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Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Published online by Cambridge University Press:  20 July 2007

ARLENE A. HIRANO
Affiliation:
Departments of Neurobiology & Medicine, Geffen School of Medicine at UCLA, Los Angeles, California VAGLAHS, Los Angeles, California
JOHANN HELMUT BRANDSTÄTTER
Affiliation:
Institute for Biology, Department of Animal Physiology, University of Erlangen-Nuernberg, Erlangen, Germany Department of Neuroanatomy, Max Planck Institute for Brain Research, Frankfurt am Main, Germany
ALEJANDRO VILA
Affiliation:
Departments of Neurobiology & Medicine, Geffen School of Medicine at UCLA, Los Angeles, California
NICHOLAS C. BRECHA
Affiliation:
Departments of Neurobiology & Medicine, Geffen School of Medicine at UCLA, Los Angeles, California Jules Stein Eye Institute, Geffen School of Medicine at UCLA, Los Angeles, California CURE Digestive Diseases Research Center, Geffen School of Medicine at UCLA, Los Angeles, California VAGLAHS, Los Angeles, California

Abstract

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.

Information

Type
Research Article
Copyright
© 2007 Cambridge University Press

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