Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes

The Muscarinic Acetylcholine M2 Receptor-Induced Nitration of p190A by eNOS Increases RhoA Activity in Cardiac Myocytes

p190RhoGAP, which exists in two paralogs, p190RhoGAP-A (p190A) and p190RhoGAP-B (p190B), is a GTPase activating protein (GAP) contributing to the regulation of the cellular activity of RhoGTPases. Recent data showed that M2 muscarinic acetylcholine receptor (M2R) stimulation in neonatal rat cardiac myocytes (NRCM) induces the binding of p190RhoGAP to the long isoform of the regulator of G protein signaling 3 (RGS3L). This complex formation alters the substrate preference of p190RhoGAP from RhoA to Rac1. By analyzing carbachol-stimulated GAP activity, we show herein that p190A, but not p190B, alters its substrate preference in NRCM. Based on data that the RhoGAP activity of p190A in endothelial cells is diminished upon nitration by endothelial nitric oxide synthase (eNOS)-derived peroxynitrite, we studied whether carbachol-induced NO/peroxynitrite formation contributes to the carbachol-induced RhoA activation in NRCM. Interestingly, the carbachol-induced RhoA activation in NRCM was suppressed by the eNOS-preferring inhibitor L-NIO as well as the non-selective NOS inhibitor L-NAME. Using L-NIO, we firstly verified the carbachol-induced NO production concurrent with eNOS activation and, secondly, the carbachol-induced nitration of p190A in NRCM. By co-immunoprecipitation, the carbachol-induced complex formation of eNOS, p190A, RGS3L and caveolin-3 was detected. We thus conclude that the NO production by M2R-induced eNOS activation in caveolae in NRCM is required for the nitration of p190A, leading to the binding to RGS3L and the change in substrate preference from RhoA to Rac1. In line with this interpretation, the disruption of caveolae in NRCM by methyl-β-cyclodextrin suppressed carbachol-induced RhoA activation in NRCM to a similar extent as the inhibition of NO production.

Opioid-Modulated Receptor Localization and Erk1/2 Phosphorylation in Cells Coexpressing μ-Opioid and Nociceptin Receptors

We attempted to examine the alterations elicited by opioids via coexpressed μ-opioid (MOP) and nociceptin/orphanin FQ (NOP) receptors for receptor localization and Erk1/2 (p44/42 MAPK) in human embryonic kidney (HEK) 293 cells. Through two-photon microscopy, the proximity of MOP and NOP receptors was verified by fluorescence resonance energy transfer (FRET), and morphine but not buprenorphine facilitated the process of MOP-NOP heterodimerization. Single-particle tracking (SPT) further revealed that morphine or buprenorphine hindered the movement of the MOP-NOP heterodimers. After exposure to morphine or buprenorphine, receptor localization on lipid rafts was detected by immunocytochemistry, and phosphorylation of Erk1/2 was determined by immunoblotting in HEK 293 cells expressing MOP, NOP, or MOP+NOP receptors. Colocalization of MOP and NOP on lipid rafts was enhanced by morphine but not buprenorphine. Morphine stimulated the phosphorylation of Erk1/2 with a similar potency in HEK 293 cells expressing MOP and MOP+NOP receptors, but buprenorphine appeared to activate Erk1/2 solely through NOP receptors. Our results suggest that opioids can fine-tune the cellular localization of opioid receptors and phosphorylation of Erk1/2 in MOP+NOP-expressing cells.

G-Protein-Coupled Receptors Mediate Modulations of Cell Viability and Drug Sensitivity by Aberrantly Expressed Recoverin 3 within A549 Cells

To elucidate the currently unknown molecular mechanisms responsible for the aberrant expression of recoverin (Rec) within cancerous cells, we examined two-dimensional (2D) and three-dimensional (3D) cultures of Rec-negative lung adenocarcinoma A549 cells which had been transfected with a plasmid containing human recoverin cDNA (A549 Rec) or an empty plasmid as a mock control (A549 MOCK). Using these cells, we measured cytotoxicity by several anti-tumor agents (2D), cellular metabolism including mitochondrial and glycolytic functions by a Seahorse bio-analyzer (2D), the physical properties, size and stiffness of the 3D spheroids, trypsin sensitivities (2D and 3D), and RNA sequencing analysis (2D). Compared with the A549 MOCK, the A549 Rec cells showed (1) more sensitivity toward anti-tumor agents (2D) and a 0.25% solution of trypsin (3D); (2) a metabolic shift from glycolysis to oxidative phosphorylation; and (3) the formation of larger and stiffer 3D spheroids. RNA sequencing analysis and bioinformatic analyses of the differentially expressed genes (DEGs) using Gene Ontology (GO) enrichment analysis suggested that aberrantly expressed Rec is most likely associated with several canonical pathways including G-protein-coupled receptor (GPCR)-mediated signaling and signaling by the cAMP response element binding protein (CREB). The findings reported here indicate that the aberrantly expressed Rec-induced modulation of the cell viability and drug sensitivity may be GPCR mediated.

A Role for Caveolin-3 in the Pathogenesis of Muscular Dystrophies

Caveolae are the cholesterol-rich small invaginations of the plasma membrane present in many cell types including adipocytes, endothelial cells, epithelial cells, fibroblasts, smooth muscles, skeletal muscles and cardiac muscles. They serve as specialized platforms for many signaling molecules and regulate important cellular processes like energy metabolism, lipid metabolism, mitochondria homeostasis, and mechano-transduction. Caveolae can be internalized together with associated cargo. The caveolae-dependent endocytic pathway plays a role in the withdrawal of many plasma membrane components that can be sent for degradation or recycled back to the cell surface. Caveolae are formed by oligomerization of caveolin proteins. Caveolin-3 is a muscle-specific isoform, whose malfunction is associated with several diseases including diabetes, cancer, atherosclerosis, and cardiovascular diseases. Mutations in Caveolin-3 are known to cause muscular dystrophies that are collectively called caveolinopathies. Altered expression of Caveolin-3 is also observed in Duchenne’s muscular dystrophy, which is likely a part of the pathological process leading to muscle weakness. This review summarizes the major functions of Caveolin-3 in skeletal muscles and discusses its involvement in the pathology of muscular dystrophies.

Increased expression of blood muscarinic receptors in patients with reflex syncope

AIMS

Pathophysiology of reflex syncope is not fully understood but a vagal overactivity might be involved in this syncope. Previously, overexpression of muscarinic M2 receptors and acetylcholinesterase was found in particular in the heart and in lymphocytes of rabbits with vagal overactivity as well as in hearts of Sudden Infant Death Syndromes. The aim of this present study was to look at M2 receptor expression in blood of patients with reflex syncope. The second objective was to measure acetylcholinesterase expression in these patients.

METHODS AND RESULTS

136 subjects were enrolled. This monocenter study pooled 45 adults exhibiting recurrent reflex syncope compared with 32 healthy adult volunteers (18-50 years) and 38 children exhibiting reflex syncope requiring hospitalization compared with 21 controls (1-17 years). One blood sample was taken from each subject and blood mRNA expression of M2 receptors was assessed by qRT-PCR. Taking into account the non-symmetric distributions of values in both groups, statistical interferences were assessed using bayesian techniques. A M2 receptor overexpression was observed in adult and pediatric patients compared to controls. The medians [q1;q3] were 0.9 [0.3;1.9] in patients versus 0.2 [0.1;1.0] in controls; the probability that M2 receptor expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.99. Acetylcholinesterase expression was also increased 0.7 [0.4;1.6] in patients versus 0.4 [0.2;1.1] in controls; the probability that acetylcholinesterase expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.97. Both in adults and children, the expression ratio of M2 receptors over acetylcholinesterase was greater in the patient group compared with the control group.

CONCLUSION

M2 receptor overexpression has been detected in the blood of both, adults and children, exhibiting reflex syncope. As in our experimental model, i.e. rabbits with vagal overactivity, acetylcholinesterase overexpression was associated with M2 receptor overexpression. For the first time, biological abnormalities are identified in vagal syncope in which only clinical signs are, so far, taken into account for differential diagnosis and therapeutic management. Further work will be needed to validate potential biomarkers of risk or severity associated with the cholinergic system.

Functional Microdomains in Heart's Pacemaker: A Step Beyond Classical Electrophysiology and Remodeling

Spontaneous beating of the sinoatrial node (SAN), the primary pacemaker of the heart, is initiated, sustained, and regulated by a complex system that integrates ion channels and transporters on the cell membrane surface (often referred to as "membrane clock") with subcellular calcium handling machinery (by parity of reasoning referred to as an intracellular "Ca²⁺ clock"). Stable, rhythmic beating of the SAN is ensured by a rigorous synchronization between these two clocks highlighted in the coupled-clock system concept of SAN timekeeping. The emerging results demonstrate that such synchronization of the complex pacemaking machinery at the cellular level depends on tightly regulated spatiotemporal signals which are restricted to precise sub-cellular microdomains and associated with discrete clusters of different ion channels, transporters, and regulatory receptors. It has recently become evident that within the microdomains, various proteins form an interacting network and work together as a part of a macromolecular signaling complex. These protein-protein interactions are tightly controlled and regulated by a variety of neurohormonal signaling pathways and the diversity of cellular responses achieved with a limited pool of second messengers is made possible through the organization of essential signal components in particular microdomains. In this review, we highlight the emerging understanding of the functionality of distinct subcellular microdomains in SAN myocytes and their functional role in the accumulation and neurohormonal regulation of proteins involved in cardiac pacemaking. We also demonstrate how changes in scaffolding proteins may lead to microdomain-targeted remodeling and regulation of pacemaker proteins contributing to SAN dysfunction.

Regulation of the NRG1/ErbB4 Pathway in the Intrinsic Cardiac Nervous System Is a Potential Treatment for Atrial Fibrillation

Background: The NRG1/ErbB4 signaling mechanism has been widely studied in the central nervous system for many years. However, the role of this pathway in modulating the intrinsic cardiac nervous system is largely unknown.

Objective: The present study investigated whether the NRG1/ErbB4 signaling system affects the activity of major atrial ganglionated plexi (GP) in a paroxysmal atrial fibrillation (AF) model by 6-h rapid atrial pacing (RAP).

Methods: Twenty-four dogs were randomly divided into (1) a control group (saline microinjections into GP), (2) RAP group (saline microinjections into GP plus 6 h-RAP), (3) NRG1 group (microinjections of neuregulin-1 into GP plus 6 h-RAP) and (4) NRG1 + ERA group (microinjections of neuregulin-1 and ErbB4 receptor antagonist-ERA into GP plus 6 h-RAP). The effective refractory period (ERP), window of vulnerability (WOV), anterior right GP (ARGP) function and neural activity were measured. ARGP tissues were excised for histological study and western blotting.

Results: When compared to the control group, 6 h-RAP produced a significant (1) decrease in ERP, an increase in ΣWOV, (2) an increase in ARGP neural activity and neural function, and (3) an increase in c-fos and nerve growth factor protein expression in the ARGP. However, microinjection of NRG1 into the ARGP prior to RAP prevented ERP shortening and AGRP activity enhancement and inhibited the expression of c-Fos and NGF proteins. Furthermore, these changes were significantly attenuated by pretreatment with an ErbB4 receptor antagonist.

Conclusion: The NRG1/ErbB4 signaling pathway may exist in the GP, and activation of this pathway suppressed RAP-induced GP activation, atrial electrical remodeling and AF.

Caveolin-3 differentially orchestrates cholinergic and serotonergic constriction of murine airways

The mechanisms of controlling airway smooth muscle (ASM) tone are of utmost clinical importance as inappropriate constriction is a hallmark in asthma and chronic obstructive pulmonary disease. Receptors for acetylcholine and serotonin, two relevant mediators in this context, appear to be incorporated in specialized, cholesterol-rich domains of the plasma membrane, termed caveolae due to their invaginated shape. The structural protein caveolin-1 partly accounts for anchoring of these receptors. We here determined the role of the other major caveolar protein, caveolin-3 (cav-3), in orchestrating cholinergic and serotonergic ASM responses, utilizing newly generated cav-3 deficient mice. Cav-3 deficiency fully abrogated serotonin-induced constriction of extrapulmonary airways in organ baths while leaving intrapulmonary airways unaffected, as assessed in precision cut lung slices. The selective expression of cav-3 in tracheal, but not intrapulmonary bronchial epithelial cells, revealed by immunohistochemistry, might explain the differential effects of cav-3 deficiency on serotonergic ASM constriction. The cholinergic response of extrapulmonary airways was not altered, whereas a considerable increase was observed in cav-3^-/- intrapulmonary bronchi. Thus, cav-3 differentially organizes serotonergic and cholinergic signaling in ASM through mechanisms that are specific for airways of certain caliber and anatomical position. This may allow for selective and site-specific intervention in hyperreactive states.

Assembly of ER-PM Junctions: A Critical Determinant in the Regulation of SOCE and TRPC1

Store-operated calcium entry (SOCE), a unique plasma membrane Ca²⁺ entry mechanism, is activated when ER-[Ca²⁺] is decreased. SOCE is mediated via the primary channel, Orai1, as well as others such as TRPC1. STIM1 and STIM2 are ER-Ca²⁺ sensor proteins that regulate Orai1 and TRPC1. SOCE requires assembly of STIM proteins with the plasma membrane channels which occurs within distinct regions in the cell that have been termed as endoplasmic reticulum (ER)-plasma membrane (PM) junctions. The PM and ER are in close proximity to each other within this region, which allows STIM1 in the ER to interact with and activate either Orai1 or TRPC1 in the plasma membrane. Activation and regulation of SOCE involves dynamic assembly of various components that are involved in mediating Ca²⁺ entry as well as those that determine the formation and stabilization of the junctions. These components include proteins in the cytosol, ER and PM, as well as lipids in the PM. Recent studies have also suggested that SOCE and its components are compartmentalized within ER-PM junctions and that this process might require remodeling of the plasma membrane lipids and reorganization of structural and scaffolding proteins. Such compartmentalization leads to the generation of spatially- and temporally-controlled Ca²⁺signals that are critical for regulating many downstream cellular functions.

Investigating Internalization and Intracellular Trafficking of GPCRs: New Techniques and Real-Time Experimental Approaches

The ability to regulate the interaction between cells and their extracellular environment is essential for the maintenance of appropriate physiological function. For G protein-coupled receptors (GPCRs), this regulation occurs through multiple mechanisms that provide spatial and temporal control for signal transduction. One of the major mechanisms for GPCR regulation involves their endocytic trafficking, which serves to internalize the receptors from the plasma membrane and thereby attenuate G protein-dependent signaling. However, there is accumulating evidence to suggest that GPCRs can signal independently of G proteins, as well as from intracellular compartments including endosomes. It is in this context that receptor internalization and intracellular trafficking have attracted renewed interest within the GPCR field. In this chapter, we will review the current understanding and methodologies that have been used to investigate internalization and intracellular signaling of GPCRs, with a particular focus on emerging real-time techniques. These recent developments have improved our understanding of the complexities of GPCR internalization and intracellular signaling and suggest that the broader biological relevance and potential therapeutic implications of these processes remain to be explored.

High Efficacy but Low Potency of δ-Opioid Receptor-G Protein Coupling in Brij-58-Treated, Low-Density Plasma Membrane Fragments

Principal Findings

HEK293 cells stably expressing PTX-insensitive δ-opioid receptor-Gi1α (C³⁵¹I) fusion protein were homogenized, treated with low concentrations of non-ionic detergent Brij-58 at 0°C and fractionated by flotation in sucrose density gradient. In optimum range of detergent concentrations (0.025–0.05% w/v), Brij-58-treated, low-density membranes exhibited 2-3-fold higher efficacy of DADLE-stimulated, high-affinity [³²P]GTPase and [³⁵S]GTPγS binding than membranes of the same density prepared in the absence of detergent. The potency of agonist DADLE response was significantly decreased. At high detergent concentrations (>0.1%), the functional coupling between δ-opioid receptors and G proteins was completely diminished. The same detergent effects were measured in plasma membranes isolated from PTX-treated cells. Therefore, the effect of Brij-58 on δ-opioid receptor-G protein coupling was not restricted to the covalently bound G_i1α within δ-opioid receptor-G_i1α fusion protein, but it was also valid for PTX-sensitive G proteins of G_i/G_o family endogenously expressed in HEK293 cells. Characterization of the direct effect of Brij-58 on the hydrophobic interior of isolated plasma membranes by steady-state anisotropy of diphenylhexatriene (DPH) fluorescence indicated a marked increase of membrane fluidity. The time-resolved analysis of decay of DPH fluorescence by the “wobble in cone” model of DPH motion in the membrane indicated that the exposure to the increasing concentrations of Brij-58 led to a decreased order and higher motional freedom of the dye.

Summary

Limited perturbation of plasma membrane integrity by low concentrations of non-ionic detergent Brij-58 results in alteration of δ-OR-G protein coupling. Maximum G protein-response to agonist stimulation (efficacy) is increased; affinity of response (potency) is decreased. The total degradation plasma membrane structure at high detergent concentrations results in diminution of functional coupling between δ-opioid receptors and G proteins.

Role of the third intracellular loop in the subtype-specific internalization and recycling of muscarinic M2 and M4 receptors

Muscarinic M2, M4, and M2-M4 chimera receptors were transiently expressed in HEK-293 tsA201 cells, and agonist-dependent internalization of these receptors and recycling of internalized receptors were examined by measuring the amount of cell-surface receptors as [3H]N-methylscopolamine (NMS) binding activity. Coexpression of a dominant negative form of dynamin (DN-dynamin,dynamin K44A) greatly reduced the agonist-dependent internalization of M4 receptors but not of M2 receptors, as was reported by Vögler et al. (J Biol Chem 273, 12155-12160, 1998).The agonist-dependent internalization of M2/M4-i3/M2 chimera receptors (M2 receptors with the i3 loop replaced by that of M4 receptors) was greatly reduced by co-expression of DN-dynamin as was the case for M4 receptors, whereas the agonist-dependent internalization of M4/M2-i3/M4 chimera receptors was hardly affected by co-expression of DN-dynamin as was the case for M2 receptors.Internalized M2/M4-i3/M2 receptors as well as internalized M4 receptors were shown to be recycled back to the cell surface after removal of agonists, whereas no recycling was observed for M4/M2-i3/M4 receptors as well as M2 receptors. These results indicate that the i3 loops of M2 and M4 receptors take a major role in their agonist-dependent internalization and recycling.

Identification of caveolar resident proteins in ventricular myocytes using a quantitative proteomic approach: dynamic changes in caveolar composition following adrenoceptor activation

The lipid raft concept proposes that membrane environments enriched in cholesterol and sphingolipids cluster certain proteins and form platforms to integrate cell signaling. In cardiac muscle, caveolae concentrate signaling molecules and ion transporters, and play a vital role in adrenergic regulation of excitation–contraction coupling, and consequently cardiac contractility. Proteomic analysis of cardiac caveolae is hampered by the presence of contaminants that have sometimes, erroneously, been proposed to be resident in these domains. Here we present the first unbiased analysis of the proteome of cardiac caveolae, and investigate dynamic changes in their protein constituents following adrenoreceptor (AR) stimulation.

Rat ventricular myocytes were treated with methyl-β-cyclodextrin (MβCD) to deplete cholesterol and disrupt caveolae. Buoyant caveolin-enriched microdomains (BCEMs) were prepared from MβCD-treated and control cell lysates using a standard discontinuous sucrose gradient. BCEMs were harvested, pelleted, and resolubilized, then alkylated, digested, and labeled with iTRAQ reagents, and proteins identified by LC-MS/MS on a LTQ Orbitrap Velos Pro. Proteins were defined as BCEM resident if they were consistently depleted from the BCEM fraction following MβCD treatment. Selective activation of α-, β1-, and β2-AR prior to preparation of BCEMs was achieved by application of agonist/antagonist pairs for 10 min in populations of field-stimulated myocytes.

We typically identified 600–850 proteins per experiment, of which, 249 were defined as high-confidence BCEM residents. Functional annotation clustering indicates cardiac BCEMs are enriched in integrin signaling, guanine nucleotide binding, ion transport, and insulin signaling clusters. Proteins possessing a caveolin binding motif were poorly enriched in BCEMs, suggesting this is not the only mechanism that targets proteins to caveolae. With the notable exception of the cavin family, very few proteins show altered abundance in BCEMs following AR activation, suggesting signaling complexes are preformed in BCEMs to ensure a rapid and high fidelity response to adrenergic stimulation in cardiac muscle.

Opioid-Receptor (OR) Signaling Cascades in Rat Cerebral Cortex and Model Cell Lines: the Role of Plasma Membrane Structure

Large number of extracellular signals is received by plasma membrane receptors which, upon activation, transduce information into the target cell interior via trimeric G-proteins (GPCRs) and induce activation or inhibition of adenylyl cyclase enzyme activity (AC). Receptors for opioid drugs such as morphine (μ-OR, δ-OR and κ-OR) belong to rhodopsin family of GPCRs. Our recent results indicated a specific up-regulation of AC I (8-fold) and AC II (2.5-fold) in plasma membranes (PM) isolated from rat brain cortex exposed to increasing doses of morphine (10-50 mg/kg) for 10 days. Increase of ACI and ACII represented the specific effect as the amount of ACIII-ACIX, prototypical PM marker Na, K-ATPase and trimeric G-protein α and β subunits was unchanged. The up-regulation of ACI and ACII faded away after 20 days since the last dose of morphine. Proteomic analysis of these PM indicated that the brain cortex of morphine-treated animals cannot be regarded as being adapted to this drug because significant up-regulation of proteins functionally related to oxidative stress and alteration of brain energy metabolism occurred. The number of δ-OR was increased 2-fold and their sensitivity to monovalent cations was altered. Characterization of δ-OR-G-protein coupling in model HEK293 cell line indicated high ability of lithium to support affinity of δ-OR response to agonist stimulation. Our studies of PM structure and function in context with desensitization of GPCRs action were extended by data indicating participation of cholesterol-enriched membrane domains in agonist-specific internalization of δ-OR. In HEK293 cells stably expressing δ-OR-Gi1α fusion protein, depletion of PM cholesterol was associated with the decrease in affinity of G-protein response to agonist stimulation, whereas maximum response was unchanged. Hydrophobic interior of isolated PM became more “fluid”, chaotically organized and accessible to water molecules. Validity of this conclusion was supported by the analysis of an immediate PM environment of cholesterol molecules in living δ-OR-Gi1α-HEK293 cells by fluorescent probes 22- and 25-NBD-cholesterol. The alteration of plasma membrane structure by cholesterol depletion made the membrane more hydrated. Understanding of the positive and negative feedback regulatory loops among different OR-initiated signaling cascades (µ-, δ-, and κ-OR) is crucial for understanding of the long-term mechanisms of drug addiction as the decrease in functional activity of µ-OR may be compensated by increase of δ-OR and/or κ-OR signaling.

Differential regulation of muscarinic M2 and M3 receptor signaling in gastrointestinal smooth muscle by caveolin-1

Caveolae act as scaffolding proteins for several G protein-coupled receptor signaling molecules to regulate their activity. Caveolin-1, the predominant isoform in smooth muscle, drives the formation of caveolae. The precise role of caveolin-1 and caveolae as scaffolds for G protein-coupled receptor signaling and contraction in gastrointestinal muscle is unclear. Thus the aim of this study was to examine the role of caveolin-1 in the regulation of Gq- and Gi-coupled receptor signaling. RT-PCR, Western blot, and radioligand-binding studies demonstrated the selective expression of M2 and M3 receptors in gastric smooth muscle cells. Carbachol (CCh) stimulated phosphatidylinositol (PI) hydrolysis, Rho kinase and zipper-interacting protein (ZIP) kinase activity, induced myosin phosphatase 1 (MYPT1) phosphorylation (at Thr(696)) and 20-kDa myosin light chain (MLC20) phosphorylation (at Ser(19)) and muscle contraction, and inhibited cAMP formation. Stimulation of PI hydrolysis, Rho kinase, and ZIP kinase activity, phosphorylation of MYPT1 and MLC20, and muscle contraction in response to CCh were attenuated by methyl β-cyclodextrin (MβCD) or caveolin-1 small interfering RNA (siRNA). Similar inhibition of PI hydrolysis, Rho kinase, and ZIP kinase activity and muscle contraction in response to CCh and gastric emptying in vivo was obtained in caveolin-1-knockout mice compared with wild-type mice. Agonist-induced internalization of M2, but not M3, receptors was blocked by MβCD or caveolin-1 siRNA. Stimulation of PI hydrolysis, Rho kinase, and ZIP kinase activities in response to other Gq-coupled receptor agonists such as histamine and substance P was also attenuated by MβCD or caveolin-1 siRNA. Taken together, these results suggest that caveolin-1 facilitates signaling by Gq-coupled receptors and contributes to enhanced smooth muscle function.

Clostridium difficile binary toxin CDT induces clustering of the lipolysis-stimulated lipoprotein receptor into lipid rafts

Clostridium difficile is the leading cause of antibiotics-associated diarrhea and pseudomembranous colitis. Hypervirulent C. difficile strains produce the binary actin-ADP-ribosylating toxin CDT (C. difficile transferase), in addition to the Rho-glucosylating toxins A and B. We recently identified the lipolysis-stimulated lipoprotein receptor (LSR) as the host receptor that mediates uptake of CDT into target cells. Here we investigated in H1-HeLa cells, which ectopically express LSR, the influence of CDT on the plasma membrane distribution of the receptor. We found by fluorescence microscopy that the binding component of CDT (CDTb) induces clustering of LSR into subcompartments of the plasma membrane. Detergent extraction of cells treated with CDTb, followed by sucrose gradient fractionation, uncovered accumulation of LSR in detergent-resistant membranes (DRMs) that contained typical marker proteins of lipid rafts. Membrane cholesterol depletion with methyl-β-cyclodextrin inhibited the association of LSR with DRMs upon addition of CDTb. The receptor-binding domain of CDTb also triggered LSR clustering into DRMs. CDTb-triggered clustering of LSR into DRMs could be confirmed in Caco-2 cells. Our data suggest that CDT forces its receptor to cluster into lipid rafts and that oligomerization of the B component might enhance but is not essential for this process.

C. difficile binary toxin CDT is a member of the iota-like, actin ADP-ribosylating toxin family. The mechanism that mediates endocytic uptake of these toxins still remains elusive. Previous studies highlighted the importance of lipid rafts for oligomerization of the binding component of these toxins and for cell entry. Recently, the host cell receptor for this toxin family, namely, the lipolysis-stimulated lipoprotein receptor (LSR), has been identified. Our study now demonstrates that the binding component of CDT (CDTb) induces clustering of LSR into lipid rafts. Importantly, LSR clustering is efficiently induced also by the receptor-binding domain of CDTb, suggesting that oligomerization of the B component of CDT is not the main trigger of this process. The current work extends our knowledge on the cooperative play between iota-like toxins and their receptor.

Non-neuronal functions of the m2 muscarinic acetylcholine receptor

Acetylcholine is an important neurotransmitter whose effects are mediated by two classes of receptors. The nicotinic acetylcholine receptors are ion channels, whereas the muscarinic receptors belong to the large family of G protein coupled seven transmembrane helix receptors. Beyond its function in neuronal systems, it has become evident that acetylcholine also plays an important role in non-neuronal cells such as epithelial and immune cells. Furthermore, many cell types in the periphery are capable of synthesizing acetylcholine and express at least some of the receptors. In this review, we summarize the non-neuronal functions of the muscarinic acetylcholine receptors, especially those of the M₂ muscarinic receptor in epithelial cells. We will review the mechanisms of signaling by the M₂ receptor but also the cellular trafficking and ARF6 mediated endocytosis of this receptor, which play an important role in the regulation of signaling events. In addition, we provide an overview of the M₂ receptor in human pathological conditions such as autoimmune diseases and cancer.

Stiffened lipid platforms at molecular force foci

How mechanical forces are sensed remains largely mysterious. The forces that gate prokaryotic and several eukaryotic channels were found to come from the lipid membrane. Our survey of animal cells found that membrane force foci all have cholesterol-gathering proteins and are reinforced with cholesterol. This result is evident in overt force sensors at the tips of stereocilia for vertebrate hearing and the touch receptor of Caenorhabditis elegans and mammalian neurons. For less specialized cells, cadherins sustain the force between neighboring cells and integrins between cells and matrix. These tension bearers also pass through and bind to a cholesterol-enriched platform before anchoring to cytoskeleton through other proteins. Cholesterol, in alliance with sphingomyelin and specialized proteins, enforces a more ordered structure in the bilayer. Such a stiffened platform can suppress mechanical noise, redirect, rescale, and confine force. We speculate that such platforms may be dynamic. The applied force may allow disordered-phase lipids to enter the platform-staging channel opening in the thinner mobile neighborhood. The platform may also contain specialized protein/lipid subdomains enclosing mechanosensitive channels to open with localized tension. Such a dynamic stage can mechanically operate structurally disparate channels or enzymes without having to tie them directly to cadherin, integrin, or other protein tethers.

A role for G-proteins in directing G-protein-coupled receptor-caveolae localization

Caveolae are membrane domains that may influence cell signaling by sequestering specific proteins such as G-protein-coupled receptors (GPCRs). While previous reports largely show that Gα_q subunits, but not other G-proteins, interact strongly with the caveolae protein, Caveolin-1 (Cav1), the inclusion of GPCRs in caveolae is controversial. Here, we have used fluorescence methods to determine the effect of caveolae on the physical and functional properties of two GPCRs that have been reported to reside in caveolae, bradykinin receptor type 2 (B₂R), which is coupled to Gα_q, and the μ-opioid receptor (μOR), which is coupled to Gα_i. While caveolae do not affect cAMP signals mediated by μOR, they prolong Ca²⁺ signals mediated by B₂R. In A10 cells that endogenously express B₂R and Cav1, downregulation of Cav1 ablates the prolonged recovery seen upon bradykinin stimulation in accord with the idea that the presence of caveolae prolongs Gα_q activation. Immunofluorescence and Förster resonance energy transfer (FRET) studies show that a significant fraction of B₂R resides at or close to caveolae domains while none or very little μOR resides in caveolae domains. The level of FRET between B₂R and caveolae is reduced by downregulation of Gα_q or by addition of a peptide that interferes with Gα_q–Caveolin-1 interactions, suggesting that Gα_q promotes localization of B₂R to caveolae domains. Our results lead to the suggestion that Gα_q can localize its associated receptors to caveolae domains to enhance their signals.

Prostaglandin E1 facilitates inotropic effects of 5-HT4 serotonin receptors and β-adrenoceptors in failing human heart

Prostaglandins have displayed both beneficial and detrimental effects in clinical studies in patients with severe heart failure. Prostaglandins are known to increase cardiac output, but the mechanism is not clarified. Here, we tested the hypothesis that prostaglandins can increase contractility in human heart by amplifying cAMP-dependent inotropic responses. Contractility was measured ex vivo in isolated left ventricular strips and phosphodiesterase (PDE) and adenylyl cyclase (AC) activity was measured in homogenates or membranes from failing human left ventricles. PGE(1) (1 µM) alone did not modify contractility, but given prior, amplified maximal serotonin (5-HT)-evoked (10 µM) contractile responses mediated by 5-HT(4) receptors several fold (24 ± 7 % with PGE(1) vs. 3 ± 2 % above basal with 5-HT alone). The 5-HT(4)-mediated inotropic response was amplified by the PDE3 inhibitor cilostamide and further amplified in combination with PGE(1) (26 ± 6 vs. 56 ± 12 % above basal). PGE(1) reduced the time to reach 90 % of both the maximal 5-HT- and isoproterenol-evoked inotropic response compared to 5-HT or isoproterenol alone. PGE(1) did not modify PDE activity in the homogenate, either alone or when given simultaneously with PDE3 and/or PDE4 inhibitors. Neither 5-HT- nor isoproterenol-stimulated AC activity was significantly amplified by PGE(1). Sensitivity of ventricular strips to Ca(2+) was not enhanced in the presence of PGE(1). Our results show that PGE(1) can enhance cAMP-mediated responses in failing human left ventricle, through a mechanism independent of PDE inhibition, amplification of AC activity or increasing sensitivity to calcium. This effect of PGE(1) possibly contributes to the increase of cardiac output, independent of decreased afterload, observed after prostaglandin administration in humans.

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

Lipid rafts and redox regulation of cellular signaling in cholesterol induced atherosclerosis

Redox mediated signaling mechanisms play crucial roles in the pathogenesis of several cardiovascular diseases. Atherosclerosis is one of the most important disorders induced mainly by hypercholesterolemia. Oxidation products and related signaling mechanisms are found within the characteristic biomarkers of atherosclerosis. Several studies have shown that redox signaling via lipid rafts play a significant role in the regulation of pathogenesis of many diseases including atherosclerosis. This review attempts to summarize redox signaling and lipid rafts in hypercholesterolemia induced atherosclerosis.

Caveolae create local signalling domains through their distinct protein content, lipid profile and morphology

Compartmentation of signalling allows multiple stimuli to achieve diverse cellular responses with only a limited pool of second messengers. This spatial control of signalling is achieved, in part, by cellular structures which bring together elements of a particular cascade. One such structure is the caveola, a flask-shaped lipid raft. Caveolae are well-recognised as signalosomes, platforms for assembly of signalling complexes of receptors, effectors and their targets, which can facilitate efficient and specific cellular responses. Here we extend this simple model and present evidence to show how the protein and lipid profiles of caveolae, as well as their characteristic morphology, define their roles in creating local signalling domains in the cardiac myocyte. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Cyclic AMP-dependent inotropic effects are differentially regulated by muscarinic G(i)-dependent constitutive inhibition of adenylyl cyclase in failing rat ventricle

BACKGROUND AND PURPOSE

β-Adrenoceptor (β-AR)-mediated inotropic effects are attenuated and G(i) proteins are up-regulated in heart failure (HF). Muscarinic receptors constitutively inhibit cAMP formation in normal rat cardiomyocytes. We determined whether constitutive activity of muscarinic receptors to inhibit adenylyl cyclase (AC) increases in HF and if so, whether it modifies the reduced β-AR- or emergent 5-HT₄-mediated cAMP-dependent inotropic effects.

EXPERIMENTAL APPROACH

Contractility and AC activity were measured and related to each other in rat ventricle with post-infarction HF and sham-operated (Sham) controls with or without blockade of muscarinic receptors by atropine and inactivation of G(i) protein by pertussis toxin (PTX).

KEY RESULTS

Isoprenaline-mediated inotropic effects were attenuated and basal, isoprenaline- and forskolin-stimulated AC activity was reduced in HF compared with Sham. Atropine or PTX pretreatment increased forskolin-stimulated AC activity in HF hearts. β-AR-stimulated AC and maximal inotropic response were unaffected by atropine in Sham and HF. In HF, the potency of serotonin (5-HT) to evoke an inotropic response was increased in the presence of atropine with no change in the maximal inotropic response. Interestingly, PTX pretreatment reduced the potency of 5-HT to evoke inotropic responses while increasing the maximal inotropic response.

CONCLUSIONS AND IMPLICATIONS

Although muscarinic constitutive inhibition of AC is increased in HF, it does not contribute to the reduced β-AR-mediated inotropic effects in rat ventricle in HF. The data support the hypothesis that there are differences in the functional compartmentation of 5-HT₄ and β-AR AC signalling in myocardium during HF.

Agonist-induced localization of Gq-coupled receptors and G protein-gated inwardly rectifying K+ (GIRK) channels to caveolae determines receptor specificity of phosphatidylinositol 4,5-bisphosphate signaling

G protein-gated inwardly rectifying K(+) (GIRK) channels are parasympathetic effectors in cardiac myocytes that act as points of integration of signals from diverse pathways. Neurotransmitters and hormones acting on the Gq protein regulate GIRK channels by phosphatidylinositol 4,5-bisphosphate (PIP(2)) depletion. In previous studies, we found that endothelin-1, but not bradykinin, inhibited GIRK channels, even though both of them hydrolyze PIP(2) in cardiac myocytes, showing receptor specificity. The present study assessed whether the spatial organization of the PIP(2) signal into caveolar microdomains underlies the specificity of PIP(2)-mediated signaling. Using biochemical analysis, we examined the localization of GIRK and Gq protein-coupled receptors (GqPCRs) in mouse atrial myocytes. Agonist stimulation induced a transient co-localization of GIRK channels with endothelin receptors in the caveolae, excluding bradykinin receptors. Such redistribution was eliminated by caveolar disruption with methyl-β-cyclodextrin (MβCD). Patch clamp studies showed that the specific response of GIRK channels to GqPCR agonists was abolished by MβCD, indicating the functional significance of the caveolae-dependent spatial organization. To assess whether low PIP(2) mobility is essential for PIP(2)-mediated signaling, we blocked the cytoskeletal restriction of PIP(2) diffusion by latrunculin B. This abolished the GIRK channel regulation by GqPCRs without affecting their targeting to caveolae. These data suggest that without the hindered diffusion of PIP(2) from microdomains, PIP(2) loses its signaling efficacy. Taken together, these data suggest that specific targeting combined with restricted diffusion of PIP(2) allows the PIP(2) signal to be compartmentalized to the targets localized closely to the GqPCRs, enabling cells to discriminate between identical PIP(2) signaling that is triggered by different receptors.

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.

Do caveolae have a role in the fidelity and dynamics of receptor activation of G-protein-gated inwardly rectifying potassium channels?

In atrial and nodal cardiac myocytes, M2 muscarinic receptors activate inhibitory G-proteins (G_i/o), which in turn stimulate G-protein-gated inwardly rectifying K⁺ channels through direct binding of the Gβγ subunit. Despite also releasing Gβγ, G_s-coupled receptors such as the β-adrenergic receptor are not able to prominently activate this current. An appealing hypothesis would be if components were sequestered in membrane domains such as caveolae/rafts. Using biochemical fractionation followed by Western blotting and/or radioligand binding experiments, we examined the distribution of the components in stable HEK293 and HL-1 cells, which natively express the transduction cascade. The channel, M2 muscarinic, and A1 adenosine receptors were located in noncaveolar/nonraft fractions. G_iα_1/2 was enriched in both caveolar/raft and noncaveolar/nonraft fractions. In contrast, G_sα was only enriched in caveolar/raft fractions. We constructed YFP-tagged caveolin-2 (YFP-Cav2) and chimeras with the M2 (M2-YFP-Cav2) and A1 (A1-YFP-Cav2) receptors. Analysis of gradient fractions showed that these receptor chimeras were now localized to caveolae-enriched fractions. Microscopy showed that M2-YFP and A1-YFP had a diffuse homogenous membrane signal. YFP-Cav2, M2-YFP-Cav2, and A1-YFP-Cav2 revealed a more punctuate pattern. Finally, we looked at the consequences for signaling. Activation via M2-YFP-Cav2 or A1-YFP-Cav2 revealed substantially slower kinetics compared with M2-YFP or A1-YFP and was reversed by the addition of methyl-β-cyclodextrin. Thus the localization of the channel signal transduction cascade in non-cholesterol rich domains substantially enhances the speed of signaling. The presence of G_sα solely in caveolae may account for signaling selectivity between G_i/o and G_s-coupled receptors.

Human urinary bladder smooth muscle is dependent on membrane cholesterol for cholinergic activation

Voiding is mediated by muscarinic receptors in urinary bladder smooth muscle cells. Lipid rafts and caveolae are cholesterol enriched membrane domains that modulate the activity of G protein-coupled receptors and second messenger systems. Conflicting findings regarding sensitivity of muscarinic signalling to cholesterol desorption, which perturbs lipid rafts and caveolae, have been reported, and no study has used human urinary bladder. Here, the dependence of human bladder muscarinic receptor signalling on plasma membrane cholesterol was examined. Nerve-mediated contraction, elicited by electrical field stimulation of human bladder strips, was impaired by desorption of cholesterol using methyl-beta-cyclodextrin, and the concentration-response curve for the muscarinic agonist carbachol was right-shifted. No effect of cholesterol desorption was observed in rat, and in mouse increased maximum contraction was seen. Expression of caveolin-1, PLCbeta1 and M3 muscarinic receptors did not differ between species in a manner that would explain the differential sensitivity to cholesterol desorption. In human bladder, threshold depolarisation eliminated the difference between cyclodextrin-treated and control preparations. Contraction elicited by depolarisation per se was not affected. M3 muscarinic receptors appeared clustered along plasma membrane profiles as shown by immunohistochemical staining of human bladder, but no redistribution in association with cholesterol reduction was seen. Thus, muscarinic receptor-induced contraction of the urinary bladder exhibits species-specific differences in its sensitivity to cholesterol desorption suggesting differential roles of lipid rafts/caveolae in muscarinic receptor signalling between species.

Muscarinic receptor-mediated bronchoconstriction is coupled to caveolae in murine airways

Cholinergic bronchoconstriction is mediated by M(2) and M(3) muscarinic receptors (MR). In heart and urinary bladder, MR are linked to caveolin-1 or -3, the structural proteins of caveolae. Caveolae are cholesterol-rich, omega-shaped invaginations of the plasma membrane. They provide a scaffold for multiple G protein receptors and membrane-bound enzymes, thereby orchestrating signaling into the cell interior. Hence, we hypothesized that airway MR signaling pathways are coupled to caveolae as well. To address this issue, we determined the distribution of caveolin isoforms and MR subtype M2R in murine and human airways and investigated protein-protein associations by fluorescence resonance energy transfer (FRET)-confocal laser scanning microscopy (CLSM) analysis in immunolabeled murine tissue sections. Bronchoconstrictor responses of murine bronchi were recorded in lung-slice preparations before and after caveolae disruption by methyl-β-cyclodextrin, with efficiency of this treatment being validated by electron microscopy. KCl-induced bronchoconstriction was unaffected after treatment, demonstrating functional integrity of the smooth muscle. Caveolae disruption decreased muscarine-induced bronchoconstriction in wild-type and abolished it in M2R(-/-) and M3R(-/-) mice. Thus M2R and M3R signaling pathways require intact caveolae. Furthermore, we identified a presumed skeletal and cardiac myocyte-specific caveolin isoform, caveolin-3, in human and murine bronchial smooth muscle and found it to be associated with M2R in situ. In contrast, M2R was not associated with caveolin-1, despite an in situ association of caveolin-1 and caveolin-3 that was detected. Here, we demonstrated that M2R- and M3R-mediated bronchoconstriction is caveolae-dependent. Since caveolin-3 is directly associated with M2R, we suggest caveolin-3 as novel regulator of M2R-mediated signaling.

Lipid raft-mediated regulation of G-protein coupled receptor signaling by ligands which influence receptor dimerization: a computational study

G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors; they activate heterotrimeric G-proteins in response to ligand stimulation. Although many GPCRs have been shown to form homo- and/or heterodimers on the cell membrane, the purpose of this dimerization is not known. Recent research has shown that receptor dimerization may have a role in organization of receptors on the cell surface. In addition, microdomains on the cell membrane termed lipid rafts have been shown to play a role in GPCR localization. Using a combination of stochastic (Monte Carlo) and deterministic modeling, we propose a novel mechanism for lipid raft partitioning of GPCRs based on reversible dimerization of receptors and then demonstrate that such localization can affect GPCR signaling. Modeling results are consistent with a variety of experimental data indicating that lipid rafts have a role in amplification or attenuation of G-protein signaling. Thus our work suggests a new mechanism by which dimerization-inducing or inhibiting characteristics of ligands can influence GPCR signaling by controlling receptor organization on the cell membrane.

Extraction of membrane cholesterol disrupts caveolae and impairs serotonergic (5-HT2A) and histaminergic (H1) responses in bovine airway smooth muscle: role of Rho-kinase

Some receptors and signaling molecules, such as Rho-kinase (ROCK), localize in caveolae. We asked whether the function of histamine receptors (H1) and 5-hydroxytryptamine (serotonin) receptors (5-HT2A) in bovine tracheal smooth muscle are modified after caveolae disruption and if so, whether the altered ROCK activity plays a role in this modification. Methyl-β-cyclodextrin (MβCD), used to deplete membrane cholesterol, was shown to disrupt caveolae and diminish sustained contractions to histamine (∼80%), 5-HT (100%), α-methyl-5-HT (100%), and KCl (∼30%). Cholesterol-loaded MβCD (CL-MβCD) restored the responses to KCl and partially restored the responses to agonists. ROCK inhibition by Y-27632 diminished contractions to histamine (∼85%) and 5-HT (∼59%). 5-HT or histamine stimulation augmented ROCK activity. These increases were reduced by MβCD and partially reestablished by CL-MβCD. The increase in intracellular Ca2+ that was induced by both agonists was reduced by MβCD. The presence of caveolin-1 (Cav-1), H1, 5-HT2A, and ROCK1 was corroborated by immunoblotting of membrane fractions from sucrose gradients and by confocal microscopy. H1 receptors coimmunoprecipitated with Cav-1 in caveolar and noncaveolar membrane fractions, whereas 5-HT2A receptors appeared to be restricted to noncaveolar membrane fractions. We conclude that caveolar and cholesterol integrity are indispensable for the proper functionality of the H1 and 5-HT2A receptors through their Rho/ROCK signaling.

Heterooligomers of the muscarinic receptor and G proteins purified from porcine atria

Muscarinic receptor extracted from porcine atria in digitonin-cholate copurified with Galpha(o), Galpha(i1-3), and caveolins. The presence of complexes was confirmed by coimmunoprecipitation of the receptor, alpha-subunits, and caveolins in various combinations. Homooligomers of alpha(i2) were detected on Western blots, and heterooligomers of alpha(i2) and alpha(o) were identified by coimmunoprecipitation; thus, a complex may contain at least two alpha-subunits. Other combinations of alpha-subunit were not detected. The ratio of total alpha-subunit to receptor was near 1, as measured by [(35)S]GTPgammaS and the antagonist [(3)H]quinuclidinylbenzilate, and the binding of [(35)S]GTPgammaS was manifestly biphasic. The ratio of alpha(o) to alpha(i1,2) also was near 1, as determined from the intensity of Western blots. Cardiac muscarinic receptors therefore can be purified as a mixture of complexes that contain caveolins and oligomers of alpha-subunit, some of which are heteromeric. Each complex would appear to contain equal numbers of alpha-subunit and the receptor.

Muscarinic M4 receptor recycling requires a motif in the third intracellular loop

The present study was performed to identify sequence(s) in the third intracellular loop (i3) of the muscarinic acetylcholine receptor M4 subtype (M4 receptor) involved in its internalization and recycling. In transiently transfected human embryonic kidney 293-tsA201 cells, 40 to 50% of cell-surface M4 receptors are internalized in an agonist-dependent manner, and approximately 65% of internalized receptors are recycled back to the cell surface after removal of the agonist. We examined the internalization and recycling of M4 receptor mutants with partial deletion in i3 and found that various mutants (M4del-K(235)-K(240), M4del-T(241)-K(271), and M4del-W(339)-N(372)) showed internalization and cell-surface recycling in a similar manner to the M4 receptor. We also found that the mutant M4del-L(272)-R(338) was internalized to only half the extent of the M4 receptor and was recycled after agonist removal, and the mutant M4del-V(373)-A(393) was also internalized to half the extent of the wild type but was not recycled back to the cell surface after agonist removal. When the sequence corresponding to Val(373)-Ala(393) was grafted onto the i3 portion of a recycling-negative mutant of muscarinic M2 receptor with deletion of almost the whole of the i3 sequence, approximately 40% of the chimeric receptor on the cell surface was internalized, and more than 65% of the internalized receptors were recycled back to the cell surface. These results indicate that the regions including Leu(272)-Arg(338) and Val(373)-Ala(393) are involved in internalization of the M4 receptor, and the region including Val(373)-Ala(393) is indispensable for its recycling, whereas the other regions of i3 are dispensable for internalization and recycling.

Disruption of the plasma membrane integrity by cholesterol depletion impairs effectiveness of TRH receptor-mediated signal transduction via G(q)/G(11)alpha proteins

We monitored the radioligand-binding characteristics of thyrotropin-releasing hormone (TRH) receptors, functional activity of G(q/11)alpha proteins, and functional status of the whole signaling cascade in HEK293 expressing high levels of TRH receptors and G(11)alpha. Our analyses indicated that disruption of plasma membrane microdomains by cholesterol depletion did not markedly influence the binding parameters of TRH receptors, but it altered efficacy of signal transduction. The functional coupling between TRH receptor and G(q/11)alpha was assessed by agonist-stimulated [(35)S]GTPgammaS binding, and results of these measurements pointed out to significantly lower potency of TRH to mediate G protein activation in the plasma membrane fraction isolated from cholesterol-depleted cells; there was a shift in sensitivity by one order of magnitude to the higher concentrations. A markedly lower sensitivity to stimulation with TRH was also observed in our experiments dealing with determination of hormone-induced Ca(2+) response. These data suggest that the intact structure of plasma membranes is an important optimum signal transduction initiated by TRH receptors and mediated by G(q/11)alpha proteins.

The internalization of the M2 and M4 muscarinic acetylcholine receptors involves distinct subsets of small G-proteins

Multiple mechanisms exist for the endocytosis of receptors from the cell surface. While the M1, M3, and M4 subtypes of muscarinic acetylcholine receptor and M4 receptors transduce their signals through the same second messengers but internalize though different pathways, we tested the ability of several small G-proteins to regulate the agonist-induced endocytosis of M2 and M4 in JEG-3 human choriocarcinoma cells. Dominant-negative Rab5 as well as both wild-type and dominant-negative Rab11 inhibited M4 but not M2 endocytosis. In contrast, a dominant-negative Arf6 as well as wild-type Rab22 increased M2 but not M4 endocytosis. We used immunocytochemistry to show that in unstimulated cells, the M2 and M4 receptors co-localize on the cell surface, whereas after stimulation M2 and M4 are in distinct vesicular compartments. In this study, we demonstrate that agonist-induced internalization of the M2 receptor utilizes an Arf6, Rab22 dependent pathway, while the M4 receptor undergoes agonist-induced internalization through a Rab5, Rab11 dependent pathway. Additionally, we show that Rab15 and RhoA are not involved in either pathway in JEG-3 cells.

Caveolae facilitate muscarinic receptor-mediated intracellular Ca2+ mobilization and contraction in airway smooth muscle

Contractile responses of airway smooth muscle (ASM) determine airway resistance in health and disease. Caveolae microdomains in the plasma membrane are marked by caveolin proteins and are abundant in contractile smooth muscle in association with nanospaces involved in Ca(2+) homeostasis. Caveolin-1 can modulate localization and activity of signaling proteins, including trimeric G proteins, via a scaffolding domain. We investigated the role of caveolae in contraction and intracellular Ca(2+) ([Ca(2+)](i)) mobilization of ASM induced by the physiological muscarinic receptor agonist, acetylcholine (ACh). Human and canine ASM tissues and cells predominantly express caveolin-1. Muscarinic M(3) receptors (M(3)R) and Galpha(q/11) cofractionate with caveolin-1-rich membranes of ASM tissue. Caveolae disruption with beta-cyclodextrin in canine tracheal strips reduced sensitivity but not maximum isometric force induced by ACh. In fura-2-loaded canine and human ASM cells, exposure to methyl-beta-cyclodextrin (mbetaCD) reduced sensitivity but not maximum [Ca(2+)](i) induced by ACh. In contrast, both parameters were reduced for the partial muscarinic agonist, pilocarpine. Fluorescence microscopy revealed that mbetaCD disrupted the colocalization of caveolae-1 and M(3)R, but [N-methyl-(3)H]scopolamine receptor-binding assay revealed no effect on muscarinic receptor availability or affinity. To dissect the role of caveolin-1 in ACh-induced [Ca(2+)](i) flux, we disrupted its binding to signaling proteins using either a cell-permeable caveolin-1 scaffolding domain peptide mimetic or by small interfering RNA knockdown. Similar to the effects of mbetaCD, direct targeting of caveolin-1 reduced sensitivity to ACh, but maximum [Ca(2+)](i) mobilization was unaffected. These results indicate caveolae and caveolin-1 facilitate [Ca(2+)](i) mobilization leading to ASM contraction induced by submaximal concentrations of ACh.

Lipid rafts and caveolae in signaling by growth factor receptors

Lipid rafts and caveolae are microdomains of the plasma membrane enriched in sphingolipids and cholesterol, and hence are less fluid than the remainder of the membrane. Caveolae have an invaginated structure, while lipid rafts are flat regions of the membrane. The two types of microdomains have different protein compositions (growth factor receptors and their downstream molecules) suggesting that lipid rafts and caveolae have a role in the regulation of signaling by these receptors. The purpose of this review is to discuss this model, and the implications that it might have regarding a potential role for lipid rafts and caveolae in human cancer. Particular attention will be paid to the epidermal growth factor receptor, for which the largest amount of information is available. It has been proposed that caveolins act as tumor suppressors. The role of lipid rafts is less clear, but they seem to be capable of acting as 'signaling platforms', in which signal initiation and propagation can occur efficiently.

Evidence for a role of caveolin-1 in neurokinin-1 receptor plasma-membrane localization, efficient signaling, and interaction with beta-arrestin 2

This study was focused on the relationship between the plasma-membrane localization of neurokinin-1 receptor (NK1-R) and its endocytic and signaling properties. First, we employed electron paramagnetic resonance (EPR) to study the domain structure of HEK-293 cells and NK1-R microlocalization. EPR spectra and the GHOST condensation routine demonstrated that NK1-R was distributed in a well-ordered domain of HEK-293 cells possibly representing lipid raft/caveolae microdomains, whereas the impairment of caveolae changed the NK1-R plasma-membrane distribution. Internalization and second messenger assays combined with bioluminescence resonance energy transfer were employed subsequently to evaluate the functional importance of the NK1-R microlocalization in lipid raft/caveolae microdomains. The internalization pattern was delineated through the use of dominant-negative mutants (DNM) of caveolin-1 S80E (Cav1 S80E), dynamin-1 K44A (Dyn K44A), and beta-arrestin (beta-arr 319-418) and by means of cell lines that expressed various endogenous levels of beta-arrestins. NK1-R displayed rapid internalization that was substantially reduced by DNMs of dynamin-1 and beta-arrestin and even more profoundly in cells lacking both beta-arrestin1 and beta-arrestin2. These internalization data were highly suggestive of the predominant use of the clathrin-mediated pathway by NK1-R, even though NK1-R tended to reside constitutively in lipid raft/caveolae microdomains. Evidence was also obtained that the proper clustering of the receptor in these microdomains was important for effective agonist-induced NK1-R signaling and for its interaction with beta-arrestin2.

Methyl-β-cyclodextrin Prevents Angiotensin II-Induced Tachyphylactic Contractile Responses in Rat Aorta

Tachyphylaxis or desensitization is frequently observed following angiotensin II type I (AT1) receptor activation by angiotensin II. One of the possible mechanisms contributing to receptor desensitization involves receptor internalization. In addition to clathrin-coated pits/vesicles, caveolae, small invaginations in the plasma membrane rich in cholesterol, may also be involved in receptor internalization. After activation, AT1 receptor partially redistributes to lipid-enriched domains. We hypothesize that AT1 receptor internalization via caveolae contributes to the tachyphylactic response observed to angiotensin II. Endothelium-denuded rat aortic rings were exposed to increasing concentrations of angiotensin II or phenylephrine, generating two cumulative concentration-effect curves (CCEC) with a 90-min interval separating each curve (CCEC-I and CCEC-II). CCEC-II was performed in the presence of either vehicle or methyl-beta-cyclodextrin (CD), a drug that depletes cholesterol from the membrane and disassembles caveolae. CCEC-II to angiotensin II, but not to phenylephrine, was blunted in aortic rings treated with vehicle. In the presence of CD, CCEC-II did not differ significantly from CCEC-I for both agonists. CCEC-I to angiotensin II was abolished when in the presence of the AT1 receptor antagonist. The presence of AT1 receptors at the aortic smooth muscle cells' membrane treated with angiotensin II was observed by immunofluorescence only in the presence of CD. In addition, caveolin-1 coimmunoprecipitated with AT1 receptor after agonist stimulation, and this interaction was inhibited by CD. Our data suggest that caveolae are involved in the tachyphylactic contractile response induced by angiotensin II in rat aorta, and this effect is related to receptor internalization.

Cholesterol as a determinant of cooperativity in the M2 muscarinic cholinergic receptor

M2 muscarinic receptor extracted from Sf9 cells in cholate-NaCl differs from that extracted from porcine sarcolemma. The latter has been shown to exhibit an anomalous pattern in which the capacity for N-[3H]methylscopolamine (NMS) is only 50% of that for [3H]quinuclidinylbenzilate (QNB), yet unlabeled NMS exhibits high affinity for all of the sites labeled by [3H]QNB. The effects can be explained in terms of cooperativity within a receptor that is at least tetravalent [Park PS, Sum CS, Pawagi AB, Wells JW. Cooperativity and oligomeric status of cardiac muscarinic cholinergic receptors. Biochemistry 2002;41:5588-604]. In contrast, M2 receptor extracted from Sf9 membranes exhibited no shortfall in the capacity for [3H]NMS at either 30 or 4 degrees C, although there was a time-dependent inactivation during incubation with [3H]NMS at 30 degrees C; also, any discrepancies in the affinity of NMS were comparatively small. The level of cholesterol in Sf9 membranes was only 4% of that in sarcolemmal membranes, and it was increased to about 100% by means of cholesterol-methyl-beta-cyclodextrin. M2 receptors extracted from treated Sf9 membranes were stable at 30 and 4 degrees C and resembled those from heart. Cholesterol induced a marked heterogeneity detected in the binding of both radioligands, including a shortfall in the apparent capacity for [3H]NMS, and there were significant discrepancies in the apparent affinity of NMS as estimated directly and via the inhibition of [3H]QNB. The data can be described quantitatively in terms of cooperative effects among six or more interacting sites. Cholesterol therefore appears to promote cooperativity in the binding of antagonists to the M2 muscarinic receptor.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Lipid rafts in health and disease

Lipid rafts are sphingolipid- and cholesterol-rich domains of the plasma membrane which contain a variety of signalling and transport proteins. Different subtypes of lipid rafts can be distinguished according to their protein and lipid composition. Caveolae are types of rafts that are rich in proteins of the caveolin family (caveolin-1, -2 and -3) which present a distinct signalling platform. The importance of lipid raft signalling in the pathogenesis of a variety of conditions, such as Alzheimer's, Parkinson's, cardiovascular and prion diseases, systemic lupus erythematosus and HIV, has been elucidated over recent years and makes these specific membrane domains an interesting target for pharmacological approaches in the cure and prevention of these diseases. This Review analyses the importance of lipid raft proteins and lipids in health and disease, with a focus on the current state of knowledge.

Disruption of endothelial caveolae is associated with impairment of both NO- as well as EDHF in acetylcholine-induced relaxation depending on their relative contribution in different vascular beds

Caveolae represent an important structural element involved in endothelial signal-transduction. The present study was designed to investigate the role of caveolae in endothelium-dependent relaxation of different vascular beds. Caveolae were disrupted by cholesterol depletion with filipin (4x10(-6) g L(-1)) or methyl-beta-cyclodextrin (MCD; 1x10(-3) mol L(-1)) and the effect on endothelium-dependent relaxation was studied in rat aorta, small renal arteries and mesenteric arteries in the absence and presence of L-NMMA. The contribution of NO and EDHF, respectively, to total relaxation in response to acetylcholine (ACh) gradually changed from aorta (71.2+/-6.1% and 28.8+/-6.1%), to renal arteries (48.6+/-6.4% and 51.4+/-6.4%) and to mesenteric arteries (9.1+/-4.0% and 90.9+/-4.1%). Electron microscopy confirmed filipin to decrease the number of endothelial caveolae in all vessels studied. Incubation with filipin inhibited endothelium-dependent relaxation induced by cumulative doses of ACh (3x10(-9)-10(-4) mol L(-1)) in all three vascular beds. In aorta, treatment with either filipin or MCD only inhibited the NO component, whereas in renal artery both NO and EDHF formation were affected. In contrast, in mesenteric arteries, filipin treatment only reduced EDHF formation. Disruption of endothelial caveolae is associated with the impairment of both NO and EDHF in acetylcholine-induced relaxation.

Role of internalization of M2muscarinic receptor via clathrin-coated vesicles in desensitization of the muscarinic K+current in heart

In the heart, ACh activates the ACh-activated K+current ( IK,ACh) via the M2muscarinic receptor. The relationship between desensitization of IK,AChand internalization of the M2receptor has been studied in rat atrial cells. On application of the stable muscarinic agonist carbachol for 2 h, IK,AChdeclined by ∼62% with time constants of 1.5 and 26.9 min, whereas ∼83% of the M2receptor was internalized from the cell membrane with time constants of 2.9 and 51.6 min. Transfection of the cells with β-adrenergic receptor kinase 1 (G protein-receptor kinase 2) and β-arrestin 2 significantly increased IK,AChdesensitization and M2receptor internalization during a 3-min application of agonist. Internalized M2receptor in cells exposed to carbachol for 2 h was colocalized with clathrin and not caveolin. It is concluded that a G protein-receptor kinase 2- and β-arrestin 2-dependent internalization of the M2receptor into clathrin-coated vesicles could play a major role in IK,AChdesensitization.