Intrinsic chiral properties of theXenopusegg cortex: an early indicator of left-right asymmetry?

Abstract

Vertebrate embryos define an anatomic plane of bilateral symmetry by establishing rudimentary anteroposterior and dorsoventral (DV) axes. A left-right (LR) axis also emerges, presaging eventual morphological asymmetries of the heart and other viscera. In the radially symmetric egg of Xenopus laevis, the earliest steps in DV axis determination are driven by microtubule-dependent localization of maternal components toward the prospective dorsal side. LR axis determination is linked in time to this DVdetermining process, but the earliest steps are unclear. Significantly, no cytoskeletal polarization has been identified in early embryos capable of lateral displacement of maternal components. Cleaving Xenopus embryos and parthenogenetically activated eggs treated with 2,3-butanedione monoxime (BDM) undergo a dramatic large-scale torsion, with the cortex of the animal hemisphere shearing in an exclusively counterclockwise direction past the vegetal cortex. Long actin fibers develop in a shear zone paralleling the equator. Drug experiments indicate that the actin is not organized by microtubules, and depends on the reorganization of preexisting f-actin fibers rather than new actin polymerization. The invariant chirality of this drug response suggests a maternally inherited, microfilament-dependent organization within the egg cortex that could play an early role in LR axis determination during the first cell cycle. Consistent with this hypothesis, brief disruption of cortical actin during the first cell cycle randomizes the LR orientation of tadpole heart and gut.

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

“…The very early presence of the relevant ion transporter proteins does not rule out this model, because, like ciliary proteins themselves which can also be expressed long before ciliogenesis, they could be present but not functioning in LR asymmetry until cilia appear. This proposal is however not consistent with the timing of the earliest consistently-asymmetrical physiological gradients (Levin et al, 2002;Adams et al, 2006), cytoskeletal structures (Danilchik et al, 2006), and protein localizations (Bunney et al, 2003;Adams et al, 2006;Morokuma et al, 2008), which clearly show that at least in Xenopus, embryos know their left from their right within an hour or two of fertilization. This model is also not consistent with the timing of perturbations that are known to disrupt asymmetry far upstream of unilateral gene expression such as left-sided Nodal (Yost, 1991;Fukumoto et al, 2005a,b;Adams et al, 2006;Danilchik et al, 2006;Vandenberg and Levin, 2010).…”

“…At the level of physiology, the right ventral blastomere in the four-cell frog embryo pumps twice as many protons out as does the left ventral blastomere (Adams et al, 2006), the L and R blastomeres of ascidian embryos exhibit asymmetric calcium signaling (Albrieux and Villaz, 2000), and left-sided depolarization is observed to the left of the early (prenode) primitive streak in the chick (Levin et al, 2002). Early biophysical asymmetries include the discovery that the Xenopus egg has consistently chiral microfilament organization (Danilchik et al, 2006), as well as a cytoskeleton that has a net right-ward orientation for the guidance of intracellular transport by means of kinesin motors . At the level of protein modification, asymmetric syndecan phosphorylation occurs in gastrulating Xenopus embryos before the time when the node has formed and motile cilia are present ; in a pathway controlled by PKCg, syndecan-2 is phosphorylated in right, but not left, ectodermal cells.…”

“…1) derives consistent asymmetry from the orientation of a chiral intracellular component, which distributes cytoplasmic ion transporter proteins in a biased manner, thus driving a variety of physiological and biophysical asymmetries at very early stages (i.e., during initial cleavages; Levin and Palmer, 2007). It is now known that the cytoskeleton of some very early vertebrate embryos has inherent chirality (Danilchik et al, 2006;. In the frog embryo, the asymmetry of the cytoskeleton drives differential localization of maternal protein cargo along the LR axis during the first cleavages.…”

Laterality defects are influenced by timing of treatments and animal model

“…A second cilia-independent model proposes that asymmetry is generated from a chirally oriented cytoskeleton (Danilchik et al, 2006; Aw et al, 2008) and the asymmetric intracellular localization of motor proteins [i.e. kinesin and LRD (Qiu et al, 2005)] and cell polarity proteins (Bunney et al, 2003a).…”

“…Many treatments affecting LR asymmetry result in 20–40% heterotaxia, a much lower level than the 50–90% expected (depending on the number of organs assessed), an issue that is rarely if ever discussed [see for example (Nagai et al, 2010; Fogelgren et al, 2011; Marszalek et al, 1999; Bajoghli et al, 2007; Amack et al, 2007; Danilchik et al, 2006)]. A considerable number of studies on the mechanisms of LR asymmetry are available and comparable endpoints (i.e.…”