Periodic traction in migrating large amoeba of Physarum polycephalum

The slime mould Physarum polycephalum is a giant multinucleated cell exhibiting well-known Ca 2+ -dependent actomyosin contractions of its vein network driving the so-called cytoplasmic shuttle streaming. Its actomyosin network forms both a filamentous cortical layer and large fibrils. In order to understand the role of each structure in the locomotory activity, we performed birefringence observations and traction force microscopy on excised fragments of Physarum . After several hours, these microplasmodia adopt three main morphologies: flat motile amoeba, chain types with round contractile heads connected by tubes and motile hybrid types. Each type exhibits oscillations with a period of about 1.5 min of cell area, traction forces and fibril activity (retardance) when fibrils are present. The amoeboid types show only peripheral forces while the chain types present a never-reported force pattern with contractile rings far from the cell boundary under the spherical heads. Forces are mostly transmitted where the actomyosin cortical layer anchors to the substratum, but fibrils maintain highly invaginated structures and contribute to forces by increasing the length of the anchorage line. Microplasmodia are motile only when there is an asymmetry in the shape and/or the force distribution.

A mathematically modelled cytogel cortex exhibits periodic Ca++-modulated contraction cycles seen in Physarum shuttle streaming

If each of many cells of an embryo (or different zones in a single cell) possess identical active cytogel machinery, having the ‘right’ mechanochemical response properties, then the collective interaction among those identical participants leads automatically to the globally coherent tissue deformations seen in embryogenesis, and to shuttle streaming in the plasmodial slime mould Physarum polycephalum. Biologically plausible, and experimentally verifiable hypotheses are proposed concerning how the tension generated by a strand of cytogel is determined by the deformation it suffers and by the concentration of a contraction trigger chemical, Ca2+, whose kinetics involve coupling to mechanical strain. The consequences of these hypotheses, deduced by solving the appropriate differential equation systems numerically, and displayed in computer-animated films, closely imitate diverse tissue deformation events seen in developing embryos. The same hypotheses on cytogel behaviour are used to model a thick-walled Physarum vein segment, and two such segments are set up to be able to pump endoplasm back and forth between them. Under certain conditions, this model exhibits spontaneous rhythmic mechanochemical oscillations, many features of which correlate well with shuttle streaming in Physarum. Small gradual variations of parameters, presumably under genetic control, are shown to cause abrupt and biologically interesting bifurcations of the qualitative behaviour of the model.

Relation of cytoplasmic calcium to contractility in Physarum polycephalum

In a dumbbell-shaped plasmodium of Physarum polycephalum showing active shuttle streaming Ca was precipitated with potassium pyroantimonate (K[Sb(OH)6]), and the distribution of Ca between the cytoplasm and cellular organelles, especially vacuoles, was examined by electron microscopy. The contracting half-mass, where many empty vacuoles were present, was rich in the small Ca precipitates located in the cytoplasm. The relaxing half-mass, where many Ca-containing vacuoles were present, was poor in the cytoplasmic Ca precipitates. One half-mass of a dumbbell-shaped plasmodium was treated with Ca ionophore, X-537A, and its effect on the motive force for endoplasmic streaming and the distribution of Ca was investigated. The motive force was increased by X-537A, but the period of shuttle streaming was not changed. X-537A also induced a significant increase in the number of the cytoplasmic Ca precipitates in the X-537A-treated contracting half-mass, so that the asymmetry of the distribution of cytoplasmic Ca precipitates was enhanced. A large portion of the vacuoles were empty in the contracting half-mass, and Ca-containing in the relaxing one as in the case of the untreated plasmodium.