When trafficking and signaling mix: How subcellular location shapes G protein‐coupled receptor activation of heterotrimeric G proteins

Abstract

G protein‐coupled receptors (GPCRs) physically connect extracellular information with intracellular signal propagation. Membrane trafficking plays a supportive role by “bookending” signaling events: movement through the secretory pathway delivers GPCRs to the cell surface where receptors can sample the extracellular environment, while endocytosis and endolysosomal membrane trafficking provide a versatile system to titrate cellular signaling potential and maintain homeostatic control. Recent evidence suggests that, in addition to these important effects, GPCR trafficking actively shapes the cellular signaling response by altering the location and timing of specific receptor‐mediated signaling reactions. Here, we review key experimental evidence underlying this expanding view, focused on GPCR signaling mediated through activation of heterotrimeric G proteins located in the cytoplasm. We then discuss lingering and emerging questions regarding the interface between GPCR signaling and trafficking.

A cellular perspective of bias at G protein‐coupled receptors

“…In addition to “what” proteins function with GPCRs, it is increasingly clear that the cellular response to an agonist is shaped by “where” and “when” protein interactions occur (Figure 1). 7–10 Yet, these biochemical parameters are not inviolable. Structurally different agonists acting on the same receptor can alter these parameters and, as a consequence, change the downstream signaling and regulatory processes of the cell 11,12 .…”

“…Ligands which show agonist bias, location bias, or kinetic bias can change these parameters compared to their unbiased counterparts. Panel 2 was adapted from Lobingier and von Zastrow 2019 10 …”

“…Two recently described types of bias involve these where and when variables: location bias and kinetic bias. Location bias is a two part phenomenon: (a) GPCRs can activate signaling events from intracellular compartments as well as the plasma membrane; (b) certain ligands can preferentially drive GPCR activity from intracellular compartments or restrict activity to the plasma membrane and, as consequence, change signaling 9,10 . One type of intracellular signaling occurs at compartments such as the Golgi or nucleus.…”

“…GPCRs at these compartments are activated by agonists which are actively transported into the cell or are inherently capable of passive diffusion through the cell membrane 31–34 . A second type of intracellular signaling occurs at endosomes following agonist‐induced endocytosis 9,10 . Importantly, intracellular GPCR activation can drive distinct signaling events compared to agonists which restrict receptor activity to the cell surface 34–40 .…”

Analyzing kinetic signaling data for G-protein-coupled receptors

“…The classic rise-and-fall time course of cAMP in S49 cells indicated desensitization of β 2 adrenoceptor signaling 11 . Sustained signaling after agonist washout by parathyroid hormone, sphingosine 1-phosphate and thyroid-stimulating hormone receptors revealed signaling by internalized receptors [12][13][14] , a new spatiotemporal paradigm of GPCR signaling of potential therapeutic utility [15][16][17][18][19] . Signaling kinetics can also affect quantification of drug activity.…”