SAP102 Switches the Mechanism of D₁R-Mediated ERK1/2 Activation from a PKA-Independent to PKA-Dependent Pathway

Abstract

Hyperactivation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) by dopamine D₁ receptor (D₁R) in the striatum is a characteristic feature of several neuropsychiatric conditions, including drug addiction and L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) within individuals suffering from Parkinson’s disease. However, the current mechanistic gap in understanding of D₁R-mediated regulation of ERK1/2, both in physiology and disease, hampers effective treatment of these conditions. One important factor that is underexplored in this regard is the role played by proteins that physically interact with the intracellular regions of D₁R. Using yeast two-hybrid screens and co-transfected human embryonic kidney 293 (HEK293) cells, our laboratory has recently characterized an interaction between the third intracellular loop of D₁R and synapse-associated protein 102 (SAP102), a member of the membrane-associated guanylate kinase family. Moreover, our lab identified endogenous D₁R-SAP102 complex within rat striatum and hippocampus. Interestingly, SAP102 regulates ERK signaling pathway within the hippocampus, and modulates adenosine A2A receptor-mediated ERK1/2 activation within transfected HEK293 cells. Capitalizing on the above findings, I hypothesized a role for SAP102 in controlling D₁R-mediated ERK1/2 activation. Herein, I demonstrate using co-transfected HEK293 cells that SAP102 alters the temporal activation of ERK1/2 by D₁R. Intriguingly, experiments using the protein kinase A (PKA) inhibitors H89 and protein kinase inhibitor 14-22 amide myristoylated show that SAP102 also facilitates a switch in D₁R-mediated ERK1/2 activation from a PKA-independent to PKA-dependent pathway. Furthermore, SAP102 reduces basal ERK1/2 activation in HEK293 cells, reminiscent of a previously documented role of hippocampal SAP102. To the best of my knowledge, my findings are the first to demonstrate scaffolding protein-mediated switching of a G protein-coupled receptor (GPCR) signaling pathway. Future studies aimed at uncovering the details of this process should provide valuable insight on the mechanisms contributing to D₁R-mediated ERK1/2 activation, and may also offer clues to the existence of similar phenomena for other GPCRs. These studies would also aid development of improved pharmacological treatment options for conditions with dysfunctional D₁R-dependent ERK1/2 activation.