Data from: Notch and Nodal control expression of a forkhead factor in the specification network of a multipotent progenitor population in sea urchin

Indirect development, where embryogenesis gives rise to a larval form, requires that some cells retain developmental potency until they contribute to the different tissues in the adult, including the germ line, in a later, post-embryonic phase. In sea urchins, the coelomic pouches are the major contributor to the adult but how coelomic pouch cells (CPCs) are specified during embryogenesis is unknown. We here identify the key signaling inputs into the CPC specification network and show the forkhead factor foxY is the first transcription factor specifically expressed in CPC progenitors. Through dissection of its cis-regulatory apparatus we determine that the foxY expression pattern is the result of two signaling inputs: First, Delta/Notch signaling activates foxY in CPC progenitors and, second, Nodal signaling restricts its expression to the left side, where the adult rudiment will form, through direct repression by the Nodal target pitx2. A third signal, Hedgehog, is required for coelomic pouch morphogenesis and institution of laterality but does not directly affect foxY transcription. Knockdown of foxY results in failure to form coelomic pouches and disrupts expression of virtually all transcription factors known to be expressed in this cell type. Our experiments place foxY at the top of the regulatory hierarchy underlying specification of a cell type maintaining developmental potency.

间接发育是指胚胎发生过程产生幼虫形态的发育模式，该模式要求部分细胞保留发育潜能，直至在后续的胚后阶段参与成体的各类组织（包括生殖系）的形成。在海胆中，体腔囊是成体结构的主要贡献来源，但胚胎发生过程中体腔囊细胞（coelomic pouch cells, CPCs）的特化机制仍不明晰。本研究阐明了调控CPC特化网络的关键信号输入，并证实叉头框转录因子foxY是首个在CPC前体中特异性表达的转录因子。通过解析其顺式调控元件（cis-regulatory apparatus），我们发现foxY的表达模式由两类信号输入共同调控：其一，Delta/Notch信号通路在CPC前体中激活foxY的转录；其二，Nodal信号通路通过其下游靶基因pitx2的直接抑制作用，将foxY的表达限定在成体原基形成的左侧区域。第三种信号分子刺猬信号通路（Hedgehog）参与体腔囊的形态发生与左右不对称性的建立，但并不直接调控foxY的转录。敲低foxY会导致体腔囊无法形成，并几乎完全阻断该细胞类型中所有已知转录因子的表达。本研究实验证实，foxY位于维持发育潜能的细胞类型特化调控网络的顶层。