Regulation of cardiovascular development and integrity by the heart of glass-cerebral cavernous malformation protein pathway

Loss of cerebral cavernous malformation 3 (Ccm3) in neuroglia leads to CCM and vascular pathology

Communication between neural cells and the vasculature is integral to the proper development and later function of the central nervous system. A mechanistic understanding of the interactions between components of the neurovascular unit has implications for various disorders, including cerebral cavernous malformations (CCMs) in which focal vascular lesions form throughout the central nervous system. Loss of function mutations in three genes with proven endothelial cell autonomous roles, CCM1/krev1 interaction trapped gene 1, CCM2, and CCM3/programmed cell death 10, cause familial CCM. By using neural specific conditional mouse mutants, we show that Ccm3 has both neural cell autonomous and nonautonomous functions. Gfap- or Emx1-Cre-mediated Ccm3 neural deletion leads to increased proliferation, increased survival, and activation of astrocytes through cell autonomous mechanisms involving activated Akt signaling. In addition, loss of neural CCM3 results in a vascular phenotype characterized by diffusely dilated and simplified cerebral vasculature along with formation of multiple vascular lesions that closely resemble human cavernomas through cell nonautonomous mechanisms. RNA sequencing of the vascular lesions shows abundant expression of molecules involved in cytoskeletal remodeling, including protein kinase A and Rho-GTPase signaling. Our findings implicate neural cells in the pathogenesis of CCMs, showing the importance of this pathway in neural/vascular interactions within the neurovascular unit.

Dissection of cardiovascular development and disease pathways in zebrafish

The use of animal models in medicine has contributed significantly to the development of drug treatments and surgical procedures for the last century, in particular for cardiovascular disease. In order to model human disease in an animal, an appreciation of the strengths and limitations of the system are required to interpret results and design the logical sequence of steps toward clinical translation. As the world's population ages, cardiovascular disease will become even more prominent and further progress will be essential to stave off what seems destined to become a massive public health issue. Future treatments will require the imaginative application of current models as well as the generation of new ones. In this review, we discuss the resources available for modeling cardiovascular disease in zebrafish and the varied attributes of this system. We then discuss current zebrafish disease models and their potential that has yet to be exploited.

Zebrafish as a model for hemorrhagic stroke

Blood vessels perform the fundamental role of providing conduits for the circulation of oxygen and nutrients and the removal of waste products throughout the body. Disruption of tissue perfusion by ischemia or hemorrhage of blood vessels has a range of devastating consequences including stroke. Stroke is a complex trait that includes both genetic and environmental risk factors. The zebrafish is an attractive model for the study of hemorrhagic stroke due to the conservation of the molecular mechanisms of blood vascular development among vertebrates and the experimental advantages that can be applied to zebrafish embryos and larva. This chapter will focus on the maintenance of vascular integrity and some of the seminal experimentation carried out in the zebrafish.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

Upregulation of transmembrane endothelial junction proteins in human cerebral cavernous malformations

OBJECT

Cerebral cavernous malformations (CCMs) are among the most prevalent cerebrovascular malformations, and endothelial cells seem to play a major role in the disease. However, the underlying mechanisms, including endothelial intercellular communication, have not yet been fully elucidated. In this article, the authors focus on the endothelial junction proteins CD31, VE-cadherin, and occludin as important factors for functional cell-cell contacts known as vascular adhesion molecules and adherence and tight junctions.

METHODS

Thirteen human CCM specimens and 6 control tissue specimens were cryopreserved and examined for the presence of VE-cadherin, occludin, and CD31 by immunofluorescence staining. Protein quantification was performed by triplicate measurements using western blot analysis.

RESULTS

Immunofluorescent analyses of the CCM sections revealed a discontinuous pattern of dilated microvessels and capillaries as well as increased expression of occludin, VE-cadherin, and CD31 in the intima and in the enclosed parenchymal tissue compared with controls. Protein quantification confirmed these findings by showing upregulation of the levels of these proteins up to 2-6 times.

CONCLUSIONS

A protocol enabling the molecular and morphological examination of the intercellular contact proteins in human CCM was validated. The abnormal and discontinuous pattern in these endothelial cell-contact proteins compared with control tissue explains the loose intercellular junctions that are considered to be one of the causes of CCM-associated bleeding or transendothelial oozing of erythrocytes. Despite the small number of specimens, this study demonstrates for the first time a quantitative analysis of endothelial junction proteins in human CCM.

Evaluating strategies for the treatment of cerebral cavernous malformations

Cerebral cavernous malformations are common vascular lesions of the central nervous system that predispose to seizures, focal neurological deficits, and potentially fatal hemorrhagic stroke. Human genetic studies have identified 3 genes associated with the disease, and biochemical studies of these proteins have identified interaction partners and possible signaling pathways. A recurring theme dominating the recent scientific literature is the causal link between mutations in the 3 cerebral cavernous malformation genes and hyperactivation of the small GTP exchange protein, RhoA, and the efficacy of reducing this hyperactivation using inexpensive and well-studied medicines, statins. Familial cerebral cavernous malformation offers a unique opportunity to use a personalized genomic medicine approach to identify a subset of patients prone to intracerebral hemorrhage that may benefit from a pharmacological therapy, where presently only neurosurgical options are available.

Electrostatic cell-surface repulsion initiates lumen formation in developing blood vessels

Blood vessels function in the uptake, transport, and delivery of gases and nutrients within the body. A key question is how the central lumen of blood vessels develops within a cord of vascular endothelial cells. Here, we demonstrate that sialic acids of apical glycoproteins localize to apposing endothelial cell surfaces and generate repelling electrostatic fields within an endothelial cell cord. Both in vitro and in vivo experiments show that the negative charge of sialic acids is required for the separation of endothelial cell surfaces and subsequent lumen formation. We also demonstrate that sulfate residues can substitute for sialic acids during lumen initiation. These results therefore reveal a key step in the creation of blood vessels, the most abundant conduits in the vertebrate body. Because negatively charged mucins and proteoglycans are often found on luminal cell surfaces, it is possible that electrostatic repulsion is a general principle also used to initiate lumen formation in other organs.

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.

CCM1 regulates vascular-lumen organization by inducing endothelial polarity

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

Differential angiogenesis function of CCM2 and CCM3 in cerebral cavernous malformations

Object

Loss-of-function mutations in CCM genes are frequently detected in familial cerebral cavernous malformations (CCMs). However, the current functional studies of the CCM genes in vitro have been performed mostly in commercially purchased normal cell lines and the results appeared discrepant. The fact that the cerebral vascular defects are rarely observed in CCM gene-deficient animals suggests the requirement of additional pathological background for the formation of vascular lesions. Consistent with these data, the authors assumed that silencing CCM genes in the endothelium derived from CCMs (CCM-ECs) serves as a unique and valuable model for investigating the function of the CCM genes in the pathogenesis of CCMs. To this end, the authors investigated the role and signaling of CCM2 and CCM3 in the key steps of angiogenesis using CCM-ECs.

Methods

Endothelial cells (ECs) derived from CCMs were isolated, purified, and cultured from the fresh operative specimens of sporadic CCMs (31 cases). The CCM2 and CCM3 genes were silenced by the specific short interfering RNAs in CCM-ECs and in control cultures (human brain microvascular ECs and human umbilical vein ECs). The efficiency of gene silencing was proven by real-time reverse transcriptase polymerase chain reaction. Cell proliferation and apoptosis, migration, tube formation, and the expression of phosphor-p38, phosphor-Akt, and phosphor-extracellular signal-regulated kinase–1 and 2 (ERK1/2) were analyzed under CCM2 and CCM3 silenced conditions in CCM-ECs.

Results

The CCM3 silencing significantly promoted proliferation and reduced apoptosis in all 3 types of endothelium, but accelerated cell migration exclusively in CCM-ECs. Interestingly, CCM2 siRNA influenced neither cell proliferation nor migration. Silencing of CCM3, and to a lesser extent CCM2, stimulated the growth and extension of sprouts selectively in CCM-ECs. Loss of CCM2 or CCM3 did not significantly influence the formation of the tubelike structure. However, the maintenance of tube stability was largely impaired by CCM2, but not CCM3, silencing. Western blot analysis revealed that CCM2 and CCM3 silencing commonly activated p38, Akt, and ERK1/2 in CCM-ECs.

Conclusions

The unique response of CCM-ECs to CCM2 or CCM3 siRNA indicates that silencing CCM genes in CCM-ECs is valuable for further studies on the pathogenesis of CCMs. Using this model system, the authors demonstrate a distinct role of CCM2 and CCM3 in modulating the different processes of angiogenesis. The stimulation of endothelial proliferation, migration, and massively growing and branching angiogenic sprouts after CCM3 silencing may potentially contribute to the formation of enriched capillary-like immature vessels in CCM lesions. The severe impairment of the tube integrity by CCM2, but not CCM3, silencing is associated with the different intracranial hemorrhage rate observed from CCM2 and CCM3 mutation carriers. The activation of p38, ERK1/2, and Akt signal proteins in CCM2- or CCM3-silenced CCM-ECs suggests a possible involvement of these common pathways in the pathogenesis of CCMs. However, the specific signaling mediating the distinct function of CCM genes in the pathogenesis of CCMs needs to be further elucidated.

Resolving cell-cell junctions: lumen formation in blood vessels

Formation of a patent vascular lumen is essential for the transport of oxygen, nutrients and waste products to and from tissues. No matter whether the blood vessel arises from vasculogenesis or angiogenesis, endothelial cells (EC) first have to form a cord, which subsequently lumenizes, in order to generate a functional vessel. During these processes, cellular junctions rearrange between adjacent ECs and are involved in EC polarization as a prerequisite for lumen formation. Here we review the role of EC junctions in vascular lumen formation within different vascular beds.

Control of endothelial barrier function by regulating vascular endothelial-cadherin

PURPOSE OF REVIEW

Proper control of endothelial cell contacts is the basis for maintenance of the vascular barrier function. Loss of this function leads to leak of fluid and protein from the vasculature and extensive leaks cause shock and death. The endothelial barrier also controls the entry of leukocytes into tissue and it is believed that leukocytes target endothelial cell contacts to reach sites of inflammation.

RECENT FINDINGS

Within the last 2 years several new molecular players and molecular interactions have been identified that either help in stabilizing the endothelial contacts or mediate their opening if triggered by the appropriate stimuli. Novel signaling mechanisms have been identified that regulate endothelial cell contacts. Whether, how and to what extent the complex of the endothelial specific adhesion molecule vascular endothelial-cadherin and its associated catenins is involved in these processes will be a major focus of this article.

SUMMARY

Endothelial cell contacts are regulated by a complex interplay between various receptors and signaling mediators that control the plasticity of the cytoskeleton and the function of junctional adhesion molecules. Knowing and understanding the essential players of this network will allow development of agents that could prevent breakdown of the vascular permeability barrier in shock or that could block leukocyte extravasation and thereby antagonize inflammation.

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.

CCM3 signaling through sterile 20-like kinases plays an essential role during zebrafish cardiovascular development and cerebral cavernous malformations

Cerebral cavernous malformation is a common human vascular disease that arises due to loss-of-function mutations in genes encoding three intracellular adaptor proteins, cerebral cavernous malformations 1 protein (CCM1), CCM2, and CCM3. CCM1, CCM2, and CCM3 interact biochemically in a pathway required in endothelial cells during cardiovascular development in mice and zebrafish. The downstream effectors by which this signaling pathway regulates endothelial function have not yet been identified. Here we have shown in zebrafish that expression of mutant ccm3 proteins (ccm3Delta) known to cause cerebral cavernous malformation in humans confers cardiovascular phenotypes identical to those associated with loss of ccm1 and ccm2. CCM3Delta proteins interacted with CCM1 and CCM2, but not with other proteins known to bind wild-type CCM3, serine/threonine protein kinase MST4 (MST4), sterile 20-like serine/threonine kinase 24 (STK24), and STK25, all of which have poorly defined biological functions. Cardiovascular phenotypes characteristic of CCM deficiency arose due to stk deficiency and combined low-level deficiency of stks and ccm3 in zebrafish embryos. In cultured human endothelial cells, CCM3 and STK25 regulated barrier function in a manner similar to CCM2, and STKs negatively regulated Rho by directly activating moesin. These studies identify STKs as essential downstream effectors of CCM signaling in development and disease that may regulate both endothelial and epithelial cell junctions.

Cerebral cavernous malformation protein CCM1 inhibits sprouting angiogenesis by activating DELTA-NOTCH signaling

Cerebral cavernous malformations (CCM) are frequent vascular abnormalities caused by mutations in one of the CCM genes. CCM1 (also known as KRIT1) stabilizes endothelial junctions and is essential for vascular morphogenesis in mouse embryos. However, cellular functions of CCM1 during the early steps of the CCM pathogenesis remain unknown. We show here that CCM1 represents an antiangiogenic protein to keep the human endothelium quiescent. CCM1 inhibits endothelial proliferation, apoptosis, migration, lumen formation, and sprouting angiogenesis in primary human endothelial cells. CCM1 strongly induces DLL4-NOTCH signaling, which promotes AKT phosphorylation but reduces phosphorylation of the mitogen-activated protein kinase ERK. Consistently, blocking of NOTCH activity alleviates CCM1 effects. ERK phosphorylation is increased in human CCM lesions. Transplantation of CCM1-silenced human endothelial cells into SCID mice recapitulates hallmarks of the CCM pathology and serves as a unique CCM model system. In this setting, the multikinase inhibitor Sorafenib can ameliorate loss of CCM1-induced excessive microvascular growth, reducing the microvessel density to levels of normal wild-type endothelial cells. Collectively, our data suggest that the origin of CCM lesions is caused by perturbed Notch signaling-induced excessive capillary sprouting, which can be therapeutically targeted.

On the death Trk

The trk family of receptor tyrosine kinases supports survival and differentiation in the nervous system. Paradoxically it has also been shown that members of the trk family can induce cell death in pediatric tumor cells of neuronal origin. Moreover, TrkA and TrkC serve as good prognostic indicators in neuroblastoma and medulloblatoma, respectively. Although the possible linkage between these observations was intriguing, until recently there was limited insight on the mechanisms involved. Recent findings suggest that TrkA might influence neuronal cell death through stimulation of p75 cleavage. An alternative p75-independent mechanism was suggested by a newly discovered interaction between TrkA and CCM2 (the protein product of the gene cerebral cavernous malformation 2). Coexpression of CCM2 with TrkA induces cell death in medulloblastoma and neuroblastoma cells, and CCM2 expression levels correlate with those of TrkA and with good prognosis in neuroblastoma patients. Thus, mechanistic clues to the enigma of trk-induced cell death have begun to emerge. Detailed elucidation of these mechanisms and their in vivo physiological significance will be of keen interest for future research.

Mechanisms of vascular stability and the relationship to human disease

PURPOSE OF REVIEW

The genetic basis for a variety of vascular malformation syndromes have been described, with an increasing functional understanding of the associated genes.

RECENT FINDINGS

Genes responsible for familial vascular malformation syndromes have increasingly been shown to be involved in the control of vascular stability.

SUMMARY

Genes involved in vascular stability pathways are good candidates for causing vascular malformation syndromes. Although these findings confirm the biologic importance of the involved pathways, further explanations are required to describe the focal nature of disease.

Stabilization of VEGFR2 signaling by cerebral cavernous malformation 3 is critical for vascular development

Cerebral cavernous malformations (CCMs) are human vascular malformations caused by mutations in three genes of unknown function: CCM1, CCM2, and CCM3. CCM3, also known as PDCD10 (programmed cell death 10), was initially identified as a messenger RNA whose abundance was induced by apoptotic stimuli in vitro. However, the in vivo function of CCM3 has not been determined. Here, we describe mice with a deletion of the CCM3 gene either ubiquitously or specifically in the vascular endothelium, smooth muscle cells, or neurons. Mice with global or endothelial cell-specific deletion of CCM3 exhibited defects in embryonic angiogenesis and died at an early embryonic stage. CCM3 deletion reduced vascular endothelial growth factor receptor 2 (VEGFR2) signaling in embryos and endothelial cells. In response to VEGF stimulation, CCM3 was recruited to and stabilized VEGFR2, and the carboxyl-terminal domain of CCM3 was required for the stabilization of VEGFR2. Indeed, the CCM3 mutants found in human patients lacking the carboxyl-terminal domain were labile and were unable to stabilize and activate VEGFR2. These results demonstrate that CCM3 promotes VEGFR2 signaling during vascular development.

CCM3/PDCD10 stabilizes GCKIII proteins to promote Golgi assembly and cell orientation

Mutations in CCM3/PDCD10 result in cerebral cavernous malformations (CCMs), a major cause of cerebral hemorrhage. Despite intense interest in CCMs, very little is known about the function of CCM3. Here, we report that CCM3 is located on the Golgi apparatus, forming a complex with proteins of the germinal center kinase III (GCKIII) family and GM130, a Golgi-resident protein. Cells depleted of CCM3 show a disassembled Golgi apparatus. Furthermore, in wound-healing assays, CCM3-depleted cells cannot reorient the Golgi and centrosome properly, and demonstrate impaired migration. Golgi disassembly after either depletion of CCM3 or dissociation of CCM3 from the GM130-GCKIII complex is the result of destabilization of GCKIII proteins and dephosphorylation of their substrate, 14-3-3zeta. Significantly, the phenotype induced by CCM3 depletion can be reverted by expression of wild-type CCM3, but not by disease-associated mutants. Our findings suggest that Golgi dysfunction and the ensuing abnormalities of cell orientation and migration resulting from CCM3 mutations contribute to CCM pathogenesis.

Cerebral cavernous malformations proteins inhibit Rho kinase to stabilize vascular integrity

Endothelial cell–cell junctions regulate vascular permeability, vasculogenesis, and angiogenesis. Familial cerebral cavernous malformations (CCMs) in humans result from mutations of CCM2 (malcavernin, OSM, MGC4607), PDCD10 (CCM3), or KRIT1 (CCM1), a Rap1 effector which stabilizes endothelial cell–cell junctions. Homozygous loss of KRIT1 or CCM2 produces lethal vascular phenotypes in mice and zebrafish. We report that the physical interaction of KRIT1 and CCM2 proteins is required for endothelial cell–cell junctional localization, and lack of either protein destabilizes barrier function by sustaining activity of RhoA and its effector Rho kinase (ROCK). Protein haploinsufficient Krit1^+/− or Ccm2^+/− mouse endothelial cells manifested increased monolayer permeability in vitro, and both Krit1^+/− and Ccm2^+/− mice exhibited increased vascular leak in vivo, reversible by fasudil, a ROCK inhibitor. Furthermore, we show that ROCK hyperactivity occurs in sporadic and familial human CCM endothelium as judged by increased phosphorylation of myosin light chain. These data establish that KRIT1–CCM2 interaction regulates vascular barrier function by suppressing Rho/ROCK signaling and that this pathway is dysregulated in human CCM endothelium, and they suggest that fasudil could ameliorate both CCM disease and vascular leak.

Rho kinase inhibition rescues the endothelial cell cerebral cavernous malformation phenotype

Cerebral cavernous malformations (CCM) are vascular lesions causing seizures and stroke. Mutations causing inactivation of one of three genes, ccm1, -2, or -3, are sufficient to induce vascular endothelial cell defects resulting in CCM. Herein, we show that loss of expression of the CCM1, -2, or -3 proteins causes a marked increase in expression of the GTPase RhoA. Live cell imaging with a RhoA-specific biosensor demonstrates increased RhoA activity with loss of CCM1, -2, or -3, with an especially pronounced RhoA activation in both the cytosol and the nucleus with loss of CCM1 expression. Increased RhoA activation was associated with Rho kinase-dependent phosphorylation of myosin light chain 2. Functionally, loss of CCM1, -2, or -3 inhibited endothelial cell vessel-like tube formation and extracellular matrix invasion, each of which is rescued by chemical inhibition or short hairpin RNA knockdown of Rho kinase. The findings, for the first time, define a signaling network for CCM1, -2, and -3 in CCM pathology, whereby loss of CCM1, -2, or -3 protein expression results in increased RhoA activity, with the activation of Rho kinase responsible for endothelial cell dysregulation. The results define Rho kinase as a therapeutic target to rescue endothelial cells from loss of CCM protein function.

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.

Recent insights into cerebral cavernous malformations: animal models of CCM and the human phenotype

Cerebral cavernous malformations are common vascular lesions of the central nervous system that predispose to seizures, focal neurologic deficits and potentially fatal hemorrhagic stroke. Human genetic studies have identified three genes associated with the disease and biochemical studies of these proteins have identified interaction partners and possible signaling pathways. A variety of animal models of CCM have been described to help translate the cellular and biochemical insights into a better understanding of disease mechanism. In this minireview, we discuss the contributions of animal models to our growing understanding of the biology of cavernous malformations, including the elucidation of the cellular context of CCM protein actions and the in vivo confirmation of abnormal endothelial cell-cell interactions. Challenges and progress towards developing a faithful model of CCM biology are reviewed.

Epac: defining a new mechanism for cAMP action

cAMP is a second messenger that is essential for relaying hormonal responses in many biological processes. The discovery of the cAMP target Epac explained various effects of cAMP that could not be attributed to the established targets PKA and cyclic nucleotide-gated ion channels. Epac1 and Epac2 function as guanine nucleotide exchange factors for the small G protein Rap. cAMP analogs that selectively activate Epac have helped to reveal a role for Epac in processes ranging from insulin secretion to cardiac contraction and vascular permeability. Advances in the understanding of the activation mechanism of Epac and its regulation by diverse anchoring mechanisms have helped to elucidate the means by which cAMP fulfills these functions via Epac.

Rap1 and its effector KRIT1/CCM1 regulate beta-catenin signaling

KRIT1, also called CCM1, is a member of a multiprotein complex that contains the products of the CCM2 and PDCD10 (also known as CCM3) loci. Heterozygous loss of any of the genes that encode these proteins leads to cerebral cavernous malformations (CCM), which are vascular lesions that are found in around 0.5% of humans. KRIT1 mediates the stabilization of beta-catenin-containing endothelial cell-cell junctions downstream of the Rap1 GTPase. Here, we report that Rap1 and KRIT1 are negative regulators of canonical beta-catenin signaling in mice and that hemizygous Krit1 deficiency exacerbates beta-catenin-driven pathologies. Depletion of endothelial KRIT1 caused beta-catenin to dissociate from vascular endothelial (VE)-cadherin and to accumulate in the nucleus with consequent increases in beta-catenin-dependent transcription. Activation of Rap1 inhibited beta-catenin-dependent transcription in confluent endothelial cells; this effect required the presence of intact cell-cell junctions and KRIT1. These effects of KRIT1 were not limited to endothelial cells; the KRIT1 protein was expressed widely and its depletion increased beta-catenin signaling in epithelial cells. Moreover, a reduction in KRIT1 expression also increased beta-catenin signaling in vivo. Hemizygous deficiency of Krit1 resulted in a ~1.5-fold increase in intestinal polyps in the Apc(Min/+) mouse, which was associated with increased beta-catenin-driven transcription. Thus, KRIT1 regulates beta-catenin signaling, and Krit1(+/-) mice are more susceptible to beta-catenin-driven intestinal adenomas.

Vascular integrity mediated by vascular endothelial cadherin and regulated by sphingosine 1-phosphate and angiopoietin-1

Development of blood vessels is coordinated by angiogenesis and stabilization of vascular endothelial cells (ECs). The vascular network is established during embryogenesis to supply oxygen and nutrients to the tissues and organs. However, after cardiac or peripheral ischemia is caused by occlusion of the vessels, new vessels must be formed to rescue the ischemic tissues. Many angiogenic growth factors and chemokines are produced in the ischemic tissue to induce angiogenic sprouting of preexisting vessels. Branched vessels must be again restabilized to form mature vessels that deliver blood to the tissues. To this end, vascular EC-cell adhesion is tightly regulated by cell-cell adhesion molecules and extracellular stimuli that activate G protein-coupled receptors and receptor tyrosine kinases exclusively expressed on vascular ECs. This review spotlights the recent studies of vascular endothelial cadherin and of sphingosine 1-phosphate signaling and angiopoietin-Tie signaling.

Biology of vascular malformations of the brain

BACKGROUND AND PURPOSE

This review discusses recent research on the genetic, molecular, cellular, and developmental mechanisms underlying the etiology of vascular malformations of the brain (VMBs), including cerebral cavernous malformation, sporadic brain arteriovenous malformation, and the arteriovenous malformations of hereditary hemorrhagic telangiectasia. Summary of Review- The identification of gene mutations and genetic risk factors associated with cerebral cavernous malformation, hereditary hemorrhagic telangiectasia, and sporadic arteriovenous malformation has enabled the development of animal models for these diseases and provided new insights into their etiology. All of the genes associated with VMBs to date have known or plausible roles in angiogenesis and vascular remodeling. Recent work suggests that the angiogenic process most severely disrupted by VMB gene mutation is that of vascular stabilization, the process whereby vascular endothelial cells form capillary tubes, strengthen their intercellular junctions, and recruit smooth muscle cells to the vessel wall. In addition, there is now good evidence that in some cases, cerebral cavernous malformation lesion formation involves a genetic 2-hit mechanism in which a germline mutation in one copy of a cerebral cavernous malformation gene is followed by a somatic mutation in the other copy. There is also increasing evidence that environmental second hits can produce lesions when there is a mutation to a single allele of a VMB gene.

CONCLUSIONS

Recent findings begin to explain how mutations in VMB genes render vessels vulnerable to rupture when challenged with other inauspicious genetic or environmental factors and have suggested candidate therapeutics. Understanding of the cellular mechanisms of VMB formation and progression in humans has lagged behind that in animal models. New knowledge of lesion biology will spur new translational work. Several well-established clinical and genetic database efforts are already in place, and further progress will be facilitated by collaborative expansion and standardization of these.

Control of cell adhesion dynamics by Rap1 signaling

Individual cells in their particular environments adhere to the extracellular matrix (ECM) and their neighbours via integrin-containing and cadherin-containing complexes, respectively. The dynamics of these interactions regulate the formation and maintenance of complex tissues. An expanding body of evidence accentuates the role of the small Rap1 GTPase and its associated signaling network in many of these processes. In this review we will discuss more recently revealed roles of Rap1 signaling by primarily focusing on functions of the Rap1 effectors RIAM, KRIT-1/CCM1 and AF-6/Afadin in junctional regulation of the vascular system and in epithelial cells. Furthermore, we will describe novel findings on the Rap activator PDZ-GEF in the regulation of cell-cell adhesion between epithelial cells and within a stem cell niche.

Emerging mechanisms of vascular stabilization

Neural guidance cues are essential for a growing axon to correctly course through the body and innervate target tissues. Interestingly, the vascular network follows a parallel trajectory along nerves, suggesting that guidance cues important for neural patterning may also be required for proper vascular patterning. However, while an axon arises from one cell, a blood vessel is composed of many endothelial cells. Recent evidence suggests that neural repulsive cues are usurped by multi-cellular blood vessels to ensure vascular stabilization cues. Additional clues into the signaling mechanisms that promote vascular stabilization are emerging from cerebral cavernous malformations, a disease characterized by headache, epilepsy, and stroke. Thus, neurobiology and neurology are providing insights into the concepts of vascular stability.

Endogenous endothelial cell signaling systems maintain vascular stability

The function of the endothelium is to provide a network to allow delivery of oxygen and nutrients to tissues throughout the body. This network comprises adjacent endothelial cells that utilize adherens junction proteins such as vascular endothelial cadherin (VE-cadherin) to maintain the appropriate level of vascular permeability. The disruption of VE-cadherin interactions during pathologic settings can lead to excessive vascular leak with adverse effects. Endogenous cell signaling systems have been defined, which help to maintain the proper level of vascular stability. Perhaps the best described system is Angiopoietin-1 (Ang-1). Ang-1 acting through its receptor Tie2 generates a well-described set of signaling events ultimately leading to enhanced vascular stability. In this review, we will focus on what is known about additional endogenous cell signaling systems that stabilize the vasculature, and using Ang-1/Tie2 as a model, we will address where our understanding of these additional systems is lacking.