Angiogenic therapy for the chronically ischemic lower limb in a rabbit model

The Buerger's rabbit model: a closer step to unravelling thromboangiitis obliterans?

OBJECTIVE

Thromboangiitis obliterans (TAO) remains clinical challenging due to its rarity and underwhelming management outcomes. This study aimed to describe a novel TAO rabbit model that demonstrates a closer resemblance to TAO.

METHODS

Thirty-six New Zealand rabbits underwent the surgical implantation of calibrated gelatin sponge particles (CGSPs) into their right femoral artery. The CGSPs were soaked in different solutions to simulate different types of thrombi: normal (NT; normal saline); inflammatory TAO thrombus (TAO; dimethylsulfoxide [DMSO]), and DMSO with methotrexate (MTX). All groups underwent clinical assessment, digital subtraction angiography (DSA) and histopathological analysis at time points day 0 (immediate), week 1 (acute), week 2 (subacute), and week 4 (chronic).

RESULTS

The TAO rabbit presented with signs of ischemia of the right digit at week 4. On DSA, the TAO rabbits exhibited formation of corkscrew collaterals starting week 1. On H&E staining, gradual CGSP degradation was observed along with increased red blood cell aggregation and inflammatory cells migration in week 1. On week 2, disorganization of the tunica media layer and vascular smooth muscle cell (VSMC) proliferation was observed. In the TAO rabbit, migrated VSMCs, inflammatory cells, and extracellular matrix with collagen-like substances gradually occluded the lumen. On week 4, the arterial lumen of the TAO rabbit was filled with relatively-organized VSMC and endothelial cell clusters with less inflammatory cells. Neorevascularization was found in the MTX-treated group.

CONCLUSION

The novel TAO rabbit model shows a closer resemblance to human TAO clinically, radiographically, and histopathologically. Histological analysis of the IT progression in the TAO model suggests that it is of VSMC origin.

Notoginsenoside R1 activates the Ang2/Tie2 pathway to promote angiogenesis

BACKGROUND

Therapeutic angiogenesis is a novel strategy for the treatment of ischemic diseases that involves promotion of angiogenesis in ischemic tissues via the use of proangiogenic agents. However, effective proangiogenic drugs that activate the Ang2/Tie2 signaling pathway remain scarce.

PURPOSE

We aimed to investigate the proangiogenic activity of notoginsenoside R1 (NR1) isolated from total saponins of Panax notoginseng with regard to activation of the Ang2/Tie2 signaling pathway.

METHODS

We examined the proangiogenic effects of NR1 by assessing the effects of NR1 on the proliferation, migration, invasion and tube formation of human umbilical vein endothelial cells (HUVECs). The aortic ring assay and vascular endothelial growth factor receptor inhibitor (VRI)-induced vascular regression in the zebrafish model were used to confirm the proangiogenic effects of NR1 ex vivo and in vivo. Furthermore, the molecular mechanism was investigated by Western blot analysis.

RESULTS

We found that NR1 promoted the proliferation, mobility and tube formation of HUVECs in vitro. NR1 also increased the number of sprouting vessels in rat aortic rings and rescued VRI-induced vascular regression in zebrafish. NR1-induced angiogenesis was dependent on Tie2 receptor activation mediated by increased autocrine Ang2 in HUVECs, and inhibition of the Ang2/Tie2 pathway abrogated the proangiogenic effects of NR1.

CONCLUSIONS

Our results suggest that NR1 promotes angiogenesis by activating the Ang2/Tie2 signaling pathway. Thus, NR1-induced activation of the Ang2/Tie2 pathway is an effective proangiogenic approach. NR1 may be useful agent for the treatment of ischemic diseases.

Peripheral artery disease: the new frontiers of imaging techniques to evaluate the evolution of regenerative medicine

Introduction: Stem cells (ESC, iPSC, MSC) are known to have intrinsic regenerative properties. In the last decades numerous findings have favored the development of innovative therapeutic protocols based on the use of stem cells (Regenerative Medicine/Cell Therapy) for the treatment of numerous diseases including PAD, with promising results in preclinical studies. So far, several clinical studies have shown a general improvement of the patient's clinical outcome, however they possess many critical issues caused by the non-randomized design of the limited number of patients examined, the type cells to be used, their dosage, the short duration of treatment and also their delivery strategy. Areas covered: In this context, the use of the most advanced molecular imaging techniques will allow the visualization of very important physio-pathological processes otherwise invisible with conventional techniques, such as angiogenesis, also providing important structural and functional data. Expert opinion: The new frontier of cell therapy applied to PAD, potentially able to stop or even the process that causes the disease, with particular emphasis on the clinical aspects that different types of cells involve and on the use of more innovative molecular imaging techniques now available.

Limb ischemia and vessel regeneration: Is there a role for VEGF?

Vascular endothelial growth factor (VEGF), as an endothelial cell-specific mitogen, is crucial for new blood vessels formation. Atherosclerosis affecting the cardiovascular system causes ischemia and functio laesa in tissues supplied by the occluded vessels. When such a situation occurs in the lower extremities, it causes critical limb ischemia (CLI) often requiring leg amputation. Low oxygen tension leads to upregulation of hypoxia-regulated genes (i.e. VEGF), that should help to restore the impaired blood flow. In CLI these rescue mechanisms are, however, often inefficient. Moreover, there are many contradictory reports showing either induction, no changes or even down-regulation of VEGF in specimens taken from patients with CLI, as well as in samples collected from animals subjected to hindlimb ischemia. Additionally, taking into account numerous experimental and clinical data demonstrating rather insufficient therapeutic potential of VEGF, we called into question the role of this protein in limb ischemia and vessel regeneration. In this review we are also summarizing several aspects which can influence VEGF expression and its measurement in the ischemic tissues.

Collateral Growth in the Peripheral Circulation: A Review

Arterial occlusive diseases are a major cause of morbidity and death in the United States. The enlargement of pre-existing vessels, which bypass the site of arterial occlusion, provide a natural way for the body to compensate for such obstructions. Individuals differ in their capacity to develop collateral vessels. In recent years much attention has been focused upon therapy to promote collateral development, primarily using individual growth factors. Such studies have had mixed results. Persistent controversies exist regarding the initiating stimuli, the processes involved in enlargement, the specific vessels that should be targeted, and the most appropriate terminology. Consequently, it is now recognized that more research is needed to extend our knowledge of the complex process of collateral growth. This basic science review addresses five questions essential in understanding current problems in collateral growth research and the development of therapeutic interventions.

Evaluation of the clinical relevance and limitations of current pre-clinical models of peripheral artery disease

Peripheral artery disease (PAD) has recognized treatment deficiencies requiring the discovery of novel interventions. This article describes current animal models of PAD and discusses their advantages and disadvantages. There is a need for models which more directly simulate the characteristics of human PAD, such as acute-on-chronic presentation, presence of established risk factors and impairment of physical activity.

The efficacy of extraembryonic stem cells in improving blood flow within animal models of lower limb ischaemia

BACKGROUND

Stem cell (SC) administration is a potential therapeutic strategy to improve blood supply in patients with peripheral artery disease (PAD). The aim of this systematic review and meta-analysis was to investigate the efficacy of extraembryonic tissue-derived SC (ETSC) in improving blood flow within animal models of hindlimb ischaemia (HLI).

METHODS

PubMed, ScienceDirect and Web of Science were searched to identify studies which investigated ETSCs within animal HLI models. A meta-analysis was performed focusing on the effect of ETSCs on limb blood flow assessed by laser Doppler imaging using a random effects model. Methodological quality was assessed using a newly devised quality assessment tool.

RESULTS

Five studies investigating umbilical cord-derived SCs (three studies), placental SCs (one study), amnion and chorionic SCs (one study) were included. A meta-analysis suggested that administration of ETSCs improved the restoration of blood flow within the HLI models used. The methodological quality of the included studies was assessed as poor. Problems identified included lack of randomised design and blinding of outcome assessors; that the animal models did not incorporate recognised risk factors for human PAD or atherosclerosis; the models used did not have established chronic ischaemia as is the cases in most patients presenting with PAD; and the studies lacked a clear rationale for the dosage and frequency of SCs administered.

CONCLUSIONS

The identified studies suggest that ETSCs improve recovery of limb blood supply within current animal HLI models. Improved study quality is, however, needed to provide support for the likelihood of translating these findings to patients with PAD.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

Stimulation of angiogenesis and survival of endothelial cells by human monoclonal Tie2 receptor antibody

Angiopoietin-1 (Ang1) and its endothelium-specific receptor, tyrosine kinase with Ig and epidermal growth factor homology domain 2 (Tie2), play critical roles in vascular development. Although the Ang1/Tie2 system has been considered a promising target for therapeutic neovascularization, several imitations of large-scale production have hampered the development of recombinant Ang1 for therapeutics. In this study, we produced a fully human agonistic antibody against Tie2, designated 1-4h, and tested the applicability of 1-4h as an alternative to native Ang1 in therapeutic angiogenesis. 1-4h significantly enhanced the phosphorylation of Tie2 in a dose- and time-dependent manner in human Tie2-expressing HEK293 cells and human umbilical vein endothelial cells. Moreover, 1-4h induced the activation of Tie2-mediated intracellular signaling such as AKT, eNOS, MAPK, and Focal Adhesion Kinase p125(FAK). In addition, 1-4h increased the chemotactic motility and capillary-like tube formation of endothelial cells in vitro and enhanced the survival of serum-deprived endothelial cells. Taken together, our data clearly suggest that a human Tie2 agonistic antibody is a potentially useful therapeutic approach for the treatment of several ischemic diseases including delayed-wound healing and ischemic heart and limb diseases.

Fabrication of polymeric biomaterials: a strategy for tissue engineering and medical devices

Fabrication of biomaterials scaffolds using various methods and techniques is discussed, utilising biocompatible, biodegradable and stimuli-responsive polymers and their composites. This review covers the lithography and printing techniques, self-organisation and self-assembly methods for 3D structural scaffolds generation, and smart hydrogels, for tissue regeneration and medical devices.

Therapeutic angiogenesis for revascularization in peripheral artery disease

Therapeutic angiogenesis for peripheral artery disease (PAD), achieved by gene and cell therapy, has recently raised a great deal of hope for patients who cannot undergo standard revascularizing treatment. Although pre-clinical studies gave very promising data, still clinical trials of gene therapy have not provided satisfactory results. On the other hand, cell therapy approach, despite several limitations, demonstrated more beneficial effects but initial clinical studies must be constantly validated by larger randomized, multi-center, double-blinded, placebo-controlled trials. This review focuses on previous and recent gene and cell therapy studies for limb ischemia, including both experimental and clinical research, and summarizes some important papers published in this field. Moreover, it provides a short comment on combined gene and cell therapy approach on the example of heme oxygenase-1 overexpressing cells with therapeutic properties.

Promoting blood vessel growth in ischemic diseases: challenges in translating preclinical potential into clinical success

Angiogenic therapy, which involves the use of an exogenous stimulus to promote blood vessel growth, is an attractive approach for the treatment of ischemic diseases. It has been shown in animal models that the stimulation of blood vessel growth leads to the growth of the whole vascular tree, improvement of ischemic tissue perfusion and improved muscle aerobic energy metabolism. However, very few positive results have been gained from Phase 2 and 3 clinical angiogenesis trials. Many reasons have been given for the failures of clinical trials, including poor transgene expression (in gene-therapy trials) and instability of the vessels induced by therapy. In this Review, we discuss the selection of preclinical models as one of the main reasons why clinical translation has been unsuccessful thus far. This issue has received little attention, but could have had dramatic implications on the expectations of clinical trials. We highlight crucial differences between human patients and animal models with regards to blood flow and pressure, as well as issues concerning the chronic nature of ischemic diseases in humans. We use these as examples to demonstrate why the results from preclinical trials might have overestimated the efficacy of angiogenic therapies developed to date. We also suggest ways in which currently available animal models of ischemic disease could be improved to better mimic human disease conditions, and offer advice on how to work with existing models to avoid overestimating the efficacy of new angiogenic therapies.

Wound healing gene therapy: cartilage regeneration induced by vascular endothelial growth factor plasmid

AIMS

The identification of growth factors and cytokines with angiogenic activity has enabled new therapeutic treatments for a variety of diseases; this concept is called therapeutic angiogenesis. The vascular endothelial growth factor (VEGF) is the most critical regulator of vascular formation. In the present study, we were interested in the therapeutic angiogenesis effect using plasmid transfer of human complementary DNA VEGF(165) (phVEGF(165)) in experimental skin and cartilage trauma.

METHODS

Ten BALB/c mice were used for cartilage injury model. At 6 weeks of age, all mice were ear-punched, resulting in 2-mm-diameter puncture through the center of both pinnae. Each mouse got phVEGF(165) injection into the first ear and vector without insert or saline injection into the second one. The healing process was followed. The hollow diameter was measured on days 0, 14, and 42. Histological sections of experimental and control pinnae were taken from days 1, 3, 5, 7, 9, 11, 13, 15, 20, and 30 after experimental injury for hematoxylin and eosin and periodic acid Schiff staining and for human VEGF immunocytochemistry. The expression of human VEGF was also checked by real-time polymerase chain reaction in formalin-fixed, paraffin-embedded tissue sections.

KEY FINDINGS

In BALB/c mouse strain, a significant angiogenesis promotion and cartilage repair were observed after phVEGF(165) injection into the punched ear area.

SIGNIFICANCE

We suggest that administering phVEGF(165) leads to faster cartilage regeneration even if not only on the angiogenic basis.

Fragmin/protamine microparticles as cell carriers to enhance viability of adipose-derived stromal cells and their subsequent effect on in vivo neovascularization

We prepared fragmin/protamine microparticles (F/P MPs) as cell carriers to enhance cell viability. Use of material consisting of a low-molecular-weight heparin (fragmin) mixed with protamine resulted in water-insoluble microparticles (about 0.5-1 microm in diameter). In this study, we investigated the capability of F/P MPs to enhance the viabilities of human microvascular endothelial cells (HMVECs), human dermal fibroblasts (fibroblasts), and adipose tissue-derived stromal cells (ATSCs) in suspension culture. F/P MPs were bound to the surfaces of these cells, and the interaction of these cells with F/P MPs induced cells/F/P MPs-aggregate formations in vitro, and maintained viabilities of those cells for at least 3 days. The ATSCs/F/P MPs-aggregates adhered to and grew on suspension culture plates in a fashion similar to those on type I collagen-coated plates. The cultured ATSCs secreted significant amounts of angiogenic heparin-binding growth factors such as FGF-2. When the ATSCs/F/P MPs-aggregates were subcutaneously injected into the back of nude mice, significant neovascularization and fibrous tissue formation were induced near the site of injection from day 3 to week 2. The ATSCs/F/P MPs-aggregates were thus useful and convenient biomaterials for cell-therapy of angiogenesis.

Cellular and molecular mechanism regulating blood flow recovery in acute versus gradual femoral artery occlusion are distinct in the mouse

BACKGROUND

Most current animal models of hindlimb ischemia use acute arterial occlusion that does not accurately reflect the pathogenesis of gradual arterial occlusion in humans. We, therefore, developed the first mouse model of gradual arterial occlusion and tested the hypothesis that the mechanisms regulating blood flow recovery are critically dependent on the rate of arterial occlusion.

METHODS

Gradual arterial occlusion was induced by placing ameroid constrictors on the proximal and distal left femoral artery, and ligating the femoral arterial branches (n = 36). Acute arterial occlusion was accomplished by excising the left femoral artery (n = 36). The blood flow recovery was studied by laser Doppler imaging. Differential gene expression between these two models was assessed by quantitative real-time polymerase chain reactions (PCR). Inflammatory and progenitor cells recruitment were determined by immunohistochemistry.

RESULTS

We found that hypoxia-related genes increased significantly in the calf, but not in the thigh, after gradual and acute femoral arterial occlusion (P < .05). Shear-stress dependent genes and inflammatory genes were upregulated immediately in the thigh only after acute femoral arterial occlusion (P < .05). These differences in gene expression were consistent with increased SDF-1alpha expression, recruitment of macrophages and hemangiocytes, and higher blood flow recovery after acute arterial occlusion than after gradual arterial occlusion (P < .05).

CONCLUSION

This is the first study to show the mechanisms that regulate blood flow recovery are critically dependent on the rate of arterial occlusion. This novel model of gradual arterial occlusion may more closely resemble the human diseases, and may provide more accurate mechanistic insights for creating novel molecular therapies.

Role of image-guided vascular intervention in therapeutic angiogenesis translational research

Therapeutic angiogenesis, the process of growing collateral blood vessels to better perfuse ischemic tissue, has been hailed as an up-and-coming treatment for symptomatic lower-extremity peripheral arterial occlusive disease. A minimally invasive durable treatment would be welcome since current treatment options for this disease carry high risk, limited efficacy or limited durability. Unfortunately, as evidenced by disappointing results in multiple clinical trials, therapeutic angiogenesis has yet to deliver in humans the success it has seen in animal models. In this review, we discuss the challenges of translating therapeutic angiogenesis into effective clinical treatments for lower-extremity peripheral arterial occlusive disease and we highlight the role that experts in image-guided vascular interventions can play in advancing the field.

Therapeutic angiogenesis induced by controlled release of fibroblast growth factor-2 from injectable chitosan/non-anticoagulant heparin hydrogel in a rat hindlimb ischemia model

The addition of non-anticoagulant heparin [periodate-oxidized (IO4) heparin] and fibroblast growth factor (FGF)-2 to a viscous water-soluble chitosan (CH-LA) aqueous solution produces an injectable FGF-2/CH-LA/IO4-heparin hydrogel. The purpose of this study was to examine the ability of the injected FGF-2/CH-LA/IO4-heparin hydrogel to induce vascularization and fibrous tissue formation. FGF-2/CH-LA/IO4-heparin hydrogels (100 microL of hydrogel consisting of 20 mg/mL of CH-LA, 2 mg/mL of IO4-heparin, and 50 microg/mL of FGF-2) were subcutaneously injected into the backs of wound healing-impaired diabetic (db/db) mice. Furthermore, the effect of percutaneous injection of FGF-2/CH-LA/IO4-heparin hydrogel at eight sites (25 microL/site) into ischemic left lower limbs of rats was examined from day 4 to at least day 28 postinjection. The injection of FGF-2/CH-LA/IO4-heparin hydrogels into the backs of db/db mice resulted in significant increases in blood vessel formation, significant vascularization, and fibrous tissue formation near the injection site. Injection of FGF-2/CH-LA/IO4-heparin hydrogel into ischemic left lower limbs of rats also significantly recovered and increased blood flow and blood oxygen in the calf and thigh. These results indicate that the controlled release of biologically active FGF-2 molecules from FGF-2/CH-LA/IO4-heparin induces angiogenesis and possibly collateral circulation in db/db mice and the ischemic limbs of rats.

Microenvironmental VEGF distribution is critical for stable and functional vessel growth in ischemia

The critical role of vascular endothelial growth factor (VEGF) expression levels in developmental angiogenesis is well established. Nonetheless, the effects of different local (microenvironmental) VEGF concentrations in ischemia have not been studied in the adult organism, and VEGF delivery to patients has been disappointing. Here, we demonstrate the existence of both lower and upper threshold levels of microenvironmental VEGF concentrations for the induction of therapeutic vessel growth in ischemia. In the ischemic hind limb, implantation of myoblasts transduced to express VEGF164 at different levels per cell increased blood flow only moderately, and vascular leakage and aberrant preangiomatous vessels were always induced. When the same total dose was uniformly distributed by implanting a monoclonal population derived from a single VEGF-expressing myoblast, blood flow was fully restored to nonischemic levels, collateral growth was induced, and ischemic damage was prevented. Hemangiomas were avoided and only normal, pericyte-covered vessels were induced persisting over 15 mo. Surprisingly, clones uniformly expressing either lower or higher VEGF levels failed to provide any functional benefit. A biphasic effect of VEGF dose on vessel number and diameter was found. Blood flow was only improved if vessels were increased both in size and in number. Microenvironmental VEGF concentrations determine efficacy and safety in a therapeutic setting.

Chitosan hydrogel as a drug delivery carrier to control angiogenesis

An aqueous solution of photocrosslinkable chitosan containing azide groups and lactose moieties (Az-CH-LA) incorporating paclitaxel formed an insoluble hydrogel within 30 s of ultraviolet light (UV) irradiation. The chitosan hydrogel showed strong potential for use as a new tissue adhesive in surgical applications and wound dressing. The fibroblast growth factor (FGF)-2 molecules retained in the chitosan hydrogel and in an injectable chitosan/IO(4)-heparin hydrogel remain biologically active, and were gradually released from the hydrogels as they biodegraded in vivo. The controlled release of biologically active FGF-2 molecules from the hydrogels caused induction of angiogenesis and collateral circulation occurred in healing-impaired diabetic (db/db) mice and in the ischemic limbs of rats. Paclitaxel, which is an antitumor reagent, was also retained in the chitosan hydrogel and remained biologically active as it was released on degradation of the hydrogel in vivo. The chitosan hydrogels incorporating paclitaxel effectively inhibited tumor growth and angiogenesis in mice. The purpose of this review is to describe the effectiveness of chitosan hydrogel as a local drug delivery carrier for agents (e.g., FGF-2 and paclitaxel) to control angiogenesis. It is thus proposed that chitosan hydrogel may be a promising new local carrier for drugs such as FGF-2 and paclitaxel to control vascularization.

Beyond angiogenesis: the cardioprotective potential of fibroblast growth factor-2

In the field of cardiovascular research, a number of independent approaches have been explored to protect the heart from acute and chronic ischemic damage. Fibroblast growth factor-2 (FGF-2) recently has received considerable attention with respect to its angiogenic potential. While therapeutic angiogenesis may serve to salvage chronically ischemic myocardium, more acute treatments are in demand to increase cardiac resistance to injury (preconditioning) and to guard against secondary injury after an acute ischemic insult. Here, we look beyond the angiogenic potential of FGF-2 and examine its acute cardioprotective activity as demonstrated under experimental conditions, both as an agent of a preconditioning-like response and for secondary injury prevention at the time of reperfusion. Factors to consider in moving to the clinical setting will be discussed, including issues of dosage, treatment duration, and routes of administration. Finally, issues of safety and clinical trial design will be considered. The prospect of such a multipotent growth factor having clinical usefulness opens the door to effective treatment of both acute and chronic ischemic heart disease, something well worth the attention of the cardiovascular community.

Efficacy of Photocrosslinkable Chitosan Hydrogel Containing Fibroblast Growth Factor-2 in a Rabbit Model of Chronic Myocardial Infarction

BACKGROUND

Therapeutic angiogenesis in ischemic myocardium has been shown to be an effective strategy to improve regional blood flow and myocardial function. However, no effective delivery system for growth factor administration is yet known to induce important therapeutic angiogenic responses in ischemic myocardium.

MATERIALS AND METHODS

FGF-2-incorporated chitosan (FGF-2/chitosan) hydrogels were immobilized on the surface of ischemic myocardium of rabbit models of chronic myocardial infarction by UV-irradiation. After 4 weeks, cardiac functional analyses by noradrenalin challenge and histopathological analyses were performed to evaluate the efficacy of a controlled release of FGF-2 from FGF-2/chitosan hydrogel immobilized on the surface of ischemic myocardium.

RESULTS

Significant improvement by application of FGF-2/chitosan hydrogels was found in systolic pressure at the left ventricle, +dp/dt maximum, and -dp/dt maximum during noradrenalin challenge at a dose of 1 microg/kg/min. Histological observations showed that a significantly larger amount of viable myocardium and CD 31 immunostained blood vessels were found in the FGF-2/chitosan hydrogel-applied group than only the chitosan-applied and control groups.

CONCLUSIONS

These preliminary results indicate that the controlled release of biologically active FGF-2 molecules from FGF-2/chitosan hydrogel induces angiogenesis and possibly collateral circulation in ischemic myocardium, thereby protecting the myocardium.

The effect of gradual or acute arterial occlusion on skeletal muscle blood flow, arteriogenesis, and inflammation in rat hindlimb ischemia

BACKGROUND

Current experimental models of critical limb ischemia are based on acute ischemia rather than on chronic ischemia. Human peripheral vascular disease is largely a result of chromic ischemia. We hypothesized that a model of chronic hindlimb ischemia would develop more collateral arteries, more blood flow, and less necrosis and inflammation than would acute hindlimb ischemia. We therefore developed a rat model of chronic hindlimb ischemia and compared the effects of chronic ischemia with those of acute ischemia on hindlimb skeletal muscle.

METHODS

Acute or chronic ischemia was induced in 36 male Sprague-Dawley rats. Chronic ischemia caused blood flow, as measured by laser Doppler scanning and confirmed by muscle oxygen tension measurements, to gradually decrease over 1 to 2 weeks after operation.

RESULTS

Histologic analysis showed chronic hindlimb ischemia better preserved muscle mass and architecture and stimulated capillary angiogenesis, while lacking the muscle necrosis and inflammatory cell infiltrate seen after acute ischemia. Surprisingly, the chronic ischemia group recovered dermal blood flow more slowly and less completely than did the acute ischemia group, as measured by laser Doppler (0.66 +/- 0.02 vs 0.76 +/- 0.04, P < .05) and tissue oxygen tension (0.61 +/- 0.06 vs 0.81 +/- 0.05, P < .05) at 40 days postoperatively. Consistent with poorer blood flow recovery, chronic ischemia resulted in smaller diameter collateral arteries (average diameter of the five largest collaterals on angiogram was 0.01 +/- 0.0003 mm vs 0.013 +/- 0.0007 mm for acute, P < .005 at 40 days postoperatively). Acute ischemia resulted in decreased tissue concentrations of vascular endothelial growth factor (VEGF) (0.96 +/- 0.23 pg/mg of muscle for acute vs 4.4 +/- 0.75 and 4.8 +/- 0.75 pg/mg of muscle for unoperated and chronic, respectively, P < .05 acute vs unoperated), and in increased tissue concentrations of interleukin (IL)-1beta (7.3 +/- 4.0 pg/mg of muscle for acute vs undetectable and 1.7 +/- 1.6 pg/mg of muscle for unoperated and chronic, respectively, P < 0.05 acute vs unoperated).

CONCLUSIONS

We describe here the first model of chronic hindlimb ischemia in the rat. Restoration of blood flow after induction of hindlimb ischemia is dependent on the rate of arterial occlusion. This difference in blood flow recovery correlates with distinct patterns of muscle necrosis, inflammatory cell infiltration, and cytokine induction in the ischemic muscle. Differences between models of acute and chronic hindlimb ischemia may have important consequences for future studies of mechanisms regulating arteriogenesis and for therapeutic approaches aimed at promoting arteriogenesis in humans suffering from critical limb ischemia.

CLINICAL RELEVANCE

Despite the substantial clinical differences between acute and chronic ischemia, researchers attempting to develop molecular therapies to treat critical limb ischemia have only tested those therapies in experimental models of acute hindlimb ischemia. We present here a novel model of chronic hindlimb ischemia in the rat. We further demonstrate that when hindlimb ischemia is developed chronically, collateral artery development is poorer than when hindlimb ischemia is developed acutely. These findings suggest that further tests of molecular therapies for critical limb ischemia should be performed in chronic hindlimb ischemia models rather than in acute hindlimb ischemia models.

Marriage of resistance and conduit arteries breeds critical limb ischemia

Atherosclerosis in a major leg artery leads to impaired blood supply, which normally progresses to critical limb ischemia. Atherosclerosis produces substantial alterations of structure and endothelial function in the large conduit arteries. Pressure unloading and ischemia in the distal vasculature bring about alterations in microvascular function. Resistance arteries undergo significant wall thinning and changes in their contractile regulation. Optimization of large artery dimensions by the small arteries through flow-mediated vasodilation is impaired. Angiogenesis is stimulated, which can result in the formation of major collateral feeder vessels in addition to small nutritive blood vessels. However, angiogenesis can also contribute to instability of atherosclerotic plaques, which ultimately leads to further deterioration in blood supply. Surgical bypass grafting to restore blood supply to the distal leg generates a sudden increase of pressure in the weakened resistance vasculature, leading to uncontrolled changes in capillary hydrostatic pressure, extravasation of fluid, and tissue edema. This review aims to highlight the importance of the resistance vasculature in critical limb ischemia and the interdependence of pathophysiological changes in the large conduit and small resistance arteries. The major unresolved question is why the physiological mechanisms that regulate vascular structure and function ultimately break down, leading to circulatory failure within the distal limb.

pO2and regional blood flow in a rabbit model of limb ischemia

Oxygen tension (pO2) in muscles and regional blood flow were measured in a rabbit model of limb ischemia. pO2 was measured repetitively by EPR oximetry with EMS char in four different muscle groups in the same animals. Blood flow in the same muscles at several time points was measured using microspheres. A linear mixed effects model was developed to analyze the data on pO2 and blood flow. The results suggest that while under normal conditions pO2 in muscles does not depend significantly on blood flow, immediately after arterial occlusion pO2 correlates linearly with blood flow. Within two weeks of occlusion the pO2 is recovered to 45% of baseline. This study demonstrates, for the first time, the applicability of EPR oximetry in animals larger than rodents.

Current perspectives in therapeutic myocardial angiogenesis

The complex mechanisms mediating the development of new blood vessels are now beginning to be unraveled. In conjunction with major biotechnology advances, this has facilitated the initiation of translational research related to a novel treatment strategy for patients with myocardial or leg ischemia due to obstructive arterial disease--therapeutic angiogenesis. At present, at least 17 clinical trials of myocardial angiogenesis have been presented involving over 900 patients. Uncertainty exists as to the optimal delivery route and angiogenic agent, and this uncertainty is reflected in the diverse methodology of the trials published thus far. The majority of patients received an angiogenic protein via the intracoronary route. Other delivery techniques--such as direct intramyocardial injection via transepicardial or transendocardial routes--and other angiogenic agents, including master genes, have also been studied. Most recently, interest has grown in the potential angiogenesis effects of cell therapy--such as autologous bone marrow cells or cultured stem cells--and there are now several groups initiating Phase I/II trials in this area. This review summarizes the current evidence pertaining to the safety, feasibility, and efficacy of various angiogenic techniques aimed at enhancing myocardial blood flow and alleviating angina.

Using gene expression profiling to identify the molecular basis of the synergistic actions of hepatocyte growth factor and vascular endothelial growth factor in human endothelial cells

Hepatocyte growth factor (HGF) and vascular endothelial cell growth factor (VEGF) are two potent endothelial mitogens with demonstrated angiogenic activities in animal models of therapeutic angiogenesis. Several recent studies suggest that these growth factors may act synergistically, although the mechanism of this interaction is not understood. Changes in the gene expression profile of human umbilical vein endothelial cells treated with HGF, VEGF or the combination of the two were analyzed with high-density oligonucleotide arrays, representing approximately 22000 genes. Notably, the genes significantly up- and downregulated by VEGF versus HGF exhibited very little overlap, indicating distinct signal transduction pathways. The combination of HGF and VEGF markedly increased the number of significantly up- and downregulated genes. At 4 h, the combination of the two growth factors induced a number of chemokine and cytokines and their receptors (IL-8, IL-6, IL-11, CCR6, CXCR1,CXC1 and IL17RC), numerous genes involved in growth factor signal transduction (egr-1, fosB, grb10, grb14,MAP2K3,MAP3K8, MAPKAP2,MPK3, DUSP4 and DUSP6), as well as a number of other growth factors (PDGFA, BMP2, Hb-EGF, FGF16, heuregulin beta 1, c-kit ligand, angiopoietin 2 and angiopoietin 4 and VEGFC). In addition, the VEGF receptors neuropilin-1 and flt-1 were also upregulated. At 24 h, a clear 'cell cycle' signature is noted, with the upregulated expression of various cell cycle control proteins and gene involved in the regulation of mitosis and mitotic spindle assembly. The receptor for HGF, c-met, is also upregulated. These data are consistent with the hypothesis that the combination of HGF and VEGF results in the cooperative upregulation of a number of different molecular pathways leading to a more robust proliferative response, that is, growth factor(s), receptors, molecules involved in growth factor signal transduction, as well as, at later time points, upregulation of the necessary cellular proteins required for cells to escape cell cycle arrest and enter the cell cycle.

Direct fibrin injection to promote new collateral growth in hind limb ischemia in a rabbit model

Local stimulation of angiogenesis is a new approach for the treatment of critical limb ischemia. Our investigation tested intramuscular (i.m.) injection of a modified fibrin meshwork in a rabbit model.

METHODS

The left external iliac and femoral arteries were excised in 24 rabbits that were divided into four groups: control; i.m. saline injection; fibrin meshwork plus low dose (2.5 mg) fibrinogen i.m.; fibrin meshwork plus high-dose (5.0 mg) fibrinogen i.m. Angiography was performed before surgery, immediately after surgery, and one month postoperatively. Lower limb-calf blood pressure was measured immediately after surgery and at postoperative days 10, 20, and 30. On day 30, conventional indirect immunostaining was performed to determine the percentage of the area occupied by capillaries.

RESULTS

Immediately after surgery, in all four groups, the number of contract-opacified arteries (COA) crossing a specific segment of a grid decreased from 5.3 +/- 1.3 to 3.2 +/- 1.0 (p < 0.05); the number of grid intersections decreased from 30.2 +/- 6.5 to 19.3 +/- 4.8 (p < 0.05); and the total number of grids with COA decreased from 18.3 +/- 3.8 to 12.2 +/- 2.5 (p < 0.05). One month after surgery, in the control group, these parameters were 6.2 +/- 1.1, 33.2 +/- 5.7 and 20.3 +/- 1.5, respectively; in the saline-treated group, these parameters were 6.1 +/- 0.8, 28.3 +/- 6.9 and 19.8 +/- 1.1, respectively (p > 0.05 versus control and versus baseline data). When fibrin containing 5.0 mg fibrinogen was used, these parameters increased to 8.5 +/- 0.9, 48.3 +/- 5.1, and 27.1 +/- 0.9, respectively (p < 0.001 versus immediately after surgery and p < 0.05 versus control). In all four series, no Doppler flow signal was detected from the posterior tibial artery by day 10. By day 30, the lower limb-calf blood pressure ratio had improved in all four series, but was significantly improved in only the two groups treated with fibrin sealant (0.3 +/- 0.05 control; 0.3 +/- 0.08 saline; 0.6 +/- 0.06 fibrinogen 2.5; 0.7 +/- 0.05 fibrinogen 5.0).

CONCLUSION

Intramuscular injection of a fibrin meshwork considerably increased angiogenesis in the severely ischemic hind limb and may be strongly recommended for clinical use in patients with limb-threatening ischemia.

Gene therapy for therapeutic angiogenesis in critically ischaemic lower limb - on the way to the clinic

Currently, no effective pharmacological treatment is available for vascularisation defects in lower limbs. Many patients presenting with persistent pain and ischaemic ulcers are not suitable candidates for surgical or endovascular approaches. Further refinement of the available methods will undoubtedly lead to a more active approach towards treatment of peripheral arterial occlusive disease (PAOD). Recently, therapeutic angiogenesis, in the form of recombinant growth factor administration or gene therapy, has emerged as a novel tool to treat these patients. However, improved gene transfer methods and better understanding of blood vessel formation are required to bring therapeutic angiogenesis to clinical practice. Here we review the clinical problem (PAOD), mechanisms of blood vessel formation (angiogenesis, vasculogenesis and arteriogenesis), experimental evidence and clinical trials for therapeutic angiogenesis in critically ischaemic lower limbs. Also, angiogenic growth factors, including vascular endothelial growth factors (VEGFs) and fibroblast growth factors (FGFs), delivery methods, and vectors for gene transfer in skeletal muscle, are discussed. In addition to vascular growth, gene transfer of growth factors may enhance regeneration, survival, and innervation of ischaemic skeletal muscle. Nitric oxide (NO) appears to be a key mediator in vascular homeostasis and growth, and a reduction in its production by age, hypercholesterolemia or diabetes leads to the impairment of ischaemic disorders.