Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

Visualization of the spatial and temporal dynamics of MAPK signaling using fluorescence imaging techniques

Conserved mitogen-activated protein kinase (MAPK) signaling pathways are major mechanisms through which cells perceive and respond properly to their surrounding environment. Such homeostatic responses maintain the life of the organism. Since errors in MAPK signaling pathways can lead to cancers and to defects in immune responses, in the nervous system and metabolism, these pathways have been extensively studied as potential therapeutic targets. Although much has been studied about the roles of MAPKs in various cellular functions, less is known regarding regulation of MAPK in living organisms. This review will focus on the latest understanding of the dynamic regulation of MAPK signaling in intact cells that was revealed by using novel fluorescence imaging techniques and advanced systems-analytical methods. These techniques allowed quantitative analyses of signal transduction in situ with high spatio-temporal resolution and have revealed the nature of the molecular dynamics that determine cellular responses and fates.

MAP Kinase Cascades in Antigen Receptor Signaling and Physiology

Mitogen-activated protein kinases (MAPKs) play roles in a cell type and context-dependent manner to convert extracellular stimuli to a variety of cellular responses, thereby directing cells to proliferation, differentiation, survival, apoptosis, and migration. Studies of genetically engineered mice or chemical inhibitors specific to each MAPK signaling pathway revealed that MAPKs have various, but non-redundant physiologically important roles among different families. MAPK cascades are obviously integrated in the B cell receptor signaling pathways as critical components to drive B cell-mediated immunity.

Analyzing the roles of multi-functional proteins in cells: The case of arrestins and GRKs

Most proteins have multiple functions. Obviously, conventional methods of manipulating the level of the protein of interest in the cell, such as over-expression, knockout or knockdown, affect all of its functions simultaneously. The key advantage of these methods is that over-expression, knockout or knockdown does not require any knowledge of the molecular mechanisms of the function(s) of the protein of interest. The disadvantage is that these approaches are inadequate to elucidate the role of an individual function of the protein in a particular cellular process. An alternative is the use of re-engineered proteins, in which a single function is eliminated or enhanced. The use of mono-functional elements of a multi-functional protein can also yield cleaner answers. This approach requires detailed knowledge of the structural basis of each function of the protein in question. Thus, a lot of preliminary structure-function work is necessary to make it possible. However, when this information is available, replacing the protein of interest with a mutant in which individual functions are modified can shed light on the biological role of those particular functions. Here, we illustrate this point using the example of protein kinases, most of which have additional non-enzymatic functions, as well as arrestins, known multi-functional signaling regulators in the cell.

Overview of different mechanisms of arrestin-mediated signaling

Arrestins are characterized by their ability to selectively bind active, phosphorylated GPCRs and suppress (arrest) receptor coupling to G proteins. Nonvisual arrestins are also signaling proteins in their own right, activating a variety of cellular pathways. Arrestins are highly flexible proteins that can assume many distinct conformations. In their receptor-bound conformation, arrestins have higher affinity for a subset of partners. This explains how receptor activation regulates certain branches of arrestin-dependent signaling via arrestin recruitment to GPCRs. However, free arrestins are also active molecular entities that act in other pathways and localize signaling proteins to particular subcellular compartments, such as cytoskeleton. These functions are regulated by the enhancement or reduction of arrestin affinity for target proteins by other binding partners and by proteolytic cleavage. Recent findings suggest that the two visual arrestins, arrestin-1 and arrestin-4, which are expressed in photoreceptor cells, do not regulate signaling solely via binding to photopigments but also interact with a variety of nonreceptor partners, critically affecting the health and survival of photoreceptor cells. Detailed in this overview are GPCR-dependent and independent modes of arrestin-mediated regulation of cellular signaling pathways.

Morphine dependence in single enteric neurons from the mouse colon requires deletion of β-arrestin2

Chronic administration of morphine results in the development of tolerance to the analgesic effects and to inhibition of upper gastrointestinal motility but not to colonic motility, resulting in persistent constipation. In this study we examined the effect of chronic morphine in myenteric neurons from the adult mouse colon. Similar to the ileum, distinct neuronal populations exhibiting afterhyperpolarization (AHP)-positive and AHP-negative neurons were identified in the colon. Acute morphine (3 μM) decreased the number of action potentials, and increased the threshold for action potential generation indicative of reduced excitability in AHP-positive neurons. In neurons from the ileum of mice that were rendered antinociceptive tolerant by morphine-pellet implantation for 5 days, the opioid antagonist naloxone precipitated withdrawal as evidenced by increased neuronal excitability. Overnight incubation of ileum neurons with morphine also resulted in enhanced excitability to naloxone. Colonic neurons exposed to long-term morphine, remained unresponsive to naloxone suggesting that precipitated withdrawal does not occur in colonic neurons. However, morphine-treated colonic neurons from β-arrestin2 knockout mice demonstrated increased excitability upon treatment with naloxone as assessed by change in rheobase, number of action potentials and input resistance. These data suggest that similar to the ileum, acute exposure to morphine in colonic neurons results in reduced excitability due to inhibition of sodium currents. However, unlike the ileum, dependence to chronic exposure of morphine develops in colonic neurons from the β-arrestin2 knockout mice. These studies corroborate the in-vivo findings of the differential role of neuronal β-arrestin2 in the development of morphine tolerance/dependence in the ileum and colon.

Gαs regulates the post-endocytic sorting of G protein-coupled receptors

The role of Gαs in G protein-coupled receptor (GPCR) signalling at the cell surface is well established. Recent evidence has revealed the presence of Gαs on endosomes and its capacity to elicit GPCR-promoted signalling from this intracellular compartment. Here, we report an unconventional role for Gαs in the endocytic sorting of GPCRs to lysosomes. Cellular depletion of Gαs specifically delays the lysosomal degradation of GPCRs by disrupting the transfer of GPCRs into the intraluminal vesicles (ILVs) of multivesicular bodies. We show that Gαs interacts with GPCR-associated binding protein-1 (GASP1) and dysbindin, two key proteins that serve as linkers between GPCRs and the endosomal-sorting complex required for transport (ESCRT) machinery involved in receptor sorting into ILVs. Our findings reveal that Gαs plays a role in both GPCR signalling and trafficking pathways, providing another piece in the intertwining molecular network between these processes.

β-Arrestin 2 negatively regulates Toll-like receptor 4 (TLR4)-triggered inflammatory signaling via targeting p38 MAPK and interleukin 10

The control of IL-10 production in Toll-like receptor (TLR) signals remains to be elucidated. Here, we report that β-arrestin 2 positively regulates TLR-triggered IL-10 production in a p38 mitogen-activated protein kinase (MAPK)-dependent mechanism. In vitro studies with cells including peritoneal macrophages and HEK293/TLR4 cells have demonstrated that β-arrestin 2 forms complexes with p38 and facilitates p38 activation after lipopolysaccharide (LPS) stimulation. Deficiency of β-arrestin 2 and inhibition of p38 MAPK activity both ameliorate TLR4-stimulated IL-10 response. Additionally, in vivo experiments show that mice lacking β-arrestin 2 produce less amount of IL-10, and are more susceptible to LPS-induced septic shock which is further enhanced by blocking IL-10 signal. These results reveal a novel mechanism by which β-arrestin 2 negatively regulates TLR4-mediated inflammatory reactions.

Arrestin-3 binds the MAP kinase JNK3α2 via multiple sites on both domains

Although arrestins bind dozens of non-receptor partners, the interaction sites for most signaling proteins remain unknown. Here we report the identification of arrestin-3 elements involved in binding MAP kinase JNK3α2. Using purified JNK3α2 and MBP fusions containing separated arrestin-3 domains and peptides exposed on the non-receptor-binding surface of arrestin-3 we showed that both domains bind JNK3α2 and identified one element on the N-domain and two on the C-domain that directly interact with JNK3α2. Using in vitro competition we confirmed that JNK3α2 engages identified N-domain element and one of the C-domain peptides in the full-length arrestin-3. The 25-amino acid N-domain element has the highest affinity for JNK3α2, suggesting that it is the key site for JNK3α2 docking. The identification of elements involved in protein-protein interactions paves the way to targeted redesign of signaling proteins to modulate cell signaling in desired ways. The tools and methods developed here to elucidate the molecular mechanism of arrestin-3 interactions with JNK3α2 are suitable for mapping of arrestin-3 sites involved in interactions with other partners.

Extensive shape shifting underlies functional versatility of arrestins

Among four vertebrate arrestins, only two are ubiquitously expressed. Arrestins specifically bind active phosphorylated G protein-coupled receptors (GPCRs), thereby precluding further G protein activation. Recent discoveries suggest that the formation of the arrestin-receptor complex initiates the second round of signaling with comparable biological importance. Despite having virtually no recognizable sequence motifs known to mediate protein-protein interactions, arrestins bind a surprising variety of signaling proteins with mind-boggling range of functional consequences. High conformational flexibility allows arrestins to show many distinct 'faces' to the world, which allows these relatively small ∼45kDa proteins to bind various partners under different physiological conditions, organizing multi-protein signaling complexes and localizing them to distinct subcellular compartments.

Non-canonical signalling and roles of the vasoactive peptides angiotensins and kinins

GPCRs (G-protein-coupled receptors) are among the most important targets for drug discovery due to their ubiquitous expression and participation in cellular events under both healthy and disease conditions. These receptors can be activated by a plethora of ligands, such as ions, odorants, small ligands and peptides, including angiotensins and kinins, which are vasoactive peptides that are classically involved in the pathophysiology of cardiovascular events. These peptides and their corresponding GPCRs have been reported to play roles in other systems and under pathophysiological conditions, such as cancer, central nervous system disorders, metabolic dysfunction and bone resorption. More recently, new mechanisms have been described for the functional regulation of GPCRs, including the transactivation of other signal transduction receptors and the activation of G-protein-independent pathways. The existence of such alternative mechanisms for signal transduction and the discovery of agonists that can preferentially trigger one signalling pathway over other pathways (called biased agonists) have opened new perspectives for the discovery and development of drugs with a higher specificity of action and, therefore, fewer side effects. The present review summarizes the current knowledge on the non-canonical signalling and roles of angiotensins and kinins.

β-adrenoceptors as molecular targets in the treatment of hypertension

Regulation of sympathoadrenal activity has been a long-time target in the management of hypertension. Regulation of β-adrenoceptor (βAR) function has been the most therapeutically important of these targets. The development of effective antihypertensive treatments based on βAR antagonism paralleled the elucidation of the molecular basis of β-adrenergic effects by the family of βARs, which are members of the G-protein-coupled receptor (GPCR) superfamily. βARs serve as the extracellular face of the transmembrane signalling pathway that results in the consequent activation of heterotrimeric G-proteins and the activation of several other newly appreciated signalling molecules that include β-arrestins and GPCR kinases (GRKs). The aggregate effect of the activation of these signalling pathways mediates the response to βAR activation. Paradoxically, the hypertensive state is characterized by impaired βAR responsiveness. This defect is common to many other receptor systems linked to the stimulator G protein (Gs) and adenylyl cyclase activation. This impairment is principally mediated by receptor-G-protein uncoupling, which has been linked to increased expression and activity of GRK2.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Minireview: More than just a hammer: ligand "bias" and pharmaceutical discovery

Conventional orthosteric drug development programs targeting G protein-coupled receptors (GPCRs) have focused on the concepts of agonism and antagonism, in which receptor structure determines the nature of the downstream signal and ligand efficacy determines its intensity. Over the past decade, the emerging paradigms of "pluridimensional efficacy" and "functional selectivity" have revealed that GPCR signaling is not monolithic, and that ligand structure can "bias" signal output by stabilizing active receptor states in different proportions than the native ligand. Biased ligands are novel pharmacologic entities that possess the unique ability to qualitatively change GPCR signaling, in effect creating "new receptors" with distinct efficacy profiles driven by ligand structure. The promise of biased agonism lies in this ability to engender "mixed" effects not attainable using conventional agonists or antagonists, promoting therapeutically beneficial signals while antagonizing deleterious ones. Indeed, arrestin pathway-selective agonists for the type 1 parathyroid hormone and angiotensin AT1 receptors, and G protein pathway-selective agonists for the GPR109A nicotinic acid and μ-opioid receptors, have demonstrated unique, and potentially therapeutic, efficacy in cell-based assays and preclinical animal models. Conversely, activating GPCRs in "unnatural" ways may lead to downstream biological consequences that cannot be predicted from prior knowledge of the actions of the native ligand, especially in the case of ligands that selectively activate as-yet poorly characterized G protein-independent signaling networks mediated via arrestins. Although much needs to be done to realize the clinical potential of functional selectivity, biased GPCR ligands nonetheless appear to be important new additions to the pharmacologic toolbox.

SIRT1 regulates MAPK pathways in vitiligo skin: insight into the molecular pathways of cell survival

Vitiligo is an acquired and progressive hypomelanotic disease that manifests as circumscribed depigmented patches on the skin. The aetiology of vitiligo remains unclear, but recent experimental data underline the interactions between melanocytes and other typical skin cells, particularly keratinocytes. Our previous results indicate that keratinocytes from perilesional skin show the features of damaged cells. Sirtuins (silent mating type information regulation 2 homolog) 1, well-known modulators of lifespan in many species, have a role in gene repression, metabolic control, apoptosis and cell survival, DNA repair, development, inflammation, neuroprotection and healthy ageing. In the literature there is no evidence for SIRT1 signalling in vitiligo and its possible involvement in disease progression. Here, biopsies were taken from the perilesional skin of 16 patients suffering from non-segmental vitiligo and SIRT1 signalling was investigated in these cells. For the first time, a new SIRT1/Akt, also known as Protein Kinase B (PKB)/mitogen-activated protein kinase (MAPK) signalling has been revealed in vitiligo. SIRT1 regulates MAPK pathway via Akt-apoptosis signal-regulating kinase-1 and down-regulates pro-apoptotic molecules, leading to decreased oxidative stress and apoptotic cell death in perilesional vitiligo keratinocytes. We therefore propose SIRT1 activation as a novel way of protecting perilesional vitiligo keratinocytes from damage.

Role of β-adrenergic receptor signaling and desensitization in heart failure: new concepts and prospects for treatment

The use of beta-blockers to antagonize beta-adrenergic receptor signaling in the heart has become a standard method of treatment for heart failure, resulting in positive clinical outcomes alone and in conjunction with other modulators of cardiomyocyte contractility. However, an entire explanation for improved cardiac function in patients using beta-blockers is unknown, and in fact may be quite complicated, considering the numerous intracellular signaling pathways associated with beta-adrenergic receptors. Stimulation of beta-adrenergic receptors during both normal conditions and during heart failure activate several distinct signaling cascades, which influence cardiomyocyte contraction, hypertrophy and apoptosis. This review explores the signaling cascades induced by beta-adrenergic receptor activation in normal and desensitized states to provide new insight into the effective treatment of cardiac dysfunction.

Arrestins in apoptosis

Programmed cell death (apoptosis) is a coordinated set of events eventually leading to the massive activation of specialized proteases (caspases) that cleave numerous substrates, orchestrating fairly uniform biochemical changes than culminate in cellular suicide. Apoptosis can be triggered by a variety of stimuli, from external signals or growth factor withdrawal to intracellular conditions, such as DNA damage or ER stress. Arrestins regulate many signaling cascades involved in life-or-death decisions in the cell, so it is hardly surprising that numerous reports document the effects of ubiquitous nonvisual arrestins on apoptosis under various conditions. Although these findings hardly constitute a coherent picture, with the same arrestin subtypes, sometimes via the same signaling pathways, reported to promote or inhibit cell death, this might reflect real differences in pro- and antiapoptotic signaling in different cells under a variety of conditions. Recent finding suggests that one of the nonvisual subtypes, arrestin-2, is specifically cleaved by caspases. Generated fragment actively participates in the core mechanism of apoptosis: it assists another product of caspase activity, tBID, in releasing cytochrome C from mitochondria. This is the point of no return in committing vertebrate cells to death, and the aspartate where caspases cleave arrestin-2 is evolutionary conserved in vertebrate, but not in invertebrate arrestins. In contrast to wild-type arrestin-2, its caspase-resistant mutant does not facilitate cell death.

Arrestin-dependent activation of ERK and Src family kinases

The four members of the mammalian arrestin family, two visual and two nonvisual, share the property of stimulus-dependent docking to G protein-coupled receptors. This conformational selectivity permits them to function in receptor desensitization, as arrestin binding sterically inhibits G protein coupling. The two nonvisual arrestins further act as adapter proteins, linking receptors to the clathrin-dependent endocytic machinery and regulating receptor sequestration, intracellular trafficking, recycling, and degradation. Arrestins also function as ligand-regulated scaffolds, recruiting catalytically active proteins into receptor-based multiprotein "signalsome" complexes. Arrestin binding thus marks the transition from a transient G protein-coupled state on the plasma membrane to a persistent arrestin-coupled state that continues to signal as the receptor internalizes. Two of the earliest discovered and most studied arrestin-dependent signaling pathways involve regulation of Src family nonreceptor tyrosine kinases and the ERK1/2 mitogen-activated kinase cascade. In each case, arrestin scaffolding imposes constraints on kinase activity that dictate signal duration and substrate specificity. Evidence suggests that arrestin-bound ERK1/2 and Src not only play regulatory roles in receptor desensitization and trafficking but also mediate longer term effects on cell growth, migration, proliferation, and survival.

β-arrestin promotes c-Jun N-terminal kinase mediated apoptosis via a GABA(B)R·β-arrestin·JNK signaling module

Evidence is growing that the GABAB receptor, which belongs to the G protein-coupled receptor (GPCR) superfamily, is involved in tumorigenesis. Recent studies have shown that β-arrestin can serve as a scaffold to recruit signaling protein c-Jun N-terminal knase (JNK) to GPCR. Here we investigated whether β-arrestin recruits JNK to the GABAB receptor and facilitates its activation to affect the growth of cancer cells. Our results showed that β-arrestin expression is decreased in breast cancer cells in comparison with controls. β-arrestin could enhance interactions of the GABABR·β-arrestin·JNK signaling module in MCF-7 and T-47D cells. Further studies revealed that increased expression of β-arrestin enhances the phosphorylation of JNK and induces cancer cells apoptosis. Collectively, these results indicate that β-arrestin promotes JNK mediated apoptosis via a GABABR·β-arrestin·JNK signaling module.

Arrestin-dependent activation of JNK family kinases

The activity of all mitogen-activated protein kinases (MAPKs) is stimulated via phosphorylation by upstream MAPK kinases (MAPKK), which are in their turn activated via phosphorylation by MAPKK kinases (MAPKKKs). The cells ensure the specificity of signaling in these cascades by employing a variety of scaffolding proteins that bind matching MAPKKKs, MAPKKs, and MAPKs. All four vertebrate arrestin subtypes bind JNK3, but only arrestin-3 serves as a scaffold, promoting JNK3 activation in intact cells. Arrestin-3-mediated JNK3 activation does not depend on arrestin-3 interaction with G protein-coupled receptors (GPCRs), as demonstrated by the ability of some arrestin mutants that cannot bind receptors to activate JNK3, whereas certain mutants with enhanced GPCR binding fail to promote JNK3 activation. Recent findings suggest that arrestin-3 directly binds both MAPKKs necessary for JNK activation and facilitates JNK3 phosphorylation at both Thr (by MKK4) and Tyr (by MKK7). JNK3 is expressed in a limited set of cell types, whereas JNK1 and JNK2 isoforms are as ubiquitous as arrestin-3. Recent study showed that arrestin-3 facilitates the activation of JNK1 and JNK2, scaffolding MKK4/7-JNK1/2/3 signaling complexes. In all cases, arrestin-3 acts by bringing the kinases together: JNK phosphorylation shows biphasic dependence on arrestin-3, being enhanced at lower and suppressed at supraoptimal concentrations. Thus, arrestin-3 regulates the activity of multiple JNK isoforms, suggesting that it might play a role in survival and apoptosis of all cell types.

Arrestin-mediated activation of p38 MAPK: molecular mechanisms and behavioral consequences

Studies of kappa opioid receptor signaling mechanisms during the last decade have demonstrated that agonist activation of the receptor results in Gβγ-dependent signaling and distinct arrestin-dependent signaling events. Gβγ-dependent signaling results in ion channel regulation causing neuronal inhibition, inhibition of transmitter release, and subsequent analgesic responses. In contrast, arrestin-dependent signaling events result in p38 MAPK activation and subsequent dysphoric and proaddictive behavioral responses. Resolution of these two branches of signaling cascades has enabled strategies designed to identify pathway-selective drugs that may have unique therapeutic utilities.

Endosomal signaling and oncogenesis

The endosomal system provides a route whereby nutrients, viruses, and receptors are internalized. During the course of endocytosis, activated receptors can accumulate within endosomal structures and certain signal-transducing molecules can be recruited to endosomal membranes. In the context of signaling and cancer, they provide platforms within the cell from which signals can be potentiated or attenuated. Regulation of the duration of receptor signaling is a pivotal means of refining growth responses in cells. In cancers, this is often considered in terms of mutations that affect receptor tyrosine kinases and maintain them in hyperactivated states of dimerization and/or phosphorylation. However, disruption to the regulatory control exerted by the assembly of protein complexes within the endosomal network can also contribute to disease among which oncogenesis is characterized in part by dysregulated growth, enhanced cell survival, and changes in the expression of markers of differentiation. In this chapter, we will discuss the role of proteins that regulate in endocytosis as tumor suppressors or oncogenes and how changing the fate of internalized receptors and concomitant endosomal signaling can contribute to cancer.

Arrestin interactions with G protein-coupled receptors

G-protein-coupled receptors (GPCRs) are the primary interaction partners for arrestins. The visual arrestins, arrestin1 and arrestin4, physiologically bind to only very few receptors, i.e., rhodopsin and the color opsins, respectively. In contrast, the ubiquitously expressed nonvisual variants β-arrestin1 and 2 bind to a large number of receptors in a fairly nonspecific manner. This binding requires two triggers, agonist activation and receptor phosphorylation by a G-protein-coupled receptor kinase (GRK). These two triggers are mediated by two different regions of the arrestins, the "phosphorylation sensor" in the core of the protein and a less well-defined "activation sensor." Binding appears to occur mostly in a 1:1 stoichiometry, involving the N-terminal domain of GPCRs, but in addition a second GPCR may loosely bind to the C-terminal domain when active receptors are abundant.Arrestin binding initially uncouples GPCRs from their G-proteins. It stabilizes receptors in an active conformation and also induces a conformational change in the arrestins that involves a rotation of the two domains relative to each other plus changes in the polar core. This conformational change appears to permit the interaction with further downstream proteins. The latter interaction, demonstrated mostly for β-arrestins, triggers receptor internalization as well as a number of nonclassical signaling pathways.Open questions concern the exact stoichiometry of the interaction, possible specificity with regard to the type of agonist and of GRK involved, selective regulation of downstream signaling (=biased signaling), and the options to use these mechanisms as therapeutic targets.

Arrestins in host-pathogen interactions

In the context of host-pathogen interaction, host cell receptors and signaling pathways are essential for both invading pathogens, which exploit them for their own profit, and the defending organism, which activates early mechanism of defense, known as innate immunity, to block the aggression. Because of their central role as scaffolding proteins downstream of activated receptors, β-arrestins are involved in multiple signaling pathways activated in host cells by pathogens. Some of these pathways participate in the innate immunity and the inflammatory response. Other β-arrestin-dependent pathways are actually hijacked by microbes and toxins to penetrate into host cells and spread in the organism.

Arrestin-3 binds c-Jun N-terminal kinase 1 (JNK1) and JNK2 and facilitates the activation of these ubiquitous JNK isoforms in cells via scaffolding

Non-visual arrestins scaffold mitogen-activated protein kinase (MAPK) cascades. The c-Jun N-terminal kinases (JNKs) are members of MAPK family. Arrestin-3 has been shown to enhance the activation of JNK3, which is expressed mainly in neurons, heart, and testes, in contrast to ubiquitous JNK1 and JNK2. Although all JNKs are activated by MKK4 and MKK7, both of which bind arrestin-3, the ability of arrestin-3 to facilitate the activation of JNK1 and JNK2 has never been reported. Using purified proteins we found that arrestin-3 directly binds JNK1α1 and JNK2α2, interacting with the latter comparably to JNK3α2. Phosphorylation of purified JNK1α1 and JNK2α2 by MKK4 or MKK7 is increased by arrestin-3. Endogenous arrestin-3 interacted with endogenous JNK1/2 in different cell types. Arrestin-3 also enhanced phosphorylation of endogenous JNK1/2 in intact cells upon expression of upstream kinases ASK1, MKK4, or MKK7. We observed a biphasic effect of arrestin-3 concentrations on phosphorylation of JNK1α1 and JNK2α2 both in vitro and in vivo. Thus, arrestin-3 acts as a scaffold, facilitating JNK1α1 and JNK2α2 phosphorylation by MKK4 and MKK7 via bringing JNKs and their activators together. The data suggest that arrestin-3 modulates the activity of ubiquitous JNK1 and JNK2 in non-neuronal cells, impacting the signaling pathway that regulates their proliferation and survival.

ERK5 activation by Gq-coupled muscarinic receptors is independent of receptor internalization and β-arrestin recruitment

G-protein-coupled receptors (GPCRs) are known to activate both G protein- and β-arrestin-dependent signalling cascades. The initiation of mitogen-activated protein kinase (MAPK) pathways is a key downstream event in the control of cellular functions including proliferation, differentiation, migration and apoptosis. Both G proteins and β-arrestins have been reported to mediate context-specific activation of ERK1/2, p38 and JNK MAPKs. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has been described to involve a direct interaction between Gαq and two novel effectors, PKCζ and MEK5. However, the possible contribution of β-arrestin towards this pathway has not yet been addressed. In the present work we sought to investigate the role of receptor internalization processes and β-arrestin recruitment in the activation of ERK5 by Gq-coupled GPCRs. Our results show that ERK5 activation is independent of M1 or M3 muscarinic receptor internalization. Furthermore, we demonstrate that phosphorylation-deficient muscarinic M1 and M3 receptors are still able to fully activate the ERK5 pathway, despite their reported inability to recruit β-arrestins. Indeed, the overexpression of Gαq, but not that of β-arrestin1 or β-arrestin2, was found to potently enhance ERK5 activation by GPCRs, whereas silencing of β-arrestin2 expression did not affect the activation of this pathway. Finally, we show that a β-arrestin-biased mutant form of angiotensin II (SII; Sar1-Ile4-Ile8 AngII) failed to promote ERK5 phosphorylation in primary cardiac fibroblasts, as compared to the natural ligand. Overall, this study shows that the activation of ERK5 MAPK by model Gq-coupled GPCRs does not depend on receptor internalization, β-arrestin recruitment or receptor phosphorylation but rather is dependent on Gαq-signalling.

A novel noncanonical signaling pathway for the μ-opioid receptor

The µ-opioid receptor (OPRM1) signals as a classic G protein-coupled receptor by activating heterotrimeric Gi/Go proteins resulting in adenylyl cyclase (AC) inhibition. Such AC inhibition is desensitized after prolonged agonist treatment. However, after receptor desensitization, the intracellular cAMP level remains regulated by OPRM1, as demonstrated by the intracellular cAMP level increase or AC superactivation upon removal of an agonist or addition of an antagonist. We now demonstrate that such intracellular cAMP regulation is mediated by a novel noncanonical signaling pathway resulting from OPRM1 being converted to a receptor tyrosine kinase (RTK)-like entity. This noncanonical OPRM1 signaling is initiated by the receptor recruiting and activating Src kinase within the receptor complex, leading to phosphorylation of the OPRM1 Tyr(336) residue. Phospho-Tyr(336) serves as the docking site for growth factor receptor-bound protein/son of sevenless, leading to the recruitment and activation of the Ras/Raf-1 and subsequent phosphorylation and activation of AC5/6 by Raf-1. Such sequence of events was established by the absence of Ras/Raf1 recruitment and activation by the OPRM1-Y336F mutant, by the presence of Src kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) or the absence of Src activity, by the presence of specific Raf-1 inhibitor GW5074 (5-iodo-3-[(3,5-dibromo-4-hydroxyphenyl) methylene]-2-indolinone) or the absence of Raf-1, or by the dominant negative RasN17 mutant. Src together with Ras activates Raf1 which was established by the inability of the Raf1-Tyr(340/341) mutant to activate AC. Hence, the phosphorylation of OPRM1 at Tyr(336) by Src serves as the trigger for the conversion of a classic Gi/Go-coupled receptor into an RTK-like entity, resulting in a noncanonical pathway even after the original Gi/Go signals are blunted.

JNK3 enzyme binding to arrestin-3 differentially affects the recruitment of upstream mitogen-activated protein (MAP) kinase kinases

Arrestin-3 was previously shown to bind JNK3α2, MKK4, and ASK1. However, full JNK3α2 activation requires phosphorylation by both MKK4 and MKK7. Using purified proteins we show that arrestin-3 directly interacts with MKK7 and promotes JNK3α2 phosphorylation by both MKK4 and MKK7 in vitro as well as in intact cells. The binding of JNK3α2 promotes an arrestin-3 interaction with MKK4 while reducing its binding to MKK7. Interestingly, the arrestin-3 concentration optimal for scaffolding the MKK7-JNK3α2 module is ∼10-fold higher than for the MKK4-JNK3α2 module. The data provide a mechanistic basis for arrestin-3-dependent activation of JNK3α2. The opposite effects of JNK3α2 on arrestin-3 interactions with MKK4 and MKK7 is the first demonstration that the kinase components in mammalian MAPK cascades regulate each other's interactions with a scaffold protein. The results show how signaling outcomes can be affected by the relative expression of scaffolding proteins and components of signaling cascades that they assemble.

Altered sympathetic-to-immune cell signaling via β₂-adrenergic receptors in adjuvant arthritis

Adjuvant-induced arthritic (AA) differentially affects norepinephrine concentrations in immune organs, and in vivo β-adrenergic receptor (β-AR) agonist treatment distinctly regulates ex vivo cytokine profiles in different immune organs. We examined the contribution of altered β-AR functioning in AA to understand these disparate findings. Twenty-one or 28 days after disease induction, we examined β₂-AR expression in spleen and draining lymph nodes (DLNs) for the arthritic limbs using radioligand binding and western blots and splenocyte β-AR-stimulated cAMP production using enzyme-linked immunoassay (EIA). During severe disease, β-AR agonists failed to induce splenocyte cAMP production, and β-AR affinity and density declined, indicating receptor desensitization and downregulation. Splenocyte β₂-AR phosphorylation (pβ₂-AR) by protein kinase A (pβ₂-AR_PKA) decreased in severe disease, and pβ₂-AR by G protein-coupled receptor kinases (pβ₂-AR_GRK) increased in chronic disease. Conversely, in DLN cells, pβ₂-AR_PKA rose during severe disease, but fell during chronic disease, and pβ₂-AR_GRK increased during both disease stages. A similar pβ₂-AR pattern in DLN cells with the mycobacterial cell wall component of complete Freund's adjuvant suggests that pattern recognition receptors (i.e., toll-like receptors) are important for DLN pβ₂-AR patterns. Collectively, our findings indicate lymphoid organ- and disease stage-specific sympathetic dysregulation, possibly explaining immune compartment-specific differences in β₂-AR-mediated regulation of cytokine production in AA and rheumatoid arthritis.

β-Arrestins 1 and 2 are critical regulators of inflammation

β-Arrestins 1 and 2 couple to seven trans-membrane receptors and regulate G protein-dependent signaling, receptor endocytosis and ubiquitylation. Recent studies have uncovered several unanticipated functions of β-arrestins, suggesting that the role of β-arrestins in cell signaling is much broader than originally thought. It is now recognized that β-arrestins can transduce receptor signaling independent of G proteins. The expression of β-arrestins is differentially regulated in immune cells and tissues in response to specific inflammatory stimuli, and β-arrestins are critical regulators of the inflammatory response. This review will focus on β-arrestins in immune cells and the impact of altered expression on the pathogenesis of specific inflammatory diseases. Understanding the role of β-arrestins in inflammation may lead to new strategies to treat inflammatory diseases, such as sepsis, rheumatoid arthritis, asthma, multiple sclerosis, inflammatory bowel disease and atherosclerosis.

Depletion of β-arrestin2 in hepatic stellate cells reduces cell proliferation via ERK pathway

β-Arrestins are multifunctional adaptor proteins. Recently, some new roles of β-arrestins in regulating intracellular signaling networks have been discovered, which regulate cell growth, proliferation, and apoptosis. Though, the role of β-arrestins expression in the pathology of hepatic fibrosis remains unclear. In this study, the possible relationship between the expression of β-arrestins with the experimental hepatic fibrosis and the proliferation of hepatic stellate cells (HSCs) were investigated. Porcine serum induced liver fibrosis was established in this study. At five time points, the dynamic expression of β-arrestin1, β-arrestin2, and α-smooth muscle actin (α-SMA) in rat liver tissues, was measured by immunohistochemical staining, double immunofluorescent staining, and Western blotting. This study showed that aggravation of hepatic fibrosis with gradually increasing expression of β-arrestin2 in the hepatic tissues, but not β-arrestin1. Further, as hepatic fibrosis worsens, β-arrestin2-expressing activated HSCs accounts for an increasingly larger percentage of all activated HSCs. And the expression of β-arrestin2 had a significant positive correlation with the expression of α-SMA, an activated HSCs marker. In vitro studies, the dynamic expression of β-arrestin1 and β-arrestin2 in platelet derived growth factor-BB (PDGF-BB) stimulated HSCs was assessed by Western blotting. The expression of β-arrestin2 was remarkably increased in PDGF-BB stimulated HSCs. Furthermore, the small interfering RNA (siRNA) technique was used to explore the effect of β-arrestins on the proliferation of HSCs and the activation of ERK1/2. Transfection of siRNA targeting β-arrestin2 mRNA (siβ-arrestin2) into HSCs led to a 68% and 70% reduction of β-arrestin2 mRNA and protein expression, respectively. siβ-arrestin2 abolished the effect of PDGF-BB on the proliferation of HSCs. In addition, siβ-arrestin2 exerted the inhibition of the activation of ERK1/2 in HSCs. The present study provided strong evidence for the participation of the β-arrestin2 in the pathogenesis of hepatic fibrosis. The β-arrestin2 depletion diminishes HSCs ERK1/2 signaling and proliferation stimulated by PDGF-BB. Selective targeting of β-arrestin2 inhibitors to HSCs might present as a novel strategy for the treatment of hepatic fibrosis.

Interaction of PICK1 with C-terminus of growth hormone-releasing hormone receptor (GHRHR) modulates trafficking and signal transduction of human GHRHR

Release of growth hormone (GH) from the somatotroph is regulated by binding GH-releasing hormone (GHRH) to its cognate receptor (GHRHR), one of the members of the G protein-coupled receptor (GPCR) superfamily. Proteins bound to the carboxy (C)-terminus of GPCR have been reported to regulate intracellular trafficking and function of the receptor; however, no functionally significant protein associated with GHRHR has been reported. We have identified a protein interacting with C-kinase 1 (PICK1) as a binding partner of GHRHR. In vitro binding assay revealed the PDZ-domain of PICK1 and the last four amino acid residues of GHRHR were prerequisite for the interaction. Further, in vivo association of these proteins was confirmed. Immunostaining data of a stable cell line expressing GHRHR with or without PICK1 suggested the C-terminus of GHRHR promoted cell surface expression of GHRHR and PICK1 affected the kinetics of the cell surface expression of GHRHR. Furthermore, cAMP production assay showed the C-terminus of GHRHR is involved in the regulation of receptor activation, and the interaction of GHRHR with PICK1 may influence intensities of the signal response after ligand stimulation. Thus, the interaction of the C-terminus of GHRHR with PICK1 has a profound role in regulating the trafficking and the signaling of GHRHR. [Supplementary Figure: available only at http://dx.doi.org/10.1254/jphs.12287FP].

Beta-arrestin2 is involved in the increase of distal colonic contraction in diabetic rats

Colonic dysmotility occurs in diabetes and the patients exhibit diarrhea or constipation. The pathogenetic mechanisms underlying colonic dysmotility in diabetic patients remain poorly understood. The effects of β-arrestin2 on colonic contraction in diabetic rats were investigated for the first time. Male SD rats were treated with a single intraperitoneally injected dose of streptozotocin, and those displaying sustained high blood glucose were selected as diabetes mellitus models. Longitudinal muscle strips of the distal colon were prepared to monitor contraction of the colon in vitro. Expression of β-arrestin2 was investigated by Western blot analysis. Anti-β-arrestin2 antibody had no direct effect on the contraction of distal colonic strips in both normal and diabetic rats. Carbachol-induced contractions of distal colonic strips were higher in diabetic rats than in normal rats. Anti-β-arrestin2 antibody partly blocked carbachol-induced increases of distal colonic strips in diabetic rats. The expression level of β-arrestin2 protein in the colon was higher in diabetic rats than in normal rats. These results suggest that β-arrestin2 is involved in the increase of distal colonic contraction in diabetic rats.

The small GTPase RALA controls c-Jun N-terminal kinase-mediated FOXO activation by regulation of a JIP1 scaffold complex

FOXO (forkhead box O) transcription factors are tumor suppressors and increase the life spans of model organisms. Cellular stress, in particular oxidative stress caused by an increase in levels of reactive oxygen species (ROS), activates FOXOs through JNK-mediated phosphorylation. Importantly, JNK regulation of FOXO is evolutionarily conserved. Here we identified the pathway that mediates ROS-induced JNK-dependent FOXO regulation. Following increased ROS, RALA is activated by the exchange factor RLF (RalGDS-like factor), which is in complex with JIP1 (C-Jun-amino-terminal-interacting protein 1) and JNK. Active RALA consequently regulates assembly and activation of MLK3, MKK4, and JNK onto the JIP1 scaffold. Furthermore, regulation of FOXO by RALA and JIP1 is conserved in C. elegans, where both ral-1 and jip-1 depletion impairs heat shock-induced nuclear translocation of the FOXO orthologue DAF16.

Gene dosage-dependent negative regulatory role of β-arrestin-2 in polymicrobial infection-induced inflammation

β-arrestin-2 (β-arr2) is a scaffolding protein of the arrestin family with a wide variety of cellular functions. Recent studies have demonstrated differential roles for β-arr2 in inflammation following endotoxemia and cecal ligation and puncture (CLP) models of sepsis. Because CLP-induced inflammation involves response to fecal contents and necrotic cecum in addition to microbial challenge, in this study, we examined the role of β-arr2 in an exclusively polymicrobial infection (PMI) model. In addition, we examined the role of gene dosage of β-arr2 in polymicrobial sepsis. Our studies demonstrate that β-arr2 is a negative regulator of systemic inflammation in response to polymicrobial infection and that one allele is sufficient for this process. Our results further reveal that loss of β-arr2 leads to increased neutrophil sequestration and overt inflammation specifically in the lungs following polymicrobial infection. Consistent with this, specific NF-κB and mitogen-activated protein kinase (MAPK) signaling pathways were differentially activated in the β-arr2 knockout (KO) mice lungs compared to the wild type (WT) following PMI. Associated with enhanced inflammation in the KO mice, PMI-induced mortality was also significantly higher in KO mice than in WT mice. To understand the differential role of β-arr2 in different sepsis models, we used cell culture systems to evaluate inflammatory cytokine production following endotoxin and polymicrobial stimulation. Our results demonstrate cell-type- as well as stimulus-specific roles for β-arr2 in inflammation. Taken together, our results reveal a negative regulatory role for β-arr2 in polymicrobial infection-induced inflammation and further demonstrate that one allele of β-arr2 is sufficient to mediate most of these effects.

Sh3tc2 deficiency affects neuregulin-1/ErbB signaling

Mutations in SH3TC2 trigger autosomal recessive demyelinating Charcot-Marie-Tooth type 4C (CMT4C) neuropathy. Sh3tc2 is specifically expressed in Schwann cells and is necessary for proper myelination of peripheral axons. In line with the early onset of neuropathy observed in patients with CMT4C, our analyses of the murine model of CMT4C revealed that the myelinating properties of Sh3tc2-deficient Schwann cells are affected at an early stage. This early phenotype is associated with changes in the canonical Nrg1/ErbB pathway involved in control of myelination. We demonstrated that Sh3tc2 interacts with ErbB2 and plays a role in the regulation of ErbB2 intracellular trafficking from the plasma membrane upon Nrg1 activation. Interestingly, both the loss of Sh3tc2 function in mice and the pathological mutations present in CMT4C patients affect ErbB2 internalization, potentially altering its downstream intracellular signaling pathways. Altogether, our results indicate that the molecular mechanism for the axonal size sensing is disturbed in Sh3tc2-deficient myelinating Schwann cells, thus providing a novel insight into the pathophysiology of CMT4C neuropathy.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

Modulation of methuselah expression targeted to Drosophila insulin-producing cells extends life and enhances oxidative stress resistance

Ubiquitously reduced signaling via Methuselah (MTH), a G-protein-coupled receptor (GPCR) required for neurosecretion, has previously been reported to extend life and enhance stress resistance in flies. Whether these effects are due to reduced MTH signalling in specific tissues remains unknown. We determined that reduced expression of mth targeted to the insulin-producing cells (IPCs) of the fly brain was sufficient to extend life and enhance oxidative stress resistance. Paradoxically, we discovered that overexpression of mth targeted to the same cells has similar phenotypic effects to reduced expression due to MTH's interaction with β-arrestin, which uncouples GPCRs from their G-proteins. We confirmed the functional relationship between MTH and β-arrestin by finding that IPC-targeted overexpression of β-arrestin alone mimics the longevity phenotype of reduced MTH signaling. As reduced MTH signaling also inhibits insulin secretion from the IPCs, the most parsimonious mechanistic explanation of its longevity and stress-resistance enhancement might be through reduced insulin/IGF signaling (IIS). However, examination of phenotypic features of long-lived IPC-mth modulated flies as well as several downstream IIS targets implicates enhanced activity of the JNK stress-resistance pathway more directly than insulin signaling in the longevity and stress-resistance phenotypes.

β-Arrestin-1 directly interacts with Gαs and regulates its function

β-Arrestins function to mediate G protein-coupled receptor (GPCR) desensitization and internalization and to initiate G protein independent signaling of GPCRs. Elucidating how β-arrestin and G protein signal pathways coordinate with each other is important to fully understand GPCR signaling. Here we report that β-arrestin-1 directly interacts with Gα(s). Purified β-arrestin-1 binds to Gα(s) in a rapid association and dissociation manner. β-Arrestin-1 promotes the binding and the release of GTPγS from Gα(s) in vitro. β-Arrestin-1 L33K mutant shows reduced interaction with Gα(s) and has no detectable effects on Gα(s) function. Our study thus reveals a direct crosstalk of β-arrestin-1 with Gα(s).

Spatial coupling of JNK activation to the B cell antigen receptor by tyrosine-phosphorylated ezrin

The ezrin-radixin-moesin proteins regulate B lymphocyte activation via their effect on BCR diffusion and microclustering. This relies on their ability to dynamically tether the plasma membrane with actin filaments that is in turn facilitated by phosphorylation of the conserved threonine residue in the actin-binding domain. In this study, we describe a novel function of ezrin in regulating JNK activation that is mediated by phosphorylation of a tyrosine (Y353) residue that is unconserved with moesin and radixin. BCR, but not CD40, TLR4, or CXCR5 stimulation, induced phosphorylation of ezrin at Y353 in mouse splenic B cells. Ezrin existed in a preformed complex with Syk in unstimulated B cells and underwent Syk-dependent phosphorylation upon anti-IgM stimulation. Y353-phosphorylated ezrin colocalized with the BCR within minutes of stimulation and cotrafficked with the endocytosed BCRs through the early and late endosomes. The T567 residue of ezrin was rephosphorylated in late endosomes and at the plasma membrane at later times of BCR stimulation. Expression of a nonphosphorylatable Y353F mutant of ezrin specifically impaired JNK activation. BCR crosslinking induced the association of Y353-phosphorylated ezrin with JNK and its kinase MAPKK7, as well as spatial colocalization with phosphorylated JNK in the endosomes. The yellow fluorescent protein-tagged Y353F mutant displayed reduced colocalization with the endocytosed BCR as compared with wild-type ezrin-yellow fluorescent protein. Taken together, our data identify a novel role for ezrin as a spatial adaptor that couples JNK signaling components to the BCR signalosome, thus facilitating JNK activation.

β-Arrestin-kinase scaffolds: turn them on or turn them off?

G-protein-coupled receptors (GPCRs) can signal through heterotrimeric G-proteins or through β-arrestins to elicit responses to a plethora of extracellular stimuli. While the mechanisms underlying G-protein signaling is relatively well understood, the mechanisms by which β-arrestins regulate the diverse set of proteins with which they associate remain unclear. Multi-protein complexes are a common feature of β-arrestin-dependent signaling. The first two such complexes discovered were the mitogen-activated kinases modules associated with extracellular regulated kinases (ERK1/2) and Jnk3. Subsequently a number of other kinases have been shown to undergo β-arrestin-dependent regulation, including Akt, phosphatidylinositol-3kinase (PI3K), Lim-domain-containing kinase (LIMK), calcium calmodulin kinase II (CAMKII), and calcium calmodulin kinase kinase β (CAMKKβ). Some are positively and some negatively regulated by β-arrestin association. One of the missing links to understanding these pathways is the molecular mechanisms by which the activity of these kinases is regulated. Do β-arrestins merely serve as scaffolds to bring enzyme and substrate together or do they have a direct effect on the enzymatic activities of target kinases? Recent evidence suggests that both mechanisms are involved and that the mechanisms by which β-arrestins regulate kinase activity varies with the target kinase. This review discusses recent advances in the field focusing on 5 kinases for which considerable mechanistic detail and specific sites of interaction have been elucidated.

β-arrestin-selective G protein-coupled receptor agonists engender unique biological efficacy in vivo

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in β-arrestin2-null mice, identifying them as downstream targets of β-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on β-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not β-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

Noncanonical GPCR signaling arising from a PTH receptor-arrestin-Gβγ complex

G protein-coupled receptors (GPCRs) participate in ubiquitous transmembrane signal transduction processes by activating heterotrimeric G proteins. In the current "canonical" model of GPCR signaling, arrestins terminate receptor signaling by impairing receptor-G-protein coupling and promoting receptor internalization. However, parathyroid hormone receptor type 1 (PTHR), an essential GPCR involved in bone and mineral metabolism, does not follow this conventional desensitization paradigm. β-Arrestins prolong G protein (G(S))-mediated cAMP generation triggered by PTH, a process that correlates with the persistence of arrestin-PTHR complexes on endosomes and which is thought to be associated with prolonged physiological calcemic and phosphate responses. This presents an inescapable paradox for the current model of arrestin-mediated receptor-G-protein decoupling. Here we show that PTHR forms a ternary complex that includes arrestin and the Gβγ dimer in response to PTH stimulation, which in turn causes an accelerated rate of G(S) activation and increases the steady-state levels of activated G(S), leading to prolonged generation of cAMP. This work provides the mechanistic basis for an alternative model of GPCR signaling in which arrestins contribute to sustaining the effect of an agonist hormone on the receptor.

The role of arrestins in visual and disease processes of the eye

Visual arrestins are well known for their function in quenching the phototransduction process in rods and cones. Perhaps not as well known is their participation in multiple other processes in the normal and disease states of the eye. This chapter covers the range of the known functions of the visual arrestins, beginning with their classical role in quenching light-activated visual pigments. The role of visual arrestins is also reviewed from the perspective of their dynamic mobility whereby they redistribute significantly between the compartments of highly polarized photoreceptor cells. Additional roles of the visual arrestins are also reviewed based on new interacting partners that have been discovered over the past decade. Finally, the contribution of the visual arrestins to diseases of the visual system is explored.