Fusion and fission events in the endocytic pathway of Dictyostelium

Starvation Induces Extracellular Accumulation of Polyphosphate in Dictyostelium discoideum to Inhibit Macropinocytosis, Phagocytosis, and Exocytosis

Dictyostelium discoideum is a soil-dwelling unicellular eukaryote that accumulates extracellular polyphosphate (polyP). At high cell densities, when the cells are about to overgrow their food supply and starve, the corresponding high extracellular concentrations of polyP allow the cells to preemptively anticipate starvation, inhibit proliferation, and prime themselves to begin development. In this report, we show that starved D. discoideum cells accumulate cell surface and extracellular polyP. Starvation reduces macropinocytosis, exocytosis, and phagocytosis, and we find that these effects require the G protein-coupled polyP receptor (GrlD) and two enzymes, Polyphosphate kinase 1 (Ppk1), which is required for synthesizing intracellular polyP, cell surface polyP, and some of the extracellular polyP, and Inositol hexakisphosphate kinase (I6kA), which is required for cell surface polyP and polyP binding to cells, and some of the extracellular polyP. PolyP reduces membrane fluidity, and we find that starvation reduces membrane fluidity; this effect requires GrlD and Ppk1, but not I6kA. Together, these data suggest that in starved cells, extracellular polyP decreases membrane fluidity, possibly as a protective measure. In the starved cells, sensing polyP appears to decrease energy expenditure from ingestion, and decrease exocytosis, and to both decrease energy expenditures and retain nutrients.

Starvation induces extracellular accumulation of polyphosphate in Dictyostelium discoideum to inhibit macropinocytosis, phagocytosis, and exocytosis

Dictyostelium discoideum is a soil-dwelling unicellular eukaryote that accumulates extracellular polyphosphate (polyP). At high cell densities, when the cells are about to overgrow their food supply and starve, the corresponding high extracellular concentrations of polyP allow the cells to preemptively anticipate starvation, inhibit proliferation, and prime themselves to begin development. In this report, we show that starved D. discoideum cells accumulate cell surface and extracellular polyP. Starvation reduces macropinocytosis, exocytosis, and phagocytosis, and we find that these effects require the G protein-coupled polyP receptor (GrlD) and two enzymes, Polyphosphate kinase 1 (Ppk1), which is required for synthesizing intracellular polyP, cell surface polyP, and some of the extracellular polyP, and Inositol hexakisphosphate kinase (I6kA), which is required for cell surface polyP and polyP binding to cells, and some of the extracellular polyP. PolyP reduces membrane fluidity, and we find that starvation reduces membrane fluidity, and this effect requires GrlD and Ppk1 but not I6kA. Together, these data suggest that in starved cells, extracellular polyP decreases membrane fluidity, possibly as a protective measure. In the starved cells, sensing polyP appears to decrease energy expenditure from ingestion, and decrease exocytosis, to both decrease energy expenditures and retain nutrients.

Placozoan fiber cells: mediators of innate immunity and participants in wound healing

Placozoa is a phylum of non-bilaterian marine animals. These small, flat organisms adhere to the substrate via their densely ciliated ventral epithelium, which mediates mucociliary locomotion and nutrient uptake. They have only six morphological cell types, including one, fiber cells, for which functional data is lacking. Fiber cells are non-epithelial cells with multiple processes. We used electron and light microscopic approaches to unravel the roles of fiber cells in Trichoplax adhaerens, a representative member of the phylum. Three-dimensional reconstructions of serial sections of Trichoplax showed that each fiber cell is in contact with several other cells. Examination of fiber cells in thin sections and observations of live dissociated fiber cells demonstrated that they phagocytose cell debris and bacteria. In situ hybridization confirmed that fiber cells express genes involved in phagocytic activity. Fiber cells also are involved in wound healing as evidenced from microsurgery experiments. Based on these observations we conclude that fiber cells are multi-purpose macrophage-like cells. Macrophage-like cells have been described in Porifera, Ctenophora, and Cnidaria and are widespread among Bilateria, but our study is the first to show that Placozoa possesses this cell type. The phylogenetic distribution of macrophage-like cells suggests that they appeared early in metazoan evolution.

Phagosome resolution regenerates lysosomes and maintains the degradative capacity in phagocytes

Phagocytes engulf unwanted particles into phagosomes that then fuse with lysosomes to degrade the enclosed particles. Ultimately, phagosomes must be recycled to help recover membrane resources that were consumed during phagocytosis and phagosome maturation, a process referred to as “phagosome resolution.” Little is known about phagosome resolution, which may proceed through exocytosis or membrane fission. Here, we show that bacteria-containing phagolysosomes in macrophages undergo fragmentation through vesicle budding, tubulation, and constriction. Phagosome fragmentation requires cargo degradation, the actin and microtubule cytoskeletons, and clathrin. We provide evidence that lysosome reformation occurs during phagosome resolution since the majority of phagosome-derived vesicles displayed lysosomal properties. Importantly, we show that clathrin-dependent phagosome resolution is important to maintain the degradative capacity of macrophages challenged with two waves of phagocytosis. Overall, our work suggests that phagosome resolution contributes to lysosome recovery and to maintaining the degradative power of macrophages to handle multiple waves of phagocytosis.

Modelling of Neuronal Ceroid Lipofuscinosis Type 2 in Dictyostelium discoideum Suggests That Cytopathological Outcomes Result from Altered TOR Signalling

The neuronal ceroid lipofuscinoses comprise a group of neurodegenerative disorders with similar clinical manifestations whose precise mechanisms of disease are presently unknown. We created multiple cell lines each with different levels of reduction of expression of the gene coding for the type 2 variant of the disease, Tripeptidyl peptidase (Tpp1), in the cellular slime mould Dictyostelium discoideum. Knocking down Tpp1 in Dictyostelium resulted in the accumulation of autofluorescent material, a characteristic trait of Batten disease. Phenotypic characterisation of the mutants revealed phenotypic deficiencies in growth and development, whilst endocytic uptake of nutrients was enhanced. Furthermore, the severity of the phenotypes correlated with the expression levels of Tpp1. We propose that the phenotypic defects are due to altered Target of Rapamycin (TOR) signalling. We show that treatment of wild type Dictyostelium cells with rapamycin (a specific TOR complex inhibitor) or antisense inhibition of expression of Rheb (Ras homologue enriched in the brain) (an upstream TOR complex activator) phenocopied the Tpp1 mutants. We also show that overexpression of Rheb rescued the defects caused by antisense inhibition of Tpp1. These results suggest that the TOR signalling pathway is responsible for the cytopathological outcomes in the Dictyostelium Tpp1 model of Batten disease.

Functions of the Dictyostelium LIMP-2 and CD36 homologues in bacteria uptake, phagolysosome biogenesis and host cell defence

Phagocytic cells take up, kill and digest microbes by a process called phagocytosis. To this end, these cells bind the particle, rearrange their actin cytoskeleton, and orchestrate transport of digestive factors to the particle-containing phagosome. The mammalian lysosomal membrane protein LIMP-2 (also known as SCARB2) and CD36, members of the class B of scavenger receptors, play a crucial role in lysosomal enzyme trafficking and uptake of mycobacteria, respectively, and generally in host cell defences against intracellular pathogens. Here, we show that the Dictyostelium discoideum LIMP-2 homologue LmpA regulates phagocytosis and phagolysosome biogenesis. The lmpA knockdown mutant is highly affected in actin-dependent processes, such as particle uptake, cellular spreading and motility. Additionally, the cells are severely impaired in phagosomal acidification and proteolysis, likely explaining the higher susceptibility to infection with the pathogenic bacterium Mycobacterium marinum, a close cousin of the human pathogen Mycobacterium tuberculosis Furthermore, we bring evidence that LmpB is a functional homologue of CD36 and specifically mediates uptake of mycobacteria. Altogether, these data indicate a role for LmpA and LmpB, ancestors of the family of which LIMP-2 and CD36 are members, in lysosome biogenesis and host cell defence.

Pycnosomes: Condensed Endosomal Structures Secreted by Dictyostelium Amoebae

Dictyostelium discoideum has been used largely as a model organism to study the organization and function of the endocytic pathway. Here we describe dense structures present in D. discoideum endocytic compartments, which we named pycnosomes. Pycnosomes are constitutively secreted in the extracellular medium, from which they can be recovered by differential centrifugation. We identified the most abundant protein present in secreted pycnosomes, that we designated SctA. SctA defines a new family of proteins with four members in D. discoideum, and homologous proteins in other protists and eumetazoa. We developed a monoclonal antibody specific for SctA and used it to further characterize secreted and intracellular pycnosomes. Within cells, immunofluorescence as well as electron microscopy identified pycnosomes as SctA-enriched dense structures in the lumen of endocytic compartments. Pycnosomes are occasionally seen in continuity with intra-endosomal membranes, particularly in U18666A-treated cells where intraluminal budding is highly enhanced. While the exact nature, origin and cellular function of pycnosomes remain to be established, this study provides a first description of these structures as well as a characterization of reagents that can be used for further studies.

Dictyostelium EHD associates with Dynamin and participates in phagosome maturation

C-terminal EHDs (Eps15 homology-domain-containing proteins) are newly identified key regulators of endosomal membrane trafficking. Here we show that D. discoideum contains a single EHD protein that localizes to endosomal compartments and newly formed phagosomes. We provide the first evidence that EHD regulates phagosome maturation. Deletion of EHD results in defects in intraphagosomal proteolysis and acidification. These defects are linked to early delivery of lysosomal enzymes and fast retrieval of the vacuolar H+-ATPase in maturing phagosomes. We also demonstrate that EHD physically interacts with DymA. Our results indicate that EHD and DymA can associate independently to endomembranes, and yet they share identical kinetics of phagosome recruitment and release during phagosome maturation. Functional analysis of ehd−, dymA−, and double dymA−/ehd− knock-out strains indicate that DymA and EHD play non-redundant and independent functions in phagosome maturation. Finally, we show that the absence of EHD leads to increase tubulation of endosomes, indicating that EHD participates in the scission of endosomal tubules as reported for DymA.

Loss of Cln3 function in the social amoeba Dictyostelium discoideum causes pleiotropic effects that are rescued by human CLN3

The neuronal ceroid lipofuscinoses (NCL) are a group of inherited, severe neurodegenerative disorders also known as Batten disease. Juvenile NCL (JNCL) is caused by recessive loss-of-function mutations in CLN3, which encodes a transmembrane protein that regulates endocytic pathway trafficking, though its primary function is not yet known. The social amoeba Dictyostelium discoideum is increasingly utilized for neurological disease research and is particularly suited for investigation of protein function in trafficking. Therefore, here we establish new overexpression and knockout Dictyostelium cell lines for JNCL research. Dictyostelium Cln3 fused to GFP localized to the contractile vacuole system and to compartments of the endocytic pathway. cln3⁻ cells displayed increased rates of proliferation and an associated reduction in the extracellular levels and cleavage of the autocrine proliferation repressor, AprA. Mid- and late development of cln3⁻ cells was precocious and cln3⁻ slugs displayed increased migration. Expression of either Dictyostelium Cln3 or human CLN3 in cln3⁻ cells suppressed the precocious development and aberrant slug migration, which were also suppressed by calcium chelation. Taken together, our results show that Cln3 is a pleiotropic protein that negatively regulates proliferation and development in Dictyostelium. This new model system, which allows for the study of Cln3 function in both single cells and a multicellular organism, together with the observation that expression of human CLN3 restores abnormalities in Dictyostelium cln3⁻ cells, strongly supports the use of this new model for JNCL research.

Defective lysosome maturation and Legionella pneumophila replication in Dictyostelium cells mutant for the Arf GAP ACAP-A

Dictyostelium discoideum ACAP-A is an Arf GTPase-activating protein (GAP) involved in cytokinesis, cell migration and actin cytoskeleton dynamics. In mammalian cells, ACAP family members regulate endocytic protein trafficking. Here, we explored the function of ACAP-A in the endocytic pathway of D. discoideum. In the absence of ACAP-A, the efficiency of fusion between post-lysosomes and the plasma membrane was reduced, resulting in the accumulation of post-lysosomes. Moreover, internalized fluid-phase markers showed extended intracellular transit times, and the transfer kinetics of phagocyted particles from lysosomes to post-lysosomes was reduced. Neutralization of lysosomal pH, one essential step in lysosome maturation, was also delayed. Whereas expression of ACAP-A-GFP in acapA(-) cells restored normal particle transport kinetics, a mutant ACAP-A protein with no GAP activity towards the small GTPase ArfA failed to complement this defect. Taken together, these data support a role for ACAP-A in maturation of lysosomes into post-lysosomes through an ArfA-dependent mechanism. In addition, we reveal that ACAP-A is required for efficient intracellular growth of Legionella pneumophila, a pathogen known to subvert the endocytic host cell machinery for replication. This further emphasizes the role of ACAP-A in the endocytic pathway.

Dictyostelium LvsB has a regulatory role in endosomal vesicle fusion

Defects in human lysosomal-trafficking regulator (Lyst) are associated with the lysosomal disorder Chediak-Higashi syndrome. The absence of Lyst results in the formation of enlarged lysosome-related compartments, but the mechanism for how these compartments arise is not well established. Two opposing models have been proposed to explain Lyst function. The fission model describes Lyst as a positive regulator of fission from lysosomal compartments, whereas the fusion model identifies Lyst as a negative regulator of fusion between lysosomal vesicles. Here, we used assays that can distinguish between defects in vesicle fusion versus fission. We compared the phenotype of Dictyostelium discoideum cells defective in LvsB, the ortholog of Lyst, with that of two known fission defect mutants (μ3- and WASH-null mutants). We found that the temporal localization characteristics of the post-lysosomal marker vacuolin, as well as vesicular acidity and the fusion dynamics of LvsB-null cells are distinct from those of both μ3- and WASH-null fission defect mutants. These distinctions are predicted by the fusion defect model and implicate LvsB as a negative regulator of vesicle fusion.

Monoubiquitination of EEA1 regulates endosome fusion and trafficking

Background

Early endosomal autoantigen 1 (EEA1) is a membrane tethering factor required for the fusion and maturation of early endosomes in endocytosis. How the activity of EEA1 is regulated in cells is unclear.

Results

Here we show that endogenous EEA1 is prone to monoubiquitination at multiple sites, owing to an intrinsic affinity to ubiquitin conjugating enzymes (E2). The E2 interactions enable a ubiquitin ligase (E3) independent mechanism that decorate EEA1 with multiple mono-ubiquitin moieties. Expression of an ubiquitin-EEA1 chimera that mimics native mono-ubiquitinated EEA1 generates giant endosomes abutting the nucleus. Several lines of evidence suggest that this phenotype is due to increased endosome fusion and a simultaneous blockade on an endosome recycling pathway. The latter is likely caused by diminished endosome fission in cells expressing ubiquitin-EEA1.

Conclusion

Our results demonstrate that ubiquitination may dramatically affect the activity of an endosome fusion factor to alter endosome morphology and trafficking pattern, and thereby implicating an unexpected role of ubiquitin signaling in endocytosis.

Secretory lysosomes in Dictyostelium: visualization, characterization, and dynamics

Dictyostelium discoideum is a unicellular eukaryotic model largely used for the study of the endocytic pathway, mainly due to its high resemblance to the mammalian pathway. Over the last years, Dictyostelium has been increasingly employed for investigating the biogenesis and secretion of secretory lysosomes, given its advantage over mammalian cells. Here we describe protocols for determination of ionic concentration and rates of maturation and exocytosis of these compartments.

Antagonistic control of lysosomal fusion by Rab14 and the Lyst-related protein LvsB

While loss of the protein Lyst causes abnormal lysosomes in patients with Chediak-Higashi syndrome, the contribution of Lyst to lysosome biology is not known. Previously we found that the Dictyostelium ortholog of Lyst, LvsB, is a cytosolic protein that associates with lysosomes and post-lysosomes to prevent their inappropriate fusion. Here we provide three lines of evidence that indicate that LvsB contributes to lysosome function by antagonizing the function of DdRab14, a protein that promotes homotypic fusion among lysosomes. (1) Instead of restricting DdRab14 to lysosomes, cells that lack LvsB expand DdRab14 localization to include post-lysosomes. (2) Expression of activated DdRab14 phenocopies the loss of LvsB, causing inappropriate heterotypic fusion between lysosomes and post-lysosomes and their subsequent enlargement. (3) Conversely, expression of inactivated DdRab14 suppresses the phenotype of LvsB null cells and restores their lysosomal size and segregation from post-lysosomes. Our data suggest a scenario where LvsB binds to late lysosomes and promotes the inactivation of DdRab14. This inactivation allows the lysosomes to mature into post-lysosomes for eventual secretion. We propose that human Lyst may function similarly to regulate Rab-dependent fusion of lysosomal compartments.

The model organism Dictyostelium discoideum

Much of our knowledge of molecular cellular functions is based on studies with a few number of model organisms that were established during the last 50 years. The social amoeba Dictyostelium discoideum is one such model, and has been particularly useful for the study of cell motility, chemotaxis, phagocytosis, endocytic vesicle traffic, cell adhesion, pattern formation, caspase-independent cell death, and, more recently, autophagy and social evolution. As nonmammalian model of human diseases D. discoideum is a newcomer, yet it has proven to be a powerful genetic and cellular model for investigating host-pathogen interactions and microbial infections, for mitochondrial diseases, and for pharmacogenetic studies. The D. discoideum genome harbors several homologs of human genes responsible for a variety of diseases, -including Chediak-Higashi syndrome, lissencephaly, mucolipidosis, Huntington disease, IBMPFD, and Shwachman-Diamond syndrome. A few genes have already been studied, providing new insights on the mechanism of action of the encoded proteins and in some cases on the defect underlying the disease. The opportunities offered by the organism and its place among the nonmammalian models for human diseases will be discussed.

Quantitative analysis of phagocytosis and phagosome maturation

Phagocytosis and phagosome maturation lead to killing and digestion of bacteria by protozoans and innate immune phagocytes. Phagocytosis of particles expressing or coupled to various fluorescent reporters and sensors can be used to monitor quantitatively various parameters of this central biological process. In this chapter we detail different labeling techniques of bacteria and latex beads used to measure adhesion and uptake by FACS analysis. We also describe methods to use fluorescent reporter dyes (FITC or DQgreen) coupled to silica beads to measure the kinetics of acidification and proteolysis. Measurements can be performed either at the single-cell level, using live microscopy, or for a whole cell population, with a fluorescence microplate reader.

Mucolipin controls lysosome exocytosis in Dictyostelium

Mucolipidosis type IV is a poorly understood lysosomal storage disease caused by alterations in the mucolipin lysosomal Ca(2+) channel. In this study, we generated mucolipin-knockout Dictyostelium cells, and observed that lysosome exocytosis was markedly increased in these cells compared with wild-type cells. In addition, mucolipin-knockout cells were more resistant to Ca(2+) deprivation, and the Ca(2+) concentration in their secretory lysosomes was decreased, suggesting that mucolipin transfers Ca(2+) ions from the cytosol to the lumen of secretory lysosomes. We speculate that mucolipin attenuates the fusogenic effect of local cytosolic increases in Ca(2+) by dissipating them into the lumen of lysosomal compartments.

Effect of starvation on the endocytic pathway in Dictyostelium cells

Dictyostelium discoideum amoebae have been used extensively to study the structure and dynamics of the endocytic pathway. Here, we show that while the general structure of the endocytic pathway is maintained in starved cells, its dynamics rapidly slow down. In addition, analysis of apm3 and lvsB mutants reveals that the functional organization of the endocytic pathway is profoundly modified upon starvation. Indeed, in these mutant cells, some of the defects observed in rich medium persist in starved cells, notably an abnormally slow transfer of endocytosed material between endocytic compartments. Other parameters, such as endocytosis of the fluid phase or the rate of fusion of postlysosomes to the cell surface, vary dramatically upon starvation. Studying the endocytic pathway in starved cells can provide a different perspective, allowing the primary (invariant) defects resulting from specific mutations to be distinguished from their secondary (conditional) consequences.

A Coronin7 homolog with functions in actin-driven processes

Dictyostelium discoideum Coronin7 (DdCRN7) together with human Coronin7 (CRN7) and Pod-1 of Drosophila melanogaster and Caenorhabditis elegans belong to the coronin family of WD-repeat domain-containing proteins. Coronin7 proteins are characterized by two WD-repeat domains that presumably fold into two beta-propeller structures. DdCRN7 shares highest homology with human CRN7, a protein with roles in membrane trafficking. DdCRN7 is present in the cytosol and accumulates in cell surface projections during movement and phago- and pinocytosis. Cells lacking CRN7 have altered chemotaxis and phagocytosis. Furthermore, loss of CRN7 affects the infection process by the pathogen Legionella pneumophila and allows a more efficient internalization of bacteria. To provide a mechanism for CNR7 action, we studied actin-related aspects. We could show that CRN7 binds directly to F-actin and protects actin filaments from depolymerization. CRN7 also associated with F-actin in vivo. It was present in the Triton X-100-insoluble cytoskeleton, colocalized with F-actin, and its distribution was sensitive to drugs affecting the actin cytoskeleton. We propose that the CRN7 role in chemotaxis and phagocytosis is through its effect on the actin cytoskeleton.

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

BACKGROUND

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

METHODOLOGY

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

PRINCIPAL FINDINGS

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

CONCLUSIONS/SIGNIFICANCE

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

The human homologue of Dictyostelium discoideum phg1A is expressed by human metastatic melanoma cells

Tumour cannibalism is a characteristic of malignancy and metastatic behaviour. This atypical phagocytic activity is a crucial survival option for tumours in conditions of low nutrient supply, and has some similarities to the phagocytic activity of unicellular microorganisms. In fact, Dictyostelium discoideum has been used widely as a model to study phagocytosis. Recently, phg1A has been described as a protein that is primarily involved in the phagocytic process of this microorganism. The closest human homologue to phg1A is transmembrane 9 superfamily protein member 4 (TM9SF4). Here, we report that TM9SF4 is highly expressed in human malignant melanoma cells deriving from metastatic lesions, whereas it is undetectable in healthy human tissues and cells. TM9SF4 is predominantly expressed in acidic vesicles of melanoma cells, in which it co-localizes with the early endosome antigens Rab5 and early endosome antigen 1. TM9SF4 silencing induced marked inhibition of cannibal activity, which is consistent with a derangement of intracellular pH gradients, with alkalinization of acidic vesicles and acidification of the cell cytosol. We propose TM9SF4 as a new marker of malignancy, representing a potential new target for anti-tumour strategies with a specific role in tumour cannibalism and in the establishment of a metastatic phenotype.

Dictyostelium discoideum: The Social Ameba

INTRODUCTIONDictyostelium discoideum is a unicellular eukaryote often referred to as a "social ameba" because it can form a multicellular structure when nutrient conditions are limiting. D. discoideum and related organisms, known as the Dictyostelia, have been studied for almost 150 years. The cellular and molecular aspects of their multicellular lifestyle have been studied in detail, and general principles for cell-to-cell communication, intracellular signaling, and cytoskeletal organization during cell motility have been derived from this work and have been found to be conserved across all eukaryotes. The bacteriovore nature of the unicellular stage provides an excellent model in which to study phagocytosis and the mechanisms of bacterial virulence. D. discoideum has also been used successfully to explore the molecular basis of various human diseases, as well as the mechanisms of drug action and the pathways that lead to resistance to certain therapeutic agents. The availability of a complete genome sequence has further widened the scope of studies using D. discoideum. A large potential for secondary metabolism has become apparent, which opens the door to discovering new compounds with potential medical applications. Numerous putative orthologs of genes responsible for diseases in humans, but whose molecular functions are still uncharacterized, are present in the D. discoideum genome. Finally, the availability of community resources, including the genome database dictyBase and the Dicty Stock Center, makes D. discoideum an easily accessible and powerful model organism to study.

Vacuole membrane protein 1 is an endoplasmic reticulum protein required for organelle biogenesis, protein secretion, and development

Vacuole membrane protein 1 (Vmp1) is membrane protein of unknown molecular function that has been associated with pancreatitis and cancer. The social amoeba Dictyostelium discoideum has a vmp1-related gene that we identified previously in a functional genomic study. Loss-of-function of this gene leads to a severe phenotype that compromises Dictyostelium growth and development. The expression of mammalian Vmp1 in a vmp1(-) Dictyostelium mutant complemented the phenotype, suggesting a functional conservation of the protein among evolutionarily distant species and highlights Dictyostelium as a valid experimental system to address the function of this gene. Dictyostelium Vmp1 is an endoplasmic reticulum protein necessary for the integrity of this organelle. Cells deficient in Vmp1 display pleiotropic defects in the secretory pathway and organelle biogenesis. The contractile vacuole, which is necessary to survive under hypoosmotic conditions, is not functional in the mutant. The structure of the Golgi apparatus, the function of the endocytic pathway and conventional protein secretion are also affected in these cells. Transmission electron microscopy of vmp1(-) cells showed the accumulation of autophagic features that suggests a role of Vmp1 in macroautophagy. In addition to these defects observed at the vegetative stage, the onset of multicellular development and early developmental gene expression are also compromised.

Legionella pathogenicity: Genome structure, regulatory networks and the host cell response

Legionella spp. the causative agent of Legionnaires' disease is naturally found in fresh water where the bacteria parasitize intracellularly within protozoa. Upon aerosol formation via man-made water systems, Legionella can enter the human lung and cause a severe form of pneumonia. Here we review results from systematic comparative genome analysis of Legionella species with different pathogenic potentials. The complete genomes reveal that horizontal gene transfer has played an important role during the evolution of Legionella and indicate the importance of secretion machineries for the intracellular lifestyle of this pathogen. Moreover, we highlight recent findings on the in vivo transcriptional program of L. pneumophila and the regulatory networks involved in the biphasic life cycle. In order to understand how Legionella effectively subvert host cell functions for its own benefit the transcriptional host cell response upon infection of the model amoeba Dictyostelium discoideum was studied. The use of this model organism made it possible to develop a roadmap of host cell factors which significantly contribute to the uptake of L. pneumophila and the establishment of an ER-associated replicative vacuole.

Flotillin and RacH modulate the intracellular immunity of Dictyostelium to Mycobacterium marinum infection

Mycobacterium marinum, a close relative of Mycobacterium tuberculosis, provides a useful model to study the pathogenesis of tuberculosis in genetically tractable model organisms. Using the amoeba Dictyostelium discoideum as a host, we show that expression of the M. marinum protein MAG24-1 is crucial to interfere with phagosome maturation. We find that two host proteins - the flotillin homologue vacuolin and p80, a predicted copper transporter - accumulate at the vacuole during pathogen replication until it finally ruptures and the bacteria are released into the host cytosol. Flotillin-1 accumulation at the replication niche and its rupture were also observed in human peripheral blood monocytes. By infecting various Dictyostelium mutants, we show that the absence of one of the two Dictyostelium vacuolin isoforms renders the host more immune to M. marinum. Conversely, the absence of the small GTPase RacH renders the host more susceptible to M. marinum proliferation but inhibits its cell-to-cell spreading.

Dictyostelium Hip1r contributes to spore shape and requires epsin for phosphorylation and localization

Clathrin-coated pits assemble on the plasma membrane to select and sequester proteins within coated vesicles for delivery to intracellular compartments. Although a host of clathrin-associated proteins have been identified, much less is known regarding the interactions between clathrin-associated proteins or how individual proteins influence the function of other proteins. In this study, we present evidence of a functional relationship between two clathrin-associated proteins in Dictyostelium, Hip1r and epsin. Hip1r-null cells form fruiting bodies that yield defective spores that lack the organized fibrils typical of wild-type spores. This spore coat defect leads to formation of round, rather than ovoid, spores in Hip1r-null cells that exhibit decreased viability. Like Hip1r-null cells, epsin-null cells also construct fruiting bodies with round spores, but these spores are more environmentally robust. Double-null cells that harbor deletions in both epsin and Hip1r form fruiting bodies, with spores identical in shape and viability to Hip1r single-null cells. In the growing amoeba, Hip1r is phosphorylated and localizes to puncta on the plasma membrane that also contain epsin. Both the phosphorylation state and localization of Hip1r into membrane puncta require epsin. Moreover, expression of the N-terminal ENTH domain of epsin is sufficient to restore both the phosphorylation and the restricted localization of Hip1r within plasma membrane puncta. The results from this study reveal a novel interaction between two clathrin-associated proteins during cellular events in both growing and developing Dictyostelium cells.

Src triggers circular ruffling and macropinocytosis at the apical surface of polarized MDCK cells

We addressed the role of Src on cortical actin dynamics and polarized endocytosis in MDCK cells harboring a thermosensitive v-src mutant. Shifting monolayers established at 40 degrees C (non-permissive temperature) to 34 degrees C (permissive temperature) rapidly reactivated v-Src kinase, but tight junctions and cell polarity resisted for >6 h. At this interval, activated v-src was recruited on apical vesicles, induced cortactin-associated apical circular ruffles productive of macropinosomes, thereby accelerating apical pinocytosis by approximately fivefold. Ruffling and macropinosome formation were selectively abrogated by inhibitors of actin polymerization, phosphoinositide 3-kinase, phospholipase C, and phospholipase D, which all returned apical pinocytosis to the level observed at 40 degrees C, underscoring the distinct control of apical micropinocytosis and macropinocytosis. Src promoted microtubule-dependent fusion of macropinosomes to the apical recycling endosome (ARE), causing its strong vacuolation. However, preservation of tubulation and apical polarity indicated that its function was not affected. The ARE was labeled for v-src, Rab11, and rabankyrin-5 but not early endosome antigen 1, thus distinguishing two separate Rab5-dependent apical pathways. The mechanisms of Src-induced apical ruffling and macropinocytosis could shed light on the triggered apical enteroinvasive pathogens entry and on the apical differentiation of osteoclasts.

Endocytosis in the shiitake mushroom Lentinula edodes and involvement of GTPase LeRAB7

Endocytosis is the process by which substrates enter a cell without passing through the plasma membrane but rather invaginate the cell membrane and form intracellular vesicles. Rab7 regulates endocytic trafficking between early and late endosomes and between late endosomes and lysosomes. LeRab7 in Lentinula edodes is strongly homologous to Rab7 in Homo sapiens. Receptors for activated C kinase-1 (LeRACK1) and Rab5 GTPase (LeRAB5) were isolated as interacting partners of LeRab7, and the interactions were confirmed by in vivo and in vitro protein interaction assays. The three genes showed differential expression in the various developmental stages of the mushroom. In situ hybridization showed that the three transcripts were localized in regions of active growth, such as the outer region of trama cells, and the subhymenium of the hymenophore of mature fruiting bodies and the prehymenophore of young fruiting bodies. The existence of endocytosis in the mycelium and hymenophores was confirmed by the internalization of FM4-64. LeRAB7 was partially colocalized with the AM4-64 and was located in the late endocytic pathway. This is the first report of the presence of endocytosis in homobasidiomycetes. LeRAB7, LeRAB5, and LeRACK1 may contribute to the growth of L. edodes and cell differentiation in hymenophores.

The BEACH Protein LvsB Is Localized on Lysosomes and Postlysosomes and Limits Their Fusion with Early Endosomes

The Chediak-Higashi syndrome (CHS) is a genetic disorder caused by the loss of the BEACH protein Lyst. Impaired lysosomal function in CHS patients results in many physiological problems, including immunodeficiency, albinism and neurological problems. Dictyostelium LvsB is the ortholog of mammalian Lyst and is also important for lysosomal function. A knock-in approach was used to tag LvsB with green fluorescent protein (GFP) and express it from its single chromosomal locus. GFP-LvsB was observed on late lysosomes and postlysosomes. Loss of LvsB resulted in enlarged postlysosomes, in the abnormal localization of proton pumps on postlysosomes and their abnormal acidification. The abnormal postlysosomes in LvsB-null cells were produced by the inappropriate fusion of early endosomal compartments with postlysosomal compartments. The intermixing of compartments resulted in a delayed transit of fluid-phase marker through the endolysosomal system. These results support the model that LvsB and Lyst proteins act as negative regulators of fusion by limiting the heterotypic fusion of early endosomes with postlysosomal compartments.

Diverse cytopathologies in mitochondrial disease are caused by AMP-activated protein kinase signaling

The complex cytopathology of mitochondrial diseases is usually attributed to insufficient ATP. AMP-activated protein kinase (AMPK) is a highly sensitive cellular energy sensor that is stimulated by ATP-depleting stresses. By antisense-inhibiting chaperonin 60 expression, we produced mitochondrially diseased strains with gene dose-dependent defects in phototaxis, growth, and multicellular morphogenesis. Mitochondrial disease was phenocopied in a gene dose-dependent manner by overexpressing a constitutively active AMPK alpha subunit (AMPKalphaT). The aberrant phenotypes in mitochondrially diseased strains were suppressed completely by antisense-inhibiting AMPKalpha expression. Phagocytosis and macropinocytosis, although energy consuming, were unaffected by mitochondrial disease and AMPKalpha expression levels. Consistent with the role of AMPK in energy homeostasis, mitochondrial "mass" and ATP levels were reduced by AMPKalpha antisense inhibition and increased by AMPKalphaT overexpression, but they were near normal in mitochondrially diseased cells. We also found that 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, a pharmacological AMPK activator in mammalian cells, mimics mitochondrial disease in impairing Dictyostelium phototaxis and that AMPKalpha antisense-inhibited cells were resistant to this effect. The results show that diverse cytopathologies in Dictyostelium mitochondrial disease are caused by chronic AMPK signaling not by insufficient ATP.

The phagosome: compartment with a license to kill

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

Selective membrane exclusion in phagocytic and macropinocytic cups

Specialized eukaryotic cells can ingest large particles and sequester them within membrane-delimited phagosomes. Many studies have described the delivery of lysosomal proteins to the phagosome, but little is known about membrane sorting during the early stages of phagosome formation. Here we used Dictyostelium discoideum amoebae to analyze the membrane composition of newly formed phagosomes. The membrane delimiting the closing phagocytic cup was essentially derived from the plasma membrane, but a subgroup of proteins was specifically excluded. Interestingly the same phenomenon was observed during the formation of macropinosomes, suggesting that the same sorting mechanisms are at play during phagocytosis and macropinocytosis. Analysis of mutant strains revealed that clathrin-associated adaptor complexes AP-1, -2 and -3 were not necessary for this selective exclusion and, accordingly, ultrastructural analysis revealed no evidence for vesicular transport around phagocytic cups. Our results suggest the existence of a new, as yet uncharacterized, sorting mechanism in phagocytic and macropinocytic cups.

The annexins of Dictyostelium

Annexins are a highly conserved ubiquitous family of Ca2+- and phospholipid-binding proteins present in nearly all eukaryotic cells. Analysis of the Dictyostelium genome revealed the presence of two annexin genes, the annexin C1 gene (nxnA) giving rise to two isoforms of 47 and 51 kDa (previously synexin), and the annexin C2 gene (nxnB) coding for a 56-kDa protein with 33% sequence identity to annexin C1. Annexin C2 is expressed at very low and constant levels throughout development. Quantification by real-time PCR indicated that it is present in about 35-fold lower amounts compared to annexin C1. We have used a GFP-tagged annexin C2 to study its cellular distribution and dynamics. In cell fractionation studies, annexin C2 cofractionates with annexin C1 and is enriched in the 100,000 g pellet. Like annexin C1, GFP-AnxC2 stains the plasma membrane. In addition it is present in the perinuclear region and overlaps to some degree with the Golgi apparatus, whereas annexin C1 is present on intracellular membranes resembling endosomal membranes and in the nucleus. Annexin C2 is not observed in the nucleus. An annexin C1 mutant (SYN-) which shows a defect during multicellular development can be rescued by full-length annexin C1, whereas overexpression of GFP-AnxC2 did not rescue the developmental defect The data support the concept that annexins, although having a highly conserved structure, participate in different functions in a cell.

Trix, a novel Rac guanine-nucleotide exchange factor from Dictyostelium discoideum is an actin-binding protein and accumulates at endosomes

Small Rho family GTPases are involved in regulation of actin cytoskeleton dynamics. These molecular switches are themselves mainly controlled by specific GTPase-activating proteins (GAPs) and guanine-nucleotide exchange factors (GEFs). We have cloned and initially characterized a novel putative RhoGEF from Dictyostelium discoideum. The predicted 135-kDa protein displays a unique domain organization in its N-terminus by harboring two type3 calponin homology (CH) domains followed by a single type1 CH domain. The C-terminal region encompasses a diffuse B-cell lymphoma homology/pleckstrin homology tandem domain that is typically found in RhoGEFs. We therefore refer to this protein as Trix (triple CH-domain array exchange factor). A recombinant N-terminal region of Trix carrying all three CH domains binds to F-actin and bundles actin filaments. Trix-null mutants are viable and display only subtle defects when compared to wild-type cells with the exception of a substantial decrease in exocytosis of a fluid-phase marker. GFP fusions with the full-length protein or the N-terminal part containing all three CH domains revealed that Trix localizes to the cortical region and strongly accumulates on late endosomes. Our results suggest that Trix is specifically involved in a Rho GTPase-signaling pathway that is required for regulation of the actin cytoskeleton during exocytosis.

Proteomics fingerprinting of phagosome maturation and evidence for the role of a Galpha during uptake

Phagocytosis, whether of food particles in protozoa or bacteria and cell remnants in the metazoan immune system, is a conserved process. The particles are taken up into phagosomes, which then undergo complex remodeling of their components, called maturation. By using two-dimensional gel electrophoresis and mass spectrometry combined with genomic data, we identified 179 phagosomal proteins in the amoeba Dictyostelium, including components of signal transduction, membrane traffic, and the cytoskeleton. By carrying out this proteomics analysis over the course of maturation, we obtained time profiles for 1,388 spots and thus generated a dynamic record of phagosomal protein composition. Clustering of the time profiles revealed five clusters and 24 functional groups that were mapped onto a flow chart of maturation. Two heterotrimeric G protein subunits, Galpha4 and Gbeta, appeared at the earliest times. We showed that mutations in the genes encoding these two proteins produce a phagocytic uptake defect in Dictyostelium. This analysis of phagosome protein dynamics provides a reference point for future genetic and functional investigations.

Exocytosis of late endosomes does not directly contribute membrane to the formation of phagocytic cups or pseudopods in Dictyostelium

Exocytosis of late endocytic compartments in Dictyostelium has mostly been studied by live microscopy. Here we show that this exocytosis is accompanied by a complete fusion of late endosomes with the plasma membrane resulting in the transient formation of membrane microdomains that can be visualized by immunofluorescence in fixed cells. This permitted to demonstrate that fusion of late endocytic compartments with the cell surface does not occur in regions of the plasma membrane engaged in the formation of pseudopods, macropinosomes or phagosomes. Our results propose that exocytosis of late endosomes and actin-driven membrane remodeling are mutually exclusive processes.

Dictyostelium RacH Regulates Endocytic Vesicular Trafficking and is Required for Localization of Vacuolin

Dictyostelium RacH localizes predominantly to membranes of the nuclear envelope, endoplasmic reticulum and Golgi apparatus. To investigate the role of this protein, we generated knockout and overexpressor strains. RacH-deficient cells displayed 50% reduced fluid-phase uptake and a moderate exocytosis defect, but phagocytosis was unaffected. Detailed examination of the endocytic pathway revealed defective acidification of early endosomes and reduced secretion of acid phosphatase in the presence of sucrose. The distribution of the post-lysosomal marker vacuolin was altered, with a high proportion of cells showing a diffuse vesicular pattern in contrast to the wild-type strain, where few intensely stained vacuoles predominate. Cytokinesis, cell motility, chemotaxis and development appeared largely unaffected. In a cell-free system, RacH stimulates actin polymerization, suggesting that this protein is involved in actin-based trafficking of vesicular compartments. We also investigated the determinants of subcellular localization of RacH by expression of green-fluorescent-protein-tagged chimeras in which the C-terminus of RacH and the plasma-membrane-targeted RacG were exchanged, the insert region was deleted or the net positive charge of the hypervariable region was increased. We show that several regions of the molecule, not only the hypervariable region, determine targeting of RacH. Overexpression of mistargeted RacH mutants did not recapitulate the phenotypes of a strain overexpressing nonmutated RacH, indicating that the function of this protein is in great part related to its subcellular localization.

Phagocyte meets prey: uptake, internalization, and killing of bacteria by Dictyostelium amoebae

Dictyostelium cells are professional phagocytes that avidly consume bacteria, their natural prey. Fluorescent probes have allowed us to monitor the initial steps in this process in living cells. Using probes that bind to F-actin, we have visualized the assembly and disassembly of actin filaments responsible for extending the phagocytic cup to engulf a bacterium, and, after the phagosome has sealed, the assembly of new actin filaments to propel the phagosome away from the site of uptake. Using bacteria expressing fluorescent proteins that are susceptible to proteolysis, we have monitored the loss of that fluorescent signal and the staining of the bacterial contents with neutral red, indicating permeabilization of the bacterial cell wall and acidification of the cytoplasm. We find that acidification occurs during a period of microtubule-based transport that promotes fusion of the phagosome with microtubule-associated acidic endosomes. Actin-powered phagosome internalization, transport of the phagosome along microtubules, proteolysis and acidification of bacterial contents, all typically occur within the first six or seven minutes after formation of the phagosome. Thus, tracking individual phagosomes has revealed that early steps in phagosome maturation occur much more rapidly than had been inferred from previous population studies.

Copine A, a calcium-dependent membrane-binding protein, transiently localizes to the plasma membrane and intracellular vacuoles in Dictyostelium

Background

Copines are soluble, calcium-dependent membrane binding proteins found in a variety of organisms. Copines are characterized as having two C2 domains at the N-terminal region followed by an "A domain" at the C-terminal region. The "A domain" is similar in sequence to the von Willebrand A (VWA) domain found in integrins. The presence of C2 domains suggests that copines may be involved in cell signaling and/or membrane trafficking pathways.

Results

We have identified six copines genes in the Dictyostelium discoideum genome, cpnA-cpnF, and have focused our studies on cpnA. CpnA is expressed throughout development and was shown to be capable of binding to membranes in a calcium-dependent manner in vitro. A GFP-tagged CpnA was also capable of binding to membranes in a calcium-dependent manner in vitro. In live wildtype Dictyostelium cells expressing GFP-CpnA, GFP-CpnA was typically found in the cytoplasm without any specific localization to membranes. However, in a small subset of starved cells, GFP-CpnA was observed to bind transiently (typically ~1–10 s) to the plasma membrane and intracellular vacuoles. In some cells, the transient membrane localization of GFP-CpnA was observed to occur multiple times in an oscillatory manner over several minutes. In plasma membrane disrupted cells, GFP-CpnA was observed to associate with the plasma membrane and intracellular vacuoles in a calcium-dependent manner. In fixed cells, GFP-CpnA labeled the plasma membrane and intracellular vacuoles, including contractile vacuoles, organelles of the endolysosomal pathway, and phagosomes.

Conclusion

Our results show that Dictyostelium has multiple copine homologs and provides an excellent system in which to study copine function. The localization of a GFP-tagged CpnA to the plasma membrane, contractile vacuoles, organelles of the endolysosomal pathway, and phagosomes suggests that CpnA may have a role in the function of these organelles or the trafficking to and from them. In addition, we hypothesize that the observed transient oscillatory membrane localization of GFP-CpnA in a small subset of starved cells is caused by fast calcium waves and speculate that CpnA may have a role in development, particularly in the differentiation of stalk cells.

Dictyostelium as host model for pathogenesis

The haploid social soil amoeba Dictyostelium discoideum has been established as a host model for several pathogens including Pseudomonas aeruginosa, Cryptococcus neoformans, Mycobacterium spp. and Legionella pneumophila. The research areas presently pursued include (i) the use of Dictyostelium wild-type cells as screening system for virulence of extracellular and intracellular pathogens and their corresponding mutants, (ii) the use of Dictyostelium mutant cells to identify genetic host determinants of susceptibility and resistance to infection and (iii) the use of reporter systems in Dictyostelium cells which allow the dissection of the complex host-pathogen cross-talk. The body of information presented in this review demonstrates that the availability of host cell markers, the knowledge of cell signalling pathways, the completion of the genome sequencing project and the tractability for genetic studies qualifies Dictyostelium for the study of fundamental cellular processes of pathogenesis.

Directed motility of phagosomes inTetrahymena thermophila requires actin and Myo1p, a novel unconventional myosin

The phagosome cycle was investigated in Tetrahymena thermophila, which had internalized fluorescent latex beads. Confocal microscopy of cells from a GFP-actin strain revealed actin filaments that extended 3-5 mum from the periphery of fluorescent phagosomes. In GFP-actin cells and in wild-type cells, motility of fluorescent phagosomes was directed from the oral cavity to the posterior end of the cell. Although 60% of fluorescent phagosomes in the MYO1-knockout strain were motile, movement of phagosomes was not directed toward the posterior end of the cell and was random. Forty percent of fluorescent phagosomes in knockout cells were non-motile in contrast to only 20% non-motile phagosomes in wild-type cells. The increased incidence of non-motile phagosomes in the knockout strain could reflect absence of Myo1p as a motor. Another myosin or other molecular motors could power random movement of phagosomes in the MYO1-knockout strain. In latrunculin-treated GFP-actin cells, movement of fluorescent phagosomes was random. Average velocity of random movement of fluorescent phagosomes in the knockout strain and in latrunculin-treated cells was statistically the same as the average velocity (2.0 +/- 1.9 microm/min) of phagosomes in GFP-actin cells. These findings are an indication that dynamic actin and Myo1p are required for directed motility of phagosomes.

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.

Mycobacteria inhibit nitric oxide synthase recruitment to phagosomes during macrophage infection

Inducible nitric oxide synthase (iNOS) is a cytoplasmic protein responsible for the generation of nitric oxide (NO. ) in macrophages. In this work, we hypothesized that the intracellular localization of iNOS is significant for effective delivery of NO. to phagosomes containing ingested microorganisms. Using immunofluorescence microscopy and Western blot analysis, iNOS was shown to localize in the vicinity of phagosomes containing latex beads in stimulated macrophages. iNOS also localized to phagosomes containing Escherichia coli. The colocalization of iNOS with ingested latex beads was an actin-dependent process, since treatment with the actin microfilament disrupter cytochalasin D prevented iNOS recruitment to latex bead phagosomes. In contrast to E. coli and inert particle phagosomes, mycobacterial phagosomes did not colocalize with iNOS. This study demonstrates that (i). iNOS can be recruited to phagosomes; (ii). this recruitment is dependent on a functional actin cytoskeleton; (iii). certain microorganisms have the ability to prevent or reduce colocalization with iNOS; and (iv). spatial exclusion of iNOS may play a role in Mycobacterium tuberculosis pathogenesis.

GFP-golvesin constructs to study Golgi tubulation and post-Golgi vesicle dynamics in phagocytosis

Dictyostelium cells are professional phagocytes that are optimally suited for the imaging of phagosome processing from particle uptake to exocytosis. In order to design fluorescent probes for monitoring membrane trafficking in the endocytic pathway, we have dissected a membrane protein, golvesin, and have linked fragments of its sequence to GFP. Endogenous golvesin is partitioned between the ER, the Golgi apparatus, endosomes, and the contractile vacuole complex. We have localized signals that are required for exit from the Golgi to post-Golgi compartments to the C-terminal region of the golvesin sequence. One GFP-tagged fragment turned out to be a highly specific Golgi marker and was used to demonstrate the interaction of Golgi tubules with phagosomes. Signals essential for the retrieval of golvesin at the end of phagosome processing were localized to the N-terminal region. A truncated golvesin construct escaping retrieval was employed in recording the delivery of a phagosomal protein to the plasma membrane. Applying this construct to a phagosome filled with multiple particles, we observed that the phagosome is segmented during exocytosis, meaning that sequential release of particles alternates with membrane fusion.