β-Adrenergic Receptors: Not Always Outside-In

Abstract

Canonical activation of G protein-coupled receptors (GPCRs) by hormone binding occurs at the plasma membrane, resulting in the diffusion of second messengers to intracellular effector sites throughout the cell. In contrast, recent evidence suggests that functional GPCRs can induce signaling from distinct intracellular domains, contributing to specificity in signaling. Functional adrenergic receptors have been identified at intracellular sites in the cardiac myocyte such as endosomes, the sarcoplasmic reticulum, the Golgi, and the inner nuclear membrane. These receptors are key regulators of cardiac physiology, mediating the response of the heart to sympathetic stimulation. Under conditions of prolonged cardiac stress leading to chronic adrenergic receptor stimulation, these receptors stimulate pathways that lead to cardiac pathophysiology such as myocyte hypertrophy, apoptosis, and fibrosis, ultimately leading to heart failure. Hence, significant work has resulted in the pharmacological modulation of β-adrenergic receptors for therapeutic benefit. Here, we discuss how the localization of β₁- and β₂-adrenergic receptors to different sites within the cardiac myocyte dictates control over specific physiological and pathological events. We discuss how therapeutically targeting receptors at these distinct sites may be used for the treatment of cardiac disease.

Keywords: compartmentation; scaffold protein; signaling; β-adrenergic receptors.

Figure 1.. β-adrenergic receptor signaling.

A) Hormone binding to βAR induces the recruitment of a heterochromatic G-protein complex consisting of Gα_s and Gβγ. Gα_s stimulates adenylyl cyclase to produce cAMP, allowing for the activation of cAMP effectors such as EPAC and PKA. PKA is a heterotetrametric protein kinase, while EPAC is a guanine nucleotide exchange factor for RAP GTPases. Other cAMP targets not discussed here are Popeye domain containing proteins (POPDC) and Hyperpolarization Activated Cyclic Nucleotide Gated Potassium Channels (HCN). Created in BioRender. Turcotte, M. (2025) https://BioRender.com/k4dbczu

Figure 2.. Termination of β-adrenergic receptor signaling

GRK phosphorylation of βAR facilitates the recruitment of β-arrestin, inducing the internalization of the receptor, desensitization of the receptor, while maintaining the possibility of prolonged β-arrestin-mediated signaling (13). Internalized receptors can recycle to the plasmalemma or shuttle to lysosomes for degradation. Created in BioRender. Turcotte, M. (2025) https://BioRender.com/y0xemad

Figure 3.. Defined internal sites of βAR localization.

Functional β₂AR (depicted in purple) are localized to endosomes while β₁AR (depicted in blue) have been found at sites such as the sarcoplasmic reticulum, the Golgi, and the inner membrane of the nuclear envelope. β₃AR (depicted in green) have been identified on inner nuclear membrane of cardiac myocytes. Created in BioRender. Turcotte, M. (2025) https://BioRender.com/zdbauge

Figure 4.. Tools to investigate intracellular βAR.

Several tools have been utilized to investigate the role of internal βAR on cardiac physiology, including 1) the use of membrane permeant and membrane impermeant βAR blockers, 2) drugs such as corticosterone that inhibit the import of the endogenous hormone by inhibiting the OCT3 transporter, 3) the use of subcellularly targeted nanobodies and pepducins that either inhibit βAR signaling or stimulate cAMP signaling in the absence of hormone binding, respectively, and 4) the use of caged agonists. Created in BioRender. Possidento, S. (2025) https://BioRender.com/jh16ki2