Contribution of arteriogenesis and angiogenesis to postocclusive hindlimb perfusion in mice

The chemokine system in arteriogenesis and hind limb ischemia

Chemokines (chemotactic cytokines) are important in the recruitment of leukocytes to injured tissues and, as such, play a pivotal role in arteriogenesis and the tissue response to ischemia. Hind limb ischemia represents a complex model with arteriogenesis (collateral artery formation) occurring in tissues with normal perfusion while areas exhibiting ischemic necrosis undergo angiogenesis and skeletal muscle regeneration; monocytes and macrophages play an important role in all three of these processes. In addition to leukocyte trafficking, chemokines are produced by and chemokine receptors are present on diverse cell types, including myoblasts, endothelial, and smooth muscle cells. Thus, the chemokine system may have direct effects as well as inflammatory-mediated effects on arteriogenesis, angiogenesis, and skeletal muscle regeneration. This article reviews the complexity of the hind limb ischemia model and the role of the chemokine system in arteriogenesis and the tissue response to ischemia. Special emphasis will be placed on the roles of monocytes/macrophages and CCL2/monocyte chemotactic protein-1 (MCP-1) in these processes.

Matrix metalloproteinases in peripheral vascular disease

Matrix metalloproteinases (MMPs) are extracellular matrix-modifying enzymes that are important in many physiologic and pathologic vascular processes. Dysregulation of MMP activity has been associated with common vascular diseases such as atherosclerotic plaque formation, abdominal aortic aneurysms, and critical limb ischemia. For this reason, MMPs have become an important focus for basic science studies and clinical investigations by vascular biology researchers. This article reviews the recent literature, summarizing our current understanding of the role of MMPs in the pathogenesis of various peripheral vascular disease states. In addition, the importance of MMPs in the future diagnosis and treatment of peripheral vascular disease is discussed.

Collateral density, remodeling, and VEGF-A expression differ widely between mouse strains

Substantial variability exists in collateral density and ischemia-induced collateral growth among species. To begin to probe the underlying mechanisms, which are unknown, we characterized two mouse strains with marked differences in both parameters. Immediately after femoral artery ligation, collateral and foot perfusion were lower in BALB/c than C57BL/6 (P < 0.05 here and below), suggesting fewer pre-existing collaterals. This was confirmed with angiography and immunohistochemistry (approximately 35% fewer collaterals in the BALB/c's thigh). Recovery of hindlimb perfusion was attenuated in BALB/c, in association with 54% less collateral remodeling, reduced angiogenesis, greater ischemia, and more impaired hindlimb use. Densities of CD45+ and CD4+ leukocytes around collaterals increased similarly, but TNF-alpha expression was 50% lower in BALB/c, which may contribute to reduced collateral remodeling. In normal tissues, compared with C57BL/6, BALB/c exhibit an altered arterial branching pattern and, like skeletal muscle above, have 30% fewer collaterals in intestine and, remarkably, almost none in pial circulation, resulting in greatly impaired perfusion after cerebral artery occlusion. Ischemic induction of VEGF-A was attenuated in BALB/c. Analysis of a C57BL/6 x BALB/c recombinant inbred strain dataset identified a quantitative trait locus for VEGF-A mRNA abundance at or near the Vegfa locus that associates with lower expression in BALB/c. This suggests a cis-acting polymorphism in the Vegfa gene in BALB/c could contribute to reduced VEGF-A expression and, in turn, the above deficiencies in this strain. These findings suggest these strains offer a model to investigate genetic determinants of collateral formation and growth in ischemia.

Nonmyeloablative stem cell therapy enhances microcirculation and tissue regeneration in murine inflammatory bowel disease

BACKGROUND AND AIMS

Reduced microcirculation has been implicated in the pathogenesis of inflammatory bowel disease (IBD). Stem cells or endothelial progenitor cells are thought to contribute to tissue regeneration through neoangiogenesis or vasculogenesis in ischemia- or inflammatory-related diseases. We therefore hypothesized that adult stem cells facilitate epithelial repair in IBD.

METHODS

Moderate-severe colitis in mice was induced by dextran sulfate sodium (DSS) and 2.0 x 10(6) immortalized CD34(-) stem cells infused twice via the tail vein during an observation period of 35 days in a nonmyeloablative setting.

RESULTS

Here, we demonstrate that adult stem cells home to the damaged digestive tract in the large intestine and facilitate mucosal repair in moderate-severe colitis. Nonmyeloablative stem cell therapy resulted in increased survival in severe colitis (P < .0001). Moreover, clinical activity and histologic evaluation of the colitis severity score were reduced significantly in moderate (P = .0003 or P = .03) and severe (P < .0001 or P < .03) colitis after 35 days, in addition to the DSS-induced shortening of colon length (P = .002 and P < .0002). Genetically marked stem cells were detected predominantly in the submucosa of the damaged colon epithelium. Epithelial repair in experimental IBD was mediated either by induction of improved vasculogenesis or by the differentiation of the transplanted stem cells into endothelial cells, as demonstrated by the promotion of Tie2 activity in the infused cells at the site of the damaged mucosa.

CONCLUSIONS

Our findings indicate that systemically administered adult stem cells respond to an adequate tissue lesion in murine IBD by enhancing microcirculation, resulting in accelerated tissue repair.

Magnetic resonance angiography reveals therapeutic enlargement of collateral vessels induced by VEGF in a murine model of peripheral arterial disease

PURPOSE

To quantify spontaneous and therapeutic arteriogenesis in vivo in a murine model of peripheral arterial disease using magnetic resonance angiography.

MATERIALS AND METHODS

Male, 8-12-week-old, C57/BL6 mice underwent femoral artery ligation; 21 days later, 2 mg/kg recombinant murine VEGF165, formulated for slow release, was injected into the ipsilateral gastrocnemius. The spontaneous (following ligation) and therapeutic (following vascular endothelial growth factor (VEGF)) formation of collateral vessels was quantified using 3D magnetic resonance angiography on a small-bore 4.7T system. Therapeutically induced angiogenesis and blood flow were quantified using an in situ anti-platelet endothelial cell adhesion molecule (PECAM) 1 radioimmunoassay and radiolabeled microsphere deposition, respectively.

RESULTS

Spontaneous arteriogenesis was visible in all animals five days after ligation. VEGF treatment doubled the arteriogenic response five days after treatment compared to vehicle (cross-sectional area of vessels: 0.96 vs. 0.46 mm2, P<0.01). VEGF also induced angiogenesis (PECAM1 levels 191% of vehicle, P<0.05) and increased blood flow specific to the injection site (57 vs. 7 mL/minute/100 g, P<0.05).

CONCLUSION

The presented methodology allowed in vivo quantification of spontaneous arteriogenesis in a murine model of peripheral arterial disease and demonstrated that therapeutic enlargement of collateral vessels is possible with VEGF.

Notch ligand Delta-like 1 is essential for postnatal arteriogenesis

Growth of functional arteries is essential for the restoration of blood flow to ischemic organs. Notch signaling regulates arterial differentiation upstream of ephrin-B2 during embryonic development, but its role during postnatal arteriogenesis is unknown. Here, we identify the Notch ligand Delta-like 1 (Dll1) as an essential regulator of postnatal arteriogenesis. Dll1 expression was specifically detected in arterial endothelial cells, but not in venous endothelial cells or capillaries. During ischemia-induced arteriogenesis endothelial Dll1 expression was strongly induced, Notch signaling activated and ephrin-B2 upregulated, whereas perivascular cells expressed proangiogenic vascular endothelial growth factor, and the ephrin-B2 activator EphB4. In heterozygous Dll1 mutant mice endothelial Notch activation and ephrin-B2 induction after hindlimb ischemia were absent, arterial collateral growth was abrogated and recovery of blood flow was severely impaired, but perivascular vascular endothelial growth factor and EphB4 expression was unaltered. In vitro, angiogenic growth factors synergistically activated Notch signaling by induction of Dll1, which was necessary and sufficient to regulate ephrin-B2 expression and to induce ephrin-B2 and EphB4-dependent branching morphogenesis in human arterial EC. Thus, Dll1-mediated Notch activation regulates ephrin-B2 expression and postnatal arteriogenesis.

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.

Murine models of myocardial and limb ischemia: Diagnostic end-points and relevance to clinical problems

Ischemic disease represents the new epidemic worldwide. Animal models of ischemic disease are useful because they can help us to understand the underlying pathogenetic mechanisms and develop new therapies. The present review article summarizes the results of a consensus conference on the status and future development of experimentation in the field of cardiovascular medicine using murine models of peripheral and myocardial ischemia. The starting point was to recognize the limits of the approach, which mainly derive from species- and disease-related differences in cardiovascular physiology. For instance, the mouse heart beats at a rate 10 times faster than the human heart. Furthermore, healing processes are more rapid in animals, as they rely on mechanisms that may have lost relevance in man. The main objective of the authors was to propose general guidelines, diagnostic end points and relevance to clinical problems.

Allometric scaling of wall shear stress from mice to humans: quantification using cine phase-contrast MRI and computational fluid dynamics

Allometric scaling laws relate structure or function between species of vastly different sizes. They have rarely been derived for hemodynamic parameters known to affect the cardiovascular system, e.g., wall shear stress (WSS). This work describes noninvasive methods to quantify and determine a scaling law for WSS. Geometry and blood flow velocities in the infrarenal aorta of mice and rats under isoflurane anesthesia were quantified using two-dimensional magnetic resonance angiography and phase-contrast magnetic resonance imaging at 4.7 tesla. Three-dimensional models constructed from anatomic data were discretized and used for computational fluid dynamic simulations using phase-contrast velocity imaging data as inlet boundary conditions. WSS was calculated along the infrarenal aorta and compared between species to formulate an allometric equation for WSS. Mean WSS along the infrarenal aorta was significantly greater in mice and rats compared with humans (87.6, 70.5, and 4.8 dyn/cm2, P < 0.01), and a scaling exponent of −0.38 ( R2 = 0.92) was determined. Manipulation of the murine genome has made small animal models standard surrogates for better understanding the healthy and diseased human cardiovascular system. It has therefore become increasingly important to understand how results scale from mouse to human. This noninvasive methodology provides the opportunity to serially quantify changes in WSS during disease progression and/or therapeutic intervention.

Arteriogenesis: Noninvasive Quantification with Multi–Detector Row CT Angiography and Three-dimensional Volume Rendering in Rodents

PURPOSE

To evaluate two-dimensional (2D) multi-detector row computed tomographic (CT) angiography and three-dimensional (3D) volume rendering for depiction of patterns of arterial growth and quantification of blood vessel density and volume.

MATERIALS AND METHODS

The institutional animal care and use committee approved this study. The right femoral artery and its branches were ligated and excised in 16 inbred Lewis rats; animals were randomly assigned to receive 70 microL Dulbecco's modified Eagle's medium (DMEM) or 1.5 x 10(7) bone marrow-derived mononuclear cells (BMC) from isogenic donor rats in 70 microL DMEM. At 2 weeks, CT angiography was performed with injection of 0.45 mL barium sulfate suspension at 0.7 mL/min, followed by silver staining. Number of blood vessels, area, mean area, volume, and blood vessel size distribution derived from digitally subtracted 2D CT angiographic sections were quantified; 3D images were reconstructed. Two-way analysis of variance and paired and unpaired Student t tests were performed.

RESULTS

CT angiography showed two patterns of arterial growth: collateral arterial formation and branching arteriogenesis. Two-way analysis of variance indicated that differences within subjects (ischemic vs nonischemic legs) and between subjects (BMC vs DMEM treatment) were significant for total blood vessel area, total blood vessel volume, and mean of blood vessel area (P < .001). In the BMC group, there were significantly more arteries (mean, 241.6 +/- 77.0 [standard deviation] vs 196.4 +/- 75.2, P = .028), but mean cross-sectional area of these arteries was smaller in ischemic versus nonischemic legs (5.4 mm(2) +/- 1.2 vs 6.8 mm(2) +/- 1.3, P = .006). Total arterial area and volume did not differ significantly between ischemic and nonischemic legs.

CONCLUSION

BMC injection had a substantial effect on arteriogenesis, with normalization of total arterial area and volume in the BMC group; this effect was successfully depicted.

Abnormal monocyte recruitment and collateral artery formation in monocyte chemoattractant protein-1 deficient mice

Monocyte chemoattractant protein 1 (MCP-1) has been shown to be effective for the stimulation of collateral artery formation in small and large animal models. The availability of a genetic knockout mouse enables evaluation of the importance of the role of MCP-1 in the natural course of collateral artery growth. In a total of 21 MCP-1 -/- as well as 13 of the appropriate genetic background controls ([129Sv/J X C57BI/6J]F1), a femoral artery ligation was performed. Subsequently, a polyethylene catheter, connected to an osmotic minipump, was inserted retrogradely into the occluded femoral artery with the tip pointing upstream. Using this technique, PBS (MCP-1 -/-: n = 13 and C57BI/6J: n = 13) or MCP-1 (JE; MCP-1 -/-: n = 8) was delivered intra-arterially. Seven days after ligation, determination of hind limb flow was assessed by controlled tissue perfusion using differently labeled fluorescent microspheres. MCP-1 -/- mice exhibited a reduction of hind limb flow of 32.9 +/- 9.2% of the unligated hind limb, compared with 55.4 +/- 6.8% in C57BI/6J mice (p<0.01). MCP-1 -/- mice that underwent a subsequent 'rescue' treatment with MCP-1 showed a restoration of flow to a level of 47.4 +/- 9.8% (p = NS compared with PBS-treated C57BI/6J). Specific immunohistochemical staining for monocytes (MOMA-2: MCP-1 -/-, n = 5 and C57BI/6J, n = 5) showed a reduced number of monocytes around developing collateral arteries in the MCP-1 -/- mice. In conclusion, our data show that the absence of MCP-1 causes a strong reduction in flow restoration after femoral artery occlusion, coinciding with a reduced monocyte attraction, emphasizing the central role of this chemokine in the multifactorial process of collateral artery formation.

The range of adaptation by collateral vessels after femoral artery occlusion

Natural adaptation to femoral artery occlusion in animals by collateral artery growth restores only approximately 35% of adenosine-recruitable maximal conductance (C(max)) probably because initially elevated fluid shear stress (FSS) quickly normalizes. We tested the hypothesis whether this deficit can be mended by artificially increasing FSS or whether anatomical restraints prevent complete restitution. We chronically increased FSS by draining the collateral flow directly into the venous system by a side-to-side anastomosis between the distal stump of the occluded femoral artery and the accompanying vein. After reclosure of the shunt collateral flow was measured at maximal vasodilatation. C(max) reached 100% already at day 7 and had, after 4 weeks, surpassed (2-fold) the C(max) of the normal vasculature before occlusion. Expression profiling showed upregulation of members of the Rho-pathway (RhoA, cofilin, focal adhesion kinase, vimentin) and the Rho-antagonist Fasudil markedly inhibited arteriogenesis. The activities of Ras and ERK-1,-2 were markedly increased in collateral vessels of the shunt experiment, and infusions of L-NAME and L-NNA strongly inhibited MAPK activity as well as shunt-induced arteriogenesis. Infusions of the peroxinitrite donor Sin-1 inhibited arteriogenesis. The radical scavengers urate, ebselen, SOD, and catalase had no effect. We conclude that increased FSS can overcome the anatomical restrictions of collateral arteries and is potentially able to completely restore maximal collateral conductance. Increased FSS activates the Ras-ERK-, the Rho-, and the NO- (but not the Akt-) pathway enabling collateral artery growth.

Identification of Sokotrasterol Sulfate As a Novel Proangiogenic Steroid

The potential to promote neovascularization in ischemic tissues using exogenous agents has become an exciting area of therapeutics. In an attempt to identify novel small molecules with angiogenesis promoting activity, we screened a library of natural products and identified a sulfated steroid, sokotrasterol sulfate, that induces angiogenesis in vitro and in vivo. We show that sokotrasterol sulfate promotes endothelial sprouting in vitro, new blood vessel formation on the chick chorioallantoic membrane, and accelerates angiogenesis and reperfusion in a mouse hindlimb ischemia model. We demonstrate that sulfation of the steroid is critical for promoting angiogenesis, as the desulfated steroid exhibited no endothelial sprouting activity. We thus developed a chemically synthesized sokotrasterol sulfate analog, 2beta,3alpha,6alpha-cholestanetrisulfate, that demonstrated equivalent activity in the hindlimb ischemia model and resulted in the generation of stable vessels that persisted following cessation of therapy. The function of sokotrasterol sulfate was dependent on cyclooxygenase-2 activity and vascular endothelial growth factor induction, as inhibition of either cyclooxygenase-2 or vascular endothelial growth factor blocked angiogenesis. Surface expression of alpha(v)beta(3) integrin was also necessary for function, as neutralization of alpha(v)beta(3) integrin, but not beta(1) integrin, binding abrogated endothelial sprouting and antiapoptotic activity in response to sokotrasterol sulfate. Our findings indicate that sokotrasterol sulfate and its analogs can promote angiogenesis in vitro and in vivo and could potentially be used for promoting neovascularization to relieve the sequelae of vasoocclusive diseases.

High-Resolution Quantitative Computed Tomography Demonstrating Selective Enhancement of Medium-Size Collaterals by Placental Growth Factor-1 in the Mouse Ischemic Hindlimb

BACKGROUND

The process of arteriogenesis after occlusion of a major artery is poorly understood. We have used high-resolution microcomputed tomography (mu-CT) imaging to define the arteriogenic response in the mouse model of hindlimb ischemia and to examine the effect of placental growth factor-1 (PlGF-1) on this process.

METHODS AND RESULTS

After common femoral artery ligation, mu-CT imaging demonstrated formation of collateral vessels originating near the ligation site in the upper limb and connecting to the ischemic calf muscle region. Three-dimensional mu-CT and quantitative image analysis revealed changes in the number of segments and the segmental volume of vessels, ranging from 8 to 160 microm in diameter. The medium-size vessels (48 to 160 microm) comprising 85% of the vascular volume were the major contributor (188%) to the change in vascular volume in response to ischemia. Intramuscular injections of Ad-PlGF-1 significantly increased Sca1+ cells in the circulation, alpha-actin-stained vessels, and perfusion of the ischemic hindlimb. These effects were predominantly associated with an increase in vascular volume contributed by the medium-size (96 to 144 microm) vessels as determined by mu-CT.

CONCLUSIONS

High-resolution mu-CT delineated the formation of medium-size collaterals representing a major vascular change that contributed to the restoration of vascular volume after ischemia. This effect is selectively potentiated by PlGF-1. Such selective enhancement of arteriogenesis by therapeutically administered PlGF-1 demonstrates a desirable biological activity for promoting the growth of functionally relevant vasculature.

Carcinoembryonic antigen-related cell adhesion molecule 1 modulates vascular remodeling in vitro and in vivo

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), a cellular adhesion molecule of the Ig superfamily, is associated with early stages of angiogenesis. In vitro, CEACAM1 regulates proliferation, migration, and differentiation of murine endothelial cells. To prove that CEACAM1 is functionally involved in the regulation of vascular remodeling in vivo, we analyzed 2 different genetic models: in Ceacam1-/- mice, the Ceacam1 gene was deleted systemically, and in CEACAM1(endo+) mice, CEACAM1 was overexpressed under the control of the endothelial cell-specific promoter of the Tie2 receptor tyrosine kinase. In Matrigel plug assays, Ceacam1-/- mice failed to establish new capillaries whereas in CEACAM1(endo+) mice the implants were vascularized extensively. After induction of hind limb ischemia by femoral artery ligation, Ceacam1-/- mice showed significantly reduced growth of arterioles and collateral blood flow compared with their WT littermates. In agreement with a causal role of CEACAM1 in vascular remodeling, CEACAM1(endo+) mice exhibited an increase in revascularization and collateral blood flow after arterial occlusion. Our findings indicate that CEACAM1 expression is important for the establishment of newly formed vessels in vivo. Hence CEACAM1 could be a future target for therapeutic manipulation of angiogenesis in disease.

Experimental models of arteriogenesis: differences and implications

Cardiovascular and cerebrovascular disease represent the two most common causes of mortality and morbidity in western countries, and the treatment for these is generally by the mechanical restoration of blood flow in the affected tissues. Stimulation of collateral artery growth (arteriogenesis) provides a potential alternative option for the treatment of patients suffering from occlusive artery disease. Therefore, researchers have established several angiogenesis and arteriogenesis animal models to investigate basic mechanisms and pharmacological modulation of collateral artery growth. The authors highlight the most important aspects of vascular growth, discuss different methods and techniques for examining the process, and review the advantages and disadvantages associated with the animal models available for studying this phenomenon.

Angiopoietin-1 promotes functional neovascularization that relieves ischemia by improving regional reperfusion in a swine chronic myocardial ischemia model

This study investigates the long-term angiogenic effects of ANG-1 and VEGF in a swine chronic myocardial ischemia model. Four-weeks after gradual occlusion of the left circumflex coronary artery by ameroid constrictor, animals were injected with recombinant adenoviral vectors carrying either human ANG-1 (n=9), human VEGF(165) (n=10) or empty vector (n=7) into the left ventricle free wall supplied by the constricted artery. Left ventricular perfusion in animals that received AdANG-1 (3.25+/-0.16 ml/min/g, p<0.05) recovered robustly 4 weeks after gene transfer while ischemia persisted in the AdVEGF (1.09+/-0.13 ml/min/g) and empty vector (1.20+/-0.03 ml/min/g) groups. Microvascular densities in the left ventricles of animals that received AdANG-1 (19.61+/-1.76/0.572 mm(2) myocardial tissue, p<0.05) and AdVEGF (18.17+/-1.43/0.572 mm(2) myocardial tissue, p<0.05) were significantly higher than animals that received empty vector (13.53+/-0.92/0.572 mm(2) myocardial tissue) 12 weeks after gene transfer. ANG-1, but not VEGF, contributed to enhanced regional perfusion by increasing arteriolar density (1.9+/-0.4/0.572 mm(2) myocardial tissue vs. 0.7+/-0.2/0.572 mm(2) myocardial tissue, p<0.05) of large-sized (50-100 microm) arterioles. These data demonstrate that gene transfer of ANG-1 and VEGF enhances angiogenesis, but ANG-1 promotes sustained improvement of ventricular perfusion that expedites recovery of ischemic myocardium via arteriogenesis.

Impact of Mouse Strain Differences in Innate Hindlimb Collateral Vasculature

Objective— To assess the importance of genetic background for collateral artery development.

Methods and Results— C57BL/6, BALB/c and 129S2/Sv mice were studied after femoral artery ligation by laser Doppler imaging, visible light oximetry, time-of-flight–magnetic resonance imaging, and treadmill testing; C57BL/6 and BALB/c also underwent electron paramagnetic resonance (EPR) oximetry, x-ray angiography, and histology. C57BL/6 had the least initial distal ischemia and most complete recovery. BALB/c had the most severe initial ischemia and poorest recovery. BALB/c had some vasodilatory reserve in their ligated limbs not seen in the other strains at 3 weeks. By in vivo TOF-magnetic resonance angiography, C57BL/6 had larger preexistent and developed collaterals. By x-ray angiography, C57BL/6 had a higher collateral-dependent filling score and number of visible collaterals immediately after femoral ligation and a higher number of visible collaterals at 1 week but not at 4 weeks. EPR oximetry and histology revealed hypoxia and tissue damage in regions of collateral growth of BALB/c but not C57BL/6 mice. In C57BL/6 BrdUrd uptake in the thigh was limited to larger vessels and isolated perivascular cells. Proliferative activity in collateral arterioles was similar in both strains.

Conclusions— Genetic differences in preexistent collateral vasculature can profoundly affect outcome and milieu for compensatory collateral artery growth after femoral artery occlusion.

MCP-1 parallels inflammatory and regenerative responses in ischemic muscle

BACKGROUND

Monocyte chemotactic protein-1 (MCP-1) is important in macrophage recruitment and activation. However, the magnitude and temporal sequence of MCP-1 expression in relation to tissue injury and regeneration following ischemic injury remains unknown.

MATERIALS AND METHODS

Hind limb ischemia was induced by femoral artery excision (FAE) in C57Bl/6J mice; a sham surgery was performed on the contralateral leg. Muscle lysates were used to measure MCP-1 and activities of creatine kinase, lactate dehydrogenase, and myeloperoxidase. Histology and immunohistochemistry were used to localize inflammation and MCP-1.

RESULTS

FAE resulted in a prolonged period of ischemia and the administration of MCP-1 did not alter the restoration of perfusion. One day after femoral artery excision, extensive muscle necrosis and neutrophils were prevalent throughout the musculature of the lower leg. By 3 days, a mononuclear cell infiltrate predominated in association with robust muscle regeneration as indicated by myoD expression. Concomitantly, myeloperoxidase was maximally increased. Muscle enzymes (creatine kinase and lactate dehydrogenase) were maximally decreased within 3 days and returned to baseline levels by day 14, a time course consistent with injury and regeneration observed by histology. In parallel with these inflammatory and regenerative events, MCP-1 in muscle was maximally increased at day 3. By immunohistochemistry, MCP-1 was within vascular endothelial cells and infiltrating macrophages in areas of ischemic injury.

CONCLUSIONS

The transient increases and selective tissue distribution of MCP-1 during early inflammation and muscle regeneration support the hypothesis that this cytokine participates in the early reparative events preceding the restoration of vascular perfusion following ischemic injury.

Enhanced arteriogenesis in mice overexpressing erythropoietin

After permanent occlusion of the femoral artery, the survival of ischemic limb tissue depends on collateral artery growth (arteriogenesis). In previous work, we have shown that shear stress triggers arteriogenesis. To test whether increased shear stress results in enhanced arteriogenesis, we compared arteriogenesis in transgenic mice overexpressing erythropoietin (EPO), which possessed increased blood viscosity through the higher hematocrit (thereby providing increased shear stress), with wild-type mice. The right femoral artery was occluded proximal to the origin of the arteria poplitea. Distal blood flow was assessed by laser Doppler imaging, and the growth and remodeling of collateral arteries was examined by light and electron microscopy and morphometry. After occlusion of the femoral artery, EPO mice demonstrated enhanced arteriogenesis: their collateral arteries developed a 1.7-fold diameter and a 2-fold wall thickness compared with wild-type. However, the blood flow recovery in EPO mice was markedly retarded. Structural remodeling and growth of collateral arteries was markedly enhanced in EPO mice, presumably as a result of increased blood viscosity and shear stress.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

The targeted delivery of intravascular drugs and genes across the endothelial barrier with only minimal side effects remains a significant obstacle in establishing effective therapies for many pathological conditions. Recent investigations have shown that contrast agent microbubbles, which are typically used for image enhancement in diagnostic ultrasound, may also be promising tools in emergent, ultrasound-based therapies. Explorations of the bioeffects generated by ultrasound-microbubble interactions indicate that these phenomena may be exploited for clinical utility such as in the targeted revascularization of flow-deficient tissues. Moreover, development of this treatment modality may also include using ultrasound-microbubble interactions to deliver therapeutic material to tissues, and reporter genes and therapeutic agents have been successfully transferred from the microcirculation to tissue in various animal models of normal and pathological function. This article reviews the recent studies aimed at using interactions between ultrasound and contrast agent microbubbles in the microcirculation for therapeutic purposes. Furthermore, the authors present investigations involving microspheres that are of a different design compared to current microbubble contrast agents, yet are acoustically active and demonstrate potential as tools for targeted delivery. Future directions necessary to address current challenges and advance these techniques to clinical practicality are also discussed.

Sustained Peripheral Arterial Insufficiency Durably Impairs Normal and Regenerating Skeletal Muscle Function

Peripheral vascular occlusive diseases are frequently observed in humans, and studies with animal models have been largely used. However the effects of sustained lower limb ischemia on normal and regenerating hindlimb skeletal muscles are not well known in the mouse model. Therefore prolonged unilateral hindlimb ligation was generated by femoral artery ligation. Normal (myotoxic-untreated) and regenerating (myotoxic-reated) ischemic muscles were studied by analyses of the in situ contractile properties and histological parameters. Concerning normal mouse muscles, we found that femoral artery ligation reduced hindlimb perfusion and altered muscle structure and function. Thus 7 days after ligation, maximal tetanic force was reduced by about 70%, (p < 0.05). By 56 days after ligation, muscle weights and cross-section areas of muscle fibers were still reduced (p < 0.05). Concerning myotoxic treated muscles, we report that ligation reduced the recovery of muscle weight and maximal tetanic force and increased fatigue resistance at 56 days (p < 0.05). In conclusion, our results demonstrate that sustained peripheral arterial insufficiency in mice induces long-term as well as acute detrimental effects in both normal and regenerating muscles.

CD8+ T lymphocytes regulate the arteriogenic response to ischemia by infiltrating the site of collateral vessel development and recruiting CD4+ mononuclear cells through the expression of interleukin-16

BACKGROUND

Previous studies have demonstrated that macrophages and CD4+ T lymphocytes play pivotal roles in collateral development. Indirect evidence suggests that CD8+ T cells also play a role. Thus, after acute cerebral ischemia, CD8+ T cells infiltrate the perivascular space and secrete interleukin-16 (IL-16), a potent chemoattractant for monocytes and CD4+ T cells. We tested whether CD8+ T lymphocytes contribute to collateral vessel development and whether the lack of circulating CD8+ T cells prevents IL-16 expression, impairs CD4+ mononuclear cell recruitment, and reduces collateral vessel growth after femoral artery ligation in CD8(-/-) mice.

METHODS AND RESULTS

After surgical excision of the femoral artery, laser Doppler perfusion imaging demonstrated reduced blood flow recovery in CD8(-/-) mice compared with C57/BL6 mice (ischemic/nonischemic limb at day 28, 0.66+/-0.04 versus 0.87+/-0.04, respectively; P<0.01). This resulted in greater calf muscle atrophy (mean fiber area, 785+/-68 versus 1067+/-69 microm2, respectively; P<0.01) and increased fibrotic tissue content (10.8+/-1.2% versus 7+/-1%, respectively; P<0.01). Moreover, CD8(-/-) mice displayed reduced IL-16 expression and decreased CD4+ T-cell recruitment at the site of collateral vessel development. Exogenous CD8+ T cells, infused into CD8(-/-) mice immediately after femoral artery ligation, selectively homed to the ischemic hind limb and expressed IL-16. The restoration of IL-16 expression resulted in significant CD4+ mononuclear cell infiltration of the ischemic limb, faster blood flow recovery, and reduced hindlimb muscle atrophy/fibrosis. When exogenous CD8+ T cells deficient in IL-16 (IL-16(-/-)) were infused into CD8(-/-) mice immediately after femoral artery ligation, they selectively homed to the ischemic hind limb but were unable to recruit CD4+ mononuclear cells and did not improve blood flow recovery.

CONCLUSIONS

These results demonstrate that CD8+ T cells importantly contribute to the early phase of collateral development. After femoral artery ligation, CD8+ T cells infiltrate the site of collateral vessel growth and recruit CD4+ mononuclear cells through the expression of IL-16. Our study provides further evidence of the significant role of the immune system in modulating collateral development in response to peripheral ischemia.

Differential Necrosis Despite Similar Perfusion in Mouse Strains after Ischemia1

BACKGROUND

Numerous mouse models have been used to study the tissue response to ischemia, but multiple technical differences make comparisons difficult. We have comprehensively characterized the mouse hind limb ischemia model and determined how different genetic backgrounds of mice affect recovery.

MATERIALS AND METHODS

Severity of tissue necrosis and restoration of perfusion after femoral artery excision or femoral artery transection, using five different surgical procedures, were evaluated using laser Doppler imaging in a mouse model of hind limb ischemia. Severity of necrosis was concurrently measured using a five-point scale.

RESULTS

Significant differences were observed depending upon the surgical procedure used to initiate ischemia as well as the strain of mouse used. First, a progressively delayed and incomplete recovery of vascular perfusion occurred in relation to the anatomical position and extent of the arterial defect. Second, among mouse strains, the severity of tissue necrosis varied despite similar restoration of perfusion. Thus, DBA/1J mice had significantly increased severity and incidence of tissue loss as compared with either C57Bl/6J (P = 0.01) or BALB/c (P = 0.01) mice. Finally, contrary to previous reports, T-cell-mediated immune events did not modify ischemia-induced hind limb perfusion and necrosis as responses in nude mice were not different than controls on either BALB/c or C57Bl/6J backgrounds.

CONCLUSIONS

Surgical approach, mouse strain, and measures of hind limb perfusion and tissue injury are crucial considerations in the study of ischemia. Understanding how different genetic backgrounds in mice can affect necrosis may provide insights into the diverse healing responses observed in humans.

Delayed Arteriogenesis in Hypercholesterolemic Mice

Background— Hypercholesterolemia has been reported to inhibit ischemia-induced angiogenesis. To address its effects on arteriogenesis, we investigated arterial growth in hypercholesterolemic low-density lipoprotein receptor −/− /ApoB-48 −/− (HCE) mice.

Methods and Results— The extent and the time course of arteriogenesis after femoral artery ligation was evaluated in HCE and strain-matched control mice. Distal limb perfusion was measured by laser Doppler imaging, whereas MRI was used to visualize arterial flow and micro-computed tomography to assess vascular growth. After femoral artery ligation, serial laser Doppler imaging demonstrated significantly delayed restoration of perfusion in untreated HCE compared with control mice (day 3, 0.09 versus 0.19, P <0.05). Treatment with Ad-PR39 in control mice led to a significant restoration of arterial blood flow and tissue perfusion at day 3, whereas in HCE mice, hindlimb perfusion began increasing only by day 7. Micro-CT analysis confirmed increased growth of smaller arterioles (16 to 63 μm in diameter) in the Ad-PR39–treated control compared with HCE mice. The delay in arteriogenesis in HCE mice correlated with delayed tissue appearance of F4/80 + cells. Analysis of gene expression after Ad-PR39 treatment demonstrated that HCE mice had significantly reduced expression of FGF receptor 1, hypoxia-inducible factor-1α, vascular cell adhesion molecule-1, macrophage scavenger receptor-1, and cyclophilin A compared with controls 3 days after arterial ligation that equalized by day 7, mimicking relative changes in arteriogenesis and tissue perfusion.

Conclusions— Hypercholesterolemia results in delayed native arteriogenesis because of reduced early monocyte/macrophage influx and delayed and impaired arterial growth response to growth factor therapy.

Akt1/protein kinase Balpha is critical for ischemic and VEGF-mediated angiogenesis

Akt, or protein kinase B, is a multifunctional serine-threonine protein kinase implicated in a diverse range of cellular functions including cell metabolism, survival, migration, and gene expression. However, the in vivo roles and effectors of individual Akt isoforms in signaling are not explicitly clear. Here we show that the genetic loss of Akt1, but not Akt2, in mice results in defective ischemia and VEGF-induced angiogenesis as well as severe peripheral vascular disease. Akt1 knockout (Akt1-/-) mice also have reduced endothelial progenitor cell (EPC) mobilization in response to ischemia, and reintroduction of WT EPCs, but not EPCs isolated from Akt1-/- mice, into WT mice improves limb blood flow after ischemia. Mechanistically, the loss of Akt1 reduces the basal phosphorylation of several Akt substrates, the migration of fibroblasts and ECs, and NO release. Reconstitution of Akt1-/- ECs with Akt1 rescues the defects in substrate phosphorylation, cell migration, and NO release. Thus, the Akt1 isoform exerts an essential role in blood flow control, cellular migration, and NO synthesis during postnatal angiogenesis.

Bradycardia Stimulates Vascular Growth During Gradual Coronary Occlusion

OBJECTIVE

In cultured endothelium, stretch induces release of growth factors that contribute to angiogenesis. We tested the hypothesis that bradycardia, which prolongs ventricular diastolic filling time and volume, promotes collateral vessel growth.

METHODS AND RESULTS

An ameroid occluder was placed on coronary arteries of dogs with normal heart rates (AM) or bradycardia (55 bpm; AM+BC). A third group had normal heart rates and no ameroid (control [CON]). Four weeks after occluder placement, myocardial blood flow at rest and maximal vasodilation (adenosine) at equivalent heart rates and vascular morphometry of hearts were measured. In AM dogs, conductance (myocardial flow/diastolic pressure) of collateral-dependent myocardium was similar to collateral-independent myocardium during rest but increased to only one third of CON during maximal vasodilation. In contrast, in AM+BC dogs, conductance was similar in collateral-dependent and -independent regions during maximal vasodilation. Arteriolar length density in collateral-dependent myocardium was 80% greater in AM+BC than AM dogs. Capillary length density in collateral-dependent region of AM dogs was lower than CON but normal in AM+BC dogs. The angiopoietin receptor Tie-2 increased in collateral-dependent regions of AM and AM+BC groups, whereas vascular endothelial growth factor increased in collateral-dependent and -independent regions only in AM+BC dogs.

CONCLUSIONS

Chronic bradycardia during gradual coronary artery occlusion facilitates angiogenesis/arteriogenesis in collateral-dependent myocardium and preserves maximal perfusion.

Ultrasound-microbubble-induced neovascularization in mouse skeletal muscle

Ultrasound-microbubble (US-MB) interactions stimulate neovascularization in rat gracilis muscle (GM). We examined microvascular remodeling (MVR) in GMs of C57BL/6 and balb/C mice following ultrasonic MB destruction. A range of MB dosages were administered IV, and exposed GMs received US. Muscles harvested 3, 7 and 14 d posttreatment were stained for vascular markers and assessed for changes in microvessel number, diameter and length. Muscles receiving a low MB dose (LMBD) and US showed significant increases in microvascular density after 3 d, returning to sham levels after one week. A MB dose producing maximum capillary disruptions was then established. This high MB dose (HMBD) facilitated significant MVR in C57BL/6 mice after one week. Balb/C GMs exhibited neovascularization 3 d, but not 7 or 14 d, following US-HMBD treatment. We conclude that HMBD in C57BL/6 mice induces a more sustained neovascularization response compared to balb/C or LMBD-treated C57BL/6 muscles; however, this response is still impermanent.

Catecholamines augment collateral vessel growth and angiogenesis in hindlimb ischemia

Catecholamine stimulation of α1-adrenoceptors exerts growth factor-like activity, mediated by generation of reactive oxygen species, on arterial smooth muscle cells and adventitial fibroblasts and contributes to hypertrophy and hyperplasia in models of vascular injury and disease. Adrenergic trophic activity also contributes to flow-mediated positive arterial remodeling by augmenting proliferation and leukocyte accumulation. To further examine this concept, we studied whether catecholamines contribute to collateral growth and angiogenesis in hindlimb insufficiency. Support for this hypothesis includes the above-mentioned studies, evidence that ischemia augments norepinephrine release from sympathetic nerves, and proposed involvement of reactive oxygen species in angiogenesis and collateral growth. Mice deficient in catecholamine synthesis [by gene deletion of dopamine β-hydroxylase (DBH−/−)] were studied. At 3 wk after femoral artery ligation, increases in adductor muscle perfusion were similar in DBH−/− and wild-type mice, whereas recovery of plantar perfusion and calf microsphere flow were attenuated, although not significantly. Preexisting collaterals in adductor of wild-type mice showed increases in lumen diameter (60%) and medial and adventitial thickness (57 and 119%, P < 0.05 here and below). Lumen diameter increased similarly in DBH−/− mice (52%); however, increases in medial and adventitial thicknesses were reduced (30 and 65%). Leukocyte accumulation in the adventitia/periadventitia of collaterals was 39% less in DBH−/− mice. Increased density of α-smooth muscle actin-positive vessels in wild-type adductor (45%) was inhibited in DBH−/− mice (2%). Although both groups experienced similar atrophy in the gastrocnemius (∼22%), the increase in capillary-to-muscle fiber ratio in wild-type mice (21%) was inhibited in DBH−/− mice (7%). These data suggest that catecholamines may contribute to collateral growth and angiogenesis in tissue ischemia.

Noninvasive Imaging of Angiogenesis With a 99m Tc-Labeled Peptide Targeted at α v β 3 Integrin After Murine Hindlimb Ischemia

Background— Noninvasive imaging strategies play a critical role in assessment of the efficacy of angiogenesis therapies. The α v β 3 integrin is activated in angiogenic vessels and represents a potential target for noninvasive imaging of angiogenesis.

Methods and Results— We evaluated a 99m Tc-labeled peptide (NC100692) targeted at α v β 3 integrin for imaging in an established murine model of angiogenesis induced by hindlimb ischemia. Control mice (n=9) or mice with surgical right femoral artery occlusion (n=29) were injected with NC100692 (1.5±0.2 mCi IV) at different times after femoral occlusion (1, 3, 7, and 14 days) for in vivo pinhole planar gamma camera imaging. Tissue from hindlimb proximal and distal to occlusion was excised for gamma well counting and for immunostaining. On in vivo pinhole images, increased focal NC100692 activity was seen distal to the occlusion at days 3 and 7. This increase in relative NC100692 activity was confirmed by gamma well counting. Lectin staining confirmed increased angiogenesis in the ischemic hindlimb at these time points. A fluorescent analogue of NC100692 was used to confirm specificity and localization of the targeted tracer in cultured endothelial cells. In addition, endothelial cell specificity was confirmed on tissue sections with the use of dual immunofluorescent staining of endothelium and the fluorescent analogue targeted at the α v β 3 integrin.

Conclusions— A 99m Tc-labeled peptide (NC100692) targeted at α v β 3 integrin selectively localized to endothelial cells in regions of increased angiogenesis and could be used for noninvasive serial “hot spot” imaging of angiogenesis. This targeted radiotracer imaging approach is a major advance in tracking therapeutic myocardial angiogenesis and has an important clinical potential.

Influence of mechanical, cellular, and molecular factors on collateral artery growth (arteriogenesis)

Growth of collateral blood vessels (arteriogenesis) is potentially able to preserve structure and function of limbs and organs after occlusion of a major artery. The success of the remodeling process depends on the following conditions: (1) existence of an arteriolar network that connects the preocclusive with the postocclusive microcirculation; (2) activation of the arteriolar endothelium by elevated fluid shear stress; (3) invasion (but not incorporation) of bone marrow-derived cells; and (4) proliferation of endothelial and smooth muscle cells. Most organs of most mammals including man can rely on the existence of interconnecting arterioles in most organs and tissues with heart being the exception in rodents and pigs. Arterial occlusion lowers the pressure in the distal vasculature thereby creating a pressure gradient favoring increased flow through preexisting collaterals. This increases fluid shear stress leading to endothelial activation with cellular edema, upregulation of adhesion molecules, mitogenic-, thrombogenic-, and fibrinolytic factors, leading to monocyte invasion with matrix digestion. Smooth muscle cells migrate and proliferate and the vessel enlarges under the influence of increasing circumferential wall stress. Growth factors involved belong to the FGF family and signaling proceeds via the Ras/Raf- and the Rho cascades. Increases in vascular radius and wall thickness restore fluid shear stress and circumferential wall stress to normal levels and growth stops. Although increases in collateral vessel size are very substantial their maximal conductance amounts to only 40% of normal. Forced increases in FSS can reach almost 100%.

Integrative signaling in angiogenesis

Angiogenesis is a complex process requiring integration of multiple signals in order to achieve successful development of the new vasculature. While individual activities of numerous growth factors are well understood, the integration of their signaling at the cellular and tissue level is just beginning to be appreciated. This review focuses on these two process using vascular endothelial growth factor (VEGF) and fibroblast growth factor 2 (FGF2) as examples.

Structural adaptation increases predicted perfusion capacity after vessel obstruction in arteriolar arcade network of pig skeletal muscle

Arteriolar arcades provide alternate pathways for blood flow after obstruction of arteries or arterioles such as occurs in stroke and coronary and peripheral vascular disease. When obstruction is prolonged, remaining vessels adjust their diameters chronically in response to altered hemodynamic and metabolic conditions. Here, the effectiveness of arcades in maintaining perfusion both immediately following obstruction and after structural adaptation was examined. Morphometric data from a vascular casting of the pig triceps brachii muscle and published data were used to develop a computational model for the hemodynamics and structural adaptation of the arcade network between two feed artery branches, FA1 and FA2. The predicted total flow to capillaries (Q(TA)) in the region initially supplied by FA2 decreased to 26% of the normal value immediately after FA2 obstruction but was restored to 78% of the normal value after adaptation. After obstruction of 1-10 randomly selected arcade segments, Q(TA) was on average 18% higher in the arcade network than in a corresponding two-tree network without arcades. Structural adaptation increased Q(TA) by an additional 16% in the arcade network but had almost no effect in the two-tree network. These results indicate that arcades can partially maintain blood flow after vascular blockage and that this effect is substantially enhanced by structural adaptation.

Simultaneous application of basic fibroblast growth factor and hepatocyte growth factor to enhance the blood vessels formation

OBJECTIVE

The present study investigated whether the simultaneous application of basic fibroblast growth factor (bFGF) and hepatocyte growth factor (HGF) enhances blood vessel formation in murine ischemic hindlimb compared with bFGF or HGF applied alone.

METHODS

Unilateral hindlimb ischemia was created in C57BL/6 mice. Hindlimb blood flow was evaluated by laser Doppler perfusion image index (LDPII) (ratio (%) of ischemic-to-normal-limb blood flow). The ischemic limbs were treated with bFGF and HGF separately, or bFGF and HGF together, and their therapeutic effects were assessed. Collagen microspheres (CM) were used as a sustained-release carrier for bFGF and HGF.

RESULTS

A single intramuscular injection of 5 microg or less of bFGF-incorporated CM (bFGF/CM) into the ischemic limb did not significantly increase the LDPII compared with the control (no treatment) 4 weeks after the treatment. Similarly, 20 microg or less of HGF/CM did not increase LDPII. Based on these results, we compared the dual release of CM incorporating 5 microg of bFGF and 20 microg of HGF with either the single release of 5 mug of bFGF/CM alone or 20 microg of HGF/CM alone. The LDPII of the dual release (94.2% +/- 10.9%) was higher than either single release (51.2% +/- 5.8% or 52.5% +/- 8.0%, P < .01). Furthermore, the LDPII in the dual release (94.2% +/- 10.9%) was equivalent to that with 80 microg of bFGF/CM (95.1% +/- 7.6%) alone or 80 microg of HGF/CM (92.8% +/- 7.6%) alone. A histologic evaluation at 4 weeks showed capillary density in the dual release (868 +/- 173 vessels/mm(2)) was higher than that in either single release (204 +/- 68 vessels/mm(2) or 185 +/- 98 vessels/mm(2) , P < .01). The percentage of mature vessels assessed by alpha-smooth muscle actin staining was also higher in the dual release (43.8% +/- 7.8% vs 9.5% +/- 3.0% or 11.7% +/- 3.8%, respectively; P < .01).

CONCLUSIONS

This study demonstrates that the sustained dual release of a lower dose of bFGF and HGF from a carrier matrix can achieve equivalent blood perfusion recovery and more mature vasculature in the ischemic limb than a higher dose of bFGF or HGF alone. This approach may be a highly promising strategy for the future treatment of peripheral vascular disease.

Structural remodeling of mouse gracilis artery after chronic alteration in blood supply

The goals of this study were to determine the time course and spatial dependence of structural diameter changes in the mouse gracilis artery after a redistribution of blood flow and to compare the observations with predictions of computational models for structural adaptation. Diameters were measured 1, 2, 7, 14, 21, 28, and 56 days after resection of one of the two blood supplies to the artery. Overall average diameter, normalized with respect to diameters in untreated vessels, increased slightly during the first 7 days, then increased more rapidly, reaching a peak around day 21, and then decreased. This transient increase in diameter was spatially nonuniform, being largest toward the point of resection. A previously developed theoretical model, in which diameter varies in response to stimuli derived from local metabolic and hemodynamic conditions, was extended to include effects of time-delayed remodeling stimuli in regions of reduced perfusion. Predictions of this model were consistent with observed diameter changes, including the transient increase in diameters near the point of resection, when a remodeling stimulus with a time delay of approximately 7 days was included. The results suggest that delayed stimuli significantly influence the dynamic characteristics of vascular remodeling resulting from reduced blood supply.

Improved Arteriogenesis with Simultaneous Skeletal Muscle Repair in Ischemic Tissue by SCL+ Multipotent Adult Progenitor Cell Clones from Peripheral Blood

BACKGROUND

The CD34(-) murine stem cell line RM26 cloned from peripheral blood mononuclear cells has been shown to generate hematopoietic progeny in lethally irradiated animals. The peripheral blood-derived cell clones expresses a variety of mesodermal and erythroid/myeloid transcription factors suggesting a multipotent differentiation potential like the bone marrow-derived 'multipotent adult progenitor cells' (MAP-C).

METHODS

SCL(+) CD34(-) RM26 cells were transfused intravenously into mice suffering from chronic hind-limb ischemia, evaluating the effect of stem cells on collateral artery growth and simultaneous skeletal muscle repair.

RESULTS

RM26 cells are capable of differentiating in vitro into endothelial cells when cultured on the appropriate collagen matrix. Activation of the SCL stem cell enhancer (SCL(+)) is mediated through the binding to two Ets and one GATA site and cells start to express milieu- and growth condition-dependent levels of the endothelial markers CD31 (PECAM) and Flt-1 (VEGF-R1). Intravenously infused RM26 cells significantly improved the collateral blood flow (arteriogenesis) and neo-angiogenesis formation in a murine hind-limb ischemia transplant model. Although transplanted RM26 cells did not integrate into the growing collateral arteries, cells were found adjacent to local arteriogenesis, but instead integrated into the ischemic skeletal muscle exclusively in the affected limb for simultaneous tissue repair.

CONCLUSION

These data suggest that molecularly primed hem-/mesangioblast-type adult progenitor cells can circulate in the peripheral blood improving perfusion of tissues with chronic ischemia and extending beyond the vascular compartment.

Elevated fluid shear stress enhances postocclusive collateral artery growth and gene expression in the pig hind limb

OBJECTIVE

The role of fluid shear stress (FSS) in collateral vessel growth remains disputed and prospective in vivo experiments to test its morphogenic power are rare. Therefore, we studied the influence of FSS on arteriogenesis in a new model with extremely high levels of collateral flow and FSS in pig and rabbit hind limbs.

METHODS AND RESULTS

A side-to-side anastomosis was created between the distal stump of one of the bilaterally occluded femoral arteries with the accompanying vein. This clamps the collateral reentry pressure at venous levels and increases collateral flow, which is directed to a large part into the venous system. This decreases circumferential wall stress and markedly increases FSS. One week after anastomosis, angiographic number and size of collaterals were significantly increased. Maximal collateral flow exceeded by 2.3-fold that obtained in the ligature-only hind limb. Capillary density increased in lower leg muscles. Immunohistochemistry revealed augmented proliferative activity of endothelial and smooth muscle cells. Intercellular adhesion molecule-1 and vascular cell adhesion molecule (VCAM)-1 were upregulated, and monocyte invasion was markedly increased. In 2-dimensional gels, actin-regulating cofilin1 and cofilin2, destrin, and transgelin2 showed the highest degree of differential regulation.

CONCLUSIONS

High levels of FSS cause a strong arteriogenic response, reinstate cellular proliferation, stimulate cytoskeletal rearrangement, and normalize maximal conductance. FSS is the initiating molding force in arteriogenesis. The role of fluid shear stress on the development of a collateral circulation was studied by abruptly increasing collateral blood flow by a distal femoral artery-to-vein anastomosis. This increased number and size of collateral vessels to a hitherto unknown degree. Fluid shear stress is the primary and strongest arteriogenic stimulus.

Quantitative microcomputed tomography analysis of collateral vessel development after ischemic injury

Transgenic mouse models are increasingly being used to investigate the functions of specific growth factors or matrix proteins to design therapeutic strategies for controlling blood vessel growth. However, the available methodologies for evaluating angiogenesis and arteriogenesis in these models are limited by animal size, user subjectivity, the power to visualize the three-dimensional vessel networks, or the capability to employ a vigorous quantitative analysis. In this study, we employed contrast-enhanced microcomputed tomography imaging to assess collateral development after induction of hindlimb ischemia in the mouse. The morphological parameters vessel volume, connectivity, number, thickness, thickness distribution, separation, and degree of anisotropy were evaluated in control and surgery limbs 0, 3, and 14 days postsurgery. Results indicate that the vascular volume of the surgically manipulated limb was reconstituted as early as 3 days after femoral artery excision through development of a series of highly connected, small caliber, closely spaced, and isotropically oriented collateral vessels. Parametric analyses were completed to assess the sensitivity of the calculated morphological parameters to variations in image binarization threshold and voxel size. Images taken at the 36-μm voxel size were found to be optimal for evaluating collateral vessel formation, whereas 8- to 16-μm voxel sizes were needed to resolve smaller vascular structures. This study demonstrates the utility of microcomputed tomography as a robust method for quantitative, three-dimensional analysis of blood vessel networks. Whereas these initial efforts focused on the mouse hindlimb ischemia model, the developed techniques may be applied to a variety of model systems to investigate mechanisms of angiogenesis and arteriogenesis.

Temporal patterns of gene expression after acute hindlimb ischemia in mice: insights into the genomic program for collateral vessel development

OBJECTIVES

We sought to understand the genomic program leading to collateral vessel formation.

BACKGROUND

Recently, technology has advanced to the point that it is now possible to elucidate the large array of genes that must be expressed, as well as the temporal expression pattern, for the development of functionally important collateral vessels. In this investigation, we used deoxyribonucleic acid array expression profiling to determine the time course of differential expression of 12,000 genes after femoral artery ligation in C57BL/6 mice.

METHODS

Ribonucleic acid was extracted from the adductor muscle, which showed no signs of ischemia. Sampling was at baseline, 6 h, and 1, 3, 7, and 14 days after femoral artery ligation or sham operation.

RESULTS

Femoral artery ligation caused the differential expression (>2-fold) of 783 genes at one or multiple time points: 518 were induced and 265 were repressed. Cluster analysis generated four temporal patterns: 1) early upregulated (6 to 24 h)-immediate early transcriptional factors, angiogenesis, inflammation, and stress-related genes; 2) mid-phase upregulated (day 3)-cell cycle and cytoskeletal and inflammatory genes; 3) late upregulated (days 7 to 14)-angiostatic, anti-inflammatory, and extracellular matrix-associated genes; and 4) downregulated-genes involved in energy metabolism, water channel, and muscle contraction. Microarray data were validated using quantitative reverse transcription polymerase chain reaction.

CONCLUSIONS

This study documents the large number of genes whose differential expression and temporal functional clustering appear to contribute to collateral formation. These results can serve as a genomic model for arteriogenesis and as a database for developing new therapeutic strategies.

Local delivery of marrow-derived stromal cells augments collateral perfusion through paracrine mechanisms

BACKGROUND

Bone marrow cell therapy is reported to contribute to collateral formation through cell incorporation into new or remodeling vessels. However, the possible role of a paracrine contribution to this effect is less well characterized.

METHODS AND RESULTS

Murine marrow-derived stromal cells (MSCs) were purified by magnetic bead separation of cultured bone marrow. The release of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), placental growth factor (PlGF), and monocyte chemoattractant protein-1 (MCP-1) was demonstrated by analysis of MSC conditioned media (MSC-CM). MSC-CM enhanced proliferation of endothelial cells and smooth muscle cells in a dose-dependent manner; anti-VEGF and anti-FGF antibodies only partly attenuated these effects. Balb/C mice (n=10) underwent distal femoral artery ligation, followed by adductor muscle injection of 1x10(6) MSCs 24 hours later. Compared with controls injected with media (n=10) or mature endothelial cells (n=8), distal limb perfusion improved, and mid-thigh conductance vessels increased in number and total cross-sectional area. MSC injection improved limb function and appearance, reduced the incidence of auto-amputation, and attenuated muscle atrophy and fibrosis. After injection, labeled MSCs were seen dispersed between muscle fibers but were not seen incorporated into mature collaterals. Injection of MSCs increased adductor muscle levels of bFGF and VEGF protein compared with controls. Finally, colocalization of VEGF and transplanted MSCs within adductor tissue was demonstrated.

CONCLUSIONS

MSCs secrete a wide array of arteriogenic cytokines. MSCs can contribute to collateral remodeling through paracrine mechanisms.

Collateral artery growth (arteriogenesis) after experimental arterial occlusion is impaired in mice lacking CC-chemokine receptor-2

Arteriogenesis has been associated with the presence of monocytes/macrophages within the collateral vessel wall. Induced macrophage migration in vivo is driven by the binding of monocyte chemoattractant protein-1 (MCP-1, or CCL2 in the new nomenclature) to the CCR2-chemokine receptor on macrophages. To determine whether the CCL2-CCR2 signaling pathway is involved in the accumulation of macrophages in growing collateral vessels, we used mice that are deficient in CCR2 in a model of experimental arterial occlusion and collateral vessel growth. In an in vitro CCL2-driven chemotaxis assay, mononuclear cells isolated from wild-type BALB/c mice exhibited CCL2 concentration-dependent migration, whereas this migration was abolished in cells from CCR2(-/-) mice on a BALB/c genetic background. In vivo, blood flow recovery as measured by laser Doppler (LDI) and MRI (MRI) was impaired in CCR2(-/-) mice on either the BALB/c or C57BL/6 genetic backgrounds. Three weeks after femoral artery ligation, LDI perfusion ratio of operated versus nonoperated distal hindlimb in BALB/c wild-type mice increased to 0.45+/-0.06 and in CCR2(-/-) animals only to 0.21+/-0.03 (P<0.01). In C57BL/6 mice, ratio increased to 0.96+/-0.09 and 0.85+/-0.08 (P<0.05), respectively. MRI at 3 weeks (0.76+/-0.06 versus 0.62+/-0.01; P<0.05) and hemoglobin oxygen saturation measurements confirmed these findings. Active foot movement score significantly decreased and gastrocnemius muscle atrophy was significantly greater in CCR2(-/-) mice. Morphometric analysis showed a lesser increase in collateral vessel diameters in CCR2(-/-) mice. Importantly, the number of invaded monocytes/macrophages in the perivascular space of collateral arteries of CCR2(-/-) animals was dramatically reduced in comparison to wild-type mice. In conclusion, our results demonstrate that the CCR2 signaling pathway is essential for efficient collateral artery growth.

Magnetic resonance angiography of collateral vessels in a murine femoral artery ligation model

The in vivo detection of growing collateral vessels following arterial occlusion is difficult in small animals. We have addressed the feasibility of performing high resolution time-of-flight angiograms to monitor the growth of collateral vessels after femoral artery occlusion in mice. We will also present a low-pass quadrature birdcage coil construction with a sufficient signal-to-noise ratio to produce high resolution. After a 4-month recovery period a C57BL/6 mouse with a surgical occlusion of the right femoral artery was used to assess the image quality and time requirements to produce magnetic resonance angiograms sufficient to assess collateral artery development using a two-dimensional gradient echo sequence. At a resolution of 100 x 100 x 100 microm and a matrix size of 256 x 128 x 256 for a 2.56 cm isometric volume, three scans were performed with one, two and four repetitions resulting in signal-to-noise ratios for the femoral artery proximal to the ligation site of 58, 126 and 194, respectively. Five C57BL/6 mice were additionally measured 4 weeks after occlusion using two repetitions and the visual collateral vessels were assessed for number and location: 2.0 +/- 1.2 in quadriceps muscle, 0.6 +/- 0.5 in adductor (deep adductor vessel), 0.0 +/- 0.0 in adductor (surface adductor vessels). The results showed a significant difference, two-sided t-test, p < 0.05, in number of vessels in all the locations. We have shown that this method can be utilized to elucidate the contribution of collateral vessels to arterial flow.

Marrow-derived stromal cells express genes encoding a broad spectrum of arteriogenic cytokines and promote in vitro and in vivo arteriogenesis through paracrine mechanisms

We recently demonstrated that marrow stromal cells (MSCs) augment collateral remodeling through release of several cytokines such as VEGF and bFGF rather than via cell incorporation into new or remodeling vessels. The present study was designed to characterize the full spectrum of cytokine genes expressed by MSCs and to further examine the role of paracrine mechanisms that underpin their therapeutic potential. Normal human MSCs were cultured under normoxic or hypoxic conditions for 72 hours. The gene expression profile of the cells was determined using Affymetrix GeneChips representing 12 000 genes. A wide array of arteriogenic cytokine genes were expressed at baseline, and several were induced >1.5-fold by hypoxic stress. The gene array data were confirmed using ELISA assays and immunoblotting of the MSC conditioned media (MSC(CM)). MSC(CM) promoted in vitro proliferation and migration of endothelial cells in a dose-dependent manner; anti-VEGF and anti-FGF antibodies only partially attenuated these effects. Similarly, MSC(CM) promoted smooth muscle cell proliferation and migration in a dose-dependent manner. Using a murine hindlimb ischemia model, murine MSC(CM) enhanced collateral flow recovery and remodeling, improved limb function, reduced the incidence of autoamputation, and attenuated muscle atrophy compared with control media. These data indicate that paracrine signaling is an important mediator of bone marrow cell therapy in tissue ischemia, and that cell incorporation into vessels is not a prerequisite for their effects.

Bone marrow-derived cells do not incorporate into the adult growing vasculature

Bone marrow-Derived cells have been proposed to form new vessels or at least incorporate into growing vessels in adult organisms under certain physiological and pathological conditions. We investigated whether bone marrow-Derived cells incorporate into vessels using mouse models of hindlimb ischemia (arteriogenesis and angiogenesis) and tumor growth. C57BL/6 wild-type mice were lethally irradiated and transplanted with bone marrow cells from littermates expressing enhanced green fluorescent protein (GFP). At least 6 weeks after bone marrow transplantation, the animals underwent unilateral femoral artery occlusions with or without pretreatment with vascular endothelial growth factor or were subcutaneously implanted with methylcholanthrene-induced fibrosarcoma (BFS-1) cells. Seven and 21 days after surgery, proximal hindlimb muscles with growing collateral arteries and ischemic gastrocnemius muscles as well as grown tumors and various organs were excised for histological analysis. We failed to colocalize GFP signals with endothelial or smooth muscle cell markers. Occasionally, the use of high-power laser scanning confocal microscopy uncovered false-positive results because of overlap of different fluorescent signals from adjacent cells. Nevertheless, we observed accumulations of GFP-positive cells around growing collateral arteries (3-fold increase versus nonoccluded side, P<0.001) and in ischemic distal hindlimbs. These cells were identified as fibroblasts, pericytes, and primarily leukocytes that stained positive for several growth factors and chemokines. Our findings suggest that in the adult organism, bone marrow-Derived cells do not promote vascular growth by incorporating into vessel walls but may function as supporting cells.