Expression and translocation of Rac2 in eosinophils during superoxide generation

Protocadherin-7 Regulates Osteoclast Differentiation through Intracellular SET-Binding Domain-Mediated RhoA and Rac1 Activation

Protocadherin-7 (Pcdh7) is a member of the non-clustered protocadherin δ1 subgroup of the cadherin superfamily. Although the cell-intrinsic role of Pcdh7 in osteoclast differentiation has been demonstrated, the molecular mechanisms of Pcdh7 regulating osteoclast differentiation remain to be determined. Here, we demonstrate that Pcdh7 contributes to osteoclast differentiation by regulating small GTPases, RhoA and Rac1, through its SET oncoprotein binding domain. Pcdh7 is associated with SET along with RhoA and Rac1 during osteoclast differentiation. Pcdh7-deficient (Pcdh7-/-) cells showed abolished RANKL-induced RhoA and Rac1 activation, and impaired osteoclast differentiation. Impaired osteoclast differentiation in Pcdh7-/- cells was restored by retroviral transduction of full-length Pcdh7 but not by a Pcdh7 mutant that lacks SET binding domain. The direct crosslink of the Pcdh7 intracellular region induced the activation of RhoA and Rac1, which was not observed when Pcdh7 lacks the SET binding domain. Additionally, retroviral transduction of the constitutively active form of RhoA and Rac1 completely restored the impaired osteoclast differentiation in Pcdh7-/- cells. Collectively, these results demonstrate that Pcdh7 controls osteoclast differentiation by regulating RhoA and Rac1 activation through the SET binding domain.

The Regulatory Role of Rho GTPases and their Substrates in Osteoclastogenesis

Pathological bone loss diseases (osteolysis, Paget's diseases) are commonly caused by the excessive differentiation and activity of osteoclasts. The Rho GTPases family members Rac1/2 (Rac1 and Rac2) have been reported for their special role in exerting multiple cellular functions during osteoclastic differentiation, which includes the most prominent function on dynamic actin cytoskeleton rearranging. Besides that, the increasing studies demonstrated that the regulating effects of Rac1/2 on the osteoclastic cytoskeletal organization are through the GEFs member Dock5. Although the amount of relevant studies on this topic is still limited, several excellent studies have been reported that extensively explored the molecular mechanisms involved in Rac1/2 and Dock5 during the osteoclastogenesis regulation, as well as their role as the therapeutic target in bone loss diseases. Herein, in this review, we aim to focus on recent advances studies for extensively understanding the role of Rho GTPases Rac1/2 and Dock5 in osteoclastogenesis, as well as their role as a potential therapeutic target in regulating osteoclastogenesis.

The role of Rho GTPases' substrates Rac and Cdc42 in osteoclastogenesis and relevant natural medicinal products study

Recently, Rho GTPases substrates include Rac (Rac1 and Rac2) and Cdc42 that have been reported to exert multiple cellular functions in osteoclasts, the most prominent of which includes regulating the dynamic actin cytoskeleton rearrangements. In addition, natural products and their molecular frameworks have a long tradition as valuable starting points for medicinal chemistry and drug discovery. Although currently, there are reports about the natural product, which could play a therapeutic role in bone loss diseases (osteoporosis and osteolysis) through the regulation of Rac1/2 and Cdc42 during osteoclasts cytoskeletal structuring. There have been several excellent studies for exploring the therapeutic potentials of various natural products for their role in inhibiting cancer cells migration and function via regulating the Rac1/2 and Cdc42. Herein in this review, we try to focus on recent advancement studies for extensively understanding the role of Rho GTPases substrates Rac1, Rac2 and Cdc42 in osteoclastogenesis, as well as therapeutic potentials of natural medicinal products for their properties on the regulation of Rac1, and/or Rac2 and Cdc42, which is in order to inspire drug discovery in regulating osteoclastogenesis.

Expression of novel "LOCGEF" isoforms of ARHGEF18 in eosinophils

Genomic, transcriptomic and proteomic databases indicate that the N-terminal 322 residues encoded by the presumptive LOC100996504 gene, which is adjacent to the ARHGEF18 guanine nucleotide exchange factor gene on chromosome 19, constitute the N-terminal portion of a 1361-residue isoform of ARHGEF18, dubbed LOCGEF-X3. LOCGEF-X3 arises from the use of a leukocyte-specific alternative transcriptional start site and splicing that bypasses the initial noncoding exon of the canonical 1015-residue ARHGEF18 isoform, p114. Eosinophil LOCGEF-X3 was amplified and cloned, recombinant LOCGEF-X3 was expressed, and anti-ARHGEF18 antibody was found to recognize a band in immunoblots of eosinophil lysates that co-migrates with recombinant LOCGEF-X3. PCR of eosinophils revealed minor amounts of transcripts for X4 and X5 isoforms of LOCGEF that arise from differential splicing and differ from the X3 isoform at their extreme N-termini. No p114 transcript or protein band was detected in eosinophils. Immunostaining with anti-ARHGEF18 antibody revealed relocalization of LOCGEF and RHOA from the periphery of round unstimulated eosinophils to the 2 poles of eosinophils polarized by treatment with IL5, CCL11, or IL33 in suspension. Canonical p114 ARHGEF18 has been implicated in maintenance of epithelial cell polarity. We suggest that the "LOC" portion of LOCGEF, which is unlike any other protein domain, has unique functions in control of polarity in activated eosinophils and other leukocytes.

Optical Tools To Study the Isoform-Specific Roles of Small GTPases in Immune Cells

Despite the 92% homology of the hematopoietic cell-specific Rac2 to the canonical isoform Rac1, these isoforms have been shown to play nonredundant roles in immune cells. To study isoform-specific dynamics of Rac in live cells, we developed a genetically encoded, single-chain FRET-based biosensor for Rac2. We also made significant improvements to our existing single-chain Rac1 biosensor. We optimized the biosensor constructs for facile expression in hematopoietic cells and performed functional validations in murine macrophage sublines of RAW264.7 cells. Rac2, Rac1, and Cdc42 have been implicated in the formation of actin-rich protrusions by macrophages, but their individual activation dynamics have not been previously characterized. We found that both Rac1 and Rac2 had similar activation kinetics, yet they had distinct spatial distributions in response to the exogenous stimulus, fMLF. Active Rac1 was mainly localized to the cell periphery, whereas active Rac2 was distributed throughout the cell, with an apparent higher concentration in the perinuclear region. We also performed an extensive morphodynamic analysis of Rac1, Rac2, and Cdc42 activities during the extension of random protrusions. We found that Rac2 appears to play a leading role in the generation of random protrusions, as we observed an initial strong activation of Rac2 in regions distal from the leading edge, followed by the activation of Rac1, a second burst of Rac2 and then Cdc42 immediately behind the leading edge. Overall, isoform-specific biosensors that have been optimized for expression should be valuable for interrogating the coordination of Rho family GTPase activities in living cells.

Agonist activation of f-actin-mediated eosinophil shape change and mediator release is dependent on Rac2

BACKGROUND

Tissue recruitment and activation of eosinophils contribute to allergic symptoms by causing airway hyperresponsiveness and inflammation. Shape changes and mediator release in eosinophils may be regulated by mammalian Rho-related guanosine triphosphatases. Of these, Rac2 is essential for F-actin formation as a central process underlying cell motility, exocytosis, and respiratory burst in neutrophils, while the role of Rac2 in eosinophils is unknown.We set out to determine the role of Rac2 in eosinophil mediator release and F-actin-dependent shape change in response to chemotactic stimuli.

METHODS

Rac2-deficient eosinophils from CD2-IL-5 transgenic mice crossed with rac2 gene knockout animals were examined for their ability to release superoxide through respiratory burst or eosinophil peroxidase by degranulation. Eosinophil shape change and actin polymerization were also assessed by flow cytometry and confocal microscopy following stimulation with eotaxin-2 or platelet-activating factor.

RESULTS

Eosinophils from wild-type mice displayed inducible superoxide release, but at a small fraction (4-5%) of human eosinophils. Rac2-deficient eosinophils showed significantly less superoxide release (p < 0.05, 26% less than wild type). Eosinophils lacking Rac2 had diminished degranulation (p < 0.05, 62% less eosinophil peroxidase) and shape changes in response to eotaxin-2 or platelet-activating factor (with 68 and 49% less F-actin formation, respectively; p < 0.02) compared with wild-type cells.

CONCLUSION

These results demonstrate that Rac2 is an important regulator of eosinophil function by contributing to superoxide production, granule protein release, and eosinophil shape change. Our findings suggest that Rho guanosine triphosphatases are key regulators of cellular inflammation in allergy and asthma.

Primary granule exocytosis in human neutrophils is regulated by Rac-dependent actin remodeling

The actin cytoskeleton regulates exocytosis in all secretory cells. In neutrophils, Rac2 GTPase has been shown to control primary (azurophilic) granule exocytosis. In this report, we propose that Rac2 is required for actin cytoskeletal remodeling to promote primary granule exocytosis. Treatment of neutrophils with low doses (< or = 10 microM) of the actin-depolymerizing drugs latrunculin B (Lat B) or cytochalasin B (CB) enhanced both formyl peptide receptor- and Ca(2+) ionophore-stimulated exocytosis. Higher concentrations of CB or Lat B, or stabilization of F-actin with jasplakinolide (JP), inhibited primary granule exocytosis measured as myeloperoxidase release but did not affect secondary granule exocytosis determined by lactoferrin release. These results suggest an obligatory role for F-actin disassembly before primary granule exocytosis. However, lysates from secretagogue-stimulated neutrophils showed enhanced actin polymerization activity in vitro. Microscopic analysis showed that resting neutrophils contain significant cortical F-actin, which was redistributed to sites of primary granule translocation when stimulated. Exocytosis and actin remodeling was highly polarized when cells were primed with CB; however, polarization was reduced by Lat B preincubation, and both polarization and exocytosis were blocked when F-actin was stabilized with JP. Treatment of cells with the small molecule Rac inhibitor NSC23766 also inhibited actin remodeling and primary granule exocytosis induced by Lat B/fMLF or CB/fMLF, but not by Ca(2+) ionophore. Therefore, we propose a role for F-actin depolymerization at the cell cortex coupled with Rac-dependent F-actin polymerization in the cell cytoplasm to promote primary granule exocytosis.

The role of Rho GTPases and SNAREs in mediator release from granulocytes

Granulocytes are defined as the population of granulated white blood cells (eosinophils, neutrophils, and basophils). These cells are involved in inflammation and contribute to the pathogenesis of allergic and inflammatory diseases. Inflammation is induced by the release of mediators from granulocytes recruited to or resident within tissues, resulting in edema, leukocyte recruitment, and tissue injury. Eosinophils and neutrophils express Rac1 and Rac2 guanosine triphosphatases (GTPases), 2 members of the Rho GTPase subfamily of ras-related GTPases. Rho GTPases are activated by receptors in the cell membrane and are proposed to function as intracellular molecular switches to regulate mediator release, including exocytosis, from granulocytes. Exocytosis involves granule fusion, which requires the binding of intracellular membrane receptors known as SNAP receptor (SNAREs; soluble N-ethylmaleimide-sensitive factor [NSF] attachment protein [SNAP] receptors). Eosinophils and neutrophils express similar SNARE isoforms that are important in granule fusion events. Together, these molecules link together to form a common signaling pathway for mediator release from granulocytes. Identifying these molecules and their function may provide novel targets for the prevention of inflammatory reactions.

The Effect of IFN-γ and TNF-α on the NADPH Oxidase System of Human Colostrum Macrophages, Blood Monocytes, and THP-1 Cells

The aim of this work was to analyze the effect of Interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on NADPH oxidase activity and gp91-phox gene expression in human colostrum macrophages (CM), peripheral blood monocytes (PBM), and myelomonocytic THP-1 cells. We also investigated the effect of IFN-gamma on the release of TNF-alpha by these cells. Our results show that under basal culture conditions, CM release more superoxide than PBM and THP-1 cells (p < 0.05). The addition of IFN-gamma, alone or in combination with TNF-alpha, increased spontaneous superoxide release by PBM and THP-1 cells (p < 0.05) and increased phorbol myristate acetate (PMA)-stimulated superoxide release by CM, PBM, and THP-1 cells (p < 0.05). The NADPH oxidase activity of THP-1 cells consistently remained lower than that of CM or PBM, despite a dramatic response to IFN-gamma and TNF-alpha. Under basal conditions, gp91-phox gene expression was significantly higher in CM and PBM compared with THP-1 cells (p < 0.05). The addition of IFN-gamma alone or in combination with TNF-alpha caused a dramatic increase in gp91-phox gene expression in THP-1 cells (p < 0.05) but not in CM or PBM. Under basal conditions or in the presence of IFN-gamma, CM released more TNF-alpha than PBM or THP-1 cells (p < 0.05). In addition, PBM released more TNF-gamma than THP-1 cells (p < 0.05). IFN-gamma did not significantly augment the release of TNF-alpha by these cells (p > 0.05). Thus, IFN-gamma and TNF-alpha induced equivalent gp91-phox gene expression in THP-1 cells compared with CM or PBM but did not bring about equivalent NADPH oxidase activity. TNF-alpha release was higher in more mature cells. This partial divergence of gp91- phox gene expression, NADPH oxidase activity, and TNF-alpha release is probably a consequence of different events of myeloid cell biology and relates at least in part to cell differentiation state.

CXCL9 inhibits eosinophil responses by a CCR3- and Rac2-dependent mechanism

Recently, inhibitory cytokine pathways for leukocyte chemoattraction and activation have been identified, but there is little insight into the operational mechanisms except for models that rely on simple receptor antagonism. We have previously identified the existence of a murine eosinophil inhibitory pathway mediated by the CXC chemokine ligand 9 (CXCL9, Mig [monokine induced by interferon-gamma]) that impressively blocks eosinophil chemoattraction and function, but the mechanism has remained elusive. We now demonstrate that Mig's inhibitory action extends beyond receptor antagonism alone. Notably, in addition to inhibiting eotaxin-induced filamentous actin (F-actin) formation and chemoattraction, Mig potently blocks platelet activating factor (PAF)- and leukotriene B4 (LTB4)-induced responses. Remarkably, Mig-treated eosinophils display an abnormal F-actin assembly in the absence of agonist stimulation. Additionally, Mig pretreatment inhibits eotaxin-induced activation of the Rho-guanosine triphosphatase (GTPase) Rac, and Rac2-deficient eosinophils demonstrate an impaired transmigration and actin polymerization response to eotaxin stimulation. Furthermore, Mig was unable to inhibit eotaxin-induced responses in Rac2-deficient eosinophils. Finally, using CCR3 gene-targeted cells, Mig's inhibitory activity is demonstrated to be mediated by CC chemokine receptor 3 (CCR3). Thus, by altering agonist-induced signaling and abrogating cytoskeletal reorganization by a Rac2-dependent mechanism, Mig markedly inhibits eosinophil responses to diverse stimuli. These results establish evidence that distinct chemokines can use CCR3 to induce opposing signals in eosinophils.

Effects of clarithromycin on inflammatory cell mediator release and survival

BACKGROUND

Clarithromycin exhibits anti-inflammatory as well as antimicrobial activity, leading to decreased symptoms of asthma and chronic sinusitis. The mode of anti-inflammatory effects of clarithromycin on inflammatory cells is not well understood. We hypothesized that clarithromycin inhibits inflammatory cell mediator release and survival.

METHODS

We investigated the effects of this drug on survival and mediator release from mast cells, eosinophils and neutrophils.

RESULTS

Human eosinophil and neutrophil respiratory burst was inhibited by up to 54% after 1-2 h pretreatment with 100 microg/ml clarithromycin. Similar doses of erythromycin did not affect respiratory burst responses in these cells. Clarithromycin at doses of up to 100 microg/ml had no effect on granule-derived mediators released from mast cells and neutrophils. However, we found that clarithromycin (100 microg/ml) induced cell death in mast cells and eosinophils after 16-48 h incubation.

CONCLUSION

Clarithromycin inhibited inflammatory cell mediator release and survival, which may enhance its ability to reduce the symptoms of chronic sinusitis and asthma.

Helicobacter pylori lipopolysaccharide activates Rac1 and transcription of NADPH oxidase Nox1 and its organizer NOXO1 in guinea pig gastric mucosal cells

Primary cultures of guinea pig gastric mucosal cells express NADPH oxidase 1 (Nox1), a homolog of gp91(phox), and produce superoxide anion (O2-) at a rate of approximately 100 nmol.mg protein(-1).h(-1) in response to Helicobacter pylori (H. pylori) lipopolysaccharide (LPS) from virulent type I strains. The upregulated O2- production also enhances H. pylori LPS-stimulated tumor necrosis factor-alpha or cyclooxygenase-2 mRNA expression, which suggests a potential role for Nox1 in the pathogenesis of H. pylori-associated diseases. The H. pylori LPS-stimulated O2- production in cultured gastric mucosal cells was inhibited by actinomycin D as well as cycloheximide, suggesting that the induction is regulated at the transcriptional level. The LPS treatment not only increased the Nox1 mRNA to a greater extent but also induced expression of the message-encoding, Nox-organizing protein 1 (NOXO1), a novel p47phox homolog required for Nox1 activity. In addition, H. pylori LPS activated Rac1; i.e., it converted Rac1 to the GTP-bound state. A phosphoinositide 3-kinase inhibitor, LY-294002, blocked H. pylori LPS-induced Rac1 activation and O2- generation without interfering with the expression of Nox1 and NOXO1 mRNA. O2- production inhibited by LY-294002 was completely restored by transfection of an adenoviral vector encoding a constitutively active Rac1 but not an inactive Rac1 or a constitutively active Cdc42. These findings indicate that Rac1 plays a crucial role in Nox1 activation. Thus the H. pylori LPS-stimulated O2- production in gastric mucosal cells appears to require two distinct events: 1) transcriptional upregulation of Nox1 and NOXO1 and 2) activation of Rac1.

Anaplasma phagocytophilum utilizes multiple host evasion mechanisms to thwart NADPH oxidase-mediated killing during neutrophil infection

Anaplasma phagocytophilum, the etiologic agent of human anaplasmosis, is a bacterial pathogen that specifically colonizes neutrophils. Neutrophils utilize the NADPH oxidase complex to generate superoxide (O(2)(-)) and initiate oxidative killing of microorganisms. A. phagocytophilum's unique tropism for neutrophils, however, indicates that it subverts and/or avoids oxidative killing. We therefore examined the effects of A. phagocytophilum infection on neutrophil NADPH oxidase assembly and reactive oxygen species (ROS) production. Following neutrophil binding, Anaplasma invasion requires at least 240 min. During its prolonged association with the neutrophil plasma membrane, A. phagocytophilum stimulates NADPH oxidase assembly, as indicated by increased cytochrome b(558) mobilization to the membrane, as well as colocalization of Rac and p22(phox). This initial stimulation taxes the host neutrophil's finite oxidase reserves, as demonstrated by time- and bacterial-dose-dependent decreases in secondary activation by N-formyl-methionyl-leucyl-phenylalanine (FMLP) or phorbol myristate acetate (PMA). This stimulation is modest, however, and does not diminish oxidase stores to nearly the extent that Escherichia coli, serum-opsonized zymosan, FMLP, or PMA do. Despite the apparent activation of NADPH oxidase, no change in ROS-dependent chemiluminescence is observed upon the addition of A. phagocytophilum to neutrophils, indicating that the bacterium may scavenge exogenous O(2)(-). Indeed, A. phagocytophilum rapidly detoxifies O(2)(-) in a cell-free system. Once internalized, the bacterium resides within a protective vacuole that excludes p22(phox) and gp91(phox). Thus, A. phagocytophilum employs at least two strategies to protect itself from neutrophil NADPH oxidase-mediated killing.

Divergence of mechanisms regulating respiratory burst in blood and sputum eosinophils and neutrophils from atopic subjects

Eosinophil respiratory burst is an important event in asthma and related inflammatory disorders. However, little is known concerning activation of the respiratory burst NADPH oxidase in human eosinophils. Conversely, neutrophils are known to assemble NADPH oxidase in intracellular and plasma membranes. We hypothesized that eosinophils and neutrophils translocate NADPH oxidase to distinct intracellular locations, consistent with their respective functions in O(2)(-)-mediated cytotoxicity. PMA-induced O(2)(-) release assayed by cytochrome c was 3.4-fold higher in atopic human eosinophils than in neutrophils, although membrane-permeable dihydrorhodamine-123 showed similar amounts of release. Eosinophil O(2)(-) release was dependent on Rac, in that it was 54% inhibited by Clostridium difficile toxin B (400-800 ng/ml). In eosinophils stimulated with PMA, a pronounced shift of cytosolic Rac to p22(phox)-positive plasma membrane was observed by confocal microscopy, whereas neutrophils directed Rac2 mainly to intracellular sites coexpressing p22(phox). Similarly, ex vivo sputum eosinophils from asthmatic subjects exhibited predominantly plasma membrane-associated immunoreactivity for Rac, whereas sputum neutrophils exhibited cytoplasmic Rac2 staining. Thus, activated sputum eosinophils, rather than neutrophils, may contribute significantly to the pathogenesis of asthma by extracellular release of tissue-damaging O(2)(-). Our findings suggest that the differential modes of NADPH oxidase assembly in these cells may have important implications for oxidant-mediated tissue injury.

Translocation of the tetraspanin CD63 in association with human eosinophil mediator release

The tetraspanin CD63 (also known as LAMP-3) has been implicated in phagocytic and intracellular lysosome-phagosome fusion events. It is also present in eosinophils, with predominant expression on crystalloid granule membrane. However, its role in eosinophil function is obscure. We hypothesized that CD63 is associated with intracellular events involved in eosinophil activation and mediator release. We used a combination of confocal immunofluorescence microscopy, flow cytometry, and secretion assays, including beta-hexosaminidase, eosinophil peroxidase, and RANTES, to examine CD63 expression, intracellular localization, and its association with cell activation and mediator release. In resting eosinophils, CD63 immunoreactivity was localized to plasma and crystalloid granule membranes. In interferon-gamma (IFN-gamma)- or C5a/CB-stimulated cells (10 minutes), intracellular CD63 appeared to shift to the cell periphery and plasma membrane, while stimulation with a cocktail of interleukin-3 (IL-3)/IL-5/granulocyte-macrophage colony-stimulating factor induced the appearance of discrete intracellular clusters of CD63 immunoreactivity. IFN-gamma induced mobilization of CD63 to the cell periphery, which coincided with selective mobilization of RANTES prior to its release, implying CD63 association with piecemeal degranulation. Agonist-induced CD63 mobilization and cell surface up-regulation was associated with beta-hexosaminidase, eosinophil peroxidase, and RANTES release. Dexamethasone as well as genistein (a broad-spectrum inhibitor of tyrosine kinases) inhibited agonist-induced intracellular mobilization of CD63 and RANTES together with cell surface up-regulation of CD63 and mediator release. This is the first report of an association between CD63 mobilization and agonist-induced selective mediator release, which may imply the involvement of CD63 in eosinophil activation and piecemeal degranulation.

Expression of eosinophil target SNAREs as potential cognate receptors for vesicle-associated membrane protein-2 in exocytosis

BACKGROUND

Exocytosis of eosinophil granule-derived mediators is thought to be an important effector response contributing to allergic inflammation. Secretion from many cell types has been shown to be dependent on the formation of a docking complex composed of soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs) located on the vesicle (v-SNAREs) and the target membrane (t-SNAREs). The SNARE isoforms VAMP-2, SNAP-23, and syntaxin-4 have been described in secretory processes in myeloid cells. Previously, we have demonstrated that the v-SNARE VAMP-2 is a candidate v-SNARE involved in eosinophil exocytosis and is localized to a pool of RANTES-positive vesicles that translocate to the cell periphery after IFN-gamma-induced degranulation.

OBJECTIVE

We sought to determine whether eosinophils express the t-SNARE isoforms SNAP-23 and syntaxin-4 as potential binding targets for VAMP-2 during exocytosis.

METHODS

Human peripheral blood eosinophils (>97%) from atopic subjects were subjected to RT-PCR and sequence analysis by using specific primers for SNAP-23 and syntaxin-4. Protein expression and localization was determined by means of Western blot analysis of eosinophil subcellular fractions and confirmed with confocal laser scanning microscopy.

RESULTS

Nucleotide sequences obtained from PCR products exhibited nearly identical (>95%) homology with reported sequences for human SNAP-23 and syntaxin-4. Both SNAP-23 and syntaxin-4 were present in plasma membranes, with some staining in endoplasmic reticulum and Golgi membranes. Negligible expression was detected in crystalloid and small secretory granules.

CONCLUSIONS

The plasma membrane-associated t-SNAREs SNAP-23 and syntaxin-4 are expressed in human eosinophils and are likely candidates for association with VAMP-2 during docking, which is followed by exocytosis. These findings support a role for SNARE molecules in eosinophil mediator release.

The functional role of rho and rho-associated coiled-coil forming protein kinase in eotaxin signaling of eosinophils

The CC chemokine eotaxin plays a pivotal role in local accumulation of eosinophils. Very little is known about the eotaxin signaling in eosinophils except the activation of the mitogen-activated protein (MAP) kinase family. The p21 G protein Rho and its substrate Rho-associated coiled-coil forming protein kinase (ROCK) regulate the formation of stress fibers and focal adhesions. In the present study, we studied the functional relevance of Rho and ROCK in eosinophils using the ROCK inhibitor (Y-27632) and exoenzyme C3, a specific Rho inhibitor. Eotaxin stimulates activation of Rho A and ROCK II in eosinophils. Exoenzyme C3 almost completely inhibited the ROCK activity, indicating that ROCK is downstream of Rho. We then examined the role of Rho and ROCK in eosinophil chemotaxis. The eotaxin-induced eosinophil chemotaxis was significantly inhibited by exoenzyme C3 or Y-27632. Because extracellular signal-regulated kinase (ERK)1/2 and p38 MAP kinases are activated by eotaxin and are critical for eosinophil chemotaxis, we investigated whether Rho and ROCK are upstream of these MAP kinases. C3 partially inhibited eotaxin-induced phosphorylation of ERK1/2 but not p38. In contrast, neither ERK1/2 nor p38 phosphorylation was abrogated by Y-27632. Both C3 and Y-27632 reduced reactive oxygen species production from eosinophils. We conclude that both Rho and ROCK are important for eosinophil chemotaxis and reactive oxygen species production. There is a dichotomy of downstream signaling pathways of Rho, namely, Rho-ROCK and Rho-ERK pathways. Taken together, eosinophil chemotaxis is regulated by multiple signaling pathways that involve at least ROCK, ERK, and p38 MAP kinase.

Interactions between NADPH oxidase-related proton and electron currents in human eosinophils

1. Proton and electron currents in human eosinophils were studied using the permeabilized-patch voltage-clamp technique, with an applied NH4+ gradient to control pH(i). 2. Voltage-gated proton channels in unstimulated human eosinophils studied with the permeabilized-patch approach had properties similar to those reported in whole-cell studies. 3. Superoxide anion (O2-) release assessed by cytochrome c reduction was compared in human eosinophils and neutrophils stimulated by phorbol myristate acetate (PMA). PMA-stimulated O2 release was more transient and the maximum rate was three times greater in eosinophils. 4. In PMA-activated eosinophils, the H+ current amplitude (I(H)) at +60 mV increased 4.7-fold, activation was 4.0 times faster, deactivation (tail current decay) was 5.4 times slower, the H+ conductance-voltage (g(H)-V) relationship was shifted -43 mV, and diphenylene iodinium (DPI)-inhibitable inward current reflecting electron flow through NADPH oxidase was activated. The data reveal that PMA activates the H+ efflux during the respiratory burst by modulating the properties of H+ channels, not simply as a result of NADPH oxidase activity. 5. The electrophysiological response of eosinophils to PMA resembled that reported in human neutrophils, but PMA activated larger proton and electron currents in eosinophils and the response was more transient. 6. ZnCl2 slowed the activation of H+ currents and shifted the g(H)-V relationship to more positive voltages. These effects occurred at similar ZnCl2 concentrations in eosinophils before and after PMA stimulation. These data are compatible with the existence of a single type of H+ channel in eosinophils that is modulated during the respiratory burst.

Fusion protein vesicle-associated membrane protein 2 is implicated in IFN-gamma-induced piecemeal degranulation in human eosinophils from atopic individuals

BACKGROUND

Exocytosis is an integral event during IFN-gamma-induced piecemeal degranulation in eosinophils. In many tissues soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), including vesicle-associated membrane protein (VAMP), act as specific intracellular receptors to allow granule fusion with the membrane during degranulation. However, the mechanisms underlying eosinophil piecemeal degranulation induced by IFN-gamma are not well understood.

OBJECTIVE

We sought to assess whether eosinophils express the vesicular SNARE protein VAMP-2 and to determine the involvement of VAMP-2 in IFN-gamma-induced piecemeal degranulation.

METHODS

Human peripheral blood eosinophils (> or =97%) from atopic subjects were subjected to RT-PCR and sequence analysis with specific primers for VAMP-2 mRNA. Western blotting and flow cytometric analysis were carried out to confirm the identity of VAMP-2 and its susceptibility to cleavage by tetanus toxin. Confocal laser scanning microscopy imaging was conducted on double-labeled cytospin preparations of eosinophils at 0, 5, 10, 30, and 60 minutes and 16 hours of IFN-gamma (500 U/mL) stimulation.

RESULTS

Eosinophils expressed VAMP-2 mRNA (n = 4 donors), which exhibited 100% homology with human VAMP-2 cDNA on sequencing. Eosinophils were also found to express tetanus toxin-sensitive VAMP-2 protein. RANTES and VAMP-2 immunofluorescence were observed to colocalize to similar intracellular structures by means of confocal imaging. IFN-gamma induced a rapid translocation of VAMP-2(+) organelles toward the cell membrane in correlation with RANTES.

CONCLUSIONS

These findings suggest that exocytosis in human eosinophils is regulated by SNAREs, with a specific role indicated for VAMP-2 in piecemeal degranulation.

Cloning and characterization of bovine low molecular weight GTPases (Rac1 and Rac2) and rho GDP-dissociation inhibitor 2 (D4-GDI)

GTPases of the Rho family play important roles in human leukocyte signal transduction pathways; however, little is known about the function of these proteins in bovine cells. In the present studies, we isolated molecular clones of bovine Rac1, Rac2, and the Rac/Rho GTPase regulatory protein D4-GDP dissociation inhibitor (D4-GDI) from a bovine bone marrow cDNA library. These clones contained complete open reading frames, encoding 192, 192, and 200 amino acids, respectively. Comparison of the bovine amino acid sequences with those of other species demonstrated a high degree of identity of these proteins across all species, suggesting that these proteins likely play conserved functional roles in bovine leukocyte signal transduction pathways. Comparative Western blotting of these proteins in human and bovine neutrophil cytosol demonstrated that Rac2 was the predominant Rac species and that D4-GDI was the predominant GDI species in bovine neutrophil cytosol. Despite the high degree of homology between human and bovine Rac2, some of the anti-peptide antibody probes prepared against human Rac2 failed to recognize the bovine homologue. We also showed by subcellular fractionation techniques that Rac2 is localized primarily to the cytosolic compartment of resting bovine neutrophils, but is translocated to the plasma membrane after stimulation with PMA. These findings suggest that Rac2 does play a role in bovine neutrophil activation. In addition, these data will be helpful in developing more specific probes for investigating the role of these proteins in bovine leukocyte signal transduction pathways and for studying various inflammatory diseases in cattle.