Paxillin-ARF GAP signaling and the cytoskeleton

Edaravone mimics sphingosine-1-phosphate-induced endothelial barrier enhancement in human microvascular endothelial cells

Edaravone is a potent scavenger of hydroxyl radicals and is quite successful in patients with acute cerebral ischemia, and several organ-protective effects have been reported. Treatment of human microvascular endothelial cells with edaravone (1.5 microM) resulted in the enhancement of transmonolayer electrical resistance coincident with cortical actin enhancement and redistribution of focal adhesion proteins and adherens junction proteins to the cell periphery. Edaravone also induced small GTPase Rac activation and focal adhesion kinase (FAK; Tyr(576)) phosphorylation associated with sphingosine-1-phosphate receptor type 1 (S1P(1)) transactivation. S1P(1) protein depletion by the short interfering RNA technique completely abolished edaravone-induced FAK (Tyr(576)) phosphorylation and Rac activation. This is the first report of edaravone-induced endothelial barrier enhancement coincident with focal adhesion remodeling and cytoskeletal rearrangement associated with Rac activation via S1P(1) transactivation. Considering the well-established endothelial barrier-protective effect of S1P, endothelial barrier enhancement as a consequence of S1P(1) transactivation may at least partly be the potent mechanisms for the organ-protective effect of edaravone and is suggestive of edaravone as a therapeutic agent against systemic vascular barrier disorder.

Paxillin-β-catenin interactions are involved in Rac/Cdc42-mediated endothelial barrier-protective response to oxidized phospholipids

Oxidized phospholipids may appear in the pulmonary circulation as a result of acute lung injury or inflammation. We have previously described barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine (OxPAPC) on human pulmonary endothelial cells (EC) mediated by small GTPases Rac and Cdc42. This work examined OxPAPC-induced focal adhesion (FA) and adherens junction (AJ) remodeling and potential interactions between FA and AJ protein complexes involved in OxPAPC-induced EC barrier enhancement. Immunofluorescence analysis, subcellular fractionation, and coimmunoprecipitation assays have shown that OxPAPC induced translocation and peripheral accumulation of FA complexes containing paxillin, focal adhesion kinase, vinculin, GIT1, and GIT2, increased association of AJ proteins vascular endothelial-cadherin, p120-catenin, α-, β-, and γ-catenins, and dramatically enhanced cell junction areas covered by AJ. Coimmunoprecipitation, pulldown assays, and confocal microscopy studies have demonstrated that OxPAPC promoted novel interactions between FA and AJ complexes via paxillin and β-catenin association, which was critically dependent on Rac and Cdc42 activities and was abolished by pharmacological or small interfering RNA (siRNA)-mediated inhibition of Rac and Cdc42. Depletion of β-catenin using the siRNA approach attenuated OxPAPC-induced paxillin translocation to the cell periphery, but also significantly decreased interaction of paxillin with AJ protein complex. In turn, paxillin knockdown by specific siRNA attenuated AJ enhancement in response to OxPAPC. These results show for the first time the novel interactions between FA and AJ protein complexes critical for EC barrier regulation by OxPAPC.

Dynamic motion of paxillin on actin filaments in living endothelial cells

Our three-dimensional (3-D) images showed that paxillin co-localized on actin filaments as fibrous structures, as well as clusters, in endothelial cells (ECs). In living ECs under flow condition, we monitored concurrently the intracellular dynamics of DsRed2-paxillin and GFP-actin by time-lapse video recording and dual-color fluorescence imaging. The results showed that the dynamic motion of paxillin as fibrous structures was associated with actin filaments, but not with microtubules. Our findings suggest that the actin network plays an important role not only in the assembly/disassembly of paxillin at focal adhesions, but also as a track for the intracellular transport of paxillin, which is involved in signaling pathway.

Homocysteine-induced biochemical stress predisposes to cytoskeletal remodeling in stretched endothelial cells

Cellular cytoskeletal remodeling reflects alterations in local biochemical and mechanical changes in terms of stress that manifests relocation of signaling molecules within and across the cell. Although stretching due to load and chemical changes by high homocysteine (HHcy) causes cytoskeletal re-arrangement, the synergism between stretch and HHcy is unclear. We investigated the contribution of HHcy in cyclic stretch-induced focal adhesion (FA) protein redistribution leading to cytoskeletal re-arrangement in mouse aortic endothelial cells (MAEC). MAEC were subjected to cyclic stretch (CS) and HHcy alone or in combination. The redistribution of FA protein, and small GTPases were determined by Confocal microscopy and Western blot techniques in membrane and cytosolic compartments. We found that each treatment induces focal adhesion kinase (FAK) phosphorylation and cytoskeletal actin polymerization. In addition, CS activates and membrane translocates small GTPases RhoA with minimal effect on Rac1, whereas HHcy alone is ineffective in both GTPases translocation. However, the combined effect of CS and HHcy activates and membrane translocates both GTPases. Free radical scavenger NAC (N-Acetyl-Cysteine) inhibits CS and HHcy-mediated FAK phosphorylation and actin stress fiber formation. Interestingly, CS also activates and membrane translocates another FA protein, paxillin in HHcy condition. Cytochalasin D, an actin polymerization blocker and PI3-kinase inhibitor Wortmannin inhibited FAK phosphorylation and membrane translocation of paxillin suggesting the involvement of PI3K pathway. Together our results suggest that CS- and HHcy-induced oxidative stress synergistically contribute to small GTPase membrane translocation and focal adhesion protein redistribution leading to endothelial remodeling.

Induction of vascular permeability: beta PIX and GIT1 scaffold the activation of extracellular signal-regulated kinase by PAK

Increased permeability of blood vessels is an important component of inflammation, but in some circumstances it contributes to tissue injury and organ failure. Previous work showed that p21-activated kinase (PAK) is a critical regulator of endothelial cell-cell junctions through effects on myosin light chain phosphorylation and cell contractility. We now show that blocking PAK function inhibits fluid leak in a mouse model of acute lung injury. In cultured endothelial cells, induction of myosin light chain phosphorylation by PAK is mediated by mitogen-activated protein kinase kinase and extracellular signal-regulated kinase (Erk). Erk in lipopolysaccharide (LPS)-treated mouse lung is activated in a PAK-dependent manner in several cell types, most prominently vascular endothelium. Activation of Erk requires the integrity of the complex between PAK, PIX, and GIT1. Several means of disrupting this complex inhibit stimulation of vascular permeability in vitro. A cell-permeant peptide that blocks binding of PAK to PIX inhibits LPS-induced fluid leak in the mouse lung injury model. We conclude that the PAK-PIX-GIT1 complex is critical for Erk-dependent myosin phosphorylation and vascular permeability.

HGF attenuates thrombin-induced endothelial permeability by Tiam1-mediated activation of the Rac pathway and by Tiam1/Rac-dependent inhibition of the Rho pathway

Reorganization of the endothelial cell (EC) cytoskeleton and cell adhesive complexes provides a structural basis for increased vascular permeability implicated in the pathogenesis of many diseases, including asthma, sepsis, and acute respiratory distress syndrome (ARDS). We have recently described the barrier-protective effects of hepatocyte growth factor (HGF) on the human pulmonary EC. In the present study, we explored the involvement of Rac-GTPase and Rac-specific nucleotide exchange factor Tiam1 in the mechanisms of EC barrier protection by HGF. HGF protected EC monolayers from thrombin-induced hyperpermeability, disruption of intercellular junctions, and formation of stress fibers and paracellular gaps by inhibiting thrombin-induced activation of Rho GTPase, Rho association with nucleotide exchange factor p115-RhoGEF, and myosin light chain phosphorylation, which was opposed by stimulation of Rac-dependent signaling. The pharmacological Rac inhibitor or silencing RNA (siRNA) based depletion of either Rac or Tiam1 significantly attenuated HGF-induced peripheral translocation of Rac effector cortactin, cortical actin ring formation, and EC barrier enhancement. Moreover, Tiam1 knockdown using the siRNA approach, attenuated the protective effect of HGF against thrombin-induced activation of Rho signaling, monolayer disruption, and EC hyperpermeability. This study demonstrates the Tiam1/Rac-dependent mechanism of HGF-induced EC barrier protection and provides novel mechanistic insights into regulation of EC permeability via dynamic interactions between Rho- and Tiam1/Rac-mediated pathways.

Kinetic analysis of GTP hydrolysis catalysed by the Arf1-GTP-ASAP1 complex

Arf (ADP-ribosylation factor) GAPs (GTPase-activating proteins) are enzymes that catalyse the hydrolysis of GTP bound to the small GTP-binding protein Arf. They have also been proposed to function as Arf effectors and oncogenes. We have set out to characterize the kinetics of the GAP-induced GTP hydrolysis using a truncated form of ASAP1 [Arf GAP with SH3 (Src homology 3) domain, ankyrin repeats and PH (pleckstrin homology) domains 1] as a model. We found that ASAP1 used Arf1-GTP as a substrate with a k(cat) of 57+/-5 s(-1) and a K(m) of 2.2+/-0.5 microM determined by steady-state kinetics and a kcat of 56+/-7 s(-1) determined by single-turnover kinetics. Tetrafluoroaluminate (AlF4-), which stabilizes complexes of other Ras family members with their cognate GAPs, also stabilized a complex of Arf1-GDP with ASAP1. As anticipated, mutation of Arg-497 to a lysine residue affected kcat to a much greater extent than K(m). Changing Trp-479, Iso-490, Arg-505, Leu-511 or Asp-512 was predicted, based on previous studies, to affect affinity for Arf1-GTP. Instead, these mutations primarily affected the k(cat). Mutants that lacked activity in vitro similarly lacked activity in an in vivo assay of ASAP1 function, the inhibition of dorsal ruffle formation. Our results support the conclusion that the Arf GAP ASAP1 functions in binary complex with Arf1-GTP to induce a transition state towards GTP hydrolysis. The results have led us to speculate that Arf1-GTP-ASAP1 undergoes a significant conformational change when transitioning from the ground to catalytically active state. The ramifications for the putative effector function of ASAP1 are discussed.

Helicobacter pylori VacA and gastric ulcer

VacA, the only protein toxin produced by Helicobacter pylori, vacuolates cultured cells. The presence of 2 VacA receptors has been demonstrated. One is the receptor-type protein tyrosine phosphatase (RPTP) zeta/beta (PTP zeta/beta), and the other is RPTP alpha. VacA binds to PTP zeta/beta, resulting in gastric epithelial detachment through the tyrosine phosphorylation of Git-1, which then leads to gastric ulceration by the direct action of gastric acid. Thus, disturbance of adhesion between gastric epithelial cells and the extracellular matrix due to an abnormal PTP zeta/beta signal is the main mechanism of gastric ulceration.

SYD-2 Liprin-α organizes presynaptic active zone formation through ELKS

A central event in synapse development is formation of the presynaptic active zone in response to positional cues. Three active zone proteins, RIM, ELKS (also known as ERC or CAST) and Liprin-alpha, bind each other and are implicated in linking active zone formation to synaptic vesicle release. Loss of function in Caenorhabditis elegans syd-2 Liprin-alpha alters the size of presynaptic specializations and disrupts synaptic vesicle accumulation. Here we report that a missense mutation in the coiled-coil domain of SYD-2 causes a gain of function. In HSN synapses, the syd-2(gf) mutation promotes synapse formation in the absence of syd-1, which is essential for HSN synapse formation. syd-2(gf) also partially suppresses the synaptogenesis defects in syg-1 and syg-2 mutants. The activity of syd-2(gf) requires elks-1, an ELKS homolog; but not unc-10, a RIM homolog. The mutant SYD-2 shows increased association with ELKS. These results establish a functional dependency for assembly of the presynaptic active zone in which SYD-2 plays a key role.

ARAP2 effects on the actin cytoskeleton are dependent on Arf6-specific GTPase-activating-protein activity and binding to RhoA-GTP

ARAP2 is a protein that contains both ArfGAP and RhoGAP domains. We found that it is a phosphatidylinositol (3,4,5)-trisphosphate-dependent Arf6 GAP that binds RhoA-GTP but lacks RhoGAP activity. In agreement with the hypothesis that ARAP2 mediates effects of RhoA, endogenous ARAP2 associated with focal adhesions (FAs) and reduction of ARAP2 expression, by RNAi, resulted in fewer FAs and actin stress fibers (SFs). In cells with reduced levels of endogenous ARAP2, FAs and SFs could be restored with wild-type recombinant ARAP2 but not mutants lacking ArfGAP or Rho-binding activity. Constitutively active Arf6 also caused a loss of SFs. The Rho effector ROKα was ineffective in restoring FAs. Conversely, overexpression of ARAP2 did not restore SFs in cells treated with a ROK inhibitor but induced punctate accumulations of paxillin. We conclude that ARAP2 is an Arf6GAP that functions downstream of RhoA to regulate focal adhesion dynamics.

ArfGAP family proteins in cell adhesion, migration and tumor invasion

The identification of several ArfGAP proteins as binding partners of paxillin, an integrin signaling and scaffolding protein, has suggested the existence of molecular links between integrin functions and intracellular traffic, as proposed by MS Bretscher long ago. Among the paxillin-binding ArfGAPs, AMAP1 has recently been strongly implicated in tumor invasion as well as malignancy, owing to its highly augmented expression in tumors and its direct involvement in invasive activities. Another ArfGAP, Git2, was found to be a component of the Gbetagamma-mediated directional sensing machinery, while simultaneously playing an essential role in the suppressive control of superoxide production, which is mediated by vesicle transport in GPCR-stimulated neutrophils. These emerging molecular mechanisms may further delineate key processes regulating intracellular traffic as principal controls of cell motility and invasive activities.

Differential regulation of pulmonary endothelial monolayer integrity by varying degrees of cyclic stretch

Ventilator-induced lung injury is a life-threatening complication of mechanical ventilation at high-tidal volumes. Besides activation of proinflammatory cytokine production, excessive lung distension directly affects blood-gas barrier and lung vascular permeability. To investigate whether restoration of pulmonary endothelial cell (EC) monolayer integrity after agonist challenge is dependent on the magnitude of applied cyclic stretch (CS) and how these effects are linked to differential activation of small GTPases Rac and Rho, pulmonary ECs were subjected to physiologically (5% elongation) or pathologically (18% elongation) relevant levels of CS. Pathological CS enhanced thrombin-induced gap formation and delayed monolayer recovery, whereas physiological CS induced nearly complete EC recovery accompanied by peripheral redistribution of focal adhesions and cortactin after 50 minutes of thrombin. Consistent with differential effects on monolayer integrity, 18% CS enhanced thrombin-induced Rho activation, whereas 5% CS promoted Rac activation during the EC recovery phase. Rac inhibition dramatically attenuated restoration of monolayer integrity after thrombin challenge. Physiological CS preconditioning (5% CS, 24 hours) enhanced EC paracellular gap resolution after step-wise increase to 18% CS (30 minutes) and thrombin challenge. These results suggest a critical role for the CS amplitude and the balance between Rac and Rho in mechanochemical regulation of lung EC barrier.

betaPIX controls cell motility and neurite extension by regulating the distribution of GIT1

Cell motility entails the reorganization of the cytoskeleton and membrane trafficking for effective protrusion. GIT1/p95-APP1 is a member of a family of GTPase-activating proteins for ARF GTPases that affect endocytosis, adhesion and migration. GIT1 associates with paxillin and a complex including the Rac/Cdc42 exchanging factors PIX/Cool and the kinase PAK. In this study, we show that overexpression of betaPIX induces the accumulation of endogenous and overexpressed GIT1 at large structures similar to those induced by an ArfGAP-defective mutant of GIT1 (p95-C2). Immunohistochemical analysis and immunoelectron microscopy reveal that these structures include the endogenous transferrin receptor. Time-lapse analysis during motogenic stimuli shows that the formation and perinuclear accumulation of the p95-C2-positive structures is paralleled by inhibition of lamellipodium formation and cell retraction. Both dimerization and a functional SH3 domain of betaPIX are required for the accumulation of GIT1 in fibroblasts, which is prevented by the monomeric PIX-PG-DeltaLZ. This mutant also prevents the formation of endocytic aggregates and inhibition of neurite outgrowth in retinal neurons expressing p95-C2. Our results indicate that betaPIX is an important regulator of the subcellular distribution of GIT1, and suggest that alteration in the level of expression of the complex affects the endocytic compartment and cell motility.

Differentially expressed genes in the prostate cancer cell line LNCaP after exposure to androgen and anti-androgen

Androgens play an important role in growth and maintenance of prostate cells. The actions of androgens are mediated by the androgen receptor (AR), a transcription factor member of the super-family of nuclear hormone receptors. Androgen regulated genes (ARGs) are potential markers for early diagnosis and treatment of prostate cancer patients. In the present study, we used DDRT-PCR (differential display reverse transcriptase polymerase chain reaction) technique in order to investigate differentially expressed genes in the prostate cancer cell line LNCaP after treatment with dihydrotestosterone and bicalutamide for 6, 24, and 48 hours. Fifty-five differentially expressed fragments were isolated, cloned, and sequenced. Sequencing analysis of these fragments revealed 56 different transcripts that showed homology to transcription factors, cell cycle regulators, metabolic enzymes, and hypothetical proteins. Among the differentially expressed genes, SPA17 and DDEF2 were further validated using quantitative real time RT-PCR (qPCR) in a series of 25 prostate tumor samples. The DDEF2 gene is involved in adhesion and cell migration of monocytes, and the SPA17 gene might be involved in cellular signal transduction. The transcripts of both, SPA17 and DDEF2 genes, showed altered pattern of expression in the group of prostate tumors analyzed by qPCR. The differentially expressed genes identified in this study might provide new insights into the androgen signaling pathways in prostate cells.

Role of the Arf6 GDP/GTP cycle and Arf6 GTPase-activating proteins in actin remodeling and intracellular transport

We have analyzed both biochemically and functionally a series of Arf6 mutants, providing new insights into the molecular mode of action of the small G protein Arf6. First, by comparing a fast-cycling mutant (Arf6(T157N)) and a GTPase-deficient mutant (Arf6(Q67L)), we established the necessity for completion of the Arf6 GDP/GTP cycle for recycling of major histocompatibility complex molecules to the plasma membrane. Second, we found that aluminum fluoride (AlF), known for inducing membrane protrusion in cells expressing exogenous wild-type Arf6, stabilized a functional wild-type Arf6.AlF(x) . GTPase-activating protein (GAP) complex in vitro and in vivo. We also found that the tandem mutation Q37E/S38I prevented the binding of two Arf GAPs, but not the effector ARHGAP10, and blocked the formation of membrane protrusion and actin reorganization. Together, our results with AlF(x) and Arf6(Q37E/S38I) demonstrate the critical role of the Arf6 GAPs as effectors for Arf6-regulated actin cytoskeleton remodeling. Finally, competition experiments conducted in vivo suggest the existence of a membrane receptor for GDP-bound Arf6.

HEF1-aurora A interactions: points of dialog between the cell cycle and cell attachment signaling networks

Regulated timing of cell division cycles, and geometrical precision in the planar orientation of cell division, are critical during organismal development and remain important for the maintenance of polarized structures in adults. Mounting evidence suggests that these processes are coordinated at the centrosome through the action of proteins that mediate both cell cycle and cell attachment. Our recent work identifying HEF1 as an activator of the Aurora A kinase suggests a novel hub for such integrated signaling. We suggest that defects in components of the machinery specifying the temporal and spatial integration of cell division may induce cancer and other diseases through pleiotropic effects on cell migration, proliferation, apoptosis, and genomic stability.

Regulation of neuroendocrine exocytosis by the ARF6 GTPase-activating protein GIT1

Neuroendocrine cells release hormones and neuropeptides by exocytosis, a highly regulated process in which secretory granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Using chromaffin and PC12 cells, we have recently described that the granule-associated GTPase ARF6 plays a crucial role in exocytosis by activating phospholipase D1 at the plasma membrane and, presumably, promoting the fusion reaction between the two membrane bilayers. ARF6 is activated by the nucleotide exchange factor ARNO following docking of granules to the plasma membrane. We show here that GIT1, a GTPase-activating protein stimulating GTP hydrolysis on ARF6, is the second molecular partner that turns over the GDP/GTP cycle of ARF6 during cell stimulation. Western blot and immunofluorescence experiments indicated that GIT1 is cytosolic in resting cells but is recruited to the plasma membrane in stimulated cells, where it co-localizes with ARF6 at the granule docking sites. Over-expression of wild-type GIT1 inhibits growth hormone secretion from PC12 cells; this inhibitory effect was not observed in cells expressing a GIT1 mutant impaired in its ARF-GTPase-activating protein (GAP) activity or in cells expressing other ARF6-GAPs. Conversely reduction of GIT1 by RNA interference increased the exocytotic activity. Using a real time assay for individual chromaffin cells, we found that microinjection of GIT1 strongly reduced the number of exocytotic events. These results provide the first evidence that GIT1 plays a function in calcium-regulated exocytosis in neuroendocrine cells. We propose that GIT1 represents part of the pathway that inactivates ARF6-dependent reactions and thereby negatively regulates and/or terminates exocytotic release.

Biochemical characterization of the Cool (Cloned-out-of-Library)/Pix (Pak-interactive exchange factor) proteins

The Cool (Cloned out of Library)/Pix (Pak interactive exchange factor) proteins have been implicated in a diversity of biological activities, ranging from pathways initiated by growth factors and chemoattractants to X-linked mental retardation. Initially discovered through yeast two-hybrid and biochemical analyses as binding partners for the Cdc42/Rac-target/effector, Pak (p21 activated kinase), the sequences for the Cool/Pix proteins revealed a DH (Dbl homology) domain. Because the DH domain is the limit functional unit for stimulating guanine nucleotide exchange on Rho family GTP-binding proteins, it was assumed that the Cool/Pix proteins would act as guanine nucleotide exchange factors (GEFs) for the Rho proteins. Of the three known isoforms, (p50Cool-1, p85Cool-1/beta-Pix, and 90Cool-2/alpha-Pix), only Cool-2/alpha-Pix has exhibited significant GEF activity. A number of experimental techniques have been used to characterize Cool-2, and in vitro analysis has revealed that its GEF activity is under tight control through intramolecular interactions involving several binding partners. Here we describe the biochemical methods used to study the Cool/Pix proteins and, in particular, the regulation of the GEF activity of Cool-2/alpha-Pix.

Characterization of the endogenous GIT1-betaPIX complex, and identification of its association to membranes

G protein-coupled receptor kinase interactors (GITs) are adaptor proteins with ADP-ribosylating factor--GTPase-activating protein (ARF-GAP) activity that form complexes with the p21-activated kinase-interacting exchange factor (PIX) guanine nucleotide exchanging factors for Rac and Cdc42. In this study we have characterized the endogenous GIT1/p95-APP1/Cat1 (GIT1)- PIX complexes in neuronal and non-neuronal cells. In COS7 cells, immunocytochemical analysis shows the localization of endogenous GIT1 in the perinuclear region of the cell, as well as at the cell periphery, where GIT1 co-localizes with filamentous actin. The perinuclear localization of endogenous GIT1 was confirmed in avian fibroblasts. In COS7 cells, immunoprecipitation and microsequencing experiments with either anti-GIT1 or anti-betaPIX antibodies unequivocally show that betaPIX is uniquely associated with GIT1 in lysates from these cells, while GIT2/PKL/p95-APP2/Cat2 (GIT2) is undetectable in the endogenous complexes. Moreover, this analysis demonstrates that betaPIX is the limiting factor for the formation of the endogenous complexes, since a small fraction of GIT1 can be co-immunoprecipitated with most betaPIX from these cells. Saponin treatment of unfixed cells indicates that betaPIX-bound GIT1 is preferentially retained in the saponin-resistant fraction when compared to betaPIX-free GIT1. Moreover, analysis by tissue fractionation shows that a significant fraction of the endogenous GIT1-betaPIX complex is firmly associated to membranes from brain homogenates. Our findings show the specific localization of the complex at intracellular membranes, and indicate a correlation between the association of GIT1 to betaPIX, and the localization of the endogenous complex at membranes.

DDEF1 is located in an amplified region of chromosome 8q and is overexpressed in uveal melanoma

PURPOSE

The molecular pathogenesis of uveal melanoma is poorly understood but is usually accompanied by amplification of chromosome 8q, suggesting the activation of one or more oncogenes. We recently identified a gene expression profile that distinguishes low-grade from high-grade melanomas. In this profile, a cluster of genes at chromosome 8q was overexpressed in high-grade tumors, providing an opportunity to search for potential oncogenes in this region.

EXPERIMENTAL DESIGN

Gene expression microarray analysis was done on 25 primary uveal melanomas. Microarray comparative genomic hybridization (CGH), quantitative PCR, and immunohistochemistry were done on a subset of these tumors. Cell motility was measured using a wound-healing assay.

RESULTS

In melanomas analyzed for microarray gene expression and CGH, gain of chromosome 8q correlated most strongly with expression of DDEF1, a gene located at 8q24. In contrast, the nearby MYC oncogene exhibited no significant change in expression. Confirming the microarray findings, DDEF1 mRNA levels and protein expression were significantly higher in high-grade melanomas. Furthermore, ectopic expression of DDEF1 in low-grade melanoma cells resulted in a significant increase in cell motility, a feature of high-grade metastasizing cells.

CONCLUSIONS

These findings suggest that DDEF1 overexpression may be a pathogenetically relevant consequence of chromosome 8q amplification, which commonly occurs in high-grade uveal melanomas. We conclude that DDEF1 may act as an oncogene in this cancer, and it may be a useful diagnostic marker and therapeutic target.

Dynamic recruitment of PAK1 to the immunological synapse is mediated by PIX independently of SLP-76 and Vav1

T cell receptor engagement activates p21-activated kinase 1 (PAK1) through a LAT-SLP-76-Nck-Vav-Rac-dependent pathway. A second independent pathway involving a GIT-PIX-PAK1 trimolecular complex is also activated by T cell receptor ligation. Here we show a Vav-independent pathway exists that leads to PAK1 activation. In addition, PAK1, PIX and GIT1 were recruited to the T cell-antigen-presenting cell contact site independently of SLP-76 and Vav1. PAK1 recruitment to the T cell-antigen-presenting cell interface required interaction with PIX, which also led to optimal PLC-gamma1 activation and T cell receptor-dependent transcriptional responses. These data indicate that a pathway involving the GIT-PIX-PAK1 complex has a crucial function in PAK1 activation by recruiting PAK1 to the immunological synapse.

Regulation of Rho and Rac signaling to the actin cytoskeleton by paxillin during Drosophila development

Paxillin is a prominent focal adhesion docking protein that regulates cell adhesion and migration. Although numerous paxillin-binding proteins have been identified and paxillin is required for normal embryogenesis, the precise mechanism by which paxillin functions in vivo has not yet been determined. We identified an ortholog of mammalian paxillin in Drosophila (Dpax) and have undertaken a genetic analysis of paxillin function during development. Overexpression of Dpax disrupted leg and wing development, suggesting a role for paxillin in imaginal disc morphogenesis. These defects may reflect a function for paxillin in regulation of Rho family GTPase signaling as paxillin interacts genetically with Rac and Rho in the developing eye. Moreover, a gain-of-function suppressor screen identified a genetic interaction between Dpax and cdi in wing development. cdi belongs to the cofilin kinase family, which includes the downstream Rho target, LIM kinase (LIMK). Significantly, strong genetic interactions were detected between Dpax and Dlimk, as well as downstream effectors of Dlimk. Supporting these genetic data, biochemical studies indicate that paxillin regulates Rac and Rho activity, positively regulating Rac and negatively regulating Rho. Taken together, these data indicate the importance of paxillin modulation of Rho family GTPases during development and identify the LIMK pathway as a critical target of paxillin-mediated Rho regulation.

VAN3 ARF-GAP-mediated vesicle transport is involved in leaf vascular network formation

Within the leaf of an angiosperm, the vascular system is constructed in a complex network pattern called venation. The formation of this vein pattern has been widely studied as a paradigm of tissue pattern formation in plants. To elucidate the molecular mechanism controlling the vein patterning process, we previously isolated Arabidopsis mutants van1 to van7, which show a discontinuous vein pattern. Here we report the phenotypic analysis of the van3 mutant in relation to auxin signaling and polar transport, and the molecular characterization of the VAN3 gene and protein. Double mutant analyses with pin1, emb30-7/gn and mp, and physiological analyses using the auxin-inducible marker DR5::GUS and an auxin transport inhibitor indicated that VAN3 may be involved in auxin signal transduction, but not in polar auxin transport. Positional cloning identified VAN3 as a gene that encodes an adenosine diphosphate (ADP)-ribosylation factor-guanosine triphosphatase (GTPase) activating protein (ARF-GAP). It resembles animal ACAPs and contains four domains: a BAR (BIN/amphiphysin/RVS) domain, a pleckstrin homology (PH) domain, an ARF-GAP domain and an ankyrin (ANK)-repeat domain. Recombinant VAN3 protein showed GTPase-activating activity and a specific affinity for phosphatidylinositols. This protein can self-associate through the N-terminal BAR domain in the yeast two-hybrid system. Subcellular localization analysis by double staining for Venus-tagged VAN3 and several green-fluorescent-protein-tagged intracellular markers indicated that VAN3 is located in a subpopulation of the trans-Golgi network (TGN). Our results indicate that the expression of this gene is induced by auxin and positively regulated by VAN3 itself, and that a specific ACAP type of ARF-GAP functions in vein pattern formation by regulating auxin signaling via a TGN-mediated vesicle transport system.

A novel GTPase-activating protein for ARF6 directly interacts with clathrin and regulates clathrin-dependent endocytosis

ADP-ribosylation factor 6 (Arf6) is a small-GTPase that regulates the membrane trafficking between the plasma membrane and endosome. It is also involved in the reorganization of the actin cytoskeleton. GTPase-activating protein (GAP) is a critical regulator of Arf function as it inactivates Arf. Here, we identified a novel species of GAP denoted as SMAP1 that preferentially acts on Arf6. Although overexpression of SMAP1 did not alter the subcellular distribution of the actin cytoskeleton, it did block the endocytosis of transferrin receptors. Knock down of endogenous SMAP1 also abolished transferrin internalization, which confirms that SMAP1 is needed for this endocytic process. SMAP1 overexpression had no effect on clathrin-independent endocytosis, however. Intriguingly, SMAP1 binds directly to the clathrin heavy chain via its clathrin-box and mutation studies revealed that its GAP domain and clathrin-box both contribute to the role SMAP1 plays in clathrin-dependent endocytosis. These observations suggest that SMAP1 may be an Arf6GAP that specifically regulates one of the multiple functions of Arf6, namely, clathrin-dependent endocytosis, and that it does so by binding directly to clathrin.

Assays and properties of the Arf GAPs AGAP1, ASAP1, and Arf GAP1

ADP-ribosylation factors (Arfs) are Ras-like GTP-binding proteins that regulate membrane traffic and actin remodeling. Arf function requires GTP hydrolysis but Arf lacks GTPase activity; consequently, Arf function is dependent on Arf GTPase-activating proteins (GAPs). The Arf GAPs are a structurally diverse group of at least 16 proteins. Several Arf GAPs use a single Arf isoform. However, due to structural differences, the conditions supporting productive interactions between Arf and different Arf GAPs vary. Here, we describe preparation and basic properties of three Arf GAPs. We use these proteins to illustrate assays for Arf GAP activity. Conditions that optimize activity for each GAP are discussed. These methods can be used for the further characterization of Arf-Arf GAP interaction that is necessary for understanding the function of Arf in cellular physiology.

ARAP3 is transiently tyrosine phosphorylated in cells attaching to fibronectin and inhibits cell spreading in a RhoGAP-dependent manner

ARAP3 is a GTPase activating protein (GAP) for Rho and Arf GTPases that is implicated in phosphoinositide 3-kinase (PI 3-kinase) signalling pathways controlling lamellipodia formation and actin stress fibre assembly. We have identified ARAP3 as a phosphorylated target of protein tyrosine kinases. In cells, ARAP3 was tyrosine phosphorylated when co-expressed with Src-family kinases (SFKs), upon stimulation with growth factors and during adhesion to the extracellular matrix (ECM) substrate fibronectin. Adhesion-induced phosphorylation of ARAP3 was suppressed by selective inhibitors of Src-family kinases and PI 3-kinase and by a Src dominant interfering mutant. Inducible expression of ARAP3 in HEK293 epithelial cells resulted in increased cell rounding, membrane process formation and cell clustering on ECM substrates. In contrast, ARAP3 dramatically slowed the kinetics of cell spreading on fibronectin but had no effect on cell adhesion. These effects of ARAP3 required a functional Rho GAP domain and were associated with reduced cellular levels of active RhoA and Rac1 but did not require the sterile alpha motif (SAM) or Arf GAP domains. Mutation of two phosphorylation sites, Y1399 and Y1404, enhanced some ARAP3 activities, suggesting that ARAP3 may be negatively regulated by phosphorylation on these tyrosine residues. These results implicate ARAP3 in integrin-mediated tyrosine kinase signalling pathways controlling Rho GTPases and cell spreading.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Epoxycyclopentenone-Containing Oxidized Phospholipids Restore Endothelial Barrier Function via Cdc42 and Rac

After an acute phase of inflammation or injury, restoration of the endothelial barrier is important to regain vascular integrity and to prevent edema formation. However, little is known about mediators that control restoration of endothelial barrier function. We show here that oxidized phospholipids that accumulate at sites of inflammation and tissue damage are potent regulators of endothelial barrier function. Oxygenated epoxyisoprostane-containing phospholipids, but not fragmented oxidized phospholipids, exhibited barrier-protective effects mediated by small GTPases Cdc42 and Rac and their cytoskeletal, focal adhesion, and adherens junction effector proteins. Oxidized phospholipid-induced cytoskeletal rearrangements resulted in a unique peripheral actin rim formation, which was mimicked by coexpression of constitutively active Cdc42 and Rac, and abolished by coexpression of dominant-negative Rac and Cdc42. Thus, oxidative modification of phospholipids during inflammation leads to the formation of novel regulators that may be critically involved in restoration of vascular barrier function.

A protein interaction network links GIT1, an enhancer of huntingtin aggregation, to Huntington's disease

Analysis of protein-protein interactions (PPIs) is a valuable approach for characterizing proteins of unknown function. Here, we have developed a strategy combining library and matrix yeast two-hybrid screens to generate a highly connected PPI network for Huntington's disease (HD). The network contains 186 PPIs among 35 bait and 51 prey proteins. It revealed 165 new potential interactions, 32 of which were confirmed by independent binding experiments. The network also permitted the functional annotation of 16 uncharacterized proteins and facilitated the discovery of GIT1, a G protein-coupled receptor kinase-interacting protein, which enhances huntingtin aggregation by recruitment of the protein into membranous vesicles. Coimmunoprecipitations and immunofluorescence studies revealed that GIT1 and huntingtin associate in mammalian cells under physiological conditions. Moreover, GIT1 localizes to neuronal inclusions, and is selectively cleaved in HD brains, indicating that its distribution and function is altered during disease pathogenesis.

AGAP1, a novel binding partner of nitric oxide-sensitive guanylyl cyclase

Nitric oxide (NO)-sensitive soluble guanylyl cyclase (sGC) is the major cytosolic receptor for NO, catalyzing the conversion of GTP to cGMP. In a search for proteins specifically interacting with human sGC, we have identified the multidomain protein AGAP1, the prototype of an ArfGAP protein with a GTPase-like domain, Ankyrin repeats, and a pleckstrin homology domain. AGAP1 binds through its carboxyl terminal portion to both the alpha1 and beta1 subunits of sGC. We demonstrate that AGAP1 mRNA and protein are co-expressed with sGC in human, murine, and rat cells and tissues and that the two proteins interact in vitro and in vivo. We also show that AGAP1 is prone to tyrosine phosphorylation by Src-like kinases and that tyrosine phosphorylation potently increases the interaction between AGAP1 and sGC, indicating that complex formation is modulated by reversible phosphorylation. Our findings may hint to a potential role of AGAP1 in integrating signals from Arf, NO/cGMP, and tyrosine kinase signaling pathways.

Tyrosine phosphoproteomics of fibroblast growth factor signaling: a role for insulin receptor substrate-4

Signal transduction by receptor tyrosine kinases is initiated by recruitment of a variety of signaling proteins to tyrosine-phosphorylated motifs in the activated receptors. Several signaling pathways are thus activated in parallel, the combination of which decides the cellular response. Here, we present a dual strategy for extensive mapping of tyrosine-phosphorylated proteins and probing of signal-dependent protein interactions of a signaling cascade. The approach relies on labeling of cells with "heavy" and "light" isotopic forms of Arg to distinguish two cell populations. First, tyrosine-phosphorylated proteins from stimulated ("heavy"-labeled) and control samples ("normal"-labeled) are isolated and subjected to high sensitivity Fourier transform ion cyclotron resonance mass spectrometry analysis. Next, phosphopeptides corresponding to tyrosine phosphorylation sites identified during the tyrosine phosphoproteomic analysis are used as baits to isolate phosphospecific protein binding partners, which are subsequently identified by mass spectrometry. We used this approach to identify 28 components of the signaling cascade induced by stimulation with the basic fibroblast growth factor. Insulin receptor substrate-4 was identified as a novel candidate in fibroblast growth factor receptor signaling, and we defined phosphorylation-dependent interactions with other components, such as adaptor protein Grb2, of the signaling cascade. Finally, we present evidence for a complex containing insulin receptor substrate-4 and ShcA in signaling by the fibroblast growth factor receptor.

Novel regulatory mechanisms for the Dbl family guanine nucleotide exchange factor Cool-2/alpha-Pix

The Cool-2 (cloned-out of library-2) protein (identical to alpha-Pix for Pak-interactive exchange factor) has been implicated in various biological responses including chemoattractant signaling and in certain forms of mental retardation. We show that when Cool-2 exists as a dimer, it functions as a Rac-specific guanine nucleotide exchange factor (GEF). Dimerization of Cool-2 enables its Dbl (diffuse B-cell lymphoma) and pleckstrin homology domains to work together (in trans) to bind specifically to Rac-GDP. Dissociation of dimeric Cool-2 into its monomeric form allows it to act as a GEF for Cdc42 as well as for Rac. The binding of either PAK (p21-activated kinase) or Cbl (Casitas B-lymphoma) to the SH3 domain of monomeric Cool-2 is necessary for the functional interactions between GDP-bound Cdc42 or Rac and the Cool-2 monomer. The betagamma subunit complex of large GTP-binding proteins, by interacting with PAK, stimulates the dissociation of the Cool-2 dimer and activates its GEF activity for Cdc42. Overall, these findings highlight novel mechanisms by which extracellular signals can direct the specific activation of Rac versus Cdc42 by Cool-2/alpha-Pix.

RNA and RNA Binding Proteins Participate in Early Stages of Cell Spreading through Spreading Initiation Centers

Focal adhesions are specialized attachment and signaling centers that form at sites of cell-matrix contacts. We employed a quantitative mass spectrometry-based method called SILAC to identify and quantify proteins interacting in an attachment-dependent manner with focal adhesion proteins. Subsequent confocal microscopy revealed a previously undescribed structure, which we have termed a spreading initiation center (SIC), existing only in early stages of cell spreading. SICs contain focal adhesion markers, appear to be surrounded by an actin sheath, and, surprisingly, contain numerous RNA binding proteins, ribosomal RNA, and perhaps other RNAs. Interfering with the function of FUS/TLS, hnRNP K, and hnRNP E1 results in increased spreading. Spreading initiation centers are ribonucleoprotein complexes distinct from focal adhesions and demonstrate a role for RNA and RNA binding proteins in the initiation of cell spreading.

Arf GAPs: multifunctional proteins that regulate membrane traffic and actin remodelling

The ADP-ribosylation factor (Arf) Arf GTPase-activating proteins (GAPs) are a family of proteins that induce hydrolysis of GTP bound to Arf. A conserved domain containing a zinc finger motif mediates catalysis. The substrate, Arf.GTP, affects membrane trafficking and actin remodelling. Consistent with activity as an Arf regulator, the Arf GAPs affect both of these pathways. However, the Arf GAPs are likely to have Arf-independent activities that contribute to their cellular functions. Structures of the Arf GAPs are diverse containing catalytic, protein-protein interaction and lipid interaction domains in addition to the Arf GAP domain. Some Arf GAPs have been identified and characterized on the basis of activities other than Arf GAP. Here, we describe the Arf GAP family, enzymology of some members of the Arf GAP family and known functions of the proteins. The results discussed illustrate roles for both Arf-dependent and -independent activities in the regulation of cellular architecture.

Rho and Rac take center stage

Many features of cell behavior are regulated by Rho family GTPases, but the most profound effects of these proteins are on the actin cytoskeleton and it was these that first drew attention to this family of signaling proteins. Focusing on Rho and Rac, we will discuss how their effectors regulate the actin cytoskeleton. We will describe how the activity of Rho proteins is regulated downstream from growth factor receptors and cell adhesion molecules by guanine nucleotide exchange factors and GTPase activating proteins. Additionally, we will discuss how there is signaling crosstalk between family members and how various bacterial pathogens have developed strategies to manipulate Rho protein activity so as to enhance their own survival.

Involvement of site-specific FAK phosphorylation in sphingosine-1 phosphate- and thrombin-induced focal adhesion remodeling: role of Src and GIT

Sphingosine-1 phosphate (S1P) and thrombin are agents with profound but divergent effects on vascular endothelial cell (EC) barrier properties. We have previously reported that S1P-induced focal adhesion (FA) remodeling involves interactions between focal adhesion kinase (FAK), paxillin, and G-protein-coupled receptor kinase-interacting proteins GIT1 and GIT2 and suggested a critical involvement of focal adhesions in the EC barrier regulation. In this study, we examined redistribution of FA proteins (FAK, paxillin, GIT1, and GIT2) and site-specific FAK tyrosine phosphorylation in human pulmonary artery endothelial cells stimulated with thrombin. In contrast to S1P, which we have shown to induce peripheral translocation of FA proteins associated with cortical actin ring formation, thrombin caused the redistribution of FA proteins to the ends of the newly formed massive stress fibers. S1P and thrombin induced distinct patterns of FAK site-specific phosphorylation with the FAK Y576 phosphorylation site targeted by SIP challenge and phosphorylation of three FAK sites (Y397, Y576, and Y925) in response to thrombin stimulation. Pharmacological inhibition of Src with Src-specific inhibitor PP2 abolished S1P-induced translocation of FA proteins, cortical actin ring formation, and FAK [Y576] phosphorylation. However, PP2 failed to alter thrombin-induced morphological changes and exhibited only partial inhibition of FAK site-specific tyrosine phosphorylation. These observations highlight the differential mechanisms of focal adhesion protein complex remodeling and FAK activation by S1P and thrombin and link differential FA remodeling to EC barrier regulation.

hRIP, a cellular cofactor for Rev function, promotes release of HIV RNAs from the perinuclear region

Human immunodeficiency virus Rev facilitates the cytoplasmic accumulation of viral RNAs that contain a Rev binding site. A human Rev-interacting protein (hRIP) was originally identified based on its ability to interact with the Rev nuclear export signal (NES) in yeast two-hybrid assays. To date, however, the function of hRIP and a role for hRIP in Rev-directed RNA export have remained elusive. Here we ablate hRIP activity with a dominant-negative mutant or RNA interference and analyze Rev function by RNA in situ hybridization. We find, unexpectedly, that in the absence of functional hRIP, Rev-directed RNAs mislocalize and aberrantly accumulate at the nuclear periphery, where hRIP is localized. In contrast, in the absence of Rev or the Rev cofactor CRM1, Rev-directed RNAs remain nuclear. We further show that the RNA mislocalization pattern resulting from loss of hRIP activity is highly specific to Rev function: the intracellular distribution of cellular poly(A)(+) mRNA, nuclear proteins, and, most important, NES-containing proteins, are unaffected. Thus, hRIP is an essential cellular Rev cofactor, which acts at a previously unanticipated step in HIV-1 RNA export: movement of RNAs from the nuclear periphery to the cytoplasm.

Cell migration: integrating signals from front to back

Cell migration is a highly integrated multistep process that orchestrates embryonic morphogenesis; contributes to tissue repair and regeneration; and drives disease progression in cancer, mental retardation, atherosclerosis, and arthritis. The migrating cell is highly polarized with complex regulatory pathways that spatially and temporally integrate its component processes. This review describes the mechanisms underlying the major steps of migration and the signaling pathways that regulate them, and outlines recent advances investigating the nature of polarity in migrating cells and the pathways that establish it.

Interaction of the ERC family of RIM-binding proteins with the liprin-alpha family of multidomain proteins

Liprin-alpha/SYD-2 is a family of multidomain proteins with four known isoforms. One of the reported functions of liprin-alpha is to regulate the development of presynaptic active zones, but the underlying mechanism is poorly understood. Here we report that liprin-alpha directly interacts with the ERC (ELKS-Rab6-interacting protein-CAST) family of proteins, members of which are known to bind RIMs, the active zone proteins that regulate neurotransmitter release. In vitro results indicate that ERC2/CAST, an active zone-specific isoform, interacts with all of the known isoforms of liprin-alpha and that liprin-alpha1 associates with both ERC2 and ERC1b, a splice variant of ERC1 that distributes to both cytosolic and active zone regions. ERC2 colocalizes with liprin-alpha1 in cultured neurons and forms a complex with liprin-alpha1 in brain. Liprin-alpha1, when expressed alone in cultured neurons, shows a partial synaptic localization. When coexpressed with ERC2, however, liprin-alpha1 is redistributed to synaptic sites. Moreover, roughly the first half of ERC2, which contains the liprin-alpha-binding region, is sufficient for the synaptic localization of liprin-alpha1 while the second half is not. These results suggest that the interaction between ERC2 and liprin-alpha may be involved in the presynaptic localization of liprin-alpha and the molecular organization of presynaptic active zones.

Matrix metalloproteinase-9 silencing by RNA interference triggers the migratory-adhesive switch in Ewing's sarcoma cells

Enhanced expression of (pro)matrix metalloproteinase-9 (MMP-9) is associated with human tumor invasion and/or metastasis. COH cells derived from a highly invasive and metastatic Ewing's sarcoma constitutively express proMMP-9. Transfection of a double stranded RNA that targets the MMP-9 mRNA into COH cells depleted the corresponding mRNA and protein as demonstrated by reverse transcriptase-PCR, enzyme-linked immunosorbent assay, and gelatin zymography. proMMP-9 extinction resulted in the following: (i) decreased spreading on extracellular matrix (fibronectin, laminin, collagen IV)-coated surfaces, (ii) inhibition of migration toward fibronectin, and (iii) induced aggregation, which was specifically disrupted by a function-blocking E-cadherin antibody. MMP-9 knockdown concomitantly resulted in increased levels of surface E-cadherin, redistribution at the plasma membrane of beta-catenin, and its physical association with E-cadherin. Moreover, induction of E-cadherin-mediated adhesion was associated with RhoA activation and changes in paxillin cytoskeleton. Finally, an inhibitor of gelatinolytic activity of pro-MMP9 did not reduce COH cell migration confirming that the enzymatic property of COH MMP-9 was not required for migration toward fibronectin. Overall, our observations define a novel critical role for proMMP-9 in providing a cellular switch between stationary and migratory cell phases.

Specific regulation of the adaptor protein complex AP-3 by the Arf GAP AGAP1

Arf1 regulates membrane trafficking at several membrane sites by interacting with at least seven different vesicle coat proteins. Here, we test the hypothesis that Arf1-dependent coats are independently regulated by specific interaction with Arf GAPs. We find that the Arf GAP AGAP1 directly associates with and colocalizes with AP-3, a coat protein complex involved in trafficking in the endosomal-lysosomal system. Binding is mediated by the PH domain of AGAP1 and the delta and sigma3 subunits of AP-3. Overexpression of AGAP1 changes the cellular distribution of AP-3, and reduced expression of AGAP1 renders AP-3 resistant to brefeldin A. AGAP1 overexpression does not affect the distribution of other coat proteins, and AP-3 distribution is not affected by overexpression of other Arf GAPs. Cells overexpressing AGAP1 also exhibit increased LAMP1 trafficking via the plasma membrane. Taken together, these results support the hypothesis that AGAP1 directly and specifically regulates AP-3-dependent trafficking.

Arf and its many interactors

Arf GTP-binding proteins regulate membrane traffic and actin remodeling. Similar to other GTP-binding proteins, a complex of Arf-GTP with an effector protein mediates Arf function. Arf interacts with at least three qualitatively different types of effectors. First, it interacts with structural proteins, the vesicle coat proteins. The second type of effector is lipid-metabolizing enzymes, and the third comprises those proteins that bind to Arf-GTP but whose biochemical or biological functions are not yet clearly defined. Arf interacts with two other families of proteins, the exchange factors and the GTPase-activating proteins. Recent work examining the functional relationships among the diverse Arf interactors has led to reconsideration of the prevailing paradigms for Arf action.

Crk associates with a multimolecular Paxillin/GIT2/beta-PIX complex and promotes Rac-dependent relocalization of Paxillin to focal contacts

We have previously demonstrated that the CrkII and CrkL adapter proteins are required for the spreading of epithelial colonies and the breakdown of adherens junctions in response to hepatocyte growth factor. When overexpressed, CrkII and CrkL promote lamellipodia formation, cell spreading, and the loss of epithelial adherens junctions in the absence of hepatocyte growth factor. The exact mechanism by which Crk proteins elicit these changes is unclear. We show that the overexpression of CrkII or CrkL, but not Src homology 2 or amino-terminal Src homology 3 domain mutant Crk proteins, promotes the relocalization of Paxillin to focal contacts throughout the cell and within lamellipodia in a Rac-dependent manner. In stable cell lines overexpressing CrkII, enhanced lamellipodia formation and cell spreading correlate with an increased association of CrkII with Paxillin, GIT2 (an ARF-GAP) and beta-PIX (a Rac1 exchange factor). Mutants of Paxillin that fail to associate with Crk or GIT2, or do not target to focal adhesions inhibit Crk-dependent cell spreading and lamellipodia formation. We conclude from these studies that the association of Crk with Paxillin is important for the spreading of epithelial colonies, by influencing the recruitment of Paxillin to focal complexes and promoting the enhanced assembly of Paxillin/GIT2/beta-PIX complexes.

Leucine-zipper-mediated homo- and hetero-dimerization of GIT family p95-ARF GTPase-activating protein, PIX-, paxillin-interacting proteins 1 and 2

ADP-ribosylation factor GTPase-activating proteins (ARFGAPs) of the G-protein-coupled receptor kinase interactor 1/p95 paxillin kinase linker/p95-ARFGAP Pak-interacting exchange factor paxillin-binding protein (APP)-1 family are multidomain proteins, which interact functionally with both ARF and Rac GTPases. These proteins are involved in the dynamic reorganization of adhesion and the cytoskeleton during cell motility. Our previous work [Di Cesare, Paris, Albertinazzi, Dariozzi, Andersen, Mann, Longhi and de Curtis (2000) Nat. Cell Biol. 2, 521-530] has pointed out a role for p95-APP1 in the regulation of ARF6-mediated membrane recycling. These proteins include different domains, and are capable of interacting stably with proteins that are supposed to play a role in the regulation of actin dynamics and adhesion. They contain a coiled-coil region comprising a putative leucine zipper, predicted to be involved in dimerization. In the present study, we have investigated the possibility that these proteins form dimers. Our results show that p95-APP1 forms homodimers and may also form heterodimers with the other member of the family, p95 paxillin kinase linker/p95-APP2. Both homo- and heterodimerization are disrupted by mutation of two leucine residues in the coiled-coil region of p95-APP1. The N-terminal portion of p95-APP1, including the ARFGAP domain, three ankyrin repeats and the Pak-interacting exchange factor-binding region, are not required for dimerization. Evidence is presented for the existence of endogenous oligomeric complexes. The implication of dimerization/oligomerization in the functioning of these proteins is discussed.

The Shank family of postsynaptic density proteins interacts with and promotes synaptic accumulation of the beta PIX guanine nucleotide exchange factor for Rac1 and Cdc42

The Shank/ProSAP family of multidomain proteins is known to play an important role in organizing synaptic multiprotein complexes. Here we report a novel interaction between Shank and beta PIX, a guanine nucleotide exchange factor for the Rac1 and Cdc42 small GTPases. This interaction is mediated by the PDZ domain of Shank and the C-terminal leucine zipper domain and the PDZ domain-binding motif at the extreme C terminus of beta PIX. Shank colocalizes with beta PIX at excitatory synaptic sites in cultured neurons. In brain, Shank forms a complex with beta PIX and beta PIX-associated signaling molecules including p21-associated kinase (PAK), an effector kinase of Rac1/Cdc42. Importantly, overexpression of Shank in cultured neurons promotes synaptic accumulation of beta PIX and PAK. Considering the involvement of Rac1 and PAK in spine dynamics, these results suggest that Shank recruits beta PIX and PAK to spines for the regulation of postsynaptic structure.

Mice deficient in protein tyrosine phosphatase receptor type Z are resistant to gastric ulcer induction by VacA of Helicobacter pylori

The vacuolating cytotoxin VacA produced by Helicobacter pylori causes massive cellular vacuolation in vitro and gastric tissue damage in vivo, leading to gastric ulcers, when administered intragastrically. Here we report that mice deficient in protein tyrosine phosphatase receptor type Z (Ptprz, also called PTP-zeta or RPTP-beta, encoded by Ptprz) do not show mucosal damage by VacA, although VacA is incorporated into the gastric epithelial cells to the same extent as in wild-type mice. Primary cultures of gastric epithelial cells from Ptprz+/+ and Ptprz-/- mice also showed similar incorporation of VacA, cellular vacuolation and reduction in cellular proliferation, but only Ptprz+/+ cells showed marked detachment from a reconstituted basement membrane 24 h after treatment with VacA. VacA bound to Ptprz, and the levels of tyrosine phosphorylation of the G protein-coupled receptor kinase-interactor 1 (Git1), a Ptprz substrate, were higher after treatment with VacA, indicating that VacA behaves as a ligand for Ptprz. Furthermore, pleiotrophin (PTN), an endogenous ligand of Ptprz, also induced gastritis specifically in Ptprz+/+ mice when administered orally. Taken together, these data indicate that erroneous Ptprz signaling induces gastric ulcers.

S1P induces FA remodeling in human pulmonary endothelial cells: role of Rac, GIT1, FAK, and paxillin

Sphingosine 1-phosphate (S1P) enhances human pulmonary endothelial monolayer integrity via Rac GTPase-dependent formation of a cortical actin ring (Garcia et al. J Clin Invest 108: 689-701, 2001). The mechanisms underlying this response are not well understood but may involve rapid redistribution of focal adhesions (FA) as attachment sites for actin filaments. We evaluate the effects of S1P on the redistribution of paxillin, FA kinase (FAK), and the G protein-coupled receptor kinase-interacting proteins (GITs). S1P induced Rac GTPase activation and cortical actin ring formation at physiological concentrations (0.5 microM), whereas 5 microM S1P caused prominent stress fiber formation and activation of Rho and Rac GTPases. S1P (0.5 microM) stimulated the tyrosine phosphorylation of FAK Y(576), and paxillin was linked to FA disruption and redistribution to the cell periphery. Furthermore, S1P induced a transient association of GIT1 with paxillin and redistribution of the GIT2-paxillin complex to the cell cortical area without affecting GIT2-paxillin association. These results suggest a role of FA rearrangement in S1P-mediated barrier enhancement via Rac- and GIT-mediated processes.

The GIT family of proteins forms multimers and associates with the presynaptic cytomatrix protein Piccolo

The cytoskeletal matrix assembled at active zones (CAZ) is implicated in defining neurotransmitter release sites. However, little is known about the molecular mechanisms by which the CAZ is organized. Here we report a novel interaction between Piccolo, a core component of the CAZ, and GIT proteins, multidomain signaling integrators with GTPase-activating protein activity for ADP-ribosylation factor small GTPases. A small region (approximately 150 amino acid residues) in Piccolo, which is not conserved in the closely related CAZ protein Bassoon, mediates a direct interaction with the Spa2 homology domain (SHD) domain of GIT1. Piccolo and GIT1 colocalize at synaptic sites in cultured neurons. In brain, Piccolo forms a complex with GIT1 and various GIT-associated proteins, including betaPIX, focal adhesion kinase, liprin-alpha, and paxillin. Point mutations in the SHD of GIT1 differentially interfere with the association of GIT1 with Piccolo, betaPIX, and focal adhesion kinase, suggesting that these proteins bind to the SHD by different mechanisms. Intriguingly, GIT proteins form homo- and heteromultimers through their C-terminal G-protein-coupled receptor kinase-binding domain in a tail-to-tail fashion. This multimerization enables GIT1 to simultaneously interact with multiple SHD-binding proteins including Piccolo and betaPIX. These results suggest that, through their multimerization and interaction with Piccolo, the GIT family proteins are involved in the organization of the CAZ.

AGAP1, an endosome-associated, phosphoinositide-dependent ADP-ribosylation factor GTPase-activating protein that affects actin cytoskeleton

We have identified three members of the AGAP subfamily of ASAP family ADP-ribosylation factor GTPase-activating proteins (Arf GAPs). In addition to the Arf GAP domain, these proteins contain GTP-binding protein-like, ankyrin repeat and pleckstrin homology domains. Here, we have characterized the ubiquitously expressed AGAP1/KIAA1099. AGAP1 had Arf GAP activity toward Arf1>Arf5>Arf6. Phosphatidylinositol 4,5-bisphosphate and phosphatidic acid synergistically stimulated GAP activity. As found for other ASAP family Arf GAPs, the pleckstrin homology domain was necessary for activity. Deletion of the GTP-binding protein-like domain affected lipid dependence of Arf GAP activity. In vivo effects of AGAP1 were distinct from other ASAP family Arf GAPs. Overexpressed AGAP1 induced the formation of and was associated with punctate structures containing the endocytic markers transferrin and Rab4. AP1 was redistributed from the trans-Golgi to the punctate structures. Like other ASAP family members, AGAP1 overexpression inhibited the formation of PDGF-induced ruffles. However, distinct from other ASAP family members, AGAP1 also induced the loss of actin stress fibers. Thus, AGAP1 is a phosphoinositide-dependent Arf GAP that impacts both the endocytic compartment and actin.

Epithelial cell cytoskeleton and intracellular trafficking V. Confluence of membrane trafficking and motility in epithelial cell models

Migration of epithelial cells occurs in a variety of important biological processes including tissue morphogenesis, wound healing, and the metastasis of epithelial tumors. In some instances, the cells remain attached to each other and migrate together as a sheet, maintaining epithelial integrity. In others (e.g., metastasis), junctional complexes are disrupted and cells migrate individually. In both cases, motility involves the extension of membranous protrusions (filopodia and lamellipodia) in the direction of movement and the transient assembly and disassembly of integrin-mediated adhesions with the extracellular matrix. The driving force for these events is provided by regulated changes in the organization of the actin cytoskeleton, which are thought to be coordinated with alterations in intracellular membrane traffic. In this themes article, I review current hypotheses about how these processes are integrated and attempt to identify fruitful areas for future research.