A Ca V1.1 Ca 2+ Channel Splice Variant with High Conductance and Voltage-Sensitivity Alters EC Coupling in Developing Skeletal Muscle

Abstract

The Ca 2+ channel α 1S subunit (Ca V1.1) is the voltage sensor in skeletal muscle excitation-contraction (EC) coupling. Upon membrane depolarization, this sensor rapidly triggers Ca 2+ release from internal stores and conducts a slowly activating Ca 2+ current. However, this Ca 2+ current is not essential for skeletal muscle EC coupling. Here, we identified a Ca V1.1 splice variant with greatly distinct current properties. The variant of the CACNA1S gene lacking exon 29 was expressed at low levels in differentiated human and mouse muscle, and up to 80% in myotubes. To test its biophysical properties, we deleted exon 29 in a green fluorescent protein (GFP)-tagged α 1S subunit and expressed it in dysgenic ( α 1S-null) myotubes. GFP- α 1SΔ29 was correctly targeted into triads and supported skeletal muscle EC coupling. However, the Ca 2+ currents through GFP- α 1SΔ29 showed a 30-mV left-shifted voltage dependence of activation and a substantially increased open probability, giving rise to an eightfold increased current density. This robust Ca 2+ influx contributed substantially to the depolarization-induced Ca 2+ transient that triggers contraction. Moreover, deletion of exon 29 accelerated current kinetics independent of the auxiliary α 2 δ-1 subunit. Thus, characterizing the Ca V1.1Δ29 splice variant revealed the structural bases underlying the specific gating properties of skeletal muscle Ca 2+ channels, and it suggests the existence of a distinct mode of EC coupling in developing muscle.