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M.K. Manglapus, D.D. Hunter, W.J. Brunken; Bridging the Gap: Laminins Play a Key Role in the Maintenance of the Photoreceptor Synapse . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5156.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Laminins provide environmental signals that influence photoreceptor morphogenesis. Specifically their expression in the outer plexiform layer suggests that they may provide an extracellular scaffold for synapse formation and stabilization. Mice lacking the laminin ß 2 chain show a significant photoreceptor dysgenesis: photoreceptor synapses are structurally disorganized and the ERG b-wave is markedly attenuated. Together these data support the hypothesis ß 2-containing laminins play stabilize the photoreceptor synapse. Here, we demonstrate that laminins act to bridge the synaptic cleft and stabilize synapses by coupling signaling complexes across the synaptic cleft. Methods: We used standard immunohistochemisty to determine the expression pattern of several synaptic proteins in the adult mouse OPL. These proteins include bassoon, kinesin, mGluR6, α PKC, α 3 integrin, Erbin, and GRIP1. Confocal microscopy and deconvolution analysis provide high-resolution, 3D reconstruction, of photoreceptor synapses. Co-immunoprocipitation and Western blotting were used to study the functional associations. Results: Integrin α 3 immunoreactivity (IR) shows an expression pattern similar to that of bassoon and reveals arc-shaped structures that are closely associated but not colocalized with bassoon. This association remains in laminin ß 2 retinae with subtle changes in organization. Co-immunoprecipitation experiments suggest that integrin α 3 binds to ß 2-containing laminins. Erbin is a novel cytomatrix adapter protein, which binds the endodomains of both integrin b4 and Erb-B2 receptors. We show that Erbin is expressed exclusively by photoreceptors in the OPL and its subcellular distribution is not altered in the ß 2 laminin null retinae. Photoreceptor cells also express GRIP1, a metabotropic glutamate receptor interacting protein. Surprisingly, GRIP IR is also associated with the arc-shaped ribbon in the OPL. Unlike other "synaptic" markers, we also find GRIP expression in photoreceptor outer segments and its expression persists in the OPL and outersegments in ß 2 laminin deficient retinae. Conclusions:Integrins function as transmembrane laminin receptors to stabilize the photoreceptor synapse. In laminin ß 2 null mice, the structure of the photoreceptor synapse is perturbed. Specifically the transynaptic organization is disrupted while the expression and association of many presynaptic elements are apparently normal. Thus, the loss in laminin results in disruptions of postsynaptic elements.
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