Slow Gating Research Articles

An Improved iMemComp OR Gate and its Applications in Logic Circuits

The iMemComp (Intelligent memristive computing) gates are a family of logic gates based on the RRAM (Resistive Random Access Memory) devices. It has potential advantage for the design of high-performance logic circuits, because the iMemComp NAND, AND, NOT, and transmission gates only consume single cycle, respectively. However, the original two-input iMemComp OR gate, which requires three cycles, is a relatively slow gate. It decreases the performance of some logic circuits. This work proposes an improved iMemComp OR gate with only one cycle and three RRAM cells. Both the circuit performance and area consumption of the full-adder and LFSR circuits are improved by the application of the proposed OR gate. Furthermore, we propose a general synthesis method of logic circuits based on the improved logic gate. The synthesis results show that the circuits generated from the proposed synthesis method outperform the previous RRAM based counterparts for most cases of the MCNC benchmark circuits.

Effect of Overburden Height on Hydraulic Fracturing of Concrete-Lined Pressure Tunnels Excavated in Intact Rock: A Numerical Study

This study investigated the impact of overburden height on the hydraulic fracturing of a concrete-lined pressure tunnel, excavated in intact rock, under steady-state and transient-state conditions. Moreover, the Norwegian design criterion that only suggests increasing the overburden height as a countermeasure against hydraulic fracturing was evaluated. The Mohr–Coulomb failure criterion was implemented to investigate failure in the rock elements adjacent to the lining. A pressure tunnel with an inner diameter of 3.6 m was modeled in Abaqus Finite Element Analysis (FEA), using the finite element method (FEM). It was assumed that transient pressures occur inside the tunnel due to control gate closure in a hydroelectric power plant, downstream of the tunnel, in three different closure modes: fast (14 s), normal (18 s), and slow (26 s). For steady-state conditions, the results indicated that resistance to the fracturing of the rock increased with increasing the rock friction angle, as well as the overburden height. However, the influence of the friction angle on the resistance to rock fracture was much larger than that of the overburden height. For transient-state conditions, the results showed that, in fast, normal, and slow control gate closure modes, the required overburden heights to failure were respectively 1.07, 0.8, and 0.67 times the static head of water in the tunnel under a steady-state condition. It was concluded that increasing the height of overburden should not be the absolute solution to prevent hydraulic fracturing in pressure tunnels.

Fidelity benchmarks for two-qubit gates in silicon.

Universal quantum computation will require qubit technology based on a scalable platform1, together with quantum error correction protocols that place strict limits on the maximum infidelities for one- and two-qubit gate operations2,3. Although various qubit systems have shown high fidelities at the one-qubit level4-10, the only solid-state qubits manufactured using standard lithographic techniques that have demonstrated two-qubit fidelities near the fault-tolerance threshold6 have been in superconductor systems. Silicon-based quantum dot qubits are also amenable to large-scale fabrication and can achieve high single-qubit gate fidelities (exceeding 99.9 per cent) using isotopically enriched silicon11,12. Two-qubit gates have now been demonstrated in a number of systems13-15, but as yet an accurate assessment of their fidelities using Clifford-based randomized benchmarking, which uses sequences of randomly chosen gates to measure the error, has not been achieved. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80 to 89 per cent, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7 per cent and anaverage controlled-rotation fidelity of 98 per cent. These fidelities are found to be limited by the relatively long gate times used here compared with the decoherence times of the qubits. Silicon qubit designs employing fast gate operations with high Rabi frequencies16,17, together with advanced pulsing techniques18, should therefore enable much higher fidelities in the near future.

Structure of the human ClC-1 chloride channel.

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue (“fast gate”) known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule (“slow gating”). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1–related diseases.

Using Scam to Investigate Reconfiguration of Molecular Determinants in D1-S6 during Slow Inactivation of hNav1.4

The goals of this research are to determine if 1) a single point mutation affects gating kinetics and 2) whether MTS accessibility changes via inactivation. This will be accomplished using the substituted cysteine accessibility method (SCAM) by substituting cysteine for each of the target amino acids and by applying methanethiosulfonate ethylammonium (MTSEA), which selectively binds to cysteine, indicating when the sulfhydryl side chain is accessible to binding. The target mutants consist of positions from the gating hinge to near the bundle crossing in domain 1 - segment 6 (D1-S6). At this time, data for all mutants have been recorded except for Y437C & L438C. Patch clamp techniques were used to record whole-cell currents from transiently transfected HEK cells to determine 1) the potential effects of the cysteine substitution on activation, fast inactivation, and slow inactivation, and 2) the potential accessibility of cysteine in the closed, fast inactivated, and slow inactivated states. Several mutations affected one or more baseline kinetics of the sodium channel, including significant differences in the voltage needed for either fast inactivation and/or slow inactivation in N440C, L441C, G434C, and I439C. However, MTS exposure in mutants L433C, G434C, S435C, F436C, I439C, N440C, L441C, I442C, and L443C were found to have had little-to-no effect on when the mutants were in either the closed state (−160 mV), the fast-inactivated state (brief depolarization at 0 mV), or the slow-inactivated state (prolonged depolarization at 0 mV). Thus, our results suggest that changes in accessibility of the substituted cysteines in D1-S6 of hNav1.4 may not occur, and therefore that D1-S6 may not undergo prominent reconfiguration during slow inactivation gating.

Comparison of ion transport determinants between a TMEM16 chloride channel and phospholipid scramblase.

Two TMEM16 family members, TMEM16A and TMEM16F, have different ion transport properties. Upon activation by intracellular Ca2+, TMEM16A-a Ca2+-activated Cl- channel-is more selective for anions than cations, whereas TMEM16F-a phospholipid scramblase-appears to transport both cations and anions. Under saturating Ca2+ conditions, the current-voltage (I-V) relationships of these two proteins also differ; the I-V curve of TMEM16A is linear, while that of TMEM16F is outwardly rectifying. We previously found that mutating a positively charged lysine residue (K584) in the ion transport pathway to glutamine converted the linear I-V curve of TMEM16A to an outwardly rectifying curve. Interestingly, the corresponding residue in the outwardly rectifying TMEM16F is also a glutamine (Q559). Here, we examine the ion transport functions of TMEM16 molecules and compare the roles of K584 of TMEM16A and Q559 of TMEM16F in controlling the rectification of their respective I-V curves. We find that rectification of TMEM16A is regulated electrostatically by the side-chain charge on the residue at position 584, whereas the charge on residue 559 in TMEM16F has little effect. Unexpectedly, mutation of Q559 to aromatic amino acid residues significantly alters outward rectification in TMEM16F. These same mutants show reduced Ca2+-induced current rundown (or desensitization) compared with wild-type TMEM16F. A mutant that removes the rundown of TMEM16F could facilitate the study of ion transport mechanisms in this phospholipid scramblase in the same way that a CLC-0 mutant in which inactivation (or closure of the slow gate) is suppressed was used in our previous studies.

Early afterdepolarizations in cardiac action potentials as mixed mode oscillations due to a folded node singularity.

Early afterdepolarizations (EADs) are pathological voltage oscillations during the repolarization phase of cardiac action potentials. They are considered as potential precursors to cardiac arrhythmias and have recently gained much attention in the context of preclinical drug safety testing under the Comprehensive in vitro Proarrhythmia Assay (CiPA) paradigm. From the viewpoint of multiple time scales theory, the onset of EADs has previously been studied by means of mathematical action potential models with one slow ion channel gating variable. In this article, we for the first time associate EADs with mixed mode oscillations in dynamical systems with two slow gating variables and present a folded node singularity of the slow flow as a novel mechanism for EADs genesis. We derive regions of the pharmacology parameter space in which EADs occur using both the folded node analysis and a full system bifurcation analysis, and we suggest the normal distance to the boundary of the EADs region as a mechanism-based risk metric to computationally estimate a drug’s proarrhythmic liability.

Extracellular protons accelerate hERG channel deactivation by destabilizing voltage sensor relaxation.

hERG channels underlie the delayed-rectifier K+ channel current (IKr), which is crucial for membrane repolarization and therefore termination of the cardiac action potential. hERG channels display unusually slow deactivation gating, which contributes to a resurgent current upon repolarization and may protect against post-depolarization-induced arrhythmias. hERG channels also exhibit robust mode shift behavior, which reflects the energetic separation of activation and deactivation pathways due to voltage sensor relaxation into a stable activated state. The mechanism of relaxation is unknown and likely contributes to slow hERG channel deactivation. Here, we use extracellular acidification to probe the structural determinants of voltage sensor relaxation and its influence on the deactivation gating pathway. Using gating current recordings and voltage clamp fluorimetry measurements of voltage sensor domain dynamics, we show that voltage sensor relaxation is destabilized at pH 6.5, causing an ∼20-mV shift in the voltage dependence of deactivation. We show that the pH dependence of the resultant loss of mode shift behavior is similar to that of the deactivation kinetics acceleration, suggesting that voltage sensor relaxation correlates with slower pore gate closure. Neutralization of D509 in S3 also destabilizes the relaxed state of the voltage sensor, mimicking the effect of protons, suggesting that acidic residues on S3, which act as countercharges to S4 basic residues, are involved in stabilizing the relaxed state and slowing deactivation kinetics. Our findings identify the mechanistic determinants of voltage sensor relaxation and define the long-sought mechanism by which protons accelerate hERG deactivation.

The hERG potassium channel intrinsic ligand regulates N- and C-terminal interactions and channel closure.

Human ether-à-go-go-related gene (hERG, KCNH2) voltage-activated potassium channels are critical for cardiac excitability. hERG channels have characteristic slow closing (deactivation), which is auto-regulated by a direct interaction between the N-terminal Per-Arnt-Sim (PAS) domain and the C-terminal cyclic nucleotide binding homology domain (CNBHD). hERG channels are not activated by the binding of extrinsic cyclic nucleotide ligands, but rather bind an "intrinsic ligand" that is composed of residues 860-862 within the CNBHD and mimics a cyclic nucleotide. The intrinsic ligand is located at the PAS-CNBHD interface, but its mechanism of action in hERG is not well understood. Here we use whole-cell patch-clamp electrophysiology and FRET spectroscopy to examine how the intrinsic ligand regulates gating. To carry out this work, we coexpress PAS (a PAS domain fused to cyan fluorescent protein) in trans with hERG "core" channels (channels with a deletion of the PAS domain fused to citrine fluorescent protein). The PAS domain in trans with hERG core channels has slow (regulated) deactivation, like that of WT hERG channels, as well as robust FRET, which indicates there is a direct functional and structural interaction of the PAS domain with the channel core. In contrast, PAS in trans with hERG F860A core channels has intermediate deactivation and intermediate FRET, indicating perturbation of the PAS domain interaction with the CNBHD. Furthermore, PAS in trans with hERG L862A core channels, or PAS in trans with hERG F860G,L862G core channels, has fast (nonregulated) deactivation and no measurable FRET, indicating abolition of the PAS and CNBHD interaction. These results indicate that the intrinsic ligand is necessary for the functional and structural interaction between the PAS domain and the CNBHD, which regulates the characteristic slow deactivation gating in hERG channels.

Finite-frequency noise of interacting single-electron emitters: Spectroscopy with higher noise harmonics

We derive the symmetrized current-noise spectrum of a quantum dot, which is weakly tunnel-coupled to an electron reservoir and driven by a slow time-dependent gate voltage. This setup can be operated as an on-demand emitter of single electrons into a mesoscopic conductor. By extending a real-time diagrammatic technique which is perturbative in the tunnel coupling, we obtain the time-resolved finite-frequency noise as well as its decomposition into noise harmonics in the presence of both strong Coulomb interaction and slow time-dependent driving. We investigate the noise over a large range of frequencies and point out where the interplay of Coulomb interaction and driving leads to unique signatures in finite-frequency noise spectra, in particular in the first harmonic. Besides that, we employ the first noise harmonic as a spectroscopic tool to access individual fluctuation processes. We discuss how the inverse noise frequency sets a time scale for fluctuations, which competes with time scales of the quantum-dot relaxation dynamics as well as the driving.

Control of Concentration of Nonhydrogen-Bonded Hydroxyl Groups in Polymer Dielectrics for Organic Field-Effect Transistors with Operational Stability.

Poly(4-vinylphenol) (PVP) is a promising gate dielectric material for organic field-effect transistors (OFETs) and circuits fabricated on plastic substrates. Thermal cross-linking of PVP with a cross-linker, such as poly(melamine- co-formaldehyde) methylated(PMF), at a high temperature (above 170 °C) is widely considered an effective method to remove residual hydroxyl groups that induce polarization effects in the dielectric bulk. However, the threshold voltage shift in transfer characteristics is still observed for an OFET with a PVP-PMF dielectric when it is operated at a slow gate voltage sweep rate. The present study examines the cause of the undesired hysteresis phenomenon and suggests a route to enable a reliable operation. We systematically investigate the effect of the PVP-PMF weight ratio and their annealing temperature on the transfer characteristics of OFETs. We discover that the size of the hysteresis is closely related to the concentration of nonhydrogen-bonded hydroxyl groups in the dielectric bulk and this is controlled by the weight ratio. At a ratio of 0.5:1, a complete elimination of hysteresis was observed irrespective of the annealing temperature. We finally demonstrate a highly reliable operation of small-molecule-based OFETs fabricated on a plastic substrate at a low temperature.

Fully Automatic Multiresolution Idealization for Filtered Ion Channel Recordings: Flickering Event Detection.

We propose a new model-free segmentation method, JULES, which combines recent statistical multiresolution techniques with local deconvolution for idealization of ion channel recordings. The multiresolution criterion takes into account scales down to the sampling rate enabling the detection of flickering events, i.e., events on small temporal scales, even below the filter frequency. For such small scales the deconvolution step allows for a precise determination of dwell times and, in particular, of amplitude levels, a task which is not possible with common thresholding methods. This is confirmed theoretically and in a comprehensive simulation study. In addition, JULES can be applied as a preprocessing method for a refined hidden Markov analysis. Our new methodology allows us to show that gramicidin A flickering events have the same amplitude as the slow gating events. JULES is available as an R function jules in the package clampSeg.

Activation of renal ClC-K chloride channels depends on an intact N terminus of their accessory subunit barttin

ClC-K channels belong to the CLC family of chloride channels and chloride/proton antiporters. They contribute to sodium chloride reabsorption in Henle's loop of the kidney and to potassium secretion into the endolymph by the stria vascularis of the inner ear. Their accessory subunit barttin stabilizes the ClC-K/barttin complex, promotes its insertion into the surface membrane, and turns the pore-forming subunits into a conductive state. Barttin mutations cause Bartter syndrome type IV, a salt-wasting nephropathy with sensorineural deafness. Here, studying ClC-K/barttin channels heterologously expressed in MDCK-II and HEK293T cells with confocal imaging and patch-clamp recordings, we demonstrate that the eight-amino-acids-long barttin N terminus is required for channel trafficking and activation. Deletion of the complete N terminus (Δ2-8 barttin) retained barttin and human hClC-Ka channels in intracellular compartments. Partial N-terminal deletions did not compromise subcellular hClC-Ka trafficking but drastically reduced current amplitudes. Sequence deletions encompassing Thr-6, Phe-7, or Arg-8 in barttin completely failed to activate hClC-Ka. Analyses of protein expression and whole-cell current noise revealed that inactive channels reside in the plasma membrane. Substituting the deleted N terminus with a polyalanine sequence was insufficient for recovering chloride currents, and single amino acid substitutions highlighted that the correct sequence is required for proper function. Fast and slow gate activation curves obtained from rat V166E rClC-K1/barttin channels indicated that mutant barttin fails to constitutively open the slow gate. Increasing expression of barttin over that of ClC-K partially recovered this insufficiency, indicating that N-terminal modifications of barttin alter both binding affinities and gating properties.

Deafness-associated mutation opens the gate to understanding.

New JGP study uncovers a connection between Connexin 26 fast and slow gates.

The syndromic deafness mutation G12R impairs fast and slow gating in Cx26 hemichannels.

Mutations in connexin 26 (Cx26) hemichannels can lead to syndromic deafness that affects the cochlea and skin. These mutations lead to gain-of-function hemichannel phenotypes by unknown molecular mechanisms. In this study, we investigate the biophysical properties of the syndromic mutant Cx26G12R (G12R). Unlike wild-type Cx26, G12R macroscopic hemichannel currents do not saturate upon depolarization, and deactivation is faster during hyperpolarization, suggesting that these channels have impaired fast and slow gating. Single G12R hemichannels show a large increase in open probability, and transitions to the subconductance state are rare and short-lived, demonstrating an inoperative fast gating mechanism. Molecular dynamics simulations indicate that G12R causes a displacement of the N terminus toward the cytoplasm, favoring an interaction between R12 in the N terminus and R99 in the intracellular loop. Disruption of this interaction recovers the fast and slow voltage-dependent gating mechanisms. These results suggest that the mechanisms of fast and slow gating in connexin hemichannels are coupled and provide a molecular mechanism for the gain-of-function phenotype displayed by the syndromic G12R mutation.

Using voltage-sensor toxins and their molecular targets to investigate NaV 1.8 gating.

Voltage-gated sodium (NaV ) channel gating is a complex phenomenon which involves a distinct contribution of four integral voltage-sensing domains (VSDI, VSDII, VSDIII and VSDIV). Utilizing accrued pharmacological and structural insights, we build on an established chimera approach to introduce animal toxin sensitivity in each VSD of an acceptor channel by transferring in portable S3b-S4 motifs from the four VSDs of a toxin-susceptible donor channel (NaV 1.2). By doing so, we observe that in NaV 1.8, a relatively unexplored channel subtype with distinctly slow gating kinetics, VSDI-III participate in channel opening whereas VSDIV can regulate opening as well as fast inactivation. These results illustrate the effectiveness of a pharmacological approach to investigate the mechanism underlying gating of a mammalian NaV channel complex.

Inactivation gating of Kv7.1 channels does not involve concerted cooperative subunit interactions

ABSTRACTInactivation is an intrinsic property of numerous voltage-gated K+ (Kv) channels and can occur by N-type or/and C-type mechanisms. N-type inactivation is a fast, voltage independent process, coupled to activation, with each inactivation particle of a tetrameric channel acting independently. In N-type inactivation, a single inactivation particle is necessary and sufficient to occlude the pore. C-type inactivation is a slower process, involving the outermost region of the pore and is mediated by a concerted, highly cooperative interaction between all four subunits. Inactivation of Kv7.1 channels does not exhibit the hallmarks of N- and C-type inactivation. Inactivation of WT Kv7.1 channels can be revealed by hooked tail currents that reflects the recovery from a fast and voltage-independent inactivation process. However, several Kv7.1 mutants such as the pore mutant L273F generate an additional voltage-dependent slow inactivation. The subunit interactions during this slow inactivation gating remain unexplored. The goal of the present study was to study the nature of subunit interactions along Kv7.1 inactivation gating, using concatenated tetrameric Kv7.1 channel and introducing sequentially into each of the four subunits the slow inactivating pore mutation L273F. Incorporating an incremental number of inactivating mutant subunits did not affect the inactivation kinetics but slowed down the recovery kinetics from inactivation. Results indicate that Kv7.1 inactivation gating is not compatible with a concerted cooperative process. Instead, adding an inactivating subunit L273F into the Kv7.1 tetramer incrementally stabilizes the inactivated state, which suggests that like for activation gating, Kv7.1 slow inactivation gating is not a concerted process.

Robust operating point for capacitively coupled singlet-triplet qubits

Singlet-triplet qubits in lateral quantum dots in semiconductor heterostructures exhibit high-fidelity single-qubit gates via exchange interactions and magnetic field gradients. High-fidelity two-qubit entangling gates are challenging to generate since weak interqubit interactions result in slow gates that accumulate error in the presence of noise. However, the interqubit electrostatic interaction also produces a shift in the local double well detunings, effectively changing the dependence of exchange on the gate voltages. We consider an operating point where the effective exchange is first order insensitive to charge fluctuations while maintaining nonzero interactions. This "sweet spot" exists only in the presence of interactions. We show that working at the interacting sweet spot can directly produce maximally entangling gates and we simulate the gate evolution under realistic 1/f noise. We report theoretical two-qubit gate fidelities above 99% in GaAs and Si systems.

Downregulation of KCNMB4 expression and changes in BK channel subtype in hippocampal granule neurons following seizure activity.

A major challenge is to understand maladaptive changes in ion channels that sets neurons on a course towards epilepsy development. Voltage- and calcium-activated K+ (BK) channels contribute to early spike timing in neurons, and studies indicate that the BK channel plays a pathological role in increasing excitability early after a seizure. Here, we have investigated changes in BK channels and their accessory β4 subunit (KCNMB4) in dentate gyrus (DG) granule neurons of the hippocampus, key neurons that regulate excitability of the hippocampus circuit. Two days after pilocarpine-induced seizures, we found that the predominant effect is a downregulation of the β4 accessory subunit mRNA. Consistent with reduced expression, single channel recording and pharmacology indicate a switch in the subtype of channels expressed; from iberiotoxin-resistant, type II BK channels (BK α/β4) that have higher channel open probability and slow gating, to iberiotoxin-sensitive type I channels (BK α alone) with low open probability and faster gating. The switch to a majority of type I channel expression following seizure activity is correlated with a loss of BK channel function on spike threshold while maintaining the channel’s contribution to increased early spike frequency. Using heterozygous β4 knockout mice, we find reduced expression is sufficient to increase seizure sensitivity. We conclude that seizure-induced downregulation of KCNMB4 is an activity dependent mechanism that increases the excitability of DG neurons. These novel findings indicate that BK channel subtypes are not only defined by cell-specific expression, but can also be plastic depending on the recent history of neuronal excitability.

Functional asymmetry and plasticity of electrical synapses interconnecting neurons through a 36-state model of gap junction channel gating.

We combined the Hodgkin–Huxley equations and a 36-state model of gap junction channel gating to simulate electrical signal transfer through electrical synapses. Differently from most previous studies, our model can account for dynamic modulation of junctional conductance during the spread of electrical signal between coupled neurons. The model of electrical synapse is based on electrical properties of the gap junction channel encompassing two fast and two slow gates triggered by the transjunctional voltage. We quantified the influence of a difference in input resistances of electrically coupled neurons and instantaneous conductance–voltage rectification of gap junctions on an asymmetry of cell-to-cell signaling. We demonstrated that such asymmetry strongly depends on junctional conductance and can lead to the unidirectional transfer of action potentials. The simulation results also revealed that voltage spikes, which develop between neighboring cells during the spread of action potentials, can induce a rapid decay of junctional conductance, thus demonstrating spiking activity-dependent short-term plasticity of electrical synapses. This conclusion was supported by experimental data obtained in HeLa cells transfected with connexin45, which is among connexin isoforms expressed in neurons. Moreover, the model allowed us to replicate the kinetics of junctional conductance under different levels of intracellular concentration of free magnesium ([Mg2+]i), which was experimentally recorded in cells expressing connexin36, a major neuronal connexin. We demonstrated that such [Mg2+]i-dependent long-term plasticity of the electrical synapse can be adequately reproduced through the changes of slow gate parameters of the 36-state model. This suggests that some types of chemical modulation of gap junctions can be executed through the underlying mechanisms of voltage gating. Overall, the developed model accounts for direction-dependent asymmetry, as well as for short- and long-term plasticity of electrical synapses. Our modeling results demonstrate that such complex behavior of the electrical synapse is important in shaping the response of coupled neurons.