Cardiac looping and the vertebrate left-right axis: antagonism of left-sided Vg1 activity by a right-sided ALK2-dependent BMP pathway

The rightward looping of the primary heart tube is dependent upon upstream patterning events that establish the vertebrate left-right axis. In Xenopus , a left-sided Vg1 signaling pathway has been implicated in instructing cel ls to adopt a 'left-sided identity'; however, it is not known whether 'righ t-sided identity' is acquired by a default pathway or by antagonism of Vg1 signaling. Here, we propose that an antagonistic, BMP/ALK2/S mad-mediated s ignaling pathway is active on the right side of the Xenopus embryo, Truncat ed ALK2 receptor expression on the right side of the blastula elicits heart reversals and altered nodal expression, Consistent with these findings, co nstitutively active ALK2 (CA-ALK2) receptor expression on the left side of the blastula also elicits heart reversals and altered nodal expression, Coe xpression of CA-ALK2 with mature Vg1 ligand results in predominantly left-s ided nodal expression patterns and normal heart looping, demonstrating that the ALK2 pathway can 'rescue' left-right reversals that otherwise occur fo llowing right-sided misexpression of mature Vg1 ligand alone. Results with chimeric precursor proteins indicate that the mature domain of BMP ligands can mimic the ability of the ALK2. signaling pathway to antagonize the Vg1 pathway, Consistent with the observed antagonism between BMP and Vg1 ligand s, left-sided ectopic expression of Xolloid results in heart reversals, Mor eover, ectopic expression of Smad1 or Smad7 identified two downstream modul ators of the BMP/ALK2 signaling pathway that also can regulate cardiac orie ntation. Collectively, these results define a BMP/ALK2-mediated pathway on the right side of the Xenopus embryo and, moreover, suggest that left-right patterning preceding cardiac morphogenesis involves the activation of two distinct and antagonistic, left- and right-sided TGF beta-related signaling pathways.