Feedback regulation of G protein-coupled receptor signaling by GRKs and arrestins

Abstract

GPCRs are ubiquitous in mammalian cells and present intricate mechanisms for cellular signaling and communication. Mechanistically, GPCR signaling was identified to occur vectorially through heterotrimeric G proteins that are negatively regulated by GRK and arrestin effectors. Emerging evidence highlights additional roles for GRK and Arrestin partners, and establishes the existence of interconnected feedback pathways that collectively define GPCR signaling. GPCRs influence cellular dynamics and can mediate pathologic development, such as cancer and cardiovascular remolding. Hence, a better understanding of their overall signal regulation is of great translational interest and research continues to exploit the pharmacologic potential for modulating their activity.

Keywords: Arrestin; Biased signaling; G protein; GPCR; GRK; Signal transduction.

Figure 1. Classical 7TMR/GPCR signaling

A) Inactive G proteins exist in a heterotrimeric state: the constituent α-subunit, bound to GDP, is associated with a βγ-subunit dimer. B) GPCR activation promotes GDP/GTP exchange on the α-subunit, which stimulates the dissociation of activated α•GTP from the βγ-subunits. Gsα•GTP and Gqα••GTP respectively activate adenylyl cyclase and phospholipase C, which increase intracellular second messengers cyclic AMP (cAMP) and inositol tris-phosphate (IP₃). Giα•GTP inhibits adenylyl cyclase thereby decreasing cAMP accumulation.

Figure 2. 7TMR/GPCR signal regulation

A) Receptor desensitization. GRKs phosphorylate the C-terminal of activated GPCRs. Arrestin binds to the phosphorylated receptor that is then sterically blocked from interacting with cognate G protein. B) Receptor sequestration. Arrestin serve as an adaptor and recruit endocytic machinery proteins to mediate GPCR internalization into vesicles. GPCR internalization sequesters the activated receptors from further stimulation. C) Arrestin may dissociate or remain bound to the receptor in vesicle. Early dissociation of arrestin promotes receptor recycling. Failure of the arrestin-GPCR complex to dissociate marks that receptor for further processing that results in either receptor degradation or delayed receptor recycling.

Figure 3. GPCR feedback regulation

A) G protein switching. Upon initial stimulation, β₂AR activates the G_s protein to increase adenylate cyclase signaling and PKA activity to propagate the cellular response. PKA initiates G protein switching by phosphorylating site-specific residues on β₂AR. PKA phosphorylation of β₂AR alters the receptor coupling from G_s to G_i. Activated G_i yields a decrease in cAMP production and PKA activity. B) Arrestins partake in G_q-coupled M1 muscarinic receptor feedback regulation. Arrestin scaffolds with diacylglycerol kinases (DGKs) and recruits them to the plasma membrane. These events activate DGKs to degrade DAG, promoting arrestin-mediated G_q negative feedback regulation. C) Upon β₂AR activation, arrestin scaffolds with PDE4D and promotes its translocation to the plasma membrane. PDE4D translocation and activation negatively regulates the β₂AR-cAMP cascade by degrading the substrate necessary for PKA activation

Figure 4. Arrestin mediated signaling

A) Arrestins extend the GPCR signaling pathway beyond G proteins by scaffolding with kinases. GPCR ligand activation induces receptor conformational changes that can result in arrestin signaling activation. Arrestin mediated signaling is independent of G protein coupling and can regulate diverse cellular processes. B) Arrestins regulate protein spatiotemporal distribution. GPCR stimulation promotes ERK activation via G protein mediated mechanisms. This pathway promotes nuclear translocation of ERK and subsequent activation of downstream transcription factors. Arrestin mediates an alternative pathway wherein ERK complexes with GPCR-bound arrestin. Vesicle-bound ERK is sequestered in the cytoplasm and activates a separate set of effectors.