Potentiation of the cardiac L-type Ca(2+) channel (alpha(1C)) by dihydropyridine agonist and strong depolarization occur via distinct mechanisms

Pore mutation N617D in the skeletal muscle DHPR blocks Ca2+ influx due to atypical high-affinity Ca2+ binding

Skeletal muscle excitation-contraction (EC) coupling roots in Ca²⁺-influx-independent inter-channel signaling between the sarcolemmal dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR1) in the sarcoplasmic reticulum. Although DHPR Ca²⁺ influx is irrelevant for EC coupling, its putative role in other muscle-physiological and developmental pathways was recently examined using two distinct genetically engineered mouse models carrying Ca²⁺ non-conducting DHPRs: DHPR(N617D) (Dayal et al., 2017) and DHPR(E1014K) (Lee et al., 2015). Surprisingly, despite complete block of DHPR Ca²⁺-conductance, histological, biochemical, and physiological results obtained from these two models were contradictory. Here, we characterize the permeability and selectivity properties and henceforth the mechanism of Ca²⁺ non-conductance of DHPR(N617). Our results reveal that only mutant DHPR(N617D) with atypical high-affinity Ca²⁺ pore-binding is tight for physiologically relevant monovalent cations like Na⁺ and K⁺. Consequently, we propose a molecular model of cooperativity between two ion selectivity rings formed by negatively charged residues in the DHPR pore region.

Distinct Components of Retrograde Ca(V)1.1-RyR1 Coupling Revealed by a Lethal Mutation in RyR1

The molecular basis for excitation-contraction coupling in skeletal muscle is generally thought to involve conformational coupling between the L-type voltage-gated Ca(2+) channel (CaV1.1) and the type 1 ryanodine receptor (RyR1). This coupling is bidirectional; in addition to the orthograde signal from CaV1.1 to RyR1 that triggers Ca(2+) release from the sarcoplasmic reticulum, retrograde signaling from RyR1 to CaV1.1 results in increased amplitude and slowed activation kinetics of macroscopic L-type Ca(2+) current. Orthograde coupling was previously shown to be ablated by a glycine for glutamate substitution at RyR1 position 4242. In this study, we investigated whether the RyR1-E4242G mutation affects retrograde coupling. L-type current in myotubes homozygous for RyR1-E4242G was substantially reduced in amplitude (∼80%) relative to that observed in myotubes from normal control (wild-type and/or heterozygous) myotubes. Analysis of intramembrane gating charge movements and ionic tail current amplitudes indicated that the reduction in current amplitude during step depolarizations was a consequence of both decreased CaV1.1 membrane expression (∼50%) and reduced channel Po (∼55%). In contrast, activation kinetics of the L-type current in RyR1-E4242G myotubes resembled those of normal myotubes, unlike dyspedic (RyR1 null) myotubes in which the L-type currents have markedly accelerated activation kinetics. Exogenous expression of wild-type RyR1 partially restored L-type current density. From these observations, we conclude that mutating residue E4242 affects RyR1 structures critical for retrograde communication with CaV1.1. Moreover, we propose that retrograde coupling has two distinct and separable components that are dependent on different structural elements of RyR1.

Properties of Na+ currents conducted by a skeletal muscle L-type Ca2+ channel pore mutant (SkEIIIK)

Four glutamate residues residing at corresponding positions within the four conserved membrane-spanning repeats of L-type Ca(2+) channels are important structural determinants for the passage of Ca(2+) across the selectivity filter. Mutation of the critical glutamate in Repeat III in the a 1S subunit of the skeletal L-type channel (Ca(v)1.1) to lysine virtually eliminates passage of Ca(2+) during step depolarizations. In this study, we examined the ability of this mutant Ca(v)1.1 channel (SkEIIIK) to conduct inward Na(+) current. When 150 mM Na(+) was present as the sole monovalent cation in the bath solution, dysgenic (Ca(v)1.1 null) myotubes expressing SkEIIIK displayed slowly-activating, non-inactivating, nifedipine-sensitive inward currents with a reversal potential (45.6 ± 2.5 mV) near that expected for Na(+). Ca(2+) block of SkEIIIK-mediated Na(+) current was revealed by the substantial enhancement of Na(+) current amplitude after reduction of Ca(2+) in the external recording solution from 10 mM to near physiological 1 mM. Inward SkEIIIK-mediated currents were potentiated by either ±Bay K 8644 (10 mM) or 200-ms depolarizing prepulses to +90 mV. In contrast, outward monovalent currents were reduced by ±Bay K 8644 and were unaffected by strong depolarization, indicating a preferential potentiation of inward Na(+) currents through the mutant Ca(v)1.1 channel. Taken together, our results show that SkEIIIK functions as a non-inactivating, junctionally-targeted Na(+) channel when Na(+) is the sole monvalent cation present and urge caution when interpreting the impact of mutations designed to ablate Ca(2+) permeability mediated by Ca(v) channels on physiological processes that extend beyond channel gating and permeability.

Phospholemman modulates the gating of cardiac L-type calcium channels

Ca(2+) entry through L-type calcium channels (Ca(V)1.2) is critical in shaping the cardiac action potential and initiating cardiac contraction. Modulation of Ca(V)1.2 channel gating directly affects myocyte excitability and cardiac function. We have found that phospholemman (PLM), a member of the FXYD family and regulator of cardiac ion transport, coimmunoprecipitates with Ca(V)1.2 channels from guinea pig myocytes, which suggests PLM is an endogenous modulator. Cotransfection of PLM in HEK293 cells slowed Ca(V)1.2 current activation at voltages near the threshold for activation, slowed deactivation after long and strong depolarizing steps, enhanced the rate and magnitude of voltage-dependent inactivation (VDI), and slowed recovery from inactivation. However, Ca(2+)-dependent inactivation was not affected. Consistent with slower channel closing, PLM significantly increased Ca(2+) influx via Ca(V)1.2 channels during the repolarization phase of a human cardiac action potential waveform. Our results support PLM as an endogenous regulator of Ca(V)1.2 channel gating. The enhanced VDI induced by PLM may help protect the heart under conditions such as ischemia or tachycardia where the channels are depolarized for prolonged periods of time and could induce Ca(2+) overload. The time and voltage-dependent slowed deactivation could represent a gating shift that helps maintain Ca(2+) influx during the cardiac action potential waveform plateau phase.

The cardiac alpha(1C) subunit can support excitation-triggered Ca2+ entry in dysgenic and dyspedic myotubes

Depolarization-induced entry of divalent ions into skeletal muscle has been attributed to a process termed Excitation-Coupled Ca(2+) Entry (ECCE), which is hypothesized to require the interaction of the ryanodine receptor (RyR1), the L-type Ca(2+) channel (DHPR) and another unidentified cation channel. Thus, ECCE is absent in myotubes lacking either the DHPR (dysgenic) or RyR1 (dyspedic). Furthermore, ECCE, as measured by Mn(2+) quench of Fura-2, is reconstituted by expression of a mutant DHPR alpha(1S) subunit (SkEIIIK) thought to be impermeable to divalent cations. Previously, we showed that the bulk of depolarization-induced Ca(2+) entry could be explained by the skeletal L-type current. Accordingly, one would predict that any Ca(2+) current similar to the endogenous current would restore such entry and that this entry would not require coupling to either the DHPR or RyR1. Here, we show that expression of the cardiac alpha(1C) subunit in either dysgenic or dyspedic myotubes does result in Ca(2+) entry similar to that ascribed to ECCE. We also demonstrate that, when potentiated by strong depolarization and Bay K 8644, SkEIIIK supports entry of Mn(2+). These results strongly support the idea that the L-type channel is the major route of Ca(2+) entry in response to repetitive or prolonged depolarization of skeletal muscle.

The skeletal L-type Ca(2+) current is a major contributor to excitation-coupled Ca(2+) entry

The term excitation-coupled Ca(2+) entry (ECCE) designates the entry of extracellular Ca(2+) into skeletal muscle cells, which occurs in response to prolonged depolarization or pulse trains and depends on the presence of both the 1,4-dihydropyridine receptor (DHPR) in the plasma membrane and the type 1 ryanodine receptor in the sarcoplasmic reticulum (SR) membrane. The ECCE pathway is blocked by pharmacological agents that also block store-operated Ca(2+) entry, is inhibited by dantrolene, is relatively insensitive to the DHP antagonist nifedipine (1 microM), and is permeable to Mn(2+). Here, we have examined the effects of these agents on the L-type Ca(2+) current conducted via the DHPR. We found that the nonspecific cation channel antagonists (2-APB, SKF 96356, La(3+), and Gd(3+)) and dantrolene all inhibited the L-type Ca(2+) current. In addition, complete (>97%) block of the L-type current required concentrations of nifedipine >10 microM. Like ECCE, the L-type Ca(2+) channel displays permeability to Mn(2+) in the absence of external Ca(2+) and produces a Ca(2+) current that persists during prolonged ( approximately 10-second) depolarization. This current appears to contribute to the Ca(2+) transient observed during prolonged KCl depolarization of intact myotubes because (1) the transients in normal myotubes decayed more rapidly in the absence of external Ca(2+); (2) the transients in dysgenic myotubes expressing SkEIIIK (a DHPR alpha(1S) pore mutant thought to conduct only monovalent cations) had a time course like that of normal myotubes in Ca(2+)-free solution and were unaffected by Ca(2+) removal; and (3) after block of SR Ca(2+) release by 200 microM ryanodine, normal myotubes still displayed a large Ca(2+) transient, whereas no transient was detectable in SkEIIIK-expressing dysgenic myotubes. Collectively, these results indicate that the skeletal muscle L-type channel is a major contributor to the Ca(2+) entry attributed to ECCE.

The alpha(1S) III-IV loop influences 1,4-dihydropyridine receptor gating but is not directly involved in excitation-contraction coupling interactions with the type 1 ryanodine receptor

In skeletal muscle, coupling between the 1,4-dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) underlies excitation-contraction (EC) coupling. The III-IV loop of the DHPR alpha(1S) subunit binds to a segment of RyR1 in vitro, and mutations in the III-IV loop alter the voltage dependence of EC coupling, raising the possibility that this loop is directly involved in signal transmission from the DHPR to RyR1. To clarify the role of the alpha(1S) III-IV loop in EC coupling, we examined the functional properties of a chimera (GFP-alpha(1S)[III-IVa]) in which the III-IV loop of the divergent alpha(1A) isoform replaced that of alpha(1S). Dysgenic myotubes expressing GFP-alpha(1S)[III-IVa] yielded myoplasmic Ca(2+) transients that activated at approximately 10 mV more hyperpolarized potentials and that were approximately 65% smaller than those of GFP-alpha(1S). A similar reduction was observed in voltage-dependent charge movements for GFP-alpha(1S)[III-IVa], indicating that the chimeric channels trafficked less well to the membrane but that those that were in the membrane functioned as efficiently in EC coupling as GFP-alpha(1S). Relative to GFP-alpha(1S), L-type currents mediated by GFP-alpha(1S)[III-IVa] were approximately 40% smaller and activated at approximately 5 mV more hyperpolarized potentials. The altered gating of GFP-alpha(1S)[III-IVa] was accentuated by exposure to +/-Bay K 8644, which caused a much larger hyperpolarizing shift in activation compared with its effect on GFP-alpha(1S). Taken together, our observations indicate that the alpha(1S) III-IV loop is not directly involved in EC coupling but does influence DHPR gating transitions important both for EC coupling and activation of L-type conductance.

Permeable ions differentially affect gating kinetics and unitary conductance of L-type calcium channels

Although ion permeation and gating of L-type Ca(2+) channels are generally considered separate processes controlled by distinct components of the channel protein, ion selectivity can vary with the kinetic state. To test this possibility, we studied single-channel currents (cell-attached) of recombinant L-type channels (Ca(V)1.2, beta(2a), and alpha(2)delta) transiently expressed in tsA201 cells in the presence of the channel agonist BayK 8644 which promotes long channel openings (Mode 2 openings). We found that both the brief (Mode 1) and long (Mode 2) mean open times in the presence of Ca(2+) were relatively longer than those with Ba(2+). The unitary slope conductance with Ba(2+) was significantly larger (p<0.05) in Mode 2 openings than for brief Mode 1 openings, whereas the conductance with Ca(2+) did not vary with mode gating. Consequently, the gamma(Ba):gamma(Ca) ratio was greater for Mode 2 than Mode 1 openings. Our findings indicate that both ion permeation and gating kinetics of the L-type channel are differentially modulated by permeable ions. Ca(2+) binding to the L-type channel may stabilize the alteration of channel ion permeability mediated by gating kinetics, and thus, play a role in preventing excessive ion entry when the activation gating of the channel is promoted to the prolonged open state.

High-voltage-activated calcium channels in Muller cells acutely isolated from tiger salamander retina

Muller cells mediate retinal function by stabilizing the ionic environment and signal glial network activity via calcium waves. Using whole-cell patch clamp recording, we describe a high-voltage-activated, slowly inactivating Ca channel current in isolated salamander Muller cells that has unusual pharmacological properties. The Ca channel current has an activation midpoint of approximately -8 mV and an inactivation midpoint of approximately -26 mV in 10 mM Ba2+. The time constant for inactivation is approximately 380 ms at potentials positive to zero. The current is blocked by Cd2+ with an EC50 of <100 nM. nisoldipine (10 microM) blocks approximately 50%, while nifedipine (1 microM), diltiazem (20 microM), and verapamil (50 microM) each block one-third of the current. In contrast to its typical actions, BayK 8644 blocks the current by approximately 25%. Blockers of other Ca channel subtypes were also tested: omega-agatoxin IVA (200 nM) blocked only 13% of the Ca channel current, while omega-conotoxin GVIA (1 microM) blocked 84% of the current. Immnohistochemistry supported the presence of alpha1A, alpha1B, alpha1C, and alpha1D Ca channel subunits. Mapping of dihydropyridine-binding sites with DM-BODIPY revealed a distribution of channels over the entire membrane of the Muller cell with a higher density at the apical region. Overall, these observations suggest either the presence of a mix of L- and N-type Ca channels or a single, unconventional HVA Ca channel subtype sharing L- and N-type Ca channel characteristics.

FPL 64176 modification of Ca(V)1.2 L-type calcium channels: dissociation of effects on ionic current and gating current

FPL 64176 (FPL) is a nondihydropyridine compound that dramatically increases macroscopic inward current through L-type calcium channels and slows activation and deactivation. To understand the mechanism by which channel behavior is altered, we compared the effects of the drug on the kinetics and voltage dependence of ionic currents and gating currents. Currents from a homogeneous population of channels were obtained using cloned rabbit Ca(V)1.2 (alpha1C, cardiac L-type) channels stably expressed in baby hamster kidney cells together with beta1a and alpha2delta1 subunits. We found a striking dissociation between effects of FPL on ionic currents, which were modified strongly, and on gating currents, which were not detectably altered. Inward ionic currents were enhanced approximately 5-fold for a voltage step from -90 mV to +10 mV. Kinetics of activation and deactivation were slowed dramatically at most voltages. Curiously, however, at very hyperpolarized voltages (< -250 mV), deactivation was actually faster in FPL than in control. Gating currents were measured using a variety of inorganic ions to block ionic current and also without blockers, by recording gating current at the reversal potential for ionic current (+50 mV). Despite the slowed kinetics of ionic currents, FPL had no discernible effect on the fundamental movements of gating charge that drive channel gating. Instead, FPL somehow affects the coupling of charge movement to opening and closing of the pore. An intriguing possibility is that the drug causes an inactivated state to become conducting without otherwise affecting gating transitions.

Modulation of single channels underlying hippocampal L-type current enhancement by agonists depends on the permeant ion

The influx of calcium (Ca(2+)) ions through L-type channels underlies many cellular processes, ranging from initiation of gene transcription to activation of Ca(2+)-activated potassium channels. L-type channels possess a diagnostic pharmacology, being enhanced by the dihydropyridine BAY K 8644 and benzoylpyrrole FPL 64176. It is assumed that the action of these compounds is independent of the ion conducted through the channel. In contrast to this assumption, modulation of L-type channel activity in acutely dissociated rat CA1 hippocampal neurons depended on the divalent ion identity. BAY K 8644 and FPL 64176 substantially increased single-channel open time only when barium (Ba(2+)) was the permeant ion. BAY K 8644 increased single-channel conductance when either Ba(2+) or Ca(2+) ions were the charge carrier, an effect not observed with FPL 64176. BAY K 8644 enhanced the whole cell L-type channel Ca(2+)- or Ba(2+)-carried current without a change in deactivation tail kinetics. In contrast, enhancement by FPL 64176 was associated with a dramatic slowing of deactivation kinetics only when Ba(2+) and not Ca(2+) was the charge carrier. Current activation was slowed by FPL 64176 with either charge carrier, an effect arising from a clustering of agonist-modified long-duration openings toward the end of the voltage step. These data indicate that agonists enhanced L-type current by distinct mechanisms dependent on the permeant ion, indicating that care must be considered when used as diagnostic tools.

Potentiated L-type Ca2+ channels rectify

Strong depolarization and dihydropyridine agonists potentiate inward currents through native L-type Ca2+ channels, but the effect on outward currents is less clear due to the small size of these currents. Here, we examined potentiation of wild-type alpha1C and two constructs bearing mutations in conserved glutamates in the pore regions of repeats II and IV (E2A/E4A-alpha1C) or repeat III (E3K-alpha1C). With 10 mM Ca2+ in the bath and 110 mM Cs+ in the pipette, these mutated channels, expressed in dysgenic myotubes, produced both inward and outward currents of substantial amplitude. For both the wild-type and mutated channels, we observed strong inward rectification of potentiation: strong depolarization had little effect on outward tail currents but caused the inward tail currents to be larger and to decay more slowly. Similarly, exposure to DHP agonist increased the amplitude of inward currents and decreased the amplitude of outward currents through both E2A/E4A-alpha1C and E3K-alpha1C. As in the absence of drug, strong depolarization in the presence of dihydropyridine agonist had little effect on outward tail currents but increased the amplitude and slowed the decay of inward tail currents. We tested whether cytoplasmic Mg2+ functions as the blocking particle responsible for the rectification of potentiated L-type Ca2+ channels. However, even after complete removal of cytoplasmic Mg2+, (-)BayK 8644 still potentiated inward current and partially blocked outward current via E2A/E4A-alpha1C. Although zero Mg2+ did not reveal potentiation of outward current by DHP agonist, it did have two striking effects, (a) a strong suppression of decay of both inward and outward currents via E2A/E4A-alpha1C and (b) a nearly complete elimination of depolarization-induced potentiation of inward tail currents. These results can be explained by postulating that potentiation exposes a binding site in the pore to which an intracellular blocking particle can bind and produce inward rectification of the potentiated channels.