Bmp and Nodal Independently Regulate lefty1 Expression to Maintain Unilateral Nodal Activity during Left-Right Axis Specification in Zebrafish

Abstract

In vertebrates, left-right (LR) axis specification is determined by a ciliated structure in the posterior region of the embryo. Fluid flow in this ciliated structure is responsible for the induction of unilateral left-sided Nodal activity in the lateral plate mesoderm, which in turn regulates organ laterality. Bmp signalling activity has been implied in repressing Nodal expression on the right side, however its mechanism of action has been controversial. In a forward genetic screen for mutations that affect LR patterning, we identified the zebrafish linkspoot (lin) mutant, characterized by cardiac laterality and mild dorsoventral patterning defects. Mapping of the lin mutation revealed an inactivating missense mutation in the Bmp receptor 1aa (bmpr1aa) gene. Embryos with a mutation in lin/bmpr1aa and a novel mutation in its paralogue, bmpr1ab, displayed a variety of dorsoventral and LR patterning defects with increasing severity corresponding with a decrease in bmpr1a dosage. In Bmpr1a-deficient embryos we observed bilateral expression of the Nodal-related gene, spaw, coupled with reduced expression of the Nodal-antagonist lefty1 in the midline. Using genetic models to induce or repress Bmp activity in combination with Nodal inhibition or activation, we found that Bmp and Nodal regulate lefty1 expression in the midline independently of each other. Furthermore, we observed that the regulation of lefty1 by Bmp signalling is required for its observed downregulation of Nodal activity in the LPM providing a novel explanation for this phenomenon. From these results we propose a two-step model in which Bmp regulates LR patterning. Prior to the onset of nodal flow and Nodal activation, Bmp is required to induce lefty1 expression in the midline. When nodal flow has been established and Nodal activity is apparent, both Nodal and Bmp independently are required for lefty1 expression to assure unilateral Nodal activation and correct LR patterning.

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

“…In the context of later heart development, high-resolution imaging in zebrafish has identified a role for BMPs during heart tube rotation; here, asymmetric BMP signaling differentially affects migratory behavior on one side (32)(33)(34). These latter observations, together with our data, implicate BMP signaling in early CPC migration and in cardiac morphogenesis in anamniotes and amniotes, respectively.…”

“…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.…”

A Nodal-independent and tissue-intrinsic mechanism controls heart-looping chirality

“…The straightforward (sfw) mutant was identified in a forward genetic screen for mutants with organ laterality defects 12 . Whereas the heart in wild-type embryos is positioned asymmetrically under the left eye at 28 h post fertilization (hpf), sfw mutants exhibit a midline heart at 28 hpf (Fig.…”

Diversity Is in My Veins

“…For instance, in mice, deletion of Alk2 or Alk3 in endothelial cells disrupts the formation of cardiac valves 88, 89 , deletion of BMPER , a context dependent modulator of BMP signaling, causes failure of coronary vessel remodeling 90 , and a lack of BMPR2 in endothelial cells predisposes pulmonary hypertension 91 . Similarly, in zebrafish, a lack of alk3a or alk3b causes severe disruption in cardiovascular development 92, 93 , and mutations in alk1 causes failure in cranial vessel remodeling. 94 More recently, it has been shown that BMP signaling also regulates the emergence of lymphatic endothelial cells during development.…”