dictyNews Electronic Edition Volume 28, number 6 March 9, 2007 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu or by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. Back issues of dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. ========= Abstracts ========= Mitochondrial Biology and Disease in Dictyostelium Christian Barth, Phuong Le and Paul R. Fisher Department of Microbiology, La Trobe University, Melbourne, Australia Int. Rev. Cytol., in press. Over the last dozen years, the social amoeba or cellular slime mould Dictyostelium discoideum has become an increasingly useful model for the study of mitochondrial biology and disease. Dictyostelium is an amoebozoan, a sister clade to the animal and fungal lineages. Like other aspects of its biology, the mitochondrial biology of Dictyostelium exhibits some features which are unique, others which are common to all eukaryotes, and still others that are otherwise found only in the plant or the animal lineages. The AT-rich mitochondrial genome of Dictyostelium is larger than its mammalian counterpart and contains 56 kb (compared to 17 kb in mammals) encoding tRNAs, rRNAs and 33 polypeptides (compared to 13 in mammals). The mode of expression is reminiscent of metazoan mitochondrial genomes and involves production of a single primary transcript that is cotranscriptionally processed into multiple mono-, di- and tricistronic mRNAs, tRNAs and rRNAs. The mitochondrial fission mechanism employed by Dictyostelium involves both the extramitochondrial dynamin-based system used by plant, animal and fungal mitochondria and the ancient FtsZ-based intramitochondrial fission process inherited from the bacterial ancestor of all modern mitochondria. The mitochondrial protein import apparatus is homologous to that of other eukaryotes. As in mammalian cells, the mitochondria in Dictyostelium play an important role in the programmed cell death pathways used by the organism. Mitochondrial disease in Dictyostelium has been created both by targeted gene disruptions and by antisense RNA and RNAi inhibition of expression of essential nuclear-encoded mitochondrial proteins. This has revealed a regular pattern of aberrant mitochondrial disease phenotypes that are caused not by ATP insufficiency per se, but by chronic activation of the universal eukaryotic energy-sensing protein kinase AMPK. This novel insight into the cytopathological mechanisms of mitochondrial dysfunction suggests new possibilities for therapeutic intervention in mitochondrial and neurodegenerative diseases. Submitted by Paul R. Fisher [P.Fisher@latrobe.edu.au] -------------------------------------------------------------------------------- Chemoattractants and chemorepellents act by inducing opposite polarity in phospholipase C and PI3-kinase signaling Ineke Keizer-Gunnink, Arjan Kortholt and Peter J.M. Van Haastert Department of Molecular Cell Biology, University of Groningen, Kerklaan 30, 9751NN Haren, the Netherlands J Cell Biology, in press During embryonic development cell movement is orchestrated by a multitude of attractants and repellents. Chemoattractants applied as a gradient, such as cAMP with Dictyostelium or fMLP with neutrophils, induce the activation of phospholipase C (PLC) and PI3-kinase at the front of the cell, leading to the localized depletion of PI(4,5)P2 and the accumulation of PI(3,4,5)P3. We show here using Dictyostelium that chemorepellent cAMP-analogues induce localized inhibition of PLC thereby reversing the polarity of PI(4,5)P2. This leads to the accumulation of PI(3,4,5)P3 at the rear of the cell and chemotaxis occurs away from the source. We conclude that a PLC polarity switch controls the response to attractants and repellents. Submitted by Peter Van Haastert [p.j.m.van.haastert@rug.nl] -------------------------------------------------------------------------------- Regulation of phagocytosis in Dictyostelium by the inositol 5-phosphatase OCRL homologue Dd5P4 Harriet M. Loovers, Arjan Kortholt, Hendrie de Groote, Leslie Whitty, Robert L. Nussbaum and Peter J.M. van Haastert Traffic, in press Phosphoinositides are involved in endocytosis in both mammalian cells and the amoeba Dictyostelium discoideum. Dd5P4 is the Dictyostelium homologue of human OCRL; both have a RhoGAP domain and a 5-phosphatase domain that acts on PI(4,5)P2/PI(3,4,5)P3. Inactivation of Dd5P4 inhibits growth on liquid medium and on bacteria. Dd5p4 null cells are impaired in phagocytosis of yeast cells. In wild-type cells PI(3,4,5)P3 is formed and converted to PI(3,4)P2 just before closure of the phagocytic cup. In dd5p4 null cells a phagocytic cup is formed upon contact with the yeast cell, and PI(3,4,5)P3 is still produced, but the phagocytic cup does not close. We suggest that Dd5P4 regulates the conversion of PI(3,4,5)P3 to PI(3,4)P2 and that this conversion is essential for closure of the phagocytic cup. Phylogenetic analysis of OCRL-like 5-phosphatases with RhoGAP domains reveal that D. discoideum Dd5P4 is a surprisingly close homologue of human OCRL, the protein responsible for Lowe syndrome. We expressed human OCRL in dd5p4 null cells. Growth on bacteria and axenic medium is largely restored, whereas the rate of phagocytosis of yeast cells is partly restored, indicating that human OCRL can functionally replace Dictyostelium Dd5P4. Submitted by Peter Van Haastert [p.j.m.van.haastert@rug.nl] -------------------------------------------------------------------------------- Attenuation of Phospholipid Signaling Provides a Novel Mechanism for the Action of Valproic Acid Xuehua Xu1, Annette Mueller-Taubenberger2, Kathryn E. Adley3, Nadine Pawolleck4, Vivian W. Lee5, Claudia Wiedemann6, Talvinder S. Sihra5, Markus Maniak4, Tian Jin7 and Robin S.B. Williams3,8* 1. Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University Medical School, Washington, DC 20057, USA. 2. Institut fuer Zellbiologie (ABI), Ludwig-Maximilians-Universitaet Muenchen, 80336 Muenchen, Germany. 3. Department of Biology and the Wolfson Institute for Biomedical Research, University College London, London, WC1E 6BT, UK. 4. Abt. Zellbiologie, Universitaet Kassel, 34132 Kassel, Germany. 5. Department of Pharmacology, University College London, London, WC1E 6BT, UK. 6. Centre for Molecular Cell Biology, Royal Free and UCL Medical School, NW3 2PF, UK. 7. Laboratories of Immunogenetics, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852, USA. 8. Present address: School of Biological Sciences, Royal Holloway University of London, Egham, TW20 0EX, UK. *Corresponding author: Euk. Cell., in press Valproic acid (VPA) is used to treat epilepsy and bipolar disorder, and to prevent migraine. It is also undergoing trials for cancer therapy. However, the biochemical and molecular biological actions of VPA are poorly understood. Using the social amoeba Dictyostelium discoideum, we show that an acute affect of VPA is the inhibition of chemotactic cell movement, a process partially dependent upon phospholipid signaling. Analysis of this process shows that VPA attenuates the signal-induced translocation of PHCrac-GFP from cytosol to membrane, suggesting the inhibition of the phosphatidylinositol-(3,4,5)-trisphosphate (PIP3) production. Direct labeling of lipids in vivo also shows a reduction in PIP and PIP2 phosphorylation following VPA treatment. We further show that VPA acutely reduces endocytosis and exocytosis - processes previously shown to be dependent upon PIP3 production. These results suggest that in Dictyostelium, VPA rapidly attenuates phospholipid signaling to reduce endocytic trafficking. To examine this effect in a mammalian model, we also tested depolarization-dependent neurotransmitter release in rat nerve terminals and show that this process is also suppressed upon application of VPA and an inhibitor of phosphatidylinositol 3-kinase (PI3K). Although a more comprehensive analysis of the effect of VPA on lipid signaling will be necessary in mammalian systems, these results suggest that VPA may function to reduce phospholipid signaling processes and thus may provide a novel therapeutic effect for this drug. Submitted by: Robin Williams [robin.williams@rhul.ac.uk] -------------------------------------------------------------------------------- Dictyostelium as a biomedical model Katrina Boeckeler, University College London, UK Robin SB Williams, Royal Holloway, University of London, Sussex, UK Encyclopedia of Life Sciences, in press The social amoeba, Dictyostelium discoideum, has been commonly used to investigate cell motility, signal transduction, cell type differentiation and development. With the recent completion of the genome and the increasing number of experimental tools available for the organism, it has now become an attractive model for examining some well defined biomedical questions. Submitted by: Robin Williams [robin.williams@rhul.ac.uk] ============================================================ [End dictyNews, volume 28, number 6]