The ubiquitous inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) Ca2+ release channel plays a central role in the generation and modulation of intracellular Ca2+ signals, and is intricately regulated by multiple mechanisms including cytoplasmic ligand (InsP3, free Ca2+, free ATP4−) binding, posttranslational modifications, and interactions with cytoplasmic and endoplasmic reticulum (ER) luminal proteins. However, regulation of InsP3R channel activity by free Ca2+ in the ER lumen ([Ca2+]ER) remains poorly understood because of limitations of Ca2+ flux measurements and imaging techniques. Here, we used nuclear patch-clamp experiments in excised luminal-side-out configuration with perfusion solution exchange to study the effects of [Ca2+]ER on homotetrameric rat type 3 InsP3R channel activity. In optimal [Ca2+]i and subsaturating [InsP3], jumps of [Ca2+]ER from 70 nM to 300 µM reduced channel activity significantly. This inhibition was abrogated by saturating InsP3 but restored when [Ca2+]ER was raised to 1.1 mM. In suboptimal [Ca2+]i, jumps of [Ca2+]ER (70 nM to 300 µM) enhanced channel activity. Thus, [Ca2+]ER effects on channel activity exhibited a biphasic dependence on [Ca2+]i. In addition, the effect of high [Ca2+]ER was attenuated when a voltage was applied to oppose Ca2+ flux through the channel. These observations can be accounted for by Ca2+ flux driven through the open InsP3R channel by [Ca2+]ER, raising local [Ca2+]i around the channel to regulate its activity through its cytoplasmic regulatory Ca2+-binding sites. Importantly, [Ca2+]ER regulation of InsP3R channel activity depended on cytoplasmic Ca2+-buffering conditions: it was more pronounced when [Ca2+]i was weakly buffered but completely abolished in strong Ca2+-buffering conditions. With strong cytoplasmic buffering and Ca2+ flux sufficiently reduced by applied voltage, both activation and inhibition of InsP3R channel gating by physiological levels of [Ca2+]ER were completely abolished. Collectively, these results rule out Ca2+ regulation of channel activity by direct binding to the luminal aspect of the channel.

INTRODUCTION

Modulating cytoplasmic free Ca2+ concentration ([Ca2+]i) is a universal intracellular signaling pathway that regulates numerous cellular physiological processes including apoptosis, gene expression, bioenergetics, secretion, immune responses, fertilization, muscle contraction, and synaptic transmission (Clapham, 1995; Marks, 1997; Berridge, 1998, 2003; Berridge et al., 2000; Bootman et al., 2001; Orrenius et al., 2003; Braet et al., 2004; Randriamampita and Trautmann, 2004; Cárdenas et al., 2010). Ubiquitous ER-localized inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) Ca2+ release channels (Foskett et al., 2007) play a central role in this pathway in many cells (Taylor and Richardson, 1991; Putney and Bird, 1993; Bezprozvanny and Ehrlich, 1995; Furuichi and Mikoshiba, 1995; Patterson et al., 2004; Foskett et al., 2007; Joseph and Hajnóczky, 2007; Cárdenas et al., 2010; Foskett, 2010). InsP3 generated in the cytoplasm in response to extracellular stimuli (Berridge, 1993) binds to and activates InsP3R channels to release Ca2+ stored in the ER lumen into the cytoplasm, generating diverse local and global [Ca2+]i signals (Berridge, 1993, 1997; Hagar and Ehrlich, 2000; Thrower et al., 2001; Taylor and Laude, 2002; Foskett et al., 2007). Whereas much is known regarding the intricate regulation of InsP3R channel gating by multiple processes—binding of cytoplasmic ligands (Ca2+, InsP3, and ATP4−), posttranslational modifications, interactions with proteins, clustering, differential localization (Joseph, 1996; MacKrill, 1999; Patel et al., 1999; Johenning and Ehrlich, 2002; Foskett et al., 2007; Betzenhauser et al., 2008; Kang et al., 2008; Wagner et al., 2008; Li et al., 2009; Taufiq-Ur-Rahman et al., 2009)—the regulation of InsP3R channel activity by free Ca2+ in the lumen of the ER ([Ca2+]ER) remains poorly understood and controversial (Irvine, 1990; Tregear et al., 1991; Ferris et al., 1992; Swillens, 1992; Kindman and Meyer, 1993; Bezprozvanny and Ehrlich, 1994; Bootman, 1994a,b; Swillens et al., 1994; Shuttleworth, 1995; Dupont and Swillens, 1996; Missiaen et al., 1996; Parys et al., 1996; Beecroft and Taylor, 1997; Caroppo et al., 2003; Dawson et al., 2003; Fraiman and Dawson, 2004; Foskett et al., 2007; McCarron et al., 2008; Yamasaki-Mann and Parker, 2011).

The main techniques for studying possible [Ca2+]ER modulation of InsP3R channel gating have been 45Ca2+ flux measurements (Nunn and Taylor, 1991, 1992; Tregear et al., 1991; Missiaen et al., 1992a,b; Parys et al., 1993; Beecroft and Taylor, 1997) and fluorescence Ca2+ imaging (Combettes et al., 1992, 1996; Missiaen et al., 1992c; Shuttleworth, 1992; Renard-Rooney et al., 1993; Short et al., 1993; Steenbergen and Fay, 1996; Tanimura and Turner, 1996; Tanimura et al., 1998; Caroppo et al., 2003; Higo et al., 2005; McCarron et al., 2008; Yamasaki-Mann and Parker, 2011). Both approaches rely on changes in [Ca2+]i or [Ca2+]ER to infer channel activity and therefore cannot rigorously control both [Ca2+]ER and [Ca2+]i simultaneously during experiments. This has made it difficult to differentiate direct effects of [Ca2+]ER on the luminal aspect of the InsP3R from feed-through effects caused by Ca2+ flux through the open channel.

Electrophysiological recordings of single InsP3R channels allow InsP3R channel activity to be determined from currents carried by K+ through open channels (Foskett et al., 2007) and therefore can be performed under rigorously controlled and defined [Ca2+]i and [Ca2+]ER. However, only two electrophysiological studies of the effects of [Ca2+]ER on InsP3R channel activity have been reported (Bezprozvanny and Ehrlich, 1994; Thrower et al., 2000). Both used InsP3R reconstituted in lipid bilayers and explored only a limited set of [Ca2+]ER, [Ca2+]i, and [InsP3]. Insights about [Ca2+]ER regulation of InsP3R channels from these studies were limited by insufficient buffering of [Ca2+]i and the use of nonphysiological concentrations of divalent cations in the luminal solutions in Bezprozvanny and Ehrlich (1994), or inappropriate Ca2+ buffering and nonphysiological [KCl] used in Thrower et al. (2000).

Here, we studied systematically the effects of [Ca2+]ER on the activity of single homotetrameric channels of recombinant rat type 3 InsP3R (r-InsP3R-3) expressed in cells with no endogenous InsP3R expression (Sugawara et al., 1997; Mak et al., 2005), using nuclear patch-clamp techniques (Mak et al., 2005; Vais et al., 2010a) that record activities of the InsP3R channel in its native membrane milieu (Foskett et al., 2007) with ionic conditions on both sides of the channel, especially [Ca2+]i and [Ca2+]ER, rigorously controlled. By comparing activities (open probability [Po]) of type 3 InsP3R channels in excised luminal-side-out (lum-out) nuclear membrane patches exposed to different [Ca2+]ER by rapid perfusion solution exchange (Vais et al., 2010a), we found that high [Ca2+]ER modulated InsP3R channel activity. However, our experiments ruled out [Ca2+]ER modulation of InsP3R channel activity through intrinsic functional Ca2+-binding sites on the luminal side of the channel. Instead, the experimental results were consistent with [Ca2+]ER affecting InsP3R channel gating solely through the rise in local [Ca2+]i in the vicinity of the open channel generated by the [Ca2+]ER-driven Ca2+ flux through the open channel itself, which modulates InsP3R channel activity through functional cytoplasmic Ca2+-binding sites of the channel.

MATERIALS AND METHODS

Nucleus isolation and nuclear patch-clamp electrophysiology

Generation and maintenance of DT40-KO-r-InsP3R-3 cells (mutant cells derived from chicken B cells with the endogenous genes for all three InsP3R isoforms knocked out and then stably transfected to express recombinant r-InsP3R-3) were described in Mak et al. (2005). Nuclear patch-clamp experiments were performed using nuclei isolated from DT40-KO-r-InsP3R-3 cells as described previously (Mak et al., 2005). Experiments investigating InsP3R activity under constant ligand conditions were performed in the on-nucleus configuration (Mak et al., 2007). Excised nuclear membrane patches in the lum-out configuration were obtained from isolated nuclei (Mak et al., 2007) using protocols analogous to those used to obtain inside-out excised patches in plasma membrane patch-clamp experiments. The solution around the excised nuclear membrane patch was rapidly switched multiple times using a solution-switching setup described in Mak et al. (2007).

InsP3R channel current traces were acquired at room temperature as described previously (Mak et al., 1998), digitized at 5 kHz, and anti-aliasing filtered at 1 kHz. All applied potentials (Vapp) were measured relative to the bath electrode. All on-nucleus experiments were performed at Vapp = −40 mV. All lum-out experiments were performed at Vapp = −30 mV unless stated otherwise.

Experimental solution composition

All experimental solutions contained 140 mM KCl and 10 mM HEPES, pH to 7.3 with KOH. Because physiological levels of free Mg2+ (0–3 mM) have no effects on channel activities (Mak et al., 1999), and to avoid the complicating effects of Mg2+ on InsP3R channel conductance (Mak and Foskett, 1998) and free [ATP] in experimental solutions, Mg2+ was not added to any of the solutions used.

All experiments were performed using the same bath solution with free Ca2+ concentration ([Ca2+]f) of 70 nM buffered by 0.5 mM BAPTA (1,2-bis(o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid). All pipette solutions contained 0.5 mM Na2ATP.

Pipette solutions used in on-nucleus patch-clamp experiments contained various [Ca2+]f buffered by 0.5 mM Ca2+ chelator and [InsP3] as specified. BAPTA was used for 20 nM < [Ca2+]f < 600 nM, diBrBAPTA (5,5′-dibromo BAPTA) for 600 nM ≤ [Ca2+]f ≤ 4 µM, and hydroxyethylethylenediaminetriacetic acid (HEDTA) for [Ca2+]f > 4 µM. ATP contributed to Ca2+ buffering in solutions with [Ca2+]f > 30 µM (Bers et al., 2010).

Pipette solutions used in lum-out patch-clamp experiments had [Ca2+]f of either 2 µM buffered by various concentrations of diBrBAPTA or HEDTA as specified, or 55 nM buffered by various concentrations of BAPTA as specified. Four perfusion solutions were used in these experiments: one had [Ca2+]f of 70 nM buffered by 0.5 mM BAPTA with no ATP; one contained 1.3 mM CaCl2 and 1.5 mM Na2ATP, so that [Ca2+]f was buffered to 300 µM according to Max Chelator freeware; and two with no Ca2+ chelator, with one containing 1 mM CaCl2 and the other containing 2 mM CaCl2, giving [Ca2+]f of 550 µM and 1.1 mM, respectively, according to activity coefficient calculations (Butler, 1968; Vais et al., 2010a).

[Ca2+]f in all solutions (<100 µM) was confirmed by Ca2+-sensitive dye fluorimetry.

Data analysis

Because there is no Ca2+ flux through open InsP3R channels in steady-state on-nucleus experiments using bath solutions containing only 70 nM [Ca2+]f, there was no possibility of cross talk between channels. Furthermore, we detected no effect of channel clustering on gating of InsP3R channels expressed in DT40-KO-r-InsP3R-3 cells (Vais et al., 2011). Thus, multi-channel and single-channel current traces were selected for channel Po and dwell-time analysis using QuB software (Qin et al., 2000a,b). However, because substantial Ca2+ flux can be driven through open InsP3R channels when some perfusion solutions were used in excised lum-out nuclear patch-clamp experiments, only single-channel current traces from such experiments were selected for analysis to avoid complications arising from Ca2+ flux through one InsP3R channel affecting gating behaviors of neighboring active channel(s). We accepted only current records long enough to allow the number of active channels observed to be accurately determined (with >99% confidence) for data analysis (Mak et al., 2001b; Ionescu et al., 2006; Vais et al., 2010b). Only single-channel current traces >15 s were used for channel Po > 0.02. Longer traces were required for lower Po, so only single-channel traces >1.5 min were used for Po ≈ 0.005. Least-square fitting and statistical analysis of data were done with IGOR Pro software (WaveMetrics).

Modeling of the [Ca2+]i profile in the vicinity of an InsP3R channel

The [Ca2+]i profile around an open InsP3R channel was calculated by considering the open channel as a circular aqueous pore with a diameter of 2.5 nm (Jiang et al., 2002; Serysheva et al., 2003; Wolfram et al., 2010). Magnitudes of the Ca2+ current through an open channel (iCa) in the presence of 140 mM KCl and various [Ca2+]i, [Ca2+]ER, and Vapp were evaluated using the general Goldman–Hodgkin–Katz current equation (Lewis, 1979; Hille, 2001):

 
iCa=PCazCa2F2VappRT{[Ca]i[Ca]ERexp(zCaFVapp/RT)1exp(zCaFVapp/RT)},
(1)

where zCa (= 2) is the valence of Ca2+, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and PCa is the effective channel permeability for Ca2+ through the InsP3R channel measured experimentally in buffers containing 140 mM KCl (Vais et al., 2010a).

Although the analytical equation obtained by using the approximation in Neher (1986, 1998), Smith (1996), and Naraghi and Neher (1997) can provide a reasonable estimation of the steady-state [Ca2+]i profile around a Ca2+ channel generated by Ca2+ flux through the channel buffered by mobile Ca2+-binding chelators (Liu et al., 2010; Vais et al., 2010a), it says nothing about the time scale of the evolution of the [Ca2+]i profile to reach the steady state. These dynamics then affect the dissipation of the Ca2+ profile after the channel closes. Once a channel closes, the profile collapses first to a level that is above basal very quickly but then dissipates toward basal on a slow time scale. Therefore, to follow the time-dependent evolution of the [Ca2+]i profiles around the channel as it gates, numerical modeling of Ca2+ diffusion around the open InsP3R channel was used to calculate the [Ca2+]i profiles in the vicinity of the channel under various [Ca2+]i, [Ca2+]ER, and Vapp combinations.

In the simulations, [Ca2+]i is controlled by spatial diffusion, with the rate equation for [Ca2+]i at distance r from the channel at time t after the channel opens, C(r,t), given as

 
C(r,t)t=DC2C+J+bt,
(2)

where J is the Ca2+ flux passing through the channel from the ER lumen, DC is the diffusion coefficient of Ca2+ in the medium (= 800 µm2s−1 [Cussler, 1997] for aqueous medium), and b is the free Ca2+ buffer concentration.

For mobile Ca2+ buffers, b/t is given as

 
bt=Db2b+koff(Bb)konCb,
(3)

where B is the total Ca2+ buffer concentration, and kon and koff are the rates of Ca2+ binding to and dissociating from the buffer, respectively, so that koff/kon = Kd (dissociation constant) of the buffer. Db is the diffusion coefficient of the mobile buffer.

Kd for diBrBAPTA, HEDTA, and BAPTA is 1.6 µM, 4.7 µM, and 180 nM, respectively. kon for diBrBAPTA, HEDTA, and BAPTA is 450, 4.5, and 450 µM−1s−1, respectively (Tsien, 1980; Naraghi, 1997). Db for diBrBAPTA, HEDTA, and BAPTA is 296, 319, and 390 µm2s−1, respectively, estimated from Db=DCMC/Mb3, where Mb and MC are the molecular weights of the buffer and Ca2+, respectively.

iCa in Eq. 1 is converted into J in Eq. 2 by

 
J={iCa/(FzCaδV)forrrch0forr>rch,
(4)

where rch is the radius of the channel pore. For simplicity, the channel is considered to be embedded in an infinite membrane, opening into a semi-infinite cytoplasmic volume. Then, δV is the volume of a hemisphere with radius rch over the channel. Propagation of Ca2+ and mobile buffer (if present) was simulated throughout a homogeneous semi-infinite 3-D cytosolic space.

With spherical symmetry around the channel, the Laplacian of C and b in spherical coordinates is

 
2X(r,t)=1r2r(r2Xr),
(5)

where X = C or b.

The differential Eqs. 2 and 3 were solved implicitly with a spatial grid size of 0.625 nm using the Tridiagonal Matrix Solver software in a hemispherical volume of a large radius of 10 µm, so that the volume was effectively semi-infinite. The channel opened at t = 0 and remained open for the duration of the simulation. Calcium profiles were evolved with 0.1-µs time steps.

To simulate the collapse of the [Ca2+]i profile after channel closure, the [Ca2+]i profile was allowed to evolve for 50 ms after the channel opened. Then, J was set equal to 0 for all r at t = 0 as the channel closed. The [Ca2+]i profile was then evolved using the same software with the same time steps for various durations.

RESULTS

Dependence of steady-state InsP3R-3 channel activity on cytoplasmic ligands

To investigate the effects of ER luminal Ca2+ concentration ([Ca2+]ER) on InsP3R-3 channel gating, we first performed on-nucleus patch-clamp experiments on nuclei isolated from DT40-KO-r-InsP3R-3 cells to characterize the gating behaviors of single recombinant homotetrameric r-InsP3R-3 channels over a wide range of [InsP3] and [Ca2+]i in the presence of a physiological level (5 mM) of free ATP, which supports InsP3R channel gating (Mak et al., 1999, 2001a). In a bath solution with [Ca2+]f = 70 nM and no MgATP to support activity of the SERCA in the outer nuclear membrane, [Ca2+]f in the perinuclear space of the isolated nuclei (equivalent to the ER lumen topologically) equilibrated with that of the bath solution so that [Ca2+]ER = 70 nM in these experiments. The channel with large conductance (∼550 pS in 140 mM KCl) observed in the outer nuclear membrane of nuclei isolated from DT40-KO-r-InsP3R-3 cells (Fig. 1) was identified as recombinant InsP3R-3 channel by its requirement of InsP3 for activation (Cheung et al., 2008; Vais et al., 2010a).

In the presence of high (10 µM) [InsP3], the InsP3R-3 channel exhibited biphasic dependence on [Ca2+]i, with channel Po increasing as [Ca2+]i increased until a maximum Po was reached at [Ca2+]i of ∼2–6 µM. Further increases in [Ca2+]i inhibited channel Po (Fig. 1, A and C).

A further increase in [InsP3] (100 µM) did not change the gating of InsP3R-3 appreciably, indicating that the channel was saturated by 10 µM InsP3 in [Ca2+]i between 1 and 20 µM (Fig. 1 C). Reduction in [InsP3] below 10 µM did not affect Ca2+ activation of the channel significantly but substantially enhanced the sensitivity of the channel to [Ca2+]i inhibition so the channel was inhibited at lower [Ca2+]i (Fig. 1, B and C). This reduced both the maximum channel Po observed and the range of [Ca2+]i over which the channel gated appreciably in low [InsP3] (notably for [InsP3] = 1 µM in Fig. 1 C).

The [Ca2+]i dependence of InsP3R-3 channel Po in all [InsP3] can be well fitted simultaneously for all [InsP3] investigated by the biphasic Hill equation (Foskett et al., 2007) (Fig. 1 C):

 
Po=Pmax{1+(Kact[Ca2+]i)Hact}1{1+([Ca2+]iKinh)Hinh}1,
(7)

with four of the five parameters retaining the same values: Pmax = 1, Kact = 940 nM, Hact (Hill coefficient for Ca2+ activation) = 1.3, and Hinh (Hill coefficient for Ca2+ inhibition) = 1.6. Only Kinh varies with [InsP3] (inset in Fig. 1 C). Pmax from the Hill equation fit of the Po data is significantly greater than the maximum Po (0.78) observed in saturating [InsP3]. This indicates that even in saturating [InsP3], the recombinant InsP3R-3 channel in DT40-KO-r-InsP3R-3 cells is not fully activated by [Ca2+]i before it begins to be inhibited by [Ca2+]i. Because the [Ca2+]i dependence of the InsP3R-3 channel does not exhibit a clear plateau with channel Po staying at Pmax over a broad range of [Ca2+]i, Kact and Kinh are not uniquely defined by the observed [Ca2+]i dependence (Foskett et al., 2007). Nevertheless, because Pmax must be ≤1, the large observed maximum Po (≈0.78) indicates that the values of Kact and Kinh derived from the biphasic Hill equation fit are reasonable indications of the apparent affinities of the activating and inhibitory cytoplasmic Ca2+-binding sites of the channel. Thus, the biphasic Hill equation fitting result that only Kinh depends on [InsP3] indicates that InsP3 modulates InsP3R-3 gating solely by changing the sensitivity of the channel to Ca2+ inhibition (Mak et al., 1998; Foskett et al., 2007). Both Hill coefficients for Ca2+ activation and inhibition are moderately >1, suggesting that both Ca2+ activation and inhibition are cooperative but not strongly so.

The dependence of Kinh on [InsP3] is well described by a simple activating Hill equation (inset in Fig. 1 C):

 
Kinh=Kinh{1+(KInsP3/[InsP3])HInsP3}1,

with Kinh (Kinh in saturating [InsP3]) ≈ 13 µM and KInsP3 (half-maximal [InsP3]) ≈ 4.5 µM. HInsP3 (Hill coefficient for modulation of Kinh by [InsP3]) is ∼2.3. This suggests that InsP3 modulation of InsP3R-3 channel activity is strongly cooperative.

These main features of ligand regulation of the activity of homotetrameric recombinant r-InsP3R-3 channel in DT40-KO-r-InsP3R-3 cells are highly reminiscent of those of a variety of InsP3R channels in different cell systems examined using the same approach: endogenous Xenopus laevis type 1 InsP3R (InsP3R-1) channel in oocytes, recombinant r-InsP3R-3 channel expressed in Xenopus oocytes, and endogenous insect InsP3R channel in Sf9 cells (Foskett et al., 2007). However, the r-InsP3R-3 channel in DT40-KO-r-InsP3R-3 cells is significantly less sensitive to [Ca2+]i and [InsP3] activation than the other channels, with significantly higher Kact and KInsP3. Furthermore, the efficacy of InsP3 to activate the channel by reducing its sensitivity to inhibition by Ca2+i is also lowest for r-InsP3R-3 channel in DT40-KO-r-InsP3R-3 cells among InsP3R channels studied (Foskett et al., 2007), as indicated by its low Kinh.

Regulation of r-InsP3R-3 channel gating kinetics by cytoplasmic ligands

The [Ca2+]i dependence of the mean open duration (to) of r-InsP3R-3 channels expressed in DT40-KO-r-InsP3R-3 cells loosely mirrors that of channel Po, with to continuously increasing as [Ca2+]i was increased from 40 nM to ≈1 µM. Within this [Ca2+]i range, to was similar in the same [Ca2+]i for all [InsP3]. to then remained high for a range of [Ca2+]i extending beyond the point where channel Po started to be reduced by higher [Ca2+]i. Beyond a threshold [Ca2+]i, to started to be reduced by higher [Ca2+]i. The threshold [Ca2+]i was ≈20 µM in 10 µM InsP3 and ≈6 µM in 3 µM InsP3. Thus, the threshold [Ca2+]i decreased as [InsP3] was reduced (Fig. 1 D).

Ligand regulation of the mean channel closed duration (tc) is more complex. In saturating [InsP3], tc decreased continuously as [Ca2+]i was increased up to 1 µM, when it reached a minimum of ∼10 ms at [Ca2+]i ≈ 900 nM, before Po attained its maximum value. tc remained at the minimum value as [Ca2+]i was increased to 6 µM. Then, tc increased when Po started to be reduced by rising [Ca2+]i. Unlike to, tc was increased in all [Ca2+]i as [InsP3] was reduced below saturating levels, except at very low [Ca2+]i (∼40 nM). As [Ca2+]i was increased, tc followed a trend that is the inverse of that of Po, decreasing until tc reached its minimum at the [Ca2+]i at which Po was maximal, and then increasing as Po decreased (Fig. 1 E).

These ligand dependencies of the gating characteristics (to and tc) of r-InsP3R-3 in DT40-KO-r-InsP3R-3 cells are markedly more complex than those of other InsP3R channels examined (Mak et al., 1998, 2001b; Ionescu et al., 2006). For the other channels, to remained effectively constant over most of the ranges of [InsP3] and [Ca2+]i examined, so that once a channel opens, the duration for which it remains open is largely independent of the local [Ca2+]i and [InsP3]. Ligand modulations of Po of those channels therefore result mostly from ligand modulations of tc.

Effects of physiological levels of [Ca2+]ER on InsP3R-3 channel gating

To study possible modulation of InsP3R-3 channel gating by [Ca2+]ER, nuclear patch-clamp experiments in the excised lum-out configuration were performed on nuclei from DT40-KO-r-InsP3R-3 cells. The luminal side of InsP3R channels in the isolated nuclear membrane patches was exposed rapidly and repeatedly to solutions containing different [Ca2+]f using rapid perfusion solution exchange (Vais et al., 2010a,b). To avoid the complication of Ca2+ moving through one active InsP3R channel affecting the activity of neighboring active channels by raising local [Ca2+]i, only single-channel current traces obtained in these lum-out nuclear patch-clamp experiments were used for analysis.

To detect possible activating or inhibitory effects of [Ca2+]ER on InsP3R channel gating, we first used a pipette solution containing subsaturating (3 µM) [InsP3] and optimal (2 µM) [Ca2+]i buffered by 0.5 mM diBrBAPTA. Active InsP3R-3 channels were exposed alternately to [Ca2+]ER of 70 nM, a sub-physiological [Ca2+]ER in which no Ca2+ flux flowed from the luminal side of the channels to the cytoplasmic side, and 300 µM, a physiological [Ca2+]ER that drives substantial Ca2+ flux through the channels (Vais et al., 2010a). Increasing [Ca2+]ER from 70 nM to 300 µM caused a reduction in the magnitude of the current passing through the active InsP3R-3 channels (Fig. 2 A) because Ca2+ acts as permeant channel blocker, reducing K+ conductance of the channel (Vais et al., 2010a). Returning [Ca2+]ER to 70 nM restored the channel current magnitude (Fig. 2 B). Such changes in channel current size were used in all experiments to mark the time when the perfusion solution exchange was completed at the luminal side of the active InsP3R channels.

Besides the reduction in channel conductance, the jump in [Ca2+]ER from 70 nM to 300 µM caused a significant decrease in channel Po (Fig. 2 A), which was reversed when [Ca2+]ER was returned to 70 nM (Fig. 2 B). This effect of [Ca2+]ER on channel Po was quantified by the ratio of the Po observed when [Ca2+]ER = 300 µM (Po(300 µM)) to that when [Ca2+]ER = 70 nM (Po(70 nM)), evaluated immediately before and after each perfusion solution switch (Fig. 2, A and B). With [InsP3] = 3 µM and [Ca2+]i = 2 µM, the InsP3R channel was significantly less active in [Ca2+]ER = 300 µM than in [Ca2+]ER = 70 nM, so the mean Po ratio (Po(300μM)/Po(70nM); the angle brackets are used to emphasize that this is the mean of the Po ratios, not the ratio of the mean Po’s at different [Ca2+]ER) was significantly lower than unity (Fig. 2 D, red bar). The change in [Ca2+]ER also caused significant changes in both to and tc, so the mean to and tc ratios (to(300μM)/to(70nM) and tc(300μM)/tc(70nM), respectively) are both significantly different from unity (Fig. 2, E and F, red bars).

The observed inhibition of InsP3R gating by elevated [Ca2+]ER can be caused by luminal free Ca2+ (Ca2+ER) binding to an inhibitory site on the luminal side of the channel. Alternatively, it could be caused by Ca2+ binding to cytoplasmic sites on the InsP3R channel because of local [Ca2+]i in the vicinity of the pore elevated by Ca2+ flux driven through the channel by the high [Ca2+]ER. Because [Ca2+]i in the pipette solution used (2 µM) optimally activates InsP3R channels in 3 µM InsP3, a rise in local [Ca2+]i near the channel is predicted to reduce Po caused by Ca2+ binding to the inhibitory Ca2+-binding sites on the cytoplasmic side of the channel (Foskett et al., 2007).

[Ca2+]ER inhibition of InsP3R channel activity is abolished by saturating [InsP3] but restored by higher [Ca2+]ER

To better characterize the observed [Ca2+]ER regulation of InsP3R channel activity, we investigated the [InsP3] dependence of the effect of [Ca2+]ER by using pipette solutions containing different [InsP3] in our lum-out nuclear patch-clamp experiments. Inhibition of InsP3R channel gating by raising [Ca2+]ER from 70 nM to 300 µM was abrogated in the presence of saturating [InsP3] (10 µM) (Fig. 2 C), so the mean Po, to, and tc ratios observed were not significantly different from unity (Fig. 2, D–F, gray bars). In contrast, even in saturating [InsP3], InsP3R channel activity was still substantially inhibited when [Ca2+]ER was raised to higher (1.1 mM) levels (Fig. 3 A), with mean Po ratio significantly less than unity (Fig. 3 B) because of both longer tc and shorter to (Fig. 3, C and D). This is similar to the suppression of channel gating by [Ca2+]ER jumping from 70 nM to 300 µM in subsaturating (3 µM) [InsP3] (Fig. 2). Accordingly, if the inhibitory effect of high [Ca2+]ER on channel Po is mediated by some luminal Ca2+-binding site(s) on the channel, the site must be allosterically coupled to the InsP3-binding sites on the cytoplasmic side of the channel so that channel activation by InsP3 binding to its cytoplasmic site and channel inhibition by Ca2+ binding to the luminal site are mutually antagonistic. Alternatively, if the [Ca2+]ER effect is mediated by the Ca2+ flux through the channel, the lack of effect on channel gating of [Ca2+]ER jump from 70 nM to 300 µM in the presence of saturating [InsP3] can be accounted for by the InsP3-induced reduction in sensitivity of InsP3R to [Ca2+]i inhibition. In this scenario, a rise in local [Ca2+]i caused by Ca2+ flux driven through the channel by [Ca2+]ER of 300 µM that inhibits InsP3R channel gating in subsaturating [InsP3] is not sufficient to affect channel gating, as saturating [InsP3] reduces the sensitivity of the channel to [Ca2+]i inhibition. However, the higher rise in local [Ca2+]i driven by a higher [Ca2+]ER of 1.1 mM can still cause suppression of channel activity, even in the presence of saturating [InsP3].

[Ca2+]i dependence of the modulation of InsP3R channel Po by [Ca2+]ER

To identify the mechanisms underlying the regulation of InsP3R channel activity by [Ca2+]ER, we investigated the [Ca2+]i dependence of the effect. If the observed [Ca2+]ER modulation of InsP3R channel activity is mediated by cytoplasmic Ca2+-binding sites of the channel, [Ca2+]ER modulation should be consistent with the biphasic [Ca2+]i regulation of InsP3R channel Po (Fig. 1 C). Accordingly, when a pipette solution containing suboptimal low [Ca2+]i is used instead of one with optimal [Ca2+]i, the rise in local [Ca2+]i caused by the Ca2+ flux driven through the open channel by high [Ca2+]ER is expected to activate instead of inhibit channel gating. Indeed, when excised lum-out experiments were performed with saturating (10 µM) [InsP3] and suboptimal low (55 nM) [Ca2+]i in the pipette solution, the activity of the InsP3R channel was significantly enhanced when [Ca2+]ER was switched from 70 nM to 300 µM (Fig. 4 A), as reflected in a mean Po ratio significantly larger than unity (Fig. 4 B) mainly caused by increase in to (Fig. 4 C) while tc remained unaltered (Fig. 4 D). This observation is difficult to account for with the hypothesis of a luminal Ca2+-binding site on the InsP3R channel regulating its activity.

[Ca2+]ER modulation of InsP3R channel Po depends on the magnitude of the Ca2+ flux

To further confirm that rise in local [Ca2+]i in the vicinity of the channel pore caused by feed-through Ca2+ flux driven by high [Ca2+]ER modulates InsP3R channel significantly by Ca2+ binding to cytoplasmic sites on the channel, we investigated whether channel activity would be affected if the magnitude of the Ca2+ flux through the InsP3R channel was altered by changing Vapp only, with [Ca2+]ER kept the same. In all previous experiments, Vapp = −30 mV. This Vapp drove Ca2+ from the bath solution through the channel to the pipette solution, in the same direction as the [Ca2+]f gradient when the perfusion solution contained [Ca2+]ER of 300 µM. When the polarity of Vapp is reversed (Vapp = +30 mV), the applied Vapp opposes the [Ca2+]f gradient. Although this change in Vapp polarity is insufficient to reverse the direction of the Ca2+ flux through the channel, it reduces the magnitude of the flux and therefore diminishes the rise in local [Ca2+]i around the channel pore. In excised lum-out nuclear patch-clamp experiments with saturating [InsP3] (10 µM) and suboptimal [Ca2+]i (70 nM), jumps of [Ca2+]ER from 70 nM to 300 µM with Vapp = +30 mV still enhanced InsP3R channel activity (Fig. 5 A) in a qualitatively similar way as in Vapp = −30 mV, giving a mean Po ratio significantly larger than unity (Fig. 5 B) solely by prolonging to (Fig. 5, C and D). However, the increases in channel Po and to were substantially less in Vapp = +30 mV than those observed with Vapp = −30 mV (Fig. 5, B and C).

[Ca2+]ER modulation of InsP3R channel Po depends on cytoplasmic Ca2+-buffering conditions

Our experiments so far have demonstrated convincingly that most of the effects of [Ca2+]ER on InsP3R channel activity are mediated by the Ca2+ flux driven by [Ca2+]ER to raise local [Ca2+]i to modify InsP3R channel activity through the cytoplasmic-activating and inhibitory Ca2+-binding sites on the channel. We next performed experiments to examine more closely the existence of an intrinsic functional regulatory Ca2+-binding site on the luminal side of the channel. We investigated the effects of cytoplasmic Ca2+-buffering conditions on [Ca2+]ER modulation of channel activity in optimal [Ca2+]i (2 µM) and subsaturating [InsP3] (3 µM). In all previous experiments involving [Ca2+]i = 2 µM, [Ca2+]i in the pipette (cytoplasmic) solution was buffered by 0.5 mM of the fast Ca2+ chelator diBrBAPTA (Ca2+ binding rate kon of ∼450 µM−1s−1; Naraghi, 1997). In weaker buffering conditions in which [Ca2+]i was buffered to 2 µM by a low concentration (0.1 mM) of the slower Ca2+ chelator HEDTA (kon of ∼4.5 µM−1s−1) (Naraghi, 1997), suppression of channel activity when [Ca2+]ER was raised from 70 nM to 300 µM was more profound than that observed in 0.5 mM diBrBAPTA (Fig. 6 A). Accordingly, the mean Po ratio was significantly lower under weak cytoplasmic Ca2+-buffering conditions than under the normally used buffering conditions, even though [Ca2+]i was kept at 2 µM (Fig. 6 C). This is solely because of a significant increase in tc (Fig. 6, D and E). In contrast, the inhibitory effects of the same [Ca2+]ER jump were completely abolished when cytoplasmic Ca2+ was strongly buffered at 2 µM with a high concentration (5 mM) of diBrBAPTA (Fig. 6 B), giving mean Po, to, and tc ratios not significantly different from unity (Fig. 6, C–E). Control experiments demonstrated that different Ca2+-buffering conditions themselves had no significant effect on Po in the absence of Ca2+ flux through the active channels ([Ca2+]ER = 70 nM) in both on-nucleus and excised lum-out configurations (Fig. 7).

If [Ca2+]ER modulation of channel Po is mediated by luminal Ca2+-binding site(s), it is difficult to conceive how such a luminal site(s) could be sensitive to Ca2+ buffering on the cytoplasmic side by artificial chemical Ca2+ chelators that are not naturally found in vivo. Furthermore, the complete abolition of the effect of physiological [Ca2+]ER (300 µM) on channel activity by sufficiently strong cytoplasmic Ca2+ buffering suggests that there is no luminal Ca2+-binding site intrinsic to the InsP3R, activating or inhibitory, that is sensitive to 300 µM [Ca2+]ER.

Complete abrogation of modulatory effects of [Ca2+]ER in the physiological range on InsP3R channel activity

To determine if [Ca2+]ER in the upper limits of the physiological range could possibly inhibit InsP3R channel activity through a luminal Ca2+-binding site on the channel, we looked for inhibitory effects on InsP3R channel activity by [Ca2+]ER jumps from 70 nM to 1.1 mM (higher than most observed [Ca2+]ER; Button and Eidsath, 1996; Bygrave and Benedetti, 1996; Meldolesi and Pozzan, 1998; Yu and Hinkle, 2000) that were independent of the rise in [Ca2+]i resulting from Ca2+ flux passing through the open InsP3R channel. In excised lum-out nuclear patch-clamp experiments with Vapp = +30 mV (to reduce the magnitude of the Ca2+ flux through the open channel) and pipette solution containing saturating (10 µM) [InsP3] and optimal (2 µM) [Ca2+]i buffered by 5 mM diBrBAPTA (to reduce the rise in [Ca2+]i caused by the Ca2+ flux), [Ca2+]ER jumps from 70 nM to 1.1 mM did not affect InsP3R channel gating (Fig. 8 A), so that the mean Po, to, and tc ratios observed were not significantly different from unity (Fig. 8, B–D).

We also checked whether the activating effects of [Ca2+]ER on InsP3R channel activity observed in suboptimal (55 nM) [Ca2+]i (Fig. 4) could be completely abrogated by a combination of positive Vapp and strong cytoplasmic Ca2+ buffering. Unlike the inhibitory effects, [Ca2+]ER jumps from 70 nM to 550 µM (maximal [Ca2+]ER observed in many cell types; Button and Eidsath, 1996; Bygrave and Benedetti, 1996; Meldolesi and Pozzan, 1998; Yu and Hinkle, 2000) still enhanced InsP3R channel activity in excised lum-out nuclear patch-clamp experiments with Vapp = +30 mV and pipette solution containing saturating (10 µM) [InsP3] and suboptimal (55 nM) [Ca2+]i buffered by 10 mM BAPTA (Fig. 9 A), so that the mean Po observed was significantly higher than unity (Fig. 9 D), mostly because of longer to (Fig. 9, E and F). However, increasing Vapp further to +50 mV substantially diminished the activating effects of the [Ca2+]ER jumps (Fig. 9 B), with a mean Po ratio that is much lower although still significantly larger than unity (Fig. 9 D), which is caused by the to ratio that is significantly greater than unity (Fig. 9, E and F). At Vapp = +70 mV, the activating effects of the [Ca2+]ER jumps were completely abrogated so that the mean Po, to, and tc ratios were not different from unity (Fig. 9, D–F).

In the experiments investigating the abrogation of activating effects of [Ca2+]ER changes, [Ca2+]ER was increased to 550 µM rather than 1.1 mM because using high concentrations of Ca2+ chelator in the pipette solution did not reduce the activating effects of [Ca2+]ER effectively. This is probably because the cytoplasmic activating Ca2+-binding site is located close to the channel pore (see Discussion). To abrogate the effects of [Ca2+]ER jumps to 1.1 mM as in other experiments would require using Vapp of >70 mV, which severely compromised the integrity of the gigaohm seal between the isolated nuclear membrane patch and the patch-clamp microelectrode.

Collectively, the total abrogation of both the inhibitory effects (in optimal [Ca2+]i = 2 µM) and the activating effects (in suboptimal [Ca2+]i = 55 nM) of physiological levels of [Ca2+]ER in conditions that only affected the magnitude of the Ca2+ flux through the channel and the changes in [Ca2+]i demonstrates that there is no regulatory Ca2+-binding site sensitive to physiological [Ca2+]ER on the luminal side of the InsP3R channel.

DISCUSSION

This study is the first investigation of the effects of [Ca2+]ER on single-channel activity of InsP3R using the nuclear patch-clamp approach. Previously, two electrophysiological studies explored the effects of [Ca2+]ER using reconstituted cerebellar type 1 InsP3R channels in planar lipid bilayers. In the first study (Bezprozvanny and Ehrlich, 1994), the cytoplasmic solution contained no permeant ion, with 55 mM Ba2+, Mg2+, Sr2+, or a combination of Ca2+ and Sr2+ in the luminal solutions used as the main charge carrier. Channel Po was significantly reduced as [Ca2+]ER was increased from 300 µM to 44 mM. It was suggested that feed-through effects of [Ca2+]ER-driven Ca2+ flux through the channel contributed to the inhibition of InsP3R channel activity, but the existence of a luminal inhibitory Ca2+ site could not be conclusively ruled out because of insufficient [Ca2+]i buffering by only 1 mM EGTA. The relevance of this study was further diminished by the nonphysiological ionic compositions used, which rendered it questionable whether the magnitude of Ca2+ flux and therefore the resulting changes in [Ca2+]i resembled those in physiological ionic conditions. In the second study (Thrower et al., 2000), 500 mM K+ was present in all solutions with [Ca2+]i buffered to 0.2–0.3 µM by either 10 mM HEDTA or 1.7 mM BAPTA. The conclusion of the study that Ca2+ER affects InsP3R channel gating through direct interaction with the luminal face of the channel was critically undermined by technical difficulties (brief and inconsistent channel activities, multiple conductance substates), a mostly qualitative description of channel activity (no quantitative Po or to-tc analysis), and the inappropriate use of Ca2+ buffers (HEDTA cannot effectively buffer [Ca2+]i at 0.2–0.3 µM despite the high concentrations used because of its low Ca2+ affinity; BAPTA has the right Ca2+ affinity, but only a low concentration was used).

In this study, modulation of InsP3R channel activity by [Ca2+]ER was examined under rigorously controlled [Ca2+]i, [Ca2+]ER, [InsP3], and cytoplasmic Ca2+-buffering conditions in excised lum-out nuclear patch-clamp experiments with perfusion solution exchange. The observed dependencies of [Ca2+]ER modulation of channel activity on [InsP3] (Fig. 2), [Ca2+]ER (Fig. 3), [Ca2+]i (Fig. 4), Vapp (Fig. 5), and cytoplasmic Ca2+-buffering conditions (Fig. 6), and the total abrogation of the effects of [Ca2+]ER on channel activity by conditions that affect the rise in local [Ca2+]i caused by the Ca2+ flux but not [Ca2+]ER itself (Figs. 6, 8, and 9), together demonstrate that InsP3R channel activity is regulated solely by the feed-through effects of the [Ca2+]ER-driven Ca2+ flux through the open channel, raising [Ca2+]i in the microdomain around the channel to alter Po of the channel through its cytoplasmic activating and inhibitory Ca2+-binding sites, and not by direct binding of Ca2+ to the luminal side of the InsP3R channel.

The observed modulation of channel activity by [Ca2+]ER-driven Ca2+ flux through the channel itself provides insights regarding the kinetics of [Ca2+]i regulation of InsP3R channel gating. From these insights and others from previous studies of ligand regulation of InsP3R channel gating, we develop below the concept of the effective time-averaged [Ca2+]i profile around the channel caused by Ca2+ flux through the channel itself when it is gating. With that concept, and using channel Po observed in the presence of various [Ca2+]ER-driven Ca2+ fluxes under various [Ca2+]i, [Ca2+]ER, and [InsP3], we then estimate the distances between the channel pore and the cytoplasmic regulatory Ca2+ sites.

Kinetics of fluctuations of local [Ca2+]i in the vicinity of the channel pore caused by Ca2+ flux through the pore

In a previous study (Vais et al., 2010a), we determined that physiological levels of [Ca2+]ER can drive substantial Ca2+ fluxes through an open InsP3R-3 channel. Numeric simulations allow us to follow the changes in the [Ca2+]i profile around the Ca2+-permeable InsP3R channel during its gating. The simulations indicate that under our experimental Ca2+-buffering conditions, the [Ca2+]i profile around an open InsP3R channel achieves steady-state levels within 100 µs after the channel opens (Fig. 10 A). Furthermore, after a channel closes, the elevated [Ca2+]i around the channel collapses rapidly and returns to the basal level within 1 ms (Fig. 10 B). Thus, the [Ca2+]i profile around an InsP3R channel fluctuates abruptly between the steady-state open- and closed-channel levels in a quasi-binary manner, with kinetics rigidly dictated by the opening and closing of the channel. This has significant implications for the kinetic properties of [Ca2+]i regulation of the channel and how [Ca2+]ER-driven Ca2+ flux through the InsP3R channel pore can modulate the activity of the channel itself.

Kinetics of cytoplasmic Ca2+ activation of InsP3R channel deduced from the observed enhancement of channel activity by [Ca2+]ER-driven Ca2+ flux

To derive insights into the kinetics of cytoplasmic Ca2+ activation of InsP3R channel from the observed channel response to [Ca2+]ER-driven Ca2+ flux through the channel itself, we first consider a hypothetical, extreme kind of response of a Ca2+i-activated channel to the increase in local [Ca2+]i at the single activating cytoplasmic Ca2+-binding site of the channel. In this extreme case, the kinetics of channel gating are rigidly dictated by the local [Ca2+]i at its Ca2+ site. To generate this kind of response, the activating latency (interval between local [Ca2+]i increasing beyond Kact and the first resulting opening of the channel, τact) and the deactivating latency (interval between local [Ca2+]i decreasing below Kact and the last closing of the channel, τdeact) must both be much less than the time scale of channel gating (to and tc), so that the channel opens/closes practically instantaneously after the rise/drop in local [Ca2+]i at its activating site. Moreover, the channel must remain open as long as its single activating Ca2+ site is occupied, and it must remain closed whenever the activating site is vacant, i.e., the gating status (open or closed) of the channel is deterministically dependent on the occupancy of the activating Ca2+ site. Given that the [Ca2+]i profile around a Ca2+-permeable channel is rigidly dictated by the gating of the channel under our experimental conditions (as discussed in the previous section), a lone Ca2+i-activated, Ca2+-permeable channel in the ER with its gating rigidly dictated by [Ca2+]i at its activating Ca2+ site as described above would not be expected to be activated by Ca2+ flux through the channel itself. This is simply because the activating site on the channel must be already occupied when the channel is open and therefore cannot further bind Ca2+. Consequently, the channel cannot be affected by the local [Ca2+]i elevated by the [Ca2+]ER-driven Ca2+ flux. Conversely, when its Ca2+-activating site is vacant and available to bind Ca2+, the channel must be closed so that there is no [Ca2+]ER-driven Ca2+ flux through that channel to activate the channel. Thus, the channel would behave as if there is no Ca2+ flux through the open channel.

However, in lum-out nuclear patch-clamp experiments with the pipette solution containing [Ca2+]f = 55 nM and [InsP3] = 10 µM, we observed significant sustained activation of the channel in the presence of high [Ca2+]ER (Figs. 4, A and B, and 5, A and B) that can be completely accounted for by Ca2+ flux through the channel raising local [Ca2+]i at its cytoplasmic activating Ca2+ site to enhance its Po (Fig. 5 A). This observation therefore has nontrivial implications for the kinetics of cytoplasmic Ca2+ activation of InsP3R channel. Most obviously, this indicates that local [Ca2+]i at the cytoplasmic site does not rigidly dictate the gating of the channel. Different mechanisms can contribute to uncoupling of the gating kinetics of the channel from changes in [Ca2+]i at the activating Ca2+ site. Most importantly, Ca2+ regulates InsP3R channel activity stochastically, so Ca2+ binding to the activating site only induces the channel to adopt a more active kinetic conformation with a higher Po, but does not always result in channel opening. The channel can open and close with no change in its Ca2+-binding status (Mak et al., 2003). Furthermore, the tetrameric structure of the channel (Foskett et al., 2007) indicates that it has multiple activating Ca2+ sites. Thus, the channel can open even when there are vacant activating Ca2+ sites on the channel available to detect the elevated local [Ca2+]i and further enhance channel activity. Another significant factor determining the degree of coupling between local [Ca2+]i at the activating Ca2+ site and the gating of the channel is the kinetics of Ca2+i activation of the channel, i.e., τact and τdeact. Previous cytoplasmic-side-out nuclear patch-clamp experiments with rapid perfusion exchange studying endogenous InsP3R channels from insect Sf9 cells (Mak et al., 2007) revealed that in saturating (10 µM) [InsP3], InsP3R channels can respond relatively slowly to abrupt changes in [Ca2+]i, with long latencies (τact of approximately tens of milliseconds and τdeact of approximately a few hundreds of milliseconds). With long response latencies (relative to channel gating to and tc), the channel can open and close, and local [Ca2+]i around the channel jumps up and down, multiple times during the time the channel takes to respond to one change in [Ca2+]i, thereby effectively uncoupling the [Ca2+]i at the activating sites from the gating of the channel.

In the other extreme case, in which the gating of the channel is completely uncoupled from the local [Ca2+]i at the activation sites, instead of responding to the instantaneous [Ca2+]i resulting from individual channel opening and closing events, the gating of the channel depends only on the time-averaged [Ca2+]i at the sites. Over a long period T (>>τact, τdeact, to, tc), a vacant activating Ca2+ site on the channel will on average be exposed to the steady-state open-channel [Ca2+]i (because of the open-channel Ca2+ current, iCa, driven by the electrochemical gradient across the channel) for a period of PoT, and to the steady-state closed-channel [Ca2+]i for a period of (1–Po)T. Thus, assuming first-order Ca2+ binding to the activating sites, the channel will exhibit steady-state gating kinetics similar to that of a channel with activating sites constantly exposed to a local [Ca2+]i equivalent to that generated by a Ca2+ current of magnitude Po iCa passing through the pore.

In reality, the coupling between InsP3R channel gating and changes of local [Ca2+]i is partial, lying between the two extremes of total rigid dictation of gating by [Ca2+]i at the activating sites and complete decoupling with gating unrelated to instantaneous [Ca2+]i at the sites. Therefore, the activating sites are effectively exposed to a [Ca2+]i that is equivalent to a time-averaged Ca2+ current of magnitude between 0 and Po iCa.

Besides deducing that InsP3R channel gating is not rigidly dictated by [Ca2+]i at the activating sites, other insights about the kinetics of Ca2+i activation of InsP3R channel can be derived from the observed activating effects of Ca2+-driven Ca2+ flux on channel gating. In the present study, in the absence of Ca2+ flux with low [Ca2+]ER (70 nM), the InsP3R-3 channel was observed to exhibit low Po (∼0.02–0.05) with short to (∼2 ms) and long tc (∼50 ms) in constant suboptimal [Ca2+]i (55 nM), even in saturating [InsP3] (10 µM) (Figs. 1 C, 4 A, and 9 A). Nevertheless, abrupt and sustained increases in channel Po and to were observed (Figs. 4 A and 9 A) in response to the onset of Ca2+ flux through the channel to the cytoplasmic side resulting from [Ca2+]ER being raised to physiological levels. This rapid response indicates that vacant cytoplasmic activating Ca2+ sites of the channel were able to capture Ca2+ during the first couple of brief channel-opening events after the [Ca2+]ER jump, when the local [Ca2+]i at the sites was raised by the Ca2+ flux and before the channel closed and terminated the Ca2+ flux. Thus, the rate of Ca2+ binding to the activating sites must be high, suggesting that the Ca2+ flux can raise the local [Ca2+]i at the activating sites to a high level, and therefore the sites are probably located close to the channel pore (see further discussion below).

Another feature of the activating effects of [Ca2+]ER on InsP3R-3 channel gating is that the increase in channel Po as a result of [Ca2+]ER jumps from 70 nM to physiological levels was mostly achieved by prolonging to, with no significant change in tc (Figs. 4 D, 5 E, and 9 F). It has been suggested that because cytoplasmic regulatory Ca2+ sites are inaccessible to [Ca2+]ER when a Ca2+-permeable, Ca2+i-regulated channel is closed, the absence of luminal Ca2+ site on the channel means that tc of the channel should not depend on [Ca2+]ER (Laver, 2007a,b). According to this statement, our observation that tc was not significantly affected by [Ca2+]ER is consistent with a conclusion that the InsP3R channel has no luminal regulatory Ca2+ site. However, the statement is only true for a Ca2+i-regulated, Ca2+-permeable channel whose gating is strongly dictated by [Ca2+]i at its cytoplasmic regulatory Ca2+ sites. Because gating of the InsP3R channel is not rigidly dictated by [Ca2+]i at its cytoplasmic Ca2+-activating sites, the observation is better interpreted as an indication that under the experimental conditions used in Figs. 4 A, 5 A, and 9 (A and B), the conformations assumed by the channel in the presence and absence of Ca2+ flux have similar tc.

Cytoplasmic inhibitory Ca2+ sites also experience an effective time-averaged local [Ca2+]i due to [Ca2+]ER-driven Ca2+ flux through the channel

In the extreme case where gating is rigidly dictated by [Ca2+]i at the regulatory sites, the situation for Ca2+ inhibition is different from that for Ca2+ activation for a Ca2+i-regulated, Ca2+-permeable channel. Whereas activating sites can never experience the flux through the channel (discussed above), the inhibitory sites will be vacant when the channel opens and occupied when the channel is closed. Thus, effectively, the vacant sites will always be exposed to the elevated open-channel local [Ca2+]i caused by the Ca2+ current of magnitude iCa through the pore.

However, this cannot be true for the InsP3R-3 channel. Even though our experimental observations clearly demonstrated that the channel has no luminal regulatory Ca2+-binding site (Fig. 8), and the rapid collapse of the [Ca2+]i profile around the channel pore after the channel closes (Fig. 10) means that the cytoplasmic inhibitory Ca2+ sites are effectively inaccessible to [Ca2+]ER when the channel is closed, channel tc still exhibited clear dependence on [Ca2+]ER when the channel was inhibited by rise in [Ca2+]i caused by Ca2+ flux through the open channel (Figs. 2 F, 3 D, and 6 E). Therefore, the gating of the channel cannot be rigidly dictated by [Ca2+]i at the inhibitory sites. Rather, the observations suggest the presence of uncoupling mechanisms: the tetrameric InsP3R channel having multiple inhibitory sites that regulate channel Po stochastically, with Ca2+ inhibition latency and the latency of channel recovery from Ca2+ inhibition both significantly longer than the time scale of channel gating (to and tc). Such mechanisms can allow the modulation of InsP3R channel gating by the high local [Ca2+]i at the inhibitory sites established during a channel opening to extend beyond the termination of that opening and the subsequent rapid collapse of the local [Ca2+]i rise.

At the other extreme, with the gating of the channel completely decoupled from the fluctuations of local [Ca2+]i caused by the openings and closings of the channel, the channel should gate with kinetics similar to those associated with one of the inhibitory sites exposed to a steady local [Ca2+]i equivalent to that generated by a Ca2+ current of magnitude Po iCa passing through the pore, the same as the situation for the activating site. For realistic partial coupling between the two extremes, the inhibitory sites are effectively exposed to a time-averaged local [Ca2+]i caused by a Ca2+ current of magnitude between Po iCa and iCa.

In summary, the observed sustained activation and inhibition of gating by Ca2+ flux through an InsP3R channel indicate that channel gating is not deterministically regulated by [Ca2+]i, and that a channel can respond to the Ca2+ flux through itself because its activation and inhibition kinetics enable it to sense an effective steady-state local flux-driven [Ca2+]i.

Estimates of the locations of functional cytoplasmic Ca2+-binding sites from Ca2+ flux–mediated modulation of InsP3R channel activity

Our experiments demonstrate that [Ca2+]ER modulates the activity of r-InsP3R-3 channels in DT40-KO-r-InsP3R-3 cells solely via the Ca2+ flux it drives through the channel that raises the local [Ca2+]i at the cytoplasmic regulatory Ca2+-binding sites. Using the steady-state [Ca2+]i dependence of the channel Po (Fig. 1), the effects of [Ca2+]ER on channel activity can provide estimates of the effective time-averaged local [Ca2+]i at the cytoplasmic activating or inhibitory Ca2+-binding sites of the channel (Table 1). [Ca2+]i profiles ([Ca2+]i at various distances from the channel pore) were numerically generated (Materials and methods) for the different Ca2+-buffering conditions, Ca2+ electrochemical gradients, and cytoplasmic ligand concentrations used in our experiments (Table 1). Checking the estimates of the effective time-averaged local [Ca2+]i at the regulatory sites against the appropriate simulated [Ca2+]i profiles, estimates can be made of the locations of the regulatory Ca2+-binding sites relative to the channel pore, which is situated at the center of the channel based on the structural symmetry of the tetrameric InsP3R channel (Foskett et al., 2007).

For the inhibitory Ca2+ sites, depending on the degree of coupling between the gating of the channel and the fluctuations in local [Ca2+]i associated with each opening and closing event, the [Ca2+]i profile suitable for estimating the location of the sites lies between the [Ca2+]i profile generated for Ca2+ current = iCa derived from the Goldman–Hodgkin–Katz current equation (Eq. 1) (for deterministic coupling between channel gating and the local [Ca2+]i at the inhibitory Ca2+ site), and the profile generated for Ca2+ current = Po iCa (for completely uncoupled channel gating and local [Ca2+]i at the inhibitory site). Because the exact degree of coupling between channel gating and local [Ca2+]i fluctuations for the experimental conditions used are not known, we use the two [Ca2+]i profiles for currents = iCa and Po iCa to derive upper and lower limits for the distance of the inhibitory Ca2+ sites from the channel pore.

In this study, we made six independent measurements of the inhibitory effects on channel gating of raising local [Ca2+]i around the r-InsP3R-3 channel beyond 2 µM (optimal [Ca2+]i) (shown in Figs. 2, A and C, 3 A, 6, A and B, and 8 A), each of which provides an independent estimate of the range for the distance of the inhibitory Ca2+-binding site from the channel pore (Fig. 11, A–F, and Table 1). The estimated upper limits of this distance range between 21 and 44 nm (Table 1), with an average of 34 ± 4 nm. The estimated lower limits range between 2 and 29 nm (Table 1), with an average of 19 ± 4 nm. These suggest that the distance from the channel pore to the inhibitory site is ∼20–30 nm.

Using image reconstructions based on electron cryomicroscopy or electron microscopy with negative staining, 3-D structures of single tetrameric InsP3R channel have been determined (Jiang et al., 2002; da Fonseca et al., 2003; Hamada et al., 2003; Serysheva et al., 2003; Sato et al., 2004; Wolfram et al., 2010; Ludtke et al., 2011). Although the details differ significantly, they generally show a large structure on the cytoplasmic side with maximum radius (r) from the channel pore axis between 10.5 and 14.2 nm, and height above the ER membrane (h) between 13.5 and 18.3 nm. Simple geometric consideration for a 3-D structure suggests that the maximum distance between the channel pore and a Ca2+-binding site on the channel is ∼24–32.5 nm (r + h). Thus, our estimate of the inhibitory site being 20–30 nm from the pore of the channel is not in conflict with the 3-D structures reported and suggests that the inhibitory site may be located in a part of the channel furthest from the pore.

For the activating Ca2+ sites, an upper limit for the distance to the channel pore can be derived from the [Ca2+]i profile for current with magnitude = Po iCa, which corresponds to the extreme case when the channel gating is completely decoupled from local [Ca2+]i at the activating sites during channel openings and closings. However, no lower limit can be deduced for the pore-to-activating-site distance from the extreme case with channel gating rigidly dictated by local [Ca2+]i at the activating sites. This is because in this case, the channel cannot be activated by [Ca2+]ER-driven Ca2+ flux through the pore, no matter where the activating Ca2+ sites are.

We made five independent measurements of the activating effects on channel gating of raising local [Ca2+]i around the channel above 70 nM (resting [Ca2+]i) (Figs. 4 A, 5 A, and 9, A–C). These provided five independent estimates of the upper limits for the activating site to channel pore distance (Fig. 11, G–K, and Table 1), suggesting that the activating site is less than ∼9–62 nm from the channel pore, which is also consistent with the 3-D structures of the channel. Moreover, 5 mM diBrBAPTA can effectively buffer the local [Ca2+]i at the inhibitory Ca2+-binding sites to abolish the inhibiting effect of Ca2+ flux driven through the channel by 1.1 mM [Ca2+]ER at Vapp = +30 mV (Fig. 8 A), whereas even 10 mM BAPTA cannot sufficiently buffer the local [Ca2+]i at the activating Ca2+-binding sites to abolish the activating effect of Ca2+ flux driven by 0.55 mM [Ca2+]ER at the same Vapp (Fig. 9 A). These observations strongly suggest that the activating Ca2+ sites are closer to the channel pore than the inhibitory sites.

It should be pointed out that these estimates of the locations of the cytoplasmic regulatory Ca2+ sites relative to the channel pore are very rough because the [Ca2+]i profiles around the channel were simulated without taking into consideration factors that can affect the distribution of Ca2+ around the channel but are not known in any detail, like the 3-D surface topology and charge distribution of the channel.

A very similar approach to that used here was applied to estimate the locations of activating and inhibitory Ca2+-binding sites in the RyR intracellular Ca2+ release channel that also exhibits [Ca2+]ER-driven Ca2+ flux modulation of channel activity (Liu et al., 2010). However, in that study, the couplings between channel gating and the local [Ca2+]i fluctuations at the regulatory sites during channel openings and closings were not taken into consideration. The distances derived from the effective time-averaged [Ca2+]i profile generated from Ca2+ current of magnitude = Po iCa was assumed to be estimates of the actual distances between the regulatory sites and the channel pore, instead of limiting values of those distances. This led to their conclusion that the inhibitory Ca2+ site was 1.2 ± 0.16 nm from the channel pore. This is probably an underestimation because that was actually the lower limit of the distance. The activating Ca2+ site to channel pore distance was calculated to be 1.7 µm, which led to a conclusion that the activating site on the open channel was shielded from the channel’s own Ca2+ flux. However, this value should be the upper limit of the activating Ca2+ site to pore distance. Accordingly, their derivation does not provide strong support for the notion that the activating site is shielded from feed-through effects of the channel’s own Ca2+ flux.

Limitation of the excised lum-out nuclear patch-clamp experiments

Using excised-patch lum-out nuclear patch clamping, we have demonstrated that all modulation by [Ca2+]ER of the gating activity of the r-InsP3R-3 channel can be attributed to feed-through effects causing a rise in local [Ca2+]i at cytoplasmic regulatory Ca2+-binding sites of the channel. We found no modulatory effects on InsP3R channel gating involving luminal Ca2+-binding site(s) on the channel. However, it must be noted that the luminal side of the excised nuclear membrane patches was perfused with various solutions to change [Ca2+]ER in our experiments. It is possible that a luminal Ca2+-binding factor(s) that could mediate effects of [Ca2+]ER on InsP3R channel activity was washed off by the perfusion. The possible existence of [Ca2+]ER regulation of InsP3R channel activity mediated by factor(s) in the ER lumen loosely associated with the channel should be investigated in the future using nuclear patch-clamp experiments in which the ER luminal milieu is preserved.

Acknowledgments

This work was supported by National Institutes of Health grants R01 MH059937 (to J.K. Foskett) and 5R01 GM065830 (to J.E. Pearson, D.-O.D. Mak, and J.K. Foskett).

Sharona E. Gordon served as editor.

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    Abbreviations used in this paper:
     
  • HEDTA

    hydroxyethylethylenediaminetriacetic acid

  •  
  • InsP3

    inositol 1,4,5-trisphosphate

  •  
  • InsP3R

    InsP3 receptor

  •  
  • lum-out

    luminal-side-out

  •  
  • Po

    open probability

  •  
  • r-InsP3R-3

    rat type 3 InsP3R

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