Aberrant RNA Processing in a Neurodegenerative Disease: the Cause for Absent EAAT2, a Glutamate Transporter, in Amyotrophic Lateral Sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that is characterized by selective upper and lower motor neuron degeneration, the pathogenesis of which is unknown. About 60%-70% of sporadic ALS patients have a 30%-95% loss of the astroglial glutamate transporter EAAT2 (excitatory amino acid transporter 2) protein in motor cortex and spinal cord. Loss of EAAT2 leads to increased extracellular glutamate and excitotoxic neuronal degeneration. Multiple abnormal EAAT2 mRNAs, including intron-retention and exon-skipping, have now been identified from the affected areas of ALS patients. The aberrant mRNAs were highly abundant and were found only in neuropathologically affected areas of ALS patients but not in other brain regions. They were found in 65% of sporadic ALS patients but were not found in nonneurologic disease or other disease controls. They were also detectable in the cerebrospinal fluid (CSF) of living ALS patients, early in the disease. In vitro expression studies suggest that proteins translated from these aberrant mRNAs may undergo rapid degradation and/ or produce a dominant negative effect on normal EAAT2 resulting in loss of protein and activity. These findings suggest that the loss of EAAT2 in ALS is due to aberrant mRNA and that these aberrant mRNAs could result from RNA processing errors. Aberrant RNA processing could be important in the pathophysiology of neurodegenerative disease and in excitotoxicity. The presence of these mRNA species in ALS CSF may have diagnostic utility.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

“…42 The variant with exon 9 skipped lacks the motif that triggers export of the transporter from the endoplasmic reticulum. 43 Retention of intron 7 introduces premature termination codons that should mark the transcript for nonsense-mediated decay.…”

Astrocytes expressing ALS‐linked mutant FUS induce motor neuron death through release of tumor necrosis factor‐alpha

“…Furthermore, we recently showed that ALS astrocytes, including mutFUS expressing patient astrocytes, drive up‐regulation of the multidrug resistance transporter P‐Glycoprotein (P‐gp) in endothelial cells of the blood‐brain barrier (Qosa et al, 2016). Astrocytes participate in disease also through a “loss of function” [reviewed (Maragakis and Rothstein, 2006)] as demonstrated by several human and rodent studies that have observed defects and/or loss of astrocytic glutamate transporter EAAT2/GLT1 for several ALS subtypes including SOD1‐ALS (Benkler, Ben‐Zur, Barhum, & Offen, 2013; Howland et al, 2002) sALS (Lin et al, 1998), and C9orf72‐ALS (Kwon et al, 2014). In contrast, studies of astrocytic contributions in TDP‐43‐mediated ALS have shown conflicting results for both gain and loss‐of‐function toxicity (Haidet‐Phillips et al, 2013; Serio et al, 2013; Tong et al, 2013).…”

Glutamate transporters in platelets: EAAT1 decrease in aging and in Alzheimer’s disease

“…The first three transporters are involved in high affinity, sodium-dependent glutamate uptake and knock-out models indicate a specific role of each transporter in preventing excitotoxicity and neuronal degeneration [46,54]. Moreover, reduced expression of GLT1 (EAAT2) in global transient ischemia animal model [56] and in ALS patients [30,47] seems responsible for the elevation of extracellular glutamate concentration and neuronal death. Molecular and/or functional characterizations of glutamate transporters were performed in different cell culture models from CNS and peripheral tissues, in order to investigate the regulation of their expression, translocation and activity by various endogenous and exogenous factors [11][12][13]21,35,38,49].…”