Interaction between the mu opioid receptor and filamin A is involved in receptor regulation and trafficking

Filamin A Is Required for NK Cell Cytotoxicity at the Expense of Cytokine Production via Synaptic Filamentous Actin Modulation

Natural killer (NK) cells are innate cytotoxic lymphocytes that efficiently eliminate malignant and virus-infected cells without prior activation via the directed and focused release of lytic granule contents for target cell lysis. This cytolytic process is tightly regulated at discrete checkpoint stages to ensure the selective killing of diseased target cells and is highly dependent on the coordinated regulation of cytoskeletal components. The actin-binding protein filamin crosslinks cortical actin filaments into orthogonal networks and links actin filament webs to cellular membranes to modulate cell migration, adhesion, and signaling. However, its role in the regulation of NK cell functions remains poorly understood. Here, we show that filamin A (FLNa), a filamin isoform with preferential expression in leukocytes, is recruited to the NK cell lytic synapse and is required for NK cell cytotoxicity through the modulation of conjugate formation with target cells, synaptic filamentous actin (F-actin) accumulation, and cytotoxic degranulation, but not granule polarization. Interestingly, we also find that the loss of FLNa augments the target cell-induced expression of IFN-γ and TNF-α by NK cells, correlating with enhanced activation signals such as Ca²⁺ mobilization, ERK, and NF-κB, and a delayed down-modulation of the NKG2D receptor. Thus, our results identify FLNa as a new regulator of NK cell effector functions during their decision to kill target cells through a balanced regulation of NK cell cytotoxicity vs cytokine production. Moreover, this study implicates the cross-linking/bundling of F-actin mediated by FLNa as a necessary process coordinating optimal NK effector functions.

Antagonism of the mu-delta opioid receptor heterodimer enhances opioid antinociception by activating Src and calcium/calmodulin-dependent protein kinase II signaling

ABSTRACT

The opioid receptors are important regulators of pain, reward, and addiction. Limited evidence suggests the mu and delta opioid receptors form a heterodimer (MDOR), which may act as a negative feedback brake on opioid-induced analgesia. However, evidence for the MDOR in vivo is indirect and limited, and there are few selective tools available. We recently published the first MDOR-selective antagonist, D24M, allowing us to test the role of the MDOR in mice. We thus cotreated CD-1 mice with D24M and opioids in tail flick, paw incision, and chemotherapy-induced peripheral neuropathy pain models. D24M treatment enhanced oxymorphone antinociception in all models by 54.7% to 628%. This enhancement could not be replicated with the mu and delta selective antagonists CTAP, naltrindole, and naloxonazine, and D24M had a mild transient effect in the rotarod test, suggesting this increase is selective to the MDOR. However, D24M had no effect on morphine or buprenorphine, suggesting that only specific opioids interact with the MDOR. To find a mechanism, we performed phosphoproteomic analysis on brainstems of mice. We found that the kinases Src and CaMKII were repressed by oxymorphone, which was restored by D24M. We were able to confirm the role of Src and CaMKII in D24M-enhanced antinociception using small molecule inhibitors (KN93 and Src-I1). Together, these results provide direct in vivo evidence that the MDOR acts as an opioid negative feedback brake, which occurs through the repression of Src and CaMKII signal transduction. These results further suggest that MDOR antagonism could be a means to improve clinical opioid therapy.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Evaluation of [Cys(ATTO 488)8]Dermorphin-NH2 as a novel tool for the study of μ-opioid peptide receptors

The μ-opioid peptide (MOP) receptor is a member of the opioid receptor family and an important clinical target for analgesia. Measuring MOP receptor location and tracking its turnover traditionally used radiolabels or antibodies with attendant problems of utility of radiolabels in whole cells and poor antibody selectivity. To address these issues we have synthesized and characterised a novel ATTO488 based fluorescent Dermorphin analogue; [Cys(ATTO 488)⁸]Dermorphin-NH₂ (Derm_ATTO488). We initially assessed the binding profile of Derm_ATTO488 in HEK cells expressing human MOP and CHO cells expressing human MOP, δ-opioid peptide (DOP), κ-opioid peptide (KOP) and Nociceptin/Orphanin FQ peptide (NOP) receptors using radioligand binding. Functional activity of the conjugated peptide was assessed by measuring (i) the ability of the ligand to engage G-protein by measuring the ability to stimulate GTPγ[³⁵S] binding and (ii) the ability to stimulate phosphorylation of ERK1/2. Receptor location was visualised using confocal scanning laser microscopy. Dermorphin and Derm_ATTO488 bound to HEK_MOP (pK_i: 8.29 and 7.00; p<0.05), CHO_MOP (pK_i: 9.26 and 8.12; p<0.05) and CHO_DOP (pK_i: 7.03 and 7.16; p>0.05). Both ligands were inactive at KOP and NOP. Dermorphin and Derm_ATTO488 stimulated the binding of GTPγ[³⁵S] with similar pEC₅₀ (7.84 and 7.62; p>0.05) and E_max (1.52 and 1.34fold p>0.05) values. Moreover, Dermorphin and Derm_ATTO488 produced a monophasic stimulation of ERK1/2 phosphorylation peaking at 5mins (6.98 and 7.64-fold; p>0.05). Finally, in confocal microscopy Derm_ATTO488 bound to recombinant MOP receptors on CHO and HEK cells in a concentration dependent manner that could be blocked by pre-incubation with unlabelled Dermorphin or Naloxone. Collectively, addition to ATTO488 to Dermorphin produced a ligand not dissimilar to Dermorphin; with ~10fold selectivity over DOP. This new ligand Derm_ATTO488 retained functional activity and could be used to visualise MOP receptor location.

Filamin A is required for somatostatin receptor type 5 expression and pasireotide-mediated signaling in pituitary corticotroph tumor cells

Somatostatin receptor type 5 (SST5) represents the main pharmacological target in the treatment of adrenocorticotroph hormone (ACTH)-secreting tumors. However, molecular predictors of responsiveness to pasireotide require further investigation. The cytoskeleton protein filamin A (FLNA) modulates the responsiveness to somatostatin analogs (SSA) treatment in other types of pituitary tumors by regulating somatostatin receptor type 2 (SST2)/dopamine receptor type 2 (DRD2) expression and activity. Here, we aimed to test the involvement of FLNA in the modulation of SST5 response to SSA in human and murine tumor corticotrophs. Western blot analysis of human corticotropinomas showed that FLNA and SST5 correlate. Both in human primary cultures and AtT-20 cells, FLNA genetic silencing caused a decrease of receptor expression level. Moreover, pasireotide-mediated SST5 downregulation observed in AtT-20 control cells was no further detected in FLNA silenced cells. In AtT-20 cells, in situ PLA experiments revealed an increased number of SST5-FLNA complexes following pasireotide incubation. Finally, FLNA knock down abolished pasireotide-induced SST5 actions on hormone secretion, cell proliferation and apoptosis. In conclusion, FLNA is implicated in SST5 expression modulation and signaling.

Hsp90β positively regulates μ-opioid receptor function

AIMS

Many μ-opioid receptor (MOR)-associated proteins can regulate the MOR signaling pathway. Using a bacterial two-hybrid screen, we found that the C-terminal of the MOR associated with heat shock protein 90 isoform β (Hsp90β). Here, we explored the effect of Hsp90β on MOR signaling transduction and function.

MAIN METHODS

The interaction of Hsp90β with MOR was detected by co-immunoprecipitation and immunofluorescence. The effects of Hsp90β on MOR signaling induced by opioids were studied in vitro and in vivo. The effects of the Hsp90β inhibitor 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) on morphine tolerance and dependence were studied via a hot plate test and CPP test.

KEY FINDINGS

Hsp90β, instead of Hsp90α, interacted with the MOR in HEK293 cells and SH-SY5Y cells, and the interaction was augmented after morphine pretreatment. The interaction of Hsp90β and MOR increased the inhibition of cAMP and decreased PKA activity under opioid treatment. The functional Hsp90β-MOR complex also promoted the phosphorylation and internalization of the MOR induced by DAMGO in MOR-CHO cells. 17-AAG blocked Hsp90β-MOR interactions and decreased the effect of Hsp90β on the MOR signal transduction. In C57BL/6 mice, 17-AAG decreased morphine-induced acute anti-nociception in the hot plate test, with an increase in phosphorylated PKA and phosphorylated JNK and a decrease in phosphorylated CREB and phosphorylated ERK in murine brains. Chronic morphine treatment induced tolerance, and dependence was inhibited by 17-AAG co-administration.

SIGNIFICANCE

Hsp90β is a positive co-regulator of the MOR via the activation of a G-protein-dependent and β-arrestin-dependent pathway. Hsp90β has the potential to improve the pharmacologic profile of existing opiates. It is conceivable that in future clinical treatments, the Hsp90β inhibitor, 17-AAG, could decrease the tolerance and dependence in cancer patients induced by opioids.

Cytoskeleton Protein Filamin A Is Required for Efficient Somatostatin Receptor Type 2 Internalization and Recycling through Rab5 and Rab4 Sorting Endosomes in Tumor Somatotroph Cells

The high expression of somatostatin receptor 2 (SST2) in growth hormone (GH)-secreting tumors represents the rationale for the clinical use of somatostatin analogs (SSAs) in acromegaly. Recently, the cytoskeletal protein Filamin A (FLNA) has emerged as key modulator of the responsiveness of GH-secreting pituitary tumors to SSAs by regulating SST2 signaling and expression. The aim of this study was to explore FLNA involvement in SST2 intracellular trafficking in tumor somatotroph cells. By biotinylation assay, we found that FLNA silencing abolished octreotide-mediated SST2 internalization in rat GH3 cell line (28.0 ± 2.7 vs. 4 ± 4.3% SST2 internalization, control versus FLNA small interfering RNAs (siRNA) cells, respectively, p < 0.001) and human GH-secreting primary cultured cells (70.3 ± 21.1 vs. 24 ± 19.2% SST2 internalization, control versus FLNA siRNA cells, respectively, p < 0.05). In addition, confocal imaging revealed impaired SST2 recycling to the plasma membrane in FLNA silenced GH3 cells. Coimmunoprecipitation and immunofluorescence experiments showed that FLNA, as well as β-arrestin2, is timely dependent recruited to octreotide-stimulated SST2 receptors both in rat and human tumor somatotroph cells. Although FLNA expression knock down did not prevent the formation of β-arrestin2-SST2 complex in GH3 cells, it significantly impaired efficient SST2 loading into cytosolic vesicles positive for the early endocytic and recycling markers Rab5 and 4, respectively (33.7 ± 8.9% down to 25.9 ± 6.9%, p < 0.05, and 28.4 ± 7.4% down to 17.6 ± 5.7%, p < 0.01, for SST2-Rab5 and SST2-Rab4 colocalization, respectively, in control versus FLNA siRNA cells). Altogether these data support an important role for FLNA in the mediation of octreotide-induced SST2 trafficking in GH-secreting pituitary tumor cells through Rab5 and 4 sorting endosomes.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Ethanol and Naltrexone Have Distinct Effects on the Lateral Nano-organization of Mu and Kappa Opioid Receptors in the Plasma Membrane

The complex spatiotemporal organization of proteins and lipids in the plasma membrane is an important determinant of receptor function. Certain substances, such as ethanol, can penetrate into the hydrophobic regions of the plasma membrane. By altering protein-lipid and protein-protein interactions, these substances can modify the dynamic lateral organization and the function of plasma membrane receptors. To assess changes in plasma membrane receptor organization, we used photoactivated localization microscopy (PALM). This single molecule localization microscopy technique was employed to quantitatively characterize the effects of pharmacologically relevant concentrations of ethanol and naltrexone (an opioid receptor antagonist and medication used to treat alcohol use disorders) on the lateral nano-organization of mu and kappa opioid receptors (MOR and KOR, respectively). Ethanol affected the lateral organization of MOR and KOR similarly: It reduced the size and occupancy of opioid receptor nanodomains and increased the fraction of opioid receptors residing outside of nanodomains. In contrast, naltrexone affected MOR and KOR lateral organization differently. It significantly increased KOR surface density, nanodomain size, and the occupancy of KOR nanodomains. However, naltrexone marginally affected these parameters for MOR. Pretreatment with naltrexone largely protected against ethanol-induced changes in MOR and KOR lateral organization. Based on these data, we propose a putative mechanism of naltrexone action that operates in addition to its canonical antagonistic effect on MOR- and KOR-mediated signaling.

Single-Molecule Microscopy Reveals Dynamic FLNA Interactions Governing SSTR2 Clustering and Internalization

The cytoskeletal protein filamin A (FLNA) has been suggested to play an important role in the responsiveness of GH-secreting pituitary tumors to somatostatin receptor subtype 2 (SSTR2) agonists by regulating SSTR2 expression and signaling. However, the underlying mechanisms are unknown. In this study, we use fast multicolor single-molecule microscopy to image individual SSTR2 and FLNA molecules at the surface of living cells with unprecedented spatiotemporal resolution. We find that SSTR2 and FLNA undergo transient interactions, which occur preferentially along actin fibers and contribute to restraining SSTR2 diffusion. Agonist stimulation increases the localization of SSTR2 along actin fibers and, subsequently, SSTR2 clustering and recruitment to clathrin-coated pits (CCPs). Interfering with FLNA-SSTR2 binding with a dominant-negative FLNA fragment increases SSTR2 mobility, hampers the formation and alignment of SSTR2 clusters along actin fibers, and impairs both SSTR2 recruitment to CCPs and SSTR2 internalization. These findings indicate that dynamic SSTR2-FLNA interactions critically control the nanoscale localization of SSTR2 at the plasma membrane and are required for coupling SSTR2 clustering to internalization. These mechanisms explain the critical role of FLNA in the control of SSTR2 expression and signaling and suggest the possibility of targeting SSTR2-FLNA interactions for the therapy of pharmacologically resistant GH-secreting pituitary tumors.

Dynamic lateral organization of opioid receptors (kappa, muwt and muN40D ) in the plasma membrane at the nanoscale level

Opioid receptors are important pharmacological targets for the management of numerous medical conditions (eg, severe pain), but they are also the gateway to the development of deleterious side effects (eg, opiate addiction). Opioid receptor signaling cascades are well characterized. However, quantitative information regarding their lateral dynamics and nanoscale organization in the plasma membrane remains limited. Since these dynamic properties are important determinants of receptor function, it is crucial to define them. Herein, the nanoscale lateral dynamics and spatial organization of kappa opioid receptor (KOP), wild type mu opioid receptor (MOPwt ), and its naturally occurring isoform (MOPN40D ) were quantitatively characterized using fluorescence correlation spectroscopy and photoactivated localization microscopy. Obtained results, supported by ensemble-averaged Monte Carlo simulations, indicate that these opioid receptors dynamically partition into different domains. In particular, significant exclusion from GM1 ganglioside-enriched domains and partial association with cholesterol-enriched domains was observed. Nanodomain size, receptor population density and the fraction of receptors residing outside of nanodomains were receptor-specific. KOP-containing domains were the largest and most densely populated, with the smallest fraction of molecules residing outside of nanodomains. The opposite was true for MOPN40D . Moreover, cholesterol depletion dynamically regulated the partitioning of KOP and MOPwt , whereas this effect was not observed for MOPN40D .

Signaling regulation and role of filamin A cleavage in Ca2+-stimulated migration of androgen receptor-deficient prostate cancer cells

Ca²⁺, a ubiquitous cellular signal, and filamin A, an actin-binding protein, play an important role in the regulation of cell adhesion, shape and motility. Using transwell filters to analyze cell migration, we found that extracellular Ca²⁺ (Ca_o²⁺) promotes the migration of androgen receptor (AR)-deficient and highly metastatic prostate cancer cell lines (DU145 and PC-3) compared to AR-positive and relatively less metastatic prostate cancer cells (LNCaP). Furthermore, we found that expression of filamin A is up-regulated in DU145 and PC-3 cells, and that Ca_o²⁺ significantly induces the cleavage of filamin A. Silencing expression of Ca²⁺-sensing receptor (CaR) and p115RhoGEF, and treating with leupeptin, a protease inhibitor, and ALLM, a calpain specific inhibitor, we further demonstrate that Ca_o²⁺-induced filamin A cleavage occurs via a CaR- p115RhoGEF-calpain dependent pathway. Our data show that Ca_o²⁺ via CaR- mediated signaling induces filamin A cleavage and promotes the migration in AR-deficient and highly metastatic prostate cancer cells.

Somatostatin Receptor Type 2 (SSTR2) Internalization and Intracellular Trafficking in Pituitary GH-Secreting Adenomas: Role of Scaffold Proteins and Implications for Pharmacological Resistance

Somatostatin receptor type 2 (SSTR2), together with SSTR5, represents the main target of medical treatment for growth hormone (GH)-secreting pituitary tumors, since it is expressed in most of these tumors and exerts both antiproliferative and cytostatic effects, and reduces hormone secretion, as well. However, clinical practice indicates a great variability in the frequency and entity of favorable responses of acromegalic patients to long-acting somatostatin analogues (SSAs), but the molecular mechanisms regulating this pharmacological resistance are not completely understood. So far, several potentially implied mechanisms have been suggested, including impaired expression of SSTRs, or post-receptor signal transduction alterations. More recently, new studies exploited the molecular factors involved in SSTRs intracellular trafficking regulation, this being a critical point for the modulation of the available active G-coupled receptors (GPCRs) amount at the cell surface. In this respect, the role of the scaffold proteins such as β-arrestins, and the cytoskeleton protein Filamin A (FLNA), have become of relevant importance for GH-secreting pituitary tumors. In fact, β-arrestins are linked to SSTR2 desensitization and internalization, and FLNA is able to regulate SSTR2 trafficking and stability at the plasma membrane. Therefore, the present review will summarize emerging evidence highlighting the role of β-arrestins and FLNA, as possible novel players in the modulation of agonist activated-SSTR2 receptor trafficking and response in GH-secreting pituitary tumors.

Structural and thermodynamic basis of a frontometaphyseal dysplasia mutation in filamin A

Filamin-mediated linkages between transmembrane receptors (TR) and the actin cytoskeleton are crucial for regulating many cytoskeleton-dependent cellular processes such as cell shape change and migration. A major TR binding site in the immunoglobulin repeat 21 (Ig21) of filamin is masked by the adjacent repeat Ig20, resulting in autoinhibition. The TR binding to this site triggers the relief of Ig20 and protein kinase A (PKA)-mediated phosphorylation of Ser-2152, thereby dynamically regulating the TR-actin linkages. A P2204L mutation in Ig20 reportedly cause frontometaphyseal dysplasia, a skeletal disorder with unknown pathogenesis. We show here that the P2204L mutation impairs a hydrophobic core of Ig20, generating a conformationally fluctuating molten globule-like state. Consequently, unlike in WT filamin, where PKA-mediated Ser-2152 phosphorylation is ligand-dependent, the P2204L mutant is readily accessible to PKA, promoting ligand-independent phosphorylation on Ser-2152. Strong TR peptide ligands from platelet GP1bα and G-protein-coupled receptor MAS effectively bound Ig21 by displacing Ig20 from autoinhibited WT filamin, but surprisingly, the capacity of these ligands to bind the P2204L mutant was much reduced despite the mutation-induced destabilization of the Ig20 structure that supposedly weakens the autoinhibition. Thermodynamic analysis indicated that compared with WT filamin, the conformationally fluctuating state of the Ig20 mutant makes Ig21 enthalpically favorable to bind ligand but with substantial entropic penalty, resulting in total higher free energy and reduced ligand affinity. Overall, our results reveal an unusual structural and thermodynamic basis for the P2204L-induced dysfunction of filamin and frontometaphyseal dysplasia disease.

RalA employs GRK2 and β-arrestins for the filamin A-mediated regulation of trafficking and signaling of dopamine D2 and D3 receptor

Filamin A (FLNA) is known to act as platform for the signaling and intracellular trafficking of various GPCRs including dopamine D2 and D3 receptors (D2R, D3R). To understand molecular mechanisms involved in the FLNA-mediated regulation of D2R and D3R, comparative studies were conducted on the signaling and intracellular trafficking of the D2R and D3R in FLNA-knockdown cells, with a specific focus on the roles of the proteins that interact with FLNA and the D2R and D3R. Lowering the level of cellular FLNA caused an elevation in RalA activity and resulted in selective interference with the normal intracellular trafficking and signaling of the D2R and D3R, through GRK2 and β-arrestins, respectively. Knockdown of FLNA or coexpression of active RalA interfered with the recycling of the internalized D2R and resulted in the development of receptor tolerance. Active RalA was found to interact with GRK2 to sequester it from D2R. Knockdown of FLNA or coexpression of active RalA prevented D3R from coupling with G protein. The selective involvement of GRK2- and β-arrestins in the RalA-mediated cellular processes of the D2R and D3R was achieved via their different modes of interactions with the receptor and their distinct functional roles in receptor regulation. Our results show that FLNA is a multi-functional protein that acts as a platform on which D2R and D3R can interact with various proteins, through which selective regulation of these receptors occurs in combination with GRK2 and β-arrestins.

Regulation of chemokine receptor CCR2 recycling by filamin a phosphorylation

Proper endosomal trafficking of ligand-activated G protein-coupled receptors (GPCRs) is essential to spatiotemporally tune their physiological responses. For the monocyte chemoattractant receptor 2 (CCR2B), endocytic recycling is important to sustain monocyte migration; while filamin A (FLNa) is essential for CCL2-induced monocyte migration. Here, we analyze the role of FLNa in the trafficking of CCR2B along the endocytic pathway. In FLNa knockdown cells, activated CCR2B accumulated in enlarged EEA-1-positive endosomes, which exhibited slow movement and fast fluorescence recovery, suggesting an imbalance between receptor entry and exit rates. Utilizing super-resolution microscopy, we observed that FLNa-GFP, CCR2B and β2-adrenergic receptor (β2AR) were present in actin-enriched endosomal microdomains. Depletion of FLNa decreased CCR2B association with these microdomains and concomitantly delayed CCR2B endosomal traffic, without apparently affecting the number of microdomains. Interestingly, CCR2B and β2AR signaling induced phosphorylation of FLNa at S2152 and this phosphorylation event was contributes to sustain receptor recycling. Thus, our data strongly suggest that CCR2B and β2AR signals to FLNa to stimulate its endocytosis and recycling to the plasma membrane.

G protein-coupled receptors directly bind filamin A with high affinity and promote filamin phosphorylation

Although interaction of a few G protein-coupled receptors (GPCRs) with Filamin A, a key actin cross-linking and biomechanical signal transducer protein, has been observed, a comprehensive structure-function analysis of this interaction is lacking. Through a systematic sequence-based analysis, we found that a conserved filamin binding motif is present in the cytoplasmic domains of >20% of the 824 GPCRs encoded in the human genome. Direct high-affinity interaction of filamin binding motif peptides of select GPCRs with the Ig domain of Filamin A was confirmed by nuclear magnetic resonance spectroscopy and isothermal titration calorimetric experiments. Engagement of the filamin binding motif with the Filamin A Ig domain induced the phosphorylation of filamin by protein kinase A in vitro. In transfected cells, agonist activation as well as constitutive activation of representative GPCRs dramatically elicited recruitment and phosphorylation of cellular Filamin A, a phenomenon long known to be crucial for regulating the structure and dynamics of the cytoskeleton. Our data suggest a molecular mechanism for direct GPCR-cytoskeleton coupling via filamin. Until now, GPCR signaling to the cytoskeleton was predominantly thought to be indirect, through canonical G protein-mediated signaling cascades involving GTPases, adenylyl cyclases, phospholipases, ion channels, and protein kinases. We propose that the GPCR-induced filamin phosphorylation pathway is a conserved, novel biochemical signaling paradigm.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

Postendocytic Sorting of Adrenergic and Opioid Receptors: New Mechanisms and Functions

The endocytic pathway tightly regulates the activity of G protein-coupled receptors (GPCRs). Much of our understanding of this relationship between GPCR endocytic trafficking and signaling comes from studies done on catecholamine and opioid receptors. After ligand-induced endocytosis, a key sorting step in the endosome determines whether receptors are recycled back to the cell surface, leading to recovery of signaling, or are degraded in the lysosome, leading to desensitization. Recycling of GPCRs, unlike that of many other proteins, is an active process driven by specific sequences on the receptor and proteins that interact with this sequence. Recent data suggest that sequence-dependent recycling plays complex roles in regulating both the timing and location of GPCR signaling. This chapter will describe our current understanding of the mechanisms regulating GPCR sorting in the endosome and discuss emerging ideas on their role in GPCR signaling, focusing on adrenergic and opioid receptors as prototypes.

Filamin A (FLNA) plays an essential role in somatostatin receptor 2 (SST2) signaling and stabilization after agonist stimulation in human and rat somatotroph tumor cells

Somatostatin receptor type 2 (SST2) is the main pharmacological target of medical therapy for GH-secreting pituitary tumors, but molecular mechanisms regulating its expression and signaling are largely unknown. The aim of this study was to investigate the role of cytoskeleton protein filamin A (FLNA) in SST2 expression and signaling in somatotroph tumor cells. We found a highly variable expression of FLNA in human GH-secreting tumors, without a correlation with SST2 levels. FLNA silencing in human tumoral cells did not affect SST2 expression and localization but abolished the SST2-induced reduction of cyclin D1 (-37% ± 15% in control cells, P < .05 vs basal) and caspase-3/7 activation (+63% ± 31% in control cells, P < .05 vs basal). Overexpression of a FLNA dominant-negative mutant that specifically prevents SST2-FLNA binding reduced SST2 expression after prolonged agonist exposure (-55% ± 5%, P < .01 vs untreated cells) in GH3 cells. Moreover, SST2-induced apoptotic effect (77% ± 54% increase of caspase activity, P < .05 vs basal) and SST2-mediated ERK1/2 inhibition (48% ± 17% reduction of ERK1/2 phosphorylation, P < .01 vs basal) were abrogated in cells overexpressing another FLNA mutant that prevents FLNA interaction with partner proteins but not with SST2, suggesting a scaffold function of FLNA in somatotrophs. In conclusion, these data demonstrate that FLNA is involved in SST2 stabilization and signaling in tumoral somatotrophs, playing both a structural and functional role.

Morphine-induced trafficking of a mu-opioid receptor interacting protein in rat locus coeruleus neurons

Opiate addiction is a devastating health problem, with approximately 2million people currently addicted to heroin or non-medical prescription opiates in the United States alone. In neurons, adaptations in cell signaling cascades develop following opioid actions at the mu opioid receptor (MOR). A novel putative target for intervention involves interacting proteins that may regulate trafficking of MOR. Morphine has been shown to induce a re-distribution of a MOR-interacting protein Wntless (WLS, a transport molecule necessary for secretion of neurotrophic Wnt proteins), from cytoplasmic to membrane compartments in rat striatal neurons. Given its opiate-sensitivity and its well-characterized molecular and cellular adaptations to morphine exposure, we investigated the anatomical distribution of WLS and MOR in the rat locus coeruleus (LC)-norepinephrine (NE) system. Dual immunofluorescence microscopy was used to test the hypothesis that WLS is localized to noradrenergic neurons of the LC and that WLS and MOR co-exist in common LC somatodendritic processes, providing an anatomical substrate for their putative interactions. We also hypothesized that morphine would influence WLS distribution in the LC. Rats received saline, morphine or the opiate agonist [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO), and tissue sections through the LC were processed for immunogold-silver detection of WLS and MOR. Statistical analysis showed a significant re-distribution of WLS to the plasma membrane following morphine treatment in addition to an increase in the proximity of gold-silver labels for MOR and WLS. Following DAMGO treatment, MOR and WLS were predominantly localized within the cytoplasmic compartment when compared to morphine and control. In a separate cohort of rats, brains were obtained from saline-treated or heroin self-administering male rats for pulldown co-immunoprecipitation studies. Results showed an increased association of WLS and MOR following heroin exposure. As the LC-NE system is important for cognition as well as decisions underlying substance abuse, adaptations in WLS trafficking and expression may play a role in modulating MOR function in the LC and contribute to the negative sequelae of opiate exposure on executive function.

Filamin A promotes dynamin-dependent internalization of hyperpolarization-activated cyclic nucleotide-gated type 1 (HCN1) channels and restricts Ih in hippocampal neurons

The actin-binding protein filamin A (FLNa) regulates neuronal migration during development, yet its roles in the mature brain remain largely obscure. Here, we probed the effects of FLNa on the regulation of ion channels that influence neuronal properties. We focused on the HCN1 channels that conduct Ih, a hyperpolarization-activated current crucial for shaping intrinsic neuronal properties. Whereas regulation of HCN1 channels by FLNa has been observed in melanoma cell lines, its physiological relevance to neuronal function and the underlying cellular pathways that govern this regulation remain unknown. Using a combination of mutational, pharmacological, and imaging approaches, we find here that FLNa facilitates a selective and reversible dynamin-dependent internalization of HCN1 channels in HEK293 cells. This internalization is accompanied by a redistribution of HCN1 channels on the cell surface, by accumulation of the channels in endosomal compartments, and by reduced Ih density. In hippocampal neurons, expression of a truncated dominant-negative FLNa enhances the expression of native HCN1. Furthermore, acute abrogation of HCN1-FLNa interaction in neurons, with the use of decoy peptides that mimic the FLNa-binding domain of HCN1, abolishes the punctate distribution of HCN1 channels in neuronal cell bodies, augments endogenous Ih, and enhances the rebound-response ("voltage-sag") of the neuronal membrane to transient hyperpolarizing events. Together, these results support a major function of FLNa in modulating ion channel abundance and membrane trafficking in neurons, thereby shaping their biophysical properties and function.

Nanoscale effects of ethanol and naltrexone on protein organization in the plasma membrane studied by photoactivated localization microscopy (PALM)

Background

Ethanol affects the signaling of several important neurotransmitter and neuromodulator systems in the CNS. It has been recently proposed that ethanol alters the dynamic lateral organization of proteins and lipids in the plasma membrane, thereby affecting surface receptor-mediated cellular signaling. Our aims are to establish whether pharmacologically relevant levels of ethanol can affect the lateral organization of plasma membrane and cytoskeletal proteins at the nanoscopic level, and investigate the relevance of such perturbations for mu-opioid receptor (MOP) function.

Methodology/Principal Findings

We used Photoactivated Localization Microscopy with pair-correlation analysis (pcPALM), a quantitative fluorescence imaging technique with high spatial resolution (15–25 nm) and single-molecule sensitivity, to study ethanol effects on protein organization in the plasma membrane. We observed that short (20 min) exposure to 20 and 40 mM ethanol alters protein organization in the plasma membrane of cells that harbor endogenous MOPs, causing a rearrangement of the lipid raft marker glycosylphosphatidylinositol (GPI). These effects could be largely occluded by pretreating the cells with the MOP antagonist naltrexone (200 nM for 3 hours). In addition, ethanol induced pronounced actin polymerization, leading to its partial co-localization with GPI.

Conclusions/Significance

Pharmacologically relevant levels of ethanol alter the lateral organization of GPI-linked proteins and induce actin cytoskeleton reorganization. Pretreatment with the MOP antagonist naltrexone is protective against ethanol action and significantly reduces the extent to which ethanol remodels the lateral organization of lipid-rafts-associated proteins in the plasma membrane. Super-resolution pcPALM reveals details of ethanol action at the nanoscale level, giving new mechanistic insight on the cellular and molecular mechanisms of its action.

Intrathecal ultra-low dose naloxone enhances the antihyperalgesic effects of morphine and attenuates tumor necrosis factor-α and tumor necrosis factor-α receptor 1 expression in the dorsal horn of rats with partial sciatic nerve transection

BACKGROUND

Glutamate homeostasis and microglia activation play an important role in the development and maintenance of neuropathic pain. We designed this investigation to examine whether ultra-low dose naloxone administered alone or in combination with morphine could alter the concentration of the excitatory amino acids (EAAs) glutamate and aspartate, as well as the expression of tumor necrosis factor-α (TNF-α) and its receptors (TNFR1 and TNFR2) in the spinal cord dorsal horn of rats with partial sciatic nerve transection (PST).

METHODS

Male Wistar rats underwent intrathecal catheter implantation for drug delivery and were divided in 7 groups: sham-operated + saline (sham), PST + saline (S), PST + 15 ng naloxone (n), PST + 15 µg naloxone (N), PST + 10 µg morphine (M), PST + 15 ng naloxone + 10 µg morphine (Mn), PST + 15 µg naloxone + 10 µg morphine (MN). Thermal withdrawal latency and mechanical withdrawal threshold, TNF-α and TNFR expression in the spinal cord and dorsal root ganglia, and EAAs glutamate and aspartate concentration in cerebrospinal fluid dialysates were measured.

RESULTS

Ten days after PST, rats developed hyperalgesia (P < 0.0001) and allodynia (P < 0.0001), and increased TNF-α (P < 0.0001) and TNFR1 expression (P = 0.0009) were measured in the ipsilateral spinal cord dorsal horn. The antihyperalgesic and antiallodynic effects of morphine (10 μg) were abolished by high-dose naloxone (15 μg; P = 0.0031) but enhanced by ultra-low dose naloxone (15 ng; P = 0.0015), and this was associated with a reduction of TNF-α (P < 0.0001) and TNFR1 (P = 0.0009) expression in the spinal cord dorsal horn and EAAs concentration (glutamate: P = 0.0001; aspartate: P = 0.004) in cerebrospinal fluid dialysate. Analysis of variance (ANOVA) or Student t test with Bonferroni correction were used for statistical analysis.

CONCLUSIONS

Ultra-low dose naloxone enhances the antihyperalgesia and antiallodynia effects of morphine in PST rats, possibly by reducing TNF-α and TNFR1 expression, and EAAs concentrations in the spinal dorsal horn. Ultra-low dose naloxone may be a useful adjuvant for increasing the analgesic effect of morphine in neuropathic pain conditions.

Differential expression of the alternatively spliced OPRM1 isoform μ-opioid receptor-1K in HIV-infected individuals

OBJECTIVE

We previously examined the expression of specific C-terminal μ-opioid receptor (MOR) splice variants in human central nervous system cell types and HIV-infected brain tissue from individuals with neurocognitive impairment ± HIV encephalitis (HIVE). In the present study, we examined the N-terminal splice variant MOR-1K, which mediates excitatory cellular signaling.

METHODS AND RESULTS

We found segregation of expression ranging from undetectable to seemingly exclusive across nervous system cell types compared to the pool of C-terminal MOR splice variants using the real-time polymerase chain reaction (RT-PCR). Expression of MOR-1K mRNA was also increased in HIV-infected individuals with combined neurocognitive impairment and HIVE compared with the other groups. MOR-1K expression correlated with the level of patient neurocognitive impairment, whereas the pool of C-terminal MOR splice variants did not. HIVE was also associated with increased expression of the inflammatory mediators MCP-1, MCP-2, and RANTES, but not the host HIV coreceptors CXCR4 and CCR5 or the CD4 receptor using qRT-PCR. Network analysis of microarray data from these same patients revealed filamin A (FLNA) as a possible interaction partner with MOR-1K, and FLNA gene expression was also found to be upregulated in HIVE using qRT-PCR. Overexpression of FLNA in HEK293 cells redistributed MOR-1K from intracellular compartments to the cell surface.

CONCLUSION

These results suggest that HIVE, and neurocognitive impairment depending on its severity, are associated with enhanced MOR-1K signaling through both increased expression and trafficking to the cell surface, which may alter the contribution of MOR receptor isoforms and exacerbate the effects of MOR activation in neuroAIDS.

Filamin-A is required for the incorporation of tissue factor into cell-derived microvesicles

We previously reported that the incorporation of tissue factor (TF) into cell-derived microvesicles (MVs) is regulated by the phosphorylation of the cytoplasmic domain of TF. Since the cytoskeletal protein filamin-A is known to bind to the cytoplasmic domain of TF in a phosphorylation-dependent manner, the involvement of filamin-A in the incorporation of TF into MVs was examined. Endothelial cells were transfected to express TF, whereas MDA-MB-231 cells were used to examine endogenously expressed TF. MV release was induced by activating protease-activated receptor-2 (PAR2). Partial suppression of filamin-A expression using two different filamin-A siRNA sequences resulted in significant reductions in the incorporation of TF antigen into MVs as determined by TF-ELISA and western blot analysis, and was reflected in reduced thrombin-generation and FXa-generation capacities of these MVs. Deletion of the cytoplasmic domain of TF also resulted in reduced incorporation of TF into MVs, whereas the suppression of filamin-A expression had no additional effect on the incorporation of truncated TF into MVs. Partial suppression of filamin-A expression had no effect on the number and size distribution of the released MVs. However, >90% suppression of filamin-A expression resulted in increased MV release, possibly as a result of increased instability of the plasma membrane and underlying cytoskeleton. In conclusion, the presence of filamin-A appears to be essential for the incorporation of TF into MVs following PAR2 activation, but is not required for the process of MV formation and release following PAR2 activation.

Cyclic AMP-Rap1A signaling mediates cell surface translocation of microvascular smooth muscle α2C-adrenoceptors through the actin-binding protein filamin-2

The second messenger cyclic AMP (cAMP) plays a vital role in vascular physiology, including vasodilation of large blood vessels. We recently demonstrated cAMP activation of Epac-Rap1A and RhoA-Rho-associated kinase (ROCK)-F-actin signaling in arteriolar-derived smooth muscle cells increases expression and cell surface translocation of functional α2C-adrenoceptors (α2C-ARs) that mediate vasoconstriction in small blood vessels (arterioles). The Ras-related small GTPAse Rap1A increased expression of α2C-ARs and also increased translocation of perinuclear α2C-ARs to intracellular F-actin and to the plasma membrane. This study examined the mechanism of translocation to better understand the role of these newly discovered mediators of blood flow control, potentially activated in peripheral vascular disorders. We utilized a yeast two-hybrid screen with human microvascular smooth muscle cells (microVSM) cDNA library and the α2C-AR COOH terminus to identify a novel interaction with the actin cross-linker filamin-2. Yeast α-galactosidase assays, site-directed mutagenesis, and coimmunoprecipitation experiments in heterologous human embryonic kidney (HEK) 293 cells and in human microVSM demonstrated that α2C-ARs, but not α2A-AR subtype, interacted with filamin. In Rap1-stimulated human microVSM, α2C-ARs colocalized with filamin on intracellular filaments and at the plasma membrane. Small interfering RNA-mediated knockdown of filamin-2 inhibited Rap1-induced redistribution of α2C-ARs to the cell surface and inhibited receptor function. The studies suggest that cAMP-Rap1-Rho-ROCK signaling facilitates receptor translocation and function via phosphorylation of filamin-2 Ser(2113). Together, these studies extend our previous findings to show that functional rescue of α2C-ARs is mediated through Rap1-filamin signaling. Perturbation of this signaling pathway may lead to alterations in α2C-AR trafficking and physiological function.

Case-control association study of WLS variants in opioid and cocaine addicted populations

The opioid receptor family is involved in the development and maintenance of drug addiction. The mu-opioid receptor (MOR) mediates the rewarding effects of multiple drugs, including opiates and cocaine. A number of proteins interact with MOR, potentially modulating MOR function and altering the physiological consequences of drug use. These mu-opioid receptor interacting proteins (MORIPs) are potential therapeutic targets for the treatment of addiction. The Wntless (WLS) protein was recently identified as a MORIP in a yeast two-hybrid screen. In this study, we conducted a case-control association analysis of 16 WLS genetic variants in opioid and cocaine addicted individuals of both African-American (opioid n=336, cocaine n=908) and European-American (opioid n=335, cocaine n=336) ancestry. Of the analyzed SNPs, three were nominally associated with opioid addiction and four were nominally associated with cocaine addiction. None of these associations were significant following multiple testing correction. These data suggest that the common variants of WLS analyzed in this study are not associated with opioid or cocaine addiction. However, this study does not exclude the possibilities that rare variants in WLS may affect susceptibility to drug addiction, or that common variants with small effect size may fall below the detection level of our analysis.

Distinct regional and subcellular localization of the actin-binding protein filamin A in the mature rat brain

Filamin A (FLNa) is an actin-binding protein that regulates cell motility, adhesion, and elasticity by cross-linking filamentous actin. Additional roles of FLNa include regulation of protein trafficking and surface expression. Although the functions of FLNa during brain development are well studied, little is known on its expression, distribution, and function in the adult brain. Here we characterize in detail the neuroanatomical distribution and subcellular localization of FLNa in the mature rat brain, by using two antisera directed against epitopes at either the N' or the C' terminus of the protein, further validated by mRNA expression. FLNa was widely and selectively expressed throughout the brain, and the intensity of immunoreactivity was region dependent. The most intensely FLNa-labeled neurons were found in discrete neuronal systems, including basal forebrain structures, anterior nuclear group of thalamus, and hypothalamic parvocellular neurons. Pyramidal neurons in neocortex and hippocampus and magnocellular cells in basolateral amygdaloid nucleus were also intensely FLNa immunoreactive, and strong FLNa labeling was evident in the pontine and medullary raphe nuclei and in sensory and spinal trigeminal nuclei. The subcellular localization of FLNa was evaluated in situ as well as in primary hippocampal neurons. Punctate expression was found in somata and along the dendritic shaft, but FLNa was not detected in dendritic spines. These subcellular distribution patterns were recapitulated in hippocampal and neocortical pyramidal neurons in vivo. The characterization of the expression and subcellular localization of FLNa may provide new clues to the functional roles of this cytoskeletal protein in the adult brain.

Fast modulation of μ-opioid receptor (MOR) recycling is mediated by receptor agonists

The μ-opioid receptor (MOR) is a member of the G protein-coupled receptor family and the main target of endogenous opioid neuropeptides and morphine. Upon activation by ligands, MORs are rapidly internalized via clathrin-coated pits in heterologous cells and dissociated striatal neurons. After initial endocytosis, resensitized receptors recycle back to the cell surface by vesicular delivery for subsequent cycles of activation. MOR trafficking has been linked to opioid tolerance after acute exposure to agonist, but it is also involved in the resensitization process. Several studies describe the regulation and mechanism of MOR endocytosis, but little is known about the recycling of resensitized receptors to the cell surface. To study this process, we induced internalization of MOR with [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) and morphine and imaged in real time single vesicles recycling receptors to the cell surface. We determined single vesicle recycling kinetics and the number of receptors contained in them. Then we demonstrated that rapid vesicular delivery of recycling MORs to the cell surface was mediated by the actin-microtubule cytoskeleton. Recycling was also dependent on Rab4, Rab11, and the Ca(2+)-sensitive motor protein myosin Vb. Finally, we showed that recycling is acutely modulated by the presence of agonists and the levels of cAMP. Our work identifies a novel trafficking mechanism that increases the number of cell surface MORs during acute agonist exposure, effectively reducing the development of opioid tolerance.

A switch of G protein-coupled receptor binding preference from phosphoinositide 3-kinase (PI3K)-p85 to filamin A negatively controls the PI3K pathway

Frequent oncogenic alterations occur in the phosphoinositide 3-kinase (PI3K) pathway, urging identification of novel negative controls. We previously reported an original mechanism for restraining PI3K activity, controlled by the somatostatin G protein-coupled receptor (GPCR) sst2 and involving a ligand-regulated interaction between sst2 with the PI3K regulatory p85 subunit. We here identify the scaffolding protein filamin A (FLNA) as a critical player regulating the dynamic of this complex. A preexisting sst2-p85 complex, which was shown to account for a significant basal PI3K activity in the absence of ligand, is disrupted upon sst2 activation. FLNA was here identified as a competitor of p85 for direct binding to two juxtaposed sites on sst2. Switching of GPCR binding preference from p85 toward FLNA is determined by changes in the tyrosine phosphorylation of p85- and FLNA-binding sites on sst2 upon activation. It results in the disruption of the sst2-p85 complex and the subsequent inhibition of PI3K. Knocking down FLNA expression, or abrogating FLNA recruitment to sst2, reversed the inhibition of PI3K and of tumor growth induced by sst2. Importantly, we report that this FLNA inhibitory control on PI3K can be generalized to another GPCR, the mu opioid receptor, thereby providing an unprecedented mechanism underlying GPCR-negative control on PI3K.

Phosphorylated filamin A regulates actin-linked caveolae dynamics

Caveolae are relatively stable membrane invaginations that compartmentalize signaling, regulate lipid metabolism and mediate viral entry. Caveolae are closely associated with actin fibers and internalize in response to diverse stimuli. Loss of cell adhesion is known to induce rapid and robust caveolae internalization and trafficking toward a Rab11-positive recycling endosome; however, pathways governing this process are poorly understood. Here, we report that filamin A is required to maintain the F-actin-dependent linear distribution of caveolin-1. High spatiotemporal resolution particle tracking of caveolin-1-GFP vesicles by total internal reflection fluorescence (TIRF) microscopy revealed that FLNa is required for the F-actin-dependent arrest of caveolin-1 vesicles in a confined area and their stable anchorage to the plasma membrane. The linear distribution and anchorage of caveolin-1 vesicles are both required for proper caveolin-1 inwards trafficking. De-adhesion-triggered caveolae inward trafficking towards a recycling endosome is impaired in FLNa-depleted HeLa and FLNa-deficient M2-melanoma cells. Inwards trafficking of caveolin-1 requires both the ability of FLNa to bind actin and cycling PKCα-dependent phosphorylation of FLNa on Ser2152 after cell detachment.

Structural and functional evaluation of C. elegans filamins FLN-1 and FLN-2

Filamins are long, flexible, multi-domain proteins composed of an N-terminal actin-binding domain (ABD) followed by multiple immunoglobulin-like repeats (IgFLN). They function to organize and maintain the actin cytoskeleton, to provide scaffolds for signaling components, and to act as mechanical force sensors. In this study, we used transcript sequencing and homology modeling to characterize the gene and protein structures of the C. elegans filamin orthologs fln-1 and fln-2. Our results reveal that C. elegans FLN-1 is well conserved at the sequence level to vertebrate filamins, particularly in the ABD and several key IgFLN repeats. Both FLN-1 and the more divergent FLN-2 colocalize with actin in vivo. FLN-2 is poorly conserved, with at least 23 IgFLN repeats interrupted by large regions that appear to be nematode-specific. Our results indicate that many of the key features of vertebrate filamins are preserved in C. elegans FLN-1 and FLN-2, and suggest the nematode may be a very useful model system for further study of filamin function.

Filamin A protein interacts with human immunodeficiency virus type 1 Gag protein and contributes to productive particle assembly

HIV-1 Gag precursor directs virus particle assembly and release. In a search for Gag-interacting proteins that are involved in late stages of the HIV-1 replication cycle, we performed yeast two-hybrid screening against a human cDNA library and identified the non-muscle actin filament cross-linking protein filamin A as a novel Gag binding partner. The 280-kDa filamin A regulates cortical actin network dynamics and participates in the anchoring of membrane proteins to the actin cytoskeleton. Recent studies have shown that filamin A facilitates HIV-1 cell-to-cell transmission by binding to HIV receptors and coreceptors and regulating their clustering on the target cell surface. Here we report a novel role for filamin A in HIV-1 Gag intracellular trafficking. We demonstrate that filamin A interacts with the capsid domain of HIV-1 Gag and that this interaction is involved in particle release in a productive manner. Disruption of this interaction eliminated Gag localization at the plasma membrane and induced Gag accumulation within internal compartments. Moreover, blocking clathrin-dependent endocytic pathways did not relieve the restriction to particle release induced by filamin A depletion. These results suggest that filamin A is involved in the distinct step of the Gag trafficking pathway. The discovery of the Gag-filamin A interaction may provide a new therapeutic target for the treatment of HIV infection.

Adapter protein SH2B1beta binds filamin A to regulate prolactin-dependent cytoskeletal reorganization and cell motility

Prolactin (PRL) regulates cytoskeletal rearrangement and cell motility. PRL-activated Janus tyrosine kinase 2 (JAK2) phosphorylates the p21-activated serine-threonine kinase (PAK)1 and the Src homology 2 (SH2) domain-containing adapter protein SH2B1β. SH2B1β is an actin-binding protein that cross-links actin filaments, whereas PAK1 regulates the actin cytoskeleton by different mechanisms, including direct phosphorylation of the actin-binding protein filamin A (FLNa). Here, we have used a FLNa-deficient human melanoma cell line (M2) and its derivative line (A7) that stably expresses FLNa to demonstrate that SH2B1β and FLNa are required for maximal PRL-dependent cell ruffling. We have found that in addition to two actin-binding domains, SH2B1β has a FLNa-binding domain (amino acids 200-260) that binds directly to repeats 17-23 of FLNa. The SH2B1β-FLNa interaction participates in PRL-dependent actin rearrangement. We also show that phosphorylation of the three tyrosines of PAK1 by JAK2, as well as the presence of FLNa, play a role in PRL-dependent cell ruffling. Finally, we show that the actin- and FLNa-binding-deficient mutant of SH2B1β (SH2B1β 3Δ) abolished PRL-dependent ruffling and PRL-dependent cell migration when expressed along with PAK1 Y3F (JAK2 tyrosyl-phosphorylation-deficient mutant). Together, these data provide insight into a novel mechanism of PRL-stimulated regulation of the actin cytoskeleton and cell motility via JAK2 signaling through FLNa, PAK1, and SH2B1β. We propose a model for PRL-dependent regulation of the actin cytoskeleton that integrates our findings with previous studies.

The interaction between the mu opioid receptor and filamin A

Our laboratory embarked on research to discover proteins the interaction of which with the mu opioid receptor (MOPr) is required for its function and regulation. We performed yeast two-hybrid screens, using the carboxy tail of the human MOPr as bait and a human brain library. This yielded a number of proteins that seemed to bind to the MOPr C-tail. The one we chose to study in detail was filamin A (FLNA). Evidence was obtained that there was indeed protein-protein binding between the C-tail of MOPr and FLNA. A human melanoma cell line (M2) lacking the gene for FLNA and a control cell line (A7) which differed from M2 only in having been transfected with the gene for FLNA and expressing the FLNA protein were made available to us. We transfected these cell lines with the gene for MOPr and used them in our studies. The absence of FLNA strongly reduced MOPr downregulation as well as desensitization of adenylyl cyclase inhibition and G protein activation. A recent finding, published here for the first time, is that FLNA is required for the activation by mu opioid agonists of the MAP kinase p38. Deletion studies indicated that the MOPr binding site on FLNA is in the 24th repeat, close to its C-terminal. It was further found that FLNA lacking the N-terminal actin binding domain is as capable as full length FLNA to restore cells to control status, suggesting that actin binding is not required. A surprising finding was that upregulation of MOPr by morphine and some agonist analogs occurs in M2 cells lacking FLNA, whereas normal receptor downregulation takes place in A7 cells.

Filamin a binds to CCR2B and regulates its internalization

The chemokine (C-C motif) receptor 2B (CCR2B) is one of the two isoforms of the receptor for monocyte chemoattractant protein-1 (CCL2), the major chemoattractant for monocytes, involved in an array of chronic inflammatory diseases. Employing the yeast two-hybrid system, we identified the actin-binding protein filamin A (FLNa) as a protein that associates with the carboxyl-terminal tail of CCR2B. Co-immunoprecipitation experiments and in vitro pull down assays demonstrated that FLNa binds constitutively to CCR2B. The colocalization of endogenous CCR2B and filamin A was detected at the surface and in internalized vesicles of THP-1 cells. In addition, CCR2B and FLNa were colocalized in lamellipodia structures of CCR2B-expressing A7 cells. Expression of the receptor in filamin-deficient M2 cells together with siRNA experiments knocking down FLNa in HEK293 cells, demonstrated that lack of FLNa delays the internalization of the receptor. Furthermore, depletion of FLNa in THP-1 monocytes by RNA interference reduced the migration of cells in response to MCP-1. Therefore, FLNa emerges as an important protein for controlling the internalization and spatial localization of the CCR2B receptor in different dynamic membrane structures.

Binding of pro-prion to filamin A: by design or an unfortunate blunder

Over the last decades, cancer research has focused on tumor suppressor genes and oncogenes. Genes in other cellular pathways has received less attention. Between 0.5% to 1% of the mammalian genome encodes for proteins that are tethered on the cell membrane via a glycosylphosphatidylinositol (GPI)-anchor. The GPI modification pathway is complex and not completely understood. Prion (PrP), a GPI-anchored protein, is infamous for being the only normal protein that when misfolded can cause and transmit a deadly disease. Though widely expressed and highly conserved, little is known about the functions of PrP. Pancreatic cancer and melanoma cell lines express PrP. However, in these cell lines the PrP exists as a pro-PrP as defined by retaining its GPI anchor peptide signal sequence (GPI-PSS). Unexpectedly, the GPI-PSS of PrP has a filamin A (FLNA) binding motif and binds FLNA. FLNA is a cytolinker protein, and an integrator of cell mechanics and signaling. Binding of pro-PrP to FLNA disrupts the normal FLNA functions. Although normal pancreatic ductal cells lack PrP, about 40% of patients with pancreatic ductal cell adenocarcinoma express PrP in their cancers. These patients have significantly shorter survival time compared with patients whose cancers lack PrP. Pro-PrP is also detected in melanoma in situ but is undetectable in normal melanocyte, and invasive melanoma expresses more pro-PrP. In this review, we will discuss the underlying mechanisms by which binding of pro-PrP to FLNA disrupts normal cellular physiology and contributes to tumorigenesis, and the potential mechanisms that cause the accumulation of pro-PrP in cancer cells.

Regulation of opioid receptors by endocytic membrane traffic: mechanisms and translational implications

Opioid neuropeptide receptors mediate diverse physiological functions and are important targets for both therapeutic and abused drugs. Opioid receptors are highly regulated in intact cells, and there is reason to believe that this regulation controls the clinical effects of opioid drugs. The present review will discuss some of this evidence, focusing specifically on the regulation of opioid receptors by endocytic membrane trafficking mechanisms. First, some basic principles of regulated endocytosis will be reviewed, and the principle of 'molecular sorting' as a means to determine the functional consequences of endocytosis will be introduced, Most of this information has been derived from studies of simplified cell models. Second, present knowledge about the operation of these mechanisms in physiologically relevant CNS neurons will be discussed, focusing on studies of neurons cultured from rodent brain. Third, recent insight into the effects of endocytic trafficking on opioid regulation in vivo will be considered, focusing on results from studies of transgenic mouse models. Much remains to be learned at these pre-clinical levels, and effects of endocytosis on opioid actions in humans remain completely unexplored. Two particular insights, which have emerged from pre-clinical studies, will be proposed for translational consideration.

Interaction of the mu-opioid receptor with GPR177 (Wntless) inhibits Wnt secretion: potential implications for opioid dependence

BACKGROUND

Opioid agonist drugs produce analgesia. However, long-term exposure to opioid agonists may lead to opioid dependence. The analgesic and addictive properties of opioid agonist drugs are mediated primarily via the mu-opioid receptor (MOR). Opioid agonists appear to alter neuronal morphology in key brain regions implicated in the development of opioid dependence. However, the precise role of the MOR in the development of these neuronal alterations remains elusive. We hypothesize that identifying and characterizing novel MOR interacting proteins (MORIPs) may help to elucidate the underlying mechanisms involved in the development of opioid dependence.

RESULTS

GPR177, the mammalian ortholog of Drosophila Wntless/Evi/Sprinter, was identified as a MORIP in a modified split ubiquitin yeast two-hybrid screen. GPR177 is an evolutionarily conserved protein that plays a critical role in mediating Wnt protein secretion from Wnt producing cells. The MOR/GPR177 interaction was validated in pulldown, coimmunoprecipitation, and colocalization studies using mammalian tissue culture cells. The interaction was also observed in rodent brain, where MOR and GPR177 were coexpressed in close spatial proximity within striatal neurons. At the cellular level, morphine treatment caused a shift in the distribution of GPR177 from cytosol to the cell surface, leading to enhanced MOR/GPR177 complex formation at the cell periphery and the inhibition of Wnt protein secretion.

CONCLUSIONS

It is known that chronic morphine treatment decreases dendritic arborization and hippocampal neurogenesis, and Wnt proteins are essential for these processes. We therefore propose that the morphine-mediated MOR/GPR177 interaction may result in decreased Wnt secretion in the CNS, resulting in atrophy of dendritic arbors and decreased neurogenesis. Our results demonstrate a previously unrecognized role for GPR177 in regulating cellular response to opioid drugs.

Regulation of mu opioid receptor internalization by the scaffold protein RanBPM

Mu opioid receptors (MOP) are transducers of the pharmacological effects of many opioid drugs, including analgesia and tolerance/dependence. Previously, we observed increased MOP signaling during postnatal development that was not associated with increased MOP or G protein expression. A yeast two-hybrid screen of a human brain cDNA library using the MOP C-terminus as bait identified RanBPM as a potential MOP-interacting protein. RanBPM has been recognized as a multi-functional scaffold protein that interacts with a variety of signaling receptors/proteins. Co-immunoprecipitation studies in HEK293 cells indicated that RanBPM constitutively associates with MOP. Functionally, RanBPM had no effect on MOP-mediated inhibition of adenylyl cyclase, yet reduced agonist-induced endocytosis of MOP. Mechanistically, RanBPM interfered with beta arrestin2-GFP translocation stimulated by MOP but not alpha(1B)-adrenergic receptor activation, indicating selectivity of action. Our findings suggest that RanBPM is a novel MOP-interacting protein that negatively regulates receptor internalization without altering MOP signaling through adenylyl cyclase.

Filamin A regulates caveolae internalization and trafficking in endothelial cells

Transcytosis via caveolae is critical for maintaining vascular homeostasis by regulating the tissue delivery of macromolecules, hormones, and lipids. In the present study, we test the hypothesis that interactions between F-actin cross-linking protein filamin A and caveolin-1 facilitate the internalization and trafficking of caveolae. Small interfering RNA-mediated knockdown of filamin A, but not filamin B, reduced the uptake and transcytosis of albumin by approximately 35 and 60%, respectively, without altering the actin cytoskeletal structure or cell-cell adherens junctions. Mobility of both intracellular caveolin-1-green fluorescent protein (GFP)-labeled vesicles measured by fluorescence recovery after photobleaching and membrane-associated vesicles measured by total internal reflection-fluorescence microscopy was decreased in cells with reduced filamin A expression. In addition, in melanoma cells that lack filamin A (M2 cells), the majority of caveolin-1-GFP was localized on the plasma membrane, whereas in cells in which filamin A expression was reconstituted (A7 cells and M2 cells transfected with filamin A-RFP), caveolin-1-GFP was concentrated in intracellular vesicles. Filamin A association with caveolin-1 in endothelial cells was confirmed by cofractionation of these proteins in density gradients, as well as by coimmunoprecipitation. Moreover, this interaction was enhanced by Src activation, associated with increased caveolin-1 phosphorylation, and blocked by Src inhibition. Taken together, these data suggest that filamin A association with caveolin-1 promotes caveolae-mediated transport by regulating vesicle internalization, clustering, and trafficking.

The Rac GTPase-activating bacterial protein toxin CNF1 induces analgesia up-regulating mu-opioid receptors

Cytotoxic Necrotizing Factor 1 (CNF1) is a protein toxin from Escherichia coli that constitutively activates the Rho, Rac and Cdc42 GTPases. These regulatory proteins oscillate between a cytosolic GDP-bound inactive form and a membrane-linked GTP-bound active form, orchestrating the actin cytoskeleton assembly and dynamics. We herein describe, for the first time, the ability of CNF1 to potently counteract the formalin-induced inflammatory pain in mice. The analgesic response due to CNF1 requires both the sustained activation of the Rac GTPase, with consequent cerebral actin cytoskeleton remodeling, and the up-regulation of the mu-opioid receptors (MORs), the most important receptors controlling pain perception. The crucial role of Rac is proved by the lack of analgesic activity in mice challenged with a recombinant CNF1, in which the enzymatic activity was abolished by substituting serine with cysteine at position 866. The importance of MORs is proved by the inability of CNF1 to induce any analgesic effect in MORs knockout mice and by the ability of naloxone to antagonize the analgesic effects. Furthermore, it is worth noting that the analgesic effect in mice occurs after both peripheral and central administration of CNF1. Hence, taken altogether, our findings provide new insights into the comprehension of intracellular mechanisms involved in pain modulation, and indicate this bacterial protein toxin as a novel tool in the field of pain control. Conceivably, this might pave the way for new therapeutic strategies.

Membrane functional organisation and dynamic of mu-opioid receptors

The activation and signalling activity of the membrane mu-opioid receptor (MOP-R) involve interactions among the receptor, G-proteins, effectors and many other membrane or cytosolic proteins. Decades of investigation have led to identification of the main biochemical processes, but the mechanisms governing the successive protein-protein interactions have yet to be established. We will need to unravel the dynamic membrane organisation of this complex and multifaceted molecular machinery if we are to understand these mechanisms. Here, we review and discuss advances in our understanding of the signalling mechanism of MOP-R resulting from biochemical or biophysical studies of the organisation of this receptor in the plasma membrane.

Novel anti-filamin-A antibody detects a secreted variant of filamin-A in plasma from patients with breast carcinoma and high-grade astrocytoma

Identification of tumor-derived proteins in the circulation may allow for early detection of cancer and evaluation of therapeutic responses. To identify circulating tumor-derived proteins, mice were immunized with concentrated culture medium conditioned by human breast cancer cells. Antibodies generated by hybridomas were screened against conditioned media from both normal epithelial cells and tumor cells. Antibody selectively reacting with tumor cell-conditioned media was further characterized. This led to the development of a monoclonal antibody (Alper-p280) that reacts with a newly identified 280-kDa secreted variant of human filamin-A. Circulating filamin-A was detected in patient plasma samples using Alper-p280 in an ELISA assay. Human plasma samples from 134 patients with brain, breast, or ovarian cancer, 15 patients with active arthritis, and 76 healthy controls were analyzed. Filamin-A protein levels in human cell lines and tissues were analyzed by western blotting, immunohistochemistry, and electron and confocal microscopy. Circulating filamin-A was detected in the plasma of 109 of 143 patients with breast cancer and primary brain tumors. Plasma levels of filamin-A showed 89.5% sensitivity (95% confidence interval [CI] = 0.67% to 0.99%) and 97.8% specificity (95% CI = 0.88% to 0.99%) for glioblastoma at a cut-off of 21.0 ng/mL. Plasma levels of filamin-A (>36.0 ng/mL) had 96.7% sensitivity (95% CI = 0.80% to 0.99%) and 67.8% specificity (95% CI = 0.54% to 0.79%) for metastatic breast cancer. Filamin-A levels were increased in malignant breast or brain tissues, but not in normal control tissues. Filamin-A localized to lysosomes in MDA.MB.231 breast cancer cells, but not in normal human mammary epithelial cells, suggesting that filamin-A may undergo cancer-specific processing. Plasma filamin-A appears to be a specific and sensitive marker for patients with high-grade astrocytoma or metastatic breast cancer. Additional novel cancer biomarkers have been identified and are being developed alongside Alper-p280 for use in diagnosis of breast carcinoma and high-grade astrocytoma, and for use in the evaluation of therapeutic responses.

Filamin A modulates kinase activation and intracellular trafficking of epidermal growth factor receptors in human melanoma cells

The actin-binding protein filamin A (FLNa) affects the intracellular trafficking of various classes of receptors and has a potential role in oncogenesis. However, it is unclear whether FLNa regulates the signaling capacity and/or down-regulation of the activated epidermal growth factor receptor (EGFR). Here it is shown that partial knockdown of FLNa gene expression blocked ligand-induced EGFR responses in metastatic human melanomas. To gain greater insights into the role of FLNa in EGFR activation and intracellular sorting, we used M2 melanoma cells that lack endogenous FLNa and a subclone in which human FLNa cDNA has been stably reintroduced (M2A7 cells). Both tyrosine phosphorylation and ubiquitination of EGFR were significantly lower in epidermal growth factor (EGF)-stimulated M2 cells when compared with M2A7 cells. Moreover, the lack of FLNa interfered with EGFR interaction with the ubiquitin ligase c-Cbl. M2 cells exhibited marked resistance to EGF-induced receptor degradation, which was very active in M2A7 cells. Despite comparable rates of EGF-mediated receptor endocytosis, internalized EGFR colocalized with the lysosomal marker lysosome-associated membrane protein-1 in M2A7 cells but not M2 cells, in which EGFR was found to be sequestered in large vesicles and subsequently accumulated in punctated perinuclear structures after EGF stimulation. These results suggest the requirement of FLNa for efficient EGFR kinase activation and the sorting of endocytosed receptors into the degradation pathway.

Naloxone's pentapeptide binding site on filamin A blocks Mu opioid receptor-Gs coupling and CREB activation of acute morphine

Chronic morphine causes the mu opioid receptor (MOR) to switch its coupling from Gi/o to Gs, resulting in excitatory signaling via both Gαs and its Gβγ dimer. Ultra-low-dose naloxone (NLX) prevents this switch and attenuates opioid tolerance and dependence. This protective effect is mediated via a high-affinity interaction of NLX to a pentapeptide region in c-terminal filamin A (FLNA), a scaffolding protein interacting with MOR. In organotypic striatal slice cultures, we now show that acute morphine induces a dose-dependent Go-to-Gs coupling switch at 5 and 15 min that resolves by 1 hr. The acute Gs coupling induced by 100 µM morphine was completely prevented by co-treatment with 100 pM NLX, (+)NLX, or naltrexone (NTX), or their pentapeptide binding site (FLNA_2561–2565), which we show can act as a decoy for MOR or bind to FLNA itself. All of these co-treatments presumably prevent the MOR–FLNA interaction. Since ultra-low-dose NTX also attenuates the addictive properties of opioids, we assessed striatal cAMP production and CREB phosphorylation at S¹³³. Correlating with the Gs coupling, acute morphine induced elevated cAMP levels and a several-fold increase in pS¹³³CREB that were also completely blocked by NLX, NTX or the FLNA pentapeptide. We propose that acute, robust stimulation of MOR causes an interaction with FLNA that allows an initially transient MOR–Gs coupling, which recovers with receptor recycling but persists when MOR stimulation is repeated or prolonged. The complete prevention of this acute, morphine-induced MOR–Gs coupling by 100 pM NLX/NTX or 10 µM pentapeptide segment of FLNA further elucidates both MOR signaling and the mechanism of action of ultra-low-dose NLX or NTX in attenuating opioid tolerance, dependence and addictive potential.