Abstract:
:Na+-dependent uptake of excitatory neurotransmitter glutamate in astrocytes increases cell energy demands primarily due to the elevated ATP consumption by glutamine synthetase and Na+, K+-ATPase. The major pool of GLAST/EAAT1, the only glutamate transporter subtype expressed by human fetal astrocytes in undifferentiated cultures, was restricted to the cytoplasmic compartment. Elevated glutamate concentrations (up to 50 microM) stimulated both glutamate uptake and Na+, K+-ATPase activity and concomitantly increased cell surface expression of GLAST and FXYD2/gamma subunit of Na+, K+-ATPase. Intracellular accumulation of glutamate or its metabolites per se was not responsible for these changes since metabolically inert transport substrate, D-aspartate, exerted the same effect. Nanomolar concentrations of TFB-TBOA, a novel nontransportable inhibitor of glutamate carriers, almost completely reversed the action of glutamate or D-aspartate. In the same conditions (i.e. block of glutamate transport) monensin, a potent Na+ ionophore, had no significant effect neither on the activation of Na+, K+-ATPase nor on the cell surface expression of gamma subunit or GLAST. In order to elucidate the roles of gamma subunit in the glutamate uptake-dependent trafficking events or the activation of the astroglial sodium pump, in some cultures gamma subunit/FXYD2 was effectively knocked down using siRNA silencing. Unlike the blocking effect of TFB-TBOA, the down-regulation of gamma subunit had no effect neither on the trafficking nor activity of GLAST. However, the loss of gamma subunit effectively abolished the glutamate uptake-dependent activation of Na+, K+-ATPase. Following withdrawal of siRNA from cultures, the expression levels of gamma subunit and the sensitivity of Na+, K+-ATPase to glutamate/aspartate uptake have been concurrently restored. Thus, the activity of GLAST directs FXYD2 protein/gamma subunit to the cell surface, that, in turn, leads to the activation of the astroglial sodium pump, presumably due to the modulatory effect of gamma subunit on the kinetic parameters of catalytic alpha subunit(s) of Na+, K+-ATPase.
journal_name
Neurochem Intjournal_title
Neurochemistry internationalauthors
Gegelashvili M,Rodriguez-Kern A,Sung L,Shimamoto K,Gegelashvili Gdoi
10.1016/j.neuint.2006.12.015subject
Has Abstractpub_date
2007-06-01 00:00:00pages
916-20issue
7-8eissn
0197-0186issn
1872-9754pii
S0197-0186(07)00021-6journal_volume
50pub_type
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