Neuronal aging leads to disrupted RNA metabolism

Aging is one of the most prominent risk factors for neurodegeneration, yet the molecular mechanisms underlying the deterioration of old neurons are mostly unknown. To efficiently study neurodegeneration in the context of aging, we transdifferentiated primary human fibroblasts from aged (i.e. >50 year-old) healthy donors directly into neurons, which retained their aging-associated phenotypes including senescence and dysregulation of CpG methylation. Here we show that aged neurons are broadly depleted of RNA-binding proteins, especially 26 spliceosome components. Intriguingly, splicing proteins – like the dementia and ALS-associated protein TDP-43 – mislocalize to the cytoplasm in aged neurons, which leads to widespread alternative splicing. Although spliceosome components can be sequestered within cytoplasmic stress granules, we find that aged neurons suffer from chronic stress that leads to the segregation of stress granule and spliceosome RNA-binding proteins. We link chronic stress to the accumulation of misfolded proteins and to poor HSP90α chaperone activity. Importantly, we also show that aged neurons respond poorly to acute stress treatments, leading to long-term retention of stress granules and failure to initiate transcription of stress-related heat shock proteins. Together our data demonstrates that dysregulation of RNA metabolism is a key driver of poor resiliency in aged neurons.

衰老是神经退行性病变（neurodegeneration）最显著的风险因素之一，但其介导衰老神经元功能衰退的分子机制仍未完全明晰。为高效研究衰老背景下的神经退行性病变，我们将年龄＞50岁的健康供体来源的原代人成纤维细胞（primary human fibroblasts）直接转分化为神经元，该类神经元保留了衰老相关表型，包括细胞衰老与CpG甲基化（CpG methylation）失调。本研究证实，衰老神经元中的RNA结合蛋白（RNA-binding proteins）广泛耗竭，尤其涉及26种剪接体（spliceosome）组分。有趣的是，与痴呆及肌萎缩侧索硬化（ALS）相关的TDP-43等剪接蛋白在衰老神经元中发生错误细胞质定位，进而引发广泛的可变剪接（alternative splicing）异常。尽管剪接体组分可被募集至细胞质应激颗粒（stress granules）中，但我们发现衰老神经元存在慢性应激，导致应激颗粒与剪接体RNA结合蛋白发生分离。我们将慢性应激与错误折叠蛋白（misfolded proteins）积累以及HSP90α伴侣蛋白活性低下建立了关联。尤为重要的是，我们还证实衰老神经元对急性应激处理的响应能力显著降低，导致应激颗粒长期滞留，且无法启动应激相关热休克蛋白（heat shock proteins）的转录。综上，本研究数据表明RNA代谢失调是衰老神经元应激弹性低下的关键驱动因素。