Ole Mathis Schütte

Fluorescence nanoscopy by polarization modulation and polarization angle narrowing

When excited with rotating linear polarized light, differently oriented fluorescent dyes emit periodic signals peaking at different times. We show that measurement of the average orientation of fluorescent dyes attached to rigid sample structures mapped to regularly defined (50 nm)2 image nanoareas can, in combination with application of the SPEED (sparsity penalty-enhanced estimation by demodulation) deconvolution algorithm, provide subdiffraction resolution (super resolution by polarization demodulation, SPoD). Because the polarization angle range for effective excitation of an oriented molecule is rather broad and unspecific, we narrowed this range by simultaneous irradiation with a second, de-excitation, beam possessing a polarization perpendicular to the excitation beam (excitation polarization angle narrowing, ExPAN). This shortened the periodic emission flashes, allowing better discrimination between molecules or nanoareas. Our method requires neither the generation of nanometric interference structures nor the use of switchable or blinking fluorescent probes. We applied the method to standard wide-field microscopy with camera detection and to two-photon scanning microscopy, imaging the fine structural details of neuronal spines.

Influence of Gb3 glycosphingolipids differing in their fatty acid chain on the phase behaviour of solid supported membranes: chemical syntheses and impact of Shiga toxin binding

The Shiga toxin B subunit (STxB), which is involved in cell membrane attachment and trafficking of Shiga holotoxin, binds specifically to the glycosphingolipid Gb3. In biological membranes, Gb3 glycosphingolipids differ in their fatty acid composition and there is strong evidence that the fatty acid alters the binding behaviour of STxB as well as the intracellular routing of the Shiga toxin/Gb3 complex. To analyse the binding of STxB to different Gb3s, we chemically synthesized saturated, unsaturated, α-hydroxylated Gb3s and a combination thereof, all based on a C24-fatty acid chain starting from monosaccharide building blocks, sphingosine and the respective fatty acids. These chemically well-defined Gb3s were inserted into solid supported phase-separated lipid bilayers composed of DOPC/sphingomyelin/cholesterol as a simple mimetic of the outer leaflet of animal cell membranes. By fluorescence- and atomic force microscopy the phase behaviour of the bilayer as well as the lateral organization of bound STxB were analysed. The fatty acid of Gb3 significantly alters the ratio between the ordered and disordered phase and induces a third intermediate phase in the presence of unsaturated Gb3. The lateral organization of STxB on the membranes varies significantly. While STxB attached to membranes with Gb3s with saturated fatty acids forms protein clusters, it is more homogeneously bound to membranes containing unsaturated Gb3s. Large interphase lipid redistribution is observed for α-hydroxylated Gb3 doped membranes. Our results clearly demonstrate that the fatty acid of Gb3 strongly influences the lateral organization of STxB on the membrane and impacts the overall membrane organization of phase-separated lipid membranes.

A DNA-Inspired Synthetic Ion Channel Based on G–C Base Pairing

A dinucleoside containing guanosine and cytidine at the end groups has been prepared using a modular one-pot azide-alkyne cycloaddition. Single channel analysis showed that this dinucleoside predominantly forms large channels with 2.9 nS conductance for the transport of potassium ions across a phospholipid bilayer. Transmission electron microscopy, atomic force microscopy, and circular dichroism spectroscopy studies reveal that this dinucleoside can spontaneously associate through Watson-Crick canonical H-bonding and π-π stacking to form stable supramolecular nanostructures. Most importantly, the ion channel activity of this G-C dinucleoside can be inhibited using the nucleobase cytosine.

Idealizing Ion Channel Recordings by a Jump Segmentation Multiresolution Filter

Based on a combination of jump segmentation and statistical multiresolution analysis for dependent data, a new approach called J-SMURF to idealize ion channel recordings has been developed. It is model-free in the sense that no a-priori assumptions about the channels characteristics have to be made; it thus complements existing methods which assume a model for the channels dynamics, like hidden Markov models. The method accounts for the effect of an analog filter being applied before the data analysis, which results in colored noise, by adapting existing muliresolution statistics to this situation. J-SMURFs ability to denoise the signal without missing events even when the signal-to-noise ratio is low is demonstrated on simulations as well as on ion current traces obtained from gramicidin A channels reconstituted into solvent-free planar membranes. When analyzing a newly synthesized acylated system of a fatty acid modified gramicidin channel, we are able to give statistical evidence for unknown gating characteristics such as subgating.

2-Hydroxy Fatty Acid Enantiomers of Gb3 Impact Shiga Toxin Binding and Membrane Organization

Shiga toxin subunit B (STxB) binding to its cellular receptor Gb3 leads to the formation of protein-lipid clusters and bending of the membrane. A newly developed synthetic route allowed synthesizing the biologically most relevant Gb3-C24:1 2OH species with both, the natural (Gb3-R) as well as the unnatural (Gb3-S) configuration of the 2OH group. The derivatives bind STxB with identical nanomolar affinity, while the propensity to induce membrane tubules in giant unilamellar vesicles is more pronounced for Gb3-S. Fluorescence and atomic force microscopy images of phase-separated supported membranes revealed differences in the lateral organization of the protein on the membrane. Gb3-R favorably induces large and tightly packed protein clusters, while a lower protein density is found on Gb3-S doped membranes.

Size and mobility of lipid domains tuned by geometrical constraints

Significance Diffusion of lipids and proteins organized in finite-sized domains plays a pivotal role in biological membranes enhancing biomolecular signaling efficiency. The impact of membrane geometry and spatial boundaries on the lateral mobility of domains in membranes remains, however, still unclear. By using pore-spanning membranes, we were able to control the length scale and dynamics of lipid domains by an underlying porous mesh serving both as a confinement and as a source of additional friction that slows down the mobility of domains by several orders of magnitude. We could show that increased hydrodynamic drag acting on liquid-ordered domains due to the confined geometry is responsible for the reduction of diffusion constants. In the plasma membrane of eukaryotic cells, proteins and lipids are organized in clusters, the latter ones often called lipid domains or “lipid rafts.” Recent findings highlight the dynamic nature of such domains and the key role of membrane geometry and spatial boundaries. In this study, we used porous substrates with different pore radii to address precisely the extent of the geometric constraint, permitting us to modulate and investigate the size and mobility of lipid domains in phase-separated continuous pore-spanning membranes (PSMs). Fluorescence video microscopy revealed two types of liquid-ordered (lo) domains in the freestanding parts of the PSMs: (i) immobile domains that were attached to the pore rims and (ii) mobile, round-shaped lo domains within the center of the PSMs. Analysis of the diffusion of the mobile lo domains by video microscopy and particle tracking showed that the domains’ mobility is slowed down by orders of magnitude compared with the unrestricted case. We attribute the reduced mobility to the geometric confinement of the PSM, because the drag force is increased substantially due to hydrodynamic effects generated by the presence of these boundaries. Our system can serve as an experimental test bed for diffusion of 2D objects in confined geometry. The impact of hydrodynamics on the mobility of enclosed lipid domains can have great implications for the formation and lateral transport of signaling platforms.

Self-Assembly of a Guanosine Derivative To Form Nanostructures and Transmembrane Channels

We herein report the self-assembly of a lipophilic bromoguanosine derivative (G1) in homogeneous solution, in the solid state and in planar bilayer membranes. The self-assembly of G1, driven by H-bonding and π-π stacking interactions can form different nano-structures depending on incubation time. The G1 nanostructure is able to bind a bioactive dye like Rose Bengal. In crystal state, it shows ribbon type H-bonding pattern and exhibits birefringence in polarized light. And further, the self-assembled nanostructure of G1 can form discrete transmembrane ion channels in lipid bilayer membranes, enabling passage of potassium ions.

Gb3 Glycosphingolipids with Fluorescent Oligoene Fatty Acids: Synthesis and Phase Behavior in Model Membranes

Glycosphingolipids are involved in a number of physiological and pathophysiological processes, and they serve as receptors for a variety of bacterial toxins and viruses. To investigate their function in lipid membranes, fluorescently labeled glycosphingolipids are highly desirable. Herein, a synthetic route to access Gb3 glycosphingolipids with fluorescently labeled fatty acids, consisting of pentaene and hexaene moieties either at the terminus or in the middle of the acyl chain, has been developed. The fluorescent properties of the Gb3 derivatives were investigated in small unilamellar vesicles composed of a raft‐like mixture. Phase‐separated giant unilamellar vesicles (GUVs) allowed the quantification of the apparent partitioning coefficients of the Gb3 compounds by means of confocal fluorescence laser scanning microscopy. The determined partition coefficients demonstrate that the Gb3 derivatives are preferentially localized in the liquid‐disordered (ld) phase. To analyze whether the compounds behave like their physiological counterparts, Cy3‐labeled (Cy: cyanine) Shiga toxin B subunits (STxB) were specifically bound to Gb3‐doped GUVs. However, the protein was favorably localized in the ld phase, in contrast to results reported for STxB bound to naturally occurring Gb3, which is discussed in terms of the packing density of the lipids in the liquid‐ordered (lo) phase.

Fully Automatic Multiresolution Idealization for Filtered Ion Channel Recordings: Flickering Event Detection

We propose a new model-free segmentation method, JULES, which combines recent statistical multiresolution techniques with local deconvolution for idealization of ion channel recordings. The multiresolution criterion takes into account scales down to the sampling rate enabling the detection of flickering events, i.e., events on small temporal scales, even below the filter frequency. For such small scales the deconvolution step allows for a precise determination of dwell times and, in particular, of amplitude levels, a task which is not possible with common thresholding methods. This is confirmed theoretically and in a comprehensive simulation study. In addition, JULES can be applied as a preprocessing method for a refined hidden Markov analysis. Our new methodology allows us to show that gramicidin A flickering events have the same amplitude as the slow gating events. JULES is available as an R function jules in the package clampSeg.