Cell-type-specific localization of protocadherin b16 at @entity140 and XXXX / @entity4176 receptor-containing synapses in the primate retina.

@entity2227 @entity7336 @entity4176 @entity140 @entity2373
Protocadherins (Pcdhs) are thought to be key features of cell-type-specific synapse formation. Here we analyzed the expression pattern of Pcdh subunit b16 (b16) in the primate retina by applying antibodies against b16, different subunits of ionotropic @entity2227 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 b16 as revealed by the strong reduction of b16 at short-wavelength-sensitive cones. b16 colocalized with the GluR subunits @entity7336 -4 at horizontal cell dendritic tips and with @entity7336 -4 and @entity2373 /7 at the desmosome-like junctions. At the latter, these @entity140 and @entity4176 receptor subunits were found to be clustered within single synaptic hot spots. Additionally, b16-labeled dendritic tips of OFF cone bipolar cells appeared in triad-associated positions at the cone pedicle base, pointing to b16 expression by OFF midget or DB3 bipolar cells. In the inner plexiform layer, b16 was localized also postsynaptically at most of the glutamatergic synapses. Overall, we provide evidence for a cell-type-specific localization of b16 together with GluRs at defined postsynaptic sites and a coexistence of @entity140 and @entity4176 receptors within single synaptic hot spots. This study supports the hypothesis that b16 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 b16 and GluRs.
