dictyNews Electronic Edition Volume 35, number 7 September 17, 2010 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. Follow dictyBase on twitter: http://twitter.com/dictybase ========= Abstracts ========= Peter J.M. Van Haastert A model for a correlated random walk based on the ordered extension of pseudopodia Department of Cell Biochemistry, University of Groningen, Kerklaan 30, 9751 NN Haren, the Netherlands PLoS Comp. Biol., in press Cell migration in the absence of external cues is well described by a correlated random walk. Most single cells move by extending protrusions called pseudopodia. To deduce how cells walk, we have analyzed the formation of pseudopodia by Dictyostelium cells. We have observed that the formation of pseudopodia is highly ordered with two types of pseudopodia: First, de novo formation of pseudopodia at random positions on the cell body, and therefore in random directions. Second, pseudopod splitting near the tip of the current pseudopod in alternating right/left directions, leading to a persistent zig-zag trajectory. Here we analyzed the probability frequency distributions of the angles between pseudopodia and used this information to design a stochastic model for cell movement. Monte Carlo simulations show that the critical elements are the ratio of persistent splitting pseudopodia relative to random de novo pseudopodia, the Left/Right alternation, the angle between pseudopodia and the variance of this angle. Experiments confirm predictions of the model, showing reduced persistence in mutants that are defective in pseudopod splitting and in mutants with an irregular cell surface. Submitted by Peter Van Haastert [p.j.m.van.haastert@rug.nl] -------------------------------------------------------------------------------- Peter J.M. Van Haastert A stochastic model for chemotaxis based on the ordered extension of pseudopods Department of Cell Biochemistry, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands Biophysical Journal, in press Many amoeboid cells move by extending pseudopods. Here we present a new stochastic model for chemotaxis that is based on pseudopod extensions by Dictyostelium cells. In the absence of external cues, pseudopod extension is highly ordered with two types of pseudopods: de novo formation of a pseudopod at the cell body in random directions, and alternating right/left splitting of an existing pseudopod that leads to a persistent zig-zag trajectory. We measured the directional probabilities of the extension of splitting and de novo pseudopods in chemoattractant gradients with different steepness. Very shallow cAMP gradients can bias the direction of splitting pseudopods, but the bias is not perfect. Orientation of de novo pseudopods require much steeper cAMP gradients and can be more precise. These measured probabilities of pseudopod directions were used to obtain an analytical model for chemotaxis of cell populations. Measured chemotaxis of wild type cells and mutants with specific defects in these stochastic pseudopod properties are similar to predictions of the model. These results show that combining splitting and de novo pseudopods is a very effective way for cells to obtain very high sensitivity to stable gradient and still be responsive to changes in the direction of the gradient. Submitted by Peter Van Haastert [p.j.m.van.haastert@rug.nl] -------------------------------------------------------------------------------- Peter J.M. Van Haastert Chemotaxis: insights from the extending pseudopod Department of Cell Biochemistry, University of Groningen, Kerklaan 30, 9751 NN Haren, the Netherlands J. Cell Sci., in press Chemotaxis is one of the most fascinating processes in cell biology. Shallow gradients of chemoattractant direct the movement of cells, and an intricate network of signalling pathways somehow instructs the movement apparatus to induce pseudopods in the direction of these gradients. Exciting new experiments have approached chemotaxis from the perspective of the extending pseudopod. These recent studies have revealed that, in the absence of external cues, cells use endogenous signals for the highly ordered extension of pseudopods, which appear mainly as alternating right and left splits. In addition, chemoattractants activate other signalling molecules that induce a positional bias of this basal system, such that the extending pseudopods are oriented towards the gradient. In this Commentary, I review the findings of these recent experiments, which together provide a new view of cell movement and chemotaxis. Submitted by Peter Van Haastert [p.j.m.van.haastert@rug.nl] ------------------------------------------------------------------------------ Regulation of Hip1r by epsin controls the temporal and spatial coupling of actin filaments to clathrin-coated pits Rebecca J. Brady (1), Cynthia K. Damer (2), John E. Heuser (3) and Theresa J. O’Halloran (1) (1) Department of Molecular Cell and Developmental Biology, University of Texas at Austin, Austin, TX 78712 (2) Department of Biology, Central Michigan University, Mount Pleasant, MI 48858 (3) Department of Cell Biology and Biophysics, Washington University, St. Louis, MO 63130 Journal of Cell Science, in press Recently, it has become clear that the actin cytoskeleton is involved in clathrin-mediated endocytosis. During clathrin-mediated endocytosis, clathrin triskelions and adaptor proteins assemble into lattices, forming clathrin-coated pits. These coated pits invaginate and detach from the membrane, a process that requires dynamic actin polymerization. We found an unexpected role for the clathrin adaptor epsin in regulating actin dynamics during this late stage of coated vesicle formation. In Dictyostelium cells, epsin is required for both the membrane recruitment and phosphorylation of the actin- and clathrin-binding protein Hip1r. Epsin-null and Hip1r-null cells exhibit deficiencies in the timing and organization of actin filaments at clathrin-coated pits. Consequently, clathrin structures persist on the membranes of epsin and Hip1r mutants and the internalization of clathrin structures is delayed. We conclude that epsin works with Hip1r to regulate actin dynamics by controlling the spatial and temporal coupling of actin filaments to clathrin coated pits. Specific residues in the ENTH domain of epsin that are required for the membrane recruitment and phosphorylation of Hip1r are also required for normal actin and clathrin dynamics at the plasma membrane. We propose that epsin promotes the membrane recruitment and phosphorylation of Hip1r, which in turn regulates actin polymerization at clathrin-coated pits. Submitted by Terry O’Halloran [t.ohalloran@mail.utexas.edu] ============================================================== [End dictyNews, volume 35, number 7]