Distinct roles of class I and class III phosphatidylinositol 3-kinases in phagosome formation and maturation

Dynamic macrophage "probing" is required for the efficient capture of phagocytic targets

Binding of ligands by immunoreceptors is thought to be a passive, stochastic process. Contrary to this notion, we found that binding of IgG-opsonized particles by Fcγ receptors was inhibited in macrophages, dendritic and microglial cells by agents that interfere with actin assembly or disassembly. Changes in the lateral mobility of the receptors--assessed by single-particle tracking--or in the microelasticity of the membrane--determined by atomic-force microscopy--could not account for the effects of actin disruption on particle binding. Instead, we found that the macrophages contact their targets by actively extending actin-rich structures. Formation of these protrusions is driven by Rac and requires phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Capture of C3bi-opsonized as well as unopsonized targets by macrophages was also dependent on actin. Thus, phagocytes continuously probe their environment for foreign particles in a manner akin to the constitutive sampling of the fluid milieu by dendritic cells. Active probing by phagocytes is most important when confronted by scarcely opsonized and/or highly mobile targets.

PtdIns3P and Rac direct the assembly of the NADPH oxidase on a novel, pre-phagosomal compartment during FcR-mediated phagocytosis in primary mouse neutrophils

The generation of reactive oxygen species (ROS) by the nicotinamide adenine dinucleotide phosphate oxidase is an important mechanism by which neutrophils kill pathogens. The oxidase is composed of a membrane-bound cytochrome and 4 soluble proteins (p67(phox), p40(phox), p47(phox), and GTP-Rac). These components form an active complex at the correct time and subcellular location through a series of incompletely understood mutual interactions, regulated, in part, by GTP/GDP exchange on Rac, protein phosphorylation, and binding to lipid messengers. We have used a variety of assays to follow the spatiotemporal assembly of the oxidase in genetically engineered primary mouse neutrophils, during phagocytosis of both serum- and immunoglobulin G-opsonized targets. The oxidase assembles directly on serum-Staphylococcus aureus-containing phagosomes within seconds of phagosome formation; this process is only partially dependent (∼ 30%) on PtdIns3P binding to p40(phox), but totally dependent on Rac1/2 binding to p67(phox). In contrast, in response to immunoglobulin G-targets, the oxidase first assembles on a tubulovesicular compartment that develops at sites of granule fusion to the base of the emerging phagosome; oxidase assembly and activation is highly dependent on both PtdIns3P-p40(phox) and Rac2-p67(phox) interactions and delivery to the phagosome is regulated by Rab27a. These results define a novel pathway for oxidase assembly downstream of FcR-activation.

Phosphoinositides differentially regulate bacterial uptake and Nramp1-induced resistance to Legionella infection in Dictyostelium

Membrane phosphatidylinositides recruit cytosolic proteins to regulate phagocytosis, macropinocytosis and endolysosomal vesicle maturation. Here, we describe effects of inactivation of PI3K, PTEN or PLC on Escherichia coli and Legionella pneumophila uptake by the professional phagocyte Dictyostelium discoideum. We show that L. pneumophila is engulfed by macropinocytosis, a process that is partially sensitive to PI3K inactivation, unlike phagocytosis of E. coli. Both processes are blocked by PLC inhibition. Whereas E. coli is rapidly digested, Legionella proliferates intracellularly. Proliferation is blocked by constitutively expressing Nramp1, an endolysosomal iron transporter that confers resistance against invasive bacteria. Inactivation of PI3K, but not PTEN or PLC, enhances Legionella infection and suppresses the protective effect of Nramp1 overexpression. PI3K activity is restricted to early infection and is not mediated by effects on the actin cytoskeleton; rather L. pneumophila, in contrast to E. coli, subverts phosphoinositide-sensitive fusion of Legionella-containing macropinosomes with acidic vesicles, without affecting Nramp1 recruitment. A model is presented to explain how Legionella escapes fusion with acidic vesicles and Nramp1-induced resistance to pathogens.

The dynamic interaction of AMBRA1 with the dynein motor complex regulates mammalian autophagy

Autophagy is an evolutionary conserved catabolic process involved in several physiological and pathological processes such as cancer and neurodegeneration. Autophagy initiation signaling requires both the ULK1 kinase and the BECLIN 1-VPS34 core complex to generate autophagosomes, double-membraned vesicles that transfer cellular contents to lysosomes. In this study, we show that the BECLIN 1-VPS34 complex is tethered to the cytoskeleton through an interaction between the BECLIN 1-interacting protein AMBRA1 and dynein light chains 1/2. When autophagy is induced, ULK1 phosphorylates AMBRA1, releasing the autophagy core complex from dynein. Its subsequent relocalization to the endoplasmic reticulum enables autophagosome nucleation. Therefore, AMBRA1 constitutes a direct regulatory link between ULK1 and BECLIN 1-VPS34, which is required for core complex positioning and activity within the cell. Moreover, our results demonstrate that in addition to a function for microtubules in mediating autophagosome transport, there is a strict and regulatory relationship between cytoskeleton dynamics and autophagosome formation.

Imaging signal transduction during phagocytosis: phospholipids, surface charge, and electrostatic interactions

Together with the development of genetically encoded fluorescent probes, digital imaging has provided great impetus to the study of cell signaling by providing enhanced sensitivity and much-improved spatial and temporal resolution. We have used phagocytosis as a paradigm of signal transduction, taking advantage of the generous size of phagosomes and of their comparatively leisurely rate of formation. Aided by the design of specific probes, we demonstrated a highly localized and elegantly choreographed sequence of changes in the level of several phosphoinositides and were able to also monitor the fate of phosphatidylserine. The net changes in the content of these anionic phospholipids are accompanied by marked alterations in the surface charge of the membrane of nascent phagosomes. These, in turn, cause the relocation of proteins that associate with the membrane by electrostatic interactions. Our studies suggest that anionic lipids control protein targeting not only through stereospecific recognition by specialized domains but also by electrostatic association mediated by polycationic motifs. The “electrostatic switch” can be turned on or off by altering the charge of the protein ligand (e.g., by phosphorylation) or, alternatively, by modifying the lipid composition of the target membrane.

SLAM is a microbial sensor that regulates bacterial phagosome functions in macrophages

Phagocytosis is a pivotal process by which macrophages eliminate microorganisms after recognition by pathogen sensors. Here we unexpectedly found that the self ligand and cell surface receptor SLAM functioned not only as a costimulatory molecule but also as a microbial sensor that controlled the killing of gram-negative bacteria by macrophages. SLAM regulated activity of the NADPH oxidase NOX2 complex and phagolysosomal maturation after entering the phagosome, following interaction with the bacterial outer membrane proteins OmpC and OmpF. SLAM recruited a complex containing the intracellular class III phosphatidylinositol kinase Vps34, its regulatory protein kinase Vps15 and the autophagy-associated molecule beclin-1 to the phagosome, which was responsible for inducing the accumulation of phosphatidylinositol-3-phosphate, a regulator of both NOX2 function and phagosomal or endosomal fusion. Thus, SLAM connects the gram-negative bacterial phagosome to ubiquitous cellular machinery responsible for the control of bacterial killing.

p47phox Phox homology domain regulates plasma membrane but not phagosome neutrophil NADPH oxidase activation

The assembly of cytosolic subunits p47(phox), p67(phox), and p40(phox) with flavocytochrome b(558) at the membrane is required for activating the neutrophil NADPH oxidase that generates superoxide for microbial killing. The p47(phox) subunit plays a critical role in oxidase assembly. Recent studies showed that the p47(phox) Phox homology (PX) domain mediates phosphoinositide binding in vitro and regulates phorbol ester-induced NADPH oxidase activity in a K562 myeloid cell model. Because the importance of the p47(phox) PX domain in neutrophils is unclear, we investigated its role using p47(phox) knock-out (KO) mouse neutrophils to express human p47(phox) and derivatives harboring R90A mutations in the PX domain that result in loss of phosphoinositide binding. Human p47(phox) proteins were expressed at levels similar to endogenous murine p47(phox), with the exception of a chronic granulomatous disease-associated R42Q mutant that was poorly expressed, and wild type human p47(phox) rescued p47(phox) KO mouse neutrophil NADPH oxidase activity. Plasma membrane NAPDH oxidase activity was reduced in neutrophils expressing p47(phox) with Arg(90) substitutions, with substantial effects on responses to either phorbol ester or formyl-Met-Leu-Phe and more modest effects to particulate stimuli. In contrast, p47(phox) Arg(90) mutants supported normal levels of intracellular NADPH oxidase activity during phagocytosis of a variety of particles and were recruited to phagosome membranes. This study defines a differential and agonist-dependent role of the p47(phox) PX domain for neutrophil NADPH oxidase activation.

Finding a fitting shoe for Cinderella: searching for an autophagy inhibitor

Vps34 is the ancestral phosphatidylinositol 3-kinase (PtdIns3K) isoform and is essential for endosomal trafficking of proteins to the vacuole/lysosome, autophagy and phagocytosis. Vps34-containing complexes associate with specific cellular compartments to produce PtdIns(3)P. Understanding the roles of Vps34 has been hampered by the lack of potent, specific inhibitors. To boost development of Vps34 inhibitors, we determined the crystal structures of Vps34 alone and in complexes with multitargeted PtdIns3K inhibitors. These structures provided a first glimpse into the uniquely constricted ATP-binding site of Vps34 and enabled us to model Vps34 regulation. We showed that the substrate-binding "activation" loop and the flexibly attached amphipathic C-terminal helix are crucial for catalysis on membranes. The C-terminal helix also suppresses ATP hydrolysis in the absence of membranes. We propose that membrane binding shifts the C-terminal helix to orient the enzyme for catalysis, and the Vps15 regulatory subunit, which binds to this and the preceding helix, may facilitate this process. This C-terminal region may also represent a target for specific, non-ATP-competitive PtdIns3K inhibitors.

A diacylglycerol-dependent signaling pathway contributes to regulation of antibacterial autophagy

Autophagy mediates the degradation of cytoplasmic contents in the lysosome and plays a significant role in immunity. Lipid second messengers have previously been implicated in the regulation of autophagy. Here, we demonstrate a signaling role for diacylglycerol (DAG) in antibacterial autophagy. DAG production was necessary for efficient autophagy of Salmonella, and its localization to bacteria-containing phagosomes preceded autophagy. The actions of phospholipase D and phosphatidic acid phosphatase were required for DAG generation and autophagy. Furthermore, the DAG-responsive delta isoform of protein kinase C was required, as were its downstream targets JNK and NADPH oxidase. Previous studies have revealed a role for the ubiquitin-binding adaptor molecules p62 and NDP52 in autophagy of S. Typhimurium. We observed bacteria-containing autophagosomes colocalizing individually with either DAG or ubiquitinated proteins, indicating that both signals can act independently to promote antibacterial autophagy. These findings reveal an important role for DAG-mediated PKC function in mammalian antibacterial autophagy.

pp60c-Src phosphorylates and activates vacuolar protein sorting 34 to mediate cellular transformation

Vacuolar protein sorting 34 (VPS34) contributes to the regulation of the mammalian target of rapamycin complex 1/S6 kinase 1 pathway downstream of nutrient signaling. However, intracellular mechanisms leading to VPS34 activation remain unclear. Here, we report that Src directly phosphorylates VPS34, and that this phosphorylation activates VPS34 lipid kinase activity, leading to Src-Y527F-mediated cellular transformation. Silencing endogenous VPS34 specifically inhibits Src-Y527F-induced colony formation in soft agar, but not Ras-G12V-induced colony formation. We have identified two novel hVPS34 mutations, which either eliminate lipid kinase activity (kinase-dead mutant) or reduce tyrosine phosphorylation by Src-Y527F. When kinase-dead mutant of hVPS34 is stably expressed in Src-Y527F-transformed cells, transformation activities are blocked, indicating that the lipid kinase activity of hVPS34 is essential for Src-mediated cellular transformation. Furthermore, stable expression of this hVPS34 kinase-dead mutant causes an increased number of binucleate and multinucleate cells, suggesting that the kinase activity of hVPS34 is also required for cytokinesis. Moreover, when the hVPS34 mutant that has reduced tyrosine phosphorylation by Src is stably expressed in Src-Y527F-transformed cells, Src-Y527F-stimulated colony formation is also reduced. Data presented here provide important evidence that VPS34 lipid kinase activity could be positively regulated by Src-mediated tyrosine phosphorylation in mammalian cells. This finding highlights a previously unappreciated relationship between VPS34, a class III phosphatidylinositol-3-kinase, and Src non-receptor tyrosine kinase. Additionally, we find that the levels of VPS34 expression and tyrosine phosphorylation are correlated with the tumorigenic activity of human breast cancer cells, indicating that Src to VPS34 signaling warrants further investigation as a pathway contributing to the development and progression of human cancers.

Multiple phosphatidylinositol 3-kinases regulate vaccinia virus morphogenesis

Poxvirus morphogenesis is a complex process that involves the successive wrapping of the virus in host cell membranes. We screened by plaque assay a focused library of kinase inhibitors for those that caused a reduction in viral growth and identified several compounds that selectively inhibit phosphatidylinositol 3-kinase (PI3K). Previous studies demonstrated that PI3Ks mediate poxviral entry. Using growth curves and electron microscopy in conjunction with inhibitors, we show that that PI3Ks additionally regulate morphogenesis at two distinct steps: immature to mature virion (IMV) transition, and IMV envelopment to form intracellular enveloped virions (IEV). Cells derived from animals lacking the p85 regulatory subunit of Type I PI3Ks (p85α^−/−β^−/−) presented phenotypes similar to those observed with PI3K inhibitors. In addition, VV appear to redundantly use PI3Ks, as PI3K inhibitors further reduce plaque size and number in p85α^−/−β^−/− cells. Together, these data provide evidence for a novel regulatory mechanism for virion morphogenesis involving phosphatidylinositol dynamics and may represent a new therapeutic target to contain poxviruses.

Sorting nexin 3 (SNX3) is a component of a tubular endosomal network induced by Salmonella and involved in maturation of the Salmonella-containing vacuole

Salmonella enterica serovar Typhimurium is an intracellular pathogen that grows within a modified endomembrane compartment, the Salmonella-containing vacuole (SCV). Maturation of nascent SCVs involves the recruitment of early endosome markers and the remodelling of phosphoinositides at the membrane of the vacuole, in particular the production of phosphatidylinositol 3-phosphate [PI(3)P]. Sorting nexins (SNXs) are a family of proteins characterized by the presence of a phox homology (PX) domain that binds to phosphoinositides and are involved in intracellular trafficking in eukaryotic cells. We therefore studied whether sorting nexins, particularly sorting nexin 3 (SNX3), play a role in Salmonella infection. We found that SNX3 transiently localized to SCVs at early times post invasion (10 min) and presented a striking tubulation phenotype in the vicinity of SCVs at later times (30-60 min). The bacterial effector SopB, which is known to promote PI(3)P production on SCVs, was required for the formation of SNX3 tubules. In addition, RAB5 was also required for the formation of SNX3 tubules. Depletion of SNX3 by siRNA impaired RAB7 and LAMP1 recruitment to the SCV. Moreover, the formation of Salmonella-induced filaments (Sifs) was altered by SNX3 knock-down. Therefore, SNX3 plays a significant role in regulating the maturation of SCVs.

The emerging mechanisms of isoform-specific PI3K signalling

Phosphoinositide 3-kinases (PI3Ks) function early in intracellular signal transduction pathways and affect many biological functions. A further level of complexity derives from the existence of eight PI3K isoforms, which are divided into class I, class II and class III PI3Ks. PI3K signalling has been implicated in metabolic control, immunity, angiogenesis and cardiovascular homeostasis, and is one of the most frequently deregulated pathways in cancer. PI3K inhibitors have recently entered clinical trials in oncology. A better understanding of how the different PI3K isoforms are regulated and control signalling could uncover their roles in pathology and reveal in which disease contexts their blockade could be most beneficial.

Pivotal Advance: Phospholipids determine net membrane surface charge resulting in differential localization of active Rac1 and Rac2

In this investigation, we used primary murine neutrophils to demonstrate that local changes in membrane phospholipid composition alter the net cytoplasmic membrane surface charge, which results in selective recruitment of Rac1 or Rac2 based on the net charge of their respective C-terminal domains. Murine neutrophils undergoing chemotaxis or carrying out phagocytosis were transfected with K-ras4B-derived membrane charge biosensors and lipid markers, which allowed us to simultaneously monitor the levels of PIP(2), PIP(3), and PS and net membrane charge of the newly developing phagosome membrane and plasma membrane. Our results indicate that the combination of PIP(2), PIP(3), and PS generates a high negative charge (-8) at the plasma membrane of actin-rich pseudopods, where active Rac1 preferentially localizes during phagosome formation. The lipid metabolism that occurs during phagosome maturation results in the localized depletion of PIP(2), PIP(3), and partial decrease in PS. This creates a moderately negative net charge that correlates with the localization of active Rac2. Conversely, the accumulation of PIP(3) at the leading-edge membrane during chemotaxis generates a polarized accumulation of negative charges that recruits Rac1. These results provide evidence that alterations in membrane lipid composition and inner-membrane surface charge are important elements for the recruitment of differentially charged proteins and localization of signaling pathways during phagocytosis and chemotaxis in neutrophils.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

A Cdc42 activation cycle coordinated by PI 3-kinase during Fc receptor-mediated phagocytosis

Fcgamma Receptor (FcR)-mediated phagocytosis by macrophages requires phosphatidylinositol 3-kinase (PI3K) and activation of the Rho-family GTPases Cdc42 and Rac1. Cdc42 is activated at the advancing edge of the phagocytic cup, where actin is concentrated, and is deactivated at the base of the cup. The timing of 3' phosphoinositide (3'PI) concentration changes in cup membranes suggests a role for 3'PIs in deactivation of Cdc42. This study examined the relationships between PI3K and the patterns of Rho-family GTPase signaling during phagosome formation. Inhibition of PI3K resulted in persistently active Cdc42 and Rac1, but not Rac2, in stalled phagocytic cups. Patterns of 3'PIs and Rho-family GTPase activities during phagocytosis of 5- and 2-mum-diameter microspheres indicated similar underlying mechanisms despite particle size-dependent sensitivities to PI3K inhibition. Expression of constitutively active Cdc42(G12V) increased 3'PI concentrations in plasma membranes and small phagosomes, indicating a role for Cdc42 in PI3K activation. Cdc42(G12V) inhibited phagocytosis at a later stage than inhibition by dominant negative Cdc42(N17). Together, these studies identified a Cdc42 activation cycle organized by PI3K, in which FcR-activated Cdc42 stimulates PI3K and actin polymerization, and the subsequent increase of 3'PIs in cup membranes inactivates Cdc42 to allow actin recycling necessary for phagosome formation.

Mycobacterial nucleoside diphosphate kinase blocks phagosome maturation in murine RAW 264.7 macrophages

Background

Microorganisms capable of surviving within macrophages are rare, but represent very successful pathogens. One of them is Mycobacterium tuberculosis (Mtb) whose resistance to early mechanisms of macrophage killing and failure of its phagosomes to fuse with lysosomes causes tuberculosis (TB) disease in humans. Thus, defining the mechanisms of phagosome maturation arrest and identifying mycobacterial factors responsible for it are key to rational design of novel drugs for the treatment of TB. Previous studies have shown that Mtb and the related vaccine strain, M. bovis bacille Calmette-Guérin (BCG), disrupt the normal function of host Rab5 and Rab7, two small GTPases that are instrumental in the control of phagosome fusion with early endosomes and late endosomes/lysosomes respectively.

Methodology/Principal Findings

Here we show that recombinant Mtb nucleoside diphosphate kinase (Ndk) exhibits GTPase activating protein (GAP) activity towards Rab5 and Rab7. Then, using a model of latex bead phagosomes, we demonstrated that Ndk inhibits phagosome maturation and fusion with lysosomes in murine RAW 264.7 macrophages. Maturation arrest of phagosomes containing Ndk-beads was associated with the inactivation of both Rab5 and Rab7 as evidenced by the lack of recruitment of their respective effectors EEA1 (early endosome antigen 1) and RILP (Rab7-interacting lysosomal protein). Consistent with these findings, macrophage infection with an Ndk knocked-down BCG strain resulted in increased fusion of its phagosome with lysosomes along with decreased survival of the mutant.

Conclusion

Our findings provide evidence in support of the hypothesis that mycobacterial Ndk is a putative virulence factor that inhibits phagosome maturation and promotes survival of mycobacteria within the macrophage.

Retrieval of the vacuolar H-ATPase from phagosomes revealed by live cell imaging

BACKGROUND

The vacuolar H+-ATPase, or V-ATPase, is a highly-conserved multi-subunit enzyme that transports protons across membranes at the expense of ATP. The resulting proton gradient serves many essential functions, among them energizing transport of small molecules such as neurotransmitters, and acidifying organelles such as endosomes. The enzyme is not present in the plasma membrane from which a phagosome is formed, but is rapidly delivered by fusion with endosomes that already bear the V-ATPase in their membranes. Similarly, the enzyme is thought to be retrieved from phagosome membranes prior to exocytosis of indigestible material, although that process has not been directly visualized.

METHODOLOGY

To monitor trafficking of the V-ATPase in the phagocytic pathway of Dictyostelium discoideum, we fed the cells yeast, large particles that maintain their shape during trafficking. To track pH changes, we conjugated the yeast with fluorescein isothiocyanate. Cells were labeled with VatM-GFP, a fluorescently-tagged transmembrane subunit of the V-ATPase, in parallel with stage-specific endosomal markers or in combination with mRFP-tagged cytoskeletal proteins.

PRINCIPAL FINDINGS

We find that the V-ATPase is commonly retrieved from the phagosome membrane by vesiculation shortly before exocytosis. However, if the cells are kept in confined spaces, a bulky phagosome may be exocytosed prematurely. In this event, a large V-ATPase-rich vacuole coated with actin typically separates from the acidic phagosome shortly before exocytosis. This vacuole is propelled by an actin tail and soon acquires the properties of an early endosome, revealing an unexpected mechanism for rapid recycling of the V-ATPase. Any V-ATPase that reaches the plasma membrane is also promptly retrieved.

CONCLUSIONS/SIGNIFICANCE

Thus, live cell microscopy has revealed both a usual route and alternative means of recycling the V-ATPase in the endocytic pathway.

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.

The application of fluorescent probes for the analysis of lipid dynamics during phagocytosis

Phagocytosis is the process whereby specialized leukocytes ingest large particles. This is an extremely dynamic and localized process that requires the recruitment to the sites of ingestion of numerous effector proteins, together with extensive lipid remodelling. To investigate such a dynamic series of events in living cells, non-invasive methods are required. The use of fluorescent probes in conjunction with spectroscopic analysis is optimally suited for this purpose. Here we describe a method to express in RAW264.7 murine macrophages genetically encoded probes that allow for the spatio-temporal analysis of lipid distribution and metabolism during phagocytosis of immunoglobulin-opsonized beads. The fluorescence of the probes is best analysed by laser scanning or spinning disc confocal microscopy. While the focus of this chapter is on phagocytic events, this general method can be employed for the analysis of lipid distribution and dynamics during a variety of biological processes in the cell type of the investigator's choice.

Cellular and nuclear degradation during apoptosis

Apoptosis ensures quick death and quiet clearance of unwanted or damaged cells, without inducing much, if any, immunological responses from the organism. In metazoan organisms, apoptotic cells are swiftly engulfed by other cells. The degradation of cellular content is initiated in apoptotic cells and completed within engulfing cells. In apoptotic cells, caspase-mediated proteolysis cleaves protein substrates into fragments; nuclear DNA is partially degraded into nucleosomal units; and autophagy potentially contributes to apoptotic cell removal. In engulfing cells, specific signaling pathways promote the sequential fusion of intracellular vesicles with phagosomes and lead to the complete degradation of apoptotic cells in an acidic environment. Phagocytic receptors that initiate the engulfment of apoptotic cells play an additional and crucial role in initiating phagosome maturation through activating these signaling pathways. Here we highlight recent discoveries made in invertebrate models and mammalian systems, focusing on the molecular mechanisms that regulate the efficient degradation of apoptotic cells.

Akt and SHIP modulate Francisella escape from the phagosome and induction of the Fas-mediated death pathway

Francisella tularensis infects macrophages and escapes phago-lysosomal fusion to replicate within the host cytosol, resulting in host cell apoptosis. Here we show that the Fas-mediated death pathway is activated in infected cells and correlates with escape of the bacterium from the phagosome and the bacterial burden. Our studies also demonstrate that constitutive activation of Akt, or deletion of SHIP, promotes phago-lysosomal fusion and limits bacterial burden in the host cytosol, and the subsequent induction of Fas expression and cell death. Finally, we show that phagosomal escape/intracellular bacterial burden regulate activation of the transcription factors sp1/sp3, leading to Fas expression and cell death. These data identify for the first time host cell signaling pathways that regulate the phagosomal escape of Francisella, leading to the induction of Fas and subsequent host cell death.

Ultrastructural analysis of lysosome reactions to inside-out cell membrane vesicles in a cell-free system

Lysosome reactions were ultrastructurally analyzed using a cell-free system with inside-out cell membrane vesicles (IOVs) prepared from rat erythrocyte ghosts in an alkaline buffer and with wheat germ agglutinin-coated colloidal gold particles (WGA-CGs). The submembranous surface coat in the ghosts was depleted from the IOVs' outer surfaces. When lysosomes from rat liver were incubated with these IOVs, some of the trilaminar membranes of the lysosomes and IOVs came into close contact and formed a five-laminar structure without an intermembranous gap. In other reactions, the membranes of both structures formed one continuous trilaminar membrane along the margin of contact and ruffling five-laminar structures in other regions. Several lysosomes exhibited invaginating hollows or projections that entrapped or encircled the IOVs. Similar five-laminar structures were seen at a few points of contact between the IOVs and the hollowing or projecting membranes. In contrast, such reactions were much rarer when IOVs with reconstituted spectrins and actins on their outer surface were used. The formation of tubuliform pits with membrane-bound WGA-CGs was also observed after the incubation of lysosomes with WGA-CGs. These observations suggest that lysosomes fuse with cytoskeleton-depleted IOVs, wrap arm-like projections around them, enclose them by invagination or incorporate their membrane-bound macromolecules through the process of tubuliform invagination. Furthermore, the fusion and wrapping processes are not necessarily independent.

A new genetic subgroup of chronic granulomatous disease with autosomal recessive mutations in p40 phox and selective defects in neutrophil NADPH oxidase activity

Chronic granulomatous disease (CGD), an immunodeficiency with recurrent pyogenic infections and granulomatous inflammation, results from loss of phagocyte superoxide production by recessive mutations in any 1 of 4 genes encoding subunits of the phagocyte NADPH oxidase. These include gp91(phox) and p22(phox), which form the membrane-integrated flavocytochrome b, and cytosolic subunits p47(phox) and p67(phox). A fifth subunit, p40(phox), plays an important role in phagocytosis-induced superoxide production via a phox homology (PX) domain that binds to phosphatidylinositol 3-phosphate (PtdIns(3)P). We report the first case of autosomal recessive mutations in NCF4, the gene encoding p40(phox), in a boy who presented with granulomatous colitis. His neutrophils showed a substantial defect in intracellular superoxide production during phagocytosis, whereas extracellular release of superoxide elicited by phorbol ester or formyl-methionyl-leucyl-phenylalanine (fMLF) was unaffected. Genetic analysis of NCF4 showed compound heterozygosity for a frameshift mutation with premature stop codon and a missense mutation predicting a R105Q substitution in the PX domain. Parents and a sibling were healthy heterozygous carriers. p40(phox)R105Q lacked binding to PtdIns(3)P and failed to reconstitute phagocytosis-induced oxidase activity in p40(phox)-deficient granulocytes, with premature loss of p40(phox)R105Q from phagosomes. Thus, p40(phox) binding to PtdIns(3)P is essential for phagocytosis-induced oxidant production in human neutrophils and its absence can be associated with disease.

Beta1 integrin-dependent engulfment of Yersinia enterocolitica by macrophages is coupled to the activation of autophagy and suppressed by type III protein secretion

Autophagy is a central lysosomal degradation process that is essential for the maintenance of cellular homeostasis. Autophagy has furthermore emerged as integral part of the host immune response. Autophagic processes promote the separation and degradation of intracellular microorganisms which contributes to the development of innate and adaptive immunity. Some pathogenic microbes have therefore evolved mechanisms to evade or impede autophagy. We analyzed the effects of the enteropathogenic bacterium Yersinia enterocolitica on autophagy in macrophages. Yersiniae use a number of defined adhesins and secreted proteins to manipulate host immune responses. Our results showed that Y. enterocolitica defective in type III protein secretion efficiently activated autophagy in macrophages. Autophagy was mediated by the Yersinia adhesins invasin and YadA and particularly depended on the engagement of beta(1) integrin receptors. Several autophagy-related events followed beta(1) integrin-mediated engulfment of the bacteria including the formation of autophagosomes, processing of the marker protein LC3, redistribution of GFP-LC3 to bacteria-containing vacuoles, and the segregation of intracellular bacteria by autophagosomal compartments. These results provide direct evidence for the linkage of beta(1) integrin-mediated phagocytosis and autophagy induction. Multiple microbes signal through integrin receptors, and our results suggest a general principle by which the sensing of an extracellular microbe triggers autophagy. Owing to the importance of autophagy as host defense response, wild-type Y. enterocolitica suppressed autophagy by mobilizing type III protein secretion. The subversion of autophagy may be part of the Y. enterocolitica virulence strategy that supports bacterial survival when beta(1) integrin-dependent internalization and autophagy activation by macrophages are deleterious for the pathogen.

Targeting and regulation of reactive oxygen species generation by Nox family NADPH oxidases

Nox family NADPH oxidases serve a variety of functions requiring reactive oxygen species (ROS) generation, including antimicrobial defense, biosynthetic processes, oxygen sensing, and redox-based cellular signaling. We explored targeting, assembly, and activation of several Nox family oxidases, since ROS production appears to be regulated both spatially and temporally. Nox1 and Nox3 are similar to the phagocytic (Nox2-based) oxidase, functioning as multicomponent superoxide-generating enzymes. Factors regulating their activities include cytosolic activator and organizer proteins and GTP-Rac. Their regulation varies, with the following rank order: Nox2 > Nox1 > Nox3. Determinants of subcellular targeting include: (a) formation of Nox-p22(phox) heterodimeric complexes allowing plasma membrane translocation, (b) phospholipids-binding specificities of PX domain-containing organizer proteins (p47(phox) or Nox organizer 1 (Noxo1 and p40(phox)), and (c) variably splicing of Noxo1 PX domains directing them to nuclear or plasma membranes. Dual oxidases (Duox1 and Duox2) are targeted by different mechanisms. Plasma membrane targeting results in H(2)O(2) release, not superoxide, to support extracellular peroxidases. Human Duox1 and Duox2 have no demonstrable peroxidase activity, despite their extensive homology with heme peroxidases. The dual oxidases were reconstituted by Duox activator 2 (Duoxa2) or two Duoxa1 variants, which dictate maturation, subcellular localization, and the type of ROS generated by forming stable complexes with Duox.

C. elegans Rab GTPase activating protein TBC-2 promotes cell corpse degradation by regulating the small GTPase RAB-5

During apoptosis, dying cells are quickly internalized by neighboring cells or phagocytes, and are enclosed in phagosomes that undergo a maturation process to generate the phagoslysosome, in which cell corpses are eventually degraded. It is not well understood how apoptotic cell degradation is regulated. Here we report the identification and characterization of the C. elegans tbc-2 gene, which is required for the efficient degradation of cell corpses. tbc-2 encodes a Rab GTPase activating protein (GAP) and its loss of function affects several events of phagosome maturation, including RAB-5 release, phosphatidylinositol 3-phosphate dynamics, phagosomal acidification, RAB-7 recruitment and lysosome incorporation, which leads to many persistent cell corpses at various developmental stages. Intriguingly, the persistent cell corpse phenotype of tbc-2 mutants can be suppressed by reducing gene expression of rab-5, and overexpression of a GTP-locked RAB-5 caused similar defects in phagosome maturation and cell corpse degradation. We propose that TBC-2 functions as a GAP to cycle RAB-5 from an active GTP-bound to an inactive GDP-bound state, which is required for maintaining RAB-5 dynamics on phagosomes and serves as a switch for the progression of phagosome maturation.

Specific role of phosphoinositide 3-kinase p110alpha in the regulation of phagocytosis and pinocytosis in macrophages

PI3K (phosphoinositide 3-kinase) alpha has been implicated in phagocytosis and fluid-phase pinocytosis in macrophages. The subtype-specific role of PI3K in these processes is poorly understood. To elucidate this issue, we made Raw 264.7 cells (a mouse leukaemic monocyte-macrophage cell line) deficient in each of the class-I PI3K catalytic subunits: p110alpha, p110beta, p110delta and p110gamma. Among these cells, only the p110alpha-deficient cells exhibited lower phagocytosis of opsonized and non-opsonized zymosan. The p110alpha-deficient cells also showed the impaired phagocytosis of IgG-opsonized erythrocytes and the impaired fluid-phase pinocytosis of dextran (molecular mass of 40 kDa). Receptor-mediated pinocytosis of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate)-labelled acetylated low-density lipoprotein and fluid-phase pinocytosis of Lucifer Yellow (molecular mass of 500 Da) were resistant to p110alpha depletion. None of these processes were impaired in cells lacking p110beta, p110delta or p110gamma, but were susceptible to a pan-PI3K inhibitor wortmannin. In cells deficient in the enzymes catalysing PtdIns(3,4,5)P3 breakdown [PTEN (phosphatase and tensin homologue deleted on chromosome 10) or SHIP-1 (Src-homology-2-domain-containing inositol phosphatase-1)], uptake of IgG-opsonized particles was enhanced. These results indicated that phagocytosis and fluid-phase pinocytosis of larger molecules are dependent on the lipid kinase activity of p110alpha, whereas pinocytosis via clathrin-coated and small non-coated vesicles may depend on subtypes of PI3Ks other than class I.

Techniques to study autophagy in plants

Autophagy (or self eating), a cellular recycling mechanism, became the center of interest and subject of intensive research in recent years. Development of new molecular techniques allowed the study of this biological phenomenon in various model organisms ranging from yeast to plants and mammals. Accumulating data provide evidence that autophagy is involved in a spectrum of biological mechanisms including plant growth, development, response to stress, and defense against pathogens. In this review, we briefly summarize general and plant-related autophagy studies, and explain techniques commonly used to study autophagy. We also try to extrapolate how autophagy techniques used in other organisms may be adapted to plant studies.

Targeting of the GTPase Irgm1 to the phagosomal membrane via PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3) promotes immunity to mycobacteria

Vertebrate immunity to infection enlists a newly identified family of 47-kilodalton immunity-related GTPases (IRGs). One IRG in particular, Irgm1, is essential for macrophage host defense against phagosomal pathogens, including Mycobacterium tuberculosis (Mtb). Here we show that Irgm1 targets the mycobacterial phagosome through lipid-mediated interactions with phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P(2)) and PtdIns(3,4,5)P(3). An isolated Irgm1 amphipathic helix conferred lipid binding in vitro and in vivo. Substitutions in this region blocked phagosome recruitment and failed to complement the antimicrobial defect in Irgm1(-/-) macrophages. Removal of PtdIns(3,4,5)P(3) or inhibition of class I phosphatidylinositol-3-OH kinase (PI(3)K) mimicked this effect in wild-type cells. Cooperation between Irgm1 and PI(3)K further facilitated the engagement of Irgm1 with its fusogenic effectors at the site of infection, thereby ensuring pathogen-directed responses during innate immunity.

Toll-like receptor signaling in the lysosomal pathways

The lysosomal pathway digests material received by two main routes, phagocytosis and autophagy. Cells use phagocytosis to ingest extracellular particles by invaginations of the plasma membrane. In autophagy, a double membrane structure isolates portions of the cytoplasm to target it for degradation. During infection, phagocytes use both of these cellular functions to restrict microbial replication and at the same time to orchestrate an appropriate response against the invader. Toll-like receptor recognition of a pathogen initiates an innate immune response against the pathogen that includes production of inflammatory cytokines, upregulation of costimulatory molecules to prime an adaptive immune response, and activation of phagocytosis and autophagy. Signaling through this family of receptors also produces a hybrid response in which proteins that participate in autophagy are recruited to phagosomes, resulting in expedited microbial elimination. In this review, we discuss recent views on how Toll-like receptors direct microbes to final destruction by regulating the different pathways that lead to the lysosome.

Phagocytosis inhibits F-actin-enriched membrane protrusions stimulated by fractalkine (CX3CL1) and colony-stimulating factor 1

Cryptococcus neoformans is the only encapsulated human-pathogenic fungus and a facultative intracellular pathogen that can reside in macrophages without host cell lysis. In the present study, we investigated how phagocytosis of C. neoformans affected the macrophage response to chemoattractants such as fractalkine (FKN) (CX3CL1) and colony-stimulating factor 1 (CSF-1). Phagocytosis of immunoglobulin G (IgG)-opsonized C. neoformans and IgG- or C3bi-opsonized sheep erythrocytes was performed using a RAW 264.7 subline (LR5 cells) and bone marrow-derived macrophages (BMM). The chemotactic response to FKN or CSF-1 was quantitated by measurement of the formation of F-actin-enriched membrane protrusions (ruffles), which showed that FKN or CSF-1 stimulated strong transient ruffling in both LR5 cells and BMM. This stimulated cell ruffling was inhibited by phagocytosis in an intracellular-pathogen-number-dependent manner. The inhibition of ruffling was not simply a result of reduced membrane availability since membrane sequestration by sucrose treatment did not inhibit the ruffling response. The phagocytosis process was required to inhibit ruffling as BMM from Fc gamma (-/-) mice that bound C. neoformans but did not ingest it retained the ability to ruffle in response to chemoattractants. These results imply that the inhibition of FKN- or CSF-1-stimulated cell ruffling was a direct consequence of the phagocytosis process. Since cell ruffling is a prelude to chemotaxis, this observation links two functions of macrophages that are critical to host defense, chemotaxis and phagocytosis. Phagocytosis-induced chemotactic suppression may enhance host defense by keeping these antimicrobial effector cells at infected sites and reduce the likelihood of microbial spread by wandering macrophages containing infectious cargo.

Contribution of phosphatidylserine to membrane surface charge and protein targeting during phagosome maturation

During phagocytosis, the phosphoinositide content of the activated membrane decreases sharply, as does the associated surface charge, which attracts polycationic proteins. The cytosolic leaflet of the plasma membrane is enriched in phosphatidylserine (PS); however, a lack of suitable probes has precluded investigation of the fate of this phospholipid during phagocytosis. We used a recently developed fluorescent biosensor to monitor the distribution and dynamics of PS during phagosome formation and maturation. Unlike the polyphosphoinositides, PS persists on phagosomes after sealing even when other plasmalemmal components have been depleted. High PS levels are maintained through fusion with endosomes and lysosomes and suffice to attract cationic proteins like c-Src to maturing phagosomes. Phagocytic vacuoles containing the pathogens Legionella pneumophila and Chlamydia trachomatis, which divert maturation away from the endolysosomal pathway, are devoid of PS, have little surface charge, and fail to recruit c-Src. These findings highlight a function for PS in phagosome maturation and microbial killing.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

Decoupling internalization, acidification and phagosomal-endosomal/lysosomal fusion during phagocytosis of InlA coated beads in epithelial cells

Background

Phagocytosis has been extensively examined in ‘professional’ phagocytic cells using pH sensitive dyes. However, in many of the previous studies, a separation between the end of internalization, beginning of acidification and completion of phagosomal-endosomal/lysosomal fusion was not clearly established. In addition, very little work has been done to systematically examine phagosomal maturation in ‘non-professional’ phagocytic cells. Therefore, in this study, we developed a simple method to measure and decouple particle internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion in Madin-Darby Canine Kidney (MDCK) and Caco-2 epithelial cells.

Methodology/Principal Findings

Our method was developed using a pathogen mimetic system consisting of polystyrene beads coated with Internalin A (InlA), a membrane surface protein from Listeria monocytogenes known to trigger receptor-mediated phagocytosis. We were able to independently measure the rates of internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion in epithelial cells by combining the InlA-coated beads (InlA-beads) with antibody quenching, a pH sensitive dye and an endosomal/lysosomal dye. By performing these independent measurements under identical experimental conditions, we were able to decouple the three processes and establish time scales for each. In a separate set of experiments, we exploited the phagosomal acidification process to demonstrate an additional, real-time method for tracking bead binding, internalization and phagosomal acidification.

Conclusions/Significance

Using this method, we found that the time scales for internalization, phagosomal acidification and phagosomal-endosomal/lysosomal fusion ranged from 23–32 min, 3–4 min and 74–120 min, respectively, for MDCK and Caco-2 epithelial cells. Both the static and real-time methods developed here are expected to be readily and broadly applicable, as they simply require fluorophore conjugation to a particle of interest, such as a pathogen or mimetic, in combination with common cell labeling dyes. As such, these methods hold promise for future measurements of receptor-mediated internalization in other cell systems, e.g. pathogen-host systems.

Antimicrobial mechanisms of phagocytes and bacterial evasion strategies

Professional phagocytes have a vast and sophisticated arsenal of microbicidal features. They are capable of ingesting and destroying invading organisms, and can present microbial antigens on their surface, eliciting acquired immune responses. To survive this hostile response, certain bacterial species have developed evasive strategies that often involve the secretion of effectors to co-opt the cellular machinery of the host. In this Review, we present an overview of the antimicrobial defences of the host cell, with emphasis on macrophages, for which phagocytosis has been studied most extensively. In addition, using Mycobacterium tuberculosis, Listeria monocytogenes, Legionella pneumophila and Coxiella burnetii as examples, we describe some of the evasive strategies used by bacteria.

A fluorescently tagged C-terminal fragment of p47phox detects NADPH oxidase dynamics during phagocytosis

The assembly of cytosolic p47(phox) and p67(phox) with flavocytochrome b(558) at the membrane is crucial for activating the leukocyte NADPH oxidase that generates superoxide for microbial killing. p47(phox) and p67(phox) are linked via a high-affinity, tail-to-tail interaction involving a proline-rich region (PRR) and a C-terminal SH3 domain (SH3b), respectively, in their C-termini. This interaction mediates p67(phox) translocation in neutrophils, but is not required for oxidase activity in model systems. Here we examined phagocytosis-induced NADPH oxidase assembly, showing the sequential recruitment of YFP-tagged p67(phox) to the phagosomal cup, and, after phagosome internalization, a probe for PI(3)P followed by a YFP-tagged fragment derived from the p47(phox) PRR. This fragment was recruited in a flavocytochrome b(558)-dependent, p67(phox)-specific, and PI(3)P-independent manner. These findings indicate that p47PRR fragment probes the status of the p67(phox) SH3b domain and suggest that the p47(phox)/p67(phox) tail-to-tail interaction is disrupted after oxidase assembly such that the p67(phox)-SH3b domain becomes accessible. Superoxide generation was sustained within phagosomes, indicating that this change does not correlate with loss of enzyme activity. This study defines a sequence of events during phagocytosis-induced NADPH oxidase assembly and provides experimental evidence that intermolecular interactions within this complex are dynamic and modulated after assembly on phagosomes.

Phosphoinositide 3-kinase delta inhibitor as a novel therapeutic agent in asthma

Multiple signal transduction pathways are involved in airway inflammation with one of the key signalling pathways being phosphoinositide 3-kinase (PI3K). Numerous components of the PI3K pathway play an important role in the expression and activation of inflammatory mediators, inflammatory cell recruitment, immune cell function, airway remodelling and corticosteroid insensitivity in asthma. More recently studies exploring the specific roles of different PI3K catalytic subunit isoforms in asthma have been initiated. Several of these have highlighted the importance of p110delta isoform as a novel target for therapeutic intervention in asthma. In this review the biological role of PI3Ks, especially PI3Kdelta, are highlighted and the therapeutic potential of selective PI3Kdelta inhibitor in asthma discussed.

Defining macropinocytosis

Macropinocytosis represents a distinct pathway of endocytosis in mammalian cells. This actin-driven endocytic process is not directly co-ordinated by the presence of cargo but can be induced upon activation of growth factor signalling pathways. The capacity to dissect the contribution of macropinocytosis to cellular processes has been hampered by a lack of unique molecular markers and defining features. While aspects of macropinosome formation and maturation are common to those shared by the other endocytic pathways, a number of key differences have recently begun to emerge and will be discussed in this study. It is now well established that macropinocytosis significantly contributes to antigen presentation by the immune system and is exploited by a range of pathogens for cellular invasion and avoidance of immune surveillance.

Differential role of the Ca(2+) sensor synaptotagmin VII in macrophages and dendritic cells

Synaptotagmin VII (Syt VII) is a Ca(2+) sensing molecule that regulates lysosomal exocytosis in several cell types. In macrophages (MØ), Syt VII is required for efficient uptake of large particle loads, by promoting the delivery of lysosomal membrane to phagocytic cups. Here we compare the phagocytic capacity of bone marrow-derived MØs and dendritic cells (DC), and show that the requirement for Syt VII correlates with the unique ability of MØs for continuous phagocytosis. In contrast to MØs, Syt VII(+/+) and Syt VII(-/-) immature DCs show similar levels of initial phagocytosis, followed by a marked decrease in particle uptake. [Ca(2+)](i) chelation and PI-3 kinase inhibition reduce particle uptake by MØs, but are markedly less inhibitory in DCs. Thus, immature DCs appear to lack the Syt VII, Ca(2+) and PI-3 kinase-dependent forms of phagocytosis that are present in MØs. Interestingly, expression of Syt VII is up-regulated during LPS-induced DC maturation, a stimulus that also induces Syt VII translocation from intracellular compartments to the plasma membrane. Syt VII(-/-) DCs show a delayed translocation of MHC class II to the cell surface during maturation, consistent with the possibility that Syt VII facilitates exocytosis and/or surface retention of molecules critical for antigen presentation.

The PtdIns 3-kinase/Akt pathway regulates macrophage-mediated ADCC against B cell lymphoma

Macrophages are important effectors in the clearance of antibody-coated tumor cells. However, the signaling pathways that regulate macrophage-induced ADCC are poorly defined. To understand the regulation of macrophage-mediated ADCC, we used human B cell lymphoma coated with Rituximab as the tumor target and murine macrophages primed with IFNγ as the effectors. Our data demonstrate that the PtdIns 3-kinase/Akt pathway is activated during macrophage-induced ADCC and that the inhibition of PtdIns 3-kinase results in the inhibition of macrophage-mediated cytotoxicity. Interestingly, downstream of PtdIns 3-kinase, expression of constitutively active Akt (Myr-Akt) in macrophages significantly enhanced their ability to mediate ADCC. Further analysis revealed that in this model, macrophage-mediated ADCC is dependent upon the release of nitric oxide (NO). However, the PtdIns 3-kinase/Akt pathway does not appear to regulate NO production. An examination of the role of the PtdIns 3-kinase/Akt pathway in regulating conjugate formation indicated that macrophages treated with an inhibitor of PtdIns 3-kinase fail to polarize the cytoskeleton at the synapse and show a significant reduction in the number of conjugates formed with tumor targets. Further, inhibition of PtdIns 3-kinase also reduced macrophage spreading on Rituximab-coated surfaces. On the other hand, Myr-Akt expressing macrophages displayed a significantly greater ability to form conjugates with tumor cells. Taken together, these findings illustrate that the PtdIns 3-kinase/Akt pathway plays a critical role in macrophage ADCC through its influence on conjugate formation between macrophages and antibody-coated tumor cells.

Phosphoinositide 3-kinase Akt signaling pathway interacts with protein kinase Cbeta2 in the regulation of physiologic developmental hypertrophy and heart function

The phosphoinositide 3-kinase (PI3-kinase)-protein kinase B (Akt) signaling pathway is essential in the induction of physiological cardiac hypertrophy. In contrast, protein kinase C beta2 (PKCbeta2) is implicated in the development of pathological cardiac hypertrophy and heart failure. Thus far, no clear association has been demonstrated between these two pathways. In this study, we examined the potential interaction between the PI3-kinase and PKCbeta2 pathways by crossing transgenic mice with cardiac specific expression of PKCbeta2, constitutively active (ca) PI3-kinase, and dominant-negative (dn) PI3-kinase. In caPI3-kinase/PKCbeta2 and dnPI3-kinase/PKCbeta2 double-transgenic mice, the heart weight-to-body weight ratios and cardiomyocyte sizes were similar to those observed in caPI3-kinase and dnPI3-kinase transgenic mice, respectively, suggesting that the regulation of physiological developmental hypertrophy via modulation of cardiomyocyte size proceeds through the PI3-kinase pathway. In addition, we observed that caPI3-kinase/PKCbeta2 mice showed improved cardiac function while the function of dnPI3-kinase/PKCbeta2 mice was similar to that of the PKCbeta2 group. PKCbeta2 protein levels in both dnPI3-kinase/PKCbeta2 and PKCbeta2 mice were significantly upregulated. Interestingly, however, PKCbeta2 protein expression was significantly attenuated in caPI3-kinase/PKCbeta2 mice. PI3-kinase activity measured by Akt phosphorylation was not affected by PKCbeta2 overexpression. These data suggest a potential interaction between these two pathways in the heart, where PI3-kinase is predominantly responsible for the regulation of physiological developmental hypertrophy and may act as an upstream modulator of PKCbeta2 with the potential for rescuing the pathological cardiac dysfunction induced by overexpression of PKCbeta.

Use of fluorescent probes to detect lipid signaling intermediates in macrophages

To fulfill their function in host defense, professional phagocytes such as neutrophils and macrophages exhibit the ability to ingest (phagocytose), kill, and dispose of pathogenic microorganisms. Recent studies have provided strong evidence for the importance of membrane lipids such as polyphosphoinositides in these processes. In turn, reversible phosphorylation events, involving protein and lipid kinases and phosphatases, regulate signaling pathways involving metabolism of membrane lipids. Our ability to study lipid signaling events has been greatly facilitated by the development of fluorescent molecular imaging techniques. In particular, the expression of recombinant fusions of derivatives of the jellyfish-derived green fluorescent proteins (GFP) coupled to reporter molecules enables real-time monitoring of signaling events in live cells. Here, we discuss methods to monitor alterations in membrane polyphosphoinositides involved in signaling events regulating phagocytosis. To illustrate the use of this technology, we will focus on the role of protein tyrosine phosphatase MEG2 in phagocytosis and its modulation by phosphatidylinositol-3,4,5-triphosphate (PIP3). This approach enables investigators to ascertain the involvement of lipid intermediates in diverse signaling pathways.

Cholesterol accumulation by macrophages impairs phagosome maturation

Macrophages are key to the pathogenesis of atherosclerosis. They take up and store excessive amounts of cholesterol associated with modified low density lipoprotein, eventually becoming foam cells that display altered immune responsiveness. We studied the effects of cholesterol accumulation on phagosome formation and maturation, using lipid transport antagonists and cholesterol transport-deficient mutants. In macrophages treated with U18666A, a transport antagonist that prevents cholesterol exit from late endosomes/lysosomes, the early stages of maturation proceeded normally; phagosomes acquired Rab5, phosphatidylinositol 3-phosphate, and EEA1 and merged with LAMP-containing vesicles. However, fusion with lysosomes was impaired. Rab7, which is required for phagolysosome formation, was acquired by phagosomes but remained inactive. Maturation was also studied in fibroblasts from Niemann-Pick type C individuals that have defective cholesterol transport. Transfection of FcgammaIIA receptors was used to confer phagocytic capability to these fibroblasts. Niemann-Pick type C phagosomes failed to fuse with lysosomes, whereas wild type fibroblasts formed normal phagolysosomes. These findings indicate that cholesterol accumulation can have a detrimental effect on phagosome maturation by impairing the activation of Rab7, sequestering it and its effectors in cholesterol-enriched multilamellar compartments.