Adrenergic signaling controls RGK-dependent trafficking of cardiac voltage-gated L-type Ca ²⁺ channels through PKD1

Rationale: The Rad-Gem/Kir-related family (RGKs) consists of small GTP-binding proteins that strongly inhibit the activity of voltage-gated calcium channels. Among RGKs, Rem1 is strongly and specifically expressed in cardiac tissue. However, the physiological role and regulation of RGKs, and Rem1 in particular, are largely unknown. Objective: To determine if Rem1 function is physiologically regulated by adrenergic signaling and thus impacts voltage-gated L-type calcium channel (VLCC) activity in the heart. Methods and Results: We found that activation of protein kinase D1, a protein kinase downstream of α1-adrenergic signaling, leads to direct phosphorylation of Rem1 at Ser18. This results in an increase of the channel activity and plasma membrane expression observed by using a combination of electrophysiology, live cell confocal microscopy, and immunohistochemistry in heterologous expression system and neonatal cardiomyocytes. In addition, we show that stimulation of α1-adrenergic receptor-protein kinase D1-Rem1 signaling increases transverse-tubule VLCC expression that results in increased L-type Ca current density in adult ventricular myocytes. Conclusion: The α1-adrenergic stimulation releases Rem1 inhibition of VLCCs through direct phosphorylation of Rem1 at Ser18 by protein kinase D1, resulting in an increase of the channel activity and transverse-tubule expression. Our results uncover a novel molecular regulatory mechanism of VLCC trafficking and function in the heart and provide the first demonstration of physiological regulation of RGK function.