Rac-MEKK3-MKK3 scaffolding for p38 MAPK activation during hyperosmotic shock

Pathogenesis of vascular anomalies

Vascular anomalies are localized defects of vascular development. Most of them occur sporadically (ie, there is no familial history of lesions, yet in a few cases clear inheritance is observed). These inherited forms are often characterized by multifocal lesions that are mainly small in size and increase in number with patients' age. The authors review the known (genetic) causes of vascular anomalies and call attention to the concept of Knudson's double-hit mechanism to explain incomplete penetrance and large clinical variation in expressivity observed in inherited vascular anomalies. The authors also discuss the identified pathophysiological pathways involved in vascular anomalies and how it has opened the doors toward a more refined classification of vascular anomalies and the development of animal models that can be tested for specific molecular therapies.

Hyperosmotic stress-induced ATF-2 activation through Polo-like kinase 3 in human corneal epithelial cells

Elevated extracellular solute concentration (hyperosmotic stress) perturbs cell function and stimulates cell responses by evoking MAPK cascades and activating AP-1 transcription complex resulting in alterations of gene expression, cell cycle arrest, and apoptosis. The results presented here demonstrate that hyperosmotic stress elicited increases in ATF-2 phosphorylation through a novel Polo-like kinase 3 (Plk3) pathway in human corneal epithelial (HCE) cells. We found in hyperosmotic stress-induced HCE cells that Plk3 transferred to the nuclear compartment and was colocalized with ATF-2 in nuclei. Kinase activity of Plk3 was significantly activated by hyperosmotic stimulation. Further downstream, active Plk3 phosphorylated ATF-2 at the Thr-71 site in vivo and in vitro. Overexpression of Plk3 and its mutants enhanced hyperosmotic stress-induced ATF-2 phosphorylation. In contrast, suppression of Plk3 by knocking down Plk3 mRNA effectively diminished the effect of hyperosmotic stress-induced ATF-2 phosphorylation. The effect of hyperosmotic stress-induced activation of Plk3 on ATF-2 transcription factor function was also examined in CRE reporter-overexpressed HCE cells. Our results for the first time reveal that hyperosmotic stress can activate the Plk3 signaling pathway that subsequently regulates the AP-1 complex by directly phosphorylating ATF-2 independent from the effects of JNK and p38 activation.

The P38alpha and P38delta MAP kinases may be gene therapy targets in the future treatment of severe burns

Microvascular barrier damage, induced by thermal injury, imposes life-threatening problems owing to the pathophysiological consequences of plasma loss and impaired perfusion that finally may lead to multiple organ failure. The aim of the present study was to define the signaling role of selected mitogen-activated protein kinases (MAPKs) in general vessel hyperpermeability caused by burns and to look for a potential gene therapy. Rearrangement of cytoskeletons and cell tight junctions were evaluated by phalloidin labeling of actin and immunocytochemical demonstration of the ZO-1 protein, whereas blood vessel permeability was evaluated by a fluorescence ratio technique. The p38 MAPK inhibitor SB203580 largely blocked burn serum-induced stress-fiber formation and tight-junction damage. Using the adenoviral approach to transfect dominant negative forms of p38 MAPKs, we found that p38alpha and p38delta had similar effects. The in vivo part of the study showed that transfection of these two constructs significantly lowered general venular hyperpermeability and enhanced the survival of burned animals. Because the p38 MAPK pathway seems to play a crucial role in burn-induced vascular hyperpermeability, general transfection with p38 MAP dominant negative constructs might become a new therapeutic method to block burn-induced plasma leakage.

PDCD10/CCM3 acts downstream of {gamma}-protocadherins to regulate neuronal survival

γ-Protocadherins (PCDH-γ) regulate neuronal survival in the vertebrate central nervous system. The molecular mechanisms of how PCDH-γ mediates this function are still not understood. In this study, we show that through their common cytoplasmic domain, different PCDH-γ isoforms interact with an intracellular adaptor protein named PDCD10 (programmed cell death 10). PDCD10 is also known as CCM3, a causative genetic defect for cerebral cavernous malformations in humans. Using RNAi-mediated knockdown, we demonstrate that PDCD10 is required for the occurrence of apoptosis upon PCDH-γ depletion in developing chicken spinal neurons. Moreover, overexpression of PDCD10 is sufficient to induce neuronal apoptosis. Taken together, our data reveal a novel function for PDCD10/CCM3, acting as a critical regulator of neuronal survival during development.

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.

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.

New insights into the biogenesis of nuclear RNA polymerases?

More than 30 years of research on nuclear RNA polymerases (RNAP I, II, and III) has uncovered numerous factors that regulate the activity of these enzymes during the transcription reaction. However, very little is known about the machinery that regulates the fate of RNAPs before or after transcription. In particular, the mechanisms of biogenesis of the 3 nuclear RNAPs, which comprise both common and specific subunits, remains mostly uncharacterized and the proteins involved are yet to be discovered. Using protein affinity purification coupled to mass spectrometry (AP-MS), we recently unraveled a high-density interaction network formed by nuclear RNAP subunits from the soluble fraction of human cell extracts. Validation of the dataset using a machine learning approach trained to minimize the rate of false positives and false negatives yielded a high-confidence dataset and uncovered novel interactors that regulate the RNAP II transcription machinery, including a set of proteins we named the RNAP II-associated proteins (RPAPs). One of the RPAPs, RPAP3, is part of an 11-subunit complex we termed the RPAP3/R2TP/prefoldin-like complex. Here, we review the literature on the subunits of this complex, which points to a role in nuclear RNAP biogenesis.

Tonicity-responsive microRNAs contribute to the maximal induction of osmoregulatory transcription factor OREBP in response to high-NaCl hypertonicity

Osmotic response element binding protein (OREBP) is a Rel-like transcription factor critical for cellular osmoresponses. Previous studies suggest that hypertonicity-induced accumulation of OREBP protein might be mediated by transcription activation as well as posttranscriptional mRNA stabilization or increased translation. However, the underlying mechanisms remain incompletely elucidated. Here, we report that microRNAs (miRNAs) play critical regulatory roles in hypertonicity-induced induction of OREBP. In renal medullary epithelial mIMCD3 cells, hypertonicity greatly stimulates the activity of the 3'-untranslated region of OREBP (OREBP-3'UTR). Furthermore, overexpression of OREBP-3'UTR or depletion of miRNAs by knocking-down Dicer greatly increases OREBP protein expression. On the other hand, significant alterations in miRNA expression occur rapidly in response to high NaCl exposure, with miR-200b and miR-717 being most significantly down-regulated. Moreover, increased miR-200b or miR-717 causes significant down-regulation of mRNA, protein and transcription activity of OREBP, whereas inhibition of miRNAs or disruption of the miRNA-3'UTR interactions abrogates the silencing effects. In vivo in mouse renal medulla, miR-200b and miR-717 are found to function to tune OREBP in response to renal tonicity alterations. Together, our results support the notion that miRNAs contribute to the maximal induction of OREBP to participate in cellular responses to osmotic stress in mammalian renal cells.

The p38 SAPK is recruited to chromatin via its interaction with transcription factors

In mammals, the stress-activated protein kinase (SAPK) p38 coordinates a rapid and complex transcriptional program to adapt to sudden changes in the extracellular environment. Although a number of genes have been reported to be under the control of p38, the basic mechanisms of transcriptional regulation by this SAPK remain uncharacterized. Here we show that in response to osmotic shock, anisomycin- or TNFα-activated p38 SAPK is recruited to stress-induced genes. The MAPKK MKK6 is also found at stress-responsive promoters. The recruitment of RNA polymerase II complex to the target promoters requires p38 activity. Moreover, when tethered to DNA as a LexA fusion protein, p38 activates transcription in a stress-regulated manner. Thus, p38 activity allows for recruitment of RNA polymerase and transcription initiation. p38 directly phosphorylates and interacts with the transcription factor Elk1. p38 activity is necessary for the recruitment of Elk1 to the c-Fos promoter, and knocking down Elk1 by siRNAs compromises both p38 recruitment to the c-Fos promoter and c-Fos transcriptional up-regulation upon osmostress. In addition, p38 recruitment to the osmoinducible gene Cox2 and the TNFα target gene IL8 is mediated by the transcription factors AP1 and NFκB, respectively. Therefore, anchoring of active SAPK to target genes is mediated by transcription factors. The presence of active p38 at open reading frames also suggests the involvement of the SAPK in elongation. Taken together, SAPK recruitment to target genes appears to be a broad mechanism to regulate transcription that has been preserved from yeast to mammals.

Mechanisms and functions of p38 MAPK signalling

The p38 MAPK (mitogen-activated protein kinase) signalling pathway allows cells to interpret a wide range of external signals and respond appropriately by generating a plethora of different biological effects. The diversity and specificity in cellular outcomes is achieved with an apparently simple linear architecture of the pathway, consisting of a core of three protein kinases acting sequentially. In the present review, we dissect the molecular mechanisms underlying p38 MAPK functions, with special emphasis on the activation and regulation of the core kinases, the interplay with other signalling pathways and the nature of p38 MAPK substrates as a source of functional diversity. Finally, we discuss how genetic mouse models are facilitating the identification of physiological functions for p38 MAPKs, which may impinge on their eventual use as therapeutic targets.

TGFβ2-mediated production of hyaluronan is important for the induction of epicardial cell differentiation and invasion

In the developing heart, the epicardium is a major source of progenitor cells that contribute to the formation of the coronary vessel system. These epicardial progenitors give rise to the different cellular components of the coronary vasculature by undergoing a number of morphological and physiological changes collectively known as epithelial to mesenchymal transformation (EMT). However, the specific signaling mechanisms that regulate epicardial EMT are yet to be delineated. In this study we investigated the role of TGFβ2 and hyaluronan (HA) during epicardial EMT and how signals from these two molecules are integrated during this important process. Here we show that TGFβ2 induces MEKK3 activation, which in turn promotes ERK1/2 and ERK5 phosphorylation. TGFβ2 also increases Has2 expression and subsequent HA production. Nevertheless, inhibition of MEKK3 kinase activity, silencing of ERK5 or pharmacological disruption of ERK1/2 activation significantly abrogates this response. Thus, TGFβ2 promotes Has2 expression and HA production through a MEKK3/ERK1/2/5-dependent cascade. Furthermore, TGFβ2 is able to induce epicardial cell invasion and differentiation but not proliferation. However, inhibition of MEKK3-dependent pathways, degradation of HA by hyaluronidases or blockade of CD44, significantly impairs the biological response to TGFβ2. Taken together, these findings demonstrate that TGFβ2 activation of MEKK3/ERK1/2/5 signaling modulates Has2 expression and HA production leading to the induction of EMT events. This is an important and novel mechanism showing how TGFβ2 and HA signals are integrated to regulate changes in epicardial cell behavior.

The Guanine Nucleotide Exchange Factor Brx: A Link between Osmotic Stress, Inflammation and Organ Physiology and Pathophysiology

Dehydration, and consequent intracellular hyperosmolarity, is a major challenge to land organisms, as it is associated with extraction of water from cells and disturbance of global cellular function. Organisms have thus developed a highly conserved regulatory mechanism that transduces the hyperosmolarity signal from the cell surface to the cell nucleus and adjusts the expression of cellular osmolarity-regulating genes. We recently found that the Rho-type guanine nucleotide exchange factor Brx, or AKAP13, is essential for osmotic stress-stimulated expression of nuclear factor of activated T-cells 5 (NFAT5), a key transcription factor of intracellular osmolarity. It accomplishes this by first attracting cJun kinase (JNK)-interacting protein (JIP) 4 and then coupling activated Rho-type small G-proteins to cascade components of the p38 MAPK signaling pathway, ultimately activating NFAT5. We describe the potential implications of osmotic stress and Brx activation in organ physiology and pathophysiology and connect activation of this system to key human homeostatic states.

CHIP-dependent termination of MEKK2 regulates temporal ERK activation required for proper hyperosmotic response

The extracellular signal-regulated kinase (ERK) pathway is an important signalling pathway that regulates a large number of cellular processes, including proliferation, differentiation and gene expression. Hyperosmotic stress activates the ERK pathway, whereas little is known about the regulatory mechanisms and physiological functions of ERK activation in hyperosmotic response. Here, we show that MAPK/ERK kinase kinase 2 (MEKK2), a member of the MAPKKK family, mediated the specific and transient activation of ERK, which was required for the induction of aquaporin 1 (AQP1) and AQP5 gene expression in response to hyperosmotic stress. Moreover, we identified the E3 ubiquitin ligase carboxyl terminus of Hsc70-interacting protein (CHIP) as a binding partner of MEKK2. Depletion of CHIP by small-interference RNA or gene targeting attenuated the degradation of MEKK2 and prolonged the ERK activity. Interestingly, hyperosmolality-induced gene expression of AQP1 and AQP5 was suppressed by CHIP depletion and was reversed by inhibition of the prolonged phase of ERK activity. These findings show that transient activation of the ERK pathway, which depends not only on MEKK2 activation, but also on CHIP-dependent MEKK2 degradation, is crucial for proper gene expression in hyperosmotic stress response.

Ccm1 assures microvascular integrity during angiogenesis

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

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.

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.

Safeguarding entry into mitosis: the antephase checkpoint

Maintenance of genomic stability is needed for cells to survive many rounds of division throughout their lifetime. Key to the proper inheritance of intact genome is the tight temporal and spatial coordination of cell cycle events. Moreover, checkpoints are present that function to monitor the proper execution of cell cycle processes. For instance, the DNA damage and spindle assembly checkpoints ensure genomic integrity by delaying cell cycle progression in the presence of DNA or spindle damage, respectively. A checkpoint that has recently been gaining attention is the antephase checkpoint that acts to prevent cells from entering mitosis in response to a range of stress agents. We review here what is known about the pathway that monitors the status of the cells at the brink of entry into mitosis when cells are exposed to insults that threaten the proper inheritance of chromosomes. We highlight issues which are unresolved in terms of our understanding of the antephase checkpoint and provide some perspectives on what lies ahead in the understanding of how the checkpoint functions.

Differential activation of p38MAPK isoforms by MKK6 and MKK3

All four members of the mammalian p38 mitogen-activated protein kinase (MAPK) family (p38alpha, p38beta, p38gamma and p38delta) are activated by dual phosphorylation in the TGY motif in the activation loop. This phosphorylation is mediated by three kinases, MKK3, MKK6 and MKK4, at least in vitro. The role of these MKK in the activation of p38alpha has been demonstrated in studies using fibroblasts that lack MKK3 and/or MKK6. Nonetheless, the physiological upstream activators of the other p38MAPK isoforms have not yet been reported using MKK knockout cells. In this study, we examined p38beta, gamma and delta activation by MKK3 and MKK6, in cells lacking MKK3, MKK6 or both. We show that MKK3 and MKK6 are both essential for the activation of p38gamma and p38beta induced by environmental stress, whereas MKK6 is the major p38gamma activator in response to TNFalpha. In contrast, p38delta activation by ultraviolet radiation, hyperosmotic shock, anisomycin or by TNFalpha is mediated by MKK3. Moreover, in response to osmotic stress, MKK3 and MKK6 are crucial in regulating the phosphorylation of the p38gamma substrate hDlg and its activity as scaffold protein. These data indicate that activation of distinct p38MAPK isoforms is regulated by the selective and synchronized action of two kinases, MKK3 and MKK6, in response to cell stress.

Rac1 mediates NaCl-induced superoxide generation in the thick ascending limb

Superoxide (O(2)(-)) produced by NADPH oxidase regulates Na absorption and renal hemodynamics. Increased NaCl in the thick ascending limb (TAL) stimulates O(2)(-) generation. However, we do not know whether physiological changes in NaCl concentration augment O(2)(-) generation, nor do we know the mediator(s) involved. In other cells, Rac1, a regulatory subunit of NADPH oxidase, is activated by elevated NaCl. We hypothesized that increasing luminal NaCl within the physiological range activates Rac1 and NADPH oxidase and, thereby, increases O(2)(-) production. We increased NaCl from 10 to 57 mM in medullary TAL suspensions and used lucigenin to measure O(2)(-) generation and Western blot to measure Rac1 activity. Increasing NaCl stimulated O(2)(-) generation from 1.41 +/- 0.16 to 2.71 +/- 0.30 nmol O(2)(-) x min(-1) x mg protein(-1) (n = 6, P < 0.05). This increase was blocked by the Na-K-2Cl cotransporter inhibitor furosemide and the NADPH oxidase inhibitor apocynin. To examine the role of Rac1 in NaCl-induced O(2)(-) production, we measured Rac1 translocation by Western blot. When we added NaCl, Rac1 in the particulate fraction increased from 6.8 +/- 0.8 to 11.7 +/- 2.4% of total Rac1 (n = 7, P < 0.05). Then we measured O(2)(-) generation in the presence and absence of the Rac1 inhibitor. In the absence of the Rac1 inhibitor, NaCl increased O(2)(-) generation from 1.07 +/- 0.24 to 2.02 +/- 0.49 nmol O(2)(-) x min(-1) x mg protein(-1), and this increase was completely blocked by the inhibitor. Similarly, in vivo treatment of TALs with adenovirus expressing dominant-negative Rac1 decreased NaCl-induced O(2)(-) generation by 60% compared with control (0.33 +/- 0.04 vs. 0.81 +/- 0.17 nmol O(2)(-) x min(-1) x mg protein(-1), n = 6, P < 0.05). We concluded that physiological increases in NaCl stimulate TAL O(2)(-) generation by activating Rac1.

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.

Stimulus-specific distinctions in spatial and temporal dynamics of stress-activated protein kinase kinase kinases revealed by a fluorescence resonance energy transfer biosensor

The stress-activated protein kinases (SAPKs), namely, p38 and JNK, are members of the mitogen-activated protein kinase family and are important determinants of cell fate when cells are exposed to environmental stresses such as UV and osmostress. SAPKs are activated by SAPK kinases (SAP2Ks), which are in turn activated by various SAP2K kinases (SAP3Ks). Because conventional methods, such as immunoblotting using phospho-specific antibodies, measure the average activity of SAP3Ks in a cell population, the intracellular dynamics of SAP3K activity are largely unknown. Here, we developed a reporter of SAP3K activity toward the MKK6 SAP2K, based on fluorescence resonance energy transfer, that can uncover the dynamic behavior of SAP3K activation in cells. Using this reporter, we demonstrated that SAP3K activation occurs either synchronously or asynchronously among a cell population and in different cellular compartments in single cells, depending on the type of stress applied. In particular, SAP3Ks are activated by epidermal growth factor and osmostress on the plasma membrane, by anisomycin and UV in the cytoplasm, and by etoposide in the nucleus. These observations revealed previously unknown heterogeneity in SAPK responses and supplied answers to the question of the cellular location in which various stresses induce stimulus-specific SAPK responses.

PAKing up to the endothelium

Angiogenesis recapitulates the growth of blood vessels that progressively expand and remodel into a highly organized and stereotyped vascular network. During adulthood, endothelial cells that formed the vascular wall retain their plasticity and can be engaged in neo-vascularization in response to physiological stimuli, such as hypoxia, wound healing and tissue repair, ovarian cycle and pregnancy. In addition, numerous human diseases and pathological conditions are characterized by an excessive, uncontrolled and aberrant angiogenesis. The signalling pathways involving the small Rho GTPase, Rac and its downstream effector the p21-activated serine/threonine kinase (PAK) had recently emerged as pleiotropic modulators in these processes. Indeed, Rac and PAK were found to modulate endothelial cell biology, such as sprouting, migration, polarity, proliferation, lumen formation, and maturation. Elucidating the Rac/PAK molecular circuitry will provide essential information for the development of new therapeutic agents designed to normalize the blood vasculature in human diseases.

Apoptosis commitment and activation of mitochondrial Bax during anoikis is regulated by p38MAPK

Most cells undergo apoptosis through the intrinsic pathway. This is dependent on mitochondrial outer membrane permeabilisation (MOMP), which is mediated by the pro-apoptotic Bcl-2 family proteins, Bax and Bak. During apoptosis, Bax translocates from the cytosol to the outer mitochondrial membrane (OMM), wherein it contributes to the formation of pores to release cytochrome-c. However, it remains unclear whether Bax translocation is sufficient to bring about MOMP or whether Bax requires further signals on the OMM to be fully activated. We have previously shown that during mammary epithelial cell anoikis, Bax translocation does not commit cells to MOMP and detached cells are rescued if survival signals from the extracellular matrix (ECM) are restored. These findings implied that a second signal is required for mitochondrial Bax to fully activate and cause MOMP. We now identify p38MAPK (mitogen-activated protein kinase) as this necessary signal to activate Bax after its translocation to mitochondria. The inhibition of p38MAPK did not prevent Bax translocation, but its activity was required for mitochondrial Bax to bring about MOMP. p38MAPK was activated and recruited to a high molecular weight mitochondrial complex after loss of ECM attachment. Artificially targeting p38MAPK to the OMM increased the kinetics of anoikis, supporting a requirement for its mitochondrial localisation to regulate Bax activation and drive commitment to apoptosis.

Hyperosmolarity regulates SOX9 mRNA posttranscriptionally in human articular chondrocytes

The transcription factor SOX9 regulates cartilage extracellular matrix gene expression and is essential for chondrocyte differentiation. We previously showed that activation of p38 MAPK by cycloheximide in human chondrocytes leads to stabilization of SOX9 mRNA (Tew SR and Hardingham TE. J Biol Chem 281: 39471–39479, 2006). In this study we investigated whether regulation of p38 MAPK caused by changes in osmotic pressure could control SOX9 mRNA levels expression by a similar mechanism. Primary human articular chondrocytes isolated from osteoarthritic cartilage at passage 2-4 showed significantly raised SOX9 mRNA levels when exposed to hyperosmotic conditions for 5 h. The effect was strongest and most reproducible when actin stress fibers were disrupted by the Rho effector kinase inhibitor Y27632, or by culturing the cells within alginate beads. Freshly isolated chondrocytes, used within 24–48 h of isolation, did not contain actin stress fibers and upregulated SOX9 mRNA in response to hyperosmolarity in the presence and absence of Y27632. In these freshly isolated chondrocytes, hyperosmolarity led to an increase in the half-life of SOX9 mRNA, which was sensitive to the p38 MAPK inhibitor SB202190. SOX9 protein levels were increased by hyperosmotic culture over 24 h, and, in passaged chondrocytes, the activity of a COL2A1 enhancer driven luciferase assay was upregulated. However, in freshly isolated chondrocytes, COL2A1 mRNA levels were reduced by hyperosmotic conditions and the half-life was decreased. The results showed that the osmotic environment regulated both SOX9 and COL2A1 mRNA posttranscriptionally, but in fresh cells resulted in increased SOX9, but decreased COL2A1.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

Protein kinase Cmu plays an essential role in hypertonicity-induced heat shock protein 70 expression

Heat shock protein 70 (HSP70), which evidences important functions as a molecular chaperone and anti-apoptotic molecule, is substantially induced in cells exposed to a variety of stresses, including hypertonic stress, heavy metals, heat shock, and oxidative stress, and prevents cellular damage under these conditions. However, the molecular mechanism underlying the induction of HSP70 in response to hypertonicity has been characterized to a far lesser extent. In this study, we have investigated the cellular signaling pathway of HSP70 induction under hypertonic conditions. Initially, we applied a variety of kinase inhibitors to NIH3T3 cells that had been exposed to hypertonicity. The induction of HSP70 was suppressed specifically by treatment with protein kinase C (PKC) inhibitors (Gö6976 and GF109203X). As hypertonicity dramatically increased the phosphorylation of PKCmu, we then evaluated the role of PKCmu in hypertonicity-induced HSP70 expression and cell viability. The depletion of PKCmu with siRNA or the inhibition of PKCmu activity with inhibitors resulted in a reduction in HSP70 induction and cell viability. Tonicity-responsive enhancer binding protein (TonEBP), a transcription factor for hypertonicity-induced HSP70 expression, was translocated rapidly into the nucleus and was modified gradually in the nucleus under hypertonic conditions. When we administered treatment with PKC inhibitors, the mobility shift of TonEBP was affected in the nucleus. However, PKCmu evidenced no subcellular co-localization with TonEBP during hypertonic exposure. From our results, we have concluded that PKCmu performs a critical function in hypertonicity-induced HSP70 induction, and finally cellular protection, via the indirect regulation of TonEBP modification.

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.

Brx shines a light on the route from hyperosmolarity to NFAT5

Nuclear factor of activated T cells 5 (NFAT5) is a member of the Rel family of transcription factors and is an essential inducer of osmoprotective gene products in mammalian cells. Its activation by hypertonicity requires p38 mitogen-activated protein kinase (MAPK) signaling and other pathways. A study now elucidates a signaling cascade regulated by the guanine nucleotide exchange factor Brx that leads to the activation of p38alpha MAPK and the induction of nfat5 messenger RNA in response to osmotic stress in lymphocytes and renal medullary cells. Brx-deficient lymphocytes showed impaired responses to hypertonicity, and brx(+/-) mice exhibited immune defects similar to those of nfat5-deficient mice. These findings support a major role for Brx in regulating the osmoprotective function of NFAT5 in different cell types.

Cerebral cavernous malformation 2 protein promotes smad ubiquitin regulatory factor 1-mediated RhoA degradation in endothelial cells

Mutation of CCM2 predisposes individuals to cerebral cavernous malformations, vascular abnormalities that cause seizures and hemorrhagic stroke. CCM2 has been proposed to regulate the activity of RhoA for maintenance of vascular integrity. Herein, we define a novel mechanism where the CCM2 phosphotyrosine binding (PTB) domain binds the ubiquitin ligase (E3) Smurf1, controlling RhoA degradation. Brain endothelial cells with knockdown of CCM2 have increased RhoA protein and display impaired directed cell migration. CCM2 binding of Smurf1 increases Smurf1-mediated degradation of RhoA. CCM2 does not significantly alter the catalytic activity of Smurf1, nor is CCM2 a Smurf1 substrate. Rather the CCM2-Smurf1 interaction functions to localize Smurf1 for RhoA degradation. These findings provide a molecular mechanism for the pathogenesis of cerebral cavernous malformations (CCM) resulting from loss of CCM2-mediated localization of Smurf1, which controls RhoA degradation required for maintenance of normal endothelial cell physiology.

The control of vascular integrity by endothelial cell junctions: molecular basis and pathological implications

Human pathologies such as vascular malformations, hemorrhagic stroke, and edema have been associated with defects in the organization of endothelial cell junctions. Understanding the molecular basis of these diseases requires different integrated approaches which include basic cell biology, clinical studies, and studies in animal models such as mice and zebrafish. In this review we discuss recent findings derived from these approaches and their possible integration in a common picture.

Trophoblast stem cell maintenance by fibroblast growth factor 4 requires MEKK4 activation of Jun N-terminal kinase

Trophoblast differentiation during placentation involves an epithelial-mesenchymal transition (EMT) with loss of E-cadherin and gain of trophoblast invasiveness. Mice harboring a point mutation that renders inactive the mitogen-activated protein kinase kinase kinase MEKK4 exhibit dysregulated placental development with increased trophoblast invasion. Isolated MEKK4 kinase-inactive trophoblast stem (TS) cells cultured under undifferentiating, self-renewing conditions in the presence of fibroblast growth factor 4 (FGF4) display increased expression of Slug, Twist, and matrix metalloproteinase 2 (MMP2), loss of E-cadherin, and hyperinvasion of extracellular matrix, each a hallmark of EMT. MEKK4 kinase-inactive TS cells show a preferential differentiation to Tpbp alpha- and Gcm1-positive trophoblasts, which are indicative of spongiotrophoblast and syncytiotrophoblast differentiation, respectively. FGF4-stimulated Jun N-terminal kinase (JNK) and p38 activity is markedly reduced in MEKK4 kinase-inactive TS cells. Chemical inhibition of JNK in wild-type TS cells induced a similar EMT response as loss of MEKK4 kinase activity, including inhibition of E-cadherin expression and increased expression of Slug, MMP2, Tpbp alpha, and Gcm1. Chromatin immunoprecipitation analyses revealed changes in AP-1 composition with increased Fra-2 and decreased Fra-1 and JunB binding to the regulatory regions of Gcm1 and MMP2 genes in MEKK4 kinase-inactive TS cells. Our results define MEKK4 as a signaling hub for FGF4 activation of JNK that is required for maintenance of TS cells in an undifferentiated state.

Apoptotic functions of PDCD10/CCM3, the gene mutated in cerebral cavernous malformation 3

BACKGROUND AND PURPOSE

Mutations in the Programmed Cell Death 10 (PDCD10) gene cause autosomal dominant familial cerebral cavernous malformations (CCM3). To date, little is known about the function of this gene and its role in disease pathogenesis.

METHODS

We examined the effects of overexpression of wild-type and 2 human disease-causing variants of PDCD10 on cell death using 3 different methods (TUNEL and MTT assays and caspase-3 activation). We analyzed expression of CCM3, activated caspase-3, and p38 in endothelial cell lines using the serum deprivation model of apoptosis induction. Finally, we assayed the effects of siRNA-mediated inhibition of endogenous PDCD10 expression on cell death in endothelial cell cultures.

RESULTS

Overexpression of wild-type CCM3, but not disease-linked mutant forms, induced apoptosis as confirmed by TUNEL and increased levels of activated caspase-3. Serum starvation of endothelial cells, an inducer of apoptosis, led to increased expression of CCM3 and activation of p38 and ultimately activated caspase-3. siRNA-mediated inhibition of CCM3 expression resulted in decreased levels of p38 and activated caspase-3, and decreased cell death.

CONCLUSIONS

CCM3 is both necessary and sufficient to induce apoptosis in vitro in well-defined cell culture systems. Even though it is currently unclear whether this effect on apoptosis is direct or indirect through modulation of cell cycle, these results led to the novel hypothesis that CCM lesions may form as a consequence of aberrant apoptosis, potentially altering the balance between the endothelium and neural cells within the neurovascular unit.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

MAP3Ks as central regulators of cell fate during development

The cytoplasmic serine/threonine kinases transduce extracellular signals into regulatory events that impact cellular responses. The induction of one kinase triggers the activation of several downstream kinases, leading to the regulation of transcription factors to affect gene function. This arrangement allows for the kinase cascade to be amplified, and integrated according to the cellular context. An upstream mitogen or growth factor signal initiates a module of three kinases: a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK; e.g., Raf) that phosphorylates and activates a MAP kinase kinase (MAPKK; e.g., MEK) and finally activation of MAP kinase (MAPK; e.g., ERK). Thus, this MAP3K-MAP2K-MAPK module represents critical effectors that regulate extracellular stimuli into cellular responses, such as differentiation, proliferation, and apoptosis all of which function during development. There are 21 characterized MAP3Ks that activate known MAP2Ks, and they function in many aspects of developmental biology. This review summarizes known transduction routes linked to each MAP3K and highlights mouse models that provide clues to their physiological functions. This perspective reveals that some of these MAP3K effectors may have redundant functions, and also serve as unique nexus depending on the context of the signaling pathway.

EGF receptor signaling is involved in expression of osmoprotective TonEBP target gene aldose reductase under hypertonic conditions

Renal medullary cells adapt to their hyperosmotic environment by enhanced expression of various osmoprotective genes. Although it is clearly established that TonEBP contributes to the expression of these genes, neither the precise signaling mechanism by which hypertonicity activates TonEBP is completely understood, nor is it known whether a membrane-bound osmosenser, corresponding to yeast and bacteria, is present in mammalian cells. We found evidence that metalloproteinase (MMP)-dependent activation of the epidermal growth factor receptor (EGFR) signals to TonEBP and stimulates the expression of the TonEBP target gene aldose reductase (AR) under hypertonic conditions. Phosphorylation of EGFR and the downstream MAP kinases ERK1/2 and p38 was significantly enhanced by high NaCl in Madin-Darby canine kidney (MDCK) cells. Conversely, the broad-spectrum MMP inhibitor GM6001 or the EGFR inhibitor AG1478 diminished phosphorylation of EGFR, p38, and ERK1/2, the induction of AR mRNA and protein, and AR promoter reporter activity in response to hypertonicity. Accordingly, neutralizing antibodies against the putative EGFR ligand transforming growth factor-alpha (TGF-alpha) abolished AR induction during osmotic stress. Furthermore, tonicity-induced phosphorylation of p38 and ERK1/2 and expression of AR were reduced significantly in MDCK cells transfected with a dominant-negative Ras construct. These effects were not caused by reduced nuclear abundance of TonEBP during osmotic stress; however, inhibition of EGFR or p38 diminished TonEBP transactivation activity under hypertonic conditions. The contribution of MMP/EGFR signaling in vivo was confirmed in C57BL/6 mice, in which treatment with GM6001 was associated with reduced AR induction following dehydration. Taken together, these results indicate that osmotic stress induces MMP-dependent activation of EGFR, likely via shedding of TGF-alpha, and downstream activation of Ras and the MAP kinases p38 and ERK1/2, which stimulate TonEBP transactivation activity. This EGFR-Ras-MAPK pathway contributes to TonEBP transcriptional activation and targets gene expression during osmotic stress, thus establishing a membrane-associated signal input that contributes to the regulation of TonEBP activity.

Brx mediates the response of lymphocytes to osmotic stress through the activation of NFAT5

Extracellular hyperosmolarity, or osmotic stress, generally caused by differences in salt and macromolecule concentrations across the plasma membrane, occurs in lymphoid organs and at inflammatory sites. The response of immune cells to osmotic stress is regulated by nuclear factor of activated T cells 5 (NFAT5), a transcription factor that induces the expression of hyperosmolarity-responsive genes and stimulates cytokine production. We report that the guanine nucleotide exchange factor (GEF) Brx [also known as protein kinase A-anchoring protein 13 (AKAP13)] is essential for the expression of nfat5 in response to osmotic stress, thus transmitting the extracellular hyperosmolarity signal and enabling differentiation of splenic B cells and production of immunoglobulin. This process required the activity of p38 mitogen-activated protein kinase (MAPK) and NFAT5 and involved a physical interaction between Brx and c-Jun N-terminal kinase (JNK)-interacting protein 4 (JIP4), a scaffold molecule specific to activation of the p38 MAPK cascade. Our results indicate that Brx integrates the responses of immune cells to osmotic stress and inflammation by elevating intracellular osmolarity and stimulating the production of cytokines.

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

Cerebral cavernous malformations (CCM) are vascular malformations of the brain that lead to cerebral hemorrhages. In 20% of CCM patients, this results from an autosomal dominant condition caused by loss-of-function mutations in one of the three CCM genes. High expression levels of the CCM genes in the neuroepithelium indicate that CCM lesions might be caused by a loss of function of these genes in neural cells rather than in vascular cells. However, their in vivo function, particularly during cerebral angiogenesis, is totally unknown. We developed mice with constitutive and tissue-specific CCM2 deletions to investigate CCM2 function in vivo. Constitutive deletion of CCM2 leads to early embryonic death. Deletion of CCM2 from neuroglial precursor cells does not lead to cerebrovascular defects, whereas CCM2 is required in endothelial cells for proper vascular development. Deletion of CCM2 from endothelial cells severely affects angiogenesis, leading to morphogenic defects in the major arterial and venous blood vessels and in the heart, and results in embryonic lethality at mid-gestation. These findings establish the essential role of endothelial CCM2 for proper vascular development and strongly suggest that the endothelial cell is the primary target in the cascade of events leading from CCM2 mutations to CCM cerebrovascular lesions.

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.

The cerebral cavernous malformation signaling pathway promotes vascular integrity via Rho GTPases

Cerebral cavernous malformation (CCM) is a common vascular dysplasia that affects both systemic and CNS blood vessels. Loss of function mutations in the CCM2 gene cause CCM. Here we show that targeted disruption of Ccm2 in mice results in failed lumen formation and early embryonic death through an endothelial cell autonomous mechanism. We demonstrate that CCM2 regulates endothelial cytoskeletal architecture, cell-cell interactions and lumen formation. Heterozygosity at Ccm2, a genotype equivalent to human CCM, results in impaired endothelial barrier function. Because our biochemical studies indicate that loss of CCM2 results in activation of RHOA GTPase, we rescued the cellular phenotype and barrier function in heterozygous mice using simvastatin, a drug known to inhibit Rho GTPases. These data offer the prospect for pharmacologic treatment of a human vascular dysplasia using a widely available and safe drug.

Hyperosmotic stress induces Rho/Rho kinase/LIM kinase-mediated cofilin phosphorylation in tubular cells: key role in the osmotically triggered F-actin response

Hyperosmotic stress induces cytoskeleton reorganization and a net increase in cellular F-actin, but the underlying mechanisms are incompletely understood. Whereas de novo F-actin polymerization likely contributes to the actin response, the role of F-actin severing is unknown. To address this problem, we investigated whether hyperosmolarity regulates cofilin, a key actin-severing protein, the activity of which is inhibited by phosphorylation. Since the small GTPases Rho and Rac are sensitive to cell volume changes and can regulate cofilin phosphorylation, we also asked whether they might link osmostress to cofilin. Here we show that hyperosmolarity induced rapid, sustained, and reversible phosphorylation of cofilin in kidney tubular (LLC-PK1 and Madin-Darby canine kidney) cells. Hyperosmolarity-provoked cofilin phosphorylation was mediated by the Rho/Rho kinase (ROCK)/LIM kinase (LIMK) but not the Rac/PAK/LIMK pathway, because 1) dominant negative (DN) Rho and DN-ROCK but not DN-Rac and DN-PAK inhibited cofilin phosphorylation; 2) constitutively active (CA) Rho and CA-ROCK but not CA-Rac and CA-PAK induced cofilin phosphorylation; 3) hyperosmolarity induced LIMK-2 phosphorylation, and 4) inhibition of ROCK by Y-27632 suppressed the hypertonicity-triggered LIMK-2 and cofilin phosphorylation.We thenexamined whether cofilin and its phosphorylation play a role in the hypertonicity-triggered F-actin changes. Downregulation of cofilin by small interfering RNA increased the resting F-actin level and eliminated any further rise upon hypertonic treatment. Inhibition of cofilin phosphorylation by Y-27632 prevented the hyperosmolarity-provoked F-actin increase. Taken together, cofilin is necessary for maintaining the osmotic responsiveness of the cytoskeleton in tubular cells, and the Rho/ROCK/LIMK-mediated cofilin phosphorylation is a key mechanism in the hyperosmotic stress-induced F-actin increase.