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C. Puller, S. Haverkamp; Protocadherin Beta16 is Localized at AMPA and Kainate Receptor Expressing Synapses in the Outer Plexiform Layer of the Primate Retina. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1044.
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© ARVO (1962-2015); The Authors (2016-present)
The largest subfamily of cadherins is comprised of the protocadherins. They are thought to be key features of cell type specific synapse formation. The Protocadherin subunit beta16 (b16) has been shown to localize postsynaptically to photoreceptors at distinct synapses and neuronal subtypes in the adult mouse retina (Junghans et al., Eur J Neurosci. 2008; 27:559-71). Here, we analyzed the expression pattern of b16 in the outer plexiform layer (OPL) of primate retina.
Antibodies against b16, different subunits of ionotropic glutamate receptors (GluRs), and against synaptic components as well as cell type specific markers were applied. Immunocytochemical localization was analyzed by confocal microscopy and pre-embedding electron microscopy.
Dendritic and axonal tips of H1-horizontal cells expressed b16. There was a strong reduction of b16 at short-wavelength sensitive cone photoreceptors. b16 was also observed at the level of desmosome-like junctions beneath the cone pedicle. b16 colocalized with the GluR subunits GluR2 - 4 (AMPA) and GluR6/7 (kainate), which are expressed by horizontal cells. We also found a colocalization of AMPA and kainate receptor subunits at these sites. Furthermore, we found b16 labeled dendritic tips of OFF-cone bipolar cells in triad-associated (TA) positions at the cone pedicle base.
Our data confirms the findings from the mouse OPL that b16 expression is restricted to distinct types of horizontal and OFF bipolar cells. The finding of b16 at TA positions points to its expression by OFF midget bipolar cells. We provide evidence for a clustering of b16 together with GluRs at postsynaptic sites and show that the expression of b16 is the same amongst different mammalian species. This study supports the hypothesis that b16 plays an important role in the formation and/or stabilization of specific glutamatergic synapses and neuronal networks. The coexistence of AMPA and kainate receptors within the same postsynaptic cluster on horizontal cell dendrites was intriguing.
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