Abstract
It has become well accepted that Huntington disease (HD) is associated with impaired glutamate uptake, resulting in a prolonged time-course of extracellular glutamate that contributes to excitotoxicity. However, the data supporting this view come largely from work in synaptosomes, which may overrepresent nerve-terminal uptake over astrocytic uptake. Here, we quantify real-time glutamate dynamics in HD mouse models by high-speed imaging of an intensity-based glutamate-sensing fluorescent reporter (iGluSnFR) and electrophysiological recordings of synaptically activated transporter currents in astrocytes. These techniques reveal a disconnect between the results obtained in synaptosomes and those in situ. Exogenous glutamate uptake is impaired in synaptosomes, whereas real-time measures of glutamate clearance in the HD striatum are normal or even accelerated, particularly in the aggressive R6/2 model. Our results highlight the importance of quantifying glutamate dynamics under endogenous release conditions, and suggest that the widely cited uptake impairment in HD does not contribute to pathogenesis.
Highlights
It has become well accepted that Huntington disease (HD) is associated with impaired glutamate uptake, resulting in a prolonged time-course of extracellular glutamate that contributes to excitotoxicity
Slow glutamate clearance as a result of dysfunctional transportermediated uptake has been associated with neuronal cell death in disease via pathogenic activation of ionotropic glutamate receptors located on neurons
We were interested in quantifying the temporal dynamics of glutamate sensed at the surface of striatal neurons (Fig. 1a) in mouse models of HD. iGluSnFR was expressed under the control of the synapsin promoter by viral delivery (AAV2/1-synapsin-iGluSnFR) into the dorsal striatum
Summary
It has become well accepted that Huntington disease (HD) is associated with impaired glutamate uptake, resulting in a prolonged time-course of extracellular glutamate that contributes to excitotoxicity. 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[2,3,4,5,6,7,8]. This is an important finding, as a much higher density of uptake sites is found on astrocytes than on neurons[1,22] and, there appears to be a much greater physiological role of astrocytic uptake in comparison with nerve terminal uptake[21] 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. Our 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
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