Spatial and temporal control of angiogenesis and arterialization using focal applications of VEGF164 and Ang-1

A Challenge for Engineering Biomimetic Microvascular Models: How do we Incorporate the Physiology?

The gap between in vitro and in vivo assays has inspired biomimetic model development. Tissue engineered models that attempt to mimic the complexity of microvascular networks have emerged as tools for investigating cell-cell and cell-environment interactions that may be not easily viewed in vivo. A key challenge in model development, however, is determining how to recreate the multi-cell/system functional complexity of a real network environment that integrates endothelial cells, smooth muscle cells, vascular pericytes, lymphatics, nerves, fluid flow, extracellular matrix, and inflammatory cells. The objective of this mini-review is to overview the recent evolution of popular biomimetic modeling approaches for investigating microvascular dynamics. A specific focus will highlight the engineering design requirements needed to match physiological function and the potential for top-down tissue culture methods that maintain complexity. Overall, examples of physiological validation, basic science discoveries, and therapeutic evaluation studies will emphasize the value of tissue culture models and biomimetic model development approaches that fill the gap between in vitro and in vivo assays and guide how vascular biologists and physiologists might think about the microcirculation.

Therapeutic angiogenesis in coronary artery disease: a review of mechanisms and current approaches

INTRODUCTION

Despite tremendous advances, the shortcomings of current therapies for coronary disease are evidenced by the fact that it remains the leading cause of death in many parts of the world. There is hence a drive to develop novel therapies to tackle this disease. Therapeutic approaches to coronary angiogenesis have long been an area of interest in lieu of its incredible, albeit unrealized potential.

AREAS COVERED

This paper offers an overview of mechanisms of native angiogenesis and a description of angiogenic growth factors. It progresses to outline the advances in gene and stem cell therapy and provides a brief description of other investigational approaches to promote angiogenesis. Finally, the hurdles and limitations unique to this particular area of study are discussed.

EXPERT OPINION

An effective, sustained, and safe therapeutic option for angiogenesis truly could be the paradigm shift for cardiovascular medicine. Unfortunately, clinically meaningful therapeutic options remain elusive because promising animal studies have not been replicated in human trials. The sheer complexity of this process means that numerous major hurdles remain before therapeutic angiogenesis truly makes its way from the bench to the bedside.

Ultrasound-targeted microbubble destruction-mediated Ang1 gene transfection improves left ventricular structural and sympathetic nerve remodeling in canines with myocardial infarction

Background

The present study aimed to determine whether ultrasound-targeted microbubble destruction (UTMD)-mediated angiopoietin 1 (Ang1) gene transfection can improve angiogenesis and potentially reverse left ventricular (LV) structural and sympathetic nerve remodeling in canines with myocardial infarction (MI).

Methods

Thirty dogs were randomly divided into groups (n=10/group) as follows: the MI group (MI dogs without UTMD treatment), the UTMD group (MI dogs with UTMD-mediated negative control plasmid treatment), and the UTMD-Ang1 group (MI dogs with UTMD-mediated Ang1 plasmid treatment). LV dimensions, systolic function, and synchrony were used to reflect the structural remodeling. The density of tyrosine hydroxylase (TH)- and growth-associated protein 43 (GAP43)-positive nerve fibers were calculated to assess the sympathetic nerve remodeling.

Results

One month after treatment, the UTMD-Ang1 group showed lower LV end-diastolic dimension (LVEDD: 31.2±2.3 mm) and higher LV ejection fraction (LVEF: 44.6%±4.3%) than the MI group (LVEDD: 34.5±2.2 mm, t=2.282, P=0.014; LVEF: 37.3%±3.1%, t=3.718, P=0.003) and the UTMD group (LVEDD: 34.1±2.8 mm, t=2.264, P=0.040; LVEF: 39.3%±4.5%, t=2.408, P=0.030). LV synchrony was higher in the UTMD-Ang1 group compared with the MI group by 2-dimensional speckle-tracking echocardiography. Angiogenic density was higher in the UTMD group than the MI group but was highest in the UTMD-Ang1 group according to immunohistochemistry of CD31 and α-smooth muscle actin staining. The density of TH- and GAP43-positive nerve fibers were decreased in the UTMD-Ang1 group (TH: 1,928.2±376.6 μm²/mm²; GAP43: 2,090.8±329.2 μm²/mm²) compared with the MI group (TH: 2916.5±558.4 μm²/mm², t=4.069, P=0.001; GAP43: 3,275.4±548.6 μm²/mm², t=5.153, P=0.000) and the UTMD group (TH: 2,552.7±408.1 μm²/mm², t=3.181, P=0.007; GAP43: 2,630.5±419.3 μm²/mm², t=2.863, P=0.013). The relative Ang1 and sarcoplasmic reticulum Ca²⁺-ATPase 2a protein levels were significantly higher in the UTMD-Ang1 group than the UTMD and MI groups by Western blot, while the phospholamban levels exhibited the opposite trend. Plasma norepinephrine and N-terminal pro-B-type-natriuretic peptide were significantly reduced in the UTMD-Ang1 group from day 1 to 1 month after MI.

Conclusions

UTMD-mediated Ang1 transfection can promote angiogenesis, reverse LV structural and sympathetic nerve remodeling, and improve LV synchrony after MI.

Tissue Engineering of the Microvasculature

The ability to generate new microvessels in desired numbers and at desired locations has been a long-sought goal in vascular medicine, engineering, and biology. Historically, the need to revascularize ischemic tissues nonsurgically (so-called therapeutic vascularization) served as the main driving force for the development of new methods of vascular growth. More recently, vascularization of engineered tissues and the generation of vascularized microphysiological systems have provided additional targets for these methods, and have required adaptation of therapeutic vascularization to biomaterial scaffolds and to microscale devices. Three complementary strategies have been investigated to engineer microvasculature: angiogenesis (the sprouting of existing vessels), vasculogenesis (the coalescence of adult or progenitor cells into vessels), and microfluidics (the vascularization of scaffolds that possess the open geometry of microvascular networks). Over the past several decades, vascularization techniques have grown tremendously in sophistication, from the crude implantation of arteries into myocardial tunnels by Vineberg in the 1940s, to the current use of micropatterning techniques to control the exact shape and placement of vessels within a scaffold. This review provides a broad historical view of methods to engineer the microvasculature, and offers a common framework for organizing and analyzing the numerous studies in this area of tissue engineering and regenerative medicine. © 2019 American Physiological Society. Compr Physiol 9:1155-1212, 2019.

Understanding angiogenesis during aging: opportunities for discoveries and new models

Microvascular network growth and remodeling are common denominators for most age-related pathologies. For multiple pathologies (myocardial infarction, stroke, hypertension), promoting microvascular growth, termed angiogenesis, would be beneficial. For others (cancer, retinopathies, rheumatoid arthritis), blocking angiogenesis would be desirable. Most therapeutic strategies, however, are motivated based on studies using adult animal models. This approach is problematic and does not account for the impaired angiogenesis or the inherent network structure changes that might result from age. Considering the common conception that angiogenesis is impaired with age, a need exists to identify the causes and mechanisms of angiogenesis in aged scenarios and for new tools to enable comparison of aged versus adult responses to therapy. The objective of this article is to introduce opportunities for advancing our understanding of angiogenesis in aging through the discovery of novel cell changes along aged microvascular networks and the development of novel ex vivo models.

Engineering a microcirculation for perfusion control of ex vivo-assembled organ systems: Challenges and opportunities

Donor organ shortage remains a clear problem for many end-stage organ patients around the world. The number of available donor organs pales in comparison with the number of patients in need of these organs. The field of tissue engineering proposes a plausible solution. Using stem cells, a patient's autologous cells, or allografted cells to seed-engineered scaffolds, tissue-engineered constructs can effectively supplement the donor pool and bypass other problems that arise when using donor organs, such as who receives the organ first and whether donor organ rejection may occur. However, current research methods and technologies have been unable to successfully engineer and vascularize large volume tissue constructs. This review examines the current perfusion methods for ex vivo organ systems, defines the different types of vascularization in organs, explores various strategies to vascularize ex vivo organ systems, and discusses challenges and opportunities for the field of tissue engineering.

Dynamic, heterogeneous endothelial Tie2 expression and capillary blood flow during microvascular remodeling

Microvascular endothelial cell heterogeneity and its relationship to hemodynamics remains poorly understood due to a lack of sufficient methods to examine these parameters in vivo at high resolution throughout an angiogenic network. The availability of surrogate markers for functional vascular proteins, such as green fluorescent protein, enables expression in individual cells to be followed over time using confocal microscopy, while photoacoustic microscopy enables dynamic measurement of blood flow across the network with capillary-level resolution. We combined these two non-invasive imaging modalities in order to spatially and temporally analyze biochemical and biomechanical drivers of angiogenesis in murine corneal neovessels. By stimulating corneal angiogenesis with an alkali burn in Tie2-GFP fluorescent-reporter mice, we evaluated how onset of blood flow and surgically-altered blood flow affects Tie2-GFP expression. Our study establishes a novel platform for analyzing heterogeneous blood flow and fluorescent reporter protein expression across a dynamic microvascular network in an adult mammal.

Nanoparticle drug delivery systems and their use in cardiac tissue therapy

Cardiovascular diseases make up one of the main causes of death today, with myocardial infarction and ischemic heart disease contributing a large share of the deaths reported. With mainstream clinical therapy focusing on palliative medicine following myocardial infarction, the structural changes that occur in the diseased heart will eventually lead to end-stage heart failure. Heart transplantation remains the only gold standard of cure but a shortage in donor organs pose a major problem that led to clinicians and researchers looking into alternative strategies for cardiac repair. This review will examine some alternative methods of treatment using chemokines and drugs carried by nanoparticles as drug delivering agents for the purposes of treating myocardial infarction through the promotion of revascularization. We will also provide an overview of existing studies involving such nanoparticulate drug delivery systems, their reported efficacy and the challenges facing their translation into ubiquitous clinical use.

Muscle weakness during aging: a deficiency state involving declining angiogenesis

This essay begins by proposing that muscle weakness of old age from sarcopenia is due in large part to reduced capillary density in the muscles, as documented in 9 reports of aged persons and animals. Capillary density (CD) is determined by local levels of various angiogenic factors, which also decline in muscles with aging, as reported in 7 studies of old persons and animals. There are also numerous reports of reduced CD in the aged brain and other studies showing reduced CD in the kidney and heart of aged animals. Thus a waning angiogenesis throughout the body may be a natural occurrence in later years and may account significantly for the lesser ailments (physical and cognitive) of elderly people. Old age is regarded here as a deficiency state which may be corrected by therapeutic angiogenesis, much as a hormonal deficiency can be relieved by the appropriate hormone therapy. Such therapy could employ recombinant angiogenic factors which are now commercially available.

Scaffold biomaterials for nano-pathophysiology

This review is intended to provide an overview of tissue engineering strategies using scaffold biomaterials to develop a vascularized tissue engineered construct for nano-pathophysiology. Two primary topics are discussed. The first is the biological or synthetic microenvironments that regulate cell behaviors in pathological conditions and tissue regeneration. Second is the use of scaffold biomaterials with angiogenic factors and/or cells to realize vascularized tissue engineered constructs for nano-pathophysiology. These topics are significantly overlapped in terms of three-dimensional (3-D) geometry of cells and blood vessels. Therefore, this review focuses on neovascularization of 3-D scaffold biomaterials induced by angiogenic factors and/or cells. The novel strategy of this approach in nano-pathophysiology is to utilize the vascularized tissue engineered construct as a tissue model to predict the distribution and subsequent therapeutic efficacy of a drug delivery system with different physicochemical and biological properties.

The Experimental Therapy on Brain Ischemia by Improvement of Local Angiogenesis with Tissue Engineering in the Mouse

Neural restoration has proven to be difficult after brain stroke, especially in its chronic stage. This is mainly due to the generation of an unpropitious niche in the injured area, including loss of vascular support but production of numerous inhibitors against neuronal regeneration. Reconstruction of a proper niche for promoting local angiogenesis, therefore, should be a key approach for neural restoration after stroke. In the present study, a new biomaterial composite that could be implanted in the injured area of the brain was created for experimental therapy of brain ischemia in the mouse. This composite was made using a hyaluronic acid (HA)-based biodegradable hydrogel scaffold, mixed with poly(lactic- co-glycolic acid) (PLGA) microspheres containing vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang1), two factors that stimulate angiogenesis. In addition, the antibody of Nogo receptor (NgR-Ab), which can bind to multiple inhibitory myelin proteins and promote neural regeneration, was covalently attached to the hydrogel, making the hydrogel more bioactive and suitable for neural survival. This composite (HA–PLGA) was implanted into the mouse model with middle cerebral artery occlusion (MCAO) to explore a new approach for restoration of brain function after ischemia. A good survival and proliferation of human umbilical artery endothelial cells (HUAECs) and neural stem cells (NSCs) were seen on the HA hydrogel with PLGA microspheres in vitro. This new material was shown to have good compatibility with the brain tissue and inhibition to gliosis and inflammation after its implantation in the normal or ischemic brain of mice. Particularly, good angiogenesis was found around the implanted HA–PLGA hydrogel, and the mouse models clearly showed a behavioral improvement. The results in this present study indicate, therefore, that the HA–PLGA hydrogel is a promising material, which is able to induce angiogenesis in the ischemic region by releasing VEGF and Ang1, thus creating a suitable niche for neural restoration in later stages of stroke. This manuscript is published as part of the International Association of Neurorestoratology (IANR) special issue of Cell Transplantation.

Enhancing microvascular formation and vessel maturation through temporal control over multiple pro-angiogenic and pro-maturation factors

Therapeutic stimulation of angiogenesis to re-establish blood flow in ischemic tissues offers great promise as a treatment for patients suffering from cardiovascular disease or trauma. Since angiogenesis is a complex, multi-step process, different signals may need to be delivered at appropriate times in order to promote a robust and mature vasculature. The effects of temporally regulated presentation of pro-angiogenic and pro-maturation factors were investigated in vitro and in vivo in this study. Pro-angiogenic factors vascular endothelial growth factor (VEGF) and angiopoietin 2 (Ang2) cooperatively promoted endothelial sprouting and pericyte detachment in a three-dimensional in vitro EC-pericyte co-culture model. Pro-maturation factors platelet-derived growth factor B (PDGF) and angiopoietin 1 (Ang1) inhibited the early stages of VEGF- and Ang2-mediated angiogenesis if present simultaneously with VEGF and Ang2, but promoted these behaviors if added subsequently to the pro-angiogenesis factors. VEGF and Ang2 were also found to additively enhance microvessel density in a subcutaneous model of blood vessel formation, while simultaneously administered PDGF/Ang1 inhibited microvessel formation. However, a temporally controlled scaffold that released PDGF and Ang1 at a delay relative to VEGF/Ang2 promoted both vessel maturation and vascular remodeling without inhibiting sprouting angiogenesis. Our results demonstrate the importance of temporal control over signaling in promoting vascular growth, vessel maturation and vascular remodeling. Delivering multiple growth factors in combination and sequence could aid in creating tissue engineered constructs and therapies aimed at promoting healing after acute wounds and in chronic conditions such as diabetic ulcers and peripheral artery disease.

A systems biology view of blood vessel growth and remodelling

Blood travels throughout the body in an extensive network of vessels – arteries, veins and capillaries. This vascular network is not static, but instead dynamically remodels in response to stimuli from cells in the nearby tissue. In particular, the smallest vessels – arterioles, venules and capillaries – can be extended, expanded or pruned, in response to exercise, ischaemic events, pharmacological interventions, or other physiological and pathophysiological events. In this review, we describe the multi-step morphogenic process of angiogenesis – the sprouting of new blood vessels – and the stability of vascular networks in vivo. In particular, we review the known interactions between endothelial cells and the various blood cells and plasma components they convey. We describe progress that has been made in applying computational modelling, quantitative biology and high-throughput experimentation to the angiogenesis process.

Phase II angiogenesis stimulators

INTRODUCTION

Therapeutic angiogenesis is a strategy of inducing new collateral vessels and stimulating new capillaries that enhance tissue oxygen exchange in ischemic cardiovascular disorders, including acute myocardial infarction, chronic cardiac ischemia, peripheral artery disease and stroke.

AREAS COVERED

Over the last 10 years, promising results of early clinical trials have generated great expectation on the potential of therapeutic angiogenesis. However, even if large randomized placebo-controlled and double-blinded Phase II clinical trials have confirmed the feasibility, safety and potential effectiveness of therapeutic angiogenesis, they provided very limited evidence of its efficacy in terms of clinical benefit.

EXPERT OPINION

Results of the latest trials on therapeutic angiogenesis have not provided satisfactory results. Much is still unknown about the optimal delivery of angiogenic factors. Trials using alternative growth factors, dose regimens and methods of delivery are needed to enhance the treatment benefit of therapeutic angiogenesis.

Angiogenic therapy for cardiac repair based on protein delivery systems

Cardiovascular diseases remain the first cause of morbidity and mortality in the developed countries and are a major problem not only in the western nations but also in developing countries. Current standard approaches for treating patients with ischemic heart disease include angioplasty or bypass surgery. However, a large number of patients cannot be treated using these procedures. Novel curative approaches under investigation include gene, cell, and protein therapy. This review focuses on potential growth factors for cardiac repair. The role of these growth factors in the angiogenic process and the therapeutic implications are reviewed. Issues including aspects of growth factor delivery are presented in relation to protein stability, dosage, routes, and safety matters. Finally, different approaches for controlled growth factor delivery are discussed as novel protein delivery platforms for cardiac regeneration.

Alginate-based strategies for therapeutic vascularization

Therapeutic stimulation of vessel growth to improve tissue perfusion has shown promise in many regenerative medicine and tissue engineering applications. Alginate-based biomaterial systems have been investigated for growth factor and/or cell delivery as tools for modulating vessel assembly. Growth factor encapsulation allows for a sustained release of protein and protection from degradation. Implantation of growth factor-loaded alginate constructs typically shows an increase in capillary density but without vascular stabilization. Delivery of multiple factors may improve these outcomes. Cell delivery approaches focus on stimulating vascularization either via cell release of soluble factors, cell proliferation and incorporation into new vessels or alginate prevascularization prior to implantation. These methods have shown some promise but routine clinical application has not been achieved. In this review, current research on the application of alginate for therapeutic neovascularization is presented, shortcomings are addressed and the future direction of these systems discussed.

Affinity improvement of a VEGF aptamer by in silico maturation for a sensitive VEGF-detection system

Systematic evolution of ligands by exponential enrichment (SELEX) is an efficient method to identify aptamers; however, it sometimes fails to identify aptamers that bind to their target with high affinity. Thus, post-SELEX optimization of aptamers is required to improve aptamer binding affinity. We developed in silico maturation based on a genetic algorithm (1) as an efficient mutagenesis method to improve aptamer binding affinity. In silico maturation was performed to improve a VEGF-binding DNA aptamer (VEap121). The VEap121 aptamer is considered to fold into a G-quadruplex structure and this structure may be important for VEGF recognition. Using in silico maturation, VEap121 was mutated with the exception of the guanine tracts that are considered to form the G-quartet. As a result, four aptamers were obtained that showed higher affinity compared with VEap121. The dissociation constant (K(d)) of the most improved aptamer (3R02) was 300 pM. The affinity of 3R02 was 16-fold higher than that of VEap121. Moreover, a bivalent aptamer was constructed by connecting two identical 3R02s through a 10-mer thymine linker for further improvement of affinity. The bivalent aptamer (3R02 Bivalent) bound to VEGF with a K(d) value of 30 pM. Finally, by constructing a VEGF-detection system using a VEGF antibody as the capture molecule and monovalent 3R02 as the detection molecule, a more sensitive assay was developed compared with the system using VEap121. These results indicate that in silico maturation could be an efficient method to improve aptamer affinity for construction of sensitive detection systems.

Advances in growth factor delivery for therapeutic angiogenesis

Therapeutic angiogenesis is a new revascularization strategy involving the administration of growth factors to induce new vessel formation. The biology and delivery of angiogenic growth factors involved in vessel formation have been extensively studied but success in translating the angiogenic capacity of growth factors into benefits for vascular disease patients is still limited. This could be attributed to issues related to patient selection, growth factor delivery methods or lack of vessel maturation. Comprehensive understanding of the cellular and molecular cross-talk during the different stages of vascular development is needed for the design of efficient therapeutic strategies. The presentation of angiogenic factors either in series or in parallel using a strategy that mimics physiological events, such as concentration and spatio-temporal profiles, is an immediate requirement for functional blood vessel formation. This review provides an overview of the recent delivery strategies of angiogenic factors and discusses targeting neovascular maturation as a promising approach to induce stable and functional vessels for therapeutic angiogenesis.

Engineering vascularized tissues using natural and synthetic small molecules

Vascular growth and remodeling are complex processes that depend on the proper spatial and temporal regulation of many different signaling molecules to form functional vascular networks. The ability to understand and regulate these signals is an important clinical need with the potential to treat a wide variety of disease pathologies. Current approaches have focused largely on the delivery of proteins to promote neovascularization of ischemic tissues, most notably VEGF and FGF. Although great progress has been made in this area, results from clinical trials are disappointing and safer and more effective approaches are required. To this end, biological agents used for therapeutic neovascularization must be explored beyond the current well-investigated classes. This review focuses on potential pathways for novel drug discovery, utilizing small molecule approaches to induce and enhance neovascularization. Specifically, four classes of new and existing molecules are discussed, including transcriptional activators, receptor selective agonists and antagonists, natural product-derived small molecules, and novel synthetic small molecules.

Sustained improvement in perfusion and flow reserve after temporally separated delivery of vascular endothelial growth factor and angiopoietin-1 plasmid deoxyribonucleic acid

OBJECTIVES

The aim of this study was to compare temporally separated vascular endothelial growth factor (VEGF) and angiopoietin (Ang)-1 delivery with concomitant delivery or single VEGF delivery, for therapeutic angiogenesis in chronic ischemia.

BACKGROUND

Single gene delivery of VEGF results in immature neovessels that ultimately regress. Endogenously, VEGF acts early to initiate angiogenesis, whereas Ang-1 acts later to induce vessel maturation. Timing VEGF and Ang-1 gene delivery to mimic endogenous angiogenesis might be more effective for sustained neovascularization.

METHODS

Unilateral hindlimb ischemia was induced in 170 rats. Ultrasound-mediated gene delivery was performed with cationic microbubbles and plasmid deoxyribonucleic acid. Groups included VEGF at 2 weeks, VEGF/Ang-1 at 2 weeks, VEGF at 2 weeks with Ang-1 at 4 weeks, and untreated control subjects. At 2, 4, and 8 weeks after ligation, blood flow and flow reserve (FR) were assessed by contrast-enhanced ultrasound. Vascular density, organization, and supporting cell coverage were assessed by fluorescent microangiography and immunohistochemistry.

RESULTS

In untreated control subjects, blood flow, FR, and vessel density remained reduced. The VEGF delivery improved flow and vessel density at 4 weeks; however, FR remained low, supporting cell coverage was poor, and flow and vessel density regressed by 8 weeks. The VEGF/Ang-1 co-delivery marginally increased flow and vessel density; however, FR and supporting cell coverage improved. After temporally separated VEGF and Ang-1 delivery, blood flow, vessel density, and FR increased and were sustained, with improved pericyte coverage at 8 weeks.

CONCLUSIONS

In conclusion, temporally separated VEGF and Ang-1 gene therapy results in sustained and functional neovascularization.

Dynamics of angiogenesis during wound healing: a coupled in vivo and in silico study

OBJECTIVE

The most critical determinant of restoration of tissue structure during wound healing is the re-establishment of a functional vasculature, which largely occurs via angiogenesis, specifically endothelial sprouting from the pre-existing vasculature.

MATERIALS AND METHODS

We used confocal microscopy to capture sequential images of perfused vascular segments within the injured panniculus carnosus muscle in the mouse dorsal skin-fold window chamber to quantify a range of microcirculatory parameters during the first nine days of healing. This data was used to inform a mathematical model of sequential growth of the vascular plexus. The modeling framework mirrored the experimental circular wound domain and incorporated capillary sprouting and endothelial cell (EC) sensing of vascular endothelial growth factor gradients.

RESULTS

Wound areas, vessel densities and vessel junction densities obtained from the corresponding virtual wound were in excellent agreement both temporally and spatially with data measured during the in vivo healing process. Moreover, by perturbing the proliferative ability of ECs in the mathematical model, this leads to a severe reduction in vascular growth and poor healing. Quantitative measures from this second set of simulations were found to correlate extremely well with experimental data obtained from animals treated with an agent that targets endothelial proliferation (TNP-470).

CONCLUSION

Our direct combination and comparison of in vivo longitudinal analysis (over time in the same animal) and mathematical modeling employed in this study establishes a useful new paradigm. The virtual wound created in this study can be used to investigate a wide range of experimental hypotheses associated with wound healing, including disorders characterized by aberrant angiogenesis (e.g., diabetic models) and the effects of vascular enhancing/disrupting agents or therapeutic interventions such as hyperbaric oxygen.

Wide-field functional imaging of blood flow and hemoglobin oxygen saturation in the rodent dorsal window chamber

The rodent dorsal window chamber is a widely used in vivo model of the microvasculature. The model consists of a 1cm region of exposed microvasculature in the rodent dorsal skin that is immobilized by surgically implanted titanium frames, allowing the skin microvasculature to be visualized. We describe a detailed protocol for surgical implantation of the dorsal window chamber which enables researchers to perform the window chamber implantation surgery. We further describe subsequent wide-field functional imaging of the chamber to obtain hemodynamic information in the form of blood oxygenation and blood flow on a cm size region of interest. Optical imaging techniques, such as intravital microscopy, have been applied extensively to the dorsal window chamber to study microvascular-related disease and conditions. Due to the limited field of view of intravital microscopy, detailed hemodynamic information typically is acquired from small regions of interest, typically on the order of hundreds of μm. The wide-field imaging techniques described herein complement intravital microscopy, allowing researchers to obtain hemodynamic information at both microscopic and macroscopic spatial scales. Compared with intravital microscopy, wide-field functional imaging requires simple instrumentation, is inexpensive, and can give detailed metabolic information over a wide field of view.

Vascular gene transfer of SDF-1 promotes endothelial progenitor cell engraftment and enhances angiogenesis in ischemic muscle

Gene therapy approaches to enhance endothelial progenitor cell (EPC) homing may augment cell engraftment to ischemic tissue and lead to a greater therapeutic response. Therefore, we assessed the effects of ultrasound-mediated (UM) transfection of the chemokine stromal cell-derived factor-1 (SDF-1) on homing and engraftment of intravenously administered EPCs and the subsequent angiogenic response in chronically ischemic skeletal muscle. Bone marrow-derived EPCs were isolated from donor Fisher 344 rats, cultured and labeled in preparation for injection into recipient animals via a jugular vein. Using a model of chronic hindlimb ischemia in rats, we demonstrated that UM destruction of intravenous carrier microbubbles loaded with SDF-1 plasmid DNA resulted in targeted transfection of the vascular endothelium within ischemic muscle and greater local engraftment of EPCs. The combination of SDF-1gene therapy and EPCs lead to the greatest increase in tissue perfusion and microvascular density within ischemic muscle, compared to no treatment or either monotherapy alone. Our results demonstrate that UM transfection of SDF-1 improves EPC targeting to chronically ischemic tissue, enhancing vascular engraftment and leading to a more robust neovascularization response.

Selective activation of sphingosine 1-phosphate receptors 1 and 3 promotes local microvascular network growth

Proper spatial and temporal regulation of microvascular remodeling is critical to the formation of functional vascular networks, spanning the various arterial, venous, capillary, and collateral vessel systems. Recently, our group has demonstrated that sustained release of sphingosine 1-phosphate (S1P) from biodegradable polymers promotes microvascular network growth and arteriolar expansion. In this study, we employed S1P receptor-specific compounds to activate and antagonize different combinations of S1P receptors to elucidate those receptors most critical for promotion of pharmacologically induced microvascular network growth. We show that S1P(1) and S1P(3) receptors act synergistically to enhance functional network formation via increased functional length density, arteriolar diameter expansion, and increased vascular branching in the dorsal skinfold window chamber model. FTY720, a potent activator of S1P(1) and S1P(3), promoted a 107% and 153% increase in length density 3 and 7 days after implantation, respectively. It also increased arteriolar diameters by 60% and 85% 3 and 7 days after implantation. FTY720-stimulated branching in venules significantly more than unloaded poly(D, L-lactic-co-glycolic acid). When implanted on the mouse spinotrapezius muscle, FTY720 stimulated an arteriogenic response characterized by increased tortuosity and collateralization of branching microvascular networks. Our results demonstrate the effectiveness of S1P(1) and S1P(3) receptor-selective agonists (such as FTY720) in promoting microvascular growth for tissue engineering applications.

Strategies for vascularization of polymer scaffolds

Biocompatible, degradable polymer scaffolds combined with cells or biological signals are being investigated as alternatives to traditional options for tissue reconstruction and transplantation. These approaches are already in clinical use as engineered tissues that enhance wound healing and skin regeneration. The continued enhancement of these material strategies is highly dependent on the ability to promote rapid and stable neovascularization (new blood vessel formation) within the scaffold. Whereas neovascularization therapies have shown some promise for the treatment of ischemic tissues, vascularization of polymer scaffolds in tissue engineering strategies provides a unique challenge owing to the volume and the complexity of the tissues targeted. In this article, we examine recent advances in research focused on promoting neovascularization in polymer scaffolds for tissue engineering applications. These approaches include the use of growth factors, cells, and novel surgical approaches to both enhance and control the nature of the vascular networks formed. The continued development of these approaches may lead to new tissue engineering strategies for the generation of skin and other tissues or organs.

Dose-dependent systolic contribution of differentiated stem cells in post-infarct ventricular function

BACKGROUND

Differentiation of bone marrow stem cells toward cardiomyocytes has been widely reported in vitro. However, optimum cell types and mechanisms leading to functional improvement in cardiac cell therapy remain unresolved. There is limited evidence showing a dose-dependent effect of transplanted cells in contributing to functional recovery. This study showed that cell transplantation of differentiated cardiomyocyte-like cells (CLCs) and undifferentiated mesenchymal stem cells (MSCs) dose-dependently improved left ventricular function in a rat myocardial infarction model.

METHODS

At 1 week after infarction in Wistar rats, 1 × 10(6) MSCs (n = 9) or CLCs (n = 9) and 5 × 10(6) MSCs (n = 18) or CLCs (n = 15) were injected into peri-infarcted myocardium to study their effect after 6 weeks.

RESULTS

High-dose CLCs exhibited a dose-response that was significantly more effective than MSCs in recovering cardiac contractility. Superiority of CLCs over MSCs was demonstrated in load-independent measurement of the end-systolic pressure-volume relationship and pre-load recruitable stroke work, but not in the end-diastolic pressure-volume relationship. These findings showed a unique systolic role of CLCs in contractility recovery. Functional improvement mediated by MSCs was mainly derived from preservation of endogenous myocyte function and restriction of chamber dilatation by enhancing intramyocardial angiogenesis during post-infarct ventricular remodeling. Engrafted CLCs showed better survival, were strategically integrated into myofiber-associated collagen V matrix, and exhibited mature sarcomeric cross-striations. Vascular differentiation, but not cardiac, was observed with MSCs.

CONCLUSION

These cell type-specific effects suggest that committing stem cells to a cardiac phenotype ex vivo promoted mechanical and functional integration of CLCs into the myofibrillar syncytium of infarcted myocardium.

Vascular tissue engineering by computer-aided laser micromachining

Many conventional technologies for fabricating tissue engineering scaffolds are not suitable for fabricating scaffolds with patient-specific attributes. For example, many conventional technologies for fabricating tissue engineering scaffolds do not provide control over overall scaffold geometry or over cell position within the scaffold. In this study, the use of computer-aided laser micromachining to create scaffolds for vascular tissue networks was investigated. Computer-aided laser micromachining was used to construct patterned surfaces in agarose or in silicon, which were used for differential adherence and growth of cells into vascular tissue networks. Concentric three-ring structures were fabricated on agarose hydrogel substrates, in which the inner ring contained human aortic endothelial cells, the middle ring contained HA587 human elastin and the outer ring contained human aortic vascular smooth muscle cells. Basement membrane matrix containing vascular endothelial growth factor and heparin was to promote proliferation of human aortic endothelial cells within the vascular tissue networks. Computer-aided laser micromachining provides a unique approach to fabricate small-diameter blood vessels for bypass surgery as well as other artificial tissues with complex geometries.

Dual delivery of VEGF and MCP-1 to support endothelial cell transplantation for therapeutic vascularization

Transplantation of endothelial cells (EC) for therapeutic vascularization is a promising approach in tissue engineering but has yet to be proven effective in clinical trials. This cell-based therapy is hindered by significant apoptosis of EC upon transplantation as well as poor recruitment of host mural cells to stabilize nascent vessels. Here, we address these deficiencies by augmenting endothelial cell transplantation with dual delivery of vascular endothelial growth factor (VEGF) - to improve survival of transplanted EC - and monocyte chemotactic protein-1 (MCP-1) - to induce mural cell recruitment. We produced alginate microparticles that deliver VEGF and MCP-1 with distinct release kinetics and that can be integrated into a collagen/fibronectin (protein) gel construct for delivery of EC. Combined delivery of VEGF and MCP-1 increased functional vessel formation from transplanted EC and also led to a higher number of smooth muscle cell-invested vessels than did EC therapy alone. Despite the well-known role of MCP-1 in inflammation, these beneficial effects were accomplished without a long-term increase in monocyte/macrophage recruitment or a shift to a pro-inflammatory (M1) macrophage phenotype. Overall, these data suggest a potential benefit of combined delivery of MCP-1 and VEGF from EC-containing hydrogels as a strategy for therapeutic vascularization.

Angiogenesis and Microvascular Remodeling: A Brief History and Future Roadmap

Angiogenesis and vessel remodeling determine the integrative control of the architectural structure and functional behaviors of the microcirculation over the lifetime of an organism. Vascular remodeling is the basis of promising therapeutic strategies, including vascularization of ischemic organs. The history of angiogenesis research is long-more than 250 years-and the Microcirculatory Society has been the birthplace of numerous techniques, assays, and scientific concepts that have stimulated massive research endeavors in the pharmaceutical and medical arena. At present, angiogenesis isa dynamic field in which the molecular genetic and proteomic components of the process are still being identified, while integrative systems approaches are once again being recognized as essential to understand microvascular assembly in vivo across multiple scales from cells to whole vessel networks. A short history of people and ideas in this field is presented, followed by discussion of emerging directions receiving intense attention today and major questions that remain unanswered. The primary conclusion is that the need for scientists trained in the integrative approaches nurtured by the Microcirculatory Society over the past 50 years has never been greater, as it is clear that a complete mechanistic understanding of vessel adaptation (based on genomic and proteomic supporting casts) will now require deeper studies of angiogenesis and microvascular remodeling in the exquisite complexity of the native microenvironment-the microcirculation.

Chronic whole-body hypoxia induces intussusceptive angiogenesis and microvascular remodeling in the mouse retina

Currently, little is known about the response of the adult retinal microvasculature to hypoxia. To test the hypothesis that chronic systemic hypoxia induces angiogenesis and microvascular remodeling in the adult mouse retina, adult 10-week old female C57Bl/6 mice were exposed to 10% O(2) for 2 or 3 weeks. After hypoxia exposure, retinas were harvested, whole-mounted, and processed for immunohistochemistry. Retinas were stained with lectin, anti-smooth muscle alpha-actin antibody, and anti-NG2 antibody to visualize microvascular networks and their cellular components. Confocal microscopy was used to obtain images of superficial retinal networks. Images were analyzed to assess vessel diameter, vascular length density, branch point density, and the presence of vascular loops, a hallmark of intussusceptive angiogenesis. Both 2 and 3 weeks of hypoxia exposure resulted in a significant increase in the diameters of arterioles and post-arteriole capillaries (p<0.003). After 3 weeks of hypoxia, vascular length density and branch point density were significantly increased in retinas exposed to hypoxia as compared to normoxic controls (p<0.001). The number of vascular loops in the superficial retinal networks was significantly greater in hypoxia-exposed retinas (p < or = 0.001). Our results demonstrate, for the first time, intussusceptive angiogenesis as a tissue-level mechanism of vascular adaptation to chronic systemic hypoxia in the adult mouse retina and contribute to our understanding of hypoxia-induced angiogenesis and microvascular remodeling in the adult animal.

Materials for engineering vascularized adipose tissue

Loss of adipose tissue can occur due to congenital and acquired lipoatrophies, trauma, tumor resection, and chronic disease. Clinically, it is difficult to regenerate or reconstruct adipose tissue. The extensive microvsacular network present in adipose, and the sensitivity of adipocytes to hypoxia, hinder the success of typical tissue transfer procedures. Materials that promote the formation of vascularized adipose tissue may offer alternatives to current clinical treatment options. A number of synthetic and natural biomaterials common in tissue engineering have been investigated as scaffolds for adipose regeneration. While these materials have shown some promise they do not account for the unique extracellular microenvironment of adipose. Adipose derived hydrogels more closely approximate the physical and chemical microenvironment of adipose tissue, promote preadipocyte differentiation and vessel assembly in vitro, and stimulate vascularized adipose formation in vivo. The combination of these materials with techniques that promote rapid and stable vascularization could lead to new techniques for engineering stable, vascularized adipose tissue for clinical application. In this review we discuss materials used for adipose tissue engineering and strategies for vascularization of these scaffolds.

CLINICAL RELEVANCE

Materials that promote formation of vascularized adipose tissue have the potential to serve as alternatives or supplements to existing treatment options, for adipose defects or deficiencies resulting from chronic disease, lipoatrophies, trauma, and tumor resection.

Primary Study on Transplantation of Endothelialized Dermal Equivalents Into Normal Rats

This study was designed to determine the ability of human umbilical vein endothelial cells (HUVEC) in dermal equivalent (DE) to form microvessel-like tubes after transplantation into normal rats. A mixture of rat fibroblasts and HUVEC was inosculated into collagen-chitosan sponges to prepare endothelialized dermal equivalents (EDE). After culture in vitro for 24 hours, inosculated cells dispersed throughout the sponges and the equivalents were transplanted subcutaneously into the back of normal Lewis rats. Anti-human specific CD31 antibody was used for immunohistochemical localization of human endothelial cells in sections of EDE excised from rats after grafting. HUVEC in EDE organized into microvessel-like tubes at the end of the first week after transplantation, which still persisted after two weeks. The host microvessels began to pervade both DE and EDE during the second week after transplantation. These results demonstrated that HUVEC in EDE was able to persist and form microvessel-like tubes after transplantation into normal rats, and this is the first time to transplant DE containing HUVEC into normal rats.

The influence of the sequential delivery of angiogenic factors from affinity-binding alginate scaffolds on vascularization

This study describes the features of tissue-engineering scaffold capable of sequentially delivering three angiogenic factors. The scaffold consists of alginate-sulfate/alginate, wherein vascular endothelial growth factor (VEGF) platelet-derived growth factor-BB (PDGF-BB) and transforming growth factor-beta1 (TGF-beta1) are bound to alginate-sulfate with an affinity similar to that realized upon their binding to heparin. Factor release rate from the scaffold was correlated with the equilibrium binding constants of the factors to the matrix, thus enabling the sequential delivery of VEGF, PDGF-BB and TGF-beta1. In alginate scaffolds lacking alginate-sulfate, release of the adsorbed proteins was instantaneous. After subcutaneous implantation for 1 and 3 months in rats, the blood vessel density and percentage of mature vessels were 3-fold greater in the triple factor-bound scaffolds than in the factor-adsorbed or untreated scaffolds. Moreover, vascularization within the triple factor-bound scaffolds was superior to that found in scaffolds delivering only basic fibroblast growth factor. Application of this novel scaffold may be extended to the combined delivery of additional heparin-binding angiogenic factors or combinations of growth factors active in different tissue regeneration processes.

Sustained release of sphingosine 1-phosphate for therapeutic arteriogenesis and bone tissue engineering

Sphingosine 1-phosphate (S1P) is a bioactive phospholipid that impacts migration, proliferation, and survival in diverse cell types, including endothelial cells, smooth muscle cells, and osteoblast-like cells. In this study, we investigated the effects of sustained release of S1P on microvascular remodeling and associated bone defect healing in vivo. The murine dorsal skinfold window chamber model was used to evaluate the structural remodeling response of the microvasculature. Our results demonstrated that 1:400 (w/w) loading and subsequent sustained release of S1P from poly(lactic-co-glycolic acid) (PLAGA) significantly enhanced lumenal diameter expansion of arterioles and venules after 3 and 7 days. Incorporation of 5-bromo-2-deoxyuridine (BrdU) at day 7 revealed significant increases in mural cell proliferation in response to S1P delivery. Additionally, three-dimensional (3D) scaffolds loaded with S1P (1:400) were implanted into critical-size rat calvarial defects, and healing of bony defects was assessed by radiograph X-ray, microcomputed tomography (muCT), and histology. Sustained release of S1P significantly increased the formation of new bone after 2 and 6 weeks of healing and histological results suggest increased numbers of blood vessels in the defect site. Taken together, these experiments support the use of S1P delivery for promoting microvessel diameter expansion and improving the healing outcomes of tissue-engineered therapies.

The role of actively released fibrin-conjugated VEGF for VEGF receptor 2 gene activation and the enhancement of angiogenesis

A major challenge for therapeutic delivery of angiogenic agents such as vascular endothelial growth factor (VEGF) is to achieve sustained, low dose signaling leading to durable neovessel formation. To this end, we recently created a variant of VEGF(121), TG-VEGF(121) that directly binds to fibrin and gets released locally in proteolysis-triggered manner. Here we combined noninvasive biophotonic monitoring of VEGF receptor 2 gene activation in transgenic VEGFR2-luc mice and histomorphometry to compare endothelial activation and long-term neovascularization by actively released TG-VEGF(121)versus passively released, diffusible wild-type VEGF(121) in subcutaneous fibrin implants. Monitoring in real-time over 3 weeks of luciferase signal driven by the VEGFR2 promoter revealed endothelial activation in skin exposed to wild-type VEGF(121), but no detectable elevation over fibrin alone by TG-VEGF(121). Histology at 3 weeks, however, demonstrated that TG-VEGF(121) promoted vessel growth significantly more effectively and reliably than wild-type VEGF(121). The majority of vessels surviving to 3 weeks contained stabilizing smooth muscle cells. Yet, by 6 weeks, no extra vessels induced by exogenous VEGF were left. In conclusion, release of fibrin-conjugated variant TG-VEGF(121) elicited lower VEGFR2-luc activation than wild-type VEGF(121) yet significantly more vascularization. In the absence of true physiological demand, even stabilized vessels are ultimately regressed.

Hydrogels for tissue engineering and delivery of tissue-inducing substances

This manuscript presents hydrogels (HGs) from a tissue engineering perspective being especially written for those who are approaching this field by offering a concise but inclusive review of hydrogel synthesis, properties, characterization methods, and applications.

Spatiotemporal control over growth factor signaling for therapeutic neovascularization

Many of the qualitative roles of growth factors involved in neovascularization have been delineated, but it is unclear yet from an engineering perspective how to use these factors as therapies. We propose that an approach that integrates quantitative spatiotemporal measurements of growth factor signaling using 3-D in vitro and in vivo models, mathematic modeling of factor tissue distribution, and new delivery technologies may provide an opportunity to engineer neovascularization on demand.

Angiopoietin: a TIE(d) balance in tumor angiogenesis

Angiopoietins (ANG-1 and ANG-2) and their TIE-2 receptor tyrosine kinase have wide-ranging effects on tumor malignancy that includes angiogenesis, inflammation, and vascular extravasation. These multifaceted pathways present a valuable opportunity in developing novel inhibition strategies for cancer treatment. However, the regulatory role of ANG-1 and ANG-2 in tumor angiogenesis remains controversial. There is a complex interplay between complementary yet conflicting roles of both the ANGs in shaping the outcome of angiogenesis. Embryonic vascular development suggests that ANG-1 is crucial in engaging interaction between endothelial and perivascular cells. However, recruitment of perivascular cells by ANG-1 has recently been implicated in its antiangiogenic effect on tumor growth. It is becoming clear that TIE-2 signaling may function in a paracrine and autocrine manner directly on tumor cells because the receptor has been increasingly found in tumor cells. In addition, alpha(5)beta(1) and alpha(v)beta(5) integrins were recently recognized as functional receptors for ANG-1 and ANG-2. Therefore, both the ligands may have wide-ranging functions in cellular activities that affect overall tumor development. Collectively, these TIE-2-dependent and TIE-2-independent activities may account for the conflicting findings of ANG-1 and ANG-2 in tumor angiogenesis. These uncertainties have impeded development of a clear strategy to target this important angiogenic pathway. A better understanding of the molecular basis of ANG-1 and ANG-2 activity in the pathophysiologic regulation of angiogenesis may set the stage for novel therapy targeting this pathway.

Dose response of angiogenesis to basic fibroblast growth factor in rat corneal pocket assay: I. Experimental characterizations

Understanding mechanisms of formation of vascular networks under different experimental conditions is essential for improving treatment of angiogenesis-dependent diseases. To this end, we investigated the dose response of angiogenesis to basic fibroblast growth factor (bFGF) using the rat corneal pocket assay. The response was quantified, in terms of (i) the migration distance of vascular networks, (ii) the total vessel length, (iii) the distribution of the projected width of vessels, (iv) the distribution of the number of vessels, and (v) the distribution of vessel diameters. The quantification was based on new image analysis methods developed in the study. It was observed that the migration distance and the total vessel length increased by 82% and 199%, respectively, when the dose of bFGF was increased from 5 ng to 50 ng. The number and the diameter of vessels increased with the dose of bFGF as well. However, the last two parameters at a given dose of bFGF were approximately independent of the location in the middle region between the pellet and the limbus. These results provided useful information for understanding mechanisms of angiogenesis induced by bFGF and important data for validating a mathematical model of angiogenesis described in the second part of the study.

Polyelectrolyte Complexes Stabilize and Controllably Release Vascular Endothelial Growth Factor

Angiogenesis has long been a desired therapeutic approach to improve clinical outcomes of conditions typified by ischemia. Vascular endothelial growth factor (VEGF) has demonstrated the ability to generate new blood vessels in vivo, but trials using intravenous delivery have not yet produced clinical success. Localized, sustained delivery of VEGF has been proven necessary to generate blood vessels as demonstrated by implantable, controlled release devices. Ultimately, nanoparticles delivered by intravenous injection may be designed to accumulate in target tissues and sustain the local VEGF concentration; however, injectable nanosuspensions that control the release of stabilized VEGF must first be developed. In this study, we utilize the heparin binding domain of VEGF to bind the polyanion dextran sulfate, resulting in an enhanced thermal stability of VEGF. Coacervation of the VEGF-bound dextran sulfate with selected polycations (chitosan, polyethylenimine, or poly-L-lysine) produced nanoparticles approximately 250 nm in diameter with high VEGF encapsulation efficiency (50-85%). Release of VEGF from these formulations persisted for >10 days and maintained high VEGF activity as determined by ELISA and a mitogenic bioassay. Chitosan-dextran sulfate complexes were preferred because of their biodegradability, desirable particle size ( approximately 250 nm), entrapment efficiency ( approximately 85%), controlled release (near linear for 10 days), and mitogenic activity.

Concurrent delivery of dexamethasone and VEGF for localized inflammation control and angiogenesis

Localized elution of corticosteroids has been used in suppressing inflammation and fibrosis associated with implantation and continuous in vivo residence of bio-medical devices. However, these agents also inhibit endogenous growth factors preventing angiogenesis at the local tissue, interface thereby delaying the healing process and negatively impacting device performance. In this work, a combination of dexamethasone and vascular endothelial growth factor (VEGF) was investigated for concurrent localized delivery using PLGA microsphere/PVA hydrogel composites. Pharmacodynamic effects were evaluated by histopathological examination of subcutaneous tissue surrounding implanted composites using a rat model. The hydrogel composites were capable of simultaneously releasing VEGF and dexamethasone with approximately zero order kinetics. Composites were successful in controlling the implant/tissue interface by suppressing inflammation and fibrosis as well as facilitating neo-angiogenesis at a fraction of their typical oral or i.v. bolus doses. Implants containing VEGF showed a significantly higher number of mature blood vessels at the end of the 4 week study irrespective of the presence of dexamethasone. Thus, localized concurrent elution of VEGF and dexamethasone can overcome the anti-angiogenic effects of the corticosteroid and can be used to engineer inflammation-free and well-vascularized tissue in the vicinity of the implant. These PLGA microsphere/PVA hydrogel composites show promise as coatings for implantable bio-medical devices to improve biocompatibility and ensure in vivo performance.

Spatio–temporal VEGF and PDGF Delivery Patterns Blood Vessel Formation and Maturation

PURPOSE

Biological mechanisms of tissue regeneration are often complex, involving the tightly coordinated spatial and temporal presentation of multiple factors. We investigated whether spatially compartmentalized and sequential delivery of factors can be used to pattern new blood vessel formation.

MATERIALS AND METHODS

A porous bi-layered poly(lactide-co-glycolide) (PLG) scaffold system was used to locally present vascular endothelial growth factor (VEGF) alone in one spatial region, and sequentially deliver VEGF and platelet-derived growth factor (PDGF) in an adjacent region. Scaffolds were implanted in severely ischemic hindlimbs of SCID mice for 2 and 6 weeks, and new vessel formation was quantified within the scaffolds.

RESULTS

In the compartment delivering a high dose of VEGF alone, a high density of small, immature blood vessels was observed at 2 weeks. Sequential delivery of VEGF and PDGF led to a slightly lower blood vessel density, but vessel size and maturity were significantly enhanced. Results were similar at 6 weeks, with continued remodeling of vessels in the VEGF and PDGF layer towards increased size and maturation.

CONCLUSIONS

Spatially localizing and temporally controlling growth factor presentation for angiogenesis can create spatially organized tissues.

VEGF and angiopoietin-1 stimulate different angiogenic phenotypes that combine to enhance functional neovascularization in adult tissue

OBJECTIVE

Therapeutic angiogenesis requires an understanding of how growth factors such as vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang-1) result in physiological neovascularization. This study determined the physiological mechanism by which adenoviral delivery of growth factor combinations alter vascular phenotype and functionality.

METHODS

Adenovirus-mediated gene transfer into the adjacent fat pad of the rat mesentery was used to characterize induction of angiogenesis by VEGF and Ang-1, in a model that permitted a detailed examination of the neovessel phenotype.

RESULTS

Ang-1 combined with VEGF resulted in a distinct vascular phenotype from either factor alone. Microvascular perfusion was significantly enhanced in all groups, but VEGF produced short, narrow, highly branched and sprouting vessels, with normal pericyte coverage. Ang-1 induced broader, longer neovessels, with no increase in branching or sprouting, yet a significantly higher pericyte ensheathment. Combination of Ang-1 and VEGF generated a significantly higher degree of functionally perfused, larger, less branched, and more mature microvessels, resulting from increased efficiency of sprout to vessel formation. Ang-1 and VEGF also caused differential effects on larger compared with smaller blood vessels, a finding reproduced in vitro.

CONCLUSIONS

Ang-1 and VEGF use different physiological mechanisms to enhance neovascularization of relatively avascular tissue. Administration of both growth factors combines these physiological mechanisms to give greater enhancement of neovascularization than either growth factor alone. These results suggest that effective revascularization therapy may require combination growth factor treatment.