STIM1L is a new actin-binding splice variant involved in fast repetitive Ca2+ release.

Cytosolic Ca(2+) signals encoded by repetitive Ca(2+) releases rely on two processes to refill Ca(2+) stores: Ca(2+) reuptake from the cytosol and activation of a Ca(2+) influx via store-operated Ca(2+) entry (SOCE). However, SOCE activation is a slow process. It is delayed by >30 s after store depletion because stromal interaction molecule 1 (STIM1), the Ca(2+) sensor of the intracellular stores, must form clusters and migrate to the membrane before being able to open Orai1, the plasma membrane Ca(2+) channel. In this paper, we identify a new protein, STIM1L, that colocalizes with Orai1 Ca(2+) channels and interacts with actin to form permanent clusters. This property allowed the immediate activation of SOCE, a characteristic required for generating repetitive Ca(2+) signals with frequencies within seconds such as those frequently observed in excitable cells. STIM1L was expressed in several mammalian tissues, suggesting that many cell types rely on this Ca(2+) sensor for their Ca(2+) homeostasis and intracellular signaling.

Human muscle economy myoblast differentiation and excitation-contraction coupling use the same molecular partners, STIM1 and STIM2.

Our recent work identified store-operated Ca(2+) entry (SOCE) as the critical Ca(2+) source required for the induction of human myoblast differentiation (Darbellay, B., Arnaudeau, S., König, S., Jousset, H., Bader, C., Demaurex, N., and Bernheim, L. (2009) J. Biol. Chem. 284, 5370-5380). The present work indicates that STIM2 silencing, similar to STIM1 silencing, reduces myoblast SOCE amplitude and differentiation. Because myoblasts in culture can be induced to differentiate into myotubes, which spontaneously contract in culture, we used the same molecular tools to explore whether the Ca(2+) mechanism of excitation-contraction coupling also relies on STIM1 and STIM2. Live cell imaging of early differentiating myoblasts revealed a characteristic clustering of activated STIM1 and STIM2 during the first few hours of differentiation. Thapsigargin-induced depletion of endoplasmic reticulum Ca(2+) content caused STIM1 and STIM2 redistribution into clusters, and co-localization of both STIM proteins. Interaction of STIM1 and STIM2 was revealed by a rapid increase in fluorescence resonance energy transfer between CFP-STIM1 and YFP-STIM2 after SOCE activation and confirmed by co-immunoprecipitation of endogenous STIM1 and STIM2. Although both STIM proteins clearly contribute to SOCE and are required during the differentiation process, STIM1 and STIM2 are functionally largely redundant as overexpression of either STIM1 or STIM2 corrected most of the impact of STIM2 or STIM1 silencing on SOCE and differentiation. With respect to excitation-contraction, we observed that human myotubes rely also on STIM1 and STIM2 to refill their endoplasmic reticulum Ca(2+)-content during repeated KCl-induced Ca(2+) releases. This indicates that STIM2 is a necessary partner of STIM1 for excitation-contraction coupling. Thus, both STIM proteins are required and interact to control SOCE during human myoblast differentiation and human myotube excitation-contraction coupling.

VSOP/Hv1 proton channels sustain calcium entry, neutrophil migration, and superoxide production by limiting cell depolarization and acidification.

Neutrophils kill microbes with reactive oxygen species generated by the NADPH oxidase, an enzyme which moves electrons across membranes. Voltage-gated proton channels (voltage-sensing domain only protein [VSOP]/Hv1) are required for high-level superoxide production by phagocytes, but the mechanism of this effect is not established. We show that neutrophils from VSOP/Hv1-/- mice lack proton currents but have normal electron currents, indicating that these cells have a fully functional oxidase that cannot conduct protons. VSOP/Hv1-/- neutrophils had a more acidic cytosol, were more depolarized, and produced less superoxide and hydrogen peroxide than neutrophils from wild-type mice. Hydrogen peroxide production was rescued by providing an artificial conductance with gramicidin. Loss of VSOP/Hv1 also aborted calcium responses to chemoattractants, increased neutrophil spreading, and decreased neutrophil migration. The migration defect was restored by the addition of a calcium ionophore. Our findings indicate that proton channels extrude the acid and compensate the charge generated by the oxidase, thereby sustaining calcium entry signals that control the adhesion and motility of neutrophils. Loss of proton channels thus aborts superoxide production and causes a severe signaling defect in neutrophils.

Glucose-regulated protein 78: a new partner of p53 in trophoblast.

Although wild-type p53 protein is overexpressed in first trimester trophoblast, it is inactive towards its target genes Metalloproteinase 2 and 9. This seems to be due to a complex mechanism of inactivation and stabilization of p53 relying on the formation of protein complexes involving the N-terminus of p53. To detect the proteins associated with this sequence, we incubated biotinylated p53 N-terminal peptide in cytotrophoblastic cell medium 24 h before lysis of cells. We purified the proteins retained on biotinylated peptide using a neutravidin affinity column. Proteins were then identified by peptide mass finger printing followed or not by peptide fragmentation sequencing. Among these proteins, we identified glucose-regulated protein 78 (GRP78) and verified its interaction with p53 in trophoblastic cells by immunoprecipitation and Western blot analysis. Moreover, the decreased expression of GRP78 induced by GRP78siRNA or versipelostatin decreased the formation of high molecular weight p53 complexes and p53 monomer and increased trophoblastic invasion. These results suggest that GRP78 is involved in inactivation and stabilization of p53 and in the regulation of trophoblastic invasion.

Local and global calcium signals associated with the opening of neuronal alpha7 nicotinic acetylcholine receptors.

Neuronal nicotinic acetylcholine receptors (nAChRs) are Ca(2+)-permeable ligand-gated channels widely expressed in the central and peripheral nervous system. One of the most Ca(2+) selective isoform is the homopentameric alpha7-nAChR implicated in schizophrenia. The activity of alpha7-nAChRs is usually recorded electrophysiologically, which limits the amount of information obtained. Here, we used fluorescence imaging to record Ca(2+) transients associated with activation of the alpha7-nAChR in neuroblastoma cells stably expressing human alpha7-nAChRs. Application of nicotine (50 microM) consistently evoked transient (30s), stereotyped Ca(2+) responses that were inhibited by the selective alpha7-nAChRs antagonists methyllycaconitine (MLA) and alpha-bungarotoxin, and greatly increased and prolonged by the allosteric modulator PNU-120596 (1 microM). Unexpectedly, brief (1-5s), repetitive Ca(2+) transients of sub-micrometric dimension were observed in filopodia of cells expressing alpha7-nAChR. PNU-120596 increased the frequency and slowed the decay kinetics of these miniature Ca(2+) elevations, which were insensitive to ryanodine, preserved during hyperpolarisation, and prevented by MLA, alpha-bungarotoxin, or Ca(2+) removal. Global Ca(2+) responses were also recorded in ganglion cells of embryo chicken retina during co-application of PNU-120596 and nicotine, together with whole-cell currents and brief current bursts. These data demonstrate that Ca(2+) signals generated by alpha7-nAChRs can be recorded optically both in cell lines and in intact tissues. The possibility to image miniature Ca(2+) signals enables to map the location of functional alpha7-nAChR channel clusters within cells and to analyze their single channel properties optically. Deciphering the rich pattern of intracellular Ca(2+) signals generated by the activity of the alpha7-nAChRs will reveal the physiological role of these receptor-channels.

STIM1- and Orai1-dependent store-operated calcium entry regulates human myoblast differentiation.

Our previous work on human myoblasts suggested that a hyperpolarization followed by a rise in [Ca(2+)](in) involving store-operated Ca(2+) entry (SOCE) channels induced myoblast differentiation. Advances in the understanding of the SOCE pathway led us to examine more precisely its role in post-natal human myoblast differentiation. We found that SOCE orchestrated by STIM1, the endoplasmic reticulum Ca(2+) sensor activating Orai Ca(2+) channels, is crucial. Silencing STIM1, Orai1, or Orai3 reduced SOCE amplitude and myoblast differentiation, whereas Orai2 knockdown had no effect. Conversely, overexpression of STIM1 with Orai1 increased SOCE and accelerated myoblast differentiation. STIM1 or Orai1 silencing decreased resting [Ca(2+)](in) and intracellular Ca(2+) store content, but correction of these parameters did not rescue myoblast differentiation. Remarkably, SOCE amplitude correlated linearly with the expression of two early markers of myoblast differentiation, MEF2 and myogenin, regardless of the STIM or Orai isoform that was silenced. Unexpectedly, we found that the hyperpolarization also depends on SOCE, placing SOCE upstream of K(+) channel activation in the signaling cascade that controls myoblast differentiation. These findings indicate that STIM1 and Orai1 are key molecules for the induction of human myoblast differentiation.

In vitro activated human T lymphocytes very efficiently attach to allogenic multipotent mesenchymal stromal cells and transmigrate under them.

The regulatory effect of human multipotent mesenchymal stromal cells (MSC) on allogenic T lymphocytes is extremely powerful and of important clinical relevance, but the mechanisms underlying this process are not fully elucidated. We report here that T lymphocytes activated with a sub-mitogenic stimulus such as phytohemaglutinin alone (PHA), or with mitogenic stimuli such as PHA + interleukin-2 (P-IL2), or immobilized anti-CD3 + anti-CD28 mAb (a3-28), tightly bound allogenic MSC and transmigrated within 4 h under them, where they remained for approximately 60 h. Allogenic MSC induced T cell proliferation in cultures containing sub-mitogenic PHA concentrations, and inhibited the mitogenic effect of P-IL2 or a3-28. Anti-gamma-IFN mAb or L-tryptophan complementation partially restored proliferation in P-IL2 and a3-28 cultures, whereby gamma-IFN-synthesizing CD3+ cells were detectable. MSC-lymphocyte contact hindrance using transwells abrogated proliferation in PHA cultures, restored it integrally in P-IL2 cultures, and partially in a3-28 cultures. These data suggest that MSC-induced T lymphocyte regulation results from the combination of various processes. Allogenic cell-cell contact, as demonstrated by the PHA co-cultures is per se stimulatory, whereas gamma-IFN synthesized by activated T lymphocytes, which activates indolamine 2,3-dioxygenase in MSC, and L-tryptophan depletion, which is induced by this enzyme, are inhibitory. Transmigration is nevertheless pivotal for the establishment of the inhibition by these mediators because it targets lymphocytes under the stroma in small extracellular spaces surrounded by MSC, where L-tryptophan is efficiently destroyed, leading to T lymphocyte proliferation arrest. In conclusion lymphocyte transmigration under allogenic MSC potentiates the inhibitory effect of soluble mediators generated by these cells.

NOX5 is expressed at the plasma membrane and generates superoxide in response to protein kinase C activation.

NOX5 is a ROS-generating NADPH oxidase which contains an N-terminal EF-hand region and can be activated by cytosolic Ca(2+) elevations. However the C-terminal region of NOX5 also contains putative phosphorylation sites. In this study we used HEK cells stably expressing NOX5 to analyze the size and subcellular localization of the NOX5 protein, its mechanisms of activation, and the characteristics of the ROS released. We demonstrate that NOX5 can be activated both by the protein kinase C activating phorbol esther PMA and by the Ca(2+) ionophore ionomycin. The PMA- but not the ionomycin-dependent activation can be inhibited by protein kinase C inhibitors. NOX5 activity is inhibited by submicromolar concentrations of diphenyl iodonium (DPI), but not by apocynin. Western blot analysis showed a lower ( approximately 70 kDa) than expected (82 kDa) molecular mass. Two arguments suggest that NOX5 is at least partially expressed on the plasma membrane: (i) the membrane-impermeant superoxide was readily detected by extracellular probes, and (ii) immunofluorescent labeling of NOX5 detected a fraction of the NOX5 protein at the plasma membrane. In summary, we demonstrate that NOX5 can be found intracellularly and at the cell surface. We also describe that it can be activated through protein kinase C, in addition to its Ca(2+) activation.

Calcium sources used by post-natal human myoblasts during initial differentiation.

Increases in cytoplasmic Ca(2+) are crucial for inducing the initial steps of myoblast differentiation that ultimately lead to fusion; yet the mechanisms that produce this elevated Ca(2+) have not been fully resolved. For example, it is still unclear whether the increase comes exclusively from membrane Ca(2+) influx or also from Ca(2+) release from internal stores. To address this, we investigated early differentiation of myoblast clones each derived from single post-natal human satellite cells. Initial differentiation was assayed by immunostaining myonuclei for the transcription factor MEF2. When Ca(2+) influx was eliminated by using low external Ca(2+) media, we found that approximately half the clones could still differentiate. Of the clones that required influx of external Ca(2+), most clones used T-type Ca(2+) channels, but others used store-operated channels as influx-generating mechanisms. On the other hand, clones that differentiated in low external Ca(2+) relied on Ca(2+) release from internal stores through IP(3) receptors. Interestingly, by following clones over time, we observed that some switched their preferred Ca(2+) source: clones that initially used calcium release from internal stores to differentiate later required Ca(2+) influx and inversely. In conclusion, we show that human myoblasts can use three alternative mechanisms to increase cytoplasmic Ca(2+) at the onset of the differentiation process: influx through T-types Ca(2+) channels, influx through store operated channels and release from internal stores through IP(3) receptors. In addition, we suggest that, probably because Ca(2+) elevation is essential during initial differentiation, myoblasts may be able to select between these alternate Ca(2+) pathways.

Pax6-induced alteration of cell fate: shape changes, expression of neuronal alpha tubulin, postmitotic phenotype, and cell migration.

The transcription factor Pax6 plays an important role in the development of the central nervous system. To understand its mechanism of action, we transduced HeLa cells with a Pax6-expressing lentiviral vector. Upon transduction, HeLa cells markedly changed shape and formed neuritelike extensions. Pax6-transduced HeLa cells expressed high levels of neuronal alpha3 tubulin, demonstrating a partial transdifferentiation towards a neuronal phenotype. Neurons are postmitotic cells. Pax6-transduced HeLa cells became postmitotic through mechanisms involving up-regulation of p53 and cyclin-dependent kinase inhibitor p21. One of the most striking effects of Pax6 was observed by time-lapse videomicroscopy: cells started to dissociate from cell clusters and displayed intense migratory activity. Migration was accompanied by dynamic and reversible shape changes. Our results identified three elements of Pax6 action: (i) expression of neuron-specific genes; (ii) establishment of a postmitotic phenotype; and (iii) involvement in the regulation of cell shape and cell migration.

Subplasmalemmal mitochondria modulate the activity of plasma membrane Ca2+-ATPases.

Mitochondria are dynamic organelles that modulate cellular Ca2+ signals by interacting with Ca2+ transporters on the plasma membrane or the endoplasmic reticulum (ER). To study how mitochondria dynamics affects cell Ca2+ homeostasis, we overexpressed two mitochondrial fission proteins, hFis1 and Drp1, and measured Ca2+ changes within the cytosol and the ER in HeLa cells. Both proteins fragmented mitochondria, decreased their total volume by 25-40%, and reduced the fraction of subplasmalemmal mitochondria by 4-fold. The cytosolic Ca2+ signals elicited by histamine were unaltered in cells lacking subplasmalemmal mitochondria as long as Ca2+ was present in the medium, but the signals were significantly blunted when Ca2+ was removed. Upon Ca2+ withdrawal, the free ER Ca2+ concentration decreased rapidly, and hFis1 cells were unable to respond to repetitive histamine stimulations. The loss of stored Ca2+ was due to an increased activity of plasma membrane Ca2+-ATPase (PMCA) pumps and was associated with an increased influx of Ca2+ and Mn2+ across store-operated Ca2+ channels. The increased Ca2+ influx compensated for the loss of stored Ca2+, and brief Ca2+ additions between successive agonist stimulations fully corrected subsequent histamine responses. We propose that the lack of subplasmalemmal mitochondria disrupts the transfer of Ca2+ from plasma membrane channels to the ER and that the resulting increase in subplasmalemmal [Ca2+] up-regulates the activity of PMCA. The increased Ca2+ extrusion promotes ER depletion and the subsequent activation of store-operated Ca2+ channels. Cells thus adapt to the lack of subplasmalemmal mitochondria by relying on external rather than on internal Ca2+ for signaling.

Gelsolin mediates calcium-dependent disassembly of Listeria actin tails.

The role of intracellular Ca2+ in the regulation of actin filament assembly and disassembly has not been clearly defined. We show that reduction of intracellular free Ca2+ concentration ([Ca2+]i) to <40 nM in Listeria monocytogenes-infected, EGFP-actin-transfected Madin-Darby canine kidney cells results in a 3-fold lengthening of actin filament tails. This increase in tail length is the consequence of marked slowing of the actin filament disassembly rate, without a significant change in assembly rate. The Ca2+-sensitive actin-severing protein gelsolin concentrates in the Listeria rocket tails at normal resting [Ca2+]i and disassociates from the tails when [Ca2+]i is lowered. Reduction in [Ca2+]i also blocks the severing activity of gelsolin, but not actin-depolymerizing factor (ADF)/cofilin microinjected into Listeria-infected cells. In Xenopus extracts, Listeria tail lengths are also calcium-sensitive, markedly shortening on addition of calcium. Immunodepletion of gelsolin, but not Xenopus ADF/cofilin, eliminates calcium-sensitive actin-filament shortening. Listeria tail length is also calcium-insensitive in gelsolin-null mouse embryo fibroblasts. We conclude that gelsolin is the primary Ca2+-sensitive actin filament recycling protein in the cell and is capable of enhancing Listeria actin tail disassembly at normal resting [Ca2+]i (145 nM). These experiments illustrate the unique and complementary functions of gelsolin and ADF/cofilin in the recycling of actin filaments.

Extracellular hypotonicity increases Na,K-ATPase cell surface expression via enhanced Na+ influx in cultured renal collecting duct cells.

In the renal collecting duct (CD), the Na,K-ATPase, which provides the driving force for Na+ absorption, is under tight multifactorial control. Because CD cells are physiologically exposed to variations of interstitial and tubular fluid osmolarities, the effects of extracellular anisotonicity on Na,K-ATPase cell surface expression were studied. Results show that hypotonic conditions increased, whereas hypertonic conditions had no effect on Na,K-ATPase cell surface expression in confluent mpkCCDcl4 cells. Incubating cells with amphotericin B, which increases [Na+]i, under isotonic or anisotonic conditions, revealed that Na,K-ATPase recruitment to the cell surface was not directly related to variations of cell volume and osmolarity. The effects of amphotericin B and extracellular hypotonicity were not additive, and both were prevented by protein kinase A and proteasome inhibitors, suggesting a common mechanism of action. In line with this hypothesis, extracellular hypotonicity induced a sustained stimulation of the amiloride-sensitive short-circuit current, indicating increased Na+ influx through the apical epithelial Na+ channel. Moreover, inhibiting apical Na+ entry by amiloride, a blocker of epithelial Na+ channel, or incubating cells in Na+ -free medium prevented the cell surface recruitment of Na,K-ATPase in response to extracellular hypotonicity. Altogether, these findings strongly suggest that extracellular hypotonicity stimulates apical Na+ influx leading to increased [Na+]i, protein kinase A activation, and recruitment of Na,K-ATPase units to the cell surface of mpkCCDcl4 cells.

Membrane hyperpolarization triggers myogenin and myocyte enhancer factor-2 expression during human myoblast differentiation.

It is widely thought that myogenin is one of the earliest detectable markers of skeletal muscle differentiation. Here we show that, during human myoblast differentiation, an inward rectifier K(+) channel (Kir2.1) and its associated hyperpolarization trigger expression and activity of the myogenic transcription factors, myogenin and myocyte enhancer factor-2 (MEF2). Furthermore, Kir2.1 current precedes and is required for the developmental increase in expression/activity of myogenin and MEF2. Drugs or antisense reducing Kir2.1 current diminished or suppressed fusion as well as expression/activity of myogenin and MEF2. In contrast, LY294002, an inhibitor of phosphatidylinositol 3-kinase (a pathway controlling initiation of the myogenic program) that inhibited both myogenin/MEF2 expression and fusion, did not affect Kir2.1 current. This non-blockade by LY294002 indicates that Kir2.1 acts upstream of myogenin and MEF2. We propose that Kir2.1 channel activation is a required key early event that initiates myogenesis by turning on myogenin and MEF2 transcription factors via a hyperpolarization-activated Ca(2+)-dependent pathway.

Intracellular transport of calcium from plasma membrane to mitochondria in adrenal H295R cells: implication for steroidogenesis.

Angiotensin II and extracellular potassium stimulate aldosterone production in adrenal glomerulosa cells by mobilizing the calcium messenger system. This response requires calcium influx across the plasma membrane, followed by calcium uptake into the mitochondria. It has been proposed that calcium is transported to the mitochondria via the lumen of the endoplasmic reticulum, acting as a kind of intracellular calcium pipeline. This hypothesis has been tested in the present study by measuring intramitochondrial calcium variations in H295R cells with a new fluorescent calcium probe, ratiometric pericam. Calyculin A, a protein phosphatase inhibitor, induced the formation of a large cortical layer of actin filaments, removing the peripheral endoplasmic reticulum away from the plasma membrane and thereby physically uncoupling the calcium channels from the pipeline. The mitochondrial calcium response to potassium was markedly reduced after calyculin treatment, but that of AngII was unaffected. Under the same conditions, potassium-stimulated pregnenolone and aldosterone production was significantly reduced, whereas the steroidogenic response to AngII remained unchanged. The inhibitory action of calyculin A on the responses to potassium was not mediated by a modification of the calcium channel activity and was not accompanied by a reduction of the cytosolic calcium response. It therefore appears that, in H295R cells, the organization of the actin cytoskeleton at the cell periphery influences the steroidogenic action of potassium, but not the response to angiotensin II. The response to potassium is proposed to be dependent on the endoplasmic reticulum-mediated transfer of calcium entering through plasma membrane calcium channels to the mitochondria.

Acceleration of human myoblast fusion by depolarization: graded Ca2+ signals involved.

We have previously shown that human myoblasts do not fuse when their voltage fails to reach the domain of a window T-type Ca(2+) current. We demonstrate, by changing the voltage in the window domain, that the Ca(2+) signal initiating fusion is not of the all-or-none type, but can be graded and is interpreted as such by the differentiation program. This was carried out by exploiting the properties of human ether-à-go-go related gene K(+) channels that we found to be expressed in human myoblasts. Methanesulfonanilide class III antiarrhythmic agents or antisense-RNA vectors were used to suppress completely ether-à-go-go related gene current. Both procedures induced a reproducible depolarization from -74 to -64 mV, precisely in the window domain where the T-type Ca(2+) current increases with voltage. This 10 mV depolarization raised the cytoplasmic free Ca(2+) concentration, and triggered a tenfold acceleration of myoblast fusion. Our results suggest that any mechanism able to modulate intracellular Ca(2+) concentration could affect the rate of myoblast fusion.

Calreticulin differentially modulates calcium uptake and release in the endoplasmic reticulum and mitochondria.

To study the role of calreticulin in Ca(2+) homeostasis and apoptosis, we generated cells inducible for full-length or truncated calreticulin and measured Ca(2+) signals within the cytosol, the endoplasmic reticulum (ER), and mitochondria with "cameleon" indicators. Induction of calreticulin increased the free Ca(2+) concentration within the ER lumen, [Ca(2+)](ER), from 306 +/- 31 to 595 +/- 53 microm, and doubled the rate of ER refilling. [Ca(2+)](ER) remained elevated in the presence of thapsigargin, an inhibitor of SERCA-type Ca(2+) ATPases. Under these conditions, store-operated Ca(2+) influx appeared inhibited but could be reactivated by decreasing [Ca(2+)](ER) with the low affinity Ca(2+) chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. In contrast, [Ca(2+)](ER) decreased much faster during stimulation with carbachol. The larger ER release was associated with a larger cytosolic Ca(2+) response and, surprisingly, with a shorter mitochondrial Ca(2+) response. The reduced mitochondrial signal was not associated with visible morphological alterations of mitochondria or with disruption of the contacts between mitochondria and the ER but correlated with a reduced mitochondrial membrane potential. Altered ER and mitochondrial Ca(2+) responses were also observed in cells expressing an N-truncated calreticulin but not in cells overexpressing calnexin, a P-domain containing chaperone, indicating that the effects were mediated by the unique C-domain of calreticulin. In conclusion, calreticulin overexpression increases Ca(2+) fluxes across the ER but decreases mitochondrial Ca(2+) and membrane potential. The increased Ca(2+) turnover between the two organelles might damage mitochondria, accounting for the increased susceptibility of cells expressing high levels of calreticulin to apoptotic stimuli.