Effects of ATP on phosphatidylinositol-phospholipase C and inositol 1-phosphate accumulation in rat brain synaptosomes

Yin-Yang Mechanisms Regulating Lipid Peroxidation of Docosahexaenoic Acid and Arachidonic Acid in the Central Nervous System

Phospholipids in the central nervous system (CNS) are rich in polyunsaturated fatty acids (PUFAs), particularly arachidonic acid (ARA) and docosahexaenoic acid (DHA). Besides providing physical properties to cell membranes, these PUFAs are metabolically active and undergo turnover through the “deacylation-reacylation (Land's) cycle”. Recent studies suggest a Yin-Yang mechanism for metabolism of ARA and DHA, largely due to different phospholipases A₂ (PLA₂s) mediating their release. ARA and DHA are substrates of cyclooxygenases and lipoxygenases resulting in an array of lipid mediators, which are pro-inflammatory and pro-resolving. The PUFAs are susceptible to peroxidation by oxygen free radicals, resulting in the production of 4-hydroxynonenal (4-HNE) from ARA and 4-hydroxyhexenal (4-HHE) from DHA. These alkenal electrophiles are reactive and capable of forming adducts with proteins, phospholipids and nucleic acids. The perceived cytotoxic and hormetic effects of these hydroxyl-alkenals have impacted cell signaling pathways, glucose metabolism and mitochondrial functions in chronic and inflammatory diseases. Due to the high levels of DHA and ARA in brain phospholipids, this review is aimed at providing information on the Yin-Yang mechanisms for regulating these PUFAs and their lipid peroxidation products in the CNS, and implications of their roles in neurological disorders.

[3H]PtdIns hydrolysis in postmortem human brain membranes is mediated by the G-proteins Gq/11 and phospholipase C-beta

A method utilizing exogenously added [3H]PtdIns incubated with membranes prepared from postmoretem human brain has been shown to provide a means of measuring agonist-induced, guanosine 5'-O-(thiotriphosphate) (GTP[S])-dependent hydrolysis of [3H]PtdIns, thus allowing investigations of the activity of the phosphoinositide second-messenger system in accessible human brain tissue. Agonists inducing [3H]PtdIns hydrolysis include carbachol, trans-1-aminocyclopentyl-1,3-dicarboxylate (ACPD; a glutamatergic metabotropic receptor agonist), serotonin and ATP, with the latter two agonists producing the largest responses. In addition to ATP, [3H]PtdIns hydrolysis was induced by ADP and by 2-methylthio-ATP, indicating that P2-purinergic receptors mediate this process. Subtype-selective antibodies we used to identify Gq/11 and phospholipase C-beta as the G-protein and phospholipase C subtypes that mediated GTP[S]-induced and agonist-induced [3H]PtdIns hydrolysis. These results demonstrate that this method reveals that agonist-induced, GTP[S]-dependent [3H]PtdIns hydrolysis is retained in postmortem human brain membranes with properties similar to rat brain. This method should allow studies of the modulation of phosphoinositide hydrolysis in human brain and investigations of potential alterations in postmortem brain from subjects with neurological and psychiatric diseases.

Purine- and pyrimidine-stimulated phosphoinositide breakdown and intracellular calcium mobilisation in astrocytes

Phosphoinositide breakdown in cultured cortical astrocytes was assessed by measuring the accumulation of [3H]inositol phosphates (IP's) following incubations with various purines and pyrimidines. Dose-response relationships gave the following order of potency: 2-methylthioadenosine triphosphate (2-MeSATP) > uridine 5'-triphosphate (UTP) > ATP = ADP > inosine 5' triphosphate (ITP). However, 2-MeSATP and UTP were only half as effective as either ATP or ADP in stimulating [3H]IP production. Astrocytes were also challenged with combined additions of maximally effective concentrations of agonists. Responses to ADP plus UTP and 2-MeSATP plus UTP were essentially additive whilst ATP plus UTP evoked a response which was only partially additive. ATP-stimulated [3H]IP accumulation was markedly reduced in the presence of 2-MeSATP suggesting that the latter may be a partial agonist at these receptors. We also examined the ability of ATP and UTP to increase intracellular Ca2+ concentrations in these cells. Greater than 90% of all cells tested responded to ATP with a release from internal Ca2+ stores but less than half of these responded similarly when challenged with UTP. Our results indicate that astrocytes possess both P2Y-purinoceptors and a population of receptors which are also coupled to phosphoinositide metabolism and intracellular Ca2+ mobilisation but recognise ATP and the pyrimidine nucleotide UTP.

Interactions between inositol phosphates and cytosolic free calcium following bradykinin stimulation in cultured human skin fibroblasts

The inositol triphosphate (IP3) that results from hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) is generally accepted to be responsible for the mobilization of intracellular calcium. However, some studies suggest that low concentrations of agonists elevate cytosolic free calcium concentration ([Ca2+]i) without IP3 formation. Thus, in the present studies, a comparison of the temporal response of inositol phosphates (IP3, IP2 and IP) and [Ca2+]i to a wide range of bradykinin concentrations was used to examine the relation of these two signal transduction events in cultured human skin fibroblasts (GM3652). In addition, the effects of alterations in internal or external calcium on the response of these second messengers to bradykinin were determined. Bradykinin stimulated accumulation of inositol phosphates and a rise of [Ca2+]i in a time- and dose-dependent manner. Decreasing the bradykinin concentration from 1 microM to 0.1 microM increased the time until the IP3 peak, and when the bradykinin concentration was reduced to 0.01 microM IP3 was not detected. [Ca2+]i was examined under parallel conditions. As the bradykinin concentration was reduced from 1 microM to 0.01 microM, the time to reach the peak of [Ca2+]i increased progressively, but the magnitude of the peak was unaltered. These two second messengers were variably dependent on external calcium. Although the bradykinin-stimulated initial spike of [Ca2+]i did not depend on extracellular calcium, the subsequent sustained levels of [Ca2+]i were abolished in calcium free medium. The bradykinin-stimulated inositol phosphate formation was not dependent on the extracellular calcium nor on the elevation of [Ca2+]i that was produced with Br-A23187. These results demonstrate that bradykinin-induced IP3 formation can be independent of [Ca2+]i and of external calcium, whereas changes in [Ca2+]i are partially dependent on external calcium.

Excitatory and inhibitory effects of adenosine on the neurotransmission in the hippocampal slices of guinea pig

Postsynaptic potentials (PSPs) were recorded from the granular cell layer of guinea pig hippocampal slices. Application of adenosine at a concentration of more than 10 microM in the incubation medium depressed the amplitude of the PSP, but adenosine at a concentration of 10 nM to 1 microM enhanced the amplitude of the PSP. The dose-response curve of the effect of adenosine showed an excitatory and inhibitory biphasic pattern. Adenosine analogues such as adenosine triphosphate (ATP), adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide (NAD) showed similar biphasic effects, however beta-gamma-methyleneadenosinetriphosphate (beta-gamma-ATP), 5'-N-ethylcarboxamidoadenosine (NECA). N6-cyclohexyladenosine (CHA) and N6-R-phenylisopropyladenosine (RPIA) revealed only inhibitory effects.

Ca2(+)-mobilizing receptors of gastrulating chick embryo

1. Gastrulating chick embryo cells (stages 3-5 by HH) possess Ca2(+)-mobilizing receptors for ACh and ATP; insulin and noradrenaline have a weaker effect on [Ca2+]i mobilization. 2. The ED50 value for ACh is 4 (+/- 0.5) X 10(-6) M and for ATP 20 (+/- 5) X 10(-6) M. 3. Addition of ACh and ATP to dissociated chick embryo cells causes rapid accumulation of IP3. 4. The stimulatory effects of ACh and ATP on [Ca2+]i mobilization and IP3 rapid formation are both additive.

ATP-induced synaptic potentiation in hippocampal slices

The purpose of this study was to investigate the influence of different adenosine triphosphate (ATP) concentrations (ranging from 400 nM to 250 microM) on hippocampal potentials recorded from pyramidal neurons. ATP applied at a concentration of 400 nM induced a 100% increase in the size of the population spike (potentiation). The potential started to increase 30-60 s after ATP application, reached a maximum after 20 min, and remained potentiated for longer than 1.5 h. Washing the slices with fresh Ringer solution did not reverse the effect. ATP applied at a concentration of 50-150 microM, temporarily depressed the potential. This depression, however, was transient, as the potential gradually recovered by itself and reached a value higher than that observed before ATP application. ATP applied at the concentration of 250 microM caused a long-lasting depression of the potential. The potential was not restored by washing the slices, but recovered after addition of 0.7 microM 3,4-diaminopyridine. These data show a concentration-dependent mode of ATP action on hippocampal neurons and suggest a role for ATP in regulating synaptic efficiency.

Phosphatidylinositol metabolism during in vitro hypoxia

The effects of in vitro histotoxic hypoxia (0.5 mM KCN) on potassium-stimulated phosphatidylinositol turnover were determined. In rat cortical slices that were prelabeled with [2-3H]inositol, depolarization with 60 mM KCl increased [2-3H]inositol monophosphate and [2-3H]inositol bisphosphate accumulation in a Ca2+-dependent manner. At early times (10 s and 1 min), histotoxic hypoxia enhanced potassium-stimulated [2-3H]inositol monophosphate and inositol bisphosphate accumulation. Under basal conditions, hypoxia did not alter the accumulation of [2-3H]inositol phosphates. These results are consistent with the following hypothesis. The hypoxic-induced increase in cytosolic free calcium that we reported previously may lead to the early stimulation of inositol phosphates formation during hypoxia through activation of phospholipase C. The impairment of inositol phosphates formation during more prolonged hypoxia may be due to negative feedback regulation of the phosphatidylinositol cascade by protein kinase C or to a reduction in ATP levels.

Muscarinic-agonist and guanine nucleotide stimulation of myo-inositol trisphosphate formation in membranes isolated from bovine iris sphincter smooth muscle: effects of short-term cholinergic desensitization

The effect of short-term cholinergic desensitization on muscarinic acetylcholine receptor (mAChR)-mediated activation of phospholipase C was investigated in membranes isolated from the bovine iris sphincter smooth muscle. Membranes prepared from normal or desensitized muscles, prelabeled with either [3H]myo-inositol or 32P from [gamma-32P]ATP, were incubated with a hydrolysis-resistant analogue of GTP, GTP gamma S, or GTP gamma S plus carbachol (CCh), and the production of [3H]myo-inositol 1,4,5-trisphosphate (IP3) and the breakdown of polyphosphoinositides were assessed. In normal membranes, GTP (greater than or equal to 1 mM), GTP gamma S (greater than 10 microM) and GTP gamma S (1 microM) plus CCh (10 microM), but not GDP or GDP beta S, increased phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and IP3 production. GTP gamma S increased IP3 accumulation in a time- and dose-dependent manner, and CCh, which had no effect on phospholipase C activity in the absence of GTP gamma S, potentiated the effects of GTP gamma S. The effect of CCh plus GTP gamma S on IP3 production was inhibited by atropine, had an absolute requirement for nM amounts of Ca2+ and was not affected by pertussis toxin. At higher concentrations (greater than 1 microM), Ca2+ alone induced PIP2 hydrolysis. Short-term exposure (less than 60 min) of the muscle to CCh (100 microM) did not affect the total number (Bmax) of mAChRs nor their affinity (KD) for [3H]-N-methylscopolamine. Desensitization did, however, result in: (1) a loss of the CCh-high affinity binding state of the sphincter mAChRs in a manner analogous to that produced by GTP gamma S; (2) a loss of the ability of GTP gamma S to affect CCh binding to the receptors; and (3) an attenuation of the GTP gamma S plus CCh-stimulated PIP2 hydrolysis. In conclusion, the data presented suggest that, in the iris smooth muscle, G-proteins are involved in the coupling of mAChRs to phospholipase C and that short-term cholinergic desensitization results in (1) the uncoupling of the receptor-G-protein complex and (2) the attenuation of mAChR-activation of phospholipase C.

Effects of in vitro hypoxia on depolarization-stimulated accumulation of inositol phosphates in synaptosomes

The effects of potassium and in vitro histotoxic hypoxia (i.e. KCN) on phosphatidylinositol turnover in rat cortical synaptosomes were determined. [2-3H] Inositol prelabelled rat synaptosomes were prepared from cerebral cortex slices that had been incubated with [2-3H] inositol. Depolarization with 60 mM KCl increased [2-3H] inositol phosphates in a time dependent manner. Depolarization with 60 mM KCl increased [2-3H] inositol trisphosphate transiently at 5 s. K+ induced rapid formation of [2-3H]-inositol bisphosphate and maintained an elevated level for at least 5 min. K+ stimulated gradual formation of [2-3H] inositol monophosphate with time. One minute of hypoxia enhanced potassium-stimulated [2-3H] inositol bisphosphate formation. However, 30 min of hypoxia impaired potassium-stimulated accumulation of [2-3H] inositol phosphates. The effects of histotoxic hypoxia were all dependent upon calcium in the medium and on K+-depolarization. Thus, hypoxia altered the K+-induced accumulation of inositol phosphates in prelabelled synaptosomes in a time dependent, biphasic manner that was calcium dependent.

G-protein involvement in central-nervous-system muscarinic-receptor-coupled polyphosphoinositide hydrolysis

Potentiation of muscarinic-agonist-stimulated polyphosphoinositide (PPI) hydrolysis was demonstrated in a rat cerebral-cortical membrane preparation prelabelled with myo-[3H]inositol. Accumulation of myo-[3H]inositol 1,4-bisphosphate ([3H]IP2) was used to assess brain [3H]phosphatidylinositol 4,5-bisphosphate hydrolysis as its immediate metabolite, myo-[3H]inositol 1,4,5-trisphosphate, was rapidly hydrolysed to [3H]IP2. Inclusion of ATP (100 microM) and Mg2+ (5 mM) in the assay medium was necessary to demonstrate the effect of GTP analogues on carbachol-stimulated brain [3H]PPI turnover. Carbachol (100 microM) induced only a small increment in [3H]IP2 accumulation (142% of control) in 1 min. However, its effect was markedly enhanced, to 800% and 300% of control, by 100 microM-guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) respectively. GTP[S] and p[NH]ppG also stimulated [3H]IP2 accumulation by over 500% and 200% of control, respectively. The GTP-analogue-potentiated carbachol effect was antagonized by 10 microM-atropine, whereas the GTP-analogue stimulation was unaffected. This report confirms the involvement of a G (GTP-binding) protein(s) in brain PPI metabolism and provides new evidence for the role of G protein(s) in the coupling of stimulated muscarinic receptors to PPI hydrolysis in the central nervous system.