Bmp4 signaling is required for outflow-tract septation and branchial-arch artery remodeling

Abstract

The Bmp4 signaling molecule is expressed in ventral splanchnic and branchial-arch mesoderm and outflow-tract (OFT) myocardium, suggesting a role for Bmp4 in OFT development. Inactivation of Bmp4 in the caudal branchial arch and splanchnic mesoderm and OFT myocardium by using a conditional null allele of Bmp4 and the Nkx2.5 cre recombinase allele resulted in abnormal morphogenesis of branchialarch arteries (BAAs) and defective OFT septation. Expression of aortic-sac myocardial markers was reduced and expression of the sm22 LacZ transgene, a smooth-muscle marker, was attenuated in BAAs and conotruncus of Nkx2.5 cre ; Bmp4 conditional mutants. Moreover, we found tissue-specific functions for Bmp4 in the regulation of cellular proliferation and apoptosis. We also demonstrate a strong genetic interaction between Bmp4 and Bmp7 in OFT development. Our findings uncover a previously uncharacterized function for Bmp4 in vascular remodeling of the BAAs, and they show definitively that Bmp4, in cooperation with Bmp7, has a central role in OFT septation. T he vertebrate heart can be subdivided into inflow, outflow, and primitive-ventricular regions (1). The cardiac outflow tract (OFT), which develops from the anterior part of the linear heart tube, forms the right-sided conotruncal region after heart looping. Initially unseptated, the OFT divides into the pulmonary trunk (PT) and aorta, and it is critical for separation of postnatal pulmonary and systemic circulation. Congenital OFT malformations are common, making an understanding of the genetic pathways regulating OFT development an important goal in developmental biology and clinical medicine.At defined areas of the OFT, endocardial cells undergo an epithelial to mesenchymal transformation (perhaps in response to a signal from overlying myocardium) and invade the intervening space to form the endocardial cushions. The cardiac neural crest also invades the forming aorto-pulmonary (AP) septum and OFT cushions (2). The OFT myocardium receives an additional input from splanchnic and branchial-arch mesoderm, the anterior or secondary heart field (SHF), which may be important for OFT lengthening and morphogenesis (3).Bmp4 is a member of the Bone morphogenetic protein (Bmp) subclass of transforming growth factor type ␤ (TGF-␤)-signaling molecules (4). Bmp4 expression in splanchnic and branchial-arch mesoderm (which contributes to OFT myocardium) and within the OFT myocardium itself suggests a role in OFT morphogenesis (ref.5 and see below). Investigation of Bmp4 function in cardiac development has been hampered by the early embryonic lethality of Bmp4 null mutant embryos (6). Recent work analyzing an allele of the ubiquitously expressed Bmp type 2 receptor (Bmpr2), containing a partial ectodomain deletion, revealed defective proximal OFT septation in mouse embryos, providing insight into Bmp function in the OFT (7). However, because Bmpr2 is broadly expressed, the developmental mechanisms responsible for the cushion defects remain unclear. Overexpression of noggin in chick embry...

Canonical Wnt signaling functions in second heart field to promote right ventricular growth

“…Conditional genetic experiments in mice support this interpretation because Mesp cre ; Smad1 f/f mutants, which delete the Bmp effector Smad1 widely in mesoderm, have elongated OFTs and elevated proliferation in SHF but not OFT myocardium (6). In addition, Nkx2.5 cre ; Bmp4 flox mutants, which have a broad Bmp4 deletion in SHF, SHF derivatives, and pharyngeal endoderm, show elevated proliferation in OFT myocardium (17). In contrast to what might have been predicted, our data indicate that ␤-catenin loss-offunction embryos with reduced Bmp4 in pharyngeal mesenchyme and OFT myocardium have less myocardial proliferation.…”

“…An enhancer trap into the Fgf10 locus provided early evidence for the existence of the SHF in mice and indicates, along with conditional loss-of-function experiments, the involvement of Fgf signaling in SHF formation (14)(15)(16). Likewise, Bmp signaling has been implicated in SHF development based on data from chick embryos and mouse conditional deletion studies (6,17,18).…”

Tbx5 overexpression favors a first heart field lineage in murine embryonic stem cells and in Xenopus laevis embryos

“…In contrast, Hand2 and BMP4 are expressed in derivatives of the SHF. Whereas the right ventricle expresses Hand2, BMP4 expression is detected in the outflow tract Liu et al, 2004). Of interest, expression of these genes is down-regulated upon Tbx5 overexpression (Figs.…”

“…In contrast, cells of the SHF lineage stay longer in a proliferative progenitor state, are integrated into the heart tube later, enter terminal differentiation later, and are fundamental for right ventricle and outflow tract formation (Buckingham et al, 2005;Abu-Issa and Kirby, 2007;Laugwitz et al, 2008). Factors expressed in the SHF lineage before onset of differentiation are among others Isl1 and Tbx1, whereas differentiated derivatives of the SHF such as the outflow tract (OFT) or the right ventricle can be labeled by BMP4 or Hand2, respectively (Cai et al, 2003;Ilagan et al, 2006;Seo and Kume, 2006;Lin et al, 1997;Liu et al, 2004).…”

Distinct cardiac malformations caused by absence of connexin 43 in the neural crest and in the non-crest neural tube

“…Furthermore, the Cx43-null mutant phenotype, characterized by infundibular bulging without septation defects, differs from other murine genetic models that affect CNC function. Genetic models including those with mutations in or knockouts of Pax3, neurotrophin 3/TrkC, TGF␤ receptor type II, BMP4, BMP receptor IA, endothelin 1 and combinations of retinoic acid receptors (Choudhary et al, 2006;Donovan et al, 1996;Epstein et al, 2000;Kurihara et al, 1995;Liu et al, 2004;Mendelsohn et al, 1994;Stottmann et al, 2004;Youn et al, 2003) commonly cause OFT septation defects similar to those resulting from chick NC ablation. Thus, during development, Cx43 may be required in tissues that contribute to heart formation other than or in addition to the CNC (Li et al, 2002;Walker et al, 2005).…”