Constitutive Desensitization: A New Paradigm for G Protein-Coupled Receptor Regulation

Abstract

GPCRs are a large family of cell-surface proteins that regulate many important biochemical pathways and physiological responses. The isolation and characterization of GPCRs represent one of the more remarkable success stories that occurred during the revolution in biology of the last quarter century. Of the many discoveries that originated in the laboratory of Robert Lefkowitz at Duke University concerning GPCR regulation, none is more fundamental than the elucidation of the families of GRKs and arrestin proteins that terminate GPCR signaling. In this essay, we will discuss how advances in microscopy and biology have made the visualization of GPCR, GRK, and arrestin activity possible in single cells. Additionally, we will discuss how imaging studies using arrestins and a naturally occurring mutant of the vasopressin receptor led to the recognition of a novel phenotypic receptor behavior, in which the receptor desensitizes in the absence of agonist. We have termed this process constitutive desensitization, and this unexpected receptor property suggests that it may be possible to develop novel classes of signal-inhibiting drugs distinct from conventional antagonists.

Constitutive Desensitization of Opioid Receptors in Peripheral Sensory Neurons

“…Constitutive receptor activity, like agonist-stimulated activity, can differentially activate multiple cellular signaling mechanisms, including those that lead to desensitization (loss of responsiveness). As a result, receptor systems can exist in a state of constitutive desensitization (Pei et al, 1994;Berg et al, 1999;Barak et al, 2001Barak et al, , 2003Sullivan et al, 2015b) where responsiveness to agonist stimulation is reduced. Prolonged reduction in constitutive receptor activity produced by treatment with an inverse agonist allows constitutively desensitized receptor systems to resensitize, thereby increasing responsiveness to agonist stimulation once occupancy by the inverse agonist is removed.…”

“…With respect to desensitization, b-arrestin binding interferes with G protein binding and activation and can result in receptor internalization and downregulation, both of which result in loss of responsiveness (for reviews, see Gainetdinov et al, 2004;DeFea, 2011). Similarly, constitutive receptor activity also can activate these desensitization mechanisms (Barak et al, 2003). Under basal conditions, opioid receptors have been found to be constitutively phosphorylated (Arden et al, 1995;Hasbi et al, 1998) and associated with b-arrestin (Walwyn et al, 2007;Audet et al, 2012).…”

“…), constitutive receptor activity, like agonist-dependent activity, has also been shown to activate desensitization mechanisms. This receptor activity results in receptor systems that are "constitutively desensitized," as evidenced by constitutive receptor phosphorylation, internalization, and decreased responsiveness to agonist stimulation (Pei et al, 1994;Berg et al, 1999;Barak et al, 2003;Sullivan et al, 2015b). Prolonged reduction of constitutive receptor activity that activates desensitization mechanisms by treatment with inverse agonists allows receptor systems to resensitize, leading to increased responsiveness to agonists (Milligan and Bond, 1997;Berg et al, 1999;MisereyLenkei et al, 2002;Chanrion et al, 2008;Nijmeijer et al, 2010;Sullivan et al, 2015b).…”

Trace Amine-Associated Receptors as Emerging Therapeutic Targets: TABLE 1

“…In our laboratory (Barak et al, 2008), in an attempt to identify agonists and antagonists of TAAR1 we initially used a high content high throughput cell assay employing ␤-arrestin2 (␤arr2) molecules labeled with green fluorescent pro- (Barak et al, 1997;Kim et al, 2001;Barak et al, 2003Barak et al, , 2008. ␤arr2-GFP binds to agonist-activated GPCRs and competitively blocks G protein-dependent receptor signaling (Lefkowitz et al, 1993;Barak et al, 1997;Barak et al, 2003).…”

“…␤arr2-GFP binds to agonist-activated GPCRs and competitively blocks G protein-dependent receptor signaling (Lefkowitz et al, 1993;Barak et al, 1997;Barak et al, 2003). This desensitization process, in which ␤arr2-GFP redistributes in response to agonists and is prevented from redistributing by coexpression of antagonists, can be readily measured using automated high-content imaging tools.…”