Helical Superstructure of Intermediate Filaments.

Intermediate filaments are the least explored among the large cytoskeletal elements. We show here that they display conformational anomalies in narrow microfluidic channels. Their unusual behavior can be understood as the consequence of a previously undetected, large-scale helically curved superstructure. Confinement in a channel orders the otherwise soft, strongly fluctuating helical filaments and enhances their structural correlations, giving rise to experimentally detectable, strongly oscillating tangent correlation functions. We propose an explanation for the detected intrinsic curving phenomenon-an elastic shape instability that we call autocoiling. The mechanism involves self-induced filament buckling via a surface stress located at the outside of the cross section. The results agree with ultrastructural findings and rationalize for the commonly observed looped intermediate filament shapes. Beyond curvature, explaining the molecular origin of the detected helical torsion remains an interesting challenge.

Direct observation of subunit exchange along mature vimentin intermediate filaments.

Actin filaments, microtubules, and intermediate filaments (IFs) are central elements of the metazoan cytoskeleton. At the molecular level, the assembly mechanism for actin filaments and microtubules is fundamentally different from that of IFs. The former two types of filaments assemble from globular proteins. By contrast, IFs assemble from tetrameric complexes of extended, half-staggered, and antiparallel oriented coiled-coils. These tetramers laterally associate into unit-length filaments; subsequent longitudinal annealing of unit-length filaments yields mature IFs. In vitro, IFs form open structures without a fixed number of tetramers per cross-section along the filament. Therefore, a central question for the structural biology of IFs is whether individual subunits can dissociate from assembled filaments and rebind at other sites. Using the fluorescently labeled IF-protein vimentin for assembly, we directly observe and quantitatively determine subunit exchange events between filaments as well as with soluble vimentin pools. Thereby we demonstrate that the cross-sectional polymorphism of donor and acceptor filaments plays an important role. We propose that in segments of donor filaments with more than the standard 32 molecules per cross-section, subunits are not as tightly bound and are predisposed to be released from the filament.

Intermediate filaments in small configuration spaces.

Intermediate filaments play a key role in cell mechanics. Apart from their great importance from a biomedical point of view, they also act as a very suitable micrometer-sized model system for semiflexible polymers. We perform a statistical analysis of the thermal fluctuations of individual filaments confined in microchannels. The small channel width and the resulting deflections at the walls give rise to a reduction of the configuration space by about 2 orders of magnitude. This circumstance enables us to precisely measure the intrinsic persistence length of vimentin intermediate filaments and to show that they behave as ideal wormlike chains; we observe that small fluctuations in perpendicular planes decouple. Furthermore, the inclusion of results for confined actin filaments demonstrates that the Odijk confinement regime is valid over at least 1 order of magnitude in persistence length.

Detection of contactin-2 in cerebrospinal fluid (CSF) of patients with Alzheimer's disease using Fluorescence Correlation Spectroscopy (FCS).

OBJECTIVES: Alzheimer's disease (AD) is the most common cause of dementia in the world. As many AD biomarkers occur at rather low abundances in CSF or blood, techniques of very high sensitivity and accuracy are important as diagnostic tools in the clinic. Here, we aimed to provide proof of concept of the use of a single molecule detection technique, Fluorescence Correlation Spectroscopy (FCS) for detection of novel candidate biomarkers for AD. DESIGN AND METHODS: FCS detects the diffusion times of the antigen-antibody complexes in highly diluted sample solutions, thus eliminating the need of large sample volumes and allows estimating the concentration of the target antigen. We developed a FCS set-up for contactin-2, a neuronal cell adhesion molecule and a ligand of beta-secretase 1 (BACE1) and amyloid precursor protein (APP), the latter proteins being important players in AD. With this method, we investigated whether contactin-2 concentrations are changed after delayed storage and in patients with Alzheimer's disease. RESULTS: The FCS set-up for measuring contactin-2 in CSF had a lower limit of quantification (LLOQ) of 0.2ng/ml and intra- and inter-assay coefficients of variation (CVs) of 12.2% and 14.6% respectively. Contactin-2 levels were stable up to one week storage of CSF (n=3) at RT and 4°C. Further, contactin-2 levels were similar in probable AD patients (n=34, p=0.27) compared to patients with subjective cognitive decline (SCD) (n=11). CONCLUSIONS: FCS is a sensitive tool, which can be used for detecting biomarkers in the clinical setting using very low sample volumes (10µl) and can measure proteins in their native conformations in the body fluid.

Vimentin networks at tunable ion-concentration in microfluidic drops.

The structure and function of biological systems, for example, cells and proteins, depend strongly on their chemical environment. To investigate such dependence, we design a polydimethylsiloxane-based microfluidic device to encapsulate biological systems in picoliter-sized drops. The content of each individual drop is tuned in a defined manner. As a key feature of our method, the individual chemical composition is determined and related to the drop content. In our case, the drop content is imaged using microscopy methods, while the drops are immobilized to allow for long-time studies. As an application of our device, we study the influence of divalent ions on vimentin intermediate filament networks in a quantitative way by tuning the magnesium concentration from drop to drop. This way we are able to directly image the effect of magnesium on the fluorescently tagged protein in a few hundreds of drops. Our study shows that with increasing magnesium concentration in the drops, the compaction of the networks becomes more pronounced. The degree of compaction is characterized by different morphologies; freely fluctuating networks are observed at comparatively low magnesium concentrations of 5-10 mM, while with increasing magnesium concentration reaching 16 mM they develop into fully aggregated networks. Our approach demonstrates how a systematic study of interactions in biological systems can benefit from the exceptional controllability of microfluidic methods.

Dynamics of intermediate filament assembly followed in micro-flow by small angle X-ray scattering.

The assembly of intermediate filaments (IFs) is a complex process that can be recapitulated through a series of distinct steps in vitro. The combination of microfluidics and small angle X-ray scattering (SAXS) provides a powerful tool to investigate the kinetics of this process on the relevant timescales. Microfluidic mixers based on the principle of hydrodynamic focusing allow for precise control of the mixing of proteins and smaller reagents like ions. Here, we present a multi-layer device that prevents proteins from adsorbing to the channel walls by engulfing the protein jet with a fluid layer of buffer. To ensure compatibility with SAXS, the device is fabricated from UV-curable adhesive (NOA 81). To demonstrate the successful prevention of contact between the protein jet and the channel walls we measure the distribution of a fluorescent dye in the device by confocal microscopy at various flow speeds and compare the results to finite element method (FEM) simulations. The prevention of contact enables the investigation of the assembly of IFs in flow by gradually increasing the salt concentration in the protein jet. The diffusion of salt into the jet can be determined by FEM simulations. SAXS data are collected at different positions in the jet, corresponding to different salt concentrations, and they reveal distinct differences between the earliest assembly states. We find that the mean square radius of gyration perpendicular to the filament axis increases from 13 nm(2) to 58 nm(2) upon assembly. Thereby we provide dynamic structural data of a complex assembly process that was amenable up to now only by microscopic techniques.