Cell-type-specific localization of protocadherin β16 at AMPA and AMPA/Kainate receptor-containing synapses in the primate retina.

Protocadherins (Pcdhs) are thought to be key features of cell-type-specific synapse formation. Here we analyzed the expression pattern of Pcdh subunit β16 (β16) in the primate retina by applying antibodies against β16, different subunits of ionotropic glutamate receptors (GluRs), and cell-type-specific markers as well as by coimmunoprecipitation and Western blots. Immunocytochemical localization was analyzed by confocal microscopy and preembedding electron microscopy. In the outer plexiform layer (OPL) H1, but not H2, horizontal cells expressed β16 as revealed by the strong reduction of β16 at short-wavelength-sensitive cones. β16 colocalized with the GluR subunits GluR2-4 at horizontal cell dendritic tips and with GluR2-4 and GluR6/7 at the desmosome-like junctions. At the latter, these AMPA and kainate receptor subunits were found to be clustered within single synaptic hot spots. Additionally, β16-labeled dendritic tips of OFF cone bipolar cells appeared in triad-associated positions at the cone pedicle base, pointing to β16 expression by OFF midget or DB3 bipolar cells. In the inner plexiform layer, β16 was localized also postsynaptically at most of the glutamatergic synapses. Overall, we provide evidence for a cell-type-specific localization of β16 together with GluRs at defined postsynaptic sites and a coexistence of AMPA and kainate receptors within single synaptic hot spots. This study supports the hypothesis that β16 plays an important role in the formation and/or stabilization of specific glutamatergic synapses, whereas our in vivo protein biochemical results argue against the existence of protein complexes formed by β16 and GluRs.