The effect of the outermost basic residues in the S4 segments of the Ca(V)3.1 T-type calcium channel on channel gating

Role of individual S4 segments in gating of Cav3.1 T-type calcium channel by voltage

Contributions of voltage sensing S4 segments in domains I – IV of Ca_V3.1 channel to channel activation were analyzed. Neutralization of the uppermost charge in individual S4 segments by exchange of arginine for cysteine was employed. Mutant channels with single exchange in domains I – IV, in two adjacent domains, and in all four domains were constructed and expressed in HEK 293 cells. Changes in maximal gating charge Q_max and the relation between Q_max and maximal conductance G_max were evaluated. Q_max was the most affected by single mutation in domain I and by double mutations in domains I + II and I + IV. The ratio G_max/Q_max proportional to opening probability of the channel was significantly decreased by the mutation in domain III and increased by mutations in domains I and II. In channels containing double mutations G_max/Q_max ratio increased significantly when the mutation in domain I was included. Mutations in domains II and III zeroed each other. Mutation in domain IV prevented the decrease caused by the mutation in domain III. Neither ion current nor gating current was observed when channels with quadruple mutations were expressed. Immunocytochemistry analysis did not reveal the presence of channel protein in the cell membrane. Likely, quadruple mutation results in a structural change that affects the channel’s trafficking mechanism. Altogether, S4 segments in domains I-IV of the Ca_V3.1 channel unequally contribute to channel gating by voltage. We suggest the most important role of the voltage sensor in the domain I and lesser roles of voltage sensors in domains II and III.

Contribution of S4 segments and S4-S5 linkers to the low-voltage activation properties of T-type CaV3.3 channels

Voltage-gated calcium channels contain four highly conserved transmembrane helices known as S4 segments that exhibit a positively charged residue every third position, and play the role of voltage sensing. Nonetheless, the activation range between high-voltage (HVA) and low-voltage (LVA) activated calcium channels is around 30–40 mV apart, despite the high level of amino acid similarity within their S4 segments. To investigate the contribution of S4 voltage sensors for the low-voltage activation characteristics of Ca_V3.3 channels we constructed chimeras by swapping S4 segments between this LVA channel and the HVA Ca_V1.2 channel. The substitution of S4 segment of Domain II in Ca_V3.3 by that of Ca_V1.2 (chimera IIS4C) induced a ~35 mV shift in the voltage-dependence of activation towards positive potentials, showing an I-V curve that almost overlaps with that of Ca_V1.2 channel. This HVA behavior induced by IIS4C chimera was accompanied by a 2-fold decrease in the voltage-dependence of channel gating. The IVS4 segment had also a strong effect in the voltage sensing of activation, while substitution of segments IS4 and IIIS4 moved the activation curve of Ca_V3.3 to more negative potentials. Swapping of IIS4 voltage sensor influenced additional properties of this channel such as steady-state inactivation, current decay, and deactivation. Notably, Domain I voltage sensor played a major role in preventing Ca_V3.3 channels to inactivate from closed states at extreme hyperpolarized potentials. Finally, site-directed mutagenesis in the Ca_V3.3 channel revealed a partial contribution of the S4-S5 linker of Domain II to LVA behavior, with synergic effects observed in double and triple mutations. These findings indicate that IIS4 and, to a lesser degree IVS4, voltage sensors are crucial in determining the LVA properties of Ca_V3.3 channels, although the accomplishment of this function involves the participation of other structural elements like S4-S5 linkers.

A Recurrent Mutation in CACNA1G Alters Cav3.1 T-Type Calcium-Channel Conduction and Causes Autosomal-Dominant Cerebellar Ataxia

Hereditary cerebellar ataxias (CAs) are neurodegenerative disorders clinically characterized by a cerebellar syndrome, often accompanied by other neurological or non-neurological signs. All transmission modes have been described. In autosomal-dominant CA (ADCA), mutations in more than 30 genes are implicated, but the molecular diagnosis remains unknown in about 40% of cases. Implication of ion channels has long been an ongoing topic in the genetics of CA, and mutations in several channel genes have been recently connected to ADCA. In a large family affected by ADCA and mild pyramidal signs, we searched for the causative variant by combining linkage analysis and whole-exome sequencing. In CACNA1G, we identified a c.5144G>A mutation, causing an arginine-to-histidine (p.Arg1715His) change in the voltage sensor S4 segment of the T-type channel protein Cav3.1. Two out of 479 index subjects screened subsequently harbored the same mutation. We performed electrophysiological experiments in HEK293T cells to compare the properties of the p.Arg1715His and wild-type Cav3.1 channels. The current-voltage and the steady-state activation curves of the p.Arg1715His channel were shifted positively, whereas the inactivation curve had a higher slope factor. Computer modeling in deep cerebellar nuclei (DCN) neurons suggested that the mutation results in decreased neuronal excitability. Taken together, these data establish CACNA1G, which is highly expressed in the cerebellum, as a gene whose mutations can cause ADCA. This is consistent with the neuropathological examination, which showed severe Purkinje cell loss. Our study further extends our knowledge of the link between calcium channelopathies and CAs.

Contrasting the roles of the I-II loop gating brake in CaV3.1 and CaV3.3 calcium channels

Low-voltage-activated CaV3 channels are distinguished among other voltage-activated calcium channels by the most negative voltage activation threshold. The voltage dependence of current activation is virtually identical in all three CaV3 channels while the current kinetics of the CaV3.3 current is one order slower than that of the CaV3.1 and CaV3.2 channels. We have analyzed the voltage dependence and kinetics of charge (Q) movement in human recombinant CaV3.3 and CaV3.1 channels. The voltage dependence of voltage sensor activation (Qon-V) of the CaV3.3 channel was significantly shifted with respect to that of the CaV3.1 channel by +18.6 mV and the kinetic of Qon activation in the CaV3.3 channel was significantly slower than that of the CaV3.1 channel. Removal of the gating brake in the intracellular loop connecting repeats I and II in the CaV3.3 channel in the ID12 mutant channel shifted the Qon-V relation to a value even more negative than that for the CaV3.1 channel. The kinetic of Qon activation was not significantly different between ID12 and CaV3.1 channels. Deletion of the gating brake in the CaV3.1 channel resulted in a GD12 channel with the voltage dependence of the gating current activation significantly shifted toward more negative potentials. The Qon kinetic was not significantly altered. ID12 and GD12 mutants did not differ significantly in voltage dependence nor in the kinetic of voltage sensor activation. In conclusion, the putative gating brake in the intracellular loop connecting repeats I and II controls the gating current of the CaV3 channels. We suggest that activation of the voltage sensor in domain I is limiting both the voltage dependence and the kinetics of CaV3 channel activation.

Tetraspanin-13 modulates voltage-gated CaV2.2 Ca2+ channels

Integration of voltage-gated Ca(2+) channels in a network of protein-interactions is a crucial requirement for proper regulation of channel activity. In this study, we took advantage of the specific properties of the yeast split-ubiquitin system to search for and characterize so far unknown interaction partners of CaV2 Ca(2+) channels. We identified tetraspanin-13 (TSPAN-13) as an interaction partner of the α1 subunit of N-type CaV2.2, but not of P/Q-type CaV2.1 or L- and T-type Ca(2+) channels. Interaction could be located between domain IV of CaV2.2 and transmembrane segments S1 and S2 of TSPAN-13. Electrophysiological analysis revealed that TSPAN-13 specifically modulates the efficiency of coupling between voltage sensor activation and pore opening of the channel and accelerates the voltage-dependent activation and inactivation of the Ba(2+) current through CaV2.2. These data indicate that TSPAN-13 might regulate CaV2.2 Ca(2+) channel activity in defined synaptic membrane compartments and thereby influences transmitter release.

Cooperative activation of the T-type CaV3.2 channel: interaction between Domains II and III

T-type CaV3 channels are important mediators of Ca(2+) entry near the resting membrane potential. Little is known about the molecular mechanisms responsible for channel activation. Homology models based upon the high-resolution structure of bacterial NaV channels predict interaction between the S4-S5 helix of Domain II (IIS4-S5) and the distal S6 pore region of Domain II (IIS6) and Domain III (IIIS6). Functional intra- and inter-domain interactions were investigated with a double mutant cycle analysis. Activation gating and channel kinetics were measured for 47 single mutants and 20 pairs of mutants. Significant coupling energies (ΔΔG(interact) ≥ 1.5 kcal mol(-1)) were measured for 4 specific pairs of mutants introduced between IIS4-S5 and IIS6 and between IIS4-S5 and IIIS6. In agreement with the computer based models, Thr-911 in IIS4-S5 was functionally coupled with Ile-1013 in IIS6 during channel activation. The interaction energy was, however, found to be stronger between Val-907 in IIS4-S5 and Ile-1013 in IIS6. In addition Val-907 was significantly coupled with Asn-1548 in IIIS6 but not with Asn-1853 in IVS6. Altogether, our results demonstrate that the S4-S5 and S6 helices from adjacent domains are energetically coupled during the activation of a low voltage-gated T-type CaV3 channel.