Co-agonists differentially tune GluN2B-NMDA receptor trafficking at hippocampal synapses

Abstract

The subunit composition of synaptic NMDA receptors (NMDAR), such as the relative content of GluN2A- and GluN2B-containing receptors, greatly influences the glutamate synaptic transmission. Receptor co-agonists, glycine and D-serine, have intriguingly emerged as potential regulators of the receptor trafficking in addition to their requirement for its activation. Using a combination of single-molecule imaging, biochemistry and electrophysiology, we show that glycine and D-serine relative availability at rat hippocampal glutamatergic synapses regulate the trafficking and synaptic content of NMDAR subtypes. Acute manipulations of co-agonist levels, both ex vivo and in vitro, unveil that D-serine alter the membrane dynamics and content of GluN2B-NMDAR, but not GluN2A-NMDAR, at synapses through a process requiring PDZ binding scaffold partners. In addition, using FRET-based FLIM approach, we demonstrate that D-serine rapidly induces a conformational change of the GluN1 subunit intracellular C-terminus domain. Together our data fuels the view that the extracellular microenvironment regulates synaptic NMDAR signaling.DOI: http://dx.doi.org/10.7554/eLife.25492.001

D-serine Measurements in Brain Slices or Other Tissue Explants

“…Therefore, detecting and measuring D-serine levels has become necessary in several subfields of neuroscience and other disciplines but has proven technically challenging. In the brain and spinal cord, assessing the occupancy of the NMDAR co-agonist binding site is an excellent first approach to assess D-serine levels (Papouin et al ., 2012; Papouin et al ., 2017b; Ferreira et al ., 2017). However, major limitations of this approach are that 1) it provides little quantitative insights into the actual concentration of D-serine, 2) it is subject to a strong ceiling effect (once the co-agonist binding site is saturated, higher levels of D-serine go undetected), 3) glycine can also bind to the NMDAR co-agonist binding site and compete with D-serine, 4) this approach is subject to changes and differences in the affinity of the NMDAR co-agonist binding site, and finally 5) this method is only useful in conditions where recording NMDAR activity is technically possible.…”

“…Two methods are currently available. The first one is electrophoresis-based, in particular high performance liquid chromatography (Papouin et al ., 2017b) or capillary electrophoresis (Ferreira et al ., 2017) which has been amply documented. While they provide high levels of precision and reliability, they also require expensive equipment and extensive technical expertise.…”

NYX‐2925 induces metabotropic N‐methyl‐d‐aspartate receptor (NMDAR) signaling that enhances synaptic NMDAR and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor

“…As reviewed by Dore and colleagues (Dore et al ), the existence and biological relevance of metabotropic NMDAR signaling is an active area of research with several investigators finding both supportive (Yang et al ; Kessels et al ; Nabavi et al ; Tamburri et al ; Dore et al ; Kim et al ; Stein et al ) and contradictory (Babiec et al ; Volianskis et al ; Sanderson et al ) evidence. Among the former, several studies have revealed ion flux‐independent trafficking of GluN2‐containing NMDARs both by NMDAR agonists (Vissel et al ; Barria and Malinow ) and by co‐agonists (Nong et al ; Ferreira et al ). Together, these data add to the rich repertoire by which neuronal communication can be shaped by a growing number of NMDAR modulatory mechanisms.…”

“…Although the population of neuronal GluN2A and GluN2B was initially thought to be enriched in the synaptic or extrasynaptic membrane, respectively (Kew et al ; Tovar and Westbrook ; Liu et al ), substantial amounts of extrasynaptic GluN2A and synaptic GluN2B have been identified (Luo et al ; Groc et al ; Thomas et al ; Harris and Pettit ; Petralia et al ), and the relative mobility of these receptor populations are subtype specific, with GluN2B being more mobile than GluN2A (Groc et al ; Ferreira et al ), and GluN2A less rapidly endocytosed than GluN2B (Lavezzari et al ). Thus, synaptic receptor levels are in a state of dynamic equilibrium being routed into and out of the synapse.…”

“…This partitioning suggests that specific NMDAR composition uniquely impacts neuronal integration of synaptic and extrasynaptic inputs (Groc et al ). Previous work has found that the GluN2A and GluN2B subtypes are dynamically trafficked into and out of the synaptic membrane by NMDAR agonists (Barria and Malinow ) and co‐agonists (Nong et al ; Ferreira et al ). Thus, NMDARs (particularly those containing the GluN2B subtype) are highly mobile and can exchange between synaptic and extrasynaptic sites (Tovar and Westbrook ; Groc et al ; Ferreira et al ).…”