Differential requirement for NMDAR activity in SAP 97 β - mediated regulation of the 1 number and strength of glutamatergic AMPAR - containing synapses

23 PSD-95-like, disc-large (DLG) family membrane-associated guanylate kinase proteins 24 (PSD/DLG-MAGUKs) are essential for regulating synaptic AMPA receptor (AMPAR) 25 function and activity-dependent trafficking of AMPARs. Using a molecular replacement 26 strategy to replace endogenous PSD-95 with SAP97β, we show that the prototypic β27 isoform of the PSD-MAGUKs, SAP97β, has distinct NMDA receptor (NMDAR)28 dependent roles in regulating basic properties of AMPAR-containing synapses. 29 SAP97β enhances the number of AMPAR-containing synapses in an NMDAR-dependent 30 manner, while its effect on the size of unitary synaptic response is not fully dependent on 31 NMDAR activity. These effects contrast with those of PSD-95α, which increases both the 32 number of AMPAR-containing synapses and the size of unitary synaptic responses, with or 33 without NMDAR activity. Our results suggest that SAP97β regulates synaptic AMPAR 34 content by increasing surface expression of GluA1-containing AMPARs, whereas PSD35 95α enhances synaptic AMPAR content presumably via increasing the synaptic scaffold 36 capacity for synaptic AMPARs. Our approach delineates discrete effects of different PSD37 MAGUKs on principal properties of glutamatergic synaptic transmission. Our results 38 suggest that the molecular diversity of PSD-MAGUKs can provide rich molecular substrates 39 for differential regulation of glutamatergic synapses in the brain. 40 41