Background information. In the embryos of various animals, the body elongates after gastrulation by morphogenetic movements involving convergent extension. The Wnt/PCP (planar cell polarity) pathway plays roles in this process, particularly mediolateral polarization and intercalation of the embryonic cells. In ascidians, several factors in this pathway, including Wnt5, have been identified and found to be involved in the intercalation process of notochord cells.
Results. In the present study, the role of the Wnt5 genes, Hr-Wnt5 alpha (Halocynthia roretzi Wnt5 alpha) and Hr-Wnt5 beta, in convergent extension was investigated in the ascidian H. roretzi by injecting antisense oligonucleotides and mRNAs into single precursor blastomeres of various tissues, including notochord, at the 64-cell stage. Hr-Wnt5 alpha is expressed in developing notochord and was essential for notochord morphogenesis. Precise quantitative control of its expression level was crucial for proper cell intercalation. Overexpression of Wnt5 proteins in notochord and other tissues that surround the notochord indicated that Wnt5a plays a role within the notochord, and is unlikely to be the source of polarizing cues arising outside the notochord. Detailed mosaic analysis of the behaviour of individual notochord cells overexpressing Wnt5 alpha indicated that a Wnt5 alpha-manipulated cell does not affect the behaviour of neighbouring notochord cells, suggesting that Wnt5 alpha works in a cell-autonomous manner. This is further supported by comparison of the results of Wnt5 alpha and Dsh (Dishevelled) knockdown experiments. In addition, our results suggest that the Wnt/PCP pathway is also involved in mediolateral intercalation of cells of the ventral row of the nerve cord (floor plate) and the endodermal strand.
Conclusion. The present study highlights the role of the Wnt5 alpha signal in notochord convergent extension movements in ascidian embryos. Our results raise the novel possibility that Wnt5 alpha functions in a cell-autonomous manner in activation of the Wnt/PCP pathway to polarize the protrusive activity that drives convergent extension.