GPCR Signaling and Trafficking: The Long and Short of It

Abstract

Emerging findings disclose unexpected components of G protein-coupled receptor (GPCR) signaling and cell biology. Select GPCRs exhibit classical signaling, that is restricted to cell membranes, as well as newly described persistent signaling that depends on internalization of the GPCR bound to β-arrestins. Termination of non-canonical endosomal signaling requires intraluminal acidification and sophisticated protein trafficking machineries. Recent studies reveal the structural determinants of the trafficking chaperones. This review summarizes advances in GPCR signaling and trafficking with a focus on the parathyroid hormone receptor (PTHR) as a prototype, and on the actin-sorting nexin 27 (SNX27)-retromer tubule (ASRT) complex, an endosomal sorting hub responsible for recycling and preservation of cell surface receptors. The findings are integrated into a model of PTHR trafficking with implications for signal transduction, bone growth, and mineral ion metabolism.

Keywords: ASRT complex; GPCR; PTHR; SNX27; endosome; retromer.

Figure 1. Classical (Brief) vs. Endosomal (Extended) GPCR Signaling

(A) (1) Binding of agonist ligand (PTH/PTHrP) to PTHR initiates signaling through the recruitment of heterotrimeric G proteins (αβγ). (2) Guanine nucleotide exchange activates the G alpha subunit (α) which transduces parallel downstream signaling cascades, either via (i) PKC (α_q) or (ii) PKA (α_s) leading to transient signaling at the cell surface (B). (3) Like many, GPCRs, PTHR is then phosphorylated by GRK leading to recruitment of and engagement with β-arrestins. (4) β-arrestin-binding initiates receptor internalization into clathrin-coated pits by interaction with endocytic machinery. (5) PTHR G-protein signaling continues upon entry into early endosomes. (6) Endosomal signaling induces prolonged cAMP responses from ligand-receptor complexes (C). (7) Termination of non-canonical signaling requires PKA driven V-ATPase-mediated intraluminal acidification and recruitment of the ASRT complex. (8) These events coincide with the release of β-arrestin and the ligand from the receptor. (9) The ASRT complex subsequently facilitates post-endocytic recycling of the receptor back to the plasma membrane thereby resensitizing the cell surface.

Figure 2. Alternative Modes of GPCR Trafficking Along the Endocytic Network

Upon ligand-induced activation GPCRs are internalized into early endosomes for sorting. At the early endosome, GPCRs containing C-terminal PDZ binding motifs are recognized by compatible PDZ-adaptor proteins such as SNX27, which works in conjunction with retromer and the WASH complex (Inset). Following sorting, receptors are packaged into recycling tubules which emanate and bud from the membranes of early endosomes. These recycling intermediaries transport GPCRs back to the plasma membrane leading to resensitization of the cell surface. Alternatively, GPCRs marked for degradation are sequestered into intraluminal vesicles (ILVs) of multivesicular endosomes that fuse with lysosomes resulting in receptor proteolysis and down-regulation at the cell surface. Membrane retrieval of endocytosed PTHR occurs via three divergent trafficking itineraries: (i) ASRT-mediated endosome-to-plasma membrane recycling (green); (ii) ASRT-independent endosome-to-PM recycling (blue) or (iii) retrograde endosome-to-TGN-PM recycling (red). This GPCR signaling-trafficking cycle is superimposed by the recruitment and functional transition of small Rab GTPases: early endosome (Rab5), late-endosome/lysosome (Rab7) and recycling endosomes (Rab4 and Rab11).