seeded with both cell types and at both seeding densities, with exposure to TGF-beta3 resulting in a clear shift toward type II collagen deposition as determined by immunohistochemical staining.

CONCLUSIONS

These findings demonstrate that the tensile properties, an important and often overlooked metric of cartilage development, increase with time in culture in engineered hydrogel-based cartilage constructs. Under the free-swelling conditions employed in the present study, tensile moduli and toughness did not match that of the native tissue, though significant time-dependent increases were observed with the inclusion of TGF-beta3. Of note, MSC-seeded constructs achieved tensile properties that were comparable to chondrocyte-seeded constructs, confirming the utility of this alternative cell source in cartilage tissue engineering. Further work, including both modulation of the chemical and mechanical culture environment, is required to optimize the deposition of collagen and its remodeling to achieve tensile properties in engineered constructs matching the native tissue.

Self-assembling peptide-polymer hydrogels designed from the coiled coil region of fibrin

Biomaterials constructed from self-assembling peptides, peptide derivatives, and peptide-polymer conjugates are receiving increasing attention as defined matrices for tissue engineering, controlled therapeutic release, and in vitro cell expansion, but many are constructed from peptide structures not typically found in the human extracellular matrix. Here we report a self-assembling biomaterial constructed from a designed peptide inspired by the coiled coil domain of human fibrin, the major protein constituent of blood clots and the provisional scaffold of wound healing. Targeted substitutions were made in the residues forming the interface between coiled coil strands for a 37-amino acid peptide from human fibrinogen to stabilize the coiled coil peptide bundle, while the solvent-exposed residues were left unchanged to provide a surface similar to that of the native protein. This peptide, which self-assembled into coiled coil dimers and tetramers, was then used to produce triblock peptide-PEG-peptide bioconjugates that self-assembled into viscoelastic hydrogel biomaterials.

Integrating novel technologies to fabricate smart scaffolds

Tissue engineering aims at restoring or regenerating a damaged tissue by combining cells, derived from a patient biopsy, with a 3D porous matrix functioning as a scaffold. After isolation and eventual in vitro expansion, cells are seeded on the 3D scaffolds and implanted directly or at a later stage in the patient's body. 3D scaffolds need to satisfy a number of requirements: (i) biocompatibility, (ii) biodegradability and/or bioresorbability, (iii) suitable mechanical properties, (iv) adequate physicochemical properties to direct cell-material interactions matching the tissue to be replaced and (v) ease in regaining the original shape of the damaged tissue and the integration with the surrounding environment. Still, it appears to be a challenge to satisfy all the aforementioned requisites with the biomaterials and the scaffold fabrication technologies nowadays available. 3D scaffolds can be fabricated with various techniques, among which rapid prototyping and electrospinning seem to be the most promising. Rapid prototyping technologies allow manufacturing scaffolds with a controlled, completely accessible pore network--determinant for nutrient supply and diffusion--in a CAD/CAM fashion. Electrospinning (ESP) allows mimicking the extracellular matrix (ECM) environment of the cells and can provide fibrous scaffolds with instructive surface properties to direct cell faith into the proper lineage. Yet, these fabrication methods have some disadvantages if considered alone. This review aims at summarizing conventional and novel scaffold fabrication techniques and the biomaterials used for tissue engineering and drug-delivery applications. A new trend seems to emerge in the field of scaffold design where different scaffolds fabrication technologies and different biomaterials are combined to provide cells with mechanical, physicochemical and biological cues at the macro-, micro- and nano-scale. If merged together, these integrated technologies may lead to the generation of a new set of 3D scaffolds that satisfies all of the scaffolds' requirements for tissue-engineering applications and may contribute to their success in a long-term scenario.

Insensitivity to salt of assembly of a rigid biopolymer aggrecan

Many polyelectrolytes, ranging from sulfonated polystyrene to DNA, exhibit a strong sensitivity of their phase behavior to salt concentration, especially to higher valence salts, which often lead to phase separation. We show that the stiff polyelectrolyte aggrecan exhibits a qualitatively different behavior. Specifically, the scattering properties of aggrecan solutions are exceptionally insensitive to the addition of calcium salt, conferring on aggrecan the role of an ion reservoir mediating calcium metabolism in cartilage and bone, and also providing osmotic resilience to compressive load.

Facile preparation of self-assembled hydrogel-like GdPO4*H2O nanorods

Of the methods employed in the preparation of one-dimensional lanthanide phosphate (LnPO(4)) nanorods/nanowires, such as GdPO(4), the hydrothermal method has been mainly used as a synthetic route. In this study, we report a facile low-temperature solution approach to prepare GdPO 4*H(2)O nanorods by simply refluxing GdCl(3) and KH(2)PO(4) for only 15 min at 88 degrees C, an approach that can easily be scaled up by increasing the reagent amounts. We observed a highly viscous macroscopic hydrogel-like material when we mixed as-prepared GdPO(4)*H(2)O nanomaterials with H(2)O. Hydrogels are an important class of biomaterials. Their building blocks, normally formed from protein-, peptide-, polymer-, and lipid-based materials, offer three-dimensional scaffolds for drug delivery, tissue engineering, and biosensors. Our preliminary results showed that GdPO(4)*H(2)O hydrogels could be used for encapsulation and drug release, and that they were biocompatible, acting as scaffolds to foster cell proliferation. These findings suggested that they might have biomedical uses. Our findings may lead to the creation of other inorganic nanomaterial-based hydrogels apart from the organic and biomolecular protein-, peptide-, polymer-, and lipid-based building blocks.

Stimuli responsive self-assembled hydrogel of a low molecular weight free dipeptide with potential for tunable drug delivery

Bottom-up fabrication by molecular self-assembly is now widely recognized as a potent method for generating interesting and functional nano- and mesoscale structures. Hydrogels from biocompatible molecules are an interesting class of mesoscale assemblies with potential biomedical applications. The self-assembly of a proteolysis resistant aromatic dipeptide containing a conformational constraining residue (DeltaPhe) into a stable hydrogel has been studied in this work. The reported dipeptide has free -N and -C termini. The hydrogel was self-supportive, was fractaline in nature, and possessed high mechanical strength. It was responsive to environmental conditions like pH, temperature, and ionic strength. The gel matrix could encapsulate and release bioactive molecules in a sustained manner. The described hydrogel showed no observable cytotoxicity to the HeLa and L929 cell lines in culture.

Elastic deformation and failure in protein filament bundles: Atomistic simulations and coarse-grained modeling

The synthetic peptide RAD16-II has shown promise in tissue engineering and drug delivery. It has been studied as a vehicle for cell delivery and controlled release of IGF-1 to repair infarcted cardiac tissue, and as a scaffold to promote capillary formation for an in vitro model of angiogenesis. The structure of RAD16-II is hierarchical, with monomers forming long beta-sheets that pair together to form filaments; filaments form bundles approximately 30-60 nm in diameter; branching networks of filament bundles form macroscopic gels. We investigate the mechanics of shearing between the two beta-sheets constituting one filament, and between cohered filaments of RAD16-II. This shear loading is found in filament bundle bending or in tensile loading of fibers composed of partial-length filaments. Molecular dynamics simulations show that time to failure is a stochastic function of applied shear stress, and that for a given loading time behavior is elastic for sufficiently small shear loads. We propose a coarse-grained model based on Langevin dynamics that matches molecular dynamics results and facilities extending simulations in space and time. The model treats a filament as an elastic string of particles, each having potential energy that is a periodic function of its position relative to the neighboring filament. With insight from these simulations, we discuss strategies for strengthening RAD16-II and similar materials.

Self-assembling Peptides: From Bio-inspired Materials to Bone Regeneration

In recent years, the development of new biomaterials with specifications for tissue and organ functional requirements—such as proper biological, structural, and biomechanical properties as well as designed control for biodegradation and therapeutic drug-release capacity—is the main aim of many academic and industrial programs. Hence, the concept of molecular self-assembly is the driving force for the development of new biomaterials that support the growth and functional differentiation of cells and tissues in a controlled manner. The discovery, properties, and development of self-assembling peptides to be used as three-dimensional (3D) scaffolds based on their similarity (in structure and mechanical features) to extracellular matrices are described. Self-assembling peptides can be used for in vitro applications for cell 3D culture as well as in vivo for tissue regeneration such as bone and optical nerve repair, as well as for drug delivery of mediators to improve therapy, as in the case of myocardial infarction. Finally, the use of self-assembling materials in combination with a bioengineering platform is proposed to assist functional bone regeneration in cases of larger bone defects, including exposed fractures due to trauma and spinal disorders dealing with high loadings, as well as replacement of big bone structures due to tumors.

Gel-like behavior in aggrecan assemblies

Aggrecan, a large biological polyelectrolyte molecule with a bottlebrush shape, forms complexes with hyaluronic acid (HA) that provide compressive resistance in cartilage. In solutions of aggrecan alone, the concentration dependence of the osmotic pressure Pi is marked by self-assembly of the molecules into aggregates. When HA is added to the solution at low aggrecan concentration c, the osmotic pressure is reduced, but in the physiological concentration range this trend is reversed. The osmotic modulus c partial differentialPi partial differentialc, which determines load bearing resistance, is enhanced in the HA-containing solutions. Dynamic light scattering (DLS) measurements show that the aggregates behave like microgels and that they become denser as the aggrecan concentration increases. The degree of densification is greatest at large distance scales in the microgels, but decreases at short distance scales. Measurements at higher resolution, involving small angle neutron scattering and small angle x-ray scattering (SAXS), confirm that at length scales shorter than 1000 angstroms, the density is independent of the concentration and that the individual bottlebrushes in the microgels retain their identity. The absence of collective diffusion modes in the relaxation spectrum, measured by DLS and neutron spin echo, corroborates the lack of interpenetration among the aggrecan subunits in the microgel. Complexation with HA modifies the long-range spatial organization of the microgels. Comparison of the scattering pattern of the individual aggrecan molecules obtained from SAXS measurements with that of the complexes measured by DLS shows that the aggrecan-HA structure is denser and is more uniform than the random microgels. This enhanced space-filling property allows higher packing densities to be attained, thus, optimizing resistance to osmotic compression.

Three-dimensional cell culture matrices: state of the art

Traditional methods of cell growth and manipulation on 2-dimensional (2D) surfaces have been shown to be insufficient for new challenges of cell biology and biochemistry, as well as in pharmaceutical assays. Advances in materials chemistry, materials fabrication and processing technologies, and developmental biology have led to the design of 3D cell culture matrices that better represent the geometry, chemistry, and signaling environment of natural extracellular matrix. In this review, we present the status of state-of-the-art 3D cell-growth techniques and scaffolds and analyze them from the perspective of materials properties, manufacturing, and functionality. Particular emphasis was placed on tissue engineering and in vitro modeling of human organs, where we see exceptionally strong potential for 3D scaffolds and cell-growth methods. We also outline key challenges in this field and most likely directions for future development of 3D cell culture over the period of 5-10 years.

Collagen tissue engineering: development of novel biomaterials and applications

Scientific investigations involving collagen have inspired tissue engineering and design of biomaterials since collagen fibrils and their networks primarily regulate and define most tissues. The collagen networks form a highly organized, three-dimensional architecture to entrap other ingredients. Biomaterials are expected to function as cell scaffolds to replace native collagen-based extracellular matrix. The composition and properties of biomaterials used as scaffold for tissue engineering significantly affect the regeneration of neo-tissues and influence the conditions of collagen engineering. The complex scenario of collagen characteristics, types, fibril arrangement, and collagen structure-related functions (in a variety of connective tissues including bone, cartilage, tendon, skin and cornea) are addressed in this review. Discussion will focus on nanofibrillar assemblies and artificial synthetic peptides that mimic either the fibrillar structure or the elemental components of type I collagen as illustrated by their preliminary applications in tissue engineering. Conventional biomaterials used as scaffolds in engineering collagen-containing tissues are also discussed. The design of novel biomaterials and application of conventional biomaterials will facilitate development of additional novel tissue engineering bioproducts by refining the currently available techniques. The field of tissue engineering will ultimately be advanced by increasing control of collagen in native tissue and by continual manipulation of biomaterials.

Modulating the mechanical properties of self-assembled peptide hydrogels via native chemical ligation

Hydrogels produced from self-assembling peptides and peptide derivatives are being investigated as synthetic extracellular matrices for defined cell culture substrates and scaffolds for regenerative medicine. In many cases, however, they are less stiff than the tissues and extracellular matrices they are intended to mimic, and they are prone to cohesive failure. We employed native chemical ligation to produce peptide bonds between the termini of fibrillized beta-sheet peptides to increase gel stiffness in a chemically specific manner while maintaining the morphology of the self-assembled fibrils. Polymerization, fibril structure, and mechanical properties were measured by SDS-PAGE, mass spectrometry, TEM, circular dichroism, and oscillating rheometry; and cellular responses to matrix stiffening were investigated in cultures of human umbilical vein endothelial cells (HUVECs). Ligation led to a fivefold increase in storage modulus and a significant enhancement of HUVEC proliferation and expression of CD31 on the surface of the gels. The approach was also orthogonal to the inclusion of unprotected RGD-functionalized self-assembling peptides, which further increased proliferation. This strategy broadens the utility of self-assembled peptide materials for applications that require enhancement or modulation of matrix mechanical properties by providing a chemoselective means for doing so without significantly disrupting the gels' fibrillar structure.

Sequence effect of self-assembling peptides on the complexation and in vitro delivery of the hydrophobic anticancer drug ellipticine

A special class of self-assembling peptides has been found to be capable of stabilizing the hydrophobic anticancer agent ellipticine in aqueous solution. Here we study the effect of peptide sequence on the complex formation and its anticancer activity in vitro. Three peptides, EAK16-II, EAK16-IV and EFK16-II, were selected to have either a different charge distribution (EAK16-II vs. EAK16-IV) or a varying hydrophobicity (EAK16-II vs. EFK16-II). Results on their complexation with ellipticine revealed that EAK16-II and EAK16-IV were able to stabilize protonated ellipticine or ellipticine microcrystals depending on the peptide concentration; EFK16-II could stabilize neutral ellipticine molecules and ellipticine microcrystals. These different molecular states of ellipticine were expected to affect ellipticine delivery. The anticancer activity of these complexes was tested against two cancer cell lines: A549 and MCF-7, and related to the cell viability. The viability results showed that the complexes with protonated ellipticine were effective in eradicating both cancer cells (viability <0.05), but their dilutions in water were not stable, leading to a fast decrease in their toxicity. In contrast, the complexes formulated with EFK16-II were relatively stable upon dilution, but their original toxicity was relatively low compared to that with protonated ellipticine. Overall, the charge distribution of the peptides seemed not to affect the complex formation and its therapeutic efficacy in vitro; however, the increase in hydrophobicity of the peptides significantly altered the state of stabilized ellipticine and increased the stability of the complexes. This work provides essential information for peptide sequence design in the development of self-assembling peptide-based delivery of hydrophobic anticancer drugs.