Synthesis and release of amino acid transmitters

Dynamic Structural Biology Experiments at XFEL or Synchrotron Sources

Macromolecular crystallography (MX) leverages the methods of physics and the language of chemistry to reveal fundamental insights into biology. Often beautifully artistic images present MX results to support profound functional hypotheses that are vital to entire life science research community. Over the past several decades, synchrotrons around the world have been the workhorses for X-ray diffraction data collection at many highly automated beamlines. The newest tools include X-ray-free electron lasers (XFELs) located at facilities in the USA, Japan, Korea, Switzerland, and Germany that deliver about nine orders of magnitude higher brightness in discrete femtosecond long pulses. At each of these facilities, new serial femtosecond crystallography (SFX) strategies exploit slurries of micron-size crystals by rapidly delivering individual crystals into the XFEL X-ray interaction region, from which one diffraction pattern is collected per crystal before it is destroyed by the intense X-ray pulse. Relatively simple adaptions to SFX methods produce time-resolved data collection strategies wherein reactions are triggered by visible light illumination or by chemical diffusion/mixing. Thus, XFELs provide new opportunities for high temporal and spatial resolution studies of systems engaged in function at physiological temperature. In this chapter, we summarize various issues related to microcrystal slurry preparation, sample delivery into the X-ray interaction region, and some emerging strategies for time-resolved SFX data collection.

Serial protein crystallography in an electron microscope

Serial X-ray crystallography at free-electron lasers allows to solve biomolecular structures from sub-micron-sized crystals. However, beam time at these facilities is scarce, and involved sample delivery techniques are required. On the other hand, rotation electron diffraction (MicroED) has shown great potential as an alternative means for protein nano-crystallography. Here, we present a method for serial electron diffraction of protein nanocrystals combining the benefits of both approaches. In a scanning transmission electron microscope, crystals randomly dispersed on a sample grid are automatically mapped, and a diffraction pattern at fixed orientation is recorded from each at a high acquisition rate. Dose fractionation ensures minimal radiation damage effects. We demonstrate the method by solving the structure of granulovirus occlusion bodies and lysozyme to resolutions of 1.55 Å and 1.80 Å, respectively. Our method promises to provide rapid structure determination for many classes of materials with minimal sample consumption, using readily available instrumentation.

Aspartate release and signalling in the hippocampus

The Ca(2+)-dependent release of aspartate from hippocampal preparations was first reported 35 years ago, but the functional significance of this process remains uncertain. Aspartate satisfies all the criteria normally required for identification of a CNS transmitter. It is synthesized in nerve terminals, is accumulated and stored in synaptic vesicles, is released by exocytosis upon nerve terminal depolarization, and activates postsynaptic NMDA receptors. Aspartate may be employed as a neuropeptide-like co-transmitter by pathways that release either glutamate or GABA as their principal transmitter. Aspartate mechanisms include vesicular transport by sialin, vesicular content sensitive to glucose concentration, release mainly outside the presynaptic active zones, and selective activation of extrasynaptic NR1-NR2B NMDA receptors. Possible neurobiological functions of aspartate in immature neurons include activation of cAMP-dependent gene transcription and in mature neurons inhibition of CREB function, reduced BDNF expression, and induction of excitotoxic neuronal death. Recent findings suggest new experimental approaches toward resolving the functional significance of aspartate release.

Postsynaptic response to stimulation of the Schaffer collaterals with properties similar to those of synaptosomal aspartate release

Aspartate satisfies all the criteria normally required for identification of a CNS neurotransmitter. Nevertheless, little electrophysiological evidence supports the existence of aspartate transmission. In studies with rat hippocampal synaptosomes, chemically evoked aspartate release differed from glutamate release in its relative sensitivity to increased Ca(2+) concentration outside the presynaptic active zones, inefficient coupling to P/Q-type Ca(2+) channels, sensitivity to KB-R7943, and resistance to native Clostridial toxins. We took advantage of these differences to search for a potential aspartate-mediated response at Schaffer collateral synapses in organotypic hippocampal slice cultures. The slice cultures were pretreated with botulinum neurotoxin C (BoNT/C) to eliminate most of the glutamate release so that an expectedly smaller aspartate-like component of the compound EPSC could be detected by whole cell patch clamp recording. In control cultures, NMDA receptor activation accounted for only 18% of the evoked EPSC and an NR2B-selective antagonist reduced the NMDA receptor-mediated component by only 20%. Block of P/Q-type Ca(2+) channels essentially eliminated the response and 0.1 muM KB-R7943 had no significant effect. In BoNT/C-pretreated cultures, however, NMDA receptor activation accounted for 77% of the evoked EPSC and an NR2B-selective antagonist reduced the NMDA receptor-mediated component by 57%. Block of P/Q-type Ca(2+) channels reduced the response by only 28%, but 0.1 muM KB-R7943 reduced it by 45%. These results suggest that part of the Schaffer collateral synaptic response has pharmacological properties similar to those of synaptosomal aspartate release and may therefore be mediated at least partly by released aspartate.

Aspartate- and Glutamate-like Immunoreactivities in Rat Hippocampal Slices: Depolarization-induced Redistribution and Effects of Precursors

The light microscopic localization of aspartate-like immunoreactivity (Asp-LI) was compared to that of glutamate-like immunoreactivity (Glu-LI) in hippocampal slices by means of specific polyclonal antibodies recognizing the amino acids fixed by glutaraldehyde. After incubation in Krebs' solution with normal (5 mM) or depolarizing concentrations of K+, and various additives, the slices were fixed with glutaraldehyde, resectioned and processed according to the peroxidase - antiperoxidase procedure. At 5 mM K+, Glu-LI was localized in nerve-terminal like dots with a conspicuous laminar distribution, the highest Glu-LI concentrations coinciding with the terminal fields of major excitatory pathways thought to use glutamate or aspartate as transmitters. The localization of Asp-LI showed some similarity to that of Glu-LI, but the laminar distribution was less differentiated and the immunoreactivity was much weaker. At 40 and 55 mM K+ the nerve terminal localizations of Glu-LI and Asp-LI were strongly reduced. Concomitantly, both immunoreactivities appeared in astroglial cells. These changes were Ca2+-dependent. The nerve ending staining patterns of Asp-LI and Glu-LI could be sustained during depolarization if the medium was supplemented with glutamine (0.5 mM). Under these conditions Asp-LI became more intense and its distribution approached that of Glu-LI. This suggests that, when stimulated, some nerve endings can increase their reservoir of releasable aspartate. The presence of glutamine during depolarization strongly reduced glial Asp-LI and Glu-LI, possibly due to its providing nitrogen for conversion of glutamate to glutamine. alpha-Ketoglutarate, another glia-derived precursor of neuronal glutamate, was virtually ineffective in supporting Glu-LI and Asp-LI in nerve endings, and did not suppress Glu-LI or Asp-LI in glia. Our findings provide morphological support for the view that excitatory nerve endings under certain conditions can contain high levels of both aspartate and glutamate (possibly in the same terminals), and that aspartate as well as glutamate can be released synaptically. Further, they underline the importance of the glial supply of the nerve endings with precursor glutamine, which allows them to build up and sustain high concentrations of transmitter amino acids during release.

Synaptic vesicular localization and exocytosis of L-aspartate in excitatory nerve terminals: a quantitative immunogold analysis in rat hippocampus

To elucidate the role of aspartate as a signal molecule in the brain, its localization and those of related amino acids were examined by light and electron microscopic quantitative immunocytochemistry using antibodies specifically recognizing the aldehyde-fixed amino acids. Rat hippocampal slices were incubated at physiological and depolarizing [K+] before glutaraldehyde fixation. At normal [K+], aspartate-like and glutamate-like immunoreactivities were colocalized in nerve terminals forming asymmetrical synapses on spines in stratum radiatum of CA1 and the inner molecular layer of fascia dentata (i.e., excitatory afferents from CA3 and hilus, respectively). During K+ depolarization there was a loss of aspartate and glutamate from these terminals. Simultaneously the immunoreactivities strongly increased in glial cells. These changes were Ca2+-dependent and tetanus toxin-sensitive and did not comprise taurine-like immunoreactivity. Adding glutamine at CSF concentration prevented the loss of aspartate and glutamate and revealed an enhancement of aspartate in the terminals at moderate depolarization. In hippocampi from animals perfused with glutaraldehyde during insulin-induced hypoglycemia (to combine a strong aspartate signal with good ultrastructure) aspartate was colocalized with glutamate in excitatory terminals in stratum radiatum of CA1. The synaptic vesicle-to-cytoplasmic matrix ratios of immunogold particle density were similar for aspartate and glutamate, significantly higher than those observed for glutamine or taurine. Similar results were obtained in normoglycemic animals, although the nerve terminal contents of aspartate were lower. The results indicate that aspartate can be concentrated in synaptic vesicles and subject to sustained exocytotic release from the same nerve endings that contain and release glutamate.

Cocaine and amphetamine preferentially stimulate glutamate release in the limbic system: studies on the involvement of dopamine

The effects of cocaine and d-amphetamine on extracellular glutamate and aspartate levels in the nucleus accumbens, prefrontal cortex, and striatum were studied by in vivo microdialysis in awake, freely moving rats. In the nucleus accumbens, glutamate levels were stimulated by cocaine (15-30 mg/kg, i.p.), GBR 12909 (15 mg/kg, i.p.), and d-amphetamine (2 mg/kg, i.p.), while aspartate levels were not affected. The increase in nucleus accumbens glutamate levels following cocaine (30 mg/kg) was calcium-dependent and was blocked by pretreatment with dopamine antagonists; haloperidol (0.2 mg/kg, i.p.), SCH 23390 (0.02 mg/kg, i.p.), and raclopride (1 mg/kg, i.p.), as well as local 6-OHDA lesions of the nucleus accumbens. In the prefrontal cortex, glutamate levels were stimulated by both cocaine (15-30 mg/kg, i.p.) and d-amphetamine (2 mg/kg, i.p.), while aspartate levels were moderately stimulated by d-amphetamine only. The increase in prefrontal cortex glutamate levels following cocaine (30 mg/kg) was calcium-dependent and was blocked by pretreatment with SCH 23390 (0.02 mg/kg, i.p.), but not haloperidol (0.2 mg/kg, i.p.) or raclopride (1 mg/kg, i.p.). In the striatum, glutamate and aspartate levels were not affected by either cocaine (15-30 mg/kg, i.p.) or d-amphetamine (2 mg/kg, i.p.). These findings demonstrate that stimulants enhance glutamate release in limbic brain structures, nucleus accumbens, and prefrontal cortex, but not extrapyamidal brain structures, striatum. Furthermore, the increase in glutamate release in the nucleus accumbens may be mediated by dopamine.

Effect of the non-NMDA receptor antagonist GYKI 52466 on the microdialysate and tissue concentrations of amino acids following transient forebrain ischaemia

The effect of the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466) on ischaemia-induced changes in the microdialysate and tissue concentrations of glutamate, aspartate, and gamma-aminobutyric acid (GABA) was studied in rats. Twenty minutes of four-vessel occlusion resulted in a transient increase in microdialysate levels of glutamate, aspartate, and GABA in striatum, cortex, and hippocampus. Administration of GYKI 52466 (10 mg/kg bolus + 10 mg/kg/60 min intravenously starting 20 min before onset of ischaemia) inhibited ischaemia-induced increases in microdialysate glutamate and GABA in striatum without affecting the increases in hippocampus or cortex. Twenty minutes of four-vessel occlusion resulted in immediate small decreases and larger delayed (72 h) decreases in tissue levels of glutamate and aspartate. Transient increases in tissue levels of GABA were shown in all three structures at the end of the ischaemic period. At 72 h, after the ischaemic period, significantly reduced GABA levels were observed in striatum and hippocampus. GYKI 52466, given under identical conditions as above, augmented the ischaemia-induced decrease in striatal tissue levels of glutamate and aspartate, without significantly affecting the decreases in hippocampus and cortex. Twenty minutes of ischaemia resulted in a large increase in microdialysate dopamine in striatum. GYKI 52466 failed to inhibit this increase. Kainic acid (500 microM infused through the probe for 20 min) caused increases in microdialysate glutamate and aspartate in the striatum. GYKI 52466 (10 mg/kg bolus + 10 mg/kg/60 min) completely inhibited the kainic acid-induced glutamate release. In conclusion, the action of the non-NMDA antagonist, GYKI 52466, in the striatum is different from that in the cortex and hippocampus. The inhibition by GYKI 52466 of ischaemia-induced and kainate-induced increases in microdialysate glutamate concentration in the striatum may be related to the neuroprotection provided by GYKI 52466 in this region.

Autoreceptor regulation of glutamate and aspartate release from slices of the hippocampal CA1 area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.

Glutamine and aspartate loading of synaptosomes: a reevaluation of effects on calcium-dependent excitatory amino acid release

Guinea-pig cerebral cortical synaptosomes were preincubated for 60 min with 100 microM D-aspartate, L-aspartate, or L-glutamate. The total D- plus L-aspartate content of the synaptosomal fraction increased to 235%, 195%, or 164%, respectively, of the control. Despite this no increase was seen in the very low KCl evoked, Ca2+-dependent release of aspartate. Preincubation with the three amino acids changed the synaptosomal glutamate content to 78% (D-aspartate), 149% (L-aspartate), or 168% (L-glutamate) of control. However there was no statistically significant effect of these preincubations on the extent of Ca2+-dependent glutamate release. Thus the Ca2+-dependent release of aspartate and glutamate is not determined by the total synaptosomal content of these amino acids. The addition of 0.1-0.5 mM glutamine to the incubation caused a massive appearance of glutamate in the extrasynaptosomal medium. Analysis of specific activities showed that glutamine was hydrolysed directly by an extrasynaptosomal glutaminase, and that intrasynaptosomal glutamate was predominantly labelled by uptake of this glutaminase-derived glutamate. No increase was seen in the extent of Ca2+-dependent release of glutamate (by fluorimetry) either after preincubation with glutamine or in the continued presence of glutamine. Thus we are unable to confirm reports that glutamine expands the transmitter pool of glutamate. The extrasynaptosomal glutaminase activity in the synaptosomal preparation was inhibited by Ca2+ and activated by phosphate. Identical kinetics were obtained with "free" brain mitochondria, confirming the origin of the glutamine-derived glutamate.

Hypoxic-ischemic damage of the basal ganglia. Case reports and a review of the literature

Three cases of movement disorders secondary to hypoxic-ischemic encephalopathy are reported. Despite similarities among the clinical events, the neurological syndromes produced were dissimilar. Cerebral hypoxia-ischemia typically produces lesions of the globus pallidus that may result in an akinetic rigid syndrome. Due to its unique blood supply, vascular insufficiency is found to be a major factor. Lesions in the putamen also occur, and these tend to be associated with dystonia. Recent evidence supports a specific neuronal sensitivity in the striatum, possibly due to afferent excitatory amino acid connections. These two components and changes in the levels of neurotransmitters during hypoxia-ischemia may interact to produce varied clinical outcomes. These factors must also be considered when planning therapeutic interventions.

Penicillin-induced convulsions have preferential effects on transmitter glutamate pools in rat neostriatum

Convulsant doses of penicillin and elevated ambient pressure of 41 bar enhance the excitability of neurons. Their effects have been studied in neostriatal tissue with methods allowing differentiation between transmitter and metabolic glutamate pools. Levels of glutamate (Glu), glutamine (Gln), aspartate (Asp); gamma-aminobutyric acid and taurine were measured in the intact and decorticated neostriatum and parieto-occipital cortex of rats with a unilateral frontal cortex ablation. Intravenous infusion of penicillin at 1 bar decreased the neostriatal Glu content in the intact but not in the decorticated hemisphere. Pressure of 41 bar significantly decreased the level of Asp in the decorticated side only. Infusion of penicillin at 41 bar reduced the levels of Glu by 20.1% and Gln by 21.0% in the intact neostriatum only, whereas it decreased the Asp level in both sides as compared to control. The cortical Glu content was decreased only after infusion of penicillin at 41 bar. The results suggest that intravenous penicillin has a more pronounced effect on transmitter than on metabolic Glu pools in rat brain.

Regulation of glutamate and aspartate release from slices of the hippocampal CA1 area: effects of adenosine and baclofen

Glutamate and/or aspartate is the probable transmitter released from synaptic terminals of the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers in area CA1 of the rat hippocampal formation. Slices of the CA1 area were employed to test the effects of adenosine- and gamma-aminobutyrate (GABA)-related compounds on the release of glutamate and aspartate from this projection. Under the conditions of these experiments, the release of glutamate and aspartate evoked by 50 mM K+ was more than 90% Ca2+-dependent and originated predominantly from the CA3-derived pathways. Adenosine reduced the K+-evoked release of glutamate and aspartate by a maximum of about 60%, but did not affect the release of GABA. This action was reversed by 1 microM 8-phenyltheophylline. The order of potency for adenosine analogues was as follows: L-N6-phenylisopropyladenosine greater than N6-cyclohexyladenosine greater than D-N6-phenylisopropyladenosine approximately equal to 2-chloroadenosine greater than adenosine much greater than 5'-N-ethylcarboxamidoadenosine. 8-Phenyltheophylline (10 microM) by itself enhanced glutamate/aspartate release, whereas dipyridamole alone depressed release. These results support the view that adenosine inhibits transmission at Schaffer collateral-commissural-ipsilateral associational synapses mainly by reducing transmitter release and that these effects involve the activation of an A1 receptor. Neither adenosine, L-N6-phenylisopropyladenosine, nor 8-phenyltheophylline affected the release of glutamate or aspartate evoked by 10 microM veratridine. The differing effects of adenosine compounds on release evoked by K+ and veratridine suggest that A1 receptor activation either inhibits Ca2+ influx through the voltage-sensitive channels or interferes with a step subsequent to Ca2+ entry that is coupled to the voltage-sensitive Ca2+ channels in an obligatory fashion. Neither baclofen nor any other agent active at GABAB or GABAA receptors affected glutamate or aspartate release evoked by elevated K+ or veratridine. Therefore, either baclofen does not inhibit transmission at these synapses by depressing transmitter release or else it does so in a way that cannot be detected when a chemical depolarizing agent is employed.

Changes in the relative amounts of aspartate and glutamate released and retained in hippocampal slices during stimulation

It has been found previously that the ratio of aspartate to glutamate released and retained by brain slices reversibly changes with changing glucose concentrations in the medium. To find out whether increased neuronal activity also results in changes in the ratio of aspartate to glutamate, in this study electrical-field stimulation was applied for 10 min to hippocampal slices in the presence of 0.2-5 mM glucose. In 5 mM glucose, the ratio of aspartate to glutamate released did not change during stimulation, but the amount of aspartate retained at the end of stimulation was reduced. In contrast, in 1 mM or less glucose, the ratio of aspartate to glutamate released increased progressively and the rate of increase was inversely proportional to the glucose content of the medium. The evoked release of aspartate and glutamate both in low and high glucose was nearly suppressed in low (0.1 mM) Ca2+ or by tetrodotoxin. In low glucose, the ratio of aspartate to glutamate contained in the slices also increased as a result of stimulation. This increase was reduced only a little in low Ca2+, but was nearly eliminated by tetrodotoxin. Results suggest that increased neuronal activity causes a shift in the ratio of aspartate to glutamate released in the presence of glucose concentrations similar to those found in the brain in normoglycemic rats. This shift, due to an increased energy demand, probably originates from terminals which release aspartate and glutamate in different proportions.