Multiple structural domains contribute to voltage-dependent inactivation of rat brain alpha(1E) calcium channels

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

Role of a conserved glutamine in the function of voltage-gated Ca2+ channels revealed by a mutation in human CACNA1D

Voltage-gated Ca_v Ca²⁺ channels play crucial roles in regulating gene transcription, neuronal excitability, and synaptic transmission. Natural or pathological variations in Ca_v channels have yielded rich insights into the molecular determinants controlling channel function. Here, we report the consequences of a natural, putatively disease-associated mutation in the CACNA1D gene encoding the pore-forming Ca_v1.3 α₁ subunit. The mutation causes a substitution of a glutamine residue that is highly conserved in the extracellular S1-S2 loop of domain II in all Ca_v channels with a histidine and was identified by whole-exome sequencing of an individual with moderate hearing impairment, developmental delay, and epilepsy. When introduced into the rat Ca_v1.3 cDNA, Q558H significantly decreased the density of Ca²⁺ currents in transfected HEK293T cells. Gating current analyses and cell-surface biotinylation experiments suggested that the smaller current amplitudes caused by Q558H were because of decreased numbers of functional Ca_v1.3 channels at the cell surface. The substitution also produced more sustained Ca²⁺ currents by weakening voltage-dependent inactivation. When inserted into the corresponding locus of Ca_v2.1, the substitution had similar effects as in Ca_v1.3. However, the substitution introduced in Ca_v3.1 reduced current density, but had no effects on voltage-dependent inactivation. Our results reveal a critical extracellular determinant of current density for all Ca_v family members and of voltage-dependent inactivation of Ca_v1.3 and Ca_v2.1 channels.

Neuronal voltage-gated calcium channels: structure, function, and dysfunction

Voltage-gated calcium channels are the primary mediators of depolarization-induced calcium entry into neurons. There is great diversity of calcium channel subtypes due to multiple genes that encode calcium channel α1 subunits, coassembly with a variety of ancillary calcium channel subunits, and alternative splicing. This allows these channels to fulfill highly specialized roles in specific neuronal subtypes and at particular subcellular loci. While calcium channels are of critical importance to brain function, their inappropriate expression or dysfunction gives rise to a variety of neurological disorders, including, pain, epilepsy, migraine, and ataxia. This Review discusses salient aspects of voltage-gated calcium channel function, physiology, and pathophysiology.

The role of a voltage-dependent Ca2+ channel intracellular linker: a structure-function analysis

Voltage-dependent calcium channels (VDCCs) allow the passage of Ca(2+) ions through cellular membranes in response to membrane depolarization. The channel pore-forming subunit, α1, and a regulatory subunit (Ca(V)β) form a high affinity complex where Ca(V)β binds to a α1 interacting domain in the intracellular linker between α1 membrane domains I and II (I-II linker). We determined crystal structures of Ca(V)β2 functional core in complex with the Ca(V)1.2 and Ca(V)2.2 I-II linkers to a resolution of 1.95 and 2.0 Å, respectively. Structural differences between the highly conserved linkers, important for coupling Ca(V)β to the channel pore, guided mechanistic functional studies. Electrophysiological measurements point to the importance of differing linker structure in both Ca(V)1 and 2 subtypes with mutations affecting both voltage- and calcium-dependent inactivation and voltage dependence of activation. These linker effects persist in the absence of Ca(V)β, pointing to the intrinsic role of the linker in VDCC function and suggesting that I-II linker structure can serve as a brake during inactivation.

The N terminus of a schistosome beta subunit regulates inactivation and current density of a Cav2 channel

The β subunit of high voltage-activated Ca(2+) (Ca(v)) channels targets the pore-forming α(1) subunit to the plasma membrane and tunes the biophysical phenotype of the Ca(v) channel complex. We used a combination of molecular biology and whole-cell patch clamp to investigate the functional role of a long N-terminal polyacidic motif (NPAM) in a Ca(v)β subunit of the human parasite Schistosoma mansoni (β(Sm)), a motif that does not occur in other known Ca(v)β subunits. When expressed in human embryonic kidney cells stably expressing Ca(v)2.3, β(Sm) accelerates Ca(2+)/calmodulin-independent inactivation of Ca(v)2.3. Deleting the first 44 amino acids of β(Sm), a region that includes NPAM, significantly slows the predominant time constant of inactivation (τ(fast)) under conditions that prevent Ca(2+)/CaM-dependent inactivation (β(Sm): τ(fast) = 66 ms; β(SmΔ2-44): τ(fast) = 111 ms, p < 0.01). Interestingly, deleting the amino acids that are N-terminal to NPAM (2-24 or 2-17) results in faster inactivation than with an intact N terminus (τ(fast) = 42 ms with β(SmΔ2-17); τ(fast) = 40 ms with β(SmΔ2-24), p < 0.01). This suggests that NPAM is the structural determinant for accelerating Ca(2+)/calmodulin-independent inactivation. We also created three chimeric subunits that contain the first 44 amino acids of β(Sm) attached to mammalian β(1b), β(2a), and β(3) subunits. For any given mammalian β subunit, inactivation was faster if it contained the N terminus of β(Sm) than if it did not. Co-expression of the mammalian α(2)δ-1 subunit resulted in doubling of the inactivation rate, but the effects of NPAM persisted. Thus, it appears that the schistosome Ca(v) channel complex has acquired a new function that likely contributes to reducing the amount of Ca(2+) that enters the cells in vivo. This feature is of potential interest as a target for new antihelminthics.

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.

Determinants of the differential gating properties of Cav3.1 and Cav3.3 T-type channels: a role of domain IV?

We have investigated the channel structural determinants that underlie the difference in gating properties of Cav3.1 and Cav3.3 T-type channels, by creating a series of chimeric channel constructs in which the major transmembrane domains were swapped. The chimeras were then expressed in tsA-201 cells and subjected to whole cell patch clamp analysis. Our data reveal that domains I and IV are major determinants of the half-activation potential. Substitution of domain IV was the most important determinant of activation time constant and time constant for recovery from inactivation, with domains I and II mediating a smaller role. In contrast, the carboxy terminal region did not appear to be involved. Determinants of the time constant for inactivation could not be localized to a specific transmembrane domain, but the concomitant substitution of domains I+IV was able to partially confer the inactivation kinetics among the two wild type channels. Our data indicate that the domain IV region mediates an important role in T-type channel activation, whereas multiple channel structural determinants appear to control T-type channel inactivation.

A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel

The inactivation of voltage-gated L-type Ca(2+) channels (Ca(V)1) regulates Ca(2+) entry and controls intracellular Ca(2+) levels that are essential for cellular activity. The molecular entities implicated in L-channel (Ca(V)1.2) inactivation are not fully identified. Here we show for the first time the functional impact of one of the two highly conserved clusters of six negatively charged glutamates and aspartate (802-807; poly ED motif) at the II-III loop of the alpha 1 subunits of rabbit of Ca(v)1.2, alpha(1)1.2 and alpha(1)1.2 DeltaN60-Delta1733) on voltage-dependent inactivation. Mutation of the poly ED motif to alanine or glutamine/asparagine greatly enhanced voltage-dependent inactivation, shifting the voltage dependence to negative potentials by >50 mV and conferring a neuronal like inactivation kinetics onto Ca(V)1.2. The large shift in the midpoint of inactivation of the steady-state inactivation kinetics was observed also in Ca(2+) or Ba(2+) and was not altered by the beta2A subunit. Missing from the fast inactivating neuronal P/Q (Ca(V)2.1)-, N (Ca(V)2.2)- or R (Ca(V)2.3)-type channels and modulating Ca(V)1.2 inactivation kinetics, the poly ED motif is likely to be a specific L-type Ca(2+) channels inactivating domain. Our results fit a model in which the poly ED either by itself or as part of a larger inactivating motif acts as Ca(V)1.2 specific built-in "stopper." In this model, Ca(V)1 accomplishes a large Ca(2+) influx during depolarization, possibly by the poly ED hindering occlusion at the pore. Furthermore, the selective designed poly ED perhaps clarifies major inactivation differences between L- and non-L-type calcium channels.

Vascular calcium channels and high blood pressure: pathophysiology and therapeutic implications

Long-lasting Ca(2+) (Ca(L)) channels of the Ca(v)1.2 gene family are heteromultimeric structures that are minimally composed of a pore-forming alpha(1C) subunit and regulatory beta and alpha(2)delta subunits in vascular smooth muscle cells. The Ca(L) channels are the primary pathways for voltage-gated Ca(2+) influx that trigger excitation-contraction coupling in small resistance vessels. Notably, vascular smooth muscle cells of hypertensive rats show an increased expression of Ca(L) channel alpha(1C) subunits, which is associated with elevated Ca(2+) influx and the development of abnormal arterial tone. Indeed, blood pressure per se appears to promote Ca(L) channel expression in small arteries, and even short-term rises in pressure may alter channel expression. Membrane depolarization has been shown to be one stimulus associated with elevated blood pressure that promotes Ca(L) channel expression at the plasma membrane. Future studies to define the molecular processes that regulate Ca(L) channel expression in vascular smooth muscle cells will provide a rational basis for designing antihypertensive therapies to normalize Ca(L) channel expression and the development of anomalous vascular tone in hypertensive pathologies.

Auxiliary subunit regulation of high-voltage activated calcium channels expressed in mammalian cells

The effects of auxiliary calcium channel subunits on the expression and functional properties of high-voltage activated (HVA) calcium channels have been studied extensively in the Xenopus oocyte expression system, but are less completely characterized in a mammalian cellular environment. Here, we provide the first systematic analysis of the effects of calcium channel beta and alpha(2)-delta subunits on expression levels and biophysical properties of three different types (Ca(v)1.2, Ca(v)2.1 and Ca(v)2.3) of HVA calcium channels expressed in tsA-201 cells. Our data show that Ca(v)1.2 and Ca(v)2.3 channels yield significant barium current in the absence of any auxiliary subunits. Although calcium channel beta subunits were in principle capable of increasing whole cell conductance, this effect was dependent on the type of calcium channel alpha(1) subunit, and beta(3) subunits altogether failed to enhance current amplitude irrespective of channel subtype. Moreover, the alpha(2)-delta subunit alone is capable of increasing current amplitude of each channel type examined, and at least for members of the Ca(v)2 channel family, appears to act synergistically with beta subunits. In general agreement with previous studies, channel activation and inactivation gating was regulated both by beta and by alpha(2)-delta subunits. However, whereas pronounced regulation of inactivation characteristics was seen with the majority of the auxiliary subunits, effects on voltage dependence of activation were only small (< 5 mV). Overall, through a systematic approach, we have elucidated a previously underestimated role of the alpha(2)-delta(1) subunit with regard to current enhancement and kinetics. Moreover, the effects of each auxiliary subunit on whole cell conductance and channel gating appear to be specifically tailored to subsets of calcium channel subtypes.

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.

Multiple Structural Elements Contribute to the Slow Kinetics of the Cav3.3 T-type Channel

Molecular cloning and expression studies established the existence of three T-type Ca(2+) channel (Ca(v)3) alpha(1) subunits: Ca(v)3.1 (alpha(1G)), Ca(v)3.2 (alpha(1H)), and Ca(v)3.3 (alpha(1I)). Although all three channels are low voltage-activated, they display considerable differences in their kinetics, with Ca(v)3.1 and Ca(v)3.2 channels activating and inactivating much faster than Ca(v)3.3 channels. The goal of the present study was to determine the structural elements that confer the distinctively slow kinetics of Ca(v)3.3 channels. To address this question, a series of chimeric channels between Ca(v)3.1 and Ca(v)3.3 channels were constructed and expressed in Xenopus oocytes. Kinetic analysis showed that the slow activation and inactivation kinetics of the Ca(v)3.3 channel were not completely abolished by substitution with any one portion of the Ca(v)3.1 channel. Likewise, the Ca(v)3.1 channel failed to acquire the slow kinetics by simply adopting one portion of the Ca(v)3.3 channel. These findings suggest that multiple structural elements contribute to the slow kinetics of Ca(v)3.3 channels.

Functional roles of cytoplasmic loops and pore lining transmembrane helices in the voltage-dependent inactivation of HVA calcium channels

Voltage-dependent inactivation of calcium channels is a key mechanism for regulating intracellular calcium levels and neuronal excitability. In sodium and potassium channels, the molecular determinants that govern fast inactivation involve pore block by a cytoplasmic gating particle. As we discuss here, there is an increasing body of evidence that is consistent with a qualitatively similar inactivation mechanism in high-voltage-activated calcium channels. Work from a number of laboratories has implicated both cytoplasmic regions and the pore-lining S6 transmembrane helices in the inactivation process. Together with our recent findings, this leads us to propose a model in which the intracellular domain I-II linker region acts as a 'hinged lid' that physically occludes the pore by docking to the cytoplasmic ends of the S6 segments. We further propose that the ancillary calcium channel Beta subunits differentially modulate inactivation kinetics by binding to and thereby regulating the mobility of the putative inactivation gate. Indeed, additional evidence suggests that the carboxy terminus, amino terminus and domain III-IV linker regions of the channel modulate inactivation rates through interactions with the I-II linker per se, or indirectly via the ancillary Beta subunits. Taken together, the fast voltage-dependent inactivation of calcium channels appears reminiscent of that of sodium channels, but appears to show a more complex regulation through intramolecular interactions between the putative inactivation gate and other cytoplasmic regions.

G Protein Modulation of Voltage-Gated Calcium Channels

Calcium influx into any cell requires fine tuning to guarantee the correct balance between activation of calcium-dependent processes, such as muscle contraction and neurotransmitter release, and calcium-induced cell damage. G protein-coupled receptors play a critical role in negative feedback to modulate the activity of the CaV2 subfamily of the voltage-dependent calcium channels, which are largely situated on neuronal and neuro-endocrine cells. The basis for the specificity of the relationships among membrane receptors, G proteins, and effector calcium channels will be discussed, as well as the mechanism by which G protein-mediated inhibition is thought to occur. The inhibition requires free G beta gamma dimers, and the cytoplasmic linker between domains I and II of the CaV2 alpha 1 subunits binds G beta gamma dimers, whereas the intracellular N terminus of CaV2 alpha 1 subunits provides essential determinants for G protein modulation. Evidence suggests a key role for the beta subunits of calcium channels in the process of G protein modulation, and the role of a class of proteins termed "regulators of G protein signaling" will also be described.

Unique structure-activity relationship for 4-isoxazolyl-1,4-dihydropyridines

A series of 4-isoxazolyl-1,4-dihydropyridines (IDs) were prepared and characterized, and their interaction with the calcium channel was studied by patch clamp analysis. The structure-activity relationship (SAR) that emerges is distinct from the 4-aryldihydropyridines (DHPs), and affinity increases dramatically at higher holding potentials. Thus, among the 3'-arylisoxazolyl analogues p-Br > p-Cl >> p-F, and p-Cl > m-Cl > o-Cl >> o-MeO. Four of the analogues were examined by single-crystal X-ray diffractometry, and all were found to adopt an O-exo conformation in the solid state. The calculated barrier to rotation, however, suggests that rotation about the juncture between the heterocyclic rings is plausible under physiological conditions. A variable-temperature NMR study confirmed the computation. With Striessnig's computational sequence homologation procedure, a working hypothesis was derived from the data that explains the unique SAR for IDs.

Trafficking of L-type calcium channels mediated by the postsynaptic scaffolding protein AKAP79

Accurate calcium signaling requires spatial and temporal coordination of voltage-gated calcium channels (VGCCs) and a variety of signal transduction proteins. Accordingly, regulation of L-type VGCCs involves the assembly of complexes that include the channel subunits, protein kinase A (PKA), protein kinase A anchoring proteins (AKAPs), and beta2-adrenergic receptors, although the molecular details underlying these interactions remain enigmatic. We show here, by combining extracellular epitope splicing into the channel pore-forming subunit and immunoassays with whole cell and single channel electrophysiological recordings, that AKAP79 directly regulates cell surface expression of L-type calcium channels independently of PKA. This regulation involves a short polyproline sequence contained specifically within the II-III cytoplasmic loop of the channel. Thus we propose a novel mechanism whereby AKAP79 and L-type VGCCs function as components of a biosynthetic mechanism that favors membrane incorporation of organized molecular complexes in a manner that is independent of PKA phosphorylation events.

The interaction between the I-II loop and the III-IV loop of Cav2.1 contributes to voltage-dependent inactivation in a beta -dependent manner

We have investigated the molecular mechanisms whereby the I-II loop controls voltage-dependent inactivation in P/Q calcium channels. We demonstrate that the I-II loop is localized in a central position to control calcium channel activity through the interaction with several cytoplasmic sequences; including the III-IV loop. Several experiments reveal the crucial role of the interaction between the I-II loop and the III-IV loop in channel inactivation. First, point mutations of two amino acid residues of the I-II loop of Ca(v)2.1 (Arg-387 or Glu-388) facilitate voltage-dependent inactivation. Second, overexpression of the III-IV loop, or injection of a peptide derived from this loop, produces a similar inactivation behavior than the mutated channels. Third, the III-IV peptide has no effect on channels mutated in the I-II loop. Thus, both point mutations and overexpression of the III-IV loop appear to act similarly on inactivation, by competing off the native interaction between the I-II and the III-IV loops of Ca(v)2.1. As they are known to affect inactivation, we also analyzed the effects of beta subunits on these interactions. In experiments in which the beta(4) subunit is co-expressed, the III-IV peptide is no longer able to regulate channel inactivation. We conclude that (i) the contribution of the I-II loop to inactivation is partly mediated by an interaction with the III-IV loop and (ii) the beta subunits partially control inactivation by modifying this interaction. These data provide novel insights into the mechanisms whereby the beta subunit, the I-II loop, and the III-IV loop altogether can contribute to regulate inactivation in high voltage-activated calcium channels.

Molecular determinants of voltage-dependent slow inactivation of the Ca2+ channel

Ba(2+) current through the L-type Ca(2+) channel inactivates essentially by voltage-dependent mechanisms with fast and slow kinetics. Here we found that slow inactivation is mediated by an annular determinant composed of hydrophobic amino acids located near the cytoplasmic ends of transmembrane segments S6 of each repeat of the alpha(1C) subunit. We have determined the molecular requirements that completely obstruct slow inactivation. Critical interventions include simultaneous substitution of A752T in IIS6, V1165T in IIIS6, and I1475T in IVS6, each preventing in additive manner a considerable fraction of Ba(2+) current from inactivation. In addition, it requires the S405I mutation in segment IS6. The fractional inhibition of slow inactivation in tested mutants caused an acceleration of fast inactivation, suggesting that fast and slow inactivation mechanisms are linked. The channel lacking slow inactivation showed approximately 45% of the sustained Ba(2+) or Ca(2+) current with no indication of decay. The remaining fraction of the current was inactivated with a single-exponential decay (pi(f) approximately 10 ms), completely recovered from inactivation within 100 ms and did not exhibit Ca(2+)-dependent inactivation properties. No voltage-dependent characteristics were significantly changed, consistent with the C-type inactivation model suggesting constriction of the pore as the main mechanism possibly targeted by Ca(2+) sensors of inactivation.

Inactivation determinants in segment IIIS6 of Ca(v)3.1

1. Low threshold, T-type, Ca(2+) channels of the Ca(v)3 family display the fastest inactivation kinetics among all voltage-gated Ca(2+) channels. The molecular inactivation determinants of this channel family are largely unknown. Here we investigate whether segment IIIS6 plays a role in Ca(v)3.1 inactivation as observed previously in high voltage-activated Ca(2+) channels. 2. Amino acids that are identical in IIIS6 segments of all Ca(2+) channel subtypes were mutated to alanine (F1505A, F1506A, N1509A, F1511A, V1512A, F1519A, FV1511/1512AA). Additionally M1510 was mutated to isoleucine and alanine. 3. The kinetic properties of the mutants were analysed with the two-microelectrode voltage-clamp technique after expression in Xenopus oocytes. The time constant for the barium current (I(Ba)) inactivation, tau(inact), of wild-type channels at -20 mV was 9.5 +/- 0.4 ms; the corresponding time constants of the mutants ranged from 9.2 +/- 0.4 ms in V1512A to 45.7 +/- 5.2 ms (4.8-fold slowing) in M1510I. Recovery at -80 mV was most significantly slowed by V1512A and accelerated by F1511A. 4. We conclude that amino acids M1510, F1511 and V1512 corresponding to previously identified inactivation determinants in IIIS6 of Ca(v)2.1 (Hering et al. 1998) have a significant role in Ca(v)3.1 inactivation. These data suggest common elements in the molecular architecture of the inactivation mechanism in high and low threshold Ca(2+) channels.

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

A defining property of L-type Ca(2+) channels is their potentiation by both 1,4-dihydropyridine agonists and strong depolarization. In contrast, non-L-type channels are potentiated by neither agonist nor depolarization, suggesting that these two processes may by linked. In this study, we have tested whether the mechanisms of agonist- and depolarization-induced potentiation in the cardiac L-type channel (alpha(1C)) are linked. We found that the mutant L-type channel GFP-alpha(1C)(TQ-->YM), bearing the mutations T1066Y and Q1070M, was able to undergo depolarization-induced potentiation but not potentiation by agonist. Conversely, the chimeric channel GFP-CACC was potentiated by agonist but not by strong depolarization. These data indicate that the mechanisms of agonist- and depolarization-induced potentiation of alpha(1C) are distinct. Since neither GFP-CACC nor GFP-CCAA was potentiated significantly by depolarization, no single repeat of alpha(1C) appears to be responsible for depolarization-induced potentiation. Surprisingly, GFP-CACC displayed a low estimated open probability similar to that of the alpha(1C), but could not support depolarization-induced potentiation, demonstrating that a relatively low open probability alone is not sufficient for depolarization-induced potentiation to occur. Thus, depolarization-induced potentiation may be a global channel property requiring participation from all four homologous repeats.

Role of Repeat I in the fast inactivation kinetics of the Ca(V)2.3 channel

The molecular basis for inactivation in Ca(V)2.3 (alpha 1E) channels was studied after expression of alpha 1E/alpha 1C (Ca(V)2.3/Ca(V)1.2) chimeras in Xenopus oocytes. In the presence of 10 mM Ba(2+), the CEEE chimera (Repeat I+part of the I-II linker from Ca(V)1.2) displayed inactivation properties similar to Ca(V)1.2 despite being more than 90% homologous to Ca(V)2.3. The transmembrane segments of Repeat I did not appear to be crucial as inactivation of EC(IS1-6)EEE was not significantly different than Ca(V)2.3. In contrast, EC(AID)EEE, with the beta-subunit binding domain from Ca(V)1.2, tended to behave like Ca(V)1.2 in terms of inactivation kinetics and voltage dependence. A detailed kinetic analysis revealed nonetheless that CEEE and EC(AID)EEE retained the fast inactivation time constant (tau(fast) approximately equal to 20-30 ms) that is a distinctive feature of Ca(V)2.3. Altogether, these data suggest that the region surrounding the AID binding site plays a pivotal albeit not exclusive role in determining the inactivation properties of Ca(V)2.3.

Identification of inactivation determinants in the domain IIS6 region of high voltage-activated calcium channels

We have recently reported that transfer of the domain IIS6 region from rapidly inactivating R-type (alpha(1E)) calcium channels to slowly inactivating L-type (alpha(1C)) calcium channel confers rapid inactivation (Stotz, S. C., Hamid, J., Spaetgens, R. L., Jarvis, S. E., and Zamponi, G. W. (2000) J. Biol. Chem. 275, 24575-24582). Here we have identified individual amino acid residues in the IIS6 regions that are responsible for these effects. In this region, alpha(1C) and alpha(1E) channels differ in seven residues, and exchanging five of those residues individually or in combination did not significantly affect inactivation kinetics. By contrast, replacement of residues Phe-823 or Ile-829 of alpha(1C) with the corresponding alpha(1E) residues significantly accelerated inactivation rates and, when substituted concomitantly, approached the rapid inactivation kinetics of R-type channels. A systematic substitution of these residues with a series of other amino acids revealed that decreasing side chain size at position 823 accelerates inactivation, whereas a dependence of the inactivation kinetics on the degree of hydrophobicity could be observed at position 829. Although these point mutations facilitated rapid entry into the inactivated state of the channel, they had little to no effect on the rate of recovery from inactivation. This suggests that the development of and recovery from inactivation are governed by separate structural determinants. Finally, the effects of mutations that accelerated alpha(1C) inactivation could still be antagonized following coexpression of the rat beta(2a) subunit or by domain I-II linker substitutions that produce ultra slow inactivation of wild type channels, indicating that the inactivation kinetics seen with the mutants remain subject to regulation by the domain I-II linker. Overall, our results provide novel insights into a complex process underlying calcium channel inactivation.

Interaction of SNX482 with domains III and IV inhibits activation gating of alpha(1E) (Ca(V)2.3) calcium channels

We have investigated the action of SNX482, a toxin isolated from the venom of the tarantula Hysterocrates gigas, on voltage-dependent calcium channels expressed in tsa-201 cells. Upon application of 200 nM SNX482, R-type alpha(1E) calcium channels underwent rapid and complete inhibition, which was only poorly reversible upon washout. However, upon application of strong membrane depolarizations, rapid and complete recovery from inhibition was obtained. Tail current analysis revealed that SNX482 mediated an approximately 70 mV depolarizing shift in half-activation potential, suggesting that the toxin inhibits alpha(1E) calcium channels by preventing their activation. Experiments involving chimeric channels combining structural features of alpha(1E) and alpha(1C) subunits indicated that the presence of the domain III and IV of alpha(1E) is a prerequisite for a strong gating inhibition. In contrast, L-type alpha(1C) channels underwent incomplete inhibition at saturating concentrations of SNX482 that was paralleled by a small shift in half-activation potential and which could be rapidly reversed, suggesting a less pronounced effect of the toxin on L-type calcium channel gating. We conclude that SNX482 does not exhibit unequivocal specificity for R-type channels, but highly effectively antagonizes their activation.

Multiple structural elements contribute to voltage-dependent facilitation of neuronal alpha 1C (CaV1.2) L-type calcium channels

Voltage- and frequency-dependent facilitation of calcium channel activity has been implicated in a number of key physiological processes. Various mechanisms have been proposed to mediate these regulations, including a switch between channel gating modes, voltage-dependent phosphorylation, and a voltage-dependent deinhibition of G-protein block. Studying such modulation on recombinant Ca channels expressed in oocytes, we previously reported that alpha(1C) L-type calcium channel contrast with non-L type Ca channels by its ability to exhibit facilitation by pre-depolarization (Voltage-dependent facilitation of a neuronal alpha(IC) L-type calcium channel, E. Bourinet et al., EMBO Journal, 1994; 13, 5032-5039). To further analyze this effect, we have investigated the molecular determinants which mediate the differences in voltage-dependent facilitation between "facilitable" alpha(1C) and "non facilitable" alpha(1E) calcium channels. We used a series of chimeras which combine the four transmembrane domains of the two channels. Results show that the four domains of alpha(1C) contribute to facilitation, with domain I being most critical. This domain is required but not sufficient alone to generate facilitation. The minimal requirement to observe the effect is the presence of domain I plus one of the three others. We conclude that similarly to activation gating, voltage-dependent facilitation of alpha(1C) is a complex process which involves multiple structural elements were domains I and III play the major role.

The [beta]2a subunit is a molecular groom for the Ca2+ channel inactivation gate

Ca(2+) channel inactivation is a key element in controlling the level of Ca(2+) entry through voltage-gated Ca(2+) channels. Interaction between the pore-forming alpha(1) subunit and the auxiliary beta subunit is known to be a strong modulator of voltage-dependent inactivation. Here, we demonstrate that an N-terminal membrane anchoring site (MAS) of the beta(2a) subunit strongly reduces alpha(1A) (Ca(V)2.1) Ca(2+) channel inactivation. This effect can be mimicked by the addition of a transmembrane segment to the N terminus of the beta(2a) subunit. Inhibition of inactivation by beta(2a) also requires a link between MAS and another important molecular determinant, the beta interaction domain (BID). Our data suggest that mobility of the Ca(2+) channel I-II loop is necessary for channel inactivation. Interaction of this loop with other identified intracellular channel domains may constitute the basis of voltage-dependent inactivation. We thus propose a conceptually novel mechanism for slowing of inactivation by the beta(2a) subunit, in which the immobilization of the channel inactivation gate occurs by means of MAS and BID.

The status of voltage-dependent calcium channels in alpha 1E knock-out mice

It has been hypothesized that R-type Ca currents result from the expression of the alpha(1E) gene. To test this hypothesis we examined the properties of voltage-dependent Ca channels in mice in which the alpha(1E) Ca channel subunit had been deleted. Application of omega-conotoxin GVIA, omega-agatoxin IVA, and nimodipine to cultured cerebellar granule neurons from wild-type mice inhibited components of the whole-cell Ba current, leaving a "residual" R current with an amplitude of approximately 30% of the total Ba current. A minor portion of this R current was inhibited by the alpha(1E)-selective toxin SNX-482, indicating that it resulted from the expression of alpha(1E). However, the majority of the R current was not inhibited by SNX-482. The SNX-482-sensitive portion of the granule cell R current was absent from alpha(1E) knock-out mice. We also identified a subpopulation of dorsal root ganglion (DRG) neurons from wild-type mice that expressed an SNX-482-sensitive component of the R current. However as with granule cells, most of the DRG R current was not blocked by SNX-482. We conclude that there exists a component of the R current that results from the expression of the alpha(1E) Ca channel subunit but that the majority of R currents must result from the expression of other Ca channel alpha subunits.

Molecular determinant for run-down of L-type Ca2+ channels localized in the carboxyl terminus of the 1C subunit

1. The role of the sequence 1572-1651 in the C-terminal tail of the alpha1C subunit in run-down of Ca2+ channels was studied by comparing functional properties of the conventional alpha1C,77 channel with those of three isoforms carrying alterations in this motif. 2. The pore-forming alpha1C subunits were co-expressed with alpha2delta and beta2a subunits in HEK-tsA201 cells, a subclone of the human embryonic kidney cell line, and studied by whole-cell and single-channel patch-clamp techniques. 3. Replacement of amino acids 1572-1651 in alpha1C,77 with 81 different amino acids leading to alpha1C,86 significantly altered run-down behaviour. Run-down of Ba2+ currents was rapid with alpha1C,77 channels, but was slow with alpha1C,86. 4. Transfer of the alpha1C,86 segments L (amino acids 1572-1598) or K (amino acids 1595-1652) into the alpha1C,77 channel yielded alpha1C,77L and alpha1C,77K channels, respectively, the run-down of which resembled more that of alpha1C,77. These results demonstrate that a large stretch of sequence between residues 1572 and 1652 of alpha1C,86 renders Ca2+ channels markedly resistant to run-down. 5. The protease inhibitor calpastatin added together with ATP was able to reverse the run-down of alpha1C,77 channels. Calpastatin expression was demonstrated in the HEK-tsA cells by Western blot analysis. 6. These results indicate a significant role of the C-terminal sequence 1572-1651 of the alpha1C subunit in run-down of L-type Ca2+ channels and suggest this sequence as a target site for a modulatory effect by endogenous calpastatin.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.

Fast inactivation of voltage-dependent calcium channels. A hinged-lid mechanism?

We recently described domains II and III as important determinants of fast, voltage-dependent inactivation of R-type calcium channels (Spaetgens, R. L., and Zamponi, G. W. (1999) J. Biol. Chem. 274, 22428-22438). Here we examine in greater detail the structural determinants of inactivation using a series of chimeras comprising various regions of wild type alpha(1C) and alpha(1E) calcium channels. Substitution of the II S6 and/or III S6 segments of alpha(1E) into the alpha(1C) backbone resulted in rapid inactivation rates that closely approximated those of wild type alpha(1E) channels. However, neither individual or combined substitution of the II S6 and III S6 segments could account for the 60 mV more negative half-inactivation potential seen with wild type alpha(1E) channels, indicating that the S6 regions contribute only partially to the voltage dependence of inactivation. Interestingly, the converse replacement of alpha(1E) S6 segments of domains II, III, or II+III with those of alpha(1C) was insufficient to significantly slow inactivation rates. Only when the I-II linker region and the domain II and III S6 regions of alpha(1E) were concomitantly replaced with alpha(1C) sequence could inactivation be abolished. Conversely, introduction of the alpha(1E) domain I-II linker sequence into alpha(1C) conferred alpha(1E)-like inactivation rates, indicating that the domain I-II linker is a key contributor to calcium channel inactivation. Overall, our data are consistent with a mechanism in which inactivation of voltage-dependent calcium channels may occur via docking of the I-II linker region to a site comprising, at least in part, the domain II and III S6 segments.

The alpha1B Ca2+ channel amino terminus contributes determinants for beta subunit-mediated voltage-dependent inactivation properties

Co-expression of auxiliary beta subunits with the alpha1B Ca2+ channel subunit in COS-7 cells resulted in an increase in current density and a hyperpolarising shift in the mid-point of activation. Amongst the beta subunits, beta2a in particular, but also beta4 and beta1b caused a significant retardation of the voltage-dependent inactivation compared to currents with alpha1B alone, whilst no significant changes in inactivation properties were seen for the beta3 subunit in this system. Prevention of beta2a palmitoylation, by introducing cysteine to serine mutations (beta2a(C3,4S)), greatly reduced the ability of beta2a to retard voltage-dependent inactivation. Deletion of the proximal half of the alpha1B cytoplasmic amino terminus (alpha1BDelta1-55) differentially affected beta subunit-mediated voltage-dependent inactivation properties. These effects were prominent with the beta2a subunit and, to a lesser extent, with beta1b. For beta2a, the major effects of this deletion were a partial reversal of beta2a-mediated retardation of inactivation and the introduction of a fast component of inactivation, not seen with full-length alpha1B. Deletion of the amino terminus had no other major effects on the measured biophysical properties of alpha1B when co-expressed with beta subunits. Transfer of the whole alpha1B amino terminus into alpha1C (alpha1bCCCC) conferred a similar retardation of inactivation on alpha1C when co-expressed with beta2a to that seen in parental alpha1B. Individual (alpha1B(Q47A) and alpha1B(R52A)) and double (alpha1B(R52,54A)) point mutations within the amino terminus of alpha1B also opposed the beta2a-mediated retardation of alpha1B inactivation kinetics. These results indicate that the alpha1B amino terminus contains determinants for beta subunit-mediated voltage-dependent inactivation properties. Furthermore, effects were beta subunit selective. As deletion of the alpha1B amino terminus only partially opposed beta subunit-mediated changes in inactivation properties, the amino terminus is likely to contribute to a complex site necessary for complete beta subunit function.