Role of protein kinase A and A kinase anchoring proteins in buffering and compartmentation of cAMP signalling in human airway smooth muscle cells

BACKGROUND AND PURPOSE

In human airway smooth muscle (hASM) cells, not all receptors stimulating cAMP production elicit the same effects. This can only be explained if cAMP movement throughout the cell is restricted, yet the mechanisms involved are not fully understood. Phosphodiesterases (PDEs) contribute to compartmentation of many cAMP responses, but PDE activity alone is predicted to be insufficient if cAMP is otherwise freely diffusible. We tested the hypothesis that buffering of cAMP by protein kinase A (PKA) associated with A kinase anchoring proteins (AKAPs) slows cAMP diffusion and that this contributes to receptor-mediated, compartmentalized responses.

EXPERIMENTAL APPROACH

Raster image correlation spectroscopy (RICS) was used to measure intracellular cAMP diffusion coefficients and evaluate the contribution of PKA-AKAP interactions. Western blotting and immunocytochemistry were used to identify the AKAPs involved. RNA interference was used to down-regulate AKAP expression and determine its effects on cAMP diffusion. Compartmentalized cAMP responses were measured using fluorescence resonance energy transfer (FRET) based biosensors.

KEY RESULTS

Cyclic AMP movement was significantly slower than that of free-diffusion in hASM cells, and disrupting PKA-AKAP interactions significantly increased the diffusion coefficient. PKA associated with the outer mitochondrial membrane appears to play a prominent role in this effect. Consistent with this idea, knocking down expression of D-AKAP2, the primary mitochondrial AKAP, increased cAMP diffusion and disrupted compartmentation of receptor-mediated responses.

CONCLUSION AND IMPLICATIONS

Our results confirm that AKAP-anchored PKA contributes to the buffering of cAMP and is consequential in the compartmentation of cAMP responses in hASM cells.

Real-Time cAMP Dynamics in Live Cells Using the Fluorescent cAMP Difference Detector In Situ

cAMP Difference Detector In Situ (cADDis) is a novel biosensor that allows for the continuous measurement of cAMP levels in living cells. The biosensor is created from a circularly permuted fluorescent protein linked to the hinge region of Epac2. This creates a single fluorophore biosensor that displays either increased or decreased fluorescence upon binding of cAMP. The biosensor exists in red and green upward versions, as well as green downward versions, and several red and green versions targeted to subcellular locations. To illustrate the effectiveness of the biosensor, the green downward version, which decreases in fluorescence upon cAMP binding, was used. Two protocols using this sensor are demonstrated: one utilizing a 96-well plate reading spectrophotometer compatible with high-throughput screening and another utilizing single-cell imaging on a fluorescent microscope. On the plate reader, HEK-293 cells cultured in 96-well plates were stimulated with 10 µM forskolin or 10 nM isoproterenol, which induced rapid and large decreases in fluorescence in the green downward version. The biosensor was used to measure cAMP levels in individual human airway smooth muscle (HASM) cells monitored under a fluorescent microscope. The green downward biosensor displayed similar responses to populations of cells when stimulated with forskolin or isoproterenol. This single-cell assay allows visualization of the biosensor location at 20x and 40x magnification. Thus, this cAMP biosensor is sensitive and flexible, allowing real-time measurement of cAMP in both immortalized and primary cells, and with single cells or populations of cells. These attributes make cADDis a valuable tool for studying cAMP signaling dynamics in living cells.

Prorelaxant E-type Prostanoid Receptors Functionally Partition to Different Procontractile Receptors in Airway Smooth Muscle

Prostaglandin E2 imparts diverse physiological effects on multiple airway cells through its actions on four distinct E-type prostanoid (EP) receptor subtypes (EP1-EP4). Gs-coupled EP2 and EP4 receptors are expressed on airway smooth muscle (ASM), yet their capacity to regulate the ASM contractile state remains subject to debate. We used EP2 and EP4 subtype-specific agonists (ONO-259 and ONO-329, respectively) in cell- and tissue-based models of human ASM contraction-magnetic twisting cytometry (MTC), and precision-cut lung slices (PCLSs), respectively-to study the EP2 and EP4 regulation of ASM contraction and signaling under conditions of histamine or methacholine (MCh) stimulation. ONO-329 was superior (<0.05) to ONO-259 in relaxing MCh-contracted PCLSs (log half maximal effective concentration [logEC50]: 4.9 × 10-7 vs. 2.2 × 10-6; maximal bronchodilation ± SE, 35 ± 2% vs. 15 ± 2%). However, ONO-259 and ONO-329 were similarly efficacious in relaxing histamine-contracted PCLSs. Similar differential effects were observed in MTC studies. Signaling analyses revealed only modest differences in ONO-329- and ONO-259-induced phosphorylation of the protein kinase A substrates VASP and HSP20, with concomitant stimulation with MCh or histamine. Conversely, ONO-259 failed to inhibit MCh-induced phosphorylation of the regulatory myosin light chain (pMLC20) and the F-actin/G-actin ratio (F/G-actin ratio) while effectively inhibiting their induction by histamine. ONO-329 was effective in reversing induced pMLC20 and the F/G-actin ratio with both MCh and histamine. Thus, the contractile-agonist-dependent differential effects are not explained by changes in the global levels of phosphorylated protein kinase A substrates but are reflected in the regulation of pMLC20 (cross-bridge cycling) and F/G-actin ratio (actin cytoskeleton integrity, force transmission), implicating a role for compartmentalized signaling involving muscarinic, histamine, and EP receptor subtypes.

Quantitative phosphoproteomic analysis reveals unique cAMP signaling pools emanating from AC2 and AC6 in human airway smooth muscle cells

Human airway smooth muscle (HASM) is the primary target of ßAR agonists used to control airway hypercontractility in asthma and chronic obstructive pulmonary disease (COPD). ßAR agonists induce the production of cAMP by adenylyl cyclases (ACs), activate PKA and cause bronchodilation. Several other G-protein coupled receptors (GPCR) expressed in human airway smooth muscle cells transduce extracellular signals through cAMP but these receptors elicit different cellular responses. Some G-protein coupled receptors couple to distinct adenylyl cyclases isoforms with different localization, partly explaining this compartmentation, but little is known about the downstream networks that result. We used quantitative phosphoproteomics to define the downstream signaling networks emanating from cAMP produced by two adenylyl cyclases isoforms with contrasting localization in uman airway smooth muscle. After a short stimulus of adenylyl cyclases activity using forskolin, phosphopeptides were analyzed by LC-MS/MS and differences between cells overexpressing AC2 (localized in non-raft membranes) or AC6 (localized in lipid raft membranes) were compared to control human airway smooth muscle. The degree of AC2 and AC6 overexpression was titrated to generate roughly equal forskolin-stimulated cAMP production. 14 Differentially phosphorylated proteins (DPPs) resulted from AC2 activity and 34 differentially phosphorylated proteins resulted from AC6 activity. Analysis of these hits with the STRING protein interaction tool showed that AC2 signaling is more associated with modifications in RNA/DNA binding proteins and microtubule/spindle body proteins while AC6 signaling is associated with proteins regulating autophagy, calcium-calmodulin (Ca2+/CaM) signaling, Rho GTPases and cytoskeletal regulation. One protein, OFD1, was regulated in opposite directions, with serine 899 phosphorylation increased in the AC6 condition 1.5-fold but decreased to 0.46-fold by AC2. In conclusion, quantitative phosphoproteomics is a powerful tool for deciphering the complex signaling networks resulting from discreet signaling events that occur in cAMP compartments. Our data show key differences in the cAMP pools generated from AC2 and AC6 activity and imply that distinct cellular responses are regulated by these two compartments.

Physiological Roles of Mammalian Transmembrane Adenylyl Cyclase Isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors. The transmembrane ACs display varying expression patterns across tissues, giving potential for them having a wide array of physiologic roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gα_s so it was long assumed that all ACs are activated by Gα_s-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Anti-inflammatory effects of α7-nicotinic ACh receptors are exerted through interactions with adenylyl cyclase-6

BACKGROUND AND PURPOSE

Nicotinic ACh receptors containing the α7 sub-unit (α7-nAChRs) suppress inflammation through a wide range of pathways in immune cells. These receptors are thus potentially involved in a number of inflammatory diseases. However, the detailed mechanisms underlying the anti-inflammatory effects of α7-nAChRs remain to be described.

EXPERIMENTAL APPROACH

Anti-inflammatory effects of α7-nAChR agonists were assessed in both murine macrophages (RAW 264.7) and bone marrow-derived macrophages (BMDM), stimulated with LPS, using immunoblotting, RT-PCR and luciferase reporter assays. The role of adenylyl cyclase-6 in the degradation of Toll-like receptor 4 (TLR4) following endocytosis, was explored via overexpression and knockdown. A mouse model of chronic obstructive pulmonary disease (COPD) induced by porcine pancreatic elastase was used to confirm key findings.

RESULTS

Anti-inflammatory effects of α7-nAChRs were largely dependent on adenylyl cyclase-6 activation, as knockdown of adenylyl cyclase-6 considerably reduced the effects of α7-nAChR agonists while adenylyl cyclase-6 overexpression promoted them. We found that α7-nAChRs and adenylyl cyclase-6 are co-localized in lipid rafts of macrophages and directly interact. Activation of adenylyl cyclase-6 led to increased degradation of TLR4. Administration of the α7-nAChR agonist PNU-282987 attenuated pathological and inflammatory end points in a mouse model of COPD.

CONCLUSION AND IMPLICATIONS

The α7-nAChRs inhibit inflammation through activating adenylyl cyclase-6 and promoting degradation of TLR4. The use of α7-nAChR agonists may represent a novel therapeutic approach for treating COPD and possibly other inflammatory diseases.

SARS-CoV-2 early infection signature identified potential key infection mechanisms and drug targets

Background

The ongoing COVID-19 outbreak has caused devastating mortality and posed a significant threat to public health worldwide. Despite the severity of this illness and 2.3 million worldwide deaths, the disease mechanism is mostly unknown. Previous studies that characterized differential gene expression due to SARS-CoV-2 infection lacked robust validation. Although vaccines are  now available, effective treatment options are still out of reach.

Results

To characterize the transcriptional activity of SARS-CoV-2 infection, a gene signature consisting of 25 genes was generated using a publicly available RNA-Sequencing (RNA-Seq) dataset of cultured cells infected with SARS-CoV-2. The signature estimated infection level accurately in bronchoalveolar lavage fluid (BALF) cells and peripheral blood mononuclear cells (PBMCs) from healthy and infected patients (mean 0.001 vs. 0.958; P < 0.0001). These signature genes were investigated in their ability to distinguish the severity of SARS-CoV-2 infection in a single-cell RNA-Sequencing dataset. TNFAIP3, PPP1R15A, NFKBIA, and IFIT2 had shown bimodal gene expression in various immune cells from severely infected patients compared to healthy or moderate infection cases. Finally, this signature was assessed using the publicly available ConnectivityMap database to identify potential disease mechanisms and drug repurposing candidates. Pharmacological classes of tricyclic antidepressants, SRC-inhibitors, HDAC inhibitors, MEK inhibitors, and drugs such as atorvastatin, ibuprofen, and ketoconazole showed strong negative associations (connectivity score < − 90), highlighting the need for further evaluation of these candidates for their efficacy in treating SARS-CoV-2 infection.

Conclusions

Thus, using the 25-gene SARS-CoV-2 infection signature, the SARS-CoV-2 infection status was captured in BALF cells, PBMCs and postmortem lung biopsies. In addition, candidate SARS-CoV-2 therapies with known safety profiles were identified. The signature genes could potentially also be used to characterize the COVID-19 disease severity in patients’ expression profiles of BALF cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07433-4.

Phosphodiesterase isoforms and cAMP compartments in the development of new therapies for obstructive pulmonary diseases

The second messenger molecule 3'5'-cyclic adenosine monophosphate (cAMP) imparts several beneficial effects in lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). While cAMP is bronchodilatory in asthma and COPD, it also displays anti-fibrotic properties that limit fibrosis. Phosphodiesterases (PDEs) metabolize cAMP and thus regulate cAMP signaling. While some existing therapies inhibit PDEs, there are only broad family specific inhibitors. The understanding of cAMP signaling compartments, some centered around lipid rafts/caveolae, has led to interest in defining how specific PDE isoforms maintain these signaling microdomains. The possible altered expression of PDEs, and thus abnormal cAMP signaling, in obstructive lung diseases has been poorly explored. We propose that inhibition of specific PDE isoforms can improve therapy of obstructive lung diseases by amplifying specific cAMP signals in discreet microdomains.

Transforming Growth Factor-β1 Decreases β2-Agonist-induced Relaxation in Human Airway Smooth Muscle

Helper T effector cytokines implicated in asthma modulate the contractility of human airway smooth muscle (HASM) cells. We have reported recently that a profibrotic cytokine, transforming growth factor (TGF)-β1, induces HASM cell shortening and airway hyperresponsiveness. Here, we assessed whether TGF-β1 affects the ability of HASM cells to relax in response to β₂-agonists, a mainstay treatment for airway hyperresponsiveness in asthma. Overnight TGF-β1 treatment significantly impaired isoproterenol (ISO)-induced relaxation of carbachol-stimulated, isolated HASM cells. This single-cell mechanical hyporesponsiveness to ISO was corroborated by sustained increases in myosin light chain phosphorylation. In TGF-β1-treated HASM cells, ISO evoked markedly lower levels of intracellular cAMP. These attenuated cAMP levels were, in turn, restored with pharmacological and siRNA inhibition of phosphodiesterase 4 and Smad3, respectively. Most strikingly, TGF-β1 selectively induced phosphodiesterase 4D gene expression in HASM cells in a Smad2/3-dependent manner. Together, these data suggest that TGF-β1 decreases HASM cell β₂-agonist relaxation responses by modulating intracellular cAMP levels via a Smad2/3-dependent mechanism. Our findings further define the mechanisms underlying β₂-agonist hyporesponsiveness in asthma, and suggest TGF-β1 as a potential therapeutic target to decrease asthma exacerbations in severe and treatment-resistant asthma.

Glucocorticoids rapidly activate cAMP production via Gαs to initiate non-genomic signaling that contributes to one-third of their canonical genomic effects

Glucocorticoids are widely used for the suppression of inflammation, but evidence is growing that they can have rapid, non-genomic actions that have been unappreciated. Diverse cell signaling effects have been reported for glucocorticoids, leading us to hypothesize that glucocorticoids alone can swiftly increase the 3',5'-cyclic adenosine monophosphate (cAMP) production. We found that prednisone, fluticasone, budesonide, and progesterone each increased cAMP levels within 3 minutes without phosphodiesterase inhibitors by measuring real-time cAMP dynamics using the cAMP difference detector in situ assay in a variety of immortalized cell lines and primary human airway smooth muscle (HASM) cells. A membrane- impermeable glucocorticoid showed similarly rapid stimulation of cAMP, implying that responses are initiated at the cell surface. siRNA knockdown of G_αs virtually eliminated glucocorticoid-stimulated cAMP responses, suggesting that these drugs activate the cAMP production via a G protein-coupled receptor. Estradiol had small effects on cAMP levels but G protein estrogen receptor antagonists had little effect on responses to any of the glucocorticoids tested. The genomic and non-genomic actions of budesonide were analyzed by RNA-Seq analysis of 24 hours treated HASM, with and without knockdown of G_αs . A 140-gene budesonide signature was identified, of which 48 genes represent a non-genomic signature that requires G_αs signaling. Collectively, this non-genomic cAMP signaling modality contributes to one-third of the gene expression changes induced by glucocorticoid treatment and shifts the view of how this important class of drugs exerts its effects.

Budesonide enhances agonist-induced bronchodilation in human small airways by increasing cAMP production in airway smooth muscle

The nongenomic mechanisms by which glucocorticoids modulate β₂ agonist-induced-bronchodilation remain elusive. Our studies aimed to elucidate mechanisms mediating the beneficial effects of glucocorticoids on agonist-induced bronchodilation. Utilizing human precision-cut lung slices (hPCLS), we measured bronchodilation to formoterol, prostaglandin E₂ (PGE₂), cholera toxin (CTX), or forskolin in the presence and absence of budesonide. Using cultured human airway smooth muscle (HASM), intracellular cAMP was measured in live cells following exposure to formoterol, PGE₂, or forskolin in the presence or absence of budesonide. We showed that simultaneous budesonide administration amplified formoterol-induced bronchodilation and attenuated agonist-induced phosphorylation of myosin light chain, a necessary signaling event mediating force generation. In parallel studies, cAMP levels were augmented by simultaneous exposure of HASM cells to formoterol and budesonide. Budesonide, fluticasone, and prednisone alone rapidly increased cAMP levels, but steroids alone had little effect on bronchodilation in hPCLS. Bronchodilation induced by PGE₂, CTX, or forskolin was also augmented by simultaneous exposure to budesonide in hPCLS. Furthermore, HASM cells expressed membrane-bound glucocorticoid receptors that failed to translocate with glucocorticoid stimulation and that potentially mediated the rapid effects of steroids on β₂ agonist-induced bronchodilation. Knockdown of glucocorticoid receptor-α had little effect on budesonide-induced and steroid-dependent augmentation of formoterol-induced cAMP generation in HASM. Collectively, these studies suggest that glucocorticoids amplify cAMP-dependent bronchodilation by directly increasing cAMP levels. These studies identify a molecular mechanism by which the combination of glucocorticoids and β₂ agonists may augment bronchodilation in diseases such as asthma or chronic obstructive pulmonary disease.

p53 Expression in Lung Fibroblasts Is Linked to Mitigation of Fibrotic Lung Remodeling

Idiopathic pulmonary fibrosis (IPF) is a debilitating, incurable, and life-threatening disease. A cardinal feature of the pathogenesis of IPF is excessive extracellular matrix deposition attributable to proliferation of activated fibrotic lung fibroblasts (fLfs). To assess the underlying mechanism, we analyzed the status of the tumor suppressor protein p53 in fLfs from the lungs of IPF patients or mice with bleomycin-induced established PF. We report that basal expression of p53 is markedly reduced in fLfs. Forced expression of caveolin-1 in fLfs increased basal p53 and reduced profibrogenic proteins, including collagen-1. Transduction of fLfs with adenovirus expressing p53 reduced expression of these proteins. Conversely, inhibition of baseline p53 in control lung fibroblasts from lung tissues increased profibrogenic protein expression. Lung transduction of adenovirus expressing p53 reduced bleomycin-induced PF in wild-type or caveolin-1-deficient mice. Furthermore, treatment of fLfs or fibrotic lung tissues with caveolin-1 scaffolding domain peptide (CSP) or its fragment, CSP7, restored p53 and reduced profibrogenic proteins. Treatment of wild-type mice with i.p. CSP or CSP7 resolved bleomycin-induced PF. These peptides failed to resolve PF in inducible conditional knockout mice lacking p53 in fLfs, indicating the induction of baseline fLf p53 as the basis of the antifibrotic effects.

Effect of Adenylyl Cyclase Type 6 on Localized Production of cAMP by β-2 Adrenoceptors in Human Airway Smooth-Muscle Cells

β ₂-Adrenoceptors (β ₂ARs) are concentrated in caveolar lipid raft domains of the plasma membrane in airway smooth-muscle (ASM) cells, along with adenylyl cyclase type 6 (AC6). This is believed to contribute to how these receptors can selectively regulate certain types of cAMP-dependent responses in these cells. The goal of the present study was to test the hypothesis that β ₂AR production of cAMP is localized to specific subcellular compartments using fluorescence resonance energy transfer-based cAMP biosensors targeted to different microdomains in human ASM cells. Epac2-MyrPalm and Epac2-CAAX biosensors were used to measure responses associated with lipid raft and nonraft regions of the plasma membrane, respectively. Activation of β ₂ARs with isoproterenol produced cAMP responses that are most readily detected in lipid raft domains. Furthermore, overexpression of AC6 somewhat paradoxically inhibited β ₂AR production of cAMP in lipid raft domains without affecting β ₂AR responses detected in other subcellular locations or cAMP responses to EP₂ prostaglandin receptor activation, which were confined primarily to nonraft domains of the plasma membrane. The inhibitory effect of overexpressing AC6 was blocked by inhibition of phosphodiesterase type 4 (PDE4) activity with rolipram, inhibition of protein kinase A (PKA) activity with H89, and inhibition of A kinase anchoring protein (AKAP) interactions with the peptide inhibitor Ht31. These results support the idea that overexpression of AC6 leads to enhanced feedback activation of PDE4 via phosphorylation by PKA that is part of an AKAP-dependent signaling complex. This provides insight into the molecular basis for localized regulation of cAMP signaling in human ASM cells.

cAMP attenuates TGF-β's profibrotic responses in osteoarthritic synoviocytes: involvement of hyaluronan and PRG4

Osteoarthritis (OA) is characterized by synovitis and synovial fibrosis. Synoviocytes are fibroblast-like resident cells of the synovium that are activated by transforming growth factor (TGF)-β to proliferate, migrate, and produce extracellular matrix. Synoviocytes secrete hyaluronan (HA) and proteoglycan-4 (PRG4). HA reduces synovial fibrosis in vivo, and the Prg4^-/- mouse exhibits synovial hyperplasia. We investigated the antifibrotic effects of increased intracellular cAMP in TGF-β-stimulated human OA synoviocytes. TGF-β1 stimulated collagen I (COL1A1), α-smooth muscle actin (α-SMA), tissue inhibitor of metalloproteinase (TIMP)-1, and procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2) expression, and procollagen I, α-SMA, HA, and PRG4 production, migration, and proliferation of OA synoviocytes were measured. Treatment of OA synoviocytes with forskolin (10 μM) increased intracellular cAMP levels and reduced TGF-β1-stimulated COL1A1, α-SMA, and TIMP-1 expression, with no change in PLOD2 expression. Forskolin also reduced TGF-β1-stimulated procollagen I and α-SMA content as well as synoviocyte migration and proliferation. Forskolin (10 μM) increased HA secretion and PRG4 expression and production. A cell-permeant cAMP analog reduced COL1A1 and α-SMA expression and enhanced HA and PRG4 secretion by OA synoviocytes. HA and PRG4 reduced α-SMA expression and content, and PRG4 reduced COL1A1 expression and procollagen I content in OA synoviocytes. Prg4^-/- synovium exhibited increased α-SMA, COL1A1, and TIMP-1 expression compared with Prg4^+/+ synovium. Prg4^-/- synoviocytes demonstrated strong α-SMA and collagen type I staining, whereas these were undetected in Prg4^+/+ synoviocytes and were reduced with PRG4 treatment. We conclude that increasing intracellular cAMP levels in synoviocytes mitigates synovial fibrosis through enhanced production of HA and PRG4, possibly representing a novel approach for treatment of OA synovial fibrosis.

PDE8 Is Expressed in Human Airway Smooth Muscle and Selectively Regulates cAMP Signaling by β2-Adrenergic Receptors and Adenylyl Cyclase 6

Two cAMP signaling compartments centered on adenylyl cyclase (AC) exist in human airway smooth muscle (HASM) cells, one containing β2-adrenergic receptor AC6 and another containing E prostanoid receptor AC2. We hypothesized that different PDE isozymes selectively regulate cAMP signaling in each compartment. According to RNA-sequencing data, 18 of 24 PDE genes were expressed in primary HASM cells derived from age- and sex-matched donors with and without asthma. PDE8A was the third most abundant of the cAMP-degrading PDE genes, after PDE4A and PDE1A. Knockdown of PDE8A using shRNA evoked twofold greater cAMP responses to 1 μM forskolin in the presence of 3-isobutyl-1-methylxanthine. Overexpression of AC2 did not alter this response, but overexpression of AC6 increased cAMP responses an additional 80%. We examined cAMP dynamics in live HASM cells using a fluorescence sensor. PF-04957325, a PDE8-selective inhibitor, increased basal cAMP concentrations by itself, indicating a significant basal level of cAMP synthesis. In the presence of an AC inhibitor to reduce basal signaling, PF-04957325 accelerated cAMP production and increased the inhibition of cell proliferation induced by isoproterenol, but it had no effect on cAMP concentrations or cell proliferation regulated by prostaglandin E2. Lipid raft fractionation of HASM cells revealed PDE8A immunoreactivity in buoyant fractions containing caveolin-1 and AC5/6 immunoreactivity. Thus, PDE8 is expressed in lipid rafts of HASM cells, where it specifically regulates β2-adrenergic receptor AC6 signaling without effects on signaling by the E prostanoid receptors 2/4-AC2 complex. In airway diseases such as asthma and chronic obstructive pulmonary disease, PDE8 may represent a novel therapeutic target to modulate HASM responsiveness and airway remodeling.

cAMP Signaling Compartmentation: Adenylyl Cyclases as Anchors of Dynamic Signaling Complexes

It is widely accepted that cAMP signaling is compartmentalized within cells. However, our knowledge of how receptors, cAMP signaling enzymes, effectors, and other key proteins form specific signaling complexes to regulate specific cell responses is limited. The multicomponent nature of these systems and the spatiotemporal dynamics involved as proteins interact and move within a cell make cAMP responses highly complex. Adenylyl cyclases, the enzymatic source of cAMP production, are key starting points for understanding cAMP compartments and defining the functional signaling complexes. Three basic elements are required to form a signaling compartment. First, a localized signal is generated by a G protein-coupled receptor paired to one or more of the nine different transmembrane adenylyl cyclase isoforms that generate the cAMP signal in the cytosol. The diffusion of cAMP is subsequently limited by several factors, including expression of any number of phosphodiesterases (of which there are 24 genes plus spice variants). Finally, signal response elements are differentially localized to respond to cAMP produced within each locale. A-kinase-anchoring proteins, of which there are 43 different isoforms, facilitate this by targeting protein kinase A to specific substrates. Thousands of potential combinations of these three elements are possible in any given cell type, making the characterization of cAMP signaling compartments daunting. This review will focus on what is known about how cells organize cAMP signaling components as well as identify the unknowns. We make an argument for adenylyl cyclases being central to the formation and maintenance of these signaling complexes.

Suppression of CHRN endocytosis by carbonic anhydrase CAR3 in the pathogenesis of myasthenia gravis

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction manifested as fatigable muscle weakness, which is typically caused by pathogenic autoantibodies against postsynaptic CHRN/AChR (cholinergic receptor nicotinic) in the endplate of skeletal muscle. Our previous studies have identified CA3 (carbonic anhydrase 3) as a specific protein insufficient in skeletal muscle from myasthenia gravis patients. In this study, we investigated the underlying mechanism of how CA3 insufficiency might contribute to myasthenia gravis. Using an experimental autoimmune myasthenia gravis animal model and the skeletal muscle cell C2C12, we find that inhibition of CAR3 (the mouse homolog of CA3) promotes CHRN internalization via a lipid raft-mediated pathway, leading to accelerated degradation of postsynaptic CHRN. Activation of CAR3 reduces CHRN degradation by suppressing receptor endocytosis. CAR3 exerts this effect by suppressing chaperone-assisted selective autophagy via interaction with BAG3 (BCL2-associated athanogene 3) and by dampening endoplasmic reticulum stress. Collectively, our study illustrates that skeletal muscle cell CAR3 is critical for CHRN homeostasis in the neuromuscular junction, and its deficiency leads to accelerated degradation of CHRN and development of myasthenia gravis, potentially revealing a novel therapeutic approach for this disorder.

Compartmentalized cAMP responses to prostaglandin EP2 receptor activation in human airway smooth muscle cells

BACKGROUND AND PURPOSE

Previous studies indicate that prostaglandin EP₂ receptors selectively couple to AC2 in non-lipid raft domains of airway smooth muscle (ASM) cells, where they regulate specific cAMP-dependent responses. The goal of the present study was to identify the cellular microdomains where EP₂ receptors stimulate cAMP production.

EXPERIMENTAL APPROACH

FRET-based cAMP biosensors were targeted to different subcellular locations of primary human ASM cells. The Epac2-camps biosensor, which expresses throughout the cell, was used to measure bulk cytoplasmic responses. Epac2-MyrPalm and Epac2-CAAX were used to measure responses associated with lipid raft and non-raft regions of the plasma membrane respectively. Epac2-NLS was used to monitor responses at the nucleus.

KEY RESULTS

Activation of AC with forskolin or β₂ -adrenoceptors with isoprenaline increased cAMP in all subcellular locations. Activation of EP₂ receptors with butaprost produced cAMP responses that were most readily detected by the non-raft and nuclear sensors, but only weakly detected by the cytosolic sensor and not detected at all by the lipid raft sensor. Exposure to rolipram, a PDE4 inhibitor, unmasked the ability of EP₂ receptors to increase cAMP levels associated with lipid raft domains. Overexpression of AC2 selectively increased EP₂ receptor-stimulated production of cAMP in non-raft membrane domains.

CONCLUSIONS AND IMPLICATIONS

EP₂ receptor activation of AC2 leads to cAMP production in non-raft and nuclear compartments of human ASMs, while β₂ adrenoceptor signalling is broadly detected across microdomains. The activity of PDE4 appears to play a role in maintaining the integrity of compartmentalized EP₂ receptor responses in these cells.

A CNS-permeable Hsp90 inhibitor rescues synaptic dysfunction and memory loss in APP-overexpressing Alzheimer's mouse model via an HSF1-mediated mechanism

Induction of neuroprotective heat-shock proteins via pharmacological Hsp90 inhibitors is currently being investigated as a potential treatment for neurodegenerative diseases. Two major hurdles for therapeutic use of Hsp90 inhibitors are systemic toxicity and limited central nervous system permeability. We demonstrate here that chronic treatment with a proprietary Hsp90 inhibitor compound (OS47720) not only elicits a heat-shock-like response but also offers synaptic protection in symptomatic Tg2576 mice, a model of Alzheimer's disease, without noticeable systemic toxicity. Despite a short half-life of OS47720 in mouse brain, a single intraperitoneal injection induces rapid and long-lasting (>3 days) nuclear activation of the heat-shock factor, HSF1. Mechanistic study indicates that the remedial effects of OS47720 depend upon HSF1 activation and the subsequent HSF1-mediated transcriptional events on synaptic genes. Taken together, this work reveals a novel role of HSF1 in synaptic function and memory, which likely occurs through modulation of the synaptic transcriptome.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Adenylyl cyclase 6 activation negatively regulates TLR4 signaling through lipid raft-mediated endocytosis

Proper intracellular localization of TLRs is essential for their signaling and biological function. Endocytosis constitutes a key step in protein turnover, as well as maintenance of TLR localization in plasma membrane and intracellular compartments, and thus provides important regulating points to their signaling. In this study, we demonstrate that adenylyl cyclase (AC) activation attenuates TLR4 signaling in a murine macrophage cell line (RAW 264.7) and bone marrow-derived macrophages when stimulated with LPS. We further show that the AC6 isoform plays a key role in negative regulation of TLR4 signaling by promoting protein degradation. TLR4 is normally endocytosed through the clathrin-mediated pathway, but concomitant AC6 activation shifts it to lipid raft-mediated endocytosis, which accelerates degradation of TLR4 and suppresses downstream signaling. Our studies unveil a new mechanism of negative regulation of TLR4 signaling through AC6-mediated endocytosis, which might provide a novel therapeutic approach for limiting inflammatory and autoimmune diseases.

Development of a high-throughput screening paradigm for the discovery of small-molecule modulators of adenylyl cyclase: identification of an adenylyl cyclase 2 inhibitor

Adenylyl cyclase (AC) isoforms are implicated in several physiologic processes and disease states, but advancements in the therapeutic targeting of AC isoforms have been limited by the lack of potent and isoform-selective small-molecule modulators. The discovery of AC isoform-selective small molecules is expected to facilitate the validation of AC isoforms as therapeutic targets and augment the study of AC isoform function in vivo. Identification of chemical probes for AC2 is particularly important because there are no published genetic deletion studies and few small-molecule modulators. The present report describes the development and implementation of an intact-cell, small-molecule screening approach and subsequent validation paradigm for the discovery of AC2 inhibitors. The NIH clinical collections I and II were screened for inhibitors of AC2 activity using PMA-stimulated cAMP accumulation as a functional readout. Active compounds were subsequently confirmed and validated as direct AC2 inhibitors using orthogonal and counterscreening assays. The screening effort identified SKF-83566 [8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrobromide] as a selective AC2 inhibitor with superior pharmacological properties for selective modulation of AC2 compared with currently available AC inhibitors. The utility of SKF-83566 as a small-molecule probe to study the function of endogenous ACs was demonstrated in C2C12 mouse skeletal muscle cells and human bronchial smooth muscle cells.

Adenylyl cyclase 2 selectively couples to E prostanoid type 2 receptors, whereas adenylyl cyclase 3 is not receptor-regulated in airway smooth muscle

Adenylyl cyclases (ACs) are important regulators of airway smooth muscle function, because β-adrenergic receptor (βAR) agonists stimulate AC activity and cAMP production. We have previously shown in a number of cell types that AC6 selectively couples to βAR and these proteins are coexpressed in lipid rafts. We overexpressed AC2, AC3, and AC6 in mouse bronchial smooth muscle cells (mBSMCs) and human embryonic kidney (HEK)-293 cells by using recombinant adenoviruses and assessed their localization and regulation by various G protein-coupled receptors (GPCRs). AC3 and AC6 were expressed primarily in caveolin-rich fractions, whereas AC2 expression was excluded from these domains. AC6 expression enhanced cAMP production in response to isoproterenol but did not increase responses to butaprost, reflecting the colocalization of AC6 with β(2)AR but not E prostanoid type 2 receptor (EP(2)R) in lipid raft fractions. AC2 expression enhanced butaprost-stimulated cAMP production but had no effect on the β(2)AR-mediated response. AC3 did not couple to any GPCR tested. Forskolin-induced arborization of mBSMCs was assessed as a functional readout of cAMP signaling. Arborization was enhanced by overexpression of AC6 and AC3, but AC2 had no effect. GPCR-stimulated arborization mirrored the selective coupling observed for cAMP production. With the addition of the phosphodiesterase 4 (PDE4) inhibitor rolipram AC2 accelerated forskolin-stimulated arborization. Thus, AC2 selectively couples to EP(2)R, but signals from this complex are limited by PDE4 activity. AC3 does not seem to couple to GPCR in either mBSMCs or HEK-293 cells, so it probably exists in a distinct signaling domain in these cells.

Melatonin membrane receptors in peripheral tissues: distribution and functions

Many of melatonin's actions are mediated through interaction with the G-protein coupled membrane bound melatonin receptors type 1 and type 2 (MT1 and MT2, respectively) or, indirectly with nuclear orphan receptors from the RORα/RZR family. Melatonin also binds to the quinone reductase II enzyme, previously defined the MT3 receptor. Melatonin receptors are widely distributed in the body; herein we summarize their expression and actions in non-neural tissues. Several controversies still exist regarding, for example, whether melatonin binds the RORα/RZR family. Studies of the peripheral distribution of melatonin receptors are important since they are attractive targets for immunomodulation, regulation of endocrine, reproductive and cardiovascular functions, modulation of skin pigmentation, hair growth, cancerogenesis, and aging. Melatonin receptor agonists and antagonists have an exciting future since they could define multiple mechanisms by which melatonin modulates the complexity of such a wide variety of physiological and pathological processes.

Human bronchial smooth muscle cells express adenylyl cyclase isoforms 2, 4, and 6 in distinct membrane microdomains

Adenylyl cyclases (AC) are important regulators of airway smooth muscle function, because β-adrenergic receptor (AR) agonists stimulate AC activity and increase airway diameter. We assessed expression of AC isoforms in human bronchial smooth muscle cells (hBSMC). Reverse transcriptase-polymerase chain reaction and immunoblot analyses detected expression of AC2, AC4, and AC6. Forskolin-stimulated AC activity in membranes from hBSMC displayed Ca(2+)-inhibited and G(βγ)-stimulated AC activity, consistent with expression of AC6, AC2, and AC4. Isoproterenol-stimulated AC activity was inhibited by Ca(2+) but unaltered by G(βγ), whereas butaprost-stimulated AC activity was stimulated by G(βγ) but unaffected by Ca(2+) addition. Using sucrose density centrifugation to isolate lipid raft fractions, we found that only AC6 localized in lipid raft fractions, whereas AC2 and AC4 localized in nonraft fractions. Immunoisolation of caveolae using caveolin-1 antibodies yielded Ca(2+)-inhibited AC activity (consistent with AC6 expression), whereas the nonprecipitated material displayed G(βγ)-stimulated AC activity (consistent with expression of AC2 and/or AC4). Overexpression of AC6 enhanced cAMP production in response to isoproterenol and beraprost but did not increase responses to prostaglandin E(2) or butaprost. β(2)AR, but not prostanoid EP(2) or EP(4) receptors, colocalized with AC5/6 in lipid raft fractions. Thus, particular G protein-coupled receptors couple to discreet AC isoforms based, in part, on their colocalization in membrane microdomains. These different cAMP signaling compartments in airway smooth muscle cells are responsive to different hormones and neurotransmitters and can be regulated by different coincident signals such as Ca(2+) and G(βγ).

The M2-muscarinic receptor inhibits the development of streptozotocin-induced neuropathy in mouse urinary bladder

We investigate the role of M(2)-muscarinic receptors in maintaining neurogenic bladder contraction during hyperglycemia. Mice were injected with a single dose of streptozotocin (125 mg/kg), and neurogenic contraction of urinary bladder from wild type and M(2)-muscarinic receptor knockout (M(2) KO) mice was measured at 8 to 24 weeks after treatment. In wild-type bladder lacking urothelium, the summation of the cholinergic (64%) and purinergic (56%) components of the electrical-field-stimulated response exceeded 100%, indicating a reserve capacity. Although the cholinergic component was slightly less in the M(2) KO mouse, the total electrical-field-stimulated contraction was the same as wild type. The cholinergic and purinergic components of contraction in wild-type bladder were minimally affected by streptozotocin treatment. In M(2) KO bladder, streptozotocin treatment reduced both the cholinergic (after 8-9 and 20-24 weeks) and purinergic (after 20-24 weeks only) components. The loss of function was approximately 50 to 70%. Similar results were observed in bladder with intact urothelium. M(2) KO bladder was more sensitive to the relaxant effect of isoproterenol compared with wild type, and this difference significantly increased at the early and late time points after streptozotocin treatment. In the presence of urothelium, however, this difference in isoproterenol sensitivity was smaller with streptozotocin treatment, but this trend reversed over time. Our results show that M(2) receptors oppose urinary bladder distension in wild-type bladder and inhibit streptozotocin-induced neuropathy.

Fibroblast-specific expression of AC6 enhances beta-adrenergic and prostacyclin signaling and blunts bleomycin-induced pulmonary fibrosis

Pulmonary fibroblasts regulate extracellular matrix production and degradation and are critical in maintenance of lung structure, function, and repair, but they also play a central role in lung fibrosis. cAMP-elevating agents inhibit cytokine- and growth factor-stimulated myofibroblast differentiation and collagen synthesis in pulmonary fibroblasts. In the present study, we overexpressed adenylyl cyclase 6 (AC6) in pulmonary fibroblasts and measured cAMP production and collagen synthesis. AC6 overexpression enhanced cAMP production and the inhibition of collagen synthesis mediated by isoproterenol and beraprost, but not the responses to butaprost or PGE(2). To examine if increased AC6 expression would impact the development of fibrosis in an animal model, we generated transgenic mice that overexpress AC6 under a fibroblast-specific promoter, FTS1. Lung fibrosis was induced in FTS1-AC6(+/-) mice and littermate controls by intratracheal instillation of saline or bleomycin. Wild-type mice treated with bleomycin showed extensive peribronchial and interstitial fibrosis and collagen deposition. By contrast, FTS1-AC6(+/-) mice displayed decreased fibrotic development, lymphocyte infiltration (as determined by pathological scoring), and lung collagen content. Thus, AC6 overexpression inhibits fibrogenesis in the lung by reducing pulmonary fibroblast-mediated collagen synthesis and myofibroblast differentiation. Because AC6 overexpression does not lead to enhanced basal or PGE(2)-stimulated levels of cAMP, we conclude that endogenous catecholamines or prostacyclin is produced during bleomycin-induced lung fibrosis and that these signals have antifibrotic potential.

The guinea pig ileum lacks the direct, high-potency, M(2)-muscarinic, contractile mechanism characteristic of the mouse ileum

We explored whether the M(2) muscarinic receptor in the guinea pig ileum elicits a highly potent, direct-contractile response, like that from the M(3) muscarinic receptor knockout mouse. First, we characterized the irreversible receptor-blocking activity of 4-DAMP mustard in ileum from muscarinic receptor knockout mice to verify its M(3) selectivity. Then, we used 4-DAMP mustard to inactivate M(3) responses in the guinea pig ileum to attempt to reveal direct, M(2) receptor-mediated contractions. The muscarinic agonist, oxotremorine-M, elicited potent contractions in ileum from wild-type, M(2) receptor knockout, and M(3) receptor knockout mice characterized by negative log EC(50) (pEC (50)) values +/- SEM of 6.75 +/- 0.03, 6.26 +/- 0.05, and 6.99 +/- 0.08, respectively. The corresponding E (max) values in wild-type and M(2) receptor knockout mice were approximately the same, but that in the M(3) receptor knockout mouse was only 36% of wild type. Following 4-DAMP mustard treatment, the concentration-response curve of oxotremorine-M in wild-type ileum resembled that of the M(3) knockout mouse in terms of its pEC (50), E (max), and inhibition by selective muscarinic antagonists. Thus, 4-DAMP mustard treatment appears to inactivate M(3) responses selectively and renders the muscarinic contractile behavior of the wild-type ileum similar to that of the M(3) knockout mouse. Following 4-DAMP mustard treatment, the contractile response of the guinea pig ileum to oxotremorine-M exhibited low potency and a competitive-antagonism profile consistent with an M(3) response. The guinea pig ileum, therefore, lacks a direct, highly potent, M(2)-contractile component but may have a direct, lower potency M(2) component.

CD82 endocytosis and cholesterol-dependent reorganization of tetraspanin webs and lipid rafts

Tetraspanin CD82 suppresses cell migration, tumor invasion, and tumor metastasis. To determine the mechanism by which CD82 inhibits motility, most studies have focused on the cell surface CD82, which forms tetraspanin-enriched microdomains (TEMs) with other transmembrane proteins, such as integrins. In this study, we found that CD82 undergoes endocytosis and traffics to endosomes and lysosomes. To determine the endocytic mechanism of CD82, we demonstrated that dynamin and clathrin are not essential for CD82 internalization. Depletion or sequestration of sterol in the plasma membrane markedly inhibited the endocytosis of CD82. Despite the demand on Cdc42 activity, CD82 endocytosis is distinct from macropinocytosis and the documented dynamin-independent pinocytosis. As a TEM component, CD82 reorganizes TEMs and lipid rafts by redistributing cholesterol into these membrane microdomains. CD82-containing TEMs are characterized by the cholesterol-containing microdomains in the extreme light- and intermediate-density fractions. Moreover, the endocytosis of CD82 appears to alleviate CD82-mediated inhibition of cell migration. Taken together, our studies demonstrate that lipid-dependent endocytosis drives CD82 trafficking to late endosomes and lysosomes, and CD82 reorganizes TEMs and lipid rafts through redistribution of cholesterol.

The C1 and C2 domains target human type 6 adenylyl cyclase to lipid rafts and caveolae

Previous data has shown that adenylyl cyclase type 6 (AC6) is expressed principally in lipid rafts or caveolae of cardiac myocytes and other cell types while certain other isoforms of AC are excluded from these microdomains. The mechanism by which AC6 is localized to lipid rafts or caveolae is unknown. In this study, we show AC6 is localized in lipid rafts of COS-7 cells (expressing caveolin-1) and in HEK-293 cells or cardiac fibroblasts isolated from caveolin-1 knock-out mice (both of which lack prototypical caveolins). To determine the region of AC6 that confers raft localization, we independently expressed each of the major intracellular domains, the N-terminus, C1 and C2 domains, and examined their localization with various approaches. The N-terminus did not associate with lipid rafts or caveolae of either COS-7 or HEK-293 cells nor did it immunoprecipitate with caveolin-1 when expressed in COS-7 cells. By contrast, the C1 and C2 domains each associated with lipid rafts to varying degrees and were present in caveolin-1 immunoprecipitates. There were no differences in the pattern of localization of either the C1 or C2 domains between COS-7 and HEK-293 cells. Further dissection of the C1 domain into four individual proteins indicated that the N-terminal half of this domain is responsible for its raft localization. To probe for a role of a putative palmitoylation motif in the C-terminal portion of the C2 domain, we expressed various truncated forms of AC6 lacking most or all of the C-terminal 41 amino acids. These truncated AC6 proteins were not altered in terms of their localization in lipid rafts or their catalytic activity, implying that this C-terminal region is not required for lipid raft targeting of AC6. We conclude that while the C1 domain may be most important, both the C1 and C2 domains of AC6 play a role in targeting AC6 to lipid rafts.

Adenylyl cyclase type 6 overexpression selectively enhances beta-adrenergic and prostacyclin receptor-mediated inhibition of cardiac fibroblast function because of colocalization in lipid rafts

Cardiac fibroblasts produce and degrade extracellular matrix and are critical in regulating cardiac remodeling and hypertrophy. Fibroblasts are activated by factors such as transforming growth factor beta and inhibited by agents that elevate 3',5'-cyclic adenosine monophosphate (cAMP) levels. cAMP signal generation and response is known to be compartmentalized in many cell types in part through the colocalization of receptors and specific adenylyl cyclase isoforms in lipid rafts and caveolae. The present study sought to define the localization of key G protein-coupled receptors with adenylyl cyclase type 6 (AC6) in lipid rafts of rat cardiac fibroblasts and to determine if this colocalization was functionally relevant. We found that cardiac fibroblasts produce cAMP in response to agonists for beta-adrenergic (isoproterenol), prostaglandin EP2 (butaprost), adenosine (adenosine-5'-N-ethylcarboxamide, NECA), and prostacyclin (beraprost) receptors. Overexpression of AC6 increased cAMP production stimulated by isoproterenol and beraprost but not by butaprost or NECA. A key function of fibroblasts is the production of collagen. Isoproterenol- and beraprostmediated inhibition of collagen synthesis was also enhanced by AC6 overexpression, while inhibition by butaprost and NECA were unaltered. Lipid raft fractions from cardiac fibroblasts contain the preponderance of beta-adrenergic receptors and AC6 but exclude EP2 receptors. While we could not determine the localization of native prostacyclin receptors, we were able to determine that epitope-tagged prostanoid IP receptors (IPR) expressed in COS7 cells did localize, in part, in lipid raft fractions. These findings indicate that IP receptors are expressed in lipid rafts and can activate raft-localized AC isoforms. AC6 is completely compartmentized in lipid raft domains where it is activated solely by coresident G protein-coupled receptors to regulate cardiac fibroblast function.

Detergent and detergent-free methods to define lipid rafts and caveolae

Lipid rafts and their related membrane vesicular structures, caveolae, are cholesterol- and sphingolipid-rich microdomains of the plasma membrane that have attracted considerable interest because of their ability to concentrate numerous signaling proteins. Efforts to define the proteins that reside in lipid rafts and caveolae as well as investigations into the functional role of these microdomains in signaling, endocytosis, and other cellular processes have led to the hypothesis that they compartmentalize or prearrange molecules involved in regulating these pathways. This chapter describes biochemical approaches for defining lipid rafts and caveolae. Included are detergent- and nondetergent-based fractionations on sucrose-density gradients that isolate buoyant lipid rafts and caveolae as well as caveolin antibody-based immunoisolation of detergent-insoluble membranes that selectively isolates caveolae and not lipid rafts. Also, a general method to disrupt lipid rafts and caveolae using beta-cyclodextrin that is useful for probing the role of these microdomains in cellular processes is described. The advantages and disadvantages of the respective approaches are discussed. Taken together, these methods are useful for defining the role of lipid rafts and caveolae in cell signaling.