cis‐Regulatory analysis for later phase of anterior neuroectoderm‐specific foxQ2 expression in sea urchin embryos

Abstract

Summary The specification of anterior neuroectoderm is controlled by a highly conserved molecular mechanism in bilaterians. A forkhead family gene, foxQ2, is known to be one of the pivotal regulators, which is zygotically expressed in this region during embryogenesis of a broad range of bilaterians. However, what controls the expression of this essential factor has remained unclear to date. To reveal the regulatory mechanism of foxQ2, we performed cis‐regulatory analysis of two foxQ2 genes, foxQ2a and foxQ2b, in a sea urchin Hemicentrotus pulcherrimus. In sea urchin embryos, foxQ2 is initially expressed in the entire animal hemisphere and subsequently shows narrower expression restricted to the anterior pole region. In this study, as a first step to understand the foxQ2 regulation, we focused on the later restricted expression and analyzed the upstream cis‐regulatory sequences of foxQ2a and foxQ2b by using the constructs fused to short half‐life green fluorescent protein. Based on deletion and mutation analyses of both foxQ2, we identified each of the five regulatory sequences, which were 4–9 bp long. Neither of the regulatory sequences contains any motifs for ectopic activation or spatial repression, suggesting that later mRNA localization is regulated in situ. We also suggest that the three amino acid loop extension‐class homeobox gene Meis is involved in the maintenance of foxQ2b, the expression of which is dominant during embryogenesis.

An ancient gene regulatory network sets the position of the forebrain in chordates

“…We see that FoxQ2 and Wnt8 are expressed in amphioxus from the blastula stage in two mutually exclusive domains, at the animal and vegetal side of the embryo respectively. This early expression greatly resembles the one seen in ambulacrarian larvae, and has no counterpart in vertebrates, as FoxQ2 has not been detected in any vertebrate embryos before organogenesis and the gene is lost in placental mammals 51 . Moreover, at this stage the expression is radial and does not show any type of DV regionalization, suggesting that it is not directly influenced by Nodal signalling acting at the posterior-dorsal side at this stage 84 .…”

“…The mutually repressive interactions between Six3 and Wnt have been also demonstrated in vertebrate nervous systems, where they are essential to establish the anterior identity of the brain 49 . However, unlike in the AO of invertebrate larvae, Six3 expression in vertebrates is not influenced by FoxQ2 , which has been lost in placental mammals 51,52 . Adding to this striking difference, Six3 in vertebrates is expressed well after gastrulation has initiated in cells already committed to a neuronal fate 53,54 .…”

ウニ幼生の神経の形成と機能

“…と３)消 化管 46,53) 16,34) 。また，脳領域を 抑制する Wnt シグナルが前端部に達するタイミングになる と，その領域で Wnt inhibitor が複数発現しはじめて，Wnt シグナルをブロックすることになり結果的にWnt -freeに なった前端部が脳領域へと specify される 16) 15) 。Zygoticに脳領 域で最も早く発現し，脳領域の Specification に必要な転写 因子はFoxQ2である 43,59) 。FoxQ2は16細胞期の動物半球 すべての割球で発現するうえ，転写調節解析の過程では植 物半球においても一時的に発現していることが示されてい る 64) 。in situ hybridizationでは動物半球における発現の みが明確に示されるが，その発現は発生が進むにつれて胚 前端部へと収束する(図３B) 。これは先に述べた「ほぼす べての細胞のもともとの運命が脳領域であり，体の後方 (植 物極側)からの Wnt シグナルによって前端部にだけ脳領域 が抑えられる」のパターンを見事に再現している。実際に， Wnt/ß-catenin シグナルを抑制した胚では，ほぼすべての 領域に FoxQ2の発現が維持される 59) 。FoxQ2は左右相称動 物の多くで体の最前端部に発現し，脳領域の形成に必要で あることがウニの研究以降に明らかになりつつあり，脊椎 動物の zebrafish においても視覚に関して重要な働きを担っ ていることが報告されている 17,27)…”