Phosphatidylcholine-specific phospholipase C regulates glutamate-induced nerve cell death

Phosphatidylcholine-Specific Phospholipase C as a Promising Drug Target

Phosphatidylcholine-specific phospholipase C (PC-PLC) is an enzyme that catalyzes the formation of the important secondary messengers phosphocholine and diacylglycerol (DAG) from phosphatidylcholine. Although PC-PLC has been linked to the progression of many pathological conditions, including cancer, atherosclerosis, inflammation and neuronal cell death, studies of PC-PLC on the protein level have been somewhat neglected with relatively scarce data. To date, the human gene expressing PC-PLC has not yet been found, and the only protein structure of PC-PLC that has been solved was from Bacillus cereus (PC-PLCBc). Nonetheless, there is evidence for PC-PLC activity as a human functional equivalent of its prokaryotic counterpart. Additionally, inhibitors of PC-PLCBc have been developed as potential therapeutic agents. The most notable classes include 2-aminohydroxamic acids, xanthates, N,N'-hydroxyureas, phospholipid analogues, 1,4-oxazepines, pyrido[3,4-b]indoles, morpholinobenzoic acids and univalent ions. However, many medicinal chemistry studies lack evidence for their cellular and in vivo effects, which hampers the progression of the inhibitors towards the clinic. This review outlines the pathological implications of PC-PLC and highlights current progress and future challenges in the development of PC-PLC inhibitors from the literature.

Membrane lipid raft homeostasis is directly linked to neurodegeneration

Age-associated neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD) are an unmet health need, with significant economic and societal implications, and an ever-increasing prevalence. Membrane lipid rafts (MLRs) are specialised plasma membrane microdomains that provide a platform for intracellular trafficking and signal transduction, particularly within neurons. Dysregulation of MLRs leads to disruption of neurotrophic signalling and excessive apoptosis which mirrors the final common pathway for neuronal death in ALS, PD and AD. Sphingomyelinase (SMase) and phospholipase (PL) enzymes process components of MLRs and therefore play central roles in MLR homeostasis and in neurotrophic signalling. We review the literature linking SMase and PL enzymes to ALS, AD and PD with particular attention to attractive therapeutic targets, where functional manipulation has been successful in preclinical studies. We propose that dysfunction of these enzymes is upstream in the pathogenesis of neurodegenerative diseases and to support this we provide new evidence that ALS risk genes are enriched with genes involved in ceramide metabolism (P=0.019, OR = 2.54, Fisher exact test). Ceramide is a product of SMase action upon sphingomyelin within MLRs, and it also has a role as a second messenger in intracellular signalling pathways important for neuronal survival. Genetic risk is necessarily upstream in a late age of onset disease such as ALS. We propose that manipulation of MLR structure and function should be a focus of future translational research seeking to ameliorate neurodegenerative disorders.

ZmCTLP1 is required for the maintenance of lipid homeostasis and the basal endosperm transfer layer in maize kernels

Maize kernel weight is influenced by the unloading of nutrients from the maternal placenta and their passage through the transfer tissue of the basal endosperm transfer layer (BETL) and the basal intermediate zone (BIZ) to the upper part of the endosperm. Here, we show that Small kernel 10 (Smk10) encodes a choline transporter-like protein 1 (ZmCTLP1) that facilitates choline uptake and is located in the trans-Golgi network (TGN). Its loss of function results in reduced choline content, leading to smaller kernels with a lower starch content. Mutation of ZmCTLP1 disrupts membrane lipid homeostasis and the normal development of wall in-growths. Expression levels of Mn1 and ZmSWEET4c, two kernel filling-related genes, are downregulated in the smk10, which is likely to be one of the major causes of incompletely differentiated transfer cells. Mutation of ZmCTLP1 also reduces the number of plasmodesmata (PD) in transfer cells, indicating that the smk10 mutant is impaired in PD formation. Intriguingly, we also observed premature cell death in the BETL and BIZ of the smk10 mutant. Together, our results suggest that ZmCTLP1-mediated choline transport affects kernel development, highlighting its important role in lipid homeostasis, wall in-growth formation and PD development in transfer cells.

Glucosamine Enhancement of BDNF Expression and Animal Cognitive Function

Brain-derived neurotrophic factor (BDNF) is an important factor for memory consolidation and cognitive function. Protein kinase A (PKA) signaling interacts significantly with BDNF-provoked downstream signaling. Glucosamine (GLN), a common dietary supplement, has been demonstrated to perform a variety of beneficial physiological functions. In the current study, an in vivo model of 7-week-old C57BL/6 mice receiving daily intraperitoneal injection of GLN (0, 3, 10 and 30 mg/animal) was subjected to the novel object recognition test in order to determine cognitive performance. GLN significantly increased cognitive function. In the hippocampus GLN elevated tissue cAMP concentrations and CREB phosphorylation, and upregulated the expression of BDNF, CREB5 and the BDNF receptor TrkB, but it reduced PDE4B expression. With the in vitro model in the HT22 hippocampal cell line, GLN exposure significantly increased protein and mRNA levels of BDNF and CREB5 and induced cAMP responsive element (CRE) reporter activity; the GLN-mediated BDNF expression and CRE reporter induction were suppressed by PKA inhibitor H89. Our current findings suggest that GLN can exert a cognition-enhancing function and this may act at least in part by upregulating the BDNF levels via a cAMP/PKA/CREB-dependent pathway.

Sinomenine inhibits amyloid beta-induced astrocyte activation and protects neurons against indirect toxicity

Amyloid beta is a major constituent of the plaques found in the brains of patients suffering from Alzheimer’s disease (AD). A growing body of research work suggests that neuroinflammation plays important roles in the development of AD. Thus, considerable efforts are directed towards identification of compounds that can reduce or inhibit neuroinflammation. Here, we show that sinomenine, a compound present in a Chinese medicinal plant, Sinomenium acutum, inhibits oligomeric amyloid beta-induced production of reactive oxygen species (ROS), nitric oxide (NO) and inflammation-related molecules from astrocytic cells. The conditioned medium from oligomeric amyloid beta-treated astrocytic cells induces cell death in the hippocampal neuronal cells. Importantly, sinomenine inhibits this cell death. In addition, this compound has inhibitory effects on the production of ROS, NO and inflammation-related factors from oligomeric amyloid-beta treated human astrocytes. Finally, the conditioned medium from oligomeric amyloid beta-treated human astrocytes induces cell death in the primary culture of human neurons, which is inhibited by sinomenine. Thus, sinomenine inhibits amyloid beta-induced production of toxic factors from astrocytes, and confers protection to hippocampal neuronal cells as well as human neurons against indirect toxicity. The results suggest that this compound could provide beneficial effects in AD and other neurodegenerative conditions by reducing inflammation and neuronal cell death.

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.

Homeostasis of the Intraparenchymal-Blood Glutamate Concentration Gradient: Maintenance, Imbalance, and Regulation

It is widely accepted that glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS). However, there is also a large amount of glutamate in the blood. Generally, the concentration gradient of glutamate between intraparenchymal and blood environments is stable. However, this gradient is dramatically disrupted under a variety of pathological conditions, resulting in an amplifying cascade that causes a series of pathological reactions in the CNS and peripheral organs. This eventually seriously worsens a patient’s prognosis. These two “isolated” systems are rarely considered as a whole even though they mutually influence each other. In this review, we summarize what is currently known regarding the maintenance, imbalance and regulatory mechanisms that control the intraparenchymal-blood glutamate concentration gradient, discuss the interrelationships between these systems and further explore their significance in clinical practice.

Sex differences in the glutamate signaling pathway in juvenile rats

Females have been found to be at lower risk for the development of neurodevelopmental disorders than males. The greater neuroprotection in females is mostly due to female sex hormones. Estrogen is hypothesized to provide neuroprotection by suppressing the neuro-excitotoxicity induced by glutamate (Glu). This study was conducted to understand the effect of sex in modulating Glu signaling in juvenile rats. Brain tissue homogenate of 15 Wistar albino rats (9 males, 6 females) weighing 60 to 80 g and aged approximately 28 days was used. Biochemical parameters related to Glu signaling, such as the absolute and relative concentrations of Glu, gamma aminobutyric acid (GABA), and glutamine, as well as glutamate transporter 1 (GLT1), glutamine synthetase (GS), glutaminase (GLN), and glutamate decarboxylase-67 (GAD-67), were measured by ELISA. The data obtained demonstrated that compared with the levels in males, female rats exhibited significantly lower levels of Glu (p = .001) and GLN/GS (p = .021). The Glu/GABA and Glu/GLT1 ratios as well as the levels of GAD-67 were also lower in female rats, although the difference was not significant. The GLN/GAD-67 ratio (p = .027) and levels of GS (p = .019) were significantly higher in female rats than in males. Multiple regression analysis confirmed the role of GLN/GS, together with the much higher affinity of GLT1 to Glu, in avoiding excitotoxicity in females. In conclusion, there was a significant difference in Glu signaling between female and male rats. The females exhibited a lower susceptibility to develop Glu-induced excitotoxicity, an etiological mechanism for multiple neurodevelopmental disorders.

Structure-activity studies of non-steroid analogues structurally-related to neuroprotective estrogens

Estrone and 17β-estradiol are phenolic steroids that are known to be neuroprotective in multiple models of neuronal injury. Previous studies have identified the importance of their phenolic steroid A-ring for neuroprotection and have identified ortho substituents at the C-2 and C-4 positions on the phenol ring that enhance this activity. To investigate the importance of the steroid ring system for neuroprotective activity, phenolic compounds having the cyclopent[b]anthracene, cyclopenta[b]phenanthrene, benz[f]indene, benz[e]indene, indenes linked to a phenol, and a phenolic spiro ring system were prepared. New synthetic methods were developed to make some of the cyclopent[b]anthracene analogues as well as the spiro ring system. Compounds were evaluated for their ability to protect HT-22 hippocampal neurons from glutamate neurotoxicity and their activity relative to a potent neuroprotective analogue of 17β-estradiol was determined. An adamantyl substituent placed ortho to the phenolic hydroxyl group gave neuroprotective analogues in all ring systems studied.

A sex difference in oxidative stress and behavioral suppression induced by ethanol withdrawal in rats

Ethanol withdrawal (EW) is referred to the abrupt termination of long-term heavy drinking, and provokes oxidative brain damage. Here, we investigated whether the cerebellum and hippocampus of female rats are less affected by prooxidant EW than male rats due to the antioxidant effect of 17β-estradiol (E2). Female and male rats received a four-week ethanol diet and three-week withdrawal per cycle for two cycles. Some female rats were ovariectomized with E2 or antioxidant (Vitamin E+Co-Q10) treatment. Measurements were cerebellum (Rotarod) and hippocampus (water-maze)-related behaviors, oxidative markers (O2(-), malondialdehyde, protein carbonyls), mitochondrial membrane swelling, and a key mitochondrial enzyme, cytochrome c oxidase (CcO). Separately, HT22 (hippocampal) cells were subjected to ethanol-exposure and withdrawal for two cycles to assess the effect of a CcO inhibitor on E2's protection for mitochondrial respiration and cell viability. Ethanol-withdrawn female rats showed a smaller increase in oxidative markers in cerebellum and hippocampus than male rats, and E2 treatment decreased the oxidative markers. Compared to male counterparts, ethanol-withdrawn female rats showed better Rotarod but poorer water-maze performance, accompanied by more severe mitochondrial membrane swelling and CcO suppression in hippocampus. E2 or antioxidant treatment improved Rotarod but not water-maze performance. In the presence of a CcO inhibitor, E2 treatment failed to protect mitochondrial respiration and cell viability from EW. These data suggest that antioxidant E2 contributes to smaller oxidative stress in ethanol-withdrawn female than male rats. They also suggest that EW-induced severe mitochondrial damage in hippocampus may blunt E2's antioxidant protection for hippocampus-related behavior.

Sub-toxic Ethanol Exposure Modulates Gene Expression and Enzyme Activity of Antioxidant Systems to Provide Neuroprotection in Hippocampal HT22 Cells

Ethanol is known to cause severe systemic damage often explained as secondary to oxidative stress. Brain is particularly vulnerable to ethanol-induced reactive oxygen species (ROS) because the high amounts of lipids, and because nerve cell membranes contain high amounts of peroxidable fatty acids. Usually these effects of ethanol are associated to high and/or chronic exposure to ethanol. However, as we show in this manuscript, a low and acute dose of ethanol trigger a completely different response in hippocampal cells. Thus, we have observed that 0.1% ethanol exposure to HT22 cells, a murine hippocampal-derived cell line, increases the transcriptional expression of different genes belonging to the classical, glutathione/glutaredoxin and thioredoxin/peroxiredoxin antioxidant systems, these including Sod1, Sod2, Gpx1, Gclc, and Txnrd1. Paralleling these changes, enzyme activities of total superoxide dismutase (tSOD), catalase, total glutathione peroxidase (tGPx), glutathione-S-reductase (GSR), and total thioredoxin reductase (tTXNRD), were all increased, while the generation of thiobarbituric acid reactive substances (TBARS), as indicators of lipid peroxidation, and glutathione levels remained unaltered. Ethanol exposure did not affect cell viability or cell growing as assessed by real-time cell culture monitoring, indicating that low ethanol doses are not deleterious for hippocampal cells, but rather prevented glutamate-induced excitotoxicity. In summary, we conclude that sub-toxic exposure to ethanol may well be neuroprotective against oxidative insults in hippocampal cells.

The Role of Presenilin-1 in the Excitotoxicity of Ethanol Withdrawal

Presenilin-1 (PS1) is a core component of γ-secretase that is involved in neurodegeneration. We have previously shown that PS1 interacts with a mitogen-activated protein kinase [(MAPK) jun-NH2-terminal-kinase], and another MAPK (p38) is activated by ethanol withdrawal (EW), abrupt termination from chronic ethanol exposure. EW is excitotoxic in nature, induces glutamate upregulation, and provokes neuronal damage. Here, we explored a potential mechanistic pathway involving glutamate, p38 (p38α isozyme), and PS1 that may mediate EW-induced excitotoxic stress. We used the prefrontal cortex of male rats withdrawn from a chronic ethanol diet. Additionally, we used ethanol-withdrawn HT22 cells (mouse hippocampal) treated with the inhibitor of glutamate receptors [dizocilpine (MK-801)], p38α (SB203580; 4-[4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-1H-imidazol-5-yl]pyridine), or γ-secretase [N-[N- (3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT)] during EW. Separately, ethanol-free HT22 cells were exposed to glutamate with or without SB203580 or DAPT. Protein levels, mRNA levels, and cell viability were assessed using immunoblotting, qualitative polymerase chain reaction, and calcein assay, respectively. The prefrontal cortex of ethanol-withdrawn rats or HT22 cells showed an increase in PS1 and p38α, which was attenuated by MK-801 and SB203580, but mimicked by glutamate treatment to ethanol-free HT22 cells. DAPT attenuated the toxic effect of EW or glutamate on HT22 cells. These results suggest that PS1 expression is triggered by glutamate through p38α, contributing to the excitotoxic stimulus of EW.

Effect of glutamate on lysosomal membrane permeabilization in primary cultured cortical neurons

Glutamate is the principal neurotransmitter in the central nervous system. Glutamate-mediated excitotoxicity is the predominant cause of cerebral damage. Recent studies have shown that lysosomal membrane permeabilization (LMP) is involved in ischemia-associated neuronal death in non-human primates. This study was designed to investigate the effect of glutamate on lysosomal stability in primary cultured cortical neurons. Glutamate treatment for 30 min induced the permeabilization of lysosomal membranes as assessed by acridine orange redistribution and immunofluorescence of cathepsin B in the cytoplasm. Inhibition of glutamate excitotoxicity by the NMDA receptor antagonist MK-801 and the calcium chelator ethylene glycolbis (2-aminoethylether)-N, N, N′, N′-tetraacetic acid, rescued lysosomes from permeabilization. The role of calpain and reactive oxygen species (ROS) in inducing LMP was also investigated. Ca²⁺ overload following glutamate treatment induced the activation of calpain and the production of ROS, which are two major contributors to neuronal death. It has been reported that lysosomal-associated membrane protein 2 (LAMP2) and heat shock protein (HSP)70 are two calpain substrates that promote LMP in cancer cells; however, it was found that calpains were activated by glutamate, but only LAMP2 was subsequently degraded. Furthermore, LMP was not alleviated by treatment with the calpain inhibitors calpeptin and SJA6017, which blocked the cleavage of the calpain substrate α-fodrin. It was demonstrated that LMP was significantly alleviated by treatment with the antioxidant N-Acetyl-L-cysteine, indicating that LMP involvement in early glutamate excitotoxicity may be mediated partly by ROS rather than calpain activation. Overall, these data shed light on the role of ROS-mediated LMP in early glutamate excitotoxicity.

More from less: high-throughput dual polarity lipid imaging of biological tissues

Here, we reveal the increased biochemical and spatial information acquired using high-speed MALDI-MSI and sequential acquisitions of positive and negative lipid-MSI data from single tissue sections.

Ferroptosis: Death by Lipid Peroxidation

Ferroptosis is a regulated form of cell death driven by loss of activity of the lipid repair enzyme glutathione peroxidase 4 (GPX4) and subsequent accumulation of lipid-based reactive oxygen species (ROS), particularly lipid hydroperoxides. This form of iron-dependent cell death is genetically, biochemically, and morphologically distinct from other cell death modalities, including apoptosis, unregulated necrosis, and necroptosis. Ferroptosis is regulated by specific pathways and is involved in diverse biological contexts. Here we summarize the discovery of ferroptosis, the mechanism of ferroptosis regulation, and its increasingly appreciated relevance to both normal and pathological physiology.

Mulberry extracts alleviate aβ 25-35-induced injury and change the gene expression profile in PC12 cells

Mulberry, which contained high amounts of anthocyanins, has been used in traditional Chinese medicine. Mulberry fruit extracts (ME) have demonstrated the antioxidant activity and neuroprotection. The study was to investigate the neuroprotective efficacy of ME against β-amyloid 25–35- (Aβ_25–35-) induced PC12 cells injury. Cells preincubated with or without ME (200 μg/mL) for 24 h were treated with Aβ_25–35 (20 μmol/L) for another 24 h. Cell viability was assessed by MTT, gene expression profiles were examined by cDNA microarrays, and RT-PCR were used to confirm the results of microarray assays. ME pretreatment was found to neutralize the cytotoxicity and prevent Aβ_25–35-induced cells injury. Analyses of gene expression profile revealed that genes involving cell adhesion, peptidase activity, cytokine activity, ion binding activity, and angiogenesis regulation were significantly modulated by ME pretreatment. Among those genes, Apaf1, Bace2, and Plcb4 were enriched in the “Alzheimer's disease-reference pathway” and downregulated after ME intervention. RT-PCR results showed that ME preincubation could significantly inhibit Aβ_25–35 increased mRNA levels of these three genes. Overall, ME pretreatment could substantially alleviate PC12 cells injury and downregulate expression of AD-related genes, such as Apaf1, Bace2, and Plcb4. This study has a great nutrigenomics interest and brings new and important light in the field of AD intervention.

Pharmacology of conjugated equine estrogens: efficacy, safety and mechanism of action

Oral conjugated equine estrogens (CEE) are the most used estrogen formulation for postmenopausal hormone therapy either alone or in combination with a progestin. CEE is most commonly used for the management of early menopausal symptoms such as hot flashes, vaginitis, insomnia, and mood disturbances. Additionally, if used at the start of the menopausal phase (age 50-59 years), CEE prevents osteoporosis and may in some women reduce the risk of cardiovascular disease (CVD) and Alzheimer's disease (AD). There appears to be a common mechanism through which estrogens can protect against CVD and AD. CEE is a natural formulation of an extract prepared from pregnant mares' urine. The product monogram lists the presence of only 10 estrogens consisting of the classical estrogens, estrone and 17β-estradiol, and a group of unique ring B unsaturated estrogens such as equilin and equilenin. The ring B unsaturated estrogens are formed by an alternate steroidogenic pathway in which cholesterol is not an obligatory intermediate. Both the route of administration and structure of these estrogens play a role in the overall pharmacology of CEE. In contrast to 17β-estradiol, ring B unsaturated estrogens express their biological effects mainly mediated by the estrogen receptor β and not the estrogen receptor α. All estrogen components of CEE are antioxidants, and some ring B unsaturated estrogens have several fold greater antioxidant activity than estrone and 17β-estradiol. The cardioprotective and neuroprotective effects of CEE appear to be, to some extent, due to its ability to prevent the formation of oxidized LDL and HDL, and by inhibiting or modulating some of the key proteases involved in programmed cell death (apoptosis) induced by the excess neurotransmitter glutamate and other neurotoxins. Selective combinations of ring B unsaturated estrogens have the potential of being developed as novel therapeutic agents for the prevention of cardiovascular disease and Alzheimer's disease in both aging women and men. This article is part of a Special Issue entitled 'Menopause'.

Role of transient receptor potential channel 1 (TRPC1) in glutamate-induced cell death in the hippocampal cell line HT22

Transient receptor potential channel 1 (TRPC1; a cation channel activated by store depletion and/or through an intracellular messenger) is expressed in a variety of tissues, including the brain. To study the physiological function of TRPC1, we investigated the role of endogenously expressed TRPC1 in glutamate-induced cell death, using the murine hippocampal cell line HT22. Knocking down TRPC1 mRNA using TRPC1-shRNA or blocking of TRPC channels using 2-APB (≥200 μM) robustly attenuated glutamate-induced cell death after 24 h of incubation with 5 mM glutamate. Glutamate toxicity in HT22 cells seems to involve metabotropic glutamate receptor mGluR5 since MPEP (2-methyl-6-(phenylethynyl)-pyridine), an mGluR5 antagonist (≥100 μM), abrogated glutamate toxicity. Furthermore, a direct activation of mGluR5 by CHPG [(RS)-chloro-5-hydroxyphenylglycine; 100 μM or 300 μM] promoted HT22 cell death. TRPC1 knock-down markedly reduced CHPG-induced cell death. These observations suggest that glutamate-induced cell death in HT22 cells activates mGluR5 receptors, which significantly increases Ca(2+) influx through TRPC1 channels. TRPC1 knock-down prevented glutamate- and CHPG-induced cell death, suggesting that glutamate-induced toxicity in HT22 cells is mediated through TRPC1 channels and an mGluR5-dependent pathway. Together, this work provides evidence for a novel receptor activation pathway of TRPC1 in glutamate-induced toxicity.

Simultaneous recovery of dual pathways for ammonia metabolism do not improve further detoxification of ammonia in HepG2 cells

BACKGROUND

Key enzyme deficiency in the dual-pathway of ammonia metabolism leads to low detoxification capacity of HepG2 cells. Previously, we established a HepG2/AFhGS cell line with overexpression of human glutamine synthetase (hGS) in pathway 1 and a HepG2/(hArgI+hOTC)4 cell line with overexpression of human arginase I (hArgI) and human ornithine transcarbamylase (hOTC) in pathway 2. The present study aimed to investigate whether simultaneous recovery of the two pathways contributes to the further improvement of ammonia detoxification in HepG2 cells.

METHODS

We adopted a recombinant retrovirus carrying the hGS gene to infect HepG2/(hArgI+hOTC)4 cells and selected a new recombinant HepG2 cell line. The capacities of ammonia tolerance and detoxification in cells were detected by biochemical methods. Cell cycle PCR chip was used to assess the changes of gene expression.

RESULTS

Introducing hGS into HepG2/(hArgI+hOTC)4 cells did not lead to hGS overexpression, but inhibited hArgI expression. The levels of synthetic glutamine and urea in HepG2/(hArgI+hOTC+AFhGS)1 cells were significantly lower than those in HepG2/(hArgI+hOTC)4 cells when cultured in the medium with 10 and 15 mmol/L glutamate (Glu) and with 60 and 180 mmol/L NH4Cl, respectively. In addition, the comparison of different cell growth showed that HepG2/AFhGS cells significantly lagged behind the other cells by the 5th and 7th day, indicating that introduction of hGS impedes HepG2 cell proliferation. Analysis of the mechanism suggested that the decreased expression of BCL2 played an important role.

CONCLUSIONS

This study demonstrated that the recovery of two ammonia metabolic pathways in HepG2 cells is not helpful in increasing ammonia metabolism. The reinforcement of the pathway of urea metabolism is more important and valuable in improving the ammonia metabolism capacity in HepG2 cells.

Phospholipase C activity increases in cerebrospinal fluid from migraineurs in proportion to the number of comorbid conditions: a case-control study

Background

Migraineurs are more often afflicted by comorbid conditions than those without primary headache disorders, though the linking pathophysiological mechanism(s) is not known. We previously reported that phosphatidylcholine-specific phospholipase C (PC-PLC) activity in cerebrospinal fluid (CSF) increased during migraine compared to the same individual’s well state. Here, we examined whether PC-PLC activity from a larger group of well-state migraineurs is related to the number of their migraine comorbidities.

Methods

In a case–control study, migraineurs were diagnosed using International Headache Society criteria, and controls had no primary headache disorder or family history of migraine. Medication use, migraine frequency, and physician-diagnosed comorbidities were recorded for all participants. Lumbar CSF was collected between the hours of 1 and 5 pm, examined immediately for cells and total protein, and stored at −80°C. PC-PLC activity in thawed CSF was measured using a fluorometric enzyme assay. Multivariable logistic regression was used to evaluate age, gender, medication use, migraine frequency, personality scores, and comorbidities as potential predictors of PC-PLC activity in CSF.

Results

A total of 18 migraineurs-without-aura and 17 controls participated. In a multivariable analysis, only the number of comorbidities was related to PC-PLC activity in CSF, and only in migraineurs [parameter estimate (standard error) = 1.77, p = 0.009].

Conclusion

PC-PLC activity in CSF increases with increasing number of comorbidities in migraine-without-aura. These data support involvement of a common lipid signaling pathway in migraine and in the comorbid conditions.

A sigma-1 receptor antagonist (NE-100) prevents tunicamycin-induced cell death via GRP78 induction in hippocampal cells

Endoplasmic reticulum (ER) stress is involved in various diseases such as ischemia, Alzheimer's disease, and Parkinson's disease. The widely used selective sigma-1 receptor antagonist, N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)-phenyl]-ethylamine monohydrochloride (NE-100), has been shown to suppress ischemia-induced neuronal cell death in the murine hippocampus. In the present study, we investigated whether NE-100 might suppress neuronal cell death that is induced by ER stress in ischemic injury. These studies show that NE-100 protected the ER stress-induced cell death of murine hippocampal HT22 cells, but not the oxidative stress-induced cell death. This suggests that NE-100 may have a protective effect on the ER. However, another sigma-1 receptor antagonist (BD1047) did not suppress ER stress-induced cell death. In addition, NE-100 attenuated the upregulation of C/EBP homologous protein (CHOP) induced by ER stress and upregulated the expression of both the 50-kDa activating transcription factor 6 (p50ATF6) and the 78-kDa glucose-regulated protein (GRP78). However, NE-100 did not impact the expression of phosphorylated eukaryotic initiation factor 2α (p-eIF2α) nor splicing of X-box-binding protein 1 (XBP-1). These findings suggest that NE-100 suppresses ER stress-induced cell death via CHOP expression by the upregulation of GRP78 through ATF6 pathway, independent sigma-1 receptor antagonist effect.

Isoliquiritigenin isolated from Glycyrrhiza uralensis protects neuronal cells against glutamate-induced mitochondrial dysfunction

Glutamate-mediated excitotoxicity, which is associated with reactive oxygen species (ROS), is hypothesized to be a major contributor to pathological cell death in the mammalian central nervous system, and to be involved in many acute and chronic brain diseases. Here, we showed that isoliquiritigenin (ISL) isolated from Glycyrrhiza uralensis (Gu), one of the most frequently prescribed oriental herbal medicines, protected HT22 hippocampal neuronal cells from glutamate-induced oxidative stress. In addition, we clarified the molecular mechanisms by which it protects against glutamate-induced neuronal cell death. ISL reversed glutamate-induced ROS production and mitochondrial depolarization, as well as glutamate-induced changes in expression of the apoptotic regulators Bcl-2 and Bax. Pretreatment of HT22 cells with ISL suppresses the release of apoptosis-inducing factor from mitochondria into the cytosol. Taken together, our results suggest that ISL may protect against mitochondrial dysfunction by limiting glutamate-induced oxidative stress. In conclusion, our results demonstrated that ISL isolated from Gu has protective effects against glutamate-induced mitochondrial damage and hippocampal neuronal cell death. We expect ISL to be useful in the development of drugs to prevent or treat neurodegenerative diseases.

Tricyclodecan-9-yl-xanthogenate (D609) mechanism of actions: a mini-review of literature

Tricyclodecan-9-yl-xanthogenate (D609) is known for its antiviral and antitumor properties. D609 actions are widely attributed to inhibiting phosphatidylcholine (PC)-specific phospholipase C (PC-PLC). D609 also inhibits sphingomyelin synthase (SMS). PC-PLC and/or SMS inhibition will affect lipid second messengers 1,2-diacylglycerol (DAG) and/or ceramide. Evidence indicates either PC-PLC and/or SMS inhibition affected the cell cycle and arrested proliferation, and stimulated differentiation in various in vitro and in vivo studies. Xanthogenate compounds are also potent antioxidants and D609 reduced Aß-induced toxicity, attributed to its antioxidant properties. Zn²⁺ is necessary for PC-PLC enzymatic activity; inhibition by D609 might be attributed to its Zn²⁺ chelation. D609 has also been proposed to inhibit acidic sphingomyelinase or down-regulate hypoxia inducible factor-1α; however these are down-stream events related to PC-PLC inhibition. Characterization of the mammalian PC-PLC is limited to inhibition of enzymatic activity (frequently measured using Amplex red assay with bacterial PC-PLC as a standard). The mammalian PC-PLC has not been cloned; sequenced and structural information is unavailable. D609 showed promise in cancer studies, reduced atherosclerotic plaques (inhibition of PC-PLC) and cerebral infarction after stroke (PC-PLC or SMS). D609 actions as an antagonist to pro-inflammatory cytokines have been attributed to PC-PLC. The purpose of this review is to comprehensively evaluate the literature and summarize the findings and relevance to cell cycle and CNS pathologies.

Protection by D609 through cell-cycle regulation after stroke

Expressions of cell-cycle regulating proteins are altered after stroke. Cell-cycle inhibition has shown dramatic reduction in infarction after stroke. Ceramide can induce cell-cycle arrest by up-regulation of cyclin-dependent kinase (Cdk) inhibitors p21 and p27 through activation of protein phosphatase 2A (PP2A). Tricyclodecan-9-yl-xanthogenate (D609)-increased ceramide levels after transient middle cerebral artery occlusion (tMCAO) in spontaneously hypertensive rat (SHR) probably by inhibiting sphingomyelin synthase (SMS). D609 significantly reduced cerebral infarction and up-regulated Cdk inhibitor p21 and down-regulated phospho-retinoblastoma (pRb) expression after tMCAO in rat. Others have suggested bFGF-induced astrocyte proliferation is attenuated by D609 due to an increase in ceramide by SMS inhibition. D609 also reduced the formation of oxidized phosphatidylcholine (OxPC) protein adducts. D609 may attenuate generation of reactive oxygen species and formation of OxPC by inhibiting microglia/macrophage proliferation after tMCAO (please also see note added in proof: D609 may prevent mature neurons from entering the cell cycle at the early reperfusion, however may not interfere with later proliferation of microglia/ macrophages that are the source of brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) in offering protection). It has been proposed that D609 provides benefit after tMCAO by attenuating hypoxia-inducible factor-1alpha and Bcl2/adenovirus E1B 19 kDa interacting protein 3 expressions. Our data suggest that D609 provides benefit after stoke through inhibition of SMS, increased ceramide levels, and induction of cell-cycle arrest by up-regulating p21 and causing hypophosphorylation of Rb (through increased protein phosphatase activity and/or Cdk inhibition).

Hydrogen sulfide increases glutathione production and suppresses oxidative stress in mitochondria

Hydrogen sulfide (H(2)S) is a synaptic modulator as well as a neuroprotectant in the brain. We recently showed that H(2)S protects neurons from oxidative stress by increasing the levels of glutathione (GSH), a major cellular antioxidant, by more than twice that of a control through enhancing the cystine transport. Here we show that H(2)S enhances the transport of cysteine to increase GSH production more than cystine transport and to redistribute the localization of GSH to mitochondria. The efficiency of GSH production enhanced by H(2)S is even greater by fourfold under oxidative stress by glutamate. H(2)S reinstated GSH levels in the fetal brain decreased by ischemia/reperfusion in utero. In addition, Neuro2a cells expressing a mitochondrial H(2)S-producing enzyme, 3-mercaptopyruvate sulfurtransferase (3MST), along with cysteine aminotransferase (CAT), showed significant resistance to oxidative stress. The present study shows that H(2)S protects cells from oxidative stress by two mechanisms. It enhances the production of GSH by enhancing cystine/cysteine transporters and redistributes GSH to mitochondria. H(2)S produced in mitochondria also may directly suppress oxidative stress. It provides a new mechanism of neuroprotection from oxidative stress by H(2)S.

GRK5 deficiency leads to reduced hippocampal acetylcholine level via impaired presynaptic M2/M4 autoreceptor desensitization

G protein-coupled receptor kinase 5 (GRK5) deficiency has been linked recently to early Alzheimer disease (AD), but the mechanism by which GRK5 deficiency may contribute to AD pathogenesis remains elusive. Here we report that overexpression of dominant negative mutant of GRK5 (dnGRK5) in a cholinergic neuronal cell line led to decreased acetylcholine (ACh) release. This reduction was fully corrected by pertussis toxin, atropine (a nonselective muscarinic antagonist), or methoctramine (a selective M2/M4 muscarinic receptor antagonist). Consistent with results in cultured cells, high potassium-evoked ACh release in hippocampal slices from young GRK5 knock-out mice was significantly reduced compared with wild type littermates, and this reduced ACh release was also fully corrected by methoctramine. In addition, following treatment with the nonselective muscarinic agonist oxotremorine-M, M2, and M4 receptors underwent significantly reduced internalization in GRK5KO slices compared with wild type slices, as assessed by plasma membrane retention of receptor immunoreactivity, whereas M1 receptor internalization was not affected by loss of GRK5 expression. Moreover, Western blotting revealed no synaptic or cholinergic degenerative changes in young GRK5 knock-out mice. Altogether, these results suggest that GRK5 deficiency leads to a reduced hippocampal ACh release and cholinergic hypofunction by selective impairment of desensitization of presynaptic M2/M4 autoreceptors. Because this nonstructural cholinergic hypofunction precedes the hippocampal cholinergic hypofunction associated with structural cholinergic degeneration and cognitive decline in aged GRK5 knock-out mice, this nonstructural alteration may be an early event contributing to cholinergic degeneration in AD.

HT22 hippocampal neuronal cell line possesses functional cholinergic properties

AIMS

Hippocampal cholinergic hypofunction is known to be involved in the cognitive deficits of Alzheimer's disease, but the detailed mechanisms remain to be elucidated. In order to establish an in vitro hippocampal cholinergic neuronal model for the relevant mechanistic studies, we have characterized a widely used hippocampal neuronal cell line, HT22, a sub-line derived from parent HT4 cells that were originally immortalized from primary mouse hippocampal neuronal culture.

MAIN METHODS

Western blot and immunocytochemistry were used to examine expression of cholinergic markers in HT22 cells. High potassium-evoked [(3)H]ACh release was used to evaluate the cholinergic functional properties of the cells.

KEY FINDINGS

We found that HT22 cells express essential cholinergic markers, such as the high affinity choline transporter, choline acetyltransferase, vesicular acetylcholine transporter, and muscarinic acetylcholine receptors. Exposure of HT22 cells to high potassium evoked [(3)H]ACh release in a dose-dependent manner. In addition, the [(3)H]ACh release was significantly potentiated when presynaptic autoreceptors were blocked.

SIGNIFICANCE

Our results suggest that HT22 cells possess functional cholinergic properties, and can be used for an in vitro model for defining the mechanisms in cognitive deficits of Alzheimer's disease.

Arginine-modified DNA aptamers that show enantioselective recognition of the dicarboxylic acid moiety of glutamic acid

We have screened glutamic acid-binding aptamers from a modified DNA pool containing arginine residues using the method of systematic evolution of ligands by exponential enrichment (SELEX). Thirty-one modified DNA molecules were obtained from the enriched pool after the 17th round of selection, and their binding affinities for the target were evaluated by binding assays using affinity gels. Three modified DNA molecules having higher affinity were sequenced and we determined their affinity and specificity for the target by surface plasmon resonance (SPR) measurements. The SPR studies indicated that two of these three aptamers distinguished the dicarboxylic acid moiety of the D-isomer from that of the L-isomer; however, the third aptamer did not show enantioselectivity.

A prototypical Sigma-1 receptor antagonist protects against brain ischemia

Previous studies indicate that the Sigma-1 ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) protects the brain from ischemia. Less clear is whether protection is mediated by agonism or antagonism of the Sigma-1 receptor, and whether drugs already in use for other indications and that interact with the Sigma-1 receptor might also prevent oxidative damage due to conditions such as cerebral ischemic stroke. The antipsychotic drug haloperidol is an antagonist of Sigma-1 receptors and in this study it potently protects against oxidative stress-related cell death in vitro at low concentrations. The protective potency of haloperidol and a number of other butyrophenone compounds positively correlate with their affinity for a cloned Sigma-1 receptor, and the protection is mimicked by a Sigma-1 receptor-selective antagonist (BD1063), but not an agonist (PRE-084). In vivo, an acute low dose (0.05 mg/kg s.c.) of haloperidol reduces by half the ischemic lesion volume induced by a transient middle cerebral artery occlusion. These in vitro and in vivo pre-clinical results suggest that a low dose of acutely administered haloperidol might have a novel application as a protective agent against ischemic cerebral stroke and other types of brain injury with an ischemic component.

Knockdown of integrin beta4 in primary cultured mouse neurons blocks survival and induces apoptosis by elevating NADPH oxidase activity and reactive oxygen species level

Recently, the specific roles of integrin beta4 in the signaling networks that drive pathological angiogenesis and tumor progression have been revealed. Our previous study showed that integrin beta4 might be involved in neuron survival signal transduction. To further our study on the role of integrin beta4 in the survival and apoptosis of primary cultured mouse neurons, we inhibited the expression of integrin beta4 by its specific small interfering RNA. Viability of the cells remarkably declined, and neurons underwent apoptosis with down-regulation of integrin beta4. Next, we investigated the effect of siRNA-mediated down-regulation of integrin beta4 on the level of intracellular reactive oxygen species and the activities of NADPH oxidase and superoxide dismutase. The level of reactive oxygen species in the neurons was elevated significantly, the activities of manganese-dependent superoxide dismutase and copper/zinc-dependent superoxide dismutase were not altered, but the activity of NADPH oxidase was increased. Furthermore, inhibition of NADPH oxidase by its specific inhibitor dibenziodolium chloride attenuated the neuronal death induced by integrin beta4 knockdown. The data suggest that integrin beta4 is a key factor in neuron survival and apoptosis and indicate that this integrin subunit might perform its action through regulating NADPH oxidase and the level of reactive oxygen species in neuronal survival and apoptosis.

Baicalein, an antioxidant 12/15-lipoxygenase inhibitor improves clinical rating scores following multiple infarct embolic strokes

The present study assessed whether baicalein (5,6,7-trihydroxyflavone), a polyphenolic antioxidant 12/15-lipoxygenase inhibitor would attenuate oxidative cell death in vitro using a mouse hippocampal HT22 cell assay. Moreover, we determined if baicalein would be useful to attenuate behavioral deficits associated with multiple infarct ischemic events in vivo using a rabbit small clot embolic stroke model (RSCEM). Using HT22 cell in vitro, baicalein was shown to significantly promote cell survival with an estimated dose for 50% cell survival of 2 muM following incubation in the presence of iodoacetic acid (20 muM), an irreversible inhibitor of the glycolytic pathway that results in the free radical production, lipid peroxidation and cell death. Since baicalein was neuroprotective and attenuated iodoacetic acid (IAA) toxicity in vitro, we studied its effects in vivo in an embolic stroke model using behavioral measures as the endpoint. Quantal analysis for each treatment in the embolism model identifies the quantity of microclots (mg) that produce neurologic dysfunction in 50% of a group of animals (P(50)), with intervention considered neuroprotective if it increases the P(50) compared with controls. Baicalein (100 mg/kg, s.c.) injected 5 and 60 min post-embolization significantly (P<0.05) improved behavioral function. The calculated P(50) values were 2.85+/-0.64 mg (n=21) and 2.15+/-0.12 mg (n=14), respectively compared with 1.37+/-0.20 mg (n=23) for the control group. In conclusion, we have shown that baicalein effectively attenuated cell death in vitro using HT22 cells and also significantly reduced behavioral deficits in rabbits when given up to 1 h following an embolic stroke. The results suggest that baicalein, or derivatives of baicalein with multiple pharmacological activities may be useful to develop as novel treatments for acute ischemic stroke.

Suppressing phosphatidylcholine-specific phospholipase C and elevating ROS level, NADPH oxidase activity and Rb level induced neuronal differentiation in mesenchymal stem cells

In the previous research, we found that D609 (tricyclodecan-9-yl-xanthogenate) could induce human marrow stromal cell (hMSC) differentiation to neuron-like cells. In this study, to understand the possible mechanism, we sequentially investigated the changes of phosphatidylcholine-specific phospholipase C (PC-PLC) activity, the expression of Rb, the intracellular reactive oxygen species (ROS) levels, NADPH oxidase and superoxide dismutase (SOD) activities when D609 induced neuronal differentiation in rat mesenchymal stem cells (MSCs). The results showed that D609 obviously inhibited the activity of PC-PLC when it induced neuronal differentiation in rat MSCs. Simultaneously, ROS level and the activity of NADPH oxidase increased significantly, but the MnSOD and Cu/ZnSOD activities were not altered. Furthermore, the level of Rb protein was evidently elevated. Our data suggested that PC-PLC mediated neuronal differentiation of rat MSCs by elevating NADPH oxidase activity, ROS level, and up-regulating the expression of Rb protein.

Involvement of Ca2+- and cyclic adenosine monophosphate-mediated signaling pathways in photodynamic injury of isolated crayfish neuron and satellite glial cells

To investigate the mechanisms of oxidative injury of neurons and glia, we studied the photodynamic effect on isolated stretch receptor that consists of only two sensory neurons enwrapped by satellite glial cells. Photodynamic therapy (PDT), a potent inducer of oxidative stress, is a prospective method for destruction of brain tumors. PDT induced functional inactivation and necrosis of neurons, necrosis, apoptosis, and proliferation of glial cells. The roles of calmodulin, calmodulin-dependent kinase II, phospholipase C, protein kinases A and C, and phosphodiesterase in these processes were studied by using their inhibitors: fluphenazine, KN-93, D-609, H89, staurosporine, and papaverine, respectively. PDT-induced firing abolishment was enhanced by H89 or papaverine, whereas staurosporine acted oppositely. Fluphenazine or KN-93 reduced necrosis of neurons and glial cells. H89 enhanced necrosis of neurons, whereas staurosporine enhanced necrosis of glial cells. Inhibition of protein kinases A and C enhanced PDT-induced glial apoptosis. Photodynamic gliosis was prevented by KN-93 or staurosporine. These data indicate possible involvement of calmodulin and calmodulin-dependent kinase II in photoinduced necrosis of neurons and glia. Protein kinase C could protect glial cells from necrosis and apoptosis and participate in photoinduced gliosis and loss of neuronal activity. Protein kinase A maintained neuronal firing and protected neurons from photoinduced necrosis and glial cells from apoptosis. Phosphodiesterase reduced necrosis of photosensitized neurons and glia. Thus, Ca(2+)- and cAMP-mediated signaling pathways were involved in photooxidative injury of neurons and glia. Their pharmacological modulation may differently change the efficacy of photodynamic injury of neurons and glial cells.

Effect of tricyclodecan-9-yl potassium xanthate (D609) on phospholipid metabolism and cell death during oxygen-glucose deprivation in PC12 cells

Alterations in lipid metabolism play an integral role in neuronal death in cerebral ischemia. Here we used an in vitro model, oxygen-glucose deprivation (OGD) of rat pheochromocytoma (PC12) cells, and analyzed changes in phosphatidylcholine (PC) and sphingomyelin (SM) metabolism. OGD (4-8 h) of PC12 cells triggered a dramatic reduction in PC and SM levels, and a significant increase in ceramide. OGD also caused increases in phosphatidylcholine-phospholipase C (PC-PLC) and phospholipase D (PLD) activities and PLD2 protein expression, and reduction in cytidine triphosphate:phosphocholine cytidylyltransferase-alpha (CCTalpha, the rate-limiting enzyme in PC synthesis) protein expression and activity. Phospholipase A2 activity and expression were unaltered during OGD. Increased neutral sphingomyelinase activity during OGD could account for SM loss and increased ceramide. Surprisingly, treatment with PC-PLC inhibitor tricyclodecan-9-yl potassium xanthate (D609) aggravated cell death in PC12 cells during OGD. D609 was cytotoxic only during OGD; cell death could be prevented by inclusion of sera, glucose or oxygen. During OGD, D609 caused further loss of PC and SM, depletion of 1,2-diacylglycerol (DAG), increase in ceramide and free fatty acids (FFA), cytochrome c release from mitochondria, increases in intracellular Ca2+ ([Ca2+]i), poly-ADP ribose polymerase (PARP) cleavage and phosphatidylserine externalization, indicative of apoptotic cell death. Exogenous PC during OGD in PC12 cells with D609 attenuated PC, SM loss, restored DAG, attenuated ceramide levels, decreased cytochrome c release, PARP cleavage, annexin V binding, attenuated the increase in [Ca2+]i, FFA release, and significantly increased cell viability. Exogenous PC may have elicited these effects by restoring membrane PC levels. A tentative scheme depicting the mechanism of action of D609 (inhibiting PC-PLC, SM synthase, PC synthesis at the CDP-choline-1,2-diacylglycerol phosphocholine transferase (CPT) step and causing mitochondrial dysfunction) has been proposed based on our observations and literature.

Phosphatidylcholine-specific phospholipase C and ROS were involved in chicken blastodisc differentiation to vascular endothelial cells

To find the key factors that were involved in the survival and vascular endothelial differentiation of chick blatodisc induced by fibroblast growth factor 2 (FGF-2), we built a chick vasculogenesis model in vitro. Subsequently, the activities of phosphatidylcholine-specific phospholipase C (PC-PLC), including Ca(2+)-dependent and -independent PC-PLC, and the level of reactive oxygen species (ROS) were evaluated during the endothelial differentiation of chick blastodisc. The results showed that Ca(2+)-indepentent PC-PLC underwent a remarkable increase in 24 h (P < 0.01), then it decreased gradually with the cell differentiation, while the Ca(2+)-depentent PC-PLC was nearly not changed in the whole process. At the same time, ROS level dramatically decreased during the cell differentiation. To understand the role of PC-PLC and how it performs its function in the vascular endothelial differentiation induced by FGF-2, we suppressed PC-PLC activity by its specific inhibitor D609 (tricyclodecan-9-yl potassium xanthate) at 24 h during the cell differentiation. As a result, the cell differentiation could not progress and the intracellular level of ROS was elevated. The data suggested that PC-PLC and ROS were involved in chicken blastodisc differentiation to vascular endothelial cells. PC-PLC was an important factor in the blastodisc cell survival and differentiation, and it might perform its function associated with ROS.

Involvement of phosphatidylcholine-selective phospholipase C in activation of mitogen-activated protein kinase pathways in imidazoline receptor antisera-selected protein

Imidazoline receptor antisera-selected protein (IRAS) is considered as a candidate for the I1-imidazoline receptor (I1R), but the signaling pathway mediated by IRAS remains unknown. In our study, the signal transduction pathways of IRAS were investigated in CHO cells stably expressing IRAS (CHO-IRAS), and compared to the native I1R signaling pathways. Rilmenidine or moxonidine (10 nM-100 microM), I1R agonists, failed to stimulate [35S]-GTPgammaS binding in CHO-IRAS cell membrane preparations, suggesting that G protein may not be involved in IRAS signaling pathway. However, incubation of CHO-IRAS with rilmenidine or moxonidine for 5 min could induce an upregulation of phosphatidylcholine-selective phospholipase C (PC-PLC) activity, and an increase in the accumulation of diacylglycerol (DAG), the hydrolysate of PC-PLC, in a concentration-dependent manner. The elevated activation of PC-PLC by rilmenidine or moxonidine (100 nM) could be blocked by efaroxan, a selective I1R antagonist. Cells treated with rilmenidine or moxonidine showed an increased level of extracellular signal-regulated kinase (ERK) phosphorylation in a concentration-dependent manner, which could be reversed by efaroxan or D609, a selective PC-PLC inhibitor. These results suggest that the signaling pathway of IRAS in response to I1R agonists coupled with the activation of PC-PLC and its downstream signal transduction molecule, ERK. These findings are similar to those in the signaling pathways of native I1R, providing some new evidence for the relationship between I1R and IRAS.

Methylmercury-induced toxicity is mediated by enhanced intracellular calcium through activation of phosphatidylcholine-specific phospholipase C

Methylmercury (MeHg) is a ubiquitous environmental toxicant to which humans can be exposed by ingestion of contaminated food. MeHg has been suggested to exert its toxicity through its high reactivity to thiols, generation of arachidonic acid and reactive oxygen species (ROS), and elevation of free intracellular Ca(2+) levels ([Ca(2+)](i)). However, the precise mechanism has not been fully defined. Here we show that phosphatidylcholine-specific phospholipase C (PC-PLC) is a critical pathway for MeHg-induced toxicity in MDCK cells. D609, an inhibitor of PC-PLC, significantly reversed the toxicity in a time- and dose-dependent manner with concomitant inhibition of the diacylglycerol (DAG) generation and the phosphatidylcholine (PC)-breakdown. MeHg activated the group IV cytosolic phospholipase A(2) (cPLA(2)) and acidic form of sphingomyelinase (A-SMase) downstream of PC-PLC, but these enzymes as well as protein kinase C (PKC) were not linked to the toxicity by MeHg. Furthermore, MeHg produced ROS, which did not affect the toxicity. Addition of EGTA to culture media resulted in partial decrease of [Ca(2+)](i) and partially blocked the toxicity. In contrast, when the cells were treated with MeHg in the presence of Ca(2+) in the culture media, D609 completely prevented cell death with parallel decrease in [Ca(2+)](i). Our results demonstrated that MeHg-induced toxicity was linked to elevation of [Ca(2+)](i) through activation of PC-PLC, but not attributable to the signaling pathways such as cPLA(2), A-SMase, and PKC, or to the generation of ROS.

Guanosine 5'-[gamma-thio]triphosphate-mediated activation of cytosol phospholipase C caused lysosomal destabilization

Lysosomal disintegration is critical for the organelle functions and cellular viability. In this study, we established that guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S)-activated cytosol of rat hepatocytes could increase lysosomal permeability to both potassium ions and protons and osmotically destabilize the lysosomes via K(+)/H(+) exchange. These results were obtained through measurements of lysosomal beta-hexosaminidase-free activity, membrane potential and intralysosomal pH. Assays of phospholipase C (PLC) activity show that cytosolic PLC was activated upon addition of GTP-gamma-S to the cytosol. The effects of cytosol on the lysosomes could be abolished by D609, an inhibitor of PLC, but not by the inhibitors of phospholipase A(2). The cytosol-treated lysosomes disintegrated markedly in hypotonic sucrose medium, reflecting that the lysosomal osmotic sensitivity increased. Microscopic observations showed that the lysosomes became more swollen in hypotonic sucrose medium. This indicates that the cytosol treatment induced osmotic shock to the lysosomes and an influx of water into the organelle.

D609 blocks cell survival and induces apoptosis in neural stem cells

In order to investigate the effects of tricyclodecane-9-yl-xanthogenate (D609) on the survival of neural stem cells (NSCs), which were isolated from rat forebrain, we treated the NSCs with D609 in the presence of basic fibroblast growth factor (bFGF). We found that when NSCs were exposed to 18.76-56.29 microM D609, the viability of the cells remarkably declined and apoptosis occurred. At the same time, the ROS level in NSCs was depressed. The data suggested that D609 was a powerful growth inhibitor and apoptosis inducer in NSCs.

Mechanism of cytosol phospholipase C and sphingomyelinase-induced lysosome destabilization

Lysosomal disintegration may cause apoptosis, necrosis and some diseases. However, mechanisms for these events are still unclear. In this study, we measured lysosomal beta-hexosaminidase free activity, membrane potential and intralysosomal pH. The results revealed that the cytosolic extracts of rat hepatocytes could increase the lysosomal permeability to both potassium ions and protons, and osmotically destabilize lysosomes via K(+)/H(+) exchange. The effects of cytosol on lysosomes could be completely abolished by D609, which inhibited both phospholipase C and sphingomyelinase, and partly prevented by sphingomyelinase inhibitor Ara-AMP, but not by the inhibitors of PLA(2). Moreover, purified phospholipase C could destabilize the lysosomes while phospholipase A(2) and phospholipase D did not produce such effects. The cytosolic phospholipases hydrolyzed lysosomal membrane phospholipids by 50%, which could be prevented by D609. Disintegration of the cytosol-treated lysosomes biphasically depended on the cytosolic [Ca(2+)]. The cytosol did not disintegrate lysosomes below 100 nM or above 10 muM cytosolic [Ca(2+)], but markedly destabilized lysosomes at about 340 nM [Ca(2+)]. The results suggest that cytosolic phospholipase C and sphingomyelinase may be responsible for the alterations in lysosomal stability by increasing the ion permeability.