Elucidating the Role of Endocytosis in cAMP-dependent Transcription
There is a growing body of evidence from the last decade that supports the ability of G protein-coupled receptors (GPCRs) to signal from the endosome. With examples of different subtypes (Gs, Gi, Gq) of GPCRs now recognized with this ability, we still have much to learn about the role of the endosomal signal. This work explores how endocytosis selectively promotes cAMP-dependent transcription by studying a Gs-coupled GPCR, the beta-2 adrenergic receptor (β2AR). To begin to investigate the mechanism of how the endosome signal is selective for transcription, I utilized recently developed tools including a cAMP fluorescence biosensor, and a knock-in cell line endogenously expressing fluorescently-labeled protein kinase A catalytic subunit (PKAcat). I examined the effects of endocytic blockade on each step of the cAMP signaling cascade using primarily spinning disk confocal microscopy, genetic manipulations, biochemical and molecular biology methods. In this study we found that endocytosis greatly affected PKAcat nuclear accumulation, and that the accumulation in the nucleus was necessary for cAMP-dependent transcription of PCK1.
Because this is still a relatively new area of GPCR cell biology, many questions remain unanswered, including how does endocytosis affect the functional relationship between two receptors? β2ARs, known to signal from the endosome, have an antagonistic relationship with the M2 muscarinic acetylcholine receptor (M2R). These Gs- and Gi-coupled receptors, respectively, control the overall cAMP produced in the cell by stimulating or inhibiting the cAMP producing enzyme. The functional relationship between these two receptors are a great example for studying the effects of endocytosis on integrative cellular signaling. This work explored the relationship between the β2AR and M2R both by examining the trafficking of each receptor and by registering the endosomal signal of the β2AR, cAMP-dependent transcription. I utilized already established imaging and molecular biology methods to probe the effect of M2R inhibition on β2AR-stimulated cAMP signaling. I additionally generated modified constructs complementary to existing nanobody biosensors to detect activated states of the M2R and Gαi protein. This work found that it was not necessary for M2R to undergo endocytosis in order to inhibit β2AR cAMP-dependent transcription.