Glutamate Gums Up Motor, Dopaminergic Neurons

Glutamate excitotoxicity has emerged as a favorite theory for the damage that motor neurons undergo in amyotrophic lateral sclerosis (ALS). Unless glia mop up used glutamate, it accumulates in the synapse and poisons neurons by inducing over-firing and calcium influx. Two recent papers add more fuel to the fire. Writing in the Acta Neuropathologica online April 7, researchers from the University of Tokyo, Japan, report that editing of the RNA for a glutamate receptor component is intimately linked with TDP-43 inclusions, a hallmark of sporadic ALS. And in the April 14 Journal of Neuroscience, scientists from the University of Rome, Italy, show that the non-protein amino acid BMAA acts as a glutamate receptor agonist, potentially causing symptoms of Parkinson disease as well as ALS.

First author Hitoshi Aizawa and senior author Shin Kwak led the Tokyo group. Kwak and colleagues previously showed that the enzyme adenosine deaminase acting on RNA 2 (ADAR2) edits RNA for the glutamate receptor subunit Glu2R, a component of calcium-dependent AMPA receptors (see ARF related news story on Kawahara et al., 2004; Kwak et al., 2008). ADAR2 catalyzes an adenosine-to-inosine switch essential to produce functional Glu2R, but this editing is incomplete in the motor neurons of people with sporadic ALS (Kawahara et al., 2004). Without properly edited GluR2, AMPA receptors are more permeable to calcium ions, disrupting membrane polarization and causing excitotoxicity. RNA regulation has also been linked to the genetics of familial ALS. Mutations in genes that encode TAR DNA-binding protein-43 (TDP-43) and fused in sarcoma (FUS)—proteins that are responsible for RNA editing and trafficking—can cause disease (see ARF related news story on Kwiatkowski et al., 2009 and
Vance et al., 2009).

Kwak and coauthors noted that ADAR2 is absent from spinal motor neurons in ALS. Since TDP-43 leaves its normal nuclear localization to form inclusions in the cytoplasm in many ALS cases as well, they tried to find a connection between these two events. The researchers stained spinal cord sections from people who had died of ALS for both ADAR2 and TDP-43. They found that all motor neurons contain ADAR2 in non-ALS, control tissue samples, but many motor neurons lacked the enzyme in ALS sections. The ADAR2-free cells—which presumably had unedited glutamate receptors—uniformly had TDP-43 inclusions in the cytoplasm, outside of the nucleus.

Because the loss of ADAR2 and TDP-43 inclusion formation coincide, the authors conclude that one likely causes the other, although they are not sure which comes first. Because both deficiency of ADAR2 and TDP-43 knockout induces neuronal death, it would be more reasonable to consider that either one of the two ALS-associated abnormalities induces neuronal death via inducing the other, rather than to consider that these two ALS-associated abnormalities occur simultaneously as a result of other upstream abnormalities, Kwak wrote in an e-mail to ARF. He plans to investigate the link between ADAR2 and TDP-43 in future studies.

The Rome-based scientists performed electrophysiology experiments to examine the effects of BMAA on neurons. The ingestion of BMAA has been linked, although tenuously, to a unique ALS syndrome among the Chamorro people of Guam (see ARF related news story on Cox et al., 2003). This Guamian ALS includes a mixture of symptoms of ALS, Parkinson disease, and Alzheimer disease.

BMAA is a glutamate receptor agonist, but little is known about its precise mechanism of action. First author Maria Letizia Cucchiaroni and joint senior authors Ezia Guatteo and Nicola Mercuri sought to understand BMAA’s effect on dopaminergic neurons from the substantia nigra of rats to see if the effects could be linked to Parkinson-like symptoms.

The researchers applied BMAA to rat brain slices, and saw that it caused increased calcium influx, with intracellular calcium concentrations nearly tripling upon BMAA treatment. This led to membrane depolarization and toxicity, with cell shrinkage and production of reactive oxygen species. To determine which parts of glutamate receptors BMAA affects, the scientists added antagonists specific for glutamate receptors subunits. They found a GluR1 antagonist blocked the toxic effects of BMAA, suggesting the amino acid binds this receptor subunit. TRPC, or transient receptor potential-like channels containing GluR1 were mostly responsible for BMAA-induced cytotoxicity, with AMPA receptors making a small, non-calcium-dependent contribution as well.

Because Guamian ALS includes parkinsonism, GluR1 excitotoxicity is worth pursuing as a mechanism for typical Parkinson’s, too, Mercuri wrote in an e-mail to ARF. The authors suggest that exposure to BMAA—now known to exist worldwide, not just in Guam—might contribute to idiopathic Parkinson disease.

References:
Aizawa H, Sawada J, Hideyama T, Yamashita T, Katayama T, Hasebe N, Kimura T, Yahara O, Kwak S. TDP-43 pathology in sporadic ALS occurs in motor neurons lacking the RNA editing enzyme ADAR2. Acta Neuropathol. 2010 Apr 7. Abstract

Cucciaroni ML, Viscomi MT, Bernardi G, Molinari M, Guatteo E, Mercuri NB. Metabotropic glutamate receptor 1 mediates the electrophysiological and toxic actions of the cycad derivative beta-N-methylamino-L-alanine on substantia nigra pars compacta DAergic neurons. J Neurosci. 2010 Apr 14;30(15):5176-5188. Abstract


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