&bgr;2-Adrenergic Stimulation Compartmentalizes &bgr;1 Signaling Into Nanoscale Local Domains by Targeting the C-Terminus of &bgr;1-Adrenoceptors

Abstract

Rationale: &bgr;ARs (&bgr;-adrenergic receptors) are prototypical GPCRs (G protein–coupled receptors) that play a pivotal role in sympathetic regulation. In heart cells, &bgr;1AR signaling mediates a global response, including both L-type Ca2+ channels in the sarcolemma/T tubules and RyRs (ryanodine receptors) in the SR (sarcoplasmic reticulum). In contrast, &bgr;2AR mediates local signaling with little effect on the function of SR proteins. Objective: To investigate the signaling relationship between &bgr;1ARs and &bgr;2ARs. Method and Results: Using whole-cell patch-clamp analyses combined with confocal Ca2+ imaging, we found that the activation of compartmentalized &bgr;2AR signaling was able to convert the &bgr;1AR signaling from global to local mode, preventing &bgr;1ARs from phosphorylating RyRs that were only nanometers away from sarcolemma/T tubules. This offside compartmentalization was eliminated by selective inhibition of &bgr;2AR, GRK2 (GPCR kinase-2), &bgr;arr1 (&bgr;-arrestin-1), and phosphodiesterase-4. A knockin rat model harboring mutations of the last 3 serine residues of the &bgr;1AR C terminus, a component of the putative &bgr;arr1 binding site and GRK2 phosphorylation site, eliminated the offside compartmentalization conferred by &bgr;2AR activation. Conclusions: &bgr;2AR stimulation compartmentalizes &bgr;1AR signaling into nanoscale local domains in a phosphodiesterase-4–dependent manner by targeting the C terminus of &bgr;1ARs. This finding reveals a fundamental negative feed-forward mechanism that serves to avoid the cytotoxicity of circulating catecholamine and to sharpen the transient &bgr;1AR response of sympathetic excitation.