FBXO22 protein is required for optimal synthesis of the N-methyl-D-aspartate (NMDA) receptor coagonist D-serine.

d-Serine is a physiological activator of NMDA receptors (NMDARs) in the nervous system that mediates several NMDAR-mediated processes ranging from normal neurotransmission to neurodegeneration. d-Serine is synthesized from l-serine by serine racemase (SR), a brain-enriched enzyme. However, little is known about the regulation of d-serine synthesis. We now demonstrate that the F-box only protein 22 (FBXO22) interacts with SR and is required for optimal d-serine synthesis in cells. Although FBXO22 is classically associated with the ubiquitin system and is recruited to the Skip1-Cul1-F-box E3 complex, SR interacts preferentially with free FBXO22 species. In vivo ubiquitination and SR half-life determination indicate that FBXO22 does not target SR to the proteasome system. FBXO22 primarily affects SR subcellular localization and seems to increase d-serine synthesis by preventing the association of SR to intracellular membranes. Our data highlight an atypical role of FBXO22 in enhancing d-serine synthesis that is unrelated to its classical effects as a component of the ubiquitin-proteasome degradation pathway.

Neuronal D-serine and glycine release via the Asc-1 transporter regulates NMDA receptor-dependent synaptic activity.

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 both D-serine and glycine from neurons, and, in turn, this modulates NMDAR synaptic activity. Asc-1 antiporter activity is enhanced by D-isoleucine (D-Ile), which releases D-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 that D-Ile enhances the long-term potentiation (LTP) in rat and mouse hippocampal CA1 by stimulating Asc-1-mediated endogenous D-serine release. D-Ile effects on synaptic plasticity are abolished by enzymatically depleting D-serine or by using serine racemase knock-out (SR-KO) mice, confirming its specificity and supporting the notion that LTP depends mostly on D-serine release. Conversely, our data also disclose a role of glycine in activating synaptic NMDARs. Although acute enzymatic depletion of D-serine also drastically decreases the isolated NMDAR synaptic potentials, these responses are still enhanced by D-Ile. Furthermore, NMDAR synaptic potentials are preserved in SR-KO mice and are also enhanced by D-Ile, indicating that glycine overlaps with D-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 of D-serine and glycine. Our data also highlight an important role of neuron-derived D-serine and glycine, indicating that astrocytic D-serine is not solely responsible for activating synaptic NMDARs.

Feedback inactivation of D-serine synthesis by NMDA receptor-elicited translocation of serine racemase to the membrane.

D-serine is a physiological coagonist of N-methyl D-aspartate receptors (NMDARs) that plays a major role in several NMDAR-dependent events. In this study we investigate mechanisms regulating D-serine production by the enzyme serine racemase (SR). We now report that NMDAR activation promotes translocation of SR to the plasma membrane, which dramatically reduces the enzyme activity. Membrane-bound SR isolated from rat brain is not extracted from the membrane by high detergent and salt concentration, indicating a strong association. Colocalization studies indicate that most membrane-bound SR is located at the plasma membrane and dendrites, with much less SR observed in other types of membrane. NMDAR activation promotes translocation of the cytosolic SR to the membrane, resulting in reduced D-serine synthesis, and this effect is averted by blockade of NMDARs. In primary neuronal cultures, SR translocation to the membrane is blocked by a palmitoylation inhibitor, indicating that membrane binding is mediated by fatty acid acylation of SR. In agreement, we found that SR is acylated in transfected neuroblastoma cells using [(3)H]palmitate or [(3)H]octanoic acid as precursors. In contrast to classical S-palmitoylation of cysteines, acylation of SR occurs through the formation of an oxyester bond with serine or threonine residues. In addition, we show that phosphorylation of Thr-227 is also required for steady-state binding of SR to the membrane under basal, nonstimulated condition. We propose that the inhibition of D-serine synthesis caused by translocation of SR to the membrane provides a fail-safe mechanism to prevent NMDAR overactivation in vicinal cells or synapses.

Double-Balloon Device for 6 Compared With 12 Hours for Cervical Ripening: A Randomized Controlled Trial.

OBJECTIVE: To evaluate whether removal of a double-balloon device for cervical ripening for 6 compared with 12 hours in women with an unfavorable cervix will result in a shorter time to delivery, similar cervical ripening, and without affecting cesarean delivery rate. METHODS: In a prospective randomized trial, cervical ripening was performed using a double-balloon device. Women were randomized to removal of the device after 6 compared with 12 hours. Primary outcome was time to delivery. Secondary outcomes included mode of delivery, Bishop score, and maternal and neonatal adverse outcomes. A sample size of 100 nulliparous and 100 parous women was required assuming a 95% CI, power of 80%, and mean decrease of 6 hours to delivery between the groups. RESULTS: From March 2017 through February 2019, 688 women were screened, 243 were found eligible, and 197 were randomized as follows: nulliparous cohort (n=101): removal after 6 hours (n=48) compared with removal after 12 hours (n=53); parous cohort (n=96): removal after 6 hours (n=49) compared with removal after 12 hours (n=47). Insertion-to-delivery interval was significantly shorter in the 6-hour group for both nulliparous (25.6±12.8 hours vs 31.4±15.2 hours, P<.04; mean difference 5.8, 95% CI 0.2-11.3), and parous cohorts (18.0±6.8 hours vs 22.6±8.2 hours, P=.003; mean difference 4.7, 95% CI 1.6-7.7). Bishop score change and cesarean delivery rate were similar between groups regardless of parity. The 12-hour group in the combined cohort was associated with higher rates of maternal intrapartum fever (2% vs 10%, P=.02; odds ratio 5.3, 95% CI 1.1-24.8). CONCLUSION: Insertion-to-delivery interval is shorter after 6 compared with 12 hours for both nulliparous and parous women. Cervical ripening with a double-balloon device may be achieved in 6 hours. The longer time was associated with a higher rate of intrapartum fever. Six hours should be considered as standard placement time for double-balloon catheters. CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, NCT03045939.

Phosphorylation of mouse serine racemase regulates D-serine synthesis.

Serine racemase (SR) catalyses the synthesis of the transmitter/neuromodulator D-serine, which plays a major role in synaptic plasticity and N-methyl D-aspartate receptor neurotoxicity. We now report that SR is phosphorylated at Thr71 and Thr227 as revealed by mass spectrometric analysis and in vivo phosphorylation assays. Thr71 phosphorylation was observed in the cytosolic and membrane-bound SR while Thr227 phosphorylation was restricted to the membrane fraction. The Thr71 site has a motif for proline-directed kinases and is the main phosphorylation site of SR. Experiments with a phosphorylation-deficient SR mutant indicate that Thr71 phosphorylation increases SR activity, suggesting a novel mechanism for regulating D-serine production.