Oligodendroglial exosomes in glia to neuron signaling

In the CNS, myelinating oligodendrocytes and axons form a functional unit based on intimate cell-cell interactions. In addition to axonal insulation serving to increase the conduction velocity of electrical impulses, oligodendrocytes provide trophic support to neurons essential for the long-term functional integrity of axons. The glial signals maintaining axonal functions are just at the beginning to become uncovered. Yet, their determination is highly relevant for all types of demyelinating diseases, where lack of glial support significantly contributes to pathology. rnThe present PhD thesis uncovers exosomes as a novel signaling entity in the CNS by which cargo can be transferred from oligodendrocytes to neurons. Exosomes are small membranous vesicles of endocytic origin, which are released by almost every cell type and have been implicated in intercellular communication. Oligodendrocytes secrete exosomes containing a distinct set of proteins as well as mRNA and microRNA. Intriguingly, oligodendroglial exosome release is stimulated by the neurotransmitter glutamate indicating that neuronal electrical activity controls glial exosome release. In this study, the role of exosomes in neuron-glia communication and their implications on glial support was examined. Cortical neurons internalized and accumulated oligodendroglial exosomes in the neuronal cell soma in a time-dependent manner. Moreover, uptake occurred likewise at the somatodendritic and axonal compartment of the neurons via dynamin and clathrin dependent endocytosis. Intriguingly, neuronal internalization of exosomes resulted in functional retrieval of exosomal cargo in vitro and in vivo upon stereotactic injection of Cre recombinase bearing exosomes. Functional recovery of Cre recombinase from transferred exosomes was indicated by acquired reporter recombination in the target cell. Electrophysiological analysis showed an increased firing rate in neurons exposed to oligodendroglial exosomes. Moreover, microarray analysis revealed differentially expressed genes after exosome treatment, indicating functional implications on neuronal gene expression and activity. rnTaken together, the results of this PhD thesis represent a proof of principle for exosome transmission from oligodendrocytes to neurons suggesting a new route of horizontal transfer in the CNS.rn

Towards a new test of time dilation: Laser spectroscopy on lithium ions stored at a velocity of 33.8 % of the speed of light

The subject of the presented thesis is the accurate measurement of time dilation, aiming at a quantitative test of special relativity. By means of laser spectroscopy, the relativistic Doppler shifts of a clock transition in the metastable triplet spectrum of ^7Li^+ are simultaneously measured with and against the direction of motion of the ions. By employing saturation or optical double resonance spectroscopy, the Doppler broadening as caused by the ions' velocity distribution is eliminated. From these shifts both time dilation as well as the ion velocity can be extracted with high accuracy allowing for a test of the predictions of special relativity. A diode laser and a frequency-doubled titanium sapphire laser were set up for antiparallel and parallel excitation of the ions, respectively. To achieve a robust control of the laser frequencies required for the beam times, a redundant system of frequency standards consisting of a rubidium spectrometer, an iodine spectrometer, and a frequency comb was developed. At the experimental section of the ESR, an automated laser beam guiding system for exact control of polarisation, beam profile, and overlap with the ion beam, as well as a fluorescence detection system were built up. During the first experiments, the production, acceleration and lifetime of the metastable ions at the GSI heavy ion facility were investigated for the first time. The characterisation of the ion beam allowed for the first time to measure its velocity directly via the Doppler effect, which resulted in a new improved calibration of the electron cooler. In the following step the first sub-Doppler spectroscopy signals from an ion beam at 33.8 %c could be recorded. The unprecedented accuracy in such experiments allowed to derive a new upper bound for possible higher-order deviations from special relativity. Moreover future measurements with the experimental setup developed in this thesis have the potential to improve the sensitivity to low-order deviations by at least one order of magnitude compared to previous experiments; and will thus lead to a further contribution to the test of the standard model.

Ab-initio study of hydrogen technology materials for hydrogen storage and proton conduction

This dissertation deals with two specific aspects of a potential hydrogen-based energy economy, namely the problems of energy storage and energy conversion. In order to contribute to the solution of these problems, the structural and dynamical properties of two promising materials for hydrogen storage (lithium imide/amide) and proton conduction (poly[vinyl phosphonic acid]) are modeled on an atomistic scale by means of first principles molecular dynamics simulation methods.rnrnrnIn the case of the hydrogen storage system lithium amide/imide (LiNH_2/Li_2NH), the focus was on the interplay of structural features and nuclear quantum effects. For these calculations, Path-Integral Molecular Dynamics (PIMD) simulations were used. The structures of these materials at room temperature were elucidated; in collaboration with an experimental group, a very good agreement between calculated and experimental solid-state 1H-NMR chemical shifts was observed. Specifically, the structure of Li_2NH features a disordered arrangement of the Li lattice, which was not reported in previous studies. In addition, a persistent precession of the NH bonds was observed in our simulations. We provide evidence that this precession is the consequence of a toroid-shaped effective potential, in which the protons in the material are immersed. This potential is essentially flat along the torus azimuthal angle, which might lead to important quantum delocalization effects of the protons over the torus.rnrnOn the energy conversion side, the dynamics of protons in a proton conducting polymer (poly[vinyl phosphonic acid], PVPA) was studied by means of a steered ab-initio Molecular Dynamics approach applied on a simplified polymer model. The focus was put on understanding the microscopic proton transport mechanism in polymer membranes, and on characterizing the relevance of the local environment. This covers particularly the effect of water molecules, which participate in the hydrogen bonding network in the material. The results indicate that these water molecules are essential for the effectiveness of proton conduction. A water-mediated Grotthuss mechanism is identified as the main contributor to proton conduction, which agrees with the experimentally observed decay on conductivity for the same material in the absence of water molecules.rnrnThe gain in understanding the microscopic processes and structures present in this materials can help the development of new materials with improved properties, thus contributing to the solution of problems in the implementation of fuel cells.