Adrenergic Ca_V1.2 Activation via Rad Phosphorylation Converges at α_1C I-II Loop

Changing activity of cardiac Ca_V1.2 channels under basal conditions, during sympathetic activation, and in heart failure is a major determinant of cardiac physiology and pathophysiology. Although cardiac Ca_V1.2 channels are prominently upregulated via activation of PKA (protein kinase A), essential molecular details remained stubbornly enigmatic.

The primary goal of this study was to determine how various factors converging at the Ca_V1.2 I-II loop interact to regulate channel activity under basal conditions, during β-adrenergic stimulation, and in heart failure.

We generated transgenic mice with expression of Ca_V1.2 α_1C subunits with (1) mutations ablating interaction between α_1C and β-subunits, (2) flexibility-inducing polyglycine substitutions in the I-II loop (GGG-α_1C), or (3) introduction of the alternatively spliced 25-amino acid exon 9* mimicking a splice variant of α_1C upregulated in the hypertrophied heart. Introducing 3 glycine residues that disrupt a rigid IS6-α-interaction domain helix markedly reduced basal open probability despite intact binding of Ca_Vβ to α_1C I-II loop and eliminated β-adrenergic agonist stimulation of Ca_V1.2 current. In contrast, introduction of the exon 9* splice variant in the α_1C I-II loop, which is increased in ventricles of patients with end-stage heart failure, increased basal open probability but did not attenuate stimulatory response to β-adrenergic agonists when reconstituted heterologously with β_2B and Rad or transgenically expressed in cardiomyocytes.

Ca²⁺ channel activity is dynamically modulated under basal conditions, during β-adrenergic stimulation, and in heart failure by mechanisms converging at the α_1C I-II loop. Ca_Vβ binding to α_1C stabilizes an increased channel open probability gating mode by a mechanism that requires an intact rigid linker between the β-subunit binding site in the I-II loop and the channel pore. Release of Rad-mediated inhibition of Ca²⁺ channel activity by β-adrenergic agonists/PKA also requires this rigid linker and β-binding to α_1C.