Dihydropyridine-sensitive calcium channels from skeletal muscle. I. Roles of subunits in channel activity

The molecular basis of the inhibition of CaV1 calcium-dependent inactivation by the distal carboxy tail

Ca2+/calmodulin-dependent inactivation (CDI) of CaV channels is a critical regulatory process that tunes the kinetics of Ca2+ entry for different cell types and physiologic responses. CDI is mediated by calmodulin (CaM), which is bound to the IQ domain of the CaV carboxy tail. This modulatory process is tailored by alternative splicing such that select splice variants of CaV1.3 and CaV1.4 contain a long distal carboxy tail (DCT). The DCT harbors an inhibitor of CDI (ICDI) module that competitively displaces CaM from the IQ domain, thereby diminishing CDI. While this overall mechanism is now well described, the detailed interactions required for ICDI binding to the IQ domain are yet to be elucidated. Here, we perform alanine-scanning mutagenesis of the IQ and ICDI domains and evaluate the contribution of neighboring regions to CDI inhibition. Through FRET binding analysis, we identify functionally relevant residues within the CaV1.3 IQ domain and the CaV1.4 ICDI and nearby A region, which are required for high-affinity IQ/ICDI binding. Importantly, patch-clamp recordings demonstrate that disruption of this interaction commensurately diminishes ICDI function resulting in the re-emergence of CDI in mutant channels. Furthermore, CaV1.2 channels harbor a homologous DCT; however, the ICDI region of this channel does not appear to appreciably modulate CaV1.2 CDI. Yet coexpression of CaV1.2 ICDI with select CaV1.3 splice variants significantly disrupts CDI, implicating a cross-channel modulatory scheme in cells expressing both channel subtypes. In all, these findings provide new insights into a molecular rheostat that fine-tunes Ca2+-entry and supports normal neuronal and cardiac function.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

Tunicamycin Dissociates Depolarization-induced Calcium Entry From Transmitter Release. Involvement of Glycosylated Protein(s) in the Process of Neurosecretion in PC-12 Cells

The process of regulated secretion in PC-12 cells is tightly coupled to calcium entry, which is absolutely dependent on extracellular Ca2+([Ca2+]ex). Tunicamycin treatment of the cells dissociated depolarization-triggered Ca2+ influx from depolarization (high K+)-induced transmitter release into two distinct and independent phases. Deplarization-evoked Ca2+ influx was not affected by tunicamycin treatment (1 microg/ml, 72 h), whereas depolarization-evoked transmitter release was strongly inhibited (> 60%), suggesting at least a two-step process, and the participation of glycosylated protein(s) in the actual fusion/secretion step. Similarly, bradykinin-mediated transmitter release was linearly related to and absolutely dependent on Ca2+ entry, and was inhibited by tunicamycin treatment (> 80%), whereas bradykinin-evoked Ca2+ entry was not impaired, indicating that glycosylated protein(s) are essential for bradykinin-evoked release at a step subsequent to Ca2+ influx. The heavily glycosylated alpha2 subunit of the dihydropyridine-sensitive channel, which was used to monitor tunicamycin inhibition of glycosylation, was not expressed in the tunicamycin-treated cells, as shown by Western blot analysis. This observation allowed us to conclude that the alpha1 subunit of the heteromeric dihydropyridine voltage-sensitive Ca2+ channel, which is responsible for Ca2+ entry, is also fully functional when not assembled with its corresponding alpha2 subunit. The molecular properties of the alpha2 subunit, whose role in the complex structure of the channel is not yet understood, are shown for the first time for the L-type Ca2+ channel of PC-12 cells. Similar to cardiac and skeletal muscle cells, the alpha2 subunit appears to be a glycosylated polypeptide of molecular weight 170 kD and to display a characteristic mobility shift to 140 kD under reducing conditions.

Molecular pharmacology of UK-118, 434-05, a permanently charged amlodipine analog

We studied the effects of UK-118, 434-05, a permanently charged form of amlodipine, on recombinant smooth muscle and cardiac L-type calcium channels to determine the distinct modulatory properties of the ionized form of amlodipine. We found that the short distance between the permanent charge group and the active dihydropyridine (DHP) ring of UK-118, 434-05 reduces the potency of this compound as an inhibitor of smooth muscle (alpha(1c-b)) L-type channels, and is similar to the effects of other charged DHP derivatives on cardiac (alpha(1c-a)) L-type channels. However, we found surprisingly that the tonic block of cardiac (alpha(1c-a)) L-type channels was more pronounced than the tonic block of smooth muscle (alpha(1c-b)) L-type channels. This result contrasts with the previously reported subunit-specificity of neutral DHP compounds, and suggests that interactions between the amlodipine charge group and site(s) on the L-type channel alpha1 subunit distinguish the action of charged from neutral DHPs and may contribute to amlodipine's unique pharmacological profile.

L-type Ca2+ channel-insulin-like growth factor-1 receptor signaling impairment in aging rat skeletal muscle

The present study investigates the modulation of skeletal muscle L-type Ca2+ channel receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation. Single extensor digitorum longus and multifiber preparations were isolated from 7- (young), 14- (middle-age) and 28-(old) month- Fisher 344 X Brown Norway rats. Calcium current was potentiated in fibers from young and middle-age rats due to a -13 mV shift in half-activation potential. Fibers from old animals failed to show current potentiation in response to IGF-1R activation. IGF-1 induced a ten-fold increase in the phosphorylation of the L-type Ca2+ channel alpha1 subunit in young and middle-age fibers but failed to induce phosphorylation in old fibers. Addition of 0.5 mM Ca2+ increased the IGF-1 induced phosphorylation in young and middle-age fibers three fold but not in old fibers. The tyrosine kinase inhibitor, genistein, and the PKC inhibitor peptide, 19-36, decreased IGF-1 induced phosphorylation of alpha1 subunit to 15% in young and middle-age fibers but failed to inhibit phosphorylation in old fibers. These results demonstrate that the IGF-1-L-type Ca2+ channel alpha1 subunit signaling is impaired in skeletal muscle fibers from old animals due to alterations in the trk-PKC pathway.

Developmental and tissue-specific regulation of rabbit skeletal and cardiac muscle calcium channels involved in excitation-contraction coupling

Two types of calcium channels signal excitation-contraction (E-C) coupling in striated muscle: dihydropyridine receptors (DHPRs, voltage-gated L-type calcium channels on the transverse tubule) and ryanodine receptors (RyRs, calcium release channels on the sarcoplasmic reticulum). Sarcolemmal depolarization activates the DHPR; subsequently, the RyR is activated and releases calcium that activates muscle contraction. We show in the present study that expression of the E-C coupling calcium channels is upregulated during myogenic development in the rabbit. Skeletal and cardiac muscle isoforms of the following genes were examined: the DHPR alpha 1, alpha 2, beta, and gamma subunits and the RyR. Distinct cardiac and skeletal muscle-specific cDNAs were isolated, encoding each of the DHPR subunits and the RyR. The skeletal muscle DHPR alpha 1, alpha 2, beta, and gamma subunits and the cardiac DHPR alpha 1 subunit mRNA levels increased on the day of birth and at the adult stage compared with fetal levels. The skeletal and cardiac RyR mRNA levels increased on the day of birth and at adult stages compared with fetal levels. Ryanodine binding sites increased in both skeletal and cardiac muscle. We now provide a molecular explanation for the physiological "maturation" of the E-C coupling apparatus observed at the day of birth and during early postnatal development in both skeletal and cardiac muscles. Low levels of calcium channel expression in fetal cardiac and skeletal muscle make these tissues more sensitive to pharmacological therapy with calcium channel blockers, a phenomenon that has been reported in human neonates.

A large scale preparative gel electrophoresis separation of alpha 1 and alpha 2 subunits of the voltage-gated Ca2+ channel from rabbit skeletal muscle

A large-scale preparative gel electrophoresis method effectively separates individual voltage-gated calcium channel (VGCC) subunits with high resolution, starting with up to 4 mg of rabbit skeletal muscle L-type VGCC complex. Using this method, we separated alpha 1 and alpha 2 subunits of rabbit skeletal muscle VGCC with a high efficiency and with protein recoveries of 83%. The separated alpha 1 and alpha 2 subunits eluted from the gel in a 1:1 molar ratio. The method should be applicable for separating the other VGCC subunits or subunits of other protein complexes.

Blockers of Ca2+ channels in the plasmalemma of perfused Characeae cells

Ionic currents in the plasmalemma of perfused Nitella syncarpa cells identified as currents through Ca2+ channels were registered for the first time. The effect of 1,4-dihydropyridine derivatives (nifedipine, nitrendipine, riodipine) and phenylalkylamines (verapamil, D600) as well as the agonist CGP-28392 on the Ca2+ channels in the plasmalemma of perfused cells of Nitellopsis obtusa and Nitella syncarpa have been studied. A blocking effect of 1,4-dihydropyridine derivatives and phenylalkylamines on the plasmalemma Ca2+ channels has been detected. Phenylalkylamines have been found to block both inward and outward Ca2+ currents. The activating effect of the agonist CGP-28392 on the Ca2+ channels of plasmalemma has been shown.

Modulation of cardiac Ca2+ channels in Xenopus oocytes by protein kinase C

L-Type calcium channel was expressed in Xenopus laevis oocytes injected with RNAs coding for different cardiac Ca2+ channel subunits, or with total heart RNA. The effects of activation of protein kinase C (PKC) by the phorbol ester PMA (4 beta-phorbol 12-myristate 13-acetate) were studied. Currents through channels composed of the main (alpha 1) subunit alone were initially increased and then decreased by PMA. A similar biphasic modulation was observed when the alpha 1 subunit was expressed in combination with alpha 2/delta, beta and/or gamma subunits, and when the channels were expressed following injection of total rat heart RNA. No effects on the voltage dependence of activation were observed. The effects of PMA were blocked by staurosporine, a protein kinase inhibitor. beta subunit moderate the enhancement caused by PMA. We conclude that both enhancement and inhibition of cardiac L-type Ca2+ currents by PKC are mediated via an effect on the alpha 1 subunit, while the beta subunit may play a mild modulatory role.