KRIT1-mediated regulation of neutrophil adhesion and motility

Loss of Krev interaction-trapped-1 (KRIT1) expression leads to the development of cerebral cavernous malformations (CCM), a disease in which abnormal blood vessel formation compromises the structure and function of the blood-brain barrier. The role of KRIT1 in regulating endothelial function is well-established. However, several studies have suggested that KRIT1 could also play a role in regulating nonendothelial cell types and, in particular, immune cells. In this study, we generated a mouse model with neutrophil-specific deletion of KRIT1 in order to investigate the effect of KRIT1 deficiency on neutrophil function. Neutrophils isolated from adult Ly6Gtm2621(cre)Arte Krit1flox/flox mice had a reduced ability to attach and spread on the extracellular matrix protein fibronectin and exhibited a subsequent increase in migration. However, adhesion to and migration on ICAM-1 was unchanged. In addition, we used a monomeric, fluorescently-labelled fragment of fibronectin to show that integrin activation is reduced in the absence of KRIT1 expression, though β1 integrin expression appears unchanged. Finally, neutrophil migration in response to lipopolysaccharide-induced inflammation in the lung was decreased, as shown by reduced cell number and myeloperoxidase activity in lavage samples from Krit1PMNKO mice. Altogether, we show that KRIT1 regulates neutrophil adhesion and migration, likely through regulation of integrin activation, which can lead to altered inflammatory responses in vivo.

β1 integrin monoclonal antibody treatment ameliorates cerebral cavernous malformations

β1 integrins are important in blood vessel formation and function, finely tuning the adhesion of endothelial cells to each other and to the extracellular matrix. The role of integrins in the vascular disease, cerebral cavernous malformation (CCM) has yet to be explored in vivo. Endothelial loss of the gene KRIT1 leads to brain microvascular defects, resulting in debilitating and often fatal consequences. We tested administration of a monoclonal antibody that enforces the active β1 integrin conformation, (clone 9EG7), on a murine neonatal CCM mouse model, Krit1flox/flox ;Pdgfb-iCreERT2 (Krit1ECKO ), and on KRIT1-silenced human umbilical vein endothelial cells (HUVECs). In addition, endothelial deletion of the master regulator of integrin activation, Talin 1 (Tln1), in Krit1ECKO mice was performed to assess the effect of completely blocking endothelial integrin activation on CCM. Treatment with 9EG7 reduced lesion burden in the Krit1ECKO model and was accompanied by a strong reduction in the phosphorylation of the ROCK substrate, myosin light chain (pMLC), in both retina and brain endothelial cells. Treatment of KRIT1-silenced HUVECs with 9EG7 in vitro stabilized cell-cell junctions. Overnight treatment of HUVECs with 9EG7 resulted in significantly reduced total surface expression of β1 integrin, which was associated with reduced pMLC levels, supporting our in vivo findings. Genetic blockade of integrin activation by Tln1ECKO enhanced bleeding and did not reduce CCM lesion burden in Krit1ECKO mice. In sum, targeting β1 integrin with an activated-specific antibody reduces acute murine CCM lesion development, which we found to be associated with suppression of endothelial ROCK activity.

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.

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.

CmP signaling network unveils novel biomarkers for triple negative breast cancer in African American women

Breast cancer is the most diagnosed cancer worldwide and remains the second leading cause of cancer death. While breast cancer mortality has steadily declined over the past decades through medical advances, an alarming disparity in breast cancer mortality has emerged between African American women (AAW) and Caucasian American women (CAW). New evidence suggests more aggressive behavior of triple-negative breast cancer (TNBC) in AAW may contribute to racial differences in tumor biology and mortality. Progesterone (PRG) can exert its cellular effects through either its classic, non-classic, or combined responses through binding to either classic nuclear PRG receptors (nPRs) or non-classic membrane PRG receptors (mPRs), warranting both pathways equally important in PRG-mediated signaling. In our previous report, we demonstrated that the CCM signaling complex (CSC) consisting of CCM1, CCM2, and CCM3 can couple both nPRs and mPRs signaling cascades to form a CSC-mPRs-PRG-nPRs (CmPn) signaling network in nPR positive(+) breast cancer cells. In this report, we furthered our research by establishing the CSC-mPRs-PRG (CmP) signaling network in nPR(-) breast cancer cells, demonstrating that a common core mechanism exists, regardless of nPR(+⁣/⁣-) status. This is the first report stating that inducible expression patterns exist between CCMs and major mPRs in TNBC cells. Furthermore, we firstly show mPRs in TNBC cells are localized in the nucleus and participate in nucleocytoplasmic shuttling in a coordinately synchronized fashion with CCMs under steroid actions, following the same cellular distribution as other well-defined steroid hormone receptors. Finally, for the first time, we deconvoluted the CmP signalosome by using systems biology and TNBC clinical data, which helped us understand key factors within the CmP network and identify 6 specific biomarkers with potential clinical applications associated with AAW-TNBC tumorigenesis. These novel biomarkers could have immediate clinical implications to dramatically improve health disparities among AAW-TNBCs.

MicroRNA-431-5p encapsulated in serum extracellular vesicles as a biomarker for proliferative diabetic retinopathy

Early diagnosis and precise monitoring of the development of proliferative diabetic retinopathy (PDR) can significantly improve therapeutic strategies and help decrease blindness caused by it. Extracellular vesicles (EVs) were recently found to be involved in intercellular communications and are a potential source for the discovery of novel biomarkers. The current study aims to investigate the effectiveness of microRNAs (miRNAs) encapsulated in small EVs (sEVs) as minimally invasive biomarkers for PDR. SEVs were extracted from plasma of healthy subjects, diabetic patients, nonPDR patients and PDR patients. Then, we performed microarray analysis to determine the miRNA expression profile. MiR-431-5p expression doubled in the PDR patients compared with the healthy controls and the diabetic patients. We further found that miR-431-5p expression was 2.3 times higher in 4-hydroxynonenal treated human retinal capillary endothelial cells (HRCECs) than the control. After transfection with miR-431-5p mimics, proliferation of HRCECs was promoted, while transfection with miR-431-5p inhibitor demonstrated the opposite effect. The present findings indicate that circulating sEVs showed a differential miRNA profile in PDR patients. MiR-431-5p was involved in the pathogenesis of PDR development and may function as a novel biomarker for PDR.

Emerging roles of CCM genes during tumorigenesis with potential application as novel biomarkers across major types of cancers

Cerebral cavernous malformations (CCMs) are microvascular anomalies in the brain that result in increased susceptibility to stroke. Three genes have been identified as causes of CCMs: cerebral cavernous malformations 1 [(CCM1) also termed Krev interaction trapped 1 (KRIT1)], cerebral cavernous malformation 2 [(CCM2) also termed MGC4607] and cerebral cavernous malformation 3 [(CCM3) also termed programmed cell death 10 (PDCD10)]. It has been demonstrated that both CCM1 and CCM3 bind to CCM2 to form a CCM signaling complex (CSC) with which to modulate multiple signaling cascades. CCM proteins have been reported to play major roles in microvascular angiogenesis in human and animal models. However, CCM proteins are ubiquitously expressed in all major tissues, suggesting an unseen broader role of the CSC in biogenesis. Recent evidence suggests the possible involvement of the CSC complex during tumorigenesis; however, studies concerning this aspect are limited. This is the first report to systematically investigate the expression patterns of CCM proteins in major human tumors using real‑time quantitative PCR, RNA‑fluorescence in situ hybridization, immunohistochemistry and multicolor immunofluorescence imaging. Our data demonstrated that differential expression patterns of the CSC complex are correlated with certain types and grades of major human cancers, indicating the potential application of CCM genes as molecular biomarkers for clinical oncology. Our data strongly suggest that more efforts are needed to elucidate the role of the CSC complex in tumorigenesis, which may have enormous clinical potential for cancer diagnostic, prognostic and therapeutic applications.

Emerging Pharmacologic Targets in Cerebral Cavernous Malformation and Potential Strategies to Alter the Natural History of a Difficult Disease: A Review

Importance

Cerebral cavernous malformations (CCMs) are vascular lesions of the brain that may lead to hemorrhage, seizures, and neurologic deficits. Most are linked to loss-of-function mutations in 1 of 3 genes, namely CCM1 (originally called KRIT1), CCM2 (MGC4607), or CCM3 (PDCD10), that can either occur as sporadic events or are inherited in an autosomal dominant pattern with incomplete penetrance. Familial forms originate from germline mutations, often have multiple intracranial lesions that grow in size and number over time, and cause an earlier and more severe presentation. Despite active preclinical research on a few pharmacologic agents, clinical translation has been slow. Open surgery and, in some cases, stereotactic radiosurgery remain the only effective treatments, but these options are limited by lesion accessibility and are associated with nonnegligible rates of morbidity and mortality.

Observations

We discuss the limits of CCM management and introduce findings from in vitro and in vivo studies that provide insight into CCM pathogenesis and indicate molecular mechanisms as potential therapeutic targets. These studies report dysregulated cellular pathways shared between CCM, cardiovascular diseases, and cancer. They also suggest the potential effectiveness of proper drug repurposing in association with, or as an alternative to, targeted interventions.

Conclusions and Relevance

We propose methods to exploit specific molecular pathways to design patient-tailored therapeutic approaches in CCM, with the aim to alter its natural progression. In this scenario, the lack of effective pharmacologic options remains a critical barrier that poses an unfulfilled and urgent medical need.

Comparative omics of CCM signaling complex (CSC)

BACKGROUND

Cerebral cavernous malformations (CCMs), a major neurosurgical condition, characterized by abnormally dilated intracranial capillaries, result in increased susceptibility to stroke. KRIT1 (CCM1), MGC4607 (CCM2), and PDCD10 (CCM3) have been identified as causes of CCMs in which at least one of them is disrupted in most familial cases. Our goal is to identify potential biomarkers and genetic modifiers of CCMs, using a global comparative omics approach across several in vitro studies and multiple in vivo animal models. We hypothesize that through analysis of the CSC utilizing various omics, we can identify potential biomarkers and genetic modifiers, by systemically evaluating effectors and binding partners of the CSC as well as second layer interactors.

METHODS

We utilize a comparative omics approach analyzing multiple CCMs deficient animal models across nine independent studies at the genomic, transcriptomic, and proteomic levels to dissect alterations in various signaling cascades.

RESULTS

Our analysis revealed a large set of genes that were validated across multiple independent studies, suggesting an important role for these identified genes in CCM pathogenesis.

CONCLUSION

This is currently one of the largest comparative omics analysis of CCM deficiencies across multiple models, allowing us to investigate global alterations among multiple signaling cascades involved in both angiogenic and non-angiogenic events and to also identify potential biomarker candidates of CCMs, which can be used for new therapeutic strategies.

Systems-wide analysis unravels the new roles of CCM signal complex (CSC)

Cerebral cavernous malformations (CCMs) are characterized by abnormally dilated intracranial capillaries that result in increased susceptibility to stroke. Three genes have been identified as causes of CCMs; KRIT1 (CCM1), MGC4607 (CCM2) and PDCD10 (CCM3); one of them is disrupted in most CCM cases. It was demonstrated that both CCM1 and CCM3 bind to CCM2 to form a CCM signaling complex (CSC) to modulate angiogenesis. In this report, we deployed both RNA-seq and proteomic analysis of perturbed CSC after depletion of one of three CCM genes to generate interactomes for system-wide studies. Our results demonstrated a unique portrait detailing alterations in angiogenesis and vascular integrity. Interestingly, only in-direct overlapped alterations between RNA and protein levels were detected, supporting the existence of multiple layers of regulation in CSC cascades. Notably, this is the first report identifying that both β4 integrin and CAV1 signaling are downstream of CSC, conveying the angiogenic signaling. Our results provide a global view of signal transduction modulated by the CSC, identifies novel regulatory signaling networks and key cellular factors associated with CSC.

Alternatively spliced isoforms reveal a novel type of PTB domain in CCM2 protein

Cerebral cavernous malformations (CCMs) is a microvascular disorder in the central nervous system. Despite tremendous efforts, the causal genetic mutation in some CCM patients has not be identified, raising the possibility of an unknown CCM locus. The CCM2/MGC4607 gene has been identified as one of three known genes causing CCMs. In this report, we defined a total of 29 novel exons and 4 novel promoters in CCM2 genomic structure and subsequently identified a total of 50 new alternative spliced isoforms of CCM2 which eventually generated 22 novel protein isoforms. Genetic analysis of CCM2 isoforms revealed that the CCM2 isoforms can be classified into two groups based on their alternative promoters and alternative start codon exons. Our data demonstrated that CCM2 isoforms not only are specific in their subcellular compartmentation but also have distinct cellular expression patterns among various tissues and cells, indicating the pleiotropic cellular roles of CCM2 through their multiple isoforms. In fact, the complexity of the CCM2 genomic structure was reflected by the multiple layers of regulation of CCM2 expression patterns. At the transcriptional level, it is accomplished by alternative promoters, alternative splicing, and multiple transcriptional start sites and termination sites; while at the translational level, it is carried out with various cellular functions with a distinguishable CCM2 protein group pattern, specified abundance and composition of selective isoforms in a cell and tissue specific fashion. Through experimentation, we discovered a unique phosphotyrosine binding (PTB) domain, namely atypical phosphotyrosine binding (aPTB) domain. Some long CCM2 isoform proteins contain both classes of PTB domains, making them a dual PTB domain-containing protein. Both CCM1 and CCM3 can bind competitively to this aPTB domain, indicating CCM2 as the cornerstone for CCM signaling complex (CSC).

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.

Novel functions of CCM1 delimit the relationship of PTB/PH domains

BACKGROUND

Three NPXY motifs and one FERM domain in CCM1 makes it a versatile scaffold protein for tethering the signaling components together within the CCM signaling complex (CSC). The cellular role of CCM1 protein remains inadequately expounded. Both phosphotyrosine binding (PTB) and pleckstrin homology (PH) domains were recognized as structurally related but functionally distinct domains.

METHODS

By utilizing molecular cloning, protein binding assays and RT-qPCR to identify novel cellular partners of CCM1 and its cellular expression patterns; by screening candidate PTB/PH proteins and subsequently structurally simulation in combining with current X-ray crystallography and NMR data to defined the essential structure of PTB/PH domain for NPXY-binding and the relationship among PTB, PH and FERM domain(s).

RESULTS

We identified a group of 28 novel cellular partners of CCM1, all of which contain either PTB or PH domain(s), and developed a novel classification system for these PTB/PH proteins based on their relationship with different NPXY motifs of CCM1. Our results demonstrated that CCM1 has a wide spectrum of binding to different PTB/PH proteins and perpetuates their specificity to interact with certain PTB/PH domains through selective combination of three NPXY motifs. We also demonstrated that CCM1 can be assembled into oligomers through intermolecular interaction between its F3 lobe in FERM domain and one of the three NPXY motifs. Despite being embedded in FERM domain as F3 lobe, F3 module acts as a fully functional PH domain to interact with NPXY motif. The most salient feature of the study was that both PTB and PH domains are structurally and functionally comparable, suggesting that PTB domain is likely evolved from PH domain with polymorphic structural additions at its N-terminus.

CONCLUSIONS

A new β1A-strand of the PTB domain was discovered and new minimum structural requirement of PTB/PH domain for NPXY motif-binding was determined. Based on our data, a novel theory of structure, function and relationship of PTB, PH and FERM domains has been proposed, which extends the importance of the NPXY-PTB/PH interaction on the CSC signaling and/or other cell receptors with great potential pointing to new therapeutic strategies.

GENERAL SIGNIFICANCE

The study provides new insight into the structural characteristics of PTB/PH domains, essential structural elements of PTB/PH domain required for NPXY motif-binding, and function and relationship among PTB, PH and FERM domains.

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.

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.

Rap1 signaling in endothelial barrier control

The small G-protein Rap1 plays an important role in the regulation of endothelial barrier function, a process controlled largely by cell–cell adhesions and their connection to the actin cytoskeleton. During the various stages of barrier dynamics, different guanine nucleotide exchange factors (GEFs) control Rap1 activity, indicating that Rap1 integrates multiple input signals. Once activated, Rap1 induces numerous signaling cascades, together responsible for the increased endothelial barrier function. Most notably, Rap1 activation results in the inhibition of Rho to decrease radial stress fibers and the activation of Cdc42 to increase junctional actin. This implies that Rap regulates endothelial barrier function by dual control of cytoskeletal tension. The molecular details of the signaling pathways are becoming to be elucidated.

The cardiovascular triad of dysfunctional angiogenesis

Cerebral cavernous malformation is a clinically well-defined microvascular disorder predisposing to stroke; however, the major phenotype observed in zebrafish is the cardiac defect, specifically an enlarged heart. Less effort has been made to explore this phenotypic discrepancy between human and zebrafish. Given the fact that the gene products from Ccm1/Ccm2 are nearly identical between the two species, the common sense has dictated that the zebrafish animal model would provide a great opportunity to dissect the detailed molecular function of Ccm1/Ccm2 during angiogenesis. We recently reported on the cellular role of the Ccm1 gene in biochemical processes that permit proper angiogenic microvascular development in the zebrafish model. In the course of this experimentation, we encountered a vast amount of recent research on the relationship between dysfunctional angiogenesis and cardiovascular defects in zebrafish. Here we compile the findings of our research with the most recent contributions in this field and glean conclusions about the effect of defective angiogenesis on the developing cardiovascular system. Our conclusion also serves as a bridge for the phenotypic discrepancy between humans and animal models, which might provide some insights into future translational research on human stroke.

Signaling pathways and the cerebral cavernous malformations proteins: lessons from structural biology

Cerebral cavernous malformations (CCM) are neurovascular dysplasias that result in mulberry-shaped lesions predominantly located in brain and spinal tissues. Mutations in three genes are associated with CCM. These genes encode for the proteins KRIT1/CCM1 (krev interaction trapped 1/cerebral cavernous malformations 1), cerebral cavernous malformations 2, osmosensing scaffold for MEKK3 (CCM2/malcavernin/OSM), and cerebral cavernous malformations 3/programmed cell death 10 (CCM3/PDCD10). There have been many significant recent advances in our understanding of the structure and function of these proteins, as well as in their roles in cellular signaling. Here, we provide an update on the current knowledge of the structure of the CCM proteins and their functions within cellular signaling, particularly in cellular adhesion complexes and signaling cascades. We go on to discuss subcellular localization of the CCM proteins, the formation and regulation of the CCM complex signaling platform, and current progress towards targeted therapy for CCM disease. Recent structural studies have begun to shed new light on CCM protein function, and we focus here on how these studies have helped inform the current understanding of these roles and how they may aid future studies into both CCM-related biology and disease mechanisms.

CCM1-ICAP-1 complex controls β1 integrin-dependent endothelial contractility and fibronectin remodeling

Loss of CCM1/2 leads to destabilization of ICAP-1 and up-regulation of β1 integrin, resulting in the destabilization of intercellular junctions due to increased cell contractility and aberrant extracellular matrix remodeling.

The endothelial CCM complex regulates blood vessel stability and permeability. Loss-of-function mutations in CCM genes are responsible for human cerebral cavernous malformations (CCMs), which are characterized by clusters of hemorrhagic dilated capillaries composed of endothelium lacking mural cells and altered sub-endothelial extracellular matrix (ECM). Association of the CCM1/2 complex with ICAP-1, an inhibitor of β1 integrin, prompted us to investigate whether the CCM complex interferes with integrin signaling. We demonstrate that CCM1/2 loss resulted in ICAP-1 destabilization, which increased β1 integrin activation and led to increased RhoA-dependent contractility. The resulting abnormal distribution of forces led to aberrant ECM remodeling around lesions of CCM1- and CCM2-deficient mice. ICAP-1–deficient vessels displayed similar defects. We demonstrate that a positive feedback loop between the aberrant ECM and internal cellular tension led to decreased endothelial barrier function. Our data support that up-regulation of β1 integrin activation participates in the progression of CCM lesions by destabilizing intercellular junctions through increased cell contractility and aberrant ECM remodeling.

Mechanism for KRIT1 release of ICAP1-mediated suppression of integrin activation

KRIT1 (Krev/Rap1 Interaction Trapped-1) mutations are observed in ∼40% of autosomal-dominant cerebral cavernous malformations (CCMs), a disease occurring in up to 0.5% of the population. We show that KRIT1 functions as a switch for β1 integrin activation by antagonizing ICAP1 (Integrin Cytoplasmic Associated Protein-1)-mediated modulation of "inside-out" activation. We present cocrystal structures of KRIT1 with ICAP1 and ICAP1 with integrin β1 cytoplasmic tail to 2.54 and 3.0 Å resolution (the resolutions at which I/σI = 2 are 2.75 and 3.0 Å, respectively). We find that KRIT1 binds ICAP1 by a bidentate surface, that KRIT1 directly competes with integrin β1 to bind ICAP1, and that KRIT1 antagonizes ICAP1-modulated integrin activation using this site. We also find that KRIT1 contains an N-terminal Nudix domain, in a region previously designated as unstructured. We therefore provide insights to integrin regulation and CCM-associated KRIT1 function.

Structural basis for small G protein effector interaction of Ras-related protein 1 (Rap1) and adaptor protein Krev interaction trapped 1 (KRIT1)

Cerebral cavernous malformations (CCMs) affect 0.1-0.5% of the population resulting in leaky vasculature and severe neurological defects. KRIT1 (Krev interaction trapped-1) mutations associate with ∼40% of familial CCMs. KRIT1 is an effector of Ras-related protein 1 (Rap1) GTPase. Rap1 relocalizes KRIT1 from microtubules to cell membranes to impact integrin activation, potentially important for CCM pathology. We report the 1.95 Å co-crystal structure of KRIT1 FERM domain in complex with Rap1. Rap1-KRIT1 interaction encompasses an extended surface, including Rap1 Switch I and II and KRIT1 FERM F1 and F2 lobes. Rap1 binds KRIT1-F1 lobe using a GTPase-ubiquitin-like fold interaction but binds KRIT1-F2 lobe by a novel interaction. Point mutagenesis confirms the interaction. High similarity between KRIT1-F2/F3 and talin is revealed. Additionally, the mechanism for FERM domains acting as GTPase effectors is suggested. Finally, structure-based alignment of each lobe suggests classification of FERM domains as ERM-like and TMFK-like (talin-myosin-FAK-KRIT-like) and that FERM lobes resemble domain "modules."

Cerebral cavernous malformations as a disease of vascular permeability: from bench to bedside with caution

Tremendous insight into the molecular and genetic pathogenesis of cerebral cavernous malformations (CCMs) has been gained over the past 2 decades. This includes the identification of 3 distinct genes involved in familial CCMs. Still, a number of unanswered questions regarding the process from gene mutation to vascular malformation remain. It is becoming more evident that the disruption of interendothelial junctions and ensuing vascular hyperpermeability play a principal role. The purpose of this review is to summarize the current understanding of CCM genes, associated proteins, and functional pathways. Promising molecular and genetic therapies targeted at identified molecular aberrations are discussed as well.

Ccm1 regulates microvascular morphogenesis during angiogenesis

Cerebral cavernous malformations (CCMs) are characterized by abnormally dilated intracranial capillaries that have a propensity to bleed. The development of some CCMs in humans has been attributed to mutations in CCM1 and CCM2 genes. In animal models, major cardiovascular defects caused by both gene mutations have been observed. However, the effects of the loss of Ccm function on the microvasculature in animal models are less defined. Using high-resolution imaging in vivo, we demonstrated that the loss of Ccm1 in zebrafish embryos leads to failed microvascular lumenization during angiogenesis due to impaired intraendothelial vacuole formation and fusion. No developmental changes during vasculogenesis and the initial stage of angiogenesis were observed, being in contrast to prior reports. In vivo zebrafish studies were further substantiated by in vitro findings in human endothelial cells that elucidated the biochemical pathways of CCM1 deficiency. We found that CCM1 regulates angiogenic microvascular lumen formation through Rac1 small GTPase. In summary, Ccm1 has been identified as a key angiogenic modulator in microvascular tubulogenesis. Additionally, the microvascular pathology observed in developing Ccm1 mutant zebrafish embryos mirrors that seen in human CCM lesions, suggesting that zebrafish might provide a superior animal model to study the pathogenesis of human CCM.

KRIT1 regulates the homeostasis of intracellular reactive oxygen species

KRIT1 is a gene responsible for Cerebral Cavernous Malformations (CCM), a major cerebrovascular disease characterized by abnormally enlarged and leaky capillaries that predispose to seizures, focal neurological deficits, and fatal intracerebral hemorrhage. Comprehensive analysis of the KRIT1 gene in CCM patients has suggested that KRIT1 functions need to be severely impaired for pathogenesis. However, the molecular and cellular functions of KRIT1 as well as CCM pathogenesis mechanisms are still research challenges. We found that KRIT1 plays an important role in molecular mechanisms involved in the maintenance of the intracellular Reactive Oxygen Species (ROS) homeostasis to prevent oxidative cellular damage. In particular, we demonstrate that KRIT1 loss/down-regulation is associated with a significant increase in intracellular ROS levels. Conversely, ROS levels in KRIT1^−/− cells are significantly and dose-dependently reduced after restoration of KRIT1 expression. Moreover, we show that the modulation of intracellular ROS levels by KRIT1 loss/restoration is strictly correlated with the modulation of the expression of the antioxidant protein SOD2 as well as of the transcriptional factor FoxO1, a master regulator of cell responses to oxidative stress and a modulator of SOD2 levels. Furthermore, we show that the KRIT1-dependent maintenance of low ROS levels facilitates the downregulation of cyclin D1 expression required for cell transition from proliferative growth to quiescence. Finally, we demonstrate that the enhanced ROS levels in KRIT1^−/− cells are associated with an increased cell susceptibility to oxidative DNA damage and a marked induction of the DNA damage sensor and repair gene Gadd45α, as well as with a decline of mitochondrial energy metabolism. Taken together, our results point to a new model where KRIT1 limits the accumulation of intracellular oxidants and prevents oxidative stress-mediated cellular dysfunction and DNA damage by enhancing the cell capacity to scavenge intracellular ROS through an antioxidant pathway involving FoxO1 and SOD2, thus providing novel and useful insights into the understanding of KRIT1 molecular and cellular functions.

Ccm1 assures microvascular integrity during angiogenesis

Cerebral cavernous malformations (CCM) are characterized by abnormal dilated intracranial capillaries that predispose to hemorrhage. The development of some CCMs in humans has been attributed to mutations in the CCM1 genes. Currently, contradictory results have been generated regarding the vascular endothelial cell population changes in Ccm1 deficiency in zebrafish. We hypothesize that the inconsistent results simply reflect the spatial and temporal difference for the observed vascular endothelial cells during zebrafish embryonic development. Using high resolution images in vivo, we demonstrated that the loss of Ccm1 in zebrafish embryos leads to marked increases in apoptosis in vascular endothelium at the end stage of microvascular angiogenesis. In vivo zebrafish studies were further substantiated by in vitro findings in human endothelial cells that elucidated the biochemical pathways of CCM1 deficiency. We found that that loss of CCM1 in vitro promotes apoptosis through decreased activation of the integrin-linked kinase survival signaling pathway. In summary, Ccm1 has been identified as a key modulator in maintaining microvascular integrity during zebrafish embryonic angiogenesis.

Recent insights into cerebral cavernous malformations: a complex jigsaw puzzle under construction

Cerebral cavernous malformations (CCM) are common vascular malformations with an unpredictable risk of hemorrhage, the consequences of which range from headache to stroke or death. Three genes, CCM1, CCM2 and CCM3, have been linked to the disease. The encoded CCM proteins interact with each other within a large protein complex. Within the past 2 years, a plethora of new data has emerged on the signaling pathways in which CCM proteins are involved. CCM proteins regulate diverse aspects of endothelial cell morphogenesis and blood vessel stability such as cell-cell junctions, cell shape and polarity, or cell adhesion to the extracellular matrix. Although fascinating, a global picture is hard to depict because little is known about how these pathways coordinate to orchestrate angiogenesis. Here we present what is known about the structural domain organization of CCM proteins, their association as a ternary complex and their subcellular localization. Numerous CCM partners have been identified using two-hybrid screens, genetic analyses or proteomic studies. We focus on the best-characterized partners and review data on the signaling pathways they regulate as a step towards a better understanding of the etiology of CCM disease.

Cerebral cavernous malformations: somatic mutations in vascular endothelial cells

OBJECTIVE

Germline mutations in 3 genes have been found in familial cases of cerebral cavernous malformations (CCMs). We previously discovered somatic and germline truncating mutations in the KRIT1 gene, supporting the "2-hit" mechanism of CCM lesion formation in a single lesion. The purpose of this study was to screen for somatic, nonheritable mutations in 3 more lesions from different patients and identify the cell type(s) in which somatic mutations occur.

METHODS

Somatic mutations were sought in DNA from 3 surgically excised, fresh-frozen CCM lesions by cloning and screening polymerase chain reaction products generated from KRIT1 or PDCD10 coding regions. Laser capture microdissection was used on isolated endothelial and nonendothelial cells to determine whether somatic mutations were found in endothelial cells.

RESULTS

CCM lesions harbor somatic and germline KRIT1 mutations on different chromosomes and are therefore biallelic. Both mutations are predicted to truncate the protein. The KRIT1 somatic mutations (novel c.1800delG mutation and previously identified 34 nucleotide deletion) in CCMs from 2 different patients were found only in the vascular endothelial cells lining caverns. No obvious somatic mutations were identified in the 2 other lesions; however, the results were inconclusive, possibly owing to the technical limitations or the fact that these specimens had a small proportion of vascular endothelial cells lining pristine caverns.

CONCLUSION

The "2-hit" mechanism occurs in vascular endothelial cells lining CCM caverns from 2 patients with somatic and Hispanic-American KRIT1 germline mutations. Methods for somatic mutation detection should focus on vascular endothelial cells lining pristine caverns.