Dicty News Electronic Edition Volume 13, number 2 July 17, 1999 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@nwu.edu. Back issues of Dicty-News, the Dicty Reference database and other useful information is available at the Dictyostelium Web Page "http://dicty.cmb.nwu.edu/dicty/dicty.html" ============= Abstracts ============= Self-organized Vortex State in Two-dimensional Dictyostelium Dynamics Wouter-Jan Rappel, Alastair Nicol, Armand Sarkissian, Herbert Levine, and William F. Loomis Depts. of Physics and Biology, University of California San Diego, La Jolla, CA 92093 Phys. Rev. Letters, in press Abstract We present results of experiments on the dynamics of Dictyostelium discoideum in a novel set-up which constrains cell motion to a plane. After aggregation, the amoebae collect into round ''pancake" structures in which the cells rotate around the center of the pancake. This vortex state persists for many hours and we have explicitly verified that the motion is not due to rotating waves of cAMP. To provide an alternative mechanism for the self organization of the Dictyostelium cells, we have developed a new model of the dynamics of self-propelled deformable objects. In this model, we show that cohesive energy between the cells, together with a coupling between the self-generated propulsive force and the cell's configuration produces a self-organized vortex state. The angular velocity profiles of the experiment and of the model are in good quantitative agreement. The mechanism for self organization reported here can possibly explain similar vortex states in other biological systems. ---------------------------------------------------------------------------- Migration and thermotaxis of Dictyostelium discoideum slugs, a model study Athanasius F.M. Marée, Alexander V. Panfilov and Paulien Hogeweg Theoretical Biology and Bioinformatics, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands. E-mail: S.Maree@bio.uu.nl. J. theor. Biol., in press ABSTRACT: Dictyostelium discoideum slugs show a pronounced thermotaxis. We have modelled the motion of the D. discoideum slug in the absence and in the presence of a thermal gradient. Our model is an extension of the hybrid cellular automata/partial differential equation model, as formulated by Savill and Hogeweg, J. theor. Biol., V.184, pp.229-235 (1997). The modelled slugs maintain their shape and crawl, with a velocity depending on slug size, as is found in experiments. Moreover, they show thermotactic behaviour: independently of the initial orientation, after some transient process the slugs start moving along the temperature gradient. The slug behaviour in our model is due to the collective behaviour of the amoebae. Individual amoebae can neither respond to a shallow temperature gradient, nor show differentiation in motion velocity. The behaviour is achieved by a modification of the cyclic AMP waves: differences in temperature alter the excitability of the cell, and therewith the shape of the cyclic AMP wave. Chemotaxis towards cyclic AMP causes the slug to turn. We show that the mechanism still functions at very low signal to noise ratios. ---------------------------------------------------------------------------- Phototaxis during the slug stage of Dictyostelium discoideum: a model study. Athanasius F.M. Marée, Alexander V. Panfilov and Paulien Hogeweg Theoretical Biology and Bioinformatics, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands. E-mail: S.Maree@bio.uu.nl. Proc. R. Soc. Lond. B, in press ABSTRACT: During the slug stage, the cellular slime mould Dictyostelium discoideum moves towards light sources. We have modelled this phototactic behaviour using a hybrid cellular automata/partial differential equation model. In our model, individual amoebae are not able to measure the direction from which the light comes, and differences in light intensity do not lead to differentiation in motion velocity among the amoebae. Nevertheless, the whole slug orients itself towards the light. This behaviour is mediated by a modification of the cyclic AMP(cAMP) waves. As an explanation for phototaxis we propose the following mechanism, which is basically characterised by four processes: (i) light is focused on the distal side of the slug as a result of the so-called `lens-effect'; (ii) differences in luminous intensity cause differences in NH_3 concentration; (iii) NH_3 alters the excitability of the cell, and thereby the shape of the cAMP wave; (iv) chemotaxis towards cAMP causes the slug to turn. We show that this mechanism can account for a number of other behaviours that have been observed in experiments, such as bidirectional phototaxis and the cancellation of bidirectionality by a decrease in the light intensity or the addition of charcoal to the medium. ---------------------------------------------------------------------------- Serpentine receptors and STAT activation: more than one way to twin a STAT Jeffrey G. Williams Department of Anatomy and Physiology, University of Dundee, MSI/WTB Complex, Dow Street, Dundee DD1 5EH TIBS, in press Summary When a cytokine binds to its receptor an associated JAK (Janus kinase) is activated and the JAK phosphorylates one or more STAT (Signal Transducer and Activator of Transcription) proteins. Cytokine receptors are single transmembrane-spanning domain proteins but recent work in mammals and in the social amoeba Dictyostelium shows that seven transmembrane spanning domain (serpentine) receptors also serve to activate STAT molecules. In both Dictyostelium and mammals this activation appears to be independent of heterotrimeric G proteins, suggesting that this, lesser-known pathway of STAT activation may be evolutionarily conserved. ---------------------------------------------------------------------------- [End Dicty News, volume 13, number 2]