Involvement of a Rab8-like protein of Dictyostelium discoideum, Sas1, in the formation of membrane extensions, secretion and adhesion during development

The Effect of Overexpressed DdRabS on Development, Cell Death, Vesicular Trafficking, and the Secretion of Lysosomal Glycosidase Enzymes

Rab GTPases are essential regulators of many cellular processes and play an important role in downstream signaling vital to proper cell function. We sought to elucidate the role of novel D. discoideum GTPase RabS. Cell lines over-expressing DdRabS and expressing DdRabS N137I (dominant negative (DN)) proteins were generated, and it was determined that DdRabS localized to endosomes, ER-Golgi membranes, and the contractile vacuole system. It appeared to function in vesicular trafficking, and the secretion of lysosomal enzymes. Interestingly, microscopic analysis of GFP-tagged DdRabS (DN) cells showed differential localization to lysosomes and endosomes compared to GFP-tagged DdRabS overexpressing cells. Both cell lines over-secreted lysosomal glycosidase enzymes, especially β-glucosidase. Furthermore, DdRabS overexpressing cells were defective in aggregation due to decreased cell–cell cohesion and sensitivity to cAMP, leading to abnormal chemotactic migration, the inability to complete development, and increased induced cell death. These data support a role for DdRabS in trafficking along the vesicular and biosynthetic pathways. We hypothesize that overexpression of DdRabS may interfere with GTP activation of related proteins essential for normal development resulting in a cascade of defects throughout these processes.

The Small GTPase MoSec4 Is Involved in Vegetative Development and Pathogenicity by Regulating the Extracellular Protein Secretion in Magnaporthe oryzae

The Rab GTPase proteins play important roles in the membrane trafficking, and consequently protein secretion and development of eukaryotic organisms. However, little is known about the function of Rab GTPases in Magnaporthe oryzae. To further explore the function of Rab GTPases, we deleted the ortholog of the yeast Sec4p protein in M. oryzae, namely MoSEC4. The ΔMosec4 mutant is defective in polarized growth and conidiation, and it displays decreased appressorium turgor pressure and attenuated pathogenicity. Notably, the biotrophic invasive hyphae produced in rice cells are more bulbous and compressed in the ΔMosec4 mutant. Further studies showed that deletion of the MoSEC4 gene resulted in decreased secretion of extracellular enzymes and mislocalization of the cytoplasmic effector PWL2-mCherry-NLS. In accordance with a role in secretion, the GFP-MoSec4 fusion protein mainly accumulates at tips of growing vegetative hyphae. Our results suggest that the MoSec4 protein plays important roles in the secretion of extracellular proteins and consequently hyphal development and pathogenicity in the rice blast fungus.

Vesicular Trafficking Defects, Developmental Abnormalities, and Alterations in the Cellular Death Process Occur in Cell Lines that Over-Express Dictyostelium GTPase, Rab2, and Rab2 Mutants

Small molecular weight GTPase Rab2 has been shown to be a resident of pre-Golgi intermediates and required for protein transport from the ER to the Golgi complex, however, the function of Rab2 in Dictyostelium has yet to be fully characterized. Using cell lines that over-express DdRab2, as well as cell lines over-expressing constitutively active (CA), and dominant negative (DN) forms of the GTPase, we report a functional role in vesicular transport specifically phagocytosis, and endocytosis. Furthermore, Rab2 like other GTPases cycles between an active GTP-bound and an inactive GDP-bound state. We found that this GTP/GDP cycle for DdRab2 is crucial for normal Dictyostelium development and cell-cell adhesion. Similar to Rab5 and Rab7 in C. elegans, we found that DdRab2 plays a role in programmed cell death, possibly in the phagocytic removal of apoptotic corpses.

Localization of phosphatidylinositol 4,5-bisphosphate to lipid rafts and uroids in the human protozoan parasite Entamoeba histolytica

Entamoeba histolytica is an intestinal protozoan parasite and is the causative agent of amoebiasis. During invasive infection, highly motile amoebae destroy the colonic epithelium, enter the blood circulation, and disseminate to other organs such as liver, causing liver abscess. Motility is a key factor in E. histolytica pathogenesis, and this process relies on a dynamic actomyosin cytoskeleton. In other systems, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is known to regulate a wide variety of cellular functions, including signal transduction, actin remodeling, and cell motility. Little is known about the role of PI(4,5)P2 in E. histolytica pathogenicity. In this study, we demonstrate that PI(4,5)P2 is localized to cholesterol-rich microdomains, lipid rafts, and the actin-rich fractions of the E. histolytica membrane. Microscopy revealed that the trailing edge of polarized trophozoites, uroids, are highly enriched in lipid rafts and their constituent lipid, PI(4,5)P2. Polarization and enrichment of uroids and rafts with PI(4,5)P2 were enhanced upon treatment of E. histolytica cells with cholesterol. Exposure to cholesterol also increased intracellular calcium, which is a downstream effector of PI(4,5)P2, with a concomitant increase in motility. Together, our data suggest that in E. histolytica, PI(4,5)P2 may signal from lipid rafts and cholesterol may play a role in triggering PI(4,5)P2-mediated signaling to enhance the motility of this pathogen.

CHARACTERIZATION OF A RABE (RAS GENE FROM RAT BRAIN E) GTPASE EXPRESSED DURING MORPHOGENESIS IN THE UNICELLULAR GREEN ALGA MICRASTERIAS DENTICULATA (ZYGNEMATOPHYCEAE, STREPTOPHYTA)(1)

Rab GTPases are central regulators of cell shape in land plants by coordinating vesicle trafficking during morphogenesis. To date, relatively little is known about the role of these ubiquitous signaling proteins during cell growth in microalgae, in particular in the related charophyte algae. This article identifies the first charophyte Rab GTPase, MdRABE1, in Micrasterias denticulata Bréb., a convenient model organism for studying morphogenesis. Its expression correlated with the onset of morphogenesis, and structural analysis indicated that it belongs to the RABE (Ras gene from rat brain E) subclass. Confocal fluorescence and immunoelectron microscopy (IEM) of transiently GFP-MdRABE1 overexpressing interphase cells demonstrated that the GFP-MdRABE1 protein was localized to the endoplasmic reticulum, dictyosomes, exocytotic vesicles, the cell margin, the membranes of cell organelles, and in the isthmus zone around the nucleus. Although overexpression phenotyping of both N- and C-terminal green fluorescent protein (GFP) fusions failed to indicate additional functional evidence of the MdRABE1 protein due to mortality of those transgenic cells, its expression profile, bioinformatics, and intracellular localization suggest a role in vesicle trafficking during morphogenesis.

Rab8a regulates the exocyst-mediated kiss-and-run discharge of the Dictyostelium contractile vacuole

A molecular dissection of contractile vacuole (CV) discharge shows that Rab8a is recruited to the CV a few seconds before the exocyst. Together they tether it to the plasma membrane and commit it to fusion. GTP hydrolysis is necessary for vacuole detethering, a process in which LvsA, a protein of the Chédiak–Higashi family, plays a crucial role.

Water expulsion by the contractile vacuole (CV) in Dictyostelium is carried out by a giant kiss-and-run focal exocytic event during which the two membranes are only transiently connected but do not completely merge. We present a molecular dissection of the GTPase Rab8a and the exocyst complex in tethering of the contractile vacuole to the plasma membrane, fusion, and final detachment. Right before discharge, the contractile vacuole bladder sequentially recruits Drainin, a Rab11a effector, Rab8a, the exocyst complex, and LvsA, a protein of the Chédiak–Higashi family. Rab8a recruitment precedes the nucleotide-dependent arrival of the exocyst to the bladder by a few seconds. A dominant-negative mutant of Rab8a strongly binds to the exocyst and prevents recruitment to the bladder, suggesting that a Rab8a guanine nucleotide exchange factor activity is associated with the complex. Absence of Drainin leads to overtethering and blocks fusion, whereas expression of constitutively active Rab8a allows fusion but blocks vacuole detachment from the plasma membrane, inducing complete fragmentation of tethered vacuoles. An indistinguishable phenotype is generated in cells lacking LvsA, implicating this protein in postfusion detethering. Of interest, overexpression of a constitutively active Rab8a mutant reverses the lvsA-null CV phenotype.

Localisation to lipid rafts correlates with increased function of the Gal/GalNAc lectin in the human protozoan parasite, Entamoeba histolytica

Entamoeba histolytica is the causative agent of dysentery and liver abscess and is prevalent in developing countries. Adhesion to the host is critical to infection and is mediated by amoebic surface receptors. One such receptor, the Gal/GalNAc lectin, binds to galactose or N-acetylgalactosamine residues on host components and consists of heavy (Hgl), light (Lgl) and intermediate (Igl) subunits. The mechanism by which the lectin assembles into a functional complex is not known. The parasite also relies on cholesterol-rich domains (lipid rafts) for adhesion. Therefore, it is conceivable that rafts regulate the assembly or function of the lectin. To test this, amoebae were loaded with cholesterol and lipid rafts were purified and characterised. Western blotting showed that cholesterol loading resulted in co-compartmentalisation of all three subunits in rafts. This co-compartmentalisation was accompanied by an increase in the ability of the amoebae to bind to host cells in a galactose-specific manner, suggesting that there is a correlation between location and function of the Gal/GalNAc lectin. Cholesterol loading did not increase the surface levels of the lectin subunits. Therefore, the cholesterol-induced increase in adhesion was not the result of externalisation of an internal pool of subunits. A mutant cell line that modestly responded to cholesterol with a slight increase in adhesion exhibited only a slight enrichment of Hgl and Lgl in rafts. This supports the connection between location and function of the Gal/GalNAc lectin. Actin can also influence the interaction of proteins with rafts. Therefore, the sub-membrane distribution of the lectin subunits was also assessed after treatment with an actin depolymerising agent, cytochalasin D. Cytochalasin D-treatment had no effect on the submembrane distribution of the subunits, suggesting that actin does not prevent the association of lectin subunits with rafts in this system. Together, these data provide insight into the molecular mechanisms regulating the location and function of this adhesin.

Endosomal and secretory markers of the Legionella-containing vacuole

The Gram-negative opportunistic pathogen Legionella pneumophila replicates in phagocytes within a specific compartment, the Legionella-containing vacuole (LCV). Formation of LCVs is a complex process requiring the bacterial Icm/Dot type IV secretion system and more than 100 translocated effector proteins, which putatively subvert cellular signaling and vesicle trafficking pathways. Phosphoinositide (PI) glycerolipids are pivotal regulators of signal transduction and membrane dynamics in eukaryotes. Recently, a number of Icm/Dot substrates were found to anchor to the LCV membrane by binding to PIs. One of these effectors, SidC, specifically interacts with phosphatidylinositol-4 phosphate [PtdIns(4)P]. Using an antibody against SidC and magnetic beads coupled to a secondary antibody, intact LCVs were purified by immuno-magnetic separation, followed by density centrifugation. This purification strategy is in principle applicable to any pathogen vacuole that carries specific markers. The LCV proteome determined by LC-MS/MS revealed 566 host proteins, including novel components of the endosomal pathway, as well as the early and late secretory trafficking pathways. Thus, LCV formation is a robust process that involves many (functionally redundant) Icm/Dot substrates, as well as the interaction with different host cell vesicle trafficking pathways.

Localization of phosphatidylinositol (3,4,5)-trisphosphate to phagosomes in entamoeba histolytica achieved using glutathione S-transferase- and green fluorescent protein-tagged lipid biosensors

Entamoeba histolytica is an intestinal protozoan parasite that causes amoebic dysentery and liver abscess. Phagocytosis by the parasite is a critical virulence process, since it is a prerequisite for tissue invasion and establishment of chronic infection. While the roles of many of the proteins that regulate phagocytosis-related signaling events in E. histolytica have been characterized, the functions of lipids in this cellular process remain largely unknown in this parasite. In other systems, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), a major product of phosphoinositide 3 kinase (PI3-kinase) activity, is essential for phagocytosis. Pleckstrin homology (PH) domains are protein domains that specifically bind to PIP(3). In this study, we utilized glutathione S-transferase (GST)- and green fluorescent protein (GFP)-labeled PH domains as lipid biosensors to characterize the spatiotemporal aspects of PIP(3) distribution during various endocytic processes in E. histolytica. PIP(3)-specific biosensors accumulated at extending pseudopodia and in phagosomal cups in trophozoites exposed to erythrocytes but did not localize to pinocytic compartments during the uptake of a fluid-phase marker, dextran. Our results suggest that PIP(3) is involved in the early stages of phagosome formation in E. histolytica. In addition, we demonstrated that PIP(3) exists at high steady-state levels in the plasma membrane of E. histolytica and that these levels, unlike those in mammalian cells, are not abolished by serum withdrawal. Finally, expression of a PH domain in trophozoites inhibited erythrophagocytosis and enhanced motility, providing genetic evidence supporting the role of PI3-kinase signaling in these processes in E. histolytica.

Overexpression of a mutant form of EhRabA, a unique Rab GTPase of Entamoeba histolytica, alters endoplasmic reticulum morphology and localization of the Gal/GalNAc adherence lectin

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. Vesicle trafficking events, such as phagocytosis and delivery of plasma membrane proteins, have been implicated in pathogenicity. Rab GTPases are proteins whose primary function is to regulate vesicle trafficking; therefore, understanding the function of Rabs in this organism may provide insight into virulence. E. histolytica possesses a number of unique Rabs that exhibit limited homology to host Rabs. In this study we examined the function of one such Rab, EhRabA, by characterizing a mutant overexpressing a constitutively GTP-bound version of the protein. Overexpression of mutant EhRabA resulted in decreased adhesion to and phagocytosis of human red blood cells and in the appearance of large tubular organelles that could be stained with endoplasmic reticulum (ER)-specific but not Golgi complex-specific antibodies. Consistent with the adhesion defect, two subunits of a cell surface adhesin, the galactose/N-acetylgalactosamine lectin, were mislocalized to the novel organelle. A cysteine protease, EhCP2, was also localized to the ER-like compartment in the mutant; however, the localization of two additional cell surface proteins, Igl and SREHP, remained unchanged in the mutant. The phenotype of the mutant could be recapitulated by treatment with brefeldin A, a cellular toxin that disrupts ER-to-Golgi apparatus vesicle traffic. This suggests that EhRabA influences vesicle trafficking pathways that are also sensitive to brefeldin A. Together, the data indicate that EhRabA directly or indirectly influences the morphology of secretory organelles and regulates trafficking of a subset of secretory proteins in E. histolytica.

Proteome analysis of Legionella vacuoles purified by magnetic immunoseparation reveals secretory and endosomal GTPases

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates in macrophages and amoebae within 'Legionella-containing vacuoles' (LCVs), which communicate with the early secretory pathway and the endoplasmic reticulum. Formation of LCVs requires the bacterial Icm/Dot type IV secretion system. The Icm/Dot-translocated effector protein SidC selectively anchors to LCVs by binding the host lipid phosphatidylinositol-4-phosphate (PtdIns(4)P). Here, we describe a novel and simple approach to purify intact vacuoles formed by L. pneumophila within Dictyostelium discoideum by using magnetic immunoseparation with an antibody against SidC, followed by density gradient centrifugation. To monitor LCV purification by fluorescence microscopy, we used Dictyostelium producing the LCV marker calnexin-GFP and L. pneumophila labeled with the red fluorescent protein DsRed. A proteome analysis of purified LCVs by liquid chromatography coupled to tandem mass spectrometry revealed 566 host proteins, including known LCV components, such as the small GTPases Arf1, Rab1 and Rab7. Rab8, an endosomal regulator of the late secretory pathway originating from the trans Golgi network, and the endosomal GTPase Rab14 were identified as novel LCV components, which were found to be present on vacuoles harboring wild-type but not Icm/Dot-deficient L. pneumophila. Thus, LCVs also communicate with the late secretory and endosomal pathways. Depletion of Rab8 or Arf1 by RNA interference reduced the amount of SidC on LCVs, indicating that the GTPases promote the recruitment of Legionella effectors by regulating the level of PtdIns(4)P.

The interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of tight junctions and adherens junctions

The assembly of tight junctions (TJs) and adherens junctions (AJs) is regulated by the transport of integral TJ and AJ proteins to and/or from the plasma membrane (PM) and it is tightly coordinated in epithelial cells. We previously reported that Rab13 and a junctional Rab13-binding protein (JRAB)/molecule interacting with CasL-like 2 (MICAL-L2) mediated the endocytic recycling of an integral TJ protein occludin and the formation of functional TJs. Here, we investigated the role of Rab13 and JRAB/MICAL-L2 in the transport of other integral TJ and AJ proteins claudin-1 and E-cadherin to the PM by using a Ca(2+)-switch model. Although knockdown of Rab13 specifically suppressed claudin-1 and occludin but not E-cadherin transport, knockdown of JRAB/MICAL-L2 and expression of its Rab13-binding domain (JRAB/MICAL-L2-C) inhibited claudin-1, occludin, and E-cadherin transport. We then identified Rab8 as another JRAB/MICAL-L2-C-binding protein. Knockdown of Rab8 inhibited the Rab13-independent transport of E-cadherin to the PM. Rab8 and Rab13 competed with each other for the binding to JRAB/MICAL-L2 and functionally associated with JRAB/MICAL-L2 at the perinuclear recycling/storage compartments and PM, respectively. These results suggest that the interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of AJs and TJs.

Naturally occurring and stress induced tubular structures from mammalian cells, a survival mechanism

Background

Tubular shaped mammalian cells in response to dehydration have not been previously reported. This may be due to the invisibility of these cells in aqueous solution, and because sugars and salts added to the cell culture for manipulation of the osmotic conditions inhibit transformation of normal cells into tubular shaped structures.

Results

We report the transformation of normal spherical mammalian cells into tubular shaped structures in response to stress. We have termed these transformed structures 'straw cells' which we have associated with a variety of human tissue types, including fresh, post mortem and frozen lung, liver, skin, and heart. We have also documented the presence of straw cells in bovine brain and prostate tissues of mice. The number of straw cells in heart, lung tissues, and collapsed straw cells in urine increases with the age of the mammal. Straw cells were also reproduced in vitro from human cancer cells (THP1, CACO2, and MCF7) and mouse stem cells (D1 and adipose D1) by dehydrating cultured cells. The tubular center of the straw cells is much smaller than the original cell; houses condensed organelles and have filamentous extensions that are covered with microscopic hair-like structures and circular openings. When rehydrated, the filaments uptake water rapidly. The straw cell walls, have a range of 120 nm to 200 nm and are composed of sulfated-glucose polymers and glycosylated acidic proteins. The transformation from normal cell to straw cells takes 5 to 8 hr in open-air. This process is characterized by an increase in metabolic activity. When rehydrated, the straw cells regain their normal spherical shape and begin to divide in 10 to 15 days. Like various types of microbial spores, straw cells are resistant to harsh environmental conditions such as UV-C radiation.

Conclusion

Straw cells are specialized cellular structures and not artifacts from spontaneous polymerization, which are generated in response to stress conditions, like dehydration. The disintegrative, mobile, disruptive and ubiquitous nature of straw cells makes this a possible physiological process that may be involved in human health, longevity, and various types of diseases such as cancer.

Characterization of the Rab8-specific membrane traffic route linked to protrusion formation

Rab8 has a drastic effect on cell shape, but the membrane trafficking route it regulates is poorly defined. Here, we show that endogenous and ectopically expressed Rab8 is associated with macropinosomes generated at ruffling membrane domains. These macropinosomes fuse or transform into tubules that move toward the cell center, from where they are recycled back to the leading edge. The biogenesis of these tubules is dependent on actin and microtubular dynamics. Expression of dominant-negative Rab8 mutants or depletion of Rab8 by RNA interference inhibit protrusion formation, but promote cell-cell adhesion and actin stress fiber formation, whereas expression of the constitutively active Rab8-Q67L has the opposite effect. Rab8 localization overlaps with both Rab11 and Arf6, and is functionally linked to Arf6. We also demonstrate that Rab8 activity is needed for the transport of transferrin and the transferrin receptor to the pericentriolar region and to cell protrusions, and that Rab8 controls the traffic of cholera toxin B to the Golgi compartment. Finally, Rab8 colocalizes and binds specifically to a synaptotagmin-like protein (Slp1/JFC1), which is involved in controlling Rab8 membrane dynamics. We propose that Rab8 regulates a membrane-recycling pathway that mediates protrusion formation.

Entamoeba histolytica: FYVE-finger domains, phosphatidylinositol 3-phosphate biosensors, associate with phagosomes but not fluid filled endosomes

Endocytosis is an important virulence function for Entamoeba histolytica, the causative agent of amoebic dysentery. Although a number of E. histolytica proteins that regulate this process have been identified, less is known about the role of lipids. In other systems, phosphatidylinositol 3-phosphate (PI3P), a product of phosphatidylinositol 3-kinase (PI 3-kinase), has been shown to be required for endocytosis. FYVE-finger domains are protein motifs that bind specifically to PI3P. Using a PI3P biosensor consisting of glutathione-S-transferase (GST) fused to two tandem FYVE-finger domains, we have localized PI3P to phagosomes but not fluid-phase pinosomes in E. histolytica, suggesting a role for PI3P in phagocytosis. Treatment of cells with PI 3-kinase inhibitors impaired GST-2 x FYVE-phagosome association supporting the authenticity of the biosensor staining. However, treatment with PI 3-kinase inhibitors did not inhibit E. histolytica-particle interaction, indicating that PI3P is not required for the initial step, but is required for subsequent steps of phagocytosis.

A unique Rab GTPase, EhRabA, is involved in motility and polarization of Entamoeba histolytica cells

Entamoeba histolytica, an enteric protozoan parasite, infects 10% of the world's population leading to 50 million cases of invasive amoebiasis annually. Motility, which requires cell polarization, is important to the virulence of this pathogen, as it may result in destruction of host tissues and invasion. To gain insight into these processes in Entamoeba, a unique Rab GTPase, EhRabA, which localizes to the leading edge of cells, was characterized. Cell lines expressing a dominant negative version of EhRabA (EhRabA-DN) were generated. These mutant cells exhibited alterations in cell shape, polarity, and motility, supporting a role for this Rab in the regulation of these processes. Consistent with the notion that a dynamic actin cytoskeleton is crucial to cell polarity and motility, these mutants also exhibited alterations in the actin cytoskeleton. Cells expressing EhRabA-DN also displayed defects in several virulence functions including the ability to adhere to host cells, destroy host cells, and release cysteine proteases. Mislocalization of a prominent adhesion molecule, the galactose/N-acetylgalactosamine (Gal/GalNAc) adherence lectin and reorganization of ordered lipid domains, known as lipid rafts, also accompanied expression of EhRabA-DN. Interestingly, several endocytic processes were unaffected by expression of EhRabA-DN. Together, these data suggest that EhRabA may be involved in the regulation of polarization, motility and actin cytoskeletal dynamics: functions that participate in the pathogenicity of Entamoeba.