Abstract
Distinct subtypes of ionotropic glutamate receptors can segregate to specific synaptic inputs in a given neuron. Using functional mapping by focal glutamate uncaging in CA3 pyramidal cells (PCs), we observe that kainate receptors (KARs) are strictly confined to the postsynaptic elements of mossy fibre (mf) synapses and excluded from other glutamatergic inputs and from extrasynaptic compartments. By molecular replacement in organotypic slices from GluK2 knockout mice, we show that the faithful rescue of KAR segregation at mf-CA3 synapses critically depends on the amount of GluK2a cDNA transfected and on a sequence in the GluK2a C-terminal domain responsible for interaction with N-cadherin. Targeted deletion of N-cadherin in CA3 PCs greatly reduces KAR content in thorny excrescences and KAR-EPSCs at mf-CA3 synapses. Hence, multiple mechanisms combine to confine KARs at mf-CA3 synapses, including a stringent control of the amount of GluK2 subunit in CA3 PCs and the recruitment/stabilization of KARs by N-cadherins.
Highlights
Distinct subtypes of ionotropic glutamate receptors can segregate to specific synaptic inputs in a given neuron
We show that the strict compartimentalization of kainate receptors (KARs) at mfCA3 synapses in CA3 pyramidal cells (PCs) is readily preserved in organotypic slices, providing a much improved model over dissociated cultured neurons, which express functional native KARs, not synaptically
Pathway-specific localization of postsynaptic KARs may be a general feature, which has been reported in pyramidal neurons of the medial entorhinal cortex[44] and at climbing fibre synapses onto Purkinje cells[45], indicating that confined expression of KARs can be observed in the absence of a morphologically specialized postsynaptic element such as thorny excrescences (TEs)
Summary
Distinct subtypes of ionotropic glutamate receptors can segregate to specific synaptic inputs in a given neuron. CA3 pyramidal cells (PCs) receive three types of glutamatergic inputs, which are precisely positioned along apical and basal dendrites, from the entorhinal cortex (perforant path), from recurrent CA3 collaterals (associational/ commissural, A/C fibres) and from the dentate gyrus (DG) through mossy fibres (mf). Glutamatergic synapses from these different inputs greatly vary in their structural and functional properties. In transfected hereologous cell lines, activation of N-cadherins by ligand-covered latex beads recruits GluK2 to N-cadherin/b-catenin complexes[16] Whether this interaction plays a role in the trafficking and stabilization of KARs at synaptic sites has not yet been explored. We have used focal glutamate uncaging combined with electrophysiology to precisely characterize the functional expression of KARs and combined this functional approach with a semi-quantitative gene replacement strategy to identify molecular mechanisms for the subcellular segregation of KARs in CA3 PCs
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