Absence of regulation of the T-type calcium current by Cav1.1, β1a and γ1 dihydropyridine receptor subunits in skeletal muscle cells

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

Botulinum Toxin Type A Possibly Affects Cav3.2 Calcium Channel Subunit in Rats with Spinal Cord Injury-Induced Muscle Spasticity

INTRODUCTION

Spinal cord injury (SCI) often causes muscle spasticity, which can be inhibited by using calcium channel blocker. Botulinum toxin type A (BoT-A) shows therapeutic efficacy on spasticity and may exert inhibitory effects on the calcium channel.

METHODS

A rat model with muscle spasticity was established after SCI via contusion and compression. Different concentrations (0, 1, 3 and 6 U/kg) of BoT-A Botox were injected in the extensor digitorum longus (EDL) muscles of the right hindlimb in the muscle spasticity model. The changes of muscle spasticity and calcium level in EDL muscles were measured after the establishment of SCI-induced spasticity. Cav3.2 calcium channel subunit and its mutant (M1560V) were analyzed using Western blot before (input) or after immunoprecipitation with anti-FLAG antibody, and their currents were measured in motoneurons by using whole-cell voltage clamp recordings.

RESULTS

SCI induced muscle spasticity, whereas calcium level in EDL muscles and expression of Cav3.2 was increased in the SCI model when compared with the sham group (p < 0.05). BoT-A Botox treatment significantly reduced muscle spasticity and calcium level in EDL muscles and Cav3.2 expression in a dose-dependent way (p < 0.05). The ratio of biotinylated to total Cav3.2 was reduced in the mutant (M1560V) of Cav3.2 and lower than that in the wild Cav3.2. BoT-A Botox intervention also reduced the current values of calcium channel and the ratio in a dose-dependent way (p < 0.05).

DISCUSSION

BoT-A Botox possibly attenuates SCI-induced muscle spasticity by affecting the expression of Cav3.2 calcium channel subunit in the rat models. There may be multiple mechanisms for the function of BoT-A Botox. Further work is needed to be done to address these issues.

L-type CaV1.2 calcium channels: from in vitro findings to in vivo function

The L-type Cav1.2 calcium channel is present throughout the animal kingdom and is essential for some aspects of CNS function, cardiac and smooth muscle contractility, neuroendocrine regulation, and multiple other processes. The L-type CaV1.2 channel is built by up to four subunits; all subunits exist in various splice variants that potentially affect the biophysical and biological functions of the channel. Many of the CaV1.2 channel properties have been analyzed in heterologous expression systems including regulation of the L-type CaV1.2 channel by Ca(2+) itself and protein kinases. However, targeted mutations of the calcium channel genes confirmed only some of these in vitro findings. Substitution of the respective serines by alanine showed that β-adrenergic upregulation of the cardiac CaV1.2 channel did not depend on the phosphorylation of the in vitro specified amino acids. Moreover, well-established in vitro phosphorylation sites of the CaVβ2 subunit of the cardiac L-type CaV1.2 channel were found to be irrelevant for the in vivo regulation of the channel. However, the molecular basis of some kinetic properties, such as Ca(2+)-dependent inactivation and facilitation, has been approved by in vivo mutagenesis of the CaV1.2α1 gene. This article summarizes recent findings on the in vivo relevance of well-established in vitro results.

The alpha1 subunit EGL-19, the alpha2/delta subunit UNC-36, and the beta subunit CCB-1 underlie voltage-dependent calcium currents in Caenorhabditis elegans striated muscle

Voltage-gated calcium channels, which play key roles in many physiological processes, are composed of a pore-forming α1 subunit associated with up to three auxiliary subunits. In vertebrates, the role of auxiliary subunits has mostly been studied in heterologous systems, mainly because of the severe phenotypes of knock-out animals. The genetic model Caenorhabditis elegans has all main types of voltage-gated calcium channels and strong loss-of-function mutations in all pore-forming and auxiliary subunits; it is therefore a useful model to investigate the roles of auxiliary subunits in their native context. By recording calcium currents from channel and auxiliary subunit mutants, we molecularly dissected the voltage-dependent calcium currents in striated muscle of C. elegans. We show that EGL-19 is the only α1 subunit that carries calcium currents in muscle cells. We then demonstrate that the α2/δ subunit UNC-36 modulates the voltage dependence, the activation kinetics, and the conductance of calcium currents, whereas another α2/δ subunit TAG-180 has no effect. Finally, we characterize mutants of the two β subunits, CCB-1 and CCB-2. CCB-1 is necessary for viability, and voltage-dependent calcium currents are abolished in the absence of CCB-1 whereas CCB-2 does not affect currents. Altogether these results show that EGL-19, UNC-36, and CCB-1 underlie voltage-dependent calcium currents in C. elegans striated muscle.