GPCR signaling is highly compartmentalized in human cardiomyocytes and severely remodeled in atrial fibrillation

Atrial fibrillation (AF) has been linked to the remodeling of membrane receptors and alterations in downstream cAMP-dependent regulation. However, to date, no study has elucidated how the increase on cAMP upon different G-protein-coupled receptors (GPCRs) can lead to different physiological compartmentalized responses. The aim of this study was to investigate the compartmentally specific effects of GPCRs on cAMP levels in human atrial myocytes (HAMs) from patients with AF and control patients without AF (Ctl), and how these compartmentalized effects are altered in AF. HAMs were isolated from 60 AF and 76 Ctl patient tissues. Cells were transduced with adenoviruses (Epac1-camps, pm-Epac1-camps and Epac1-JNC) and cultured for 48 hours to express the FRET-based cAMP sensor in the cytosolic, membrane, and RYR2 nanodomains. Förster-resonance energy transfer (FRET) was used to measure cAMP levels in 525 HAMs stimulated with isoprenaline (100 µM), serotonin (100 µM), or the A2AR agonist CGS (200 nM). A desensitization to β-adrenergic receptor stimulation was exclusively found in the cytosol of AF myocytes, while no difference was seen in the RYR2 or LTCC compartment. Similar effects were observed upon serotonin stimulation with a significant desensitization in the cytosol, and no difference in the RYR2 compartment. In response to A2ARs stimulation AF myocytes displayed a significantly higher cytosolic increase in cAMP levels. However, no response was seen in the LTCC compartment in response to serotonin or A2AR stimulation. Collectively, our data show that cAMP levels are highly compartmentalized and differentially regulated by GPCRs. Furthermore, these results provide a mechanistic insight for the previously reported functional effects seen upon stimulation of these three receptors.