Pore structure influences gating properties of the T-type Ca2+ channel alpha1G

A historical biophysical dogma vs. an understanding of the structure and function of voltage-gated tetrameric ion channels. A review

The outstanding work of several eminent biophysicists has allowed the functional features of voltage-gated tetrameric ion channels to be disclosed using ingenious and sophisticated electrophysiological techniques. However, the kinetics and mechanism underlying these functions have been heavily conditioned by an arbitrary interpretation of the groundbreaking results obtained by Hodgkin and Huxley (HH) in their investigation of sodium and potassium currents using the voltage clamp technique. Thus, the heavy parametrization of their results was considered to indicate that any proposed sequence of closed states terminates with a single open state. This 'dogma' of HH parametrization has influenced the formulation of countless mechanistic models, mainly stochastic, requiring a high number of free parameters and of often unspecified conformational states. This note aims to point out the advantages of a deterministic kinetic model that simulates the main features of tetrameric ion channels using only two free parameters by assuming their stepwise opening accompanied by a progressively increasing cation flow. This model exploits the electrostatic attractive interactions stemming from the charge distribution shared by all tetrameric ion channels, providing a close connection between their structure and function. Quite significantly, a stepwise opening of all ligand-gated tetrameric ion channels, such as glutamate receptors (GluRs), with concomitant ion flow, is nowadays generally accepted, not having been influenced by this dogma. This provides a unified picture of both voltage-gated and ligand-gated tetrameric ion channels.

Activation of Drosophila melanogaster TRPA1 Isoforms by Citronellal and Menthol

BACKGROUND

The transient receptor potential ankyrin 1 (TRPA1) cation channels function as broadly-tuned sensors of noxious chemicals in many species. Recent studies identified four functional TRPA1 isoforms in Drosophila melanogaster (dTRPA1(A) to (D)), but their responses to non-electrophilic chemicals are yet to be fully characterized.

METHODS

We determined the behavioral responses of adult flies to the mammalian TRPA1 non-electrophilic activators citronellal and menthol, and characterized the effects of these compounds on all four dTRPA1 channel isoforms using intracellular Ca²⁺ imaging and whole-cell patch-clamp recordings.

RESULTS

Wild type flies avoided citronellal and menthol in an olfactory test and this behavior was reduced in dTrpA1 mutant flies. Both compounds activate all dTRPA1 isoforms in the heterologous expression system HEK293T, with the following sensitivity series: dTRPA1(C) = dTRPA1(D) > dTRPA1(A) ≫ dTRPA1(B) for citronellal and dTRPA1(A) > dTRPA1(D) > dTRPA1(C) > dTRPA1(B) for menthol.

CONCLUSIONS

dTrpA1 was required for the normal avoidance of Drosophila melanogaster towards citronellal and menthol. All dTRPA1 isoforms are activated by both compounds, but the dTRPA1(B) is consistently the least sensitive. We discuss how these findings may guide further studies on the physiological roles and the structural bases of chemical sensitivity of TRPA1 channels.

Divalent cations permeation in a Ca2+ non-conducting skeletal muscle dihydropyridine receptor mouse model

In response to excitation of skeletal muscle fibers, trains of action potentials induce changes in the configuration of the dihydropyridine receptor (DHPR) anchored in the tubular membrane which opens the Ca²⁺ release channel in the sarcoplasmic reticulum membrane. The DHPR also functions as a voltage-gated Ca²⁺ channel that conducts L-type Ca²⁺ currents routinely recorded in mammalian muscle fibers, which role was debated for more than four decades. Recently, to allow a closer look into the role of DHPR Ca²⁺ influx in mammalian muscle, a knock-in (ki) mouse model (ncDHPR) carrying mutation N617D (adjacent to domain II selectivity filter E) in the DHPRα_1S subunit abolishing Ca²⁺ permeation through the channel was generated [Dayal et al., 2017]. In the present study, the Mn²⁺ quenching technique was initially intended to be used on voltage-clamped muscle fibers from this mouse to determine whether Ca²⁺ influx through a pathway distinct from DHPR may occur to compensate for the absence of DHPR Ca²⁺ influx. Surprisingly, while N617D DHPR muscle fibers of the ki mouse do not conduct Ca²⁺, Mn²⁺ entry and subsequent quenching did occur because Mn²⁺ was able to permeate and produce L-type currents through N617D DHPR. N617D DHPR was also found to conduct Ba²⁺ and Ba²⁺ currents were strongly blocked by external Ca²⁺. Ba²⁺ permeation was smaller, current kinetics slower and Ca²⁺ block more potent than in wild-type DHPR. These results indicate that residue N617 when replaced by the negatively charged residue D is suitably located at entrance of the pore to trap external Ca²⁺ impeding in this way permeation. Because Ba²⁺ binds with lower affinity to D, Ba²⁺ currents occur, but with reduced amplitudes as compared to Ba²⁺ currents through wild-type channels. We conclude that mutations located outside the selectivity filter influence channel permeation and possibly channel gating in a fully differentiated skeletal muscle environment.

Deterministic model of Cav3.1 Ca2+ channel and a proposed sequence of its conformations

A family of current-time curves of T-type Ca_v3.1 Ca²⁺ channels available in the literature is simulated by a kinetic model differing from that used for the interpretation of all salient features of Na⁺ and Shaker K⁺ channels by the insertion of a multiplying factor expressing the difference between the working potential ϕ and the reversal potential ϕ_r. This deterministic model is also used to simulate experimental curves taken from the literature for steady-state 'fast inactivation' and for a gradual passage from fast to 'slow inactivation'. A depolarizing pulse induces fast or slow inactivation depending on whether it lasts 100-500 ms or about 1 min, and is believed to cause a collapse of the central pore near the selectivity filter (SF). A number of features of fast and slow inactivation of Ca_v3.1 Ca²⁺ channels are qualitatively interpreted on the basis of a sequence of conformational states. Briefly, the conformation responsible for 'fast inactivation' is assumed to have the activation gate open and the inactivation gate (i.e., the SF) inactive. Immediately after a depolarizing pulse, this conformation is inactive and requires a sufficiently long rest time at a far negative holding potential to recover from inactivation. 'Slow inactivation' is ascribed to a different conformation with the activation gate closed and the SF inactive.

Interaction of MDIMP with the Voltage-Gated Calcium Channels

Amino acid-derived isoindolines are synthetic compounds that were created with the idea of investigating their biological actions. The amino acid moiety was included on the grounds that it may help to avoid toxic effects. Recently, the isoindoline MDIMP was shown to inhibit both cardiac excitation-contraction coupling and voltage-dependent calcium channels. Here, we revealed that MDIMP binds preferentially to low-voltage-activated (LVA) channels. Using a holding potential of -90 mV, the following IC₅₀ values were found (in micromolars): >1000 (Ca_V2.3), 957 (Ca_V1.3), 656 (Ca_V1.2), 219 (Ca_V3.2), and 132 (Ca_V3.1). Moreover, the isoindoline also promoted both accelerated inactivation kinetics of high-voltage-activated Ca²⁺ channels and a modest upregulation of Ca_V1.3 and Ca_V2.3. Additional data indicate that although MDIMP binds to the closed state of the channels, it has more preference for the inactivated one. Concerning Ca_V3.1, the compound did not alter the shape of the instantaneous current-voltage curve, and substituting one or two residues in the selectivity filter drastically increased the IC₅₀ value, suggesting that MDIMP binds to the extracellular side of the pore. However, an outward current failed in removing the inhibition, which implies an alternative mechanism may be involved. The enantiomer (R)-MDIMP [methyl (R)-2-(1,3-dihydroisoindol-2-yl)-4-methylpentanoate], on the other hand, was synthesized and evaluated, but it did not improve the affinity to LVA channels. Implications of these findings are discussed in terms of the possible underlying mechanisms and pharmacological relevance. SIGNIFICANCE STATEMENT: We have studied the regulation of voltage-gated calcium channels by MDIMP, which disrupts excitation-contraction coupling in cardiac myocytes. The latter effect is more potent in atrial than ventricular myocytes, and this could be explained by our results showing that MDIMP preferentially blocks low-voltage-activated channels. Our data also provide mechanistic insights about the blockade and suggest that MDIMP is a promising member of the family of Ca²⁺ channel blockers, with possible application to the inhibition of subthreshold membrane depolarizations.

The Citrus Flavonoid Hesperetin Has an Inadequate Anti-Arrhythmic Profile in the ΔKPQ NaV1.5 Mutant of the Long QT Type 3 Syndrome

Type 3 long QT syndromes (LQT3) are associated with arrhythmogenic gain-of-function mutations in the cardiac voltage-gated Na⁺ channel (hNa_V1.5). The citrus flavanone hesperetin (HSP) was previously suggested as a template molecule to develop new anti-arrhythmic drugs, as it blocks slowly-inactivating currents carried by the LQT3-associated hNa_V1.5 channel mutant R1623Q. Here we investigated whether HSP also has potentially beneficial effects on another LQT3 hNa_V1.5 channel variant, the ΔKPQ, which is associated to lethal ventricular arrhythmias. We used whole-cell patch-clamp to record Na⁺ currents (I_Na) in HEK293T cells transiently expressing hNa_V1.5 wild type or ΔKPQ mutant channels. HSP blocked peak I_Na and the late I_Na carried by ΔKPQ mutant channels with an effective concentration of ≈300 μM. This inhibition was largely voltage-independent and tonic. HSP decreased the rate of inactivation of ΔKPQ channels and, consequently, was relatively weak in reducing the intracellular Na⁺ load in this mutation. We conclude that, although HSP has potential value for the treatment of the R1623Q LQT3 variant, this compound is inadequate to treat the LQT3 associated to the ΔKPQ genetic variant. Our results underscore the precision medicine rationale of better understanding the basic pathophysiological and pharmacological mechanisms to provide phenotype- genotype-directed individualization of treatment.

CaV3.1 channel pore pseudo-symmetry revealed by selectivity filter mutations in its domains I/II

There is growing evidence indicating that the pore structure of voltage-gated ion channels (VGICs) influences gating besides their conductance. Regarding low voltage-activated (LVA) Ca²⁺ channels, it has been demonstrated that substitutions of the pore aspartate (D) by a glutamate (D-to-E substitution) in domains III and IV alter channel gating properties such as a positive shift in the channel activation voltage dependence. In the present report, we evaluated the effects of E-to-D substitution in domains I and II on the Ca_V3.1 channel gating properties. Our results indicate that substitutions in these two domains differentially modify the gating properties of Ca_V3.1 channels. The channel with a single mutation in domain I (DEDD) presented slower activation and faster inactivation kinetics and a slower recovery from inactivation, as compared with the WT channel. In contrast, the single mutant in domain II (EDDD) presented a small but significant negative shift of activation voltage dependence with faster activation and slower inactivation kinetics. Finally, the double mutant channel (DDDD) presented somehow intermediate properties with respect to the two single mutants but with fastest deactivation kinetics. Overall, our results indicate that single amino acid modification of the selectivity filter of LVA Ca²⁺ channels in distinct domains differentially influence their gating properties, supporting a pore pseudo-symmetry.

A compartmentalized mathematical model of mouse atrial myocytes

Various experimental mouse models are extensively used to research human diseases, including atrial fibrillation, the most common cardiac rhythm disorder. Despite this, there are no comprehensive mathematical models that describe the complex behavior of the action potential and [Ca²⁺]_i transients in mouse atrial myocytes. Here, we develop a novel compartmentalized mathematical model of mouse atrial myocytes that combines the action potential, [Ca²⁺]_i dynamics, and β-adrenergic signaling cascade for a subpopulation of right atrial myocytes with developed transverse-axial tubule system. The model consists of three compartments related to β-adrenergic signaling (caveolae, extracaveolae, and cytosol) and employs local control of Ca²⁺ release. It also simulates ionic mechanisms of action potential generation and describes atrial-specific Ca²⁺ handling as well as frequency dependences of the action potential and [Ca²⁺]_i transients. The model showed that the T-type Ca²⁺ current significantly affects the later stage of the action potential, with little effect on [Ca²⁺]_i transients. The block of the small-conductance Ca²⁺-activated K⁺ current leads to a prolongation of the action potential at high intracellular Ca²⁺. Simulation results obtained from the atrial model cells were compared with those from ventricular myocytes. The developed model represents a useful tool to study complex electrical properties in the mouse atria and could be applied to enhance the understanding of atrial physiology and arrhythmogenesis.NEW & NOTEWORTHY A new compartmentalized mathematical model of mouse right atrial myocytes was developed. The model simulated action potential and Ca²⁺ dynamics at baseline and after stimulation of the β-adrenergic signaling system. Simulations showed that the T-type Ca²⁺ current markedly prolonged the later stage of atrial action potential repolarization, with a minor effect on [Ca²⁺]_i transients. The small-conductance Ca²⁺-activated K⁺ current block resulted in prolongation of the action potential only at the relatively high intracellular Ca²⁺.

Different roles for aspartates and glutamates for cation permeation in bacterial sodium channels

A key driving force for ion channel selectivity is represented by the negative charge of the Selectivity Filter carried by aspartate (D) and glutamate (E) residues. However, the structural effects and specific properties of D and E residues have not been extensively studied. In order to investigate this issue we studied the mutants of NaChBac channel with all possible combinations of D and E in the charged rings in position 191 and 192. Electrophysiological measurements showed significant Ca²⁺ currents only when position 191 was occupied by E. Equilibrium Molecular Dynamics simulations revealed the existence of two binding sites, corresponding to the charged rings and another one, more internal, at the level of L190. The simulations showed that the ion in the innermost site can interact with the residue in position 191 only when this is glutamate. Based on the MD simulations, we suggest that a D in position 191 leads to a high affinity Ca²⁺ block site resulting from a significant drop in the free energy of binding for an ion moving between the binding sites; in contrast, the free energy change is more gradual when an E residue occupies position 191, resulting in Ca²⁺ permeability. This scenario is consistent with the model of ion channel selectivity through stepwise changes in binding affinity proposed by Dang and McCleskey. Our study also highlights the importance of the structure of the selectivity filter which should contribute to the development of more detailed physical models for ion channel selectivity.

Selectivity filters and cysteine-rich extracellular loops in voltage-gated sodium, calcium, and NALCN channels

How nature discriminates sodium from calcium ions in eukaryotic channels has been difficult to resolve because they contain four homologous, but markedly different repeat domains. We glean clues from analyzing the changing pore region in sodium, calcium and NALCN channels, from single-cell eukaryotes to mammals. Alternative splicing in invertebrate homologs provides insights into different structural features underlying calcium and sodium selectivity. NALCN generates alternative ion selectivity with splicing that changes the high field strength (HFS) site at the narrowest level of the hourglass shaped pore where the selectivity filter is located. Alternative splicing creates NALCN isoforms, in which the HFS site has a ring of glutamates contributed by all four repeat domains (EEEE), or three glutamates and a lysine residue in the third (EEKE) or second (EKEE) position. Alternative splicing provides sodium and/or calcium selectivity in T-type channels with extracellular loops between S5 and P-helices (S5P) of different lengths that contain three or five cysteines. All eukaryotic channels have a set of eight core cysteines in extracellular regions, but the T-type channels have an infusion of 4–12 extra cysteines in extracellular regions. The pattern of conservation suggests a possible pairing of long loops in Domains I and III, which are bridged with core cysteines in NALCN, Cav, and Nav channels, and pairing of shorter loops in Domains II and IV in T-type channel through disulfide bonds involving T-type specific cysteines. Extracellular turrets of increasing lengths in potassium channels (Kir2.2, hERG, and K2P1) contribute to a changing landscape above the pore selectivity filter that can limit drug access and serve as an ion pre-filter before ions reach the pore selectivity filter below. Pairing of extended loops likely contributes to the large extracellular appendage as seen in single particle electron cryo-microscopy images of the eel Na_v1 channel.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Three-dimensional structure of CaV3.1: comparison with the cardiac L-type voltage-gated calcium channel monomer architecture

Calcium entry through voltage-gated calcium channels has widespread cellular effects upon a host of physiological processes including neuronal excitability, muscle excitation-contraction coupling, and secretion. Using single particle analysis methods, we have determined the first three-dimensional structure, at 23 A resolution, for a member of the low voltage-activated voltage-gated calcium channel family, CaV3.1, a T-type channel. CaV3.1 has dimensions of approximately 115x85x95 A, composed of two distinct segments. The cytoplasmic densities form a vestibule below the transmembrane domain with the C terminus, unambiguously identified by the presence of a His tag being approximately 65 A long and curling around the base of the structure. The cytoplasmic assembly has a large exposed surface area that may serve as a signaling hub with the C terminus acting as a "fishing rod" to bind regulatory proteins. We have also determined a three-dimensional structure, at a resolution of 25 A, for the monomeric form of the cardiac L-type voltage-gated calcium (high voltage-activated) channel with accessory proteins beta and alpha2delta bound to the ion channel polypeptide CaV1.2. Comparison with the skeletal muscle isoform finds a good match particularly with respect to the conformation, size, and shape of the domain identified as that formed by alpha2. Furthermore, modeling of the CaV3.1 structure (analogous to CaV1.2 at these resolutions) into the heteromeric L-type voltage-gated calcium channel complex volume reveals multiple interaction sites for beta-CaV1.2 binding and for the first time identifies the size and organization of the alpha2delta polypeptides.

Permeation and gating in CaV3.1 (alpha1G) T-type calcium channels effects of Ca2+, Ba2+, Mg2+, and Na+

We examined the concentration dependence of currents through Ca(V)3.1 T-type calcium channels, varying Ca(2+) and Ba(2+) over a wide concentration range (100 nM to 110 mM) while recording whole-cell currents over a wide voltage range from channels stably expressed in HEK 293 cells. To isolate effects on permeation, instantaneous current-voltage relationships (IIV) were obtained following strong, brief depolarizations to activate channels with minimal inactivation. Reversal potentials were described by P(Ca)/P(Na) = 87 and P(Ca)/P(Ba) = 2, based on Goldman-Hodgkin-Katz theory. However, analysis of chord conductances found that apparent K(d) values were similar for Ca(2+) and Ba(2+), both for block of currents carried by Na(+) (3 muM for Ca(2+) vs. 4 muM for Ba(2+), at -30 mV; weaker at more positive or negative voltages) and for permeation (3.3 mM for Ca(2+) vs. 2.5 mM for Ba(2+); nearly voltage independent). Block by 3-10 muM Ca(2+) was time dependent, described by bimolecular kinetics with binding at approximately 3 x 10(8) M(-1)s(-1) and voltage-dependent exit. Ca(2+)(o), Ba(2+)(o), and Mg(2+)(o) also affected channel gating, primarily by shifting channel activation, consistent with screening a surface charge of 1 e(-) per 98 A(2) from Gouy-Chapman theory. Additionally, inward currents inactivated approximately 35% faster in Ba(2+)(o) (vs. Ca(2+)(o) or Na(+)(o)). The accelerated inactivation in Ba(2+)(o) correlated with the transition from Na(+) to Ba(2+) permeation, suggesting that Ba(2+)(o) speeds inactivation by occupying the pore. We conclude that the selectivity of the "surface charge" among divalent cations differs between calcium channel families, implying that the surface charge is channel specific. Voltage strongly affects the concentration dependence of block, but not of permeation, for Ca(2+) or Ba(2+).

Differential interactions of Na+ channel toxins with T-type Ca2+ channels

Two types of voltage-dependent Ca²⁺ channels have been identified in heart: high (I_CaL) and low (I_CaT) voltage-activated Ca²⁺ channels. In guinea pig ventricular myocytes, low voltage–activated inward current consists of I_CaT and a tetrodotoxin (TTX)-sensitive I_Ca component (I_Ca(TTX)). In this study, we reexamined the nature of low-threshold I_Ca in dog atrium, as well as whether it is affected by Na⁺ channel toxins. Ca²⁺ currents were recorded using the whole-cell patch clamp technique. In the absence of external Na⁺, a transient inward current activated near −50 mV, peaked at −30 mV, and reversed around +40 mV (HP = −90 mV). It was unaffected by 30 μM TTX or micromolar concentrations of external Na⁺, but was inhibited by 50 μM Ni²⁺ (by ∼90%) or 5 μM mibefradil (by ∼50%), consistent with the reported properties of I_CaT. Addition of 30 μM TTX in the presence of Ni²⁺ increased the current approximately fourfold (41% of control), and shifted the dose–response curve of Ni²⁺ block to the right (IC₅₀ from 7.6 to 30 μM). Saxitoxin (STX) at 1 μM abolished the current left in 50 μM Ni²⁺. In the absence of Ni²⁺, STX potently blocked I_CaT (EC₅₀ = 185 nM) and modestly reduced I_CaL (EC₅₀ = 1.6 μM). While TTX produced no direct effect on I_CaT elicited by expression of hCa_V3.1 and hCa_V3.2 in HEK-293 cells, it significantly attenuated the block of this current by Ni²⁺ (IC₅₀ increased to 550 μM Ni²⁺ for Ca_V3.1 and 15 μM Ni²⁺ for Ca_V3.2); in contrast, 30 μM TTX directly inhibited hCa_V3.3-induced I_CaT and the addition of 750 μM Ni²⁺ to the TTX-containing medium led to greater block of the current that was not significantly different than that produced by Ni²⁺ alone. 1 μM STX directly inhibited Ca_V3.1-, Ca_V3.2-, and Ca_V3.3-mediated I_CaT but did not enhance the ability of Ni²⁺ to block these currents. These findings provide important new implications for our understanding of structure–function relationships of I_CaT in heart, and further extend the hypothesis of a parallel evolution of Na⁺ and Ca²⁺ channels from an ancestor with common structural motifs.

I-II loop structural determinants in the gating and surface expression of low voltage-activated calcium channels

The intracellular loops that interlink the four transmembrane domains of Ca²⁺- and Na⁺-channels (Ca_v, Na_v) have critical roles in numerous forms of channel regulation. In particular, the intracellular loop that joins repeats I and II (I–II loop) in high voltage-activated (HVA) Ca²⁺ channels possesses the binding site for Ca_vβ subunits and plays significant roles in channel function, including trafficking the α₁ subunits of HVA channels to the plasma membrane and channel gating. Although there is considerable divergence in the primary sequence of the I–II loop of Ca_v1/Ca_v2 HVA channels and Ca_v3 LVA/T-type channels, evidence for a regulatory role of the I–II loop in T-channel function has recently emerged for Ca_v3.2 channels. In order to provide a comprehensive view of the role this intracellular region may play in the gating and surface expression in Ca_v3 channels, we have performed a structure-function analysis of the I–II loop in Ca_v3.1 and Ca_v3.3 channels using selective deletion mutants. Here we show the first 60 amino acids of the loop (post IS6) are involved in Ca_v3.1 and Ca_v3.3 channel gating and kinetics, which establishes a conserved property of this locus for all Ca_v3 channels. In contrast to findings in Ca_v3.2, deletion of the central region of the I–II loop in Ca_v3.1 and Ca_v3.3 yielded a modest increase (+30%) and a reduction (−30%) in current density and surface expression, respectively. These experiments enrich our understanding of the structural determinants involved in Ca_v3 function by highlighting the unique role played by the intracellular I–II loop in Ca_v3.2 channel trafficking, and illustrating the prominent role of the gating brake in setting the slow and distinctive slow activation kinetics of Ca_v3.3.

Characterization of the gating brake in the I-II loop of Ca(v)3.2 T-type Ca(2+) channels

Mutations in the I-II loop of Ca(v)3.2 channels were discovered in patients with childhood absence epilepsy. All of these mutations increased the surface expression of the channel, whereas some mutations, and in particular C456S, altered the biophysical properties of channels. Deletions around C456S were found to produce channels that opened at even more negative potentials than control, suggesting the presence of a gating brake that normally prevents channel opening. The goal of the present study was to identify the minimal sequence of this brake and to provide insights into its structure. A peptide fragment of the I-II loop was purified from bacteria, and its structure was analyzed by circular dichroism. These results indicated that the peptide had a high alpha-helical content, as predicted from secondary structure algorithms. Based on homology modeling, we hypothesized that the proximal region of the I-II loop may form a helix-loop-helix structure. This model was tested by mutagenesis followed by electrophysiological measurement of channel gating. Mutations that disrupted the helices, or the loop region, had profound effects on channel gating, shifting both steady state activation and inactivation curves, as well as accelerating channel kinetics. Mutations designed to preserve the helical structure had more modest effects. Taken together, these studies showed that any mutations in the brake, including C456S, disrupted the structural integrity of the brake and its function to maintain these low voltage-activated channels closed at resting membrane potentials.

Molecular determinant for specific Ca/Ba selectivity profiles of low and high threshold Ca2+ channels

Voltage-gated Ca²⁺ channels (VGCC) play a key role in many physiological functions by their high selectivity for Ca²⁺ over other divalent and monovalent cations in physiological situations. Divalent/monovalent selection is shared by all VGCC and is satisfactorily explained by the existence, within the pore, of a set of four conserved glutamate/aspartate residues (EEEE locus) coordinating Ca²⁺ ions. This locus however does not explain either the choice of Ca²⁺ among other divalent cations or the specific conductances encountered in the different VGCC. Our systematic analysis of high- and low-threshold VGCC currents in the presence of Ca²⁺ and Ba²⁺ reveals highly specific selectivity profiles. Sequence analysis, molecular modeling, and mutational studies identify a set of nonconserved charged residues responsible for these profiles. In HVA (high voltage activated) channels, mutations of this set modify divalent cation selectivity and channel conductance without change in divalent/monovalent selection, activation, inactivation, and kinetics properties. The Ca_V2.1 selectivity profile is transferred to Ca_V2.3 when exchanging their residues at this location. Numerical simulations suggest modification in an external Ca²⁺ binding site in the channel pore directly involved in the choice of Ca²⁺, among other divalent physiological cations, as the main permeant cation for VGCC. In LVA (low voltage activated) channels, this locus (called DCS for divalent cation selectivity) also influences divalent cation selection, but our results suggest the existence of additional determinants to fully recapitulate all the differences encountered among LVA channels. These data therefore attribute to the DCS a unique role in the specific shaping of the Ca²⁺ influx between the different HVA channels.

Zn2+ Slows Down CaV3.3 Gating Kinetics: Implications for Thalamocortical Activity

We employed whole cell patch-clamp recordings to establish the effect of Zn2+ on the gating the brain specific, T-type channel isoform CaV3.3 expressed in HEK-293 cells. Zn2+ (300 μM) modified the gating kinetics of this channel without influencing its steady-state properties. When inward Ca2+ currents were elicited by step depolarizations at voltages above the threshold for channel opening, current inactivation was significantly slowed down while current activation was moderately affected. In addition, Zn2+ slowed down channel deactivation but channel recovery from inactivation was only modestly changed. Zn2+ also decreased whole cell Ca2+ permeability to 45% of control values. In the presence of Zn2+, Ca2+ currents evoked by mock action potentials were more persistent than in its absence. Furthermore, computer simulation of action potential generation in thalamic reticular cells performed to model the gating effect of Zn2+ on T-type channels (while leaving the kinetic parameters of voltage-gated Na+ and K+ unchanged) revealed that Zn2+ increased the frequency and the duration of burst firing, which is known to depend on T-type channel activity. In line with this finding, we discovered that chelation of endogenous Zn2+ decreased the frequency of occurrence of ictal-like epileptiform discharges in rat thalamocortical slices perfused with medium containing the convulsant 4-aminopyridine (50 μM). These data demonstrate that Zn2+ modulates CaV3.3 channel gating thus leading to increased neuronal excitability. We also propose that endogenous Zn2+ may have a role in controlling thalamocortical oscillations.

Ca2+-dependent inactivation of CaV1.2 channels prevents Gd3+ block: does Ca2+ block the pore of inactivated channels?

Lanthanide gadolinium (Gd(3+)) blocks Ca(V)1.2 channels at the selectivity filter. Here we investigated whether Gd(3+) block interferes with Ca(2+)-dependent inactivation, which requires Ca(2+) entry through the same site. Using brief pulses to 200 mV that relieve Gd(3+) block but not inactivation, we monitored how the proportions of open and open-blocked channels change during inactivation. We found that blocked channels inactivate much less. This is expected for Gd(3+) block of the Ca(2+) influx that enhances inactivation. However, we also found that the extent of Gd(3+) block did not change when inactivation was reduced by abolition of Ca(2+)/calmodulin interaction, showing that Gd(3+) does not block the inactivated channel. Thus, Gd(3+) block and inactivation are mutually exclusive, suggesting action at a common site. These observations suggest that inactivation causes a change at the selectivity filter that either hides the Gd(3+) site or reduces its affinity, or that Ca(2+) occupies the binding site at the selectivity filter in inactivated channels. The latter possibility is supported by previous findings that the EEQE mutation of the selectivity EEEE locus is void of Ca(2+)-dependent inactivation (Zong Z.Q., J.Y. Zhou, and T. Tanabe. 1994. Biochem. Biophys. Res. Commun. 201:1117-11123), and that Ca(2+)-inactivated channels conduct Na(+) when Ca(2+) is removed from the extracellular medium (Babich O., D. Isaev, and R. Shirokov. 2005. J. Physiol. 565:709-717). Based on these results, we propose that inactivation increases affinity of the selectivity filter for Ca(2+) so that Ca(2+) ion blocks the pore. A minimal model, in which the inactivation "gate" is an increase in affinity of the selectivity filter for permeating ions, successfully simulates the characteristic U-shaped voltage dependence of inactivation in Ca(2+).

Selectivity signatures of three isoforms of recombinant T-type Ca2+ channels

Voltage-gated Ca(2+) channels (VGCCs) are recognized for their superb ability for the preferred passage of Ca(2+) over any other more abundant cation present in the physiological saline. Most of our knowledge about the mechanisms of selective Ca(2+) permeation through VGCCs was derived from the studies on native and recombinant L-type representatives. However, the specifics of the selectivity and permeation of known recombinant T-type Ca(2+)-channel alpha1 subunits, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are still poorly defined. In the present study we provide comparative analysis of the selectivity and permeation Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3 functionally expressed in Xenopus oocytes. Our data show that all Ca(v)3 channels select Ca(2+) over Na(+) by affinity. Ca(v)3.1 and Ca(v)3.2 discriminate Ca(2+), Sr(2+) and Ba(2+) based on the ion's effects on the open channel probability, whilst Ca(v)3.3 discriminates based on the ion's intrapore binding affinity. All Ca(v)3s were characterized by much smaller difference in the K(D) values for Na(+) current blockade by Ca(2+) (K(D1) approximately 6 microM) and for Ca(2+) current saturation (K(D2) approximately 2 mM) as compared to L-type channels. This enabled them to carry notable mixed Na(+)/Ca(2+) current at close to physiological Ca(2+) concentrations, which was the strongest for Ca(v)3.3, smaller for Ca(v)3.2 and the smallest for Ca(v)3.1. In addition to intrapore Ca(2+) binding site(s) Ca(v)3.2, but not Ca(v)3.1 and Ca(v)3.3, is likely to possess an extracellular Ca(2+) binding site that controls channel permeation. Our results provide novel functional tests for identifying subunits responsible for T-type Ca(2+) current in native cells.

The ion channel inverse problem: neuroinformatics meets biophysics

Ion channels are the building blocks of the information processing capability of neurons: any realistic computational model of a neuron must include reliable and effective ion channel components. Sophisticated statistical and computational tools have been developed to study the ion channel structure-function relationship, but this work is rarely incorporated into the models used for single neurons or small networks. The disjunction is partly a matter of convention. Structure-function studies typically use a single Markov model for the whole channel whereas until recently whole-cell modeling software has focused on serial, independent, two-state subunits that can be represented by the Hodgkin-Huxley equations. More fundamentally, there is a difference in purpose that prevents models being easily reused. Biophysical models are typically developed to study one particular aspect of channel gating in detail, whereas neural modelers require broad coverage of the entire range of channel behavior that is often best achieved with approximate representations that omit structural features that cannot be adequately constrained. To bridge the gap so that more recent channel data can be used in neural models requires new computational infrastructure for bringing together diverse sources of data to arrive at best-fit models for whole-cell modeling. We review the current state of channel modeling and explore the developments needed for its conclusions to be integrated into whole-cell modeling.

Evidence for common structural determinants of activation and inactivation in T-type Ca2+ channels

One of the most distinctive features of T-type Ca(2+) channels is their fast inactivation. Recent structure-function studies indicate that the rate of macroscopic inactivation of these channels is influenced by several structural components, including intracellular linkers, transmembrane segments, and pore loops. The macroscopic inactivation of T-type channels is partially coupled to activation. It is therefore possible that changes in the rate of macroscopic inactivation after alteration in the structure of these channels might actually result from changes in activation kinetics. In this study, we use kinetic simulations to illustrate how the alteration of the rate of channel activation may lead to changes in the rate of macroscopic inactivation. By examining data pooled from several structure-function studies we demonstrate that gating modifications induced by alteration in the channel structure unveils a correlation between the time constants of macroscopic inactivation and activation. This analysis underscores the relevance of considering the inactivation-activation coupling when analyzing the structural determinants of T-type channel inactivation. Furthermore, we demonstrate that slow-inactivating mutants, with modifications in the IIIS6 segment and the proximal C terminus, display significant alterations in the voltage dependencies of activation and deactivation with respect to the wild type channel Ca(V)3.1. Our results indicate that common structures, most likely the S6 transmembrane segments, are involved in the conformational changes occurring during both channel activation and inactivation.

Role of Domain IV/S4 outermost arginines in gating of T-type calcium channels

The role of the outermost three charged residues of Domain IV/S4 in controlling gating of Ca(v)3.2 was investigated using single substitutions of each arginine with glutamine, cysteine, histidine, and lysine in a Flp-In-293 cell line, in which expression levels could be compared. Channel density, based on gating charge measurements, was ~125,000 channels/cell (10 fC/pF), except for R2Q and R3C, which expressed at lower levels. Channels substituted at Arg-1715 (R1C, R1Q, R1H) demonstrated such modest changes that a role in voltage sensing could not be determined. Arg-1718 (R2) made a contribution to activation voltage sensing, and the channel was sensitive to the geometry of side-chain substitutions at this position. Arg-1721 (R3) substitutions produced complex kinetic changes that together suggested that geometry made a larger contribution than charge. Current decay at positive potentials (O-->I) exponentially approached a constant value for all mutants except R2K channels, which were biphasically dependent on potential. R2K channels also displayed slowed deactivation with reduced voltage dependence despite near control values for conductance. Voltage-dependent accessibility of R to C mutants, evaluated with intracellularly and extracellularly applied methanthiosulfonate (MTS) reagents, showed that both R2 and R3 were exposed only when cells were depolarized, although it was not necessary for channels to open. Together, the data indicate that Domain IV/S4 is an activation domain and is not involved in inactivation from the open state.

Molecular regions underlying the activation of low- and high-voltage activating calcium channels

We have studied two aspects of calcium channel activation. First, we investigated the molecular regions that are important in determining differences in activation between low- and high-voltage activated channels. For this, we made chimeras between the low-voltage activating Ca(V)3.1 channel and the high-voltage activating Ca(V)1.2 channel. Chimeras were expressed in oocytes, and calcium channel currents recorded by voltage clamp. For domain I, we found that the molecular region that is important in determining the voltage dependence of activation comprises the pore regions S5-P as well as P-S6, but surprisingly not the voltage sensor S1-S4 region, which might have been expected to play a major part. By contrast, the smaller, but still significant, modulating effects of domain II on activation properties were due to effects involving both S1-S4 and S5-S6 but not the I/II linker. Second, during channel activation we studied movement of the S4 segment in domain I of one of the chimeras, using cysteine-scanning mutagenesis. The reagent parachloromercuribenzensulfonate inhibited currents for mutants V263, A265, L266 and A268, but not for F269 and V271, and voltage dependence of inhibition for residue V263 indicated S4 movement, which occurred before channel opening. The data indicate movement outwards upon depolarisation so as to expose amino acids up to residue 268 in S4.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Mutations in the Pore Region Modify Epithelial Sodium Channel Gating by Shear Stress

Previous studies have shown that epithelial Na+ channels (ENaCs) are activated by laminar shear stress (LSS). ENaCs with a high intrinsic open probability because of a mutation (betaS518K) or covalent modification of an introduced Cys residue (alphaS580C) in the pre-second transmembrane domain (pre-M2) were not activated by LSS, suggesting that the pre-M2 region participates in conformational rearrangements during channel activation. We examined the role of the pore region of the alpha-subunit in channel gating by studying the kinetics of activation by LSS of wild-type ENaC and channels with Cys mutations in the tract Ser576-Ser592. Whole cell Na+ currents were monitored in oocytes expressing wild-type or mutant ENaCs prior to and following application of LSS. Following a 2.2-s delay, a monoexponential increase in Na+ currents was observed with a time constant (tau) of 8.1 s in oocytes expressing wild-type ENaC. Cys substitutions within the alpha-subunit in the tract Ser580-Ser589 resulted in: (i) a reduction (Ser580-Trp585, Gly587) or increase (Ser589) in delay times preceding channel activation by LSS, (ii) an increase (Gln581, Leu584, Trp585, Phe586, Ser588) or decrease (Ser589) in the rate of channel activation, or (iii) a decrease in the magnitude of the response (Ser583, Gly587, Leu584). Cys substitutions at a putative amiloride-binding site (alphaSer583 or betaGly525) or within the selectivity filter (alphaGly587) resulted in a reduction in the LSS response, and exhibited a multiexponential time course of activation. The corresponding gamma-subunit mutant (alphabetagammaG542C) had a minimal response to LSS and exhibited a high intrinsic open probability. These data suggest that residues in the pore region participate in the sensing and/or transduction of the mechanical stimulus that results in channel activation and are consistent with the hypothesis that the ENaC pore region has a key role in modulating channel gating.

Block of tetrodotoxin-resistant Na+ channel pore by multivalent cations: gating modification and Na+ flow dependence

Tetrodotoxin-resistant (TTX-R) Na(+) channels are much less susceptible to external TTX but more susceptible to external Cd(2+) block than tetrodotoxin-sensitive (TTX-S) Na(+) channels. Both TTX and Cd(2+) seem to block the channel near the "DEKA" ring, which is probably part of a multi-ion single-file region adjacent to the external pore mouth and is involved in the selectivity filter of the channel. In this study we demonstrate that other multivalent transitional metal ions such as La(3+), Zn(2+), Ni(2+), Co(2+), and Mn(2+) also block the TTX-R channels in dorsal root ganglion neurons. Just like Cd(2+), the blocking effect has little intrinsic voltage dependence, but is profoundly influenced by Na(+) flow. The apparent dissociation constants of the blocking ions are always significantly smaller in inward Na(+) currents than those in outward Na(+) current, signaling exit of the blocker along with the Na(+) flow and a high internal energy barrier for "permeation" of these multivalent blocking ions through the pore. Most interestingly, the activation and especially the inactivation kinetics are slowed by the blocking ions. Moreover, the gating changes induced by the same concentration of a blocking ion are evidently different in different directions of Na(+) current flow, but can always be correlated with the extent of pore block. Further quantitative analyses indicate that the apparent slowing of channel activation is chiefly ascribable to Na(+) flow-dependent unblocking of the bound La(3+) from the open Na(+) channel, whereas channel inactivation cannot happen with any discernible speed in the La(3+)-blocked channel. Thus, the selectivity filter of Na(+) channel is probably contiguous to a single-file multi-ion region at the external pore mouth, a region itself being nonselective in terms of significant binding of different multivalent cations. This region is "open" to the external solution even if the channel is "closed" ("deactivated"), but undergoes imperative conformational changes during the gating (especially the inactivation) process of the channel.

Mechanism of arachidonic acid modulation of the T-type Ca2+ channel alpha1G

Arachidonic acid (AA) modulates T-type Ca(2+) channels and is therefore a potential regulator of diverse cell functions, including neuronal and cardiac excitability. The underlying mechanism of modulation is unknown. Here we analyze the effects of AA on the T-type Ca(2+) channel alpha(1G) heterologously expressed in HEK-293 cells. AA inhibited alpha(1G) currents within a few minutes, regardless of preceding exposure to inhibitors of AA metabolism (ETYA and 17-ODYA). Current inhibition was also observed in cell-free inside-out patches, indicating a membrane-delimited interaction of AA with the channel. AA action was consistent with a decrease of the open probability without changes in the size of unitary currents. AA shifted the inactivation curve to more negative potentials, increased the speed of macroscopic inactivation, and decreased the extent of recovery from inactivation at -80 mV but not at -110 mV. AA induced a slight increase of activation near the threshold and did not significantly change the deactivation kinetics or the rectification pattern. We observed a tonic current inhibition, regardless of whether the channels were held in resting or inactivated states during AA perfusion, suggesting a state-independent interaction with the channel. Model simulations indicate that AA inhibits T-type currents by switching the channels into a nonavailable conformation and by affecting transitions between inactivated states, which results in the negative shift of the inactivation curve. Slow-inactivating alpha(1G) mutants showed an increased affinity for AA with respect to the wild type, indicating that the structural determinants of fast inactivation are involved in the AA-channel interaction.

Roles of Molecular Regions in Determining Differences between Voltage Dependence of Activation of CaV3.1 and CaV1.2 Calcium Channels

Voltage-dependent calcium channels are classified into low voltage-activated and high voltage-activated channels. We have investigated the molecular basis for this difference in voltage dependence of activation by constructing chimeras between a low voltage-activated channel (Ca(V)3.1) and a high voltage-activated channel (Ca(V)1.2), focusing on steady-state activation properties. Wild type and chimeras were expressed in oocytes, and two-electrode voltage clamp recordings were made of calcium channel currents. Replacement of domains I, III, or IV of the Ca 3.1 channel with the corresponding domain of Ca(V)1.2 led (V)to high voltage-activated channels; for these constructs the current/voltage (I/V) curves were similar to those for Ca(V)1.2 wild type. However, replacement of domain II gave only a small shift to the right of the I/V curve and modulation of the activation kinetics but did not lead to a high voltage-activating channel with an I/V curve like Ca 1.2. We also investigated the role of the voltage sensor (V)S4 by replacing the S4 segment of Ca(V)3.1 with that of Ca 1.2. For domain I, there was no shift in the I/V curve (V)as compared with Ca(V)3.1, and there were relatively small shifts to the right for domains III and IV. Taken together, these results suggest that domains I, III, and IV (rather than domain II) are apparently critical for channel opening and, therefore, contribute strongly to the difference in voltage dependence of activation between Ca 3.1 and Ca(V)1.2. However, the S4 segments in domains I, (V)III, and IV did not account for this difference in voltage dependence.

Slow inactivation of the Ca(V)3.1 isotype of T-type calcium channels

T-type calcium channels (the Ca(V)3 channel family) are involved in defining the resting membrane potential and in neuronal activities such as oscillations and rebound depolarization. Their physiological roles depend upon the channel activation and inactivation kinetics. A fast inactivation that stops the ionic flux of calcium in tens of milliseconds has already been described in both native and heterologously expressed channels. Here, using HEK 293 cells expressing the rat Ca(V)3.1 channel and whole-cell voltage clamp, we investigate an additional inactivation process, which can be distinguished from the previously described fast inactivation by its slow time course of recovery from inactivation (tau= 1 s) and by its sensitivity to external calcium. Steady-state slow inactivation is voltage dependent around the resting membrane potential (the potential of half-inactivation (V(0.5)) =-70 mV, slope factor = 7.4 mV) and can reduce the calcium current by up to 50%. Near resting potential, the slow inactivation displays a half-time of induction of tens of seconds. The slow inactivation therefore modulates the availability of T-type calcium channels depending upon recent cell history, providing a mechanism to store information in a time scale of seconds.

Extracellular Ca2+ modulates the effects of protons on gating and conduction properties of the T-type Ca2+ channel alpha1G (CaV3.1)

Since Ca2+ is a major competitor of protons for the modulation of high voltage-activated Ca2+ channels, we have studied the modulation by extracellular Ca2+ of the effects of proton on the T-type Ca2+ channel alpha1G (CaV3.1) expressed in HEK293 cells. At 2 mM extracellular Ca2+ concentration, extracellular acidification in the pH range from 9.1 to 6.2 induced a positive shift of the activation curve and increased its slope factor. Both effects were significantly reduced if the concentration was increased to 20 mM or enhanced in the absence of Ca2+. Extracellular protons shifted the voltage dependence of the time constant of activation and decreased its voltage sensitivity, which excludes a voltage-dependent open pore block by protons as the mechanism modifying the activation curve. Changes in the extracellular pH altered the voltage dependence of steady-state inactivation and deactivation kinetics in a Ca2+-dependent manner, but these effects were not strictly correlated with those on activation. Model simulations suggest that protons interact with intermediate closed states in the activation pathway, decreasing the gating charge and shifting the equilibrium between these states to less negative potentials, with these effects being inhibited by extracellular Ca2+. Extracellular acidification also induced an open pore block and a shift in selectivity toward monovalent cations, which were both modulated by extracellular Ca2+ and Na+. Mutation of the EEDD pore locus altered the Ca2+-dependent proton effects on channel selectivity and permeation. We conclude that Ca2+ modulates T-type channel function by competing with protons for binding to surface charges, by counteracting a proton-induced modification of channel activation and by competing with protons for binding to the selectivity filter of the channel.