The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Neurobiology, Campus Gasthuisberg O&N2, PB1022, Herestraat 49, B-3000 Leuven, Belgium.
Biochimica et Biophysica Acta (Impact Factor: 4.66). 11/2006; 1762(11-12):1068-82. DOI: 10.1016/j.bbadis.2006.05.002
Source: PubMed


Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.

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Available from: Ludo Van Den Bosch
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    • "First, MNs preferentially express the ionotropic glutamate receptors AMPA, NMDA and kainate. AMPA receptor subunit alteration has been largely demonstrated in ALS rodent models (Spalloni et al., 2004; Zhao et al., 2008; Kwak et al., 2010) leading to abnormal Ca 2+ influx and neurodegeneration (Van Den Bosch et al., 2006). In addition, MNs in ALS patients lack the intracellular expression of Ca 2+ binding proteins, parvalbumin and calbindin D28k, both required to buffer intracellular Ca 2+ (Ince et al., 1993). "
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    ABSTRACT: Amyotrophic lateral sclerosis (ALS) is now recognized as a multisystem disorder, in which the primary pathology is the degeneration of motor neurons, with cognitive and/or behavioral dysfunctions that constitutes the non-motor manifestations of ALS. The combination of clinical, neuroimaging, and neuropathological data, and detailed genetic studies suggest that ALS and frontotemporal dementia (FTD) might form part of a disease continuum, with pure ALS and pure FTD at the two extremes. Mutations in the superoxide dismutase 1 (SOD1) gene were the first genetic mutations linked to the insurgence of ALS. Since that discovery numerous animal models carrying SOD1 mutations have been created. Despite their limitations these animal models, particularly the mice, have broaden our knowledge on the system alterations occurring in the ALS spectrum of disorders. The present review aims at providing an overview of the data obtained with the SOD1 animal models first and foremost on the cortical and subcortical regions, the cortico-striatal and hippocampal synaptic plasticity, dendritic branching and glutamate receptors function.
    Full-text · Article · Nov 2015 · Neuroscience & Biobehavioral Reviews
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    • "The Ca 2þ permeability of AMPA receptors has been suggested to be governed by its GluR2 subunit that undergoes post translational modifications at Q/R site (Corona and Tapia, 2007; Duncan, 2009; Kawahara and Kwak, 2005; Van Damme et al., 2002; Van Den Bosch et al., 2006). It was proposed that motor neurons possess Ca 2þ permeable AMPA receptor but there are contradictory reports which show abundant expression of edited GluR2 subunit in spinal cord motor neurons and their relative Ca 2þ permeability does not differ from those of other spinal neurons (Greig et al., 2000; Vandenberghe et al., 2000a, 2000b). "
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    ABSTRACT: It is well established that motor neurons are highly vulnerable to glutamate induced excitotoxicity. The selective vulnerability of these neurons has been attributed to AMPA receptor mediated excessive rise in cytosolic calcium and consequent mitochondrial Ca(2+) loading. Earlier we have reported that in motor neurons a generic rise in [Ca(2+)]i does not always lead to mitochondrial Ca(2+) loading and membrane depolarization but it occurs upon AMPA receptor activation. The mechanism of such specific mitochondrial involvement upon AMPA receptor activation is not known. The present study examines the mitochondrial Ca(2+) regulation and oxidative stress in spinal cord neurons upon AMPA subtype of glutamate receptor activation. Stimulating the spinal neurons with AMPA exhibited a sharp rise in [Ca(2+)]m in both motor and other spinal neurons that was sustained up to the end of recording time of 30min. The rise in [Ca(2+)]m was substantially higher in motor neurons than in other spinal neurons which could be due to the differential mitochondrial homeostasis in two types of neurons. To examine this possibility, we measured AMPA induced [Ca(2+)]m loading in presence of mitochondrial inhibitors. In both cell types the AMPA induced [Ca(2+)]m loading was blocked by mitochondrial calcium uniporter blocker ruthenium red. Moreover, in motor neurons it was also inhibited substantially by CGP37157 and cyclosporine-A, the blockers of Na(+)/Ca(2+) exchanger and mitochondrial permeability transition pore (MPTP) respectively, whereas no effect of these agents was observed in other spinal neurons. Thus in motor neurons the Ca(2+) sequestration by mitochondria occurs through mitochondrial calcium uniporter as well as due to reversal of Na(+)/Ca(2+) exchanger, in contrast the latter pathway does not contribute in other spinal neurons. The ROS formation was inhibited by nitric oxide synthase (NOS) inhibitor L-NAME in both types of neurons, however the mitochondrial complex-I inhibitor rotenone suppressed the ROS formation only in motor neurons. It appears that activation of cytoplasmic nNOS leads to ROS formation in both types of spinal neurons but mitochondria is the major source of ROS in motor neurons. Spinal neurons exhibited a significant time dependent fall in glutathione (GSH) level. The GSH level in motor neurons did not recover even at 24h after AMPA exposure, whereas the other spinal neurons exhibited a tendency to maintain the GSH after a certain level suggesting that the oxidative stress is arrested in other spinal neurons but it continues to increase in motor neurons. Thus our results demonstrate that upon AMPA receptor stimulation the motor neurons employ some additional pathways for regulation of mitochondrial calcium and oxidative stress as compared to other spinal neurons. It is suggested that such differential signaling mechanisms in motor neurons could be crucial for their selective vulnerability to excitotoxicity. Copyright © 2015. Published by Elsevier B.V.
    Full-text · Article · May 2015 · Brain research
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    • "ALS is an adult-onset neurodegenerative disorder characterized by selective loss of upper and lower motor neurons. Glutamate excitotoxicity is implicated in its pathogenesis (Bruijn et al., 2004; Van Den Bosch et al., 2006), and the accumulation of D-serine due to reduced activity of DAO possibly exacerbates hyperactivity of motor neurons via NMDA receptors (Sasabe et al., 2007, 2012; Paul et al., 2014). On the other hand, schizophrenia is a severely debilitating psychiatric condition, characterized by psychotic features, negative symptoms, and cognitive defects. "
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    ABSTRACT: It has been proposed that D-amino acid oxidase (DAO) plays an essential role in degrading D-serine, an endogenous coagonist of N-methyl-D-aspartate (NMDA) glutamate receptors. DAO shows genetic association with amyotrophic lateral sclerosis (ALS) and schizophrenia, in whose pathophysiology aberrant metabolism of D-serine is implicated. Although the pathology of both essentially involves the forebrain, in rodents, enzymatic activity of DAO is hindbrain-shifted and absent in the region. Here, we show activity-based distribution of DAO in the central nervous system (CNS) of humans compared with that of mice. DAO activity in humans was generally higher than that in mice. In the human forebrain, DAO activity was distributed in the subcortical white matter and the posterior limb of internal capsule, while it was almost undetectable in those areas in mice. In the lower brain centers, DAO activity was detected in the gray and white matters in a coordinated fashion in both humans and mice. In humans, DAO activity was prominent along the corticospinal tract, rubrospinal tract, nigrostriatal system, ponto-/olivo-cerebellar fibers, and in the anterolateral system. In contrast, in mice, the reticulospinal tract and ponto-/olivo-cerebellar fibers were the major pathways showing strong DAO activity. In the human corticospinal tract, activity-based staining of DAO did not merge with a motoneuronal marker, but colocalized mostly with excitatory amino acid transporter 2 and in part with GFAP, suggesting that DAO activity-positive cells are astrocytes seen mainly in the motor pathway. These findings establish the distribution of DAO activity in cerebral white matter and the motor system in humans, providing evidence to support the involvement of DAO in schizophrenia and ALS. Our results raise further questions about the regulation of D-serine in DAO-rich regions as well as the physiological/pathological roles of DAO in white matter astrocytes.
    Full-text · Article · Jun 2014 · Frontiers in Synaptic Neuroscience
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