Novel Function of cardiac protein kinase D1 as a dynamic regulator of Ca²⁺ sensitivity of contraction

Although the function of protein kinase D1 (PKD) in cardiac cells has remained enigmatic, recent work has shown that PKD phosphorylates the nuclear regulators HDAC5/7 (histone deacetylase 5/7) and CREB, implicating this kinase in the development of dysfunction seen in heart failure. Additional studies have shown that PKD also phosphorylates multiple sarcomeric substrates to regulate myofilament function. Initial studies examined PKD through adenoviral vector expression of wild type PKD, constitutively active PKD (caPKD), or dominant negative PKD in cultured adult rat ventricular myocytes. Confocal immunofluorescent images of these cells reveal a predominant distribution of all PKD forms in a non-nuclear, Z-line localized, striated reticular pattern, suggesting the importance of PKD in Ca²⁺ signaling in heart. Consistent with an established role of PKD in targeting cardiac troponin I (cTnI), caPKD expression led to a marked decrease in contractile myofilament Ca²⁺ sensitivity with an unexpected electrical stimulus dependence to this response. This desensitization was accompanied by stimulus-dependent increases in cTnI phosphorylation in control and caPKD cells with a more pronounced effect in the latter. Electrical stimulation also provoked phosphorylation of regulatory site Ser⁹¹⁶ on PKD. The functional importance of this phospho-Ser⁹¹⁶ event is demonstrated in experiments with a phosphorylation-defective mutant, caPKD-S916A, which is functionally inactive and blocks stimulus-dependent increases in cTnI phosphorylation. Dominant negative PKD expression resulted in sensitization of the myofilaments to Ca²⁺ and blocked stimulus-dependent increases in cTnI phosphorylation. Taken together, these data reveal that localized PKD may play a role as a dynamic regulator of Ca²⁺ sensitivity of contraction in cardiac myocytes.