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

Abstract

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Fig. 1.

Schematic of cytoplasmic model of left–right (LR) asymmetry. This schematization, based on events characterized in Xenopus laevis, shows how intracellular chirality can be imposed upon multicellular cell fields, and initiate asymmetric gene expression relative to the midline, by a physiological mechanism driven by maternal, biophysical events. A: Eggs are initially loaded with maternal mRNAs and proteins (blue) that are symmetrically (radially) distributed around the animal–vegetal axis. B: When the egg is fertilized, the location of the sperm entry point dictates the location of the first cleavage plane, which usually coincides with the midline of the developing animal (Scharf and Gerhart, 1983; Ubbels et al., 1983; Klein, 1987; Masho, 1990; Marrari et al., 2004) . The crucial event at this stage is the complex set of known cytoskeletal rearrangements that include the hypothetical orientation of an organizing center (e.g., the centrioles existing at right angles to each other) with respect to the animal–vegetal and dorsal–ventral axes (the enantiomeric “F-molecule,” here represented by a hand). Maternal cargo proteins/mRNAs can then begin to be distributed in a consistently-asymmetric manner by cytoplasmic motor transport (dependent on kinesin, dynein, and microtubule arrays). C: By the four-cell stage, maternal mRNAs are now largely localized to the right ventral blastomere. These mRNAs encode ion transporters including two potassium channels (Aw et al., 2008; Morokuma et al., 2008) and two proton pumps, the H⁺-K⁺-ATPase (Levin et al., 2002) and the H⁺-V-ATPase (Adams et al., 2006). D: Together, the asymmetric localization of these transporters (blue circles) leads to a circuit establishing physiological asymmetries such as an increased pumping of positively charged ions out of the right side cell (+), leading to a difference in transmembrane potential between the L and R blastomeres across the ventral midline. The blastomeres of the early embryo next become connected by means of open gap junctions (orange channels), with the exception of the cells between the left and right ventral cells (Levin and Mercola, 1998b), which are junctionally-isolated (dark line). E: Serotonin and perhaps other small charged morphogens (yellow dots) are initially present in all blastomeres. F: Subject to selectivity of the gap junctions, some are then driven toward the right-most ventral cell by an electrophoretic force maintained by the battery at the ventral edge. G,H: Through an as-yet uncharacterized pathway, the localization of morphogens such as serotonin to the right side of the embryo suppresses downstream expression of genes (i.e., Nodal), which are thus expressed only on the left side (G), which eventually leads to asymmetric organ morphogenesis (H). This model can be readily extended to bodyplans where the midline is defined after the initial cleavages are complete by the spread of LR orientation information from coordinator cell(s) by means of planar cell polarity pathways (Aw and Levin, 2009; Vandenberg and Levin, 2010).

Fig. 2.

Logical schematization of the three major models of left–right (LR) asymmetry. The three fundamental models of asymmetry generation all derive the initial chirality from the centrosome or other cytoskeletal organizing center. The ciliary models propose that, having formed a cilium with a molecular chirality determined by its physical structure and the nucleating element, planar polarity aligns its rotary motion with the anterior–posterior (AP) and dorsal–ventral (DV) axes. Thereupon, calcium fluxes generated by mechanosensory cilia or specific receptors for bulk transport of some extracellular morphogen provide the first difference between the left and right sides; this would occur on the Node during gastrulation, and shortly thereafter turned into asymmetric gene expression of Nodal on the left side. The differential strand segregation model proposes that the two DNA strands during the earliest cell divisions are not identical in terms of their imprinting or in terms of the mRNAs associating with them (Lambert and Nagy, 2002). The cytokinesis machinery, coupled to planar polarity or another mechanism for orienting DNA segregation with respect to the existing two axes, ensures that the right and left sides acquire DNA with distinct gene expression profiles in subsequent stages. The intracellular/physiological model proposes that early asymmetries of the cytoskeleton allow ion channels and pumps to be localized by intracellular motor proteins to one side of blastomeres also oriented within the anatomical polarity of the embryo during the first few cleavages. These bioelectrical asymmetries result in the redistribution of a small molecule morphogen (maternal serotonin) that subsequently initiates repression of Nodal on the right side. All three models feed into the less controversial steps of the asymmetric gene cascade that ultimately controls the patterning of the heart and visceral organs.