Left-right asymmetry: cilia stir up new surprises in the node

Abstract

Cilia are microtubule-based hair-like organelles that project from the surface of most eukaryotic cells. They play critical roles in cellular motility, fluid transport and a variety of signal transduction pathways. While we have a good appreciation of the mechanisms of ciliary biogenesis and the details of their structure, many of their functions demand a more lucid understanding. One such function, which remains as intriguing as the time when it was first discovered, is how beating cilia in the node drive the establishment of left-right asymmetry in the vertebrate embryo. The bone of contention has been the two schools of thought that have been put forth to explain this phenomenon. While the 'morphogen hypothesis' believes that ciliary motility is responsible for the transport of a morphogen preferentially to the left side, the 'two-cilia model' posits that the motile cilia generate a leftward-directed fluid flow that is somehow sensed by the immotile sensory cilia on the periphery of the node. Recent studies with the mouse embryo argue in favour of the latter scenario. Yet this principle may not be generally conserved in other vertebrates that use nodal flow to specify their left-right axis. Work with the teleost fish medaka raises the tantalizing possibility that motility as well as sensory functions of the nodal cilia could be residing within the same organelle. In the end, how ciliary signalling is transmitted to institute asymmetric gene expression that ultimately induces asymmetric organogenesis remains unresolved.

Keywords: Pkd2; calcium signalling; immotile cilia; left–right asymmetry; motile cilia; node.

Figure 1.

Left–right (L–R) asymmetry in man. (a) In the wild-type, also known as ‘situs solitus’, the heart, stomach and spleen are oriented to the left side, whereas the liver is present on the right side. (b) In KS patients with ‘situs inversus’, transposition of the visceral organs occurs in a mirror-image along the L–R axis. R indicates the right side, while L indicates the left side.

Figure 2.

Nodal pathway activity in the determination of L–R asymmetry. A simplified schematic depicting asymmetric Nodal expression in the node, and the essential elements of asymmetric Nodal signalling in the left LPM.

Figure 3.

Models to explain the function of nodal flow in L–R asymmetry. (a) The ‘morphogen’ hypothesis. Clockwise beating of motile cilia transports a morphogen or NVPs towards the left side of the node. (b) The ‘two-cilia’ hypothesis. Fluid flow generated by the motile cilia is sensed by immotile cilia on the perinodal crown cells (shown here as deflections; blue arrow). Nodal pit cells are depicted in light brown, whereas perinodal crown cells are in dark brown. The motile cilia are tilted posteriorly. Basal bodies are indicated with red dots and the direction of nodal flow is shown with the black arrow. A, anterior; P, posterior.

Figure 4.

Crown cells sense nodal flow through Pkd2. (a) Pkd2 expression throughout the node (pit cells and crown cells) ensures correct inception of L–R asymmetry. The majority of pit cell cilia are motile, whereas the majority of crown cell cilia are immotile. (b) Pkd2 expression exclusively in the pit cells is not sufficient for the correct determination of L–R asymmetry. (c) Pkd2 expression only in crown cells is sufficient for proper determination of L–R asymmetry.

Figure 5.

Motile cilia in the node may sense flow. (a) KV cilia of medaka fish embryo labelled with antibodies to Lrd. (b) KV cilia labelled with antibodies to acetylated tubulin (ace tub). (c) Merged image of (a,b) showing that all cilia in KV contain Lrd, and hence are motile. (d) KV cilia labelled with antibodies to Pkd1l1. (e) KV cilia labelled with antibodies to Pkd2. (f) KV cilia labelled with antibodies to acetylated tubulin. (g) Merged image of (d–f) showing that all cilia in KV contain Pkd1l1–Pkd2 complex. Insets in (a–g) highlight a single motile cilium. (h) showing that the cilia in medaka KV contain Lrd as well as the Pkd1l1–Pkd2 complex. (a–g) Reproduced from Kamura et al. [50] with permission from the Company of Biologists.