Consistent left-right asymmetry cannot be established by late organizers inXenopusunless the late organizer is a conjoined twin

Abstract

SUMMARYHow embryos consistently orient asymmetries of the left-right (LR) axis is an intriguing question, as no macroscopic environmental cues reliably distinguish left from right. Especially unclear are the events coordinating LR patterning with the establishment of the dorsoventral (DV) axes and midline determination in early embryos. In frog embryos, consistent physiological and molecular asymmetries manifest by the second cell cleavage; however, models based on extracellular fluid flow at the node predict correct de novo asymmetry orientation during neurulation. We addressed these issues in Xenopus embryos by manipulating the timing and location of dorsal organizer induction: the primary dorsal organizer was ablated by UV irradiation, and a new organizer was induced at various locations, either early, by mechanical rotation, or late, by injection of lithium chloride (at 32 cells) or of the transcription factor XSiamois (which functions after mid-blastula transition). These embryos were then analyzed for the position of three asymmetric organs. Whereas organizers rescued before cleavage properly oriented the LR axis 90% of the time, organizers induced in any position at any time after the 32-cell stage exhibited randomized laterality. Late organizers were unable to correctly orient the LR axis even when placed back in their endogenous location. Strikingly, conjoined twins produced by late induction of ectopic organizers did have normal asymmetry. These data reveal that although correct LR orientation must occur no later than early cleavage stages in singleton embryos, a novel instructive influence from an early organizer can impose normal asymmetry upon late organizers in the same cell field.

Laterality defects are influenced by timing of treatments and animal model

“…We showed previously that the penetrance of LR defects increased from 25% to 40% when Xenopus tadpoles with slight axial defects were included for analysis (Vandenberg and Levin, 2010a) and similar results have been reported by others as well (Danos and Yost, 1996; 1995; Lohr et al, 1997). Here, I found that many LR phenotypes occur concurrently with malformations of midline structures (i.e.…”

“…Previous studies have shown that in Xenopus, developmental defects that affect patterning of the other axes can influence orientation of the LR axis (Lohr et al, 1997; Lohr et al, 1998; Vandenberg and Levin, 2010a). To determine whether this is the case for all treatments, I first determined the frequency of developmental problems in EARLY and CILIA treatments.…”

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

“…PCP is an attractive mechanism for LR asymmetry because it can impose the initial orientation of cells or small groups of blastomeres to entire cell fields (Amonlirdviman et al, 2005), as was originally proposed in the ''LR Coordinator'' model (Hyatt and Yost, 1998). A similar mechanism was recently proposed to explain the instructive influence by means of which primary organizers (existing during the initial cytoskeletal rearrangements) can impose correct asymmetry upon secondary organizers induced later in development (which can otherwise correctly pattern all of their axes except the LR; Vandenberg and Levin, 2010). Consistently, it is known that reversal of the blastocoel roof can randomize asymmetry (Yost, 1992), as predicted by the proposal that the embryonic blastoderm polarizes long before neurulation.…”

“…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).…”

“…Because it is a true visceral laterality reversal, this phenotype is a more striking example than the long-known bookending (mirroring) of hair whorls and craniofacial defects in human monozygotic twins (Levin, 1999), and shows that in amphibia, later ciliary motions and nodal flow cannot overcome the disruption of LR information at early cleavage stages. The second line of evidence comes from recent work analyzing the ability of lateinduced organizers to pattern the LR axis in Xenopus (Vandenberg and Levin, 2010). When the endogenous organizer is ablated and embryonic patterning is rescued by induction of an organizer as early as the 32-cell stage, embryos are normal except that their LR axis is destabilized, showing that an organizer deprived of the orienting events occurring during the first few cleavages cannot orient asymmetry properly, despite the lack of any disruption at the later ciliated node.…”