p110-related PI 3-kinases regulate phagosome-phagosome fusion and phagosomal pH through a PKB/Akt dependent pathway in Dictyostelium

Zooming in on common immune evasion mechanisms of pathogens in phagolysosomes: potential broad-spectrum therapeutic targets against infectious diseases

The intracellular viral, bacterial, or parasitic pathogens evade the host immune challenges to propagate and cause fatal diseases. The microbes overpower host immunity at various levels including during entry into host cells, phagosome formation, phagosome maturation, phagosome-lysosome fusion forming phagolysosomes, acidification of phagolysosomes, and at times after escape into the cytosol. Phagolysosome is the final organelle in the phagocyte with sophisticated mechanisms to degrade the pathogens. The immune evasion strategies by the pathogens include the arrest of host cell apoptosis, decrease in reactive oxygen species, the elevation of Th2 anti-inflammatory response, avoidance of autophagy and antigen cross-presentation pathways, and escape from phagolysosomal killing. Since the phagolysosome organelle in relation to infection/cure is seldom discussed in the literature, we summarize here the common host as well as pathogen targets manipulated or utilized by the pathogens established in phagosomes and phagolysosomes, to hijack the host immune system for their benefit. These common molecules or pathways can be broad-spectrum therapeutic targets for drug development for intervention against infectious diseases caused by different intracellular pathogens.

Endocytosis of collagen by hepatic stellate cells regulates extracellular matrix dynamics

Hepatic stellate cells (HSCs) generate matrix, which in turn may also regulate HSCs function during liver fibrosis. We hypothesized that HSCs may endocytose matrix proteins to sense and respond to changes in microenvironment. Primary human HSCs, LX2, or mouse embryonic fibroblasts (MEFs) [wild-type; c-abl(-/-); or Yes, Src, and Fyn knockout mice (YSF(-/-))] were incubated with fluorescent-labeled collagen or gelatin. Fluorescence-activated cell sorting analysis and confocal microscopy were used for measuring cellular internalization of matrix proteins. Targeted PCR array and quantitative real-time PCR were used to evaluate gene expression changes. HSCs and LX2 cells endocytose collagens in a concentration- and time-dependent manner. Endocytosed collagen colocalized with Dextran 10K, a marker of macropinocytosis, and 5-ethylisopropyl amiloride, an inhibitor of macropinocytosis, reduced collagen internalization by 46%. Cytochalasin D and ML7 blocked collagen internalization by 47% and 45%, respectively, indicating that actin and myosin are critical for collagen endocytosis. Wortmannin and AKT inhibitor blocked collagen internalization by 70% and 89%, respectively, indicating that matrix macropinocytosis requires phosphoinositide-3-kinase (PI3K)/AKT signaling. Overexpression of dominant-negative dynamin-2 K44A blocked matrix internalization by 77%, indicating a role for dynamin-2 in matrix macropinocytosis. Whereas c-abl(-/-) MEF showed impaired matrix endocytosis, YSF(-/-) MEF surprisingly showed increased matrix endocytosis. It was also associated with complex gene regulations that related with matrix dynamics, including increased matrix metalloproteinase 9 (MMP-9) mRNA levels and zymographic activity. HSCs endocytose matrix proteins through macropinocytosis that requires a signaling network composed of PI3K/AKT, dynamin-2, and c-abl. Interaction with extracellular matrix regulates matrix dynamics through modulating multiple gene expressions including MMP-9.

Interactions between Mytilus galloprovincialis hemocytes and the bivalve pathogens Vibrio aestuarianus 01/032 and Vibrio splendidus LGP32

Marine bivalves can accumulate large numbers of bacteria, in particular Vibrio species, whose persistence in bivalve tissues largely depends on their sensitivity to the bactericidal activity of circulating hemocytes and hemolymph soluble factors. The interactions between vibrios and hemolymph have been investigated, in particular in bivalve species susceptible to infection by certain Vibrio spp. and strains. In this work, the effects of two bivalve pathogens, Vibrio splendidus LGP32 (V.s.) and Vibrio aestuarianus 01/032 (V.a.), isolated from oyster mortality outbreaks, on the hemocytes of Mytilus galloprovincialis were investigated. In vitro, V.s., but not V.a., induced a dramatic decrease in lysosomal membrane stability-LMS in the hemocytes; both vibrios induced a moderate lysozyme release, with V.s. > V.a.. The V.s.-induced decrease in LMS was mediated by activation of PI-3Kinase, as shown by use of different kinase inhibitors. TEM analysis showed rapid internalization of both vibrios; however, V.s. lead to cellular and lysosomal damage and was able to survive within the hemocytes, whereas significant killing of V.a. was observed. In vivo, in mussels challenged with either vibrio and sampled at 6, 24 and 96 h post-injection, transient decreases in hemocyte LMS and progressive increases in serum lysozyme activity were observed, with V.s. > V.a.. Moreover, whereas V.a. was efficiently cleared from hemolymph, V.s. showed significant growth, that was maximal at 24 h p.i. when lowest LMS values were recorded in the hemocytes. Both vibrios also induced significant decreases in LMS in the digestive gland, again with V.s. > V.a.. The results indicate distinct interactions between mussel hemocytes and the two vibrio strains tested. The effects of V.s. may be due to the capacity of this strain to interfere with the signaling pathways involved in hemocyte function, thus escaping the bactericidal activity of the host cell, as observed for certain mammalian pathogens. Although V.s. is considered not pathogenic to Mytilus, this vibrio strain can affect the lysosomal function at the cellular and tissue level, thus leading to stressful conditions.

FYVE-dependent endosomal targeting of an arrestin-related protein in amoeba

Background

Visual and β-arrestins are scaffolding proteins involved in the regulation of receptor-dependent intracellular signaling and their trafficking. The arrestin superfamilly includes several arrestin domain-containing proteins and the structurally related protein Vps26. In Dictyostelium discoideum, the arrestin-domain containing proteins form a family of six members, namely AdcA to -F. In contrast to canonical arrestins, Dictyostelium Adc proteins show a more complex architecture, as they possess, in addition to the arrestin core, other domains, such as C2, FYVE, LIM, MIT and SAM, which potentially mediate selective interactions with either lipids or proteins.

Methodology and Principal Findings

A detailed analysis of AdcA has been performed. AdcA extends on both sides of the arrestin core, in particular by a FYVE domain which mediates selective interactions with PI(3)P, as disclosed by intrinsic fluorescence measurements and lipid overlay assays. Localization studies showed an enrichment of tagged- and endogenous AdcA on the rim of early macropinosomes and phagosomes. This vesicular distribution relies on a functional FYVE domain. Our data also show that the arrestin core binds the ADP-ribosylation factor ArfA, the unique amoebal Arf member, in its GDP-bound conformation.

Significance

This work describes one of the 6 arrestin domain-containing proteins of Dictyostelium, a novel and atypical member of the arrestin clan. It provides the basis for a better understanding of arrestin-related protein involvement in trafficking processes and for further studies on the expanding roles of arrestins in eukaryotes.

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

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

Dictyostelium as a model for human lysosomal and trafficking diseases

Dictyostelium cells are genetically haploid and therefore easily analyzed for mutant phenotypes. In the past, many tools and molecular markers have been developed for a quantitative and qualitative analysis of the endocytic pathway in these amoebae. This review outlines parallels and discrepancies between mutants in Dictyostelium, the corresponding mammalian cells and the symptoms of human patients affected by lysosomal and trafficking defects. Situations where knowledge from Dictyostelium may potentially help understand human disease and vice versa are also addressed.

Intracellular bacterial growth is controlled by a kinase network around PKB/AKT1

With the emergence of multidrug resistant (MDR) bacteria, it is imperative to develop new intervention strategies. Current antibiotics typically target pathogen rather than host-specific biochemical pathways. Here we have developed kinase inhibitors that prevent intracellular growth of unrelated pathogens such as Salmonella typhimurium and Mycobacterium tuberculosis. An RNA interference screen of the human kinome using automated microscopy revealed several host kinases capable of inhibiting intracellular growth of S. typhimurium. The kinases identified clustered in one network around AKT1 (also known as PKB). Inhibitors of AKT1 prevent intracellular growth of various bacteria including MDR-M. tuberculosis. AKT1 is activated by the S. typhimurium effector SopB, which promotes intracellular survival by controlling actin dynamics through PAK4, and phagosome-lysosome fusion through the AS160 (also known as TBC1D4)-RAB14 pathway. AKT1 inhibitors counteract the bacterial manipulation of host signalling processes, thus controlling intracellular growth of bacteria. By using a reciprocal chemical genetics approach, we identified kinase inhibitors with antibiotic properties and their host targets, and we determined host signalling networks that are activated by intracellular bacteria for survival.

Phagocytosis and host-pathogen interactions in Dictyostelium with a look at macrophages

Research into phagocytosis and host-pathogen interactions in the lower eukaryote Dictyostelium discoideum has flourished in recent years. This chapter presents a glimpse of where this research stands, with emphasis on the cell biology of the phagocytic process and on the wealth of molecular genetic data that have been gathered. The basic mechanistic machinery and most of the underlying genes appear to be evolutionarily conserved, reflecting the fact that phagocytosis arose as an efficient way to ingest food in single protozoan cells devoid of a rigid cell wall. In spite of some differences, the signal transduction pathways regulating phagosome biogenesis are also emerging as ultimately similar between Dictyostelium and macrophages. Both cell types are hosts for many pathogenic invasive bacteria, which exploit phagocytosis to grow intracellularly. We present an overwiew, based on the analysis of mutants, on how Dictyostelium contributes as a genetic model system to decipher the complexity of host-pathogen interactions.

G protein-independent Ras/PI3K/F-actin circuit regulates basic cell motility

Phosphoinositide 3-kinase (PI3K)gamma and Dictyostelium PI3K are activated via G protein-coupled receptors through binding to the Gbetagamma subunit and Ras. However, the mechanistic role(s) of Gbetagamma and Ras in PI3K activation remains elusive. Furthermore, the dynamics and function of PI3K activation in the absence of extracellular stimuli have not been fully investigated. We report that gbeta null cells display PI3K and Ras activation, as well as the reciprocal localization of PI3K and PTEN, which lead to local accumulation of PI(3,4,5)P(3). Simultaneous imaging analysis reveals that in the absence of extracellular stimuli, autonomous PI3K and Ras activation occur, concurrently, at the same sites where F-actin projection emerges. The loss of PI3K binding to Ras-guanosine triphosphate abolishes this PI3K activation, whereas prevention of PI3K activity suppresses autonomous Ras activation, suggesting that PI3K and Ras form a positive feedback circuit. This circuit is associated with both random cell migration and cytokinesis and may have initially evolved to control stochastic changes in the cytoskeleton.

Attenuation of phospholipid signaling provides a novel mechanism for the action of valproic acid

Valproic acid (VPA) is used to treat epilepsy and bipolar disorder and to prevent migraine. It is also undergoing trials for cancer therapy. However, the biochemical and molecular biological actions of VPA are poorly understood. Using the social amoeba Dictyostelium discoideum, we show that an acute effect of VPA is the inhibition of chemotactic cell movement, a process partially dependent upon phospholipid signaling. Analysis of this process shows that VPA attenuates the signal-induced translocation of PH(Crac)-green fluorescent protein from cytosol to membrane, suggesting the inhibition of phosphatidylinositol-(3,4,5)-trisphosphate (PIP(3)) production. Direct labeling of lipids in vivo also shows a reduction in PIP and PIP(2) phosphorylation following VPA treatment. We further show that VPA acutely reduces endocytosis and exocytosis-processes previously shown to be dependent upon PIP(3) production. These results suggest that in Dictyostelium, VPA rapidly attenuates phospholipid signaling to reduce endocytic trafficking. To examine this effect in a mammalian model, we also tested depolarization-dependent neurotransmitter release in rat nerve terminals, and we show that this process is also suppressed upon application of VPA and an inhibitor of phosphatidylinositol 3-kinase. Although a more comprehensive analysis of the effect of VPA on lipid signaling will be necessary in mammalian systems, these results suggest that VPA may function to reduce phospholipid signaling processes and thus may provide a novel therapeutic effect for this drug.

Phagosome extrusion and host-cell survival after Cryptococcus neoformans phagocytosis by macrophages

Cryptococcus neoformans (Cn) is an encapsulated yeast that is a facultative intracellular pathogen and a frequent cause of human disease. The interaction of Cn with alveolar macrophages is critical for containing the infection , but Cn can also replicate intracellularly and lyse macrophages . Cn has a unique intracellular pathogenic strategy that involves cytoplasmic accumulation of polysaccharide-containing vesicles and intracellular replication leading to the formation of spacious phagosomes in which multiple cryptococcal cells are present . The Cn intracellular pathogenic strategy in macrophages and amoebas is similar, leading to the proposal that it originated as a mechanism for survival against phagocytic predators in the environment . Here, we report that under certain conditions, including phagosomal maturation, possible actin depolymerization, and homotypic phagosome fusion, Cn can exit the macrophage host through an extrusion of the phagosome, while both the released pathogen and host remain alive and able to propagate. The phenomenon of "phagosomal extrusion" indicates the existence of a previously unrecognized mechanism whereby a fungal pathogen can escape the intracellular confines of mammalian macrophages to continue propagation and, possibly, dissemination.

Thioredoxin reductase is required for growth and regulates entry into culmination of Dictyostelium discoideum

The thioredoxin system, consisting of thioredoxin, thioredoxin reductase and NADPH, has been well established to be critical for the redox regulation of protein function and signalling. To investigate the role of thioredoxin reductase (Trr) in Dictyostelium discoideum, we generated mutant cells that underexpress or overexpress Trr. Trr-underexpressing cells exhibited severe defects in axenic growth and development. Trr-overexpressing (TrrOE) cells formed very tiny plaques on a bacterial lawn and had a lower rate of bacterial uptake. When developed in the dark, TrrOE cells exhibited a slugger phenotype, defined by a prolonged migrating slug stage. Like other slugger mutants, they were hypersensitive to ammonia, which has been known to inhibit culmination by raising the pH of intracellular acidic compartments. Interestingly, TrrOE cells showed defective acidification of intracellular compartments and decreased activity of vacuolar H+-ATPase which functions in the acidification of intracellular compartments. Moreover, biochemical studies revealed that the thioredoxin system can directly reduce the catalytic subunit of vacuolar H+-ATPase whose activity is regulated by reversible disulphide bond formation. Taken together, these results suggest that Dictyostelium Trr may be essential for growth and play a role in regulation of phagocytosis and culmination, possibly through the modulation of vacuolar H+-ATPase activity.

Disruption of inositol biosynthesis through targeted mutagenesis in Dictyostelium discoideum: generation and characterization of inositol-auxotrophic mutants

myo-Inositol and its downstream metabolites participate in diverse physiological processes. Nevertheless, considering their variety, it is likely that additional roles are yet to be uncovered. Biosynthesis of myo-inositol takes place via an evolutionarily conserved metabolic pathway and is strictly dependent on inositol-3-phosphate synthase (EC 5.5.1.4). Genetic manipulation of this enzyme will disrupt the cellular inositol supply. Two methods, based on gene deletion and antisense strategy, were used to generate mutants of the cellular slime mould Dictyostelium discoideum. These mutants are inositol-auxotrophic and show phenotypic changes under inositol starvation. One remarkable attribute is their inability to live by phagocytosis of bacteria, which is the exclusive nutrient source in their natural environment. Cultivated on fluid medium, the mutants lose their viability when deprived of inositol for longer than 24 h. Here, we report a study of the alterations in the first 24 h in cellular inositol, inositol phosphate and phosphoinositide concentrations, whereby a rapidly accumulating phosphorylated compound was detected. After its identification as 2,3-BPG (2,3-bisphosphoglycerate), evidence could be found that the internal disturbances of inositol homoeostasis trigger the accumulation. In a first attempt to characterize this as a physiologically relevant response, the efficient in vitro inhibition of a D. discoideum inositol-polyphosphate 5-phosphatase (EC 3.1.3.56) by 2,3-BPG is presented.

Legionella pneumophila exploits PI(4)P to anchor secreted effector proteins to the replicative vacuole

The causative agent of Legionnaires' disease, Legionella pneumophila, employs the intracellular multiplication (Icm)/defective organelle trafficking (Dot) type IV secretion system (T4SS) to upregulate phagocytosis and to establish a replicative vacuole in amoebae and macrophages. Legionella-containing vacuoles (LCVs) do not fuse with endosomes but recruit early secretory vesicles. Here we analyze the role of host cell phosphoinositide (PI) metabolism during uptake and intracellular replication of L. pneumophila. Genetic and pharmacological evidence suggests that class I phosphatidylinositol(3) kinases (PI3Ks) are dispensable for phagocytosis of wild-type L. pneumophila but inhibit intracellular replication of the bacteria and participate in the modulation of the LCV. Uptake and degradation of an icmT mutant strain lacking a functional Icm/Dot transporter was promoted by PI3Ks. We identified Icm/Dot-secreted proteins which specifically bind to phosphatidylinositol(4) phosphate (PI(4)P) in vitro and preferentially localize to LCVs in the absence of functional PI3Ks. PI(4)P was found to be present on LCVs using as a probe either an antibody against PI(4)P or the PH domain of the PI(4)P-binding protein FAPP1 (phosphatidylinositol(4) phosphate adaptor protein-1). Moreover, the presence of PI(4)P on LCVs required a functional Icm/Dot T4SS. Our results indicate that L. pneumophila modulates host cell PI metabolism and exploits the Golgi lipid second messenger PI(4)P to anchor secreted effector proteins to the LCV.

Chronic activation of protein kinase Bβ/Akt2 leads to multinucleation and cell fusion in human epithelial kidney cells: events associated with tumorigenesis

Most cancers arise from the stepwise accumulation of genetic changes. There is also evidence for defects in the machinery and checkpoints for maintenance of normal diploid chromosome complements, resulting in genetic instability that helps fuel the accumulation of mutations that contribute to the development of cancer. The proto-oncogene protein kinase B (PKB/Akt), and its regulators including phosphatidylinositol 3' kinase and PTEN, has been shown to play critical roles in the regulation of multiple cellular functions such as transcription, cell survival, cell cycle progression, angiogenesis and cell motility--all of which are important to the malignant process. Here, we report the use of a membrane targeted PKBbeta, the activation of which is under the control of a 4-hydroxy-Tamoxifen-responsive estrogen-receptor (ER) ligand binding domain. Induction of PKBbeta-ER activity in human kidney epithelial cells (HEK293) resulted in changes in cellular growth, size, and in the appearance of aneuploid cells. Over time, in a PKBbeta-dependent manner, cells also underwent extensive multinucleation caused due to a combination of both endomitosis and cell fusion. These findings suggest that chronic activation of PKBbeta may contribute to genetic instability and autophagy, properties commonly found in tumor cells.

Cross-Talk between CD14 and Complement Receptor 3 Promotes Phagocytosis of Mycobacteria: Regulation by Phosphatidylinositol 3-Kinase and Cytohesin-1

The glycosylphosphatidyl anchored molecule CD14 to the monocyte membrane plays a prominent role in innate immunity, and the paradigms for CD14 selective signaling are beginning to be elucidated. In this study, transfected human monocytic cell line THP-1 and Chinese hamster ovary (CHO) fibroblastic cells were used to examine phagocytosis of Mycobacterium bovis bacillus Calmette-Guerin (BCG). Flow cytometry was combined with molecular and biochemical approaches to demonstrate a dual mechanism for BCG internalization involving either CD14 alone or a CD14-regulated complement receptor (CR)3-dependent pathway. Phagocytosis by CD14-positive THP-1 cells was attenuated by phosphatidylinositol-3 inhibitors LY294002 and wortmannin and experiments using transfected CHO cells showed substantial accumulation of phosphatidylinositol-3,4,5-trisphosphate at the BCG attachment site in CHO cells expressing CD14 and TLR2 suggesting that bacteria bind to CD14 and use TLR2 to initiate a PI3K signaling pathway. Additional experiments using blocking Abs showed that anti-TLR2 Abs inhibit phagocytosis of BCG by THP-1 cells. Furthermore, knockdown of cytohesin-1, a PI3K-regulated adaptor molecule for beta(2) integrin activation, specifically abrogated CD14-regulated CR3 ingestion of BCG consistent with the observation of physical association between CR3 and cytohesin-1 in cells stimulated with mycobacterial surface components. These findings reveal that mycobacteria promote their uptake through a process of "inside-out" signaling involving CD14, TLR2, PI3K, and cytohesin-1. This converts low avidity CR3 into an active receptor leading to increased bacterial internalization.

Differences in morphology of phagosomes and kinetics of acidification and degradation in phagosomes between the pathogenicEntamoeba histolytica and the non-pathogenicEntamoeba dispar

Phagocytosis plays an important role in the pathogenicity of the intestinal protozoan parasite Entamoeba histolytica. We compared the morphology of phagosomes and the kinetics of phagosome maturation using conventional light and electron microscopy and live imaging with video microscopy between the virulent E. histolytica and the closely-related, but non-virulent E. dispar species. Electron micrographs showed that axenically cultivated trophozoites of the two Entamoeba species revealed morphological differences in the number of bacteria contained in a single phagosome and the size of phagosomes. Video microscopy using pH-sensitive fluorescein isothiocynate-conjugated yeasts showed that phagosome acidification occurs within 2 min and persists for >12 h in both species. The acidity of phagosomes significantly differed between two species (4.58 +/- 0.36 or 5.83 +/- 0.38 in E. histolytica or E. dispar, respectively), which correlated well with the differences in the kinetics of degradation of promastigotes of GFP-expressing Leishmania amazonensis. The acidification of phagosomes was significantly inhibited by a myosin inhibitor, whereas it was only marginally inhibited by microtubules or actin inhibitors. A specific inhibitor of vacuolar ATPase, concanamycin A, interrupted both the acidification and degradation in phagosomes in both species, suggesting the ubiquitous role of vacuolar ATPase in the acidification and degradation in Entamoeba. In contrast, inhibitors against microtubules or cysteine proteases (CP) showed distinct effects on degradation in phagosomes between these two species. Although depolymerization of microtubules severely inhibited degradation in phagosomes of E. histolytica, it did not affect degradation in E. dispar. Similarly, the inhibition of CP significantly reduced degradation in phagosomes of E. histolytica, but not in E. dispar. These data suggest the presence of biochemical or functional differences in the involvement of microtubules and proteases in phagosome maturation and degradation between the two species.

Dynamics of endocytic traffic ofEntamoeba histolyticarevealed by confocal microscopy and flow cytometry

Entamoeba histolytica, the protozoan parasite of humans, manifests constitutive endocytosis to obtain nutrients and, when induced to express invasive behavior, as a means of ingesting and processing host cells and tissue debris. E. histolytica trophozoites were grown in liquid axenic medium that contained fluorescently labeled fluid-phase markers, so that the kinetics of uptake, the transit of loaded endosomes through the cytoplasm, and the time of release of the markers could be monitored by flow cytometry. Confocal microscopy of live trophozoites revealed uptake of fluid by avid macropinocytosis and the occurrence of fusion between young and older endosomes, as well as between pinosomes and phagosomes containing bacteria. Endosomes were rapidly acidified, then gradually neutralized; finally, indigestible material was released. Transit of endosomes containing fluid-phase markers required about 2 h. Uptake and release of fluid-phase markers were impaired by drugs that inhibited actin dynamics and actin-myosin interaction; uptake was also impaired by inhibition of PI 3-kinase. A striking feature of the trophozoites was the great heterogeneity of their endocytic behavior.

In vivo analysis of 3-phosphoinositide dynamics during Dictyostelium phagocytosis and chemotaxis

Phagocytosis and chemotaxis are receptor-mediated processes that require extensive rearrangements of the actin cytoskeleton, and are controlled by lipid second messengers such as phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P2]. We used a panel of pleckstrin homology (PH) domains with distinct binding specificities for PtdIns(3,4,5)P3 and PtdIns(3,4)P2 to study the spatiotemporal dynamics of these phosphoinositides in vivo. During phagocytosis and macropinocytosis PtdIns(3,4,5)P3 levels transiently increased at sites of engulfment, followed by a rapid PtdIns(3,4)P2 production round the phagosome/macropinosome upon its internalisation, suggesting that PtdIns(3,4,5)P3 is degraded to PtdIns(3,4)P2. PTEN null mutants, which are defective in phagocytosis, showed normal rates of PtdIns(3,4,5)P3 degradation, but unexpectedly an accelerated PtdIns(3,4)P2 degradation. During chemotaxis to cAMP only PtdIns(3,4,5)P3 was formed in the plasma membrane, and no PtdIns(3,4)P2 was detectable, showing that all PtdIns(3,4,5)P3 was degraded by PTEN to PtdIns(4,5)P2. Furthermore, we showed that different PtdIns(3,4,5)P3 binding PH domains gave distinct spatial and temporal readouts of the same underlying PtdIns(3,4,5)P3 signal, enabling distinct biological responses to one signal.

A coat of filamentous actin prevents clustering of late-endosomal vacuoles in vivo

The endocytic pathway depends on the actin cytoskeleton. Actin contributes to internalization at the plasma membrane and to subsequent trafficking steps like propulsion through the cytoplasm, fusion of phagosomes with early endosomes, and transport from early to late endosomes. In vitro studies with mammalian endosomes and yeast vacuoles implicate actin in membrane fusion. Here, we investigate the function of the actin coat that surrounds late endosomes in Dictyostelium. Latrunculin treatment leads to aggregation of these endosomes into grape-like clusters and completely blocks progression of endocytic marker. In addition, the cells round up and stop moving. Because this drug treatment perturbs all actin assemblies in the cell simultaneously, we used a novel targeting approach to specifically study the function of the cytoskeleton in one subcellular location. To this end, we constructed a hybrid protein targeting cofilin, an actin depolymerizing protein, to late endosomes. As a consequence, the endosomal compartments lost their actin coats and aggregated, but these cells remained morphologically normal, and the kinetics of endocytic marker trafficking were unaltered. Therefore, the actin coat prevents the clustering of endosomes, which could be one safeguard mechanism precluding their docking and fusion.

Shear flow-induced motility of Dictyostelium discoideum cells on solid substrate

Application of a mild hydrodynamic shear stress to Dicytostelium discoideum cells, unable to detach cells passively from the substrate, triggers a cellular response consisting of steady membrane peeling at the rear edge of the cell and periodic cell contact extensions at its front edge. Both processes require an active actin cytoskeleton. The cell movement induced by the hydrodynamic forces is very similar to amoeboid cell motion during chemotaxis, as for its kinematic parameters and for the involvement of phosphatidylinositol(3,4,5)-trisphosphate internal gradient to maintain cell polarity. Inhibition of phosphoinositide 3-kinases by LY294002 randomizes the orientation of cell movement with respect to the flow without modifying cell speed. Two independent signaling pathways are, therefore, induced in D. discoideum in response to external forces. The first increases the frequency of pseudopodium extension, whereas the second redirects the actin cytoskeleton polymerization machinery to the edge opposite to the stressed side of the cell.

Conserved features of endocytosis in Dictyostelium

Endocytosis in protozoa is often regarded as largely different from the pathways operating in mammalian cells. Experiments in the amoeba Dictyostelium, one of the genetically tractable single-celled organisms, have allowed us to manipulate the flow through endocytic compartments and to study the dynamic distribution of molecules by means of green fluorescent protein fusions. This review attempts to compile the molecular data available from Dictyostelium and assign them to specific steps of internalization by phagocytosis or macropinocytosis and to subsequent stages of the endocytic pathway. Parallels to phagocytes of the mammalian immune system are emphasized. The major distinctive feature between mammalian phagocytes and free-living cells is the need for osmoregulation. Therefore Dictyostelium cells possess a contractile vacuole that has occasionally obscured analysis of endocytosis but is now found to be entirely separate from endocytic organelles. In conclusion, the potential of Dictyostelium amoebas to provide a model system of mammalian phagocytes is ever increasing.

Fusion and fission events in the endocytic pathway of Dictyostelium

The endocytic pathway in Dictyostelium appears as a short circuit between endocytosis and exocytosis. Within the hour that elapses between internalization of nutrients and release of remnants, digestion by lysosomal enzymes occurs. Meanwhile, the maturing endosome undergoes a complex series of fusion and fission events, which change its character profoundly and which are far from being fully understood. This review attempts to order the dynamic events into a sequence of stages that is most consistent with present knowledge.

RabD, a Dictyostelium Rab14-related GTPase, regulates phagocytosis and homotypic phagosome and lysosome fusion

RabD, a Dictyostelium Rab14-related GTPase, localizes in the endo-lysosomal pathway and contractile vacuole system of membranes. Cell lines expressing dominant-negative RabD were defective in endocytosis, endosomal membrane flow and homotypic lysosome fusion. In support of a role for RabD in fusion, cells overexpressing constitutively active RabD(Q67L) accumulated enlarged hydrolase-rich acidic vesicles ringed with GFP-RabD, consistent with RabD directly regulating lysosome fusion. To determine whether RabD also regulated phagocytosis and/or homotypic phagosome fusion (a process stimulated by many intracellular pathogens), cells overexpressing dominant-active (RabD(Q67L)) or dominant-negative (Rab(N121I)) RabD were analyzed microscopically and biochemically. The rate of phagocytosis was increased two-fold in RabD(Q67L)-expressing cells and reduced by 50% in RabD(N121I)-expressing cells compared with control cells. To examine the role of RabD in the formation of multiparticle phagosomes, we performed a series of pulse-chase experiments using fluorescently labeled bacteria and fluorescent latex beads. The rate of fusion of newly formed phagosomes was five times higher in the RabD(Q67L)-expressing cells and reduced by over 50% in RabD(N121I)-expressing cells as compared with control cells. GFP-RabD(Q67L) was found to ring multiparticle spacious phagosomes, which supports a direct role for this protein in regulating fusion. Inhibition of PI 3-kinase activity, which is known to regulate phagosome fusion in the wild-type cells, reduced the rate of phagosome fusion in RabD(Q67L+) cells, indicating that RabD acted upstream of or parallel with PI 3-kinase. We hypothesize that RabD and, possibly, Rab14, a related GTPase that associates with phagosomes in mammalian cells, are important regulators of homotypic phagosome and endo-lysosome fusion.

Dynamics of the vacuolar H+-ATPase in the contractile vacuole complex and the endosomal pathway ofDictyosteliumcells

The vacuolar H+-ATPase (V-ATPase) is a multi-subunit enzyme that plays important roles in eukaryotic cells. In Dictyostelium, it is found primarily in membranes of the contractile vacuole complex, where it energizes fluid accumulation by this osmoregulatory organelle and also in membranes of endolysosomes, where it serves to acidify the endosomal lumen. In the present study, a fusion was created between vatM, the gene encoding the 100 kDa transmembrane subunit of the V-ATPase, and the gene encoding Green Fluorescent Protein (GFP). When expressed in Dictyostelium cells, this fusion protein, VatM-GFP, was correctly targeted to contractile vacuole and endolysosomal membranes and was competent to direct assembly of the V-ATPase enzyme complex. Protease treatment of isolated endosomes indicated that the GFP moiety, located on the C-terminus of VatM, was exposed to the cytoplasmic side of the endosomal membrane rather than to the lumenal side. VatM-GFP labeling of the contractile vacuole complex revealed clearly the dynamics of this pleiomorphic vesiculotubular organelle. VatM-GFP labeling of endosomes allowed direct visualization of the trafficking of vacuolar proton pumps in this pathway, which appeared to be entirely independent from the contractile vacuole membrane system. In cells whose endosomes were pre-labeled with TRITC-dextran and then fed yeast particles,VatM-GFP was delivered to newly formed yeast phagosomes with the same time course as TRITC-dextran, consistent with transfer via a direct fusion of endosomes with phagosomes. Several minutes were required before the intensity of the VatM-GFP labeling of new phagosomes reached the level observed in older phagosomes, suggesting that this fusion process was progressive and continuous. VatM-GFP was retrieved from the phagosome membrane prior to exocytosis of the indigestible remnants of the yeast particle. These data suggest that vacuolar proton pumps are recycled by fusion of advanced with newly formed endosomes.

Dictyostelium LvsB mutants model the lysosomal defects associated with Chediak-Higashi syndrome

Chediak-Higashi syndrome is a genetic disorder caused by mutations in a gene encoding a protein named LYST in humans ("lysosomal trafficking regulator") or Beige in mice. A prominent feature of this disease is the accumulation of enlarged lysosome-related granules in a variety of cells. The genome of Dictyostelium discoideum contains six genes encoding proteins that are related to LYST/Beige in amino acid sequence, and disruption of one of these genes, lvsA (large volume sphere), results in profound defects in cytokinesis. To better understand the function of this family of proteins in membrane trafficking, we have analyzed mutants disrupted in lvsA, lvsB, lvsC, lvsD, lvsE, and lvsF. Of all these, only lvsA and lvsB mutants displayed interesting phenotypes in our assays. lvsA-null cells exhibited defects in phagocytosis and contained abnormal looking contractile vacuole membranes. Loss of LvsB, the Dictyostelium protein most similar to LYST/Beige, resulted in the formation of enlarged vesicles that by multiple criteria appeared to be acidic lysosomes. The rates of endocytosis, phagocytosis, and fluid phase exocytosis were normal in lvsB-null cells. Also, the rates of processing and the efficiency of targeting of lysosomal alpha-mannosidase were normal, although lvsB mutants inefficiently retained alpha-mannosidase, as well as two other lysosomal cysteine proteinases. Finally, results of pulse-chase experiments indicated that an increase in fusion rates accounted for the enlarged lysosomes in lvsB-null cells, suggesting that LvsB acts as a negative regulator of fusion. Our results support the notion that LvsB/LYST/Beige function in a similar manner to regulate lysosome biogenesis.

Sequential activities of phosphoinositide 3-kinase, PKB/Aakt, and Rab7 during macropinosome formation in Dictyostelium

Macropinocytosis plays an important role in the internalization of antigens by dendritic cells and is the route of entry for many bacterial pathogens; however, little is known about the molecular mechanisms that regulate the formation or maturation of macropinosomes. Like dendritic cells, Dictyostelium amoebae are active in macropinocytosis, and various proteins have been identified that contribute to this process. As described here, microscopic analysis of null mutants have revealed that the class I phosphoinositide 3-kinases, PIK1 and PIK2, and the downstream effector protein kinase B (PKB/Akt) are important in regulating completion of macropinocytosis. Although actin-rich membrane protrusions form in these cell lines, they recede without forming macropinosomes. Imaging of cells expressing green fluorescent protein (GFP) fused to the pleckstrin homology domain (PH) of PKB (GFP-PHPKB) indicates that D3 phosphoinositides are enriched in the forming macropinocytic cup and remain associated with newly formed macropinosomes for <1 minute. A fusion protein, consisting of GFP fused to an F-actin binding domain, overlaps with GFP-PHPKB in the timing of association with forming macropinosomes. Although macropinocytosis is reduced in cells expressing dominant negative Rab7, microscopic imaging studies reveal that GFP-Rab7 associates only with formed macropinosomes at approximately the time that F-actin and D3 phosphoinositide levels decrease. These results support a model in which F-actin modulating proteins and vesicle trafficking proteins coordinately regulate the formation and maturation of macropinosomes.

Rab7 regulates phagosome maturation in Dictyostelium

A Dictyostelium Rab7 homolog has been demonstrated to regulate fluid-phase influx, efflux, retention of lysosomal hydrolases and phagocytosis. Since Rab7 function appeared to be required for efficient phagocytosis, we sought to further characterize the role of Rab7 in phagosomal maturation. Expression of GFP-Rab7 resulted in labeling of both early and late phagosomes containing yeast, but not forming phagocytic cups. In order to determine if Rab7 played a role in regulating membrane traffic between the endo/lysosomal system and maturing phagosomes, latex bead containing (LBC) phagosomes were purified from wild-type cells at various times after internalization. Glycosidases, cysteine proteinases, Rab7 and lysosomally associated membrane proteins were delivered rapidly to nascent phagosomes in control cells. LBC phagosomes isolated from cells overexpressing dominant negative (DN) Rab7 contained very low levels of LmpA (lysosomal integral membrane protein) and α-mannosidase was not detectable. Interestingly, cysteine proteinases were delivered to phagosomes as apparent pro-forms in cells overexpressing DN Rab7. Despite these defects, phagosomes in cells overexpressing DN Rab7 matured to form multi-particle spacious phagosomes, except that these phagosomes remained significantly more acidic than control phagosomes. These results suggested that Rab7 regulates both an early and late steps of phagosomal maturation, similar to its role in the endo/lysosomal system.