Ptbp2 modulates axon growth in motoneurons through axonal localization and translation of Hnrnpr

The neuronal RNA-binding protein Ptbp2 regulates neuronal differentiation by modulating alternative splicing programs in the nucleus. Such programs contribute to axonogenesis by adjusting the levels of protein isoforms involved in axon growth and branching. While its functions in alternative splicing have been described in detail, cytosolic roles of Ptbp2 for axon growth have remained elusive. Here, we show that Ptbp2 is located in the cytosol and in axons of motoneurons, and that depletion of Ptbp2 affects axon growth. We identified Ptbp2 as a major interactor of the 3' UTR of Hnrnpr mRNA. Axonal localization of Hnrnpr mRNA and local synthesis of hnRNP R protein are strongly reduced when Ptbp2 is depleted, leading to defective axon growth. Ptbp2 regulates hnRNP R translation by mediating the association of Hnrnpr with ribosomes in a manner dependent on the translation factor eIF5A2. Our data thus, suggest a mechanism whereby Ptbp2 modulates axon growth by fine-tuning the mRNA transport and local synthesis of an RNA-binding protein.

神经元RNA结合蛋白（neuronal RNA-binding protein）Ptbp2通过调控细胞核内的可变剪接（alternative splicing）程序调控神经元分化。此类程序通过调整参与轴突生长与分支的蛋白质同工型（protein isoforms）的表达水平，参与轴突发生（axonogenesis）过程。尽管其在可变剪接中的功能已被详细阐释，但Ptbp2在细胞质中调控轴突生长的功能仍不明晰。本研究发现，Ptbp2定位于运动神经元（motoneurons）的细胞质与轴突中，且Ptbp2的耗竭会影响轴突生长。我们鉴定出Ptbp2是Hnrnpr mRNA的3'非翻译区（3' Untranslated Region，3' UTR）的主要互作蛋白。当Ptbp2被耗竭时，Hnrnpr mRNA的轴突定位以及hnRNP R蛋白的局部合成都显著降低，进而导致轴突生长缺陷。Ptbp2通过介导Hnrnpr mRNA与核糖体（ribosome）的结合，依赖翻译因子eIF5A2的方式调控hnRNP R的翻译。综上，我们的数据揭示了一种机制：Ptbp2通过精准调控一种RNA结合蛋白的mRNA运输与局部合成，从而调节轴突生长。