The role of monoamine metabolism in oxidative glutamate toxicity

Lutein inhibits glutamate-induced apoptosis in HT22 cells via the Nrf2/HO-1 signaling pathway

Introduction

Excessive glutamate levels induce oxidative stress, resulting in neuronal damage, and cell death. While natural antioxidants show promise for neuroprotection, their effectiveness in the central nervous system (CNS) is limited by the blood -brain barrier. Lutein, a neuroprotective carotenoid, has gained attention for its ability to traverse this barrier and accumulate in various brain regions. This study aimed to elucidate the mechanisms underlying the protective effects of lutein against glutamateinduced cell death in HT22 cells.

Methods

HT22 cells were treated with lutein (1.25-20 μM) for 24 hours. Cell viability, ROS levels, apoptosis, and mitochondrial membrane potential were assessed following lutein pretreatment and glutamate exposure. Protein expression of apoptotic markers was analyzed using Western blotting.

Results

Lutein effectively attenuated glutamate-induced apoptosis due to its antioxidant properties. Additionally, lutein inhibited glutamate-induced mitochondrial-mediated apoptosis. We observed that lutein modulated the nuclear translocation of nuclear factor erythroid 2 -related factor 2 (Nrf2) and upregulated the expression of heme oxygenase-1 (HO-1). Inhibition of HO-1 by tin protoporphyrin (SnPP), a synthetic inhibitor, weakened the protective effect of lutein. Furthermore, we demonstrated that lutein prevented the aberrant activation of MAPKs induced by glutamate, including ERK1/2, p38, and JNK, thereby conferring oxidative protection.

Discussion

Our study highlights the potent antioxidant properties of lutein, which effectively safeguards against glutamate-induced mitochondrial apoptotic cell death through the Nrf2/HO-1 signaling pathway and inhibition of MAPK activation. These findings demonstrate that lutein exerts a neuroprotective effect against glutamate-induced neuronal cell damage.

The Neuroprotective Flavonoids Sterubin and Fisetin Maintain Mitochondrial Health under Oxytotic/Ferroptotic Stress and Improve Bioenergetic Efficiency in HT22 Neuronal Cells

The global increase in the aging population has led to a rise in many age-related diseases with continuing unmet therapeutic needs. Research into the molecular mechanisms underlying both aging and neurodegeneration has identified promising therapeutic targets, such as the oxytosis/ferroptosis cell death pathway, in which mitochondrial dysfunction plays a critical role. This study focused on sterubin and fisetin, two flavonoids from the natural pharmacopeia previously identified as strong inhibitors of the oxytosis/ferroptosis pathway. Here, we investigated the effects of the compounds on the mitochondrial physiology in HT22 hippocampal nerve cells under oxytotic/ferroptotic stress. We show that the compounds can restore mitochondrial homeostasis at the level of redox regulation, calcium uptake, biogenesis, fusion/fission dynamics, and modulation of respiration, leading to the enhancement of bioenergetic efficiency. However, mitochondria are not required for the neuroprotective effects of sterubin and fisetin, highlighting their diverse homeostatic impacts. Sterubin and fisetin, thus, provide opportunities to expand drug development strategies for anti-oxytotic/ferroptotic agents and offer new perspectives on the intricate interplay between mitochondrial function, cellular stress, and the pathophysiology of aging and age-related neurodegenerative disorders.

SREBP1 deficiency diminishes glutamate-mediated HT22 cell damage and hippocampal neuronal pyroptosis induced by status epilepticus

Status epilepticus (SE) is a life-threatening disorder that can result in death or severe brain damage, and there is a substantial body of evidence suggesting a strong association between pyroptosis and SE. Sterol regulatory element binding protein 1 (SREBP1) is a significant transcription factor participating in both lipid homeostasis and glucose metabolism. However, the function of SREBP1 in pyroptosis during SE remains unknown. In this study, we established a SE rat model by intraperitoneal injection of lithium chloride and pilocarpine in vivo. Additionally, we treated HT22 hippocampal cells with glutamate to create neuronal injury models in vitro. Our results demonstrated a significant induction of SREBP1, inflammasomes, and pyroptosis in the hippocampus of SE rats and glutamate-treated HT22 cells. Moreover, we found that SREBP1 is regulated by the mTOR signaling pathway, and inhibiting mTOR signaling contributed to the amelioration of SE-induced hippocampal neuron pyroptosis, accompanied by a reduction in SREBP1 expression. Furthermore, we conducted siRNA-mediated knockdown of SREBP1 in HT22 cells and observed a significant reversal of glutamate-induced cell death, activation of inflammasomes, and pyroptosis. Importantly, our confocal immunofluorescence analysis revealed the co-localization of SREBP1 and NLRP1. In conclusion, our findings suggest that deficiency of SREBP1 attenuates glutamate-induced HT22 cell injury and hippocampal neuronal pyroptosis in rats following SE. Targeting SREBP1 may hold promise as a therapeutic strategy for SE.

Negative modulation of mitochondrial calcium uniporter complex protects neurons against ferroptosis

Ferroptosis is an iron- and reactive oxygen species (ROS)-dependent form of regulated cell death, that has been implicated in Alzheimer's disease and Parkinson's disease. Inhibition of cystine/glutamate antiporter could lead to mitochondrial fragmentation, mitochondrial calcium ([Ca2+]m) overload, increased mitochondrial ROS production, disruption of the mitochondrial membrane potential (ΔΨm), and ferroptotic cell death. The observation that mitochondrial dysfunction is a characteristic of ferroptosis makes preservation of mitochondrial function a potential therapeutic option for diseases associated with ferroptotic cell death. Mitochondrial calcium levels are controlled via the mitochondrial calcium uniporter (MCU), the main entry point of Ca2+ into the mitochondrial matrix. Therefore, we have hypothesized that negative modulation of MCU complex may confer protection against ferroptosis. Here we evaluated whether the known negative modulators of MCU complex, ruthenium red (RR), its derivative Ru265, mitoxantrone (MX), and MCU-i4 can prevent mitochondrial dysfunction and ferroptotic cell death. These compounds mediated protection in HT22 cells, in human dopaminergic neurons and mouse primary cortical neurons against ferroptotic cell death. Depletion of MICU1, a [Ca2+]m gatekeeper, demonstrated that MICU is protective against ferroptosis. Taken together, our results reveal that negative modulation of MCU complex represents a therapeutic option to prevent degenerative conditions, in which ferroptosis is central to the progression of these pathologies.

Novel Probucol Analogue, 4,4'-Diselanediylbis (2,6-Di-tert-Butylphenol), Prevents Oxidative Glutamate Neurotoxicity In Vitro and Confers Neuroprotection in a Rodent Model of Ischemic Stroke

Oxidative glutamate toxicity is regarded as one of the injurious mechanisms associated with ischemic stroke, which represents a major health problem and requires improved pharmacological treatments. We designed and synthesized two new probucol analogues [2,6-di-tert-butyl-4-selenocyanatophenol (C1) and 4,4'-diselanediylbis (2,6-di-tert-butylphenol) (C2)] and investigated their effects against glutamate-induced neuronal oxidative toxicity in vitro in cultured HT22 cells, compared with their parental compound (probucol). In addition, C2, which exhibited the lowest toxicity, was investigated in an in vivo rodent model of ischemic stroke. Glutamate caused concentration- and time-dependent cytotoxicity in HT22 neuronal cells, which was preceded by increased levels of oxidants and depletion of the antioxidant glutathione. The analogues (C1 and C2), but not probucol, significantly decreased the levels of oxidants (including mitochondrial superoxide anion and lipid reactive oxygen species (ROS)) and protected against glutamate-induced cytotoxicity. In the in vivo model of ischemic stroke, which was based on central injections of the vasoconstrictor agent endothelin-1 (800 pmol/site), C2 (20 or 50 mg/kg/day, intraperitoneally, for 4 consecutive days after stroke) displayed significant beneficial effects against ischemic injury in vivo, improving rats' motor-related behavioral skills and decreasing stroke-related striatal gliosis. This is the first study to design, synthesize, and present a probucol analogue (C2) with in vivo beneficial effects against ischemic stroke. This novel compound, which was able to mitigate glutamate-induced oxidative toxicity in vitro, represents a promising neuroprotective drug.

Antiferroptotic Activities of Oxindole GIF-0726-r Derivatives: Involvement of Ferrous Iron Coordination and Free-Radical Scavenging Capacities

Ferroptosis and oxytosis are iron- and oxidative stress-dependent cell death pathways strongly implicated in neurodegenerative diseases, cancers, and metabolic disorders. Therefore, specific inhibitors may have broad clinical applications. We previously reported that 3-[4-(dimethylamino)benzyl]-2-oxindole (GIF-0726-r) and derivatives protected the mouse hippocampal cell line HT22 against oxytosis/ferroptosis by suppressing reactive oxygen species (ROS) accumulation. In this study, we evaluated the biological activities of GIF-0726-r derivatives with modifications at the oxindole skeleton and other positions. The addition of a methyl, nitro, or bromo group to C-5 of the oxindole skeleton enhanced antiferroptotic efficacy on HT22 cells during membrane cystine-glutamate antiporter inhibition and ensued intracellular glutathione depletion. In contrast, the substitution of the dimethylamino group on the side chain phenyl ring with a methyl, nitro, or amine group dramatically suppressed antiferroptotic activity regardless of other modifications. Compounds with antiferroptotic activity also directly scavenged ROS and decreased free ferrous ions in both HT22 cells and cell-free reactions while those compounds without antiferroptotic activity had little effect on either ROS or ferrous-ion concentration. Unlike oxindole compounds, which we have previously reported, the antiferroptotic compounds had little effect on the nuclear factor erythroid-2-related factor 2-antioxidant response element pathway. Oxindole GIF-0726-r derivatives with a 4-(dimethylamino)benzyl moiety at C-3 and some types of bulky group at C-5 (whether electron-donating or electron-withdrawing) can suppress ferroptosis, warranting safety and efficacy evaluations in animal models of disease.

Haloperidol Prevents Oxytosis/Ferroptosis by Targeting Lysosomal Ferrous Ions in a Manner Independent of Dopamine D2 and Sigma-1 Receptors

Haloperidol is a widely used antipsychotic agent that exerts antipsychotic effects through a strong antagonism of dopamine D2 receptors. In addition, haloperidol is classified as a sigma-1 receptor (S1R) antagonist that prevents endogenous oxidative stress in cultured cells. However, pharmacological activities of haloperidol against oxidative stress remain unclear. Oxytosis/ferroptosis are iron-dependent nonapoptotic oxidative cell deaths that are regarded as two names for the same cell death pathway and the potential physiological relevance of oxytosis/ferroptosis in multiple diseases is suggested. In the present study, the effects of haloperidol on oxytosis/ferroptosis were investigated in S1R-knockdown mouse hippocampal HT22 cells. The results indicate that haloperidol is a strong inhibitor of oxytosis/ferroptosis independent of S1R. Imaging of HT22 cells with a newly developed fluorescent probe showed that haloperidol was localized to late endosomes and lysosomes and reduced the accumulation of lysosomal ferrous ions, resulting in reduced production of intracellular reactive oxygen species and inhibition of cell death. These results indicate that haloperidol is useful not only as an antipsychotic agent but also as a neuroprotective agent against endogenous oxidative stress via distinct mechanisms. Furthermore, lysosome-targeting ferroptosis inhibitors could be useful for the treatment of various diseases, including cancers, ischemia-reperfusion injury, and neurodegenerative disorders, which have been associated with ferroptosis.

Quercetin ameliorates glutamate toxicity-induced neuronal cell death by controlling calcium-binding protein parvalbumin

Background

Glutamate is the main excitatory neurotransmitter. Excessive glutamate causes excitatory toxicity and increases intracellular calcium, leading to neuronal death. Parvalbumin is a calcium-binding protein that regulates calcium homeostasis. Quercetin is a polyphenol found in plant and has neuroprotective effects against neurodegenerative diseases.

Objectives

We investigated whether quercetin regulates apoptosis by modulating parvalbumin expression in glutamate induced neuronal damage.

Methods

Glutamate was treated in hippocampal-derived cell line, and quercetin or vehicle was treated 1 h before glutamate exposure. Cells were collected for experimental procedure 24 h after glutamate treatment and intracellular calcium concentration and parvalbumin expression were examined. Parvalbumin small interfering RNA (siRNA) transfection was performed to detect the relation between parvalbumin and apoptosis.

Results

Glutamate reduced cell viability and increased intracellular calcium concentration, while quercetin preserved calcium concentration and neuronal damage. Moreover, glutamate reduced parvalbumin expression and quercetin alleviated this reduction. Glutamate increased caspase-3 expression, and quercetin attenuated this increase in both parvalbumin siRNA transfected and non-transfected cells. The alleviative effect of quercetin was statistically significant in non-transfected cells. Moreover, glutamate decreased bcl-2 and increased bax expressions, while quercetin alleviated these changes. The alleviative effect of quercetin in bcl-2 family protein expression was more remarkable in non-transfected cells.

Conclusions

These results demonstrate that parvalbumin contributes to the maintainace of intracellular calcium concentration and the prevention of apoptosis, and quercetin modulates parvalbumin expression in glutamate-exposed cells. Thus, these findings suggest that quercetin performs neuroprotective function against glutamate toxicity by regulating parvalbumin expression.

Metabolically Activated Proteostasis Regulators Protect against Glutamate Toxicity by Activating NRF2

The extracellular accumulation of glutamate is a pathologic hallmark of numerous neurodegenerative diseases including ischemic stroke and Alzheimer's disease. At high extracellular concentrations, glutamate causes neuronal damage by promoting oxidative stress, which can lead to cellular death. This has led to significant interest in developing pharmacologic approaches to mitigate the oxidative toxicity caused by high levels of glutamate. Here, we show that the small molecule proteostasis regulator AA147 protects against glutamate-induced cell death in a neuronal-derived cell culture model. While originally developed as an activator of the activating transcription factor 6 (ATF6) arm of the unfolded protein response, this AA147-dependent protection against glutamate toxicity is primarily mediated through activation of the NRF2-regulated oxidative stress response. We demonstrate that AA147 activates NRF2 selectively in neuronal-derived cells through a mechanism involving metabolic activation to a reactive electrophile and covalent modification of KEAP1─a mechanism analogous to that involved in the AA147-dependent activation of ATF6. These results define the potential for AA147 to protect against glutamate-induced oxidative toxicity and highlight the potential for metabolically activated proteostasis regulators like AA147 to activate both protective ATF6 and NRF2 stress-responsive signaling pathways to mitigate oxidative damage associated with diverse neurologic diseases.

Synergistic gene regulation by thyroid hormone and glucocorticoid in the hippocampus

The hippocampus is considered the center for learning and memory in the brain, and its development and function is greatly affected by the thyroid and stress axes. Thyroid hormone (TH) and glucocorticoids (GC) are known to have a synergistic effect on developmental programs across several vertebrate species, and their effects on hippocampal structure and function are well-documented. However, there are few studies that focus on the processes and genes that are cooperatively regulated by the two hormone axes. Cross-regulation of the thyroid and stress axes in the hippocampus occurs on multiple levels such that TH can regulate the expression of the GC receptor (GR) while GC can modulate tissue sensitivity to TH by controlling the expression of TH receptor (TR) and enzymes involved in TH biosynthesis. Thyroid hormone and GC are also known to synergistically regulate the transcription of genes associated with neuronal function and development. Synergistic gene regulation by TH and GC may occur through the direct, cooperative action of TR and GR on common target genes, or by indirect mechanisms involving gene regulatory cascades activated by TR and GR. In this chapter, we describe the known physiological effects and underlying molecular mechanisms of TH and GC synergistic gene regulation in the hippocampus.

Identification of novel neuroprotective N,N-dimethylaniline derivatives that prevent oxytosis/ferroptosis and localize to late endosomes and lysosomes

Oxidative stress has been implicated in the aging process and the progression of many neurodegenerative disorders. We previously reported that a novel oxindole compound, GIF-0726-r, effectively prevents endogenous oxidative stress, such as oxytosis/ferroptosis, an iron-dependent form of non-apoptotic cell death, in mouse hippocampal cells. In this study, using two hundred compounds that were developed based on the structure-activity relationship of GIF-0726-r, we screened for the most potent compounds that prevent glutamate- and erastin-induced oxytosis and ferroptosis. Using submicromolar concentrations, we identified nine neuroprotective compounds that have N,N-dimethylaniline as a common structure but no longer contain an oxindole ring. The most potent derivatives, GIF-2114 and GIF-2197-r (the racemate of GIF-2115 and GIF-2196), did not affect glutathione levels, had no antioxidant activity in vitro, or ability to activate the Nrf2 pathway, but prevented oxytosis/ferroptosis via reducing reactive oxygen production and decreasing ferrous ions. Furthermore, we developed fluorescent probes of GIF-2114 and GIF-2197-r to image their distribution in live cells and found that they preferentially accumulated in late endosomes/lysosomes, which play a central role in iron metabolism. These results suggest that GIF-2114 and GIF-2197-r protect hippocampal cells from oxytosis/ferroptosis by targeting late endosomes and lysosomes, as well as decreasing ferrous ions.

Identification of regulated proteins by resveratrol in glutamate-induced cortical injury of newborn rats

Glutamate induces neuronal damage by generating oxidative stress and neurotoxicities. The neurological damage caused by glutamate is more severe during brain development in newborns than in adults. Resveratrol is naturally present in a variety of fruits and medicinal plants and exerts a neuroprotective effect against brain damage. The goal of this study was to evaluate the neuroprotective effects of resveratrol and to identify changed proteins in response to resveratrol treatment during glutamate-induced neonatal cortical damage. Sprague-Dawley rat pups (7 days old) were randomly divided into vehicle, resveratrol, glutamate, and glutamate and resveratrol groups. The animals were intraperitoneally injected with glutamate (10 mg/kg) and/or resveratrol (20 mg/kg) and their brain tissue was collected 4 hr after drug administration. Glutamate exposure caused severe histopathological changes, while resveratrol attenuated this damage. We identified regulated proteins by resveratrol in glutamate-induced cortical damaged tissue using two-dimensional gel electrophoresis and mass spectrometry. Among identified proteins, we focused on eukaryotic initiation factor 4A2, γ-enolase, protein phosphatase 2A subunit B, and isocitrate dehydrogenase. These proteins decreased in the glutamate-treated group, whereas the combination treatment of glutamate and resveratrol attenuated these protein reductions. These proteins are anti-oxidant proteins and anti-apoptotic proteins. These results suggest that glutamate induces brain cortical damage in newborns; resveratrol exerts a neuroprotective effect by controlling expression of various proteins with anti-oxidant and anti-apoptotic functions.

Using the Oxytosis/Ferroptosis Pathway to Understand and Treat Age-Associated Neurodegenerative Diseases

Oxytosis was first described over 30 years ago in nerve cells as a non-excitotoxic pathway for glutamate-induced cell death. The key steps of oxytosis, including glutathione depletion, lipoxygenase activation, reactive oxygen species accumulation, and calcium influx, were identified using a combination of chemical and genetic tools. A pathway with the same characteristics as oxytosis was identified in transformed fibroblasts in 2012 and named ferroptosis. Importantly, the pathophysiological changes seen in oxytosis and ferroptosis are also observed in multiple neurodegenerative diseases as well as in the aging brain. This led to the hypothesis that this pathway could be used as a screening tool to identify novel drug candidates for the treatment of multiple age-associated neurological disorders, including Alzheimer's disease (AD). Using this approach, we have identified several AD drug candidates, one of which is now in clinical trials, as well as new target pathways for AD.

Ubiquitination and the proteasome rather than caspase-3-mediated C-terminal cleavage are involved in the EAAT2 degradation by staurosporine-induced cellular stress

Diminished glutamate (Glu) uptake via the excitatory amino acid transporter EAAT2, which normally accounts for ~90% of total forebrain EAAT activity, may contribute to neurodegeneration via Glu-mediated excitotoxicity. C-terminal cleavage by caspase-3 (C3) was reported to mediate EAAT2 inactivation and down-regulation in the context of neurodegeneration. For a detailed analysis of C3-dependent EAAT2 degradation, we employed A172 glioblastoma as well as hippocampal HT22 cells and murine astrocytes over-expressing VSV-G-tagged EAAT2 constructs. C3 activation was induced by staurosporine (STR). In HT22 cells, STR-induced C3 activation-induced rapid EAAT2 protein degradation. The mutation of asparagine 504 to aspartate (D504N), which should inactivate the putative C3 cleavage site, increased EAAT2 activity in A172 cells. In contrast, the D504N mutation did not protect EAAT2 protein against STR-induced degradation in HT22 cells, whereas inhibition of caspases, ubiquitination and the proteasome did. Similar results were obtained in astrocytes. Phylogenetic analysis showed that C-terminal ubiquitin acceptor sites-but not the putative C3 cleavage site-exhibit a high degree of conservation. Moreover, C-terminal truncation mimicking C3 cleavage increased rather than decreased EAAT2 activity and stability as well as protected EAAT2 against STR-induced ubiquitination-dependent degradation. We conclude that cellular stress associated with endogenous C3 activation degrades EAAT2 via a pathway involving ubiquitination and the proteasome but not direct C3-mediated cleavage. In addition, C3 cleavage of EAAT2, described to occur in other models, is unlikely to inactivate EAAT2. However, mutation of the highly conserved D504 within the putative C3 cleavage site increases EAAT2 activity via an unknown mechanism.

Neuroprotective effects of natural compounds on neurotoxin-induced oxidative stress and cell apoptosis

OBJECTIVES

Overproduction of reactive species, notably reactive oxygen (ROS) and nitrogen (RNS) species, along with the failure of balancing effects of endogenous antioxidant defenses result in destruction of cellular structures, lipids, proteins, and genetic material, which lead to oxidative stress. Oxidative stress-induced neuronal apoptosis plays a pivotal role in pathogenesis of neurodegeneration. Antioxidants represent one of the medical choice strategies for protecting against this unbalanced oxidation-antioxidation status. Recently, natural compounds with neuroprotective potential that can scavenge free radicals and protect cells from oxidative damage have received extensive attention.

METHODS

In this review, we summarized the detailed research progress on the medicinal plants-derived natural compounds with potential anti-oxidation effects and their molecular mechanisms on modulating the neurotoxin (6-OHDA, H₂O₂, glutamate, Aβ)-induced oxidative stress and cell apoptosis.

RESULTS

The natural compounds that efficacious in modulating reactive species production and mitochondrial function include flavonoids, glucosides, alkaloids, polyphenols, lignans, coumarins, terpenoids, quinones and others. They decreased the neurotoxin-induced oxidative damage and apoptosis by (1) decreasing ROS/RNS generation, lipid peroxidation, caspase-3 and caspase-9 activities, LDH release, the ratio of Bax/Bcl-2, Ca²⁺ influx and cytochrome c release, (2) elevating MMP, and (3) restoring endogenous antioxidant enzymatic activities (CAT, GSH-Px, GSR, SOD). And they exerted neuroprotective effects against cell damages and apoptosis by modulating the oxidative cascades of different signaling pathways (Nrf2/HO-1, NF-κB, MAPKs, PI3K/Akt, GSK-3β) and preventing mitochondria-dependent apoptosis pathways.

DISCUSSION

The present work reviews the role of oxidative stress in neurodegeneration, highlighting the potential anti-oxidation effects of natural compounds as a promising approach to develop innovative neuroprotective strategy.

Decrease of protein phosphatase 2A subunit B by glutamate exposure in the cerebral cortex of neonatal rats

Glutamate induces neurotoxicity during brain development, causing nerve damage. Protein phosphatase 2A (PP2A) is a type of serine/threonine phosphatase that regulates various biological functions. Among the PP2A subunit types, subunit B is abundant in brain tissue and plays an essential role in the nervous system. This study investigated changes in PP2A subunit B expression through glutamate exposure in the cerebral cortex of newborn rats. Sprague-Dawley rat pups (7 days after birth) were injected intraperitoneally with vehicle or glutamate (10 mg/kg). After 4 h of drug treatment, the brain tissue was isolated and fixed for morphological study. In addition, the cerebral cortex was collected for RNA and protein works. We observed severe histopathological changes including swollen neuron and atrophied dendrite in the glutamate exposed cerebral cortex. Glutamate exposure leads to a decrease in PP2A subunit B. Reverse-transcription PCR and Western blot analyses confirmed that glutamate induces a decrease of PP2A subunit B in the cerebral cortex of newborn rats. Moreover, immunohistochemical study showed a decrease in PP2A subunit B positive cells. The reduction of PP2A subunit B expression is considered an indicator of neurodegenerative damage. These results suggest that glutamate exposure causes neuronal damage in the cerebral cortex of new born rats through a decrease in PP2A subunit B.

Peroxiredoxin 4 attenuates glutamate-induced neuronal cell death through inhibition of endoplasmic reticulum stress

High concentrations of glutamate induce neurotoxicity by eliciting reactive oxygen species (ROS) generation and intracellular Ca²⁺ influx. The disruption of Ca²⁺ homeostasis in the endoplasmic reticulum (ER) evokes ER stress, ultimately resulting in neuronal dysfunction. Additionally, glutamate participates in the development of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. Peroxiredoxins (Prxs) are members of a family of antioxidant enzymes that protect cells from neurotoxic factor-induced apoptosis by scavenging hydrogen peroxide (H₂O₂). Prx4 is located in the ER and controls the redox condition within the ER. The present study investigated the protective effects of Prx4 against glutamate-induced neurotoxicity linked to ER stress. HT22 cells in which Prx4 was either overexpressed or silenced were used to elucidate the protective role of Prx4 against glutamate toxicity. The expression of Prx4 in HT22 cells was significantly increased in response to glutamate treatment, while ROS scavengers and ER chemical chaperones reduced Prx4 levels. Moreover, Prx4 overexpression reduces glutamate-induced apoptosis of HT22 cells by inhibiting ROS formation, Ca²⁺ influx, and ER stress. Therefore, we conclude that Prx4 has protective effects against glutamate-induced HT22 cell damage. Collectively, these results suggest that Prx4 could contribute to the treatment of neuronal disorders.

The Paralogous Krüppel-like Factors 9 and 13 Regulate the Mammalian Cellular Circadian Clock Output Gene Dbp

An intricate transcription-translation feedback loop (TTFL) governs cellular circadian rhythms in mammals. Here, we report that the zinc finger transcription factor Krüppel-like factor 9 (KLF9) is regulated by this TTFL, it associates in chromatin at the core circadian clock and clock-output genes, and it acts to modulate transcription of the clock-output gene Dbp. Our earlier genome-wide analysis of the mouse hippocampus-derived cell line HT22 showed that KLF9 associates in chromatin with Per1, Per3, Dbp, Tef, Bhlhe40, Bhlhe41, Nr1d1, and Nr1d2. Of the 3514 KLF9 peaks identified in HT22 cells, 1028 contain E-box sequences to which the transcriptional activators CLOCK and BMAL1 may bind, a frequency significantly greater than expected by chance. Klf9 mRNA showed circadian oscillation in synchronized HT22 cells, mouse hippocampus, and liver. At the clock-output gene Dbp, KLF9 exhibited circadian rhythmicity in its association in chromatin in HT22 cells and hippocampus. Forced expression of KLF9 in HT22 cells repressed basal Dbp transcription and strongly inhibited CLOCK+BMAL1-dependent transcriptional activation of a transfected Dbp reporter. Mutational analysis showed that this action of KLF9 depended on 2 intact KLF9-binding motifs within the Dbp locus that are in close proximity to E-boxes. Knockout of Klf9 or the paralogous gene Klf13 using CRISPR/Cas9 genome editing in HT22 cells had no effect on Dbp expression, but combined knockout of both genes strongly impaired circadian Dbp mRNA oscillation. Like KLF9, KLF13 also showed association in chromatin with clock- and clock-output genes, and forced expression of KLF13 inhibited the actions of CLOCK+BMAL1 on Dbp transcription. Our results suggest novel and partly overlapping roles for KLF9 and KLF13 in modulating cellular circadian clock output by a mechanism involving direct interaction with the core TTFL.

Decrease of 14-3-3 proteins by glutamate exposure in the cerebral cortex of newborn rats

Glutamate is a representative excitatory neurotransmitter. However, excessive glutamate exposure causes neuronal cell damage by generating neuronal excitotoxicity. Excitotoxicity in neonates caused by glutamate treatment induces neurological deficits in adults. The 14-3-3 family proteins are conserved proteins that are expressed ubiquitously in a variety of tissues. These proteins contribute to cellular processes, including signal transduction, protein synthesis, and cell cycle control. We proposed that glutamate induces neuronal cell damage by regulating 14-3-3 protein expression in newborn animals. In this study, we investigated the histopathological changes and 14-3-3 proteins expressions as a result of glutamate exposure in the neonatal cerebral cortex. Rat pups at post-natal day 7 were intraperitoneally administrated with vehicle or glutamate (10 mg/kg). Animals were sacrificed 4 h after treatment, and brain tissues were fixed for histological study. Cerebral cortices were isolated and frozen for proteomic study. We observed serious histopathological damages including shrunken dendrites and atypical neurons in glutamate-treated cerebral cortices. In addition, we identified that 14-3-3 family proteins decreased in glutamate-exposed cerebral cortices using a proteomic approach. Moreover, Western blot analysis provided results that glutamate treatment in neonates decreased 14-3-3 family proteins expressions, including the β/α, ζ/δ, γ, ε, τ, and η isoforms. 14-3-3 proteins are involved in signal transduction, metabolism, and anti-apoptotic functions. Thus, our findings suggest that glutamate induces neonatal neuronal cell damage by modulating 14-3-3 protein expression.

Potentiation of morphine-induced antinociception by harmaline: involvement of μ-opioid and ventral tegmental area NMDA receptors

RATIONAL

Morphine is one of the most well-known and potent analgesic agents; however, it can also induce various side effects. Thus, finding drugs and mechanisms which can potentiate the analgesic effects of low doses of morphine will be a good strategy for pain management.

OBJECTIVE

The involvement of μ-opioid receptors and ventral tegmental area (VTA) glutamatergic system in harmaline and morphine combination on the nociceptive response were investigated. Also, we examined reward efficacy and tolerance expression following the drugs.

METHODS

Animals were bilaterally cannulated in the VTA by stereotaxic instrument. A tail-flick (TF) apparatus and conditioned place preference (CPP) paradigm were used to measure nociceptive response and rewarding effects in male NMRI mice respectively.

RESULTS

Morphine (2 mg/kg, i.p.) had no effect in TF test. Also, harmaline (1.25 and 5 mg/kg, i.p.) could not change pain threshold. Combination of a non-effective dose of harmaline (5 mg/kg) and morphine (2 mg/kg) produced antinociception and also prevented morphine tolerance but had no effect on the acquisition of CPP. Systemic administration of naloxone (0.5 and 1 mg/kg) and intra-VTA microinjection of NMDA (0.06 and 0.1 μg/mouse) before harmaline (5 mg/kg) plus morphine (2 mg/kg) prevented antinociception induced by the drugs. D-AP5 (0.5 and 1 μg/mouse, intra-VTA) potentiated the effect of low-dose harmaline (1.25 mg/kg) and morphine (2 mg/kg) and induced antinociception. Microinjection of the same doses of NMDA or D-AP5 into the VTA alone had no effect on pain threshold.

CONCLUSION

The findings showed that harmaline potentiated the analgesic effect of morphine and reduced morphine tolerance. Glutamatergic and μ-opioidergic system interactions in the VTA seem to have a modulatory role in harmaline plus morphine-induced analgesia.

Discovery and Biological Evaluation of New Selective Acetylcholinesterase Inhibitors with Anti-Aβ Aggregation Activity through Molecular Docking-Based Virtual Screening

Discovery of novel multifunctional inhibitors targeting acetylcholinesterase (AChE) has becoming a hot spot in anti-Alzheimer's disease (AD) drug development. In the present study, four potent small molecule inhibitors (A01, A02, A03 and A04) of AChE with new chemical scaffold were identified. Inhibitor A03 displayed the most potent inhibition activity on AChE at enzymatic level with IC₅₀ value of 180 nM, and high selectivity towards AChE over butyrylcholinesterase (BChE) by more than 100-fold. The binding modes of compounds A01-A04 were carefully analyzed by molecular docking and molecular dynamics (MD) simulation to provide informative clues for further structure modification. Finally, the anti-amyloid beta (Aβ) aggregation and neuroprotective activity were also well investigated. Our findings highlighted the therapeutic promise of AChE inhibitors A01-A04 for AD treatment.

Reduced Sulfation Enhanced Oxytosis and Ferroptosis in Mouse Hippocampal HT22 Cells

Sulfation is a common modification of extracellular glycans, tyrosine residues on proteins, and steroid hormones, and is important in a wide variety of signaling pathways. We investigated the role of sulfation on endogenous oxidative stress, such as glutamate-induced oxytosis and erastin-induced ferroptosis, using mouse hippocampal HT22 cells. Sodium chlorate competitively inhibits the formation of 3′-phosphoadenosine 5′-phosphosulfate, the high energy sulfate donor in cellular sulfation reactions. The treatment of HT22 cells with sodium chlorate decreased sulfation of heparan sulfate proteoglycans and chondroitin sulfate proteoglycans. Sodium chlorate and β-d-xyloside, which prevents proteoglycan glycosaminoglycan chain attachment, exacerbated both glutamate- and erastin-induced cell death, suggesting that extracellular matrix influenced oxytosis and ferroptosis. Moreover, sodium chlorate enhanced the generation of reactive oxygen species and influx of extracellular Ca²⁺ in the process of oxytosis and ferroptosis. Interestingly, sodium chlorate did not affect antioxidant glutathione levels. Western blot analysis revealed that sodium chlorate enhanced erastin-induced c-Jun N-terminal kinase phosphorylation, which is preferentially activated by cell stress-inducing signals. Collectively, our findings indicate that sulfation is an important modification for neuroprotection against oxytosis and ferroptosis in neuronal hippocampal cells.

Identification of proteins differentially expressed by glutamate treatment in cerebral cortex of neonatal rats

Glutamate leads to neuronal cell damage by generating neurotoxicity during brain development. The objective of this study is to identify proteins that differently expressed by glutamate treatment in neonatal cerebral cortex. Sprague-Dawley rat pups (post-natal day 7) were intraperitoneally injected with vehicle or glutamate (10 mg/kg). Brain tissues were isolated 4 h after drug treatment and fixed for morphological study. Moreover, cerebral cortices were collected for protein study. Two-dimensional gel electrophoresis and mass spectrometry were carried out to identify specific proteins. We observed severe histopathological changes in glutamate-exposed cerebral cortex. We identified various proteins that differentially expressed by glutamate exposure. Identified proteins were thioredoxin, peroxiredoxin 5, ubiquitin carboxy-terminal hydrolase L1, proteasome subunit alpha proteins, isocitrate dehydrogenase, and heat shock protein 60. Heat shock protein 60 was increased in glutamate exposed condition. However, other proteins were decreased in glutamate-treated animals. These proteins are related to anti-oxidant, protein degradation, metabolism, signal transduction, and anti-apoptotic function. Thus, our findings can suggest that glutamate leads to neonatal cerebral cortex damage by regulation of specific proteins that mediated with various functions.

In Vitro and in Silico Human Monoamine Oxidase Inhibitory Potential of Anthraquinones, Naphthopyrones, and Naphthalenic Lactones from Cassia obtusifolia Linn Seeds

In recent years, Cassia seed extract has been reported as a neuroprotective agent in various models of neurodegeneration, mainly via an antioxidant mechanism. However, no one has previously reported the effects of Cassia seed extract and its phytochemicals on human monoamine oxidase (hMAO) enzyme activity. The seed methanol extract, the solvent-soluble fractions, and almost all isolated compounds displayed selective inhibition of hMAO-A isozyme activity. Interestingly, compounds obtusin (3), alaternin (8), aloe-emodin (9), questin (12), rubrofusarin (13), cassiaside (15), toralactone 9-O-β-gentiobioside (26), and (3S)-9,10-dihydroxy-7-methoxy-3-methyl-1-oxo-3,4-dihydro-1H-benzo[g]isochromene-3-carboxylic acid 9-O-β-d-glucopyranoside (38) showed the most promising inhibition of the hMAO-A isozyme with IC₅₀ values of 0.17-11 μM. The kinetic study characterized their mode of inhibition and molecular docking simulation predicted interactions with Ile-335 and Tyr-326 in support of the substrate/inhibitor selectivity in respective isozymes. These results demonstrate that Cassia seed extract and its constituents inhibit hMAO-A enzyme activity with high selectivity and suggest that they could play a preventive role in neurodegenerative diseases, especially anxiety and depression.

Coordinated transcriptional regulation by thyroid hormone and glucocorticoid interaction in adult mouse hippocampus-derived neuronal cells

The hippocampus is a well-known target of thyroid hormone (TH; e.g., 3,5,3’-triiodothyronine—T₃) and glucocorticoid (GC; e.g., corticosterone—CORT) action. Despite evidence that TH and GC play critical roles in neural development and function, few studies have identified genes and patterns of gene regulation influenced by the interaction of these hormones at a genome-wide scale. In this study we investigated gene regulation by T₃, CORT, and T₃ + CORT in the mouse hippocampus-derived cell line HT-22. We treated cells with T₃, CORT, or T₃ + CORT for 4 hr before cell harvest and RNA isolation for microarray analysis. We identified 9 genes regulated by T₃, 432 genes by CORT, and 412 genes by T₃ + CORT. Among the 432 CORT-regulated genes, there were 203 genes that exhibited an altered CORT response in the presence of T₃, suggesting that T₃ plays a significant role in modulating CORT-regulated genes. We also found 80 genes synergistically induced, and 73 genes synergistically repressed by T₃ + CORT treatment. We performed in silico analysis using publicly available mouse neuronal chromatin immunoprecipitation-sequencing datasets and identified a considerable number of synergistically regulated genes with TH receptor and GC receptor peaks mapping within 1 kb of chromatin marks indicative of hormone-responsive enhancer regions. Functional annotation clustering of synergistically regulated genes reveal the relevance of proteasomal-dependent degradation, neuroprotective effect of growth hormones, and neuroinflammatory responses as key pathways to how TH and GC may coordinately influence learning and memory. Taken together, our transcriptome data represents a promising exploratory dataset for further study of common molecular mechanisms behind synergistic TH and GC gene regulation, and identify specific genes and their role in processes mediated by cross-talk between the thyroid and stress axes in a mammalian hippocampal model system.

Neuroprotective effects of Brazilian green propolis on oxytosis/ferroptosis in mouse hippocampal HT22 cells

Propolis is a sticky dark-colored substance produced by honey bees and comprises resin, balsam, wax, essential and aromatic oils, pollen, and several other substances; it is used in food and beverages to improve health and prevent diseases. We studied the neuroprotective effects of extracts of Brazilian green propolis in the mouse hippocampal cell line HT22. Ethanol extracts of Brazilian green propolis had a more potent preventive effect on oxidative stress-induced cell death, oxytosis/ferroptosis, in HT22 cells than water extracts of Brazilian green propolis, whereas it did not protect against anticancer drug-induced apoptotic cell death. Among the primary constituents of ethanol extracts of Brazilian green propolis, only artepillin C, kaempferide, and kaempferol demonstrated neuroprotective effects against oxytosis/ferroptosis. The flavonoid derivatives kaempferide and kaempferol are antioxidants with radical-scavenging abilities that additionally induce antioxidant response element-mediated transcriptional activity, suggesting that upregulation of endogenous antioxidant defense protects against oxidative stress. In contrast, artepillin C attenuated reactive oxygen species production; however, it did not induce antioxidant response element activation. These findings indicate that the ethanol extracts of Brazilian green propolis help to prevent oxidative stress-related neuronal cell death that is involved in the pathogenesis of several neurodegenerative diseases.

Anti-oxidative effects of catechins and theaflavins on glutamate-induced HT22 cell damage

Background: Glutamate is an excitatory neurotransmitter that is involved in cell stress caused by oxidation. Polyphenolic compounds display various potential neuroprotective properties due to their ability to donate electrons and hydrogen atoms. Method: In this study, we evaluate the protective effect towards glutamate-induced HT22 cell damage. Two families of polyphenolic compounds are investigated, including the monomer polyphenol catechins, as well as the dimerized theaflavins. The cell apoptosis and intercellular ROS production are quantified by flow cytometry, and the protective mechanism is evaluated by quantifying the expression of cell apoptosis and energy related proteins. Result: Both sets of compounds protect cells against glutamate-induced oxidative stress, partially restore the cell viability, and prevent cells from apoptosis via bcl-2 and bax regulation, and attenuate intercellular ROS production. We demonstrate here that the protective effect is mediated by multiple factors, including reducing intracellular Ca²⁺ concentration, increasing glutathione level and related enzyme activity. Thus, the phosphorylation of AMP-activated protein kinase (AMPK) and extracellular signal-regulated kinase (ERK) show inverse correlation of activity after catechins and theaflavins stimulation. Conclusion: These results suggest both catechins and theaflavins compounds protect cells from glutamate-induced damage via cell apoptosis-related proteins and indirect regulation of cellular energy enzymes. These natural sourced antioxidants provide potential therapeutic agents for glutamate accumulation and toxicity related diseases.

Müller cells in pathological retinal angiogenesis

Müller cells are the major glial cells spanning the entire layer of the retina and maintaining retinal structure. Under pathological conditions, Müller cells are involved in retinal angiogenesis, a process of growing new blood vessels from pre-existing capillaries. In response to hypoxia, high glucose, and inflammation conditions, multiple signaling pathways are activated in Müller cells, followed by the increased production of proangiogenic factors including vascular endothelial growth factor, basic fibroblast growth factor, matrix metalloproteinases, Netrin-4, and angiopoietin-like 4. Expression of antiangiogenic factors is also downregulated in Müller cells. Besides, proliferation and dedifferentiation of Müller cells facilitates retinal angiogenesis. In this review, we summarized molecular mechanisms of Müller cells-related retinal angiogenesis. The potential of Müller cells as a therapeutic target for retinal angiogenesis was also discussed.

Venlafaxine Mitigates Depressive-Like Behavior in Ovariectomized Rats by Activating the EPO/EPOR/JAK2 Signaling Pathway and Increasing the Serum Estradiol Level

Reduced estradiol levels are associated with depression in women during the transition to and after menopause. A considerable number of studies focusing on the theme of treating depression through the activation of erythropoietin (EPO)-induced signaling pathways have been published. Venlafaxine is an approved antidepressant drug that inhibits both serotonin and norepinephrine transporters. The aim of the present study was to investigate the effects of venlafaxine on the depressive-like behaviors and serum estradiol levels in female rats following ovariectomy (OVX) and the possible roles of EPO-induced signaling pathways. Venlafaxine (10 mg/kg/day) was orally administered to OVX rats over a period of 4 weeks using two different treatment regimens: either starting 24 h or 2 weeks after OVX. Venlafaxine showed a superior efficacy in inducing antidepressant-like effects after an acute treatment (24 h post-OVX) than after the delayed treatment (2 weeks post-OVX) and was characterized by a decreased immobility time in the forced swimming test. In parallel, venlafaxine induced EPO and EPO receptor mRNA expression and increased levels of phospho-Janus kinase 2 (p-JAK2), phospho-signal transducer and activator of transcription 5, and phospho-extracellular signal-regulated kinase 1/2 in the hippocampus of OVX rats. Meanwhile, rats exhibited a marked reduction in the hippocampal Bax/Bcl2 ratio, caspase-3 activity, and tumor necrosis factor alpha levels after venlafaxine treatment. Venlafaxine also increased the hippocampal brain-derived neurotrophic factor and serum estradiol levels. Based on these findings, venlafaxine exerts a neuroprotective effect on OVX rats that is at least partially attributed to the activation of EPO/EPOR/JAK2 signaling pathways, anti-apoptotic activities, anti-inflammatory activities, and neurotrophic activities, as well as an increase in serum estradiol level. Graphical Abstract ᅟ.

Frontiers in Probing Alzheimer's Disease Biomarkers with Fluorescent Small Molecules

Alzheimer's disease (AD) is the most common form of dementia. The pathogenesis of the disease is associated with aggregated amyloid-β, hyperphosphorylated tau, a high level of metal ions, abnormal enzyme activities, and reactive astrocytes. This outlook gives an overview of fluorescent small molecules targeting AD biomarkers for ex vivo and in vivo imaging. These chemical imaging probes are categorized based on the potential biomarkers, and their pros and cons are discussed. Guidelines for designing new sensing strategies as well as the desirable properties to be pursued for AD fluorescence imaging are also provided.

Inhibition of double-stranded RNA-dependent protein kinase prevents oxytosis and ferroptosis in mouse hippocampal HT22 cells

Double-stranded RNA-dependent protein kinase (PKR) is a component of signal transduction pathways mediating various stress signals including oxidative stress and endoplasmic reticulum (ER) stress and is suggested to be implicated in several neurodegenerative diseases. Cell death in neurodegenerative conditions has been linked to oxidative stress; however, the involvement of PKR in endogenous oxidative stress such as oxytosis and ferroptosis which is quite distinct from classical apoptosis remains unknown. We investigated here the effect of a PKR inhibitor C16 (an imidazole-oxindole derivative) on oxytosis and ferroptosis in cultured HT22 mouse hippocampal cells. C16 prevented glutamate- and erastin-induced cell death, reactive oxygen species accumulation, Ca²⁺ influx, phosphorylation of inositol-requiring enzyme 1 (IRE1), one of the three branches of ER stress signaling and its downstream signaling components. On the other hand, C16 did not prevent oxidative stress-induced heme oxygenase-1 expression; instead, C16 activated the extracellular signal-regulated kinase pathway. The protective effect of C16 is diminished in PKR knockout HT22 cells. Real time measurements of the oxygen consumption rate and extracellular acidification rate over a long period of time leading to cell death showed that C16 partially prevented erastin-induced mitochondrial and glycolytic dysfunction. These results suggest that PKR is an important component of oxytosis and ferroptosis and the inhibition of PKR is neuroprotective against endogenous oxidative stress-induced cell death and provide an effective strategy for neuroprotection.

A Focused Library of Psychotropic Analogues with Neuroprotective and Neuroregenerative Potential

Overcoming the lack of effective treatments and the continuous clinical trial failures in neurodegenerative drug discovery might require a shift from the prevailing paradigm targeting pathogenesis to the one targeting simultaneously neuroprotection and neuroregeneration. In the studies reported herein, we sought to identify small molecules that might exert neuroprotective and neuroregenerative potential as tools against neurodegenerative diseases. In doing so, we started from the reported neuroprotective/neuroregenerative mechanisms of psychotropic drugs featuring a tricyclic alkylamine scaffold. Thus, we designed a focused-chemical library of 36 entries aimed at exploring the structural requirements for efficient neuroprotective/neuroregenerative cellular activity, without the manifestation of toxicity. To this aim, we developed a synthetic protocol, which overcame the limited applicability of previously reported procedures. Next, we evaluated the synthesized compounds through a phenotypic screening pipeline, based on primary neuronal systems. Phenothiazine 2Bc showed improved neuroregenerative and neuroprotective properties with respect to reference drug desipramine (2Aa). Importantly, we have also shown that 2Bc outperformed currently available drugs in cell models of Alzheimer's and Parkinson's diseases and attenuates microglial activation by reducing iNOS expression.

Neuroprotective Mitochondrial Remodeling by AKAP121/PKA Protects HT22 Cell from Glutamate-Induced Oxidative Stress

Protein kinase A (PKA) is a ser/thr kinase that is critical for maintaining essential neuronal functions including mitochondrial homeostasis, bioenergetics, neuronal development, and neurotransmission. The endogenous pool of PKA is targeted to the mitochondrion by forming a complex with the mitochondrial scaffold A-kinase anchoring protein 121 (AKAP121). Enhanced PKA signaling via AKAP121 leads to PKA-mediated phosphorylation of the fission modulator Drp1, leading to enhanced mitochondrial networks and thereby blocking apoptosis against different toxic insults. In this study, we show for the first time that AKAP121/PKA confers neuroprotection in an in vitro model of oxidative stress induced by exposure to excess glutamate. Unexpectedly, treating mouse hippocampal progenitor neuronal HT22 cells with an acute dose or chronic exposure of glutamate robustly elevates PKA signaling, a beneficial compensatory response that is phenocopied in HT22 cells conditioned to thrive in the presence of excess glutamate but not in parental HT22 cells. Secondly, redirecting the endogenous pool of PKA by transiently transfecting AKAP121 or transfecting a constitutively active mutant of PKA targeted to the mitochondrion (OMM-PKA) or of an isoform of AKAP121 that lacks the KH and Tudor domains (S-AKAP84) are sufficient to significantly block cell death induced by glutamate toxicity but not in an oxygen deprivation/reperfusion model. Conversely, transient transfection of HT22 neuronal cells with a PKA-binding-deficient mutant of AKAP121 is unable to protect against oxidative stress induced by glutamate toxicity suggesting that the catalytic activity of PKA is required for AKAP121's protective effects. Mechanistically, AKAP121 promotes neuroprotection by enhancing PKA-mediated phosphorylation of Drp1 to increase mitochondrial fusion, elevates ATP levels, and elicits an increase in the levels of antioxidants GSH and superoxide dismutase 2 leading to a reduction in the level of mitochondrial superoxide. Overall, our data supports AKAP121/PKA as a new molecular target that confers neuroprotection against glutamate toxicity by phosphorylating Drp1, to stabilize mitochondrial networks and mitochondrial function and to elicit antioxidant responses.

Reduction of Mitophagy-Related Oxidative Stress and Preservation of Mitochondria Function Using Melatonin Therapy in an HT22 Hippocampal Neuronal Cell Model of Glutamate-Induced Excitotoxicity

Recent evidence indicates that autophagy-mediated mitochondrial homeostasis is crucial for oxidative stress-related brain damage and repair. The highest concentration of melatonin is in the mitochondria of cells, and melatonin exhibits well-known antioxidant properties. We investigated the impact and mechanism involved in mitochondrial function and the mitochondrial oxidative stress/autophagy regulator parameters of glutamate cytotoxicity in mouse HT22 hippocampal neurons. We tested the hypothesis that melatonin confers neuroprotective effects via protecting against mitochondrial impairment and mitophagy. Cells were divided into four groups: the control group, melatonin alone group, glutamate injury group, and melatonin pretreatment group. We found that glutamate induced significant changes in mitochondrial function/oxidative stress-related parameters. Leptin administration preserved mitochondrial function, and this effect was associated with increased superoxide dismutase, glutathione (GSH), and mitochondrial membrane potential and decreased GSSG (oxidized glutathione) and mitochondrial reactive oxygen species. Melatonin significantly reduced the fluorescence intensity of mitophagy via the Beclin-1/Bcl-2 pathway, which involves Beclin-1 and Bcl-2 proteins. The mitophagy inhibitor CsA corrected these glutamate-induce changes, as measured by the fluorescence intensity of Mitophagy-Tracker Red CMXROS, mitochondrial ROS, and mitochondrial membrane potential changes. These findings indicate that melatonin exerts neuroprotective effects against glutamate-induced excitotoxicity by reducing mitophagy-related oxidative stress and maintaining mitochondrial function.

Astrocytes enhance the tolerance of rat cortical neurons to glutamate excitotoxicity

Glutamate excitotoxicity is responsible for neuronal death in acute neurological disorders, including stroke, trauma and neurodegenerative diseases. Astrocytes are the main cells for the removal of glutamate in the synaptic cleft and may affect the tolerance of neurons to the glutamate excitotoxicity. Therefore, the present study aimed to investigate the tolerance of rat cortical neurons to glutamate excitotoxicity in the presence and absence of astrocytes. Rat cortical neurons in the presence or absence of astrocytes were exposed to different concentrations of glutamate (10‑2,000 µM) and 10 µM glycine for different incubation periods. After 24 h, the Cell Counting kit‑8 (CCK‑8) assay was used to measure the cytotoxicity to neurons in the presence or absence of astrocytes. According to the results, in the absence of astrocytes, glutamate induced a concentration‑dependent decrease of neuronal survival rate compared with the control rat cortical neurons, and the neurotoxic half‑maximal inhibitory concentration (IC50) at 15, 30 and 60 min was 364.5, 258.5 and 138.3 µM, respectively. Furthermore, in the presence of astrocytes, glutamate induced a concentration‑dependent decrease of neuronal survival rate compared with the control rat cortical neurons, and the neurotoxic IC50 at 15, 30 and 60 min was 1,935, 932.8 and 789.3 µM, respectively. However, astrocytic toxicity was not observed when the rat cortical astrocytes alone were exposed to different concentrations of glutamate (500, 1,000 and 2,000 µM) for 6, 12 and 24 h. In conclusion, the glutamate‑induced neurotoxic IC50 values at 15, 30 and 60 min were respectively higher in the presence of astrocytes as compared with those in the absence of astrocytes, suggesting that astrocytes can protect against rat cortical neuronal acute damage induced by glutamate.

Taurine Chloramine Prevents Neuronal HT22 Cell Damage Through Nrf2-Related Heme Oxygenase-1

Oxidative cell damages are able to contribute to neuronal degeneration in several diseases of the central nervous system (CNS) including stroke as well as ischemia. Heme oxygenase (HO)-1 plays a major role in the pathogenesis of neuronal disorder. Taurine chloramine (TauCl) has been shown to possess strong neuronal activities; however, the direct effects of TauCl on neuronal cell death remain to be determined. Therefore, this study was designed to assess the neuroprotective effect of TauCl using oxidative stress-stimulated mouse hippocampal HT22 cells. TauCl showed protective effects against oxidative stress-induced neurotoxicity and inhibited the reactive oxygen species (ROS) production by inducing the heme oxygenase (HO)-1 expression in HT22 cells. TauCl upregulated HO-1 expression and it also increased the nuclear factor E2-related factor 2 (Nrf2) translocation to nuclear. Using an inhibitor of HO-1 activity, we verified that the oxidative stress-related HT22 cell death was significantly suppressed by TauCl. In addition, we found reduced TauCl-induced HO-1 expression and cytoprotection following treatment of the cells with an extracellular signal-regulated kinase (ERK) inhibitor (PD98059) or a p38 inhibitor (SB203580), but not following treatment with a SP600125 as a c-Jun NH2-terminal kinase (JNK) inhibitor. These findings suggest that TauCl improves cellular damage induced by glutamate or H2O2 through ERK and p38, Nrf2, and HO-1 pathways in HT22 cells.

Sulfite protects neurons from oxidative stress

BACKGROUND AND PURPOSE

Hydrogen sulfide (H₂ S) and polysulfides (H₂ S_n ) are signalling molecules that mediate various physiological responses including cytoprotection. Their oxidized metabolite sulfite (SO₃ ^2- ) is found in blood and tissues. However, its physiological role remains unclear. In this study, we investigated the cytoprotective effect of sulfite on neurons exposed to oxidative stress caused by high concentrations of the neurotransmitter glutamate, known as oxytosis.

EXPERIMENTAL APPROACH

Concentrations of sulfite as well as those of cysteine and GSH in rats were measured by HPLC. Cytoprotective effects of sulfite on primary cultures of rat neurons against oxytosis was examined by WST-8 cytoprotective and LDH cytotoxicity assays and compared with that of H₂ S, H₂ S_n and thiosulfate.

KEY RESULTS

Free sulfite, present at approximately 2 μM in the rat brain, converts cystine to cysteine more efficiently than H₂ S and H₂ S_n and facilitates transport of cysteine into cells. Physiological concentrations of sulfite protected neurons from oxytosis and were accompanied by increased intracellular concentrations of cysteine and GSH probably due to converting extracellular cystine to cysteine, more efficiently than H₂ S and H₂ S_n . In contrast, thiosulfate only slightly protected neurons from oxytosis.

CONCLUSIONS AND IMPLICATIONS

Our present data have shown sulfite to be a novel cytoprotective molecule against oxytosis, through maintaining cysteine levels in the extracellular milieu, leading to increased intracellular cysteine and GSH. Although there may be adverse clinical effects in sensitive individuals, our results provide a new insight into the therapeutic application of sulfite to neuronal diseases caused by oxidative stress.

LINKED ARTICLES

This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Alzheimer Disease and Selected Risk Factors Disrupt a Co-regulation of Monoamine Oxidase-A/B in the Hippocampus, but Not in the Cortex

Monoamine oxidase-A (MAO-A) and MAO-B have both been implicated in the pathology of Alzheimer disease (AD). We examined 60 autopsied control and AD donor brain samples to determine how well MAO function aligned with two major risk factors for AD, namely sex and APOE ε4 status. MAO-A activity was increased in AD cortical, but not hippocampal, samples. In contrast, MAO-B activity was increased in both regions (with a strong input from female donors) whether sample means were compared based on: (a) diagnosis alone; (b) diagnosis-by-APOE ε4 status (i.e., carriers vs. non-carriers of the ε4 allele); or (c) APOE ε4 status alone (i.e., ignoring ‘diagnosis’ as a variable). Sample means strictly based on the donor’s sex did not reveal any difference in either MAO-A or MAO-B activity. Unexpectedly, we found that cortical MAO-A and MAO-B activities were highly correlated in both males and females (if focussing strictly on the donor’s sex), while in the hippocampus, any correlation was lost in female samples. Stratifying for sex-by-APOE ε4 status revealed a strong correlation between cortical MAO-A and MAO-B activities in both non-carriers and carriers of the allele, but any correlation in hippocampal samples was lost in carriers of the allele. A diagnosis of AD disrupted the correlation between MAO-A and MAO-B activities in the hippocampus, but not the cortex. We observed a novel region-dependent co-regulation of MAO-A and MAO-B mRNAs (but not proteins), while a lack of correlation between MAO activities and the respective proteins corroborated previous reports. Overexpression of human APOE4 increased MAO activity (but not mRNA/protein) in C6 and in HT-22 cell cultures. We identified a novel co-regulation of MAO-A and MAO-B activities that is spared from any influence of risk factors for AD or AD itself in the cortex, but vulnerable to these same factors in the hippocampus. Sex- and region-dependent abilities to buffer influences on brain MAO activities could have significant bearing on ambiguous outcomes when monoaminergic systems are targeted in clinical populations.

Oxytosis/Ferroptosis-(Re-) Emerging Roles for Oxidative Stress-Dependent Non-apoptotic Cell Death in Diseases of the Central Nervous System

Although nerve cell death is the hallmark of many neurological diseases, the processes underlying this death are still poorly defined. However, there is a general consensus that neuronal cell death predominantly proceeds by regulated processes. Almost 30 years ago, a cell death pathway eventually named oxytosis was described in neuronal cells that involved glutathione depletion, reactive oxygen species production, lipoxygenase activation, and calcium influx. More recently, a cell death pathway that involved many of the same steps was described in tumor cells and termed ferroptosis due to a dependence on iron. Since then there has been a great deal of discussion in the literature about whether these are two distinct pathways or cell type- and insult-dependent variations on the same pathway. In this review, we compare and contrast in detail the commonalities and distinctions between the two pathways concluding that the molecular pathways involved in the regulation of ferroptosis and oxytosis are highly similar if not identical. Thus, we suggest that oxytosis and ferroptosis should be regarded as two names for the same cell death pathway. In addition, we describe the potential physiological relevance of oxytosis/ferroptosis in multiple neurological diseases.

Knockdown of peroxiredoxin V increases glutamate‑induced apoptosis in HT22 hippocampal neuron cells

High concentrations of glutamate may mediate neuronal cell apoptosis by increasing intracellular reactive oxygen species (ROS) levels. Peroxiredoxin V (Prx V), a member of the Prx family, serves crucial roles in protecting cells from oxidative stress. The present study investigated the regulatory effect of Prx V on glutamate‑induced effects on viability and apoptosis in HT22 cells. Western blotting was used for protein expression analysis and Annexin V/PI staining and flow cytometry for determination of apoptosis. The results demonstrated that glutamate may ROS‑dependently increase HT22 cell apoptosis and upregulate Prx V protein levels. Furthermore, knockdown of Prx V protein expression with a lentivirus significantly enhanced HT22 cell apoptosis mediated by glutamate, which was reversed by inhibition of ROS with N‑acetyl‑L‑cysteine. Inhibiting the extracellular signal‑regulated kinase (ERK) signaling pathway with PD98059, a specific inhibitor for ERK phosphorylation, markedly decreased glutamate‑induced HT22 cell apoptosis in Prx V knockdown cells, indicating the potential involvement of ERK signaling in glutamate‑induced HT22 cell apoptosis. In addition, an increase in nuclear apoptosis‑inducing factor was observed in Prx V knockdown HT22 cells following glutamate treatment, compared with mock cells, whereas no differences in B‑cell lymphoma‑2 and cleaved‑caspase‑3 protein expression levels were observed between mock and Prx V knockdown cells. The results of the present study indicated that Prx V may have potential as a therapeutic molecular target for glutamate‑induced neuronal cell death and provide novel insight into the role of Prx V in oxidative‑stress induced neuronal cell death.

Protective effect of casuarinin against glutamate-induced apoptosis in HT22 cells through inhibition of oxidative stress-mediated MAPK phosphorylation

Glutamate is the major excitatory neurotransmitter in the central nervous system and is involved in oxidative stress during neurodegeneration. In the present study, casuarinin prevented glutamate-induced HT22 murine hippocampal neuronal cell death by inhibiting intracellular reactive oxygen species (ROS) production. Moreover, casuarinin reduced chromatin condensation and annexin-V-positive cell production induced by glutamate. We also confirmed the underlying protective mechanism of casuarinin against glutamate-induced neurotoxicity. Glutamate markedly increased the phosphorylation of extracellular signal regulated kinase (ERK)-1/2 and p38, which are crucial in oxidative stress-mediated neuronal cell death. Conversely, treatment with casuarinin diminished the phosphorylation of ERK1/2 and P38. In conclusion, the results of this study suggest that casuarinin, obtained from natural products, acts as potent neuroprotective agent by suppressing glutamate-mediated apoptosis through the inhibition of ROS production and activation of the mitogen activated protein kinase (MAPK) pathway. Thus, casuarinin can be a potential therapeutic agent in the treatment of neurodegenerative diseases.

Cerebral ischemic injury decreases α-synuclein expression in brain tissue and glutamate-exposed HT22 cells

α-Synuclein is abundantly expressed in neuronal tissue, plays an essential role in the pathogenesis of neurodegenerative disorders, and exerts a neuroprotective effect against oxidative stress. Cerebral ischemia causes severe neurological disorders and neuronal dysfunction. In this study, we examined α-synuclein expression in middle cerebral artery occlusion (MCAO)-induced cerebral ischemic injury and neuronal cells damaged by glutamate treatment. MCAO surgical operation was performed on male Sprague-Dawley rats, and brain samples were isolated 24 hours after MCAO. We confirmed neurological behavior deficit, infarction area, and histopathological changes following MCAO injury. A proteomic approach and Western blot analysis demonstrated a decrease in α-synuclein in the cerebral cortices after MCAO injury. Moreover, glutamate treatment induced neuronal cell death and decreased α-synuclein expression in a hippocampal-derived cell line in a dose-dependent manner. It is known that α-synuclein regulates neuronal survival, and low levels of α-synuclein expression result in cytotoxicity. Thus, these results suggest that cerebral ischemic injury leads to a reduction in α-synuclein and consequently causes serious brain damage.

Prevention of oxytosis-induced c-Raf down-regulation by (arylthio)cyclopentenone prostaglandins is neuroprotective

Prolonged exposure to high concentrations of glutamate leads to cell type specific glutathione depletion and resulting oxidative stress, known as oxytosis. As a result of glutathione depletion, accumulation of reactive oxygen species and Ca²⁺ influx are increased; however, the specific target of oxytosis has yet to be identified. In the present study, we focused on the effect of glutamate-induced oxidative stress on the extracellular-regulated protein kinase (ERK) pathway using the murine hippocampal HT22 cell line. Although the contribution of the ERK pathway to glutamate-induced oxytosis in HT22 cells is controversial, Western blot analysis revealed that glutamate caused down-regulation of mitogen-activated protein kinase kinase kinase (c-Raf) and a resulting decrease in the phosphorylation of c-Raf, as well as of mitogen-activated protein kinase kinase1/2 (MEK1/2) and ERK1/2, downstream components of the c-Raf/MEK/ERK pathway. Furthermore, neuroprotective (arylthio)cyclopentenone prostaglandins prevented glutamate-induced c-Raf down-regulation and consequently maintained the basal activity of c-Raf and its downstream signaling components. A pull-down assay using biotin-labeled cyclopentenone prostaglandins revealed that they preferentially bound to c-Raf relative to other signaling molecules of the ERK pathway, including Ras, MEK1/2, and ERK. These results suggest that neuroprotective (arylthio)cyclopentenone prostaglandins directly bind to c-Raf protein and protect cells from down-regulation of the c-Raf protein itself, resulting in neuroprotection against oxidative stress.

Hyperglycemia reduces PP2A subunit B expression in a middle cerebral artery occlusion animal model

Diabetes is a metabolic disorder that worsens clinical outcome following cerebral ischemia. Protein phosphatase 2A (PP2A) is a conserved, heterotrimeric, serine/threonine phosphatase with various cellular functions. PP2A subunit B is abundant in brain tissue and modulates PP2A function. The aim of this study was to investigate PP2A subunit B protein expression in the cerebral cortex of non-diabetic and diabetic animals with middle cerebral artery occlusion (MCAO) injury. Sprague-Dawley rats were injected with streptozotocin (40 mg/kg, i.p.) to induce diabetic conditions. After 4 weeks of streptozotocin treatment, the rats underwent MCAO to induce focal cerebral ischemia. The cerebral cortex tissue was collected 24 hr after MCAO. Body weight and blood glucose were measured, and Western blot analysis was performed to elucidate the expression of PP2A subunit B. We confirmed decreased body weight and increased blood glucose in diabetic animals. Reverse transcription-PCR and Western blot analyses showed decreased PP2A subunit B expression in the cerebral cortices of MCAO-injured animals. Moreover, diabetic animals with MCAO showed more severe decreases in PP2A subunit B protein levels than non-diabetic animals following MCAO. The decline of PP2A subunit B indicates degradation of neuronal function. These findings suggest that conspicuous decreases in PP2A subunit B may exacerbate cerebral ischemia under diabetic conditions following MCAO.

Mitochondrial Ca2+-activated K+ channels and their role in cell life and death pathways

Ca²⁺-activated K⁺ channels (K_Ca) are expressed at the plasma membrane and in cellular organelles. Expression of all K_Ca channel subtypes (BK, IK and SK) has been detected at the inner mitochondrial membrane of several cell types. Primary functions of these mitochondrial K_Ca channels include the regulation of mitochondrial ROS production, maintenance of the mitochondrial membrane potential and preservation of mitochondrial calcium homeostasis. These channels are therefore thought to contribute to cellular protection against oxidative stress through mitochondrial mechanisms of preconditioning. In this review, we summarize the current knowledge on mitochondrial KCa channels, and their role in mitochondrial function in relation to cell death and survival pathways. More specifically, we systematically discuss studies on the role of these mitochondrial K_Ca channels in pharmacological preconditioning, and according protective effects on ischemic insults to the brain and the heart.