今天繼續(xù)給大家介紹circRNA其他類型的分子機制，希望大家多看多學(xué)。

1.Structure and Degradation of Circular RNAs Regulate PKR Activation in Innate Immunity.

Cell31.3981區(qū). 2019 May 2

分子機制：本文的circRNA研究的是和固有免疫相關(guān)。本文作者測序SLE紅斑狼瘡病人或者是病毒感染的PBMC細胞和正常人的PBMC細胞，發(fā)現(xiàn)兩種狀態(tài)下（病理和正常），細胞內(nèi)的circRNA表達譜變化較大，所有的circRNA都會被RNase L大量降解，而這個RNase L正是在在dsRNA激活的PKR激活時發(fā)生的事件。在正常情況下，circRNA傾向于形成16-26 bp大小的RNA雙聚合體，可發(fā)揮PKR的抑制劑，而PKR正是機體固有免疫激活的效應(yīng)分子。而當(dāng)發(fā)生病毒感染或者自體免疫疾病時，PBMC中的circRNA被降解，而PKR受體則被大量磷酸化激活。功能實驗時發(fā)現(xiàn)，發(fā)現(xiàn)在PBMC以及T-cell中過表達具有dsRNA結(jié)構(gòu)的circRNA時，其可結(jié)合抑制PKR，減弱了PKR激活的級聯(lián)放大效應(yīng)，提供了一種全新的circRNA參與自體免疫疾病以及病毒感染導(dǎo)致機體發(fā)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區(qū). 2019 May 2

分子機制：自然界中存在很多的RNA病毒，機體為有效抵御這些病毒的感染，進化出了復(fù)雜的免疫系統(tǒng)，其中針對RNA分子的識別受體是激活相關(guān)免疫機制的觸發(fā)器。已知RNA感受器包括RIG-I，TLR-3/7/8等分子，它們可以識別不同的RNA或其降解產(chǎn)物。在利用長鏈RNA進行外源基因?qū)牒捅磉_的實驗中，常引入特殊的RNA修飾以降低這些RNA感受器分子介導(dǎo)的免疫效應(yīng)，這些修飾包括假尿苷修飾，N1-甲基假尿，5-甲氧基尿苷（5moU）修飾等等。本文作者報道了未經(jīng)修飾的外源的circRNA可以繞過小鼠機體中含有RIG-I以及TLR成分的細胞，避免激活免疫反應(yīng)，但是線性的RNA可以激活強烈的免疫反應(yīng)。而在體外的功能實驗顯示，同樣的環(huán)狀的circRNA的免疫刺激反應(yīng)更弱。

體外環(huán)化的circRNA可在體內(nèi)翻譯：為分析高純度的體外環(huán)化circRNA能否在體內(nèi)被有效翻譯，作者在小鼠體內(nèi)進行了分析。作者構(gòu)建了環(huán)化的人促紅細胞生成素的circRNA和帶N1-甲基假尿修飾的線性mRNA，體外細胞實驗表明兩者翻譯效率相當(dāng)。注射小鼠后分析血清中游離分子的半衰期，表明circRNA存在的時間更持久。小鼠體內(nèi)能夠檢測到注射體外環(huán)化的circRNA的翻譯產(chǎn)物，表明高純度的體外環(huán)化circRNA可以在體內(nèi)實現(xiàn)有效，穩(wěn)定的蛋白表達。作者最后還嘗試脂質(zhì)體進行體外環(huán)化circRNA的包裹和體內(nèi)給藥，結(jié)果表明體外環(huán)化的高純度circRNA能夠兼容納米脂質(zhì)體載體。

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.

3.Loss of Super-Enhancer-Regulated CircRNA Nfix Induces Cardiac Regeneration After Myocardial Infarction in Adult Mice.

Circulation18.881區(qū). 2019 Apr 5

分子機制：本文作者通過測序發(fā)現(xiàn)一個circRNA，名叫circNfix，可以被超級啟動子（Super-enhancer）調(diào)控，其在人類、大鼠、小鼠的心臟中高表達。而轉(zhuǎn)錄因子Meis1被發(fā)現(xiàn)可結(jié)合該circNfix的SE序列上，增加其表達水平。在體內(nèi)外實驗中發(fā)現(xiàn)，當(dāng)把circNfix敲減后，心肌細胞增殖促進，否則反之。進一步發(fā)現(xiàn)敲減circNfix可促進血管生成，而抑制心臟缺血后的心肌細胞的凋亡，減弱心臟功能失調(diào)，促進了病人的預(yù)后。機制上，circNfix促進了YBX1和NEDD4I（E3泛素連接酶），引起了YBX1的泛素化降解，抑制了cyclin蛋白A2以及B1的表達。另外還有一條分子機制，那就是其可吸附miR-214，從而促進了Gsk3β的表達，進一步抑制了WNT/β-catenin通路，抑制了心肌細胞的增殖，促進其凋亡。

英文摘要：We identified a circRNA, circNfix, that was regulated by a SE and overexpressed in the adult heart in humans, rats and mice. The transcription factor Meis1 bound to the SE at the circNfix locus and increased its expression. In vitro and in vivo, CM proliferation was increased by knockdown of circNfix, while it was inhibited by circNfix overexpression. Moreover, circNfix downregulation promoted CM proliferation and angiogenesis and inhibited CM apoptosis after MI, attenuating cardiac dysfunction and improving the prognosis. Mechanistically, circNfix reinforced the interaction of Ybx1 with Nedd4l, an E3 ubiquitin ligase, and induced Ybx1 degradation through ubiquitination, repressing cyclin A2 and cyclin B1 expression. In addition, circNfix acted as a sponge for miR-214 to promote Gsk3β expression and repress β-catenin activity.