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      Relating structure and function of inner hair cell ribbon synapses

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          Abstract

          In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus–secretion coupling, exocytosis mechanisms, exo–endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

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          Most cited references 238

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          Far-field optical nanoscopy.

           Stefan Hell (2007)
          In 1873, Ernst Abbe discovered what was to become a well-known paradigm: the inability of a lens-based optical microscope to discern details that are closer together than half of the wavelength of light. However, for its most popular imaging mode, fluorescence microscopy, the diffraction barrier is crumbling. Here, I discuss the physical concepts that have pushed fluorescence microscopy to the nanoscale, once the prerogative of electron and scanning probe microscopes. Initial applications indicate that emergent far-field optical nanoscopy will have a strong impact in the life sciences and in other areas benefiting from nanoscale visualization.
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            Visualizing secretion and synaptic transmission with pH-sensitive green fluorescent proteins.

            In neural systems, information is often carried by ensembles of cells rather than by individual units. Optical indicators provide a powerful means to reveal such distributed activity, particularly when protein-based and encodable in DNA: encodable probes can be introduced into cells, tissues, or transgenic organisms by genetic manipulation, selectively expressed in anatomically or functionally defined groups of cells, and, ideally, recorded in situ, without a requirement for exogenous cofactors. Here we describe sensors for secretion and neurotransmission that fulfil these criteria. We have developed pH-sensitive mutants of green fluorescent protein ('pHluorins') by structure-directed combinatorial mutagenesis, with the aim of exploiting the acidic pH inside secretory vesicles to monitor vesicle exocytosis and recycling. When linked to a vesicle membrane protein, pHluorins were sorted to secretory and synaptic vesicles and reported transmission at individual synaptic boutons, as well as secretion and fusion pore 'flicker' of single secretory granules.
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              Channelrhodopsin-2, a directly light-gated cation-selective membrane channel.

              Microbial-type rhodopsins are found in archaea, prokaryotes, and eukaryotes. Some of them represent membrane ion transport proteins such as bacteriorhodopsin, a light-driven proton pump, or channelrhodopsin-1 (ChR1), a recently identified light-gated proton channel from the green alga Chlamydomonas reinhardtii. ChR1 and ChR2, a related microbial-type rhodopsin from C. reinhardtii, were shown to be involved in generation of photocurrents of this green alga. We demonstrate by functional expression, both in oocytes of Xenopus laevis and mammalian cells, that ChR2 is a directly light-switched cation-selective ion channel. This channel opens rapidly after absorption of a photon to generate a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light to a smaller steady-state conductance. Recovery from desensitization is accelerated by extracellular H+ and negative membrane potential, whereas closing of the ChR2 ion channel is decelerated by intracellular H+. ChR2 is expressed mainly in C. reinhardtii under low-light conditions, suggesting involvement in photoreception in dark-adapted cells. The predicted seven-transmembrane alpha helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. Finally, we demonstrate that ChR2 may be used to depolarize small or large cells, simply by illumination.
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                Author and article information

                Contributors
                cwichma@gwdg.de
                tmoser@gwdg.de
                Journal
                Cell Tissue Res
                Cell Tissue Res
                Cell and Tissue Research
                Springer Berlin Heidelberg (Berlin/Heidelberg )
                0302-766X
                1432-0878
                22 January 2015
                22 January 2015
                2015
                : 361
                : 1
                : 95-114
                Affiliations
                [ ]Molecular Architecture of Synapses Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany
                [ ]Collaborative Research Center 889, University Medical Center Göttingen, Göttingen, Germany
                [ ]Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany
                [ ]Center for Nanoscale Microscopy and Molecular Physiology of the Brain, University of Göttingen, Göttingen, Germany
                [ ]Bernstein Center for Computational Neuroscience, University of Göttingen, Göttingen, Germany
                Article
                2102
                10.1007/s00441-014-2102-7
                4487357
                25874597
                © The Author(s) 2015

                Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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                © Springer-Verlag Berlin Heidelberg 2015

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