Antagonistic Interaction between Adenosine A_2AReceptors and Na⁺/K⁺-ATPase-α₂Controlling Glutamate Uptake in Astrocytes

Abstract

Astrocytic glutamate transporter-1 (GLT-I) is critical to control the bulk of glutamate uptake and, thus, to regulate synaptic plasticity and excitotoxicity. GLT-I glutamate uptake is driven by the sodium gradient implemented by Na ϩ /K ϩ -ATPases (NKAs) and the ␣ 2 subunit of NKA (NKA-␣2) is actually linked to GLT-I to regulate astrocytic glutamate transport. We recently found that adenosine A 2A receptors (A 2A Rs), which control synaptic plasticity and neurodegeneration, regulate glutamate uptake through unknown mechanisms. Here we report that A 2A R activation decreases NKA activity selectively in astrocytes to inhibit glutamate uptake. Furthermore, we found a physical association of A 2A Rs with NKA-␣2s in astrocytes, as gauged by coimmunoprecipitation and in situ proximity ligation assays, in the cerebral cortex and striatum, two brain regions where A 2A Rs inhibit the astrocytic glutamate uptake. Moreover, the selective deletion of A 2A Rs in astrocytes (using Gfa2-A 2A R-KO mice) leads to a concurrent increase of both astrocytic glutamate uptake and NKA-␣ 2 levels and activity in the striatum and cortex. This coupling of astrocytic A 2A Rs to the regulation of glutamate transport through modulation of NKA-␣ 2 activity provides a novel mechanism linking neuronal activity to ion homeostasis controlling glutamatergic activity, all of which are processes intricately associated with the etiology of several brain diseases.

Adenosine A2A receptor and ecto-5′-nucleotidase/CD73 are upregulated in hippocampal astrocytes of human patients with mesial temporal lobe epilepsy (MTLE)

“…Augmentation of the A 2A receptor protein in the hippocampus of MTLE patients is much more evident (P < 0.01) in total lysates than in synaptosomal fractions. These results suggest that the A 2A receptor is localized predominantly in nonneuronal cells, most probably glial cells [11][12][13]19], which are the most abundant cell contributor to hippocampal total lysates. Two protein species were recognized by the A 2A receptor antibody from Alpha Diagnostic (San Antonio, TX, USA).…”

“…While A 1 receptors are widely expressed throughout the brain, A 2A receptors are most abundant in the basal ganglia, but these receptors are also present at lower density in other brain regions, namely the hippocampus. The molecular mechanisms by which the A 2A receptor controls neuronal excitability and synaptic plasticity have been extensively studied [10][11][12][13][14]. Activation of the A 2A receptor stimulates glutamate release and prevents its uptake, thus resulting in increased synaptic levels of this excitatory amino acid [11][12][13].…”

“…The molecular mechanisms by which the A 2A receptor controls neuronal excitability and synaptic plasticity have been extensively studied [10][11][12][13][14]. Activation of the A 2A receptor stimulates glutamate release and prevents its uptake, thus resulting in increased synaptic levels of this excitatory amino acid [11][12][13]. Excitatory synaptic transmission may be further unbalanced because A 2A receptor activation favors desensitization of presynaptic inhibitory modulation via A 1 receptors [15,16].…”

“…Although it was believed that the adenosine A 2A receptor was mostly located on hippocampal glutamatergic nerve terminals [27,28], recent evidences demonstrated that this receptor is also highly expressed in glial cells, both in astrocytes and microglia [11][12][13]19]. Astrocytes modulate the activity of neuronal networks upon sustained and intense activity, but it has been recently evidenced that astrocytes can also modulate basal synaptic transmission by the release of purines [29].…”

“…Interestingly, γ-aminobutyric acid (GABA) uptake in astrocytes has been attributed to the expression of A 1 /A 2A heteromers and to a specific mechanism by which these heteromers signal via inhibitory G i or facilitatory G s depending on the extracellular concentration of adenosine [30]. The astrocytic A 2A receptor also seems to be critical for the modulation of glutamate transport, either by decreasing the uptake [12,13] or increasing the release [11] of this amino acid. In view of this, astrocytic A 2A receptors have been implicated in memory formation [19].…”