Golgi apparatus is involved in intracellular Ca2+ regulation in epithelial LLC-PK1 cells

The role of CaMKK2 in Golgi-associated vesicle trafficking

Calcium/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a serine/threonine-protein kinase, that is involved in maintaining various physiological and cellular processes within the cell that regulate energy homeostasis and cell growth. CaMKK2 regulates glucose metabolism by the activation of downstream kinases, AMP-activated protein kinase (AMPK) and other calcium/calmodulin-dependent protein kinases. Consequently, its deregulation has a role in multiple human metabolic diseases including obesity and cancer. Despite the importance of CaMKK2, its signalling pathways and pathological mechanisms are not completely understood. Recent work has been aimed at broadening our understanding of the biological functions of CaMKK2. These studies have uncovered new interaction partners that have led to the description of new functions that include lipogenesis and Golgi vesicle trafficking. Here, we review recent insights into the role of CaMKK2 in membrane trafficking mechanisms and discuss the functional implications in a cellular context and for disease.

Contribution of intracellular Ca2+ stores to Ca2+ signaling during chemokinesis of human neutrophil granulocytes

Extracellular agonists increase the cytosolic free Ca2+ concentration ([Ca2+]c) by Ca2+ influx and by stimulating Ca2+ release from intracellular stores, mainly the endoplasmic reticulum and to a lesser extent also later compartments of the secretory pathway, particularly the Golgi. The Golgi takes up Ca2+ via Sarco/Endoplasmic Reticulum Ca2+ATPases (SERCAs) and the Secretory-Pathway Ca2+ATPases (SPCAs). The endogenous expression of SERCAs and SPCAs neutrophils was demonstrated by Western blotting and immunocytochemistry. Up till now, all cytosolic Ca2+ transients due to intracellular Ca2+ release have been found to originate from SERCA-dependent stores. We found that human neutrophils also present Ca2+ release from a SERCA-independent store. Changes in [Ca2+]c of neutrophils were investigated during chemokinesis induced by chemotactic factors in Ca2+-free solution with and without the SERCA-specific inhibitor thapsigargin. Using N-formyl-methionyl-leucyl-phenylalanine or interleukin-8 as agonists, Ca2+ release from intracellular stores was observed in respectively about 40% and 20% of the neutrophils pre-treated with Ca2+-free solution and thapsigargin. In the latter condition, 20-30% of the cells preserved migratory behaviour. These results indicate that both SERCA-dependent and SERCA-independent (presumably SPCA-dependent) intracellular Ca2+ stores contribute to Ca2+ signaling during chemokinesis of human neutrophil granulocytes.

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+and Mn2+homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+“spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+in transporting epithelia: to transport or store Ca2+in bulk without compromising its use as a signal.

Quantitative imaging of subcellular calcium stores in mammalian LLC-PK1 epithelial cells undergoing mitosis by SIMS ion microscopy

Quantitative 3-D total calcium gradients, representing subcellular stored calcium, were imaged with a CAMECA IMS-3f SIMS ion microscope in cryogenically prepared frozen freeze-dried LLC-PK1 cells captured in interphase and various stages of mitosis. 39K and 23Na concentrations were also measured in the same cells. Correlative optical (or SEM) and SIMS analysis of cells revealed a redistribution of the interphase Golgi calcium store in prophase and prometaphase cells. In metaphase cells, simultaneous SIMS imaging of total calcium in both the spindle and the non-spindle cytoplasm of individual cells revealed a gradual and dynamic alignment of calcium stores in both half-spindles prior to the onset of anaphase. The anaphase cells revealed the highest local total calcium concentrations in the spindle regions behind the daughter chromosomes and the lowest in the central spindle region. The pericentriolar material in telophase cells contained calcium stores. Quantitatively, a typical metaphase cell with well-aligned calcium stores in the spindle region contained 1.1 mM total calcium in each half-spindle, 0.8 mM total calcium in the non-spindle cytoplasm, and 0.5mM total calcium in the chromosomes. At the submicron scale, the distribution of total calcium was heterogeneous in the chromosomes, metaphase spindle, and non-spindle cytoplasm. An increased binding of calcium to chromosomes is not a physiological requirement for chromosomal condensation in mitosis, since interphase nuclei and mitotic chromosomes contained comparable total calcium concentrations measured per unit volume. A significant reduction of total calcium in the non-spindle cytoplasm was observed in the metaphase, anaphase, and telophase cells, which is indicative of the limited storage of the releasable calcium pool in these specific stages of mitosis. Direct total calcium measurements in subcellular regions confirmed that both the spindle and the non-spindle cytoplasm of metaphase cells contained inositol 1,4,5-trisphosphate (IP3)-sensitive calcium stores sensitive to arginine vasopressin, thapsigargin, and calcium ionophore A23187. The dynamic alignment of calcium stores in both half-spindles may be an integral part of the time-dependent process of a cell's overall preparation for exiting the metaphase stage in mammalian LLC-PK1 cells.

Calcium release from the Golgi apparatus and the endoplasmic reticulum in HeLa cells stably expressing targeted aequorin to these compartments

Extracellular agonists mobilize Ca2+ from SERCA-comprising intracellular Ca2+ stores located in both the Golgi apparatus and the endoplasmic reticulum. Ca2+ release from both these compartments was studied in HeLa cells stably expressing the luminescent Ca2+ indicator aequorin specifically targeted to these compartments. Changes in lumenal [Ca2+] as detected by the aequorin measurements were correlated with parallel changes in total Ca2+ content of the stores. The latencies and initial rates of Ca2+ release from the Golgi apparatus and the endoplasmic reticulum were quite similar. However, maximal Ca2+ release measured with Golgi-targeted aequorin terminated faster than that from the endoplasmic reticulum. The rate and extent of Ca2+ depletion from both compartments correlated well with the peak amplitude of the cytosolic [Ca2+] rise. Time-course experiments further revealed that the peak of the cytosolic Ca2+ response occurred before the lumenal [Ca2+] reached its lowest level. We conclude that both the Golgi apparatus and the endoplasmic reticulum contribute to the rise in cytosolic [Ca2+] upon agonist stimulation, but the kinetics of the Ca2+ release are different.

SPCA1 pumps and Hailey-Hailey disease

Both the endoplasmic reticulum and the Golgi apparatus are agonist-sensitive intracellular Ca2+ stores. The Golgi apparatus has Ca2+-release channels and a Ca2+-uptake mechanism consisting of sarco(endo)plasmic-reticulum Ca2+-ATPases (SERCA) and secretory-pathway Ca2+-ATPases (SPCA). SPCA1 has been shown to transport both Ca2+ and Mn2+ in the Golgi lumen and therefore plays an important role in the cytosolic and intra-Golgi Ca2+ and Mn2+ homeostasis. Human genetic studies have provided new information on the physiological role of SPCA1. Loss of one functional copy of the SPCA1 (ATP2C1) gene causes Hailey-Hailey disease, a skin disorder arising in the adult age with recurrent vesicles and erosions in the flexural areas. Here, we review recent experimental evidence showing that the Golgi apparatus plays a much more important role in intracellular ion homeostasis than previously anticipated.

Inositol trisphosphate producing agonists do not mobilize the thapsigargin-insensitive part of the endoplasmic-reticulum and Golgi Ca2+ store

Non-mitochondrial intracellular Ca2+ stores contain both thapsigargin-sensitive sarco(endo)plasmic-reticulum Ca2+-ATPases (SERCA) and thapsigargin-insensitive secretory-pathway Ca2+-ATPases (SPCA1). We now have studied the Ca2+-release properties of the compartments associated with these pumps in intact, i.e. non-permeabilized, cells of different origin (HeLa, keratinocytes, 16HBE14o-, COS-1, A7r5) and with different approaches (45Ca2+ fluxes, Ca2+ imaging and measurements of the free luminal [Ca2+] in the endoplasmic-reticulum and the Golgi apparatus using targeted aequorin). Application of an extracellular agonist in the absence of thapsigargin induced in all cells a Ca2+ release from both the endoplasmic-reticulum and the Golgi apparatus. The agonists were not able to release Ca2+ in the presence of 10 microM thapsigargin, except in COS-1 cells overexpressing SPCA1, where this pump not only appeared in the Golgi compartment but also overflowed into the agonist-sensitive part of the endoplasmic-reticulum. We conclude that the subcompartments of the endoplasmic-reticulum and of the Golgi complex that endogenously express SPCA1 are insensitive to agonist stimulation.

Perlecan-induced suppression of smooth muscle cell proliferation is mediated through increased activity of the tumor suppressor PTEN

We were interested in the elucidation of the interaction between the heparan sulfate proteoglycan, perlecan, and PTEN in the regulation of vascular smooth muscle cell (SMC) growth. We verified serum-stimulated DNA synthesis, and Akt and FAK phosphorylation were significantly reduced in SMCs overexpressing wild-type PTEN. Our previous studies showed perlecan is a potent inhibitor of serum-stimulated SMC growth. We report in the present study, compared with SMCs plated on fibronectin, serum-stimulated SMCs plated on perlecan exhibited increased PTEN activity, decreased FAK and Akt activities, and high levels of p27, consistent with SMC growth arrest. Adenoviral-mediated overexpression of constitutively active Akt reversed perlecan-induced SMC growth arrest while morpholino antisense-mediated loss of endogenous PTEN resulted in increased growth and phosphorylation of FAK and Akt of SMCs on perlecan. Immunohistochemical and Western analyses of balloon-injured rat carotid artery tissues showed a transient increase in phosphoPTEN (inactive) after injury, correlating to high rates of neointimal cell replication; phosphoPTEN was largely limited to actively replicating SMCs. Similarly, in the developing rat aorta, we found increased PTEN activity associated with increased perlecan deposition and decreased SMC replication rates. However, significantly decreased PTEN activity was detected in aortas of perlecan-deficient mouse embryos, consistent with SMC hyperplasia observed in these animals, compared with E17.5 heterozygous controls that produce abundant amounts of perlecan at this developmental time point. Our data show PTEN is a potent endogenously produced inhibitor of SMC growth and increased PTEN activity mediates perlecan-induced suppression of SMC proliferation.

Similar Ca(2+)-signaling properties in keratinocytes and in COS-1 cells overexpressing the secretory-pathway Ca(2+)-ATPase SPCA1

Mutations in the ubiquitously expressed secretory-pathway Ca(2+)-ATPase (SPCA1) Ca(2+) pump result in Hailey-Hailey disease, which almost exclusively affects the epidermal part of the skin. We have studied Ca(2+) signaling in human keratinocytes by measuring the free Ca(2+) concentration in the cytoplasm and in the lumen of both the Golgi apparatus and the endoplasmic reticulum. These signals were compared with those recorded in SPCA1-overexpressing and control COS-1 cells. Both the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA) and SPCA1 can mediate Ca(2+) uptake into the Golgi stacks. Our results indicate that keratinocytes mainly used the SPCA1 Ca(2+) pump to load the Golgi complex with Ca(2+) whereas the SERCA Ca(2+) pump was mainly used in control COS-1 cells. Cytosolic Ca(2+) signals in keratinocytes induced by extracellular ATP or capacitative Ca(2+) entry were characterized by an unusually long latency reflecting extra Ca(2+) buffering by an SPCA1-containing Ca(2+) store, similarly as in SPCA1-overexpressing COS-1 cells. Removal of extracellular Ca(2+) elicited spontaneous cytosolic Ca(2+) transients in keratinocytes, similarly as in SPCA1-overexpressing COS-1 cells. With respect to Ca(2+) signaling keratinocytes and SPCA1-overexpressing COS-1 cells therefore behaved similarly but differed from control COS-1 cells. The relatively large contribution of the SPCA1 pumps for loading the Golgi stores with Ca(2+) in keratinocytes may, at least partially, explain why mutations in the SPCA1 gene preferentially affect the skin in Hailey-Hailey patients.

The contribution of the SPCA1 Ca2+ pump to the Ca2+ accumulation in the Golgi apparatus of HeLa cells assessed via RNA-mediated interference

The secretory-pathway Ca(2+)-ATPase SPCA1 is a thapsigargin-insensitive intracellular Ca(2+) pump found mostly in the Golgi compartment. We have explored the contribution of this Ca(2+) pump to cytosolic Ca(2+) signaling in HeLa cells by using RNA-mediated interference to disrupt its expression. Removal of SPCA1 was confirmed by immunofluorescence with specific anti-SPCA1 antibodies. Measurements of the free Ca(2+) concentration in the lumen of the Golgi apparatus by specifically targeting the Ca(2+)-sensitive luminescent protein aequorin to this organelle revealed that endogenous SPCA1 was responsible for Ca(2+) uptake in a subfraction of the Golgi apparatus. HeLa cells lacking SPCA1 could still set up baseline Ca(2+) spiking when stimulated with histamine, indicating that the SPCA1-containing Ca(2+) store was not absolutely needed to set up these oscillations. However, baseline Ca(2+) oscillations occurred less frequently than in control cells, pointing to a contribution of SPCA1 in the shaping of the cytosolic Ca(2+) signal in HeLa cells.

Ca(2+) signals in Pmr1-GFP-expressing COS-1 cells with functional endoplasmic reticulum

We studied the role of the Pmr1-containing Ca(2+) store in COS-1 cells endowed with a functional endoplasmic reticulum. Transfected cells could be recognized by using a green-fluorescent-protein (GFP)-tagged form of Pmr1. Pmr1-GFP fluorescence showed a typical juxtanuclear Golgi-like distribution. Pmr1-GFP-containing cells with functional endoplasmic reticulum responded to 100 microM ATP with baseline Ca(2+) spiking, while non-transfected cells produced an initial Ca(2+) peak followed by a long-lasting plateau. The Ca(2+) signal often appeared after a long latency in Pmr1-GFP-expressing cells. ATP-stimulated Pmr1-GFP-expressing cells with functional endoplasmic reticulum responded after a latency period to extracellular Ca(2+) with a regenerative Ca(2+) signal, while non-transfected control cells responded with an immediate slow rise in free cytosolic Ca(2+) concentration. These results demonstrate the importance of the Pmr1-containing Ca(2+) store in generating or modifying cellular Ca(2+) signals.

Ca2+ uptake and release properties of a thapsigargin-insensitive nonmitochondrial Ca2+ store in A7r5 and 16HBE14o- cells

In a previous study we overexpressed the thapsigargin (tg)-insensitive Pmr1 Ca(2+) pump of the Golgi apparatus of Caenorhabditis elegans in COS-1 cells and studied the properties of the Ca(2+) store into which it was integrated. Here we assessed the properties of an endogenous tg-insensitive nonmitochondrial Ca(2+) store in A7r5 and 16HBE14o- cells, which express a mammalian homologue of Pmr1. The tg-insensitive Ca(2+) store was considerably less leaky for Ca(2+) than the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA)-containing Ca(2+) store. Moreover like for the worm Pmr1 Ca(2+) pump expressed in COS-1 cells, Ca(2+) accumulation into the endogenous tg-insensitive store showed a 2 orders of magnitude lower sensitivity to cyclopiazonic acid than the SERCA-mediated transport. 2,5-Di-(tert-butyl)-1,4-benzohydroquinone was only a very weak inhibitor of the tg-insensitive Ca(2+) uptake in A7r5 and 16HBE14o- cells and in COS-1 cells overexpressing the worm Pmr1. Inositol 1,4,5-trisphosphate released 11% of the Ca(2+) accumulated in permeabilized A7r5 cells pretreated with tg with an EC(50) that was 5 times higher than for the SERCA-containing Ca(2+) store but failed to release Ca(2+) in 16HBE14o- cells. In the presence of tg, 15% of intact A7r5 cells responded to 10 microm arginine-vasopressin with a small rise in cytosolic Ca(2+) concentration after a long latency. In conclusion, A7r5 and 16HBE14o- cells express a Pmr1-containing Ca(2+) store with properties that differ substantially from the SERCA-containing Ca(2+) store.

Studies of cell division (mitosis and cytokinesis) by dynamic secondary ion mass spectrometry ion microscopy: LLC-PK1 epithelial cells as a model for subcellular isotopic imaging

The feasibility of the renal epithelial LLC-PK1 cell line as a model for cell division studies with secondary ion mass spectrometry (SIMS) was tested. In this cell line, cells undergoing all stages of mitosis and cytokinesis remained firmly attached to the substrate and could be cryogenically prepared. Fractured freeze-dried mitotic cells showed well-preserved organelles as revealed by fluorescence imaging of rhodamine-123 and C6-NBD-ceramide by confocal laser scanning microscopy. Secondary electron microscopy analysis of fractured freeze-dried dividing cells revealed minimal surface topography that does not interfere in isotopic imaging of both positive (39K, 23Na, 24Mg, 40Ca, etc.) and negative (31P, 35Cl, etc.) secondaries with a CAMECA IMS-3f ion microscope. Mitotic cells revealed well-preserved intracellular ionic composition of even the most diffusible ions (total concentrations of 39K+ and 23Na+) as revealed by K : Na ratios of approximately 10. Structurally damaged mitotic cells could be identified by their reduced K : Na ratios and an excessive loading of calcium. Quantitative three-dimensional SIMS analysis was required for studying subcellular calcium distribution in dividing cells. The LLC-PK1 model also allowed SIMS studies of M-phase arrested cells with mitosis-arresting drugs (taxol, monastrol and nocodazole). This study opens new avenues of cell division research related to ion fluxes and chemical composition with SIMS.

Baseline cytosolic Ca2+ oscillations derived from a non-endoplasmic reticulum Ca2+ store

Cytosolic Ca(2+) oscillations can be due to cycles of release and re-uptake of internally stored Ca(2+). To investigate the nature of these Ca(2+) stores, we expressed the Pmr1 Ca(2+) pump of Caenorhabditis elegans in COS-1 cells and pretreated the cells with thapsigargin to prevent Ca(2+) uptake by the sarco(endo)plasmic reticulum Ca(2+)-ATPase. Pmr1 co-localized with the Golgi-specific 58K protein and was targeted to a Ca(2+) store that was less leaky for Ca(2+) than the endoplasmic reticulum and whose inositol trisphosphate receptors were less sensitive to inositol trisphosphate and ATP than those in the endoplasmic reticulum. ATP-stimulated Pmr1-overexpressing cells responded after a latency to extracellular Ca(2+) with a regenerative Ca(2+) signal, which could be prevented by caffeine. They also produced very stable ilimaquinone-sensitive baseline Ca(2+) spikes, even in the presence of thapsigargin. Such responses never occurred in non-transfected cells or in cells that overexpressed the type-1 sarco(endo)plasmic reticulum Ca(2+)-ATPase. Abortive Ca(2+) spikes also occurred in histamine-stimulated untransfected HeLa cells pretreated with thapsigargin, and they too were inhibited by ilimaquinone. We conclude that the Pmr1-induced Ca(2+) store, which probably corresponds to the Golgi compartment, can play a crucial role in setting up baseline Ca(2+) spiking.

A role for calcium in stabilizing transport vesicle coats

Calcium has been implicated in regulating vesicle fusion reactions, but its potential role in regulating other aspects of protein transport, such as vesicle assembly, is largely unexplored. We find that treating cells with the membrane-permeable calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM), leads to a dramatic redistribution of the vesicle coat protein, coatomer, in the cell. We have used the cell-free reconstitution of coat-protomer I (COPI) vesicle assembly to characterize the mechanisms of this redistribution. We find that the recovery of COPI-coated Golgi vesicles is inhibited by the addition of BAPTA to the cell-free vesicle budding assay. When coatomer-coated membranes are incubated in the presence of calcium chelators, the membranes "uncoat," indicating that calcium is necessary for maintaining the integrity of the coat. This uncoating is reversed by the addition of calcium. Interestingly, BAPTA, a calcium chelator with fast binding kinetics, is more potent at uncoating the coatomer-coated membrane than EGTA, suggesting that a calcium transient or a calcium gradient is important for stabilizing COPI vesicle coat. The primary target for the effects of calcium on coatomer recruitment is a step that occurs after ADP-ribosylation factor binding to the membrane. We suggest that a calcium gradient may serve to regulate the timing of vesicle uncoating.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

The Golgi PMR1 P-type ATPase of Caenorhabditis elegans. Identification of the gene and demonstration of calcium and manganese transport

In recent years, it has been well established that the Ca(2+) concentration in the lumen of intracellular organelles is a key determinant of cell function. Despite the fact that essential functions of the Golgi apparatus depend on the Ca(2+) and Mn(2+) concentration in its lumen, little is known on the transport system responsible for ion accumulation. The Golgi ion pump PMR1 has been functionally studied only in yeast. In humans, mutations in the orthologous gene ATP2C1 cause Hailey-Hailey disease. We report here the identification of the PMR1 homologue in the model organism Caenorhabditis elegans and after ectopic expression the direct study of its ion transport in permeabilized COS-1 cells. The C. elegans genome is predicted to contain a single PMR1 orthologue on chromosome I. We found evidence for alternative splicing in the 5'-untranslated region, but no indication for the generation of different protein isoforms. C. elegans PMR1 overexpressed in COS-1 cells transports Ca(2+) and Mn(2+) with high affinity into the Golgi apparatus in a thapsigargin-insensitive manner. Part of the accumulated Ca(2+) can be released by inositol 1,4,5-trisphosphate, in agreement with the idea that the Golgi apparatus is an inositol 1,4,5-trisphosphate-sensitive Ca(2+) store.

Molecular mechanisms in intimal hyperplasia

Intimal hyperplasia is the process by which the cell population increases within the innermost layer of the arterial wall, such as occurs physiologically during closure of the ductus arteriosus and during involution of the uterus. It also occurs pathologically in pulmonary hypertension, atherosclerosis, after angioplasty, in transplanted organs, and in vein grafts. The underlying causes of intimal hyperplasia are migration and proliferation of vascular smooth muscle cells provoked by injury, inflammation, and stretch. This review discusses, at a molecular level, both the final common pathways leading to smooth muscle migration and proliferation and their (patho)-physiological triggers. It emphasizes the key roles played by growth factors and extracellular matrix-degrading metalloproteinases, which act in concert to remodel the extracellular matrix and permit cell migration and proliferation.

Transient reexpression of an embryonic autonomous growth phenotype by adult carotid artery smooth muscle cells after vascular injury

High rates of vascular smooth muscle cell (SMC) replication are observed, at least transiently, after injury to the arterial wall and contribute to the formation of a neointima. Neutralizing antibodies designed to inhibit growth of SMC have only been variably successful in inhibiting neointima formation, raising the possibility that neointimal cell proliferation involves unique growth mechanisms. This study examined the possibility that SMC isolated from injured rat carotid arteries would express an autonomous, mitogen-independent growth phenotype similar to that utilized by embryonic vascular SMC during periods of rapid growth. We found that primary cultures of SMC isolated 7 and 14 days after injury, times at which high in vivo replication rates were observed, demonstrated high intrinsic DNA synthetic rates compared to SMC isolated from uninjured arteries or at 2, 4, 21, and 28 days after injury where in vivo replication rates were far less. Subcultured SMC isolated from 7-day injured vessels (Neo7 SMC) exhibited a stable, autonomous growth phenotype, did not secrete detectable mitogenic activity, and had decreased alpha-actin and myosin expression compared to mitogen-dependent SMC. Heterokaryons constructed between autonomous Neo7 SMC and mitogen-dependent SMC exhibited a mitogen-dependent growth phenotype suggesting that nonautonomous SMC produce factors that actively inhibit autonomous growth. In contrast, heterokaryons constructed between Neo7 SMC and autonomous embryonic SMC retained an autonomous growth phenotype. We examined the expression of known tumor suppressors to determine if any of these factors played a role in inhibiting SMC autonomous growth. p27, p53, pRb, and PTEN were abundantly expressed by Neo7 SMC and e17 SMC under both basal and serum stimulated conditions. The data suggest that the mechanisms driving SMC replication during neointimal formation are self-driven and self-regulated, and that at specific times after injury, SMC escape normal growth suppressive mechanisms through the loss of intracellular growth suppressor activity.

Confocal laser scanning microscopy of calcium dynamics in living cells

Confocal laser scanning microscopy (CLSM) is widely used to monitor intracellular calcium levels in living cells loaded with calcium-sensitive fluorophores. This review examines the basic advantages and limitations of CLSM in in vivo imaging analyses of calcium dynamics. The benefits of utilizing ratioed images and dextran-conjugated fluorophores are addressed, and practical aspects of handling confocal datasets are outlined. After considering some relatively new microscopical methods that can be used in conjunction with conventional CLSM, possible future applications of confocal techniques in analyses of intracellular calcium dynamics are discussed.

Ultrastructural localization of nitric oxide synthase and endothelin in the renal and mesenteric arteries of the golden hamster: differences during and after arousal from hibernation

This is a study of the electron-immunocytochemical localization of nitric oxide synthase (type III) and endothelin in renal and mesenteric artery endothelial cells of normal (active) and hibernating hamsters, as well as hamsters exposed to the cold but not hibernating, and hamsters aroused for 2h following hibernation. In the renal artery of hibernating hamsters and cold-exposed hamsters, a subpopulation of nitric oxide synthase-positive endothelial cells displayed immunoprecipitate predominantly in the vicinity of the Golgi complex indicating intracellular translocation from the cytoplasm to the Golgi complex. In hibernating animals, the percentages of both nitric oxide synthase-positive and endothelin-positive endothelial cells were notably lower than those observed either in active, cold-exposed or aroused animals. These changes may reflect a reduced endothelial contribution to the maintenance of vascular tone in these vessels during hibernation and an upregulation of expression of nitric oxide synthase and endothelin in the endothelium early on during arousal from hibernation.

Imaging of total calcium in urediospore germlings of Uromyces by ion microscopy

Calcium has been implicated in growth and appressorium formation of urediospore germlings of the bean rust fungus, Uromyces appendiculatus. Using ion microscopy, a mass spectrometry-based imaging technique, intracellular stores of calcium were analyzed by direct imaging of total calcium in frozen freeze-dried germlings. Calcium concentration was calculated by ratioing and spatially registering (40)Ca to (12)C signals. Intracellular distributions of total potassium, sodium, magnesium, and carbon were similarly imaged in the same germlings for a direct comparison of their localizations to total calcium. Calcium was remarkably heterogeneous with highest concentrations (2 to 10 mM) in the mid-region of the germling between the nuclei and the apex. A similar distribution of Ca(2+) (assessed using Fluo-3) was also noted sequestered in organelles in live germlings. Distributions of remaining elements (K, Na, Mg, and C) were mostly homogeneous throughout the cytoplasm and nuclei of the fungal cell. The K/Na ratio ranged from 17 to 31.

Functional characterization of thapsigargin and agonist-insensitive acidic Ca2+ stores in Drosophila melanogaster S2 cell lines

The role of acidic intracellular calcium stores in calcium homeostasis was investigated in the Drosophila Schneider cell line 2 (S2) by means of free cytosolic calcium ([Ca2+]i) and intracellular pH (pHi) imaging together with measurements of total calcium concentrations within intracellular compartments. Both a weak base (NH4Cl, 15 mM) and a Na+/H+ ionophore (monensin, 10 microM) evoked cytosolic alkalinization followed by Ca2+ release from acidic intracellular Ca2+ stores. Pretreatment of S2 cells with either thapsigargin (1 microM), an inhibitor of endoplasmic reticulum Ca(2+)-ATPases, or with the Ca2+ ionophore ionomycin (10 microM) was without effect on the amplitude of Ca2+ release evoked by alkalinization. Application of the cholinergic agonist carbamylcholine (100 microM) to transfected S2-DM1 cells expressing a Drosophila muscarinic acetylcholine receptor (DM1) emptied the InsP3-sensitive Ca2+ store but failed to affect the amplitude of alkalinization-evoked Ca2+ release. Glycyl-L-phenylalanine-beta-naphthylamide (200 microM), a weak hydrophobic base known to permeabilize lysosomes by osmotic swelling, triggered Ca2+ release from internal stores, while application of brefeldin A (10 microM), an antibiotic which disperses the Golgi complex, resulted in a smaller increase in [Ca2+]i. These results suggest that the alkali-evoked calcium release is largely attributable to lysosomes, a conclusion that was confirmed by direct measurements of total calcium content of S2 organelles. Lysosomes and endoplasmic reticulum were the only organelles found to have concentrations of total calcium significantly higher than the cytosol. However, NH4Cl (15 mM) reduced the level of total calcium only in lysosomes. Depletion of acidic Ca2+ stores did not elicit depletion-operated Ca2+ entry. They were refilled upon re-exposure of cells to normal saline ([Ca2+]o = 2 mM), but not by thapsigargin-induced [Ca2+]i elevation in Ca(2+)-free saline.

Threshold-dependent DNA synthesis by pure pressure in human aortic smooth muscle cells: Gialpha-dependent and -independent pathways

Mechanical forces related to pressure and flow are important for the etiology of atherosclerosis and hypertension. We hypothesized the presence of mechanosensors that were solely sensitive to pure atmospheric pressure in the absence of shear and tensile stresses. A pressure-loading apparatus was set up to examine the effects of atmospheric pressure on human aortic smooth muscle cells (HASMC). Pressure application of 140 to 180 mmHg produced DNA synthesis in a pressure-dependent manner. In contrast, pressure of 120 mmHg or less produced no significant change. Both extracellular signal-regulated kinase and c-Jun N-terminal kinase activities, but not p38 activity, were stimulated by pressures of more than 160 mmHg. Pertussis toxin (PTx) completely inhibited the pressure-induced increase of DNA synthesis under the high pressure of 200 mmHg. These data suggest that HASMC have a mechanosensing cellular switch for DNA synthesis which is sensitive to pure atmospheric pressure, and that the molecular switch is activated by pressure of more than 140 mmHg. The activation mechanism consists of PTx-sensitive and -insensitive pathways, and the former is activated by high pure pressure.

The Golgi apparatus is an inositol 1,4,5-trisphosphate-sensitive Ca2+ store, with functional properties distinct from those of the endoplasmic reticulum

In the past few years, intracellular organelles, such as the endoplasmic reticulum, the nucleus and the mitochondria, have emerged as key determinants in the generation and transduction of Ca2+ signals of high spatio-temporal complexity. Little is known about the Golgi apparatus, despite the fact that Ca2+ within its lumen controls essential processes, such as protein processing and sorting. We report the direct monitoring of the [Ca2+] in the Golgi lumen ([Ca2+]Golgi) of living HeLa cells, using a specifically targeted Ca2+-sensitive photoprotein. With this probe, we show that, in resting cells, [Ca2+]Golgi is approximately 0.3 mM and that Ca2+ accumulation by the Golgi has properties distinct from those of the endoplasmic reticulum (as inferred by the sensitivity to specific inhibitors). Upon stimulation with histamine, an agonist coupled to the generation of inositol 1,4,5-trisphosphate (IP3), a large, rapid decrease in [Ca2+]Golgi is observed. The Golgi apparatus can thus be regarded as a bona fide IP3-sensitive intracellular Ca2+ store, a notion with major implications for the control of organelle function, as well as for the generation of local cytosolic Ca2+ signals.

Measurement of cytosolic, mitochondrial, and Golgi pH in single living cells with green fluorescent proteins

Many cellular events depend on a tightly compartmentalized distribution of H+ ions across membrane-bound organelles. However, measurements of organelle pH in living cells have been scarce. Several mutants of the Aequorea victoria green fluorescent protein (GFP) displayed a pH-dependent absorbance and fluorescent emission, with apparent pKa values ranging from 6.15 (mutations F64L/S65T/H231L) and 6.4 (K26R/F64L/S65T/Y66W/N146I/M153T/ V163A/N164H/H231L) to a remarkable 7.1 (S65G/S72A/T203Y/H231L). We have targeted these GFPs to the cytosol plus nucleus, the medial/trans-Golgi by fusion with galactosyltransferase, and the mitochondrial matrix by using the targeting signal from subunit IV of cytochrome c oxidase. Cells in culture transfected with these cDNAs displayed the expected subcellular localization by light and electron microscopy and reported local pH that was calibrated in situ with ionophores. We monitored cytosolic and nuclear pH of HeLa cells, and mitochondrial matrix pH in HeLa cells and in rat neonatal cardiomyocytes. The pH of the medial/trans-Golgi was measured at steady-state (calibrated to be 6.58 in HeLa cells) and after various manipulations. These demonstrated that the Golgi membrane in intact cells is relatively permeable to H+, and that Cl- serves as a counter-ion for H+ transport and likely helps to maintain electroneutrality. The amenability to engineer GFPs to specific subcellular locations or tissue targets using gene fusion and transfer techniques should allow us to examine pH at sites previously inaccessible.

Myogenic tone attenuates pressure-induced gene expression in isolated small arteries

This study was designed to determine whether pressure-induced expression of early response genes in the arterial wall is dependent on an increase in cell stretch or an increase in wall stress. Mesenteric arteries (245 to 385 microm in diameter) were isolated from Wistar rats and subjected to static pressures of either 90 mm Hg (control), 140 mm Hg, or 165 mm Hg for a period of 3 hours. Arteries developed a range of myogenic tone such that wall stresses in the 140 and 165 mm Hg arteries (1.60 to 4.44x10(6) dynes/cm2) were equivalent in some cases to those of controls (1.76 to 2.63x10(6) dynes/cm2). Vessels subjected to 140 or 165 mm Hg intraluminal pressure had diameters ranging from 74% to 104% of their relaxed diameter at 90 mm Hg, whereas control vessel diameters ranged from 88% to 100%. At the end of each experiment, vessels were fixed in 10% formalin, embedded in paraffin, and sectioned for in situ hybridization. Wall stress significantly correlated with c-myc mRNA and 18S rRNA expression. Gene expression did not correlate with vessel diameter, expressed as a percentage of the relaxed diameter at 90 mm Hg, ie, cell stretch. The expression of beta-actin mRNA did not differ between vessels and showed no correlation with wall stress, suggesting that the induction of c-myc mRNA and 18S rRNA was part of a specific response. These findings show that in an isolated artery, a pressure stimulus can be perceived as an increase in wall stress, independently of cell stretch. Therefore, wall stress may be the signaling parameter in hypertension where arteries are tonically constricted. The inhibition of gene expression by myogenic constriction may explain why hypertrophy takes place in large arteries during hypertension but not in arterioles where increased tone reduces wall stress.

Vasorelaxant properties of brefeldin A in rat aorta

The effects of brefeldin A, a putative specific agent that disassembles the Golgi apparatus were assessed on the contractility of de-endothelised rat aorta. Brefeldin A inhibited, either as pre- or as post-treatment, the contractions elicited by K+ (75 mM) or phenylephrine (10 microM), being significantly more potent upon the latter. The thapsigargin (1 microM)-induced rat aorta contraction was less sensitive to brefeldin A inhibition. Pre-treatment with brefeldin A (30-100 microM) did not affect phenylephrine-induced transient contractions in Ca2+-free medium, but strongly inhibited the phenylephrine-induced sustained contractions upon re-admission of Ca2+ to the medium. Brefeldin A was unable to prevent Ca2+ stores refilling. We concluded that brefeldin A inhibits Ca2+ entry but not the pathways activated after Ca2+ stores depletion or the pathways responsible for replenishment of these stores in rat aorta, presumably by disassembling the Golgi apparatus network.

PMR1, a Ca2+-ATPase in yeast Golgi, has properties distinct from sarco/endoplasmic reticulum and plasma membrane calcium pumps

PMR1, a P-type ATPase cloned from the yeast Saccharomyces cerevisiae, was previously localized to the Golgi, and shown to be required for normal secretory processes (Antebi, A., and Fink, G.R. (1992) Mol. Biol. Cell 3, 633-654). We provide biochemical evidence that PMR1 is a Ca2+-transporting ATPase in the Golgi, a hitherto unusual location for a Ca2+ pump. As a starting point for structure-function analysis using a mutagenic approach, we used the strong and inducible heat shock promoter to direct high level expression of PMR1 from a multicopy plasmid. Yeast lysates were separated on sucrose density gradients, and fractions assayed for organellar markers. PMR1 is found in fractions containing the Golgi marker guanosine diphosphatase, and is associated with an ATP-dependent, protonophore-insensitive 45Ca2+ uptake activity. This activity is virtually abolished in the absence of the expression plasmid. Furthermore, replacement of the active site aspartate within the phosphorylation domain had the expected effect of abolishing Ca2+ transport activity entirely. Interestingly, the mutant enzymes (Asp-371 --> Glu and Asp-371 --> Asn) demonstrated proper targeting to the Golgi, unlike analogous mutations in the related yeast H+-ATPase. Detailed characterization of calcium transport by PMR1 showed that sensitivity to inhibitors (vanadate, thapsigargin, and cyclopiazonic acid) and affinity for substrates (MgATP and Ca2+) were different from the previously characterized sarco/endoplasmic reticulum and plasma membrane Ca2+-ATPases. PMR1 therefore represents a new and distinct P-type Ca2+-ATPase. Because close homologs of PMR1 have been cloned from rat and other organisms, we suggest that Ca2+-ATPases in the Golgi will form a discrete subgroup that are important for functioning of the secretory pathway.

Nicotinic acid-adenine dinucleotide phosphate mobilizes Ca2+ from a thapsigargin-insensitive pool

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is a novel intracellular Ca2+ releasing agent recently described in sea-urchin eggs and egg homogenates. Ca2+ release by NAADP is independent of that induced by either inositol trisphosphate (InsP3) or cyclic adenosine dinucleotide phosphate (cADPR). We now report that in sea urchin egg homogenates, NAADP releases Ca2+ from a Ca2+ pool that is distinct from those that are sensitive to InsP3 and cADPR. This organelle has distinct Ca2+ uptake characteristics: it is insensitive to thapsigargin and cyclopiazoic acid, but maintenance of the pool shows some requirement for ATP. Although the different Ca2+ pools have different characteristics, there appears to be some degree of overlap or cross-talk between the NAADP- and cADPR/InsP3-sensitive Ca2+ pools. Ca(2+)-induced Ca2+ release is unlikely to account for the apparent overlap between stores, since NAADP-induced Ca2+ release, in contrast with that stimulated by cADPR, is not potentiated by bivalent cations.

Evidence for vacuolar-type proton pumps in nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores of insulin-secreting cells

This study examines whether acidic, vacuolar-type, proton-pump-carrying organelles of insulin-secreting cells (clonal endocrine pancreatic cell line INS-1) function as rapidly exchanging, inositol 1,4,5-trisphosphate-sensitive calcium stores. Calcium uptake into calcium stores will be modulated by the proton concentration within the stores, since calcium pumps in general appear to mediate a countertransport of calcium with protons. We therefore tested for sensitivity of calcium sequestration by nonmitochondrial stores (inhibition of mitochondrial calcium uptake by 2 microM ruthenium red) in saponin-permeabilized cells to proton-conducting ionophores and proton pump inhibition, using this as a marker for involvement of acidic organelles. Calcium sequestration was partially inhibited by the protonophores nigericin (10-50 microM) and carbonylcyanide m-chlorophenylhydrazone (CCCP; 20-50 microM), as well as by inclusion of 30 mM NH4Cl. Bafilomycin A1, a potent and selective inhibitor of vacuolar-type proton pumps, alone (1 - 500 nM) had no effect on calcium sequestration. however, it induced an inhibitory effect in the presence of nigericin or CCCP, even at low concentrations (5 microM) of these ionophores, lacking itself an inhibitory action on calcium sequestration. Bafilomycin A1 then was already maximally active at a concentration as low as 10 nM. Corres ponding to inhibition of total nonmitochondrial calcium sequestration, filling of inositol 1,4,5-trisphosphate-sensitive stores was decreased or even abolished by the protonophores alone or the protonophores combined with bafilomycin A1. We conclude that vacuolar-type proton pumps are present in at least a part of nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores in INS-1 cells. This assigns these stores to organelles such as secretory granules, the trans Golgi network, or endosomes. Luminal acidity of these stores will stimulate calcium sequestration by providing more protons for countertransport of calcium by calcium pumps. High concentrations of protonophores may be required for inhibitory effects because otherwise the proton pumps may be able to compensate sufficiently for ionophore-mediated proton loss. The lack of effect of bafilomycin A1 without protonophores may be due to a sufficient luminal buffering capacity or to preceding inhibition of the pump by an inside-positive transmembrane potential.

Agonist-releasable intracellular calcium stores and the phenomenon of store-dependent calcium entry. A novel hypothesis based on calcium stores in organelles of the endo- and exocytotic apparatus

Store-dependent calcium entry represents a little characterized calcium permeation pathway that is present in a variety of cell types. It is activated in an unknown way by depletion of intracellular calcium stores, for example in the course of phospholipase C stimulation. Current hypotheses propose that depleted calcium stores signal their filling state to this permeation pathway either by direct, protein-mediated interaction or by release of a small, diffusible messenger. The further characterization of store-dependent calcium entry will benefit from progress in the identification of the intracellular calcium storing compartments. Recent findings reviewed here suggest that these compartments include parts of the organelle system that is involved in endo- and exocytosis. This commentary describes a novel model of store-dependent calcium entry based on calcium stores belonging to the endo- and exocytotic organelle system. Such calcium stores could establish a tubule-like connection with the extracellular space, in analogy to the cellular compartments that contain the insulin-sensitive glucose transporter or the gastric proton pump. This connection will provide a pathway for store-dependent calcium entry. Under store depletion, extracellular calcium will permeate through the tubule-like connection into the store lumen and from there into the cytosol. The consequences of this model for the development of drugs modulating store-dependent calcium entry are discussed.

Aequorin targeted to the endoplasmic reticulum reveals heterogeneity in luminal Ca++ concentration and reports agonist- or IP3-induced release of Ca++

A chimeric protein (ERaeq) comprised of the invariant chain (Ii) of class II major histocompatability complex (MHC-II) and aequorin was localized in the endoplasmic reticulum (ER) of transfected human embryonal kidney 293 cells. The targeted aequorin resided in the lumen of the ER membrane system, including the nuclear cistern, and following addition of the chromophore coelenterazine underwent Ca(++)-activated chemiluminescence. The majority of chemiluminescence produced by coelenterazine treatment of ERaeq-expressing 293 cells was consumed rapidly (within 2-4 min) upon re-addition of Ca++ to coelenterazine-loaded cells, a finding consistent with very high Ca++ concentrations (approximately 10(-5)-10(-3) M Ca++ ion) inside the ER. However, following the initial rapid consumption of ERaeq chemiluminescence, the activity that remained (10-30% of total sample luminescence of permeabilized cells or 50-70% of total sample luminescence of intact cells) was found to produce a stable baseline corresponding to a Ca++ ion concentration < or = 1-2 microM. The stable baseline of luminescence observed following rapid consumption of the majority of the sample's activity was not derived from re-binding of fresh chromophore to spent photoprotein, suggesting that a minority fraction of the ER membrane system within which the ERaeq chimera was distributed contained a relatively low Ca++ concentration. Addition of IP3 to digitonin-permeabilized cells, or agonist treatment of intact cells decreased this residual signal. Luminescence recordings from cells expressing an ER-targeted aequorin with relatively high affinity for Ca++ thus reveal heterogeneity in luminal ER Ca++ concentration and permit observation of receptor- and IP3-activated release of Ca++ from the ER membrane system.