Umeda Kentaro, Iwasawa Nariaki, Negishi Manabu, Oinuma Izumi
Laboratory of Molecular Neurobiology, Graduate School of Biostudies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan.
Laboratory of Molecular Neurobiology, Graduate School of Biostudies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
Mol Biol Cell. 2015 May 15;26(10):1957-70. doi: 10.1091/mbc.E15-01-0039. Epub 2015 Mar 25.
Precise wiring patterns of axons are among the remarkable features of neuronal circuit formation, and establishment of the proper neuronal network requires control of outgrowth, branching, and guidance of axons. R-Ras is a Ras-family small GTPase that has essential roles in multiple phases of axonal development. We recently identified afadin, an F-actin-binding protein, as an effector of R-Ras mediating axon branching through F-actin reorganization. Afadin comprises two isoforms--l-afadin, having the F-actin-binding domain, and s-afadin, lacking the F-actin-binding domain. Compared with l-afadin, s-afadin, the short splicing variant of l-afadin, contains RA domains but lacks the F-actin-binding domain. Neurons express both isoforms; however, the function of s-afadin in brain remains unknown. Here we identify s-afadin as an endogenous inhibitor of cortical axon branching. In contrast to the abundant and constant expression of l-afadin throughout neuronal development, the expression of s-afadin is relatively low when cortical axons branch actively. Ectopic expression and knockdown of s-afadin suppress and promote branching, respectively. s-Afadin blocks the R-Ras-mediated membrane translocation of l-afadin and axon branching by inhibiting the binding of l-afadin to R-Ras. Thus s-afadin acts as a dominant-negative isoform in R-Ras-afadin-regulated axon branching.
轴突精确的布线模式是神经元回路形成的显著特征之一，而建立适当的神经元网络需要控制轴突的生长、分支和导向。R-Ras是一种Ras家族的小GTP酶，在轴突发育的多个阶段发挥着重要作用。我们最近鉴定出一种F-肌动蛋白结合蛋白afadin，它是R-Ras的效应器，通过F-肌动蛋白重组介导轴突分支。Afadin包含两种异构体——具有F-肌动蛋白结合结构域的l-afadin和缺乏F-肌动蛋白结合结构域的s-afadin。与l-afadin相比，s-afadin是l-afadin的短剪接变体，包含RA结构域但缺乏F-肌动蛋白结合结构域。神经元表达这两种异构体；然而，s-afadin在大脑中的功能仍然未知。在这里，我们将s-afadin鉴定为皮质轴突分支的内源性抑制剂。与l-afadin在整个神经元发育过程中丰富且持续的表达不同，当皮质轴突活跃分支时，s-afadin的表达相对较低。s-afadin的异位表达和敲低分别抑制和促进分支。s-Afadin通过抑制l-afadin与R-Ras的结合，阻断R-Ras介导的l-afadin膜转位和轴突分支。因此，s-afadin在R-Ras-afadin调节的轴突分支中作为显性负性异构体发挥作用。
