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AMPA receptor calcium permeability, GluR2 expression, and selective motoneuron vulnerability
W Vandenberghe1, W Robberecht, J R Brorson
1Department of Neurology, The University of Chicago, Chicago, Illinois 60637, USA.
Abstract:
AMPA receptor-mediated excitotoxicity is proposed to play a major pathogenic role in the selective motoneuron death of amyotrophic lateral sclerosis. Motoneurons have been shown in various models to be more susceptible to AMPA receptor-mediated injury than other spinal neurons. It has been hypothesized that this selective vulnerability of motoneurons is caused by the expression of highly Ca(2+)-permeable AMPA receptors and a complete or relative lack of the AMPA receptor subunit Glu receptor 2 (GluR2). The aim of this study was to quantify the relative Ca(2+) permeability of AMPA receptors and the fractional expression of GluR2 in motoneurons by combining whole-cell patch-clamp electrophysiology and single-cell RT-PCR and to compare these properties with those of dorsal horn neurons. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. As in previous studies, motoneurons were significantly more vulnerable to AMPA and kainate than dorsal horn neurons. However, all motoneurons expressed GluR2 mRNA ( approximately 40% of total AMPA receptor subunit mRNA), and their AMPA receptors had intermediate whole-cell relative Ca(2+) permeability (P(Ca(2+))/P(Cs(+)) approximately 0. 4). AMPA receptor P(Ca(2+))/P(Cs(+)) and the relative abundance of GluR2 varied more widely in dorsal horn neurons than in motoneurons, but the mean values did not differ significantly between the two cell populations. GluR2 was virtually completely edited at the Q/R site both in motoneurons and dorsal horn neurons. These results indicate that the selective vulnerability of motoneurons to AMPA receptor agonists is not determined solely by whole-cell relative Ca(2+) permeability of AMPA receptors.
Insights
Selective motoneuron death in ALS may involve AMPA receptor excitotoxicity. However, motoneurons express GluR2 and have intermediate Ca2+ permeability, suggesting other factors contribute to their vulnerability.
Area of Science:
- Neuroscience
- Neurobiology
- Cellular Neuroscience
Background:
- AMPA receptor-mediated excitotoxicity is implicated in motoneuron death in amyotrophic lateral sclerosis (ALS).
- Motoneurons exhibit greater susceptibility to AMPA receptor-mediated injury compared to other spinal neurons.
- Hypotheses suggest this vulnerability stems from highly Ca2+-permeable AMPA receptors and low expression of the GluR2 subunit.
Purpose of the Study:
- To quantify the Ca2+ permeability of AMPA receptors in motoneurons.
- To determine the fractional expression of the GluR2 subunit in motoneurons.
- To compare these properties with those of dorsal horn neurons.
Main Methods:
- Isolation and culture of rat spinal motoneurons and dorsal horn neurons.
- Whole-cell patch-clamp electrophysiology to measure Ca2+ permeability.
- Single-cell reverse transcription polymerase chain reaction (RT-PCR) to quantify GluR2 mRNA expression.
Main Results:
- Motoneurons showed significantly higher vulnerability to AMPA and kainate than dorsal horn neurons.
- All motoneurons expressed GluR2 mRNA (approx. 40% of total AMPA receptor subunit mRNA).
- Motoneuron AMPA receptors exhibited intermediate Ca2+ permeability (P(Ca2+)/P(Cs+) approx. 0.4).
- GluR2 expression and Ca2+ permeability showed greater variability in dorsal horn neurons but no significant mean difference compared to motoneurons.
- GluR2 was almost completely edited at the Q/R site in both neuron types.
Conclusions:
- The selective vulnerability of motoneurons to AMPA receptor agonists is not solely determined by the Ca2+ permeability of their AMPA receptors.
- The presence of GluR2 and intermediate Ca2+ permeability in motoneurons challenge previous hypotheses regarding their unique susceptibility.
- Further research is needed to elucidate the precise mechanisms underlying motoneuron excitotoxicity in ALS.