Dorsal activity of maternal squint is mediated by a non-coding function of the RNA

Abstract

Despite extensive study, the earliest steps of vertebrate axis formation are only beginning to be elucidated. We previously showed that asymmetric localization of maternal transcripts of the conserved zebrafish TGFβ factor Squint (Sqt) in 4-cell stage embryos predicts dorsal, preceding nuclear accumulation of β-catenin. Cell ablations and antisense oligonucleotides that deplete Sqt lead to dorsal deficiencies, suggesting that localized maternal sqt functions in dorsal specification. However, based upon analysis of sqt and Nodal signaling mutants, the function and mechanism of maternal sqt was debated. Here, we show that sqt RNA may function independently of Sqt protein in dorsal specification. sqt insertion mutants express localized maternal sqt RNA. Overexpression of mutant/non-coding sqt RNA and, particularly, the sqt 3'UTR, leads to ectopic nuclear β-catenin accumulation and expands dorsal gene expression. Dorsal activity of sqt RNA requires Wnt/β-catenin but not Oep-dependent Nodal signaling. Unexpectedly, sqt ATG morpholinos block both sqt RNA localization and translation and abolish nuclear β-catenin, providing a mechanism for the loss of dorsal identity in sqt morphants and placing maternal sqt RNA upstream of β-catenin. The loss of early dorsal gene expression can be rescued by the sqt 3'UTR. Our findings identify new non-coding functions for the Nodal genes and support a model wherein sqt RNA acts as a scaffold to bind and deliver/sequester maternal factors to future embryonic dorsal.