The structural basis of arrestin-mediated regulation of G-protein-coupled receptors

Structural plasticity of arrestin-G protein coupled receptor complexes as a molecular determinant of signaling

G protein coupled receptors (GPCRs) are critically regulated by arrestins. In this study, high-resolution data was combined with molecular dynamics simulations to infer the determinants of β-arrestin 1 (βarr1)-GPCR coupling, using the V2 vasopressin receptor (V2R) as a model system. The study highlighted the extremely high plasticity of βarr1-GPCR complexes, dependent on receptor type, state, and membrane environment. The multiple functions of receptor-bound βarr1 are likely determined by the interplay of intrinsic flexibility and collective motions both as a bi-domain protein and as a whole. The two major collective motions of the whole βarr1, consisting in rotation parallel to the membrane plane and inclination with respect to the receptor main axis, are distinctly linked to the two intermolecular interfaces involved in tail and core interactions. The intermolecular dynamic coupling between βarr1 and V2R depends on the allosteric effect of the agonist arginine-vasopressin (AVP). In the absence of AVP the dynamic coupling concerns only tail interactions, while in the presence of AVP it involves both tail and core interactions. This suggests that constitutive and agonist-induced arrestin-receptor dynamic coupling is linked to distinct arrestin functions.

Receptor Determinants for β-Arrestin Functional Specificity at C-X-C Chemokine Receptor 5

β-arrestins are multifaceted adaptor proteins that mediate G protein-coupled receptor (GPCR) desensitization, internalization, and signaling. It is emerging that receptor-specific determinants specify these divergent functions at GPCRs, yet this remains poorly understood. Here, we set out to identify the receptor determinants responsible for β-arrestin-mediated regulation of the chemokine receptor C-X-C motif chemokine receptor 5 (CXCR5). Using bioluminescence resonance energy transfer, we show that β-arrestin1 and β-arrestin2 are dose-dependently recruited to CXCR5 by its cognate ligand C-X-C motif chemokine ligand 13 (CXCL13). The carboxy-terminal tail of CXCR5 contains several serine/threonine residues that can be divided into three discrete phospho-site clusters based on their position relative to transmembrane domain 7. Mutagenesis experiments revealed that the distal and medial phospho-site clusters, but not the proximal, are required for agonist-stimulated β-arrestin1 or β-arrestin2 recruitment to CXCR5. Consistent with this, we provide evidence that the distal and medial, but not proximal, phospho-site clusters are required for receptor desensitization. Surprisingly, the individual phospho-site clusters are not required for agonist-stimulated internalization of CXCR5. Further, we show that CXCL13-stimulated CXCR5 internalization and ERK1/2 phosphorylation, but not desensitization, remain intact in human embryonic kidney 293 cells lacking β-arrestin1 and β-arrestin2. Our study provides evidence that β-arrestins are recruited to CXCR5 and are required for desensitization but are dispensable for internalization or signaling, suggesting that discrete receptor determinants specify the divergent functions of β-arrestins. SIGNIFICANCE STATEMENT: C-X-C motif ligand 13 (CXCL13) and C-X-C motif chemokine receptor 5 (CXCR5) are important in the immune system and are linked to diseases, yet regulation of CXCR5 signaling remains poorly understood. We provide evidence that a phospho-site cluster located at the extreme distal carboxyl-terminal tail of the receptor is responsible for β-arrestin recruitment and receptor desensitization. β-arrestins are not required for CXCL13-stimulated internalization or signaling, indicating that β-arrestins perform only one of their functions at CXCR5 and that discrete receptor determinants specify the divergent functions of β-arrestins.

Mechanisms of amphibian arrestin 1 self-association and dynamic distribution in retinal photoreceptors

Visual arrestin 1 (Arr1) is an essential protein for termination of the light response in photoreceptors. While mammalian Arr1s form dimers and tetramers at physiological concentrations in vitro, oligomerization in other vertebrates has not been studied. Here we examine self-association of Arr1 from two amphibian species, Xenopus laevis (xArr1) and Ambystoma tigrinum (salArr1). Sedimentation velocity analytical ultracentrifugation showed that xArr1 and salArr1 oligomerization is limited to dimers. The KD for dimer formation was 53 μM for xArr1 and 44 μM for salArr1, similar to the 69 μM KD for bovine Arr1 (bArr1) dimers. Mutations of orthologous amino acids important for mammalian Arr1 oligomerization had no impact on xArr1 dimerization. Crystallography showed that the fold of xArr1 closely resembles that of bArr1 and crystal structures in different space groups revealed two potential xArr1 dimer forms: a symmetric dimer with a C-domain interface (CC dimer), resembling the bArr1 solution dimer, and an asymmetric dimer with an N-domain/C-domain interface. Mutagenesis of residues predicted to interact in either of these two dimer forms yielded modest reduction in dimer affinity, suggesting that the dimer interfaces compete or are not unique. Indeed, small-angle X-ray scattering and protein painting data were consistent with a symmetric anti-parallel solution dimer (AP dimer) distinct from the assemblies observed by crystallography. Finally, a computational model evaluating xArr1 binding to compartment-specific partners and partitioning based on heterogeneity of available cytoplasmic spaces shows that Arr1 distribution in dark-adapted photoreceptors is largely explained by the excluded volume effect together with tuning by oligomerization.

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.

Mapping the interaction site for β-arrestin-2 in the prokineticin 2 receptor

G protein-coupled receptors (GPCRs) are a family of cell membrane receptors that couple and activate heterotrimeric G proteins and their associated intracellular signalling processes after ligand binding. Although the carboxyl terminal of the receptors is essential for this action, it can also serve as a docking site for regulatory proteins such as the β-arrestins. Prokineticin receptors (PKR1 and PKR2) are a new class of GPCRs that are able to activate different classes of G proteins and form complexes with β-arrestins after activation by the endogenous agonists PK2. The aim of this work was to define the molecular determinants within PKR2 that are required for β-arrestin-2 binding and to investigate the role of β-arrestin-2 in the signalling pathways induced by PKR2 activation. Our data show that PKR2 binds constitutively to β-arrestin-2 and that this process occurs through the core region of the receptor without being affected by the carboxy-terminal region. Indeed, a PKR2 mutant lacking the carboxy-terminal amino acids retains the ability to bind constitutively to β-arrestin-2, whereas a mutant lacking the third intracellular loop does not. Overall, our data suggest that the C-terminus of PKR2 is critical for the stability of the β-arrestin-2-receptor complex in the presence of PK2 ligand. This leads to the β-arrestin-2 conformational change required to initiate intracellular signalling that ultimately leads to ERK phosphorylation and activation.

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.

ArreSTick motif controls β-arrestin-binding stability and extends phosphorylation-dependent β-arrestin interactions to non-receptor proteins

The binding and function of β-arrestins are regulated by specific phosphorylation motifs present in G protein-coupled receptors (GPCRs). However, the exact arrangement of phosphorylated amino acids responsible for establishing a stable interaction remains unclear. We employ a 1D sequence convolution model trained on GPCRs with established β-arrestin-binding properties. With this approach, amino acid motifs characteristic of GPCRs that form stable interactions with β-arrestins can be identified, a pattern that we name "arreSTick." Intriguingly, the arreSTick pattern is also present in numerous non-receptor proteins. Using proximity biotinylation assay and mass spectrometry analysis, we demonstrate that the arreSTick motif controls the interaction between many non-receptor proteins and β-arrestin2. The HIV-1 Tat-specific factor 1 (HTSF1 or HTATSF1), a nuclear transcription factor, contains the arreSTick pattern, and its subcellular localization is influenced by β-arrestin2. Our findings unveil a broader role for β-arrestins in phosphorylation-dependent interactions, extending beyond GPCRs to encompass non-receptor proteins as well.

MRAP2 Inhibits β-Arrestin-2 Recruitment to the Prokineticin Receptor 2

Melanocortin receptor accessory protein 2 (MRAP2) is a membrane protein that binds multiple G protein-coupled receptors (GPCRs) involved in the control of energy homeostasis, including prokineticin receptors. These GPCRs are expressed both centrally and peripherally, and their endogenous ligands are prokineticin 1 (PK1) and prokineticin 2 (PK2). PKRs couple all G-protein subtypes, such as Gαq/11, Gαs, and Gαi, and recruit β-arrestins upon PK2 stimulation, although the interaction between PKR2 and β-arrestins does not trigger receptor internalisation. MRAP2 inhibits the anorexigenic effect of PK2 by binding PKR1 and PKR2. The aim of this work was to elucidate the role of MRAP2 in modulating PKR2-induced β-arrestin-2 recruitment and β-arrestin-mediated signalling. This study could allow the identification of new specific targets for potential new drugs useful for the treatment of the various pathologies correlated with prokineticin, in particular, obesity.

Gαs is dispensable for β-arrestin coupling but dictates GRK selectivity and is predominant for gene expression regulation by β2-adrenergic receptor

β-arrestins play a key role in G protein-coupled receptor (GPCR) internalization, trafficking, and signaling. Whether β-arrestins act independently of G protein-mediated signaling has not been fully elucidated. Studies using genome-editing approaches revealed that whereas G proteins are essential for mitogen-activated protein kinase activation by GPCRs., β-arrestins play a more prominent role in signal compartmentalization. However, in the absence of G proteins, GPCRs may not activate β-arrestins, thereby limiting the ability to distinguish G protein from β-arrestin-mediated signaling events. We used β2-adrenergic receptor (β2AR) and its β2AR-C tail mutant expressed in human embryonic kidney 293 cells wildtype or CRISPR-Cas9 gene edited for Gαs, β-arrestin1/2, or GPCR kinases 2/3/5/6 in combination with arrestin conformational sensors to elucidate the interplay between Gαs and β-arrestins in controlling gene expression. We found that Gαs is not required for β2AR and β-arrestin conformational changes, β-arrestin recruitment, and receptor internalization, but that Gαs dictates the GPCR kinase isoforms involved in β-arrestin recruitment. By RNA-Seq analysis, we found that protein kinase A and mitogen-activated protein kinase gene signatures were activated by stimulation of β2AR in wildtype and β-arrestin1/2-KO cells but absent in Gαs-KO cells. These results were validated by re-expressing Gαs in the corresponding KO cells and silencing β-arrestins in wildtype cells. These findings were extended to cellular systems expressing endogenous levels of β2AR. Overall, our results support that Gs is essential for β2AR-promoted protein kinase A and mitogen-activated protein kinase gene expression signatures, whereas β-arrestins initiate signaling events modulating Gαs-driven nuclear transcriptional activity.

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.

Modulation of Neuron and Astrocyte Dopamine Receptors via Receptor-Receptor Interactions

Dopamine neurotransmission plays critical roles in regulating complex cognitive and behavioral processes including reward, motivation, reinforcement learning, and movement. Dopamine receptors are classified into five subtypes, widely distributed across the brain, including regions responsible for motor functions and specific areas related to cognitive and emotional functions. Dopamine also acts on astrocytes, which express dopamine receptors as well. The discovery of direct receptor-receptor interactions, leading to the formation of multimeric receptor complexes at the cell membrane and providing the cell decoding apparatus with flexible dynamics in terms of recognition and signal transduction, has expanded the knowledge of the G-protein-coupled receptor-mediated signaling processes. The purpose of this review article is to provide an overview of currently identified receptor complexes containing dopamine receptors and of their modulatory action on dopamine-mediated signaling between neurons and between neurons and astrocytes. Pharmacological possibilities offered by targeting receptor complexes in terms of addressing neuropsychiatric disorders associated with altered dopamine signaling will also be briefly discussed.

In-Cell Arrestin-Receptor Interaction Assays

G protein-coupled receptors (GPCRs) represent ∼30% of current drug targets. Ligand binding to these receptors activates G proteins and arrestins, which function in different signaling pathways. Given that functionally selective or biased ligands preferentially activate one of these two groups of pathways, they may be superior medications for certain disease states. The identification of such ligands requires robust drug screening assays for both G protein and arrestin activity. This unit describes protocols for assays that monitor reversible arrestin recruitment to GPCRs in living cells using either bioluminescence resonance energy transfer (BRET) or nanoluciferase complementation (NanoLuc). Two types of assays can be used: one configuration directly measures arrestin recruitment to a GPCR fused to a protein tag at its intracellular C-terminus, whereas the other configuration detects arrestin translocation to the plasma membrane in response to activation of an unmodified GPCR. Together, these assays are powerful tools for studying dynamic interactions between GPCRs and arrestins. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Receptor-arrestin BRET assay to measure ligand-induced recruitment of arrestin to receptors Basic Protocol 2: Receptor-arrestin NANOBIT assay to measure ligand-induced recruitment of arrestin to receptors Alternative Protocol 1: BRET assay to measure ligand-induced recruitment of arrestin to the plasma membrane Alternative Protocol 2: NANOBIT assay to measure ligand-induced recruitment of arrestin to the plasma membrane Support Protocol 1: Optimization of polyethylenimine (PEI) concentration for transfection.

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.

Tail engagement of arrestin at the glucagon receptor

Arrestins have pivotal roles in regulating G protein-coupled receptor (GPCR) signalling by desensitizing G protein activation and mediating receptor internalization1,2. It has been proposed that the arrestin binds to the receptor in two different conformations, 'tail' and 'core', which were suggested to govern distinct processes of receptor signalling and trafficking3,4. However, little structural information is available for the tail engagement of the arrestins. Here we report two structures of the glucagon receptor (GCGR) bound to β-arrestin 1 (βarr1) in glucagon-bound and ligand-free states. These structures reveal a receptor tail-engaged binding mode of βarr1 with many unique features, to our knowledge, not previously observed. Helix VIII, instead of the receptor core, has a major role in accommodating βarr1 by forming extensive interactions with the central crest of βarr1. The tail-binding pose is further defined by a close proximity between the βarr1 C-edge and the receptor helical bundle, and stabilized by a phosphoinositide derivative that bridges βarr1 with helices I and VIII of GCGR. Lacking any contact with the arrestin, the receptor core is in an inactive state and loosely binds to glucagon. Further functional studies suggest that the tail conformation of GCGR-βarr governs βarr recruitment at the plasma membrane and endocytosis of GCGR, and provides a molecular basis for the receptor forming a super-complex simultaneously with G protein and βarr to promote sustained signalling within endosomes. These findings extend our knowledge about the arrestin-mediated modulation of GPCR functionalities.

Spatiotemporal control of genome engineering in cone photoreceptors

BACKGROUND

Cones are essential for color recognition, high resolution, and central vision; therefore cone death causes blindness. Understanding the pathophysiology of each cell type in the retina is key to developing therapies for retinal diseases. However, studying the biology of cone cells in the rod-dominant mammalian retina is particularly challenging. In this study, we used a bacterial artificial chromosome (BAC) recombineering method to knock in the "CreER^T2" sequence into the Gnat2 and Arr3 genes, respectively and generated three novel inducible CreER^T2 mice with different cone cell specificities.

RESULTS

These models (Gnat2^CreERT2, Arr3^T2ACreERT2, and Arr3^P2ACreERT2) express temporally controllable Cre recombinase that achieves conditional alleles in cone photoreceptors. Cre-LoxP recombination can be induced as early as postnatal day (PD) two upon tamoxifen injection at varying efficiencies, ranging from 10 to 15% in Gnat2^CreERT2, 40% in Arr3^T2ACreERT2, and 100% in Arr3^P2ACreERT2. Notably, knocking in the P2A-CreERT2 cassette does not affect cone cell morphology and functionality. Most cone-phototransduction enzymes, including Opsins, CNGA3, etc. are not altered except for a reduction in the Arr3 transcript.

CONCLUSIONS

The Arr3^P2ACreERT2 mouse, an inducible cone-specific Cre driver, is a valuable line in studying cone cell biology, function, as well as its relationship with rod and other retinal cells. Moreover, the Cre activity can be induced by delivering tamoxifen intragastrically as early as PD2, which will be useful for studying retinal development or in rapid degenerative mouse models.

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.

The complement receptor C5aR2 regulates neutrophil activation and function contributing to neutrophil-driven epidermolysis bullosa acquisita

Introduction

The function of the second receptor for the complement cleavage product C5a, C5aR2, is poorly understood and often neglected in the immunological context. Using mice with a global deficiency of C5aR2, we have previously reported an important role of this receptor in the pathogenesis of the neutrophil-driven autoimmune disease epidermolysis bullosa acquisita (EBA). Based on in vitro analyses, we hypothesized that the absence of C5aR2 specifically on neutrophils is the cause of the observed differences. Here, we report the generation of a new mouse line with a LysM-specific deficiency of C5aR2.

Methods

LysM-specific deletion of C5aR2 was achieved by crossing LysMcre mice with tdTomato-C5ar2fl/fl mice in which the tdTomato-C5ar2 gene is flanked by loxP sites. Passive EBA was induced by subcutaneous injection of rabbit anti-mouse collagen type VII IgG. The effects of targeted deletion of C5ar2 on C5a-induced effector functions of neutrophils were examined in in vitro assays.

Results

We confirm the successful deletion of C5aR2 at both the genetic and protein levels in neutrophils. The mice appeared healthy and the expression of C5aR1 in bone marrow and blood neutrophils was not negatively affected by LysM-specific deletion of C5aR2. Using the antibody transfer mouse model of EBA, we found that the absence of C5aR2 in LysM-positive cells resulted in an overall amelioration of disease progression, similar to what we had previously found in mice with global deficiency of C5aR2. Neutrophils lacking C5aR2 showed decreased activation after C5a stimulation and increased expression of the inhibitory Fcγ receptor FcγRIIb.

Discussion

Overall, with the data presented here, we confirm and extend our previous findings and show that C5aR2 in neutrophils regulates their activation and function in response to C5a by potentially affecting the expression of Fcγ receptors and CD11b. Thus, C5aR2 regulates the finely tuned interaction network between immune complexes, Fcγ receptors, CD11b, and C5aR1 that is important for neutrophil recruitment and sustained activation. This underscores the importance of C5aR2 in the pathogenesis of neutrophil-mediated autoimmune diseases.

Plasma membrane preassociation drives β-arrestin coupling to receptors and activation

β-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-β-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in β-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that β-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes β-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of β-arrestin function at the plasma membrane, revealing a critical role for β-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

Model of ligand-triggered information transmission in G-protein coupled receptor complexes

We present a model for the effects of ligands on information transmission in G-Protein Coupled Receptor (GPCR) complexes. The model is built ab initio entirely on principles of statistical mechanics and tenets of information transmission theory and was validated in part using agonist-induced effector activity and signaling bias for the angiotensin- and adrenergic-mediated signaling pathways, with in vitro observations of phosphorylation sites on the C tail of the GPCR complex, and single-cell information-transmission experiments. The model extends traditional kinetic models that form the basis for many existing models of GPCR signaling. It is based on maximizing the rates of entropy production and information transmission through the GPCR complex. The model predicts that (1) phosphatase-catalyzed reactions, as opposed to kinase-catalyzed reactions, on the C-tail and internal loops of the GPCR are responsible for controlling the signaling activity, (2) signaling favors the statistical balance of the number of switches in the ON state and the number in the OFF state, and (3) biased-signaling response depends discontinuously on ligand concentration.

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.

Mechanistic-Based Classification of Endocytosis-Related Inhibitors: Does It Aid in Assigning Drugs against SARS-CoV-2?

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) canonically utilizes clathrin-mediated endocytosis (CME) and several other endocytic mechanisms to invade airway epithelial cells. Endocytic inhibitors, particularly those targeting CME-related proteins, have been identified as promising antiviral drugs. Currently, these inhibitors are ambiguously classified as chemical, pharmaceutical, or natural inhibitors. However, their varying mechanisms may suggest a more realistic classification system. Herein, we present a new mechanistic-based classification of endocytosis inhibitors, in which they are segregated among four distinct classes including: (i) inhibitors that disrupt endocytosis-related protein-protein interactions, and assembly or dissociation of complexes; (ii) inhibitors of large dynamin GTPase and/or kinase/phosphatase activities associated with endocytosis; (iii) inhibitors that modulate the structure of subcellular components, especially the plasma membrane, and actin; and (iv) inhibitors that cause physiological or metabolic alterations in the endocytosis niche. Excluding antiviral drugs designed to halt SARS-CoV-2 replication, other drugs, either FDA-approved or suggested through basic research, could be systematically assigned to one of these classes. We observed that many anti-SARS-CoV-2 drugs could be included either in class III or IV as they interfere with the structural or physiological integrity of subcellular components, respectively. This perspective may contribute to our understanding of the relative efficacy of endocytosis-related inhibitors and support the optimization of their individual or combined antiviral potential against SARS-CoV-2. However, their selectivity, combined effects, and possible interactions with non-endocytic cellular targets need more clarification.

Migration mediated by the oxysterol receptor GPR183 depends on arrestin coupling but not receptor internalization

The chemotactic G protein-coupled receptor GPR183 and its most potent endogenous oxysterol ligand 7α,25-dihydroxycholesterol (7α,25-OHC) are important for immune cell positioning in secondary lymphoid tissues. This receptor-ligand pair is associated with various diseases, in some cases contributing favorably and in other cases adversely, making GPR183 an attractive target for therapeutic intervention. We investigated the mechanisms underlying GPR183 internalization and the role of internalization in the main biological function of the receptor, chemotaxis. We found that the C terminus of the receptor was important for ligand-induced internalization but less so for constitutive (ligand-independent) internalization. β-arrestin potentiated ligand-induced internalization but was not required for ligand-induced or constitutive internalization. Caveolin and dynamin were the main mediators of both constitutive and ligand-induced receptor internalization in a mechanism independent of G protein activation. Clathrin-mediated endocytosis also contributed to constitutive GPR183 internalization in a β-arrestin-independent manner, suggesting the existence of different pools of surface-localized GPR183. Chemotaxis mediated by GPR183 depended on receptor desensitization by β-arrestins but could be uncoupled from internalization, highlighting an important biological role for the recruitment of β-arrestin to GPR183. The role of distinct pathways in internalization and chemotaxis may aid in the development of GPR183-targeting drugs for specific disease contexts.

Role of Monomer/Tetramer Equilibrium of Rod Visual Arrestin in the Interaction with Phosphorylated Rhodopsin

The phototransduction cascade in vertebrate rod visual cells is initiated by the photoactivation of rhodopsin, which enables the activation of the visual G protein transducin. It is terminated by the phosphorylation of rhodopsin, followed by the binding of arrestin. Here we measured the solution X-ray scattering of nanodiscs containing rhodopsin in the presence of rod arrestin to directly observe the formation of the rhodopsin/arrestin complex. Although arrestin self-associates to form a tetramer at physiological concentrations, it was found that arrestin binds to phosphorylated and photoactivated rhodopsin at 1:1 stoichiometry. In contrast, no complex formation was observed for unphosphorylated rhodopsin upon photoactivation, even at physiological arrestin concentrations, suggesting that the constitutive activity of rod arrestin is sufficiently low. UV-visible spectroscopy demonstrated that the rate of the formation of the rhodopsin/arrestin complex well correlates with the concentration of arrestin monomer rather than the tetramer. These findings indicate that arrestin monomer, whose concentration is almost constant due to the equilibrium with the tetramer, binds to phosphorylated rhodopsin. The arrestin tetramer would act as a reservoir of monomer to compensate for the large changes in arrestin concentration in rod cells caused by intense light or adaptation.

Structural details of a Class B GPCR-arrestin complex revealed by genetically encoded crosslinkers in living cells

Understanding the molecular basis of arrestin-mediated regulation of GPCRs is critical for deciphering signaling mechanisms and designing functional selectivity. However, structural studies of GPCR-arrestin complexes are hampered by their highly dynamic nature. Here, we dissect the interaction of arrestin-2 (arr2) with the secretin-like parathyroid hormone 1 receptor PTH1R using genetically encoded crosslinking amino acids in live cells. We identify 136 intermolecular proximity points that guide the construction of energy-optimized molecular models for the PTH1R-arr2 complex. Our data reveal flexible receptor elements missing in existing structures, including intracellular loop 3 and the proximal C-tail, and suggest a functional role of a hitherto overlooked positively charged region at the arrestin N-edge. Unbiased MD simulations highlight the stability and dynamic nature of the complex. Our integrative approach yields structural insights into protein-protein complexes in a biologically relevant live-cell environment and provides information inaccessible to classical structural methods, while also revealing the dynamics of the system.

Computational investigation of functional water molecules in GPCRs bound to G protein or arrestin

G protein-coupled receptors (GPCRs) are membrane proteins constituting the largest family of drug targets. The activated GPCR binds either the heterotrimeric G proteins or arrestin through its activation cycle. Water molecules have been reported to play a role in GPCR activation. Nevertheless, reported studies are focused on the hydrophobic helical bundle region. How water molecules function in GPCR bound either G protein or arrestin is rarely studied. To address this issue, we carried out computational studies on water molecules in both GPCR/G protein complexes and GPCR/arrestin complexes. Using inhomogeneous fluid theory (IFT), we locate all possible hydration sites in GPCRs binding either to G protein or arrestin. We observe that the number of water molecules on the interaction surface between GPCRs and signal proteins are correlated with the insertion depths of the α5-helix from G-protein or "finger loop" from arrestin in GPCRs. In three out of the four simulation pairs, the interfaces of Rhodopsin, M₂R and NTSR1 in the G protein-associated systems show more water-mediated hydrogen-bond networks when compared to these in arrestin-associated systems. This reflects that more functionally relevant water molecules may probably be attracted in G protein-associated structures than that in arrestin-associated structures. Moreover, we find the water-mediated interaction networks throughout the NPxxY region and the orthosteric pocket, which may be a key for GPCR activation. Reported studies show that non-biased agonist, which can trigger both GPCR-G protein and GPCR-arrestin activation signal, can result in pharmacologically toxicities. Our comprehensive studies of the hydration sites in GPCR/G protein complexes and GPCR/arrestin complexes may provide important insights in the design of G-protein biased agonists.

Surveying nonvisual arrestins reveals allosteric interactions between functional sites

Arrestins are important scaffolding proteins that are expressed in all vertebrate animals. They regulate cell-signaling events upon binding to active G-protein coupled receptors (GPCR) and trigger endocytosis of active GPCRs. While many of the functional sites on arrestins have been characterized, the question of how these sites interact is unanswered. We used anisotropic network modeling (ANM) together with our covariance compliment techniques to survey all the available structures of the nonvisual arrestins to map how structural changes and protein-binding affect their structural dynamics. We found that activation and clathrin binding have a marked effect on arrestin dynamics, and that these dynamics changes are localized to a small number of distant functional sites. These sites include α-helix 1, the lariat loop, nuclear localization domain, and the C-domain β-sheets on the C-loop side. Our techniques suggest that clathrin binding and/or GPCR activation of arrestin perturb the dynamics of these sites independent of structural changes.

Do arrestin oligomers have specific functions?

Arrestins are a small family of versatile regulators of cell signaling. Arrestins regulate signaling and trafficking of G protein-coupled receptors, regulate and direct to particular subcellular compartments numerous protein kinases, ubiquitin ligases, etc. Three out of four arrestin subtypes expressed in vertebrates self-associate, each forming oligomers of a distinct size and shape. While the structures of the solution oligomers of arrestin-1, -2, and -3 have been elucidated, no function specific for the oligomeric form of either of these three subtypes has been identified thus far. Considering how multi-functional average-sized (~45 kDa) arrestin proteins were found to be, it appears likely that certain functions are predominantly or exclusively fulfilled by monomeric and oligomeric forms of each subtype.

Independent and combined effects of liraglutide and aerobic interval training on glycemic control and cardiac protection in diabetic cardiomyopathy rats

OBJECTIVE

This study intended to explore the hypoglycemic and cardioprotective effects of 8-week aerobic interval training combined with liraglutide and elucidate the underlying mechanisms.

METHOD

Male Wistar rats were randomly divided into 5 groups - normal control group (CON), diabetic cardiomyopathy group (DCM), high-dose liraglutide group (DH), low-dose liraglutide group (DL), and aerobic interval training combined with liraglutide group (DLE). High-fat diet and streptozotocin (STZ) were used to induce the DCM model, and both the liraglutide administration group and combination therapy group allocated to 8 weeks of either liraglutide or liraglutide and exercise intervention. Cardiac functions were analyzed by electrocardiography. Blood biochemical parameters were measured to judge glycemic control conditions. Hematoxylin and eosin (HE) staining and Sirus red staining was used to identify cardiac morphology and collagen accumulation, respectively. Advanced glycation end products (AGEs) were determined by enzymatic methods. The mRNA expression of myocardial remodeling genes (BNP, GSK3β, α-MHC, β-MHC and PPARα) and the protein expression of GLP-1, GLP-1R were analyzed.

RESULTS

DCM rats developed hyperglycemia, impaired cardiac function with accumulation of AGEs and collagen (P < 0.05). The development of hyperglycemia and cardiac dysfunction was significantly attenuated with all interventions, as reduced cardiac fibrosis and improved cardiac function (P < 0.05). Cardiac remodeling genes were normalized after all interventions, these positive modifications were due to increased GLP-1 and GLP-1R expression in DCM heart (P < 0.05). Liraglutide combined with AIT significantly increased the diameters of cardiomyocytes, increased the α-MHC expressionx, reduced PPARαexpression and reduced the fluctuation of blood glucose level, which showed the safety and effective of medicine combined with exercise.

CONCLUSION

Liraglutide combined with AIT intervention normalized blood glucose alleviates myocardial fibrosis and improves cardiac contractile function in DCM rats, supporting the efficacy and safety of the combination therapy.

Thromboxane-induced contractile response of mesenteric arterioles is diminished in the older rats and the older hypertensive rats

Nearly all physiological processes are controlled at some level by G-protein-coupled receptor (GPCR) signaling activity. The thromboxane A2 (TXA2) receptor (TP) is a member of the GPCR family. The ultimate effect of TP receptor activation depends on the availability of specific G proteins, which in turn depend on the cell type, tissue, and disease state. However, the roles of the TXA2-TP signaling pathway executed under disease states are poorly defined. In this study, 16-week-spontaneously hypertensive rats (SHR), the 18-month-SHR (OldSHR), and the age-matched Wistar-Kyoto (WKY) rats were used to study the vasoconstriction of mesenteric resistance artery induced by TP-specific agonist, U-46619. Vasoconstriction induced by U-46619 was significantly attenuated in OldWKY and OldSHR rats, and mesenteric arteries with impaired response to U-46619 responded strongly to the adrenergic receptor agonist, phenylephrine. Similar vascular responses to U-46619 were obtained in endothelium-denuded mesenteric arteries. Accordingly, the expression of TP membrane proteins in mesenteric vessels was decreased, and the endogenous TP competitor, 8, 9-EET, in serum was increased, which was partly responsible for the decreased vascular reactivity of U-46619. Decreased TP membrane expression was associated with TP endocytosis, which involved actin cytoskeletal remodeling, including increased ratio of F-actin/G-actin in OldWKY and OldSHR rats. Hence, we studied the effects of TXA2 and its receptors on blood vessels and found that the TXA2-TP prostaglandin signaling pathway was impaired in older adults, which would facilitate the creation of “precision therapeutics” that possess selective efficacy in diseases.

Signaling snapshots of a serotonin receptor activated by the prototypical psychedelic LSD

Serotonin (5-hydroxytryptamine [5-HT]) 5-HT2-family receptors represent essential targets for lysergic acid diethylamide (LSD) and all other psychedelic drugs. Although the primary psychedelic drug effects are mediated by the 5-HT2A serotonin receptor (HTR2A), the 5-HT2B serotonin receptor (HTR2B) has been used as a model receptor to study the activation mechanisms of psychedelic drugs due to its high expression and similarity to HTR2A. In this study, we determined the cryo-EM structures of LSD-bound HTR2B in the transducer-free, Gq-protein-coupled, and β-arrestin-1-coupled states. These structures provide distinct signaling snapshots of LSD's action, ranging from the transducer-free, partially active state to the transducer-coupled, fully active states. Insights from this study will both provide comprehensive molecular insights into the signaling mechanisms of the prototypical psychedelic LSD and accelerate the discovery of novel psychedelic drugs.

G protein-coupled receptor signaling: transducers and effectors

G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.

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.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

Solo vs. Chorus: Monomers and Oligomers of Arrestin Proteins

Three out of four subtypes of arrestin proteins expressed in mammals self-associate, each forming oligomers of a distinct kind. Monomers and oligomers have different subcellular localization and distinct biological functions. Here we summarize existing evidence regarding arrestin oligomerization and discuss specific functions of monomeric and oligomeric forms, although too few of the latter are known. The data on arrestins highlight biological importance of oligomerization of signaling proteins. Distinct modes of oligomerization might be an important contributing factor to the functional differences among highly homologous members of the arrestin protein family.

Cathepsin D interacts with adenosine A2A receptors in mouse macrophages to modulate cell surface localization and inflammatory signaling

Adenosine A_2A receptor (A_2AR)–dependent signaling in macrophages plays a key role in the regulation of inflammation. However, the processes regulating A_2AR targeting to the cell surface and degradation in macrophages are incompletely understood. For example, the C-terminal domain of the A_2AR and proteins interacting with it are known to regulate receptor recycling, although it is unclear what role potential A_2AR-interacting partners have in macrophages. Here, we aimed to identify A_2AR-interacting partners in macrophages that may effect receptor trafficking and activity. To this end, we performed a yeast two-hybrid screen using the C-terminal tail of A_2AR as the “bait” and a macrophage expression library as the “prey.” We found that the lysosomal protease cathepsin D (CtsD) was a robust hit. The A_2AR–CtsD interaction was validated in vitro and in cellular models, including RAW 264.7 and mouse peritoneal macrophage (IPMΦ) cells. We also demonstrated that the A_2AR is a substrate of CtsD and that the blockade of CtsD activity increases the density and cell surface targeting of A_2AR in macrophages. Conversely, we demonstrate that A_2AR activation prompts the maturation and enzymatic activity of CtsD in macrophages. In summary, we conclude that CtsD is a novel A_2AR-interacting partner and thus describe molecular and functional interplay that may be crucial for adenosine-mediated macrophage regulation in inflammatory processes.

Molecular Insights into Phosphorylation-Induced Allosteric Conformational Changes in a β2-Adrenergic Receptor

The large conformational flexibility of G protein-coupled receptors (GPCRs) has been a puzzle in structural and pharmacological studies for the past few decades. Apart from structural rearrangements induced by ligands, enzymatic phosphorylations by GPCR kinases (GRKs) at the carboxy-terminal tail (C-tail) of a GPCR also make conformational alterations to the transmembrane helices and facilitates the binding of one of its transducer proteins named β-arrestin. The phosphorylation-induced conformational transition of the receptor that causes specific binding to β-arrestin but prevents the association of other transducers such as G proteins lacks atomistic understanding and is elusive to experimental studies. Using microseconds of all-atom conventional and Gaussian accelerated molecular dynamics (GaMD) simulations, we investigate the allosteric mechanism of phosphorylation induced-conformational changes in β₂-adrenergic receptor, a well-characterized GPCR model system. Free energy profiles reveal that the phosphorylated receptor samples a new conformational state in addition to the canonical active state corroborating with recent nuclear magnetic resonance experimental findings. The new state has a smaller intracellular cavity that is likely to accommodate β-arrestin better than G protein. Using contact map and inter-residue interaction energy calculations, we found the phosphorylated C-tail adheres to the cytosolic surface of the transmembrane domain of the receptor. Transfer entropy calculations show that the C-tail residues drive the correlated motions of TM residues, and the allosteric signal is relayed via several residues at the cytosolic surface. Our results also illustrate how the redistribution of inter-residue nonbonding interaction couples with the allosteric communication from the phosphorylated C-tail to the transmembrane. Atomistic insight into phosphorylation-induced β-arrestin specific conformation is therapeutically important to design drugs with higher efficacy and fewer side effects. Our results, therefore, open novel opportunities to fine-tune β-arrestin bias in GPCR signaling.

Quantifying the Kinetics of Signaling and Arrestin Recruitment by Nervous System G-Protein Coupled Receptors

Neurons integrate inputs over different time and space scales. Fast excitatory synapses at boutons (ms and μm), and slow modulation over entire dendritic arbors (seconds and mm) are all ultimately combined to produce behavior. Understanding the timing of signaling events mediated by G-protein-coupled receptors is necessary to elucidate the mechanism of action of therapeutics targeting the nervous system. Measuring signaling kinetics in live cells has been transformed by the adoption of fluorescent biosensors and dyes that convert biological signals into optical signals that are conveniently recorded by microscopic imaging or by fluorescence plate readers. Quantifying the timing of signaling has now become routine with the application of equations in familiar curve fitting software to estimate the rates of signaling from the waveform. Here we describe examples of the application of these methods, including (1) Kinetic analysis of opioid signaling dynamics and partial agonism measured using cAMP and arrestin biosensors; (2) Quantifying the signaling activity of illicit synthetic cannabinoid receptor agonists measured using a fluorescent membrane potential dye; (3) Demonstration of multiplicity of arrestin functions from analysis of biosensor waveforms and quantification of the rates of these processes. These examples show how temporal analysis provides additional dimensions to enhance the understanding of GPCR signaling and therapeutic mechanisms in the nervous system.

Targeting the Ghrelin Receptor as a Novel Therapeutic Option for Epilepsy

Epilepsy is a neurological disease affecting more than 50 million individuals worldwide. Notwithstanding the availability of a broad array of antiseizure drugs (ASDs), 30% of patients suffer from pharmacoresistant epilepsy. This highlights the urgent need for novel therapeutic options, preferably with an emphasis on new targets, since “me too” drugs have been shown to be of no avail. One of the appealing novel targets for ASDs is the ghrelin receptor (ghrelin-R). In epilepsy patients, alterations in the plasma levels of its endogenous ligand, ghrelin, have been described, and various ghrelin-R ligands are anticonvulsant in preclinical seizure and epilepsy models. Up until now, the exact mechanism-of-action of ghrelin-R-mediated anticonvulsant effects has remained poorly understood and is further complicated by multiple downstream signaling pathways and the heteromerization properties of the receptor. This review compiles current knowledge, and discusses the potential mechanisms-of-action of the anticonvulsant effects mediated by the ghrelin-R.

Biased M1 muscarinic receptor mutant mice show accelerated progression of prion neurodegenerative disease

The M1 muscarinic acetylcholine receptor (M1-receptor) plays a crucial role in learning and memory and is a validated drug target for the treatment of Alzheimer’s disease (AD). Furthermore, M1-receptor ligands have been demonstrated to display disease-modifying effects in preclinical models of neurodegenerative disease. By employing a genetic mouse model expressing a G protein–biased M1-receptor in combination with a mouse model of terminal neurodegenerative disease, we demonstrate here that the M1-receptor exerts an inherent neuroprotective activity that is dependent on its phosphorylation status. Thus, in AD drug development programs, M1-receptor ligands that maintain the receptor phosphorylation status will be more likely to lead to beneficial neuroprotective outcomes.

There are currently no treatments that can slow the progression of neurodegenerative diseases, such as Alzheimer’s disease (AD). There is, however, a growing body of evidence that activation of the M1 muscarinic acetylcholine receptor (M1-receptor) can not only restore memory loss in AD patients but in preclinical animal models can also slow neurodegenerative disease progression. The generation of an effective medicine targeting the M1-receptor has however been severely hampered by associated cholinergic adverse responses. By using genetically engineered mouse models that express a G protein–biased M1-receptor, we recently established that M1-receptor mediated adverse responses can be minimized by ensuring activating ligands maintain receptor phosphorylation/arrestin-dependent signaling. Here, we use these same genetic models in concert with murine prion disease, a terminal neurodegenerative disease showing key hallmarks of AD, to establish that phosphorylation/arrestin-dependent signaling delivers neuroprotection that both extends normal animal behavior and prolongs the life span of prion-diseased mice. Our data point to an important neuroprotective property inherent to the M1-receptor and indicate that next generation M1-receptor ligands designed to drive receptor phosphorylation/arrestin-dependent signaling would potentially show low adverse responses while delivering neuroprotection that will slow disease progression.

β-arrestin1 promotes tauopathy by transducing GPCR signaling, disrupting microtubules and autophagy

GPCRs regulator, β-arrestin1, is increased in FTLD-tau patients, is required for β2-adrenergic receptor and metabotropic glutamate receptor 2-induced tau phosphorylation, promotes tau aggregation by impairing autophagy, and destabilizes microtubule dynamics, whereas genetic reduction in β-arrestin1 mitigates tauopathy and cognitive impairments.

G protein–coupled receptors (GPCRs) have been shown to play integral roles in Alzheimer’s disease pathogenesis. However, it is unclear how diverse GPCRs similarly affect Aβ and tau pathogenesis. GPCRs share a common mechanism of action via the β-arrestin scaffolding signaling complexes, which not only serve to desensitize GPCRs by internalization, but also mediate multiple downstream signaling events. As signaling via the GPCRs, β2-adrenergic receptor (β2AR), and metabotropic glutamate receptor 2 (mGluR2) promotes hyperphosphorylation of tau, we hypothesized that β-arrestin1 represents a point of convergence for such pathogenic activities. Here, we report that β-arrestins are not only essential for β2AR and mGluR2-mediated increase in pathogenic tau but also show that β-arrestin1 levels are increased in brains of Frontotemporal lobar degeneration (FTLD-tau) patients. Increased β-arrestin1 in turn drives the accumulation of pathogenic tau, whereas reduced ARRB1 alleviates tauopathy and rescues impaired synaptic plasticity and cognitive impairments in PS19 mice. Biochemical and cellular studies show that β-arrestin1 drives tauopathy by destabilizing microtubules and impeding p62/SQSTM1 autophagy flux by interfering with p62 body formation, which promotes pathogenic tau accumulation.

Constitutive activation of S1P receptors at the trans-Golgi network is required for surface transport carrier formation

The importance of the G-protein βγ subunits in the regulation of cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PM) is well accepted; however, the molecular mechanism underlying the G-protein activation at the TGN remains unclear. We show here that sphingosine 1-phosphate (S1P) receptors at the PM were trafficked to the TGN in response to a surface transport cargo, temperature-sensitive vesicular stomatitis virus glycoprotein tagged with green fluorescent protein accumulation in the Golgi. The receptor internalization occurred in an S1P-independent manner but required phosphorylation by G-protein receptor kinase 2 and β-arrestin association before internalization. Continuously activated S1P receptors in a manner dependent on S1P at the TGN kept transmitting G-protein signals including the βγ subunits supply necessary for transport carrier formation at the TGN destined for the PM.

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S1P receptors traffic from the PM to Golgi in a surface cargo-dependent manner

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S1PR trafficking follows GRK2-dependent phosphorylation and β-arrestin binding

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S1PRs at the Golgi are continuously activated by S1P while sending G-protein signals

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S1PR/Gβγ signals at the Golgi are indispensable for surface transport carrier formation

Structural Basis of Arrestin Selectivity for Active Phosphorylated G Protein-Coupled Receptors

Arrestins are a small family of proteins that bind G protein-coupled receptors (GPCRs). Arrestin binds to active phosphorylated GPCRs with higher affinity than to all other functional forms of the receptor, including inactive phosphorylated and active unphosphorylated. The selectivity of arrestins suggests that they must have two sensors, which detect receptor-attached phosphates and the active receptor conformation independently. Simultaneous engagement of both sensors enables arrestin transition into a high-affinity receptor-binding state. This transition involves a global conformational rearrangement that brings additional elements of the arrestin molecule, including the middle loop, in contact with a GPCR, thereby stabilizing the complex. Here, we review structural and mutagenesis data that identify these two sensors and additional receptor-binding elements within the arrestin molecule. While most data were obtained with the arrestin-1-rhodopsin pair, the evidence suggests that all arrestins use similar mechanisms to achieve preferential binding to active phosphorylated GPCRs.

Intrinsic bias at non-canonical, β-arrestin-coupled seven transmembrane receptors

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors (7TMRs), typically interact with two distinct signal-transducers, i.e., G proteins and β-arrestins (βarrs). Interestingly, there are some non-canonical 7TMRs that lack G protein coupling but interact with barrs, although an understanding of their transducer coupling preference, downstream signaling, and structural mechanism remains elusive. Here, we characterize two such non-canonical 7TMRs, namely, the decoy D6 receptor (D6R) and the complement C5a receptor subtype 2 (C5aR2), in parallel with their canonical GPCR counterparts. We discover that D6R and C5aR2 efficiently couple to βarrs, exhibit distinct engagement of GPCR kinases (GRKs), and activate non-canonical downstream signaling pathways. We also observe that βarrs adopt distinct conformations for D6R and C5aR2, compared to their canonical GPCR counterparts, in response to common natural agonists. Our study establishes D6R and C5aR2 as βarr-coupled 7TMRs and provides key insights into their regulation and signaling with direct implication for biased agonism.

The pivotal protein profile between the conjoined twins and normal mosquitofish Gambusia affinis based on iTRAQ proteomic analysis

The fish abnormal embryonic development has attracted public attention in the recent few years. In this study, an iTRAQ proteomic analysis of mosquitofish between conjoined twins and normal fishes is applied for the first time by using the genome database of mosquitofish. Three thousand four hundred ninety proteins were identified with 304 differentially expressed proteins (DEPs). One hundred six differentially upregulated proteins (DUPs) and 198 differentially downregulated proteins (DDPs) were identified between the conjoined twins and normal mosquitofish groups. Notably, the proteins related to lipid and proteolysis were the important GO terms for the DUPs while response to light stimulus and response to radiation were the most enriched GO terms for the DDPs. The proteins related to lysosome, apoptosis, autophagy, and phagosome were the functional KEGG pathway for the DUPs while most of the pathways were related to cardiovascular for the DDPs. This study expatiated a pivotal protein profile between the conjoined twins and normal mosquitofish which can provide a conference for fish embryonic development.

Assays for detecting arrestin interaction with GPCRs

The four vertebrate arrestins play a key role in the desensitization and internalization of G protein-coupled receptors (GPCRs) and also mediate receptor-dependent signaling. Recent work has shown that bias for arrestin vs G protein signaling could offer certain therapeutic advantages (or disadvantages) in different systems, making assays that measure arrestin binding to receptors important for drug discovery efforts. Herein, we briefly review several commonly used techniques for measuring arrestin binding to receptors, as well as provide an in-depth and methodologically focused review of two methods that do not require receptor modification. The first approach measures direct binding between purified arrestin and rhodopsin, and the second measures the recruitment of arrestin to receptors in living cells.

Bioactive peptide apelin rescues acute kidney injury by protecting the function of renal tubular mitochondria

Mitochondrial dysfunction in proximal tubular epithelial cells is a key event in acute kidney injury (AKI), which is a risk factor for the development of chronic kidney disease (CKD). Apelin is a bioactive peptide that protects against AKI by alleviating inflammation, inhibiting apoptosis, and preventing lipid oxidation, but its role in protecting against mitochondrial damage remains unknown. Herein, we examined the protective effects of apelin on mitochondria in cisplatin-stimulated human renal proximal tubular epithelial cells and evaluated its therapeutic efficacy in cisplatin-induced AKI mice. In vitro, apelin inhibited the cisplatin-induced mitochondrial fission factor (MFF) upregulation and the fusion-promoting protein optic atrophy 1 (OPA1) downregulation. Apelin co-treatment reversed the decreased levels of the deacetylase, Sirt3, and the increased levels of protein acetylation in mitochondria of cisplatin-stimulated cells. Overall, apelin improved the mitochondrial morphology and membrane potential in vitro. In the AKI model, apelin administration significantly attenuated mitochondrial damage, as evidenced by longer mitochondrial profiles and increased ATP levels in the renal cortex. Suppression of MFF expression, and maintenance of Sirt3 and OPA1 expression in apelin-treated AKI mice was also observed. Finally, exogenous administration of apelin normalized the serum level of creatinine and urea nitrogen and the urine levels of NGAL and Kim-1. We also confirmed a regulatory pathway that drives mitochondrial homeostasis including PGC-1α, ERRα and Sirt3. In conclusion, we demonstrated that apelin ameliorates renal functions by protecting tubular mitochondria through Sirt3 upregulation, which is a novel protective mechanism of apelin in AKI. These results suggest that apelin has potential renoprotective effects and may be an effective agent for AKI treatment to significantly retard CKD progression.