The beta2-adrenergic receptor interacts with the Na+/H+-exchanger regulatory factor to control Na+/H+ exchange

Stimulation of beta 2-adrenergic receptor increases cystic fibrosis transmembrane conductance regulator expression in human airway epithelial cells through a cAMP/protein kinase A-independent pathway

PSD-95/Dlg-A/ZO-1 (PDZ) domains play an essential role in determining cell polarity. The Na(+)/H(+) exchanger regulatory factor (NHERF), also known as EBP50, contains two PDZ domains that mediate the assembly of transmembrane and cytosolic proteins into functional signal transduction complexes. Moreover, it has been shown that cystic fibrosis transmembrane conductance regulator (CFTR) and beta(2)-adrenergic receptor (beta(2)AR) bind equally well to the PDZ1 domain of EBP50. We hypothesized that beta(2)AR activation may regulate CFTR protein expression. To verify this, we evaluated the effects of a pharmacologically relevant concentration of salmeterol (2.10(-7) m), a long acting beta(2)AR agonist, on CFTR expression in primary human airway epithelial cells (HAEC). beta(2)AR stimulation induced a time-dependent increase in apical CFTR protein expression, with a maximal response reached after treatment for 24 h. This effect was post-transcriptional, dependent upon the beta(2)AR agonist binding to beta(2)AR and independent of the known beta(2)AR agonist-mediated cAMP/PKA pathway. We demonstrated by immunohistochemistry that CFTR, beta(2)AR, and EBP50 localize to the apical membrane of HAEC. Analyses of anti-EBP50 protein immunoprecipitate showed that salmeterol induced an increase in the amount of CFTR that binds to EBP50. These data suggest that beta(2)AR activation regulates the association of CFTR with EBP50 in polarized HAEC.

The venerable inveterate invertebrate TRP channels

The transient receptor potential (TRP) superfamily is subdivided into four main classes of cation channels, TRPC, TRPV, TRPM and TRPN, each of which includes members in worms, flies, mice and humans. While the biophysical features of many of the mammalian channels have been described, relatively little is known concerning the biological roles of these channels. Forward genetic screens in Drosophila melanogaster and Caenorhabditis elegans have led to the identification of the founding members of each of these four subfamilies. Moreover, phenotypic analyses of invertebrate mutants have contributed greatly to our understanding of the roles of TRP proteins. A recurring theme is that many of these proteins function in sensory signaling processes ranging from vision to olfaction, osmosensation, light touch, social feeding, and temperature- and mechanically-induced nociception. In addition, at least one invertebrate TRP protein is required for cell division. As many of these functions may be conserved among the mammalian TRPs, the invertebrate TRPs offer valuable genetic handles for characterizing the functions of these cation channels in vivo.

PDZ domain proteins: plug and play!

Protein-protein interactions are key elements in building functional protein complexes. Among the plethora of domains identified during the last 10 years, PDZ domains are one of the most commonly found protein-protein interaction domains in organisms from bacteria to humans. Although they may be the sole protein interaction domain within a cytoplasmic protein, they are most often found in combination with other protein interaction domains (for instance, SH3, PTB, WW) participating in complexes that facilitate signaling or determine the localization of receptors. Diversity of PDZ-containing protein function is provided by the large number of PDZ proteins that Mother Nature has distributed in the genome and implicates this protein family in the wiring of a huge number of molecules in molecular networks from the plasma membrane to the nucleus. Although at first sight their binding specificity appeared rather monotonous, involving only binding to the carboxyl-terminus of various proteins, it is now recognized that PDZ domains interact with greater versatility through PDZ-PDZ domain interaction; they bind to internal peptide sequences and even to lipids. Furthermore, PDZ domain-mediated interactions can sometimes be modulated in a dynamic way through target phosphorylation. In this review, we attempt to describe the structural basis of PDZ domain recognition and to give some functional insights into their role in the scaffolding of protein complexes implicated in normal and pathological biological processes.

Role of the PDZ scaffolding protein in tubule cells in maintenance of polarised function

Polarized tubule epithelial cell functions are dependent on correct delivery of effector proteins to the target apical or basolateral plasma membrane and associated cortical cytoskeleton. PDZ (Postsynaptic density protein 95/Drosophila Disks large/Zona occludens-1) domain-containing proteins have been identified as playing a critical role in membrane trafficking and sorting of ion transporters, receptors and other signalling proteins. These scaffolding proteins coordinate the assembly of functional plasma membrane multiprotein complexes, through PDZ domain binding to a consensus amino acid motif within the carboxyl-terminus of target proteins. The organization of these proteins into submembranous complexes may facilitate downstream signalling. Although several epithelial PDZ proteins that bind to a number of important mammalian proteins have been isolated, in many cases the significance of these interactions is unclear. However, the epithelial PDZ domain-containing Na(+)/H(+) exchanger regulatory factor tethers the Na(+)/H(+) exchanger and cystic fibrosis transmembrane regulator Cl(-) channel within an apical plasma membrane signalling complex, and has been shown to regulate the activity of these proteins. This article reviews the current evidence that supports a central role for the PDZ protein in the regulation of polarized tubule cell functions, such as vectorial solute transport.

Allosteric regulation and spatial distribution of kainate receptors bound to ancillary proteins

A diverse range of accessory proteins regulates the behaviour of most ligand- and voltage-gated ion channels. For glutamate receptor 6 (GluR6) kainate receptors, two unrelated proteins, concanavalin-A (Con-A) and postsynaptic density protein 95 (PSD-95), bind to extra- and intracellular domains, respectively, but are reported to exert similar effects on GluR6 desensitization behaviour. We have tested the hypothesis that distinct allosteric binding sites control GluR6 receptors via a common transduction pathway. Rapid agonist application to excised patches revealed that neither Con-A nor PSD-95 affect the onset of desensitization. The rate of desensitization elicited by 10 mM L-glutamate was similar in control (taufast = 5.5 +/- 0.4 ms), Con-A-treated patches (taufast = 6.1 +/- 0.5 ms) and patches containing PSD-95 and GluR6 receptors (taufast = 4.7 +/- 0.6 ms). Likewise, the time course of recovery from GluR6 desensitization was similar in both control and Con-A conditions, whereas PSD-95 accelerated recovery almost twofold. Peak and steady-state (SS) dose-response relationships to glutamate were unchanged by lectin treatment (e.g. control, EC50(SS) = 31 +/- 28 microM vs Con-A, EC50(SS) = 45 +/- 9 microM, n = 6), suggesting that Con-A does not convert non-conducting channels with high agonist affinity into an open conformation. Instead, we demonstrate that the effects of Con-A on macroscopic responses reflect a shift in the relative contribution of different open states of the channel. In contrast, the effect of PSD-95 on recovery behaviour suggests that the association between kainate receptors and cytoskeletal proteins regulates signalling at glutamatergic synapses. Our results show that Con-A and PSD-95 regulate kainate receptors via distinct allosteric mechanisms targeting selective molecular steps in the transduction pathway.

The COOH termini of NBC3 and the 56-kDa H+-ATPase subunit are PDZ motifs involved in their interaction

The electroneutral sodium bicarbonate cotransporter 3 (NBC3) coimmunoprecipitates from renal lysates with the vacuolar H(+)-ATPase. In renal type A and B intercalated cells, NBC3 colocalizes with the vacuolar H(+)-ATPase. The involvement of the COOH termini of NBC3 and the 56-kDa subunit of the proton pump in the interaction of these proteins was investigated. The intact and modified COOH termini of NBC3 and the 56-kDa subunit of the proton pump were synthesized, coupled to Sepharose beads, and used to pull down kidney membrane proteins. Both the 56- and the 70-kDa subunits of the proton pump, as well as a PDZ domain containing protein Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1), were bound to the intact 18 amino acid NBC3 COOH terminus. A peptide truncated by five COOH-terminal amino acids did not bind these proteins. Replacement of the COOH-terminal leucine with glycine blocked binding of both the proton pump subunits but did not affect binding of NHERF-1. The 18 amino acid COOH terminus of the 56-kDa subunit of the proton pump bound NHERF-1 and NBC3, but the truncated and modified peptide did not. A complex of NBC3, the 56-kDa subunit of the proton pump, and NHERF-1 was identified in rat kidney. The data indicate that the COOH termini of NBC3 and the 56-kDa subunit of the vacuolar proton pump are PDZ-interacting motifs that are necessary for the interaction of these proteins. NHERF-1 is involved in the interaction of NBC3 and the vacuolar proton pump.

Antidepressant-induced switch of beta 1-adrenoceptor trafficking as a mechanism for drug action

Reduction in surface beta(1)-adrenoceptor (beta1AR) density is thought to play a critical role in mediating the therapeutic long term effects of antidepressants. Since antidepressants are neither agonists nor antagonists for G protein-coupled receptors, receptor density must be regulated through processes independent of direct receptor activation. Endocytosis and recycling of the beta1AR fused to green fluorescent protein at its carboxyl-terminus (beta1AR-GFP) were analyzed by confocal fluorescence microscopy of live cells and complementary ligand binding studies. In stably transfected C6 glioblastoma cells, beta1AR-GFP displayed identical ligand-binding isotherms and adenylyl cyclase activation as native beta1AR. Upon exposure to isoproterenol, a fraction of beta1AR-GFP (10-15%) internalized rapidly and colocalized with endocytosed transferrin receptors in an early endosomal compartment in the perinuclear region. Chronic treatment with the tricyclic antidepressant desipramine (DMI) did not affect internalization characteristics of beta1AR-GFP when challenged with isoproterenol. However, internalized receptors were not able to recycle back to the cell surface in DMI-treated cells, whereas recycling of transferrin receptors was not affected. Endocytosed receptors were absent from structures that stained with fluorescently labeled dextran, and inhibition of lysosomal protease activity did not restore receptor recycling, indicating that beta1AR-GFP did not immediately enter the lysosomal compartment. The data suggest a new mode of drug action causing a "switch" of receptor fate from a fast recycling pathway to a slowly exchanging perinuclear compartment. Antidepressant-induced reduction of receptor surface expression may thus be caused by modulation of receptor trafficking routes.

A macromolecular complex of beta 2 adrenergic receptor, CFTR, and ezrin/radixin/moesin-binding phosphoprotein 50 is regulated by PKA

It has been demonstrated previously that both the cystic fibrosis transmembrane conductance regulator (CFTR) and beta(2) adrenergic receptor (beta(2)AR) can bind ezrinradixinmoesin-binding phosphoprotein 50 (EBP50, also referred to as NHERF) through their PDZ motifs. Here, we show that beta(2) is the major adrenergic receptor isoform expressed in airway epithelia and that it colocalizes with CFTR at the apical membrane. beta(2)AR stimulation increases CFTR activity, in airway epithelial cells, that is glybenclamide sensitive. Deletion of the PDZ motif from CFTR uncouples the channel from the receptor both physically and functionally. This uncoupling is specific to the beta(2)AR receptor and does not affect CFTR coupling to other receptors (e.g., adenosine receptor pathway). Biochemical studies demonstrate the existence of a macromolecular complex involving CFTR-EBP50-beta(2)AR through PDZ-based interactions. Assembly of the complex is regulated by PKA-dependent phosphorylation. Deleting the regulatory domain of CFTR abolishes PKA regulation of complex assembly. This report summarizes a macromolecular signaling complex involving CFTR, the implications of which may be relevant to CFTR-dysfunction diseases.

Troglitazone reduces activity of the Na+/H+ exchanger in fructose-fed borderline hypertensive rats

Activation of the Na+/H+ exchanger (NHE) is known to be related to elevated blood pressure in hyperinsulinemia. We previously demonstrated that a fructose-enriched diet induced hyperinsulinemia and hypertriglyceridemia, elevated NHE activity, increased intracellular calcium concentrations ([Ca2+]i), and increased blood pressure in borderline hypertensive rats (BHR). This study examines whether pharmacologically reducing plasma triglyceride or insulin concentrations lowers blood pressure and reduces NHE activity in fructose-fed BHR. Eicosapentaenoic acid (EPA), bezafibrate (BEZ), and troglitazone (TRO) were administered to treat hypertriglyceridemia and/or hyperinsulinemia. Rats were fed a 60% fructose diet or a control diet for 4 weeks, followed by a diet with either vehicle, EPA, BEZ, or TRO for 4 weeks. Intracellular pH (pHi) was measured in platelets by fluorescent dye. Platelet NHE activity was evaluated by the recovery of pHi following addition of sodium propionate (Vmax). [Ca2+]i in platelets were measured fluorometrically. In fructose-fed rats, EPA prevented further increase in blood pressure, and reduced triglyceride concentration and [Ca2+]i without affecting Vmax or plasma insulin concentrations. BEZ reduced triglyceride concentrations without affecting blood pressure, Vmax, [Ca2+]i, or insulin concentrations. TRO prevented an increase in blood pressure, and reduced Vmax, [Ca2+]i, and insulin, but not triglycerides. Plasma insulin and Vmax were positively correlated. In conclusion, improvement of hyperinsulinemia can decrease NHE activity and blood pressure in fructose-fed BHR.

G Protein-Coupled Receptors in Invertebrates: A State of the Art

G protein-coupled receptors (GPCRs) constitute one of the largest and most ancient superfamilies of membrane-spanning proteins. We focus on neuropeptide GPCRs, in particular on those of invertebrates. In general, such receptors mediate the responses of signaling molecules that constitute the highest hierarchical position in the regulation of physiological processes. Until recently, only a few of these receptors were identified in invertebrates. However, the availability of a plethora of genomic information has boosted the discovery of novel members in several invertebrate species, such as Drosophila, in which 18 neuropeptide GPCRs have been characterized. The finalization of genomic projects in other invertebrates will lead to a similar expansion of GPCR understanding. Many new insights regarding neuropeptide regulation have followed from the discovery of their cognate receptors. Furthermore, information on GPCR signaling is still fragmentary and the elucidation of these pathways in model insects such as Drosophila will lead to further insights in other species, including mammals. In this review we present the current status of what is known about invertebrate GPCRs, discuss some novel perceptions that follow from the identified members, and, finally, present some future prospects.

Concerted roles of SGK1 and the Na+/H+ exchanger regulatory factor 2 (NHERF2) in regulation of NHE3

Na+/H+ exchanger regulatory factors, NHERF1 and NHERF2, are structurally related proteins and highly expressed in epithelial cells. These proteins are initially identified as accessory proteins in the regulation of Na+/H+ exchanger isoform 3, NHE3. In addition to regulation of NHE3, recent studies demonstrate the importance of NHERF1 and NHERF2 in recycling and localization of membrane receptors, ion channels and transporters. Recent studies show that serum- and glucocorticoid-induced kinase 1 (SGK1) specifically interacts with NHERF2 but not with NHERF1, adding to the growing number of differences between the two proteins. The association of SGK1 with NHERF2 is necessary for stimulation of NHE3 activity by glucocorticoids. In addition, SGK1 together with NHERF2 stimulates the K+ channel ROMK1, suggesting a broader role of SGK1 in regulation of ion transport.

A proteomic approach based on peptide affinity chromatography, 2-dimensional electrophoresis and mass spectrometry to identify multiprotein complexes interacting with membrane-bound receptors

There is accumulating evidence that membrane-bound receptors interact with many intracellular proteins. Multiprotein complexes associated with ionotropic receptors have been extensively characterized, but the identification of proteins interacting with G protein-coupled receptors (GPCRs) has so far only been achieved in a piecemeal fashion, focusing on one or two protein species. We describe a method based on peptide affinity chromatography, two-dimensional electrophoresis, mass spectrometry and immunoblotting to identify the components of multiprotein complexes interacting directly or indirectly with intracellular domains of GPCRs or, more generally, any other membrane-bound receptor. Using this global approach, we have characterized multiprotein complexes that bind to the carboxy-terminal tail of the 5-hydroxytryptamine type 2C receptor and are important for its subcellular localization in CNS cells (Bécamel et al., EMBO J., 21(10): 2332, 2002).

Beta-adrenergic receptor activation inhibits keratinocyte migration via a cyclic adenosine monophosphate-independent mechanism

There is increasing evidence that G-protein-coupled receptors cross-talk with growth factor receptor-mediated signal transduction in a variety of cell types. We have investigated mechanisms by which the activation of beta-adrenergic receptors, classically GTP-binding proteins coupled receptors, influence the migration of cultured human keratinocytes. We found that iso-proterenol, a beta-adrenergic receptor-selective agonist, inhibited cell migration stimulated by either epidermal growth factor, or extracellular Ca2+ in a concentration-dependent manner. This was prevented by pretreatment of the cells with the beta-adrenergic receptor-selective antagonist timolol. Interestingly, isoproterenol, at a concentration of 1 nm, did not measurably increase intracellular cyclic adenosine monophosphate concentrations yet inhibited cell migration by 50%. To test further if isoproterenol's actions were mediated via activation of adenylyl cyclase, two inhibitors of its activity, 2'5'-dideoxyadenosine and SQ22536, were used. Both compounds significantly diminished iso-proterenol-induced increases in intracellular cyclic adenosine monophosphate concentrations but did not attenuate isoproterenol-induced inhibition of cell migration. Also, forskolin (1 microm) markedly increased intracellular cyclic adenosine monophosphate concentrations but did not significantly inhibit cell migration. As mitogen-activated protein kinases are known to signal growth factor-stimulated cell migration, we examined whether beta-adrenergic receptor-mediated inhibition of keratinocyte migration might occur via inactivation of mitogen-activated protein kinases. We found that isoproterenol inhibited phosphorylation of extracellular signal-regulated kinase mitogen-activated protein kinase in a concentration-dependent manner but had no effect on the phosphorylation of the stress mitogen-activated protein kinases c-jun N-terminal kinase and stress-activated protein kinase-2. Neither forskolin nor a membrane permeable cyclic adenosine monophosphate analog inhibited phosphorylation of any of these mitogen-activated protein kinases. These findings suggest that beta-adrenergic receptor-induced inhibition of keratinocyte migration is mediated through inhibition of the extracellular signal-regulated kinase mitogen-activated protein kinase signaling in a cyclic adenosine monophosphate-independent manner.

Novel beta2-adrenergic receptor signaling pathways

The beta(2)-adrenergic receptor (beta(2)AR) is perhaps the most thoroughly investigated of all G-protein-coupled receptors. Although the classical pathway of beta(2)AR signaling involves agonist-promoted binding of the receptor to the heterotrimeric guanosine triphosphate-binding protein G(s), activation of adenylyl cyclase, and production of cyclic adenosine monophosphate (cAMP), current evidence suggests that beta(2)AR signaling is regulated by interaction with multiple proteins. These interactions fall into 3 major groups: guanosine triphosphate-binding proteins such as G(s) and G(i); protein kinases such as the cAMP-dependent protein kinase, protein kinase C, G-protein-coupled receptor kinases, and tyrosine kinases; and adaptor proteins such as arrestins, A-kinase anchoring proteins, and the Na(+)/H(+)-exchanger regulatory factor. This review discusses these various interactions with particular emphasis on their role in regulating beta(2)AR signaling and trafficking.

Update on current concepts of the molecular basis of beta2-adrenergic receptor signaling

The proposed manner by which beta(2)-adrenergic receptors signal has dramatically changed from earlier concepts that centered on a lock-and-key mechanism in which the receptor acts as a simple switch. We now know that beta(2)-adrenergic receptors spontaneously toggle to an activated state (R*) and that the equilibrium between R (the inactive state) and R* can be altered by ligands. In addition, the R* conformation is likely to consist of multiple subspecies that may favor certain signaling pathways or regulatory events. Changes in agonist structure alter the abundance of certain subspecies of R*. Indeed, multifunctional coupling is common with many G-protein-coupled receptors and can be modulated pharmacologically to attain specific outcomes. In addition to providing the basis for development of new beta-agonists for unique signaling, these properties can be extended such that beta(2)-adrenergic receptors, or highly modified "designer" receptors, can be used for gene therapy with highly specific effects.

Direct binding of the beta1 adrenergic receptor to the cyclic AMP-dependent guanine nucleotide exchange factor CNrasGEF leads to Ras activation

G-protein-coupled receptors (GPCRs) can indirectly activate Ras primarily through the betagamma subunits of G proteins, which recruit c-Src, phosphatidylinositol 3-kinase, and Grb2-SOS. However, a direct interaction between a Ras activator (guanine nucleotide exchange factor [GEF]) and GPCRs that leads to Ras activation has never been demonstrated. We report here a novel mechanism for a direct GPCR-mediated Ras activation. The beta1 adrenergic receptor (beta1-AR) binds to the PDZ domain of the cyclic AMP (cAMP)-dependent Ras exchange factor, CNrasGEF, via its C-terminal SkV motif. In cells heterologously expressing beta1-AR and CNrasGEF, Ras is activated by the beta1-AR agonist isoproterenol, and this activation is abolished in beta1-AR mutants that cannot bind CNrasGEF or in CNrasGEF mutants lacking the catalytic CDC25 domain or cAMP-binding domain. Moreover, the activation is transduced via Gsalpha and not via Gbetagamma. In contrast to beta1-AR, the beta2-AR neither binds CNrasGEF nor activates Ras via CNrasGEF after agonist stimulation. These results suggest a model whereby the physical interaction between the beta1-AR and CNrasGEF facilitates the transduction of Gsalpha-induced cAMP signal into the activation of Ras. The present study provides the first demonstration of direct physical association between a Ras activator and a GPCR, leading to agonist-induced Ras activation

Beta(2)-adrenergic receptor lacking the cyclic AMP-dependent protein kinase consensus sites fully activates extracellular signal-regulated kinase 1/2 in human embryonic kidney 293 cells: lack of evidence for G(s)/G(i) switching

Stimulation of the beta(2)-adrenergic receptor (beta(2)AR) in human embryonic kidney (HEK) 293 cells causes a transient activation of Extracellular Signal-Regulated Kinase (ERK) 1/2. One of the mechanisms proposed for this activation is a PKA-mediated phosphorylation of the beta(2)AR that switches receptor coupling from G(s) to G(i) and triggers internalization of the receptor. To examine these phenomena, we characterized agonist activation of ERK1/2 in HEK293 cells by the endogenous beta(2)AR and in HEK293 cells stably overexpressing either the wild-type beta(2)AR or a substitution mutant beta(2)AR (PKA(-)) that lacks the cyclic AMP-dependent protein kinase (PKA) consensus phosphorylation sites (S261A, S262A and S345A, S346A). As the baseline, we established that epinephrine stimulation of the endogenous beta(2)AR in HEK293 cells (20-30 fmol/mg) caused a rapid and transient activation of ERK1/2 with an EC(50) of 5 to 6 nM. In contrast, the potency of epinephrine stimulation of ERK1/2 in cells stably overexpressing WTbeta(2)AR and PKA(-) (2-4 pmol of beta(2)AR/mg) was increased by over 100-fold relative to HEK293 cells, the EC(50) values being 20 to 60 pM. The nearly identical 100-fold shift in EC(50) for ERK1/2 activation in the PKA(-) and WTbeta(2)AR relative to that in the HEK293 showed that the PKA(-) are fully capable of activating ERK1/2. We also found maximal activation of ERK1/2 in the overexpressing cell lines at concentrations of epinephrine that cause no internalization (i.e., the EC(50) for internalization was 75 nM). Pertussis toxin pretreatment caused only a weak inhibition of epinephrine activation of ERK1/2 in the HEK293 (7-16%) and no inhibition in the PKA(-) cells. Finally we found that the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (10 microM) caused a >90% inhibition of epinephrine or forskolin activation of ERK1/2 in both cell lines. Our results indicate that the dominant mechanism of beta(2)AR activation of ERK1/2 does not require PKA phosphorylation of the beta(2)AR, receptor internalization or switching from activation of G(s) to G(i) but clearly requires activation of a Src family member that may be downstream of PKA.

Regulation of G protein-coupled receptor signaling by scaffold proteins

The actions of many hormones and neurotransmitters are mediated through stimulation of G protein-coupled receptors. A primary mechanism by which these receptors exert effects inside the cell is by association with heterotrimeric G proteins, which can activate a wide variety of cellular enzymes and ion channels. G protein-coupled receptors can also interact with a number of cytoplasmic scaffold proteins, which can link the receptors to various signaling intermediates and intracellular effectors. The multicomponent nature of G protein-coupled receptor signaling pathways makes them ideally suited for regulation by scaffold proteins. This review focuses on several specific examples of G protein-coupled receptor-associated scaffolds and the roles they may play in organizing receptor-initiated signaling pathways in the cardiovascular system and other tissues.

Functional characterization of alternatively spliced 5-HT2 receptor isoforms from the pharynx and muscle of the parasitic nematode, Ascaris suum

Serotonin (5-HT) receptors play key regulatory roles in nematodes and alternatively spliced 5-HT2 receptor isoforms have been identified in the parasitic nematode, Ascaris suum. 5-HT2As1 and 5-HT2As2 contain different C-termini, and 5-HT2As1Delta4 lacks 42 amino acids at the C-terminus of the third intracellular loop. 5-HT2As1 and 5-HT2As2 exhibited identical pharmacological profiles when stably expressed in human embryonic kidney (HEK) 293 cells. Both 5-HT2As isoforms had higher affinity for 5-HT than their closely related Caenorhabditis elegans homolog (5-HT2Ce). This increased 5-HT affinity was not related to the substitution in 5-HT2As1 of F120 for Y in the highly conserved DRY motif found in the second intracellular loop of other 5-HT receptors, since a 5-HT2As1F120Y mutant actually exhibited increased 5-HT affinity compared with that of 5-HT2As1. As predicted, cells expressing either 5-HT2As1 or 5-HT2As2 exhibited a 5-HT-dependent increase in phosphatidylinositol (PI) turnover. In contrast, although 5-HT2As1Delta4 displayed a 10-fold higher affinity for 5-HT and 5-HT agonists than either 5-HT2As1 or 5-HT2As2, 5-HT2As1Delta4 did not couple to either PI turnover or adenyl cyclase activity. Based on RT-PCR, 5-HT2As1 and 5-HT2As2 were more highly expressed in pharynx and body wall muscle and 5-HT2As1Delta4 in nerve cord/hypodermis. This is the first report of different alternatively spliced 5-HT2 receptor isoforms from any system.

The PDZ binding motif of the beta 1 adrenergic receptor modulates receptor trafficking and signaling in cardiac myocytes

Beta(1) and beta(2) adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse cardiac myocytes through distinct signaling pathways. It has been shown that stimulation of beta(1)ARs leads to a protein kinase A-dependent increase in contraction rate. In contrast, stimulation of beta(2)ARs has a biphasic effect on contraction rate, with an initial protein kinase A-independent increase followed by a sustained decrease that is blocked by pertussis toxin. The beta(2)AR undergoes agonist-induced endocytosis in cardiac myocytes while the beta(1)AR remains on the cell surface. It has been shown that a PDZ domain binding motif at the carboxyl terminus of beta(1)AR interacts with the postsynaptic density protein PSD-95 when both are expressed in HEK293 cells. We found that mutation of this PDZ binding motif in the beta(1)AR (beta(1)AR-PDZ) enabled agonist-induced internalization in cardiac myocytes. Moreover, stimulation of beta(1)AR-PDZ had a biphasic effect on the myocyte contraction rate similar to that observed following stimulation of the beta(2)AR. The secondary decrease in the contraction rate was mediated by G(i) and could be blocked by pertussis toxin. Furthermore, a non-selective endocytosis inhibitor, concanavalin A, inhibited the internalization of wild type beta(2)AR and the mutated beta(1)AR-PDZ, and blocked the coupling of both receptors to G(i). Finally, treating myocytes with a membrane-permeable peptide representing beta(1)AR PDZ motif caused the endogenous beta(1)AR to behave like beta(1)AR-PDZ. These studies suggest that association of the beta(1)AR with PSD-95 or a related protein dictates signaling specificity by retaining the receptor at the cell surface and preventing interaction with G(i).

Epithelial inducible nitric-oxide synthase is an apical EBP50-binding protein that directs vectorial nitric oxide output

Nitric oxide (NO), produced via inducible NO synthase (iNOS), can modulate polarized epithelial processes such as solute transport. Given the high reactivity of NO, we hypothesized that optimal NO regulation of polarized epithelial functions is achieved through compartmentalization of iNOS, allowing local NO delivery to its molecular targets. Here, we show that iNOS localizes to the apical domain of epithelial cells within a submembranous protein complex tightly bound to cortical actin. We further show that iNOS can bind to the apical PDZ protein, EBP50 (ezrin-radixin-moesin-binding phosphoprotein 50), an interaction that is dependent on the last three COOH-terminal amino acids of iNOS, SAL, but requires the presence of additional unknown cellular proteins. Mutation of these three COOH-terminal residues abolishes the iNOS-EBP50 interaction and disrupts the apical association of iNOS in transfected cells, showing that this COOH-terminal motif is essential for the correct localization of iNOS in epithelial cells. Apically localized iNOS directs vectorial NO production at the apical proximal tubule epithelial cell surface. These studies define human epithelial iNOS as an apical EBP50-binding protein and suggest that the physical association of iNOS with EBP50 might allow precise NO modulation of EBP50-associated protein functions.

The adenosine 2b receptor is recruited to the plasma membrane and associates with E3KARP and Ezrin upon agonist stimulation

We have previously shown that adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5'-AMP. Acting through the adenosine A2b receptor (A2bR), the luminally derived adenosine induces vectorial chloride secretion and a polarized secretion of interleukin-6 to the intestinal lumen. Although some G protein-coupled receptors interact with anchoring or signaling molecules, not much is known in this critical area for the A2bR. We used the model intestinal epithelial cell line, T84, and Caco2-BBE cells stably transfected with GFP-A2b receptor to study the intestinal A2bR. The A2bR is present in both the apical and basolateral membranes of intestinal epithelia. Apical or basolateral stimulation of the A2bR induces recruitment of the receptor to the plasma membrane and caveolar fractions. The A2bR co-immunoprecipitates with E3KARP and ezrin upon agonist stimulation. Ezrin interacts with E3KARP and PKA and the interaction between ezrin and E3KARP is enhanced by agonist stimulation. Our data suggest that the A2bR is recruited to the plasma membrane upon apical or basolateral agonist stimulation and interacts with E3KARP and ezrin. We speculate that such an interaction may not only anchor the A2bR to the plasma membrane but may also function to stabilize the receptor in a signaling complex in the plasma membrane.

Seven-transmembrane receptors

Seven-transmembrane receptors, which constitute the largest, most ubiquitous and most versatile family of membrane receptors, are also the most common target of therapeutic drugs. Recent findings indicate that the classical models of G-protein coupling and activation of second-messenger-generating enzymes do not fully explain their remarkably diverse biological actions.

Targeted disruption of the mouse NHERF-1 gene promotes internalization of proximal tubule sodium-phosphate cotransporter type IIa and renal phosphate wasting

Na+/H+ exchanger regulatory factor (NHERF)-1 and NHERF-2, two structurally related protein adapters containing tandem PSD-95/Discs large/ZO-1 (PDZ) domains, were identified as essential factors for protein kinase A-mediated inhibition of the sodium-hydrogen exchanger, NHE3. NHERF-1 and NHERF-2 also bound other cellular targets including the sodium-phosphate cotransporter type IIa encoded by the NPT2 gene. Targeted disruption of the mouse NHERF-1 gene eliminated NHERF-1 expression in kidney and other tissues of the mutant mice without altering NHERF-2 levels in these tissues. NHERF-1 (+/-) and (-/-) male mice maintained normal blood electrolytes but showed increased urinary excretion of phosphate when compared with wild-type (+/+) animals. Although the overall levels of renal NHERF-1 targets, NHE3 and Npt2, were unchanged in the mutant mice, immunocytochemistry showed that the Npt2 protein was aberrantly localized at internal sites in the renal proximal tubule cells. The mislocalization of Npt2 paralleled a reduction in the transporter protein in renal brush-border membranes isolated from the mutant mice. In contrast, NHE3 was appropriately localized at the apical surface of proximal tubules in both wild-type and mutant mice. These data suggested that NHERF-1 played a unique role in the apical targeting and/or trafficking of Npt2 in the mammalian kidney, a function not shared by NHERF-2 or other renal PDZ proteins. Phosphate wasting seen in the NHERF-1(-/-) null mice provided a new experimental system for defining the role of PDZ adapters in the hormonal control of ion transport and renal disease.

ERM proteins and merlin: integrators at the cell cortex

A fundamental property of many plasma-membrane proteins is their association with the underlying cytoskeleton to determine cell shape, and to participate in adhesion, motility and other plasma-membrane processes, including endocytosis and exocytosis. The ezrin-radixin-moesin (ERM) proteins are crucial components that provide a regulated linkage between membrane proteins and the cortical cytoskeleton, and also participate in signal-transduction pathways. The closely related tumour suppressor merlin shares many properties with ERM proteins, yet also provides a distinct and essential function.

Ezrin-radixin-moesin-binding phosphoprotein-50/Na+/H+ exchanger regulatory factor (EBP50/NHERF) blocks U50,488H-induced down-regulation of the human kappa opioid receptor by enhancing its recycling rate

We have investigated whether Ezrin-radixin-moesin (ERM)-binding phosphoprotein-50/Na(+)/H(+) exchanger regulatory factor (EBP50/NHERF), a PDZ domain-containing phosphoprotein, is associated with the human kappa opioid receptor (hkor) and whether it regulates the trafficking and signaling of the hkor. When expressed in CHO cells stably transfected with the FLAG-tagged hkor (FLAG-hkor), EBP50/NHERF co-immunoprecipitated with FLAG-hkor, and the PDZ domain I, but not the PDZ domain II, of EBP50/NHERF was involved in the interaction. Treatment with the agonist (-)-(trans)-3,4- dichloro-N-methyl-N-[2-(1-pyrrolidiny)cyclohexyl]benzeneacetamide (U50,488H) enhanced the association of EBP50/NHERF with FLAG-hkor. Expression of EBP50/NHERF, but not a truncated form lacking the ERM-binding domain, abolished U50,488H-induced down-regulation of FLAG-hkor, which was apparently due to an increase in the recycling rate of internalized receptors. However, expression of EBP50/NHERF did not affect U50,488H binding affinity and U50,488H-stimulated [(35)S]guanosine 5'-3-O-(thio)triphosphate binding and p42/p44 MAP kinase activation, nor did it affect U50,488H-induced desensitization and internalization of FLAG-hkor. To determine the motif of FLAG-hkor involved in EBP50/NHERF binding, we generated two mutants, FLAG-hkor-A and FLAG-hkor-EE, in which one Ala or two Glu residues were added to the C terminus, respectively. Neither FLAG-hkor-A nor FLAG-hkor-EE co-immunoprecipitated with EBP50/NHERF, and U50,488H-induced down-regulation of FLAG-hkor-A and FLAG-hkor-EE were not affected by expression of EBP50/NHERF. Thus, EBP50/NHERF binds to the C terminus of FLAG-hkor and blocks the down-regulation of FLAG-hkor. The C-terminal sequence of the hkor, NKPV, is distinctly different from the sequence D(S/T)XL, the optimal C-terminal motif in the beta(2)-adrenergic receptor for EBP50/NHERF binding. EBP50/NHERF may have a broader binding specificity and may interact with a subset of G protein-coupled receptors to serve as a recycling signal for these receptors.

Ca(2+)-dependent inhibition of Na+/H+ exchanger 3 (NHE3) requires an NHE3-E3KARP-alpha-actinin-4 complex for oligomerization and endocytosis

Two PDZ domain-containing proteins, NHERF and E3KARP are necessary for cAMP-dependent inhibition of Na(+)/H(+) exchanger 3 (NHE3). In this study, we demonstrate a specific role of E3KARP, which is not duplicated by NHERF, in Ca(2+)-dependent inhibition of NHE3 activity. NHE3 activity is inhibited by elevation of intracellular Ca(2+) ([Ca(2+)](i)) in PS120 fibroblasts stably expressing E3KARP but not those expressing NHERF. In addition, this Ca(2+)-dependent inhibition requires Ca(2+)-dependent association between alpha-actinin-4 and E3KARP. NHE3 is indirectly connected to alpha-actinin-4 in a protein complex through Ca(2+)-dependent interaction between alpha-actinin-4 and E3KARP, which occurs through the actin-binding domain plus spectrin repeat domain of alpha-actinin-4. Elevation of [Ca(2+)](i) results in oligomerization and endocytosis of NHE3 as well as in inhibition of NHE3 activity. Overexpression of alpha-actinin-4 potentiates the inhibitory effect of ionomycin on NHE3 activity by accelerating the oligomerization and endocytosis of NHE3. In contrast, overexpression of the actin-binding domain plus spectrin repeat domain acts as a dominant-negative mutant and prevents the inhibitory effect of ionomycin on NHE3 activity as well as the oligomerization and internalization of NHE3. From these results, we propose that elevated Ca(2+) inhibits NHE3 activity through oligomerization and endocytosis of NHE3, which occurs via formation of an NHE3-E3KARP-alpha-actinin-4 complex.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

A novel PDZ protein regulates the activity of guanylyl cyclase C, the heat-stable enterotoxin receptor

Secretory diarrhea is the leading cause of infectious diarrhea in humans. Secretory diarrhea may be caused by binding of heat-stable enterotoxins to the intestinal receptor guanylyl cyclase C (GCC). Activation of GCC catalyzes the formation of cGMP, initiating a signaling cascade that opens the cystic fibrosis transmembrane conductance regulator chloride channel at the apical cell surface. To identify proteins that regulate the trafficking or function of GCC, we used the unique COOH terminus of GCC as the "bait" to screen a human intestinal yeast two-hybrid library. We identified a novel protein, IKEPP (intestinal and kidney-enriched PDZ protein) that associates with the COOH terminus of GCC in biochemical assays and by co-immunoprecipitation. IKEPP is expressed in the intestinal epithelium, where it is preferentially accumulated at the apical surface. The GCC-IKEPP interaction is not required for the efficient targeting of GCC to the apical cell surface. Rather, the association with IKEPP significantly inhibits heat-stable enterotoxin-mediated activation of GCC. Our findings are the first to identify a regulatory protein that associates with GCC to modulate the catalytic activity of the enzyme and provides new insights in mechanisms that regulate GCC activity in response to bacterial toxin.

PICK1 is required for the control of synaptic transmission by the metabotropic glutamate receptor 7

Both postsynaptic density and presynaptic active zone are structural matrix containing scaffolding proteins that are involved in the organization of the synapse. Little is known about the functional role of these proteins in the signaling of presynaptic receptors. Here we show that the interaction of the presynaptic metabotropic glutamate (mGlu) receptor subtype, mGlu7a, with the postsynaptic density-95 disc-large zona occludens 1 (PDZ) domain-containing protein, PICK1, is required for specific inhibition of P/Q-type Ca(2+) channels, in cultured cerebellar granule neurons. Furthermore, we show that activation of the presynaptic mGlu7a receptor inhibits synaptic transmission and this effect also requires the presence of PICK1. These results indicate that the scaffolding protein, PICK1, plays an essential role in the control of synaptic transmission by the mGlu7a receptor complex.

Selectivity and promiscuity of the first and second PDZ domains of PSD-95 and synapse-associated protein 102

PDZ domains typically interact with the very carboxyl terminus of their binding partners. Type 1 PDZ domains usually require valine, leucine, or isoleucine at the very COOH-terminal (P(0)) position, and serine or threonine 2 residues upstream at P(-2). We quantitatively defined the contributions of carboxyl-terminal residues to binding selectivity of the prototypic interactions of the PDZ domains of postsynaptic density protein 95 (PSD-95) and its homolog synapse-associated protein 90 (SAP102) with the NR2b subunit of the N-methyl-d-aspartate-type glutamate receptor. Our studies indicate that all of the last five residues of NR2b contribute to the binding selectivity. Prominent were a requirement for glutamate or glutamine at P(-3) and for valine at P(0) for high affinity binding and a preference for threonine over serine at P(-2), in the context of the last 11 residues of the NR2b COOH terminus. This analysis predicts a COOH-terminal (E/Q)(S/T)XV consensus sequence for the strongest binding to the first two PDZ domains of PSD-95 and SAP102. A search of the human genome sequences for proteins with a COOH-terminal (E/Q)(S/T)XV motif yielded 50 proteins, many of which have not been previously identified as PSD-95 or SAP102 binding partners. Two of these proteins, brain-specific angiogenesis inhibitor 1 and protein kinase Calpha, co-immunoprecipitated with PSD-95 and SAP102 from rat brain extracts.

TRH signal transduction in melanotrope cells of Xenopus laevis

TRH is a neuropeptide that activates phospholipase C and, when acting on secretory cells, usually induces a biphasic response consisting of a transitory increase in secretion (due to IP(3) mobilization of Ca(2+) from intracellular stores), followed by a sustained plateau phase of stimulated secretion (by protein kinase C-dependent influx of extracellular Ca(2+) through voltage-operated Ca(2+) channels). The melanotrope cell of the amphibian Xenopus laevis displays a unique secretory response to TRH, namely a broad transient but no sustained second phase, consistent with the observation that TRH induces a single Ca(2+) transient rather than the classic biphasic increase in [Ca(2+)](i). The purpose of the present study was to determine the signal transduction mechanism utilized by TRH in generating this Ca(2+) signaling response. Our hypothesis was that the transient reflects the operation of only one of the two signaling arms of the lipase (i.e., either IP(3)-induced mobilization of internal Ca(2+) or PKC-dependent influx of external Ca(2+)). Using video-imaging microscopy it is shown that the TRH-induced Ca(2+) transient is dramatically attenuated under Ca(2+)-free conditions and that thapsigargin has no noticeable effect on the TRH-induced transient. These observations indicate that an IP(3)-dependent mechanism plays no important role in the action of TRH. PKC also does not seem to be involved because an activator of PKC did not induce a Ca(2+) transient and an inhibitor of PKC did not affect the TRH response. Experiments with a bis-oxonol membrane potential probe showed that the TRH response also does not underlie a PKC-independent mechanism that would induce membrane depolarization. We conclude that the action of TRH on the Xenopus melanotrope does not rely on the classical phospholipase C-dependent mechanism.

Down-regulation of protease-activated receptor-1 is regulated by sorting nexin 1

Degradation or "down-regulation" of protease-activated receptor-1 (PAR1), a G protein-coupled receptor for thrombin, is critical for termination of receptor signaling. Toward understanding the molecular mechanisms by which activated PAR1 is internalized, sorted to lysosomes, and degraded, we investigated whether PAR1 interacted with sorting nexin 1 (SNX1). SNX1 is a membrane-associated protein that functions in lysosomal sorting of the epidermal growth factor receptor. In vitro biochemical binding assays revealed a specific interaction between a glutathione S-transferase fusion of SNX1 and PAR1. In HeLa cells, activated PAR1 colocalized with endogenous SNX1 and coimmunoprecipitated SNX1. SNX1 contains a phox homology domain predicted to bind phosphatidylinositol-3-phosphate and a C-terminal coiled-coil region. To assess SNX1 function, we examined the effects of SNX1 deletion mutants on PAR1 trafficking. Neither the N terminus nor phox homology domain of SNX1 affected PAR1 trafficking. By contrast, overexpression of SNX1 C-terminal domain markedly inhibited agonist-induced degradation of PAR1, whereas internalization remained virtually intact. Immunofluorescence microscopy studies revealed substantial PAR1 accumulation in an early endosome antigen-1-positive compartment in agonist-treated cells expressing SNX1 C terminus. By contrast, lysosome-associated membrane protein-1 distribution was unperturbed. Together, these findings strongly suggest a role for SNX1 in sorting of PAR1 from early endosomes to lysosomes. Moreover, this study provides the first example of a protein involved in lysosomal sorting of a G protein-coupled receptor in mammalian cells.

Structural determinants of the Na+/H+ exchanger regulatory factor interaction with the beta 2 adrenergic and platelet-derived growth factor receptors

The Na(+)/H(+) exchanger regulatory factor (NHERF) binds through its PDZ1 domain to the carboxyl-terminal sequences NDSLL and EDSFL of the beta(2) adrenergic receptor (beta(2)AR) and platelet-derived growth factor receptor, respectively, and plays a critical role in the membrane localization and physiological regulation of these receptors. The crystal structures of the human NHERF PDZ1 domain bound to the sequences NDSLL and EDSFL have been determined at 1.9- and 2.2-A resolution, respectively. The beta(2)AR and platelet-derived growth factor receptor ligands insert into the PDZ1 binding pocket by a beta-sheet augmentation process and are stabilized by largely similar networks of hydrogen bonds and hydrophobic contacts. In the PDZ1-beta(2)AR complex, the side chain of asparagine at position -4 in the beta(2)AR peptide forms two additional hydrogen bonds with Gly(30) of PDZ1, which contribute to the higher affinity of this interaction. Remarkably, both complexes are further stabilized by hydrophobic interactions involving the side chains of the penultimate amino acids of the peptide ligands, whereas the PDZ1 residues Asn(22) and Glu(43) undergo conformational changes to accommodate these side chains. These results provide structural insights into the mechanisms by which different side chains at the position -1 of peptide ligands interact with PDZ domains and contribute to the affinity of the PDZ-ligand interaction.

Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach

Membrane-bound receptors such as tyrosine kinases and ionotropic receptors are associated with large protein networks structured by protein-protein interactions involving multidomain proteins. Although these networks have emerged as a general mechanism of cellular signalling, much less is known about the protein complexes associated with G-protein-coupled receptors (GPCRs). Using a proteomic approach based on peptide affinity chromatography followed by mass spectrometry and immunoblotting, we have identified 15 proteins that interact with the C- terminal tail of the 5-hydroxytryptamine 2C (5-HT(2C)) receptor, a GPCR. These proteins include several synaptic multidomain proteins containing one or several PDZ domains (PSD95 and the proteins of the tripartite complex Veli3-CASK-Mint1), proteins of the actin/spectrin cytoskeleton and signalling proteins. Coimmunoprecipitation experiments showed that 5-HT(2C) receptors interact with PSD95 and the Veli3-CASK-Mint1 complex in vivo. Electron microscopy also indicated a synaptic enrichment of Veli3 and 5-HT(2C) receptors and their colocalization in microvilli of choroidal cells. These results indicate that the 5-HT(2C) receptor is associated with protein networks that are important for its synaptic localization and its coupling to the signalling machinery.

G protein-coupled receptor signalling and cross-talk: achieving rapidity and specificity

Activation of a given type of G protein-coupled receptor (GPCR) triggers a limited set of signalling events in a very rapid and specific manner. The classical paradigm of GPCR signalling was rather linear and sequential. Emerging evidence, however, has revealed that this is only a part of the complex signalling mediated by GPCR. Propagation of GPCR signalling involves cross-regulation of many but specific pathways, including cross-talks between different GPCRs as well as with other signalling pathways. Moreover, it is increasingly apparent that GPCRs can activate both heterotrimeric G protein-dependent and G protein-independent signalling pathways. In this review, we discuss how the signallings initiated by GPCRs achieve rapidity as well as specificity, and how the GPCRs can cross-regulate other specific signalling pathways at the same time. New concepts regarding GPCR signalling have been arising to address this issue, which include multiprotein signalling complex and signalling compartment in microdomain concepts that enable close colocalization or even contact among the proteins engaged in the specific signal transduction. The final outcome of a stimulation of GPCR will thus be the sum of its own specific set of intracellular signalling pathways it regulates.

G protein-coupled receptor interacting proteins: emerging roles in localization and signal transduction

The mechanism by which G protein-coupled receptors (GPCRs) translate extracellular signals into cellular changes initially was envisioned as a simple linear model: activation of the receptor by agonist binding leads to dissociation of the heterotrimeric GTP-binding G protein into its alpha and betagamma subunits, both of which can activate or inhibit various downstream effector molecules. The plethora of recently described multidomain scaffolding proteins and accessory/chaperone molecules that interact with GPCR, including GPCR themselves as homo- or heterodimers, provides for diverse molecular mechanisms for ligand recognition, signalling specificity, and receptor trafficking. This review will summarize the recently described GPCR-interacting proteins and their individual functional roles, as understood. Implicit in the search for the functional relevance of these interactions is the expectation that enhancement or disruption of target cell-specific events could serve as highly selective therapeutic opportunities.

Plasma membrane Ca2+ ATPase isoform 2b interacts preferentially with Na+/H+ exchanger regulatory factor 2 in apical plasma membranes

Spatial and temporal regulation of Ca(2+) signaling require the assembly of multiprotein complexes linking molecules involved in Ca(2+) influx, sensing, buffering, and extrusion. Recent evidence indicates that plasma membrane Ca(2+) ATPases (PMCAs) participate in the control of local Ca(2+) fluxes, but the mechanism of multiprotein complex formation of specific PMCAs is poorly understood. Using the PMCA2b COOH-terminal tail as bait in a yeast two-hybrid screen, we identified the PSD-95, Dlg, ZO-1 (PDZ) domain-containing Na(+)/H(+) exchanger regulatory factor-2 (NHERF2) as an interacting partner. Protein pull-down and coimmunoprecipitation experiments using recombinant PMCA2b and PMCA4b as well as NHERF1 and NHERF2 showed that the interaction of PMCA2b with NHERF2 was specific and selective. PMCA4b did not interact with either of the NHERFs, and PMCA2b selectively preferred NHERF2 over NHERF1. Green fluorescent protein-tagged PMCA2b was expressed at the apical membrane in Madin-Darby canine kidney epithelial cells, where it colocalized with apically targeted NHERF2. Our study identifies NHERF2 as the first specific PDZ partner for PMCA2b not shared with PMCA4b, and demonstrates that PMCA splice forms differing only minimally in their COOH-terminal residues interact with unique PDZ proteins. NHERFs have been implicated in the targeting, retention and regulation of membrane proteins including the beta(2)-adrenergic receptor, cystic fibrosis transmembrane conductance regulator, and Trp4 Ca(2+) channel, and NHERF2 is now shown to also interact with PMCA2b. This interaction may allow the functional assembly of PMCA2b in a multiprotein Ca(2+) signaling complex, facilitating integrated cross-talk between local Ca(2+) influx and efflux.

The human 5-HT7 serotonin receptor splice variants: constitutive activity and inverse agonist effects

1. Using membranes from stably or transiently transfected HEK293 cells cultured in 5-HT-free medium and expressing the recombinant human 5-HT(7) receptor splice variants (h5-HT(7(a)), h5-HT(7(b)) and h5-HT(7(d))), we compared their abilities to constitutively activate adenylyl cyclase (AC). 2. All h5-HT(7) splice variants elevated basal and forskolin-stimulated AC. The basal AC activity was reduced by the 5-HT(7) antagonist methiothepin and this effect was blocked by mesulergine (neutral 5-HT(7) antagonist) indicating that the inhibitory effect of methiothepin is inverse agonism at the 5-HT(7) receptor. 3. Receptor density correlated poorly with constitutive AC activity in stable clonal cell lines and transiently transfected cells. Mean constitutive AC activity as a percentage of forskolin-stimulated AC was significantly higher for the h5-HT(7(b)) splice variant compared to the h5-HT(7(a)) and h5-HT(7(d)) splice variants but only in stable cell lines. 4. All eight 5-HT antagonists tested inhibited constitutive AC activity of all splice variants in a concentration-dependent manner. No differences in inverse agonist potencies (pIC(50)) were observed between the splice variants. The rank order of potencies was in agreement and highly correlated with antagonist potencies (pK(b)) determined by antagonism of 5-HT-stimulated AC activity (methiothepin >metergoline> mesulergine > or = clozapine > or = spiperone > or = ritanserin > methysergide > ketanserin). 5. The efficacy of inverse agonism was not receptor level dependent and varied for several 5-HT antagonists between membrane preparations of transiently and stably transfected cells. 6. It is concluded that the h5-HT(7) splice variants display similar constitutive activity and inverse agonist properties.

Association of dopamine D(3) receptors with actin-binding protein 280 (ABP-280)

Proteins that bind to G protein-coupled receptors have been identified as regulators of receptor localization and signaling. In our previous studies, a cytoskeletal protein, actin-binding protein 280 (ABP-280), was found to associate with the third cytoplasmic loop of dopamine D(2) receptors. In this study, we demonstrate that ABP-280 also interacts with dopamine D(3) receptors, but not with D(4) receptors. Similar to the dopamine D(2) receptor, the D(3)/ABP-280 association is of signaling importance. In human melanoma M2 cells lacking ABP-280, D(3) receptors were unable to inhibit forskolin-stimulated cyclic AMP (cAMP) production significantly. D(4) receptors, however, exhibited a similar degree of inhibition of forskolin-stimulated cAMP production in ABP-280-deficient M2 cells and ABP-280-replent M2 subclones (A7 cells). Further experiments revealed that the D(3)/ABP-280 interaction was critically dependent upon a 36 amino acid carboxyl domain of the D(3) receptor third loop, which is conserved in the D(2) receptor but not in the D(4) receptor. Our results demonstrate a subtype-specific regulation of dopamine D(2)-family receptor signaling by the cytoskeletal protein ABP-280.

Identification of novel G protein-coupled receptor-interacting proteins

Biochemical and genetic methods utilizing G protein-coupled receptor fragments have been used successfully to identify G protein-coupled receptor-interacting proteins. As noted earlier, these methods may be unable to detect interactions that require certain conformations of the native receptor protein, but have nevertheless proven quite useful in expanding our understanding of receptor regulation to include interactions with proteins other than G proteins, G protein-coupled receptor kinases, and arrestins. Undoubtedly, it is likely that all G protein-coupled receptors have their own unique constellations of associated cytoplasmic proteins, and the techniques described here should prove useful in identifying these.

Efficacy at G-protein-coupled receptors

At present, the drug-discovery process centres on ligands that either block or produce physiological responses. However, there are therapeutic uses for ligands that do neither of these things, but which still affect receptors in other ways. This review discusses the intimate relationship between the affinity of a ligand for its receptor, and the probability that the binding of the ligand will produce some change in the receptor, resulting in efficacy. This, in turn, argues that ligands that have affinity should be tested more broadly, for a wider range of efficacies, to detect hidden therapeutic activities.

Distinct cell type-specific expression of scaffolding proteins EBP50 and E3KARP: EBP50 is generally expressed with ezrin in specific epithelia, whereas E3KARP is not

The ezrin/radixin/moesin (ERM) proteins are regulated microfilament membrane linking proteins. Previous tissue localization studies have revealed that the three related proteins show distinct tissue distributions, with ezrin being found predominantly in polarized epithelial cells, whereas moesin is enriched in endothelial cells and lymphocytes. EBP50 and E3KARP are two related scaffolding proteins that bind to the activated form of ERM proteins in vitro, and through their PDZ domains to the cytoplasmic domains of specific membrane proteins, including the Na+/H+ exchanger isoform (NHE3) present in kidney proximal tubules and the beta2-adrenergic receptor. Using specific antibodies to EBP50 and E3KARP for localization in murine tissues, we find that the cellular distribution of EBP50 and E3KARP is mutually exclusive. Epithelial cells expressing ezrin generally co-express EBP50, such as intestinal epithelial cells, gastric parietal cells, the epithelial cells of the kidney proximal tubule, the terminal bronchiole of the lung, and in mesothelia. This correlation is not absolute as cells of the mucous epithelium of the stomach and in the renal corpuscle, express ezrin but no detectable EBP50, whereas the bile canaliculi of hepatocytes express EBP50 and not ezrin. E3KARP has a restricted tissue distribution with the highest expression being found in lung. It is largely colocalized with moesin and radixin, especially in the alveoli of the lung, as well as being highly enriched in the renal corpuscle. These results document a preference for co-expression of EBP50, but not E3KARP, with ezrin in polarized epithelia. These results place constraints on the physiological roles that can be proposed for these scaffolding molecules.

Cutting edge: negative regulation of immune synapse formation by anchoring lipid raft to cytoskeleton through Cbp-EBP50-ERM assembly

Ag recognition by T lymphocytes induces immune synapse formation and recruitment of signaling molecules into a lipid raft. Cbp/PAG is a Csk-associated membrane adapter protein exclusively localized in a lipid raft. We identified NHERF/EBP50 as a Cbp-binding molecule, which connects the membrane raft and cytoskeleton by binding to both Cbp through its PDZ domain and ezrin-radixin-moesin through the C terminus. Overexpression of Cbp reduced the mobility of the raft on the cell surface of unstimulated T cells and prevented synapse formation and subsequent T cell activation, whereas a mutant incapable of EBP50 binding restored both synapse formation and activation. These results suggest that anchoring of lipid raft to the cytoskeleton through Cbp-EBP50-ezrin-radixin-moesin assembly regulates membrane dynamism for synapse formation and T cell activation.

G protein-coupled receptor kinase 5 regulates beta 1-adrenergic receptor association with PSD-95

We previously reported that the beta(1)-adrenergic receptor (beta(1)AR) associates with PSD-95 through a PDZ domain-mediated interaction, by which PSD-95 modulates beta(1)AR function and facilitates the physical association of beta(1)AR with other synaptic proteins such as N-methyl-d-aspartate receptors. Here we demonstrate that beta(1)AR association with PSD-95 is regulated by G protein-coupled receptor kinase 5 (GRK5). When beta(1)AR and PSD-95 were coexpressed with either GRK2 or GRK5 in COS-7 cells, GRK5 alone dramatically decreased the association of beta(1)AR with PSD-95, although GRK2 and GRK5 both could be co-immunoprecipitated with beta(1)AR and both could enhance receptor phosphorylation in vivo. Increasing expression of GRK5 in the cells led to further decreased beta(1)AR association with PSD-95. Stimulation with the beta(1)AR agonist isoproterenol further decreased PSD-95 binding to beta(1)AR. In addition, GRK5 protein kinase activity was required for this regulatory effect since a kinase-inactive GRK5 mutant had no effect on PSD-95 binding to beta(1)AR. Moreover, the regulatory effect of GRK5 on beta(1)AR association with PSD-95 was observed only when GRK5 was expressed together with the receptor, but not when GRK5 was coexpressed with PSD-95. Thus, we propose that GRK5 regulates beta(1)AR association with PSD-95 through phosphorylation of beta(1)AR. Regulation of protein association through receptor phosphorylation may be a general mechanism used by G protein-coupled receptors that associate via PDZ domain-mediated protein/protein interactions.

Usher syndrome: from genetics to pathogenesis

Usher syndrome (USH) is defined by the association of sensorineural deafness and visual impairment due to retinitis pigmentosa. The syndrome has three distinct clinical subtypes, referred to as USH1, USH2, and USH3. Each subtype is genetically heterogeneous, and 12 loci have been detected so far. Four genes have been identified, namely, USH1B, USH1C, USH1D, and USH2A. USH1B, USH1C, and USH1D encode an unconventional myosin (myosin VIIA), a PDZ domain-containing protein (harmonin), and a cadherin-like protein (cadherin-23), respectively. Mutations of these genes cause primary defects of the sensory cells in the inner ear, and probably also in the retina. In the inner ear, the USH1 genes, I propose, are involved in the same signaling pathway, which may control development and/or maintenance of the hair bundles of sensory cells via an adhesion force (a) at the junctions between these cells and supporting cells and (b) at the level of the lateral links that interconnect the stereocilia. In contrast, the molecular pathogenesis of USH2A, which is owing to a defect of a novel extracellular matrix protein, is likely to be different from that of USH1.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.