Nodal activity in the node governs left-right asymmetry

Abstract

Nodal is expressed at the lateral edges of the mouse node, but its function in this "organizer" tissue remains unknown due to the early lethality of Nodal mutant embryos. Here we used a genetic strategy to selectively remove Nodal activity from the node. Embryos lacking Nodal in the node fail to initiate molecular asymmetry in the left lateral plate mesoderm and exhibit multiple left-right patterning defects. Nodal may also act as a short-range signal to establish a functional midline barrier. Our findings confirm that the mouse node is instrumental in initiating left-right axis specification and identify Nodal as the key morphogen regulating this process. Genetic pathways controlling invariant situs of the major internal organ systems have been described in several vertebrate model systems (for review, see Capdevila et al. 2000;Hamada et al. 2002). In all cases, expression of the TGF-␤ signaling molecule, Nodal, is restricted to the left side of the embryonic axis, where its activity is critical for specifying the embryonic left-right (LR) axis. The upstream cellular and molecular mechanisms that direct asymmetric gene expression have been extensively described in chick. However, relatively little is known about how embryonic symmetry is broken, and how the LR signaling cascade that ultimately controls the positioning and morphogenesis of the internal organs is activated in mammalian embryos.The mouse node is a bilaminar structure found at the rostral end of the primitive streak and has equivalent secondary axes inducing properties as Spemann's organizer in Xenopus (for review, see Beddington and Robertson 1999). It consists dorsally of epiblast and ventrally of the most caudal aspect of the notochordal plate. Several recent studies indicate that the mouse node plays an important role in the establishment of LR asymmetry (Nonaka et al. 1998;Okada et al. 1999). Ultrastructural studies have shown that monocilia present on cells of the exposed ventral surface of the node rotate in a counterclockwise direction and have been suggested to generate a net leftward flow of extraembryonic fluid known as the nodal flow (Sulik et al. 1994;Nonaka et al. 1998). Laterality defects documented in several mouse mutants that lack cilia, or whose cilia motility is compromised, have lead to the hypothesis that the nodal flow results in the localized accumulation of morphogen(s) on the left side of the node, thereby initiating the LR signaling cascade . In keeping with this, artificially reversing the direction of nodal flow in cultured embryos is sufficient to reverse situs (Nonaka et al. 2002). Surgical ablation of the node (Davidson et al. 1999) and mutations affecting node formation (Dufort et al. 1998) both lead to embryonic laterality defects, implying that the node functions as a signaling center during LR axis formation. The node gives rise to midline structures such as the notochord and floor plate that act as a midline barrier necessary for maintaining correct laterality (Dufort et al. 1998;King et al. 1998;Meno et al. 199...

Gap junctions relay FGF8‐mediated right‐sided repression of Nodal in rabbit

“…7C), in which Nodal protein synthesized in the bilateral PNC domain diffuses as a morphogen to the LPM (Nakamura et al, 2006, Ohi andWright, 2007). Rabbit LP explant cultures isolated before the onset of PNC Nodal transcription never initiated LPM Nodal (not shown), in agreement with corresponding genetic data in mouse (Brennan et al, 2002;Saijoh et al, 2003). In that setting, the role of an inhibitor on both sides falls upon FGF8, which acts to prevent the initiation of the autoregulatory Nodal feedback loop, in which Nodal signaling induces Nodal transcription (Saijoh et al, 2000;Hamada et al, 2002, Ohi andWright, 2007).…”

Massively parallel sequencing identifies the gene Megf8 with ENU-induced mutation causing heterotaxy

“…Genetic analyses have suggested that Nodal expression at the node is required for the activation of left-determinant transcription cascade in the left LPM (15,16). Our data, would suggest the molecular defect underlying the Megf8 m/m phenotype is either an interruption of signal transfer from the node to left LPM and/or a disruption in the initiation and expansion of Nodal expression in the LPM.…”

Wnt Signaling in Mammalian Development: Lessons from Mouse Genetics

“…In mice, L-R determination relies on Nodal expression at the periphery of the node, a specialized structure located immediately rostral to the primitive streak ( Fig. 2) (Brennan et al 2002). Nodal is initially expressed symmetrically along both the right and left side of the node, but a leftward flow generated by node cilia rotation (see below) causes enhanced Nodal signaling on the left node, eventually breaking the L-R symmetry by up-regulating Nodal expression in the left node and left LPM.…”