Effect of phosphomonoesters, phosphodiesters, and phosphocreatine on glutamate uptake by synaptic vesicles

L-Glutamate, a major excitatory amino acid, plays an important role in learning and memory. L-Glutamate uptake into synaptic vesicles is an ATP-dependent process. Exposure of neurons to high, sustained extracellular concentrations of glutamate results in excitotoxicity. Elevated levels of phosphomonoesters (PMEs), phosphodiesters (PDEs), and phosphocreatine (PCr) have been reported in Alzheimer disease (AD). In this article, the effects of selected PMEs, PDEs, and PCr on vesicular L-[3H]glutamate uptake into isolated bovine synaptic vesicles are investigated. D-myo-Inositol-1-monophosphate (I1P), D-myo-inositol-2-monophosphate (I2P), sn-glycero-3-phosphate, (alpha-GP) and PCr significantly stimulated L-[3H]glutamate uptake into synaptic vesicles. Phosphoethanolamine (PE), phosphocholine (PC), L-phosphoserine (L-PS) sn-glycero-3-phosphocholine (GPC), and sn-glycero-3-phosphoethanolamine (GPE) had little or no effect on vesicular L-glutamate uptake. These observations suggested that the vesicular uptake of glutamate can be regulated by endogenous PMEs and PCr. The mechanism of activation by I1P, I2P, and alpha-GP appears to be stimulation of Mg(2+)-ATPase activity. These effects on vesicular glutamate uptake may be important in diseases in which the levels of these metabolites are altered, as they are in AD.

Aggregation of beta-amyloid peptide is promoted by membrane phospholipid metabolites elevated in Alzheimer's disease brain

Increased amounts of beta-amyloid (A beta) peptide deposits are found in Alzheimer's disease brain. These amyloid deposits have been implicated in the pathophysiology of this common dementing illness. A beta peptides have been shown to be toxic to neurons in cell culture, and this toxicity is critically dependent on the aggregation of the peptide into cross-beta-pleated sheet fibrils. Also, in vivo and postmortem NMR studies have shown changes in certain brain membrane phospholipid metabolites in normal aging and more extensive alterations in patients with Alzheimer's disease. The finding that membrane phospholipids affect the aggregation of A beta suggests that the abnormalities in membrane metabolism found in Alzheimer's disease could affect the deposition of A beta in vivo. Therefore, we examined the effect of membrane phospholipid metabolites that are altered in Alzheimer's disease brain on the aggregation of A beta(1-40) using a light scattering method. Certain metabolites (glycerophosphocholine, glycerophosphoethanolamine, and alpha-glycerophosphate) augment the aggregation of A beta. Other membrane phospholipid metabolites (phosphocholine, phosphoethanolamine, and inositol-1-phosphate) have no effect. We conclude that increased membrane phospholipid metabolite concentrations may play a role in the deposition of A beta seen in normal aging and the even greater deposition of A beta observed in Alzheimer's disease.

Elevated phosphocholine and phosphatidylcholine following rat entorhinal cortex lesions

At early stages of Alzheimer's disease, phosphomonoesters (PMEs) including phosphocholine (P-choline) are present at elevated levels. PMEs also are elevated in the developing brain during the period of neurite extension. To determine if the elevation of PMEs in AD could reflect neuritic sprouting, 31P-NMR was used to examine phospholipid metabolites and membrane phospholipids at various times following unilateral lesions of the entorhinal cortex, a well-defined model of neuritic sprouting. Two to 7 days postlesion, P-choline levels were elevated 48% in the hippocampus ipsilateral to the entorhinal cortex lesion, but not in the contralateral hippocampus or cerebral cortex. P-choline levels declined by day 15, and reached control levels 45 days following the lesion. The lesion-induced elevation in P-choline could result from increased P-choline synthesis via choline kinase, decreased activity of CTP:phosphocholine cytidylyltransferase, or breakdown of phosphatidylcholine (PC). To distinguish between these possibilities, the membrane phospholipids PC and phosphatidylethanolamine (PE) were measured. Both phospholipids were maintained at or above control levels at each of the postlesion time points, arguing against membrane breakdown or decreased PC synthesis contributing to the elevation of P-choline levels. Other alterations included a widespread elevation in inositol phosphate 2 days postlesion, but not at later time points. The alterations in phospholipid metabolites observed in the rat hippocampus following entorhinal cortex lesions closely resemble those observed in the human brain in the early stages of AD.

Phospholipid metabolism in Alzheimer's disease and in a human cholinergic cell

There is evidence available suggesting that membrane alterations occur in Alzheimer's disease including the metabolism of membrane phospholipids. We have quantitated in vitro the phospholipase D activity of homogenates from Alzheimer's disease brain tissue. There was a significant increase of this enzyme activity as compared to controls. Amyloid beta protein is the predominant protein of the characteristic senile plaques found in Alzheimer's disease. Treatment of LA-N-2 cells, a human cholinergic neuroblastoma clone, with amyloid beta protein results in an activation of phospholipases A, C and D.

A 1-year multicenter placebo-controlled study of acetyl-L-carnitine in patients with Alzheimer's disease

A 1-year, double-blind, placebo-controlled, randomized, parallel-group study compared the efficacy and safety of acetyl-L-carnitine hydrochloride (ALCAR) with placebo in patients with probable Alzheimer's disease (AD). Subjects with mild to moderate probable AD, aged 50 or older, were treated with 3 g/day of ALCAR or placebo (1 g tid) for 12 months. Four hundred thirty-one patients entered the study, and 83% completed 1 year of treatment. The Alzheimer's Disease Assessment Scale cognitive component and the Clinical Dementia Rating Scale were the primary outcome measures. Overall, both ALCAR- and placebo-treated patients declined at the same rate on all primary and most secondary measures during the trial. In a subanalysis by age that compared early-onset patients (aged 65 years or younger at study entry) with late-onset patients (older than 66 at study entry), we found a trend for early-onset patients on ALCAR to decline more slowly than early-onset AD patients on placebo on both primary endpoints. In addition, early-onset patients tended to decline more rapidly than older patients in the placebo groups. Conversely, late-onset AD patients on ALCAR tended to progress more rapidly than similarly treated early-onset patients. The drug was very well tolerated during the trial. The study suggests that a subgroup of AD patients aged 65 or younger may benefit from treatment with ALCAR whereas older individuals might do more poorly. However, these preliminary findings are based on past hoc analyses. A prospective trial of ALCAR in younger patients is underway to test the hypothesis that young, rapidly progressing subjects will benefit from ALCAR treatment.

Phosphocreatine-dependent glutamate uptake by synaptic vesicles. A comparison with atp-dependent glutamate uptake

ATP-dependent uptake of glutamate into synaptic vesicles has been well documented. Stimulation of glutamate uptake into synaptic vesicles by other high-energy phosphates has not been described. In this paper, we examine the stimulation of phosphocreatine (PCr)-induced glutamate uptake and determine whether this stimulation is secondary to conversion of PCr to ATP. We found the following. 1) PCr stimulates glutamate uptake into synaptic vesicles in the absence of added ATP. 2) At a glutamate concentration of 50 microM, no concentration of added ATP could produce the degree of stimulation seen in the presence of PCr. 3) 0.5 mM iodoacetamide completely inhibits synaptic vesicle creatine kinase activity but does not inhibit PCr-stimulated glutamate uptake. 4) PCr-dependent glutamate uptake, unlike ATP-dependent uptake, is not magnesium- or chloride-dependent. 5) 0.5 mM N-ethylmaleimide, a selective H+-ATPase inhibitor, completely inhibits ATP-dependent glutamate uptake but only slightly inhibits PCr-dependent glutamate uptake. 6) PCr-dependent glutamate uptake is sensitive to valinomycin, a K+/H+ translocator, whereas the ATP-dependent uptake is not. Therefore, it appears that in addition to the well-known ATP-dependent glutamate uptake system, there is a previously unreported PCr-dependent glutamate uptake system in synaptic vesicles. The total glutamate uptake by synaptic vesicles is likely the sum of both ATP- and PCr-dependent glutamate uptake.

Quantitative 1H and 31P MRS of PCA extracts of postmortem Alzheimer's disease brain

Several previous studies have shown metabolic abnormalities in perchloric acid extracts of postmortem Alzheimer's disease (AD) brain by both proton (1H) and phosphorus-31 (31P) magnetic resonance spectroscopy (MRS). In all of these studies the results were expressed in relative terms, in units of mol percent. The results of this study, expressed in the absolute units of mumol/g wet weight, verify the previous 1H and 31P MRS studies. Absolute increases were found for myo-inositol, aspartate, L-glutamate, alanine, phosphocholine, and the phosphodiesters,. Absolute decreases were found for phosphoethanolamine and N-acetyl-l-aspartate. Many of these changes also were observed in non-AD dementia brain extracts, but changes in myo-inositol, inositol-l-phosphate, aspartate, and L-glutamate appeared to be more specific for AD in extracts of many brain areas. These results suggest that compounds related to membrane degradation and excitatory neuro-transmission increase in Alzheimer's disease while compounds related to neuronal integrity and inhibitory neurotransmission are decreased.

Molecular membrane interactions of a phospholipid metabolite. Implications for Alzheimer's disease pathophysiology

Alzheimer's disease is characterized by changes in phospholipid metabolism leading to a perturbation in the levels of phosphomonoesters, including L-Phosphoserine (L-PS). These early changes in lipid metabolism may result in a defect in membrane bilayer structure, leading to increased rates of beta-amyloid formation. To investigate the effect of L-PS on membrane lipid bilayers, small angle x-ray diffraction and high resolution differential scanning calorimetry (DSC) approaches were used with liposomes composed of lecithin and cholesterol. A one-dimensional electron density profile of a control dimyristoyl phosphatidylcholine (DMPC)/cholesterol lipid bilayer with a unit cell dimension of 52 A at 37 degrees C was generated from the x-ray diffraction data. Following incubation with 2.0 mM L-PS, a broad decrease in electron density +/- 4.12A from the lipid bilayer center was observed concomitant with an increase in the width of the phospholipid headgroup electron density and a 3A reduction in lipid bilayer width. The interactions of L-PS with DMPC lipid bilayers were concentration-dependent, highly affected by cholesterol content and reproduced in egg phosphatidylcholine/cholesterol liposomes. DSC analysis showed that millimolar (1.0-5.0 mM) L-PS levels decreased the phase transition cooperative unit size of DMPC liposomes in a highly concentration-dependent manner which was significantly greater in preparations containing 10 mol% cholesterol. These data provide direct evidence that phosphomonoester levels modulate the biophysical properties of the membrane lipid bilayer which may, in turn, lead to altered structure/function relationships in AD.

Changes in brain membrane phospholipid and high-energy phosphate metabolism precede dementia

A 52-year-old Caucasian male was followed with Mattis and 31P MRS examinations every 6 months for 33 months. At entry into the study, the subject had a normal clinical examination and normal Mattis scores but had alterations in MRS measures of membrane phospholipid and high-energy phosphate metabolism indistinguishable from those previously reported in mildly demented AD patients. After 33 months of follow-up, the subject had clinical and Mattis findings suggestive of possible incipient dementia and after 46 months of follow-up there was sufficient cognitive decline to make the diagnosis of dementia with a frontal lobe preponderance. The findings in this subject support the contention that alterations in brain membrane phospholipid and high-energy metabolism can be noninvasively detected by 31P MRS years before any clinical manifestations of the disease.

Structural determinants of activity at the GABAB receptor. A comparison of phosphoethanolamine and related GABA analogs

Phosphoethanolamine is a phosphomonoester that is reduced in Alzheimer disease brain. Despite its close structural similarity to GABA and the GABAB partial agonist 3-aminopropylphosphonic acid, phosphoethanolamine binds very poorly to GABAB receptors (IC50 = 7.5 +/- 0.8 mM). In this study, we examined whether the marked decrease in binding affinity associated with the presence of an ester oxygen in place of the alpha-CH2 group of GABAergic compounds also occurred in sulfonates and used high resolution solution NMR and molecular mechanics calculations to determine the structural basis of this decrease in activity. The sulfonate analog of GABA, 3-amino-propylsulfonic acid, became > 2500-fold less potent when the alpha-CH2 was replaced by an ester oxygen. Structural studies showed that the active alpha-CH2 compounds (GABA, 3-aminopropylphosphonic acid, and 3-aminopropylsulfonic acid) prefer a fully extended conformation. The inactive compounds, phosphoethanolamine and ethanolamine-O-sulfate, exist in a gauche conformation around the C beta-C gamma bond. This study, which suggests conformational differences, may explain how PE can be so efficiently excluded from GABAB receptors, despite being present in millimolar concentrations in brain. Exclusion of phosphoethanolamine from GABAB receptors may be an important physiologic control mechanism in the regulation of inhibitory neurotransmission.

Biological correlates of slow wave sleep deficits in functional psychoses: 31P-magnetic resonance spectroscopy

Deficits in slow wave sleep (SWS) are consistently seen in schizophrenia and related psychotic disorders. However, the pathophysiological significance of this finding is uncertain. In 19 patients with psychotic illness, sleep and 31P-magnetic resonance spectroscopy (MRS) studies were carried out before the patients began medication treatment. Polysomnographic studies were carried out in 2-3 consecutive nights. MRS studies were performed with a surface coil and a depth-resolved pulse sequence focusing on the dorsal prefrontal cortex. Phosphomonoesters were correlated with visually scored delta and Stage 4 sleep, as well as with automated delta wave counts. An inverse relation was also seen between negative symptoms scores and SWS. The association between decreases brain anabolic processes (reflected by decreased PME) and decreased SWS may be related either to processes of accelerated aging or to developmentally mediated alterations in cortical synaptic pruning, postulated to underlie the pathophysiology of functional psychoses.

Membrane interactions of a phosphomonoester elevated early in Alzheimer's disease

Phosphoserine (L-PS) is among several phosphomonoesters found to be elevated in autopsied Alzheimer's disease (AD) brain tissue. To investigate the molecular interactions of L-PS with membrane lipid bilayers, small angle X-ray diffraction and high resolution differential scanning calorimetry (DSC) approaches were used with liposomes composed of lecithin and cholesterol. A one-dimensional electron density profile of a control dimyristoyl phosphatidylcholine (DMPC)/cholesterol lipid bilayer with a unit cell dimension of 52 A at 37 degrees C was generated from the X-ray diffraction data. Following incubation with 2.0 mM L-PS, a broad decrease in electron density +/- 4-12 A from the lipid bilayer center was observed concomitant with an increase in the width of the phospholipid headgroup electron density and a 3 A reduction in lipid bilayer width. The interactions of L-PS with DMPC lipid bilayers were concentration-dependent, highly affected by cholesterol content and reproduced in egg phosphatidylcholine/cholesterol liposomes. DSC analysis showed that millimolar (1.0-5.0 mM) L-PS levels decrease the phase transition cooperative unit size of DMPC liposomes in a highly concentration-dependent manner which was significantly greater in preparations containing cholesterol. The endotherm width at half-maximum doubled at 5.0 mM and 1.25 mM L-PS, respectively, for DMPC and DMPC/cholesterol liposomes. These data provide direct evidence that elevated phosphomonoester levels modulate the biophysical properties of the membrane lipid bilayer which may, in turn, lead to altered structure/function relationships in membranes during AD.

Chrysamine-G binding to Alzheimer and control brain: autopsy study of a new amyloid probe

Chrysamine-G (CG) is a carboxylic acid analogue of Congo red, a histologic dye which stains amyloid. CG binds to the beta-amyloid protein of Alzheimer's disease (AD) in vitro and partitions into the brain of normal mice. In this study, we demonstrate increased binding of [14C]CG to homogenates of several regions of AD brain as compared to control. The total binding of CG to AD brain was approximately two- to three-fold that of control brain. The cerebellum could be used as an internal standard for each brain as CG binding to cerebellum did not differ between AD and control. The binding of [14C]CG correlated with numbers of senile plaques and neurofibrillary tangles. In addition, CG could be used to stain cerebrovascular amyloid in tissue sections. These results suggest that CG may prove useful as an in vivo probe of amyloid deposition in AD.

A comparison of stereology and segmentation techniques for volumetric measurements of lateral ventricles in magnetic resonance imaging

Lateral ventricular volumes were measured on magnetic resonance imaging (MRI) scans by independent raters in 18 subjects (11 psychotic patients and 7 healthy control subjects) with two different approaches: a point-counting stereological (PCS) technique and a computerized technique based on segmentation algorithms. The correlation between the two techniques was very high (r = 0.96), and phantom studies showed good validity for both approaches. These findings and the technical simplicity of the PCS technique support its potential use for MRI morphometric measurements.

Inactivity of phosphoethanolamine, an endogenous GABA analog decreased in Alzheimer's disease, at GABA binding sites

Phosphoethanolamine (PE) is a metabolite of the phospholipid metabolism which is decreased in Alzheimer's disease brain. PE shows a strong structural similarity to the inhibitory neurotransmitter, GABA, and the GABAB receptor partial agonist, 3-amino-propylphosphonic acid. The ability of PE to compete for binding to GABAA and GABAB binding sites was investigated. GABAA sites were studied using [3H]SR-95531 and [3H]muscimol. GABAB sites were studied using [3H]GABA in the presence of isoguvacine to saturate GABAA sites. Total [3H]GABA binding was also examined. PE showed little activity at any of the GABA binding sites investigated. PE was most potent at GABAB sites, but the IC50 of 7.5 +/- 0.75 mM was considerably higher than its maximal physiologic concentration of approximately 1.5 mM. The efficient exclusion of PE from GABA binding sites may be an important physiologic mechanism in the control of inhibitory neurotransmission. The structural basis for this exclusion is discussed in reference to the GABAB partial agonist 3-amino-propylphosphonic acid.

Magnetic resonance spectroscopy and its application to aging and Alzheimer's disease

This is a review of magnetic resonance (MR) spectroscopy and its application to aging and Alzheimer's disease (AD). Examinations of perchloric acid extracts of AD brain tissue by MR spectroscopy reveal elevated levels of phosphomonoesters, phosphodiesters, and glutamate accompanied by reduced levels of N-acetyl-L-aspartate compared with extracts from controls. These metabolicalterations may be an indication of accelerated membrane phospholipid metabolism, glutamate neurotoxicity, and neuronal loss in AD brain that is not seen in normal aging. In vivo 31P MR spectroscopy studies of AD indicate that levels of phosphomonoesters are elevated early in the course of AD, which may be a causative molecular neuropathologic event. In vivo MR spectroscopy is a powerful technique to investigate the molecular neuropathology of the disease, to follow the progression of AD, and to assess the efficacy of experimental therapies.

Clinical and neurochemical effects of acetyl-L-carnitine in Alzheimer's disease

In a double-blind, placebo study, acetyl-L-carnitine was administered to 7 probable Alzheimer's disease patients who were then compared by clinical and 31P magnetic resonance spectroscopic measures to 5 placebo-treated probable AD patients and 21 age-matched healthy controls over the course of 1 year. Compared to AD patients on placebo, acetyl-L-carnitine-treated patients showed significantly less deterioration in their Mini-Mental Status and Alzheimer's Disease Assessment Scale test scores. Furthermore, the decrease in phosphomonoester levels observed in both the acetyl-L-carnitine and placebo AD groups at entry was normalized in the acetyl-L-carnitine-treated but not in the placebo-treated patients. Similar normalization of high-energy phosphate levels was observed in the acetyl-L-carnitine-treated but not in the placebo-treated patients. This is the first direct in vivo demonstration of a beneficial effect of a drug on both clinical and CNS neurochemical parameters in AD.

Development of small molecule probes for the beta-amyloid protein of Alzheimer's disease

This study describes the synthesis and in vitro testing of small molecule probes that may eventually prove useful as markers of amyloid deposition in living patients. The prototype agent, Chrysamine G (CG), is a derivative of Congo red. CG binds synthetic beta-amyloid well in vitro, as does a fluorinated derivative. The mechanism of binding appears to be the same as Congo red--through a bidentate attachment spanning several amyloid peptide chains. CG is much more lipophilic than Congo red and crosses the blood-brain barrier in normal mice, achieving a brain/blood ratio over 10/1. There was no acute toxicity in mice at doses 10 times those used in the distribution studies. CG appears to be a relatively high affinity probe for beta-amyloid that appears to have low toxicity and can cross the blood-brain barrier. These characteristics are promising for development of in vivo amyloid probes similar to CG.

Endogenous inhibitors of human choline acetyltransferase present in Alzheimer's brain: preliminary observation

We have previously demonstrated the presence in Alzheimer's disease brain of an endogenous inhibitor of choline acetyltransferase activity. Selected properties of these compounds were investigated. There appear to be two distinct classes of inhibitor present, both phosphomonoesters and nonphosphorylated substances. They are not proteins, pass through 500 mm dialyses membranes and are not lipoidal. There are both different sensitivities of individual control cytosotic activity to inhibition and differences in intrinsic inhibitory activity present in individual Alzheimer's disease brain samples. There is a competitive type of inhibition with respect to acetyl CoA as substrate and a noncompetitive type with respect to choline as substrate.

Is schizophrenia due to excessive synaptic pruning in the prefrontal cortex? The Feinberg hypothesis revisited

Several lines of evidence support the notion that a substantial reorganization of cortical connections, involving a programmed synaptic pruning, takes place during adolescence in humans. A review of neurobiological abnormalities in schizophrenia indicates that the neurobiological parameters that undergo peripubertal regressive changes may be abnormal in this disorder. An excessive pruning of the prefrontal corticocortical, and corticosubcortical synapses, perhaps involving the excitatory glutamatergic inputs to pyramidal neurons, may underlie schizophrenia. A reciprocal failure of pruning in certain subcortical structures, such as lenticular nuclei, may also occur. Several developmental trajectories, related to early brain insults as well as genetic factors affecting postnatal neurodevelopment, could lead to the illness. These models would have heuristic value and may be consistent with several known facts of the schizophrenic illness, such as its onset in adolescence and the gender differences in its onset and natural course. The relationship between these models and other etiological models of schizophrenia are summarized and approaches to test relevant hypotheses are discussed.

NMR identification of the formic acid-modified residue in Alzheimer's amyloid protein

The beta/A4-amyloid protein (beta/A4) and many synthetic fragments of this protein have proved to be very difficult to solubilize, leading to the use of relatively harsh chemical methods, most notably, formic acid. This treatment has previously been shown to cause a covalent modification of this peptide. In this study, one- and two-dimensional NMR techniques are used to show that the nature of this covalent modification is formation of a formate ester to a serine residue. This finding is consistent with our previously reported kinetic studies of formic acid-induced modification of beta/A4 and further illustrates the potential danger of solubilizing fragments of beta/A4 in formic acid. Alternative methods of solubilization are discussed.

Analysis of magnetic resonance spectra by mole percent: comparison to absolute units

A variety of metabolites present in perchloric acid extracts of brain tissue were measured by 1H and 31P magnetic resonance spectroscopy (MRS) and HPLC in the same tissue sample and the MRS results were expressed both in terms of mole % and mumole/g based on an internal standard. The levels of 16 metabolites were compared by linear regression analysis and the mole % results were found to correlate very well with the results expressed as mumole/g. To compare the two units under typical experimental conditions, the percent change in metabolites in a group of Alzheimer's disease brains was compared to a control group using both units. The results were essentially identical for the mole % and mumole/g methods. We conclude that the use of the mole % method of expressing MRS data yields results which are equivalent to those expressed in absolute units and suggest that, for in vivo MRS studies, use of the mole % method is preferable because fewer artifacts, such as partial volume effects, are introduced.

Alterations of cerebral metabolism in probable Alzheimer's disease: a preliminary study

Previous in vitro and in vivo 31P MRS studies of Alzheimer's disease patients have revealed alterations in membrane phospholipid metabolism and PET studies have shown alterations in glucose and oxidative metabolism. This study of probable Alzheimer's disease patients demonstrates severity dependent alterations in measures of both high-energy phosphate and membrane phospholipid metabolism. Mildly demented Alzheimer's patients compared to the controls, have increases in the levels of phosphomonoesters, decreases in the levels of phosphocreatine and probably adenosine diphosphate, and an increased oxidative metabolic rate. As the dementia worsens, the levels of phosphocreatine and adenosine diphosphate increase, the levels of phosphomonoesters decrease, and the oxidative metabolic rate decreases. The phosphomonoester findings replicate previous findings and provide a new dimension to the molecular pathology of Alzheimer's disease, implicating basic defects in membrane metabolism. The changes in oxidative metabolic rate suggest the AD brain is under energetic stress. The changes in energy metabolites with increasing dementia could be a consequence of nerve terminal degeneration and are consistent with previous PET findings. 31P MRS provides new diagnostic and metabolic insights into this disease and would be a noninvasive method to follow the progression of the disease and the metabolic response to therapeutic interventions.