Gamma-protocadherin localization at the synapse corresponds to parameters of synaptic maturation

Clustered protocadherins (Pcdhs) are a large family of ~60 cadherin-like proteins (divided into the subclasses , {beta}, and {gamma}) that compose a surface "barcode" in individual neurons. The code is generated through combinatorial expression via epigenetic regulation at a large gene cluster that encodes the molecules. During early neural development, Pcdhs were shown to mediate dendrite self-avoidance in some neuronal types through a still uncharacterized anti-adhesive mechanism. Pcdhs were also shown to be important for dendritic complexity in cortical neurons likely through a pro-adhesive mechanism. Pcdhs have also been postulated to participate in synaptogenesis and the specificity of connectivity. Some synaptic defects were noted in knockout animals, including synaptic number and physiology, but the role of these molecules in synaptic development is not understood. The effects of Pcdh knockout on dendritic patterning may present a confound to studying synaptogenesis. We have shown previously in vivo and in cultures that Pcdh-{gamma}s are highly enriched in intracellular compartments located in dendrites and spines with localization at only a few synaptic clefts. To gain insight into how Pcdh-{gamma}s might affect synapses, we compared synapses that harbored endogenous Pcdh-{gamma}s versus those that did not for parameters of synaptic maturation including pre- and postsynaptic size, postsynaptic perforations, and spine morphology by light microscopy in cultured hippocampal neurons and by serial section immuno-electron microscopy in hippocampal CA1. In mature neurons, synapses immunopositive for Pcdh-{gamma}s were found to be larger in diameter with more frequent perforations. Analysis of spines in cultured neurons revealed that mushroom spines were more frequently immunopositive for Pcdh-{gamma}s at their tips than thin spines. Taken together, these results suggest that Pcdh-{gamma} function at the synapse may be related to promotion of synaptic maturation and stabilization.