Popeye domain containing proteins are essential for stress-mediated modulation of cardiac pacemaking in mice.

Cardiac pacemaker cells create rhythmic pulses that control heart rate; pacemaker dysfunction is a prevalent disorder in the elderly, but little is known about the underlying molecular causes. Popeye domain containing (Popdc) genes encode membrane proteins with high expression levels in cardiac myocytes and specifically in the cardiac pacemaking and conduction system. Here, we report the phenotypic analysis of mice deficient in Popdc1 or Popdc2. ECG analysis revealed severe sinus node dysfunction when freely roaming mutant animals were subjected to physical or mental stress. In both mutants, bradyarrhythmia developed in an age-dependent manner. Furthermore, we found that the conserved Popeye domain functioned as a high-affinity cAMP-binding site. Popdc proteins interacted with the potassium channel TREK-1, which led to increased cell surface expression and enhanced current density, both of which were negatively modulated by cAMP. These data indicate that Popdc proteins have an important regulatory function in heart rate dynamics that is mediated, at least in part, through cAMP binding. Mice with mutant Popdc1 and Popdc2 alleles are therefore useful models for the dissection of the mechanisms causing pacemaker dysfunction and could aid in the development of strategies for therapeutic intervention.

Comparative analysis of mRNA and protein expression of Popdc1 (Bves) during early development in the chick embryo.

The isolation of the Popeye gene family was based on its preferential expression in striated muscle tissue. Recently, a monoclonal antibody against chick Popdc1 (also known as Bves) became available and was used in this study to comparatively analyze the expression pattern of Popdc1 at both the protein and mRNA level during early chick embryogenesis. Using whole-mount immunohistochemistry, expression in the heart was first observed at Hamburger and Hamilton (HH) stage 10 in the presumptive left ventricular segment. Cardiac expression was confined to differentiated cardiac myocytes, and undifferentiated myocytes at the anterior and posterior pole showed little expression. After looping, the outer curvature myocardium showed prominent Popdc1 staining, whereas the inner curvature was unlabeled. Despite previous reports, Popdc1 protein was not detectable at any time point in the proepicardium, epicardium, or the smooth muscle layer of the coronary vessels. Whole-mount in situ hybridization using a full-length Popdc1 probe detected novel expression domains, which have not been described previously. Popdc1 mRNA was found in Hensen's node at HH stage 4, and by HH stage 5+, expression became asymmetric. In addition, Popdc1 mRNA was found in pharyngeal endoderm and in the notochordal plate. Subsequently, beginning at HH stage 9, Popdc1 mRNA expression was found in the cardiac mesoderm and expression was maintained in the heart in a pattern very similar to the one observed by antibody staining.

Popeye domain containing gene 2 (Popdc2) is a myocyte-specific differentiation marker during chick heart development.

The Popeye domain containing (popdc) genes constitute a novel gene family encoding proteins of the plasma membrane in muscle cells, with three N-terminal transmembrane domains and a cytoplasmic carboxy terminus. In vertebrates, three members of the Popdc gene family have been described. However, in the chick system only two cDNAs, Popdc1 and Popdc3, have been cloned previously. By screening a chick expressed sequence tag database, we report here the identification of five alternatively spliced chick Popdc2 cDNAs with different carboxy termini. Northern blot analysis revealed expression of Popdc2 predominantly in the myocardium and weaker expression in skeletal muscle. By whole-mount in situ hybridization, chick Popdc2 was first detected at Hamburger and Hamilton (HH) stage 7 within the anterior part of the heart fields. In the tubular heart, atrial and ventricular precursor cells stained positively for Popdc2. Weaker expression was observed in myocardium of the outflow tract and sinus venosus. By HH stage 18, the outer curvature myocardium was strongly stained, whereas expression in myocardium of the inner curvature was negligible. Popdc2 expression was absent from the endocardium and propepicardial organ. At HH stage 36, Popdc2 expression was confined to the compact layer myocardium. In addition to the heart, Popdc2 expression was also observed in the myotome and in the muscle-forming fields of the limbs. Our results indicate that Popdc2 is highly expressed in the developing heart and may serve as a novel marker of myocardial differentiation in the chick embryo.