G protein-coupled receptors stimulation and the control of cell migration

Cell migration is a fundamental biological process involved in normal physiology. Altered motile phenotypes are however often associated with the development and progression of diseases such as cancer and atherosclerosis. Remodeling of the actin cytoskeleton is required for cell shape changes and is controlled by a broad variety of cellular proteins. Interestingly, several extracellular stimuli can promote actin reorganization and result in enhanced cell migration. Namely, G protein-coupled receptors (GPCRs), which are activated by factors ranging from small amines, to hormones, and chemokines, initiate signalling cascades resulting in cell shape changes, formation of a migrating front (leading edge) and altered adhesion. GPCRs are heptahelical membrane proteins, which classically transmit signal via the activation of heterotrimeric G proteins. Sustained stimulation leads to the activation of G protein-coupled receptor kinases (GRKs) and the recruitment of arrestin proteins, which engage alternative signalling pathways. In this review, we will discuss the role of GPCR mediated signal transduction and review their importance in the regulation of actin remodeling leading to cell migration.     

Reviews in Molecular Biology and Biotechnology: Transmembrane Signaling by G Protein-Coupled Receptors

As the most diverse type of cell surface receptor, the importance heptahelical G protein-coupled receptors (GPCRs) to clinical medicine cannot be overestimated. Visual, olfactory and gustatory sensation, intermediary metabolism, cell growth and differentiation are all influenced by GPCR signals. The basic receptor-G protein-effector mechanism of GPCR signaling is tuned by a complex interplay of positive and negative regulatory events that amplify the effect of a hormone binding the receptor or that dampen cellular responsiveness. The association of heptahelical receptors with a variety of intracellular partners other than G proteins has led to the discovery of potential mechanisms of GPCR signaling that extend beyond the classical paradigms. While the physiologic relevance of many of these novel mechanisms of GPCR signaling remains to be established, their existence suggests that the mechanisms of GPCR signaling are even more diverse than previously imagined.     

Clathrin-dependent mechanisms of G protein-coupled receptor endocytosis

The heptahelical G protein-coupled receptor (GPCR) family includes approximately 900 members and is the largest family of signaling receptors encoded in the mammalian genome. G protein-coupled receptors elicit cellular responses to diverse extracellular stimuli at the plasma membrane and some internalized receptors continue to signal from intracellular compartments. In addition to rapid desensitization, receptor trafficking is critical for regulation of the temporal and spatial aspects of GPCR signaling. Indeed, GPCR internalization functions to control signal termination and propagation as well as receptor resensitization. Our knowledge of the mechanisms that regulate mammalian GPCR endocytosis is based predominantly on arrestin regulation of receptors through a clathrin- and dynamin-dependent pathway. However, multiple clathrin adaptors, which recognize distinct endocytic signals, are now known to function in clathrin-mediated endocytosis of diverse cargo. Given the vast number and diversity of GPCRs, the complexity of clathrin-mediated endocytosis and the discovery of multiple clathrin adaptors, a single universal mechanism controlling endocytosis of all mammalian GPCRs is unlikely. Indeed, several recent studies now suggest that endocytosis of different GPCRs is regulated by distinct mechanisms and clathrin adaptors. In this review, we discuss the diverse mechanisms that regulate clathrin-dependent GPCR endocytosis.     

Activation and targeting of mitogen-activated protein kinases by G-protein-coupled receptors

Over the past decade, it has become apparent that many G-protein-coupled receptors (GPCRs) generate signals that control cellular differentiation and growth, including stimulation of Ras family GTPases and activation of mitogen-activated protein (MAP) kinase pathways. The mechanisms that GPCRs use to control the activity of MAP kinases vary between receptor and cell type but fall broadly into one of three categories: signals initiated by classical G protein effectors, e.g., protein kinase (PK)A and PKC, signals initiated by cross-talk between GPCRs and classical receptor tyrosine kinases, e.g., "transactivation" of epidermal growth factor (EGF) receptors, and signals initiated by direct interaction between beta-arrestins and components of the MAP kinase cascade, e.g., beta-arrestin "scaffolds". While each of these pathways results in increased cellular MAP kinase activity, emerging data suggest that they are not functionally redundant. MAP kinase activation occurring via PKC-dependent pathways and EGF receptor transactivation leads to nuclear translocation of the kinase and stimulates cell proliferation, while MAP kinase activation via beta-arrestin scaffolds primarily increases cytosolic kinase activity. By controlling the spatial and temporal distribution of MAP kinase activity within the cell, the consequences of GPCR-stimulated MAP kinase activation may be determined by the mechanism by which they are activated.     

Intracellular α(2C)-adrenoceptors: storage depot, stunted development or signaling domain?

G-protein coupled receptors (GPCRs) are generally considered to function as cell surface signaling structures that respond to extracellular mediators, many of which do not readily access the cell's interior. Indeed, most GPCRs are preferentially targeted to the plasma membrane. However, some receptors, including α(2C)-Adrenoceptors, challenge conventional concepts of GPCR activity by being preferentially retained and localized within intracellular organelles. This review will address the issues associated with this unusual GPCR localization and discuss whether it represents a novel sub-cellular niche for GPCR signaling, whether these receptors are being stored for rapid deployment to the cell surface, or whether they represent immature or incomplete receptor systems.     

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

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.      

The activation mechanism of class-C G-protein coupled receptors

Class-C G-protein coupled receptors (GPCRs) represent a distant group among the large family of GPCRs. This class includes the receptors for the main neurotransmitters, glutamate and gamma-aminobutyric acid (GABA), and the receptors for Ca(2+), some taste and pheromone molecules, as well as some orphan receptors. Like any other GPCRs, class-C receptors possess a heptahelical domain (HD) involved in heterotrimeric G-protein activation, but most of them also have a large extracellular domain (ECD) responsible for agonist recognition and binding. In addition, it is now well accepted that these receptors are dimers, either homo or heterodimers. This complex architecture raises a number of important questions. Here we will discuss our view of how agonist binding within the large ECD triggers the necessary change of conformation, or stabilize a specific conformation, of the heptahelical domain leading to G-protein activation. How ligands acting within the heptahelical domain can change the properties of these complex macromolecules.     

Intracellular α2C-Adrenoceptors: Storage depot, stunted development or signaling domain?

G-protein coupled receptors (GPCRs) are generally considered to function as cell surface signaling structures that respond to extracellular mediators, many of which do not readily access the cell's interior. Indeed, most GPCRs are preferentially targeted to the plasma membrane. However, some receptors, including α_2C-Adrenoceptors, challenge conventional concepts of GPCR activity by being preferentially retained and localized within intracellular organelles. This review will address the issues associated with this unusual GPCR localization and discuss whether it represents a novel sub-cellular niche for GPCR signaling, whether these receptors are being stored for rapid deployment to the cell surface, or whether they represent immature or incomplete receptor systems.     

ADAMs as mediators of EGF receptor transactivation by G protein-coupled receptors

A disintegrin and metalloprotease (ADAM) is a membrane-anchored metalloprotease implicated in the ectodomain shedding of cell surface proteins, including the ligands for epidermal growth factor (EGF) receptors (EGFR)/ErbB. It has been well documented that the transactivation of the EGFR plays critical roles for many cellular functions, such as proliferation and migration mediated through multiple G protein-coupled receptors (GPCRs). Recent accumulating evidence has suggested that ADAMs are the key metalloproteases activated by several GPCR agonists to produce a mature EGFR ligand leading to the EGFR transactivation. In this review, we describe the current knowledge on ADAMs implicated in mediating EGFR transactivation. The major focus of the review will be on the possible upstream mechanisms of ADAM activation by GPCRs as well as downstream signal transduction and the pathophysiological significances of ADAM-dependent EGFR transactivation. ectodomain shedding; angiotensin II     

A family of heptahelical receptors with adhesion-like domains: a marriage between two super families

Some G protein-coupled receptors (GPCRs) are regulators of cell adhesion via inside-out effector signaling pathways. Such is the case with leukocyte chemokine receptors which stimulate intracellular second messenger pathways resulting in upregulation of integrin adhesion to ligands present in the extracellular matrix or on opposing cells resulting in chemotaxis and extravasation during immune surveillance. Remarkably, a family of GPCRs has recently been discovered that may themselves be triggered by cell-cell or cell-matrix interactions. Along with a canonical heptahelical membrane-spanning region, these intriguing proteins contain putative cell adhesion-like modules. The evidence to date suggests that they are involved in lymphocyte activation, macrophage biology, synaptic exocytosis and planar polarization during organogenesis.     

When a G protein-coupled receptor does not couple to a G protein

Classically, G protein-coupled receptors (GPCRs) relay signals by directly activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Increasing evidence indicates that GPCRs may also signal through G protein-independent pathways. JAK/STATs, Src-family tyrosine kinases, GRKs/beta-arrestins, and PDZ domain-containing proteins have been suggested to directly relay signals from GPCRs independent of G proteins. In addition, our laboratory recently reported that the beta(2) adrenergic receptor (beta(2)AR) could switch from G protein-coupled to G protein-independent ERK (extracellular signal-regulated kinase) activation in an agonist dosage-dependent manner. This finding provides a novel mechanism for G protein-independent GPCR signaling. This review focuses on recent progress in understanding the mechanisms by which G protein-independent GPCR signaling occurs.     

A Family of Heptahelical Receptors With Adhesion-Like Domains: A Marriage Between Two Super Families

Abstract

Some G protein-coupled receptors (GPCRs) are regulators of cell adhesion via inside-out effector signaling pathways. Such is the case with leukocyte chemokine receptors which stimulate intracellular second messenger pathways resulting in upregulation of integrin adhesion to ligands present in the extracellular matrix or on opposing cells resulting in chemotaxis and extravasation during immune surveillance. Remarkably, a family of GPCRs has recently been discovered that may themselves be triggered by cell-cell or cell-matrix interactions. Along with a canonical heptahelical membrane-spanning region, these intriguing proteins contain putative cell adhesionlike modules. The evidence to date suggests that they are involved in lymphocyte activation, macrophage biology, synaptic exocytosis and planar polarization during organogenesis.     

Regulation of G protein-coupled receptor export trafficking

G protein-coupled receptors (GPCRs) constitute a superfamily of cell-surface receptors which share a common topology of seven transmembrane domains and modulate a variety of cell functions through coupling to heterotrimeric G proteins by responding to a vast array of stimuli. The magnitude of cellular response elicited by a given signal is dictated by the level of GPCR expression at the plasma membrane, which is the balance of elaborately regulated endocytic and exocytic trafficking. This review will cover recent advances in understanding the molecular mechanism underlying anterograde transport of the newly synthesized GPCRs from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane. We will focus on recently identified motifs involved in GPCR exit from the ER and the Golgi, GPCR folding in the ER and the rescue of misfolded receptors from within, GPCR-interacting proteins that modulate receptor cell-surface targeting, pathways that mediate GPCR traffic, and the functional role of export in controlling GPCR signaling.     

Regulation of G protein-coupled receptor export trafficking

G protein-coupled receptors (GPCRs) constitute a superfamily of cell-surface receptors which share a common topology of seven transmembrane domains and modulate a variety of cell functions through coupling to heterotrimeric G proteins by responding to a vast array of stimuli. The magnitude of cellular response elicited by a given signal is dictated by the level of GPCR expression at the plasma membrane, which is the balance of elaborately regulated endocytic and exocytic trafficking. This review will cover recent advances in understanding the molecular mechanism underlying anterograde transport of the newly synthesized GPCRs from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane. We will focus on recently identified motifs involved in GPCR exit from the ER and the Golgi, GPCR folding in the ER and the rescue of misfolded receptors from within, GPCR-interacting proteins that modulate receptor cell-surface targeting, pathways that mediate GPCR traffic, and the functional role of export in controlling GPCR signaling.     

The expanding repertoire of receptor activity modifying protein (RAMP) function

Receptor activity modifying proteins (RAMPs) associate with G-protein-coupled receptors (GPCRs) at the plasma membrane and together bind a variety of peptide ligands, serving as a communication interface between the extracellular and intracellular environments. The collection of RAMP-interacting GPCRs continues to expand and now consists of GPCRs from families A, B and C, suggesting that RAMP activity is extremely prevalent. RAMP association with GPCRs can regulate GPCR function by altering ligand binding, receptor trafficking and desensitization, and downstream signaling pathways. Here, we elaborate on these RAMP-dependent mechanisms of GPCR regulation, which provide opportunities for pharmacological intervention.     

Structures of rhodopsin kinase in different ligand states reveal key elements involved in G protein-coupled receptor kinase activation

G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate activated heptahelical receptors, leading to their uncoupling from G proteins. Here we report six crystal structures of rhodopsin kinase (GRK1), revealing not only three distinct nucleotide-binding states of a GRK but also two key structural elements believed to be involved in the recognition of activated GPCRs. The first is the C-terminal extension of the kinase domain, which was observed in all nucleotide-bound GRK1 structures. The second is residues 5-30 of the N terminus, observed in one of the GRK1.(Mg2+)2.ATP structures. The N terminus was also clearly phosphorylated, leading to the identification of two novel phosphorylation sites by mass spectral analysis. Co-localization of the N terminus and the C-terminal extension near the hinge of the kinase domain suggests that activated GPCRs stimulate kinase activity by binding to this region to facilitate full closure of the kinase domain.     

Activation of rhodopsin: new insights from structural and biochemical studies

G-protein-coupled receptors (GPCRs) are involved in a vast variety of cellular signal transduction processes from visual, taste and odor perceptions to sensing the levels of many hormones and neurotransmitters. As a result of agonist-induced conformation changes, GPCRs become activated and catalyze nucleotide exchange within the G proteins, thus detecting and amplifying the signal. GPCRs share a common heptahelical transmembrane structure as well as many conserved key residues and regions. Rhodopsins are prototypical GPCRs that detect photons in retinal photoreceptor cells and trigger a phototransduction cascade that culminates in neuronal signaling. Biophysical and biochemical studies of rhodopsin activation, and the recent crystal structure determination of bovine rhodopsin, have provided new information that enables a more complete mechanism of vertebrate rhodopsin activation to be proposed. In many aspects, rhodopsin might provide a structural and functional template for other members of the GPCR family.