Release of choline from rat brain under hypoxia: contribution from phospholipase A2 but not from phospholipase D

Utility of Magnetic Resonance Spectroscopy for the Progression of Neurological Symptoms in Lenticulostriate Artery Territory Infarction

OBJECTIVES

The present study aimed to examine the effectiveness of proton magnetic resonance spectroscopy (1HMRS) in determining the progression of neurological symptoms resulting in acute ischemic stroke in patients with lenticulostriate artery (LSA) infarction.

MATERIALS AND METHODS

1HMRS was performed within 72 h after neurological symptom onset. Voxel of interest was placed in tissue that included the pyramidal tract and identified diffusion weighted echo planar spin-echo sequence (DWI) coronal images. Infarct volume in DWI was calculated using the ABC/2 method. 1HMRS data (tNAA, tCr, Glx, tCho, and Ins) were analyzed using LCModel. Progressive neurological symptoms were defined as an increase of 1 or more in the NIHSS score. Patients who underwent 1HMRS after progressive neurological symptoms were excluded.

RESULTS

In total, 77 patients were enrolled. Of these, 19 patients had progressive neurological symptoms. The patients with progressive neurological symptoms were significantly more likely to be female and had higher tCho/tCr values, higher rates of axial slices ≥ 3 slices on DWI, higher infarct volume on DWI, higher maximum diameter of infarction of axial slice on DWI, and higher SBP on admission compared to those without. Multivariable logistic analysis revealed that higher tCho/tCr values were independently associated with progressive neurological symptoms after adjusting for age, sex, and initial DWI infarct volume (tCho/tCr per 0.01 increase, OR 1.26, 95% CI 1.03-1.52, P = 0.022).

CONCLUSIONS

Increased tCho/tCr score were associated with progressive neurological symptoms in patients with LSA ischemic stroke. Quantitative evaluation of 1HMRS parameters may be useful for predicting the progression of neurological symptoms.

Role of Phospholipase D-Derived Phosphatidic Acid in Regulated Exocytosis and Neurological Disease

Lipids play a vital role in numerous cellular functions starting from a structural role as major constituents of membranes to acting as signaling intracellular or extracellular entities. Accordingly, it has been known for decades that lipids, especially those coming from diet, are important to maintain normal physiological functions and good health. On the other side, the exact molecular nature of these beneficial or deleterious lipids, as well as their precise mode of action, is only starting to be unraveled. This recent improvement in our knowledge is largely resulting from novel pharmacological, molecular, cellular, and genetic tools to study lipids in vitro and in vivo. Among these important lipids, phosphatidic acid plays a unique and central role in a great variety of cellular functions. This review will focus on the proposed functions of phosphatidic acid generated by phospholipase D in the last steps of regulated exocytosis with a specific emphasis on hormonal and neurotransmitter release and its potential impact on different neurological diseases.

Functional expression of choline transporter like-protein 1 (CTL1) and CTL2 in human brain microvascular endothelial cells

In this study, we examined the molecular and functional characterization of choline transporter in human brain microvascular endothelial cells (hBMECs). Choline uptake into hBMECs was a saturable process that was mediated by a Na(+)-independent, membrane potential and pH-dependent transport system. The cells have two different [(3)H]choline transport systems with Km values of 35.0 ± 4.9 μM and 54.1 ± 8.1 μM, respectively. Choline uptake was inhibited by choline, acetylcholine (ACh) and the choline analog hemicholinium-3 (HC-3). Various organic cations also interacted with the choline transport system. Choline transporter-like protein 1 (CTL1) and CTL2 mRNA were highly expressed, while mRNA for high-affinity choline transporter 1 (CHT1) and organic cation transporters (OCTs) were not expressed in hBMECs. CTL1 and CTL2 proteins were localized to brain microvascular endothelial cells in human brain cortical sections. Both CTL1 and CTL2 proteins were expressed on the plasma membrane and mitochondria. CTL1 and CTL2 proteins are mainly expressed in plasma membrane and mitochondria, respectively. We conclude that choline is mainly transported via an intermediate-affinity choline transport system, CTL1 and CTL2, in hBMECs. These transporters are responsible for the uptake of extracellular choline and organic cations. CTL2 participate in choline transport mainly in mitochondria, and may be the major site for the control of choline oxidation.

A positive allosteric modulator of α7 nAChRs augments neuroprotective effects of endogenous nicotinic agonists in cerebral ischaemia

BACKGROUND AND PURPOSE

Activation of α7 nicotinic acetylcholine receptors (nAChRs) can be neuroprotective. However, endogenous choline and ACh have not been regarded as potent neuroprotective agents because physiological levels of choline/ACh do not produce neuroprotective levels of α7 activation. This limitation may be overcome by the use of type-II positive allosteric modulators (PAMs-II) of α7 nAChRs, such as 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea (PNU-120596). This proof-of-concept study presents a novel neuroprotective paradigm that converts endogenous choline/ACh into potent neuroprotective agents in cerebral ischaemia by inhibiting α7 nAChR desensitization using PNU-120596.

EXPERIMENTAL APPROACH

An electrophysiological ex vivo cell injury assay (to quantify the susceptibility of hippocampal neurons to acute injury by complete oxygen and glucose deprivation; COGD) and an in vivo middle cerebral artery occlusion model of ischaemia were used in rats.

KEY RESULTS

Choline (20-200 μM) in the presence, but not absence of 1 μM PNU-120596 significantly delayed anoxic depolarization/injury of hippocampal CA1 pyramidal neurons, but not CA1 stratum radiatum interneurons, subjected to COGD in acute hippocampal slices and these effects were blocked by 20 nM methyllycaconitine, a selective α7 antagonist, thus, activation of α7 nAChRs was required. PNU-120596 alone was ineffective ex vivo. In in vivo experiments, both pre- and post-ischaemia treatments with PNU-120596 (30 mg·kg(-1) , s.c. and 1 mg·kg(-1) , i.v., respectively) significantly reduced the cortical/subcortical infarct volume caused by transient focal cerebral ischaemia. PNU-120596 (1 mg·kg(-1) , i.v., 30 min post-ischaemia) remained neuroprotective in rats subjected to a choline-deficient diet for 14 days prior to experiments.

CONCLUSIONS AND IMPLICATIONS

PNU-120596 and possibly other PAMs-II significantly improved neuronal survival in cerebral ischaemia by augmenting neuroprotective effects of endogenous choline/ACh.

Serial plasma choline measurements after cardiac arrest in patients undergoing mild therapeutic hypothermia: a prospective observational pilot trial

OBJECTIVE

Choline is related to phospholipid metabolism and is a marker for global ischaemia with a small reference range in healthy volunteers. The aim of our study was to characterize the early kinetics of plasma free choline in patients after cardiac arrest. Additionally, we investigated the potential of plasma free choline to predict neurological outcome.

METHODS

Twenty patients admitted to our medical intensive care unit were included in this prospective, observational trial. All patients were enrolled between May 2010 and May 2011. They received post cardiac arrest treatment including mild therapeutic hypothermia which was initiated with a combination of cold fluid and a feedback surface cooling device according to current guidelines. Sixteen blood samples per patient were analysed for plasma free choline levels within the first week after resuscitation. Choline was detected by liquid chromatography-tandem mass spectrometry.

RESULTS

Most patients showed elevated choline levels on admission (median 14.8 µmol/L; interquartile range; IQR 9.9-20.1) which subsequently decreased. 48 hours after cardiac arrest choline levels in all patients reached subnormal levels at a median of 4.0 µmol/L (IQR 3-4.9; p = 0.001). Subsequently, choline levels normalized within seven days. There was no significant difference in choline levels when groups were analyzed in relation to neurological outcome.

CONCLUSIONS

Our data indicate a choline deficiency in the early postresucitation phase. This could potentially result in impaired cell membrane recovery. The detailed characterization of the early choline time course may aid in planning of choline supplementation trials. In a limited number of patients, choline was not promising as a biomarker for outcome prediction.

A type-II positive allosteric modulator of α7 nAChRs reduces brain injury and improves neurological function after focal cerebral ischemia in rats

In the absence of clinically-efficacious therapies for ischemic stroke there is a critical need for development of new therapeutic concepts and approaches for prevention of brain injury secondary to cerebral ischemia. This study tests the hypothesis that administration of PNU-120596, a type-II positive allosteric modulator (PAM-II) of α7 nicotinic acetylcholine receptors (nAChRs), as long as 6 hours after the onset of focal cerebral ischemia significantly reduces brain injury and neurological deficits in an animal model of ischemic stroke. Focal cerebral ischemia was induced by a transient (90 min) middle cerebral artery occlusion (MCAO). Animals were then subdivided into two groups and injected intravenously (i.v.) 6 hours post-MCAO with either 1 mg/kg PNU-120596 (treated group) or vehicle only (untreated group). Measurements of cerebral infarct volumes and neurological behavioral tests were performed 24 hrs post-MCAO. PNU-120596 significantly reduced cerebral infarct volume and improved neurological function as evidenced by the results of Bederson, rolling cylinder and ladder rung walking tests. These results forecast a high therapeutic potential for PAMs-II as effective recruiters and activators of endogenous α7 nAChR-dependent cholinergic pathways to reduce brain injury and improve neurological function after cerebral ischemic stroke.

Acetylcholine precursor choline evokes NMDA-dependent epileptoid activity in rat hippocampal CA1 area

Application of choline (5 and 10 mM) to electrically stimulated (1 Hz) rat hippocampal slices evoked epileptoid activity manifested by generation of extra population spikes. Application of methyllycaconitine (10 nM), a specific agonist for α7-subunit of nicotinic acetylcholine receptors, did not prevent generation of extra population spikes. In contrast, pretreatment of slices with Mg(2+) (5 mM) or blockade of NMDA-type glutamate receptors with MK-801 (100 μM) prevented generation of the extra population spikes. It was hypothesized that elevation of choline concentration during cerebral pathology can promote activation of NMDA-receptors and provoke epileptoid activity not related to activation of α7-subunit of nicotinic acetylcholine receptor.

Subtype-specific inhibition of nicotinic acetylcholine receptors by choline: a regulatory pathway

Choline is an essential nutrient and a precursor of neurotransmitter acetylcholine (ACh) and is produced at synapses during depolarization, upon hydrolysis of ACh via acetylcholinesterase, and under conditions of injury and trauma. Animal studies have shown that supplementation with choline during early development results in long-lasting improvement in memory in adults; however, the mechanisms underlying this effect are poorly defined. Previous studies revealed that choline interacts with type IA (alpha7*) nicotinic acetylcholine receptors (nAChRs) as a full agonist and as a desensitizing agent and is a weak agonist of type III (alpha3beta4*) nAChRs. Because nAChRs play a role in learning and memory and are generally inhibited by agonists at low concentrations, we investigated in this study the inhibitory effects of choline on non-alpha7 nAChRs such as type II (alpha4beta2*) and type III nAChRs. Using whole-cell patch-clamp recordings from neurons of rat hippocampal and dorsal striatal slices, we demonstrate that choline inhibited type III nAChR-mediated glutamate excitatory postsynaptic currents (EPSCs). Choline inhibited ACh-induced N-methyl-D-aspartate (NMDA) EPSCs in CA1 stratum radiatum (SR) interneurons of rat hippocampal slices with an IC50 of approximately 15 microM. Choline did not inhibit NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in CA1 SR interneurons. Choline inhibited type II nAChRs in CA1 SR interneurons with an IC50 of approximately 370 microM. The present results reveal an order of inhibitory potency for choline type III>type IA>type II nAChRs. It is concluded that brain nAChRs, but not glutamate receptors, are the primary targets for the regulatory actions of choline.

Functions and pathophysiological roles of phospholipase D in the brain

Ten years after the isoforms of mammalian phospholipase D (PLD), PLD1 and 2, were cloned, their roles in the brain remain speculative but several lines of evidence now implicate these enzymes in basic cell functions such as vesicular trafficking as well as in brain development. Many mitogenic factors, including neurotransmitters and growth factors, activate PLD in neurons and astrocytes. Activation of PLD downstream of protein kinase C seems to be a required step for astroglial proliferation. The characteristic disruption of the PLD signaling pathway by ethanol probably contributes to the delay of brain growth in fetal alcohol syndrome. The post-natal increase of PLD activities concurs with synapto- and myelinogenesis in the brain and PLD is apparently involved in neurite formation. In the adult and aging brain, PLD activity has antiapoptotic properties suppressing ceramide formation. Increased PLD activities in acute and chronic neurodegeneration as well as in inflammatory processes are evidently due to astrogliosis and may be associated with protective responses of tissue repair and remodeling. ARF-regulated PLD participates in receptor endocytosis as well as in exocytosis of neurotransmitters where PLD seems to favor vesicle fusion by modifications of the shape and charge of lipid membranes. Finally, PLD activities contribute free choline for the synthesis of acetylcholine in the brain. Novel tools such as RNA interference should help to further elucidate the roles of PLD isoforms in brain physiology and pathology.

Release of choline in the isolated heart, an indicator of ischemic phospholipid degradation and its protection by ischemic preconditioning: No evidence for a role of phospholipase D

The release of choline as a water-soluble product of phospholipid hydrolysis was measured in the perfusate of rat hearts to monitor ischemic membrane degradation and its protection by ischemic preconditioning (IPC). Hearts were subjected to global ischemia (GI; 30 min of no-flow) followed by 60 min of reperfusion. To induce IPC, GI was preceded by four no-flow episodes of 5 min each. Deleterious consequences of GI and reperfusion, namely coronary flow reduction, incidence of arrhythmias and release of cardiac troponin T, were significantly attenuated by IPC. The release of choline increased during reperfusion in a biphasic manner: a first phase peaked immediately after GI and was followed by a second, delayed phase indicating choline release caused during reperfusion. Only the second phase was blocked by both IPC and by AACOCF3 (5 microM), an inhibitor of cytosolic phospholipase A2. The activity of phospholipase D (PLD) was unchanged after GI or IPC or GI plus IPC. In conclusion, choline release into heart perfusate was found to be a useful real-time indicator of phospholipid degradation caused by GI and by reperfusion and its protection by IPC. The results supplement previous observations on the accumulation of fatty acids in the phospholipid pool. There was no evidence for PLD activation by GI or IPC.

Intestinal ischaemia during cardiac arrest and resuscitation: comparative analysis of extracellular metabolites by microdialysis

Intestinal ischaemia is a major complication of shock syndromes causing translocation of bacteria and endotoxins and multiple organ failure in intensive care patients. The present study was designed to use microdialysis as a tool to monitor intestinal ischaemia after cardiac arrest and resuscitation in pigs. For this purpose, microdialysis probes were implanted in pig jejunal wall, peritoneum, skeletal muscle and brain, and interstitial fluid was obtained during circulatory arrest (induced by ventricular fibrillation) and after return of spontaneous circulation (ROSC). Cardiac arrest for 4 min caused a prolonged (60 min) reduction of blood flow in jejunal wall, muscle and brain as determined by the ethanol technique. This was accompanied by cellular damage in heart muscle and brain as indicated by increased levels of troponin-I and protein S-100, respectively. Plasma levels of glucose, lactate and choline were increased at 15-60 min following cardiac arrest. In contrast, cardiac arrest induced a rapid but variable decrease of interstitial glucose levels in all monitored organs; this decrease was followed by an increase over baseline during reperfusion. In the intestine, lactate, glutamate and choline levels were increased during ischaemia and reperfusion for 60-120 min; intestinal and peritoneal samples yielded parallel changes of lactate levels. Brain and muscle samples showed similar changes as in intestinum and peritoneum except for glutamate, which was increased in brain but not in muscle. We conclude that intestinal ischaemia occurs as a consequence of cardiac arrest and resuscitation and can be monitored by in vivo microdialysis. Comparative analysis by multi-site microdialysis reveals that the intestine is equally or even more sensitive to ischaemia than brain or muscle.

Putative neuroprotective actions of N-acyl-ethanolamines

N-Acyl-ethanolamines (NAEs) and their precursors, N-acyl-ethanolamine phospholipids (NAPEs), are present in the mammalian brain at levels of a few hundred picomoles/gram tissue and a few nanomoles/gram tissue, respectively. NAE-containing arachidonic acid is called anandamide, and it has attracted particular attention since it is a partial agonist for the cannabinoid receptors, for which 2-arachidonoylglycerol is the full agonist. In addition, anandamide may also activate the vanilloid receptor. Anandamide usually amounts to 1-10% of NAEs, as the vast majority of N-acyl groups are saturated and monounsaturated fatty acids. Formation of NAPE and NAE is catalyzed by an N-acyltransferase and an NAPE-hydrolyzing phospholipase D, respectively, two enzymes that have been characterized only preliminary. Interestingly, NAPEs and NAEs accumulate in the brain in response to neurodegenerative insults at a time when other phospholipids are subjected to rapid degradation. This is an important biosynthetic aspect of NAPE and NAE, as NAEs may be neuroprotective by a number of different mechanisms involving both receptor activation and non-receptor-mediated effects, e.g. by binding to cannabinoid receptors and interfering with ceramide turnover, respectively.

Acceleration of phosphatidylcholine synthesis and breakdown by inhibitors of mitochondrial function in neuronal cells: a model of the membrane defect of Alzheimer's disease

Brain cells in Alzheimer's disease (AD) exhibit a membrane defect characterized by accelerated phospholipid turnover. The mechanism responsible for this defect remains unknown. Recent studies indicate that impairment of mitochondrial function is frequently observed in AD and may be responsible for certain aspects of its pathophysiology. We show that when PC12 cells are exposed to inhibitors of mitochondrial bioenergetics, the turnover of their major membrane phospholipid, phosphatidylcholine, is accelerated, producing a pattern of metabolic changes that mimics that observed in brains of AD patients. Abnormalities of mitochondrial function may therefore underlie the membrane defect in AD.

Role of phospholipase A(2) on the variations of the choline signal intensity observed by 1H magnetic resonance spectroscopy in brain diseases

Phospholipase A(2) catalyzes the hydrolysis of membrane glycerophospholipids leading to the production of metabolites observable by both 1H and 31P magnetic resonance spectroscopy. The signal of choline-containing compounds (Cho) observed by 1H magnetic resonance spectroscopy is constituted of metabolites of phosphatidylcholine, especially phosphocholine (PCho) and glycerophosphocholine (GPCho). The phosphomonoester (PME) and phosphodiester (PDE) signals observed by 31P magnetic resonance spectroscopy are, respectively, precursors and catabolites of phospholipids. A large number of brain diseases have been reported to cause variations in the intensity of the Cho, PME and PDE signals. Changes in the activity of phospholipase A(2) have been measured in many brain diseases. In this review, the relationships between the results of 1H and 31P magnetic resonance spectroscopy and the phospholipase A(2) assays are analyzed. In many brain diseases, the variation in the Cho signal intensity can be correlated with a stimulation or inhibition of the phospholipase A(2) activity.

Region-specific and calcium-dependent increase in dialysate choline levels by NMDA

NMDA receptor-induced excitotoxicity has been hypothesized to mediate abnormal choline (Cho) metabolism that is involved in alterations in membrane permeability and cell death in certain neurodegenerative disorders. To determine whether NMDA receptor overactivation modulates choline metabolism in vivo, we investigated the effects of NMDA on interstitial choline concentrations using microdialysis. Perfusion of NMDA by retrodialysis increased dialysate choline (approximately 400%) and reduced dialysate acetylcholine (Ach) (approximately 40%). Choline levels remained increased for at least 2.5 hr, but acetylcholine returned to pretreatment values 75 min after NMDA perfusion. The NMDA-evoked increase in dialysate choline was calcium and concentration dependent and was prevented with 1 mM AP-5, a competitive NMDA antagonist, but was not altered by mepacrine, a phospholipase A2 inhibitor. NMDA increased extracellular choline levels four- to fivefold in prefrontal cortex and hippocampus, produced a slight increase in neostriatum, and did not modify dialysate choline in cerebellum. Perfusion with NMDA for 2 hr produced a delayed, but not acute, reduction in choline acetyltransferase activity in the area surrounding the dialysis probe. Consistent with a lack of acute cholinergic neurotoxicity evoked by this treatment, basal acetylcholine levels were unaltered by 2 hr of continuous NMDA perfusion. Prolonged NMDA perfusion produced a 34% decrease in phosphatidylcholine content in the lipid fraction of the tissue surrounding the dialysis probe. These results show that NMDA modulates choline metabolism, eliciting a receptor-mediated, calcium-dependent, and region-specific increase in extracellular choline from membrane phospholipids that is not mediated by phospholipase A2 and precedes delayed excitotoxic neuronal cell death.

Brain choline has a typical precursor profile

Choline is product and precursor to both acetylcholine and membrane phospholipids, and, in the brain, is ultimately provided by the circulation. The brain is protected from excess choline and choline deprivation by a refined system of homeostatic mechanisms that maintain a level of extracellular choline that, for its role as precursor, meets saturation criteria under normal conditions. The kinetic and activity profiles of choline are typical for a biosynthetic precursor.

Regulation of free choline in rat brain: dietary and pharmacological manipulations

The present study analyses, comparatively, the kinetics of free choline in the brain of rats during dietary and pharmacological manipulations. Low-choline diet halved the choline plasma level but did not cause significant changes of CSF choline. High-choline diet, hypoxia and treatment with nicotinamide increased brain choline availability through a central site of action and increased the CSF choline concentration. CSF choline concentrations were more effectively elevated by nicotinamide treatment (20-25 microM) than by acute choline administration (13-15 microM). Increases of CSF choline, due to brain choline mobilization, were consistently associated with a net release of choline from the brain as reflected by strongly negative arterio-venous differences (AVD) of brain choline. The balance between release and uptake of brain choline was controlled by the arterial plasma choline level in all treatment groups; however, the normal 'reversal point' of 15 microM--representing the plasma choline level where uptake and release of brain choline are balanced--was shifted to more than 40 microM by high-choline diet and nicotinamide. In conclusion, our data characterize the release of choline into the venous blood as an important component of brain choline homeostasis. Furthermore, we demonstrate that the concentration of brain choline (e.g. as a precursor of acetylcholine) can be enhanced more efficiently by manipulating choline homeostatic mechanisms than by acute choline administration.

Phospholipid breakdown and choline release under hypoxic conditions: inhibition by bilobalide, a constituent of Ginkgo biloba

A marked increase of choline release from rat hippocampal slices was observed when the slices were superfused with oxygen-free buffer, indicating hypoxia-induced hydrolysis of choline-containing phospholipids. This increase of choline release was suppressed by bilobalide, an ingredient of Ginkgo biloba, but not by a mixture of ginkgolides. The EC50 value for bilobalide was 0.38 microM. In ex vivo experiments, bilobalide also inhibited hypoxia-induced choline release when given p.o. in doses of 2-20 mg/kg 1 h prior to slice preparation. The half-maximum effect was observed with 6 mg/kg bilobalide. A similar effect was noted after p.o. administration of 200 mg/kg EGb 761, a ginkgo extract containing approximately 3% of bilobalide. We conclude that ginkgo extracts can suppress hypoxia-induced membrane breakdown in the brain, and that bilobalide is the active constituent for this effect.

Biosynthesis of prostaglandin D2 by motoneurons and dorsal horn microneurons: a biochemical and high resolution immunocytochemical study in chick spinal cord

Prostaglandin D2 is one of the major prostanoids formed from [14C]arachidonic acid by the central nervous system. The aim of the present study is to specify the prostaglandin D2 biosynthetic capacity in the chick spinal cord and to identify the cell type involved in this synthesis. A highly specific and sensitive enzyme immunoassay allowed us to demonstrate that the amount of newly formed prostaglandin D2 increases proportionally with the concentration of free arachidonic acid of either exogenous or endogenous origin and reaches concentration values ranging from 10(-9) to 10(-6) M. The sites of prostaglandin D2 synthesis were localized in Vibratome sections of spinal cord after incubation with antibodies raised against glutathione-independent prostaglandin D synthase; controls were performed with anti-glutathione-dependent prostaglandin D synthase antibodies and non-immune rabbit or goat serum. After immunoprocessing, electron microscope examination revealed that the specific immunoreactivity was confined to small neurons of laminae II and III in the dorsal horn and to motoneurons in the ventral horn of the spinal cord. The immunodeposits were associated with rough endoplasmic reticulum profiles distributed throughout the dorsal horn neurons or restricted to limited subsurface areas of perikarya and dendrites in motoneurons. Since the immunoreactive neurons in the dorsal horn were closely related to blood capillaries, prostaglandin D2 may be suspected to play a role in the regulation of the microcirculation. The accumulation of prostaglandin D synthase in motoneuron areas facing astrocytic membrane stacks suggests that prostaglandin D2 could interact with astrocytic functions.