chongmaguereveals an essential role for laminin-mediated boundary formation in chordate convergence and extension movements

Abstract

Although cell intercalation driven by non-canonical Wnt/planar cell polarity (PCP) pathway-dependent mediolateral cell polarity is important for notochord morphogenesis, it is likely that multiple mechanisms shape the notochord as it converges and extends. Here we show that the recessive short-tailed Ciona savignyi mutation chongmague (chm) has a novel defect in the formation of a morphological boundary around the developing notochord. chm notochord cells initiate intercalation normally, but then fail to maintain their polarized cell morphology and migrate inappropriately to become dispersed in the larval tail. This is unlike aimless (aim), a mutation in the PCP pathway component Prickle, which has a severe defect in early mediolateral intercalation but forms a robust notochord boundary. Positional cloning identifies chm as a mutation in the C. savignyi ortholog of the vertebrate alpha 3/4/5 family of laminins. Cs-lam␣3/4/5 is highly expressed in the developing notochord, and Cs-lam␣3/4/5 protein is specifically localized to the outer border of the notochord. Notochord convergence and extension, reduced but not absent in both chm and aim, are essentially abolished in the aim/aim; chm/chm double mutant, indicating that laminin-mediated boundary formation and PCPdependent mediolateral intercalation are each able to drive a remarkable degree of tail morphogenesis in the absence of the other. These mechanisms therefore initially act in parallel, but we also find that PCP signaling has an important later role in maintaining the perinotochordal/intranotochordal polarity of Cs-lam␣3/4/5 localization.

Exploiting the Extraordinary Genetic Polymorphism ofCionafor Developmental Genetics with Whole Genome Sequencing

“…To test this, we developed a bioinformatics strategy and applied it to two novel mutant lines, one in C. intestinalis and the other in C. savignyi. The C. intestinalis mutant had a short-tail phenotype ( Figure 1, A and B) closely resembling the chongmague (chm) mutation previously isolated from the related species C. savignyi (Veeman et al 2008) and was named chm-like. A cross-species fertilization test indicated that the C. intestinalis mutant was in the same complementation group as the C. savignyi chm mutant (data not shown).…”

“…Because establishing inbred lines of Ciona has proven to be very difficult, we maintain mutant and transgenic lines by outbreeding them to wild-caught stocks (Veeman et al 2011). Despite this limitation, we have been able to isolate, maintain, and map naturally occurring mutations in C. intestinalis and C. savignyi that disrupt processes such as notochord morphogenesis and neural plate development (Jiang et al 2005;Veeman et al 2008;Tresser et al 2010;Hackley et al 2013). The high level of genetic variation between individuals has actually aided mapping via bulk segregant analysis by providing an abundance of single-nucleotide variation (SNV) markers, although our current mapping procedures are very time consuming.…”

Mouse prickle1 , the homolog of a PCP gene, is essential for epiblast apical-basal polarity

“…S6D). Recent studies also suggest an important link between PCP signaling and the asymmetric deposition of ECM (23,24). We hypothesized that the disorganized epiblast seen in mpk1 Ϫ/Ϫ mutant reflects a failure in ECM deposition or spindle orientation, on which proper organization of the epiblast, which has the typical characteristics of a pseudostratified epithelium, depends.…”

Morphogenesis of the primitive gut tube is generated by Rho/ROCK/myosin II–mediated endoderm rearrangements

“…During gastrulation, the net directionality of cell movement is conferred by the stable ''capture'' of rearranging cells at the fibronectin-and lamininrich notochord-somite boundary (reviewed in Wallingford et al, 2002;reviewed in Keller et al, 2008;Veeman et al, 2008). These captured cells then exert traction on neighboring cells, likely facilitated by myosinmediated contractility , to progressively drive convergence toward the axial midline.…”