Tissue-specific conditional CCM2 knockout mice establish the essential role of endothelial CCM2 in angiogenesis: implications for human cerebral cavernous malformations

Impaired retinoic acid signaling in cerebral cavernous malformations

The capillary-venous pathology cerebral cavernous malformation (CCM) is caused by loss of CCM1/Krev interaction trapped protein 1 (KRIT1), CCM2/MGC4607, or CCM3/PDCD10 in some endothelial cells. Mutations of CCM genes within the brain vasculature can lead to recurrent cerebral hemorrhages. Pharmacological treatment options are urgently needed when lesions are located in deeply-seated and in-operable regions of the central nervous system. Previous pharmacological suppression screens in disease models of CCM led to the discovery that treatment with retinoic acid improved CCM phenotypes. This finding raised a need to investigate the involvement of retinoic acid in CCM and test whether it has a curative effect in preclinical mouse models. Here, we show that components of the retinoic acid synthesis and degradation pathway are transcriptionally misregulated across disease models of CCM. We complemented this analysis by pharmacologically modifying retinoic acid levels in zebrafish and human endothelial cell models of CCM, and in acute and chronic mouse models of CCM. Our pharmacological intervention studies in CCM2-depleted human umbilical vein endothelial cells (HUVECs) and krit1 mutant zebrafish showed positive effects when retinoic acid levels were increased. However, therapeutic approaches to prevent the development of vascular lesions in adult chronic murine models of CCM were drug regiment-sensitive, possibly due to adverse developmental effects of this hormone. A treatment with high doses of retinoic acid even worsened CCM lesions in an adult chronic murine model of CCM. This study provides evidence that retinoic acid signaling is impaired in the CCM pathophysiology and suggests that modification of retinoic acid levels can alleviate CCM phenotypes.

Genetics of brain arteriovenous malformations and cerebral cavernous malformations

Cerebrovascular malformations comprise abnormal development of cerebral vasculature. They can result in hemorrhagic stroke due to rupture of lesions as well as seizures and neurological defects. The most common forms of cerebrovascular malformations are brain arteriovenous malformations (bAVMs) and cerebral cavernous malformations (CCMs). They occur in both sporadic and inherited forms. Rapidly evolving molecular genetic methodologies have helped to identify causative or associated genes involved in genesis of bAVMs and CCMs. In this review, we highlight the current knowledge regarding the genetic basis of these malformations.

mPR-Specific Actions Influence Maintenance of the Blood–Brain Barrier (BBB)

Cerebral cavernous malformations (CCMs) are characterized by abnormally dilated intracranial microvascular sinusoids that result in increased susceptibility to hemorrhagic stroke. It has been demonstrated that three CCM proteins (CCM1, CCM2, and CCM3) form the CCM signaling complex (CSC) to mediate angiogenic signaling. Disruption of the CSC will result in hemorrhagic CCMs, a consequence of compromised blood–brain barrier (BBB) integrity. Due to their characteristically incomplete penetrance, the majority of CCM mutation carriers (presumed CCM patients) are largely asymptomatic, but when symptoms occur, the disease has typically reached a clinical stage of focal hemorrhage with irreversible brain damage. We recently reported that the CSC couples both classic (nuclear; nPRs) and nonclassic (membrane; mPRs) progesterone (PRG)-receptors-mediated signaling within the CSC-mPRs-PRG (CmP) signaling network in nPR(−) breast cancer cells. In this report, we demonstrate that depletion of any of the three CCM genes or treatment with mPR-specific PRG actions (PRG/mifepristone) results in the disruption of the CmP signaling network, leading to increased permeability in the nPR(−) endothelial cells (ECs) monolayer in vitro. Finally, utilizing our in vivo hemizygous Ccm mutant mice models, we demonstrate that depletion of any of the three CCM genes, in combination with mPR-specific PRG actions, is also capable of leading to defective homeostasis of PRG in vivo and subsequent BBB disruption, allowing us to identify a specific panel of etiological blood biomarkers associated with BBB disruption. To our knowledge, this is the first report detailing the etiology to predict the occurrence of a disrupted BBB, an indication of early hemorrhagic events.

Cellular protrusions in 3D: Orchestrating early mouse embryogenesis

Cellular protrusions generated by the actin cytoskeleton are central to the process of building the body of the embryo. Problems with cellular protrusions underlie human diseases and syndromes, including implantation defects and pregnancy loss, congenital birth defects, and cancer. Cells use protrusive activity together with actin-myosin contractility to create an ordered body shape of the embryo. Here, I review how actin-rich protrusions are used by two major morphological cell types, epithelial and mesenchymal cells, during collective cell migration to sculpt the mouse embryo body. Pre-gastrulation epithelial collective migration of the anterior visceral endoderm is essential for establishing the anterior-posterior body axis. Gastrulation mesenchymal collective migration of the mesoderm wings is crucial for body elongation, and somite and heart formation. Analysis of mouse mutants with disrupted cellular protrusions revealed the key role of protrusions in embryonic morphogenesis and embryo survival. Recent technical approaches have allowed examination of the mechanisms that control cell and tissue movements in vivo in the complex 3D microenvironment of living mouse embryos. Advancing our understanding of protrusion-driven morphogenesis should provide novel insights into human developmental disorders and cancer metastasis.

NOGOB receptor deficiency increases cerebrovascular permeability and hemorrhage via impairing histone acetylation-mediated CCM1/2 expression

The loss function of cerebral cavernous malformation (CCM) genes leads to most CCM lesions characterized by enlarged leaking vascular lesions in the brain. Although we previously showed that NOGOB receptor (NGBR) knockout in endothelial cells (ECs) results in cerebrovascular lesions in the mouse embryo, the molecular mechanism by which NGBR regulates CCM1/2 expression has not been elucidated. Here, we show that genetic depletion of Ngbr in ECs at both postnatal and adult stages results in CCM1/2 expression deficiency and cerebrovascular lesions such as enlarged vessels, blood-brain-barrier hyperpermeability, and cerebral hemorrhage. To reveal the molecular mechanism, we used RNA-sequencing analysis to examine changes in the transcriptome. Surprisingly, we found that the acetyltransferase HBO1 and histone acetylation were downregulated in NGBR-deficient ECs. The mechanistic studies elucidated that NGBR is required for maintaining the expression of CCM1/2 in ECs via HBO1-mediated histone acetylation. ChIP-qPCR data further demonstrated that loss of NGBR impairs the binding of HBO1 and acetylated histone H4K5 and H4K12 on the promotor of the CCM1 and CCM2 genes. Our findings on epigenetic regulation of CCM1 and CCM2 that is modulated by NGBR and HBO1-mediated histone H4 acetylation provide a perspective on the pathogenesis of sporadic CCMs.

Cerebral Cavernous Malformation Pathogenesis: Investigating Lesion Formation and Progression with Animal Models

Cerebral cavernous malformation (CCM) is a cerebromicrovascular disease that affects up to 0.5% of the population. Vessel dilation, decreased endothelial cell-cell contact, and loss of junctional complexes lead to loss of brain endothelial barrier integrity and hemorrhagic lesion formation. Leakage of hemorrhagic lesions results in patient symptoms and complications, including seizures, epilepsy, focal headaches, and hemorrhagic stroke. CCMs are classified as sporadic (sCCM) or familial (fCCM), associated with loss-of-function mutations in KRIT1/CCM1, CCM2, and PDCD10/CCM3. Identifying the CCM proteins has thrust the field forward by (1) revealing cellular processes and signaling pathways underlying fCCM pathogenesis, and (2) facilitating the development of animal models to study CCM protein function. CCM animal models range from various murine models to zebrafish models, with each model providing unique insights into CCM lesion development and progression. Additionally, these animal models serve as preclinical models to study therapeutic options for CCM treatment. This review briefly summarizes CCM disease pathology and the molecular functions of the CCM proteins, followed by an in-depth discussion of animal models used to study CCM pathogenesis and developing therapeutics.

Cerebral cavernous malformations do not fall in the spectrum of PIK3CA-related overgrowth

Somatic gain-of-function (GOF) mutations in phosphatidylinositol-4, 5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), the catalytic subunit of phosphoinositide 3-kinase (PI3K), have been recently discovered in cerebral cavernous malformations (CCMs), raising the possibility that the activation of PI3K pathways is a possible universal regulator of vascular morphogenesis. However, there have been contradicting data presented among various groups and studies. To enhance the current understanding of vascular anomalies, it is essential to explore this possible relationship between altered PI3K signalling pathways and its influence on the pathogenesis of CCMs. GOF PIK3CA-mutants have been linked to overgrowth syndromes, allowing this group of disorders, resulting from somatic activating mutations in PIK3CA, to be collectively named as PIK3CA-related overgrowth spectrum disorders. This paper reviews and attempts to conceptualise the relationships and differences among clinical presentations, genotypic and phenotypic correlations and possible coexistence of PIK3CA and CCM mutations/phenotypes in CCM lesions. Finally, we present a model reflecting our hypothetical understanding of CCM pathogenesis based on a systematic review and conceptualisation of data obtained from other studies.

CCM3 Loss-Induced Lymphatic Defect Is Mediated by the Augmented VEGFR3-ERK1/2 Signaling

OBJECTIVE

Cerebral cavernous malformations (CCMs) can happen anywhere in the body, although they most commonly produce symptoms in the brain. The role of CCM genes in other vascular beds outside the brain and retina is not well-examined, although the 3 CCM-associated genes (CCM1, CCM2, and CCM3) are ubiquitously expressed in all tissues. We aimed to determine the role of CCM gene in lymphatics. Approach and Results: Mice with an inducible pan-endothelial cell (EC) or lymphatic EC deletion of Ccm3 (Pdcd10ECKO or Pdcd10LECKO) exhibit dilated lymphatic capillaries and collecting vessels with abnormal valve structure. Morphological alterations were correlated with lymphatic dysfunction in Pdcd10LECKO mice as determined by Evans blue dye and fluorescein isothiocyanate(FITC)-dextran transport assays. Pdcd10LECKO lymphatics had increased VEGFR3 (vascular endothelial growth factor receptor-3)-ERK1/2 (extracellular signal-regulated kinase 1/2) signaling with lymphatic hyperplasia. Mechanistic studies suggested that VEGFR3 is primarily regulated at a transcriptional level in Ccm3-deficient lymphatic ECs, in an NF-κB (nuclear factor κB)-dependent manner. CCM3 binds to importin alpha 2/KPNA2 (karyopherin subunit alpha 2), and a CCM3 deletion releases KPNA2 to activate NF-κB P65 by facilitating its nuclear translocation and P65-dependent VEGFR3 transcription. Moreover, increased VEGFR3 in lymphatic EC preferentially activates ERK1/2 signaling, which is critical for lymphatic EC proliferation. Importantly, inhibition of VEGFR3 or ERK1/2 rescued the lymphatic defects in structure and function.

CONCLUSIONS

Our data demonstrate that CCM3 deletion augments the VEGFR3-ERK1/2 signaling in lymphatic EC that drives lymphatic hyperplasia and malformation and warrant further investigation on the potential clinical relevance of lymphatic dysfunction in patients with CCM.

The multifaceted PDCD10/CCM3 gene

The programmed cell death 10 (PDCD10) gene was originally identified as an apoptosis-related gene, although it is now usually known as CCM3, as the third causative gene of cerebral cavernous malformation (CCM). CCM is a neurovascular disease that is characterized by vascular malformations and is associated with headaches, seizures, focal neurological deficits, and cerebral hemorrhage. The PDCD10/CCM3 protein has multiple subcellular localizations and interacts with several multi-protein complexes and signaling pathways. Thus PDCD10/CCM3 governs many cellular functions, which include cell-to-cell junctions and cytoskeleton organization, cell proliferation and apoptosis, and exocytosis and angiogenesis. Given its central role in the maintenance of homeostasis of the cell, dysregulation of PDCD10/CCM3 can result in a wide range of altered cell functions. This can lead to severe diseases, including CCM, cognitive disability, and several types of cancers. Here, we review the multifaceted roles of PDCD10/CCM3 in physiology and pathology, with a focus on its functions beyond CCM.

Calm the raging hormone - A new therapeutic strategy involving progesterone-signaling for hemorrhagic CCMs

Cerebral cavernous malformations (CCMs), one of the most common vascular malformations, are characterized by abnormally dilated intracranial microvascular capillaries resulting in increased susceptibility to hemorrhagic stroke. As an autosomal dominant disorder with incomplete penetrance, the majority of CCMs gene mutation carriers are largely asymptomatic but when symptoms occur, the disease has typically reached the stage of focal hemorrhage with irreversible brain damage, while the molecular "trigger" initiating the occurrence of CCM pathology remain elusive. Currently, the invasive neurosurgery removal of CCM lesions is the only option for the treatment, despite the recurrence of the worse symptoms frequently occurring after surgery. Therefore, there is a grave need for identification of molecular targets for therapeutic treatment and biomarkers as risk predictors for hemorrhagic stroke prevention. Based on reported various perturbed angiogenic signaling cascades mediated by the CCM signaling complex (CSC), there have been many proposed candidate drugs, targeting potentially angiogenic-relevant signaling pathways dysregulated by loss of function of one of the CCM proteins, which might not be enough to correct the pathological phenotype, hemorrhagic CCMs. In this review, we describe a new paradigm for the mechanism of hemorrhagic CCM lesions, and propose a new concept for the assurance of the CSC-stability to prevent the devastating outcome of hemorrhagic CCMs.

Cerebral Cavernous Malformation: From Mechanism to Therapy

Cerebral cavernous malformations are acquired vascular anomalies that constitute a common cause of central nervous system hemorrhage and stroke. The past 2 decades have seen a remarkable increase in our understanding of the pathogenesis of this vascular disease. This new knowledge spans genetic causes of sporadic and familial forms of the disease, molecular signaling changes in vascular endothelial cells that underlie the disease, unexpectedly strong environmental effects on disease pathogenesis, and drivers of disease end points such as hemorrhage. These novel insights are the integrated product of human clinical studies, human genetic studies, studies in mouse and zebrafish genetic models, and basic molecular and cellular studies. This review addresses the genetic and molecular underpinnings of cerebral cavernous malformation disease, the mechanisms that lead to lesion hemorrhage, and emerging biomarkers and therapies for clinical treatment of cerebral cavernous malformation disease. It may also serve as an example for how focused basic and clinical investigation and emerging technologies can rapidly unravel a complex disease mechanism.

Mouse models of vascular development and disease

PURPOSE OF REVIEW

The use of genetic models has facilitated the study of the origins and mechanisms of vascular disease. Mouse models have been developed to specifically target endothelial cell populations, with the goal of pinpointing when and where causative mutations wreck their devastating effects. Together, these approaches have propelled the development of therapies by providing an in-vivo platform to evaluate diagnoses and treatment options. This review summarizes the most widely used mouse models that have facilitated the study of vascular disease, with a focus on mouse models of vascular malformations and the road ahead.

RECENT FINDINGS

Over the past 3 decades, the vascular biology scientific community has been steadily generating a powerful toolkit of useful mouse lines that can be used to tightly regulate gene ablation, or to express transgenic genes, in the murine endothelium. Some of these models inducibly (constitutively) alter gene expression across all endothelial cells, or within distinct subsets, by expressing either Cre recombinase (or inducible versions such as CreERT), or the tetracycline controlled transactivator protein tTA (or rtTA). This now relatively standard technology has been used to gain cutting edge insights into vascular disorders, by allowing in-vivo modeling of key molecular pathways identified as dysregulated across the vast spectrum of vascular anomalies, malformations and dysplasias. However, as sequencing of human patient samples expands, the number of interesting candidate molecular culprits keeps increasing. Consequently, there is now a pressing need to create new genetic mouse models to test hypotheses and to query mechanisms underlying vascular disease.

SUMMARY

The current review assesses the collection of mouse driver lines that have been instrumental is identifying genes required for blood vessel formation, remodeling, maintenance/quiescence and disease. In addition, the usefulness of these driver lines is underscored here by cataloguing mouse lines developed to experimentally assess the role of key candidate genes in vascular malformations. Despite this solid and steady progress, numerous new candidate vascular malformation genes have recently been identified for which no mouse model yet exists.

Molecular Genetic Features of Cerebral Cavernous Malformations (CCM) Patients: An Overall View from Genes to Endothelial Cells

Cerebral cavernous malformations (CCMs) are vascular lesions that affect predominantly microvasculature in the brain and spinal cord. CCM can occur either in sporadic or familial form, characterized by autosomal dominant inheritance and development of multiple lesions throughout the patient's life. Three genes associated with CCM are known: CCM1/KRIT1 (krev interaction trapped 1), CCM2/MGC4607 (encoding a protein named malcavernin), and CCM3/PDCD10 (programmed cell death 10). All the mutations identified in these genes cause a loss of function and compromise the protein functions needed for maintaining the vascular barrier integrity. Loss of function of CCM proteins causes molecular disorganization and dysfunction of endothelial adherens junctions. In this review, we provide an overall vision of the CCM pathology, starting with the genetic bases of the disease, describing the role of the proteins, until we reach the cellular level. Thus, we summarize the genetics of CCM, providing a description of CCM genes and mutation features, provided an updated knowledge of the CCM protein structure and function, and discuss the molecular mechanisms through which CCM proteins may act within endothelial cells, particularly in endothelial barrier maintenance/regulation and in cellular signaling.

Production of KRIT1-knockout and KRIT1-knockin Mouse Embryonic Fibroblasts as Cellular Models of CCM Disease

The development of distinct cellular and animal models has allowed the identification and characterization of molecular mechanisms underlying the pathogenesis of cerebral cavernous malformation (CCM) disease. This is a major cerebrovascular disorder of proven genetic origin, affecting 0.5% of the population. Three disease genes have been identified: CCM1/KRIT1, CCM2, and CCM3. These genes encode for proteins implicated in the regulation of major cellular structures and mechanisms, such as cell-cell and cell-matrix adhesion, actin cytoskeleton dynamics, and endothelial-to-mesenchymal transition, suggesting that they may act as pleiotropic regulators of cellular homeostasis. Indeed, accumulated evidence in cellular and animal models demonstrates that emerged pleiotropic functions of CCM proteins are mainly due to their ability to modulate redox-sensitive pathways and mechanisms involved in adaptive responses to oxidative stress and inflammation, thus contributing to the preservation of cellular homeostasis and stress defenses. In particular, we demonstrated that KRIT1 loss-of-function affects master regulators of cellular redox homeostasis and responses to oxidative stress, including major redox-sensitive transcriptional factors and antioxidant proteins, and autophagy, suggesting that altered redox signaling and oxidative stress contribute to CCM pathogenesis, and opening novel preventive and therapeutic perspectives.In this chapter, we describe materials and methods for isolation of mouse embryonic fibroblast (MEF) cells from homozygous KRIT1-knockout mouse embryos, and their transduction with a lentiviral vector encoding KRIT1 to generate cellular models of CCM disease that contributed significantly to the identification of pathogenetic mechanisms.

Dissection of the Role of CCM Genes in Tubulogenesis Using the Drosophila Tracheal System as a Model

Embryos deficient for an essential gene may show complex phenotypes that reflect pleiotropic functions and non-cell-autonomous requirements for the encoded protein. The generation of mosaic animals, where most cells are wild type, but a few cells are mutant, is a powerful tool permitting the detailed analysis of the cell autonomous function of a gene, in a particular cell type, at cellular and subcellular resolutions. Here we apply this method to the analysis of the Cerebral Cavernous Malformations 3 (CCM3) pathway in Drosophila.The conserved CCM3 protein functions together with its binding partner, Germinal Center Kinase III (Wheezy/GckIII in Drosophila, MST3, STK24, and STK25 in human) in the regulation of tube morphogenesis (Bergametti et al. Am J Hum Genet. 76:42-51, 2005; Fidalgo et al. J Cell Sci. 123:1274-1284, 2010; Guclu et al. Neurosurgery. 57:1008-1013, 2005; Lant et al. Nat Commun. 6:6449, 2015; Song et al. Dev Cell. 25:507-519, 2013; Ceccarelli et al. J Biol Chem. 286:25056-25064, 2011; Rehain-Bell et al. Curr Biol. 27:860-867, 2017; Xu et al. Structure. 21:1059-1066, 2013; Zhang et al. Front Biosci. 17:2295-2305, 2012; Zhang et al. Dev Cell. 27:215-226, 2013; Zheng et al. J Clin Invest. 120:2795-2804, 2010). The Drosophila proteins play a role in the regulation of tube shape in the tracheal (respiratory) system, analogous to the role of the human proteins in the vascular system. To understand the cellular basis for tube dilation defects caused by loss of pathway function, we describe techniques for the generation and analysis of positively marked homozygous mutant GckIII tracheal cells, coupled with an "open book" preparation that can be subjected to immunofluorescent analysis. Dozens of mutant tracheal cells are generated per mosaic animal, and neighboring heterozygous cells in the same animal serve as ideal internal controls.

From Genes and Mechanisms to Molecular-Targeted Therapies: The Long Climb to the Cure of Cerebral Cavernous Malformation (CCM) Disease

Cerebral cavernous malformation (CCM) is a rare cerebrovascular disorder of genetic origin consisting of closely clustered, abnormally dilated and leaky capillaries (CCM lesions), which occur predominantly in the central nervous system. CCM lesions can be single or multiple and may result in severe clinical symptoms, including focal neurological deficits, seizures, and intracerebral hemorrhage. Early human genetic studies demonstrated that CCM disease is linked to three chromosomal loci and can be inherited as autosomal dominant condition with incomplete penetrance and highly variable expressivity, eventually leading to the identification of three disease genes, CCM1/KRIT1, CCM2, and CCM3/PDCD10, which encode for structurally unrelated intracellular proteins that lack catalytic domains. Biochemical, molecular, and cellular studies then showed that these proteins are involved in endothelial cell-cell junction and blood-brain barrier stability maintenance through the regulation of major cellular structures and mechanisms, including endothelial cell-cell and cell-matrix adhesion, actin cytoskeleton dynamics, autophagy, and endothelial-to-mesenchymal transition, suggesting that they act as pleiotropic regulators of cellular homeostasis, and opening novel therapeutic perspectives. Indeed, accumulated evidence in cellular and animal models has eventually revealed that the emerged pleiotropic functions of CCM proteins are mainly due to their ability to modulate redox-sensitive pathways and mechanisms involved in adaptive responses to oxidative stress and inflammation, thus contributing to the preservation of cellular homeostasis and stress defenses.In this introductory review, we present a general overview of 20 years of amazing progress in the identification of genetic culprits and molecular mechanisms underlying CCM disease pathogenesis, and the development of targeted therapeutic strategies.

Caveolae-mediated Tie2 signaling contributes to CCM pathogenesis in a brain endothelial cell-specific Pdcd10-deficient mouse model

Cerebral cavernous malformations (CCMs) are vascular abnormalities that primarily occur in adulthood and cause cerebral hemorrhage, stroke, and seizures. CCMs are thought to be initiated by endothelial cell (EC) loss of any one of the three Ccm genes: CCM1 (KRIT1), CCM2 (OSM), or CCM3 (PDCD10). Here we report that mice with a brain EC-specific deletion of Pdcd10 (Pdcd10BECKO) survive up to 6-12 months and develop bona fide CCM lesions in all regions of brain, allowing us to visualize the vascular dynamics of CCM lesions using transcranial two-photon microscopy. This approach reveals that CCMs initiate from protrusion at the level of capillary and post-capillary venules with gradual dissociation of pericytes. Microvascular beds in lesions are hyper-permeable, and these disorganized structures present endomucin-positive ECs and α-smooth muscle actin-positive pericytes. Caveolae in the endothelium of Pdcd10BECKO lesions are drastically increased, enhancing Tie2 signaling in Ccm3-deficient ECs. Moreover, genetic deletion of caveolin-1 or pharmacological blockade of Tie2 signaling effectively normalizes microvascular structure and barrier function with attenuated EC-pericyte disassociation and CCM lesion formation in Pdcd10BECKO mice. Our study establishes a chronic CCM model and uncovers a mechanism by which CCM3 mutation-induced caveolae-Tie2 signaling contributes to CCM pathogenesis.

Signalling through cerebral cavernous malformation protein networks

Cerebral cavernous malformations (CCMs) are neurovascular abnormalities characterized by thin, leaky blood vessels resulting in lesions that predispose to haemorrhages, stroke, epilepsy and focal neurological deficits. CCMs arise due to loss-of-function mutations in genes encoding one of three CCM complex proteins, KRIT1, CCM2 or CCM3. These widely expressed, multi-functional adaptor proteins can assemble into a CCM protein complex and (either alone or in complex) modulate signalling pathways that influence cell adhesion, cell contractility, cytoskeletal reorganization and gene expression. Recent advances, including analysis of the structures and interactions of CCM proteins, have allowed substantial progress towards understanding the molecular bases for CCM protein function and how their disruption leads to disease. Here, we review current knowledge of CCM protein signalling with a focus on three pathways which have generated the most interest—the RhoA–ROCK, MEKK3–MEK5–ERK5–KLF2/4 and cell junctional signalling pathways—but also consider ICAP1-β1 integrin and cdc42 signalling. We discuss emerging links between these pathways and the processes that drive disease pathology and highlight important open questions—key among them is the role of subcellular localization in the control of CCM protein activity.

Novel Murine Models of Cerebral Cavernous Malformations

Cerebral cavernous malformations (CCMs) are ectatic capillary-venous malformations that develop in approximately 0.5% of the population. Patients with CCMs may develop headaches, focal neurologic deficits, seizures, and hemorrhages. While symptomatic CCMs, depending upon the anatomic location, can be surgically removed, there is currently no pharmaceutical therapy to treat CCMs. Several mouse models have been developed to better understand CCM pathogenesis and test therapeutics. The most common mouse models induce a large CCM burden that is anatomically restricted to the cerebellum and contributes to lethality in the early days of life. These inducible models thus have a relatively short period for drug administration. We developed an inducible CCM3 mouse model that develops CCMs after weaning and provides a longer period for potential therapeutic intervention. Using this new model, three recently proposed CCM therapies, fasudil, tempol, vitamin D₃, and a combination of the three drugs, failed to substantially reduce CCM formation when treatment was administered for 5 weeks, from postnatal day 21 (P21) to P56. We next restricted Ccm3 deletion to the brain vasculature and provided greater time (121 days) for CCMs to develop chronic hemorrhage, recapitulating the human lesions. We also developed the first model of acute CCM hemorrhage by injecting mice harboring CCMs with lipopolysaccharide. These efficient models will enable future drug studies to more precisely target clinically relevant features of CCM disease: CCM formation, chronic hemorrhage, and acute hemorrhage.

Mural Cell-Specific Deletion of Cerebral Cavernous Malformation 3 in the Brain Induces Cerebral Cavernous Malformations

OBJECTIVE

Cerebral cavernous malformations (CCM), consisting of dilated capillary channels formed by a single layer of endothelial cells lacking surrounding mural cells. It is unclear why CCM lesions are primarily confined to brain vasculature, although the 3 CCM-associated genes (CCM1, CCM2, and CCM3) are ubiquitously expressed in all tissues. We aimed to determine the role of CCM gene in brain mural cell in CCM pathogenesis. Approach and Results: SM22α-Cre was used to drive a specific deletion of Ccm3 in mural cells, including pericytes and smooth muscle cells (Ccm3smKO). Ccm3smKO mice developed CCM lesions in the brain with onset at neonatal stages. One-third of Ccm3smKO mice survived upto 6 weeks of age, exhibiting seizures, and severe brain hemorrhage. The early CCM lesions in Ccm3smKO neonates were loosely wrapped by mural cells, and adult Ccm3smKO mice had clustered and enlarged capillary channels (caverns) formed by a single layer of endothelium lacking mural cell coverage. Importantly, CCM lesions throughout the entire brain in Ccm3smKO mice, which more accurately mimicked human disease than the current endothelial cell-specific CCM3 deletion models. Mechanistically, CCM3 loss in brain pericytes dramatically increased paxillin stability and focal adhesion formation, enhancing ITG-β1 (integrin β1) activity and extracellular matrix adhesion but reducing cell migration and endothelial cell-pericyte associations. Moreover, CCM3-wild type, but not a paxillin-binding defective mutant, rescued the phenotypes in CCM3-deficient pericytes.

CONCLUSIONS

Our data demonstrate for the first time that deletion of a CCM gene in the brain mural cell induces CCM pathogenesis.

Blocking Signalopathic Events to Treat Cerebral Cavernous Malformations

Cerebral cavernous malformations (CCMs) are pathologies of the brain vasculature characterized by capillary-venous angiomas that result in recurrent cerebral hemorrhages. Familial forms are caused by a clonal loss of any of three CCM genes in endothelial cells, which causes the activation of a novel pathophysiological pathway involving mitogen-activated protein kinase and Krüppel-like transcription factor KLF2/4 signaling. Recent work has shown that cavernomas can undergo strong growth when CCM-deficient endothelial cells recruit wild-type neighbors through the secretion of cytokines. This suggests a treatment strategy based on targeting signalopathic events between CCM-deficient endothelial cells and their environment. Such approaches will have to consider recent evidence implicating 'third hits' from hypoxia-induced angiogenesis signaling or the microbiome in modulating the development of cerebral hemorrhages.

Novel Chronic Mouse Model of Cerebral Cavernous Malformations

Background and Purpose- Cerebral cavernous malformations (CCMs) are vascular malformations of the brain that lead to cerebral hemorrhages. A pharmacological treatment is needed especially for patients with nonoperable deep-seated lesions. We and others obtained CCM mouse models that were useful for mechanistic studies and rapid trials testing the preventive effects of candidate drugs. The shortened lifespan of acute mouse models hampered evaluation of compounds that would not only prevent lesion appearance but also cure preexisting lesions. Indirubin-3'-monoxime previously demonstrated its efficacy to reverse the cardiac phenotype of ccm2^m201 zebrafish mutants and to prevent lesion development in an acute CCM2 mouse model. In the present article, we developed and characterized a novel chronic CCM2 mouse model and evaluated the curative therapeutic effect of indirubin-3'-monoxime after CCM lesion development. Methods- The chronic mouse model was obtained by a postnatal induction of brain-endothelial-cell-specific ablation of the Ccm2 gene using the inducible Slco1c1-CreER^T2 mouse line. Results- We obtained a fully penetrant novel CCM chronic mouse model without any obvious off-target phenotypes and compatible with long-term survival. By 3 months of age, CCM lesions ranging in size from small isolated lesions to multiple caverns developed throughout the brain. Lesion burden was quantified in animals from 1 week to 5 months of age. Clear signs of intracerebral hemorrhages were noticed in brain-endothelial-cell-specific ablation of the Ccm2 gene. In contrast with its preventive effect in the acute CCM2 mouse model, a 20 mg/kg indirubin-3'-monoxime treatment for 3 weeks in 3-month old animals neither had any beneficial effect on the lesion burden nor alleviated cerebral hemorrhages. Conclusions- The brain-endothelial-cell-specific ablation of the Ccm2 gene chronic model is a strongly improved disease model for the CCM community whose challenge today is to decipher which candidate drugs might have a curative effect on patients' preexisting lesions. Visual Overview- An online visual overview is available for this article.

Blood Flow Suppresses Vascular Anomalies in a Zebrafish Model of Cerebral Cavernous Malformations

Rationale:

Pathological biomechanical signaling induces vascular anomalies including cerebral cavernous malformations (CCM), which are caused by a clonal loss of CCM1/KRIT1 (Krev interaction trapped protein 1), CCM2/MGC4607, or CCM3/PDCD10. Why patients typically experience lesions only in lowly perfused venous capillaries of the cerebrovasculature is completely unknown.

Objective:

In contrast, animal models with a complete loss of CCM proteins lack a functional heart and blood flow and exhibit vascular anomalies within major blood vessels as well. This finding raises the possibility that hemodynamics may play a role in the context of this vascular pathology.

Methods and Results:

Here, we used a genetic approach to restore cardiac function and blood flow in a zebrafish model of CCM1. We find that blood flow prevents cardiovascular anomalies including a hyperplastic expansion within a large Ccm1-deficient vascular bed, the lateral dorsal aorta.

Conclusions:

This study identifies blood flow as an important physiological factor that is protective in the cause of this devastating vascular pathology.

Systematic pharmacological screens uncover novel pathways involved in cerebral cavernous malformations

Cerebral cavernous malformations (CCMs) are vascular lesions in the central nervous system causing strokes and seizures which currently can only be treated through neurosurgery. The disease arises through changes in the regulatory networks of endothelial cells that must be comprehensively understood to develop alternative, non-invasive pharmacological therapies. Here, we present the results of several unbiased small-molecule suppression screens in which we applied a total of 5,268 unique substances to CCM mutant worm, zebrafish, mouse, or human endothelial cells. We used a systems biology-based target prediction tool to integrate the results with the whole-transcriptome profile of zebrafish CCM2 mutants, revealing signaling pathways relevant to the disease and potential targets for small-molecule-based therapies. We found indirubin-3-monoxime to alleviate the lesion burden in murine preclinical models of CCM2 and CCM3 and suppress the loss-of-CCM phenotypes in human endothelial cells. Our multi-organism-based approach reveals new components of the CCM regulatory network and foreshadows novel small-molecule-based therapeutic applications for suppressing this devastating disease in patients.

CCM3 and cerebral cavernous malformation disease

Cerebral cavernous malformations (CCMs) are vascular lesions characterised by enlarged and irregular structure of small blood vessels in the brain, which can result in increased risk of stroke, focal neurological defects and seizures. Three different genes, CCM1/Krev/Rap1 Interacting Trapped 1, CCM2/MGC4607 and CCM3/PDCD10, are associated with the CCMs’ progression, and mutations in one of three CCM genes cause CCM disease. These three CCM proteins have similar function in maintaining the normal structure of small blood vessels. However, CCM3 mutation results in a more severe form of the disease which may suggest that CCM3 has unique biological function in the vasculature. The current review focuses on the signalling pathways mediated by CCM3 in regulating endothelial cell junction, proliferation, migration and permeability. These findings may offer potential therapeutic strategies for the treatment of CCMs.

Phenotypic characterization of murine models of cerebral cavernous malformations

Cerebral cavernous malformations (CCMs) are clusters of dilated capillaries that affect around 0.5% of the population. CCMs exist in two forms, sporadic and familial. Mutations in three documented genes, KRIT1(CCM1), CCM2, and PDCD10(CCM3), cause the autosomal dominant form of the disease, and somatic mutations in these same genes underlie lesion development in the brain. Murine models with constitutive or induced loss of respective genes have been applied to study disease pathobiology and therapeutic manipulations. We aimed to analyze the phenotypic characteristic of two main groups of models, the chronic heterozygous models with sensitizers promoting genetic instability, and the acute neonatal induced homozygous knockout model. Acute model mice harbored a higher lesion burden than chronic models, more localized in the hindbrain, and largely lacking iron deposition and inflammatory cell infiltrate. The chronic model mice showed a lower lesion burden localized throughout the brain, with significantly greater perilesional iron deposition, immune B- and T-cell infiltration, and less frequent junctional protein immunopositive endothelial cells. Lesional endothelial cells in both models expressed similar phosphorylated myosin light chain immunopositivity indicating Rho-associated protein kinase activity. These data suggest that acute models are better suited to study the initial formation of the lesion, while the chronic models better reflect lesion maturation, hemorrhage, and inflammatory response, relevant pathobiologic features of the human disease.

Cerebrovascular disorders associated with genetic lesions

Cerebrovascular disorders are underlain by perturbations in cerebral blood flow and abnormalities in blood vessel structure. Here, we provide an overview of the current knowledge of select cerebrovascular disorders that are associated with genetic lesions and connect genomic findings with analyses aiming to elucidate the cellular and molecular mechanisms of disease pathogenesis. We argue that a mechanistic understanding of genetic (familial) forms of cerebrovascular disease is a prerequisite for the development of rational therapeutic approaches, and has wider implications for treatment of sporadic (non-familial) forms, which are usually more common.

Exploring Coronary Artery Disease GWAs Targets With Functional Links to Immunometabolism

Finding genetic variants that cause functional disruption or regulatory change among the many implicated GWAs variants remains a key challenge to translating the findings from GWAs to therapeutic treatments. Defining the causal mechanisms behind the variants require functional screening experiments that can be complex and costly. Prioritizing variants for functional characterization using techniques that capture important functional and regulatory elements can assist this. The genetic architecture of complex traits such as cardiovascular disease and type II diabetes comprise an enormously large number of variants of small effect contributing to heritability and spread throughout the genome. This makes it difficult to distinguish which variants or core genes are most relevant for prioritization and how they contribute to the regulatory networks that become dysregulated leading to disease. Despite these challenges, recent GWAs for CAD prioritized genes associated with lipid metabolism, coagulation and adhesion along with novel signals related to innate immunity, adipose tissue and, vascular function as important core drivers of risk. We focus on three examples of novel signals associated with CAD which affect risk through missense or UTR mutations indicating their potential for therapeutic modification. These variants play roles in adipose tissue function vascular function and innate immunity which form the cornerstones of immuno-metabolism. In addition we have explored the putative, but potentially important interactions between the environment, specifically food and nutrition, with respect to key processes.

Pathology of cavernous malformations

Cavernous malformations (CMs) are low-pressure angiographically occult lesions, composed of blood-filled sinusoidal locules known as "caverns." Although these lesions were once believed to be congenital in nature, there is compelling evidence to support de novo formation of CMs as well. They can occur as sporadic lesions or be inherited in an autosomal-dominant phenotype in familial forms of the disease. The pathophysiology of CMs is commonly believed to be due to abnormal vascular pathology. Three genes, CCM1, CCM2, and CCM3, have been extensively studied for their role in vascular pathology, resulting in abnormal angiogenesis and compromising the structural integrity of vessel endothelial cell. The expression of growth factors has been researched to gain insight into the dynamic behavior of CM lesions. Gross and microscopic images are utilized in this chapter to illustrate the pathologic findings of these lesions. Ultrastructural analysis demonstrates the aberrations in CM endothelial cells and structural integrity that may provide better understanding into how and why these lesions have a propensity to hemorrhage.

The pathobiology of vascular malformations: insights from human and model organism genetics

Vascular malformations may arise in any of the vascular beds present in the human body. These lesions vary in location, type, and clinical severity of the phenotype. In recent years, the genetic basis of several vascular malformations has been elucidated. This review will consider how the identification of the genetic factors contributing to different vascular malformations, with subsequent functional studies in animal models, has provided a better understanding of these factors that maintain vascular integrity in vascular beds, as well as their role in the pathogenesis of vascular malformations. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

A Novel CCM2 Gene Mutation Associated with Familial Cerebral Cavernous Malformation

Background: Cerebral cavernous malformations (CCMs) are common vascular malformations that predominantly arise in the central nervous system and are mainly characterized by enlarged vascular cavities without intervening brain parenchyma. Familial CCMs (FCCMs) is inherited in an autosomal dominant pattern with incomplete penetrance and variable symptoms.

Methods: Mutations of three pathogenic genes, CCM1, CCM2, and CCM3, were investigated by direct DNA sequencing in a Chinese family with multiple CCM lesions.

Results: Four heterozygous variants in the CCM2 gene, including one deletion (c.95delC), a missense mutation (c.358G>A, p.V120I), one silent mutation (c.915G>A, p.T305T), and a substitution (c. ^*1452 T>C), were identified in the subjects with multiple CCM lesions, but not in a healthy sibling. Among these variants, the c.95delC deletion is a novel mutation which is expected to cause a premature termination codon. It is predicted to produce a truncated CCM2 protein lacking the PTB and C-terminal domains, thus disrupting the molecular functions of CCM2.

Conclusions: The novel truncating mutation in the CCM2 gene, c.95delC, may be responsible for multiple CCM lesions in a part of FCCM. In addition, it may represent a potential genetic biomarker for early diagnosis of FCCM.

Oxidative stress and inflammation in cerebral cavernous malformation disease pathogenesis: Two sides of the same coin

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CCM proteins play pleiotropic roles in various redox-sensitive signaling pathways.

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CCM proteins modulate the crosstalk between redox signaling and autophagy that govern cell homeostasis and stress responses.

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Oxidative stress and inflammation are emerging as key focal determinants of CCM lesion formation, progression and severity.

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The pleiotropic functions of CCM proteins may prevent vascular dysfunctions triggered by local oxidative stress and inflammatory events.

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The distinct therapeutic compounds proposed so far for CCM disease share the ability to modulate redox signaling and autophagy.

Cerebral Cavernous Malformation (CCM) is a vascular disease of proven genetic origin, which may arise sporadically or is inherited as an autosomal dominant condition with incomplete penetrance and highly variable expressivity. CCM lesions exhibit a range of different phenotypes, including wide inter-individual differences in lesion number, size, and susceptibility to intracerebral hemorrhage (ICH). Lesions may remain asymptomatic or result in pathological conditions of various type and severity at any age, with symptoms ranging from recurrent headaches to severe neurological deficits, seizures, and stroke. To date there are no direct therapeutic approaches for CCM disease besides the surgical removal of accessible lesions. Novel pharmacological strategies are particularly needed to limit disease progression and severity and prevent de novo formation of CCM lesions in susceptible individuals.

Useful insights into innovative approaches for CCM disease prevention and treatment are emerging from a growing understanding of the biological functions of the three known CCM proteins, CCM1/KRIT1, CCM2 and CCM3/PDCD10. In particular, accumulating evidence indicates that these proteins play major roles in distinct signaling pathways, including those involved in cellular responses to oxidative stress, inflammation and angiogenesis, pointing to pathophysiological mechanisms whereby the function of CCM proteins may be relevant in preventing vascular dysfunctions triggered by these events. Indeed, emerging findings demonstrate that the pleiotropic roles of CCM proteins reflect their critical capacity to modulate the fine-tuned crosstalk between redox signaling and autophagy that govern cell homeostasis and stress responses, providing a novel mechanistic scenario that reconciles both the multiple signaling pathways linked to CCM proteins and the distinct therapeutic approaches proposed so far. In addition, recent studies in CCM patient cohorts suggest that genetic susceptibility factors related to differences in vascular sensitivity to oxidative stress and inflammation contribute to inter-individual differences in CCM disease susceptibility and severity.

This review discusses recent progress into the understanding of the molecular basis and mechanisms of CCM disease pathogenesis, with specific emphasis on the potential contribution of altered cell responses to oxidative stress and inflammatory events occurring locally in the microvascular environment, and consequent implications for the development of novel, safe, and effective preventive and therapeutic strategies.

Endothelial exocytosis of angiopoietin-2 resulting from CCM3 deficiency contributes to cerebral cavernous malformation

Cerebral cavernous malformations (CCMs) are vascular malformations that affect the central nervous system and result in cerebral hemorrhage, seizure and stroke. CCMs arise from loss-of-function mutations in one of three genes: KRIT1 (also known as CCM1), CCM2 or PDCD10 (also known as CCM3). PDCD10 mutations in humans often result in a more severe form of the disease relative to mutations in the other two CCM genes, and PDCD10-knockout mice show severe defects, the mechanistic basis for which is unclear. We have recently reported that CCM3 regulates exocytosis mediated by the UNC13 family of exocytic regulatory proteins. Here, in investigating the role of endothelial cell exocytosis in CCM disease progression, we found that CCM3 suppresses UNC13B- and vesicle-associated membrane protein 3 (VAMP3)-dependent exocytosis of angiopoietin 2 (ANGPT2) in brain endothelial cells. CCM3 deficiency in endothelial cells augments the exocytosis and secretion of ANGPT2, which is associated with destabilized endothelial cell junctions, enlarged lumen formation and endothelial cell-pericyte dissociation. UNC13B deficiency, which blunts ANGPT2 secretion from endothelial cells, or treatment with an ANGPT2-neutralizing antibody normalizes the defects in the brain and retina caused by endothelial-cell-specific CCM3 deficiency, including the disruption of endothelial cell junctions, vessel dilation and pericyte dissociation. Thus, enhanced secretion of ANGPT2 in endothelial cells contributes to the progression of CCM disease, providing a new therapeutic approach for treating this devastating pathology.

Micro-CT Imaging Reveals Mekk3 Heterozygosity Prevents Cerebral Cavernous Malformations in Ccm2-Deficient Mice

Mutations in CCM1 (aka KRIT1), CCM2, or CCM3 (aka PDCD10) gene cause cerebral cavernous malformation in humans. Mouse models of CCM disease have been established by deleting Ccm genes in postnatal animals. These mouse models provide invaluable tools to investigate molecular mechanism and therapeutic approaches for CCM disease. However, the full value of these animal models is limited by the lack of an accurate and quantitative method to assess lesion burden and progression. In the present study we have established a refined and detailed contrast enhanced X-ray micro-CT method to measure CCM lesion burden in mouse brains. As this study utilized a voxel dimension of 9.5μm (leading to a minimum feature size of approximately 25μm), it is therefore sufficient to measure CCM lesion volume and number globally and accurately, and provide high-resolution 3-D mapping of CCM lesions in mouse brains. Using this method, we found loss of Ccm1 or Ccm2 in neonatal endothelium confers CCM lesions in the mouse hindbrain with similar total volume and number. This quantitative approach also demonstrated a rescue of CCM lesions with simultaneous deletion of one allele of Mekk3. This method would enhance the value of the established mouse models to study the molecular basis and potential therapies for CCM and other cerebrovascular diseases.

The force within: endocardial development, mechanotransduction and signalling during cardiac morphogenesis

Endocardial cells are cardiac endothelial cells that line the interior of the heart tube. Historically, their contribution to cardiac development has mainly been considered from a morphological perspective. However, recent studies have begun to define novel instructive roles of the endocardium, as a sensor and signal transducer of biophysical forces induced by blood flow, and as an angiocrine signalling centre that is involved in myocardial cellular morphogenesis, regeneration and reprogramming. In this Review, we discuss how the endocardium develops, how endocardial-myocardial interactions influence the developing embryonic heart, and how the dysregulation of blood flow-responsive endocardial signalling can result in pathophysiological changes.

Cytochrome P450 and matrix metalloproteinase genetic modifiers of disease severity in Cerebral Cavernous Malformation type 1

BACKGROUND

Familial Cerebral Cavernous Malformation type 1 (CCM1) is an autosomal dominant disease caused by mutations in the Krev Interaction Trapped 1 (KRIT1/CCM1) gene, and characterized by multiple brain lesions. CCM lesions manifest across a range of different phenotypes, including wide differences in lesion number, size and susceptibility to intracerebral hemorrhage (ICH). Oxidative stress plays an important role in cerebrovascular disease pathogenesis, raising the possibility that inter-individual variability in genes related to oxidative stress may contribute to the phenotypic differences observed in CCM1 disease. Here, we investigated whether candidate oxidative stress-related cytochrome P450 (CYP) and matrix metalloproteinase (MMP) genetic markers grouped by superfamilies, families or genes, or analyzed individually influence the severity of CCM1 disease.

METHODS

Clinical assessment and cerebral susceptibility-weighted magnetic resonance imaging (SWI) were performed to determine total and large (≥5mm in diameter) lesion counts as well as ICH in 188 Hispanic CCM1 patients harboring the founder KRIT1/CCM1 'common Hispanic mutation' (CCM1-CHM). Samples were genotyped on the Affymetrix Axiom Genome-Wide LAT1 Human Array. We analyzed 1,122 genetic markers (both single nucleotide polymorphisms (SNPs) and insertion/deletions) grouped by CYP and MMP superfamily, family or gene for association with total or large lesion count and ICH adjusted for age at enrollment and gender. Genetic markers bearing the associations were then analyzed individually.

RESULTS

The CYP superfamily showed a trend toward association with total lesion count (P=0.057) and large lesion count (P=0.088) in contrast to the MMP superfamily. The CYP4 and CYP8 families were associated with either large lesion count or total lesion count (P=0.014), and two other families (CYP46 and the MMP Stromelysins) were associated with ICH (P=0.011 and 0.007, respectively). CYP4F12 rs11085971, CYP8A1 rs5628, CYP46A1 rs10151332, and MMP3 rs117153070 single SNPs, mainly bearing the above-mentioned associations, were also individually associated with CCM1 disease severity.

CONCLUSIONS

Overall, our candidate oxidative stress-related genetic markers set approach outlined CYP and MMP families and identified suggestive SNPs that may impact the severity of CCM1 disease, including the development of numerous and large CCM lesions and ICH. These novel genetic risk factors of prognostic value could serve as early objective predictors of disease outcome and might ultimately provide better options for disease prevention and treatment.

Nogo-B receptor deficiency causes cerebral vasculature defects during embryonic development in mice

Nogo-B receptor (NgBR) was identified as a receptor specific for Nogo-B. Our previous work has shown that Nogo-B and its receptor (NgBR) are essential for chemotaxis and morphogenesis of endothelial cells in vitro and intersomitic vessel formation via Akt pathway in zebrafish. Here, we further demonstrated the roles of NgBR in regulating vasculature development in mouse embryo and primitive blood vessel formation in embryoid body culture systems, respectively. Our results showed that NgBR homozygous knockout mice are embryonically lethal at E7.5 or earlier, and Tie2Cre-mediated endothelial cell-specific NgBR knockout (NgBR ecKO) mice die at E11.5 and have severe blood vessel assembly defects in embryo. In addition, mutant embryos exhibit dilation of cerebral blood vessel, resulting in thin-walled endothelial caverns. The similar vascular defects also were detected in Cdh5(PAC)-CreERT2 NgBR inducible ecKO mice. Murine NgBR gene-targeting embryonic stem cells (ESC) were generated by homologous recombination approaches. Homozygous knockout of NgBR in ESC results in cell apoptosis. Heterozygous knockout of NgBR does not affect ESC cell survival, but reduces the formation and branching of primitive blood vessels in embryoid body culture systems. Mechanistically, NgBR knockdown not only decreases both Nogo-B and VEGF-stimulated endothelial cell migration by abolishing Akt phosphorylation, but also decreases the expression of CCM1 and CCM2 proteins. Furthermore, we performed immunofluorescence (IF) staining of NgBR in human cerebral cavernous malformation patient tissue sections. The quantitative analysis results showed that NgBR expression levels in CD31 positive endothelial cells is significantly decreased in patient tissue sections. These results suggest that NgBR may be one of important genes coordinating the cerebral vasculature development.

Small GTPase Rap1 Is Essential for Mouse Development and Formation of Functional Vasculature

Background

Small GTPase Rap1 has been implicated in a number of basic cellular functions, including cell-cell and cell-matrix adhesion, proliferation and regulation of polarity. Evolutionarily conserved, Rap1 has been studied in model organisms: yeast, Drosophila and mice. Mouse in vivo studies implicate Rap1 in the control of multiple stem cell, leukocyte and vascular cell functions. In vitro, several Rap1 effectors and regulatory mechanisms have been proposed. In particular, Rap1 has been implicated in maintaining epithelial and endothelial cell junction integrity and linked with cerebral cavernous malformations.

Rationale

How Rap1 signaling network controls mammalian development is not clear. As a first step in addressing this question, we present phenotypes of murine total and vascular-specific Rap1a, Rap1b and double Rap1a and Rap1b (Rap1) knockout (KO) mice.

Results and Conclusions

The majority of total Rap1 KO mice die before E10.5, consistent with the critical role of Rap1 in epithelial morphogenesis. At that time point, about 50% of Tie2-double Rap1 KOs appear grossly normal and develop normal vasculature, while the remaining 50% suffer tissue degeneration and show vascular abnormalities, including hemorrhages and engorgement of perineural vessels, albeit with normal branchial arches. However, no Tie2-double Rap1 KO embryos are present at E15.5, with hemorrhages a likely cause of death. Therefore, at least one Rap1 allele is required for development prior to the formation of the vascular system; and in endothelium–for the life-supporting function of the vasculature.

ANKS1B Interacts with the Cerebral Cavernous Malformation Protein-1 and Controls Endothelial Permeability but Not Sprouting Angiogenesis

Cerebral cavernous malformations are fragile blood vessel conglomerates in the central nervous system that are caused by mutations in the CCM1/KRIT1, CCM2 or CCM3 genes. The gene products form a protein complex at adherens junctions and loss of either CCM protein disrupts endothelial cell quiescence leading to increased permeability and excessive angiogenesis. We performed a yeast 2-hybrid screen to identify novel proteins directly interacting with KRIT1. The ankyrin repeat and sterile alpha motif domain-containing protein 1B (ANKS1B) was identified as a novel binding partner of KRIT1. Silencing of ANKS1B or the related gene ANKS1A in primary human endothelial cells had no significant effects on cellular proliferation, migration and sprouting angiogenesis. However, silencing of ANKS1B expression disturbed endothelial cell barrier functions leading to increased permeability. Forced ANKS1B expression reduced permeability. This was independent of Rho kinase activity and the presence of KRIT1. Taken together, ANKS1B was identified as a novel KRIT1-interacting protein that selectively controls endothelial permeability but not angiogenesis.

The cerebral cavernous malformation proteins CCM2L and CCM2 prevent the activation of the MAP kinase MEKK3

Three genes, CCM1, CCM2, and CCM3, interact genetically and biochemically and are mutated in cerebral cavernous malformations (CCM). A recently described member of this CCM family of proteins, CCM2-like (CCM2L), has high homology to CCM2. Here we show that its relative expression in different tissues differs from that of CCM2 and, unlike CCM2, the expression of CCM2L in endothelial cells is regulated by density, flow, and statins. In vitro, both CCM2L and CCM2 bind MEKK3 in a complex with CCM1. Both CCM2L and CCM2 interfere with MEKK3 activation and its ability to phosphorylate MEK5, a downstream target. The in vivo relevance of this regulation was investigated in zebrafish. A knockdown of ccm2l and ccm2 in zebrafish leads to a more severe "big heart" and circulation defects compared with loss of function of ccm2 alone, and also leads to substantial body axis abnormalities. Silencing of mekk3 rescues the big heart and body axis phenotype, suggesting cross-talk between the CCM proteins and MEKK3 in vivo. In endothelial cells, CCM2 deletion leads to activation of ERK5 and a transcriptional program that are downstream of MEKK3. These findings suggest that CCM2L and CCM2 cooperate to regulate the activity of MEKK3.

Expression of CCM2 and CCM3 during mouse gonadogenesis

PURPOSE

Three cerebral cavernous malformation (CCM) proteins, CCM1, CCM2, and CCM3, regulate cell-cell adhesion, cell shape and polarity, and most likely cell adhesion to extracellular matrix. Recently, CCM2 and CCM3 are known to be expressed in control and varicocele-induced rat testes, but little is known about these proteins during gonadogenesis. This led us to study the CCM proteins during the mouse gonadogenesis.

METHODS

Neonatal (PND 0), postnatal, and adult mice testes and ovaries were obtained from mice. CCM2 and CCM3 expression were analyzed during mouse testicular and ovarian development by immunohistochemistry and quantitative real-time PCR.

RESULTS

The results showed that in both sexes, Ccm2 and Ccm3 mRNA and protein were first detectable after gonadogenesis when the gonads were well differentiated and remained present until the adult stage. In the testis, CCM2 and CCM3 expression were restricted to the nuclei of Sertoli cells, suggesting a conserved role in testicular differentiation. In the ovary, the CCM2 and CCM3 proteins were localized in the cytoplasm of oocytes, suggesting an unexpected role during oogenesis. Quantitative real-time PCR (qRT-PCR) results showed that expression of Ccm2 and Ccm3 genes could play a role in the regulation of mouse gonadogenesis translational activation upon testicular and ovarian development.

CONCLUSIONS

The localization of CCM2 and CCM3 proteins show their different functions for CCM2 and CCM3 which may have important roles in testicular and ovarian differentiation. In conclusion, CCM2 and CCM3 may be involved in establishing the differential expression pattern in developing mouse testis and ovary.

Structure and vascular function of MEKK3-cerebral cavernous malformations 2 complex

Cerebral cavernous malformations 2 (CCM2) loss is associated with the familial form of CCM disease. The protein kinase MEKK3 (MAP3K3) is essential for embryonic angiogenesis in mice and interacts physically with CCM2, but how this interaction is mediated and its relevance to cerebral vasculature are unknown. Here we report that Mekk3 plays an intrinsic role in embryonic vascular development. Inducible endothelial Mekk3 knockout in neonatal mice is lethal due to multiple intracranial haemorrhages and brain blood vessels leakage. We discover direct interaction between CCM2 harmonin homology domain (HHD) and the N terminus of MEKK3, and determine a 2.35 Å cocrystal structure. We find Mekk3 deficiency impairs neurovascular integrity, which is partially dependent on Rho-ROCK signalling, and that disruption of MEKK3:CCM2 interaction leads to similar neurovascular leakage. We conclude that CCM2:MEKK3-mediated regulation of Rho signalling is required for maintenance of neurovascular integrity, unravelling a mechanism by which CCM2 loss leads to disease.

CCM1 and the second life of proteins in adhesion complexes

It is well recognized that a number of proteins present within adhesion complexes perform discrete signaling functions outside these adhesion complexes, including transcriptional control. In this respect, β-catenin is a well-known example of an adhesion protein present both in cadherin complexes and in the nucleus where it regulates the TCF transcription factor. Here we discuss nuclear functions of adhesion complex proteins with a special focus on the CCM-1/KRIT-1 protein, which may turn out to be yet another adhesion complex protein with a second life.

Endothelial cell-cell adhesion during zebrafish vascular development

The vertebrate vasculature is an essential organ network with major roles in health and disease. The establishment of balanced cell-cell adhesion in the endothelium is crucial for the functionality of the vascular system. Furthermore, the correct patterning and integration of vascular endothelial cell-cell adhesion drives the morphogenesis of new vessels, and is thought to couple physical forces with signaling outcomes during development. Here, we review insights into this process that have come from studies in zebrafish. First, we describe mutants in which endothelial adhesion is perturbed, second we describe recent progress using in vivo cell biological approaches that allow the visualization of endothelial cell-cell junctions. These studies underline the profound potential of this model system to dissect in great detail the function of both known and novel regulators of endothelial cell-cell adhesion.

The cerebral cavernous malformation pathway controls cardiac development via regulation of endocardial MEKK3 signaling and KLF expression

The cerebral cavernous malformation (CCM) pathway is required in endothelial cells for normal cardiovascular development and to prevent postnatal vascular malformations, but its molecular effectors are not well defined. Here we show that loss of CCM signaling in endocardial cells results in mid-gestation heart failure associated with premature degradation of cardiac jelly. CCM deficiency dramatically alters endocardial and endothelial gene expression, including increased expression of the Klf2 and Klf4 transcription factors and the Adamts4 and Adamts5 proteases that degrade cardiac jelly. These changes in gene expression result from increased activity of MEKK3, a mitogen-activated protein kinase that binds CCM2 in endothelial cells. MEKK3 is both necessary and sufficient for expression of these genes, and partial loss of MEKK3 rescues cardiac defects in CCM-deficient embryos. These findings reveal a molecular mechanism by which CCM signaling controls endothelial gene expression during cardiovascular development that may also underlie CCM formation.