Synaptic Trafficking of Glutamate Receptors by MAGUK Scaffolding Proteins in the Mammalian Hippocampus
At Schaffer collateral synapses in area CA1 of the hippocampus, presynaptically released glutamate activates two types of postsynaptic ionotropic receptors, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPAR) and N-methyl-D-asparate (NMDAR) receptors. The trafficking of AMPA and NMDA receptors to and from synapses controls excitatory synaptic transmission. The molecular mechanisms regulating this trafficking remain largely unknown. I show that members of the family of PSD-95-like membrane associated guanylate kinases (PSD-MAGUK) scaffolding protein mediate the synaptic targeting of AMPARs in the adult hippocampus, and uncover a remarkable functional redundancy within this protein family. PSD-95 and PSD-93 independently mediate AMPAR trafficking at mature synapses. These studies also reveal unanticipated synapse heterogeneity as loss of either PSD-95 or PSD-93 silences largely non-overlapping populations of excitatory synapses. In adult mice lacking PSD-95 and PSD-93, SAP102 is up-regulated and compensates for the loss of synaptic AMPARs. These studies establish a PSD-MAGUK-specific regulation of AMPAR synaptic trafficking that maintains synaptic transmission in the adult mammalian brain. The development of synapses involves a change in the number and type of glutamate receptors. To elucidate the molecular mechanisms controlling this process in vivo I combine, for the first time, in utero intraventricular injection and electroporation of constructs that alter the molecular composition of developing synapses, with simultaneous dual whole-cell recordings in acute hippocampal slices. I find that SAP102 mediates synaptic trafficking of AMPARs and NR2B-containing NMDARs (NR2B-NMDARs) during synaptogenesis. After synaptogenesis, PSD-95 assumes the functions of SAP102 and is necessary for both the developmental increase in AMPAR-mediated transmission and the replacement of NR2B-NMDARs with NR2A-NMDARs. In mice lacking PSD-95 and PSD-93 the maturational replacement of NR2B- with NR2A-NMDARs fails to occur, and PSD-95 expression in these mice fully rescues this deficit. Thus, SAP102 and PSD-95 regulate the synaptic trafficking of distinct glutamate receptor subtypes at different developmental stages thereby playing a necessary role in excitatory synapse development. These studies establish an essential role for PSD-MAGUK scaffolding proteins in the synaptic targeting of glutamate receptors during the development and maintenance of excitatory synaptic transmission in a mammalian brain circuit for learning and memory.