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

Development of Phase-Separating Microfiber Network Hydrogels to Promote In Vitro Vascularization

Engineered vascularized tissues in vitro exhibit the potential for transplantation therapy and disease modeling. Despite efforts to design hydrogels as cell culture platforms for in vitro vascularization, development of vascularized tissues recapitulating the natural structures and functions remains difficult due to a poor understanding of the relationships between the matrix microstructures and tube formation of endothelial cells. Herein, we developed microfiber network hydrogels with microporous structures by controlling the liquid-liquid phase separation (LLPS) of proteins and matrix structures in hydrogels. Extracellular matrix protein gelatin was modified with hydrogen-bonding moieties and mixed with hyaluronic acid sodium salt to form microfiber network structures. Gelatin gelation and hyaluronic acid sodium salt dissolution led to the formation of a microporous microfiber network hydrogel formation. Matrix structures of hydrogels were modified by controlling LLPS that affects endothelial cell tube formation. Vascularization was improved using laminin peptides and coculturing with mesenchymal stem cells. Overall, our approach exhibits the potential to induce in vitro vascularization for regenerative medicine and disease modeling applications.

Bioactive Polyurethane-Poly(ethylene Glycol) Diacrylate Hydrogels for Applications in Tissue Engineering

Polyurethanes (PUs) are a highly adaptable class of biomaterials that are among some of the most researched materials for various biomedical applications. However, engineered tissue scaffolds composed of PU have not found their way into clinical application, mainly due to the difficulty of balancing the control of material properties with the desired cellular response. A simple method for the synthesis of tunable bioactive poly(ethylene glycol) diacrylate (PEGDA) hydrogels containing photocurable PU is described. These hydrogels may be modified with PEGylated peptides or proteins to impart variable biological functions, and the mechanical properties of the hydrogels can be tuned based on the ratios of PU and PEGDA. Studies with human cells revealed that PU-PEG blended hydrogels support cell adhesion and viability when cell adhesion peptides are crosslinked within the hydrogel matrix. These hydrogels represent a unique and highly tailorable system for synthesizing PU-based synthetic extracellular matrices for tissue engineering applications.

Treatment with YIGSR peptide ameliorates mouse tail lymphedema by 67 kDa laminin receptor (67LR)-dependent cell-cell adhesion

Impaired microcirculation can cause lymphatic leakage which leads to a chronic swelling in the tissues of the body. However, no successful treatment gives any protection against lymphedema due to the lack of well-revealed pathophysiology of secondary lymphedema. Binary image of laminin immunohistochemical expression revealed that distribution of laminin expression localized during surgically induced lymphedema. 67 kDa laminin receptor (67LR) mRNA expression showed a peak at during lymphedema exacerbation. Since the response of 67LR molecules may affect the prevention of inflammation and edema, here we have hypothesized that 67LR ligand of YIGSR peptide could permit reconstructive environment for amelioration of lymphedema and evaluated the effect of YIGSR in a mouse tail model of lymphedema. Indeed, intra-abdominal injections of YIGSR for the first 3 days after inducing lymphedema in the mouse tail model reduced the tail lymphedema on day 14 by 27% (P = 0.035). Histology showed that YIGSR treatment protected lymphedema impairment in epidermis and dermis, and it also inhibited the expansion of intercellular spaces and enhanced especially cell adhesion in the basement membrane as revealed by transmission electron microscopy. Interestingly, the treatment also reduced the local expression of transforming growth factor (TGF)β. Further elucidation of the mechanisms of 67LR-facilitated lymphangiogenesis contributes to find potential targets for the treatment of lymphedema.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

Role of functionalized self-assembled peptide hydrogels in in vitro vasculogenesis

Fabrication of vascularized tissue constructs plays an integral role in creating clinically relevant tissues. Scaffold materials should be sufficiently vascularized to mimic functional and complex native tissues. Herein, we report the development of bioactive and biomimetic self-assembled peptide (SAP) hydrogels that allow the rapid formation of a vascular structure in vitro. The KLDLKLDLKLDL (KLD peptide) SAP was functionalized with laminin derived peptides IKVAV (V1) and YIGSR (V2) through direct coupling to mimic the natural extracellular matrix (ECM) and human umbilical endothelial cells (HUVECs) and mesenchymal stem cells (MSCs) cultured in 0.5% and 1% SAP hydrogels organized into vascularized structures. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images proved the molecular integration of the nanofibrous structure in SAP hydrogels. The stability of SAP hydrogels was confirmed by rheological and degradation measurements. Bioactive peptide scaffolds enhanced significantly HUVEC/hMSC proliferation depicted by MTT analysis compared to KLD. Furthermore, the real time quantitative polymerase chain reaction (rt-PCR) was performed to analyse vascular gene expressions such as platelet/endothelial cell adhesion molecule-1 (PECAM-1), von Willebrand factor (vWF), and vascular endothelial cadherin (VE-cadherin). The results indicated that the KLD-V2 hydrogel significantly induced vasculogenesis in hMSC/HUVEC co-culture compared to KLD-V1, Biogelx and KLD because YIGSR in KLD-V2 promoted cell population and ECM secretion by the interaction with cells and increased vasculogenesis. Overall, the designed SAP hydrogel represents an effective scaffold for vascularization of tissue constructs with useful tissue engineering applications.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

Poly(ethylene glycol)-crosslinked gelatin hydrogel substrates with conjugated bioactive peptides influence endothelial cell behavior

The basement membrane is a specialized extracellular matrix substrate responsible for support and maintenance of epithelial and endothelial structures. Engineered basement membrane-like hydrogel systems have the potential to advance understanding of cell-cell and cell-matrix interactions by allowing precise tuning of the substrate or matrix biochemical and biophysical properties. In this investigation, we developed tunable hydrogel substrates with conjugated bioactive peptides to modulate cell binding and growth factor signaling by endothelial cells. Hydrogels were formed by employing a poly(ethylene glycol) crosslinker to covalently crosslink gelatin polymers and simultaneously conjugate laminin-derived YIGSR peptides or vascular endothelial growth factor (VEGF)-mimetic QK peptides to the gelatin. Rheological characterization revealed rapid formation of hydrogels with similar stiffnesses across tested formulations, and swelling analysis demonstrated dependency on peptide and crosslinker concentrations in hydrogels. Levels of phosphorylated VEGF Receptor 2 in cells cultured on hydrogel substrates revealed that while human umbilical vein endothelial cells (HUVECs) responded to both soluble and conjugated forms of the QK peptide, conditionally-immortalized human glomerular endothelial cells (GEnCs) only responded to the conjugated presentation of the peptide. Furthermore, whereas HUVECs exhibited greatest upregulation in gene expression when cultured on YIGSR- and QK-conjugated hydrogel substrates after 5 days, GEnCs exhibited greatest upregulation when cultured on Matrigel control substrates at the same time point. These results indicate that conjugation of bioactive peptides to these hydrogel substrates significantly influenced endothelial cell behavior in cultures but with differential responses between HUVECs and GEnCs.

A Nitric Oxide-Releasing Self-Assembled Peptide Amphiphile Nanomatrix for Improving the Biocompatibility of Microporous Hollow Fibers

Oxygenators are critical components of extracorporeal circuits used frequently in cardiopulmonary bypass and intensive care, but platelet activation and induction of a complex inflammatory response are usually observed with their use. To improve the biocompatibility of oxygenators, we developed a nitric oxide (NO)-releasing, self-assembled peptide amphiphile nanomatrix. The nanomatrix formed a homogenous coating over the microporous hollow fibers as demonstrated by scanning electron microscopy. We quantitated platelet adhesion to the artificial fibers by measuring absorbance/area of platelets (Abs/A; nm/m2) using acid phosphatase assay. There was a 17-fold decrease in platelet adhesion to the nanomatrix (Abs/A = 0.125) compared with collagen controls (Abs/A = 2.07; p < 0.05) and a 22-fold decrease compared with uncoated fibers (Abs/A = 2.75; p < 0.05). Importantly, the nanomatrix coating did not impede oxygen transfer in water through coated fiber modules (p > 0.05) in a benchtop test circuit at different flow rates as estimated by change in partial pressure of oxygen in relation to water velocity through fibers. These findings demonstrate the feasibility of coating microporous hollow fibers with a NO-releasing self-assembled amphiphile nanomatrix that may improve the biocompatibility of the hollow fibers without affecting their gas exchange capacity.

Surface modification and endothelialization of biomaterials as potential scaffolds for vascular tissue engineering applications

This review highlights the recent developments of surface modification and endothelialization of biomaterials in vascular tissue engineering applications.

Biomimetic materials for medical application through enzymatic modification

Living organisms synthesize functional materials, based on proteins and polysaccharides, using enzyme-catalyzed reactions. According to the biomimetic approach, biomaterial matrices for tissue engineering are designed to be able to mimic the properties and the functions of the extracellular matrix (ECM). In this chapter, the most significant research efforts dedicated to the study and the preparation of biomimetic materials through enzymatic modifications were reviewed. The functionalizations of different polymeric matrices obtained through the catalytic activity of two enzymes (Transglutaminase, TGase and Tyrosinase, TYRase) were discussed. Specifically, the biomimetic applications of TGase and TYRase to confer appropriate biomimetic properties to the biomaterials, such as the possibility to obtain in situ gelling hydrogels and the incorporation of bioactive molecules (growth factors) and cell-binding peptides into the scaffolds, were reviewed.

Biocompatibility of helicoidal multilamellar arginine-glycine-aspartic acid-functionalized silk biomaterials in a rabbit corneal model

Silk proteins represent a unique choice in the selection of biomaterials that can be used for corneal tissue engineering and regenerative medical applications. We implanted helicoidal multilamellar arginine-glycine-aspartic acid-functionalized silk biomaterials into the corneal stroma of rabbits, and evaluated its biocompatibility. The corneal tissue was examined after routine hematoxylin-eosin staining, immunofluorescence for collagen I and III, and fibronectin, and scanning electron microscopy. The silk films maintained their integrity and transparency over the 180-day experimental period without causing immunogenic and neovascular responses or degradation of the rabbit corneal stroma. Collagen I increased, whereas Collagen III and fibronectin initially increased and then gradually decreased. The extracellular matrix deposited on the surface of the silk films, tightly adhered to the biomaterial. We have shown this kind of silk film graft has suitable biocompatibility with the corneal stroma and is an initial step for clinical trials to evaluate this material as a transplant biomaterial for keratoplasty tissue constructs.

Peptide-grafted poly(ethylene glycol) hydrogels support dynamic adhesion of endothelial progenitor cells

This study investigated the dynamic adhesion of endothelial progenitor cells (EPCs) to peptide-grafted poly(ethylene glycol) diacrylate (PEGDA) hydrogels and determined the relative ability of RGDS, REDV and YIGSRG peptides to reduce the velocity of EPC rolling. Circulating EPCs are key mediators of endothelium repair and have been shown to accelerate re-endothelialization, which is important in reducing the incidence of restenosis following stent placement and occlusion of small diameter vascular grafts. However, to exploit these capabilities for tissue engineering applications, more knowledge is needed about EPC binding to the vascular wall under shear and, in particular, whether the incorporation of peptide ligands into biomaterials can support the process of EPC rolling or maintain EPC adhesion. This study specifically examined one type of EPCs endothelial colony forming cells (ECFCs), based on their ability to be expanded in culture and differentiate into mature endothelial cells. The amount of grafted PEG-peptide was shown to be dependent on the concentration of PEG-peptide grafting solution photopolymerized onto the hydrogel surface. The ECFC strength of adhesion on PEG-RDGS grafted hydrogels exceeded 350 dyn cm(-2) for 85% of adherent cells. PEG-RGDS grafted hydrogels supported ECFC rolling, whereas ECFC velocity on the negative control PEG-RGES grafted hydrogels and on the "blank slate" PEGDA hydrogels was substantially higher than the cutoff velocity for cell rolling. The ECFC rolling velocity on PEG-RDGS grafted hydrogels depended on the shear rate; as shear rate was increased from 20 s(-1) to 120 s(-1), ECFC rolling velocity increased from 103±3 μm s(-1) to 741±28 μm s(-1). REDV and YIGSRG, which are known to preferentially support endothelial cell adhesion, also supported ECFC rolling. Interestingly, the rolling velocity of ECFCs on PEG-REDV grafted hydrogels was significantly lower than on PEG-YIGSRG or on PEG-RGDS grafted hydrogels. Understanding the dynamic adhesion of ECFCs to peptide-grafted hydrogels is the first step towards understanding the similarities and differences of EPCs from mature endothelial cells and improving the ability to sequester EPCs to biomaterial surfaces in order to promote intravascular re-endothelialization.

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.

Chitosan-functionalized silk fibroin 3D scaffold for keratocyte culture

The goal of this study was to evaluate the potential suitability of an artificial membrane composed of silk fibroin (SF) functionalized by different ratios of chitosan (CS) as a substrate for the stroma of the cornea. Keratocytes were cultured on translucent membranes made of SF and CS with different ratios. The biophysical properties of the silk fibroin and chitosan (SF/CS) membrane were examined. The SF/CS showed tensile strengths that increased as the CS concentration increased, but the physical and mechanical properties of chitosan-functionalized silk fibroin scaffolds weakened significantly compared with those of native corneas. The resulting cell scaffolds were evaluated using western blot in addition to light and electron microscopy. The cell attachment and proliferation on the scaffold were similar to those on a plastic plate. Keratocytes cultured in serum on SF/CS exhibited stellate morphology along with a marked increase in the expression of keratocan compared with identical cultures on tissue culture plastics. The biocompatibility was tested by transplanting the acellular membrane into rabbit corneal stromal pockets. There was no inflammatory complication detected at any time point on the macroscopic level. Taken together, these results indicate that SF/CS holds promise as a substrate for corneal reconstruction.

Improvement in endothelial cell adhesion and retention under physiological shear stress using a laminin-apatite composite layer on titanium

Apatite (Ap), laminin-apatite composite (L5Ap, L10Ap, L20Ap and L40Ap) and albumin-apatite (AlbAp) composite layers were prepared on titanium (Ti) using a supersaturated calcium phosphate solution supplemented with laminin (0, 5, 10, 20 and 40 μg ml(-1)) or albumin (800 μg ml(-1)). With an increase in the concentrations of laminin in the supersaturated calcium phosphate solutions, the amounts of laminin immobilized on the Ti increased. The number of human umbilical vein endothelial cells (HUVECs) adhered to the laminin-apatite composite layers were remarkably higher than those to the untreated Ti, Ap layer and AlbAp composite layer. The number of cells adhered to the L40Ap was 4.3 times the untreated Ti. Moreover, cells adhered to the laminin-apatite composite layers showed significantly higher cell retention under the physiological shear stress for 1 h and 2 h than those to the untreated Ti, Ap layer and AlbAp composite layer. The number of cells remaining on the L40Ap under the physiological shear stress for 2 h was 9.5 times that of the untreated Ti. The laminin-apatite composite layer is a promising interfacial layer for endothelialization of blood-contacting materials.

In vitro and in vivo studies of ePTFE vascular grafts treated with P15 peptide

The purpose of this study is to evaluate the effectiveness of P15 cell-binding peptide treated ePTFE vascular grafts in vitro and in vivo. The P15 peptide was covalently immobilized onto ePTFE vascular grafts by an atmospheric plasma coating method. In vitro cell growth properties were studied using primary human umbilical vein endothelial cells (HUVECs) and primary human umbilical artery smooth muscle cells (HUASMCs). X-ray photoelectron spectroscopy and amino-acid analysis were used to analyze the surface characteristics of the peptide treated and untreated grafts. The cell growth study showed that the P15 peptide effectively promoted the adhesion and proliferation of endothelial cells. 700% more endothelial cells were proliferated on the P15-treated ePTFE grafts compared to the untreated ePTFE controls. In contrast, the P15 peptide was significantly less effective for promoting the adhesion and proliferation of smooth muscle cells than endothelial cells; only about 100% more smooth muscle cells proliferated on the P15-treated samples compared to the untreated control samples. The sheep model was used in the in vivo study. The amount of neointimal hyperplasia present at the arterial and venous sides of the anastomosis and the degree of endothelialization on the luminal surface of the grafts were assessed. Four P15-treated grafts and two control grafts were implanted as arteriovenous grafts between the femoral artery and vein or the carotid artery and jugular vein in two sheep (n = 6). The in vivo study showed that the thickness of the neointimal hyperplasia of untreated grafts was 3-times thicker than that of P15-treated grafts (P < 0.05) at the venous side of the anastomosis. P15-treated grafts also had a higher degree of endothelialization on the graft lumen.

Adhesion and growth of human umbilical vein endothelial cells on collagen-treated PU/PEGDA IPNs

For ideal non-thrombogenicity under normal physiologic conditions, we propose endothelialization. Endothelialization means that synthetic biomaterials are seeded by endothelial cells to mimic natural blood vessels. In our study, we synthesized amphiphilic polyurethane (PU)/poly(ethyleneglycol)diacrylate (PEGDA) interpenetrating polymer networks (IPNs) with different levels of surface energy to investigate the effect of adhesion and the growth of human umbilical vein endothelial cells (HUVECs). Collagen with cell-binding molecules was adsorbed on the surface of PU/PEGDA IPNs to enhance the adhesion of HUVECs. The morphology of collagens adsorbed on the IPN surfaces depends highly on the surface energy of the IPNs. As the surface becomes hydrophilic, there is greater aggregation of the adsorbed collagens on the IPN surface. The HUVECs successfully adhere to the collagen-immobilized IPN surface. The morphology of the endothelial cells (ECs) that adhere to IPN 2k-C and IPN 2k after 1 day and after 3 days incubation shows that ECs were successfully spread. The adhesion and the proliferation of ECs increase on non-treated IPN surfaces as the hydrophobicity of the IPNs increases. The surface energy of IPN 2k-C is suitable for the adhesion and proliferation of ECs. Therefore, platelet adhesion is significantly reduced on the EC-hybridized surface of IPNs.

Biomimetic modification of metallic cardiovascular biomaterials: from function mimicking to endothelialization in vivo

Biosystem-surface interactions play an important role in various biological events and determine the ultimate functionality of implanted devices. Endothelialization or mimicking of endothelium on the surface of cardiovascular materials is a promising way to solve the problems of material-induced thrombosis and restenosis. Meanwhile, a multifunctional surface design is needed as antithrombotic properties should be considered in the period when the implants are not yet completely endothelialized. In this article, we summarize some successful approaches used in our laboratory for constructing multifunctional endothelium-like surfaces on metallic cardiovascular biomaterials through chemical modification of the surface or by the introduction of specific biological molecules to induce self-endothelialization in vivo. Some directions on future research in these areas are also presented.

Multifactorial optimization of endothelial cell growth using modular synthetic extracellular matrices

Extracellular matrices (ECMs) are complex materials, containing at least dozens of different macromolecules that are assembled together, thus complicating their optimization towards applications in 3D cell culture or tissue engineering. The natural complexity of ECMs has limited cell-matrix investigations predominantly to experiments where only one matrix component is adjusted at a time, making it difficult to uncover interactions between different matrix components or to efficiently determine optimal matrix compositions for specific desired biological responses. Here we have developed modular synthetic ECMs based on peptide self-assembly whose incorporation of multiple different peptide ligands can be adjusted. The peptides can co-assemble in a wide range of combinations to form hydrogels of uniform morphology and consistent mechanical properties, but with precisely varied mixtures of peptide ligands. The modularity of this system in turn enabled multi-factorial experimental designs for investigating interactions between these ligands and for determining a multi-peptide matrix formulation that maximized endothelial cell growth. In cultures of HUVECs, we observed a previously unknown antagonistic interaction between the laminin-derived peptide YIGSR and RGDS-mediated cell attachment and growth. We also identified an optimized combination of self-assembled peptides bearing the ligands RGDS and IKVAV that led to endothelial cell growth equivalent to that on native full-length fibronectin. Both of these findings would have been challenging to uncover using more traditional one-factor-at-a-time analyses.

Helicoidal multi-lamellar features of RGD-functionalized silk biomaterials for corneal tissue engineering

RGD-coupled silk protein-biomaterial lamellar systems were prepared and studied with human cornea fibroblasts (hCFs) to match functional requirements. A strategy for corneal tissue engineering was pursued to replicate the structural hierarchy of human corneal stroma within thin stacks of lamellae-like tissues, in this case constructed from scaffolds constructed with RGD-coupled, patterned, porous, mechanically robust and transparent silk films. The influence of RGD-coupling on the orientation, proliferation, ECM organization, and gene expression of hCFs was assessed. RGD surface modification enhanced cell attachment, proliferation, alignment and expression of both collagens (type I and V) and proteoglycans (decorin and biglycan). Confocal and histological images of the lamellar systems revealed that the bio-functionalized silk human cornea 3D constructs exhibited integrated corneal stroma tissue with helicoidal multi-lamellar alignment of collagen-rich and proteoglycan-rich extracellular matrix, with transparency of the construct. This biomimetic approach to replicate corneal stromal tissue structural hierarchy and architecture demonstrates a useful strategy for engineering human cornea. Further, this approach can be exploited for other tissue systems due to the pervasive nature of such helicoids in most human tissues.

Biomimeticity in tissue engineering scaffolds through synthetic peptide modifications-altering chemistry for enhanced biological response

Biomimetic and bioactive biomaterials are desirable as tissue engineering scaffolds by virtue of their capability to mimic natural environments of the extracellular matrix. Biomimeticity has been achieved by the incorporation of synthetic short peptide sequences into suitable materials either by surface modification or by bulk incorporation. Research in this area has identified several novel synthetic peptide segments, some of them with cell-specific interactions, which may serve as potential candidates for use in explicit tissue applications. This review focuses on the developments and prospective directions of incorporating short synthetic peptide sequences onto scaffolds for tissue engineering, with emphasis on the chemistry of peptide immobilization and subsequent cell responses toward modified scaffolds. The article provides a decision-tree-type flow chart indicating the most probable cellular events on a given peptide-modified scaffold along with the consolidated list of synthetic peptide sequences, supports as well as cell types used in various tissue engineering studies, and aims to serve as a quick reference guide to peptide chemists and material scientists interested in the field.

Effect of endothelium mimicking self-assembled nanomatrices on cell adhesion and spreading of human endothelial cells and smooth muscle cells

The goal of this study is to develop unique native endothelium mimicking nanomatrices and evaluate their effects on adhesion and spreading of human umbilical vein endothelial cells (HUVECs) and aortic smooth muscle cells (AoSMCs). These nanomatrices were developed by self-assembly of peptide amphiphiles (PAs) through a solvent evaporation technique. Three PAs, one containing the Tyr-Ile-Gly-Ser-Arg (YIGSR) ligand, the second containing the Val-Ala-Pro-Gly (VAPG) ligand, and a third without cell adhesive ligands, were developed. Cell adhesion and spreading were evaluated by a PicoGreen-DNA assay and live/dead assay, respectively. Our results show that PA-YIGSR significantly enhances HUVEC adhesion (26,704 +/- 2708), spreading (84 +/- 8%), and proliferation (50 +/- 2%) compared with that of other PAs. PA-VAPG and PA-YIGSR showed significantly greater AoSMC adhesion compared with that of PA-S. PA-VAPG also showed significantly greater spreading of AoSMCs (63 +/- 11%) compared with that of other PAs. Also, all the PAs showed significantly reduced platelet adhesion compared with that of collagen I (control). These findings would facilitate the development of novel vascular grafts, heart valves, and cell-based therapies for cardiovascular diseases.

FROM THE CLINICAL EDITOR

The goal of this study was to develop unique native endothelium mimicking nanomatrices and evaluate their effects on adhesion and spreading of human umbilical vein endothelial cells (HUVECs) and aortic smooth muscle cells (AoSMCs). These nanomatrices were developed by self-assembly of peptide amphiphiles through a solvent evaporation technique. The findings are expected to facilitate the development of novel vascular grafts, heart valves, and cell based therapies for cardiovascular diseases.

Vascular Grafts and the Endothelium

This article discusses the importance of the endothelium for successful vascular grafts derived from both native arteries and synthetic materials. It also discusses the fundamental strategies to endothelialize synthetic grafts in animal experiments and in the clinic, as well as the use of endothelial progenitor cells (EPCs), bone marrow-derived cells, and mesothelium as endothelial substitutes.

A hybrid biomimetic scaffold composed of electrospun polycaprolactone nanofibers and self-assembled peptide amphiphile nanofibers

Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural advantages, but lack of bioactivity has limited their usefulness in biomedical applications. Thus, here we report the development of a hybrid, nanostructured, extracellular matrix (ECM) mimicking scaffold by a combination of ePCL nanofibers and self-assembled peptide amphiphile (PA) nanofibers. The PAs have ECM mimicking characteristics including a cell adhesive ligand (RGDS) and matrix metalloproteinase-2 (MMP-2) mediated degradable sites. Transmission electron microscope imaging verified successful PA self-assembly into nanofibers (diameters of 8-10 nm) using a solvent evaporation method. This evaporation method was then used to successfully coat PAs onto ePCL nanofibers (diameters of 300-400 nm), to develop hybrid, bioactive scaffolds. Scanning electron microscope characterization showed that the PA coatings did not interfere with the porous ePCL nanofiber network. Human mesenchymal stem cells (hMSCs) were seeded onto the hybrid scaffolds to evaluate their bioactivity. Significantly greater attachment and spreading of hMSCs were observed on ePCL nanofibers coated with PA-RGDS as compared to ePCL nanofibers coated with PA-S (no cell adhesive ligand) and uncoated ePCL nanofibers. Overall, this novel strategy presents a new solution to overcome the current bioactivity challenges of electrospun scaffolds and combines the unique characteristics of ePCL nanofibers and self-assembled PA nanofibers to provide an ECM mimicking environment. This has great potential to be applied to many different electrospun scaffolds for various biomedical applications.

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.

The differential effect of scaffold composition and architecture on chondrocyte response to mechanical stimulation

This study aims to explore the differential effect of scaffold composition and architecture on chondrogenic response to dynamic strain stimulation using encapsulating PEG-based hydrogels and primary bovine chondrocytes. Proteins and proteoglycans were conjugated to functionalized poly(ethylene glycol) (PEG) and immobilized in PEG hydrogels to create bio-synthetic materials to be used as scaffolds. Four different compositions were tested, including: PEG-Proteoglycan (PP), PEG-Fibrinogen (PF), PEG-Albumin (PA), and PEG only. Primary articular chondrocytes were encapsulated in the hydrogel scaffolds and subjected to 15% dynamic compressive strain stimulation at 1-Hz frequency for 28 days. Stimulation of PP, PF, PA and PEG constructs resulted in a respective increase in the unconfined true compressive modulus by 32%, 45.4%, 33.6%, and 28.2%, compared to their static controls. The PF showed a significantly larger relative increase in the modulus in comparison to all other scaffolds tested. These results support the hypothesis that mechanical stimulation and material bioactivity have a significant effect on the reported chondrocyte response. Similar trends were observed with the swelling ratio of the constructs. These findings indicate that while stimulation causes metabolic changes in chondrocytes seeded in PEG hydrogels, the matrix bioactivity has a significant role in enhancing chondrocyte mechanotransduction in encapsulating scaffolds subjected to physical deformations.

Surface tailoring of poly(ethylene terephthalate) via ligand-tethered comb-like PEG to enhance endothelialization

The comb-like PEG (CPEG) end-tethered with L-lysine was explored to surface modification of PET to enhance endothelialization. The hydroxyl end groups of CPEG were oxygenated into aldehyde groups. The CPEG-CHO was grafted onto the aminolysized PET. The L-lysine was then end-tethered onto surface via the residual aldehyde groups. The surface modification was confirmed by ATR-FTIR, contact angle and XPS measurements. The endothelial cell adhesion, proliferation and viability results indicated that the PET-CPEG resisted cell adhesion and growth, where as PET-CPEG-lysine promoted cell adhesion and growth. The MTT assay and total cell protein tests indicated that the endothelial cells on PET-CPEG-lysine had high viability. Cell spread uniformly and covered completely on the PET-CPEG-lysine. The CPEG end tethered with L-lysine could regulate cell adhesion and growth and enhance surface endothelialization.

Nanofabrication and Microfabrication of Functional Materials for Tissue Engineering

The burgeoning field of regenerative medicine promises significant progress in the treatment of cardiac ischemia, liver disease, and spinal cord injury. Key to its success will be the ability to engineer tissue safely and reliably. Tissue functionality must be recapitulated in the laboratory and then integrated into surrounding tissue upon transfer to the patient. Scaffolding materials must be chosen such that the microenvironment surrounding the cells is a close analog of the native environment. In the early days of tissue engineering, these materials were largely borrowed from other fields, with much of the focus on biocompatibility and biodegradation. However, attention has shifted recently to cell-cell and cell-surface interactions, largely because of enabling technologies at the nanoscale and microscale. Studies on cellular behavior in response to various stimuli are now easily realized by using microfabrication techniques and devices (e.g., biomedical microelectromechanical systems). These experiments are reproducible and moderate in cost, and often can be accomplished at high throughput, providing the fundamental knowledge required to design biomaterials that closely mimic the biological system. It is our opinion that these novel materials and technologies will bring engineered tissues one step closer to practical application in the clinic. This review discusses their application to cardiac, liver, and nerve tissue engineering.

Nitric oxide-releasing polyurethane–PEG copolymer containing the YIGSR peptide promotes endothelialization with decreased platelet adhesion

Thrombosis and intimal hyperplasia are the principal causes of small-diameter vascular graft failure. To improve the long-term patency of polyurethane vascular grafts, we have incorporated both poly(ethylene glycol) and a diazeniumdiolate nitric oxide (NO) donor into the backbone of polyurethane to improve thromboresistance. Additionally, we have incorporated the laminin-derived cell adhesive peptide sequence YIGSR to encourage endothelial cell adhesion and migration, while NO release encourages endothelial cell proliferation. NO production by polyurethane films under physiological conditions demonstrated biphasic release, in which an initial burst of 70% of the incorporated NO was released within 2 days, followed by sustained release over 2 months. Endothelial cell proliferation in the presence of the NO-releasing material was increased as compared to control polyurethane, and platelet adhesion to polyethylene glycol-containing polyurethane was decreased significantly with the addition of the NO donor.

α2(VIII) Collagen Substrata Enhance Endothelial Cell Retention Under Acute Shear Stress Flow via an α2β1Integrin–Dependent Mechanism

BACKGROUND

Essential to tissue-engineered vascular grafts is the formation of a functional endothelial monolayer capable of resisting the forces of blood flow. This study targeted alpha2(VIII) collagen, a major component of the subendothelial matrix, and examined the ability of and mechanisms by which endothelial cells attach to this collagen under static and dynamic conditions both in vitro and in vivo.

METHODS AND RESULTS

Attachment of human endothelial cells to recombinant alpha2(VIII) collagen was assessed in vitro under static and shear conditions of up to 100 dyne/cm2. Alpha2(VIII) collagen supported endothelial cell attachment in a dose-dependent manner, with an 18-fold higher affinity for endothelial cells compared with fibronectin. Cell attachment was significantly inhibited by function-blocking anti-alpha2 (56%) and -beta1 (98%) integrin antibodies but was not RGD dependent. Under flow, endothelial cells were retained at significantly higher levels on alpha2(VIII) collagen (53% and 51%) than either fibronectin (23% and 16%) or glass substrata (7% and 1%) at shear rates of 30 and 60 dyne/cm2, respectively. In vivo studies, using endothelialized polyurethane grafts, demonstrated significantly higher cell retention rates to alpha2(VIII) collagen-coated than to fibronectin-coated prostheses in the midgraft area (P < 0.05) after 24 hours' implantation in the caprine carotid artery.

CONCLUSIONS

These studies demonstrate that alpha2(VIII) collagen has the potential to improve both initial cell attachment and retention of endothelial cells on vascular grafts in vivo, which opens new avenues of research into the development of single-stage endothelialized prostheses and the next generation of tissue-engineered vascular grafts.

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.

[Conditioning of adolescents--why and how?]

Bioactive polyurethaneurea modified with polyethylene glycol (PEG) and the endothelial cell-adhesive peptide YIGSR was synthesized and fabricated into microporous scaffolds. This material has shown appropriate mechanical properties for vascular graft applications, resists platelet adhesion, and promotes endothelialization. In the current study, microporous scaffolds were formed by a gasfoaming and salt-leaching method. The scaffolds showed highly interconnected open pores throughout the matrices, with porosity of approximately 78% and pore sizes of 20-200 microm. The peptide modified scaffolds showed superior mechanical properties over peptide-free scaffolds (tensile strength, 1.4 +/- 0.03 versus 0.19 +/- 0.01 MPa; p < 0.01). Bovine aortic endothelial cells were seeded on the scaffolds, and cell attachment, proliferation, extracellular matrix production, and migration were investigated. Histological and scanning electron microscopy analysis showed that few cells adhered on peptide-free scaffolds, whereas confluent endothelial cell monolayers formed along the pores in peptide-modified scaffolds. DNA content, hydroxyproline production, and cell migration were also significantly greater in peptide-modified scaffolds.