The axenic mutations and endocytosis in Dictyostelium

Leep2A and Leep2B function as a RasGAP complex to regulate macropinosome formation

Macropinocytosis mediates the non-selective bulk uptake of extracellular fluid, enabling cells to survey the environment and obtain nutrients. A conserved set of signaling proteins orchestrates the actin dynamics that lead to membrane ruffling and macropinosome formation across various eukaryotic organisms. At the center of this signaling network are Ras GTPases, whose activation potently stimulates macropinocytosis. However, how Ras signaling is initiated and spatiotemporally regulated during macropinocytosis is not well understood. By using the model system Dictyostelium and a proteomics-based approach to identify regulators of macropinocytosis, we uncovered Leep2, consisting of Leep2A and Leep2B, as a RasGAP complex. The Leep2 complex specifically localizes to emerging macropinocytic cups and nascent macropinosomes, where it modulates macropinosome formation by regulating the activities of three Ras family small GTPases. Deletion or overexpression of the complex, as well as disruption or sustained activation of the target Ras GTPases, impairs macropinocytic activity. Our data reveal the critical role of fine-tuning Ras activity in directing macropinosome formation.

Oligopeptide transporter Slc15A modulates macropinocytosis in Dictyostelium by maintaining intracellular nutrient status

Macropinocytosis mediates non-selective bulk uptake of extracellular fluid. It is the major route by which axenic Dictyostelium cells obtain nutrients and has emerged as a nutrient-scavenging pathway for mammalian cells. How environmental and cellular nutrient status modulates macropinocytic activity is not well understood. By developing a high-content imaging-based genetic screen in Dictyostelium, we identified Slc15A, an oligopeptide transporter localized at the plasma membrane and early macropinosome, as a novel macropinocytosis regulator. We show that deletion of slc15A, but not two other related slc15 genes, leads to reduced macropinocytosis, slower cell growth, and aberrantly increased autophagy in cells grown in nutrient-rich medium. Expression of Slc15A or supplying cells with free amino acids rescues these defects. In contrast, expression of transport-defective Slc15A or supplying cells with amino acids in their di-peptide forms fails to rescue these defects. Therefore, Slc15A modulates the level of macropinocytosis by maintaining the intracellular availability of key amino acids via oligopeptide extraction from the early macropinocytic pathway. We propose that Slc15A constitutes part of a positive feedback mechanism coupling cellular nutrient status and macropinocytosis.

The Amoebal Model for Macropinocytosis

Macropinocytosis is a relatively unexplored form of large-scale endocytosis driven by the actin cytoskeleton. Dictyostelium amoebae form macropinosomes from cups extended from the plasma membrane, then digest their contents and absorb the nutrients in the endo-lysosomal system. They use macropinocytosis for feeding, maintaining a high rate of fluid uptake that makes assay and experimentation easy. Mutants collected over the years identify cytoskeletal and signalling proteins required for macropinocytosis. Cups are organized around plasma membrane domains of intense PIP3, Ras and Rac signalling, proper formation of which also depends on the RasGAPs NF1 and RGBARG, PTEN, the PIP3-regulated protein kinases Akt and SGK and their activators PDK1 and TORC2, Rho proteins, plus other components yet to be identified. This PIP3 domain directs dendritic actin polymerization to the extending lip of macropinocytic cups by recruiting a ring of the SCAR/WAVE complex around itself and thus activating the Arp2/3 complex. The dynamics of PIP3 domains are proposed to shape macropinocytic cups from start to finish. The role of the Ras-PI3-kinase module in organizing feeding structures in unicellular organisms most likely predates its adoption into growth factor signalling, suggesting an evolutionary origin for growth factor signalling.

The pervasiveness of macropinocytosis in oncological malignancies

In tumour cells, macropinocytosis functions as an amino acid supply route and supports cancer cell survival and proliferation. Initially demonstrated in oncogenic KRAS-driven models of pancreatic cancer, macropinocytosis triggers the internalization of extracellular proteins via discrete endocytic vesicles called macropinosomes. The incoming protein cargo is targeted for lysosome-dependent degradation, causing the intracellular release of amino acids. These protein-derived amino acids support metabolic fitness by contributing to the intracellular amino acid pools, as well as to the biosynthesis of central carbon metabolites. In this way, macropinocytosis represents a novel amino acid supply route that tumour cells use to survive the nutrient-poor conditions of the tumour microenvironment. Macropinocytosis has also emerged as an entry mechanism for a variety of nanomedicines, suggesting that macropinocytosis regulation in the tumour setting can be harnessed for the delivery of anti-cancer therapeutics. A slew of recent studies point to the possibility that macropinocytosis is a pervasive feature of many different tumour types. In this review, we focus on the role of this important uptake mechanism in a variety of cancers and highlight the main molecular drivers of macropinocytosis in these malignancies. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Neurofibromin controls macropinocytosis and phagocytosis in Dictyostelium

Cells use phagocytosis and macropinocytosis to internalise bulk material, which in phagotrophic organisms supplies the nutrients necessary for growth. Wildtype Dictyostelium amoebae feed on bacteria, but for decades laboratory work has relied on axenic mutants that can also grow on liquid media. We used forward genetics to identify the causative gene underlying this phenotype. This gene encodes the RasGAP Neurofibromin (NF1). Loss of NF1 enables axenic growth by increasing fluid uptake. Mutants form outsized macropinosomes which are promoted by greater Ras and PI3K activity at sites of endocytosis. Relatedly, NF1 mutants can ingest larger-than-normal particles using phagocytosis. An NF1 reporter is recruited to nascent macropinosomes, suggesting that NF1 limits their size by locally inhibiting Ras signalling. Our results link NF1 with macropinocytosis and phagocytosis for the first time, and we propose that NF1 evolved in early phagotrophs to spatially modulate Ras activity, thereby constraining and shaping their feeding structures.

DOI:http://dx.doi.org/10.7554/eLife.04940.001

Dictyostelium amoebae are microbes that feed on bacteria living in the soil. They are unusual in that the amoebae can survive and grow in a single-celled form, but when food is scarce, many individual cells can gather together to form a simple multicellular organism.

To feed on bacteria, the amoebae use a process called phagocytosis, which starts with the membrane that surrounds the cell growing outwards to completely surround the bacteria. This leads to the bacteria entering the amoeba within a membrane compartment called a vesicle, where they are broken down into small molecules by enzymes. The cells can also take up fluids and dissolved molecules using a similar process called macropinocytosis.

With its short and relatively simple lifestyle, Dictyostelium is often used in research to study phagocytosis, cell movement and other processes that are also found in larger organisms. For example, some immune cells in animals use phagocytosis to capture and destroy invading microbes. Most studies using Dictyostelium as a model have used amoebae with genetic mutations that allow them to be grown in liquid cultures in the laboratory without needing to feed on bacteria. The mutations allow the ‘mutant’ amoebae to take up more liquid and dissolved nutrients by macropinocytosis, but it is not known where in the genome these mutations are.

Here, Bloomfield et al. used genome sequencing to reveal that these mutations alter a gene that encodes a protein called Neurofibromin. The experiments show that the loss of Neurofibromin increases the amount of fluid taken up by the amoebae through macropinocytosis, and also enables the amoebae to take up larger-than-normal particles during phagocytosis.

The experiments suggest that Neurofibromin controls both phagocytosis and macropinocytosis by inhibiting the activity of another protein called Ras. Neurofibromin is found in animals and many other organisms so Bloomfield et al. propose that it is an ancient protein that evolved in early single-celled organisms to control the size and shape of their feeding structures.

In humans, mutations in the gene that encodes the Neurofibromin protein can lead to the development of a severe disorder—called Neurofibromatosis type 1—in which tumours form in the nervous system. Given that tumour cells can use phagocytosis and macropinocytosis to gain nutrients as they grow, understanding how this protein works in the Dictyostelium amoebae may help to inform future efforts to develop treatments for this human disease.

DOI:http://dx.doi.org/10.7554/eLife.04940.002

Delivery of CSF-1R to the lumen of macropinosomes promotes its destruction in macrophages

Activation of the macrophage colony stimulating factor-1 receptor (CSF-1R) by CSF-1 stimulates pronounced macropinocytosis and drives proliferation of macrophages. Although the role of macropinocytosis in CSF-1R signaling remains unknown, we show here that, despite internalizing large quantities of plasma membrane, macropinosomes contribute little to the internalization of the CSF-1-CSF-1R complex. Rather, internalization of the CSF-1R in small endocytic vesicles that are sensitive to clathrin disruption, outcompetes macropinosomes for CSF-1R endocytosis. Following internalization, small vesicles carrying the CSF-1R underwent homotypic fusion and then trafficked to newly formed macropinosomes bearing Rab5. As these macropinosomes matured, acquiring Rab7, the CSF-1R was transported into their lumen and degraded. Inhibition of macropinocytosis delayed receptor degradation despite no disruption to CSF-1R endocytosis. These data indicate that CSF-1-stimulated macropinosomes are sites of multivesicular body formation and accelerate CSF-1R degradation. Furthermore, we demonstrate that macropinocytosis and cell growth have a matching dose dependence on CSF-1, suggesting that macropinosomes might be a central mechanism coupling CSF-1R signaling and macrophage growth.

RasG signaling is important for optimal folate chemotaxis in Dictyostelium

BACKGROUND

Signaling pathways linking receptor activation to actin reorganization and pseudopod dynamics during chemotaxis are arranged in complex networks. Dictyostelium discoideum has proven to be an excellent model system for studying these networks and a body of evidence has indicated that RasG and RasC, members of the Ras GTPase subfamily function as key chemotaxis regulators. However, recent evidence has been presented indicating that Ras signaling is not important for Dictyostelium chemotaxis. In this study, we have reexamined the role of Ras proteins in folate chemotaxis and then, having re-established the importance of Ras for this process, identified the parts of the RasG protein molecule that are involved.

RESULTS

A direct comparison of folate chemotaxis methodologies revealed that rasG-C- cells grown in association with a bacterial food source were capable of positive chemotaxis, only when their initial position was comparatively close to the folate source. In contrast, cells grown in axenic medium orientate randomly regardless of their distance to the micropipette. Folate chemotaxis is restored in rasG-C- cells by exogenous expression of protein chimeras containing either N- or C- terminal halves of the RasG protein.

CONCLUSIONS

Conflicting data regarding the importance of Ras to Dictyostelium chemotaxis were the result of differing experimental methodologies. Both axenic and bacterially grown cells require RasG for optimal folate chemotaxis, particularly in weak gradients. In strong gradients, the requirement for RasG is relaxed, but only in bacterially grown cells. Both N- and C- terminal portions of the RasG protein are important for folate chemotaxis, suggesting that there are functionally important amino acids outside the well established switch I and switch II interaction surfaces.

PIP₃-dependent macropinocytosis is incompatible with chemotaxis

In eukaryotic chemotaxis, the mechanisms connecting external signals to the motile apparatus remain unclear. The role of the lipid phosphatidylinositol 3,4,5-trisphosphate (PIP₃) has been particularly controversial. PIP₃ has many cellular roles, notably in growth control and macropinocytosis as well as cell motility. Here we show that PIP₃ is not only unnecessary for Dictyostelium discoideum to migrate toward folate, but actively inhibits chemotaxis. We find that macropinosomes, but not pseudopods, in growing cells are dependent on PIP₃. PIP₃ patches in these cells show no directional bias, and overall only PIP₃-free pseudopods orient up-gradient. The pseudopod driver suppressor of cAR mutations (SCAR)/WASP and verprolin homologue (WAVE) is not recruited to the center of PIP₃ patches, just the edges, where it causes macropinosome formation. Wild-type cells, unlike the widely used axenic mutants, show little macropinocytosis and few large PIP₃ patches, but migrate more efficiently toward folate. Tellingly, folate chemotaxis in axenic cells is rescued by knocking out phosphatidylinositide 3-kinases (PI 3-kinases). Thus PIP₃ promotes macropinocytosis and interferes with pseudopod orientation during chemotaxis of growing cells.

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.

Cell motility and SCAR localisation in axenically growing Dictyostelium cells

Dictyostelium is a popular experimental organism, in particular for studies of actin dynamics, cell motility and chemotaxis. We find that the motility of axenic cells is unexpectedly different from other strains during growth. In particular, vegetative AX3 cells do not show detectable localisation of SCAR and its regulatory complex to actin-rich protrusions such as filopodia and pseudopodia. Similarly, a range of different mutations, in particular knockouts of members of the SCAR complex and Ras proteins, cause different phenotypes during vegetative growth in different parental strains. Development reverses this unusual behaviour; aggregation-competent AX3 cells localise SCAR in the same way as cells of other strains and species. Studies on cell motility using vegetative cells should therefore be interpreted with caution.

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.

Origin, originality, functions, subversions and molecular signalling of macropinocytosis

Macropinocytosis refers to the formation of primary large endocytic vesicles of irregular size and shape, generated by actin-driven evaginations of the plasma membrane, whereby cells avidly incorporate extracellular fluid. Macropinosomes resemble "empty" phagosomes and show no difference with the "spacious phagosomes" triggered by the enteropathogenic bacteria Salmonella and Shigella. Macropinosomes may fuse with lysosomes or regurgitate their content back to the extracellular space. In multiple cell types, macropinocytosis is a transient response to growth factors. When amoebas are cultured under axenic conditions, macropinocytosis is induced so as to fulfil nutritional requirements. In immature dendritic cells, macropinocytosis allows for extensive sampling of soluble antigens; after a few days of maturation, this activity vanishes as processed peptides are being presented. Macropinosomes are also formed at the leading edge of motile leukocytes or neurons. In all these examples, macropinocytosis appears tightly regulated. Transformation of fibroblasts by Src or Ras also results in constitutive formation of macropinosomes at "ruffling" zones, that could be related to accelerated cell motility. Like phagocytosis, macropinocytosis depends on signalling to the actin cytoskeleton. We have explored this signalling in transformed cells. v-Src and K-Ras activate PI3K and PLC, as demonstrated by in situ production of the corresponding lipid products. Pharmacological inhibitors of PI3K and PLC and stable transfection leading to a dominant-negative PI3-kinase construct in transformed fibroblasts abolish macropinocytosis, demonstrating that both enzyme activities are essential. Conversely, stable transfection leading to a dominant-positive P13K in non-transformed fibroblasts is sufficient to induce macropinocytosis. Combination of experiments allows to conclude that P13K and PLC act in sequential order. In non-polarized cells expressing a thermosensitive v-Src mutant, v-Src kinase activation accelerates fluid-phase endocytosis. In polarized MDCK cells, this stimulation occurs selectively at the apical domain and the response is selectively abrogated by pharmacological inhibitors of P13K and PLC. Thus, two paradigmatic oncogenes cause constitutive macropinocytosis. For v-Src, this response is polarized at the apical membrane. It is suggested that, in enterocytes that do not normally phagocytose, the P13K-PLC signalling pathway leading to selective induction of macropinocytosis at the luminal surface has been subverted by enteropathogenic bacteria to penetrate via "spacious phagosomes".

Phagocytosis and macropinocytosis in Dictyostelium: phosphoinositide-based processes, biochemically distinct

Phagocytosis and macropinocytosis are actin-dependent clathrin-independent processes primarily performed by cells like neutrophils and macrophages that result in the internalization of particles or the formation of fluid-filled macropinosomes, respectively. Phagocytosis consists of a number of stages, including attachment of particles to cell surface receptors, engulfment of the particle dependent on actin polymerization and membrane exocytosis, and formation of phago-lysosomes. In contrast, the molecular steps regulating macropinocytosis are only just now being deciphered. Much remains to be learned concerning the signaling pathways that regulate these processes. Dictyostelium is a genetically and biochemically tractable professional phagocyte that has proven to be a powerful system with which to determine the nature of the molecular steps involved in regulating these internalization processes. This review summarizes what is currently understood concerning the molecular mechanisms governing phagocytosis and macropinocytosis in Dictyostelium and describes recent data concerning the common and distinct pathways that regulate these processes.

Fluid-phase uptake and transit in axenic Dictyostelium cells

The main route for fluid-phase uptake in Dictyostelium is macropinocytosis, a process powered by the actin cytoskeleton. Nutrients within the endocytosed fluid are digested and resorbed, disposal of remnants follows by exocytosis. Along the endocytic pathway, membrane fusion and fission events take place at multiple steps. The regulator and effector molecules involved in uptake and transit are largely conserved between higher and lower eukaryotes. This feature, together with its accessibility by molecular genetics, recommend Dictyostelium as a valuable model system for mammalian cells.

Regulation of phagocytosis and endo-phagosomal trafficking pathways in Dictyostelium discoideum

Phagocytosis, a critically important process employed by leukocytes against invading pathogens, is an actin-dependent clathrin-independent process that results in the internalization of particles >0.5 microm in diameter. Phagocytosis consists of a number of stages, including the binding of particles to the cell surface via interaction with a receptor, engulfment of the particle by pseudopod extension, and fission and fusion reactions to form phago-lysosomes. Much remains to be learned concerning the molecular mechanisms that regulate particle internalization and phagosome maturation. Dictyostelium is a genetically tractable professional phagocyte that has proven useful in determining the molecular steps involved in these processes. We will summarize, in this chapter, what we currently understand concerning the molecular mechanisms that regulate the process of phagocytosis in Dictyostelium, and we will compare and contrast this body of information with that available describing phagocytosis in higher organisms. We will also present current information that suggests that macropinocytosis, a process morphologically similar to phagocytosis, utilizes a different signaling pathway than phagocytosis. Finally, we will discuss the process of maturation of phagosomes, which requires membrane trafficking events, and we will summarize data that support the use of Dictyostelium as a model to determine how intracellular pathogens survive.

Small GTPases in Dictyostelium: lessons from a social amoeba

Although the process of sequencing the Dictyostelium genome is not complete, it is already producing surprises, including an unexpectedly large number of Ras- and Rho-subfamily GTPases. Members of these families control a wide variety of cellular processes in eukaryotes, including proliferation, differentiation, cell motility and cell polarity. Comparison of small GTPases from Dictyostelium with those from higher eukaryotes provides an intriguing view of their cellular and evolutionary roles. In particular, although mammalian Ras proteins interact with several signalling pathways, the Dictyostelium pathways appear more linear, with each Ras apparently performing a specific cellular function.

Real-time high-resolution optical sectioning suggests biphasic cytokinetic mechanism in Dictyostelium discoideum

Despite its biological significance, much of the mechanism of cytokinesis is not yet resolved. The problems include: (1) signaling mechanism determining the position of the cleavage furrow, (2) molecular and mechanistic nature of the contractile ring, and (3) the origin of forces responsible for cleavage. Using high-resolution imaging technique, the present study analyzes morphometric changes of cytokinesis in wild type (NC4) Dictyostelium discoideum amoeba. A sample was prepared by the agar-overlay method, creating 3-mm-thick, nearly two-dimensional cells; and high-resolution image was acquired at 16.7 milliseconds' temporal, 234 nm x, y-, and 100 nm z-axis resolutions. Under this condition, the formation of cleavage furrow initiates at mitotic telophase, and daughter cells separate 18-22 minutes after the furrow initiation. We found that the compression of cells and the room temperature need to be carefully controlled for cytokinesis to proceed in an orderly manner. The results demonstrate that the pole-to-pole distance increases by 83% during the initial 5 minutes of cytokinesis, while the distance of equator only decreases by 56%. In contrast, during the subsequent 5 minutes, the pole-to-pole distance only increases by 17%, while the equator distance decreases as much as by 44%. This study indicates that cytokinesis consists of at least two different phases, each of which results from a different mechanism.

Characterization of Dictyostelium discoideum cathepsin D

Previous studies using magnetic purification of Dictyostelium discoideum endocytic vesicles led us to the identification of some major vesicle proteins. Using the same purification procedure, we have now focused our interest on a 44 kDa soluble vesicle protein. Microsequencing of internal peptides and subsequent cloning of the corresponding cDNA identified this protein as the Dictyostelium homolog of mammalian cathepsins D. The only glycosylation detected on Dictyostelium cathepsin D (CatD) is common antigen 1, a cluster of mannose 6-sulfate residues on N-linked oligosaccharide chains. CatD intracellular trafficking has been studied, showing the presence of the protein throughout the entire endocytic pathway. During the differentiation process, the catD gene presents a developmental regulation, which is also observed at the protein level. catD gene disruption does not alter significantly the cell behaviour, either in the vegetative form or the differentiation stage. However, modifications in the SDS-PAGE profiles of proteins bearing common antigen 1 were detected, when comparing parental and catD(-) cells. These modifications point to a possible role of CatD in the maturation of a few Dictyostelium lysosomal proteins.

Cytoskeletal association of an esterase in Dictyostelium discoideum

A 70-kDa glycoprotein, gp70, was found enriched in the detergent-insoluble cytoskeletal fraction of axenically grown Dictyostelium discoideum cells. Its N-terminal amino acid sequence identified it as 'crystal protein' (L. Bomblies et al., 1990, J. Cell Biol. 110, 669-679). This finding was corroborated when antibody to crystal protein cross-reacted with gp70 and its deglycosylated form. The postulated esterase activity of gp70/crystal protein was verified through comparative enzyme assays of extracts derived from cells that either overexpressed or lacked gp70. Gp70 cosedimented with cytoskeletons on sucrose gradients, suggesting an interaction with the cytoskeleton. Coisolation of gp70 with detergent-extracted cells, observed by immunofluorescence microscopy, also implied a gp70-cytoskeletal association. These data supported the idea that the localization or secretion of gp70, or both, was cytoskeletally mediated. Although axenically grown cells contained high levels of gp70, the same cell lines had reduced levels of gp70 when grown in bacterial suspension or in nutrient media containing bacteria. Bacterially grown cells, compared to axenically grown cells, had lower fluid-phase uptake rates even when nutrient media was present, indicating that phagocytosis was a preferred mode of feeding. Thus, bacteria inhibited gp70 expression, which suggested a role for prestarvation factor, in regulating its synthesis.

A new collection of thermosensitive endocytosis mutants in the cellular slime mold Dictyostelium discoideum

We used a photoactivatable fluid-phase marker to isolate a new collection of thermosensitive endocytosis mutants in the cellular slime mold Dictyostelium discoideum. All the strains were thermosensitive for growth on bacteria or axenic medium at 27 degrees C. Initial rates of endocytosis rapidly decreased upon incubation at the restrictive temperature, but surprisingly most of the strains showed a transient recovery of activity with prolonged exposure to 27 degrees C. Endocytosis and exocytosis activities were uncoupled for some of the cell lines at 27 degrees C whereas the others had to be shifted to 29 degrees C. Further molecular analysis of these mutants could lead to the discovery of new proteins involved in endocytosis and its regulation.

Inhibition of growth of Dictyostelium discoideum amoebae by bisphosphonate drugs is dependent on cellular uptake

PURPOSE

The aim of the study was to determine whether bisphosphonates are internalised by Dictyostelium amoebae and whether cellular uptake is required for their growth-inhibitory effects. Bisphosphonates inhibit growth of amoebae of the slime mould Dictyostelium discoideum, by mechanisms that appear to be similar to those that cause inhibition of osteoclastic bone resorption.

METHODS

Cell-free extracts prepared from amoebae that had been incubated with bisphosphonates were analysed by 31P-n.m.r, spectroscopy or ion-exchange f.p.l.c., to identify the presence of bisphosphonates or bisphosphonate metabolites respectively. The growth-inhibitory effect of bisphosphonates towards Dictyostelium amoebae was also examined under conditions in which pinocytosis was inhibited.

RESULTS

All of the bisphosphonates studied were internalised by Dictyostelium amoebae, probably by fluid-phase pinocytosis, and could be detected in cell-free extracts. Amoebae that were prevented from internalising bisphosphonates by pinocytosis were markedly resistant to the growth-inhibitory effects of these compounds. In addition, bisphosphonates encapsulated within liposomes were more potent growth inhibitors of Dictyostelium owing to enhanced intracellular delivery of bisphosphonates.

CONCLUSIONS

All bisphosphonates inhibit Dictyostelium growth by intracellular mechanisms following internalisation of bisphosphonates by fluid-phase pinocytosis. It is therefore likely that bisphosphonates also affect osteoclasts by interacting with intracellular, rather than extracellular, processes.

Coronin and vacuolin identify consecutive stages of a late, actin-coated endocytic compartment in Dictyostelium

Cells of the unicellular eukaryote Dictyostelium discoideum take up all their nutrients by endocytosis. Both particle- and fluid-containing vacuoles are transiently surrounded by a cytoskeletal coat [1] [2]. When this coat has dissociated, acidification and digestion of the vesicle contents occur, followed by exocytosis of the indigestible remnants after 60-90 minutes. At least nine compartments are needed for mathematical modelling of endocytic transit [3], suggesting that markers associate for only a few minutes with a specific endocytic compartment. Among the proteins that have been identified as components of endocytic vesicles are actin, subunits of the V-H+ ATPase and small GTP-binding proteins of the Rab family [4] [5] [6] [7]. Using a monoclonal antibody produced against Dictyostelium endocytic vesicles, we have isolated a cDNA corresponding to a novel protein that we have named vacuolin. In order to determine the precise step along the endocytic pathway that involves vacuolin, we generated a fusion protein of the green fluorescent protein (GFP) and vacuolin. GFP-vacuolin-decorated vesicles were identified as a post-lysosomal compartment that acquires endocytic markers shortly before exocytosis. At earlier stages, this post-lysosomal compartment was identified by the binding of a tagged cytoskeletal protein, coronin-GFP. Vacuoles were coated with filamentous actin along the entire post-lysosomal pathway, and the integrity of the actin coat was required for exocytosis.

Fluid-phase uptake by macropinocytosis in Dictyostelium

To study fluid-phase endocytosis in living cells and its relationship to changes in the cell cortex, we have used a green fluorescent protein (GFP)-tagged version of coronin, an actin-associated protein that localises to dynamic regions of the Dictyostelium cell cortex. In the confocal microscope, internalisation of fluorescently labelled dextran as a fluid-phase marker can be recorded simultaneously with the recruitment of the coronin-GFP fusion-protein from the cytoplasm of the phagocyte. At crown-shaped surface protrusions, extracellular medium is taken up into vesicles with an average diameter of 1.6 microns, which is significantly larger than the 0.1 microns diameter of clathrin-coated pinosomes. The observed frequency of macropinosome formation can account for a large portion, if not all, of the fluid-phase uptake. The redistribution of coronin-GFP strongly resembles cytoskeletal rearrangements during phagocytosis. Scanning-electron micrographs indicate that crown-shaped cell-surface extensions can undergo shape changes, without a particle bound, that are similar to shape changes that occur during phagocytosis. In quantitative assays, the uptake of particles and fluid are about equally dependent on F-actin and coronin.

Genetic and physiologic modulation of the prestarvation response in Dictyostelium discoideum

Throughout vegetative growth, Dictyostelium amoebae secrete an autocrine factor, prestarvation factor, PSF, which accumulates in proportion to cell density. During late exponential growth, PSF induces the expression of several genes whose products are needed for cAMP signaling and cell aggregation. Among these genes are discoidin-I and the 2.4-kb transcript of cyclic nucleotide phosphodiesterase (PDE). We have identified several parameters that modulate expression of one or both of these prestarvation response genes; all effects were monitored in cells growing exponentially on bacteria. Under these conditions, axenic mutants produce higher levels of PSF activity than wild-type cells. Consistent with the high PSF levels, the 2.4-kb PDE transcript is more abundant in axenic strains than wild-type cells at the same cell density. In contrast, the density-dependent induction of discoidin-I is greatly delayed in axenic strains, occurring only at the very end of exponential growth. Analysis of axenic strains of independent origin suggested that this negative effect on discoidin-I expression is attributable to the axenic mutations themselves. The effects of two environmental factors that inhibit the prestarvation response (the bacteria upon which the cells feed and a bacterial product, folic acid) were also analyzed. We found that folate does not account for the inhibitory effect of bacteria. Cells deficient in the G-protein beta subunit, which is thought to be common to all heterotrimeric G-proteins in Dictyostelium, respond to PSF in the same manner as G beta+ cells, and this response is inhibited by bacteria. However, folate has no inhibitory effect on g beta- cells, indicating that folate inhibition is mediated by a heterotrimeric G-protein. In cells lacking the catalytic subunit of protein kinase A, the prestarvation response is severely impaired, but about 3% of the pka- cells manifest an apparently normal density-dependent induction of discoidin-I. This behavior and the heterogeneity of the prestarvation response in wild-type cells lead us to speculate that protein kinase A may not be required for PSF signal transduction per se, but rather may render the cells responsive to PSF. Based on analysis of adenylyl cyclase mutants (aca-), the effect of protein kinase A is not cAMP-dependent.

A natural-abundance 13C-NMR study of Dictyostelium discoideum metabolism. Monitoring of the spore germination process

Amoebae and spores of the cellular slime mold Dictyostelium discoideum have been investigated by natural-abundance proton-decoupled 13C-NMR spectroscopy. Axenically grown vegetative amoebae have been found to contain, as prominent metabolites, the polyamines 1,3-diaminopropane (3.2 mM), putrescine (9.4 mM) and spermidine (1.7 mM). We also detected lactic acid (4.4 mM) and the following amino acids as free metabolites in concentrations ranging over 1-3 mM: glycine, alanine, glutamine and glutamate. The glycogen level is highly dependent upon growth state, being below the level of NMR detection in early-exponential cells and reaching about 110 mM glucose equivalents in plateau-phase cells. Dormant spores contained high amounts of trehalose (50 mM), glutamine (73 mM) and glutamate (20 mM). The latter two compounds were not reported previously to be present in such high concentrations in Dictyostelium spores. Germination induced by heat-shock activation was monitored by 13C NMR. No change in the major components occurred during the activation step. The progressive disappearance of trehalose during germination correlated with the decrease of glutamine and glutamate. In general, the data suggest that germinated spores contain a composition of free metabolites very similar to that of starved vegetative amoebae.