Ubiquitin ligase parkin promotes Mdm2-arrestin interaction but inhibits arrestin ubiquitination

Intranasal delivery of liposome encapsulated flavonoids ameliorates l-DOPA induced dyskinesia in hemiparkinsonian mice

In the present study, we explored the development of a novel noninvasive liposomal drug delivery material for use in intranasal drug delivery applications in human diseases. We used drug entrapment into liposomal nanoparticle assembly to efficiently deliver the drugs to the nasal mucosa to be delivered to the brain. The naturally occurring flavonoid 7,8-dihydroxyflavone (7,8-DHF) has previously been shown to have beneficial effects in ameliorating Parkinson's disease (PD). We used both naturally occurring 7,8-DHF and the chemically modified form of DHF, the DHF-ME, to be used as a drug candidate for the treatment of PD and l-DOPA induced dyskinesia (LID), which is the debilitating side effect of l-DOPA therapy in PD. The ligand-protein interaction behavior for 7,8-DHF and 6,7-DHF-ME was found to be more effective with molecular docking and molecular stimulation studies of flavonoid compounds with TrkB receptor. Our study showed that 7,8-DHF delivered via intranasal route using a liposomal formulation ameliorated LID in hemiparkinsonian mice model when these mice were chronically administered with l-DOPA, which is the only current medication for relieving the clinical symptoms of PD. The present study also demonstrated that apart from reducing the LID, 7,8-DHF delivery directly to the brain via the intranasal route also corrected some long-term signaling adaptations involving ΔFosB and α Synuclein in the brain of dopamine (DA) depleted animals.

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.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Arrestin-3 binds parkin and enhances parkin-dependent mitophagy

Arrestins were discovered for their role in homologous desensitization of G-protein-coupled receptors (GPCRs). Later non-visual arrestins were shown to regulate several signaling pathways. Some of these pathways require arrestin binding to GPCRs, the regulation of others is receptor independent. Here, we demonstrate that arrestin-3 binds the E3 ubiquitin ligase parkin via multiple sites, preferentially interacting with its RING0 domain. Identification of the parkin domains involved suggests that arrestin-3 likely relieves parkin autoinhibition and/or stabilizes the enzymatically active "open" conformation of parkin. Arrestin-3 binding enhances ubiquitination by parkin of the mitochondrial protein mitofusin-1 and facilitates parkin-mediated mitophagy in HeLa cells. Furthermore, arrestin-3 and its mutant with enhanced parkin binding rescue mitofusin-1 ubiquitination and mitophagy in the presence of the Parkinson's disease-associated R275W parkin mutant, which is defective in both functions. Thus, modulation of parkin activity via arrestin-3 might be a novel strategy of anti-parkinsonian therapy.

Arrestin-3-assisted activation of JNK3 mediates dopaminergic behavioral and signaling plasticity in vivo

In rodents with unilateral ablation of the substantia nigra neurons supplying dopamine to the striatum, chronic treatment with the dopamine precursor L-DOPA or dopamine agonists induces a progressive increase of behavioral responses, a process known as behavioral sensitization. The sensitization is blunted in arrestin-3 knockout mice. Using virus-mediated gene delivery to the dopamine-depleted striatum of arrestin-3 knockout mice, we found that the restoration of arrestin-3 fully rescued behavioral sensitization, whereas its mutant defective in JNK activation did not. A 25-residue arrestin-3-derived peptide that facilitates JNK3 activation in cells, expressed ubiquitously or selectively in the direct pathway striatal neurons, fully rescued sensitization, whereas an inactive homologous arrestin-2-derived peptide did not. Behavioral rescue was accompanied by the restoration of JNK3 activity and of JNK-dependent phosphorylation of the transcription factor c-Jun in the dopamine-depleted striatum. Thus, arrestin-3-dependent JNK3 activation in direct pathway neurons is a critical element of the molecular mechanism underlying sensitization.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

Expression of Untagged Arrestins in E. coli and Their Purification

Purified arrestin proteins are necessary for biochemical, biophysical, and structural studies of these versatile regulators of cell signaling. Described herein is a basic protocol for arrestin expression in Escherichia coli and purification of tag-free wild-type and mutant arrestins. The method includes ammonium sulfate precipitation of arrestins from cell lysates, followed by Heparin-Sepharose chromatography. Depending on the arrestin type and/or mutations, the next step is Q-Sepharose or SP-Sepharose chromatography. In many cases, the nonbinding column is used as a filter to bind contaminants without retaining arrestin. In some cases, both chromatographic steps must be performed sequentially to achieve high purity. Purified arrestins can be concentrated up to 10 mg/ml, remain fully functional, and withstand several cycles of freezing and thawing, provided that the overall salt concentration is maintained at or above physiological levels. © 2023 Wiley Periodicals LLC. Basic Protocol: Large-scale expression and purification of arrestins Alternate Protocol: Purification of arrestin-3 and truncated form of arrestin-1-(1-378) Support Protocol: Small-scale test expression of wild-type and mutant arrestins in E. coli.

GPCR Binding and JNK3 Activation by Arrestin-3 Have Different Structural Requirements

Arrestins bind active phosphorylated G protein-coupled receptors (GPCRs). Among the four mammalian subtypes, only arrestin-3 facilitates the activation of JNK3 in cells. In available structures, Lys-295 in the lariat loop of arrestin-3 and its homologue Lys-294 in arrestin-2 directly interact with the activator-attached phosphates. We compared the roles of arrestin-3 conformational equilibrium and Lys-295 in GPCR binding and JNK3 activation. Several mutants with enhanced ability to bind GPCRs showed much lower activity towards JNK3, whereas a mutant that does not bind GPCRs was more active. The subcellular distribution of mutants did not correlate with GPCR recruitment or JNK3 activation. Charge neutralization and reversal mutations of Lys-295 differentially affected receptor binding on different backgrounds but had virtually no effect on JNK3 activation. Thus, GPCR binding and arrestin-3-assisted JNK3 activation have distinct structural requirements, suggesting that facilitation of JNK3 activation is the function of arrestin-3 that is not bound to a GPCR.

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.

GPCR binding and JNK3 activation by arrestin-3 have different structural requirements

Arrestins bind active phosphorylated G protein-coupled receptors (GPCRs). Among the four mammalian subtypes, only arrestin-3 facilitates the activation of JNK3 in cells. In available structures, Lys-295 in the lariat loop of arrestin-3 and its homologue Lys-294 in arrestin-2 directly interact with the activator-attached phosphates. We compared the role of arrestin-3 conformational equilibrium and of Lys-295 in GPCR binding and JNK3 activation. Several mutants with enhanced ability to bind GPCRs showed much lower activity towards JNK3, whereas a mutant that does not bind GPCRs was more active. Subcellular distribution of mutants did not correlate with GPCR recruitment or JNK3 activation. Charge neutralization and reversal mutations of Lys-295 differentially affected receptor binding on different backgrounds, but had virtually no effect on JNK3 activation. Thus, GPCR binding and arrestin-3-assisted JNK3 activation have distinct structural requirements, suggesting that facilitation of JNK3 activation is the function of arrestin-3 that is not bound to a GPCR.

The Absence of FAIM Leads to a Delay in Dark Adaptation and Hampers Arrestin-1 Translocation upon Light Reception in the Retina

The short and long isoforms of FAIM (FAIM-S and FAIM-L) hold important functions in the central nervous system, and their expression levels are specifically enriched in the retina. We previously described that Faim knockout (KO) mice present structural and molecular alterations in the retina compatible with a neurodegenerative phenotype. Here, we aimed to study Faim KO retinal functions and molecular mechanisms leading to its alterations. Electroretinographic recordings showed that aged Faim KO mice present functional loss of rod photoreceptor and ganglion cells. Additionally, we found a significant delay in dark adaptation from early adult ages. This functional deficit is exacerbated by luminic stress, which also caused histopathological alterations. Interestingly, Faim KO mice present abnormal Arrestin-1 redistribution upon light reception, and we show that Arrestin-1 is ubiquitinated, a process that is abrogated by either FAIM-S or FAIM-L in vitro. Our results suggest that FAIM assists Arrestin-1 light-dependent translocation by a process that likely involves ubiquitination. In the absence of FAIM, this impairment could be the cause of dark adaptation delay and increased light sensitivity. Multiple retinal diseases are linked to deficits in photoresponse termination, and hence, investigating the role of FAIM could shed light onto the underlying mechanisms of their pathophysiology.

Interactions between β-arrestin proteins and the cytoskeletal system, and their relevance to neurodegenerative disorders

β-arrestins, which have multiple cellular functions, were initially described as proteins that desensitize rhodopsin and other G protein-coupled receptors. The cytoskeletal system plays a role in various cellular processes, including intracellular transport, cell division, organization of organelles, and cell cycle. The interactome of β-arrestins includes the major proteins of the three main cytoskeletal systems: tubulins for microtubules, actins for the actin filaments, and vimentin for intermediate filaments. β-arrestins bind to microtubules and regulate their activity by recruiting signaling proteins and interacting with assembly proteins that regulate the actin cytoskeleton and the intermediate filaments. Altered regulation of the cytoskeletal system plays an essential role in the development of Alzheimer's, Parkinson's and other neurodegenerative diseases. Thus, β-arrestins, which interact with the cytoskeleton, were implicated in the pathogenesis progression of these diseases and are potential targets for the treatment of neurodegenerative disorders in the future.

WIPI2 positively regulates mitophagy by promoting mitochondrial recruitment of VCP

The mammalian Atg18 ortholog WIPI2 is a key regulator of LC3 lipidation to promote autophagosome biogenesis during nonselective macroautophagy, while its functions in selective autophagy such as mitophagy remain largely unexplored. In this study, we explored the role of WIPI2 in PINK1-PRKN/parkin-mediated mitophagy. First, we found that WIPI2 is recruited to damaged mitochondria upon mitophagy induction. Second, loss of WIPI2 impedes mitochondrial damaging agents-induced mitophagy. Third, at molecular level, WIPI2 binds to and promotes AAA-ATPase VCP/p97 (valosin containing protein) to damaged mitochondria; and WIPI2 depletion blunts the recruitment of VCP to damaged mitochondria, leading to reduction in degradation of outer mitochondrial membrane (OMM) proteins and mitophagy. Finally, WIPI2 is implicated in cell fate decision as cells deficient in WIPI2 are largely resistant to cell death induced by mitochondrial damage. In summary, our study reveals a critical regulatory role of WIPI2 in mitochondrial recruitment of VCP to promote OMM protein degradation and eventual mitophagy.Abbreviations: ATG, autophagy related; CALCOCO2/NDP52, calcium binding and coiled-coil domain 2; CCCP, carbonyl cyanide chlorophenylhydrazone; CYCS, cytochrome c, somatic; HSPD1/HSP60, heat shock protein family D (Hsp60) member 1; IMM, inner mitochondrial membrane; MAP1LC3/LC3, microtubule associated protein 1 light chain 3; NPLOC4, NPL4 homolog, ubiquitin recognition factor; OMM, outer mitochondrial membrane; OPTN, optineurin; PtdIns3P, phosphatidylinositol-3-phosphate; PINK1, PTEN induced kinase 1; PRKN/Parkin, parkin RBR E3 ubiquitin protein ligase; UBXN6/UBXD1, UBX domain protein 6; UFD1, ubiquitin recognition factor in ER associated degradation 1; VCP/p97, valosin containing protein; WIPI2, WD repeat domain, phosphoinositide interacting 2.

A Modified Arrestin1 Increases Lactate Production in the Retina and Slows Retinal Degeneration

Glucose metabolism in the retina is carefully orchestrated, with glucose being delivered to photoreceptors from the choroidal circulation through the retinal pigmented epithelium (RPE). In photoreceptors, glucose is processed principally by aerobic glycolysis, from which the lactate byproduct is provided to the RPE and Müller glia for their energetic needs. In this study, we utilize a modified arrestin1 protein to enhance the glycolytic output of lactate from rod photoreceptors through disinhibition of enolase1 activity with the goal being to use this increased lactate production as a gene-agnostic approach to slowing retinal degeneration. Mouse arrestin1 with E362G/D363G amino acid substitutions (referred to as "ArrGG") was packaged into AAV and tested for safety and for efficacy in increasing retinal lactate production. Overexpression of ArrGG in C57BL/6J mice did not result in any detectable changes in either electroretinogram (ERG) function or photoreceptor survival as measured by outer nuclear layer (ONL) thickness. However, mouse retinas expressing ArrGG showed a ∼25% increase in the rate of lactate secretion. Therefore, AAV-ArrGG was delivered intravitreally to heterozygous P23H rhodopsin knockin mice (RhoP23H/+) to determine if enhancing glycolysis in photoreceptors can slow retinal degeneration in this animal model of retinitis pigmentosa. We found that the expression of ArrGG in these mice slowed the decline of both scotopic and photopic ERG function. Correspondingly, there was significant preservation of ONL thickness in RhoP23H/+ mice treated with ArrGG compared with controls. In conclusion, our studies show that expressing ArrGG in C57BL/6J mouse retina results in an increase in lactate production, consistent with an upregulation of glycolysis. In the P23H rhodopsin model of retinitis pigmentosa, the expression of ArrGG led to significant preservation of photoreceptor function and slowing of retinal degeneration. These findings suggest that enhancing glycolysis by targeting increased enolase1 activity with a modified arrestin1 in photoreceptors may offer a therapeutic approach to slowing retinal degeneration.

A single phenylalanine residue in β-arrestin2 critically regulates its binding to G protein-coupled receptors

Arrestins and their yeast homologs, arrestin-related trafficking adaptors (ARTs), share a stretch of 29 amino acids called the ART motif. However, the functionality of that motif is unknown. We now report that deleting this motif prevents agonist-induced ubiquitination of β-arrestin2 (β-arr2) and blocks its association with activated G protein–coupled receptors (GPCRs). Within the ART motif, we have identified a conserved phenylalanine residue, Phe116, that is critical for the formation of β-arr2–GPCR complexes. β-arr2 Phe116Ala mutant has negligible effect on blunting β₂-adrenergic receptor–induced cAMP generation unlike β-arr2, which promotes rapid desensitization. Furthermore, available structures for inactive and inositol hexakisphosphate 6–activated forms of bovine β-arr2 revealed that Phe116 is ensconced in a hydrophobic pocket, whereas the adjacent Phe117 and Phe118 residues are not. Mutagenesis of Phe117 and Phe118, but not Phe116, preserves GPCR interaction of β-arr2. Surprisingly, Phe116 is dispensable for the association of β-arr2 with its non-GPCR partners. β-arr2 Phe116Ala mutant presents a significantly reduced protein half-life compared with β-arr2 and undergoes constitutive Lys-48-linked polyubiquitination, which tags proteins for proteasomal degradation. We also found that Phe116 is critical for agonist-dependent β-arr2 ubiquitination with Lys-63-polyubiquitin linkages that are known mediators of protein scaffolding and signal transduction. Finally, we have shown that β-arr2 Phe116Ala interaction with activated β₂-adrenergic receptor can be rescued with an in-frame fusion of ubiquitin. Taken together, we conclude that Phe116 preserves structural stability of β-arr2, regulates the formation of β-arr2–GPCR complexes that inhibit G protein signaling, and promotes subsequent ubiquitin-dependent β-arr2 localization and trafficking.

Structural evidence for visual arrestin priming via complexation of phosphoinositols

Visual arrestin (Arr1) terminates rhodopsin signaling by blocking its interaction with transducin. To do this, Arr1 translocates from the inner to the outer segment of photoreceptors upon light stimulation. Mounting evidence indicates that inositol phosphates (InsPs) affect Arr1 activity, but the Arr1-InsP molecular interaction remains poorly defined. We report the structure of bovine Arr1 in a ligand-free state featuring a near-complete model of the previously unresolved C-tail, which plays a crucial role in regulating Arr1 activity. InsPs bind to the N-domain basic patch thus displacing the C-tail, suggesting that they prime Arr1 for interaction with rhodopsin and help direct Arr1 translocation. These structures exhibit intact polar cores, suggesting that C-tail removal by InsP binding is insufficient to activate Arr1. These results show how Arr1 activity can be controlled by endogenous InsPs in molecular detail.

Targeting β-Arrestins in the Treatment of Psychiatric and Neurological Disorders

Therapies for psychiatric and neurological disorders have been in the development and refinement process for the past 5 decades. Yet, most of these therapies lack optimal therapeutic efficacy and have multiple debilitating side effects. Recent advances in understanding the pathophysiological processes of psychiatric and neurological disorders have revealed an important role for β-arrestins, which are important regulators of G-protein-coupled receptor (GPCR) function, including desensitization and intracellular signaling. These findings have pushed β-arrestins to the forefront as potential therapeutic targets. Here, we highlight current knowledge on β-arrestin functions in certain psychiatric and neurological disorders (schizophrenia, Parkinson's disease, and substance abuse disorders), and how this has been leveraged to develop new therapeutic strategies. Furthermore, we discuss the obstacles impacting the field of β-arrestin-based therapeutic development and future approaches that might help advance strategies to develop optimal β-arrestin-based therapies.

Dissecting the structural features of β-arrestins as multifunctional proteins

β-arrestins bind active G protein-coupled receptors (GPCRs) and play a crucial role in receptor desensitization and internalization. The classical paradigm of arrestin function has been expanded with the identification of many non-receptor-binding partners, which indicated the multifunctional role of β-arrestins in cellular functions. To elucidate the molecular mechanism of β-arrestin-mediated signaling, the structural features of β-arrestins were investigated using X-ray crystallography and cryogenic electron microscopy (cryo-EM). However, the intrinsic conformational flexibility of β-arrestins hampers the elucidation of structural interactions between β-arrestins and their binding partners using conventional structure determination tools. Therefore, structural information obtained using complementary structure analysis techniques would be necessary in combination with X-ray crystallography and cryo-EM data. In this review, we describe how β-arrestins interact with their binding partners from a structural point of view, as elucidated by both traditional methods (X-ray crystallography and cryo-EM) and complementary structure analysis techniques.

Mdm2 enhances ligase activity of parkin and facilitates mitophagy

Loss-of-function mutations in the E3 ubiquitin ligase parkin have been implicated in the death of dopaminergic neurons in the substantia nigra, which is the root cause of dopamine deficit in the striatum in Parkinson's disease. Parkin ubiquitinates proteins on mitochondria that lost membrane potential, promoting the elimination of damaged mitochondria. Neuroprotective activity of parkin has been linked to its critical role in the mitochondria maintenance. Here we report a novel regulatory mechanism: another E3 ubiquitin ligase Mdm2 directly binds parkin and enhances its enzymatic activity in vitro and in intact cells. Mdm2 translocates to damaged mitochondria independently of parkin, enhances parkin-dependent ubiquitination of the outer mitochondria membrane protein mitofusin1. Mdm2 facilitates and its knockdown reduces parkin-dependent mitophagy. Thus, ubiquitously expressed Mdm2 might enhance cytoprotective parkin activity. The data suggest that parkin activation by Mdm2 could be targeted to increase its neuroprotective functions, which has implications for anti-parkinsonian therapy.

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.

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.

β-Arrestins: Multitask Scaffolds Orchestrating the Where and When in Cell Signalling

The β-arrestins (β-arrs) were initially appreciated for the roles they play in the desensitization and endocytosis of G protein-coupled receptors (GPCRs). They are now also known to act as multifunctional adaptor proteins binding many non-receptor protein partners to control multiple signalling pathways. β-arrs therefore act as key regulatory hubs at the crossroads of external cell inputs and functional outputs in cellular processes ranging from gene transcription to cell growth, survival, cytoskeletal regulation, polarity, and migration. An increasing number of studies have also highlighted the scaffolding roles β-arrs play in vivo in both physiological and pathological conditions, which opens up therapeutic avenues to explore. In this introductory review chapter, we discuss the functional roles that β-arrs exert to control GPCR function, their dynamic scaffolding roles and how this impacts signal transduction events, compartmentalization of β-arrs, how β-arrs are regulated themselves, and how the combination of these events culminates in cellular regulation.

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.

Arrestin mutations: Some cause diseases, others promise cure

Arrestins play a key role in homologous desensitization of G protein-coupled receptors (GPCRs) and regulate several other vital signaling pathways in cells. Considering the critical roles of these proteins in cellular signaling, surprisingly few disease-causing mutations in human arrestins were described. Most of these are loss-of-function mutations of visual arrestin-1 that cause excessive rhodopsin signaling and hence night blindness. Only one dominant arrestin-1 mutation was discovered so far. It reduces the thermal stability of the protein, which likely results in photoreceptor death via unfolded protein response. In case of the two nonvisual arrestins, only polymorphisms were described, some of which appear to be associated with neurological disorders and altered response to certain treatments. Structure-function studies revealed several ways of enhancing arrestins' ability to quench GPCR signaling. These enhanced arrestins have potential as tools for gene therapy of disorders associated with excessive signaling of mutant GPCRs.

β-arrestin 2 mediates cardiac ischemia-reperfusion injury via inhibiting GPCR-independent cell survival signalling

Aims

Ischemic heart disease is a leading cause of morbidity and mortality worldwide. Although timely restoration of coronary blood flow (reperfusion) is the most effective therapeutics of myocardial infarction, reperfusion causes further cardiac damage, i.e. ischemia-reperfusion (I/R) injury. β-arrestins (Arrbs) have been traditionally defined as negative regulators of G protein-coupled receptor (GPCR) signalling, but recent studies have shown that they are essential for G protein-independent, GPCR-mediated biased signalling. Several ligands have been reported to be cardioprotective via Arrbs dependent pathway. However, it is unclear whether Arrbs exert receptor-independent physiological or pathological functions in the heart. Here, we sought to determine whether and how Arrbs play a role in regulating cardiomyocyte viability and myocardial remodelling following I/R injury.

Methods and results

The expression of β-arrestin 2 (Arrb2), but not β-arrestin 1 (Arrb1), is upregulated in rat hearts subjected to I/R injury, or in cultured neonatal rat cardiomyocytes treated with hypoxia-reoxygenation (H/R) injury. Deficiency of Arrb2 in cultured neonatal rat cardiomyocytes alleviates H/R-induced cardiomyocyte death and Arrb2-/- mice are resistant to myocardial damage caused by I/R injury. In contrast, upregulation of Arrb2 triggers cardiomyocyte death and exaggerates I/R (or H/R)-induced detrimental effects. Mechanically, Arrb2 induces cardiomyocyte death by interacting with the p85 subunit of PI3K, and negatively regulating the formation of p85-PI3K/CaV3 survival complex, thus blocking activation of PI3K-Akt-GSK3β cell survival signalling pathway.

Conclusion

We define an upregulation of Arrb2 as a pathogenic factor in cardiac I/R injury, and also reveal a novel GPCR-independent mechanism of Arrb2-mediated cell death signalling in the heart.

The binding of captopril to angiotensin I-converting enzyme triggers activation of signaling pathways

Hypertension is a global health problem, and angiotensin I (ANG I)-converting enzyme (ACE) inhibitors are largely used to control this pathology. Recently, it has been shown that ACE can also act as a transducer signal molecule when its inhibitors or substrates bind to it. This new role of ACE could contribute to understanding some of the effects not explained by its catalytic activity only. In this study, we investigated signaling pathway activation in Chinese hamster ovary (CHO) cells stably expressing ACE (CHO-ACE) under different conditions. We also investigated gene modulation after 4 h and 24 h of captopril treatment. Our results demonstrated that CHO-ACE cells when stimulated with ANG I, ramipril, or captopril led to JNK and ERK1/2 phosphorylation. To verify any physiological role at the endogenous level, we made use of primary cultures of mesangial cells from spontaneously hypertensive rats (SHR) and Wistar rats. Our results showed that ERK1/2 activation occurred mainly in primary cultures of mesangial cells from SHR rats upon captopril stimulation, suggesting that this signaling pathway could be differentially regulated during hypertension. Our results also showed that captopril treatment leads to a decrease of cyclooxygenase 2, interleukin-1β, and β-arrestin2 and a significant increase of AP2 gene expression levels. Our findings strengthen the fact that, in addition to the blockage of enzymatic activity, ACE inhibitors also trigger signaling pathway activation, and this may contribute to their beneficial effects in the treatment of hypertension and other pathologies.

Structural Basis of Arrestin-Dependent Signal Transduction

Arrestins are a small family of proteins with four isoforms in humans. Remarkably, two arrestins regulate signaling from >800 G protein-coupled receptors (GPCRs) or nonreceptor activators by simultaneously binding an activator and one out of hundreds of other signaling proteins. When arrestins are bound to GPCRs or other activators, the affinity for these signaling partners changes. Thus, it is proposed that an activator alters arrestin's ability to transduce a signal. The comparison of all available arrestin structures identifies several common conformational rearrangements associated with activation. In particular, it identifies elements that are directly involved in binding to GPCRs or other activators, elements that likely engage distinct downstream effectors, and elements that likely link the activator-binding sites with the effector-binding sites.

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.

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.

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-2 is an essential regulator of pancreatic β-cell function under physiological and pathophysiological conditions

β-arrestins are critical signalling molecules that regulate many fundamental physiological functions including the maintenance of euglycemia and peripheral insulin sensitivity. Here we show that inactivation of the β-arrestin-2 gene, barr2, in β-cells of adult mice greatly impairs insulin release and glucose tolerance in mice fed with a calorie-rich diet. Both glucose and KCl-induced insulin secretion and calcium responses were profoundly reduced in β-arrestin-2 (barr2) deficient β-cells. In human β-cells, barr2 knockdown abolished glucose-induced insulin secretion. We also show that the presence of barr2 is essential for proper CAMKII function in β-cells. Importantly, overexpression of barr2 in β-cells greatly ameliorates the metabolic deficits displayed by mice consuming a high-fat diet. Thus, our data identify barr2 as an important regulator of β-cell function, which may serve as a new target to improve β-cell function.

Beta-arrestins have key roles in development and metabolic functions as euglycaemic control and insulin sentitivity. Here Zhu et al. show that beta-arrestin-2 regulates insulin secretion and glucose tolerance in mice by promoting CAMKII functions in beta cells.

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.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.

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.

GRK3 suppresses L-DOPA-induced dyskinesia in the rat model of Parkinson's disease via its RGS homology domain

Degeneration of dopaminergic neurons causes Parkinson’s disease. Dopamine replacement therapy with L-DOPA is the best available treatment. However, patients develop L-DOPA-induced dyskinesia (LID). In the hemiparkinsonian rat, chronic L-DOPA increases rotations and abnormal involuntary movements modeling LID, via supersensitive dopamine receptors. Dopamine receptors are controlled by G protein-coupled receptor kinases (GRKs). Here we demonstrate that LID is attenuated by overexpression of GRK3 in the striatum, whereas knockdown of GRK3 by microRNA exacerbated it. Kinase-dead GRK3 and its separated RGS homology domain (RH) suppressed sensitization to L-DOPA, whereas GRK3 with disabled RH did not. RH alleviated LID without compromising anti-akinetic effect of L-DOPA. RH binds striatal Gq. GRK3, kinase-dead GRK3, and RH inhibited accumulation of ∆FosB, a marker of LID. RH-dead mutant was ineffective, whereas GRK3 knockdown exacerbated ∆FosB accumulation. Our findings reveal a novel mechanism of GRK3 control of the dopamine receptor signaling and the role of Gq in LID.

Arrestins: Critical Players in Trafficking of Many GPCRs

Arrestins specifically bind active phosphorylated G protein-coupled receptors (GPCRs). Receptor binding induces the release of the arrestin C-tail, which in non-visual arrestins contains high-affinity binding sites for clathrin and its adaptor AP2. Thus, serving as a physical link between the receptor and key components of the internalization machinery of the coated pit is the best-characterized function of non-visual arrestins in GPCR trafficking. However, arrestins also regulate GPCR trafficking less directly by orchestrating their ubiquitination and deubiquitination. Several reports suggest that arrestins play additional roles in receptor trafficking. Non-visual arrestins appear to be required for the recycling of internalized GPCRs, and the mechanisms of their function in this case remain to be elucidated. Moreover, visual and non-visual arrestins were shown to directly bind N-ethylmaleimide-sensitive factor, an important ATPase involved in vesicle trafficking, but neither molecular details nor the biological role of these interactions is clear. Considering how many different proteins arrestins appear to bind, we can confidently expect the elucidation of additional trafficking-related functions of these versatile signaling adaptors.

Identification and molecular characterization of Parkin in Clonorchis sinensis

Clonorchis sinensis habitating in the bile duct of mammals causes clonorchiasis endemic in East Asian countries. Parkin is a RING-between-RING protein and has E3-ubiquitin ligase activity catalyzing ubiquitination and degradation of substrate proteins. A cDNA clone of C. sinensis was predicted to encode a polypeptide homologous to parkin (CsParkin) including 5 domains (Ubl, RING0, RING1, IBR, and RING2). The cysteine and histidine residues binding to Zn(2+) were all conserved and participated in formation of tertiary structural RINGs. Conserved residues were also an E2-binding site in RING1 domain and a catalytic cysteine residue in the RING2 domain. Native CsParkin was determined to have an estimated molecular weight of 45.7 kDa from C. sinensis adults by immunoblotting. CsParkin revealed E3-ubiquitin ligase activity and higher expression in metacercariae than in adults. CsParkin was localized in the locomotive and male reproductive organs of C. sinensis adults, and extensively in metacercariae. Parkin has been found to participate in regulating mitochondrial function and energy metabolism in mammalian cells. From these results, it is suggested that CsParkin play roles in energy metabolism of the locomotive organs, and possibly in protein metabolism of the reproductive organs of C. sinensis.

The dichotomy of the Insulin-like growth factor 1 receptor: RTK and GPCR: friend or foe for cancer treatment?

The prime position of the insulin-like growth factor 1 receptor (IGF-1R), at the head of the principle mitogenic and anti-apoptotic signalling cascades, along with the resilience to transformation of IGF-1R deficient cells fuelled great excitement for its anti-cancer targeting. Yet its potential has not been fulfilled, as clinical trial results fell far short of expectations. Advancements in understanding of other receptors' function have now begun to shed light on this incongruity, with the now apparent parallels highlighting the immaturity of our understanding of IGF-1R biology, with the model used for drug development now recognised as having been too simplistic. Gathering together the many advancements of the field of IGF-1R research over the past decade, alongside those in the GPCR field, advocates for a major paradigm shift in our appreciation of the subtle workings of this receptor. This review will emphasise the updating of the IGF-1R's classification from an RTK, to an RTK/GPCR functional hybrid, which integrates both canonical kinase signalling with many functions characteristic of a GPCR. Recognition of the shortcomings of IGF-1R inhibitor drug development programs and the models used not only allows us to reignite the initial interest in the IGF-1R as an anti-cancer therapeutic target, but also points to the possibility of biased ligand therapeutics, which together may hold a very powerful key to unlocking the true potential of IGF-1R modulation.

Arrestins regulate cell spreading and motility via focal adhesion dynamics

Cells lacking both nonvisual arrestins show excessive spreading, defects in focal adhesion disassembly, and sensitivity to microtubules. This phenotype is rescued by wild-type arrestins but not mutants deficient in clathrin binding, suggesting that arrestins regulate focal adhesion disassembly by linking microtubules and clathrin.

Focal adhesions (FAs) play a key role in cell attachment, and their timely disassembly is required for cell motility. Both microtubule-dependent targeting and recruitment of clathrin are critical for FA disassembly. Here we identify nonvisual arrestins as molecular links between microtubules and clathrin. Cells lacking both nonvisual arrestins showed excessive spreading on fibronectin and poly-d-lysine, increased adhesion, and reduced motility. The absence of arrestins greatly increases the size and lifespan of FAs, indicating that arrestins are necessary for rapid FA turnover. In nocodazole washout assays, FAs in arrestin-deficient cells were unresponsive to disassociation or regrowth of microtubules, suggesting that arrestins are necessary for microtubule targeting–dependent FA disassembly. Clathrin exhibited decreased dynamics near FA in arrestin-deficient cells. In contrast to wild-type arrestins, mutants deficient in clathrin binding did not rescue the phenotype. Collectively the data indicate that arrestins are key regulators of FA disassembly linking microtubules and clathrin.

Arrestin expression in E. coli and purification

Purified arrestin proteins are necessary for biochemical, biophysical, and crystallographic studies of these versatile regulators of cell signaling. Described herein is a basic protocol for arrestin expression in E. coli and purification of the tag-free wild-type and mutant arrestins. The method includes ammonium sulfate precipitation of arrestins from cell lysates, followed by heparin-Sepharose chromatography. Depending on the arrestin type and/or mutations, the next step is Q-Sepharose or SP-Sepharose chromatography. In many cases the nonbinding column is used as a filter to bind contaminants without retaining arrestin. In some cases both chromatographic steps must be performed sequentially to achieve high purity. Purified arrestins can be concentrated up to 10 mg/ml, remain fully functional, and withstand several cycles of freezing and thawing, provided that overall salt concentration is maintained at or above physiological levels.

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-dependent localization of phosphodiesterases

Many G-protein-coupled receptors trigger the synthesis of cAMP in order to transduce signals from the membrane into the cell cytoplasm. As stimulation of each receptor type results in a specific physiological outcome, compartmentalization of proteins that make, break, and are activated by cAMP underpin receptor-specific responses. Until 2002, it was thought that static compartmentalization of phosphodiesterase 4 (PDE4), conferred by N-terminal targeting sequences, was one way to shape intricate cAMP gradients that formed after receptor activation. Discovery of the PDE4-β-arrestin complex represented a major breakthrough in cAMP signaling, as it spurred the initial realization that PDE4s could be transported to sites of high cAMP to orchestrate destruction of the second messenger at the same time as the receptor's signal to the G-protein is silenced. This chapter charts the scientific process that led to the discovery and characterization of the PDE4-β-arrestin interaction and discusses the known functions of this signaling complex.

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.