CHONDROITIN SULFATE PROTEOGLYCAN SPECIFIC TO RETINAL HORIZONTAL NEURONS

Proteoglycans (PGs) are a diverse group of highly glycosylated macromo lecules that are implicated in the development and maintenance of neur onal circuitry. With its highly ordered, layered structure, the retina ideally serves to define the synthesis, processing, and distribution of these molecules within a specific cellular subpopulation. In retina l sections, monoclonal antibody (MAb) 6A2 immunostained a horizontal c ell-specific antigen. Antigen 6A2 was expressed within abundant proces ses in the outer plexiform layer and in rare neurites that extend acro ss the inner nuclear layer to the inner plexiform layer. Ultrastructur ally, the antigen was localized to cisternae within horizontal cell so mata, along tubulovesicular structures in dendrites, and in the perisy naptic space encircling presynaptic terminals of the cone photorecepto r triad. These findings suggest that this PG is synthesized within the horizontal cells, transported to the terminals, and released into the extracellular spaces just proximal to the synapse. Based on the focal stain in the adjacent photoreceptor cell, it is possible that antigen is pinocytosed by this cell and is concentrated at the ribbon synapse . In Western immunoblots of retinal homogenates, MAb 6A2 recognized a heterogeneous chondroitin sulfate (CS) PG (CSPG) of approximately 400- 500 kDa. After sequential enzymatic removal of CS glycosaminoglycans, a major broad band of 300-500 kDa was identified by MAb 1B5, which det ects CSPGs that bear uronic acid linked to unsulfated N-acetylgalactos amine as the initial disaccharide in the CS chain. Localization of thi s PG around presynaptic terminals of the horizontal neuron and at the ribbon synapse suggests that it may play a modulatory and sustaining r ole at the synapse. (C) 1998 Wiley-Liss, Inc.