Non-visual arrestins are constitutively associated with the centrosome and regulate centrosome function

Chemokine receptor CXCR7 activates Aurora Kinase A and promotes neuroendocrine prostate cancer growth

CXCR7 is an atypical chemokine receptor that recruits β-arrestin (ARRB2) and internalizes into clathrin-coated intracellular vesicles where the complex acts as a scaffold for cytoplasmic kinase assembly and signal transduction. Here, we report that CXCR7 was elevated in the majority of prostate cancer (PCa) cases with neuroendocrine features (NEPC). CXCR7 markedly induced mitotic spindle and cell cycle gene expression. Mechanistically, we identified Aurora Kinase A (AURKA), a key regulator of mitosis, as a novel target that was bound and activated by the CXCR7-ARRB2 complex. CXCR7 interacted with proteins associated with microtubules and golgi, and, as such, the CXCR7-ARRB2-containing vesicles trafficked along the microtubules to the pericentrosomal golgi apparatus, where the complex interacted with AURKA. Accordingly, CXCR7 promoted PCa cell proliferation and tumor growth, which was mitigated by AURKA inhibition. In summary, our study reveals a critical role of CXCR7-ARRB2 in interacting and activating AURKA, which can be targeted by AURKA inhibitors to benefit a subset of patients with NEPC.

Ubiquitin chains earmark GPCRs for BBSome-mediated removal from cilia

Regulated trafficking of G protein-coupled receptors (GPCRs) controls cilium-based signaling pathways. β-Arrestin, a molecular sensor of activated GPCRs, and the BBSome, a complex of Bardet-Biedl syndrome (BBS) proteins, are required for the signal-dependent exit of ciliary GPCRs, but the functional interplay between β-arrestin and the BBSome remains elusive. Here we find that, upon activation, ciliary GPCRs become tagged with ubiquitin chains comprising K63 linkages (UbK63) in a β-arrestin-dependent manner before BBSome-mediated exit. Removal of ubiquitin acceptor residues from the somatostatin receptor 3 (SSTR3) and from the orphan GPCR GPR161 demonstrates that ubiquitination of ciliary GPCRs is required for their regulated exit from cilia. Furthermore, targeting a UbK63-specific deubiquitinase to cilia blocks the exit of GPR161, SSTR3, and Smoothened (SMO) from cilia. Finally, ubiquitinated proteins accumulate in cilia of mammalian photoreceptors and Chlamydomonas cells when BBSome function is compromised. We conclude that Ub chains mark GPCRs and other unwanted ciliary proteins for recognition by the ciliary exit machinery.

Monitoring β-Arrestin 2 Targeting to the Centrosome, Basal Body, and Primary Cilium by Fluorescence Microscopy

Primary cilia (PC) are microtubule-based organelles that behave like a cellular antenna controlling key signaling pathways during development and tissue homeostasis. The ciliary membrane is highly enriched for G protein-coupled receptors (GPCRs), and PC are a crucial signaling compartment for this large receptor family. Downstream effectors of GPCR signaling are also present in cilia, and evidence obtained by our labs and others demonstrated that β-arrestin (βarr) family members are differentially recruited to PC and have investigated the role of GPCR activation in this process. In this chapter, we provide methods based on fluorescence microscopy on fixed or live cells suitable for investigating targeting and recruitment of βarrs at PC.

Endothelin A receptor drives invadopodia function and cell motility through the β-arrestin/PDZ-RhoGEF pathway in ovarian carcinoma

The endothelin-1 (ET-1)/ET A receptor (ETAR) signalling pathway is a well-established driver of epithelial ovarian cancer (EOC) progression. One key process promoted by ET-1 is tumor cell invasion, which requires the scaffolding functions of β-arrestin-1 (β-arr1) downstream of the receptor; however, the potential role of ET-1 in inducing invadopodia, which are crucial for cellular invasion and tumor metastasis, is completely unknown. We describe here that ET-1/ETAR, through β-arr1, activates RhoA and RhoC GTPase and downstream ROCK (Rho-associated coiled coil-forming kinase) kinase activity, promoting actin-based dynamic remodelling and enhanced cell invasion. This is accomplished by the direct interaction of β-arr1 with PDZ-RhoGEF (postsynaptic density protein 95/disc-large/zonula occludens-RhoGEF). Interestingly, ETAR-mediated invasive properties are related to the regulation of invadopodia, as evaluated by colocalization of actin with cortactin, as well as with TKS5 and MT1-MMP (membrane type 1-matrix metalloproteinase) with areas of matrix degradation, and activation of cofilin pathway, which is crucial for regulating invadopodia activity. Depletion of PDZ-RhoGEF, or β-arr1, or RhoC, as well as the treatment with the dual ET-1 receptor antagonist macitentan, significantly impairs invadopodia function, MMP activity and invasion, demonstrating that β-arr1/PDZ-RhoGEF interaction mediates ETAR-driven ROCK-LIMK-cofilin pathway through the control of RhoC activity. In vivo, macitentan is able to inhibit metastatic dissemination and cofilin phosphorylation. Collectively, our data unveil a noncanonical activation of the RhoC/ROCK pathway through the β-arr1/PDZ-RhoGEF complex as a regulator of ETAR-induced motility and metastasis, establishing ET-1 axis as a novel regulator of invadopodia protrusions through the RhoC/ROCK/LIMK/cofilin pathway during the initial steps of EOC invasion.

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.

Arrestins regulate cell spreading and motility via focal adhesion dynamics

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.

Regulation of Rho-GEF Rgf3 by the arrestin Art1 in fission yeast cytokinesis

The arrestin Art1 and the Rho1 guanine nucleotide exchange factor Rgf3 are interdependent for their localizations to the division site during fission yeast cytokinesis. Art1 physically interacts with Rgf3 to modulate active Rho1 GTPase levels for successful septal formation.

Rho GTPases, activated by guanine nucleotide exchange factors (GEFs), are essential regulators of polarized cell growth, cytokinesis, and many other cellular processes. However, the regulation of Rho-GEFs themselves is not well understood. Rgf3 is an essential GEF for Rho1 GTPase in fission yeast. We show that Rgf3 protein levels and localization are regulated by arrestin-related protein Art1. art1∆ cells lyse during cell separation with a thinner and defective septum. As does Rgf3, Art1 concentrates to the contractile ring starting at early anaphase and spreads to the septum during and after ring constriction. Art1 localization depends on its C-terminus, and Art1 is important for maintaining Rgf3 protein levels. Biochemical experiments reveal that the Rgf3 C-terminus binds to Art1. Using an Rgf3 conditional mutant and mislocalization experiments, we found that Art1 and Rgf3 are interdependent for localization to the division site. As expected, active Rho1 levels at the division site are reduced in art1∆ and rgf3 mutant cells. Taken together, these data reveal that the arrestin family protein Art1 regulates the protein levels and localization of the Rho-GEF Rgf3, which in turn modulates active Rho1 levels during fission yeast cytokinesis.

Identification of chemokine receptors as potential modulators of endocrine resistance in oestrogen receptor-positive breast cancers

Introduction

Endocrine therapies target oestrogenic stimulation of breast cancer (BC) growth, but resistance remains problematic. Our aims in this study were (1) to identify genes most strongly associated with resistance to endocrine therapy by intersecting global gene transcription data from patients treated presurgically with the aromatase inhibitor anastrazole with those from MCF7 cells adapted to long-term oestrogen deprivation (LTED) (2) to assess the clinical value of selected genes in public clinical data sets and (3) to determine the impact of targeting these genes with novel agents.

Methods

Gene expression and Ki67 data were available from 69 postmenopausal women with oestrogen receptor–positive (ER+) early BC, at baseline and 2 weeks after anastrazole treatment, and from cell lines adapted to LTED. The functional consequences of target genes on proliferation, ER-mediated transcription and downstream cell signalling were assessed.

Results

By intersecting genes predictive of a poor change in Ki67 with those upregulated in LTED cells, we identified 32 genes strongly correlated with poor antiproliferative response that were associated with inflammation and/or immunity. In a panel of LTED cell lines, C-X-C chemokine receptor type 7 (CXCR7) and CXCR4 were upregulated compared to their wild types (wt), and CXCR7, but not CXCR4, was associated with reduced relapse-free survival in patients with ER+ BC. The CXCR4 small interfering RNA variant (siCXCR4) had no specific effect on the proliferation of wt-SUM44, wt-MCF7 and their LTED derivatives. In contrast, siCXCR7, as well as CCX733, a CXCR7 antagonist, specifically suppressed the proliferation of MCF7-LTED cells. siCXCR7 suppressed proteins associated with G₁/S transition and inhibited ER transactivation in MCF7-LTED, but not wt-MCF7, by impeding association between ER and proline-, glutamic acid– and leucine-rich protein 1, an ER coactivator.

Conclusions

These data highlight CXCR7 as a potential therapeutic target warranting clinical investigation in endocrine-resistant BC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-014-0447-1) contains supplementary material, which is available to authorized users.

Protein adaptation: mitotic functions for membrane trafficking proteins

Membrane trafficking and mitosis are two essential processes in eukaryotic cells. Surprisingly, many proteins best known for their role in membrane trafficking have additional 'moonlighting' functions in mitosis. Despite having proteins in common, there is insufficient evidence for a specific connection between these two processes. Instead, these phenomena demonstrate the adaptability of the membrane trafficking machinery that allows its repurposing for different cellular functions.

G protein-coupled receptor kinase 2 (GRK2) is localized to centrosomes and mediates epidermal growth factor-promoted centrosomal separation

G protein-coupled receptor kinases (GRKs) play a central role in regulating receptor signaling, but recent studies suggest a broader role in modulating normal cellular functions. For example, GRK5 has been shown to localize to centrosomes and regulate microtubule nucleation and cell cycle progression. Here we demonstrate that GRK2 is also localized to centrosomes, although it has no role in centrosome duplication or microtubule nucleation. Of interest, knockdown of GRK2 inhibits epidermal growth factor receptor (EGFR)-mediated separation of duplicated centrosomes. This EGFR/GRK2-mediated process depends on the protein kinases mammalian STE20-like kinase 2 (Mst2) and Nek2A but does not involve polo-like kinase 1. In vitro analysis and dominant-negative approaches reveal that GRK2 directly phosphorylates and activates Mst2. Collectively these findings demonstrate that GRK2 is localized to centrosomes and plays a central role in mitogen-promoted centrosome separation most likely via its ability to phosphorylate Mst2.

β-Arrestins and G protein-coupled receptor trafficking

Arrestins are adaptor proteins that function to regulate G protein-coupled receptor (GPCR) signaling and trafficking. There are four mammalian members of the arrestin family, two visual and two nonvisual. The visual arrestins (arrestin-1 and arrestin-4) are localized in rod and cone cells, respectively, and function to quench phototransduction by inhibiting receptor/G protein coupling. The nonvisual arrestins (β-arrestin1 and β-arrestin2, a.k.a. arrestin-2 and arrestin-3) are ubiquitously expressed and function to inhibit GPCR/G protein coupling and promote GPCR trafficking and arrestin-mediated signaling. Arrestin-mediated endocytosis of GPCRs requires the coordinated interaction of β-arrestins with clathrin, adaptor protein 2, and phosphoinositides such as PIP(2)/PIP(3). These interactions are facilitated by a conformational change in β-arrestin that is thought to occur upon binding to a phosphorylated activated GPCR. In this chapter, we provide an overview of the reagents and techniques used to study β-arrestin-mediated receptor trafficking.

True arrestins and arrestin-fold proteins: a structure-based appraisal

Arrestin-clan proteins are folded alike, a feature responsible for their recent grouping in a single clan. In human, it includes the well-characterized visual and β-arrestins, the arrestin domain-containing proteins (ARRDCs), isoforms of the retromer subunit VPS26, and DSCR3, a protein involved in Down syndrome. A new arrestin-fold-predicted protein, RGP1, described here may join the clan. Unicellular organisms like the yeast Saccharomyces cerevisiae or the amoeba Dictyostelium discoideum harbor VPS26, DSCR3, and RGP1 isoforms as well as arrestin-related trafficking adaptors or ADCs, but true arrestins are missing. Functionally, members of the arrestin clan have generally a scaffolding role in various membrane protein trafficking events. Despite their similar structure, the mechanism of cargo recognition and internalization and the nature of recruited partners differ for the different members. Based on the recent literature, true arrestins (visual and β-arrestins), ARRDCs, and yeast ARTS are the closest from a functional point of view.

Regulatory role of β-arrestin-2 in cholesterol processing in cystic fibrosis epithelial cells

Cystic fibrosis (CF) cells exhibit an increase in the protein expression of β-arrestin-2 (βarr2) coincident with perinuclear accumulation of free cholesterol. Arrestins are proteins that both serve as broad signaling regulators and contribute to G-protein coupled receptor internalization after agonist stimulation. The hypothesis of this study is that βarr2 is an important component in the mechanisms leading to cholesterol accumulation characteristic of CF cells. To test this hypothesis, epithelial cells stably expressing GFP-tagged βarr2 (βarr2-GFP) and respective GFP-expressing control cells (cont-GFP) were analyzed by filipin staining. The βarr2-GFP cells show a late endosomal/lysosomal cholesterol accumulation that is identical to that seen in CF cells. This βarr2-mediated accumulation is sensitive to Rp-cAMPS treatment, and depleting βarr2 expression in CF-model cells by shRNA alleviates cholesterol accumulation compared with controls. Cftr/βarr2 double knockout mice also exhibit wild-type (WT) levels of cholesterol synthesis, and WT profiles of signaling protein expression have previously been shown to be altered in CF due to cholesterol-related pathways. These data indicate a significant regulatory role for βarr2 in the development of CF-like cholesterol accumulation and give further insight into cholesterol processing mechanisms. An impact of βarr2 expression on Niemann-Pick type C-1 (NPC1)-containing organelle movement is proposed as the mechanism of βarr2-mediated alterations on cholesterol processing. It is concluded that βarr2 expression contributes to altered cholesterol trafficking observed in CF cells.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

G Protein-coupled receptor kinase 5 is localized to centrosomes and regulates cell cycle progression

G protein-coupled receptor kinases (GRKs) are important regulators of G protein-coupled receptor function and mediate receptor desensitization, internalization, and signaling. While GRKs also interact with and/or phosphorylate many other proteins and modify their function, relatively little is known about the cellular localization of endogenous GRKs. Here we report that GRK5 co-localizes with γ-tubulin, centrin, and pericentrin in centrosomes. The centrosomal localization of GRK5 is observed predominantly at interphase and although its localization is not dependent on microtubules, it can mediate microtubule nucleation of centrosomes. Knockdown of GRK5 expression leads to G2/M arrest, characterized by a prolonged G2 phase, which can be rescued by expression of wild type but not catalytically inactive GRK5. This G2/M arrest appears to be due to increased expression of p53, reduced activity of aurora A kinase and a subsequent delay in the activation of polo-like kinase 1. Overall, these studies demonstrate that GRK5 is localized in the centrosome and regulates microtubule nucleation and normal cell cycle progression.

Receptor heteromerization expands the repertoire of cannabinoid signaling in rodent neurons

A fundamental question in G protein coupled receptor biology is how a single ligand acting at a specific receptor is able to induce a range of signaling that results in a variety of physiological responses. We focused on Type 1 cannabinoid receptor (CB₁R) as a model GPCR involved in a variety of processes spanning from analgesia and euphoria to neuronal development, survival and differentiation. We examined receptor dimerization as a possible mechanism underlying expanded signaling responses by a single ligand and focused on interactions between CB₁R and delta opioid receptor (DOR). Using co-immunoprecipitation assays as well as analysis of changes in receptor subcellular localization upon co-expression, we show that CB₁R and DOR form receptor heteromers. We find that heteromerization affects receptor signaling since the potency of the CB₁R ligand to stimulate G-protein activity is increased in the absence of DOR, suggesting that the decrease in CB₁R activity in the presence of DOR could, at least in part, be due to heteromerization. We also find that the decrease in activity is associated with enhanced PLC-dependent recruitment of arrestin3 to the CB₁R-DOR complex, suggesting that interaction with DOR enhances arrestin-mediated CB₁R desensitization. Additionally, presence of DOR facilitates signaling via a new CB₁R-mediated anti-apoptotic pathway leading to enhanced neuronal survival. Taken together, these results support a role for CB₁R-DOR heteromerization in diversification of endocannabinoid signaling and highlight the importance of heteromer-directed signal trafficking in enhancing the repertoire of GPCR signaling.

Differential expression of arrestins is a predictor of breast cancer progression and survival

Emerging evidence has implicated G protein-coupled receptors, such as CXCR4 and PAR2, in breast cancer progression and the development of metastatic breast cancer. However, the role of proteins that regulate the function of these receptors, such as arrestins, in breast cancer has yet to be determined. Examination of the expression of the two nonvisual arrestins, arrestin2 and 3, in various breast cancer cell lines revealed comparable expression of arrestin3 in basal and luminal lines while arrestin2 expression was much higher in the luminal lines compared to the more aggressive basal lines. Analysis of normal human breast tissue revealed that arrestin2 and 3 were expressed in both luminal and myoepithelial cells of mammary epithelia with arrestin2 highest in myoepithelial cells and arrestin3 comparable in both cell types. Quantitative immunofluorescence-based examination of primary breast tumors revealed that arrestin2 expression significantly decreased with cancer progression from ductal carcinoma in situ to invasive carcinoma and further to lymph node metastasis (P < 0.001). Moreover, decreased arrestin2 expression was associated with decreased survival (P = 0.0007) as well as positive lymph node status and increased tumor size and nuclear grade. In contrast, arrestin3 expression significantly increased during breast cancer progression (P < 0.001) and increased expression was associated with decreased survival (P = 0.014). Arrestin3 was also an independent prognostic marker of breast cancer with a hazard ratio of 1.65. Overall, these studies demonstrate that arrestin2 levels decrease while arrestin3 levels increase during breast cancer progression and these changes correlate with a poor clinical outcome.

Molecular pathways for intracellular cholesterol accumulation: common pathogenic mechanisms in Niemann-Pick disease Type C and cystic fibrosis

It has been less than two decades since the underlying genetic defects in Niemann-Pick disease Type C were first identified. These defects impair function of two proteins with a direct role in lipid trafficking, resulting in deposition of free cholesterol within late endosomal compartments and a multitude of effects on cell function and clinical manifestations. The rapid pace of research in this area has vastly improved our overall understanding of intracellular cholesterol homeostasis. Excessive cholesterol buildup has also been implicated in clinical manifestations associated with a number of genetically unrelated diseases including cystic fibrosis. Applying knowledge about anomalous cell signaling behavior in cystic fibrosis opens prospects for identifying similar previously unrecognized disease pathways in Niemann-Pick disease Type C. Recognition that Niemann-Pick disease Type C and cystic fibrosis both impair cholesterol regulatory pathways also provides a rationale for identifying common therapeutic targets.

Identification of arrestin-3-specific residues necessary for JNK3 kinase activation

Arrestins bind active phosphorylated G protein-coupled receptors, blocking G protein activation and channeling the signaling to G protein-independent pathways. Free arrestin-3 and receptor-bound arrestin-3 scaffold the ASK1-MKK4-JNK3 module, promoting JNK3 phosphorylation, whereas highly homologous arrestin-2 does not. Here, we used arrestin-2/3 chimeras and mutants to identify key residues of arrestin-3 responsible for its ability to facilitate JNK3 activation. Our data demonstrate that both arrestin domains are involved in JNK3 activation, with the C-terminal domain being more important than the N-terminal domain. We found that Val-343 is the key contributor to this function, whereas Leu-278, Ser-280, His-350, Asp-351, His-352, and Ile-353 play supporting roles. We also show that the arrestin-3-specific difference in the arrangement of the β-strands in the C-terminal domain that underlies its lower selectivity for active phosphoreceptors does not play an appreciable role in its ability to enhance JNK3 activation. Importantly, the strength of the binding of ASK1 or JNK3, as revealed by the efficiency of co-immunoprecipitation, does not correlate with the ability of arrestin proteins to promote ASK1-dependent JNK3 phosphorylation. Thus, multiple residues on the non-receptor-binding side of arrestin-3 are crucial for JNK3 activation, and this function and the receptor-binding characteristics of arrestin can be manipulated independently by targeted mutagenesis.

β-arrestin-2 regulation of the cAMP response element binding protein

Previous work demonstrated that cystic fibrosis (CF) cells exhibit an increase in cAMP-mediated signaling as a characteristic response to lost CFTR function. Evidence for increased cAMP-mediated signaling in CF included increased phosphorylation of the cAMP response element binding protein (CREB) and elevated β-arrestin-2 (βarr2) expression. However, subsequent studies reveal that CREB activation in CF cells is independent of protein kinase-A (PKA). The goal of this study is to test the hypothesis that elevated βarr2 expression leads to increased CREB activation in a PKA-independent mechanism. βarr2-GFP expressing tracheal epithelial cells (βarr2-GFP) exhibit an increase of pCREB content and subsequent CRE activation compared to GFP expressing control cells. βarr2 activation of the ERK cascade represents a candidate mechanism leading to CREB activation. ERK exhibits increased activation in βarr2-GFP cells compared to cont-GFP cells, and ERK inhibition diminishes CRE activation in both GFP and βarr2-GFP cells. To test directly whether CREB regulation in CF is βarr2-dependent, nasal epithelium excised from wt mice (Cftr +/+; βarr2 +/+), CF mice (Cftr -/-; βarr2 +/+), and DKO mice (Cftr -/-; βarr2 -/-) were analyzed for pCREB protein content. Removal of βarr2 expression from CF mice reduces both pCREB and pERK content to wt levels. These data indicate that CF-related CREB regulation is mediated directly through βarr2 expression via the ERK pathway.

Receptors, signaling networks, and disease

Over the past years, a holistic approach has been applied to the study of the field of receptor signaling, permitting the analysis of how the interaction between receptors and their cellular environment determines receptor function and the study of the role of these receptors, under both normal and pathophysiological conditions, in whole organisms. This has been facilitated by the development of high-resolution microscopy techniques, which allow single-molecule or spatiotemporal resolution, or both, of signaling processes at the cellular and organismal levels. Concurrently, the role of these signaling pathways can be tested in increasingly sophisticated murine disease models. Finally, computational approaches aid in predicting and understanding receptor behavior. The program of the Madrid meeting reflected this integrated approach, highlighting signaling by and dynamics and regulation of immune cell receptors, the T cell receptor and B cell receptor, and signaling by and regulation of G protein-coupled receptors.

Beta-arrestins as regulators of signal termination and transduction: how do they determine what to scaffold?

Over the last decade β-arrestins have emerged as pleiotropic scaffold proteins, capable of mediating numerous diverse responses to multiple agonists. Most well characterized are the G-protein-coupled receptor (GPCR) stimulated β-arrestin signals, which are sometimes synergistic with, and sometimes independent of, heterotrimeric G-protein signals. β-arrestin signaling involves the recruitment of downstream signaling moieties to β-arrestins; in many cases specific sites of interaction between β-arrestins and the downstream target have been identified. As more information unfolds about the nature of β-arrestin scaffolding interactions, it is evident that these proteins are capable of adopting multiple conformations which in turn reveal a specific set of interacting domains. Recruitment of β-arrestin to a specific GPCR can promote formation of a specific subset of available β-arrestin scaffolds, allowing for a higher level of specificity to given agonists. This review discusses recent advances in β-arrestin signaling, discussing the molecular details of a subset of known β-arrestin scaffolds and the significance of specific binding interactions on the ultimate cellular response.