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NEUROPHARMACOLOGY
Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock, Arkansas
Received for publication
September 9, 2004
Accepted
October 20, 2004.
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Abstract |
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). Overactivation of NMDA receptors, causing calcium overload in neurons, is implicated in a variety of pathological conditions, such as stroke (Dirnagl et al., 1999; Lee et al., 1999), limbic seizures (Meldrum et al., 1999), head trauma (Obrenovitch and Urenjak, 1997), and degenerative neurological diseases (Olney et al., 2001). Hypoactivity of NMDA receptors has been suggested to play a role in psychosis (Olney et al., 1997). The involvement of NMDA receptors in such diverse physiological and pathological processes has lead to considerable interest in developing a detailed understanding of cellular and molecular mechanisms involved in the regulation of NMDA receptor function.
The NMDA receptor current is modulated by a number of extracellular allosteric modulators, including proton, zinc, and polyamines. The extracellular proton site has emerged as the most critical site because the effects of several allosteric regulators of NMDA receptor are mediated by the proton site. The high-affinity Zn2+ inhibition of NR2A-containing receptors is due to enhancement of tonic proton inhibition (Choi and Lipton, 1999; Low et al., 2000). A change in tonic proton inhibition is also the underlying mechanism for polyamine potentiation of NR2B-containing receptors (Traynelis et al., 1995) and for ifenprodil inhibition of NR2B-containing receptors (Park and Williams, 1997; Mott et al., 1998). To fully understand the gating of NMDA receptors, detailed knowledge of the molecular determinants of the proton-sensitive gating of NMDA receptors is essential.
Drawing analogy to pH-sensitive gating of potassium channels (Schulte and Fakler, 2000; Yang et al., 2000), we hypothesized that the molecular determinants for the proton site are localized near the extracellular end of the M3 transmembrane domain of NMDA receptors that contains the lurcher motif (SYTANLAAF). The lurcher motif is conserved in all glutamate receptors and is thought to play a critical role in gating (Kohda et al., 2000; Taverna et al., 2000; Jones et al., 2002). We have identified residues in the lurcher motif that can shift the proton IC50 by more than 200-fold in combination (Low et al., 2003). However, a residue with an ionizable side chain is the most likely candidate to participate directly in proton-sensitive gating. All residues identified in the lurcher motif so far lack ionizable side chains. It remains unclear how these residues alter proton sensitivity of NMDA receptors.
One possibility is that the effects of all these mutants in the lurcher motif on proton sensitivity are indirect. As in the case of apparent agonist affinity (Colquhoun, 1998), the apparent proton sensitivity of a mutant NMDA receptor can be reduced by a drastic increase in the open probability, since more receptors would be shifted into the open state. Such change in open probability would also result in significantly slower deactivation time constant (Colquhoun, 1998). A previous study has reported changes in the deactivation time constants by the lurcher mutation in NMDA receptors, suggesting that the lurcher mutations may change channel open probability (Kohda et al., 2000). However, the slower deactivation could also be explained by the presence of ambient zinc. We have shown previously that an extracellular zinc site in the amino-terminal domain of NR2A is allosterically coupled to the glutamate binding site in the S1/S2 domain of NR2A (Zheng et al., 2001), and the binding of zinc to that site inhibits NMDA channels by enhancing the proton sensitivity of NMDA receptors (Low et al., 2000; Zheng et al., 2001). When the proton sensitivity is significantly reduced by a point mutation, zinc-bound but unprotonated receptors can transit into the open state. The deactivation of these zinc-bound receptors will be slower since they exhibit higher affinity for glutamate due to the positive allosteric interaction between the glutamate binding site and the zinc binding site. In the present study, we attempted to determine whether mutations with reduced proton sensitivity in the lurcher motif exhibit changes in the gating properties. Our data suggests that, although ambient zinc contributes to the slower deactivation time constants of the lurcher mutants, most of the mutations with reduced proton sensitivity exhibit significantly slower rise time and deactivation time constant. Our data suggest that the change in proton sensitivity caused by these mutants may be due to the change of pH-independent gating properties of the NMDA receptors, at least partially. However, NR2A(A653T) exhibits normal macroscopic currents, suggesting that the shift of proton sensitivity caused by this point mutation could reflect a direct alteration of the proton binding.
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Materials and Methods |
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TopAbstractMaterials and MethodsResultsDiscussionReferences |
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).
Transfection of HEK Cells. HEK 293tsa cells (Rick Horn, Tuft University, Philadelphia, PA) were maintained at 37°C and 5% CO2 in Dulbecco's modified Eagle's medium supplemented with L-glutamine (200 µM), sodium pyruvate (100 µM), penicillin/streptomycin (100 units/ml), and 10% fetal bovine serum (Invitrogen, Calsbad, CA). Low-confluency cells were transfected by the calcium phosphate precipitation method (Chen and Okayama, 1987). Cells were cotransfected with a mixture containing NR1, NR2, and green fluorescent protein plasmids (1, 2, and 0.3 µg per 12-mm-diameter coverslip, respectively). After transfection, D-AP5 (100–200 µM) and 7-Cl-kynurenic acid (50–100 µM) were added to the culture medium. For some mutants, MgCl2 (2 mM) was also needed to reduce excitotoxicity. NMDA antagonists were purchased from either Sigma-Aldrich (St. Louis, MO) or Tocris Cookson Inc. (Ellisville, MO).
Whole-Cell Patch-Clamp Recordings and Outside-Out Patch Recording from HEK 293 Cells. Patch-clamp recording in the whole-cell configuration and outside-out patch recording were made as described previously with Axopatch 200B amplifier (Axon Instruments Inc., Union City, CA). Recording electrodes (5–12 M
) were filled with: 140 mM Cs-gluconate, 5 mM HEPES, 4 mM NaCl, 2 mM MgCl2, 0.5 mM CaCl2, 1 mM ATP, 0.3 mM GTP, and 5 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (pH 7.4). The recording chamber was continually perfused with recording solution comprised of: 150 mM NaCl, 10 mM HEPES, 1.0 mM CaCl2, 3 mM KCl, and 20 mM mannitol. Glutamate (50–100 µM) and glycine (30–60 µM) were applied using a multibarrel pipette driven by a piezo-based fast solution jumping system (Burleigh LSS 3100; EXFO Burleigh, Victor, NY). The rise time of the junction potential change produced by this system is 0.66 ms. The N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine (Tricine)-buffered Zn2+ solutions were prepared according to the empirically established binding constant 10–5 M (Paoletti et al., 1997) by adding into 10 mM Tricine 1.23 µM Zn2+ to obtain 100 nM free Zn2+ at pH 7.4.
Data Analysis and Statistics. Five current traces were collected at a sampling rate of 5 to 20 kHz with pClamp 9 (Axon Instruments Inc.) and averaged with Clampfit 9 (Axon Instruments Inc.). The averaged current traces were then analyzed with ChannelLab (S. T. Traynelis, Emory University, Atlanta, GA). The rise time was determined by linear regression of the rising phase of the current responses. The deactivation time constant was determined by fitting the decay of the current responses with a single exponential component. All pooled data are expressed as mean ± S.E.M. One-way analysis of variance and Tukey post hoc test were used to determine significance (p < 0.05) unless stated otherwise.
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Results |
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), we identified three residues in the lurcher motif of NR1 subunits that greatly reduce proton sensitivity of NMDA receptors when these residues are mutated (Fig. 1). Point mutations of the corresponding residues in the NR2A subunit have similar effects (Fig. 1). To determine whether these point mutations alter the kinetic properties of NMDA receptors, we measured the rise time and the deactivation time constant of these mutants expressed in HEK 293 cells. To eliminate any potential effects of extracellular free zinc on the deactivation time constants, the whole-cell NMDA current responses were recorded in the presence of metal chelators (10 µM EDTA or 10 mM Tricine). The free zinc concentration was reduced to nominally zero (less than 10 pM) in the presence of either one of the chelators used in our experiments.
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The first pair of point mutations tested was the lurcher point mutation of NR1a and NR2A that reduce proton sensitivity of NR1/NR2A receptors by approximately 5-fold. The lurcher mutation of NR2A [NR2A(A651T)] showed no detectable changes in the macroscopic current (Fig. 2, A and B; Table 1). On the other hand, the rise time and deactivation time constant for the NR1a (A653T) mutant were significantly slower than the wild type (Fig. 2, C and D; Table 1). However, the deactivation time constant was only 0.4 s, not as high as the value reported in a previous report (Kohda et al., 2000). The difference is likely due to the presence of ambient zinc in that study as indicated by the presence of the fast desensitization caused by zinc.
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We next determined the effects of the point mutations of NR1a(T648C) or NR2A(T646C) on the macroscopic currents. In our previous study (Low et al., 2003), these two mutants caused comparable reductions in proton sensitivity to either NR1a(A653T) or NR2A(A651T) mutations. Like NR1a(A653T), these two mutants exhibited a slower rise time and a greater deactivation time constant (Fig. 3; Table 1). The substitution of the neighboring residue to cysteine caused even greater change in macroscopic currents (Fig. 4). The deactivation of NR1a(A649C)/NR2A required tens of seconds that we were unable to quantify by the regular curve fitting routine (n = 4), and the exact deactivation time constant remains undetermined. The deactivation of NR1a/NR2A(A647C) was equally slow (n = 4). The slow deactivation time course caused failure in the curve fitting routine, so the deactivation time constant was not determined. We have also recorded NMDA receptor currents in HEK293 cells expressing other cysteine substitutions in the lurcher motif of NR2A [i.e., NR2A(Y645C), NR2A(N648C), NR2A(L649C), NR2A(A650C), NR2A(A651C), and NR2A(F652C)]. The deactivation of these mutants appears normal (data not shown, n = 3–5). We have also recorded NMDA receptors currents for two additional NR1 cysteine substitutions in the lurcher motif [NR1a(A652C) and NR1a(F654C)]. The deactivation of these mutants also appears normal (data not shown, n = 3–5). In summary, we observed slower rise time and deactivation time constants for most point mutations in the lurcher motif of either NR1 or NR2A that reduce proton sensitivity. The only exception is NR2A(A651T), which exhibits normal kinetics for whole-cell currents. A change in the deactivation time constant could be caused by a change in the affinity of the agonist binding site or a change in the channel open probability. Since the lurcher motif is outside the glutamate binding domain of NR2 or the glycine binding domain of NR1 (Armstrong et al., 1998; Furukawa and Gouaux, 2003), it is unlikely that these mutants in the lurcher motif could directly alter the agonist binding site. Therefore, our data suggest that the channel open probability may be altered by NR1a(T648C), NR1a(A649C), NR1a(A653T), NR2A(A646C), and NR2A(A647C).
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In a final set of experiments, we investigated the effects of zinc on the time constant for the deactivation of lurcher mutants. As outlined earlier, our working hypothesis regarding zinc regulation of NMDA receptor activity predicts that zinc should slow the deactivation of any mutant receptors with significantly reduced affinity for proton binding. Since we had shown earlier that zinc can not modulate gating directly, and its effects are mediated by the proton site (Low et al., 2000), we assume that zinc-bound, unprotonated receptors can transit into open state. For wild-type receptors, most zinc-bound receptors will be protonated at physiological pHs so the zinc-bound, unprotonated receptors make little contribution to the macroscopic currents. For a mutant receptor with greatly reduced affinity for the proton binding site, the proportion of zinc-bound, unprotonated receptors will increase significantly and will make greater contribution to the macroscopic currents. Since glutamate affinity is higher for zinc-bound receptors, the deactivation for these receptors should be slower. On the other hand, zinc should have no significant effects on the deactivation time constants for mutants that reduce proton sensitivity indirectly by increasing channel open probability. This is due to the notion that the proportion of zinc-bound, unprotonated receptors will not change, and the contribution to the macroscopic currents by these receptors will remain the same. The effect of zinc on the deactivation of NR1a(A653T)/NR2A could explain the discrepancy between our results and a previous report (Kohda et al., 2000). We measured the deactivation time constants in the presence and absence of extracellular free zinc from the same HEK 293 cells for NR1a(A653T)/NR2A receptors and NR1a/NR2A(A651T) receptors (Fig. 5). The deactivation time constant of NR1a/NR2A(A651T) was doubled in the presence of zinc, which is consistent with the hypothesis that NR2A(A651T) alters proton dissociation constant directly. The deactivation time constant for NR1a(A653T)/NR2A was also significantly greater in the presence of zinc, which suggests that NR1a(A653T) may reduce proton sensitivity by two separate mechanisms, i.e., by reducing the channel closing rate and by reducing the dissociation constant for proton. Zinc has no detectable effects on the deactivation time constant of NR1a(T648C)/NR2A or NR1a/NR2A(T646C). The deactivation time constants for NR1a(T648C)/NR2A or NR1a/NR2A(T646C) were 667 ± 132 (n = 3) and 337 ± 25 (n = 8) ms, respectively. These values are not significantly different from the deactivation time constant for these mutants under zinc free conditions (Table 1; p > 0.05, Student's t test). The effects of zinc on deactivation for NR1a(A649C) and NR2A(A647C) were not determined since the deactivation is too slow to be reliably fitted.
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Discussion |
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; Taverna et al., 2000; Jones et al., 2002). The changes in current kinetics caused by these mutants in the lurcher motif also have clear implications on data interpretation for some earlier works (Beck et al., 1999; Jones et al., 2002; Sobolevsky et al., 2002a,b; Low et al., 2003). The role of the lurcher motif in gating has been investigated before using the substituted cysteine accessibility method. There is contradicting evidence as to whether Ala649 is aqueous accessible (Beck et al., 1999; Jones et al., 2002). The modification rate of substituted cysteine residues will be influenced by the gating rates of NMDA receptors. If the gating rates are altered by cysteine substitution as suggested by our present data, the data from most substituted cysteine accessibility method studies (Beck et al., 1999; Jones et al., 2002; Sobolevsky et al., 2002a,b) will need to be reevaluated. The conclusions in our earlier publication (Low et al., 2003) need to be reevaluated in light of our new data since we have not considered the potential effects on proton sensitivity caused by changes in gating rates produced by mutations in the lurcher motif.
Figure 6A shows the simplified kinetic scheme for proton-sensitive gating of NMDA receptors. The key presumption of this scheme is that protonated receptors cannot proceed to open state, and receptors in open state cannot be protonated. Single-channel analysis of proton inhibition of NR1/NR2B receptors appears to support this presumption since proton cannot facilitate closure, and there are little changes in the shut time distribution (Banke et al., 2004). Since the lurcher motif is located outside the binding domain for glutamate (Armstrong et al., 1998), mutations in the lurcher motif could not directly alter glutamate binding. Therefore, the apparent proton sensitivity could be altered in two distinct ways. The first way is to alter the proton dissociation constant directly by altering either the Kon or Koff for the proton-binding site. The second way is to drastically alter the channel open probability by changing the channel open rate or the close rate. Such changes in open probability could also result in changes in apparent proton sensitivity since it would alter the distribution of receptors in various states. Therefore, a point mutation could reduce the proton sensitivity of NMDA receptors by either directly reducing the dissociation constant for proton, increasing the open probability and subsequently reducing the number of protonated receptors, or a combination of the first two. The mutants in the lurcher motif that reduce proton sensitivity could fall into these three categories.
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We simulated the NMDA receptor currents based on the kinetic scheme in Fig. 6A. As shown in Fig. 6B, reducing the close rate by 10-fold is sufficient to reduce apparent proton sensitivity by 4-fold. Such changes in gating also result in changes in the rise time and deactivation time constant as shown in our experimental data for several mutants in the lurcher motif (Fig. 6B). In contrast, direct changes in the proton dissociation constant will not result in detectable kinetic changes in the macroscopic current (Fig. 6C). Thus, the simple kinetic scheme is sufficient to account for our experimental findings.
Could the changes in gating rates that would result in the observed deactivation time course account for the shift in proton dose-response curve reported previously? The mutants in the lurcher motif reported in Low et al. (2003) appear to be a heterogenous group. At one end of the broad spectrum are the cysteine substitutions of NR1a A649 and NR2A A647. These two mutants exhibit very slow deactivation time constants that would require the channel closing rates to be reduced by approximately 100-fold. Consistent with the greatest change in gating as suggested by the slow deactivation, NR1a(A649C) produces the greatest shift in proton sensitivity by any single point mutation. NR2A(A647C) also produces the greatest shift in proton sensitivity by a single point mutation (IC50, 1473 nM, P. Le and S. F. Traynelis, personal communication). The reported proton IC50 for NR1a/NR2A(A647C) in a previous study (Low et al., 2003) is incorrect. It is possible that the effects of these two mutants on proton sensitivity could be entirely due to the suggested changes in gating rates, and this residue may not play any direct role in proton-sensitive gating of NMDA receptors.
At the other end of the spectrum is the lurcher mutation of NR2A [NR2A(A651T)]. NR2A(A651T) exhibits normal rise time and deactivation time constant, whereas it produces a similar reduction in proton sensitivity as other point mutations in the lurcher motif. Our data suggest that the change in proton sensitivity caused by NR2A(A651T) cannot be accounted for by changes in gating rates. Other lines of evidence also support our hypothesis that NR2A(A651T) could alter proton sensitivity directly. Zinc slows the deactivation of this mutant as predicted for a mutant that reduces proton affinity. Finally, the effects of NR2A(A651T) are additive to the effects of NR1a(A649C) that may be due to change of open probability (Low et al., 2003). If both mutants cause reduction of proton sensitivity by the same mechanism, i.e., changing channel open probability, the effects could not be independent as suggested by the coupling coefficient (Low et al., 2003).
The lurcher mutant of NR1a [NR1a(A653T)] may be in the middle of the spectrum. We observed a significant change in the current kinetics that suggests significant changes in the channel gating rates. The changes in gating clearly would contribute to the reduction of proton sensitivity by this mutation. However, NR1a(A653T) may also have some direct effects on proton affinity. Zinc slowed the deactivation of this mutant just as it slowed the deactivation of NR2A(A651T). The change of gating rate suggested by deactivation time constant for this mutant appears also to be insufficient to produce the observed shift in proton IC50.
One clear implication of our present data is the need of abundant caution in interpreting mutagenesis data. Our present data clearly demonstrated that the reduction of proton sensitivity by some mutants in the lurcher motif is caused by changes of gating rates, at least partially. It is also possible that reduction in proton sensitivity produced by some mutants in other regions of NR1 could also be accounted for by changes in channel gating rates. Those mutations with great effects on proton sensitivity will need to be reevaluated just as the mutants in the lurcher motif. The gating properties of these mutants as well as mutants in the lurcher motif need to carefully be investigated with single channel analysis or nonstationary variance analysis. A kinetic model on proton inhibition of NMDA receptors, validated by experimental data, is also critical. The simple model presented in Fig. 6A has not been rigorously tested with experimental data. However, it shares the many features proposed in a recent study that has been tested more rigorously (Banke et al., 2004). We only intended to use it as a starting point in our attempt to evaluate the true impact of mutants in the lurcher motif on proton-sensitive gating. Detailed analyses of gating guided by kinetic modeling are needed to determine whether any of the mutants reported earlier (Low et al., 2003) alter proton sensitivity directly.
Our data also suggest that the lurcher mutant of NR2A is distinct from other lurcher mutants that reduce proton sensitivity since it is the only one with normal kinetics for macroscopic currents. Since Ala and Thr both lack ionizable side chains, it seems unlikely that Ala651 of NR2A acts directly as the proton sensor. However, Ala651 of NR2A could modulate the proton binding site by participating in hydrogen bond with the proton sensor through the peptide backbone. Alternatively, the oxygen in the side chain for threonine could be interacting with a protonated side chain through hydrogen bond. Systematic substitution of side chains of this residue may yield additional information that could improve our understanding about the molecular determinants of pH-sensitive gating of NMDA receptors.
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Footnotes |
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ABBREVIATION: NMDA, N-methyl-D-aspartate.
Address correspondence to: Dr. Fang Zheng, Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, 4310 W. Markham Street, Little Rock, AR 72205. E-mail: zhengfang{at}uams.edu
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, 250 s–1. Macroscopic currents were simulated with ChannelLab (S.T. Traynelis). Changes in the open probability (a reduction of