Ultrastructural analysis of vascular features in cerebral cavernous malformations

Lectin-type oxidized LDL receptor-1 as a potential therapeutic target for cerebral cavernous malformations treatment

Introduction

Cerebral cavernous malformations (CCMs) are pathologic lesions comprised of clusters of thin-walled capillaries characterized by abnormal proliferation, angiogenesis, and bleeding secondary to somatic or germline mutations in endothelial cells. CCMs can cause headaches, seizures and/or neurological defects. There is a clinical need to develop better tools to detect CCMs and follow their progression in conjunction with the current use of neuroimaging techniques. Here we present data supporting the utility of LOX-1 (lectin-type oxidized LDL receptor 1), a 50 kDa transmembrane protein implicated in endothelial cell dysfunction and ischemia, as a putative biomarker for CCM.

Methods

CCM urine samples (n = 23) were collected from pediatric CCM patients. Matched healthy controls (n = 24) were collected from pediatric patients with either Chiari I malformation or fatty filum terminale, and otherwise normal findings. All samples were collected with patient/family consent and institutional review board approval.Samples were analyzed with Olink Proteomic Proximity Extension Assay (PEA). Differences in expression for 2,925 unique proteins were quantified between healthy control urine samples and CCM urine samples. The results were normalized, validated, and analyzed for demographic bias. In addition to urine samples, CCM tissue from patients was harvested and used to create primary cell lines for in vitro analysis of LOX-1 expression, in addition to immunofluorescence of lesional tissue excised at surgery.

Results

ANOVA analysis of the CCM urine samples showed a statistically significant increase in LOX-1 compared to the control samples, with CCM patients exhibiting a > 5-fold increase in urinary expression. Corroborating these elevated levels of circulating marker, analysis of source tissue from surgically resected CCMs revealed that LOX-1 is increased in both CCM patient cavernoma primary cell lines and operative specimens.

Conclusion

LOX-1 is involved with pathways implicated in CCM pathogenesis and our data here reveals that LOX-1 expression is significantly elevated in CCM patients as compared to matched healthy control individuals, including both source tissue from surgically excised CCMs and in analysis of samples collected from outside of the central nervous system, particularly urine. This proof-of-principle data suggests that LOX-1 may have potential utility as a target for CCM treatment and supports further investigation related to its potential mechanistic impact on CCM pathogenesis.

mTORC1 Signaling in Brain Endothelial Progenitors Contributes to CCM Pathogenesis

BACKGROUND

Cerebral vascular malformations (CCMs) are primarily found within the brain, where they result in increased risk for stroke, seizures, and focal neurological deficits. The unique feature of the brain vasculature is the blood-brain barrier formed by the brain neurovascular unit. Recent studies suggest that loss of CCM genes causes disruptions of blood-brain barrier integrity as the inciting events for CCM development. CCM lesions are proposed to be initially derived from a single clonal expansion of a subset of angiogenic venous capillary endothelial cells (ECs) and respective resident endothelial progenitor cells (EPCs). However, the critical signaling events in the subclass of brain ECs/EPCs for CCM lesion initiation and progression are unclear.

METHODS

Brain EC-specific CCM3-deficient (Pdcd10BECKO) mice were generated by crossing Pdcd10fl/fl mice with Mfsd2a-CreERT2 mice. Single-cell RNA-sequencing analyses were performed by the chromium single-cell platform (10× genomics). Cell clusters were annotated into EC subtypes based on visual inspection and GO analyses. Cerebral vessels were visualized by 2-photon in vivo imaging and tissue immunofluorescence analyses. Regulation of mTOR (mechanistic target of rapamycin) signaling by CCM3 and Cav1 (caveolin-1) was performed by cell biology and biochemical approaches.

RESULTS

Single-cell RNA-sequencing analyses from P10 Pdcd10BECKO mice harboring visible CCM lesions identified upregulated CCM lesion signature and mitotic EC clusters but decreased blood-brain barrier-associated EC clusters. However, a unique EPC cluster with high expression levels of stem cell markers enriched with mTOR signaling was identified from early stages of the P6 Pdcd10BECKO brain. Indeed, mTOR signaling was upregulated in both mouse and human CCM lesions. Genetic deficiency of Raptor (regulatory-associated protein of mTOR), but not of Rictor (rapamycin-insensitive companion of mTOR), prevented CCM lesion formation in the Pdcd10BECKO model. Importantly, the mTORC1 (mTOR complex 1) pharmacological inhibitor rapamycin suppressed EPC proliferation and ameliorated CCM pathogenesis in Pdcd10BECKO mice. Mechanistic studies suggested that Cav1/caveolae increased in CCM3-depleted EPC-mediated intracellular trafficking and complex formation of the mTORC1 signaling proteins.

CONCLUSIONS

CCM3 is critical for maintaining blood-brain barrier integrity and CCM3 loss-induced mTORC1 signaling in brain EPCs initiates and facilitates CCM pathogenesis.

Behavioral impairments are linked to neuroinflammation in mice with Cerebral Cavernous Malformation disease

Background

Cerebral Cavernous Malformations (CCMs) are neurovascular abnormalities in the central nervous system (CNS) caused by loss of function mutations in KRIT1 (CCM1), CCM2, or PDCD10 (CCM3) genes. One of the most common symptoms in CCM patients is associated with motor disability, weakness, seizures, stress, and anxiety, and the extent of the symptom or symptoms may be due to the location of the lesion within the CNS or whether multiple lesions are present. Previous studies have primarily focused on understanding the pathology of CCM using animal models. However, more research has yet to explore the potential impact of CCM lesions on behavioral deficits in animal models, including effects on short-term and long-term memory, motor coordination, and function.

Methods

We used the accelerating RotaRod test to assess motor and coordination deficits. We also used the open field test to assess locomotor activity and pathology-related behavior and Pavlovian fear conditioning to assess short-and long-term memory deficits. Our behavioral studies were complemented by proteomics, histology, immunofluorescence, and imaging techniques. We found that neuroinflammation is crucial in behavioral deficits in male and female mice with neurovascular CCM lesions (Slco1c1-iCreERT2; Pdcd10 fl/fl ; Pdcd10 BECKO ).

Results

Functional behavior tests in male and female Pdcd10 BECKO mice revealed that CCM lesions cause sudden motor coordination deficits associated with the manifestation of profound neuroinflammatory lesions. Our findings indicate that maturation of CCM lesions in Pdcd10 BECKO mice also experienced a significant change in short- and long-term memory compared to their littermate controls, Pdcd10 fl/fl mice. Proteomic experiments reveal that as CCM lesions mature, there is an increase in pathways associated with inflammation, coagulation, and angiogenesis, and a decrease in pathways associated with learning and plasticity. Therefore, our study shows that Pdcd10 BECKO mice display a wide range of behavioral deficits due to significant lesion formation in their central nervous system and that signaling pathways associated with neuroinflammation and learning impact behavioral outcomes.

Conclusions

Our study found that CCM animal models exhibited behavioral impairments such as decreased motor coordination and amnesia. These impairments were associated with the maturation of CCM lesions that displayed a neuroinflammatory pattern.

Defining the Functional Influence of Endothelial Cell-Expressed Oncogenic Activating Mutations on Vascular Morphogenesis and Capillary Assembly

This study sought to define key molecules and signals controlling major steps in vascular morphogenesis, and how these signals regulate pericyte recruitment and pericyte-induced basement membrane deposition. The morphogenic impact of endothelial cell (EC) expression of activating mutants of Kirsten rat sarcoma virus (kRas), mitogen-activated protein kinase 1 (Mek1), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), Akt serine/threonine kinase 1 (Akt1), Ras homolog enriched in brain (Rheb) Janus kinase 2 (Jak2), or signal transducer and activator of transcription 3 (Stat3) expression versus controls was evaluated, along with EC signaling events, pharmacologic inhibitor assays, and siRNA suppression experiments. Primary stimulators of EC lumen formation included kRas, Akt1, and Mek1, whereas PIK3CA and Akt1 stimulated a specialized type of cystic lumen formation. In contrast, the key drivers of EC sprouting behavior were Jak2, Stat3, Mek1, PIK3CA, and mammalian target of rapamycin (mTor). These conclusions are further supported by pharmacologic inhibitor and siRNA suppression experiments. EC expression of active Akt1, kRas, and PIK3CA led to markedly dysregulated lumen formation coupled to strongly inhibited pericyte recruitment and basement membrane deposition. For example, activated Akt1 expression in ECs excessively stimulated lumen formation, decreased EC sprouting behavior, and showed minimal pericyte recruitment with reduced mRNA expression of platelet-derived growth factor-BB, platelet-derived growth factor-DD, and endothelin-1, critical EC-derived factors known to stimulate pericyte invasion. The study identified key signals controlling fundamental steps in capillary morphogenesis and maturation and provided mechanistic details on why EC activating mutations induced a capillary deficiency state with abnormal lumens, impaired pericyte recruitment, and basement deposition: predisposing stimuli for the development of vascular malformations.

Focused Ultrasound Blood-Brain Barrier Opening Arrests the Growth and Formation of Cerebral Cavernous Malformations

BACKGROUND

Cerebral cavernous malformations (CCM) are vascular lesions within the central nervous system, consisting of dilated and hemorrhage-prone capillaries. CCMs can cause debilitating neurological symptoms, and surgical excision or stereotactic radiosurgery are the only current treatment options. Meanwhile, transient blood-brain barrier opening (BBBO) with focused ultrasound (FUS) and microbubbles is now understood to exert potentially beneficial bioeffects, such as stimulation of neurogenesis and clearance of amyloid-β. Here, we tested whether FUS BBBO could be deployed therapeutically to control CCM formation and progression in a clinically-representative murine model.

METHODS

CCMs were induced in mice by postnatal, endothelial-specific Krit1 ablation. FUS was applied for BBBO with fixed peak-negative pressures (PNPs; 0.2-0.6 MPa) or passive cavitation detection-modulated PNPs. Magnetic resonance imaging (MRI) was used to target FUS treatments, evaluate safety, and measure longitudinal changes in CCM growth after BBBO.

RESULTS

FUS BBBO elicited gadolinium accumulation primarily at the perilesional boundaries of CCMs, rather than lesion cores. Passive cavitation detection and gadolinium contrast enhancement were comparable in CCM and wild-type mice, indicating that Krit1 ablation does not confer differential sensitivity to FUS BBBO. Acutely, CCMs exposed to FUS BBBO remained structurally stable, with no signs of hemorrhage. Longitudinal MRI revealed that FUS BBBO halted the growth of 94% of CCMs treated in the study. At 1 month, FUS BBBO-treated lesions lost, on average, 9% of their pre-sonication volume. In contrast, non-sonicated control lesions grew to 670% of their initial volume. Lesion control with FUS BBBO was accompanied by a marked reduction in the area and mesenchymal appearance of Krit mutant endothelium. Strikingly, in mice receiving multiple BBBO treatments with fixed PNPs, de novo CCM formation was significantly reduced by 81%. Mock treatment plans on MRIs of patients with surgically inaccessible lesions revealed their lesions are amenable to FUS BBBO with current clinical technology.

CONCLUSIONS

Our results establish FUS BBBO as a novel, non-invasive modality that can safely arrest murine CCM growth and prevent their de novo formation. As an incisionless, MR image-guided therapy with the ability to target eloquent brain locations, FUS BBBO offers an unparalleled potential to revolutionize the therapeutic experience and enhance the accessibility of treatments for CCM patients.

Cerebral cavernous malformation proteins, CCM1, CCM2 and CCM3, are decreased in metastatic lesions in a murine breast carcinoma model

Three genes are associated with cerebral cavernous malformations (CCMs): CCM1, CCM2 and CCM3. These genes participate in microvascular angiogenesis, cell-to-cell junctions, migration and apoptosis. We evaluated the expression in vivo of CCM genes in primary tumors and metastastases in a murine model of metastatic breast carcinoma. We used cell lines obtained from metastasis of 4T1, 4TLM and 4THM breast cancer to liver and heart. These cells were injected into the mammary ridge of Balb/C female mice. After 27 days, the primary tumors, liver and lung were removed and CCM proteins were assessed using immunohistochemistry and western blot analysis. CCM proteins were expressed in primary tumor tissues of all tumor-injected animals; however, no CCM protein was expressed in metastatic tumor cells that migrated into other tissues. CCM proteins still were observed in the lung and liver tissue cells. Our findings suggest that CCM proteins are present during primary tumor formation, but when these cells develop metastatic potential, they lose CCM protein expression. CCM protein expression was lost or reduced in metastatic tissues compared to the primary tumor, which indicates that CCM proteins might participate in tumorigenesis and metastasis.

Tunneling Nanotube: An Enticing Cell-Cell Communication in the Nervous System

The field of neuroscience is rapidly progressing, continuously uncovering new insights and discoveries. Among the areas that have shown immense potential in research, tunneling nanotubes (TNTs) have emerged as a promising subject of study. These minute structures act as conduits for the transfer of cellular materials between cells, representing a mechanism of communication that holds great significance. In particular, the interplay facilitated by TNTs among various cell types within the brain, including neurons, astrocytes, oligodendrocytes, glial cells, and microglia, can be essential for the normal development and optimal functioning of this complex organ. The involvement of TNTs in neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease, and Parkinson's disease, has attracted significant attention. These disorders are characterized by the progressive degeneration of neurons and the subsequent decline in brain function. Studies have predicted that TNTs likely play critical roles in the propagation and spread of pathological factors, contributing to the advancement of these diseases. Thus, there is a growing interest in understanding the precise functions and mechanisms of TNTs within the nervous system. This review article, based on our recent work on Rhes-mediated TNTs, aims to explore the functions of TNTs within the brain and investigate their implications for neurodegenerative diseases. Using the knowledge gained from studying TNTs could offer novel opportunities for designing targeted treatments that can stop the progression of neurodegenerative disorders.

A novel KRIT1/CCM1 mutation accompanied by a NOTCH3 mutation in a Chinese family with multiple cerebral cavernous malformations

Family cerebral cavernous malformations (FCCMs) are mainly inherited through the mutation of classical CCM genes, including CCM1/KRIT1, CCM2/MGC4607, and CCM3/PDCD10. FCCMs can cause severe clinical symptoms, including epileptic seizures, intracranial hemorrhage (ICH), or functional neurological deficits (FNDs). In this study, we reported a novel mutation in KRIT1 accompanied by a NOTCH3 mutation in a Chinese family. This family consists of 8 members, 4 of whom had been diagnosed with CCMs using cerebral MRI (T1WI, T2WI, SWI). The proband (II-2) and her daughter (III-4) had intracerebral hemorrhage and refractory epilepsy, respectively. Based on whole-exome sequencing (WES) data and bioinformatics analysis from 4 patients with multiple CCMs and 2 normal first-degree relatives, a novel KRIT1 mutation, NG_012964.1 (NM_194456.1): c.1255-1G > T (splice-3), in intron 13 was considered a pathogenic gene in this family. Furthermore, based on 2 severe and 2 mild CCM patients, we found an SNV missense mutation, NG_009819.1 (NM_000435.2): c.1630C > T (p.R544C), in NOTCH3. Finally, the KRIT1 and NOTCH3 mutations were validated in 8 members using Sanger sequencing. This study revealed a novel KRIT1 mutation, NG_012964.1 (NM_194456.1): c.1255-1G > T (splice-3), in a Chinese CCM family, which had not been reported previously. Moreover, the NOTCH3 mutation NG_009819.1 (NM_000435.2): c.1630C > T (p.R544C) might be a second hit and associated with the progression of CCM lesions and severe clinical symptoms.

Endothelial Cell-Pericyte Interactions in the Pathogenesis of Cerebral Cavernous Malformations (CCMs)

Cerebral cavernous malformations (CCMs), consisting of multiple, dilated capillary channels formed by a single layer of endothelium and lacking parenchymal cells, are exclusively to the brain. Patients with inherited autosomal-dominant CCMs carry loss-of-function mutations in one of three genes: CCM1, CCM2, and CCM3. It is not known why CCM lesions are confined to brain vasculature despite the ubiquitous expression of CCM proteins in all tissues, and whether cell types other than endothelial cells (ECs) contribute to CCM lesion formation. The prevailing view is that the primary defects in CCMs in humans are EC-intrinsic, such that EC-specific deletion of any one of the three genes in mice results in similar CCM lesions. An unexpected finding is that Ccm3 deletion in pericytes (PCs) also induces CCM lesions. CCM3 deletion in ECs or PCs destabilizes PC-EC associations, highlighting the importance of these interactions in CCM formation.

Multidrug-Loaded Lipid Nanoemulsions for the Combinatorial Treatment of Cerebral Cavernous Malformation Disease

Cerebral cavernous malformation (CCM) or cavernoma is a major vascular disease of genetic origin, whose main phenotypes occur in the central nervous system, and is currently devoid of pharmacological therapeutic strategies. Cavernomas can remain asymptomatic during a lifetime or manifest with a wide range of symptoms, including recurrent headaches, seizures, strokes, and intracerebral hemorrhages. Loss-of-function mutations in KRIT1/CCM1 are responsible for more than 50% of all familial cases, and have been clearly shown to affect cellular junctions, redox homeostasis, inflammatory responses, and angiogenesis. In this study, we investigated the therapeutic effects of multidrug-loaded lipid nanoemulsions in rescuing the pathological phenotype of CCM disease. The pro-autophagic rapamycin, antioxidant avenanthramide, and antiangiogenic bevacizumab were loaded into nanoemulsions, with the aim of reducing the major molecular dysfunctions associated with cavernomas. Through Western blot analysis of biomarkers in an in vitro CCM model, we demonstrated that drug-loaded lipid nanoemulsions rescue antioxidant responses, reactivate autophagy, and reduce the effect of pro-angiogenic factors better than the free drugs. Our results show the importance of developing a combinatorial preventive and therapeutic approach to reduce the risk of lesion formation and inhibit or completely revert the multiple hallmarks that characterize the pathogenesis and progression of cavernomas.

Immunothrombosis and vascular heterogeneity in cerebral cavernous malformation

Cerebral cavernous malformation (CCM) is a neurovascular disease that results in various neurological symptoms. Thrombi have been reported in surgically resected CCM patient biopsies, but the molecular signatures of these thrombi remain elusive. Here, we investigated the kinetics of thrombi formation in CCM and how thrombi affect the vasculature and contribute to cerebral hypoxia. We used RNA sequencing to investigate the transcriptome of mouse brain endothelial cells with an inducible endothelial-specific Ccm3 knock-out (Ccm3-iECKO). We found that Ccm3-deficient brain endothelial cells had a higher expression of genes related to the coagulation cascade and hypoxia when compared with wild-type brain endothelial cells. Immunofluorescent assays identified key molecular signatures of thrombi such as fibrin, von Willebrand factor, and activated platelets in Ccm3-iECKO mice and human CCM biopsies. Notably, we identified polyhedrocytes in Ccm3-iECKO mice and human CCM biopsies and report it for the first time. We also found that the parenchyma surrounding CCM lesions is hypoxic and that more thrombi correlate with higher levels of hypoxia. We created an in vitro model to study CCM pathology and found that human brain endothelial cells deficient for CCM3 expressed elevated levels of plasminogen activator inhibitor-1 and had a redistribution of von Willebrand factor. With transcriptomics, comprehensive imaging, and an in vitro CCM preclinical model, this study provides experimental evidence that genes and proteins related to the coagulation cascade affect the brain vasculature and promote neurological side effects such as hypoxia in CCMs. This study supports the concept that antithrombotic therapy may be beneficial for patients with CCM.

Distribution of Neuroglobin in Pericytes is Associated with Blood-Brain Barrier Leakage against Cerebral Ischemia in Mice

With emerging data on the various functions of neuroglobin (Ngb), such as neuroprotection and neurogenesis, we investigated the role of Ngb in the neurovascular unit (NVU) of the brain. To study the distribution and function of Ngb after cerebral ischemia, transient middle cerebral artery occlusion (tMCAO) was performed in mice. Brain immunostaining and fluorescence-activated cell sorting were used to analyze the role of Ngb according to the location and cell type. In normal brain tissue, it was observed that Ngb was distributed not only in neurons but also around the brain’s blood vessels. Interestingly, Ngb was largely expressed in platelet-derived growth factor receptor β (PDGFRβ)-positive pericytes in the NVU. After tMCAO, Ngb levels were significantly decreased in the core of the infarct, and Ngb and PDGFRβ-positive pericytes were detached from the vasculature. In contrast, in the penumbra of the infarct, PDGFRβ-positive pericytes expressing Ngb were increased compared with that in the core of the infarct. Moreover, the cerebral blood vessels, which have Ngb-positive PDGFRβ pericytes, showed reduced blood-brain barrier (BBB) leakage after tMCAO. It showed that Ngb-positive PDGFRβ pericytes stayed around the endothelial cells and reduced the BBB leakage in the NVU. Our results indicate that Ngb may play a role in attenuating BBB leakage in part by its association with PDGFRβ. In this study, the distribution and function of Ngb in the pericytes of the cerebrovascular system have been elucidated, which contributes to the treatment of stroke through a new function of Ngb.

Dysregulated Hemostasis and Immunothrombosis in Cerebral Cavernous Malformations

Cerebral cavernous malformation (CCM) is a neurovascular disease that affects 0.5% of the general population. For a long time, CCM research focused on genetic mutations, endothelial junctions and proliferation, but recently, transcriptome and proteome studies have revealed that the hemostatic system and neuroinflammation play a crucial role in the development and severity of cavernomas, with some of these publications coming from our group. The aim of this review is to give an overview of the latest molecular insights into the interaction between CCM-deficient endothelial cells with blood components and the neurovascular unit. Specifically, we underscore how endothelial dysfunction can result in dysregulated hemostasis, bleeding, hypoxia and neurological symptoms. We conducted a thorough review of the literature and found a field that is increasingly poised to regard CCM as a hemostatic disease, which may have implications for therapy.

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.

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.

Cerebral cavernous malformations are driven by ADAMTS5 proteolysis of versican

Cerebral cavernous malformations (CCMs) form following loss of the CCM protein complex in brain endothelial cells due to increased endothelial MEKK3 signaling and KLF2/4 transcription factor expression, but the downstream events that drive lesion formation remain undefined. Recent studies have revealed that CCM lesions expand by incorporating neighboring wild-type endothelial cells, indicative of a cell nonautonomous mechanism. Here we find that endothelial loss of ADAMTS5 reduced CCM formation in the neonatal mouse model. Conversely, endothelial gain of ADAMTS5 conferred early lesion genesis in the absence of increased KLF2/4 expression and synergized with KRIT1 loss of function to create large malformations. Lowering versican expression reduced CCM burden, indicating that versican is the relevant ADAMTS5 substrate and that lesion formation requires proteolysis but not loss of this extracellular matrix protein. These findings identify endothelial secretion of ADAMTS5 and cleavage of versican as downstream mechanisms of CCM pathogenesis and provide a basis for the participation of wild-type endothelial cells in lesion formation.

ATPIF1 maintains normal mitochondrial structure which is impaired by CCM3 deficiency in endothelial cells

Background

Numerous signaling pathways have been demonstrated experimentally to affect the pathogenesis of cerebral cavernous malformations (CCM), a disease that can be caused by CCM3 deficiency. However, the understanding of the CCM progression is still limited. The objective of the present work was to elucidate the role of CCM3 by RNA-seq screening of CCM3 knockout mice.

Results

We found that ATPIF1 was decreased in siCCM3-treated Human Umbilical Vein Endothelial Cells (HUVECs), and the overexpression of ATPIF1 attenuated the changes in cell proliferation, adhesion and migration caused by siCCM3. The probable mechanism involved the conserved ATP concentration in mitochondria and the elongated morphology of the organelles. By using the CRISPR-cas9 system, we generated CCM3-KO Endothelial Progenitor Cells (EPCs) and found that the knockout of CCM3 destroyed the morphology of mitochondria, impaired the mitochondrial membrane potential and increased mitophagy. Overexpression of ATPIF1 contributed to the maintenance of normal structure of mitochondria, inhibiting activation of mitophagy and other signaling proteins (e.g., KLF4 and Tie2). The expression of KLF4 returned to normal in CCM3-KO EPCs after 2 days of re-overexpression of CCM3, but not other signaling proteins.

Conclusion

ATPIF1 maintains the normal structure of mitochondria, inhibiting the activation of mitophagy and other signaling pathway in endothelial cells. Loss of CCM3 leads to the destruction of mitochondria and activation of signaling pathways, which can be regulated by KLF4.

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.

Antibodies in cerebral cavernous malformations react with cytoskeleton autoantigens in the lesional milieu

Previous studies have reported robust inflammatory cell infiltration, synthesis of IgG, B-cell clonal expansion, deposition of immune complexes and complement within cerebral cavernous malformation (CCM) lesions. B-cell depletion has also been shown to reduce the maturation of CCM in murine models. We hypothesize that antigen(s) within the lesional milieu perpetuate the pathogenetic immune responses in CCMs. This study aims to identify those putative antigen(s) using monoclonal antibodies (mAbs) derived from plasma cells found in surgically removed human CCM lesions. We produced human mAbs from laser capture micro-dissected plasma cells from four CCM patients, and also germline-reverted versions. CCM mAbs were assayed using immunofluorescence on central nervous system (CNS) tissues and immunocytochemistry on human primary cell lines. Antigen characterization was performed using a combination of confocal microscopy, immunoprecipitation and mass spectrometry. Affinity was determined by enzyme-linked immunosorbent assay, and specificity by multi-color confocal microscopy and quantitative co-localization. CCM mAbs bound CNS tissue, especially endothelial cells and astrocytes. Non-muscle myosin heavy chain IIA (NMMHCIIA), vimentin and tubulin are three cytoskeleton proteins that were commonly targeted. Selection of cytoskeleton proteins by plasma cells was supported by a high frequency of immunoglobulin variable region somatic hypermutations, high affinity and selectivity of mAbs in their affinity matured forms, and profoundly reduced affinity and selectivity in the germline reverted forms. Antibodies produced by plasma cells in CCM lesions commonly target cytoplasmic and cytoskeletal autoantigens including NMMHCIIA, vimentin and tubulin that are abundant in endothelial cells and astrocytes. Binding to, and selection on autoantigen(s) in the lesional milieu likely perpetuates the pathogenetic immune response in CCMs. Blocking this in situ autoimmune response may yield a novel treatment for CCM.

Targeting miR-27a/VE-cadherin interactions rescues cerebral cavernous malformations in mice

Cerebral cavernous malformations (CCMs) are vascular lesions predominantly developing in the central nervous system (CNS), with no effective treatments other than surgery. Loss-of-function mutation in CCM1/krev interaction trapped 1 (KRIT1), CCM2, or CCM3/programmed cell death 10 (PDCD10) causes lesions that are characterized by abnormal vascular integrity. Vascular endothelial cadherin (VE-cadherin), a major regulator of endothelial cell (EC) junctional integrity is strongly disorganized in ECs lining the CCM lesions. We report here that microRNA-27a (miR-27a), a negative regulator of VE-cadherin, is elevated in ECs isolated from mouse brains developing early CCM lesions and in cultured ECs with CCM1 or CCM2 depletion. Furthermore, we show miR-27a acts downstream of kruppel-like factor (KLF)2 and KLF4, two known key transcription factors involved in CCM lesion development. Using CD5-2 (a target site blocker [TSB]) to prevent the miR-27a/VE-cadherin mRNA interaction, we present a potential therapy to increase VE-cadherin expression and thus rescue the abnormal vascular integrity. In CCM1- or CCM2-depleted ECs, CD5-2 reduces monolayer permeability, and in Ccm1 heterozygous mice, it restores dermal vessel barrier function. In a neonatal mouse model of CCM disease, CD5-2 normalizes vasculature and reduces vascular leakage in the lesions, inhibits the development of large lesions, and significantly reduces the size of established lesions in the hindbrain. Furthermore, CD5-2 limits the accumulation of inflammatory cells in the lesion area. Our work has established that VE-cadherin is a potential therapeutic target for normalization of the vasculature and highlights that targeting miR-27a/VE-cadherin interaction by CD5-2 is a potential novel therapy for the devastating disease, CCM.

Cerebral cavernous malformation (CCM) is a disease for which, hitherto, surgery has been the only option. This study shows that a potential therapeutic, CD5-2, inhibits lesion development and vascular leak in the brains of CCM neonatal mice by targeting the endothelial cell–specific adhesion molecule VE-cadherin and restoring the vascular integrity of CCM lesions.

Immunofluorescence of Cell-Cell and Cell-Extracellular Matrix Adhesive Defects in In Vitro Endothelial CCM Model: Juxtacrine Role of Mutant Extracellular Matrix on Wild-Type Endothelial Cells

Endothelial cells lining cerebral cavernous malformations (CCM) present strong adhesive and mechanical defects. Increased cell contractility is a driver to the onset and the expansion of the CCM lesions. 2D in vitro endothelial models have been developed from either endothelial cells isolated from ccm1-3 knock-out mice or CCM1-3-silenced primary endothelial cells. These in vitro models faithfully recapitulate the adhesive and contractile defects of the CCM-deficient endothelial cells such as increased cell-extracellular matrix (ECM) adhesion through β1 integrin-anchored actin stress fibers, abnormal remodeling of the ECM, and destabilized VE-cadherin-dependent cell-cell junctions. Using such 2D in vitro CCM models, we have shown that the ECM remodeled by CCM-depleted endothelial cells can propagate CCM-like adhesive defects to wild-type endothelial cells, a process potentially pertinent to CCM lesion expansion. Here, we detail methods for studying the morphology of focal adhesions, actomyosin cytoskeleton, and VE-cadherin-dependent Adherens junctions by immunofluorescence and morphometric analyses. Moreover, we detail the protocols to produce and purify remodeled ECM and to test its effect on endothelial cell adhesion.

NIR-II window tracking of hyperglycemia induced intracerebral hemorrhage in cerebral cavernous malformation deficient mice

Left panel: Pseudocolor map of 3 principle components from NIR-II kinetic imaging, Right panel (top to bottom): In vivo Ag₂S QD NIR-II fluorescence, ex vivo iodine micro-CT, FITC dextran perfusion, and H&E staining in control vs CCM1+/− mice brain.

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.

Cerebral cavernous malformations form an anticoagulant vascular domain in humans and mice

Cerebral cavernous malformations (CCMs) are common brain vascular dysplasias that are prone to acute and chronic hemorrhage with significant clinical sequelae. The pathogenesis of recurrent bleeding in CCM is incompletely understood. Here, we show that central nervous system hemorrhage in CCMs is associated with locally elevated expression of the anticoagulant endothelial receptors thrombomodulin (TM) and endothelial protein C receptor (EPCR). TM levels are increased in human CCM lesions, as well as in the plasma of patients with CCMs. In mice, endothelial-specific genetic inactivation of Krit1 (Krit1 ^ECKO ) or Pdcd10 (Pdcd10 ^ECKO ), which cause CCM formation, results in increased levels of vascular TM and EPCR, as well as in enhanced generation of activated protein C (APC) on endothelial cells. Increased TM expression is due to upregulation of transcription factors KLF2 and KLF4 consequent to the loss of KRIT1 or PDCD10. Increased TM expression contributes to CCM hemorrhage, because genetic inactivation of 1 or 2 copies of the Thbd gene decreases brain hemorrhage in Pdcd10 ^ECKO mice. Moreover, administration of blocking antibodies against TM and EPCR significantly reduced CCM hemorrhage in Pdcd10 ^ECKO mice. Thus, a local increase in the endothelial cofactors that generate anticoagulant APC can contribute to bleeding in CCMs, and plasma soluble TM may represent a biomarker for hemorrhagic risk in CCMs.

Recent advances in cerebral cavernous malformation research

Cerebral cavernous malformations (CCM) are manifested by microvascular lesions characterized by leaky endothelial cells with minimal intervening parenchyma predominantly in the central nervous system predisposed to hemorrhagic stroke, resulting in focal neurological defects. Till date, three proteins are implicated in this condition: CCM1 (KRIT1), CCM2 (MGC4607), and CCM3 (PDCD10). These multi-domain proteins form a protein complex via CCM2 that function as a docking site for the CCM signaling complex, which modulates many signaling pathways. Defects in the formation of this signaling complex have been shown to affect a wide range of cellular processes including cell-cell contact stability, vascular angiogenesis, oxidative damage protection and multiple biogenic events. In this review we provide an update on recent advances in structure and function of these CCM proteins, especially focusing on the signaling cascades involved in CCM pathogenesis and the resultant CCM cellular phenotypes in the past decade.

Connexin 43 gap junctions contribute to brain endothelial barrier hyperpermeability in familial cerebral cavernous malformations type III by modulating tight junction structure

Familial cerebral cavernous malformations type III (fCCM3) is a disease of the cerebrovascular system caused by loss-of-function mutations in ccm3 that result in dilated capillary beds that are susceptible to hemorrhage. Before hemorrhage, fCCM3 lesions are characterized by a hyperpermeable blood-brain barrier (BBB), the key pathologic feature of fCCM3. We demonstrate that connexin 43 (Cx43), a gap junction (GJ) protein that is incorporated into the BBB junction complex, is up-regulated in lesions of a murine model of fCCM3. Small interfering RNA-mediated ccm3 knockdown (CCM3KD) in brain endothelial cells in vitro increased Cx43 protein expression, GJ plaque size, GJ intracellular communication (GJIC), and barrier permeability. CCM3KD hyperpermeability was rescued by GAP27, a peptide gap junction and hemichannel inhibitor of Cx43 GJIC. Tight junction (TJ) protein, zonula occludens 1 (ZO-1), accumulated at Cx43 GJs in CCM3KD cells and displayed fragmented staining at TJs. The GAP27-mediated inhibition of Cx43 GJs in CCM3KD cells restored ZO-1 to TJ structures and reduced plaque accumulation at Cx43 GJs. The TJ protein, Claudin-5, was also fragmented at TJs in CCM3KD cells, and GAP27 treatment lengthened TJ-associated fragments and increased Claudin 5-Claudin 5 transinteraction. Overall, we demonstrate that Cx43 GJs are aberrantly increased in fCCM3 and regulate barrier permeability by a TJ-dependent mechanism.-Johnson, A. M., Roach, J. P., Hu, A., Stamatovic, S. M., Zochowski, M. R., Keep, R. F., Andjelkovic, A. V. Connexin 43 gap junctions contribute to brain endothelial barrier hyperpermeability in familial cerebral cavernous malformations type III by modulating tight junction structure.

Thrombospondin1 (TSP1) replacement prevents cerebral cavernous malformations

KRIT1 mutations are the most common cause of cerebral cavernous malformation (CCM). Acute Krit1 gene inactivation in mouse brain microvascular endothelial cells (BMECs) changes expression of multiple genes involved in vascular development. These changes include suppression of Thbs1, which encodes thrombospondin1 (TSP1) and has been ascribed to KLF2- and KLF4-mediated repression of Thbs1 In vitro reconstitution of TSP1 with either full-length TSP1 or 3TSR, an anti-angiogenic TSP1 fragment, suppresses heightened vascular endothelial growth factor signaling and preserves BMEC tight junctions. Furthermore, administration of 3TSR prevents the development of lesions in a mouse model of CCM1 (Krit1ECKO ) as judged by histology and quantitative micro-computed tomography. Conversely, reduced TSP1 expression contributes to the pathogenesis of CCM, because inactivation of one or two copies of Thbs1 exacerbated CCM formation. Thus, loss of Krit1 function disables an angiogenic checkpoint to enable CCM formation. These results suggest that 3TSR, or other angiogenesis inhibitors, can be repurposed for TSP1 replacement therapy for CCMs.

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.

Nuclear Localization of Integrin Cytoplasmic Domain-associated Protein-1 (ICAP1) Influences β1 Integrin Activation and Recruits Krev/Interaction Trapped-1 (KRIT1) to the Nucleus

Binding of ICAP1 (integrin cytoplasmic domain-associated protein-1) to the cytoplasmic tails of β1 integrins inhibits integrin activation. ICAP1 also binds to KRIT1 (Krev interaction trapped-1), a protein whose loss of function leads to cerebral cavernous malformation, a cerebrovascular dysplasia occurring in up to 0.5% of the population. We previously showed that KRIT1 functions as a switch for β1 integrin activation by antagonizing ICAP1-mediated inhibition of integrin activation. Here we use overexpression studies, mutagenesis, and flow cytometry to show that ICAP1 contains a functional nuclear localization signal and that nuclear localization impairs the ability of ICAP1 to suppress integrin activation. Moreover, we find that ICAP1 drives the nuclear localization of KRIT1 in a manner dependent upon a direct ICAP1/KRIT1 interaction. Thus, nuclear-cytoplasmic shuttling of ICAP1 influences both integrin activation and KRIT1 localization, presumably impacting nuclear functions of KRIT1.

Review of familial cerebral cavernous malformations and report of seven additional families

Cerebral cavernous malformations are vascular anomalies of the central nervous system characterized by clusters of enlarged, leaky capillaries. They are caused by loss-of-function mutations in KRIT1, CCM2, or PDCD10. The proteins encoded by these genes are involved in four partially interconnected signaling pathways that control angiogenesis and endothelial permeability. Cerebral cavernous malformations can occur sporadically, or as a familial autosomal dominant disorder (FCCM) with incomplete clinical and neuroradiological penetrance and great inter-individual variability. Although the clinical course is unpredictable, symptoms typically present during adult life and include headaches, focal neurological deficits, seizures, and potentially fatal stroke. In addition to neural lesions, extraneural cavernous malformations have been described in familial disease in several tissues, in particular the skin. We here present seven novel FCCM families with neurologic and cutaneous lesions. We review histopathological and clinical features and provide an update on the pathophysiology of cerebral cavernous malformations and associated cutaneous vascular lesions. © 2016 Wiley Periodicals, Inc.

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.

PDCD10 (CCM3) regulates brain endothelial barrier integrity in cerebral cavernous malformation type 3: role of CCM3-ERK1/2-cortactin cross-talk

Impairment of brain endothelial barrier integrity is critical for cerebral cavernous malformation (CCM) lesion development. The current study investigates changes in tight junction (TJ) complex organization when PDCD10 (CCM3) is mutated/depleted in human brain endothelial cells. Analysis of lesions with CCM3 mutation and brain endothelial cells transfected with CCM3 siRNA (CCM3-knockdown) showed little or no increase in TJ transmembrane and scaffolding proteins mRNA expression, but proteins levels were generally decreased. CCM3-knockdown cells had a redistribution of claudin-5 and occludin from the membrane to the cytosol with no alterations in protein turnover but with diminished protein-protein interactions with ZO-1 and ZO-1 interaction with the actin cytoskeleton. The most profound effect of CCM3 mutation/depletion was on an actin-binding protein, cortactin. CCM3 depletion caused cortactin Ser-phosphorylation, dissociation from ZO-1 and actin, redistribution to the cytosol and degradation. This affected cortical actin ring organization, TJ complex stability and consequently barrier integrity, with constant hyperpermeability to inulin. A potential link between CCM3 depletion and altered cortactin was tonic activation of MAP kinase ERK1/2. ERK1/2 inhibition increased cortactin expression and incorporation into the TJ complex and improved barrier integrity. This study highlights the potential role of CCM3 in regulating TJ complex organization and brain endothelial barrier permeability.

Cerebral Cavernous Malformation-1 Protein Controls DLL4-Notch3 Signaling Between the Endothelium and Pericytes

BACKGROUND AND PURPOSE

Cerebral cavernous malformation (CCM) is a neurovascular dysplasia characterized by conglomerates of enlarged endothelial channels in the central nervous system, which are almost devoid of pericytes or smooth muscle cells. This disease is caused by loss-of-function mutations in CCM1, CCM2, or CCM3 genes in endothelial cells, making blood vessels highly susceptible to angiogenic stimuli. CCM1- and CCM3-silenced endothelial cells have a reduced expression of the Notch ligand Delta-like 4 (DLL4) resulting in impaired Notch signaling and irregular sprouting angiogenesis. This study aimed to address if DLL4, which is exclusively expressed on endothelial cells, may influence interactions of endothelial cells with pericytes, which express Notch3 as the predominant Notch receptor.

METHODS

Genetic manipulation of primary human endothelial cells and brain pericytes. Transgenic mouse models were also used.

RESULTS

Endothelial cell-specific ablation of Ccm1 and Ccm2 in different mouse models led to the formation of CCM-like lesions, which were poorly covered by periendothelial cells. CCM1 silencing in endothelial cells caused decreased Notch3 activity in cocultured pericytes. DLL4 proteins stimulated Notch3 receptors on human brain pericytes. Active Notch3 induced expression of PDGFRB2, N-Cadherin, HBEGF, TGFB1, NG2, and S1P genes. Notch3 signaling in pericytes enhanced the adhesion strength of pericytes to endothelial cells, limited their migratory and invasive behavior, and enhanced their antiangiogenic function. Pericytes silenced for Notch3 expression were more motile and could not efficiently repress angiogenesis.

CONCLUSIONS

The data suggest that Notch signaling in pericytes is important to maintain the quiescent vascular phenotype. Deregulated Notch signaling may, therefore, contribute to the pathogenesis of CCM.

Rho kinase as a target for cerebral vascular disorders

The development of novel pharmaceutical treatments for disorders of the cerebral vasculature is a serious unmet medical need. These vascular disorders are typified by a disruption in the delicate Rho signaling equilibrium within the blood vessel wall. In particular, Rho kinase overactivation in the smooth muscle and endothelial layers of the vessel wall results in cytoskeletal modifications that lead to reduced vascular integrity and abnormal vascular growth. Rho kinase is thus a promising target for the treatment of cerebral vascular disorders. Indeed, preclinical studies indicate that Rho kinase inhibition may reduce the formation/growth/rupture of both intracranial aneurysms and cerebral cavernous malformations.

Cerebral cavernous malformation proteins at a glance

Loss-of-function mutations in genes encoding KRIT1 (also known as CCM1), CCM2 (also known as OSM and malcavernin) or PDCD10 (also known as CCM3) cause cerebral cavernous malformations (CCMs). These abnormalities are characterized by dilated leaky blood vessels, especially in the neurovasculature, that result in increased risk of stroke, focal neurological defects and seizures. The three CCM proteins can exist in a trimeric complex, and each of these essential multi-domain adaptor proteins also interacts with a range of signaling, cytoskeletal and adaptor proteins, presumably accounting for their roles in a range of basic cellular processes including cell adhesion, migration, polarity and apoptosis. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of current models of CCM protein function focusing on how known protein-protein interactions might contribute to cellular phenotypes and highlighting gaps in our current understanding.