4,417 research outputs found
Molecular Electroporation and the Transduction of Oligoarginines
Certain short polycations, such as TAT and polyarginine, rapidly pass through
the plasma membranes of mammalian cells by an unknown mechanism called
transduction as well as by endocytosis and macropinocytosis. These
cell-penetrating peptides (CPPs) promise to be medically useful when fused to
biologically active peptides. I offer a simple model in which one or more CPPs
and the phosphatidylserines of the inner leaflet form a kind of capacitor with
a voltage in excess of 180 mV, high enough to create a molecular electropore.
The model is consistent with an empirical upper limit on the cargo peptide of
40--60 amino acids and with experimental data on how the transduction of a
polyarginine-fluorophore into mouse C2C12 myoblasts depends on the number of
arginines in the CPP and on the CPP concentration. The model makes three
testable predictions.Comment: 15 pages, 5 figure
Behavioral biology of mammalian reproduction and development for a space station
Space Station research includes two kinds of adaption to space: somatic (the adjustments made by an organism, within its lifetime, in response to local conditions), and transgenerational adaption (continuous exposure across sequential life cycles of genetic descendents). Transgenerational effects are akin to evolutionary process. Areas of a life Sciences Program in a space station address the questions of the behavioral biology of mammalian reproduction and development, using the Norway rat as the focus of experimentation
Insight into Resonant Activation in Discrete Systems
The resonant activation phenomenon (RAP) in a discrete system is studied
using the master equation formalism. We show that the RAP corresponds to a
non-monotonic behavior of the frequency dependent first passage time
probability density function (pdf). An analytical expression for the resonant
frequency is introduced, which, together with numerical results, helps
understand the RAP behavior in the space spanned by the transition rates for
the case of reflecting and absorbing boundary conditions. The limited range of
system parameters for which the RAP occurs is discussed. We show that a minimum
and a maximum in the mean first passage time (MFPT) can be obtained when both
boundaries are absorbing. Relationships to some biological systems are
suggested.Comment: 5 pages, 5 figures, Phys. Rev. E., in pres
Fluctuation spectrum of quasispherical membranes with force-dipole activity
The fluctuation spectrum of a quasi-spherical vesicle with active membrane
proteins is calculated. The activity of the proteins is modeled as the proteins
pushing on their surroundings giving rise to non-local force distributions.
Both the contributions from the thermal fluctuations of the active protein
densities and the temporal noise in the individual active force distributions
of the proteins are taken into account. The noise in the individual force
distributions is found to become significant at short wavelengths.Comment: 9 pages, 2 figures, minor changes and addition
Undulation instability in a bilayer lipid membrane due to electric field interaction with lipid dipoles
Bilayer lipid membranes [BLMs] are an essential component of all biological
systems, forming a functional barrier for cells and organelles from the
surrounding environment. The lipid molecules that form membranes contain both
permanent and induced dipoles, and an electric field can induce the formation
of pores when the transverse field is sufficiently strong (electroporation).
Here, a phenomenological free energy is constructed to model the response of a
BLM to a transverse static electric field. The model contains a continuum
description of the membrane dipoles and a coupling between the headgroup
dipoles and the membrane tilt. The membrane is found to become unstable through
buckling modes, which are weakly coupled to thickness fluctuations in the
membrane. The thickness fluctuations, along with the increase in interfacial
area produced by membrane buckling, increase the probability of localized
membrane breakdown, which may lead to pore formation. The instability is found
to depend strongly on the strength of the coupling between the dipolar
headgroups and the membrane tilt as well as the degree of dipolar ordering in
the membrane.Comment: 29 pages 8 fig
Orientational Ordering and Dynamics of Rodlike Polyelectrolytes
The interplay between electrostatic interactions and orientational
correlations is studied for a model system of charged rods positioned on a
chain, using Monte Carlo simulation techniques. It is shown that the coupling
brings about the notion of {\em electrostatic frustration}, which in turn
results in: (i) a rich variety of novel orientational orderings such as chiral
phases, and (ii) an inherently slow dynamics characterized by
stretched-exponential behavior in the relaxation functions of the system.Comment: 7 pages, 10 figure
Dynamics of non-equilibrium membrane bud formation
The dynamical response of a lipid membrane to a local perturbation of its
molecular symmetry is investigated theoretically. A density asymmetry between
the two membrane leaflets is predominantly released by in-plane lipid diffusion
or membrane curvature, depending upon the spatial extent of the perturbation.
It may result in the formation of non-equilibrium structures (buds), for which
a dynamical size selection is observed. A preferred size in the micrometer
range is predicted, as a signature of the crossover between membrane and
solvent dominated dynamical membrane response.Comment: 7 pages 3 figure
Alternative mechanisms of structuring biomembranes: Self-assembly vs. self-organization
We study two mechanisms for the formation of protein patterns near membranes
of living cells by mathematical modelling. Self-assembly of protein domains by
electrostatic lipid-protein interactions is contrasted with self-organization
due to a nonequilibrium biochemical reaction cycle of proteins near the
membrane. While both processes lead eventually to quite similar patterns, their
evolution occurs on very different length and time scales. Self-assembly
produces periodic protein patterns on a spatial scale below 0.1 micron in a few
seconds followed by extremely slow coarsening, whereas self-organization
results in a pattern wavelength comparable to the typical cell size of 100
micron within a few minutes suggesting different biological functions for the
two processes.Comment: 4 pages, 5 figure
Comparison of Simulator Wear Measured by Gravimetric vs Optical Surface Methods for Two Million Cycles
Understanding wear mechanisms are key for better implants
Critical to the success of the simulation
Small amount of metal wear can have catastrophic effects in the patient such as heavy metal poisoning or deterioration of the bone/implant interface leading to implant failure
Difficult to measure in heavy hard-on-hard implants (metal-on-metal or ceramic-on-ceramic)
May have only fractions of a milligram of wear on a 200 g component
At the limit of detection of even high-end balances when the component is 200 g and the change in weight is on the order of 0.000 1 grams
Here we compare the standard gravimetric wear estimate with
A non-contact 3D optical profiling method at each weighing stop
A coordinate measuring machine (CMM) at the beginning and end of the ru
Generic phase diagram of active polar films
We study theoretically the phase diagram of compressible active polar gels
such as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations, we perform a linear stability analysis of the uniform states in the
case of an infinite bidimensional active gel to obtain the dynamic phase
diagram of active polar films. We predict in particular modulated flowing
phases, and a macroscopic phase separation at high activity. This qualitatively
accounts for experimental observations of various active systems, such as
acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming
bacterial colonies.Comment: 4 pages, 1 figur
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