Is left-right asymmetry a form of planar cell polarity?

Abstract

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

Fig. 1.

Overview of LR patterning phases and proposed role for PCP. Consistent LR asymmetry includes sequential phases of symmetry breaking (providing cells with a consistent orientation along the LR axis), asymmetric gene expression, and organogenesis. (A) In our model, the initial orientation occurs via a cytoskeletal structure (F) in very early blastomeres that we term the LR `organizer'. This involves the coordination of the apical-basal, LR and dorsoventral (DV) axes and is imposed across the embryonic epithelial field by a PCP mechanism (Aw et al., 2008). (B) An alternative cilia-based model is also compatible with this hypothesis as PCP is required to orient the cilia, the chiral flow of which has been proposed to initiate asymmetry (Brueckner, 2001). In the intracellular model (A), the directional information is converted to LR position relative to the midline by the establishment of physiological asymmetries in membrane voltage (V_mem) that redistribute intracellular LR morphogens (Fukumoto et al., 2005). In the ciliary models (B), LR position is dictated to cells by the redistribution of an extracellular signal (e.g. Ca²⁺). In both models, these steps are followed by (C) cascades of asymmetric gene expression that (D) drive organogenesis. L, left; R, right.

Fig. 2.

PCP establishment in the Drosophila wing. (A) Polarity establishment in the Drosophila wing epithelium as a classic model of PCP. PCP establishment first requires key proteins [e.g. Strabismus (Stbm) and Frizzled (Fz)] to be apically distributed and then localized to the proximal and distal sides of the apical surface, respectively, in the plane that lies orthogonal to the apical-basal axis. See Zallen (Zallen, 2007). (B) Stbm then recruits Prickled to the proximal side of the cell, which antagonizes Dishevelled (Dsh), hence restricting the location of the Dsh-Fz complex to the opposite, distal side. At the same time, interactions between the extracellular domains of Stbm and Fz at adjacent proximal and distal membranes of neighboring cells stabilize the polarized configurations, allowing a small clone of cells to be polarized. (C) A speculative model of PCP (see Tree et al., 2002b), in which the organized asymmetric distribution of core PCP proteins spreads from a small group of polarized organizer cells. A wave of polarized interactions emanates from an initial site of polarization outwards in all directions through the plane of the tissue, establishing a coordinated array of polarized cells (indicated here by the decreasing intensity of blue shading from the center to outer cells). See Jones and Chen (Jones and Chen, 2007).

Fig. 3.

LR patterning and PCP steps share similar logic. Three phases of patterning required for LR asymmetry (left, green) and PCP (right, blue). The mutant vertebrate LR and Drosophila PCP eye phenotypes that result from disruption (red circle with a cross) of each phase are described. For simplicity, we focus on phenotypes that result from mutations in PCP genes in the ommatidia of the Drosophila eye. (The details of analogous phenotypes between the phases of LR and PCP establishment might differ in other planar-polarized epithelia.) (A) In the first phase, symmetry breaking and polarity initiation occur. Defects in this phase abolish the directional cue and lead to random selection of polarity direction, resulting in mutants that retain asymmetric, but randomly oriented, expression of key downstream genes such as Nodal and fz. Mutants develop with randomized asymmetry/polarity that follows from the direction of asymmetric gene expression. For example, 50:50 situs inversus:situs solitus in LR left-right dynein (Lrd) mutant or randomized clockwise/counterclockwise rotation of ommatidia in PCP fat (ft), dachsous (ds) and four-jointed (fj) mutants. (B) In the second phase, the asymmetry/polarity cue is amplified and refined over the cell field. In LR patterning, this might occur via asymmetric ion flux or movement of extracellular morphogens by cilia that are ultimately transduced into the left-sided Nodal transcriptional cascade. In PCP, this occurs via the asymmetric subcellular localization of PCP proteins. Mutations in this phase often cause loss of asymmetric gene expression and a spectrum of LR and PCP phenotypes [loss of, or bilateral, Nodal expression and heterotaxia in LR patterning, or loss of asymmetric Frizzled (Fz) and loss of asymmetric rotation in ommatidia, respectively]. (C) In the final phase, the molecular asymmetries are differentially interpreted in the individual tissues to produce the required morphologies. Mechanisms and phenotypes are reviewed elsewhere (Levin, 2006; Seifert and Mlodzik, 2007; Tree et al., 2002a; Wang and Nathans, 2007; Zallen, 2007). pk, prickled; dsh, dishevelled; fmi, flamingo; R, photoreceptor.

Fig. 4.

Innate chirality allows polarity determination in single cells. (A) A Xenopus egg viewed from the animal pole; the animal-vegetal axis lies perpendicular to the plane of the page. A consistent `East-West' or counterclockwise chirality has been identified in the actin cytoskeleton around the egg's periphery (black dashed line) (Danilchik et al., 2006), which provides different LR directional cues at distinct tangent points (black arrowheads along the dashed line). These cues offer no unique LR orientation because each point is equivalent to the others (red arrowheads show that cues point rightwards on one side and leftwards on the opposite side). (B) At fertilization, sperm entry breaks the radial symmetry and determines a specific point on the circumference through which the midline axis of bilateral symmetry passes. The chiral orientation of the actin cytoskeleton at this point converts the bilateral symmetry into LR asymmetry through a linear cue (red arrowhead defined by sperm entry) along the LR axis. Thus, circumferential chirality can be converted into an organism-wide linear directionality (LR) once the DV axis is determined. (C) Sperm entry point magnified, where a putative chiral `F-molecule' (Brown and Wolpert, 1990) in the microtubule-organizing center (MTOC) can be oriented with respect to the actin cortex and nucleate microtubule transport paths that have a true LR directionality. (D) Intrinsic polarity of a differentiated HL60 cell in culture (Xu et al., 2007). Cells extend pseudopodia preferentially to the left of an arrow pointing from the nucleus to the centrosome (yellow circle), revealing how HL60 cells are intrinsically chiral, utilizing cytoplasmic structures and a polarized axis (adhesion to coverslip versus free medium) to consistently orient the LR axis. (D′) A chick embryo blastoderm is analogous to the cell in D, in that its DV axis is fixed and a leftward signal must be determined to establish sonic hedgehog (Shh) expression on the left side of the node (brown).

Fig. 5.

Prediction by intracellular early models of LR patterning of asymmetry phenotypes in embryo-splitting experiments. (A) If asymmetry is initiated by the action of cilia during gastrulation, very early blastomere separation should result in no loss of LR information and in normal nodal cilia. The resulting embryos are expected to exhibit 100% correct LR patterning. (B) In intracellular models, LR information provided by early, asymmetrically localized determinants is lost or altered in some daughter blastomeres upon early splitting, leading to the prediction of LR patterning defects, as is observed in human monozygotic twins and in experiments in amphibians.