Intracellular calcium translocation during contraction in vertebrate and invertebrate smooth muscles as studied by the pyroantimonate method

FRET-based sensor analysis reveals caveolae are spatially distinct Ca2+ stores in endothelial cells

Ca2+-regulating and Ca2+-dependent molecules enriched in caveolae are typically shaped as plasmalemmal invaginations or vesicles. Caveolae structure and subcellular distribution are critical for Ca2+ release from endoplasmic reticulum Ca2+ stores and for Ca2+ influx from the extracellular space into the cell. However, Ca2+ dynamics inside caveolae have never been directly measured and remain uncharacterized. To target the fluorescence resonance energy transfer (FRET)-based Ca2+ sensing protein D1, a mutant of cameleon, to the intra-caveolar space, we made a cDNA construct encoding a chimeric protein of lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1) and D1 (LOXD1). Immunofluorescence and immunoelectron microscopy confirmed that a significant portion of LOXD1 was localized with caveolin-1 at morphologically apparent caveolar vesicles in endothelial cells. LOXD1 detected ATP-induced transient Ca2+ decreases by confocal FRET imaging in the presence or absence of extracellular Ca2+. This ATP-induced Ca2+ decrease was abolished following knockdown of caveoin-1, suggesting an association with caveolae. The X-ray spectra obtained by the spot analysis of electron-opaque pyroantimonate precipitates further confirmed that ATP-induced calcium decreases in intra-caveolar vesicles. In conclusion, subplasmalemmal caveolae function as Ca2+-releasable Ca2+ stores in response to ATP. This intracellular local Ca2+ delivery system may contribute to the complex spatiotemporal organization of Ca2+ signaling.

Transport across the endothelium: regulation of endothelial permeability

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways controlling endothelial barrier function have been identified. The transcellular pathway transports plasma proteins of the size of albumin or greater via the process of transcytosis in vesicle carriers originating from cell surface caveolae. Specific signalling cues are able to induce the internalisation of caveolae and their movement to the basal side of the endothelium. Caveolin-1, the primary structural protein required for the formation of caveolae, is also important in regulating vesicle trafficking through the cell by controlling the activity and localisation of signalling molecules that mediate vesicle fission, endocytosis, fusion and finally exocytosis. An important function of the transcytotic pathways is to regulate the delivery of albumin and immunoglobulins, thereby controlling tissue oncotic pressure and host-defence. The paracellular pathway induced during inflammation is formed by gaps between endothelial cells at the level of adherens and tight junctional complexes. Paracellular permeability is increased by second messenger signalling pathways involving Ca2+ influx via activation of store-operated channels, protein kinase Calpha (PKCalpha), and Rho kinase that together participate in the stimulation of myosin light chain phosphorylation, actin-myosin contraction, and disruption of the junctions. In this review of the field, we discuss the current understanding of the signalling pathways regulating paracellular and transcellular endothelial permeability.

SIGNALING NETWORKS IN LIVING CELLS

Recent advances in cell signaling research suggest that multiple sets of signal transducing molecules are preorganized and sequestered in distinct compartments within the cell. These compartments are assembled and maintained by specific cellular machinery. The molecular ecology within a compartment creates an environment that favors the efficient and accurate integration of signaling information arriving from humoral, mechanical, and nutritional sources. The functional organization of these compartments suggests they are the location of signaling networks that naturally organize into hierarchical interconnected sets of molecules through their participation in different classes of interacting units. An important goal is to determine the contribution of the compartment to the function of these networks in living cells.

Function of caveolae in Ca2+ entry and Ca2+-dependent signal transduction

The correct spatial and temporal control of Ca2+ signaling is essential for such cellular activities as fertilization, secretion, motility, and cell division. There has been a long-standing interest in the role of caveolae in regulating intracellular Ca2+ concentration. In this review we provide an updated view of how caveolae may regulate both Ca2+ entry into cells and Ca2+-dependent signal transduction

Caveolin regulation of endothelial function

Caveolae are the sites in the cell membrane responsible for concentrating an array of signaling molecules critical for cell function. Recent studies have begun to identify the functions of caveolin-1, the 22-kDa caveolar protein that oligomerizes and inserts into the cytoplasmic face of the plasma membrane. Caveolin-1 appears to regulate caveolar internalization by stabilizing caveolae at the plasma membrane rather than controlling the shape of the membrane invagination. Because caveolin-1 is a scaffolding protein, it has also been hypothesized to function as a "master regulator" of signaling molecules in caveolae. Deletion of the caveolin-1 gene in mice resulted in cardiac hypertrophy and lung fibrosis, indicating its importance in cardiac and lung development. In the endothelium, caveolin-1 regulates nitric oxide signaling by binding to and inhibiting endothelial nitric oxide synthase (eNOS). Increased cytosolic Ca2+ or activation of the kinase Akt leads to eNOS activation and its dissociation from caveolin-1. Caveolae have also been proposed as the vesicle carriers responsible for transcellular transport (transcytosis) in endothelial cells. Transcytosis, the primary means of albumin transport across continuous endothelia, occurs by fission of caveolae from the membrane. This event is regulated by tyrosine phosphorylation of caveolin-1 and dynamin. As Ca2+ influx channels and pumps are localized in caveolae, caveolin-1 is also an important determinant of Ca2+ signaling in endothelial cells. Many of these findings were presented in San Diego, CA, at the 2003 Experimental Biology symposium "Caveolin Regulation of Endothelial Function" and are reviewed in this summary.

A molecular sensor detects signal transduction from caveolae in living cells

Biochemical and cell fractionation studies suggest caveolae contain functionally organized sets of signaling molecules that are capable of transmitting specific signals to the cell. It is not known, however, whether any signals actually originate from caveolae in living cells. To address this question, we have engineered the calcium sensor yellow cameleon so that it is targeted either to the plasma membrane, caveolae, or the cytoplasm of endothelial cells. Quantitative measurements of the three Ca2+ pools detected by these probes indicate that caveolae are preferred sites of Ca2+ entry when Ca2+ stores in the endoplasmic reticulum are depleted. These results suggest that the signaling machinery in control of Ca2+ entry is functionally organized in the caveolae of living cells.

Calcium signal transduction from caveolae

Caveolae are specialized membrane microdomains that are found on the plasma membrane of most cells. Recent studies indicate that a variety of signaling molecules are highly organized in caveolae, where their interactions initiate specific signaling cascades. Molecules enriched in this membrane include G protein-coupled receptors, heterotrimeric GTP binding proteins, IP3 receptor-like protein, Ca2+ ATPase, eNOS, and several PKC isoforms. Direct measurements of calcium changes in endothelial cells suggest that caveolae may be sites that regulate intracellular Ca2+ concentration and Ca2+ dependent signal transduction. This review will focus on the role of caveolae in controlling the spatial and temporal pattern of intracellular Ca2+ signaling.

Role of plasmalemmal caveolae in signal transduction

Caveolae are specialized plasmalemmal microdomains originally studied in numerous cell types for their involvement in the transcytosis of macromolecules. They are enriched in glycosphingolipids, cholesterol, sphingomyelin, and lipid-anchored membrane proteins, and they are characterized by a light buoyant density and resistance to solubilization by Triton X-100 at 4 degreesC. Once the identification of the marker protein caveolin made it possible to purify this specialized membrane domain, it was discovered that caveolae also contain a variety of signal transduction molecules. This includes G protein-coupled receptors, G proteins and adenylyl cyclase, molecules involved in the regulation of intracellular calcium homeostasis, and their effectors including the endothelial isoform of nitric oxide synthase, multiple components of the tyrosine kinase-mitogen-activated protein kinase pathway, and numerous lipid signaling molecules. More recent work has indicated that caveolae further serve to compartmentalize, modulate, and integrate signaling events at the cell surface. This specialized plasmalemmal domain warrants direct consideration in future investigations of both normal and pathological signal transduction in pulmonary cell types.

The caveolae membrane system

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

In vitro capsaicin-induced cytological changes and alteration in calcium distribution in giant serotonergic neurons of the snail Helix pomatia: a light- and electron-microscopic study

Morphological changes induced by capsaicin were studied in the serotonergic metacerebral giant neurons of the cerebral ganglia of Helix pomatia under in vitro conditions. Capsaicin at a concentration of 10(-4)M caused characteristic structural alterations in the giant serotonergic neurons but did not significantly influence serotonin immunoreactivity in the neurons. At the light-microscopic level, the most conspiciuous structural alterations were swelling of the cell bodies, which contained a swollen pale nucleus. Under the electron microscope, the nuclei, mitochondria and the cisternae of the endoplasmic reticulum were swollen in the capsaicin-affected metacerebral giant neurons. Electron-microscopic cytochemical techniques for calcium demonstration revealed electron-dense deposits in the swollen mitochondria and in the cisternae of the endoplasmic reticulum, suggesting an increased Ca2+ influx. The serotonergic metacerebral giant neurons could be labelled by cobalt (1mM) in the presence of capsaicin (10(-4)M) suggesting that capsaicin opens the cation chanels of the capsaicin-sensitive neuronal membrane. The morphological and cytochemical alterations induced by capsaicin in the serotonergic metacerebral giant neurons of Helix pomatia closely resemble those induced in sensory neurons of mammalian dorsal root ganglion.

Caveolae: where incoming and outgoing messengers meet

Plasmalemmal caveolae were first identified as an endocytic compartment in endothelial cells, where they appear to move molecules across the cell by transcytosis. More recently, they have been found to be sites where small molecules are concentrated and internalized by a process called potocytosis. A growing body of biochemical and morphological evidence indicates that a variety of molecules known to function directly or indirectly in signal transduction are enriched in caveolae. This raises the possibility that a third function for caveolae is to process hormonal and mechanical signals for the cell. Insights gained from studying potocytosis suggest several different ways that this membrane specialization might function to integrate incoming and outgoing cellular messages.

Subcellular calcium localization in Toxoplasma gondii by electron microscopy and by X-ray and electron energy loss spectroscopies

The localization of calcium in Toxoplasma gondii tachyzoites was studied at the ultrastructural level, with a cytochemical pyroantimonate precipitation method (PA) and controlled by EGTA chelating and EDX and EELS microanalyses. Appropriate conditions for material preparation, fixation and embedding, were defined. The proportion of precipitates that were either free or inside vacuoles and their distribution inside Toxoplasma appeared to be PA dose-dependent. Precipitation mainly occurred in the anterior pole of the Epon-embedded tachyzoites. EDX and EELS analyses showed that out of 30 PA precipitates inside tachyzoites, 78% contained Ca. In Melamine sections, 96% of the tachyzoites had intracellular precipitates and the membrane complex was stained; 25% of the tachyzoites inside host cells contained PA-Ca precipitates, but most of them were retained in the reticular network of the parasitophorous vacuole. Melamine embedding appeared to improve the preservation of calcium pyroantimonate precipitates.

Evidence for extracellular localization of activator calcium in dog coronary artery smooth muscle as studied by the pyroantimonate method

Correlated physiological and electron-microscopic studies were made on the source of calcium activating the contractile system (activator calcium) in dog coronary artery smooth muscle fibers. The magnitude of contracture tension induced by 100 mM K+ was dependent on external Ca2+ concentration and reduced or eliminated by factors known to reduce the Ca2+ spike or Ca2+ influx. Little or no mechanical response was elicited by treatments known to cause release of intracellularly stored calcium. These results indicated that the contractile system is mainly activated by the inward movement of extracellular calcium. In accordance with the physiological experiments, electronopaque pyroantimonate precipitate containing calcium was found in the lumina of caveolae, but not in any intracellular structures close to the plasma membrane, when the relaxed fibers were fixed in a 1% osmium tetroxide solution containing 2% potassium pyroantimonate. If the contracted fibers were fixed in the same solution, the pyroantimonate precipitate was diffusely distributed in the myoplasm in the form of numerous particles, while the precipitate in the caveolar lumina was scarcely seen. These findings are discussed in connection with the regulation of intracellular Ca2+ concentration in dog coronary artery smooth muscle.

Evaluation of the pyroantimonate method for detecting intracellular calcium localization in smooth muscle fibers by the X-ray microanalysis of cryosections

The validity of the pyroantimonate method, which has been used for detecting intracellular Ca localization and translocation in smooth muscles, was examined by making cryosections of the relaxed anterior byssal retractor muscle (ABRM) of Mytilus edulis at various stages of procedures for preparing ordinary Epon-embedded sections and determining the elemental concentration ratios of the pyroantimonate precipitate, localized along the inner surface of the plasma membrane, with an energy dispersive X-ray microanalyzer. The concentration of Ca (relative to that of Sb) in the precipitate stayed constant after the procedures of fixation, dehydration and Epon-embedding, while the concentrations of K, Mg, Na and Os showed their respective characteristic changes after the above procedures, being lower than that of Ca in the Epon-embedded sections. The presence of Ca in the precipitate was also demonstrated with an electron energy-loss spectrometer. The localization of Ca underneath the plasma membrane was also observed in the cryosections of the ABRM fibers prepared after mild fixation with acrolein vapor without using pyroantimonate. These results indicate that the pyroantimonate precipitate serves as a valid measure of intracellular Ca localization.

Effects of verapamil and sodium nitroprusside on acetylcholine-induced contraction of the rabbit detrusor muscle

Effects of extracellular and intracellular Ca2+ on acetylcholine-induced contraction of the bladder detrusor muscle were studied in vitro, utilizing two types of Ca2+ antagonists of different mechanisms of action; verapamil and sodium nitroprusside (NP). Acetylcholine (10(-8) to 10(-2) M) caused dose-dependent contractions of the detrusor muscle strips. Pretreatment of the strips with verapamil (10(-7), 10(-6) M) significantly inhibited the acetylcholine-induced contraction in a dose-dependent manner, whereas NP (10(-7) to 10(-5) M) failed so suppress the contraction. The contraction of the strips once elicited by acetylcholine (10(-6) M) could be completely relaxed by verapamil (10(-5) M) addition, but only incompletely by NP (10(-5), 10(-4) M). In Ca2+-free solution containing 0.01 mM EGTA, replenishment of Ca2+ (2.5 mM) to the medium caused contractions of the strips. Addition of acetylcholine (10(-6) M) to the medium enhanced the Ca2+-induced contraction, which was significantly inhibited by pretreatment with verapamil (10(-6) M), but not affected by NP (10(-6) M). In Ca2+-free medium containing 0.1 mM EGTA, acetylcholine caused a slight degree of tension increase of the strips in a dose-dependent fashion, at higher concentrations exceeding 10(-6) M. These results suggest that the detrusor muscle contraction induced by acetylcholine is mostly dependent of extracellular Ca2+ influx both in its initiation and maintenance. It is also supposed, however, that intracellular Ca2+ fractions will partly participate in the acetylcholine-induced contraction and possibly in its maintenance.

In vivo induced malignant hyperthermia in pigs. III. Localization of calcium in skeletal muscle mitochondria by means of electronmicroscopy and microprobe analysis

Biceps femoris muscle biopsies of malignant hyperthermia susceptible (MH+) and non-susceptible (MH-) Dutch Landrace pigs were studied ultrastructurally, and exchangeable calcium was demonstrated, using the antimonate precipitation technique in combination with electron probe x-ray microanalysis. Biopsies were taken before and during the administration of halothane-plus-succinylcholine and after dantrolene sodium treatment of the animals. MH+ muscle, taken before the MH triggering, showed a high proportion (about 35%) of cells with supercontraction. Both MH+ and MH- muscle had broad but nearly identical ranges of cell diameter. Core-like structures were occasionally present in muscle from MH+ pigs. Muscle mitochondria from the MH+ pigs accumulated large amounts of calcium in their matrix compartment during the halothane-plus-succinylcholine induced MH crisis. This calcium loading in the course of time caused swelling and structural damage to the mitochondria. Skeletal muscle mitochondria from MH- pigs did not show such a reaction pattern on challenge with halothane and succinylcholine. It is concluded that in MH+ pigs the challenge brings about an increase in myoplasmic free calcium, which is predominantly due to calcium influx from the extracellular fluid. This rise in cytosolic calcium causes the mitochondria to accumulate the cation in an energy-dependent way. These findings are discussed in relation to the diverging halothane and caffeine contraction responses of aerobic type I and anaerobic type II muscle fibres.