A syntaxin 7 homologue is present in Dictyostelium discoideum endosomes and controls their homotypic fusion

The Polarized Redistribution of the Contractile Vacuole to the Rear of the Cell is Critical for Streaming and is Regulated by PI(4,5)P2-Mediated Exocytosis

Dictyostelium discoideum amoebae align in a head to tail manner during the process of streaming during fruiting body formation. The chemoattractant cAMP is the chemoattractant regulating cell migration during this process and is released from the rear of cells. The process by which this cAMP release occurs has eluded investigators for many decades, but new findings suggest that this release can occur through expulsion during contractile vacuole (CV) ejection. The CV is an organelle that performs several functions inside the cell including the regulation of osmolarity, and discharges its content via exocytosis. The CV localizes to the rear of the cell and appears to be part of the polarity network, with the localization under the influence of the plasma membrane (PM) lipids, including the phosphoinositides (PIs), among those is PI(4,5)P2, the most abundant PI on the PM. Research on D. discoideum and neutrophils have shown that PI(4,5)P2 is enriched at the rear of migrating cells. In several systems, it has been shown that the essential regulator of exocytosis is through the exocyst complex, mediated in part by PI(4,5)P2-binding. This review features the role of the CV complex in D. discoideum signaling with a focus on the role of PI(4,5)P2 in regulating CV exocytosis and localization. Many of the regulators of these processes are conserved during evolution, so the mechanisms controlling exocytosis and membrane trafficking in D. discoideum and mammalian cells will be discussed, highlighting their important functions in membrane trafficking and signaling in health and disease.

Evolution and Natural History of Membrane Trafficking in Eukaryotes

The membrane-trafficking system is a defining facet of eukaryotic cells. The best-known organelles and major protein families of this system are largely conserved across the vast diversity of eukaryotes, implying both ancient organization and functional unity. Nonetheless, intriguing variation exists that speaks to the evolutionary forces that have shaped the endomembrane system in eukaryotes and highlights ways in which membrane trafficking in protists differs from that in our well-understood models of mammalian and yeast cells. Both parasites and free-living protists possess specialized trafficking organelles, some lineage specific, others more widely distributed - the evolution and function of these organelles begs exploration. Novel members of protein families are present across eukaryotes but have been lost in humans. These proteins may well hold clues to understanding differences in cellular function in organisms that are of pressing importance for planetary health.

The key differentially expressed genes and proteins related to immune response in the spleen of pufferfish (Takifugu obscurus) infected by Aeromonas hydrophila

The immune mechanism elicited in pufferfish (Takifugu obscurus) against the invasion of Aeromonas hydrophila is still poorly understood. We examined the spleen of pufferfish at the transcriptome and proteome levels by using Illumina-seq and TMT coupled mass spectrometry after 12 h infection by A. hydrophila, respectively. A total of 2,339 genes (1,512 up-regulated and 827 down-regulated) and 537 (237 up-regulated and 300 down-regulated) proteins were identified. GO and KEGG analyses revealed that the responses to stimulus were the main biological processes, intestinal immune network for IgT production and calcium signaling pathway. Fourteen genes (8 up-regulated and 6 down-regulated) and proteins (5 up-regulated and 9 down-regulated) involved immune responses or signal transduction were validated by qRT-PCR and parallel reaction monitoring to confirm the reliability of the transcriptomic and proteomic analyses, respectively. Moreover, qRT-PCR and flow cytometry were used to detect dynamics of the genes in calcium signaling pathway and changes of concentration of cytoplasm Ca²⁺ in spleen cells within a 72 h challenge. This study provides the findings regarding immune response, especially intestinal immune network for IgT production pathway and calcium signaling pathway at the molecular, protein and cellular in pufferfish after infection by A. hydrophila. These results would provide a new insight and molecular targets into the response to pathogenic infection in pufferfish.

Dictyostelium as a model for human lysosomal and trafficking diseases

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

Evolution of specificity in the eukaryotic endomembrane system

Two hundred years after Darwin's birth, our understanding of genetic mechanisms and cell biology has advanced to a level unimaginable in the 19th century. We now know that eukaryotic cells contain a huge variety of internal compartments, each with their own function, identity and history. For the compartments that together form the membrane-trafficking system, one of the central questions is how that identity is encoded and how it evolved. Here we review the key components involved in membrane-trafficking events, including SNAREs, Rabs, vesicle coats, and tethers and what is known about their evolutionary history. Our current understanding suggests a possible common mechanism by which the membrane-trafficking organelles might have evolved. This model of increased organellar complexity by gene duplication and co-evolution of multiple, interacting, specificity-encoding proteins could well be applicable to other non-endosymbiotic organelles as well. The application of basic evolutionary principles well beyond their original scope has been exceedingly powerful not only in reconstructing the history of cellular compartments, but for medical and applied research as well, and underlines the contributions of Darwin's ideas in modern biology.

Rab5-mediated endosome-endosome fusion regulates hemoglobin endocytosis in Leishmania donovani

To understand the trafficking of endocytosed hemoglobin (Hb) in Leishmania, we investigated the characteristics of in vitro fusion between endosomes containing biotinylated Hb (BHb) and avidin-horseradish peroxidase (AHRP). We showed that early endosome fusion in Leishmania is temperature and cytosol dependent and is inhibited by ATP depletion, ATPgammaS, GTPgammaS and N-ethylmaleimide treatment. The Rab5 homolog from Leishmania donovani, LdRab5, was cloned and expressed. Our results showed that homotypic fusion between the early endosomes in Leishmania is Rab5 dependent. Early endosomes containing BHb fused efficiently with late endosomes in a process regulated by Rab7, whereas no fusion between early and late endosomes was detected using fluid phase markers. Pre-treatment of early endosomes containing BHb with monoclonal antibody specific for the C-terminus of the Hb receptor (HbR) or the addition of the C-terminal cytoplasmic fragment of the HbR specifically inhibited the fusion with late endosomes, suggesting that signal(s) mediated through the HbR cytoplasmic tail promotes the fusion of early endosomes containing Hb with late endosomes.

The regulation of phagosome maturation in Dictyostelium

Macropinocytosis (fluid uptake) and phagocytosis (particle uptake) are processes that result in the formation of intracellular membrane enclosed vacuoles termed macropinosomes and phagosomes, respectively. Macropinosomes and phagosomes are modified by fission and fusion reactions with the endo-lysosomal pathway that eventually transform these vacuoles into a lysosomal environment. Many human bacterial pathogens, including species of Mycobacteria, Legionella, and Chlamydia, are thought to survive by disrupting the normal membrane trafficking events that usually result in the formation of phago-lysosomes and death of the microorganism. In addition, a number of important pathogens facilitate homotypic phagosome fusion in order to generate an intracellular environment conducive for survival. A greater understanding of the regulation of phagosomal maturation and fusion will be critical in designing new therapies to treat infections caused by intracellular pathogens. The genetically tractable phagocyte, D. discoideum, has proven extremely useful in dissecting the signaling pathways regulating macropinocytosis, phagocytosis, phagosomal maturation and phagosome-phagosome fusion. A body of knowledge has accumulated and demonstrates important roles for Rab GTPases, the cytoskeleton, phosphoinositide metabolism and pH regulation in regulating phagosome maturation. This review will summarize the current state of knowledge.

Conserved features of endocytosis in Dictyostelium

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

The AP-1 clathrin-adaptor is required for lysosomal enzymes sorting and biogenesis of the contractile vacuole complex in Dictyostelium cells

Adaptor protein complexes (AP) are major components of the cytoplasmic coat found on clathrin-coated vesicles. Here, we report the molecular and functional characterization of Dictyostelium clathrin-associated AP-1 complex, which in mammalian cells, participates mainly in budding of clathrin-coated vesicles from the trans-Golgi network (TGN). The gamma-adaptin AP-1 subunit was cloned and shown to belong to a Golgi-localized 300-kDa protein complex. Time-lapse analysis of cells expressing gamma-adaptin tagged with the green-fluorescent protein demonstrates the dynamics of AP-1-coated structures leaving the Golgi apparatus and rarely moving toward the TGN. Targeted disruption of the AP-1 medium chain results in viable cells displaying a severe growth defect and a delayed developmental cycle compared with parental cells. Lysosomal enzymes are constitutively secreted as precursors, suggesting that protein transport between the TGN and lysosomes is defective. Although endocytic protein markers are correctly localized to endosomal compartments, morphological and ultrastructural studies reveal the absence of large endosomal vacuoles and an increased number of small vacuoles. In addition, the function of the contractile vacuole complex (CV), an osmoregulatory organelle is impaired and some CV components are not correctly targeted.

The role of BEACH proteins in Dictyostelium

The BEACH family of proteins is a novel group of proteins with diverse roles in eukaryotic cells. The identifying feature of these proteins is the BEACH domain named after the founding members of this family, the mouse beige and the human Chediak-Higashi syndrome proteins. Although all BEACH proteins share a similar structural organization, they appear to have very distinct cellular roles, ranging from lysosomal traffic to apoptosis and cytokinesis. Very little is currently known about the function of most of these proteins, few binding-partner proteins have been identified, and no molecular mechanism for any of these proteins has been discovered. Thus, it is important to establish good model systems for the study of these novel proteins. Dictyostelium contains six BEACH proteins that can be classified into four subclasses. Two of them, LvsA and LvsB, have clearly distinct roles in the cell. LvsA is localized on the contractile vacuole membrane and is essential for cytokinesis and osmoregulation. LvsB is most similar in sequence to the mammalian beige/Chediak-Higashi syndrome proteins and shares with them a common function in lysosomal trafficking. Structural and functional analysis of these proteins in Dictyostelium will help elucidate the function of this enigmatic novel family of proteins.

Fusion and fission events in the endocytic pathway of Dictyostelium

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

Syntaxin 7, syntaxin 8, Vti1 and VAMP7 (vesicle-associated membrane protein 7) form an active SNARE complex for early macropinocytic compartment fusion in Dictyostelium discoideum

The macropinocytic pathway in Dictyostelium discoideum is organized linearly. After actin-driven internalization, fluid material passes sequentially from endosomes to lysosomes, where molecules are degraded and absorbed. Residual material is exocytosed via post-lysosomal compartments. Syntaxin 7 is a SNARE (soluble N -ethylmaleimide-sensitive fusion protein attachment protein receptor) protein that is present and active in D. discoideum endosomes [Bogdanovic, Bruckert, Morio and Satre (2000) J. Biol. Chem. 275, 36691-36697]. Here we report the identification of its main SNARE partners by co-immunoprecipitation and MS peptide sequencing. The syntaxin 7 complex contains two co-t-SNAREs [Vti1 (Vps10p tail interactor 1) and syntaxin 8] and a v-SNARE [VAMP7 (vesicle-associated membrane protein 7)] (where t-SNAREs are SNAREs of the target compartment and v-SNAREs are SNAREs present in donor vesicles). In endosomes and in vitro, syntaxin 7, Vti1 and syntaxin 8 form a complex that is able to bind VAMP7. Antibodies to syntaxin 8 and a soluble recombinant VAMP7 fragment both inhibit in vitro reconstituted D. discoideum endosome fusion. The lysosomal content of syntaxin 7, Vti1, syntaxin 8 and VAMP7 is low compared with that in endosomes, implying a highly active recycling or retention mechanism. A likely model is that VAMP7 is a v-SNARE present on vesicles carrying lysosomal enzymes, and that the syntaxin 7-Vti1-syntaxin 8 t-SNARE complex is associated with incoming endocytic material.

High-resolution dissection of phagosome maturation reveals distinct membrane trafficking phases

Molecular mechanisms of endocytosis in the genetically and biochemically tractable professional phagocyte Dictyostelium discoideum reveal a striking degree of similarity to higher eukaryotic cells. Pulse-chase feeding with latex beads allowed purification of phagosomes at different stages of maturation. Gentle ATP stripping of an actin meshwork entrapping contaminating organelles resulted in a 10-fold increase in yield and purity, as confirmed by electron microscopy. Temporal profiling of signaling, cytoskeletal, and trafficking proteins resulted in a complex molecular fingerprint of phagosome biogenesis and maturation. First, nascent phagosomes were associated with coronin and rapidly received a lysosomal glycoprotein, LmpB. Second, at least two phases of delivery of lysosomal hydrolases (cathepsin D [CatD] and cysteine protease [CPp34]) were accompanied by removal of plasma membrane components (PM4C4 and biotinylated surface proteins). Third, a phase of late maturation, preparing for final exocytosis of undigested material, included quantitative recycling of hydrolases and association with vacuolin. Also, lysosomal glycoproteins of the Lmp family showed distinct trafficking kinetics. The delivery and recycling of CatD was directly visualized by confocal microscopy. This heavy membrane traffic of cargos was precisely accompanied by regulatory proteins such as the Rab7 GTPases and the endosomal SNAREs Vti1 and VAMP7. This initial molecular description of phagocytosis demonstrates the feasibility of a comprehensive analysis of phagosomal lipids and proteins in genetically modified strains.

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.