Dopamine Receptor Signaling in MIN6 β-Cells Revealed by Fluorescence Fluctuation Spectroscopy

Insulin secretion defects are central to the development of type II diabetes mellitus. Glucose stimulation of insulin secretion has been extensively studied, but its regulation by other stimuli such as incretins and neurotransmitters is not as well understood. We investigated the mechanisms underlying the inhibition of insulin secretion by dopamine, which is synthesized in pancreatic β-cells from circulating L-dopa. Previous research has shown that this inhibition is mediated primarily by activation of the dopamine receptor D3 subtype (DRD3), even though both DRD2 and DRD3 are expressed in β-cells. To understand this dichotomy, we investigated the dynamic interactions between the dopamine receptor subtypes and their G-proteins using two-color fluorescence fluctuation spectroscopy (FFS) of mouse MIN6 β-cells. We show that proper membrane localization of exogenous G-proteins depends on both the G_β and G_γ subunits being overexpressed in the cell. Triple transfections of the dopamine receptor subtype and G_β and G_γ subunits, each labeled with a different-colored fluorescent protein (FP), yielded plasma membrane expression of all three FPs and permitted an FFS evaluation of interactions between the dopamine receptors and the G_βγ complex. Upon dopamine stimulation, we measured a significant decrease in interactions between DRD3 and the G_βγ complex, which is consistent with receptor activation. In contrast, dopamine stimulation did not cause significant changes in the interactions between DRD2 and the G_βγ complex. These results demonstrate that two-color FFS is a powerful tool for measuring dynamic protein interactions in living cells, and show that preferential DRD3 signaling in β-cells occurs at the level of G-protein release.