Vascular system defects and neuronal apoptosis in mice lacking ras GTPase-activating protein

Neurocutaneous vascular syndromes

There have been significant recent advances in the past several years in the field of neurocutaneous vascular syndromes, including the development of more stringent diagnostic criteria for PHACE syndrome, the renaming of macrocephaly-cutis marmorata telangiectatica congenita to macrocephaly-capillary malformation to accurately reflect the true nature of the syndrome, and discovery of new genetic mutations such as RASA-1. There have also been advances in the understanding and management of Sturge-Weber syndrome.PHACE syndrome is a constellation of neurologic, arterial, cardiac, ophthalmologic, and sternal abnormalities associated with infantile hemangiomas. PHACE is an acronym for Posterior fossa malformation, Hemangioma, Arterial anomalies, Coarctation of the aorta, Eye abnormalities. Some authors include an "S" for PHACE(S) to denote the association of ventral defects including Sternal clefting and Supraumbilical raphe.The accurate diagnosis and work-up of these patients require coordination of care across multiple disciplines, including neuroradiology, radiology, dermatology, neurology, surgery, and interventional radiology, among others.This paper is meant to update clinicians and researchers about important advances in these rare, important vascular syndromes, to improve care, and lay the foundation for future directions for research.

MicroRNA-132-mediated loss of p120RasGAP activates the endothelium to facilitate pathological angiogenesis

Although it is well established that tumors initiate an angiogenic switch, the molecular basis of this process remains incompletely understood. Here we show that the miRNA miR-132 acts as an angiogenic switch by targeting p120RasGAP in the endothelium and thereby inducing neovascularization. We identified miR-132 as a highly upregulated miRNA in a human embryonic stem cell model of vasculogenesis and found that miR-132 was highly expressed in the endothelium of human tumors and hemangiomas but was undetectable in normal endothelium. Ectopic expression of miR-132 in endothelial cells in vitro increased their proliferation and tube-forming capacity, whereas intraocular injection of an antagomir targeting miR-132, anti-miR-132, reduced postnatal retinal vascular development in mice. Among the top-ranking predicted targets of miR-132 was p120RasGAP, which we found to be expressed in normal but not tumor endothelium. Endothelial expression of miR-132 suppressed p120RasGAP expression and increased Ras activity, whereas a miRNA-resistant version of p120RasGAP reversed the vascular response induced by miR-132. Notably, administration of anti-miR-132 inhibited angiogenesis in wild-type mice but not in mice with an inducible deletion of Rasa1 (encoding p120RasGAP). Finally, vessel-targeted nanoparticle delivery of anti-miR-132 restored p120RasGAP expression in the tumor endothelium, suppressed angiogenesis and decreased tumor burden in an orthotopic xenograft mouse model of human breast carcinoma. We conclude that miR-132 acts as an angiogenic switch by suppressing endothelial p120RasGAP expression, leading to Ras activation and the induction of neovascularization, whereas the application of anti-miR-132 inhibits neovascularization by maintaining vessels in the resting state.

Knock-in mutation reveals an essential role for focal adhesion kinase activity in blood vessel morphogenesis and cell motility-polarity but not cell proliferation

Focal adhesion kinase (FAK) associates with both integrins and growth factor receptors in the control of cell motility and survival. Loss of FAK during mouse development results in lethality at embryonic day 8.5 (E8.5) and a block in cell proliferation. Because FAK serves as both a scaffold and signaling protein, gene knock-outs do not provide mechanistic insights in distinguishing between these modes of FAK function. To determine the role of FAK activity during development, a knock-in point mutation (lysine 454 to arginine (R454)) within the catalytic domain was introduced by homologous recombination. Homozygous FAK(R454/R454) mutation was lethal at E9.5 with defects in blood vessel formation as determined by lack of yolk sac primary capillary plexus formation and disorganized endothelial cell patterning in FAK(R454/R454) embryos. In contrast to the inability of embryonic FAK(-/-) cells to proliferate ex vivo, primary FAK(R454/R454) mouse embryo fibroblasts (MEFs) were established from E8.5 embryos. R454 MEFs exhibited no difference in cell growth compared with normal MEFs, and R454 FAK localized to focal adhesions but was not phosphorylated at Tyr-397. In E8.5 embryos and primary MEFs, FAK R454 mutation resulted in decreased c-Src Tyr-416 phosphorylation. R454 MEFs exhibited enhanced focal adhesion formation, decreased migration, and defects in cell polarity. Within immortalized MEFs, FAK activity was required for fibronectin-stimulated FAK-p190RhoGAP association and p190RhoGAP tyrosine phosphorylation linked to decreased RhoA GTPase activity, focal adhesion turnover, and directional motility. Our results establish that intrinsic FAK activity is essential for developmental processes controlling blood vessel formation and cell motility-polarity but not cell proliferation. This work supports the use of FAK inhibitors to disrupt neovascularization.

The roles of Dok family adapters in immunoreceptor signaling

The mammalian Dok protein family has seven members (Dok-1-Dok-7). The Dok proteins share structural similarities characterized by the NH2-terminal pleckstrin homology and phosphotyrosine-binding domains followed by SH2 target motifs in the COOH-terminal moiety, indicating an adapter function. Indeed, Dok-1 was originally identified as a 62 kDa protein that binds with p120 rasGAP, a potent inhibitor of Ras, upon tyrosine phosphorylation by a variety of protein tyrosine kinases. Among the Dok family, only Dok-1, Dok-2, and Dok-3 are preferentially expressed in hematopoietic/immune cells. Dok-1 and its closest relative Dok-2 act as negative regulators of the Ras-Erk pathway downstream of many immunoreceptor-mediated signaling systems, and it is believed that recruitment of p120 rasGAP by Dok-1 and Dok-2 is critical to their negative regulation. By contrast, Dok-3 does not bind with p120 rasGAP. However, accumulating evidence has demonstrated that Dok-3 is a negative regulator of the activation of JNK and mobilization of Ca2+ in B-cell receptor-mediated signaling, where the interaction of Dok-3 with SHIP-1 and Grb2 appears to be important. Here, we review the physiological roles and underlying mechanisms of Dok family proteins.

Outcome of array CGH analysis for 255 subjects with intellectual disability and search for candidate genes using bioinformatics

Array CGH enables the detection of pathogenic copy number variants (CNVs) in 5-15% of individuals with intellectual disability (ID), making it a promising tool for uncovering ID candidate genes. However, most CNVs encompass multiple genes, making it difficult to identify key disease gene(s) underlying ID etiology. Using array CGH we identified 47 previously unreported unique CNVs in 45/255 probands. We prioritized ID candidate genes using five bioinformatic gene prioritization web tools. Gene priority lists were created by comparing integral genes from each CNV from our ID cohort with sets of training genes specific either to ID or randomly selected. Our findings suggest that different training sets alter gene prioritization only moderately; however, only the ID gene training set resulted in significant enrichment of genes with nervous system function (19%) in prioritized versus non-prioritized genes from the same de novo CNVs (7%, p < 0.05). This enrichment further increased to 31% when the five web tools were used in concert and included genes within mitogen-activated protein kinase (MAPK) and neuroactive ligand-receptor interaction pathways. Gene prioritization web tools enrich for genes with relevant function in ID and more readily facilitate the selection of ID candidate genes for functional studies, particularly for large CNVs.

Activation of endothelial ras signaling bypasses senescence and causes abnormal vascular morphogenesis

Angiogenesis is crucial for embryogenesis, reproduction, and wound healing and is a critical determinant of tumor growth and metastasis. The multifunctional signal transducer Ras is a proto-oncogene and frequently becomes mutated in a variety of human cancers, including angiosarcomas. Regulation of Ras is important for endothelial cell function and angiogenesis. Hyperactivation of Ras is linked with oncogene-induced senescence in many cell types. Given links between vascular malformations and angiosarcoma with activated Ras signaling, we sought to determine the consequence of sustained Ras activation on endothelial cell function. We find that sustained Ras activation in primary endothelial cells leads to prolonged activation of progrowth signaling, accompanied by a senescence bypass, enhanced proliferation, autonomous growth, and increased survival. Moreover, Ras severely compromises the ability of these cells to organize into vascular structures, instead promoting formation of planar endothelial sheets. This abnormal phenotype is regulated by phosphoinositide 3-kinase signaling, highlighting the therapeutic potential of agents targeting this axis in dealing with vascular morphogenic disorders and vascular normalization of tumors.

H-, N- and Kras cooperatively regulate lymphatic vessel growth by modulating VEGFR3 expression in lymphatic endothelial cells in mice

Mammalian Ras, which is encoded by three independent genes, has been thought to be a versatile component of intracellular signalling. However, when, where and how Ras signalling plays essential roles in development and whether the three Ras genes have overlapping functions in particular cells remain unclear. Here, we show that the three Ras proteins dose-dependently regulate lymphatic vessel growth in mice. We find that lymphatic vessel hypoplasia is a common phenotype in Ras compound knockout mice and that overexpressed normal Ras in an endothelial cell lineage selectively causes lymphatic vessel hyperplasia in vivo. Overexpression of normal Ras in lymphatic endothelial cells leads to sustained MAPK activation, cellular viability and enhanced endothelial network formation under serum-depleted culture conditions in vitro, and knockdown of endogenous Ras in lymphatic endothelial cells impairs cell proliferation, MAPK activation, cell migration and endothelial network formation. Ras overexpression and knockdown result in up- and downregulation of vascular endothelial growth factor receptor (VEGFR) 3 expression, respectively, in lymphatic endothelial cells in vitro. The close link between Ras and VEGFR3 in vitro is consistent with the result that Ras knockout and transgenic alleles are genetic modifiers in lymphatic vessel hypoplasia caused by Vegfr3 haploinsufficiency. Our findings demonstrate a cooperative function of the three Ras proteins in normal development, and also provide a novel aspect of VEGFR3 signalling modulated by Ras in lymphangiogenesis.

Design of versatile biochemical switches that respond to amplitude, duration, and spatial cues

Cells often mount ultrasensitive (switch-like) responses to stimuli. The design principles underlying many switches are not known. We computationally studied the switching behavior of GTPases, and found that this first-order kinetic system can show ultrasensitivity. Analytical solutions indicate that ultrasensitive first-order reactions can yield switches that respond to signal amplitude or duration. The three-component GTPase system is analogous to the physical fermion gas. This analogy allows for an analytical understanding of the functional capabilities of first-order ultrasensitive systems. Experiments show amplitude- and time-dependent Rap GTPase switching in response to Cannabinoid-1 receptor signal. This first-order switch arises from relative reaction rates and the concentrations ratios of the activator and deactivator of Rap. First-order ultrasensitivity is applicable to many systems where threshold for transition between states is dependent on the duration, amplitude, or location of a distal signal. We conclude that the emergence of ultrasensitivity from coupled first-order reactions provides a versatile mechanism for the design of biochemical switches.

Erk1 and Erk2 regulate endothelial cell proliferation and migration during mouse embryonic angiogenesis

Angiogenesis is a complex process orchestrated by both growth factors and cell adhesion and is initiated by focal degradation of the vascular basement membrane with subsequent migration and proliferation of endothelial cells. The Ras/Raf/MEK/ERK pathway is required for EC function during angiogenesis. Although in vitro studies implicate ERK1 and ERK2 in endothelial cell survival, their precise role in angiogenesis in vivo remains poorly defined. Cre/loxP technology was used to inactivate Erk1 and Erk2 in endothelial cells during murine development, resulting in embryonic lethality due to severely reduced angiogenesis. Deletion of Erk1 and Erk2 in primary endothelial cells resulted in decreased cell proliferation and migration, but not in increased apoptosis. Expression of key cell cycle regulators was diminished in the double knockout cells, and decreased DNA synthesis could be observed in endothelial cells during embryogenesis. Interestingly, both Paxillin and Focal Adhesion Kinase were expressed at lower levels in endothelial cells lacking Erk1 and Erk2 both in vivo and in vitro, leading to defects in the organization of the cytoskeleton and in cell motility. The regulation of Paxillin and Focal Adhesion Kinase expression occurred post-transcriptionally. These results demonstrate that ERK1 and ERK2 coordinate endothelial cell proliferation and migration during angiogenesis.

A novel association between RASA1 mutations and spinal arteriovenous anomalies

BACKGROUND AND PURPOSE

CM-AVM is a recently recognized autosomal dominant disorder associated with mutations in RASA1. Arteriovenous lesions have been reported in the brain, limbs, and the face in 18.5% of patients. We report a novel association between RASA1 mutations and spinal arteriovenous anomalies.

MATERIALS AND METHODS

In a collaborative study, 5 index patients (2 females, 3 males) with spinal AVMs or AVFs and cutaneous multifocal capillary lesions were investigated for the RASA1 gene mutation.

RESULTS

All 5 patients were found to have RASA1 mutation (2 de novo, 3 familial), and all had multifocal capillary malformations at birth. Neurologic deficits developed at ages ranging from infancy to early adulthood. All spinal anomalies (2 AVMs at the conus, 1 AVM at the lumbosacral junction, and 1 cervical and 1 cervicothoracic AVF) were complex, extensive, and fast-flow lesions. All patients required treatment based on the clinical and/or radiologic appearance of the lesions.

CONCLUSIONS

To our knowledge, an association of RASA1 mutation and spinal AVM/AVF has not been described. MR imaging screening of patients with characteristic CMs and neurologic symptoms presenting at a young age may be useful in detecting the presence of fast-flow intracranial or intraspinal arteriovenous anomalies before potentially significant neurologic insult has occurred.

Cardiac and vascular functions of the zebrafish orthologues of the type I neurofibromatosis gene NFI

Von Recklinghausen neurofibromatosis is a common autosomal dominant genetic disorder characterized by benign and malignant tumors of neural crest origin. Significant progress in understanding the pathophysiology of this disease has occurred in recent years, largely aided by the development of relevant animal models. Von Recklinghausen neurofibromatosis is caused by mutations in the NF1 gene, which encodes neurofibromin, a large protein that modulates the activity of Ras. Here, we describe the identification and characterization of zebrafish nf1a and nf1b, orthologues of NF1, and show neural crest and cardiovascular defects resulting from morpholino knockdown, including vascular and cardiac valvular abnormalities. Development of a zebrafish model of von Recklinghausen neurofibromatosis will allow for structure-function analysis and genetic screens in this tractable vertebrate system.

From germline towards somatic mutations in the pathophysiology of vascular anomalies

The localized structural abnormalities that arise during vasculogenesis, angiogenesis and lymphangiogenesis, the developmental processes which give rise to the adult vasculature, are collectively termed vascular anomalies. The last 2 years have seen an explosion of studies that underscore paradominant inheritance, the combination of inherited changes with somatic second-hits to the same genes, as underlying rare familial forms. Moreover, local, somatic genetic defects that cause some of the common sporadic forms of these malformations have been unraveled. This highlights the importance of assessing for tissue-based genetic changes, especially acquired genetic changes, as possible pathophysiological causes, which have been largely overlooked except in the area of cancer research. Large-scale somatic screens will therefore be essential in uncovering the nature and prevalence of such changes, and their downstream effects. The identification of disease genes combined with exhaustive, precise clinical delineations of the entire spectra of associated phenotypes guides better management and genetic counseling. Such a synthesis of information on functional and phenotypic effects will enable us to make and use animal models to test less invasive, targeted, perhaps locally administered, biological therapies.

Rasip1 is required for endothelial cell motility, angiogenesis and vessel formation

Ras proteins are small GTPases that regulate cellular growth and differentiation. Components of the Ras signaling pathway have been shown to be important during embryonic vasculogenesis and angiogenesis. Here, we report that Rasip1, which encodes a novel Ras-interacting protein, is strongly expressed in vascular endothelial cells throughout development, in both mouse and frog. Similar to the well-characterized vascular markers VEGFR2 and PECAM, Rasip1 is specifically expressed in angioblasts prior to vessel formation, in the initial embryonic vascular plexus, in the growing blood vessels during angiogenesis and in the endothelium of mature blood vessels into the postnatal period. Rasip1 expression is undetectable in VEGFR2 null embryos, which lack endothelial cells, suggesting that Rasip1 is endothelial specific. siRNA-mediated reduction of Rasip1 severely impairs angiogenesis and motility in endothelial cell cultures, and morpholino knockdown experiments in frog embryos demonstrate that Rasip1 is required for embryonic vessel formation in vivo. Together, these data identify Rasip1 as a novel endothelial factor that plays an essential role in vascular development.

Swiss cheese et allii, some of the first neurodegenerative mutants isolated in Drosophila

Drosophila has only recently become a model organism to study progressive neurodegeneration, mainly using transgenic flies expressing human disease genes. However, classical forward genetics isolating and characterizing fly mutants that show characteristic features of progressive neurodegeneration can also provide a useful tool to get insights into the mechanisms of neurodegeneration. Interestingly, the first such mutants have been already isolated in the 1970s, and this review focuses on the description of four such mutants originally isolated by Martin Heisenberg.

AIP1 functions as an endogenous inhibitor of VEGFR2-mediated signaling and inflammatory angiogenesis in mice

ASK1-interacting protein-1 (AIP1), a recently identified member of the Ras GTPase-activating protein family, is highly expressed in vascular ECs and regulates EC apoptosis in vitro. However, its function in vivo has not been established. To study this, we generated AIP1-deficient mice (KO mice). Although these mice showed no obvious defects in vascular development, they exhibited dramatically enhanced angiogenesis in 2 models of inflammatory angiogenesis. In one of these models, the enhanced angiogenesis observed in the KO mice was associated with increased VEGF-VEGFR2 signaling. Consistent with this, VEGF-induced ear, cornea, and retina neovascularization were greatly augmented in KO mice and the enhanced retinal angiogenesis was markedly diminished by overexpression of AIP1. In vitro, VEGF-induced EC migration was inhibited by AIP1 overexpression, whereas it was augmented by both AIP1 knockout and knockdown, with the enhanced EC migration caused by AIP1 knockdown being associated with increased VEGFR2 signaling. We present mechanistic data that suggest AIP1 is recruited to the VEGFR2-PI3K complex, binding to both VEGFR2 and PI3K p85, at a late phase of the VEGF response, and that this leads to inhibition of VEGFR2 signaling. Taken together, our data demonstrate that AIP1 functions as an endogenous inhibitor in VEGFR2-mediated adaptive angiogenesis in mice.

p120-Ras GTPase activating protein (RasGAP): a multi-interacting protein in downstream signaling

p120-RasGAP (Ras GTPase activating protein) plays a key role in the regulation of Ras-GTP bound by promoting GTP hydrolysis via its C-terminal catalytic domain. The p120-RasGAP N-terminal part contains two SH2, SH3, PH (pleckstrin homology) and CaLB/C2 (calcium-dependent phospholipid-binding domain) domains. These protein domains allow various functions, such as anti-/pro-apoptosis, proliferation and also cell migration depending of their distinct partners. The p120-RasGAP domain participates in protein-protein interactions with Akt, Aurora or RhoGAP to regulate functions described bellow. Here, we summarize, in angiogenesis and cancer, the various functional roles played by p120-RasGAP domains and their effector partners in downstream signaling.

Tissue-specific functions of the Caenorhabditis elegans p120 Ras GTPase activating protein GAP-3

All metazoan genomes encode multiple RAS GTPase activating proteins (RasGAPs) that negatively regulate the conserved RAS/MAPK signaling pathway. In mammals, several RasGAPs exhibit tumor suppressor activity by preventing excess RAS signal transduction. We have identified gap-3 as the to date missing Caenorhabditiselegans member of the p120 RasGAP family. By studying the genetic interaction of gap-3 with the two previously identified RasGAPs gap-1 and gap-2, we find that different combinations of RasGAPs are used to repress LET-60 RAS signaling depending on the cellular context. GAP-3 is the predominant negative regulator of RAS during meiotic progression of the germ cells, while GAP-1 is the key inhibitor of RAS during vulval induction. In other tissues such as the sex myoblasts or the chemosensory neurons, all three RasGAPs act in concert. The C. elegans RasGAPs have thus undergone partial specialization after gene duplication to allow the differential regulation of the RAS/MAPK signaling pathway in different cell types. A similar tissue specialization of the human tumor suppressor genes may explain the strong bias in the type of cancer they promote when mutated.

Parkes Weber syndrome, vein of Galen aneurysmal malformation, and other fast-flow vascular anomalies are caused by RASA1 mutations

Capillary malformation-arteriovenous malformation (CM-AVM) is a newly recognized autosomal dominant disorder, caused by mutations in the RASA1 gene in six families. Here we report 42 novel RASA1 mutations and the associated phenotype in 44 families. The penetrance and de novo occurrence were high. All affected individuals presented multifocal capillary malformations (CMs), which represent the hallmark of the disorder. Importantly, one-third had fast-flow vascular lesions. Among them, we observed severe intracranial AVMs, including vein of Galen aneurysmal malformation, which were symptomatic at birth or during infancy, extracranial AVM of the face and extremities, and Parkes Weber syndrome (PKWS), previously considered sporadic and nongenetic. These fast-flow lesions can be differed from the other two genetic AVMs seen in hereditary hemorrhagic telangiectasia (HHT) and in phosphatase and tensin homolog (PTEN) hamartomatous tumor syndrome. Finally, some CM-AVM patients had neural tumors reminiscent of neurofibromatosis type 1 or 2. This is the first extensive study on the phenotypes associated with RASA1 mutations, and unravels their wide heterogeneity.

H-ras regulates angiogenesis and vascular permeability by activation of distinct downstream effectors

Angiogenesis and vascular permeability occur following endothelium activation by vascular endothelial growth factor (VEGF). Downstream mechanisms that define these vascular responses remain unknown. H-Ras activation has been associated with the angiogenic response. However, active H-Ras initiates a wide spectrum of other biological responses through multiple downstream effectors. To identify vascular signaling by H-Ras and the immediate effectors we activated the extracellular signal regulated kinase/mitogen-activated protein kinase or phosphatidylinositol 3-kinase (PI3K) pathways in chicken and mouse endothelial tissues by ectopic expression of the Ras effector mutants H-RasV12S35 or H-RasV12C40, respectively. Constitutive activation of the extracellular signal-regulate kinase/mitogen-activated protein kinase pathway by H-RasV12S35 was sufficient to induce angiogenesis and not vascular permeability, whereas activation of the PI3K pathway by H-RasV12C40 was required for both angiogenesis and vascular permeability. Pharmacological inhibition of PI3K (alpha/beta) suppressed both Ras- or VEGF-mediated vascular response in vivo and survival of primary human endothelial cells in vitro. However, inhibition of PI3K (gamma/delta) suppressed Ras- or VEGF-mediated vascular permeability in vivo, with no effect on survival of primary endothelial cells. This was supported by genetic studies because PI3K p110gamma knockout mice showed impaired vascular permeability response to VEGF or H-RasV12C40 treatment yet produced a wild-type angiogenic response to H-RasV12S35. We conclude that downstream of VEGF, H-Ras serves as a cellular switch that controls neovascularization and vascular permeability by activation of distinct effectors.

Generation of mice with a conditional allele of the p120 Ras GTPase-activating protein

p120 Ras GTPase-activating protein (RasGAP) encoded by the rasa1 gene in mice is a prototypical member of the RasGAP family of proteins involved in negative-regulation of the p21 Ras proto-oncogene. RasGAP has been implicated in signal transduction through a number of cell surface receptors. In humans, inactivating mutations in the coding region of the RASA1 gene cause capillary malformation arteriovenous malformation. In mice, generalized disruption of the rasa1 gene results in early embryonic lethality associated with defective vasculogenesis and increased apoptosis of neuronal cells. The early lethality in this mouse model precludes its use to further study the importance of RasGAP as a regulator of cell function. Therefore, to circumvent this problem, we have generated a conditional rasa1 knockout mouse. In this mouse, an exon that encodes a part of the RasGAP protein essential for catalytic activity has been flanked by loxP recognition sites. With the use of different constitutive and inducible Cre transgenic mouse lines, we show that deletion of this exon from the rasa1 locus results in effective loss of expression of catalytically-active RasGAP from a variety of adult tissues. The conditional rasa1 mouse will be useful for the analysis of the role of RasGAP in mature cell types.

RASA1 mutations may cause hereditary capillary malformations without arteriovenous malformations

BACKGROUND

Capillary malformation (CM), a common vascular abnormality, is often present among family members. Recently a rare form of hereditary vascular malformation termed capillary malformation-arteriovenous malformation (CM-AVM) was shown to be caused by heterozygous mutations in RASA1, encoding RAS p21 protein activator 1. CM-AVM is characterized by multiple, small CMs associated with either AVM or arteriovenous fistula (AVF) in affected individuals or at least one of their family members.

OBJECTIVES

The purpose of the study was to find out whether CMs in the absence of AVM/AVF are associated with RASA1 mutations.

PATIENTS/METHODS

We assessed three families comprising 14 affected individuals with CMs. Linkage to the RASA1 locus was evaluated using microsatellite markers. The RASA1 gene was scrutinized for pathogenic mutations using denaturing high-performance liquid chromatography screening and direct sequencing.

RESULTS

AVM/AVF was identified in one of three affected families. CM without AVM/AVF was found to map in one large kindred to the RASA1 locus. Direct sequencing revealed novel heterozygous mutations segregating with CM in all three families. The mutations are predicted to result in premature termination of translation and RASA1 haplo-insufficiency.

CONCLUSIONS

We have demonstrated that the spectrum of clinical manifestations due to mutations in RASA1 is wider than previously thought and also includes typical CMs not associated with AVM/AVF.

Primitive neuroectodermal tumors of the central nervous system

Controversial issues relating to the pathobiology and classification of central nervous system primitive neuroectodermal tumors (PNETs) have plagued neuropathologists for more than 70 years. Hypotheses advanced in the mid-1920's have remained as fixed concepts in contemporary literature, largely consequent to repetitious support by a small number of neuropathologists despite a growing body of information discrediting these ideas from neuroembryologists, oncologists, neuroscientists and pathologists. Attention has largely focused upon PNETs arising in the cerebellum (commonly known as medulloblastomas ([MBs]), because about 80% of central nervous system (CNS) PNETs originate in this site. It has been asserted that the 20% which do not are biologically different, although most individuals agree that the histological features of PNETs that occur in different sites throughout the CNS are indistinguishable from those growing in the cerebellum. The historical aspects of this controversy are examined in the face of evidence that there is, in fact, a unique class of CNS tumors which should appropriately be regarded as primitive neuroectodermal in nature. Specifically, a number of different approaches to the problem have yielded data supporting this hypothesis. These approaches include the identification of patterns of expression among a variety of cellular antigens (demonstrated by the use of immunopathological techniques), molecular analyses of cell lines derived from these tumors, experimental production of PNETs and molecular genetic analyses. Differences of opinion among surgeons, oncologists and radiotherapists are typically resolved by conducting cooperative studies of patients with these tumors who are diagnosed and treated at multiple centers.

RasGAPs: a crucial regulator of extracellular stimuli for homeostasis of cellular functions

Ras and its GTPase activating proteins (GAPs) are among the crucial regulators of extracelluar ligands. Information about these regulators has been elucidated during the course of studies in signal transduction over the last two decades. RasGAPs such as p120GAP and neurofibromin have been studied extensively for their roles as either "negative" regulators or effectors of Ras. Accumulating evidence suggests that these molecules are crucial regulators of extracellular stimuli that serve to maintain the homeostasis of cellular functions. This compendium highlights cellular functions of RasGAPs and their signaling characteristics from the viewpoint of homeostasis, including our recent finding of the phenotype of R-RasGAP mutant mice whose GAP activity is down-regulated.

Partial functional overlap of the three ras genes in mouse embryonic development

In mammals, three ras genes, H-ras, N-ras and K-ras, encode homologous but distinct 21-kDa Ras proteins. We examined the in vivo functional relationship of the three ras genes in mouse embryonic development by investigating the phenotypes of mice deficient in one or multiple ras genes. H-ras-/- mice and N-ras-/- mice as well as a substantial proportion of H-ras-/-/N-ras-/- mice expressing only the K-ras gene were viable, while K-ras-/- mice were embryonically lethal, as have been reported previously. N-ras-/-/K-ras+/- mice died neonatally, while H-ras-/-/K-ras-/- embryos died much earlier than K-ras homozygous mutant fetuses. To further investigate the functional relationship of the ras genes in embryonic development, we introduced a human H-ras transgene into single or multiple ras mutant mice and found that the transgene rescued mice, including triple ras mutants, from embryonic lethality in association with correction of thin ventricular walls of the heart in null K-ras mutant mice. In situ hybridization revealed that the expression of the H-ras transgene on embryonic day E13.5 and E15.5 was more intense in major organs, including the heart, than those of endogenous ras genes. We therefore conclude that the functions of the ras genes are partially overlapping in mouse embryonic development.

Mutant huntingtin can paradoxically protect neurons from death

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a mutation in the gene huntingtin and characterized by motor, cognitive and psychiatric symptoms. Huntingtin contains a CAG repeat in exon 1. An expansion of this CAG repeat above 35 results in misfolding of Huntingtin, giving rise to protein aggregates and neuronal cell death. There are several transgenic HD mouse models that reproduce most of the features of the human disorder, for example protein inclusions, some neurodegeneration as well as motor and cognitive symptoms. At the same time, a subgroup of the HD transgenic mouse models exhibit dramatically reduced susceptibility to excitotoxicity. The mechanism behind this is unknown. Here, we review the literature regarding this phenomenon, attempt to explain what protein domains are crucial for this phenomenon and point toward a putative mechanism. We suggest, that the C-terminal domain of exon 1 Huntingtin, namely the proline rich domain, is responsible for mediating a neuroprotective effect against excitotoxicity. Furthermore, we point out the possible importance of this mechanism for future therapies in neurological disorders that have been suggested to be associated with excitotoxicity, for example Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.

Biochemical and functional characterization of germ line KRAS mutations

Germ line missense mutations in HRAS and KRAS and in genes encoding molecules that function up- or downstream of Ras in cellular signaling networks cause a group of related developmental disorders that includes Costello syndrome, Noonan syndrome, and cardiofaciocutaneous syndrome. We performed detailed biochemical and functional studies of three mutant K-Ras proteins (P34R, D153V, and F156L) found in individuals with Noonan syndrome and cardiofaciocutaneous syndrome. Mutant K-Ras proteins demonstrate a range of gain-of-function effects in different cell types, and biochemical analysis supports the idea that the intrinsic Ras guanosine nucleotide triphosphatase (GTPase) activity, the responsiveness of these proteins to GTPase-activating proteins, and guanine nucleotide dissociation all regulate developmental programs in vivo.

Up-regulation of ras-GAP genes is reversed by a MEK inhibitor and doxorubicin in v-Ki-ras-transformed NIH/3T3 fibroblasts

Ras-GTPase-activating proteins (Ras-GAPs) have been implicated both as suppressors of Ras and as effectors in regulating cellular activities. To study whether Ras-GAPs have roles in tumor cell survival or not, mRNA levels of ras-related genes were measured in v-Ki-ras-transformed (DT) and the parental NIH/3T3 cells, using real-time PCR. mRNA levels of p120-Gap, Gap1(m), and PIK3CA were increased in DT cells compared with NIH/3T3 cells. p120-Gap and PIK3CA genes were induced by addition of serum or epidermal growth factor to serum-starved DT cells. Three anti-cancer drugs, an ERK kinase (MEK) inhibitor PD98059, a topoisomerase II poison doxorubicin (adriamycin), and a histone deacetylase inhibitor trichostatin A, selectively blocked the overexpression of p120-Gap and Gap1(m) genes in DT cells. These drugs also caused reversion of DT cells to the adherent shape associated with growth arrest. Our results suggest that p120-Gap and Gap1(m) genes provide important biomarkers for cancer therapies.

ERK1 and ERK2 mitogen-activated protein kinases affect Ras-dependent cell signaling differentially

Background

The mitogen-activated protein (MAP) kinases p44^ERK1 and p42^ERK2 are crucial components of the regulatory machinery underlying normal and malignant cell proliferation. A currently accepted model maintains that ERK1 and ERK2 are regulated similarly and contribute to intracellular signaling by phosphorylating a largely common subset of substrates, both in the cytosol and in the nucleus.

Results

Here, we show that ablation of ERK1 in mouse embryo fibroblasts and NIH 3T3 cells by gene targeting and RNA interference results in an enhancement of ERK2-dependent signaling and in a significant growth advantage. By contrast, knockdown of ERK2 almost completely abolishes normal and Ras-dependent cell proliferation. Ectopic expression of ERK1 but not of ERK2 in NIH 3T3 cells inhibits oncogenic Ras-mediated proliferation and colony formation. These phenotypes are independent of the kinase activity of ERK1, as expression of a catalytically inactive form of ERK1 is equally effective. Finally, ectopic expression of ERK1 but not ERK2 is sufficient to attenuate Ras-dependent tumor formation in nude mice.

Conclusion

These results reveal an unexpected interplay between ERK1 and ERK2 in transducing Ras-dependent cell signaling and proliferation. Whereas ERK2 seems to have a positive role in controlling normal and Ras-dependent cell proliferation, ERK1 probably affects the overall signaling output of the cell by antagonizing ERK2 activity.

Versatile Roles of R-Ras GAP in Neurite Formation of PC12 Cells and Embryonic Vascular Development

Ras GTPase-activating proteins (GAP) are negative regulators of Ras that convert active Ras-GTP to inactive Ras-GDP. R-Ras GAP is a membrane-associated molecule with stronger GAP activity for R-Ras, an activator of integrin, than H-Ras. We found that R-Ras GAP is down-regulated during neurite formation in rat pheochromocytoma PC12 cells by nerve growth factor (NGF), which is blocked by the transient expression of R-Ras gap or dominant negative R-ras cDNA. By establishing a PC12 subclone that stably expresses exogenous R-Ras GAP, it was found that NGF reduced endogenous R-Ras GAP but not exogenous R-Ras GAP, suggesting that down-regulation of R-Ras GAP occurs at the transcription level. To clarify the physiological role of R-Ras GAP, we generated mice that express mutant Ras GAP with knocked down activity. While heterozygotes are normal, homozygous mice die at E12.5-13.5 of massive subcutaneous and intraparenchymal bleeding, probably due to underdeveloped adherens junctions between capillary endothelial cells. These results show essential roles of R-Ras GAP in development and differentiation: its expression is needed for embryonic development of blood vessel barriers, whereas its down-regulation facilitates NGF-induced neurite formation of PC12 cells via maintaining activated R-Ras.

Improved embryonic stem cell technologies

Murine embryonic stem (ES) cells have become an indispensable tool for investigating genetic function both in vitro and, importantly, in vivo. Recent advances, including tetraploid aggregation, new site-specific recombinases and RNAi, have enabled more sophisticated manipulation of the ES cell genome. For instance, it is now possible to control gene expression in both a temporally and spatially restricted manner. Such new technologies are answering complex questions surrounding the function and interaction of an increasing number of genes. This chapter will review both the history and recent technological progress that has been made in mouse ES cell derivation, genetic manipulation and the generation of ES cell-derived chimaeric animals.

The Ras inhibitor farnesylthiosalicylic acid as a potential therapy for neurofibromatosis type 1

PURPOSE

Farnesylthiosalicylic acid (FTS) is a Ras inhibitor that dislodges all active Ras isoforms from the membrane. We assessed the ability of FTS to reverse the transformed phenotype of neurofibromatosis type 1 (NF1)-associated tumor cell lines of malignant peripheral nerve sheath tumor (MPNST).

EXPERIMENTAL DESIGN

nf1 mutations were genotyped, allelic losses were analyzed, and neurofibromin expression levels were determined in MPNST cell lines ST88-14, S265P21, and 90-8. The effects of FTS on GTP-bound Ras (Ras-GTP) and its prominent downstream targets, as well as on cell morphology, anchorage-dependent and anchorage-independent growth, and tumor growth in mice, were assessed.

RESULTS

The MPNST cell lines were biallelic, NF1 inactive, and neurofibromin deficient. We show that FTS treatment shortened the relatively long duration of Ras activation and signaling to extracellular signal-regulated kinase, Akt, and RalA in all NF1-deficient MPNST cell lines (NF1 cells) to that observed in a non-NF1, normally expressing neurofibromin MPNST cell line. These effects of FTS led to lower steady-state levels of Ras-GTP and its activated targets. Both anchorage-dependent and anchorage-independent growth of NF1 cells were dose dependently inhibited by FTS, and the inhibition correlated positively with Ras-GTP levels. NF1 cells were found to possess strong actin stress fibers, and this phenotype was also corrected by FTS. NF1 tumor growth in a nude mouse model was inhibited by oral FTS.

CONCLUSIONS

FTS treatment of NF1 cells normalized Ras-GTP levels, resulting in reversal of the transformed phenotype and inhibition of tumor growth. FTS may therefore be considered as a potential drug for the treatment of NF1.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Genes critical to vasculogenesis as defined by systematic analysis of vascular defects in knockout mice

To identify genes important to the process of vasculogenesis, we evaluated embryonic vascular anomalies from 100 mouse knockout studies using a novel meta-analysis approach. By applying this method, termed approach for ranking of embryonic vascular anomalies (AREVA), rank scores were calculated for each knockout based on the occurrence of vascular defects during periods of vasculogenesis in specific embryonic regions. As a result, 12 genes (fibronectin, VEGFR-1/Flt-1, VEGFR-2/Flk-1, alpha 5 integrin, Tek/Tie2, VE-cadherin, VEGFA, connexin 45, ShcA, cytochrome P450 reductase, CD148/DEP-1, and EphrinB2) were determined to play critical roles in vasculogenesis. Functional categorization of these genes revealed the fundamental importance of VEGF signaling since 10 of the 12 genes (fibronectin, VEGFR-1/Flt-1, VEGFR-2/Flk-1, alpha 5 integrin, VE-cadherin, VEGFA, ShcA, cytochrome P450 reductase, CD148/DEP-1, and EphrinB2) relate to this pathway. Furthermore, the findings highlight a potential network for regulating VEGF signaling involving integration of fibronectin, EphrinB2, Tie2, and connexin 45 signaling pathways via the ShcA/Ras/Raf/Mek/Erk cascade. In addition to retrospective application of AREVA as done herein, AREVA can be used prospectively to determine the relevancy to vasculogenesis of newly inactivated genes.

RASA1: variable phenotype with capillary and arteriovenous malformations

Capillary malformation-arteriovenous malformation (CM-AVM) is a newly discovered hereditary disorder. Its defining features are atypical cutaneous multifocal capillary malformations often in association with high-flow lesions: cutaneous, subcutaneous, intramuscular, intraosseous and cerebral arteriovenous malformations and arteriovenous fistulas. Some patients have Parkes Weber syndrome - a large congenital cutaneous vascular stain in an extremity, with bony and soft tissue hypertrophy and microscopic arteriovenous shunting. In the past, arteriovenous malformations and arteriovenous fistulas had been considered non-hereditary. A classical genetic approach was used to identify the locus. Candidate gene screening pinpointed mutations in RASA1 (p120-RASGAP) - a RasGTPase. RASA1 reverts active GTP-bound Ras into inactive GDP-bound form. Murine Rasa1 knockout and tetraploid-aggregated embryos with RNA interference exhibited abnormal vascular development. Lack of RASA1 activity caused inhibition of cell motility, possibly through p190-RhoGAP. Thus, RASA1 defects probably cause abnormal angiogenic remodeling of the primary capillary plexus that cannot be compensated for by other RasGAPs: RASA2, RASAL and NF1. Signaling pathways involving RASA1 might offer novel targets for treatment of high-flow vascular anomalies.

Neurodegenerative mutants in Drosophila: a means to identify genes and mechanisms involved in human diseases?

There are 50 ways to leave your lover (Simon 1987) but many more to kill your brain cells. Several neurodegenerative diseases in humans, like Alzheimer's disease, have been intensely studied but the underlying cellular and molecular mechanisms are still unknown for most of them. For those syndromes where associated gene products have been identified their biochemistry and physiological as well as pathogenic function is often still under debate. This is in part due to the inherent limitations of genetic analyses in humans and other mammals and therefore experimentally accessible invertebrate in vivo models, such as Caenorhabditis elegans and Drosophila melanogaster, have recently been introduced to investigate neurodegenerative syndromes. Several laboratories have used transgenic approaches in Drosophila to study the human genes associated with neurodegenerative diseases. This has added substantially to our understanding of the mechanisms leading to neurodegenerative diseases in humans. The isolation and characterization of Drosophila mutants, which display a variety of neurodegenerative phenotypes, also provide valuable insights into genes, pathways, and mechanisms causing neurodegeneration. So far only about two dozen such mutants have been described but already their characterization reveals an involvement of various cellular functions in neurodegeneration, ranging from preventing oxidative stress to RNA editing. Some of the isolated genes can already be associated with human neurodegenerative diseases and hopefully the isolation and characterization of more of these mutants, together with an analysis of homologous genes in vertebrate models, will provide insights into the genetic and molecular basis of human neurodegenerative diseases.

A role for synGAP in regulating neuronal apoptosis

The brain-specific Ras/Rap GTPase-activating protein synGAP is a major component of the postsynaptic density at glutamatergic synapses. It is a target for phosphorylation by Ca(2+)/calmodulin-dependent protein kinase II, which up-regulates its GTPase-activating activity. Thus, SynGAP may play an important role in coupling N-methyl-D-aspartate-type glutamate receptor activation to signaling pathways downstream of Ras or Rap. Homozygous deletion of synGAP is lethal within the first few days after birth. Therefore, to study the functions of synGAP, we used the cre/loxP recombination system to produce conditional mice mutants in which gradual loss of synGAP begins at approximately 1 week, and usually becomes maximal by 3 weeks, after birth. The resulting phenotypes fall into two groups. In a small group, the level of synGAP protein is reduced to 20-25% of wild type, and they die at 2-3 weeks of age. In a larger group, the levels remain higher than approximately 40% of wild type, and they survive and remain healthy. In all mutants, however, an abnormally high number of neurons in the hippocampus and cortex undergo apoptosis, as detected by caspase-3 activation. The effect is cell autonomous, occurring only in neuronal types in which the synGAP gene is eliminated. The level of caspase-3 activation in neurons correlates inversely with the level of synGAP protein measured at 2 and 8 weeks after birth, indicating that neuronal apoptosis is enhanced by reduction of synGAP. These data show that synGAP plays a role in regulation of the onset of apoptotic neuronal death.

GAP control: regulating the regulators of small GTPases

The small GTPases of the Ras superfamily mediate numerous biological processes through their ability to cycle between an inactive GDP-bound and an active GTP-bound form. Among the key regulators of GTPase cycling are the GTPase-activating proteins (GAPs), which stimulate the weak intrinsic GTP-hydrolysis activity of the GTPases, thereby inactivating them. Despite the abundance of GAPs and the fact that mutations in GAP-encoding genes underlie several human diseases, these proteins have received relatively little attention. Recent studies have addressed the regulatory mechanisms that influence GAP activity. So far, findings suggest that GAP activity is regulated by several mechanisms, including protein-protein interactions, phospholipid interactions, phosphorylation, subcellular translocation and proteolytic degradation.

Partial cleavage of RasGAP by caspases is required for cell survival in mild stress conditions

Tight control of apoptosis is required for proper development and maintenance of homeostasis in multicellular organisms. Cells can protect themselves from potentially lethal stimuli by expressing antiapoptotic factors, such as inhibitors of apoptosis, FLICE (caspase 8)-inhibitory proteins, and members of the Bcl2 family. Here, we describe a mechanism that allows cells to survive once executioner caspases have been activated. This mechanism relies on the partial cleavage of RasGAP by caspase 3 into an amino-terminal fragment called fragment N. Generation of this fragment leads to the activation of the antiapoptotic Akt kinase, preventing further amplification of caspase activity. Partial cleavage of RasGAP is required for cell survival under stress conditions because cells expressing an uncleavable RasGAP mutant cannot activate Akt, cannot prevent amplification of caspase 3 activity, and eventually undergo apoptosis. Executioner caspases therefore control the extent of their own activation by a feedback regulatory mechanism initiated by the partial cleavage of RasGAP that is crucial for cell survival under adverse conditions.

A vascular gene trap screen defines RasGRP3 as an angiogenesis-regulated gene required for the endothelial response to phorbol esters

We identified Ras guanine-releasing protein 3 (RasGRP3) as a guanine exchange factor expressed in blood vessels via an embryonic stem (ES) cell-based gene trap screen to identify novel vascular genes. RasGRP3 is expressed in embryonic blood vessels, down-regulated in mature adult vessels, and reexpressed in newly formed vessels during pregnancy and tumorigenesis. This expression pattern is consistent with an angiogenic function for RasGRP3. Although a loss-of-function mutation in RasGRP3 did not affect viability, RasGRP3 was up-regulated in response to vascular endothelial growth factor (VEGF) stimulation of human umbilical vein endothelial cells, placing RasGRP3 regulation downstream of VEGF signaling. Phorbol esters mimic the second messenger diacylglycerol (DAG) in activating both protein kinase C (PKC) and non-PKC phorbol ester receptors such as RasGRP3. ES cell-derived wild-type blood vessels exposed to phorbol myristate acetate (PMA) underwent extensive aberrant morphogenesis that resulted in the formation of large endothelial sheets rather than properly branched vessels. This response to PMA was completely dependent on the presence of RasGRP3, as mutant vessels were refractory to the treatment. Taken together, these findings show that endothelial RasGRP3 is up-regulated in response to VEGF stimulation and that RasGRP3 functions as an endothelial cell phorbol ester receptor in a pathway whose stimulation perturbs normal angiogenesis. This suggests that RasGRP3 activity may exacerbate vascular complications in diseases characterized by excess DAG, such as diabetes.

Ras: the other pro-aging pathway

Studies in worms, flies, and mice point to the insulin/insulin-like growth factor-1 (IGF-1)/phosphatidylinositol 3-kinase/Akt-like pathway as a central regulator of longevity. A similar pathway, which includes Sch9, a functional mammalian Akt/protein kinase B homolog, regulates longevity in yeast. Chronological aging in yeast is also regulated by a second pathway that includes Ras, adenylate cyclase, protein kinase A, the transcription factors Msn2 and Msn4, and Sod2. Although Ras proteins have not been implicated in longevity regulation in worms or flies, the major role of Ras in mammalian IGF-1 signaling raises the possibility that homologs of yeast Ras2 might accelerate aging in mammals. Here I review the data from experiments at both the organismal and cellular levels that support a role for Ras in the regulation of stress resistance and life span in eukaryotes.

Murine Models of Life Span Extension

Mice are excellent experimental models for genetic research and are being used to investigate the genetic component of organismal aging. Several mutant mice are known to possess defects in the growth hormone/insulin-like growth factor 1 (GH/IGF-1) neurohormonal pathway and exhibit dwarfism together with extended life span. Their phenotypes resemble those of mice subjected to caloric restriction. Targeted mutations that affect components of this pathway, including the GH receptor, p66Shc, and the IGF-1 receptor (IGF-1R), also extend life span; mutations that affect IGF-1R or downstream components of the pathway decouple longevity effects from dwarfism. These effects on life span may result from an increased capacity to resist oxidative damage.

A Titin mutation defines roles for circulation in endothelial morphogenesis

Morphogenesis of the developing vascular network requires coordinated regulation of an extensive array of endothelial cell behaviors. Precisely regulated signaling molecules such as vascular endothelial growth factor (VEGF) direct some of these endothelial behaviors. Newly forming blood vessels also become subjected to novel biomechanical forces upon initiation of cardiac contractions. We report here the identification of a recessive mouse mutation termed shrunken-head (shru) that disrupts function of the Titin gene. Titin was found to be required for the initiation of proper heart contractions as well as for maintaining the correct overall shape and orientation of individual cardiomyocytes. Cardiac dysfunction in shrunken-head mutant embryos provided an opportunity to study the effects of lack of blood circulation on the morphogenesis of endothelial cells. Without blood flow, differentiating endothelial cells display defects in their shapes and patterns of cell-cell contact. These endothelial cells, without exposure to blood circulation, have an abnormal distribution within vasculogenic vessels. Further effects of absent blood flow include abnormal spatial regulation of angiogenesis and elevated VEGF signaling. The shrunken-head mutation has provided an in vivo model to precisely define the roles of circulation on cellular and network aspects of vascular morphogenesis.

Tyrosine phosphorylated Disabled 1 recruits Crk family adapter proteins

Disabled 1 (Dab1) functions as a critical adapter protein in the Reelin signaling pathway to direct proper positioning of neurons during brain development. Reelin stimulates phosphorylation of Dab1 on tyrosines 198 and 220, and phosphorylated Dab1 is likely to interact with downstream signaling proteins that contain Src homology 2 (SH2) domains. To search for such proteins, we used a Sepharose-conjugated peptide containing phosphotyrosine 220 (PTyr-220) of Dab1, as an affinity matrix to capture binding proteins from mouse brain extracts. Mass spectrometric analysis of bound proteins revealed that Crk family adapter proteins selectively associated with this phosphorylation site. We further show that Crk-I and Crk-II, but not CrkL, stimulate phosphorylation of Dab1 on tyrosine 220 in a Src-dependent manner. Our results suggest that Crk family adapter proteins may play an important role in the Reelin signaling pathway during brain development.

Altered Arterial Homeostasis and Cerebral Aneurysms: A Review of the Literature and Justification for a Search of Molecular Biomarkers

DESPITE THE CATASTROPHIC consequence of ruptured intracranial aneurysms, very little is understood regarding their pathogenesis, and there are no reliable predictive markers for identifying at-risk individuals. Given that intracranial aneurysms have a strong but complex genetic component and well-characterized modifiable risk factors, it seems likely that the most valuable approach to developing minimally invasive diagnostic and prognostic tools will involve a multifactorial model that includes both genetic and environmental risk factors. Unfortunately, the genetic basis of intracranial aneurysms is poorly described, and reports describing the association of nonrandom deoxyribonucleic acid sequence variation with intracranial aneurysms have been limited to a handful of ad hoc studies that have focused on a variety of markers in small populations. One reason for this lack of coordinated analysis of the genetic basis of intracranial aneurysms is that the molecular pathogenesis and pathobiological characteristics of the disease are poorly described, so candidate marker selection has been problematic. Few studies have addressed the molecular pathological basis of intracranial aneurysms or the possible mechanisms of intracranial aneurysm formation. In this regard, candidate gene selection strategies have relied almost exclusively on limited knowledge of monogenic disorders such as Ehlers-Danlos syndrome and Marfan's syndrome, in which intracranial aneurysm is a feature of a spectrum of syndromic phenotypes. Without exception, these approaches have not affected the clinical identification and/or management of intracranial aneurysms significantly. Therefore, it is imperative that coordinated large-scale efforts in genetics, molecular biology, and genetic epidemiology are implemented to overcome these obstacles and drive developments in the field. In this review, we summarize the current screening modalities for intracranial aneurysms, review the current state of understanding relating to the genetic basis of intracranial aneurysms, and suggest a broader theory of aneurysm pathogenesis to form the foundation of a coordinated molecular search for biological markers that may be associated with aneurysm formation and rupture.

Ras GTPase-activating Protein Binds to Akt and Is Required for Its Activation

RasGAP (Ras GTPase-activating protein) is a negative regulator as well as a downstream effector of Ras. To identify partners of RasGAP we used it as the bait in a yeast two-hybrid screen. This resulted in discovering its interaction with Akt. Overexpression of RasGAP or a mutant lacking the GTPase-activating domain (nGAP) enhanced phosphorylation and activity of Akt, which was dependent on the upstream integrin-linked kinase. Also, nGAP protected the cells against staurosporin-induced apoptosis through an Akt-dependent pathway. To determine the role of RasGAP in receptor-mediated activation of Akt, we used short hairpin RNA interference to knock out endogenous RasGAP expression. Although this procedure resulted in enhanced Ras activity, it inhibited Akt phosphorylation. Thus, we propose that Ras-GAP interacts with Akt and is necessary for its activation, possibly via integrin-linked kinase-mediated phosphorylation of Ser-473. The data suggest that this effect is independent of Ras activity.

Activated Fps/Fes partially rescues the in vivo developmental potential of Flk1-deficient vascular progenitor cells

Relatively little is known about the modulators of the vascular endothelial growth factor A (VEGF-A)/Flk1 signaling cascade. To functionally characterize this pathway, VEGF-A stimulation of endothelial cells was performed. VEGF-A–mediated Flk1 activation resulted in increased translocation of the endogenous Fps/Fes cytoplasmic tyrosine kinase to the plasma membrane and increased tyrosine phosphorylation, suggesting a role for Fps/Fes in VEGF-A/Flk1 signaling events. Addition of a myristoylation consensus sequence to Fps/Fes resulted in VEGF-A–independent membrane localization of Fps/Fes in endothelial cells. Expression of the activated Fps/Fes protein in Flk1-deficient embryonic stem (ES) cells rescued their contribution to the developing vascular endothelium in vivo by using ES cell–derived chimeras. Activated Fps/Fes contributed to this rescue event by restoring the migratory potential to Flk1 null progenitors, which is required for movement of hemangioblasts from the primitive streak region into the yolk sac proper. Activated Fps/Fes in the presence of Flk1 increased the number of hemangioblast colonies in vitro and increased the number of mesodermal progenitors in vivo. These results suggest that Fps/Fes may act synergistically with Flk1 to modulate hemangioblast differentiation into the endothelium. We have also demonstrated that activated Fps/Fes causes hemangioma formation in vivo, independently of Flk1, as a result of increasing vascular progenitor density.