Integration of Nodal and BMP Signals in the Heart Requires FoxH1 to Create Left–Right Differences in Cell Migration Rates That Direct Cardiac Asymmetry

Abstract

Failure to properly establish the left–right (L/R) axis is a major cause of congenital heart defects in humans, but how L/R patterning of the embryo leads to asymmetric cardiac morphogenesis is still unclear. We find that asymmetric Nodal signaling on the left and Bmp signaling act in parallel to establish zebrafish cardiac laterality by modulating cell migration velocities across the L/R axis. Moreover, we demonstrate that Nodal plays the crucial role in generating asymmetry in the heart and that Bmp signaling via Bmp4 is dispensable in the presence of asymmetric Nodal signaling. In addition, we identify a previously unappreciated role for the Nodal-transcription factor FoxH1 in mediating cell responsiveness to Bmp, further linking the control of these two pathways in the heart. The interplay between these TGFβ pathways is complex, with Nodal signaling potentially acting to limit the response to Bmp pathway activation and the dosage of Bmp signals being critical to limit migration rates. These findings have implications for understanding the complex genetic interactions that lead to congenital heart disease in humans.

Nodal signalling and asymmetry of the nervous system

“…Notably, symmetry in both cases is produced through different ontogenic pathways. Symmetrically decreased migration induces no jog in the absence of Nodal signalling, while the lack of jog in bilateral Nodal signalling results from symmetrically enhanced motility [66]. Rotation of the heart cone during jogging might be directing the later torsion of the heart tube leading to D-loop in normal conditions.…”

“…For example, the sided migration of the PpO in zebrafish [40,50], the clockwise rotation of the neural tube in ascidians [12,39] and the conversion of left-right into dorsoventral seen in the projections from the Hb to the interpeduncular nucleus of zebrafish [45]. Symmetrical Nodal (absent and bilateral) result in the lack of jog (symmetry) [65][66][67], thus suggesting a role of Nodal as asymmetry inducer in this particular aspect of heart asymmetry. Notably, symmetry in both cases is produced through different ontogenic pathways.…”

Transcriptional Control by the SMADs

“…This is a mechanism whereby activated SMAD complexes induce expression of a transcriptional regulator that subsequently interacts with the SMAD complexes at the enhancers of The first SMAD-interacting transcription factor identified was Xenopus Foxh1 (originally called FAST-1), which was shown to be absolutely required in Xenopus embryos for recruitment of an activated Smad2 -Smad4 complex to an activin-responsive element in the Xenopus mix.2 gene (Chen et al 1996(Chen et al , 1997. Binding sites for Foxh1 -Smad complexes have subsequently been found in numerous mouse genes during embryonic development and in NODAL-responsive genes in human and fish (Silvestri et al 2008;Yoon et al 2011;Lenhart et al 2013). The concept that different SMAD-interacting transcription factors could dictate the cell-type specificity of signals from TGF-b family ligands then emerged when members of the AP1 family of transcription factors (FOS and JUN) were shown to cooperate with SMAD3 -SMAD4 complexes to activate transcription from 12-O-tetradecanoyl-13-acetate (TPA)-responsive gene promoter elements (TREs) Zhang et al 1998).…”

Smad1 transcription factor integrates BMP2 and Wnt3a signals in migrating cardiac progenitor cells

“…This restricts Smad1 activity and facilitates recognition by the ubiquitin ligase Smurf1 (SMAD ubiquitination regulatory factor 1), causing degradation or, alternatively, cytoplasmic retention (24)(25)(26) (29)(30)(31), as well as during the initiation of cardiac looping (32)(33)(34). Thus, we determined whether manipulation of BMP signaling activity would alter the migration behavior of prospective cardiac cells.…”