Ca2+-Dependent Excitation-Contraction Coupling Triggered by the Heterologous Cardiac/Brain DHPR β2a-Subunit in Skeletal Myotubes

Abstract

Molecular determinants essential for skeletal-type excitation-contraction (EC) coupling have been described in the cytosolic loops of the dihydropyridine receptor (DHPR) alpha1S pore subunit and in the carboxyl terminus of the skeletal-specific DHPR beta1a-subunit. It is unknown whether EC coupling domains present in the beta-subunit influence those present in the pore subunit or if they act independent of each other. To address this question, we investigated the EC coupling signal that is generated when the endogenous DHPR pore subunit alpha1S is paired with the heterologous heart/brain DHPR beta2a-subunit. Studies were conducted in primary cultured myotubes from beta1 knockout (KO), ryanodine receptor type 1 (RyR1) KO, ryanodine receptor type 3 (RyR3) KO, and double RyR1/RyR3 KO mice under voltage clamp with simultaneous monitoring of confocal fluo-4 fluorescence. The beta2a-mediated Ca2+ current recovered in beta1 KO myotubes lacking the endogenous DHPR beta1a-subunit verified formation of the alpha1S/beta1a pair. In myotube genotypes which express no or low-density L-type Ca2+ currents, namely beta1 KO and RyR1 KO, beta2a overexpression recovered a wild-type density of nifedipine-sensitive Ca2+ currents with a slow activation kinetics typical of skeletal myotubes. Concurrent with Ca2+ current recovery, there was a drastic reduction of voltage-dependent, skeletal-type EC coupling and emergence of Ca2+ transients triggered by the Ca2+ current. A comparison of beta2a overexpression in RyR3 KO, RyR1 KO, and double RyR1/RyR3 KO myotubes concluded that both RyR1 and RyR3 isoforms participated in Ca2+-dependent Ca2+ release triggered by the beta2a-subunit. In beta1 KO and RyR1 KO myotubes, the Ca2+-dependent EC coupling promoted by beta2a overexpression had the following characteristics: 1), L-type Ca2+ currents had a wild-type density; 2), Ca2+ transients activated much slower than controls overexpressing beta1a, and the rate of fluorescence increase was consistent with the activation kinetics of the Ca2+ current; 3), the voltage dependence of the Ca2+ transient was bell-shaped and the maximum was centered at approximately +30 mV, consistent with the voltage dependence of the Ca2+ current; and 4), Ca2+ currents and Ca2+ transients were fully blocked by nifedipine. The loss in voltage-dependent EC coupling promoted by beta2a was inferred by the drastic reduction in maximal Ca2+ fluorescence at large positive potentials (DeltaF/Fmax) in double dysgenic/beta1 KO myotubes overexpressing the pore mutant alpha1S (E1014K) and beta2a. The data indicate that beta2a, upon interaction with the skeletal pore subunit alpha1S, overrides critical EC coupling determinants present in alpha1S. We propose that the alpha1S/beta pair, and not the alpha1S-subunit alone, controls the EC coupling signal in skeletal muscle.

Bidirectional signaling between calcium channels of skeletal muscle requires multiple direct and indirect interactions

“…3a and Table 3). This enhancement of the L-type Ca 2ϩ current may be a result of the overexpression of RyR3, because double RyR1/RyR3 KO skeletal myotubes produced Ca 2ϩ currents at levels similar to dyspedic myotubes, which include low levels of RyR3 (25,26).…”

Reconstitution of local Ca2+ signaling between cardiac L-type Ca2+ channels and ryanodine receptors: insights into regulation by FKBP12.6

“…The additional variable kЈ is a scaling factor that varies with (⌬F/F)max (33). Pooled currentvoltage (I-V) and fluorescence-voltage (⌬F/F-V) data in Table 1 are expressed as means Ϯ SE.…”

Triadin Binding to the C-Terminal Luminal Loop of the Ryanodine Receptor is Important for Skeletal Muscle Excitation–Contraction Coupling

“…The additional variable k' is a scaling factor that varies with (∆F/F) max (Sheridan et al, 2003;Goonasekera et al, 2005). Pooled current-voltage (I-V) and fl uorescence-voltage (∆F/F-V) data in Table I are expressed as mean ± SEM.…”