Single channel properties and regulated expression of Ca(2+) release-activated Ca(2+) (CRAC) channels in human T cells

Passive transport of Ca2+ ions through lipid bilayers imaged by widefield second harmonic microscopy

In biology, release of Ca2+ ions in the cytosol is essential to trigger or control many cell functions. Calcium signaling acutely depends on lipid membrane permeability to Ca2+. For proper understanding of membrane permeability to Ca2+, both membrane hydration and the structure of the hydrophobic core must be taken into account. Here, we vary the hydrophobic core of bilayer membranes and observe different types of behavior in high-throughput wide-field second harmonic imaging. Ca2+ translocation is observed through mono-unsaturated (DOPC:DOPA) membranes, reduced upon the addition of cholesterol, and completely inhibited for branched (DPhPC:DPhPA) and poly-unsaturated (SLPC:SLPA) lipid membranes. We propose, using molecular dynamics simulations, that ion transport occurs through ion-induced transient pores, which requires nonequilibrium membrane restructuring. This results in different rates at different locations and suggests that the hydrophobic structure of lipids plays a much more sophisticated regulating role than previously thought.

Neglected wardens: T lymphocyte ryanodine receptors

Ryanodine receptors (RyRs) are intracellular Ca²⁺ release channels ubiquitously expressed in various cell types. RyRs were extensively studied in striated muscle cells due to their crucial role in muscle contraction. In contrast, the role of RyRs in Ca²⁺ signalling and functions in non-excitable cells, such as T lymphocytes, remains poorly understood. Expression of different isoforms of RyRs was shown in primary T cells and T cell lines. In T cells, RyRs co-localize with the plasmalemmal store-operated Ca²⁺ channels of the Orai family and endoplasmic reticulum Ca²⁺ sensing Stim family proteins and are activated by store-operated Ca²⁺ entry and pyridine nucleotide metabolites, the intracellular second messengers generated upon stimulation of T cell receptors. Experimental data indicate that together with d-myo-inositol 1,4,5-trisphosphate receptors, RyRs regulate intercellular Ca²⁺ dynamics by controlling Ca²⁺ concentration within the lumen of the endoplasmic reticulum and, consequently, store-operated Ca²⁺ entry. Gain-of-function mutations, genetic deletion or pharmacological inhibition of RyRs alters T cell Ca²⁺ signalling and effector functions. The picture emerging from the collective data shows that RyRs are the essential regulators of T cell Ca²⁺ signalling and can be potentially used as molecular targets for immunomodulation or T cell-based diagnostics of the disorders associated with RyRs dysregulation.

The intermediate-conductance calcium-activated potassium channel KCa3.1 contributes to alkalinization-induced vascular calcification in vitro

OBJECTIVE

In order to find new strategies for the prevention of vascular calcification in uremic individuals especially treated by dialysis and develop novel therapeutic targets in vascular calcification, we explore the role of KCa3.1 in alkalinization-induced VSMCs calcification in vitro.

METHOD

Rat VSMCs calcification model was established by beta-glycerophosphate (β-GP, 10 mM) induction. The pH of Dulbecco's modified Eagle's medium (DMEM) was adjusted every 24 h with 10 mM HCl or 10 mM NaHCO3 . The mineralization was measured by Alizarin Red staining and O-cresolphthalein complex one method. mRNA and protein expression were detected by RT-PCR and Western blot or immunofluorescence. Ca2+ influx was measured by Elisa.

RESULT

The results indicated that alkalization induced an increase in Ca2+ influx to enhance VSMCs calcification. Furthermore, the increase of calcification was associated with the expression of KCa3.1 via advanced expression of osteoblastic differentiation markers alkaline phosphatase (ALP) and Runt-related transcription factor 2 (Runx2). Blocking KCa3.1 with TRAM-34 or shRNA vector can significantly lowered the effects of calcification in the activity of ALP and Runx2 expression.

CONCLUSION

Together all, our studies suggested that alkalinization can promote vascular calcification by upregulating KCa3.1 channel and enhancing osteogenic/chondrogenic differentiation by upregulating Runx2. The specific inhibitor TRAM-34 and KCa3.1-shRNA ameliorated VSMCs calcification by downregulating KCa3.1.

A549 in-silico 1.0: A first computational model to simulate cell cycle dependent ion current modulation in the human lung adenocarcinoma

Lung cancer is still a leading cause of death worldwide. In recent years, knowledge has been obtained of the mechanisms modulating ion channel kinetics and thus of cell bioelectric properties, which is promising for oncological biomarkers and targets. The complex interplay of channel expression and its consequences on malignant processes, however, is still insufficiently understood. We here introduce the first approach of an in-silico whole-cell ion current model of a cancer cell, in particular of the A549 human lung adenocarcinoma, including the main functionally expressed ion channels in the plasma membrane as so far known. This hidden Markov-based model represents the electrophysiology behind proliferation of the A549 cell, describing its rhythmic oscillation of the membrane potential able to trigger the transition between cell cycle phases, and it predicts membrane potential changes over the cell cycle provoked by targeted ion channel modulation. This first A549 in-silico cell model opens up a deeper insight and understanding of possible ion channel interactions in tumor development and progression, and is a valuable tool for simulating altered ion channel function in lung cancer electrophysiology.

Intracellular K+ and water content in human blood lymphocytes during transition from quiescence to proliferation

Many evidence shows that K⁺ ions are required for cell proliferation, however, changes in intracellular K⁺ concentration during transition of cells from quiescence to cycling are insufficiently studied. Here, we show using flame emission assay that a long-term increase in cell K⁺ content per g cell protein is a mandatory factor for transition of quiescent human peripheral blood lymphocytes (PBL) to proliferation induced by phytohemagglutinin, phorbol ester with ionomycin, and anti-CD3 antibodies with interleukin-2 (IL-2). The long-term increase in K⁺ content is associated with IL-2-dependent stage of PBL activation and accompanies the growth of small lymphocytes and their transformation into blasts. Inhibition of PBL proliferation with drugs specific for different steps of G0/G1/S transit prevented both blast-transformation and an increase in K⁺ content per cell protein. Determination of the water content in cells by measuring the density of cells in the Percoll gradient showed that, unlike the K⁺ content, the concentration of K⁺ in cell water remains unchanged, since water and K⁺ change in parallel. Correlation of proliferation with high cell K⁺ and water content has been confirmed by the data obtained in comparative study of PBL and permanently cycling Jurkat cells. Our data suggest that K⁺ is important for successful proliferation as the main intracellular ion that participates in regulation of cell water content during cell transition from quiescence to proliferation. We concluded that high K⁺ content in cells and the associated high water content is a characteristic feature of proliferating cells.

Proliferation-related changes in K+ content in human mesenchymal stem cells

Intracellular monovalent ions have been shown to be important for cell proliferation, however, mechanisms through which ions regulate cell proliferation is not well understood. Ion transporters may be implicated in the intracellular signaling: Na⁺ and Cl⁻ participate in regulation of intracellular pH, transmembrane potential, Ca²⁺ homeostasis. Recently, it is has been suggested that K⁺ may be involved in “the pluripotency signaling network”. Our study has been focused on the relations between K⁺ transport and stem cell proliferation. We compared monovalent cation transport in human mesenchymal stem cells (hMSCs) at different passages and at low and high densities of culture as well as during stress-induced cell cycle arrest and revealed a decline in K⁺ content per cell protein which was associated with accumulation of G1 cells in population and accompanied cell proliferation slowing. It is suggested that cell K⁺ may be important for successful cell proliferation as the main intracellular ion that participates in regulation of cell volume during cell cycle progression. It is proposed that cell K⁺ content as related to cell protein is a physiological marker of stem cell proliferation and may be used as an informative test for assessing the functional status of stem cells in vitro.

Three-dimensional spatio-temporal modelling of store operated Ca2+ entry: Insights into ER refilling and the spatial signature of Ca2+ signals

The spatial organisation of Orai channels and SERCA pumps within ER-PM junctions is important for enhancing the versatility and specificity of sub-cellular Ca²⁺ signals generated during store operated Ca²⁺ entry (SOCE). In this paper, we present a novel three dimensional spatio-temporal model describing Ca²⁺ dynamics in the ER-PM junction and sub-PM ER during SOCE. We investigate the role of Orai channel and SERCA pump location to provide insights into how these components shape the Ca²⁺ signals generated and affect ER refilling. We find that the organisation of Orai channels within the ER-PM junction controls the amplitude and shape of the Ca²⁺ profile but does not enhance ER refilling. The model shows that ER refilling is only weakly affected by the location of SERCA2b pumps within the ER-PM junction and that the placement of SERCA2a pumps within the ER-PM junction has much greater control over ER refilling.

Computational properties of mitochondria in T cell activation and fate

In this article, we review how mitochondrial Ca²⁺ transport (mitochondrial Ca²⁺ uptake and Na⁺/Ca²⁺ exchange) is involved in T cell biology, including activation and differentiation through shaping cellular Ca²⁺ signals. Based on recent observations, we propose that the Ca²⁺ crosstalk between mitochondria, endoplasmic reticulum and cytoplasm may form a proportional–integral–derivative (PID) controller. This PID mechanism (which is well known in engineering) could be responsible for computing cellular decisions. In addition, we point out the importance of analogue and digital signal processing in T cell life and implication of mitochondrial Ca²⁺ transport in this process.

Human T cells in silico: Modelling their electrophysiological behaviour in health and disease

Although various types of ion channels are known to have an impact on human T cell effector functions, their exact mechanisms of influence are still poorly understood. The patch clamp technique is a well-established method for the investigation of ion channels in neurons and T cells. However, small cell sizes and limited selectivity of pharmacological blockers restrict the value of this experimental approach. Building a realistic T cell computer model therefore can help to overcome these kinds of limitations as well as reduce the overall experimental effort. The computer model introduced here was fed off ion channel parameters from literature and new experimental data. It is capable of simulating the electrophysiological behaviour of resting and activated human CD4(+) T cells under basal conditions and during extracellular acidification. The latter allows for the very first time to assess the electrophysiological consequences of tissue acidosis accompanying most forms of inflammation.

The microfluidic multitrap nanophysiometer for hematologic cancer cell characterization reveals temporal sensitivity of the calcein-AM efflux assay

Cytometric studies utilizing flow cytometry or multi-well culture plate fluorometry are often limited by a deficit in temporal resolution and a lack of single cell consideration. Unfortunately, many cellular processes, including signaling, motility, and molecular transport, occur transiently over relatively short periods of time and at different magnitudes between cells. Here we demonstrate the multitrap nanophysiometer (MTNP), a low-volume microfluidic platform housing an array of cell traps, as an effective tool that can be used to study individual unattached cells over time with precise control over the intercellular microenvironment. We show how the MTNP platform can be used for hematologic cancer cell characterization by measuring single T cell levels of CRAC channel modulation, non-translational motility, and ABC-transporter inhibition via a calcein-AM efflux assay. The transporter data indicate that Jurkat T cells exposed to indomethacin continue to accumulate fluorescent calcein for over 60 minutes after calcein-AM is removed from the extracellular space.

Cardenolide glycosides from Elaeodendron australe var. integrifolium

Extracts from dried leaf and stems of Elaeodendron australe var. integrifolium (Celastraceae) collected in South East Queensland, Australia, were active in an assay that measured Ca(2+) driven expression of IL-2/luciferase designed to identify inhibitors of the ICRAC channel. Bioassay-guided isolation using C18 and polyamide column chromatography, HPLC (Phenyl and C18) and centrifugal partition chromatography (CPC) led to the isolation of digitoxigenin (1) and three cardenolide glycosides, glucoside 2, quinovoside 3 and the new natural product xyloside 4, as the active components with low nM activity in the reporter assay.

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

CRAC ion channels and airway defense reflexes in experimental allergic inflammation

Calcium release-activated calcium channels (CRAC) play unambiguous role in secretory functions of mast cells, T cells, and eosinophils. Less knowledge exists about the role of CRAC, widely distributed in airway smooth muscle (ASM) cells, in airway contractility. The presented study seeks to determine the possible participation of CRAC in ASM-based inflammatory airway disorders in guinea pigs. The acute and long-term administration (14 days) of the CRAC antagonist 3-fluoropyridine-4-carboxylic acid was used to examine the ASM contractility and associated reflexes in the guinea pig model of allergic airway inflammation by the following methods: (i) evaluation of specific airway resistance in vivo; (ii) evaluation of the contractile response of isolated ASM strips in vitro; and (iii) citric acid-induced cough reflex; (iv) measurement of exhaled NO levels (E(NO)). Allergic airway inflammation was induced by repetitive exposure of guinea pigs to ovalbumin (10(-6) M). The CRAC antagonist administered in a single dose to guinea pigs with confirmed allergic inflammation significantly reduced the cough response and the airway resistance, which corresponded with the findings in vitro. Long-term application of the CRAC antagonist had more strongly expressed effects. The results confirm the role of CRAC in the pathophysiology of experimental animal asthma and have a potential meaning for anti-asthma therapy.

Bidirectional coupling between ryanodine receptors and Ca2+ release-activated Ca2+ (CRAC) channel machinery sustains store-operated Ca2+ entry in human T lymphocytes

The expression and functional significance of ryanodine receptors (RyR) were investigated in resting and activated primary human T cells. RyR1, RyR2, and RyR3 transcripts were detected in human T cells. RyR1/2 transcript levels increased, whereas those of RyR3 decreased after T cell activation. RyR1/2 protein immunoreactivity was detected in activated but not in resting T cells. The RyR agonist caffeine evoked Ca(2+) release from the intracellular store in activated T cells but not in resting T cells, indicating that RyR are functionally up-regulated in activated T cells compared with resting T cells. In the presence of store-operated Ca(2+) entry (SOCE) via plasmalemmal Ca(2+) release-activated Ca(2+) (CRAC) channels, RyR blockers reduced the Ca(2+) leak from the endoplasmic reticulum (ER) and the magnitude of SOCE, suggesting that a positive feedback relationship exists between RyR and CRAC channels. Overexpression of fluorescently tagged RyR2 and stromal interaction molecule 1 (STIM1), an ER Ca(2+) sensor gating CRAC channels, in HEK293 cells revealed that RyR are co-localized with STIM1 in the puncta formed after store depletion. These data indicate that in primary human T cells, the RyR are coupled to CRAC channel machinery such that SOCE activates RyR via a Ca(2+)-induced Ca(2+) release mechanism, which in turn reduces the Ca(2+) concentration within the ER lumen in the vicinity of STIM1, thus facilitating SOCE by reducing store-dependent CRAC channel inactivation. Treatment with RyR blockers suppressed activated T cell expansion, demonstrating the functional importance of RyR in T cells.

Modulation of T cell activation by localized K⁺ accumulation at the immunological synapse--a mathematical model

The response of T cells to antigens (T cell activation) is marked by an increase in intracellular Ca²⁺ levels. Voltage-gated and Ca²⁺-dependent K⁺ channels control the membrane potential of human T cells and regulate Ca²⁺ influx. This regulation is dependent on proper accumulation of K⁺ channels at the immunological synapse (IS) a signaling zone that forms between a T cell and antigen presenting cell. It is believed that the IS provides a site for regulation of the activation response and that K⁺ channel inhibition occurs at the IS, but the underlying mechanisms are unknown. A mathematical model was developed to test whether K⁺ efflux through K⁺ channels leads to an accumulation of K⁺ in the IS cleft, ultimately reducing K⁺ channel function and intracellular Ca²⁺ concentration ([Ca²⁺](i)). Simulations were conducted in models of resting and activated T cell subsets, which express different levels of K⁺ channels, by varying the K⁺ diffusion constant and the spatial localization of K⁺ channels at the IS. K⁺ accumulation in the IS cleft was calculated to increase K⁺ concentration ([K⁺]) from its normal value of 5.0 mM to 5.2-10.0 mM. Including K⁺ accumulation in the model of the IS reduced calculated K⁺ current by 1-12% and consequently, reduced calculated [Ca²⁺](i) by 1-28%. Significant reductions in K⁺ current and [Ca²⁺](i) only occurred in activated T cell simulations when most K⁺ channels were centrally clustered at the IS. The results presented show that the localization of K⁺ channels at the IS can produce a rise in [K⁺] in the IS cleft and lead to a substantial decrease in K⁺ currents and [Ca²⁺](i) in activated T cells thus providing a feedback inhibitory mechanism during T cell activation.

Density of functional Ca2+ release-activated Ca2+ (CRAC) channels declines after T-cell activation

CRAC channel-mediated Ca(2+) entry plays a crucial role in T lymphocyte activation. Activated T cells display enhanced Ca(2+) signaling compared with resting T cells; this is partially attributed to activation-induced upregulation of CRAC channel expression. Orai and Stim family genes encode CRAC channel structural elements and regulatory proteins, respectively, but studies of their expression in T cells have led to controversial results. We re-examined Orai and Stim gene expression in resting, activated, and Jurkat T cells. Levels of Orai1 transcripts, encoding the human T cell CRAC channel subunit, were not significantly different between resting T and activated T cells. The total amount of all Orai transcripts was 2-fold higher in activated T cells than in resting T cells. Orai1 and total Orai transcript levels were significantly higher in Jurkat T cells than those in resting T cells. Stim expression did not vary significantly among cell types. Maximal whole-cell CRAC current amplitudes were 1.4-fold and 2.3-fold higher in activated and Jurkat T cells, respectively, than in resting T cells. Due to the small size of resting T cells, the surface CRAC channel density was 2.5-fold and 1.6-fold higher in resting T cells than in activated and Jurkat T cells, respectively. Predicted the rates of cytosolic Ca(2+) elevation calculated using the average values of CRAC channel currents and cell volumes showed that < 2-fold increase in the functional CRAC channel expression level cannot account for the enhanced rate of store-operated Ca(2+) entry in activated T cells compared with resting T cells.

Whole-cell recording of calcium release-activated calcium (CRAC) currents in human T lymphocytes

In T lymphocytes, depletion of Ca(2+) from the intracellular Ca(2+) store leads to activation of plasmalemmal Ca(2+) channels, called Calcium Release-Activated Calcium (CRAC) channels. CRAC channels play important role in regulation of T cell proliferation and gene expression. Abnormal CRAC channel function in T cells has been linked to severe combined immunodeficiency and autoimmune diseases. Studying CRAC channel function in human T cells may uncover new molecular mechanisms regulating normal immune responses and unravel the causes of related human diseases. Electrophysiological recordings of membrane currents provide the most accurate assessment of functional channel properties and their regulation. Electrophysiological assessment of CRAC channel currents in Jurkat T cells, a human leukemia T cell line, was first performed more than 20 years ago, however, CRAC current measurements in normal human T cells remains a challenging task. The difficulties in recording CRAC channel currents in normal T cells are compounded by the fact that blood-derived T lymphocytes are much smaller in size than Jurkat T cells and, therefore, the endogenous whole-cell CRAC currents are very low in amplitude. Here, we give a step-by-step procedure that we routinely use to record the Ca(2+) or Na(+) currents via CRAC channels in resting human T cells isolated from the peripheral blood of healthy volunteers. The method described here was adopted from the procedures used for recording the CRAC currents in Jurkat T cells and activated human T cells.

The functional network of ion channels in T lymphocytes

For more than 25 years, it has been widely appreciated that Ca2+ influx is essential to trigger T-lymphocyte activation. Patch clamp analysis, molecular identification, and functional studies using blockers and genetic manipulation have shown that a unique contingent of ion channels orchestrates the initiation, intensity, and duration of the Ca2+ signal. Five distinct types of ion channels--Kv1.3, KCa3.1, Orai1+ stromal interacting molecule 1 (STIM1) [Ca2+-release activating Ca2+ (CRAC) channel], TRPM7, and Cl(swell)--comprise a network that performs functions vital for ongoing cellular homeostasis and for T-cell activation, offering potential targets for immunomodulation. Most recently, the roles of STIM1 and Orai1 have been revealed in triggering and forming the CRAC channel following T-cell receptor engagement. Kv1.3, KCa3.1, STIM1, and Orai1 have been found to cluster at the immunological synapse following contact with an antigen-presenting cell; we discuss how channels at the synapse might function to modulate local signaling. Immuno-imaging approaches are beginning to shed light on ion channel function in vivo. Importantly, the expression pattern of Ca2+ and K+ channels and hence the functional network can adapt depending upon the state of differentiation and activation, and this allows for different stages of an immune response to be targeted specifically.

Intracellular calcium signalling patterns reflect the differentiation status of human T cells

Stimulation of T lymphocytes results in the calcium-dependent activation and repression of a large number of genes. However, the functional response made by different T cell subsets is heterogeneous, as their differentiation results in alterations in their sensitivity to activation and in the secretion of cytokines. Here we have investigated the patterns of calcium responses in CD4 and CD8 T cell subsets to help explain their different responses to activation. CD4(+) CD45RA(+) T cells isolated freshly from human blood gave a sustained calcium signal after stimulation, but this was smaller than elicited in CD4(+) CD45RO(+) cells. On in vitro differentiation of CD4(+) CD45RA(+) cells to CD45RO(+), the level of the cytoplasmic calcium response rose initially, but then declined steadily during further rounds of differentiation. The proportion producing an oscillatory calcium response or not responding was increased and differentiation was accompanied by a shift in the calcium between intracellular pools. CD8(+) T cells gave a smaller calcium response than paired CD4(+) T cells and showed a difference in the numbers of cells giving a transient, rather than sustained, calcium signal. The increase in oscillating cells in the CD4(+) CD45RO(+) population may reflect the heterogeneity of this population, particularly in terms of cytokine production. The changing patterns of calcium responses in T cells as they differentiate may explain variation in the cellular response to activation at different stages in their lifespan and emphasize the importance of the both the quantity and the quality of the calcium signal in determining the outcome of T cell activation.

Wogonin preferentially kills malignant lymphocytes and suppresses T-cell tumor growth by inducing PLCγ1- and Ca2+-dependent apoptosis

Herbs have successfully been used in traditional Chinese medicine for centuries. However, their curative mechanisms remain largely unknown. In this study, we show that Wogonin, derived from the traditional Chinese medicine Huang-Qin (Scutellaria baicalensis Georgi), induces apoptosis in malignant T cells in vitro and suppresses growth of human T-cell leukemia xenografts in vivo. Importantly, Wogonin shows almost no toxicity on T lymphocytes from healthy donors. Wogonin induces prolonged activation of PLCγ1 via H2O2 signaling in malignant T cells, which leads to sustained elevation of cytosolic Ca2+ in malignant but not normal T cells. Subsequently, a Ca2+ overload leads to disruption of the mitochondrial membrane. The selective effect of Wogonin is due to its differential regulation of the redox status of malignant versus normal T cells. In addition, we show that the L-type voltage-dependent Ca2+ channels are involved in the intracellular Ca2+ mobilization in T cells. Furthermore, we show that malignant T cells possess elevated amounts of voltage-dependent Ca2+ channels compared with normal T cells, which further enhance the cytotoxicity of Wogonin for malignant T cells. Taken together, our data show a therapeutic potential of Wogonin for the treatment of hematologic malignancies.

Orai1 and STIM1 move to the immunological synapse and are up-regulated during T cell activation

For efficient development of an immune response, T lymphocytes require long-lasting calcium influx through calcium release-activated calcium (CRAC) channels and the formation of a stable immunological synapse (IS) with the antigen-presenting cell (APC). Recent RNAi screens have identified Stim and Orai in Drosophila cells, and their corresponding mammalian homologs STIM1 and Orai1 in T cells, as essential for CRAC channel activation. Here, we show that STIM1 and Orai1 are recruited to the immunological synapse between primary human T cells and autologous dendritic cells. Both STIM1 and Orai1 accumulated in the area of contact between either resting or super-antigen (SEB)-pretreated T cells and SEB-pulsed dendritic cells, where they were colocalized with T cell receptor (TCR) and costimulatory molecules. In addition, imaging of intracellular calcium signaling in T cells loaded with EGTA revealed significantly higher Ca2+ concentration near the interface, indicating Ca2+ influx localized at the T cell/dendritic cell contact area. Expression of a dominant-negative Orai1 mutant blocked T cell Ca2+ signaling but did not interfere with the initial accumulation of STIM1, Orai1, and CD3 in the contact zone. In activated T cell blasts, mRNA expression for endogenous STIM1 and all three human homologs of Orai was up-regulated, accompanied by a marked increase in Ca2+ influx through CRAC channels. These results imply a positive feedback loop in which an initial TCR signal favors up-regulation of STIM1 and Orai proteins that would augment Ca2+ signaling during subsequent antigen encounter.

Mechanisms of host cell exit by the intracellular bacterium Chlamydia

The mechanisms that mediate the release of intracellular bacteria from cells are poorly understood, particularly for those that live within a cellular vacuole. The release pathway of the obligate intracellular bacterium Chlamydia from cells is unknown. Using a GFP-based approach to visualize chlamydial inclusions within cells by live fluorescence videomicroscopy, we identified that Chlamydia release occurred by two mutually exclusive pathways. The first, lysis, consisted of an ordered sequence of membrane permeabilizations: inclusion, nucleus and plasma membrane rupture. Treatment with protease inhibitors abolished inclusion lysis. Intracellular calcium signaling was shown to be important for plasma membrane breakdown. The second release pathway was a packaged release mechanism, called extrusion. This slow process resulted in a pinching of the inclusion, protrusion out of the cell within a cell membrane compartment, and ultimately detachment from the cell. Treatment of Chlamydia-infected cells with specific pharmacological inhibitors of cellular factors demonstrated that extrusion required actin polymerization, neuronal Wiskott-Aldrich syndrome protein, myosin II and Rho GTPase. The participation of Rho was unique in that it functioned late in extrusion. The dual nature of release characterized for Chlamydia has not been observed as a strategy for intracellular bacteria.

TRPM7 channel is sensitive to osmotic gradients in human kidney cells

TRPM7 (transient-receptor-potential melastatin 7) is an ion channel with alpha-kinase function. TRPM7 is divalent-selective and regulated by a range of receptor-stimulated second messenger pathways, intracellular Mg-nucleotides, divalent and polyvalent cations and pH. TRPM7 is ubiquitously found in mammalian cells, including kidney, the responsible organ for osmolyte regulation, posing the question whether the channel is osmosensitive. Recent reports investigated the sensitivity of native TRPM7-like currents to cell swelling with contradictory results. Here, we assess the sensitivity of TRPM7 to both hypo- and hyperosmotic conditions and explored the involvement of the channel's kinase domain. We find that hypotonicity facilitates TRPM7 at elevated intracellular magnesium and Mg.ATP (3-4 mm), but has no effect in the absence of these solutes. Hypertonic conditions, in contrast, inhibit TRPM7 with an IC(50) of 430 mosmol l(-1). This inhibitory effect is maintained in the complete absence of intra- and extracellular divalent ions, although shifted to higher osmolarities (IC(50) = 510 mosmol l(-1)). TRPM7 senses osmotic gradients rather than ionic strength and this is independent of cAMP or not affected by cytochalasin D treatment. Furthermore, the kinase-domain deletion mutant of TRPM7 shows a similar behaviour to osmolarity as the wild-type protein, both in the presence and absence of divalent ions. This indicates that at least part of the osmosensitivity resides in the channel domain. Physiologically, TRPM7 channels do not seem to play an active role in regulatory volume changes, but rather those volume changes modulate TRPM7 activity through changes in the cytosolic concentrations of free Mg, Mg-nucleotides and a further unidentified factor. We conclude that TRPM7 senses osmotically induced changes primarily through molecular crowding of solutes that affect channel activity.

A search engine to identify pathway genes from expression data on multiple organisms

Background

The completion of several genome projects showed that most genes have not yet been characterized, especially in multicellular organisms. Although most genes have unknown functions, a large collection of data is available describing their transcriptional activities under many different experimental conditions. In many cases, the coregulatation of a set of genes across a set of conditions can be used to infer roles for genes of unknown function.

Results

We developed a search engine, the Multiple-Species Gene Recommender (MSGR), which scans gene expression datasets from multiple organisms to identify genes that participate in a genetic pathway. The MSGR takes a query consisting of a list of genes that function together in a genetic pathway from one of six organisms: Homo sapiens, Drosophila melanogaster, Caenorhabditis elegans, Saccharomyces cerevisiae, Arabidopsis thaliana, and Helicobacter pylori. Using a probabilistic method to merge searches, the MSGR identifies genes that are significantly coregulated with the query genes in one or more of those organisms. The MSGR achieves its highest accuracy for many human pathways when searches are combined across species. We describe specific examples in which new genes were identified to be involved in a neuromuscular signaling pathway and a cell-adhesion pathway.

Conclusion

The search engine can scan large collections of gene expression data for new genes that are significantly coregulated with a pathway of interest. By integrating searches across organisms, the MSGR can identify pathway members whose coregulation is either ancient or newly evolved.

The Mg2+ and Mg(2+)-nucleotide-regulated channel-kinase TRPM7

TRPM7 is a member of the melastatin-related subfamily of TRP channels and represents a protein that contains both an ion channel and a kinase domain. The protein is ubiquitously expressed and represents the only ion channel known that is essential for cellular viability. TRPM7 is a divalent cation-selective ion channel that is permeable to Ca2+ and Mg2+, but also conducts essential metals such as Zn2+, Mn2+, and Co2+, as well as nonphysiologic or toxic metals such as Ni2+, Cd2+, Ba2+, and Sr2+. The channel is constitutively open but strongly downregulated by intracellular levels of Mg2+ and MgATP and other Mg-nucleotides. Reducing the cellular levels of these regulators leads to activation of TRPM7-mediated currents that exhibit a characteristic nonlinear current-voltage relationship with pronounced outward rectification due to divalent influx at physiologically negative voltages and monovalent outward fluxes at positive voltages. TRPM7 channel activity is also actively regulated following receptor-mediated changes in cyclic AMP (cAMP) and protein kinase A activity. This regulation as well as that by Mg-nucleotides requires a functional endogenous kinase domain. The function of the kinase domain is not completely understood, but may involve autophosphorylation of TRPM7 as well as phosphorylation of other target proteins such as annexin and myosin IIA heavy chain. Based on these properties, TRPM7 is currently believed to represent a ubiquitous homeostatic mechanism that regulates Ca2+ and Mg2+ fluxes based on the metabolic state of the cell. Physiologically, the channel may serve as a regulated transport mechanism for these ions that could affect cell adhesion, cell growth and proliferation, and even cell death under pathological stress such as anoxia.

Extracellular Ca2+ uptake by T cells might help to make a diagnosis of acute rejection

To more non-invasively diagnose acute rejection, we focused on the uptake of extracellular Ca2+ by T cells as a result of the activation of Ca2+ release activated Ca2+ channels. A full thickness of the skin allograft model was established using BN rats as the donors and LEW rats as the recipients, and similar LEW rats as both donors and recipients in the control group. After transplantation, the grafts were staged histopathologically in both rats. The uptake of extaracellular 45Ca pre T cell was measured in the macrophage-treated and non-treated groups, and the ratios between the two groups were calculated and the results were compared according to the post-operative day. No histopathological findings of acute rejection were observed in the control group. The allograft model group showed acute rejection histopathologically beginning on day 2 and increased through day 5. The macrophage-treated model/non-treated model 45Ca uptake ratio (CAR) was significantly higher in the allograft rats on day 2. No significant difference was observed on day 4. Measuring the uptake of extracellular Ca2+ by recipient T cells using donor macrophages might be useful for making a diagnosis of acute rejection.

Regulation of CRAC channel activity by recruitment of silent channels to a high open-probability gating mode

CRAC (calcium release-activated Ca²⁺) channels attain an extremely high selectivity for Ca²⁺ from the blockade of monovalent cation permeation by Ca²⁺ within the pore. In this study we have exploited the blockade by Ca²⁺ to examine the size of the CRAC channel pore, its unitary conductance for monovalent cations, and channel gating properties. The permeation of a series of methylammonium compounds under divalent cation-free conditions indicates a minimum pore diameter of 3.9 Å. Extracellular Ca²⁺ blocks monovalent flux in a manner consistent with a single intrapore site having an effective K_i of 20 μM at −110 mV. Block increases with hyperpolarization, but declines below −100 mV, most likely due to permeation of Ca²⁺. Analysis of monovalent current noise induced by increasing levels of block by extracellular Ca²⁺ indicates an open probability (P_o) of ∼0.8. By extrapolating the variance/mean current ratio to the condition of full blockade (P_o = 0), we estimate a unitary conductance of ∼0.7 pS for Na⁺, or three to fourfold higher than previous estimates. Removal of extracellular Ca²⁺ causes the monovalent current to decline over tens of seconds, a process termed depotentiation. The declining current appears to result from a reduction in the number of active channels without a change in their high open probability. Similarly, low concentrations of 2-APB that enhance I_CRAC increase the number of active channels while open probability remains constant. We conclude that the slow regulation of whole-cell CRAC current by store depletion, extracellular Ca²⁺, and 2-APB involves the stepwise recruitment of silent channels to a high open-probability gating mode.

Effect of malnutrition on K+ current in T lymphocytes

Severe malnutrition in children is frequently associated with infectious diseases. Animal models have been useful for studying the effects of malnutrition. One of the immunosuppressive mechanisms of malnutrition is inhibition of the activation of T lymphocytes. The voltage-dependent K(V) potassium channels are vital for the activation of T lymphocytes. The blockade of K(V) channels inhibits the activation of T lymphocytes. Malnutrition could affect the suitable synthesis of K(V) channels in T lymphocytes, producing changes in the magnitude and/or dependency of the voltage of the K+ current. We reported a significant decrease in the K+ current and activation to a 20 mV more positive membrane potential in T lymphocytes of rats with severe malnutrition. These results indicate that the diminution in the K+ conductance by alteration of K(V) channels in severe malnutrition is one of the mechanisms that inhibit the activation of T lymphocytes.

Potentiation of TRPM7 inward currents by protons

TRPM7 is unique in being both an ion channel and a protein kinase. It conducts a large outward current at +100 mV but a small inward current at voltages ranging from −100 to −40 mV under physiological ionic conditions. Here we show that the small inward current of TRPM7 was dramatically enhanced by a decrease in extracellular pH, with an ∼10-fold increase at pH 4.0 and 1–2-fold increase at pH 6.0. Several lines of evidence suggest that protons enhance TRPM7 inward currents by competing with Ca²⁺ and Mg²⁺ for binding sites, thereby releasing blockade of divalent cations on inward monovalent currents. First, extracellular protons significantly increased monovalent cation permeability. Second, higher proton concentrations were required to induce 50% of maximal increase in TRPM7 currents when the external Ca²⁺ and Mg²⁺ concentrations were increased. Third, the apparent affinity for Ca²⁺ and Mg²⁺ was significantly diminished at elevated external H⁺ concentrations. Fourth, the anomalous-mole fraction behavior of H⁺ permeation further suggests that protons compete with divalent cations for binding sites in the TRPM7 pore. Taken together, it appears that at physiological pH (7.4), Ca²⁺ and Mg²⁺ bind to TRPM7 and inhibit the monovalent cationic currents; whereas at high H⁺ concentrations, the affinity of TRPM7 for Ca²⁺ and Mg²⁺ is decreased, thereby allowing monovalent cations to pass through TRPM7. Furthermore, we showed that the endogenous TRPM7-like current, which is known as Mg²⁺-inhibitable cation current (MIC) or Mg nucleotide–regulated metal ion current (MagNuM) in rat basophilic leukemia (RBL) cells was also significantly potentiated by acidic pH, suggesting that MIC/MagNuM is encoded by TRPM7. The pH sensitivity represents a novel feature of TRPM7 and implies that TRPM7 may play a role under acidic pathological conditions.

Interleukin-2-dependent regulation of Na/K pump in human lymphocytes

The present study provides the first evidence that the abundance of catalytic alpha1-subunit of Na,K-ATPase increases in the course of T cell blast transformation. Immunodepressant cyclosporin A at anti-proliferative doses diminished the induction of alpha1 protein in activated lymphocytes. Furthermore, in competent T cells, IL-2 increases both the transport activity of Na/K pump and the content of Na,K-ATPase alpha1 protein in a time-dependent manner. A correlation was found between the long-term elevation in ouabain-sensitive Rb influxes and the increase in alpha1 protein content in late activated T cells. These results suggest that (1) the increased expression of Na,K-ATPase proteins underlie the cell cycle-dependent upregulation of ion pump during T cell transformation, and (2) IL-2 is involved in the regulated expression of Na,K-ATPase in human lymphocytes.

The mitochondrial ADPR link between Ca2+ store release and Ca2+ influx channel opening in immune cells

The regulation of non-voltage-operated Ca2+ channels in the plasma membrane remains unclear. However, there is often a link between the physiological release of Ca2+ from intracellular stores and opening of Ca2+ influx channels on the plasma membrane. This route has been referred to variously as store-operated Ca2+ entry (SOC), capacitative Ca2+ entry, and Ca2+ release-activated channel opening (CRAC), and often underlies the large changes in cytosolic free Ca2+ that accompany many stimuli in a wide variety of cell types. The linkage between Ca2+ store release and opening of Ca2+ channels on the plasma membrane has remained elusive for a number of years, perhaps in part because different mechanisms exist for this linkage, and are used to differing extents by different cells. We suggest here that one of the mechanisms that may operate in cells of the immune system, but that may be important elsewhere, involves the release of mitochondrial adenosine diphosphate ribose (ADPR) or nicotinamide adenine dinucleotide (NAD+). There is accumulating evidence to support each of the steps necessary for a complete description of this "Ca2+ store release to plasma membrane channel opening" link, but to our knowledge they have not been connected before to make a coherent model.-

Ceramide increase cytoplasmic Ca2+ concentration in Jurkat T cells by liberation of calcium from intracellular stores and activation of a store-operated calcium channel

The effect of ceramide on the cytoplasmic Ca2+ concentration ([Ca2+]i) varies depending on the cell type. We have found that in Jurkat human T cells ceramide increases the [Ca2+]i from a thapsigargin-sensitive calcium pool and the subsequent activation of a capacitative Ca2+ entry. This effect occurs both in the presence and in the absence of extracellular calcium. Addition of ceramine, a non-hydrolysable analogue of ceramide, reproduced its effect on the [Ca2+]i ruling out that this is due to the conversion of ceramide to sphingosine. The effect of ceramide was additive to that obtained by sphingosine, but not to the Jurkat T cells specific antibody OKT3. However, different to the latter, ceramide do not induced an elevation of InsP3. The opening of a store operated Ca2+ channel by ceramide was corroborated by experiments of Fura-2 quenching, using Mn2+ as a surrogate for Ca2+ and confirmed by whole-cell recording patch clamp techniques.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

The blocking of capacitative calcium entry by 2-aminoethyl diphenylborate (2-APB) and carboxyamidotriazole (CAI) inhibits proliferation in Hep G2 and Huh-7 human hepatoma cells

Calcium entry is a component of the processes regulating the proliferative phenotype of some types of cancer. In non-excitable cells, capacitative calcium entry (CCE) and non-capacitative calcium entry (NCCE) are thought to be the main pathways of Ca2+ influx into cells. Thus, blocking calcium entry may prevent normal and pathological cell proliferation and there is evidence to suggest that molecules blocking calcium entry also have antiproliferative properties. Carboxyamidotriazole (CAI), a novel inhibitor of the non-voltage-dependent calcium entry has been shown to have such properties in model systems in vitro and in vivo. We used Hep G2 and Huh-7 human hepatoma cells to investigate the effects of calcium entry blockers on cell proliferation. CAI (10 microM) and 2-APB (20 microM) completely blocked CCE in thapsigargin-treated Huh-7, and CAI and 2-APB inhibited cell proliferation with IC50 of 4.5 and 43 microM, respectively. The plateau phase of the [Ca2+]i increases triggered by 10% FCS were abolished in the absence of external Ca2+ and in the presence of CAI or 2-APB. We, therefore, suggest that CCE is the main pathway involved in regulation of the processes leading to cell proliferation.

Coligation of the B Cell Receptor with Complement Receptor Type 2 (CR2/CD21) Using Its Natural Ligand C3dg: Activation without Engagement of an Inhibitory Signaling Pathway

C3dg is a cleavage product of the C3 component of complement that can facilitate the coligation of the complement receptor 2 (CR2/CD21) with the BCR via C3dg/Ag complexes. This interaction can greatly amplify BCR-mediated signaling events and acts to lower the threshold for B cell activation. Although previous studies have used anti-CR2 Abs or used chimeric Ags in the context of BCR transgenic mice as surrogate C3d-containing ligands, we have used a physiological form of C3d to study signaling in B cells from wild-type C57BL/6 mice. We find that while CR2-enhanced BCR signaling causes intracellular Ca2+ mobilization and total pTyr phosphorylation of an intensity comparable to optimal BCR ligation using anti-IgM Abs, it does so with limited activation of inhibitory effectors (such as CD22, Src homology region 2 domain containing phosphatase 1, and SHIP-1) and without substantial receptor cross-linking. In summary, we demonstrate that CR2-enhanced BCR signaling may proceed not only through the previously described amplification of positive signaling pathways, but is potentially augmented by a lack of normal inhibitory/feedback signaling.

Intracellular Ca(2+) release triggers translocation of membrane marker FM1-43 from the extracellular leaflet of plasma membrane into endoplasmic reticulum in T lymphocytes

Stimulation of T cell receptor in lymphocytes enhances Ca(2+) signaling and accelerates membrane trafficking. The relationships between these processes are not well understood. We employed membrane-impermeable lipid marker FM1-43 to explore membrane trafficking upon mobilization of intracellular Ca(2+) in Jurkat T cells. We established that liberation of intracellular Ca(2+) with T cell receptor agonist phytohemagglutinin P or with Ca(2+)-mobilizing agents ionomycin or thapsigargin induced accumulation of FM1-43 within the lumen of the endoplasmic reticulum (ER), nuclear envelope (NE), and Golgi. FM1-43 loading into ER-NE and Golgi was not mediated via the cytosol because other organelles such as mitochondria and multivesicular bodies located in close proximity to the FM1-43-containing ER were free of dye. Intralumenal FM1-43 accumulation was observed even when Ca(2+) signaling in the cytosol was abolished by the removal of extracellular Ca(2+). Our findings strongly suggest that release of intracellular Ca(2+) may create continuity between the extracellular leaflet of the plasma membrane and the lumenal membrane leaflet of the ER by a mechanism that does not require global cytosolic Ca(2+) elevation.

Properties of Mg2+-dependent cation channels in human leukemia K562 cells

The endogenous Mg(2+)-inhibited cation (MIC) current was recently described in different cells of hematopoietic lineage and was implicated in the regulation of Mg2+ homeostasis. Here we present a single channel study of endogenously expressed Mg(2+)-dependent cation channels in the human myeloid leukemia K562 cells. Inwardly directed unitary currents were activated in cell-attached experiments in the absence of Ca2+ and Mg2+ in the pipette solution. The current-voltage (I-V) relationships displayed strong inward rectification and yielded a single channel slope conductance of approximately 30 pS at negative potentials. The I-V relationships were not altered by patch excision into divalent-free solution. Channel open probability (P(o)) and mean closed time constant (tau(C)) were strongly voltage-dependent, indicating that gating mechanisms may underlie current inward rectification. Millimolar concentrations of Ca2+ or Mg2+ applied to the cytoplasmic side of the membrane produced slow irreversible inhibition of channel activity. The Mg(2+)-dependent cation channels described in this study differ from the MIC channels described in human T-cells, Jurkat, and rat basophilic leukemia (RBL) cells in their I-V relationships, kinetic parameters and dependence on intracellular divalent cations. Our results suggested that endogenously expressed Mg(2+)-dependent cation channels in K562 cells and the MIC channels in other hematopoietic cells might be formed by different channel proteins.

Signalling shutdown strategies in aging immune cells

It is important for the resolution of inflammation that the number and activity of immune cells are reduced. Clearance of immune cells may be achieved by apoptosis and phagocytosis of cell fragments by macrophages. However, signalling shutdown by immune cells committed to apoptosis occurs early in the progression of these cells towards fragmentation and, it could be argued, is a key feature of apoptosis. There is surprisingly little known about the mechanisms that underlie this signalling shutdown, in particular the shutdown of Ca(2+) influx. The consequences and the potential mechanisms by which Ca(2+) influx shutdown is achieved are discussed. In addition, the potential consequences for cell signalling of cytochrome c release from mitochondria and of phosphatidyl-serine externalization are discussed. The aim of the review is therefore to highlight the evidence for various signalling shutdown strategies in immune cells that may limit their activity during progression towards apoptosis.

Regulation of calcium signalling by 1-O-alkylglycerols in human Jurkat T lymphocytes

We studied the role of natural occurring 1-O-alkylglycerols on the calcium signalling in Jurkat T-cells. Alkylglycerols evoked an increase in free intracellular calcium concentration [Ca2+]i, in a dose-dependent manner. When the experiments were performed in calcium-free buffer, the alkylglycerol response on the rise of [Ca2+]i was wholly abolished compared with the one in calcium-containing buffer, suggesting that these etherlipids induce a calcium influx by the opening of Ca2+ channels. We further employed inhibitors of voltage-gated calcium channels. We observed that omega-conotoxin, a blocker of N-type voltage-activated Ca2+ channels, but not verapamil, a blocker of L-type voltage-activated Ca2+ channels, curtailed significantly the calcium rise evoked by the lipid agents. Alkylglycerols also induced plasma membrane depolarisation, known to be involved in the opening of the voltage-gated calcium channels. Our study shows that alkylglycerols increase [Ca2+]i influx in human Jurkat T-cells possibly by modulating the permeability of calcium channels.

Identification and functional characterization of voltage-dependent calcium channels in T lymphocytes

In T lymphocytes, sustained calcium (Ca2+) influx through Ca2+ channels localized in the plasma membrane is critical for T cell activation and proliferation. Previous studies indicated that voltage-dependent Ca2+ channels (VDCCs) play a role in Ca2+ mobilization during T lymphocyte activation. However, the role of VDCCs in otherwise nonexcitable cells is still poorly understood. We used RT-PCR to identify a transcript encoding the pore-forming alpha1F-subunit of an L-type Ca2+ channel in T lymphocytes. Its identity was confirmed by DNA sequencing. To further investigate the contribution of Ca2+ influx through VDCCs, we assessed the effects of the 1,4-dihydropyridine L-type Ca2+ channel agonist, (+/-) Bay K 8644, and antagonist, nifedipine, on the human Jurkat T cell leukemia line, human peripheral blood T lymphocytes and mouse splenocytes. We found that treatment of T lymphocytes with (+/-) Bay K 8644 increased intracellular Ca2+ and induced the activation of phosphoextracellular-regulated kinase 1/2 (Erk1/2), whereas nifedipine blocked Ca2+ influx, the activity of Erk1/2 and nuclear factor of activated T cells (NFAT), interleukin-2 (IL-2) production, and IL-2 receptor expression. Nifedipine also significantly suppressed splenocyte proliferation in an in vitro mixed lymphocyte reaction and the proliferation of male antigen (H-Y)-specific T cell receptor-transgenic CD8+ T cells in transplanted male mice in vivo. Taken together these novel findings indicate that an L-type Ca2+ channel plays a significant role in the Ca2+ influx pathways mediating T lymphocyte activation and proliferation in vitro and in vivo.

Regulation of membrane trafficking and subcellular organization of endocytic compartments revealed with FM1-43 in resting and activated human T cells

FM1-43, a fluorescent styryl dye that penetrates into and stains membranes, was used to investigate kinetics of constitutive endocytosis and to visualize the fate of endocytic organelles in resting and activated human T lymphocytes. The rate of dye accumulation was strongly temperature dependent and approximately 10-fold higher in activated than in resting T cells. Elevation of cytosolic free Ca2+ concentration with thapsigargin or ionomycin further accelerated the rate of FM1-43 accumulation associated with cytosolic actin polymerization. Direct modulation of actin polymerization affected membrane trafficking. Actin condensation beneath the plasma membrane with calyculin A abolished FM1-43 internalization, whereas actin depolymerization with cytochalasin D had no effect. Photoconversion of DAB by FM1-43 revealed altered endocytic compartment targeting associated with T cell activation. Internalized cargo was carried to lysosome-like compartments in resting T cells and to multivesicular bodies (MVB) in activated T cells. Externalization of exosomes from MVB occurred commonly in activated but not in resting T cells. T cell exosomes contained raft-associated CD3 proteins, GM1 glycosphingolipids, and phosphatidylserine at the outer membrane leaflet. The present study demonstrates the utility of FM1-43 as a marker of membrane trafficking in T cells and reveals possible mechanisms of its modulation during T cell activation.

A stochastic view of lymphocyte motility and trafficking within the lymph node

Two-photon microscopy is providing literal insight into the cellular dynamics of lymphoid organs and, guided by analysis of three-dimensional images, into mechanisms that underlie cell migration and antigen recognition in vivo. This review describes lymphocyte motility and antigen recognition in the native tissue environment and compares these results with a much more extensive literature on lymphocyte motility, signaling, and chemotaxis in vitro. We discuss the in vitro literature on dynamic aspects of lymphocyte motility, chemotaxis, and the response to antigen and present the view that random migration of lymphocytes may drive a stochastic mechanism of antigen recognition in lymphoid organs, rather than being guided by chemotaxis.

Antisense-mediated loss of calcium homoeostasis endoplasmic reticulum protein (CHERP; ERPROT213-21) impairs Ca2+ mobilization, nuclear factor of activated T-cells (NFAT) activation and cell proliferation in Jurkat T-lymphocytes

We recently discovered a novel gene on chromosome 19p13.1 and its product, an integral endoplasmic reticulum (ER) membrane protein, termed CHERP (calcium homoeostasis endoplasmic reticulum protein). A monoclonal antibody against its C-terminal domain inhibits Ins(1,4,5) P (3)-induced Ca(2+) release from ER membrane vesicles of many cell types, and an antisense-mediated knockdown of CHERP in human erythroleukemia (HEL) cells greatly impaired Ca(2+) mobilization by thrombin. In the present paper, we explore further CHERP's function in Jurkat T-lymphocytes. Confocal laser immunofluorescence microscopy showed that CHERP was co-localized with the Ins(1,4,5) P (3) receptor throughout the cytoplasmic and perinuclear region, as previously found in HEL cells. Transfection of Jurkat cells with a lac I-regulated mammalian expression vector containing CHERP antisense cDNA caused a knockdown of CHERP and impaired the rise of cytoplasmic Ca(2+) (measured by fura-2 acetoxymethyl ester fluorescence) caused by phytohaemagglutinin (PHA) and thrombin. A 50% fall of CHERP decreased the PHA-induced rise of the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)), but Ca(2+) influx was unaffected. Greater depletion of CHERP (>70%) did not affect the concentration of Ins(1,4,5) P (3) receptors, but diminished the rise of [Ca(2+)](i) in response to PHA to </=30% of that in control cells, decreased Ca(2+) influx and slowed the initial rate of [Ca(2+)](i) rise caused by thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase, suggesting there was also some deficit in ER Ca(2+) stores. In CHERP-depleted cells the Ca(2+)-dependent activation and translocation of the key transcription factor NFAT (nuclear factor of activated T-cells) from cytoplasm to nucleus was suppressed. Furthermore, cell proliferation was greatly slowed (as in HEL cells) along with a 60% decrease in cyclin D1, a key regulator of progression through the G(1) phase of the cell cycle. These findings provide further evidence that CHERP is an important component of the ER Ca(2+)-mobilizing system in cells, and its loss impairs Ca(2+)-dependent biochemical pathways and progression through the cell cycle.

Noncatalytic domain of uPA stimulates human extravillous trophoblast migration by using phospholipase C, phosphatidylinositol 3-kinase and mitogen-activated protein kinase

The serine protease urokinase-type plasminogen activator (uPA) promotes matrix degradation by many cell types, including the invasive extravillous trophoblast (EVT) of the human placenta. The noncatalytic amino-terminal end of uPA binds to uPA receptors (uPARs) expressed by these cells. A highly polarized expression of uPAR-bound uPA at the migration front of EVT cells in situ suggests a functional role of uPA:uPAR interaction in EVT cell migration. The present study examined whether uPA stimulates EVT cell migration, independent of proteolytic function, and investigated some of the signaling pathways involved. Using in vitro-propagated EVT cells in Transwell migration assays, both uPA and its noncatalytic amino-terminal fragment (ATF) were shown to stimulate migration through multiporous polycarbonate (pore size 8 microm) membranes. A uPAR-blocking antibody inhibited basal and ATF-stimulated migration. Migration was found to be stimulated by hypoxic conditions, which upregulates uPAR expression; this stimulation was abrogated with the uPAR-blocking antibody, indicating the role of endogenous uPA in EVT cell migration. Spectrofluorometric measurement of cytosolic calcium in cells treated with uPA and ATF demonstrated a rapid rise in [Ca2+](i), which was prevented by pretreatment of cells with thapsigargin, indicating a release from intracellular stores. Both basal and ATF-mediated migratory responses were suppressed in the presence of selective pharmacological inhibitors LY294002, U73122, and U0126, implicating the respective roles of phosphatidinylinositol 3-kinase (PI 3-K), phospholipase C (PLC), and MEK1/2 in basal and ATF-stimulated migratory capacity. Taken together, these results demonstrate that uPA:uPAR interaction stimulates EVT cell migration, independent of uPA enzymatic activity, using the mitogen-activated protein kinase pathway and calcium signaling events including the participation of PI 3-K and PLC. These findings are relevant to clinical conditions of aberrant trophoblast migration, including spontaneous abortion, preeclampsia, and choriocarcinoma.