Synthesis, accumulation, and release of d-aspartate in the Aplysia californica CNS

Microchemical identification of enantiomers in early-branching animals: Lineage-specific diversification in the usage of D-glutamate and D-aspartate

D-amino acids are unique and essential signaling molecules in neural, hormonal, and immune systems. However, the presence of D-amino acids and their recruitment in early animals is mostly unknown due to limited information about prebilaterian metazoans. Here, we performed the comparative survey of L-/D-aspartate and L-/D-glutamate in representatives of four phyla of early-branching Metazoa: cnidarians (Aglantha); placozoans (Trichoplax), sponges (Sycon) and ctenophores (Pleurobrachia, Mnemiopsis, Bolinopsis, and Beroe), which are descendants of ancestral animal lineages distinct from Bilateria. Specifically, we used high-performance capillary electrophoresis for microchemical assays and quantification of the enantiomers. L-glutamate and L-aspartate were abundant analytes in all species studied. However, we showed that the placozoans, cnidarians, and sponges had high micromolar concentrations of D-aspartate, whereas D-glutamate was not detectable in our assays. In contrast, we found that in ctenophores, D-glutamate was the dominant enantiomer with no or trace amounts of D-aspartate. This situation illuminates prominent lineage-specific diversifications in the recruitment of D-amino acids and suggests distinct signaling functions of these molecules early in the animal evolution. We also hypothesize that a deep ancestry of such recruitment events might provide some constraints underlying the evolution of neural and other signaling systems in Metazoa.

Investigating brain d-serine: Advocacy for good practices

The last two decades have witnessed remarkable advance in our understanding the role of d-amino acids in the mammalian nervous system: from the unknown, to known molecules with unknown functions, to potential central players in health and disease. d-Amino acids have emerged as an important class of signaling molecules. In particular, the exploration of the roles of d-serine in brain physiopathology is a vibrant field that is growing at an accelerating pace. However, disentangling the functions of a chiral molecule in a complex chemical matrice as the brain requires specific measurement and detection methods but is also a challenging task as many molecular tools and models investigators are using can lead to confounded observations. Thus, study of d-amino acids demands accurate methodologies and specific controls, and these have often been lacking. Here we outline best practices for d-amino acid research, with a special emphasis on d-serine. We hope these concepts help move the field to greater rigor and reproducibility, allowing the field to advance.

Single Cell Neurometabolomics

Metabolomics, the characterization of metabolites and their changes within biological systems, has seen great technological and methodological progress over the past decade. Most metabolomic experiments involve the characterization of the small-molecule content of fluids or tissue homogenates. While these microliter and larger volume metabolomic measurements can characterize hundreds to thousands of compounds, the coverage of molecular content decreases as sample sizes are reduced to the nanoliter and even to the picoliter volume range. Recent progress has enabled the ability to characterize the major molecules found within specific individual cells. Especially within the brain, a myriad of cell types are colocalized, and oftentimes only a subset of these cells undergo changes in both healthy and pathological states. Here we highlight recent progress in mass spectrometry-based approaches used for single cell metabolomics, emphasizing their application to neuroscience research. Single cell studies can be directed to measuring differences between members of populations of similar cells (e.g., oligodendrocytes), as well as characterizing differences between cell types (e.g., neurons and astrocytes), and are especially useful for measuring changes occurring during different behavior states, exposure to diets and drugs, neuronal activity, and disease. When combined with other omics approaches such as transcriptomics, and with morphological and physiological measurements, single cell metabolomics aids fundamental neurochemical studies, has great potential in pharmaceutical development, and should improve the diagnosis and treatment of brain diseases.

Chiral Measurement of Aspartate and Glutamate in Single Neurons by Large-Volume Sample Stacking Capillary Electrophoresis

d-Amino acids (d-AAs) are endogenous molecules found throughout the metazoan, the functions of which remain poorly understood. Measurements of low abundance and heterogeneously distributed d-AAs in complex biological samples, such as cells and multicellular structures of the central nervous system (CNS), require the implementation of sensitive and selective analytical approaches. In order to measure the d- and l-forms of aspartate and glutamate, we developed and applied a stacking chiral capillary electrophoresis (CE) with laser-induced fluorescence detection method. The achieved online analyte preconcentration led to a 480-fold enhancement of detection sensitivity relative to capillary zone electrophoresis, without impacting separation resolution or analysis time. Additionally, the effects of inorganic ions on sample preconcentration and CE separation were evaluated. The approach enabled the relative quantification of d-aspartate and d-glutamate in individual neurons mechanically isolated from the CNS of the sea slug Aplysia californica, a well characterized neurobiological model. Levels of these structurally similar d-AAs were significantly different in subpopulations of cells collected from the investigated neuronal clusters.

Bioanalytical Application of Amino Acid Detection by Capillary Electrophoresis

This chapter illustrates the usefulness of capillary electrophoresis (CE) for the analysis of amino acids, and both normal and chiral separations are covered. In order to provide a general description of the main results and challenges in the biomedical field, some relevant applications and reviews on CE of amino acids are tabulated. Furthermore, some detailed experimental procedures are shown, regarding the CE analysis of amino acids in body fluids, in microdialysate, and released upon hydrolysis of proteins. In particular, the protocols will deal with the following compounds: (1) underivatized aminoacids in blood; (2) γ-Aminobutyric acid, glutamate, and L-Aspartate derivatized with Naphthalene-2,3-dicarboxaldehyde; (3) hydrolysate from bovine serum albumine derivatized with phenylisothiocyanate. By examining these applications on real matrices, the capillary electrophoresis efficiency as tool for Amino Acid analysis can be ascertained.

D-Alanine in the islets of Langerhans of rat pancreas

Relatively high levels of D-alanine (D-Ala), an endogenous D-amino acid, have been found in the endocrine systems of several animals, especially in the anterior pituitary; however, its functional importance remains largely unknown. We observed D-Ala in islets of Langerhans isolated from rat pancreas in significantly higher levels than in the anterior/intermediate pituitary; specifically, 180±60 fmol D-Ala per islet (300±100 nmol/gislet), and 10±2.5 nmol/g of wet tissue in pituitary. Additionally, 12±5% of the free Ala in the islets was in the d form, almost an order of magnitude higher than the percentage of D-Ala found in the pituitary. Surprisingly, glucose stimulation of the islets resulted in D-Ala release of 0.6±0.5 fmol per islet. As D-Ala is stored in islets and released in response to changes in extracellular glucose, D-Ala may have a hormonal role.

Small-volume analysis of cell-cell signaling molecules in the brain

Modern science is characterized by integration and synergy between research fields. Accordingly, as technological advances allow new and more ambitious quests in scientific inquiry, numerous analytical and engineering techniques have become useful tools in biological research. The focus of this review is on cutting edge technologies that aid direct measurement of bioactive compounds in the nervous system to facilitate fundamental research, diagnostics, and drug discovery. We discuss challenges associated with measurement of cell-to-cell signaling molecules in the nervous system, and advocate for a decrease of sample volumes to the nanoliter volume regimen for improved analysis outcomes. We highlight effective approaches for the collection, separation, and detection of such small-volume samples, present strategies for targeted and discovery-oriented research, and describe the required technology advances that will empower future translational science.

Storage and uptake of D-serine into astrocytic synaptic-like vesicles specify gliotransmission

Glial cells are increasingly recognized as active players that profoundly influence neuronal synaptic transmission by specialized signaling pathways. In particular, astrocytes have been shown recently to release small molecules, such as the amino acids l-glutamate and d-serine as "gliotransmitters," which directly control the efficacy of adjacent synapses. However, it is still controversial whether gliotransmitters are released from a cytosolic pool or by Ca(2+)-dependent exocytosis from secretory vesicles, i.e., by a mechanism similar to the release of synaptic vesicles in synapses. Here we report that rat cortical astrocytes contain storage vesicles that display morphological and biochemical features similar to neuronal synaptic vesicles. These vesicles share some, but not all, membrane proteins with synaptic vesicles, including the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) synaptobrevin 2, and contain both l-glutamate and d-serine. Furthermore, they show uptake of l-glutamate and d-serine that is driven by a proton electrochemical gradient. d-Serine uptake is associated with vesicle acidification and is dependent on chloride. Whereas l-serine is not transported, serine racemase, the synthesizing enzyme for d-serine, is anchored to the membrane of the vesicles, allowing local generation of d-serine. Finally, we reveal a previously unexpected mutual vesicular synergy between d-serine and l-glutamate filling in glia vesicles. We conclude that astrocytes contain vesicles capable of storing and releasing d-serine, l-glutamate, and most likely other neuromodulators in an activity-dependent manner.

Rethinking data collection and signal processing. 1. Real-time oversampling filter for chemical measurements

Minimizing noise in chemical measurements is critical to achieve low limits of detection and accurate measurements. We describe a real-time oversampling filter that offers a method to reduce stochastic noise in a time-dependent chemical measurement. The power of this technique is demonstrated in its application to the separation of dopamine and serotonin by micellar electrokinetic chromatography with amperometric detection. Signal-to-noise ratios were increased by almost an order of magnitude, allowing for limits of detection of 100 and 120 amol, respectively. Real-time oversampling filters can be implemented using simple software algorithms and require no change to existing experimental apparatus. The application is not limited to analytical separations, and this technique can be used to improve the signal-to-noise ratio in any experiment where the necessary sampling rate is less than the maximum sampling rate of the analog-to-digital converter. Theory, implementation, and the performance of this filter are described. We propose that this technique should be the default mode of operation for an analog-to-digital converter.

D-Aspartate acts as a signaling molecule in nervous and neuroendocrine systems

D-Aspartate (D-Asp) is an endogenous amino acid in the central nervous and reproductive systems of vertebrates and invertebrates. High concentrations of D-Asp are found in distinct anatomical locations, suggesting that it has specific physiological roles in animals. Many of the characteristics of D-Asp have been documented, including its tissue and cellular distribution, formation and degradation, as well as the responses elicited by D-Asp application. D-Asp performs important roles related to nervous system development and hormone regulation; in addition, it appears to act as a cell-to-cell signaling molecule. Recent studies have shown that D-Asp fulfills many, if not all, of the definitions of a classical neurotransmitter-that the molecule's biosynthesis, degradation, uptake, and release take place within the presynaptic neuron, and that it triggers a response in the postsynaptic neuron after its release. Accumulating evidence suggests that these criteria are met by a heterogeneous distribution of enzymes for D-Asp's biosynthesis and degradation, an appropriate uptake mechanism, localization within synaptic vesicles, and a postsynaptic response via an ionotropic receptor. Although D-Asp receptors remain to be characterized, the postsynaptic response of D-Asp has been studied and several L-glutamate receptors are known to respond to D-Asp. In this review, we discuss the current status of research on D-Asp in neuronal and neuroendocrine systems, and highlight results that support D-Asp's role as a signaling molecule.

Recent advances in amino acid analysis by capillary electrophoresis

This paper describes the most important articles that have been published on amino acid analysis using CE during the period from June 2009 to May 2011 and follows the format of the previous articles of Smith (Electrophoresis 1999, 20, 3078-3083), Prata et al. (Electrophoresis 2001, 22, 4129-4138) and Poinsot et al. (Electrophoresis 2003, 24, 4047-4062; Electrophoresis 2006, 27, 176-194; Electrophoresis 2008, 29, 207-223; Electrophoresis 2010, 31, 105-121). We present new developments in amino acid analysis with CE, which are reported describing the use of lasers or light emitting diodes for fluorescence detection, conductimetry electrochemiluminescence detectors, mass spectrometry applications, and lab-on-a-chip applications using CE. In addition, we describe articles concerning clinical studies and neurochemical applications of these techniques.

Dating silk by capillary electrophoresis mass spectrometry

A new capillary electrophoresis mass spectrometry (CE-MS) technique is introduced for age estimation of silk textiles based on amino acid racemization rates. With an L to D conversion half-life of ~2500 years for silk (B. mori) aspartic acid, the technique is capable of dating silk textiles ranging in age from several decades to a few-thousand-years-old. Analysis required only ~100 μg or less of silk fiber. Except for a 2 h acid hydrolysis at 110 °C, no other sample preparation is required. The CE-MS analysis takes ~20 min, consumes only nanoliters of the amino acid mixture, and provides both amino acid composition profiles and D/L ratios for ~11 amino acids.

Physiological evidence that D-aspartate activates a current distinct from ionotropic glutamate receptor currents in Aplysia californica neurons

D-Aspartate (D-Asp) activates an excitatory current in neurons of Aplysia californica. Although D-Asp is presumed to activate a subset of L-glutamate (L-Glu) channels, the identities of putative d-Asp receptors and channels are unclear. Whole cell voltage- and current-clamp studies using primary cultures of Aplysia buccal S cluster (BSC) neurons were executed to characterize D-Asp-activated ion channels. Both D-Asp and L-Glu evoked currents with similar current-voltage relationships, amplitudes, and relatively slow time courses of activation and inactivation when agonists were pressure applied. D-Asp-induced currents, however, were faster and desensitized longer, requiring 40 s to return to full amplitude. Of cells exposed to both agonists, 25% had D-Asp- but not L-Glu-induced currents, suggesting a receptor for D-Asp that was independent of l-Glu receptors. D-Asp channels were permeable to Na(+) and K(+), but not Ca²⁺, and were vulnerable to voltage-dependent Mg²⁺ block similarly to vertebrate NMDA receptor (NMDAR) channels. d-Asp may activate both NMDARs and non-l-Glu receptors in the nervous system of Aplysia.

Nutritional and medicinal aspects of D-amino acids

This paper reviews and interprets a method for determining the nutritional value of D-amino acids, D-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as L-lysine (L-Lys), L-methionine (L-Met), L-phenylalanine (L-Phe), and L-tryptophan (L-Trp) as well as the semi-essential amino acids L-cysteine (L-Cys) and L-tyrosine (L-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding L-amino acid. Because the organism is forced to use the D-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and pharmacological aspects of 27 individual D-amino acids, D-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results provide a valuable record and resource for further progress on the multifaceted aspects of D-amino acids in food and biological samples.

A novel pyridoxal 5'-phosphate-dependent amino acid racemase in the Aplysia californica central nervous system

D-aspartate (D-Asp) is found in specific neurons, transported to neuronal terminals and released in a stimulation-dependent manner. Because D-Asp formation is not well understood, determining its function has proved challenging. Significant levels of D-Asp are present in the cerebral ganglion of the F- and C-clusters of the invertebrate Aplysia californica, and D-Asp appears to be involved in cell-cell communication in this system. Here, we describe a novel protein, DAR1, from A. californica that can convert aspartate and serine to their other chiral form in a pyridoxal 5'-phosphate (PLP)-dependent manner. DAR1 has a predicted length of 325 amino acids and is 55% identical to the bivalve aspartate racemase, EC 5.1.1.13, and 41% identical to the mammalian serine racemase, EC 5.1.1.18. However, it is only 14% identical to the recently reported mammalian aspartate racemase, DR, which is closely related to glutamate-oxaloacetate transaminase, EC 2.6.1.1. Using whole-mount immunohistochemistry staining of the A. californica central nervous system, we localized DAR1-like immunoreactivity to the medial region of the cerebral ganglion where the F- and C-clusters are situated. The biochemical and functional similarities between DAR1 and other animal serine and aspartate racemases make it valuable for examining PLP-dependent racemases, promising to increase our knowledge of enzyme regulation and ultimately, D-serine and D-Asp signaling pathways.