Physical and Functional Interaction Between Calcineurin and the Cardiac L-Type Calcium Channel
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The L-type Ca2+ channel is the major mediator of Ca2+ influx in cardiomyocytes leading to both mechanical contraction and activation of signaling cascades. Among these cascades is calcineurin, a protein phosphatase that promotes hypertrophic growth of the heart. We previously reported from in vivo models of pressure-overload that calcineurin regulates Ca2+ channel function in the heart, such that, inhibition of calcineurin activity results in a decrease in channel function. Based on this, we hypothesize that calcineurin participates in the channel macromolecular complex. Initial immunohistochemical data demonstrated evidence for co-localization of calcineurin to alpha1C, the pore-forming subunit of the L-type Ca2+ channel in mouse ventricular tissue. Additionally, co-immunoprecipitation biochemical experiments revealed evidence for binding between calcineurin and alpha1C in native mouse and rat heart tissues. Pulldown assays using GST-fusion proteins of all intracellular alpha1C regions provided evidence for direct binding of calcineurin at the N- and C-termini of alpha1C. At the C-terminus, calcineurin bound to aa 1909-2029 overlapping the well-characterized PKA/PKC site Ser-1928. In vitro kinase/phosphatase assays revealed Ser-1928 as a substrate for calcineurin dephosphorylation. Voltage-clamp recordings of L-type Ca2+ currents from cultured cardiomyocytes expressing constitutively-active calcineurin revealed significant up-regulation of channel function, similar to our previous observations from cardiac hypertrophy in vivo. Conversely, acute suppression of calcineurin, both pharmacologically or with specific peptide-inhibitors, induced a significant decrease in L-type channel function, while neither intervention had an effect on channel function in the absence of calcineurin activity. These data provide evidence for direct interaction between the L-type Ca2+ channel and calcineurin, and insights into the regulation of the channel by calcineurin. Furthermore, they highlight the specific role of calcineurin as a potential mediator of pathophysiological electrical remodeling in cardiac hypertrophy and failure.