Arrestin-independent internalization of G protein-coupled receptors.

A feature common to most G protein-coupled receptors (GPCRs) is the cyclic process of signaling, desensitization, internalization, resensitization, and recycling to the plasma membrane. These coordinated events prevent cells from undergoing excessive receptor stimulation or periods of prolonged inactivity. In the “canonical” pathway of GPCR regulation, agonist-activated GPCR induces receptor phosphorylation mediated by GPCR kinases (GRKs), which phosphorylate serine and threonine residues in the third intracellular loop and/or the carboxyl terminus of the receptor (Ferguson, 2001) (Fig. 1). Receptor-G protein interaction, however, is not inhibited by GRK-mediated phosphorylation. For this, the phosphorylated receptor recruits another molecule, -arrestin. -Arrestin sterically hinders further coupling of the receptor with the heterotrimeric G proteins, and this interruption of signaling generates receptor desensitization. In the case of Gs-coupled 2-adrenergic receptors, -arrestins may also recruit cAMP phosphodiesterases of the PDE4D family into a complex with the activated receptor, where they are positioned to degrade cAMP at enhanced rate (Perry et al., 2002). Upon receptor phosphorylation and -arrestin binding, most GPCRs internalize into clathrin-coated vesicles. -Arrestins also play a central role in this process. By serving as an adaptor molecule, -arrestin links GPCRs to several proteins of the endocytotic machinery, including clathrin (Goodman et al., 1996) and the clathrin adaptor complex AP-2 (Laporte et al., 1999). In the endosomes, most GPCRs are dephosphorylated and return to the plasma membrane as resensitized receptors to undergo another round of signal transduction. Receptor resensitization requires dissociation of -arrestin before the receptors can be dephosphorylated. Internalized GPCRs may also be down-regulated via lysosomal degradation or may direct activation of additional intracellular signaling pathways. In these processes, -arrestins play regulatory roles as well. GPCRs that display strong and persistent -arrestin binding and cointernalize with -arrestins recycle relatively slowly or are more likely to be transported to the lysosomes for receptor degradation. In contrast, GPCRs that bind -arrestin transiently and dissociate from -arrestin shortly after movement of the receptor into the clathrin-coated vesicles recycle rapidly (Oakley et al., 2000; Shenoy and Lefkowitz, 2003). -Arrestins may also function as scaffolds for GPCR-mediated activation of Src family tyrosine kinases, extracellular signal-regulated kinase 1/2, c-Jun NH2-terminal kinase 3, and p38 mitogenactivated protein kinases (Lefkowitz and Whalen, 2004). Thus, -arrestins are important multifunctional adaptor molecules regulating desensitization, internalization, intracellular signaling, and recycling of a large number of GPCRs. It has become increasingly clear that these molecular mechanisms are by no means universal for all GPCRs. Several GPCRs do not require -arrestins for internalization, at least in certain cell types. These include the metabotropic glutamate receptor 1 (Dhami et al., 2004), secretin receptor (Walker et al., 1999), serotonin 5-HT2A receptor (Gray et al., 2003), protease-activated receptor-1 (Paing et al., 2002), prostacyclin receptor (Smyth et al., 2000), formyl peptide receptor (Bennett et al., 2000; Vines et al., 2003), and M2 muscarinic cholinergic receptor (van Koppen and Kaiser, 2003). In this issue of Molecular Pharmacology, Chen et al. (2004) elegantly show another example of -arrestin-independent internalization of a GPCR: the high-affinity leukotriene B4 receptor BLT1. Signaling by this GPCR involves the activation of phospholipase C via pertussis toxin-sensitive (Gi/Go) and -insensitive (G16/G14) G proteins (Gaudreau et al., 2002). Chen et al. show that BLT1 readily internalizes after recepArticle, publication date, and citation information can be found at http://molpharm.aspetjournals.org. doi:10.1124/mol.104.003822.