dictyNews Electronic Edition Volume 30, number 6 February 15, 2008 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 ========= Dictyostelium Sun-1 connects the centrosome to chromatin and ensures genome stability Huajiang Xiong, Francisco Rivero, Ursula Euteneuer, Subhanjan Mondal, Sebastian Mana-Capelli, Denis Larochelle, Annette Vogel, Berthold Gassen, and Angelika A. Noegel Traffic, in press The centrosome-nucleus attachment is a prerequisite for faithful chromosome segregation during mitosis. We addressed the function of the nuclear envelope protein Sun-1 in centrosome-nucleus connection and maintenance of genome stability in Dictyostelium discoideum. We provide evidence that Sun-1 requires direct chromatin binding for its inner nuclear membrane targeting. Truncation of the cryptic N-terminal chromatin-binding domain of Sun-1 induces dramatic separation of the inner from the outer nuclear membrane and deformations in nuclear morphology, which are also observed using a Sun-1 RNAi construct. Thus, chromatin binding of Sun-1 defines the integrity of the nuclear architecture. In addition to its role as a nuclear envelope scaffold, we find that abrogation of the chromatin binding of Sun-1 dissociates the centrosome-nucleus connection, demonstrating that Sun-1 provides an essential link between chromatin and the centrosome. Moreover, loss of the centrosome-nucleus connection causes severe centrosome hyperamplification and defective spindle formation, which enhances aneuploidy and cell death significantly. We highlight an important new aspect for Sun-1 in coupling the centrosome and nuclear division during mitosis to ensure faithful chromosome segregation. Submitted by: Angelika Noegel [noegel@uni-koeln.de] -------------------------------------------------------------------------------- "Reversal of Cell Polarity and Actin-Myosin Cytoskeleton Reorganization under Mechanical and Chemical Stimulation" Jeremie Dalous*, Emmanuel Burghardt+, Annette Mueller-Taubenberger+, Franz Bruckert*$, Guenther Gerisch+, and Till Bretschneider+ * CEA-Grenoble, Departement Reponse et Dynamique Cellulaires, Laboratoire de Biochimie et Biophysique des Systemes Integres, Grenoble, France; + Max-Planck-Institut fuer Biochemie, Martinsried, Germany; $ Institut National Polytechnique de Grenoble, Laboratoire des Materiaux et du Genie Physique, Grenoble, France Biophysical Journal, Volume 94, Issue 3, February 1, 2008 ABSTRACT To study reorganization of the actin system in cells that invert their polarity, we stimulated Dictyostelium cells by mechanical forces from alternating directions. The cells oriented in a fluid flow by establishing a protruding front directed against the flow and a retracting tail. Labels for polymerized actin and filamentous myosin-II marked front and tail. At 2.1 Pa, actin first disassembled at the previous front before it began to polymerize at the newly induced front. In contrast, myosin-II slowly disappeared from the previous tail and continuously redistributed to the new tail. Front specification was myosin-II independent and accumulation of polymerized actin was even more focused in mutants lacking myosin-II heavy chains. We conclude that under mechanical stimulation, the inversion of cell polarity is initiated by a global internal signal that turns down actin polymerization in the entire cell. It is thought to be elicited at the most strongly stimulated site of the cell, the incipient front region, and to be counterbalanced by a slowly generated, short-range signal that locally activates actin polymerization at the front. Similar pattern of front and tail interconversion were observed in cells reorienting in strong gradients of the chemoattractant cyclic AMP. Submitted by: Till Bretschneider [T.Bretschneider@warwick.ac.uk] -------------------------------------------------------------------------------- Mechano-chemical Signaling Maintains the Rapid Movement of Dictyostelium Cells. M. L. Lombardi, D. A. Knecht, and J. Lee* Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269. * Author for correspondence (e-mail: juliet.lee@uconn.edu; phone number: 860-486-4332; fax: 860-486-4331) Experimental Cell Research, in press The survival of Dictyostelium cells depends on their ability to efficiently chemotax, either towards food or to form multicellular aggregates. Although the involvement of Ca2+ signaling during chemotaxis is well known, it is not clear how this regulates cell movement. Previously, fish epithelial keratocytes have been shown to display transient increases in intracellular calcium ([Ca2+]i) that are mediated by stretch-activated calcium channels (SACs), which play a role in retraction of the cell body. To investigate the involvement of SACs in Dictyostelium movement we performed high resolution calcium imaging in wild-type (NC4A2) Dictyostelium cells to detect changes in [Ca2+]i. We observed small, brief, Ca2+ transients in randomly moving wild-type cells that were dependent on both intracellular and extracellular sources of calcium. Treatment of cells with the SAC blocker gadolinium (Gd3+) inhibited transients and decreased cell speed, consistent with the involvement of SACs in regulating Dictyostelium motility. Additional support for SAC activity was given by the increase in frequency of Ca2+ transients when Dictyostelium cells were moving on a more adhesive substratum or when they were mechanically stretched. We conclude that mechano-chemical signaling via SACs, plays a major role in maintaining the rapid movement of Dictyostelium cells. Submitted by: Maria Lombardi [marialucia.lombardi@gmail.com] ============================================================== [End dictyNews, volume 30, number 6]