The role of phosphoinositides and phosphorylation in regulation of NADPH oxidase

Sestrin2 suppression aggravates oxidative stress and apoptosis in endothelial cells subjected to pharmacologically induced endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is an inflammatory response that contributes to endothelial cell dysfunction, a hallmark of cardiovascular diseases, in close interplay with oxidative stress. Recently, Sestrin2 (SESN2) emerged as a novel stress-inducible protein protecting cells from oxidative stress. We investigated here, for the first time, the impact of SESN2 suppression on oxidative stress and cell survival in human endothelial cells subjected to pharmacologically (thapsigargin)-induced ER stress and studied the underlying cellular pathways. We found that SESN2 silencing, though did not specifically induce ER stress, it aggravated the effects of thapsigargin-induced ER stress on oxidative stress and cell survival. This was associated with a dysregulation of Nrf-2, AMPK and mTORC1 signaling pathways. Furthermore, SESN2 silencing aggravated, in an additive manner, apoptosis caused by thapsigargin. Importantly, SESN2 silencing, unlike thapsigargin, caused a dramatic decrease in protein expression and phosphorylation of Akt, a critical pro-survival hub and component of the AMPK/Akt/mTORC1 axis. Our findings suggest that patients with conditions characterized by ER stress activation, such as diabetes, may be at higher risk for cardiovascular complications if their endogenous ability to stimulate and/or maintain expression levels of SESN2 is disturbed or impaired. Therefore, identifying novel or repurposing existing pharmacotherapies to enhance and/or maintain SESN2 expression levels would be beneficial in these conditions.

N-chlorination mediates protective and immunomodulatory effects of oxidized human plasma proteins

Hypochlorous acid (HOCl), a powerful antimicrobial oxidant, is produced by neutrophils to fight infections. Here, we show that N-chlorination, induced by HOCl concentrations encountered at sites of inflammation, converts blood plasma proteins into chaperone-like holdases that protect other proteins from aggregation. This chaperone-like conversion was reversible by antioxidants and was abrogated by prior methylation of basic amino acids. Furthermore, reversible N-chlorination of basic amino acid side chains is the major factor that converts plasma proteins into efficient activators of immune cells. Finally, HOCl-modified serum albumin was found to act as a pro-survival molecule that protects neutrophils from cell death induced by highly immunogenic foreign antigens. We propose that activation and enhanced persistence of neutrophils mediated by HOCl-modified plasma proteins, resulting in the increased and prolonged generation of ROS, including HOCl, constitutes a potentially detrimental positive feedback loop that can only be attenuated through the reversible nature of the modification involved.

Crosstalk Between Oxidative Stress and Endoplasmic Reticulum (ER) Stress in Endothelial Dysfunction and Aberrant Angiogenesis Associated With Diabetes: A Focus on the Protective Roles of Heme Oxygenase (HO)-1

Type-2 diabetes prevalence is continuing to rise worldwide due to physical inactivity and obesity epidemic. Diabetes and fluctuations of blood sugar are related to multiple micro- and macrovascular complications, that are attributed to oxidative stress, endoplasmic reticulum (ER) activation and inflammatory processes, which lead to endothelial dysfunction characterized, among other features, by reduced availability of nitric oxide (NO) and aberrant angiogenic capacity. Several enzymatic anti-oxidant and anti-inflammatory agents have been found to play protective roles against oxidative stress and its downstream signaling pathways. Of particular interest, heme oxygenase (HO) isoforms, specifically HO-1, have attracted much attention as major cytoprotective players in conditions associated with inflammation and oxidative stress. HO operates as a key rate-limiting enzyme in the process of degradation of the iron-containing molecule, heme, yielding the following byproducts: carbon monoxide (CO), iron, and biliverdin. Because HO-1 induction was linked to pro-oxidant states, it has been regarded as a marker of oxidative stress; however, accumulating evidence has established multiple cytoprotective roles of the enzyme in metabolic and cardiovascular disorders. The cytoprotective effects of HO-1 depend on several cellular mechanisms including the generation of bilirubin, an anti-oxidant molecule, from the degradation of heme; the induction of ferritin, a strong chelator of free iron; and the release of CO, that displays multiple anti-inflammatory and anti-apoptotic actions. The current review article describes the major molecular mechanisms contributing to endothelial dysfunction and altered angiogenesis in diabetes with a special focus on the interplay between oxidative stress and ER stress response. The review summarizes the key cytoprotective roles of HO-1 against hyperglycemia-induced endothelial dysfunction and aberrant angiogenesis and discusses the major underlying cellular mechanisms associated with its protective effects.

Antenatal melatonin modulates an enhanced antioxidant/pro-oxidant ratio in pulmonary hypertensive newborn sheep

Chronic hypobaric hypoxia during fetal and neonatal life induces neonatal pulmonary hypertension. Hypoxia and oxidative stress are driving this condition, which implies an increase generation of reactive oxygen species (ROS) and/or decreased antioxidant capacity. Melatonin has antioxidant properties that decrease oxidative stress and improves pulmonary vascular function when administered postnatally. However, the effects of an antenatal treatment with melatonin in the neonatal pulmonary function and oxidative status are unknown. Therefore, we hypothesized that an antenatal therapy with melatonin improves the pulmonary arterial pressure and antioxidant status in high altitude pulmonary hypertensive neonates. Twelve ewes were bred at high altitude (3600 m); 6 of them were used as a control group (vehicle 1.4% ethanol) and 6 as a melatonin treated group (10 mg d^-1 melatonin in vehicle). Treatments were given once daily during the last third of gestation (100–150 days). Lambs were born and raised with their mothers until 12 days old, and neonatal pulmonary arterial pressure and resistance, plasma antioxidant capacity and the lung oxidative status were determined. Furthermore, we measured the pulmonary expression and activity for the antioxidant enzymes superoxide dismutase, catalase and glutathione peroxidase, and the oxidative stress markers 8-isoprostanes, 4HNE and nitrotyrosine. Finally, we assessed pulmonary pro-oxidant sources by the expression and function of NADPH oxidase, mitochondria and xanthine oxidase. Melatonin decreased the birth weight. However, melatonin enhanced the plasma antioxidant capacity and decreased the pulmonary antioxidant activity, associated with a diminished oxidative stress during postnatal life. Interestingly, melatonin also decreased ROS generation at the main pro-oxidant sources. Our findings suggest that antenatal administration of melatonin programs an enhanced antioxidant/pro-oxidant status, modulating ROS sources in the postnatal lung.

Priming and de-priming of neutrophil responses in vitro and in vivo

The activation status of neutrophils can cycle from basal through primed to fully activated ("green-amber-red"), and at least in vitro, primed cells can spontaneously revert to a near basal phenotype. This broad range of neutrophil responsiveness confers extensive functional flexibility, allowing neutrophils to respond rapidly and appropriately to varied and evolving threats throughout the body. Primed and activated cells display dramatically enhanced bactericidal capacity (including augmented respiratory burst activity, degranulation and longevity), but this enhancement also confers the capacity for significant unintended tissue injury. Neutrophil priming and its consequences have been associated with adverse outcomes in a range of disease states, hence understanding the signalling processes that regulate the transition between basal and primed states (and back again) may offer new opportunities for therapeutic intervention in pathological settings. A wide array of host- and pathogen-derived molecules is able to modulate the functional status of these versatile cells. Reflecting this extensive repertoire of potential mediators, priming can be established by a range of signalling pathways (including mitogen-activated protein kinases, phosphoinositide 3-kinases, phospholipase D and calcium transients) and intracellular processes (including endocytosis, vesicle trafficking and the engagement of adhesion molecules). The signalling pathways engaged, and the exact cellular phenotype that results, vary according to the priming agent(s) to which the neutrophil is exposed and the precise environmental context. Herein we describe the signals that establish priming (in particular for enhanced respiratory burst, degranulation and prolonged lifespan) and describe the recently recognised process of de-priming, correlating in vitro observations with in vivo significance.

Down-regulation of NOX2 activity in phagocytes mediated by ATM-kinase dependent phosphorylation

NADPH oxidases (NOX) have many biological roles, but their regulation to control production of potentially toxic ROS molecules remains unclear. A previously identified insertion sequence of 21 residues (called NIS) influences NOX activity, and its predicted flexibility makes it a good candidate for providing a dynamic switch controlling the NOX active site. We constructed NOX2 chimeras in which NIS had been deleted or exchanged with those from other NOXs (NIS1, 3 and 4). All contained functional heme and were expressed normally at the plasma membrane of differentiated PLB-985 cells. However, NOX2-ΔNIS and NOX2-NIS1 had neither NADPH-oxidase nor reductase activity and exhibited abnormal translocation of p47^phox and p67^phox to the phagosomal membrane. This suggested a functional role of NIS. Interestingly after activation, NOX2-NIS3 cells exhibited superoxide overproduction compared with wild-type cells. Paradoxically, the V_max of purified unstimulated NOX2-NIS3 was only one-third of that of WT-NOX2. We therefore hypothesized that post-translational events regulate NOX2 activity and differ between NOX2-NIS3 and WT-NOX2. We demonstrated that Ser486, a phosphorylation target of ataxia telangiectasia mutated kinase (ATM kinase) located in the NIS of NOX2 (NOX2-NIS), was phosphorylated in purified cytochrome b₅₅₈ after stimulation with phorbol 12-myristate-13-acetate (PMA). Moreover, ATM kinase inhibition and a NOX2 Ser486Ala mutation enhanced NOX activity whereas a Ser486Glu mutation inhibited it. Thus, the absence of Ser486 in NIS3 could explain the superoxide overproduction in the NOX2-NIS3 mutant. These results suggest that PMA-stimulated NOX2-NIS phosphorylation by ATM kinase causes a dynamic switch that deactivates NOX2 activity. We hypothesize that this downregulation is defective in NOX2-NIS3 mutant because of the absence of Ser486.

Heme oxygenase (HO)-1 induction prevents Endoplasmic Reticulum stress-mediated endothelial cell death and impaired angiogenic capacity

Most of diabetic cardiovascular complications are attributed to endothelial dysfunction and impaired angiogenesis. Endoplasmic Reticulum (ER) and oxidative stresses were shown to play a pivotal role in the development of endothelial dysfunction in diabetes. Hemeoxygenase-1 (HO-1) was shown to protect against oxidative stress in diabetes; however, its role in alleviating ER stress-induced endothelial dysfunction remains not fully elucidated. We aim here to test the protective role of HO-1 against high glucose-mediated ER stress and endothelial dysfunction and understand the underlying mechanisms with special emphasis on oxidative stress, inflammation and cell death. Human Umbilical Vein Endothelial Cells (HUVECs) were grown in either physiological or intermittent high concentrations of glucose for 5days in the presence or absence of Cobalt (III) Protoporphyrin IX chloride (CoPP, HO-1 inducer) or 4-Phenyl Butyric Acid (PBA, ER stress inhibitor). Using an integrated cellular and molecular approach, we then assessed ER stress and inflammatory responses, in addition to apoptosis and angiogenic capacity in these cells. Our results show that HO-1 induction prevented high glucose-mediated increase of mRNA and protein expression of key ER stress markers. Cells incubated with high glucose exhibited high levels of oxidative stress, activation of major inflammatory and apoptotic responses [nuclear factor (NF)-κB and c-Jun N-terminal kinase (JNK)] and increased rate of apoptosis; however, cells pre-treated with CoPP or PBA were fully protected. In addition, high glucose enhanced caspases 3 and 7 cleavage and activity and augmented cleaved poly ADP ribose polymerase (PARP) expression whereas HO-1 induction prevented these effects. Finally, HO-1 induction and ER stress inhibition prevented high glucose-induced reduction in NO release and impaired the angiogenic capacity of HUVECs, and enhanced vascular endothelial growth factor (VEGF)-A expression. Altogether, we show here the critical role of ER stress-mediated cell death in diabetes-induced endothelial dysfunction and impaired angiogenesis and underscore the role of HO-1 induction as a key therapeutic modulator for ER stress response in ischemic disorders and diabetes. Our results also highlight the complex interplay between ER stress response and oxidative stress.

Extracellular phosphates enhance activities of voltage-gated proton channels and production of reactive oxygen species in murine osteoclast-like cells

Osteoclasts are highly differentiated bone-resorbing cells and play a significant role in bone remodelling. In the resorption pit, inorganic phosphate (Pi) concentrations increase because of degradation of hydroxyapatite. We studied effects of extracellular Pi on voltage-gated H⁺ channels in osteoclast-like cells derived from a macrophage cell line (RAW264). Extracellular Pi (1.25-20 mM) increased the H⁺ channel currents dose dependently and reversibly. The Pi-induced increases were attenuated by removal of extracellular Na⁺ and by phosphonoformic acid, a blocker of Na⁺-dependent Pi transporters. Pi increased the maximal conductance, decreased activation time constant, increased deactivation time constant, and shifted the conductance-voltage relationship to more negative voltages. The most marked change was enhanced gating which was mainly caused by elevation of intracellular Pi levels. The Pi-induced enhanced gating was partially inhibited by protein kinase C (PKC) inhibitors, GF109203X and staurosporine, indicating that PKC-mediated phosphorylation was involved in part. The increase in the maximal conductance was mainly due to accompanying decrease in intracellular pH. These effects of Pi were not affected by intracellular Mg²⁺, bafilomycin A₁ (V-ATPase inhibitor) and removal of intracellular ATP. Extracellular Pi also upregulated reactive oxygen species (ROS). Diphenyleneiodonium chloride, an inhibitor of NADPH oxidases, decreased ROS production and partially attenuated the enhanced gating. In the cells during later passages where osteoclastogenesis declined, H⁺ channel activities and ROS production were both modest. These results suggest that, in osteoclasts, ambient Pi is a common enhancer for H⁺ channels and ROS production and that potentiation of H⁺ channels may help ROS production.

Phosphatidylinositol-3,4,5-triphosphate and cellular signaling: implications for obesity and diabetes

Phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P₃) is one of the most important phosphoinositides and is capable of activating a wide range of proteins through its interaction with their specific binding domains. Localization and activation of these effector proteins regulate a number of cellular functions, including cell survival, proliferation, cytoskeletal rearrangement, intracellular vesicle trafficking, and cell metabolism. Phosphoinositides have been investigated as an important agonist-dependent second messenger in the regulation of diverse physiological events depending upon the phosphorylation status of their inositol group. Dysregulation in formation as well as metabolism of phosphoinositides is associated with various pathophysiological disorders such as inflammation, allergy, cardiovascular diseases, cancer, and metabolic diseases. Recent studies have demonstrated that the impaired metabolism of PtdIns(3,4,5)P₃ is a prime mediator of insulin resistance associated with various metabolic diseases including obesity and diabetes. This review examines the current status of the role of PtdIns(3,4,5)P₃ signaling in the regulation of various cellular functions and the implications of dysregulated PtdIns(3,4,5)P₃ signaling in obesity, diabetes, and their associated complications.

Molecular insights of p47phox phosphorylation dynamics in the regulation of NADPH oxidase activation and superoxide production

Phagocyte superoxide production by a multicomponent NADPH oxidase is important in host defense against microbial invasion. However inappropriate NADPH oxidase activation causes inflammation. Endothelial cells express NADPH oxidase and endothelial oxidative stress due to prolonged NADPH oxidase activation predisposes many diseases. Discovering the mechanism of NADPH oxidase activation is essential for developing novel treatment of these diseases. The p47^phox is a key regulatory subunit of NADPH oxidase; however, due to the lack of full protein structural information, the mechanistic insight of p47^phox phosphorylation in NADPH oxidase activation remains incomplete. Based on crystal structures of three functional domains, we generated a computational structural model of the full p47^phox protein. Using a combination of in silico phosphorylation, molecular dynamics simulation and protein/protein docking, we discovered that the C-terminal tail of p47^phox is critical for stabilizing its autoinhibited structure. Ser-379 phosphorylation disrupts H-bonds that link the C-terminal tail to the autoinhibitory region (AIR) and the tandem Src homology 3 (SH3) domains, allowing the AIR to undergo phosphorylation to expose the SH3 pocket for p22^phox binding. These findings were confirmed by site-directed mutagenesis and gene transfection of p47^phox−/− coronary microvascular cells. Compared with wild-type p47^phox cDNA transfected cells, the single mutation of S379A completely blocked p47^phox membrane translocation, binding to p22^phox and endothelial O₂^⨪ production in response to acute stimulation of PKC. p47^phox C-terminal tail plays a key role in stabilizing intramolecular interactions at rest. Ser-379 phosphorylation is a molecular switch which initiates p47^phox conformational changes and NADPH oxidase-dependent superoxide production by cells.

Generation of reactive oxygen species by leukocytes of Prochilodus lineatus

Prochilodus lineatus (curimbatá), from the Procholodontidae family, is a Brazilian freshwater fish, which is important commercially, nutritionally and ecologically. It is encountered in the Rio da Prata Bay in Southern South America. Studies on the immune system of this fish are scarce, but the physiological mechanisms of the species are analogous to those of other vertebrates. Thus, this work discusses the present study, which correlates P. lineatus leukocytes and the generation of reactive oxygen species after modulatory stimuli. Leukocytes were characterized by light and electron transmission microscopy and investigated by the generation of H2O2 and O2 (-), using phenol red, flow-cytometry and electron transmission histochemistry. The study determined that monocytes and neutrophils are the main cells responsible for generating O2 after stimulation with phorbol myristate acetate. Superoxide dismutase successfully inhibited the generation of reactive oxygen species in neutrophils and monocytes, but stimulated generation when in association with phorbol myristate acetate. Fish leukocyte samples from P. lineatus showed cross-reactivity with antibodies directed against human NADPH-oxidase antibody subunits (p47(phox) and p67(phox)). Thus, catalase enhanced the presence of p47(phox). Neutrophil mitochondria were shown to be generators of H2O2 (charged by cerium precipitate), being enlarged and changing their format. The present study contributes to a better understanding of the respiratory burst pathways in this species and suggests mitochondria as the organelle responsible for generation of reactive oxygen species.

Phosphatidylinositol 3-phosphate, a lipid that regulates membrane dynamics, protein sorting and cell signalling

Phosphatidylinositol 3-phosphate (PtdIns3P) is generated on the cytosolic leaflet of cellular membranes, primarily by phosphorylation of phosphatidylinositol by class II and class III phosphatidylinositol 3-kinases. The bulk of this lipid is found on the limiting and intraluminal membranes of endosomes, but it can also be detected in domains of phagosomes, autophagosome precursors, cytokinetic bridges, the plasma membrane and the nucleus. PtdIns3P controls cellular functions through recruitment of specific protein effectors, many of which contain FYVE or PX domains. Cellular processes known to be controlled by PtdIns3P and its effectors include endosomal fusion, sorting and motility, autophagy, cytokinesis, regulated exocytosis and signal transduction. Here we discuss how Ptdins3P is generated on specific cellular membranes, how its localizations and functions can be studied, and how its effectors serve to control cellular functions.

Diacylglycerol kinases terminate diacylglycerol signaling during the respiratory burst leading to heterogeneous phagosomal NADPH oxidase activation

It is commonly assumed that all phagosomes have identical molecular composition. This assumption has remained largely unchallenged due to a paucity of methods to distinguish individual phagosomes. We devised an assay that extends the utility of nitro blue tetrazolium for detection and quantification of NAPDH oxidase (NOX) activity in individual phagosomes. Implementation of this assay revealed that in murine macrophages there is heterogeneity in the ability of individual phagosomes to generate superoxide, both between and within cells. To elucidate the molecular basis of the variability in NOX activation, we employed genetically encoded fluorescent biosensors to evaluate the uniformity in the distribution of phospholipid mediators of the oxidative response. Despite variability in superoxide generation, the distribution of phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, and phosphatidic acid was nearly identical in all phagosomes. In contrast, diacylglycerol (DAG) was not generated uniformly across the phagosomal population, varying in a manner that directly mirrored superoxide production. Modulation of DAG levels suggested that NOX activation is precluded when phagosomes fail to reach a critical DAG concentration. In particular, forced expression of diacylglycerol kinase β abrogated DAG accumulation at the phagosome, leading to impaired respiratory burst. Conversely, pharmacological inhibition of DAG kinases or expression of an inactive diacylglycerol kinase β mutant increased the proportion of DAG-positive phagosomes, concomitantly potentiating phagosomal NOX activity. Our data suggest that diacylglycerol kinases limit the extent of NADPH oxidase activation, curtailing the production of potentially harmful reactive oxygen species. The resulting heterogeneity in phagosome responsiveness could enable the survival of a fraction of invading microorganisms.

Activation of the neutrophil NADPH oxidase by Aspergillus fumigatus

Upon infection of the respiratory system with the fungus Aspergillus fumigatus various leukoctytes, in particular neutrophils, are recruited to the lung to mount an immune response. Neutrophils respond by both phagocytosing conidia and mediating extracellular killing of germinated, invasive hyphae. Of paramount importance to an appropriate immune response is the neutrophil NADPH oxidase enzyme, which mediates the production of various reactive oxygen species (ROS). This is evidenced by the acute sensitivity of both oxidase-deficient humans and mice to invasive aspergillosis. Herein we briefly review the mechanisms and functions of oxidase activation and discuss our recent work identifying at least some of the important players in hyphal-induced oxidase activation and neutrophil function. Among these we define the phosphoinositide 3-kinase enzyme and the regulatory protein Vav to be of critical importance and allude to a kinase-independent role for Syk.

Synaptotagmin XI regulates phagocytosis and cytokine secretion in macrophages

Synaptotagmins (Syts) are a group of type I membrane proteins that regulate vesicle docking and fusion in processes such as exocytosis and phagocytosis. All Syts possess a single transmembrane domain, and two conserved tandem Ca(2+)-binding C2 domains. However, Syts IV and XI possess a conserved serine in their C2A domain that precludes these Syts from binding Ca(2+) and phospholipids, and from mediating vesicle fusion. Given the importance of vesicular trafficking in macrophages, we investigated the role of Syt XI in cytokine secretion and phagocytosis. We demonstrated that Syt XI is expressed in murine macrophages, localized in recycling endosomes, lysosomes, and recruited to phagosomes. Syt XI had a direct effect on phagocytosis and on the secretion of TNF and IL-6. Whereas small interfering RNA-mediated knockdown of Syt XI potentiated secretion of these cytokines and particle uptake, overexpression of an Syt XI construct suppressed these processes. In addition, Syt XI knockdown led to decreased recruitment of gp91(phox) and lysosomal-associated membrane protein-1 to phagosomes, suggesting attenuated microbicidal activity. Remarkably, knockdown of Syt XI ensued in enhanced bacterial survival. Our data reveal a novel role for Syt XI as a regulator of cytokine secretion, particle uptake, and macrophage microbicidal activity.

Role of phosphatidylinositol 3,4,5-trisphosphate in cell signaling

Many lipids present in cellular membranes are phosphorylated as part of signaling cascades and participate in the recruitment, localization, and activation of downstream protein effectors. Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) is one of the most important second messengers and is capable of interacting with a variety of proteins through specific PtdIns(3,4,5)P3 binding domains. Localization and activation of these effector proteins controls a myriad of cellular functions including cell survival, proliferation, cytoskeletal rearrangement, and gene expression. Aberrations in the production and metabolism of PtdIns(3,4,5)P3 have been implicated in many human diseases including cancer, diabetes, inflammation, and heart disease. This chapter provides an overview of the role of PtdIns(3,4,5)P3 in cellular regulation and the implications of PtdIns(3,4,5)P3 dysregulation in human diseases. Additionally, recent attempts at targeting PtdIns(3,4,5)P3 signaling via small molecule inhibitors are summarized.

Rational design of small molecule inhibitors targeting the Rac GTPase-p67(phox) signaling axis in inflammation

The NADPH oxidase enzyme complex, NOX2, is responsible for reactive oxygen species production in neutrophils and has been recognized as a key mediator of inflammation. Here, we have performed rational design and in silico screen to identify a small molecule inhibitor, Phox-I1, targeting the interactive site of p67(phox) with Rac GTPase, which is a necessary step of the signaling leading to NOX2 activation. Phox-I1 binds to p67(phox) with a submicromolar affinity and abrogates Rac1 binding and is effective in inhibiting NOX2-mediated superoxide production dose-dependently in human and murine neutrophils without detectable toxicity. Medicinal chemistry characterizations have yielded promising analogs and initial information of the structure-activity relationship of Phox-I1. Our studies suggest the potential utility of Phox-I class inhibitors in NOX2 oxidase inhibition and present an application of rational targeting of a small GTPase-effector interface.

Microglial MAC1 receptor and PI3K are essential in mediating β-amyloid peptide-induced microglial activation and subsequent neurotoxicity

Background

β-Amyloid peptide (Aβ) is a major protein in the brain associated with Alzheimer's and Parkinson's diseases. The purpose of this study was to investigate the role of macrophage antigen-1 (MAC1) receptor, an integrin scavenger receptor in microglia, and subsequent signaling events in mediating Aβ-induced neurotoxicity. We have previously reported that NADPH oxidase (PHOX) on microglia and superoxide produced by PHOX are critical for Aβ-induced loss of dopaminergic neurons. However, the upstream signaling pathway of superoxide production remains unclear.

Methods

For the in vitro study, mesencephalic neuron-glia cultures and microglia-enriched cultures from mice deficient in the MAC1 receptor (MAC1^-/-) and wild type controls were used to investigate the role of MAC1 receptor in Aβ-induced neurotoxicity and the role of phosphoinositide-3 kinase (PI3K) in the signal pathway between MAC1 receptor and PHOX. For the in vivo study, Aβ was injected into the substantia nigra of MAC1^-/- mice and wild type mice to confirm the role of MAC1 receptor.

Results

We found that Aβ-induced activation of microglia, activation of PHOX, generation of superoxide and other reactive oxygen species, and loss of dopaminergic neurons were decreased in MAC1^-/- cultures compared to MAC1^+/+ cultures. In MAC1^-/- mice, dopaminergic neuron loss in response to Aβ injection into the substantia nigra was reduced relative to MAC1^+/+ mice. Thus, MAC1 receptor-mediated PHOX activation and increased superoxide production are associated with Aβ-induced neurotoxicity. PI3K activation was one downstream step in MAC1 signaling to PHOX and played an important role in Aβ-induced neurotoxicity. In microglia-enriched cultures from MAC1^-/- mice, Aβ-induced activation of PI3K (phosphorylation of target proteins and PIP₃ production) was reduced relative to MAC1^+/+ cultures.

Conclusions

Taken together, our data demonstrate that Aβ activates MAC1 receptor to increase the activity of PI3K, which in turn phosphorylates p47^phox, triggers the translocation of cytosolic subunits of PHOX to microglia membrane, increases PHOX activation and the subsequent production of superoxide and causes neurotoxicity.

Regulation of NOXO1 activity through reversible interactions with p22 and NOXA1

Reactive oxygen species (ROS) have been known for a long time to play important roles in host defense against microbial infections. In addition, it has become apparent that they also perform regulatory roles in signal transduction and cell proliferation. The source of these chemicals are members of the NOX family of NADPH oxidases that are found in a variety of tissues. NOX1, an NADPH oxidase homologue that is most abundantly expressed in colon epithelial cells, requires the regulatory subunits NOXO1 (NOX organizing protein 1) and NOXA1 (NOX activating protein 1), as well as the flavocytochrome component p22(phox) for maximal activity. Unlike NOX2, the phagocytic NADPH oxidase whose activity is tightly repressed in the resting state, NOX1 produces superoxide constitutively at low levels. These levels can be further increased in a stimulus-dependent manner, yet the molecular details regulating this activity are not fully understood. Here we present the first quantitative characterization of the interactions made between the cytosolic regulators NOXO1 and NOXA1 and membrane-bound p22(phox). Using isothermal titration calorimetry we show that the isolated tandem SH3 domains of NOXO1 bind to p22(phox) with high affinity, most likely adopting a superSH3 domain conformation. In contrast, complex formation is severely inhibited in the presence of the C-terminal tail of NOXO1, suggesting that this region competes for binding to p22(phox) and thereby contributes to the regulation of superoxide production. Furthermore, we provide data indicating that the molecular details of the interaction between NOXO1 and NOXA1 is significantly different from that between the homologous proteins of the phagocytic oxidase, suggesting that there are important functional differences between the two systems. Taken together, this study provides clear evidence that the assembly of the NOX1 oxidase complex can be regulated through reversible protein-protein interactions.

PI3K signaling in neutrophils

PI3Ks play important roles in the signaling pathways used by a wide variety of cell surface receptors on neutrophils. Class IB PI3K plays a major role in the initial generation of PtdIns(3,4,5)P₃ by Gi-coupled G-protein coupled receptors (GPCRs) (e.g., receptors for fMLP, C5a, LTB₄). Class IA PI3Ks generate PtdIns(3,4,5)P₃ downstream of receptors which directly or indirectly couple to protein tyrosine kinases such as integrins, FcγRs, cytokine receptors, and GPCRs. The PtdIns(3,4,5)P₃ made by Class I PI3Ks regulates the activity of several different effector proteins, many of which are plasma membrane GEFs or GAPs for small GTPases. Class III PI3K generates PtdIns(3)P in the phagosome membrane and plays an important role in efficient assembly of the NADPH oxidase at this location. Much still remains to be discovered about the molecular details that govern activation of PI3Ks and the mechanisms by which these enzymes regulate complex cellular processes, such as neutrophil spreading, chemotaxis, phagocytosis, and killing of pathogens. However, it is clear from recent use of transgenic mouse models and isoform-selective PI3K inhibitors that these pathways are important in regulating neutrophil recruitment to sites of infection and damage in vivo. Thus, PI3K pathways may present novel opportunities for selective inhibition in some inflammatory pathologies.

Phosphorylation of p22phox on threonine 147 enhances NADPH oxidase activity by promoting p47phox binding

NADPH oxidase comprises both cytosolic and membrane-bound subunits, which, when assembled and activated, initiate the transfer of electrons from NADPH to molecular oxygen to form superoxide. This activity, known as the respiratory burst, is extremely important in the innate immune response as indicated by the disorder chronic granulomatous disease. The regulation of this enzyme complex involves protein-protein and protein-lipid interactions as well as phosphorylation events. Previously, our laboratory demonstrated that the small membrane subunit of the oxidase complex, p22(phox), is phosphorylated in neutrophils and that its phosphorylation correlates with NADPH oxidase activity. In this study, we utilized site-directed mutagenesis in a Chinese hamster ovarian cell system to determine the phosphorylation sites within p22(phox). We also explored the mechanism by which p22(phox) phosphorylation affects NADPH oxidase activity. We found that mutation of threonine 147 to alanine inhibited superoxide production in vivo by more than 70%. This mutation also blocked phosphorylation of p22(phox) in vitro by both protein kinase C-alpha and -delta. Moreover, this mutation blocked the p22(phox)-p47(phox) interaction in intact cells. When phosphorylation was mimicked in vivo through mutation of Thr-147 to an aspartyl residue, NADPH oxidase activity was recovered, and the p22(phox)-p47(phox) interaction in the membrane was restored. Maturation of gp91(phox) was not affected by the alanine mutation, and phosphorylation of the cytosolic component p47(phox) still occurred. This study directly implicates threonine 147 of p22(phox) as a critical residue for efficient NADPH oxidase complex formation and resultant enzyme activity.

The angiotensin II-AT1 receptor stimulates reactive oxygen species within the cell nucleus

We and others have reported significant expression of the Ang II Type 1 receptor (AT1R) on renal nuclei; thus, the present study assessed the functional pathways and distribution of the intracellular AT1R on isolated nuclei. Ang II (1nM) stimulated DCF fluorescence, an intranuclear indicator of reactive oxygen species (ROS), while the AT1R antagonist losartan or the NADPH oxidase (NOX) inhibitor DPI abolished the increase in ROS. Dual labeling of nuclei with antibodies against nucleoporin 62 (Nup62) and AT1R or the NADPH oxidase isoform NOX4 revealed complete overlap of the Nup62 and AT1R (99%) by flow cytometry, while NOX4 was present on 65% of nuclei. Treatment of nuclei with a PKC agonist increased ROS while the PKC inhibitor GF109203X or PI3 kinase inhibitor LY294002 abolished Ang II stimulation of ROS. We conclude that the Ang II-AT1R-PKC axis may directly influence nuclear function within the kidney through a redox sensitive pathway.

CD18-dependent activation of the neutrophil NADPH oxidase during phagocytosis of Escherichia coli or Staphylococcus aureus is regulated by class III but not class I or II PI3Ks

Phagocytosis and activation of the NADPH oxidase are important mechanisms by which neutrophils and macrophages engulf and kill microbial pathogens. We investigated the role of PI3K signaling pathways in the regulation of the oxidase during phagocytosis of Staphylococcus aureus and Escherichia coli by mouse and human neutrophils, a mouse macrophage-like cell line and a human myeloid-like cell line. Phagocytosis of these bacteria was promoted by serum, independent of serum-derived antibodies, and effectively abolished in mouse neutrophils lacking the β2-integrin common chain, CD18. A combination of PI3K isoform-selective inhibitors, mouse knock-outs, and RNA-interference indicated CD18-dependent activation of the oxidase was independent of class I and II PI3Ks, but substantially dependent on the single class III isoform (Vps34). Class III PI3K was responsible for the synthesis of PtdIns(3)P on phagosomes containing either bacteria. The use of mouse neutrophils carrying an appropriate knock-in mutation indicated that PtdIns(3)P binding to the PX domain of their p40phox oxidase subunit is important for oxidase activation in response to both S aureus and E coli. This interaction does not, however, account for all the PI3K sensitivity of these responses, particularly the oxidase response to E coli, suggesting that additional mechanisms for PtdIns(3)P-regulation of the oxidase must exist.

Melatonin impairs NADPH oxidase assembly and decreases superoxide anion production in microglia exposed to amyloid-beta1-42

Melatonin shows significant protective effects in Alzheimer's disease (AD) models in vitro and in vivo; these effects are related to its function as an antioxidant. The source of reactive oxygen species (ROS) generation in the AD brain is primarily the amyloid-beta (Abeta)- activated microglial nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. However, the effects of melatonin on the activation of NADPH oxidase remain unclear. In the present study, the cultures of microglia were incubated in the presence of fibrillar Abeta(1-42), which induces the assembly and the activation of NADPH oxidase, and triggers the production of superoxide anion-derived ROS. Pretreatment of microglia with melatonin dose-dependently prevents the activation of NADPH oxidase and decreases the production of ROS. Melatonin inhibits the phosphorylation of the p47(phox) subunit of NADPH oxidase via a PI3K/Akt-dependent signalling pathway, blocks the translocation of p47(phox) and p67(phox) subunit to the membrane, down-regulates the binding of p47(phox) to gp91(phox), and impairs the assembly of NADPH oxidase. Our data offer new insights into the mechanism of inhibiting ROS generation by melatonin in Abeta-activated microglia. Inhibition of ROS production indirectly might be the underlying mechanism for the neuroprotection by melatonin in the AD brain.

Fc gamma R-stimulated activation of the NADPH oxidase: phosphoinositide-binding protein p40phox regulates NADPH oxidase activity after enzyme assembly on the phagosome

The phagocyte NADPH oxidase generates superoxide for microbial killing, and includes a membrane-bound flavocytochrome b(558) and cytosolic p67(phox), p47(phox), and p40(phox) subunits that undergo membrane translocation upon cellular activation. The function of p40(phox), which binds p67(phox) in resting cells, is incompletely understood. Recent studies showed that phagocytosis-induced superoxide production is stimulated by p40(phox) and its binding to phosphatidylinositol-3-phosphate (PI3P), a phosphoinositide enriched in membranes of internalized phagosomes. To better define the role of p40(phox) in FcgammaR-induced oxidase activation, we used immunofluorescence and real-time imaging of FcgammaR-induced phagocytosis. YFP-tagged p67(phox) and p40(phox) translocated to granulocyte phagosomes before phagosome internalization and accumulation of a probe for PI3P. p67(phox) and p47(phox) accumulation on nascent and internalized phagosomes did not require p40(phox) or PI3 kinase activity, although superoxide production before and after phagosome sealing was decreased by mutation of the p40(phox) PI3P-binding domain or wortmannin. Translocation of p40(phox) to nascent phagosomes required binding to p67(phox) but not PI3P, although the loss of PI3P binding reduced p40(phox) retention after phagosome internalization. We conclude that p40(phox) functions primarily to regulate FcgammaR-induced NADPH oxidase activity rather than assembly, and stimulates superoxide production via a PI3P signal that increases after phagosome internalization.

Dissociation of Rac1(GDP).RhoGDI complexes by the cooperative action of anionic liposomes containing phosphatidylinositol 3,4,5-trisphosphate, Rac guanine nucleotide exchange factor, and GTP

Rac plays a pivotal role in the assembly of the superoxide-generating NADPH oxidase of phagocytes. In resting cells, Rac is found in the cytosol in complex with Rho GDP dissociation inhibitor (RhoGDI). NADPH oxidase assembly involves dissociation of the Rac.RhoGDI complex and translocation of Rac to the membrane. We reported that liposomes containing high concentrations of monovalent anionic phospholipids cause Rac.RhoGDI complex dissociation ( Ugolev, Y., Molshanski-Mor, S., Weinbaum, C., and Pick, E. (2006) J. Biol. Chem. 281, 19204-19219 ). We now designed an in vitro model mimicking membrane phospholipid remodeling during phagocyte stimulation in vivo. We showed that liposomes of "resting cell membrane" composition (less than 20 mol % monovalent anionic phospholipids), supplemented with 1 mol % of polyvalent anionic phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) in conjunction with constitutively active forms of the guanine nucleotide exchange factors (GEFs) for Rac, Trio, or Tiam1 and a non-hydrolyzable GTP analogue, cause dissociation of Rac1(GDP).RhoGDI complexes, GDP to GTP exchange on Rac1, and binding of Rac1(GTP) to the liposomes. Complexes were not dissociated in the absence of GEF and GTP, and optimal dissociation required the presence of PtdIns(3,4,5)P(3) in the liposomes. Dissociation of Rac1(GDP).RhoGDI complexes was correlated with the affinity of particular GEF constructs, via the N-terminal pleckstrin homology domain, for PtdIns(3,4,5)P(3) and involved GEF-mediated GDP to GTP exchange on Rac1. Phagocyte membranes enriched in PtdIns(3,4,5)P(3) responded by NADPH oxidase activation upon exposure in vitro to Rac1(GDP).RhoGDI complexes, p67(phox), GTP, and Rac GEF constructs with affinity for PtdIns(3,4,5)P(3) at a level superior to that of native membranes.

Conventional protein kinase C isoenzymes undergo dephosphorylation in neutrophil-like HL-60 cells treated by chelerythrine or sanguinarine

The quaternary benzo[c]phenanthridine alkaloid chelerythrine is widely used as an inhibitor of protein kinase C (PKC). However, in biological systems chelerythrine interacts with an array of proteins. In this study, we examined the effects of chelerythrine and sanguinarine on conventional PKCs (cPKCs) and PKC upstream kinase, phosphoinositide-dependent protein kinase 1 (PDK1), under complete inhibition conditions of PKC-dependent oxidative burst. In neutrophil-like HL-60 cells, sanguinarine and chelerythrine inhibited N-formyl-Met-Leu-Phe, phorbol 12-myristate 13-acetate (PMA)-, and A23187-induced oxidative burst with IC(50) values not exceeding 4.6 micromol/L, but the inhibition of PMA-stimulated cPKC activity in intact cells required at least fivefold higher alkaloid concentrations. At concentrations below 10 micromol/L, sanguinarine and chelerythrine prevented phosphorylation of approximately 80 kDa protein and sequestered approximately 60 kDa phosphoprotein in cytosol. Moreover, neither sanguinarine nor chelerythrine impaired PMA-stimulated translocation of autophosphorylated PKCalpha/betaII isoenzymes, but both alkaloids induced dephosphorylation of the turn motif in PKCalpha/betaII. The dephosphorylation did not occur in unstimulated cells and it was not accompanied by PKC degradation. Furthermore, cell treatment with sanguinarine or chelerythrine resulted in phosphorylation of approximately 70 kDa protein by PDK1. We conclude that PKC-dependent cellular events are affected by chelerythrine primarily by multiple protein interactions rather than by inhibition of PKC activity.

Differential redox regulation within the PTP superfamily

The Protein Tyrosine Phosphatase (PTP) family comprises a large and diverse group of enzymes, regulating a range of biological processes through de-phosphorylation of many proteins and lipids. These enzymes share a catalytic mechanism that requires a reduced and reactive cysteine nucleophile, making them potentially sensitive to inactivation and regulation by oxidation. Analysis of ten PTPs identified substantial differences in the sensitivity of these enzymes to oxidation in vitro. More detailed experiments confirmed the following rank order of sensitivity: PTEN and Sac1>PTPL1/FAP-1>>myotubularins. When the apparent sensitivity to oxidation of these PTPs in cells treated with hydrogen peroxide was analysed, this correlated well with the observed sensitivities to oxidation in vitro. These data suggested that different PTPs may fall into at least three different classes with respect to mechanisms of cellular redox regulation. 1. PTEN and Sac1 were readily and reversibly oxidised in vitro and in cells treated with hydrogen peroxide 2. PTPL1 appeared to be resistant to oxidation in cells, correlating with its sensitivity to reduction by glutathione in vitro 3. The myotubularin family of lipid phosphatases was almost completely resistant to oxidation in vitro and in cells. Our results show that sensitivity to reversible oxidation is not a necessary characteristic of the PTPs and imply that such sensitivity has evolved as a regulatory mechanism for some of this large family, but not others.

Tripartite chimeras comprising functional domains derived from the cytosolic NADPH oxidase components p47phox, p67phox, and Rac1 elicit activator-independent superoxide production by phagocyte membranes: an essential role for anionic membrane phospholipids

The superoxide-generating NADPH oxidase is converted to an active state by the assembly of a membrane-localized cytochrome b(559) with three cytosolic components: p47(phox), p67(phox), and GTPase Rac1 or Rac2. Assembly involves two sets of protein-protein interactions: among cytosolic components and among cytosolic components and cytochrome b(559) within its lipid habitat. We circumvented the need for interactions among cytosolic components by constructing a recombinant tripartite chimera (trimera) consisting of the Phox homology (PX) and Src homology 3 (SH3) domains of p47(phox), the tetratricopeptide repeat and activation domains of p67(phox), and full-length Rac1. Upon addition to phagocyte membrane, the trimera was capable of oxidase activation in vitro in the presence of an anionic amphiphile. The trimera had a higher affinity (lower EC(50)) for and formed a more stable complex (longer half-life) with cytochrome b(559) compared with the combined individual components, full-length or truncated. Supplementation of membrane with anionic but not neutral phospholipids made activation by the trimera amphiphile-independent. Mutagenesis, truncations, and domain replacements revealed that oxidase activation by the trimera was dependent on the following interactions: 1) interaction with anionic membrane phospholipids via the poly-basic stretch at the C terminus of the Rac1 segment; 2) interaction with p22(phox) via Trp(193) in the N-terminal SH3 domain of the p47(phox) segment, supplementing the electrostatic attraction; and 3) an intrachimeric bond among the p67(phox) and Rac1 segments complementary to their physical fusion. The PX domain of the p47(phox) segment and the insert domain of the Rac1 segment made only minor contributions to oxidase assembly.

PI3K delta and PI3K gamma: partners in crime in inflammation in rheumatoid arthritis and beyond?

Dysregulated signal transduction in innate and adaptive immune cells is known to be associated with the development of various autoimmune and inflammatory diseases. Consequently, targeting intracellular signalling of the pro-inflammatory cytokine network heralds hope for the next generation of anti-inflammatory drugs. Phosphoinositide 3-kinases (PI3Ks) generate lipid-based second messengers that control an array of intracellular signalling pathways that are known to have important roles in leukocytes. In light of the recent progress in the development of selective PI3K inhibitors, and the beneficial effects of these inhibitors in models of acute and chronic inflammatory disorders, we discuss the therapeutic potential of blocking PI3K isoforms for the treatment of rheumatoid arthritis and other immune-mediated diseases.

Signalling through Class I PI3Ks in mammalian cells

It is now accepted that activation of Class I PI3Ks (phosphoinositide 3-kinases) is one of the most important signal transduction pathways used by cell-surface receptors to control intracellular events. The receptors which access this pathway include those that recognize growth factors, hormones, antigens and inflammatory stimuli, and the cellular events known to be regulated include cell growth, survival, proliferation and movement. We have learnt a great deal about the family of Class I PI3K enzymes themselves and the structural adaptations which allow a variety of cell-surface receptors to regulate their activity. Class I PI3Ks synthesize the phospholipid PtdIns(3,4,5)P3 in the membranes in which they are activated, and it is now accepted that PtdIns(3,4,5)P3 and its dephosphorylation product PtdIns(3,4)P2 are messenger molecules which regulate the localization and function of multiple effectors by binding to their specific PH (pleckstrin homology) domains. The number of direct PtdIns(3,4,5)P3/PtdIns(3,4)P2 effectors which exist, even within a single cell, creates an extremely complex signalling web downstream of PI3K activation. Some key players are beginning to emerge, however, linking PI3K activity to specific cellular responses. These include small GTPases for the Rho and Arf families which regulate the cytoskeletal and membrane rearrangements required for cell movement, and PKB (protein kinase B), which has important regulatory inputs into the regulation of cell-cycle progression and survival. The importance of the PI3K signalling pathway in regulating the balance of decisions in cell growth, proliferation and survival is clear from the prevalence of oncogenes (e.g. PI3Kalpha) and tumour suppressors [e.g. the PtdIns(3,4,5)P3 3-phosphatase, PTEN (phosphatase and tensin homologue deleted on chromosome 10)] found in this pathway. The recent availability of transgenic mouse models with engineered defects in Class I PI3K signalling pathways, and the development of PI3K isoform-selective inhibitors by both academic and pharmaceutical research has highlighted the importance of specific isoforms of PI3K in whole-animal physiology and pathology, e.g. PI3Kalpha in growth and metabolic regulation, PI3Kbeta in thrombosis, and PI3Kdelta and PI3Kgamma in inflammation and asthma. Thus the Class I PI3K signalling pathway is emerging as an exciting new area for the development of novel therapeutics.

Leishmania donovani lipophosphoglycan blocks NADPH oxidase assembly at the phagosome membrane

Phagocytosis of Leishmania donovani promastigotes is characterized by an inhibition of phagolysosome biogenesis mediated by the surface glycolipid lipophosphoglycan (LPG). However, the consequences of this inhibition on macrophage function remain to be determined. In this study, we investigated the impact of LPG-mediated phagosome remodelling on the assembly and function of the NADPH oxidase complex. Phagocytosis of both wild-type and LPG-defective L. donovani promastigotes triggered the release of similar levels of superoxide. However, wild-type promastigotes, but not LPG-defective mutants, inhibited generation of superoxide at the phagosome. Confocal microscopy imaging revealed that the membrane component gp91(phox) and the Rho-family GTPase Rac1 were present on phagosomes containing either wild-type or LPG-defective promastigotes. In contrast, the NADPH oxidase cytosolic components p47(phox) and p67(phox) were excluded from phagosomes in a LPG-dependent fashion. This inhibition is not the consequence of a general defect in the initiation of the NADPH oxidase activation process because both wild-type and LPG-defective promastigotes induced p47(phox) phosphorylation and the formation of complexes containing p47(phox) and p67(phox). Thus, by remodelling their intracellular habitat, L. donovani promastigotes prevent the assembly of a functional phagosomal NADPH oxidase complex, thereby evading an important host innate defence mechanism.

Phagocytosis of Leishmania donovani amastigotes is Rac1 dependent and occurs in the absence of NADPH oxidase activation

Macrophages produce little superoxide during phagocytosis of Leishmania donovani amastigotes. In this study, we characterized molecular events associated with L. donovani amastigotes uptake by mouse macrophages, to further define the mechanisms by which they are internalized without triggering superoxide production. Using transient transfections, we first showed that internalization of L. donovani amastigotes is mediated by the GTPases Rac1 and Arf6, of which Rac1 is recruited and retained on parasite-containing phagosomes. Next, we showed that, whereas internalization of amastigotes induced no superoxide release, co-internalization of serum-opsonized zymozan and amastigotes resulted in superoxide production. Furthermore, in co-internalization experiments, we detected superoxide production in over 95% of phagosomes containing IgG-opsonized SRBC compared to 5% of amastigote-harboring phagosomes. These results suggest that amastigotes evade the ability of macrophages to produce superoxide during phagocytosis. Consistently, we observed that amastigotes induced barely detectable phosphorylation of the NADPH oxidase component p47phox, leading to a defective phagosomal recruitment of p67phox and p47phox. Finally, we showed that amastigotes disrupt phagosomal lipid raft integrity, potentially interfering with NADPH oxidase assembly. Collectively, our results indicate that internalization of L. donovani amastigotes is a Rac1- and Arf6-dependent process that occurs in the absence of significant NADPH oxidase activation.

PtdIns3P binding to the PX domain of p40phox is a physiological signal in NADPH oxidase activation

The production of reactive oxygen species by the NADPH oxidase complex of phagocytes plays a critical role in our defence against bacterial and fungal infections. The PX domains of two oxidase components, p47(phox) and p40(phox), are known to bind phosphoinositide products of PI3Ks but the physiological roles of these interactions are unclear. We have created mice which carry an R58A mutation in the PX domain of their p40(phox) gene, which selectively prevents binding to PtdIns3P. p40(phoxR58A/R58A) embryos do not develop normally but p40(phoxR58A/-) mice are viable and neutrophils from these animals exhibit significantly reduced oxidase responses compared to those from their p40(phox+/-) siblings (e.g. 60% reduced in response to phagocytosis of Staphylococcus aureus). Wortmannin inhibition of the S. aureus oxidase response correlates with inhibition of phagosomal PtdIns3P accumulation and overlaps with the reduction in this response caused by the R58A mutation, suggesting PI3K regulation of this response is substantially dependent on PtdIns3P-binding to p40(phox). p40(phoxR58A/-) mice are significantly compromised in their ability to kill S. aureus in vivo, defining the physiological importance of this interaction.

Single-cell optical imaging of the phagocyte NADPH oxidase

The phagocyte NADPH oxidase is a key component of the innate immune response against invading microorganisms, because the generation of superoxide (O(2)(-)) inside the phagocytic vacuole by this enzyme is responsible for microbial killing by mechanisms that are directly or indirectly dependent on reactive oxygen species (ROS) formation. Most of what is known about the membrane-embedded and cytosolic NADPH oxidase subunits and their intricate network of interactions on assembly and activation has been derived from biochemical and biophysical studies involving subcellular fractionation or reconstituted cell-free systems. Such investigations can be complemented by single-cell microscopy on phagocytes, which may reveal spatial and/or temporal details about NADPH oxidase assembly that cannot be obtained from fractionated-cell assays. In recent years, we have investigated the NADPH oxidase in neutrophils using two complementary optical imaging techniques: Raman microscopy, a vibrational spectroscopic technique that does not require protein labeling, and live-cell fluorescence microscopy, which sheds light on the dynamics of NADPH oxidase assembly in individual cells. Here, we briefly introduce these techniques, compare their characteristics, and show their potential for studying NADPH oxidase at the single-cell level. New microscopy data are presented to illustrate the versatility of Raman and fluorescence microscopy on intact neutrophils.

Liposomes comprising anionic but not neutral phospholipids cause dissociation of Rac(1 or 2) x RhoGDI complexes and support amphiphile-independent NADPH oxidase activation by such complexes

Activation of the phagocyte NADPH oxidase involves the assembly of a membrane-localized cytochrome b559 with the cytosolic components p47(phox), p67(phox), p40(phox), and the GTPase Rac (1 or 2). In resting phagocytes, Rac is found in the cytosol as a prenylated protein in the GDP-bound form, associated with the Rho GDP dissociation inhibitor (RhoGDI). In the process of NADPH oxidase activation, Rac is dissociated from RhoGDI and translocates to the membrane, in concert with the other cytosolic components. The mechanism responsible for dissociation of Rac from RhoGDI is poorly understood. We generated Rac(1 or 2) x RhoGDI complexes in vitro from recombinant Rac(1 or 2), prenylated enzymatically, and recombinant RhoGDI, and purified these by anion exchange chromatography. Exposing Rac(1 or 2)(GDP) x RhoGDI complexes to liposomes containing four different anionic phospholipids caused the dissociation of Rac(1 or 2)(GDP) from RhoGDI and its binding to the anionic liposomes. Rac2(GDP) x RhoGDI complexes were more resistant to dissociation, reflecting the lesser positive charge of Rac2. Liposomes consisting of neutral phospholipid did not cause dissociation of Rac(1 or 2) x RhoGDI complexes. Rac1 exchanged to the hydrolysis-resistant GTP analogue, GMPPNP, associated with RhoGDI with lower affinity than Rac1(GDP) and Rac1(GMPPNP) x RhoGDI complexes were more readily dissociated by anionic liposomes. Rac1(GMPPNP) x RhoGDI complexes elicited NADPH oxidase activation in native phagocyte membrane liposomes in the presence of p67(phox), without the need for an anionic amphiphile, as activator. Both Rac1(GDP) x RhoGDI and Rac1(GMPPNP) x RhoGDI complexes elicited amphiphile-independent, p67(phox)-dependent NADPH oxidase activation in phagocyte membrane liposomes enriched in anionic phospholipids but not in membrane liposomes enriched in neutral phospholipids.

Regulation of membrane traffic by phosphoinositide 3-kinases

Phosphoinositide (PI) 3-kinases control essential cellular functions such as cytoskeletal dynamics, signal transduction and membrane trafficking. FYVE, PX and PH domains mediate the binding of effector proteins to the lipid products of PI 3-kinases. Recent studies have provided significant insights into the roles of PI 3-kinases, their catalytic products and their downstream effectors in membrane trafficking. Class I and II PI 3-kinases trigger receptor-induced trafficking processes, such as phagocytosis, macropinocytosis and regulated exocytosis. Class I PI 3-kinases also function to inhibit autophagy. By contrast, class III PI 3-kinases mainly mediate receptor-independent trafficking events, which mostly are related to endocytic membrane traffic, phagosome maturation and autophagy.

Ca2+ signalling and pancreatitis: effects of alcohol, bile and coffee

Ca2+ is a universal intracellular messenger that controls a wide range of cellular processes. In pancreatic acinar cells, acetylcholine and cholecystokinin regulate secretion via generation of repetitive local cytosolic Ca2+ signals in the apical pole. Bile acids and non-oxidative alcohol metabolites can elicit abnormal cytosolic Ca2+ signals that are global and sustained and result in necrosis. Necrosis results from excessive loss of Ca2+ from the endoplasmic reticulum, which is mediated by Ca2+ release through specific channels and inhibition of Ca2+ pumps in intracellular stores, followed by entry of extracellular Ca2+. Reduction of the cellular ATP level has a major role in this process. These abnormal Ca2+ signals, which can be inhibited by caffeine, explain how excessive alcohol intake and biliary disease cause acute pancreatitis, an often-fatal human disease in which the pancreas digests itself and its surroundings.

Single-cell Raman and fluorescence microscopy reveal the association of lipid bodies with phagosomes in leukocytes

Cellular imaging techniques based on vibrational spectroscopy have become powerful tools in cell biology because the molecular composition of subcellular compartments can be visualized without the need for labeling. Using high-resolution, nonresonant confocal Raman microscopy on individual cells, we demonstrate here that lipid bodies (LBs) rich in arachidonate as revealed by their Raman spectra associate with latex bead-containing phagosomes in neutrophilic granulocytes. This finding was corroborated in macrophages and in PLB-985 cells, which can be induced to differentiate into neutrophil-like cells, by selective staining of LBs and visualization by confocal fluorescence microscopy. We further show that the accumulation of LBs near phagosomes is mediated at least in part by the flavohemoprotein gp91phox (in which "phox" is phagocyte oxidase), because different LB distributions around phagocytosed latex beads were observed in WT and gp91phox-deficient PLB-985 cells. gp91phox, which accumulates in the phagosomal membrane, is the catalytic subunit of the leukocyte NADPH oxidase, a critical enzyme in the innate immune response. Finally, time-lapse fluorescence microscopy experiments on neutrophils revealed that the LB-phagosome association is transient, similar to the "kiss-and-run" behavior displayed by endosomes involved in phagosome maturation. Because arachidonic acid (AA) has been shown to be involved in NADPH oxidase activation and phagosome maturation in neutrophils and macrophages, respectively, the findings reported here suggest that LBs may provide a reservoir of AA for local activation of these essential leukocyte functions.