Two modes of action by which Xenopus hairy2b establishes tissue demarcation in the Spemann-Mangold organizer

Abstract

The Hairy and Enhancer-of-Split (HES) family of transcriptional repressors plays important roles in pattern formation during development throughout the animal kingdom. Generally, HES proteins repress the expression of genes specific for neighboring tissues to maintain the nature of cells expressing HES proteins, resulting in pattern formation. Xhairy2b, a Xenopus HES, establishes the prospective anterior prechordal mesoderm identity in the Spemann-Mangold organizer by both inducing specific genes and repressing the genes specific for neighboring tissues. Here we report that Xhairy2b has two modes of action, each of which corresponds to inductive and repressive functions. We show that the inductive function is independent of direct transcriptional regulation and is exhibited by the C-terminal WRPW tetrapeptide motif alone, although it induces the expression of a wide variety of the organizer genes that Xhairy2b represses. The transcriptional repression by Xhairy2b is responsible for only the repressive function. We propose that the activity of the WRPW motif intrinsically induces the expression of genes specific for the organizer in a rather non-specific manner to ensure the organizer environment. Then, the transcriptional repression selectively down-regulates the expression of some of these genes, resulting in the regionalization of the axial mesoderm. Our study provides new insight into how a region of the vertebrate embryo is demarcated by one dual-functional transcription factor in the early stages of development.

The Xenopus homologue of Down syndrome critical region protein 6 drives dorsoanterior gene expression and embryonic axis formation by antagonising polycomb group proteins

“…For example, Hairyrelated proteins may function as dual transcriptional regulators (activator and repressor) in embryonic axis formation (Murato et al, 2006). In the ascidian embryo, Posterior end mark (PEM), which has a WRPW motif and lacks a DNA-binding domain, plays a role in anteroposterior patterning (Yoshida et al, 1996).…”

Xenopus pancreas development

“…In the pancreas, Hes1 is involved in antagonizing the function of B-class bHLH proteins such as Ngn3, and is expressed in undifferentiated precursor cells (Jorgensen et al, 2007). In Xenopus, Hes1 is known as hairy2b and it has been extensively studied in the context of neural development (Yamaguti et al, 2005; Murato et al, 2006). Within the pancreas, it is expressed in a punctate fashion from the earliest stages, most likely marking pancreatic progenitor cells (data not shown).…”

Xhairy2 functions in Xenopus lens development by regulating p27xic1 expression

“…This, in turn, means that Xhairy2 overexpression around the LF region should increase the LF expression of pax6 and six3. However, neither wildtype Xhairy2 overexpression nor engrailed-fused form of Xhairy2 overexpression (Murato et al, 2006) increased the LF expression of six3 (data not shown). Moreover, our data showed that Xhairy2 knockdown did not alter the expression of fgf8 or fgfr4c (data not shown).…”

“…Since both Xhairy2a and Xhairy2b function as transcriptional repressors, similar to other Hes proteins (Murato et al, 2006(Murato et al, , 2007, we assumed that the up-regulation of genes that are not expressed in the future LF region at gastrula or early neurula stage might block LF formation at mid-neurula stage. In our previous study of the functions of Xhairy2 in neural crest formation , we showed that Xhairy2 downregulated subsets of proneural marker genes, such as X-delta-1 (Chitnis et al, 1995) and X-ngnr-1 .…”