Genetic Pathways Directing Ventral Folding Morphogenesis in Mammalian Embryos

描述（由申请人提供）：本项目的总体目标是阐明介导腹侧折叠形态发生的遗传途径和细胞机制。人类胚胎在妊娠第四周经历腹侧折叠形态发生。尽管人类和小鼠早期胚胎的外胚层具有不同的拓扑结构（前者为扁平盘状，后者为空心圆柱形），但腹侧折叠形态发生的一些关键方面是保守的：肠道内化、线状心管的形成、腹侧体壁的闭合以及胎儿在羊膜中的包裹。腹侧形态发生异常是许多与体壁不完全闭合相关的出生缺陷的基础（胃裂，腹壁外肠发育，心异位，心脏位于胸腔外）；这种腹侧折叠畸变约占活产婴儿的1/2000。由于缺乏对哺乳动物腹侧折叠形态发生的信号通路和细胞群的了解，阻碍了对这一发育过程背后的遗传学和细胞生物学的系统研究。我们提出的研究直接建立在我们最近的发现之上，即前内脏内胚层（AVE）是条纹前（PS）和早期条纹（ES）小鼠胚胎中一个明确的信号中心，在启动和指导前腹折叠形态发生中起着核心作用。组织特异性小鼠突变分析表明，骨形态发生蛋白（BMP）途径是前腹折叠形态发生的关键调节因子。BMP2在原肠胚形成过程中通过AVE信号向外胚层衍生物表达，以协调腔内形态发生的初始阶段；包括前肠内陷的形成以及心脏在头部尾侧的位置。我们的中心假设是，ave表达的BMP2通过向一种或多种不同的外胚层细胞类型（肠内胚层、前神经外胚层和/或心脏中胚层）发出信号，指导前肠内陷和头襞形态发生。在接收到av衍生的BMP2信号后，靶组织在细胞行为上发生形态发生变化，协调前肠内翻的产生，并在位置上进行喙尾移动

DESCRIPTION (provided by applicant): The overall goal of this project is to elucidate the genetic pathways and cellular mechanism that mediate ventral folding morphogenesis. Human embryos undergo ventral folding morphogenesis during the fourth week of gestation. Despite the different topologies of the epiblast in early human and mouse embryos (a flat disc in the former, a hollow cylinder in the latter), a number of key aspects of ventral folding morphogenesis are conserved: internalization of the gut, formation of a linear heart tube, closure of the ventral body wall, and encasement of the fetus in the amniotic membrane. Abnormalities in ventral morphogenesis underlie a number of birth defects associated with incomplete body wall closure (gastroschisis, development of intestines outside the abdominal wall; ectopia cordis, location of heart outside the thoracic cavity); such aberrations of ventral folding occur in ~ 1/2000 live births. An absence of knowledge about the signaling pathways and cell populations directing ventral folding morphogenesis in mammals has precluded systematic study of the genetics and cell biology underlying this developmental process. The proposed studies build directly on our recent finding that the Anterior Visceral Endoderm (AVE), a well defined signaling center in the pre-streak (PS) and Early Streak (ES) mouse embryo, plays a central role in initiating and directing anterior ventral folding morphogenesis. Tissue-specific mouse mutant analyses have shown that the Bone Morphogenetic Protein (BMP) pathway is a key regulator of anterior ventral folding morphogenesis. BMP2 expressed by the AVE signals to epiblast derivatives during gastrulation to orchestrate the initial stages of ventra morphogenesis; including formation of the foregut invagination and placement of the heart caudal to the head. Our central hypothesis is that AVE-expressed BMP2 directs foregut invagination and head fold morphogenesis by signaling to one or more distinct epiblast-derived cell types: gut endoderm, anterior neuroectoderm, and/or cardiac mesoderm. Upon receipt of the AVE-derived BMP2 signal, target tissues undergo morphogenetic changes in cell behaviors that coordinate the generation of foregut invagination with a rostral-caudal shift in the positions of head and heart. Three specific aims will investigate this hypothesis. Aim 1 uses lineage-specific Cre transgenes and a conditional allele of Bmpr1a to determine if AVE-expressed Bmp2 signals to definitive endoderm, ectoderm and/or mesoderm to initiate ventral folding morphogenesis. Aim 2 investigates whether a VE specific null allele of Bmp2 genetically interacts with null alleles of Gata4 and Hgs/Hrs, mutants known to also disrupt ventral folding morphogenesis. Aim3 maps cis-acting elements that target transcription to the proximal AVE using Bmp2-LacZ BAC reporter transgenic mouse lines; this will allow us to develop expression vectors for in vivo studies on the functional properties of the BMP2-expressing population in the proximal AVE.