Pathophysiology of collateral development

Cerebral haemodynamics in symptomatic intracranial atherosclerotic disease: a narrative review of the assessment methods and clinical implications

Intracranial atherosclerotic disease (ICAD) is a common cause of ischaemic stroke and transient ischaemic attack (TIA) with a high recurrence rate. It is often referred to as intracranial atherosclerotic stenosis (ICAS), when the plaque has caused significant narrowing of the vessel lumen. The lesion is usually considered 'symptomatic ICAD/ICAS' (sICAD/sICAS) when it has caused an ischaemic stroke or TIA. The severity of luminal stenosis has long been established as a prognostic factor for stroke relapse in sICAS. Yet, accumulating studies have also reported the important roles of plaque vulnerability, cerebral haemodynamics, collateral circulation, cerebral autoregulation and other factors in altering the stroke risks across patients with sICAS. In this review article, we focus on cerebral haemodynamics in sICAS. We reviewed imaging modalities/methods in assessing cerebral haemodynamics, the haemodynamic metrics provided by these methods and application of these methods in research and clinical practice. More importantly, we reviewed the significance of these haemodynamic features in governing the risk of stroke recurrence in sICAS. We also discussed other clinical implications of these haemodynamic features in sICAS, such as the associations with collateral recruitment and evolution of the lesion under medical treatment, and indications for more individualised blood pressure management for secondary stroke prevention. We then put forward some knowledge gaps and future directions on these topics.

Hemodynamics derived from computational fluid dynamics based on magnetic resonance angiography is associated with functional outcomes in atherosclerotic middle cerebral artery stenosis

BACKGROUND

To investigate the relationship between fluid-attenuated inversion recovery (FLAIR) vascular hyperintensity (FVH), hemodynamics, and functional outcome in atherosclerotic middle cerebral artery (MCA) stenosis using a computational fluid dynamics (CFD) model based on magnetic resonance angiography (MRA), according to a modified Rankin Scale (mRS) at 3 months.

METHODS

A total of 120 patients with 50-99% atherosclerotic MCA stenosis were included. The training and internal validation groups were composed of 99 participants and 21 participants, respectively. Demographic, imaging data, and functional outcome (mRS at 3 months) were collected. Hemodynamic parameters were obtained from the CFD model. The FVH score was based on the number of territories where FVH is positive, according to the spatial distribution in the Alberta Stroke Program Early Computed Tomography Score (ASPECTS). The prediction models were constructed according to clinical and hemodynamic parameters using multivariate logistic analysis. The DeLong test compared areas under the curves (AUCs) of the models.

RESULTS

The multivariable logistic regression analysis showed that the National Institute of Health Stroke Scale (NIHSS) at admission, hypertension, hyperlipidemia, the ratio of wall shear stress before treatment (WSSR_before), and difference in the ratio of wall shear stress (WSSR) were independently associated with functional outcome (all P<0.05). In the training group before treatment, the AUC of model 1a (only clinical variables) and 2a (clinical variables with addition of WSSR_before) were 0.750 and 0.802. After treatment, the AUC of model 1b (only clinical variables) and 2b (clinical variables with addition of difference in WSSR) were 0.815 and 0.883, respectively. The AUC of models with hemodynamic parameters was significantly higher than the models based on clinical variables only (all P<0.05, DeLong test). In the internal validation group before treatment, the AUC of the model (clinical variables) was 0.782, and that of the model (clinical variables and WSSR_before) was 0.800. After treatment, the AUC of the model (clinical variables) was 0.833, and that of the model (clinical variables and difference in WSSR) was 0.861. There were no significant differences between the good and the poor functional outcome group concerning FVH_before scores before treatment (0.30±0.81 vs. 0.26±0.97; P=0.321) and FVH_after scores after treatment (0.08±0.39 vs. 0.00±0.00; P=0.244).

CONCLUSIONS

Hemodynamics was associated with functional outcomes in patients with ischemic stroke attributed to atherosclerotic MCA stenosis, while FVH was not. Hemodynamic parameters were of great importance in the prediction models.

Intermittent appearance of right coronary fistula and collateral circulation: A case report

BACKGROUND

Congenital coronary artery fistula can lead to symptoms of chest tightness, chest pain, or exertional dyspnea, which is a congenital vascular malformation that should not be ignored. Patients who have such malformations are frequently observed with different concurrent abnormal anatomic structures. Collateral circulation may have a positive effect on improving the patients' symptoms.

CASE SUMMARY

A 53-year-old female experienced episodic chest discomfort for the past month with symptoms manifesting when she was agitated or overexerted. After a positive treadmill test, the patient underwent coronary angiography. “Ghostlike” intermittent appearance of coronary ventricular fistula and collateral branching were observed. The patient was diagnosed with a right coronary ventricular fistula and collateral circulation.

CONCLUSION

This case shows the likelihood of collateral circulation in patients with coronary artery fistula. This may provide medical staff with novel solutions to treat insufficiency of myocardial blood supply induced by cardiovascular malformations.

Importance of Choke Vessels in Injectable Fillers

Knowledge of facial anatomy is indispensable for dermatologists and plastic surgeons practicing aesthetic medicine, especially for those using fillers, as injection of fillers may be associated with serious complications such as vascular occlusion and blindness. Angiosome and choke vessels play an important role in vascular incidents occurring after filler injections. The objective of this article was to outline the anatomy and pathophysiology of choke vessels, a concept which is not well known to dermatologists.

Epigenetic regulators of the revascularization response to chronic arterial occlusion

Peripheral arterial disease (PAD) is the leading cause of lower limb amputation and estimated to affect over 202 million people worldwide. PAD is caused by atherosclerotic lesions that occlude large arteries in the lower limbs, leading to insufficient blood perfusion of distal tissues. Given the severity of this clinical problem, there has been long-standing interest in both understanding how chronic arterial occlusions affect muscle tissue and vasculature and identifying therapeutic approaches capable of restoring tissue composition and vascular function to a healthy state. To date, the most widely utilized animal model for performing such studies has been the ischaemic mouse hindlimb. Despite not being a model of PAD per se, the ischaemic hindlimb model does recapitulate several key aspects of PAD. Further, it has served as a valuable platform upon which we have built much of our understanding of how chronic arterial occlusions affect muscle tissue composition, muscle regeneration and angiogenesis, and collateral arteriogenesis. Recently, there has been a global surge in research aimed at understanding how gene expression is regulated by epigenetic factors (i.e. non-coding RNAs, histone post-translational modifications, and DNA methylation). Thus, perhaps not unexpectedly, many recent studies have identified essential roles for epigenetic factors in regulating key responses to chronic arterial occlusion(s). In this review, we summarize the mechanisms of action of these epigenetic regulators and highlight several recent studies investigating the role of said regulators in the context of hindlimb ischaemia. In addition, we focus on how these recent advances in our understanding of the role of epigenetics in regulating responses to chronic arterial occlusion(s) can inform future therapeutic applications to promote revascularization and perfusion recovery in the setting of PAD.

Arteriogenesis of the Spinal Cord-The Network Challenge

Spinal cord ischemia (SCI) is a clinical complication following aortic repair that significantly impairs the quality and expectancy of life. Despite some strategies, like cerebrospinal fluid drainage, the occurrence of neurological symptoms, such as paraplegia and paraparesis, remains unpredictable. Beside the major blood supply through conduit arteries, a huge collateral network protects the central nervous system from ischemia—the paraspinous and the intraspinal compartment. The intraspinal arcades maintain perfusion pressure following a sudden inflow interruption, whereas the paraspinal system first needs to undergo arteriogenesis to ensure sufficient blood supply after an acute ischemic insult. The so-called steal phenomenon can even worsen the postoperative situation by causing the hypoperfusion of the spine when, shortly after thoracoabdominal aortic aneurysm (TAAA) surgery, muscles connected with the network divert blood and cause additional stress. Vessels are a conglomeration of different cell types involved in adapting to stress, like endothelial cells, smooth muscle cells, and pericytes. This adaption to stress is subdivided in three phases—initiation, growth, and the maturation phase. In fields of endovascular aortic aneurysm repair, pre-operative selective segmental artery occlusion may enable the development of a sufficient collateral network by stimulating collateral vessel growth, which, again, may prevent spinal cord ischemia. Among others, the major signaling pathways include the phosphoinositide 3 kinase (PI3K) pathway/the antiapoptotic kinase (AKT) pathway/the endothelial nitric oxide synthase (eNOS) pathway, the Erk1, the delta-like ligand (DII), the jagged (Jag)/NOTCH pathway, and the midkine regulatory cytokine signaling pathways.

An Emerging Role for isomiRs and the microRNA Epitranscriptome in Neovascularization

Therapeutic neovascularization can facilitate blood flow recovery in patients with ischemic cardiovascular disease, the leading cause of death worldwide. Neovascularization encompasses both angiogenesis, the sprouting of new capillaries from existing vessels, and arteriogenesis, the maturation of preexisting collateral arterioles into fully functional arteries. Both angiogenesis and arteriogenesis are highly multifactorial processes that require a multifactorial regulator to be stimulated simultaneously. MicroRNAs can regulate both angiogenesis and arteriogenesis due to their ability to modulate expression of many genes simultaneously. Recent studies have revealed that many microRNAs have variants with altered terminal sequences, known as isomiRs. Additionally, endogenous microRNAs have been identified that carry biochemically modified nucleotides, revealing a dynamic microRNA epitranscriptome. Both types of microRNA alterations were shown to be dynamically regulated in response to ischemia and are able to influence neovascularization by affecting the microRNA’s biogenesis, or even its silencing activity. Therefore, these novel regulatory layers influence microRNA functioning and could provide new opportunities to stimulate neovascularization. In this review we will highlight the formation and function of isomiRs and various forms of microRNA modifications, and discuss recent findings that demonstrate that both isomiRs and microRNA modifications directly affect neovascularization and vascular remodeling.

Translesional pressure gradient and leptomeningeal collateral status in symptomatic middle cerebral artery stenosis

BACKGROUND AND PURPOSE

Leptomeningeal collateral (LMC) status governs the prognosis of large artery occlusive stroke, although factors determining LMC status are not fully elucidated. The aim was to investigate metrics affecting LMC status in such patients by using computational fluid dynamics (CFD) models based on computed tomography angiography (CTA).

METHODS

In this cross-sectional study, patients with recent ischaemic stroke or transient ischaemic attack attributed to atherosclerotic M1 middle cerebral artery (MCA) stenosis (50%-99%) were recruited. Demographic, clinical and imaging data of these patients were collected. Ipsilesional LMC status was graded as good or poor by assessing the laterality of anterior and posterior cerebral arteries in CTA. A CFD model based on CTA was constructed to reflect focal hemodynamics in the distal internal carotid artery, M1 MCA and A1 anterior cerebral artery. Pressure gradients were calculated across culprit MCA stenotic lesions in CFD models. Predictors for good LMC status were sought in univariate and multivariate analyses.

RESULTS

Amongst the 85 patients enrolled (mean age 61.5 ± 10.9 years), 38 (44.7%) had good ipsilesional LMC status. The mean pressure gradient across MCA lesions was 14.8 ± 18.1 mmHg. Advanced age (P = 0.030) and a larger translesional pressure gradient (P = 0.029) independently predicted good LMCs. A lower fasting blood glucose level also showed a trend for good LMCs (P = 0.058).

CONCLUSIONS

Our study suggested a correlation between translesional pressure gradient and maturation of LMCs in intracranial atherosclerotic disease. Further studies with more exquisite and dynamic monitoring of cerebral hemodynamics and LMC evolution are needed to verify the current findings.

DNA Methyltransferase 1-Dependent DNA Hypermethylation Constrains Arteriogenesis by Augmenting Shear Stress Set Point

Background

Arteriogenesis is initiated by increased shear stress and is thought to continue until shear stress is returned to its original “set point.” However, the molecular mechanism(s) through which shear stress set point is established by endothelial cells (ECs) are largely unstudied. Here, we tested the hypothesis that DNA methyltransferase 1 (DNMT1)–dependent EC DNA methylation affects arteriogenic capacity via adjustments to shear stress set point.

Methods and Results

In femoral artery ligation–operated C57BL/6 mice, collateral artery segments exposed to increased shear stress without a change in flow direction (ie, nonreversed flow) exhibited global DNA hypermethylation (increased 5‐methylcytosine staining intensity) and constrained arteriogenesis (30% less diameter growth) when compared with segments exposed to both an increase in shear stress and reversed‐flow direction. In vitro, ECs exposed to a flow waveform biomimetic of nonreversed collateral segments in vivo exhibited a 40% increase in DNMT1 expression, genome‐wide hypermethylation of gene promoters, and a DNMT1‐dependent 60% reduction in proarteriogenic monocyte adhesion compared with ECs exposed to a biomimetic reversed‐flow waveform. These results led us to test whether DNMT1 regulates arteriogenic capacity in vivo. In femoral artery ligation–operated mice, DNMT1 inhibition rescued arteriogenic capacity and returned shear stress back to its original set point in nonreversed collateral segments.

Conclusions

Increased shear stress without a change in flow direction initiates arteriogenic growth; however, it also elicits DNMT1‐dependent EC DNA hypermethylation. In turn, this diminishes mechanosensing, augments shear stress set point, and constrains the ultimate arteriogenic capacity of the vessel. This epigenetic effect could impact both endogenous collateralization and treatment of arterial occlusive diseases.

von Willebrand factor deficiency leads to impaired blood flow recovery after ischaemia in mice

Neovascularisation, i. e. arteriogenesis and angiogenesis, is an inflammatory process. Therefore attraction and extravasation of leukocytes is essential for effective blood flow recovery after ischaemia. Previous studies have shown that von Willebrand factor (VWF) is a negative regulator of angiogenesis. However, it has also been shown that VWF facilitates leukocyte attraction and extravasation. We aimed to investigate the role of VWF in arteriogenesis and angiogenesis during post-ischaemic neovascularisation. Wild-type (WT) and VWF deficient (VWF^-/-) C57BL/6 mice were subjected to hindlimb ischaemia via double ligation of the left femoral artery, and blood flow recovery was followed over time, using Laser Doppler Perfusion Imaging. Blood flow recovery was impaired in VWF^-/- mice. After 10 days, VWF^-/- mice showed a 43 ± 5 % recovery versus 68 ± 5 % in WT. Immunohistochemistry revealed that both arteriogenesis in the adductor muscles and angiogenesis in the gastrocnemius muscles were reduced in VWF^-/- mice. Furthermore, leukocyte infiltration in the affected adductor muscles was reduced in VWF^-/- mice. Residual paw perfusion directly after artery ligation was also reduced in VWF^-/- mice, indicating a decrease in pre-existing collateral arteriole density. When we quantified collateral arterioles, we observed a 31 % decrease in the average number of collateral arterioles in the pia mater compared to WT mice (57 ± 3 in WT vs 40 ± 4 pial collaterals in VWF^-/-). We conclude that VWF facilitates blood flow recovery in mice. VWF deficiency hampers both arteriogenesis and angiogenesis in a hindlimb ischaemia model. This is associated with impaired leukocytes recruitment and decreased pre-existing collateral density in the absence of VWF.

Neovascularization Potential of Blood Outgrowth Endothelial Cells From Patients With Stable Ischemic Heart Failure Is Preserved

Background

Blood outgrowth endothelial cells (BOECs) mediate therapeutic neovascularization in experimental models, but outgrowth characteristics and functionality of BOECs from patients with ischemic cardiomyopathy (ICMP) are unknown. We compared outgrowth efficiency and in vitro and in vivo functionality of BOECs derived from ICMP with BOECs from age‐matched (ACON) and healthy young (CON) controls.

Methods and Results

We isolated 3.6±0.6 BOEC colonies/100×10⁶ mononuclear cells (MNCs) from 60‐mL blood samples of ICMP patients (n=45; age: 66±1 years; LVEF: 31±2%) versus 3.5±0.9 colonies/100×10⁶MNCs in ACON (n=32; age: 60±1 years) and 2.6±0.4 colonies/100×10⁶MNCs in CON (n=55; age: 34±1 years), P=0.29. Endothelial lineage (VEGFR2⁺/CD31⁺/CD146⁺) and progenitor (CD34⁺/CD133⁻) marker expression was comparable in ICMP and CON. Growth kinetics were similar between groups (P=0.38) and not affected by left ventricular systolic dysfunction, maladaptive remodeling, or presence of cardiovascular risk factors in ICMP patients. In vitro neovascularization potential, assessed by network remodeling on Matrigel and three‐dimensional spheroid sprouting, did not differ in ICMP from (A)CON. Secretome analysis showed a marked proangiogenic profile, with highest release of angiopoietin‐2 (1.4±0.3×10⁵ pg/10⁶ICMP‐BOECs) and placental growth factor (5.8±1.5×10³ pg/10⁶ICMP BOECs), independent of age or ischemic disease. Senescence‐associated β‐galactosidase staining showed comparable senescence in BOECs from ICMP (5.8±2.1%; n=17), ACON (3.9±1.1%; n=7), and CON (9.0±2.8%; n=13), P=0.19. High‐resolution microcomputed tomography analysis in the ischemic hindlimb of nude mice confirmed increased arteriogenesis in the thigh region after intramuscular injections of BOECs from ICMP (P=0.025; n=8) and CON (P=0.048; n=5) over vehicle control (n=8), both to a similar extent (P=0.831).

Conclusions

BOECs can be successfully culture‐expanded from patients with ICMP. In contrast to impaired functionality of ICMP‐derived bone marrow MNCs, BOECs retain a robust proangiogenic profile, both in vitro and in vivo, with therapeutic potential for targeting ischemic disease.

RGS5 promotes arterial growth during arteriogenesis

Arteriogenesis—the growth of collateral arterioles—partially compensates for the progressive occlusion of large conductance arteries as it may occur as a consequence of coronary, cerebral or peripheral artery disease. Despite being clinically highly relevant, mechanisms driving this process remain elusive. In this context, our study revealed that abundance of regulator of G-protein signalling 5 (RGS5) is increased in vascular smooth muscle cells (SMCs) of remodelling collateral arterioles. RGS5 terminates G-protein-coupled signalling cascades which control contractile responses of SMCs. Consequently, overexpression of RGS5 blunted Gα_q/11-mediated mobilization of intracellular calcium, thereby facilitating Gα_12/13-mediated RhoA signalling which is crucial for arteriogenesis. Knockdown of RGS5 evoked opposite effects and thus strongly impaired collateral growth as evidenced by a blockade of RhoA activation, SMC proliferation and the inability of these cells to acquire an activated phenotype in RGS5-deficient mice after the onset of arteriogenesis. Collectively, these findings establish RGS5 as a novel determinant of arteriogenesis which shifts G-protein signalling from Gα_q/11-mediated calcium-dependent contraction towards Gα_12/13-mediated Rho kinase-dependent SMC activation.

Subject Categories Vascular Biology & Angiogenesis

Transplantation-free survival and interventions at 3 years in the single ventricle reconstruction trial

BACKGROUND

In the Single Ventricle Reconstruction (SVR) trial, 1-year transplantation-free survival was better for the Norwood procedure with right ventricle-to-pulmonary artery shunt (RVPAS) compared with a modified Blalock-Taussig shunt (MBTS). At 3 years, we compared transplantation-free survival, echocardiographic right ventricular ejection fraction, and unplanned interventions in the treatment groups.

METHODS AND RESULTS

Vital status and medical history were ascertained from annual medical records, death indexes, and phone interviews. The cohort included 549 patients randomized and treated in the SVR trial. Transplantation-free survival for the RVPAS versus MBTS groups did not differ at 3 years (67% versus 61%; P=0.15) or with all available follow-up of 4.8±1.1 years (log-rank P=0.14). Pre-Fontan right ventricular ejection fraction was lower in the RVPAS group than in the MBTS group (41.7±5.1% versus 44.7±6.0%; P=0.007), and right ventricular ejection fraction deteriorated in RVPAS (P=0.004) but not MBTS (P=0.40) subjects (pre-Fontan minus 14-month mean, -3.25±8.24% versus 0.99±8.80%; P=0.009). The RVPAS versus MBTS treatment effect had nonproportional hazards (P=0.004); the hazard ratio favored the RVPAS before 5 months (hazard ratio=0.63; 95% confidence interval, 0.45-0.88) but the MBTS beyond 1 year (hazard ratio=2.22; 95% confidence interval, 1.07-4.62). By 3 years, RVPAS subjects had a higher incidence of catheter interventions (P<0.001) with an increasing HR over time (P=0.005): <5 months, 1.14 (95% confidence interval, 0.81-1.60); from 5 months to 1 year, 1.94 (95% confidence interval, 1.02-3.69); and >1 year, 2.48 (95% confidence interval, 1.28-4.80).

CONCLUSIONS

By 3 years, the Norwood procedure with RVPAS compared with MBTS was no longer associated with superior transplantation-free survival. Moreover, RVPAS subjects had slightly worse right ventricular ejection fraction and underwent more catheter interventions with increasing hazard ratio over time.

CLINICAL TRIAL REGISTRATION URL

http://www.clinicaltrials.gov. Unique identifier: NCT00115934.

The role of mitochondrial bioenergetics and reactive oxygen species in coronary collateral growth

Coronary collateral growth is a process involving coordination between growth factors expressed in response to ischemia and mechanical forces. Underlying this response is proliferation of vascular smooth muscle and endothelial cells, resulting in an enlargement in the caliber of arterial-arterial anastomoses, i.e., a collateral vessel, sometimes as much as an order of magnitude. An integral element of this cell proliferation is the process known as phenotypic switching in which cells of a particular phenotype, e.g., contractile vascular smooth muscle, must change their phenotype to proliferate. Phenotypic switching requires that protein synthesis occurs and different kinase signaling pathways become activated, necessitating energy to make the switch. Moreover, kinases, using ATP to phosphorylate their targets, have an energy requirement themselves. Mitochondria play a key role in the energy production that enables phenotypic switching, but under conditions where mitochondrial energy production is constrained, e.g., mitochondrial oxidative stress, this switch is impaired. In addition, we discuss the potential importance of uncoupling proteins as modulators of mitochondrial reactive oxygen species production and bioenergetics, as well as the role of AMP kinase as an energy sensor upstream of mammalian target of rapamycin, the master regulator of protein synthesis.

Effects of high glucose on vascular endothelial growth factor synthesis and secretion in aortic vascular smooth muscle cells from obese and lean Zucker rats

Type 1 diabetes is characterized by insulin deficiency, type 2 by both insulin deficiency and insulin resistance: in both conditions, hyperglycaemia is accompanied by an increased cardiovascular risk, due to increased atherosclerotic plaque formation/instabilization and impaired collateral vessel formation. An important factor in these phenomena is the Vascular Endothelial Growth Factor (VEGF), a molecule produced also by Vascular Smooth Muscle Cells (VSMC). We aimed at evaluating the role of high glucose on VEGF-A₁₆₄ synthesis and secretion in VSMC from lean insulin-sensitive and obese insulin-resistant Zucker rats (LZR and OZR). In cultured aortic VSMC from LZR and OZR incubated for 24 h with d-glucose (5.5, 15 and 25 mM) or with the osmotic controls l-glucose and mannitol, we measured VEGF-A₁₆₄ synthesis (western, blotting) and secretion (western blotting and ELISA). We observed that: (i) d-glucose dose-dependently increases VEGF-A₁₆₄ synthesis and secretion in VSMC from LZR and OZR (n = 6, ANOVA p = 0.002–0.0001); (ii) all the effects of 15 and 25 mM d-glucose are attenuated in VSMC from OZR vs. LZR (p = 0.0001); (iii) l-glucose and mannitol reproduce the VEGF-A₁₆₄ modulation induced by d-glucose in VSMC from both LZR and OZR. Thus, glucose increases via an osmotic mechanism VEGF synthesis and secretion in VSMC, an effect attenuated in the presence of insulin resistance.

Induction of Smooth Muscle Cell Migration During Arteriogenesis Is Mediated by Rap2

Objective—

Collateral artery growth or arteriogenesis is the primary means of the circulatory system to maintain blood flow in the face of major arterial occlusions. Arteriogenesis depends on activation of fibroblast growth factor (FGF) receptors, but relatively little is known about downstream mediators of FGF signaling.

Methods and Results—

We screened for signaling components that are activated in response to administration of FGF-2 to cultured vascular smooth muscle cells (VSMCs) and detected a significant increase of Rap2 but not of other Ras family members, which corresponded to a strong upregulation of Rap2 and C-Raf in growing collaterals from rabbits with femoral artery occlusion. Small interfering RNAs directed against Rap2 did not affect FGF-2 induced proliferation of VSMC but strongly inhibited their migration. Inhibition of FGF receptor-1 (FGFR1) signaling by infusion of a sulfonic acid polymer or infection with a dominant-negative FGFR1 adenovirus inhibited Rap2 upregulation and collateral vessel growth. Similarly, expression of dominant-negative Rap2 blocked arteriogenesis, whereas constitutive active Rap2 enhanced collateral vessel growth.

Conclusion—

Rap2 is part of the arteriogenic program and acts downstream of the FGFR1 to stimulate VSMC migration. Specific modulation of Rap2 might be an attractive target to manipulate VSMC migration, which plays a role in numerous pathological processes.

Endothelial progenitor cells in cardiovascular disease and hypoxia--potential implications to obstructive sleep apnea

In recent years, endothelial progenitor cells (EPCs) have gained a central role in vascular regeneration and endothelial repair capacity through angiogenesis and restoring endothelial function of injured blood vessels. These bone-marrow-derived cells are capable of promoting neovascularization, improving blood perfusion, and facilitating the recovery of ischemic tissues through differentiation into functional endothelial cells and secretion of angiogenic mediators. Obstructive sleep apnea (OSA) syndrome is characterized by recurrent episodes of intermittent hypoxia (IH), which can lead to endothelial dysfunction, atherosclerosis, as well as cardiovascular morbidity and mortality. However, IH also may contribute to cardioprotection and the development of collateral vessels by mobilizing progenitor cells to the circulation and damaged myocardium. Accumulating evidence in recent years suggests that EPCs are decreased in patients with endothelial dysfunction and underlie an increased risk for cardiovascular morbidity in OSA. The current review highlights the potential role of EPCs in the pathogenesis of vascular diseases that is pertinent to OSA.

Diabetes impairs arteriogenesis in the peripheral circulation: review of molecular mechanisms

Patients suffering from both diabetes and PAD (peripheral arterial disease) are at risk of developing critical limb ischaemia and ulceration, and potentially requiring limb amputation. In addition, diabetes complicates surgical treatment of PAD and impairs arteriogenesis. Arteriogenesis is defined as the remodelling of pre-existing arterioles into conductance vessels to restore the perfusion distal to the occluded artery. Several strategies to promote arteriogenesis in the peripheral circulation have been devised, but the mechanisms through which diabetes impairs arteriogenesis are poorly understood. The present review provides an overview of the current literature on the deteriorating effects of diabetes on the key players in the arteriogenesis process. Diabetes affects arteriogenesis at a number of levels. First, it elevates vasomotor tone and attenuates sensing of shear stress and the response to vasodilatory stimuli, reducing the recruitment and dilatation of collateral arteries. Secondly, diabetes impairs the downstream signalling of monocytes, without decreasing monocyte attraction. In addition, EPC (endothelial progenitor cell) function is attenuated in diabetes. There is ample evidence that growth factor signalling is impaired in diabetic arteriogenesis. Although these defects could be restored in animal experiments, clinical results have been disappointing. Furthermore, the diabetes-induced impairment of eNOS (endothelial NO synthase) strongly affects outward remodelling, as NO signalling plays a key role in several remodelling processes. Finally, in the structural phase of arteriogenesis, diabetes impairs matrix turnover, smooth muscle cell proliferation and fibroblast migration. The review concludes with suggestions for new and more sophisticated therapeutic approaches for the diabetic population.

Depressed expression of angiogenic growth factors in the subacute phase of myocardial ischemia: a mechanism behind the remodeling plateau?

BACKGROUND AND AIMS

To investigate whether, in the subacute phase of acute myocardial infarction, in the peri-infarcted area the expressions of the vascular endothelial growth factor (VEGF-A) and angiopoietin (Ang) ligand receptors are depressed, and whether overexpression of these angiogens counteracts a downregulation of myocardial function.

METHODS

Acute myocardial infarction was induced by left anterior descending artery ligation and overexpression through injection of human VEGF-A165 and Ang-1 plasmids. The capillary and arteriolar densities, Akt-1 phosphorylation and citrate synthase activity were measured concurrent with the expression of VEGF-A, VEGFR1 and R2, Ang-1, Ang-2 and Tie-2.

RESULTS

One day after AMI, VEGR-2 was unchanged but all other measured factors in the two families were upregulated. After day 2, the Ang-2 expression increased but other measured factors decreased. After gene transfer, the vascular supply, Akt phosphorylation and citrate synthase activity were higher in the peri-infarcted area, where also the endogenous angiogenic growth factor expressions were increased.

CONCLUSION

A rapid decrease in angiogenic stimulating factors occurs in the subacute phase of AMI and is related to a progressive decrease in myocardial contraction. A negative consequence of such a circuit is a successive reduction in the vascular supply and contractility in areas with reduced perfusion. These negative adaptations can be counteracted by angiogen overexpression.

Agent-based model of therapeutic adipose-derived stromal cell trafficking during ischemia predicts ability to roll on P-selectin

Intravenous delivery of human adipose-derived stromal cells (hASCs) is a promising option for the treatment of ischemia. After delivery, hASCs that reside and persist in the injured extravascular space have been shown to aid recovery of tissue perfusion and function, although low rates of incorporation currently limit the safety and efficacy of these therapies. We submit that a better understanding of the trafficking of therapeutic hASCs through the microcirculation is needed to address this and that selective control over their homing (organ- and injury-specific) may be possible by targeting bottlenecks in the homing process. This process, however, is incredibly complex, which merited the use of computational techniques to speed the rate of discovery. We developed a multicell agent-based model (ABM) of hASC trafficking during acute skeletal muscle ischemia, based on over 150 literature-based rules instituted in Netlogo and MatLab software programs. In silico, trafficking phenomena within cell populations emerged as a result of the dynamic interactions between adhesion molecule expression, chemokine secretion, integrin affinity states, hemodynamics and microvascular network architectures. As verification, the model reasonably reproduced key aspects of ischemia and trafficking behavior including increases in wall shear stress, upregulation of key cellular adhesion molecules expressed on injured endothelium, increased secretion of inflammatory chemokines and cytokines, quantified levels of monocyte extravasation in selectin knockouts, and circulating monocyte rolling distances. Successful ABM verification prompted us to conduct a series of systematic knockouts in silico aimed at identifying the most critical parameters mediating hASC trafficking. Simulations predicted the necessity of an unknown selectin-binding molecule to achieve hASC extravasation, in addition to any rolling behavior mediated by hASC surface expression of CD15s, CD34, CD62e, CD62p, or CD65. In vitro experiments confirmed this prediction; a subpopulation of hASCs slowly rolled on immobilized P-selectin at speeds as low as 2 µm/s. Thus, our work led to a fundamentally new understanding of hASC biology, which may have important therapeutic implications.

Ischemic pathologies, such as acute myocardial infarction and peripheral vascular disease, continue to be associated with high morbidities and mortalities. Recently, therapies wherein adult stem cells are injected into the circulation have been shown to increase blood flow and help to restore tissue function following injury. Pre-clinical animal models and human trials have shown successes utilizing this approach, but variable trafficking efficiencies and low incorporation of cells into the injured tissue severely limit effectiveness and may preclude clinical adoption. To address this, we sought to study the complex process of how injected stem cells traffic through the microcirculation and home to sites of injury, in an effort to identify bottlenecks in this process that could be manipulated for therapeutic gain. We developed an agent-based computer model to speed the rate of discovery, and we identified a key cell–cell adhesion interaction that could be targeted to enhance stem cell homing efficiencies during injectable stem cell therapies.

Biomechanical Regulation of Endothelium-dependent Events Critical for Adaptive Remodeling

Alterations in hemodynamic shear stress acting on the vascular endothelium are critical for adaptive arterial remodeling. The molecular mechanisms regulating this process, however, remain largely uncharacterized. Here, we sought to define the responses evoked in endothelial cells exposed to shear stress waveforms characteristic of coronary collateral vessels and the subsequent paracrine effects on smooth muscle cells. A lumped parameter model of the human coronary collateral circulation was used to simulate normal and adaptive remodeling coronary collateral shear stress waveforms. These waveforms were then applied to cultured human endothelial cells (EC), and the resulting differences in EC gene expression were assessed by genome-wide transcriptional profiling to identify genes distinctly regulated by collateral flow. Analysis of these transcriptional programs identified several genes to be differentially regulated by collateral flow, including genes important for endothelium-smooth muscle interactions. In particular, the transcription factor KLF2 was up-regulated by the adaptive remodeling coronary collateral waveform, and several of its downstream targets displayed the expected modulation, including the down-regulation of connective tissue growth factor. To assess the effect of endothelial KLF2 expression on smooth muscle cell migration, a three-dimensional microfluidic assay was developed. Using this three-dimensional system, we showed that KLF2-expressing EC co-cultured with SMC significantly reduce SMC migration compared with control EC and that this reduction can be rescued by the addition of exogenous connective tissue growth factor. Collectively, these results demonstrate that collateral flow evokes distinct EC gene expression profiles and functional phenotypes that subsequently influence vascular events important for adaptive remodeling.

Stretch-Induced Activation of the Transcription Factor Activator Protein-1 Controls Monocyte Chemoattractant Protein-1 Expression During Arteriogenesis

Cerebral, coronary, and peripheral artery diseases combined represent the most frequent cause of death in developed nations. The underlying progressive occlusion of large conductance arteries can partially be compensated for by transformation of preexisting collateral arterioles to small artery bypasses, a process referred to as arteriogenesis. Because biomechanical forces have been implicated in the initiation of arteriogenesis, we have investigated the mechanosensitive expression of a pivotal proarteriogenic molecule, monocyte chemoattractant protein (MCP)-1, which governs the recruitment of circulating monocytes to the wall of the remodeling collateral arterioles. Using a new ear artery ligation model and the classic hindlimb ischemia model in mice, we noted that MCP-1 expression is significantly increased in collateral arterioles undergoing arteriogenesis already 24 hours after its onset. By mimicking proarteriogenic perfusion conditions in small mouse arteries, we observed that MCP-1 expression is predominantly upregulated in the smooth muscle cells, which solely sense changes in circumferential wall tension or stretch. Subsequent analyses of cultured endothelial and smooth muscle cells confirmed that cyclic stretch but not shear stress upregulates MCP-1 expression in these cells. Blockade of the mechanosensitive transcription factor activator protein-1 by using a specific decoy oligodeoxynucleotide abolished this stretch-induced MCP-1 expression. Likewise, topical administration of the decoy oligodeoxynucleotide to the mouse ear abrogated arteriogenesis through downregulation of MCP-1 expression and monocyte recruitment. Collectively, these findings point toward a stretch-induced activator protein-1–mediated rise in MCP-1 expression in vascular smooth muscle cells as a critical determinant for the initiation of arteriogenesis.

Emerging hurdles in stem cell therapy for peripheral vascular disease

Peripheral vascular disease (PVD) is a growing medical problem in Western societies and presents itself mainly in two different clinical forms. Intermittent claudication is an early moderate manifestation, while patients with critical limb ischemia suffer from severe muscle tissue loss or ulcers and are at high risk for limb amputation. Unfortunately, many patients cannot be helped with currently available surgical or endovascular revascularization procedures because of the complex anatomy of the vascular occlusion and/or the presence of other risk factors. Noninvasive stem cell therapy has been proposed as an alternative for such patients. Although pioneering clinical experience with stem cell-related therapy seems promising, it is too early for general clinical use of this technique, since many questions remain unanswered. Indeed, while questions about safety, dose, and administration route/timing/frequency are the first ones to be addressed when designing a stem cell-based clinical approach, there is accumulating evidence from recent (pre-)clinical studies that other issues may also be at stake. For instance, the choice of stem cells to be used and its precise mechanism of action, the need/possibility for concurrent tissue regeneration in case of irreversible tissue loss, the differentiation degree and specific vascular identity of the transplanted cells, and the long-term survival of engrafted cells in the absence of a normal supportive tissue environment should be well considered. Here, rather than presenting a comprehensive and extensive overview on the current literature on stem/progenitor cells and revascularization, we highlight some of the outstanding issues emerging from the recent (pre-)clinical literature that may codetermine the successful application of stem cells in a wide range of PVD patients in the future.

Growth factor-induced therapeutic neovascularization for ischaemic vascular disease: time for a re-evaluation?

PURPOSE OF REVIEW

Therapeutic angiogenesis and arteriogenesis represent an alternative treatment modality for patients with advanced ischaemic coronary or peripheral artery occlusive disease, who are unsuitable for standard revascularization procedures.

RECENT DEVELOPMENTS

Proof-of-concept evidence for therapeutic growth factor, both gene and protein-mediated neovascularization was provided in animal models of chronic myocardial and hindlimb ischaemia. Early human, phase I, trials utilizing the prototypical growth factor families, vascular endothelial growth factor and fibroblast growth factor, documented safety and suggested improvements in anginal symptoms and functional status. Large, randomized, placebo-controlled phase II/III clinical trials have, however, yielded variable results as such studies have suffered from significant limitations in therapeutic approach or design, which limits the ability to draw firm conclusions.

SUMMARY

Future trials must incorporate robust delivery strategies and address issues of study design including proper patient selection. Laboratory-based refinements in therapy, including a focus on the promotion of arteriogenesis and the modification of patient 'endotheliopathy', will all further enhance the potential of therapeutic neovascularization strategies.

Ischemia is not required for arteriogenesis in zebrafish embryos

OBJECTIVE

The role of ischemia in collateral vessel development (arteriogenesis) is a contentious issue that cannot be addressed using mammalian models. To investigate this, we developed models of arteriogenesis using the zebrafish embryo, which gains sufficient oxygenation via diffusion to prevent ischemia in response to arterial occlusion.

METHODS AND RESULTS

We studied gridlock mutant embryos that suffer a permanently occluded aorta and show that these restore aortic blood flow by collateral vessels. We phenocopied gridlock mutants by laser-induced proximal aortic occlusion in transgenic Fli1:eGFP/GATA1:dsRED embryos. Serial imaging showed these restore aortic blood flow via collateral vessels by recruitment of preexisting endothelium in a manner similar to gridlocks. Collateral aortic blood flow in gridlock mutants was dependent on both nitric oxide and myeloid cells. Confocal microscopy of transgenic gridlock/Fli1:eGFP mutants demonstrated no aberrant angiogenic response to the aortic occlusion. qPCR of HIF1alpha expression confirmed the absence of hypoxia in this model system.

CONCLUSIONS

We conclude that NO and myeloid cell-dependent collateral vessel development is an evolutionarily ancient response to arterial occlusion and is able to proceed in the absence of ischemia.

Insulin activates hypoxia-inducible factor-1alpha in human and rat vascular smooth muscle cells via phosphatidylinositol-3 kinase and mitogen-activated protein kinase pathways: impairment in insulin resistance owing to defects in insulin signalling

AIMS/HYPOTHESIS

We previously demonstrated that insulin stimulates vascular endothelial growth factor (VEGF) synthesis and secretion via phosphatidylinositol-3 kinase (PI3-K) and mitogen-activated protein kinase (MAPK) pathways in vascular smooth muscle cells (VSMC) from humans and from insulin-sensitive lean Zucker fa/+ rats. We also showed that this effect is attenuated in VSMC from insulin-resistant obese Zucker fa/fa rats. As it is not known whether the effects of insulin on VEGF involve activation of hypoxia-inducible factor-1 (HIF-1), we aimed to evaluate: (1) whether insulin modulates HIF-1alpha protein synthesis and activity; (2) the insulin signalling pathways involved; and (3) the role of insulin resistance.

METHODS

Using aortic VSMC taken from humans and Zucker rats and cultured in normoxia, the following were evaluated: (1) dose-dependent (0.5, 1, 2 nmol/l) and time-dependent (2, 4, 6 h) effects exerted by insulin on HIF-1alpha content in both nucleus and cytosol, measured by Western blots; (2) insulin effects on HIF-1 DNA-binding activity on the VEGF gene, measured by electrophoretic mobility shift assay; and (3) involvement of the insulin signalling molecules in these insulin actions, by using the following inhibitors: LY294002 (PI3-K), PD98059 (extracellular signal regulated kinase [ERK]), SP600125 (Jun N terminal kinase [JNK]), SB203580 (p38 mitogen-activated protein kinase) and rapamycin (mammalian target of rapamycin), and by detecting the insulin signalling molecules by Western blots.

RESULTS

In aortic VSMC from humans and Zucker fa/+ rats cultured in normoxia insulin increases the HIF-1alpha content in cytosol and nucleus via dose- and time-dependent mechanisms, and HIF-1 DNA-binding activity on the VEGF gene. The insulin-induced increase of HIF-1alpha is blunted by the translation inhibitor cycloheximide, LY294002, PD98059, SP600125 and rapamycin, but not by SB203580. It is also reduced in Zucker fa/fa rats, which present an impaired ability of insulin to induce Akt, ERK-1/2 and JNK-1/2 phosphorylation.

CONCLUSIONS/INTERPRETATION

These results provide a biological mechanism for the impaired collateral vessel formation in obesity.

Increased arterial load alters aortic structural and functional properties during embryogenesis

As in the adult dorsal aorta, the embryonic dorsal aorta is an important determinant of cardiovascular function, and increased stiffness may have secondary effects on cardiac and microcirculatory development. We previously showed that acutely and chronically increased arterial load via vitelline artery ligation (VAL) increases systemic arterial stiffness. To test the hypothesis that local dorsal aortic stiffness also increases, we measured aortic pulse-wave velocity (PWV) and assessed the active and passive properties (stress and strain) of isolated aortic segments. PWV along the dorsal aorta increased acutely and chronically after VAL. Analysis of isolated aortic active properties suggests that load-exposed aortas experienced higher stress, but not strain, at similar intraluminal pressures. When smooth muscle tone was relaxed, strain decreased in VAL vessels, whereas stress became similar to control vessels. Immunohistochemical analysis revealed that although aortic smooth muscle alpha-actin content was similar between groups, more cell layers expressed smooth muscle alpha-actin, and myocyte cell shape was markedly rounder in VAL embryos. Additionally, aortic and perivascular collagen type I and III content significantly increased in load-exposed VAL vessels. Increased production of these proteins is consistent with the observed increase in aortic PWV and decreased strain in VAL passive aortic segments. Thus the embryonic dorsal aorta is sensitive to increased arterial load and adapts by altering its material properties via changes in collagen content.