Altered expression of the glutamate transporter EAAT2b in neurological disease

Abstract

Functional studies suggest that up to 95% of all glutamate transport is handled by the glutamate transporter EAAT2. Amino and C-terminal antibodies demonstrate that under normal conditions EAAT2 is specific to astrocytes. A truncated splice variant of EAAT2, known as EAAT2b, also has been identified in astrocytes and some neurons. In vitro studies suggest EAAT2b transports glutamate similar to EAAT2, although the contribution of EAAT2b to normal clearance of extracellular glutamate is unknown. To investigate EAAT2b biology in pathological conditions, we examined the cellular and regional distribution of EAAT2b in amyotrophic lateral sclerosis. Using epitope-specific, affinity purified antibodies, we found that EAAT2b tissue levels were increased by more than twofold in amyotrophic lateral sclerosis motor cortex, whereas EAAT2 levels were decreased by up to 95%. EAAT2b distribution in normal human cortex was largely confined to the neuropil-like EAAT2, with occasional faint neuronal expression. In contrast, amyotrophic lateral sclerosis motor cortex had an obvious qualitative increase in neuropil EAAT2b staining and a drastic increase in neuronal soma and dendritic EAAT2b immunostaining. Despite these increases in EAAT2b immunostaining, functional transporter studies demonstrated a large loss of EAAT2 function. These studies clearly document altered regulation and splicing of the dominant glutamate transporter EAAT2 under conditions of neurological stress.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

“…Notably, increased expression of the molecule occurs primarily in the somatodendritic compartment of pyramidal cells rather than astrocytes where this isoform is found in the normal brain. 69 We have recently shown that a variant of EAAT2 with a short alternative 3 0 -UTR and EAAT2b are actually increased in ALS motor cortex at the transcript level (unpublished observations). It remains to be determined whether an increase in an EAAT2 transcript with a shorter 3 0 -UTR is physiologically meaningful in this condition or whether the transcript is associated with an aberrant splice variant that has not yet been determined.…”

“…96 In another study, a 50% decrease in EAAT2b immunoreactivity was observed in motor cortex of AD patients, although only five subjects were included in this study. 69 One study found decreased EAAT2 immunoreactivity and glutamate transporter activity in the frontal cortex of AD patients without a corresponding change in the mRNA level. 97 Likewise, transgenic mice that overexpress mutant amyloid precursor protein (APP) exhibit reduced expression of GLT1 protein with no change in mRNA levels.…”

Glutamate Transporters and the Excitotoxic Path to Motor Neuron Degeneration in Amyotrophic Lateral Sclerosis

“…Changes in expression and activity of glutamate transporters have been reported in many neurodegenerative diseases such as Huntington's disease, Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis (ALS), but also in astrogliomas, epilepsy, and in more-acute neuropathologic events like stroke and ischemia (90). In ALS, the role of the glutamate transporter EAAT2 has been investigated more thoroughly.…”

Functional Modulation of the Glutamate Transporter Variant GLT1b by the PDZ Domain Protein PICK1

“…Although there is disagreement regarding neuronal expression of GLT1b (17)(18)(19)(20) or lack thereof (15,21), it appears that there is robust neuronal expression of the GLT1b variant under pathological conditions associated with amyotrophic lateral sclerosis and hypoxia (18,22). PICK1-induced GLT1b-dependent leak currents of the relative magnitude observed in the present study would most likely translate to a significant effect on the neuronal membrane potential with associated consequences on the excitability of glutamatergic neurons.…”

“…Neuronal expression of both variants has been observed (15)(16)(17)(18)(19)(20), although other research groups failed to detect GLT1b expression in neurons (15,21). Notably, induction of neuronal GLT1b expression occurs in association with amyotrophic lateral sclerosis and hypoxia, which may be a compensatory reaction to the loss of glial GLT1a that is associated with these pathologies (18,22). The intrinsic functional properties of GLT1b are not different from those of GLT1a (21), but interestingly, GLT1b con-tains a PDZ domain binding sequence in its C terminus that is absent in GLT1a (13).…”