Glycerophosphocholine is elevated in cerebrospinal fluid of Alzheimer patients

Abstract

Experimental and clinical studies give evidence for breakdown of membrane phospholipids during neurodegeneration. In the present study, we measured the levels of glycerophosphocholine (GPCh), phosphocholine (PCh), and choline, that is, water-soluble metabolites of phosphatidylcholine (PtdCho), in human cerebrospinal fluid (CSF). Among 30 cognitively normal patients the average CSF levels of GPCh, phosphocholine and choline were 3.64, 1.28, and 1.93 μM, respectively; metabolite levels did not change with increasing age. When compared with age-matched controls, patients with Alzheimer’s disease had elevated levels of all choline metabolites: GPCh was significantly increased by 76% (P<0.01), phosphocholine by 52% (P<0.05), and free choline (Ch) by 39%. Six patients with vascular dementia had lower choline and elevated phosphocholine levels, when compared to controls, but normal levels of GPCh. These data demonstrate that Alzheimer’s disease is accompanied by an increased PtdCho hydrolysis in the brain. PtdCho breakdown seems to be mediated by phospholipase A₂ and leads to significantly elevated levels of GPCh in CSF.

Introduction

Alzheimer’s disease and vascular dementia are neurodegenerative diseases characterized by cognitive loss and dementia. The underlying processes of neurodegeneration are partially characterized. Multiple focal ischemias are typical for the progression of vascular dementias and are accompanied by lack of oxygen and nutrients, reduction of ATP, loss of membrane potential, and release of massive amounts of glutamate. Excessive glutamate levels in the brain cause influx of large amounts of calcium into neurons, for example, through NMDA receptor channels, which cannot be sequestered and cause breakdown of essential cellular structures such as membrane phospholipids leading to cell death [23]. In Alzheimer’s disease, formation of amyloid peptides from amyloid precursor protein (APP) and deposition of amyloid plaques are often held responsible for progressive deterioration [25], but cellular calcium overload, lipid peroxidation, and membrane breakdown have also been suggested as final pathways of neuronal degeneration [18].

A direct consequence of cellular calcium influx and calcium overload is the activation of calcium-dependent phospholipase A₂ (cPLA₂) and breakdown of membrane phospholipids such as phosphatidylcholine (PtdCho). There is ample experimental and clinical evidence for increased PtdCho hydrolysis during the progression of neurodegenerative diseases (reviewed in [15]). In the course of events, PtdCho hydrolysis by phospholipase A₂ (PLA₂) yields lyso-PtdCho, glycerophosphocholine (GPCh), phosphocholine (PCh), and free choline (Ch) (Fig. 1). The contribution of PLA₂ activation to ischemic damage is demonstrated by the effectiveness of PLA₂ inhibitors to reduce tissue damage following ischemic stroke [8] and by the reduced susceptibility of cPLA₂ knockout mice to ischemic neurodegeneration [5]. In Alzheimer’s disease, activation of PLA₂ followed by PtdCho breakdown has been demonstrated by post-mortem analysis of Alzheimer brains showing increases of GPCh tissue levels [3], [19], [20]. Phosphodiesters including GPCh have also been found to be increased in live Alzheimer brain by magnetic resonance imaging (MRI) [29]. Moreover, the arterio-venous difference of choline across the brain is increased in Alzheimer patients [2], another piece of evidence that disease progression is accompanied by PtdCho breakdown.

In the present study, we hypothesized that PtdCho breakdown in the brain may be reflected in elevated concentrations of PtdCho breakdown products in cerebrospinal fluid (CSF), a compartment which is accessible in humans for diagnostic purposes. While previous studies concentrated on CSF choline levels (see Section 4), we extended the analysis to include all water-soluble choline metabolites, specifically phosphocholine and GPCh which have not been determined before in human CSF. While choline can leave the cell, phosphocholine and GPCh can only be released when the cellular membrane becomes dysfunctional (Fig. 1), a phenomenon which may be expected during neurodegenerative disease. Moreover, changes of CSF choline levels are not specific to membrane breakdown because choline can also be absorbed from circulating blood or may be formed from the hydrolysis of acetylcholine. GPCh, in contrast, is a specific indicator of PtdCho breakdown because it cannot be formed by anabolic routes [15], [32]. Therefore, we measured the metabolites in CSF samples obtained from healthy humans of different ages as well as in samples obtained from patients suffering from Alzheimer disease and vascular dementia.