Fragile X mental retardation protein (FMRP) is an RNA-binding protein that regulates local translation in dendrites and spines for synaptic plasticity. In axons, FMRP is implicated in axonal extension and axon guidance. We previously demonstrated the involvement of FMRP in growth cone collapse via a translation-dependent response to Semaphorin-3A (Sema3A), a repulsive axon guidance factor. In the case of attractive axon guidance factors, RNA-binding proteins such as zipcode binding protein 1 (ZBP1) accumulate towards the stimulated side of growth cones for local translation. However, it remains unclear how Sema3A effects FMRP localization in growth cones. Here, we show that levels of FMRP in growth cones of hippocampal neurons decreased after Sema3A stimulation. This decrease in FMRP was suppressed by the ubiquitin-activating enzyme E1 enzyme inhibitor PYR-41 and proteasome inhibitor MG132, suggesting that the ubiquitin-proteasome pathway is involved in Sema3A-induced FMRP degradation in growth cones. Moreover, the E1 enzyme or proteasome inhibitor suppressed Sema3A-induced increases in microtubule-associated protein 1B (MAP1B) in growth cones, suggesting that the ubiquitin-proteasome pathway promotes local translation of MAP1B, whose translation is mediated by FMRP. These inhibitors also blocked the Sema3A-induced growth cone collapse. Collectively, our results suggest that Sema3A promotes degradation of FMRP in growth cones through the ubiquitin-proteasome pathway, leading to growth cone collapse via local translation of MAP1B. These findings reveal a new mechanism of axon guidance regulation: degradation of the translational suppressor FMRP via the ubiquitin-proteasome pathway.

中文翻译：

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Semaphorin-3A通过泛素-蛋白酶体途径促进生长锥中易碎X智力减退蛋白的降解。

脆性X智力低下蛋白（FMRP）是一种RNA结合蛋白，可调节树突和棘突中的局部翻译以实现突触可塑性。在轴突中，FMRP与轴突延伸和轴突引导有关。我们以前通过对Semaphorin-3A（Sema3A）（一种排斥轴突的引导因子）的翻译依赖性应答，证明了FMRP参与了生长锥的塌陷。在有吸引力的轴突引导因子的情况下，RNA结合蛋白（例如邮政编码结合蛋白1（ZBP1））会向生长锥的受激侧积聚，以进行局部翻译。但是，尚不清楚Sema3A如何影响FMRP在生长锥中的定位。在这里，我们显示Sema3A刺激后，海马神经元生长锥中的FMRP水平降低。FMRP的这种降低被泛素激活酶E1酶抑制剂PYR-41和蛋白酶体抑制剂MG132抑制，表明泛素-蛋白酶体途径与Sema3A诱导的生长锥中FMRP降解有关。此外，E1酶或蛋白酶体抑制剂抑制了Sema3A诱导的生长锥中微管相关蛋白1B（MAP1B）的增加，表明泛素-蛋白酶体途径促进了MAP1B的局部翻译，而MAP1B的翻译是由FMRP介导的。这些抑制剂还阻止了Sema3A诱导的生长锥塌陷。总体而言，我们的结果表明Sema3A通过泛素-蛋白酶体途径促进生长锥中FMRP的降解，从而通过MAP1B的局部翻译导致生长锥塌陷。这些发现揭示了轴突引导调节的新机制：