The Ciliary Protein Ftm Is Required for Ventricular Wall and Septal Development

Abstract

Ventricular septal defects (VSDs) are the most common congenital heart defects in humans. Despite several studies of the molecular mechanisms involved in ventricular septum (VS) development, very little is known about VS-forming signaling. We observed perimembranous and muscular VSDs in Fantom (Ftm)-negative mice. Since Ftm is a ciliary protein, we investigated presence and function of cilia in murine hearts. Primary cilia could be detected at distinct positions in atria and ventricles at embryonic days (E) 10.5–12.5. The loss of Ftm leads to shortened cilia and a reduced proliferation in distinct atrial and ventricular ciliary regions at E11.5. Consequently, wall thickness is diminished in these areas. We suggest that ventricular proliferation is regulated by cilia-mediated Sonic hedgehog (Shh) and platelet-derived growth factor receptor α (Pdgfrα) signaling. Accordingly, we propose that primary cilia govern the cardiac proliferation which is essential for proper atrial and ventricular wall development and hence for the fully outgrowth of the VS. Thus, our study suggests ciliopathy as a cause of VSDs.

Cilia and coordination of signaling networks during heart development

“…Components in this pathway localize to primary cilia in embryonic hearts as well as in cardiomyocytes differentiating from stem cells in vitro (Fig. 4), 21,22,142 and defective ciliary assembly is associated with defective cardiomyogenesis 21 as well as Hh-related heart defects that might be independent of nodal cilia, including ventricular and endocardial cushion-derived defects. 122,142,154 Further, 60% of patients with mutations in genes encoding the EVC proteins, EVC1/EVC2, which interact with Smo at the primary cilium to transduce Hh signaling [155][156][157][158][159] , display CHD, including AVSD and ASD.…”

“…Indeed, the regulation of signaling networks during heart development has in some cases been linked to the function of cardiac primary cilia, [20][21][22] indicating the complexity by which different populations of cilia may contribute to the formation the heart in a spatiotemporal manner. However, we still know little as to how cardiac primary cilia contribute to heart development independently of nodal cilia and downstream of situs establishment, and whether CHD may arise exclusively by defects in the sensory function of primary cilia in the tissues of the developing heart.…”

“…Consequently, defects in ciliary assembly or turnover of Hh components in the cilium lead to a plethora of developmental disorders and diseases, including CHD. 21,22,136,142 A number of observations have specifically linked cardiac primary cilia to the regulation of Hh signaling. Components in this pathway localize to primary cilia in embryonic hearts as well as in cardiomyocytes differentiating from stem cells in vitro (Fig.…”

“…47,51 In line with these findings, EVC proteins are coexpressed in the OFT and the mesenchymal structures of the atrial septa and AV cushions during heart development in mice 51 ; areas that are known to be ciliated during cardiogenesis. 136,142 Septation of the ventricles is also dependent on the gene Fantom (Ftm) 22 that encodes the ciliary base protein Rpgrip1l/Nphp8, which is required for proper ciliogenesis, PCP, and Shh signaling, and when mutated leads to a series of ciliopathies. 22,[52][53][54][160][161][162][163][164][165] In Ftm null mouse embryos, 33% of all analyzed embryos had VSD in the membranous part of the septum, 81.5% had defects in the muscular part, and all analyzed embryos had reduced atrial and ventricular wall thickness.…”

“…[11][12][13][14][15][16][17][18][19] In addition, primary cilia that are present in cardiomyocytes and within the developing heart, also known as cardiac primary cilia (Fig. 1B), are compartmentalized with a series of receptor systems, [20][21][22] which take part in regulating cellular signaling pathways important for the progressive differentiation, morphogenesis and maturation of the heart. This suggests that also cardiac primary cilia play a role in coordinating the signaling networks that are required for proper heart development.…”