Single Channel Properties and Regulated Expression of Ca2 Release-Act

杨柳

a Department of Physiology and Biophysics, University of California Irvine, Irvine, California 92697-4561
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Correspondence to: Michael D. Cahalan, Department of Physiology and Biophysics, University of California Irvine, Irvine, CA 92697-4561. Tel:(949) 824-7776 Fax:(949) 824-3143
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Abstract
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Although the crucial role of Ca2  influx in lymphocyte activation has been well documented, little is known about the properties or expression levels of Ca2  channels in normal human T lymphocytes. The use of Na  as the permeant ion in divalent-free solution permitted Ca2  release-activated Ca2  (CRAC) channel activation, kinetic properties, and functional expression levels to be investigated with single channel resolution in resting and phytohemagglutinin (PHA)-activated human T cells. Passive Ca2  store depletion resulted in the opening of 41-pS CRAC channels characterized by high open probabilities, voltage-dependent block by extracellular Ca2  in the micromolar range, selective Ca2  permeation in the millimolar range, and inactivation that depended upon intracellular Mg2  ions. The number of CRAC channels per cell increased greatly from 15 in resting T cells to 140 in activated T cells. Treatment with the phorbol ester PMA also increased CRAC channel expression to 60 channels per cell, whereas the immunosuppressive drug cyclosporin A (1 μM) suppressed the PHA-induced increase in functional channel expression. Capacitative Ca2  influx induced by thapsigargin was also significantly enhanced in activated T cells. We conclude that a surprisingly low number of CRAC channels are sufficient to mediate Ca2  influx in human resting T cells, and that the expression of CRAC channels increases 10-fold during activation, resulting in enhanced Ca2  signaling.
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2 t; X; S" }7 v# n. iKey Words: T lymphocyte, Ca2  channel, CRAC channel, T cell activation, Ca2  signaling
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T lymphocytes play fundamental and diverse roles in the immune response including the recognition of antigens, secretion of cytokines, and killing of foreign or virus-infected cells. T cell receptor (TCR)1 engagement by specific antigen/major histocompatibility complex proteins on an antigen-presenting cell initiates a program of intracellular signaling within the T cell. By cross-linking surface receptors, the mitogenic lectin phytohemagglutinin (PHA) mimics the early antigen evoked events in a heterogeneous population of resting T cells. A sustained or oscillatory rise in intracellular Ca2  concentration (i) following TCR engagement is required to drive many of the subsequent events of lymphocyte activation, including the activation of transcription factors, modulation of cytoskeletal elements and motility, production and release of cytokines, and cell proliferation (Metcalfe et al. 1980 ; Crabtree 1989 ; Crabtree and Clipstone 1994 ; Negulescu et al. 1994 , Negulescu et al. 1996 ; Rao 1994 ). The rise in i results initially from IP3-induced Ca2  release from internal stores and is sustained by Ca2  influx across the plasma membrane (Donnadieu et al. 1992 ; Hess et al. 1993 ; Dolmetsch and Lewis 1994 ; Lewis and Cahalan 1995 ; Lewis 1999 ). In parallel with the Ca2  signaling pathway, contact with an antigen-presenting cell triggers a PKC-dependent pathway leading to transcriptional activation of additional genes via the Fos/Jun transcription factor AP-1. The Ca2  and PKC pathways interact at several levels, resulting in amplification of downstream signaling. The crucial role of Ca2  for lymphocyte activation is underscored by clinical use of the immunosuppressive drug, cyclosporin A (CsA), which inhibits calcineurin, a i-dependent phosphatase (Schreiber and Crabtree 1992 ; Rao et al. 1997 ; Crabtree 1999 ).' i" a6 f$ L3 R- O: _
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In Jurkat T cells dialyzed with a strong Ca2  buffer or stimulated acutely with PHA, Ca2  enters via Ca2 -selective, store-operated channels (Lewis and Cahalan 1989 ). These channels, also seen in mast cells and named Ca2  release-activated Ca2  (CRAC) channels, can be activated by treatment with thapsigargin (Tg) to inhibit Ca2  pumps located in the ER, thus bypassing proximal receptor signaling and IP3 generation while depleting the Ca2  store (Hoth and Penner 1992 ; Zweifach and Lewis 1993 ). The resulting Ca2  influx through CRAC channels produces a sustained i signal that is capable of triggering gene expression (Negulescu et al. 1994 ; Fanger et al. 1995 ). CRAC channels are highly selective for Ca2  under physiological conditions, but exhibit permeability to Na  and other monovalent cations when extracellular divalent ions are withdrawn, with a size cutoff for permeation of 6? (Hoth and Penner 1993 ; Lepple-Wienhues and Cahalan 1996 ; Kerschbaum and Cahalan 1998 ). Although the mechanism linking Ca2 store depletion to the opening of CRAC channels remains elusive, progress has been made in characterizing CRAC channels at the single channel level, using Na  as a charge carrier to amplify the single channel conductance (Kerschbaum and Cahalan 1999 ). Using this approach, single CRAC channels with 36–40-pS Na  conductance were observed in Jurkat T cells. The unitary CRAC channel phenotype provides a unique single channel signature for identifying CRAC channels in other cell types. Here, with single channel resolution during whole-cell recording, we describe the properties and regulated expression of CRAC channels in resting and activated T lymphocytes from human donors.1 I9 a$ G8 E8 p1 y/ d) [) D
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Materials and Methods
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Cell Culture5 l" C9 l9 G. V. w% ~6 u% \# ~8 U. V

5 L3 R0 e1 Y1 B  V5 qT lymphocytes were purified from freshly drawn peripheral blood of healthy volunteers using a nylon-wool column and placed into culture in modified RPMI 1640 medium in a 5% CO2 incubator at 37°C according to standard procedure (Hess et al. 1993 ). Purified human T cells (>90% CD3 , containing CD4  and CD8  T cells) were pretreated in vitro with 4 μg/ml phytohemagglutinin P (PHA; Difco), 40 nM phorbol 12-myristate 13-acetate (PMA; Calbiochem), or 500 nM ionomycin (Calbiochem) for varying times; cells from the PHA-treated population are referred to as activated T cells. In some experiments, 0.1–1 μM CsA from a 1-mM stock solution in DMSO (Sigma-Aldrich) was added simultaneously with PHA to inhibit activation and proliferation. For electrophysiological and i-imaging experiments, resting, activated, or PMA-treated T cells were placed on poly-L-lysine–coated glass coverslips and mounted on the stage of an inverted microscope.
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Electrophysiology' ]* K2 O% n8 E; v$ O; [
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Whole-cell recording was performed using an EPC-9 amplifier (HEKA). The resistance of sylgard-coated, fire-polished glass microelectrodes varied from 2 to 4 M. Unless otherwise indicated, currents were sampled at 5 kHz and digitally filtered off-line at 1.4 kHz. After establishing the whole-cell recording configuration, cells were held at 0 mV and square pulses to -120 mV for 200 ms or ramp voltages from -120 to  50 mV were applied every 0.7 s. The short-pulse protocol was chosen to avoid the damaging effect of strong hyperpolarization on membrane integrity. In some experiments, long pulses (up to 2,500 ms duration) were applied to voltages ranging from -120 to -10 mV. Liquid junction potentials were measured for all internal and external solutions and corrections were made during data acquisition. All recordings were performed at room temperature (21°C). In some experiments, cells were pretreated with 2 μM Tg (Alexis Chemical) in Ringer solution for 15–45 min on the microscope stage. Cell capacitance (Cm) was monitored to determine the membrane surface area, based upon a specific Cm value of 1 μF/cm2./ M- N3 j' d. K% {( t# {
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Solutions8 q5 @  G: _9 y. ]8 {2 [9 Q+ X& ^
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CRAC channels were activated by passive Ca2  store depletion with the Ca2  chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) included in the pipette solution. The standard internal solution was composed of 128 mM Cs–aspartate, 10 mM Cs–Hepes, 12 mM BAPTA, and 0.9 mM CaCl2. In some experiments, either 3.2 mM Mg2  or 30 μM IP3 (Sigma-Aldrich) was added to standard solution. To detect single channel CRAC currents, the concentration of external divalent cations was lowered below 1 μM to enable Na  to permeate and serve as the charge carrier. The standard external solution contained 145 mM Na-methane sulfonate, 5 mM NaCl, 10 mM N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA), 10 mM Hepes, and 10 mM D-glucose. For some experiments, Na  was replaced with N-methyl-D-glucamine (NMDG ). Ca2  currents were detected in Ca2 -containing solution: 155 mM Na-methane sulfonate, 5 mM NaCl, 2 mM Ca-methane sulfonate, 10 mM Hepes, and 10 mM D-glucose. Aliquots from Ca2 -saturated HEDTA solution were added to standard solution to yield the desired concentration of extracellular free Ca2  (1, 10, and 50 μM). The pH of all solutions was 7.2, and osmolarity was 300–305 mOsm. The HEDTA stock solution saturated with Ca2  was prepared using a pH-metric method (Neher 1988 ). Extracellular solutions and drugs were applied using a gravity-driven perfusion system with output tip diameter of 50 μm placed 50 μm from the cell. Five barrels were inserted close to the output tip, and solution exchange controlled manually by valves. A complete local solution exchange was achieved within 2 s. For i imaging, cells were bathed in normal Ringer solution consisting of 155 mM NaCl, 4.5 mM KCl, 1 mM MgCl2, 2 mM CaCl2, 10 mM D-glucose, 5 mM Hepes, pH 7.4. Ca2 -free Ringer solution contained 1 mM EGTA in place of CaCl2 and a total of 3 mM MgCl2. Ca2  concentration in normal Ringer solution was lowered to 300 μM to reduce the rate of capacitative Ca2  influx in some measurements. In K  Ringer, KCl replaced NaCl.
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i Imaging8 V5 e  i/ h2 v5 J

) X% p; {5 f: Q2 t! `/ L% uCa2  imaging experiments were performed as described previously (Fanger et al. 2000 ). In brief, resting or PHA-activated T cells were incubated for 30 min at 21–24°C with 1 μM fura-2 acetoxymethyl ester (Molecular Probes) in medium. Cells were then washed, resuspended, adhered to poly-L-lysine–coated glass coverslips, and mounted in a chamber permitting rapid (1 s) solution exchange by a syringe-driven perfusion system. A Videoprobe Ca2  imaging system (ETM Systems) was used for i measurement. Tg was used to deplete Ca2  stores and elicit Ca2  influx. Intracellular Ca2  concentrations were calculated using the standard equation i = K* (R - Rmin)/(Rmax - R), where values of K*, Rmin, and Rmax, determined by in vitro calibrations in a glass chamber, were adjusted to the anticipated in vivo values by correction factors derived from T cells dialyzed with known Ca2  concentration standards (Molecular Probes) as described previously (Fanger et al. 2000 ).( _7 h0 ]+ y1 ]; g. Q& Z
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Cytokine Assay
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After 2 d of activation under various conditions, brefeldin A was added for 4–12 h. Cells were fixed, permeabilized, and IL-2 content assessed by staining with FITC–conjugated anti–human IL-2 antibody according to Cytofix/Cytoperm Plus kit instructions (BD PharMingen). Stained cells were analyzed using a FACScanTM (Becton Dickinson) and Cell QUESTTM software.! M. s2 @! J& M4 E4 h8 ~
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Data Analysis0 H8 Q4 W- h8 R( n) s1 K
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Whole-cell currents were initially processed with PULSETM analysis software (HEKA) and then exported to Microcal Origin 5.0 software (Microcal Software, Inc.) or TAC (Bruxton Corp.) for further analysis. Traces recorded before CRAC channel activation were used as a template for leak subtraction. Initial analyses of imaging data were performed using IGOR Pro software (WaveMetrics, Inc.) with home-written macros and extensions. Two population Student's t test and Mann-Whitney U test were employed for statistical analysis. Data were considered statistically significant at P & [* f$ ?; {! a2 f: G7 h

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CRAC Channels in Resting and Activated Human T Cells! X  Z3 S; m" }, f' {# V  r+ ?: R  }- f

9 E6 ?& |3 d* {# gFig 1 shows inward Na  current through single CRAC channels in a resting T cell. After patch rupture (break in) to initiate whole-cell recording, the first indication of channel activity was the appearance of very brief opening events,
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+ S6 }% o9 s; `: R* c8 \9 K  XFigure 1. Na  current through CRAC channels in a resting T cell. All currents recorded in divalent-free solution using a Cs -aspartate pipette solution containing BAPTA with no Mg2 . (A) Sequence of channel activation displaying currents at -120 mV at the indicated times following break in. Horizontal lines are multiples of 4.8 pA, with the number of open channels shown on the left. Please note baseline shifts in channel numbers for the middle and right columns. (B) All-points amplitude histogram (bin size = 0.2 pA) accumulated during nine consecutive current traces recorded at -120 mV, beginning 135 s after break in. Note that the histogram peaks are equally spaced with intervals of 4.8 pA, corresponding to the sequential and sustained opening of eight channels. (C) Time course of whole-cell current recorded from the same resting T cell. The current amplitude at each time point was measured off-line by averaging 5-ms intervals randomly selected within square-pulse traces. (D) Open probability (Po) obtained from current traces with one channel active at the beginning of current activation (a) and after currents ran down to a single remaining active channel (b). a and b correspond to time intervals marked on C. Po values were measured by integrating current traces containing one active channel. Missing points within the horizontal scale break represent skipped traces with more than one channel active. (E) Activation by passive store dialysis, IP3, and Tg. Average time course of CRAC channel currents from representative resting T cells dialyzed with either normal BAPTA-containing pipette solution (Control, ?, n = 4), with 30 μM IP3 added to the normal pipette solution (IP3, , n = 3), or after preincubation with 2 μM Tg for 15–45 min (Tg, , n = 3). All current traces were normalized to maximal current amplitude and then averaged.
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2 {  i: n' h) `9 c) ~5 J9 eCRAC channels in activated T cells opened in a similar sequential manner, with a shorter delay before observing the first channel activity in comparison to resting T cells. Channel opening and stabilization typically began within seconds after break in, followed by rapid activation of several channels. Discrete inward current steps of 4.8 pA at -120 mV were resolved at the beginning of the experiment, similar to those observed in resting T cells (Fig 2A and Fig B). CRAC channels in activated T cells also exhibited very high open probabilities (Po = 0.95 at -120 mV), and similar rates of macroscopic current activation (act = 114 ± 9 s, n = 24; Fig 2C and Fig D), followed by slow run-down (r = 1940 ± 490 s, n = 11). Peak currents typically represented the summed activity of 50–500 CRAC channels, averaging tenfold higher than in resting T cells.
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3 ?  Q" E1 P( |3 U& LFigure 2. Na  currents through CRAC channels in an activated T cell. Same recording conditions as described in the legend to Fig 1 A. (A) Sequence of channel activation at the indicated times. Horizontal lines are multiples of 4.8 pA, with the number of open channels shown on the left. Again, please note baseline shifts that indicate the number of open channels. (B) Amplitude histogram showing equally spaced peaks corresponding to the first nine channels that opened. (C and D) Time course of CRAC channel activation. C represents the initial time course marked with bar on D on an expanded time scale. Horizontal lines represent equally spaced intervals of 4.8 pA to illustrate the stepwise activation of channels.
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Ion Permeation and Divalent Block$ y  P% d& r. o: `3 s7 r
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Several features of ion permeation and divalent block demonstrate that the single channel activity reported here in human T cells is closely similar to CRAC channels described previously in Jurkat T cells (Lepple-Wienhues and Cahalan 1996 ; Kerschbaum and Cahalan 1998 , Kerschbaum and Cahalan 1999 ). Upon development of macroscopic Na  currents representing the activation of many CRAC channels, I–V curves rectified weakly and had a reversal potential of 0 mV (Fig 3 A). Introduction of physiological levels of external Ca2  resulted in much smaller macroscopic Ca2  currents with more pronounced inward rectification and a positive reversal potential typical of ICRAC. As expected for current through a common channel, Na  and Ca2  current amplitudes in different cells were strongly correlated (Fig 3 B), with a slope determined by linear regression of 80, implying a much larger single channel conductance for Na  traversing the CRAC channel compared with Ca2  (measured at 2 mM external ). External Ca2  ions in the micromolar range blocked Na  current through CRAC channels. In Fig 3 C, with a single stably open CRAC channel, buffering extracellular Ca2  to 50 μM produced burst-like ("flickering") block and unblock events at -120 mV. Ca2  block reduced the mean open time to
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Figure 3. Ionic selectivity, rectification, and divalent block of CRAC channels in human T cells. (A) Na  and Ca2  currents recorded during a voltage ramp from -120 mV to 50 mV in the absence and presence of 2 mM extracellular Ca2  in an activated T cell. Note the 10-fold difference in scales for INa and ICa. External solutions were changed rapidly between Ca2 -free and Ca2 -containing external solutions. (B) Ratio of Na  to Ca2  current. Na  current amplitudes measured immediately after application of Ca2  -free solution plotted against Ca2  current amplitude (2 mM external Ca2 ). Each point was obtained from an individual PHA-activated cell (n = 14) from different donors. The straight line is a linear regression fit with a slope factor of 81.4 ± 2.4 and a correlation coefficient of 0.99. (C) Example of single channel currents recorded at -120 mV from a resting T cell. Top trace recorded in Ca2 -free external solution (10 mM HEDTA, no Ca2  added). Bottom trace recorded after application of 50 μM Ca2  external solution. (D) External Ca2  blocks Na  current through CRAC channels in resting and activated T cells. Currents at different external Ca2  concentrations were normalized to the current in Ca2 -free external solution. Averaged data from several resting (open symbols) and activated (closed symbols) T cells (n = 4–11 cells), fitted with the equation: Fractional Current = 1 / (1   o / Kd). Apparent Kd values were 4 μM and 20 μM at -90 mV and -120 mV, respectively. (E) Voltage dependence of Ca2  block. I–V curves recorded in the presence and absence of 10 μM external Ca2  were divided point by point and fitted with the Boltzmann equation: Fractional Current = 1 / {1   exp }, with Vh = -7.3 mV and k = 13.3 mV. The steepness factor k is equivalent to movement of a single Ca2  ion 93% of the distance from the outside across the electric field of the membrane to the blocking site within the channel. (F) Ca2  and Na  currents recorded at -120 mV from a cell dialyzed with 3.6 mM Mg2 . External solution containing 2 mM Ca2  was exchanged with Ca2 -free HEDTA-containing solution as indicated. Note the reversible inactivation of Na  current.5 L4 `* o: E# |8 B/ D
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Table 1. CRAC Channel Properties in Resting and Activated Human T Lymphocytes and in Jurkat T Cells1 X1 s2 {4 m; r: k9 _; y. V

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& R7 S: M/ ~8 Q* Q. m# j  K7 d! |Resting T cells were favorable for detailed kinetic analysis of single CRAC channels. Single channel recordings during voltage ramps (Fig 4 A) and continuous recordings at various potentials (Fig 4 B) suggested that the lifetimes of both open and closed channel states increase upon membrane depolarization, resulting in fewer event transitions. Single channel closed times (c) were consistently ! x6 b) L) m# ~9 r+ i
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Figure 4. Voltage-dependent properties of single channel currents in resting T cells. (A) Single channel activity during a voltage ramp from -120 to 50 mV. Note the very brief transitions that occur between –120 and –80 mV and longer transitions at more depolarized potentials. (B) Single channel currents recorded at -120, -80, and -40 mV. Again, note that depolarization promotes longer open and closed events. (C) Distributions of open (o) and closed (c) times at -120 and -40 mV. For single channel analysis, traces with stable opening of only one channel were selected. The kinetics of transitions between the open and closed levels were idealized by fitting manually controlled cursors to the two levels and setting a discriminator at 50% of the current between levels. Time histograms computed from a minimum of 60 intervals. Histograms of open and closed event durations were fitted by single Gaussian distributions; two or more components did not provide a significantly better fit. Same cell as in B. (D) Voltage dependence of mean open (o, ) and mean closed (c, ) times (n = 4). Values of o and c for individual cells were calculated by fitting duration histograms with a single Gaussian function, as described in C. Data are fitted with a single exponential function with a steepness factor k = 35 mV for open times and 50 mV for closed times. (E) Single channel current amplitudes () and open probabilities (Po, ) at varying potentials. Averages from four cells are shown; if not shown, standard error bars are smaller than symbol size. The smooth curve through Po data was obtained from exponential fits of c and o (smooth lines on D) according to equation Po = o/(o  c).
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Upregulated CRAC Channel Expression and Capacitative Ca2  Influx in Activated T Cells
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' E% n1 n' E1 @+ q& pProperties of single CRAC channels were identical in resting and activated T cells, but the number of CRAC channels increased substantially following treatment with PHA to induce T cell activation. We confirmed that this treatment also activated IL-2 production by flow cytometric analysis of fixed and permeabilized T cells (data not shown). To evaluate the number and surface density of CRAC channels, peak currents were normalized to membrane capacitance (Cm) in cells selected from resting and PHA-treated T cell populations. Measurements from 15 different donors were combined. Capacitance values were 1.7 ± 0.1 pF in 43 resting T cells and 3.6 ± 0.2 pF in 44 PHA-activated T cells. CRAC channel current density in PHA-treated T cells increased by a factor of four compared with resting T cells (Fig 5 A). The number of channels per cell increased from 14.9 ± 2.4 (n = 40), in resting T cells, to 136.0 ± 19.2 (n = 37), in activated T cells (Fig 5 B). CsA (1 μM) substantially inhibited this increase in functional CRAC channel expression, while also inhibiting PHA-stimulated cell enlargement (Cm = 2.3 ± 0.3 pF, n = 9), cell proliferation assessed by uptake of tritiated thymidine (90% block), and IL-2 production (90% block). At lower concentration (100 nM), CsA was less effective in blocking IL-2 production and CRAC channel upregulation. Acute application of PHA or CsA during whole-cell recording had no effect on the amplitude of monovalent CRAC current (n = 4, data not shown). These data demonstrate that PHA stimulation to induce T cell activation increases functional expression of CRAC channels. Pretreatment of resting T cells with the phorbol ester PMA (40 nM for 2–4 d) to stimulate PKC-dependent pathways also increased the CRAC channel current density (Fig 5 A) and the average number of channels per cell (63.6 ± 13.0, n = 12), while causing moderate cell enlargement with Cm values averaging 2.8 ± 0.2 pF in the cells selected for recording.
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Figure 5. PHA induces functional upregulation of CRAC channels in human T cells. (A) Average CRAC channel Na  current densities in resting, PHA-activated, PHA   CsA-treated, and PMA-treated T cells. Resting T cells were maintained in culture for 24–72 h (n = 40). Activated T cells were incubated with PHA for 48–96 h (n = 37). Combined data are presented from cells incubated with PHA   CsA for 75 and 96 h (n = 9). PMA-treated cells were incubated with 40 nM PMA for 48–96 h (n = 12). Between 48 and 96 h, there was no statistically significant difference in CRAC channel expression. Average current densities in resting and PHA   CsA-treated cells are significantly different than those in PHA-activated or PMA-treated T cells (Student's t test, P - n1 i. B. T1 b% ?4 i

$ i2 W* M- C2 r3 B) s& vTo confirm that upregulated expression of CRAC channels produces larger Ca2  influx upon re-stimulation of intact T cells, we measured the rate of capacitative Ca2  influx in resting and activated T cells using Tg to deplete intracellular Ca2  stores. Fig 6 A shows averaged i traces upon removal of external Ca2 , application of Tg, and reintroduction of external Ca2 ; for these experiments, extracellular  was reduced to 300 μM so that the upstroke of the i signal could be resolved clearly. i signaling in PHA-activated T cells was enhanced, relative to resting T cells, in three respects. First, Tg-induced release from intracellular Ca2  stores in the absence of external Ca2  was increased, from a release transient peaking at 190 ± 2.7 nM (n = 320) in resting T cells to 375 ± 16.5 nM (n = 163) in activated T cells, suggesting an increased capacity of intracellular Ca2  stores in activated T cells. Second, upon reintroduction of extracellular Ca2 , the rate of rise in i was 1.60 ± 0.05 times larger in activated than in resting T cells (Fig 6A and Fig C). Third, plateau levels of i were also significantly higher in activated T cells (775 ± 22 nM for resting T cells, and 1150 ± 30 nM for activated T cells; Fig 6 A). Expression levels of both voltage-gated and Ca2 -activated K  channels are known to increase in activated versus resting T cells (Grissmer et al. 1993 ). To confirm that the observed changes in Ca2  signaling were caused by upregulation of CRAC channels rather than changes in expression of K  channels or membrane potential, we repeated these experiments in the presence of K  Ringer solution (159 mM K ) to null out a possible contribution of the membrane potential to Ca2  influx. As expected for a depolarized condition, mean influx rates were much lower in K  Ringer than in normal Ringer (9.24 ± 0.25 nM/s in K  Ringer and 24.4 ± 0.9 nM/s in normal Ringer for resting cells). Interestingly, the rate of Ca2  influx in activated cells was still 1.65 ± 0.05 times that observed in resting cells (Fig 6B and Fig C). In addition, Ca2 -imaging experiments confirmed that stimulation with PMA alone (40 nM), but not ionomycin alone (200 nM), caused upregulation of CRAC channels. These results are summarized in Fig 6 C. We conclude that the increased functional expression of CRAC channels results in a significant increase in capacitative Ca2  influx.
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Figure 6. Enhanced capacitative Ca2  entry in activated human T cells. (A) Average time course of changes in i upon removal of external Ca2 , and consequent reintroduction of 300 μM external Ca2  in resting (n = 56, dashed line) and PHA-activated (72 h in PHA; n = 32, solid line) T cells. Tg (1 μM) was applied in Ca2 -free solution as indicated. Results shown are typical of three to four experiments for each condition with two donors. (B) In this experiment, extracellular solutions were changed to K -Ringer at the end of the store depletion transient, as indicated. Results shown are from 65 activated and 127 resting cells in a typical experiment. (C) The maximal rate of initial Ca2  influx was calculated for each cell by finding the maximal slope between each pair of data points. To prevent bias from donor to donor variability, influx rates for activated cells were normalized to the mean influx rate for resting cells from the same donor on the same day. Influx rates in activated cells and in cells stimulated with PMA alone (but not ionomycin alone) are significantly different from those of resting cells (P
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CRAC Channels in Human T Lymphocytes
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Store-operated Ca2  channels play important roles in the regulation of diverse cellular functions in a wide variety of cells. A highly Ca2 -selective type of store-operated channel, the CRAC channel, was originally described in Jurkat T cells, mast cells, and the related RBL cell line (for reviews see Lewis and Cahalan 1995 ; Parekh and Penner 1997 ; Lewis 1999 ). CRAC channels mediate Ca2  influx following receptor stimulation, resulting in i signaling that in turn regulates gene transcription and secretion of biologically active molecules in hematopoietic cells (Lewis and Cahalan 1989 ; Negulescu et al. 1994 ; Fanger et al. 1995 ; Zhang and McCloskey 1995 ). Ca2  currents through CRAC channels were reported previously in human T cells and shown to be lacking in a patient suffering from a primary immunodeficiency (Partiseti et al. 1994 ). However, no data have been available on the single channel properties or functional expression levels of CRAC channels, or any other Ca2  channels, in normal human T lymphocytes. The use of Na  as the permeant ion permitted CRAC channels to be investigated in Jurkat T cells with single channel resolution (Kerschbaum and Cahalan 1999 ). In this study, we demonstrate that Ca2  store depletion in human T cells activates unitary Na  currents through CRAC channels with biophysical properties similar to those described previously in Jurkat T cells (Table 1). Single channels with a conductance of 41 pS were visualized during whole-cell recording, using Na  as the permeant ion in divalent-free external solution. External Ca2  in the micromolar range blocked the monovalent current, and at millimolar concentrations exhibited selective permeation with small current magnitudes. The ratio of Na  to Ca2  current was constant from cell to cell, indicating a common channel mechanism. In addition, Na  current exhibited inactivation in the presence of intracellular Mg2 . A bulky cation, NMDG , was unable to carry current, as shown previously for CRAC channel currents in Jurkat T cells. These results demonstrate that human T cells express CRAC channels with identical biophysical characteristics to those of Jurkat T cells., x  L/ x' [, {: F( a: d5 K8 E

% k1 D1 `: J' V7 I1 F  [& }Single Channel Kinetics: Activation and Steady-State Gating3 x# B# P- |& z9 V+ l

" M, R  l# p' jThe small number of CRAC channels present in resting T cells provided increased observation time for detailed kinetic investigation of single channel properties, compared with Jurkat or activated human T cells. During whole-cell recording, CRAC channels first exhibited very short-duration openings that later stabilized to an open state with intrinsic short-closing events. In resting T cells dialyzed with BAPTA to deplete Ca2  stores passively, the lag time before the first channel opening was relatively long (1 min), suggesting that the latency to the first channel opening event is rate-limited by the number of functional channels, rather than by the rate of BAPTA dialysis or Ca2  store depletion, which should occur more rapidly in smaller resting T cells. Strong intracellular Ca2  buffering, addition of IP3 to the pipette, or pretreatment of intact cells with Tg activated the same population of channels by depleting Ca2  stores, similar to results for Ca2  current through CRAC channels (Hoth and Penner 1992 ; Zweifach and Lewis 1993 ). IP3 significantly reduced the latency to current activation (Fig 1 E), similar to that described for ICRAC (Parekh et al. 1997 ). In addition, CRAC channels opened by Tg pretreatment were detected immediately following break in to initiate whole-cell recording. Thus, Na  current through CRAC channels exhibits normal activation kinetics upon depletion of Tg-sensitive intracellular Ca2  stores.3 F6 @) _) Z& a0 }- q6 j9 x
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The mechanism of CRAC channel activation upon Ca2  store depletion remains uncertain. Recent evidence on other store-operated Ca2  channels favors a mechanism that involves close physical contact between the store membrane and the surface membrane, with activation resulting either from channel delivery by vesicle fusion (Patterson et al. 1999 ; Yao et al. 1999 ), or by conformational coupling between IP3 receptors in the store and channels in the surface membrane (Kiselyov et al. 1998 ; Ma et al. 2000 ). In principle, a channel delivery mechanism might deliver several channels simultaneously upon vesicle fusion. However, as observed previously in Jurkat T cells (Kerschbaum and Cahalan 1999 ), CRAC channel current increased in unitary stepwise increments as individual CRAC channels opened. In these experiments, we never observed the simultaneous initial activation of more than one channel. Furthermore, during the period of CRAC channel activation, no consistent changes in Cm were observed in resting or activated T cells; on average, Cm changed by
( x. z, I2 C2 X/ ]( j- \1 p; e4 Y( X( Z$ O# x  E3 a' X. x- o5 d
After channel stabilization, the mean open time was 10 ms, and the mean closed time was 0.95. At more depolarized potentials, both mean open and mean closed times increased, and the open probability declined to 0.85 at -40 mV. Are these channel gating events intrinsic to the channel protein, or might they be the result of ionic block? With external Ca2  in the micromolar range, individual block and unblock events were visualized as a rapid process that significantly reduced mean open times (/ R' H  x3 |  j' `1 K% b

  h9 T- I" q9 a: m1 oCRAC Channel Expression and Ca2  Influx Rates in Resting and Activated T Cells
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Ca2  influx through CRAC channels underlies i signaling in Jurkat T cells, and, by extension, is thought to trigger antigen-induced activation of resting human T cells. Whole-cell recordings with single channel resolution enable the number of CRAC channels per cell to be determined in resting and activated cells. Despite their importance for lymphocyte activation, CRAC channels in resting T cells are expressed at surprisingly low copy number, averaging 15 functional CRAC channels per cell. Is this small number of channels sufficient to account for tracer Ca2  influx measurements and for the rate of rise of i during activation? To address this question, we calculated the expected influx of Ca2  under physiological conditions during a stimulus that opens all of the available CRAC channels (Fig 7). Macroscopic Na  currents were consistently 80-fold larger than Ca2  currents recorded with 2 mM external , leading to an estimated single channel conductance of 0.5 pS at -120 mV for Ca2  current through the CRAC channel with physiological levels of external Ca2 . This estimate is based upon the assumption that the number of open channels remains constant during rapid solution exchange. To calculate Ca2  influx in the range of membrane potentials found normally in intact T cells, we scaled the measured single channel Na  current at –60 mV by the factor 80 and converted to flux units. In a cell with 15 activated CRAC channels, the calculated Ca2  influx at a resting membrane potential of -60 mV would be 1.9 amol/s, sufficient to account for previous measurements of 45Ca uptake in resting T cells upon mitogen activation, which ranged from 0.4 to 2.2 amol/s (Metcalfe et al. 1980 ; Hesketh et al. 1983 ). Can this amount of Ca2  influx produce the upstroke of the i signal? The expected rise in i due to Ca2  influx depends upon cell volume and buffer capacity. Cell volume was estimated to be 200 fl (200 x 10-15 liter) from measured values of membrane capacitance. An endogenous Ca2  buffer was assumed to capture 99% of the Ca2  entering the cell (Neher and Augustine 1992 ; Neher 1995 ). Given these estimates and scaling external  to 0.3 mM for comparison with our measurements, the Ca2  influx through 15 CRAC channels would produce a rise in i of 28 nM/s in resting T cells, in close agreement with direct measurements of capacitative Ca2  influx in resting T cells averaging 24.4 ± 0.9 nM/s. Thus, despite their small number, CRAC channels would carry sufficient Ca2  into resting T cells to account for measured tracer Ca2  influx and the rates of rise of i.  T1 P% F6 ]: @5 e6 j% y
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Figure 7. Channel expression and calculated Ca2  influx in resting and activated T lymphocytes. The two cells depicted in cartoon form represent typical resting (top) and activated (bottom) T cells. Diameters (d), surface areas (s), and volumes (v) are representative of cells selected for recording. Within each cell, a signal transduction pathway leads from antigen binding to the TCR, activating tyrosine kinases (TK), phosphorylating phospholipase C (PLC), generating IP3, and releasing Ca2  from the store. Ca2  influx through CRAC channels is activated by the depletion of Ca2  from the IP3-sensitive intracellular Ca2  store via an unknown mechanism. Functional expression levels are represented by the average number of channels per cell (in parentheses), for three channel types: voltage-gated K  channels encoded by Kv1.3 (DeCoursey et al. 1984 ; Deutsch et al. 1986 ), Ca2 -activated K  channels encoded by IKCa1 together with preassociated calmodulin (CaM) (Grissmer et al. 1993 ; Fanger et al. 1999 ; Ghanshani et al. 2000 ), and CRAC channels (this paper, Fig 5 B). The calculated Ca2  influx (ions/sec and amol/s) is based upon the number of CRAC channels per cell, the measured single CRAC channel conductance for Na , the measured ratio of ICa/INa, and an assumed membrane potential of -60 mV. The maximal rate of i increase (di/dt in nM/s), resulting from the Ca2  influx with external Ca2  of 2 mM is based upon the estimated cell volume and the buffer capacity.6 Y& Y' @/ e- N: [' ~6 r+ A' J

$ n  c( a* |9 p% ?2 G$ KCRAC channel properties were identical in resting and activated T cells. However, activated T cells expressed substantially more functional channels, averaging 136 CRAC channels per cell. This functional upregulation of CRAC channels in human T lymphocytes paralleled the growth of small lymphocytes to T cell blasts. In our experiments, average membrane capacitance values increased from 1.7 pF in resting T cells to 3.6 pF in activated T cells, indicating an increase in volume to 700 fl. As calculated for resting cells above, the expected maximal rise in i due to Ca2  entry via CRAC channels in a typical activated T cell was 73 nM/s, roughly twofold larger than, but still in reasonable agreement with, the average rate of 43.7 ± 1.6 nM/s obtained by direct measurements of i rise induced by capacitative Ca2  influx. A higher Ca2  buffering capacity or increased pump activity in activated T cells may account for the difference between those numbers. Thus, the increase in number of functional channels in activated T cells not only compensates for cell enlargement but also enhances Ca2  signaling in activated T cells.& N. \/ q% J, w$ M
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Significance of Channel Upregulation for T Cell Function
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During T cell activation, numerous transcription factors are activated, resulting in changes in the expression level of many different genes (Lanzavecchia and Sallusto 2000 ). Some gene products, such as IL-2 and other secreted lymphokines, help to carry out the effector functions of T cells, and others, such as the Ca2 -activated K  (KCa) channel encoded by IKCa1, help to augment i signaling and activation responses upon future restimulation. Increased expression of IL-2 is regulated by several transcription factors, but can be inhibited by the immunosuppressive drug CsA at doses that prevent dephosphorylation and nuclear translocation of the transcription factor NF-AT by the phosphatase calcineurin (Rao et al. 1997 ; Crabtree 1999 ). In contrast, enhanced expression of KCa channels is transcriptionally regulated by AP-1 and Ikaros-2 sites in the promoter region and is independent of NF-AT, since enhanced expression can be stimulated by PMA alone and is unaffected either by CsA treatment or by deletion of an NF-AT-binding element in the promoter region of the IKCa1 gene (Ghanshani et al. 2000 ). The increased number of CRAC channels in activated T cells may also result from transcriptional upregulation of the unknown gene or genes that encode CRAC channels. Both CRAC and KCa channel expression increase greatly in T cells stimulated for at least 24 h with PHA or PMA alone, but not with ionomycin alone (Fig 5 Fig 6 Fig 7), suggesting a similar mechanism. In addition, CsA, at a concentration reported to block calcineurin-mediated nuclear translocation of NF-AT (Rao et al. 1997 ), did not inhibit CRAC channel upregulation. However, PHA-induced IL-2 secretion, T cell proliferation, and upregulation of CRAC channels were inhibited by CsA at a concentration of 1 μM, a level that may have nonspecific effects. Thus, a possible direct role of calcineurin and NF-AT in CRAC channel upregulation requires further investigation. A detailed understanding of the mechanism awaits molecular identification of the CRAC channel.
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Two levels of positive feedback involving CRAC and KCa channels augment i signaling, as suggested by Fig 7. Before TCR engagement, i is low (100 nM), CRAC and KCa channels are closed, and the resting membrane potential is maintained by voltage-gated Kv1.3 channels. After TCR engagement, i begins to rise, initially due to IP3-induced release of Ca2 . The depletion of IP3-sensitive Ca2  stores opens CRAC channels, further elevating i. As soon as i reaches 200–300 nM, KCa channels begin to open, hyperpolarizing the membrane and thereby increasing Ca2  influx through CRAC channels, in turn augmenting the rise in i and opening the remaining KCa channels. Thus, rapid positive feedback among i, KCa channels, and CRAC channels contributes to the upstroke of the Ca2  signal. In addition, slow positive feedback involving changes in gene expression is initiated by the rise in i, along with other signaling pathways inside the cell. Activated T cells exhibit enhanced Ca2  signaling in response to either TCR engagement or Tg stimulation (Hess et al. 1993 ; Verheugen et al. 1997 ) (Fig 6). Increased expression of CRAC channels in activated T cells occurs in parallel with a comparable increase in expression of Ca2 -activated K  channels encoded by IKCa1 (Grissmer et al. 1993 ; Ghanshani et al. 2000 ). Relative to resting T cells, Tg-induced Ca2  influx is augmented in activated T cells by the same amount in normal Ringer and in K  Ringer (1.65 ± 0.06-fold upregulation in normal Ringer vs. 1.60 ± 0.05-fold upregulation in K  Ringer). This implies that upregulation of CRAC channels augments Ca2  signaling, and that parallel KCa channel upregulation is adequate to provide the necessary driving force and to compensate for the increase in cell size. Without accompanying KCa channel upregulation, Ca2  influx through the increased number of CRAC channels would depolarize the membrane, causing Ca2  influx to be self-limiting.
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6 `% |" c  I8 D! {4 p; K' V. {Amplification of the Ca2  signal may be responsible for increased sensitivity to TCR stimulation observed previously in secondary T cell activation (Manger et al. 1985 ; Byrne et al. 1988 ). In addition, upregulation of Ca2  influx through CRAC channels may sustain mitogenesis. The number of CRAC channels in activated T cells (140 per cell) is close to that in proliferating Jurkat T cells (100–400 per cell), suggesting a requirement for larger numbers of CRAC channels for clonal expansion. Sustained i elevation is required for calcineurin-dependent translocation of NF-AT into the nucleus, resulting in activation of NF-AT–dependent transcription of lymphokines, and presumably other immune response genes (Rao et al. 1997 ; Crabtree 1999 ). Therefore, functional upregulation of CRAC channels may play a crucial role for continued cell proliferation and more vigorous secondary T cell responses.
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