Phosphoinositides regulate membrane-dependent actin assembly by latex bead phagosomes

An In Vitro System to Analyze Generation and Degradation of Phagosomal Phosphatidylinositol Phosphates

Phagosomes are formed when phagocytic cells take up large particles, and they develop into phagolysosomes where the particles are degraded. The transformation of nascent phagosomes into phagolysosomes is a complex multi-step process, and the precise timing of these steps depends at least in part on phosphatidylinositol phosphates (PIPs). Some such-called "intracellular pathogens" are not delivered to microbicidal phagolysosomes and manipulate the PIP composition of the phagosomes they reside in. Studying the dynamic changes of the PIP composition of inert-particle phagosomes will help to understand why the pathogens' manipulations reprogram phagosome maturation.We here describe a method to detect and to follow generation and degradation of PIPs on purified phagosomes. To this end, phagosomes formed around inert latex beads are purified from J774E macrophages and incubated in vitro with PIP-binding protein domains or PIP-binding antibodies. Binding of such PIP sensors to phagosomes indicates presence of the cognate PIP and is quantified by immunofluorescence microscopy. When phagosomes are incubated with PIP sensors and ATP at a physiological temperature, the generation and degradation of PIPs can be followed, and PIP-metabolizing enzymes can be identified using specific inhibitory agents.

Nanoparticle entry into cells; the cell biology weak link

Nanoparticles (NP) are attractive options for the therapeutic delivery of active pharmaceutical drugs, proteins and nucleic acids into cells, tissues and organs. Research into the development and application of NP most often starts with a diverse group of scientists, including chemists, bioengineers and material and pharmaceutical scientists, who design, fabricate and characterize NP in vitro (Stage 1). The next step (Stage 2) generally investigates cell toxicity as well as the processes by which NP bind, are internalized and deliver their cargo to appropriate model tissue culture cells. Subsequently, in Stage 3, selected NP are tested in animal systems, mostly mouse. Whereas the chemistry-based development and analysis in Stage 1 is increasingly sophisticated, the investigations in Stage 2 are not what could be regarded as 'state-of-the-art' for the cell biology field and the quality of research into NP interactions with cells is often sub-standard. In this review we describe our current understanding of the mechanisms by which particles gain entry into mammalian cells via endocytosis. We summarize the most important areas for concern, highlight some of the most common mis-conceptions, and identify areas where NP scientists could engage with trained cell biologists. Our survey of the different mechanisms of uptake into cells makes us suspect that claims for roles for caveolae, as well as macropinocytosis, in NP uptake into cells have been exaggerated, whereas phagocytosis has been under-appreciated.

Better Together: Current Insights Into Phagosome-Lysosome Fusion

Following phagocytosis, the nascent phagosome undergoes maturation to become a phagolysosome with an acidic, hydrolytic, and often oxidative lumen that can efficiently kill and digest engulfed microbes, cells, and debris. The fusion of phagosomes with lysosomes is a principal driver of phagosomal maturation and is targeted by several adapted intracellular pathogens. Impairment of this process has significant consequences for microbial infection, tissue inflammation, the onset of adaptive immunity, and disease. Given the importance of phagosome-lysosome fusion to phagocyte function and the many virulence factors that target it, it is unsurprising that multiple molecular pathways have evolved to mediate this essential process. While the full range of these pathways has yet to be fully characterized, several pathways involving proteins such as members of the Rab GTPases, tethering factors and SNAREs have been identified. Here, we summarize the current state of knowledge to clarify the ambiguities in the field and construct a more comprehensive phagolysosome formation model. Lastly, we discuss how other cellular pathways help support phagolysosome biogenesis and, consequently, phagocyte function.

Calcium Signaling Commands Phagosome Maturation Process

Phagosome-lysosome (P-L) fusion is one of the central immune-effector responses of host. It is known that phagosome maturation process is associated with numerous signaling cascades and among these, important role of calcium (Ca²⁺) signaling has been realized recently. Ca²⁺ plays key roles in actin rearrangement, activation of NADPH oxidase and protein kinase C (PKC). Involvement of Ca²⁺ in these cellular processes directs phagosomal maturation process. Some of the intracellular pathogens have acquired the strategies to modulate Ca²⁺ associated pathways to block P-L fusion process. In this review we have described the mechanism of Ca²⁺ signals that influence P-L fusion by controlling ROS, actin and PKC signaling cascades. We have also discussed the strategies implemented by the intracellular pathogens to manipulate Ca²⁺ signaling to consequently subvert P-L fusion. A detail study of factors associated in manipulating Ca²⁺ signaling may provide new insights for the development of therapeutic tools for more effective treatment options against infectious diseases.

Expression and Functional Role of TRPV4 in Bone Marrow-Derived CD11c+ Cells

The increase in cytosolic Ca²⁺ is essential in key effector functions of dendritic cells (DCs), including differentiation, maturation, cytokine expression, and phagocytosis. Although several Ca²⁺-permeable ion channels have been described in DCs, the contribution of transient receptor potential (TRP) channels remains poorly understood. Here, we investigated whether TRPV4 plays a role in the differentiation, maturation, and phagocytosis of granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced mouse bone marrow-derived cells (BMDCs). Using intracellular Ca²⁺ imaging experiments, we found that TRPV4 was functionally expressed in the plasma membrane of immature CD11c⁺ BMDCs and that its activity and expression were downregulated in CD11c⁺ BMDCs matured with lipopolysaccharide (LPS). Comparative analysis of the GM-CSF-stimulated cells showed that Trpv4 knockout and wild-type bone marrow cultures had a similar distribution of differentiated cells, generating a heterogenous culture population rich in CD11c⁺, CD11b⁺ cells, and low levels of F4/80⁺ cells. The lack of TRPV4 did not prevent the LPS-induced nuclear translocation of NF-κB, the upregulation of the proinflammatory cytokines IL-6 and IL-12, or the upregulation of the maturation markers CD40, CD80, and CD86. In contrast, TRPV4-deficient CD11c⁺ BMDCs exhibited a significantly reduced endocytic capacity of IgG-coated beads, but the internalization of uncoated beads in the absence of TRPV4 was not affected. Taken together, our results demonstrate that TRPV4 was dispensable in the differentiation and maturation of mouse CD11c⁺ BMDCs but contributed to the mechanism underlying Fc receptor-mediated phagocytosis. Overall, our results further strengthen the role of TRPV4 in immune-related processes.

Osmotic activation of a Ca2+-dependent phospholipase C pathway that regulates ∆N TRPV1-mediated currents in rat supraoptic neurons

The magnocellular neurosecretory cells (MNCs) of the hypothalamus regulate body fluid balance by releasing the hormones vasopressin (VP) and oxytocin (OT) in an osmolality‐dependent manner. Elevations of external osmolality increase MNC firing and hormone release. MNC osmosensitivity is largely due to activation of a mechanosensitive non‐selective cation current that responds to osmotically‐evoked changes in MNC volume and is mediated by an N‐terminal variant of the TRPV1 channel (∆N TRPV1). We report a novel mechanism by which increases in osmolality may modulate ∆N TRPV1‐mediated currents and thus influence MNC electrical behaviour. We showed previously that acute elevations of external osmolality activate the enzyme phospholipase C (PLC) in isolated MNCs. We now show that the osmotic activation of PLC has a time course and dose‐dependence that is consistent with a role in MNC osmosensitivity and that it contributes to the osmotically‐evoked increase in non‐selective cation current in MNCs through a protein kinase C‐dependent pathway. We furthermore show that the mechanism of osmotic activation of PLC requires an increase in internal Ca²⁺ that depends on influx through L‐type Ca²⁺ channels. Our data therefore suggest that MNCs possess an osmotically‐activated Ca²⁺‐dependent PLC that contributes to the osmotic activation of ∆N TRPV1 and may therefore be important in MNC osmosensitivity and in central osmoregulation.

Macrophage-Bacteria Interactions-A Lipid-Centric Relationship

Macrophages are professional phagocytes at the front line of immune defenses against foreign bodies and microbial pathogens. Various bacteria, which are responsible for deadly diseases including tuberculosis and salmonellosis, are capable of hijacking this important immune cell type and thrive intracellularly, either in the cytoplasm or in specialized vacuoles. Tight regulation of cellular metabolism is critical in shaping the macrophage polarization states and immune functions. Lipids, besides being the bulk component of biological membranes, serve as energy sources as well as signaling molecules during infection and inflammation. With the advent of systems-scale analyses of genes, transcripts, proteins, and metabolites, in combination with classical biology, it is increasingly evident that macrophages undergo extensive lipid remodeling during activation and infection. Each bacterium species has evolved its own tactics to manipulate host metabolism toward its own advantage. Furthermore, modulation of host lipid metabolism affects disease susceptibility and outcome of infections, highlighting the critical roles of lipids in infectious diseases. Here, we will review the emerging roles of lipids in the complex host-pathogen relationship and discuss recent methodologies employed to probe these versatile metabolites during the infection process. An improved understanding of the lipid-centric nature of infections can lead to the identification of the Achilles' heel of the pathogens and host-directed targets for therapeutic interventions. Currently, lipid-moderating drugs are clinically available for a range of non-communicable diseases, which we anticipate can potentially be tapped into for various infections.

Moesin and myosin IIA modulate phagolysosomal biogenesis in macrophages

Biogenesis of phagolysosomes is central to the elimination of pathogens by macrophages. We previously showed that Src homology region 2 domain-containing phosphatase 1 (SHP-1) participates in the regulation of phagosome maturation. Through proteomics, we identified moesin and the non-muscle myosin-IIA as proteins interacting with SHP-1 during phagocytosis. Silencing of either moesin or myosin IIA with small interfering RNA inhibited phagosomal acidification and recruitment of LAMP-1. Moreover, the intraphagosomal oxidative burst was impaired in the absence of either SHP-1 or myosin IIA but not moesin. Finally, absence of either SHP-1, moesin, or myosin IIA ablated the capacity of macrophages to clear bacterial infection. Collectively, these results implicate both moesin and myosin IIA in the regulation of phagolysosome biogenesis and in host defense against infections.

Quantification of Phosphatidylinositol Phosphate Species in Purified Membranes

Phosphoinositide lipids (PIPs) are required for various processes during macroautophagy, such as phagophore formation and autophagosome-lysosome fusion. Hence, quantification of the seven PIP species in autophagosome membranes is an important tool to understand how these lipids govern the transition of autophagosomes into autolysosomes. Here, we describe microscopic and mass spectrometry methods which, although designed to quantify the different PIP species on purified lysosomes, can also be applied to analyze autophagosomal PIPs.

Deciphering the roles of phosphoinositide lipids in phagolysosome biogenesis

Professional phagocytes engulf microbial invaders into plasma membrane-derived phagosomes. These mature into microbicidal phagolysosomes, leading to killing of the ingested microbe. Phagosome maturation involves sequential fusion of the phagosome with early endosomes, late endosomes, and the main degradative compartments in cells, lysosomes. Some bacterial pathogens manipulate the phosphoinositide (PIP) composition of phagosome membranes and are not delivered to phagolysosomes, pointing at a role of PIPs in phagosome maturation. This hypothesis is supported by comprehensive microscopic studies. Recently, cell-free reconstitution of fusion between phagosomes and endo(lyso)somes identified phosphatidylinositol 4-phosphate [PI(4)P] and phosphatidylinositol 3-phosphate [PI(3)P] as key regulators of phagolysosome biogenesis. Here, we describe the emerging roles of PIPs in phagosome maturation and we present tools to study PIP involvement in phagosome trafficking using intact cells or purified compartments.

U-73122 reduces the cell growth in cultured MG-63 ostesarcoma cell line involving Phosphoinositide-specific Phospholipases C

The definition of the number and nature of the signal transduction pathways involved in the pathogenesis and the identification of the molecules promoting metastasis spread might improve the knowledge of the natural history of osteosarcoma, also allowing refine the prognosis and opening the way to novel therapeutic strategies. Phosphatydil inositol (4,5) bisphosphate (PIP2), belonging to the Phosphoinositide (PI) signal transduction pathway, was related to the regulation of ezrin, an ezrin-radixin-moesin protein involved in metastatic osteosarcoma spread. The levels of PIP2 are regulated by means of the PI-specific Phospholipase C (PLC) enzymes. Recent literature data suggested that in osteosarcoma the panel of expression of PLC isoforms varies in a complex and unclear manner and is related to ezrin, probably networking with Ras GTPases, such as RhoA and Rac1. We analyzed the expression and the subcellular localization of PLC enzymes in cultured human osteosarcoma MG-63 cells, commonly used as an experimental model for human osteoblasts, using U-73122 PLC inhibitor, U-73343 inactive analogue, and by silencing ezrin. The treatment with U-73122 significantly reduces the number of MG-63 viable cells and contemporarily modifies the expression and the subcellular localization of selected PLC isoforms. U-73122 reduces the cell growth in cultured MG-63 ostesarcoma cell line involving PI-specific Phospholipases C.

Phosphatidylinositol 4-phosphate and phosphatidylinositol 3-phosphate regulate phagolysosome biogenesis

Professional phagocytic cells ingest microbial intruders by engulfing them into phagosomes, which subsequently mature into microbicidal phagolysosomes. Phagosome maturation requires sequential fusion of the phagosome with early endosomes, late endosomes, and lysosomes. Although various phosphoinositides (PIPs) have been detected on phagosomes, it remained unclear which PIPs actually govern phagosome maturation. Here, we analyzed the involvement of PIPs in fusion of phagosomes with various endocytic compartments and identified phosphatidylinositol 4-phosphate [PI(4)P], phosphatidylinositol 3-phosphate [PI(3)P], and the lipid kinases that generate these PIPs, as mediators of phagosome-lysosome fusion. Phagosome-early endosome fusion required PI(3)P, yet did not depend on PI(4)P. Thus, PI(3)P regulates phagosome maturation at early and late stages, whereas PI(4)P is selectively required late in the pathway.

Ezrin-related Phosphoinositide pathway modifies RhoA and Rac1 in human osteosarcoma cell lines

Selected Phosphoinositide-specific Phospholipase C (PI-PLC) enzymes occupy the convergence point of the broad range of pathways that promote Rho and Ras GTPase mediated signalling, which also regulate the activation of ezrin, a member of the ezrin-radixin-moesin (ERM) proteins family involved in the metastatic osteosarcoma spread. Previous studies described that in distinct human osteosarcoma cell lines ezrin networks the PI-PLC with complex interplay controlling the expression of the PLC genes, which codify for PI-PLC enzymes. In the present study, we analyzed the expression and the sub-cellular distribution of RhoA and Rac1 respectively after ezrin silencing and after PI-PLC ε silencing, in order to investigate whether ezrin-RhoGTPAses signalling might involve one or more specific PI-PLC isoforms in cultured 143B and Hs888 human osteosarcoma cell lines. In the present experiments, both ezrin and PLCE gene silencing had different effects upon RhoA and Rac1 expression and sub-cellular localization. Displacements of Ezrin and of RhoA localization were observed, probably playing functional roles.

Ezrin silencing remodulates the expression of Phosphoinositide-specific Phospholipase C enzymes in human osteosarcoma cell lines

Ezrin, a protein belonging to the Ezrin, radixin and moesin (ERM) family, was engaged in the metastatic spread of osteosarcoma. The Protein 4.1, Ezrin, radixin, moesin (FERM) domain of Ezrin binds the membrane Phosphatydil inositol (4,5) bisphosphate (PIP2), a crucial molecule belonging to the Phosphoinositide (PI) signal transduction pathway. The cytoskeleton cross-linker function of Ezrin largely depends on membrane PIP2 levels, and thus upon the activity of related enzymes belonging to the PI-specific phospholipase C (PI-PLC) family. Based on the role of Ezrin in tumour progression and metastasis, we silenced the expression of Vil2 (OMIM *123900), the gene which codifies for Ezrin, in cultured human osteosarcoma 143B and Hs888 cell lines. After Ezrin silencing, the growth rate of both cell lines was significantly reduced and morphogical changes were observed. We also observed moderate variations both of selected PI-PLC enzymes within the cell and of expression of the corresponding PLC genes. In 143B cell line the transcription of PLCB1 decreased, of PLCG2 increased and of PLCE differed in a time-dependent manner. In Hs888, the expression of PLCB1 and of PLCD4 significantly increased, of PLCE moderately increased in a time dependent manner; the expression of PLCG2 was up-regulated. These observations indicate that Ezrin silencing affects the transcription of selected PLC genes, suggesting that Ezrin might influence the expression regulation of PI-PLC enzymes.

Interaction between Bacillus cereus and cultured human enterocytes: effect of calcium, cell differentiation, and bacterial extracellular factors

Bacillus cereus interaction with cultured human enterocytes and the signaling pathways responsible for the biological effects of the infection were investigated. Results demonstrate that calcium depletion increases the ability of strains T1 and 2 to invade cells. Bacteria associated in greater extent to undifferentiated enterocytes and extracellular factors from strain 2 increased its own association and invasion. Inhibitors of signaling pathways related to phosphorylated lipids (U73122 and wortmannin) were able to significantly reduce cytoskeleton disruption induced by B. cereus infection. Adhesion of strain T1 decreased in the presence of U73122 and of wortmannin, as well as when those inhibitors were used together. In contrast, invasion values were diminished only by U73122. Results show that different factors are involved in the interaction between B. cereus and cultured human enterocytes. Following infection, disruption of the cytoskeleton could facilitate invasion of the eukaryotic cells.

Epigallocatechin gallate (EGCG) inhibits type II phosphatidylinositol 4-kinases: a key component in pathways of phosphoinositide turnover

Type II phosphatidylinositol (PtdIns) 4-kinases produce PtdIns 4-phosphate, an early key signaling molecule in phosphatidylinositol cycle, which is indispensable for T cell activation. Type II PtdIns 4-kinase alpha and beta have similar biochemical properties. To distinguish these isoforms Epigallocatechin gallate (EGCG) has been evaluated as a specific inhibitor. EGCG is the major active catechin in green tea having anti-inflammatory, antiatherogenic and cancer chemopreventive properties. The precise mechanism of actions and molecular targets of EGCG in early signaling cascades are not well understood. In the present study, we have shown that EGCG inhibits type II PtdIns 4-kinases (α and β isoforms) and PtdIns 3-kinase activity in vitro. EGCG directly bind to both alpha and beta isoforms of type II PtdIns 4-kinases with a Kd of 2.62 μM and 1.02 μM, respectively. Type II PtdIns 4-kinase-EGCG complex have different binding pattern at its excited state. Both isoforms showed significant change in helicity upon binding with EGCG. EGCG modulates its effect by interacting with ATP binding pocket; the residues likely to be involved in EGCG binding were predicted by Autodock. Our findings suggest that EGCG inhibits two isoforms and could be a key to regulate T cell activation.

Gelsolin and non-muscle myosin IIA interact to mediate calcium-regulated collagen phagocytosis

The formation of adhesion complexes is the rate-limiting step for collagen phagocytosis by fibroblasts, but the role of Ca(2+) and the potential interactions of actin-binding proteins in regulating collagen phagocytosis are not well defined. We found that the binding of collagen beads to fibroblasts was temporally and spatially associated with actin assembly at nascent phagosomes, which was absent in gelsolin null cells. Analysis of tryptic digests isolated from gelsolin immunoprecipitates indicated that non-muscle (NM) myosin IIA may bind to gelsolin. Immunostaining and immunoprecipitation showed that gelsolin and NM myosin IIA associated at collagen adhesion sites. Gelsolin and NM myosin IIA were both required for collagen binding and internalization. Collagen binding to cells initiated a prolonged increase of [Ca(2+)](i), which was absent in cells null for gelsolin or NM myosin IIA. Collagen bead-induced increases of [Ca(2+)](i) were associated with phosphorylation of the myosin light chain, which was dependent on gelsolin. NM myosin IIA filament assembly, which was dependent on myosin light chain phosphorylation and increased [Ca(2+)](i), also required gelsolin. Ionomycin-induced increases of [Ca(2+)](i) overcame the block of myosin filament assembly in gelsolin null cells. We conclude that gelsolin and NM myosin IIA interact at collagen adhesion sites to enable NM myosin IIA filament assembly and localized, Ca(2+)-dependent remodeling of actin at the nascent phagosome and that these steps are required for collagen phagocytosis.

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.

Role of lipid rafts in innate immunity and phagocytosis of polystyrene latex microspheres

Understanding of the association of phagocytosis of polymers with signaling of innate immunity of macrophages is the major purpose of this study. Polymer conjugates have been utilized for clinical therapy of cancer and infections, such as Mycobacterium tuberculosis, as effective vectors of drug-delivery systems. They are incorporated through phagocytosis into macrophages and activate innate immunity signaling, which plays a crucial role in its therapeutic and side effects. Macrophage phagocytosis of polystyrene latex microspheres was examined and assayed by treatment of macrophages with the cholesterol depletor methyl-β-cyclodextrin (MβCD) or the sphingolipid depletor n-octyl-β-D-glucopyranoside (OGP). Expressions of various mRNAs during phagocytosis were quantified by real-time PCR. Phagocytosis of polystyrene latex microspheres by various macrophages, such as murine monocyte-derived macrophage J774, rat alveolar macrophage NR8383, and murine Kupffer cell KC13-2, was suppressed by treatment with MβCD or OGP in a concentration-dependent manner. The expression of mRNAs of TNFα, IL-1β, IL-6 and CXCL10 genes induced by lipopolysaccharide (LPS) was not suppressed by treatment with MβCD in J774 cells. Moreover, genes that were induced by LPS were up-regulated even in the absence of LPS by the phagocytosis of polymer conjugates, but such up-regulations were not suppressed by the treatment with MβCD. It was shown that lipid rafts play a significant role in incorporation of polymer conjugates through phagocytosis of macrophages, but their association with signal transduction in innate immunity is very limited.

Initial receptor-ligand interactions modulate gene expression and phagosomal properties during both early and late stages of phagocytosis

The receptors engaged during recognition and phagocytic uptake of microorganisms and particles influence signaling events and diverse subcellular responses that occur during phagosome formation and maturation. However, pathogens generally have multiple ligands on their surface, making it difficult to dissect the roles of individual receptors during phagocytosis. Moreover, it remains elusive to which extent receptor-ligand interactions and early binding events define the subsequent intracellular fate of phagosomes. Here, we used latex beads coupled to single ligands, focusing on immunoglobulin G, mannan, bacterial lipopolysaccharides and avidin, and monitored: (1) phagocytic uptake rates, (2) fusion of phagosomes with lysosomal compartments, (3) the gene expression profile during phagocytosis, (4) the protein composition of mature phagosomes and (5) time-dependent dynamics of protein association with phagosomes in J774.A1 mouse macrophages. The differently coated latex beads were internalized at different rates and exhibited different kinetics of phagolysosomal fusion events dependent on their specific ligand. Furthermore, less than 60% of identified phagosomal proteins and only 10-15% of changes in gene expression were common to all investigated ligands. These findings demonstrate that each single ligand induced a distinct pattern of genes and a different protein composition of phagosomes. Taken together, our data argue that phagocytic receptor-specific programs of signaling events direct phagosomes to different physiological states and support the existence of a specific receptor-ligand 'signature' during the whole process of phagocytosis.

The platelet actin cytoskeleton associates with SNAREs and participates in alpha-granule secretion

Following platelet activation, platelets undergo a dramatic shape change mediated by the actin cytoskeleton and accompanied by secretion of granule contents. While the actin cytoskeleton is thought to influence platelet granule secretion, the mechanism for this putative regulation is not known. We found that disruption of the actin cytoskeleton by latrunculin A inhibited alpha-granule secretion induced by several different platelet agonists without significantly affecting activation-induced platelet aggregation. In a cell-free secretory system, platelet cytosol was required for alpha-granule secretion. Inhibition of actin polymerization prevented alpha-granule secretion in this system, and purified platelet actin could substitute for platelet cytosol to support alpha-granule secretion. To determine whether SNAREs physically associate with the actin cytoskeleton, we isolated the Triton X-100 insoluble actin cytoskeleton from platelets. VAMP-8 and syntaxin-2 associated only with actin cytoskeletons of activated platelets. Syntaxin-4 and SNAP-23 associated with cytoskeletons isolated from either resting or activated platelets. When syntaxin-4 and SNAP-23 were tested for actin binding in a purified protein system, only syntaxin-4 associated directly with polymerized platelet actin. These data show that the platelet cytoskeleton interacts with select SNAREs and that actin polymerization facilitates alpha-granule release.

TRPV2 has a pivotal role in macrophage particle binding and phagocytosis

Macrophage phagocytosis is critical for defense against pathogens. Whereas many steps of phagocytosis involve ionic flux, the underlying ion channels remain ill defined. Here we show that zymosan-, immunoglobulin G (IgG)- and complement-mediated particle binding and phagocytosis were impaired in macrophages lacking the cation channel TRPV2. TRPV2 was recruited to the nascent phagosome and depolarized the plasma membrane. Depolarization increased the synthesis of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)), which triggered the partial actin depolymerization necessary for occupancy-elicited phagocytic receptor clustering. TRPV2-deficient macrophages were also defective in chemoattractant-elicited motility. Consequently, TRPV2-deficient mice showed accelerated mortality and greater organ bacterial load when challenged with Listeria monocytogenes. Our data demonstrate the participation of TRPV2 in early phagocytosis and its fundamental importance in innate immunity.

Actin dynamics and Rho GTPases regulate the size and formation of parasitophorous vacuoles containing Coxiella burnetii

Q fever is a disease caused by Coxiella burnetii. In the host cell, this pathogen generates a large parasitophorous vacuole (PV) with lysosomal characteristics. Here we show that F-actin not only is recruited to but also is involved in the formation of the typical PV. Treatment of infected cells with F-actin-depolymerizing agents alters PV development. The small PVs formed in latrunculin B-treated cells were loaded with transferrin and Lysotracker and labeled with an antibody against cathepsin D, suggesting that latrunculin B did not affect vacuole cargo and its lysosomal characteristics. Nevertheless, the vacuoles were unable to fuse with latex bead phagosomes. It is known that actin dynamics are regulated by the Rho family GTPases. To assess the role of these GTPases in PV formation, infected cells were transfected with pEGFP expressing wild-type and mutant Rac1, Cdc42, and RhoA proteins. Rac1 did not show significant PV association. In contrast, PVs were decorated by both the wild types and constitutively active mutants of Cdc42 and RhoA. This association was inhibited by treatment of infected cells with chloramphenicol, suggesting a role for bacterial protein synthesis in the recruitment of these proteins. Interestingly, a decrease in vacuole size was observed in cells expressing dominant-negative RhoA; however, these small vacuoles accumulated transferrin, Lysotracker, and DQ-BSA. In summary, these results suggest that actin, likely modulated by the GTPases RhoA and Cdc42 and by bacterial proteins, is involved in the formation of the typical PV.

Sphingosine-1-phosphate receptors stimulate macrophage plasma-membrane actin assembly via ADP release, ATP synthesis and P2X7R activation

Eukaryotic plasma membranes assemble actin filaments within seconds of activation of many receptors, especially during chemotaxis. Here, serum or sphingosine-1-phosphate stimulation of J774 and RAW macrophages released ADP within seconds into the extracellular medium, along with an adenylate kinase activity that converted ADP to ATP. ATP then activated the P2X7 receptor (P2X7R) that was necessary for a peak of plasma-membrane actin assembly within 5 to 10 seconds in P2X7R-expressing J774, RAW and primary macrophages. Neither actin assembly nor characteristic P2X7R channel activity was seen in response to ATP in P2X7R-knockout macrophages, as detected by patch-clamp analysis. Since P2X7R has been shown previously to form a macromolecular complex with actin we propose that it is involved in the membrane assembly of actin. Our data reveal a surprisingly rapid and complex relay of signaling and externalization events that precede and control actin assembly induced by sphingosine-1-phosphate. The overall model we present is strongly supported by the data presented in the accompanying paper that focuses on latex bead phagosomes.

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.

Modelling phagosomal lipid networks that regulate actin assembly

Background

When purified phagosomes are incubated in the presence of actin under appropriate conditions, microfilaments start growing from the membrane in a process that is affected by ATP and the lipid composition of the membrane. Isolated phagosomes are metabolically active organelles that contain enzymes and metabolites necessary for lipid interconversion. Hence, addition of ATP, lipids, and actin to the system alter the steady-state composition of the phagosomal membrane at the same time that the actin nucleation is initiated. Our aim was to model all these processes in parallel.

Results

We compiled detailed experimental data on the effects of different lipids and ATP on actin nucleation and we investigated experimentally lipid interconversion and ATP metabolism in phagosomes by using suitable radioactive compounds.

In a first step, a complex lipid network interconnected by chemical reactions catalyzed by known enzymes was modelled in COPASI (Complex Pathway Simulator). However, several lines of experimental evidence indicated that only the phosphatidylinositol branch of the network was active, an observation that dramatically reduced the number of parameters in the model. The results also indicated that a lipid network-independent ATP-consuming activity should be included in the model. When this activity was introduced, the set of differential equations satisfactorily reproduced the experimental data. On the other hand, a molecular mechanism connecting membrane lipids, ATP, and the actin nucleation process is still missing. We therefore adopted a phenomenological (black-box) approach to represent the empirical observations. We proposed that lipids and ATP influence the dynamic interconversion between active and inactive actin nucleation sites. With this simple model, all the experimental data were satisfactorily fitted with a single positive parameter per lipid and ATP.

Conclusion

By establishing an active 'dialogue' between an initial complex model and experimental observations, we could narrow the set of differential equations and parameters required to characterize the time-dependent changes of metabolites influencing actin nucleation on phagosomes. For this, the global model was dissected into three sub-models: ATP consumption, lipid interconversion, and nucleation of actin on phagosomal membranes. This scheme allowed us to describe this complex system with a relatively small set of differential equations and kinetic parameters that satisfactorily reproduced the experimental data.

Actin and phosphoinositide recruitment to fully formed Candida albicans phagosomes in mouse macrophages

Candida albicans is a dimorphic yeast that enters macrophages (Mphi) via the beta-glucan receptor dectin-1. Phagocytosis of C. albicans is characterized by actin polymerization, Syk kinase activation and rapid acquisition of phagolysosomal markers. In mice, C. albicans are able to resist the harsh environment of the phagosome and form pseudohyphae inside the phagolysosomal compartment, eventually extending from the Mphi. In this study, we investigated these unique C. albicans phagosomes and found that actin localized dynamically around the phagosomes, before disintegrating. Membrane phosphoinositides, PI(4,5)P(2), PI(3,4,5)P(3), PI(3,4)P(2), and PI(3)P also localized to the phagosomes. Localization was not related to actin polymerization, and inhibitor studies showed that polymerization of actin on the C. albicans phagosome was independent of PI3K. The ability of mature C. albicans phagosomes to stimulate actin polymerization could facilitate the escape of the growing yeast from the Mphi.

The protein kinase A-anchoring protein moesin is bound to pigment granules in melanophores

Major signaling cascades have been shown to play a role in the regulation of intracellular transport of organelles. In Xenopus melanophores, aggregation and dispersion of pigment granules are regulated by the second messenger cyclic AMP through the protein kinase A (PKA) signaling pathway. PKA is bound to pigment granules where it forms complexes with molecular motors involved in pigment transport. Association of PKA with pigment granules occurs through binding to A-kinase-anchoring proteins (AKAPs), whose identity remains largely unknown. In this study, we used mass spectrometry to examine an 80 kDa AKAP detected in preparations of purified pigment granules. We found that tryptic digests of granule protein fractions enriched in the 80 kDa AKAP contained peptides that corresponded to the actin-binding protein moesin, which has been shown to function as an AKAP in mammalian cells. We also found that recombinant Xenopus moesin interacted with PKA in vitro, copurified with pigment granules and bound to pigment granules in cells. Overexpression in melanophores of a mutant moesin lacking conserved PKA-binding domain did not affect aggregation of pigment granules but partially inhibited their dispersion. We conclude that Xenopus moesin is an AKAP whose PKA-scaffolding activity plays a role in the regulation of pigment dispersion in Xenopus melanophores.

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.

Mammalian septins are required for phagosome formation

Septins are members of a highly conserved family of filamentous proteins that are required in many organisms for the completion of cytokinesis. In addition, septins have been implicated in a number of important cellular processes and have been suggested to have roles in regulating membrane traffic. Given the proposed role of septins in cell membrane dynamics, we investigated the function of septins during FcgammaR-mediated phagocytosis. We show that several septins are expressed in RAW264.7 and J774 mouse macrophage cell lines and that SEPT2 and SEPT11 are colocalized with submembranous actin-rich structures during the early stages of FcgammaR-mediated phagocytosis. In addition, SEPT2 accumulation is seen in primary human neutrophils and in nonprofessional phagocytes. The time course of septin accumulation mirrors actin accumulation and is inhibited by latrunculin and genistein, but not other inhibitors of phagocytosis. Inhibition of septin function by transient expression of the BD3 domain of BORG3, known to cause septin aggregation, or depletion of SEPT2 or SEPT11 by RNAi, significantly inhibited FcgammaR-mediated phagocytosis of IgG-coated latex beads. Interestingly, this occurred without affecting the accumulation of actin or the actin-associated protein coronin-1. These observations show that, although not necessary for actin recruitment, septins are required for efficient FcgammaR-mediated phagocytosis.

Four distinct pathways of hemoglobin uptake in the malaria parasite Plasmodium falciparum

During the bloodstage of malaria infection, the parasite internalizes and degrades massive amounts of hemoglobin from the host red blood cell. Using serial thin-section electron microscopy and three-dimensional reconstruction, we demonstrate four independent, but partially overlapping, hemoglobin-uptake processes distinguishable temporally, morphologically, and pharmacologically. Early ring-stage parasites undergo a profound morphological transformation in which they fold, like a cup, onto themselves and in so doing take a large first gulp of host cell cytoplasm. This event, which we term the "Big Gulp," appears to be independent of actin polymerization and marks the first step in biogenesis of the parasite's lysosomal compartment-the food vacuole. A second, previously identified uptake process, uses the cytostome, a well characterized and morphologically distinct structure at the surface of the parasite. This process is more akin to classical endocytosis, giving rise to small (<0.004 fl) vesicles that are marked by the early endosomal regulatory protein Rab5a. A third process, also arising from cytostomes, creates long thin tubes previously termed cytostomal tubes in an actin-dependent manner. The fourth pathway, which we term phagotrophy, is similar to the Big Gulp in that it more closely resembles phagocytosis, except that phagotrophy does not require actin polymerization. Each of these four processes has aspects that are unique to Plasmodium, thus opening avenues to antimalarial therapy.

Quantitative analysis of the binding of ezrin to large unilamellar vesicles containing phosphatidylinositol 4,5 bisphosphate

The plasma membrane-cytoskeleton interface is a dynamic structure participating in a variety of cellular events. Among the proteins involved in the direct linkage between the cytoskeleton and the plasma membrane is the ezrin/radixin/moesin (ERM) family. The FERM (4.1 ezrin/radixin/moesin) domain in their N-terminus contains a phosphatidylinositol 4,5 bisphosphate (PIP(2)) (membrane) binding site whereas their C-terminus binds actin. In this work, our aim was to quantify the interaction of ezrin with large unilamellar vesicles (LUVs) containing PIP(2). For this purpose, we produced human recombinant ezrin bearing a cysteine residue at its C-terminus for subsequent labeling with Alexa488 maleimide. The functionality of labeled ezrin was checked by comparison with that of wild-type ezrin. The affinity constant between ezrin and LUVs was determined by cosedimentation assays and fluorescence correlation spectroscopy. The affinity was found to be approximately 5 microM for PIP(2)-LUVs and 20- to 70-fold lower for phosphatidylserine-LUVs. These results demonstrate, as well, that the interaction between ezrin and PIP(2)-LUVs is not cooperative. Finally, we found that ezrin FERM domain (area of approximately 30 nm(2)) binding to a single PIP(2) can block access to neighboring PIP(2) molecules and thus contributes to lower the accessible PIP(2) concentration. In addition, no evidence exists for a clustering of PIP(2) induced by ezrin addition.

Phagosome proteomes open the way to a better understanding of phagosome function

Phagocytic cells take up microbes and other particles into membrane-bounded organelles called phagosomes. Studies on the protein and lipid composition of model phagosomes containing latex beads are the first step in a systems-biology approach to understanding how these organelles function.

Adipokines and the blood-brain barrier

Just as the blood-brain barrier (BBB) is not a static barrier, the adipocytes are not inert storage depots. Adipokines are peptides or polypeptides produced by white adipose tissue; they play important roles in normal physiology as well as in the metabolic syndrome. Adipokines secreted into the circulation can interact with the BBB and exert potent CNS effects. The specific transport systems for two important adipokines, leptin and tumor necrosis factor alpha, have been characterized during the past decade. By contrast, transforming growth factor beta-1 and adiponectin do not show specific permeation across the BBB, but modulate endothelial functions. Still others, like interleukin-6, may reach the brain but are rapidly degraded. This review summarizes current knowledge and recent findings of the rapidly growing family of adipokines and their interactions with the BBB.

cAMP synthesis and degradation by phagosomes regulate actin assembly and fusion events: consequences for mycobacteria

We showed recently that actin assembly by phagosomal membranes facilitates fusion with late endocytic organelles in macrophages. Moreover, lipids that induced phagosomal actin also stimulated this fusion process. In macrophages infected with pathogenic mycobacteria actin-stimulatory lipids led to an increase in pathogen destruction, whereas inhibitors facilitated their growth. A model was proposed whereby phagosomal membrane actin assembly provides tracks for lysosomes to move towards phagosomes, thereby facilitating fusion. Here, we investigated how cAMP affected phagosomal actin assembly in vitro, and phagosomal actin, acidification and late fusion events in J774 macrophages. Latex bead phagosomes are shown to possess adenylyl cyclase activity, which synthesizes cAMP, and phosphodiesterase activity, which degrades cAMP. The system is regulated by protein kinase A (PKA). Increasing cAMP levels inhibited, whereas decreasing cAMP levels stimulated, actin assembly in vitro and within cells. Increasing cAMP levels also inhibited phagosome-lysosome fusion and acidification in cells, whereas reducing cAMP had the opposite effect. High cAMP levels induced an increase in intraphagosomal growth in macrophages of both the non-pathogenic Mycobacterium smegmatis and the pathogenic Mycobacterium tuberculosis, whereas low cAMP levels or inhibition of PKA correlated with increased bacterial destruction. We argue that the phagosome cAMP-PKA system behaves as a molecular switch that regulates phagosome actin and maturation in macrophages.

Phosphatidylinositol transfer proteins and cellular nanoreactors for lipid signaling

Membrane lipids function as structural molecules, reservoirs for second messengers, membrane platforms that scaffold protein assembly and regulators of enzymes and ion channels. Such diverse lipid functions contribute substantially to cellular mechanisms for fine-tuning membrane-signaling events. Meaningful coordination of these events requires exquisite spatial and temporal control of lipid metabolism and organization, and reliable mechanisms for specifically coupling these parameters to dedicated physiological processes. Recent studies suggest such integration is linked to the action of phosphatidylinositol transfer proteins that operate at the interface of the metabolism, trafficking and organization of specific lipids.

Dynamic life and death interactions between Mycobacterium smegmatis and J774 macrophages

After internalization into macrophages non-pathogenic mycobacteria are killed within phagosomes. Pathogenic mycobacteria can block phagosome maturation and grow inside phagosomes but under some conditions can also be killed by macrophages. Killing mechanisms are poorly understood, although phago-lysosome fusion and nitric oxide (NO) production are implicated. We initiated a systematic analysis addressing how macrophages kill 'non-pathogenic'Mycobacterium smegmatis. This system was dynamic, involving periods of initial killing, then bacterial multiplication, followed by two additional killing stages. NO synthesis represented the earliest killing factor but its synthesis stopped during the first killing period. Phagosome actin assembly and fusion with late endocytic organelles coincided with the first and last killing phase, while recycling of phagosome content and membrane coincided with bacterial growth. Phagosome acidification and acquisition of the vacuolar (V) ATPase followed a different pattern coincident with later killing phases. Moreover, V-ATPase localized to vesicles distinct from classical late endosomes and lysosomes. Map kinase p38 is a crucial regulator of all processes investigated, except NO synthesis, that facilitated the host for some functions while being usurped by live bacteria for others. A mathematical model argues that periodic high and low cellular killing activity is more effective than is a continuous process.

Differential requirements for actin polymerization, calmodulin, and Ca2+ define distinct stages of lysosome/phagosome targeting

Fusion of phagosomes with late endocytic organelles is essential for cellular digestion of microbial pathogens, senescent cells, apoptotic bodies, and retinal outer segment fragments. To further elucidate the biochemistry of the targeting process, we developed a scintillation proximity assay to study the stepwise association of lysosomes and phagosomes in vitro. Incubation of tritium-labeled lysosomes with phagosomes containing scintillant latex beads led to light emission in a reaction requiring cytosol, ATP, and low Ca(2+) concentrations. The nascent complex was sensitive to disruption by alkaline carbonate, indicating that the organelles had "docked" but not fused. Through inhibitor studies and fluorescence microscopy we show that docking is preceded by a tethering step that requires actin polymerization and calmodulin. In the docked state ongoing actin polymerization and calmodulin are no longer necessary. The tethering/docking activity was purified to near homogeneity from rat liver cytosol. Major proteins in the active fractions included actin, calmodulin and IQGAP2. IQGAPs are known to bind calmodulin and cross-link F-actin, suggesting a key coordinating role during lysosome/phagosome attachment. The current results support the conclusion that lysosome/phagosome interactions proceed through distinct stages and provide a useful new approach for further experimental dissection.

Mycobacterial manipulation of the host cell

Phagosome biogenesis, the process by which macrophages neutralize ingested pathogens and initiate antigen presentation, has entered the field of cellular mycobacteriology research largely owing to the discovery 30 years ago that phagosomes harboring mycobacteria are refractory to fusion with lysosomes. In the past decade, the use of molecular genetics and biology in different model systems to study phagosome biogenesis have made significant advances in understanding subtle mechanisms by which mycobacteria inhibit the maturation of its phagosome. Thus, we are beginning to appreciate the extent to which these pathogens are able to interfere with innate immune responses and manipulate defense mechanisms to enhance their survival within the human host cell. Here, we summarize current knowledge about phagosome maturation arrest in infected macrophages and the subsequent attenuation of the macrophage-initiated adaptive anti-mycobacterial immune defenses.

PHAGOCYTOSIS: At the Crossroads of Innate and Adaptive Immunity

Phagocytosis, the process by which cells engulf large particles, requires a substantial contribution of membranes. Recent studies have revealed that intracellular compartments, including endocytic organelles and the endoplasmic reticulum (ER), can engage in fusion events with the plasma membrane at the sites of nascent phagosomes. The finding that ER proteins are delivered to phagosomes, where degraded peptides are loaded onto major histocompatibility complex (MHC) class II molecules, has significantly enhanced our understanding of the immune functions associated with these organelles. Although it is well known that pathogens are killed in phagosomes, the contribution of ER proteins to phagosomes has provided a novel pathway for the loading of exogenous peptides onto MHC class I molecules, a process known as cross-presentation. Thus, phagocytosis has evolved from a nutritional function in unicellular organisms to play key roles in both innate and adaptive immunity in vertebrates.

Separate functions of gelsolin mediate sequential steps of collagen phagocytosis

Collagen phagocytosis is a critical mediator of extracellular matrix remodeling. Whereas the binding step of collagen phagocytosis is facilitated by Ca2+-dependent, gelsolin-mediated severing of actin filaments, the regulation of the collagen internalization step is not defined. We determined here whether phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] regulation of gelsolin is required for collagen internalization. In gelsolin null fibroblasts transfected with gelsolin severing mutants, actin severing and collagen binding were strongly impaired but internalization and actin monomer addition at collagen bead sites were much less affected. PI(4,5)P2 accumulated around collagen during internalization and was associated with gelsolin. Cell-permeable peptides mimicking the PI(4,5)P2 binding site of gelsolin blocked actin monomer addition, the association of gelsolin with actin at phagosomes, and collagen internalization but did not affect collagen binding. Collagen beads induced recruitment of type 1 gamma phosphatidylinositol phosphate kinase (PIPK1gamma661) to internalization sites. Dominant negative constructs and RNA interference demonstrated a requirement for catalytically active PIPK1gamma661 for collagen internalization. We conclude that separate functions of gelsolin mediate sequential stages of collagen phagocytosis: Ca2+-dependent actin severing facilitates collagen binding, whereas PI(4,5)P2-dependent regulation of gelsolin promotes the actin assembly required for internalization of collagen fibrils.

Up-regulation of phospholipase Cgamma1 and phospholipase D during the differentiation of human monocytes to dendritic cells

Phospholipase C (PLC)gamma and phospholipase D (PLD) play pivotal roles in the signal transduction required for various cellular responses, including cell proliferation and differentiation. Dendritic cells (DCs), which are professional antigen-presenting cells, can be generated from human monocytes by stimulating the cells with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4). We investigated whether PLCgamma and PLD expression levels can be changed during the differentiation of the human monocytes into DCs. The enzymatic activity and protein level of PLC gamma1 were significantly increased in the human monocyte-derived DCs by GM-CSF/IL-4, but the protein levels of PLC gamma2 were unaltered. Moreover, the enzymatic activity and protein level of PLD1b and PLD2 were up-regulated during the differentiation of human monocytes to DCs, but those of PLD1a were not changed. A higher phagocytic activity of DCs was found to be correlated with the up-regulations of PLCgamma1 and PLD, and the phagocytic activity of DCs was inhibited by a PLC-specific inhibitor (U73122) and by a phosphatidic acid acceptor (n-butanol), but to be increased by phosphatidic acid. Thus, suggesting that PLC and PLD participate in the process. This study suggests that the up-regulations of PLCgamma1 and PLD are accompanied by the differentiation of monocytes into DCs, which results in increased phagocytic activity.

Signaling and membrane dynamics during phagocytosis: many roads lead to the phagos(R)ome

Phagocytosis is the mechanism used by specialized cells such as macrophages, dendritic cells and neutrophils to internalize, degrade and eventually present peptides derived from particulate antigens. This process relies on profound rearrangements of the actin cytoskeleton and the plasma membrane to engulf particles. Recent work has highlighted the early recruitment of internal membranes derived from endocytic compartments and from the endoplasmic reticulum to allow plasma membrane extension at the onset of phagocytosis. This ensures that the phagosome is rapidly provided with the machinery appropriate for later phagocytic functions, including particle degradation and antigen presentation.

Repeated cycles of rapid actin assembly and disassembly on epithelial cell phagosomes

We have found that early in infection of the intracellular pathogen Listeria monocytogenes in Madin-Darby canine kidney epithelial cells expressing actin conjugated to green fluorescent protein, F-actin rapidly assembles (approximately 25 s) and disassembles (approximately 30 s) around the bacteria, a phenomenon we call flashing. L. monocytogenes strains unable to perform actin-based motility or unable to escape the phagosome were capable of flashing, suggesting that the actin assembly occurs on the phagosome membrane. Cycles of actin assembly and disassembly could occur repeatedly on the same phagosome. Indirect immunofluorescence showed that most bacteria were fully internalized when flashing occurred, suggesting that actin flashing does not represent phagocytosis. Escherichia coli expressing invA, a gene product from Yersinia pseudotuberculosis that mediates cellular invasion, also induced flashing. Furthermore, polystyrene beads coated with E-cadherin or transferrin also induced flashing after internalization. This suggests that flashing occurs downstream of several distinct molecular entry mechanisms and may be a general consequence of internalization of large objects by epithelial cells.

Subversion of phosphoinositide metabolism by intracellular bacterial pathogens

Phosphoinositides are short-lived lipids, whose production at specific membrane locations in the cell enables the tightly controlled recruitment or activation of diverse cellular effectors involved in processes such as cell motility or phagocytosis. Bacterial pathogens have evolved molecular mechanisms to subvert phosphoinositide metabolism in host cells, promoting (or blocking) their internalization into target tissues, and/or modifying the maturation fate of their proliferating compartments within the intracellular environment.

A rapid method for assessing the distribution of gold labeling on thin sections

Particulate gold labeling on ultrathin sections is in widespread use for antigen localization at the EM level. To extend the usefulness of gold labeling technology, we are evaluating different methods for sampling and estimating quantities of gold labeling. Here we present a simple, rapid, and unbiased method for assessing the relative pool sizes of immunogold labeling distributed over different cell compartments. The method uses a sampling approach developed for stereology in which a regular array of microscopic fields or linear scans is positioned randomly on labeled sections. From these readouts, gold particles are counted and assigned to identifiable cell structures to construct a gold labeling frequency distribution of those labeled compartments. Here we use ultrathin cryosections labeled for a range of different proteins and for a signaling lipid. We show by scanning labeled sections at the electron microscope that counting 100-200 particles on each of two grids is sufficient to obtain a reproducible and rapid assessment of the pattern of labeling proportions over 10-16 compartments. If more precise estimates of labeling proportions over individual compartments are required (e.g., to achieve coefficients of error of 10-20%), then 100-200 particles need to be counted over each compartment of interest.