From the
Institut für experimentelle und
klinische Pharmakologie und Toxikologie and the
¶Institut für Physiologie, Gebäude 58,
Universität des Saarlandes, D-66421 Homburg/Saar, Germany
Received for publication, April 17, 2003 , and in revised form, May 5, 2003.
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ABSTRACT |
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INTRODUCTION |
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SOC channels are defined as cation or Ca2+ entry channels considered to open as a direct result of store depletion. They can be activated by passive store depletion independently of PLC-coupled receptor activation using endoplasmic reticulum Ca2+ pump blockers like TG. The mechanisms linking store depletion to Ca2+ entry are not fully understood, and it has been proposed that multiple mechanisms for signaling store operated Ca2+ entry may exist, including a conformational coupling of SOC channels to molecules located in the endoplasmic reticulum as well as a diffusible factor released from the stores (2).
In T-cells, store-operated Ca2+ channels resemble Ca2+ release-activated Ca2+ channels (CRAC) found in mast cells, (3, 4), rat basophilic leukemia cells (5), and DT40 B lymphocytes (6, 7). CRAC channels are the prototype of highly Ca2+-selective SOC channels, and their importance in T-cells is underscored by the devastating consequences of their functional impairment in some patients with severe-combined immunodeficiencies (810).
SOC channels could be viewed as a subfamily of receptor-operated Ca2+/cation channels, which are activated in a PLC-dependent manner following receptor stimulation and which depend on store depletion. Different PLC-dependent Ca2+ entry pathways, including activation by second messengers like InsP3 or arachidonic acid (reviewed in Ref. 11) coexist in one cell and might influence each other (12, 13). In DT40 B lymphocytes, it was shown that cation entry involves not only store-operated channels but also a population of Ca2+ channels requiring functional InsP3Rs (14).
Because receptor-operated Ca2+ channels are localized in the plasma membrane and can be easily accessed, they would constitute an excellent pharmacological target for drugs modulating immune function. The problem, however, is that in many cell types, including T-cells, the molecular composition and identity of those channels is still unknown. One approach to identify the molecular nature of TCR-dependent Ca2+ channels in T-lymphocytes was to generate random Jurkat T-cell mutants and select those with specific defects in Ca2+ entry and Ca2+-dependent gene expression (15). Five of those independent mutants, CJ-1 to CJ-5, have been characterized in detail. To varying degrees, they all show a reduction of Ca2+ channel activity and, correspondingly, a reduction of Ca2+ influx as well as Ca2+-dependent gene expression, strongly suggesting that mutations of the TCR-dependent Ca2+ channels underlie the mutant phenotypes. The molecular defects, however, were never identified.
Members of the family of mammalian TRP-related proteins (for nomenclature see Ref. 16) have been implicated in both receptor- and store-operated Ca2+ entry, but data linking a specific TRP protein with an endogenous Ca2+ influx channel is sparse (reviewed in Ref. 17). TRPC3 is one of the best studied members of the mammalian TRP family, but the analysis of its functional properties relies mostly upon the characterization of heterologously expressed proteins. Most reports agree that TRPC3 can be activated in a PLC-dependent manner and forms non-selective cation channels when expressed in heterologous expression systems (18). However, there is also evidence that, depending on the cell type used for overexpression, TRPC3 channels can be activated spontaneously, by InsP3, DAG, Ca2+, and G-proteins, and following store depletion. Transient expression of TRPC3 in DT40 B lymphocytes for example revealed both InsP3R-dependent and InsP3R-independent channel activity (19, 20).
In other cell types, there is evidence that TRPC3 channel activity depends
upon interaction with InsP3 and/or ryanodine receptors
(2126).
This activity seems to be modulated by the binding of
Ca2+/calmodulin
(24,
27) and by the binding of
phospholipase C (28).
The different findings regarding TRPC3 might at least in part be reconciled by
the fact that TRPC3 is able to build heterooligomers with other members of the
TRPC subfamily (29) that are
to various degrees expressed endogenously in many cell types.
In this report, we describe the molecular and functional analysis of mutant T-cell lines with specific defects in Ca2+ entry and Ca2+-dependent gene expression (15) focusing on potential defects of TRPC proteins. We found the TRPC3 gene to be damaged in five independent mutant cell lines when compared with wild type cells, whereas TRPC1, TRPC4, TRPC5, and TRPC6 were unaffected. The alteration of the TRPC3 genotype was accompanied by a highly increased expression of truncated TRPC3 transcripts. These findings led to the hypothesis that reduced Ca2+ channel activity could be explained by defects in the TRPC3 gene. This was tested by transient transfection of the human wild type TRPC3 in two of the mutant cell lines. TRPC3 was able to restore TCR-activated Ca2+ channel activity and the resulting Ca2+ signals. We conclude that TRPC3 is part of endogenous Ca2+ channels in human T-lymphocytes and contributes to TCR-dependent Ca2+ entry in human T-lymphocytes.
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EXPERIMENTAL PROCEDURES |
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Southern Blot AnalysisGenomic DNA was prepared according to
standard protocols (30). 20
µg of DNA from Jurkat parental cells and the mutants CJ-1, CJ-2, CJ-3,
CJ-4, and CJ-5 were each digested with EcoRI, BamHI,
HindIII, BglI, or XbaI. Half of the material was
loaded onto one lane of each of two 0.7% agarose gels, separated by
electrophoresis, and transferred to nylon membranes by capillary transfer to
obtain blots 1 and 2 (Fig. 1).
The probes were labeled by random priming with [-32P]dCTP
(6000 Ci/mmol). Hybridizations were performed at 68 °Cin0.9 M
NaCl, 90 mM sodium citrate, 0,1% bovine serum albumin, 0.1% Ficoll
Type 400, 0.1% polyvinylpyrrolidone, and 0.5% SDS overnight and washed under
stringent conditions (68 °C, 15 mM NaCl, 1.5 mM
sodium citrate, 0.1% SDS). In a first round of hybridization we used the
coding cDNA of human TRPC3 (accession number U47050
[GenBank]
) as a probe for blot 1 and
the complete cDNA of human TRPC4 (accession number AF175406
[GenBank]
) for blot 2. The
filters were stripped, and blot 1 was sequentially hybridized to nucleotides
20694773 of human INADL (accession number AJ 224747) and to the coding
cDNA of human TRPC6 (accession number AJ271066
[GenBank]
). Blot 2 was hybridized to the
coding cDNAs of human TRPC1 (accession number NM003304) and mouse TRPC5
(accession number AJ006204
[GenBank]
). Results were confirmed in an independent second
experiment. Filters were exposed for 310 days to x-ray films using
intensifying screens at 80 °C.
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Northern Blot AnalysisNorthern blots were performed as described previously (31) using 10 µg of poly(A)+ RNA from Jurkat T-cells, rabbit brain and lung, human salivary gland adenocarcinoma cells HSG (kindly provided by Dr. Mitsunobu Sato, Tokushima University school of Dentistry, Japan), human choriocarcinoma JEG-3 cells (ATCC HTB-36), and human placenta. For detection of TRPC transcripts the same probes as for Southern blot analysis were used. TRPV6 mRNA was detected as described (32). The filters were exposed to x-ray films with intensifying screens at 80 °C. One filter was used for the analysis of TRPC3 and TRPC1 expression, and another filter was used for the analysis of TRPC5, TRPC4, and TRPC6 (in that order). For that purpose, filters were stripped, exposed to x-ray films to control residual signals, and hybridized with the next probe and, finally, with a 239-bp cDNA fragment of the human glyceraldehyde-3-phosphate dehydrogenase. Analysis of TRPC3 transcripts in the T-cell mutants was performed in two independent experiments with identical results.
Generation and Purification of TRPC3 AntibodiesTo generate polyclonal anti-TRPC3 antibodies, we expressed a fusion protein consisting of the N-terminal 213 amino acids of TRPC3 (accession number U47050 [GenBank] ) fused to the maltose-binding protein (MBP) using the pMAL system (New England BioLabs) similar as described (33). The MBP-TRPC3 fusion proteins were bound on amylose resin columns and eluted as described by the manufacturer before they were fractionated by SDS-PAGE, electroeluted (Bio-Rad), lyophilized, and used to immunize rabbits. Serum obtained before and after immunization was tested for its reactivity against TRPC3 fusion proteins. To purify the anti-TRPC3 antibodies, MBP and MBP-TRPC3 fusion proteins were linked to N-hydroxysuccinimide-activated-Sepharose resin columns (Amersham Biosciences). Antibodies directed against MBP alone were removed from the serum by chromatography on MBP-Sepharose. TRPC3 antibodies (AT3) were then enriched on MBP-TRPC3-Sepharose and checked for their reactivity against MBP-TRPC3 and MBP on Western blots. Specificity of the antibodies was determined as described under "Results."
Immunoprecipitation and ImmunoblottingFor immunoprecipitations, 107 Jurkat cells were washed twice with ice-cold phosphate-buffered saline (136 mM NaCl, 2.7 mM KCl, 1.47 mM KH2PO4, 8 mM Na2HPO4, pH 7.4), suspended in 1.5 ml of radioimmune precipitation assay buffer (150 mM NaCl, 50 mM Tris-Cl, pH 8.0, 0,5% sodium deoxycholate, 1% (octylphenoxy)polyethoxyethanol (Igepal CA-630), 0.1% SDS, 5 mM EDTA, 0.1 mM phenylmethylsulfonyl fluoride, 1 mM benzamidine, 1 µg/ml aprotinin, 10 µM leupeptin, 1 µM soybean protease inhibitor). The suspension was homogenized at 4 °C for 2 min using a Polytron homogenizer and by subsequent pressing through a 26-gauge cannula. After centrifugation for 15 min at 12,000 x g the supernatant was transferred to a fresh tube. Nonspecific bindings were removed by a preincubation with 100 µl of protein-A-Sepharose (Amersham Biosciences) for 1 h followed by centrifugation for 20 s at 12,000 x g. 100 µl of the supernatant was kept to quantify the protein concentration using the BCA determination method. 1 ml of the supernatant was then incubated with 10 µg of AT3 or the polyclonal anti-TRPC4 antibody AT4 overnight at 4 °C (AK236 (34)). Immunocomplexes were precipitated by incubation with 100 µl of protein-A-Sepharose for 1 h, centrifuged, and washed three times with radioimmune precipitation assay buffer buffer. Complexes bound to protein-A-Sepharose were eluted by incubation in 100 µl of 2x Laemmli buffer at 95 °C for 5 min. The samples were centrifuged briefly at 12,000 x g. 515 µl of each sample, corresponding to the same amount of protein before precipitation, were subjected to SDS-PAGE.
In vitro translation of bovine TRPC4 was performed using the
TNT® system (Promega). Immunoblots and preparations of lysates
from HEK 293 cells and HEK 293 cells stably expressing human TRPC3 (accession
number U47050
[GenBank]
, clone T3-9
(35)) or the short variant of
human TRPC4 (accession number AF063823
[GenBank]
) were performed as described previously
(34). Labeled proteins were
detected using a horseradish peroxidase-labeled secondary antibody and the
Renaissance Western blot chemiluminescence reagent (PerkinElmer Life
Sciences). The blot shown in Fig.
2B (upper panel) was reprobed with antibody AT4
(4 µg/ml, Fig. 2B,
lower panel). For that purpose, the filter was stripped by 30-min
incubation in 62.5 mM Tris-Cl, pH 6.8, 2% SDS, 100 mM
-mercaptoethanol at 50 °C and checked for remaining signals before
application of AT4 and the secondary antibody.
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ElectroporationJurkat T-lymphocytes were always prepared to have a density of 0.50.8 x 106/ml on the day of transfection, which we found to be optimal to obtain high transfection rates. 1.2 x 107 cells were transfected in 300 µl of culture medium by electroporation using a Bio-Rad Gene Pulser II (0.23 kV, 925 microfarads, 4-mm cuvette) and were analyzed after 3648 h. The cDNA encoding the protein of interest (X) was cloned in the pCAGGS vector (36), and 6 µg was used for co-transfection with 2 µg of EGFP-pCAGGS to allow identification of transfected cells by their green fluorescence. In addition, we used 6 µg of X-IRES-EGFP constructs for transfection allowing a dicistronic expression of X and EGFP from the same transcript. Whereas the number of green cells after transfection of IRES constructs was below 5% we obtained between 30 and 50% green cells in co-transfection experiments. The efficiency of co-transfection was tested by electroporation of 2 µg of EGFP-pCAGGS and 6 µg of EBFP-pCAGGS encoding the green and the blue fluorescent proteins, respectively. Analyzing more than a hundred green cells we found only one without blue fluorescence, indicating that under the chosen experimental conditions the rate of co-transfection is near 100%. For most of the patch-clamp experiments, we used the IRES constructs, because only a few transfected cells were needed, whereas for Ca2+ imaging we routinely used the co-transfection protocol to get a sufficient number of transfected cells. Imaging experiments were also repeated with the IRES constructs giving similar results.
ElectrophysiologyThe standard pipette solution for
whole-cell patch-clamp recordings contained (in mM): 0.05
InsP3, 140 cesium aspartate, 10 NaCl, 3 MgCl2, 10 EGTA,
and 10 Hepes (pH 7.2 with CsOH). The bath solution contained (in
mM): 155 NaCl, 4.5 KCl, 22 CaCl2, 1 MgCl2, 10
D-glucose, and 5 Hepes (pH 7.4 with NaOH). Patch-clamp experiments
were performed at 2225 °C in the tight-seal whole-cell
configuration using fire-polished patch pipettes (25 M
uncompensated series resistance). Pipette and cell capacitance were
electronically cancelled before each voltage ramp with an EPC-9 patch-clamp
amplifier controlled by Pulse 8.3 software (Heka Elektronik). Membrane
currents were filtered at 1.52.3 kHz and digitized at a sampling rate
of 510 kHz. Whole-cell currents were elicited by 160-ms voltage-clamp
ramps from 100 to +60 mV from a holding potential of 0 mV. To measure
leak currents before activation of store-operated currents, 20 voltage ramps
were applied within the first 5 s after establishment of the whole-cell
configuration, followed by voltage ramps applied every second. All voltages
were corrected for a liquid junction potential of 12 mV between
internal solutions and the bath solution. Transfected green cells were
identified by their EGFP fluorescence.
Fluorescence MeasurementsCells were loaded at room
temperature (2223 °C) for 20 min with 1 µM fura-2/AM
in culture medium with 10 mM HEPES added (to buffer pH), washed
with fresh medium, and stored in the dark at room temperature until use.
Loaded cells were allowed to adhere to poly-L-ornithine-coated
glass coverslip chambers on the stage of an Olympus IX 70 microscope equipped
with a x20 UApo 340 (numerical aperture (N.A.) 0.75) or a x40
Uplan/Apo (N.A. 1.0) objective. Cells were alternately illuminated at 340 and
380 nm with the Polychrome IV Monochromator (TILL Photonics). The fluorescence
emissions at > 440 nm were captured with a charge-coupled device
camera (TILL Imago), digitized, and analyzed using TILL Vision software. Ratio
images were recorded at intervals of 5 s.
[Ca2+]i was estimated from the
relation, [Ca2+]i =
K*(R
Rmin)/(Rmax R), where
the values of K*, Rmin, and
Rmax were determined from an in situ calibration
of fura-2 in Jurkat T-cells as described previously
(37).
Ca2+ Ringer's solution contained (in mM): 155
NaCl, 4.5 KCl, 02 CaCl2, 1 MgCl2, 10
D-glucose, and 5 Hepes (pH 7.4 with NaOH). CaCl2 was
replaced by MgCl2 in the 0-Ca2+ Ringer's
solution with 1 mM EGTA added. A sandwiched self-made chamber was
used for all measurements, which allowed for a complete solution exchange of
<1 s. All experiments were conducted at 2223 °C.
EGFP-transfected cells were identified using 480-nm excitation (excitation
filter: HQ 470/40, dichroic mirror Q 495 LP, emission filter HQ 525/50; all
from AHF Analysentechnik, Germany). To determine an objective criteria to
identify a green (= EGFP-expressing) cell, we used the following approach:
Cells only transfected with vector or a construct not coding for EGFP were
analyzed first resulting in the mean ± S.D. arbitrary fluorescence of
non-green cells following 480-nm excitation. Cells were considered to be
EGFP-positive if the arbitrary fluorescence was higher than the mean of
non-transfected cells plus three times the standard deviation. Cells of the
same experiment were considered EGFP-negative (and called control cells) if
the arbitrary fluorescence was within the standard deviation of the mean of
non-EGFP-transfected cells. Cells with an intermediate fluorescence (between
one and three times the standard deviation) were disregarded for the
analysis.
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RESULTS |
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Expression Pattern of TRPC Channels in Jurkat
T-lymphocytesTo investigate which TRPC genes are expressed in
Jurkat T-lymphocytes, we performed Northern blot analysis
(Fig. 2A) and found
TRPC3 transcripts of 4.0 kb corresponding in size to transcripts present
in human brain (35) likely to
encode the functional TRPC3 channel protein. Shorter transcripts of
3.6
and
1.2 kb were also detected, which might arise from alternative
splicing. In addition, we found weak expression of TRPC1, and possibly TRPC4
transcripts, whereas TRPC5 and TRPC6 mRNA could not be detected. Recently,
TRPV6 (CaT1, CaT-L) has been described to contribute to store-operated calcium
current in Jurkat T-cells
(39). We could not detect the
TRPV6 transcripts of the expected size of 3.0 kb in Jurkat T-cells, whereas
3.0-kb TRPV6 transcripts are readily detected in human placenta and the human
JEG-3 cells (Fig.
2A, right panel) as well as in pancreas and
prostate carcinoma (32).
We also looked for the presence of TRPC channel proteins by Western blot (Fig. 2B). The polyclonal antibodies AT3 and AT4 raised against the N terminus of TRPC3 and the C-terminal peptide of TRPC4 (34), respectively, detected TRPC3 as well as TRPC4 in crude protein extracts of HEK293 cells stably overexpressing recombinant TRPC3 or TRPC4 channels but failed to detect endogenous TRPC proteins in HEK 293 cells (Fig. 2B) or in microsomal membranes of Jurkat parental cells (data not shown). To allow detection of intermediate or low protein expression, we used the antibodies for immunoprecipitation. With AT4 we could not detect any protein in Jurkat cells indicating that TRPC4 is not expressed or its abundance is very low. In contrast, we found endogenous TRPC3 proteins to be present in Jurkat parental cells. These proteins migrate in protein gels as a sharp band different to recombinant TRPC3 proteins overexpressed in HEK293 cells, which could be explained by the different levels of TRPC3 glycosylation occurring in these cells (40).
Changes of TRPC3 Gene Expression in Mutants Defective in Store-operated
CalciumWe investigated if the differences of the mutant
TRPC3 genes are linked to changes of TRPC3 gene expression.
Therefore, we analyzed TRPC3 transcripts in mutant and parental cells
(Fig. 2C). Similar to
the parental cells, the CJ-5 mutants expressed transcripts of 3.6 and
1.2 kb in size. In contrast,
4.0-kb transcripts were missing and
additional shorter transcripts of
3.1 and
2.1 kb were present. In
CJ-1 cells we also found additional mRNA species of
3.1 and
1.6 kb
in length. These additional transcripts likely reflect the expression of an
altered TRPC3 allele. Moreover, we found a strong enhancement of
TRPC3 gene expression in the mutant cells. Similar changes of
TRPC3 gene expression were also found in the mutants CJ-2, CJ-3, and
CJ-4 (data not shown). Thus, the mutant phenotype of reduced TCR-dependent
Ca2+ entry is apparently linked to changes of
TRPC3 gene expression.
Analyzing TRPC3 on the protein level, we found expression in the parental
cells and at a slightly higher density in CJ-1 and CJ-5 mutants
(Fig. 2D). Potentially
truncated TRPC3 proteins, corresponding to the additional transcripts observed
in the mutated cell lines, could not be detected by this approach. Apparently,
the epitopes recognized by the AT3 antibody were absent in truncated versions
of TRPC3 or these variants are similar in size to the heavy chains of the AT3
antibody (55 kDa) and thereby escaped detection after
immunoprecipitation.
Jurkat is a pseudodiploid cell line with polyploidy occurring at various
degrees. In an initial attempt to clone human TRPC3 by RT-PCR, 13 independent
clones were isolated and
sequenced.2 Among
these 13 cDNAs, 7 different TRPC3 splice variants were identified, 5 of which
yielded truncated TRPC3 proteins. In the parental cell line, the most abundant
transcripts were 1.2 kb in length. Apparently, the parental Jurkat cells
already express many truncated transcripts that encode incomplete TRPC3
proteins. Rather than to clone additional modified transcripts from the mutant
cell line, we decided to test the involvement of TRPC3 in TCR-dependent
Ca2+ entry on a functional level.
TCR Stimulation Enhances Ca2+ Signals in TRPC3-transfected Mutant T-cellsIn the absence of external Ca2+, stimulation of TCR with phytohemagglutinin (PHA) resulted in typical Ca2+ spikes due to Ca2+ release, which were similar in CJ-5 and parental cells (Fig. 3A). The Ca2+ influx across the plasma membrane following re-addition of external Ca2+, however, was significantly decreased in the CJ-5 mutants as shown for representative single cells as well as for the average of all analyzed cells (Fig. 3, B and C).
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To test our hypothesis with a more specific stimulus of the TCR, we used the anti-CD3 antibody OKT3, previously used to characterize the defects of Ca2+ influx in the T-cell mutants (15). Following stimulation with OKT3, Ca2+ influx was significantly increased in TRPC3-transfected CJ-5 cells (Fig. 4, AC). Both, the OKT3 and the PHA experiments indicate that TRPC3 is able to restore TCR-dependent Ca2+ influx in the mutants.
Restoration of Ca2+ influx was also analyzed after transfection of TRPV6, recently suggested to participate in store-operated Ca2+ entry in Jurkat cells (39) and after transfection of TRPC4 previously shown to participate in receptor-dependent Ca2+ entry in other cell systems (34, 41, 42). Introduction of human TRPC4 did not enhance TCR-dependent Ca2+ signals in CJ-5 T-cells (Fig. 4D). Overexpression of TRPV6 was analyzed in CJ-1 mutants, because the endogenous Ca2+ entry is much smaller than in CJ-5, allowing a better comparison of Ca2+ signals before and after TCR stimulation (Fig. 4E). TRPV6 increased the levels of resting [Ca2+]i as has been previously reported (32). Comparing Ca2+ entry before and after TCR stimulation, however, no difference could be detected (compare arrows in Fig. 4E). These results indicate that TRPC3 but, under our experimental conditions, not TRPC4 or TRPV6 enhance TCR-dependent Ca2+ signals in human T-lymphocytes.
Thapsigargin Increases Ba2+ but Not Ca2+ Influx in TRPC3-transfected CellsIn the current picture of T-cell activation, Ca2+ entry is solely activated in a Ca2+ store-dependent manner (43). Thus, one would predict that TRPC3 should also enhance Ca2+ signals following stimulation with thapsigargin (TG), which depletes Ca2+ stores through inhibition of the SERCA Ca2+-ATPase. To our surprise, TG-dependent Ca2+ influx was not significantly enhanced in TRPC3-transfected CJ-5 and CJ-1 cells (data not shown). In DT40 lymphocytes, it has been shown that TG-dependent TRPC3 activity can be resolved when using Ba2+ as a surrogate for Ca2+ (44). Endogenous Ca2+ channels in lymphocytes are also permeable to Ba2+ (45), the corresponding fura-2 ratio signal changes (340/380), however, are quite small when compared with the changes in fura-2 ratios with Ca2+ as the charge carrier (46). We analyzed Ba2+ entry in T-cells and confirmed the small changes in fura-2 ratios following store depletion with TG. In TRPC3-transfected CJ-5 cells, we found a significant enhancement of Ba2+ entry compared with non-transfected cells (Fig. 5A). The subsequent Ca2+ rise as well as the plateau were not increased confirming that TG did not increase Ca2+ signals in TRPC3-transfected cells. To account for cell-to-cell variation of the endogenous Ca2+ channel number, the initial rate of the Ba2+ rise was normalized against the initial rate of the Ca2+ rise for each individual cell (Fig. 5B), revealing a significant difference of Ba2+ entry between TRPC3-transfected CJ-5 cells and controls (non-transfected or EGFP-transfected). We conclude that TRPC3 can participate in store-operated Ba2+ entry in T-cells.
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It is surprising that TG enhances Ba2+ but not Ca2+ entry in TRPC3-transfected cells. Because it has been shown that Ca2+ channels in T-cells are regulated by mitochondrial Ca2+ uptake (47, 48) and that TRPC3 activity depends on functional mitochondria (49), we analyzed the role of mitochondria on TRPC3-dependent Ca2+ entry. Fig. 5C shows that, in the presence of 1 µM CCCP, which completely inhibits mitochondrial Ca2+ uptake in T-cells (47), Ca2+ entry was enhanced in TRPC3-transfected CJ-5 cells compared with controls. This result is consistent with an interaction between TRPC3 and mitochondria in T-cells in a way that mitochondria take up much of the Ca2+, which enters through TRPC3 channels activated by TG. This model could explain the differences found between TCR- and TG-activated TRPC3-dependent Ca2+ entry in T-cells.
TRPC3 Rescues InsP3-induced Currents in
the Mutant Cell LinesTCR stimulation is coupled to the activation
of PLC resulting in the formation of the second messengers
InsP3 and DAG, both of which have been implicated to activate TRPC3
channels (18). Application of
the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (100 µM)
induced a small Ca2+ influx in parental cells and
non-transfected CJ-5 control cells
(46). However, no
Ca2+ increase compared with control CJ-5 cells was
observed in TRPC3-transfected CJ-5 cells (three experiments,
n = 194 green cells, data not shown) indicating that TRPC3 channels
in Jurkat T-cells are not activated by DAGs, which is in contrast to other
studies overexpressing TRPC3 in Chinese hamster ovary cells
(50) and HEK 293 cells
(23).
The involvement of InsP3 in TRPC3 activation was analyzed in whole-cell patch-clamp experiments following transient transfection of CJ-1 and CJ-5 mutant cells. InsP3 activated a small inwardly rectifying current (Fig. 6A) in mock transfected CJ-5 control cells (only EGFP) resembling reduced CRAC currents previously described (15). The amplitude of the currents in the TRPC3-transfected CJ-5 cells was significantly enhanced as shown in single cells (Fig. 6B) and summarized in Fig. 6C. In both CJ-1 and CJ-5, inward currents were increased to a value very close to the amplitude found in mock transfected parental cells. In contrast to other studies, TRPC3 currents were found to be inwardly rectifying with no significant outward component resembling endogenous CRAC currents. Identical results were obtained using the co-transfection protocol in CJ-5 mutants (current density, 2.73 ± 0.44 pA/picofarad, n = 8). We conclude that InsP3 activates TRPC3-dependent inward currents in T-lymphocytes.
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Selectivity and Properties of the TRPC3-dependent CurrentTo test whether the TRPC3 currents were carried by Ca2+ or Na+, ion replacement experiments were performed in CJ-5 cells transfected with TRPC3. A typical experiment is shown in Fig. 7A in which the inward current is depicted as a function of time. Current responses to voltage ramps of the same cell are represented in Fig. 7B, and the statistical analysis of all experiments is summarized in Fig. 7C. On average, replacement of Na+ by N-methyl-D-glucamine only led to a minor reduction of the current amplitude, whereas removal of extracellular Ca2+ drastically reduced the current amplitude typical for CRAC channels. We conclude that TRPC3-dependent channels in T-lymphocytes have a high selectivity for Ca2+ very similar to endogenous CRAC channels (1:1000 for Ca2+:Na+ (45)).
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DISCUSSION |
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The molecular analysis of mutant T-cell lines unmasked TRPC3 as a very good candidate to participate in TCR-dependent Ca2+ entry. The altered TRPC3 genotype in the mutant cell lines results, however, in increased expression of transcripts and, consecutively, slightly higher levels of the TRPC3 protein compared with the parental cell line. The additional smaller transcripts present in the mutant cell lines might give rise to additional smaller TRPC3 variants, which, apparently, may lack the epitopes recognized by the available antibodies or which are hidden behind the antibody's heavy chains after immunoprecipitation. The phenotype of defective TCR-dependent Ca2+ entry therefore cannot be explained by the absence of TRPC3 but may well arise from the expression of dominant negative proteins that disturb the correct channel function. A similar dominant negative effect of truncated TRPC3 proteins upon endogenous Ca2+ entry has already been shown after heterologous overexpression of an N-terminal fragment of TRPC3 in human umbilical vein endothelial cells (51).
We tried to overcome the potential dominant negative effect of truncated TRPC3 proteins by overexpressing full-length TRPC3 cDNA, and we indeed found increased Ca2+ channel activity following stimulation of the cells. High Ca2+ selectivity is a hallmark of Ca2+ channels in T-lymphocytes, and we found TRPC3 to form highly Ca2+-selective channels similar to endogenous Ca2+ channels. This is in contrast to recent findings describing TRPC3 to form non-selective cation channels in primary pontine neurons (33) and, following overexpression, in HEK293 cells (22, 52, 53), Chinese hamster ovary cells (54), and bovine pulmonary artery endothelial cells (CPAE) (55). Because there is evidence that TRPC3 is able to form heteromultimers with other proteins of the TRPC family (29), the differences of selectivity between TRPC3-dependent channels in T-cells and other cells could be explained by their different subunit compositions. A good candidate is TRPC1, which regulates store operated Ca2+ entry in DT40 B-lymphocytes (56) and is also expressed in T-cells.
Our molecular and physiological experiments identify TRPC3 as a protein important for TCR activated Ca2+ entry. This activation is InsP3-dependent, which is consistent with previous studies reporting an interaction of the C terminus of TRPC3 with the InsP3 receptor (21, 22, 24, 25) and the activation of a TRPC3-dependent cation conductance by InsP3 in HEK293 cells (22, 25) and in bovine pulmonary artery endothelial cells (55).
TCR-dependent activation of PLC and the subsequent formation of
InsP3 is the physiological pathway to activate
Ca2+ channels in T-cells but Ca2+
entry can also be activated by store depletion without the generation of
InsP3 following application of thapsigargin (TG). In DT40
B-lymphocytes it has even been shown that the endogenous
Ca2+ channels, which have very similar properties to the
ones in T-cells, can be activated independently of InsP3 receptors
(6,
7,
57). We could not detect a
significant increase of Ca2+ entry following store
depletion with TG in TRPC3-transfected T-cells, which compares well with some
TRPC3 studies (54,
55,
58) but not with others
(19,
22,
35,
44).
Ba2+ has been used as a surrogate for
Ca2+ in recent TRPC3 studies and therefore we tried
Ba2+ in T-cells and observed that TG is able to activate
TRPC3-dependent Ba2+ entry. Because
Ba2+ is a poor substrate for
Ca2+-dependent regulatory mechanisms, these findings
might reflect a strong Ca2+-dependent regulation of
TRPC3 channels. This hypothesis is supported by the finding that TRPC3 channel
activity is tightly regulated by the interaction of TRPC3 with both the InsP3
receptor and Ca2+/calmodulin
(24). Furthermore,
mitochondrial Ca2+ uptake is likely to regulate
Ca2+ entry through TRPC3 channels
(49, this study), a mechanism
very important for Ca2+ entry through CRAC channels in
T-cells (47,
48).
Recently, Putney and colleagues (19, 44) have made an important step toward understanding the activation of TRPC3 by showing that receptor-dependent TRPC3 channels differ in their properties from store-operated TRPC3 channels in DT40 B lymphocytes. From their data, it appears likely that TRPC3 can be part of independent ion channels important for Ca2+ entry, which are activated and regulated in different ways. How this would fit in with the present picture of T-cell activation, can, at the moment, not be resolved. However, stimulation of the TCR is the physiological way to activate T-cells, and because TRPC3, which is endogenously expressed in T-cells, is activated through TCR-induced signaling, we have to conclude that TRPC3 mediates Ca2+ entry in T-cells and is important for the subsequent Ca2+-dependent activation of the cells.
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FOOTNOTES |
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To whom correspondence may be addressed. Tel.: 49-6841-162-6266; Fax:
49-6841-162-6060; E-mail:
stephan.philipp{at}uniklinik-saarland.de.
|| To whom correspondence may be addressed. Tel.: 49-6841-162-6266; Fax: 49-6841-162-6060; E-mail: markus.hoth{at}uniklinik-saarland.de.
1 The abbreviations used are: TCR, T-cell receptor; PLC, phospholipase
C
; InsP3, inositol 1,4,5-trisphosphate; DAG, diacylglycerol;
InsP3R, InsP3 receptor; SOC, store-operated
Ca2+/cation; TG, thapsigargin; TRP, transient receptor
potential; EGFP, enhanced green fluorescent protein; CCCP, carbonyl cyanide
m-chlorophenylhydrazone; CRAC, Ca2+
release-activated Ca2+ channels; PHA,
phytohemagglutinin; MBP, maltose-binding protein; IRES, internal ribosomal
entry site; OKT3, a CD3-specific monoclonal antibody.
2 U. Wissenbach, S. Philipp, and V. Flockerzi, unpublished data.
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ACKNOWLEDGMENTS |
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REFERENCES |
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