Endothelial Notch4 signaling induces hallmarks of brain arteriovenous malformations in mice

Developing zebrafish models of Notch-related CNS pathologies

Notch signaling is an evolutionarily conserved cellular pathway that regulates various stem cell functions, including fate determination, differentiation, proliferation, and apoptosis. This crucial signaling mechanism also plays an important role in the brain, regulating neurogenesis, cell differentiation, and homeostasis, whereas disrupted Notch signaling is linked to various neurodegenerative diseases and brain cancers. Here, we review the central nervous system (CNS) pathologies associated with aberrant Notch signaling, and summarize the available experimental (animal) models used to study these pathologies, with a special focus on zebrafish (Danio rerio). As genetic, pharmacological, and behavioral models in zebrafish have significantly advanced our understanding of Notch-related CNS disorders, future research is expected to further link Notch signaling to brain disorders and, eventually, lead to their more specific and targeted therapeuties.

From bench to bedside: murine models of inherited and sporadic brain arteriovenous malformations

Brain arteriovenous malformations are abnormal vascular structures in which an artery shunts high pressure blood directly to a vein without an intervening capillary bed. These lesions become highly remodeled over time and are prone to rupture. Historically, brain arteriovenous malformations have been challenging to treat, using primarily surgical approaches. Over the past few decades, the genetic causes of these malformations have been uncovered. These can be divided into (1) familial forms, such as loss of function mutations in TGF-β (BMP9/10) components in hereditary hemorrhagic telangiectasia, or (2) sporadic forms, resulting from somatic gain of function mutations in genes involved in the RAS-MAPK signaling pathway. Leveraging these genetic discoveries, preclinical mouse models have been developed to uncover the mechanisms underlying abnormal vessel formation, and thus revealing potential therapeutic targets. Impressively, initial preclinical studies suggest that pharmacological treatments disrupting these aberrant pathways may ameliorate the abnormal pathologic vessel remodeling and inflammatory and hemorrhagic nature of these high-flow vascular anomalies. Intriguingly, these studies also suggest uncontrolled angiogenic signaling may be a major driver in bAVM pathogenesis. This comprehensive review describes the genetics underlying both inherited and sporadic bAVM and details the state of the field regarding murine models of bAVM, highlighting emerging therapeutic targets that may transform our approach to treating these devastating lesions.

Arterial endothelial deletion of hereditary hemorrhagic telangiectasia 2/ Alk1 causes epistaxis and cerebral microhemorrhage with aberrant arteries and defective smooth muscle coverage

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant vascular disorder with manifestations including severe nose bleeding and microhemorrhage in brains. Despite being the second most common inherited bleeding disorder, the pathophysiological mechanism underlying HHT-associated hemorrhage is poorly understood. HHT pathogenesis is thought to follow a Knudsonian two-hit model, requiring a second somatic mutation for lesion formation. Mutations in activin receptor-like kinase 1 ( ALK1 ) gene cause HHT type 2. We hypothesize that somatic mutation of Alk1 in arterial endothelial cells (AECs) leads to arterial defects and hemorrhage. Here, we mutated Alk1 in AECs in postnatal mice using Bmx(PAC)-Cre ERT2 and found that somatic arterial endothelial mutation of Alk1 was sufficient to induce spontaneous epistaxis and multifocal cerebral microhemorrhage. This bleeding occurred in the presence of tortuous and enlarged blood vessels, loss of arterial molecular marker Efnb2 , disorganization of vascular smooth muscle, and impaired vasoregulation. Our data suggest that arterial endothelial deletion of Alk1 leading to reduced arterial identity and disrupted vascular smooth muscle cell coverage is a plausible molecular mechanism for HHT-associated severe epistaxis. This work provides the first evidence that somatic Alk1 mutation in AECs can cause hemorrhagic vascular lesions, offering a novel preclinical model critically needed for studying HHT-associated epistaxis, and delineating an arterial mechanism to HHT pathophysiology.

Practical immunomodulatory landscape of glioblastoma multiforme (GBM) therapy

Glioblastoma multiforme (GBM) is the most common harmful high-grade brain tumor with high mortality and low survival rate. Importantly, besides routine diagnostic and therapeutic methods, modern and useful practical techniques are urgently needed for this serious malignancy. Correspondingly, the translational medicine focusing on genetic and epigenetic profiles of glioblastoma, as well as the immune framework and brain microenvironment, based on these challenging findings, indicates that key clinical interventions include immunotherapy, such as immunoassay, oncolytic viral therapy, and chimeric antigen receptor T (CAR T) cell therapy, which are of great importance in both diagnosis and therapy. Relatively, vaccine therapy reflects the untapped confidence to enhance GBM outcomes. Ongoing advances in immunotherapy, which utilizes different methods to regenerate or modify the resistant body for cancer therapy, have revealed serious results with many different problems and difficulties for patients. Safe checkpoint inhibitors, adoptive cellular treatment, cellular and peptide antibodies, and other innovations give researchers an endless cluster of instruments to plan profoundly in personalized medicine and the potential for combination techniques. In this way, antibodies that block immune checkpoints, particularly those that target the program death 1 (PD-1)/PD-1 (PD-L1) ligand pathway, have improved prognosis in a wide range of diseases. However, its use in combination with chemotherapy, radiation therapy, or monotherapy is ineffective in treating GBM. The purpose of this review is to provide an up-to-date overview of the translational elements concentrating on the immunotherapeutic field of GBM alongside describing the molecular mechanism involved in GBM and related signaling pathways, presenting both historical perspectives and future directions underlying basic and clinical practice.

Astrocytic DLL4-NOTCH1 signaling pathway promotes neuroinflammation via the IL-6-STAT3 axis

Under neuroinflammatory conditions, astrocytes acquire a reactive phenotype that drives acute inflammatory injury as well as chronic neurodegeneration. We hypothesized that astrocytic Delta-like 4 (DLL4) may interact with its receptor NOTCH1 on neighboring astrocytes to regulate astrocyte reactivity via downstream juxtacrine signaling pathways. Here we investigated the role of astrocytic DLL4 on neurovascular unit homeostasis under neuroinflammatory conditions. We probed for downstream effectors of the DLL4-NOTCH1 axis and targeted these for therapy in two models of CNS inflammatory disease. We first demonstrated that astrocytic DLL4 is upregulated during neuroinflammation, both in mice and humans, driving astrocyte reactivity and subsequent blood-brain barrier permeability and inflammatory infiltration. We then showed that the DLL4-mediated NOTCH1 signaling in astrocytes directly drives IL-6 levels, induces STAT3 phosphorylation promoting upregulation of astrocyte reactivity markers, pro-permeability factor secretion and consequent blood-brain barrier destabilization. Finally we revealed that blocking DLL4 with antibodies improves experimental autoimmune encephalomyelitis symptoms in mice, identifying a potential novel therapeutic strategy for CNS autoimmune demyelinating disease. As a general conclusion, this study demonstrates that DLL4-NOTCH1 signaling is not only a key pathway in vascular development and angiogenesis, but also in the control of astrocyte reactivity during neuroinflammation.

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).

Inhibition of endothelial histone deacetylase 2 shifts endothelial-mesenchymal transitions in cerebral arteriovenous malformation models

Cerebral arteriovenous malformations (AVMs) are the most common vascular malformations worldwide and the leading cause of hemorrhagic strokes that may result in crippling neurological deficits. Here, using recently generated mouse models, we uncovered that cerebral endothelial cells (ECs) acquired mesenchymal markers and caused vascular malformations. Interestingly, we found that limiting endothelial histone deacetylase 2 (HDAC2) prevented cerebral ECs from undergoing mesenchymal differentiation and reduced cerebral AVMs. We found that endothelial expression of HDAC2 and enhancer of zeste homolog 1 (EZH1) was altered in cerebral AVMs. These alterations changed the abundance of H4K8ac and H3K27me in the genes regulating endothelial and mesenchymal differentiation, which caused the ECs to acquire mesenchymal characteristics and form AVMs. This investigation demonstrated that the induction of HDAC2 altered specific histone modifications, which resulted in mesenchymal characteristics in the ECs and cerebral AVMs. The results provide insight into the epigenetic impact on AVMs.

Astrogliosis in multiple sclerosis and neuro-inflammation: what role for the notch pathway?

Multiple sclerosis is an autoimmune inflammatory disease of the central nervous system leading to neurodegeneration. It affects 2.3 million people worldwide, generally younger than 50. There is no known cure for the disease, and current treatment options - mainly immunotherapies to limit disease progression - are few and associated with serious side effects. In multiple sclerosis, disruption of the blood-brain barrier is an early event in the pathogenesis of lesions, predisposing to edema, excito-toxicity and inflammatory infiltration into the central nervous system. Recently, the vision of the blood brain barrier structure and integrity has changed and include contributions from all components of the neurovascular unit, among which astrocytes. During neuro-inflammation, astrocytes become reactive. They undergo morphological and molecular changes named "astrogliosis" driving the conversion from acute inflammatory injury to a chronic neurodegenerative state. Astrogliosis mechanisms are minimally explored despite their significance in regulating the autoimmune response during multiple sclerosis. Therefore, in this review, we take stock of the state of knowledge regarding astrogliosis in neuro-inflammation and highlight the central role of NOTCH signaling in the process of astrocyte reactivity. Indeed, a very detailed nomenclature published in nature neurosciences in 2021, listing all the reactive astrocyte markers fully identified in the literature, doesn't cover the NOTCH signaling. Hence, we discuss evidence supporting NOTCH1 receptor as a central regulator of astrogliosis in the pathophysiology of neuro-inflammation, notably multiple sclerosis, in human and experimental models.

Pericytes and vascular smooth muscle cells in central nervous system arteriovenous malformations

Previously considered passive support cells, mural cells-pericytes and vascular smooth muscle cells-have started to garner more attention in disease research, as more subclassifications, based on morphology, gene expression, and function, have been discovered. Central nervous system (CNS) arteriovenous malformations (AVMs) represent a neurovascular disorder in which mural cells have been shown to be affected, both in animal models and in human patients. To study consequences to mural cells in the context of AVMs, various animal models have been developed to mimic and predict human AVM pathologies. A key takeaway from recently published work is that AVMs and mural cells are heterogeneous in their molecular, cellular, and functional characteristics. In this review, we summarize the observed perturbations to mural cells in human CNS AVM samples and CNS AVM animal models, and we discuss various potential mechanisms relating mural cell pathologies to AVMs.

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.

Arteriovenous Malformations: An Update on Models and Therapeutic Targets

Arteriovenous malformations (AVMs) are an anomaly of the vascular system where feeding arteries are directly connected to the venous drainage network. While AVMs can arise anywhere in the body and have been described in most tissues, brain AVMs are of significant concern because of the risk of hemorrhage which carries significant morbidity and mortality. The prevalence of AVM's and the mechanisms underlying their formation are not well understood. For this reason, patients who undergo treatment for symptomatic AVM's remain at increased risk of subsequent bleeds and adverse outcomes. The cerebrovascular network is delicate and novel animal models continue to provide insight into its dynamics in the context of AVM's. As the molecular players in the formation of familial and sporadic AVM's are better understood, novel therapeutic approaches have been developed to mitigate their associated risks. Here we discuss the current literature surrounding AVM's including the development of models and therapeutic targets which are currently being investigated.

Neurodevelopmental and genetic findings in neonates with intracranial arteriovenous shunts: A case series

Background

Despite the latest advances in prenatal diagnosis and postnatal embolization procedures, intracranial arteriovenous shunts (AVSs) are still associated with high mortality and morbidity rates. Our aim was to evaluate the presentation and clinical course, the neurodevelopmental outcome, and the genetic findings of neonates with AVSs.

Methods

In this retrospective observational study, medical records of neonates with cerebral AVSs admitted to our hospital from January 2020 to July 2022 were revised. In particular, we evaluated neuroimaging characteristics, endovascular treatment, neurophysiological features, neurodevelopmental outcomes, and genetic findings.

Results

We described the characteristics of 11 patients with AVSs. Ten infants (90.9%) required embolization during the first three months of life. In 5/9 infants, pathological electroencephalography findings were observed; of them, two patients presented seizures. Eight patients performed Median Nerve Somatosensory Evoked Potentials (MN-SEPs): of them, six had an impaired response. We found normal responses at Visual Evoked Potentials and Brainstem Auditory Evoked Potentials. Eight patients survived (72.7%) and were enrolled in our multidisciplinary follow-up program. Of them, 7/8 completed the Bayley-III Scales at 6 months of corrected age: none of them had cognitive and language delays; conversely, a patient had a moderate delay on the Motor scale. The remaining survivor patient developed cerebral palsy and could not undergo Bayley-III evaluation because of the severe psychomotor delay. From the genetic point of view, we found a novel pathogenic variant in the NOTCH3 gene and three additional genomic defects of uncertain pathogenicity.

Conclusion

We propose SEPs as an ancillary test to discern the most vulnerable infants at the bedside, particularly to identify possible future motor impairment in follow-up. The early identification of a cognitive or motor delay is critical to intervene with personalized rehabilitation treatment and minimize future impairment promptly. Furthermore, the correct interpretation of identified genetic variants could provide useful information, but further studies are needed to investigate the role of these variants in the pathogenesis of AVSs.

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.

Brain arteriovenous malformation in hereditary hemorrhagic telangiectasia: Recent advances in cellular and molecular mechanisms

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disorder characterized by vessel dilatation, such as telangiectasia in skin and mucosa and arteriovenous malformations (AVM) in internal organs such as the gastrointestinal tract, lungs, and brain. AVMs are fragile and tortuous vascular anomalies that directly connect arteries and veins, bypassing healthy capillaries. Mutations in transforming growth factor β (TGFβ) signaling pathway components, such as ENG (ENDOGLIN), ACVRL1 (ALK1), and SMAD4 (SMAD4) genes, account for most of HHT cases. 10-20% of HHT patients develop brain AVMs (bAVMs), which can lead to vessel wall rupture and intracranial hemorrhages. Though the main mutations are known, mechanisms leading to AVM formation are unclear, partially due to lack of animal models. Recent mouse models allowed significant advances in our understanding of AVMs. Endothelial-specific deletion of either Acvrl1, Eng or Smad4 is sufficient to induce AVMs, identifying endothelial cells (ECs) as primary targets of BMP signaling to promote vascular integrity. Loss of ALK1/ENG/SMAD4 signaling is associated with NOTCH signaling defects and abnormal arteriovenous EC differentiation. Moreover, cumulative evidence suggests that AVMs originate from venous ECs with defective flow-migration coupling and excessive proliferation. Mutant ECs show an increase of PI3K/AKT signaling and inhibitors of this signaling pathway rescue AVMs in HHT mouse models, revealing new therapeutic avenues. In this review, we will summarize recent advances and current knowledge of mechanisms controlling the pathogenesis of bAVMs, and discuss unresolved questions.

The Beauty and Complexity of Blood Vessel Patterning

This review highlights new concepts in vascular patterning in the last 10 years, with emphasis on its beauty and complexity. Endothelial cell signaling pathways that respond to molecular or mechanical signals are described, and examples of vascular patterning that use these pathways in brain, skin, heart, and kidney are highlighted. The pathological consequences of patterning loss are discussed in the context of arteriovenous malformations (AVMs), and prospects for the next 10 years presented.

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.

Endothelial k-RasV12 Expression Induces Capillary Deficiency Attributable to Marked Tube Network Expansion Coupled to Reduced Pericytes and Basement Membranes

OBJECTIVE

We sought to determine how endothelial cell (EC) expression of the activating k-Ras (kirsten rat sarcoma 2 viral oncogene homolog) mutation, k-RasV12, affects their ability to form lumens and tubes and interact with pericytes during capillary assembly Approach and Results: Using defined bioassays where human ECs undergo observable tubulogenesis, sprouting behavior, pericyte recruitment to EC-lined tubes, and pericyte-induced EC basement membrane deposition, we assessed the impact of EC k-RasV12 expression on these critical processes that are necessary for proper capillary network formation. This mutation, which is frequently seen in human ECs within brain arteriovenous malformations, was found to markedly accentuate EC lumen formation mechanisms, with strongly accelerated intracellular vacuole formation, vacuole fusion, and lumen expansion and with reduced sprouting behavior, leading to excessively widened tube networks compared with control ECs. These abnormal tubes demonstrate strong reductions in pericyte recruitment and pericyte-induced EC basement membranes compared with controls, with deficiencies in fibronectin, collagen type IV, and perlecan deposition. Analyses of signaling during tube formation from these k-RasV12 ECs reveals strong enhancement of Src (Src proto-oncogene, non-receptor tyrosine kinase), Pak2 (P21 [RAC1 (Rac family small GTPase 1)] activated kinase 2), b-Raf (v-raf murine sarcoma viral oncogene homolog B1), Erk (extracellular signal-related kinase), and Akt (AK strain transforming) activation and increased expression of PKCε (protein kinase C epsilon), MT1-MMP (membrane-type 1 matrix metalloproteinase), acetylated tubulin and CDCP1 (CUB domain-containing protein 1; most are known EC lumen regulators). Pharmacological blockade of MT1-MMP, Src, Pak, Raf, Mek (mitogen-activated protein kinase) kinases, Cdc42 (cell division cycle 42)/Rac1, and Notch markedly interferes with lumen and tube formation from these ECs.

CONCLUSIONS

Overall, this novel work demonstrates that EC expression of k-RasV12 disrupts capillary assembly due to markedly excessive lumen formation coupled with strongly reduced pericyte recruitment and basement membrane deposition, which are critical pathogenic features predisposing the vasculature to develop arteriovenous malformations.

Primary cilia and ciliary signaling pathways in aging and age-related brain disorders

Brain disorders are characterized by the progressive loss of structure and function of the brain as a consequence of progressive degeneration and/or death of nerve cells. Aging is a major risk factor for brain disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and stroke. Various cellular and molecular events have been shown to play a role in the progress of neurodegenerative diseases. Emerging studies suggest that primary cilia could be a key regulator in brain diseases. The primary cilium is a singular cellular organelle expressed on the surface of many cell types, such as astrocytes and neurons in the mature brain. Primary cilia detect extracellular cues, such as Sonic Hedgehog (SHH) protein, and transduce these signals into cells to regulate various signaling pathways. Abnormalities in ciliary length and frequency (ratio of ciliated cells) have been implicated in various human diseases, including brain disorders. This review summarizes current findings and thoughts on the role of primary cilia and ciliary signaling pathways in aging and age-related brain disorders.

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.

Sonic hedgehog is expressed in human brain arteriovenous malformations and induces arteriovenous malformations in vivo

Abnormalities in arterial versus venous endothelial cell identity and dysregulation of angiogenesis are deemed important in the pathophysiology of brain arteriovenous malformations (AVMs). The Sonic hedgehog (Shh) pathway is crucial for both angiogenesis and arterial versus venous differentiation of endothelial cells, through its dual role on the vascular endothelial growth factor/Notch signaling and the nuclear orphan receptor COUP-TFII. In this study, we show that Shh, Gli1 (the main transcription factor of the Shh pathway), and COUP-TFII (a target of the non-canonical Shh pathway) are aberrantly expressed in human brain AVMs. We also show that implantation of pellets containing Shh in the cornea of Efnb2/LacZ mice induces growth of distinct arteries and veins, interconnected by complex sets of arteriovenous shunts, without an interposed capillary bed, as seen in AVMs. We also demonstrate that injection in the rat brain of a plasmid containing the human Shh gene induces the growth of tangles of tortuous and dilated vessels, in part positive and in part negative for the arterial marker αSMA, with direct connections between αSMA-positive and -negative vessels. In summary, we show that the Shh pathway is active in human brain AVMs and that Shh-induced angiogenesis has characteristics reminiscent of those seen in AVMs in humans.

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.

Epigenetics of glioblastoma multiforme: From molecular mechanisms to therapeutic approaches

Glioblastoma multiforme (GBM) is the most common form of brain cancer and one of the most aggressive cancers found in humans. Most of the signs and symptoms of GBM can be mild and slowly aggravated, although other symptoms might demonstrate it as an acute ailment. However, the precise mechanisms of the development of GBM remain unknown. Due to the improvement of molecular pathology, current researches have reported that glioma progression is strongly connected with different types of epigenetic phenomena, such as histone modifications, DNA methylation, chromatin remodeling, and aberrant microRNA. Furthermore, the genes and the proteins that control these alterations have become novel targets for treating glioma because of the reversibility of epigenetic modifications. In some cases, gene mutations including P16, TP53, and EGFR, have been observed in GBM. In contrast, monosomies, including removals of chromosome 10, particularly q23 and q25-26, are considered the standard markers for determining the development and aggressiveness of GBM. Recently, amid the epigenetic therapies, histone deacetylase inhibitors (HDACIs) and DNA methyltransferase inhibitors have been used for treating tumors, either single or combined. Specifically, HDACIs are served as a good choice and deliver a novel pathway to treat GBM. In this review, we focus on the epigenetics of GBM and the consequence of its mutations. We also highlight various treatment approaches, namely gene editing, epigenetic drugs, and microRNAs to combat GBM.

Cellular and Molecular Mechanisms of Spinal Cord Vascularization

During embryonic central nervous system (CNS) development, the neural and the vascular systems communicate with each other in order to give rise to a fully functional and mature CNS. The initial avascular CNS becomes vascularized by blood vessel sprouting from different vascular plexus in a highly stereotypical and controlled manner. This process is similar across different regions of the CNS. In particular for the developing spinal cord (SC), blood vessel ingression occurs from a perineural vascular plexus during embryonic development. In this review, we provide an updated and comprehensive description of the cellular and molecular mechanisms behind this stereotypical and controlled patterning of blood vessels in the developing embryonic SC, identified using different animal models. We discuss how signals derived from neural progenitors and differentiated neurons guide the SC growing vasculature. Lastly, we provide a perspective of how the molecular mechanisms identified during development could be used to better understand pathological situations.

Deficiency of Notch signaling in pericytes results in arteriovenous malformations

Arteriovenous malformations (AVMs) are high-flow lesions directly connecting arteries and veins. In the brain, AVM rupture can cause seizures, stroke, and death. Patients with AVMs exhibit reduced coverage of the vessels by pericytes, the mural cells of microvascular capillaries; however, the mechanism underlying this pericyte reduction and its association with AVM pathogenesis remains unknown. Notch signaling has been proposed to regulate critical pericyte functions. We hypothesized that Notch signaling in pericytes is crucial to maintain pericyte homeostasis and prevent AVM formation. We inhibited Notch signaling specifically in perivascular cells and analyzed the vasculature of these mice. The retinal vessels of mice with deficient perivascular Notch signaling developed severe AVMs, together with a significant reduction in pericytes and vascular smooth muscle cells (vSMC) in the arteries, while vSMCs were increased in the veins. Vascular malformations and pericyte loss were also observed in the forebrain of embryonic mice deficient for perivascular Notch signaling. Moreover, the loss of Notch signaling in pericytes downregulated Pdgfrb levels and increased pericyte apoptosis, pointing to a critical role for Notch in pericyte survival. Overall, our findings reveal a mechanism of AVM formation and highlight the Notch signaling pathway as an essential mediator in this process.

miR-18a Inhibits BMP4 and HIF-1α Normalizing Brain Arteriovenous Malformations

RATIONALE

Brain arteriovenous malformations (AVMs) are abnormal tangles of vessels where arteries and veins directly connect without intervening capillary nets, increasing the risk of intracerebral hemorrhage and stroke. Current treatments are highly invasive and often not feasible. Thus, effective noninvasive treatments are needed. We previously showed that AVM-brain endothelial cells (BECs) secreted higher VEGF (vascular endothelial growth factor) and lower TSP-1 (thrombospondin-1) levels than control BEC; and that microRNA-18a (miR-18a) normalized AVM-BEC function and phenotype, although its mechanism remained unclear.

OBJECTIVE

To elucidate the mechanism of action and potential clinical application of miR-18a as an effective noninvasive treatment to selectively restore the phenotype and functionality of AVM vasculature.

METHODS AND RESULTS

The molecular pathways affected by miR-18a in patient-derived BECs and AVM-BECs were determined by Western blot, RT-qPCR (quantitative reverse transcription polymerase chain reaction), ELISA, co-IP, immunostaining, knockdown and overexpression studies, flow cytometry, and luciferase reporter assays. miR-18a was shown to increase TSP-1 and decrease VEGF by reducing PAI-1 (plasminogen activator inhibitor-1/SERPINE1) levels. Furthermore, miR-18a decreased the expression of BMP4 (bone morphogenetic protein 4) and HIF-1α (hypoxia-inducible factor 1α), blocking the BMP4/ALK (activin-like kinase) 2/ALK1/ALK5 and Notch signaling pathways. As determined by Boyden chamber assays, miR-18a also reduced the abnormal AVM-BEC invasiveness, which correlated with a decrease in MMP2 (matrix metalloproteinase 2), MMP9, and ADAM10 (ADAM metallopeptidase domain 10) levels. In vivo pharmacokinetic studies showed that miR-18a reaches the brain following intravenous and intranasal administration. Intranasal co-delivery of miR-18a and NEO100, a good manufacturing practices-quality form of perillyl alcohol, improved the pharmacokinetic profile of miR-18a in the brain without affecting its pharmacological properties. Ultra-high-resolution computed tomography angiography and immunostaining studies in an Mgp^-/- AVM mouse model showed that miR-18a decreased abnormal cerebral vasculature and restored the functionality of the bone marrow, lungs, spleen, and liver.

CONCLUSIONS

miR-18a may have significant clinical value in preventing, reducing, and potentially reversing AVM.

Overexpression of Activin Receptor-Like Kinase 1 in Endothelial Cells Suppresses Development of Arteriovenous Malformations in Mouse Models of Hereditary Hemorrhagic Telangiectasia

RATIONALE

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disease caused by mutations in ENG, ALK1, or SMAD4. Since proteins from all 3 HHT genes are components of signal transduction of TGF-β (transforming growth factor β) family members, it has been hypothesized that HHT is a disease caused by defects in the ENG-ALK1-SMAD4 linear signaling. However, in vivo evidence supporting this hypothesis is scarce.

OBJECTIVE

We tested this hypothesis and investigated the therapeutic effects and potential risks of induced-ALK1 or -ENG overexpression (OE) for HHT.

METHODS AND RESULTS

We generated a novel mouse allele (ROSA26^Alk1) in which HA (human influenza hemagglutinin)-tagged ALK1 and bicistronic eGFP expression are induced by Cre activity. We examined whether ALK1-OE using the ROSA26^Alk1 allele could suppress the development of arteriovenous malformations (AVMs) in wounded adult skin and developing retinas of Alk1- and Eng-inducible knockout (iKO) mice. We also used a similar approach to investigate whether ENG-OE could rescue AVMs. Biochemical and immunofluorescence analyses confirmed the Cre-dependent OE of the ALK1-HA transgene. We could not detect any pathological signs in ALK1-OE mice up to 3 months after induction. ALK1-OE prevented the development of retinal AVMs and wound-induced skin AVMs in Eng-iKO as well as Alk1-iKO mice. ALK1-OE normalized expression of SMAD and NOTCH target genes in ENG-deficient endothelial cells (ECs) and restored the effect of BMP9 (bone morphogenetic protein 9) on suppression of phosphor-AKT levels in these endothelial cells. On the other hand, ENG-OE could not inhibit the AVM development in Alk1-iKO models.

CONCLUSIONS

These data support the notion that ENG and ALK1 form a linear signaling pathway for the formation of a proper arteriovenous network during angiogenesis. We suggest that ALK1 OE or activation can be an effective therapeutic strategy for HHT. Further research is required to study whether this therapy could be translated into treatment for humans.

Deciphering the vascular labyrinth: role of microRNAs and candidate gene SNPs in brain AVM development - literature review

Background: Brain arteriovenous malformations (AVMs) are a relatively infrequent vascular pathology of unknown etiology that, despite their rarity, cause the highest number of hemorrhagic strokes under the age of 30 years. They pose a challenge to all forms of treatment due to their variable morphology, location, size, and, last but not least, evolving nature. MicroRNAs (miRNAs) are non-coding RNA strands that may suppress the expression of target genes by binding completely or partially to their complementary sequences. Single nucleotide polymorphisms (SNPs), as the name implies, are variations in a single nucleotide in the DNA, usually found in the non-coding segments. Although the majority of SNPs are harmless, some located in the proximity of candidate genes may result in altered expression or function of these genes and cause diseases or affect how different pathologies react to treatment. The roles miRNAs and certain SNPs play in the development and growth of AVMs are currently uncertain, yet progress in deciphering the minutiae of this pathology is already visible. Methods and Results: We performed an electronic Medline (PubMed, PubMed Central) and Google Academic exploration using permutations of the terms: "arteriovenous malformations," "single nucleotide polymorphisms," "microRNA," "non-coding RNA," and "genetic mutations." The findings were then divided into two categories, namely the miRNAs and the candidate gene SNPs associated with AVMs respectively. 6 miRNAs and 12 candidate gene SNPs were identified and discussed. Conclusions: The following literature review focuses on the discoveries made in ascertaining the different implications of miRNAs and candidate gene SNPs in the formation and evolution of brain AVMs, as well as highlighting the possible directions of future research and biological treatment. Abbreviations: ACVRL1/ALK1: activin receptor-like kinase 1; Akt: protein kinase B; ANGPTL4: angiopoietin-like 4; ANRIL: antisense noncoding RNA in the INK4 locus; AVM: arteriovenous malformation; AVM-BEC: arteriovenous malformation brain endothelial cell; BRCA1: breast cancer type 1 susceptibility protein; CCS: case-control study; CDKN2A/B: cyclin-dependent kinase inhibitor 2A/B; CLTC: clathrin heavy chain; DNA: deoxyribonucleic acid; ERK: extracellular signal-regulated kinase; GPR124: probable G-protein coupled receptor 124; GWAS: genome-wide association study; HHT: hereditary hemorrhagic telangiectasia; HIF1A: hypoxia-inducible factor 1A; IA: intracranial aneurysm; ICH: intracranial hemorrhage; Id-1: inhibitor of DNA-binding protein A; IL-17: interleukin 17; MAP4K3: mitogen-activated protein kinase kinase kinase kinase 3; miRNA: microRNA; MMP: matrix metalloproteinase; NFkB: nuclear factor kappa-light-chain of activated B cells; NOTCH: neurogenic locus notch homolog; p38MAPK: p38 mitogen-activated protein kinase; PI3K: phosphoinositide 3-kinase; RBBP8: retinoblastoma-binding protein 8; RNA: ribonucleic acid; SNAI1: Snail Family Transcriptional Repressor 1; SNP: single nucleotide polymorphism; SOX-17: SRY-related HMG-box; TGF-β: transformation growth factor β; TGFR: transformation growth factor receptor; TIMP-4, tissue inhibitor of metalloproteinase 4; TSP-1: thrombospondin-1; UTR: untranslated region; VEGF: Vascular Endothelial Growth Factor; VSMC: vascular smooth muscle cell; Wnt1: Wnt family member 1.

Somatic Gain of KRAS Function in the Endothelium Is Sufficient to Cause Vascular Malformations That Require MEK but Not PI3K Signaling

Supplemental Digital Content is available in the text.

Rationale:

We previously identified somatic activating mutations in the KRAS (Kirsten rat sarcoma viral oncogene homologue) gene in the endothelium of the majority of human sporadic brain arteriovenous malformations; a disorder characterized by direct connections between arteries and veins. However, whether this genetic abnormality alone is sufficient for lesion formation, as well as how active KRAS signaling contributes to arteriovenous malformations, remains unknown.

Objective:

To establish the first in vivo models of somatic KRAS gain of function in the endothelium in both mice and zebrafish to directly observe the phenotypic consequences of constitutive KRAS activity at a cellular level in vivo, and to test potential therapeutic interventions for arteriovenous malformations.

Methods and Results:

Using both postnatal and adult mice, as well as embryonic zebrafish, we demonstrate that endothelial-specific gain of function mutations in Kras (G12D or G12V) are sufficient to induce brain arteriovenous malformations. Active KRAS signaling leads to altered endothelial cell morphogenesis and increased cell size, ectopic sprouting, expanded vessel lumen diameter, and direct connections between arteries and veins. Furthermore, we show that these lesions are not associated with altered endothelial growth dynamics or a lack of proper arteriovenous identity but instead seem to feature exuberant angiogenic signaling. Finally, we demonstrate that KRAS-dependent arteriovenous malformations in zebrafish are refractory to inhibition of the downstream effector PI3K but instead require active MEK (mitogen-activated protein kinase kinase 1) signaling.

Conclusions:

We demonstrate that active KRAS expression in the endothelium is sufficient for brain arteriovenous malformations, even in the setting of uninjured adult vasculature. Furthermore, the finding that KRAS-dependent lesions are reversible in zebrafish suggests that MEK inhibition may represent a promising therapeutic treatment for arteriovenous malformation patients.

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.

Skip is essential for Notch signaling to induce Sox2 in cerebral arteriovenous malformations

Notch signaling and Sry-box (Sox) family transcriptional factors both play critical roles in endothelial cell (EC) differentiation in vascularization. Recent studies have shown that excessive Notch signaling induces Sox2 to cause cerebral arteriovenous malformations (AVMs). Here, we examine human pulmonary AVMs and find no induction of Sox2. Results of epigenetic studies also show less alteration of Sox2-DNA binding in pulmonary AVMs than in cerebral AVMs. We identify high expression of ski-interacting protein (Skip) in brain ECs, a Notch-associated chromatin-modifying protein that is lacking in lung ECs. Knockdown of Skip abolished Notch-induction of Sox2 in brain ECs, while restoration of Skip in lung ECs enabled Notch-mediated Sox2 induction. The results suggest that Skip is a key factor for induction of Sox2 in cerebral AVMs.

The Notch pathway in CNS homeostasis and neurodegeneration

The role of the Notch signaling pathway in neural development has been well established over many years. More recent studies, however, have demonstrated that Notch continues to be expressed and active throughout adulthood in many areas of the central nervous system. Notch signals have been implicated in adult neurogenesis, memory formation, and synaptic plasticity in the adult organism, as well as linked to acute brain trauma and chronic neurodegenerative conditions. NOTCH3 mutations are responsible for the most common form of hereditary stroke, the progressive disorder cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. Notch has also been associated with several progressive neurodegenerative diseases, including Alzheimer's disease, multiple sclerosis, and amyotrophic lateral sclerosis. Although numerous studies link Notch activity with CNS homeostasis and neurodegenerative diseases, the data thus far are primarily correlative, rather than functional. Nevertheless, the evidence for Notch pathway activity in specific neural cellular contexts is strong, and certainly intriguing, and points to the possibility that the pathway carries therapeutic promise. This article is categorized under: Nervous System Development > Flies Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: General Principles.

Elevated endothelial Sox2 causes lumen disruption and cerebral arteriovenous malformations

Lumen integrity in vascularization requires fully differentiated endothelial cells (ECs). Here, we report that endothelial-mesenchymal transitions (EndMTs) emerged in ECs of cerebral arteriovenous malformation (AVMs) and caused disruption of the lumen or lumen disorder. We show that excessive Sry-box 2 (Sox2) signaling was responsible for the EndMTs in cerebral AVMs. EC-specific suppression of Sox2 normalized endothelial differentiation and lumen formation and improved the cerebral AVMs. Epigenetic studies showed that induction of Sox2 altered the cerebral-endothelial transcriptional landscape and identified jumonji domain-containing protein 5 (JMJD5) as a direct target of Sox2. Sox2 interacted with JMJD5 to induce EndMTs in cerebral ECs. Furthermore, we utilized a high-throughput system to identify the β-adrenergic antagonist pronethalol as an inhibitor of Sox2 expression. Treatment with pronethalol stabilized endothelial differentiation and lumen formation, which limited the cerebral AVMs.

Crosstalk between BMP and Notch Induces Sox2 in Cerebral Endothelial Cells

Bone morphogenetic protein (BMP) and Notch signaling are critical for endothelial cell (EC) differentiation in vascular development. Recent studies have shown that excess BMP activity induces Notch signaling in cerebral ECs resulting in arteriovenous malformation (AVMs). However, it is unclear how the crosstalk between BMP and Notch signaling affects cerebral EC differentiation at the gene regulatory level. Here, we report that BMP6 activates the activin receptor-like kinase (ALK) 3, a BMP type 1 receptor, to induce Notch1 receptor and Jagged1 and Jagged2 ligands. We show that increased expression of the Notch components alters the transcriptional regulatory complex in the SRY-Box 2 (Sox2) promoter region so as to induce its expression in cerebral ECs. Together, our results identify Sox2 as a direct target of BMP and Notch signaling and provide information on how altered BMP and Notch signaling affects the endothelial transcriptional landscape.

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.

Role of Notch Signaling Pathway in Glioblastoma Pathogenesis

Notch signaling is an evolutionarily conserved pathway that regulates important biological processes, such as cell proliferation, apoptosis, migration, self-renewal, and differentiation. In mammals, Notch signaling is composed of four receptors (Notch1–4) and five ligands (Dll1-3–4, Jagged1–2) that mainly contribute to the development and maintenance of the central nervous system (CNS). Neural stem cells (NSCs) are the starting point for neurogenesis and other neurological functions, representing an essential aspect for the homeostasis of the CNS. Therefore, genetic and functional alterations to NSCs can lead to the development of brain tumors, including glioblastoma. Glioblastoma remains an incurable disease, and the reason for the failure of current therapies and tumor relapse is the presence of a small subpopulation of tumor cells known as glioma stem cells (GSCs), characterized by their stem cell-like properties and aggressive phenotype. Growing evidence reveals that Notch signaling is highly active in GSCs, where it suppresses differentiation and maintains stem-like properties, contributing to Glioblastoma tumorigenesis and conventional-treatment resistance. In this review, we try to give a comprehensive view of the contribution of Notch signaling to Glioblastoma and its possible implication as a target for new therapeutic approaches.

NOTCH regulation of the endothelial cell phenotype

Purpose of review

The formation of a hierarchical vascular network is a complex process that requires precise temporal and spatial integration of several signaling pathways. Amongst those, Notch has emerged as a key regulator of multiple steps that expand from endothelial sprouting to arterial specification and remains relevant in the adult. This review aims to summarize major concepts and rising hypotheses on the role of Notch signaling in the endothelium.

Recent findings

A wealth of new information has helped to clarify how Notch signaling cooperates with other pathways to orchestrate vascular morphogenesis, branching, and function. Endothelial vascular endothelial growth factor, C-X-C chemokine receptor type 4, and nicotinamide adenine dinucleotide phosphate oxidase 2 have been highlighted as key regulators of the pathway. Furthermore, blood flow forces during vascular development induce Notch1 signaling to suppress endothelial cell proliferation, enhance barrier function, and promote arterial specification. Importantly, Notch1 has been recently recognized as an endothelial mechanosensor that is highly responsive to the level of shear stress to enable differential Notch activation in distinct regions of the vessel wall and suppress inflammation.

Summary

Although it is well accepted that the Notch signaling pathway is essential for vascular morphogenesis, its contributions to the homeostasis of adult endothelium were uncovered only recently. Furthermore, its exquisite regulation by flow and impressive interface with multiple signaling pathways indicates that Notch is at the center of a highly interactive web that integrates both physical and chemical signals to ensure vascular stability.

Molecular Pathological Differences in Global Gene Expression between Two Sustained Proliferative Lesions, Nodular Regenerative Hepatocellular Hyperplasia and Hepatocellular Adenoma, in Mice

Long-term exposure to piperonyl butoxide (PBO) induces multiple nodular masses along with hepatocellular tumors in the liver of mice. The histopathological features of the nodules led to our diagnosis of nodular regenerative hepatocellular hyperplasia (NRH). However, because of the lack of data on the biological characteristics of NRH, whether this lesion is truly nonneoplastic remains unknown. In this study, the molecular characteristics of NRH were compared with those of hepatocellular adenoma (HCA) by global gene expression analysis. Six-week-old male ICR mice were fed a diet containing 6,000 ppm PBO for 43 weeks to induce NRH and HCA development. Complementary DNA microarray analysis was performed using messenger RNA extracted from NRH and HCA frozen sections collected by laser microdissection. Hierarchical cluster analysis showed that all NRH samples clustered together but were separate from the HCA cluster. Pathway analysis revealed activation of the cell cycle and Delta-Notch signaling in both lesions, but the latter was more upregulated in HCA. Downregulation of cytochrome p450 enzymes was observed in NRH, but not in HCA. These results imply that NRH differs from HCA in terms of not only morphological but also molecular characteristics.

Beyond endothelial cells: Vascular endothelial growth factors in heart, vascular anomalies and placenta

Vascular endothelial growth factors regulate vascular and lymphatic growth. Dysregulation of VEGF signaling is connected to many pathological states, including hemangiomas, arteriovenous malformations and placental abnormalities. In heart, VEGF gene transfer induces myocardial angiogenesis. Besides vascular and lymphatic endothelial cells, VEGFs affect multiple other cell types. Understanding VEGF biology and its paracrine signaling properties will offer new targets for novel treatments of several diseases.

Gene expression analysis of nidus of cerebral arteriovenous malformations reveals vascular structures with deficient differentiation and maturation

Objective

Arteriovenous malformations (AVMs) are characterised by tangles of dysplastic blood vessels which shunt blood from arteries to veins with no intervening capillary bed. It is not known at what stage of development and differentiation, AVM vessels became aberrant. To address this, we have analysed the expression of vascular differentiation, vascular maturation and brain capillary specific genes in AVM nidus.

Methodology

We performed immunohistochemistry and western blot analysis of vascular differentiation (HEY2, DLL4, EFNB2, and COUP-TFII), vascular maturation (ENG and KLF2) and brain capillary specific genes (GGTP and GLUT1) on ten surgically excised human brain AVMs and ten normal human brain tissues.

Results

Immunohistochemical analysis revealed that AVM vessels co-express both artery and vein differentiation genes. H-score analysis revealed that there is statistically significant (P < 0.0001) increase in expression of these proteins in AVM vessels compared to control vessels. These findings were further confirmed by western blot analysis and found to be statistically significant (P < 0.0001 and P < 0.001) for all proteins except Hey2. Both immunostaining and western blot analysis revealed that AVM vessels express GGTP and GLUT1, markers specific to brain capillaries. Immunofluorescent staining demonstrated that expression of KLF2, a vascular maturation marker is significantly (P <0.001) decreased in AVM vessels and was further confirmed by western blot analysis (P < 0.001). Immunohistochemical and western blot analysis demonstrated that another vascular maturation protein Endoglin had high expression in AVM vessels compared to control vessels. The results were found to be statistically significant (P < 0.0001).

Summary

Our findings suggest that vascular structures of AVMs co-express markers specific for arteries, veins and capillaries. We conclude that AVM nidus constitutes of aberrant vessels which are not terminally differentiated and inadequately matured.

A novel reporter allele for monitoring Dll4 expression within the embryonic and adult mouse

Canonical Notch signaling requires the presence of a membrane bound ligand and a corresponding transmembrane Notch receptor. Receptor engagement induces multiple proteolytic cleavage events culminating in the nuclear accumulation of the Notch intracellular domain and its binding to a transcriptional co-factor to mediate gene expression. Notch signaling networks are essential regulators of vascular patterning and angiogenesis, as well as myriad other biological processes. Delta-like 4 (Dll4) encodes the earliest Notch ligand detected in arterial cells, and is enriched in sprouting endothelial tip cells. Dll4 expression has often been inferred by proxy using a lacZ knockin reporter allele. This is problematic, as a single copy of Dll4 is haploinsufficient. Additionally, Notch activity regulates Dll4 transcription, making it unclear whether these reporter lines accurately reflect Dll4 expression. Accordingly, precisely defining Dll4 expression is essential for determining its role in development and disease. To address these limitations, we generated a novel BAC transgenic allele with a nuclear-localized β-galactosidase reporter (Dll4-BAC-nlacZ). Through a comparative analysis, we show the BAC line overcomes previous issues of haploinsufficiency, it recapitulates Dll4 expression in vivo, and allows superior visualization and imaging. As such, this novel Dll4 reporter is an important addition to the growing Notch toolkit.

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.

ALK1 signaling in development and disease: new paradigms

Activin A receptor like type 1 (ALK1) is a transmembrane serine/threonine receptor kinase in the transforming growth factor-beta receptor family that is expressed on endothelial cells. Defects in ALK1 signaling cause the autosomal dominant vascular disorder, hereditary hemorrhagic telangiectasia (HHT), which is characterized by development of direct connections between arteries and veins, or arteriovenous malformations (AVMs). Although previous studies have implicated ALK1 in various aspects of sprouting angiogenesis, including tip/stalk cell selection, migration, and proliferation, recent work suggests an intriguing role for ALK1 in transducing a flow-based signal that governs directed endothelial cell migration within patent, perfused vessels. In this review, we present an updated view of the mechanism of ALK1 signaling, put forth a unified hypothesis to explain the cellular missteps that lead to AVMs associated with ALK1 deficiency, and discuss emerging roles for ALK1 signaling in diseases beyond HHT.

Blood flow boosts BMP signaling to keep vessels in shape

Bone morphogenic proteins (BMPs) and blood flow regulate vascular remodeling and homeostasis. In this issue, Baeyens et al. (2016. J. Cell Biol http://dx.doi.org/10.1083/jcb.201603106) show that blood flow sensitizes endothelial cells to BMP9 signaling by triggering Alk1/ENG complexing to suppress cell proliferation and to recruit mural cells, thereby establishing endothelial quiescence.

Notch4 and mhc class II polymorphisms are associated with hcv-related benign and malignant lymphoproliferative diseases

Mixed cryoglobulinemia (MC), is a HCV-related, clinically benign, lymphoproliferative disorder (LPD) that may evolve into a non Hodgkin's lymphoma (NHL). Significant associations were found between two single nucleotide polymorphisms near NOTCH4 (rs2071286) and the HLA class II (rs9461776) genes and HCV-related MC syndrome (MCS). We analyzed NOTCH4 rs2071286 and HLA-II rs9461776 in 3 HCV-related LPD groups (asymptomatic MC, MCS, NHL) with HCV infection without lymphoproliferative disorders.

We found a positive relationship between NOTCH4 rs207186 T minor allele frequency (MAF) and patients with HCV-related LPDs at risk of NHL (Chi-square test for trend = 14.84 p = 0.0001), in accordance with an over-dominant model in the NHL group (CT vs CC + TT, OR=1.88, 95% CI 1.24–2.83, p = 0.0026).

Regarding HLA II rs9461776, G MAF increased in patients with HCV-related LPDs at risk of NHL (Chi-square test for trend = 8.40 p = 0.0038), in accordance with a recessive genotypic model in the NHL group (G/G vs A/A + A/G, OR = 11.07, 95% CI 2.37–51.64, p = 0.0022).

Both NOTCH4 rs2071286 and HLA-II rs9461776 were present on chromosome 6 and showed D’ and r values of linkage disequilibrium (LD) of about 0.5 values, thereby suggesting there is no extensive LD in the HCV+ population.

This data shows that the previously demonstrated association between NOTCH4 rs2071286 and HLA-II rs9461776 polymorphisms and HCV-related MCS could be extended to overall patients with HCV-related LPDs. The significant relationship between rs2071286 and rs9461776 MAF and the increased risk for NHL, suggests their use as non-invasive markers to categorize patients at risk of lymphoma.