Trends in Pharmacological Sciences
ReviewRole of endocytosis in mediating downregulation of G-protein-coupled receptors
Section snippets
Rapid processes of desensitization, sequestration and resensitization
GPCR-mediated signal transduction can be attenuated with relatively fast kinetics (within seconds to minutes after agonist-induced activation) by a process called rapid desensitization. Rapid desensitization is characterized by functional uncoupling of receptors from heterotrimeric G proteins, which occurs without any detectable change in the total number of receptors present in cells or tissues 1. A highly conserved mechanism of rapid desensitization involves ligand-dependent phosphorylation
Mechanisms of GPCR proteolysis
Studies of agonist-induced regulation of certain GPCRs, such as the delta opioid peptide (DOP) receptor, provided early support for the hypothesis that agonist-induced downregulation is mediated by proteolysis of receptors in lysosomes 23 and suggested that opioid peptide receptors traffic via clathrin-coated vesicles 24. More recent studies using fluorescence microscopy support this idea and have allowed visual confirmation of receptor trafficking to lysosomes in some cases 25. Similar
GPCR trafficking to lysosomes
Although there is little doubt that certain GPCRs can undergo downregulation by endocytic trafficking to lysosomes, at least in some cell types, there has been considerable controversy about the precise membrane pathways that mediate this trafficking. This controversy has focused in large part on whether rapid internalization (sequestration) and slower downregulation of GPCRs are mediated by separate or shared membrane pathways. This question is of great interest precisely because of the
Multiple mechanisms of GPCR endocytosis
Many GPCRs undergo ligand-induced endocytosis via clathrin-coated pits, which involves a highly conserved endocytic adaptor function of nonvisual (β-) arrestins 36., 37.. However, there is also strong evidence for the existence of considerable receptor-specific and cell-type-specific differences in mechanisms of GPCR endocytosis. For example, although the β2-adrenoceptor is endocytosed by clathrin-coated pits in several cell types 36., 38., morphological studies suggest that this receptor can
Targeting of GPCRs by clathrin-coated pits
Strong support for the hypothesis that sequestration and downregulation of certain GPCRs can involve a shared endocytic mechanism (Fig. 1a) came from the study of β2-adrenoceptor regulation in transfected HEK293 cells 15. Rapid internalization (sequestration) of the β2-adrenoceptor in these cells is mediated by clathrin-coated pits 38. Consistent with this, overexpression of K44A mutant dynamin strongly inhibits agonist-induced sequestration 37. Gagnon et al. 15 showed that K44A mutant dynamin,
Molecular sorting of GPCRs after endocytosis
Additional evidence that supports divergent trafficking of GPCRs after endocytosis came from studies comparing membrane trafficking of the β2-adrenoceptor and the DOP receptor in the same (HEK293) cells 50. Agonist-induced endocytosis of both receptors occurs with similarly rapid kinetics, is mediated by clathrin-coated pits and involves the trafficking of receptors via transferrin-containing early endosomes 38., 51., 52.. However, coexpressed β2-adrenoceptors and DOP receptors differ
Insight into a specific mechanism of β2-adrenoceptor sorting
Mechanisms of endocytic sorting have been examined most extensively in studies of receptor-mediated endocytosis of nutrient ligands. In this case it is possible to understand endocytic sorting as a consequence of the irregular geometry of endosomal membranes that segregate soluble (dissociated ligand) and membrane-bound (receptor) cargo according to differences in the surface area: volume ratio of different populations of endocytic vesicles 57., 58.. Indeed, such a model is consistent with the
Receptor-specific and cell-type-specific differences in GPCR membrane trafficking
The PDZ-domain-mediated interaction between the cytoplasmic tail of the β2-adrenoceptor and NHERF family proteins requires a sequence in the carboxyl-terminal cytoplasmic domain of the β2-adrenoceptor that diverges greatly among different GPCRs (61). Thus, the role of NHERF family proteins in controlling sorting of the β2-adrenoceptor is likely to be specific for this receptor alone or apply to a very small number of GPCRs. Nevertheless, the basic itinerary of β2-adrenoceptor membrane
Concluding remarks
The field of GPCR membrane trafficking is currently in an exciting stage of rapid development. The ideas proposed in the present review might be subject to substantial modification pending further studies, and many important questions in this field remain completely unresolved. For example, precisely how do the protein interactions implicated in the mechanism of β2-adrenoceptor sorting actually control receptor trafficking? What mechanism(s) are responsible for targeting receptors to the
Acknowledgments
We thank many colleagues for valuable discussion and sharing unpublished data. In particular, we thank Heather Deacon, Robert M. Gage, Kyung-Ah Kim, Michael Tanowitz and Jennifer Whistler for specific comments and suggestions regarding the manuscript. We regret that we are only able to cite a limited number of important studies relevant to GPCR endocytosis and downregulation in this short paper. The reader is directed to other reviews (cited in the text) for additional discussion and
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