GPCR sorting at multivesicular endosomes

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

The Emerging Role of Rab5 in Membrane Receptor Trafficking and Signaling Pathways

Ras analog in brain (Rab) proteins are small guanosine triphosphatases (GTPases) that belong to the Ras-like GTPase superfamily, and they can regulate vesicle trafficking. Rab proteins alternate between an activated (GTP-bound) state and an inactivated (GDP-bound) state. Early endosome marker Rab5 GTPase, a key member of the Rab family, plays a crucial role in endocytosis and membrane transport. The activated-state Rab5 recruits its effectors and regulates the internalization and trafficking of membrane receptors by regulating vesicle fusion and receptor sorting in the early endosomes. In this review, we summarize the role of small Rab GTPases Rab5 in membrane receptor trafficking and the activation of signaling pathways, such as Ras/MAPK and PI3K/Akt, which ultimately affect cell growth, apoptosis, tumorigenesis, and tumor development. This review may provide some insights for our future research and novel therapeutic targets for diseases.

Advances in Membrane Trafficking and Endosomal Signaling of G Protein-Coupled Receptors

The integration of GPCR signaling with membrane trafficking, as a single orchestrated system, is a theme increasingly evident with the growing reports of GPCR endosomal signaling. Once viewed as a mechanism to regulate cell surface heterotrimeric G protein signaling, the endocytic trafficking system is complex, highly compartmentalized, yet deeply interconnected with cell signaling. The organization of receptors into distinct plasma membrane signalosomes, biochemically distinct endosomal populations, endosomal microdomains, and its communication with distinct subcellular organelles such as the Golgi provides multiple unique signaling platforms that are critical for specifying receptor function physiologically and pathophysiologically. In this chapter I discuss our emerging understanding in the endocytic trafficking systems employed by GPCRs and their novel roles in spatial control of signaling. Given the extensive roles that GPCRs play in vivo, these evolving models are starting to provide mechanistic understanding of distinct diseases and provide novel therapeutic avenues that are proving to be viable targets.

ALIX Regulates the Ubiquitin-Independent Lysosomal Sorting of the P2Y1 Purinergic Receptor via a YPX3L Motif

Endocytic sorting and lysosomal degradation are integral to the regulation of G protein-coupled receptor (GPCR) function. Upon ligand binding, classical GPCRs are activated, internalized and recycled or sorted to lysosomes for degradation, a process that requires receptor ubiquitination. However, recent studies have demonstrated that numerous GPCRs are sorted to lysosomes independent of receptor ubiquitination. Here, we describe an ubiquitin-independent lysosomal sorting pathway for the purinergic GPCR P2Y₁. After activation, P2Y₁ sorts to lysosomes for degradation independent of direct ubiquitination that is mediated by a YPX₃L motif within the second intracellular loop that serves as a binding site for the adaptor protein ALIX. Depletion of ALIX or site-directed mutation of the YPX₃L motif inhibits P2Y₁ sorting into the lumen of multivesicular endosomes/lysosomes and degradation. These findings confirm the function of YPX₃L motifs as lysosomal targeting sequences for GPCRs and demonstrate that ALIX mediates the ubiquitin-independent degradation of certain GPCRs.