Xu Nan-Jie, Henkemeyer Mark
Department of Developmental Biology, Kent Waldrep Center for Basic Research on Nerve Growth and Regeneration, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9133, USA.
Nat Neurosci. 2009 Mar;12(3):268-76. doi: 10.1038/nn.2254. Epub 2009 Feb 1.
It has been suggested that ephrin-B proteins have receptor-like roles in the control of axon pathfinding by repulsion, although it is largely unknown how the reverse signals are coupled to downstream intracellular molecules and how they induce cytoskeletal reorganization at the axon terminal. We found that ephrin-B3 (EB3) was able to function as a repulsive guidance receptor and mediate stereotyped pruning of murine hippocampal mossy fiber axons during postnatal development. Targeted intracellular point mutants showed that axon pruning requires tyrosine phosphorylation-dependent reverse signaling and coupling to the SH2/SH3 adaptor protein Grb4 (also known as Nckbeta/Nck2). Furthermore, we found that the second SH3 domain of Grb4 is required and sufficient for axon pruning/retraction by mediating interactions with Dock180 and PAK to bring about guanine nucleotide exchange and signaling downstream of Rac, respectively. Our results reveal a previously unknown pathway that controls axon pruning and elucidate the biochemical mechanism by which ephrin-B reverse signals regulate actin dynamics to bring about the retraction of growth cones.
有人提出，ephrin-B蛋白在通过排斥作用控制轴突导向中具有类似受体的作用，尽管在很大程度上尚不清楚反向信号如何与下游细胞内分子偶联，以及它们如何在轴突末端诱导细胞骨架重组。我们发现，ephrin-B3（EB3）能够作为一种排斥性导向受体发挥作用，并在出生后发育过程中介导小鼠海马苔藓纤维轴突的定型修剪。靶向细胞内点突变体表明，轴突修剪需要酪氨酸磷酸化依赖性反向信号，并与SH2/SH3衔接蛋白Grb4（也称为Nckbeta/Nck2）偶联。此外，我们发现Grb4的第二个SH3结构域通过介导与Dock180和PAK的相互作用分别实现鸟嘌呤核苷酸交换和Rac下游信号传导，从而对于轴突修剪/回缩是必需且足够的。我们的结果揭示了一条以前未知的控制轴突修剪的途径，并阐明了ephrin-B反向信号调节肌动蛋白动力学以导致生长锥回缩的生化机制。
