Identification of an N-methyl-D-aspartate receptor in isolated nervous system mitochondria

Abnormal levels of mitochondrial Ca2+ channel proteins in plasma neuron-derived extracellular vesicles of early schizophrenia

Structural alterations or quantitative abnormalities of some mitochondrial ion channels and exchangers are associated with altered neuronal functions and increased susceptibility to mental illness. Here we have assessed levels of functionally prominent mitochondrial calcium ion channel proteins in plasma neuron-derived extracellular vesicles (NDEVs) of living patients with first episodes of psychosis (FP) and matched controls (Cs). NDEVs were enriched with an established method of precipitation and immunoabsorption by anti-human CD171 neural adhesion protein (L1CAM) antibody and extracted proteins quantified with ELISAs. CD81 exosome marker-normalized NDEV levels of leucine zipper EF-hand containing transmembrane 1 protein (LETM1), transient receptor potential cation channel subfamily M, member 4 (TRPM4), and solute carrier family 8 member B1 (SLC24A6) or mitochondrial Na⁺ /Ca²⁺ exchanger (NCLX) were significantly lower for FP patients (n = 10) than Cs (n = 10), whereas NDEV levels of voltage-dependent L-type calcium channel subunit α-1C (CACNA-1C) were significantly higher for FP patients than Cs. Abnormal structures or mitochondrial levels of LETM1, NCLX, and CACNA-1C have been linked through analyses of individual proteins, genome-wide association studies, and whole exome protein-coding sequence studies to neurodevelopmental disorders, mental retardation, schizophrenia, and major depressive diseases. A greater understanding of the altered calcium homeostasis in schizophrenia, that is attributable to underlying mitochondrial calcium channel abnormalities, will lead to improved diagnosis and treatment.

Biochemical Properties and Physiological Functions of pLG72: Twenty Years of Investigations

In 2002, the novel human gene G72 was associated with schizophrenia susceptibility. This gene encodes a small protein of 153 amino acids, named pLG72, which represents a rare case of primate-specific protein. In particular, the rs2391191 single nucleotide polymorphism (resulting in in the R30K substitution) was robustly associated to schizophrenia and bipolar disorder. In this review, we aim to summarize the results of 20 years of biochemical investigations on pLG72. The main known role of pLG72 is related to its ability to bind and inactivate the flavoenzyme d-amino acid oxidase, i.e., the enzyme that controls the catabolism of d-serine, the main NMDA receptor coagonist in the brain. pLG72 was proposed to target the cytosolic form of d-amino acid oxidase for degradation, preserving d-serine and protecting the cell from oxidative stress generated by hydrogen peroxide produced by the flavoenzyme reaction. Anyway, pLG72 seems to play additional roles, such as affecting mitochondrial functions. The level of pLG72 in the human body is still a controversial issue because of its low expression and challenging detection. Anyway, the intriguing hypothesis that pLG72 level in blood could represent a suitable marker of Alzheimer’s disease progression (a suggestion not sufficiently established yet) merits further investigations.

Protective Effect of D-Alanine Against Acute Kidney Injury

BACKGROUND

Recent studies revealed the connection between amino acid chirality and diseases. We previously reported that the gut microbiota produced various D-amino acids in a murine acute kidney injury (AKI) model. Here, we further explore the pathophysiological role of D-Alanine (Ala) in AKI.

METHODS

We analyzed the transcripts of the N-methyl-D-aspartate (NMDA) receptor, a receptor for D-Ala, in tubular epithelial cells (TECs). Then, the therapeutic effect of D-Ala was assessed in vivo and in vitro. Lastly, the plasma level of D-Ala was evaluated in AKI patients.

RESULTS

The Grin genes encoding NMDA receptor subtypes were expressed in TECs. Hypoxia condition changes the gene expressions of Grin1, Grin2A and Grin2B. D-Ala protected TECs from hypoxia-related cell injury and induced proliferation after hypoxia. These protective effects are associated with the chirality of D-Ala. D-Ala inhibits ROS production and improves mitochondrial membrane potential, through NMDA receptor signaling. The ratio of D-Ala/L-Ala was increased in feces, plasma, and urine after the induction of I/R. Moreover, enterobacteriaceae, such as Escherichia coli, Klebsiella oxytoca produced D-Ala. The oral administration of D-Ala ameliorated kidney injury after I/R induction in mice. The deficiency of NMDA subunit NR1 on tubular cell worsened kidney damage in AKI. In addition, the plasma level of D-Ala was increased and reflected the level of renal function in AKI patients.

CONCLUSIONS

D-Ala has protective effects on I/R-induced kidney injury. Moreover, the plasma level of D-Ala reflects the eGFR in AKI patients. D-Ala could be a promising therapeutic target and potential biomarker for AKI.

SARS-CoV-2 and mitochondrial proteins in neural-derived exosomes of COVID-19

Objective

As SARS‐CoV‐2 is known to invade neural cell mitochondria, a plasma system for quantifying central nervous system proteins in living humans was used to investigate neuropathogenic mechanisms of long‐COVID‐19.

Methods

SARS‐CoV‐2 proteins and mitochondrial proteins (MPs) in enriched plasma neuron‐derived extracellular vesicles (NDEVs) and astrocyte‐derived EVs (ADEVs) were quantified in resolved acute COVID‐19 without post‐acute sequelae of SARS‐CoV‐2 (PASC), PASC without neuropsychiatric manifestations (NP), PASC with NP and healthy controls.

Results

NDEV and ADEV mean levels of SARS‐CoV‐2 S1 and nucleocapsid (N) proteins were higher in all PASC sub‐groups than controls, but only N levels were higher in PASC with than without NP. Exosome marker CD81‐normalized NDEV mean levels of subunit 6 of MP respiratory chain complex I and subunit 10 of complex III, and neuroprotective MPs Humanin and mitochondrial open‐reading frame of the 12S rRNA‐c (MOTS‐c) all were decreased significantly in PASC with NP but not in PASC without NP relative to controls. NDEV levels of MPs voltage‐dependent anion‐selective channel protein 1 (VDAC1) and N‐methyl‐D‐aspartate receptor 1 (NMDAR1) were decreased in PASC without and with NP, whereas those of calcium channel MPs mitochondrial calcium uniporter (MCU), sodium/calcium exchanger (NCLX) and leucine zipper EF‐hand containing transmembrane 1 protein (LETM1) were decreased only in PASC with NP. ADEV levels of MCU and NCLX only were increased in PASC without and with NP.

Interpretation

Abnormal NDEV and ADEV levels of SARS‐CoV‐2 N and S1 protein and MPs correlate with NP and may be biomarkers for long‐COVID prognostics and therapeutic trials. ANN NEUROL 2022;91:772–781

Mitochondria-plasma membrane interactions and communication

Mitochondria are known as the powerhouses of eukaryotic cells; however, they perform many other functions besides oxidative phosphorylation, including Ca²⁺ homeostasis, lipid metabolism, antiviral response, and apoptosis. Although other hypotheses exist, mitochondria are generally thought as descendants of an α-proteobacteria that adapted to the intracellular environment within an Asgard archaebacteria, which have been studied for decades as an organelle subdued by the eukaryotic cell. Nevertheless, several early electron microscopy observations hinted that some mitochondria establish specific interactions with certain plasma membrane (PM) domains in mammalian cells. Furthermore, recent findings have documented the direct physical and functional interaction of mitochondria and the PM, the organization of distinct complexes, and their communication through vesicular means. In yeast, some molecular players mediating this interaction have been elucidated, but only a few works have studied this interaction in mammalian cells. In addition, mitochondria can be translocated among cells through tunneling nanotubes or by other mechanisms, and free, intact, functional mitochondria have been reported in the blood plasma. Together, these findings challenge the conception of mitochondria as organelles subdued by the eukaryotic cell. This review discusses the evidence of the mitochondria interaction with the PM that has been long disregarded despite its importance in cell function, pathogenesis, and evolution. It also proposes a scheme of mitochondria–PM interactions with the intent to promote research and knowledge of this emerging pathway that promises to shift the current paradigms of cell biology.

Dual-process brain mitochondria isolation preserves function and clarifies protein composition

The brain requires continuously high energy production to maintain ion gradients and normal function. Mitochondria critically undergird brain energetics, and mitochondrial abnormalities feature prominently in neuropsychiatric disease. However, many unique aspects of brain mitochondria composition and function are poorly understood. Developing improved neuroprotective therapeutics thus requires more comprehensively understanding brain mitochondria, including accurately delineating protein composition and channel-transporter functional networks. However, obtaining pure mitochondria from the brain is especially challenging due to its distinctive lipid and cell structure properties. As a result, conflicting reports on protein localization to brain mitochondria abound. Here we illustrate this problem with the neuropsychiatric disease-associated L-type calcium channel Cav1.2α1 subunit previously observed in crude mitochondria. We applied a dual-process approach to obtain functionally intact versus compositionally pure brain mitochondria. One branch utilizes discontinuous density gradient centrifugation to isolate semipure mitochondria suitable for functional assays but unsuitable for protein localization because of endoplasmic reticulum (ER) contamination. The other branch utilizes self-forming density gradient ultracentrifugation to remove ER and yield ultrapure mitochondria that are suitable for investigating protein localization but functionally compromised. Through this process, we evaluated brain mitochondria protein content and observed the absence of Cav1.2α1 and other previously reported mitochondrial proteins, including the NMDA receptor, ryanodine receptor 1, monocarboxylate transporter 1, excitatory amino acid transporter 1, and glyceraldehyde 3-phosphate dehydrogenase. Conversely, we confirmed mitochondrial localization of several plasma membrane proteins previously reported to also localize to mitochondria. We expect this dual-process isolation procedure will enhance understanding of brain mitochondria in both health and disease.

NMDAR in cultured astrocytes: Flux-independent pH sensor and flux-dependent regulator of mitochondria and plasma membrane-mitochondria bridging

Glutamate N-methyl-D-aspartate (NMDA) receptor (NMDAR) is critical for neurotransmission as a Ca²⁺ channel. Nonetheless, flux-independent signaling has also been demonstrated. Astrocytes express NMDAR distinct from its neuronal counterpart, but cultured astrocytes have no electrophysiological response to NMDA. We recently demonstrated that in cultured astrocytes, NMDA at pH6 (NMDA/pH6) acting through the NMDAR elicits flux-independent Ca²⁺ release from the Endoplasmic Reticulum (ER) and depletes mitochondrial membrane potential (mΔΨ). Here we show that Ca²⁺ release is due to pH6 sensing by NMDAR, whereas mΔΨ depletion requires both: pH6 and flux-dependent NMDAR signaling. Plasma membrane (PM) NMDAR guard a non-random distribution relative to the ER and mitochondria. Also, NMDA/pH6 induces ER stress, endocytosis, PM electrical capacitance reduction, mitochondria-ER, and -nuclear contacts. Strikingly, it also produces the formation of PM invaginations near mitochondria along with structures referred to here as PM-mitochondrial bridges (PM-m-br). These and earlier data strongly suggest PM-mitochondria communication. As proof of the concept of mass transfer, we found that NMDA/pH6 provoked mitochondria labeling by the PM dye FM-4-64FX. NMDA/pH6 caused PM depolarization, cell acidification, and Ca²⁺ release from most mitochondria. Finally, the MCU and microtubules were not involved in mΔΨ depletion, while actin cytoskeleton was partially involved. These findings demonstrate that NMDAR has concomitant flux-independent and flux-dependent actions in cultured astrocytes.

Mitochondria as a promising target for developing novel agents for treating Alzheimer's disease

The mitochondria-targeting drugs can be conventionally divided into the following groups: those compensating for the energy deficit involved in neurodegeneration, including stimulants of mitochondrial bioenergetics and activators of mitochondrial biogenesis; and neuroprotectors, that are compounds increasing the resistance of mitochondria to opening of mitochondrial permeability transition (MPT) pores. Although compensating for the energy deficit and inhibition of MPT are obvious targets for drugs used in the very early stages of Alzheimer-like pathology, but their use as the monotherapy for patients with severe symptoms is unlikely to be sufficiently effective. It would be optimal to combine targets that would provide the cognitive-stimulating, the neuroprotective effects and the ability to affect specific disease-forming mechanisms. In the design of such drugs, assessment of their potential mitochondrial-targeted effects is of particular importance. The possibility of targeted drug design for simultaneous action on mitochondrial and neurotransmitter's receptors targets is, in particularly, based on the known interplay of various cellular pathways and the presence of common structural components. Of particular interest is directed search for multitarget drugs that would act simultaneously on mitochondrial calcium-dependent functions, the targets (receptors, enzymes, etc.) facilitating neurotransmission, and the molecular targets related to the action of so-called disease-modifying factors, in particular, the formation and overcoming of the toxicity of β-amyloid or hyperphosphorylated tau protein. The examples of such approaches realized on the level of preclinical and clinical trials are presented below.

Monitoring of glutamate-induced excitotoxicity by mitochondrial oxygen consumption

Dysfunction of mitochondrial activity is often associated with the onset and progress of neurodegenerative diseases. Membrane depolarization induced by Na⁺ influx increases intracellular Ca²⁺ levels in neurons, which upregulates mitochondrial activity. However, overlimit of Na⁺ influx and its prolonged retention ultimately cause excitotoxicity leading to neuronal cell death. To return the membrane potential to the normal level, Na⁺ /K⁺ -ATPase exchanges intracellular Na⁺ with extracellular K⁺ by consuming a large amount of ATP. This is a reason why mitochondria are important for maintaining neurons. In addition, astrocytes are thought to be important for supporting neighboring neurons by acting as energy providers and eliminators of excessive neurotransmitters. In this study, we examined the meaning of changes in the mitochondrial oxygen consumption rate (OCR) in primary mouse neuronal populations. By varying the medium constituents and using channel modulators, we found that pyruvate rather than lactate supported OCR levels and conferred on neurons resistance to glutamate-mediated excitotoxicity. Under a pyruvate-restricted condition, our OCR monitoring could detect excitotoxicity induced by glutamate at only 10 μM. The OCR monitoring also revealed the contribution of the N-methyl-D-aspartate receptor and Na⁺ /K⁺ -ATPase to the toxicity, which allowed evaluating spontaneous excitation. In addition, the OCR monitoring showed that astrocytes preferentially used glutamate, not glutamine, for a substrate of the tricarboxylic acid cycle. This mechanism may be coupled with astrocyte-dependent protection of neurons from glutamate-mediated excitotoxicity. These results suggest that OCR monitoring would provide a new powerful tool to analyze the mechanisms underlying neurotoxicity and protection against it.

NMDA and GABA receptor presence in rat heart mitochondria

The purpose of this study is to demonstrate the presence of three more receptors in mitochondria. Two N-methyl-d-aspartate receptor (NMDAR) subunits (NR1 and NR2B) are found by protein immunoblot and immunogold labeling in mitochondria fraction isolated from rat heart. These data allow supposing NMDAR presence and functioning in the inner mitochondrial membrane. There are no signs of receptor presence obtained in heart tissue lysate, that indicates the receptor localization exactly in mitochondria. The possible receptor functions discussed are its participation in calcium transport and in excitation-metabolism coupling. Besides, preliminary evidence is obtained of GABA_A and GABA_B receptors presence in heart mitochondria. One can surmise their role in metabolism regulation and their possible co-operation with NMDAR just as in the nervous system.

HIPK2-Mediated Transcriptional Control of NMDA Receptor Subunit Expression Regulates Neuronal Survival and Cell Death

NMDA receptors are critical for neuronal communication. Dysfunction in NMDA receptors has been implicated in neuropsychiatric diseases. While it is well recognized that the composition of NMDA receptors undergoes a GluN2B-to-GluN2A switch in early postnatal life, the mechanism regulating this switch remains unclear. Using transcriptomic and functional analyses in brain tissues from male and female Hipk2^+/+ and Hipk2^-/- mice, we showed that the HIPK2-JNK-c-Jun pathway is important in suppressing the transcription of Grin2a and Grin2c, which encodes the GluN2A and GluN2C subunits of the NMDA receptors, respectively. Loss of HIPK2 leads to a significant decrease in JNK-c-Jun signaling, which in turn derepresses the transcription of Grin2a and Grin2c mRNA and upregulates GluN2A and GluN2C protein levels. These changes result in a significant increase of GluN2A/GluN2B ratio in synapse and mitochondria, a persistent activation of the ERK-CREB pathway and the upregulation of synaptic activity-regulated genes, which collectively contribute to the resistance of Hipk2^-/- neurons to cell death induced by mitochondrial toxins.SIGNIFICANCE STATEMENT We identify HIPK2-JNK-c-Jun signaling as a key mechanism that regulates the transcription of NMDA receptor subunits GluN2A and GluN2C in vivo Our results provide insights into a previously unrecognized molecular mechanism that control the switch of NMDA receptor subunits in early postnatal brain development. Furthermore, we provide evidence that changes in the ratio of NMDA subunits GluN2A/GluN2B can also be detected in the synapse and mitochondria, which contributes to a persistent activation of the prosurvival ERK-CREB pathway and its downstream target genes. Collectively, these changes protect HIPK2 deficient neurons from mitochondrial toxins.

A Novel Tetramethylpyrazine Derivative Prophylactically Protects against Glutamate-Induced Excitotoxicity in Primary Neurons through the Blockage of N-Methyl-D-aspartate Receptor

The over-activation of NMDA receptor via the excessive glutamate is believed to one of the most causal factors associated with Alzheimer’s disease (AD), a progressive neurodegenerative brain disorder. Molecules that could protect against glutamate-induced neurotoxicity may hold therapeutic values for treating AD. Herein, the neuroprotective mechanisms of dimeric DT-010, a novel derivative of naturally occurring danshensu and tetramethylpyrazine, were investigated using primary rat cerebellar granule neurons (CGNs) and hippocampal neurons. It was found that DT-010 (3–30 μM) markedly prevented excitotoxicity of CGNs caused by glutamate, as evidenced by the promotion of neuronal viability as well as the reversal of abnormal morphological changes. While its parent molecules did not show any protective effects even when their concentration reached 50 μM. Additionally, DT-010 almost fully blocked intracellular accumulation of reactive oxygen species caused by glutamate and exogenous oxidative stimulus. Moreover, Western blot results demonstrated that DT-010 remarkably attenuated the inhibition of pro-survival PI3K/Akt/GSK3β pathway caused by glutamate. Ca²⁺ imaging with Fluo-4 fluorescence analysis further revealed that DT-010 greatly declined glutamate-induced increase in intracellular Ca²⁺. Most importantly, with the use of whole-cell patch clamp electrophysiology, DT-010 directly inhibited NMDA-activated whole-cell currents in primary hippocampal neurons. Molecular docking simulation analysis further revealed a possible binding mode that inhibited NMDA receptor at the ion channel, showing that DT-010 favorably binds to Asn602 of NMDA receptor via arene hydrogen bond. These results suggest that DT-010 could be served as a novel NMDA receptor antagonist and protect against glutamate-induced excitotoxicity from blocking the upstream NMDA receptors to the subsequent Ca²⁺ influx and to the downstream GSK3β cascade.

Combination Drug Therapy of Pioglitazone and D-cycloserine Attenuates Chronic Orofacial Neuropathic Pain and Anxiety by Improving Mitochondrial Function Following Trigeminal Nerve Injury

OBJECTIVES

The study aim was to determine how peripheral trigeminal nerve injury affects mitochondrial respiration and to test efficacy of combined treatment with 2 Federal Drug Administration approved drugs with potential for improving mitochondrial bioenergetics, pain and anxiety-related behaviors in a chronic orofacial neuropathic pain mouse model.

METHODS

Efficacy of (R)-(+)-4-amino-3-isoxazolidinone (D-cycloserine, DCS), an N-Methyl-D-aspartate antagonist/agonist, and Pioglitazone (PIO), a selective agonist of nuclear receptor peroxisome proliferator-activated receptor gamma was investigate in the trigeminal inflammatory compression (TIC) neuropathic nerve injury mouse model. Combined low doses of these drugs (80 mg/kg DCS and 100 mg/kg PIO) were given as a single bolus or daily for 7 days post-TIC to test ability to attenuate neuropathic nociceptive and associated cognitive dependent anxiety behaviors. In addition, beneficial effects of the DCS/PIO drug combination were explored ex vivo in isolated cortex/brainstem mitochondria at 28 weeks post-TIC.

RESULTS

The DCS/PIO combination not only attenuated orofacial neuropathic pain and anxiety-related behaviors associated with trigeminal nerve injury, but it also improved mitochondrial bioenergetics.

DISCUSSION

The DCS/PIO combination uncoupled mitochondrial respiration in the TIC model to improve cortical mitochondrial dysfunction, as well as reduced nociceptive and anxiety behaviors present in mice with centralized chronic neuropathic nerve injury. Combining these drugs could be a beneficial treatment for patients with depression, anxiety, or other psychological conditions due to their chronic pain status.

D-Amino Acid Oxidase-pLG72 Interaction and D-Serine Modulation

pLG72 is a small, primate-specific protein of 153 amino acids. It is the product of the G72 gene, expressed in testis, spinal cord, and brain. The presence of G72 transcript and pLG72 has recurrently been called into question, however G72 mRNA and pLG72 protein levels were higher in blood and brain of patients with schizophrenia than in healthy controls. On the one hand, the SNP rs2391191 corresponding to the R30K substitution in pLG72 was genetically linked to schizophrenia, reduced thickness of the brain cortex in schizophrenia-affected individuals, and altered memory function. Various lines of evidence indicated that pLG72 is a mitochondrial protein, specifically an extrinsic protein bound on the outer membrane. Over the years, pLG72 was proposed to be involved in different functions: (a) overexpression induces mitochondria fragmentation, increasing the numbers of shorter and more mobile ones which could be delivered faster to regions of intense growth and facilitating the dendritic complexity; (b) it might induce oxidative stress by interacting with methionine-R-sulfoxide reductase B2; and (c) it binds and modulates the activity of FMN-containing oxidoreductase of the respiratory complex I. The main role of this protein, however, is related to its binding to the human flavoenzyme D-amino acid oxidase (hDAAO), i.e., the main catabolic enzyme for D-enantiomer of serine. This D-amino acid is a main endogenous coagonist of the N-methyl-D-aspartate type glutamate receptor (NMDAR) involved in main functions such as synaptic plasticity, learning, memory, and excitotoxicity. For this work, we reviewed the recent literature concerning the hDAAO-pLG72 interaction, focusing on the molecular details of the interaction, the effect of hDAAO function and stability, and the cellular effects, especially on D-serine concentration. The main effects related to the pathological R30K substitution are also reported. We have highlighted the gaps in our knowledge of this human protein as well as the relevance of clarifying the molecular details of hDAAO-pLG72 interaction in order to design molecules to modulate hDAAO activity/stability and thus NMDAR function acting at the D-serine cellular level.

An evolutionary switch in ND2 enables Src kinase regulation of NMDA receptors

The non-receptor tyrosine kinase Src is a key signalling hub for upregulating the function of N-methyl D-aspartate receptors (NMDARs). Src is anchored within the NMDAR complex via NADH dehydrogenase subunit 2 (ND2), a mitochondrially encoded adaptor protein. The interacting regions between Src and ND2 have been broadly identified, but the interaction between ND2 and the NMDAR has remained elusive. Here we generate a homology model of ND2 and dock it onto the NMDAR via the transmembrane domain of GluN1. This interaction is enabled by the evolutionary loss of three helices in bilaterian ND2 proteins compared to their ancestral homologues. We experimentally validate our model and demonstrate that blocking this interaction with an ND2 fragment identified in our experimental studies prevents Src-mediated upregulation of NMDAR currents in neurons. Our findings establish the mode of interaction between an NMDAR accessory protein with one of the core subunits of the receptor.

N-methyl D-aspartate receptor (NMDAR) activity is modulated by Src tyrosine kinase via the mitochondrial protein NADH dehydrogenase subunit 2 (ND2). Here the authors show that ND2 interacts with the transmembrane region of NMDAR GluN1 subunit, a process that is crucial for Src regulation of NMDAR activity.

On the regulative role of the glutamate receptor in mitochondria

The purpose of this work was to study the regulative role of the glutamate receptor found earlier in the brain mitochondria. In the present work a glutamate-dependent signaling system with similar features was detected in mitochondria of the heart. The glutamate-dependent signaling system in the heart mitochondria was shown to be suppressed by γ-aminobutyric acid (GABA). The GABA receptor presence in the heart mitochondria was shown by golding with the use of antibodies to α- and β-subunits of the receptor. The activity of glutamate receptor was assessed according to the rate of synthesis of hydrogen peroxide. The glutamate receptor in mitochondria could be activated only under conditions of hypoxic stress, which in model experiments was imitated by blocking Complex I by rotenone or fatty acids. The glutamate signal in mitochondria was shown to be calcium- and potential-dependent and the activation of the glutamate cascade was shown to be accompanied by production of hydrogen peroxide. It was discovered that H2O2 synthesis involves two complexes of the mitochondrial electron transfer system - succinate dehydrogenase (SDH) and fatty acid dehydrogenase (ETF:QO). Thus, functions of the glutamate signaling system are associated with the system of respiration-glycolysis switching (the Pasteur-Crabtree) under conditions of hypoxia.

Regulation of mitochondrial calcium in plants versus animals

Ca(2+) acts as an important cellular second messenger in eukaryotes. In both plants and animals, a wide variety of environmental and developmental stimuli trigger Ca(2+) transients of a specific signature that can modulate gene expression and metabolism. In animals, mitochondrial energy metabolism has long been considered a hotspot of Ca(2+) regulation, with a range of pathophysiology linked to altered Ca(2+) control. Recently, several molecular players involved in mitochondrial Ca(2+) signalling have been identified, including those of the mitochondrial Ca(2+) uniporter. Despite strong evidence for sophisticated Ca(2+) regulation in plant mitochondria, the picture has remained much less clear. This is currently changing aided by live imaging and genetic approaches which allow dissection of subcellular Ca(2+) dynamics and identification of the proteins involved. We provide an update on our current understanding in the regulation of mitochondrial Ca(2+) and signalling by comparing work in plants and animals. The significance of mitochondrial Ca(2+) control is discussed in the light of the specific metabolic and energetic needs of plant and animal cells.

G72 primate-specific gene: a still enigmatic element in psychiatric disorders

Numerous studies have demonstrated a link between genetic markers on chromosome 13 and schizophrenia, bipolar affective disorder, and other psychiatric phenotypes. The G72/G30 genes (transcribed in opposite directions) are located on chromosome 13q33, a region demonstrating strong evidence for linkage with various neuropsychiatric disorders. G72/G30 was identified in 2002 as a schizophrenia susceptibility locus; however, subsequent association studies did not reach consensus on single SNPs within the locus. Simultaneously, a new vision for the genetic architecture of psychiatric disorders suggested that schizophrenia was a quantitative trait, therefore ascribable to potentially hundreds of genes and subjected to the vagaries of the environment. The main protein product of G72 gene is named pLG72 or D-amino acid oxidase activator DAOA (153 amino acids) and its function is still debated. Functional analyses, also showing controversial results, indicate that pLG72 contributes to N-methyl-D-aspartate receptor modulation by affecting activity of the flavoprotein D-amino acid oxidase, the enzyme responsible for degrading the neuromodulator D-serine. In this review we, for the first time, summarize findings from molecular genetic linkage and association studies concerning G72 gene, cellular and molecular studies on pLG72, and investigations performed on G72/G30 transgenic mice. This will help elucidate the role of psychosis susceptibility genes, which will have a major impact on our understanding of disease pathophysiology and thus change classification and treatment.

Dysregulated mitochondrial and chloroplast bioenergetics from a translational medical perspective (Review)

Mitochondria and chloroplasts represent endosymbiotic models of complex organelle development, driven by intense evolutionary pressure to provide exponentially enhanced ATP-dependent energy production functionally linked to cellular respiration and photosynthesis. Within the realm of translational medicine, it has become compellingly evident that mitochondrial dysfunction, resulting in compromised cellular bioenergetics, represents a key causative factor in the etiology and persistence of major diseases afflicting human populations. As a pathophysiological consequence of enhanced oxygen utilization that is functionally uncoupled from the oxidative phosphorylation of ADP, significant levels of reactive oxygen species (ROS) may be generated within mitochondria and chloroplasts, which may effectively compromise cellular energy production following prolonged stress/inflammatory conditions. Empirically determined homologies in biochemical pathways, and their respective encoding gene sequences between chloroplasts and mitochondria, suggest common origins via entrapped primordial bacterial ancestors. From evolutionary and developmental perspectives, the elucidation of multiple biochemical and molecular relationships responsible for errorless bioenergetics within mitochondrial and plastid complexes will most certainly enhance the depth of translational approaches to ameliorate or even prevent the destructive effects of multiple disease states. The selective choice of discussion points contained within the present review is designed to provide theoretical bases and translational insights into the pathophysiology of human diseases from a perspective of dysregulated mitochondrial bioenergetics with special reference to chloroplast biology.

Alternative splicing-mediated targeting of the Arabidopsis GLUTAMATE RECEPTOR3.5 to mitochondria affects organelle morphology

Since the discovery of 20 genes encoding for putative ionotropic glutamate receptors in the Arabidopsis (Arabidopsis thaliana) genome, there has been considerable interest in uncovering their physiological functions. For many of these receptors, neither their channel formation and/or physiological roles nor their localization within the plant cells is known. Here, we provide, to our knowledge, new information about in vivo protein localization and give insight into the biological roles of the so-far uncharacterized Arabidopsis GLUTAMATE RECEPTOR3.5 (AtGLR3.5), a member of subfamily 3 of plant glutamate receptors. Using the pGREAT vector designed for the expression of fusion proteins in plants, we show that a splicing variant of AtGLR3.5 targets the inner mitochondrial membrane, while the other variant localizes to chloroplasts. Mitochondria of knockout or silenced plants showed a strikingly altered ultrastructure, lack of cristae, and swelling. Furthermore, using a genetically encoded mitochondria-targeted calcium probe, we measured a slightly reduced mitochondrial calcium uptake capacity in the knockout mutant. These observations indicate a functional expression of AtGLR3.5 in this organelle. Furthermore, AtGLR3.5-less mutant plants undergo anticipated senescence. Our data thus represent, to our knowledge, the first evidence of splicing-regulated organellar targeting of a plant ion channel and identify the first cation channel in plant mitochondria from a molecular point of view.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Glutamate induces H2O2 synthesis in nonsynaptic brain mitochondria

Mitochondrial reactive oxygen species regulate many important biological processes. We studied H2O2 formation by nonsynaptic brain mitochondria in response to the addition of low concentrations of glutamate, an excitatory neurotransmitter. We demonstrated that glutamate at concentrations from 10 to 50 μM stimulated the H2O2 generation in mitochondria up to 4-fold, in a dose-dependent manner. The effect of glutamate was observed only in the presence of Ca(2+) (20 μM) in the incubation medium, and the rate of calcium uptake by the brain mitochondria was increased by up to 50% by glutamate. Glutamate-dependent effects were sensitive to the NMDA receptor inhibitors MK-801 (10 μM) and D-AP5 (20 μM) and the inhibitory neurotransmitter glycine (5mM). We have shown that the H2O2 formation caused by glutamate is associated with complex II and is dependent on the mitochondrial potential. We have found that nonsynaptic brain mitochondria are a target of direct glutamate signaling, which can specifically activate H2O2 formation through mitochondrial respiratory chain complex II. The H2O2 formation induced by glutamate can be blocked by glycine, an inhibitory neurotransmitter that prevents the deleterious effects of glutamate in brain mitochondria.

Selective inhibition by ethanol of mitochondrial calcium influx mediated by uncoupling protein-2 in relation to N-methyl-D-aspartate cytotoxicity in cultured neurons

BACKGROUND

We have shown the involvement of mitochondrial uncoupling protein-2 (UCP2) in the cytotoxicity by N-methyl-D-aspartate receptor (NMDAR) through a mechanism relevant to the increased mitochondrial Ca(2+) levels in HEK293 cells with acquired NMDAR channels. Here, we evaluated pharmacological profiles of ethanol on the NMDA-induced increase in mitochondrial Ca(2+) levels in cultured murine neocortical neurons.

METHODOLOGY/PRINCIPAL FINDINGS

In neurons exposed to glutamate or NMDA, a significant increase was seen in mitochondrial Ca(2+) levels determined by Rhod-2 at concentrations of 0.1 to 100 µM. Further addition of 250 mM ethanol significantly inhibited the increase by glutamate and NMDA in Rhod-2 fluorescence, while similarly potent inhibition of the NMDA-induced increase was seen after exposure to ethanol at 50 to 250 mM in cultured neurons. Lentiviral overexpression of UCP2 significantly accelerated the increase by NMDA in Rhod-2 fluorescence in neurons, without affecting Fluo-3 fluorescence for intracellular Ca(2+) levels. In neurons overexpressing UCP2, exposure to ethanol resulted in significantly more effective inhibition of the NMDA-induced increase in mitochondrial free Ca(2+) levels than in those without UCP2 overexpression, despite a similarly efficient increase in intracellular Ca(2+) levels irrespective of UCP2 overexpression. Overexpression of UCP2 significantly increased the number of dead cells in a manner prevented by ethanol in neurons exposed to glutamate. In HEK293 cells with NMDAR containing GluN2B subunit, more efficient inhibition was similarly induced by ethanol at 50 and 250 mM on the NMDA-induced increase in mitochondrial Ca(2+) levels than in those with GluN2A subunit. Decreased protein levels of GluN2B, but not GluN2A, subunit were seen in immunoprecipitates with UCP2 from neurons with brief exposure to ethanol at concentrations over 50 mM.

CONCLUSIONS/SIGNIFICANCE

Ethanol could inhibit the interaction between UCP2 and NMDAR channels to prevent the mitochondrial Ca(2+) incorporation and cell death after NMDAR activation in neurons.

Intracellular ASIC1a regulates mitochondrial permeability transition-dependent neuronal death

Acid-sensing ion channel 1a (ASIC1a) is the key proton receptor in nervous systems, mediating acidosis-induced neuronal injury in many neurological disorders, such as ischemic stroke. Up to now, functional ASIC1a has been found exclusively on the plasma membrane. Here, we show that ASIC1a proteins are also present in mitochondria of mouse cortical neurons where they are physically associated with adenine nucleotide translocase. Moreover, purified mitochondria from ASIC1a(-/-) mice exhibit significantly enhanced Ca(2+) retention capacity and accelerated Ca(2+) uptake rate. When challenged with hydrogen peroxide (H2O2), ASIC1a(-/-) neurons are resistant to cytochrome c release and inner mitochondrial membrane depolarization, suggesting an impairment of mitochondrial permeability transition (MPT) due to ASIC1a deletion. Consistently, H2O2-induced neuronal death, which is MPT dependent, is reduced in ASIC1a(-/-) neurons. Additionally, significant increases in mitochondrial size and oxidative stress levels are detected in ASIC1a(-/-) mouse brain, which also displays marked changes (>2-fold) in the expression of mitochondrial proteins closely related to reactive oxygen species signal pathways, as revealed by two-dimensional difference gel electrophoresis followed by mass spectrometry analysis. Our data suggest that mitochondrial ASIC1a may serve as an important regulator of MPT pores, which contributes to oxidative neuronal cell death.

Oligodendrocyte plasticity with an intact cell body in vitro

Demyelination is generally regarded as a consequence of oligodendrocytic cell death. Oligodendrocyte processes that form myelin sheaths may, however, degenerate and regenerate independently of the cell body, in which case cell death does not necessarily occur. We provide here the first evidence of retraction and regeneration of oligodendrocyte processes with no cell death in vitro, using time-lapse imaging. When processes were severed mechanically in vitro, the cells did not undergo cell death and the processes regenerated in 36 h. In a separate experiment, moderate N-methyl-D-aspartate (NMDA) stimuli caused process retraction without apparent cell death, and the processes regained their elaborate morphology after NMDA was removed from the culture medium. These results strongly suggest that demyelination and remyelination can take place without concomitant cell death, at least in vitro. Process regeneration may therefore become a target for future therapy of demyelinating disorders.