N-Formyl peptide receptor ligation induces rac-dependent actin reorganization through Gbeta gamma subunits and class Ia phosphoinositide 3-kinases

Resolvin D1 modulates periodontal ligament fibroblast function

BACKGROUND

The resolution of inflammation is an active process mediated by specialized lipid mediators called lipoxins and resolvins. Periodontal ligament fibroblasts (PDLFs) play a significant role in periodontal regeneration. The purpose of the current study was to determine the impact of resolvin D1 (RvD1) on human PDLF cell wound healing and proliferation, receptor expression (G-protein-coupled receptor 32 [GPR32] and formyl peptide receptor 2 [ALX/FPR2]), and cytokine expression and release.

METHODS

PDLFs were stimulated with interleukin-1β (IL-1β) (500 pg/ml) with and without RvD1 (100 nM). RvD1 receptor expression was determined by quantitative real-time polymerase chain reaction (qPCR), immunofluorescence microscopy, and fluorescence-activated cell sorting. Wound closure was measured by a scratch assay, and proliferation was determined by bromodeoxyuridine incorporation. Interleukin-6 (IL-6), interleukin-8 (IL-8), monocyte chemoattractant protein-1, cyclooxygenase-2, matrix metalloproteinases-1, -2, and -3 (MMP-1, -2, and -3), tissue inhibitors of metalloproteinases-1 and -2 (TIMP-1 and -2), prostaglandin E2, and osteoprotegerin (OPG) gene expression and production were measured using qPCR and Western blotting, multiplex immunoassay, and enzyme-linked immunosorbent assay.

RESULTS

PDLF expressed GPR32 and ALX/FPR2. RvD1 reversed IL-1β-induced inhibition of wound healing and proliferation of PDLF. IL-1β also induced the production of proinflammatory cytokines and MMPs. This effect was reversed by RvD1. RvD1 reversed IL-1β-induced inhibition of TIMP-1, TIMP-2, and OPG.

CONCLUSION

The data suggested that RvD1 has a pro-wound healing, proliferative, and anti-inflammatory impact on the PDLF that favors periodontal regeneration.

Transgelin-2 in immunity: Its implication in cell therapy

Transgelin-2 is a small 22-kDa actin-binding protein implicated in actin dynamics, which stabilizes actin structures and participates in actin-associated signaling pathways. Much curiosity regarding transgelin-2 has centered around its dysregulation in tumor development and associated diseases. However, recent studies have shed new light on the functions of transgelin-2, the only transgelin family member present in leukocytes, in the context of various immune responses. In this review, we outlined the biochemical properties of transgelin-2 and its physiological functions in T cells, B cells, and macrophages. Transgelin-2 regulates T cell activation by stabilizing the actin cytoskeleton at the immunological synapse. Transgelin-2 in B cells also participates in the stabilization of T cell-B cell conjugates. While transgelin-2 is expressed at trace levels in macrophages, its expression is highly upregulated upon lipopolysaccharide stimulation and plays an essential role in macrophage phagocytosis. Since transgelin-2 increases T cell adhesion to target cells via boosting the "inside-out" costimulatory activation of leukocyte function-associated antigen 1, transgelin-2 could be a suitable candidate to potentiate the antitumor response of cytotoxic T cells by compensating for the lack of costimulation in tumor microenvironment. We discussed the feasibility of using native or engineered transgelin-2 as a synergistic molecule in cell-based immunotherapies, without inducing off-target disturbance in actin dynamics in other cells.

Review of the neutrophil response to Bordetella pertussis infection

The nature and timing of the neutrophil response to infection with Bordetella pertussis is influenced by multiple virulence factors expressed by the bacterium. After inoculation of the host airway, the recruitment of neutrophils signaled by B. pertussis lipooligosaccharide (LOS) is suppressed by pertussis toxin (PTX). Over the next week, the combined activities of PTX, LOS and adenylate cyclase toxin (ACT) result in production of cytokines that generate an IL-17 response, promoting neutrophil recruitment which peaks at 10-14 days after inoculation in mice. Arriving at the site of infection, neutrophils encounter the powerful local inhibitory activity of ACT, in conjunction with filamentous hemagglutinin. With the help of antibodies, neutrophils contribute to clearance of B. pertussis, but only after 28-35 days in a naïve mouse. Studies of the lasting, antigen-specific IL-17 response to infection in mice and baboons has led to progress in vaccine development and understanding of pathogenesis. Questions remain about the mediators that coordinate neutrophil recruitment and the mechanisms by which neutrophils overcome B. pertussis virulence factors.

Rho GTPases in platelet function

The Rho family of GTP binding proteins, also commonly referred to as the Rho GTPases, are master regulators of the platelet cytoskeleton and platelet function. These low-molecular-weight or 'small' GTPases act as signaling switches in the spatial and temporal transduction, and amplification of signals from platelet cell surface receptors to the intracellular signaling pathways that drive platelet function. The Rho GTPase family members RhoA, Cdc42 and Rac1 have emerged as key regulators in the dynamics of the actin cytoskeleton in platelets and play key roles in platelet aggregation, secretion, spreading and thrombus formation. Rho GTPase regulators, including GEFs and GAPs and downstream effectors, such as the WASPs, formins and PAKs, may also regulate platelet activation and function. In this review, we provide an overview of Rho GTPase signaling in platelet physiology. Previous studies of Rho GTPases and platelets have had a shared history, as platelets have served as an ideal, non-transformed cellular model to characterize Rho function. Likewise, recent studies of the cell biology of Rho GTPase family members have helped to build an understanding of the molecular regulation of platelet function and will continue to do so through the further characterization of Rho GTPases as well as Rho GAPs, GEFs, RhoGDIs and Rho effectors in actin reorganization and other Rho-driven cellular processes.

Heterotrimeric G protein betagamma subunits stimulate FLJ00018, a guanine nucleotide exchange factor for Rac1 and Cdc42

We previously reported that Gbetagamma signaling regulates cell spreading or cell shape change through activation of a Rho family small GTPase, suggesting the existence of a Gbetagamma-regulated Rho guanine-nucleotide exchange factor (RhoGEF). In this study we examined various RhoGEF clones, found FLJ00018 to beaGbetagamma-activated RhoGEF, and investigated the molecular mechanism of Gbetagamma-induced activation of Rho family GTPases. Co-expression of the genes for FLJ00018 and Gbetagamma enhanced serum response element-mediated gene transcription in HEK-293 cells. Combined expression of Gbetagamma and FLJ00018 significantly induced activation of Rac and Cdc42 but not RhoA. FLJ00018 also enhanced gene transcription induced by carbachol-stimulated m2 muscarinic acetylcholine receptor, and this enhancement was blocked by pertussis toxin. Furthermore, we demonstrated Gbetagamma to interact directly with the N-terminal region of FLJ00018 and the N-terminal fragment of this molecule to inhibit serum response element-dependent transcription induced by Gbetagamma/FLJ00018 and carbachol. In NIH3T3 cells, FLJ00018 enhanced lysophosphatidic acid-induced cell spreading, which was also blocked by the N-terminal fragment of FLJ00018. These results provide evidence for a signaling pathway by which G(i)-coupled receptor specifically induces Rac and Cdc42 activation through direct interaction of Gbetagamma with FLJ00018.

Analysis of a missense variant of the human N-formyl peptide receptor that is associated with agonist-independent β-arrestin association and indices of inflammation

Formyl-Met-Leu-Phe (fMLP) is a potent chemoattractant molecule released from both bacteria and damaged mitochondria that activates fMLP receptors (FPR) leading to neutrophil chemotaxis, degranulation and superoxide production. A common missense single nucleotide polymorphism in the human FPR1 gene at nucleotide c.32C>T results in the amino-acid substitution, p.I11T, in the FPR1 extracellular amino-terminus. The minor (c.32T) allele frequencies were 0.25, 0.27, 0.25, 0.15 and 0.14 in healthy Caucasian, African, East Indian, Chinese and Native Canadian individuals, respectively. In subjects homozygous for the p.T11 allele, we find elevated serum concentrations of C-reactive protein, increased absolute counts of blood leukocytes and neutrophils, and erythrocyte sedimentation rates. When expressed in HEK 293 and RBL-2H3 cells a substantial proportion of FPR1 p.I11T variant is retained intracellularly and agonist-independent internalization of the FPR1 p.I11T variant, but not the wild-type FPR1, is constitutively associated with beta-arrestin2-GFP in vesicles. Moreover, basal N-acetyl-D-glucosaminidase release is increased in primary neutrophils isolated from subjects either heterozygous or homozygous for the FPR1 p.T11 allele. Taken together, the data suggest an increased receptor activity and phenotypic expression of increased inflammatory indices in subjects with the p.T11 allele.

Vav activation and function as a rac guanine nucleotide exchange factor in macrophage colony-stimulating factor-induced macrophage chemotaxis

Signal transduction mediated by phosphatidylinositol 3-kinase (PI 3-kinase) is regulated by hydrolysis of its products, a function performed by the 145-kDa SH2 domain-containing inositol phosphatase (SHIP). Here, we show that bone marrow macrophages of SHIP(-/-) animals have elevated levels of phosphatidylinositol 3,4,5-trisphosphate [PI (3,4,5)P(3)] and displayed higher and more prolonged chemotactic responses to macrophage colony-stimulating factor (M-CSF) and elevated levels of F-actin relative to wild-type macrophages. We also found that the small GTPase Rac was constitutively active and its upstream activator Vav was constitutively phosphorylated in SHIP(-/-) macrophages. Furthermore, we show that Vav in wild-type macrophages is recruited to the membrane in a PI 3-kinase-dependent manner through the Vav pleckstrin homology domain upon M-CSF stimulation. Dominant inhibitory mutants of both Rac and Vav blocked chemotaxis. We conclude that Vav acts as a PI 3-kinase-dependent activator for Rac activation in macrophages stimulated with M-CSF and that SHIP regulates macrophage M-CSF-triggered chemotaxis by hydrolysis of PI (3,4,5)P(3).

Gene delivery of l-caldesmon protects cytoskeletal cell membrane integrity against adenovirus infection independently of myosin ATPase and actin assembly

The cytoskeleton is critical to the viral life cycle. Agents like cytochalasin inhibit viral infections but cannot be used for antiviral therapy because of their toxicity. We report the efficacy, safety, and mechanisms by which gene delivery of human wild-type low-molecular-weight caldesmon (l-CaD) protects cell membrane integrity from adenovirus infection in a DF-1 cell line, an immortalized avian fibroblast that is null for l-CaD. Transfection with an adenovirus (Ad)-controlled construct mediated a dose-dependent decline in transcellular resistance. In accordance with a computational model of cytoskeletal membrane properties, Ad disturbed cell-cell and cell-matrix adhesion and membrane capacitance. Transfection with the Ad-l-CaD construct attenuated adenovirus-mediated loss in transcellular resistance. Quantitation of vinculin-stained plaques revealed an increase in total focal contact mass in monolayers transfected with the Ad-l-CaD construct. Expression of l-CaD protected transcellular resistance through primary effects on membrane capacitance and independently of actin solubility and effects on prestress, as measured by the decline in isometric tension in response to cytochalasin D. Expression of l-CaD exhibited less Trypan blue cell toxicity than cytochalasin, and, unlike cytochalasin, it did not interfere with wound closure or adversely effect transcellular resistance. These findings demonstrate the gene delivery of wild-type human l-CaD as a potentially efficacious and safe agent that inhibits some of the cytopathic effects of adenovirus.

Disturbed granulocyte macrophage-colony stimulating factor priming of phosphatidylinositol 3,4,5-trisphosphate accumulation and Rac activation in fMLP-stimulated neutrophils from patients with myelodysplasia

The production of reactive oxygen species (ROS) by human neutrophils is imperative for their bactericidal activity. Proinflammatory agents such as granulocyte macrophage-colony stimulating factor (GM-CSF) can prime ROS production in response to chemoattractants such as N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP). In neutrophils from patients suffering from Myelodysplastic syndromes (MDS), a clonal, hematological disorder characterized by recurrent bacterial infections, this GM-CSF priming is severely impaired. In this study, we set out to further delineate the defects in neutrophils from MDS patients. We examined the effect of GM-CSF priming on fMLP-triggered activation of Rac, a small GTPase implicated in neutrophil ROS production. In contrast to healthy neutrophils, activation of Rac in response to fMLP was not enhanced by GM-CSF pretreatment in MDS neutrophils. Furthermore, activation of Rac was attenuated by pretreatment of neutrophils with the phosphatidylinositol 3-kinase (PI-3K) inhibitor LY294002. Unlike healthy neutrophils, fMLP-induced accumulation of the PI-3K lipid product PI(3,4,5)trisphosphate was not increased by GM-CSF pretreatment in MDS neutrophils. The disturbed Rac and PI-3K activation observed in MDS neutrophils did not appear to reflect a general GM-CSF or fMLP receptor-signaling defect, as fMLP-triggered Ras activation could be primed by GM-CSF in MDS and healthy neutrophils. Moreover, fMLP-induced activation of the GTPase Ral was also normal in neutrophils from MDS patients. Taken together, our data suggest that in neutrophils from MDS patients, a defect in priming of the PI-3K-Rac signaling pathway, located at the level of PI-3K, results in a decreased GM-CSF priming of ROS production.

Activation of group IV cytosolic phospholipase A2 in human eosinophils by phosphoinositide 3-kinase through a mitogen-activated protein kinase-independent pathway

Activation of group IV cytosolic phospholipase A(2) (gIV-PLA(2)) is the essential first step in the synthesis of inflammatory eicosanoids and in integrin-mediated adhesion of leukocytes. Prior investigations have demonstrated that phosphorylation of gIV-PLA(2) results from activation of at least two isoforms of mitogen-activated protein kinase (MAPK). We investigated the potential role of phosphoinositide 3-kinase (PI3K) in the activation of gIV-PLA(2) and the hydrolysis of membrane phosphatidylcholine in fMLP-stimulated human blood eosinophils. Transduction into eosinophils of Deltap85, a dominant negative form of class IA PI3K adaptor subunit, fused to an HIV-TAT protein transduction domain (TAT-Deltap85) concentration dependently inhibited fMLP-stimulated phosphorylation of protein kinase B, a downstream target of PI3K. FMLP caused increased arachidonic acid (AA) release and secretion of leukotriene C(4) (LTC(4)). TAT-Deltap85 and LY294002, a PI3K inhibitor, blocked the phosphorylation of gIV-PLA(2) at Ser(505) caused by fMLP, thus inhibiting gIV-PLA(2) hydrolysis and production of AA and LTC(4) in eosinophils. FMLP also caused extracellular signal-related kinases 1 and 2 and p38 MAPK phosphorylation in eosinophils; however, neither phosphorylation of extracellular signal-related kinases 1 and 2 nor p38 was inhibited by TAT-Deltap85 or LY294002. Inhibition of 1) p70 S6 kinase by rapamycin, 2) protein kinase B by Akt inhibitor, or 3) protein kinase C by Ro-31-8220, the potential downstream targets of PI3K for activation of gIV-PLA(2), had no effect on AA release or LTC(4) secretion caused by fMLP. We find that PI3K is required for gIV-PLA(2) activation and hydrolytic production of AA in activated eosinophils. Our data suggest that this essential PI3K independently activates gIV-PLA(2) through a pathway that does not involve MAPK.

Expression and function of formyl peptide receptors on human fibroblast cells

The migration of polymorphonuclear leukocytes from the blood to sites of infection in tissues is a hallmark of the innate immune response. Formylated peptides produced as a byproduct of bacterial protein synthesis are powerful chemoattractants for leukocytes. Formyl peptides bind to two different G protein-coupled receptors (formyl peptide receptor (FPR) and the low affinity formyl peptide receptor-like-1 (FPRL1)) to initiate a signal transduction cascade leading to cell activation and migration. Our analysis of expressed sequences from many cDNA libraries draws attention to the fact that FPRs are widely expressed in nonlymphoid tissues. Here we demonstrate that FPRs are expressed by normal human lung and skin fibroblasts and the human fibrosarcoma cell line HT-1080. The expression on fibroblasts of receptors for bacteria-derived peptides raises questions about the possible function of these receptors in nonleukocyte cells. We studied the function of FPRs on fibroblasts and find that stimulation with fMLP triggers dose-dependent migration of these cells. Furthermore, fMLP induces signal transduction including intracellular calcium flux and a transient increase in F-actin. The fMLP-induced adhesion and motility of fibroblasts on fibronectin require functional protein kinase C and phosphatidylinositol 3-kinase. This first report of a functional formyl peptide receptor in cells of fibroblast origin opens new possibilities for the role of fibroblasts in innate immune responses.

Beta-adrenergic receptor-mediated DNA synthesis in neonatal rat cardiac fibroblasts proceeds via a phosphatidylinositol 3-kinase dependent pathway refractory to the antiproliferative action of cyclic AMP

The following study was undertaken to elucidate the cytoskeletal phenotype of neonatal rat cardiac fibroblasts (NNCF) and the signaling pathways coupled to beta-adrenergic receptor stimulated DNA synthesis. The cytoskeletal proteins vimentin, and smooth muscle alpha-actin were detected in NNCF, suggestive of a myofibroblast phenotype. Isoproterenol (ISO) treatment stimulated (3)H-thymidine uptake, and concomitantly increased intracellular cyclic AMP levels. However, cyclic AMP-elevating agents markedly decreased DNA synthesis. Coincident with growth, ISO-stimulated phosphatidylinositol 3-kinase (PI3-K) activity, and the PI3-K inhibitor LY294002 abrogated enzyme activity, and DNA synthesis. Unexpectedly, the serine/threonine kinase protein kinase Balpha (PKBalpha), a putative downstream target of PI3-K, was dephosphorylated following ISO treatment. Despite PKBalpha inactivation, the phosphorylation of its putative downstream target, the pro-apoptotic enzyme glycogen synthase kinase-3alpha was significantly increased in response to ISO. These latter effects of ISO were mimicked by the cyclic AMP-elevating agent forskolin. Lastly, ISO treatment increased p70 ribosomal S6 kinase (p70S6K) phosphorylation, as reflected by an upward electrophoretic mobility shift. The pretreatment with rapamycin abrogated the ISO-mediated mobility shift of p70S6K, and DNA synthesis. Collectively, these data demonstrate that NNCF express a myofibroblast phenotype, and beta-adrenergic agonists promote DNA synthesis via a PI3-K-dependent pathway involving p70S6K. Although unable to suppress ISO-stimulated DNA synthesis, cyclic AMP can influence specific downstream targets of PI3-K highlighting a novel crosstalk between these signaling pathways.

Intracellular trafficking of human CXCR1 and CXCR2: regulation by receptor domains and actin-related kinases

In this study we investigated the regulation of CXCR1 and CXCR2 intracellular trafficking. First, we produced a chimeric CXCR2 receptor that contained the internalization motifs of both CXCR2 and CXCR1 (CXCR2: LLKIL sequence; CXCR1: C-terminal phosphorylation sites). Elevated levels of internalization were induced by different ELR-expressing CXC chemokines on the chimeric receptor, as compared to wild-type CXCR2. Analysis of inter-relationships between CXCR1 and CXCR2 during internalization indicated that the exposure of cells that expressed both CXCR1 and CXCR2 to CXCL8 or CXCL6 resulted in decreased levels of CXCR1 internalization as compared to those in cells that expressed only CXCR1. To characterize the role of actin-related components in CXCR1 and CXCR2 trafficking, wortmannin, a potent inhibitor of phosphatidylinositol kinases, was used. The presence of wortmannin during receptor recycling inhibited CXCR1 and CXCR2 re-expression following CXCL8-induced internalization, and resulted in a marked disruption of the proper organization of actin filaments. The kinase-dependent recycling process required CXCR2 C-terminal phosphorylation sites. Our results suggest that actin-related kinases are required for the proper functionality of actin filaments, which are the instrumental factors needed for receptor recycling. In all, CXCR1 and CXCR2 internalization and recycling are tightly regulated by receptor domains and by actin-related kinases.

Microvascular cell death in spontaneously hypertensive rats during experimental inflammation

OBJECTIVE

Chronic hypertension is associated with an increased risk for tissue injury that may be mediated in part by endothelium and inflammatory cells. To clarify a possible underlying mechanisms, we examined leukocyte migration in the microcirculation and concomitant parenchymal cell death.

METHODS

The mesentery of spontaneously hypertensive rats (SHRs) and their normotensive controls, Wistar Kyoto (WKY) rats, was examined with digital fluorescence microscopy after topical stimulation with an inflammatory mediator (f-met-leu-phe, 10(-8)M). The migratory pathways of individual leukocytes were traced, and at the same time cell death was detected by use of a life-death indicator (propidium iodide) over a period of 3 hours.

RESULTS

Both WKY and SHR had a progressively increasing number of leukocytes migrating across the endothelium in postcapillary venules into the tissue parenchyma. But parenchymal cell death was detected in a random pattern in the mesentery tissue, without correlation to the migratory positions of the leukocytes. Although mature SHR rats (about 17 weeks) exhibited the same level of cell death as age-matched WKY rats, older WKY rats (about 30 weeks) had significantly lower levels of cell death, whereas the SHR rats maintained the same number of parenchymal cell death as mature animals.

CONCLUSIONS

These results suggest that in the presence of an inflammatory mediator, the SHR may exhibit a stronger response to an inflammatory mediator than normotensive WKY rats in a fashion that is age, but not blood pressure, dependent. Parenchymal cell death does not correlate with migration of activated leukocytes at the microvascular level.

Defective lymphocyte chemotaxis in beta-arrestin2- and GRK6-deficient mice

Lymphocyte chemotaxis is a complex process by which cells move within tissues and across barriers such as vascular endothelium and is usually stimulated by chemokines such as stromal cell-derived factor-1 (CXCL12) acting via G protein-coupled receptors. Because members of this receptor family are regulated ("desensitized") by G protein-coupled receptor kinase (GRK)-mediated receptor phosphorylation and beta-arrestin binding, we examined signaling and chemotactic responses in splenocytes derived from knockout mice deficient in various beta-arrestins and GRKs, with the expectation that these responses might be enhanced. Knockouts of beta-arrestin2, GRK5, and GRK6 were examined because all three proteins are expressed at high levels in purified mouse CD3+ T and B220+ B splenocytes. CXCL12 stimulation of membrane GTPase activity was unaffected in splenocytes derived from GRK5-deficient mice but was increased in splenocytes from the beta-arrestin2- and GRK6-deficient animals. Surprisingly, however, both T and B cells from beta-arrestin2-deficient animals and T cells from GRK6-deficient animals were strikingly impaired in their ability to respond to CXCL12 both in transwell migration assays and in transendothelial migration assays. Chemotactic responses of lymphocytes from GRK5-deficient mice were unaffected. Thus, these results indicate that beta-arrestin2 and GRK6 actually play positive regulatory roles in mediating the chemotactic responses of T and B lymphocytes to CXCL12.

Differential regulation of Rho and Rac through heterotrimeric G-proteins and cyclic nucleotides

Platelets were used to study the activation of Rho and Rac through G-protein-coupled receptors and its regulation by cyclic nucleotides. The thromboxane A(2) (TXA(2)) mimetic rapidly activated both small GTPases independently of integrin alpha(IIb)beta(3) activation., which leads to the activation of G(12)/G(13) and G(q) did not induce Rac activation in G alpha(q)-deficient platelets but was able to activate Rho, to stimulate actin polymerization and phosphatidylinositol 4,5-bisphosphate formation, and to induce shape change. Rac activation by in wild-type platelets could be blocked by chelation of intracellular Ca(2+) and was partially sensitive to apyrase and AR-C69931MX, an antagonist of the G(i)-coupled ADP receptor. Cyclic AMP, which completely blocks platelet function, inhibited the -induced activation of G(q) and G(12)/G(13) as well as of Rac and Rho. In contrast, cGMP, which has no effect on platelet shape change blocked only activation of G(q) and Rac. These data demonstrate that Rho and Rac are differentially regulated through heterotrimeric G-proteins. The G(12)/G(13)-mediated Rho activation is involved in the shape change response, whereas Rac is activated through G(q) and is not required for shape change. Cyclic AMP and cGMP differentially interfere with -induced Rho and Rac activation at least in part by selective effects on the regulation of individual G-proteins through the TXA(2) receptor.

Characterisation of Rac activation in thrombin- and collagen-stimulated human blood platelets

In this study, we characterised the mechanisms of Rac GTPase activation in human platelets stimulated by two physiological agonists, either thrombin, acting through membrane receptors coupled to heterotrimeric G-proteins, or collagen which is known to mobilise a tyrosine kinase-dependent pathway. Both agonists induced a rapid activation of Rac that was not significantly affected by the inhibition of integrin alpha(IIb)beta(3) engagement. Using pharmacological inhibitors, we found that phospholipase C activation and calcium mobilisation were essential for platelet Rac activation by either thrombin or collagen whereas protein kinase C inhibition was without effect. In contrast to Rac, Cdc42 activation was independent of phospholipase C activation, indicating that the two GTPases are differently regulated. We also found that phosphoinositide 3-kinase was not required for Rac activation in response to thrombin but was involved in its activation by collagen.

A role for Wiskott-Aldrich syndrome protein in T-cell receptor-mediated transcriptional activation independent of actin polymerization

Wiskott-Aldrich syndrome protein (WASP) plays a key role in cytoskeletal rearrangement and transcriptional activation in T-cells. Recent evidence links WASP and related proteins to actin polymerization by the Arp2/3 complex. To study whether the role of WASP in actin polymerization is coupled to T-cell receptor (TCR)-mediated transcriptional activation, we made a series of WASP deletion mutants and tested them for actin co-localization, actin polymerization, and transcriptional activation of NFAT. A WASP mutant with a deletion in the C-terminal region (WASPDeltaC) that is defective in actin polymerization potentiated NFAT transcription following TCR activation by anti-CD3 and anti-CD3/CD28 antibodies, but not by phorbol 12-myristate 13-acetate/ionomycin. Furthermore, cotransfection of a dominant-active mutant (WASP-WH2-C) for Arp2/3 polymerization did not inhibit NFAT activation. Finally, by analyzing a series of WASP double-domain deletion mutants, we determined that the WASP homology-1 domain is responsible for NFAT transcriptional activation. Our results suggest that WASP activates transcription following TCR stimulation in a manner that is independent of its role in Arp2/3-directed actin polymerization.