The agonist-induced activation of GPCRs leads to a number of processes that can attenuate GPCR signaling. The long-term activation of GPCRs via agonist administration has been shown to desensitize receptors and down-regulate receptor protein levels in a variety of receptor systems. The process of desensitization is defined as a reduced capability of agonist-induced second messenger production. Two major patterns of rapid GPCR desensitization have been characterized, homologous or agonist-specific, and heterologous or agonist-nonspecific. Homologous desensitization is a process whereby stimulation of a particular GPCR leads to desensitization of that specific GPCR. Heterologous desensitization, on the other hand, is where GPCR stimulation leads to desensitization of other types of GPCRs. Alternatively, down-regulation is defined as the reduction in total specific receptor binding sites (Bmax) without a change in apparent affinity (Kd), indicating a loss of total cell receptors. Chronic antagonist exposure can also lead to supersensitivity, whereby there is an increase in second messenger production in response to agonist exposure.
Desensitization is the result of the convergence of a number of different cellular processes including receptor-phosphorylation, the internalization of cell surface receptors, the down-regulation of receptors due to reduced receptor mRNA or protein synthesis, or increases in the lysosomal and plasma membrane degradation of preexisting receptors. Receptor phosphorylation via the action of intracellular protein kinases is the most rapid mechanism of receptor desensitization, resulting in the uncoupling of GPCRs from their cognate heterotrimeric G proteins. A number of protein kinases have been discovered to phosphorylate serine and threonine residues within the intracellular domains of GPCRs following agonist exposure, including the second messenger-dependent kinases such as cAMP-dependent protein kinase (PKA) and protein kinase C (PKC), as well as specific G protein-coupled receptor kinases (GRKs). The GRK family members have been demonstrated to selectively phosphorylate agonist-activated receptors and are therefore involved in mediating homologous desensitization. Alternatively, the second messenger-dependent protein kinases not only phosphorylate agonist-activated GPCRs, but also indiscriminately phosphorylate receptors that have not been exposed to agonist, hence these protein kinases are involved in the process of heterologous desensitization.
The current model for agonist-induced trafficking of GPCRs based primarily on work on the β2-adrenergic receptor and other GPCRs. Following activation, receptors are feedback phosphorylated by kinases such as G protein coupled receptors kinases (GRKs) or second messenger kinases such as protein kinase A (PKA) resulting in rapid desensitization due to G protein uncoupling. G protein uncoupling is further promoted by the binding of arrestins to the phosphorylated third intracellular loops and carboxy-terminal tails of agonist-activated GPCRs. In addition to their role in desensitization, arrestins act as scaffolding proteins promoting the targeting of desensitized receptors to clathrin coated pits for their subsequent internalization by the interaction of the carboxy-terminal portions of arrestin with both the clathrin heavy chain and the β2-adaptin subunit of AP-2. The GTPase dynamin induces neck formation of coated pits and their release into the cytoplasm as clathrin-coated vesicles. These coated vesicles fuse with early endosomes where the receptors may be dephosphorylated by specific phosphatases and recycled back to the plasma membrane fully resensitized or targeted to lysosomes for degradation.
G protein-coupled receptor kinases (GRKs) are consist of a family of protein kinases that regulate the activity of GPCRs by phosphorylating their intracellular domains after their associated G proteins have been released and activated.
GRKs and arrestins are key participants in the canonical pathways leading to phosphorylation-dependent GPCR desensitization, endocytosis, intracellular trafficking and resensitization as well as in the modulation of important intracellular signaling cascades by GPCR.