Regulation of Wnt‐mediated developmental competency by Pygo‐Bcl9 during body axis specification in Xenopus

Abstract

The early Xenopus embryo responds to the Wnt/β‐catenin intracellular signaling pathway by specifying dorsoanterior body structures. The loss of competence to specify the dorsal side at midblastula is followed closely by commitment to the dorsal lineage by transcriptional activation of dorsoanterior genes. The objective of our study is to determine the role of a core component duet of the β‐catenin transcription complex, made up of Pygopus and Legless/Bcl9 (Pygo‐Bcl9) during body axis specification in Xenopus embryos. Pygo‐Bcl9 recruits chromatin‐remodeling factors to the complex located at promoters of actively transcribing Wnt target genes. Maternal depletion of Bcl9 mRNA prevents body axis formation while co‐expression of Pygo‐Bcl9 ectopically activates dorsoanterior gene expression, inducing a supernumerary embryonic axis. Interestingly Bcl9 mRNA is present in the embryo during the period of Wnt/β‐catenin competence, but is rapidly translated at the end of the competency period, when dorsoanterior gene transcription occurs. We hypothesize that translation of Bcl9 defines the end of the period of competency. We propose that the identification of the factors that regulate Bcl9 translation will provide further insight into the molecular mechanisms of developmental competence.