Langdon Yvette G, Fuentes Ricardo, Zhang Hong, Abrams Elliott W, Marlow Florence L, Mullins Mary C
University of Pennsylvania Perelman School of Medicine, Department of Cell and Developmental Biology, 421 Curie Blvd., Philadelphia, PA 19104, USA Millsaps College, Department of Biology, Jackson, MS 39210, USA.
University of Pennsylvania Perelman School of Medicine, Department of Cell and Developmental Biology, 421 Curie Blvd., Philadelphia, PA 19104, USA.
Development. 2016 Mar 15;143(6):1016-28. doi: 10.1242/dev.128900. Epub 2016 Feb 18.
The vertebrate embryonic dorsoventral axis is established and patterned by Wnt and bone morphogenetic protein (BMP) signaling pathways, respectively. Whereas Wnt signaling establishes the dorsal side of the embryo and induces the dorsal organizer, a BMP signaling gradient patterns tissues along the dorsoventral axis. Early Wnt signaling is provided maternally, whereas BMP ligand expression in the zebrafish is zygotic, but regulated by maternal factors. Concomitant with BMP activity patterning dorsoventral axial tissues, the embryo also undergoes dramatic morphogenetic processes, including the cell movements of gastrulation, epiboly and dorsal convergence. Although the zygotic regulation of these cell migration processes is increasingly understood, far less is known of the maternal regulators of these processes. Similarly, the maternal regulation of dorsoventral patterning, and in particular the maternal control of ventral tissue specification, is poorly understood. We identified split top, a recessive maternal-effect zebrafish mutant that disrupts embryonic patterning upstream of endogenous BMP signaling. Embryos from split top mutant females exhibit a dorsalized embryonic axis, which can be rescued by BMP misexpression or by derepressing endogenous BMP signaling. In addition to dorsoventral patterning defects, split top mutants display morphogenesis defects that are both BMP dependent and independent. These morphogenesis defects include incomplete dorsal convergence, delayed epiboly progression and an early lysis phenotype during gastrula stages. The latter two morphogenesis defects are associated with disruption of the actin and microtubule cytoskeleton within the yolk cell and defects in the outer enveloping cell layer, which are both known mediators of epiboly movements. Through chromosomal mapping and RNA sequencing analysis, we identified the lysosomal endopeptidase cathepsin Ba (ctsba) as the gene deficient in split top embryos. Our results identify a novel role for Ctsba in morphogenesis and expand our understanding of the maternal regulation of dorsoventral patterning.
脊椎动物胚胎的背腹轴分别由Wnt信号通路和骨形态发生蛋白（BMP）信号通路建立并形成模式。Wnt信号通路确立胚胎的背侧并诱导背侧组织者，而BMP信号梯度则沿背腹轴对组织进行模式化。早期Wnt信号由母体提供，而斑马鱼中BMP配体的表达是合子性的，但受母体因子调控。在BMP活性对背腹轴组织进行模式化的同时，胚胎也经历显著的形态发生过程，包括原肠胚形成、外包和背侧会聚等细胞运动。尽管对这些细胞迁移过程的合子调控越来越了解，但对这些过程的母体调节因子知之甚少。同样，背腹模式的母体调控，尤其是对腹侧组织特化的母体控制，也了解不足。我们鉴定出了“分裂顶”（split top），这是一种隐性母体效应斑马鱼突变体，它在内源性BMP信号上游破坏胚胎模式。来自“分裂顶”突变体雌性的胚胎表现出背化的胚胎轴，可通过BMP的错误表达或通过解除对内源性BMP信号的抑制来挽救。除了背腹模式缺陷外，“分裂顶”突变体还表现出依赖BMP和不依赖BMP的形态发生缺陷。这些形态发生缺陷包括背侧会聚不完全、外包进程延迟以及原肠胚阶段的早期裂解表型。后两种形态发生缺陷与卵黄细胞内肌动蛋白和微管细胞骨架的破坏以及外层包被细胞层的缺陷有关，而这两者都是已知的外包运动介质。通过染色体定位和RNA测序分析，我们确定溶酶体肽链内切酶组织蛋白酶Ba（ctsba）是“分裂顶”胚胎中缺失的基因。我们的结果确定了Ctsba在形态发生中的新作用，并扩展了我们对背腹模式母体调控的理解。
