N-Methyl-d-aspartate receptor subunit proteins and their phosphorylation status are altered selectively in Alzheimer’s disease
Introduction
Glutamate is the major excitatory neurotransmitter in the central nervous system and functions in synaptic neurotransmission by activating glutamate receptors. One subtype of glutamate receptor that is critical for synaptic function is the N-methyl-d-aspartate (NMDA) receptor. Activation of NMDA receptors participates in the mechanism of long-term potentiation (LTP), the best-understood synaptic model of learning and memory [1]. However, perturbations in glutamate neurotransmission and glutamate receptor activation may be fundamental to a variety of neurological disorders [2], [3], [4]. Excessive release of glutamate contributing to excitotoxic neuronal death has been implicated as an etiologic mechanism in anoxic-ischemic injury, epilepsy, and chronic neurodegenerative disorders, including amyotrophic lateral sclerosis and Alzheimer’s disease (AD) [5], [6], [7], [8].
Progressive impairments in memory and cognition are the clinical hallmarks of AD. These functional deficits correlate with the neuropathologic features of the disease, including neuronal loss, senile plaque and neurofibrillary tangle formation, and loss of synaptic integrity. Previous studies have shown neuronal degeneration and synaptic loss in the hippocampus and entorhinal cortex occur in AD, and these deficits correlate with neuropsychological scores that reflect impaired memory and cognitive function in AD [18], [19], [26], [29], [30].
Recent studies on AD have suggested that amyloid precursor protein may play an important role in regulating glutamate levels at synapses by modulating the activity of glutamate receptors [9]. Furthermore, β-amyloid protein, a key component of senile plaques, can enhance the neurotoxicity of glutamate [10], [11]. In this study we evaluated one major subtype of glutamate receptor, the NMDA receptor, because of its integral involvement in learning and memory, processes that are strikingly compromised in AD, and because of its important role in excitotoxic neuronal death.
Previous studies using autoradiographic ligand binding assays to measure NMDA receptors in AD brain have shown conflicting results. Some studies have reported that NMDA receptors are lost from cortical and hippocampal areas [12], [13], while others have found hippocampal NMDA receptors to be relatively stable [14], [15]. These studies provide valuable information on NMDA binding site localization in the human brain. Several NMDA receptor subunits have been identified. Some subunits appear to have a greater role in learning and memory [16]. Little is known in AD, however, about the levels of the various NR1, NR2A, and NR2B subunits that comprise the NMDA receptor, or whether alterations in these subunits of the receptor have any clinical significance. In addition, the phosphorylation status of NMDA receptor subunits is important in ischemic neuronal injury [17]. We therefore tested the hypothesis that NMDA receptor subunits and their phosphorylation status are selectively and differentially affected in AD patients who have variable cognitive impairments.
Section snippets
Subjects and clinical evaluation
The subjects selected for this study were described previously [18], [19]. The groups of subjects were characterized as definite AD (n=6), possible AD or early AD (n=4), and normal controls (n=6) (Table 1) [20] using the criteria from the Braak and Braak staging scheme and the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) [21], [22]. Subjects without clinical manifestation of dementia but with sufficient numbers of neuritic plaques in neocortex for the diagnosis of AD by
Results
The quantitative densitometric results of the regional levels of NMDA receptor subunit protein levels in immunoblots (Fig. 1) are summarized (Table 2). Western blots of AD versus controls showed that the percentage (%) reductions compared to controls among receptor subunits are differentially and regionally selective in AD. Specifically, reductions in NR1 (53%, P<0.05) and NR2B (40%, P<0.05) were found in hippocampus, and reductions in NR2A (39%, P<0.05) and NR2B (31%, P<0.01) were noted in
Discussion
We found that specific subunits of NMDA receptor are abnormal in individuals with definite AD, but not in individuals with early AD. Levels of NR1 and NR2B were profoundly reduced in the hippocampus, and levels of NR2A and NR2B were decreased in the entorhinal cortex. Interestingly, presynaptic defects have previously been shown to occur in these regions [18], [19] and in the caudate nucleus [19] early in AD, but in this study no statistically significant changes were detected in NMDA receptor
Acknowledgements
We thank Drs. Claudia Kawas, Juan Troncoso, the ADRC and the BLSA staff, and the ADRC and BLSA participants for the clinical information. We gratefully acknowledge Mr. Bob McCullough for assistance with photographs and figures and Ginger Woodward for secretarial support. This work was supported in part by funds from the Maria Teresa Jones Alzheimer’s Disease Research Award, UCHSC (CIS) and by a NIH-NIA grant (LJM, AG16282).
References (39)
Glutamate neurotoxicity and diseases of the nervous system
Neuron
(1988)- et al.
β-Amyloid protein increases the vulnerability of cultured cortical neurons to excitotoxic damage
Brain Res
(1990) - et al.
Mechanism and prevention of neurotoxicity caused by beta-amyloid peptides: relation to Alzheimer’s disease
Brain Res
(1997) - et al.
[3H]TCP binding sites in Alzheimer’s disease
Neurosci Lett
(1987) - et al.
Selective regional loss of exocytotic presynaptic vesicle proteins in Alzheimer’s disease brain
J Neurol Sci
(2000) - et al.
Glutamate receptor modulation by protein phosphorylation
J Physiol Paris
(1994) - et al.
Neuronal and volume loss in CA1 of the hippocampal formation uniquely predicts duration and severity of Alzheimer’s disease
Brain Res
(1998) - et al.
Expression of endogenous NMDAR1 transcripts without receptor protein suggests post-transcriptional control in PC-12 cells
J Biol Chem
(1993) - et al.
The glutamate synapse in neuropsychiatric disorders. Focus on schizophrenia and Alzheimer’s disease
Role of NMDA receptors in memory
TiPS
(1991)
The role of protein kinase C in long-term potentiation: a testable model
Brain Res Rev
A synaptic model of memory: long-term potentiation in the hippocampus
Nature
The toxic effect of sodium l-glutamate on the inner layers of the retina
Arch Ophthal
Glutamate-induced neuronal necrosis in the infant mouse hypothalamus. An electron microscopic study
J Neuropathol Exp Neurol
Kainic acid: a powerful neurotoxic analogue of glutamate
Brain Res
Excitotoxic cell death
J Neurobiol
Role of excitotoxins in heredito-degenerative neurological diseases
Excitatory transmitter neurotoxicity
Neurobiol Aging
A secreted form of amyloid precursor protein enhances basal glucose and glutamate transport and protects against oxidative impairment of glucose and glutamate transport in synaptosome by a cyclic GMP-mediated mechanism
J Neurochem
Cited by (122)
The glutamatergic system in Alzheimer’s disease: a systematic review with meta-analysis
2024, Molecular PsychiatryBiological Effects and Mechanisms of Taurine in Various Therapeutics
2023, Current Drug Discovery TechnologiesN-methyl-D-aspartate receptor hypofunction as a potential contributor to the progression and manifestation of many neurological disorders
2023, Frontiers in Molecular NeuroscienceThe Pathological Effects of Circulating Hydrophobic Bile Acids in Alzheimer's Disease
2023, Journal of Alzheimer's Disease Reports