G-protein coupled receptor resensitization-appreciating the balancing act of receptor function

Abstract

G-protein coupled receptors (GPCRs) are seven transmembrane receptors that are pivotal regulators of cellular responses including vision, cardiac contractility, olfaction, and platelet activation. GPCRs have been a major target for drug discovery due to their role in regulating a broad range of physiological and pathological responses. GPCRs mediate these responses through a cyclical process of receptor activation (initiation of downstream signals), desensitization (inactivation that results in diminution of downstream signals), and resensitization (receptor reactivation for next wave of activation). Although these steps may be of equal importance in regulating receptor function, significant advances have been made in understanding activation and desensitization with limited effort towards resensitization. Inadequate importance has been given to resensitization due to the understanding that resensitization is a homeostasis maintaining process and is not acutely regulated. Evidence indicates that resensitization is a critical step in regulating GPCR function and may contribute towards receptor signaling and cellular responses. In light of these observations, it is imperative to discuss resensitization as a dynamic and mechanistic regulator of GPCR function. In this review we discuss components regulating GPCR function like activation, desensitization, and internalization with special emphasis on resensitization. Although we have used β-adrenergic receptor as a proto-type GPCR to discuss mechanisms regulating receptor function, other GPCRs are also described to put forth a view point on the universality of such mechanisms.

Fig. (1). Classical view of desensitization, resensitization, and recycling of GPCRs

When a ligand binds to the GPCR it causes a conformational change in the GPCR. The GPCR activates an associated G-protein by exchanging its bound GDP for a GTP. The G-protein's α subunit dissociates from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins. The dissociated βγ subunits facilitate the recruitment of GRK and PI3K to the receptor complex. The GRK phosphorylates C-terminal tail of the receptor which results in increased affinity and binding of the receptors to β-arrestins. Once bound, β-arrestins prevent G-protein coupling (desensitization) and may recruit other proteins AP-2 and clathrin required for receptor endocytosis (internalization). PI3K plays a role in generation of D3 phosphoinositides required for recruitment of AP-2 to the receptor complex. The membrane buds inwardly and pinched off from the membrane to form endocytic vesicles. Once internalized the receptors are trafficked to recycling endosomes or targeted to lysosomes for degradation. The receptors in the recycling endosomes are subsequently dephosphorylated by PP2A and recycled back to the plasma membrane as resensitized receptors.

Fig. (2). Shifting paradigm in GPCR resensitization

Activation of GPCR by an agonist leads to the recruitment of GRK and PI3K to the receptor complex at the plasma membrane. While GRK phosphorylates the C-terminal tail of the receptor, PI3K inhibits the activity of the receptor dephosphorylating enzyme PP2A. PI3K phosphorylates an endogenous inhibitor of PP2A (I2PP2A) resulting in increased binding of I2PP2A to PP2A reducing latter's activity. Pharmacological or genetic inhibition of PI3K manifests in higher PP2A activity and rapid dephosphorylation of the receptors at the plasma membrane. Several recent studies suggest that GPCRs can be efficiently dephosphorylated and resensitized at the plasma membrane without necessarily undergoing internalization.