Four cAMP effector proteins are expressed in the heart. Norepinephrine (NE) secreted by sympathetic neurons binds to the β-adrenergic receptor leading to G_s activation and synthesis of cAMP by adenylate cyclase (AC). Acetylcholine (ACh) is secreted by parasympathetic neurons and binds to muscarinergic ACh receptors leading to an activation of G_i causing AC inhibition. The balance of sympathetic and parasympathetic input therefore determines the level of cAMP production. cAMP production in cells is compartmentalized and this is mainly achieved by phoshodiesterases (PDE), which limits cAMP diffusion through degradation. Four effector proteins sense cAMP levels. The best-characterized effector protein is protein kinase A (PKA), which plays a role in cardiac pacemaking, excitation/contraction coupling and cardiac metabolism. The exchange factor directly activated by cAMP (EPAC) has been linked to cardiac hypertrophy, Ca²⁺-signaling and apoptosis. Often, PKA and EPAC are bound by the same anchor protein (AKAP) along with protein substrates and other enzymes. The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are important for cardiac pacemaking and ventricular repolarization. Finally, the Popeye domain containing (POPDC) proteins are the most recently identified class of effector proteins and important for cardiac pacemaking, the survival of cardiac myocytes after ischemia/reperfusion and membrane trafficking.

Structure of the Popeye domain containing proteins. (a) Schematic structure of the human POPDC isoforms. The three POPDC proteins share a similar protein structure. In each case a 20–40 amino acid long extracellular domain (ECD), which harbors one or two N-glycosylation sites (asterisks) is followed by three transmembrane (TM) domains (I–III, black boxes). The cytoplasmic portion of the POPDC proteins consists of the Popeye domain (red box) and the carboxyterminal domain (CTD, yellow box). The phosphate-binding cassette (PBC), which is thought to bind cAMP, consists of the two tetrapeptides DSPE and FQVT. In case of POPDC1, the locations for the KCNK2 (TREK-1) [28] and CAV3 [33] binding sites and the putative dimerization motif [34] are indicated (blue bars). POPDC2 generates three alternative splice products, which differ in their carboxy-terminus labeled by a grey box. Mutations in POPDC1 and POPDC2, which have been identified in patients with muscle and heart disease are indicated above each protein model. (b) 3-D structure of POPDC3. A homology-based structural model was generated with the help of the Phyre 2 algorithm [35]. The resulting structure was visualized with the help of First Glance in JMOL [36].

Structure of the Popeye domain. A homology-based structural model of human POPDC1 was generated. A space-filling model of cAMP was placed in the putative phosphate-binding cassette. The DSPE and FQVT motifs, depicted as yellow halos, surround the cAMP molecule and as mutagenesis studies suggest, seem to be directly involved in cAMP-binding.

Expression of POPDC genes in the human body. RNA seq data of POPDC1, POPDC2 and POPDC3 in human tissue. RNA expression is quantified as reads per kilogram per million mapped reads (RPKM). Data were copied from the Human Protein Atlas website [46,47,48].

Expression of POPDC1 and POPDC2 in cardiac myocytes. Isolated adult cardiac myocytes were immunostained with (a) POPDC1 and (b) POPDC2 antibodies, respectively.