Heterozygous deficiency of PHD2 restores tumor oxygenation and inhibits metastasis via endothelial normalization.

A key function of blood vessels, to supply oxygen, is impaired in tumors because of abnormalities in their endothelial lining. PHD proteins serve as oxygen sensors and may regulate oxygen delivery. We therefore studied the role of endothelial PHD2 in vessel shaping by implanting tumors in PHD2(+/-) mice. Haplodeficiency of PHD2 did not affect tumor vessel density or lumen size, but normalized the endothelial lining and vessel maturation. This resulted in improved tumor perfusion and oxygenation and inhibited tumor cell invasion, intravasation, and metastasis. Haplodeficiency of PHD2 redirected the specification of endothelial tip cells to a more quiescent cell type, lacking filopodia and arrayed in a phalanx formation. This transition relied on HIF-driven upregulation of (soluble) VEGFR-1 and VE-cadherin. Thus, decreased activity of an oxygen sensor in hypoxic conditions prompts endothelial cells to readjust their shape and phenotype to restore oxygen supply. Inhibition of PHD2 may offer alternative therapeutic opportunities for anticancer therapy.

Fgfbp1 promotes blood-brain barrier development by regulating collagen IV deposition and maintaining Wnt/ß-catenin signaling.

Central nervous system (CNS) blood vessels contain a functional blood-brain barrier (BBB) that is necessary for neuronal survival and activity. Although Wnt/ß-catenin signaling is essential for BBB development, its downstream targets within the neurovasculature remain poorly understood. To identify targets of Wnt/ß-catenin signaling underlying BBB maturation, we performed a microarray analysis that identified Fgfbp1 as a novel Wnt/ß-catenin-regulated gene in mouse brain endothelial cells (mBECs). Fgfbp1 is expressed in the CNS endothelium and secreted into the vascular basement membrane during BBB formation. Endothelial genetic ablation of Fgfbp1 results in transient hypervascularization but delays BBB maturation in specific CNS regions, as evidenced by both upregulation of Plvap and increased tracer leakage across the neurovasculature due to reduced Wnt/ß-catenin activity. In addition, collagen IV deposition in the vascular basement membrane is reduced in mutant mice, leading to defective endothelial cell-pericyte interactions. Fgfbp1 is required cell-autonomously in mBECs to concentrate Wnt ligands near cell junctions and promote maturation of their barrier properties in vitro Thus, Fgfbp1 is a crucial extracellular matrix protein during BBB maturation that regulates cell-cell interactions and Wnt/ß-catenin activity.

Inflammation and neutrophil extracellular traps in cerebral cavernous malformation.

Cerebral Cavernous Malformation (CCM) is a brain vascular disease with various neurological symptoms. In this study, we describe the inflammatory profile in CCM and show for the first time the formation of neutrophil extracellular traps (NETs) in rodents and humans with CCM. Through RNA-seq analysis of cerebellum endothelial cells from wild-type mice and mice with an endothelial cell-specific ablation of the Ccm3 gene (Ccm3iECKO), we show that endothelial cells from Ccm3iECKO mice have an increased expression of inflammation-related genes. These genes encode proinflammatory cytokines and chemokines, as well as adhesion molecules, which promote recruitment of inflammatory and immune cells. Similarly, immunoassays showed elevated levels of these cytokines and chemokines in the cerebellum of the Ccm3iECKO mice. Consistently, both flow cytometry and immunofluorescence analysis showed infiltration of different subsets of leukocytes into the CCM lesions. Neutrophils, which are known to fight against infection through different strategies, including the formation of NETs, represented the leukocyte subset within the most pronounced increase in CCM. Here, we detected elevated levels of NETs in the blood and the deposition of NETs in the cerebral cavernomas of Ccm3iECKO mice. Degradation of NETs by DNase I treatment improved the vascular barrier. The deposition of NETs in the cavernomas  of patients with CCM confirms the clinical relevance of NETs in CCM.

JAM-A Acts via C/EBP-α to Promote Claudin-5 Expression and Enhance Endothelial Barrier Function.

RATIONALE: Intercellular tight junctions are crucial for correct regulation of the endothelial barrier. Their composition and integrity are affected in pathological contexts, such as inflammation and tumor growth. JAM-A (junctional adhesion molecule A) is a transmembrane component of tight junctions with a role in maintenance of endothelial barrier function, although how this is accomplished remains elusive. OBJECTIVE: We aimed to understand the molecular mechanisms through which JAM-A expression regulates tight junction organization to control endothelial permeability, with potential implications under pathological conditions. METHODS AND RESULTS: Genetic deletion of JAM-A in mice significantly increased vascular permeability. This was associated with significantly decreased expression of claudin-5 in the vasculature of various tissues, including brain and lung. We observed that C/EBP-α (CCAAT/enhancer-binding protein-α) can act as a transcription factor to trigger the expression of claudin-5 downstream of JAM-A, to thus enhance vascular barrier function. Accordingly, gain-of-function for C/EBP-α increased claudin-5 expression and decreased endothelial permeability, as measured by the passage of fluorescein isothiocyanate (FITC)-dextran through endothelial monolayers. Conversely, C/EBP-α loss-of-function showed the opposite effects of decreased claudin-5 levels and increased endothelial permeability. Mechanistically, JAM-A promoted C/EBP-α expression through suppression of ß-catenin transcriptional activity, and also through activation of EPAC (exchange protein directly activated by cAMP). C/EBP-α then directly binds the promoter of claudin-5 to thereby promote its transcription. Finally, JAM-A-C/EBP-α-mediated regulation of claudin-5 was lost in blood vessels from tissue biopsies from patients with glioblastoma and ovarian cancer. CONCLUSIONS: We describe here a novel role for the transcription factor C/EBP-α that is positively modulated by JAM-A, a component of tight junctions that acts through EPAC to up-regulate the expression of claudin-5, to thus decrease endothelial permeability. Overall, these data unravel a regulatory molecular pathway through which tight junctions limit vascular permeability. This will help in the identification of further therapeutic targets for diseases associated with endothelial barrier dysfunction. Graphic Abstract: An graphic abstract is available for this article.

Propranolol Reduces the Development of Lesions and Rescues Barrier Function in Cerebral Cavernous Malformations: A Preclinical Study.

[Figure: see text].

Deregulated TGF-ß/BMP Signaling in Vascular Malformations.

Correct organization of the vascular tree requires the balanced activities of several signaling pathways that regulate tubulogenesis and vascular branching, elongation, and pruning. When this balance is lost, the vessels can be malformed and fragile, and they can lose arteriovenous differentiation. In this review, we concentrate on the transforming growth factor (TGF)-ß/bone morphogenetic protein (BMP) pathway, which is one of the most important and complex signaling systems in vascular development. Inactivation of these pathways can lead to altered vascular organization in the embryo. In addition, many vascular malformations are related to deregulation of TGF-ß/BMP signaling. Here, we focus on two of the most studied vascular malformations that are induced by deregulation of TGF-ß/BMP signaling: hereditary hemorrhagic telangiectasia (HHT) and cerebral cavernous malformation (CCM). The first of these is related to loss-of-function mutation of the TGF-ß/BMP receptor complex and the second to increased signaling sensitivity to TGF-ß/BMP. In this review, we discuss the potential therapeutic targets against these vascular malformations identified so far, as well as their basis in general mechanisms of vascular development and stability.

EndMT contributes to the onset and progression of cerebral cavernous malformations.

Cerebral cavernous malformation (CCM) is a vascular dysplasia, mainly localized within the brain and affecting up to 0.5% of the human population. CCM lesions are formed by enlarged and irregular blood vessels that often result in cerebral haemorrhages. CCM is caused by loss-of-function mutations in one of three genes, namely CCM1 (also known as KRIT1), CCM2 (OSM) and CCM3 (PDCD10), and occurs in both sporadic and familial forms. Recent studies have investigated the cause of vascular dysplasia and fragility in CCM, but the in vivo functions of this ternary complex remain unclear. Postnatal deletion of any of the three Ccm genes in mouse endothelium results in a severe phenotype, characterized by multiple brain vascular malformations that are markedly similar to human CCM lesions. Endothelial-to-mesenchymal transition (EndMT) has been described in different pathologies, and it is defined as the acquisition of mesenchymal- and stem-cell-like characteristics by the endothelium. Here we show that endothelial-specific disruption of the Ccm1 gene in mice induces EndMT, which contributes to the development of vascular malformations. EndMT in CCM1-ablated endothelial cells is mediated by the upregulation of endogenous BMP6 that, in turn, activates the transforming growth factor-ß (TGF-ß) and bone morphogenetic protein (BMP) signalling pathway. Inhibitors of the TGF-ß and BMP pathway prevent EndMT both in vitro and in vivo and reduce the number and size of vascular lesions in CCM1-deficient mice. Thus, increased TGF-ß and BMP signalling, and the consequent EndMT of CCM1-null endothelial cells, are crucial events in the onset and progression of CCM disease. These studies offer novel therapeutic opportunities for this severe, and so far incurable, pathology.

Sulindac metabolites decrease cerebrovascular malformations in CCM3-knockout mice.

Cerebral cavernous malformation (CCM) is a disease of the central nervous system causing hemorrhage-prone multiple lumen vascular malformations and very severe neurological consequences. At present, the only recommended treatment of CCM is surgical. Because surgery is often not applicable, pharmacological treatment would be highly desirable. We describe here a murine model of the disease that develops after endothelial-cell-selective ablation of the CCM3 gene. We report an early, cell-autonomous, Wnt-receptor-independent stimulation of ß-catenin transcription activity in CCM3-deficient endothelial cells both in vitro and in vivo and a triggering of a ß-catenin-driven transcription program that leads to endothelial-to-mesenchymal transition. TGF-ß/BMP signaling is then required for the progression of the disease. We also found that the anti-inflammatory drugs sulindac sulfide and sulindac sulfone, which attenuate ß-catenin transcription activity, reduce vascular malformations in endothelial CCM3-deficient mice. This study opens previously unidentified perspectives for an effective pharmacological therapy of intracranial vascular cavernomas.

Endothelial cell disease: emerging knowledge from cerebral cavernous malformations.

PURPOSE OF REVIEW: Endothelial cells dysfunctions are crucial determinants of several human diseases. We review here the most recent reports on endothelial cell defects in cerebral cavernous malformations (CCMs), particularly focusing on adherens junctions. CCM is a vascular disease that affects specifically the venous microvessels of the central nervous system and which is caused by loss-of-function mutation in any one of the three CCM genes (CCM1, 2 or 3) in endothelial cells. The phenotypic result of these mutations are focal vascular malformations that are permeable and fragile causing neurological symptoms and occasionally haemorrhagic stroke. RECENT FINDINGS: CCM is still an incurable disease, as no pharmacological treatment is available, besides surgery. The definition of the molecular alterations ensuing loss of function mutation of CCM genes is contributing to orientate the testing of targeted pharmacological tools.Several signalling pathways are altered in the three genotypes in a similar way and concur in the acquisition of mesenchymal markers in endothelial cells. However, also genotype-specific defects are reported, in particular for the CCM1 and CCM3 mutation. SUMMARY: Besides the specific CCM disease, the characterization of endothelial alterations in CCM has the potentiality to shed light on basic molecular regulations as the acquisition and maintenance of organ and vascular site specificity of endothelial cells.

Endothelial Cells Lining Sporadic Cerebral Cavernous Malformation Cavernomas Undergo Endothelial-to-Mesenchymal Transition.

BACKGROUND AND PURPOSE: Cerebral cavernous malformation (CCM) is characterized by multiple lumen vascular malformations in the central nervous system that can cause neurological symptoms and brain hemorrhages. About 20% of CCM patients have an inherited form of the disease with ubiquitous loss-of-function mutation in any one of 3 genes CCM1, CCM2, and CCM3. The rest of patients develop sporadic vascular lesions histologically similar to those of the inherited form and likely mediated by a biallelic acquired mutation of CCM genes in the brain vasculature. However, the molecular phenotypic features of endothelial cells in CCM lesions in sporadic patients are still poorly described. This information is crucial for a targeted therapy. METHODS: We used immunofluorescence microscopy and immunohistochemistry to analyze the expression of endothelial-to-mesenchymal transition markers in the cavernoma of sporadic CCM patients in parallel with human familial cavernoma as a reference control. RESULTS: We report here that endothelial cells, a cell type critically involved in CCM development, undergo endothelial-to-mesenchymal transition in the lesions of sporadic patients. This switch in endothelial phenotype has been described only in genetic CCM patients and in murine models of the disease. In addition, TGF-ß/p-Smad- and ß-catenin-dependent signaling pathways seem activated in sporadic cavernomas as in familial ones. CONCLUSIONS: Our findings support the use of common therapeutic strategies for both sporadic and genetic CCM malformations.

VE-cadherin at a glance.

Although being a monolayer the vascular endothelium controls fundamental vessel functions such as permeability, leukocyte extravasation and angiogenesis. The endothelial selective transmembrane constituent of adherens junctions, Vascular Endothelial- (VE-) cadherin plays a crucial role in the regulation of such activities. The signaling pathways controlled by VE-cadherin as well as the ones that regulate VE-cadherin activity start to be elucidated. This delineates a complex network of molecular and functional interactions that can be altered in pathologies.

Overlapping and divergent signaling pathways of N-cadherin and VE-cadherin in endothelial cells.

Endothelial cells (ECs) express 2 members of the cadherin family, VE and N-cadherin. Although VE-cadherin induces EC homotypic adhesion, N-cadherin function in ECs remains largely unknown. EC-specific inactivation of either VE or N-cadherin leads to early fetal lethality suggesting that these cadherins play a nonredundant role in vascular development. We report here that VE-cadherin negatively controls junctional localization and expression of N-cadherin by limiting p120-catenin availability and reducing ß-catenin transcriptional activity. Using EC lines expressing either VE or N-cadherin we found that both cadherins inhibit cell proliferation and apoptosis. Both trigger the phosphatidylinositol-3-OH-kinase (PI3K)-AKT-Forkhead-box protein-O1 (FoxO1) pathway and reduce ß-catenin transcriptional activity. The extent of signaling correlates with the total level of cadherins regardless of the type of cadherin expressed. In contrast, basal and fibroblast growth factor (FGF)-induced cell motility is promoted by N-cadherin and strongly inhibited by VE-cadherin. This opposite effect is partly because of the ability of VE-cadherin to associate with FGF receptor and the density-enhanced phosphatase-1 (Dep-1) which, in turn, inhibits receptor signaling. We conclude that VE and N-cadherin have both additive and divergent effects on ECs. Differences in signaling are due, in part, to cadherin association with growth factor receptors and modulation of their downstream signaling.

CCM1 regulates vascular-lumen organization by inducing endothelial polarity.

Little is known about the molecular mechanisms that regulate the organization of vascular lumen. In this paper we show that lumen formation correlates with endothelial polarization. Adherens junctions (AJs) and VE-cadherin (VEC, encoded by CDH5) are required for endothelial apicobasal polarity in vitro and during embryonic development. Silencing of CDH5 gene expression leads to abrogation of endothelial polarity accompanied by strong alterations in lumenal structure. VEC co-distributes with members of the Par polarity complex (Par3 and PKCzeta) and is needed for activation of PKCzeta. CCM1 is encoded by the CCM1 gene, which is mutated in 60% of patients affected by cerebral cavernous malformation (CCM). The protein interacts with VEC and directs AJ organization and AJ association with the polarity complex, both in cell-culture models and in human CCM1 lesions. Both VEC and CCM1 control Rap1 concentration at cell-cell junctions. We propose that VEC, CCM1 and Rap1 form a signaling complex. In the absence of any of these proteins, AJs are dismantled, cell polarity is lost and vascular lumenal structure is severely altered.

Vascular endothelial cadherin controls VEGFR-2 internalization and signaling from intracellular compartments.

Receptor endocytosis is a fundamental step in controlling the magnitude, duration, and nature of cell signaling events. Confluent endothelial cells are contact inhibited in their growth and respond poorly to the proliferative signals of vascular endothelial growth factor (VEGF). In a previous study, we found that the association of vascular endothelial cadherin (VEC) with VEGF receptor (VEGFR) type 2 contributes to density-dependent growth inhibition (Lampugnani, G.M., A. Zanetti, M. Corada, T. Takahashi, G. Balconi, F. Breviario, F. Orsenigo, A. Cattelino, R. Kemler, T.O. Daniel, and E. Dejana. 2003. J. Cell Biol. 161:793-804). In the present study, we describe the mechanism through which VEC reduces VEGFR-2 signaling. We found that VEGF induces the clathrin-dependent internalization of VEGFR-2. When VEC is absent or not engaged at junctions, VEGFR-2 is internalized more rapidly and remains in endosomal compartments for a longer time. Internalization does not terminate its signaling; instead, the internalized receptor is phosphorylated, codistributes with active phospholipase C-gamma, and activates p44/42 mitogen-activated protein kinase phosphorylation and cell proliferation. Inhibition of VEGFR-2 internalization reestablishes the contact inhibition of cell growth, whereas silencing the junction-associated density-enhanced phosphatase-1/CD148 phosphatase restores VEGFR-2 internalization and signaling. Thus, VEC limits cell proliferation by retaining VEGFR-2 at the membrane and preventing its internalization into signaling compartments.

Mapping endothelial-cell diversity in cerebral cavernous malformations at single-cell resolution.

Cerebral cavernous malformation (CCM) is a rare neurovascular disease that is characterized by enlarged and irregular blood vessels that often lead to cerebral hemorrhage. Loss-of-function mutations to any of three genes results in CCM lesion formation; namely, KRIT1, CCM2, and PDCD10 (CCM3). Here, we report for the first time in-depth single-cell RNA sequencing, combined with spatial transcriptomics and immunohistochemistry, to comprehensively characterize subclasses of brain endothelial cells (ECs) under both normal conditions and after deletion of Pdcd10 (Ccm3) in a mouse model of CCM. Integrated single-cell analysis identifies arterial ECs as refractory to CCM transformation. Conversely, a subset of angiogenic venous capillary ECs and respective resident endothelial progenitors appear to be at the origin of CCM lesions. These data are relevant for the understanding of the plasticity of the brain vascular system and provide novel insights into the molecular basis of CCM disease at the single cell level.

Propranolol for familial cerebral cavernous malformation (Treat_CCM): study protocol for a randomized controlled pilot trial.

BACKGROUND: Cerebral cavernous malformations (CCMs) are vascular malformations characterized by clusters of enlarged leaky capillaries in the central nervous system. They may result in intracranial haemorrhage, epileptic seizure(s), or focal neurological deficits, and potentially lead to severe disability. Globally, CCMs represent the second most common intracranial vascular malformation in humans, and their familial form (FCCMs) accounts for one-fifth of cases. Neurosurgical excision, and perhaps stereotactic radiosurgery, is the only available therapeutic option. Case reports suggest that propranolol might modify disease progression. METHODS: Treat_CCM is a prospective, randomized, open-label, blinded endpoint (PROBE), parallel-group trial involving six Italian clinical centres with central reading of brain magnetic resonance imaging (MRI) and adverse events. Patients with symptomatic FCCMs are randomized (2:1 ratio) either to propranolol (40-80 mg twice daily) in addition to standard care or to standard care alone (i.e. anti-epileptic drugs or headache treatments). The primary outcome is intracranial haemorrhage or focal neurological deficit attributable to CCMs. The secondary outcomes are MRI changes over time (i.e. de novo CCM lesions, CCM size and signal characteristics, iron deposition, and vascular leakage as assessed by quantitative susceptibility mapping and dynamic contrast enhanced permeability), disability, health-related quality of life, depression severity, and anxiety (SF-36, BDI-II, State-Trait Anxiety Inventory). DISCUSSION: Treat_CCM will evaluate the safety and efficacy of propranolol for CCMs following promising case reports in a randomized controlled trial. The direction of effect on the primary outcome and the consistency of effects on the secondary outcomes (even if none of them yield statistically significant differences) of this external pilot study may lead to a larger sample size in a definitive phase 2 trial. TRIAL REGISTRATION: ClinicalTrails.gov, NCT03589014. Retrospectively registered on 17 July 2018.

Endothelial cell clonal expansion in the development of cerebral cavernous malformations.

Cerebral cavernous malformation (CCM) is a neurovascular familial or sporadic disease that is characterised by capillary-venous cavernomas, and is due to loss-of-function mutations to any one of three CCM genes. Familial CCM follows a two-hit mechanism similar to that of tumour suppressor genes, while in sporadic cavernomas only a small fraction of endothelial cells shows mutated CCM genes. We reported that in mouse models and in human patients, endothelial cells lining the lesions have different features from the surrounding endothelium, as they express mesenchymal/stem-cell markers. Here we show that cavernomas originate from clonal expansion of few Ccm3-null endothelial cells that express mesenchymal/stem-cell markers. These cells then attract surrounding wild-type endothelial cells, inducing them to express mesenchymal/stem-cell markers and to contribute to cavernoma growth. These characteristics of Ccm3-null cells are reminiscent of the tumour-initiating cells that are responsible for tumour growth. Our data support the concept that CCM has benign tumour characteristics.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease.

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.

The control of endothelial cell functions by adherens junctions.

Cell to cell junctions are important regulators of endothelial responses both in quiescent and angiogenic vessels. Endothelial cells express tight and adherens junctional structures. Although different in their specific molecular composition, these junctional complexes present a relatively similar general arrangement. Both types of junctions are formed by transmembrane adhesive proteins that bind homophilically to identical proteins on an adjacent cell and start a sequence of signalling events. Signal transmission is mediated by interaction with cytoplasmic and transmembrane partners. Adherens junctions are ubiquitous along the vascular tree. In these structures adhesion is mediated by VE-cadherin and its intracellular partners. In vitro and in vivo data show that VE-cadherin is required for endothelial integrity in quiescent vessels and for the correct organization of new vessels. VE-cadherin regulates endothelial functions through different mechanisms that include: (i) direct activation of signalling molecules such as PI3 kinase and Rac, to sustain survival and organization of the actin cytoskeleton; (ii) regulation of gene transcription, possibly modulating the nuclear level of transcription co-factors such as beta-catenin and p120; (iii) formation of complexes with growth factor receptors, such as the type 2 receptor of VEGF (VEGFR2) and modulation of their signalling properties.