A unified model for left-right asymmetry? Comparison and synthesis of molecular models of embryonic laterality

Abstract

Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.

Figure 1. Schematic outlining three major models of LR asymmetry

For each model, predictions about the origin of asymmetry, the mechanism for aligning the LR axis, the intermediate and amplification steps, and information about how early asymmetries influence asymmetric gene expression are described. Most noteworthy is that all three models agree on an intracellular cytoskeletal origin of asymmetry: the centriole.

Figure 2. Sections through early cleavage stage embryos reveal open gap junctions between blastomeres

Embryos were co-injected with small and large molecular weight dyes (red labeled rhodamine dextran, 10 kD, and green labeled Lucifer Yellow, ~450 D). The small molecular weight dyes are transferred to neighboring blastomeres via open gap junctions, but large molecular weight dyes do not, ruling out incomplete cleavage or cytoplasmic bridges. Although it has been suggested that previous studies (Levin and Mercola, 1998) reporting these findings were influenced by imaging artifacts of the whole embryo, these sections through the early cleavage stage embryo show the same results – true gap-junctional connectivity.

Figure 3. Asymmetric gene expression is a poor predictor of asymmetric organ situs

Hundreds of treatments and mutants that were analyzed for both asymmetric expression of Nodal, Lefty or Pitx2 that also reported the effect of treatment on organ situs were examined from the published literature (Vandenberg, 2012). Overwhelmingly, these studies indicate that measures of incorrect gene expression (i.e. a left-sided gene expressed on the right, on both sides, or completely absent) overestimate the effect of a treatment or mutation on organ position. The regression of the data, as indicated on the graph, suggests that ~30% of embryos could have incorrect gene expression, but no problems with organ situs – the definitive readout of embryonic LR asymmetry.

Figure 4. Two new models of LR asymmetry pathways

A) This model proposes that LR asymmetry is initiated during the early cleavage stages of development, but that LR information is then amplified via several mechanisms including the asymmetric distribution of serotonin and the asymmetric motion of cilia to distribute other morphogens and ions. According to this model, LR asymmetry is best established when all mechanisms are working properly. Animals that do not have or utilize cilia for LR patterning (i.e. chick, pig, Drosophila, etc.) depend only on early mechanisms. B) This model proposes that there are several independent ways that embryos can achieve LR asymmetry. Within a single population, some embryos use one pathway while other embryos use another pathway in a kind of multi-pathway stochasticity. Thus, constructs and pharmaceuticals that target early pathways can only influence LR asymmetry in those embryos that ‘chose’ a pathway that includes early mechanisms; similarly, treatments that alter fluid flow at the node can only influence LR asymmetry in those embryos that ‘chose’ the cilia pathway, resulting in the observed incomplete penetrance observed in most studies.