Changes In Expression Of Glutamate Transporters Research Articles

Regulation of extrasynaptic glutamate levels as a pathophysiological mechanism in disorders of motivation and addiction.

A growing body of evidence suggests that changes in glutamate transporter expression may be a factor that is common to many neuropsychiatric disorders. As an example, reduced glutamate reuptake capacity has been linked to a wide variety of conditions such as depression, schizophrenia, and addiction. Mathematical models suggest that under physiological conditions glutamate may diffuse and activate NMDA receptors within a radius of 0.5 μm from the release point (Tzingounis and Wadiche, 2007). Excitatory amino-acid transporters (EAATs) bind and transport glutamate, limiting spillover from synapses due to their dense perisynaptic expression primarily on astroglia. Thus, the spatial arrangement of glutamate synapses, their glutamate transporter buffering zones, and extrasynaptic glutamate receptors will determine the extent and effects of glutamate spillover (Tzingounis and Wadiche, 2007). Increased glutamate spillover may lead to a loss of input specificity, degrading the spatial precision of synaptic transmission. Decreased glutamate spillover, particularly in regions with high levels of physiologic spillover such as the hippocampus, could also disrupt plasticity by limiting spillover transmission. Disruption of glutamate reuptake with genetic models or pharmacological agents yields region- and mechanism-specific phenotypes. For example, the homozygous GLAST (called EAAT1 in the human) KO exhibits locomotor hyperactivity, social withdrawal, and abnormal acoustic startle—deficits analogous to the positive, negative, and cognitive symptoms observed in schizophrenia (Karlsson et al, 2009).

Changes in glutamate transporter expression in mouse forebrain areas following focal ischemia

Dysfunction of glutamate transporters has been proposed to promote neuronal death in modelled cerebral ischemia. However, these studies have produced conflicting results and the changes in glutamate transporter expression have not yet been examined in a mouse focal ischemic stroke model. This study used quantitative real-time reverse-transcription polymerase chain reaction to examine glutamate transporter mRNA expression in the hippocampus, cortex and striatum in a mouse model of focal ischemic stroke induced by middle cerebral artery occlusion (MCAO). Effects on mRNA expression of glial (GLT-1, GLAST) and neuronal (EAAC1) glutamate transporters in these brain areas were assessed by comparing MCAO brains with sham-operated control brains. Changes in transporter proteins were also assessed by immunohistochemistry using specific antibodies to GLT-1 and GLAST. Following focal ischemia, GLT-1 mRNA expression was decreased significantly in the ipsilateral hippocampus and cortex compared to the sham-operated brains (p < 0.05). There were no significant differences in GLAST or EAAC1 mRNA expression between MCAO and sham-operated brains. Immunohistochemistry also confirmed a marked reduction in GLT-1 immunoreactivity in the cortex and hippocampus. Down regulation of GLT-1 in these brain areas may impair normal clearance of synaptically-released glutamate and contribute to neural damage following focal ischemic insult.

Glutamate transporter expression and function in human glial progenitors.

Glutamate is the major neurotransmitter of the brain, whose extracellular levels are tightly controlled by glutamate transporters. Five glutamate transporters in the human brain (EAAT1-5) are present on both astroglia and neurons. We characterize the profile of three different human astroglial progenitors in vitro: human glial restricted precursors (HGRP), human astrocyte precursors (HAPC), and early-differentiated astrocytes. EAAT 1, EAAT3, and EAAT4 are all expressed in GRPs with a subsequent upregulation of EAAT1 following differentiation of GRPs into GRP-derived astrocytes in the presence of bone morphogenic protein (BMP-4). This corresponds to a significant increase in the glutamate transport capacity of these cells. EAAT2, the transporter responsible for the bulk of glutamate transport in the adult brain, is not expressed as a full-length protein, nor does it appear to have functional significance (as determined by the EAAT2 inhibitor dihydrokainate) in these precursors. A splice variant of EAAT2, termed EAAT2b, does appear to be present in low levels, however. EAAT3 and EAAT4 expression is reduced as glial maturation progresses both in astrocyte precursors and early-differentiated astrocytes and is consistent with their role in adult tissues as primarily neuronal glutamate transporters. These human glial precursors offer several advantages as tools for understanding glial biology because they can be passaged extensively in the presence of mitogens, afford the potential to study the temporal changes in glutamate transporter expression in a tightly controlled fashion, and are cultured in the absence of neuronal coculture, allowing for the independent study of astroglial biology.