Mechanism of Nodal Flow: A Conserved Symmetry Breaking Event in Left-Right Axis Determination

Abstract

The leftward flow in extraembryonic fluid is critical for the initial determination of the left-right axis of mouse embryos. It is unclear if this is a conserved mechanism among other vertebrates and how the directionality of the flow arises from the motion of cilia. In this paper, we show that rabbit and medakafish embryos also exhibit a leftward fluid flow in their ventral nodes. In all cases, primary monocilia present a clockwise rotational-like motion. Observations of defective ciliary dynamics in mutant mouse embryos support the idea that the posterior tilt of the cilia during rotational-like beating can explain the leftward fluid flow. Moreover, we show that this leftward flow may produce asymmetric distribution of exogenously introduced proteins, suggesting morphogen gradients as a subsequent mechanism of left-right axis determination. Finally, we experimentally and theoretically characterize under which conditions a morphogen gradient can arise from the flow.

Far from solved: A perspective on what we know about early mechanisms of left–right asymmetry

“…Not surprisingly, this has been suggested to be due to the actions of PCP in setting up the positioning of cilia in the node (Okada et al, 2005;Hashimoto et al, 2010). Our alternative proposal, of PCP as primary to the spread of LR orientation throughout the blastoderm in a manner unrelated to nodal cilia, is supported by the requirement for the PCP protein Vangl2 in the asymmetry of the chick embryo, where cilia play no role in asymmetry (Zhang and Levin, 2009).…”

Three types of cilia including a novel 9+4 axoneme on the notochordal plate of the rabbit embryo

“…We, therefore, termed this structure, which is bordered by bilateral nodal expression, the posterior notochord (PNC; Blum et al, 2006). The 9ϩ0 axonemes of PNC cilia are indeed motile (Sulik et al, 1994), but probably due to the lack of a regulatory CP, they show a conical rotation rather than planar beating (Nonaka et al, 1998(Nonaka et al, , 2005Okada et al, 2005).…”

“…It has become increasingly clear that almost any defect in ciliary components that disturbs the function of the PNC monocilia disrupts correct situs specification in mice as well as humans Fliegauf and Omran, 2006;Hirokawa et al, 2006;Marshall and Nonaka, 2006;Raya and Belmonte, 2006;Shiratori and Hamada, 2006). In zebrafish and medaka, a homologous cilia-driven fluid flow was discovered in Kupffer's vesicle, which was required for laterality determination as well, indicating an evolutionary conserved mechanism of LR axis determination (Essner et al, 2005;Kramer-Zucker et al, 2005;Okada et al, 2005). Interestingly, Kupffer's vesicle cilia, while performing rotational beat patterns in all cases, display 9ϩ2 axonemes in zebrafish (KramerZucker et al, 2005) and 9ϩ0 axonemes in medaka (Okada et al, 2005).…”

“…In zebrafish and medaka, a homologous cilia-driven fluid flow was discovered in Kupffer's vesicle, which was required for laterality determination as well, indicating an evolutionary conserved mechanism of LR axis determination (Essner et al, 2005;Kramer-Zucker et al, 2005;Okada et al, 2005). Interestingly, Kupffer's vesicle cilia, while performing rotational beat patterns in all cases, display 9ϩ2 axonemes in zebrafish (KramerZucker et al, 2005) and 9ϩ0 axonemes in medaka (Okada et al, 2005).…”

Mice with mutations in Mahogunin ring finger‐1 (Mgrn1) exhibit abnormal patterning of the left–right axis

“…This flow is thought to cause a build up of Nodal on the left side of the node, which is then transferred to the lateral plate mesoderm (LPM) where it sets off a cascade of signaling on the left side of the developing embryo that is maintained by a midline barrier (Nonaka et al, 1998(Nonaka et al, , 2002. While this theory of nodal flow has been widely accepted and modified to include means for both chemical (Nonaka et al, 1998;Okada et al, 2005) and physical (McGrath et al, 2003) modes of generating a left-specific signaling event, other theories have been pre-sented, including a prolonged and stable up-regulation of Ca 2ϩ in the left side of the node, which has been proposed to be involved in regulating the left-sided signaling cascade (McGrath et al, 2003;Tanaka et al, 2005). Mice lacking the membrane cation channel Polycystin-2 (PKD2) have impaired left-right axis determination and do not show left-sided elevation of Ca 2ϩ in the node (Pennekamp et al, 2002;McGrath et al, 2003).…”