Immunohistochemical demonstration of L-serine distribution in the rat brain

Tonsil-Derived Mesenchymal Stem Cells Differentiate into a Schwann Cell Phenotype and Promote Peripheral Nerve Regeneration

Schwann cells (SCs), which produce neurotropic factors and adhesive molecules, have been reported previously to contribute to structural support and guidance during axonal regeneration; therefore, they are potentially a crucial target in the restoration of injured nervous tissues. Autologous SC transplantation has been performed and has shown promising clinical results for treating nerve injuries and donor site morbidity, and insufficient production of the cells have been considered as a major issue. Here, we performed differentiation of tonsil-derived mesenchymal stem cells (T-MSCs) into SC-like cells (T-MSC-SCs), to evaluate T-MSC-SCs as an alternative to SCs. Using SC markers such as CAD19, GFAP, MBP, NGFR, S100B, and KROX20 during quantitative real-time PCR we detected the upregulation of NGFR, S100B, and KROX20 and the downregulation of CAD19 and MBP at the fully differentiated stage. Furthermore, we found myelination of axons when differentiated SCs were cocultured with mouse dorsal root ganglion neurons. The application of T-MSC-SCs to a mouse model of sciatic nerve injury produced marked improvements in gait and promoted regeneration of damaged nerves. Thus, the transplantation of human T-MSCs might be suitable for assisting in peripheral nerve regeneration.

Neuroprotective effect of L-serine against temporary cerebral ischemia in rats

To investigate the neuroprotective effect of L-serine and its underlying mechanisms, focal cerebral ischemia was induced in rats by occlusion of middle cerebral artery (MCAO) with a suture, and reperfusion was given by filament withdrawal 2 hr later. Meanwhile, rat hippocampal neurons were primarily cultured, and incubated in serum-free medium in an incubator containing 1% O(2) for hypoxic exposure of 5 hr, or incubated in serum-free medium containing 1 mM glutamate for glutamate exposure of 2 hr. Brain tissue injury and cell damage were then measured. L-serine dose-dependently decreased the neurology deficit score and infarct volume, elevated the cell viability and inhibited the leakage of lactate dehydrogenase. These effects were blocked by strychnine in both MCAO rats and cultured hippocampal neurons. Furthermore, L-serine (168 mg.kg(-1)) reduced the brain water content, permeability of blood-brain barrier, neuronal loss and the expression of activated caspase-3 in the cortex. In addition, L-serine effectively protected the brain from damage when it was administered within 6 hr after the end of MCAO. It is suggested that L-serine could exert a neuroprotective effect on the ischemic-reperfused brain and on the hypoxia- or glutamate-exposed hippocampal neurons, which may be mediated by activating glycine receptors.

Metabolism of the neuromodulator D-serine

Over the past years, accumulating evidence has indicated that D-serine is the endogenous ligand for the glycine-modulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. D-Serine is synthesized in glial cells and neurons by the pyridoxal-5' phosphate-dependent enzyme serine racemase, and it is released upon activation of glutamate receptors. The cellular concentration of this novel messenger is regulated by both serine racemase isomerization and elimination reactions, as well as by its selective degradation catalyzed by the flavin adenine dinucleotide-containing flavoenzyme D-amino acid oxidase. Here, we present an overview of the current knowledge of the metabolism of D-serine in human brain at the molecular and cellular levels, with a specific emphasis on the brain localization and regulatory pathways of D-serine, serine racemase, and D-amino acid oxidase. Furthermore, we discuss how D-serine is involved with specific pathological conditions related to N-methyl-D-aspartate receptors over- or down-regulation.

D-Serine Regulation of NMDA Receptor Activity

The N-Methyl-D-aspartate-type glutamate receptor (NMDAR) plays a key role in several important processes involving the nervous system, including brain development, synaptic plasticity, and learning. Unlike other neurotransmitter receptors, which are activated by individual neurotransmitters, activation of NMDARs requires the binding of a coagonist (D-serine or glycine) in addition to glutamate. Although previously considered an "unnatural" amino acid, D-serine is a key regulator of NMDAR activity and may be the main physiological ligand at the coagonist site. D-Serine is synthesized in the mammalian brain and is enriched in astrocytes, a class of glial cells that ensheath synapses in the brain. Astrocytes physiologically affect NMDAR neurotransmission by releasing D-serine, suggesting that D-serine acts as a gliotransmitter. However, recent findings indicate that D-serine signaling does not depend solely on glia, because D-serine and its biosynthetic enzyme are also present in substantial amounts in neurons. Here, we discuss these new findings, which begin to shed light on the relative roles of glia and neurons in D-serine signaling.

A critical role for the glial-derived neuromodulator D-serine in the age-related deficits of cellular mechanisms of learning and memory

Age-associated deficits in learning and memory are closely correlated with impairments of synaptic plasticity. Analysis of N-methyl-D-aspartate receptor (NMDAr)-dependent long-term potentiation (LTP) in CA1 hippocampal slices indicates that the glial-derived neuromodulator D-serine is required for the induction of synaptic plasticity. During aging, the content of D-serine and the expression of its synthesizing enzyme serine racemase are significantly decreased in the hippocampus. Impaired LTP and NMDAr-mediated synaptic potentials in old rats are rescued by exogenous D-serine. These results highlight the critical role of glial cells and presumably astrocytes, through the availability of D-serine, in the deficits of synaptic mechanisms of learning and memory that occur in the course of aging.

Immunocytochemical analysis of D-serine distribution in the mammalian brain reveals novel anatomical compartmentalizations in glia and neurons

D-Serine is a co-agonist at the NMDA receptor glycine-binding site. Early studies have emphasized a glial localization for D-serine. However the nature of the glial cells has not been fully resolved, because previous D-serine antibodies needed glutaraldehyde-fixation, precluding co-localization with fixation-sensitive antigens. We have raised a new D-serine antibody optimized for formaldehyde-fixation. Light and electron microscopic observations indicated that D-serine was concentrated into vesicle-like compartments in astrocytes and radial glial cells, rather than being distributed uniformly in the cytoplasm. In aged animals, patches of cortex and hippocampus were devoid of immunolabeling for D-serine, suggesting that impaired glial modulation of forebrain glutamatergic signaling might occur. Dual immunofluorescence labeling for glutamate and D-serine revealed D-serine in a subset of glutamatergic neurons, particularly in brainstem regions and in the olfactory bulbs. Microglia also contain D-serine. We suggest that some D-serine may be derived from the periphery. Collectively, our data suggest that the cellular compartmentation and distribution of D-serine may be more complex and extensive than previously thought and may have significant implications for our understanding of the role of D-serine in disease states including hypoxia and schizophrenia.

Functional identification of ASCT1 neutral amino acid transporter as the predominant system for the uptake of l-serine in rat neurons in primary culture

The uptake of L-serine, a nonessential amino acid known to be transported by the neutral amino acid transporter system ASC, was studied in primary cultures of rat neurons and astrocytes, and compared with that in human embryonic kidney (HEK293) cells transfected with rat ASCT1 cDNA. We first cloned neutral amino acid transporter ASCT1 from rat neurons in primary culture as a transporter candidate for L-serine uptake in the brain. The predicted amino acid sequence from rat ASCT1 exhibited significant homology with mouse and human ASCT1s. The amino acid sequence of rat ASCT1 was 92 and 84% identical to that of mouse and of human ASCT1, respectively. HEK293 cells expressing the rat ASCT1 cDNA showed a saturable dose-dependent and Na(+)-dependent increase in L-[(3)H] serine uptake by high affinity ( K(m) = 67 microM). The substrate selectivity of rat ASCT1 was the same as those of the mouse and human transporter. Northern blot analysis revealed that ASCT1 mRNA was ubiquitously expressed in the brain, with its highest concentration in the striatum and hippocampus. When the uptake of L -[(3)H] serine into rat primary neurons or astrocytes was compared with that of HEK293 cells expressing rat ASCT1 or rat ASCT2 cDNA, the inhibition profile of amino acids for the rat neurons quite resembled that for HEK293 cells expressing rat ASCT1. In contrast, the profile for rat astrocytes was a mixture of that for HEK293 cells expressing rat ASCT1 and that for the cells expressing rat ASCT2. Furthermore, L-[(3)H] serine uptake in neurons was fully Na(+)-dependent. ASCT1 mRNA was expressed in both primary neurons and astrocytes, whereas ASCT2 mRNA was expressed only in astrocytes, as determined by using RT-PCR with primers specific for the rat ASCT1 or rat ASCT2 transporter. Taken together, these findings indicate that ASCT1 predominantly contributes to the uptake of L-serine in primary neurons.

Uptake of [3H]L-serine in rat brain synaptosomal fractions

Accumulation of [3H]L-serine in crude synaptosomal fractions freshly prepared from rat brain has been found to be temperature-sensitive and to consist of both Na(+)-dependent and Na(+)-independent components. The accumulation of [3H]L-serine measured at submicromolar concentrations had a distinct substrate selectivity, different from the uptake of [3H]L-proline, [3H]L-glutamate and [3H]GABA. It was fully inhibited by L-glutamine, L-asparagine, L-cysteine, L-alanine, L-leucine, L-isoleucine, L-tyrosine, L-phenylalanine, L-threonine and by the synthetic marker for the large neutral amino acid transport systems 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid, but not influenced by beta-alanine, taurine, glycine nor was it inhibited by the marker for the A system, L-2-methylamino isobutyric acid. D-Serine at 1 mM concentration produced no significant inhibition of the accumulation of 10 nM [3H]L-serine. We conclude that L-serine uptake observed in the present study is mediated by at least two distinct transport systems: a Na(+)-dependent one of lower affinity (K(m) in mM range) and a Na(+)-independent system of higher affinity (K(m) approximately 20-100 micro M). Characteristics of [3H]L-serine accumulation displayed at low substrate concentrations suggest that it was mediated neither by the typical 'A', nor by the 'large neutral', amino acid transport systems but predominantly by transporters belonging to the recently identified LAT (L-amino acid transporter) family.

Novel neuroglial and glioglial relationships mediated by L-serine metabolism

L-Serine is a non-essential amino acid that can be synthesized in the body. It derives from an intermediate of the glycolytic pathway, 3-phosphoglycerate, and utilized for the syntheses of proteins, other amino acids, membrane lipids, heme, and nucleotides. Emerging evidence indicates that L-serine functions as a glia-derived trophic factor, which strongly promotes the survival and differentiation of cultured neurons. L-Serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) and small neutral amino acid transporter ASCT1 have been revealed to be expressed preferentially in the radial glia-astrocyte lineage and olfactory ensheathing glia of both adult and developing rodent brains. In contrast, these biosynthetic and transporter molecules for L-serine are faint or undetectable in neurons and phagocytic cells. In this review, we summarize recent progress to propose that L-serine synthesis in these glial cells and its supply to nearby neurons and other glia constitute a novel metabolic unit in the brain. Based on these neuroglial and glioglial relationships, glucose in neurons and phogocytes can be strategically used for energy production, while a variety of L-serine-derived biomolecules required for their proliferaton, survival, differentiation, and function are synthesized in and supplied from the radial glia-astrocyte lineage and olfactory ensheathing glia. A transient capillary expression of ASCT1 in fetal and neonatal brains further suggests that, in addition to the glia-borne L-serine, an active transport of blood-borne L-serine would play an essential role in neural development.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

Neutral amino acid transporter ASCT1 is preferentially expressed in L-Ser-synthetic/storing glial cells in the mouse brain with transient expression in developing capillaries

Nonessential amino acid L-Ser plays an essential role in neuronal survival and differentiation, through preferential expression of the L-Ser biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH), in particular in glial cells but not in neurons. To seek the molecular candidates responsible for glia-borne L-Ser transport, we performed histochemical analyses on amino acid transporter ASCT1, which prefers small neutral amino acids, such as Ala, Ser, Cys, and Thr, and mediates their obligatory exchange. At early developmental stages, neuroepithelial cells constituting the ventricular zone expressed ASCT1 mRNA and protein ubiquitously. Thereafter, ASCT1 expression was gradually downregulated in neuronal populations during the late embryonic and neonatal periods, whereas its high expression was transmitted to radial glial cells and then to astrocytes. High levels of ASCT1 were also detected in the olfactory ensheathing glia. The preferential glial expression of ASCT1 was consistent with that of 3PGDH, and their extensive colocalization was demonstrated at the cellular level. Moreover, high cellular contents of L-Ser were revealed in these glial cells by using a specific antibody to L-Ser. These results strongly suggest that a large amount of L-Ser is synthesized and stored in these glial cells and is released through ASCT1 in exchange for other extracellular substrates. In addition, we observed prominent expression of ASCT1 in capillary endothelial cells of embryonic and neonatal brains. Therefore, ASCT1 appears to be regulated to meet metabolic demands by differentiating and mature neurons through the transport of glia- and blood-borne small neutral amino acids.

Neurobiology through the looking-glass: D-serine as a new glial-derived transmitter

D-Amino acids have been known to be present in bacteria for more than 50 years, but only recently they were identified in mammals. The occurrence of D-amino acids in mammals challenge classic concepts in biology in which only L-amino acids would be present or thought to play important roles. Recent discoveries uncovered a role of endogenous D-serine as a putative glial-derived transmitter that regulates glutamatergic neurotransmission in mammalian brain. Free D-serine levels in the brain are about one third of L-serine values and its extracellular concentration is higher than many common L-amino acids. D-Serine occurs in protoplasmic astrocytes, a class of glial cells that ensheath the synapses and modulate neuronal activity. Biochemical and electrophysiological studies suggest that endogenous D-serine is a physiological modulator at the co-agonist site of NMDA-type of glutamate receptors. We previously showed that D-serine is synthesized by a glial serine racemase, a novel enzyme converting L- to D-serine in mammalian brain. The enzyme requires pyridoxal 5'-phosphate and it was the first racemase to be cloned from eucaryotes. Inhibitors of serine racemase have therapeutic implications for pathological processes in which over-stimulation of NMDA receptors takes place, such as stroke and neurodegenerative diseases. Here, we review the role of endogenous D-serine in modulating NMDA neurotransmission, its biosynthetic apparatus and the potential usefulness of serine racemase inhibitors as a novel neuroprotective strategy to decrease glutamate/NMDA excitotoxicity.

Ultrafilter co-culture, a new method for estimating the molecular mass of bioactive substances, indicates a small molecule neurotrophic substance is released from cultured cerebellar granule neurons of the BALB/c mouse

Cultured rat cerebellar granule neurons (CGNs), which require a depolarizing agent in the medium for long-term survival, are widely used for the analysis of mechanisms underlying the activity-dependent survival of neurons. It was recently found that this is not the case for BALB/c mouse CGNs, which survive without a depolarizing agent. Co-culture experiments indicated that the mouse cells release a neurotrophic substance. However, the substance is apparently short-living in the medium, making its molecular identification difficult. Here a novel co-culture method was devised for estimating the relative molecular masses of biologically active substances, using a commercially available dialysis membrane filter unit to separate substance-donor from substance-recipient cells. By this simultaneous fractionation/bioassay, the molecular mass of the assumed neurotrophic substance was estimated to be <3 kDa. Neurotrophic substances previously reported to be effective in rat CGNs, including neurotrophins, pituitary adenylate cyclase-activating polypeptide, parathyroid hormone-related polypeptide, glutamic acid, gamma-aminobutyric acid, and D-serine, were excluded as candidate molecules. Estrogen, however, remained a candidate. It should be stressed that the requirements for the activity-dependent survival of CGNs are species-dependent. Care should be taken in the analysis of activity-dependent neuronal survival using transgenic animals.

Glial transport of the neuromodulator D-serine

D-Serine is an endogenous agonist of NMDA receptors that occurs in astrocytes in gray matter areas of the brain. D-Serine is synthesized from L-serine by the activity of a glial enriched serine racemase, but little is known on the properties of D-serine transport and factors regulating its synaptic concentration. In the present report we characterize the transport of D-serine in astrocytes. In primary astrocyte cultures, D-serine uptake is dependent on sodium ions and exhibits both low affinity and low specificity for D-serine. The kinetics of D-serine transport resembles that of ASCT type transporters as several small neutral amino acids strongly inhibit the uptake of D-serine. D-Serine fluxes are coupled to counter-movement of L-serine and to a less extent to other small neutral amino acids. Thus, addition of D-serine to cell cultures elicits robust efflux of intracellular L-serine. Conversely, physiological concentrations of L-serine induce efflux of preloaded D-serine from astrocytes. L-Serine was more effective than kainate, which have been previously shown to induce D-serine release from astrocytes upon stimulation of non-NMDA type of glutamate receptors. The features of D-serine transport we describe reveal possible new mechanisms controlling the synaptic concentration of D-serine.