Stable expression of the neuronal BI (class A) calcium channel in baby hamster kidney cells

Rab3 interacting molecule 3 mutations associated with autism alter regulation of voltage-dependent Ca²⁺ channels

Autism is a neurodevelopmental psychiatric disorder characterized by impaired reciprocal social interaction, disrupted communication, and restricted and stereotyped patterns of interests. Autism is known to have a strong genetic component. Although mutations in several genes account for only a small proportion of individuals with autism, they provide insight into potential biological mechanisms that underlie autism, such as dysfunction in Ca(2+) signaling, synaptic dysfunction, and abnormal brain connectivity. In autism patients, two mutations have been reported in the Rab3 interacting molecule 3 (RIM3) gene. We have previously demonstrated that RIM3 physically and functionally interacts with voltage-dependent Ca(2+) channels (VDCCs) expressed in neurons via the β subunits, and increases neurotransmitter release. Here, by introducing corresponding autism-associated mutations that replace glutamic acid residue 176 with alanine (E176A) and methionine residue 259 with valine (M259V) into the C2B domain of mouse RIM3, we demonstrate that both mutations partly cancel the suppressive RIM3 effect on voltage-dependent inactivation of Ba(2+) currents through P/Q-type CaV2.1 recombinantly expressed in HEK293 cells. In recombinant N-type CaV2.2 VDCCs, the attenuation of the suppressive RIM3 effect on voltage-dependent inactivation is conserved for M259V but not E176A. Slowing of activation speed of P/Q-type CaV2.1 currents by RIM3 is abolished in E176A, while the physical interaction between RIM3 and β subunits is significantly attenuated in M259V. Moreover, increases by RIM3 in depolarization-induced Ca(2+) influx and acetylcholine release are significantly attenuated by E176A in rat pheochromocytoma PC12 cells. Thus, our data raise the interesting possibility that autism phenotypes are elicited by synaptic dysfunction via altered regulation of presynaptic VDCC function and neurotransmitter release.

BARP suppresses voltage-gated calcium channel activity and Ca2+-evoked exocytosis

Voltage-gated calcium channels (VGCCs) are key regulators of cell signaling and Ca(2+)-dependent release of neurotransmitters and hormones. Understanding the mechanisms that inactivate VGCCs to prevent intracellular Ca(2+) overload and govern their specific subcellular localization is of critical importance. We report the identification and functional characterization of VGCC β-anchoring and -regulatory protein (BARP), a previously uncharacterized integral membrane glycoprotein expressed in neuroendocrine cells and neurons. BARP interacts via two cytosolic domains (I and II) with all Cavβ subunit isoforms, affecting their subcellular localization and suppressing VGCC activity. Domain I interacts at the α1 interaction domain-binding pocket in Cavβ and interferes with the association between Cavβ and Cavα1. In the absence of domain I binding, BARP can form a ternary complex with Cavα1 and Cavβ via domain II. BARP does not affect cell surface expression of Cavα1 but inhibits Ca(2+) channel activity at the plasma membrane, resulting in the inhibition of Ca(2+)-evoked exocytosis. Thus, BARP can modulate the localization of Cavβ and its association with the Cavα1 subunit to negatively regulate VGCC activity.

Physical and functional interaction of the active zone protein CAST/ERC2 and the β-subunit of the voltage-dependent Ca(2+) channel

In the nerve terminals, the active zone protein CAST/ERC2 forms a protein complex with the other active zone proteins ELKS, Bassoon, Piccolo, RIM1 and Munc13-1, and is thought to play an organizational and functional role in neurotransmitter release. However, it remains obscure how CAST/ERC2 regulates the Ca(2+)-dependent release of neurotransmitters. Here, we show an interaction of CAST with voltage-dependent Ca(2+) channels (VDCCs), which are essential for regulating neurotransmitter release triggered by depolarization-induced Ca(2+) influx at the active zone. Using a biochemical assay, we showed that CAST was coimmunoprecipitated with the VDCC β(4)-subunit from the mouse brain. A pull-down assay revealed that the VDCC β(4)-subunit interacted directly with at least the N- and C-terminal regions of CAST. The II-III linker of VDCC α(1)-subunit also interacted with C-terminal regions of CAST; however, the interaction was much weaker than that of β(4)-subunit. Furthermore, coexpression of CAST and VDCCs in baby hamster kidney cells caused a shift in the voltage dependence of activation towards the hyperpolarizing direction. Taken together, these results suggest that CAST forms a protein complex with VDCCs, which may regulate neurotransmitter release partly through modifying the opening of VDCCs at the presynaptic active zones.

Rab3-interacting molecule gamma isoforms lacking the Rab3-binding domain induce long lasting currents but block neurotransmitter vesicle anchoring in voltage-dependent P/Q-type Ca2+ channels

Assembly of voltage-dependent Ca(2+) channels (VDCCs) with their associated proteins regulates the coupling of VDCCs with upstream and downstream cellular events. Among the four isoforms of the Rab3-interacting molecule (RIM1 to -4), we have previously reported that VDCC beta-subunits physically interact with the long alpha isoform of the presynaptic active zone scaffolding protein RIM1 (RIM1alpha) via its C terminus containing the C(2)B domain. This interaction cooperates with RIM1alpha-Rab3 interaction to support neurotransmitter exocytosis by anchoring vesicles in the vicinity of VDCCs and by maintaining depolarization-triggered Ca(2+) influx as a result of marked inhibition of voltage-dependent inactivation of VDCCs. However, physiological functions have not yet been elucidated for RIM3 and RIM4, which exist only as short gamma isoforms (gamma-RIMs), carrying the C-terminal C(2)B domain common to RIMs but not the Rab3-binding region and other structural motifs present in the alpha-RIMs, including RIM1alpha. Here, we demonstrate that gamma-RIMs also exert prominent suppression of VDCC inactivation via direct binding to beta-subunits. In the pheochromocytoma PC12 cells, this common functional feature allows native RIMs to enhance acetylcholine secretion, whereas gamma-RIMs are uniquely different from alpha-RIMs in blocking localization of neurotransmitter-containing vesicles near the plasma membrane. Gamma-RIMs as well as alpha-RIMs show wide distribution in central neurons, but knockdown of gamma-RIMs attenuated glutamate release to a lesser extent than that of alpha-RIMs in cultured cerebellar neurons. The results suggest that sustained Ca(2+) influx through suppression of VDCC inactivation by RIMs is a ubiquitous property of neurons, whereas the extent of vesicle anchoring to VDCCs at the plasma membrane may depend on the competition of alpha-RIMs with gamma-RIMs for VDCC beta-subunits.

Two novel alleles of tottering with distinct Ca(v)2.1 calcium channel neuropathologies

The calcium channel CACNA1A gene encodes the pore-forming, voltage-sensitive subunit of the voltage-dependent calcium Ca(v)2.1 type channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine, episodic ataxia 2 and spinocerebellar ataxia type 6. The mouse homologue, Cacna1a, is associated with the tottering, Cacna1a(tg), mutant series. Here we describe two new missense mutant alleles, Cacna1a(tg-4J) and Cacna1a(Tg-5J). The Cacna1a(tg-4J) mutation is a valine to alanine mutation at amino acid 581, in segment S5 of domain II. The recessive Cacna1a(tg-4J) mutant exhibited the ataxia, paroxysmal dyskinesia and absence seizures reminiscent of the original tottering mouse. The Cacna1a(tg-4J) mutant also showed altered activation and inactivation kinetics of the Ca(v)2.1 channel, not previously reported for other tottering alleles. The semi-dominant Cacna1a(Tg-5J) mutation changed a conserved arginine residue to glutamine at amino acid 1252 within segment S4 of domain III. The heterozygous mouse was ataxic and homozygotes rarely survived. The Cacna1a(Tg-5J) mutation caused a shift in both voltage activation and inactivation to lower voltages, showing that this arginine residue is critical for sensing Ca(v)2.1 voltage changes. These two tottering mouse models illustrate how novel allelic variants can contribute to functional studies of the Ca(v)2.1 calcium channel.

Effects of Calmodulin and Ca2+ Channel Blockers on ω-conotoxin GVI A Binding to Crude Membranes from α1B Subunit (Cav2.2) Expressed BHK Cells and Mice Brain Lacking the α1B Subunits

Characteristics for the specific binding of 125I-omega-CTX GVIA and 125I-omega-CTX MVIIC to crude membranes from BHKN101 cells expressing the alpha1B subunits of Cav2.2 channels and from mice brain lacking the alpha1B subunits of Cav2.2 channels, particularly, the effects of CaM and various Ca2+ channel blockers on these specific bindings were investigated. Specific binding of 125I-omega-CTX GVIA to the crude membranes from BHKN101 cells was observed, but not from control BHK6 cells. omega-CTX GVIA, omega-CTX MVIIC and omega-CTX SVIB inhibited the specific binding of 125I-omega-CTX GVIA to crude membranes from BHKN101 cells, and the IC50 values for omega-CTXGVIA, omega-CTX MVIIC and omega-CTX SVIB were 0.07, 8.5 and 1.7 nM, respectively. However, omega-agatoxin IVA and calciseptine at concentrations of 10(-9)-10(-6) M did not inhibit specific binding. Specific binding was also about 80% inhibited by 20 microg protein/ml CaM. The amount of 125I-omega-CTX GVIA (30 pM) specifically bound to membranes from brain of knockout mice lacking alpha1B subunits of Cav2.2 channels was about 30% of that to the crude membranes from brain of wild-type. On the other hand, specific binding of 125I-omega-CTX MVIIC (200 pM) was observed on the crude membranes of both BHKN101 and control BHK6 cells. The specific binding of 125I-omega-CTX MVIIC (200 pM) was not inhibited by omega-CTX GVIA and omega-CTX SVIB, and also omega-Aga IVA and calciseptine at concentrations of 10(-9)-10(-7) M, although specific binding was almost completely dose dependently inhibited by non-radiolabeled omega-CTX MVIIC (IC50 value was about 0.1 nM). 20 microg protein/ml CaM did not inhibit specific binding. Therefore, these results suggest that BHKN101 cells have a typical Cav2.2 channels which are also inhibited by CaM and have not specific binding sites for omega-CTX MVIIC, although omega-CTX MVIIC is a blocker for both Cav2.1 (alpha1A; P/Q-type) and Cav2.2 channels.

Polyglutamine repeats of spinocerebellar ataxia 6 impair the cell-death-preventing effect of CaV2.1 Ca2+ channel—loss-of-function cellular model of SCA6

Spinocerebellar ataxia (SCA) 6 is caused by small expansion of a polyglutamine sequence, encoded by CAG trinucleotide repeats, at the C-terminal end of the human CaV2.1 (P/Q-type) Ca2+ channel alpha12.1 subunit and it manifests itself as slowly progressive cerebellar ataxia. To elucidate the pathogenic mechanisms underlying SCA6, we introduced CAG repeats of various lengths into the Ca2+ channel alpha12.1 subunit cDNA and expressed them in baby hamster kidney cells stably expressing the auxiliary subunits (alpha2delta and beta4). The occurrence of cell death differed between cells transfected with the normal and mutant Ca2+ channels under the condition of serum starvation plus potassium-induced depolarization, and Cdk inhibition elucidated the differences more clearly. The CaV2.1 (P/Q-type) Ca2+ channel-specific blocker omega-agatoxin IVA abolished the cell-death-preventing effect of the normal Ca2+ channel. Together with our previous finding that the polyglutamine expansion in SCA6 interferes with the Ca2+ channel to reduce Ca2+ influx, these results indicate that impaired function of the mutant Ca2+ channels rendered them unable to prevent cell death.

FPL 64176 modification of Ca(V)1.2 L-type calcium channels: dissociation of effects on ionic current and gating current

FPL 64176 (FPL) is a nondihydropyridine compound that dramatically increases macroscopic inward current through L-type calcium channels and slows activation and deactivation. To understand the mechanism by which channel behavior is altered, we compared the effects of the drug on the kinetics and voltage dependence of ionic currents and gating currents. Currents from a homogeneous population of channels were obtained using cloned rabbit Ca(V)1.2 (alpha1C, cardiac L-type) channels stably expressed in baby hamster kidney cells together with beta1a and alpha2delta1 subunits. We found a striking dissociation between effects of FPL on ionic currents, which were modified strongly, and on gating currents, which were not detectably altered. Inward ionic currents were enhanced approximately 5-fold for a voltage step from -90 mV to +10 mV. Kinetics of activation and deactivation were slowed dramatically at most voltages. Curiously, however, at very hyperpolarized voltages (< -250 mV), deactivation was actually faster in FPL than in control. Gating currents were measured using a variety of inorganic ions to block ionic current and also without blockers, by recording gating current at the reversal potential for ionic current (+50 mV). Despite the slowed kinetics of ionic currents, FPL had no discernible effect on the fundamental movements of gating charge that drive channel gating. Instead, FPL somehow affects the coupling of charge movement to opening and closing of the pore. An intriguing possibility is that the drug causes an inactivated state to become conducting without otherwise affecting gating transitions.

Phoneutria nigriventer omega-phonetoxin IIA blocks the Cav2 family of calcium channels and interacts with omega-conotoxin-binding sites

omega-Phonetoxin IIA (omegaPtxIIA), a peptide from spider venom (Phoneutria nigriventer), inhibits high threshold voltage-dependent calcium currents in neurons. To define its pharmacological specificity, we have used patch-clamp methods in cell lines expressing recombinant Ca(v)2.1, Ca(v)2.2, and Ca(v)2.3 channels (P/Q-, N-, and R-type currents, respectively). Calcium currents generated by Ca(v)2.1 and Ca(v)2.2 were blocked almost irreversibly by 3 nm omegaPtxIIA, whereas Ca(v)2.3 showed partial and readily reversible inhibition. Binding assays with mono[(125)I]iodo-omegaPtxIIA indicated that membranes expressing recombinant Ca(v)2.1 or Ca(v)2.2 channels showed a single class of sites with similar affinity (K(D) approximately 50 pm), whereas low affinity interactions were detectable with Ca(v)2.3. Kinetic, saturation, and displacement assays demonstrated that rat brain synaptosomes displayed multiple classes of binding sites for (125)I-omegaPtxIIA. High affinity binding of (125)I-omegaPtxIIA was totally displaced by omegaPtxIIA (K(i) = 100 pm), but only partially by omega-conotoxin GVIA (25% inhibition) and omega-conotoxin MVIIC (50% inhibition at 0.3 microm). (125)I-omegaPtxIIA thus defines a unique high affinity binding site that is predominantly associated with Ca(v)2.1 or Ca(v)2.2 channels.

Nitric oxide augments voltage-gated P/Q-type Ca(2+) channels constituting a putative positive feedback loop

P/Q-type Ca(2+) channels, which are postulated to play major roles in synaptic transmission, are regulated in a variety of ways. Ca(2+) currents through P/Q-type Ca(2+) channels (Ca(v)2.1/beta(1a)/alpha(2)delta) heterologously expressed in mammalian cells were recorded using the whole-cell patch clamp method. The oxidant H(2)O(2) increased the current amplitude and the effect was reversed by the reducing agent dithiothreitol (DTT). The stimulatory effect of H(2)O(2) on the Ca(2+) current was mimicked by the NO donors, SNAP, and diethylamine NONOate, and reversed by the reducing agent DTT. The presence of a soluble guanylate cyclase inhibitor did not abolish the ability of SNAP to increase the Ca(2+) current. Adenovirus-mediated overexpression of nitric oxide synthase in combination with application of the Ca(2+) ionophore A23187 also increased the Ca(2+) current amplitude and the effect was again reversed by DTT. The NOS inhibitor L-NAME abolished the stimulatory effect of A23187, and A23187 did not change the Ca(2+) currents in the cells treated with control adenovirus particles. The time course of the decline of the Ca(2+) current, but not of the Ba(2+) current, in response to repeated depolarization was markedly slowed by adenovirus-mediated overexpression of nitric oxide synthase. The results demonstrate that nitric oxide enhances the channel activity by promoting oxidation and suggest that Ca(2+), nitric oxide synthase, and nitric oxide could constitute a positive feedback loop for regulation of voltage-gated P/Q-type Ca(2+) channels.

Regulation of Ca2+ channel expression at the cell surface by the small G-protein kir/Gem

Voltage-dependent calcium (Ca2+) channels are involved in many specialized cellular functions, and are controlled by intracellular signals such as heterotrimeric G-proteins, protein kinases and calmodulin (CaM). However, the direct role of small G-proteins in the regulation of Ca2+ channels is unclear. We report here that the GTP-bound form of kir/Gem, identified originally as a Ras-related small G-protein that binds CaM, inhibits high-voltage-activated Ca2+ channel activities by interacting directly with the beta-subunit. The reduced channel activities are due to a decrease in alpha1-subunit expression at the plasma membrane. The binding of Ca2+/CaM to kir/Gem is required for this inhibitory effect by promoting the cytoplasmic localization of kir/Gem. Inhibition of L-type Ca2+ channels by kir/Gem prevents Ca2+-triggered exocytosis in hormone-secreting cells. We propose that the small G-protein kir/Gem, interacting with beta-subunits, regulates Ca2+ channel expression at the cell surface.

Role of Thr(11) in the binding of omega-conotoxin MVIIC to N-type Ca2+ channels

As replacement of Thr(11) of omega-conotoxin MVIIC with Ala significantly reduced the affinity for both N- and P/Q-type calcium channels, we examined the effect of substitution at this position with other residues. Binding assays using rat cerebellar P2 membranes showed that the affinity is in the order of Leu>Val, aminobutyric acid, Thr>Asn&z.Gt;Ser, Ala, Asp, Phe, Tyr for N-type channels and Thr>Leu, Val, aminobutyric acid, Asn, Ser>Ala&z.Gt;Asp, Phe, Tyr for P/Q-type channels, suggesting that aliphatic amino acids with longer side chains are favorable for block of N-type channels. The effects of substitution were examined electrophysiologically in BHK cells expressing N-type Ca2+ channels. Inhibition of Ba2+ current by the analogs did not completely correlate with binding affinity, although binding to BHK cells was comparable to rat cerebellar membranes.

Low-affinity blockade of neuronal N-type Ca channels by the spider toxin omega-agatoxin-IVA

The recognition of neuronal Ca channel diversity has led to considerable efforts to identify useful classification criteria. Here, we revisit the pharmacological definition of P- and Q-type Ca channels, which is based on their respective high and low sensitivity to the spider omega-agatoxin-IVA (omega-Aga-IVA), using whole-cell recordings of the Ca channel currents carried by 5 mM Ba(2+) in isolated rat subthalamic and sympathetic neurons. In subthalamic neurons, omega-Aga-IVA (1 microM) targeted multiple Ca channels. One population was blocked with high potency. These channels carried 50.4 +/- 3.4% (n = 5) of the control current and showed the same inactivation kinetics and voltage-dependent high affinity for omega-Aga-IVA as do prototypic P-type Ca channels. Other Ca channels were targeted with weaker potency. This heterogeneous population contributed to 14.0 +/- 1.7% (n = 5) of the control current. It included N-type Ca channels as well as high-threshold Ca channels that displayed the pharmacological signature of Q-type Ca channels but resembled P-type Ca channels in their gating properties. N-type Ca current block by omega-Aga-IVA (1 microM) was further investigated in sympathetic neurons, which mainly express this Ca channel type. Block was incomplete ( approximately 30% of the control current). Its relief at positive potentials was consistent with omega-Aga-IVA acting as a channel-gating modifier. These effects did not reflect a complete loss of selectivity, because omega-Aga-IVA (1 microM) had no effect on subthalamic Na and K currents or their T- and L-type Ca currents. Our data confirm that omega-Aga-IVA is a selective P-type Ca channel blocker. However, its diminished selectivity in the micromolar range limits its usefulness for functional studies of Q-type Ca channels.

Reduced voltage sensitivity of activation of P/Q-type Ca2+ channels is associated with the ataxic mouse mutation rolling Nagoya (tg(rol))

Recent genetic analyses have revealed an important association of the gene encoding the P/Q-type voltage-dependent Ca(2+) channel alpha(1A) subunit with hereditary neurological disorders. We have identified the ataxic mouse mutation, rolling Nagoya (tg(rol)), in the alpha(1A) gene that leads to a charge-neutralizing arginine-to-glycine substitution at position 1262 in the voltage sensor-forming segment S4 in repeat III. Ca(2+) channel currents in acutely dissociated Purkinje cells, where P-type is the dominant type, showed a marked decrease in slope and a depolarizing shift by 8 mV of the conductance-voltage curve and reduction in current density in tg(rol) mouse cerebella, compared with those in wild-type. Compatible functional change was induced by the tg(rol) mutation in the recombinant alpha(1A) channel, indicating that a defect in voltage sensor of P/Q-type Ca(2+) channels is the direct consequence of the tg(rol) mutation. Furthermore, somatic whole-cell recording of mutant Purkinje cells displayed only abortive Na(+) burst activity and hardly exhibited Ca(2+) spike activity in cerebellar slices. Thus, in tg(rol) mice, reduced voltage sensitivity, which may derive from a gating charge defect, and diminished activity of the P-type alpha(1A) Ca(2+) channel significantly impair integrative properties of Purkinje neurons, presumably resulting in locomotor deficits.

Ablation of P/Q-type Ca(2+) channel currents, altered synaptic transmission, and progressive ataxia in mice lacking the alpha(1A)-subunit

The Ca(2+) channel alpha(1A)-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca(2+) channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. alpha(1A)-Subunits are thought to support both P- and Q-type Ca(2+) channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca(2+) delivery system at excitatory nerve terminals. We generated alpha(1A)-deficient mice (alpha(1A)(-/-)) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying approximately 3-4 weeks after birth. P-type currents in Purkinje neurons and P- and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca(2+) channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in alpha(1A)(-/-) hippocampal slices persisted despite the lack of P/Q-type channels but showed enhanced reliance on N-type and R-type Ca(2+) entry. The alpha(1A)(-/-) mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in alpha(1A).

Phosphodiesterase 4 conformers: preparation of recombinant enzymes and assay for inhibitors

Cyclic nucleotides are key regulators of many cellular processes. Their immediate action is terminated through the activity of phosphodiesterases, a diverse family of enzymes. This diversity has given rise to drug discovery opportunities, and assay technology is therefore of key importance. Inhibitors of the cyclic-AMP-specific phosphodiesterases (the PDE4 family) are drug candidates for a variety of inflammatory disorders. However, PDE4 inhibitors, besides their immunomodulatory effects, also cause side effects including nausea and emesis. Recently, it has been suggested that PDE4 exists in two different conformations with respect to inhibition by the prototypical compound rolipram. Inhibition of the low-affinity conformer is thought to give rise to anti-inflammatory effects, and inhibition of the high-affinity conformer to side effects. Therefore, a selective inhibitor of the low-affinity conformer may have clinical utility. Methods are described to prepare recombinant forms of PDE4B that allow screening for compounds that could preferentially inhibit the low-affinity conformer. Furthermore, conditions for an efficient, scintillation proximity, microtiter plate-based assay are described, providing a considerable advance over previous assays in terms of throughput and automatability.

Properties of Q-type calcium channels in neostriatal and cortical neurons are correlated with beta subunit expression

In brain neurons, P- and Q-type Ca(2+) channels both appear to include a class A alpha1 subunit. In spite of this similarity, these channels differ pharmacologically and biophysically, particularly in inactivation kinetics. The molecular basis for this difference is unclear. In heterologous systems, alternative splicing and ancillary beta subunits have been shown to alter biophysical properties of channels containing a class A alpha1 subunit. To test the hypothesis that similar mechanisms are at work in native systems, P- and Q-type currents were characterized in acutely isolated rat neostriatal, medium spiny neurons and cortical pyramidal neurons using whole-cell voltage-clamp techniques. Cells were subsequently aspirated and subjected to single-cell RT-PCR (scRT-PCR) analysis of calcium channel alpha(1) and beta (beta(1-4)) subunit expression. In both cortical and neostriatal neurons, P- and Q-type currents were found in cells expressing class A alpha(1) subunit mRNA. Although P-type currents in cortical and neostriatal neurons were similar, Q-type currents differed significantly in inactivation kinetics. Notably, Q-type currents in neostriatal neurons were similar to P-type currents in inactivation rate. The variation in Q-type channel biophysics was correlated with beta subunit expression. Neostriatal neurons expressed significantly higher levels of beta(2a) mRNA and lower levels of beta(1b) mRNA than cortical neurons. These findings are consistent with the association of beta(2a) and beta(1b) subunits with slow and fast inactivation, respectively. Analysis of alpha(1A) splice variants in the linker between domains I and II failed to provide an alternative explanation for the differences in inactivation rates. These findings are consistent with the hypothesis that the biophysical properties of Q-type channels are governed by beta subunit isoforms and are separable from toxin sensitivity.

Potentiation of recombinant L-type Ca channel currents by alpha1-adrenoceptors coexpressed in baby hamster kidney (BHK) cells

In cardiac myocytes, the effect of alpha1-adrenergic stimulation on L-type Ca current remains to be clarified. We examined this issue by the transient coexpression of alpha1-adrenoceptors on BHKC12 cells, where recombinant Ca channels composed of cardiac alpha1 subunit and skeletal beta, gamma, alpha2/delta subunits were stably expressed. After transfection of plasmid DNA encoding bovine alpha1C-adrenoceptors, bath-applied phenylephrine potentiated the cloned Ca channel current during perforated-patch whole-cell recording by 26+/-6% in 6 out of 12 cells. The potentiation was elicited also by methoxamine, and was blocked by prazosin. Phenylephrine also increased the channel open probability during cell-attached single channel recording in 7 out of 15 cells. The ratio of successful modulation of Ca channels was in accordance with the ratio of successful expression of alpha1-adrenoceptors, as estimated by beta-galactosidase staining. These results suggest that the stimulation of alpha1C-adrenoceptors is linked to potentiation of cardiac L-type Ca current. BHK cells provide a valuable expression system to study the modulation of Ca channels evoked by a receptor stimulation.

Tissue factor pathway inhibitor-2 is a novel mitogen for vascular smooth muscle cells

A mitogen for growth-arrested cultured bovine aortic smooth muscle cells was purified to homogeneity from the supernatant of cultured human umbilical vein endothelial cells by heparin affinity chromatography and reverse-phase high performance liquid chromatography. This mitogen was revealed to be tissue factor pathway inhibitor-2 (TFPI-2), which is a Kunitz-type serine protease inhibitor. TFPI-2 was expressed in baby hamster kidney cells using a mammalian expression vector. Recombinant TFPI-2 (rTFPI-2) stimulated DNA synthesis and cell proliferation in a dose-dependent manner (1-500 nM). rTFPI-2 activated mitogen-activated protein kinase (MAPK) activity and stimulated early proto-oncogene c-fos mRNA expression in smooth muscle cells. MAPK, c-fos expression and the mitogenic activity were inhibited by a specific inhibitor of MAPK kinase, PD098059. Thus, the mitogenic function of rTFPI-2 is considered to be mediated through MAPK pathway. TFPI has been reported to exhibit antiproliferative action after vascular smooth muscle injury in addition to the ability to inhibit activation of the extrinsic coagulation cascade. However, structurally similar TFPI-2 was found to have a mitogenic activity for the smooth muscle cell.

Single tottering mutations responsible for the neuropathic phenotype of the P-type calcium channel

Recent genetic and molecular biological analyses have revealed many forms of inherited channelopathies. Homozygous ataxic mice, tottering (tg) and leaner (tgla) mice, have mutations in the P/Q-type Ca2+ channel alpha1A subunit gene. Although their clinical phenotypes, histological changes, and locations of gene mutations are known, it remains unclear what phenotypes the mutant Ca2+ channels manifest, or whether the altered channel properties are the primary consequence of the mutations. To address these questions, we have characterized the electrophysiological properties of Ca2+ channels in cerebellar Purkinje cells, where the P-type is the dominant Ca2+ channel, dissociated from the normal, tg, and tgla mice, and compared them with the properties of the wild-type and mutant alpha1A channels recombinantly expressed with the alpha2 and beta subunits in baby hamster kidney cells. The most striking feature of Ca2+ channel currents of mutant Purkinje cells was a marked reduction in current density, being reduced to approximately 60 and approximately 40% of control in tg and tgla mice, respectively, without changes of cell size. The Ca2+ channel currents in the tg Purkinje cells showed a relative increase in non-inactivating component in voltage-dependent inactivation. Besides the same change, those of the tgla mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by approximately 10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tgla aberrant splicing products, tgla (short) channel, showed a significant reduction in current density, while the other form of the tgla channels, tgla (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tgla (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.

The mouse stargazer gene encodes a neuronal Ca2+-channel gamma subunit

Stargazer mice have spike-wave seizures characteristic of absence epilepsy, with accompanying defects in the cerebellum and inner ear. We describe here a novel gene, Cacng2, whose expression is disrupted in two stargazer alleles. It encodes a 36-kD protein (stargazin) with structural similarity to the gamma subunit of skeletal muscle voltage-gated calcium (Ca2+) channels. Stargazin is brain-specific and, like other neuronal Ca2+-channel subunits, is enriched in synaptic plasma membranes. In vitro, stargazin increases steady-state inactivation of alpha1 class A Ca2+ channels. The anticipated effect in stargazer mutants, inappropriate Ca2+ entry, may contribute to their more pronounced seizure phenotype compared with other mouse absence models with Ca2+-channel defects. The discovery that the stargazer gene encodes a gamma subunit completes the identification of the major subunit types for neuronal Ca2+ channels, namely alpha1, alpha2delta, beta and gamma, providing a new opportunity to understand how these channels function in the mammalian brain and how they may be targeted in the treatment of neuroexcitability disorders.

Class A calcium channel variants in pancreatic islets and their role in insulin secretion

The initiation of insulin release from rat islet beta cells relies, in large part, on calcium influx through dihydropyridine-sensitive (alpha1D) voltage-gated calcium channels. Components of calcium-dependent insulin secretion and whole cell calcium current, however, are resistant to L-type channel blockade, as well as to omega-conotoxin GVIA, a potent inhibitor of alpha1B channels, suggesting the expression of additional exocytotic calcium channels in the islet. We used a reverse transcription-polymerase chain reaction-based strategy to ascertain at the molecular level whether the alpha1A calcium channel isoform was also present. Results revealed two new variants of the rat brain alpha1A channel in the islet with divergence in a putative extracellular domain and in the carboxyl terminus. Using antibodies and cRNA probes specific for alpha1A channels, we found that the majority of cells in rat pancreatic islets were labeled, indicating expression of the alpha1A channels in beta cells, the predominant islet cell type. Electrophysiologic recording from isolated islet cells demonstrated that the dihydropyridine-resistant current was sensitive to the alpha1A channel blocker, omega-agatoxin IVA. This toxin also inhibited the dihydropyridine-resistant component of glucose-stimulated insulin secretion, suggesting functional overlap among calcium channel classes. These findings confirm the presence of multiple high voltage-activated calcium channels in the rat islet and implicate a physiologic role for alpha1A channels in excitation-secretion coupling in beta cells.

Functional characterization of ion permeation pathway in the N-type Ca2+ channel

Multiple types of high-voltage-activated Ca2+ channels, including L-, N-, P-, Q- and R-types have been distinguished from each other mainly employing pharmacological agents that selectively block particular types of Ca2+ channels. Except for the dihydropyridine-sensitive L-type Ca2+ channels, electrophysiological characterization has yet to be conducted thoroughly enough to biophysically distinguish the remaining Ca2+ channel types. In particular, the ion permeation properties of N-type Ca2+ channels have not been clarified, although the kinetic properties of both the L- and N-type Ca2+ channels are relatively well described. To establish ion conducting properties of the N-type Ca2+ channel, we examined a homogeneous population of recombinant N-type Ca2+ channels expressed in baby hamster kidney cells, using a conventional whole cell patch-clamp technique. The recombinant N-type Ca2+ channel, composed of the alpha1B, alpha2a, and beta1a subunits, displayed high-voltage-activated Ba2+ currents elicited by a test pulse more positive than -30 mV, and were strongly blocked by the N-type channel blocker omega-conotoxin-GVIA. In the presence of 110 mM Ba2+, the unitary current showed a slope conductance of 18.2 pS, characteristic of N-type channels. Ca2+ and Sr2+ resulted in smaller ion fluxes than Ba2+, with the ratio 1.0:0. 72:0.75 of maximum conductance in current-voltage relationships of Ba2+, Ca2+, and Sr2+ currents, respectively. In mixtures of Ba2+ and Ca2+, where the Ca2+ concentration was steadily increased in place of Ba2+, with the total concentration of Ba2+ and Ca2+ held constant at 3 mM, the current amplitude went through a clear minimum when 20% of the external Ba2+ was replaced by Ca+2. This anomalous mole fraction effect suggests an ion-binding site where two or more permeant ions can sit simultaneously. By using an external solution containing 110 mM Na+ without polyvalent cations, inward Na+ currents were evoked by test potentials more positive than -50 mV. These currents were activated and inactivated in a kinetic manner similar to that of Ba2+ currents. Application of inorganic Ca2+ antagonists blocked Ba2+ currents through N-type channels in a concentration-dependent manner. The rank order of inhibition was La3+ >/= Cd2+ >> Zn2+ > Ni2+ >/= Co2+. When a short strong depolarization was applied before test pulses of moderate depolarizing potentials, relief from channel blockade by La3+ and Cd2+ and subsequent channel reblocking was observed. The measured rate (2 x 10(8) M-1 s-1) of reblocking approached the diffusion-controlled limit. These results suggest that N-type Ca2+ channels share general features of a high affinity ion-binding site with the L-type Ca2+ channel, and that this site is easily accessible from the outside of the channel pore.

A novel type of calcium channel sensitive to omega-agatoxin-TK in cultured rat cerebral cortical neurons

We characterized the electrophysiological properties of calcium channels in cultured rat cerebral cortical neurons using omega-agatoxin-TK (omega-Aga-TK) by a patch-clamp technique. Two types of slowly inactivating calcium channels sensitive to omega-Aga-TK were detected. The first type showed high sensitivity to omega-Aga-TK and low recovery from the omega-Aga-TK-induced blockade during washout, corresponding to the P-type channel. The second type showed low sensitivity to omega-Aga-TK and high recovery, resembling the Q-type channel, although it was distinct from the Q-type in terms of slower inactivation kinetics. We designate this channel as Q(L)-type (long-lasting Q channel). The omega-Aga-TK-sensitive calcium channels involved in the glutamatergic synaptic transmission were also divided into two types based on the sensitivity to omega-Aga-TK and reversibility of omega-Aga-TK-induced blockade. We conclude that the Q(L)-type is a novel type of channel, and that both P-type and Q(L)-type channels play a significant role in the cerebral cortical synaptic transmission.

Properties of cloned rat alpha1A calcium channels transiently expressed in the COS-7 cell line

The rat brain alpha1A calcium channel clone has been expressed in COS-7 cells together with the neuronal accessory subunits beta1b and alpha2-delta. From reverse transcriptase polymerase chain reaction (RT-PCR), immunocytochemistry and electrophysiology experiments, we have obtained no evidence that these cells contain any endogenous calcium channels. Transfected cells were identified by co-expression of a cDNA for the reporter Green Fluorescent Protein. From immunocytochemical evidence, a high degree of co-expression was obtained between Green Fluorescent Protein and individual calcium channel subunits. When all three calcium channel subunits (alpha1, alpha2-delta and beta1b) were co-expressed, evidence was obtained that all subunits were present at the cell membrane. Voltage-dependent calcium currents were observed between 24 and 72 h after transfection with the three calcium channel subunits. The current density for the combination alpha1A/alpha2-delta/beta1b was 4.19 +/- 0.69 pA.pF(-1) and the current produced was slowly inactivating. The time constant of inactivation of the maximum I(Ba) was 332 +/- 46 ms (n = 5). The voltage-dependence of activation and steady-state inactivation had voltages of half activation and inactivation of 9.5 +/- 2.5 mV and -30.4 +/- 1.5 mV respectively, and there was little overlap between the two curves. The alpha1A current was completely blocked by 100 microM Cd2+ and was also blocked by omega-conotoxin MVIIC (500 nM). Dose-inhibition curves and analysis of k(on) and k(off) for omega-agatoxin IVA both revealed apparent K(D) values of approximately 11 nM for alpha1A currents, with a k(on) of 7.8 x 10(4) M(-1).s(-1). The results suggest that alpha1A expressed in these cells has some resemblance to the P type component of calcium current observed in native neurons, although it shows a somewhat greater degree of inactivation than native P current, more similar to the Q type current component. It also has an affinity for omega-agatoxin IVA 2-5 fold lower than reported for P current, but approximately 9-fold higher than reported for Q current.

Megakaryocytopoietic activity of a truncated variant of mouse thrombopoietin

Thrombopoietin (TPO) is a hemopoietic cytokine that specifically stimulates the growth and development of megakaryocytes. In addition to full-length TPO (corresponding to TPO-1), cDNAs of three truncated variants (TPO-2 to 4) have been isolated. However, whether or not these variants have biological activities has not been clarified presumably because of poor secretion into extracellular milieu. To examine the biological significance of the shortest variants, TPO-4, we prepared two fusion cDNAs of TPO-1 and TPO-4, each of which was combined with the finger-growth factor-kringle 1 (FGK1) domains of tissue plasminogen activator (tPA) as a tag and transfected them into baby hamster kidney (BHK) cells. An immunodetection assay for FGK1 antigen revealed that the amount of secreted TPO-4-FGK1 fusion protein was about 2% of that of TPO-1-FGK1. These fusion proteins after affinity purification had comparable activities on a molar basis in both megakaryocyte colony stimulating activity towards mouse bone marrow cells and growth promotion activity towards the TPO-responsive cell line UT-7/GM. These results indicate that the TPO-4 is biologically active in vitro, although its primary structure of the C-terminal half after the 111th amino acid residues is completely different from authentic TPO-1 due to frame shift.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Modulation of the alpha 1A Ca2+ channel by beta subunits at physiological Ca2+ concentration

The class A Ca2+ channel alpha 1 subunit (alpha 1A) was expressed in Xenopus oocytes alone or in combination with the beta 1b, beta 2a, beta 3, or beta 4 subunit. Analysis of voltage-dependent activation and inactivation in the presence of 1.8 mM external Ca2+ showed an hyperpolarising shift of both relations when compared to similar recordings performed in the presence of 40 mM Ba2+. These shifts, which differed for activation and inactivation, were strongly modulated by the nature of the coexpressed beta subunit. On the other hand, for each combination, the kinetics of inactivation were similar in 1.8 mM Ca2+ and 40 mM Ba2+ (for example co-expression of the beta 2a subunit reduced inactivation using either 40 mM Ba2+ or 1.8 mM Ca2+). Thus, modulation of channel properties by the beta subunit is different in physiological Ca2+ or high Ba2+ concentrations. These results must be taken into consideration to extrapolate the role of the beta subunit in native cells.