D-Serine Signaling and NMDAR-Mediated Synaptic Plasticity Are Regulated by System A-Type of Glutamine/D-Serine Dual Transporters

Abstract

D-serine is a physiologic coagonist of NMDA receptors (NMDARs) required for synaptic plasticity, but mechanisms that terminate D-serine signaling are unclear. In particular, the identity of unidirectional plasma membrane transporters that mediate D-serine reuptake has remained elusive. We report that D-serine and glutamine share the same neuronal transport system, consisting of the classic system A transporters Slc38a1 and Slc38a2. We show that these transporters are not saturated with glutamine in vivo and regulate the extracellular levels of D-serine and NMDAR activity. Glutamine increased the NMDAR-dependent long-term potentiation and the isolated NMDAR potentials at the Schaffer collateral-CA1 synapses, but without affecting basal neurotransmission in male mice. Glutamine did not increase the NMDAR potentials in slices from serine racemase knock-out mice, which are devoid of D-serine, indicating that the effect of glutamine is caused by outcompeting D-serine for a dual glutamine-D-serine transport system. Inhibition of the system A reduced the uptake of D-serine in synaptosomes and neuronal cultures of mice of either sex, while increasing the extracellular D-serine concentration in slices and in vivo by microdialysis. When compared with Slc38a2, the Slc38a1 transporter displayed more favorable kinetics toward the Denantiomer. Biochemical experiments with synaptosomes from Slc38a1 knock-down mice of either sex further support its role as a D-serine reuptake system. Our study identifies the first concentrative and electrogenic transporters mediating D-serine reuptake in vivo. In addition to their classical role in the glutamine-glutamate cycle, system A transporters regulate the synaptic turnover of D-serine and its effects on NMDAR synaptic plasticity.

Serine Racemase Deletion Affects the Excitatory/Inhibitory Balance of the Hippocampal CA1 Network

“…Together, these results highlight that genetically modified models must be studied carefully because compensatory mechanisms can mask changes in synapses by maintaining overall network activity. Nevertheless, SR-KO mice are still an excellent animal model to validate our understanding of mechanisms that are dependent on d -serine availability at the synapse, as recently shown for the role of the system A-type of glutamine transporters in terminating d -serine signaling [ 41 ] or the Asc1 subtype of neutral amino acid transporters in mediating d -serine release [ 42 , 43 ].…”

NMDARs, Coincidence Detectors of Astrocytic and Neuronal Activities

“…Studies employing synaptosomes and competitive inhibitors have identified the following d -serine transporters: the Na + -independent transporter asc-1 [ 142 , 176 , 177 , 178 ] and the Na + -dependent transporters ASCT1 [ 145 ], ASCT2 [ 178 ], and System A transporters (SAT1 and SAT2) [ 176 ]. The immunohistochemistry indicates that, within the CNS, Asc-1 is a neuronal protein concentrated in the presynaptic complexes [ 177 , 179 , 180 , 181 ]; ASCT2 is limited to neuronal dendrites and, except for retinal glia, is not found in glia [ 142 ]; ASCT1 is present on astrocytes [ 142 , 145 ]; SAT1 is expressed mainly in GABAergic neurons [ 176 , 182 , 183 , 184 , 185 , 186 , 187 , 188 , 189 ], pyramidal neurons lack appreciable immunoreactivity [ 176 , 183 , 184 , 190 ], and SAT2 is expressed in the somatodendritic compartments of glutamatergic neurons, barely detected in interneurons, and potentially glia [ 185 , 189 , 191 , 192 , 193 , 194 , 195 ]. Although helpful in identifying transporters, synaptosomes and inhibitors, which are themselves substrates for transport, have limited use for studying d -serine regulation in vivo [ 196 ].…”

“…So far, only System A appears to accumulate intracellular d -serine in vivo. However, this relied on inhibition by the System A substrate MeAIB [ 176 ], which is also a substrate of the novel glycine transporter, SLC6A20A [ 173 ]; as such, confirmation awaits the development of a specific non-competitive inhibitor.…”

“…As the external d -serine is low, 5–8 µM [ 26 , 198 ], the high affinity of asc-1 for d -serine (19 µM) and rapid uptake [ 178 ] have made it the primary candidate for d -serine transport in vivo. In contrast, the low affinity of System A (2.3 mM) [ 176 ], ASCT1 (150 µM) [ 199 ] and ASCT2 (110–700 µM) [ 142 , 199 , 200 , 201 , 202 ] has indicated that they are unlikely to contribute significantly to d -serine uptake in vivo [ 178 ]. A more detailed study of the spatial–temporal uptake of extracellular d -serine may provide clarity.…”

“…From the published data, it is apparent that synaptic d -serine may be regulated differentially from bulk extracellular d -serine. Briefly, inhibition of asc-1 and System A, respectively, increased and decreased synaptic NMDAR function [ 176 , 197 ], indicating that, in contrast to bulk extracellular d -serine measurements, at the synaptic level, asc-1 takes up d -serine, and System A releases d -serine. Although blocking asc-1 only modestly enhanced synaptic NMDAR function, this is consistent with the observation that the sub-saturated co-agonist sites are held close to saturation [ 9 , 10 ].…”