Molecular characterization of phagosomes

Analysis of phagosomal proteomes: from latex-bead to bacterial phagosomes

Phagosomal proteome characterization has contributed significantly to the understanding of host-pathogen interaction and the mechanism of infectious diseases caused by intracellular bacteria. The latex bead-containing phagosome has been widely used as a model system to study phagosomal proteomes at a global level. In contrast, the study of bacteria-containing phagosomes at a similar level has just begun. A number of intracellular microbial species are studied for their proteomes during the invasion of a host, providing insight into their metabolic adaptation in host cells and interaction with host-cell antimicrobial environments. In this review, we attempt to summarize the most recent advancements in the proteomic study of microbial phagosomes, especially those originating from mouse or human cells. We also briefly describe the proteomics of latex bead-containing phagosomes because they are often used as model phagosomes for study. We provide descriptions on major biological and technological components in phagosomal proteome studies. We also discuss the role of phagosomal proteome study in the broader horizon of systems biology and the technological challenges in phagosomal proteome characterization.

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural Wiskott-Aldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.

Molecular characterization of the evolution of phagosomes

First large-scale comparative proteomics/phosphoproteomics study characterizing some of the key steps that contributed to the remodeling of phagosomes that occurred during evolution. Comparison of profiling analyses of isolated phagosomes from three distant organisms (Dictyostelium, Drosophila, and mouse) revealed a protein core that defines a potential ‘ancient' phagosome and a set of 50 proteins that emerged while adaptive immunity was already well established.

Gene duplication events of mouse phagosome paralogs occurred mostly in Bilateria and Euteleostomi, coinciding with the emergence of innate and adaptive immunity, and thus, provided the functional innovations needed for the establishment of these two crucial evolutionary steps of the immune system.

Phosphoproteomics of isolated phagosomes from the same three distant species indicate that the phagosome phosphoproteome has been extensively modified during evolution. Still, some phosphosites have been maintained for >1.2 billion years, and thus, highlight their particular significance in the regulation of key phagosomal functions.

Phagocytosis is the process by which multiple cell types internalize large particulate material from the external milieu. The functional properties of phagosomes are acquired through a complex maturation process, referred to as phagolysosome biogenesis. This pathway involves a series of rapid interactions with organelles of the endocytic apparatus, enabling the gradual transformation of newly formed phagosomes into phagolysosomes in which proteolytic degradation occurs. The degradative environment encountered in the phagosome lumen has enabled the use of phagocytosis as a predation mechanism for feeding (phagotrophy) in amoeba, whereas multicellular organisms utilize this process as a defense mechanism to kill microbes and, in jawed vertebrates (fish), initiate a sustained immune response.

High-throughput proteomics profiling of isolated phagosomes has been tremendously helpful for the molecular comprehension of this organelle. This approach is achieved by feeding low buoyancy latex beads to phagocytic cells, enabling the subsequent isolation of latex bead-containing phagosomes, away from all the other cell organelles, by a single-isopicnic centrifugation in sucrose gradient. In order to characterize some of the key steps that contributed to the remodeling of phagosomes during evolution, we isolated this organelle from three distant organisms: the amoeba Dictyostelium discoideum, the fruit fly Drosophila melanogaster, and mouse (Mus musculus) that use phagocytosis for different purposes, and performed detailed proteomics and phosphoproteomics analyses with unparallel protein coverage for this organelle (two- to four-fold enhancements in identified proteins).

In order to establish the origin of the mouse phagosome proteome, we performed comparative analyses among 39 taxa including plants/algea, unicellular organisms, fungi, and more complex animal multicellular organisms. These genomic comparisons indicated that a large proportion of the mouse phagosome proteome is of ancient origin (73.1% of the proteome is conserved in eukaryotic organisms) (Figure 2A). This stresses the fact that phagocytosis is a very ancient process, as shown by its possible involvement in the emergence of eukaryotic cells (eukaryogenesis). Indeed, we identified close to 300 phagosome mouse proteins also present on Drosophila and Dictyostelium phagosomes, defining a potential ‘ancient' core of proteins from which the immune functions of phagosomes likely evolved. Around 16.7% of the mouse phagosome proteins appeared in organisms that use phagocytosis for innate immunity (Bilateria to Chordata), whereas 10.2% appeared in Euteleostomi or Tetrapoda where phagosomes have an important function in linking the killing of microorganisms with the development of a specific sustained immune response following antigen recognition. The phagosome is made of molecules taken from a variety of sources within the cell, including the cytoplasm, the cytoskeleton and membrane organelles. Despite the evolution and diversification of these various cellular systems, the mammalian phagosome proteome is made preferentially of ancient proteins (Figure 2B). Comparison of functional annotation during evolution highlighted the emergence of specific phagosomal functions at various steps during evolution (Figure 2C). Some of these proteins and their point of origin during evolution are highlighted in Figure 2D. Strikingly, we identified in Tetrapods a set of 50 proteins that arose while adaptive immunity was already well established in teleosts (fish), indicating that the phagocytic system is still evolving.

Our study highlights the fact that the functional properties of phagosomes emerged by the remodeling of ancient molecules, the addition of novel components, and the duplication of existing proteins (paralogs) leading to the formation of molecular machines of mixed origin. Gene duplication is a process that contributed continuously to the complexification of the mouse proteome during evolution. In sharp contrast, paralog analysis indicated that the phagosome proteome was mainly reorganized through two periods of gene duplication, in Bilateria and Euteleostomi, coinciding with the emergence of adaptive immunity (in jawed fish), and innate immunity (at the split between Metazoa and Bilateria). These results strongly suggest that selective constraints may have favored the maintenance of phagosome paralogs to ensure the establishment of novel functions associated with this organelle at these two crucial evolutionary steps of the immune system.

The emergence of genes associated to the MHC locus in mammals that appeared originally in the genome of jawed fishes, contributed to the development of complex molecular mechanisms linking innate (our immune system that defends the host from infection in a non-specific manner) and adaptive immunity (the part of the immune system triggered specifically after antigen recognition). Several of the genes of this locus encode proteins known to have important functions in antigen presentation, such as subunits of the immunoproteasome (LMP2 and LMP7), MHC class I and class II molecules, as well as tapasin and the transporter associated with antigen processing (TAP1 and TAP2), involved in the transport and loading of peptides on MHC class I molecules (Figure 6). In addition to their ability to present peptides on MHC class II molecules, phagosomes of vertebrates have been shown to be competent for the presentation of exogenous peptides on MHC class I molecules, a process referred to as cross-presentation. From a functional point of view, the involvement of phagosomes in antigen cross-presentation is the outcome of the successful integration of a wide range of multimolecular components that emerged throughout evolution (Figure 6). The trimming of exogenous proteins into small peptides that can be loaded on MHC class I molecules is inherited from the phagotrophic properties of unicellular organisms, where internalized bacteria are degraded into basic molecules and used as a source of nutrients. Ancient processes have therefore been co-opted (the use of an existing biological structure or feature for a new function) for new functionalities. A summarizing model of the various steps that enabled phagosome antigen presentation is presented in Figure 6. This model highlights the fact that although antigen presentation is unique to evolutionary recent phagosomes (starting in jawed fishes about 450 million years ago), it uses and integrates molecular machines composed of proteins that emerged throughout evolution.

In summary, we present here the first large-scale comparative proteomics/phosphoproteomics study characterizing some of the key evolutionary steps that contributed to the remodeling of phagosomes during evolution. Functional properties of this organelle emerged by the remodeling of ancient molecules, the addition of novel components, the extensive adaption of protein phosphorylation sites and the duplication of existing proteins leading to the formation of molecular machines of mixed origin.

Amoeba use phagocytosis to internalize bacteria as a source of nutrients, whereas multicellular organisms utilize this process as a defense mechanism to kill microbes and, in vertebrates, initiate a sustained immune response. By using a large-scale approach to identify and compare the proteome and phosphoproteome of phagosomes isolated from distant organisms, and by comparative analysis over 39 taxa, we identified an ‘ancient' core of phagosomal proteins around which the immune functions of this organelle have likely organized. Our data indicate that a larger proportion of the phagosome proteome, compared with the whole cell proteome, has been acquired through gene duplication at a period coinciding with the emergence of innate and adaptive immunity. Our study also characterizes in detail the acquisition of novel proteins and the significant remodeling of the phagosome phosphoproteome that contributed to modify the core constituents of this organelle in evolution. Our work thus provides the first thorough analysis of the changes that enabled the transformation of the phagosome from a phagotrophic compartment into an organelle fully competent for antigen presentation.

Annexin A1 is a new functional linker between actin filaments and phagosomes during phagocytosis

Remodelling of the actin cytoskeleton plays a key role in particle internalisation and the phagosome maturation processes. Actin-binding proteins (ABPs) are the main players in actin remodelling but the precise role of these proteins in phagocytosis needs to be clarified. Annexins, a group of ABPs, are known to be present on phagosomes. Here, we identified annexin A1 as a factor that binds to isolated latex bead phagosomes (LBPs) in the presence of Ca(2+) and facilitates the F-actin-LBP interaction in vitro. In macrophages the association of endogenous annexin A1 with LBP membranes was strongly correlated with the spatial and temporal accumulation of F-actin at the LBP. Annexin A1 was found on phagocytic cups and around early phagosomes, where the F-actin was prominently concentrated. After uptake was completed, annexin A1, along with F-actin, dissociated from the nascent LBP surface. At later stages of phagocytosis annexin A1 transiently concentrated only around those LBPs that showed transient F-actin accumulation ('actin flashing'). Downregulation of annexin A1 expression resulted in impaired phagocytosis and actin flashing. These data identify annexin A1 as an important component of phagocytosis that appears to link actin accumulation to different steps of phagosome formation.

Kinesin 5B is necessary for delivery of membrane and receptors during FcγR-mediated phagocytosis

FcγR-mediated phagocytosis is a cellular event that is evolutionary conserved to digest IgG-opsonized pathogens. Pseudopod formation during phagocytosis is a limiting step in managing the uptake of particles, and in this paper, we show that the conventional kinesin is involved in both receptor and membrane delivery to the phagocytic cup. Expression of a mutant kinesin isoform (GFP dominant negative mutant of kinesin H chain [EGFP-Kif5B-DN]) in RAW264.7 cells significantly reduced binding of IgG-sheep RBCs when macrophages were faced with multiple encounters with opsonized particles. Scanning electron microscopy analysis of EGFP-Kif5B-DN-expressing cells challenged with two rounds of IgG-sheep RBCs showed sparse, extremely thin pseudopods. We saw disrupted Rab11 trafficking to the phagocytic cup in EGFP-Kif5B-DN-transfected cells. Our particle overload assays also implicated phagosome membrane recycling in pseudopod formation. We observed reduced phagosome fission and trafficking in mutant kinesin-expressing cells, as well as reduced cell surface expression of FcγRs and Mac-1 receptors. In conclusion, anterograde trafficking via kinesin is essential for both receptor recycling from the phagosome and delivery of Rab11-containing membrane stores to effect broad and functional pseudopods during FcγR-mediated phagocytosis.

Subcellular phosphoproteomics

Protein phosphorylation represents one of the most extensively studied post-translational modifications, primarily due to the emergence of sensitive methods enabling the detection of this modification both in vitro and in vivo. The availability of enrichment methods combined with sensitive mass spectrometry instrumentation has played a crucial role in uncovering the dynamic changes and the large expanding repertoire of this reversible modification. The structural changes imparted by the phosphorylation of specific residues afford exquisite mechanisms for the regulation of protein functions by modulating new binding sites on scaffold proteins or by abrogating protein-protein interactions. However, the dynamic interplay of protein phosphorylation is not occurring randomly within the cell but is rather finely orchestrated by specific kinases and phosphatases that are unevenly distributed across subcellular compartments. This spatial separation not only regulates protein phosphorylation but can also control the activity of other enzymes and the transfer of other post-translational modifications. While numerous large-scale phosphoproteomics studies highlighted the extent and diversity of phosphoproteins present in total cell lysates, the further understanding of their regulation and biological activities require a spatio-temporal resolution only achievable through subcellular fractionation. This review presents a first account of the emerging field of subcellular phosphoproteomics where cell fractionation approaches are combined with sensitive mass spectrometry methods to facilitate the identification of low abundance proteins and to unravel the intricate regulation of protein phosphorylation.

The human fungal pathogen Cryptococcus neoformans escapes macrophages by a phagosome emptying mechanism that is inhibited by Arp2/3 complex-mediated actin polymerisation

The lysis of infected cells by disease-causing microorganisms is an efficient but risky strategy for disseminated infection, as it exposes the pathogen to the full repertoire of the host's immune system. Cryptococcus neoformans is a widespread fungal pathogen that causes a fatal meningitis in HIV and other immunocompromised patients. Following intracellular growth, cryptococci are able to escape their host cells by a non-lytic expulsive mechanism that may contribute to the invasion of the central nervous system. Non-lytic escape is also exhibited by some bacterial pathogens and is likely to facilitate long-term avoidance of the host immune system during latency. Here we show that phagosomes containing intracellular cryptococci undergo repeated cycles of actin polymerisation. These actin ‘flashes’ occur in both murine and human macrophages and are dependent on classical WASP-Arp2/3 complex mediated actin filament nucleation. Three dimensional confocal imaging time lapse revealed that such flashes are highly dynamic actin cages that form around the phagosome. Using fluorescent dextran as a phagosome membrane integrity probe, we find that the non-lytic expulsion of Cryptococcus occurs through fusion of the phagosome and plasma membranes and that, prior to expulsion, 95% of phagosomes become permeabilised, an event that is immediately followed by an actin flash. By using pharmacological agents to modulate both actin dynamics and upstream signalling events, we show that flash occurrence is inversely related to cryptococcal expulsion, suggesting that flashes may act to temporarily inhibit expulsion from infected phagocytes. In conclusion, our data reveal the existence of a novel actin-dependent process on phagosomes containing cryptococci that acts as a potential block to expulsion of Cryptococcus and may have significant implications for the dissemination of, and CNS invasion by, this organism.

Cryptococcus neoformans is fatal fungal pathogen of HIV-positive and other immunocompromised patients that causes an estimated 650 000 deaths per annum. Cryptococcus is able to undermine our immune system by growing within and escaping from immune cells called macrophages. In this study we describe how macrophage cells may be able to prevent this escape by forming a transient ‘cage’ of the protein actin around the intracellular pathogen. Blocking escape from within the macrophage in this way may help prevent the spread of disease around the body, especially into the brain. Thus actin flashes may represent an important host defence against diverse human pathogens.

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.

Intracellular life of Coxiella burnetii in macrophages

Coxiella burnetii, the agent of Q fever, is an obligate intracellular bacterium that is considered a potential biological weapon of category B. C. burnetii survives within myeloid cells by subverting receptor-mediated phagocytosis and preventing phagosome maturation. The intracellular fate of C. burnetii also depends on the functional state of myeloid cells. This review describes the mechanisms used by C. burnetii to circumvent uptake and trafficking events, and the role of cytokines on C. burnetii survival in myeloid cells.

Candida albicans actively modulates intracellular membrane trafficking in mouse macrophage phagosomes

The intracellular trafficking/survival strategies of the opportunistic human pathogen Candida albicans are poorly understood. Here we investigated the infection of RAW264.7 macrophages with a virulent wild-type (WT) filamentous C. albicans strain and a hyphal signalling-defective mutant (efg1Delta/cph1Delta). A comparative analysis of the acquisition by phagosomes of actin, and of early/late endocytic organelles markers of the different fungal strains was performed and related to Candida's survival inside macrophages. Our results show that both fungal strains have evolved a similar mechanism to subvert the 'lysosomal' system, as seen by the inhibition of the phagosome fusion with compartments enriched in the lysobisphosphatidic acid and the vATPase, and thereby the acquisition of a low pH from the outset of infection. Besides, the virulent WT strain displayed additional specific survival strategies to prevent its targeting to compartmentsdisplaying late endosomal/lysosomal features, such as induction of active recycling out of phagosomes of the lysosomal membrane protein LAMP-1, the lysosomal protease cathepsin D and preinternalized colloidal gold. Finally, both virulent and efg1Delta/cph1Delta mutant fungal strains actively suppressed the production of macrophage nitric oxide (NO), although their cell wall extracts were potent inducers of NO.

Lipids regulate P2X7-receptor-dependent actin assembly by phagosomes via ADP translocation and ATP synthesis in the phagosome lumen

Latex bead phagosomes isolated from J774 macrophages polymerize actin. We show here that five lipids--phosphatidylinositol-4-phosphate, phosphatidylinositol-(4,5)-bisphosphate, sphingosine-1-phosphate (S1P), ceramide-1-phosphate and phosphatidic acid--stimulate both actin assembly and transport of ADP across the phagosomal membrane into the lumen. Once there, this ADP is converted to ATP by adenylate kinase activity. High luminal ATP concentrations correlated well with phagosome actin assembly under different conditions. The ATP-binding P2X7 receptor (P2X7R) was detected in phagosomes. Although S1P stimulated actin assembly by phagosomes from P2X7R-containing bone marrow macrophages, S1P-stimulated actin assembly was inhibited in phagosomes from cells lacking P2X7R. We propose that luminal ATP accumulates in response to selected lipids and activates the P2X7R that signals across the phagosomal membrane to trigger actin assembly on the cytoplasmic membrane surface. In the accompanying paper by Kuehnel et al. (doi:10.1242/jcs.034207), more evidence is provided in support of this model from the analysis of actin assembly at the plasma membrane of intact macrophages.

Large scale phagosome preparation

Phagocytosis is the process by which cells engulf and destroy large particles such as pathogens or apoptotic cells. In this way, macrophages play a pivotal role in the resolution of microbial infections. However, many microorganisms have evolved efficient strategies to preempt the weaponry of macrophages. A better understanding of the components engaged in the phagosome formation and maturation is necessary to devise novel approaches aimed at counteracting these microbial strategies. Recently, large-scale approaches have been used to improve our understanding of phagosome functional properties by the identification of hundreds of proteins and by studying each of them. Presently, purification of pathogen-containing phagosomes presents several technical challenges, whereas the use of latex beads to isolate phagosomes presents many advantages because this system can mimic host-pathogen interactions during phagocytosis. This system thus remains the best approach to advance our knowledge of phagosome biology, notably when used in conjunction with functional approaches. In this chapter, we outline an approach for the isolation of large-scale phagosome preparations with high degrees of purity.

Membrane proteomics of phagosomes suggests a connection to autophagy

Phagocytosis is the central process by which macrophage cells internalize and eliminate infectious microbes as well as apoptotic cells. During maturation, phagosomes containing engulfed particles fuse with various endosomal compartments through the action of regulatory molecules on the phagosomal membrane. In this study, we performed a proteomic analysis of the membrane fraction from latex bead-containing (LBC) phagosomes isolated from macrophages. The profile, which comprised 546 proteins, suggests diverse functions of the phagosome and potential connections to secretory processes, toll-like receptor signaling, and autophagy. Many identified proteins were not previously known to reside in the phagosome. We characterized several proteins in LBC phagosomes that change in abundance on induction of autophagy, a process that has been previously implicated in the host defense against microbial pathogens. These observations suggest crosstalk between autophagy and phagocytosis that may be relevant to the innate immune response of macrophages.

Analysis of PRA1 and its relationship to Candida albicans- macrophage interactions

Phagocytosis of Candida albicans by either primary bone marrow-derived mouse macrophages or RAW 264.7 cells upregulated transcription of PRA1, which encodes a cell wall/membrane-associated antigen previously described as a fibrinogen binding protein. However, a pra1 null mutant was still able to bind fibrinogen, showing that Pra1p is not uniquely required for fibrinogen binding. As well, Pra1 tagged with green fluorescent protein did not colocalize with AlexaFluor 546-labeled human fibrinogen, and while PRA1 expression was inhibited when Candida was grown in fetal bovine serum-containing medium, Candida binding to fibrinogen was activated by these conditions. Therefore, it appears that Pra1p can play at most a minor role in fibrinogen binding to C. albicans. PRA1 gene expression is induced in vitro by alkaline pH, and therefore its activation in phagosomes suggested that phagosome maturation was suppressed by the presence of Candida cells. LysoTracker red-labeled organelles failed to fuse with phagosomes containing live Candida, while phagosomes containing dead Candida underwent a normal phagosome-to-phagolysosome maturation. Immunofluorescence staining with the early/recycling endosomal marker transferrin receptor (CD71) suggested that live Candida may escape macrophage destruction through the inhibition of phagolysosomal maturation.

Pathogen destruction versus intracellular survival: the role of lipids as phagosomal fate determinants

Phagocytosis is a key component of the innate immune response and of the clearance of apoptotic bodies. Phagosome formation and subsequent maturation require extensive cytoskeletal rearrangement and precisely choreographed vesicular fusion and fission events. The objectives of this review are to highlight the functional importance of lipids in the phagocytic process, to discuss how pathogenic microorganisms can in some cases manipulate host lipid metabolism to either co-opt or disrupt phagosome maturation and promote their own survival, and to describe how defective phagosomal lipid metabolism can result in disease.

Hemophagocytic macrophages harbor Salmonella enterica during persistent infection

Salmonella enterica subspecies can establish persistent, systemic infections in mammals, including human typhoid fever. Persistent S. enterica disease is characterized by an initial acute infection that develops into an asymptomatic chronic infection. During both the acute and persistent stages, the bacteria generally reside within professional phagocytes, usually macrophages. It is unclear how salmonellae can survive within macrophages, cells that evolved, in part, to destroy pathogens. Evidence is presented that during the establishment of persistent murine infection, macrophages that contain S. enterica serotype Typhimurium are hemophagocytic. Hemophagocytic macrophages are characterized by the ingestion of non-apoptotic cells of the hematopoietic lineage and are a clinical marker of typhoid fever as well as certain other infectious and genetic diseases. Cell culture assays were developed to evaluate bacterial survival in hemophagocytic macrophages. S. Typhimurium preferentially replicated in macrophages that pre-phagocytosed viable cells, but the bacteria were killed in macrophages that pre-phagocytosed beads or dead cells. These data suggest that during persistent infection hemophagocytic macrophages may provide S. Typhimurium with a survival niche.

Mechanism of inducible nitric oxide synthase exclusion from mycobacterial phagosomes

Mycobacterium tuberculosis is sensitive to nitric oxide generated by inducible nitric oxide synthase (iNOS). Consequently, to ensure its survival in macrophages, M. tuberculosis inhibits iNOS recruitment to its phagosome by an unknown mechanism. Here we report the mechanism underlying this process, whereby mycobacteria affect the scaffolding protein EBP50, which normally binds to iNOS and links it to the actin cytoskeleton. Phagosomes harboring live mycobacteria showed reduced capacity to retain EBP50, consistent with lower iNOS recruitment. EBP50 was found on purified phagosomes, and its expression increased upon macrophage activation, paralleling expression changes seen with iNOS. Overexpression of EBP50 increased while EBP50 knockdown decreased iNOS recruitment to phagosomes. Knockdown of EBP50 enhanced mycobacterial survival in activated macrophages. We tested another actin organizer, coronin-1, implicated in mycobacterium-macrophage interaction for contribution to iNOS exclusion. A knockdown of coronin-1 resulted in increased iNOS recruitment to model latex bead phagosomes but did not increase iNOS recruitment to phagosomes with live mycobacteria and did not affect mycobacterial survival. Our findings are consistent with a model for the block in iNOS association with mycobacterial phagosomes as a mechanism dependent primarly on reduced EBP50 recruitment.

Mycobacterium tuberculosis infects one third of the world's population, with the majority of infected individuals being asymptomatic while running a lifetime risk of developing active disease. The key to the success of M. tuberculosis as a recalcitrant human pathogen is its ability to parasitize macrophages and persist in these cells or their derivatives for long periods of time. We still do not have complete knowledge of the full repertoire of M. tuberculosis determinants that allow it to evade bactericidal mechanisms of the macrophage. Here we report the mechanism by which M. tuberculosis eludes effective elimination by nitric oxide, a radical with antimycobacterial properties that is generated by the inducible form of nitric oxide synthase. It was generally assumed that nitric oxide synthase, upon induction by the major anti-tuberculosis cytokine interferon gamma, simply homogeneously fills up the macrophage like a sack and generates nitric oxide throughout the cell. The present study shows that nitric oxide synthase is not randomly distributed in macrophages, and that its positioning in the cell is dictated by interactions with the scaffolding protein EBP50, shown here to be induced during macrophage activation. Thus, not only do the phagocytic cells increase the amount of nitric oxide synthase, but they also have a system to deliver and keep this enzyme in the vicinity of phagosomes. This is of significance, as nitric oxide is a highly reactive radical, and its generation somewhere else in the cell would lead to it being spent by the time it diffuses to the site of intended action, such as mycobacterium-laden phagosomes. It turns out, as this study shows, that M. tuberculosis interferes with the process of EBP50-guided positioning of the inducible nitric oxide synthase, thus avoiding delivery and accumulation of this enzyme and its noxious products near the phagosome where nitric oxide would have the best chance of inhibiting intracellular mycobacteria.

Manipulation of rab GTPase function by intracellular bacterial pathogens

Intracellular bacterial pathogens have evolved highly specialized mechanisms to enter and survive within their eukaryotic hosts. In order to do this, bacterial pathogens need to avoid host cell degradation and obtain nutrients and biosynthetic precursors, as well as evade detection by the host immune system. To create an intracellular niche that is favorable for replication, some intracellular pathogens inhibit the maturation of the phagosome or exit the endocytic pathway by modifying the identity of their phagosome through the exploitation of host cell trafficking pathways. In eukaryotic cells, organelle identity is determined, in part, by the composition of active Rab GTPases on the membranes of each organelle. This review describes our current understanding of how selected bacterial pathogens regulate host trafficking pathways by the selective inclusion or retention of Rab GTPases on membranes of the vacuoles that they occupy in host cells during infection.

Contributions of proteomics to understanding phagosome maturation

In metazoans macrophage cells use phagocytosis, the process of engulfing large particles, to control the spread of pathogens in the body, to clear dead or dying cells, and to aid in tissue remodelling, while the same process is also used by unicellular eukaryotes to ingest food. Phagocytosing cells essentially swallow the particles, trapping them in vacuoles called phagosomes that go through a series of maturation steps, culminating in the destruction of the internalized cargo. Because of their central role in innate immunity and their relatively simple structure (one membrane bilayer surrounding a single particle), phagosomes have been a popular subject for organelle proteomics studies. Qualitative proteomic technologies are now very sensitive so hundreds of different proteins have been identified in phagosomes from several species, revealing new properties of these intriguing compartments. More recently, quantitative proteomic approaches have also been applied, shedding new light on the dynamics and composition of maturing phagosomes. In this review we summarize the studies that have applied proteomic technologies to phagosomes and how they have changed our understanding of phagosome biology.

Integrin beta 1 regulates phagosome maturation in macrophages through Rac expression

Phagocytosis and subsequent phagosome maturation by professional phagocytes are essential in the clearance of infectious microbial pathogens. The molecular regulation of phagosome maturation is largely unknown. We show that integrin beta(1) plays critical roles in the phagocytosis of microbial pathogens and phagosome maturation. Macrophages lacking integrin beta(1) expression exhibit reduced phagocytosis of bacteria, including group B streptococcus and Staphylococcus aureus. Furthermore, phagosomes from macrophages lacking integrin beta(1) show lowered maturation rate, defective acquisition of lysosome membrane markers, and reduced F-actin accumulation in the periphagosomal region. Integrin beta(1)-deficient macrophages exhibit impaired bactericidal activity. We found that the expression of the Rho family GTPases Rac1, Rac2, and Cdc42 was reduced in integrin beta(1)-deficient macrophages. Ectopic expression of Rac1, but not Cdc42, in integrin beta(1)-deficient macrophages restored defective phagosome maturation and F-actin accumulation in the periphagosomal region. Importantly, macrophages lacking Rac1/2 also exhibit defective maturation of phagosomes derived from opsonized Escherichia coli or IgG beads. Taken together, these results suggest that integrin beta(1) regulates phagosome maturation in macrophages through Rac expression.

Modulation of the phagosome proteome by interferon-gamma

Macrophages are immune cells that function in the clearance of infectious particles. This process involves the engulfment of microbes into phagosomes where these particles are lysed and degraded. In the current study, we used a large scale quantitative proteomics approach to analyze the changes in protein abundance induced on phagosomes by interferon-gamma (IFN-gamma), an inflammatory cytokine that activates macrophages. Our analysis identified 167 IFN-gamma-modulated proteins on phagosomes of which more than 90% were up-regulated. The list of phagosomal proteins regulated by IFN-gamma includes proteins expected to alter phagosome maturation, enhance microbe degradation, trigger the macrophage immune response, and promote antigen loading on major histocompatibility complex (MHC) class I molecules. A dynamic analysis of IFN-gamma-sensitive proteins by Western blot indicated that newly formed phagosomes display a delayed proteolytic activity coupled to an increased recruitment of the MHC class I peptide-loading complex. These phagosomal conditions may favor antigen presentation by MHC class I molecules on IFN-gamma-activated macrophages.

Expression of phospholipase D, the major virulence factor of Corynebacterium pseudotuberculosis, is regulated by multiple environmental factors and plays a role in macrophage death

Corynebacterium pseudotuberculosis is the aetiological agent of caseous lymphadenitis, a disease affecting sheep and goats. Phospholipase D (Pld), a major virulence determinant of C. pseudotuberculosis, is believed to play a critical role in dissemination of bacteria from the site of infection to the lymph nodes. Although the pld gene has been studied for some time, it is only recently that it has been identified as being down-regulated following heat shock from 37 to 43 degrees C. To gain insights into the mechanisms of Pld action, this study investigated how it was regulated under varying environmental conditions. Studies measuring pld mRNA levels or utilizing a reporter construct containing the pld promoter upstream of a gfp gene were performed. These showed that pld was upregulated in a cell-density-dependent manner, was regulated by heat shock at all cell-culture densities, and was highly expressed in a tissue-culture macrophage-infection model. Finally, the expression of Pld by intracellular C. pseudotuberculosis was shown to play a small but significant role in the reduction of macrophage viability following infection. This study demonstrates that the regulation of C. pseudotuberculosis pld is complex. This regulatory complexity may play an important role in allowing the pathogen to successfully adapt to the changing host environment during infection, migration, establishment and disease progression.

Annexin A6-induced alterations in cholesterol transport and caveolin export from the Golgi complex

Annexin A6 (AnxA6) belongs to a family of Ca(2+)-dependent membrane-binding proteins and is involved in the regulation of endocytic and exocytic pathways. We previously demonstrated that AnxA6 regulates receptor-mediated endocytosis and lysosomal targeting of low-density lipoproteins and translocates to cholesterol-enriched late endosomes (LE). As cholesterol modulates the membrane binding and the cellular location of AnxA6, but also affects the intracellular distribution of caveolin, we investigated the localization and trafficking of caveolin in AnxA6-expressing cells. Here, we show that cells expressing high levels of AnxA6 are characterized by an accumulation of caveolin-1 (cav-1) in the Golgi complex. This is associated with a sequestration of cholesterol in the LE and lower levels of cholesterol in the Golgi and the plasma membrane, both likely contributing to retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. Further strengthening these findings, knock down of AnxA6 and the ectopic expression of the Niemann-Pick C1 protein in AnxA6-overexpressing cells restore the cellular distribution of cav-1 and cholesterol, respectively. In summary, this study demonstrates that elevated expression levels of AnxA6 perturb the intracellular distribution of cholesterol, which indirectly inhibits the exit of caveolin from the Golgi complex.

Leishmania donovani: Inhibition of phagosomal maturation is rescued by nitric oxide in macrophages

Leishmania donovani promastigotes, the causative agent of visceral leishmaniasis, survive inside macrophages by inhibiting phagosomal maturation. The main surface glycoconjugate on promastigotes, lipophosphoglycan (LPG), is crucial for parasite survival. LPG has several detrimental effects on macrophage function, including inhibition of periphagosomal filamentous actin (F-actin) breakdown during phagosomal maturation. However, in RAW 264.7 macrophages pre-stimulated with lipopolysaccharide (LPS) and interferon gamma (IFNgamma), known to up-regulate inducible nitric oxide synthase (iNOS) and nitric oxide (NO) production, L. donovani promastigotes are unable to inhibit periphagosomal F-actin breakdown and phagosomal maturation proceeds normally. Moreover, the iNOS inhibitor aminoguanidine, blocked the positive effects of LPS/IFNgamma suggesting that NO is a key player in F-actin remodeling. In conclusion, production of NO by stimulated macrophages seems to allow phagosomal maturation following uptake of L. donovani promastigotes, suggesting a novel mechanism whereby NO facilitates killing of an intracellular pathogen.

High-throughput assays of phagocytosis, phagosome maturation, and bacterial invasion

Ingestion of foreign particles by macrophages and neutrophils and the fate of the vacuole that contains the ingested material are generally monitored by optical microscopy. Invasion of host cells by pathogenic bacteria and their intracellular proliferation are similarly studied by microscopy or by plating assays. These labor-intensive and time-consuming methods limit the number of assays that can be performed. The effort required to test multiple reagents or conditions can be prohibitive. We describe high-throughput assays of phagocytosis and of phagosomal maturation. An automated fluorescence microscope-based platform and associated analysis software were used to study Fcγ receptor-mediated phagocytosis of IgG-opsonized particles by cultured murine macrophages. Phagosomal acidification was measured as an index of maturation. The same platform was similarly used to implement high-throughput assays of invasion of mammalian cells by pathogenic bacteria. The invasion of HeLa cells by Salmonella and the subsequent intracellular proliferation of the bacteria were measured rapidly and reliably in large populations of cells. These high-throughput methods are ideally suited for the efficient screening of chemical libraries to select potential drugs and of small interference RNA libraries to identify essential molecules involved in critical steps of the immune response.

The phagosome: compartment with a license to kill

Phagosomes are fascinating subcellular structures. After all, there are only a few compartments that are born before our very eyes and whose development we can follow in a light microscope until their contents disintegrate and are completely absorbed. Yet, some phagosomes are taken advantage of by pathogenic microorganisms, which change their fate. Research into phagosome biogenesis has flourished in recent years - the purpose of this review is to give a glimpse of where this research stands, with emphasis on the cell biology of macrophage phagosomes, on new model organisms for the study of phagosome biogenesis and on intracellular pathogens and their interference with normal phagosome function.

A role for the endoplasmic reticulum protein retrotranslocation machinery during crosspresentation by dendritic cells

Crosspresentation of exogenous antigens (Ags) to CD8(+) T cells by dendritic cells generally requires their entry into the cytosol. Here we show that both soluble and phagocytosed extracellular Ags accessed the cytosol via molecular components required for endoplasmic reticulum (ER)-associated degradation (ERAD). Exogenous Pseudomonas aeruginosa Exotoxin A, which inhibits protein translocation from the ER to the cytosol, abrogated crosspresentation. Exotoxin A also prevented the transporter associated with antigen processing (TAP) inhibitor, ICP47, from entering the cytosol and blocking TAP-mediated peptide transport. In an in vitro model of retrotranslocation, the AAA ATPase p97, an enzyme critical for ERAD, was the only cytosolic cofactor required for protein export from isolated phagosomes. Functional p97 was also required for crosspresentation but not conventional presentation. Thus, crosspresentation appears to result from an adaptation of the retrotranslocation mechanisms involved in the degradation of misfolded ER proteins.

Involvement of syntaxin 18, an endoplasmic reticulum (ER)-localized SNARE protein, in ER-mediated phagocytosis

The endoplasmic reticulum (ER) is thought to play an important structural and functional role in phagocytosis. According to this model, direct membrane fusion between the ER and the plasma or phagosomal membrane must precede further invagination, but the exact mechanisms remain elusive. Here, we investigated whether various ER-localized SNARE proteins are involved in this fusion process. When phagosomes were isolated from murine J774 macrophages, we found that ER-localized SNARE proteins (syntaxin 18, D12, and Sec22b) were significantly enriched in the phagosomes. Fluorescence and immuno-EM analyses confirmed the localization of syntaxin 18 in the phagosomal membranes of J774 cells stably expressing this protein tagged to a GFP variant. To examine whether these SNARE proteins are required for phagocytosis, we generated 293T cells stably expressing the Fc gamma receptor, in which phagocytosis occurs in an IgG-mediated manner. Expression in these cells of dominant-negative mutants of syntaxin 18 or D12 lacking the transmembrane domain, but not a Sec22b mutant, impaired phagocytosis. Syntaxin 18 small interfering RNA (siRNA) selectively decreased the efficiency of phagocytosis, and the rate of phagocytosis was markedly enhanced by stable overexpression of syntaxin 18 in J774 cells. Therefore, we conclude that syntaxin 18 is involved in ER-mediated phagocytosis, presumably by regulating the specific and direct fusion of the ER and plasma or phagosomal membranes.

Integrated microfluidic devices with enhanced separation performance: Application to phosphoproteome analyses of differentiated cell model systems

This work reports on the application of a microfluidic device integrating nanoscale LC to nanoelectrospray MS (nano-LC-chip-MS) for the analysis of complex protein digests. Peak profile analyses of more than 700 peptide ions, reproducibly detected across replicate nano-LC-chip-MS runs (n = 5), indicated that the system provided RSD values of 0.24% on retention time, +/- 30 ppm on m/z measurement and +/- 30% variation on intensity over three orders of magnitude. RP adsorbant media with different alkyl chains and particle size packed in both trapping and separation channels were investigated to improve the chromatographic performance of this system. A two-fold improvement in chromatographic peak capacity was achieved using microfluidic devices comprising a 5 mircrom C3 trap with 2.5 microm C18 trap separation channel compared to the traditional 5 microm C18 stationary phase. Enhanced sample selectivity for the identification of phosphopeptides was obtained by combining immobilized metal affinity media prior to peptide separation on the RP microfluidic device. This system was evaluated in the context of differential phosphoproteome analyses to identify changes in signaling events and protein expression of human monocytes following the administration of phorbol ester.

Staphylococcus aureus escapes more efficiently from the phagosome of a cystic fibrosis bronchial epithelial cell line than from its normal counterpart

Staphylococcus aureus is frequently the initial bacterium isolated from young cystic fibrosis (CF) patients, and yet its role in CF disease progression has not been determined. Recent data from our lab demonstrates that S. aureus can invade and replicate within the CF tracheal epithelial cell line (CFT-1). Here we describe the finding that the fate of internalized S. aureus in CFT-1 cells differs from its complemented counterpart (LCFSN). S. aureus strain RN6390 was able to replicate within the mutant CFT-1 cells after invasion but not in the complemented LCFSN cells. At 1 h postinvasion, S. aureus containing vesicles within both cell lines acquired vacuolar-ATPase, lysosomal markers LAMP 1 and 2, and the lysomotrophic dye LysoTracker to a similar degree. However, at 4 h postinvasion, the percentage of S. aureus within CFT-1 cells associated with these markers decreased significantly compared to LCFSN, where the association approached 100%. Transmission electron microscopic analysis revealed that the majority of bacteria within CFT-1 cells were free in the cytosol at 4 h after invasion, whereas most S. aureus bacteria internalized by LCFSN cells remained within vesicles. These results demonstrate a fundamental difference in the fate of live S. aureus after invasion of CFT-1 versus LCFSN cell lines and may explain the propensity of S. aureus to cause chronic lung infection in CF patients.

Maturation of Human Neutrophil Phagosomes Includes Incorporation of Molecular Chaperones and Endoplasmic Reticulum Quality Control Machinery

A Human neutrophils are an essential component of the innate immune response. Although significant progress has been made toward understanding mechanisms of phagocytosis and microbicidal activity, a comprehensive analysis of proteins comprising neutrophil phagosomes has not been conducted. To that end, we used subcellular proteomics to identify proteins associated with human neutrophil phagosomes following receptor-mediated phagocytosis. Proteins (n = 411 spots) resolved from neutrophil phagosome fractions were identified by MALDI-TOF MS and/or LC-MS/MS analysis. Those associated with phagocytic vacuoles originated from multiple subcellular compartments, including the cytosol, plasma membrane, specific and azurophilic granules, and cytoskeleton. Unexpectedly several enzymes typically associated with mitochondria were identified in phagosome fractions. Furthermore proteins characteristic of the endoplasmic reticulum, including 11 molecular chaperones, were resolved from phagosome preparations. Confocal microscopy confirmed that proteins representing these major subcellular compartments were enriched on phagosomes of intact neutrophils. Notably calnexin and glucose-regulated protein 78 co-localized with gp91(phox) in human neutrophils and were thus likely delivered to phagosomes by fusion of specific granules. We conclude that neutrophil phagosomes have heretofore unrecognized complexity and function, which includes potential for antigen processing events.

Reconstitution of recycling from the phagosomal compartment in streptolysin O-permeabilized macrophages: role of Rab11

By phagocytosis, macrophages engulf large particles, microorganisms and senescent cells in vesicles called phagosomes. Many internalized proteins rapidly shuttle back to the plasma membrane following phagosome biogenesis. Here, we report a new approach to the study of recycling from the phagosomal compartment: streptolysin O- (SLO) permeabilized macrophages. In this semi-intact cell system, energy and cytosol are required to efficiently reconstitute recycling transport. Addition of GDPbetaS strongly inhibits this transport step, suggesting that a GTP-binding protein modulates the dynamics of cargo exit from the phagosomal compartment. GTPases of the Rab family control vesicular trafficking, and Rab11 is involved in transferrin receptor recycling. To unravel the role of Rab11 in the phagocytic pathway, we added recombinant proteins to SLO-permeabilized macrophages. Rab11:S25N, a negative mutant, strongly diminishes the release of recycled proteins from phagosomes. In contrast, wild type Rab11 and its positive mutant (Rab11:Q70L) favor this vesicular transport event. Using biochemical and morphological assays, we confirm that overexpression of Rab11:S25N substantially decreases recycling from phagosomes in intact cells. These findings show the requirement of a functional Rab11 for the retrieval to the plasma membrane of phagosomal content. SLO-permeabilized macrophages likely constitute a useful tool to identify new molecules involved in regulating transport along the phagocytic pathway.

Quantitative and dynamic assessment of the contribution of the ER to phagosome formation

Phagosomes were traditionally thought to originate from an invagination and scission of the plasma membrane to form a distinct intracellular vacuole. An alternative model implicating the endoplasmic reticulum (ER) as a major component of nascent and maturing phagosomes was recently proposed (Gagnon et al., 2002). To reconcile these seemingly disparate hypotheses, we used a combination of biochemical, fluorescence imaging, and electron microscopy techniques to quantitatively and dynamically assess the contribution of the plasmalemma and of the ER to phagosome formation and maturation. We could not verify even a transient physical continuity between the ER and the plasma membrane, nor were we able to detect a significant contribution of the ER to forming or maturing phagosomes in either macrophages or dendritic cells. Instead, our data indicate that the plasma membrane is the main constituent of nascent and newly formed phagosomes, which are progressively remodeled by fusion with endosomal and eventually lysosomal compartments as phagosomes mature into acidic, degradative organelles.

Within the cell: analytical techniques for subcellular analysis

This review covers recent developments in the preparation, manipulation, and analyses of subcellular environments. In particular, it highlights approaches for (1) separation and detection of individual organelles, (2) preparation of ultra-pure organelle fractions, and (3) utilization of novel labeling strategies. These approaches, based on innovative technologies such as microfluidics, immunoisolation, mass spectrometry and electrophoresis, suggest that subcellular analyses will soon become as commonplace as single cell and bulk cellular assays.

The nature of the phagosomal membrane: endoplasmic reticulum versus plasmalemma

For decades, the vacuole that surrounds particles engulfed by phagocytosis was believed to originate from the plasma membrane. Conversion of the nascent phagosome into a microbicidal organelle was thought to result from the subsequent, orderly fusion of early endosomes, late endosomes, and ultimately, lysosomes with the original plasma membrane-derived vacuole. This conventional model has been challenged, if not superseded, by a revolutionary model that regards phagosome formation as resulting from the particle sliding into the endoplasmic reticulum via an opening at the base of the phagocytic cup. The merits and implications of these two hypotheses are summarized here and analyzed in light of recent results.

Functional microtubules are required for antigen processing by macrophages and dendritic cells

Antigen-presenting cells readily phagocytose antigens and channel them through various membrane-bound organelles within the cell. In previous studies, we demonstrated that macrophages concentrated and localized particulate antigens to the trans-Golgi prior to displaying the MHC-class I-antigenic peptides on the cell surface. In this study, we evaluated the importance of cytoskeletal elements in the intracellular trafficking of soluble and liposome-encapsulated ovalbumin in murine bone marrow-derived macrophages and human dendritic cells. F-actin, as identified by staining with fluorescein phalloidin, was observed at the point of contact between soluble or liposomal antigen and the cell membrane, suggesting that a rearrangement of the cytoskeleton occurs to facilitate the uptake of the antigens. Cells were incubated with colchicine, a microtubule depolymerizing agent, or paclitaxel, a microtubule polymerizing agent, before the addition of Texas Red-labeled ovalbumin or liposome-encapsulated Texas Red-labeled ovalbumin. Colchicine disrupted the trans-Golgi, whereas the trans-Golgi complexes were intact in paclitaxel treated cells. In either paclitaxel or colchicine-treated macrophages, internalized liposomal ovalbumin was not concentrated in the area of the trans-Golgi as determined by staining with fluorescent ceramide. In contrast, soluble ovalbumin was concentrated in the region of the trans-Golgi in 15% of the dendritic cells treated with paclitaxel, whereas 6% of the dendritic cells were able to concentrate liposomal antigen. In colchicine-treated dendritic cells, both soluble and liposomal antigens were internalized but did not localize to the area of the trans-Golgi. These data suggest that trafficking of soluble and liposome-encapsulated ovalbumin requires a functional microtubule-dependent translocation system.

Rab5-associated Vacuoles Play a Unique Role in Phagocytosis of the Enteric Protozoan Parasite Entamoeba histolytica

In mammals, Rab5 and Rab7 play a specific and coordinated role in a sequential process during phagosome maturation. Here, we report that Rab5 and Rab7 in the enteric protozoan parasite Entamoeba histolytica, EhRab5 and EhRab7A, are involved in steps that are distinct from those known for mammals. EhRab5 and EhRab7A were localized to independent small vesicular structures at steady state. Priming with red blood cells induced the formation of large vacuoles associated with both EhRab5 and EhRab7A ("prephagosomal vacuoles (PPV)") in the amoeba within an incubation period of 5-10 min. PPV emerged de novo physically and distinct from phagosomes. PPV were gradually acidified and matured by fusion with lysosomes containing a digestive hydrolase, cysteine proteinase, and a membrane-permeabilizing peptide amoebapore. After EhRab5 dissociated from PPV, 5-10 min later, the EhRab7A-PPV fused with phagosomes, and EhRab7A finally dissociated from the phagosomes. Immunoelectron and light micrographs showed that PPV contained small vesicle-like structures containing fluid-phase markers and amoebapores, which were not evenly distributed within PPV, suggesting that the mechanism was similar to multivesicular body formation in PPV generation. In contrast to Rab5 from other organisms, EhRab5 was involved exclusively in phagocytosis, but not in endocytosis. Overexpression of wild-type EhRab5 enhanced phagocytosis and the transport of amoebapore to phagosomes. Conversely, expression of an EhRab5Q67L GTP form mutant impaired the formation of PPV and phagocytosis. Altogether, we propose that the amoebic Rab5 plays an important role in the formation of unique vacuoles, which is essential for engulfment of erythrocytes and important for packaging of lysosomal hydrolases, prior to the targeting to phagosomes.

Induction and antimicrobial activity of platelet basic protein derivatives in human monocytes

The antimicrobial activity of a number of chemokines has recently come into focus of research about innate immunity. We have previously shown that platelet basic protein (PBP), which gives rise to several antimicrobial peptides of platelets, is also expressed in human monocytes. In the present studies, we show that exposure of human monocytes to bacteria or microbial components (lipopolysaccharide and zymosan) induces a several-fold greater expression of derivates of PBP. Also, activation of proteinase-activated receptors (PARs) by thrombin or the synthetic peptide ligand SFLLRN of PAR-1 significantly increased PBP expression, presumably on the transcriptional level, as evidenced by higher mRNA levels. Derivates of PBP appeared to reach phago-lysosomes, as higher concentration was found in latex phagosomes isolated by a flotation method. By the gel-overlay technique, two bactericidal derivatives of PBP could be visualized, which were immunoreactive with anti-PBP antibody in Western blots. By matrix-assisted laser desorption/ionization time of flight and surface-enhanced laser desorption and ionization techniques, it was confirmed that the bands corresponded to PBP derivates. After immunofixation with a monoclonal antibody to PBP, the major peptide in zymosan-stimulated monocytes was identified to correspond by molecular weight to connective tissue-activating peptide III, which has been reported to be a major antimicrobial PBP derivate also in platelets. Our observations indicate that PBP and its derivates are constituents of the antimicrobial arsenal of human monocytes. Their increased expression after exposure to microorganisms allows a rapid host response to pathogens.

Proteomic analysis reveals a role for protein kinase C-alpha in phagosome maturation

Acquisition of microbicidal properties by phagosomes requires the action of molecules which regulate the interactions between phagosomes and endocytic organelles. Members of the protein kinase C (PKC) superfamily of serine/threonine kinases are recruited to phagosomes with various kinetics during phagolysosome biogenesis. To study the role of PKC-alpha in this process, we compared the composition of latex bead-containing phagosomes isolated from control and dominant-negative (DN) PKC-alpha-overexpressing RAW 264.7 macrophages. Western blot analysis indicated that the levels of both lysosomal-associated membrane protein-1 and flotillin-1, which are acquired through interactions with late endosomes and lysosomes, are reduced in phagosomes from DN PKC-alpha-overexpressing macrophages. Proteomic characterization of latex bead-containing phagosomes revealed that recruitment of the small GTPase Rab7, cathepsin D, and cathepsin S is inhibited by DN PKC-alpha. Collectively, these data provide evidence that PKC-alpha plays a role in phagolysosome biogenesis, a critical process of the innate immune response against infections.

Ezrin/radixin/moesin proteins and Rho GTPase signalling in leucocytes

The ezrin/radixin/moesin (ERM) family of actin-binding proteins act both as linkers between the actin cytoskeleton and plasma membrane proteins and as signal transducers in responses involving cytoskeletal remodelling. The Rho family of GTPases also regulate cytoskeletal organisation, and several molecular pathways linking ERM proteins and Rho GTPases have been described. This review discusses recent findings on ERM protein function in leucocytes and how these may be integrated with Rho GTPase signalling.

Acquisition of Hrs, an essential component of phagosomal maturation, is impaired by mycobacteria

Pathogenic mycobacteria survive within macrophages by precluding the fusion of phagosomes with late endosomes or lysosomes. Because the molecular determinants of normal phagolysosome formation are poorly understood, the sites targeted by mycobacteria remain unidentified. We found that Hrs, an adaptor molecule involved in protein sorting, associates with phagosomes prior to their fusion with late endosomes or lysosomes. Recruitment of Hrs required the interaction of its FYVE domain with phagosomal phosphatidylinositol 3-phosphate, but two other attachment sites were additionally involved. Depletion of Hrs by use of small interfering RNA impaired phagosomal maturation, preventing the acquisition of lysobisphosphatidic acid and reducing luminal acidification. As a result, the maturation of phagosomes formed in Hrs-depleted cells was arrested at an early stage, characterized by the acquisition and retention of sorting endosomal markers. This phenotype is strikingly similar to that reported to occur in phagosomes of cells infected by mycobacteria. We therefore tested whether Hrs is recruited to phagosomes containing mycobacteria. Hrs associated readily with phagosomes containing inert particles but poorly with mycobacterial phagosomes. Moreover, Hrs was found more frequently in phagosomes containing avirulent Mycobacterium smegmatis than in phagosomes with the more virulent Mycobacterium marinum. These findings suggest that the inability to recruit Hrs contributes to the arrest of phagosomal maturation induced by pathogenic mycobacteria.

Virulent and avirulent strains of Francisella tularensis prevent acidification and maturation of their phagosomes and escape into the cytoplasm in human macrophages

Francisella tularensis, the agent of tularemia, is an intracellular pathogen, but little is known about the compartment in which it resides in human macrophages. We have examined the interaction of a recent virulent clinical isolate of F. tularensis subsp. tularensis and the live vaccine strain with human macrophages by immunoelectron and confocal immunofluorescence microscopy. We assessed the maturation of the F. tularensis phagosome by examining its acquisition of the lysosome-associated membrane glycoproteins (LAMPs) CD63 and LAMP1 and the acid hydrolase cathepsin D. Two to four hours after infection, vacuoles containing live F. tularensis cells acquired abundant staining for LAMPs but little or no staining for cathepsin D. However, after 4 h, the colocalization of LAMPs with live F. tularensis organisms declined dramatically. In contrast, vacuoles containing formalin-killed bacteria exhibited intense staining for all of these late endosomal/lysosomal markers at all time points examined (1 to 16 h). We examined the pH of the vacuoles 3 to 4 h after infection by quantitative immunogold staining and by fluorescence staining for lysosomotropic agents. Whereas phagosomes containing killed bacteria stained intensely for these agents, indicating a marked acidification of the phagosomes (pH 5.5), phagosomes containing live F. tularensis did not concentrate these markers and thus were not appreciably acidified (pH 6.7). An ultrastructural analysis of the F. tularensis compartment revealed that during the first 4 h after uptake, the majority of F. tularensis bacteria reside within phagosomes with identifiable membranes. The cytoplasmic side of the membranes of approximately 50% of these phagosomes was coated with densely staining fibrils of approximately 30 nm in length. In many cases, these coated phagosomal membranes appeared to bud, vesiculate, and fragment. By 8 h after infection, the majority of live F. tularensis bacteria lacked any ultrastructurally discernible membrane separating them from the host cell cytoplasm. These results indicate that F. tularensis initially enters a nonacidified phagosome with LAMPs but without cathepsin D and that the phagosomal membrane subsequently becomes morphologically disrupted, allowing the bacteria to gain direct access to the macrophagic cytoplasm. The capacity of F. tularensis to alter the maturation of its phagosome and to enter the cytoplasm is likely an important element of its capacity to parasitize macrophages and has major implications for vaccine development.

Expression pattern and raft association of NIPSNAP3 and NIPSNAP4, highly homologous proteins encoded by genes in close proximity to the ATP-binding cassette transporter A1

The highly homologous genes NIPSNAP3 and NIPSNAP4, with 87% amino acid identity, are members of the NIPSNAP family with putative roles in vesicular trafficking. NIPSNAP3 mRNA and NIPSNAP4 mRNA and protein were detected in multiple tissues and cells at varying degrees. Interestingly, NIPSNAP3 is most highly expressed in skeletal muscle, where NIPSNAP4 has a low mRNA abundance. NIPSNAP4 was found associated with membranes and partly localized in rafts. The ubiquitous expression of the highly conserved NIPSNAPs and their association with membranes further support an important cellular function of these proteins probably linked to vesicular trafficking. The NIPSNAP3 and NIPSNAP4 genes are located in close proximity to the 3' end of the ATP-binding cassette transporter A1 (ABCA1), whose mutations cause familial high-density lipoprotein deficiency syndromes. The adjacent genomic location and the finding that ABCA1 is a regulator of vesicular trafficking may indicate a functional relation of these proteins, even though NIPSNAP4 does not interact directly with ABCA1 nor is its expression altered in cells with mutated ABCA1.

Dynein-mediated vesicle transport controls intracellular Salmonella replication

Salmonella typhimurium survives and replicates intracellular in a membrane-bound compartment, the Salmonella-containing vacuole (SCV). In HeLa cells, the SCV matures through interactions with the endocytic pathway, but Salmonella avoids fusion with mature lysosomes. The exact mechanism of the inhibition of phagolysosomal fusion is not understood. Rab GTPases control several proteins involved in membrane fusion and vesicular transport. The small GTPase Rab7 regulates the transport of and fusion between late endosomes and lysosomes and associates with the SCV. We show that the Rab7 GTPase cycle is not affected on the SCV. We then manipulated a pathway downstream of the small GTPase Rab7 in HeLa cells infected with Salmonella. Expression of the Rab7 effector RILP induces recruitment of the dynein/dynactin motor complex to the SCV. Subsequently, SCV fuse with lysosomes. As a result, the intracellular replication of Salmonella is inhibited. Activation of dynein-mediated vesicle transport can thus control intracellular survival of Salmonella.