Serine racemase deletion abolishes light-evoked NMDA receptor currents in retinal ganglion cells

Physiopathological Relevance of D-Serine in the Mammalian Cochlea

NMDA receptors (NMDARs) populate the complex between inner hair cell (IHC) and spiral ganglion neurons (SGNs) in the developing and mature cochlea. However, in the mature cochlea, activation of NMDARs is thought to mainly occur under pathological conditions such as excitotoxicity. Ototoxic drugs such as aspirin enable cochlear arachidonic-acid-sensitive NMDAR responses, and induced chronic tinnitus was blocked by local application of NMDAR antagonists into the cochlear fluids. We largely ignore if other modulators are also engaged. In the brain, D-serine is the primary physiological co-agonist of synaptic NMDARs. Whether D-serine plays a role in the cochlea had remained unexplored. We now reveal the presence of D-serine and its metabolic enzymes prior to, and at hearing onset, in the sensory and non-neuronal cells of the cochlea of several vertebrate species. In vivo intracochlear perfusion of D-serine in guinea pigs reduces sound-evoked activity of auditory nerve fibers without affecting the receptor potentials, suggesting that D-serine acts specifically on the postsynaptic auditory neurons without altering the functional state of IHC or of the stria vascularis. Indeed, we demonstrate in vitro that agonist-induced activation of NMDARs produces robust calcium responses in rat SGN somata only in the presence of D-serine, but not of glycine. Surprisingly, genetic deletion in mice of serine racemase (SR), the enzyme that catalyzes D-serine, does not affect hearing function, but offers protection against noise-induced permanent hearing loss as measured 3 months after exposure. However, the mechanisms of activation of NMDA receptors in newborn rats may be different from those in adult guinea pigs. Taken together, these results demonstrate for the first time that the neuro-messenger D-serine has a pivotal role in the cochlea by promoting the activation of silent cochlear NMDAR in pathological situations. Thus, D-serine and its signaling pathway may represent a new druggable target for treating sensorineural hearing disorders (i.e., hearing loss, tinnitus).

Effects of D-serine treatment on outer retinal function

The role of the N-Methyl-D-Aspartate Receptor (NMDAR) in the outer retina is unclear despite expression of the NMDAR-complex and its subunits in the outer retina. The flash-electroretinogram (fERG) offers a non-invasive measurement of the retinal field potentials of the outer retina that can serve to clarify NMDAR contribution to early retinal processing. The role of the NMDAR in retinal function was assessed using a genetic mouse model for NMDAR hypofunction (SR^-/-), where the absence of the enzyme serine racemase (SR) results in an 85% reduction of retinal D-serine. NMDAR hypo- and hyperfunction in the retina results in alterations in the components of the fERG. The fERG was examined after application of exogenous D-serine to the eye in order to determine whether pre- and post-topical delivery of D-serine would alter the fERG in SR^-/- mice and their littermate WT controls. Amplitude and implicit time of the low-frequency components, the a- and b-wave, were conducted. Reduced NMDAR function resulted in a statistically significantly delayed a-wave and reduced b-wave in SR^-/- animals. The effect of NMDAR deprivation was more prominent in male SR^-/- mice. A hyperfunction of the NMDAR, through exogenous topical delivery of 5 mM D-serine, in WT mice caused a significantly delayed a-wave implicit time and reduced b-wave amplitude. These changes were not observed in female WT mice. There were temporal delays in the a-wave and amplitude and a decrease in the b-wave amplitude and implicit time in both hypo- and NMDAR hyperfunctional male mice. These results suggest that NMDAR and D-serine are involved in the retinal field potentials of the outer retina that interact based on the animal's sex. This implicates the involvement of gonadal hormones and D-serine in retinal functional integrity.

Different glutamate sources and endogenous co-agonists activate extrasynaptic NMDA receptors on amacrine cells of the rod pathway microcircuit

The NMDA receptors (NMDARs) expressed by AII and A17 amacrine cells, the two main inhibitory interneurons of the rod pathway microcircuit in the mammalian retina, are exclusively extrasynaptic, activated by ambient levels of glutamate, and molecularly distinct, with AII and A17 amacrines expressing GluN2B- and GluN2A-containing receptors, respectively. This important sensory microcircuit thus provides a unique model to study the activation and function of extrasynaptic NMDARs. Here, we investigated the sources of glutamate and the endogenous co-agonists (d-serine or glycine) that activate these distinct populations of NMDARs. With acute slices from rat retina, we used whole-cell voltage-clamp recording and measurement of current noise to monitor levels of NMDAR activity. Pre-incubation of retina with bafilomycin A1 (an inhibitor of neurotransmitter uptake into synaptic vesicles) abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a vesicular source of glutamate activates AII NMDARs, whereas a non-vesicular source activates A17 NMDARs. Pre-incubation of retina with l-methionine sulfoximine (an inhibitor of glutamine synthetase) also abolished NMDAR-mediated noise in AII, but not A17 amacrines, suggesting a neuronal source of glutamate activates AII NMDARs, whereas a glial source activates A17 NMDARs. Enzymatic breakdown of d-serine reduced NMDAR-mediated noise in AII, but not A17 amacrines, suggesting d-serine is the endogenous co-agonist at AII, but not A17 NMDARs. Our results reveal unique characteristics of these two populations of extrasynaptic NMDARs. The differential and independent activation of these receptors is likely to provide specific contributions to the signal processing and plasticity of the cellular components of the rod pathway microcircuit.

Overexpression of D-amino acid oxidase prevents retinal neurovascular pathologies in diabetic rats

AIMS/HYPOTHESIS

Diabetic retinopathy is characterised by retinal neurodegeneration and retinal vascular abnormalities, affecting one third of diabetic patients with disease duration of more than 10 years. Accumulated evidence suggests that serine racemase (SR) and D-serine are correlated with the pathogenesis of diabetic retinopathy and the deletion of the Srr gene reverses neurovascular pathologies in diabetic mice. Since D-serine content is balanced by SR synthesis and D-amino acid oxidase (DAAO) degradation, we examined the roles of DAAO in diabetic retinopathy and further explored relevant therapy.

METHODS

Rats were used as a model of diabetes by i.p. injection of streptozotocin at the age of 2 months and blood glucose was monitored with a glucometer. Quantitative real-time PCR was used to examine Dao mRNA and western blotting to examine targeted proteins in the retinas. Bisulphite sequencing was used to examine the methylation of Dao mRNA promoter in the retinas. Intravitreal injection of DAAO-expressing adenovirus (AAV8-DAAO) was conducted one week before streptozotocin administration. Brain specific homeobox/POU domain protein 3a (Brn3a) immunofluorescence was conducted to indicate retinal ganglion cells at 3 months after virus injection. The permeability of the blood-retinal barrier was examined by Evans blue leakage from retinal capillaries. Periodic acid-Schiff staining and haematoxylin counterstaining were used to indicate retinal vasculature, which was further examined with double immunostaining at 7 months after virus injection.

RESULTS

At the age of 12 months, DAAO mRNA and protein levels in retinas from diabetic animals were reduced to 66.2% and 70.4% of those from normal (control) animals, respectively. The Dao proximal promoter contained higher levels of methylation in diabetic than in normal retinas. Consistent with the observation, DNA methyltransferase 1 was increased in diabetic retinas. Injection of DAAO-expressing virus completely prevented the loss of retinal ganglion cells and the disruption of blood-retinal barrier in diabetic rats. Diabetic retinas contained retinal ganglion cells at a density of 54 ± 4/mm2, which was restored to 68 ± 9/mm2 by DAAO overexpression, similar to the levels in normal retinas. The ratio between the number of endothelial cells and pericytes in diabetic retinas was 6.06 ± 1.93/mm2, which was reduced to 3.42 ± 0.55/mm2 by DAAO overexpression; the number of acellular capillaries in diabetic retinas was 10 ± 5/mm2, which was restored to 6 ± 2/mm2 by DAAO overexpression, similar to the levels in normal retinas. Injection of the DAAO-expressing virus increased the expression of occludin and reduced gliosis, which were examined to probe the mechanism by which the disrupted blood-retinal barrier in diabetic rats was rescued and retinal neurodegeneration was prevented.

CONCLUSIONS/INTERPRETATION

Altogether, overexpression of DAAO before the onset of diabetes protects against neurovascular abnormalities in retinas from diabetic rats, which suggests a novel strategy for preventing diabetic retinopathy. Graphical abstract.

Electroretinographic Abnormalities and Sex Differences Detected with Mesopic Adaptation in a Mouse Model of Schizophrenia: A and B Wave Analysis

Purpose

Mesopic flash electroretinography (fERG) as a tool to identify N-methyl-d-aspartate receptor (NMDAR) hypofunction in subjects with schizophrenia shows great potential. We report the first fERG study in a genetic mouse model of schizophrenia characterized by NMDAR hypofunction from gene silencing of serine racemase (SR) expression (SR-/-), an established risk gene for schizophrenia. We analyzed fERG parameters under various background light adaptations to determine the most significant variables to allow for early identification of people at risk for schizophrenia, prior to onset of psychosis. SR is a risk gene for schizophrenia, and negative and cognitive symptoms antedate the onset of psychosis that is required for diagnosis.

Methods

The scotopic, photopic, and mesopic fERGs were analyzed in male and female mice in both SR-/- and wild-type (WT) mice and also analyzed for sex differences. Amplitude and implicit time of the a- and b-wave components, b-/a-wave ratio, and Fourier transform analysis were analyzed.

Results

Mesopic a- and b-wave implicit times were significantly delayed, and b-wave amplitudes, b/a ratios, and Fourier transform were significantly decreased in the male SR-/- mice compared to WT, but not in female SR-/- mice. No significant differences were observed in photopic or scotopic fERGs between genotype.

Conclusions

The fERG prognostic capability may be improved by examination of background light adaptation, a larger array of light intensities, considering sex as a variable, and performing Fourier transform analyses of all waveforms. This should improve the ability to differentiate between controls and subjects with schizophrenia characterized by NMDAR hypofunction.

Loss-of-function mutation of serine racemase attenuates retinal ganglion cell loss in diabetic mice

Consistent results suggest the promoting roles of serine racemase (SR)/D-serine in retinal neurodegeneration in diabetic retinopathy (DR). However, the direct evidence connecting SR deficiency with retinal neuroprotection in genetic model of diabetes mellitus has not been reported. In this investigation, we explore the effect of absence of functional SR on the degeneration of retinal ganglion cells (RGCs) with a diabetic murine model, Ins2^Akita mice. We established a murine strain with double mutation, termed Ins2^Akita-Srr, by mating heterozygous Ins2^Akita mice with homozygous Srr^ochre269 mice. Ins2^Akita retained less RGC in posterior, middle, and peripheral retinae than the counterpart from non-diabetic sibling mice at the age of five or seven months. Ins2^Akita-Srr mice retained more RGC in middle and peripheral--but not in posterior-- retinae than the counterpart from Ins2^Akita sibling mice at the age of five months. By contrast, at the age of seven months, Ins2^Akita-Srr mice contained more RGC in peripheral, middle, and posterior retinae than the counterpart from Ins2^Akita. RGCs were identified with retrograde labeling in vivo or with immunolabeling against a RGC-specific transcription factor, Brn3a, in retinal flat mounts. Correspondingly, the aqueous humor of Ins2^Akita-Srr contained less amount of D-serine than sibling Ins2^Akita mice. Thus, SR deficiency significantly prevented RGC loss in diabetic mice. We conclude that D-serine is a critical factor in the degeneration of RGC in DR. Targeting SR expression or activity may be a strategy for ameliorating RGC loss in DR.

Serine racemase deficiency attenuates choroidal neovascularization and reduces nitric oxide and VEGF levels by retinal pigment epithelial cells

Choroidal neovascularization (CNV) is a leading cause of blindness in age-related macular degeneration. Production of vascular endothelial growth factor (VEGF) and macrophage recruitment by retinal pigment epithelial cells (RPE) significantly contributes to the process of CNV in an experimental CNV model. Serine racemase (SR) is expressed in retinal neurons and glial cells, and its product, d-serine, is an endogenous co-agonist of N-methyl-d-aspartate receptor. Activation of the receptor results in production of nitric oxide (^. NO), a molecule that promotes retinal and choroidal neovascularization. These observations suggest possible roles of SR in CNV. With laser-injured CNV mice, we found that inactivation of SR-coding gene (Srr_null ) significantly reduced CNV volume, neovascular density, and invading macrophages. We exploited the underlying mechanism in vivo and ex vivo. RPE from wild-type (WT) mice expressed SR. To explore the possible downstream target of SR inactivation, we showed that choroid/RPE homogenates extracted from laser-injured Srr_null mice contained less inducible nitric oxide synthase and decreased phospho-VEGFR2 compared to amounts in WT mice. In vitro, inflammation-primed WT RPEs expressed more inducible NOS, produced more^. NO and VEGF than did inflammation-primed Srr_null RPEs. When co-cultured with inflammation-primed Srr_null RPE, significantly fewer RF/6A-a cell line of choroidal endothelial cell, migrated to the opposite side of the insert membrane than did cells co-cultured with pre-treated WT RPE. Altogether, SR deficiency reduces RPE response to laser-induced inflammatory stimuli, resulting in decreased production of a cascade of pro-angiogenic cytokines, including^. NO and VEGF, and reduced macrophage recruitment, which contribute synergistically to attenuated angiogenesis.

Phenylglycine analogs are inhibitors of the neutral amino acid transporters ASCT1 and ASCT2 and enhance NMDA receptor-mediated LTP in rat visual cortex slices

The N-methyl-d-aspartate receptor (NMDA) co-agonist d-serine is a substrate for the neutral amino acid transporters ASCT1 (SLC1A4) and ASCT2 (SLC1A5). We identified l-phenylglycine (PG) and its analogs as inhibitors of ASCT1 and ASCT2. PG analogs were shown to be non-substrate inhibitors of ASCT1 and ASCT2 with a range of activities relative to other amino acid transport systems, including sodium-dependent glutamate transporters, the sodium-independent d-serine transporter asc-1 and system L. L-4-chloroPG was the most potent and selective ASCT1/2 inhibitor identified. The PG analogs facilitated theta-burst induced long-term potentiation in rat visual cortex slices in a manner that was dependent on extracellular d-serine. For structurally-related PG analogs, there was an excellent correlation between ASCT1/2 transport inhibition and enhancement of LTP which was not the case for inhibition of asc-1 or system L. The ability of PG analogs to enhance LTP is likely due to inhibition of d-serine transport by ASCT1/2, leading to elevated extracellular levels of d-serine and increased NMDA receptor activity. These results suggest that ASCT1/2 may play an important role in regulating extracellular d-serine and NMDA receptor-mediated physiological effects and that ASCT1/2 inhibitors have the potential for therapeutic benefit.

Serine racemase is expressed in islets and contributes to the regulation of glucose homeostasis

NMDA receptors (NMDARs) have recently been discovered as functional regulators of pancreatic β-cell insulin secretion. While these excitatory receptor channels have been extensively studied in the brain for their role in synaptic plasticity and development, little is known about how they work in β-cells. In neuronal cells, NMDAR activation requires the simultaneous binding of glutamate and a rate-limiting co-agonist, such as D-serine. D-serine levels and availability in most of the brain rely on endogenous synthesis by the enzyme serine racemase (Srr). Srr transcripts have been reported in human and mouse islets but it is not clear whether Srr is functionally expressed in β-cells or what its role in the pancreas might be. In this investigation, we reveal that Srr protein is highly expressed in primary human and mouse β-cells. Mice with whole body deletion of Srr (Srr KO) show improved glucose tolerance through enhanced insulin secretory capacity, possibly through Srr-mediated alterations in islet NMDAR expression and function. We observed elevated insulin sensitivity in some animals, suggesting Srr metabolic regulation in other peripheral organs as well. Srr expression in neonatal and embryonic islets, and adult deficits in Srr KO pancreas weight and islet insulin content, point toward a potential role for Srr in pancreatic development. These data reveal the first evidence that Srr may regulate glucose homeostasis in peripheral tissues and provide circumstantial evidence that D-serine may be an endogenous islet NMDAR co-agonist in β-cells.

The postnatal development of D-serine in the retinas of two mouse strains, including a mutant mouse with a deficiency in D-amino acid oxidase and a serine racemase knockout mouse

D-Serine, an N-methyl D-aspartate receptor coagonist, and its regulatory enzymes, D-amino acid oxidase (DAO; degradation) and serine racemase (SR; synthesis), have been implicated in crucial roles of the developing central nervous system, yet the functional position that they play in regulating the availability of d-serine throughout development of the mammalian retina is not well-known. Using capillary electrophoresis and a sensitive method of enantiomeric amino acid separation, we were able to determine total levels of d-serine at specific ages during postnatal development of the mouse retina in two different strains of mice, one of which contained a loss-of-function point mutation for DAO while the other was a SR knockout line. Each mouse line was tested against conspecific wild type (WT) mice for each genetic strain. The universal trend in all WT and transgenic mice was a large amount of total retinal d-serine at postnatal age 2 (P2), followed by a dramatic decrease as the mice matured into adulthood (P70-80). SR knockout mice retinas had 41% less D-serine than WT retinas at P2, and 10 times less as an adult. DAO mutant mice retinas had significantly elevated levels of d-serine when compared to WT retinas at P2 (217%), P4 (223%), P8 (194%), and adulthood (227%).

Retinal NMDA receptor function and expression are altered in a mouse lacking D-amino acid oxidase

D-serine is present in the vertebrate retina and serves as a coagonist for the N-methyl-D-aspartate (NMDA) receptors of ganglion cells. Although the enzyme D-amino acid oxidase (DAO) has been implicated as a pathway for d-serine degradation, its role in the retina has not been established. In this study, we investigated the role of DAO in regulating D-serine levels using a mutant mouse line deficient in DAO (ddY/DAO(-)) and compared these results with their wild-type counterparts (ddY/DAO(+)). Our results show that DAO is functionally present in the mouse retina and normally serves to reduce the background levels of D-serine. The enzymatic activity of DAO was restricted to the inner plexiform layer as determined by histochemical analysis. Using capillary electrophoresis, we showed that mutant mice had much higher levels of D-serine. Whole cell recordings from identified retinal ganglion cells demonstrated that DAO-deficient animals had light-evoked synaptic activity strongly biased toward a high NMDA-to-AMPA receptor ratio. In contrast, recordings from wild-type ganglion cells showed a more balanced ratio between the two receptor subclasses. Immunostaining for AMPA and NMDA receptors was carried out to compare the two receptor ratios by quantitative immunofluorescence. These studies revealed that the mutant mouse had a significantly higher representation of NMDA receptors compared with the wild-type controls. We conclude that 1) DAO is an important regulatory enzyme and normally functions to reduce D-serine levels in the retina, and 2) D-serine levels play a role in the expression of NMDA receptors and the NMDA-to-AMPA receptor ratio.

Glycine transporter 1 modulates GABA release from amacrine cells by controlling occupancy of coagonist binding site of NMDA receptors

The occupancy of coagonist binding sites of NMDA receptors (NMDARs) by glycine or d-serine has been thought to mediate NMDAR-dependent excitatory signaling, as simultaneous binding of glutamate and a coagonist is obligatory for NMDAR activation. Amacrine cells (ACs) mediating GABAergic feedback inhibition of mixed bipolar cells (Mbs) in the goldfish retina have been shown to express NMDARs. Here we studied whether NMDAR-mediated GABAergic inhibitory currents (IGABA) recorded from the axon terminals of Mbs are influenced by experimental manipulations altering retinal glycine and d-serine levels. Feedback IGABA in Mb axon terminals was triggered by focal NMDA application or by synaptically released glutamate from depolarized Mb terminals. In both cases, blocking the coagonist binding sites of NMDARs eliminated the NMDAR-dependent IGABA, demonstrating that coagonist binding is critical in mediating NMDAR activity-triggered GABA release. Glycine transporter 1 (GLYT1) inhibition increased IGABA, indicating that coagonist binding sites of NMDARs on ACs providing GABAergic feedback inhibition to Mbs were not saturated. Focal glycine application, in the presence of the ionotropic glycine receptor blocker strychnine, triggered a GLYT1-dependent current in ACs, suggesting that GLYT1 expressed by putative glycinergic ACs controls the saturation level of NMDARs' coagonist sites. External d-serine also increased NMDAR activation-triggered IGABA in Mbs, further substantiating that the coagonist sites were unsaturated. Together, our findings demonstrate that coagonist modulation of glutamatergic input to GABAergic ACs via NMDARs is strongly reflected in the AC neuronal output (i.e., transmitter release) and thus is critical in GABAergic signal transfer function in the inner retina.

Pathogenic disruption of DISC1-serine racemase binding elicits schizophrenia-like behavior via D-serine depletion

Perturbation of Disrupted-In-Schizophrenia-1 (DISC1) and D-serine/NMDA receptor hypofunction have both been implicated in the pathophysiology of schizophrenia and other psychiatric disorders. In the present study, we demonstrate that these two pathways intersect with behavioral consequences. DISC1 binds to and stabilizes serine racemase (SR), the enzyme that generates D-serine, an endogenous co-agonist of the NMDA receptor. Mutant DISC1 fails to bind to SR, facilitating ubiquitination and degradation of SR and a decrease in D-serine production. To elucidate DISC1-SR interactions in vivo, we generated a mouse model of selective and inducible expression of mutant DISC1 in astrocytes, the main source of D-serine in the brain. Expression of mutant DISC1 downregulates endogenous DISC1 and decreases protein but not mRNA levels of SR, resulting in diminished production of D-serine. In contrast, mutant DISC1 does not alter levels of ALDH1L1, connexins, GLT-1 or binding partners of DISC1 and SR, LIS1 or PICK1. Adult male and female mice with lifelong expression of mutant DISC1 exhibit behavioral abnormalities consistent with hypofunction of NMDA neurotransmission. Specifically, mutant mice display greater responses to an NMDA antagonist, MK-801, in open field and pre-pulse inhibition of the acoustic startle tests and are significantly more sensitive to the ameliorative effects of D-serine. These findings support a model wherein mutant DISC1 leads to SR degradation via dominant negative effects, resulting in D-serine deficiency that diminishes NMDA neurotransmission thus linking DISC1 and NMDA pathophysiological mechanisms in mental illness.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

Astrocyte-derived thrombospondins mediate the development of hippocampal presynaptic plasticity in vitro

Astrocytes contribute to many neuronal functions, including synaptogenesis, but their role in the development of synaptic plasticity remains unclear. Presynaptic muting of hippocampal glutamatergic terminals defends against excitotoxicity. Here we studied the role of astrocytes in the development of presynaptic muting at glutamatergic synapses in rat hippocampal neurons. We found that astrocytes were critical for the development of depolarization-dependent and G(i/o)-dependent presynaptic muting. The ability of cAMP analogues to modulate presynaptic function was also impaired by astrocyte deficiency. Although astrocyte deprivation resulted in postsynaptic glutamate receptor deficits, this effect appeared independent of astrocytes' role in presynaptic muting. Muting was restored with chronic, but not acute, treatment with astrocyte-conditioned medium, indicating that a soluble factor is permissive for muting. Astrocyte-derived thrombospondins (TSPs) are likely responsible because TSP1 mimicked the effect of conditioned medium, and gabapentin, a high-affinity antagonist of TSP binding to the α2δ-1 calcium channel subunit, mimicked astrocyte deprivation. We found evidence that protein kinase A activity is abnormal in astrocyte-deprived neurons but restored by TSP1, so protein kinase A dysfunction may provide a mechanism by which muting is disrupted during astrocyte deficiency. In summary our results suggest an important role for astrocyte-derived TSPs, acting through α2δ-1, in maturation of a potentially important form of presynaptic plasticity.

Serine racemase: an unconventional enzyme for an unconventional transmitter

The discovery of large amounts of D-serine in the brain challenged the dogma that only L-amino acids are relevant for eukaryotes. The levels of D-serine in the brain are higher than many L-amino acids and account for as much as one-third of L-serine levels. Several studies in the last decades have demonstrated a role of D-serine as an endogenous agonist of N-methyl-D-aspartate receptors (NMDARs). D-Serine is required for NMDAR activity during normal neurotransmission as well as NMDAR overactivation that takes place in neurodegenerative conditions. Still, there are many unanswered questions about D-serine neurobiology, including regulation of its synthesis, release and metabolism. Here, we review the mechanisms of D-serine synthesis by serine racemase and discuss the lessons we can learn from serine racemase knockout mice, focusing on the roles attributed to D-serine and its cellular origin.

AMPA receptor-dependent, light-evoked D-serine release acts on retinal ganglion cell NMDA receptors

NMDA receptor (NMDAR) activation requires coincident binding of the excitatory neurotransmitter glutamate and a coagonist, either glycine or D-serine. Changes in NMDAR currents during neural transmission are typically attributed to glutamate release against a steady background of coagonist, excluding the possibility of coagonist release. AMPA receptor (AMPAR) stimulation evokes D-serine release, but it is unknown whether this is a physiological phenomenon capable of influencing synaptic responses. In this study, we utilized the intact retina to determine whether light-evoked synaptic activity in retinal ganglion cells (RGCs) is shaped by a dynamic pool of coagonist. The application of AMPAR antagonist abolished light-evoked NMDAR currents, which were rescued by adding coagonist to the bath. When NMDA was globally applied to RGCs via bath or picospritzing, the coagonist occupancy was also dependent on AMPARs but to a lesser extent than that observed during light responses, suggesting a difference in extrasynaptic coagonist regulation. By saturating the glutamate binding site of NMDARs, we were able to detect released coagonist reaching RGCs during light-evoked responses. Mutant mice lacking the d-serine-synthesizing enzyme serine racemase were deficient in coagonist release. Coagonist release in wild-type retinas was notably greater in ON than in OFF responses and depended on AMPARs. These findings suggest activity-dependent modulation of coagonist availability, particularly D-serine, and may add an extra dimension to NMDAR coincidence detection in the retina.