1,298 research outputs found
The mean velocity of two-state models of molecular motor
The motion of molecular motor is essential to the biophysical functioning of
living cells. In principle, this motion can be regraded as a multiple chemical
states process. In which, the molecular motor can jump between different
chemical states, and in each chemical state, the motor moves forward or
backward in a corresponding potential. So, mathematically, the motion of
molecular motor can be described by several coupled one-dimensional hopping
models or by several coupled Fokker-Planck equations. To know the basic
properties of molecular motor, in this paper, we will give detailed analysis
about the simplest cases: in which there are only two chemical states.
Actually, many of the existing models, such as the flashing ratchet model, can
be regarded as a two-state model. From the explicit expression of the mean
velocity, we find that the mean velocity of molecular motor might be nonzero
even if the potential in each state is periodic, which means that there is no
energy input to the molecular motor in each of the two states. At the same
time, the mean velocity might be zero even if there is energy input to the
molecular motor. Generally, the velocity of molecular motor depends not only on
the potentials (or corresponding forward and backward transition rates) in the
two states, but also on the transition rates between the two chemical states
Cell organization in soft media due to active mechanosensing
Adhering cells actively probe the mechanical properties of their environment
and use the resulting information to position and orient themselves. We show
that a large body of experimental observations can be consistently explained
from one unifying principle, namely that cells strengthen contacts and
cytoskeleton in the direction of large effective stiffness. Using linear
elasticity theory to model the extracellular environment, we calculate optimal
cell organization for several situations of interest and find excellent
agreement with experiments for fibroblasts, both on elastic substrates and in
collagen gels: cells orient in the direction of external tensile strain, they
orient parallel and normal to free and clamped surfaces, respectively, and they
interact elastically to form strings. Our method can be applied for rational
design of tissue equivalents. Moreover our results indicate that the concept of
contact guidance has to be reevaluated. We also suggest that cell-matrix
contacts are upregulated by large effective stiffness in the environment
because in this way, build-up of force is more efficient.Comment: Revtex, 7 pages, 4 Postscript files include
Scientific collaboration in a multidisciplinary organization revealed by network science.
Multidisciplinary scientific organizations have sought to face the challenges of digital transformation through new govern-ance models that optimize network collaboration and innovation. We studied the collaboration network from the long-term coauthoring system of a Brazilian multidisciplinary organization (Embrapa). The study shows that nodes degree distribution of the network is scale free and degree correlation analysis suggests a disassortative regime from competition and minimal but sufficient control that emerges as a hub-and-spoke pattern. The jobs of controller and researcher are twice as many occupied by males, except for the jobs of analyst, who act like network gatekeeper. With the largest number of individuals in productunits, the southern region of the country is more likely to form clusters. Alternatively, hubs in thematic and ecoregionalunits in the Midwest have greater gravitational attraction, positioning themselves in the inner core of the giant component. The optimization of innovation by the organization should combine greater individual autonomy through improved human capital, with a universal labeling of units as, for instance, centers of innovation
NF kappa B induces overexpression of bovine FcRn: a novel mechanism that further contributes to the enhanced immune response in genetically modified animals carrying extra copies of FcRn
Among the many functions of the neonatal Fc receptor (FcRn) for
IgG, it binds to IgG-opsonized antigen complexes and propagates
their traffic into lysosomes where antigen processing occurs. We
previously reported that transgenic (Tg) mice and rabbits that
carry multiple copies and overexpress FcRn have augmented
humoral immune responses. Nuclear factor-kappa B (NFκB) is a
critical molecule in the signaling cascade in the immune
response. NFκB induces human FcRn expression and our previous in
silico analysis suggested NFκB binding sites in the promoter
region of the bovine (b) FcRn α-chain gene (FCGRT). Here, we
report the identification of three NFκB transcription binding
sites in the promoter region of this gene using luciferase
reporter gene technology, electromobility shift assay and
supershift analysis. Stimulation of primary bovine endothelial
cells with the Toll like receptor-4 ligand lipopolysaccharide
(LPS), which mediates its effect via NFκB, resulted in rapid
upregulation of the bFcRn expression and a control gene, bovine
E-selectin. This rapid bFcRn gene induction was also observed in
the spleen of bFcRn Tg mice treated with intraperitoneally
injected LPS, analyzed by northern blot analysis. Finally, NFκB-
mediated bFcRn upregulation was confirmed at the protein level
in macrophages isolated from the bFcRn Tg mice using flow
cytometry with a newly developed FcRn specific monoclonal
antibody that does not cross-react with the mouse FcRn. We
conclude that NFκB regulates bFcRn expression and thus optimizes
its functions, e.g., in the professional antigen presenting
cells, and contributes to the much augmented humoral immune
response in the bFcRn Tg mice
Fluctuation theorem for currents and Schnakenberg network theory
A fluctuation theorem is proved for the macroscopic currents of a system in a
nonequilibrium steady state, by using Schnakenberg network theory. The theorem
can be applied, in particular, in reaction systems where the affinities or
thermodynamic forces are defined globally in terms of the cycles of the graph
associated with the stochastic process describing the time evolution.Comment: new version : 16 pages, 1 figure, to be published in Journal of
Statistical Physic
Stability analysis of non-autonomous reaction-diffusion systems: the effects of growing domains
By using asymptotic theory, we generalise the Turing diffusively-driven instability conditions for reaction-diffusion systems with slow, isotropic domain growth. There are two fundamental biological differences between the Turing conditions on fixed and growing domains, namely: (i) we need not enforce cross nor pure kinetic conditions and (ii) the restriction to activator-inhibitor kinetics to induce pattern formation on a growing biological system is no longer a requirement. Our theoretical findings are confirmed and reinforced by numerical simulations for the special cases of isotropic linear, exponential and logistic growth profiles. In particular we illustrate an example of a reaction-diffusion system which cannot exhibit a diffusively-driven instability on a fixed domain but is unstable in the presence of slow growth
Observations of Non-radial Pulsations in Radio Pulsars
We introduce a model for pulsars in which non-radial oscillations of high
spherical degree (l) aligned to the magnetic axis of a spinning neutron star
reproduce the morphological features of pulsar beams. In our model, rotation of
the pulsar carries a pattern of pulsation nodes underneath our sightline,
reproducing the longitude stationary structure seen in average pulse profiles,
while the associated time-like oscillations reproduce "drifting
subpulses"--features that change their longitude between successive pulsar
spins. We will show that the presence of nodal lines can account for observed
180 degree phase jumps in drifting subpulses and their otherwise poor phase
stability, even if the time-like oscillations are strictly periodic. Our model
can also account for the "mode changes" and "nulls" observed in some pulsars as
quasiperiodic changes between pulsation modes of different l or radial overtone
n, analogous to pulsation mode changes observed in oscillating white dwarf
stars. We will discuss other definitive and testable requirements of our model
and show that they are qualitatively supported by existing data. While
reserving judgment until the completion of quantitative tests, we are inspired
enough by the existing observational support for our model to speculate about
the excitation mechanism of the non-radial pulsations, the physics we can learn
from them, and their relationship to the period evolution of pulsars.Comment: 28 pages, 9 figures (as separate png files), Astrophysical Journal,
in pres
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