Pore stability and gating in voltage-activated calcium channels

Permeabilizing Cell Membranes with Electric Fields

The biological impact of exogenous, alternating electric fields (AEFs) and direct-current electric fields has a long history of study, ranging from effects on embryonic development to influences on wound healing. In this article, we focus on the application of electric fields for the treatment of cancers. In particular, we outline the clinical impact of tumor treating fields (TTFields), a form of AEFs, on the treatment of cancers such as glioblastoma and mesothelioma. We provide an overview of the standard mechanism of action of TTFields, namely, the capability for AEFs (e.g., TTFields) to disrupt the formation and segregation of the mitotic spindle in actively dividing cells. Though this standard mechanism explains a large part of TTFields' action, it is by no means complete. The standard theory does not account for exogenously applied AEFs' influence directly upon DNA nor upon their capacity to alter the functionality and permeability of cancer cell membranes. This review summarizes the current literature to provide a more comprehensive understanding of AEFs' actions on cell membranes. It gives an overview of three mechanistic models that may explain the more recent observations into AEFs' effects: the voltage-gated ion channel, bioelectrorheological, and electroporation models. Inconsistencies were noted in both effective frequency range and field strength between TTFields versus all three proposed models. We addressed these discrepancies through theoretical investigations into the inhomogeneities of electric fields on cellular membranes as a function of disease state, external microenvironment, and tissue or cellular organization. Lastly, future experimental strategies to validate these findings are outlined. Clinical benefits are inevitably forthcoming.

Functional impact of a congenital stationary night blindness type 2 mutation depends on subunit composition of Cav1.4 Ca2+ channels

Voltage-gated Ca_v1 and Ca_v2 Ca²⁺ channels are comprised of a pore-forming α₁ subunit (Ca_v1.1-1.4, Ca_v2.1-2.3) and auxiliary β (β_1-4) and α₂δ (α₂δ-1-4) subunits. The properties of these channels vary with distinct combinations of Ca_v subunits and alternative splicing of the encoding transcripts. Therefore, the impact of disease-causing mutations affecting these channels may depend on the identities of Ca_v subunits and splice variants. Here, we analyzed the effects of a congenital stationary night blindness type 2 (CSNB2)-causing mutation, I745T (IT), in Ca_v1.4 channels typical of those in human retina: Ca_v1.4 splice variants with or without exon 47 (Ca_v1.4+ex47 and Ca_v1.4Δex47, respectively), and the auxiliary subunits, β_2X13 and α₂δ-4. We find that IT caused both Ca_v1.4 splice variants to activate at significantly more negative voltages and with slower deactivation kinetics than the corresponding WT channels. These effects of the IT mutation, along with unexpected alterations in ion selectivity, were generally larger in channels lacking exon 47. The weaker ion selectivity caused by IT led to hyperpolarizing shifts in the reversal potential and large outward currents that were evident in channels containing the auxiliary subunits β_2X13 and α₂δ-4 but not in those with β_2A and α₂δ-1. We conclude that the IT mutation stabilizes channel opening and alters ion selectivity of Ca_v1.4 in a manner that is strengthened by exclusion of exon 47 and inclusion of β_2X13 and α₂δ-4. Our results reveal complex actions of IT in modifying the properties of Ca_v1.4 channels, which may influence the pathological consequences of this mutation in retinal photoreceptors.

Ca2+ Channels in Anterior Pituitary Somatotrophs: A Therapeutic Perspective

Ca2+ influx through voltage-gated Ca2+ channels (VGCCs) plays a key role in GH secretion. In this review, we summarize the current state of knowledge regarding the physiology and molecular machinery of VGCCs in pituitary somatotrophs. We next discuss the possible involvement of Ca2+ channelopathies in pituitary disease and the potential use of Ca2+ channel blockers to treat pituitary disease. Various types of VGCCs exist in pituitary cells. However, because L-type Ca2+ channels (LTCCs) contribute the major component to Ca2+ influx in somatotrophs, lactotrophs, and corticotrophs, we focused on these channels. An increasing number of studies in recent years have linked genetic missense mutations in LTCCs to diseases of the human cardiovascular, nervous, and endocrine systems. These disease-associated genetic mutations occur at homologous functional positions (activation gates) in LTCCs. Thus, it is plausible that similar homologous missense mutations in pituitary LTCCs can cause abnormal hormone secretion and underlying pituitary disorders. The existence of LTCCs in pituitary cells opens questions about their sensitivity to dihydropyridines, a group of selective LTCC blockers. The dihydropyridine sensitivity of pituitary cells, as with any other excitable cell, depends primarily on two parameters: the pattern of their electrical activity and the dihydropyridine sensitivity of their LTCC isoforms. These two parameters are discussed in detail in relation to somatotrophs. These discussions are also relevant to lactotrophs and corticotrophs. High dihydropyridine sensitivity may facilitate their use as drugs to treat pituitary oversecretion disorders such as acromegaly, hyperprolactinemia, and Cushing disease.

Redox regulation of the yeast voltage-gated calcium channel homolog, Cch1p, by glutathionylation of specific cysteines

CCH1, the yeast homolog of the pore-forming subunit α1 of the mammalian Voltage-gated calcium channel (VGCC) located on the plasma membrane mediates the redox-dependent influx of calcium. Cch1p is known to undergo both rapid activation (oxidative stress, high pH) and slow activation (ER stress, mating pheromone activation), but the mechanism of activation is not known. We demonstrate here that the fast activation, as well as the slow activation (tunicamycin or α-factor) is mediated through a common redox-dependant manner. Further, through mutational analysis of all 18 exposed cysteines in the Cch1p protein, we show that four of these mutants, C587A, C606A, C636A and C642A, which are clustered together in a common cytoplasmic loop region were functionally defective during both fast and slow activations and also showed reduced glutathionylation. These four cysteines are also conserved across phyla suggesting a conserved mechanism of activation. Investigations into the enzymes involved in the activation reveal that the yeast glutathione-s-transferase, Gtt1p is involved in the glutathionylation of Cch1p, while the thioredoxin, Trx2p plays a role in the Cch1p deglutathionylation.

Somatic and germline CACNA1D calcium channel mutations in aldosterone-producing adenomas and primary aldosteronism

Adrenal aldosterone-producing adenomas (APAs) constitutively produce the salt-retaining hormone aldosterone and are a common cause of severe hypertension. Recurrent mutations in the potassium channel KCNJ5 that result in cell depolarization and Ca²⁺ influx cause ~40% of these tumors¹. We found five somatic mutations (four altering glycine 403, one altering isoleucine 770) in CACNA1D, encoding a voltage-gated calcium channel, among 43 non-KCNJ5-mutant APAs. These mutations lie in S6 segments that line the channel pore. Both result in channel activation at less depolarized potentials, and glycine 403 mutations also impair channel inactivation. These effects are inferred to cause increased Ca²⁺ influx, the sufficient stimulus for aldosterone production and cell proliferation in adrenal glomerulosa². Remarkably, we identified de novo mutations at the identical positions in two children with a previously undescribed syndrome featuring primary aldosteronism and neuromuscular abnormalities. These findings implicate gain of function Ca²⁺ channel mutations in aldosterone-producing adenomas and primary aldosteronism.

Physicochemical properties of pore residues predict activation gating of Ca V1.2: a correlation mutation analysis

Single point mutations in pore-forming S6 segments of calcium channels may transform a high-voltage-activated into a low-voltage-activated channel, and resulting disturbances in calcium entry may cause channelopathies (Hemara-Wahanui et al., Proc Natl Acad Sci U S A 102(21):7553–7558, 16). Here we ask the question how physicochemical properties of amino acid residues in gating-sensitive positions on S6 segments determine the threshold of channel activation of Ca_V1.2. Leucine in segment IS6 (L434) and a newly identified activation determinant in segment IIIS6 (G1193) were mutated to a variety of amino acids. The induced leftward shifts of the activation curves and decelerated current activation and deactivation suggest a destabilization of the closed and a stabilisation of the open channel state by most mutations. A selection of 17 physicochemical parameters (descriptors) was calculated for these residues and examined for correlation with the shifts of the midpoints of the activation curve (ΔV_act). ΔV_act correlated with local side-chain flexibility in position L434 (IS6), with the polar accessible surface area of the side chain in position G1193 (IIIS6) and with hydrophobicity in position I781 (IIS6). Combined descriptor analysis for positions I781 and G1193 revealed that additional amino acid properties may contribute to conformational changes during the gating process. The identified physicochemical properties in the analysed gating-sensitive positions (accessible surface area, side-chain flexibility, and hydrophobicity) predict the shifts of the activation curves of Ca_V1.2.

Different pathways for activation and deactivation in CaV1.2: a minimal gating model

Point mutations in pore-lining S6 segments of Ca_V1.2 shift the voltage dependence of activation into the hyperpolarizing direction and significantly decelerate current activation and deactivation. Here, we analyze theses changes in channel gating in terms of a circular four-state model accounting for an activation R–A–O and a deactivation O–D–R pathway. Transitions between resting-closed (R) and activated-closed (A) states (rate constants x(V) and y(V)) and open (O) and deactivated-open (D) states (u(V) and w(V)) describe voltage-dependent sensor movements. Voltage-independent pore openings and closures during activation (A–O) and deactivation (D–R) are described by rate constants α and β, and γ and δ, respectively. Rate constants were determined for 16-channel constructs assuming that pore mutations in IIS6 do not affect the activating transition of the voltage-sensing machinery (x(V) and y(V)). Estimated model parameters of 15 Ca_V1.2 constructs well describe the activation and deactivation processes. Voltage dependence of the “pore-releasing” sensor movement ((x(V)) was much weaker than the voltage dependence of “pore-locking” sensor movement (y(V)). Our data suggest that changes in membrane voltage are more efficient in closing than in opening Ca_V1.2. The model failed to reproduce current kinetics of mutation A780P that was, however, accurately fitted with individually adjusted x(V) and y(V). We speculate that structural changes induced by a proline substitution in this position may disturb the voltage-sensing domain.

Coupled and independent contributions of residues in IS6 and IIS6 to activation gating of CaV1.2

Voltage dependence and kinetics of Ca(V)1.2 activation are affected by structural changes in pore-lining S6 segments of the alpha(1)-subunit. Significant effects are induced by either proline or threonine substitutions in the lower third of segment IIS6 ("bundle crossing region"), where S6 segments are likely to seal the channel in the closed conformation (Hohaus, A., Beyl, S., Kudrnac, M., Berjukow, S., Timin, E. N., Marksteiner, R., Maw, M. A., and Hering, S. (2005) J. Biol. Chem. 280, 38471-38477). Here we report that S435P in IS6 results in a large shift of the activation curve (-25.9 +/- 1.2 mV) and slower current kinetics. Threonine substitutions at positions Leu-429 and Leu-434 induced a similar kinetic phenotype with shifted activation curves (L429T by -6.6 +/- 1.2 and L434T by -12.1 +/- 1.7 mV). Inactivation curves of all mutants were shifted to comparable extents as the activation curves. Interdependence of IS6 and IIS6 mutations was analyzed by means of mutant cycle analysis. Double mutations in segments IS6 and IIS6 induce either additive (L429T/I781T, -34.1 +/- 1.4 mV; L434T/I781T, -40.4 +/- 1.3 mV; L429T/L779T, -12.6 +/- 1.3 mV; and L434T/L779T, -22.4 +/- 1.3 mV) or nonadditive shifts of the activation curves along the voltage axis (S435P/I781T, -33.8 +/- 1.4 mV). Mutant cycle analysis revealed energetic coupling between residues Ser-435 and Ile-781, whereas other paired mutations in segments IS6 and IIS6 had independent effects on activation gating.