Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis

Persistent holes in a fluid

We observe stable holes in a vertically oscillated 0.5 cm deep aqueous suspension of cornstarch for accelerations a above 10g. Holes appear only if a finite perturbation is applied to the layer. Holes are circular and approximately 0.5 cm wide, and can persist for more than 10^5 cycles. Above a = 17g the rim of the hole becomes unstable producing finger-like protrusions or hole division. At higher acceleration, the hole delocalizes, growing to cover the entire surface with erratic undulations. We find similar behavior in an aqueous suspension of glass microspheres.Comment: 4 pages, 6 figure

Shocks in supersonic sand

We measure time-averaged velocity, density, and temperature fields for steady granular flow past a wedge and calculate a speed of granular pressure disturbances (sound speed) equal to 10% of the flow speed. The flow is supersonic, forming shocks nearly identical to those in a supersonic gas. Molecular dynamics simulations of Newton's laws and Monte Carlo simulations of the Boltzmann equation yield fields in quantitative agreement with experiment. A numerical solution of Navier-Stokes-like equations agrees with a molecular dynamics simulation for experimental conditions excluding wall friction.Comment: 4 pages, 5 figure

Myosin II activity regulates vinculin recruitment to focal adhesions through FAK-mediated paxillin phosphorylation

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. The definitive version was published in Journal of Cell Biology 188 (2010): 877-890, doi:10.1083/jcb.200906012.Focal adhesions (FAs) are mechanosensitive adhesion and signaling complexes that grow and change composition in response to myosin II–mediated cytoskeletal tension in a process known as FA maturation. To understand tension-mediated FA maturation, we sought to identify proteins that are recruited to FAs in a myosin II–dependent manner and to examine the mechanism for their myosin II–sensitive FA association. We find that FA recruitment of both the cytoskeletal adapter protein vinculin and the tyrosine kinase FA kinase (FAK) are myosin II and extracellular matrix (ECM) stiffness dependent. Myosin II activity promotes FAK/Src-mediated phosphorylation of paxillin on tyrosines 31 and 118 and vinculin association with paxillin. We show that phosphomimic mutations of paxillin can specifically induce the recruitment of vinculin to adhesions independent of myosin II activity. These results reveal an important role for paxillin in adhesion mechanosensing via myosin II–mediated FAK phosphorylation of paxillin that promotes vinculin FA recruitment to reinforce the cytoskeletal ECM linkage and drive FA maturation.This work was supported by NHLBI (C.M. Waterman and A.M. Pasapera; and grant HL093156 to D.D. Schlaepfer) and the Burroughs Wellcome Fund (E. Rericha)

Shocks in rapid granular flows

textThe speed of a pressure wave (the speed of sound) in rapid granular flows is typically only a few centimeters per second while the collective streaming motion of the particles is on the order of meters per second. In this supersonic regime, shocks form when a granular flow encounters an obstacles. This work examines the shocks formed in three geometries: the surface wake behind a cylinder, the oblique shock formed at a wedge and a normal shock propagating through a funnel. In each case we evaluate the applicability of a hydrodynamic description to shocks in rapid granular flows. We study the V-shaped wake formed by a cylindrical rod moving through a vertically vibrated granular layer. The wake appears for rod velocities vR greater than a critical velocity c. We measure the half-angle θ of the wake as a function of vR and layer depth h. We find that c and θ can be described by a hydrodynamic description applied to shallow fluids, where c = √gh is the speed of a gravitational wave on a shallow fluid and sinθ = c/vR is the Mach relation. We find the decrease in the height of the wake hmax as it propagates away from the rod agrees with Landau’s theory for the decay of shock waves far from their origin. We measure the time-averaged velocity, density and temperature fields for a gravity driven granular flow past a wedge. The flow is supersonic with a sound speed less than 10% of the flow speed. We find the shock formed at the wedge tip is nearly identical to oblique shocks found in a supersonic, elastic gas. Molecular dynamics simulations of Newton’s laws yield fields in quantitative agreement with experiment. A numerical solution of granular hydrodynamic equations is only in qualitatively accord with experiment. We show that hydrodynamic theory fails because it does not include friction. We use molecular dynamics simulations to examine the effect of friction on the dissipation of energy and scattering angles in collisions. We examine the propagation of a normal shock formed in a quasi-two dimensional funnel. For shocks propagating without change in a fluid, one can use the Rankine-Hugoniot approximation to predict the velocity of the shock and the difference in flow values across the shock. We show that inelastic collisions between particles cause the shock to continuously evolve, hence the Rankine-Hugoniot predictions are inadequate for describing the evolution of granular shocks.Physic

Myosin II activity regulates vinculin recruitment to focal adhesions through FAK-mediated paxillin phosphorylation

Focal adhesions (FAs) are mechanosensitive adhesion and signaling complexes that grow and change composition in response to myosin II-mediated cytoskeletal tension in a process known as FA maturation. To understand tension-mediated FA maturation, we sought to identify proteins that are recruited to FAs in a myosin II-dependent manner and to examine the mechanism for their myosin II-sensitive FA association. We find that FA recruitment of both the cytoskeletal adapter protein vinculin and the tyrosine kinase FA kinase (FAK) are myosin II and extracellular matrix (ECM) stiffness dependent. Myosin II activity promotes FAK/Src-mediated phosphorylation of paxillin on tyrosines 31 and 118 and vinculin association with paxillin. We show that phosphomimic mutations of paxillin can specifically induce the recruitment of vinculin to adhesions independent of myosin II activity. These results reveal an important role for paxillin in adhesion mechanosensing via myosin II-mediated FAK phosphorylation of paxillin that promotes vinculin FA recruitment to reinforce the cytoskeletal ECM linkage and drive FA maturation.This is an article from Journal of Cell Biology, 188 (2010): 877, doi: 10.1083/jcb.200906012. Posted with permission</p

Simulation parameters.

<p>Parameters used in the numerical simulations. Except for the force constant , all the cell parameters in this table (i.e., , , , and ) are obtained from experiments. The response time is obtained from the autocorrelations of the displacement vector. The noise amplitude was calculated from the variance of the distribution, where the angle represents the orientation of the associated displacement vector.</p

Modeling and Measuring Signal Relay in Noisy Directed Migration of Cell Groups

<div><p>We develop a coarse-grained stochastic model for the influence of signal relay on the collective behavior of migrating <i>Dictyostelium discoideum</i> cells. In the experiment, cells display a range of collective migration patterns, including uncorrelated motion, formation of partially localized streams, and clumping, depending on the type of cell and the strength of the external, linear concentration gradient of the signaling molecule cyclic adenosine monophosphate (cAMP). From our model, we find that the pattern of migration can be quantitatively described by the competition of two processes, the secretion rate of cAMP by the cells and the degradation rate of cAMP in the gradient chamber. Model simulations are compared to experiments for a wide range of strengths of an external linear-gradient signal. With degradation, the model secreting cells form streams and efficiently transverse the gradient, but without degradation, we find that model secreting cells form clumps without streaming. This indicates that the observed effective collective migration in streams requires not only signal relay but also degradation of the signal. In addition, our model allows us to detect and quantify precursors of correlated motion, even when cells do not exhibit obvious streaming.</p></div

Cell tracks from simulations for the three representative modes of collective motion, uncorrelated motion, streaming, and aggregation.

<p>(A) For a relatively slow cAMP secretion rate () the cells move independently, showing no sign of collective motion. (B) If the cAMP secretion is moderate () cells form streams. (C) For high relative cAMP secretion rate () cells exhibit aggregation and therefore form clumps. Figs. (D–F) are snapshots from the same simulations exhibiting the spatial organization of the cells.</p

Time lapse images during the chemotaxis of wild-type and mutant cells in linear cAMP gradient.

<p>(A) Wild-type cells can relay the signal by secreting cAMP from their tails. They form streams which are unstable towards swirling clumps. (B) The mutant cells (aca-) lacking the ACA enzyme cannot secrete cAMP and thus undergo uniform motion in the direction of the external cAMP gradient. (C) Some representative tracks of aca- cells obtained with the tracking algorithm. Vector displacements along the tracks are color coded according to real time. (D) Distributions of the angle representing the displacement of cells exposed to different constant gradient amplitudes with respect to the vertical axis. The panel labels (5 nM to 5 µ<i>M</i>) denote the cAMP concentration in the reservoir.</p