Impaired collateral vessel development associated with reduced expression of vascular endothelial growth factor in ApoE-/- mice

Adaptation process of decellularized vascular grafts as hemodialysis access in vivo

Arteriovenous grafts (AVGs) have emerged as the preferred option for constructing hemodialysis access in numerous patients. Clinical trials have demonstrated that decellularized vascular graft exhibits superior patency and excellent biocompatibility compared to polymer materials; however, it still faces challenges such as intimal hyperplasia and luminal dilation. The absence of suitable animal models hinders our ability to describe and explain the pathological phenomena above and in vivo adaptation process of decellularized vascular graft at the molecular level. In this study, we first collected clinical samples from patients who underwent the construction of dialysis access using allogeneic decellularized vascular graft, and evaluated their histological features and immune cell infiltration status 5 years post-transplantation. Prior to the surgery, we assessed the patency and intimal hyperplasia of the decellularized vascular graft using non-invasive ultrasound. Subsequently, in order to investigate the in vivo adaptation of decellularized vascular grafts in an animal model, we attempted to construct an AVG model using decellularized vascular grafts in a small animal model. We employed a physical-chemical-biological approach to decellularize the rat carotid artery, and histological evaluation demonstrated the successful removal of cellular and antigenic components while preserving extracellular matrix constituents such as elastic fibers and collagen fibers. Based on these results, we designed and constructed the first allogeneic decellularized rat carotid artery AVG model, which exhibited excellent patency and closely resembled clinical characteristics. Using this animal model, we provided a preliminary description of the histological features and partial immune cell infiltration in decellularized vascular grafts at various time points, including Day 7, Day 21, Day 42, and up to one-year post-implantation. These findings establish a foundation for further investigation into the in vivo adaptation process of decellularized vascular grafts in small animal model.

Translational Relevance of Advanced Age and Atherosclerosis in Preclinical Trials of Biotherapies for Peripheral Artery Disease

Approximately 6% of adults worldwide suffer from peripheral artery disease (PAD), primarily caused by atherosclerosis of lower limb arteries. Despite optimal medical care and revascularization, many PAD patients remain symptomatic and progress to critical limb ischemia (CLI) and risk major amputation. Delivery of pro-angiogenic factors as proteins or DNA, stem, or progenitor cells confers vascular regeneration and functional recovery in animal models of CLI, but the effects are not well replicated in patients and no pro-angiogenic biopharmacological procedures are approved in the US, EU, or China. The reasons are unclear, but animal models that do not represent clinical PAD/CLI are implicated. Consequently, it is unclear whether the obstacles to clinical success lie in the toxic biochemical milieu of human CLI, or in procedures that were optimized on inappropriate models. The question is significant because the former case requires abandonment of current strategies, while the latter encourages continued optimization. These issues are discussed in the context of relevant preclinical and clinical data, and it is concluded that preclinical mouse models that include age and atherosclerosis as the only comorbidities that are consistently present and active in clinical trial patients are necessary to predict clinical success. Of the reviewed materials, no biopharmacological procedure that failed in clinical trials had been tested in animal models that included advanced age and atherosclerosis relevant to PAD/CLI.

Peripheral Ischemia Imprints Epigenetic Changes in Hematopoietic Stem Cells to Propagate Inflammation and Atherosclerosis

BACKGROUND

Peripheral ischemia caused by peripheral artery disease is associated with systemic inflammation, which may aggravate underlying comorbidities such as atherosclerosis and heart failure. However, the mechanisms of increased inflammation and inflammatory cell production in patients with peripheral artery disease remain poorly understood.

METHODS

We used peripheral blood collected from patients with peripheral artery disease and performed hind limb ischemia (HI) in Apoe-/- mice fed a Western diet and C57BL/6J mice with a standard laboratory diet. Bulk and single-cell RNA sequencing analysis, whole-mount microscopy, and flow cytometry were performed to analyze hematopoietic stem and progenitor cell (HSPC) proliferation, differentiation, and relocation.

RESULTS

We observed augmented numbers of leukocytes in the blood of patients with peripheral artery disease and Apoe-/- mice with HI. RNA sequencing and whole-mount imaging of the bone marrow revealed HSPC migration into the vascular niche from the osteoblastic niche and their exaggerated proliferation and differentiation. Single-cell RNA sequencing demonstrated alterations in the genes responsible for inflammation, myeloid cell mobilization, and HSPC differentiation after HI. Heightened inflammation in Apoe-/- mice after HI aggravated atherosclerosis. Surprisingly, bone marrow HSPCs expressed higher amounts of the receptors for IL (interleukin)-1 and IL-3 after HI. Concomitantly, the promoters of Il1r1 and Il3rb had augmented H3K4me3 and H3K27ac marks after HI. Genetic and pharmacological inhibition of these receptors resulted in suppressed HSPC proliferation, reduced leukocyte production, and ameliorated atherosclerosis.

CONCLUSIONS

Our findings demonstrate increased inflammation, HSPC abundance in the vascular niches of the bone marrow, and elevated IL-3Rb and IL-1R1 (IL-1 receptor 1) expression in HSPC following HI. Furthermore, the IL-3Rb and IL-1R1 signaling plays a pivotal role in HSPC proliferation, leukocyte abundance, and atherosclerosis aggravation after HI.

Inflammatory cells dynamics control neovascularization and tissue healing after localized radiation induced injury in mice

Local overexposure to ionizing radiation leads to chronic inflammation, vascular damage and cachexia. Here we investigate the kinetics of inflammatory cells from day (D)1 to D180 after mouse hindlimb irradiation and analyze the role of monocyte (Mo) subsets in tissue revascularization. At D1, we find that Mo and T cells are mobilized from spleen and bone marrow to the blood. New vessel formation during early phase, as demonstrated by ~1.4- and 2-fold increased angiographic score and capillary density, respectively, correlates with an increase of circulating T cells, and Mo^hi and type 1-like macrophages in irradiated muscle. At D90 vascular rarefaction and cachexia are observed, associated with decreased numbers of circulating Mo^lo and Type 2-like macrophages in irradiated tissue. Moreover, CCR2- and CX3CR1-deficency negatively influences neovascularization. However adoptive transfer of Mo^hi enhances vessel growth. Our data demonstrate the radiation-induced dynamic inflammatory waves and the major role of inflammatory cells in neovascularization.

Promoting Angiogenesis Using Immune Cells for Tissue-Engineered Vascular Grafts

Implantable tissue-engineered vascular grafts (TEVGs) usually trigger the host reaction which is inextricably linked with the immune system, including blood-material interaction, protein absorption, inflammation, foreign body reaction, and so on. With remarkable progress, the immune response is no longer considered to be entirely harmful to TEVGs, but its therapeutic and impaired effects on angiogenesis and tissue regeneration are parallel. Although the implicated immune mechanisms remain elusive, it is certainly worthwhile to gain detailed knowledge about the function of the individual immune components during angiogenesis and vascular remodeling. This review provides a general overview of immune cells with an emphasis on macrophages in light of the current literature. To the extent possible, we summarize state-of-the-art approaches to immune cell regulation of the vasculature and suggest that future studies are needed to better define the timing of the activity of each cell subpopulation and to further reveal key regulatory switches.

Praliciguat Promotes Ischemic Leg Reperfusion in Leptin Receptor-Deficient Mice

BACKGROUND

Lower-limb peripheral artery disease is one of the major complications of diabetes. Peripheral artery disease is associated with poor limb and cardiovascular prognoses, along with a dramatic decrease in life expectancy. Despite major medical advances in the treatment of diabetes, a substantial therapeutic gap remains in the peripheral artery disease population. Praliciguat is an orally available sGC (soluble guanylate cyclase) stimulator that has been reported both preclinically and in early stage clinical trials to have favorable effects in metabolic and hemodynamic outcomes, suggesting that it may have a potential beneficial effect in peripheral artery disease.

METHODS

We evaluated the effect of praliciguat on hind limb ischemia recovery in a mouse model of type 2 diabetes. Hind limb ischemia was induced in leptin receptor-deficient (Lepr^db/db) mice by ligation and excision of the left femoral artery. Praliciguat (10 mg/kg/day) was administered in the diet starting 3 days before surgery.

RESULTS

Twenty-eight days after surgery, ischemic foot perfusion and function parameters were better in praliciguat-treated mice than in vehicle controls. Improved ischemic foot perfusion was not associated with either improved traditional cardiovascular risk factors (ie, weight, glycemia) or increased angiogenesis. However, treatment with praliciguat significantly increased arteriole diameter, decreased ICAM1 (intercellular adhesion molecule 1) expression, and prevented the accumulation of oxidative proangiogenic and proinflammatory muscle fibers. While investigating the mechanism underlying the beneficial effects of praliciguat therapy, we found that praliciguat significantly downregulated Myh2 and Cxcl12 mRNA expression in cultured myoblasts and that conditioned medium form praliciguat-treated myoblast decreased ICAM1 mRNA expression in endothelial cells. These results suggest that praliciguat therapy may decrease ICAM1 expression in endothelial cells by downregulating Cxcl12 in myocytes.

CONCLUSIONS

Our results demonstrated that praliciguat promotes blood flow recovery in the ischemic muscle of mice with type 2 diabetes, at least in part by increasing arteriole diameter and by downregulating ICAM1 expression.

Inhibition of transient receptor potential cation channel 6 promotes capillary arterialization during post-ischaemic blood flow recovery

BACKGROUND AND PURPOSE

Capillary arterialization, characterized by the coverage of pre-existing or nascent capillary vessels with vascular smooth muscle cells (VSMCs), is critical for the development of collateral arterioles to improve post-ischaemic blood flow. We previously demonstrated that the inhibition of transient receptor potential 6 subfamily C, member 6 (TRPC6) channels facilitate contractile differentiation of VSMCs under ischaemic stress. We here investigated whether TRPC6 inhibition promotes post-ischaemic blood flow recovery through capillary arterialization in vivo.

EXPERIMENTAL APPROACH

Mice were subjected to hindlimb ischaemia by ligating left femoral artery. The recovery rate of peripheral blood flow was calculated by the ratio of ischaemic left leg to non-ischaemic right one. The number and diameter of blood vessels were analysed by immunohistochemistry. Expression and phosphorylation levels of TRPC6 proteins were determined by western blotting and immunohistochemistry.

KEY RESULTS

Although the post-ischaemic blood flow recovery is reportedly dependent on endothelium-dependent relaxing factors, systemic TRPC6 deletion significantly promoted blood flow recovery under the condition that nitric oxide or prostacyclin production were inhibited, accompanying capillary arterialization. Cilostazol, a clinically approved drug for peripheral arterial disease, facilitates blood flow recovery by inactivating TRPC6 via phosphorylation at Thr69 in VSMCs. Furthermore, inhibition of TRPC6 channel activity by pyrazole-2 (Pyr2; BTP2; YM-58483) promoted post-ischaemic blood flow recovery in Apolipoprotein E-knockout mice.

CONCLUSION AND IMPLICATIONS

Suppression of TRPC6 channel activity in VSMCs could be a new strategy for the improvement of post-ischaemic peripheral blood circulation.

C1q/TNF-related protein-2 improved angiogenesis to protect myocardial function during ischaemia‒reperfusion

BACKGROUND

Collateral growth plays an important role in the recovery of acute myocardial infarction. C1q/TNF-related protein-2 (CTRP2), a CTRP family member, showed some protective effects on cell survival. In this study, the relationship between CTRP2 and collateral growth was examined.

METHODS

C57BL/6 mice were subjected to myocardial ischaemia/reperfusion (I/R), and the expression of CTRP2 and the effect of CTRP2 on infarction size, cardiac function and angiogenesis were examined. The ischaemic hindlimb model was also used to examine the effect of CTRP2. In vitro, CTRP2-mediated regulation of angiogenesis, AKT activation and VEGFR2 expression in endothelial cells was examined. The CTRP2 level associated with good collateral growth was observed in a cohort.

RESULTS

I/R reduced CTRP2 expression, and intraperitoneal injection of recombinant CTRP2 protein improved infarction size, cardiac function and angiogenesis. Overexpression of CTRP2 promoted blood refusion and collateral growth in ischaemic hindlimb mice. In vitro, CTRP2 enhanced tube formation and migration in a dose-dependent manner, while CTRP2 increased AKT phosphorylation and VEGFR2 expression. In an observational clinical cohort, CTRP2 levels were significantly increased in patients with good collateral growth, and CTRP2 was negatively associated with poor collateral growth in patients.

CONCLUSION

CTRP2 improved cardiac function by promoting collateral growth by promoting AKT-VEGFR2.

A TRPC3/6 Channel Inhibitor Promotes Arteriogenesis after Hind-Limb Ischemia

Retarded revascularization after progressive occlusion of large conductance arteries is a major cause of bad prognosis for peripheral artery disease (PAD). However, pharmacological treatment for PAD is still limited. We previously reported that suppression of transient receptor potential canonical (TRPC) 6 channel activity in vascular smooth muscle cells (VSMCs) facilitates VSMC differentiation without affecting proliferation and migration. In this study, we found that 1-benzilpiperadine derivative (1-BP), a selective inhibitor for TRPC3 and TRPC6 channel activities, induced VSMC differentiation. 1-BP-treated mice showed increased capillary arterialization and improvement of peripheral circulation and skeletal muscle mass after hind-limb ischemia (HLI) in mice. 1-BP had no additive effect on the facilitation of blood flow recovery after HLI in TRPC6-deficient mice, suggesting that suppression of TRPC6 underlies facilitation of the blood flow recovery by 1-BP. 1-BP also improved vascular nitric oxide bioavailability and blood flow recovery after HLI in hypercholesterolemic mice with endothelial dysfunction, suggesting the retrograde interaction from VSMCs to endothelium. These results suggest that 1-BP becomes a potential seed for PAD treatments that target vascular TRPC6 channels.

Low-flow intussusception and metastable VEGFR2 signaling launch angiogenesis in ischemic muscle

Efforts to promote sprouting angiogenesis in skeletal muscles of individuals with peripheral artery disease have not been clinically successful. We discovered that, contrary to the prevailing view, angiogenesis following ischemic muscle injury in mice was not driven by endothelial sprouting. Instead, real-time imaging revealed the emergence of wide-caliber, primordial conduits with ultralow flow that rapidly transformed into a hierarchical neocirculation by transluminal bridging and intussusception. This process was accelerated by inhibiting vascular endothelial growth factor receptor-2 (VEGFR2). We probed this response by developing the first live-cell model of transluminal endothelial bridging using microfluidics. Endothelial cells subjected to ultralow shear stress could reposition inside the flowing lumen as pillars. Moreover, the low-flow lumen proved to be a privileged location for endothelial cells with reduced VEGFR2 signaling capacity, as VEGFR2 mechanosignals were boosted. These findings redefine regenerative angiogenesis in muscle as an intussusceptive process and uncover a basis for its launch.

Peripheral vascular disease: preclinical models and emerging therapeutic targeting of the vascular endothelial growth factor ligand-receptor system

Introduction: Vascular endothelial growth factor (VEGF)-A is a sought therapeutic target for PAD treatment because of its potent role in angiogenesis. However, no therapeutic benefit was achieved in VEGF-A clinical trials, suggesting that our understanding of VEGF-A biology and ischemic angiogenic processes needs development. Alternate splicing in VEGF-A produces pro- and anti-angiogenic VEGF-A isoforms; the only difference being a 6-amino acid switch in the C-terminus of the final 8th exon of the gene. This finding has changed our understanding of VEGF-A biology and may explain the lack of benefit in VEGF-A clinical trials. It presents new therapeutic opportunities for peripheral arterial disease (PAD) treatment.Areas covered: Literature search was conducted to include: 1) predicted mechanism by which the anti-angiogenic VEGF-A isoform would inhibit angiogenesis, 2) unexpected mechanism of action, and 3) how this mechanism revealed novel signaling pathways that may enhance future therapeutics in PAD.Expert opinion: Inhibiting a specific anti-angiogenic VEGF-A isoform in ischemic muscle promotes perfusion recovery in preclinical PAD. Additional efforts focused on the production of these isoforms, and the pathways altered by modulating different VEGF receptor-ligand interactions, and how this new data may allow bedside progress offers new approaches to PAD are discussed.I.

Impaired angiogenesis and extracellular matrix metabolism in autosomal-dominant hyper-IgE syndrome

There are more than 7000 described rare diseases, most lacking specific treatment. Autosomal-dominant hyper-IgE syndrome (AD-HIES, also known as Job's syndrome) is caused by mutations in STAT3. These patients present with immunodeficiency accompanied by severe nonimmunological features, including skeletal, connective tissue, and vascular abnormalities, poor postinfection lung healing, and subsequent pulmonary failure. No specific therapies are available for these abnormalities. Here, we investigated underlying mechanisms in order to identify therapeutic targets. Histological analysis of skin wounds demonstrated delayed granulation tissue formation and vascularization during skin-wound healing in AD-HIES patients. Global gene expression analysis in AD-HIES patient skin fibroblasts identified deficiencies in a STAT3-controlled transcriptional network regulating extracellular matrix (ECM) remodeling and angiogenesis, with hypoxia-inducible factor 1α (HIF-1α) being a major contributor. Consistent with this, histological analysis of skin wounds and coronary arteries from AD-HIES patients showed decreased HIF-1α expression and revealed abnormal organization of the ECM and altered formation of the coronary vasa vasorum. Disease modeling using cell culture and mouse models of angiogenesis and wound healing confirmed these predicted deficiencies and demonstrated therapeutic benefit of HIF-1α-stabilizing drugs. The study provides mechanistic insights into AD-HIES pathophysiology and suggests potential treatment options for this rare disease.

Interleukin 35 Delays Hindlimb Ischemia-Induced Angiogenesis Through Regulating ROS-Extracellular Matrix but Spares Later Regenerative Angiogenesis

Interleukin (IL) 35 is a novel immunosuppressive heterodimeric cytokine in IL-12 family. Whether and how IL-35 regulates ischemia-induced angiogenesis in peripheral artery diseases are unrevealed. To fill this important knowledge gap, we used loss-of-function, gain-of-function, omics data analysis, RNA-Seq, in vivo and in vitro experiments, and we have made the following significant findings: i) IL-35 and its receptor subunit IL-12RB2, but not IL-6ST, are induced in the muscle after hindlimb ischemia (HLI); ii) HLI-induced angiogenesis is improved in Il12rb2-/- mice, in ApoE-/-/Il12rb2-/- mice compared to WT and ApoE-/- controls, respectively, where hyperlipidemia inhibits angiogenesis in vivo and in vitro; iii) IL-35 cytokine injection as a gain-of-function approach delays blood perfusion recovery at day 14 after HLI; iv) IL-35 spares regenerative angiogenesis at the late phase of HLI recovery after day 14 of HLI; v) Transcriptome analysis of endothelial cells (ECs) at 14 days post-HLI reveals a disturbed extracellular matrix re-organization in IL-35-injected mice; vi) IL-35 downregulates three reactive oxygen species (ROS) promoters and upregulates one ROS attenuator, which may functionally mediate IL-35 upregulation of anti-angiogenic extracellular matrix proteins in ECs; and vii) IL-35 inhibits human microvascular EC migration and tube formation in vitro mainly through upregulating anti-angiogenic extracellular matrix-remodeling proteins. These findings provide a novel insight on the future therapeutic potential of IL-35 in suppressing ischemia/inflammation-triggered inflammatory angiogenesis at early phase but sparing regenerative angiogenesis at late phase.

Recruitment and maturation of the coronary collateral circulation: Current understanding and perspectives in arteriogenesis

The coronary collateral circulation is a rich anastomotic network of primitive vessels which have the ability to augment in size and function through the process of arteriogenesis. In this review, we evaluate the current understandings of the molecular and cellular mechanisms by which this process occurs, specifically focussing on elevated fluid shear stress (FSS), inflammation, the redox state and gene expression along with the integrative, parallel and simultaneous process by which this occurs. The initiating step of arteriogenesis occurs following occlusion of an epicardial coronary artery, with an increase in FSS detected by mechanoreceptors within the endothelium. This must occur within a 'redox window' where an equilibrium of oxidative and reductive factors are present. These factors initially result in an inflammatory milieu, mediated by neutrophils as well as lymphocytes, with resultant activation of a number of downstream molecular pathways resulting in increased expression of proteins involved in monocyte attraction and adherence; namely vascular cell adhesion molecule 1 (VCAM-1), monocyte chemoattractant protein 1 (MCP-1) and transforming growth factor beta (TGF-β). Once monocytes and other inflammatory cells adhere to the endothelium they enter the extracellular matrix and differentiate into macrophages in an effort to create a favourable environment for vessel growth and development. Activated macrophages secrete inflammatory cytokines such as tumour necrosis factor-α (TNF-α), growth factors such as fibroblast growth factor-2 (FGF-2) and matrix metalloproteinases. Finally, vascular smooth muscle cells proliferate and switch to a contractile phenotype, resulting in an increased diameter and functionality of the collateral vessel, thereby allowing improved perfusion of the distal myocardium subtended by the occluded vessel. This simultaneously reduces FSS within the collateral vessel, inhibiting further vessel growth.

Regulatory T cells in ischemic cardiovascular injury and repair

Ischemic injury triggers a heightened inflammatory response that is essential for tissue repair, but excessive and chronic inflammatory responses contribute to the pathogenesis of ischemic cardiovascular disease. Regulatory T cells (Tregs), a major regulator of self-tolerance and immune suppression, control innate and adaptive immune responses, modulate specific immune cell subsets, prevent excessive inflammation, and participate in tissue repair after ischemia. Herein, we summarize the multiple potential mechanisms by which Tregs exert suppressor functions including modulation of cytokine production, alteration of cell-cell interactions, and disruption of metabolic pathways. Furthermore, we review the role of Tregs implicated in ischemic injury and repair including myocardial, limb, and cerebral ischemia. We conclude with a perspective on the therapeutic opportunities and future challenges of Treg biology in understanding the pathogenesis of ischemic cardiovascular disease states.

CD8+ T-cell plasticity regulates vascular regeneration in type-2 diabetes

In this study, we observe that the ischemic tissues of type-2 diabetic (T2D) patients and mice have significantly more CD8⁺ T-cells than that of their normoglycemic counterparts, respectively. However, the role of CD8⁺ T-cells in the pathogenesis of diabetic vascular complication has been less studied. Methods: We employed loss-of-function studies in mouse models using the non-lytic anti-CD8 antibody that blocks tissue infiltration of CD8⁺ T-cells into the injured tissue. We also performed genome-wide, single-cell RNA-sequencing of CD8⁺ T-cells to uncover their role in the pathogenesis of diabetic vascular diseases. Results: The vascular density is negatively correlated with the number of CD8⁺ T-cells in the ischemic tissues of patients and mice after injury. CD8⁺ T-cells or their supernatant can directly impair human and murine angiogenesis. Compared to normoglycemic mice that can regenerate their blood vessels after injury, T2D mice fail in this regeneration. Treatment with the CD8 checkpoint blocking antibody increases the proliferation and function of endothelial cells in both Lepr^db/db mice and diet-induced diabetic Cdh5-Cre;Rosa-YFP lineage-tracing mice after ischemic injury. Furthermore, single-cell transcriptomic profiling reveals that CD8⁺ T-cells of T2D mice showed a de novo cell fate change from the angiogenic, tissue-resident memory cells towards the effector and effector memory cells after injury. Functional revascularization by CD8 checkpoint blockade is mediated through unleashing such a poised lineage commitment of CD8⁺ T-cells from T2D mice. Conclusion: Our results reveal that CD8⁺ T-cell plasticity regulates vascular regeneration; and give clinically relevant insights into the potential development of immunotherapy targeting vascular diseases associated with obesity and diabetes.

Development of a two-stage limb ischemia model to better simulate human peripheral artery disease

Peripheral arterial disease (PAD) develops due to the narrowing or blockage of arteries supplying blood to the lower limbs. Surgical and endovascular interventions are the main treatments for advanced PAD but alternative and adjunctive medical therapies are needed. Currently the main preclinical experimental model employed in PAD research is based on induction of acute hind limb ischemia (HLI) by a 1-stage procedure. Since there are concerns regarding the ability to translate findings from this animal model to patients, we aimed to develop a novel clinically relevant animal model of PAD. HLI was induced in male Apolipoprotein E (ApoE-/-) deficient mice by a 2-stage procedure of initial gradual femoral artery occlusion by ameroid constrictors for 14 days and subsequent excision of the femoral artery. This 2-stage HLI model was compared to the classical 1-stage HLI model and sham controls. Ischemia severity was assessed using Laser Doppler Perfusion Imaging (LDPI). Ambulatory ability was assessed using an open field test, a treadmill test and using established scoring scales. Molecular markers of angiogenesis and shear stress were assessed within gastrocnemius muscle tissue samples using quantitative polymerase chain reaction. HLI was more severe in mice receiving the 2-stage compared to the 1-stage ischemia induction procedure as assessed by LDPI (p = 0.014), and reflected in a higher ischemic score (p = 0.004) and lower average distance travelled on a treadmill test (p = 0.045). Mice undergoing the 2-stage HLI also had lower expression of angiogenesis markers (vascular endothelial growth factor, p = 0.004; vascular endothelial growth factor- receptor 2, p = 0.008) and shear stress response mechano-transducer transient receptor potential vanilloid 4 (p = 0.041) within gastrocnemius muscle samples, compared to animals having the 1-stage HLI procedure. Mice subjected to the 2-stage HLI receiving an exercise program showed significantly greater improvement in their ambulatory ability on a treadmill test than a sedentary control group. This study describes a novel model of HLI which leads to more severe and sustained ischemia than the conventionally used model. Exercise therapy, which has established efficacy in PAD patients, was also effective in this new model. This new model maybe useful in the evaluation of potential novel PAD therapies.

Regulatory T Cells Promote Apelin-Mediated Sprouting Angiogenesis in Type 2 Diabetes

The role of CD4⁺ T cells in the ischemic tissues of T2D patients remains unclear. Here, we report that T2D patients' vascular density was negatively correlated with the number of infiltrating CD4⁺ T cells after ischemic injury. Th1 was the predominant subset, and Th1-derived IFN-γ and TNF-α directly impaired human angiogenesis. We then blocked CD4⁺ T cell infiltration into the ischemic tissues of both Lepr^db/db and diet-induced obese T2D mice. Genome-wide RNA sequencing shows an increased proliferative and angiogenic capability of diabetic ECs in ischemic tissues. Moreover, wire myography shows enhanced EC function and laser Doppler imaging reveals improved post-ischemic blood reperfusion. Mechanistically, functional revascularization after CD4 coreceptor blockade was mediated by Tregs. Genetic lineage tracing via Cdh5-CreER and Apln-CreER and coculture assays further illustrate that Tregs increased vascular density and induced de novo sprouting angiogenesis in a paracrine manner. Taken together, our results reveal that Th1 impaired while Tregs promoted functional post-ischemic revascularization in obesity and diabetes.

Cilostazol Mediates Immune Responses and Affects Angiogenesis During the Acute Phase of Hind Limb Ischemia in a Mouse Model

BACKGROUND

Cilostazol is a drug of choice for the treatment of intermittent claudication that also affects innate and adaptive immune cells. The purpose of our study was the evaluation of cilostazol's impact on the immune and angiogenic response in murine models of hind limb ischemia.

METHODS

We used 108 immunodeficient NOD.CB17-Prkdcscid/J mice and 108 wild-type CB17 mice. At day 0 (D0), all animals underwent hind limb ischemia. Half of them in both groups received daily cilostazol starting at D0 and for the next 7 postoperative days, while the rest of them served as controls, receiving vehicle. Interleukin (IL) 2, IL-4, IL-6, IL-10, IL-17A, tumor necrosis factor α (TNF-α), and interferon γ (IFN-γ) serum concentrations were measured by flow cytometry on postsurgery days D1, D3, D5, and D7. On D7, both groups underwent positron emission tomography scan with ⁶⁸Ga-RGD. Mice were euthanatized and gastrocnemius muscles were obtained for histological evaluation.

RESULTS

There was a statistically significant augmentation (P < .05) in IL-4, IL-10, IL-6, and IFN-γ concentrations in treated CB17 animals, while IL-2 was significantly suppressed. Significant difference was detected between the CiBisch and Bisch groups on D1 and D7 (P < .05) in CD31 staining. In treated NOD.CB17 animals, TNF-α, IL-6, and IFN-γ presented significant augmentation, while ⁶⁸Ga-NODAGA-RGDfK uptake and CD31 expression were found significantly lower for both legs in comparison to the control.

CONCLUSION

Cilostazol seems to significantly increase angiogenesis in wild-type animals during the first postoperational week. It also influences immune cells, altering the type of immune response by promoting anti-inflammatory cytokine production in wild-type animals, while it helps toward inflammation regression in immunodeficient animals.

Targeting endothelial thioredoxin-interacting protein (TXNIP) protects from metabolic disorder-related impairment of vascular function and post-ischemic revascularisation

INTRODUCTION

Although thioredoxin-interacting protein (TXNIP) is involved in a variety of biological functions, the contribution of endothelial TXNIP has not been well-defined in regards to endothelial and vascular function or in post-ischemic revascularisation. We postulated that inhibition of endothelial TXNIP with siRNA or in a Cre-LoxP system could be involved in protection from high fat, high protein, low carbohydrate (HFHPLC) diet-induced oxidative stress and endothelial dysfunction, leading to vascular damage and impaired revascularisation in vivo.

METHODS AND RESULTS

To investigate the role of endothelial TXNIP, the TXNIP gene was deleted in endothelial cells using anti-TXNIP siRNA treatment or the Cre-LoxP system. Murine models were fed a HFHPLC diet, known to induce metabolic disorders. Endothelial TXNIP targeting resulted in protection against metabolic disorder-related endothelial oxidative stress and endothelial dysfunction. This protective effect mitigates media cell loss induced by metabolic disorders and hampered metabolic disorder-related vascular dysfunction assessed by aortic reactivity and distensibility. In aortic ring cultures, metabolic disorders impaired vessel sprouting and this alteration was alleviated by deletion of endothelial TXNIP. When subjected to ischemia, mice fed a HFHPLC diet exhibited defective post-ischemic angiogenesis and impaired blood flow recovery in hind limb ischemia. However, reducing endothelial TXNIP rescued metabolic disorder-related impairment of ischemia-induced revascularisation.

CONCLUSION

Collectively, these results show that targeting endothelial TXNIP in metabolic disorders is essential to maintaining endothelial function, vascular function and improving ischemia-induced revascularisation, making TXNIP a potential therapeutic target for therapy of vascular complications related to metabolic disorders.

Chronic Hindlimb Ischemia Assessment; Quantitative Evaluation Using Laser Doppler in a Rodent Model of Surgically Induced Peripheral Arterial Occlusion

Therapeutic neoangiogenesis (TNA) holds promise as a treatment for peripheral arterial disease. Nevertheless, proper tools for in vivo pre-clinical investigation of different TNA approaches and their effects are still lacking. Here we describe a chronic ischemic hindlimb model in rats using laser Doppler quantitative evaluation of tissue perfusion. Male Wistar rats (n = 20), aged between 6–8 months, with an average weight of 287 ± 26.74 g, were used. Animals were divided into two experimental groups: group A (n = 17; hindlimb chronic ischemia model) and group B (n = 3; control). Hindlimb ischemia was induced by concomitant ligation of the right femoral and popliteal artery. Evaluation of tissue perfusion was quantified in perfusion units (PU) on a scale from 0 to 500 (500 PU = maximal detectable perfusion) by laser Doppler analysis at day 0, day 15 and day 30 after induction of ischemia. Induction of chronic ischemia in the rat hindlimb by concomitant ligation of the femoral and popliteal artery can be readily obtained but requires basic microsurgical skills. Laser Doppler analysis has shown unaltered ischemia levels throughout the study (129,17 PU ± 3.13 day 0 vs. 130,33 PU day 30 ± 3,27, p = not significant (n.s.)). We demonstrate a simple and reproducible model of chronic hindlimb ischemia in rats, with stable tissue perfusion levels that are accurately quantified using laser Doppler technology. Hence, this model can represent a valid tool for further studies involving therapeutic neoangiogenesis.

Lowering circulating apolipoprotein E levels improves aged bone fracture healing

Age is a well-established risk factor for impaired bone fracture healing. Here, we identify a role for apolipoprotein E (ApoE) in age-associated impairment of bone fracture healing and osteoblast differentiation, and we investigate the mechanism by which ApoE alters these processes. We identified that, in both humans and mice, circulating ApoE levels increase with age. We assessed bone healing in WT and ApoE-/- mice after performing tibial fracture surgery: bone deposition was higher within fracture calluses from ApoE-/- mice. In vitro recombinant ApoE (rApoE) treatment of differentiating osteoblasts decreased cellular differentiation and matrix mineralization. Moreover, this rApoE treatment decreased osteoblast glycolytic activity while increasing lipid uptake and fatty acid oxidation. Using parabiosis models, we determined that circulating ApoE plays a strong inhibitory role in bone repair. Using an adeno-associated virus-based siRNA system, we decreased circulating ApoE levels in 24-month-old mice and demonstrated that, as a result, fracture calluses from these aged mice displayed enhanced bone deposition and mechanical strength. Our results demonstrate that circulating ApoE as an aging factor inhibits bone fracture healing by altering osteoblast metabolism, thereby identifying ApoE as a new therapeutic target for improving bone repair in the elderly.

Forced expression of microRNA-146b reduces TRAF6-dependent inflammation and improves ischemia-induced neovascularization in hypercholesterolemic conditions

BACKGROUND AND AIMS

MicroRNA (miR)-146 is a key regulator of inflammation, endothelial activation and atherosclerosis. This study sought to define its potential role for the modulation of ischemia-induced neovascularization in atherosclerotic conditions.

METHODS

Next generation sequencing and qRT-PCR analyses were used to compare microRNA expression in the ischemic muscles of hypercholesterolemic ApoE-deficient (ApoE^-/-) mice vs. wild type mice, and in HUVECs exposed or not to oxLDL. Neovascularization was investigated in a mouse model of hindlimb ischemia and the functional activities of HUVECs and pro-angiogenic cells (PACs) were assessed in vitro.

RESULTS

We found that miR-146b (but not miR-146a) is significantly reduced in the ischemic muscles of ApoE^-/- mice, and in HUVECs exposed to oxLDL. Inhibition of miR-146b reduces angiogenesis in vitro, whereas forced expression of miR-146b rescues oxLDL-mediated impairment of endothelial cell proliferation and tube formation. Mechanistically, miR146b directly targets tumor necrosis factor-alpha (TNFa) Receptor Associated Factor 6 (TRAF6) to inhibit inflammation. We found that hypercholesterolemia and oxLDL exposure are associated with higher levels of TRAF6, and increased expression of TNFa. However, forced expression of miR-146b in high cholesterol conditions reduces the expression of these inflammatory factors. In vivo, intramuscular injection of miR-146b mimic reduces ischemic damages and restores blood flow recuperation and capillary density in the ischemic muscles of ApoE^-/- mice. Treatment with miR-146b also increases the number and functional activities of pro-angiogenic cells (PACs).

CONCLUSIONS

Hypercholesterolemia is associated with reduced expression of miR-146b, which increases TRAF6-dependent inflammation and is associated with poor neovascularization in response to ischemia. Forced expression of miR-146b using a miR mimic could constitute a novel therapeutic strategy to improve ischemia-induced neovascularization in atherosclerotic conditions.

Placental growth factor levels in quadriceps muscle are reduced by a Western diet in association with advanced glycation end products

In peripheral artery disease (PAD), atherosclerotic occlusion chronically impairs limb blood flow. Arteriogenesis (collateral artery remodeling) is a vital adaptive response to PAD that protects tissue from ischemia. People with type II diabetes have a high risk of developing PAD and would benefit from arteriogenesis. However, arteriogenesis is suppressed in people with diabetes by a multifaceted mechanism which remains incompletely defined. Upregulation of placental growth factor (PLGF) is a key early step in arteriogenesis. Therefore, we hypothesized that metabolic dysfunction would impair PLGF expression in skeletal muscle. We tested this hypothesis in C57BL/6J and ApoE^-/- mice of both sexes fed a Western diet (WD) for 24 wk. We first assessed baseline levels of PLGF, vascular endothelial growth factor (VEGF-A), and VEGF receptor 1 (VEGFR1) protein in hindlimb skeletal muscle. Only PLGF was consistently decreased by the WD. We next investigated the effect of 24 wk of the WD on the response of PLGF, VEGF-A, VEGFR1, and monocyte chemoattractant protein-1 (MCP-1) to the physiological stimulus of vascular occlusion. Hindlimb ischemia was induced in mice by gradual femoral artery occlusion using an ameroid constrictor. Growth factor levels were measured 3-28 days postsurgery. In C57BL/6J mice, the WD decreased and delayed upregulation of PLGF and abolished upregulation of VEGF-A and VEGFR1 but had no effect on MCP-1. In ApoE^-/- mice fed either diet, all factors tested failed to respond to occlusion. Metabolic phenotyping of mice and in vitro studies suggest that an advanced glycation end product/TNFα-mediated mechanism could contribute to the effects observed in vivo.NEW & NOTEWORTHY In this study, we tested the effect of a Western diet on expression of the arteriogenic growth factor placental growth factor (PLGF) in mouse skeletal muscle. We provide the first demonstration that a Western diet interferes with both baseline expression and hindlimb ischemia-induced upregulation of PLGF. We further identify a potential role for advanced glycation end product/TNFα signaling as a negative regulator of PLGF. These studies provide insight into one possible mechanism by which type II diabetes may limit collateral growth.

Treating ischemia via recruitment of antigen-specific T cells

Ischemic diseases are a leading cause of mortality and can result in autoamputation of lower limbs. We explored the hypothesis that implantation of an antigen-releasing scaffold, in animals previously vaccinated with the same antigen, can concentrate T_H2 T cells and enhance vascularization of ischemic tissue. This approach may be clinically relevant, as all persons receiving childhood vaccines recommended by the Centers for Disease Control and Prevention have vaccines that contain aluminum, a T_H2 adjuvant. To test the hypothesis, mice with hindlimb ischemia, previously vaccinated with ovalbumin (OVA) and aluminum, received OVA-releasing scaffolds. Vaccinated mice receiving OVA-releasing scaffolds locally concentrated antigen-specific T_H2 T cells in the surrounding ischemic tissue. This resulted in local angiogenesis, increased perfusion in ischemic limbs, and reduced necrosis and enhanced regenerating myofibers in the muscle. These findings support the premise that antigen depots may provide a treatment for ischemic diseases in patients previously vaccinated with aluminum-containing adjuvants.

Nitric Oxide and Hydrogen Sulfide Regulation of Ischemic Vascular Growth and Remodeling

Ischemic vascular remodeling occurs in response to stenosis or arterial occlusion leading to a change in blood flow and tissue perfusion. Altered blood flow elicits a cascade of molecular and cellular physiological responses leading to vascular remodeling of the macro- and micro-circulation. Although cellular mechanisms of vascular remodeling such as arteriogenesis and angiogenesis have been studied, therapeutic approaches in these areas have had limited success due to the complexity and heterogeneous constellation of molecular signaling events regulating these processes. Understanding central molecular players of vascular remodeling should lead to a deeper understanding of this response and aid in the development of novel therapeutic strategies. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are gaseous signaling molecules that are critically involved in regulating fundamental biochemical and molecular responses necessary for vascular growth and remodeling. This review examines how NO and H₂ S regulate pathophysiological mechanisms of angiogenesis and arteriogenesis, along with important chemical and experimental considerations revealed thus far. The importance of NO and H₂ S bioavailability, their synthesis enzymes and cofactors, and genetic variations associated with cardiovascular risk factors suggest that they serve as pivotal regulators of vascular remodeling responses. © 2019 American Physiological Society. Compr Physiol 9:1213-1247, 2019.

Review of the Current Basic Science Strategies to Treat Critical Limb Ischemia

Critical limb ischemia (CLI) is a highly morbid disease with many patients considered poor surgical candidates. The lack of treatment options for CLI has driven interest in developing molecular therapies within recent years. Through these translational medicine studies in CLI, much has been learned about the pathophysiology of the disease. Here, we present an overview of the macrovascular and microvascular changes that lead to the development of CLI, including impairment of angiogenesis, vasculogenesis, and arteriogenesis. We summarize the randomized clinical controlled trials that have used molecular therapies in CLI, and discuss the novel imaging modalities being developed to assess the efficacy of these therapies.

Time-dependent LXR/RXR pathway modulation characterizes capillary remodeling in inflammatory corneal neovascularization

Inflammation in the normally immune-privileged cornea can initiate a pathologic angiogenic response causing vision-threatening corneal neovascularization. Inflammatory pathways, however, are numerous, complex and are activated in a time-dependent manner. Effective resolution of inflammation and associated angiogenesis in the cornea requires knowledge of these pathways and their time dependence, which has, to date, remained largely unexplored. Here, using a model of endogenous resolution of inflammation-induced corneal angiogenesis, we investigate the time dependence of inflammatory genes in effecting capillary regression and the return of corneal transparency. Endogenous capillary regression was characterized by a progressive thinning and remodeling of angiogenic capillaries and inflammatory cell retreat in vivo in the rat cornea. By whole-genome longitudinal microarray analysis, early suppression of VEGF ligand-receptor signaling and inflammatory pathways preceded an unexpected later-phase preferential activation of LXR/RXR, PPARα/RXRα and STAT3 canonical pathways, with a concurrent attenuation of LPS/IL-1 inhibition of RXR function and Wnt/β-catenin signaling pathways. Potent downstream inflammatory cytokines such as Cxcl5, IL-1β, IL-6 and Ccl2 were concomitantly downregulated during the remodeling phase. Upstream regulators of the inflammatory pathways included Socs3, Sparc and ApoE. A complex and coordinated time-dependent interplay between pro- and anti-inflammatory signaling pathways highlights a potential anti-inflammatory role of LXR/RXR, PPARα/RXRα and STAT3 signaling pathways in resolving inflammatory corneal angiogenesis.

Temporal remodeling of pial collaterals and functional deficits in a murine model of ischemic stroke

BACKGROUND

Leptomeningeal anastomoses play a critical role in regulating reperfusion following cerebrovascular obstruction; however, methods to evaluate their temporospatial remodeling remains under investigation.

NEW METHOD

We combined arteriole-specific vessel painting with histological evaluation to assess the density and diameter of inter-collateral vessels between the middle cerebral artery and anterior cerebral artery (MCA-ACA) or posterior cerebral artery (MCA-PCA) in a murine model of permanent middle cerebral artery occlusion (pMCAO).

RESULTS

While the overall density was not influenced by pMCAO, the size of MCA-ACA and MCA-PCA vessels had significantly increased 2days post-pMCAO and peaked by 4days compared to the un-injured hemisphere. Using a combination of vessel painting and immunofluorescence, we uniquely observed an induction of cellular division and a remodeling of the smooth muscle cells within the collateral niche following post-pMCAO on whole mount tissue sections. Vessel painting was also applied to pMCAO-injured Cx3cr1GFP mice, in order to identify the spatial relationship between Cx3cr1-positive peripheral-derived monocyte/macrophages and the vessel painted collaterals. Our histological findings were supplemented with analysis of cerebral blood flow using laser Doppler imaging and behavioral changes following pMCAO.

COMPARISON WITH EXISTING METHODS

Compared to polyurethane and latex methods for collateral labeling, this new method provides detailed cell-type specific analysis within the collateral niche at the microscopic level, which has previously been unavailable.

CONCLUSIONS

This simple and reproducible combination of techniques is the first to dissect the temporospatial remodeling of pial collateral arterioles. The method will advance investigations into the underlying mechanisms governing the intricate processes of arteriogenesis.

A novel targeted angiogenesis technique using VEGF conjugated magnetic nanoparticles and in-vitro endothelial barrier crossing

Background

Vascular endothelial growth factor is well known for its angiogenesis potential. The study was performed to determine the possible pro-angiogenic role of magnetic nanoparticles coupled to VEGF in vitro and their capacity to cross an endothelial monolayer. This novel treatment technique for angiogenesis could be potentially useful for therapeutic purposes using magnetic nanoparticles.

Methods

Magnetic nanoparticles (MN) were synthesized and were conjugated with the vascular endothelial growth factor. The particles were tested in vitro in a 2D to 3D culture system. MN was seeded in different positions in relation to an HUVEC spheroid to assess a preferential migration.

To evaluate the MN capacity to cross the endothelial barrier, a confluent monolayer of HUVEC cells was seeded on top of a collagen gel. MN was placed in dissolution on the cell culture media, and the MN position was determined by confocal microscopy for 24 h.

Results

HUVEC spheroids were able to generate a preferential sprouting depending on the MN position. Meanwhile, there was random migration when the MN’s were placed all over the collagen gel and no sprouting when no MN was added. The trans-endothelial migration capacity of the MN was observed after 20 h in culture in the absence of external stimuli.

Conclusion

Here we show in vitro angiogenesis following the distribution of the MN conjugated with growth factors. These nanoparticles could be controlled with a magnet to place them in the ischemic area of interest and speed up vascular recovery. Also, MN has potentials to cross endothelium, opening the doors to a possible intravascular and extravascular treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12872-017-0643-x) contains supplementary material, which is available to authorized users.

Redox regulation of ischemic limb neovascularization - What we have learned from animal studies

Mouse hindlimb ischemia has been widely used as a model to study peripheral artery disease. Genetic modulation of the enzymatic source of oxidants or components of the antioxidant system reveal that physiological levels of oxidants are essential to promote the process of arteriogenesis and angiogenesis after femoral artery occlusion, although mice with diabetes or atherosclerosis may have higher deleterious levels of oxidants. Therefore, fine control of oxidants is required to stimulate vascularization in the limb muscle. Oxidants transduce cellular signaling through oxidative modifications of redox sensitive cysteine thiols. Of particular importance, the reversible modification with abundant glutathione, called S-glutathionylation (or GSH adducts), is relatively stable and alters protein function including signaling, transcription, and cytoskeletal arrangement. Glutaredoxin-1 (Glrx) is an enzyme which catalyzes reversal of GSH adducts, and does not scavenge oxidants itself. Glrx may control redox signaling under fluctuation of oxidants levels. In ischemic muscle increased GSH adducts through Glrx deletion improves in vivo limb revascularization, indicating endogenous Glrx has anti-angiogenic roles. In accordance, Glrx overexpression attenuates VEGF signaling in vitro and ischemic vascularization in vivo. There are several Glrx targets including HIF-1α which may contribute to inhibition of vascularization by reducing GSH adducts. These animal studies provide a caution that excess antioxidants may be counter-productive for treatment of ischemic limbs, and highlights Glrx as a potential therapeutic target to improve ischemic limb vascularization.

Monocytes/Macrophages Mobilization Orchestrate Neovascularization after Localized Colorectal Irradiation

In patients undergoing radiotherapy for cancer, radiation dose to healthy tissue can occur, causing microvascular damage. Monocytes that have been shown to promote tissue revascularization comprise the subsets: CD11b⁺Ly6G^-7/4^hi/monocytes^hi and CD11b⁺Ly6G^-7/4^lo/monocytes^lo. We hypothesized that monocytes were implicated in postirradiation blood vessel formation. C57Bl6 mice underwent localized colorectal irradiation and were sacrificed at different times after exposure. Bone marrow, spleen, blood and colon were collected. Fourteen days postirradiation, colons expressed proangiogenic actors and adhesion molecules. Monocytes^hi, which were the main subset of infiltrating monocytes, mobilized to the blood from spleen and bone marrow, peaking at day 14 postirradiation, and were associated with lymphocyte Th1 polarization. At day 28 postirradiation, angiographic score and capillary density increased by ∼1.8-fold, and then returned to nonirradiated levels at day 60. Clodronate-mediated depletion of circulating monocytes prior to irradiation resulted in a ∼1.4-fold decrease in angiographic score and capillary density compared to the nontreated control. Histological analysis of the colon in clodronate-treated mice revealed a massive decrease of macrophage and lymphocyte infiltration as well as reduced collagen deposition in crypt area at day 21. However, late depletion of monocytes from day 25 postirradiation had no effect on fibrotic process. These findings demonstrate a central role for monocyte/macrophage activation in the orchestration of a neovascularization mechanism after localized colorectal irradiation.

Human Placenta-Derived Mesenchymal Stromal-Like Cells Enhance Angiogenesis via T Cell-Dependent Reprogramming of Macrophage Differentiation

Peripheral arterial disease (PAD) is a leading cause of limb loss and mortality worldwide with limited treatment options. Mesenchymal stromal cell (MSC) therapy has demonstrated positive effects on angiogenesis in preclinical models and promising therapeutic efficacy signals in early stage clinical studies; however, the mechanisms underlying MSC-mediated angiogenesis remain largely undefined. Here, we investigated the mechanism of action of human placenta-derived MSC-like cells (PDA-002) in inducing angiogenesis using mice hind limb ischemia model. We showed that PDA-002 improved blood flow and promoted collateral vessel formation in the injured limb. Histological analysis demonstrated that PDA-002 increased M2-like macrophages in ischemic tissue. Analysis of the changes in functional T cell phenotype in the draining lymph nodes revealed that PDA-002 treatment was associated with the induction of cytokine and gene expression signatures of Th2 response. Angiogenic effect of PDA-002 was markedly reduced in Balb/c nude mice compared with wild type. This reduction in efficacy was reversed by T cell reconstitution, suggesting T cells are essential for PDA-002-mediated angiogenesis. Furthermore, effect of PDA-002 on macrophage differentiation was also T cell-dependent as a PDA-002-mediated M2-like macrophage skewing was only observed in wild type and T cell reconstituted nude mice, but not in nude mice. Finally, we showed that PDA-002-treated animals had enhanced angiogenic recovery in response to the second injury when PDA-002 no longer persisted in vivo. These results suggest that PDA-002 enhances angiogenesis through an immunomodulatory mechanism involving T cell-dependent reprogramming of macrophage differentiation toward M2-like phenotype. Stem Cells 2017;35:1603-1613.

Angiogenesis in the atherosclerotic plaque

Atherosclerosis is a multifocal alteration of the vascular wall of medium and large arteries characterized by a local accumulation of cholesterol and non-resolving inflammation. Atherothrombotic complications are the leading cause of disability and mortality in western countries. Neovascularization in atherosclerotic lesions plays a major role in plaque growth and instability. The angiogenic process is mediated by classical angiogenic factors and by additional factors specific to atherosclerotic angiogenesis. In addition to its role in plaque progression, neovascularization may take part in plaque destabilization and thromboembolic events. Anti-angiogenic agents are effective to reduce atherosclerosis progression in various animal models. However, clinical trials with anti-angiogenic drugs, mainly anti-VEGF/VEGFR, used in anti-cancer therapy show cardiovascular adverse effects, and require additional investigations.

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.

ADMA regulates angiogenesis: genetic and metabolic evidence

Endothelium-derived nitric oxide (NO) plays an important role in transducing the effects of angiogenic factors. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase (NOS). We used a murine model of hindlimb ischemia to investigate whether genetic or metabolic changes in ADMA levels could impair angiogenic response in vivo. Hindlimb ischemia was surgically induced in C57BL/6J mice, apo E-deficient mice, or transgenic mice overexpressing dimethylarginine dimethylaminohydrolase (DDAH). Some animals were also treated with the NOS antagonist L-nitro-arginine, or the NO precursor L-arginine. Angiogenesis was quantified in the hindlimb skeletal muscle by capillary/myocyte ratio. Plasma or tissue ADMA levels were measured by HPLC. In normal mice, hindlimb ischemia increased tissue ADMA twofold, and reduced DDAH and NOS expression. This was associated with a reduced NOS activity (by over 80%) three days following surgery. On day seven, a threefold increase in DDAH expression and a fall in tissue ADMA levels were associated with a sevenfold increase in NOS activity, whereas NOS expression did not increase above baseline. In DDAH transgenic mice, the elevation of ADMA and decrement in NOS activity was blunted during hindlimb ischemia. Plasma ADMA levels were increased in apo E-mice (1.79 ± 0.45 versus 1.07 ± 0.08 μmol/l; p = 0.008). Capillary index was significantly reduced in apo E-mice up to seven weeks after surgery (0.25 ± 0.05 versus 0.62 ± 0.08; p < 0.001). The effect of hypercholesterolemia on capillary index was reversed by L-arginine, and (in wild-type mice) mimicked by administration of the NOS antagonist L-nitro-arginine. In conclusion, metabolic or genetic changes in plasma and tissue ADMA levels affect tissue NO production and angiogenic response to ischemia.

Transplanted Endothelial Progenitor Cells Improve Ischemia Muscle Regeneration in Mice by Diffusion Tensor MR Imaging

Endothelial progenitor cells (EPCs) play an important role in repairing ischemia tissues. Diffusion tensor imaging (DTI) was applied to detect the architectural organization of skeletal muscle. This study investigated the feasibility and accuracy of using the DTI to evaluate effectiveness of EPCs treatment. Mouse bone marrow-derived EPCs were isolated, cultured, characterized, and transplanted to hindlimb ischemia mice model. DTI was performed on the hindlimb at postischemia time points. The edema regions of diffusion restriction (high signal in diffusion weighted imaging) were decreased in the ischemic muscle of EPCs treated mice after 14 days compared with the controls. These results from DTI show the lower apparent diffusion coefficient and eigenvalues (λ1, λ2, and λ3) and the higher fractional anisotropy and fiber counts of ischemic muscle on 7 and 14 days after EPCs treatment compared to the controls. There was a significant correlation between fiber counts calculated by DTI and survival fibers evaluated by histological section (r = 0.873, P < 0.01). Our study demonstrated that the time frame for muscle fiber regeneration after EPCs transplantation was significantly shortened in vivo. DTI could be a useful tool for noninvasive evaluation of muscle tissue damage and repair in animal models and patient with ischemic diseases.

Mutually Supportive Mechanisms of Inflammation and Vascular Remodeling

Chronic inflammation is often accompanied by angiogenesis, the development of new blood vessels from existing ones. This vascular response is a response to chronic hypoxia and/or ischemia, but is also contributory to the progression of disorders including atherosclerosis, arthritis, and tumor growth. Proinflammatory and proangiogenic mediators and signaling pathways form a complex and interrelated network in these conditions, and many factors exert multiple effects. Inflammation drives angiogenesis by direct and indirect mechanisms, promoting endothelial proliferation, migration, and vessel sprouting, but also by mediating extracellular matrix remodeling and release of sequestered growth factors, and recruitment of proangiogenic leukocyte subsets. The role of inflammation in promoting angiogenesis is well documented, but by facilitating greater infiltration of leukocytes and plasma proteins into inflamed tissues, angiogenesis can also propagate chronic inflammation. This review examines the mutually supportive relationship between angiogenesis and inflammation, and considers how these interactions might be exploited to promote resolution of chronic inflammatory or angiogenic disorders.

Methods for Acute and Subacute Murine Hindlimb Ischemia

Peripheral artery disease (PAD) is a leading cause of cardiovascular morbidity and mortality in developed countries, and animal models that reliably reproduce the human disease are necessary to develop new therapies for this disease. The mouse hindlimb ischemia model has been widely used for this purpose, but the standard practice of inducing acute limb ischemia by ligation of the femoral artery can result in substantial tissue necrosis, compromising investigators' ability to study the vascular and skeletal muscle tissue responses to ischemia. An alternative approach to femoral artery ligation is the induction of gradual femoral artery occlusion through the use of ameroid constrictors. When placed around the femoral artery in the same or different locations as the sites of femoral artery ligation, these devices occlude the artery over 1 - 3 days, resulting in more gradual, subacute ischemia. This results in less substantial skeletal muscle tissue necrosis, which may more closely mimic the responses seen in human PAD. Because genetic background influences outcomes in both the acute and subacute ischemia models, consideration of the mouse strain being studied is important in choosing the best model. This paper describes the proper procedure and anatomical placement of ligatures or ameroid constrictors on the mouse femoral artery to induce subacute or acute hindlimb ischemia in the mouse.

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.

Injury-Mediated Vascular Regeneration Requires Endothelial ER71/ETV2

OBJECTIVE

Comprehensive understanding of the mechanisms regulating angiogenesis might provide new strategies for angiogenic therapies for treating diverse physiological and pathological ischemic conditions. The E-twenty six (ETS) factor Ets variant 2 (ETV2; aka Ets-related protein 71) is essential for the formation of hematopoietic and vascular systems. Despite its indispensable function in vessel development, ETV2 role in adult angiogenesis has not yet been addressed. We have therefore investigated the role of ETV2 in vascular regeneration.

APPROACH AND RESULTS

We used endothelial Etv2 conditional knockout mice and ischemic injury models to assess the role of ETV2 in vascular regeneration. Although Etv2 expression was not detectable under steady-state conditions, its expression was readily observed in endothelial cells after injury. Mice lacking endothelial Etv2 displayed impaired neovascularization in response to eye injury, wounding, or hindlimb ischemic injury. Lentiviral Etv2 expression in ischemic hindlimbs led to improved recovery of blood perfusion with enhanced vessel formation. After injury, fetal liver kinase 1 (Flk1), aka VEGFR2, expression and neovascularization were significantly upregulated by Etv2, whereas Flk1 expression and vascular endothelial growth factor response were significantly blunted in Etv2-deficient endothelial cells. Conversely, enforced Etv2 expression enhanced vascular endothelial growth factor-mediated endothelial sprouting from embryoid bodies. Lentiviral Flk1 expression rescued angiogenesis defects in endothelial Etv2 conditional knockout mice after hindlimb ischemic injury. Furthermore, Etv2(+/-); Flk1(+/-) double heterozygous mice displayed a more severe hindlimb ischemic injury response compared with Etv2(+/-) or Flk1(+/-) heterozygous mice, revealing an epistatic interaction between ETV2 and FLK1 in vascular regeneration.

CONCLUSIONS

Our study demonstrates a novel obligatory role for the ETV2 in postnatal vascular repair and regeneration.

A systematic approach to assess locoregional differences in angiogenesis

Skeletal muscle tissue differs with regard to the abundance of glycolytic and oxidative fiber types. In this context, capillary density has been described to be higher in muscle tissue with more oxidative metabolism as compared to that one with more glycolytic metabolism, and the highest abundance of capillaries has been found in boneward-oriented moieties of skeletal muscle tissue. Importantly, capillary formation is often analyzed as a measure for angiogenesis, a process that describes neo-vessel formation emanating from preexisting vessels, occurring, i.e., after arterial occlusion. However, a standardized way for investigation of calf muscle capillarization after surgically induced unilateral hind limb ischemia in mice, especially considering these locoregional differences, has not been provided so far. In this manuscript, a novel, methodical approach for reliable analysis of capillary density was established using anatomic-morphological reference points, and a software-assisted way of capillary density analysis is described. Thus, the systematic approach provided conscientiously considers intra-layer differences in capillary formation and therefore guarantees for a robust, standardized analysis of capillary density as a measure for angiogenesis. The significance of the methodology is further supported by the observation that capillary density in the calf muscle layers analyzed negatively correlates with distal lower limb perfusion measured in vivo.

Comparative Analysis of Human Mesenchymal Stem Cells Derived From Bone Marrow, Placenta, and Adipose Tissue as Sources of Cell Therapy

Various source-derived mesenchymal stem cells (MSCs) with multipotent capabilities were considered for cell therapeutics of incurable diseases. The applicability of MSCs depends on the cellular source and on their different in vivo functions, despite having similar phenotypic and cytological characteristics. We characterized MSCs from different sources, including human bone marrow (BM), placenta (PL), and adipose tissue (AT), in terms of the phenotype, surface antigen expression, differentiation ability, proteome reference map, and blood flow recovery in a hindlimb ischemic disease model. The MSCs exhibit different differentiation potentials depending on the cellular source despite having similar phenotypic and surface antigen expression. We identified approximately 90 differentially regulated proteins. Most up- or down-regulated proteins show cytoskeletal or oxidative stress, peroxiredoxin, and apoptosis roles according to their functional involvement. In addition, the PL-MSCs retained a higher therapeutic efficacy than the BM- and AT-MSCs in the hindlimb ischemic disease model. In summary, we examined differentially expressed key regulatory factors for MSCs that were obtained from several cellular sources and demonstrated their differentially expressed proteome profiles. Our results indicate that primitive PL-MSCs have biological advantages relative to those from other sources, making PL-MSCs a useful model for clinical applications of cell therapy.

Horizontal gene transfer from macrophages to ischemic muscles upon delivery of naked DNA with Pluronic block copolymers

Intramuscular administration of plasmid DNA (pDNA) with non-ionic Pluronic block copolymers increases gene expression in injected muscles and lymphoid organs. We studied the role of immune cells in muscle transfection upon inflammation. Local inflammation in murine hind limb ischemia model (MHLIM) drastically increased DNA, RNA and expressed protein levels in ischemic muscles injected with pDNA/Pluronic. The systemic inflammation (MHLIM or peritonitis) also increased expression of pDNA/Pluronic in the muscles. When pDNA/Pluronic was injected in ischemic muscles the reporter gene, Green Fluorescent Protein (GFP) co-localized with desmin(+) muscle fibers and CD11b(+) macrophages (MØs), suggesting transfection of MØs along with the muscle cells. P85 enhanced (∼ 4 orders) transfection of MØs with pDNA in vitro. Moreover, adoptively transferred MØs were shown to pass the transgene to inflamed muscle cells in MHLIM. Using a co-culture of myotubes (MTs) and transfected MØs expressing a reporter gene under constitutive (cmv-luciferase) or muscle specific (desmin-luciferase) promoter we demonstrated that P85 enhances horizontal gene transfer from MØ to MTs. Therefore, MØs can play an important role in muscle transfection with pDNA/Pluronic during inflammation, with both inflammation and Pluronic contributing to the increased gene expression. pDNA/Pluronic has potential for therapeutic gene delivery in muscle pathologies that involve inflammation.

Subacute limb ischemia induces skeletal muscle injury in genetically susceptible mice independent of vascular density

OBJECTIVE

The primary preclinical model of peripheral artery disease, which involves acute limb ischemia (ALI), can result in appreciable muscle injury that is attributed to the acuity of the ischemic injury. A less acute model of murine limb ischemia using ameroid constrictors (ACs) has been developed in an attempt to mimic the chronic nature of human disease. However, there is currently little understanding of how genetics influence muscle injury following subacute arterial occlusion in the mouse.

METHODS

We investigated the influence of mouse genetics on skeletal muscle tissue survival, blood flow, and vascular density by subjecting two different mouse strains, C57BL/6 (BL6) and BALB/c, to ALI or subacute limb ischemia using single (1AC) or double (2AC) AC placement on the femoral artery.

RESULTS

Similar to ALI, the 2AC model resulted in significant tissue necrosis and limb perfusion deficits in genetically susceptible BALB/c but not BL6 mice. In the 1AC model, no outward evidence of tissue necrosis was observed, and there were no differences in limb blood flow between BL6 and BALB/c. However, BALB/c mice displayed significantly greater muscle injury, as evidenced by increased inflammation and myofiber atrophy, despite having no differences in CD31(+) and SMA(+) vascular density and area. BALB/c mice also displayed significantly greater centralized myonuclei, indicating increased muscle regeneration.

CONCLUSIONS

The susceptibility of skeletal muscle to ischemia-induced injury is at least partly independent of muscle blood flow and vascular density, consistent with a muscle cell autonomous response that is genetically determined. Further development of preclinical models of peripheral artery disease that more accurately reflect the nature of the human disease may allow more accurate identification of genetic targets for therapeutic intervention.

The future of collateral artery research

In the event of obstructive coronary artery disease, collateral arteries have been deemed an alternative blood source to preserve myocardial tissue perfusion and function. Monocytes play an important role in modulating this process, by local secretion of growth factors and extracellular matrix degrading enzymes. Extensive efforts have focused on developing compounds for augmenting the growth of collateral vessels (arteriogenesis). Nonetheless, clinical trials investigating the therapeutic potential of these compounds resulted in disappointing outcomes. Previous studies focused on developing compounds that stimulated collateral vessel growth by enhancing monocyte survival and activity. The limited success of these compounds in clinical studies, led to a paradigm shift in arteriogenesis research. Recent studies have shown genetic heterogeneity between CAD patients with sufficient and insufficient collateral vessels. The genetic predispositions in patients with poorly developed collateral vessels include overexpression of arteriogenesis inhibiting signaling pathways. New directions of arteriogenesis research focus on attempting to block such inhibitory pathways to ultimately promote arteriogenesis. Methods to detect collateral vessel growth are also critical in realizing the therapeutic potential of newly developed compounds. Traditional invasive measurements of intracoronary derived collateral flow index remain the gold standard in quantifying functional capacity of collateral vessels. However, advancements made in hybrid diagnostic imaging modalities will also prove to be advantageous in detecting the effects of pro-arteriogenic compounds.