Activation of cAMP-dependent protein kinase is necessary for actin rearrangements in human neutrophils during phagocytosis

Uncovering the Cyclic AMP Signaling Pathway of the Protozoan Parasite Entamoeba histolytica and Understanding Its Role in Phagocytosis

Second messenger signaling controls a surprisingly diverse range of processes in several eukaryotic pathogens. Molecular machinery and pathways involving these messengers thus hold tremendous opportunities for therapeutic interventions. Relative to Ca²⁺ signaling, the knowledge of a crucial second messenger cyclic AMP (cAMP) and its signaling pathway is very scant in the intestinal parasite Entamoeba histolytica. In the current study, mining the available genomic resources, we have for the first time identified the cAMP signal transduction pathway of E. histolytica. Three heptahelical proteins with variable G-protein-coupled receptor domains, heterotrimeric G-proteins (Gα, Gβ, and Gγ subunits), soluble adenylyl cyclase, cyclase-associated protein, and enzyme carbonic anhydrase were identified in its genome. We could also identify several putative candidate genes for cAMP downstream effectors such as protein kinase A, A-kinase anchoring proteins, and exchange protein directly activated by the cAMP pathway. Using specific inhibitors against key identified targets, we could observe changes in the intracellular cAMP levels as well as defect in the rate of phagocytosis of red blood cells by the parasite E. histolytica. We thus strongly believe that characterization of some of these unexplored crucial signaling determinants will provide a paradigm shift in understanding the pathogenicity of this organism.

Neuroendocrine immune-regulatory of a neuropeptide ChGnRH from the Hongkong oyster, Crassostrea Hongkongensis

It is increasingly appreciated that neuroendocrine-immune interactions hold the key to understand the complex immune system. In this study, we explored the role of a reproductive regulation-related hormone, GnRH, in the regulation of immunity in Hong Kong oysters. We found that vibrio bacterial strains injection increased the expression of ChGnRH. Moreover, ChGnRH neuropeptide promotes the phagocytic ability and bacterial clearance effect of hemocytes which regarded to be the central immune organ. The content of cAMP after incubation with ChGnRH peptide was increased, which could be blocked by adenylyl cyclase inhibitor SQ 22,536. Furthermore, the stimulated effect of ChGnRH peptide on the phagocytosis and bacterial clearance was also blocked by SQ 22,536, H89 and enzastaurin, strongly demonstrating that cAMP dependent PKA and PKC signaling pathway was involved in ChGnRH mediated immune regulation. In conclusion, this study confirms the presence of neuroendocrine-immune regulatory system in marine invertebrates, which contributes to understand the complexity of oyster immune defense system.

Lipid Mediators in Inflammation

Lipids are potent signaling molecules that regulate a multitude of cellular responses, including cell growth and death and inflammation/infection, via receptor-mediated pathways. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. This diversity arises from their synthesis, which occurs via discrete enzymatic pathways and because they elicit responses via different receptors. This review will collate the bioactive lipid research to date and summarize the major pathways involved in their biosynthesis and role in inflammation. Specifically, lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins, and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins, and maresins) will be discussed herein.

Activation of endogenous anti-inflammatory mediator cyclic AMP attenuates acute pyelonephritis in mice induced by uropathogenic Escherichia coli

The pathogenesis of pyelonephritis caused by uropathogenic Escherichia coli (UPEC) is not well understood. Here, we show that besides UPEC virulence, the severity of the host innate immune response and invasion of renal epithelial cells are important pathogenic factors. Activation of endogenous anti-inflammatory mediator cAMP significantly attenuated acute pyelonephritis in mice induced by UPEC. Administration of forskolin (a potent elevator of intracellular cAMP) reduced kidney infection (ie, bacterial load, tissue destruction); this was associated with attenuated local inflammation, as evidenced by the reduction of renal production of proinflammatory mediators, renal infiltration of inflammatory cells, and renal myeloperoxidase activity. In primary cell culture systems, forskolin not only down-regulated UPEC-stimulated production of proinflammatory mediators by renal tubular epithelial cells and inflammatory cells (eg, monocyte/macrophages) but also reduced bacterial internalization by renal tubular epithelial cells. Our findings clearly indicate that activation of endogenous anti-inflammatory mediator cAMP is beneficial for controlling UPEC-mediated acute pyelonephritis in mice. The beneficial effect can be explained at least in part by limiting excessive inflammatory responses through acting on both renal tubular epithelial cells and inflammatory cells and by inhibiting bacteria invasion of renal tubular epithelial cells.

Myosins 1 and 6, myosin light chain kinase, actin and microtubules cooperate during antibody-mediated internalisation and trafficking of membrane-expressed viral antigens in feline infectious peritonitis virus infected monocytes

Monocytes infected with feline infectious peritonitis virus, a coronavirus, express viral proteins in their plasma membranes. Upon binding of antibodies, these proteins are quickly internalised through a new clathrin- and caveolae-independent internalisation pathway. By doing so, the infected monocytes can escape antibody-dependent cell lysis. In the present study, we investigated which kinases and cytoskeletal proteins are of importance during internalisation and subsequent intracellular transport. The experiments showed that myosin light chain kinase (MLCK) and myosin 1 are crucial for the initiation of the internalisation. With co-localisation stainings, it was found that MLCK and myosin 1 co-localise with antigens even before internalisation started. Myosin 6 co-localised with the internalising complexes during passage through the cortical actin, were it might play a role in moving or disintegrating actin filaments, to overcome the actin barrier. One minute after internalisation started, vesicles had passed the cortical actin, co-localised with microtubules and association with myosin 6 was lost. The vesicles were further transported over the microtubules and accumulated at the microtubule organising centre after 10 to 30 min. Intracellular trafficking over microtubules was mediated by MLCK, myosin 1 and a small actin tail. Since inhibiting MLCK with ML-7 was so efficient in blocking the internalisation pathway, this target can be used for the development of a new treatment for FIPV.

Combined activity of post-exercise concentrations of NA and eHsp72 on human neutrophil function: role of cAMP

Extracellular heat shock proteins of 72 kDa (eHsp72) and noradrenaline (NA) can act as "danger signals" during exercise-induced stress by activating neutrophil function (chemotaxis, phagocytosis, and fungicidal capacity). In addition, post-exercise concentrations of NA increase the expression and release of Hsp72 by human neutrophils, and adrenoreceptors and cAMP are involved in the stimulation of neutrophils by eHsp72. This suggests an interaction between the two molecules in the modulation of neutrophils during exercise-induced stress. Given this context, the aim of the present investigation was to study the combined activity of post-exercise circulating concentrations of NA and eHsp72 on the neutrophil phagocytic process, and to evaluate the role of cAMP as intracellular signal in these effects. Results showed an accumulative stimulation of chemotaxis induced by NA and eHsp72. However, while NA and eHsp72, separately, stimulate the phagocytosis and fungicidal activity of neutrophils, when they act together they do not modify these capacities of neutrophils. Similarly, post-exercise concentrations of NA and eHsp72 separately increased the intracellular level of cAMP, but NA and eHsp72 acting together did not modify the intracellular concentration of cAMP. These results confirm that cAMP can be involved in the autocrine/paracrine physiological regulation of phagocytosis and fungicidal capacity of human neutrophils mediated by NA and eHsp72 in the context of exercise-induced stress.

Regulation of cytotoxic T lymphocyte antigen 4 by cyclic AMP

Recent studies indicate that cyclic AMP (cAMP) induces cytotoxic T lymphocyte antigen (CTLA) 4. CTLA4 is expressed in T cells, and is a negative regulator of T cell activation. CTLA4 expression is regulated by T cell receptor plus CD28 (adaptive immune signaling) at both the transcriptional and post-transcriptional level. Here, we examine the pathways by which cAMP regulates CTLA4 expression, focusing on transcriptional activation. Elevating intracellular cAMP levels by cell-permeable cAMP analogs, the adenylyl cyclase activator, forskolin, or phosphodiesterase inhibitors increases CTLA4 mRNA expression in EL4 murine T cells and primary CD4(+) T cells. Activation of protein kinase A (using the protein kinase A-selective agonist, N6-phenyladenosine-cAMP), but not exchange proteins activated by cAMP (using the exchange proteins activated by cAMP-selective 8-pCPT-2Me-cAMP), increases CTLA4 promoter activity. Mutation constructs of the CTLA4 promoter uncover an enhancer binding site located within the -150 to -130 bp region relative to the transcription start site. Promoter analysis and chromatin immunoprecipitation assays suggest that cAMP response element-binding is a putative transcription factor induced by cAMP. We have previously shown that CTLA4 mediates decreased pulmonary inflammation in an LPS-induced murine model of acute lung injury (ALI). We observed that LPS can induce CTLA4 transcription via the same cAMP-inducible promoter region. The immunosuppressant, rapamycin, decreases cAMP and LPS-induced CTLA4 transcription in vitro. In vivo, LPS induces cAMP accumulation in bronchoalveolar lavage fluid, bronchoalveolar lavage cells, and lung tissues in ALI. We demonstrate that rapamycin decreases cAMP accumulation and CTLA4 expression in ALI. Together, these data suggest that cAMP may negatively regulate pulmonary inflammatory responses in vivo and in vitro by altering CTLA4 expression.

PTEN directly activates the actin depolymerization factor cofilin-1 during PGE2-mediated inhibition of phagocytosis of fungi

Macrophage ingestion of the yeast Candida albicans requires its recognition by multiple receptors and the activation of diverse signaling programs. Synthesis of the lipid mediator prostaglandin E(2) (PGE(2)) and generation of cyclic adenosine monophosphate (cAMP) also accompany this process. Here, we characterized the mechanisms underlying PGE(2)-mediated inhibition of phagocytosis and filamentous actin (F-actin) polymerization in response to ingestion of C. albicans by alveolar macrophages. PGE(2) suppressed phagocytosis and F-actin formation through the PGE(2) receptors EP2 and EP4, cAMP, and activation of types I and II protein kinase A. Dephosphorylation and activation of the actin depolymerizing factor cofilin-1 were necessary for these inhibitory effects of PGE(2). PGE(2)-dependent activation of cofilin-1 was mediated by the protein phosphatase activity of PTEN (phosphatase and tensin homolog deleted on chromosome 10), with which it directly associated. Because enhanced production of PGE(2) accompanies many immunosuppressed states, the PTEN-dependent pathway described here may contribute to impaired antifungal defenses.

Adrenoreceptors are involved in the stimulation of neutrophils by exercise-induced circulating concentrations of Hsp72: cAMP as a potential "intracellular danger signal"

Recently, the terms "stress mediators" or "danger signals" have come to be used to describe endogenous molecules that can be released in stress situations and activate the innate immune system even in the absence of antigenic stimuli. There is evidence suggesting that extracellular heat shock proteins of 72  kDa (eHsp72), together with noradrenaline (NA), are candidates as danger signals during exercise-induced stress, interacting in the activation of neutrophils. Previous studies have shown that the post-exercise circulating concentration of eHsp72 activates the phagocytic process of neutrophils with the participation of toll-like receptor 2, but that other receptors must also be involved. The present investigation evaluates the role of adrenoreceptors in the activation of the chemotaxis, phagocytosis, and fungicidal capacity of neutrophils by the post-exercise circulating concentration of eHsp72. The results showed that intact α- and β-adrenoreceptors are necessary for the stimulation of all stages of the phagocytic process by eHsp72. Also, eHsp72 increased the intracellular levels of cAMP, suggesting that it is an "intracellular danger signal" during stress-induced activation of neutrophils mediated by extracellular heat shock proteins. These results can contribute to better understanding the mechanisms involved in the regulation of the innate immune response mediated by "danger signals" during exercise, and probably during other stress situations.

Old and new generation lipid mediators in acute inflammation and resolution

Originally regarded as just membrane constituents and energy storing molecules, lipids are now recognised as potent signalling molecules that regulate a multitude of cellular responses via receptor-mediated pathways, including cell growth and death, and inflammation/infection. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. The diversity of their actions arises because such metabolites are synthesised via discrete enzymatic pathways and because they elicit their response via different receptors. This review will collate the bioactive lipid research to date and summarise the findings in terms of the major pathways involved in their biosynthesis and their role in inflammation and its resolution. It will include lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins and maresins).

Adenosine suppresses lipopolysaccharide-induced tumor necrosis factor-alpha production by murine macrophages through a protein kinase A- and exchange protein activated by cAMP-independent signaling pathway

Adenosine is generated during tissue hypoxia and stress, which reduces inflammation by suppressing the activity of most immune cells. Among its various actions, adenosine suppresses the production of proinflammatory cytokines including tumor necrosis factor (TNF)-alpha, through the cAMP-elevating A(2A) adenosine receptor (AR) subtype. In this study, we examined the signaling mechanisms by which A(2A)AR activation inhibits TNF-alpha production in thioglycollate-elicited mouse peritoneal macrophages. Pretreating murine macrophages with the nonselective AR agonist adenosine-5'-N-ethylcarboxamide (NECA), the A(2A)AR agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), or the cAMP-elevating agent forskolin reduced TNF-alpha production in response to lipopolysaccharide (LPS) by greater than 60%. All of these agents increased cAMP production in macrophages and activated protein kinase A (PKA). However, we were surprised to find that treating macrophages with three different PKA inhibitors or small interfering RNA-mediated knockdown of the exchange protein activated by cAMP (Epac-1) failed to block the suppressive actions of NECA or forskolin on LPS-induced TNF-alpha release. Instead, okadaic acid was effective at low concentrations that selectively inhibit protein serine/threonine phosphatases. Subsequent studies showed that NECA and forskolin decreased LPS-induced steady-state TNF-alpha mRNA levels; this effect was due to a decreased rate of transcription based on assays examining the rate of generation of primary TNF-alpha transcripts. Treatment with NECA or forskolin did not interfere with LPS-induced translocation or DNA binding of the RelA/p65 subunit of nuclear factor-kappaB or phosphorylation of inhibitor of nuclear factor-kappaB-alpha, extracellular signal-regulated kinase 1/2, c-Jun NH(2)-terminal kinase, or p38 kinase. Our results suggest that AR activation inhibits LPS-induced TNF-alpha production by murine macrophages at the level of gene transcription through a unique cAMP-dependent, but PKA- and Epac-independent, signaling pathway involving protein phosphatase activity.

The cannabinoid receptor 2 is critical for the host response to sepsis

Leukocyte function can be modulated through the cannabinoid receptor 2 (CB2R). Using a cecal ligation and puncture (CLP) model of sepsis, we examined the role of the CB2R during the immune response to an overwhelming infection. CB2R-knock out (KO) mice showed decreased survival as compared with wild-type mice. CB2R-KO mice also had increased serum IL-6 and bacteremia. Twenty-four hours after CLP, the CB2R-deficient mice had increased lung injury. Additionally, CB2R-deficiency led to increased neutrophil recruitment, decreased neutrophil activation, and decreased p38 activity at the site of infection. Consistent with a novel role for CB2R in sepsis, CB2R-agonist treatment in wild-type mice increased the mean survival time in response to CLP. Treatment with CB2R-agonist also decreased serum IL-6 levels, bacteremia, and damage to the lungs compared with vehicle-treated mice. Finally, the CB2R agonist decreased neutrophil recruitment, while increasing neutrophil activation and p38 activity at the site of infection compared with vehicle-treated mice. These data suggest that CB2R is a critical regulator of the immune response to sepsis and may be a novel therapeutic target.

Bacillus anthracis edema toxin impairs neutrophil actin-based motility

Inhalation anthrax results in high-grade bacteremia and is accompanied by a delay in the rise of the peripheral polymorphonuclear neutrophil (PMN) count and a paucity of PMNs in the infected pleural fluid and mediastinum. Edema toxin (ET) is one of the major Bacillus anthracis virulence factors and consists of the adenylate cyclase edema factor (EF) and protective antigen (PA). Relatively low concentrations of ET (100 to 500 ng/ml of PA and EF) significantly impair human PMN chemokinesis, chemotaxis, and ability to polarize. These changes are accompanied by a reduction in chemoattractant-stimulated PMN actin assembly. ET also causes a significant decrease in Listeria monocytogenes intracellular actin-based motility within HeLa cells. These defects in actin assembly are accompanied by a >50-fold increase in intracellular cyclic AMP and a >4-fold increase in the phosphorylation of protein kinase A. We have previously shown that anthrax lethal toxin (LT) also impairs neutrophil actin-based motility (R. L. During, W. Li, B. Hao, J. M. Koenig, D. S. Stephens, C. P. Quinn, and F. S. Southwick, J. Infect. Dis. 192:837-845, 2005), and we now find that LT combined with ET causes an additive inhibition of PMN chemokinesis, polarization, chemotaxis, and FMLP (N-formyl-met-leu-phe)-induced actin assembly. We conclude that ET alone or combined with LT impairs PMN actin assembly, resulting in paralysis of PMN chemotaxis.

Crosstalk between prostaglandin E2 and leukotriene B4 regulates phagocytosis in alveolar macrophages via combinatorial effects on cyclic AMP

Eicosanoid lipid mediators, including prostaglandin E(2) (PGE(2)) and leukotrienes (LTs) B(4) and D(4), are produced in abundance in the infected lung. We have previously demonstrated that individually, PGE(2) suppresses while both classes of LTs augment alveolar macrophage (AM) innate immune functions. In this study, we sought to more appropriately model the milieu at a site of infection by studying the in vitro effects of these lipid mediators on Fc gammaR-mediated phagocytosis when they are present in combination. Consistent with their individual actions, both LTB(4) and LTD(4) opposed the suppressive effect of PGE(2) on phagocytosis, but only LTB(4) did so by mitigating the stimulatory effect of PGE(2) on intracellular cAMP production. Unexpectedly, we observed that IgG-opsonized targets themselves elicited a dose-dependent reduction in intracellular cAMP in AMs, but this was not observed in peritoneal macrophages or elicited peritoneal neutrophils; this effect in AMs was completely abolished by treatment with the LT synthesis inhibitor AA861, the BLT receptor 1 antagonist CP 105,696, and the G alpha i inhibitor pertussis toxin. Of two downstream cAMP effectors, protein kinase A and exchange protein activated by cAMP, the ability of PGE(2) to activate the latter but not the former was abrogated by both LTs B(4) and D(4). Taken together, our results indicate that both classes of LTs oppose the immune suppressive actions of PGE(2), with the stimulatory actions of LTB(4) reflecting combinatorial modulation of intracellular cAMP and those of LTD(4) being cAMP independent.

Cyclic AMP: master regulator of innate immune cell function

Cyclic adenosine monophosphate (cAMP) was the original "second messenger" to be discovered. Its formation is promoted by adenylyl cyclase activation after ligation of G protein-coupled receptors by ligands including hormones, autocoids, prostaglandins, and pharmacologic agents. Increases in intracellular cAMP generally suppress innate immune functions, including inflammatory mediator generation and the phagocytosis and killing of microbes. The importance of the host cAMP axis in regulating antimicrobial defense is underscored by the fact that microbes have evolved virulence-enhancing strategies that exploit it. Many clinical situations that predispose to infection are associated with increases in cAMP, and therapeutic strategies to interrupt cAMP generation or actions have immunostimulatory potential. This article reviews the anatomy of the cAMP axis, the mechanisms by which it controls phagocyte immune function, microbial strategies to dysregulate it, and its clinical relevance.

Activation of phosphatase and tensin homolog on chromosome 10 mediates the inhibition of FcgammaR phagocytosis by prostaglandin E2 in alveolar macrophages

PGE2 has important inhibitory effects on the macrophage host defense functions of phagocytosis and killing, yet the molecular mechanisms involved remain to be fully elucidated. PGE2 causes an elevation of cAMP in alveolar macrophages (AMs), which in turn activates the cAMP effector targets, protein kinase A and the exchange protein activated by cAMP (Epac)-1. We now report that FcgammaR-induced PI3K/Akt and ERK-1/2 activation are inhibited by PGE2 in AMs. By specifically inhibiting the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) in AMs, we attenuated the inhibitory effects of both PGE2 and a specific Epac-1 agonist (8-pCPT-2'-O-Me-cAMP) on FcgammaR-mediated phagocytosis and Akt/ERK-1/2 activation; PTEN inhibition also decreased PGE2-induced suppression of bacterial killing by AMs. Moreover, PGE2 and the Epac-1 agonist induced an increase in PTEN lipid phosphatase activity, and this was associated with decreased tyrosine phosphorylation on PTEN-a mechanism known to regulate PTEN activity. Using a pharmacological approach, we demonstrated a role for Src homology 2-containing protein tyrosine phosphatase-1 in the PGE2-induced tyrosine dephosphorylation of PTEN. Collectively, these data reveal that PGE2, via Epac-1 activation, enhances SHP-1 activity, resulting in increased PTEN activity. We suggest that this mechanism contributes to the ability of PGE2 to inhibit PI3K-dependent innate immune signaling in primary macrophages.

Regulation of VASP serine 157 phosphorylation in human neutrophils after stimulation by a chemoattractant

Vasodilator-stimulated phosphoprotein (VASP) is a cAMP-dependent protein kinase A (PKA) substrate, which links cellular signaling to cytoskeletal organization and cellular movement. VASP is phosphorylated by PKA on serine 157 (Ser 157), which is required for VASP function in platelet adhesion and fibroblast motility. Our hypothesis is that PKA regulates neutrophil migration through VASP Ser 157 phosphorylation. The objective of this study was to characterize VASP Ser 157 phosphorylation in chemoattractant-stimulated neutrophils. fMLF, IL-8, leukotriene B(4), or platelet-activating factor stimulation resulted in an initial increase in VASP Ser 157 phosphorylation, which was maximal by 30 s and was followed by a return to baseline Ser 157 phosphorylation by 10 min. In contrast, stimulation with the nonchemoattractant, proinflammatory cytokine TNF-alpha did not affect Ser 157 phosphorylation. The kinetics of fMLF-induced VASP Ser 157 phosphorylation levels closely matched the kinetics of the fold-change in F-actin levels in fMLF-stimulated neutrophils. fMLF-induced Ser 157 phosphorylation was abolished by pretreatment with the PKA inhibitor H89 and the adenylyl cyclase inhibitor SQ22536. In contrast, fMLF-induced Ser 157 phosphorylation was unaffected by the PKC inhibitors calphostin and staurosporine, the PKG inhibitors Rp-8-pCPT-cGMP and KT5823, and the calmodulin-dependent protein kinase II inhibitor KN-62. Inhibition of adhesion with EDTA or the anti-beta2-integrin antibody IB4 did not alter fMLF-induced VASP phosphorylation or dephosphorylation. These data show that chemoattractant stimulation of human neutrophils induces a rapid and transient PKA-dependent VASP Ser 157 phosphorylation. Adhesion does not appear to be an important regulator of the state of VASP Ser 157 phosphorylation in chemoattractant-stimulated neutrophils.

Inactivation of Cdc42 Is Necessary for Depolymerization of Phagosomal F-Actin and Subsequent Phagosomal Maturation

Phagocytosis is a complex process involving the activation of various signaling pathways, such as the Rho GTPases, and the subsequent reorganization of the actin cytoskeleton. In neutrophils, Rac and Cdc42 are activated during phagocytosis but less is known about the involvement of these GTPases during the different stages of the phagocytic process. The aim of this study was to elucidate the role of Cdc42 in phagocytosis and the subsequent phagosomal maturation. Using a TAT-based protein transduction technique, we introduced dominant negative and constitutively active forms of Cdc42 into neutrophil-like HL60 (human leukemia) cells that were allowed to phagocytose IgG-opsonized yeast particles. Staining of cellular F-actin in cells transduced with constitutively active Cdc42 revealed that the activation of Cdc42 induced sustained accumulation of periphagosomal actin. Moreover, the fusion of azurophilic granules with the phagosomal membrane was prevented by the accumulated F-actin. In contrast, introducing dominant negative Cdc42 impaired the translocation per se of azurophilic granules to the periphagosomal area. These results show that efficient phagosomal maturation and the subsequent eradication of ingested microbes in human neutrophils is dependent on a strictly regulated Cdc42. To induce granule translocation, Cdc42 must be in its active state but has to be inactivated to allow depolymerization of the F-actin cage around the phagosome, a process essential for phagolysosome formation.

Glycosyl-phosphatidyl-inositol-defective granulocytes from paroxysmal nocturnal haemoglobinuria patients show increased bacterial ingestion but reduced respiratory burst induction

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by the emergence of a GPI-defective clonal hematopoiesis. Its clinical features are hemolytic anemia, cytopenia, and thrombosis. Circulating monocytes and granulocytes are largely GPI-defective in PNH patients. This study aims to investigate the granulocyte functional properties in PNH. We analyzed bacterial-dependent intracellular ingestion and the consequent activation of oxidative burst in GPI-defective granulocytes from four neutropenic PNH patients. Our data show a significant increase in the ability of GPI-defective granulocytes to ingest opsonized bacteria. In addition, an impaired respiratory burst effectiveness in response to two independent bacterial stimuli, the N-formyl-MetLeuPhe (fMLP) synthetic bacterial peptide and E. coli, was revealed. The occurrence of neutropenia and the severe impairment of oxidative burst, occurring in chronic granulomatosis disease, were unable to significantly affect phagocytosis. Thus, additional mechanisms, able to differentially affect ingestion ability and respiratory burst effectiveness, have to be hypothesized. The reduced burst effectiveness of GPI-defective granulocytes was maintained after treatment with phorbol 12-myristate 13-acetate, a pharmacological stimulus able to extensively recruit and to trigger intracellular protein kinase C (PKC). Moreover, blocking of PKC has been observed to severely affect granulocyte respiratory burst with a mild effect on the phagocytosis. These data suggest a role for a modulation of intracellular PKC in the pathogenesis of the impaired granulocyte oxidative burst.

CFTR regulates phagosome acidification in macrophages and alters bactericidal activity

Acidification of phagosomes has been proposed to have a key role in the microbicidal function of phagocytes. Here, we show that in alveolar macrophages the cystic fibrosis transmembrane conductance regulator Cl- channel (CFTR) participates in phagosomal pH control and has bacterial killing capacity. Alveolar macrophages from Cftr-/- mice retained the ability to phagocytose and generate an oxidative burst, but exhibited defective killing of internalized bacteria. Lysosomes from CFTR-null macrophages failed to acidify, although they retained normal fusogenic capacity with nascent phagosomes. We hypothesize that CFTR contributes to lysosomal acidification and that in its absence phagolysosomes acidify poorly, thus providing an environment conducive to bacterial replication.

cAMP cascade (PKA, Epac, adenylyl cyclase, Gi, and phosphodiesterases) regulates myelin phagocytosis mediated by complement receptor-3 and scavenger receptor-AI/II in microglia and macrophages

The removal by phagocytosis of degenerated myelin is central for repair in Wallerian degeneration that follows traumatic injury to axons and in autoimmune demyelinating diseases (e.g., multiple sclerosis). We tested for roles played by the cAMP cascade in the regulation of myelin phagocytosis mediated by complement receptor-3 (CR3/MAC-1) and scavenger receptor-AI/II (SRAI/II) separately and combined in mouse microglia and macrophages. Components of the cAMP cascade tested are cAMP, adenylyl cyclase (AC), Gi, protein kinase A (PKA), exchange protein directly activated by cAMP (Epac), and phosphodiesterases (PDE). PKA inhibitors H-89 and PKI(14-22) amide inhibited phagocytosis at normal operating cAMP levels (i.e., those occurring in the absence of reagents that alter cAMP levels), suggesting activation of phagocytosis through PKA at normal cAMP levels. Phagocytosis was inhibited by reagents that elevate endogenous cAMP levels to above normal: Gi-inhibitor Pertussis toxin (PTX), AC activator Forskolin, and PDE inhibitors IBMX and Rolipram. Phagocytosis was inhibited also by cAMP analogues whose addition mimics abnormal elevations in endogenous cAMP levels: nonselective 8-bromo-cAMP, PKA-specific 6-Benz-cAMP, and Epac-specific 8-CPT-2'-O-Me-cAMP, suggesting that abnormal high cAMP levels inhibit phagocytosis through PKA and Epac. Altogether, observations suggest a dual role for cAMP and PKA in phagocytosis: activation at normal cAMP levels and inhibition at higher. Furthermore, a balance between Gi-controlled cAMP production by AC and cAMP degradation by PDE maintains normal operating cAMP levels that enable efficient phagocytosis.

Phagocytosis by Lymnaea stagnalis haemocytes: A potential role for phosphatidylinositol 3-kinase but not protein kinase A

The molecular events that regulate phagocytosis, an important innate immune response, in invertebrate defence cells (haemocytes) are poorly understood. Lymnaea stagnalis haemocytes were used as a model to elucidate the role of cell signalling pathways in phagocytosis by molluscan defence cells. The phosphatidylinositol 3-kinase (PI3-K) inhibitor, LY294002, significantly impaired haemocyte phagocytic activity in a dose-responsive manner with 10 microM LY294002 reducing internalization of fluorescent-conjugated Escherichia coli by 62% (P < or = 0.001). In contrast, the protein kinase A (PKA) inhibitor KT5720 was without effect. Therefore, PI3-K, but not PKA, appears to control phagocytosis by haemocytes in these gastropod molluscs.

Signalization and cytoskeleton activity through myosin IB during the early steps of phagocytosis in Entamoeba histolytica: a proteomic approach

Phagocytosis of human cells is a crucial activity for the virulence of the human parasite Entamoeba histolytica. This protozoan invades and destroys the intestine by killing and phagocytosing epithelial cells, erythrocytes and cells from the immune system. In this study, we used magnetic beads covered with proteins from human serum as a model system to study the early events involved in phagocytosis by E. histolytica. We validated the system showing that the beads uptake triggered the activation of the actin-myosin cytoskeleton and involved a PI3-kinase as previously described for erythrophagocytosis. We purified early phagosomes from wild-type (WT) amoeba and from parasites that overproduced myosin IB (MyoIB+), the unique unconventional myosin of E. histolytica. The MyoIB+ cells exhibit a slower and more synchronized uptake process than the WT strain. Proteomic analysis by liquid chromatography and tandem mass spectroscopy (LC-MS/MS) of the WT and MyoIB+ phagosomes allowed us to identify, for the first time, molecular actors involved in the early step of the uptake process. These include proteins involved in cytoskeleton activity, signalling, endocytosis, lytic activity and cell surface proteins. Interestingly, the proteins that we found specifically recruited on the phagosomes from the MyoIB+ strain were previously described in other eukarytotic cells, as involved in the regulation of cortical F-actin dynamics, such as alpha-actinin and formins. This proteomics approach allows a step further towards the understanding of the molecular mechanisms involved in phagocytosis in E. histolytica that revealed some interesting differences compared with phagocytosis in macrophages or Dictyostelium discoideum, and allowed to identify putative candidates for proteins linked to myosin IB activity during the phagocytic process.

Cutting edge: macrophage inhibition by cyclic AMP (cAMP): differential roles of protein kinase A and exchange protein directly activated by cAMP-1

cAMP has largely inhibitory effects on components of macrophage activation, yet downstream mechanisms involved in these effects remain incompletely defined. Elevation of cAMP in alveolar macrophages (AMs) suppresses FcgammaR-mediated phagocytosis. We now report that protein kinase A (PKA) inhibitors (H-89, KT-5720, and myristoylated PKA inhibitory peptide 14-22) failed to prevent this suppression in rat AMs. We identified the expression of the alternative cAMP target, exchange protein directly activated by cAMP-1 (Epac-1), in human and rat AMs. Using cAMP analogs that are highly specific for PKA (N6-benzoyladenosine-3',5'-cAMP) or Epac-1 (8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cAMP), we found that activation of Epac-1, but not PKA, dose-dependently suppressed phagocytosis. By contrast, activation of PKA, but not Epac-1, suppressed AM production of leukotriene B(4) and TNF-alpha, whereas stimulation of either PKA or Epac-1 inhibited AM bactericidal activity and H(2)O(2) production. These experiments now identify Epac-1 in primary macrophages, and define differential roles of Epac-1 vs PKA in the inhibitory effects of cAMP.

PDE4 inhibitors as new anti-inflammatory drugs: effects on cell trafficking and cell adhesion molecules expression

Phosphodiesterase 4 (PDE4) is a major cyclic AMP-hydrolyzing enzyme in inflammatory and immunomodulatory cells. The wide range of inflammatory mechanisms under control by PDE4 points to this isoenzyme as an attractive target for new anti-inflammatory drugs. Selective inhibitors of PDE4 have demonstrated a broad spectrum of anti-inflammatory activities including the inhibition of cellular trafficking and microvascular leakage, cytokine and chemokine release from inflammatory cells, reactive oxygen species production, and cell adhesion molecule expression in a variety of in vitro and in vivo experimental models. The initially detected side effects, mainly nausea and emesis, appear at least partially overcome by the 'second generation' PDE4 inhibitors, some of which like roflumilast and cilomilast are in the later stages of clinical development for treatment of chronic obstructive pulmonary disease. These new drugs may also offer opportunities for treatment of other inflammatory diseases.

Norepinephrine as mediator in the stimulation of phagocytosis induced by moderate exercise

During intensive exercise the stimulation of phagocytosis is mediated by "stress hormones". During moderate exercise, however, such mediation is less clear. The influence of moderate exercise (45 min at 55% maximal oxygen uptake) on the phagocytic capacity of neutrophils was evaluated in sedentary men. The exercise stimulated phagocytosis of Candida albicans, and the stimulation was maintained for at least 24 h. The possible neuroendocrine mediators were then investigated. Stimulation of phagocytosis was found after incubating neutrophils from sedentary individuals, who were in a basal state, with plasma from exercised individuals. Immediately after exercise, there was a significant increase in the concentration of norepinephrine, but not of epinephrine or cortisol. Incubation of neutrophils with this post-exercise physiological concentration of norepinephrine also stimulated phagocytosis, and the effect was blocked by both propranolol and phentolamine. The norepinephrine-augmented phagocytosis was accompanied by an increase in intracellular levels of cAMP, but not of cGMP or calcium. In conclusion, moderate exercise performed by sedentary people stimulates the phagocytic capacity of neutrophils, and the stimulation lasts for at least 24 h. Norepinephrine mediates the stimulation, although other mechanisms could be involved during the recovery period.

Prostaglandin E2Inhibits Alveolar Macrophage Phagocytosis through an E-Prostanoid 2 Receptor-Mediated Increase in Intracellular Cyclic AMP

Prostaglandin E(2) is a potent lipid mediator of inflammation that effects changes in cell functions through ligation of four distinct G protein-coupled receptors (E-prostanoid (EP)1, EP2, EP3, and EP4). During pneumonia, PGE(2) production is enhanced. In the present study, we sought to assess the effect of endogenously produced and exogenously added PGE(2) on FcRgamma-mediated phagocytosis of bacterial pathogens by alveolar macrophages (AMs), which are critical participants in lung innate immunity. We also sought to characterize the EP receptor signaling pathways responsible for these effects. PGE(2) (1-1000 nM) dose-dependently suppressed the phagocytosis by rat AMs of IgG-opsonized erythrocytes, immune serum-opsonized Klebsiella pneumoniae, and IgG-opsonized Escherichia coli. Conversely, phagocytosis was stimulated by pretreatment with the cyclooxygenase inhibitor indomethacin. PGE(2) suppression of phagocytosis was associated with enhanced intracellular cAMP production. Experiments using both forskolin (adenylate cyclase activator) and rolipram (phosphodiesterase IV inhibitor) confirmed the inhibitory effect of cAMP stimulation. Immunoblot analysis of rat AMs identified expression of only EP2 and EP3 receptors. The selective EP2 agonist butaprost, but neither the EP1/EP3 agonist sulprostone nor the EP4-selective agonist ONO-AE1-329, mimicked the effects of PGE(2) on phagocytosis and cAMP stimulation. Additionally, the EP2 antagonist AH-6809 abrogated the inhibitory effects of both PGE(2) and butaprost. We confirmed the specificity of our results by showing that AMs from EP2-deficient mice were resistant to the inhibitory effects of PGE(2). Our data support a negative regulatory role for PGE(2) on the antimicrobial activity of AMs, which has important implications for future efforts to prevent and treat bacterial pneumonia.

Local activation of protein kinase A inhibits morphogenetic movements during Xenopus gastrulation

cAMP-dependent protein kinase (PKA) has various biological roles in many organisms. However, little is known about its role in the developmental processes of vertebrates. In this study, we describe the functional analysis of PKA during gastrulation movements in Xenopus laevis. Overexpression of constitutively active PKA (cPKA) in the dorsal equatorial region of the embryo affects morphogenetic movement during gastrulation. We also show that intrinsic differences of PKA activities along the dorsoventral axis are set up and the level of PKA activity on the dorsal region is lower than that on the ventral region from late blastula to gastrula stages. In addition, PKA activation in animal explants inhibits activin-induced elongation. In cPKA-injected embryos, there were no changes in the expressions of markers involved in mesoderm specification, although the correct expression domains of these genes were altered. The effects of PKA activation can be restored by coexpression of PKI, a pseudosubstrate of PKA. We further analyzed the effects of PKA activation on the behavior of migratory gastrulating cells in vitro. Expression of cPKA in head mesoderm cells causes less polarized and/or randomized migration as demonstrated by a directional cell migration assay. Finally, we show that RhoA GTPase lies downstream of PKA, affecting activin-induced convergent extension movements. Taken together, these results suggest that overexpressed PKA can modulate a pathway responsible for morphogenetic movements during Xenopus gastrulation.

Phagocytosis and innate immunity

Phagocytosis is an evolutionarily conserved process utilized by many cells to ingest microbial pathogens, and apoptotic and necrotic corpses. Recent investigation has revealed a fundamental requirement for two co-ordinated cellular processes--cytoskeletal alterations and membrane trafficking--in the phagocytic event. Some elements of this machinery are co-opted by certain pathogens to gain entry into host cells.

Inhibition of PKA blocks fibroblast migration in response to growth factors

Cell migration requires precise coordination of many signaling pathways to achieve directed motility. We report here that NIH3T3 fibroblasts expressing a dominant negative PKA subunit (dnPKA) show diminished migration in response to serum or growth factors. This effect is not a general effect on cell motility, but rather a decreased capacity to enhance migration in response to stimuli. Control (neo) and dnPKA cells show very similar haptotactic migration toward fibronectin, but dnPKA cells show reduced stimulation of migration in response to EGF/PDGF or serum. These effects were not due to alterations in cell growth or adhesion to fibronectin. Forskolin, which elevates cyclic adenosine monophosphate (cAMP) levels, dramatically inhibited neo cell motility in a scrape migration assay, although dnPKA cell migration was unaffected. The MEK selective inhibitor U0126 and the phosphatidyl-inositol-3 kinase (PI3K) inhibitor LY294002 inhibited migrating neo cells and were able to further inhibit residual dnPKA cell migration. Our data show that intermediate or well-controlled levels of PKA activity are required for optimal growth factor-stimulated migration in fibroblasts. PKA may play an important role in the signaling processes that lead to motility.

Role of the parasite-derived prostaglandin D2 in the inhibition of epidermal Langerhans cell migration during schistosomiasis infection

Epidermal Langerhans cells (LCs) play a key role in immune defense mechanisms and in numerous immunological disorders. In this report, we show that percutaneous infection of C57BL/6 mice with the helminth parasite Schistosoma mansoni leads to the activation of LCs but, surprisingly, to their retention in the epidermis. Moreover, using an experimental model of LC migration induced by tumor necrosis factor (TNF)-α, we show that parasites transiently impair the departure of LCs from the epidermis and their subsequent accumulation as dendritic cells in the draining lymph nodes. The inhibitory effect is mediated by soluble lipophilic factors released by the parasites and not by host-derived antiinflammatory cytokines, such as interleukin-10. We find that prostaglandin (PG)D₂, but not the other major eicosanoids produced by the parasites, specifically impedes the TNF-α–triggered migration of LCs through the adenylate cyclase–coupled PGD₂ receptor (DP receptor). Moreover, the potent DP receptor antagonist BW A868C restores LC migration in infected mice. Finally, in a model of contact allergen-induced LC migration, we show that activation of the DP receptor not only inhibits LC emigration but also dramatically reduces the contact hypersensitivity responses after challenge. Taken together, we propose that the inhibition of LC migration could represent an additional stratagem for the schistosomes to escape the host immune system and that PGD₂ may play a key role in the control of cutaneous immune responses.

The vasodilator-stimulated phosphoprotein is regulated by cyclic GMP-dependent protein kinase during neutrophil spreading

The expression and phosphorylation state of the vasodilator-stimulated phosphoprotein (VASP), a membrane-associated focal adhesion protein, was investigated in human neutrophils. Adhesion and spreading of neutrophils induced the rapid phosphorylation of VASP. The phosphorylation of VASP was dependent on cell spreading, as VASP was expressed as a dephosphorylated protein in round adherent cells and was phosphorylated at the onset of changes in cell shape from round to spread cells. Immunofluorescence microscopy demonstrated that VASP was localized at the cell cortex in round cells and redistributed to focal adhesions at the ventral surface of the cell body during cell spreading. Dual labeling of spread cells indicated that VASP was colocalized with F-actin in filopodia and in focal adhesions, suggesting that the phosphorylation of VASP during cell spreading may be involved in focal adhesion complex organization and actin dynamics. VASP is a prominent substrate for both cGMP-dependent protein kinase (cGK) and cAMP-dependent protein kinase. Evidence suggested that cGK regulated neutrophil spreading, as both VASP phosphorylation and neutrophil spreading were inhibited by Rp-8-pCPT-cGMPS (cGK inhibitor), but not KT5720 (cAMP-dependent protein kinase inhibitor). In contrast, neutrophil spreading was accelerated when cGMP levels were elevated with 8-Br-cGMP, a direct activator of cGK. Furthermore, the same conditions that lead to VASP phosphorylation during neutrophil adherence and spreading induced significant elevations of cGMP in neutrophils. These results indicate that cGMP/cGK signal transduction is required for neutrophil spreading, and that VASP is a target for cGK regulation.