Compartmentation of G-protein-coupled receptors and their signalling components in lipid rafts and caveolae

Reduced incretin receptor trafficking upon activation enhances glycemic control and reverses obesity in diet-induced obese mice

Activation of incretin receptors by their cognate agonist augments sustained cAMP generation both from the plasma membrane as well as from the endosome. To address the functional outcome of this spatiotemporal signaling, we developed a nonacylated glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor dual agonist I-M-150847 that reduced receptor internalization following activation of the incretin receptors. The incretin receptor dual agonist I-M-150847 was developed by replacing the tryptophan cage of exendin-4 tyrosine substituted at the amino terminus with the C-terminal undecapeptide sequence of oxyntomodulin that placed lysine 30 of I-M-150847 in frame with the corresponding lysine residue of GIP. The peptide I-M-150847 is a partial agonist of GLP-1R and GIPR; however, the receptors, upon activation by I-M-150847, undergo reduced internalization that promotes agonist-mediated iterative cAMP signaling and augments glucose-stimulated insulin exocytosis in pancreatic β cells. Chronic administration of I-M-150847 improved glycemic control, enhanced insulin sensitivity, and provided profound weight loss in diet-induced obese (DIO) mice. Our results demonstrated that despite being a partial agonist, I-M-150847, by reducing the receptor internalization upon activation, enhanced the incretin effect and reversed obesity.NEW & NOTEWORTHY Replacement of the tryptophan cage (Trp-cage) with the C-terminal oxyntomodulin undecapeptide along with the tyrosine substitution at the amino terminus converts the selective glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 to a novel GLP-1R and GIPR dual agonist I-M-150847. Reduced internalization of incretin receptors upon activation by the GLP-1R and GIPR dual agonist I-M-150847 promotes iterative receptor signaling that enhances the incretin effect and reverses obesity.

The Role of Lipids in Allosteric Modulation of Dopamine D2 Receptor-In Silico Study

The dopamine D₂ receptor, belonging to the class A G protein-coupled receptors (GPCRs), is an important drug target for several diseases, including schizophrenia and Parkinson’s disease. The D₂ receptor can be activated by the natural neurotransmitter dopamine or by synthetic ligands, which in both cases leads to the receptor coupling with a G protein. In addition to receptor modulation by orthosteric or allosteric ligands, it has been shown that lipids may affect the behaviour of membrane proteins. We constructed a model of a D₂ receptor with a long intracellular loop (ICL3) coupled with G_iα1 or G_iα2 proteins, embedded in a complex asymmetric membrane, and simulated it in complex with positive, negative or neutral allosteric ligands. In this study, we focused on the influence of ligand binding and G protein coupling on the membrane–receptor interactions. We show that there is a noticeable interplay between the cell membrane, G proteins, D₂ receptor and its modulators.

Sphingolipid Metabolism and Signaling in Endothelial Cell Functions

The endothelium, inner layer of blood vessels, constitutes a metabolically active paracrine, endocrine, and autocrine organ, able to sense the neighboring environment and exert a variety of biological functions important to preserve the health of vasculature, tissues, and organs. Sphingolipids are both fundamental structural components of the eukaryotic membranes and signaling molecules regulating a variety of biological functions. Ceramide and sphingosine-1-phosphate (S1P), bioactive sphingolipids, have emerged as important regulators of cardiovascular functions in health and disease. In this review we discuss recent insights into the role of ceramide and S1P biosynthesis and signaling in regulating endothelial cell functions, in health and diseases. We also highlight advances into the mechanisms regulating serine palmitoyltransferase, the first and rate-limiting enzyme of de novo sphingolipid biosynthesis, with an emphasis on its inhibitors, ORMDL and NOGO-B. Understanding the molecular mechanisms regulating the sphingolipid de novo biosynthesis may provide the foundation for therapeutic modulation of this pathway in a variety of conditions, including cardiovascular diseases, associated with derangement of this pathway.

EH Domain-Containing 2 Deficiency Restricts Adipose Tissue Expansion and Impairs Lipolysis in Primary Inguinal Adipocytes

Lipid uptake can be facilitated via caveolae, specific plasma membrane invaginations abundantly expressed in adipocytes. The dynamin-related protein EH domain-containing 2 (EHD2) stabilizes caveolae at the cell surface. Here, we have examined the importance of EHD2 for lipid handling using primary adipocytes isolated from EHD2 knockout (Ehd2^−/−) C57BL6/N mice. Following high-fat diet (HFD) feeding, we found a clear impairment of epididymal, but not inguinal, adipose tissue expansion in Ehd2^−/− compared with Ehd2^+/+ (WT) mice. Cell size distribution analysis revealed that Ehd2^−/− mice had a lower proportion of small adipocytes, and an accumulation of medium-sized adipocytes in both epididymal and inguinal adipose tissue. Further, PPARγ activity, FABP4 and caveolin-1 expression were decreased in adipocytes isolated from Ehd2^−/− mice. Inguinal adipocytes isolated from Ehd2^−/− mice displayed reduced lipolysis in response to beta adrenergic receptor agonist, which was associated with reduced phosphorylation of perilipin-1 and hormone sensitive lipase (HSL). This impairment could not be rescued using a cAMP analog, indicating that impaired lipolysis in Ehd2^−/− primary adipocytes likely occurs at the level of, or downstream of, protein kinase A (PKA). Altogether, these findings pinpoint the importance of EHD2 for maintained intracellular lipid metabolism, and emphasize differences in mechanisms regulating lipid handling in various adipose-tissue depots.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Identification of Novel Adenylyl Cyclase 5 (AC5) Signaling Networks in D1 and D2 Medium Spiny Neurons using Bimolecular Fluorescence Complementation Screening

Adenylyl cyclase type 5 (AC5), as the principal isoform expressed in striatal medium spiny neurons (MSNs), is essential for the integration of both stimulatory and inhibitory midbrain signals that initiate from dopaminergic G protein-coupled receptor (GPCR) activation. The spatial and temporal control of cAMP signaling is dependent upon the composition of local regulatory protein networks. However, there is little understanding of how adenylyl cyclase protein interaction networks adapt to the multifarious pressures of integrating acute versus chronic and inhibitory vs. stimulatory receptor signaling in striatal MSNs. Here, we presented the development of a novel bimolecular fluorescence complementation (BiFC)-based protein-protein interaction screening methodology to further identify and characterize elements important for homeostatic control of dopamine-modulated AC5 signaling in a neuronal model cell line and striatal MSNs. We identified two novel AC5 modulators: the protein phosphatase 2A (PP2A) catalytic subunit (PPP2CB) and the intracellular trafficking associated protein-NSF (N-ethylmaleimide-sensitive factor) attachment protein alpha (NAPA). The effects of genetic knockdown (KD) of each gene were evaluated in several cellular models, including D₁- and D₂-dopamine receptor-expressing MSNs from CAMPER mice. The knockdown of PPP2CB was associated with a reduction in acute and sensitized adenylyl cyclase activity, implicating PP2A is an important and persistent regulator of adenylyl cyclase activity. In contrast, the effects of NAPA knockdown were more nuanced and appeared to involve an activity-dependent protein interaction network. Taken together, these data represent a novel screening method and workflow for the identification and validation of adenylyl cyclase protein-protein interaction networks under diverse cAMP signaling paradigms.

The Effects of Aging on the Regulation of T-Tubular ICa by Caveolin in Mouse Ventricular Myocytes

Aging is associated with diminished cardiac function in males. Cardiac excitation-contraction coupling in ventricular myocytes involves Ca influx via the Ca current (I_Ca) and Ca release from the sarcoplasmic reticulum, which occur predominantly at t-tubules. Caveolin-3 regulates t-tubular I_Ca, partly through protein kinase A (PKA), and both I_Ca and caveolin-3 decrease with age. We therefore investigated I_Ca and t-tubule structure and function in cardiomyocytes from male wild-type (WT) and caveolin-3-overexpressing (Cav-3OE) mice at 3 and 24 months of age. In WT cardiomyocytes, t-tubular I_Ca-density was reduced by ~50% with age while surface I_Ca density was unchanged. Although regulation by PKA was unaffected by age, inhibition of caveolin-3-binding reduced t-tubular I_Ca at 3 months, but not at 24 months. While Cav-3OE increased cardiac caveolin-3 protein expression ~2.5-fold at both ages, the age-dependent reduction in caveolin-3 (WT ~35%) was preserved in transgenic mice. Overexpression of caveolin-3 reduced t-tubular I_Ca density at 3 months but prevented further I_Ca loss with age. Measurement of Ca release at the t-tubules revealed that the triggering of local Ca release by t-tubular I_Ca was unaffected by age. In conclusion, the data suggest that the reduction in I_Ca density with age is associated with the loss of a caveolin-3-dependent mechanism that augments t-tubular I_Ca density.

Study of G-Protein Coupled Receptor Signaling in Membrane Environment by Plasmon Waveguide Resonance

Here we describe an experimental technique, termed plasmon waveguide resonance (PWR) spectroscopy that enables the characterization of molecular interactions occurring at the level of anisotropic thin films as lipid membranes and therein inserted or interacting molecules. PWR allows one to characterize such molecular interactions at different levels: (1) acquire binding curves and calculate dissociation constants; (2) obtain kinetic information; (3) obtain information about associated anisotropy changes and changes in membrane thickness; (4) obtain insight about lateral homogeneity (formation of domains). Points 1, 2, and 4 can be directly obtained from the data. Point 3 requires spectral fitting procedures so that the different optical parameters characterizing thin films as proteolipid membranes, namely refractive index and extinction coefficient for both p- (TM component of light that is parallel to the incident light) and s- (TE component of light that is perpendicular to the incident light) polarizations and thickness, can be determined. When applied to membrane proteins as the G-protein coupled receptor (GPCR) family, both ligand-induced conformational changes of the receptor can be followed as well as interactions with effectors (e.g., G-proteins). Additionally, by either altering the lipid composition in cellular membranes or specifically controlling its composition in the case of lipid model membranes with reconstituted proteins, the role of the lipid environment in receptor activation and signaling can be determined. Additionally, the eventual partition of receptors in different lipid microdomains (e.g., lipid rafts) can be followed. Such information can be obtained  ex cellulo with mammalian cell membrane fragments expressing the protein of interest and/or in vitro with lipid model systems where the protein under investigation has been reconstituted. Moreover, PWR can also be applied to directly follow the reconstitution of membrane proteins in lipid model membranes. The measurements are performed directly (no labeling of molecular partners), in real time and with very high sensitivity. Here we will discuss different aspects of GPCR activation and signaling where PWR brought important information in parallel with other approaches. The utility of PWR is not limited to GPCRs but can be applied to any membrane protein. PWR is also an excellent tool to characterize the interaction of membrane active molecules (as cell penetrating, antimicrobial, viral and amyloid peptides) with lipids. A brief section is dedicated to such applications, with particular emphasis on amyloid peptides. To finalize, as PWR is a homemade technology, ongoing instrument developments aiming at breaking current experimental limitations are briefly discussed, namely, the coupling of PWR with electrochemical measurements and the expansion of measurements from the visible to the infrared region.

Functional organization of postsynaptic glutamate receptors

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in the brain, responsible for mediating the vast majority of excitatory transmission in neuronal networks. The AMPA- and NMDA-type ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the fast synaptic responses, while metabotropic glutamate receptors (mGluRs) are coupled to downstream signaling cascades that act on much slower timescales. These functionally distinct receptor sub-types are co-expressed at individual synapses, allowing for the precise temporal modulation of postsynaptic excitability and plasticity. Intriguingly, these receptors are differentially distributed with respect to the presynaptic release site. While iGluRs are enriched in the core of the synapse directly opposing the release site, mGluRs reside preferentially at the border of the synapse. As such, to understand the differential contribution of these receptors to synaptic transmission, it is important to not only consider their signaling properties, but also the mechanisms that control the spatial segregation of these receptor types within synapses. In this review, we will focus on the mechanisms that control the organization of glutamate receptors at the postsynaptic membrane with respect to the release site, and discuss how this organization could regulate synapse physiology.

Disruption of lipid-raft localized Gαs/tubulin complexes by antidepressants: a unique feature of HDAC6 inhibitors, SSRI and tricyclic compounds

Current antidepressant therapies meet with variable therapeutic success and there is increasing interest in therapeutic approaches not based on monoamine signaling. Histone deacetylase 6 (HDAC6), which also deacetylates α-tubulin shows altered expression in mood disorders and HDAC6 knockout mice mimic traditional antidepressant treatments. Nonetheless, a mechanistic understanding for HDAC6 inhibitors in the treatment of depression remains elusive. Previously, we have shown that sustained treatment of rats or glioma cells with several antidepressants translocates Gαs from lipid rafts toward increased association with adenylyl cyclase (AC). Concomitant with this is a sustained increase in cAMP production. While Gαs modifies microtubule dynamics, tubulin also acts as an anchor for Gαs in lipid-rafts. Since HDAC-6 inhibitors potentiate α-tubulin acetylation, we hypothesize that acetylation of α-tubulin disrupts tubulin-Gαs raft-anchoring, rendering Gαs free to activate AC. To test this, C6 Glioma (C6) cells were treated with the HDAC-6 inhibitor, tubastatin-A. Chronic treatment with tubastatin-A not only increased α-tubulin acetylation but also translocated Gαs from lipid-rafts, without changing total Gαs. Reciprocally, depletion of α-tubulin acetyl-transferase-1 ablated this phenomenon. While escitalopram and imipramine also disrupt Gαs/tubulin complexes and translocate Gαs from rafts, they evoke no change in tubulin acetylation. Finally, two indicators of downstream cAMP signaling, cAMP response element binding protein phosphorylation (pCREB) and expression of brain-derived-neurotrophic-factor (BDNF) were both elevated by tubastatin-A. These findings suggest HDAC6 inhibitors show a cellular profile resembling traditional antidepressants, but have a distinct mode of action. They also reinforce the validity of antidepressant-induced Gαs translocation from lipid-rafts as a biosignature for antidepressant response that may be useful in the development of new antidepressant compounds.

Regulation of heterotrimeric G-protein signaling by NDPK/NME proteins and caveolins: an update

Heterotrimeric G proteins are pivotal mediators of cellular signal transduction in eukaryotic cells and abnormal G-protein signaling plays an important role in numerous diseases. During the last two decades it has become evident that the activation status of heterotrimeric G proteins is both highly localized and strongly regulated by a number of factors, including a receptor-independent activation pathway of heterotrimeric G proteins that does not involve the classical GDP/GTP exchange and relies on nucleoside diphosphate kinases (NDPKs). NDPKs are NTP/NDP transphosphorylases encoded by the nme/nm23 genes that are involved in a variety of cellular events such as proliferation, migration, and apoptosis. They therefore contribute, for example, to tumor metastasis, angiogenesis, retinopathy, and heart failure. Interestingly, NDPKs are translocated and/or upregulated in human heart failure. Here we describe recent advances in the current understanding of NDPK functions and how they have an impact on local regulation of G-protein signaling.

Receptor-specific crosstalk between prostanoid E receptor 3 and bombesin receptor subtype 3

Bombesin receptor subtype 3 (BRS-3) is a GPCR that is expressed in the CNS, peripheral tissues, and tumors. Our understanding of BRS-3's role in physiology and pathophysiology is limited because its natural ligand is unknown. In an attempt to identify this ligand, we screened toad skin ( Bufo bufo gargarizans Cantor) extracts and identified prostaglandins as putative ligands. In BRS-3-transfected human embryonic kidney (HEK) cells, we found that prostaglandins, with prostaglandin E2 (PGE2) being the most potent, fulfill the pharmacologic criteria of affinity, selectivity, and specificity to be considered as agonists to the BRS-3 receptor. However, PGE2 is unable to activate BRS-3 in different cellular environments. We speculated that EP receptors might be the cause of this cellular selectivity, and we found that EP₃ is the receptor primarily responsible for the differential PGE2 effect. Consequently, we reconstituted the HEK environment in Chinese hamster ovary (CHO) cells and found that BRS-3 and EP₃ interact to potentiate PGE2 signaling. This potentiating effect is receptor specific, and it occurs only when BRS-3 is paired to EP₃. Our study represents an example of functional crosstalk between two distantly related GPCRs and may be of clinical importance for BRS-3-targeted therapies.-Zhang, Y., Liu, Y., Wu, L., Fan, C., Wang, Z., Zhang, X., Alachkar, A., Liang, X., Civelli, O. Receptor-specific crosstalk between prostanoid E receptor 3 and bombesin receptor subtype 3.

Single-molecule imaging reveals receptor-G protein interactions at cell surface hot spots

G-protein-coupled receptors mediate the biological effects of many hormones and neurotransmitters and are important pharmacological targets. They transmit their signals to the cell interior by interacting with G proteins. However, it is unclear how receptors and G proteins meet, interact and couple. Here we analyse the concerted motion of G-protein-coupled receptors and G proteins on the plasma membrane and provide a quantitative model that reveals the key factors that underlie the high spatiotemporal complexity of their interactions. Using two-colour, single-molecule imaging we visualize interactions between individual receptors and G proteins at the surface of living cells. Under basal conditions, receptors and G proteins form activity-dependent complexes that last for around one second. Agonists specifically regulate the kinetics of receptor-G protein interactions, mainly by increasing their association rate. We find hot spots on the plasma membrane, at least partially defined by the cytoskeleton and clathrin-coated pits, in which receptors and G proteins are confined and preferentially couple. Imaging with the nanobody Nb37 suggests that signalling by G-protein-coupled receptors occurs preferentially at these hot spots. These findings shed new light on the dynamic interactions that control G-protein-coupled receptor signalling.

The use of fluorescence correlation spectroscopy to characterize the molecular mobility of fluorescently labelled G protein-coupled receptors

The membranes of living cells have been shown to be highly organized into distinct microdomains, which has spatial and temporal consequences for the interaction of membrane bound receptors and their signalling partners as complexes. Fluorescence correlation spectroscopy (FCS) is a technique with single cell sensitivity that sheds light on the molecular dynamics of fluorescently labelled receptors, ligands or signalling complexes within small plasma membrane regions of living cells. This review provides an overview of the use of FCS to probe the real time quantification of the diffusion and concentration of G protein-coupled receptors (GPCRs), primarily to gain insights into ligand–receptor interactions and the molecular composition of signalling complexes. In addition we document the use of photon counting histogram (PCH) analysis to investigate how changes in molecular brightness (ε) can be a sensitive indicator of changes in molecular mass of fluorescently labelled moieties.

Real time monitoring of membrane GPCR reconstitution by plasmon waveguide resonance: on the role of lipids

G-protein coupled receptors (GPCRs) are important therapeutic targets since more than 40% of the drugs on the market exert their action through these proteins. To decipher the molecular mechanisms of activation and signaling, GPCRs often need to be isolated and reconstituted from a detergent-solubilized state into a well-defined and controllable lipid model system. Several methods exist to reconstitute membrane proteins in lipid systems but usually the reconstitution success is tested at the end of the experiment and often by an additional and indirect method. Irrespective of the method used, the reconstitution process is often an intractable and time-consuming trial-and-error procedure. Herein, we present a method that allows directly monitoring the reconstitution of GPCRs in model planar lipid membranes. Plasmon waveguide resonance (PWR) allows following GPCR lipid reconstitution process without any labeling and with high sensitivity. Additionally, the method is ideal to probe the lipid effect on receptor ligand binding as demonstrated by antagonist binding to the chemokine CCR5 receptor.

Membrane-Mediated Oligomerization of G Protein Coupled Receptors and Its Implications for GPCR Function

The dimerization or even oligomerization of G protein coupled receptors (GPCRs) causes ongoing, controversial debates about its functional role and the coupled biophysical, biochemical or biomedical implications. A continously growing number of studies hints to a relation between oligomerization and function of GPCRs and strengthens the assumption that receptor assembly plays a key role in the regulation of protein function. Additionally, progress in the structural analysis of GPCR-G protein and GPCR-ligand interactions allows to distinguish between actively functional and non-signaling complexes. Recent findings further suggest that the surrounding membrane, i.e., its lipid composition may modulate the preferred dimerization interface and as a result the abundance of distinct dimeric conformations. In this review, the association of GPCRs and the role of the membrane in oligomerization will be discussed. An overview of the different reported oligomeric interfaces is provided and their capability for signaling discussed. The currently available data is summarized with regard to the formation of GPCR oligomers, their structures and dependency on the membrane microenvironment as well as the coupling of oligomerization to receptor function.

Antidepressants Accumulate in Lipid Rafts Independent of Monoamine Transporters to Modulate Redistribution of the G Protein, Gαs

Depression is a significant public health problem for which currently available medications, if effective, require weeks to months of treatment before patients respond. Previous studies have shown that the G protein responsible for increasing cAMP (Gαs) is increasingly localized to lipid rafts in depressed subjects and that chronic antidepressant treatment translocates Gαs from lipid rafts. Translocation of Gαs, which shows delayed onset after chronic antidepressant treatment of rats or of C6 glioma cells, tracks with the delayed onset of therapeutic action of antidepressants. Because antidepressants appear to specifically modify Gαs localized to lipid rafts, we sought to determine whether structurally diverse antidepressants accumulate in lipid rafts. Sustained treatment of C6 glioma cells, which lack 5-hydroxytryptamine transporters, showed marked concentration of several antidepressants in raft fractions, as revealed by increased absorbance and by mass fingerprint. Closely related molecules without antidepressant activity did not concentrate in raft fractions. Thus, at least two classes of antidepressants accumulate in lipid rafts and effect translocation of Gαs to the non-raft membrane fraction, where it activates the cAMP-signaling cascade. Analysis of the structural determinants of raft localization may both help to explain the hysteresis of antidepressant action and lead to design and development of novel substrates for depression therapeutics.

Neuronal Orphan G-Protein Coupled Receptor Proteins Mediate Plasmalogens-Induced Activation of ERK and Akt Signaling

The special glycerophospholipids plasmalogens (Pls) are enriched in the brain and reported to prevent neuronal cell death by enhancing phosphorylation of Akt and ERK signaling in neuronal cells. Though the activation of Akt and ERK was found to be necessary for the neuronal cells survival, it was not known how Pls enhanced cellular signaling. To answer this question, we searched for neuronal specific orphan GPCR (G-protein coupled receptor) proteins, since these proteins were believed to play a role in cellular signal transduction through the lipid rafts, where both Pls and some GPCRs were found to be enriched. In the present study, pan GPCR inhibitor significantly reduced Pls-induced ERK signaling in neuronal cells, suggesting that Pls could activate GPCRs to induce signaling. We then checked mRNA expression of 19 orphan GPCRs and 10 of them were found to be highly expressed in neuronal cells. The knockdown of these 10 neuronal specific GPCRs by short hairpin (sh)-RNA lentiviral particles revealed that the Pls-mediated phosphorylation of ERK was inhibited in GPR1, GPR19, GPR21, GPR27 and GPR61 knockdown cells. We further found that the overexpression of these GPCRs enhanced Pls-mediated phosphorylation of ERK and Akt in cells. Most interestingly, the GPCRs-mediated cellular signaling was reduced significantly when the endogenous Pls were reduced. Our cumulative data, for the first time, suggest a possible mechanism for Pls-induced cellular signaling in the nervous system.

DT(270-326) , a Truncated Diphtheria Toxin, Increases Blood-Tumor Barrier Permeability by Upregulating the Expression of Caveolin-1

AIM

The nontoxic mutant of diphtheria toxin (DT) has been demonstrated to act as a receptor-specific carrier protein to delivery drug into brain. Recent research showed that the truncated "receptorless" DT was still capable of being internalized into cells. This study investigated the effects and potential mechanisms of DT(270-326) , a truncated "receptorless" DT, on the permeability of the blood-tumor barrier (BTB).

METHODS

BTB and GECs were subjected to DT(270-326) treatment. HRP flux assays, immunofluorescent, co-immunoprecipitation, Western blot, CCK-8, and Flow cytometry analysis were used to evaluate the effects of DT(270-326) administration.

RESULTS

Our results revealed that 5 μM of DT(270-326) significantly increased the permeability of BTBin vitro, which reached its peak at 6 h. The permeability was reduced by pretreatment with filipinIII. DT(270-326) co-localized and interacted with caveolin-1 via its caveolin-binding motif. The mRNA and protein expression levels of caveolin-1 were identical with the changes of BTB permeability. The upregulated expression of caveolin-1 was associated with Src kinase-dependent tyrosine phosphorylation of caveolin-1, which subsequently induced phosphorylation and inactivation of the transcription factor Egr-1. The combination of DT(270-326) with doxorubicin significantly enhanced the loss of cell viability and apoptosis of U87 glioma cells in contrast to doxorubicin alone.

CONCLUSIONS

DT(270-326) might provide a novel strategy to increase the delivery of macromolecular therapeutic agents across the BTB.

Mechanotransduction in cardiac hypertrophy and failure

Cardiac muscle cells have an intrinsic ability to sense and respond to mechanical load through a process known as mechanotransduction. In the heart, this process involves the conversion of mechanical stimuli into biochemical events that induce changes in myocardial structure and function. Mechanotransduction and its downstream effects function initially as adaptive responses that serve as compensatory mechanisms during adaptation to the initial load. However, under prolonged and abnormal loading conditions, the remodeling processes can become maladaptive, leading to altered physiological function and the development of pathological cardiac hypertrophy and heart failure. Although the mechanisms underlying mechanotransduction are far from being fully elucidated, human and mouse genetic studies have highlighted various cytoskeletal and sarcolemmal structures in cardiac myocytes as the likely candidates for load transducers, based on their link to signaling molecules and architectural components important in disease pathogenesis. In this review, we summarize recent developments that have uncovered specific protein complexes linked to mechanotransduction and mechanotransmission within the sarcomere, the intercalated disc, and at the sarcolemma. The protein structures acting as mechanotransducers are the first step in the process that drives physiological and pathological cardiac hypertrophy and remodeling, as well as the transition to heart failure, and may provide better insights into mechanisms driving mechanotransduction-based diseases.

Cyclic Adenosine 3',5'-Monophosphate Elevation and Biological Signaling through a Secretin Family Gs-Coupled G Protein-Coupled Receptor Are Restricted to a Single Adenylate Cyclase Isoform

PC12 cells express five adenylate cyclase (AC) isoforms, most abundantly AC6 and AC7. These two ACs were individually silenced using lentiviral short hairpin RNAs, which lead to a decrease (≥80%) of the protein product of each transcript. These stable PC12 sublines were then used to examine potential AC isoform preference for signaling through a family B G protein-coupled receptor (GPCR). Cells were challenged with the endogenous agonist of the pituitary adenylate cyclase-activating polypeptide type I receptor (PAC1), pituitary adenylate cyclase-activating polypeptide (PACAP)-38, or the diterpene forskolin as an AC-proximal control. Intracellular cAMP levels were elevated by forskolin about equally in wild-type, AC6, and AC7 knockdown cells. The ability of PACAP-38 and forskolin to activate three cAMP sensors downstream of AC [protein kinase A (PKA), exchange protein activated by cAMP (Epac) 2/Rapgef4, and neuritogenic cAMP sensor (NCS)/Rapgef2] was examined by monitoring the phosphorylation status of their respective targets, cAMP response element-binding protein, p38, and extracellular signal-regulated kinase. Forskolin stimulation of each downstream target of cAMP was unaffected by knockdown of either AC6 or AC7. PACAP-38 activation of all downstream targets of cAMP was unaffected by AC7 knockdown, but abolished following AC6 knockdown. Membrane cholesterol depletion with methyl-β-cyclodextrin mimicked the effects of AC6 silencing on PACAP signaling, without attenuating forskolin signaling. These data suggest that vicinal constraint of the GPCR PAC1 and AC6 determines the exclusive requirement for this AC in PACAP signaling, but that the coupling of the cAMP sensors PKA, Epac2/Rapgef4, and NCS/Rapgef2, to their respective downstream signaling targets, determines how cAMP signaling is parcellated to physiologic responses, such as neuritogenesis, upon GPCR-Gs activation in neuroendocrine cells.

Caveolin modulates integrin function and mechanical activation in the cardiomyocyte

β1 integrins (β1) transduce mechanical signals in many cells, including cardiac myocytes (CM). Given their close localization, as well as their role in mechanotransduction and signaling, we hypothesized that caveolin (Cav) proteins might regulate integrins in the CM. β1 localization, complex formation, activation state, and signaling were analyzed using wild-type, Cav3 knockout, and Cav3 CM-specific transgenic heart and myocyte samples. Studies were performed under basal and mechanically loaded conditions. We found that: (1) β1 and Cav3 colocalize in CM and coimmunoprecipitate from CM protein lysates; (2) β1 is detected in a subset of caveolae; (3) loss of Cav3 caused reduction of β1D integrin isoform and active β1 integrin from the buoyant domains in the heart; (4) increased expression of myocyte Cav3 correlates with increased active β1 integrin in adult CM; (5) in vivo pressure overload of the wild-type heart results in increased activated integrin in buoyant membrane domains along with increased association between active integrin and Cav3; and (6) Cav3-deficient myocytes have perturbed basal and stretch mediated signaling responses. Thus, Cav3 protein can modify integrin function and mechanotransduction in the CM and intact heart.

Potential of caveolae in the therapy of cardiovascular and neurological diseases

Caveolae are membrane micro-domains enriched in cholesterol, sphingolipids and caveolins, which are transmembrane proteins with a hairpin-like structure. Caveolae participate in receptor-mediated trafficking of cell surface receptors and receptor-mediated signaling. Furthermore, caveolae participate in clathrin-independent endocytosis of membrane receptors. On the one hand, caveolins are involved in vascular and cardiac dysfunction. Also, neurological abnormalities in caveolin-1 knockout mice and a link between caveolin-1 gene haplotypes and neurodegenerative diseases have been reported. The aim of this article is to present the rationale for considering caveolae as potential targets in cardiovascular and neurological diseases.

Role of membrane microdomains in compartmentation of cAMP signaling

Spatially restricting cAMP production to discrete subcellular locations permits selective regulation of specific functional responses. But exactly where and how cAMP signaling is confined is not fully understood. Different receptors and adenylyl cyclase isoforms responsible for cAMP production are not uniformly distributed between lipid raft and non-lipid raft domains of the plasma membrane. We sought to determine the role that these membrane domains play in organizing cAMP responses in HEK293 cells. The freely diffusible FRET-based biosensor Epac2-camps was used to measure global cAMP responses, while versions of the probe targeted to lipid raft (Epac2-MyrPalm) and non-raft (Epac2-CAAX) domains were used to monitor local cAMP production near the plasma membrane. Disruption of lipid rafts by cholesterol depletion selectively altered cAMP responses produced by raft-associated receptors. The results indicate that receptors associated with lipid raft as well as non-lipid raft domains can contribute to global cAMP responses. In addition, basal cAMP activity was found to be significantly higher in non-raft domains. This was supported by the fact that pharmacologic inhibition of adenylyl cyclase activity reduced basal cAMP activity detected by Epac2-CAAX but not Epac2-MyrPalm or Epac2-camps. Responses detected by Epac2-CAAX were also more sensitive to direct stimulation of adenylyl cyclase activity, but less sensitive to inhibition of phosphodiesterase activity. Quantitative modeling was used to demonstrate that differences in adenylyl cyclase and phosphodiesterase activities are necessary but not sufficient to explain compartmentation of cAMP associated with different microdomains of the plasma membrane.

CaV1.2 signaling complexes in the heart

L-type Ca(2+) channels (LTCCs) are essential for generation of the electrical and mechanical properties of cardiac muscle. Furthermore, regulation of LTCC activity plays a central role in mediating the effects of sympathetic stimulation on the heart. The primary mechanism responsible for this regulation involves β-adrenergic receptor (βAR) stimulation of cAMP production and subsequent activation of protein kinase A (PKA). Although it is well established that PKA-dependent phosphorylation regulates LTCC function, there is still much we do not understand. However, it has recently become clear that the interaction of the various signaling proteins involved is not left to completely stochastic events due to random diffusion. The primary LTCC expressed in cardiac muscle, CaV1.2, forms a supramolecular signaling complex that includes the β2AR, G proteins, adenylyl cyclases, phosphodiesterases, PKA, and protein phosphatases. In some cases, the protein interactions with CaV1.2 appear to be direct, in other cases they involve scaffolding proteins such as A kinase anchoring proteins and caveolin-3. Functional evidence also suggests that the targeting of these signaling proteins to specific membrane domains plays a critical role in maintaining the fidelity of receptor mediated LTCC regulation. This information helps explain the phenomenon of compartmentation, whereby different receptors, all linked to the production of a common diffusible second messenger, can vary in their ability to regulate LTCC activity. The purpose of this review is to examine our current understanding of the signaling complexes involved in cardiac LTCC regulation.

Caveolins: targeting pro-survival signaling in the heart and brain

The present review discusses intracellular signaling moieties specific to membrane lipid rafts (MLRs) and the scaffolding proteins caveolin and introduces current data promoting their potential role in the treatment of pathologies of the heart and brain. MLRs are discreet microdomains of the plasma membrane enriched in gylcosphingolipids and cholesterol that concentrate and localize signaling molecules. Caveolin proteins are necessary for the formation of MLRs, and are responsible for coordinating signaling events by scaffolding and enriching numerous signaling moieties in close proximity. Specifically in the heart and brain, caveolins are necessary for the cytoprotective phenomenon termed ischemic and anesthetic preconditioning. Targeted overexpression of caveolin in the heart and brain leads to induction of multiple pro-survival and pro-growth signaling pathways; thus, caveolins represent a potential novel therapeutic target for cardiac and neurological pathologies.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Receptor-G protein interaction studied by bioluminescence resonance energy transfer: lessons from protease-activated receptor 1

Since its development, the bioluminescence resonance energy transfer (BRET) approach has been extensively applied to study G protein-coupled receptors (GPCRs) in real-time and in live cells. One of the major aspects of GPCRs investigated in considerable details is their physical coupling to the heterotrimeric G proteins. As a result, new concepts have emerged, but few questions are still a matter of debate illustrating the complexity of GPCR-G protein interactions and coupling. Here, we summarized the recent advances on our understanding of GPCR-G protein coupling based on BRET approaches and supported by other FRET-based studies. We essentially focused on our recent studies in which we addressed the concept of preassembly vs. the agonist-dependent interaction between the protease-activated receptor 1 (PAR1) and its cognate G proteins. We discussed the concept of agonist-induced conformational changes within the preassembled PAR1-G protein complexes as well as the critical question how the multiple coupling of PAR1 with two different G proteins, Gαi1 and Gα12, but also β-arrestin 1, can be regulated.

Adenosine monophosphate-activated kinase alpha1 promotes endothelial barrier repair

The vascular endothelium responds to damage through activation of multiple signaling events that restore cell-cell adhesion and vascular integrity. However, the molecular mechanisms that integrate these events are not clearly defined. Herein, we identify a previously unexpected role for adenosine monophosphate-activated protein kinase (AMPK) in pulmonary microvascular endothelial cell (PMVEC) repair. PMVECs selectively express the AMPKα1 catalytic subunit, pharmacological and short hairpin RNA-mediated inhibition of which attenuates Ca(2+) entry in these cells induced by the inflammatory Ca(2+)-signaling mimetic thapsigargin. We find that AMPKα1 activity is required for the formation of PMVEC cell-cell networks in a prorepair environment and for monolayer resealing after wounding. Decreasing AMPKα1 expression reduces barrier resistance in PMVEC monolayers, results consistent with a role for AMPKα1 in cell-cell adhesion. AMPKα1 colocalizes and coimmunoprecipitates with the adherens junction protein N-cadherin and cofractionates with proteins selectively expressed in caveolar membranes. Assessment of permeability, by measuring the filtration coefficient (K(f)) in isolated perfused lungs, confirmed that AMPK activation contributes to barrier repair in vivo. Our findings thus provide novel evidence for AMPKα1 in Ca(2+) influx-mediated signaling and wound repair in the endothelium.

Sphingosine 1-phosphate-induced motility and endocytosis of dendritic cells is regulated by SWAP-70 through RhoA

The phospholipid mediator sphingosine 1-phosphate (S1P) enhances motility and endocytosis of mature dendritic cells (DCs). We show that in vitro migration of Swap-70(-/-) bone marrow-derived DCs (BMDCs) in response to S1P and S1P-induced upregulation of endocytosis are significantly reduced. S1P-stimulated movement of Swap-70(-/-) BMDCs, specifically retraction of their trailing edge, in a collagen three-dimensional environment is impaired. These in vitro observations correlate with delayed entry into lymphatic vessels and migration to lymph nodes of skin DCs in Swap-70(-/-) mice. Expression of S1P receptors (S1P(1-3)) by wild-type and Swap-70(-/-) BMDCs is similar, but Swap-70(-/-) BMDCs fail to activate RhoA and to localize Rac1 and RhoA into areas of actin polymerization after S1P stimulus. The Rho-activating G protein Gα(i) interacts with SWAP-70, which also supports the localization of Gα(13) to membrane rafts in BMDCs. LPS-matured Swap-70(-/-) BMDCs contain significantly more active RhoA than wild-type DCs. Preinhibition of Rho activation restored migration to S1P, S1P-induced upregulation of endocytosis in mature Swap-70(-/-) BMDCs, and localization of Gα(13) to membrane rafts. These data demonstrate SWAP-70 as a novel regulator of S1P signaling necessary for DC motility and endocytosis.

Unanticipated signaling events associated with cardiac adenylyl cyclase gene transfer

The published papers on the effects of increased cardiac expression of adenylyl cyclase type 6 (AC6) are reviewed. These include the effects of AC on normal and failing left ventricle in several pathophysiological models in mice and pigs. In addition, the effects of increased expression of AC6 in cultured neonatal and adult rat cardiac myocytes are discussed in the context of attempting to establish mechanisms for the unanticipated beneficial effects of AC6 on the failing heart. This article is part of a Special Section entitled "Special Section: Cardiovascular Gene Therapy".

Membrane rafts and GnRH receptor signaling

The binding of hypothalamic gonadotropin-releasing hormone (GnRH) to the pituitary GnRH receptor (GnRHR) is essential for reproductive function by stimulating the synthesis and secretion of gonadotropic hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH). Engagement of the GnRHR by GnRH initiates a complex series of signaling events that include the activation of various mitogen-activated protein kinase (MAPK) pathways, including extracellular signal-regulated kinase (ERK). GnRHR signaling is thought to initiate within specialized microdomains in the plasma membrane termed membrane rafts. These microdomains are enriched in sphingolipid and cholesterol and are believed to be highly dynamic organizing centers for receptors and their cognate signaling molecules associated with the plasma membrane. Within this review we discuss the composition and role of membrane rafts in cell signaling and examine evidence that the mammalian type I GnRHR is constitutively and exclusively localized to these membrane microdomains in various experimental models. We conclude that membrane raft composition and organization potentially underlie the functional ability of GnRH to elicit the assembly of multi-protein signaling complexes necessary for downstream signaling to the ERK pathway that ultimately is critical for controlling fertility.

Sympathetic stimulation of adult cardiomyocytes requires association of AKAP5 with a subpopulation of L-type calcium channels

RATIONALE

Sympathetic stimulation of the heart increases the force of contraction and rate of ventricular relaxation by triggering protein kinase (PK)A-dependent phosphorylation of proteins that regulate intracellular calcium. We hypothesized that scaffolding of cAMP signaling complexes by AKAP5 is required for efficient sympathetic stimulation of calcium transients.

OBJECTIVE

We examined the function of AKAP5 in the β-adrenergic signaling cascade.

METHODS AND RESULTS

We used calcium imaging and electrophysiology to examine the sympathetic response of cardiomyocytes isolated from wild type and AKAP5 mutant animals. The β-adrenergic regulation of calcium transients and the phosphorylation of substrates involved in calcium handling were disrupted in AKAP5 knockout cardiomyocytes. The scaffolding protein, AKAP5 (also called AKAP150/79), targets adenylyl cyclase, PKA, and calcineurin to a caveolin 3-associated complex in ventricular myocytes that also binds a unique subpopulation of Ca(v)1.2 L-type calcium channels. Only the caveolin 3-associated Ca(v)1.2 channels are phosphorylated by PKA in response to sympathetic stimulation in wild-type heart. However, in the AKAP5 knockout heart, the organization of this signaling complex is disrupted, adenylyl cyclase 5/6 no longer associates with caveolin 3 in the T-tubules, and noncaveolin 3-associated calcium channels become phosphorylated after β-adrenergic stimulation, although this does not lead to an enhanced calcium transient. The signaling domain created by AKAP5 is also essential for the PKA-dependent phosphorylation of ryanodine receptors and phospholamban.

CONCLUSIONS

These findings identify an AKAP5-organized signaling module that is associated with caveolin 3 and is essential for sympathetic stimulation of the calcium transient in adult heart cells.

Differential involvement of caveolin-1 in brown adipocyte signaling: impaired beta3-adrenergic, but unaffected LPA, PDGF and EGF receptor signaling

Caveolae and caveolin have been implicated as being involved in the signal transduction of many receptors, including the EGF, PDGF, LPA and beta3-adrenergic receptors. To investigate the role of caveolin-1 (Cav1) in these signaling pathways in brown adipose tissue, primary brown adipocyte cultures from Cav1-ablated mice and wild-type mice were investigated. In pre-adipocytes, Cav1-ablation affected neither the G-protein coupled LPA receptor signaling to Erk1/2, nor the receptor tyrosine kinases PDGF- or EGF-receptor signaling to Erk1/2. Mature primary Cav1-/- brown adipocytes accumulated lipids and expressed aP2 to the same extent as did wild-type cells. However, the cAMP levels induced by the beta3-adrenergic receptor agonist CL316,243 were lower in the Cav1-/- cultures, with an unchanged EC50 for CL316,243. Also the response to the direct adenylyl cyclase agonist forskolin was reduced. Thus, in brown adipocytes, Cav1 is apparently required for an intact response to adenylyl cyclase-linked agonists/activators, whereas other signaling pathways examined function without Cav1.

Isoflurane via TGF-beta1 release increases caveolae formation and organizes sphingosine kinase signaling in renal proximal tubules

We previously showed that the inhalational anesthetic isoflurane protects against renal proximal tubule necrosis via isoflurane-mediated stimulation and translocation of sphingosine kinase-1 (SK1) with subsequent synthesis of sphingosine-1-phosphate (S1P) in renal proximal tubule cells (Kim M, Kim M, Kim N, D'Agati VD, Emala CW Sr, Lee HT. Am J Physiol Renal Physiol 293: F1827-F1835, 2007). We also demonstrated that the anti-necrotic and anti-inflammatory effect of isoflurane is due in part to phosphatidylserine (PS) externalization and subsequent release of transforming growth factor-beta1 (TGF-beta1) (Lee HT, Kim M, Kim J, Kim N, Emala CW. Am J Nephrol 27: 416-424, 2007). In this study, we tested the hypothesis that isoflurane, via TGF-beta1 release, increases caveolae formation in the buoyant fraction of the cell membrane of human renal proximal tubule (HK-2) cells to organize SK1 and S1P signaling. To detect SK1 protein in the caveolae/caveolin fractions, we overexpressed human SK1 in HK-2 cells (SK1-HK-2). SK1-HK-2 cells exposed to isoflurane increased caveolae/caveolin formation in the buoyant membrane fractions which contained key signaling intermediates involved in isoflurane-mediated renal tubule protection, including S1P, SK1, ERK MAPK, and TGF-beta1 receptors. Furthermore, treating SK1-HK-2 cells with recombinant TGF-beta1 or PS liposome mixture increased caveolae formation, mimicking the effects of isoflurane. Conversely, TGF-beta1-neutralizing antibody blocked the increase in caveolae formation induced by isoflurane in SK1-HK-2 cells. The increase in SK1 activity in the caveolae-enriched fractions from isoflurane-treated nonlentivirus-infected HK-2 cells, while smaller in magnitude, was qualitatively similar to that found in the SK1-HK-2 cell line. Finally, isoflurane also increased caveolae formation in the kidneys of TGF-beta1 +/+ mice but not in TGF-beta1 +/- mice (mice with reduced levels of TGF-beta1). Our study demonstrates that isoflurane organizes several key cytoprotective signaling intermediates including TGF-beta1 receptors, SK1 and ERK, within the caveolae fraction of the plasma membrane. Our findings may help to unravel the cellular signaling pathways of volatile anesthetic-mediated renal protection and lead to new therapeutic applications of inhalational anesthetics during the perioperative period.

beta(1)-Adrenergic receptor vs adenylyl cyclase 6 expression in cardiac myocytes: differences in transgene localization and intracellular signaling

Adenylyl cyclase type 6 (AC6) and the beta(1) adrenergic receptor (beta(1)AR) are pivotal proteins in transmembrane betaAR-signaling in cardiac myocytes. Increased expression of AC6 has beneficial effects on the heart, but increased beta(1)AR expression has marked deleterious effects. Why do these two elements of the betaAR pathway have such different effects? Using adenovirus-mediated gene transfer of the two transgenes in neonatal rat cardiac myocytes, we assessed cellular distribution and performed selected biochemical assays. beta(1)AR was found predominantly in the plasma membrane. In contrast, AC6 was found in the plasma membrane but also was associated with the nuclear envelope, sarcoplasmic reticulum, mitochondria, and cytoplasm. Increased beta(1)AR, but not AC6, increased follistatin expression, p38 phosphorylation, phosphatidylserine translocation to the PM, and apoptosis. In contrast, increased AC6, but not beta(1)AR, inhibited PHLPP2 activity, activated PI3K and Akt, and increased p70S6 kinase phosphorylation and Bcl-2 expression; apoptosis was unchanged. The distribution of AC6 to multiple cellular compartments appears to enable interactions with other proteins (e.g., PHLPP2) and activates cardioprotective signaling (PI3K/Akt). In contrast, beta(1)AR, confined to the plasma membrane, increased phosphatidylserine translocation and apoptosis. These data provide a potential underlying mechanism for the beneficial vs deleterious effects of these two related betaAR-signaling elements.

Heteromerization of dopamine D2 receptors with dopamine D1 or D5 receptors generates intracellular calcium signaling by different mechanisms

The repertoire of signal transduction pathways activated by dopamine in brain includes the increase of intracellular calcium. However the mechanism(s) by which dopamine activated this important second messenger system was/were unknown. Although we showed that activation of the D5 dopamine receptor increased calcium concentrations, the restricted anatomic distribution of this receptor made this unlikely to be the major mechanism in brain. We have identified novel heteromeric dopamine receptor complexes that are linked to calcium signaling. The calcium pathway activated through the D1-D2 receptor heteromer involved coupling to Gq, through phospholipase C and IP(3) receptors to result in a rise in intracellular calcium. The calcium rise activated through the D2-D5 receptor heteromer involved a small rise in intracellular calcium through the Gq pathway that triggered a store-operated channel mediated influx of extracellular calcium. These novel receptor heteromeric complexes, for the first time, establish the link between dopamine action and rapid calcium signaling.

CaMKII uses GTP as a phosphate donor for both substrate and autophosphorylation

The vast majority of serine/threonine protein kinases have a strong preference for ATP over GTP as a phosphate donor. CK2 (Casein kinase 2) is an exception to this rule and in this study we investigate whether calcium/calmodulin-dependent protein kinase II (CaMKII) has the same extended nucleotide range. Using the Drosophila enzyme, we have shown that CaMKII uses Mg(2+)GTP with a higher K(m) and V(max) compared to Mg(2+)ATP. Substitution of Mn(2+) for Mg(2+) resulted in a much lower K(m) for GTP, while nearly abolishing the ability of CaMKII to use ATP. These similar results were obtained with rat alphaCaMKII, showing the ability to use GTP to be a general property of CaMKII. The V(max) difference between Mg(2+)ATP and Mg(2+)GTP was found to be due to the fact that ADP is a potent inhibitor of phosphorylation, while GDP has modest effects. There were no differences found between sites autophosphorylated by ATP and GTP, either by partial proteolysis or mass spectrometry. Phosphorylation of fly head extract revealed that similar proteins are substrates for CaMKII whether using Mg(2+)ATP or Mg(2+)GTP. This new information confirms that CaMKII can use both ATP and GTP, and opens new avenues for the study of regulation of this kinase.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.

Role of lipid rafts in virus infection

Rafts are domains of the plasma membrane, enriched in cholesterol and sphingolipids; they form a platform for signaling proteins and receptors. The lipid rafts are utilized in the replication cycle of numerous viruses. Internalization receptors of many viruses localize to rafts or are recruited there after virus binding. Arrays of signal transduction proteins found in rafts contribute to efficient trafficking and productive infection. Some viruses are dependent on raft domains for the biogenesis of their membranous replication structures. Finally, rafts are often important in virus assembly and budding. Subsequently, raft components in the viral envelope may be vital for the entry to a new host cell. Here, we summarize the current knowledge of the involvement of rafts in virus infection.

G protein-coupled receptor kinase 4 (GRK4) regulates the phosphorylation and function of the dopamine D3 receptor

During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance by engendering natriuresis. Dopamine exerts its effects on dopamine receptors, including the dopamine D(3) receptor. G protein-coupled receptor kinase 4 (GRK4), whose gene locus (4p16.3) is linked to essential hypertension, desensitizes the D(1) receptor, another dopamine receptor. This study evaluated the role of GRK4 on D(3) receptor function in human proximal tubule cells. D(3) receptor co-segregated in lipid rafts and co-immunoprecipitated and co-localized in human proximal tubule cells and in proximal and distal tubules and glomeruli of kidneys of Wistar Kyoto rats. Bimolecular fluorescence complementation and confocal microscopy revealed that agonist activation of the receptor initiated the interaction between D(3) receptor and GRK4 at the cell membrane and promoted it intracellularly, presumably en route to endosomal trafficking. Of the four GRK4 splice variants, GRK4-gamma and GRK4-alpha mediated a 3- and 2-fold increase in the phosphorylation of agonist-activated D(3) receptor, respectively. Inhibition of GRK activity with heparin or knockdown of GRK4 expression via RNA interference completely abolished p44/42 phosphorylation and mitogenesis induced by D(3) receptor stimulation. These data demonstrate that GRK4, specifically the GRK4-gamma and GRK4-alpha isoforms, phosphorylates the D(3) receptor and is crucial for its signaling in human proximal tubule cells.

Nanoliposomal short-chain ceramide inhibits agonist-dependent translocation of neurotensin receptor 1 to structured membrane microdomains in breast cancer cells

Neurotensin (NTS) receptor 1 (NTSR1) is a G protein-coupled receptor that has been recently identified as a mediator of tumorigenicity and metastasis. NTSR1, as well as its endogenous ligand, NTS, are coexpressed in several breast cancer cell lines and breast cancer tumor samples but not in normal breast tissue. We have previously published that ceramide mimetics could inhibit breast cancer growth in vitro and in vivo. Thus, understanding the biochemical and biophysical regulation of NTSR1 by ceramide can help further define NTSR1 as a novel target in breast cancer. Our results show that nanoliposomal formulations of ceramide inhibit NTSR1-mediated MDA-MB-231 breast cancer progression (mitogenesis, migration, and matrix metalloproteinase-9 activity). In addition, liposomal ceramide inhibited NTSR1-mediated, but not phorbol 12-myristate 13-acetate-mediated, activation of the mitogen-activated protein kinase pathway. Mechanistically, nanoliposomal short-chain ceramide reduces NTSR1 interaction with Galphaq/11 subunits within structured membrane microdomains, consistent with diminished NTS-induced translocation of NTSR1 into membrane microdomains. Collectively, our findings suggest that exogenous short-chain ceramide has the potential to be used as an adjuvant therapy to inhibit NTS-dependent breast cancer progression.

Dynamic protein kinase a activities induced by beta-adrenoceptors dictate signaling propagation for substrate phosphorylation and myocyte contraction

cAMP/protein kinase (PK)A activation represents a key signaling mechanism for neurohormonal stimulation of diversified physiological processes. Using real-time, fluorescence resonance energy transfer-based imaging of PKA activity in neonatal cardiac myocytes, we report that sustained activation of PKA induced by beta-adrenoceptor (betaAR) dictates signaling propagation for substrate phosphorylation and myocyte contraction. Activation of betaARs in wild-type myocytes induces strong and sustained PKA activities, which are rapidly attenuated on washing away agonist or adding antagonist to the cells. The sustained PKA activities promote signaling propagation to the sarcoplasmic reticulum for phosphorylation of phospholamban and increases in myocyte contraction. Addition of antagonist after betaAR stimulation significantly attenuates PKA phosphorylation of phospholamban and rapidly reduces contraction rate increases. Moreover, stimulation of beta(1)AR subtype induces PKA activities similar to those in wild-type cells. In contrast, stimulation of beta(2)AR subtype induces strong initial activation of PKA similar to those induced by beta(1)AR; however, the activities are rapidly decreased to baseline levels. The transient PKA activities are sufficient for phosphorylation of the overexpressed beta(2)ARs under agonist stimulation, but not phospholamban. Further analysis reveals that phosphodiesterase 4 is the major family that shapes PKA activities under betaAR stimulation. Inhibition of phosphodiesterase 4 extends beta(2)AR-induced PKA activities, promotes PKA phosphorylation of phospholamban, and ultimately enhances myocyte contraction responses. Together, our data have revealed insights into kinetics of PKA activities in signaling propagation under neurohormonal stimulation.

Catecholamines relax detrusor through beta 2-adrenoceptors in mouse and beta 3-adrenoceptors in man

(-)-Isoproterenol [4-[1-hydroxy-2-[(1-methylethyl)amino]ethyl]-1,2-benzene diol hydrochloride] relaxes murine detrusor through beta-adrenoceptors (ARs); however, the beta-AR subtypes involved are unknown. beta(2)-ARs have been associated with caveolae, plasma-lemmal scaffolding domains that are absent in caveolin-1 (cav-1) knockout (KO) mice. Here, we studied detrusor responses in the absence and presence of beta-AR subtype-selective antagonists in wild-type (WT) and cav-1 KO mice. To inquire whether the murine detrusor model is relevant to man, beta-AR subtypes that mediate (-)-isoproterenol-evoked human detrusor relaxation were investigated. In WT mice, (-)-isoproterenol concentration-dependently relaxed the KCl (40 mM)-precontracted detrusor (-logEC(50)M = 8.04, E(max) = 62%). The effects of (-)-isoproterenol were surmountably antagonized by the beta(2)-AR-selective antagonist ICI 118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol] (pK(B) = 9.28) but not affected by the beta(1)-AR-selective antagonist CGP 20712 [1-[2-((3-carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol] and beta(3)-AR-selective L-748,337 [(S)-M-[4-[2-[3-[3-[acetamidomethyl)phenoxy)-2-hydroxypropyl]-amino]-ethyl]-phenylbenzsulfonamide)], suggesting involvement of beta(2)-AR only. The cav-1 KO detrusor displayed significant contractile dysfunction. (-)-Isoproterenol was less potent and efficient in relaxing detrusor from cav-1 KO (-logEC(50)M, 7.76; E(max) = 44%), but ICI 118,551 caused similar antagonism (pK(B) = 9.15), suggesting that beta(2)-AR function persisted in cav-1 KO. The beta(3)-AR-selective antagonist L-748,337 in the presence of ICI 118,551 and CGP 20712 caused additional blockade of (-)-isoproterenol effects in cav-1 KO, consistent with a beta(3)-AR involvement during relaxation and suppression of this effect in WT. (-)-Isoproterenol relaxed human detrusor muscle precontracted with carbachol (-logEC(50)M = 6.39, E(max) = 52%). However, the effects of (-)-isoproterenol in human detrusor were not blocked by CGP 20712 or ICI 118,551 but antagonized by L-748,337 (pK(B) = 7.65). We conclude that murine detrusor relaxation occurs via beta(2)-AR, and loss of caveolae does not perturb beta(2)-AR function but unmasks an additional activation of beta(3)-AR. In contrast, detrusor relaxation in man is mediated exclusively via beta(3)-AR.

Oxycodone plus ultra-low-dose naltrexone attenuates neuropathic pain and associated mu-opioid receptor-Gs coupling

Both peripheral nerve injury and chronic opioid treatment can result in hyperalgesia associated with enhanced excitatory neurotransmission at the level of the spinal cord. Chronic opioid administration leads to a shift in mu-opioid receptor (MOR)-G protein coupling from G(i/o) to G(s) that can be prevented by cotreatment with an ultra-low-dose opioid antagonist. In this study, using lumbar spinal cord tissue from rats with L(5)/L(6) spinal nerve ligation (SNL), we demonstrated that SNL injury induces MOR linkage to G(s) in the damaged (ipsilateral) spinal dorsal horn. This MOR-G(s) coupling occurred without changing G(i/o) coupling levels and without changing the expression of MOR or Galpha proteins. Repeated administration of oxycodone alone or in combination with ultra-low-dose naltrexone (NTX) was assessed on the SNL-induced MOR-G(s) coupling as well as on neuropathic pain behavior. Repeated spinal oxycodone exacerbated the SNL-induced MOR-G(s) coupling, whereas ultra-low-dose NTX cotreatment slightly but significantly attenuated this G(s) coupling. Either spinal or oral administration of oxycodone plus ultra-low-dose NTX markedly enhanced the reductions in allodynia and thermal hyperalgesia produced by oxycodone alone and minimized tolerance to these effects. The MOR-G(s) coupling observed in response to SNL may in part contribute to the excitatory neurotransmission in spinal dorsal horn in neuropathic pain states. The antihyperalgesic and antiallodynic effects of oxycodone plus ultra-low-dose NTX (Oxytrex, Pain Therapeutics, Inc., San Mateo, CA) suggest a promising new treatment for neuropathic pain.

PERSPECTIVE

The current study investigates whether Oxytrex (oxycodone with an ultra-low dose of naltrexone) alleviates mechanical and thermal hypersensitivities in an animal model of neuropathic pain over a period of 7 days, given locally or systemically. In this report, we first describe an injury-induced shift in mu-opioid receptor coupling from G(i/o) to G(s), suggesting why a mu-opioid agonist may have reduced efficacy in the nerve-injured state. These data present a novel approach to neuropathic pain therapy.

Caveolae as organizers of pharmacologically relevant signal transduction molecules

Caveolae, a subset of membrane (lipid) rafts, are flask-like invaginations of the plasma membrane that contain caveolin proteins, which serve as organizing centers for cellular signal transduction. Caveolins (-1, -2, and -3) have cytoplasmic N and C termini, palmitolylation sites, and a scaffolding domain that facilitates interaction and organization of signaling molecules so as to help provide coordinated and efficient signal transduction. Such signaling components include upstream entities (e.g., G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and steroid hormone receptors) and downstream components (e.g., heterotrimeric and low-molecular-weight G proteins, effector enzymes, and ion channels). Diseases associated with aberrant signaling may result in altered localization or expression of signaling proteins in caveolae. Caveolin-knockout mice have numerous abnormalities, some of which may reflect the impact of total body knockout throughout the life span. This review provides a general overview of caveolins and caveolae, signaling molecules that localize to caveolae, the role of caveolae/caveolin in cardiac and pulmonary pathophysiology, pharmacologic implications of caveolar localization of signaling molecules, and the possibility that caveolae might serve as a therapeutic target.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.