d-Serine inhibits AMPA receptor-mediated current in rat hippocampal neurons

Astrocytes control hippocampal synaptic plasticity through the vesicular-dependent release of D-serine

Astrocytes, the most abundant glial cells in the central nervous system (CNS), sense synaptic activity and respond through the release of gliotransmitters, a process mediated by intracellular Ca2+ level changes and SNARE-dependent mechanisms. Ionotropic N-methyl-D-aspartate (NMDA) receptors, which are activated by glutamate along with D-serine or glycine, play a crucial role in learning, memory, and synaptic plasticity. However, the precise impact of astrocyte-released D-serine on neuronal modulation remains insufficiently characterized. To address this, we have used the dominant negative SNARE (dnSNARE) mouse model, which selectively inhibits SNARE-dependent exocytosis from astrocytes. We recorded field excitatory postsynaptic potentials (fEPSPs) in CA3-CA1 synapses within hippocampal slices obtained from dnSNARE mice and wild-type (Wt) littermates. Our results demonstrate that hippocampal θ-burst long-term potentiation (LTP), a critical form of synaptic plasticity, is impaired in hippocampal slices from dnSNARE mice. Notably, this LTP impairment was rescued upon incubation with D-serine. To further investigate the involvement of astrocytes in D-serine-mediated mechanisms of LTP maintenance, we perfused hippocampal slices with L-serine - a substrate used by both neurons and astrocytes for D-serine production. The enhancement in LTP observed in dnSNARE mice was exclusively associated with D-serine presence, with no effects evident in the presence of L-serine. Additionally, both D- and L-serine reduced basal synaptic strength in the hippocampal slices of both Wt and dnSNARE mice. These results provide compelling evidence that distinct processes underlie the modulation of basal synaptic transmission and LTP through D-serine. Our findings underscore the pivotal contribution of astrocytes in D-serine-mediated processes that govern LTP establishment and basal transmission. This study not only provides essential insights into the intricate interplay between neurons and astrocytes but also emphasizes their collective role in shaping hippocampal synaptic function.

The t-N-methyl-d-aspartate receptor: Making the case for d-Serine to be considered its inverse co-agonist

The N-methyl-d-aspartate receptor (NMDAR) is an enigmatic macromolecule that has garnered a good deal of attention on account of its involvement in the cellular processes that underlie learning and memory, following its discovery in the mid twentieth century (Baudry and Davis, 1991). Yet, despite advances in knowledge about its function, there remains much more to be uncovered regarding the receptor's biophysical properties, subunit composition, and role in CNS physiology and pathophysiology. The motivation for this review stems from the need for synthesizing new information gathered about these receptors that sheds light on their role in synaptic plasticity and their dichotomous relationship with the amino acid d-serine through which they influence the pathogenesis of neurodegenerative diseases like temporal lobe epilepsy (TLE), the most common type of adult epilepsies (Beesley et al., 2020a). This review will outline pertinent ideas relating structure and function of t-NMDARs (GluN3 subunit-containing triheteromeric NMDARs) for which d-serine might serve as an inverse co-agonist. We will explore how tracing d-serine's origins blends glutamate-receptor biology with glial biology to help provide fresh perspectives on how neurodegeneration might interlink with neuroinflammation to initiate and perpetuate the disease state. Taken together, we envisage the review to deepen our understanding of endogenous d-serine's new role in the brain while also recognizing its therapeutic potential in the treatment of TLE that is oftentimes refractory to medications.

Racemization in Post-Translational Modifications Relevance to Protein Aging, Aggregation and Neurodegeneration: Tip of the Iceberg

Homochirality of DNA and prevalent chirality of free and protein-bound amino acids in a living organism represents the challenge for modern biochemistry and neuroscience. The idea of an association between age-related disease, neurodegeneration, and racemization originated from the studies of fossils and cataract disease. Under the pressure of new results, this concept has a broader significance linking protein folding, aggregation, and disfunction to an organism's cognitive and behavioral functions. The integrity of cognitive function is provided by a delicate balance between the evolutionarily imposed molecular homo-chirality and the epigenetic/developmental impact of spontaneous and enzymatic racemization. The chirality of amino acids is the crucial player in the modulation the structure and function of proteins, lipids, and DNA. The collapse of homochirality by racemization is the result of the conformational phase transition. The racemization of protein-bound amino acids (spontaneous and enzymatic) occurs through thermal activation over the energy barrier or by the tunnel transfer effect under the energy barrier. The phase transition is achieved through the intermediate state, where the chirality of alpha carbon vanished. From a thermodynamic consideration, the system in the homo-chiral (single enantiomeric) state is characterized by a decreased level of entropy. The oscillating protein chirality is suggesting its distinct significance in the neurotransmission and flow of perceptual information, adaptive associative learning, and cognitive laterality. The common pathological hallmarks of neurodegenerative disorders include protein misfolding, aging, and the deposition of protease-resistant protein aggregates. Each of the landmarks is influenced by racemization. The brain region, cell type, and age-dependent racemization critically influence the functions of many intracellular, membrane-bound, and extracellular proteins including amyloid precursor protein (APP), TAU, PrP, Huntingtin, α-synuclein, myelin basic protein (MBP), and collagen. The amyloid cascade hypothesis in Alzheimer's disease (AD) coexists with the failure of amyloid beta (Aβ) targeting drug therapy. According to our view, racemization should be considered as a critical factor of protein conformation with the potential for inducing order, disorder, misfolding, aggregation, toxicity, and malfunctions.

The DAO gene is associated with schizophrenia and interacts with other genes in the Taiwan Han Chinese population

BACKGROUND

Schizophrenia is a highly heritable disease with a polygenic mode of inheritance. Many studies have contributed to our understanding of the genetic underpinnings of schizophrenia, but little is known about how interactions among genes affect the risk of schizophrenia. This study aimed to assess the associations and interactions among genes that confer vulnerability to schizophrenia and to examine the moderating effect of neuropsychological impairment.

METHODS

We analyzed 99 SNPs from 10 candidate genes in 1,512 subject samples. The permutation-based single-locus, multi-locus association tests, and a gene-based multifactorial dimension reduction procedure were used to examine genetic associations and interactions to schizophrenia.

RESULTS

We found that no single SNP was significantly associated with schizophrenia. However, a risk haplotype, namely A-T-C of the SNP triplet rsDAO7-rsDAO8-rsDAO13 of the DAO gene, was strongly associated with schizophrenia. Interaction analyses identified multiple between-gene and within-gene interactions. Between-gene interactions including DAO*DISC1 , DAO*NRG1 and DAO*RASD2 and a within-gene interaction for CACNG2 were found among schizophrenia subjects with severe sustained attention deficits, suggesting a modifying effect of impaired neuropsychological functioning. Other interactions such as the within-gene interaction of DAO and the between-gene interaction of DAO and PTK2B were consistently identified regardless of stratification by neuropsychological dysfunction. Importantly, except for the within-gene interaction of CACNG2, all of the identified risk haplotypes and interactions involved SNPs from DAO.

CONCLUSIONS

These results suggest that DAO, which is involved in the N-methyl-d-aspartate receptor regulation, signaling and glutamate metabolism, is the master gene of the genetic associations and interactions underlying schizophrenia. Besides, the interaction between DAO and RASD2 has provided an insight in integrating the glutamate and dopamine hypotheses of schizophrenia.

Functional evidence for D-serine inhibition of non-N-methyl-D-aspartate ionotropic glutamate receptors in retinal neurons

D-Serine is an important signaling molecule throughout the central nervous system, acting as an N-methyl-D-aspartate (NMDA) receptor coagonist. This study investigated the D-serine modulation of non-NMDA ionotropic glutamate receptors expressed by inner retinal neurons. We first identified that the degradation of endogenous retinal D-serine, by application of D-amino acid oxidase, caused an enhancement of kainate- and α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor-mediated calcium responses from the ganglion cell layer of the isolated rat retina and light-evoked responses obtained by multi-electrode array recordings from the guinea pig retina. Approximately 30-45% of cells were endogenously inhibited by D-serine, as suggested by the effect of D-amino acid oxidase. Conversely, bath application of D-serine caused a reduction in multi-electrode array recorded responses and decreased kainate, but not potassium-induced calcium responses, in a concentration-dependent manner (IC(50), 280 μm). Using cultured retinal ganglion cells to reduce network influences, D-serine reduced kainate-induced calcium responses and AMPA induced whole-cell currents. Finally, the inhibitory effect of D-serine on the kainate-induced calcium response was abolished by IEM 1460, thereby identifying calcium-permeable AMPA receptors as a potential target for D-serine. To our knowledge, this is the first study to address specifically the effect of D-serine on AMPA/kainate receptors in intact central nervous system tissue, to identify its effect on calcium permeable AMPA receptors and to report the endogenous inhibition of AMPA/kainate receptors.

The neurobiology of D-amino acid oxidase and its involvement in schizophrenia

D-amino acid oxidase (DAO) is a flavoenzyme that metabolizes certain D-amino acids, notably the endogenous N-methyl D-aspartate receptor (NMDAR) co-agonist, D-serine. As such, it has the potential to modulate the function of NMDAR and to contribute to the widely hypothesized involvement of NMDAR signalling in schizophrenia. Three lines of evidence now provide support for this possibility: DAO shows genetic associations with the disorder in several, although not all, studies; the expression and activity of DAO are increased in schizophrenia; and DAO inactivation in rodents produces behavioural and biochemical effects, suggestive of potential therapeutic benefits. However, several key issues remain unclear. These include the regional, cellular and subcellular localization of DAO, the physiological importance of DAO and its substrates other than D-serine, as well as the causes and consequences of elevated DAO in schizophrenia. Herein, we critically review the neurobiology of DAO, its involvement in schizophrenia, and the therapeutic value of DAO inhibition. This review also highlights issues that have a broader relevance beyond DAO itself: how should we weigh up convergent and cumulatively impressive, but individually inconclusive, pieces of evidence regarding the role that a given gene may have in the aetiology, pathophysiology and pharmacotherapy of schizophrenia?

Glutamatergic modulatory therapy for Tourette syndrome

Tourette syndrome (TS) is a neuropsychiatric disorder characterized by the presence of chronic, fluctuating motor and vocal (phonic) tics. The disorder is commonly associated with a variety of comorbidities including obsessive-compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), school problems, anxiety, and depression. Therapeutically, if tics are causing psychosocial or physical problems, symptomatic medications are often prescribed, typically alpha-adrenergic agonists or dopamine antagonists. Recognizing that therapy is often ineffective and frequently associated with unacceptable side-effects, there is an ongoing effort to identify new tic-suppressing therapies. Several lines of evidence are presented that support the use of glutamate modulators in TS including glutamate's major role in cortico-striatal-thalamo-cortical circuits (CSTC), the recognized extensive interaction between glutamate and dopamine systems, results of familial genetic studies, and data from neurochemical analyses of postmortem brain samples. Since insufficient data is available to determine whether TS is definitively associated with a hyper- or hypo-glutamatergic state, potential treatment options using either glutamate antagonists or agonists are reviewed. Data from studies using these agents in the treatment of OCD are presented. If validated, modulation of the glutamate system could provide a valuable new pharmacological approach in the treatment of tics associated with Tourette syndrome.

D-serine signalling as a prominent determinant of neuronal-glial dialogue in the healthy and diseased brain

Rather different from their initial image as passive supportive cells of the CNS, the astrocytes are now considered as active partners at synapses, able to release a set of gliotransmitter-like substances to modulate synaptic communication within neuronal networks. Whereas glutamate and ATP were first regarded as main determinants of gliotransmission, growing evidence indicates now that the amino acid D-serine is another important player in the neuronal-glial dialogue. Through the regulation of glutamatergic neurotransmission through both N-methyl-D-aspartate (NMDA-R) and non-NMDA-R, D-serine is helping in modelling the appropriate connections in the developing brain and influencing the functional plasticity within neuronal networks throughout lifespan. The understanding of D-serine signalling, which has increased linearly in the last few years, gives new insights into the critical role of impaired neuronal-glial communication in the diseased brain, and offers new opportunities for developing relevant strategies to treat cognitive deficits associated to brain disorders.