Dual pathways of internalization of the cholecystokinin receptor.

Abstract

Abstract. Receptor molecules play a major role in the desensitization of agonist-stimulated cellular responses. For G protein-coupled receptors, rapid desensitization occurs via receptor phosphorylation, sequestration, and internalization, yet the cellular compartments in which these events occur and their interrelationships are unclear. In this work, we focus on the cholecystokinin (CCK) receptor, which has been well characterized with respect to phosphorylation. We have used novel fluorescent and electron-dense CCK receptor ligands and an antibody to probe receptor localization in a CCK receptor-bearing CHO cell line. In the unstimulated state, receptors were diffusely distributed over the plasmalemma. Agonist occupation stimulated endocytosis via both clathrin-dependent and independent pathways. The former was predominant, leading to endosomal and lysosomal compartments, as well as recycling to the plasmalemma. The clathrin-independent processes led to a smooth vesicular compartment adjacent to the plasmalemma resembling caveolae, which did not transport ligand deeper within the cell. Potassium depletion largely eliminated clathrin-dependent endocytosis, while not interfering with agoniststimulated receptor movement into subplasmalemmal smooth vesicle compartments. These cellular endocytic events can be related to the established cycle of CCK receptor phosphorylation and dephosphorylation, which we have previously described (Klueppelberg,

Plasma membrane and lysosomal localization of CB1 cannabinoid receptor are dependent on lipid rafts and regulated by anandamide in human breast cancer cells

“…Hence, our finding that CB1R shows a lysosomal localization could represent fundamental basis to further studies about receptor internalization, downregulation and eventual resensitization which constitute essential steps to analyze receptors function and signalling within the cells. For several receptors, the cellular responses to effectors have been shown to be regulated in part by the compartmentalization of the receptors and their effectors within the cells [40], therefore elucidating the mechanism underlying the trafficking of CB1R is critical for understanding the physiological response to a variety of ligands.…”

Structural basis of cholecystokinin receptor binding and regulation

“…With a signaling system having a soluble ligand that cannot cross the lipid bilayer and that is delivered from the extracellular milieu, any movement of the receptor off the cell surface functionally desensitizes it. Two such internalization pathways have been described for the CCK 1 receptor, clathrin-mediated endocytosis and potocytosis via caveolae (Roettger et al, 1995b). Similarly, coupling between receptor and G protein requires lateral mobility in the plasma membrane and access for the two molecules to meet each other (Roettger et al, 2001).…”

The cytoplasmic domain close to the transmembrane region of the glucagon-like peptide-1 receptor contains sequence elements that regulate agonist-dependent internalisation

“…The group of G-proteincoupled receptors, whose main function is to transduce information from the extracellular environment to the interior of the cell, initiates this process after a specific ligand has been bound to its receptor, thereby facilitating transmembrane signalling. Both clathrin-coated pits (Ghinea et al 1992, Von Zastrow & Kobilka 1992, Hoxie et al 1993, Von Zastrow et al 1993, Roettger et al 1995 and uncoated endocytic pathways (Raposo et al 1989, Roettger et al 1995 have been suggested to be involved in the internalisation of G-protein-coupled receptors. Clathrin-coated pits require cytoplasmic receptor motifs in order to interact with proteins of the endocytic machinery (Trowbridge et al 1993, Clague 1998.…”

“…It is not clear whether both clathrin-coated pits and uncoated endocytic pathways play a similar role. The cholecystokinin receptor-ligand complex internalised by uncoated pits has been purported to mediate rapid resensitisation of the receptor (Roettger et al 1995).…”

“…These data could also explain the faster rate of resensitisation in cells expressing GLPR 418R receptors. The cholecystokinin A receptor has been reported to be internalised through coated pits and uncoated pits and the authors of that paper suggested that the uncoated pit pathway might be involved in the rapid resensitisation of those receptors (Roettger et al 1995).…”