Roles of afadin in functional differentiations of hippocampal mossy fiber synapse

A hippocampal mossy fiber synapsehas a complex structure and is implicated in learning and memory. In this synapse, the mossy fiber boutons attach to the dendritic shaft by puncta adherentia junctions and wrap around a multiply-branched spine, forming synaptic junctions. We have recently shown using transmission electron microscopy, immunoelectron microscopy and serial block face-scanning electron microscopythat atypical puncta adherentia junctions are formed in the afadin-deficient mossy fiber synapseand that the complexity of postsynaptic spines and mossy fiber boutons, the number of spine heads, the area of postsynaptic densitiesand the density of synaptic vesiclesdocked to active zonesare decreased in the afadin-deficient synapse. We investigated here the roles of afadin in the functional differentiations of the mossy fiber synapseusing the afadin-deficient mice. The electrophysiological studiesshowed that both the release probability of glutamate and the postsynaptic responsiveness to glutamate were markedly reduced, but not completely lost, in the afadin-deficient mossy fiber synapse, whereas neither long-term potentiationnor long-term depressionwas affected. These results indicate that afadin plays roles in the functional differentiations of the presynapse and the postsynapse of the hippocampal mossy fiber synapse.