Self-organized stress patterns drive state transitions in actin cortices.

Biological functions rely on ordered structures and intricately controlled collective dynamics. This order in living systems is typically established and sustained by continuous dissipation of energy. The emergence of collective patterns of motion is unique to nonequilibrium systems and is a manifestation of dynamic steady states.

A symmetrical method to obtain shear moduli from microrheology.

Passive microrheology typically deduces shear elastic loss and storage moduli from displacement time series or mean-squared displacements (MSD) of thermally fluctuating probe particles in equilibrium materials. Common data analysis methods use either Kramers-Kronig (KK) transformation or functional fitting to calculate frequency-dependent loss and storage moduli. We propose a new analysis method for passive microrheology that avoids the limitations of both of these approaches.

Axial Colocalization of Single Molecules with Nanometer Accuracy Using Metal-Induced Energy Transfer.

Single-molecule localization based super-resolution microscopy has revolutionized optical microscopy and routinely allows for resolving structural details down to a few nanometers. However, there exists a rather large discrepancy between lateral and axial localization accuracy, the latter typically three to five times worse than the former. Here, we use single-molecule metal-induced energy transfer (smMIET) to localize single molecules along the optical axis, and to measure their axial distance with an accuracy of 5 nm.

Topology determines force distributions in one-dimensional random spring networks.

Networks of elastic fibers are ubiquitous in biological systems and often provide mechanical stability to cells and tissues. Fiber-reinforced materials are also common in technology. An important characteristic of such materials is their resistance to failure under load.

Myotubularin related protein-2 and its phospholipid substrate PIP control Piezo2-mediated mechanotransduction in peripheral sensory neurons.

Piezo2 ion channels are critical determinants of the sense of light touch in vertebrates. Yet, their regulation is only incompletely understood. We recently identified myotubularin related protein-2 (Mtmr2), a phosphoinositide (PI) phosphatase, in the native Piezo2 interactome of murine dorsal root ganglia (DRG).

Topology Counts: Force Distributions in Circular Spring Networks.

Filamentous polymer networks govern the mechanical properties of many biological materials. Force distributions within these networks are typically highly inhomogeneous, and, although the importance of force distributions for structural properties is well recognized, they are far from being understood quantitatively. Using a combination of probabilistic and graph-theoretical techniques, we derive force distributions in a model system consisting of ensembles of random linear spring networks on a circle.

Fluorescence lifetime correlation spectroscopy: Basics and applications.

This chapter presents a concise introduction into the method of Fluorescence Lifetime Correlation Spectroscopy (FLCS). This is an extension of Fluorescence Correlation Spectroscopy (FCS) that analyses fluorescence intensity fluctuations from small detection volumes in samples of ultra-low concentration. FCS has been widely used for investigating diffusion, conformational changes, molecular binding/unbinding equilibria, or chemical reaction kinetics, at single molecule sensitivity.

Dual-color metal-induced and Förster resonance energy transfer for cell nanoscopy.

We report a novel method, dual-color axial nanometric localization by metal--induced energy transfer, and combine it with Förster resonance energy transfer (FRET) for resolving structural details in cells on the molecular level. We demonstrate the capability of this method on cytoskeletal elements and adhesions in human mesenchymal stem cells. Our approach is based on fluorescence-lifetime-imaging microscopy and allows for precise determination of the three-dimensional architecture of stress fibers anchoring at focal adhesions, thus yielding crucial information to understand cell-matrix mechanics.

Experimental Observation of Two Features Unexpected from the Classical Theories of Rubber Elasticity.

Although the elastic modulus of a Gaussian chain network is thought to be successfully described by classical theories of rubber elasticity, such as the affine and phantom models, verification experiments are largely lacking owing to difficulties in precisely controlling of the network structure. We prepared well-defined model polymer networks experimentally, and measured the elastic modulus G for a broad range of polymer concentrations and connectivity probabilities, p. In our experiment, we observed two features that were distinct from those predicted by classical theories.

Rhombic organization of microvilli domains found in a cell model of the human intestine.

Symmetry is rarely found on cellular surfaces. An exception is the brush border of microvilli, which are essential for the proper function of transport epithelia. In a healthy intestine, they appear densely packed as a 2D-hexagonal lattice.

Single-Walled Carbon Nanotubes Probed with Insulator-Based Dielectrophoresis.

Single-walled carbon nanotubes (SWNTs) offer unique electrical and optical properties. Common synthesis processes yield SWNTs with large length polydispersity (several tens of nanometers up to centimeters) and heterogeneous electrical and optical properties. Applications often require suitable selection and purification.

A Neurocomputational Model of Goal-Directed Navigation in Insect-Inspired Artificial Agents.

Despite their small size, insect brains are able to produce robust and efficient navigation in complex environments. Specifically in social insects, such as ants and bees, these navigational capabilities are guided by orientation directing vectors generated by a process called path integration. During this process, they integrate compass and odometric cues to estimate their current location as a vector, called the home vector for guiding them back home on a straight path.

Robust Heterogeneous Hydrogels with Dynamic Nanocrystal-Polymer Interface.

A kind of novel heterogeneous composite hydrogel with dynamic nanocrosslinkers is designed, which is built via the preorganized host-guest interaction on the surface of cellulose nanocrystals. The reversible β-cyclodextrin/adamantane conjunctions and their gradual dissociation on the nanocrystal-polymer interface guarantee the compressibility and stretchability of the composite hydrogels. While the sacrificed toughening mechanism can be rebuilt in the as-prepared hydrogels, it fails to be regenerated in the swollen hydrogels.

2D pair distribution function analysis of anisotropic small-angle scattering patterns from elongated nano-composite hydrogels.

Small angle scattering (SAS) on polymer nanocomposites under elongation or shear flow is an important experimental method to investigate the reinforcement effects of the mechanical properties by fillers. However, the anisotropic scattering patterns that appear in SAS are very complicated and difficult to interpret. A representative example is a four-spot scattering pattern observed in the case of polymer materials containing silica nanoparticles, the origin of which is still in debate because of the lack of quantitative analysis.

A computational model of conditioning inspired by Drosophila olfactory system.

Recent studies have demonstrated that Drosophila melanogaster (briefly Drosophila) can successfully perform higher cognitive processes including second order olfactory conditioning. Understanding the neural mechanism of this behavior can help neuroscientists to unravel the principles of information processing in complex neural systems (e.g.

Limits of Applicability of the Voronoi Tessellation Determined by Centers of Cell Nuclei to Epithelium Morphology.

It is well accepted that cells in the tissue can be regarded as tiles tessellating space. A number of approaches were developed to find an appropriate mathematical description of such cell tiling. A particularly useful approach is the so called Voronoi tessellation, built from centers of mass of the cell nuclei (CMVT), which is commonly used for estimating the morphology of cells in epithelial tissues.

Sulfo-SMCC Prevents Annealing of Taxol-Stabilized Microtubules In Vitro.

Microtubule structure and functions have been widely studied in vitro and in cells. Research has shown that cysteines on tubulin play a crucial role in the polymerization of microtubules. Here, we show that blocking sulfhydryl groups of cysteines in taxol-stabilized polymerized microtubules with a commonly used chemical crosslinker prevents temporal end-to-end annealing of microtubules in vitro.

Opposing Effects of Neuronal Activity on Structural Plasticity.

The connectivity of the brain is continuously adjusted to new environmental influences by several activity-dependent adaptive processes. The most investigated adaptive mechanism is activity-dependent functional or synaptic plasticity regulating the transmission efficacy of existing synapses. Another important but less prominently discussed adaptive process is structural plasticity, which changes the connectivity by the formation and deletion of synapses.

Game of Zones: how actin-binding proteins organize muscle contraction.

Locomotion of C. elegans requires coordinated, efficient transmission of forces generated on the molecular scale by myosin and actin filaments in myocytes to dense bodies and the hypodermis and cuticle enveloping body wall muscles. The complex organization of the acto-myosin scaffold with its accessory proteins provides a fine-tuned machinery regulated by effectors that guarantees that sarcomere units undergo controlled, reversible cycles of contraction and relaxation.

Phosphorylation of FEZ1 by Microtubule Affinity Regulating Kinases regulates its function in presynaptic protein trafficking.

Adapters bind motor proteins to cargoes and therefore play essential roles in Kinesin-1 mediated intracellular transport. The regulatory mechanisms governing adapter functions and the spectrum of cargoes recognized by individual adapters remain poorly defined. Here, we show that cargoes transported by the Kinesin-1 adapter FEZ1 are enriched for presynaptic components and identify that specific phosphorylation of FEZ1 at its serine 58 regulatory site is mediated by microtubule affinity-regulating kinases (MARK/PAR-1).

Probe-SAXS on hydrogels under elongation.

We have investigated the effect of polymer/filler interaction on the displacements of silica nanoparticles in gels by introducing them into poly(N,N-dimethylacrylamide) gel (PDAM-NP gel) and polyacrylamide gel (PAM-NP gel). It is well known that PDAM chains are strongly adsorbed onto silica nanoparticles, while PAM chains are not. We carried out SAXS measurements on these gels under uniaxial elongation.

MD simulations and FRET reveal an environment-sensitive conformational plasticity of importin-β.

The nuclear pore complex mediates nucleocytoplasmic transport of macromolecules in eukaryotic cells. Transport through the pore is restricted by a hydrophobic selectivity filter comprising disordered phenylalanine-glycine-rich repeats of nuclear pore proteins. Exchange through the pore requires specialized transport receptors, called exportins and importins, that interact with cargo proteins in a RanGTP-dependent manner.

The filament sensor for near real-time detection of cytoskeletal fiber structures.

A reliable extraction of filament data from microscopic images is of high interest in the analysis of acto-myosin structures as early morphological markers in mechanically guided differentiation of human mesenchymal stem cells and the understanding of the underlying fiber arrangement processes. In this paper, we propose the filament sensor (FS), a fast and robust processing sequence which detects and records location, orientation, length, and width for each single filament of an image, and thus allows for the above described analysis. The extraction of these features has previously not been possible with existing methods.

Force fluctuations in three-dimensional suspended fibroblasts.

Cells are sensitive to mechanical cues from their environment and at the same time generate and transmit forces to their surroundings. To test quantitatively forces generated by cells not attached to a substrate, we used a dual optical trap to suspend 3T3 fibroblasts between two fibronectin-coated beads. In this simple geometry, we measured both the cells' elastic properties and the force fluctuations they generate with high bandwidth.

A surprising twist.

X-ray crystallography has revealed an unusual structural element in kinesin-5 motor proteins.