Differential expression of adenosine A₃ receptors controls adenosine A_2A receptor-mediated inhibition of TLR responses in microglia

Microglia activation is a prominent feature in many neuroinflammatory disorders. Unrestrained activation can generate a chronic inflammatory environment that might lead to neurodegeneration and autoimmunity. Extracellular adenosine modulates cellular activation through adenosine receptor (ADORA)-mediated signaling. There are four ADORA subtypes that can either increase (A_2A and A_2B receptors) or decrease (A ₁ and A₃ receptors) intracellular cyclic AMP levels. The expression pattern of the subtypes thus orchestrates the cellular response to extracellular adenosine. We have investigated the expression of ADORA subtypes in unstimulated and TLR-activated primary rhesus monkey microglia. Activation induced an up-regulation of A_2A and a down-regulation of A ₃ receptor (A₃R) levels. The altered ADORA-expression pattern sensitized microglia to A_2A receptor (A_2AR)- mediated inhibition of subsequent TLR-induced cytokine responses. By using combinations of subtype-specific agonists and antagonists, we revealed that in unstimulated microglia, A_2AR-mediated inhibitory signaling was effectively counteracted by A₃R-mediated signaling. In activated microglia, the decrease in A₃R-mediated signaling sensitized them to A_2AR-mediated inhibitory signaling. We report a differential, activation state-specific expression of ADORA in microglia and uncover a role for A₃R as dynamically regulated suppressors of A_2AR- mediated inhibition of TLR-induced responses. This would suggest exploration of combinations of A_2AR agonists and A₃R antagonists to dampen microglial activation during chronic neuroinflammatory conditions.