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| Friday, January 5 — Session 8: Cell Dynamics |
| 1:50-2:30 pm |
Herbert Levine
University of California, San Diego
Eukaryotic Chemotaxis: How cells use nonlinear pde's to decide where to go
Many types of eukaryotic cells are able to detect chemical gradients and move accordingly. Unlike the case for bacteria, these cells are large enough for the gradient detection to rely on differential receptor binding probabilities on the cell membrane. It is not yet understood how this input data is processed by the cell to make the motion decision; thus we cannot a priori predict the detection threshold, the response kinetics and the plasticity to changing stimuli. This talk will focus on some recent nonlinear models of this cellular information processing system and on experiments in progress on the amoeba Dictyostelium discoideum to test some of the resulting expectations.
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| 2:30-2:50 pm |
Wolfgang Losert
University of Maryland
Email: wlosert@umd.edu
authors: Ron Skupsky (a,b), Colin McCann (b), Ralph Nossal (a), Wolfgang Losert (b)
(a) NICHD, National Institutes of Health
(b) Physics Department, University of Maryland
Bias in the Gradient Sensing Response of Chemotactic Cells
We apply linear-stability theory and perform perturbation studies to better characterize, and to generate new experimental predictions from, a model of chemotactic gradient sensing in eukaryotic cells. The model uses reaction-diffusion equations to describe 3' phosphoinositide signaling and its regulation at the plasma membrane.
An analysis of the stability of polarized steady-state solutions indicates
that the model is most sensitive to off-axis perturbations. This biased
sensitivity leads to a clear experimental prediction, namely, that a cell
which is polarized in a background gradient will be most sensitive to
transient stimuli lying within a range of 40 - 80 degrees with respect to
its polarization axis. Stimuli at angles below this range will elicit responses whose directions overshoot the stimulus angle, while responses to stimuli applied at larger angles will undershoot the stimulus angle. We argue that such a bias is likely to be a general feature of gradient sensing in highly motile cells, particularly if they are optimized to respond to small gradients. Finally, an angular bias in gradient sensing might lead to preferred turn angles and zigzag movements of cells moving up chemotactic gradients, as has been noted under certain experimental conditions.
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| 2:50-3:10 p m |
Troy Shinbrot
Rutgers University
Email: shinbrot@soemail.rutgers.edu
author: Troy Shinbrot*, Carlos Caicedo, Ramsey Foty
Simulated Morphogenesis applied to Cerebellar Development and in Ductal Carcinoma
In 1963, MS Steinberg proposed that differential adhesion strengths between sets of cells could induce the morphogenesis of concentric
spheres of cells, with the less strongly attracting species innermost.
Since that time, advances in laboratory techniques have allowed us to
establish, with unprecedented control and detail, how cells will respond to mechanical, electrical and chemical stimulae. Thus we can today obtain data that permits us to develop comprehensive models to simulate how groups of cells will behave under many prescribed conditions. In this talk, we describe how two types of cells in silico interact to generate self-assembled structures under well regulated conditions, with the goal of making specific predictions for future in vitro and even in vivo comparisons. As we will show, it is straightforward to simulate cellular self-assembly using established computational techniques, and we find that both obvious and unexpected patterns of cells can be produced.
We compare simulated patterns with known morphogenesis in two
well studied model systems: development of folds in the cerebellum, and production of patterned states in ductal breast cancers.
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| 3:10-3:50 pm |
Raymond Goldstein
Department of Applied Mathematics and Theoretical Physics, University of Cambridge
Email: R.E.Goldstein@damtp.cam.ac.uk
Physical Aspects of Evolutionary Transitions to Multicellularity
An important issue in evolutionary biology is the emergence of multicellular organisms from unicellular individuals. The accompanying differentiation from motile totipotent unicellular organisms to multicellular ones having cells specialized into reproductive (germ) and vegetative (soma) functions, such as motility, implies both costs and benefits, the analysis of which involves the physics of buoyancy, diffusion, and mixing. In this talk, I discuss recent results on this transition in a model lineage: the volvocine green algae. Particle Imaging Velocimetry of fluid flows generated by these organisms show that they exist in the regime of very large Peclet numbers, where the scaling of nutrient uptake rates with organism size is highly nontrivial.
In concert with metabolic studies of deflagellated colonies, investigations of phenotypic plasticity under nutrient-deprived conditions, and theoretical studies of transport in the high-Peclet number regime, we find that f! lagella-generated fluid flows enhance the nutrient uptake rate per cell, and thereby provide a driving force for evolutionary transitions to multicellularity. Thus, there is a link between motility, mixing, and multicellularity. |
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