Serine racemase and the serine shuttle between neurons and astrocytes

Exocytosis in astrocytes: transmitter release and membrane signal regulation

Astrocytes, a type of glial cells in the brain, are eukaryotic cells, and a hallmark of these are subcellular organelles, such as secretory vesicles. In neurons vesicles play a key role in signaling. Upon a stimulus-an increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](i))-the membrane of vesicle fuses with the presynaptic plasma membrane, allowing the exit of neurotransmitters into the extracellular space and their diffusion to the postsynaptic receptors. For decades it was thought that such vesicle-based mechanisms of gliotransmitter release were not present in astrocytes. However, in the last 30 years experimental evidence showed that astrocytes are endowed with mechanisms for vesicle- and non-vesicle-based gliotransmitter release mechanisms. The aim of this review is to focus on exocytosis, which may play a role in gliotransmission and also in other forms of cell-to-cell communication, such as the delivery of transporters, ion channels and antigen presenting molecules to the cell surface.

D-Serine: a key to synaptic plasticity?

Two discoveries have put D-serine in the spotlight of neuroscience. First, D-serine was detected in brain tissue at high levels. Second, it was found to act on the N-methyl-D-aspartate receptor (NMDAR). This receptor is central to use-dependent synaptic plasticity, the cellular process which is widely believed to underlie learning. The ensuing quest for the mechanisms of D-serine synthesis, release and clearance, as well as for its physiological significance has provided a wealth of experimental evidence implicating D-serine in synaptic plasticity. However some key questions remain unanswered. Which cells release D-serine and upon what stimuli? Is D-serine supply dynamically regulated? What is the fate of released D-serine? Answering these questions appears to be an essential step in our understanding of how NMDARs trigger synaptic plasticity and learning. This review will highlight some recent advances and avenues of enquiry in dynamic D-serine signaling in the mammalian brain with emphasis on neurophysiology.

d-Serine diffusion through the blood-brain barrier: effect on d-serine compartmentalization and storage

d-Serine is a co-agonist of N-methyl-d-aspartate (NMDA) receptors. It has been implicated in the etiology of schizophrenia and has shown efficacy as an adjuvant to reduce positive and negative symptoms of schizophrenia. In addition, d-serine can modulate cognition in animals when administered alone. However, the neurochemical effects of exogenous d-serine on extra- and intra-cellular d-serine brain levels are poorly understood. In this study, we used both high performance liquid chromatography (HPLC) and enzyme-based microelectrode biosensors to quantify d-serine in the rat brain. We demonstrated levels of 2.3-2.8μM in the extracellular medium, 4μM in plasma and 188pmol/mg in brain tissue samples. An intraperitoneal (i.p.) d-serine injection (1g/kg) produced a slow increase in extracellular d-serine concentration in the cortex despite a surge in d-serine up to 13mM in the plasma, indicating poor diffusion through the blood-brain barrier. Using the respective volume fractions of blood, extracellular and intracellular spaces published in the literature, we estimated that d-serine intracellular stores represented more than 99% of total d-serine. These intracellular stores almost doubled 3h after d-serine administration. Overall, our data indicate that d-serine administration increases brain extra- and intra-cellular concentrations despite weak diffusion through the blood-brain barrier. These results pave the way for a better understanding of the neurochemical mechanisms by which d-serine administration modulates cognition.

Cell selective conditional null mutations of serine racemase demonstrate a predominate localization in cortical glutamatergic neurons

D-serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N-methyl-D-aspartate receptor (NMDAR). Crucial to an understanding of the signaling functions of D-serine is defining the sites responsible for its synthesis and release. In order to quantify the contributions of astrocytes and neurons to SR and D-serine localization, we used recombinant DNA techniques to effect cell type selective suppression of SR expression in astrocytes (aSRCKO) and in forebrain glutamatergic neurons (nSRCKO). The majority of SR is expressed in neurons: SR expression was reduced by ~65% in nSRCKO cerebral cortex and hippocampus, but only ~15% in aSRCKO as quantified by western blots. In contrast, nSRCKO is associated with only modest decreases in D-serine levels as quantified by HPLC, whereas D-serine levels were unaffected in aSRCKO mice. Liver expression of SR was increased by 35% in the nSRCKO, suggesting a role for peripheral SR in the maintenance of brain D-serine. Electrophysiologic studies of long-term potentiation (LTP) at the Schaffer collateral-CA1 pyramidal neuron synapse revealed no alterations in the aSRCKO mice versus wild-type. LTP induced by a single tetanic stimulus was reduced by nearly 70% in the nSRCKO mice. Furthermore, the mini-excitatory post-synaptic currents mediated by NMDA receptors but not by AMPA receptors were significantly reduced in nSRCKO mice. Our findings indicate that in forebrain, where D-serine appears to be the endogenous co-agonist at NMDA receptors, SR is predominantly expressed in glutamatergic neurons, and co-release of glutamate and D-serine is required for optimal activation of post-synaptic NMDA receptors.

Defragged Binary I Ching Genetic Code Chromosomes Compared to Nirenberg's and Transformed into Rotating 2D Circles and Squares and into a 3D 100% Symmetrical Tetrahedron Coupled to a Functional One to Discern Start From Non-Start Methionines through a Stella Octangula

BACKGROUND: Three binary representations of the genetic code according to the ancient I Ching of Fu-Xi will be presented, depending on their defragging capabilities by pairing based on three biochemical properties of the nucleic acids: H-bonds, Purine/Pyrimidine rings, and the Keto-enol/Amino-imino tautomerism, yielding the last pair a 32/32 single-strand self-annealed genetic code and I Ching tables. METHODS: Our working tool is the ancient binary I Ching's resulting genetic code chromosomes defragged by vertical and by horizontal pairing, reverse engineered into non-binaries of 2D rotating 4×4×4 circles and 8×8 squares and into one 3D 100% symmetrical 16×4 tetrahedron coupled to a functional tetrahedron with apical signaling and central hydrophobicity (codon formula: 4[1(1)+1(3)+1(4)+4(2)]; 5:5, 6:6 in man) forming a stella octangula, and compared to Nirenberg's 16×4 codon table (1965) pairing the first two nucleotides of the 64 codons in axis y. RESULTS: One horizontal and one vertical defragging had the start Met at the center. Two, both horizontal and vertical pairings produced two pairs of 2×8×4 genetic code chromosomes naturally arranged (M and I), rearranged by semi-introversion of central purines or pyrimidines (M' and I') and by clustering hydrophobic amino acids; their quasi-identity was disrupted by amino acids with odd codons (Met and Tyr pairing to Ile and TGA Stop); in all instances, the 64-grid 90° rotational ability was restored. CONCLUSIONS: We defragged three I Ching representations of the genetic code while emphasizing Nirenberg's historical finding. The synthetic genetic code chromosomes obtained reflect the protective strategy of enzymes with a similar function, having both humans and mammals a biased G-C dominance of three H-bonds in the third nucleotide of their most used codons per amino acid, as seen in one chromosome of the i, M and M' genetic codes, while a two H-bond A-T dominance was found in their complementary chromosome, as seen in invertebrates and plants. The reverse engineering of chromosome I' into 2D rotating circles and squares was undertaken, yielding a 100% symmetrical 3D geometry which was coupled to a previously obtained genetic code tetrahedron in order to differentiate the start methionine from the methionine that is acting as a codifying non-start codon.

Glial D-serine gates NMDA receptors at excitatory synapses in prefrontal cortex

N-methyl-D-aspartate receptors (NMDARs) subserve numerous neurophysiological and neuropathological processes in the cerebral cortex. Their activation requires the binding of glutamate and also of a coagonist. Whereas glycine and D-serine (D-ser) are candidates for such a role at central synapses, the nature of the coagonist in cerebral cortex remains unknown. We first show that the glycine-binding site of NMDARs is not saturated in acute slices preparations of medial prefrontal cortex (mPFC). Using enzymes that selectively degrade either D-ser or glycine, we demonstrate that under the present conditions, D-ser is the principle endogenous coagonist of synaptic NMDARs at mature excitatory synapses in layers V/VI of mPFC where it is essential for long-term potentiation (LTP) induction. Furthermore, blocking the activity of glia with the metabolic inhibitor, fluoroacetate, impairs NMDAR-mediated synaptic transmission and prevents LTP induction by reducing the extracellular levels of D-serine. Such deficits can be restored by exogenous D-ser, indicating that the D-amino acid mainly originates from glia in the mPFC, as further confirmed by double-immunostaining studies for D-ser and anti-glial fibrillary acidic protein. Our findings suggest that D-ser modulates neuronal networks in the cerebral cortex by gating the activity of NMDARs and that altering its levels is relevant to the induction and potentially treatment of psychiatric and neurological disorders.

D-serine: the right or wrong isoform?

Only recently, d-amino acids have been identified in mammals. Of these, d-serine has been most extensively studied. d-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl-d-aspartate receptor (NMDAr), similar to glycine. Therefore, d-serine may well play a role in all physiological and pathological processes in which NMDArs have been implied. In this review, we discuss the findings implying an important role for d-serine in human physiology (CNS development and memory and learning) and pathology (excitotoxicity, perinatal asphyxia, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, epilepsy, schizophrenia and bipolar disorder). We will debate on the relative contribution of d-serine versus glycine and conclude with clinical applications derived from these results and future directions to progress in this field. In general, adequate concentrations of d-serine are required for normal CNS development and function, while both decreased and increased concentrations can lead to CNS pathology. Therefore, d-serine appears to be the right isoform when present in the right concentrations.