Perspectives and open problems in the early phases of left-right patterning

Abstract

Embryonic left-right (LR) patterning is a fascinating aspect of embryogenesis. The field currently faces important questions about the origin of LR asymmetry, the mechanisms by which consistent asymmetry is imposed on the scale of the whole embryo, and the degree of conservation of early phases of LR patterning among model systems. Recent progress on planar cell polarity and cellular asymmetry in a variety of tissues and species provides a new perspective on the early phases of LR patterning. Despite the huge diversity in body-plans over which consistent LR asymmetry is imposed, and the apparent divergence in molecular pathways that underlie laterality, the data reveal conservation of physiological modules among phyla and a basic scheme of cellular chirality amplified by a planar cell polarity-like pathway over large cell fields.

Figure 1

Conceptual phases of LR patterning and their later readouts (A) LR symmetry breaking requires that a midline be established, and one side be made different from the other. This difference needs to be consistently oriented within the population. The information needs to be amplified and transmitted to multiple organ systems; midline structures must perform a restriction function to side-specific signals from crossing over. Lack of coordination results in heterotaxia, where organs make independent decisions resulting in a spectrum of random placement. Red arrows indicate phenotypes arising from disruption of each step. (B) In situ hybridization of a brain section from a songbird in which chromosomal aberrations during the first cleavages resulted in a gynandromorphy (chimera of male and female cells). The division between the female chromosome cells (dark signal) and the male chromosome cells (no signal) is precisely down the anatomical midline of the brain, suggesting that the embryonic midline is determined long prior to streak development in bird in development. Taken from Fig. 6 of (Agate et al 2003), copyright held by National Academy of Sciences. (C) Hair whorls indicate the presence of chirality distinct from the situs of the body organs. In monozygotic twins, such hair whorls are mirror images, revealing that splitting of mammalian embryos results in subtle asymmetries initiated very early and not reversed by later embryonic events. (D) Immunohistochemistry of a 2-cell frog embryo reveals the first step of a polarization process, where an intracellular protein is localized to one side of each cell (revealing direction and subcellular planar polarity along the LR axis, red arrowheads). (E) During the orientation and amplification phases, cells must convert intracellular knowledge of direction along the LR axis (the same in all cells) into position relative to the midline (different in L vs. R cells), here illustrated by the expression of Nodal in lateral plate cells only on the left side of the chick embryo.

Figure 2

Elaboration of asymmetry: from single cells to cell fields Consistent “East-West” (counterclockwise) chirality (Danilchik et al 2006) exists in the actin cytoskeleton around the periphery of the Xenopus egg (black dashed line; the egg is viewed from the animal pole with the animal-vegetal axis perpendicular to the plane of the page). (A) This provides different LR directional cues at distinct tangent points along the periphery (black arrowheads along the dashed line) and offers no unique LR orientation because each point on the circumference is equivalent to the others (red arrowheads show that this cue points right-ward on one side and left-ward on the opposite side). (A′) 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 that unique point converts the bilateral symmetry into LR asymmetry through a linear cue along the LR axis (red arrowhead at the tangent point defined by the sperm entry). This shows how circumferential chirality (East-West) can be converted into an organism-wide linear directionality (Left-Right) once the dorsoventral axis is determined. Close-up is in (A″), where a putative chiral “F-molecule” in the MTOC can be oriented with respect to the actin cortex and nucleate microtubule transport paths that have a true LR directionality. (B) Goosecoid expression designates the frog organizer; this identifies the future dorsal side, but the midline is an imaginary line drawn through the middle of this region and it is unclear how it is determined in most species. (B′) In Xenopus, the midline is defined by the lack of gap junctional communication between the L and R ventral cells (blue cylinders represent gap junctions). (C) Asymmetric cutaneous pigmentation pattern with a sharp midplane demarcation in the X-linked CHILD syndrome of humans (reproduced with permission from John Wiley and Sons, from (Konig et al 2000)). (C′) A gynandromorphic swallowtail butterfly, Papilio glaucas (dorsal view; brightly-colored left side of body is male; courtesy of James Adams). (C″) Gynandromorph chicken (courtesy of Michael Clinton). (E) Coherence of consistent orientation of eye cells in Drosophila is disrupted after overexpression of the planar cell polarity protein Diego. Taken with permission from (Moeller et al 2006). (E″) This system shares remarkable similarity with the F-molecule model of (Brown & Wolpert 1990), where plane-polarized elements coupled to directed transport allow intracellular chirality to impose asymmetry across cell fields.

Figure 3

Conserved mechanisms of LR patterning among phyla Numerous mechanisms have been discovered that pattern the left-right axis in both vertebrates and invertebrates (Levin 2006): each of the organisms has at least one molecular component in common with one or more of the others (schematized by colored ovals encompassing the species that are known to utilize each mechanism).