Functional Indicators of Glutamate Transport in Single Striatal Astrocytes and the Influence of Kir4.1 in Normal and Huntington Mice

Abstract

This study evaluates single-cell indicators of glutamate transport in sulforhodamine 101-positive astrocytes of Q175 mice, a knock-in model of Huntington's disease (HD). Transport-related fluorescent ratio signals obtained with sodium-binding benzofuran isophtalate (SBFI) AM from unperturbed or voltage-clamped astrocytes and respective glutamate transporter currents (GTCs) were induced by photolytic or synaptic glutamate release and isolated pharmacologically. The HD-induced deficit ranged from Ϫ27% (GTC maximum at Ϫ100 mV in Ba 2ϩ ) to Ϫ41% (sodium transients in astrocytes after loading SBFI-AM). Our specific aim was to clarify the mechanism(s) by which Kir4.1 channels can influence glutamate transport, as determined by either Na ϩ imaging or transport-associated electrical signals. A decrease of Kir4.1 conductance was mimicked with Ba 2ϩ (200 M), and an increase of Kir4.1 expression was obtained by intravenous administration of AAV9 -gfaABC1D-Kir4.1-EGFP. The decrease of Kir4.1 conductance reduced the sodium transients but increased the amplitudes of somatic GTCs. Accordingly, after genetic upregulation of Kir4.1, somatic GTCs were found to be decreased. In individual cells, there was a negative correlation between Kir4.1 currents and GTCs. The relative effect of the Kir4.1 conductance was higher in the astrocyte periphery. These and other results suggest that the Kir4.1 conductance affects glutamate transporter activity in a dual manner: (1) by providing the driving force (voltage dependency of the transport itself) and (2) by limiting the lateral charge transfer (thereby reducing the interference with other electrogenic transporter functions). This leads to the testable prediction that restoring the high conductance state of passive astrocytes will not only normalize glutamate uptake but also restore other astrocytic transporter activities afflicted with HD.

Differential electrophysiological and morphological alterations of thalamostriatal and corticostriatal projections in the R6/2 mouse model of Huntington's disease

“…Postmortem HD tissue shows a reduction of GLT1 and decreased glutamate reuptake (Arzberger et al, 1997; Hassel et al, 2008; Faideau et al, 2010). Failure in glutamate reuptake and the resulting excitotoxicity also have been proposed in HD models (Lievens et al, 2001; NicNiocaill et al, 2001; Miller et al, 2008; Estrada-Sanchez et al, 2009; Tong et al, 2014; Dvorzhak et al, 2016), however, see (Parsons et al, 2016). Further, mutant HTT expression restricted to astrocytes is sufficient to decrease GLT1 levels and elicit the HD behavioral phenotype (Bradford et al, 2009).…”

Unravelling and Exploiting Astrocyte Dysfunction in Huntington’s Disease

“…In Q175 mice, EPSC decay was slowed [93], notably when the EPSC included NMDAR-mediated currents at membrane potentials of +50 mV and higher. This observation is in line with the results from normal mice and rats where pharmacological block of GLT1 with TBOA induced EPSC prolongation [94] and changes in the preferentially expressed type of plasticity [82].…”

“…This question was addressed by recording glutamate transporter currents (GTC; Figure 4C–F) in single sulphorhodamine-labelled astrocytes [93]. Addition of Ba 2+ to the recording solution decreased the impact of Kir4.1, thereby unmasking a functional deficit of GLT1 itself.…”

Early dysfunction and progressive degeneration of the subthalamic nucleus in mouse models of Huntington's disease

“…Several reports suggest that glutamate uptake is impaired due to reduced expression of the glutamate transporter EAAT2 (GLT-1) and/or GLT-1 dysfunction (Arzberger et al, 1997; Liévens et al, 2001; Behrens et al, 2002; Miller et al, 2008; Bradford et al, 2009; Faideau et al, 2010; Huang et al, 2010; Menalled et al, 2012; Dvorzhak et al, 2016; Jiang et al, 2016). However, others have found no evidence for deficient glutamate uptake (Parsons et al, 2016), suggesting that abnormal NMDAR-mediated transmission is caused by increased expression of extrasynaptic receptors and/or aberrant coupling to signaling pathways (e.g., Parsons and Raymond, 2014).…”