Cell‐demanded release of VEGF from synthetic, biointeractive cell‐ingrowth matrices for vascularized tissue growth

Defining and designing polymers and hydrogels for neural tissue engineering

The use of biomaterials, such as hydrogels, as neural cell delivery devices is becoming more common in areas of research such as stroke, traumatic brain injury, and spinal cord injury. When reviewing the available research there is some ambiguity in the type of materials used and results are often at odds. This review aims to provide the neuroscience community who may not be familiar with fundamental concepts of hydrogel construction, with basic information that would pertain to neural tissue applications, and to describe the use of hydrogels as cell and drug delivery devices. We will illustrate some of the many tunable properties of hydrogels and the importance of these properties in obtaining reliable and consistent results. It is our hope that this review promotes creative ideas for ways that hydrogels could be adapted and employed for the treatment of a broad range of neurological disorders.

Matrix-Bound VEGF Mimetic Peptides: Design and Endothelial Cell Activation in Collagen Scaffolds

Long term survival and success of artificial tissue constructs depend greatly on vascularization. Endothelial cell (EC) differentiation and vasculature formation are dependent on spatio-temporal cues in the extracellular matrix that dynamically interact with cells, a process difficult to reproduce in artificial systems. Here we present a novel bifunctional peptide that mimics matrix-bound vascular endothelial growth factor (VEGF) and can be used to encode spatially controlled angiogenic signals in collagen scaffolds. The peptide is comprised of a collagen mimetic domain that was previously reported to bind to type I collagen by a unique hybridization mechanism, and a VEGF mimetic domain with pro-angiogenic activity. Circular dichroism and collagen binding studies confirm the triple helical structure and the collagen binding affinity of the collagen mimetic domain, and EC culture studies demonstrate the peptide's ability to induce endothelial cell morphogenesis and network formation as a matrix-bound factor in 2D and 3D collagen scaffolds. We also show spatial modification of collagen substrates with this peptide that allows localized EC activation and network formation. These results demonstrate that the peptide can be used to present spatially directed angiogenic cues in collagen scaffolds, which may be useful for engineering organized microvasculature.

FTY720-loaded poly(DL-lactide-co-glycolide) electrospun scaffold significantly increases microvessel density over 7 days in streptozotocin-induced diabetic C57b16/J mice: preliminary results

BACKGROUND

Nanofiber scaffolds could improve islet transplant success by physically mimicking the shape of extracellular matrix and by acting as a drug-delivery vehicle. Scaffolds implanted in alternate transplant sites must be prevascularized or very quickly vascularized following transplantation to prevent hypoxia-induced islet necrosis. The local release of the S1P prodrug FTY720 induces diameter enlargement and increases in length density. The objective of this preliminary study was to evaluate length and diameter differences between diabetic and nondiabetic animals implanted with FTY720-containing electrospun scaffolds using intravital imaging of dorsal skinfold window chambers.

METHODS

Electrospun mats of randomly oriented fibers we created from polymer solutions of PLAGA (50:50 LA:GA) with and without FTY720 loaded at a ratio of 1:200 (FTY720:PLAGA by wt). The implanted fiber mats were 4 mm in diameter and ∼0.2 mm thick. Increases in length density and vessel diameter were assessed by automated analysis of images over 7 days in RAVE, a Matlab program.

RESULTS

Image analysis of repeated measures of microvessel metrics demonstrated a significant increase in the length density from day 0 to day 7 in the moderately diabetic animals of this preliminary study (P < .05). Furthermore, significant differences in length density at day 0 and day 3 were found between recently STZ-induced moderately diabetic and nondiabetic animals in response to FTY720 local release (P < .05, Student t test).

CONCLUSIONS

Driving the islet revascularization process using local release of factors, such as FTY720, from biodegradable polymers makes an attractive system for the improvement of islet transplant success. Preliminary study results suggest that a recently induced moderately diabetic state may potentiate the mechanism by which local release of FTY720 from polymer fibers increases length density of microvessels. Therefore, local release of S1P receptor-targeted drugs is under further investigation for improvement of transplanted islet function.

Application of microtechnologies for the vascularization of engineered tissues

Recent advances in medicine and healthcare allow people to live longer, increasing the need for the number of organ transplants. However, the number of organ donors has not been able to meet the demand, resulting in an organ shortage. The field of tissue engineering has emerged to produce organs to overcome this limitation. While tissue engineering of connective tissues such as skin and blood vessels have currently reached clinical studies, more complex organs are still far away from commercial availability due to pending challenges with in vitro engineering of 3D tissues. One of the major limitations of engineering large tissue structures is cell death resulting from the inability of nutrients to diffuse across large distances inside a scaffold. This task, carried out by the vasculature inside the body, has largely been described as one of the foremost important challenges in engineering 3D tissues since it remains one of the key steps for both in vitro production of tissue engineered construct and the in vivo integration of a transplanted tissue. This short review highlights the important challenges for vascularization and control of the microcirculatory system within engineered tissues, with particular emphasis on the use of microfabrication approaches.

The matrix-binding domain of microfibril-associated glycoprotein-1 targets active connective tissue growth factor to a fibroblast-produced extracellular matrix

It is advantageous to use biomaterials in tissue engineering that stimulate extracellular matrix (ECM) production by the cellular component. Connective tissue growth factor (CTGF) stimulates type I collagen (COL1A1) transcription, but is functionally limited as a free molecule. Using a matrix-binding domain (MBD) from microfibril-associated glycoprotein-1, the fusion protein MBD-CTGF was targeted to the ECM and tested for COL1A1 transcriptional activation. MBD-CTGF produced by the ECM-synthesizing fibroblasts, or provided exogenously, localized to the elastic fiber ECM. MBD-CTGF, but not CTGF alone, led to a two-fold enhancement of COL1A1 expression. This study introduces a targeting technology that can be used to elevate collagen transcription in engineered tissues and thereby improve tissue mechanics.

Collagen Mimetic Peptides: Progress Towards Functional Applications

Traditionally, collagen mimetic peptides (CMPs) have been used for elucidating the structure of the collagen triple helix and the factors responsible for its stabilization. The wealth of fundamental knowledge on collagen structure and cell-extracellular matrix (ECM) interactions accumulated over the past decades has led to a recent burst of research exploring the potential of CMPs to recreate the higher order assembly and biological function of natural collagens for biomedical applications. Although a large portion of such research is still at an early stage, the collagen triple helix has become a promising structural motif for engineering self-assembled, hierarchical constructs similar to natural tissue scaffolds which are expected to exhibit unique or enhanced biological activities. This paper reviews recent progress in the field of collagen mimetic peptides that bears both direct and indirect implications to engineering collagen-like materials for potential biomedical use. Various CMPs and collagen-like proteins that mimic either structural or functional characteristics of natural collagens are discussed with particular emphasis on providing helpful information to bioengineers and biomaterials scientists interested in collagen engineering.

VEGF internalization is not required for VEGFR-2 phosphorylation in bioengineered surfaces with covalently linked VEGF

Vascular endothelial growth factor (VEGF) is known to activate proliferation, migration, and survival pathways in endothelial cells through phosphorylation of VEGF receptor-2 (VEGFR-2). VEGF has been incorporated into biomaterials through encapsulation, electrostatic sequestration, and covalent attachment, but the effect of these immobilization strategies on VEGF signaling has not been thoroughly investigated. Further, although growth factor internalization along with the receptor generally occurs in a physiological setting, whether this internalization is needed for receptor phosphorylation is not entirely clear. Here we show that VEGF covalently bound through a modified heparin molecule elicits an extended response of pVEGFR-2 in human umbilical vein endothelial cells (HUVECs) and that the covalent linkage reduces internalization of the growth factor during receptor endocytosis. Optical tweezer measurements show that the rupture force required to disrupt the heparin-VEGF-VEGFR-2 interaction increases from 3-8 pN to 6-12 pN when a covalent bond is introduced between VEGF and heparin. Importantly, by covalently binding VEGF to a heparin substrate, the stability (half-life) of VEGF is extended over three-fold. Here, mathematical models support the biological conclusions, further suggesting that VEGF internalization is significantly reduced when covalently bound, and indicating that VEGF is available for repeated phosphorylation events.

Supramolecular nanostructures that mimic VEGF as a strategy for ischemic tissue repair

There is great demand for the development of novel therapies for ischemic cardiovascular disease, a leading cause of morbidity and mortality worldwide. We report here on the development of a completely synthetic cell-free therapy based on peptide amphiphile nanostructures designed to mimic the activity of VEGF, one of the most potent angiogenic signaling proteins. Following self-assembly of peptide amphiphiles, nanoscale filaments form that display on their surfaces a VEGF-mimetic peptide at high density. The VEGF-mimetic filaments were found to induce phosphorylation of VEGF receptors and promote proangiogenic behavior in endothelial cells, indicated by an enhancement in proliferation, survival, and migration in vitro. In a chicken embryo assay, these nanostructures elicited an angiogenic response in the host vasculature. When evaluated in a mouse hind-limb ischemia model, the nanofibers increased tissue perfusion, functional recovery, limb salvage, and treadmill endurance compared to controls, which included the VEGF-mimetic peptide alone. Immunohistological evidence also demonstrated an increase in the density of microcirculation in the ischemic hind limb, suggesting the mechanism of efficacy of this promising potential therapy is linked to the enhanced microcirculatory angiogenesis that results from treatment with these polyvalent VEGF-mimetic nanofibers.

The effect of vascular endothelial growth factor (VEGF) presentation within fibrin matrices on endothelial cell branching

Vascular endothelial growth factor (VEGF) has been extensively investigated to promote vascularization at damaged or diseased sites and in tissue implants. Here we are interested in determining if the manner in which VEGF is presented from a scaffold to endothelial cells influences the architecture of the blood vessels formed. We bound VEGF to nanoparticles and placed these nanoparticles inside fibrin hydrogels, which contained human umbilical vein endothelial cells (HUVECs) bound to cytodex beads. Fibroblast cells are plated on top of the fibrin gel to further mimic a physiologic environment. In addition, we used a chorioallantoic membrane (CAM) assay to determine the role of VEGF presentation on angiogenesis in vivo. We tested VEGF bound in high density and low density to study differences between growth factor presentation in heterogeneous nanodomains and homogenous distribution. VEGF covalently bound to nanoparticles at high density led to an increase in HUVEC tube branching, thickness, and total vessel network length compared to soluble VEGF. While VEGF bound electrostatically exhibited no significant difference with covalently bound VEGF in the tube formation assay, this method failed to promote host vessel infiltration into the fibrin implant on the CAM. Together our data suggest that the mode of VEGF presentation to endothelial cells influences the vessel architecture and vascularization of implants in vivo.

Total chemical synthesis of biologically active vascular endothelial growth factor

Efficient access: the 204-residue covalent-dimer vascular endothelial growth factor with full mitogenic activity was prepared from three unprotected peptide segments by one-pot native chemical ligations. The covalent structure of the synthetic VEGF was confirmed by precise mass measurement, and the three-dimensional structure of the synthetic protein was determined by high-resolution X-ray crystallography.

An agent-based model for the investigation of neovascularization within porous scaffolds

The ability to control blood vessel assembly in polymer scaffolds is important for clinical success in tissue engineering. A mathematical and computational representation of the relationship between scaffold properties and neovascularization may provide a better understanding of the fundamental process itself and help guide the design of new therapeutic approaches. This article proposes a multilayered, multiagent framework to model sprouting angiogenesis in porous scaffolds and examines the impact of pore structure on vessel invasion and network structure. We have defined the speed of vessel sprouting in the agent-based model based on in vivo results in the absence of a polymer scaffold. A number of cases were run to investigate the effect of scaffold pore size on angiogenesis. The simulation results indicate that the rate of scaffold vascularization increases with pore size. Pores of larger size (160-270 μm) support rapid and extensive angiogenesis throughout the scaffold. Model predictions were compared to experimental results of vascularization in porous poly(ethylene glycol) hydrogels to validate the results. This model can be used to provide insight into optimal scaffold properties that support vascularization of engineered tissues.

Drug delivery in soft tissue engineering

INTRODUCTION

Tissue defects, sustained through disease or trauma, present enormous challenges in regenerative medicine. Modern tissue engineering (TE) aims at replacing or repairing these defects through a combined approach of biodegradable scaffolds, suitable cell sources and appropriate environmental cues, such as biomolecules presented on scaffold surfaces or sustainably released from within.

AREAS COVERED

This review provides a brief overview of the various drugs and bioactive molecules of interest to TE, as well as a selection of materials that have been proposed for TE scaffolds and matrices in the past. It then proceeds to discuss encapsulation, immobilization and controlled release strategies for bioactive proteins, before discussing recent advances in this area with a special focus on soft TE.

EXPERT OPINION

Overall, minimal clinical success has been achieved so far in using growth factor, morphogen, or adhesion factor modified scaffolds and matrices; only one growth factor delivery system (Regranex Gel), has been approved by the FDA for clinical use, with only a handful of other growth factors being approved for human use so far. However, many more growth factors are currently in clinical Phase I - II or preclinical trials and many delivery systems utilize materials already approved by the FDA for other purposes. With respect to drug delivery in soft TE, a combination of increased research efforts in hydrogel and support material development as well as growth factor development is needed before clinical success is realized.

Hydrogels as a platform for stem cell delivery to the heart

Stem cell therapy offers great promise to repair the injured or failing heart. The outcomes of clinical trials to date, however, have shown that the actual benefit realized falls far short of the promise. A number of factors may explain why that is the case, but poor stem cell retention and engraftment in the hostile environment of the injured heart would seem to be a major factor. Improving stem cell retention and longevity once delivered would seem a logical means to enhance their reparative function. One way to accomplish this goal may be injectable hydrogels, which would serve to fix stem cells in place while providing a sheltering environment. Hydrogels also provide a means to allow for the paracrine factors produced by encapsulated stem cells to diffuse into the injured myocardium. Alternatively, hydrogels themselves can be used for the sustained delivery of reparative factors. Here the authors discuss chitosan-based hydrogels.

Inhibition of VEGF signaling pathway attenuates hemorrhage after tPA treatment

An angiogenic factor, vascular endothelial growth factor (VEGF), might be associated with the blood-brain barrier (BBB) disruption after focal cerebral ischemia; however, it remains unknown whether hemorrhagic transformation (HT) after tissue plasminogen activator (tPA) treatment is related to the activation of VEGF signaling pathway in BBB. Here, we hypothesized that inhibition of VEGF signaling pathway can attenuate HT after tPA treatment. Rats subjected to thromboembolic focal cerebral ischemia were assigned to a permanent ischemia group and groups treated with tPA at 1 or 4 hours after ischemia. Anti-VEGF neutralizing antibody or control antibody was administered simultaneously with tPA. At 24 hours after ischemia, we evaluated the effects of the antibody on the VEGF expression, matrix metalloproteinase-9 (MMP-9) activation, degradation of BBB components, and HT. Delayed tPA treatment at 4 hours after ischemia promoted expression of VEGF in BBB, MMP-9 activation, degradation of BBB components, and HT. Compared with tPA and control antibody, combination treatment with tPA and the anti-VEGF neutralizing antibody significantly attenuated VEGF expression in BBB, MMP-9 activation, degradation of BBB components, and HT. It also improved motor outcome and mortality. Inhibition of VEGF signaling pathway may be a promising therapeutic strategy for attenuating HT after tPA treatment.

The promotion of microvasculature formation in poly(ethylene glycol) diacrylate hydrogels by an immobilized VEGF-mimetic peptide

Microvascularization of tissue engineered constructs was achieved by utilizing a VEGF-mimicking peptide, QK, covalently bound to a poly(ethylene glycol) hydrogel matrix. The 15-amino acid peptide, developed by D'Andrea et al., was modified with a PEG-succinimidyl ester linker on the N-terminus of the peptide, then photocrosslinked onto the surface or throughout PEG hydrogels. PEGylation of the peptide increased its solubility and bioactivity, as evidenced by endothelial cell proliferation. PEG-QK showed equal or superior ability to promote angiogenesis in vitro, on the surface of hydrogels and within three-dimensional collagenase-degradable hydrogels, compared to RGDS only or PEG-VEGF hydrogels. Endothelial cells were shown to form tubule structures, migrate, and make cell-cell contacts in response to covalently-bound PEG-QK. In vivo in a mouse cornea micropocket angiogenesis assay, PEG-QK hydrogels promoted more complete coverage of host microvasculature within the hydrogel. PEG-QK was shown to enhance vessel branch points and vessel density as well as space filling properties of fractal dimension and lacunarity. This report shows the ability to promote angiogenesis in tissue engineered constructs using a covalently-bound small peptide rather than a large protein and may point to an advance in designing biomimetic cellular environments.

Biomimetic approaches to control soluble concentration gradients in biomaterials

Soluble concentration gradients play a critical role in controlling tissue formation during embryonic development. The importance of soluble signaling in biology has motivated engineers to design systems that allow precise and quantitative manipulation of gradient formation in vitro. Engineering techniques have increasingly moved to the third dimension in order to provide more physiologically relevant models to study the biological role of gradient formation and to guide strategies for controlling new tissue formation for therapeutic applications. This review provides an overview of efforts to design biomimetic strategies for soluble gradient formation, with a focus on microfluidic techniques and biomaterials approaches for moving gradient generation to the third dimension.

Dermal substitute-assisted healing: enhancing stem cell therapy with novel biomaterial design

The use of dermal substitutes is increasingly widespread but the outcomes of substitute-assisted healing remain functionally deficient. Presently, the most successful scaffolds are acellular polymer matrices, prepared through lyophilization and phase separation techniques, designed to mimic the dermal extracellular matrix. The application of scaffolds containing viable cells has proven to be problematic due to short shelf-life, high cost and death of transplanted cells as a result of immune rejection and apoptosis. Recent advances in biomaterial science have made new techniques available capable of increasing scaffold complexity, allowing the creation of 3D microenvironments that actively control cell behaviour. Importantly, it may be possible through these sophisticated novel techniques, including bio-printing and electrospinning, to accurately direct stem cell behaviour. This complex proposal involves the incorporation of cell-matrix, cell-cell, mechanical cues and soluble factors delivered in a spatially and temporally pertinent manner. This requires accurate modelling of three-dimensional stem cell interactions within niche environments to identify key signalling molecules and mechanisms. The application of stem cells within substitutes containing such environments may result in greatly improved transplanted cell viability. Ultimately this may increase cellular organization and complexity of skin substitutes. This review discusses progress made in improving the efficacy of cellular dermal substitutes for the treatment of cutaneous defects and the potential of evolving new technology to improve current results.

In vivo evaluation of MMP sensitive high-molecular weight HA-based hydrogels for bone tissue engineering

Hyaluronic acid (170 kDa)-based hydrogel was synthesized using acrylated hyaluronic acid (HA) and matrix metalloproteinase (MMP) sensitive HA-based hydrogels were then prepared by conjugation with two different peptides: cell adhesion peptides containing integrin-binding domains (Arg-Gly-Asp: RGD) and a cross-linker with MMP degradable peptides to mimic the remodeling characteristics of natural extracellular matrices by cell-derived MMPs. Mechanical properties of these hydrogels were evaluated with different weight percentages (2.5 and 3.5 wt %) by measuring elastic modulus, viscous modulus, and swelling ratio. Human mesenchymal stem cells (hMSCs) were then cultured in MMP-sensitive or insensitive HA-based hydrogels and/or immobilized cell adhesive RGD peptides in vitro. Actin staining and image analysis proved that cells cultured in the MMP-sensitive hydrogel with RGD peptides showed extensive cell spreading and sprouting. Gene expression analysis showed that bone specific genes such as alkaline phosphatase, osteocalcin, and osteopontin increased in MMP-sensitive hydrogels as biomolecules such as BMPs and cells were added in the gels. For in vivo calvarial defect regeneration, five different samples (MMP insensitive hydrogel, MMP sensitive hydrogel, MMP sensitive hydrogel with BMP-2, MMP sensitive hydrogel with hMSC, and MMP sensitive hydrogel with BMP-2 and hMSC) were prepared. After 4 weeks of implantation, the Masson-Trichrome staining and micro computed tomography scan results demonstrated that the MMP sensitive hydrogels with BMP-2 and hMSCs have the highest mature bone formation. The MMP sensitive HA-based hydrogel could become useful scaffolds in bone tissue engineering with improvements on tissue remodeling rates and regeneration activity.

Growth factor delivery-based tissue engineering: general approaches and a review of recent developments

The identification and production of recombinant morphogens and growth factors that play key roles in tissue regeneration have generated much enthusiasm and numerous clinical trials, but the results of many of these trials have been largely disappointing. Interestingly, the trials that have shown benefit all contain a common denominator, the presence of a material carrier, suggesting strongly that spatio-temporal control over the location and bioactivity of factors after introduction into the body is crucial to achieve tangible therapeutic effect. Sophisticated materials systems that regulate the biological presentation of growth factors represent an attractive new generation of therapeutic agents for the treatment of a wide variety of diseases. This review provides an overview of growth factor delivery in tissue engineering. Certain fundamental issues and design strategies relevant to the material carriers that are being actively pursued to address specific technical objectives are discussed. Recent progress highlights the importance of materials science and engineering in growth factor delivery approaches to regenerative medicine.

Development of functional fibrous matrices for the controlled release of basic fibroblast growth factor to improve therapeutic angiogenesis

In this study, novel fibrous matrices were developed as a depot to store and liberate growth factors in a controlled manner. Specifically, heparin was covalently conjugated onto the surface of fibrous matrices (composites of poly[caprolactone] and gelatin crosslinked with genipin), and basic fibroblast growth factor (bFGF) was then reversibly immobilized. The immobilization of bFGF was controlled as a function of the amount of conjugated heparin. The sustained release of bFGF from the fibrous matrices was successfully achieved over 4 weeks whereas physical adsorption of bFGF released quickly. The bFGF released from the fibrous matrices significantly enhanced in vitro proliferation of human umbilical vein endothelial cells. From the in vivo study, the group implanted with a higher amount of immobilized bFGF significantly facilitated neo-blood vessel formation as compared with other implantation groups. These results indicate that the sustained release of bFGF is important for the formation of blood vessels and that our fibrous matrices could be useful for regulation of tissue damage requiring angiogenesis. Further, our system can be combined with other growth factors with heparin binding domains, representing a facile depot for spatiotemporal control over the delivery of bioactive molecules in regenerative medicine.

The effect of pore size on tissue ingrowth and neovascularization in porous bioceramics of controlled architecture in vivo

The purpose of this study was to investigate the role of pore size on tissue ingrowth and neovascularization in porous bioceramics under the accurate control of the pore parameters. For that purpose, β-tricalcium phosphate (β-TCP) cylinders with four different macropore sizes (300-400, 400-500, 500-600 and 600-700 µm) but the same interconnection size (120 µm) and unchangeable porosity were implanted into fascia lumbodorsalis in rabbits. The fibrous tissues and blood vessels formed in scaffolds were observed histologically and histomorphometrically. The vascularization of the porous bioceramics was analyzed by single-photon emission computed tomography (SPECT). It is found that pore size as an important parameter of a porous structure plays an important role in tissue infiltration into porous biomaterial scaffolds. The amount of fibrous tissue ingrowth increases with the decrease of the pore size. In four kinds of scaffolds with different macropore sizes (300-400, 400-500, 500-600 and 600-700 µm) and a constant interconnection size of 120 µm, the areas of fibrous tissue (%) were 60.5%, 52.2%, 41.3% and 37.3%, respectively, representing a significant decrease at 4 weeks (P < 0.01). The pore size of a scaffold is closely related to neovascularization of macroporous biomaterials implanted in vivo. A large pore size is beneficial for the growth of blood vessels, and the diameter of a pore smaller than 400 µm limits the growth of blood vessels and results in a smaller blood vessel diameter.

Micron-scale spatially patterned, covalently immobilized vascular endothelial growth factor on hydrogels accelerates endothelial tubulogenesis and increases cellular angiogenic responses

Spontaneous formation of endothelial tubules was restricted to patterned micron-scale regions presenting cell adhesion ligands and angiogenic signaling protein on poly(ethylene glycol) hydrogels. Arginine-glycine-aspartic acid-serine (RGDS), an integrin ligand, and vascular endothelial growth factor (VEGF), a rate-limiting signaling protein involved in angiogenesis, were covalently bound through photopolymerization via laser scanning lithography to the surface of poly(ethylene glycol) hydrogels in patterned micron-scale regions. Endothelial cells cultured in this restricted environment underwent accelerated tubulogenesis, forming endothelial tubes within 2 days, whereas cells cultured on larger patterned areas remained spread and did not form tubules by day 2. Tubules formed in 2 days on RGDS and VEGF patterns were observed to possess lumens; however, tubule-like structures formed in 2 days on RGDS-only control patterns did not have observable lumens. Additionally, tubules that formed on restricted areas of RGDS and VEGF expressed more VEGF receptor 1, VEGF receptor 2, and ephA7 surface markers, in addition to higher expression of laminin, than cells remaining spread on wide patterned lines. This work reports spatial control and acceleration of endothelial tubule formation using biocompatible hydrogel materials to allow the formation of highly organized vascularized tissues.

Enhanced neovascular formation in a novel hydrogel matrix consisting of citric Acid and collagen

BACKGROUND

Three-dimensional regenerative tissue with large bulk generally requires blood perfusion through a vascular network to maintain its viability, and one promising approach is induction of neovascular growth from the recipient bed into the tissue. To induce ingrowth of a vascular network, it is necessary to furnish the regenerative tissue with a scaffold structure for neovasculature and a delivery system for an angiogenic growth factor. As such a scaffold structure, the present study created novel hydrogel materials by chemically cross-linking alkali-treated collagen (AlCol) with trisuccinimidyl citrate (TSC).

MATERIALS AND METHODS

Many prototypes, consisting of several concentrations of TSC and AlCol, were implanted into the subfascial space of the rat rectus muscle, and 7 days later, the implanted materials were excised for histological analysis. Cross-sections were stained and neovascular development in the materials was evaluated by measuring vessel density, length and number of joints and branches.

RESULTS

Significant ingrowth of vascularized granulation was observed in some materials, which surpassed the angiogenic ability of Matrigel(TM). Further, combination with basic fibroblast growth factor (bFGF) significantly increased the vascular formation in these gels.

CONCLUSIONS

The TSC-AlCol gel functioned as a favorable scaffold for neovascular formation and also as a reservoir for controlled delivery of bFGF.

Vascularization of the dermal support enhances wound re-epithelialization by in situ delivery of epidermal keratinocytes

Despite significant advances in management of severe wounds such as burns and chronic ulcers, autologous split-thickness skin grafts are still the gold standard of care. The main problems with this approach include pain and discomfort associated with harvesting autologous tissue, limited availability of donor sites, and the need for multiple surgeries. Although tissue engineering has great potential to provide alternative approaches for tissue regeneration, several problems have hampered progress in translating technological advances to clinical reality. Specifically, engineering of skin substitutes requires long culture times and delayed vascularization after implantation compromises graft survival. To address these issues we developed a novel two-prong strategy for tissue regeneration in vivo: (1) vascularization of acellular dermal scaffolds by infiltration of angiogenic factors; and (2) generation of stratified epidermis by in situ delivery of epidermal keratinocytes onto the prevascularized dermal support. Using athymic mouse as a model system, we found that incorporation of angiogenic factors within acellular human dermis enhanced the density and diameter of infiltrating host blood vessels. Increased vascularization correlated with enhanced proliferation and stratification of the neoepidermis originating from the fibrin-keratinocyte cell suspension. This strategy promoted tissue regeneration in vivo with no need for engineering skin substitutes; therefore, it may be useful for treatment of major wounds when skin donor sites are scarce and rapid wound coverage is required.

The enhancement of VEGF-mediated angiogenesis by polycaprolactone scaffolds with surface cross-linked heparin

This study investigates the effect of surface cross-linked heparin on vascular endothelial growth factor (VEGF)-mediated angiogenesis in porous polycaprolactone (PCL) scaffolds in vivo. We tested the hypothesis that VEGF delivered by scaffolds coated with a sub-micron thick layer of immobilized heparin would accelerate angiogenesis. The bioactivity of retained VEGF was confirmed by its phosphorylation of VEGF receptor-2. After 7 and 14 days of subcutaneous implantation in mice, the heparin-PCL scaffolds loaded with VEGF displayed significantly higher infiltration of blood vessels which traversed the entire scaffold thickness (2 mm). The stability and function of the newly formed vessels were confirmed by smooth muscle cell coverage and vessel perfusability, respectively. The contribution of individual components was assessed by varying the VEGF dose and heparin thickness. Prolonging the cross-linking reaction on PCL scaffolds resulted in higher heparin content, thicker heparin layer, and higher VEGF retention. While a dose dependent angiogenic response was observed with VEGF, higher amount of cross-linked heparin did not translate into additional improvement in angiogenesis for a given dose of VEGF. The synergism of immobilized heparin and VEGF in stimulating angiogenesis was observed in vivo.

Biomimetic hydrogels with VEGF induce angiogenic processes in both hUVEC and hMEC

Angiogenesis is the process by which new blood vessels arise from the pre-existing vasculature. Human endothelial cells are known to be involved in three key cellular processes during angiogenesis: increased cell proliferation, degradation of the extracellular matrix during cell migration, and the survival of apoptosis. The above processes depend upon the presence of growth factors, such as vascular endothelial growth factor isoform 165 (VEGF(165)) that is released from the extracellular matrix as it is being degraded or secreted from activated endothelial cells. Thus, the goal of the current study is to develop a system with a backbone of polyethylene glycol (PEG) and grafted angiogenic signals to compare the initial angiogenic response of human umbilical vein endothelial cells (hUVEC) or human microvascular endothelial cells (hMEC). Adhesion ligands (PEG-RGDS) for cell attachment and PEG-modified VEGF(165) (PEG-VEGF(165)) are grafted into the hydrogels to encourage the angiogenic response. Our data suggest that our biomimetic system is equally effective in stimulating proliferation, migration, and survival of apoptosis in hMEC as compared to the response to hUVEC.

SPARC-derived protease substrates to enhance the plasmin sensitivity of molecularly engineered PEG hydrogels

Bioactive hydrogels formed from the Michael-type addition reactions of end-functionalized poly (ethylene glycol) macromers with thiol-containing protease-sensitive peptide crosslinkers have previously been described as matrices for cell-induced enzymatic remodeling. In this study, we sought to develop materials formulations with different degradation profiles by evaluating peptides derived from secreted protein acidic and rich in cysteine (SPARC) as potential substrates for plasmin, matrix metalloproteinase (MMP)-1, and MMP-2. Michaelis-Menten analysis showed that different peptides could provide a range of k(cat) values for each enzyme. In most cases, hydrogels formed with crosslinker peptides that had higher k(cat) values degraded faster when exposed to the appropriate enzyme(s), and fibroblasts showed increased cell proliferation and cell spreading when cultured in the faster degrading hydrogels. Further, greater cell invasion was observed from aortic ring segments embedded in the faster degrading hydrogels. The addition of the SPARC-derived peptides to the repertoire of protease-sensitive crosslinkers increases the potential application of these materials by providing enhanced susceptibility to plasmin. Further, the graded increases in k(cat) and the differential responses for plasmin, MMP-1, and MMP-2 can be used to engineer hydrogels with degradation properties tuned to the enzymes produced by particular cell types, allowing for broader in vivo application.

Tissue-engineered three-dimensional tumor models to study tumor angiogenesis

Cell-microenvironment interactions play a critical role in the transformation of normal cells into cancer; however, the underlying mechanisms and effects are far from being well understood. Tissue Engineering provides innovative culture tools and strategies to study tumorigenesis under pathologically relevant culture conditions. Specifically, integration of biomaterials, scaffold fabrication, and micro/nano-fabrication techniques offers great promise to reveal the dynamic role of chemical, cell-cell, cell-extracellular matrix, and mechanical interactions in the pathogenesis of cancer. Due to the central importance of blood vessel formation in tumor growth, progression, and drug response, this review will discuss specific design parameters for the development of culture microenvironments to study tumor angiogenesis. Tumor engineering approaches have the potential to revolutionize our understanding of cancer, provide new platforms for testing of anti-cancer drugs, and may ultimately result in improved treatment strategies.

Hyaluronic acid hydrogels with controlled degradation properties for oriented bone regeneration

Non-healing fractures can result from trauma, disease, or age-related bone loss. While many treatments focus on restoring bone volume, few try to recapitulate bone organization. However, the native architecture of bone is optimized to provide its necessary mechanical properties. Hyaluronic acid (HA) hydrogel scaffold systems with tunable degradation properties were developed for the controlled delivery of osteoinductive and angiogenic growth factors, thus affecting the quantity and quality of regenerated tissue. HA hydrogels were designed to degrade at fast, intermediate, and slow rates due to hydrolysis and further provided controlled release of cationic proteins due to electrostatic interactions. Scaffolds delivering bone morphogenetic protein-2 (BMP-2) were evaluated in a rat calvarial bone critical size defect model. BMP-2 delivery from the HA hydrogels had a clear osteoinductive effect in vivo and, for all hydrogel types, BMP-2 delivery resulted in significant mineralization compared to control hydrogels. The temporal progression of this effect could be modulated by altering the degradation rate of the scaffold. All three degradation rates tested resulted in similar amounts of mineral formation at the latest (six week) time point examined. Interestingly, however, the fastest and slowest degrading scaffolds seemed to result in more organized bone than the intermediate degrading scaffold, which was designed to degrade in 6-8 weeks to match the healing time. Additionally, healing could be enhanced by co-delivery of vascular endothelial growth factor along with BMP-2.

Peptide-Functionalized Click Hydrogels with Independently Tunable Mechanics and Chemical Functionality for 3D Cell Culture

Multifunctionalized macromers react via a copper-free click chemistry to form an idealized 3D hydrogel. Subsequently, thiol-containing biomolecules are spatially patterned within the material with precise control over the amount and location of functionalization. Both the network formation and subsequent patterning reactions are fully cytocompatible, allowing these systems to be used to study individual cell behavior at user-defined locations throughout the material.

Wound Healing Versus Regeneration: Role of the Tissue Environment in Regenerative Medicine

One of the major challenges in the field of regenerative medicine is how to optimize tissue regeneration in the body by therapeutically manipulating its natural ability to form scar at the time of injury or disease. It is often the balance between tissue regeneration, a process that is activated at the onset of disease, and scar formation, which develops as a result of the disease process that determines the ability of the tissue or organ to be functional. Using biomaterials as scaffolds often can provide a "bridge" for normal tissue edges to regenerate over small distances, usually up to 1 cm. Larger tissue defect gaps typically require both scaffolds and cells for normal tissue regeneration to occur without scar formation. Various strategies can help to modulate the scar response and can potentially enhance tissue regeneration. Understanding the mechanistic basis of such multivariate interactions as the scar microenvironment, the immune system, extracellular matrix, and inflammatory cytokines may enable the design of tissue engineering and wound healing strategies that directly modulate the healing response in a manner favorable to regeneration.

Quantifying the proteolytic release of extracellular matrix-sequestered VEGF with a computational model

Background

VEGF proteolysis by plasmin or matrix metalloproteinases (MMPs) is believed to play an important role in regulating vascular patterning in vivo by releasing VEGF from the extracellular matrix (ECM). However, a quantitative understanding of the kinetics of VEGF cleavage and the efficiency of cell-mediated VEGF release is currently lacking. To address these uncertainties, we develop a molecular-detailed quantitative model of VEGF proteolysis, used here in the context of an endothelial sprout.

Methodology and Findings

To study a cell's ability to cleave VEGF, the model captures MMP secretion, VEGF-ECM binding, VEGF proteolysis from VEGF₁₆₅ to VEGF₁₁₄ (the expected MMP cleavage product of VEGF₁₆₅) and VEGF receptor-mediated recapture. Using experimental data, we estimated the effective bimolecular rate constant of VEGF₁₆₅ cleavage by plasmin to be 328 M⁻¹s⁻¹ at 25°C, which is relatively slow compared to typical MMP-ECM proteolysis reactions. While previous studies have implicated cellular proteolysis in growth factor processing, we show that single cells do not individually have the capacity to cleave VEGF to any appreciable extent (less than 0.1% conversion). In addition, we find that a tip cell's receptor system will not efficiently recapture the cleaved VEGF due to an inability of cleaved VEGF to associate with Neuropilin-1.

Conclusions

Overall, VEGF₁₆₅ cleavage in vivo is likely to be mediated by the combined effect of numerous cells, instead of behaving in a single-cell-directed, autocrine manner. We show that heparan sulfate proteoglycans (HSPGs) potentiate VEGF cleavage by increasing the VEGF clearance time in tissues. In addition, we find that the VEGF-HSPG complex is more sensitive to proteases than is soluble VEGF, which may imply its potential relevance in receptor signaling. Finally, according to our calculations, experimentally measured soluble protease levels are approximately two orders of magnitude lower than that needed to reconcile levels of VEGF cleavage seen in pathological situations.

Enhanced proteolytic degradation of molecularly engineered PEG hydrogels in response to MMP-1 and MMP-2

Bioactive hydrogels formed by Michael-type addition reactions of end-functionalized poly(ethylene glycol) macromers with cysteine-containing peptides have been described as extracellular matrix mimetics and tissue engineering scaffolds. Although these materials have shown favorable behavior in vivo in tissue repair, we sought to develop materials formulations that would be more rapidly responsive to cell-induced enzymatic remodeling. In this study, protease-sensitive peptides that have increased k(cat) values were characterized and evaluated for their effects on gel degradability. Biochemical properties for soluble peptides and hydrogels were examined for matrix metalloproteinase (MMP)-1 and MMP-2. The most efficient peptide substrates in some cases overlap and in other cases differ between the two enzymes tested, and a range of k(cat) values was obtained. For each enzyme, hydrogels formed using the peptides with higher k(cat) values degraded faster than a reference with lower k(cat). Fibroblasts showed increased cell spreading and proliferation when cultured in 3D hydrogels with faster degrading peptides, and more cell invasion from aortic ring segments embedded in the hydrogels was observed. These faster degrading gels should provide matrices that are easier for cells to remodel and lead to increased cellular infiltration and potentially more robust healing in vivo.

Cell-mediated Delivery and Targeted Erosion of Vascular Endothelial Growth Factor-Crosslinked Hydrogels

We have previously reported a novel polymeric delivery vehicle that is assembled via interaction between heparin and the vascular endothelial growth factor (VEGF). Here, the cell-responsiveness of this hydrogel-including the delivery of VEGF in response to VEGFR-2 overexpressing PAE/KDR cells (porcine aortic endothelial cells (PAE) equipped with the transcript for the kinase insert domain receptor (KDR)), consequent erosion of the hydrogel matrix, and cellular response-are highlighted. The release of VEGF and hydrogel erosion reached 100% only in the presence of PAE/KDR. The [PEG-LMWH/VEGF] hydrogel (PEG = poly(ethylene glycol), LMWH = low molecular weight heparin) correspondingly prompted increases in VEGFR-2 phosphorylation and proliferation of PAE/KDR cells. This study proves that growth factor-crosslinked hydrogels can liberate VEGF in response to specific receptors, causing gel erosion and desired cell responses. The promise of these approaches in therapeutic applications, including targeted delivery, is suggested.

Biomimetic PEG hydrogels crosslinked with minimal plasmin-sensitive tri-amino acid peptides

Semi-synthetic, proteolytically degradable polymer hydrogels have proven effective as scaffolds to augment bone and skin regeneration in animals. However, high costs due to expensive peptide building blocks pose a significant hurdle towards broad clinical usage of these materials. Here we demonstrate that tri-amino acid peptides bearing lysine (or arginine), flanked by two cysteine residues for crosslinking, are adequate as minimal plasmin-sensitive peptides in poly(ethylene glycol)-based hydrogels formed via Michael-type addition. Substitution of lysine (or arginine) with serine rendered the matrices insensitive to the action of plasmin. This was demonstrated in vitro by performing gel degradation experiments in the presence of plasmin (0.1 U/mL), and in the in vivo situation of regeneration of critical-sized bone defects. When placed as implants into rat calvaria, gels formed from the minimal plasmin substrates showed clear signs of cell infiltration and gel remodeling that coincided with extensive bone formation.

Engineering surfaces for site-specific vascular differentiation of mouse embryonic stem cells

Differentiation of stem and progenitor cells routinely relies on the application of soluble growth factors, an approach that enables temporal control of cell fate but enables no spatial control of the differentiation process. Angiogenic progenitor cells derived from mouse embryonic stem cells (ESCs) were differentiated here according to the pattern of immobilized vascular endothelial growth factor-A (VEGF). Mouse ESCs engineered to express green fluorescent protein (eGFP) under control of promoter for the receptor tyrosine kinase Flk1 were used. The Flk1+ angiogenic progenitors were selected from day 3 differentiating embryoid bodies based on their expression of eGFP using fluorescence activated cell sorting. Mouse VEGF(165) was covalently immobilized onto collagen IV (ColIV) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) chemistry. A non-cell adhesive layer of photocrosslinkable chitosan was first created, after which VEGF-ColIV was stamped as 100mum wide lanes on top of the chitosan layer and the Flk1+ angiogenic progenitors were seeded for site-specific differentiation. Lanes stamped with only ColIV served as controls. The results presented here demonstrate that the cultivation of Flk1+ progenitors on surfaces with immobilized VEGF yielded primarily endothelial cells (53+/-13% CD31 positive and 17+/-2% smooth muscle actin positive), whereas surfaces without VEGF favored vascular smooth muscle-like cell differentiation (26+/-17% CD31 positive and 38+/-9% smooth muscle actin positive).

Smart biomaterials - regulating cell behavior through signaling molecules

Important advances in the field of tissue engineering are arising from increased interest in novel biomaterial designs with bioactive components that directly influence cell behavior. Following the recent work of Mitchell and co-workers published in BMC Biology, we review how spatial and temporal control of signaling molecules in a matrix material regulates cellular responses for tissue-specific applications.

See research article http://www.biomedcentral.com/1741-7007/8/57