干货：circRNA高分文章分子机制第二弹，开拓眼界！
今天继续给大家介绍circRNA其他类型的分子机制，希望大家多看多学。
1.Structure and Degradation of Circular RNAs Regulate PKR Activation in Innate Immunity.
Cell31.3981区. 2019 May 2
分子机制：本文的circRNA研究的是和固有免疫相关。本文作者测序SLE红斑狼疮病人或者是病毒感染的PBMC细胞和正常人的PBMC细胞，发现两种状态下（病理和正常），细胞内的circRNA表达谱变化较大，所有的circRNA都会被RNase L大量降解，而这个RNase L正是在在dsRNA激活的PKR激活时发生的事件。在正常情况下，circRNA倾向于形成16-26 bp大小的RNA双聚合体，可发挥PKR的抑制剂，而PKR正是机体固有免疫激活的效应分子。而当发生病毒感染或者自体免疫疾病时，PBMC中的circRNA被降解，而PKR受体则被大量磷酸化激活。功能实验时发现，发现在PBMC以及T-cell中过表达具有dsRNA结构的circRNA时，其可结合抑制PKR，减弱了PKR激活的级联放大效应，提供了一种全新的circRNA参与自体免疫疾病以及病毒感染导致机体发病的研究。
英文摘要：Circular RNAs (circRNAs) produced from back-splicing of exons of pre-mRNAs are widely expressed, but current understanding of their functions is limited. Here, we show that endogenous circRNAs tend to form 16-26 bp imperfect RNA duplexes and act as inhibitors of double-stranded RNA (dsRNA)-activated protein kinase (PKR) related to innate immunity. Upon poly(I:C) stimulation or viral infection, circRNAs are globally degraded by RNase L, a process required for PKR activation in early cellular innate immune responses. Augmented PKR phosphorylation and circRNA reduction are found in peripheral blood mononuclear cells (PBMCs) derived from patients with autoimmune disease systemic lupus erythematosus (SLE). Importantly, overexpression of the dsRNA-containing circRNA in PBMCs or T cells derived from SLE can alleviate the aberrant PKR activation cascade, thus providing a connection between circRNAs and SLE.
2.RNA Circularization Diminishes Immunogenicity and Can Extend Translation Duration In Vivo.
Mol Cell14.2481区. 2019 May 2
分子机制：自然界中存在很多的RNA病毒，机体为有效抵御这些病毒的感染，进化出了复杂的免疫系统，其中针对RNA分子的识别受体是激活相关免疫机制的触发器。已知RNA感受器包括RIG-I，TLR-3/7/8等分子，它们可以识别不同的RNA或其降解产物。在利用长链RNA进行外源基因导入和表达的实验中，常引入特殊的RNA修饰以降低这些RNA感受器分子介导的免疫效应，这些修饰包括假尿苷修饰，N1-甲基假尿，5-甲氧基尿苷（5moU）修饰等等。本文作者报道了未经修饰的外源的circRNA可以绕过小鼠机体中含有RIG-I以及TLR成分的细胞，避免激活免疫反应，但是线性的RNA可以激活强烈的免疫反应。而在体外的功能实验显示，同样的环状的circRNA的免疫刺激反应更弱。
proliferation
体外环化的circRNA可在体内翻译：为分析高纯度的体外环化circRNA能否在体内被有效翻译，作者在小鼠体内进行了分析。作者构建了环化的人促红细胞生成素的circRNA和带N1-甲基假尿修饰的线性mRNA，体外细胞实验表明两者翻译效率相当。注射小鼠后分析血清中游离分子的半衰期，表明circRNA存在的时间更持久。小鼠体内能够检测到注射体外环化的circRNA的翻译产物，表明高纯度的体外环化circRNA可以在体内实现有效，稳定的蛋白表达。作者最后还尝试脂质体进行体外环化circRNA的包裹和体内给药，结果表明体外环化的高纯度circRNA能够兼容纳米脂质体载体。
英文摘要：Here, we show that unmodified exogenous circRNA is able to bypass cellular RNA sensors and thereby avoid provoking an immune response in RIG-I and Toll-like receptor (TLR) competent cells and in mice. The immunogenicity and protein expression stability of circRNA preparations are found to be dependent on purity, with small amounts of contaminating linear RNA leading to robust cellular immune responses. Unmodified circRNA is less immunogenic than unmodified linear mRNA in vitro, in part due to the evasion of TLR sensing. Finally, we provide the first demonstration to our knowledge of exogenous circRNA delivery and translation in vivo, and we show that circRNA translation
is extended in adipose tissue in comparison to unmodified and uridine-modified linear mRNAs.

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