L-aspartate as an amino acid neurotransmitter: mechanisms of the depolarization-induced release from cerebrocortical synaptosomes

Transplant of fecal microbiota from healthy young mice relieves cognitive defects in late-stage diabetic mice by reducing metabolic disorders and neuroinflammation

Fecal microbiota transplant (FMT) is becoming as a promising area of interest for treating refractory diseases. In this study, we investigated the effects of FMT on diabetes-associated cognitive defects in mice as well as the underlying mechanisms. Fecal microbiota was prepared from 8-week-aged healthy mice. Late-stage type 1 diabetics (T1D) mice with a 30-week history of streptozotocin-induced diabetics were treated with antibiotics for 7 days, and then were transplanted with bacterial suspension (100 μL, i.g.) once a day for 14 days. We found that FMT from healthy young mice significantly alleviated cognitive defects of late-stage T1D mice assessed in Morris water maze test. We revealed that FMT significantly reduced the relative abundance of Gram-negative bacteria in the gut microbiota and enhanced intestinal barrier integrity, mitigating LPS translocation into the bloodstream and NLRP3 inflammasome activation in the hippocampus, thereby reducing T1D-induced neuronal loss and astrocytic proliferation. FMT also reshaped the metabolic phenotypes in the hippocampus of T1D mice especially for alanine, aspartate and glutamate metabolism. Moreover, we showed that application of aspartate (0.1 mM) significantly inhibited NLRP3 inflammasome activation and IL-1β production in BV2 cells under a HG/LPS condition. We conclude that FMT can effectively relieve T1D-associated cognitive decline via reducing the gut-brain metabolic disorders and neuroinflammation, providing a potential therapeutic approach for diabetes-related brain disorders in clinic.

Changes in aspartate metabolism in the medial-prefrontal cortex of nicotine addicts based on J-edited magnetic resonance spectroscopy

This study aims to explore the changes of the aspartate (Asp) level in the medial-prefrontal cortex (mPFC) of subjects with nicotine addiction (nicotine addicts [NAs]) using the J-edited 1 H MR spectroscopy (MRS), which may provide a positive imaging evidence for intervention of NA. From March to August 2022, 45 males aged 40-60 years old were recruited from Henan Province, including 21 in NA and 24 in nonsmoker groups. All subjects underwent routine magnetic resonance imaging (MRI) and J-edited MRS scans on a 3.0 T MRI scanner. The Asp level in mPFC was quantified with reference to the total creatine (Asp/Cr) and water (Aspwater-corr , with correction of the brain tissue composition) signals, respectively. Two-tailed independent samples t-test was used to analyze the differences in levels of Asp and other coquantified metabolites (including total N-acetylaspartate [tNAA], total cholinine [tCho], total creatine [tCr], and myo-Inositol [mI]) between the two groups. Finally, the correlations of the Asp level with clinical characteristic assessment scales were performed using the Spearman criteria. Compared with the control group (n = 22), NAs (n = 18) had higher levels of Asp (Asp/Cr: p = .005; Aspwater-corr : p = .004) in the mPFC, and the level of Asp was positively correlated with the daily smoking amount (Asp/Cr: p < .001; Aspwater-corr : p = .004). No significant correlation was found between the level of Asp and the years of nicotine use, Fagerstrom Nicotine Dependence (FTND), Russell Reason for Smoking Questionnaire (RRSQ), or Barratt Impulsivity Scale (BIS-11) score. The elevated Asp level was observed in mPFC of NAs in contrast to nonsmokers, and the Asp level was positively correlated with the amount of daily smoking, which suggests that nicotine addiction may result in elevated Asp metabolism in the human brain.

Aspartic Acid in Health and Disease

Aspartic acid exists in L- and D-isoforms (L-Asp and D-Asp). Most L-Asp is synthesized by mitochondrial aspartate aminotransferase from oxaloacetate and glutamate acquired by glutamine deamidation, particularly in the liver and tumor cells, and transamination of branched-chain amino acids (BCAAs), particularly in muscles. The main source of D-Asp is the racemization of L-Asp. L-Asp transported via aspartate-glutamate carrier to the cytosol is used in protein and nucleotide synthesis, gluconeogenesis, urea, and purine-nucleotide cycles, and neurotransmission and via the malate-aspartate shuttle maintains NADH delivery to mitochondria and redox balance. L-Asp released from neurons connects with the glutamate-glutamine cycle and ensures glycolysis and ammonia detoxification in astrocytes. D-Asp has a role in brain development and hypothalamus regulation. The hereditary disorders in L-Asp metabolism include citrullinemia, asparagine synthetase deficiency, Canavan disease, and dicarboxylic aminoaciduria. L-Asp plays a role in the pathogenesis of psychiatric and neurologic disorders and alterations in BCAA levels in diabetes and hyperammonemia. Further research is needed to examine the targeting of L-Asp metabolism as a strategy to fight cancer, the use of L-Asp as a dietary supplement, and the risks of increased L-Asp consumption. The role of D-Asp in the brain warrants studies on its therapeutic potential in psychiatric and neurologic disorders.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Mitochondrial respiratory quiescence: A new model for examining the role of mitochondrial metabolism in development

Mitochondria are vital organelles with a central role in all aspects of cellular metabolism. As a means to support the ever-changing demands of the cell, mitochondria produce energy, drive biosynthetic processes, maintain redox homeostasis, and function as a hub for cell signaling. While mitochondria have been widely studied for their role in disease and metabolic dysfunction, this organelle has a continually evolving role in the regulation of development, wound repair, and regeneration. Mitochondrial metabolism dynamically changes as tissues transition through distinct phases of development. These organelles support the energetic and biosynthetic demands of developing cells and function as key structures that coordinate the nutrient status of the organism with developmental progression. This review will examine the mechanisms that link mitochondria to developmental processes. We will also examine the process of mitochondrial respiratory quiescence (MRQ), a novel mechanism for regulating cellular metabolism through the biochemical and physiological remodeling of mitochondria. Lastly, we will examine MRQ as a system to discover the mechanisms that drive mitochondrial remodeling during development.

Assessment of neurotransmitter release in human iPSC-derived neuronal/glial cells: a missing in vitro assay for regulatory developmental neurotoxicity testing

Human induced pluripotent stem cell (hiPSC)-derived neural stem cells (NSCs) and their differentiated neuronal/glial derivatives have been recently considered suitable to assess in vitro developmental neurotoxicity (DNT) triggered by exposure to environmental chemicals. The use of human-relevant test systems combined with in vitro assays specific for different neurodevelopmental events, enables a mechanistic understanding of the possible impact of environmental chemicals on the developing brain, avoiding extrapolation uncertainties associated with in vivo studies. Currently proposed in vitro battery for regulatory DNT testing accounts for several assays suitable to study key neurodevelopmental processes, including NSC proliferation and apoptosis, differentiation into neurons and glia, neuronal migration, synaptogenesis, and neuronal network formation. However, assays suitable to measure interference of compounds with neurotransmitter release or clearance are at present not included, which represents a clear gap of the biological applicability domain of such a testing battery. Here we applied a HPLC-based methodology to measure the release of neurotransmitters in a previously characterized hiPSC-derived NSC model undergoing differentiation towards neurons and glia. Glutamate release was assessed in control cultures and upon depolarization, as well as in cultures repeatedly exposed to some known neurotoxicants (BDE47 and lead) and chemical mixtures. Obtained data indicate that these cells have the ability to release glutamate in a vesicular manner, and that both glutamate clearance and vesicular release concur in the maintenance of extracellular glutamate levels. In conclusion, analysis of neurotransmitter release is a sensitive readout that should be included in the envisioned battery of in vitro assays for DNT testing.

Cerebrolysin restores balance between excitatory and inhibitory amino acids in brain following concussive head injury. Superior neuroprotective effects of TiO2 nanowired drug delivery

Concussive head injury (CHI) often associated with military personnel, soccer players and related sports personnel leads to serious clinical situation causing lifetime disabilities. About 3-4k head injury per 100k populations are recorded in the United States since 2000-2014. The annual incidence of concussion has now reached to 1.2% of population in recent years. Thus, CHI inflicts a huge financial burden on the society for rehabilitation. Thus, new efforts are needed to explore novel therapeutic strategies to treat CHI cases to enhance quality of life of the victims. CHI is well known to alter endogenous balance of excitatory and inhibitory amino acid neurotransmitters in the central nervous system (CNS) leading to brain pathology. Thus, a possibility exists that restoring the balance of amino acids in the CNS following CHI using therapeutic measures may benefit the victims in improving their quality of life. In this investigation, we used a multimodal drug Cerebrolysin (Ever NeuroPharma, Austria) that is a well-balanced composition of several neurotrophic factors and active peptide fragments in exploring its effects on CHI induced alterations in key excitatory (Glutamate, Aspartate) and inhibitory (GABA, Glycine) amino acids in the CNS in relation brain pathology in dose and time-dependent manner. CHI was produced in anesthetized rats by dropping a weight of 114.6g over the right exposed parietal skull from a distance of 20cm height (0.224N impact) and blood-brain barrier (BBB), brain edema, neuronal injuries and behavioral dysfunctions were measured 8, 24, 48 and 72h after injury. Cerebrolysin (CBL) was administered (2.5, 5 or 10mL/kg, i.v.) after 4-72h following injury. Our observations show that repeated CBL induced a dose-dependent neuroprotection in CHI (5-10mL/kg) and also improved behavioral functions. Interestingly when CBL is delivered through TiO₂ nanowires superior neuroprotective effects were observed in CHI even at a lower doses (2.5-5mL/kg). These observations are the first to demonstrate that CBL is effectively capable to attenuate CHI induced brain pathology and behavioral disturbances in a dose dependent manner, not reported earlier.

Hydrolyzed Chicken Meat Extract Attenuates Neuroinflammation and Cognitive Impairment in Middle-Aged Mouse by Regulating M1/M2 Microglial Polarization

Aging is the most common cause of several neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. The pathological hallmarks of age-dependent neuropathology consist of chronic neuroinflammation, oxidative stress, gliosis, learning disability, and cognitive decline. A novel hydrolyzed bioactive peptide mixture extracted from chicken meat, that is, hydrolyzed chicken extract (HCE) has been previously demonstrated to exert neuroprotective effects in rodents and humans. However, the mechanism of HCE on age-related neurological disorders remains unclear. Herein, we aimed to clarify the impact and mechanism of isolated bioactive components (BCs) from HCE on age-dependent neuroinflammation and cognitive impairment in middle-aged mice. We found that both BC and HCE supplementation ameliorated age-induced memory loss, alleviated hippocampal neuroinflammation and oxidative stress, followed by promoting hippocampal neurogenesis in mice. BC and HCE treatment also ameliorated age-dependent morphological anomalies and alleviated microgliosis and astrogliosis. In parallel, BC and HCE treatment showed a significant decrease in the NF-κB p65 and p38 MAPK signaling, which were associated with the enhancement of antioxidative enzymes activities. Furthermore, BC treatment attenuated the neuroinflammatory phenotypes by the decrease in M1-polarized microglia and the increase in M2-polarized microglia in vivo and in vitro. In addition, we found that cyclo(Phe-Phe), one of the cyclopeptides purified from BC, showed notable anti-inflammatory effects in BV2 cells. Taken together, BC might be used as a dietary supplement for alleviating age-dependent neuropathology in middle-aged individuals.

Effects of a monocarboxylate transport 1 inhibitor, AZD3965, on retinal and visual function in the rat

BACKGROUND AND PURPOSE

Inhibition of monocarboxylate transport 1 (MCT1) is of interest in targeting highly glycolytic tumours. However, MCT1 is expressed in retina, and so inhibition of MCT1 could affect retinal function.

EXPERIMENTAL APPROACH

AZD3965, an MCT1 inhibitor selected for clinical development, and two additional MCT1 inhibitors were evaluated for effects on visual acuity in albino (Han Wistar) rats. The effects of AZD3965 on visual acuity and electroretinography (ERG) were further investigated in pigmented (Long-Evans) rats, with dosing for up to 7 days.

KEY RESULTS

All three MCT1 inhibitors reduced visual acuity within 2 h of dosing, suggesting a class effect. The deficit caused by AZD3965 (1,000 mg·kg^-1 p.o. per day for 4 days) in Long Evans rats recovered to pre-dose levels 7 days after cessation of dosing. AZD3965 (50 to 1,000 mg·kg^-1 p.o.) reduced the amplitude of scotopic a- and b-waves, and photopic b-wave of the ERG in a dose-related fashion, within 2 h of dosing. The effects on the scotopic ERG had diminished by Day 7 of dosing, demonstrating partial restoration of function despite continued treatment. Seven days after cessation of dosing at the highest dose tested (1,000 mg·kg^-1 ), there was recovery of both scotopic a- and b- waves and, to a lesser extent, photopic b-wave. ERG was affected at lower plasma exposures than was visual function.

CONCLUSIONS AND IMPLICATIONS

This study clarifies the role of the MCT1 transporter in retinal function. The monitorability of the functional effects on the retina enabled safe clinical use of AZD3965.

Maladaptive consequences of repeated intermittent exposure to uncertainty

Recently we reported that nucleus accumbens (NAcc) dopamine (DA) tracks uncertainty during operant responding for non-caloric saccharin. We also showed that repeated intermittent exposure to this uncertainty, like exposure to drugs of abuse, leads to sensitization of the locomotor and NAcc DA effects of amphetamine and promotes the subsequent self-administration of the drug. Here we review these findings together with others showing that NAcc glutamate signaling is similarly affected by uncertainty. Extracellular levels of glutamate in this site also track uncertainty in a task in which nose poking for saccharin on an escalating variable ratio schedule of reinforcement is associated with progressively increasing variance between performance of the operant and payout. Furthermore, sensitized behavioral responding to and for amphetamine following exposure to uncertainty is accompanied by increased levels of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) phosphorylation as well as altered protein levels of the transcription factor ∆FosB (increased) and glutamate transporter 1 (GLT1; decreased) in NAcc tissues. Notably, phosphorylation by CaMKII and PKC regulates AMPA receptor trafficking and function in this site, is elevated following psychostimulant exposure, and is necessary for the expression of enhanced drug taking. Increased ∆FosB and decreased GLT1 levels are observed following psychostimulant exposure, are associated with increased drug taking and seeking, and are known to modulate AMPA receptors and extracellular glutamate levels respectively. These adaptations in glutamate transmission as well as those observed with DA following repeated intermittent exposure to uncertainty are similar to those produced by exposure to abused drugs. Together, they point to the recruitment of both DA and glutamate signaling pathways in the NAcc in both drug and behavioral addictions. As uncertainty is central to games of chance, these findings have particular relevance for gambling disorders known to exhibit comorbidity with drug abuse.

Kinetic Modeling of the Blood Oxygenation Level Dependent (BOLD) Signals and Biocatalytic Reactions Observed in the Human Brain Using MRI: An Analysis of Normal and Pathological Conditions

A kinetic model describing the pulse of increased oxygen concentrations and the subsequent changes in the concentration of N-acetylaspartate in the excited nervous tissue of the human brain in response to an external signal is presented. The model is based on biochemical data, a multistage and nonlinear dynamic process the BOLD signal and N-acetylaspartate. The existence of multiple steady states explains the triggering effect of the system. The inhibitory effect of the substrate is a necessary factor for the autostabilization of N-acetylaspartate. The kinetic model allows the dynamic behavior of previously unmeasurable metabolites, namely, products of the hydrolysis of N-acetylaspartate, such as acetic and aspartic acid, and glutamic acid to be predicted. Kinetic modeling of the BOLD signal and the subsequent hydrolysis of N-acetylaspartate provides information about the biochemical and dynamic characteristics of some pathological conditions (schizophrenia, Canavan disease, and the superexcitation of the neural network).

A Review of Neurotransmitters Sensing Methods for Neuro-Engineering Research

Neurotransmitters as electrochemical signaling molecules are essential for proper brain function and their dysfunction is involved in several mental disorders. Therefore, the accurate detection and monitoring of these substances are crucial in brain studies. Neurotransmitters are present in the nervous system at very low concentrations, and they mixed with many other biochemical molecules and minerals, thus making their selective detection and measurement difficult. Although numerous techniques to do so have been proposed in the literature, neurotransmitter monitoring in the brain is still a challenge and the subject of ongoing research. This article reviews the current advances and trends in neurotransmitters detection techniques, including in vivo sampling and imaging techniques, electrochemical and nano-object sensing techniques for in vitro and in vivo detection, as well as spectrometric, analytical and derivatization-based methods mainly used for in vitro research. The document analyzes the strengths and weaknesses of each method, with the aim to offer selection guidelines for neuro-engineering research.

Antagonizing α7 nicotinic receptors with methyllycaconitine (MLA) potentiates receptor activity and memory acquisition

α7 nicotinic acetylcholine receptors (α7nAChRs) have been targeted to improve cognition in different neurological and psychiatric disorders. Nevertheless, no α7nAChR activating ligand has been clinically approved. Here, we investigated the effects of antagonizing α7nAChRs using the selective antagonist methyllycaconitine (MLA) on receptor activity in vitro and cognitive functioning in vivo. Picomolar concentrations of MLA significantly potentiated receptor responses in electrophysiological experiments mimicking the in vivo situation. Furthermore, microdialysis studies showed that MLA administration substantially increased hippocampal glutamate efflux which is related to memory processes. Accordingly, pre-tetanus administration of low MLA concentrations produced longer lasting potentiation (long-term potentiation, LTP) in studies examining hippocampal plasticity. Moreover, low doses of MLA improved acquisition, but not consolidation memory processes in rats. While the focus to enhance cognition by modulating α7nAChRs lies on agonists and positive modulators, antagonists at low doses should provide a novel approach to improve cognition in neurological and psychiatric disorders.

Binding and transport of D-aspartate by the glutamate transporter homolog GltTk

Mammalian glutamate transporters are crucial players in neuronal communication as they perform neurotransmitter reuptake from the synaptic cleft. Besides L-glutamate and L-aspartate, they also recognize D-aspartate, which might participate in mammalian neurotransmission and/or neuromodulation. Much of the mechanistic insight in glutamate transport comes from studies of the archeal homologs Glt_Ph from Pyrococcus horikoshii and Glt_Tk from Thermococcus kodakarensis. Here, we show that Glt_Tk transports D-aspartate with identical Na⁺: substrate coupling stoichiometry as L-aspartate, and that the affinities (K_d and K_m) for the two substrates are similar. We determined a crystal structure of Glt_Tk with bound D-aspartate at 2.8 Å resolution. Comparison of the L- and D-aspartate bound Glt_Tk structures revealed that D-aspartate is accommodated with only minor rearrangements in the structure of the binding site. The structure explains how the geometrically different molecules L- and D-aspartate are recognized and transported by the protein in the same way.

Blockade of Glutamate Receptors within the Prelimbic Cortex Attenuate Concentration of Excitatory Amino Acids in the Morphine Self-administration in Rats

Background

The attitude of research on addiction has been done on the key role of glutamate. As a regard, the prelimbic cortex (PrL) has an important role in addiction, learning, and memory. We tried to investigate the level of glutamate and aspartate concentration after glutamate receptors blockade in this region in the morphine-addicted rats.

Materials and Methods

In this study, we examined the effects of local infusion of the N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonists, 2-amino-5-phosphonovaleric acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), into the PrL cortex on the level of excitatory amino acids (EAAs) and glycine. After 11 days of self-administration, the prelimbic area of the brain was taken out, and the EAAs and glycine concentration was measured by high-performance liquid chromatography.

Results

Morphine resulted in the significant increase in the EAAs concentration within this area (P ≤ 0.001). Microinjection of AP5 into this region before using of morphine significantly decreased the morphine-induced glutamate and aspartate concentration (P ≤ 0.001). CNQX had the same effect and significantly reduced the EAAs concentration compared to the morphine group (P ≤ 0.001). In addition, microinjection of AP5 and CNQX simultaneously increased glycine concentration (P ≤ 0.001).

Conclusions

These results show that morphine stimulates the EAAs release in the prelimbic area. It seems that microinjection of AP5 or CNQX in this region is effective in reducing morphine-induced EAA. It is suggested that EAA transmission in the PrL cortex may be a possible target for treatment of morphine addiction.

Chiral halogen and chalcogen bonding receptors for discrimination of stereo- and geometric dicarboxylate isomers in aqueous media

Dicarboxylate isomer discrimination and fluorescence sensing properties of a chalcogen bonding receptor differs considerably from halogen and hydrogen bonding analogues.

Effects of serotonin 2A/1A receptor stimulation on social exclusion processing

Social ties are crucial for physical and mental health. However, psychiatric patients frequently encounter social rejection. Moreover, an increased reactivity to social exclusion influences the development, progression, and treatment of various psychiatric disorders. Nevertheless, the neuromodulatory substrates of rejection experiences are largely unknown. The preferential serotonin (5-HT) 2A/1A receptor agonist, psilocybin (Psi), reduces the processing of negative stimuli, but whether 5-HT2A/1A receptor stimulation modulates the processing of negative social interactions remains unclear. Therefore, this double-blind, randomized, counterbalanced, cross-over study assessed the neural response to social exclusion after the acute administration of Psi (0.215 mg/kg) or placebo (Pla) in 21 healthy volunteers by using functional magnetic resonance imaging (fMRI) and resting-state magnetic resonance spectroscopy (MRS). Participants reported a reduced feeling of social exclusion after Psi vs. Pla administration, and the neural response to social exclusion was decreased in the dorsal anterior cingulate cortex (dACC) and the middle frontal gyrus, key regions for social pain processing. The reduced neural response in the dACC was significantly correlated with Psi-induced changes in self-processing and decreased aspartate (Asp) content. In conclusion, 5-HT2A/1A receptor stimulation with psilocybin seems to reduce social pain processing in association with changes in self-experience. These findings may be relevant to the normalization of negative social interaction processing in psychiatric disorders characterized by increased rejection sensitivity. The current results also emphasize the importance of 5-HT2A/1A receptor subtypes and the Asp system in the control of social functioning, and as prospective targets in the treatment of sociocognitive impairments in psychiatric illnesses.

Copper(ii) complexes of macrocyclic and open-chain pseudopeptidic ligands: synthesis, characterization and interaction with dicarboxylates

Cu(ii) complexes of pseudopeptides interact with very similar dicarboxylates of biological interest (malate and aspartate) affording strikingly different outcomes.

Human iPSC neurons display activity-dependent neurotransmitter secretion: aberrant catecholamine levels in schizophrenia neurons

This study investigated human-induced pluripotent stem cell (hiPSC) -derived neurons for their ability to secrete neurotransmitters in an activity-dependent manner, the fundamental property required for chemical neurotransmission. Cultured hiPSC neurons showed KCl stimulation of activity-dependent secretion of catecholamines—dopamine (DA), norepinephrine (NE), and epinephrine (Epi)—and the peptide neurotransmitters dynorphin and enkephlain. hiPSC neurons express the biosynthetic enzymes for catecholamines and neuropeptides. Because altered neurotransmission contributes to schizophrenia (SZ), we compared SZ to control cultures of hiPSC neurons and found that SZ cases showed elevated levels of secreted DA, NE, and Epi. Consistent with increased catecholamines, the SZ neuronal cultures showed a higher percentage of tyrosine hydroxylase (TH)-positive neurons, the first enzymatic step for catecholamine biosynthesis. These findings show that hiPSC neurons possess the fundamental property of activity-dependent neurotransmitter secretion and can be advantageously utilized to examine regulation of neurotransmitter release related to brain disorders.

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hiPSC neurons show activity-dependent secretion of catecholamines and neuropeptides

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hiPSC neurons express enzymes for production of catecholamines and neuropeptides

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SZ hiPSC neurons show changes in catecholamines secreted

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SZ hiPSC neuronal cultures display increased percentage of TH-positive neurons

Development of a novel fluorescent protein construct by genetically fusing green fluorescent protein to the N-terminal of aspartate dehydrogenase

We developed a fluorescent protein construct by genetically fusing green fluorescent protein (GFP) to aspartate dehydrogenase from Thermotoga maritima. The fusion protein was cloned, heterologously expressed in Escherichia coli cells, and purified by Ni-chelate affinity chromatography. It was then introduced into a measurement cuvette to monitor its fluorescence signal. Aspartate dehydrogenase functioned as the biorecognition element, and aspartate-induced conformational change was converted to a fluorescence signal by GFP. The recombinant protein responded to l-aspartate (l-Asp) linearly within the concentration range of 1-50 mM, and it was capable of giving a fluorescence signal in 1 Min. Although a linear response was also observed for l-Glu, the fluorescence signal was 2.7 times lower than that observed for l-Asp. In the present study, we describe two novelties: development of a genetically encoded fluorescent protein construct for monitoring of l-Asp in vitro, and employment of aspartate dehydrogenase scaffold as a biorecognition element. A few genetically encoded amino-acid biosensors have been described in the literature, but to our knowledge, a protein has not been constructed solely for determination of l-Asp. Periplasmic ligand binding proteins offer high binding affinity in the micromolar range, and they are frequently used as biorecognition elements. Instead of choosing a periplasmic l-Asp binding protein, we attempted to use the substrate specificity of aspartate dehydrogenase enzyme.

Surface modified fluorescent quantum dots with neurotransmitter ligands for potential targeting of cell signaling applications

The possibility of combining nanotechnology with nanomedicine opens a broad field of research which may truly revolutionize our society. The neural system plays a crucial role in the human body, and most related diseases can dramatically change the quality of life. Thus, the present study reports a novel approach for using neurotransmitters as ligands in the synthesis of surface-modified fluorescent nanocrystals for potential use in cell labeling applications. Briefly, CdS quantum dots (QDs) were prepared using L-glutamic and L-aspartic as surface capping agents via a one-step chemical processing method, which resulted in stable aqueous colloidal systems at room temperature and ambient pressure. UV-visible spectroscopy, photoluminescence spectroscopy (PL), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM) were used to characterize the synthesis and relative stability of peptide-capped CdS nanocrystals. The results demonstrate that both ligands were effective in nucleating and stabilizing CdS quantum dots in colloidal aqueous suspensions, with an estimated dimension below 3.3 nm and with fluorescence activity. Thus, novel nanohybrids were developed based on QDs bioconjugated to surface-active neurotransmitter moieties suitable for investigation as potential biomarkers in cell targeting and signaling applications.

Vesicular uptake and exocytosis of L-aspartate is independent of sialin

The mechanism of release and the role of l-aspartate as a central neurotransmitter are controversial. A vesicular release mechanism for l-aspartate has been difficult to prove, as no vesicular l-aspartate transporter was identified until it was found that sialin could transport l-aspartate and l-glutamate when reconstituted into liposomes. We sought to clarify the release mechanism of l-aspartate and the role of sialin in this process by combining l-aspartate uptake studies in isolated synaptic vesicles with immunocyotchemical investigations of hippocampal slices. We found that radiolabeled l-aspartate was taken up into synaptic vesicles. The vesicular l-aspartate uptake, relative to the l-glutamate uptake, was twice as high in the hippocampus as in the whole brain, the striatum, and the entorhinal and frontal cortices and was not inhibited by l-glutamate. We further show that sialin is not essential for exocytosis of l-aspartate, as there was no difference in ATP-dependent l-aspartate uptake in synaptic vesicles from sialin-knockout and wild-type mice. In addition, expression of sialin in PC12 cells did not result in significant vesicle uptake of l-aspartate, and depolarization-induced depletion of l-aspartate from hippocampal nerve terminals was similar in hippocampal slices from sialin-knockout and wild-type mice. Further, there was no evidence for nonvesicular release of l-aspartate via volume-regulated anion channels or plasma membrane excitatory amino acid transporters. This suggests that l-aspartate is exocytotically released from nerve terminals after vesicular accumulation by a transporter other than sialin.

Transient increase in neuronal chloride concentration by neuroactive aminoacids released from glioma cells

Neuronal chloride concentration ([Cl⁻]_i) is known to be dynamically modulated and alterations in Cl⁻ homeostasis may occur in the brain at physiological and pathological conditions, being also likely involved in glioma-related seizures. However, the mechanism leading to changes in neuronal [Cl⁻]_i during glioma invasion are still unclear. To characterize the potential effect of glioma released soluble factors on neuronal [Cl⁻]_i, we used genetically encoded CFP/YFP-based ratiometric Cl-(apical) Sensor transiently expressed in cultured hippocampal neurons. Exposition of neurons to glioma conditioned medium (GCM) caused rapid and transient elevation of [Cl⁻]_i, resulting in the increase of fluorescence ratio, which was strongly reduced by blockers of ionotropic glutamate receptors APV and NBQX. Furthermore, in HEK cells expressing GluR1-AMPA receptors, GCM activated ionic currents with efficacy similar to those caused by glutamate, supporting the notion that GCM contains glutamate or glutamatergic agonists, which cause neuronal depolarization, activation of NMDA and AMPA/KA receptors leading to elevation of [Cl⁻]_i. Chromatographic analysis of the GCM showed that it contained several aminoacids, including glutamate, whose release from glioma cells did not occur via the most common glial mechanisms of transport, or in response to hypoosmotic stress. GCM also contained glycine, whose action contrasted the glutamate effect. Indeed, strychnine application significantly increased GCM-induced depolarization and [Cl⁻]_i rise. GCM-evoked [Cl⁻]_i elevation was not inhibited by antagonists of Cl⁻ transporters and significantly reduced in the presence of anion channels blocker NPPB, suggesting that Cl⁻ selective channels are a major route for GCM-induced Cl⁻ influx. Altogether, these data show that glioma released aminoacids may dynamically alter Cl⁻ equilibrium in surrounding neurons, deeply interfering with their inhibitory balance, likely leading to physiological and pathological consequences.

Analysis of changes in gene expression and metabolic profiles induced by silica-coated magnetic nanoparticles

Magnetic nanoparticles (MNPs) have proven themselves to be useful in biomedical research; however, previous reports were insufficient to address the potential dangers of nanoparticles. Here, we investigated gene expression and metabolic changes based on the microarray and gas chromatography-mass spectrometry with human embryo kidney 293 cells treated with MNPs@SiO(2)(RITC), a silica-coated MNP containing Rhodamine B isothiocyanate (RITC). In addition, measurement of reactive oxygen species (ROS) and ATP analysis were performed to evaluate the effect of MNPs@SiO(2)(RITC) on mitochondrial function. Compared to the nontreated control, glutamic acid was increased by more than 2.0-fold, and expression of genes related to the glutamic acid metabolic pathway was also disturbed in 1.0 μg/μL of MNPs@SiO(2)(RITC)-treated cells. Furthermore, increases in ROS concentration and mitochondrial damage were observed in this MNPs@SiO(2)(RITC) concentration. The organic acids related to the Krebs cycle were also disturbed, and the capacity of ATP synthesis was decreased in cell treated with an overdose of MNPs@SiO(2)(RITC). Collectively, these results suggest that overdose (1.0 μg/μL) of MNPs caused transcriptomic and metabolic disturbance. In addition, we suggest that a combination of gene expression and metabolic profiles will provide more detailed and sensitive toxicological evaluation for nanoparticles.

Pre-synaptic nicotinic receptors evoke endogenous glutamate and aspartate release from hippocampal synaptosomes by way of distinct coupling mechanisms

BACKGROUND AND PURPOSE

The present work aimed to investigate whether and through which mechanisms selective α7 and α4β2 nicotinic receptor (nAChR) agonists stimulate endogenous glutamate (GLU) and aspartate (ASP) release in rat hippocampus.

EXPERIMENTAL APPROACH

Rat hippocampal synaptosomes were purified on Percoll gradients and superfused in vitro to study endogenous GLU and ASP release. The synaptosomes were superfused with selective α7 and α4β2 nAChR agonists and antagonists. The excitatory amino acid (EAA) content of the samples of superfusate was determined by HPLC after pre-column derivatization and separation on a chromatographic column coupled with fluorimetric detection.

KEY RESULTS

Choline (Ch), a selective α7 receptor agonist, elicited a significant release of both GLU and ASP which was blocked by the α7 receptor antagonist methyllycaconitine (MLA), but was unaltered by the α4β2 receptor antagonist dihydro-β-erythroidine (DHβE). The stimulant effect of Ch was strongly reduced in a Ca(2+) -free medium, was not inhibited by Cd(2+) and tetrodotoxin (TTX), but was antagonized by dantrolene, xestospongin C and thapsigargin. 5-Iodo-A-85380 dihydrochloride (5IA85380), a selective α4β2 receptor agonist, elicited EAA release in a DHβE-sensitive, MLA-insensitive fashion. The 5IA85380-evoked release was dependent on extracellular Ca(2+) , blocked by Cd(2+) and TTX, but unaffected by dantrolene.

CONCLUSIONS AND IMPLICATIONS

Our study shows for the first time that rat hippocampal synaptosomes possess α7 and α4β2 nAChR subtypes, which can enhance the release of endogenous GLU and ASP via two distinct mechanisms of action. These results extend our knowledge of the nicotinic modulation of excitatory synaptic transmission in the hippocampus.

D-Aspartic acid is a novel endogenous neurotransmitter

D-aspartic acid (D-Asp) is present in invertebrate and vertebrate neuroendocrine tissues, where it carries out important physiological functions and is implicated in nervous system development. We show here that D-Asp is a novel endogenous neurotransmitter in two distantly related animals, a mammal (Rattus norvegicus) and a mollusk (Loligo vulgaris). Our main findings demonstrate that D-Asp is present in high concentrations in the synaptic vesicles of axon terminals; synthesis for this amino acid occurs in neurons by conversion of L-Asp to D-Asp via D-aspartate racemase; depolarization of nerve endings with K(+) ions evokes an immediate release of D-Asp in a Ca(2+) dependent manner; specific receptors for D-Asp occur at the postsynaptic membrane, as demonstrated by binding assays and by the expansion of squid skin chromatophores; D-aspartate oxidase, the specific enzyme that oxidizes D-Asp, is present in the postsynaptic membranes; and stimulation of nerve endings with D-Asp triggers signal transduction by increasing the second messenger cAMP. Taken together, these data demonstrate that D-Asp fulfills all criteria necessary to be considered a novel endogenous neurotransmitter. Given its known role in neurogenesis, learning, and neuropathologies, our results have important implications for biomedical and clinical research.

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.

Ionic dysregulations typical of ischemia provoke release of glycine and GABA by multiple mechanisms

Energy deprivation during ischemia causes dysregulations of ions, particularly sodium, potassium and calcium. Under these conditions, release of neurotransmitters is often enhanced and can occur by multiple mechanisms. The aim of this work was to characterize the modes of exit of glycine and GABA from nerve endings exposed to stimuli known to reproduce some of the ionic changes typical of ischemic conditions. Their approach was chosen instead of application of ischemic conditions because the release evoked during ischemia is mechanistically too heterogeneous. Mouse hippocampus and spinal cord synaptosomes, pre-labeled with [(3)H]glycine or [(3)H]GABA, were exposed in superfusion to 50 mM KCl or to 10 microM veratridine. The evoked overflows differed greatly between the two transmitters and between the two regions examined. Significant portions of the K(+)- and the veratridine-evoked overflows occurred by classical exocytosis. Carrier-mediated release of GABA, but not of glycine, was evoked by high K(+); GABA and, less so, glycine were released through transporter reversal by veratridine. External calcium-dependent overflows were only in part sensitive to omega-conotoxins; significant portions occurred following reversal of the plasmalemmal Na(+)/Ca(2+) exchanger. Finally, a relevant contribution to the overall transmitter overflows came from cytosolic calcium originating through the mitochondrial Na(+)/Ca(2+) exchanger. To conclude, ionic dysregulations typical of ischemia cause neurotransmitter release by heterogeneous mechanisms that differ depending on the transmitters and the CNS regions examined.

Intercellular glutamate signaling in the nervous system and beyond

Most intercellular glutamate signaling in the nervous system occurs at synapses. Some intercellular glutamate signaling occurs outside synapses, however, and even outside the nervous system where high ambient extracellular glutamate might be expected to preclude the effectiveness of glutamate as an intercellular signal. Here, I briefly review the types of intercellular glutamate signaling in the nervous system and beyond, with emphasis on the diversity of signaling mechanisms and fundamental unanswered questions.

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.