We have examined the effect of temperature on the electrophysiological properties of three neuronal nicotinic acetylcholine receptor (nAChR) subtypes: the rapidly desensitizing homomeric α7 nAChR, the more slowly desensitizing heteromeric α4β2 nAChR and on α7 nAChRs containing a transmembrane mutation (L247T) that results in dramatically reduced desensitization. In all cases, the functional properties of receptors expressed in Xenopus oocytes at room temperature (RT; 21°C) were compared to those recorded at either physiological temperature (37°C) or at lower temperature (4°C). Alterations in temperature had dramatically differing effects on the amplitude of whole-cell responses detected with these three nAChR subtypes. Compared to responses at RT, the amplitude of agonist-evoked responses with α4β2 nAChRs was increased at high temperature (125±9%, n = 6, P<0.01) and reduced at low temperature (47±5%, n = 6, P<0.01), whereas the amplitude of α7 responses was reduced at high temperature (27±7%, n = 11, P<0.001) and increased at low temperatures (224±16%, n = 10, P<0.001). In contrast to the effects of temperature on α4β2 and wild type α7 nAChRs, the amplitude of α7 nAChRs containing the L247T mutation was unaffected by changes in temperature. In addition, changes in temperature had little or no effect on current amplitude when α7 nAChRs were activated by the largely non-desensitizing allosteric agonist 4BP-TQS. Despite these differing effects of temperature on the amplitude of agonist-evoked responses in different nAChRs, changes in temperature had a consistent effect on the rate of receptor desensitization on all subtypes examined. In all cases, higher temperature resulted in increased rates of desensitization. Thus, it appears that the differing effects of temperature on the amplitudes of whole-cell responses cannot be explained by temperature-induced changes in receptor desensitization rates.
Citation: Jindrichova M, Lansdell SJ, Millar NS (2012) Changes in Temperature Have Opposing Effects on Current Amplitude in α7 and α4β2 Nicotinic Acetylcholine Receptors. PLoS ONE 7(2): e32073. doi:10.1371/journal.pone.0032073
Editor: Jon T. Brown, University of Bristol, United Kingdom
Received: November 8, 2011; Accepted: January 19, 2012; Published: February 16, 2012
Copyright: © 2012 Jindrichova et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: MJ was supported by a Short Term Fellowship from Federation of European Biochemical Societies (FEBS). SJL was supported by a grant awarded to NSM from The Wellcome Trust (WT085141). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of ligand-gated ion channels which also includes receptors for 5-hydroxytryptamine (5-HT), γ-aminobutyric acid (GABA) and glycine . In common with other Cys-loop receptors, nAChRs are oligomeric transmembrane proteins in which five subunits co-assemble to form a central ion-channel pore . In addition, subunits of Cys-loop receptors share a common transmembrane topology, containing an extracellular N-terminal region and four α-helical transmembrane domains .
Seventeen nAChRs subunits (α1–α10, β1–β4, γ, δ and ε) have been identified in vertebrate species and can co-assemble into a large number of nAChR subtypes with considerable diversity in subunit composition . In most cases, nAChRs are heteromeric complexes (containing two, three or four different subunit subtypes) but some subunits, such as α7, are capable of forming homomeric receptors (containing five copies of a single subunit). Within the mammalian brain, two nAChR subtypes (heteromeric α4β2 nAChRs and homomeric α7 nAChRs) have attracted particular attention as targets for pharmaceutical drug discovery. Homomeric α7 nAChRs have high calcium permeability and very rapid desensitization. They have been identified as potential drug targets in treatment of disorders such as Alzheimer's disease and schizophrenia , , . Heteromeric α4β2 nAChRs have lower calcium permeability and display less agonist-induced desensitization. Receptors containing α4 and β2 subunits mediate the effects of nicotine associated with tobacco smoking and are the site of action of drugs used to assist with smoking cessation . In addition, α4β2 nAChRs are targets for drug discovery in areas such as cognition, attention and pain , , .
A variety of experimental approaches have confirmed that conventional orthosteric agonists such as acetylcholine bind at an extracellular site located at the interface of two subunits , . More recently, studies with α7 nAChRs have demonstrated that nAChRs can also be activated by agonists binding to an allosteric site located in the transmembrane region , a site that has previously been proposed as the binding site for a range of allosteric modulators of α7 nAChRs , . Whereas activation of α7 nAChRs by acetylcholine results in rapid desensitization , activation by allosteric agonists such as 4BP-TQS results in very low levels of desensitization , consistent with these two agonists having different mechanisms of action. Previous studies have also demonstrated that the rapid rate of desensitization observed when α7 nAChRs is activated by orthosteric agonists such as acetylcholine can be reduced dramatically by the introduction of a single point mutation (L247T) located within the second transmembrane domain .
A particular advantage of studies conducted with recombinant nAChRs in artificial expression systems is the ability to examine the properties of receptors with defined subunit composition, as well as the ability to examine the effects of alterations in amino acid composition by means of techniques such as site-directed mutagenesis . Typically, electrophysiological studies of recombinant nAChRs are conducted at room temperature, as are studies of native receptors from isolated cell and tissue preparations. In the present study we have examined the influence of conducting electrophysiological recordings at temperatures above and below room temperature (37°C and 4°C). By means of expression studies in Xenopus oocytes, we have examined heteromeric α4β2 and homomeric α7 nAChRs. In addition, we have examined the effect of changes in temperature on responses evoked by both orthosteric and allosteric agonists, as well as on α7 nAChRs containing a mutation that slows the rate of desensitization caused by orthosteric agonists such as acetylcholine. Changes in temperature resulted in changes in the magnitude of agonist-evoked responses. However, opposing effects were observed on different nAChR subtypes. Changes in temperature were also associated with changes in rates of receptor desensitization, but this does not appear to explain the differences observed in current amplitudes at different temperatures.
Effect of temperature on current amplitude in α7 nAChRs
Expression of the human α7 nAChR in Xenopus oocytes was examined by two-electrode voltage-clamp recording. When acetylcholine-evoked responses were recorded at room temperature (RT; 21°C), rapidly desensitizing currents were observed (Fig. 1), typical of α7 nAChRs . When responses were recorded from the same oocytes at physiological temperature (37°C), agonist-evoked current amplitudes were significantly smaller (P<0.001; Fig. 1, Table 1). This was the case, irrespective of whether a maximal concentration (3 mM; Fig. 1A) or an EC50 concentration (100 µM; Fig. 1B) of acetylcholine or was used. Conversely, when responses were recorded at lower temperature (4°C), currents were significantly larger (P<0.001; Fig. 1, Table 1). Again, this was the case, irrespective of whether a maximal concentration or an EC50 concentration of acetylcholine or was used (Fig. 1, Table 1).
Bar charts illustrate responses (mean ± SEM) from α7 nAChRs expressed in Xenopus oocytes in response to a maximal (3 mM) and EC50 (100 µM) concentration of acetylcholine (A and B, respectively) at room temperature (RT; 21°C), higher temperature (37°C) and lower temperature (4°C). Data are means of 7–11 responses, each from a different oocyte, in which responses obtained at either 4°C or 37°C are normalized to responses obtained from the same oocyte at RT. C) Representative traces illustrating responses obtained at RT (upper trace) and 4°C (lower trace) from a single oocyte. D) Representative traces illustrating responses obtained at RT (upper trace) and 37°C (lower trace) from a single oocyte.
Effect of temperature on current amplitude in α4β2 nAChRs
A similar series of experiments was performed with the human α4β2 nAChR subtype, which displays lower levels of desensitization than homomeric α7 nAChRs (Fig. 2). With α4β2 nAChRs, the effect of temperature on current amplitude was the opposite of that observed with α7 nAChRs. When responses were recorded from oocytes at 37°C, agonist-evoked current amplitudes were significantly larger than at RT (P = 0.002; Fig. 2, Table 1). Conversely, when responses were recorded at 4°C, currents were significantly smaller than at RT (P = 0.002; Fig. 2, Table 1).
Bar charts illustrate responses (mean ± SEM) from α4β2 nAChRs expressed in Xenopus oocytes in response to a maximal (1 mM) concentration of acetylcholine in either calcium-contanining (A) or barium-containing Ringer solution (B) at room temperature (RT; 21°C), higher temperature (37°C) and lower temperature (4°C). Data are means of 6–9 responses, each from a different oocyte, in which responses obtained at either 4°C or 37°C are normalized to responses obtained from the same oocyte at RT. C) Representative traces illustrating responses obtained at RT (upper trace) and 4°C (lower trace) from a single oocyte. D) Representative traces illustrating responses obtained at RT (upper trace) and 37°C (lower trace) from a single oocyte. Representative traces are from calcium-containing saline but similar responses were obtained with barium-containing saline.
Whereas oocyte recordings with α4β2 nAChRs were conducted in calcium-containing saline, recordings with α7 nAChRs were conducted in barium-containing (calcium-free) saline. A calcium-free saline is typically used with α7 nAChRs to minimize the possibility of calcium-activated chloride channels (due to the higher calcium permeability of α7 nAChRs). In order to examine whether differences in saline composition might explain differences in the effect of temperature on current amplitudes, experiments with α4β2 nAChRs were repeated in calcium-free saline. The effect of temperature on current amplitude with α4β2 nAChRs was consistent between the two saline solutions (Fig. 2, Table 1). In addition, further experiments were performed with α7 nAChRs to examine the effects of temperature on responses recorded in calcium-containing saline, rather than in barium-containing saline. The effect of both high (37°C) and low (4°C) temperature on current amplitude of α7 nAChRs was not significantly different to that observed in barium-containing saline (n = 5).
Effect of temperature on current amplitude in α7L247T nAChRs
A further series of experiments was performed with the human α7 nAChR containing a single point mutation (L247T) in the second transmembrane domain. As has been described previously , one consequence of this mutation is a reduction in the extent of receptor desensitization (Fig. 3). In contrast to the situation with the wild-type α7 nAChR (Fig. 1), no significant difference was observed in the effect of temperature on the amplitude of responses evoked by acetylcholine on α7 nAChRs containing the L247T mutation (Fig. 3). This was the case, irrespective of whether a maximal concentration (30 µM) or an EC50 concentration (0.4 µM) of acetylcholine or was used (Fig. 3, Table 1).
Bar charts illustrate responses (mean ± SEM) from α7L247T nAChRs expressed in Xenopus oocytes in response to a maximal (30 µM) and EC50 (0.4 µM) concentration of acetylcholine (A and B, respectively) at room temperature (RT; 21°C), higher temperature (37°C) and lower temperature (4°C). Data are means of 5–9 responses, each from a different oocyte, in which responses obtained at either 4°C or 37°C are normalized to responses obtained from the same oocyte at RT. C) Representative traces illustrating responses obtained at RT (upper trace) and 4°C (lower trace) from a single oocyte. D) Representative traces illustrating responses obtained at RT (upper trace) and 37°C (lower trace) from a single oocyte.
Effect of temperature on α7 nAChRs activated by 4BP-TQS
Whereas activation of wild-type α7 nAChRs by acetylcholine results in rapidly desensitizing responses, activation of α7 nAChRs by the allosteric agonist 4BP-TQS has a much slower onset and causes much less desensitization . The effect of changes in temperature was examined on α7 nAChRs after activation by 4BP-TQS (Fig. 4). As reported previously , slowly activating responses were observed with minimal levels of desensitization. In contrast to the marked effect of temperature changes on the amplitude of currents evoked by acetylcholine on α7 nAChRs (Fig. 1), temperature changes had little or no effect on responses to 4BP-TQS (Fig. 4). Lower temperature had no significant effect and higher temperature caused a relatively small but statistically significant (P = 0.002), reduction in current amplitude.
A) A bar chart illustrates responses (mean ± SEM) from α7 nAChRs expressed in Xenopus oocytes in response to a maximal (10 µM) concentration of the allosteric agonist 4BP-TQS at room temperature (RT; 21°C), higher temperature (37°C) and lower temperature (4°C). Data are means of 5–22 responses, each from a different oocyte, in which responses obtained at either 4°C or 37°C are normalized to responses obtained from the same oocyte at RT. B) Representative traces illustrating responses obtained at RT (upper trace) and 4°C (lower trace) from a single oocyte. C) Representative traces illustrating responses obtained at RT (upper trace) and 37°C (lower trace) from a single oocyte.
One possible explanation for these results is that the relatively small effect of high temperature on current amplitude in α7 nAChRs activated by 4BP-TQS might be due to instability of the compound at 37°C. To examine this possibility, control experiments were performed with solutions of 4BP-TQS that had been stored at 37°C for three hours and then cooled to room temperature. No differences in agonist responses were observed, when compared with freshly prepared solutions (data not shown).
Effect of temperature on receptor desensitization and deactivation
In addition to examining changes in current amplitude, the effect of altering temperature was examined on the rate of receptor desensitization (Fig. 5). Despite both high and low temperatures having opposing effects on current amplitudes on α4β2 and α7 nAChRs, changes in temperature had a consistent effect on the rate of receptor desensitization for α4β2 and α7 nAChRs. For both receptor subtypes (α4β2 and α7), an increase in the rate of desensitization was observed at 37°C and a decrease in the rate of desensitization at 4°C (Fig. 5). For both α7L247T nAChRs and for wild-type α7 nAChRs activated by 4BP-TQS, despite changes in temperature having little or no effect on current amplitude (Figs. 3 and 4), an increase in the rate of receptor desensitization was observed at 37°C, consistent with the effects seen with α4β2 and wild-type α7 nAChRs (Fig. 5). There was also evidence of a reduction in the rate of desensitization at 4°C for both α7L247T nAChRs and for wild-type α7 nAChRs activated by 4BP-TQS, consistent with the effects seen with α4β2 and wild-type α7 nAChRs, but this was not statistically significant (Fig. 5, Table 2). The influence of changes in temperature was also examined on the rate of receptor deactivation after removal of agonist (Fig. 5E). It was not possible to measure this parameter for of α7 nAChRs activated by acetylcholine, due to the very rapid desensitization but, in all other cases, changes in temperature had a consistent effect (Table 2), with an increased rate of deactivation observed at 37°C and a reduced rate at 4°C.
Representative traces are shown illustrating responses obtained at RT (black), 4°C (blue) and 37°C (red). Current traces obtained at each temperature have been normalized to the same peak response. In each case, the response showing the fastest rate of desensitization was observed at 37°C and the slowest rate of desensitization was observed at 4°C. Responses are from α7 nAChRs with 3 mM acetylcholine (A), α4β2 nAChRs with 1 mM acetylcholine in calcium-containing saline (B), α7L247T nAChRs with 30 µM acetylcholine (C) and α7 nAChRs with 10 µM 4BP-TQS (D). Rates of receptor deactivation after removal of agonist were also influenced in a consistent manner by changes in temperature (faster at 37°C and slower at 4°C). Representative traces from α7L247T nAChRs with 30 µM acetylcholine are illustrated (E) and are typical of results from all receptor/agonist combinations studied (see Tables 1 and 2 for details).
Effects of temperature are reversible
For all experiments, the effects of raising or lowering temperature on agonist responses were normalized to responses recorded at RT on the same oocyte. In addition, after responses had been recorded at a temperature above or below RT, further responses were measured on the same oocyte at RT. In all cases, changes in current amplitude or in the rate of receptor desensitization were found to be reversible (data not shown).
It has been known for several decades that changes in temperature can influence the properties of nAChRs. For example, a series of studies conducted in the early 1970s demonstrated that end-plate currents recorded at the frog neuromuscular junction are influenced by temperature , , , , . Typically, the effects observed include increased rates of decay of end-plate currents with increased temperature. This is consistent with the increase in the rate of receptor desensitization that we have observed with both α4β2 and α7 nAChRs. Similarly, previous studies conducted with rat diaphragm preparations reported similar effects of temperature on rates of decay of end-plate currents . Studies of native neuronal nAChRs expressed in cultured PC12 cells also demonstrated that rates of recovery from desensitization were faster at higher temperatures . Thus, it seems that changes in temperature appear to have a consistent effect on the rate of agonist-induced desensitization and the decay of end-plate potentials.
An increase in current amplitude with increased temperature, similar to the effect we have observed with α4β2 nAChRs, has been reported recently for recombinant P2X3 receptors . However, in contrast to the findings reported here, the development of desensitization in P2X3 receptors has been reported to be independent of temperature, at least between 25°C and 40°C . A similar increase in current amplitude with increased temperature has also been reported for NMDA-type glutamate receptors . Conversely, an increase in current amplitude in response to decreased temperature, similar to the effect that we observe with α7 nAChRs, has been reported with an acetylcholine-gated chloride channel from the parietal ganglion of the pond snail Lymnaea . Thus, there are precedents for changes in temperature having opposing effects on the amplitude of ion channel currents, albeit from different classes of ligand-gated ion channels. In addition, we have recently become aware of a paper, published after the submission of this manuscript, examining the influence of higher temperature on α7 nAChRs . Higher temperature resulted in reduced responses to acetylcholine when co-applied with the positive allosteric modulator PNU-120597, consistent with our results obtained with either acetylcholine or 4BP-TQS applied alone.
We have found that introduction of a single point mutation (L247T) in the α7 nAChR completely abolishes the effect of temperature on current amplitude, whilst retaining an effect of temperature on the rate of desensitization of the macroscopic response. This observation adds to the many effects that have been reported for the L247T mutation in α7 nAChRs. These include an increase in agonist potency, reduction in receptor desensitization, increased spontaneous openings, the conversion of competitive antagonists into agonists and the conversion of allosteric potentiators into allosteric agonists , , , . As has been discussed previously , the non-desensitizing responses observed when α7 is activated by 4BP-TQS resembles that of acetylcholine responses on α7L247T nAChRs. Changes in temperature had a greatly reduced effect on the amplitude of α7 responses in response to 4BP-TQS compared to responses to acetylcholine. Indeed, no significant effect was observed when responses at 4°C were compared to those at RT (Fig. 4). This is consistent with previous evidence  indicating that acetylcholine and 4BP-TQS cause activation of α7 nAChRs through different mechanisms of action.
It has been reported previously that exposure of some nAChRs to low temperature for several hours can facilitate more efficient protein folding and assembly. This was first demonstrated for the muscle-type nAChR from the marine ray Torpedo, expressed in cultured mammalian cell lines  and can be explained by the fact that proteins in cold water fish have not evolved to fold efficiently at 37°C. Similar effects have been reported for insect nAChRs expressed in cultured cell lines , , . In fact, even mammalian nAChRs have been reported to fold and assemble more efficiently at lower temperature , . However, in contrast to these long-term effects occurring over several hours, it is unlikely that increased efficiency of subunit folding and assembly explains the effects reported here. The effects of temperature on current amplitude examine in this study are both very rapid (occurring within seconds) and are reversible, suggesting that the effects observed are a consequence of a change in the thermodynamic properties of already assembled cell-surface receptors, rather than a change in the efficiency of receptor folding and assembly. In contrast to the experiments examining agonist responses at physiological temperature, the rationale for examining responses of human nAChRs at 4°C is probably less obvious. Although responses measured at 4°C do not have direct physiological relevance for human receptors, the fact that lowering temperature to 4°C has opposing effects on α4β2 and α7 nAChRs provides evidence that these two nAChR subtypes have differing biophysical properties.
It is unclear why changes in temperature should have opposing effects on current amplitude in two closely related ion channels (α4β2 and α7 nAChRs) or why these effects should be largely or completely abolished by either a single point mutation or by activation by an allosteric agonist, rather than by the conventional orthosteric agonist, acetylcholine. It appears, however, that these effects are not a consequence of changes in the rate of receptor desensitization observed during macroscopic oocyte responses, since changes in temperature were found to have a broadly consistent effect on this parameter. In addition, changes in temperature had a consistent effect on the rate of receptor deactivation after removal of agonist (faster at 37°C and slower at 4°C), presumably reflecting changes in the off-rate of agonists from their binding sites. It is possible that receptors can adopt multiple open or desensitized states, and that entry into these various states occurs at different rates and can be affected differently by phenomena such as changes in temperature, mutagenesis or by allosteric modulators. This idea is consistent with models that have been advanced to explain mechanisms of allosteric modulation of nAChRs .
It is possible that the changes in current amplitude that we have observed with α4β2 and α7 nAChRs may be a consequence of temperature-induced changes in single-channel conductance. Indeed, it has been reported that the conductance of muscle-type nAChRs increases with increasing temperature , , , . As has been discussed previously in connection with the effects of temperature on functional properties of acetylcholine-gated chloride channels from the pond snail Lymnaea , alternative possibilities for the effects that we have observed include temperature-dependent changes in single-channel kinetics and/or changes in the affinity of agonist binding. Recently, evidence has been obtained indicating that the gating rate constants of muscle nAChRs are altered by temperature . Further work will be required to establish the precise mechanism of action of the temperature dependent effects that we have observed on the amplitudes of whole-cell responses in different subtypes of neuronal nAChRs. We can conclude, however, that changes in temperature can have opposing effects on the amplitude of nAChR whole-cell responses, whilst having a similar effect on the rate of desensitization of the macroscopic agonist-evoked response.
Materials and Methods
Plasmids, site-directed mutagenesis and cRNA synthesis
Plasmid pSP64GL constructs containing human α4, α7 and β2 cDNA have been described previously  as has a plasmid containing the α7 cDNA with the L247T mutation . All pSP64GL plasmids were linearized with BamHI and purified with QIAquick PCR purification kit (Qiagen). In vitro synthesis of cRNA was performed using the SP6 mMessage mMachine SP6 kit (Ambion).
Two-electrode voltage-clamp recording in Xenopus oocytes
Adult female Xenopus laevis frogs were obtained from the European Xenopus Resource Centre (University of Portsmouth). Oocytes were isolated and defolliculated as described previously  following procedures that have been approved by both UCL's Biological Services Management Group and the UK Home Office (under licences PIL70/23585 and PPL70/06819). Heterologous expression was achieved by injection of cRNA (8–12 ng) into the oocyte cytoplasm. Oocytes were injected in a volume of 32.2 nl using a variable volume Nanoject II microinjector (Drummond Scientific). cRNAs encoding α4 and β2 subunits were injected in a 1∶1 ratio in a total injection volume of 32.2 nl/oocyte. After injection, oocytes were incubated at 18°C in a modified Barth's solution containing: 88 mM NaCl, 1 mM KCl, 0.82 mM MgCl2, 0.77 mM CaCl2, 2.4 mM NaHCO3, and 15 mM Tris, 50 mg/l tetracycline, 50 mg/l penicillin and 50 mg/l streptomycin (pH 7.5 with HCl). Experiments were performed on oocytes 3 to 5 days of injection. Oocytes were placed in a recording chamber (internal diameter, 3 mm), which was continuously perfused with a modified Ringer solution (115 mM NaCl, 2.5 mM KCl, 10 mM HEPES, 1.8 mM BaCl2 or CaCl2). Oocytes were impaled by two microelectrodes filled with 3 M KCl (0.5–1.5 MΩ) and voltage-clamped at −60 mV using a GeneClamp 500 amplifier (Axon Instruments). Drugs were applied by switching between control and drug-containing solution using a BPS-8 valve control system (ALA Scientific). Agonists were applied for either 5 seconds (acetylcholine) or up to 40 seconds (4BP-TQS) with 2 minute (acetylcholine) or 8 minute (4BP-TQS) intervals between applications. The experiments were performed at room temperature (RT; approx. 21°C). To detect effects of changed temperature on kinetics of nAChRs the modified Ringer solution was warmed/cooled to 37°C or 4°C in water bath or on ice. Agonist solutions were prepared fresh and warmed or cooled for 10–15 min immediately prior to use. In control experiment, it was confirmed that warming agonist solutions, even for 3 hours did not alter agonist potency. To examine effects of temperature, oocytes were continuously perfused with warm/cold modified Ringer solution for 40 sec prior to agonist application. Peak current amplitudes evoked by warm/cold acetylcholine were normalized to the peak amplitudes at RT from the same oocyte. To examine reversibility of any temperature induced changes, oocytes were then perfused with RT Ringer for 5 mins, followed by application of acetylcholine at RT. Peak amplitudes, the rise time and time to decay to half of the maximum amplitude were measured using Clampfit 9.2 (Molecular Devices). Effects of temperature on current amplitude were determined by measuring peak responses, however, results were not significantly different when responses were measure as area above the curve/total charge transfer. Statistical significance was determined using with SigmaStat (Aspire Software International) using non-paired Student's t-test. The effects of raising or lowering temperature on agonist-evoked responses were normalized to responses recorded at RT on the same oocyte. All data are reported as mean ± S.E.M.
Conceived and designed the experiments: MJ SJL NSM. Performed the experiments: MJ SJL. Analyzed the data: MJ SJL NSM. Wrote the paper: MJ SJL NSM.
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