Glutamate transporters are responsible for the rapid uptake of extracellular glutamate following synaptic release. This ensures a low basal level of extracellular glutamate necessary to achieve a high signal-to-noise ratio during neurotransmission, and prevents overactivation of neuronal glutamate receptors that can promote cell-death signalling.
Various lines of evidence, notably from biochemical uptake assays in synaptosomal preparations, have suggested that several neurological conditions are characterized by impaired transporter-mediated glutamate uptake. This reduced capacity of synaptosomes to take up exogenous glutamate has been extrapolated to indicate a prolonged temporal profile of extracellular glutamate following synaptic release, thereby enhancing neuronal susceptibility to excitotoxic cell death. As a result, the enhancement of transporter-mediated uptake is understood to be a viable therapeutic approach for a number of conditions, particularly Huntington disease (HD).
HD is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding the huntingtin protein. This mutation gives rise to a clinical triad of motor, cognitive and psychiatric symptoms as well as progressive brain atrophy that is particularly striking in the striatum.
Huntingtin interacts with hundreds of proteins, and the mutant protein has been implicated in altered protein and organelle trafficking, changes in cellular metabolism, disrupted mitochondrial function and calcium homeostasis, transcriptional dysregulation and synaptic dysfunction. In addition, the earliest animal models of HD relied on intrastriatal injections of glutamate receptor agonists, and evidence indicates that striatal neurons show increased susceptibility to glutamate-mediated excitotoxicity in early HD.
Several studies demonstrate a reduced uptake capacity when HD striatal tissue is exposed to exogenous glutamate or aspartate on a timescale of minutes. These data have promulgated the view that glutamate uptake, particularly astrocytic uptake mediated by glutamate transporter-1 (GLT-1), is impaired in HD, resulting in extracellular glutamate build-up and excitotoxic signalling.
However, emerging data convincingly demonstrate that the uptake of externally supplied substrate in the brains slices and synaptosomal preparations largely occurs in the nerve terminals rather than in astrocytes. This is an important finding, as a much higher density of uptake sites is found on astrocytes than on neurons and, accordingly, there appears to be a much greater physiological role of astrocytic uptake in comparison with nerve terminal uptake.
Together, these data highlight the need to revisit the well-accepted view of an uptake impairment in HD, as no study to date has tested whether the HD mutation influences the time course of extracellular glutamate following synaptic release.
Researchers use a rapid, intensity-based glutamate-sensing fluorescent reporter (iGluSnFR) to visualize the spatiotemporal dynamics of extracellular glutamate in the HD striatum following evoked synaptic release. By virally expressing iGluSnFR under the control of a neuronal promoter, authors demonstrate that striatal neurons in HD are not exposed to a prolonged time course of extracellular glutamate following synaptic release, despite a significant reduction in 3H-labelled glutamate uptake in HD striatal synaptosomes as quantified by a standard biochemical assay.
Surprisingly, endogenous glutamate clearance in the brain slices, as measured both by high-speed iGluSnFR imaging and electrophysiological measures of transporter-mediated currents recorded from striatal astrocytes, is consistently accelerated in the well-characterized aggressive R6/2mouse model of HD.
The data suggest that biochemical measurements of exogenous glutamate uptake capacity do not necessarily correlate with glutamate clearance dynamics in situ, and highlight the need to re-evaluate our views on the contribution of, and therapeutic potential of targeting, glutamate transporters in disease.