High resolution studies of Xnr1 signaling and left-right asymmetry in Xenopus

Abstract

The spatiotemporally dynamic distribution of instructive ligands within embryonic tissue, and their feedback antagonists, including inherent stabilities and rates of clearance, are affected by interactions with cell surfaces or extracellular matrix (ECM). Nodal (Xnr1 in Xenopus) and Lefty interact in a cross-regulatory relationship in mesendoderm induction, and are the conserved instructors of left-right (L-R) asymmetry in early-somitogenesis-stage embryos. Expressing Xnr1 and Lefty proproteins that produce mature functional epitope-tagged ligands in vivo, we found that ECM is a principal surface of Nodal and Lefty accumulation. We detected Lefty moving faster than Nodal, with evidence that intact sulfated proteoglycans in the ECM facilitate the remarkable long distance movement of Nodal. We propose that Nodal autoregulation substantially aided by rapid ligand transport underlies the anteriorward shift of Nodal expression in the L LPM (lateral plate mesoderm), and speculate that the higher levels of chondroitin-sulfate proteoglycan (CSPG) in more mature anterior regions provide directional transport cues. Immunodetection and biochemical analysis showed transfer of Lefty from L LPM to R LPM, providing direct evidence that L-side-derived Lefty is a significant influence in ensuring the continued suppression of R-sided expression of Nodal, maintaining unilateral expression of this conserved determinant of asymmetry. Downstream effectors of Nodal such as Pitx2c subsequently drive asymmetric morphogenesis potentially through actin cytoskeletal alterations within L LPM cells.