Biophysical Reviews and Letters

Cell migration depends on the rapid changes of the organization of actin filaments and generation of force by motor proteins. Vertebrate cells use two different mechanisms: mesenchymal or amoeboid migration. Cells migrating in mesenchymal mode are elongated and move over a two-dimensional substratum. They extend thin veil-like extensions at their leading face — lamellipodia, whose protrusion depend on polymerization and depolymerization processes of actin. During mesenchymal migration actin filaments are firmly connected by integrins to the extracellular matrix (ECM) at focal contacts, which serve as points of fixation for subsequent cell body traction by force producing actomyosin interactions. Cells migrating in amoeboid fashion are rounded and move through a three-dimensional ECM-network undergoing considerable shape changes and generating vesicle-like surface extensions — so-called blebs. These blebs and the migrating cells exhibit no or strongly reduced affinity to the ECM. Bleb formation depends on a transient decrease of plasma membrane stiffness and locally increased hydrostatic pressure, which is generated by actin-myosin interactions. Formation of numerous surface blebs is also typical of cells that undergo apoptotic cell death. Since these share a number of properties to blebs of amoeboid cells, an analysis is given of the distribution of some cytoskeletal components in apoptotic blebs.

Animal cells export the content of vesicles by exocytosis, a process in which a vesicle and the plasma membrane fuse and ultimately form a pore. The initial event leading to the breakthrough of the membranes for pore formation is not well understood. Here we present a mathematical approach which suggests that this process is initiated by single electrons which could be present in the immediate proximity of the fusing membranes in aromatic amino acids. The electron can be regarded as non-classical; it takes up energy and transfers it to the membrane barrier, thereby eliciting conformational changes in the proteo-lipid complex leading to membrane fusion and thus initiating the opening of the fusion pore.

Biological membranes have properties and behavior that emerge from the propagation of the molecular characteristics of their components across many scales. Artificial smart materials, such as drug delivery vehicles and nanoparticles, often rely on modifying naturally-occurring soft matter, such as polymers and lipid vesicles, so that they possess useful behavior. Mesoscopic simulations allow in silico experiments to be easily and cheaply performed on complex, soft materials requiring as input only the molecular structure of the constituents at a coarse-grained level. They can therefore act as a guide to experimenters prior to performing costly assays. Additionally, mesoscopic simulations provide the only currently feasible window on the length and time scales relevant to important biophysical processes such as vesicle fusion. We describe here recent work using Dissipative Particle Dynamics simulations to explore the structure and behavior of amphiphilic membranes, the fusion of vesicles, and the interactions between rigid nanoparticles and soft surfaces.

The Hodgkin-Huxley model of nerve pulse propagation relies on ion currents through specific resistors called ion channels. We discuss a number of classical thermodynamic findings on nerves that are not contained within this classical theory. In particular striking is the finding of reversible heat changes, thickness and phase changes of the membrane during the action potential. Data on various nerves rather suggest that a reversible density pulse accompanies the action potential of nerves. Here, we attempted to explain these phenomena by propagating solitons that depend on the presence of cooperative phase transitions in the nerve membrane. The transitions, however, are strongly influenced by the presence of anesthetics. Therefore, the thermodynamic theory of nerve pulses suggests an explanation for the famous Meyer-Overton rule that states that the critical anesthetic dose is linearly related to the solubility of the drug in the membranes.

Neural field models have a long tradition in mathematical neuroscience, and in the present tutorial paper we outline the neurobiological and biophysical origin of such models, in particular two-population field models describing excitatory and inhibitory neurons interacting via nonlocal spatial connections. Results from investigations of such models on the existence and stability of stationary localized activity pulses ('bumps') and generation of stationary spatial and spatiotemporal oscillations through Turing-type instabilities are described.

Luminescent semiconductor nanocrystals of CdTe and CdSe were derivatised by mercaptocarboxylic acids. A direct synthesis in the solution of the ligand and a post-synthesis ligand exchange method was used. The two methods were compared and evaluated by isotachophoresis. The electrophoretic mobility as well as the colloidal stability and the coverage with the ligand, were determined. An alternative method for the concentration estimation of nanocrystals by means of isotachophoresis was introduced.

The synthesis of nanosized beads of superparamagnetic iron oxide nanoparticles coated with 3-(aminopropyl)triethoxysilane is reported in this paper. The resulting beads are agglomerates of SPIONs with an open fractal like structure. These beads have been examined by means of transmission electron microscopy (TEM), photon correlation spectroscopy (PCS), and attenuated total reflection Fourier transformation infrared spectroscopy (ATR-FTIR). The resulting beads with a size of 88 nm have been further biofunctionalized with avidin and biotinylated oligo(dT).