Cowan Chad A, Yokoyama Nobuhiko, Saxena Ankur, Chumley Michael J, Silvany Robert E, Baker Linda A, Srivastava Deepak, Henkemeyer Mark
Center for Developmental Biology and Kent Waldrep Center for Basic Research on Nerve Growth and Regeneration, University of Texas Southwestern Medical Center, Dallas, TX 75390-9133, USA.
Dev Biol. 2004 Jul 15;271(2):263-71. doi: 10.1016/j.ydbio.2004.03.026.
Vascular development begins with the formation of a primary vascular plexus that is rapidly remodeled by angiogenesis into the interconnected branched patterns characteristic of mature vasculature. Several receptor tyrosine kinases and their ligands have been implicated to control early development of the vascular system. These include the vascular endothelial growth factor receptors (VEGFR-1 and VEGFR-2) that bind VEGF, the Tie-1 and Tie-2 receptors that bind the angiopoietins, and the EphB4 receptor that binds the membrane-anchored ligand ephrin-B2. Targeted mutations in the mouse germline have revealed essential functions for these molecules in vascular development. In particular, protein-null mutations that delete either EphB4 or ephrin-B2 from the mouse have been shown to result in early embryonic lethality due to failed angiogenic remodeling. The venous expression of EphB4 and arterial expression of ephrin-B2 has lead to the speculation that the interaction of these two molecules leads to bidirectional signaling into both the receptor-expressing cell and the ligand-expressing cell, and that both forward and reverse signals are required for proper development of blood vessels in the embryo. Indeed, targeted removal of the ephrin-B2 carboxy-terminal cytoplasmic tail by another group was shown to perturb vascular development and result in the same early embryonic lethality as the null mutation, leading the authors to propose that ephrin-B2 reverse signaling directs early angiogenic remodeling of the primary vascular plexus [Cell 104 (2001) 57]. However, we show here that the carboxy-terminal cytoplasmic domain of ephrin-B2, and hence reverse signaling, is not required during early vascular development, but it is necessary for neonatal survival and functions later in cardiovascular development in the maturation of cardiac valve leaflets. We further show that ephrin-B2 reverse signaling is required for the pathfinding of axons that form the posterior tract of the anterior commissure. Our results thus indicate that ephrin-B2 functions in the early embryo as a typical instructive ligand to stimulate EphB4 receptor forward signaling during angiogenic remodeling and that later in embryonic development ephrin-B2 functions as a receptor to transduce reverse signals involved in cardiac valve maturation and axon pathfinding.
血管发育始于初级血管丛的形成，该血管丛通过血管生成迅速重塑为成熟脉管系统特有的相互连接的分支模式。几种受体酪氨酸激酶及其配体被认为可控制血管系统的早期发育。这些包括结合血管内皮生长因子（VEGF）的血管内皮生长因子受体（VEGFR - 1和VEGFR - 2）、结合血管生成素的Tie - 1和Tie - 2受体，以及结合膜锚定配体ephrin - B2的EphB4受体。小鼠种系中的靶向突变揭示了这些分子在血管发育中的重要功能。特别是，从小鼠中删除EphB4或ephrin - B2的蛋白质缺失突变已被证明会由于血管生成重塑失败而导致早期胚胎致死。EphB4的静脉表达和ephrin - B2的动脉表达引发了这样的推测，即这两种分子的相互作用导致双向信号传导进入表达受体的细胞和表达配体的细胞，并且正向和反向信号对于胚胎中血管的正常发育都是必需的。事实上，另一组研究表明，靶向去除ephrin - B2的羧基末端细胞质尾巴会扰乱血管发育，并导致与缺失突变相同的早期胚胎致死，这使得作者提出ephrin - B2反向信号传导指导初级血管丛的早期血管生成重塑[《细胞》104(2001)57]。然而，我们在此表明，ephrin - B2的羧基末端细胞质结构域以及因此的反向信号传导在早期血管发育过程中并非必需，但对于新生儿存活以及在心血管发育后期心脏瓣膜小叶成熟过程中发挥作用是必需的。我们进一步表明，ephrin - B2反向信号传导对于形成前连合后束的轴突的路径寻找是必需的。因此，我们的结果表明，ephrin - B2在早期胚胎中作为一种典型的诱导性配体发挥作用，在血管生成重塑过程中刺激EphB4受体的正向信号传导，而在胚胎发育后期，ephrin - B2作为一种受体发挥作用，转导参与心脏瓣膜成熟和轴突路径寻找的反向信号。
