Departments of 1 Anatomy, Histology, and Forensic Medicine, 2 Physiological Sciences, and 3 Biochemical Sciences, University of Florence, 50134 Florence; and 4 Biophotonics Lab, National Institute of Applied Optics, 50125 Florence, Italy
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ABSTRACT |
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In many cell systems, sphingosine 1-phosphate (SPP) increases cytosolic Ca2+ concentration ([Ca2+]i) by acting as intracellular mediator and extracellular ligand. We recently demonstrated (Meacci E, Cencetti F, Formigli L, Squecco R, Donati C, Tiribilli B, Quercioli F, Zecchi-Orlandini S, Francini F, and Bruni P. Biochem J 362: 349-357, 2002) involvement of endothelial differentiation gene (Edg) receptors (Rs) specific for SPP in agonist-mediated Ca2+ response of a mouse skeletal myoblastic (C2C12) cell line. Here, we investigated the Ca2+ sources of SPP-mediated Ca2+ transients in C2C12 cells and the possible correlation of ion response to cytoskeletal rearrangement. Confocal fluorescence imaging of C2C12 cells preloaded with Ca2+ dye fluo 3 revealed that SPP elicited a transient Ca2+ increase propagating as a wave throughout the cell. This response required extracellular and intracellular Ca2+ pool mobilization. Indeed, it was significantly reduced by removal of external Ca2+, pretreatment with nifedipine (blocker of L-type plasma membrane Ca2+ channels), and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-mediated Ca2+ pathway inhibitors. Involvement of EdgRs was tested with suramin (specific inhibitor of Edg-3). Fluorescence associated with Ins(1,4,5)P3Rs and L-type Ca2+ channels was evident in C2C12 cells. SPP also induced C2C12 cell contraction. This event, however, was unrelated to [Ca2+]i increase, because the two phenomena were temporally shifted. We propose that SPP may promote C2C12 cell contraction through Ca2+-independent mechanisms.
calcium ion transients; cytoskeleton; cell contraction; confocal microscopy
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INTRODUCTION |
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SPHINGOSINE 1-PHOSPHATE (SPP) is a bioactive lysophospholipid mediator that is recognized as a highly versatile molecule capable of affecting many cellular processes, including cell proliferation and differentiation, apoptotic cell death, cell motility, and cytoskeletal organization (19). Some of these biological effects of SPP have long been attributed to its action as second messenger. Indeed, the mitogenic responses to several growth factors, such as platelet-derived growth factor, epidermal growth factor, nerve growth factor, and insulin, as well as the inhibition of apoptosis induced by antiblastic drugs have been related to the activation of sphingosine kinase and to the subsequent enhanced production of intracellular SPP (35, 36, 38). More recently, the identification of a subset of receptors belonging to the endothelial differentiation gene (Edg) receptor (R) family has suggested that SPP may also act as an extracellular lipid mediator. In agreement with this suggestion, SPP is released on platelet activation and is an important constituent of serum (18). Moreover, extracellular SPP stimulation is required for inhibition of cell motility in vascular smooth muscle and melanoma cells, neurite retraction, and stimulation of DNA synthesis in 3T3 fibroblasts (8, 39, 50, 52). Only very recently has a role for exogenous SPP in the pathogenesis of inflammatory diseases such as asthma also been proposed (2).
It is becoming apparent that both modes of action of SPP may involve Ca2+ mobilization from intracellular stores and/or from the external pool. However, at present, the mechanisms by which SPP affects cytosolic Ca2+ concentration ([Ca2+]i) are far from being fully delineated. Several studies have suggested that SPP, when acting as a second messenger, can directly promote Ca2+ release from the endoplasmic reticulum (ER) through mechanisms independent of the activation of the ryanodine (Ry)R or inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]R pathways (5, 54). Consistent with this, a novel sphingolipid-gated Ca2+-permeable channel has been discovered on isolated ER vesicles of Xenopus oocytes (26). In contrast, dissection of the Ca2+ signaling pathways triggered by the interaction of SPP with its G protein-coupled receptors is of great difficulty and complexity considering that multiple effector systems, including phospholipase C (PLC) and protein kinase C (PKC), may be involved (3, 37, 40). In particular, although activation of phospholipases has been reported in several cell types after SPP stimulation, only limited data have provided conclusive evidence for a role of Ins(1,4,5)P3 in the SPP-induced Ca2+ signaling pathway. Moreover, we recently found (28) in a myoblastic C2C12 cell line that SPP elicits a Ca2+ response in the form of Ca2+ waves that are dependent on extracellular Ca2+, thus suggesting a role for the lipid metabolite in mediating the influx of the ion through plasma membrane Ca2+ channels. Furthermore, in the same study we also showed that inhibition of Edg-3R and Edg-5R by specific antisense oligodeoxyribonucleotides totally abolished SPP-induced Ca2+ response (28). Voltage-dependent dihydropyridine receptors (L-type Ca2+ channels), located on the plasma membrane, represent the major Ca2+ entry pathway in excitable cells. In particular, Ca2+ influx through these channels is critical for the activation of Ca2+-induced Ca2+ release via RyR channels of the sarcoplasmic reticulum (SR) and for contractility stimulation in cardiac muscle cells (5). In contrast, L-type Ca2+ channels were shown to couple conformationally with RyRs on depolarization to release Ca2+ during contraction in mature skeletal muscle cells (33, 41, 49). Nevertheless, even though numerous studies exist on the physiological significance of L-type Ca2+ channels in striated muscle cells, their possible role in SPP-induced Ca2+ response remains to be studied.
Because a better understanding of the molecular basis of SPP action may be of crucial importance in understanding the physiological significance and possible pathological implications of this metabolite, it seemed worthwhile, in the present study, to further characterize the Ca2+ response elicited by exogenous SPP in skeletal muscle cells, particularly in view of the crucial role exerted by Ca2+ effector molecules in skeletal muscle development and differentiation. Confocal laser scanning microscopy equipped with Time Course software was then used to determine the spatiotemporal distribution of SPP-mediated Ca2+ transients in a myoblastic C2C12 cell line and the extracellular and intracellular sources of Ca2+ mobilization and to characterize the pattern of expression of voltage- and ligand-gated plasma membrane and intracellular Ca2+ channels in these cells. The effects of SPP-induced Ca2+ transients on the cytoskeletal reorganization were also considered in view of the well-known role that this ion plays in the regulation of cell contractility.
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MATERIALS AND METHODS |
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Cell Cultures
Mouse skeletal C2C12 myoblasts (51) were obtained from American Type Culture Collection (Manassas, VA) and maintained in Dulbecco's modified minimum essential medium (DMEM) with 10% fetal calf serum, penicillin (100 U/ml), and streptomycin (100 µg/ml) (Sigma, Milan, Italy) at 37°C and exposed to a humidified atmosphere of 5% CO2.Confocal Analysis of Calcium Transients
To reveal variations in intracellular concentrations of calcium in C2C12 cells incubated with SPP (Calbiochem, San Diego, CA), ~2 × 104 cells were plated on glass coverslips and incubated at room temperature for 10 min in serum-free DMEM with 0.1% bovine serum albumin (BSA) containing fluo 3-acetoxymethyl ester as fluorescent calcium indicator at a final concentration of 10 µM and 0.1% anhydrous dimethyl sulfoxide and Pluronic F-127 (0.01% wt/vol) as dispersing agent (Molecular Probes, Eugene, OR). The cells were then washed and maintained in fresh medium for 10 min to allow complete deesterification of fluo 3. After that, the cells were placed in open slide flow-loading chambers that were mounted on the stage of a confocal Bio-Rad MRC 1024 ES scanning microscope (Bio-Rad, Hercules, CA) equipped with a krypton/argon laser source (15 mW) for fluorescence measurements. The microscope was also equipped with differential interference contrast (DIC) optics. The fluorescence of fluo 3-loaded cells was monitored by using a 488-nm wavelength and collecting the emitted fluorescence with a Nikon Plan Apo ×60 oil-immersion objective through a 510-nm long-wave pass filter. The time course analysis of Ca2+ waves after SPP stimulation was performed with Time Course Kinetic software (Bio-Rad).Some experiments were performed in Ca2+-free, 2 mM
Mg2+-containing medium and/or after pretreatment of
C2C12 cells with various modulators of known
voltage- and ligand-gated calcium channels. In particular, caffeine
(100 mM; Sigma), 2-aminoethyldiphenylborate (2-APB, 100 µM;
Alexis, San Diego, CA), heparin (50 mM), and
1-[6-([(17)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl]-1H-pyrrole-2,5-dione (U-73122, 10 µM; Alexis) were used to inhibit any potential
Ins(1,4,5)P3R-mediated release, Ry (100 µM;
Sigma) to inhibit RyR-Ca2+ release channels, and
nifedipine (100 nM; Sigma) to inhibit Ca2+ influx
through L-type Ca2+ channels. To test the involvement of
EdgRs in the Ca2+ response, the cells were treated with
suramin (100 µM) before stimulation. SPP was dissolved in the medium
by fast perfusion; the small size of the chamber used (0.2 ml) and the
perfusion flux of ~0.2 ml/s allowed a complete replacement of the
bathing medium in ~1 s.
Usually, cells did not reach confluence on coverslips; a single
coverslip with adherent cells was used for only one experiment. For
each cell preparation a variable number of cells ranging from 10 to 22 were analyzed. Multiple regions of interest (ROIs) of 25 µm2 were selected in single cells to monitor the
spatiotemporal distribution of Ca2+ transients.
Fluorescence signals are expressed as fractional changes above the
resting baseline, F/F, where F is the averaged baseline fluorescence
before the application of SPP and
F represents the fluorescence
changes from the baseline. The latency (T0) of the Ca2+ wave was measured as the lag between the addition
of the agonist and the beginning of the fluorescence increase over the
basal noise. The time to peak (Tp) was measured
as the time interval between T0 and the peak
level. The time to half-decay (T0.5) of fluorescence was measured as the time for the fluorescence to decay
from the peak to half its peak value. The temporal delay to peak
amplitude between adjacent ROIs was used to calculate the extent of
synchronization vs. propagation of Ca2+ transients.
Usually, two ROIs were placed within the nucleus, whereas a variable
number ranging from 3 to 10 ROIs were placed inside the cytoplasm.
Confocal fluorescence images were also used to evaluate intracellular
Ca2+ spatial distribution with a purpose-developed software
running under Interactive Data Language (Research Systems, Boulder, CO).
Determination of Inositol Phosphate Production
Serum-starved C2C12 myoblasts were incubated for 24 h in inositol-free DME in the presence of 5 µCi/ml myo-[2-3H(N)]inositol (25 Ci/mmol; NEN, Dreiech, Germany). Two hours before the beginning of the experiment the medium was changed, and 30 min before the addition of the agonist (1 µM SPP) the cells were incubated with 20 mM LiCl. Incubation was stopped by aspirating the medium and washing the monolayer twice with PBS. Inositol phosphate (InsP) accumulated in the cells was extracted with 5% ice-cold perchloric acid for 30 min. Cell extracts were neutralized with K2CO3, and InsP was separated onto Dowex (Bio-Rad) formiate form (1 ml) and quantified essentially as described previously (45).Determination of Diacylglycerol Production
C2C12 cells were labeled with 5 µCi/ml [2-3H]glycerol (14.2 Ci/mmol; NEN) for 24 h and then incubated for 30 s with 1 µM SPP. Lipid extraction and [3H]diacylglycerol (DAG) separation by thin-layer chromatography was performed as described previously (31).Confocal Immunofluorescence
C2C12 cells grown on coverslips were fixed in 4% buffered paraformaldehyde for 10 min at room temperature. The cells were then washed, permeabilized with 0.2% Triton X-100 in PBS for 5 min, and blocked with a solution containing 0.5% BSA and 3% glycerol in PBS.Calcium channel immunodetection.
Cells were incubated with the following primary antibodies diluted in
BSA-PBS: rabbit anti-1c L-type channels (recognizing
1c-subunit of voltage-gated Ca2+ channel)
and rabbit anti-
1D L-type channels (reacting with all forms of
1D-subunit of voltage-gated Ca2+
channel; 1:100; Chemicon International), mouse
anti-Ins(1,4,5)P3R [recognizing COOH-terminal
cytoplasmic domain of Ins(1,4,5)P3R types 1, 2, and 3; 1:200; Chemicon], and mouse anti-RyR (reacting with
COOH-terminal domain of RyR; 500 kDa, 1:50; Chemicon) for 1 h at
room temperature. After incubation with the primary antibodies, the
cells were washed to remove unbound antibodies (Abs) and incubated with
Alexa 488-conjugated anti-mouse or anti-rabbit secondary Abs (1:200
dilution; Chemicon). Counterstaining for F-actin was performed
with rhodamine-phalloidin (Sigma).
Cytoskeletal protein immunodetection. C2C12 cells were incubated with a monoclonal anti-myosin Ab (1:50 dilution; Sigma) and a monoclonal anti-vinculin Ab (1:100 dilution; Sigma) for 1 h at room temperature. The cells were subsequently incubated with Alexa 488-labeled anti-mouse IgG (Molecular Probes). Actin filaments were stained with tetramethylrhodamine-isothiocyanate-labeled phalloidin. After a series of washes the coverslips containing the immunostained cells were mounted with an antifade mounting medium (Biomeda; Electron Microscopic Sciences). Negative controls were performed by substituting blocking solution for the primary Abs. The fluorescence signals were revealed by a confocal laser scanning microscope (Bio-Rad). To this purpose, a series of optical sections (512 × 512 pixels) was taken through the depth of the cells with a thickness of 1 µm at intervals of 0.8 µm. Twenty optical sections for each examined sample were then projected as a single composite image by superimposition.
Imaging of Dynamic Changes of Actin Cytoskeleton
Alexa 488-labeled G-actin monomers were used as probes for live cell cytoskeleton. The monomers were introduced into C2C12 cells by the scrape-loading technique. Briefly, C2C12 cells grown to confluence on 100-mm petri dishes were washed twice with PBS and 2 ml of scraping buffer (in mM: 114 KCl, 1 NaCl, 5.5 MgCO3, and 10 Tris · HCl). Fluorescent G-actin monomers (250 µg/ml; Molecular Probes) were added in 0.5 ml of scraping buffer, and the cells were gently scraped, suspended in DMEM-10% FCS, and split among six-well dishes on coverslips. After 1 h of incubation, we estimated that ~70% of the treated cells were viable. Labeled cells were then observed by confocal microscope and used to test the effect of SPP on dynamic changes of actin cytoskeleton.Statistics
All data are expressed as means ± SD. Data were analyzed by one-way ANOVA with Bonferroni's correction for multiple comparisons. The ![]() |
RESULTS |
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SPP-Induced Ca2+ Transients in C2C12 Myoblasts
C2C12 cells cultured on coverslips and observed under light microscopy showed variable morphology, being rounded or spindle-shaped with several cytoplasmic projections emanating from the cell surfaces and anchoring to the substrate. We first verified the responsiveness of the C2C12 myoblasts. To this aim, the cells were loaded with fluorescent Ca2+ dye fluo 3 and ATP (1 mM) was added to Ca2+-containing medium. In accordance with previous reports (24), ATP induced a significant intracellular Ca2+ elevation in all cells (data not shown). The application of exogenous SPP (1 µM) to C2C12 cells also promoted a substantial increase in intracellular Ca2+ that was evident in both the cytoplasmic and nuclear compartments. This increase was concentration dependent, with an EC50 of ~50 nM. The SPP-induced Ca2+ increase was transient and was followed by a return to near resting levels within 1 min (Fig. 1). In particular, a synchronous Ca2+ increase was observed in some cells, whereas the Ca2+ response propagated as a wave in others (Figs. 1 and 2A). As shown in Fig. 3A, almost 60% of the examined cells were responsive to SPP, exhibiting relative fluorescence changes significantly (P < 0.001) higher in the nuclear than in the cytoplasmic region (Fig. 3B). Moreover, differences in the time course of the fluorescence signal were found between the cytoplasmic and nuclear regions; in fact, Tp and T0.5 of the nucleus were significantly higher (P < 0.001) and lower (P < 0.05), respectively, compared with those of the cytoplasm (Fig. 3, C-E). The Ca2+ response elicited by exogenous SPP could consist of at least two components: Ca2+ influx across the plasma membrane and Ca2+ release from the endogenous stores. To better investigate this issue, we stimulated C2C12 cells in Ca2+-free, Mg2+-containing medium (Fig. 2B). Under these particular conditions, the number of cells responsive to SPP was reduced to ~40% (Fig. 3A), and the cytosolic and nuclear
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A role for extracellular Ca2+ influx was further confirmed
in two-step experimental protocols consisting of addition of SPP to
cells cultured in Ca2+-free medium and subsequent
readministration of Ca2+ to the medium once the ion
transients had occurred (Fig. 4,
A and B). Indeed, Ca2+
readministration caused a faster elevation of intracellular
Ca2+ in all cells in both the cytoplasmic and nuclear ROIs
(Fig. 4, A, B, F, and G).
The Ca2+ increase was transient but, in contrast to that
elicited by SPP, it rapidly decayed (Fig. 4H) to
steady-state intracellular Ca2+ levels that remained
elevated above the baseline. This latter response was absent when the
two-step procedure was applied to cells not previously stimulated by
SPP (data not shown), thus suggesting the existence of a SPP-dependent
Ca2+ influx pathway mediated by the activation of putative
plasma membrane Ca2+ channels.
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To verify this latter hypothesis, C2C12 myoblasts were treated with nifedipine (100 nM), a prototypical blocker of plasma membrane L-type Ca2+ channels, 20 min before stimulation with SPP (Fig. 2C). The presence of nifedipine in Ca2+-containing medium significantly reduced the Ca2+ transients, which became similar to those observed in Ca2+-free medium (Figs. 2B and 3), supporting a role for nifedipine-sensitive receptors in SPP-mediated Ca2+ influx.
We next tested C2C12 myoblasts for the presence of voltage-dependent ionic channels in C2C12 cells by adding KCl (100 mM) to the medium. High extracellular K+ was not able to elicit any Ca2+ response in undifferentiated C2C12 cells. This finding indicated the absence of voltage-dependent Ca2+ channels and demonstrated that the observed nifedipine-sensitive Ca2+ influx in response to SPP was independent from the existence of functional voltage-dependent L-type Ca2+ channels.
Subsequently, we explored the involvement of intracellular receptors in the Ca2+ mobilization elicited by SPP by adding caffeine (100 mM), a known agonist of RyRs, to the medium. Caffeine was without effect on induction of Ca2+ mobilization in all C2C12 cells examined. Moreover, pretreatment with Ry (100 µM) did not affect SPP-mediated intracellular Ca2+ transients (Figs. 2D and 3), suggesting that RyR-mediated Ca2+ mobilization was absent in C2C12 myoblasts.
Interestingly, pretreatment with caffeine 1 min before SPP stimulation
(Fig. 5A) significantly
reduced the number of responsive cells (P < 0.05) and
the peak of Ca2+ transients in both the cytoplasmic and
nuclear ROIs by ~35% (P < 0.001; Fig.
6, A and B).
Moreover, T0 and T0.5 of
the Ca2+ response were significantly increased with respect
to controls in both the cytoplasm and nuclear ROIs (P < 0.001; Fig. 6). Because it has been reported that caffeine, besides
activating RyRs, inhibits Ins(1,4,5)P3Rs
(53), these latter data are suggestive for an involvement
of the Ins(1,4,5)P3 signaling pathway in the
cytosolic Ca2+ response. As it occurred in the cells
cultured in Ca2+-free medium, the pretreatment with
caffeine before stimulation with SPP elicited a Ca2+
response in the form of a synchronous wave (Fig. 5A). Quite
similar results were obtained by pretreating
C2C12 cells with other inhibitors of the
Ins(1,4,5)P3 signaling Ca2+
pathway such as heparin and 2-APB, both known blockers of
Ins(1,4,5) P3Rs (Figs. 5, B and
C, and 6). In contrast, pretreatment with U-73122, a
specific inhibitor of PLC activation, completely blocked the occurrence
of Ca2+ transients elicited by SPP (Figs. 5D and
6). These data are in favor of a role of PLC activation not only in the
Ins(1,4,5)P3 signaling pathway but also in
mediating plasma membrane Ca2+ influx.
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The involvement of EdgRs in the Ca2+ signaling pathway induced by SPP was tested by pretreating C2C12 cells with suramin, an inhibitor of Edg-3R (Ref. 4; Fig. 5E). Pretreatment with suramin, although significantly (P < 0.001) reducing the number of responsive cells (by ~40%), only slightly affected the time course of the intracellular Ca2+ transients evoked by SPP in C2C12 myoblasts (Figs. 5E and 6).
The dual origin, extracellular and intracellular, of Ca2+ transients elicited by SPP was further supported by the finding of a complete inhibition of SPP-mediated Ca2+ response after pretreatment with caffeine in Ca2+-free medium (Fig. 4C). The subsequent readministration of Ca2+ caused fast elevation of intracellular Ca2+ in all cells examined (Fig. 4D). The Ca2+ increase was transient and decreased to steady-state intracellular Ca2+ levels that remained elevated over baseline. A complete inhibition of the SPP-mediated Ca2+ response in C2C12 cells was also observed after pretreatment with caffeine and nifedipine in Ca2+-containing medium (data not shown), strongly suggesting that the plasma membrane Ca2+ channels implicated in the Ca2+ influx were sensitive to nifedipine.
InsP and DAG Production
To further study the characteristics and mechanisms of the Ca2+ response elicited by SPP, we measured the production of radiolabeled total InsP as well as [3H]DAG after SPP stimulation. Treatment of C2C12 cells with 1 µM SPP increased the cellular levels of both InsP and DAG after 30 s by ~25% ± 3 [10,141 ± 697 (control) vs. 13,747 ± 1,107 disintegrations/min (dpm) InsP/106 dpm labeled phospholipid (SPP); n = 4, P = 0.05] and 41% ± 7 [18,756 ± 2,014 (control) vs. 27,530 ± 2,950 dpm DAG/106 dpm labeled phospholipid (SPP); n = 3, P < 0.01], respectively.Confocal Immunocytochemistry
To further probe the Ca2+ sources implicated in SPP-induced Ca2+ mobilization, we evaluated, by confocal immunofluorescence, the expression of several known plasma membrane as well as intracellular Ca2+ channels in C2C12 myoblastic cells. We found that some cells abundantly expressed L-type Ca2+ channels in the form of small, granular fluorescence bodies, whereas others displayed virtually no reactivity (Fig. 7, A and B). When present, both the
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Cytoskeletal Modifications Induced by SPP
We finally examined whether the SPP-dependent effects on intracellular Ca2+ mobilization were associated with variations in cytoskeletal organization. We first analyzed the organization of the cytoskeletal apparatus in fixed C2C12 cells and found that actin myofilaments represented the main cytoskeletal components in myoblastic cells at this stage of differentiation; actin filaments were arranged in a weblike structure that invaded all the cytoplasm, anchoring to the plasma membrane, and terminated in typical focal adhesion sites containing vinculin immunostaining (Fig. 8). A less defined reaction was, however, observed for myosin filaments, which appeared as scattered fluorescent small cytoplasmic aggregates (data not shown). These results indicated that the cytoskeletal organization of these cells was quite different from the orderly arrays of myofilaments forming the sarcomeric units of mature skeletal muscle cells. Interestingly, in Ca2+-containing medium, SPP stimulated contraction and shortening of living C2C12 cells, but, unexpectedly, this phenomenon did not correlate with the onset of Ca2+ rise in these cells. In fact, by comparing time-lapse video imaging obtained by DIC with the fluorescence Ca2+ images, we found that 88.4% of the cells underwent cytoskeletal contraction whereas only 64% displayed Ca2+ transients in response to SPP stimulation. Moreover, whenever the two phenomena coexisted in the same cell, a clear temporal shift existed between SPP-stimulated cell contractility and SPP-stimulated intracellular Ca2+ increase (Fig. 9). In fact, cell contraction was a very rapid event, occurring within 3-5 s, whereas the rise in intracellular Ca2+ became evident within ~14-35 s (mean value 24 ± 1.5 s; Figs. 1, 2A, and 3C) from SPP stimulation. These data were further confirmed by statistical analyses of the time behaviors of the intracellular spatial Ca2+ distribution (Fig. 10). By plotting together the time course of the area occupied by fluo 3 fluorescence with the temporal behavior of the total fluo 3 fluorescence, it was found that significant modifications in the fluorescence area (i.e., an initial decrease followed by a return to basal levels, indicating cell contraction and cell relaxation, respectively) occurred before the beginning of Ca2+ transients . The small increase in the fluorescence signal (
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To confirm that SPP was able to affect the cytoskeletal reorganization,
C2C12 cells were preloaded with fluorescent
probes for actin. With a time interval quite similar to that observed with DIC video imaging (~5 s), a remarkable reorganization of the
actin cytoskeleton could be visualized (Fig.
11). Indeed, small fluorescent dots,
representing G-actin short polymers, tended to move coordinately and
concentrate toward the nucleus in response to SPP.
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DISCUSSION |
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In the present study we have shown that SPP is capable of
producing intracellular Ca2+ transients in myoblastic
C2C12 cells. In light of our previous findings
(28), this response was mediated by the interaction of the
bioactive lipid with specific EdgRs. Evidence is reported here that the
Ca2+ response elicited by SPP involved both the cytoplasmic
and nuclear compartments as propagated or synchronous waves and
required contributions from intracellular and extracellular
Ca2+ sources. In trying to dissect the Ca2+
signaling pathway we also found that intracellular
Ins(1,4,5)P3-sensitive Ca2+
release channels [Ins(1,4,5) P3Rs] and
nifedipine-sensitive Ca2+ channels
(undifferentiated/non-voltage-dependent L-type channels) of the plasma
membrane were likely involved in the SPP action on myoblasts. Our
experimental evidence to support this hypothesis includes the 40%
reduction of the SPP-mediated Ca2+ response in cells in the
absence of external Ca2+; the requirement of functional PLC
cascade and Ins(1,4,5)P3Rs for this response,
as evidenced by its reduction up to 60% on pretreatment with
inhibitors of Ins(1,4,5)P3Rs, such as heparin, caffeine
and 2-APB, or with U-73122, an inhibitor of PLC; the ability of
nifedipine, a prototypical blocker of L-type Ca2+ channels,
to affect significantly the Ca2+ transients elicited by
SPP; and, finally, the complete inhibition of the Ca2+
response in cells pretreated with caffeine and nifedipine in Ca2+-containing medium or with caffeine in
Ca2+-free medium.
Both the cytosolic and nuclear Ca2+ signals elicited by SPP action were dependent on intracellular Ca2+ increase, regardless of whether it originated from external or endogenous stores. Indeed, removal of external Ca2+ and pretreatment with nifedipine or caffeine significantly affected the latency time of the SPP-mediated Ca2+ response. In these particular circumstances, in fact, T0 was about twofold that of control myoblasts. A possible explanation of these results may be that Ca2+ influx, namely through nifedipine-sensitive channels, and Ca2+ release from the endogenous stores in the cytoplasm, perinuclear, and nuclear regions may be a Ca2+-sensitive, autocatalytic processes. Moreover, under these particular experimental conditions the decrease in the number of cells responsive to SPP with respect to that of control conditions may be linked to the expression of different levels of Ca2+ channels in these cells.
The involvement of Ins(1,4,5)P3R and L-type Ca2+ channels in the SPP-mediated Ca2+ response in both the cytoplasmic and nuclear compartments was further confirmed by confocal immunofluorescence studies. Ins(1,4,5)P3Rs were, in fact, found throughout the cytoplasm and in association with the nuclear envelope, whereas L-type channels were localized both at the plasma membrane and inside the cytoplasm. However, not all the cells revealed the same degree of fluorescence labeling. SPP did not affect Ca2+ release through RyR channels in the myoblastic cells, in agreement with the absence of any detectable immunostaining associated with anti-RyR antibody in C2C12 cells and with previous findings on the absence of RyRs in proliferating undifferentiated skeletal muscle cells (1, 23, 25, 27).
The role for Ins(1,4,5)P3R in the generation
of Ca2+ signals has been well established in several cell
types, including immature and developing skeletal muscle cells
(12, 20). In particular, C2C12
myoblasts have been shown to express several isoforms of G
protein-coupled SPP receptors (Edg-1, Edg-3, and Edg-5 ; Ref. 30), and numerous studies on signal transduction have
demonstrated that these receptors, activated by SPP, stimulate PLC and,
in turn, promote Ins(1,4,5)P3 generation
(34, 37, 40). In contrast, Ca2+ influx through
L-type Ca2+ channels in myoblastic cells has not been
investigated in depth. Voltage-dependent, dihydropyridine-sensitive
L-type Ca2+ channels are multisubunit transmembrane
proteins that allow Ca2+ influx necessary for the
excitation-contraction coupling of cardiac fibers and for modulation of
RyR Ca2+-release channels of skeletal muscle cells
(14). Their expression is developmentally regulated in
embryos and in muscle cell lines, because their plasma membrane density
sharply increases on muscle differentiation (23, 44). In
such a view, the presence of the 1-subunit of
nifedipine-sensitive L-type Ca2+ channels in the
cytoplasm of C2C12 cells probably denotes that they are on course to be transferred to the cell surface during this
stage of myogenic differentiation. Moreover, the presence of these
receptors at the cell surface, where no voltage-gated Ca2+
currents were ever seen, further indicates the existence of
"immature" L-type channels in undifferentiated
C2C12 myoblasts. Nevertheless, a possible
effect of SPP on these channels during muscle cell differentiation
should be taken into account in view of the findings that L-type
Ca2+ channels of cardiac muscle cells (21) and
developing skeletal muscle cells (10, 46) are modulated by
PKC activation, which is a critical step in the signal transduction of
exogenous SPP in C2C12 cells (30).
Moreover, activation of L-type Ca2+ channels, which
involves phosphorylation of both
- and
-subunits, may occur
whether or not the auxiliary subunits are coexpressed (42). Therefore, it is likely that SPP interacting with
Edg cell surface receptors expressed on myoblastic cells triggers a
signaling pathway that, through DAG production and PKC activation, targets regulation of immature plasma membrane L-type Ca2+
channels, thus promoting extracellular Ca2+ influx in
C2C12 cells.
In searching for a possible morphological-functional correlation of the SPP-mediated Ca2+ transients, we examined whether the Ca2+ response was associated with corresponding changes in the cytoskeletal organization in the myoblastic cells, particularly in view of the well-known effects that the sphingolipid has on cytoskeletal remodeling in several cell types (2, 6, 39, 48). Interestingly, we found that SPP promoted cell contractility in the myoblastic cells, but this event did not require intracellular Ca2+ mobilization. Indeed, cell contraction was a rapid event that occurred early after stimulation, whereas the intracellular Ca2+ changes become evident only after longer times, thus suggesting that binding of SPP to Edg receptors was able to activate multiple signaling pathways. Indeed, differential coupling to G proteins and effector systems have been shown for Edg-3 and Edg-5 receptors, which are involved in Ca2+ mobilization and cytoskeletal remodeling, respectively (17, 32, 37, 40). In particular, Edg-5 receptors ligated by SPP stimulate Rho proteins and Rho-dependent kinases have been shown to play an important role in the regulation of smooth muscle and nonmuscle cell contractility by modulating the levels of phosphorylation of myosin light chain (16, 22, 43). Interestingly, in a previous study we demonstrated (29) that RhoA activation also occurs in C2C12 myoblasts after SPP stimulation. In view of that finding and in consideration of the findings reported here that C2C12 cytoskeleton has structural similarities to that of nonmuscle cells, being formed by an extensive network of actin filaments rather than a highly ordered array of myofilaments forming the contractile units of striated fibers, it may be speculated that Ca2+-independent/Rho-dependent contraction of the actin cytoskeleton may also occur in undifferentiated skeletal muscle cells. Although not directly addressed here, it seems likely, on the basis of these data, that Ca2+ transients elicited by exogenous SPP in C2C12 cells may play a role in the myogenic differentiation program rather than in the regulation of cell contractility. This hypothesis would be consistent with an involvement of calcineurin in skeletal muscle differentiation (15) and with recent data showing that C2C12 myoblasts subjected to genetic and metabolic mitochondrial stress release high Ca2+ transients that are capable of enhancing the expression of RyR-1 and of modifying the activity of several Ca2+-dependent transcription factor pathways (7). In keeping with all these findings, it is becoming apparent that Ca2+ released from the muscle Ins(1,4,5)P3Rs may not be significantly involved in muscle cell contraction (20, 47).
In conclusion, the present study, taking advantage of updated techniques and instrumentation, substantially contributes to the understanding of the molecular and functional properties of the Ca2+ signaling pathway triggered by exogenous SPP in myoblastic cells. It appears that SPP exerts profound biological effects on myoblasts that could have physiological and pathophysiological implications. Indeed, very recently, it was shown that cystic fibrosis transmembrane regulator (CFTR), also expressed in skeletal muscle (13), mediates cellular uptake of SPP, thus attenuating SPP signaling (9). In view of this finding, it is tempting to speculate that alterations of the SPP signaling and skeletal muscle cell response may occur in cystic fibrosis, accounting for the peripheral muscle weakness observed in this pathology (11).
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ACKNOWLEDGEMENTS |
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The technical assistance of Dr. Alessia Tani and Dr. Ferdinando Paternostro is gratefully acknowledged.
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FOOTNOTES |
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This work was supported by Telethon Grants no. 945 to F. Francini and no. 1086 to P. Bruni and by Ministero dell' Università e della Ricerca Scientifica e Tecnologica grants (Cofin-1999) to S. Zecchi Orlandini, F. Francini, and P. Bruni.
Address for reprint requests and other correspondence: S. Zecchi Orlandini, Dept. of Anatomy, Histology, and Forensic Medicine, Univ. of Florence, Viale Morgagni, 85, 50134 Florence, Italy (E-mail: zecchi{at}unifi.it).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published January 30, 2002;10.1152/ajpcell.00378.2001
Received 6 August 2001; accepted in final form 25 January 2002.
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