15,506 research outputs found
Anomalous transport in the crowded world of biological cells
A ubiquitous observation in cell biology is that diffusion of macromolecules
and organelles is anomalous, and a description simply based on the conventional
diffusion equation with diffusion constants measured in dilute solution fails.
This is commonly attributed to macromolecular crowding in the interior of cells
and in cellular membranes, summarising their densely packed and heterogeneous
structures. The most familiar phenomenon is a power-law increase of the MSD,
but there are other manifestations like strongly reduced and time-dependent
diffusion coefficients, persistent correlations, non-gaussian distributions of
the displacements, heterogeneous diffusion, and immobile particles. After a
general introduction to the statistical description of slow, anomalous
transport, we summarise some widely used theoretical models: gaussian models
like FBM and Langevin equations for visco-elastic media, the CTRW model, and
the Lorentz model describing obstructed transport in a heterogeneous
environment. Emphasis is put on the spatio-temporal properties of the transport
in terms of 2-point correlation functions, dynamic scaling behaviour, and how
the models are distinguished by their propagators even for identical MSDs.
Then, we review the theory underlying common experimental techniques in the
presence of anomalous transport: single-particle tracking, FCS, and FRAP. We
report on the large body of recent experimental evidence for anomalous
transport in crowded biological media: in cyto- and nucleoplasm as well as in
cellular membranes, complemented by in vitro experiments where model systems
mimic physiological crowding conditions. Finally, computer simulations play an
important role in testing the theoretical models and corroborating the
experimental findings. The review is completed by a synthesis of the
theoretical and experimental progress identifying open questions for future
investigation.Comment: review article, to appear in Rep. Prog. Phy
The macroscopic effects of microscopic heterogeneity
Over the past decade, advances in super-resolution microscopy and
particle-based modeling have driven an intense interest in investigating
spatial heterogeneity at the level of single molecules in cells. Remarkably, it
is becoming clear that spatiotemporal correlations between just a few molecules
can have profound effects on the signaling behavior of the entire cell. While
such correlations are often explicitly imposed by molecular structures such as
rafts, clusters, or scaffolds, they also arise intrinsically, due strictly to
the small numbers of molecules involved, the finite speed of diffusion, and the
effects of macromolecular crowding. In this chapter we review examples of both
explicitly imposed and intrinsic correlations, focusing on the mechanisms by
which microscopic heterogeneity is amplified to macroscopic effect.Comment: 20 pages, 5 figures. To appear in Advances in Chemical Physic
Molecular modeling to study dendrimers for biomedical applications
© 2014 by the authors; licensee MDPI; Basel; Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/). Date of Acceptance: 17/11/2014Molecular modeling techniques provide a powerful tool to study the properties of molecules and their interactions at the molecular level. The use of computational techniques to predict interaction patterns and molecular properties can inform the design of drug delivery systems and therapeutic agents. Dendrimers are hyperbranched macromolecular structures that comprise repetitive building blocks and have defined architecture and functionality. Their unique structural features can be exploited to design novel carriers for both therapeutic and diagnostic agents. Many studies have been performed to iteratively optimise the properties of dendrimers in solution as well as their interaction with drugs, nucleic acids, proteins and lipid membranes. Key features including dendrimer size and surface have been revealed that can be modified to increase their performance as drug carriers. Computational studies have supported experimental work by providing valuable insights about dendrimer structure and possible molecular interactions at the molecular level. The progress in computational simulation techniques and models provides a basis to improve our ability to better predict and understand the biological activities and interactions of dendrimers. This review will focus on the use of molecular modeling tools for the study and design of dendrimers, with particular emphasis on the efforts that have been made to improve the efficacy of this class of molecules in biomedical applications.Peer reviewedFinal Published versio
Cholesterol modulates acetylcholine receptor diffusion by tuning confinement sojourns and nanocluster stability
Translational motion of neurotransmitter receptors is key for determining receptor number at the synapse and hence, synaptic efficacy. We combine live-cell STORM superresolution microscopy of nicotinic acetylcholine receptor (nAChR) with single-particle tracking, mean-squared displacement (MSD), turning angle, ergodicity, and clustering analyses to characterize the lateral motion of individual molecules and their collective behaviour. nAChR diffusion is highly heterogeneous: subdiffusive, Brownian and, less frequently, superdiffusive. At the single-track level, free walks are transiently interrupted by ms-long confinement sojourns occurring in nanodomains of ~36 nm radius. Cholesterol modulates the time and the area spent in confinement. Turning angle analysis reveals anticorrelated steps with time-lag dependence, in good agreement with the permeable fence model. At the ensemble level, nanocluster assembly occurs in second-long bursts separated by periods of cluster disassembly. Thus, millisecond-long confinement sojourns and second-long reversible nanoclustering with similar cholesterol sensitivities affect all trajectories; the proportion of the two regimes determines the resulting macroscopic motional mode and breadth of heterogeneity in the ensemble population.Fil: Mosqueira, Alejo. Pontificia Universidad CatĂłlica Argentina "Santa MarĂa de los Buenos Aires". Instituto de Investigaciones BiomĂ©dicas. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de Investigaciones BiomĂ©dicas; ArgentinaFil: Camino, Pablo A.. Pontificia Universidad CatĂłlica Argentina "Santa MarĂa de los Buenos Aires". Instituto de Investigaciones BiomĂ©dicas. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de Investigaciones BiomĂ©dicas; ArgentinaFil: Barrantes, Francisco Jose. Pontificia Universidad CatĂłlica Argentina "Santa MarĂa de los Buenos Aires". Instituto de Investigaciones BiomĂ©dicas. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de Investigaciones BiomĂ©dicas; Argentin
Through the Eye of the Needle: Recent Advances in Understanding Biopolymer Translocation
In recent years polymer translocation, i.e., transport of polymeric molecules
through nanometer-sized pores and channels embedded in membranes, has witnessed
strong advances. It is now possible to observe single-molecule polymer dynamics
during the motion through channels with unprecedented spatial and temporal
resolution. These striking experimental studies have stimulated many
theoretical developments. In this short theory-experiment review, we discuss
recent progress in this field with a strong focus on non-equilibrium aspects of
polymer dynamics during the translocation process.Comment: 29 pages, 6 figures, 3 tables, to appear in J. Phys.: Condens. Matter
as a Topical Revie
The buckling instability of aggregating red blood cells
Plasma proteins such as fibrinogen induce the aggregation of red blood cells
(RBC) into rouleaux, which are responsible for the pronounced shear thinning
behavior of blood, control the erythro- cyte sedimentation rate (ESR) a common
hematological test and are involved in many situations of physiological
relevance such as structuration of blood in the microcirculation or clot
formation in pathological situations. Confocal microscopy is used to
characterize the shape of RBCs within rouleaux at equilibrium as a function of
macromolecular concentration, revealing the diversity of contact zone
morphology. Three different configurations that have only been partly predicted
before are identified, namely parachute, male-female and sigmoid shapes, and
quantitatively recovered by numerical simulations. A detailed experimental and
theoretical analysis of clusters of two cells shows that the deformation
increases nonlinearly with the interaction energy. Models indicate a forward
bifurcation in which the contacting membrane undergoes a buckling instability
from a flat to a de- formed contact zone at a critical value of the interaction
energy. These results are not only relevant for the understanding of the
morphology and stability of RBC aggregates, but also for a whole class of
interacting soft deformable objects such as vesicles, capsules or cells in
tissues.Comment: 22 pages, 12 figure
- …