title: 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 -27% (GTC maximum at -100 mV in Ba(2+)) to -41% (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(+) imaging or transport-associated electrical signals. A decrease of Kir4.1 conductance was mimicked with Ba(2+) (200 μm), 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.
  
  
  
  Insufficiency of astrocytic glutamate uptake is a major element in the pathophysiology of neurodegenerative diseases. Considering the heterogeneity of astrocytes and their differential susceptibility to therapeutic interventions, it becomes necessary to evaluate the determinants of transport activity in individual astroglial cells. We have examined intracellular Na(+) transients and glutamate transporter currents as the most telling indicators of glutamate clearance after synaptic or photolytic release of glutamate in striatal slices. The results show that, in Huntington's disease, glutamate uptake activity critically depends on Kir4.1. These channels enable the high conductance state of the astrocytic plasma membrane, which ensures the driving force for glutamate transport and dumps the transport-associated depolarization along the astrocyte processes. This has significant implications for developing therapeutic targets.
authors.affiliation: Cluster of Excellence NeuroCure Berlin and.
authors.forename: Anton
authors.initials: A
authors.lastname: Dvorzhak
authors.affiliation: Cluster of Excellence NeuroCure Berlin and.
authors.forename: Tatyana
authors.initials: T
authors.lastname: Vagner
authors.affiliation: Hans Berger Department of Neurology, University Hospital Jena, Jena D-07747, Germany, and.
authors.forename: Knut
authors.initials: K
authors.lastname: Kirmse
authors.affiliation: Cluster of Excellence NeuroCure Berlin and Department of Experimental Neurology, University Medicine Charité Berlin, Berlin D-10115, Germany rosemarie.grantyn@charite.de.
authors.forename: Rosemarie
authors.initials: R
authors.lastname: Grantyn
mesh_terms: D027322:Amino Acid Transport System X-AG / Q000378:metabolism; D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001572:Benzofurans / Q000494:pharmacology; D027342:Excitatory Amino Acid Transporter 2 / Q000235:genetics / Q000378:metabolism; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D017629:Gap Junctions / Q000187:drug effects / Q000378:metabolism; D055879:Gene Knock-In Techniques; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism*; D006816:Huntington Disease / Q000378:metabolism*; D051379:Mice; D017072:Neostriatum / Q000166:cytology / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D010795:Phthalic Acids / Q000494:pharmacology; D024661:Potassium Channels, Inwardly Rectifying / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000098854:Kcnj10 Channel
chemical_list: D027322:Amino Acid Transport System X-AG; D001572:Benzofurans; D027342:Excitatory Amino Acid Transporter 2; D005904:Glial Fibrillary Acidic Protein; D000098854:Kcnj10 Channel; D010795:Phthalic Acids; D024661:Potassium Channels, Inwardly Rectifying; C493396:Slc1a2 protein, mouse; C528900:glial fibrillary astrocytic protein, mouse; C103805:sodium binding benzofuran isophthalate acetoxymethyl ester; D018698:Glutamic Acid
keywords: SBFI imaging; adenoviral transduction; glutamate uptake; neuron-glia; paired recording; photolytic uncaging