Hu Guangwei, Li Guang, Wang Yiquan
Jiangsu Key Laboratory of Marine Bioresources and Environment /Jiangsu Key Laboratory of Marine Biotechnology, School of Marine Science and Fisheries, Jiangsu Ocean University, Lianyungang, 222005, China.
Co-Innovation Center of Jiangsu Marine Bio-industry Technology, Jiangsu Ocean University, Lianyungang, 222005, China.
Zoological Lett. 2021 Dec 24;7(1):16. doi: 10.1186/s40851-021-00186-8.
The left-sided position of the mouth in amphioxus larvae has fascinated researchers for a long time. Despite the fundamental importance of mouth development in the amphioxus, the molecular regulation of its development is almost unknown. In our previous study, we showed that Hh mutation in the amphioxus leads to no mouth opening, indicating a requirement of Hh signaling for amphioxus mouth formation. Nevertheless, since the Hh mutant also exhibits defects in early left-right (LR) patterning, it remains currently unknown whether the loss of mouth opening is affected directly by Hh deficiency or a secondary effect of its influence on LR establishment.
We demonstrated that knockout of the Smo gene, another key component of the Hh signaling pathway, in the amphioxus resulted in the absence of mouth opening, but caused no effects on LR asymmetry development. Upregulation of Hh signaling led to a dramatic increase in mouth size. The inability of Smo mutation to affect LR development is due to Smo's high maternal expression in amphioxus eggs and cleavage-stage embryos. In Smo mutants, Pou4 and Pax2/5/8 expression at the primordial oral site is not altered before mouth opening.
Based on these results and our previous study, we conclude that Hh signal is necessary for amphioxus mouth formation and that the Hh-mediated regulation of mouth development is specific to the mouth. Our data suggest that Hh signaling regulates mouth formation in the amphioxus in a similar way as that in vertebrates, indicating the conserved role of Hh signaling in mouth formation.
文昌鱼幼体中口位于左侧的现象长期以来一直吸引着研究人员。尽管口的发育在文昌鱼中具有根本重要性，但其发育的分子调控几乎未知。在我们之前的研究中，我们表明文昌鱼中的Hh突变导致无法开口，这表明Hh信号对于文昌鱼口的形成是必需的。然而，由于Hh突变体在早期左右（LR）模式形成中也表现出缺陷，目前尚不清楚开口缺失是直接受Hh缺乏的影响还是其对LR建立影响的次生效应。
我们证明，在文昌鱼中敲除Hh信号通路的另一个关键成分Smo基因会导致无法开口，但对LR不对称发育没有影响。Hh信号的上调导致口的大小显著增加。Smo突变无法影响LR发育是由于Smo在文昌鱼卵和卵裂期胚胎中母源表达较高。在Smo突变体中，原口部位的Pou4和Pax2/5/8表达在开口前未改变。
基于这些结果和我们之前的研究，我们得出结论，Hh信号对于文昌鱼口的形成是必需的，并且Hh介导的口发育调控对口是特异的。我们的数据表明，Hh信号以与脊椎动物类似的方式调节文昌鱼口的形成，这表明Hh信号在口形成中具有保守作用。
