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

Flavors of GPCR signaling bias

GPCRs are inherently flexible molecules existing in an equilibrium of multiple conformations. Binding of GPCR agonists shifts this equilibrium. Certain agonists can increase the fraction of active-like conformations that predispose the receptor to coupling to a particular signal transducer or a select group of transducers. Such agonists are called biased, in contrast to balanced agonists that facilitate signaling via all transducers the receptor couples to. These biased agonists preferentially channel the signaling of a GPCR to particular G proteins, GRKs, or arrestins. Preferential activation of particular G protein or arrestin subtypes can be beneficial, as it would reduce unwanted on-target side effects, widening the therapeutic window. However, biasing GPCRs has two important limitations: a) complete bias is impossible due to inherent flexibility of GPCRs; b) receptor-independent functions of signal transducer proteins cannot be directly affected by GPCR ligands or differential receptor barcoding by GRK phosphorylation. This article is part of the Special Issue on "Ligand Bias".

Hormonal and Allosteric Regulation of the Luteinizing Hormone/Chorionic Gonadotropin Receptor

Luteinizing hormone (LH) and human chorionic gonadotropin (CG), like follicle-stimulating hormone, are the most important regulators of the reproductive system. They exert their effect on the cell through the LH/CG receptor (LHCGR), which belongs to the family of G protein-coupled receptors. Binding to gonadotropin induces the interaction of LHCGR with various types of heterotrimeric G proteins (Gs, Gq/11, Gi) and β-arrestins, which leads to stimulation (Gs) or inhibition (Gi) of cyclic adenosine monophosphate-dependent cascades, activation of the phospholipase pathway (Gq/11), and also to the formation of signalosomes that mediate the stimulation of mitogen-activated protein kinases (β-arrestins). The efficiency and selectivity of activation of intracellular cascades by different gonadotropins varies, which is due to differences in their interaction with the ligand-binding site of LHCGR. Gonadotropin signaling largely depends on the status of N- and O-glycosylation of LH and CG, on the formation of homo- and heterodimeric receptor complexes, on the cell-specific microenvironment of LHCGR and the presence of autoantibodies to it, and allosteric mechanisms are important in the implementation of these influences, which is due to the multiplicity of allosteric sites in different loci of the LHCGR. The development of low-molecular-weight allosteric regulators of LHCGR with different profiles of pharmacological activity, which can be used in medicine for the correction of reproductive disorders and in assisted reproductive technologies, is promising. These and other issues regarding the hormonal and allosteric regulation of LHCGR are summarized and discussed in this review.

Arrestin-3-assisted activation of JNK3 mediates dopaminergic behavioral sensitization

In rodents with unilateral ablation of neurons supplying dopamine to the striatum, chronic treatment with the dopamine precursor L-DOPA induces a progressive increase of behavioral responses, a process known as behavioral sensitization. This sensitization is blunted in arrestin-3 knockout mice. Using virus-mediated gene delivery to the dopamine-depleted striatum of these mice, we find that the restoration of arrestin-3 fully rescues behavioral sensitization, whereas its mutant defective in c-Jun N-terminal kinase (JNK) activation does not. A 25-residue arrestin-3-derived peptide that facilitates JNK3 activation in cells, expressed ubiquitously or selectively in direct pathway striatal neurons, also fully rescues sensitization, whereas an inactive homologous arrestin-2-derived peptide does not. Behavioral rescue is accompanied by the restoration of JNK3 activity, as reflected by JNK-dependent phosphorylation of the transcription factor c-Jun in the dopamine-depleted striatum. Thus, arrestin-3-assisted JNK3 activation in direct pathway neurons is a critical element of the molecular mechanism underlying sensitization upon dopamine depletion and chronic L-DOPA treatment.

GPCR-dependent and -independent arrestin signaling

Biological activity of free arrestins is often overlooked. Based on available data, we compare arrestin-mediated signaling that requires and does not require binding to G-protein-coupled receptors (GPCRs). Receptor-bound arrestins activate ERK1/2, Src, and focal adhesion kinase (FAK). Yet, arrestin-3 regulation of Src family member Fgr does not appear to involve receptors. Free arrestin-3 facilitates the activation of JNK family kinases, preferentially binds E3 ubiquitin ligases Mdm2 and parkin, and facilitates parkin-dependent mitophagy. The binding of arrestins to microtubules and calmodulin and their function in focal adhesion disassembly and apoptosis also do not involve receptors. Biased GPCR ligands and the phosphorylation barcode can only affect receptor-dependent arrestin signaling. Thus, elucidation of receptor dependence or independence of arrestin functions has important scientific and therapeutic implications.

Hepatic β-arrestins: potential roles in liver health and disease

Β-arrestins are intracellular scaffolding proteins that have multifaceted roles in different types of disorders. In this review article, we gave a summary about the discovery, characterization and classification of these proteins and their intracellular functions. Moreover, this review article focused on the hepatic expression of β-arrestins and their hepatocellular distribution and function in each liver cell type. Also, we showed that β-arrestins are key regulators of distinct types of hepatic disorders. On the other hand, we addressed some important points that have never been studied before regarding the role of β-arrestins in certain types of hepatic disorders which needs more research efforts to cover.

Arrestin-3-Dependent Activation of c-Jun N-Terminal Kinases (JNKs)

Only 1 out of 4 mammalian arrestin subtypes, arrestin-3, facilitates the activation of c-Jun N-terminal kinase (JNK) family kinases. Here, we describe two different sets of protocols used for elucidating the mechanisms involved. One is based on reconstitution of signaling modules from the following purified proteins: arrestin-3, MKK4, MKK7, JNK1, JNK2, and JNK3. The main advantage of this method is that it unambiguously establishes which effects are direct because only intended purified proteins are present in these assays. The key drawback is that the upstream-most kinases of these cascades, ASK1 or other MAP3Ks, are not available in purified form, limiting reconstitution to incomplete two-kinase modules. The other approach is used for analyzing the effects of arrestin-3 on JNK activation in intact cells. In this case, signaling modules include ASK1 and/or other MAP3Ks. However, as every cell expresses thousands of different proteins, their possible effects on the readout cannot be excluded. Nonetheless, the combination of in vitro reconstitution from purified proteins and cell-based assays makes it possible to elucidate the mechanisms of arrestin-3-dependent activation of JNK family kinases. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Construction of arrestin-3-scaffolded MKK4/7-JNK1/2/3 signaling modules in vitro using purified proteins Alternate Protocol 1: Characterization of arrestin-3-mediated JNK1/2 activation by MKK4/7 by measurement of JNK1/2 phosphorylation using immunoblotting with anti-phospho-JNK antibody Support Protocol 1: Expression, purification, and activation of GST-MKK4 Support Protocol 2: Expression, purification, and activation of GST-MKK7-His6 Support Protocol 3: Expression, purification, and activation of tagless JNK1Α1 Support Protocol 4: Expression, purification, and activation of tagless JNK2Α2 Basic Protocol 2: Analysis of the role of arrestin-3 in ASK1/MKK4/MKK7-induced JNK activation in intact cells Alternate Protocol 2: Analysis of the role of arrestin-3 in MKK4-induced JNK activation in intact cells Basic Protocol 3: Characterization of the biphasic effect of arrestin-3 on ASK1/MKK7-stimulated JNK phosphorylation in cells.

JNK Cascade-Induced Apoptosis-A Unique Role in GqPCR Signaling

The response of cells to extracellular signals is mediated by a variety of intracellular signaling pathways that determine stimulus-dependent cell fates. One such pathway is the cJun-N-terminal Kinase (JNK) cascade, which is mainly involved in stress-related processes. The cascade transmits its signals via a sequential activation of protein kinases, organized into three to five tiers. Proper regulation is essential for securing a proper cell fate after stimulation, and the mechanisms that regulate this cascade may involve the following: (1) Activatory or inhibitory phosphorylations, which induce or abolish signal transmission. (2) Regulatory dephosphorylation by various phosphatases. (3) Scaffold proteins that bring distinct components of the cascade in close proximity to each other. (4) Dynamic change of subcellular localization of the cascade's components. (5) Degradation of some of the components. In this review, we cover these regulatory mechanisms and emphasize the mechanism by which the JNK cascade transmits apoptotic signals. We also describe the newly discovered PP2A switch, which is an important mechanism for JNK activation that induces apoptosis downstream of the Gq protein coupled receptors. Since the JNK cascade is involved in many cellular processes that determine cell fate, addressing its regulatory mechanisms might reveal new ways to treat JNK-dependent pathologies.

Mechanisms of Arrestin-Mediated Signaling

Arrestins were first discovered as proteins that selectively bind active phosphorylated GPCRs and suppress (arrest) their G protein-mediated signaling. Nonvisual arrestins are also recognized as signaling proteins regulating a variety of cellular pathways. Arrestins are highly flexible; they can assume many different conformations. In their receptor-bound conformation, arrestins have higher affinity for a subset of binding 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 regulate other signaling pathways and localize signaling proteins to particular subcellular compartments. Recent findings suggest that the two visuals, arrestin-1 and arrestin-4, which are expressed in photoreceptor cells, not only regulate signaling via binding to photopigments but also interact with several 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. © 2023 Wiley Periodicals LLC.

Short Arrestin-3-Derived Peptides Activate JNK3 in Cells

Arrestins were first discovered as suppressors of G protein-mediated signaling by G protein-coupled receptors. It was later demonstrated that arrestins also initiate several signaling branches, including mitogen-activated protein kinase cascades. Arrestin-3-dependent activation of the JNK family can be recapitulated with peptide fragments, which are monofunctional elements distilled from this multi-functional arrestin protein. Here, we use maltose-binding protein fusions of arrestin-3-derived peptides to identify arrestin elements that bind kinases of the ASK1-MKK4/7-JNK3 cascade and the shortest peptide facilitating JNK signaling. We identified a 16-residue arrestin-3-derived peptide expressed as a Venus fusion that leads to activation of JNK3α2 in cells. The strength of the binding to the kinases does not correlate with peptide activity. The ASK1-MKK4/7-JNK3 cascade has been implicated in neuronal apoptosis. While inhibitors of MAP kinases exist, short peptides are the first small molecule tools that can activate MAP kinases.

The Two Non-Visual Arrestins Engage ERK2 Differently

Arrestin binding to active phosphorylated G protein-coupled receptors terminates G protein coupling and initiates another wave of signaling. Among the effectors that bind directly to receptor-associated arrestins are extracellular signal-regulated kinases 1/2 (ERK1/2), which promote cellular proliferation and survival. Arrestins may also engage ERK1/2 in isolation in a pre- or post-signaling complex that is likely in equilibrium with the full signal initiation complex. Molecular details of these binary complexes remain unknown. Here, we investigate the molecular mechanisms whereby arrestin-2 and arrestin-3 (a.k.a. β-arrestin1 and β-arrestin2, respectively) engage ERK1/2 in pairwise interactions. We find that purified arrestin-3 binds ERK2 more avidly than arrestin-2. A combination of biophysical techniques and peptide array analysis demonstrates that the molecular basis in this difference of binding strength is that the two non-visual arrestins bind ERK2 via different parts of the molecule. We propose a structural model of the ERK2-arrestin-3 complex in solution using size-exclusion chromatography coupled to small angle X-ray scattering (SEC-SAXS). This binary complex exhibits conformational heterogeneity. We speculate that this drives the equilibrium either toward the full signaling complex with receptor-bound arrestin at the membrane or toward full dissociation in the cytoplasm. As ERK1/2 regulates cell migration, proliferation, and survival, understanding complexes that relate to its activation could be exploited to control cell fate.

[β-arrestins, their mechanisms of action and multiple roles in the biology of G protein-coupled receptors]

The stimulation of G protein-coupled receptors (GPCRs) induces biological responses to a wide range of extracellular cues. The heterotrimeric G proteins, which are recruited to the active conformation of GPCRs, lead to the generation of various diffusible second messengers. Only two other families of proteins exhibit the remarkable characteristic of recognizing and binding to the active conformation of most GPCRs: GPCR kinases (GRKs) and β-arrestins. These two families of proteins were initially identified as key players in the desensitization of G protein activation by GPCRs. Over the years, β-arrestins have been implicated in an increasing number of interactions with non-receptor proteins, expanding the range of cellular functions in which they are involved. It is now well established that β-arrestins, by scaffolding and recruiting protein complexes in an agonist-dependent manner, directly regulate the trafficking and signaling of GPCRs. Remarkable advances have been made in recent years which have made it possible i) to identify biased ligands capable, by stabilizing particular conformations of a growing number of GPCRs, of activating or blocking the action of β-arrestins independently of that of G proteins, some of these ligands holding great therapeutic interest; ii) to demonstrate β-arrestins' role in the compartmentalization of GPCR signaling within the cell, and iii) to understand the molecular details of their interaction with GPCRs and of their activation through structural and biophysical approaches.

A Model for the Signal Initiation Complex Between Arrestin-3 and the Src Family Kinase Fgr

Arrestins regulate a wide range of signaling events, most notably when bound to active G protein-coupled receptors (GPCRs). Among the known effectors recruited by GPCR-bound arrestins are Src family kinases, which regulate cellular growth and proliferation. Here, we focus on arrestin-3 interactions with Fgr kinase, a member of the Src family. Previous reports demonstrated that Fgr exhibits high constitutive activity, but can be further activated by both arrestin-dependent and arrestin-independent pathways. We report that arrestin-3 modulates Fgr activity with a hallmark bell-shaped concentration-dependence, consistent with a role as a signaling scaffold. We further demonstrate using NMR spectroscopy that a polyproline motif within arrestin-3 interacts directly with the SH3 domain of Fgr. To provide a framework for this interaction, we determined the crystal structure of the Fgr SH3 domain at 1.9 Å resolution and developed a model for the GPCR-arrestin-3-Fgr complex that is supported by mutagenesis. This model suggests that Fgr interacts with arrestin-3 at multiple sites and is consistent with the locations of disease-associated Fgr mutations. Collectively, these studies provide a structural framework for arrestin-dependent activation of Fgr.

Scaffolding of Mitogen-Activated Protein Kinase Signaling by β-Arrestins

β-arrestins were initially identified to desensitize and internalize G-protein-coupled receptors (GPCRs). Receptor-bound β-arrestins also initiate a second wave of signaling by scaffolding mitogen-activated protein kinase (MAPK) signaling components, MAPK kinase kinase, MAPK kinase, and MAPK. In particular, β-arrestins facilitate ERK1/2 or JNK3 activation by scaffolding signal cascade components such as ERK1/2-MEK1-cRaf or JNK3-MKK4/7-ASK1. Understanding the precise molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly would deepen our understanding of GPCR-mediated MAPK activation and provide clues for the selective regulation of the MAPK signaling cascade for therapeutic purposes. Over the last decade, numerous research groups have attempted to understand the molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly. Although not providing the complete mechanism, these efforts suggest potential binding interfaces between β-arrestins and MAPK signaling components and the mechanism for MAPK signal amplification by β-arrestin-mediated scaffolding. This review summarizes recent developments of cellular and molecular works on the scaffolding mechanism of β-arrestin for MAPK signaling cascade.

Scaffolding mechanism of arrestin-2 in the cRaf/MEK1/ERK signaling cascade

Arrestins were initially identified for their role in homologous desensitization and internalization of G protein-coupled receptors. Receptor-bound arrestins also initiate signaling by interacting with other signaling proteins. Arrestins scaffold MAPK signaling cascades, MAPK kinase kinase (MAP3K), MAPK kinase (MAP2K), and MAPK. In particular, arrestins facilitate ERK1/2 activation by scaffolding ERK1/2 (MAPK), MEK1 (MAP2K), and Raf (MAPK3). However, the structural mechanism underlying this scaffolding remains unknown. Here, we investigated the mechanism of arrestin-2 scaffolding of cRaf, MEK1, and ERK2 using hydrogen/deuterium exchange-mass spectrometry, tryptophan-induced bimane fluorescence quenching, and NMR. We found that basal and active arrestin-2 interacted with cRaf, while only active arrestin-2 interacted with MEK1 and ERK2. The ATP binding status of MEK1 or ERK2 affected arrestin-2 binding; ATP-bound MEK1 interacted with arrestin-2, whereas only empty ERK2 bound arrestin-2. Analysis of the binding interfaces suggested that the relative positions of cRaf, MEK1, and ERK2 on arrestin-2 likely facilitate sequential phosphorylation in the signal transduction cascade.

Chronic Treatment with Morphine Disrupts Acute Kinase-Dependent Desensitization of GPCRs

Based on studies using mutations of the µ-opioid receptor (MOR), phosphorylation of multiple sites on the C-terminus has been recognized as a critical step underlying acute desensitization and the development of cellular tolerance. The aim of this study is to explore which kinases mediate desensitization of MOR in brain slices from drug-naïve and morphine-treated animals. Whole-cell recordings from locus coeruleus neurons were made, and the agonist-induced increase in potassium conductance was measured. In slices from naïve animals, pharmacological inhibition of G-protein receptor kinase (GRK2/3) with compound 101 blocked acute desensitization. Following chronic treatment with morphine, compound 101 was less effective at blocking acute desensitization. Compound 101 blocked receptor internalization in tissue from both naïve and morphine-treated animals, suggesting that GRK2/3 remained active. Kinase inhibitors aimed at blocking protein kinase C and c-Jun N-terminal kinase had no effect on desensitization in tissue taken from naïve animals. However, in slices taken from morphine-treated animals, the combination of these blockers along with compound 101 was required to block acute desensitization. Acute desensitization of the potassium conductance induced by the somatostatin receptor was also blocked by compound 101 in slices from naïve but not morphine-treated animals. As was observed with MOR, it was necessary to use the combination of kinase inhibitors to block desensitization of the somatostatin receptor in slices from morphine-treated animals. The results show that chronic treatment with morphine results in a surprising and heterologous adaptation in kinase-dependent desensitization. SIGNIFICANCE STATEMENT: The results show that chronic treatment with morphine induced heterologous adaptations in kinase regulation of G protein coupled receptor (GPCR) desensitization. Although the canonical mechanism for acute desensitization through phosphorylation by G protein-coupled receptor kinase is supported in tissue taken from naïve animals, following chronic treatment with morphine, the acute kinase-dependent desensitization of GPCRs is disrupted such that additional kinases, including protein kinase C and c-Jun N-terminal kinase, contribute to desensitization.

Map kinase signaling as therapeutic target for neurodegeneration

Aging is known to be one of the major risk factors in many neurodegenerative diseases (ND) whose prevalence is estimated to rise in the coming years due to the increase in life expectancy. Examples of neurodegenerative diseases include Huntington's, Parkinson's, and Alzheimer's diseases, along with Amyotrophic Lateral Sclerosis, Spinocerebellar ataxias and Frontotemporal Dementia. Given that so far these ND do not have effective pharmacological therapies, a better understanding of the molecular and cellular mechanisms can contribute to development of effective treatments. During the previous decade, the data indicated that dysregulation of MAP kinases [which included c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1 and 2 (ERK1/2), and p38] are associated with several stages of the inflammatory process which in turn contributes to age-related neurodegenerative diseases. This evidence suggests that control of inflammation through regulation of MAP kinase could be a worthwhile approach against neurodegenerative diseases. In this review we summarize the pathways of MAP kinase signal transduction and different pharmacological inhibitors that can be used in its modulation against ND.

Biased GPCR signaling: Possible mechanisms and inherent limitations

G protein-coupled receptors (GPCRs) are targeted by about a third of clinically used drugs. Many GPCRs couple to more than one type of heterotrimeric G proteins, become phosphorylated by any of several different GRKs, and then bind one or more types of arrestin. Thus, classical therapeutically active drugs simultaneously initiate several branches of signaling, some of which are beneficial, whereas others result in unwanted on-target side effects. The development of novel compounds to selectively channel the signaling into the desired direction has the potential to become a breakthrough in health care. However, there are natural and technological hurdles that must be overcome. The fact that most GPCRs are subject to homologous desensitization, where the active receptor couples to G proteins, is phosphorylated by GRKs, and then binds arrestins, suggest that in most cases the GPCR conformations that facilitate their interactions with these three classes of binding partners significantly overlap. Thus, while partner-specific conformations might exist, they are likely low-probability states. GPCRs are inherently flexible, which suggests that complete bias is highly unlikely to be feasible: in the conformational ensemble induced by any ligand, there would be some conformations facilitating receptor coupling to unwanted partners. Things are further complicated by the fact that virtually every cell expresses numerous G proteins, several GRK subtypes, and two non-visual arrestins with distinct signaling capabilities. Finally, novel screening methods for measuring ligand bias must be devised, as the existing methods are not specific for one particular branch of signaling.

Targeting arrestin interactions with its partners for therapeutic purposes

Most vertebrates express four arrestin subtypes: two visual ones in photoreceptor cells and two non-visuals expressed ubiquitously. The latter two interact with hundreds of G protein-coupled receptors, certain receptors of other types, and numerous non-receptor partners. Arrestins have no enzymatic activity and work by interacting with other proteins, often assembling multi-protein signaling complexes. Arrestin binding to every partner affects cell signaling, including pathways regulating cell survival, proliferation, and death. Thus, targeting individual arrestin interactions has therapeutic potential. This requires precise identification of protein-protein interaction sites of both participants and the choice of the side of each interaction which would be most advantageous to target. The interfaces involved in each interaction can be disrupted by small molecule therapeutics, as well as by carefully selected peptides of the other partner that do not participate in the interactions that should not be targeted.

Arrestin-3 interaction with maternal embryonic leucine-zipper kinase

Maternal embryonic leucine-zipper kinase (MELK) overexpression impacts survival and proliferation of multiple cancer types, most notably glioblastomas and breast cancer. This makes MELK an attractive molecular target for cancer therapy. Yet the molecular mechanisms underlying the involvement of MELK in tumorigenic processes are unknown. MELK participates in numerous protein-protein interactions that affect cell cycle, proliferation, apoptosis, and embryonic development. Here we used both in vitro and in-cell assays to identify a direct interaction between MELK and arrestin-3. A part of this interaction involves the MELK kinase domain, and we further show that the interaction between the MELK kinase domain and arrestin-3 decreases the number of cells in S-phase, as compared to cells expressing the MELK kinase domain alone. Thus, we describe a new mechanism of regulation of MELK function, which may contribute to the control of cell fate.

Plethora of functions packed into 45 kDa arrestins: biological implications and possible therapeutic strategies

Mammalian arrestins are a family of four highly homologous relatively small ~ 45 kDa proteins with surprisingly diverse functions. The most striking feature is that each of the two non-visual subtypes can bind hundreds of diverse G protein-coupled receptors (GPCRs) and dozens of non-receptor partners. Through these interactions, arrestins regulate the G protein-dependent signaling by the desensitization mechanisms as well as control numerous signaling pathways in the G protein-dependent or independent manner via scaffolding. Some partners prefer receptor-bound arrestins, some bind better to the free arrestins in the cytoplasm, whereas several show no apparent preference for either conformation. Thus, arrestins are a perfect example of a multi-functional signaling regulator. The result of this multi-functionality is that reduction (by knockdown) or elimination (by knockout) of any of these two non-visual arrestins can affect so many pathways that the results are hard to interpret. The other difficulty is that the non-visual subtypes can in many cases compensate for each other, which explains relatively mild phenotypes of single knockouts, whereas double knockout is lethal in vivo, although cultured cells lacking both arrestins are viable. Thus, deciphering the role of arrestins in cell biology requires the identification of specific signaling function(s) of arrestins involved in a particular phenotype. This endeavor should be greatly assisted by identification of structural elements of the arrestin molecule critical for individual functions and by the creation of mutants where only one function is affected. Reintroduction of these biased mutants, or introduction of monofunctional stand-alone arrestin elements, which have been identified in some cases, into double arrestin-2/3 knockout cultured cells, is the most straightforward way to study arrestin functions. This is a laborious and technically challenging task, but the upside is that specific function of arrestins, their timing, subcellular specificity, and relations to one another could be investigated with precision.

β2 Adrenergic Receptor Complexes with the L-Type Ca2+ Channel CaV1.2 and AMPA-Type Glutamate Receptors: Paradigms for Pharmacological Targeting of Protein Interactions

Formation of signaling complexes is crucial for the orchestration of fast, efficient, and specific signal transduction. Pharmacological disruption of defined signaling complexes has the potential for specific intervention in selected regulatory pathways without affecting organism-wide disruption of parallel pathways. Signaling by epinephrine and norepinephrine through α and β adrenergic receptors acts on many signaling pathways in many cell types. Here, we initially provide an overview of the signaling complexes formed between the paradigmatic β₂ adrenergic receptor and two of its most important targets, the L-type Ca²⁺ channel Ca_V1.2 and the AMPA-type glutamate receptor. Importantly, both complexes contain the trimeric G_s protein, adenylyl cyclase, and the cAMP-dependent protein kinase, PKA. We then discuss the functional implications of the formation of these complexes, how those complexes can be specifically disrupted, and how such disruption could be utilized in the pharmacological treatment of disease.

JNK and cardiometabolic dysfunction

Cardiometabolic syndrome (CMS) describes the cluster of metabolic and cardiovascular diseases that are generally characterized by impaired glucose tolerance, intra-abdominal adiposity, dyslipidemia, and hypertension. CMS currently affects more than 25% of the world’s population and the rates of diseases are rapidly rising. These CMS conditions represent critical risk factors for cardiovascular diseases including atherosclerosis, heart failure, myocardial infarction, and peripheral artery disease (PAD). Therefore, it is imperative to elucidate the underlying signaling involved in disease onset and progression. The c-Jun N-terminal Kinases (JNKs) are a family of stress signaling kinases that have been recently indicated in CMS. The purpose of this review is to examine the in vivo implications of JNK as a potential therapeutic target for CMS. As the constellation of diseases associated with CMS are complex and involve multiple tissues and environmental triggers, carefully examining what is known about the JNK pathway will be important for specificity in treatment strategies.

The structural basis of the arrestin binding to GPCRs

G protein-coupled receptors (GPCRs) are the largest family of signaling proteins targeted by more clinically used drugs than any other protein family. GPCR signaling via G proteins is quenched (desensitized) by the phosphorylation of the active receptor by specific GPCR kinases (GRKs) followed by tight binding of arrestins to active phosphorylated receptors. Thus, arrestins engage two types of receptor elements: those that contain GRK-added phosphates and those that change conformation upon activation. GRKs attach phosphates to serines and threonines in the GPCR C-terminus or any one of the cytoplasmic loops. In addition to these phosphates, arrestins engage the cavity that appears between trans-membrane helices upon receptor activation and several other non-phosphorylated elements. The residues that bind GPCRs are localized on the concave side of both arrestin domains. Arrestins undergo a global conformational change upon receptor binding (become activated). Arrestins serve as important hubs of cellular signaling, emanating from activated GPCRs and receptor-independent.

GPCR Signaling Regulation: The Role of GRKs and Arrestins

Every animal species expresses hundreds of different G protein-coupled receptors (GPCRs) that respond to a wide variety of external stimuli. GPCRs-driven signaling pathways are involved in pretty much every physiological function and in many pathologies. Therefore, GPCRs are targeted by about a third of clinically used drugs. The signaling of most GPCRs via G proteins is terminated by the phosphorylation of active receptor by specific kinases (GPCR kinases, or GRKs) and subsequent binding of arrestin proteins, that selectively recognize active phosphorylated receptors. In addition, GRKs and arrestins play a role in multiple signaling pathways in the cell, both GPCR-initiated and receptor-independent. Here we focus on the mechanisms of GRK- and arrestin-mediated regulation of GPCR signaling, which includes homologous desensitization and redirection of signaling to additional pathways by bound arrestins.

Cleavage of arrestin-3 by caspases attenuates cell death by precluding arrestin-dependent JNK activation

The two non-visual subtypes, arrestin-2 and arrestin-3, are ubiquitously expressed and bind hundreds of G protein-coupled receptors. In addition, these arrestins also interact with dozens of non-receptor signaling proteins, including c-Src, ERK and JNK, that regulate cell death and survival. Arrestin-3 facilitates the activation of JNK family kinases, which are important players in the regulation of apoptosis. Here we show that arrestin-3 is specifically cleaved at Asp366, Asp405 and Asp406 by caspases during the apoptotic cell death. This results in the generation of one main cleavage product, arrestin-3-(1-366). The formation of this fragment occurs in a dose-dependent manner with the increase of fraction of apoptotic cells upon etoposide treatment. In contrast to a caspase-resistant mutant (D366/405/406E) the arrestin-3-(1-366) fragment reduces the apoptosis of etoposide-treated cells. We found that caspase cleavage did not affect the binding of the arrestin-3 to JNK3, but prevented facilitation of its activation, in contrast to the caspase-resistant mutant, which facilitated JNK activation similar to WT arrestin-3, likely due to decreased binding of the upstream kinases ASK1 and MKK4/7. The data suggest that caspase-generated arrestin-3-(1-366) prevents the signaling in the ASK1-MKK4/7-JNK1/2/3 cascade and protects cells, thereby suppressing apoptosis.

Arrestin-3 scaffolding of the JNK3 cascade suggests a mechanism for signal amplification

Scaffold proteins tether and orient components of a signaling cascade to facilitate signaling. Although much is known about how scaffolds colocalize signaling proteins, it is unclear whether scaffolds promote signal amplification. Here, we used arrestin-3, a scaffold of the ASK1-MKK4/7-JNK3 cascade, as a model to understand signal amplification by a scaffold protein. We found that arrestin-3 exhibited >15-fold higher affinity for inactive JNK3 than for active JNK3, and this change involved a shift in the binding site following JNK3 activation. We used systems biochemistry modeling and Bayesian inference to evaluate how the activation of upstream kinases contributed to JNK3 phosphorylation. Our combined experimental and computational approach suggested that the catalytic phosphorylation rate of JNK3 at Thr-221 by MKK7 is two orders of magnitude faster than the corresponding phosphorylation of Tyr-223 by MKK4 with or without arrestin-3. Finally, we showed that the release of activated JNK3 was critical for signal amplification. Collectively, our data suggest a "conveyor belt" mechanism for signal amplification by scaffold proteins. This mechanism informs on a long-standing mystery for how few upstream kinase molecules activate numerous downstream kinases to amplify signaling.

Using In Vitro Pull-Down and In-Cell Overexpression Assays to Study Protein Interactions with Arrestin

Nonvisual arrestins (arrestin-2/arrestin-3) interact with hundreds of G protein-coupled receptor (GPCR) subtypes and dozens of non-receptor signaling proteins. Here we describe the methods used to identify the interaction sites of arrestin-binding partners on arrestin-3 and the use of monofunctional individual arrestin-3 elements in cells. Our in vitro pull-down assay with purified proteins demonstrates that relatively few elements in arrestin engage each partner, whereas cell-based functional assays indicate that certain arrestin elements devoid of other functionalities can perform individual functions in living cells.

Methods to Characterize Protein Interactions with β-Arrestin In Cellulo

β-Arrestins 1 and 2 (β-arr1 and β-arr2) are ubiquitous proteins with common and distinct functions. They were initially identified as proteins recruited to stimulated G protein-coupled receptors (GPCRs), regulating their desensitization and internalization. The discovery that β-arrs could also interact with more than 400 non-GPCR protein partners brought to light their central roles as multifunctional scaffold proteins regulating multiple signalling pathways from the plasma membrane to the nucleus, downstream of GPCRs or independently from these receptors. Through the regulation of the activities and subcellular localization of their binding partners, β-arrs control various cell processes such as proliferation, cytoskeletal rearrangement, cell motility, and apoptosis. Thus, the identification of β-arrs binding partners and the characterization of their mode of interaction in cells are central to the understanding of their function. Here we provide methods to explore the molecular interaction of β-arrs with other proteins in cellulo.

Arrestin-mediated signaling: Is there a controversy?

The activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK)1/2 was traditionally used as a readout of signaling of G protein-coupled receptors (GPCRs) via arrestins, as opposed to conventional GPCR signaling via G proteins. Several recent studies using HEK293 cells where all G proteins were genetically ablated or inactivated, or both non-visual arrestins were knocked out, demonstrated that ERK1/2 phosphorylation requires G protein activity, but does not necessarily require the presence of non-visual arrestins. This appears to contradict the prevailing paradigm. Here we discuss these results along with the recent data on gene edited cells and arrestin-mediated signaling. We suggest that there is no real controversy. G proteins might be involved in the activation of the upstream-most MAP3Ks, although in vivo most MAP3K activation is independent of heterotrimeric G proteins, being initiated by receptor tyrosine kinases and/or integrins. As far as MAP kinases are concerned, the best-established role of arrestins is scaffolding of the three-tiered cascades (MAP3K-MAP2K-MAPK). Thus, it seems likely that arrestins, GPCR-bound and free, facilitate the propagation of signals in these cascades, whereas signal initiation via MAP3K activation may be independent of arrestins. Different MAP3Ks are activated by various inputs, some of which are mediated by G proteins, particularly in cell culture, where we artificially prevent signaling by receptor tyrosine kinases and integrins, thereby favoring GPCR-induced signaling. Thus, there is no reason to change the paradigm: Arrestins and G proteins play distinct non-overlapping roles in cell signaling.

Arrestins: structural disorder creates rich functionality

Arrestins are soluble relatively small 44-46 kDa proteins that specifically bind hundreds of active phosphorylated GPCRs and dozens of non-receptor partners. There are binding partners that demonstrate preference for each of the known arrestin conformations: free, receptor-bound, and microtubule-bound. Recent evidence suggests that conformational flexibility in every functional state is the defining characteristic of arrestins. Flexibility, or plasticity, of proteins is often described as structural disorder, in contrast to the fixed conformational order observed in high-resolution crystal structures. However, protein-protein interactions often involve highly flexible elements that can assume many distinct conformations upon binding to different partners. Existing evidence suggests that arrestins are no exception to this rule: their flexibility is necessary for functional versatility. The data on arrestins and many other multi-functional proteins indicate that in many cases, "order" might be artificially imposed by highly non-physiological crystallization conditions and/or crystal packing forces. In contrast, conformational flexibility (and its extreme case, intrinsic disorder) is a more natural state of proteins, representing true biological order that underlies their physiologically relevant functions.

β-Arrestin Based Receptor Signaling Paradigms: Potential Therapeutic Targets for Complex Age-Related Disorders

G protein coupled receptors (GPCRs) were first characterized as signal transducers that elicit downstream effects through modulation of guanine (G) nucleotide-binding proteins. The pharmacotherapeutic exploitation of this signaling paradigm has created a drug-based field covering nearly 50% of the current pharmacopeia. Since the groundbreaking discoveries of the late 1990s to the present day, it is now clear however that GPCRs can also generate productive signaling cascades through the modulation of β-arrestin functionality. β-Arrestins were first thought to only regulate receptor desensitization and internalization - exemplified by the action of visual arrestin with respect to rhodopsin desensitization. Nearly 20 years ago, it was found that rather than controlling GPCR signal termination, productive β-arrestin dependent GPCR signaling paradigms were highly dependent on multi-protein complex formation and generated long-lasting cellular effects, in contrast to G protein signaling which is transient and functions through soluble second messenger systems. β-Arrestin signaling was then first shown to activate mitogen activated protein kinase signaling in a G protein-independent manner and eventually initiate protein transcription - thus controlling expression patterns of downstream proteins. While the possibility of developing β-arrestin biased or functionally selective ligands is now being investigated, no additional research has been performed on its possible contextual specificity in treating age-related disorders. The ability of β-arrestin-dependent signaling to control complex and multidimensional protein expression patterns makes this therapeutic strategy feasible, as treating complex age-related disorders will likely require therapeutics that can exert network-level efficacy profiles. It is our understanding that therapeutically targeting G protein-independent effectors such as β-arrestin will aid in the development of precision medicines with tailored efficacy profiles for disease/age-specific contextualities.

β-arrestins and biased signaling in gonadotropin receptors

Gonadotropin receptors include the follicle stimulating hormone receptor (FSHR) and the luteinizing hormone/choriogonadotropin receptor (LHCGR), both belong to the G protein-coupled receptor (GPCR) superfamily and are essential to reproduction. FSHR is activated by follicle stimulating hormone (FSH) while LHCGR is activated by either luteinizing hormone (LH) or choriogonadotropin (CG). Upon ligand binding, gonadotropin receptors undergo conformational changes that lead to the activation of the heterotrimeric G protein, resulting in the production of different second messengers. Gonadotropin receptors can also recruit and bind β-arrestins. This particular class of scaffold proteins were initially identified to mediate GPCRs desensitization and recycling, but it is now well established that β-arrestins can also initiate Gs-independent signaling by assembling signaling modules. Furthermore, new advances in structural biology and biophysical techniques have revealed novel activation mechanisms allowing β-arrestins and G proteins to control signaling in time and space. The ability of different ligands to preferentially elicit G- or β-arrestin-mediated signaling is known as functional selectivity or biased signaling. This new concept has switched the view of pharmacology efficacy from monodimensional to multidimensional. Biased signaling offers the possibility to separate therapeutic benefits of a drug from its adverse effects. The proof of concept that gonadotropin receptors can be subjected to biased signaling is now established. The challenge will now be the design of molecules that can specifically activate beneficial signaling pathway at gonadotropin receptors while reducing or abolishing those leading to side effects. Such strategy could for instance lead to improved treatments for infertility.

Subcellular Organization of GPCR Signaling

G protein-coupled receptors (GPCRs) comprise a large and diverse class of signal-transducing receptors that undergo dynamic and isoform-specific membrane trafficking. GPCRs thus have an inherent potential to initiate or regulate signaling reactions from multiple membrane locations. This review discusses emerging insights into the subcellular organization of GPCR function in mammalian cells, focusing on signaling transduced by heterotrimeric G proteins and β-arrestins. We summarize recent evidence indicating that GPCR-mediated activation of G proteins occurs not only from the plasma membrane (PM) but also from endosomes and Golgi membranes and that β-arrestin-dependent signaling can be transduced from the PM by β-arrestin trafficking to clathrin-coated pits (CCPs) after dissociation from a ligand-activated GPCR.

Lack of beta-arrestin signaling in the absence of active G proteins

G protein-independent, arrestin-dependent signaling is a paradigm that broadens the signaling scope of G protein-coupled receptors (GPCRs) beyond G proteins for numerous biological processes. However, arrestin signaling in the collective absence of functional G proteins has never been demonstrated. Here we achieve a state of “zero functional G” at the cellular level using HEK293 cells depleted by CRISPR/Cas9 technology of the Gs/q/12 families of Gα proteins, along with pertussis toxin-mediated inactivation of Gi/o. Together with HEK293 cells lacking β-arrestins (“zero arrestin”), we systematically dissect G protein- from arrestin-driven signaling outcomes for a broad set of GPCRs. We use biochemical, biophysical, label-free whole-cell biosensing and ERK phosphorylation to identify four salient features for all receptors at “zero functional G”: arrestin recruitment and internalization, but—unexpectedly—complete failure to activate ERK and whole-cell responses. These findings change our understanding of how GPCRs function and in particular of how they activate ERK1/2.

Arrestins terminate signaling from GPCRs, but several lines of evidence suggest that they are also able to transduce signals independently of G proteins. Here, the authors systematically ablate G proteins in cell lines, and show that arrestins are unable to act as genuine signal initiators.

Non-visual arrestins regulate the focal adhesion formation via small GTPases RhoA and Rac1 independently of GPCRs

Arrestins recruit a variety of signaling proteins to active phosphorylated G protein-coupled receptors in the plasma membrane and to the cytoskeleton. Loss of arrestins leads to decreased cell migration, altered cell shape, and an increase in focal adhesions. Small GTPases of the Rho family are molecular switches that regulate actin cytoskeleton and affect a variety of dynamic cellular functions including cell migration and cell morphology. Here we show that non-visual arrestins differentially regulate RhoA and Rac1 activity to promote cell spreading via actin reorganization, and focal adhesion formation via two distinct mechanisms. Arrestins regulate these small GTPases independently of G-protein-coupled receptor activation.

Regulation of ASK1 signaling by scaffold and adaptor proteins

The mitogen-activated protein kinase (MAPK) signaling pathway is a three-tiered kinase cascade where mitogen-activated protein kinase kinase kinases (MAP3Ks) lead to the activation of mitogen-activated protein kinase kinases (MAP2K), and ultimately MAPK proteins. MAPK signaling can promote a diverse set of biological outcomes, ranging from cell death to proliferation. There are multiple mechanisms which govern MAPK output, such as the duration and strength of the signal, cellular localization to upstream and downstream binding partners, pathway crosstalk and the binding to scaffold and adaptor molecules. This review will focus on scaffold and adaptor proteins that bind to and regulate apoptosis signal-regulating kinase 1 (ASK1), a MAP3K protein with a critical role in mediating stress response pathways.

WITHDRAWN: JIP3 deficiency protects mice from high fat diet-induced liver injury

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Arrestin-2 and arrestin-3 differentially modulate locomotor responses and sensitization to amphetamine

Arrestins play a prominent role in shutting down signaling via G protein-coupled receptors. In recent years, a signaling role for arrestins independent of their function in receptor desensitization has been discovered. Two ubiquitously expressed arrestin isoforms, arrestin-2 and arrestin-3, perform similarly in the desensitization process and share many signaling functions, enabling them to substitute for one another. However, signaling roles specific to each isoform have also been described. Mice lacking arrestin-3 (ARR3KO) were reported to show blunted acute responsiveness to the locomotor stimulatory effect of amphetamine (AMPH). It has been suggested that mice with deletion of arrestin-2 display a similar phenotype. Here we demonstrate that the AMPH-induced locomotion of male ARR3KO mice is reduced over the 7-day treatment period and during AMPH challenge after a 7-day withdrawal. The data are consistent with impaired locomotor sensitization to AMPH and suggest a role for arrestin-3-mediated signaling in the sensitization process. In contrast, male ARR2KO mice showed enhanced early responsiveness to AMPH and the lack of further sensitization, suggesting a role for impaired receptor desensitization. The comparison of mice possessing one allele of arrestin-3 and no arrestin-2 with ARR2KO littermates revealed reduced activity of the former line, consistent with a contribution of arrestin-3-mediated signaling to AMPH responses. Surprisingly, ARR3KO mice with one arrestin-2 allele showed significantly reduced locomotor responses to AMPH combined with lower novelty-induced locomotion, as compared to the ARR3KO line. These data suggest that one allele of arrestin-2 is unable to support normal locomotor behavior due to signaling and/or developmental defects.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

Novel Structural Insights into GPCR-β-Arrestin Interaction and Signaling

G protein-coupled receptors (GPCRs) are major signal recognition and transmission units in the plasma membrane. The interaction of activated and phosphorylated GPCRs with the multifunctional adaptor proteins β-arrestins (βarrs) is crucial for regulation of their signaling and functional outcomes. Over the past few years, a range of structural, biochemical, and cellular studies have revealed novel insights into GPCR-βarr interaction and signaling. Some of these findings have come as a surprise and therefore have the potential to significantly refine the conceptual framework of the GPCR-βarr system. Here we discuss these recent advances with particular emphasis on biphasic GPCR-βarr interaction, the formation of GPCR-G-protein-βarr supercomplexes, and receptor-specific conformational signatures in βarrs. We also underline the emerging research areas that are likely to be at the center stage of investigations in the coming years.

The β-Arrestins: Multifunctional Regulators of G Protein-coupled Receptors

The β-arrestins (βarrs) are versatile, multifunctional adapter proteins that are best known for their ability to desensitize G protein-coupled receptors (GPCRs), but also regulate a diverse array of cellular functions. To signal in such a complex fashion, βarrs adopt multiple conformations and are regulated at multiple levels to differentially activate downstream pathways. Recent structural studies have demonstrated that βarrs have a conserved structure and activation mechanism, with plasticity of their structural fold, allowing them to adopt a wide array of conformations. Novel roles for βarrs continue to be identified, demonstrating the importance of these dynamic regulators of cellular signaling.

Peptide mini-scaffold facilitates JNK3 activation in cells

Three-kinase mitogen-activated protein kinase (MAPK) signaling cascades are present in virtually all eukaryotic cells. MAPK cascades are organized by scaffold proteins, which assemble cognate kinases into productive signaling complexes. Arrestin-3 facilitates JNK activation in cells, and a short 25-residue arrestin-3 peptide was identified as the critical JNK3-binding element. Here we demonstrate that this peptide also binds MKK4, MKK7, and ASK1, which are upstream JNK3-activating kinases. This peptide is sufficient to enhance JNK3 activity in cells. A homologous arrestin-2 peptide, which differs only in four positions, binds MKK4, but not MKK7 or JNK3, and is ineffective in cells at enhancing activation of JNK3. The arrestin-3 peptide is the smallest MAPK scaffold known. This peptide or its mimics can regulate MAPKs, affecting cellular decisions to live or die.

Mu opioid receptor stimulation activates c-Jun N-terminal kinase 2 by distinct arrestin-dependent and independent mechanisms

G protein-coupled receptor desensitization is typically mediated by receptor phosphorylation by G protein-coupled receptor kinase (GRK) and subsequent arrestin binding; morphine, however, was previously found to activate a c-Jun N-terminal kinase (JNK)-dependent, GRK/arrestin-independent pathway to produce mu opioid receptor (MOR) inactivation in spinally-mediated, acute anti-nociceptive responses [Melief et al.] [1]. In the current study, we determined that JNK2 was also required for centrally-mediated analgesic tolerance to morphine using the hotplate assay. We compared JNK activation by morphine and fentanyl in JNK1(-/-), JNK2(-/-), JNK3(-/-), and GRK3(-/-) mice and found that both compounds specifically activate JNK2 in vivo; however, fentanyl activation of JNK2 was GRK3-dependent, whereas morphine activation of JNK2 was GRK3-independent. In MOR-GFP expressing HEK293 cells, treatment with either arrestin siRNA, the Src family kinase inhibitor PP2, or the protein kinase C (PKC) inhibitor Gö6976 indicated that morphine activated JNK2 through an arrestin-independent Src- and PKC-dependent mechanism, whereas fentanyl activated JNK2 through a Src-GRK3/arrestin-2-dependent and PKC-independent mechanism. This study resolves distinct ligand-directed mechanisms of JNK activation by mu opioid agonists and understanding ligand-directed signaling at MOR may improve opioid therapeutics.

Arrestin-3-Dependent Activation of c-Jun N-Terminal Kinases (JNKs)

Only one out of four mammalian arrestin subtypes, arrestin-3, facilitates the activation of JNK family kinases. Here we describe two different protocols used for elucidating the mechanisms involved. One is based on reconstitution of signaling modules from purified proteins: arrestin-3, MKK4, MKK7, JNK1, JNK2, and JNK3. The main advantage of this method is that it unambiguously establishes which effects are direct because only intended purified proteins are present in these assays. The key drawback is that the upstream-most kinases of these cascades, ASK1 or other MAPKKKs, are not available in purified form, limiting reconstitution to incomplete two-kinase modules. The other approach is used for analyzing the effects of arrestin-3 on JNK activation in intact cells. In this case, signaling modules include ASK1 and/or other MAPKKKs. However, as every cell expresses thousands of different proteins their possible effects on the readout cannot be excluded. Nonetheless, the combination of in vitro reconstitution from purified proteins and cell-based assays makes it possible to elucidate the mechanisms of arrestin-3-dependent activation of JNK family kinases.

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.

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.