Abstract 226: Mutual Interactive Factors β-Catenin and Oct4 Regulate Mesp1 Transcription

Abstract

The b-HLH family protein Mesp1 sits on the top of the regulatory hierarchy of factors that govern cardiac specification. Ectopic expression of Mesp1 drives precocious appearance of cardiac progenitors, while ablation of Mesp1 and its close genetic relative Mesp2 completely blocked the emergence of heart in mouse embryos. Despite its central importance in cardiac specification, the regulation of Mesp1 is largely unknown. In this study, we show several lines of evidences supporting that beta-catenin and Oct4 cooperate in regulating Mesp1 induction. (1) A conserved TCF/Lef-Oct4 composite binding site is identified near the transcription start site of Mesp1. (2) beta-catenin and Oct4 each activates a Mesp1-Luciferase reporter, with beta-catenin being more potent; while together, beta-catenin and Oct4 synergize in activating the Mesp1-Luciferase reporter. Point mutations within the TCF/Lef-Oct4 binding site impair reporter activation by beta-catenin. (3) beta-catenin and Oct4 individually binds to an oligo covering the putative TCF/Lef-Oct4 binging site on Mesp1 promoter, and together they form a ternary complex, as detected by electrophoresis mobility shift assays. (4) Mesp1 induction is absent in cells with null alleles of beta-catenin, or conditional null alleles of Oct4. (5) While a 6K promoter of Mesp1 effectively drives expression of a lacZ reporter at E7.5, mutants affecting the TCF/Lef-Oct4 binding site fail to do so, in transient transgenic assays. Together, our results show that the cooperation between beta-catenin and Oct4, for Mesp1 transcription and cardiac specification substantiate a novel regulatory mechanism governing cell fate determination and cardiac development, which also explain the mechanism of well-established roles of each factor in mesoderm formation and heart development. Such knowledge may be used to establish new technology to generate cardiac cells for therapeutic purposes.