Department of Neuroscience, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States of America.
PLoS Genet. 2018 Apr 4;14(4):e1007314. doi: 10.1371/journal.pgen.1007314. eCollection 2018 Apr.
Commissural axons must cross the midline to establish reciprocal connections between the two sides of the body. This process is highly conserved between invertebrates and vertebrates and depends on guidance cues and their receptors to instruct axon trajectories. The DCC family receptor Frazzled (Fra) signals chemoattraction and promotes midline crossing in response to its ligand Netrin. However, in Netrin or fra mutants, the loss of crossing is incomplete, suggesting the existence of additional pathways. Here, we identify Brain Tumor (Brat), a tripartite motif protein, as a new regulator of midline crossing in the Drosophila CNS. Genetic analysis indicates that Brat acts independently of the Netrin/Fra pathway. In addition, we show that through its B-Box domains, Brat acts cell autonomously to regulate the expression and localization of Adenomatous polyposis coli-2 (Apc2), a key component of the Wnt canonical signaling pathway, to promote axon growth across the midline. Genetic evidence indicates that the role of Brat and Apc2 to promote axon growth across the midline is independent of Wnt and Beta-catenin-mediated transcriptional regulation. Instead, we propose that Brat promotes midline crossing through directing the localization or stability of Apc2 at the plus ends of microtubules in navigating commissural axons. These findings define a new mechanism in the coordination of axon growth and guidance at the midline.
连合轴突必须穿越中线，才能在身体的两侧建立相互连接。这一过程在无脊椎动物和脊椎动物之间高度保守，依赖于导向线索及其受体来指导轴突轨迹。DCC 家族受体 Frazzled（Fra）通过其配体 Netrin 发出趋化吸引信号，促进中线穿越。然而，在 Netrin 或 fra 突变体中，穿越的缺失并不完全，这表明存在额外的途径。在这里，我们确定 Brain Tumor（Brat），一种三联基序蛋白，是果蝇中枢神经系统中线穿越的一个新的调节因子。遗传分析表明 Brat 独立于 Netrin/Fra 途径发挥作用。此外，我们还表明，通过其 B-Box 结构域，Brat 以细胞自主性的方式调节腺瘤性结肠息肉病-2（Apc2）的表达和定位，Apc2 是 Wnt 经典信号通路的关键组成部分，以促进轴突中线穿越。遗传证据表明，Brat 和 Apc2 促进轴突中线穿越的作用独立于 Wnt 和 Beta-catenin 介导的转录调控。相反，我们提出 Brat 通过指导导航连合轴突中微管末端的 Apc2 的定位或稳定性来促进中线穿越。这些发现定义了中线处轴突生长和导向协调的一个新机制。
