Requirement of both NR3A and NR3B subunits for dominant negative properties on Ca2+ mobilization mediated by acquired N-methyl-d-aspartate receptor channels into mitochondria

Extrasynaptic NMDA receptors in acute and chronic excitotoxicity: implications for preventive treatments of ischemic stroke and late-onset Alzheimer's disease

Stroke and late-onset Alzheimer's disease (AD) are risk factors for each other; the comorbidity of these brain disorders in aging individuals represents a significant challenge in basic research and clinical practice. The similarities and differences between stroke and AD in terms of pathogenesis and pathophysiology, however, have rarely been comparably reviewed. Here, we discuss the research background and recent progresses that are important and informative for the comorbidity of stroke and late-onset AD and related dementia (ADRD). Glutamatergic NMDA receptor (NMDAR) activity and NMDAR-mediated Ca²⁺ influx are essential for neuronal function and cell survival. An ischemic insult, however, can cause rapid increases in glutamate concentration and excessive activation of NMDARs, leading to swift Ca²⁺ overload in neuronal cells and acute excitotoxicity within hours and days. On the other hand, mild upregulation of NMDAR activity, commonly seen in AD animal models and patients, is not immediately cytotoxic. Sustained NMDAR hyperactivity and Ca²⁺ dysregulation lasting from months to years, nevertheless, can be pathogenic for slowly evolving events, i.e. degenerative excitotoxicity, in the development of AD/ADRD. Specifically, Ca²⁺ influx mediated by extrasynaptic NMDARs (eNMDARs) and a downstream pathway mediated by transient receptor potential cation channel subfamily M member (TRPM) are primarily responsible for excitotoxicity. On the other hand, the NMDAR subunit GluN3A plays a "gatekeeper" role in NMDAR activity and a neuroprotective role against both acute and chronic excitotoxicity. Thus, ischemic stroke and AD share an NMDAR- and Ca²⁺-mediated pathogenic mechanism that provides a common receptor target for preventive and possibly disease-modifying therapies. Memantine (MEM) preferentially blocks eNMDARs and was approved by the Federal Drug Administration (FDA) for symptomatic treatment of moderate-to-severe AD with variable efficacy. According to the pathogenic role of eNMDARs, it is conceivable that MEM and other eNMDAR antagonists should be administered much earlier, preferably during the presymptomatic phases of AD/ADRD. This anti-AD treatment could simultaneously serve as a preconditioning strategy against stroke that attacks ≥ 50% of AD patients. Future research on the regulation of NMDARs, enduring control of eNMDARs, Ca²⁺ homeostasis, and downstream events will provide a promising opportunity to understand and treat the comorbidity of AD/ADRD and stroke.

Igf1 and Pacap rescue cerebellar granule neurons from apoptosis via a common transcriptional program

A shift of the delicate balance between apoptosis and survival-inducing signals determines the fate of neurons during the development of the central nervous system and its homeostasis throughout adulthood. Both pathways, promoting or protecting from apoptosis, trigger a transcriptional program. We conducted whole-genome expression profiling to decipher the transcriptional regulatory elements controlling the apoptotic/survival switch in cerebellar granule neurons following the induction of apoptosis by serum and potassium deprivation or their rescue by either insulin-like growth factor-1 (Igf1) or pituitary adenylyl cyclase-activating polypeptide (Pacap). Although depending on different upstream signaling pathways, the survival effects of Igf1 and Pacap converged into common transcriptional cascades, thus suggesting the existence of a general transcriptional program underlying neuronal survival.

Redistribution of subcellular calcium and its effect on apoptosis in primary cultures of rat proximal tubular cells exposed to lead

Previous studies have shown that cytosolic Ca(2+) ([Ca(2+)]c) overload was involved in Pb-induced apoptosis in primary cultures of rat proximal tubular (rPT) cells, but the source of elevated Ca(2+) and the effect of potential subcellular Ca(2+) redistribution on apoptosis are still unknown. In this study, variations of [Ca(2+)]c in two culture media (Ca(2+)-containing and Ca(2+)- free) were analyzed, indicating that Pb-induced elevation of [Ca(2+)]c was primarily generated intracellularly. Fluo-4-AM, dihydro-Rhod-2-AM and Mag-Fluo-4-AM was loaded to Pb-exposed rPT cells to monitor the imaging of Ca(2+) concentrations in the cytoplasm ([Ca(2+)]c), mitochondria ([Ca(2+)]mit) and endoplasmic reticulum (ER) ([Ca(2+)]ER), respectively, under the confocal microscope. Data indicate that elevations of [Ca(2+)]c and [Ca(2+)]mit with depletion of [Ca(2+)]ER were revealed in Pb-treated rPT cells, but this subcellular Ca(2+) redistribution could be significantly suppressed by 2-APB, a specific inhibitor of inositol 1,4,5-trisphosphate receptor (IP3R) that functions to release Ca(2+) from ER stores. Simultaneously, Pb-mediated mitochondrial Ca(2+) overload can be partially suppressed by the cytosolic Ca(2+) chelator BAPTA-AM, suggesting that Ca(2+) uptake into mitochondria occurs via diverse pathways and ER Ca(2+) storage was the chief source. Furthermore, Pb-induced apoptosis was markedly inhibited by 2-APB and BAPTA-AM, respectively. Additionally, elevated IP3 levels with up-regulated IP3R-1 and IP3R-2 (mRNA and protein) levels were revealed in Pb-exposed rPT cells. In summary, IP3R-mediated ER Ca(2+) release promoted the elevations of [Ca(2+)]c and [Ca(2+)]mit in Pb-exposed rPT cells, which played a chief role in apoptosis induced by impaired calcium homeostasis.

A naturally occurring null variant of the NMDA type glutamate receptor NR3B subunit is a risk factor of schizophrenia

Hypofunction of the N-methyl-D-aspartate type glutamate receptor (NMDAR) has been implicated in the pathogenesis of schizophrenia. Here, we investigated the significance of a common human genetic variation of the NMDAR NR3B subunit that inserts 4 bases within the coding region (insCGTT) in the pathogenesis of schizophrenia. The cDNA carrying this polymorphism generates a truncated protein, which is electrophysiologically non-functional in heterologous expression systems. Among 586 schizophrenia patients and 754 healthy controls, insCGTT was significantly overrepresented in patients compared to controls (odds ratio = 1.37, p = 0.035). Among 121 schizophrenia patients and 372 healthy controls, genetic analyses of normal individuals revealed that those carrying insCGTT have a predisposition to schizotypal personality traits (F_1,356 = 4.69, p = 0.031). Furthermore, pre-pulse inhibition, a neurobiological trait disturbed in patients with schizophrenia, was significantly impaired in patients carrying insCGTT compared with those with the major allele (F_1,116 = 5.72, p = 0.018, F_1,238 = 4.46, p = 0.036, respectively). These results indicate that a naturally occurring null variant in NR3B could be a risk factor of schizophrenia.

Cigarette smoke-induced Ca2+ release leads to cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction

Chronic obstructive pulmonary disease affects 64 million people and is currently the fourth leading cause of death worldwide. Chronic obstructive pulmonary disease includes both emphysema and chronic bronchitis, and in the case of chronic bronchitis represents an inflammatory response of the airways that is associated with mucus hypersecretion and obstruction of small airways. Recently, it has emerged that exposure to cigarette smoke (CS) leads to an inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, causing airway surface liquid dehydration, which may play a role in the development of chronic bronchitis. CS rapidly clears CFTR from the plasma membrane and causes it to be deposited into aggresome-like compartments. However, little is known about the mechanism(s) responsible for the internalization of CFTR following CS exposure. Our studies revealed that CS triggered a rise in cytoplasmic Ca(2+) that may have emanated from lysosomes. Furthermore, chelation of cytoplasmic Ca(2+), but not inhibition of protein kinases/phosphatases, prevented CS-induced CFTR internalization. The macrolide antibiotic bafilomycin A1 inhibited CS-induced Ca(2+) release and prevented CFTR clearance from the plasma membrane, further linking cytoplasmic Ca(2+) and CFTR internalization. We hypothesize that CS-induced Ca(2+) release prevents normal sorting/degradation of CFTR and causes internalized CFTR to reroute to aggresomes. Our data provide mechanistic insight into the potentially deleterious effects of CS on airway epithelia and outline a hitherto unrecognized signaling event triggered by CS that may affect the long term transition of the lung into a hyper-inflammatory/dehydrated environment.

Neuroprotective effect of calcitriol on ischemic/reperfusion injury through the NR3A/CREB pathways in the rat hippocampus

Calcitriol has been demonstrated to provide neuroprotection against ischemia/reperfusion (I/R) injury. However, the exact mechanism of this protection remains unknown. In the present study, the neuroprotective effect of calcitriol was investigated in rats exposed to cerebral I/R injury induced by middle cerebral artery occlusion (MCAO). In addition, the involvement of NR3A, extracellular signal‑regulated kinase 1/2 (ERK1/2), and phosphorylated cAMP/Ca2+‑response element binding protein (p‑CREB) in this protective action was determined in the hippocampal neurons. Western blot analysis was conducted to analyze the protein levels of NR3A, mitogen‑activated protein kinase kinase (MEK) and p‑CREB. The immunoreactivity of p‑CREB and NR3A were measured by quantum dot‑based immunofluorescence analysis. Results showed that MCAO rats exhibited large cortical infarct volumes. By contrast, intraperitoneal administration of calcitriol significantly reduced infarct volumes seven days following reperfusion, and these results were accompanied by elevated NR3A and p‑CREB activity in the hippocampal neurons. The inhibition of MEK by the addition of PD98059 led to attenuation of the neuroprotective effects of calcitriol and a correlated decrease in CREB activity. The results also demonstrated that calcitriol protected the brain from I/R injury through the NR3A‑MEK/ERK‑CREB pathway.

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.

NR2-reactive antibody decreases cell viability through augmentation of Ca(2+) influx in systemic lupus erythematosus

OBJECTIVE

Anti-N-methyl-D-aspartate (anti-NMDA) receptor subunit NR2-reactive antibody may play a crucial role in neuronal manifestations of systemic lupus erythematosus (SLE). However, how NR2-reactive antibody acts as a critical modulator of the NMDA receptor is unknown. This study was undertaken to investigate the biologic function of NR2-reactive antibody in patients with SLE.

METHODS

The study included 14 patients with SLE, 9 of whom had NR2-reactive antibody. We analyzed the effects of NR2-reactive antibody on cell viability and intracellular Ca(2+) level. We also investigated the efficacy of zinc as a modulator of the intracellular Ca(2+) level in the presence of NR2-reactive antibody.

RESULTS

There was a significant inverse correlation between the NR2-reactive antibody titer and cell viability (R(2) = 0.67, P < 0.0001; n = 23), and there was a significant association between the NR2-reactive antibody titer and the intracellular Ca(2+) level in NR1/NR2a-transfected HEK 293 cells (R(2) = 0.69, P < 0.0001). Intracellular Ca(2+) levels were significantly higher in cells incubated with IgG derived from NR2-reactive antibody-positive SLE patients than in those incubated with IgG derived from NR2-reactive antibody-negative SLE patients (P = 0.0002). The addition of zinc decreased the intracellular Ca(2+) level in a dose-dependent manner. NR2-reactive antibody-positive SLE IgG weakened the efficacy of zinc as a negative modulator of the intracellular Ca(2+) level.

CONCLUSION

Our findings indicate that NR2-reactive antibody decreases cell viability by Ca(2+) influx in SLE through inhibition of the binding capacity of zinc.

A possible pivotal role of mitochondrial free calcium in neurotoxicity mediated by N-methyl-d-aspartate receptors in cultured rat hippocampal neurons

We have previously shown that mitochondrial membrane potential disruption is involved in mechanisms underlying differential vulnerabilities to the excitotoxicity mediated by N-methyl-d-aspartate (NMDA) receptors between primary cultured neurons prepared from rat cortex and hippocampus. To further elucidate the role of mitochondria in the excitotoxicity after activation of NMDA receptors, neurons were loaded with the fluorescent dye calcein diffusible in the cytoplasm and organelles for determination of the activity of mitochondrial permeability transition pore (mPTP) responsible for the leakage of different mitochondrial molecules. The addition of CoCl(2) similarly quenched the intracellular fluorescence except mitochondria in both cultured neurons, while further addition of NMDA led to a leakage of the dye into the cytoplasm in hippocampal neurons only. An mPTP inhibitor prevented the NMDA-induced loss of viability in hippocampal neurons, while an activator of mPTP induced a similarly potent loss of viability in cortical and hippocampal neurons. Although NMDA was more effective in increasing rhodamine-2 fluorescence as a mitochondrial calcium indicator in hippocampal than cortical neurons, a mitochondrial calcium uniporter inhibitor significantly prevented the NMDA-induced loss of viability in hippocampal neurons. Expression of mRNA was significantly higher for the putative uniporter uncoupling protein-2 in hippocampal than cortical neurons. These results suggest that mitochondrial calcium uniporter would be at least in part responsible for the NMDA neurotoxicity through a mechanism relevant to promotion of mPTP orchestration in hippocampal neurons.