Separation of the CaV 1.2-CaV 1.3 calcium channel duo prevents type 2 allergic airway inflammation

BACKGROUND

Voltage-gated calcium (Ca_v 1) channels contribute to T-lymphocyte activation. Ca_v 1.2 and Ca_v 1.3 channels are expressed in Th2 cells but their respective roles are unknown, which is investigated herein.

METHODS

We generated mice deleted for Ca_v 1.2 in T cells or Ca_v 1.3 and analyzed TCR-driven signaling. In this line, we developed original fast calcium imaging to measure early elementary calcium events (ECE). We also tested the impact of Ca_v 1.2 or Ca_v 1.3 deletion in models of type 2 airway inflammation. Finally, we checked whether the expression of both Ca_v 1.2 and Ca_v 1.3 in T cells from asthmatic children correlates with Th2-cytokine expression.

RESULTS

We demonstrated non-redundant and synergistic functions of Ca_v 1.2 and Ca_v 1.3 in Th2 cells. Indeed, the deficiency of only one channel in Th2 cells triggers TCR-driven hyporesponsiveness with weakened tyrosine phosphorylation profile, a strong decrease in initial ECE and subsequent reduction in the global calcium response. Moreover, Ca_v 1.3 has a particular role in calcium homeostasis. In accordance with the singular roles of Ca_v 1.2 and Ca_v 1.3 in Th2 cells, deficiency in either one of these channels was sufficient to inhibit cardinal features of type 2 airway inflammation. Furthermore, Ca_v 1.2 and Ca_v 1.3 must be co-expressed within the same CD4⁺ T cell to trigger allergic airway inflammation. Accordingly with the concerted roles of Ca_v 1.2 and Ca_v 1.3, the expression of both channels by activated CD4⁺ T cells from asthmatic children was associated with increased Th2-cytokine transcription.

CONCLUSIONS

Thus, Ca_v 1.2 and Ca_v 1.3 act as a duo, and targeting only one of these channels would be efficient in allergy treatment.

Cav2.3 channels contribute to dopaminergic neuron loss in a model of Parkinson's disease

Degeneration of dopaminergic neurons in the substantia nigra causes the motor symptoms of Parkinson's disease. The mechanisms underlying this age-dependent and region-selective neurodegeneration remain unclear. Here we identify Cav2.3 channels as regulators of nigral neuronal viability. Cav2.3 transcripts were more abundant than other voltage-gated Ca2+ channels in mouse nigral neurons and upregulated during aging. Plasmalemmal Cav2.3 protein was higher than in dopaminergic neurons of the ventral tegmental area, which do not degenerate in Parkinson's disease. Cav2.3 knockout reduced activity-associated nigral somatic Ca2+ signals and Ca2+-dependent after-hyperpolarizations, and afforded full protection from degeneration in vivo in a neurotoxin Parkinson's mouse model. Cav2.3 deficiency upregulated transcripts for NCS-1, a Ca2+-binding protein implicated in neuroprotection. Conversely, NCS-1 knockout exacerbated nigral neurodegeneration and downregulated Cav2.3. Moreover, NCS-1 levels were reduced in a human iPSC-model of familial Parkinson's. Thus, Cav2.3 and NCS-1 may constitute potential therapeutic targets for combatting Ca2+-dependent neurodegeneration in Parkinson's disease.

Enhancing VTA Cav1.3 L-type Ca2+ channel activity promotes cocaine and mood-related behaviors via overlapping AMPA receptor mechanisms in the nucleus accumbens

Genetic factors significantly influence susceptibility for substance abuse and mood disorders. Rodent studies have begun to elucidate a role of Ca_v1.3 L-type Ca²⁺ channels in neuropsychiatric-related behaviors, such as addictive and depressive-like behaviors. Human studies have also linked the CACNA1D gene, which codes for the Ca_v1.3 protein, with bipolar disorder. However, the neurocircuitry and the molecular mechanisms underlying the role of Ca_v1.3 in neuropsychiatric phenotypes are not well established. In the present study, we directly manipulated Ca_v1.3 channels in Ca_v1.2 dihydropyridine insensitive mutant mice and found that ventral tegmental area (VTA) Ca_v1.3 channels mediate cocaine-related and depressive-like behavior through a common nucleus accumbens (NAc) shell calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (CP-AMPAR) mechanism that requires GluA1 phosphorylation at S831. Selective activation of VTA Ca_v1.3 with (±)-BayK-8644 (BayK) enhanced cocaine conditioned place preference and cocaine psychomotor activity while inducing depressive-like behavior, an effect not observed in S831A phospho-mutant mice. Infusion of the CP-AMPAR-specific blocker Naspm into the NAc shell reversed the cocaine and depressive-like phenotypes. In addition, activation of VTA Ca_v1.3 channels resulted in social behavioral deficits. In contrast to the cocaine- and depression-related phenotypes, GluA1/A2 AMPARs in the NAc core mediated social deficits, independent of S831-GluA1 phosphorylation. Using a candidate gene analysis approach, we also identified single-nucleotide polymorphisms in the CACNA1D gene associated with cocaine dependence in human subjects. Together, our findings reveal novel, overlapping mechanisms through which VTA Ca_v1.3 mediates cocaine-related, depressive-like and social phenotypes, suggesting that Ca_v1.3 may serve as a target for the treatment of neuropsychiatric symptoms.

Aldosterone-Producing Adenomas

Mutations in KCNJ5 , ATP1A1 , ATP2B3 , CACNA1D , and CTNNB1 are thought to cause the excessive autonomous aldosterone secretion of aldosterone-producing adenomas (APAs). The histopathology of KCNJ5 mutant APAs, the most common and largest, has been thoroughly investigated and shown to have a zona fasciculata–like composition. This study aims to characterize the histopathologic spectrum of the other genotypes and document the proliferation rate of the different sized APAs. Adrenals from 39 primary aldosteronism patients were immunohistochemically stained for CYP11B2 to confirm diagnosis of an APA. Twenty-eight adenomas had sufficient material for further analysis and were target sequenced at hot spots in the 5 causal genes. Ten adenomas had a KCNJ5 mutation (35.7%), 7 adenomas had an ATP1A1 mutation (25%), and 4 adenomas had a CACNA1D mutation (14.3%). One novel mutation in exon 28 of CACNA1D (V1153G) was identified. The mutation caused a hyperpolarizing shift of the voltage-dependent activation and inactivation and slowed the channel’s inactivation kinetics. Immunohistochemical stainings of CYP17A1 as a zona fasciculata cell marker and Ki67 as a proliferation marker were used. KCNJ5 mutant adenomas showed a strong expression of CYP17A1, whereas ATP1A1 / CACNA1D mutant adenomas had a predominantly negative expression ( P value =1.20×10 −4 ). ATP1A1 / CACNA1D mutant adenomas had twice the nuclei with intense staining of Ki67 than KCNJ5 mutant adenomas (0.7% [0.5%–1.9%] versus 0.4% [0.3%–0.7%]; P value =0.04). Further, 3 adenomas with either an ATP1A1 mutation or a CACNA1D mutation had >30% nuclei with moderate Ki67 staining. In summary, similar to KCNJ5 mutant APAs, ATP1A1 and CACNA1D mutant adenomas have a seemingly specific histopathologic phenotype.

L-type Cav1.3 channels regulate ryanodine receptor-dependent Ca2+ release during sino-atrial node pacemaker activity

AIMS

Sino-atrial node (SAN) automaticity is an essential mechanism of heart rate generation that is still not completely understood. Recent studies highlighted the importance of intracellular Ca(2+) ([Ca(2+)]i) dynamics during SAN pacemaker activity. Nevertheless, the functional role of voltage-dependent L-type Ca(2+) channels in controlling SAN [Ca(2+)]i release is largely unexplored. Since Cav1.3 is the predominant L-type Ca(2+) channel isoform in SAN cells, we studied [Ca(2+)]i dynamics in isolated cells and ex vivo SAN preparations explanted from wild-type (WT) and Cav1.3 knockout (KO) mice (Cav1.3(-/-)).

METHODS AND RESULTS

We found that Cav1.3 deficiency strongly impaired [Ca(2+)]i dynamics, reducing the frequency of local [Ca(2+)]i release events and preventing their synchronization. This impairment inhibited the generation of Ca(2+) transients and delayed spontaneous activity. We also used action potentials recorded in WT SAN cells as voltage-clamp commands for Cav1.3(-/-) cells. Although these experiments showed abolished Ca(2+) entry through L-type Ca(2+) channels in the diastolic depolarization range of KO SAN cells, their sarcoplasmic reticulum Ca(2+) load remained normal. β-Adrenergic stimulation enhanced pacemaking of both genotypes, though, Cav1.3(-/-) SAN cells remained slower than WT. Conversely, we rescued pacemaker activity in Cav1.3(-/-) SAN cells and intact tissues through caffeine-mediated stimulation of Ca(2+)-induced Ca(2+) release.

CONCLUSIONS

Cav1.3 channels play a critical role in the regulation of [Ca(2+)]i dynamics, providing an unanticipated mechanism for triggering local [Ca(2+)]i releases and thereby controlling pacemaker activity. Our study also provides an additional pathophysiological mechanism for congenital SAN dysfunction and heart block linked to Cav1.3 loss of function in humans.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Compensatory T-type Ca2+ channel activity alters D2-autoreceptor responses of Substantia nigra dopamine neurons from Cav1.3 L-type Ca2+ channel KO mice

The preferential degeneration of Substantia nigra dopamine midbrain neurons (SN DA) causes the motor-symptoms of Parkinson's disease (PD). Voltage-gated L-type calcium channels (LTCCs), especially the Cav1.3-subtype, generate an activity-related oscillatory Ca(2+) burden in SN DA neurons, contributing to their degeneration and PD. While LTCC-blockers are already in clinical trials as PD-therapy, age-dependent functional roles of Cav1.3 LTCCs in SN DA neurons remain unclear. Thus, we analysed juvenile and adult Cav1.3-deficient mice with electrophysiological and molecular techniques. To unmask compensatory effects, we compared Cav1.3 KO mice with pharmacological LTCC-inhibition. LTCC-function was not necessary for SN DA pacemaker-activity at either age, but rather contributed to their pacemaker-precision. Moreover, juvenile Cav1.3 KO but not WT mice displayed adult wildtype-like, sensitised inhibitory dopamine-D2-autoreceptor (D2-AR) responses that depended upon both, interaction of the neuronal calcium sensor NCS-1 with D2-ARs, and on voltage-gated T-type calcium channel (TTCC) activity. This functional KO-phenotype was accompanied by cell-specific up-regulation of NCS-1 and Cav3.1-TTCC mRNA. Furthermore, in wildtype we identified an age-dependent switch of TTCC-function from contributing to SN DA pacemaker-precision in juveniles to pacemaker-frequency in adults. This novel interplay of Cav1.3 L-type and Cav3.1 T-type channels, and their modulation of SN DA activity-pattern and D2-AR-sensitisation, provide new insights into flexible age- and calcium-dependent activity-control of SN DA neurons and its pharmacological modulation.

Generation of a neuro-specific microarray reveals novel differentially expressed noncoding RNAs in mouse models for neurodegenerative diseases

We have generated a novel, neuro-specific ncRNA microarray, covering 1472 ncRNA species, to investigate their expression in different mouse models for central nervous system diseases. Thereby, we analyzed ncRNA expression in two mouse models with impaired calcium channel activity, implicated in Epilepsy or Parkinson's disease, respectively, as well as in a mouse model mimicking pathophysiological aspects of Alzheimer's disease. We identified well over a hundred differentially expressed ncRNAs, either from known classes of ncRNAs, such as miRNAs or snoRNAs or which represented entirely novel ncRNA species. Several differentially expressed ncRNAs in the calcium channel mouse models were assigned as miRNAs and target genes involved in calcium signaling, thus suggesting feedback regulation of miRNAs by calcium signaling. In the Alzheimer mouse model, we identified two snoRNAs, whose expression was deregulated prior to amyloid plaque formation. Interestingly, the presence of snoRNAs could be detected in cerebral spine fluid samples in humans, thus potentially serving as early diagnostic markers for Alzheimer's disease. In addition to known ncRNAs species, we also identified 63 differentially expressed, entirely novel ncRNA candidates, located in intronic or intergenic regions of the mouse genome, genomic locations, which previously have been shown to harbor the majority of functional ncRNAs.

Cav1.3 channels control D2-autoreceptor responses via NCS-1 in substantia nigra dopamine neurons

Dopamine midbrain neurons within the substantia nigra are particularly prone to degeneration in Parkinson's disease. Their selective loss causes the major motor symptoms of Parkinson's disease, but the causes for the high vulnerability of SN DA neurons, compared to neighbouring, more resistant ventral tegmental area dopamine neurons, are still unclear. Consequently, there is still no cure available for Parkinson's disease. Current therapies compensate the progressive loss of dopamine by administering its precursor l-DOPA and/or dopamine D2-receptor agonists. D2-autoreceptors and Cav1.3-containing L-type Ca(2+) channels both contribute to Parkinson's disease pathology. L-type Ca(2+) channel blockers protect SN DA neurons from degeneration in Parkinson's disease and its mouse models, and they are in clinical trials for neuroprotective Parkinson's disease therapy. However, their physiological functions in SN DA neurons remain unclear. D2-autoreceptors tune firing rates and dopamine release of SN DA neurons in a negative feedback loop through activation of G-protein coupled potassium channels (GIRK2, or KCNJ6). Mature SN DA neurons display prominent, non-desensitizing somatodendritic D2-autoreceptor responses that show pronounced desensitization in PARK-gene Parkinson's disease mouse models. We analysed surviving human SN DA neurons from patients with Parkinson's disease and from controls, and detected elevated messenger RNA levels of D2-autoreceptors and GIRK2 in Parkinson's disease. By electrophysiological analysis of postnatal juvenile and adult mouse SN DA neurons in in vitro brain-slices, we observed that D2-autoreceptor desensitization is reduced with postnatal maturation. Furthermore, a transient high-dopamine state in vivo, caused by one injection of either l-DOPA or cocaine, induced adult-like, non-desensitizing D2-autoreceptor responses, selectively in juvenile SN DA neurons, but not ventral tegmental area dopamine neurons. With pharmacological and genetic tools, we identified that the expression of this sensitized D2-autoreceptor phenotype required Cav1.3 L-type Ca(2+) channel activity, internal Ca(2+), and the interaction of the neuronal calcium sensor NCS-1 with D2-autoreceptors. Thus, we identified a first physiological function of Cav1.3 L-type Ca(2+) channels in SN DA neurons for homeostatic modulation of their D2-autoreceptor responses. L-type Ca(2+) channel activity however, was not important for pacemaker activity of mouse SN DA neurons. Furthermore, we detected elevated substantia nigra dopamine messenger RNA levels of NCS-1 (but not Cav1.2 or Cav1.3) after cocaine in mice, as well as in remaining human SN DA neurons in Parkinson's disease. Thus, our findings provide a novel homeostatic functional link in SN DA neurons between Cav1.3- L-type-Ca(2+) channels and D2-autoreceptor activity, controlled by NCS-1, and indicate that this adaptive signalling network (Cav1.3/NCS-1/D2/GIRK2) is also active in human SN DA neurons, and contributes to Parkinson's disease pathology. As it is accessible to pharmacological modulation, it provides a novel promising target for tuning substantia nigra dopamine neuron activity, and their vulnerability to degeneration.

Somatic mutations in ATP1A1 and CACNA1D underlie a common subtype of adrenal hypertension

At least 5% of individuals with hypertension have adrenal aldosterone-producing adenomas (APAs). Gain-of-function mutations in KCNJ5 and apparent loss-of-function mutations in ATP1A1 and ATP2A3 were reported to occur in APAs. We find that KCNJ5 mutations are common in APAs resembling cortisol-secreting cells of the adrenal zona fasciculata but are absent in a subset of APAs resembling the aldosterone-secreting cells of the adrenal zona glomerulosa. We performed exome sequencing of ten zona glomerulosa-like APAs and identified nine with somatic mutations in either ATP1A1, encoding the Na(+)/K(+) ATPase α1 subunit, or CACNA1D, encoding Cav1.3. The ATP1A1 mutations all caused inward leak currents under physiological conditions, and the CACNA1D mutations induced a shift of voltage-dependent gating to more negative voltages, suppressed inactivation or increased currents. Many APAs with these mutations were <1 cm in diameter and had been overlooked on conventional adrenal imaging. Recognition of the distinct genotype and phenotype for this subset of APAs could facilitate diagnosis.

Distinct localization and modulation of Cav1.2 and Cav1.3 L-type Ca2+ channels in mouse sinoatrial node

Dysregulation of L-type Ca(2+) currents in sinoatrial nodal (SAN) cells causes cardiac arrhythmia. Both Ca(v)1.2 and Ca(v)1.3 channels mediate sinoatrial L-type currents. Whether these channels exhibit differences in modulation and localization, which could affect their contribution to pacemaking, is unknown. In this study, we characterized voltage-dependent facilitation (VDF) and subcellular localization of Ca(v)1.2 and Ca(v)1.3 channels in mouse SAN cells and determined how these properties of Ca(v)1.3 affect sinoatrial pacemaking in a mathematical model. Whole cell Ba(2+) currents were recorded from SAN cells from mice carrying a point mutation that renders Ca(v)1.2 channels relatively insensitive to dihydropyridine antagonists. The Ca(v)1.2-mediated current was isolated in the presence of nimodipine (1 μm), which was subtracted from the total current to yield the Ca(v)1.3 component. With strong depolarizations (+80 mV), Ca(v)1.2 underwent significantly stronger inactivation than Ca(v)1.3. VDF of Ca(v)1.3 was evident during recovery from inactivation at a time when Ca(v)1.2 remained inactivated. By immunofluorescence, Ca(v)1.3 colocalized with ryanodine receptors in sarcomeric structures while Ca(v)1.2 was largely restricted to the delimiting plasma membrane. Ca(v)1.3 VDF enhanced recovery of pacemaker activity after pauses and positively regulated pacemaking during slow heart rate in a numerical model of mouse SAN automaticity, including preferential coupling of Ca(v)1.3 to ryanodine receptor-mediated Ca(2+) release. We conclude that strong VDF and colocalization with ryanodine receptors in mouse SAN cells are unique properties that may underlie a specific role for Ca(v)1.3 in opposing abnormal slowing of heart rate.

Cav 1.3 L-type Ca ( 2+) channels mediate long-term adaptation in dopamine D2L-mediated GluA1 trafficking in the dorsal striatum following cocaine exposure

AMPA receptor (AMPAR) plasticity at glutamatergic synapses in the mesostriatal dopaminergic pathway has been implicated in persistent cocaine-induced behavioral responses; however, the precise mechanism underlying these changes remains unknown. Utilizing cocaine psychomotor sensitization in mice we find that repeated cocaine results in a basal reduction of Ser 845 GluA1 and cell surface GluA1 levels in the dorsal striatum (dStr) following a protracted withdrawal period, an adaptation that is dependent on Cav 1.3 channels but not those expressed in the VTA. We find that the basally-induced decrease in this phosphoprotein is the result of recruitment of the striatal dopamine D2 pathway, as evidenced by enhanced levels of D2 receptor (D2R) mRNA expression and D2R function as examined using the D2R antagonist, eticlopride, as well as alterations in the phosphorylation status of several downstream molecular targets of D2R's, including CREB, DARPP-32, Akt and GSK3β. Taken together with our recently published findings examining similar phenomena in the nucleus accumbens (NAc), these results underscore the utilization of divergent molecular mechanisms in the dStr, in mediating cocaine-induced persistent behavioral changes.

Iron overload decreases CaV1.3-dependent L-type Ca2+ currents leading to bradycardia, altered electrical conduction, and atrial fibrillation

BACKGROUND

Chronic iron overload (CIO) is associated with blood disorders such as thalassemias and hemochromatosis. A major prognostic indicator of survival in patients with CIO is iron-mediated cardiomyopathy characterized by contractile dysfunction and electrical disturbances, including slow heart rate (bradycardia) and heart block.

METHODS AND RESULTS

We used a mouse model of CIO to investigate the effects of iron on sinoatrial node (SAN) function. As in humans, CIO reduced heart rate (≈20%) in conscious mice as well as in anesthetized mice with autonomic nervous system blockade and in isolated Langendorff-perfused mouse hearts, suggesting that bradycardia originates from altered intrinsic SAN pacemaker function. Indeed, spontaneous action potential frequencies in SAN myocytes with CIO were reduced in association with decreased L-type Ca(2+) current (I(Ca,L)) densities and positive (rightward) voltage shifts in I(Ca,L) activation. Pacemaker current (I(f)) was not affected by CIO. Because I(Ca,L) in SAN myocytes (as well as in atrial and conducting system myocytes) activates at relatively negative potentials due to the presence of Ca(V)1.3 channels (in addition to Ca(V)1.2 channels), our data suggest that elevated iron preferentially suppresses Ca(V)1.3 channel function. Consistent with this suggestion, CIO reduced Ca(V)1.3 mRNA levels by ≈40% in atrial tissue (containing SAN) and did not lower heart rate in Ca(V)1.3 knockout mice. CIO also induced PR-interval prolongation, heart block, and atrial fibrillation, conditions also seen in Ca(V)1.3 knockout mice.

CONCLUSIONS

Our results demonstrate that CIO selectively reduces Ca(V)1.3-mediated I(Ca,L), leading to bradycardia, slowing of electrical conduction, and atrial fibrillation as seen in patients with iron overload.

Role of voltage-gated L-type Ca2+ channel isoforms for brain function

Voltage-gated LTCCs (L-type Ca2+ channels) are established drug targets for the treatment of cardiovascular diseases. LTCCs are also expressed outside the cardiovascular system. In the brain, LTCCs control synaptic plasticity in neurons, and DHP (dihydropyridine) LTCC blockers such as nifedipine modulate brain function (such as fear memory extinction and depression-like behaviour). Voltage-sensitive Ca2+ channels Cav1.2 and Cav1.3 are the predominant brain LTCCs. As DHPs and other classes of organic LTCC blockers inhibit both isoforms, their pharmacological distinction is impossible and their individual contributions to defined brain functions remain largely unknown. Here, we summarize our recent experiments with two genetically modified mouse strains, which we generated to explore the individual biophysical features of Cav1.2 and Cav1.3 LTCCs and to determine their relative contributions to various physiological peripheral and neuronal functions. The results described here also allow predictions about the pharmacotherapeutic potential of isoform-selective LTCC modulators.

Up-regulation of dopamine D(2)L mRNA levels in the ventral tegmental area and dorsal striatum of amphetamine-sensitized C57BL/6 mice: role of Ca(v)1.3 L-type Ca(2+) channels

Dopamine D(2) long (D(2)L) and D(2) short (D(2)S) isoforms of the D(2) receptor play an important role in psychostimulant-induced neuronal adaptations. In this study, we used quantitative real-time PCR to specifically amplify these two splice variants to examine their mRNA expression in the dorsal striatum (dStr), nucleus accumbens (NAc) and the ventral tegmental area (VTA) of amphetamine-sensitized C57BL/6 mice. We found a significant increase in D(2)L mRNA in the VTA and dStr of amphetamine-treated mice that positively correlated with the sensitized locomotor response. We also found a significant increase in D(2)S mRNA in the VTA. We further examined the role of the Ca(v)1.3 subtype of L-type Ca(2+) channels in up-regulation of D(2)L and D(2)S mRNA in the VTA. Amphetamine-pretreated Ca(v)1.3 wild-type (Ca(v)1.3(+/+)) mice exhibited sensitized behavior and a significant increase in D(2)L and D(2)S mRNA compared with saline-pretreated mice Amphetamine-pretreated homozygous Ca(v)1.3 knockout (Ca(v)1.3(-/-)) mice did not exhibit sensitized behavior. There was a significant increase in D(2)S mRNA, but not D(2)L mRNA. In conclusion, our results find that amphetamine increases D(2)L mRNA expression in the dStr and the VTA, an adaptation that correlates with expression of sensitized behavior and dependence on Ca(v)1.3 Ca(2+) channels.

International Union of Pharmacology. XLVIII. Nomenclature and structure-function relationships of voltage-gated calcium channels

The family of voltage-gated calcium channels serves as the key transducers of cell surface membrane potential changes into local intracellular calcium transients that initiate many different physiological events. There are 10 members of the voltage-gated calcium channel family that have been characterized in mammals, and they serve distinct roles in cellular signal transduction. This article presents the molecular relationships and physiological functions of these calcium channel proteins and provides comprehensive information on their molecular, genetic, physiological, and pharmacological properties.

Brain activation pattern induced by stimulation of L-type Ca2+-channels: Contribution of CaV1.3 and CaV1.2 isoforms

Ca(V)1.2 and Ca(V)1.3, are the main dihydropyridine-sensitive L-type calcium channel isoforms in the brain. To reveal the contribution of each isoform to the neuronal activation pattern elicited by the dihydropyridine L-type calcium channel activator BayK 8644, we utilized Fos expression as a marker of neuronal activation in mutant mice (Ca(V)1.2(DHP-/-) mice) expressing dihydropyridine-insensitive Ca(V)1.2 L-type calcium channels. BayK 8644-treated wildtype mice displayed intense and widespread Fos expression throughout the neuroaxis in 77 of 80 brain regions quantified. The Fos response in Ca(V)1.2(DHP-/-) mice was greatly attenuated or absent in most of these areas, suggesting that a major part of the widespread Fos induction including most cortical areas was mediated by Ca(V)1.2 L-type calcium channels. BayK 8644-induced Fos expression in Ca(V)1.2(DHP-/-) mice indicating predominantly Ca(V)1.3 L-type calcium channel-mediated activation was noted in more restricted neuronal populations (20 of 80), in particular in the central amygdala, the bed nucleus of the stria terminalis, paraventricular hypothalamic nucleus, lateral preoptic area, locus coeruleus, lateral parabrachial nucleus, central nucleus of the inferior colliculus, and nucleus of the solitary tract. Our data indicate that selective stimulation of other than Ca(V)1.2 L-type calcium channels, mostly Ca(V)1.3, causes neuronal activation in a specific set of mainly limbic, hypothalamic and brainstem areas, which are associated with functions including integration of emotion-related behavior. Hence, selective modulation of Ca(V)1.3 L-type calcium channels could represent a novel (pharmacotherapeutic) tool to influence these CNS functions.

International Union of Pharmacology. XL. Compendium of Voltage-Gated Ion Channels: Calcium Channels

This summary article presents an overview of the molecular relationships among the voltage-gated calcium channels and a standard nomenclature for them, which is derived from the IUPHAR Compendium of Voltage-Gated Ion Channels. The complete Compendium, including data tables for each member of the calcium channel family can be found at http://www.iuphar-db.org/iuphar-ic/.

CaV1.3 channels are essential for development and presynaptic activity of cochlear inner hair cells

Cochlear inner hair cells (IHCs) release neurotransmitter onto afferent auditory nerve fibers in response to sound stimulation. During early development, afferent synaptic transmission is triggered by spontaneous Ca2+ spikes of IHCs, which are under efferent cholinergic control. Around the onset of hearing, large-conductance Ca2+-activated K+ channels are acquired, and Ca2+ spikes as well as the cholinergic innervation are lost. Here, we performed patch-clamp measurements in IHCs of mice lacking the CaV1.3 channel (CaV1.3-/-) to investigate the role of this prevailing voltage-gated Ca2+ channel in IHC development and synaptic function. The small Ca2+ current remaining in IHCs from 3-week-old CaV1.3-/- mice was mainly mediated by L-type Ca2+ channels, because it was sensitive to dihydropyridines but resistant to inhibitors of non-L-type Ca2+ channels such as omega-conotoxins GVIA and MVIIC and SNX-482. Depolarization induced only marginal exocytosis in CaV1.3-/- IHC, which was solely mediated by L-type Ca2+ channels, whereas robust exocytic responses were elicited by photolysis of caged Ca2+. Secretion triggered by short depolarizations was reduced proportionally to the Ca2+ current, suggesting that the coupling of the remaining channels to exocytosis was unchanged. CaV1.3-/- IHCs lacked the Ca2+ action potentials and displayed a complex developmental failure. Most strikingly, we observed a continued presence of efferent cholinergic synaptic transmission and a lack of functional large-conductance Ca2+-activated K+ channels up to 4 weeks after birth. We conclude that CaV1.3 channels are essential for normal hair cell development and synaptic transmission.

Neurological phenotype and synaptic function in mice lacking the CaV1.3 alpha subunit of neuronal L-type voltage-dependent Ca2+ channels

Neuronal L-type calcium channels have been implicated in pain perception and neuronal synaptic plasticity. To investigate this we have examined the effect of disrupting the gene encoding the CaV1.3 (alpha 1D) alpha subunit of L-type Ca2+ channels on neurological function, acute nociceptive behavior, and hippocampal synaptic function in mice. CaV1.3 alpha 1 subunit knockout (CaV1.3 alpha 1(-/-)) mice had relatively normal neurological function with the exception of reduced auditory evoked behavioral responses and lower body weight. Baseline thermal and mechanical thresholds were unaltered in these animals. CaV1.3 alpha 1(-/-) mice were also examined for differences in N-methyl-D-aspartate (NMDA) receptor-dependent (100 Hz tetanization for 1 s) and NMDA receptor-independent (200 Hz in 100 microM DL-2-amino-5-phosphopentanoic acid) long-term potentiation within the CA1 region of the hippocampus. Both NMDA receptor-dependent and NMDA receptor-independent forms of long-term potentiation were expressed normally. Radioligand binding studies revealed that the density of (+)[3H]isradipine binding sites in brain homogenates was reduced by 20-25% in CaV1.3 alpha 1(-/-) mice, without any detectable change in CaV1.2 (alpha 1C) protein levels as detected using Western blot analysis. Taken together these data indicate that following loss of CaV1.3 alpha 1 subunit expression there is sufficient residual activity of other Ca2+ channel subtypes to support NMDA receptor-independent long-term potentiation and some forms of sensory behavior/function.

Visualization of the domain structure of an L-type Ca2+ channel using electron cryo-microscopy

The three-dimensional structure of the skeletal muscle voltage-gated L-type calcium channel (Ca(v)1.1; dihydropyridine receptor, DHPR) was determined using electron cryo-microscopy and single-particle averaging. The structure shows a single channel complex with an approximate total molecular mass of 550 kDa, corresponding to the five known subunits of the DHPR, and bound detergent and lipid. Features visible in our structure together with antibody labeling of the beta and alpha(2) subunits allowed us to assign locations for four of the five subunits within the structure. The most striking feature of the structure is the extra-cellular alpha(2) subunit that protrudes from the membrane domain in close proximity to the alpha(1) subunit. The cytosolic beta subunit is located close to the membrane and adjacent to subunits alpha(1), gamma and delta. Our structure correlates well with the functional and biochemical data available for this channel and suggests a three-dimensional model for the excitation-contraction coupling complex consisting of DHPR tetrads and the calcium release channel.

Role of class D L-type Ca2+ channels for cochlear morphology

Voltage-gated Ca(2+) channels formed by subunits (class D Ca(2+) channels) tightly regulate neurotransmitter release from cochlear inner hair cells (IHCs) by controlling the majority of depolarisation-induced Ca(2+) entry. We have recently shown that the absence of these channels can cause deafness and degeneration of outer hair cells (OHCs) and IHCs in alpha1D-deficient mice (alpha1D(-/-)) (Platzer et al., 2000. Cell 102, 89-97). We investigated the time-dependent patterns of degeneration during postnatal development in the alpha1D(-/-) mouse cochlea using light and electron microscopy. At postnatal day 3 (P3), electron microscopy revealed no morphological aberrations in sensory cells, in afferent as well as in efferent nerve endings. But at P7 we observed a beginning degeneration of afferent nerve fibres by electron microscopy. By P15, we found a loss of OHCs in apical turns but electron microscopy revealed no ultrastructural changes in IHCs and efferent axons as compared to C57 black control animals (C57BL). We demonstrated by serial ultrathin sectioning of 15 days old alpha1D(-/-) mice that intact efferent nerve fibres formed direct contacts with IHCs as the degeneration of afferent nerve fibres progressed. We also saw a notable degeneration of spiral ganglion cells at P15. By 8 months, nearly all spiral ganglion and sensory cells of the organ of Corti were absent. Random ultrathin sectioning gave the impression that synaptic bodies abundant in wild-type animals were absent in nearly all alpha1D(-/-) mice investigated. We conclude that besides presumably reduced synaptic bodies the absence of class D L-type Ca(2+) channels does not prevent morphological development of the cochlea until P3 but may cause cochlear degeneration thereafter. The observed pattern of degeneration involves afferent nerve fibres (P7) followed by cell bodies in the spiral ganglion (P15), OHCs (P15) and IHCs (after P15).

Familial hemiplegic migraine mutations increase Ca(2+) influx through single human CaV2.1 channels and decrease maximal CaV2.1 current density in neurons

Insights into the pathogenesis of migraine with aura may be gained from a study of human Ca(V)2.1 channels containing mutations linked to familial hemiplegic migraine (FHM). Here, we extend the previous single-channel analysis to human Ca(V)2.1 channels containing mutation V1457L. This mutation increased the channel open probability by shifting its activation to more negative voltages and reduced both the unitary conductance and the density of functional channels in the membrane. To investigate the possibility of changes in Ca(V)2.1 function common to all FHM mutations, we calculated the product of single-channel current and open probability as a measure of Ca(2+) influx through single Ca(V)2.1 channels. All five FHM mutants analyzed showed a single-channel Ca(2+) influx larger than wild type in a broad voltage range around the threshold of activation. We also expressed the FHM mutants in cerebellar granule cells from Ca(V)2.1alpha(1)-/- mice rather than HEK293 cells. The FHM mutations invariably led to a decrease of the maximal Ca(V)2.1 current density in neurons. Current densities were similar to wild type at lower voltages because of the negatively shifted activation of FHM mutants. Our data show that mutational changes of functional channel densities can be different in different cell types, and they uncover two functional effects common to all FHM mutations analyzed: increase of single-channel Ca(2+) influx and decrease of maximal Ca(V)2.1 current density in neurons. We discuss the relevance of these findings for the pathogenesis of migraine with aura.

Stimulation of 5-HT(2) receptors in prefrontal pyramidal neurons inhibits Ca(v)1.2 L type Ca(2+) currents via a PLCbeta/IP3/calcineurin signaling cascade

There is growing evidence linking alterations in serotonergic signaling in the prefrontal cortex to the etiology of schizophrenia. Prefrontal pyramidal neurons are richly innervated by serotonergic fibers and express high levels of serotonergic 5-HT(2)-class receptors. It is unclear, however, how activation of these receptors modulates cellular activity. To help fill this gap, whole cell voltage-clamp and single-cell RT-PCR studies of acutely isolated layer V-VI prefrontal pyramidal neurons were undertaken. The vast majority (>80%) of these neurons had detectable levels of 5-HT(2A) or 5-HT(2C) receptor mRNA. Bath application of 5-HT(2) agonists inhibited voltage-dependent Ca(2+) channel currents. L-type Ca(2+) channels were a particularly prominent target of this signaling pathway. The L-type channel modulation was blocked by disruption of G(alphaq) signaling or by inhibition of phospholipase Cbeta. Antagonism of intracellular inositol trisphosphate signaling, chelation of intracellular Ca(2+), or depletion of intracellular Ca(2+) stores also blocked this modulation. Inhibition of the Ca(2+)-dependent phosphatase calcineurin prevented receptor-mediated modulation of L-type currents. Last, the 5-HT(2) receptor modulation was robustly expressed in neurons from Ca(v)1.3 knockout mice. These findings argue that 5-HT(2) receptors couple through G(alphaq) proteins to trigger a phospholipase Cbeta/inositol trisphosphate signaling cascade resulting in the mobilization of intracellular Ca(2+), activation of calcineurin, and inhibition of Ca(v)1.2 L-type Ca(2+) currents. This modulation and its blockade by atypical neuroleptics could have wide-ranging effects on synaptic integration and long-term gene expression in deep-layer prefrontal pyramidal neurons.

Fast exocytosis with few Ca(2+) channels in insulin-secreting mouse pancreatic B cells

The association of L-type Ca(2+) channels to the secretory granules and its functional significance to secretion was investigated in mouse pancreatic B cells. Nonstationary fluctuation analysis showed that the B cell is equipped with <500 alpha1(C) L-type Ca(2+) channels, corresponding to a Ca(2+) channel density of 0.9 channels per microm(2). Analysis of the kinetics of exocytosis during voltage-clamp depolarizations revealed an early component that reached a peak rate of 1.1 pFs(-1) (approximately 650 granules/s) 25 ms after onset of the pulse and is completed within approximately 100 ms. This component represents a subset of approximately 60 granules situated in the immediate vicinity of the L-type Ca(2+) channels, corresponding to approximately 10% of the readily releasable pool of granules. Experiments involving photorelease of caged Ca(2+) revealed that the rate of exocytosis was half-maximal at a cytoplasmic Ca(2+) concentration of 17 microM, and concentrations >25 microM are required to attain the rate of exocytosis observed during voltage-clamp depolarizations. The rapid component of exocytosis was not affected by inclusion of millimolar concentrations of the Ca(2+) buffer EGTA but abolished by addition of exogenous L(C753-893), the 140 amino acids of the cytoplasmic loop connecting the 2(nd) and 3(rd) transmembrane region of the alpha1(C) L-type Ca(2+) channel, which has been proposed to tether the Ca(2+) channels to the secretory granules. In keeping with the idea that secretion is determined by Ca(2+) influx through individual Ca(2+) channels, exocytosis triggered by brief (15 ms) depolarizations was enhanced 2.5-fold by the Ca(2+) channel agonist BayK8644 and 3.5-fold by elevating extracellular Ca(2+) from 2.6 to 10 mM. Recordings of single Ca(2+) channel activity revealed that patches predominantly contained no channels or many active channels. We propose that several Ca(2+) channels associate with a single granule thus forming a functional unit. This arrangement is important in a cell with few Ca(2+) channels as it ensures maximum usage of the Ca(2+) entering the cell while minimizing the influence of stochastic variations of the Ca(2+) channel activity.

alpha 1D (Cav1.3) subunits can form l-type Ca2+ channels activating at negative voltages

In cochlea inner hair cells (IHCs), L-type Ca(2+) channels (LTCCs) formed by alpha1D subunits (D-LTCCs) possess biophysical and pharmacological properties distinct from those of alpha1C containing C-LTCCs. We investigated to which extent these differences are determined by alpha1D itself by analyzing the biophysical and pharmacological properties of cloned human alpha1D splice variants in tsA-201 cells. Variant alpha1D(8A,) containing exon 8A sequence in repeat I, yielded alpha1D protein and L-type currents, whereas no intact protein and currents were observed after expression with exon 8B. In whole cell patch-clamp recordings (charge carrier 15-20 mm Ba(2+)), alpha1D(8A) - mediated currents activated at more negative voltages (activation threshold, -45.7 versus -31.5 mV, p < 0.05) and more rapidly (tau(act) for maximal inward currents 0.8 versus 2.3 ms; p < 0.05) than currents mediated by rabbit alpha1C. Inactivation during depolarizing pulses was slower than for alpha1C (current inactivation after 5-s depolarizations by 90 versus 99%, p < 0.05) but faster than for LTCCs in IHCs. The sensitivity for the dihydropyridine (DHP) L-type channel blocker isradipine was 8.5-fold lower than for alpha1C. Radioligand binding experiments revealed that this was not due to a lower affinity for the DHP binding pocket, suggesting that differences in the voltage-dependence of DHP block account for decreased sensitivity of D-LTCCs. Our experiments show that alpha1D(8A) subunits can form slowly inactivating LTCCs activating at more negative voltages than alpha1C. These properties should allow D-LTCCs to control physiological processes, such as diastolic depolarization in sinoatrial node cells, neurotransmitter release in IHCs and neuronal excitability.

Mechanism of dihydropyridine interaction with critical binding residues of L-type Ca2+ channel alpha 1 subunits

We investigated the mechanism of interaction of individual L-type channel amino acid residues with dihydropyridines within a dihydropyridine-sensitive alpha1A subunit (alpha1A(DHP)). Mutation of individual residues in repeat III and expression in Xenopus oocytes revealed that Thr(1393) is not required for dihydropyridine interaction but that bulky side chains (tyrosine, phenylalanine) in this position sterically inhibit dihydropyridine coordination. In position 1397 a side chain carbonyl group was required for high antagonist sensitivity. Agonist function required the complete amide group of a glutamine residue. Val(1516) and Met(1512) side chains were required for agonist (Val(1516)) and antagonist (Val(1516), Met(1512)) sensitivity. Replacement of Ile(1504) and Ile(1507) by alpha1A phenylalanines was tolerated. Substitution of Thr(1393) by phenylalanine or Val(1516) by alanine introduced voltage dependence of antagonist action into alpha1A(DHP), suggesting that these residues form part of a mechanism mediating voltage dependence of dihydropyridine sensitivity. Our data provide important insight into dihydropyridine binding to alpha1A(DHP) which could facilitate the development of alpha1A-selective modulators. By modulating P/Q-type Ca(2+) channels such drugs could serve as new anti-migraine therapeutics.

Down-regulation of L-type calcium channels in inflamed circular smooth muscle cells of the canine colon

BACKGROUND & AIMS

Circular smooth muscle phasic contractions and tone are suppressed during colonic inflammation, but the contributing factors are poorly understood. This study investigated if the expression level of voltage-gated long-lasting (L-type) Ca(2+) channel protein and functional Ca(2+) channel current are down-regulated in the circular muscle cells of the inflamed canine colon.

METHODS

L-type Ca(2+) channel expression was compared between normal and inflamed smooth muscle cells by Western immunoblots using an antibody directed against the pore-forming alpha 1C-subunit, and patch-clamp methods were used to evaluate Ca(2+) channel current density.

RESULTS

The expression of the L-type Ca(2+) channel protein was significantly reduced in inflamed compared with normal circular smooth muscle cell membranes, and this finding was associated with suppressed levels of Ca(2+) channel current in patch-clamped cells. The L-type Ca(2+) channel current in normal and inflamed cells increased proportionately in response to Bay K 8644, but the maximal current density was still lower in the inflamed cells. Acetylcholine increased the L-type Ca(2+) channel current in normal but not in inflamed cells.

CONCLUSIONS

The expression level of L-type Ca(2+) channels is down-regulated in the circular smooth muscle cell membranes of the inflamed colon, which may result in reduced Ca(2+) influx. The functional and pharmacologic properties of the channels seem normal. Although some Ca(2+) channels are still present in the inflamed cells, acetylcholine does not activate these channels, which may be caused by additional upstream defects in the receptor signaling cascade. The down-regulation of L-type Ca(2+) channel expression may suppress circular smooth muscle contractions in the inflamed colon and contribute to the abnormalities in motility and digestion observed during inflammatory disorders.

Functional embryonic cardiomyocytes after disruption of the L-type alpha1C (Cav1.2) calcium channel gene in the mouse

The L-type alpha(1C) (Ca(v)1.2) calcium channel is the major calcium entry pathway in cardiac and smooth muscle. We inactivated the Ca(v)1.2 gene in two independent mouse lines that had indistinguishable phenotypes. Homozygous knockout embryos (Ca(v)1. 2-/-) died before day 14.5 postcoitum (p.c.). At day 12.5 p.c., the embryonic heart contracted with identical frequency in wild type (+/+), heterozygous (+/-), and homozygous (-/-) Ca(v)1.2 embryos. Beating of isolated embryonic cardiomyocytes depended on extracellular calcium and was blocked by 1 microm nisoldipine. In (+/+), (+/-), and (-/-) cardiomyocytes, an L-type Ba(2+) inward current (I(Ba)) was present that was stimulated by Bay K 8644 in all genotypes. At a holding potential of -80 mV, nisoldipine blocked I(Ba) of day 12.5 p.c. (+/+) and (+/-) cells with two IC(50) values of approximately 0.1 and approximately 1 microm. Inhibition of I(Ba) of (-/-) cardiomyocytes was monophasic with an IC(50) of approximately 1 microm. The low affinity I(Ba) was also present in cardiomyocytes of homozygous alpha(1D) (Ca(v)1.3) knockout embryos at day 12.5 p.c. These results indicate that, up to day 14 p.c., contraction of murine embryonic hearts requires an unidentified, low affinity L-type like calcium channel.

Congenital deafness and sinoatrial node dysfunction in mice lacking class D L-type Ca2+ channels

Voltage-gated L-type Ca2+ channels (LTCCs) containing a pore-forming alpha1D subunit (D-LTCCs) are expressed in neurons and neuroendocrine cells. Their relative contribution to total L-type Ca2+ currents and their physiological role and significance as a drug target remain unknown. Therefore, we generated D-LTCC deficient mice (alpha1D-/-) that were viable with no major disturbances of glucose metabolism. alpha1D-/-mice were deaf due to the complete absence of L-type currents in cochlear inner hair cells and degeneration of outer and inner hair cells. In wild-type controls, D-LTCC-mediated currents showed low activation thresholds and slow inactivation kinetics. Electrocardiogram recordings revealed sinoatrial node dysfunction (bradycardia and arrhythmia) in alpha1D-/- mice. We conclude that alpha1D can form LTCCs with negative activation thresholds essential for normal auditory function and control of cardiac pacemaker activity.

Pharmacodynamic interaction between mibefradil and other calcium channel blockers

Briefly after withdrawal of the (T-type) calcium channel blocker mibefradil from the market, four cases of life-threatening interaction of mibefradil with dihydropyridines were reported. We investigated in vitro whether mibefradil interacts with a dihydropyridine, as described for other non-dihydropyridine compounds. Rat working hearts were used to examine functional interactions between amlodipine and mibefradil. Gallopamil and another T-type-channel blocker, ethosuximide, were included for comparison. Effects of mibefradil, (+)- and (-)-gallopamil on [3H](+)-isradipine binding were studied in membranes from tsA201-cells transfected with alpha(1c)-, alpha(2)delta-, and beta(1a)- or beta(2a)-calcium channel subunits. Mibefradil increased negative inotropic effect of amlodipine, but not of gallopamil. Gallopamil and ethosuximide showed no influence on contractile effects of amlodipine. Furthermore, mibefradil concentration-dependently caused bradycardic rhythm disturbance. The same type of arrhythmia was observed combining low concentrations of mibefradil with amlodipine, or with gallopamil, respectively. Amlodipine alone, or the combination of gallopamil or ethosuximide with amlodipine did not cause any arrhythmia. Binding studies showed a concentration-dependent positive allosteric interaction between [3H](+)-isradipine and mibefradil, but not with [3H](+)-isradipine and gallopamil enantiomers. Molecular and functional evidence points to an interaction between a dihydropyridine and mibefradil. Mibefradil caused rhythm disturbances and potentiation of negative inotropy when combined with amlodipine.

Pharmacology, structure and function of cardiac L-type Ca(2+) channels

Voltage-gated L-type Ca(2+) channels control depolarization-induced Ca(2+) entry in different electrically excitable cells, including mammalian heart. Important molecular and functional details providing new insight into L-type channel structure and modulation are reviewed in this article. This includes the identification of amino acid residues responsible for drug binding, the role of accessory subunits and alternative splicing for fine-tuning channel activity and modulation by protein kinases (A, C, tyrosine kinases), cGMP-dependent pathways, calmodulin and Ca(2+). Alterations in Ca(2+) channel activity under pathological conditions such as in heart failure or during ischemia could provide new clues for the development of drugs to treat cardiovascular diseases.

Conserved Ca2+-antagonist-binding properties and putative folding structure of a recombinant high-affinity dihydropyridine-binding domain

Sensitivity to 1,4-dihydropyridines (DHPs) can be transferred from L-type (alpha1C) to non-L-type (alpha1A) Ca(2+) channel alpha1 subunits by the mutation of nine pore-associated non-conserved amino acid residues, yielding mutant alpha1A(DHP). To determine whether the hallmarks of reversible DHP binding to L-type Ca(2+) channels (nanomolar dissociation constants, stereoselectivity and modulation by other chemical classes of Ca(2+) antagonist drugs) were maintained in alpha1A(DHP), we analysed the pharmacological properties of (+)-[(3)H]isradipine-labelled alpha1A(DHP) Ca(2+) channels after heterologous expression. Binding of (+)-isradipine (K(i) 7.4 nM) and the non-benzoxadiazole DHPs nifedipine (K(i) 86 nM), (+/-)-nitrendipine (K(i) 33 nM) and (+/-)-nimodipine (K(i) 67 nM) to alpha1A(DHP) occurred at low nanomolar K(i) values. DHP binding was highly stereoselective [25-fold higher affinity for (+)-isradipine]. As with native channels it was stimulated by (+)-cis-diltiazem, (+)-tetrandrine and mibefradil. This suggested that the three-dimensional architecture of the channel pore was maintained within the non-L-type alpha1A subunit. To predict the three-dimensional arrangement of the DHP-binding residues we exploited the X-ray structure of a recently crystallized bacterial K(+) channel (KcsA) as a template. Our model is based on the assumption that the Ca(2+) channel S5 and S6 segments closely resemble the KcsA transmembrane folding architecture. In the absence of three-dimensional structural data for the alpha1 subunit this is currently the most reasonable approach for modelling this drug-interaction domain. Our model predicts that the previously identified DHP-binding residues form a binding pocket large enough to co-ordinate a single DHP molecule. It also implies that the four homologous Ca(2+) channel repeats are arranged in a clockwise manner.

Three new familial hemiplegic migraine mutants affect P/Q-type Ca(2+) channel kinetics

Missense mutations in the pore-forming human alpha(1A) subunit of neuronal P/Q-type Ca(2+) channels are associated with familial hemiplegic migraine. We studied the functional consequences on P/Q-type Ca(2+) channel function of three recently identified mutations, R583Q, D715E, and V1457L after introduction into rabbit alpha(1A) and expression in Xenopus laevis oocytes. The potential for half-maximal channel activation of Ba(2+) inward currents was shifted by > 9 mV to more negative potentials in all three mutants. The potential for half-maximal channel inactivation was shifted by > 7 mV in the same direction in R583Q and D715E. Biexponential current inactivation during 3-s test pulses was significantly faster in D715E and slower in V1457L than in wild type. Mutations R583Q and V1457L delayed the time course of recovery from channel inactivation. The decrease of peak current through R583Q (30.2%) and D715E (30. 1%) but not V1457L (18.7%) was more pronounced during 1-Hz trains of 15 100-ms pulses than in wild type (18.2%). Our data demonstrate that the mutations R583Q, D715E, and V1457L, like the previously reported mutations T666M, V714A, and I1819L, affect P/Q-type Ca(2+) channel gating. We therefore propose that altered channel gating represents a common pathophysiological mechanism in familial hemiplegic migraine.

beta subunit heterogeneity of L-type Ca(2+) channels in smooth muscle tissues

Various beta subunit isoforms stabilize different gating properties of voltage-gated L-type Ca(2+) channels. We therefore investigated the expression of Ca(2+) channel beta subunit isoforms in different smooth muscle types on the protein level by immunoblotting and immunoprecipitation employing beta subunit-selective sequence-directed antibodies. From the four known beta subunit isoforms only beta2 and beta3 were detected in porcine uterus, bovine trachea and bovine aorta membranes. Multiple immunoreactive beta2 bands were detected in a tissue-selective manner indicating structural heterogeneity of beta2. Immunoprecipitation of (+)-[(3)H]isradipine-prelabeled channels revealed that beta2 and beta3 participate in Ca(2+) channel formation in uterus and trachea, and beta3 in aortic smooth muscle. We conclude that beta2 and beta3 subunits form L-type Ca(2+) channels in smooth muscle tissues. This subunit heterogeneity may be important to fine-tune channel function.

Current modulation and membrane targeting of the calcium channel alpha1C subunit are independent functions of the beta subunit

1. The beta subunits of voltage-sensitive calcium channels facilitate the incorporation of channels into the plasma membrane and modulate calcium currents. In order to determine whether these two effects of the beta subunit are interdependent or independent of each other we studied plasma membrane incorporation of the channel subunits with green fluorescent protein and immunofluorescence labelling, and current modulation with whole-cell and single-channel patch-clamp recordings in transiently transfected human embryonic kidney tsA201 cells. 2. Coexpression of rabbit cardiac muscle alpha1C with rabbit skeletal muscle beta1a, rabbit heart/brain beta2a or rat brain beta3 subunits resulted in the colocalization of alpha1C with beta and in a marked translocation of the channel complexes into the plasma membrane. In parallel, the whole-cell current density and single-channel open probability were increased. Furthermore, the beta2a isoform specifically altered the voltage dependence of current activation and the inactivation kinetics. 3. A single amino acid substitution in the beta subunit interaction domain of alpha1C (alpha1CY467S) disrupted the colocalization and plasma membrane targeting of both subunits without affecting the beta subunit-induced modulation of whole-cell currents and single-channel properties. 4. These results show that the modulation of calcium currents by beta subunits can be explained by beta subunit-induced changes of single-channel properties, but the formation of stable alpha1C-beta complexes and their increased incorporation into the plasma membrane appear not to be necessary for functional modulation.

Subcellular localization of chromogranins, calcium channels, amine carriers, and proteins of the exocytotic machinery in bovine splenic nerve

Subcellular fractionation of bovine splenic nerves, which consist mainly of sympathetic nerve fibers, has been useful for characterizing cellular organelles en route to the terminal. In the present study we have characterized the subcellular distribution of both secretory and membrane proteins. A newly discovered chromogranin-like protein, NESP55, was found in large dense-core vesicles. The endogenous processing of NESP55 was comparable to that of chromogranins but more limited than that of secretogranin II and chromogranin B. For membrane proteins three major types of distribution were found. The amine carrier VMAT2 was confined to large dense-core vesicles. VAMP or synaptobrevin was present both in large dense-core vesicles and in lighter vesicles, whereas SNAP-25, syntaxin, and two types (N and L) of Ca2+ channels were found in a special population of lighter vesicles but were not present in large dense-core vesicles or at the most in very low concentrations. The plasma membrane norepinephrine transporter was apparently present in a separate type of vesicle, but this requires further study. These results further characterize vesicles en route to the terminal and establish for the first time that peptides involved in exocytosis (syntaxin, SNAP-25, and N- and L-type Ca2+ channels) are apparently transported to the terminal in a special type of vesicle. The exclusive presence of the amine carrier in large dense-core vesicles indicates that the formation of small dense-core vesicles in the terminals requires a reuse of membrane components of large dense-core vesicles.

Molecular mechanism of diltiazem interaction with L-type Ca2+ channels

Benzothiazepine Ca2+ antagonists (such as (+)-cis-diltiazem) interact with transmembrane segments IIIS6 and IVS6 in the alpha1 subunit of L-type Ca2+ channels. We investigated the contribution of individual IIIS6 amino acid residues for diltiazem sensitivity by employing alanine scanning mutagenesis in a benzothiazepine-sensitive alpha1 subunit chimera (ALDIL) expressed in Xenopus laevis oocytes. The most dramatic decrease of block by 100 microM diltiazem (ALDIL 45 +/- 4.8% inhibition) during trains of 100-ms pulses (0.1 Hz, -80 mV holding potential) was found after mutation of adjacent IIIS6 residues Phe1164(21 +/- 3%) and Val1165 (8.5 +/- 1.4%). Diltiazem delayed current recovery by promoting a slowly recovering current component. This effect was similar in ALDIL and F1164A but largely prevented in V1165A. Both mutations slowed inactivation kinetics during a pulse. The reduced diltiazem block can therefore be explained by slowing of inactivation kinetics (F1164A and V1165A) and accelerated recovery from drug block (V1165A). The bulkier diltiazem derivative benziazem still efficiently blocked V1165A. From these functional and from additional radioligand binding studies with the dihydropyridine (+)-[3H]isradipine we propose a model in which Val1165 controls dissociation of the bound diltiazem molecule, and where bulky substituents on the basic nitrogen of diltiazem protrude toward the adjacent dihydropyridine binding domain.