Abstract 14933: Beta1-Adrenergic Receptor Polymorphisms Modulate Receptor Processing and Function

The beta1-adrenergic receptor (beta1AR) is a G protein-coupled receptor that modulates the sympathetic activation of the heart. The extracellular N-terminal domain of the beta1AR, that undergoes extensive post-translational modifications, is generally overlooked as a domain influencing receptor signaling and regulation. Previously, we showed that the N-terminal domain is cleaved in an activation dependent manner, and more recently, we demonstrated that O-glycosylation inhibits the N-terminal cleavage and modulates signaling of the receptor. This suggests that processing of the N-terminal domain has a complex role in the regulation of beta1AR function. Here, we investigated the three human beta1AR (hbeta1AR) N-terminal single-nucleotide polymorphisms (SNPs) that reside close to the previously identified cleavage and O-glycosylation sites. These SNPs, S49G, R31Q and A29T, have allele frequencies of ~15%, <1% and <1%, respectively. The aim was to investigate if these SNPs can modulate the O-glycosylation -mediated receptor regulation. We studied the impact of the SNPs on hbeta1AR O-glycosylation, proteolytic processing and cell surface expression using cultured cell lines and isolated neonatal rat cardiomyocytes. We find that the proteolytic cleavage of the receptor is attenuated for the T29 and Q31 variants and enhanced for the G49 variant. The attenuated cleavage of T29 and Q31 variants leads to an increase in the number of full-length receptors at the cell surface. For the Q31 variant, the attenuated cleavage is caused by the disruption of the receptor cleavage site at position R31↓L32, whereas cleavage of the T29 variant is inhibited due to an additional O-glycan found at position 29. In comparison, the existing O-glycan is eliminated at position 49 in the G49 variant. We conclude that the hbeta1AR SNPs affect receptor processing via disrupting the proteolytic cleavage site at R31↓L32 (Q31 variant) or through altered O-glycosylation (T29 and G49 variants). Since the altered glycosylation/cleavage of the beta1AR variants affects the number of functional receptors at the cell surface, it can offer new strategies for drug design and help in understanding the mechanism for the decreased cardiomyocyte beta1AR density in heart failure.