Brain expression and 3H-Guanosine binding analysis of novel G protein-coupled receptor for guanosine (GPR23/LPA4)

Several studies have shown that guanine-based purines exert biological effects on the central nervous system, possibly through membrane receptor. In a parallel work, we have identified the first guanosine G protein-coupled receptor GPR23, known as LPA4 receptor, involved in the modulation of guanosine-mediated antiproliferative effects in human glioma cell lines. Here, we performed in different brain areas the following studies: by PCR, the expression levels of GPR23; by [3H]-Guanosine radioligand binding assay, the binding properties of GPR23; by [35S] GTPγS binding assay, the receptor activation properties of guanosine. Among the examined areas, the cerebral cortex showed the highest GPR23 expression levels and affinity binding site for [3H]-Guanosine (KD 143.8 nM and Bmax 3713 nM) as compared to other brain regions with the following rank order: cerebral cortex>hippocampus>striatum>spinal cord. T! he activation of a G protein-coupled receptor in response to guanosine showed in the cerebral cortex an EC50 92 nM. The binding site for [3H]-guanosine was highly specific and both lysophosphatidic acid and guanine agonists were 10 times less effective than guanosine in displacing 50 nM [3H]-guanosine binding. Overall these data demonstrate the existence of different levels of GPR23 mRNAs and of guanosine binding in the brain areas examined. Nevertheless at this stage of study the guanosine binding observed could include, in addition to GPR23, other unidentified receptors