Caveolae from canine airway smooth muscle contain the necessary components for a role in Ca(2+) handling

Caveolin-1 abolishment attenuates the myogenic response in murine cerebral arteries

Intravascular pressure-induced vasoconstriction (the "myogenic response") is intrinsic to smooth muscle cells, but mechanisms that underlie this response are unresolved. Here we investigated the physiological function of arterial smooth muscle cell caveolae in mediating the myogenic response. Since caveolin-1 (cav-1) ablation abolishes caveolae formation in arterial smooth muscle cells, myogenic mechanisms were compared in cerebral arteries from control (cav-1(+/+)) and cav-1-deficient (cav-1(-/-)) mice. At low intravascular pressure (10 mmHg), wall membrane potential, intracellular calcium concentration ([Ca(2+)](i)), and myogenic tone were similar in cav-1(+/+) and cav-1(-/-) arteries. In contrast, pressure elevations to between 30 and 70 mmHg induced a smaller depolarization, [Ca(2+)](i) elevation, and myogenic response in cav-1(-/-) arteries. Depolarization induced by 60 mM K(+) also produced an attenuated [Ca(2+)](i) elevation and constriction in cav-1(-/-) arteries, whereas extracellular Ca(2+) removal and diltiazem, an L-type Ca(2+) channel blocker, similarly dilated cav-1(+/+) and cav-1(-/-) arteries. N(omega)-nitro-l-arginine, an nitric oxide synthase inhibitor, did not restore myogenic tone in cav-1(-/-) arteries. Iberiotoxin, a selective Ca(2+)-activated K(+) (K(Ca)) channel blocker, induced a similar depolarization and constriction in pressurized cav-1(+/+) and cav-1(-/-) arteries. Since pressurized cav-1(-/-) arteries are more hyperpolarized and this effect would reduce K(Ca) current, these data suggest that cav-1 ablation leads to functional K(Ca) channel activation, an effect that should contribute to the attenuated myogenic constriction. In summary, data indicate that cav-1 ablation reduces pressure-induced depolarization and depolarization-induced Ca(2+) influx, and these effects combine to produce a diminished arterial wall [Ca(2+)](i) elevation and constriction.

Caveolar nanospaces in smooth muscle cells

Caveolae, specialized membrane nanodomains, have a key role in signaling processes, including calcium handling in smooth muscle cells (SMC). We explored the three-dimensional (3D) architecture of peripheral cytoplasmic space at the nanoscale level and the close spatial relationships between caveolae, sarcoplasmic reticulum (SR), and mitochondria, as ultrastructural basis for an excitation-contraction coupling system and, eventually, for excitation - transcription coupling. About 150 electron micrographs of SMC showed that superficial SR and peripheral mitochondria are rigorously located along the caveolar domains of plasma membrane, alternating with plasmalemmal dense plaques. Electron micrographs made on serial ultrathin sections were digitized, then computer-assisted organellar profiles were traced on images, and automatic 3D reconstruction was obtained using the ‘Reconstruct’ software. The reconstruction was made for 1 μm³ in rat stomach (muscularis mucosae) and 10 μm³ in rat urinary bladder (detrusor smooth muscle). The close appositions (about 15 nm distance) of caveolae, peripheral SR, and mitochondria create coherent cytoplasmic nanoscale subdomains. Apparently, 80% of caveolae establish close contacts with SR and about 10% establish close contacts with mitochondria in both types of SMC. Thus, our results show that caveolae and peripheral SR build Ca²⁺release units in which mitochondria often could play a part. The caveolae-SR couplings occupy 4.19% of the cellular volume in stomach and 3.10% in rat urinary bladder, while caveolae-mitochondria couplings occupy 3.66% and 3.17%, respectively. We conclude that there are strategic caveolae-SR or caveolae-mitochondria contacts at the nanoscale level in the cortical cytoplasm of SMC, presumably responsible for a vectorial control of free Ca²⁺ cytoplasmic concentrations in definite nanospaces. This may account for slective activation of specific Ca²⁺ signaling pathways.

Caveolae and calcium handling, a review and a hypothesis

Caveolae are associated with molecules crucial for calcium handling. This review considers the roles of caveolae in calcium handling for smooth muscle and interstitial cells of Cajal (ICC). Structural studies showed that the plasma membrane calcium pump (PMCA), a sodium-calcium exchanger (NCX1), and a myogenic nNOS appear to be colocalized with caveolin I, the main constituent of these caveolae. Voltage dependent calcium channels (VDCC) are associated but not co-localized with caveolin 1, as are proteins of the peripheral sarcoplasmic reticulum (SR) such as calreticulin. Only the nNOS is absent from caveolin 1 knockout animals. Functional studies in calcium free media sugest that a source of calcium in tonic smooth muscles exists, partly sequestered from extracellular EGTA. This source supported sustained contractions to carbachol using VDCC and dependent on activity of the SERCA pump. This source is postulated to be caveolae, near peripheral SR. New evidence, presented here, suggests that a similar source exists in phasic smooth muscle of the intestine and its ICC. These results suggest that caveolae and peripheral SR are a functional unit recycling calcium through VDCC and controlling its local concentration. Calcium handling molecules associated with caveolae in smooth muscle and ICC were identified and their possible functions also reviewed.

Subcellular targeting and trafficking of nitric oxide synthases

Unlike most other endogenous messengers that are deposited in vesicles, processed on demand and/or secreted in a regulated fashion, NO (nitric oxide) is a highly active molecule that readily diffuses through cell membranes and thus cannot be stored inside the producing cell. Rather, its signalling capacity must be controlled at the levels of biosynthesis and local availability. The importance of temporal and spatial control of NO production is highlighted by the finding that differential localization of NO synthases in cardiomyocytes translates into distinct effects of NO in the heart. Thus NO synthases belong to the most tightly controlled enzymes, being regulated at transcriptional and translational levels, through co- and post-translational modifications, by substrate availability and not least via specific sorting to subcellular compartments, where they are in close proximity to their target proteins. Considerable efforts have been made to elucidate the molecular mechanisms that underlie the intracellular targeting and trafficking of NO synthases, to ultimately understand the cellular pathways controlling the formation and function of this powerful signalling molecule. In the present review, we discuss the mechanisms and triggers for subcellular routing and dynamic redistribution of NO synthases and the ensuing consequences for NO production and action.

Role of caveolin-1 in p42/p44 MAP kinase activation and proliferation of human airway smooth muscle

Chronic airways diseases, including asthma, are associated with an increased airway smooth muscle (ASM) mass, which may contribute to chronic airway hyperresponsiveness. Increased muscle mass is due, in part, to increased ASM proliferation, although the precise molecular mechanisms for this response are not completely clear. Caveolae, which are abundant in smooth muscle cells, are membrane microdomains where receptors and signaling effectors can be sequestered. We hypothesized that caveolae and caveolin-1 play an important regulatory role in ASM proliferation. Therefore, we investigated their role in p42/p44 MAPK signaling and proliferation using human ASM cell lines. Disruption of caveolae using methyl-beta-cyclodextrin and small interfering (si)RNA-knockdown of caveolin-1 caused spontaneous p42/p44 MAPK activation; additionally, caveolin-1 siRNA induced ASM proliferation in mitogen deficient conditions, suggesting a key role for caveolae and caveolin-1 in maintaining quiescence. Moreover, caveolin-1 accumulates twofold in myocytes induced to a contractile phenotype compared with proliferating ASM cells. Caveolin-1 siRNA failed to increase PDGF-induced p42/p44 MAPK activation and cell proliferation, however, indicating that PDGF stimulation actively reversed the antimitogenic control by caveolin-1. Notably, the PDGF induced loss of antimitogenic control by caveolin-1 coincided with a marked increase in caveolin-1 phosphorylation. Furthermore, the strong association of PDGF receptor-beta with caveolin-1 that exists in quiescent cells was rapidly and markedly reduced with agonist addition. This suggests a dynamic relationship in which mitogen stimulation actively reverses caveolin-1 suppression of p42/p44 MAPK signal transduction. As such, caveolae and caveolin-1 coordinate PDGF receptor signaling, leading to myocyte proliferation, and inhibit constitutive activity of p42/p44 MAPK to sustain cell quiescence.

The association of caveolae, actin, and the dystrophin-glycoprotein complex: a role in smooth muscle phenotype and function?

Smooth muscle cells exhibit phenotypic and mechanical plasticity. During maturation, signalling pathways controlling actin dynamics modulate contractile apparatus-associated gene transcription and contractile apparatus remodelling resulting from length change. Differentiated myocytes accumulate abundant caveolae that evolve from the structural association of lipid rafts with caveolin-1, a protein with domains that confer unique functional properties. Caveolae and caveolin-1 modulate and participate in receptor-mediated signalling, and thus contribute to functional diversity of phenotypically similar myocytes. In mature smooth muscle, caveolae are partitioned into discrete linear domains aligned with structural proteins that tether actin to the extracellular matrix. Caveolin-1 binds with beta-dystroglycan, a subunit of the dystrophin glycoprotein complex (DGC), and with filamin, an actin binding protein that organizes cortical actin, to which integrins and focal adhesion complexes are anchored. The DGC is linked to the actin cytoskeleton by a dystrophin subunit and is a receptor for extracellular laminin. Thus, caveolae and caveolin-associated signalling proteins and receptors are linked via structural proteins to a dynamic filamentous actin network. Despite development of transgenic models to investigate caveolins and membrane-associated actin-linking proteins in skeletal and cardiac muscle function, only superficial understanding of this association in smooth muscle phenotype and function has emerged.

Genetic ablation of caveolin-1 modifies Ca2+ spark coupling in murine arterial smooth muscle cells

L-type, voltage-dependent calcium (Ca(2+)) channels, ryanodine-sensitive Ca(2+) release (RyR) channels, and large-conductance Ca(2+)-activated potassium (K(Ca)) channels comprise a functional unit that regulates smooth muscle contractility. Here, we investigated whether genetic ablation of caveolin-1 (cav-1), a caveolae protein, alters Ca(2+) spark to K(Ca) channel coupling and Ca(2+) spark regulation by voltage-dependent Ca(2+) channels in murine cerebral artery smooth muscle cells. Caveolae were abundant in the sarcolemma of control (cav-1(+/+)) cells but were not observed in cav-1-deficient (cav-1(-/-)) cells. Ca(2+) spark and transient K(Ca) current frequency were approximately twofold higher in cav-1(-/-) than in cav-1(+/+) cells. Although voltage-dependent Ca(2+) current density was similar in cav-1(+/+) and cav-1(-/-) cells, diltiazem and Cd(2+), voltage-dependent Ca(2+) channel blockers, reduced transient K(Ca) current frequency to approximately 55% of control in cav-1(+/+) cells but did not alter transient K(Ca) current frequency in cav-1(-/-) cells. Furthermore, although K(Ca) channel density was elevated in cav-1(-/-) cells, transient K(Ca) current amplitude was similar to that in cav-1(+/+) cells. Higher Ca(2+) spark frequency in cav-1(-/-) cells was not due to elevated intracellular Ca(2+) concentration, sarcoplasmic reticulum Ca(2+) load, or nitric oxide synthase activity. Similarly, Ca(2+) spark amplitude and spread, the percentage of Ca(2+) sparks that activated a transient K(Ca) current, the amplitude relationship between sparks and transient K(Ca) currents, and K(Ca) channel conductance and apparent Ca(2+) sensitivity were similar in cav-1(+/+) and cav-1(-/-) cells. In summary, cav-1 ablation elevates Ca(2+) spark and transient K(Ca) current frequency, attenuates the coupling relationship between voltage-dependent Ca(2+) channels and RyR channels that generate Ca(2+) sparks, and elevates K(Ca) channel density but does not alter transient K(Ca) current activation by Ca(2+) sparks. These findings indicate that cav-1 is required for physiological Ca(2+) spark and transient K(Ca) current regulation in cerebral artery smooth muscle cells.

Caveolins in vascular smooth muscle: form organizing function

Caveolae are becoming increasingly recognized as an important organizational structure for a variety of signal and energy-transducing systems in vascular smooth muscle (VSM). In this review, we discuss the emerging role of the caveolins in organizing and modulating the basic functions of smooth muscle: contraction, growth/proliferation, and the energetic support systems that support these functions. With clear alterations in cell metabolism and function in VSM with altered caveolin-1 (Cav-1) protein expression and with cardiovascular abnormalities associated with Cav-1 null mice, the caveolin family of proteins may play an important role in the function and dysfunction of VSM.

A membrane defect in the pathogenesis of the Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an often lethal birth defect resulting from mutations in the gene responsible for the synthesis of the enzyme 3beta-hydroxy-steroid-Delta7-reductase, which catalyzes the reduction of the double bond at carbon 7 on 7-dehydrocholesterol (7-DHC) to form unesterified cholesterol. We hypothesize that the deficiency in cholesterol biosynthesis and subsequent accumulation of 7-DHC in the cell membrane leads to defective composition, organization, dynamics, and function of the cell membrane. Using skin fibroblasts obtained from SLOS patients, we demonstrate that the SLOS membrane has increased 7-DHC and reduced cholesterol content and abnormal membrane fluidity. X-ray diffraction analyses of synthetic membranes prepared to mimic SLOS membranes revealed atypical membrane organization. In addition, calcium permeability is markedly augmented, whereas membrane-bound Na+/K+ATPase activity, folate uptake, inositol-1,4,5-trisphosphate signaling, and cell proliferation rates are markedly suppressed. These data indicate that the disturbance in membrane sterol content in SLOS, likely at the level of membrane caveolae, directly contributes to the widespread tissue abnormalities in this disease.

Colocalization between caveolin isoforms in the intestinal smooth muscle and interstitial cells of Cajal of the Cav1(+/+) and Cav1 (-/-) mouse

Confocal microscopic images were obtained from the immunohistochemical sections of jejeunum to determine the localization/colocalization between caveolin-1, caveolin-2 and caveolin-3 in intestinal smooth muscle cells (SMCs) and interstitial cells of Cajal (ICC) of Cav1(+/+) and Cav1(-/-) mouse. Intestinal regions were segmented [inner circular muscle (icm), outer circular muscle (ocm), myenteric plexus region (mp), and longitudinal muscle (lm)] by LSM 5 and analyzed by ImageJ to show Pearson's correlation (r (p)) and overlap coefficient (r) of colocalization. In the intestine of Cav1(+/+), caveolin-1 (cav1) was colocalized with caveolin-2 (cav2) and caveolin-3 (cav3). Cav2 also was well colocalized with cav3. In the intestine of Cav1(-/-), cav1 and cav2 were absent in all images, but reduced cav3 was expressed in ocm. Caveolae were present in cell types with cav1 in Cav1(+/+), and present with cav3 in ocm of Cav1(-/-). C-kit occurred in deep muscular plexus (ICC-DMP) and myenteric plexus (ICC-MP), in both Cav1(+/+) and Cav1(-/-), and colocalized with cav1 and cav2 in the intestine of Cav1(+/+). Cav3 was absent/present at low immunoreactivity in ICC-DMP and ICC-MP of the intestines of Cav1(+/+) and Cav1(-/-). To conclude, cav1 is necessary for the expression of cav2 in SMC and ICC of intestine and facilitates, but is not necessary for the expression of cav3.

Plasma membrane calcium pumps in smooth muscle: from fictional molecules to novel inhibitors

Plasma membrane Ca2+pumps (PMCA pumps) are Ca2+-Mg2+ATPases that expel Ca2+from the cytosol to extracellular space and are pivotal to cell survival and function. PMCA pumps are encoded by the genes PMCA1, -2, -3, and -4. Alternative splicing results in a large number of isoforms that differ in their kinetics and activation by calmodulin and protein kinases A and C. Expression by 4 genes and a multifactorial regulation provide redundancy to allow for animal survival despite genetic defects. Heterozygous mice with ablation of any of the PMCA genes survive and only the homozygous mice with PMCA1 ablation are embryolethal. Some PMCA isoforms may also be involved in other cell functions. Biochemical and biophysical studies of PMCA pumps have been limited by their low levels of expression. Delineation of the exact physiological roles of PMCA pumps has been difficult since most cells also express sarco/endoplasmic reticulum Ca2+pumps and a Na+-Ca2+-exchanger, both of which can lower cytosolic Ca2+. A major limitation in the field has been the lack of specific inhibitors of PMCA pumps. More recently, a class of inhibitors named caloxins have emerged, and these may aid in delineating the roles of PMCA pumps.Key words: ATPases, hypertension, caloxin, protein kinase A, protein kinase C, calmodulin.

Maxi-K channels localize to caveolae in human myometrium: a role for an actin-channel-caveolin complex in the regulation of myometrial smooth muscle K+ current

Multiple cell-signaling pathways converge to modulate large-conductance, voltage- and Ca2+-sensitive K+ channel (maxi-K channel) activity and buffer cell excitability in human myometrial smooth muscle cells (hMSMCs). Recent evidence indicates that maxi-K channel proteins can target to membrane microdomains; however, their association with other proteins within these macromolecular complexes has not been elucidated. Biochemical isolation of detergent-resistant membrane fractions from human myometrium demonstrates the presence of maxi-K channels in lipid raft microdomains, which cofractionate with caveolins. In both nonpregnant and late-pregnant myometrium, maxi-K channels associate and colocalize with caveolar scaffolding proteins caveolin-1 and caveolin-2, but not caveolin-3. Disruption of cultured hMSMC caveolar complexes by cholesterol depletion with cyclodextrin increases an iberiotoxin-sensitive K+ current. Coimmunoprecipitations have indicated that the maxi-K channel also is associated with both α- and γ-actin. Immunocytochemical analysis indicates colocalization of maxi-K channels, actin, and caveolin-1 in primary cultures of hMSMCs. Further experiments using immunoelectron microscopy have shown the proximity of both actin and the maxi-K channel within the same cell surface caveolar structures. Functionally, disruption of the actin cytoskeleton in cultured hMSMCs by cytochalasin D and latrunculin A greatly increased the open-state probability of the channel, while stabilization of actin cytoskeleton with jasplakinolide abolished the effect of latrunculin A. These data indicate that the actin cytoskeleton is involved as part of a caveolar complex in the regulation of myometrial maxi-K channel function.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.

Proteins of interstitial cells of Cajal and intestinal smooth muscle, colocalized with caveolin-1

The murine jejunum and lower esophageal sphincter (LES) were examined to determine the locations of various signaling molecules and their colocalization with caveolin-1 and one another. Caveolin-1 was present in punctate sites of the plasma membranes (PM) of all smooth muscles and diffusely in all classes of interstitial cells of Cajal (ICC; identified by c-kit immunoreactivity), ICC-myenteric plexus (MP), ICC-deep muscular plexus (DMP), ICC-serosa (ICC-S), and ICC-intramuscularis (IM). In general, all ICC also contained the L-type Ca(2+) (L-Ca(2+)) channel, the PM Ca(2+) pump, and the Na(+)/Ca(2+) exchanger-1 localized with caveolin-1. ICC in various sites also contained Ca(2+)-sequestering molecules such as calreticulin and calsequestrin. Calreticulin was present also in smooth muscle, frequently in the cytosol, whereas calsequestrin was present in skeletal muscle of the esophagus. Gap junction proteins connexin-43 and -40 were present in circular muscle of jejunum but not in longitudinal muscle or in LES. In some cases, these proteins were associated with ICC-DMP. The large-conductance Ca(2+)-activated K(+) channel was present in smooth muscle and skeletal muscle of esophagus and some ICC but was not colocalized with caveolin-1. These findings suggest that all ICC have several Ca(2+)-handling and -sequestering molecules, although the functions of only the L-Ca(2+) channel are currently known. They also suggest that gap junction proteins are located at sites where ultrastructural gap junctions are know to exist in circular muscle of intestine but not in other smooth muscles. These findings also point to the need to evaluate the function of Ca(2+) sequestration in ICC.

Caveolae-associated signalling in smooth muscle

Caveolae are flask-shaped invaginations in the membrane that depend on the contents of cholesterol and on the structural protein caveolin. The organisation of caveolae in parallel strands between dense bands in smooth muscle is arguably unique. It is increasingly recognised, bolstered in large part by recent studies in caveolae deficient animals, that caveolae sequester and regulate a variety of signalling intermediaries. The role of caveolae in smooth muscle signal transduction, as inferred from studies on transgenic animals and in vitro approaches, is the topic of the current review. Both G-protein coupled receptors and tyrosine kinase receptors are believed to cluster in caveolae, and the exciting possibility that caveolae provide a platform for interactions between the sarcoplasmic reticulum and plasmalemmal ion channels is emerging. Moreover, messengers involved in Ca2+ sensitization of myosin phosphorylation and contraction may depend on caveolae or caveolin. Caveolae thus appear to constitute an important signalling domain that plays a role not only in regulation of smooth muscle tone, but also in proliferation, such as seen in neointima formation and atherosclerosis.

Cellular Functions of Endoplasmic Reticulum Chaperones Calreticulin, Calnexin, and ERp57

Glycosylated proteins destined for the cell surface or to be secreted from the cell are trafficked through the endoplasmic reticulum during synthesis and folding. Correct folding is determined in large part by the sequence of the protein, but it is also assisted by interaction with enzymes and chaperones of the endoplasmic reticulum. Calreticulin, calnexin, and ERp57 are among the endoplasmic chaperones that interact with partially folded glycoproteins and determine if the proteins are to be released from the endoplasmic reticulum to be expressed, or alternatively, if they are to be sent to the proteosome for degradation. Studies on the effect of alterations in the expression and function of these proteins are providing information about the importance of this quality control system, as well as uncovering other important functions these proteins play outside of the endoplasmic reticulum.

Loss of caveolin-1 expression is associated with disruption of muscarinic cholinergic activities in the urinary bladder

Caveolin-1 (Cav1), a structural protein of caveolae, plays cell- and context-dependent roles in signal transduction pathway regulation. We have generated a knockout mouse homozygous for a null mutation of the Cav1 gene. Cav1 knockout mice exhibited impaired urinary bladder contractions in vivo during cystometry. Contractions of male bladder strips were evoked with electric and pharmacologic stimulation (5-40 Hz, 1-10 microM carbachol, 10 mM alpha,beta-methylene ATP, 100 mM KCl). Acetylcholine (ACh) and norepinephrine (NE) release from bladder strips were measured with a radiochemical method by incubating the strips with 14C-choline and 3H-NE prior to electric stimulation, whereas ATP release was measured using the luciferin-luciferase assay with a luminometer. A 60-75% decline in contractility was observed when Cav1 knockout muscle strips were stimulated with electric current or carbachol, compared to wildtype muscle strips. No difference in contractility was noted when contractions were evoked either by the purinergic agonist alpha,beta-methylene ATP, or by extracellular potassium. To investigate the relative contribution of non-cholinergic activity to bladder contractility, the amplitude of the electric stimulation-evoked contractions was compared in the presence of the muscarinic antagonist atropine (1 microM). While the non-muscarinic (purinergic) response was unaltered, muscarinic cholinergic response was principally disrupted in Cav1 knockout mice. The loss of Cav1 gene expression was also associated with a 70% reduction in ACh release. NE and ATP release was not altered. It is concluded that the loss of caveolin-1 is associated with disruption of M3 muscarinic cholinergic activity in the bladder. Both pre-junctional (acetylcholine neurotransmitter release from neuromuscular junctions) and post-junctional (M3 receptor-mediated signal transduction in bladder smooth muscles) mechanisms are disrupted, resulting in impaired bladder contraction.

Basal calcium entry in vascular smooth muscle

Basal calcium leak into smooth muscle was identified 30 years ago yet remains poorly understood. We characterized this leak measuring 45Ca2+ uptake into cultured rat aortic smooth muscle cells. Wash solution (0 degrees C) containing lanthanum (3 mM) removed extracellular tracer and increased cellular 45Ca2+ retention more effectively than EGTA (0.2 mM). Basal Ca2+ entry was 1.45 x 10(9) Ca2+ x cell(-1) x min(-1). This translated to approximately 250 micromol(-1) x min(-1) given cell volumes of 4-15 pl as determined by 3-D image reconstruction. Gadolinium (100 microM) blocked 80% of the leak and exhibited a biphasic concentration-response relation (IC50s=1 microM and 2 mM). Organic ion channel blockers also inhibited approximately 80% of the leak; 45% by nifedipine (10 microM), 7% was exclusively blocked by SKF 96365 (1-[b-[3-(4-Methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole) (50 microM) and 23% was exclusively sensitive to 2-aminoethoxy-diphenylborate (2-APB, 75 microM). Reverse transcriptase polymerase chain reaction revealed TrpC1, 4 and 6 mRNA, and we propose that 2-APB may selectively block TrpC4-containing channels. We conclude that basal Ca2+ entry is mainly due to a basal open probability of excitable Ca2+ -channels.

Caveolae localize protein kinase A signaling to arterial ATP-sensitive potassium channels

Arterial ATP-sensitive K+ (K(ATP)) channels are critical regulators of vascular tone, forming a focal point for signaling by many vasoactive transmitters that alter smooth muscle contractility and so blood flow. Clinically, these channels form the target of antianginal and antihypertensive drugs, and their genetic disruption leads to hypertension and sudden cardiac death through coronary vasospasm. However, whereas the biochemical basis of K(ATP) channel modulation is well-studied, little is known about the structural or spatial organization of the signaling pathways that converge on these channels. In this study, we use discontinuous sucrose density gradients and Western blot analysis to show that K(ATP) channels localize with an upstream signaling partner, adenylyl cyclase, to smooth muscle membrane fractions containing caveolin, a protein found exclusively in cholesterol and sphingolipid-enriched membrane invaginations known as caveolae. Furthermore, we show that an antibody against the K(ATP) pore-forming subunit, Kir6.1 co-immunoprecipitates caveolin from arterial homogenates, suggesting that Kir6.1 and caveolin exist together in a complex. To assess whether the colocalization of K(ATP) channels and adenylyl cyclase to smooth muscle caveolae has functional significance, we disrupt caveolae with the cholesterol-depleting agent, methyl-beta-cyclodextrin. This reduces the cAMP-dependent protein kinase A-sensitive component of whole-cell K(ATP) current, indicating that the integrity of caveolae is important for adenylyl cyclase-mediated channel modulation. These results suggest that to be susceptible to protein kinase A-dependent activation, arterial K(ATP) channels need to be localized in the same lipid compartment as adenylyl cyclase; the results also provide the first indication of the spatial organization of signaling pathways that regulate K(ATP) channel activity.

The evolving role of lipid rafts and caveolae in G protein-coupled receptor signaling: implications for molecular pharmacology

The many components of G-protein-coupled receptor (GPCR) signal transduction provide cells with numerous combinations with which to customize their responses to hormones, neurotransmitters, and pharmacologic agonists. GPCRs function as guanine nucleotide exchange factors for heterotrimeric (alpha, beta, gamma) G proteins, thereby promoting exchange of GTP for GDP and, in turn, the activation of 'downstream' signaling components. Recent data indicate that individual cells express mRNA for perhaps over 100 different GPCRs (out of a total of nearly a thousand GPCR genes), several different combinations of G-protein subunits, multiple regulators of G-protein signaling proteins (which function as GTPase activating proteins), and various isoforms of downstream effector molecules. The differential expression of such protein combinations allows for modulation of signals that are customized for a specific cell type, perhaps at different states of maturation or differentiation. In addition, in the linear arrangement of molecular interactions involved in a given GPCR-G-protein-effector pathway, one needs to consider the localization of receptors and post-receptor components in subcellular compartments, microdomains, and molecular complexes, and to understand the movement of proteins between these compartments. Co-localization of signaling components, many of which are expressed at low overall concentrations, allows cells to tailor their responses by arranging, or spatially organizing in unique and kinetically favorable ways, the molecules involved in GPCR signal transduction. This review focuses on the role of lipid rafts and a subpopulation of such rafts, caveolae, as a key spatial compartment enriched in components of GPCR signal transduction. Recent data suggest cell-specific patterns for expression of those components in lipid rafts and caveolae. Such domains likely define functionally important, cell-specific regions of signaling by GPCRs and drugs active at those GPCRs.

Modulation of plasma membrane calcium-ATPase activity by local calcium microdomains near CRAC channels in human T cells

The spatial distribution of Ca(2+) signalling molecules is critical for establishing specific interactions that control Ca(2+) signal generation and transduction. In many cells, close physical coupling of Ca(2+) channels and their targets enables precise and robust activation of effector molecules through local [Ca(2+)](i) elevation in microdomains. In T cells, the plasma membrane Ca(2+)-ATPase (PMCA) is a major target of Ca(2+) influx through Ca(2+) release-activated Ca(2+) (CRAC) channels. Elevation of [Ca(2+)](i) slowly modulates pump activity to ensure the stability and enhance the dynamic nature of Ca(2+) signals. In this study we probed the functional organization of PMCA and CRAC channels in T cells by manipulating Ca(2+) microdomains near CRAC channels and measuring the resultant modulation of PMCAs. The amplitude and spatial extent of microdomains was increased by elevating the rate of Ca(2+) entry, either by raising extracellular [Ca(2+)], by increasing the activity of CRAC channels with 2-aminoethoxyborane (2-APB), or by hyperpolarizing the plasma membrane. Surprisingly, doubling the rate of Ca(2+) influx does not further increase global [Ca(2+)](i) in a substantial fraction of cells, due to a compensatory increase in PMCA activity. The enhancement of PMCA activity without changes in global [Ca(2+)](i) suggests that local [Ca(2+)](i) microdomains near CRAC channels effectively promote PMCA modulation. These results reveal an intimate functional association between CRAC channels and Ca(2+) pumps in the plasma membrane which may play an important role in governing the time course and magnitude of Ca(2+) signals in T cells.

Targeting of ion channels to membrane microdomains: localization of KV channels to lipid rafts

Voltage-gated K(+) channels are an important determinant of cellular excitability and key components of multiple signal transduction pathways. However, relatively little is known about the mechanisms of K(V) channel localization or their membrane partitioning. Lipid rafts are specialized membrane microdomains that are rich in sphingolipids and cholesterol. These rafts have been implicated in the organization of many membrane-associated signaling pathways and are currently the focus of intense interest in the scientific community. Biochemical and functional evidence indicate that K(V) channels, in addition to other ion channels, localize to lipid raft microdomains on the cell surface. Although several important questions regarding specific mechanisms of channel localization remain, emerging data indicate that protein-lipid interactions should be considered as a new mechanism of ion channel localization and compartmentation that might permit the therapeutic modulation of channel properties via alteration in membrane lipids.

Relationship between asynchronous Ca2+ waves and force development in intact smooth muscle bundles of the porcine trachea

Fluctuations in intracellular calcium concentration ([Ca2+]i) constitute the main link in excitation-contraction coupling (E-C coupling) in airway smooth muscle cells (ASMC). It has recently been reported that ACh induces asynchronous recurring Ca2+ waves in intact ASMC of murine bronchioles. With the use of a novel technique allowing us to simultaneously measure subcellular [Ca2+]i and force generation in ASMC located within an intact tracheal muscle bundle, we examined a similar pattern of Ca2+ signaling in the trachea. We found that application of ACh resulted in the generation of recurring intracellular Ca2+ waves progressing along the longitudinal axis of the ribbon-shaped intact ASMC. These Ca2+ waves were not synchronized between neighboring cells, and induction of wave-like [Ca2+]i oscillations was temporally associated with development of force by the tracheal muscle bundle. By comparing the concentration dependence of force generation and the parameters characterizing the [Ca2+]i oscillations, we found that the concentration-dependent increase in ACh-induced force development by the tracheal smooth muscle bundle is achieved by differential recruitment of intact ASMC to initiate Ca2+ waves and by enhancement in the frequency of [Ca2+]i oscillations and elevation of interspike [Ca2+]i once the cells are recruited. Our findings demonstrate that asynchronous recurring Ca2+ waves underlie E-C coupling in ACh-induced contraction of the intact tracheal smooth muscle bundle. Furthermore, in contrast to what was reported in enzymatically dissociated ASMC, Ca2+ influx through the L-type voltage-gated Ca2+ channel was not an obligatory requirement for the generation of [Ca2+]i oscillations and development of force in ACh-stimulated intact ASMC.

Central role of connexin40 in the propagation of electrically activated vasodilation in mouse cremasteric arterioles in vivo

When a short segment of arteriole is stimulated, vasomotor responses spread bidirectionally along the vessel axis purportedly via gap junctions. We used connexin40 knockout (Cx40-/-) mice to study vasomotor responses induced by 10-second trains of electrical stimulation (30 Hz, 1 ms, 30 to 50 V) in 2nd or 3rd order arterioles of the cremaster muscle. Measurements were made at the stimulation site (local) and at conducted sites (500, 1000, and 2000 microm upstream). In wild-type (Cx40+/+) animals, electrical stimulation evoked a local vasoconstriction and a conducted vasodilation that spread very rapidly along the vessel length without detectable decay. In Cx40-/- mice, the conducted dilation was converted into either vasoconstriction or a slowly developing vasodilation that decayed along the vessel length. Tetrodotoxin (TTX, 1 micromol/L) had no effect on the local vasoconstriction in either Cx40+/+ or Cx40-/- mice, but enhanced the conducted vasodilation in Cx40+/+ animals. In Cx40-/- mice, TTX abolished the conducted vasoconstriction when present and revealed a small vasodilation that decayed with distance. In the group of Cx40-/- mice in which electrical stimulation elicited a conducted vasodilation, TTX had no effect. Immunocytochemistry revealed Cx40 only in the endothelial layer of arterioles from Cx40+/+ mice and complete elimination of this connexin in the Cx40-/- animals. These results indicate that focal current stimulation causes vasoconstriction by a combination of perivascular nerve stimulation and smooth muscle activation. Moreover, electrical stimulation activates a nonneuronal, Cx40-dependent vasodilator response that spreads along the vessel length without decay.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Roles of guanylate cyclase in responses to myogenic and neural nitric oxide in canine lower esophageal sphincter

Whether cGMP and cytosolic guanylate cyclase (cGC) mediate responses of canine lower esophageal sphincter (LES) to nitric oxide (NO) released from nerves, produced in muscle, or added exogenously was evaluated in vitro. 1-H-(1,2,4)oxadiazole(4,3-alpha)quinoxalin-1-1 (ODQ), inhibitor of cGC, reduced relaxations to nerve stimulation and sodium nitroprusside but not to nitric-oxide synthase activity-dependent outward K(+)-currents in isolated muscle cells. ODQ also failed to increase tone after nerve blockade. Nonspecific K(+) channel blocker, TEA ion at 20 mM was previously shown to increase tone, occlude NO-mediated modulation of tone, and inhibit NO-dependent outward currents but not neural relaxation in LES cells. In this study, TEA abolished neural relaxation and nearly abolished relaxation to sodium nitroprusside when present with ODQ. We conclude that mechanisms coupling NO in canine LES to responses vary with the source of NO. ODQ-dependent mechanisms, presumably involving cGC, mediate actions of NO from nerves, but NO from muscle utilizes TEA-sensitive but not ODQ-dependent mechanisms to modulate tone and outward currents. Exogenous NO utilizes both TEA- and ODQ-dependent mechanisms.

Connexin family members target to lipid raft domains and interact with caveolin-1

Lipid rafts are cholesterol-sphingolipid-rich microdomains that function as platforms for membrane trafficking and signal transduction. Caveolae are specialized lipid raft domains that contain the structural proteins known as the caveolins. Connexins are a family of transmembrane proteins that self-associate to form cell-cell connections known as gap junctions and that are linked to cytosolic proteins, forming a protein complex or Nexus. To determine the extent to which these intracellular compartments intersect, we have systematically evaluated whether connexins are associated with lipid rafts and caveolin-1. We show that connexin 43 (Cx43) colocalizes, cofractionates, and coimmunoprecipitates with caveolin-1. A mutational analysis of Cx43 reveals that the hypothesized PDZ- and presumptive SH2/SH3-binding domains within the Cx43 carboxyl terminus are not required for this targeting event or for its stable interaction with caveolin-1. Furthermore, Cx43 appears to interact with two distinct caveolin-1 domains, i.e., the caveolin-scaffolding domain (residues 82-101) and the C-terminal domain (135-178). We also show that other connexins (Cx32, Cx36, and Cx46) are targeted to lipid rafts, while Cx26 and Cx50 are specifically excluded from these membrane microdomains. Interestingly, recombinant coexpression of Cx26 with caveolin-1 recruits Cx26 to lipid rafts, where it colocalizes with caveolin-1. This trafficking event appears to be unique to Cx26, since the other connexins investigated in this study do not require caveolin-1 for targeting to lipid rafts. Our results provide the first evidence that connexins interact with caveolins and partition into lipid raft domains and indicate that these interactions are connexin specific.

Ca(2+) oscillations, gradients, and homeostasis in vascular smooth muscle

Vascular smooth muscle shows both plasticity and heterogeneity with respect to Ca(2+) signaling. Physiological perturbations in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) may take the form of a uniform maintained rise, a transient uniform [Ca(2+)](i) elevation, a transient localized rise in [Ca(2+)](i) (also known as spark and puff), a transient propagated wave of localized [Ca(2+)](i) elevation (Ca(2+) wave), recurring asynchronous Ca(2+) waves, or recurring synchronized Ca(2+) waves dependent on the type of blood vessel and the nature of stimulation. In this overview, evidence is presented which demonstrates that interactions of ion transporters located in the membranes of the cell, sarcoplasmic reticulum, and mitochondria form the basis of this plasticity of Ca(2+) signaling. We focus in particular on how the junctional complexes of plasmalemma and superficial sarcoplasmic reticulum, through the generation of local cytoplasmic Ca(2+) gradients, maintain [Ca(2+)](i) oscillations, couple these to either contraction or relaxation, and promote Ca(2+) cycling during homeostasis.

nNOS in canine lower esophageal sphincter: colocalized with Cav-1 and Ca2+-handling proteins?

Immunochemical studies with light microscopy, confocal microscopy, and electron microscopy were used to examine proteins associated with caveolin (Cav) in canine lower esophageal sphincter. The main Cav was Cav-1. It appeared to be colocalized at the cell periphery, in punctate sites, with immunoreactivity to antibodies against different COOH- and NH2-terminal epitopes of neuronal nitric oxide (NO) synthase (nNOS). One COOH-terminal-directed antibody, made in guinea pig, was used to colocalize other immunoreactivities. Those that apparently colocalized with nNOS were L-Ca2+ channels, the PM Ca2+ pump, and, in part, calreticulin and calsequestrin. The large-conductance Ca2+-activated K+ (BK(Ca)) channels were located in discrete peripheral sites, some with Cav. Immunoreactivities not fully colocalized with nNOS were to the sarcoplasmic reticulum Ca2+ pump, connexins 43, 40, and 45, and vinculin. In patch-clamp studies, NO-driven outward currents, mainly through BK(Ca) channels, were inhibited by antibodies to Cav-1 and not by calmodulin and were restored by an NO donor. Several Ca2+-handling molecules are localized at the PM with and/or near Cav. This may allow intracellular calcium concentration levels to be controlled differently than those in the cytosol near caveolae.

Two distinct caveolin-1 domains mediate the functional interaction of caveolin-1 with protein kinase A

Numerous components of the cAMP-based signaling cascade, namely G-proteins and G- protein coupled receptors, adenylyl cyclase, and protein kinase A (PKA) have been localized to caveolae and shown to be regulated by the caveolar marker proteins, the caveolins. In order to gain mechanistic insights into these processes in vivo, we have assessed the functional interaction of caveolin-1 (Cav-1) with PKA using mutational analysis. As two regions of Cav-1 had previously been implicated in PKA signaling in vitro, we constructed Cav-1 molecules with mutations/deletions in one or both of these domains. Examination of these mutants shows that Cav-1 requires the presence of either the scaffolding domain or the COOH-terminal domain (but not both) to functionally interact with and inhibit PKA. Interestingly, in contrast to the wild-type protein, these Cav-1 mutants are not localized to caveolae microdomains. However, upon coexpression with wild-type Cav-1, a substantial amount of the mutants was recruited to the caveolae membrane fraction. Using the Cav-1 double mutant with both disrupted scaffolding and COOH-terminal domains, we show that wild-type Cav-1's inhibition of PKA signaling can be partially abrogated in a dose-responsive manner; i.e., the mutant acts in a dominant-negative fashion. Thus, this dominant-negative caveolin-1 mutant will be extremely valuable for assessing the functional role of endogenous caveolin-1 in regulating a variety of other signaling cascades.

Invited review: arteriolar smooth muscle mechanotransduction: Ca(2+) signaling pathways underlying myogenic reactivity

The smooth muscle of arterioles responds to an increase in intraluminal pressure with vasoconstriction and with vasodilation when pressure is decreased. Such myogenic vasoconstriction provides a level of basal tone that enables arterioles to appropriately adjust diameter in response to neurohumoral stimuli. Key in this process of mechanotransduction is the role of changes in intracellular Ca(2+). However, it is becoming clear that considerable complexity exists in the spatiotemporal characteristics of the Ca(2+) signal and that changes in intracellular Ca(2+) may play roles other than direct effects on the contractile process via activation of myosin light-chain phosphorylation. The involvement of Ca(2+) may extend to modulation of ion channels and release of Ca(2+) from the sarcoplasmic reticulum, alterations in Ca(2+) sensitivity, and coupling between cells within the vessel wall. The purpose of this brief review is to summarize the current literature relating to Ca(2+) and the arteriolar myogenic response. Consideration is given to coupling of Ca(2+) changes to the mechanical stimuli, sources of Ca(2+), involvement of ion channels, and spatiotemporal aspects of intracellular Ca(2+) signaling.

The de-adhesive activity of matricellular proteins: is intermediate cell adhesion an adaptive state?

The process of cellular de-adhesion is potentially important for the ability of a cell to participate in morphogenesis and to respond to injurious stimuli. Cellular de-adhesion is induced by the highly regulated matricellular proteins TSP1 and 2, tenascin-C, and SPARC. These proteins induce a rapid transition to an intermediate state of adhesiveness characterized by loss of actin-containing stress fibers and restructuring of the focal adhesion plaque that includes loss of vinculin and alpha-actinin, but not of talin or integrin. This process involves intracellular signaling mediators, which are engaged in response to matrix protein-receptor interactions. Each of these proteins employs different receptors and signaling pathways to achieve this common morphologic endpoint. What is the function of this intermediate adhesive state and what is the physiologic significance of this action of the matricellular proteins? Given that matricellular proteins are expressed in response to injury and during development, one can speculate that the intermediate adhesive state is an adaptive condition that facilitates expression of specific genes that are involved in repair and adaptation. Since cell shape is maintained in weakly adherent cells, this state might induce survival signals to prevent apoptosis due to loss of strong cell adhesion, but yet allow for cell locomotion. The three matricellular proteins considered here might each preferentially facilitate one or more aspects of this adaptive response rather than all of these equally. Currently, we have only preliminary data to support the specific ideas proposed in this article. It will be interesting in the next several years to continue to elucidate the biological roles of the intermediate adhesive state induced by these matricellular proteins. and focal adhesions in a cell that nevertheless maintains a spread, extended morphology and integrin clustering. TSP1, tenascin-C, and SPARC induce the intermediate adhesive state, as shown by the red arrows. The significance of each adhesive state for cell behavior is indicated beneath the cells. The weak adhesive state would be consistent with cells undergoing apoptosis during remodeling or those undergoing cytokinesis. The strong adhesive state is characteristic of a differentiated, quiescent cell, whereas cells in the intermediate adhesive state would include those involved in responding to injury during wound healing or in tissue remodeling during morphogenesis.