N-methyl-d-aspartate receptors (NMDARs) are located in neuronal cell membranes at synaptic and extrasynaptic locations, where they are believed to mediate distinct physiological and pathological processes. Activation of NMDARs requires glutamate and a coagonist whose nature and impact on NMDAR physiology remain elusive. We report that synaptic and extrasynaptic NMDARs are gated by different endogenous coagonists, d-serine and glycine, respectively. The regionalized availability of the coagonists matches the preferential affinity of synaptic NMDARs for d-serine and extrasynaptic NMDARs for glycine. Furthermore, glycine and d-serine inhibit NMDAR surface trafficking in a subunit-dependent manner, which is likely to influence NMDARs subcellular location. Taking advantage of this coagonist segregation, we demonstrate that long-term potentiation and NMDA-induced neurotoxicity rely on synaptic NMDARs only. Conversely, long-term depression requires both synaptic and extrasynaptic receptors. Our observations provide key insights into the operating mode of NMDARs, emphasizing functional distinctions between synaptic and extrasynaptic NMDARs in brain physiology.

AMPA glutamate receptors (AMPARs) mediate fast excitatory synaptic transmission. Upon fast consecutive synaptic stimulation, transmission can be depressed. Recuperation from fast synaptic depression has been attributed solely to recovery of transmitter release and/or AMPAR desensitization. We show that AMPAR lateral diffusion, observed in both intact hippocampi and cultured neurons, allows fast exchange of desensitized receptors with naïve functional ones within or near the postsynaptic density. Recovery from depression in the tens of millisecond time range can be explained in part by this fast receptor exchange. Preventing AMPAR surface movements through cross-linking, endogenous clustering, or calcium rise all slow recovery from depression. Physiological regulation of postsynaptic receptor mobility affects the fidelity of synaptic transmission by shaping the frequency dependence of synaptic responses.The fidelity of synaptic transmission between coupled neurons depends on their ability to transmit activity over a wide range of frequencies. Because of the relative slowness of chemical transmission, synaptic transmission acts as a low-pass filter with a cutoff between 10 and 100 Hz (1). When a presynaptic cell is stimulated at repetitive short intervals, the postsynaptic response usually decreases over time, the rate of depression being faster as the stimulus frequency increases (2). Most studies explain paired-pulse depression (PPD) as a combination of depression of presynaptic glutamate release and intrinsic kinetic properties of postsynaptic AMPARs upon agonist binding (2). Return from depression is believed to arise from recovery of release, together with AMPAR exit from desensitization. This assumes that AMPARs are stable within the postsynaptic density (PSD). Dynamic imaging has shown that AMPARs are not static but diffuse rapidly at the surface of neurons, traveling micrometer distances per second by random movements both in the synaptic and extrasynaptic membranes (3-8). Traffic of AMPARs from and to synapses through endo/exocytosis takes place in tens of minutes (9, 10). However, lateral diffusion allows AMPARs to explore the synapse in the second range (6,8,11), which suggests that surface AMPAR trafficking might be implicated in faster processes.

Accumulation of AMPA receptors at synapses is a fundamental feature of glutamatergic synaptic transmission. Stargazin, a member of the TARP family, is an AMPAR auxiliary subunit allowing interaction of the receptor with scaffold proteins of the postsynaptic density, such as PSD-95. How PSD-95 and Stargazin regulate AMPAR number in synaptic membranes remains elusive. We show, using single quantum dot and FRAP imaging in live hippocampal neurons, that exchange of AMPAR by lateral diffusion between extrasynaptic and synaptic sites mostly depends on the interaction of Stargazin with PSD-95 and not upon the GluR2 AMPAR subunit C terminus. Disruption of interactions between Stargazin and PSD-95 strongly increases AMPAR surface diffusion, preventing AMPAR accumulation at postsynaptic sites. Furthermore, AMPARs and Stargazin diffuse as complexes in and out synapses. These results propose a model in which the Stargazin-PSD-95 interaction plays a key role to trap and transiently stabilize diffusing AMPARs in the postsynaptic density.

The basis for differences in activity-dependent trafficking of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) remains unclear. Using single-molecule tracking, we found different lateral mobilities for AMPARs and NMDARs: changes in neuronal activity modified AMPAR but not NMDAR mobility, whereas protein kinase C activation modified both. Differences in mobility were mainly detected for extrasynaptic AMPARs, suggesting that receptor diffusion between synaptic and extrasynaptic domains is involved in plasticity processes.

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The NR2 subunit composition of NMDA receptors (NMDARs) varies during development, and this change is important in NMDARdependent signaling. In particular, synaptic NMDAR switch from containing mostly NR2B subunit to a mixture of NR2B and NR2A subunits. The pathways by which neurons differentially traffic NR2A-and NR2B-containing NMDARs are poorly understood. Using single-particle and -molecule approaches and specific antibodies directed against NR2A and NR2B extracellular epitopes, we investigated the surface mobility of native NR2A and NR2B subunits at the surface of cultured neurons. The surface mobility of NMDARs depends on the NR2 subunit subtype, with NR2A-containing NMDARs being more stable than NR2B-containing ones, and NR2A subunit overexpression stabilizes surface NR2B-containing NMDARs. The developmental change in the synaptic surface content of NR2A and NR2B subunits was correlated with a developmental change in the time spent by the subunits within synapses. This suggests that the switch in synaptic NMDAR subtypes depends on the regulation of the receptor surface trafficking.development ͉ glutamate receptor ͉ lateral mobility N MDA receptors (NMDARs) are heterotetrameric cation channels composed of NR1 and NR2͞3 subunits (1). NMDARs are assembled early in the endoplasmic reticulum, and both NR1 and NR2 subunits are necessary for their association and their successful cell surface targeting (2). In addition to glutamate and glycine, NMDARs require membrane depolarization to open with high probability (3), making this receptor a pre-and postsynaptic activity coincident detector involved in the induction of Hebbian synaptic plasticity. The functional properties of NMDARs depend also on the subunit composition, and such subunit heterogeneity of synaptic NMDARs is thought to play an important role during synaptic development, maturation, and plasticity processes (4). During synaptic development, the subunit composition of synaptic NMDARs changes from heterodimers containing predominantly NR2B subunits at early stages to heterodimers containing NR1͞NR2B, NR1͞NR2A, and NR1͞NR2A͞NR2B subunits at mature stage (1,(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). This change often is associated with the refinement of neuronal connections within cortical areas, although this model has been challenged and, thus, is likely incomplete (4). The pathways by which neurons differentially traffic NR2A-and NR2B-containing NMDARs remain, however, an open question of crucial importance to understand the shaping of synaptic maturation and plasticity.Changes in NR2 subunit composition of NMDARs within synapses can be triggered by mechanisms that include differences in insertion (15), internalization (16, 17), and͞or lateral diffusion. Interestingly, NMDARs diffuse laterally at the neuronal surface (18,19). In immature neurons, synaptic NMDARs are replaced rapidly by extrasynaptic ones through lateral diffusion (18), suggesting that surface mobility of NMDARs may be involved in shaping mature NMDAR synaptic components. In this study, we i...

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Autoimmune synaptic encephalitides are recently described human brain diseases leading to psychiatric and neurological syndromes through inappropriate brain-autoantibody interactions. The most frequent synaptic autoimmune encephalitis is associated with autoantibodies against extracellular domains of the glutamatergic N-methyl-d-aspartate receptor, with patients developing psychotic and neurological symptoms in an autoantibody titre-dependent manner. Although N-methyl-d-aspartate receptors are the primary target of these antibodies, the cellular and molecular pathway(s) that rapidly lead to N-methyl-d-aspartate receptor dysfunction remain poorly understood. In this report, we used a unique combination of high-resolution nanoparticle and bulk live imaging approaches to demonstrate that anti-N-methyl-d-aspartate receptor autoantibodies from patients with encephalitis strongly alter, in a time-dependent manner, the surface content and trafficking of GluN2-NMDA receptor subtypes. Autoantibodies laterally displaced surface GluN2A-NMDA receptors out of synapses and completely blocked synaptic plasticity. This loss of extrasynaptic and synaptic N-methyl-d-aspartate receptor is prevented both in vitro and in vivo, by the activation of EPHB2 receptors. Indeed, the anti-N-methyl-d-aspartate receptor autoantibodies weaken the interaction between the extracellular domains of the N-methyl-d-aspartate and Ephrin-B2 receptors. Together, we demonstrate that the anti-N-methyl-d-aspartate receptor autoantibodies from patients with encephalitis alter the dynamic retention of synaptic N-methyl-d-aspartate receptor through extracellular domain-dependent mechanism(s), shedding new light on the pathology of the neurological and psychiatric disorders observed in these patients and opening possible new therapeutic strategies.