Molecular and functional interactions between adenosine and dopamine receptors. New therapeutic targets for the treatment of Parkinson's disease.

Abstract

Adenosine and dopamine receptors are members of the GPCRs family for which a significant segregation between their A2A and D2 subtypes has been demonstrated in the GABAergic striopallidal neurons of the basal ganglia. In addition, several reports suggest an antagonistic interaction between these subtypes at different levels. For this reason the first study has been focused in the characterization of the molecular and functional interaction between A2AR and D2R. Colocalization of these receptors and clustering upon agonist stimulation has been observed by double immunocytochemistry in neuroblastoma SH-SY5Y cells as well as in primary cultures of striatal neurons. In addition, in the neuroblastoma cell line, it has been demonstrated that the internalization of both receptors takes place following their clusterization. Despite the A2AR-D2R heteromerization has been determined by coimmunoprecipitation, we have also studied if this interaction occurs in living cells by using FRET (Fluorescence Resonance Energy Transfer) and BRET (Bioluminescence Resonance Energy Transfer) approaches. Finally, using both, experimental and theoretical modeling data, it is suggested that the helix 5 and 6 and the 3rd intracellular loop of the D2R and the C-terminal tail of the A2AR are important domains for the formation of the A2AR-D2R heteromers. The homodimerization of the adenosine A2AR has also been studied by FRET and BRET. Furthermore, TR-FRET (Time-Resolved FRET) and biotinylation experiments have suggested that the homodimer, not the monomer, is the functional form of the receptor that reaches the cell surface. The last part of this study has been focused in the identification of the adenosine receptors that mediate the regulation of neuronal differentiation and the molecular mechanisms involved in this effect. Agonist-induced stimulation of A1Rs and A2ARs induces neurite outgrowth processes in the human neuroblastoma SH-SY5Y cell line and also in striatal neuronal precursor cells in primary cultures. The triggering of the expression of TrkB receptor and the arrest of cells in the G1 phase by the activation of adenosine receptors suggest that adenosine may participate in early steps of neuronal differentiation. Furthermore, the signaling transduction pathway involved in these effects have been shown to require both, the MAPK and the PKC activation but in an independent manner. The results presented in this work, therefore, suggest that adenosine not only acts as a key neuromodulator in information handling but also exerts trophic effects that can enhance neuronal differentiation and neuronal repair.