Circadian clock regulation by compartmentalized protein translation

Cells exquisitely compartmentalize many biochemical reactions through phase-separated membrane-less organelles. Besides spatial organization, many biological processes are temporally regulated. While both transcription and translation oscillate in a rhythmic manner, the regulatory mechanism of the latter is understudied. Here we report that the translational regulation during circadian cycles is associated with oscillating phase separation controlled by Ataxin-2 and Ataxin-2L. Ataxin-2/2L forms rhythmic condensates through phase separation in the mammalian central clock suprachiasmatic nucleus to selectively concentrate the transcripts of the core clock genes and recruit the mRNA translation machinery. The rhythmic translational activation is abolished in cells and mouse circadian behavior is altered in the absence of Ataxin-2/2L. During aging or neurodegeneration, Ataxin-2/2L form large, irreversible puncta that no longer regulate translation. Overall, our results unveil Ataxin-2/2L’s role as the master regulators of rhythmic translation via a oscillating phase-separation, and explained the reason for their dysfunction in disease states. Ribosome profiling of U2OS WT, Ataxin-2 KO, Ataxin-2L KO and DKO cells at 4 hours and 12 hours after synchronization with DXMS.

细胞通过经相分离（phase separation）形成的无膜细胞器（membrane-less organelles），精巧地分隔了众多生化反应。除空间层面的组织调控外，诸多生命过程还受到时序性调控。尽管转录与翻译均以节律方式振荡，但针对翻译过程的调控机制研究仍较为匮乏。本研究发现，昼夜节律周期中的翻译调控，与共济失调蛋白2（Ataxin-2）和共济失调蛋白2L（Ataxin-2L）介导的节律性相分离密切相关。共济失调蛋白2/2L可通过相分离在哺乳动物中枢生物钟视交叉上核中形成节律性凝聚体，选择性富集核心生物钟基因的转录本，并招募mRNA翻译机器。当缺失共济失调蛋白2/2L时，细胞内的节律性翻译激活会被消除，小鼠的昼夜节律行为也会发生改变。在衰老或神经退行性病变过程中，共济失调蛋白2/2L会形成大型不可逆点状聚集体，不再具备翻译调控功能。综上，本研究结果揭示了共济失调蛋白2/2L通过节律性相分离作为节律性翻译的核心调控因子发挥作用，并阐明了其在疾病状态下功能异常的原因。本研究对经DXMS同步化处理4小时和12小时后的U2OS野生型（WT）、共济失调蛋白2敲除（Ataxin-2 KO）、共济失调蛋白2L敲除（Ataxin-2L KO）及双敲除（DKO）细胞进行了核糖体谱分析（Ribosome profiling）。