548 research outputs found
Dynamical order, disorder and propagating defects in homogeneous system of relaxation oscillators
Reaction-diffusion (RD) mechanisms in chemical and biological systems can
yield a variety of patterns that may be functionally important. We show that
diffusive coupling through the inactivating component in a generic model of
coupled relaxation oscillators give rise to a wide range of spatio-temporal
phenomena. Apart from analytically explaining the genesis of anti-phase
synchronization and spatially patterned oscillatory death regimes in the model
system, we report the existence of a chimera state, characterized by spatial
co-occurrence of patches with distinct dynamics. We also observe propagating
phase defects in both one- and two-dimensional media resembling persistent
structures in cellular automata, whose interactions may be used for computation
in RD systems.Comment: 6 pages, 4 figure
Turbulence near cyclic fold bifurcations in birhythmic media
We show that at the onset of a cyclic fold bifurcation, a birhythmic medium
composed of glycolytic oscillators displays turbulent dynamics. By computing
the largest Lyapunov exponent, the spatial correlation function, and the
average transient lifetime, we classify it as a weak turbulence with transient
nature. Virtual heterogeneities generating unstable fast oscillations are the
mechanism of the transient turbulence. In the presence of wavenumber
instability, unstable oscillations can be reinjected leading to stationary
turbulence. We also find similar turbulence in a cell cycle model. These
findings suggest that weak turbulence may be universal in biochemical
birhythmic media exhibiting cyclic fold bifurcations.Comment: 14 pages 10 figure
From ballistic to Brownian vortex motion in complex oscillatory media
We show that the breaking of the rotation symmetry of spiral waves in
two-dimensional complex (period-doubled or chaotic) oscillatory media by
synchronization defect lines (SDL) is accompanied by an intrinsic drift of the
pattern. Single vortex motion changes from ballistic flights at a well-defined
angle from the SDL to Brownian-like diffusion when the turbulent character of
the medium increases. It gives rise, in non-turbulent multi-spiral regimes, to
a novel ``vortex liquid''.Comment: 5 pages, 4 figure
Robustness of circadian clocks to daylight fluctuations: hints from the picoeucaryote Ostreococcus tauri
The development of systemic approaches in biology has put emphasis on
identifying genetic modules whose behavior can be modeled accurately so as to
gain insight into their structure and function. However most gene circuits in a
cell are under control of external signals and thus quantitative agreement
between experimental data and a mathematical model is difficult. Circadian
biology has been one notable exception: quantitative models of the internal
clock that orchestrates biological processes over the 24-hour diurnal cycle
have been constructed for a few organisms, from cyanobacteria to plants and
mammals. In most cases, a complex architecture with interlocked feedback loops
has been evidenced. Here we present first modeling results for the circadian
clock of the green unicellular alga Ostreococcus tauri. Two plant-like clock
genes have been shown to play a central role in Ostreococcus clock. We find
that their expression time profiles can be accurately reproduced by a minimal
model of a two-gene transcriptional feedback loop. Remarkably, best adjustment
of data recorded under light/dark alternation is obtained when assuming that
the oscillator is not coupled to the diurnal cycle. This suggests that coupling
to light is confined to specific time intervals and has no dynamical effect
when the oscillator is entrained by the diurnal cycle. This intringuing
property may reflect a strategy to minimize the impact of fluctuations in
daylight intensity on the core circadian oscillator, a type of perturbation
that has been rarely considered when assessing the robustness of circadian
clocks
Equivalence of the Siegert-pseudostate and Lagrange-mesh R-matrix methods
Siegert pseudostates are purely outgoing states at some fixed point expanded
over a finite basis. With discretized variables, they provide an accurate
description of scattering in the s wave for short-range potentials with few
basis states. The R-matrix method combined with a Lagrange basis, i.e.
functions which vanish at all points of a mesh but one, leads to simple
mesh-like equations which also allow an accurate description of scattering.
These methods are shown to be exactly equivalent for any basis size, with or
without discretization. The comparison of their assumptions shows how to
accurately derive poles of the scattering matrix in the R-matrix formalism and
suggests how to extend the Siegert-pseudostate method to higher partial waves.
The different concepts are illustrated with the Bargmann potential and with the
centrifugal potential. A simplification of the R-matrix treatment can usefully
be extended to the Siegert-pseudostate method.Comment: 19 pages, 1 figur
The Dynamics of Zeroth-Order Ultrasensitivity: A Critical Phenomenon in Cell Biology
It is well known since the pioneering work of Goldbeter and Koshland [Proc.
Natl. Acad. Sci. USA, vol. 78, pp. 6840-6844 (1981)] that cellular
phosphorylation- dephosphorylation cycle (PdPC), catalyzed by kinase and
phosphatase under saturated condition with zeroth order enzyme kinetics,
exhibits ultrasensitivity, sharp transition. We analyse the dynamics aspects of
the zeroth order PdPC kinetics and show a critical slowdown akin to the phase
transition in condensed matter physics. We demonstrate that an extremely
simple, though somewhat mathematically "singular" model is a faithful
representation of the ultrasentivity phenomenon. The simplified mathematical
model will be valuable, as a component, in developing complex cellular
signaling network theory as well as having a pedagogic value.Comment: 8 pages, 3 figure
Threshold responses to morphogen gradients by zero-order ultrasensitivity
Translating a graded morphogen distribution into tight response borders is central to all developmental processes. Yet, the molecular mechanisms generating such behavior are poorly understood. During patterning of the Drosophila embryonic ventral ectoderm, a graded mitogen-activated protein kinase (MAPK) activation is converted into an all-or-none degradation switch of the Yan transcriptional repressor. Replacing the cardinal phosphorylated amino acid of Yan by a phosphomimetic residue allowed its degradation in a MAPK-independent manner, consistent with Yan phosphorylation being the critical event in generating the switch. Several alternative threshold mechanisms that could, in principle, be realized by this phosphorylation, including first order, cooperativity, positive feedback and zero-order ultrasensitivity, were analyzed. We found that they can be distinguished by their kinetics and steady-state responses to Yan overexpression. In agreement with the predictions for zero-order kinetics, an increase in Yan levels did not shift the degradation border, but significantly elevated the time required to reach steady state. We propose that a reversible loop of Yan phosphorylation implements a zero-order ultrasensitivity-like threshold mechanism, with the capacity to form sharp thresholds that are independent of the level of Yan
Competing Patterns of Signaling Activity in Dictyostelium discoideum
Quantitative experiments are described on spatio-temporal patterns of
coherent chemical signaling activity in populations of {\it Dictyostelium
discoideum} amoebae. We observe competition between spontaneously firing
centers and rotating spiral waves that depends strongly on the overall cell
density. At low densities, no complete spirals appear and chemotactic
aggregation is driven by periodic concentric waves, whereas at high densities
the firing centers seen at early times nucleate and are apparently entrained by
spiral waves whose cores ultimately serve as aggregation centers. Possible
mechanisms for these observations are discussed.Comment: 10 pages, RevTeX, 4 ps figures, accepted in PR
The statistical mechanics of complex signaling networks : nerve growth factor signaling
It is becoming increasingly appreciated that the signal transduction systems
used by eukaryotic cells to achieve a variety of essential responses represent
highly complex networks rather than simple linear pathways. While significant
effort is being made to experimentally measure the rate constants for
individual steps in these signaling networks, many of the parameters required
to describe the behavior of these systems remain unknown, or at best,
estimates. With these goals and caveats in mind, we use methods of statistical
mechanics to extract useful predictions for complex cellular signaling
networks. To establish the usefulness of our approach, we have applied our
methods towards modeling the nerve growth factor (NGF)-induced differentiation
of neuronal cells. Using our approach, we are able to extract predictions that
are highly specific and accurate, thereby enabling us to predict the influence
of specific signaling modules in determining the integrated cellular response
to the two growth factors. We show that extracting biologically relevant
predictions from complex signaling models appears to be possible even in the
absence of measurements of all the individual rate constants. Our methods also
raise some interesting insights into the design and possible evolution of
cellular systems, highlighting an inherent property of these systems wherein
particular ''soft'' combinations of parameters can be varied over wide ranges
without impacting the final output and demonstrating that a few ''stiff''
parameter combinations center around the paramount regulatory steps of the
network. We refer to this property -- which is distinct from robustness -- as
''sloppiness.''Comment: 24 pages, 10 EPS figures, 1 GIF (makes 5 multi-panel figs + caption
for GIF), IOP style; supp. info/figs. included as brown_supp.pd
Vortex Glass and Vortex Liquid in Oscillatory Media
We study the disordered, multi-spiral solutions of two-dimensional
homogeneous oscillatory media for parameter values at which the single
spiral/vortex solution is fully stable. In the framework of the complex
Ginzburg-Landau (CGLE) equation, we show that these states, heretofore believed
to be static, actually evolve on ultra-slow timescales. This is achieved via a
reduction of the CGLE to the evolution of the sole vortex position and phase
coordinates. This true defect-mediated turbulence occurs in two distinct
phases, a vortex liquid characterized by normal diffusion of individual
spirals, and a slowly relaxing, intermittent, ``vortex glass''.Comment: 4 pages, 2 figures, submitted to Physical Review Letter
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