ABSTRACT
Title
Glutamatergic alterations and mitochondrial impairment in a murine model of Alzheimer’s disease
Authors
A. Romano1, T. Cassano2, P. Dipasquale1, S. Cianci1,3, A. Petrella4, S. Cimino2,3, S. Gaetani1, V. Cuomo1
1Dept. of Physiology and Pharmacology “V. Erspamer”, Sapienza University of Rome, Italy; 2Dept. of Biomedical Sciences, University of Foggia, Italy; 3Dept. of Pharmacology and Human Physiology, University of Bari, Italy; 4Istituto Zooprofilattico di Puglia e Basilicata, Foggia, Italy
1Dept. of Physiology and Pharmacology “V. Erspamer”, Sapienza University of Rome, Italy; 2Dept. of Biomedical Sciences, University of Foggia, Italy; 3Dept. of Pharmacology and Human Physiology, University of Bari, Italy; 4Istituto Zooprofilattico di Puglia e Basilicata, Foggia, Italy
Abstract
Alzheimer’s disease (AD) is characterized clinically by progressive cognitive decline, and pathologically by the presence in the brain of senile plaques composed primarily of amyloid-β peptide (Aβ) and neurofibrillary tangles (NFTs) containing hyperphosphorylated tau protein. About 5-10% of cases are familial, being associated with inheritable mutations of genes encoding for amyloid precursor protein (APP), presenilin 1 (PS1) and presenilin 2 (PS2).
Although plaques and NFTs are pathognomic, other significant pathological changes also occur in the AD brain, including numerous structural and functional alterations such as changes in neurotransmitter levels and mitochondrial injury. Although cholinergic depletion is an established hallmark in AD, less is known about the effects on glutamatergic neurotransmission. Altered expression of glutamate transport proteins in the brains of AD patients have been linked to reduced glutamate uptake activity and these alterations were suggested as responsible for the elevation of extracellular glutamate concentration, which, in turn, renders neurons for excitotoxicity.
Increasing evidence suggests that mitochondrial dysfunction might be a primary event in glutamate excitotoxicity. In this regard, both human and animal studies demonstrated that the accumulation of Aβ can be observed also in mitochondria, where it is associated with diminished enzymatic activity of respiratory chain complexes III and IV, and a reduced rate of oxygen consumption. These observations may help to explain the multitude of mitochondrial defects described in AD and mouse models of the disease, although the role of mitochondria in glutamate-induced neuronal cell death remains unclear to date.
Over the past decade, there have been considerable advances in developing transgenic mouse models of AD. The triple transgenic model of AD (3×Tg-AD), which harbours three mutant human genes (APPswe, PS1M146V, and tauP301L), has been one of the most thoroughly characterized. The 3×Tg-AD mice develop amyloid plaques and neurofibrillary pathology in a hierarchical manner in AD-relevant brain regions, manly the HIPP, cortex and amygdala. The 3×Tg-AD mice also develop age-related cognitive decline, impairment in synaptic plasticity and closely mimic the disease progression in humans.
In this study, deficits in glutamate neurotransmission and mitochondrial functions were detected in the frontal cortex (FC) and hippopcampus (HIPP) of aged 3×Tg-AD mice, compared to their wild-type littermates (Non-Tg). Likewise, basal levels of glutamate and vesicular glutamate transporter 1 (VGLUT1) expression were reduced in both areas. Cortical glutamate release responded to K+-stimulation, whereas no peak release was observed in the HIPP of mutant mice. Synaptosomal-associated protein 25 (SNAP-25) and other glutamate transporters such as glutamate/aspartate transporter (GLAST), glutamate transporter 1 (GLT1) and excitatory amino acid carrier 1 (EAAC1) were reduced in HIPP homogenates, and the ATP content was also found to be lower; in contrast, GLT1 and glial fibrillary acidic protein (GFAP) were found to be higher in the FC. The rate of complex-I and complex-II respiration was reduced in mitochondria in the FC, as was the membrane potential, but no proton leak and no alteration of F0F1-ATPase activity were observed. In contrast, mitochondrial complex-I respiration was significantly increased in the HIPP of 3×Tg-AD mice, together with mitochondrial proton leak and marked loss of F0F1-ATPase activity. These data suggest that impairments of mitochondrial bioenergetics might sustain the failure in the energy-requiring glutamatergic transmission.
Although plaques and NFTs are pathognomic, other significant pathological changes also occur in the AD brain, including numerous structural and functional alterations such as changes in neurotransmitter levels and mitochondrial injury. Although cholinergic depletion is an established hallmark in AD, less is known about the effects on glutamatergic neurotransmission. Altered expression of glutamate transport proteins in the brains of AD patients have been linked to reduced glutamate uptake activity and these alterations were suggested as responsible for the elevation of extracellular glutamate concentration, which, in turn, renders neurons for excitotoxicity.
Increasing evidence suggests that mitochondrial dysfunction might be a primary event in glutamate excitotoxicity. In this regard, both human and animal studies demonstrated that the accumulation of Aβ can be observed also in mitochondria, where it is associated with diminished enzymatic activity of respiratory chain complexes III and IV, and a reduced rate of oxygen consumption. These observations may help to explain the multitude of mitochondrial defects described in AD and mouse models of the disease, although the role of mitochondria in glutamate-induced neuronal cell death remains unclear to date.
Over the past decade, there have been considerable advances in developing transgenic mouse models of AD. The triple transgenic model of AD (3×Tg-AD), which harbours three mutant human genes (APPswe, PS1M146V, and tauP301L), has been one of the most thoroughly characterized. The 3×Tg-AD mice develop amyloid plaques and neurofibrillary pathology in a hierarchical manner in AD-relevant brain regions, manly the HIPP, cortex and amygdala. The 3×Tg-AD mice also develop age-related cognitive decline, impairment in synaptic plasticity and closely mimic the disease progression in humans.
In this study, deficits in glutamate neurotransmission and mitochondrial functions were detected in the frontal cortex (FC) and hippopcampus (HIPP) of aged 3×Tg-AD mice, compared to their wild-type littermates (Non-Tg). Likewise, basal levels of glutamate and vesicular glutamate transporter 1 (VGLUT1) expression were reduced in both areas. Cortical glutamate release responded to K+-stimulation, whereas no peak release was observed in the HIPP of mutant mice. Synaptosomal-associated protein 25 (SNAP-25) and other glutamate transporters such as glutamate/aspartate transporter (GLAST), glutamate transporter 1 (GLT1) and excitatory amino acid carrier 1 (EAAC1) were reduced in HIPP homogenates, and the ATP content was also found to be lower; in contrast, GLT1 and glial fibrillary acidic protein (GFAP) were found to be higher in the FC. The rate of complex-I and complex-II respiration was reduced in mitochondria in the FC, as was the membrane potential, but no proton leak and no alteration of F0F1-ATPase activity were observed. In contrast, mitochondrial complex-I respiration was significantly increased in the HIPP of 3×Tg-AD mice, together with mitochondrial proton leak and marked loss of F0F1-ATPase activity. These data suggest that impairments of mitochondrial bioenergetics might sustain the failure in the energy-requiring glutamatergic transmission.