Hoxa2 Selectively Enhances Meis Binding to Change a Branchial Arch Ground State

Shilu Amin,Ian J Donaldson,Denise A Zannino,James Hensman,Magnus Rattray,Marta Losa,François Spitz,Franck Ladam,Charles Sagerström,Nicoletta Bobola

doi:10.1016/j.devcel.2014.12.024

Abstract

SummaryHox transcription factors (TFs) are essential for vertebrate development, but how these evolutionary conserved proteins function in vivo remains unclear. Because Hox proteins have notoriously low binding specificity, they are believed to bind with cofactors, mainly homeodomain TFs Pbx and Meis, to select their specific targets. We mapped binding of Meis, Pbx, and Hoxa2 in the branchial arches, a series of segments in the developing vertebrate head. Meis occupancy is largely similar in Hox-positive and -negative arches. Hoxa2, which specifies second arch (IIBA) identity, recognizes a subset of Meis prebound sites that contain Hox motifs. Importantly, at these sites Meis binding is strongly increased. This enhanced Meis binding coincides with active enhancers, which are linked to genes highly expressed in the IIBA and regulated by Hoxa2. These findings show that Hoxa2 operates as a tissue-specific cofactor, enhancing Meis binding to specific sites that provide the IIBA with its anatomical identity.

Highlights

The body plan of vertebrates involves the formation of similar repetitive segments, which subsequently diversify to give rise to different body parts
We extracted chromatin from the IIBA of E11.5 embryos and confirmed that Hoxa2 binds to Meis1 and Meis2 (Figures 1A and 1B)
At the same embryonic stage (E11.5), whole-mount in situ hybridization (ISH) revealed high expression of Meis1 and Meis2 in the main Hoxa2 domain of expression, the IIBA, and in the posterior margin of the in the anterior first branchial arch (IBA) (Figure S1 available online). We found that both transcripts were downregulated, and the proximal domains of Meis1 and Meis2 expression were absent in the IIBA of Hoxa2 mutant embryos (Figures 1C– 1F, arrows)

Summary

Introduction

The body plan of vertebrates involves the formation of similar repetitive segments, which subsequently diversify to give rise to different body parts. Branchial arches are a useful model of segmental specification. This transient, metameric series of tissue bands appears in the head region of vertebrate embryos. Hox proteins reprogram the execution of this first arch-like program in the subsequent arches. In embryos that lack Hoxa function, the second branchial arch (IIBA) forms mirror image copies of first arch skeletal derivatives (Gendron-Maguire et al, 1993; Rijli et al, 1993). Simultaneous inactivation of all HoxA cluster genes in the CNC leads to partial replacement of second, third, and fourth arch skeletal derivatives with multiple first arch-like structures (Minoux et al, 2009). Hox proteins appear to instruct arch-specific morphologies by overriding a ground-patterning program. In IIBA territory, Hoxa represses the expression of genes that are expressed in the anterior first branchial arch (IBA) (Bobola et al, 2003; Geisen et al, 2008; Kirilenko et al, 2011; Kutejova et al, 2005, 2008)

Results

Discussion

Conclusion