Endothelial Notch signaling is upregulated in human brain arteriovenous malformations and a mouse model of the disease

Pathophysiology in Brain Arteriovenous Malformations: Focus on Endothelial Dysfunctions and Endothelial-to-Mesenchymal Transition

Brain arteriovenous malformations (bAVMs) substantially increase the risk for intracerebral hemorrhage (ICH), which is associated with significant morbidity and mortality. However, the treatment options for bAVMs are severely limited, primarily relying on invasive methods that carry their own risks for intraoperative hemorrhage or even death. Currently, there are no pharmaceutical agents shown to treat this condition, primarily due to a poor understanding of bAVM pathophysiology. For the last decade, bAVM research has made significant advances, including the identification of novel genetic mutations and relevant signaling in bAVM development. However, bAVM pathophysiology is still largely unclear. Further investigation is required to understand the detailed cellular and molecular mechanisms involved, which will enable the development of safer and more effective treatment options. Endothelial cells (ECs), the cells that line the vascular lumen, are integral to the pathogenesis of bAVMs. Understanding the fundamental role of ECs in pathological conditions is crucial to unraveling bAVM pathophysiology. This review focuses on the current knowledge of bAVM-relevant signaling pathways and dysfunctions in ECs, particularly the endothelial-to-mesenchymal transition (EndMT).

Understanding the pathogenesis of brain arteriovenous malformation: genetic variations, epigenetics, signaling pathways, and immune inflammation

Brain arteriovenous malformation (BAVM) is a rare but serious cerebrovascular disease whose pathogenesis has not been fully elucidated. Studies have found that epigenetic regulation, genetic variation and their signaling pathways, immune inflammation, may be the cause of BAVM the main reason. This review comprehensively analyzes the key pathways and inflammatory factors related to BAVMs, and explores their interplay with epigenetic regulation and genetics. Studies have found that epigenetic regulation such as DNA methylation, non-coding RNAs and m6A RNA modification can regulate endothelial cell proliferation, apoptosis, migration and damage repair of vascular malformations through different target gene pathways. Gene defects such as KRAS, ACVRL1 and EPHB4 lead to a disordered vascular environment, which may promote abnormal proliferation of blood vessels through ERK, NOTCH, mTOR, Wnt and other pathways. PDGF-B and PDGFR-β were responsible for the recruitment of vascular adventitial cells and smooth muscle cells in the extracellular matrix environment of blood vessels, and played an important role in the pathological process of BAVM. Recent single-cell sequencing data revealed the diversity of various cell types within BAVM, as well as the heterogeneous expression of vascular-associated antigens, while neutrophils, macrophages and cytokines such as IL-6, IL-1, TNF-α, and IL-17A in BAVM tissue were significantly increased. Currently, there are no specific drugs targeting BAVMs, and biomarkers for BAVM formation, bleeding, and recurrence are lacking clinically. Therefore, further studies on molecular biological mechanisms will help to gain insight into the pathogenesis of BAVM and develop potential therapeutic strategies.

GNAQ R183Q somatic mutation contributes to aberrant arteriovenous specification in Sturge-Weber syndrome through Notch signaling

Episcleral vasculature malformation is a significant feature of Sturge-Weber syndrome (SWS) secondary glaucoma, the density and diameter of which are correlated with increased intraocular pressure. We previously reported that the GNAQ R183Q somatic mutation was located in the SWS episclera. However, the mechanism by which GNAQ R183Q leads to episcleral vascular malformation remains poorly understood. In this study, we investigated the correlation between GNAQ R183Q and episcleral vascular malformation via surgical specimens, human umbilical vein endothelial cells (HUVECs), and the HUVEC cell line EA.hy926. Our findings demonstrated a positive correlation between episcleral vessel diameter and the frequency of the GNAQ R183Q variant. Furthermore, the upregulation of genes from the Notch signaling pathway and abnormal coexpression of the arterial marker EphrinB2 and venous marker EphB4 were demonstrated in the scleral vasculature of SWS. Analysis of HUVECs overexpressing GNAQ R183Q in vitro confirmed the upregulation of Notch signaling and arterial markers. In addition, knocking down of Notch1 diminished the upregulation of arterial markers induced by GNAQ R183Q. Our findings strongly suggest that GNAQ R183Q leads to malformed episcleral vasculatures through Notch-induced aberrant arteriovenous specification. These insights into the molecular basis of episcleral vascular malformation will provide new pathways for the development of effective treatments for SWS secondary glaucoma.

Rbpj Deficiency Disrupts Vascular Remodeling via Abnormal Apelin and Cdc42 (Cell Division Cycle 42) Activity in Brain Arteriovenous Malformation

BACKGROUND

Brain arteriovenous malformations (bAVM) are characterized by enlarged blood vessels, which direct blood through arteriovenous shunts, bypassing the artery-capillary-vein network and disrupting blood flow. Clinically, bAVM treatments are invasive and not routinely applicable. There is critical need to understand mechanisms of bAVM pathologies and develop pharmacological therapies.

METHODS

We used an in vivo mouse model of Rbpj-mediated bAVM, which develops pathologies in the early postnatal period and an siRNA in vitro system to knockdown RBPJ in human brain microvascular endothelial cells (ECs). To understand molecular events regulated by endothelial Rbpj, we conducted RNA-Seq and chromatin immunoprecipitation-Seq analyses from isolated brain ECs.

RESULTS

Rbpj-deficient (mutant) brain ECs acquired abnormally rounded shape (with no change to cell area), altered basement membrane dynamics, and increased endothelial cell density along arteriovenous shunts, compared to controls, suggesting impaired remodeling of neonatal brain vasculature. Consistent with impaired endothelial cell dynamics, we found increased Cdc42 (cell division cycle 42) activity in isolated mutant ECs, suggesting that Rbpj regulates small GTPase (guanosine triphosphate hydrolase)-mediated cellular functions in brain ECs. siRNA-treated, RBPJ-deficient human brain ECs displayed increased Cdc42 activity, disrupted cell polarity and focal adhesion properties, and impaired migration in vitro. RNA-Seq analysis from isolated brain ECs identified differentially expressed genes in mutants, including Apelin, which encodes a ligand for G protein-coupled receptor signaling known to influence small GTPase activity. Chromatin immunoprecipitation-Seq analysis revealed chromatin loci occupied by Rbpj in brain ECs that corresponded to G-protein and Apelin signaling molecules. In vivo administration of a competitive peptide antagonist against the Apelin receptor (Aplnr/Apj) attenuated Cdc42 activity and restored endothelial cell morphology and arteriovenous connection diameter in Rbpj-mutant brain vessels.

CONCLUSIONS

Our data suggest that endothelial Rbpj promotes rearrangement of brain ECs during cerebrovascular remodeling, through Apelin/Apj-mediated small GTPase activity, and prevents bAVM. By inhibiting Apelin/Apj signaling in vivo, we demonstrated pharmacological prevention of Rbpj-mediated bAVM.

Nitric oxide synthase and reduced arterial tone contribute to arteriovenous malformation

Mechanisms underlying arteriovenous malformations (AVMs) are poorly understood. Using mice with endothelial cell (EC) expression of constitutively active Notch4 (Notch4*EC), we show decreased arteriolar tone in vivo during brain AVM initiation. Reduced vascular tone is a primary effect of Notch4*EC, as isolated pial arteries from asymptomatic mice exhibited reduced pressure-induced arterial tone ex vivo. The nitric oxide (NO) synthase (NOS) inhibitor NG-nitro-l-arginine (L-NNA) corrected vascular tone defects in both assays. L-NNA treatment or endothelial NOS (eNOS) gene deletion, either globally or specifically in ECs, attenuated AVM initiation, assessed by decreased AVM diameter and delayed time to moribund. Administering nitroxide antioxidant 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl also attenuated AVM initiation. Increased NOS-dependent production of hydrogen peroxide, but not NO, superoxide, or peroxynitrite was detected in isolated Notch4*EC brain vessels during AVM initiation. Our data suggest that eNOS is involved in Notch4*EC-mediated AVM formation by up-regulating hydrogen peroxide and reducing vascular tone, thereby permitting AVM initiation and progression.

Oscillatory shear stress modulates Notch-mediated endothelial mesenchymal plasticity in cerebral arteriovenous malformations

BACKGROUND

Cerebral arteriovenous malformations (cAVM) are a significant cause of intracranial hemorrhagic stroke and brain damage. The arteriovenous junctions in AVM nidus are known to have hemodynamic disturbances such as altered shear stress, which could lead to endothelial dysfunction. The molecular mechanisms coupling shear stress and endothelial dysfunction in cAVMs are poorly understood. We speculated that disturbed blood flow in artery-vein junctions activates Notch receptors and promotes endothelial mesenchymal plasticity during cAVM formation.

METHODS

We investigated the expression profile of endothelial mesenchymal transition (EndMT) and cell adhesion markers, as well as activated Notch receptors, in 18 human cAVM samples and 15 control brain tissues, by quantitative real-time PCR (qRT-PCR) and immunohistochemical evaluation. Employing a combination of a microfluidic system, qRT-PCR, immunofluorescence, as well as invasion and inhibitor assays, the effects of various shear stress conditions on Notch-induced EndMT and invasive potential of human cerebral microvascular endothelial cells (hCMEC/d3) were analyzed.

RESULTS

We found evidence for EndMT and enhanced expression of activated Notch intracellular domain (NICD3 and NICD4) in human AVM nidus samples. The expression of transmembrane adhesion receptor integrin α9/β1 is significantly reduced in cAVM nidal vessels. Cell-cell adhesion proteins such as VE-cadherin and N-cadherin were differentially expressed in AVM nidus compared with control brain tissues. Using well-characterized hCMECs, we show that altered fluid shear stress steers Notch3 nuclear translocation and promotes SNAI1/2 expression and nuclear localization. Oscillatory flow downregulates integrin α9/β1 and VE-cadherin expression, while N-cadherin expression and endothelial cell invasiveness are augmented. Gamma-secretase inhibitor RO4929097, and to a lesser level DAPT, prevent the mesenchymal transition and invasiveness of cerebral microvascular endothelial cells exposed to oscillatory fluid flow.

CONCLUSIONS

Our study provides, for the first time, evidence for the role of oscillatory shear stress in mediating the EndMT process and dysregulated expression of cell adhesion molecules, especially multifunctional integrin α9/β1 in human cAVM nidus. Concomitantly, our findings indicate the potential use of small-molecular inhibitors such as RO4929097 in the less-invasive therapeutic management of cAVMs.

Notch Signaling in the Vasculature: Angiogenesis and Angiocrine Functions

Formation of a functional blood vessel network is a complex process tightly controlled by pro- and antiangiogenic signals released within the local microenvironment or delivered through the bloodstream. Endothelial cells precisely integrate such temporal and spatial changes in extracellular signals and generate an orchestrated response by modulating signaling transduction, gene expression, and metabolism. A key regulator in vessel formation is Notch signaling, which controls endothelial cell specification, proliferation, migration, adhesion, and arteriovenous differentiation. This review summarizes the molecular biology of endothelial Notch signaling and how it controls angiogenesis and maintenance of the established, quiescent vasculature. In addition, recent progress in the understanding of Notch signaling in endothelial cells for controlling organ homeostasis by transcriptional regulation of angiocrine factors and its relevance to disease will be discussed.

Epigenome-Wide Analysis of DNA Methylation and Optimism in Women and Men

OBJECTIVE

Higher optimism is associated with reduced mortality and a lower risk of age-related chronic diseases. DNA methylation (DNAm) may provide insight into mechanisms underlying these relationships. We hypothesized that DNAm would differ among older individuals who are more versus less optimistic.

METHODS

Using cross-sectional data from two population-based cohorts of women with diverse races/ethnicities ( n = 3816) and men (only White, n = 667), we investigated the associations of optimism with epigenome-wide leukocyte DNAm. Random-effects meta-analyses were subsequently used to pool the individual results. Significantly differentially methylated cytosine-phosphate-guanines (CpGs) were identified by the "number of independent degrees of freedom" approach: effective degrees of freedom correction using the number of principal components (PCs), explaining >95% of the variation of the DNAm data (PC-correction). We performed regional analyses using comb-p and pathway analyses using the Ingenuity Pathway Analysis software.

RESULTS

We found that essentially all CpGs (total probe N = 359,862) were homogeneous across sex and race/ethnicity in the DNAm-optimism association. In the single CpG site analyses based on homogeneous CpGs, we identified 13 significantly differentially methylated probes using PC-correction. We found four significantly differentially methylated regions and two significantly differentially methylated pathways. The annotated genes from the single CpG site and regional analyses are involved in psychiatric disorders, cardiovascular disease, cognitive impairment, and cancer. Identified pathways were related to cancer, and neurodevelopmental and neurodegenerative disorders.

CONCLUSION

Our findings provide new insights into possible mechanisms underlying optimism and health.

Endothelial Rbpj deletion normalizes Notch4-induced brain arteriovenous malformation in mice

Upregulation of Notch signaling is associated with brain arteriovenous malformation (bAVM), a disease that lacks pharmacological treatments. Tetracycline (tet)-regulatable endothelial expression of constitutively active Notch4 (Notch4*tetEC) from birth induced bAVMs in 100% of mice by P16. To test whether targeting downstream signaling, while sustaining the causal Notch4*tetEC expression, induces AVM normalization, we deleted Rbpj, a mediator of Notch signaling, in endothelium from P16, by combining tet-repressible Notch4*tetEC with tamoxifen-inducible Rbpj deletion. Established pathologies, including AV connection diameter, AV shunting, vessel tortuosity, intracerebral hemorrhage, tissue hypoxia, life expectancy, and arterial marker expression were improved, compared with Notch4*tetEC mice without Rbpj deletion. Similarly, Rbpj deletion from P21 induced advanced bAVM regression. After complete AVM normalization induced by repression of Notch4*tetEC, virtually no bAVM relapsed, despite Notch4*tetEC re-expression in adults. Thus, inhibition of endothelial Rbpj halted Notch4*tetEC bAVM progression, normalized bAVM abnormalities, and restored microcirculation, providing proof of concept for targeting a downstream mediator to treat AVM pathologies despite a sustained causal molecular lesion.

Molecular and genetic mechanisms in brain arteriovenous malformations: new insights and future perspectives

Brain arteriovenous malformations (bAVMs) are rare vascular lesions made of shunts between cerebral arteries and veins without the interposition of a capillary bed. The majority of bAVMs are asymptomatic, but some may be revealed by seizures and potentially life-threatening brain hemorrhage. The management of unruptured bAVMs remains a matter of debate. Significant progress in the understanding of their pathogenesis has been made during the last decade, particularly using genome sequencing and biomolecular analysis. Herein, we comprehensively review the recent molecular and genetic advances in the study of bAVMs that not only allow a better understanding of the genesis and growth of bAVMs, but also open new insights in medical treatment perspectives.

Cellular loci involved in the development of brain arteriovenous malformations

Brain arteriovenous malformations (bAVMs) are abnormal vessels that are prone to rupture, causing life-threatening intracranial bleeding. The mechanism of bAVM formation is poorly understood. Nevertheless, animal studies revealed that gene mutation in endothelial cells (ECs) and angiogenic stimulation are necessary for bAVM initiation. Evidence collected through analyzing bAVM specimens of human and mouse models indicate that cells other than ECs also are involved in bAVM pathogenesis. Both human and mouse bAVMs vessels showed lower mural cell-coverage, suggesting a role of pericytes and vascular smooth muscle cells (vSMCs) in bAVM pathogenesis. Perivascular astrocytes also are important in maintaining cerebral vascular function and take part in bAVM development. Furthermore, higher inflammatory cytokines in bAVM tissue and blood demonstrate the contribution of inflammatory cells in bAVM progression, and rupture. The goal of this paper is to provide our current understanding of the roles of different cellular loci in bAVM pathogenesis.

A human model of arteriovenous malformation (AVM)-on-a-chip reproduces key disease hallmarks and enables drug testing in perfused human vessel networks

Brain arteriovenous malformations (AVMs) are a disorder wherein abnormal, enlarged blood vessels connect arteries directly to veins, without an intervening capillary bed. AVMs are one of the leading causes of hemorrhagic stroke in children and young adults. Most human sporadic brain AVMs are associated with genetic activating mutations in the KRAS gene. Our goal was to develop an in vitro model that would allow for simultaneous morphological and functional phenotypic data capture in real time during AVM disease progression. By generating human endothelial cells harboring a clinically relevant mutation found in most human patients (activating mutations within the small GTPase KRAS) and seeding them in a dynamic microfluidic cell culture system that enables vessel formation and perfusion, we demonstrate that vessels formed by KRAS4A^G12V mutant endothelial cells (ECs) were significantly wider and more leaky than vascular beds formed by wild-type ECs, recapitulating key structural and functional hallmarks of human AVM pathogenesis. Immunofluorescence staining revealed a breakdown of adherens junctions in mutant KRAS vessels, leading to increased vascular permeability, a hallmark of hemorrhagic stroke. Finally, pharmacological blockade of MEK kinase activity, but not PI3K inhibition, improved endothelial barrier function (decreased permeability) without affecting vessel diameter. Collectively, our studies describe the creation of human KRAS-dependent AVM-like vessels in vitro in a self-assembling microvessel platform that is amenable to phenotypic observation and drug delivery.

De novo brain AVM following radiotherapy for cerebral cavernous malformation in a child: A 15-year clinical course

Multiple de novo brain arteriovenous malformations (bAVM) have been reported in the literature, raising questions about the contended purely congenital nature of these lesions. We present the 15-year course of a pediatric patient, who initially presented at age 5 with a thalamic cavernous malformation and was treated with radiosurgery, and then subsequently developed a thalamic de novo bAVM immediately adjacent to the initial lesion location, discovered 2 years later. Treatment of the bAVM entailed two transarterial embolizations and one radiosurgery session which ultimately led to complete angiographic resolution. Finally, this patient's course was complicated by intraparenchymal hemorrhage and acute obstructive hydrocephalus, and further imaging revealed two newly formed cavernous malformations, also associated with the initial lesion's location, that have remained stable since their formation. This case likely represents the second-hit model for the formation of vascular malformations, as sparsely supported by the current literature. According to this, genetically aberrant, yet quiescent, brain areas might promote the de novo formation of vascular malformations after brain injury, including radiation.

De novo brain arteriovenous malformation formation and development: A case report

BACKGROUND

Brain arteriovenous malformation (AVM), an aberrant vascular development during the intrauterine period, is traditionally considered a congenital disease. Sporadic reports of cases of de novo AVM formation in children and adults have challenged the traditional view of its congenital origin.

CASE SUMMARY

In this report, we have presented the case of a child with a de novo brain AVM. Magnetic resonance imaging and magnetic resonance angiography of the brain showed no AVM at the age of 5 years and 2 mo. Brain AVM was first detected in this child at the age of 7 years and 4 mo. The brain AVM was significantly advanced, and hemorrhage was seen for the first time at the age of 12 years and 8 mo. There was further progression in the AVM, and hemorrhage occurred again at the age of 13 years and 5 mo. Genetic analysis of this patient revealed a mutation in the NOTCH2 (p.Asp473Val) gene.

CONCLUSION

In short, our case has once again confirmed the view that brain AVM is an acquired disease and is the result of the interaction of genes, environment, and molecules.

Rethinking growth factors: the case of BMP9 during vessel maturation

Angiogenesis is an essential process for correct development and physiology. This mechanism is tightly regulated by many signals that activate several pathways, which are constantly interacting with each other. There is mounting evidence that BMP9/ALK1 pathway is essential for a correct vessel maturation. Alterations in this pathway lead to the development of hereditary haemorrhagic telangiectasias. However, little was known about the BMP9 signalling cascade until the last years. Recent reports have shown that while BMP9 arrests cell cycle, it promotes the activation of anabolic pathways to enhance endothelial maturation. In light of this evidence, a new criterion for the classification of cytokines is proposed here, based on the physiological objective of the activation of anabolic routes. Whether this activation by a growth factor is needed to sustain mitosis or to promote a specific function such as matrix formation is a critical characteristic that needs to be considered to classify growth factors. Hence, the state-of-the-art of BMP9/ALK1 signalling is reviewed here, as well as its implications in normal and pathogenic angiogenesis.

Review of treatment and therapeutic targets in brain arteriovenous malformation

Brain arteriovenous malformations (bAVM) are an important cause of intracranial hemorrhage (ICH), especially in younger patients. The pathogenesis of bAVM are largely unknown. Current understanding of bAVM etiology is based on studying genetic syndromes, animal models, and surgically resected specimens from patients. The identification of activating somatic mutations in the Kirsten rat sarcoma viral oncogene homologue (KRAS) gene and other mitogen-activated protein kinase (MAPK) pathway genes has opened up new avenues for bAVM study, leading to a paradigm shift to search for somatic, de novo mutations in sporadic bAVMs instead of focusing on inherited genetic mutations. Through the development of new models and understanding of pathways involved in maintaining normal vascular structure and functions, promising therapeutic targets have been identified and safety and efficacy studies are underway in animal models and in patients. The goal of this paper is to provide a thorough review or current diagnostic and treatment tools, known genes and key pathways involved in bAVM pathogenesis to summarize current treatment options and potential therapeutic targets uncovered by recent discoveries.

Molecular Biomarkers and Drug Targets in Brain Arteriovenous and Cavernous Malformations: Where Are We?

Vascular malformations of the brain (VMB) comprise abnormal development of blood vessels. A small fraction of VMBs causes hemorrhages with neurological morbidity and risk of mortality in patients. Most often, they are symptomatically silent and are detected at advanced stages of disease progression. The most common forms of VMBs are arteriovenous and cavernous malformations in the brain. Radiopathological features of these diseases are complex with high phenotypic variability. Early detection of these malformations followed by preclusion of severe neurological deficits such as hemorrhage and stroke is crucial in the clinical management of patients with VMBs. The technological advances in high-throughput omics platforms have currently infused a zest in translational research in VMBs. Besides finding novel biomarkers and therapeutic targets, these studies have withal contributed significantly to the understanding of the etiopathogenesis of VMBs. Here we discuss the recent advances in predictive and prognostic biomarker research in sporadic and familial arteriovenous malformations as well as cerebral cavernous malformations. Furthermore, we analyze the clinical applicability of protein and noncoding RNA-based molecular-targeted therapies which may have a potentially key role in disease management.

Molecular Signature of Brain Arteriovenous Malformation Hemorrhage: A Systematic Review

BACKGROUND

The mechanisms of brain arteriovenous malformation (bAVM) development, formation, and progress are still poorly understood. By gaining more knowledge about the molecular signature of bAVM in relation to hemorrhage, we might be able to find biomarkers associated with this serious complication, which can function as a goal for further research and can be a potential target for gene therapy.

AIMS

To provide a comprehensive overview of the molecular signature of bAVM-related hemorrhage We conducted a systematic review, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, of articles published in Embase, Medline, Cochrane central, Scopus, and Chinese databases (CNKI, Wanfang).

SUMMARY OF REVIEW

Our search identified 3944 articles, of which 3108 remained after removal of duplicates. After title, abstract, and full-text screening, 31 articles were included for analysis. The results show an overview of molecular characteristics. Several genetic polymorphisms are identified that increase the risk of bAVM rupture by increasing the expression of certain inflammatory cytokines (interleukin [IL]-6, IL-17A, IL-1β, and tumor necrosis factor-α), NOTCH pathways, matrix metalloproteinase-9, and vascular endothelial growth factor-α.

CONCLUSIONS

Several molecular factors are associated with the risk of bAVM-related hemorrhage. These factors are associated with increased inflammation on the cellular level and changes in the endothelium leading to instability of the vessel wall. Further investigation of these biomarkers regarding hemorrhage rates, together with their relationship with noninvasive diagnostic methods, should be a goal of future studies to improve the patient specific risk estimation and future treatment options.

Progress and prospects of mechanotransducers in shear stress-sensitive signaling pathways in association with arteriovenous malformation

Arteriovenous malformations are congenital vascular lesions characterized by a direct and tangled connection between arteries and veins, which disrupts oxygen circulation and normal blood flow. Arteriovenous malformations often occur in the patient with hereditary hemorrhagic telangiectasia. The attempts to elucidate the causative factors and pathogenic mechanisms of arteriovenous malformations are now still in progress. Some studies reported that shear stress in blood flow is one of the factors involved in arteriovenous malformations manifestation. Through several mechanotransducers harboring the endothelial cells membrane, the signal from shear stress is transduced towards the responsible signaling pathways in endothelial cells to maintain cell homeostasis. Any disruption in this well-established communication will give rise to abnormal endothelial cells differentiation and specification, which will later promote arteriovenous malformations. In this review, we discuss the update of several mechanotransducers that have essential roles in shear stress-induced signaling pathways, such as activin receptor-like kinase 1, Endoglin, Notch, vascular endothelial growth factor receptor 2, Caveolin-1, Connexin37, and Connexin40. Any disruption of these signaling potentially causes arteriovenous malformations. We also present some recent insights into the fundamental analysis, which attempts to determine potential and alternative solutions to battle arteriovenous malformations, especially in a less invasive and risky way, such as gene treatments.

Effects of serum starvation and vascular endothelial growth factor stimulation on the expression of Notch signalling pathway components

Brain arteriovenous malformation (BAVM) is an abnormality in the cerebral vascular system. Although the upregulation of the Notch signalling pathway is a deterministic factor in BAVM, the mechanism by which this pathway is upregulated in patients with BAVM is uncertain. The effects of serum starvation and vascular endothelial growth factor (VEGF) stimulation on the Notch signalling pathway in brain microvascular endothelial cells (MECs) and mouse embryonic stem (mES)/embryoid body (EB)-derived endothelial cells were investigated in this study. The duration of serum starvation and VEGF concentration were changed, cell viability was measured, and reasonable time and concentration gradients were selected for subsequent studies. Protein and mRNA expression levels of Notch signalling pathway components in both MECs and mES/EB-derived endothelial cells were detected using western blotting and real-time PCR, respectively. Expression levels of the Notch1, Notch4, Jagged1, delta-like ligand 4 (Dll4) and Hes1 proteins and mRNAs were upregulated by lower VEGF concentrations and shorter-term serum starvation but inhibited by higher VEGF concentrations and longer-term serum starvation. This study revealed effects of changes in the duration of serum starvation and VEGF concentration on the expression of Notch signalling pathway components in both MECs and mES/EB-derived endothelial cells, potentially contributing to BAVM formation.

An Update on Medications for Brain Arteriovenous Malformations

Despite a variety of treatment options for brain arteriovenous malformations (bAVMs), many lesions remain challenging to treat and present significant ongoing risk for hemorrhage. In Vitro investigations have recently led to a greater understanding of the formation, growth, and rupture of bAVMs. This has, in turn, led to the development of therapeutic targets for medications for bAVMs, some of which have begun testing in clinical trials in humans. These include bevacizumab, targeting the vascular endothelial growth factor driven angiogenic pathway; thalidomide or lenalidomide, targeting blood-brain barrier impairment; and doxycycline, targeting matrix metalloproteinase overexpression. A variety of other medications appear promising but either requires adaptation from other disease states or development from early bench studies into the clinical realm. This review aims to provide an overview of the current state of development of medications targeting bAVMs and to highlight their likely applications in the future.

Aberrant Activation of Notch1 Signaling in Glomerular Endothelium Induces Albuminuria

RATIONALE

Glomerular capillaries are lined with a highly specialized fenestrated endothelium and contribute to the glomerular filtration barrier. The Notch signaling pathway is involved in regulation of glomerular filtration barrier, but its role in glomerular endothelium has not been investigated due to the embryonic lethality of animal models with genetic modification of Notch pathway components in the endothelium.

OBJECTIVE

To determine the effects of aberrant activation of the Notch signaling in glomerular endothelium and the underlying molecular mechanisms.

METHODS AND RESULTS

We established the ZEG-NICD1 (notch1 intracellular domain)/Tie2-tTA/Tet-O-Cre transgenic mouse model to constitutively activate Notch1 signaling in endothelial cells of adult mice. The triple transgenic mice developed severe albuminuria with significantly decreased VE-cadherin (vascular endothelial cadherin) expression in the glomerular endothelium. In vitro studies showed that either NICD1 (Notch1 intracellular domain) lentiviral infection or treatment with Notch ligand DLL4 (delta-like ligand 4) markedly reduced VE-cadherin expression and increased monolayer permeability of human renal glomerular endothelial cells. In addition, Notch1 activation or gene knockdown of VE-cadherin reduced the glomerular endothelial glycocalyx. Further investigation demonstrated that activated Notch1 suppression of VE-cadherin was through the transcription factors SNAI1 (snail family transcriptional repressor 1) and ERG (Ets related gene), which bind to the -373 E-box and the -134/-118 ETS (E26 transformation-specific) element of the VE-cadherin promoter, respectively.

CONCLUSIONS

Our results reveal novel regulatory mechanisms whereby endothelial Notch1 signaling dictates the level of VE-cadherin through the transcription factors SNAI1 and ERG, leading to dysfunction of glomerular filtration barrier and induction of albuminuria. Graphic Abstract: A graphic abstract is available for this article.

Brain vascular biology

The complex development of the brain vascular system can be broken down by embryonic stages and anatomic locations, which are tightly regulated by different factors and pathways in time and spatially. The adult brain is relatively quiescent in angiogenesis. However, under disease conditions, such as trauma, stroke, or tumor, angiogenesis can be activated in the adult brain. Disruption of any of the factors or pathways may lead to malformed vessel development. In this chapter, we will discuss factors and pathways involved in normal brain vasculogenesis and vascular maturation, and the pathogenesis of several brain vascular malformations.

Notch in mechanotransduction - from molecular mechanosensitivity to tissue mechanostasis

Tissue development and homeostasis are controlled by mechanical cues. Perturbation of the mechanical equilibrium triggers restoration of mechanostasis through changes in cell behavior, while defects in these restorative mechanisms lead to mechanopathologies, for example, osteoporosis, myopathies, fibrosis or cardiovascular disease. Therefore, sensing mechanical cues and integrating them with the biomolecular cell fate machinery is essential for the maintenance of health. The Notch signaling pathway regulates cell and tissue fate in nearly all tissues. Notch activation is directly and indirectly mechanosensitive, and regulation of Notch signaling, and consequently cell fate, is integral to the cellular response to mechanical cues. Fully understanding the dynamic relationship between molecular signaling, tissue mechanics and tissue remodeling is challenging. To address this challenge, engineered microtissues and computational models play an increasingly large role. In this Review, we propose that Notch takes on the role of a 'mechanostat', maintaining the mechanical equilibrium of tissues. We discuss the reciprocal role of Notch in the regulation of tissue mechanics, with an emphasis on cardiovascular tissues, and the potential of computational and engineering approaches to unravel the complex dynamic relationship between mechanics and signaling in the maintenance of cell and tissue mechanostasis.

N6-methyladenosine methyltransferase METTL3 affects the phenotype of cerebral arteriovenous malformation via modulating Notch signaling pathway

Background

Cerebral arteriovenous malformation (AVM) is a serious life-threatening congenital cerebrovascular disease. Specific anatomical features, such as nidus size, location, and venous drainage, have been validated to affect treatment outcomes. Until recently, molecular biomarkers and corresponding molecular mechanism related to anatomical features and treatment outcomes remain unknown.

Methods

RNA N6-methyladenosine (m⁶A) Methyltransferase METTL3 was identified as a differentially expressed gene in groups with different lesion sizes by analyzing the transcriptome sequencing (RNA-seq) data. Tube formation and wound healing assays were performed to investigate the effect of METTL3 on angiogenesis. In addition, Methylated RNA Immunoprecipitation Sequencing technology (MeRIP-seq) was performed to screen downstream targets of METTL3 in endothelial cells and to fully clarify the specific underlying molecular mechanisms affecting the phenotype of cerebral AVM.

Results

In the current study, we found that the expression level of METTL3 was reduced in the larger pathological tissues of cerebral AVMs. Moreover, knockdown of METTL3 significantly affected angiogenesis of the human endothelial cells. Mechanistically, down-regulation of METTL3 reduced the level of heterodimeric Notch E3 ubiquitin ligase formed by DTX1 and DTX3L, thereby continuously activating the Notch signaling pathway. Ultimately, the up-regulated downstream genes of Notch signaling pathway dramatically affected the angiogenesis of endothelial cells. In addition, we demonstrated that blocking Notch pathway with DAPT could restore the phenotype of METTL3 deficient endothelial cells.

Conclusions

Our findings revealed the mechanism by which m⁶A modification regulated the angiogenesis and might provide potential biomarkers to predict the outcome of treatment, as well as provide suitable pharmacological targets for preventing the formation and progression of cerebral AVM.

Pathogenesis of non-hereditary brain arteriovenous malformation and therapeutic implications

Brain arteriovenous malformations have a high risk of intracranial hemorrhage, which is a substantial cause of morbidity and mortality in patients with brain arteriovenous malformations. Although a variety of genetic factors leading to hereditary brain arteriovenous malformations have been extensively investigated, their pathogenesis is still not well elucidated, especially in sporadic brain arteriovenous malformations. The authors have reviewed the updated data of not only the genetic aspects of sporadic brain arteriovenous malformations, but also the architecture of microvasculature, the roles of the angiogenic factors, and the signaling pathways. This knowledge may allow us to infer the pathogenesis of sporadic brain arteriovenous malformations and develop pre-emptive treatments for them.

Defective vascular signaling & prospective therapeutic targets in brain arteriovenous malformations

The neurovascular unit is composed of endothelial cells, vascular smooth muscle cells, pericytes, astrocytes and neurons. Through tightly regulated multi-directional cell signaling, the neurovascular unit is responsible for the numerous functionalities of the cerebrovasculature - including the regulation of molecular and cellular transport across the blood-brain barrier, angiogenesis, blood flow responses to brain activation and neuroinflammation. Historically, the study of the brain vasculature focused on endothelial cells; however, recent work has demonstrated that pericytes and vascular smooth muscle cells - collectively known as mural cells - play critical roles in many of these functions. Given this emerging data, a more complete mechanistic understanding of the cellular basis of brain vascular malformations is needed. In this review, we examine the integrated functions and signaling within the neurovascular unit necessary for normal cerebrovascular structure and function. We then describe the role of aberrant cell signaling within the neurovascular unit in brain arteriovenous malformations and identify how these pathways may be targeted therapeutically to eradicate or stabilize these lesions.

Integrated Analysis of Whole Exome Sequencing and Copy Number Evaluation in Parkinson's Disease

Genetic studies of the familial forms of Parkinson’s disease (PD) have identified a number of causative genes with an established role in its pathogenesis. These genes only explain a fraction of the diagnosed cases. The emergence of Next Generation Sequencing (NGS) expanded the scope of rare variants identification in novel PD related genes. In this study we describe whole exome sequencing (WES) genetic findings of 60 PD patients with 125 variants validated in 51 of these cases. We used strict criteria for variant categorization that generated a list of variants in 20 genes. These variants included loss of function and missense changes in 18 genes that were never previously linked to PD (NOTCH4, BCOR, ITM2B, HRH4, CELSR1, SNAP91, FAM174A, BSN, SPG7, MAGI2, HEPHL1, EPRS, PUM1, CLSTN1, PLCB3, CLSTN3, DNAJB9 and NEFH) and 2 genes that were previously associated with PD (EIF4G1 and ATP13A2). These genes either play a critical role in neuronal function and/or have mouse models with disease related phenotypes. We highlight NOTCH4 as an interesting candidate in which we identified a deleterious truncating and a splice variant in 2 patients. Our combined molecular approach provides a comprehensive strategy applicable for complex genetic disorders.

Notch signaling pathway is a potential therapeutic target for extracranial vascular malformations

Notch expression has been shown to be aberrant in brain arteriovenous malformations (AVM), and targeting Notch has been suggested as an approach to their treatment. It is unclear whether extracranial vascular malformations follow the same patterning and Notch pathway defects. In this study, we examined human extracranial venous (VM) (n = 3), lymphatic (LM) (n = 10), and AV (n = 6) malformations, as well as sporadic brain AVMs (n = 3). In addition to showing that extracranial AVMs demonstrate interrupted elastin and that AVMs and LMs demonstrate abnormal α-smooth muscle actin just as brain AVMS do, our results demonstrate that NOTCH1, 2, 3 and 4 proteins are overexpressed to varying degrees in both the endothelial and mural lining of the malformed vessels in all types of malformations. We further show that two gamma secretase inhibitors (GSIs), DAPT (GSI-IX) and RO4929097, cause dose-dependent inhibition of Notch target gene expression (Hey1) and rate of migration of monolayer cultures of lymphatic endothelial cells (hLECs) and blood endothelial cells (HUVEC). GSIs also inhibit HUVEC network formation. hLECs are more sensitive to GSIs compared to HUVEC. GSIs have been found to be safe in clinical trials in patients with Alzheimer’s disease or cancer. Our results provide further rationale to support testing of Notch inhibitors in patients with extracranial vascular malformations.

De novo brain arteriovenous malformation after tumor resection: case report and literature review

The congenital origin of brain arteriovenous malformations (bAVMs) has been increasingly challenged by reports of de novo bAVMs in patients previously confirmed to have no vascular malformation. We describe the oldest patient reported in the English language literature harboring a de novo bAVM. An uneventful frontal convexity meningioma resection was performed for a 60-year-old woman, and at 67 years of age, a bAVM was detected by MRI and confirmed by digital subtraction angiography at the site of the previous meningioma resection. This case adds to the growing literature that the etiology of bAVMs is most likely multifactorial.

Extracellular bone morphogenetic protein modulator BMPER and twisted gastrulation homolog 1 preserve arterial-venous specification in zebrafish blood vessel development and regulate Notch signaling in endothelial cells

The bone morphogenetic protein (BMP) signaling pathway plays a central role during vasculature development. Mutations or dysregulation of the BMP pathway members have been linked to arteriovenous malformations. In the present study, we investigated the effect of the BMP modulators bone morphogenetic protein endothelial precursor-derived regulator (BMPER) and twisted gastrulation protein homolog 1 (TWSG1) on arteriovenous specification during zebrafish development and analyzed downstream Notch signaling pathway in human endothelial cells. Silencing of bmper and twsg1b in zebrafish embryos by morpholinos resulted in a pronounced enhancement of venous ephrinB4a marker expression and concomitant dysregulated arterial ephrinb2a marker expression detected by in situ hybridization. As arteriovenous specification was disturbed, we assessed the impact of BMPER and TWSG1 protein stimulation on the Notch signaling pathway on endothelial cells from different origin. Quantitative real-time PCR (qRT-PCR) and western blot analysis showed increased expression of Notch target gene hairy and enhancer of split, HEY1/2 and EPHRINB2. Consistently, silencing of BMPER in endothelial cells by siRNAs decreased Notch signaling and downstream effectors. BMP receptor antagonist DMH1 abolished BMPER and BMP4 induced Notch signaling pathway activation. In conclusion, we found that in endothelial cells, BMPER and TWSG1 are necessary for regular Notch signaling activity and in zebrafish embryos BMPER and TWSG1 preserve arteriovenous specification to prevent malformations.

Somatic Activating KRAS Mutations in Arteriovenous Malformations of the Brain

BACKGROUND

Sporadic arteriovenous malformations of the brain, which are morphologically abnormal connections between arteries and veins in the brain vasculature, are a leading cause of hemorrhagic stroke in young adults and children. The genetic cause of this rare focal disorder is unknown.

METHODS

We analyzed tissue and blood samples from patients with arteriovenous malformations of the brain to detect somatic mutations. We performed exome DNA sequencing of tissue samples of arteriovenous malformations of the brain from 26 patients in the main study group and of paired blood samples from 17 of those patients. To confirm our findings, we performed droplet digital polymerase-chain-reaction (PCR) analysis of tissue samples from 39 patients in the main study group (21 with matching blood samples) and from 33 patients in an independent validation group. We interrogated the downstream signaling pathways, changes in gene expression, and cellular phenotype that were induced by activating KRAS mutations, which we had discovered in tissue samples.

RESULTS

We detected somatic activating KRAS mutations in tissue samples from 45 of the 72 patients and in none of the 21 paired blood samples. In endothelial cell-enriched cultures derived from arteriovenous malformations of the brain, we detected KRAS mutations and observed that expression of mutant KRAS (KRAS^G12V) in endothelial cells in vitro induced increased ERK (extracellular signal-regulated kinase) activity, increased expression of genes related to angiogenesis and Notch signaling, and enhanced migratory behavior. These processes were reversed by inhibition of MAPK (mitogen-activated protein kinase)-ERK signaling.

CONCLUSIONS

We identified activating KRAS mutations in the majority of tissue samples of arteriovenous malformations of the brain that we analyzed. We propose that these malformations develop as a result of KRAS-induced activation of the MAPK-ERK signaling pathway in brain endothelial cells. (Funded by the Swiss Cancer League and others.).

Control of Blood Vessel Formation by Notch Signaling

Blood vessels span throughout the body to nourish tissue cells and to provide gateways for immune surveillance. Endothelial cells that line capillaries have the remarkable capacity to be quiescent for years but to switch rapidly into the activated state once new blood vessels need to be formed. In addition, endothelial cells generate niches for progenitor and tumor cells and provide organ-specific paracrine (angiocrine) factors that control organ development and regeneration, maintenance of homeostasis and tumor progression. Recent data indicate a pivotal role for blood vessels in responding to metabolic changes and that endothelial cell metabolism is a novel regulator of angiogenesis. The Notch pathway is the central signaling mode that cooperates with VEGF, WNT, BMP, TGF-β, angiopoietin signaling and cell metabolism to orchestrate angiogenesis, tip/stalk cell selection and arteriovenous specification. Here, we summarize the current knowledge and implications regarding the complex roles of Notch signaling during physiological and tumor angiogenesis, the dynamic nature of tip/stalk cell selection in the nascent vessel sprout and arteriovenous differentiation. Furthermore, we shed light on recent work on endothelial cell metabolism, perfusion-independent angiocrine functions of endothelial cells in organ-specific vascular beds and how manipulation of Notch signaling may be used to target the tumor vasculature.

NOTCH4 gene polymorphisms as potential risk factors for brain arteriovenous malformation development and hemorrhagic presentation

OBJECTIVE

Arteriovenous malformations (AVMs) of the brain are a frequent and important cause of intracranial hemorrhage in young adults. Little is known about the molecular-genetic pathomechanisms underlying AVM development. Genes of the NOTCH family control the normal development of vessels and proper arteriovenous specification. Transgenic mice with constitutive expression of active NOTCH4 frequently develop AVMs. Here, the authors report a genetic association study investigating possible associations between NOTCH4 gene polymorphisms and formation and clinical presentation of AVMs.

METHODS

After PCR amplification and direct DNA sequencing or restriction digests, 10 single-nucleotide polymorphisms (SNPs) of the NOTCH4 gene were used for genotyping 153 AVM patients and 192 healthy controls (i.e., blood donors). Pertinent clinical data were available for 129 patients. Uni- and multivariate single-marker and explorative haplotype analyses were performed to identify potential genetic risk factors for AVM development and for hemorrhagic or epileptic presentation.

RESULTS

Eleven calculated haplotypes consisting of 3–4 SNPs (most of which were located in the epidermal growth factor–like domain of the NOTCH4 gene) were observed significantly more often among AVM patients than among controls. Univariate analysis indicated that rs443198_TT and rs915895_AA genotypes both were significantly associated with hemorrhage and that an rs1109771_GG genotype was associated with epilepsy. The association between rs443198_TT and AVM bleeding remained significant in the multivariate regression analysis.

CONCLUSIONS

The authors' results suggest NOTCH4 SNPs as possible genetic risk factors for the development and clinical presentation of AVMs and a role of NOTCH4 in the pathogenesis of this disease.

Vascular Integrity in the Pathogenesis of Brain Arteriovenous Malformation

Brain arteriovenous malformation (bAVM) is an important cause of intracranial hemorrhage (ICH), particularly in the young population. ICH is the first clinical symptom in about 50 % of bAVM patients. The vessels in bAVM are fragile and prone to rupture, causing bleeding into the brain. About 30 % of unruptured and non-hemorrhagic bAVMs demonstrate microscopic evidence of hemosiderin in the vascular wall. In bAVM mouse models, vascular mural cell coverage is reduced in the AVM lesion, accompanied by vascular leakage and microhemorrhage. In this review, we discuss possible signaling pathways involved in abnormal vascular development in bAVM.

Arteriovenous malformations in hereditary haemorrhagic telangiectasia: looking beyond ALK1-NOTCH interactions

Hereditary haemorrhagic telangiectasia (HHT) is characterized by the development of arteriovenous malformations--enlarged shunts allowing arterial flow to bypass capillaries and enter directly into veins. HHT is caused by mutations in ALK1 or Endoglin; however, the majority of arteriovenous malformations are idiopathic and arise spontaneously. Idiopathic arteriovenous malformations differ from those due to loss of ALK1 in terms of both location and disease progression. Furthermore, while arteriovenous malformations in HHT and Alk1 knockout models have decreased NOTCH signalling, some idiopathic arteriovenous malformations have increased NOTCH signalling. The pathogenesis of these lesions also differs, with loss of ALK1 causing expansion of the shunt through proliferation, and NOTCH gain of function inducing initial shunt enlargement by cellular hypertrophy. Hence, we propose that idiopathic arteriovenous malformations are distinct from those of HHT. In this review, we explore the role of ALK1-NOTCH interactions in the development of arteriovenous malformations and examine a possible role of two signalling pathways downstream of ALK1, TMEM100 and IDs, in the development of arteriovenous malformations in HHT. A nuanced understanding of the precise molecular mechanisms underlying idiopathic and HHT-associated arteriovenous malformations will allow for development of targeted treatments for these lesions.

Animal Models in Studying Cerebral Arteriovenous Malformation

Brain arteriovenous malformation (AVM) is an important cause of hemorrhagic stroke. The etiology is largely unknown and the therapeutics are controversial. A review of AVM-associated animal models may be helpful in order to understand the up-to-date knowledge and promote further research about the disease. We searched PubMed till December 31, 2014, with the term “arteriovenous malformation,” limiting results to animals and English language. Publications that described creations of AVM animal models or investigated AVM-related mechanisms and treatments using these models were reviewed. More than 100 articles fulfilling our inclusion criteria were identified, and from them eight different types of the original models were summarized. The backgrounds and procedures of these models, their applications, and research findings were demonstrated. Animal models are useful in studying the pathogenesis of AVM formation, growth, and rupture, as well as in developing and testing new treatments. Creations of preferable models are expected.

The molecular regulation of arteriovenous specification and maintenance

The formation of a hierarchical vascular network, composed of arteries, veins, and capillaries, is essential for embryogenesis and is required for the production of new functional vasculature in the adult. Elucidating the molecular mechanisms that orchestrate the differentiation of vascular endothelial cells into arterial and venous cell fates is requisite for regenerative medicine, as the directed formation of perfused vessels is desirable in a myriad of pathological settings, such as in diabetes and following myocardial infarction. Additionally, this knowledge will enhance our understanding and treatment of vascular anomalies, such as arteriovenous malformations (AVMs). From studies in vertebrate model organisms, such as mouse, zebrafish, and chick, a number of key signaling pathways have been elucidated that are required for the establishment and maintenance of arterial and venous fates. These include the Hedgehog, Vascular Endothelial Growth Factor (VEGF), Transforming Growth Factor-β (TGF-β), Wnt, and Notch signaling pathways. In addition, a variety of transcription factor families acting downstream of, or in concert with, these signaling networks play vital roles in arteriovenous (AV) specification. These include Notch and Notch-regulated transcription factors (e.g., HEY and HES), SOX factors, Forkhead factors, β-Catenin, ETS factors, and COUP-TFII. It is becoming apparent that AV specification is a highly coordinated process that involves the intersection and carefully orchestrated activity of multiple signaling cascades and transcriptional networks. This review will summarize the molecular mechanisms that are involved in the acquisition and maintenance of AV fate, and will highlight some of the limitations in our current knowledge of the molecular machinery that directs AV morphogenesis.

VEGF, Notch and TGFβ/BMPs in regulation of sprouting angiogenesis and vascular patterning

The blood vasculature is constantly adapting to meet the demand from tissue. In so doing, branches may form, reorganize or regress. These complex processes employ integration of multiple signalling cascades, some of them being restricted to endothelial and mural cells and, hence, suitable for targeting of the vasculature. Both genetic and drug targeting experiments have demonstrated the requirement for the vascular endothelial growth factor (VEGF) system, the Delta-like-Notch system and the transforming growth factor β (TGFβ)/bone morphogenetic protein (BMP) cascades in vascular development. Although several of these signalling cascades in part converge into common downstream components, they differ in temporal and spatial regulation and expression. For example, the pro-angiogenic VEGFA is secreted by cells in need of oxygen, presented to the basal side of the endothelium, whereas BMP9 and BMP10 are supplied via the bloodstream in constant interaction with the apical side to suppress angiogenesis. Delta-like 4 (DLL4), on the other hand, is provided as an endothelial membrane bound ligand. In the present article, we discuss recent data on the integration of these pathways in the process of sprouting angiogenesis and vascular patterning and malformation.

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

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

Context-specific interactions between Notch and ALK1 cannot explain ALK1-associated arteriovenous malformations

AIMS

Notch and activin receptor-like kinase 1 (ALK1) have been implicated in arterial specification, angiogenic tip/stalk cell differentiation, and development of arteriovenous malformations (AVMs), and ALK1 can cooperate with Notch to up-regulate expression of Notch target genes in cultured endothelial cells. These findings suggest that Notch and ALK1 might collaboratively program arterial identity and prevent AVMs. We therefore sought to investigate the interaction between Notch and Alk1 signalling in the developing vertebrate vasculature.

METHODS AND RESULTS

We modulated Notch and Alk1 activities in zebrafish embryos and examined effects on Notch target gene expression and vascular morphology. Although Alk1 is not necessary for expression of Notch target genes in arterial endothelium, loss of Notch signalling unmasks a role for Alk1 in supporting hey2 and ephrinb2a expression in the dorsal aorta. In contrast, Notch and Alk1 play opposing roles in hey2 expression in cranial arteries and dll4 expression in all arterial endothelium, with Notch inducing and Alk1 repressing these genes. Although alk1 loss increases expression of dll4, AVMs in alk1 mutants could neither be phenocopied by Notch activation nor rescued by Dll4/Notch inhibition.

CONCLUSION

Control of Notch targets in arterial endothelium is context-dependent, with gene-specific and region-specific requirements for Notch and Alk1. Alk1 is not required for arterial identity, and perturbations in Notch signalling cannot account for alk1 mutant-associated AVMs. These data suggest that AVMs associated with ALK1 mutation are not caused by defective arterial specification or altered Notch signalling.

Biology of cerebral arteriovenous malformations with a focus on inflammation

Cerebral arteriovenous malformations (AVMs) entail a significant risk of intracerebral hemorrhage owing to the direct shunting of arterial blood into the venous vasculature without the dissipation of the arterial blood pressure. The mechanisms involved in the growth, progression and rupture of AVMs are not clearly understood, but a number of studies point to inflammation as a major contributor to their pathogenesis. The upregulation of proinflammatory cytokines induces the overexpression of cell adhesion molecules in AVM endothelial cells, resulting in enhanced recruitment of leukocytes. The increased leukocyte-derived release of metalloproteinase-9 is known to damage AVM walls and lead to rupture. Inflammation is also involved in altering the AVM angioarchitecture via the upregulation of angiogenic factors that affect endothelial cell proliferation, migration and apoptosis. The effects of inflammation on AVM pathogenesis are potentiated by certain single-nucleotide polymorphisms in the genes of proinflammatory cytokines, increasing their protein levels in the AVM tissue. Furthermore, studies on metalloproteinase-9 inhibitors and on the involvement of Notch signaling in AVMs provide promising data for a potential basis for pharmacological treatment of AVMs. Potential therapeutic targets and areas requiring further investigation are highlighted.

Constitutively active Notch4 receptor elicits brain arteriovenous malformations through enlargement of capillary-like vessels

Arteriovenous (AV) malformation (AVM) is a devastating condition characterized by focal lesions of enlarged, tangled vessels that shunt blood from arteries directly to veins. AVMs can form anywhere in the body and can cause debilitating ischemia and life-threatening hemorrhagic stroke. The mechanisms that underlie AVM formation remain poorly understood. Here, we examined the cellular and hemodynamic changes at the earliest stages of brain AVM formation by time-lapse two-photon imaging through cranial windows of mice expressing constitutively active Notch4 (Notch4*). AVMs arose from enlargement of preexisting microvessels with capillary diameter and blood flow and no smooth muscle cell coverage. AV shunting began promptly after Notch4* expression in endothelial cells (ECs), accompanied by increased individual EC areas, rather than increased EC number or proliferation. Alterations in Notch signaling in ECs of all vessels, but not arteries alone, affected AVM formation, suggesting that Notch functions in the microvasculature and/or veins to induce AVM. Increased Notch signaling interfered with the normal biological control of hemodynamics, permitting a positive feedback loop of increasing blood flow and vessel diameter and driving focal AVM growth from AV connections with higher blood velocity at the expense of adjacent AV connections with lower velocity. Endothelial expression of constitutively active Notch1 also led to brain AVMs in mice. Our data shed light on cellular and hemodynamic mechanisms underlying AVM pathogenesis elicited by increased Notch signaling in the endothelium.

Human brain arteriovenous malformations express lymphatic-associated genes

Objective

Brain arteriovenous malformations (AVMs) are devastating, hemorrhage-prone, cerebrovascular lesions characterized by well-defined feeding arteries, draining vein(s) and the absence of a capillary bed. The endothelial cells (ECs) that comprise AVMs exhibit a loss of arterial and venous specification. Given the role of the transcription factor COUP-TFII in vascular development, EC specification, and pathological angiogenesis, we examined human AVM tissue to determine if COUP-FTII may have a role in AVM disease biology.

Methods

We examined 40 human brain AVMs by immunohistochemistry (IHC) and qRT-PCR for the expression of COUP-TFII as well as other genes involved in venous and lymphatic development, maintenance, and signaling. We also examined proliferation and EC tube formation with human umbilical ECs (HUVEC) following COUP-TFII overexpression.

Results

We report that AVMs expressed COUP-TFII, SOX18, PROX1, NFATC1, FOXC2, TBX1, LYVE1, Podoplanin, and vascular endothelial growth factor (VEGF)-C, contained Ki67-positive cells and heterogeneously expressed genes involved in Hedgehog, Notch, Wnt, and VEGF signaling pathways. Overexpression of COUP-TFII alone in vitro resulted in increased EC proliferation and dilated tubes in an EC tube formation assay in HUVEC.

Interpretation

This suggests AVM ECs are further losing their arterial/venous specificity and acquiring a partial lymphatic molecular phenotype. There was significant correlation of gene expression with presence of clinical edema and acute hemorrhage. While the precise role of these genes in the formation, stabilization, growth and risk of hemorrhage of AVMs remains unclear, these findings have potentially important implications for patient management and treatment choice, and opens new avenues for future work on AVM disease mechanisms.

Deletion of Rbpj from postnatal endothelium leads to abnormal arteriovenous shunting in mice

Arteriovenous malformations (AVMs) are tortuous vessels characterized by arteriovenous (AV) shunts, which displace capillaries and shunt blood directly from artery to vein. Notch signaling regulates embryonic AV specification by promoting arterial, as opposed to venous, endothelial cell (EC) fate. To understand the essential role of endothelial Notch signaling in postnatal AV organization, we used inducible Cre-loxP recombination to delete Rbpj, a mediator of canonical Notch signaling, from postnatal ECs in mice. Deletion of endothelial Rbpj from birth resulted in features of AVMs by P14, including abnormal AV shunting and tortuous vessels in the brain, intestine and heart. We further analyzed brain AVMs, as they pose particular health risks. Consistent with AVM pathology, we found cerebral hemorrhage, hypoxia and necrosis, and neurological deficits. AV shunts originated from capillaries (and possibly venules), with the earliest detectable morphological abnormalities in AV connections by P8. Prior to AV shunt formation, alterations in EC gene expression were detected, including decreased Efnb2 and increased Pai1, which encodes a downstream effector of TGFβ signaling. After AV shunts had formed, whole-mount immunostaining showed decreased Efnb2 and increased Ephb4 expression within AV shunts, suggesting that ECs were reprogrammed from arterial to venous identity. Deletion of Rbpj from adult ECs led to tortuosities in gastrointestinal, uterine and skin vascular beds, but had mild effects in the brain. Our results demonstrate a temporal requirement for Rbpj in postnatal ECs to maintain proper artery, capillary and vein organization and to prevent abnormal AV shunting and AVM pathogenesis.

De novo cerebral arteriovenous malformations: is epileptic seizure a potential trigger?

MATERIAL AND METHODS

The pathogenesis of cerebral arteriovenous malformations (cAVMs) is still not well understood. Generally, cAVMs are thought to be congenital lesions originating prenatally. We report a 7-year-old boy diagnosed with a de novo cAVM after 3 years of recurrent epileptic seizures.

RESULTS

MR imaging at 4 years of age was normal. Follow-up MR imaging 3 years later demonstrated a de novo 2-cm cAVM in the right occipital lobe, confirmed by conventional angiography. We reviewed five previously reported cases of de novo cAVMs who did not have a previous neurovascular abnormality. Including our case, recurrent epileptic seizures are the major presentation (83.3 %) before de novo cAVM occurrence.

CONCLUSION

We suggest that epileptic seizure is a potential trigger of de novo cAVMs.