Calcineurin finds a new partner in the L-type Ca2+ channel.

Calcineurin is a Ca 2 /calmodulin-sensitive phosphatase that sits near the top of signaling pathways leading to pathological cardiac hypertrophy.1 Pathological stressors activate calcineurin, for which nuclear factor of activated T cells (NFAT) is a principal substrate. Dephosphorylation unmasks a nuclear localization signal on NFAT, which then translocates to the nucleus and, there, serves as part of a transcription factor complex able to initiate gene expression cascades that induce cardiac hypertrophy and subsequent heart failure. Determining how calcineurin is specifically activated in response to hypertrophic stressors by the same Ca signal used for excitation–contraction coupling has been a major challenge. Triggered by Ca influx through CaV1.2 voltage-gated Ca 2 channels at the sarcolemmal membrane, intracellular calcium briefly increases 10-fold with each heart beat to generate myocyte contraction. Almost as quickly, intracellular calcium falls to its baseline level, allowing myocyte relaxation and preparation for the next beat. There are 2 leading proposals for how Ca could activate the calcineurin/NFAT pathway or function as a versatile regulator of multiple other signaling cascades in the face of the large Ca oscillations driving each cycle of contraction and relaxation. One is that modulation of the amplitude and/or frequency of the oscillatory contractile Ca signal triggers particular Ca sensors that then activate particular downstream signaling pathways, such as calcineurin activation.2 The alternative suggests that local Ca signaling microdomains, secluded from the contractile oscillations, feed into specific signaling cascades, such as calcineurin activation.3,4 In this issue of Circulation Research, Tandan et al5 propose that calcineurin binds directly to the sarcolemmal CaV1.2 voltage-gated Ca 2 channel and thereby modulates its function. By localizing calcineurin at the site of the major source of Ca entry, this report could provide more support for the Ca -signaling microdomain hypothesis and offer a new wrinkle into the understanding of hypertrophic signaling. We already knew that the CaV1.2 Ca 2 channel and calcineurin could reside in the same neighborhood. In neurons, for example, the A-kinase anchoring protein AKAP79/ 150 has been shown to act as a scaffold for both the CaV1.2 channel and calcineurin.6 CaV1.2 also functions as part of a complex with PKC and calcineurin in vascular smooth muscle cells; here, AKAP79/150 is also thought to be the scaffold.7 In both cases, the coupled calcineurin activates NFAT via Ca influx through CaV1.2. Now, Tandan et al5 report that in cardiac myocytes, calcineurin and CaV1.2 can shed their AKAP chaperone and cohabit. They identify direct calcineurin-binding sites on the CaV1.2 N and C termini. Nevertheless, CaV1.2 is a strange bedfellow. Both calcineurin-binding sites (the most extensively characterized in the distal C terminus [amino acids 1943 to 1971] and another, unidentified site in the N terminus) lack a consensus VIVIT-like motif found in many validated calcineurinbinding proteins, such as NFAT8 and AKAP79/150.9 Tandan et al also report that the interaction with the CaV1.2 C terminus renders the channel a calcineurin substrate and that calcineurin reverses PKC phosphorylation. Thus, this report may define a new calcineurin interaction motif and a novel mode of action. A particularly notable finding is that calcineurin positively regulates CaV1.2 in neonatal myocytes. Although adenoviral overexpression of calcineurin increased CaV1.2 currents,5 consistent with previous reports that calcineurin-induced hypertrophy augmented CaV1.2 currents in myocytes,10,11 none of these experiments distinguished whether calcineurin directly potentiated the CaV1.2 currents or if the increased current amplitude was an indirect outcome of the resulting cellular hypertrophy. Here, Tandan et al5 report that calcineurin inhibition (via cyclosporin) induced an immediate and partially reversible potentiation of CaV1.2 currents in neonatal myocytes. The rapidity and reversibility of these effects suggest a direct effect of calcineurin on CaV1.2 rather than transcriptional regulation via actions on NFAT. The potentiating effects of calcineurin on CaV1.2 in neonatal myocytes are surprising, however, because a recent study suggests that the predominant effect of phosphorylation on cardiac CaV1.2 by PKC, which calcineurin is proposed to antagonize, is potentiation.12 This mechanism of action for calcineurin also provides a stark contrast to that reported in neurons,6 where AKAP79/150-bound calcineurin opposes potentiation of CaV1.2 by PKA. In neonatal myocytes, Tandan et al found that neither adenoviral overexpression of calcineurin nor RCAN1 (a calcineurin inhibitor) affected isoproterenol-stimulated increase in CaV1.2 current amplitude. Thus, the specific means by which calcineurin produces this intriguing rapid and reversible CaV1.2 potentiation are not yet apparent. So, does this CaV1.2-bound calcineurin with newly defined properties activate NFAT, as the AKAP-bound calcineurin The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Ion Channel Research Unit, Department of Medicine, Duke University Medical Center, Durham, NC. Correspondence to Geoffrey S. Pitt, Department of Medicine, Duke University Medical Center, Box 103030 Medical Center, Durham, NC 27710. E-mail [email protected] (Circ Res. 2009;105:7-8.) © 2009 American Heart Association, Inc.