Lippincott Williams & Wilkins, Inc., Philadelphia
Altered Intracellular Calcium Homeostasis in Cerebellar Granule Cells of Prion Protein-Deficient Mice
Version of Record online: 4 JAN 2002
Journal of Neurochemistry
Volume 75, Issue 4, pages 1487–1492, October 2000
How to Cite
Herms, J. W., Korte, S., Gall, S., Schneider, I., Dunker, S. and Kretzschmar, H. A. (2000), Altered Intracellular Calcium Homeostasis in Cerebellar Granule Cells of Prion Protein-Deficient Mice. Journal of Neurochemistry, 75: 1487–1492. doi: 10.1046/j.1471-4159.2000.0751487.x
Abbreviations used: [Ca2+]i, intracellular free calcium concentration; ω-CgTx, ω-conotoxin; DIV, days in vitro; Prnp0/0, deficient in cellular prion protein; PrPC, cellular prion protein; PrPSc pathological isoform of PrPC; PrP106-126, prion protein peptide corresponding to residue 106-126 of the human prion protein; SOD, superoxide dismutase; VGCC, voltage-gated calcium channel.
- Issue online: 4 JAN 2002
- Version of Record online: 4 JAN 2002
- Prion protein;
- Cerebellar granule cells;
- Whole-cell patch clamp
Abstract: Previous studies have indicated that recombinant cellular prion protein (PrPC), as well as a synthetic peptide of PrPC, affects intracellular calcium homeostasis. To analyze whether calcium homeostasis in neurons is also affected by a loss of PrPC, we performed microfluorometric calcium measurements on cultured cerebellar granule cells derived from prion protein-deficient (Prnp0/0) mice. The resting concentration of intracellular free calcium ([Ca2+]i) was found to be slightly, but significantly, reduced in Prnp0/0 mouse granule cell neurites. Moreover, we observed a highly significant reduction in the [Ca2+]i increase after high potassium depolarization. Pharmacological studies further revealed that the L-type specific blocker nifedipine, which reduces the depolarization-induced [Ca2+]i increase by 66% in wild-type granule cell somas, has no effect on [Ca2+]i in Prnp0/0 mouse granule cells. Patch-clamp measurements, however, did not reveal a reduced calcium influx through voltage-gated calcium channels in Prnp0/0 mice. These data clearly indicate that loss of PrPC alters the intracellular calcium homeostasis of cultured cerebellar granule cells. There is no evidence, though, that this change is due to a direct alteration of voltage-gated calcium channels.
The prion protein (PrPC) is a copper-binding glyco-protein of the cell surface predominantly expressed by neurons and glia (Moser et al., 1995; Brown et al., 1997b; Herms et al., 1999). An abnormal isoform of this protein (PrPSc) is widely believed to be the transmissible agent in prion diseases (transmissible spongiform encephalopathies) (DeArmond and Prusiner, 1995). Prion diseases include Creutzfeldt-Jakob disease, Gerstmann—Sträussler—Scheinker syndrome, fatal familial insomnia in humans, scrapie in sheep, and bovine spongiform encephalopathy in cattle. Prion diseases are characterized by neuronal loss, gliosis, and the extracellular accumulation of PrPSc (Kretzschmar et al., 1996). Electrophysiological studies on scrapie-infected mice and hamsters suggest that alterations in excitability and nerve cell loss in scrapie might be due to a decreased Ca2+ entry and a reduced intracellular free Ca2+ concentration ([Ca2+]i) (Jefferys et al., 1994; Barrow et al., 1999). Indeed, studies with cultured scrapie-infected cells indicate that Ca2+ homeostasis may be compromised (Kristensson et al., 1993; Wong et al., 1996). Moreover, the neurotoxic prion protein peptide corresponding to residue 106-126 of the human prion protein (PrP106-126) (Forloni et al., 1993) has been shown to alter the [Ca2+] in neuronal and glial cells (Florio et al., 1996, 1998; Herms et al., 1997; Combs et al., 1999; Silei et al., 1999). The neurotoxic effect of the peptide was found to be reduced by blockers of voltage-gated Ca2+ channels (VGCCs) (Brown et al., 1997a), and further studies gave evidence of a direct alteration of L-type VGCCs by PrP106-126 (Florio et al., 1998). The normal PrPC may also interact directly with VGCC subunits to stabilize membrane permeability (Whatley et al., 1995)
The aim of the present study was to clarify whether loss of PrPC expression in neurons affects intracellular Ca2+ homeostasis. Previous electrophysiological data on PrPC-deficient (Prnp0/0) mice have indeed given some evidence of an alteration of the intracellular Ca2+ homeostasis due to the loss of PrPC. Alterations in Ca2+-dependent K+ currents have been observed in hippocampal pyramidal cells, as well as in cerebellar Purkinje cells, which can best be explained by an altered intracellular Ca2+ homeostasis (Colling et al., 1996; Herms et al., 1998). Here we studied the spatiotemporal changes of [Ca2+]i in cultured cerebellar granule cells of Prnp0/0 mice. Our results show that the loss of PrPC affects basal [Ca2+]i as well as the high K+ depolarization-induced rise in [Ca2+]i.
MATERIALS AND METHODS
The Prnp0/0 mice used in this study were described originally by Büeler et al. (1992). The wild-type mice used in this study are descendants of an F1 generation mouse produced by interbreeding the original parental strain [C57BL/6J and 129/Sv(ev) mice] used to generate Prnp0/0 mice initially.
ω-Conotoxin (ω-CgTx) GVIA and nifedipine were purchased from Tocris (Bristol, U.K.), and fura-2, all salts, and other reagents were from Sigma (St. Louis, MO, U.S.A.) if not otherwise stated.
Neuronal cell culture
Cerebellar granule cells were prepared from 5-day-old mice as described previously (Brown et al., 1997a). Mice were killed by cervical dislocation. Following dissociation in Hanks' solution (Bio Whittaker, Verviers, Belgium) and digestion in 0.5% trypsin, cerebella were plated at 1-2 × 106 cells/cm2 in poly-D-lysine (50 μg/ml)-coated four-well trays (Nunc, Wiesbaden, Germany). Cultures were maintained in Dulbecco's minimal essential medium (Bio Whittaker) supplemented with 10% fetal calf serum, 2 mM glutamine, and 1% antibiotics (penicillin, streptomycin). Trypsin, Dulbecco's minimal essential medium, glutamine, penicillin, and streptomycin were purchased from Seromed Biochrom KG (Berlin, Germany). Cultures were maintained at 37°C with 10% CO2. The K+ concentration within the medium was 5 mM as in previous studies (Brown et al., 1997a,b,c).
Over ∼80% of the cerebellar cells in culture are granule neurons that were identified as being spherical or ovoid cells 5-8 μm in diameter with bipolar neurites. We used small solitary cells for further recordings.
Microfluorometric Ca2+ measurements
Dye loading and subsequent experiments were performed in experimental medium containing the following (in mM): NaCl, 130; HEPES, 10; KCl, 5.4; CaCl2, 1.8; MgCl2, 1.0; D-glucose, 25 (at pH 7.4). Solutions containing 50 mM caffeine were osmotically balanced by lowering the NaCl concentration to 105 mM. Coverslips with attached cells were loaded with the Ca2+-sensitive dye fura-2 acetoxymethyl ester (2 μM; 45 min; Molecular Probes, Göttingen, Germany) at 37°C as described previously (Herms et al., 1997). After washing, cultures were viewed on an upright microscope (BX50 WI, Olympus, Hamburg, Germany). Approximate intracellular Ca2+ concentrations were calculated from the ratio of fura-2 emission evoked by 360- and 380-nm light from a 75-W argon lamp using a digital imaging system (Till Photonics, Munich, Germany). Measurements were acquired at 1—3-s intervals at dual excitation wavelengths (360 and 380 nm), and digital fluorescence images were constructed. They were displayed online as pseudocolour images on a monitor and stored on hard disk for later analysis. Measurements were made on 10-15 single cells, and processes in the stored images were analyzed using the Vision software program (Till Photonics). [Ca2+]i was calculated for each pixel in the frame with fluorescence intensities over a threshold according to Grynkiewicz et al. (1985). The parameters dissociation constant (KD), Rmin, and Rmax characterizing the system were 290 nM, 1,020, and 2,030, respectively. These parameters correspond to average values obtained fromin vivo calibrations. Each experiment was performed on at least four sets of cultures, prepared from different mice. Cells were studied after 2-6 days in culture. Experiments were performed at room temperature, and drugs were applied by bath superfusion (4 ml/min). High K+ solution [in mM: NaCl, 110; HEPES, 10; KCl, 25; CaCl2, 1.8; MgCl2, 1.0; D-glucose, 25 (at pH 7.4)] was applied with a rapid perfusion system (SPM-8, List, Darmstadt, Germany), which enabled us to control the temporal profile in a range of <1 s. Ca2+-free experiments used the same buffer, but CaCl2 was omitted and 20 μM EGTA was added.
Experiments were performed in voltage and current clamp using conventional whole-cell patch-clamp methods (Hamill et al., 1981; Herms et al., 1999). Cells grown on coverslips were placed in the recording chamber and superfused with a solution containing 148 mM tetraethylammonium chloride, 4 mM KCl, 10 mM CaCl2, 1 mM MgCl2, and 10 mM HEPES at room temperature. A granule cell was selected using an upright microscope (Axioscope FS, Zeiss, Göttingen, Germany) with a ×63 water-immersion lens (Zeiss). Electrodes were pulled from borosilicate glass capillaries and filled with a solution containing 120 mM CsCl, 20 mM CsF, 4 mM MgCl2, 4 mM ATP, 10 mM EGTA, and 10 mM HEPES (pH 7.3 adjusted with tetraethylammonium hydroxide; pipette resistance, 3-5 MW). All chemicals were purchased from Sigma (Deisenhofen, Germany). Single-electrode voltage-clamp recordings were performed with a patch-clamp amplifier (EPC-9, HEKA Elektronik, Lambrecht, Germany) using optimal series-resistance compensation as recommended (Llano et al., 1991). The initial series resistance of granule cells before compensation typically was 3-10 MΩ. Values of series resistance and membrane capacitance were obtained from settings of the capacitance cancellation circuitry of the patch-clamp amplifier. Series resistance was set to 70-80%. Recordings were stopped when the holding current increased over 40 pA or any sign of oscillations occurred in the current trace.
The cells were held at a holding voltage of -70 mV and stimulated by 300-ms depolarizing voltage steps at a frequency of 0.67 Hz. Leak and residual capacitance artifacts were estimated and subtracted to give the voltage-activated currents. Peak currents for the on-line analysis were obtained by measuring the maximum current within a period of 15 ms after onset of the depolarizing pulses.
Patch-clamp measurements were performed at room temperature (20-22°C).
The wild-type and Prnp0/0 mouse neonatal cerebellar granule cells investigated in the present work were culturedin vitro for time periods of up to 7 days. During this period, the cells underwent profound changes in phenotype, from a spherical shape on the first day of culture to a complex neurite network organization at the later stages of the culture. Morphologically, no differences were observed between wild-type and Prnp0/0 mouse cerebellar granule cells in culture. However, as described previously, Prnp0/0 mouse granule cells die significantly faster in culture than granule cells of control mice (Brown et al., 1997c)
Basal Ca2+ levels
To assess whether the basal Ca2+ level is independent of the culture period, [Ca2+]i was studied in 1-day intervals from 2 to 6 daysin vitro (DIV). In the cells incubated in the experimental medium, the resting [Ca2+]i was apparently constant (i.e., spontaneous oscillations never occurred). Basal [Ca2+]i levels measured at the somata had a stable value between 54 and 58 nM up to 4 days of culture and increased to 112 nM on the sixth day in the wild-type granule cell culture (Fig. 1A). These values are similar to those described in studies on rat cerebellar granule cells (Ciardo and Meldolesi, 1991). The basal Ca2+ levels in the neurites were in the same range between the second and the fourth day (62 nM) and ∼100 nM the fifth and sixth day in culture (Fig. 1B). In Prnp0/0 mouse cells, a similar increase of the basal Ca2+ level with culture period was observed (Fig. 1A). The mean values in Prnp0/0 mouse granule cell somata were slightly reduced at most time points; however, a significant level was reached only in granule cells at 2 DIV (p < 0.05, Mann-Whitney test). In Prnp0/0 mouse granule cell neurites, however, the resting [Ca2+]i was found to be significantly decreased compared with that in wild-type granule cell neurites at 2, 3, 5, and 6 DIV (Fig. 1B, Mann-Whitney test, p < 0.05).
Effects of high K+
Addition of high K+ media resulted in a rapid [Ca2+]i increase. Typical measurements in wild-type granule cells are shown in Fig. 2A. In the cell soma, [Ca2+]i rose very quickly with the application of the high K+ solution, reaching a peak value 3 s after the beginning of the stimulation. The initial peak was followed by a rapid decline to a plateau (upper trace in Fig. 2A). In the neurite, the peak value was found to be lower than at the soma (bottom traces in Fig. 2A). Also, the biphasic shape of the Ca2+ transient was attenuated in the neurite. Washout of the high K+ solution with the bath medium resulted in a progressive decline of [Ca2+]i (t1/2 = 44 ± 2 s in the soma and 55 ± 4 s in the neurites), with an eventual return to the initial resting level. The high K+-induced responses disappeared in Ca2+-free medium (data not shown). In wild-type granule cells, the high K+-induced rise in [Ca2+]i was blocked by the L-type channel blocker nifedipine (2 μM) by 63% (n = 20).
The peak value of [Ca2+]i after high K+ depolarization at the soma was found to depend on the time in culture (Fig. 2C and D). Whereas granule cells at 3 DIV showed a somatic peak value of 213 ± 21 nM (n = 93), granule cells at 5 DIV demonstrated a peak value of 409 ± 52 nM (n = 78; Fig. 2C). Similar observations were made in rat cerebellar granule cells (Ciardo and Meldolesi, 1991). The neuritic peak [Ca2+]i value, however, was not found to depend significantly on culture time (Fig. 2D). It was found to be slightly lower in granule cells at 5 DIV.
In Prnp0/0 mouse granule cells, depolarization also led to a Ca2+ influx (Fig. 2B). The maximal [Ca2+]i amplitude was reached shortly after the application of the high K+ solution just as in wild-type granule cells. However, the analysis of 122 Prnp0/0 mouse granule cells at 3 DIV and 93 cells at 5 DIV from four different preparations revealed a significant reduction in the peak [Ca2+]i value after high K+ depolarization compared with wild-type cultures (Fig. 2C and D).
To elucidate the relative involvement of L-type VGCCs in [Ca2+]i transient, we blocked L-type Ca2+ channels pharmacologically by using the L-type specific blocker nifedipine. In wild-type cerebellar granule cells, 2 μM nifedipine reduced the maximal amplitude of the [Ca2+]i transient evoked by high K+ solution from 442 ± 14 nM to 145 ± 17 nM (n = 20; Fig. 3), indicating that in our preparation the increase was brought about by an influx of Ca2+ mainly through L-type Ca2+ channels. In Prnp0/0 mouse granule cells, however, nifedipine was not found to have an effect on the high K+ depolarization-induced rise in [Ca2+]i [187 ± 21 nM without nifedipine, 194 ± 30 nM with nifedipine (n = 35), Fig. 3].
Effects of caffeine
To analyze whether the Ca2+-induced Ca2+ release contributes to the observed differences in [Ca2+]i between wild-type and Prnp0/0 mouse granule cells, the effect of caffeine was studied. To correct for differences in the filling state of the intracellular Ca2+ pools, the effect of caffeine was tested after exposure of the cells to a high K+ solution to fill up Ca2+ stores. In wild-type granule cells at 3 DIV, the [Ca2+]i transient elicited by the high K+ depolarization was followed by a rapid [Ca2+]i transient that appeared shortly after the administration of 50 mM caffeine (Fig. 4A). The addition of 3 mMN-hydroxyethylethylenediaminotriacetic acid (pH 7.4) to the medium, decreasing free [Ca2+]i from 1.8 mM to an estimated 10 μM, did not affect the response to caffeine (data not shown).
In Prnp0/0 mouse granule cells at 3 DIV, caffeine increased [Ca2+]i in the same manner as in wild type (Fig. 4B). The peak amplitudes were not found to be significantly different from control either (Fig. 4C). Both in wild-type and in Prnp0/0 mouse granule cells, the caffeine-induced response was strongly dependent on the time in culture: In granule cells at 5 DIV, the application of caffeine was found to have no effect on [Ca2+]i both in wild-type and in Prnp0/0 mouse granule cells (data not shown). This phenomenon of a decrease of the caffeine response after the third day of culturing is already well known from rat cerebellar granule cells (Irving et al., 1992).
To analyze if the altered increase of [Ca2+]i in Prnp0/0 mouse granule cells after high K+ depolarization is due to a reduced Ca2+ influx through VGCCs, we performed patch-clamp measurements. No differences were observed in the resting membrane potential between wildtype (63 ± 1.9 mV; n = 9) and Prnp0/0 mouse granule cells (63 ± 2.0 mV; n = 9) at 3-5 days in culture. Strong depolarization steps from a negative holding potential (300 ms, -70 mV to 20 mV) produced an inward current that activated rapidly and decayed to a nonzero level by the end of the voltage step (Fig. 5A). Concerning the kinetics of this inward current, no differences were found between Prnp0/0 mice and control mice. The threshold for activation of the Ca2+ currents as estimated from current-voltage curves was -29 ± 6 mV (n = 17) in control and -26 ± 5 mV (n = 19) in Prnp0/0 mouse granule cells. Mean holding potential of the maximum current was 27 ± 3 mV in wild-type and 23 ± 3 mV in knockout cells. The sequential block of L- and N-type Ca2+ channels by nifedipine and ω-CgTx GVIA, respectively, is shown in Fig. 5. An addition of 2 μM nifedipine blocked ∼27% of the current (n = 17) in granule neurons from wild-type mice. Further application of ω-CgTx GVIA resulted in an additional decrease down to 34% of the initial current. The non-L/non-N component was blocked completely by 100 μM Cd2+. The residual current that could not be blocked by Cd2+ was subtracted. In contrast to the microfluorometric [Ca2+]i measurements, we did not observe differences in current threshold or kinetics between granule cells at 3 DIV and 5 DIV (data not shown). As depicted in Fig. 5B, Prnp0/0 mouse granule cell VGCCs showed no significant differences in threshold as well as peak current voltage when compared with the current-voltage curves of VGCCs in wild type.
Previous studies have indicated that PrPC may interact with L-type VGCCs and that it may increase Ca2+ influx (Whatley et al., 1995). To evaluate whether the loss of PrPC in Prnp knockout mice alters the intracellular Ca2+ homeostasis, we performed microfluorometric and patchclamp measurements on cultured cerebellar granule cells.
The microfluorometric experiments revealed alterations both in the basal [Ca2+]i and in the changes of [Ca2+]i with high K+ depolarization. The differences in the effect of high K+ depolarization were found to be most prominent at the soma of cells after 5 days in culture. Nifedipine was found to have no effect on the rise in [Ca2+]i with high K+ depolarization in Prnp0/0 mouse granule cells, indicating an alteration of L-type VGCCs. Patch-clamp studies, however, did not reveal a reduced Ca2+ influx through VGCCs in cultured Prnp0/0 mouse granule cells. This indicates that the reduced rise in [Ca2+]i with high K+ depolarization in Prnp0/0 mouse granule cells is not due to direct alterations of VGCCs. We therefore looked at other mechanisms that could lead to a reduced rise in [Ca2+]i with high K+ depolarization in Prnp0/0 mouse granule cells. We excluded the possibility that differences in the resting membrane potential are the cause of the reduced rise in [Ca2+]i with high K+ depolarization. Both wild-type and Prnp0/0 mouse granule cells were found to have a resting membrane potential of -63 mV. Nor was there evidence of a reduced Ca2+-induced Ca2+ release from intracellular stores that could serve as an explanation of a reduced rise in [Ca2+]i with high K+ depolarization even when the amount of Ca2+ that enters the cell through VGCC is the same. We could not exclude, however, the possibility of differences in the intracellular buffer capacity and extrusion mechanism, which are known to modulate [Ca2+]i. An increased buffer or extrusion capacity could indeed explain a reduced basal [Ca2+]i, as well as a reduced rise [Ca2+]i with high K+ depolarization, in Prnp0/0 mouse granule cells independently of the Ca2+ influx through VGCCs. However, there are so far no clues how the loss of PrPC could affect these mechanisms.
What needs to be taken into account, though, is the fact that changes in the Ca2+ influx through VGCCs might be masked by the use of the patch-clamp technique. The Ca2+ influx through VGCCs is subject to regulation by various intracellular mechanisms, some of which are altered when patch-clamp measurements are performed. If the differences in [Ca2+]i between Prnp0/0 mouse granule cells and wild-type cells are due to a modulation of VGCCs by cytoplasmatic substances that are diluted into the patch pipette when performing patch-clamp measurements, this might cause similar Ca2+ currents in wild-type and Prnp0/0 mouse granule cells. We have shown previously that the copper-depending enzyme superoxide dismutase (SOD) shows a significantly reduced activity in Prnp0/0 mouse granule cells (Brown et al., 1997c). The SOD is prominently involved in the reduction of intracellularly produced oxygen radicals, which are known to affect VGCCs. Oxygen radicals alter L-type VGCCs, an effect that can be prevented by the application of SOD (Guerra et al., 1996). If therefore a higher oxygen radical load in Prnp0/0 mouse granule cells causes the reduced rise in [Ca2+]i with high K+ depolarization in the microfluorometic measurements, this could be masked in patch-clamp measurements, where the intracellular fluid is rapidly diluted by the solution that is within the patch pipette.
In conclusion, microfluorometric measurements in cultured cerebellar granule cells of Prnp0/0 mice reveal a prominent alteration of the [Ca2+]i. This finding could serve as an explanation of previously observed alterations in Ca2+-activated K+ currents in hippocampal CA1 pyramidal cells (Colling et al., 1996), as well as cerebellar Purkinje cells of Prnp0/0 mice (Herms et al., 1998), and supports the notion that these alterations may be secondary effects due to alterations in [Ca2+]i. It remains to be clarified if the reduced Ca2+-activated K+ currents that have been observed in scrapie-infected hamster (Barrow et al., 1999) might also be due to a loss of the function of PrpC in Ca2+ homeostasis. We have no indication that a loss of PrpC directly alters VGCCs. Therefore, we cannot support the notion that PrpC is directly involved in the modulation of VGCC subunits as suggested previously (Whatley et al., 1995). Supported by our previous studies (Brown et al., 1997b,c; Herms et al., 1999), we favor the hypothesis that alterations observed in [Ca2+]i in Prnp0/0 mouse granule cells are due to alterations in oxygen-radical clearance.
The authors thank Charles Weissmann for providing the Prnp0/0 mice. This study was supported by research grants from the Wilhelm Sander-Stiftung (9343008) and the Deutsche Forschungsgemeinschaft (SFB 406, A10) to J.W.H. and H.A.K.
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