Calcineurin in the heart: New horizons for an old friend

Abstract

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca²⁺/CaM) and therefore acts as a node for integrating Ca²⁺ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Keywords: Calcineurin; Calcium signaling; Cardiac development; Heart failure; Hypertrophy; Intracellular signaling.

Figure 1:. Schematic shows the domain details of calcineurin catalytic and regulatory subunits, as well conserved domains in RCAN proteins using the RCAN1.4 isoform as an example.

The location of calpain cleavage sites are indicated by red arrow points below CnAα. CnAα* depicts the constitutively active calcineurin expressed in αMHC-CnA* transgenic mice. CnB binding domain (BBD), calmodulin binding domain (CBD), autoinhibitory sequence (AIS), Autoinhibitory domain (AID), polyproline rich domain (PP), membrane localization sequence 9MLS), LxVP calcineurin docking motif (LxVP), PxIxIT calcineurin docking motif (PxIxIT), EF-hand calcium-binding domain (EF), RCAN1 motif (SPPASPP), RCAN1 calcineurin inhibitory domain (TxxP).

Figure 2:. Model of calcineurin domain interactions

in the inactive state (A), activated following binding of Ca²⁺ and Ca²⁺/Calmodulin to disengage the AID from the catalytic site (B), NFAT docked with the activated form of calcineurin, small red circles indicate phosphorylated amino acids in the NFAT regulatory domain that are dephosphorylated by calcineurin (C), RCAN1 docked with calcineurin showing the TxxP motif blocking the catalytic site and inhibiting calcineurin activity (D), and docking of immunophilin bound drug complexes (E).

Figure 3:. Schematic of calcineurin (CN) functional and physical distribution in adult cardiomyocytes.

AKAPs and other proteins that act as calcineurin scaffolds are depicted in green. Interactions that are isoform-specific are indicated. Direct and indirect calcineurin substrates are depicted in blue. Calcineurin inhibitors are yellow. Note that CN is primarily tethered near known sites of Ca²⁺ release. A kinase anchor proteins (AKAP), β-adrenergic receptor (β-AR), BCL2 associated agonist of cell death (BAD), calcineurin (CN), carabin/TBC1D10C (Carabin), cardiomyopathy associated 5 (CMYA5), component of oligomeric goli complex 8 (COG8), connexin 43 (Cx43), dynamin related protein 1 (DRP1), inhibitor 1 (I1), LIM and cysteine-rich domains 1 (LMCD1), L-type calcium channel (LTCC), mammalian target of rapamycin complex 2 (mTROC2), muscle LIM protein (MLP), Myozenin 2 (MYOZ2), nuclear factor of activated T cells (NFAT), nuclear pore complex (NCP), protein phosphatase 1 (PP1), regulator of calcineurin 1 (RCAN1.4), ryanodine receptors (RyR), sodium-calcium exchanger (NCX1), sodium-potassium exchanger (NHE1), thyroid hormone receptor interactor 10 (TRIP10), transient receptor potential canonical (TRPC).