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

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.

Interactions between calcium regulatory pathways and mechanosensitive channels in airways

INTRODUCTION

Asthma is a chronic lung disease influenced by environmental and inflammatory triggers and involving complex signaling pathways across resident airway cells such as epithelium, airway smooth muscle, fibroblasts, and immune cells. While our understanding of asthma pathophysiology is continually progressing, there is a growing realization that cellular microdomains play critical roles in mediating signaling relevant to asthma in the context of contractility and remodeling. Mechanosensitive pathways are increasingly recognized as important to microdomain signaling, with Piezo and transient receptor protein (TRP) channels at the plasma membrane considered important for converting mechanical stimuli into cellular behavior. Given their ion channel properties, particularly Ca2+ conduction, a question becomes whether and how mechanosensitive channels contribute to Ca2+ microdomains in airway cells relevant to asthma.

AREAS COVERED

Mechanosensitive TRP and Piezo channels regulate key Ca2+ regulatory proteins such as store operated calcium entry (SOCE) involving STIM and Orai channels, and sarcoendoplasmic (SR) mechanisms such as IP3 receptor channels (IP3Rs), and SR Ca2+ ATPase (SERCA) that are important in asthma pathophysiology including airway hyperreactivity and remodeling.

EXPERT OPINION

Physical and/or functional interactions between Ca2+ regulatory proteins and mechanosensitive channels such as TRP and Piezo can toward understanding asthma pathophysiology and identifying novel therapeutic approaches.

PD 102807 Induces M3 mAChR-Dependent GRK-/Arrestin-Biased Signaling in Airway Smooth Muscle Cells

G protein-coupled receptors (GPCRs) not only are turned on or off to control canonical G protein signaling but also may be fine-tuned to promote qualitative/biased signaling. Qualitative signaling by M3 muscarinic acetylcholine receptors (mAChRs) has been proposed, but its impact on physiologic systems remains unclear, and currently no biased M3 mAChR ligands have been described. Herein, we identify PD 102807 as a biased M3 ligand and delineate its signaling and function in human airway smooth muscle (ASM) cells. PD 102807 induced M3-mediated β-arrestin recruitment but not calcium mobilization. PD 102807 inhibited methacholine (MCh)-induced calcium mobilization in (M3-expressing) ASM cells. PD 102807 induced phosphorylation of AMP-activated protein kinase (AMPK) and the downstream effector acetyl-coenzyme A carboxylase (ACC). PD 102807- induced phosphorylated (p)-AMPK levels were greatly reduced in ASM cells with minimal M3 expression and were not inhibited by the Gq inhibitor YM-254890. Induction of p-AMPK and p-ACC was inhibited by β-arrestin 1 or GRK2/3 knockdown. Similarly, MCh induced phosphorylation of AMPK/ACC, but these effects were Gq dependent and unaffected by GRK2/3 knockdown. Consistent with the known ability of AMPK to inhibit transforming growth factor β (TGF-β)-mediated functions, PD 102807 inhibited TGF-β-induced SMAD-Luc activity, sm-α-actin expression, actin stress fiber formation, and ASM cell hypercontractility. These findings reveal that PD 102807 is a biased M3 ligand that inhibits M3-transduced Gq signaling but promotes Gq protein-independent, GRK-/arrestin-dependent, M3-mediated AMPK signaling, which in turn regulates ASM phenotype and contractile function. Consequently, biased M3 ligands hold significant promise as therapeutic agents capable of exploiting the pleiotropic nature of M3 signaling.

Mitochondrial Transfer Regulates Bioenergetics in Healthy and Chronic Obstructive Pulmonary Disease Airway Smooth Muscle

Mitochondrial dysfunction has been reported in chronic obstructive pulmonary disease (COPD). Transfer of mitochondria from mesenchymal stem cells to airway smooth muscle cells (ASMCs) can attenuate oxidative stress-induced mitochondrial damage. It is not known whether mitochondrial transfer can occur between structural cells in the lungs or what role this may have in modulating bioenergetics and cellular function in healthy and COPD airways. Here, we show that ASMCs from both healthy ex-smokers and subjects with COPD can exchange mitochondria, a process that happens, at least partly, via extracellular vesicles. Exposure to cigarette smoke induces mitochondrial dysfunction and leads to an increase in the donation of mitochondria by ASMCs, suggesting that the latter may be a stress response mechanism. Healthy ex-smoker ASMCs that receive mitochondria show increases in mitochondrial biogenesis and respiration and a reduction in cell proliferation, irrespective of whether the mitochondria are transferred from healthy ex-smoker or COPD ASMCs. Our data indicate that mitochondrial transfer between structural cells is a homeostatic mechanism for the regulation of bioenergetics and cellular function within the airways and may represent an endogenous mechanism for reversing the functional consequences of mitochondrial dysfunction in diseases such as COPD.

Vitamin D3 Attenuates Viral-Induced Inflammation and Fibrotic Responses in Bronchial Smooth Muscle Cells

Toll-like receptor 3 (TLR3) activation by viral infections plays a key role in promoting inflammatory immune responses that contribute to pulmonary fibrosis in chronic inflammatory respiratory diseases. Vitamin D3 has been shown to be beneficial to patients with asthma and chronic obstructive pulmonary disease (COPD) through its anti-inflammatory and anti-fibrotic properties. Smooth muscle cells are one of the major contributors to airway remodeling in asthma and COPD. We therefore aimed to investigate the effect of vitamin D3 treatment on viral-induced TLR3 responses in Bronchial Smooth Muscle Cells (BSMCs) as a mechanism contributing to pulmonary fibrosis in asthma and COPD. Primary BSMCs from patients with asthma (n=4), COPD (n=4), and healthy control subjects (n=6) were treated with polyinosinic: polycytidylic acid (polyI:C), TLR3 agonist in the presence or absence of vitamin D3 (1,25D3). Here we report the mRNA expression and protein levels of pro-inflammatory and pro-fibrotic markers (IL-6, IFN-β1, CCL2/MCP-1, fibronectin 1 and type I collagen) among BSMCs groups: asthma, COPD, and healthy controls. We show that at the baseline, prior to polyI:C stimulation, asthma and COPD BSMCs presented increased pro-inflammatory and pro-fibrotic state compared to healthy control subjects, as measured by quantitative PCR and immunoassays (ELISA/Flow Cytometry. Ligation of TLR3 by polyI:C in BSMCs was associated with increased TLR3 mRNA expression, and 1,25D3 treatment significantly reduced its expression. In addition, 1,25D3 decreased the expression of IL-6, IFN-β1, CCL2, FN1 and COL1A1 induced by polyI:C in BSMCs. The regulatory effect of 1,25D3 treatment on polyI:C-stimulated BSMCs was further confirmed at protein levels. Our findings suggest that vitamin D3 attenuates TLR3 agonist-induced inflammatory and fibrotic responses in BSMCs and support the clinical relevance of vitamin D3 supplementation in patients with viral infections having chronic respiratory diseases, such as asthma and COPD.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Mechanical Regulation of Endocytosis: New Insights and Recent Advances

Endocytosis is a mechanosensitive process. It involves remodeling of the plasma membrane from a flat shape to a budded morphology, often at the sub-micrometer scale. This remodeling process is energy-intensive and is influenced by mechanical factors such as membrane tension, membrane rigidity, and physical properties of cargo and extracellular surroundings. The cellular responses to a variety of mechanical factors by distinct endocytic pathways are important for cells to counteract rapid and extreme disruptions in the mechanohomeostasis of cells. Recent advances in microscopy and mechanical manipulation at the cellular scale have led to new discoveries of mechanoregulation of endocytosis by the aforementioned factors. While factors such as membrane tension and membrane rigidity are generally shown to inhibit endocytosis, other mechanical stimuli have complex relationships with endocytic pathways. At this juncture, it is now possible to utilize experimental techniques to interrogate theoretical predictions on mechanoregulation of endocytosis in cells and even living organisms.

Caveolae, caveolin-1 and lung diseases of aging

INTRODUCTION

Flask-shaped plasma membrane (PM) invaginations called caveolae and their constitutive caveolin and cavin proteins regulate cellular function via plasma membrane and intracellular signal transduction pathways. Caveolae are present in a variety of cells in the lung including airway smooth muscle (ASM) where they interact with other proteins, receptors, and ion channels and thereby have the potential to affect both normal and disease processes such as inflammation, contractility, and fibrosis. Given their involvement in cell signaling, caveolae may play important roles in mediating and modulating aging processes, and contribute to lung diseases of aging. Areas covered: This review provides a broad overview of the current state of knowledge regarding caveolae and their constituent proteins in lung diseases in the elderly and identifies potential mechanisms that can be targeted for future therapies. Expert Commentary: Caveolin-1 may play a protective role in lung disease. What is less clear is whether altered caveolin-1 with aging is a natural process, or a biomarker of disease progression in the elderly.

NF-κB and GATA-Binding Factor 6 Repress Transcription of Caveolins in Bladder Smooth Muscle Hypertrophy

Caveolins (CAVs) are structural proteins of caveolae that function as signaling platforms to regulate smooth muscle contraction. Loss of CAV protein expression is associated with impaired contraction in obstruction-induced bladder smooth muscle (BSM) hypertrophy. In this study, microarray analysis of bladder RNA revealed down-regulation of CAV1, CAV2, and CAV3 gene transcription in BSM from models of obstructive bladder disease in mice and humans. We identified and characterized regulatory regions responsible for CAV1, CAV2, and CAV3 gene expression in mice with obstruction-induced BSM hypertrophy, and in men with benign prostatic hyperplasia. DNA affinity chromatography and chromatin immunoprecipitation assays revealed a greater increase in binding of GATA-binding factor 6 (GATA-6) and NF-κB to their cognate binding motifs on CAV1, CAV2, and CAV3 promoters in obstructed BSM relative to that observed in control BSM. Knockout of NF-κB subunits, shRNA-mediated knockdown of GATA-6, or pharmacologic inhibition of GATA-6 and NF-κB in BSM increased CAV1, CAV2, and CAV3 transcription and promoter activity. Conversely, overexpression of GATA-6 decreased CAV2 and CAV3 transcription and promoter activity. Collectively, these data provide new insight into the mechanisms by which CAV gene expression is repressed in hypertrophied BSM in obstructive bladder disease.

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.

Metabolic re-patterning in COPD airway smooth muscle cells

Chronic obstructive pulmonary disease (COPD) airways are characterised by thickening of airway smooth muscle, partly due to airway smooth muscle cell (ASMC) hyperplasia. Metabolic reprogramming involving increased glycolysis and glutamine catabolism supports the biosynthetic and redox balance required for cellular growth. We examined whether COPD ASMCs show a distinct metabolic phenotype that may contribute to increased growth.We performed an exploratory intracellular metabolic profile analysis of ASMCs from healthy nonsmokers, healthy smokers and COPD patients, under unstimulated or growth conditions of transforming growth factor (TGF)-β and fetal bovine serum (FBS).COPD ASMCs showed impaired energy balance and accumulation of the glycolytic product lactate, glutamine, fatty acids and amino acids compared to controls in unstimulated and growth conditions. Fatty acid oxidation capacity was reduced under unstimulated conditions. TGF-β/FBS-stimulated COPD ASMCs showed restoration of fatty acid oxidation capacity, upregulation of the pentose phosphate pathway product ribose-5-phosphate and of nucleotide biosynthesis intermediates, and increased levels of the glutamine catabolite glutamate. In addition, TGF-β/FBS-stimulated COPD ASMCs showed a higher reduced-to-oxidised glutathione ratio and lower mitochondrial oxidant levels. Inhibition of glycolysis and glutamine depletion attenuated TGF-β/FBS-stimulated growth of COPD ASMCs.Changes in glycolysis, glutamine and fatty acid metabolism may lead to increased biosynthesis and redox balance, supporting COPD ASMC growth.

Cryopreserved Human Precision-Cut Lung Slices as a Bioassay for Live Tissue Banking. A Viability Study of Bronchodilation with Bitter-Taste Receptor Agonists

Human precision-cut lung slices (hPCLSs) provide a unique ex vivo model for translational research. However, the limited and unpredictable availability of human lung tissue greatly impedes their use. Here, we demonstrate that cryopreservation of hPCLSs facilitates banking of live human lung tissue for routine use. Our results show that cryopreservation had little effect on overall cell viability and vital functions of immune cells, including phagocytes and T lymphocytes. In addition, airway contraction and relaxation in response to specific agonists and antagonists, respectively, were unchanged after cryopreservation. At the subcellular level, cryopreserved hPCLSs maintained Ca(2+)-dependent regulatory mechanisms for the control of airway smooth muscle cell contractility. To exemplify the use of cryopreserved hPCLSs in smooth muscle research, we provide evidence that bitter-taste receptor (TAS2R) agonists relax airways by blocking Ca(2+) oscillations in airway smooth muscle cells. In conclusion, the banking of cryopreserved hPCLSs provides a robust bioassay for translational research of lung physiology and disease.

Caveolin-1: Functional Insights into Its Role in Muscarine- and Serotonin-Induced Smooth Muscle Constriction in Murine Airways

An increased bronchoconstrictor response is a hallmark in the progression of obstructive airway diseases. Acetylcholine and 5-hydroxytryptamine (5-HT, serotonin) are the major bronchoconstrictors. There is evidence that both cholinergic and serotonergic signaling in airway smooth muscle (ASM) involve caveolae. We hypothesized that caveolin-1 (cav-1), a structural protein of caveolae, plays an important regulatory role in ASM contraction. We analyzed airway contraction in different tracheal segments and extra- and intrapulmonary bronchi in cav-1 deficient (cav-1−/−) and wild-type mice using organ bath recordings and videomorphometry of methyl-beta-cyclodextrin (MCD) treated and non-treated precision-cut lung slices (PCLS). The presence of caveolae was investigated by electron microscopy. Receptor subtypes driving 5-HT-responses were studied by RT-PCR and videomorphometry after pharmacological inhibition with ketanserin. Cav-1 was present in tracheal epithelium and ASM. Muscarine induced a dose dependent contraction in all airway segments. A significantly higher Emax was observed in the caudal trachea. Although, caveolae abundancy was largely reduced in cav-1−/− mice, muscarine-induced airway contraction was maintained, albeit at diminished potency in the middle trachea, in the caudal trachea and in the bronchus without changes in the maximum efficacy. MCD-treatment of PLCS from cav-1−/− mice reduced cholinergic constriction by about 50%, indicating that cholesterol-rich plasma domains account for a substantial portion of the muscarine-induced bronchoconstriction. Notably, cav-1-deficiency fully abrogated 5-HT-induced contraction of extrapulmonary airways. In contrast, 5-HT-induced bronchoconstriction was fully maintained in cav-1-deficient intrapulmonary bronchi, but desensitization upon repetitive stimulation was enhanced. RT-PCR analysis revealed 5-HT1B, 5-HT2A, 5-HT6, and 5-HT7 receptors as the most prevalent subtypes in the airways. The 5-HT-induced-constriction in PCLS could be antagonized by ketanserin, a 5-HT2A receptor inhibitor. In conclusion, the role of cav-1, caveolae, and cholesterol-rich plasma domains in regulation of airway tone are highly agonist-specific and dependent on airway level. Cav-1 is indispensable for serotonergic contraction of extrapulmonary airways and modulates cholinergic constriction of the trachea and main bronchus. Thus, cav-1/caveolae shall be considered in settings such as bronchial hyperreactivity in common airway diseases and might provide an opportunity for modulation of the constrictor response.

Moderate hyperoxia induces extracellular matrix remodeling by human fetal airway smooth muscle cells

BACKGROUND

Premature infants are at increased risk for airway diseases, such as wheezing and asthma, because of early exposure to risk factors including hyperoxia. As in adult asthma, airway remodeling and increased extracellular matrix (ECM) deposition is involved.

METHODS

We assessed the impact of 24-72 h of moderate hyperoxia (50%) on human fetal airway smooth muscle (fASM) ECM deposition through western blot, modified in-cell western, and zymography techniques.

RESULTS

Hyperoxia exposure significantly increased collagen I and collagen III deposition, increased pro- and cleaved matrix metalloproteinase 9 (MMP9) activity, and decreased endogenous MMP inhibitor, TIMP1, expression. Hyperoxia-induced change in caveolin-1 (CAV1) expression was assessed as a potential mechanism for the changes in ECM deposition. CAV1 expression was decreased following hyperoxia. Supplementation of CAV1 activity with caveolar scaffolding domain (CSD) peptide abrogated the hyperoxia-mediated ECM changes.

CONCLUSION

These results demonstrate that moderate hyperoxia enhances ECM deposition in developing airways by altering the balance between MMPs and their inhibitors (TIMPs), and by increasing collagen deposition. These effects are partly mediated by a hyperoxia-induced decrease in CAV1 expression. In conjunction with prior data demonstrating increased fASM proliferation with hyperoxia, these data further demonstrate that hyperoxia is an important instigator of remodeling in developing airways.

Caveolae - mechanosensitive membrane invaginations linked to actin filaments

An essential property of the plasma membrane of mammalian cells is its plasticity, which is required for sensing and transmitting of signals, and for accommodating the tensional changes imposed by its environment or its own biomechanics. Caveolae are unique invaginated membrane nanodomains that play a major role in organizing signaling, lipid homeostasis and adaptation to membrane tension. Caveolae are frequently associated with stress fibers, a major regulator of membrane tension and cell shape. In this Commentary, we discuss recent studies that have provided new insights into the function of caveolae and have shown that trafficking and organization of caveolae are tightly regulated by stress-fiber regulators, providing a functional link between caveolae and stress fibers. Furthermore, the tension in the plasma membrane determines the curvature of caveolae because they flatten at high tension and invaginate at low tension, thus providing a tension-buffering system. Caveolae also regulate multiple cellular pathways, including RhoA-driven actomyosin contractility and other mechanosensitive pathways, suggesting that caveolae could couple mechanotransduction pathways to actin-controlled changes in tension through their association with stress fibers. Therefore, we argue here that the association of caveolae with stress fibers could provide an important strategy for cells to deal with mechanical stress.

Antigen-induced airway hyperresponsiveness and obstruction is related to caveolin-1 expression in airway smooth muscle in a guinea pig asthma model

BACKGROUND

Caveolin-1 is a fundamental signalling scaffold protein involved in contraction; however, the role of caveolin-1 in airway responsiveness remains unclear. We evaluated the relationship between caveolin-1 expression in airway smooth muscle (ASM) and antigen-induced airway responsiveness and obstruction in a guinea pig asthma model.

METHODS

Airway obstruction in sensitised guinea pigs, induced by antigenic (ovalbumin) challenges administered every 10 days, was measured. Antigen-induced responsiveness to histamine and the expression of caveolin-1 and cavin 1, 2 and 3 were evaluated at the third ovalbumin challenge. The control group received saline solution instead of ovalbumin.

RESULTS

After the first challenge, antigen exposure induced a transient airway obstruction and airway hyperresponsiveness, high levels of IL-4 and IL-5 in lung and airway globet cells proliferation at the third antigenic challenge. Caveolin-1 mRNA levels in total lung decreased in the experimental group compared with controls. Flow cytometric analysis of ASM from the experimental group showed a high number of cells expressing caveolin-1 compared with controls. This increase was confirmed by western blot. Airway obstruction and hyperresponsiveness correlated with the degree of increased caveolin-1 expression in ASM cells (P < 0.05; r = 0.69 and -0.52, respectively). The expression of cavins 1, 2 and 3 in ASM also increased in the experimental group compared to controls. Immunohistochemical findings reveal that differences in ASM caveolin-1 were not evident between groups. Nevertheless, a marked decrease in caveolin-1 and caspase 3 was observed in the pulmonary vascular smooth muscle of asthma model compared with controls. Histological analysis did not reveal differences in smooth muscles mass or subepithelial fibrosis levels in airways between groups. However, an enlargement of smooth muscle mass was observed in the pulmonary microvessels of experimental animals. This enlargement did not induce changes in pulmonary or systemic arterial pressures.

CONCLUSIONS

Our data suggest that caveolin-1 expression in ASM has a crucial role in the development of antigen-induced airway obstruction and hyperresponsiveness in a guinea pig asthma model. In addition, the asthma model in guinea pigs appears to induce a contractile smooth muscle phenotype in the airways and a proliferative smooth muscle phenotype in pulmonary vessels.

Atherosclerosis differentially affects calcium signalling in endothelial cells from aortic arch and thoracic aorta in Apolipoprotein E knockout mice

Apolipoprotein‐E knockout (ApoE^−/−) mice develop hypercholesterolemia and are a useful model of atherosclerosis. Hypercholesterolemia alters intracellular Ca²⁺ signalling in vascular endothelial cells but our understanding of these changes, especially in the early stages of the disease process, is limited. We therefore determined whether carbachol‐mediated endothelial Ca²⁺ signals differ in plaque‐prone aortic arch compared to plaque‐resistant thoracic aorta, of wild‐type and ApoE^−/− mice, and how this is affected by age and the presence of hypercholesterolemia. The extent of plaque development was determined using en‐face staining with Sudan IV. Tissues were obtained from wild‐type and ApoE^−/− mice at 10 weeks (pre‐plaques) and 24 weeks (established plaques). We found that even before development of plaques, significantly increased Ca²⁺ responses were observed in arch endothelial cells. Even with aging and plaque formation, ApoE^−/− thoracic responses were little changed, however a significantly enhanced Ca²⁺ response was observed in arch, both adjacent to and away from lesions. In wild‐type mice of any age, 1–2% of cells had oscillatory Ca²⁺ responses. In young ApoE^−/− and plaque‐free regions of older ApoE^−/−, this is unchanged. However a significant increase in oscillations (~13–15%) occurred in thoracic and arch cells adjacent to lesions in older mice. Our data suggest that Ca²⁺ signals in endothelial cells show specific changes both before and with plaque formation, that these changes are greatest in plaque‐prone aortic arch cells, and that these changes will contribute to the reported deterioration of endothelium in atherosclerosis.

We have investigated aortic endothelial cell calcium signalling changes in the Apolipoprotein E knockout mouse model of atherosclerosis. Our data show that calcium signals in endothelial cells undergo specific changes both before and with plaque formation, that these changes are greater in plaque‐prone aortic arch than in plaque‐resistant thoracic aorta, and that these changes will contribute to the reported deterioration of endothelium in atherosclerosis.

Role of dystrophin in airway smooth muscle phenotype, contraction and lung function

Dystrophin links the transmembrane dystrophin-glycoprotein complex to the actin cytoskeleton. We have shown that dystrophin-glycoprotein complex subunits are markers for airway smooth muscle phenotype maturation and together with caveolin-1, play an important role in calcium homeostasis. We tested if dystrophin affects phenotype maturation, tracheal contraction and lung physiology. We used dystrophin deficient Golden Retriever dogs (GRMD) and mdx mice vs healthy control animals in our approach. We found significant reduction of contractile protein markers: smooth muscle myosin heavy chain (smMHC) and calponin and reduced Ca2+ response to contractile agonist in dystrophin deficient cells. Immunocytochemistry revealed reduced stress fibers and number of smMHC positive cells in dystrophin-deficient cells, when compared to control. Immunoblot analysis of Akt1, GSK3β and mTOR phosphorylation further revealed that downstream PI3K signaling, which is essential for phenotype maturation, was suppressed in dystrophin deficient cell cultures. Tracheal rings from mdx mice showed significant reduction in the isometric contraction to methacholine (MCh) when compared to genetic control BL10ScSnJ mice (wild-type). In vivo lung function studies using a small animal ventilator revealed a significant reduction in peak airway resistance induced by maximum concentrations of inhaled MCh in mdx mice, while there was no change in other lung function parameters. These data show that the lack of dystrophin is associated with a concomitant suppression of ASM cell phenotype maturation in vitro, ASM contraction ex vivo and lung function in vivo, indicating that a linkage between the DGC and the actin cytoskeleton via dystrophin is a determinant of the phenotype and functional properties of ASM.

Atherosclerosis affects calcium signalling in endothelial cells from apolipoprotein E knockout mice before plaque formation

Little is known about how hypercholesterolaemia affects Ca²⁺ signalling in the vasculature of ApoE^−/− mice, a model of atherosclerosis. Our objectives were therefore to determine (i) if hypercholesterolaemia alters Ca²⁺ signalling in aortic endothelial cells before overt atherosclerotic lesions occur, (ii) how Ca²⁺ signals are affected in older plaque-containing mice, and (iii) whether Ca²⁺ signalling changes were translated into contractility differences. Using confocal microscopy we found agonist-specific Ca²⁺ changes in endothelial cells. ATP responses were unchanged in ApoE^−/− cells and methyl-β-cyclodextrin, which lowers cholesterol, was without effect. In contrast, Ca²⁺ signals to carbachol were significantly increased in ApoE^−/− cells, an effect methyl-β-cyclodextrin reversed. Ca²⁺ signals were more oscillatory and store-operated Ca²⁺ entry decreased as mice aged and plaques formed. Despite clearly increased Ca²⁺ signals, aortic rings pre-contracted with phenylephrine had impaired relaxation to carbachol. This functional deficit increased with age, was not related to ROS generation, and could be partially rescued by methyl-β-cyclodextrin. In conclusion, carbachol-induced calcium signalling and handling are significantly altered in endothelial cells of ApoE^−/− mice before plaque development. We speculate that reduction in store-operated Ca²⁺ entry may result in less efficient activation of eNOS and thus explain the reduced relaxatory response to CCh, despite the enhanced Ca²⁺ response.

The role of caveolae in the pathophysiology of lung diseases

Caveolae are flask-shaped plasma membrane invaginations formed by constitutive caveolin proteins and regulatory cavin proteins. Caveolae harbor a range of signaling components such as receptors, ion channels and regulatory molecules. There is now increasing evidence that caveolins and cavins play an important role in a variety of diseases. However, the mechanisms by which these caveolar proteins affect lung health and disease are still under investigation, with emerging data suggesting complex roles in disease pathophysiology. This review summarizes the current state of understanding of how caveolar proteins contribute to lung structure and function and how their altered expression and/or function can influence lung diseases.

PECAM-1 and caveolae form the mechanosensing complex necessary for NOX2 activation and angiogenic signaling with stopped flow in pulmonary endothelium

We showed that stop of flow triggers a mechanosignaling cascade that leads to the generation of reactive oxygen species (ROS); however, a mechanosensor coupled to the cytoskeleton that could potentially transduce flow stimulus has not been identified. We showed a role for KATP channel, caveolae (caveolin-1), and NADPH oxidase 2 (NOX2) in ROS production with stop of flow. Based on reports of a mechanosensory complex that includes platelet endothelial cell adhesion molecule-1 (PECAM-1) and initiates signaling with mechanical force, we hypothesized that PECAM-1 could serve as a mechanosensor in sensing disruption of flow. Using lungs in situ, we observed that ROS production with stop of flow was significantly reduced in PECAM-1(-/-) lungs compared with lungs from wild-type (WT) mice. Lack of PECAM-1 did not affect NOX2 activation machinery or the caveolin-1 expression or caveolae number in the pulmonary endothelium. Stop of flow in vitro triggered an increase in angiogenic potential of WT pulmonary microvascular endothelial cells (PMVEC) but not of PECAM-1(-/-) PMVEC. Obstruction of flow in lungs in vivo showed that the neutrophil infiltration as observed in WT mice was significantly lowered in PECAM-1(-/-) mice. With stop of flow, WT lungs showed higher expression of the angiogenic marker VEGF compared with untreated (sham) and PECAM-1(-/-) lungs. Thus PECAM-1 (and caveolae) are parts of the mechanosensing machinery that generates superoxide with loss of shear; the resultant ROS potentially drives neutrophil influx and acts as an angiogenic signal.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

Airway hyperresponsiveness is associated with airway remodeling but not inflammation in aging Cav1-/- mice

Background

Airway inflammation and airway remodeling are the key contributors to airway hyperresponsiveness (AHR), a characteristic feature of asthma. Both processes are regulated by Transforming Growth Factor (TGF)-β. Caveolin 1 (Cav1) is a membrane bound protein that binds to a variety of receptor and signaling proteins, including the TGF-β receptors. We hypothesized that caveolin-1 deficiency promotes structural alterations of the airways that develop with age will predispose to an increased response to allergen challenge.

Methods

AHR was measured in Cav1-deficient and wild-type (WT) mice 1 to 12 months of age to examine the role of Cav1 in AHR and the relative contribution of inflammation and airway remodeling. AHR was then measured in Cav1^-/- and WT mice after an ovalbumin-allergen challenge performed at either 2 months of age, when remodeling in Cav1^-/- and WT mice was equivalent, and at 6 months of age, when the Cav1^-/- mice had established airway remodeling.

Results

Cav1^-/- mice developed increased thickness of the subepithelial layer and a correspondingly increased AHR as they aged. In addition, allergen-challenged Cav1^-/- mice had an increase in AHR greater than WT mice that was largely independent of inflammation. Cav1^-/- mice challenged at 6 months of age have decreased AHR compared to those challenged at 2 months with correspondingly decreased BAL IL-4 and IL-5 levels, inflammatory cell counts and percentage of eosinophils. In addition, in response to OVA challenge, the number of goblet cells and α-SMA positive cells in the airways were reduced with age in response to OVA challenge in contrast to an increased collagen deposition further enhanced in absence of Cav1.

Conclusion

A lack of Cav1 contributed to the thickness of the subepithelial layer in mice as they aged resulting in an increase in AHR independent of inflammation, demonstrating the important contribution of airway structural changes to AHR. In addition, age in the Cav1^-/- mice is a contributing factor to airway remodeling in the response to allergen challenge.

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.

Cyclic AMP directs inositol (1,4,5)-trisphosphate-evoked Ca2+ signalling to different intracellular Ca2+ stores

Cholesterol depletion reversibly abolishes carbachol-evoked Ca(2+) release from inositol (1,4,5)-trisphosphate (IP3)-sensitive stores, without affecting the distribution of IP3 receptors (IP3R) or endoplasmic reticulum, IP3 formation or responses to photolysis of caged IP3. Receptors that stimulate cAMP formation do not alone evoke Ca(2+) signals, but they potentiate those evoked by carbachol. We show that these potentiated signals are entirely unaffected by cholesterol depletion and that, within individual cells, different IP3-sensitive Ca(2+) stores are released by carbachol alone and by carbachol combined with receptors that stimulate cAMP formation. We suggest that muscarinic acetylcholine receptors in lipid rafts deliver IP3 at high concentration to associated IP3R, stimulating them to release Ca(2+). Muscarinic receptors outside rafts are less closely associated with IP3R and provide insufficient local IP3 to activate IP3R directly. These IP3R, probably type 2 IP3R within a discrete Ca(2+) store, are activated only when their sensitivity is increased by cAMP. Sensitization of IP3R by cAMP extends the effective range of signalling by phospholipase C, allowing muscarinic receptors that are otherwise ineffective to recruit additional IP3-sensitive Ca(2+) stores.

Epithelium-dependent modulation of responsiveness of airways from caveolin-1 knockout mice is mediated through cyclooxygenase-2 and 5-lipoxygenase

BACKGROUND AND PURPOSE

Acute silencing of caveolin-1 (Cav-1) modulates receptor-mediated contraction of airway smooth muscle. Moreover, COX-2- and 5-lipoxygenase (5-LO)-derived prostaglandin and leukotriene biosynthesis can influence smooth muscle reactivity. COX-2 half-life can be prolonged through association with Cav-1. We suggested that lack of Cav-1 modulated levels of COX-2 which in turn modulated tracheal contraction, when arachidonic acid signalling was disturbed by inhibition of COX-2.

EXPERIMENTAL APPROACH

Using tracheal rings from Cav-1 knockout (KO) and wild-type mice (B6129SF2/J), we measured isometric contractions to methacholine and used PCR, immunoblotting and immunohistology to monitor expression of relevant proteins.

KEY RESULTS

Tracheal rings from Cav-1 KO and wild-type mice exhibited similar responses, but the COX-2 inhibitor, indomethacin, increased responses of tracheal rings from Cav-1 KO mice to methacholine. The phospholipase A₂ inhibitor, eicosatetraynoic acid, which inhibits formation of both COX-2 and 5-LO metabolites, had no effect on wild-type or Cav-1 KO tissues. Indomethacin-mediated hyperreactivity was ablated by the LTD₄ receptor antagonist (montelukast) and 5-LO inhibitor (zileuton). The potentiating effect of indomethacin on Cav-1 KO responses to methacholine was blocked by epithelial denudation. Immunoprecipitation showed that COX-2 binds Cav-1 in wild-type lungs. Immunoblotting and qPCR revealed elevated levels of COX-2 and 5-LO protein, but not COX-1, in Cav-1 KO tracheas, a feature that was prevented by removal of the epithelium.

CONCLUSION AND IMPLICATIONS

The indomethacin-induced hypercontractility observed in Cav-1 KO tracheas was linked to increased expression of COX-2 and 5-LO, which probably enhanced arachidonic acid shunting and generation of pro-contractile leukotrienes when COX-2 was inhibited.

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.

Muscarinic receptor stimulation augments TGF-β1-induced contractile protein expression by airway smooth muscle cells

Acetylcholine (ACh) is the primary parasympathetic neurotransmitter in the airways. Recently, it was established that ACh, via muscarinic receptors, regulates airway remodeling in animal models of asthma and chronic obstructive pulmonary disease (COPD). The mechanisms involved are not well understood. Here, we investigated the functional interaction between muscarinic receptor stimulation and transforming growth factor (TGF)-β(1) on the expression of contractile proteins in human airway smooth muscle (ASM) cells. ASM cells expressing functional muscarinic M(2) and M(3) receptors were stimulated with methacholine (MCh), TGF-β(1), or their combination for up to 7 days. Western blot analysis revealed a strong induction of sm-α-actin and calponin by TGF-β(1), which was increased by MCh in ASM cells. Immunocytochemistry confirmed these results and revealed that the presence of MCh augmented the formation of sm-α-actin stress fibers by TGF-β(1). MCh did not augment TGF-β(1)-induced gene transcription of contractile phenotype markers. Rather, translational processes were involved in the augmentation of TGF-β(1)-induced contractile protein expression by muscarinic receptor stimulation, including phosphorylation of glycogen synthase kinase-3β and 4E-binding protein 1, which was enhanced by MCh. In conclusion, muscarinic receptor stimulation augments functional effects of TGF-β(1) in human ASM cells on cellular processes that underpin ASM remodeling in asthma and COPD.

Integrin and GPCR Crosstalk in the Regulation of ASM Contraction Signaling in Asthma

Airway hyperresponsiveness (AHR) is one of the cardinal features of asthma. Contraction of airway smooth muscle (ASM) cells that line the airway wall is thought to influence aspects of AHR, resulting in excessive narrowing or occlusion of the airway. ASM contraction is primarily controlled by agonists that bind G protein-coupled receptor (GPCR), which are expressed on ASM. Integrins also play a role in regulating ASM contraction signaling. As therapies for asthma are based on symptom relief, better understanding of the crosstalk between GPCRs and integrins holds good promise for the design of more effective therapies that target the underlying cellular and molecular mechanism that governs AHR. In this paper, we will review current knowledge about integrins and GPCRs in their regulation of ASM contraction signaling and discuss the emerging concept of crosstalk between the two and the implication of this crosstalk on the development of agents that target AHR.

Models to study airway smooth muscle contraction in vivo, ex vivo and in vitro: implications in understanding asthma

Asthma is a chronic obstructive airway disease characterised by airway hyperresponsiveness (AHR) and airway wall remodelling. The effector of airway narrowing is the contraction of airway smooth muscle (ASM), yet the question of whether an inherent or acquired dysfunction in ASM contractile function plays a significant role in the disease pathophysiology remains contentious. The difficulty in determining the role of ASM lies in limitations with the models used to assess contraction. In vivo models provide a fully integrated physiological response but ASM contraction cannot be directly measured. Ex vivo and in vitro models can provide more direct assessment of ASM contraction but the loss of factors that may modulate ASM responsiveness and AHR, including interaction between multiple cell types and disruption of the mechanical environment, precludes a complete understanding of the disease process. In this review we detail key advantages of common in vivo, ex vivo and in vitro models of ASM contraction, as well as emerging tissue engineered models of ASM and whole airways. We also highlight important findings from each model with respect to the pathophysiology of asthma.

Caveolin-1 knockout mice exhibit airway hyperreactivity

Caveolae are flask-shaped plasma membrane invaginations expressing the scaffolding caveolin proteins. Although caveolins have been found in endothelium and epithelium (where they regulate nitric oxide synthase activity), their role in smooth muscle is still under investigation. We and others have previously shown that caveolae of human airway smooth muscle (ASM), which express caveolin-1, contain Ca(2+) and force regulatory proteins and are involved in mediating the effects of inflammatory cytokines such as TNF-α on intracellular Ca(2+) concentration responses to agonist. Accordingly, we tested the hypothesis that in vivo, absence of caveolin-1 leads to reduced airway hyperresponsiveness, using a knockout (KO) (Cav1 KO) mouse and an ovalbumin-sensitized/challenged (OVA) model of allergic airway hyperresponsiveness. Surprisingly, airway responsiveness to methacholine, tested by use of a FlexiVent system, was increased in Cav1 KO control (CTL) as well as KO OVA mice, which could not be explained by a blunted immune response to OVA. In ASM of wild-type (WT) OVA mice, expression of caveolin-1, the caveolar adapter proteins cavins 1-3, and caveolae-associated Ca(2+) and force regulatory proteins such as Orai1 and RhoA were all increased, effects absent in Cav1 KO CTL and OVA mice. However, as with WT OVA, both CTL and OVA Cav1 KO airways showed signs of enhanced remodeling, with high expression of proliferation markers and increased collagen. Separately, epithelial cells from airways of all three groups displayed lower endothelial but higher inducible nitric oxide synthase and arginase expression. Arginase activity was also increased in these three groups, and the inhibitor nor-NOHA (N-omega-nor-l-arginine) enhanced sensitivity of isolated tracheal rings to ACh, especially in Cav1 KO mice. On the basis of these data disproving our original hypothesis, we conclude that caveolin-1 has complex effects on ASM vs. epithelium, resulting in airway hyperreactivity in vivo mediated by altered airway remodeling and bronchodilation.

Muscarinic receptors on airway mesenchymal cells: novel findings for an ancient target

Since ancient times, anticholinergics have been used as a bronchodilator therapy for obstructive lung diseases. Targets of these drugs are G-protein-coupled muscarinic M(1), M(2) and M(3) receptors in the airways, which have long been recognized to regulate vagally-induced airway smooth muscle contraction and mucus secretion. However, recent studies have revealed that acetylcholine also exerts pro-inflammatory, pro-proliferative and pro-fibrotic actions in the airways, which may involve muscarinic receptor stimulation on mesenchymal, epithelial and inflammatory cells. Moreover, acetylcholine in the airways may not only be derived from vagal nerves, but also from non-neuronal cells, including epithelial and inflammatory cells. Airway smooth muscle cells seem to play a major role in the effects of acetylcholine on airway function. It has become apparent that these cells are multipotent cells that may reversibly adopt (hyper)contractile, proliferative and synthetic phenotypes, which are all under control of muscarinic receptors and differentially involved in bronchoconstriction, airway remodeling and inflammation. Cholinergic contractile tone is increased by airway inflammation associated with asthma and COPD, resulting from exaggerated acetylcholine release as well as increased expression of contraction related proteins in airway smooth muscle. Moreover, muscarinic receptor stimulation promotes proliferation of airway smooth muscle cells as well as fibroblasts, and regulates cytokine, chemokine and extracellular matrix production by these cells, which may contribute to airway smooth muscle growth, airway fibrosis and inflammation. In line, animal models of chronic allergic asthma and COPD have recently demonstrated that tiotropium may potently inhibit airway inflammation and remodeling. These observations indicate that muscarinic receptors have a much larger role in the pathophysiology of obstructive airway diseases than previously thought, which may have important therapeutic implications.

Caveolin-1 regulation of store-operated Ca(2+) influx in human airway smooth muscle

Caveolae, plasma membrane invaginations with constitutive caveolin proteins, harbour proteins involved in intracellular calcium ([Ca(2+)](i)) regulation. In human airway smooth muscle (ASM), store-operated Ca(2+) entry (SOCE) is a key component of [Ca(2+)](i) regulation, and contributes to increased [Ca(2+)](i) in inflammation. SOCE involves proteins Orai1 and stromal interaction molecule (STIM)1. We investigated the link between caveolae, SOCE and inflammation in ASM. [Ca(2+)](i) was measured in human ASM cells using fura-2. Small interference RNA (siRNA) or overexpression vectors were used to alter expression of caveolin-1 (Cav-1), Orai1 or STIM1. Tumour necrosis factor (TNF)-α was used as a representative pro-inflammatory cytokine. TNF-α increased SOCE following sarcoplasmic reticulum Ca(2+) depletion, and increased whole-cell and caveolar Orai1 (but only intracellular STIM1). Cav-1 siRNA decreased caveolar and whole-cell Orai1 (but not STIM1) expression, and blunted SOCE, even in the presence of TNF-α. STIM1 overexpression substantially enhanced SOCE: an effect only partially reversed by Cav-1 siRNA. In contrast, Orai1 siRNA substantially blunted SOCE even in the presence of TNF-α. Cav-1 overexpression significantly increased Orai1 expression and SOCE, especially in the presence of TNF-α. These results demonstrate that caveolar expression and regulation of proteins such as Orai1 are important for [Ca(2+)](i) regulation in human ASM cells and its modulation during inflammation.

Increased Nitric Oxide Production Prevents Airway Hyperresponsiveness in Caveolin-1 Deficient Mice Following Endotoxin Exposure

BACKGROUND

Caveolin-1, the hallmark protein of caveolae, is highly expressed within the lung in the epithelium, endothelium, and in immune cells. In addition to its classical roles in cholesterol metabolism and endocytosis, caveolin-1 has also been shown to be important in inflammatory signaling pathways. In particular, caveolin-1 is known to associate with the nitric oxide synthase enzymes, downregulating their activity. Endotoxins, which are are composed mainly of lipopolysaccharide (LPS), are found ubiquitously in the environment and can lead to the development of airway inflammation and increased airway hyperresponsiveness (AHR).

METHODS

We compared the acute responses of wild-type and caveolin-1 deficient mice after LPS aerosol, a well-accepted mode of endotoxin exposure, to investigate the role of caveolin-1 in the development of environmental lung injury.

RESULTS

Although the caveolin-1 deficient mice had greater lung inflammatory indices compared to wild-type mice, they exhibited reduced AHR following LPS exposure. The uncoupling of inflammation and AHR led us to investigate the role of caveolin-1 in the production of nitric oxide, which is known to act as a bronchodilator. The absence of caveolin-1 resulted in increased nitrite levels in the lavage fluid in both sham and LPS treated mice. Additionally, inducible nitric oxide synthase expression was increased in the lung tissue of caveolin-1 deficient mice following LPS exposure and administration of the potent and specific inhibitor 1400W increased AHR to levels comparable to wild-type mice.

CONCLUSIONS

We attribute the relative airway hyporesponsiveness in the caveolin-1 deficient mice after LPS exposure to the specific role of caveolin-1 in mediating nitric oxide production.

Caveolin-1 is required for contractile phenotype expression by airway smooth muscle cells

Airway smooth muscle cells exhibit phenotype plasticity that underpins their ability to contribute both to acute bronchospasm and to the features of airway remodelling in chronic asthma. A feature of mature, contractile smooth muscle cells is the presence of abundant caveolae, plasma membrane invaginations that develop from the association of lipid rafts with caveolin-1, but the functional role of caveolae and caveolin-1 in smooth muscle phenotype plasticity is unknown. Here, we report a key role for caveolin-1 in promoting phenotype maturation of differentiated airway smooth muscle induced by transforming growth factor (TGF)-β(1). As assessed by Western analysis and laser scanning cytometry, caveolin-1 protein expression was selectively enriched in contractile phenotype airway myocytes. Treatment with TGF-β(1) induced profound increases in the contractile phenotype markers sm-α-actin and calponin in cells that also accumulated abundant caveolin-1; however, siRNA or shRNAi inhibition of caveolin-1 expression largely prevented the induction of these contractile phenotype marker proteins by TGF-β(1). The failure by TGF-β(1) to adequately induce the expression of these smooth muscle specific proteins was accompanied by a strongly impaired induction of eukaryotic initiation factor-4E binding protein(4E-BP)1 phosphorylation with caveolin-1 knockdown, indicating that caveolin-1 expression promotes TGF-β(1) signalling associated with myocyte maturation and hypertrophy. Furthermore, we observed increased expression of caveolin-1 within the airway smooth muscle bundle of guinea pigs repeatedly challenged with allergen, which was associated with increased contractile protein expression, thus providing in vivo evidence linking caveolin-1 expression with accumulation of contractile phenotype myocytes. Collectively, we identify a new function for caveolin-1 in controlling smooth muscle phenotype; this mechanism could contribute to allergic asthma.

Real-time dynamic movement of caveolin-1 during smooth muscle contraction of human colon and aged rat colon transfected with caveolin-1 cDNA

Caveolin-1 (cav-1) plays a key role in PKC-α and RhoA signaling pathways during acetylcholine (ACh)-induced contraction of colonic smooth muscle cells (CSMC). Aged rat CSMC showed sluggish contractility, concomitant with reduced expression of cav-1 with an associated reduction in activation of PKC-α and RhoA signaling pathway. Real-time monitoring of live human CSMC transfected with yellow fluorescent protein-tagged wild-type caveolin 1 cDNA (YFP-wt-cav-1) cDNA in the present study suggests that cav-1 cycles within and along the membrane in a synchronized, highly organized cytoskeletal path. These studies provide, for the first time, the advantages of real-time monitoring of the dynamic movement of caveolin in living cells. Rapid movement of cav-1 in response to ACh suggests its dynamic role in CSMC contraction. Human CSMC transfected with YFP-ΔTFT-cav-1 dominant negative cDNA show fluorescence in the cytosol of the CSMC and no movement of fluorescent cav-1 in response to ACh mimicking the response shown by aged rat CSMC. Transfection of CSMC from aged rat with YFP-wt-cav-1 cDNA restored the physiological contractile response to ACh as well as the dynamic movement of cav-1 along the organized cytoskeletal path observed in normal adult CSMC. To study the force generation by CSMC, three-dimensional colonic rings were bioengineered. Colonic bioengineered rings from aged CSMC showed reduced force generation compared with colonic bioengineered rings from adult CSMC. Colonic bioengineered rings from aged CSMC transfected with wt-cav-1 cDNA showed force generation similar to colonic bioengineered rings from adult rat CSMC. The data suggest that contraction in CSMC is dependent on cav-1 reorganization dynamics, which restores the physiological contractile response in aged CSMC. We hypothesize that dynamic movement of cav-1 is essential for physiological contractile response of colonic smooth muscle.

Biomechanical properties and innervation of the female caveolin-1-deficient detrusor

BACKGROUND AND PURPOSE

Caveolin-1-deficiency is associated with substantial urogenital alterations. Here, a mechanical, histological and biochemical characterization of female detrusors from wild-type and caveolin-1-deficient (KO) mice was made to increase the understanding of detrusor changes caused by lack of caveolae.

EXPERIMENTAL APPROACH

Length-tension relationships were generated, and we recorded responses to electrical field stimulation, the muscarinic receptor agonist carbachol and the purinoceptor agonist ATP. Tyrosine nitration and the contents of caveolin-1, cavin-1, muscarinic M₃ receptors, phospholipase C(β1), muscle-specific kinase (MuSK) and L-type Ca(2+) channels were determined by immunoblotting. Innervation was assessed by immunohistochemistry.

KEY RESULTS

Bladder to body weight ratio was not changed, nor was there any change in the optimum circumference for force development. Depolarization- and ATP-induced stress was reduced, as was carbachol-induced stress between 0.1 and 3 µM, but the supramaximal relative (% K(+)) response to carbachol was increased, as was M₃ expression. The scopolamine-sensitive component of the electrical field stimulation response was impaired, and yet bladder nerves contained little caveolin-1. The density of cholinergic nerves was unchanged, whereas CART- and CGRP-positive nerves were reduced. Immunoblotting revealed loss of MuSK.

CONCLUSIONS AND IMPLICATIONS

Ablation of caveolae in the female detrusor leads to generalized impairment of contractility, ruling out prostate hypertrophy as a contributing factor. Cholinergic neuroeffector transmission is impaired without conspicuous changes in the density of cholinergic nerves or morphology of their terminals, but correlating with reduced expression of MuSK.

Caveolin-1 and force regulation in porcine airway smooth muscle

Caveolae are specialized membrane microdomains expressing the scaffolding protein caveolin-1. We recently demonstrated the presence of caveolae in human airway smooth muscle (ASM) and the contribution of caveolin-1 to intracellular calcium ([Ca(2+)](i)) regulation. In the present study, we tested the hypothesis that caveolin-1 regulates ASM contractility. We examined the role of caveolins in force regulation of porcine ASM under control conditions as well as TNF-α-induced airway inflammation. In porcine ASM strips, exposure to 10 mM methyl-β-cyclodextrin (CD) or 5 μM of the caveolin-1 specific scaffolding domain inhibitor peptide (CSD) resulted in time-dependent decrease in force responses to 1 μM ACh. Overnight exposure to the cytokine TNF-α (50 ng/ml) accelerated and increased caveolin-1 expression and enhanced force responses to ACh. Suppression of caveolin-1 with small interfering RNA mimicked the effects of CD or CSD. Regarding mechanisms by which caveolae contribute to contractile changes, inhibition of MAP kinase with 10 μM PD98059 did not alter control or TNF-α-induced increases in force responses to ACh. However, inhibiting RhoA with 100 μM fasudil or 10 μM Y27632 resulted in significant decreases in force responses, with lesser effects in TNF-α exposed samples. Furthermore, Ca(2+) sensitivity for force generation was substantially reduced by fasudil or Y27632, an effect even more enhanced in the absence of caveolin-1 signaling. Overall, these results indicate that caveolin-1 is a critical player in enhanced ASM contractility with airway inflammation.

Direct evidence for functional smooth muscle myosin II in the 10S self-inhibited monomeric conformation in airway smooth muscle cells

The 10S self-inhibited monomeric conformation of myosin II has been characterized extensively in vitro. Based upon its structural and functional characteristics, it has been proposed to be an assembly-competent myosin pool in equilibrium with filaments in cells. It is known that myosin filaments can assemble and disassemble in nonmuscle cells, and in some smooth muscle cells, but whether or not the disassembled pool contains functional 10S myosin has not been determined. Here we address this question using human airway smooth muscle cells (hASMCs). Using two antibodies against different epitopes on smooth muscle myosin II (SMM), two distinct pools of SMM, diffuse, and stress-fiber-associated, were visualized by immunocytochemical staining. The two SMM pools were functional in that they could be interconverted in two ways: (i) by exposure to 10S- versus filament-promoting buffer conditions, and (ii) by exposure to a peptide that shifts the filament-10S equilibrium toward filaments in vitro by a known mechanism that requires the presence of the 10S conformation. The effect of the peptide was not due to a trivial increase in SMM phosphorylation, and its specificity was demonstrated by use of a scrambled peptide, which had no effect. Based upon these data, we conclude that hASMCs contain a significant pool of functional SMM in the 10S conformation that can assemble into filaments upon changing cellular conditions. This study provides unique direct evidence for the presence of a significant pool of functional myosin in the 10S conformation in cells.

beta-Dystroglycan binds caveolin-1 in smooth muscle: a functional role in caveolae distribution and Ca2+ release

The dystrophin-glycoprotein complex (DGC) links the extracellular matrix and actin cytoskeleton. Caveolae form membrane arrays on smooth muscle cells; we investigated the mechanism for this organization. Caveolin-1 and beta-dystroglycan, the core transmembrane DGC subunit, colocalize in airway smooth muscle. Immunoprecipitation revealed the association of caveolin-1 with beta-dystroglycan. Disruption of actin filaments disordered caveolae arrays, reduced association of beta-dystroglycan and caveolin-1 to lipid rafts, and suppressed the sensitivity and responsiveness of methacholine-induced intracellular Ca2+ release. We generated novel human airway smooth muscle cell lines expressing shRNA to stably silence beta-dystroglycan expression. In these myocytes, caveolae arrays were disorganized, caveolae structural proteins caveolin-1 and PTRF/cavin were displaced, the signaling proteins PLCbeta1 and G(alphaq), which are required for receptor-mediated Ca2+ release, were absent from caveolae, and the sensitivity and responsiveness of methacholine-induced intracellular Ca2+ release, was diminished. These data reveal an interaction between caveolin-1 and beta-dystroglycan and demonstrate that this association, in concert with anchorage to the actin cytoskeleton, underpins the spatial organization and functional role of caveolae in receptor-mediated Ca2+ release, which is an essential initiator step in smooth muscle contraction.

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.

Cholesterol depletion alters coronary artery myocyte Ca(2+) signalling in a stimulus-specific manner

Although there is evidence that caveolae and cholesterol play an important role in myocyte signalling processes, details of the mechanisms involved remain sparse. In this paper we have studied for the first time the clinically relevant intact coronary artery and measured in situ Ca(2+) signals in individual myocytes using confocal microscopy. We have examined the effect of the cholesterol-depleting agents, methyl-cyclodextrin (MCD) and cholesterol oxidase, on high K(+), caffeine and agonist-induced Ca(2+) signals. We find that cholesterol depletion produces a stimulus-specific alteration in Ca(2+) responses; with 5-HT (10microM) and endothelin-1 (10nM) responses being selectively decreased, the phenylephrine response (100microM) increased and the responses to high K(+) (60mM) and caffeine (10mM) unaffected. Agonist-induced Ca(2+) signals were restored when cholesterol was replenished using cholesterol-saturated MCD. In additional experiments, enzymatically isolated myocytes were patch clamped. We found that cholesterol depletion caused a selective modification of ion channel function, with whole cell inward Ca(2+) current being unaltered, whereas outward K(+) current was increased, due to BK(Ca) channel activation. There was also a significant decrease in cell capacitance. These data are discussed in terms of the involvement of caveolae in receptor localisation, Ca(2+) entry pathways and SR Ca(2+) release, and the role of these in agonist signalling.

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.

Ethanol attenuates contraction of primary cultured rat airway smooth muscle cells

Airway smooth muscle cells are the main effector cells involved in airway narrowing and have been used to study the signaling pathways involved in asthma-induced airway constriction. Our previous studies demonstrated that ethanol administration to mice attenuated methacholine-stimulated increases in airway responsiveness. Because ethanol administration attenuates airway responsiveness in mice, we hypothesized that ethanol directly blunts the ability of cultured airway smooth muscle cells to shorten. To test this hypothesis, we measured changes in the size of cultured rat airway smooth muscle (RASM) cells exposed to ethanol (100 mM) after treatment with methacholine. Ethanol markedly attenuated methacholine-stimulated cell shortening (methacholine-stimulated length change = 8.3 ± 1.2% for ethanol versus 43.9 ± 1.5% for control; P < 0.001). Ethanol-induced inhibition of methacholine-stimulated cell shortening was reversible 24 hours after removal of alcohol. To determine if ethanol acts through a cGMP-dependent pathway, incubation with ethanol for as little as 15 minutes produced a doubling of cGMP-dependent protein kinase (PKG) activity. Furthermore, treatment with the PKG antagonist analog Rp-8Br-cGMPS (10 μM) inhibited ethanol-induced kinase activation when compared with control-treated cells. In contrast to the effect of ethanol on PKG, ethanol pretreatment did not activate a cAMP-dependent protein kinase. These data demonstrate that brief ethanol exposure reversibly prevents methacholine-stimulated RASM cell contraction. In addition, it appears that this effect is the result of activation of the cGMP/PKG kinase pathway. These findings implicate a direct effect of ethanol on airway smooth muscle cells as the basis for in vivo ethanol effects.

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.

Expression of the dystrophin-glycoprotein complex is a marker for human airway smooth muscle phenotype maturation

Airway smooth muscle (ASM) cells may contribute to asthma pathogenesis through their capacity to switch between a synthetic/proliferative and a contractile phenotype. The multimeric dystrophin-glycoprotein complex (DGC) spans the sarcolemma, linking the actin cytoskeleton and extracellular matrix. The DGC is expressed in smooth muscle tissue, but its functional role is not fully established. We tested whether contractile phenotype maturation of human ASM is associated with accumulation of DGC proteins. We compared subconfluent, serum-fed cultures and confluent cultures subjected to serum deprivation, which express a contractile phenotype. Western blotting confirmed that beta-dystroglycan, beta-, delta-, and epsilon-sarcoglycan, and dystrophin abundance increased six- to eightfold in association with smooth muscle myosin heavy chain (smMHC) and calponin accumulation during 4-day serum deprivation. Immunocytochemistry showed that the accumulation of DGC subunits was specifically localized to a subset of cells that exhibit robust staining for smMHC. Laminin competing peptide (YIGSR, 1 microM) and phosphatidylinositol 3-kinase (PI3K) inhibitors (20 microM LY-294002 or 100 nM wortmannin) abrogated the accumulation of smMHC, calponin, and DGC proteins. These studies demonstrate that the accumulation of DGC is an integral feature for phenotype maturation of human ASM cells. This provides a strong rationale for future studies investigating the role of the DGC in ASM smooth muscle physiology in health and disease.