Coronary collateral growth--back to the future

Expanding landscape of coronary microvascular disease in co-morbid conditions: Metabolic disease and beyond

Coronary microvascular disease (CMD) and impaired coronary blood flow control are defects that occur early in the pathogenesis of heart failure in cardiometabolic conditions, prior to the onset of atherosclerosis. In fact, recent studies have shown that CMD is an independent predictor of cardiac morbidity and mortality in patients with obesity and metabolic disease. CMD is comprised of functional, structural, and mechanical impairments that synergize and ultimately reduce coronary blood flow in metabolic disease and in other co-morbid conditions, including transplant, autoimmune disorders, chemotherapy-induced cardiotoxicity, and remote injury-induced CMD. This review summarizes the contemporary state-of-the-field related to CMD in metabolic and these other co-morbid conditions based on mechanistic data derived mostly from preclinical small- and large-animal models in light of available clinical evidence and given the limitations of studying these mechanisms in humans. In addition, we also discuss gaps in current understanding, emerging areas of interest, and opportunities for future investigations in this field.

Resolvin D2 and its receptor GPR18 in cardiovascular and metabolic diseases: A promising biomarker and therapeutic target

Accumulating evidence suggests that inflammation plays an important role in the pathophysiology of the initiation and progression of cardiovascular and metabolic diseases (CVMDs). Anti-inflammation strategies and those that promote inflammation resolution have gradually become potential therapeutic approaches for CVMDs. Resolvin D2 (RvD2), a specialized pro-resolving mediator, exerts anti-inflammatory and pro-resolution effects through its receptor GPR18, a G protein-coupled receptor. Recently, the RvD2/GPR18 axis has received more attention due to its protective role in CVMDs, including atherosclerosis, hypertension, ischaemiareperfusion, and diabetes. Here, we introduce basic information about RvD2 and GPR18, summarize their roles in different immune cells, and review the therapeutic potential of the RvD2/GPR18 axis in CVMDs. In summary, RvD2 and its receptor GPR18 play an important role in the occurrence and development of CVMDs and are potential biomarkers and therapeutic targets.

The Roles of Bone Marrow-Derived Stem Cells in Coronary Collateral Growth Induced by Repetitive Ischemia

Many clinical trials have attempted to use stem cells to treat ischemic heart diseases (IHD), but the benefits have been modest. Though coronary collaterals can be a "natural bypass" for IHD patients, the regulation of coronary collateral growth (CCG) and the role of endogenous stem cells in CCG are not fully understood. In this study, we used a bone marrow transplantation scheme to study the role of bone marrow stem cells (BMSCs) in a rat model of CCG. Transgenic GFP rats were used to trace BMSCs after transplantation; GFP bone marrow was harvested or sorted for bone marrow transplantation. After recovering from transplantation, the recipient rats underwent 10 days of repetitive ischemia (RI), with echocardiography before and after RI, to measure cardiac function and myocardial blood flow. At the end of RI, the rats were sacrificed for the collection of bone marrow for flow cytometry or heart tissue for imaging analysis. Our study shows that upon RI stimulation, BMSCs homed to the recipient rat hearts' collateral-dependent zone (CZ), proliferated, differentiated into endothelial cells, and engrafted in the vascular wall for collateral growth. These RI-induced collaterals improved coronary blood flow and cardiac function in the recipients' hearts during ischemia. Depletion of donor CD34+ BMSCs led to impaired CCG in the recipient rats, indicating that this cell population is essential to the process. Overall, these results show that BMSCs contribute to CCG and suggest that regulation of the function of BMSCs to promote CCG might be a potential therapeutic approach for IHD.

Case Series: Extracorporeal Shockwave Myocardial Revascularization Therapy Improves Ischemic Response, Functional Capacity, and Quality of Life in Indicated CABG-Stable Angina Pectoris Patients

Introduction

Extracorporeal shockwave myocardial revascularization (ESMR) is included in the guidelines only for patients with refractory angina pectoris having no option for invasive revascularization. We intend to report a case series with ESMR therapy is indicated patients with coronary artery bypass grafting-stable angina pectoris (CABG-SAP) who refuse the surgery, irrespective of angina symptoms.

Methods

We review medical records of patients with SAP admitted to ESMR therapy in Dr. Hasan Sadikin General Hospital, Bandung, Indonesia from January 2018 to December 2019. Recorded variables at baseline and after therapy extracted, namely, (1) ischemic response, double product, and (2) functional capacity measured as metabolic equivalent (MET) using treadmill test; (3) six-minute walking test distance achieved; and (4) quality of life using SF-36 Questionnaire.

Results

A total of four indicated patients with CABG-SAP from 50 to 75 years old were included in this study. At baseline, one patient is CCS class I and two patients are CCS class II with SDS ranging from 3 to 17. Ischemic response improved in all the patients. The double product improved in patient 1 9,600–14,872 mm Hg × bpm, patient 2 9,460–10,640 mm Hg × bpm, and patient 4 17,220–20,480 mm Hg × bpm. The functional capacity improved in Patient 1 8.07–8.91 METs, patient 2 1.91–4.01 METs, patient 3 3.45–6.39 METs, and patient 4 3.9–4.43 METs. The 6-min walking distance improved in patient 1 540–570 m and patient 2 345–405 m. The CCS class, bodily pain, and general health domain scores improved in all patients.

Conclusion

ESMR therapy might be beneficial for indicated patients with CABG-SAP to improve ischemic response, functional capacity, and physical component of quality of life.

The essential role for endothelial cell sprouting in coronary collateral growth

RATIONALE

Coronary collateral growth is a natural bypass for ischemic heart diseases. It offers tremendous therapeutic benefit, but the process of coronary collateral growth isincompletely understood due to limited preclinical murine models that would enable interrogation of its mechanisms and processes via genetic modification and lineage tracing. Understanding the processes by which coronary collaterals develop can unlock new therapeutic strategies for ischemic heart disease.

OBJECTIVE

To develop a murine model of coronary collateral growth by repetitive ischemia and investigate whether capillary endothelial cells could contribute to the coronary collateral formation in an adult mouse heart after repetitive ischemia by lineage tracing.

METHODS AND RESULTS

A murine model of coronary collateral growth was developed using short episodes of repetitive ischemia. Repetitive ischemia stimulation resulted in robust collateral growth in adult mouse hearts, validated by high-resolution micro-computed tomography. Repetitive ischemia-induced collateral formation compensated ischemia caused by occlusion of the left anterior descending artery. Cardiac function improved during ischemia after repetitive ischemia, suggesting the improvement of coronary blood flow. A capillary-specific Cre driver (Apln-CreER) was used for lineage tracing capillary endothelial cells. ROSA mT/mG reporter mice crossed with the Apln-CreER transgene mice underwent a 17 days' repetitive ischemia protocol for coronary collateral growth. Two-photon and confocal microscopy imaging of heart slices revealed repetitive ischemia-induced coronary collateral growth initiated from sprouting Apelin⁺ endothelial cells. Newly formed capillaries in the collateral-dependent zone expanded in diameter upon repetitive ischemia stimulation and arterialized with smooth muscle cell recruitment, forming mature coronary arteries. Notably, pre-existing coronary arteries and arterioles were not Apelin⁺, and all Apelin⁺ collaterals arose from sprouting capillaries. Cxcr4, Vegfr2, Jag1, Mcp1, and Hif1⍺ mRNA levels in the repetitive ischemia-induced hearts were also upregulated at the early stage of coronary collateral growth, suggesting angiogenic signaling pathways are activated for coronary collaterals formation during repetitive ischemia.

CONCLUSIONS

We developed a murine model of coronary collateral growth induced by repetitive ischemia. Our lineage tracing study shows that sprouting endothelial cells contribute to coronary collateral growth in adult mouse hearts. For the first time, sprouting angiogenesis is shown to give rise to mature coronary arteries in response to repetitive ischemia in the adult mouse hearts.

A simple coronary blood flow model to study the collateral flow index

In this work, we present a novel modeling framework to investigate the effects of collateral circulation into the coronary blood flow physiology. A prototypical model of the coronary tree, integrated with the concept of Collateral Flow Index (CFI), is employed to gain insight about the role of model parameters associated with the collateral circuitry, which results in physically-realizable solutions for specific CFI data. Then, we discuss the mathematical feasibility of pressure-derived CFI, anatomical implications and practical considerations involving the estimation of model parameters in collateral connections. A sensitivity analysis is carried out, and the investigation of the impact of the collateral circulation on FFR values is also addressed.

Coronary remodeling and biomechanics: Are we going with the flow in 2020?

Under normal conditions, coronary blood flow (CBF) provides critical blood supply to the myocardium so that it can appropriately meet the metabolic demands of the body. Dogmatically, there exist several known regulators and modulators of CBF that include local metabolites and neurohormonal factors that can influence the function of the coronary circulation. In disease states such as diabetes and myocardial ischemia, these regulators are impaired or shifted such that CBF is reduced. Although functional considerations have been and continued to be well studied, more recent evidence builds upon established studies that collectively suggest that the relative roles of coronary structure, biomechanics, and the influence of cardiac biomechanics via extravascular compression may also play a significant role in dictating CBF. In this mini review, we discuss these regulators of CBF under normal and pathophysiological conditions and their potential influence on the control of CBF.

Requirement of β1 integrin for endothelium-dependent vasodilation and collateral formation in hindlimb ischemia

An acute increase in blood flow triggers flow-mediated dilation (FMD), which is mainly mediated by endothelial nitric oxide synthase (eNOS). A long-term increase in blood flow chronically enlarges the arterial lumen, a process called arteriogenesis. In several common human diseases, these processes are disrupted for as yet unknown reasons. Here, we asked whether β1 integrin, a mechanosensory protein in endothelial cells, is required for FMD and arteriogenesis in the ischemic hindlimb. Permanent ligation of the femoral artery in C57BL/6 J mice enlarged pre-existing collateral arteries and increased numbers of arterioles in the thigh. In the lower leg, the numbers of capillaries increased. Notably, injection of β1 integrin-blocking antibody or tamoxifen-induced endothelial cell-specific deletion of the gene for β1 integrin (Itgb1) inhibited both arteriogenesis and angiogenesis. Using high frequency ultrasound, we demonstrated that β1 integrin-blocking antibody or endothelial cell-specific depletion of β1 integrin attenuated FMD of the femoral artery, and blocking of β1 integrin function did not further decrease FMD in eNOS-deficient mice. Our data suggest that endothelial β1 integrin is required for both acute and chronic widening of the arterial lumen in response to hindlimb ischemia, potentially via functional interaction with eNOS.

Pigment Epithelium-Derived Factor Increases Native Collateral Blood Flow to Improve Cardiac Function and Induce Ventricular Remodeling After Acute Myocardial Infarction

Background

We previously found that the structural defects of the coronary collateral microcirculation reserve (CCMR) prevent these preformed collateral vessels from continuously delivering the native collateral blood and supporting the ischemic myocardium in rats. Here, we tested whether these native collaterals can be remodeled by artificially increasing pigment epithelium–derived factor (PEDF) expression and demonstrated the mechanism for this stimulation.

Methods and Results

We performed intramyocardial gene delivery (PEDF‐lentivirus, 2×10⁷ TU) along the left anterior descending coronary artery to artificially increase the expression of PEDF in the tissue of the region for 2 weeks. By blocking the left anterior descending coronary artery, we examined the effects of PEDF on native collateral blood flow and CCMR. The results of positron emission tomography perfusion imaging showed that PEDF increased the native collateral blood flow and significantly inhibited its decline during acute myocardial infarction. In addition, the number of CCMR vessels decreased and the size increased. Similar results were obtained from in vitro experiments. We tested whether PEDF induces CCMR remodeling in a fluid shear stress–like manner by detecting proteins and signaling pathways that are closely related to fluid shear stress. The nitric oxide pathway and the Notch‐1 pathway participated in the process of CCMR remodeling induced by PEDF.

Conclusions

PEDF treatment activates the nitric oxide pathway, and the Notch‐1 pathway enabled CCMR remodeling. Increasing the native collateral blood flow can promote the ventricular remodeling process and improve prognosis after acute myocardial infarction.

Clinical significance of chronic myocardial ischemia in coronary artery disease patients

Myocardial ischemia is considered the cornerstone of the treatment of patients with coronary artery disease (CAD). Although the deleterious effects of myocardial infarction, the maximum expression of ischemia, have been extensively studied and described, the clinical effects of chronic, documented myocardial ischemia are not completely clarified. The first studies that compared therapies for coronary disease focused on the presence of anatomical features and assessed ischemia based on the interpretation of the findings of obstructive atherosclerotic lesions. They suggested that revascularization interventions did not confer any clinical advantage over medical therapy (MT), in terms of cardiac or overall death. Other retrospective studies that were dedicated to assessing the impact of documented stress-induced ischemia on cardiovascular outcomes have suggested a prognostic impact of chronic ischemia. However, this has been questioned in recent studies. Moreover, the previous understanding that chronic ischemia could lead to worsening of ventricular function was not confirmed in a recent study. Thus, the prognostic significance of stress-induced ischemia has been questioned. Regarding treatment options, although some previous analyses have suggested that interventional therapies would reduce cardiovascular events in CAD patients with documented ischemia, recent post-hoc studies and metanalysis have shown distinct results. In this review article, the authors discuss myocardial ischemia, the different responses of the myocardium to ischemic insults, ischemic preconditioning, and the main findings of recent studies about the clinical aspects and treatment of patients with chronic, documented myocardial ischemia.

Cardioprotection during ischemia by coronary collateral growth

Ischemic heart diseases (IHD) cause millions of deaths around the world annually. While surgical and pharmacological interventions are commonly used to treat patients with IHD, their efficacy varies from patient to patient and is limited by the severity of the disease. One promising, at least theoretically, approach for treating IHD is induction of coronary collateral growth (CCG). Coronary collaterals are arteriole-to-arteriole anastomoses that can undergo expansion and remodeling in the setting of coronary disease when the disease elicits myocardial ischemia and creates a pressure difference across the collateral vessel that creates unidirectional flow. Well-developed collaterals can restore blood flow in the ischemic area of the myocardium and protect the myocardium at risk. Moreover, such collaterals are correlated to reduced mortality and infarct size and better cardiac function during occlusion of coronary arteries. Therefore, understanding the process of CCG is highly important as a potentially viable treatment of IHD. While there are several excellent review articles on this topic, this review will provide a unified overview of the various aspects related to CCG as well as an update of the advancements in the field. We also call for more detailed studies with an interdisciplinary approach to advance our knowledge of CCG. In this review, we will describe growth of coronary collaterals, the various factors that contribute to CCG, animal models used to study CCG, and the cardioprotective effects of coronary collaterals during ischemia. We will also discuss the impairment of CCG in metabolic syndrome and the therapeutic potentials of CCG in IHD.

The relationship of plasma miR-503 and coronary collateral circulation in patients with coronary artery disease

OBJECTIVE

Although angiogenesis plays an important role in coronary collateral circulation (CCC) formation and there are many determinants of coronary angiogenesis, they cannot fully explain the mechanism of CCC formation or as potent biomarker for CCC status. Therefore, there is of great clinical significance to identify the novel molecules associated with CCC. Previously, miR-503 exerts anti-angiogenesis effect via inhibition of VEGF-A and its expression is associated with many angiogenesis-related factors. Thus, we aimed to investigate the relationship of plasma miR-503 with CCC formation as well as its predictive power for CCC status in patients with coronary artery disease.

METHODS

Among patients who underwent coronary angiography with coronary artery disease and a stenosis of ≥90% were included in our study. Collateral degree was graded according to Rentrop Cohen classification. The patients were divided to good CCC group (grade 2 or 3) and poor CCC group (grade 0 or 1) according to Rentrop grade. We investigated the plasma levels of miR-503 and VEGF-A by ELISA or q RT-PCR, respectively. In addition, we assayed the correlations of plasma miR-503 with VEGF-A or Rentrop grade using the spearman correlation test and its predictive power by receiver operating characteristic (ROC) and binary logistical regression analysis.

RESULTS

Our data showed that plasma VEGF-A was significantly higher in good CCC group than that in poor group. Plasma miR-503 was lower in CAD patients with good CCC or poor CCC compared with control subjects and lowest in good CCC group. In addition, miR-503 negatively correlated with VEGF-A and Rentrop grade, respectively. Moreover, miR-503 displayed more potent predictive power for CCC status than VEGF-A, but its sensitivity and specificity for CCC status were only 72.4 or 60.9%, respectively.

CONCLUSIONS

Lower plasma miR-503 level was related to better CCC formation, accompanied by up-regulation of VEGF-A. In addition, miR-503 displayed potent predictive power for CCC status, but its sensitivity and specificity were not high enough, indicating that miR-503 might be as an additional prognosis biomarker for CCC.

Donor artery stenosis interactions with diastolic blood pressure on coronary collateral flow in type 2 diabetic patients with chronic total occlusion

Background

We investigated whether and to what extent stenosis of predominant collateral donor artery (PCDA) affects coronary collateral flow in relation to blood pressure (BP) in type 2 diabetic patients with chronic total occlusion (CTO).

Methods

Collateral flow index (CFI) as derived from intracoronary pressure distal to occluded segment and mean aortic pressure in 220 type 2 diabetic patients and 220 propensity score matched non-diabetic controls undergoing percutaneous coronary intervention for CTO. The severity of PCDA stenosis was graded according to lumen diameter narrowing.

Results

CFI decreased stepwise from mild to severe stenosis of the PCDA and was lower in diabetic patients with moderate or severe PCDA stenosis than in non-diabetic controls (0.36 ± 0.10 vs. 0.45 ± 0.08, P < 0.001; 0.29 ± 0.09 vs. 0.35 ± 0.08, P = 0.008). When the PCDA was mildly stenotic, CFI increased initially along with a reduction in diastolic BP, and decreased when diastolic BP was below 60 mmHg in diabetic patients (0.38 ± 0.16 vs. 0.57 ± 0.09, P < 0.001). In the presence of moderate PCDA stenosis, diabetic patients had significantly lower CFI compared to non-diabetic controls, with a relative reduction of 19.8% at diastolic BP 70–79 mmHg, 28.2% at 60–69 mmHg and 38.2% below 60 mmHg (all P < 0.05). A severe PCDA stenosis resulted in a more pronounced decrease in CFI, with a relative reduction of 37.3% for diabetics compared to non-diabetics when diastolic BP was below 60 mmHg (P = 0.050).

Conclusions

In the setting of CTO, donor artery stenosis confers greater risk for reduced coronary collateral flow when diastolic BP is decreased. Even a moderate stenosis in the PCDA may be associated with lower collateral flow as diastolic BP decreases below 80 mmHg in type 2 diabetic than in non-diabetic patients.

The role of coronary artery collaterals in the preservation of left ventricular function: a study to address a long-standing controversy

Introduction:

The functional significance of coronary artery collateral (CAC) vasculature in humans has been debated for decades and this has been compounded by the lack of a standard, systematic, objective method of grading and documenting CAC flow in man. CACs serve as alternative conduits for blood in obstructive coronary artery disease. This study aimed to evaluate the impact of CACs on left ventricular function in the presence of total coronary arterial occlusion.

Methods:

The study group included the coronary angiographic records of 97 patients (mean age: 59 ± 8 years). CACs were graded from 0–3 based on the collateral connection between the donor and recipient arteries. Left ventricular function was computed from the ventriculogram and expressed as ejection fraction (EF).

Results:

The mean EF of the patients with grades 0, 1, 2 and 3 CACs were calculated as 50.4, 47, 60.5 and 70%, respectively. A significant difference was recorded in the mean EF calculated for the different CAC grades (p = 0.001). There was a significant positive correlation (p < 0.001; r = 0.478) between the mean EF and the CAC grades.

Conclusion:

The patients with better coronary collateral grades had a higher mean EF. Therefore, as the grade of CACs increased, there was an improvement in their ability to preserve left ventricular function.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Specialized pro-resolving mediators in cardiovascular diseases

The resolution of inflammation is a highly regulated process enacted by endogenous mediators including specialized pro-resolving lipid mediators (SPMs): the lipoxins, resolvins, protectins and maresins. SPMs activate specific cellular receptors to temper the production of pro-inflammatory mediators, diminish the recruitment of neutrophils, and promote the clearance of dead cells by macrophages. These mediators also enhance host-defense and couple resolution of inflammation to subsequent phases of tissue repair. Given that unresolved inflammation plays a causal role in the development of cardiovascular diseases, an understanding of these endogenous pro-resolving processes is critical for determining why cardiovascular inflammation does not resolve. Here, we discuss the receptor-dependent actions of resolvins and related pro-resolving mediators and highlight their emerging roles in the cardiovascular system. We propose that stimulating resolution could be a novel approach for treating chronic cardiovascular inflammation without promoting immunosuppression.

Resolvin D2 Enhances Postischemic Revascularization While Resolving Inflammation

BACKGROUND

Resolvins are lipid mediators generated by leukocytes during the resolution phase of inflammation. They have been shown to regulate the transition from inflammation to tissue repair; however, it is unknown whether resolvins play a role in tissue revascularization following ischemia.

METHODS

We used a murine model of hind limb ischemia (HLI), coupled with laser Doppler perfusion imaging, microcomputed tomography, and targeted mass spectrometry, to assess the role of resolvins in revascularization and inflammation resolution.

RESULTS

In mice undergoing HLI, we identified resolvin D2 (RvD2) in bone marrow and skeletal muscle by mass spectrometry (n=4-7 per group). We also identified RvD2 in skeletal muscle biopsies from humans with peripheral artery disease. Monocytes were recruited to skeletal muscle during HLI and isolated monocytes produced RvD2 in a lipoxygenase-dependent manner. Exogenous RvD2 enhanced perfusion recovery in HLI and microcomputed tomography of limb vasculature revealed greater volume, with evidence of tortuous arterioles indicative of arteriogenesis (n=6-8 per group). Unlike other treatment strategies for therapeutic revascularization that exacerbate inflammation, RvD2 did not increase vascular permeability, but reduced neutrophil accumulation and the plasma levels of tumor necrosis factor-α and granulocyte macrophage colony-stimulating factor. In mice treated with RvD2, histopathologic analysis of skeletal muscle of ischemic limbs showed more regenerating myocytes with centrally located nuclei. RvD2 enhanced endothelial cell migration in a Rac-dependent manner, via its receptor, GPR18, and Gpr18-deficient mice had an endogenous defect in perfusion recovery following HLI. Importantly, RvD2 rescued defective revascularization in diabetic mice.

CONCLUSIONS

RvD2 stimulates arteriogenic revascularization during HLI, suggesting that resolvins may be a novel class of mediators that both resolve inflammation and promote arteriogenesis.

Angiogenic gene therapy for refractory angina

INTRODUCTION

Stimulation of coronary collateral vessel growth by therapeutic angiogenesis (TA) offers an alternative treatment option for patients with refractory angina. Several TA modalities, including delivery to the heart of angiogenic growth factors (proteins or genes) and cells have been tested in clinical trials in the past two decades, but so far none of them resulted in significant therapeutic efficacy in large scale studies. This review attempts to identify the main obstacles hindering clinical success and recommends measures to overcome them in the future.

AREAS COVERED

After stating the medical need and rational for TA, and listing and briefly discussing past and current TA clinical trials, three main areas of obstacles are described: conceptual questions, technical limitations and clinical design uncertainties. Based on scientific and technical advances and lessons learned in past clinical trials, potential solutions to overcome some of these obstacles are proposed.

EXPERT OPINION

Several success criteria are identified, which apply to any TA approach of choice. It is emphasized, that each of these criteria needs to be met in future clinical trials to have a chance of therapeutic success.

De-novo collateral formation following acute myocardial infarction: Dependence on CCR2⁺ bone marrow cells

Wide variation exists in the extent (number and diameter) of native pre-existing collaterals in tissues of different strains of mice, with supportive indirect evidence recently appearing for humans. This variation is a major determinant of the wide variation in severity of tissue injury in occlusive vascular disease. Whether such genetic-dependent variation also exists in the heart is unknown because no model exists for study of mouse coronary collaterals. Also owing to methodological limitations, it is not known if ischemia can induce new coronary collaterals to form ("neo-collaterals") versus remodeling of pre-existing ones. The present study sought to develop a model to study coronary collaterals in mice, determine whether neo-collateral formation occurs, and investigate the responsible mechanisms. Four strains with known rank-ordered differences in collateral extent in brain and skeletal muscle were studied: C57BLKS>C57BL/6>A/J>BALB/c. Unexpectedly, these and 5 additional strains lacked native coronary collaterals. However after ligation, neo-collaterals formed rapidly within 1-to-2 days, reaching their maximum extent in ≤7 days. Rank-order for neo-collateral formation differed from the above: C57BL/6>BALB/c>C57BLKS>A/J. Collateral network conductance, infarct volume(-1), and contractile function followed this same rank-order. Neo-collateral formation and collateral conductance were reduced and infarct volume increased in MCP1(-/-) and CCR2(-/-) mice. Bone-marrow transplant rescued collateral formation in CCR2(-/-) mice. Involvement of fractalkine➔CX3CR1 signaling and endothelial cell proliferation were also identified. This study introduces a model for investigating the coronary collateral circulation in mice, demonstrates that neo-collaterals form rapidly after coronary occlusion, and finds that MCP➔CCR2-mediated recruitment of myeloid cells is required for this process.

Molecular controls of arterial morphogenesis

Formation of arterial vasculature, here termed arteriogenesis, is a central process in embryonic vascular development as well as in adult tissues. Although the process of capillary formation, angiogenesis, is relatively well understood, much remains to be learned about arteriogenesis. Recent discoveries point to the key role played by vascular endothelial growth factor receptor 2 in control of this process and to newly identified control circuits that dramatically influence its activity. The latter can present particularly attractive targets for a new class of therapeutic agents capable of activation of this signaling cascade in a ligand-independent manner, thereby promoting arteriogenesis in diseased tissues.

What can we learn about treating heart failure from the heart's response to acute exercise? Focus on the coronary microcirculation

Coronary microvascular function and cardiac function are closely related in that proper cardiac function requires adequate oxygen delivery through the coronary microvasculature. Because of the close proximity of cardiomyocytes and coronary microvascular endothelium, cardiomyocytes not only communicate their metabolic needs to the coronary microvasculature, but endothelium-derived factors also directly modulate cardiac function. This review summarizes evidence that the myocardial oxygen balance is disturbed in the failing heart because of increased extravascular compressive forces and coronary microvascular dysfunction. The perturbations in myocardial oxygen balance are exaggerated during exercise and are due to alterations in neurohumoral influences, endothelial function, and oxidative stress. Although there is some evidence from animal studies that the myocardial oxygen balance can partly be restored by exercise training, it is largely unknown to what extent the beneficial effects of exercise training include improvements in endothelial function and/or oxidative stress in the coronary microvasculature and how these improvements are impacted by risk factors such as diabetes, obesity, and hypercholesterolemia.

miR-21-mediated decreased neutrophil apoptosis is a determinant of impaired coronary collateral growth in metabolic syndrome

Coronary collateral growth (CCG) is impaired in metabolic syndrome. microRNA-21 (miR-21) is a proproliferative and antiapoptotic miR, which we showed to be elevated in metabolic syndrome. Here we investigate whether impaired CCG in metabolic syndrome involved miR-21-mediated aberrant apoptosis. Normal Sprague-Dawley (SD) and metabolic syndrome [J. C. Russel (JCR)] rats underwent transient, repetitive coronary artery occlusion [repetitive ischemia (RI)]. Antiapoptotic Bcl-2, phospho-Bad, and Bcl-2/Bax dimers were increased on days 6 and 9 RI, and proapoptotic Bax and Bax/Bax dimers and cytochrome-c release concurrently decreased in JCR versus SD rats. Active caspases were decreased in JCR versus SD rats (~50%). Neutrophils increased transiently on day 3 RI in the collateral-dependent zone of SD rats but remained elevated in JCR rats, paralleling miR-21 expression. miR-21 downregulation by anti-miR-21 induced neutrophil apoptosis and decreased Bcl-2 and Bcl-2/Bax dimers (~75%) while increasing Bax/Bax dimers, cytochrome-c release, and caspase activation (~70, 400, and 400%). Anti-miR-21 also improved CCG in JCR rats (~60%). Preventing neutrophil infiltration with blocking antibodies resulted in equivalent CCG recovery, confirming a major role for deregulated neutrophil apoptosis in CCG impairment. Neutrophil and miR-21-dependent CCG inhibition was in significant part mediated by increased oxidative stress. We conclude that neutrophil apoptosis is integral to normal CCG and that inappropriate prolonged miR-21-mediated survival of neutrophils plays a major role in impaired CCG, in part via oxidative stress generation.

Effect of combined treatment with rosuvastatin and protein kinase Cβ2 inhibitor on angiogenesis following myocardial infarction in diabetic rats

The aim of the present study was to investigate the effects of combined treatment with rosuvastatin and LY333531, a selective protein kinase C (PKC)β2 inhibitor, on angiogenesis under hyperglycemic conditions. Human umbilical vein endothelial cells (HUVECs) cultured in medium containing a normal or high concentration of glucose (33.3 mmol/l) were treated with rosuvastatin (0.1 µmol/l) alone or in combination with LY333531 (10 nmol/l). HUVEC migration and tube formation were assessed. Furthermore, rats with streptozotocin-induced diabetes were randomly divided into groups and treated with either rosuvastatin alone (5 mg/kg/day) or in combination with LY333531 (10 mg/kg/day) for 4 weeks following the induction of myocardial infarction (MI). Echocardiographic patterns, the extent of myocardial fibrosis, capillary density in myocardial tissue, the phosphorylation of Akt and endothelial nitric oxide synthase (eNOS), as well as the expression levels of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor 1-α (HIF‑1α) were assessed. The results from the in vitro experiment revealed that the tube-forming and migration ability of the HUVECs exposed to high-glucose medium was significantly improved in the group treated with the combination of rosuvastatin and LY333531. In vivo, the combination of rosuvastatin and LY333531 significantly improved left ventricular function, reduced the extent of myocardial fibrosis and increased myocardial capillary density compared to treatment with rosuvastatin alone. In addition, the expression levels of VEGF, and Akt and eNOS phosphorylation were significantly higher in the group exposed to the combination treatment than in the group treated with rosuvastatin alone. The results of the present study indicate that, compared to treatment with rosuvastatin alone, combined treatment with rosuvastatin and LY333531 promotes a greater level of angiogenesis in diabetic rats with MI. This effect is likely mediated through the upregulation of the VEGF‑dependent Akt/eNOS signaling pathway.

Cerebral neovascularization in diabetes: implications for stroke recovery and beyond

Neovascularization is an innate physiologic response by which tissues respond to various stimuli through collateral remodeling (arteriogenesis) and new vessel formation from existing vessels (angiogenesis) or from endothelial progenitor cells (vasculogenesis). Diabetes has a major impact on the neovascularization process but the response varies between different organ systems. While excessive angiogenesis complicates diabetic retinopathy, impaired neovascularization contributes to coronary and peripheral complications of diabetes. How diabetes influences cerebral neovascularization remained unresolved until recently. Diabetes is also a major risk factor for stroke and poor recovery after stroke. In this review, we discuss the impact of diabetes, stroke, and diabetic stroke on cerebral neovascularization, explore potential mechanisms involved in diabetes-mediated neovascularization as well as the effects of the diabetic milieu on poststroke neovascularization and recovery, and finally discuss the clinical implications of these effects.

Despite normal arteriogenic and angiogenic responses, hind limb perfusion recovery and necrotic and fibroadipose tissue clearance are impaired in matrix metalloproteinase 9-deficient mice

OBJECTIVE

The relative contributions of arteriogenesis, angiogenesis, and ischemic muscle tissue composition toward reperfusion after arterial occlusion are largely unknown. Differential loss of bone marrow-derived cell (BMC) matrix metalloproteinase 9 (MMP9), which has been implicated in all of these processes, was used to assess the relative contributions of these processes during limb reperfusion.

METHODS

We compared collateral growth (arteriogenesis), capillary growth (angiogenesis), and ischemic muscle tissue composition after femoral artery ligation in FVB/NJ mice that had been reconstituted with bone marrow from wild-type or MMP9(-/-) mice.

RESULTS

Laser Doppler perfusion imaging confirmed decreased reperfusion capacity in mice with BMC-specific loss of MMP9; however, collateral arteriogenesis was not affected. Furthermore, when accounting for the fact that muscle tissue composition changes markedly with ischemia (ie, necrotic, fibroadipose, and regenerating tissue regions are present), angiogenesis was also unaffected. Instead, BMC-specific loss of MMP9 caused an increase in the proportion of necrotic and fibroadipose tissue, which showed the strongest correlation with poor perfusion recovery. Similarly, the reciprocal loss of MMP9 from non-BMCs showed similar deficits in perfusion and tissue composition without affecting arteriogenesis.

CONCLUSIONS

By concurrently analyzing arteriogenesis, angiogenesis, and ischemic tissue composition, we determined that the loss of BMC-derived or non-BMC-derived MMP9 impairs necrotic and fibroadipose tissue clearance after femoral artery ligation, despite normal arteriogenic and angiogenic vascular growth. These findings imply that therapeutic revascularization strategies for treating peripheral arterial disease may benefit from additionally targeting necrotic tissue clearance or skeletal muscle regeneration, or both.

Integrative modeling of small artery structure and function uncovers critical parameters for diameter regulation

Organ perfusion is regulated by vasoactivity and structural adaptation of small arteries and arterioles. These resistance vessels are sensitive to pressure, flow and a range of vasoactive stimuli. Several strongly interacting control loops exist. As an example, the myogenic response to a change of pressure influences the endothelial shear stress, thereby altering the contribution of shear-dependent dilation to the vascular tone. In addition, acute responses change the stimulus for structural adaptation and vice versa. Such control loops are able to maintain resistance vessels in a functional and stable state, characterized by regulated wall stress, shear stress, matched active and passive biomechanics and presence of vascular reserve. In this modeling study, four adaptation processes are identified that together with biomechanical properties effectuate such integrated regulation: control of tone, smooth muscle cell length adaptation, eutrophic matrix rearrangement and trophic responses. Their combined action maintains arteries in their optimal state, ready to cope with new challenges, allowing continuous long-term vasoregulation. The exclusion of any of these processes results in a poorly regulated state and in some cases instability of vascular structure.

Coronary microvascular dysfunction: an update

Many patients undergoing coronary angiography because of chest pain syndromes, believed to be indicative of obstructive atherosclerosis of the epicardial coronary arteries, are found to have normal angiograms. In the past two decades, a number of studies have reported that abnormalities in the function and structure of the coronary microcirculation may occur in patients without obstructive atherosclerosis, but with risk factors or with myocardial diseases as well as in patients with obstructive atherosclerosis; furthermore, coronary microvascular dysfunction (CMD) can be iatrogenic. In some instances, CMD represents an epiphenomenon, whereas in others it is an important marker of risk or may even contribute to the pathogenesis of cardiovascular and myocardial diseases, thus becoming a therapeutic target. This review article provides an update on the clinical relevance of CMD in different clinical settings and also the implications for therapy.

Endothelial cell-dependent regulation of arteriogenesis

RATIONALE

Arteriogenesis is the process of formation of arterial conduits. Its promotion is an attractive therapeutic strategy in occlusive atherosclerotic diseases. Despite the functional and clinical importance of arteriogenesis, the biology of the process is poorly understood. Synectin, a gene previously implicated in the regulation of vascular endothelial cell growth factor signaling, offers a unique opportunity to determine relative contributions of various cell types to arteriogenesis.

OBJECTIVE

We investigated the cell-autonomous effects of a synectin knockout in arterial morphogenesis.

METHODS AND RESULTS

A floxed synectin knockin mouse line was crossbred with endothelial-specific (Tie2, Cdh5, Pdgfb) and smooth muscle myosin heavy chain-specific Cre driver mouse lines to produce cell type-specific deletions. Ablation of synectin expression in endothelial, but not smooth muscle cells resulted in the presence of developmental arterial morphogenetic defects (smaller size of the arterial tree, reduced number of arterial branches and collaterals) and impaired arteriogenesis in adult mice.

CONCLUSIONS

Synectin modulates developmental and adult arteriogenesis in an endothelial cell-autonomous fashion. These findings show for the first time that endothelial cells are central to both developmental and adult arteriogenesis and provide a model for future studies of factors involved in this process.

Myoglobin overexpression inhibits reperfusion in the ischemic mouse hindlimb through impaired angiogenesis but not arteriogenesis

Adaptive vascular remodeling in response to arterial occlusion takes the form of capillary growth (angiogenesis) and outward remodeling of pre-existing collateral arteries (arteriogenesis). However, the relative contributions of angiogenesis and arteriogenesis toward the overall reperfusion response are both highly debated and poorly understood. Here, we tested the hypothesis that myoglobin overexpressing transgenic mice (MbTg(+)) exhibit impaired angiogenesis in the setting of normal arteriogenesis in response to femoral artery ligation, and thereby serve as a model for disconnecting these two vascular growth processes. After femoral artery ligation, MbTg(+) mice were characterized by delayed distal limb reperfusion (by laser Doppler perfusion imaging), decreased foot use, and impaired distal limb muscle angiogenesis in both glycolytic and oxidative muscle fiber regions at day 7. Substantial arteriogenesis occurred in the primary collaterals supplying the ischemic limb in both wild-type and MbTg(+) mice; however, there were no significant differences between groups, indicating that myoglobin overexpression does not affect arteriogenesis. Together, these results uniquely demonstrate that functional collateral arteriogenesis alone is not necessarily sufficient for adequate reperfusion after arterial occlusion. Angiogenesis is a key component of an effective reperfusion response, and clinical strategies that target both angiogenesis and arteriogenesis could yield the most efficacious treatments for peripheral arterial disease.

Mechanistic, technical, and clinical perspectives in therapeutic stimulation of coronary collateral development by angiogenic growth factors

Stimulation of collateral vessel development in the heart by angiogenic growth factor therapy has been tested in animals and humans for almost two decades. Discordance between the outcome of preclinical studies and clinical trials pointed to the difficulties of translation from animal models to patients. Lessons learned in this process identified specific mechanistic, technical, and clinical hurdles, which need to be overcome. This review summarizes current understanding of the mechanisms leading to the establishment of a functional coronary collateral network and the biological processes growth factor therapies should stimulate even under conditions of impaired natural adaptive vascular response. Vector delivery methods are recommended to maximize angiogenic gene therapy efficiency and reduce side effects. Optimization of clinical trial design should include the choice of clinical end points which provide mechanistic proof-of-concept and also reflect clinical benefits (e.g., surrogates to assess increased collateral flow reserve, such as myocardial perfusion imaging). Guidelines are proposed to select patients who may respond to the therapy with high(er) probability. Both short and longer term strategies are outlined which may help to make therapeutic angiogenesis (TA) work in the future.

Macrophages in collateral arteriogenesis

Arteriosclerotic vascular disease is the most common cause of death and a major cause of disability in the developed world. Adverse outcomes of arteriosclerotic vascular disease are related to consequences of tissue ischemia and necrosis affecting the heart, brain, limbs, and other organs. Collateral artery growth or arteriogenesis occurs naturally and can help restore perfusion to ischemic tissues. Understanding the mechanisms of collateral artery growth may provide therapeutic options for patients with ischemic vascular disease. In this review, we examine the evidence for a role of monocytes and macrophages in collateral arteriogenesis.