Cell Function on Substrates Containing Immobilized Bioactive Peptides

Biomimetic Strategies to Develop Bioactive Scaffolds for Myocardial Tissue Engineering

The aim of this paper is to provide an overview of the results of the research activity carried out in our laboratories, over the last 10 years, in relation to the development of strategies for the production of biomimetic and bioactive scaffolds for myocardial tissue engineering. Biomimetic and bioactive polymeric scaffolds for cardiac regeneration were designed and manufactured in our laboratories and their morphological, physicochemical, mechanical and biological properties were investigated by different techniques, such as scanning electron microscopy, infrared chemical imaging, swelling test, in vitro degradation assessment, dynamic mechanical analysis, in vitro and in vivo biological tests. Biomimetic scaffolds, able to favor tissue regeneration by mimicking nature, were engineered by different strategies, comprising: (i) the imitation of the composition and interactions among components of the natural extracellular matrix (ECM), by mixing of proteins and polysaccharides; (ii) the material surface modification, using both traditional and innovative techniques, such as molecular imprinting; (iii) the incorporation and release of specific active agents and (iv) the production of scaffolds with a microarchitecture similar to that of native ECM. All the developed strategies were found to be effective in creating materials able to influence cellular behavior and therefore to favor the process of new tissue formation. In particular, the approach based on the combination of different strategies aimed at creating a system capable of communicating with the cells and promoting specific cellular responses, as the ECM does, has appeared particularly promising, in view to favor the formation of a tissue equivalent to the cardiac one.

Development of Biomimetic Alginate/Gelatin/Elastin Sponges with Recognition Properties toward Bioactive Peptides for Cardiac Tissue Engineering

In recent years, there has been an increasing interest toward the covalent binding of bioactive peptides from extracellular matrix proteins on scaffolds as a promising functionalization strategy in the development of biomimetic matrices for tissue engineering. A totally new approach for scaffold functionalization with peptides is based on Molecular Imprinting technology. In this work, imprinted particles with recognition properties toward laminin and fibronectin bioactive moieties were synthetized and used for the functionalization of biomimetic sponges, which were based on a blend of alginate, gelatin, and elastin. Functionalized sponges underwent a complete morphological, physicochemical, mechanical, functional, and biological characterization. Micrographs of functionalized sponges showed a highly porous structure and a quite homogeneous distribution of imprinted particles on their surface. Infrared and thermal analyses pointed out the presence of interactions between blend components. Biodegradation and mechanical properties appeared adequate for the aimed application. The results of recognition tests showed that the deposition on sponges did not alter the specific recognition and binding behavior of imprinted particles. In vitro biological characterization with cardiac progenitor cells showed that early cell adherence was promoted. In vivo analysis showed that developed scaffolds improved cardiac progenitor cell adhesion and differentiation toward myocardial phenotypes.

Surface chemical immobilization of bioactive peptides on synthetic polymers for cardiac tissue engineering

The aim of this work was the development of new synthetic polymeric systems, functionalized by surface chemical modification with bioactive peptides, for myocardial tissue engineering. Polycaprolactone and a poly(ester-ether-ester) block copolymer synthesized in our lab, polycaprolactone-poly(ethylene oxide)-polycaprolactone (PCL-PEO-PCL), were used as the substrates to be modified. Two pentapeptides, H-Gly-Arg-Gly-Asp-Ser-OH (GRGDS) from fibronectin and H-Tyr-Ile-Gly-Ser-Arg-OH (YIGSR) from laminin, were used for the functionalization. Polymeric membranes were obtained by casting from solutions and then functionalized by means of alkaline hydrolysis and subsequent coupling of the bioactive molecules through 1-(3-dimethylaminopropyl)-3-ethylcarbodimide hydrochloride/N-hydroxysuccinimide chemistry. The hydrolysis conditions, in terms of hydrolysis time, temperature, and sodium hydroxide concentration, were optimized for the two materials. The occurrence of the coupling reaction was demonstrated by infrared spectroscopy, as the presence on the functionalized materials of the absorption peaks typical of the two peptides. The peptide surface density was determined by chromatographic analysis and the distribution was studied by infrared chemical imaging. The results showed a nearly homogeneous peptide distribution, with a density above the minimum value necessary to promote cell adhesion. Preliminary in vitro cell culture studies demonstrated that the introduction of the bioactive molecules had a positive effect on improving C2C12 myoblasts growth on the synthetic materials.

Immobilization of Cell-Adhesive Laminin Peptides in Degradable PEGDA Hydrogels Influences Endothelial Cell Tubulogenesis

Attachment, spreading, and organization of endothelial cells into tubule networks are mediated by interactions between cells in the extracellular microenvironment. Laminins are key extracellular matrix components and regulators of cell adhesion, migration, and proliferation. In this study, laminin-derived peptides were conjugated to poly(ethylene glycol) (PEG) monoacrylate and covalently incorporated into degradable PEG diacrylate (PEGDA) hydrogels to investigate the influence of these peptides on endothelial cellular adhesion and function in organizing into tubule networks. Degradable PEGDA hydrogels were synthesized by incorporating a matrix metalloproteinase (MMP)–sensitive peptide, GGGPQGIWGQGK (abbreviated PQ), into the polymer backbone. The secretion of MMP-2 and MMP-9 by endothelial cells promotes polymer degradation and consequently cell migration. We demonstrate the formation of extensive networks of tubule-like structures by encapsulated human umbilical vein endothelial cells in hydrogels with immobilized synthetic peptides. The resulting structures were stabilized by pericyte precursor cells (10T1/2s) in vitro. During tubule formation and stabilization, extracellular matrix proteins such as collagen IV and laminin were deposited. Tubules formed in the matrix of metalloproteinase sensitive hydrogels were visualized from 7 days to 4 weeks in response to different combination of peptides. Moreover, hydrogels functionalized with laminin peptides and transplanted in a mouse cornea supported the ingrowth and attachment of endothelial cells to the hydrogel during angiogenesis. Results of this study illustrate the use of laminin-derived peptides as potential candidates for modification of biomaterials to support angiogenesis.

Development of a YIGSR-peptide-modified polyurethaneurea to enhance endothelialization

Polyurethanes have been investigated for use as vascular grafts due to their excellent mechanical properties and relatively good biocompatibility. However, poor retention of endothelial cells and thrombogenicity in vivo remain problematic for vascular graft applications. The peptide YIGSR has been shown to increase endothelial cell adhesion but not attachment of platelets, suggesting its possible utility for vascular graft applications. In this study, a bioactive polyurethaneurea has been synthesized by incorporating GGGYIGSRGGGK peptide sequences into the polymer backbone. Successful incorporation of the peptides was confirmed by NMR, contact angle measurement and ESCA. Uniform distribution of peptides on the surface was observed using a fluorescent probe capable of reacting with tyrosine residues on the peptides. Hard segment domains were visualized using tapping mode AFM. Endothelial cell adhesion, spreading, proliferation, migration and extra-cellular matrix production were improved on bioactive polyurethaneurea compared to control polyurethaneurea. Competitive inhibition of endothelial cell attachment and spreading by soluble YIGSR peptides indicated that cell adhesion and spreading were specifically mediated by YIGSR-sensitive cell adhesion receptor, not just by changed surface properties. There was no significant difference in the number of adherent platelets. Therefore, this bioactive polyurethanurea may improve vascular graft endothelialization without increasing thrombogenicity.

RGD peptide-immobilized electrospun matrix of polyurethane for enhanced endothelial cell affinity

An Arg-Gly-Asp (RGD) peptide-immobilized electrospun matrix of polyurethane (PU) was developed for the enhanced affinity of endothelial cells (EC). The novel PU matrix was fabricated as a vascular shape using the electrospinning technique. Then, poly(ethylene glycol) (PEG) was immobilized on the porous PU matrix as a spacer, followed by conjugating RGD peptide to the amino end group of the PEG chain. In the proliferation test of human umbilical vein endothelial cells (HUVEC) on the modified PU matrix, the RGD-immobilized porous matrix showed enhanced viability of HUVEC as compared with an unmodified surface, demonstrating that the presence of RGD peptide promoted HUVEC proliferation. In addition, the RGD-immobilized PU porous matrix revealed higher cell viability than the RGD-immobilized PU film because of the porous structure with higher surface area, indicating an advantageous property of the porous matrix for HUVEC proliferation.

A biomimetic peptide fluorosurfactant polymer for endothelialization of ePTFE with limited platelet adhesion

Endothelialization of expanded polytetrafluoroethylene (ePTFE) has the potential to improve long-term patency for small-diameter vascular grafts. Successful endothelialization requires ePTFE surface modification to permit cell attachment to this otherwise non-adhesive substrate. We report here on a peptide fluorosurfactant polymer (FSP) biomimetic construct that promotes endothelial cell (EC)-selective attachment, growth, shear stability, and function on ePTFE. The peptide FSP consists of a flexible poly(vinyl amine) backbone with EC-selective peptide ligands for specific cell adhesion and pendant fluorocarbon branches for stable anchorage to underlying ePTFE. The EC-selective peptide (primary sequence: Cys-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys, CRRETAWAC) has demonstrated high binding affinity for the alpha(5)beta(1) integrin found on ECs. Here, we demonstrate low affinity of CRRETAWAC for platelets and platelet integrins, thus providing it with EC-selectivity. This EC-selectivity could potentially facilitate rapid in vivo endothelialization and healing without thrombosis for small-diameter ePTFE vascular grafts.

The effect of RGD fluorosurfactant polymer modification of ePTFE on endothelial cell adhesion, growth, and function

We have synthesized and characterized a novel peptide fluorosurfactant polymer (PFSP) modification that facilitates the adhesion and growth of endothelial cells on expanded polytetrafluoroetheylene (ePTFE) vascular graft material. This PFSP consists of a poly(vinyl amine) (PVAm) backbone with integrin binding Arg-Gly-Asp (RGD) peptides and perfluorocarbon pendant branches for adsorption and stable adhesion to underlying ePTFE. Aqueous PFSP solution was used to modify the surface of fluorocarbon substrates. Following subconfluent seeding, endothelial cell (EC) adhesion and growth on PFSP was assessed by determining cell population at different time points. Spectroscopic results indicated successful synthesis of PFSP. PFSP modification of ePTFE reduced the receding water contact angle measurement from 120 degrees to 6 degrees , indicating successful surface modification. Quantification of cell population demonstrated reduced EC attachment efficiency but increased growth rate on RGD PFSP compared with fibronectin (FN). Actin staining revealed a well-developed cytoskeleton for ECs on RGD PFSP indicative of stable adhesion. Uptake of acetylated low-density lipoprotein and positive staining for VE-Cadherin confirm EC phenotype for adherent cells. Production of prostacyclin, a potent antiplatelet agent, was equivalent between ECs on FN and RGD PFSP surfaces. Our results indicate successful synthesis and surface modification with PFSP; this is a simple, quantitative, and effective approach to modifying ePTFE to encourage endothelial cell attachment, growth, and function.

Osteogenic differentiation of rat bone marrow stromal cells cultured on Arg–Gly–Asp modified hydrogels without dexamethasone and β-glycerol phosphate

In this study, we investigated the effect of signaling peptides incorporated into oligo(poly(ethylene glycol) fumarate) (OPF) hydrogels on in vitro differentiation and mineralization of marrow stromal cells (MSCs) cultured in media without soluble osteogenic supplements (dexamethasone and beta-glycerol phosphate). When MSCs were cultured for 16 days on OPF hydrogels modified with Arg-Gly-Asp (RGD) containing peptides, the normalized cell number was dependent on the peptide concentration between days 0 and 5 and reached comparable values at day 10 regardless of the concentration. The alkaline phosphatase (ALP) activity of MSCs on the peptide-modified OPF hydrogels was also concentration-dependent: ALP activity showed peaks on day 10 or day 13 on OPF hydrogels modified with 2.0 and 1.0 micromol peptide/g, which were significantly greater than those on the OPF hydrogels modified with 0.1 micromol peptides/g or no peptide. A characteristic marker of osteoblastic differentiation, osteopontin (OPN), was detected for all the test groups. However, OPN secretion between days 0 and 10 was significantly higher on the peptide modified hydrogels compared to that on tissue culture-treated polystyrene. Taken together, the results indicate that the presence of signaling peptide allows for a favorable microenvironment for MSCs to differentiate into osteoblasts and produce mineralized matrix, although the soluble factors may further enhance calcium deposition. These findings further support the usefulness of OPF hydrogels as scaffolds for guided bone regeneration, and represent an initial step in exploring the complex relationship between soluble and insoluble factors in osteogenic differentiation on biodegradable materials.

Human microvascular endothelial cell growth and migration on biomimetic surfactant polymers

Successful engineering of a tissue-incorporated vascular prosthesis requires cells to proliferate and migrate on the scaffold. Here, we report on a series of "ECM-like" biomimetic surfactant polymers that exhibit quantitative control over the proliferation and migrational properties of human microvascular endothelial cells (HMVEC). The biomimetic polymers consist of a poly(vinyl amine) (PVAm) backbone with hexanal branches and varying ratios of cell binding peptide (RGD) to carbohydrate (maltose). Proliferation and migration behavior of HMVEC was investigated using polymers containing RGD: maltose ratios of 100:0, 75:25 and 50:50, and compared with fibronectin (FN) coated glass (1 microg/cm2). A radial Teflon fence migration assay was used to examine HMVEC migration at 12 h intervals over a 48 h period. Migration was quantified using an inverted optical microscope, and HMVEC were examined by confocal microscopy for actin and focal adhesion organization/ arrangement. Over the range of RGD ligand density studied (approximately 0.19-0.6 peptides/nm2), our results show HMVEC migration decreases with increasing RGD density in the polymer. HMVEC were least motile on the 100% RGD polymer (approximately 0.38-0.6 peptides/nm2) with an average migration of 0.20 mm2/h in area covered, whereas HMVEC showed the fastest migration of 0.48+/-0.06 mm2/h on the 50% RGD surface ( approximately 0.19-0.30 peptides/nm2). In contrast, cell proliferation increased with increasing surface peptide density; proliferation on the 50% RGD surface was 1.5%+/-0.06/h compared with 2.2%+/-0.07/h on the 100% RGD surface. Our results show that surface peptide density affects cellular functions such as growth and migration, with the highest peptide density supporting the most proliferation but the slowest migration.

Osteoblast-like cell adhesion to bone sialoprotein peptides

A number of studies have suggested that biomimetic peptides can be used in the design of a new generation of prosthetic implants to promote the successful biointegration of the implant materials. In the current study, the in vitro bioactivities of several peptides representing RGD (Arg-Gly-Asp)-containing sequences of bone sialoprotein (BSP) toward an osteoblast-like cell line (MC3T3-E1) were examined to provide insight into the molecular basis of BSP's interaction with bone cells. BSP residues 283-288, 281-290, 278-293 and 278-302 were coated on polystyrene surfaces in 96-well non-tissue (untreated) culture plates, and their osteoblast adhesive properties compared to intact BSP and fibronectin as positive controls. BSP peptides 278-302 and 278-293 were found to be the most potent in their adhesive activity, increasing the number of adherent cells to 350% of control levels at an added concentration of 1 microM. Since these two peptides were equivalent in potency, it is suggested that the region 294-302 beyond the RGD domain is not necessary for cell binding. In comparison, peptides 283-288 and 281-290 were only active at concentrations greater than 200 microM. 50-70% of the peptide-stimulated adhesion was inhibited by the pretreatment of cell suspensions with solution phase RGD, suggesting that a portion of the peptides' adhesive effects was specific and integrin-mediated, although other non-RGD flanking regions were probably also involved in the mechanism of adhesion. Importantly, a modified BSP peptide, in which an aspartic acid residue at position 288 of the RGD sequence was replaced by a glutamic acid residue to form RGE, was completely inactive as a cell adhesion stimulus at concentrations up to 200 microM. Thus, despite the potential role of non-RGD flanking regions, an intact RGD tripeptide was essential for all of the adhesive activity of the BSP peptides.

Modification of polyurethaneurea with PEG and YIGSR peptide to enhance endothelialization without platelet adhesion

Improved endothelialization without platelet adhesion is essential to enhance the long-term patency of synthetic vascular grafts and other blood-contacting devices. We have developed a dually modified polyurethaneurea by incorporating endothelial cell adhesive YIGSR peptide sequences as chain extenders and nonthrombogenic PEG as a soft segment (PUUYIGSR-PEG) in the polymer backbone. PUUYIGSR-PEG was successfully synthesized and characterized by proton NMR, FTIR, GPC, DSC, ESCA, and contact angle measurement. Despite having similar molecular weight, the peptide/PEG-modified polyurethaneurea (PUUYIGSR-PEG) showed superior mechanical properties compared to the control PEG-modified polyurethaneurea (PUUPPD-PEG). Virtually no platelet adhesion was observed on PUUYIGSR-PEG, while endothelial cell adhesion, spreading, and migration were significantly greater on PUUYIGSR-PEG compared to PUUPPD-PEG. Thus, this bioactive polymer may be an appropriate biomaterial for small diameter vascular grafts.

Biomimetic materials for tissue engineering

The development of biomaterials for tissue engineering applications has recently focused on the design of biomimetic materials that are capable of eliciting specific cellular responses and directing new tissue formation mediated by biomolecular recognition, which can be manipulated by altering design parameters of the material. Biomolecular recognition of materials by cells has been achieved by surface and bulk modification of biomaterials via chemical or physical methods with bioactive molecules such as a native long chain of extracellular matrix (ECM) proteins as well as short peptide sequences derived from intact ECM proteins that can incur specific interactions with cell receptors. The biomimetic materials potentially mimic many roles of ECM in tissues. For example, biomimetic scaffolds can provide biological cues for cell-matrix interactions to promote tissue growth, and the incorporation of peptide sequences into materials can also make the material degradable by specific protease enzymes. This review discusses the surface and bulk modification of biomaterials with cell recognition molecules to design biomimetic materials for tissue engineering. The criteria to design biomimetic materials such as the concentration and spatial distribution of modified bioactive molecules are addressed. Recent advances for the development of biomimetic materials in bone, nerve, and cardiovascular tissue engineering are also summarized.

Cell adhesion peptides alter smooth muscle cell adhesion, proliferation, migration, and matrix protein synthesis on modified surfaces and in polymer scaffolds

The effects of cell adhesion peptides (RGDS, KQAGDV, VAPG) on vascular smooth muscle cells grown on modified surfaces and in tissue-engineering scaffolds were examined. Cells were more strongly adhered to surfaces modified with adhesive ligands than to control surfaces (no ligand or a nonadhesive ligand). Cell migration was higher on surfaces with 0.2 nmol/cm(2) of adhesive ligand than on control surfaces, but it was lower on surfaces with 2.0 nmol/cm(2) of adhesive ligand than it was on control surfaces. Further, cell proliferation was lower on adhesive surfaces than it was on control surfaces, and it decreased as the ligand density increased. Similarly, in the peptide-grafted hydrogel scaffolds, cell proliferation was lower in scaffolds containing the adhesive peptides than it was in control scaffolds. After 7 days of culture, more collagen per cell was produced in control scaffolds than in scaffolds containing adhesive peptides. In addition, collagen production decreased in the scaffolds as the ligand concentration increased. While modification of a surface or scaffold material with adhesive ligands initially increases cell attachment, it may be necessary to optimize cell adhesion simultaneously with proliferation, migration, and matrix production.

Tethered-TGF-beta increases extracellular matrix production of vascular smooth muscle cells

Biomaterials developed for tissue engineering and wound healing applications need to support robust cell adhesion, yet also need to be replaced by new tissue synthesized by those cells. In order to maintain mechanical integrity of the tissue, the cells must generate sufficient extracellular matrix before the scaffold is degraded. We have previously shown that materials containing cell adhesive ligands to promote or improve cell adhesion can decrease extracellular matrix production (Mann et al., Modification of surfaces with cell adhesion peptides alters extracellular matrix deposition. Biomaterials 1999;20:2281-6). Such decreased matrix production by cells in tissue engineering scaffolds may result in tissue failure. However, we have found that TGF-beta1 can be used in scaffolds to dramatically increase matrix production. Matrix production by vascular smooth muscle cells grown on adhesive ligand-modified glass surfaces and in PEG hydrogels containing covalently bound adhesive ligands was increased in the presence of 0.04 pmol/ml (1 ng/ml) TGF-beta1. TGF-beta1 can counteract the effect of these adhesive ligands on matrix production; matrix production could be increased even above that observed in the absence of adhesive peptides. Further, TGF-beta1 covalently immobilized to PEG retained its ability to increase matrix production. Tethering TGF-beta1 to the polymer scaffold resulted in a significant increase in matrix production over the same amount of soluble TGF-beta1.