Jes Dreier

Texture defects in lipid membrane domains

Important aspects of lateral organization in biomembranes can be addressed in model systems. Recently, it has become clear from polarized fluorescence imaging and X-ray scattering that gel domains in bilayers may contain orientational texture related to the tilted acyl chains. Such internal structure of domains can be imaged in polarized 2-photon fluorescence microscopy using the Laurdan probe that aligns with the lipids. By imaging intensity variations as a function of the polarization angle, we map the lateral variations within domains. A Fourier analysis of the signal enables the texture to be obtained in single pixels. Here we show that gel domains display a complex pattern containing line defects and a pair of |m| = 1/2 point disclinations in the domain center. We perform a detailed image analysis of the line and point defects using gradient calculations. In contrast to results from Langmuir monolayers, the membrane texture vary radially and is continuous in the center and segmented near the periphery. A possible explanation is that bilayer domains are grown thermally whereas Langmuir monolayer domains grow isothermally upon compression. The defect lines show significant variation in the angle jump across the lines. We simulate the structure of the central disclination pair and the fit to the experimental data yields an offset angle of 60.5° ± 6.5°. This indicates that the central defect has a conserved structure with an intermediate character between bend and splay.

Superresolution and Fluorescence Dynamics Evidence Reveal That Intact Liposomes Do Not Cross the Human Skin Barrier

In this study we use the combination of super resolution optical microscopy and raster image correlation spectroscopy (RICS) to study the mechanism of action of liposomes as transdermal drug delivery systems in human skin. Two different compositions of liposomes were applied to newly excised human skin, a POPC liposome and a more flexible liposome containing the surfactant sodium cholate. Stimulated emission depletion microscopy (STED) images of intact skin and cryo-sections of skin treated with labeled liposomes were recorded displaying an optical resolution low enough to resolve the 100 nm liposomes in the skin. The images revealed that virtually none of the liposomes remained intact beneath the skin surface. RICS two color cross correlation diffusion measurements of double labeled liposomes confirmed these observations. Our results suggest that the liposomes do not act as carriers that transport their cargo directly through the skin barrier, but mainly burst and fuse with the outer lipid layers of the stratum corneum. It was also found that the flexible liposomes showed a greater delivery of the fluorophore into the stratum corneum, indicating that they functioned as chemical permeability enhancers.

Systematic variation of gel-phase texture in phospholipid membranes.

The tilted gel phase of lipid bilayers can display in-plane orientational texture due to long-range alignment of the molecular director. We explore systematic variations of texture defects in a series of binary phospholipid membranes. Using polarized two-photon fluorescence microscopy, the texture pattern of single domains is revealed. The appearance of a central vortex-type defect in each domain correlates with a particular range of hydrophobic mismatch values h > 1 nm at the domain border while domains with h < 1 nm correlate with uniformly aligned texture. The central vortex defect is characterized by a defect angle, indicating its bend or splay nature. Using image analysis, we measure the defect angle and find that it has primarily bend character for small mismatch values (h ≈ 1 nm) and primarily splay nature for larger values of h. For domains containing a vortex, the domain shape is decoupled from the texture while for uniformly textured domains there is a preferred texture orientation of ≃45° along the domain border. The results establish a foundation for understanding texture phenomena in compositionally complex membranes.

Hydrophobic Mismatch Triggering Texture Defects in Membrane Gel Domains

The orientational texture of gel-phase lipid bilayers is a phenomenon that can structure membrane domains. Using two-photon polarized fluorescence microscopy and image analysis, we map the lateral variation of the lipid orientation (the texture) in single domains. With this method, we uncover a lipid-induced transition between vortex and uniform textures in binary phospholipid bilayers. By tuning the lipid composition, the hydrophobic mismatch at the domain boundary can be varied systematically as monitored by AFM. Low hydrophobic mismatch correlates with domains having uniform texture, while higher mismatch values correlate with a vortex-type texture. The defect pattern created during early growth persists in larger domains, and a minimal model incorporating the anisotropic line tension and the vortex energy can rationalize this finding. The results suggest that the lipid composition and the domain nucleation process are critical factors that determine the texture pattern of membrane domains.

Vocal motor experiences consolidate the vocal motor circuitry and accelerate future vocal skill development

Complex motor skills take considerable time and practice to learn. Without continued practice the level of skill performance quickly degrades, posing a problem for the timely utilization of skilled motor responses. Here we quantified the recurring development of vocal motor skills and the accompanying changes in synaptic connectivity in the brain of a songbird, while manipulating skill performance by consecutively administrating and withdrawing testosterone. We demonstrate that a songbird with prior singing experience can significantly accelerate the re-acquisition of vocal performance. We further demonstrate that an increase in vocal performance is accompanied by a pronounced synaptic pruning in the forebrain vocal motor area HVC, a reduction that is not reversed when birds stop singing. These results provide evidence that lasting synaptic changes in the motor circuitry are associated with the savings of motor skills, enabling a rapid recovery of motor performance under environmental time constraints.