Pulmonary Vein, Dorsal Atrial Wall and Atrial Septum Abnormalities in Podoplanin Knockout Mice With Disturbed Posterior Heart Field Contribution

Abstract

ABSTRACT:The developing sinus venosus myocardium, derived from the posterior heart field, contributes to the atrial septum, the posterior atrial wall, the sino-atrial node, and myocardium lining the pulmonary and cardinal veins, all expressing podoplanin, a coelomic and myocardial marker. We compared development and differentiation of the myocardium and vascular wall of the pulmonary veins (PV), left atrial dorsal wall, and atrial septum in wild type with podoplanin knockout mouse embryos (E10.5-E18.5) by 3D reconstruction and immunohistochemistry. Expression of Nkx2.5 in the pulmonary venous myocardium changes from mosaic to positive during development pointing out a high proliferative rate compared with Nkx2.5 negative myocardium of the sino-atrial node and cardinal veins. In mutants, myocardium of the PVs, dorsal atrial wall and atrial septum was hypoplastic. The atrial septum and right-sided wall of the PV almost lacked interposed mesenchyme. Extension of smooth muscle cells into the left atrial body was diminished. We conclude that myocardium of the PVs, dorsal atrial wall, and atrial septum, as well as the smooth muscle cells, are derived from the posterior heart field regulated by podoplanin. (Pediatr Res 65: 27-32, 2009) T he developmental origin and molecular mechanisms controlling the formation of the myocardium of the pulmonary veins (PV) and the smooth muscle cells (SMCs) of the PV media are a matter of debate. The PV myocardial sleeve develops by either migration of myocardial cells from the left atrium (LA) (1) or by recruitment of extracardiac mesenchymal cells, which differentiate into myocardial cells (2). Based on expression patterns of Nkx2.5, Islet1, and Pitx2c the LA and PV myocardium was suggested to be formed by the addition of myocardial cells from the second heart field at the venous pole (3).The second heart field, part of the splanchnic mesoderm, is involved in the addition of (myocardial) cells to the primary heart tube at both the arterial (the secondary and anterior heart field) and the venous pole (the posterior heart field) (4). Islet1, a marker of undifferentiated cardiac progenitor cells, is expressed throughout the second heart field (5). Myocardial cells can differentiate from second heart field derived mesenchymal cells, and SMCs may also differentiate from mesenchyme (6).The transmembrane glycoprotein podoplanin, a coelomic and myocardial marker expressed in the posterior heart field (7), has gained interest for its role in epithelial-tomesenchymal transformation (EMT) and in formation of the myocardium and coronary SMCs at the venous pole of the heart (8). It is found in the proepicardial organ, epicardium, and sinus venosus myocardium including the sino-atrial node, the PVs and cardinal veins, the dorsal atrial wall and the base of the atrial septum extending into the developing atrial and ventricular cardiac conduction system. Podoplanin promotes EMT by binding ezrin, radixin, moesin (ERM) proteins that activate RhoA, and by down-regulation of the cell-to-cell adhesion mo...

Podoplanin deficient mice show a rhoa‐related hypoplasia of the sinus venosus myocardium including the sinoatrial node

“…Recently, Manner and Merkel (Manner and Merkel, 2007) have described the pulmonary pit as a bilaterally paired structure. We have described that the early common pulmonary vein is surrounded by Nkx2.5 mosaic cells which are positive for MLC-2a and podoplanin Douglas et al, 2008). Part of this myocardium has also been reported as "mediastinal myocardium" (Soufan et al, 2004).…”

“…In the cardiomyocyte population podoplanin staining was seen in major parts of the developing atrioventricular cardiac conduction system, in sinus venosus myocardium including the sinoatrial node, the venous valves, the dorsal mesocardium, the dorsal atrial wall and primary atrial septum. Also the myocardium surrounding the cardinal veins and the common pulmonary vein belongs to this population Douglas et al, 2008;Mahtab et al, 2008). In earlier publications, podoplanin, a 43-kDa mucin type transmembrane glycoprotein, first named E11 antigen as a new marker for an osteoblastic cell line , was also reported in the nervous system; the epithelia of lung, eye, esophagus, and intestine (Williams et al, 1996); the mesothelium of the visceral peritoneum ; the coelomic wall (pericardium) lining the pericardial cavity and the epicardium ; the podocytes of the kidney (Breiteneder-Geleff et al, 1997); and the lymphatic endothelium (Schacht et al, 2003).…”

“…As a coelomic and myocardial marker, podoplanin is specifically expressed in the mesenchyme and in the myocardium at the venous pole Douglas et al, 2008;Mahtab et al, 2008). Studying the mesenchymal population, podoplanin expression was observed in the proepicardial organ and epicardium Lie-Venema et al, 2007;Mahtab et al, 2008).…”

Expression of Id2 in the second heart field and cardiac defects in Id2 knock‐out mice

“…Id2 knockout embryos demonstrated hypoplasia of the atrial myocardium and larger interatrial communications than observed in wildtype animals, a phenotype that was also observed in other models with impaired myocardial contributions from the posterior heart field, including the mice lacking podoplanin, Pdgfr-a, and Shox2 (Blaschke et al, 2007;Douglas et al, 2009;Bax et al, 2010). However, in contrast to observations in podoplanin and Pdgfra knockout embryos, atrioventricular septal defects were not observed in Id2 knockouts.…”

“…This is in line with the fact that the DMP was not affected in the Id2 knockout. An interesting observation was the small left atrium in the Id2 knockout embryos, which was also observed in the podoplanin knockout mouse (Douglas et al, 2009). The intracardiac part of the pulmonary vein runs in the DMP, itself PHF-derived (Snarr et al, 2007b), which expresses Id2 throughout development.…”