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International Journal of Alzheimer's Disease
Volume 2011, Article ID 706576, 6 pages
http://dx.doi.org/10.4061/2011/706576
Research Article

The Cellular Prion Protein Prevents Copper-Induced Inhibition of P2 Receptors

1Departamento de Fisiología, Centro de Envejecimiento y Regeneración (CARE), Facultad de Ciencias Biológicas, P. Universidad Católica de Chile, Santiago 8331150, Chile
2Departamento de Biología Celular y Molecular, Centro de Envejecimiento y Regeneración (CARE), Facultad de Ciencias Biológicas, P. Universidad Católica de Chile, Santiago 8331150, Chile

Received 1 June 2011; Accepted 16 August 2011

Academic Editor: Rosanna Squitti

Copyright © 2011 Ramón A. Lorca et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Although the physiological function of the cellular prion protein (PrPC) remains unknown, several evidences support the notion of its role in copper homeostasis. PrPC binds Cu2+ through a domain composed by four to five repeats of eight amino acids. Previously, we have shown that the perfusion of this domain prevents and reverses the inhibition by Cu2+ of the adenosine triphosphate (ATP)-evoked currents in the P2X4 receptor subtype, highlighting a modulatory role for PrPC in synaptic transmission through regulation of Cu2+ levels. Here, we study the effect of full-length PrPC in Cu2+ inhibition of P2X4 receptor when both are coexpressed. PrPC expression does not significantly change the ATP concentration-response curve in oocytes expressing P2X4 receptors. However, the presence of PrPC reduces the inhibition by Cu2+ of the ATP-elicited currents in these oocytes, confirming our previous observations with the Cu2+ binding domain. Thus, our observations suggest a role for PrPC in modulating synaptic activity through binding of extracellular Cu2+.

1. Introduction

Prion diseases are a group of fatal neurodegenerative disorders that are sporadic, inherited, or transmissible [1]. These include kuru and Creutzfeldt-Jakob disease in humans, scrapie in sheep and bovine spongiform encephalopathy in cattle. These pathologies are caused by the conformational transition of the native and predominantly α-helical cellular prion protein (PrPC) into a significantly more β-sheet-containing pathogenic isoform (PrPSc) [2], which unlike PrPC, is insoluble in mild detergents and partially resistant to digestion with proteinase K [3]. PrPC is a cell surface glycosylphosphatidylinositol-anchored protein that is mainly expressed in neurons and glial cells and to a lesser extent in several peripheral tissues [4, 5]. The normal physiological function of PrPC remains elusive, although it has been related to signaling, neuroprotection, neuritogenesis, synaptic transmission, oxidative stress, and copper metabolism [611].

PrPC binds copper ions with low micromolar affinity via histidine and glycine-containing peptide repeats in its N-terminal region [1217]. This Cu2+ binding domain is located between residues 60–91 and consists of four identical repeats of the peptide sequence Pro-His-Gly-Gly-Gly-Trp-Gly-Gln. Although the number of octapeptide repeats varies in different species, in mammals this region is one of the most highly conserved [18] and therefore, very likely defines a functional domain of PrPC. In vitro, the octarepeat region has the capacity to reduce Cu(II) to Cu(I) [19, 20]. In addition, there is another Cu2+ binding site outside the octarepeat region [2124] of higher affinity, in the order of nanomolar, that involves His96 and His111 [24]. PrPC is localized presynaptically at central synapses [2527] and is found in synaptic membranes and in synaptic vesicles [9, 28]. Furthermore, PrPC-null mice show an impaired long-term potentiation, suggesting that PrPC is involved in normal synaptic function [10], and moreover, it has been shown that PrPC is involved in regulating the presynaptic Cu2+ concentration and synaptic transmission [9].

The P2X family of nucleotide receptors forms non-selective cationic channels activated by extracellular adenosine triphosphate (ATP) [29]. These receptors are widely expressed in the central nervous system (CNS) [3032] and are involved in synaptic transmission and plasticity including long-term potentiation as recently shown by us [33]. Interestingly, trace metals modulate P2X receptors, particularly, the P2X4 receptor subtype is differentially modulated by trace metals at physiological concentrations [3437]. While Zn2+ facilitates the ATP-evoked currents, Cu2+ inhibits it in a concentration-dependent manner [37]. Previously, we demonstrated that the N-terminal octarepeat fragment of the PrPC prevents and reverses the inhibitory action of Cu2+ on the P2X4 receptor when added to the media [38]. Herein, in an attempt to determine whether the PrPC-Cu2+ interaction is relevant to synaptic activity, we extended our investigations to test whether the full-length PrPCco-expressed with the P2X4 receptor may modulate in situ the Cu2+-induced inhibition of the ATP current gated by the P2X4 receptor.

2. Materials and Methods

2.1. Drugs and Chemicals

Copper chloride, ATP (as the tetrasodium salt), collagenase IA, and penicillin-streptomycin were purchased from Sigma Chemical Co (St Louis, Mo). All the salts used to prepare the Barth’s incubation media and the recording solutions were analytically graded and were purchased from Merck (Darmstadt, Germany).

2.2. Oocyte Preparation, Injection, and Electrophysiological Recordings

A segment of the Xenopus laevis ovary lobe was surgically removed from adult anesthetized frogs; stages V-VI oocytes were manually defolliculated and then incubated with collagenase IA (1 mg/mL) for 30 min. Oocytes were manually injected with 7.5–12.5 ng cDNA coding for the rat P2X4 receptor with or without cDNA coding for the hamster prion protein (PrP-3F4), both cDNAs in plasmid pcDNA3, at 250 ng/μL. After 48–72 h of incubation at 15°C in Barth’s solution (in mM): 88 NaCl, 1 KCl, 2.4 NaHCO3, 10 HEPES, 0.82 MgSO4, 0.33 Ca(NO3)2, pH 7.5, supplemented with 10 IU/L penicillin/10 mg streptomycin, oocytes were clamped at −70 mV using the two-electrode voltage clamp technique with an OC-725C oocyte clamper (Warner Instrument Corp, Hamden, CT). ATP and CuCl2, dissolved in Barth’s solution, were superfused at 2 ml/min. ATP-evoked currents were recorded with a 10 s ATP exposure applied regularly at 10–15 min intervals. These intervals were increased up to 25 min for maximal ATP concentrations in concentration-response curves protocols to decrease desensitization. Copper was applied for 30 s prior 10 μM ATP (coapplied with CuCl2).

2.3. Confocal Microscopy

To study the distribution of PrP, oocytes were coinjected with the cDNA coding for the rat P2X4 receptor with the cDNA coding for mouse PrP-GFP (MmPrP-EGFP[25-266]-cDNA3). Oocytes, where P2X4 receptor expression was validated electrophysiologically, were directly analyzed on a Zeiss LSM 5 Pascal confocal microscope.

2.4. Western Blotting

After electrophysiological protocols, each oocyte injected with cDNa coding for P2X4 and PrP-3F4 was homogenized for 30 min in ice, using 40 μL of lysis buffer per oocyte (100 mM NaCl, 20 mM Tris-HCl pH 7.4, 1% Triton X-100) supplemented with a protease inhibitors cocktail [39]. The extracts were centrifuged for 30 s at 14000 r.p.m. at 4°C and the supernatant was removed and resolved by 12% SDS-PAGE and transferred to nitrocellulose. Nonspecific binding sites were blocked with 5% (w/v) milk in Tris-Buffered Saline (TBS) 0.1% Tween (TBST) for 1 h. After blocking, blots were incubated with monoclonal anti-3F4 antibody [40], diluted 1 : 5000 in 3% (w/v) milk in TBST for 1 h at room temperature, followed by three 15 min washes in TBST at room temperature. The reactions were followed by incubation with anti-mouse antibody peroxidase labeled (Pierce, Rockford, IL) and developed by enhanced chemiluminescence.

2.5. Data Analysis

The average reduction of the ATP-gated current was normalized. The ATP and Cu2+ concentration-response curves were fitted to a sigmoid function using the GraphPad Prism software (San Diego, Cal). The median effective (EC50) or median inhibitory concentrations (IC50) for ATP or copper, respectively, were interpolated from these curves. Each protocol was performed in separate oocytes coming from at least two separate batches of oocytes. Mann-Whitney nonparametric Student’s t-test was used for statistical analysis. A was considered significant.

3. Results

3.1. The Expression of PrP-3F4 Did Not Change the ATP Concentration-Response Curve of P2X4 Receptors

To evaluate whether the expression of PrPC modulates the inhibition of the P2X4 receptor by Cu2+, we first evaluated the expression of PrPC in oocytes co-injected with the cDNA coding for the hamster prion protein (PrP-3F4) and the cDNA coding for the rat P2X4 receptor. Figure 1(a) shows the detection by western blot of P2X4 receptor and PrP-3F4 using an antibody that recognizes the 3F4 epitope [40]. β-Tubulin detection was used as a loading control. As observed, both proteins are strongly detected in an injected oocyte and not in the control noninjected oocyte. Then we analyzed the distribution of PrPC in oocytes co-injected with the cDNA coding for the rat P2X4 receptor and the cDNA coding for PrP-GFP. Oocytes in which the expression of P2X4 receptor was verified electrophysiologically were analyzed in a confocal microscope to study the localization of PrP-GFP. As observed in Figure 1(b), PrP-GFP is located on the surface of injected oocytes.

fig1
Figure 1: Coexpression of P2X4 and PrPC in X. laevis oocytes. (a) Western blot of total lysate fractions from a non-injected oocyte (left lane, 1) and from an oocyte co-expressing P2X4 receptor and PrP-3F4 (right lane, 2). Numbers on the right are molecular weights in kDa. (b) Fluorescence microscopy of an oocyte co-expressing P2X4 receptor and PrP-GFP (green), bar = 10 μM.

Then, we evaluated the ATP concentration-response curves in oocytes expressing the P2X4 receptor and coexpressing the P2X4 receptor and PrP-3F4. The presence of PrP-3F4 caused a slight, but not significant, reduction in the potency of ATP, reflected as an increase in its EC50 from 11.2 ± 1.1 μM for P2X4 alone to 45.2 ± 9.4 μM for P2X4/PrP-3F4 ( , , Figure 2), this slight displacement of ATP concentration-response curve in the presence of PrP-3F4 could represent a minor regulation of PrP-3F4 on P2X4 receptor activity.

706576.fig.002
Figure 2: ATP concentration-response curves from oocytes expressing P2X4 receptor (open circles) or co-expressing P2X4 receptor and PrP-3F4 (closed circles). Symbols are mean values ± SEM, numbers in parenthesis are number of oocytes.
3.2. The Co-Expression of P2X4 Receptors and PrP-3F4 Partially Prevents the Copper-Induced Inhibition of the ATP-Evoked Currents

We assess the Cu2+-induced inhibition of 10 μM ATP currents in oocytes expressing P2X4 receptors. The magnitude of the inhibition by 10 μM Cu2+, preapplied during 30 s, was 51.5 ± 5.3% of the 10 μM ATP-evoked currents ( , Figures 3(a) and 3(b)). However, the 10 μM Cu2+-induced inhibition was reduced only to 71.9 ± 5% of the 10 μM ATP-evoked currents in oocytes co-expressing P2X4 receptors and the PrP-3F4 ( , compared to P2X4 alone, Figures 3(a) and 3(b)), showing that PrP-3F4 prevented the Cu2+-induced inhibition of P2X4 receptors compared to the Cu2+ inhibition elicited in oocytes expressing only this receptor. Furthermore, the presence of PrP-3F4 in the oocytes caused a rightward displacement of the Cu2+ concentration-response curve obtained in oocytes expressing only P2X4 receptor, an IC50 of 11.5 ± 1.9 μM was obtained for P2X4 and 34.1 ± 7.6 μM for P2X4/PrP-3F4 ( , , Figure 3(c)), confirming that PrP-3F4 prevented the Cu2+-induced inhibition not only at low micromolar concentrations of Cu2+, but even at higher physiological concentrations of the metal.

fig3
Figure 3: PrPC prevents Cu2+-induced inhibition of P2X4 receptor. (a) Representative recordings obtained from oocytes expressing P2X4 receptor (left traces, P2X4) or coexpressing P2X4 receptor and PrP-3F4 (right traces, P2X4/PrP-3F4) showing 10 μM ATP-evoked currents (open bars) and its inhibition by 10 μM Cu2+ (closed bars). (b) Statistical analysis of Cu2+ inhibition showed in (a), performed in different oocytes ( , * versus ATP, # versus P2X4 alone). Bars are mean values ± SEM. (c), Cu2+ concentration-response curves of 10 μM ATP inhibition in oocytes expressing P2X4 receptor (open circles) or co-expressing P2X4 receptor and PrP-3F4 (closed circles). Symbols are mean values ± SEM, numbers in parenthesis are number of oocytes.

4. Discussion

Several functions have been attributed to PrPC, including immunoregulation, signal transduction, copper binding, neurite outgrowth, induction of apoptosis or prevention of apoptosis against apoptotic stimuli, and others [41]. In addition, PrPC has been related to synapse formation and maintenance and synaptic transmission [9, 10, 42], although the mechanisms by which it exerts its role is still unknown. One of the proposed targets for PrPC in synapse is to modulate Cu2+ homeostasis, based on a highly conserved Cu2+-binding sequence located on its N-terminal domain, which includes four identical repeats of the peptide sequence Pro-His-Gly-Gly-Gly-Trp-Gly-Gln [12, 15, 16]. It is known that PrPC binds Cu2+ with high affinity [1417], and the octarepeat region of the human PrPC (PrP59-91) reduces Cu(II) to Cu(I) in vitro, which depends on the tryptophan residues present in the octapeptide repeats [19, 20]. Cu2+ modulates synaptic transmission at micromolar concentrations by a wide range of mechanisms, be one of the most relevanting modulations of neurotransmitter receptors within glutamatergic, gabaergic, and purinergic synapses, among others [43, 44]. In a previous study, we demonstrated that Cu2+ at micromolar concentrations inhibits the ATP-evoked currents of P2X4 receptors [37]. Here we show that the full-length prion protein-expressed in Xenopus oocytes localizes in the cell surface and modulates the Cu2+ interaction with P2X4 receptor; oocytes which coexpressed PrP-3F4 and P2X4 receptors have a diminished Cu2+-induced inhibition of the ATP-evoked currents compared with oocytes which only expressed the P2X4 receptor. This reduced inhibition by Cu2+ was observed on Cu2+ concentration-response curves, where the IC50 of Cu2+ was significantly increased in the presence of PrP-3F4, indicating that PrP-3F4 can exert its modulatory role even at high micromolar concentrations of Cu2+, reached in the synaptic cleft after depolarization [45]. These results, together with our previous findings showing that coapplication of Cu2+ with the N-terminal PrP fragment (PrP59-91) prevents the inhibitory effect of copper on P2X4 receptors and even reverts the established Cu2+-induced inhibition of the P2X4 receptors [38], strongly support the idea that PrPC could modulate synaptic copper and therefore affect the function of P2X4 receptors and synaptic transmission.

In addition to the potential synaptic role of PrPC driven by its ability to bind Cu2+, a known modulator of neuronal excitability [43, 44], there is increasing evidence of direct interaction between PrPC and neurotransmitter receptors. PrPC directly interacts with the NR2D subunit of the NMDA receptor, inhibiting it and preventing NMDA-induced excitoxicity in the hippocampus [46]. On the other hand, PrPC also exerts a neuroprotective role against kainate-induced neurotoxicity in the hippocampus, probably by regulating differentially the expression of GluR6 and GluR7 kainate receptor subunits [47]. Moreover, PrPC can modulate the activity of serotoninergic receptors signaling pathways in 1C115-HT cells [48]. We observed a slight, although not significant, reduction on ATP affinity of P2X4 receptor in the presence of PrP-3F4, this might suggest an interference with ATP binding or stabilization of closed states, although further experiments are required to evaluate this hypothesis. Altogether, these studies and the presented here highlight the modulatory role of PrPC at synaptic transmission in CNS, involving direct regulation of neurotransmitter receptors and/or their signaling cascade, or indirectly, by controlling the synaptic levels of Cu2+.

The understanding of the physiological function of PrPC on synaptic transmission may clarify the pathogenic processes underlying prion diseases. Based on our results, it is possible to suggest that the resulting cognitive deterioration of prion diseases could involve a loss of the modulatory role of PrPC on brain function, as it is converted to the pathogenic isoform.

Abbreviations

PrPC: Cellular prion protein
ATP:Adenosine triphosphate
CNS:Central nervous system
EC50:Median effective concentration
IC50:Median inhibitory concentration.

Acknowledgments

The authors would like to thank Dr. Claudio Soto (Department of Neurology, The University of Texas Medical School, Tex, USA) for his kind gift of the PrP-GFP construct, and Dr. Richard Kascsak (New York State Institute for Basic Research, Staten Island, NY, USA) for mouse anti-3F4 antibody. This work was supported by Basal Center of Excellence in Aging and Regeneration (CONICYT-PFB12/2007). Ramón A. Lorca is currently with Department of Obstetrics and Gynecology, Center for Women's Reproductive Sciences Research, BJC Institute of Health, Washington University in St. Louis, St. Louis, MO, USA.

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