D-Serine and glycine are coagonists of NMDA receptors (NMDARs), but their relative contributions for several NMDAR-dependent processes are unclear. We now report that the alanine–serine–cysteine transporter-1 (Asc-1) mediates release of bothd-serine and glycine from neurons, and, in turn, this modulates NMDAR synaptic activity. Asc-1 antiporter activity is enhanced byd-isoleucine (d-Ile), which releasesd-serine and glycine from Asc-1-transfected cells, primary neuronal cultures, and hippocampal slices.d-Ile has no effect on astrocytes, which do not express Asc-1. We show thatd-Ile enhances the long-term potentiation (LTP) in rat and mouse hippocampal CA1 by stimulating Asc-1-mediated endogenousd-serine release.d-Ile effects on synaptic plasticity are abolished by enzymatically depletingd-serine or by using serine racemase knock-out (SR-KO) mice, confirming its specificity and supporting the notion that LTP depends mostly ond-serine release. Conversely, our data also disclose a role of glycine in activating synaptic NMDARs. Although acute enzymatic depletion ofd-serine also drastically decreases the isolated NMDAR synaptic potentials, these responses are still enhanced byd-Ile. Furthermore, NMDAR synaptic potentials are preserved in SR-KO mice and are also enhanced byd-Ile, indicating that glycine overlaps withd-serine binding at synaptic NMDARs. Altogether, our results disclose a novel role of Asc-1 in regulating NMDAR-dependent synaptic activity by mediating concurrent non-vesicular release ofd-serine and glycine. Our data also highlight an important role of neuron-derivedd-serine and glycine, indicating that astrocyticd-serine is not solely responsible for activating synaptic NMDARs.

Significance: Phgdh is a crucial enzyme in the astrocytic synthesis of l -serine from glucose. Our study uncovers the role of glial-neuronal cross-talk in regulating NMDAR synaptic activation through the astrocytic export of Phgdh-derived l -serine to generate the coagonists d -serine and glycine. We clarify the relative roles of d -serine and glycine by showing that like d -serine, Phgdh-derived glycine mediates NMDAR synaptic activation in the mature hippocampus. We show that glycine is a significant regulator of d -serine synthesis and release, revealing a cross-talk between glycine and d -serine metabolism that fine tunes NMDAR activation. Our data reveal a multifaceted mechanism regulating NMDAR activity, which ultimately depends on a Phgdh-dependent serine shuttle.

D -serine is a physiologic coagonist of NMDA receptors, but little is known about the regulation of its synthesis and synaptic turnover. The amino acid exchangers ASCT1 (Slc1a4) and ASCT2 (Slc1a5) are candidates for regulating d -serine levels. Using ASCT1 and ASCT2 KO mice, we report that ASCT1, rather than ASCT2, is a physiologic regulator of d -serine metabolism. ASCT1 is a major d -serine uptake system in astrocytes and can also export l -serine via heteroexchange, supplying neurons with the substrate for d -serine synthesis. ASCT1-KO mice display lower levels of brain d -serine along with higher levels of l -alanine, l -threonine, and glycine. Deletion of ASCT1 was associated with neurodevelopmental alterations including lower hippocampal and striatal volumes and changes in the expression of neurodevelopmental-relevant genes. Furthermore, ASCT1-KO mice exhibited deficits in motor function, spatial learning, and affective behavior, along with changes in the relative contributions of d -serine vs. glycine in mediating NMDA receptor activity. In vivo microdialysis demonstrated lower levels of extracellular d -serine in ASCT1-KO mice, confirming altered d -serine metabolism. These alterations are reminiscent of some of the neurodevelopmental phenotypes exhibited by patients with ASCT1 mutations. ASCT1-KO mice provide a useful model for potential therapeutic interventions aimed at correcting the metabolic impairments in patients with ASCT1 mutations.

Summary: Oxidative stress (OS) resulting from an imbalance between antioxidant defenses and the intracellular accumulation of reactive oxygen species (ROS) contributes to age‐related memory deficits. While impaired synaptic plasticity in neuronal networks is thought to underlie cognitive deficits during aging, whether this process is targeted by OS and what the mechanisms involved are still remain open questions. In this study, we investigated the age‐related effects of the reducing agent N‐acetyl‐L‐cysteine (L‐NAC) on the activation of the N‐methyl‐ d‐aspartate receptor (NMDA‐R) by its co‐agonist d‐serine, because alterations in this mechanism contribute greatly to synaptic plasticity deficits in aged animals. Long‐term dietary supplementation with L‐NAC prevented oxidative damage in the hippocampus of aged rats. Electrophysiological recordings in the CA1 of hippocampal slices indicated that NMDA‐R‐mediated synaptic potentials and theta‐burst‐induced long‐term potentiation (LTP) were depressed in aged animals, deficits that could be reversed by exogenous d‐serine. Chronic treatment with L‐NAC, but not acute application of the reducing agent, restored potent d‐serine‐dependent NMDA‐R activation and LTP induction in aged rats. In addition, it is also revealed that the age‐related decrease in d‐serine levels and in the expression of the synthesizing enzyme serine racemase, which underlies the decrease in NMDA‐R activation by the amino acid, was rescued by long‐term dietary treatment with L‐NAC. Our results indicate that protecting redox status in aged animals could prevent injury to the cellular mechanisms underlying cognitive aging, in part by maintaining potent NMDA‐R activation through the d‐serine‐dependent pathway.