Three articles on Immunity Z-RNA editing restrict innate immunity from recognizing endogenous RNA

Written | King Koala's Eucalyptus The human innate immune system monitors the abnormal nucleic acid molecules in the cells to detect the invasion of pathogens
.

The double-stranded nucleic acid molecule in the cell will form an abnormal conformation of Z-DNA and Z-RNA, which in turn activates the innate immune response
.

Unlike the classic B-DNA (right-handed double helix), Z-DNA is a left-handed double helix with a zigzag phosphodiester skeleton structure [1]
.

The biological function of Z nucleic acid, especially Z-RNA, is still unknown
.

A small number of proteins involved in innate immunity contain Z-DNA and Z-RNA binding domains called Zα [2], which specifically bind and stabilize Z-DNA and Z-RNA [3]
.

There are two proteins with Zα domains in mammals: Z-DNA binding protein 1 (ZBP1) and RNA adenosine deaminase 1 (ADAR1)
.

ADAR1 has two splicing subtypes: ADAR1-p110, which is continuously expressed and localized in the nucleus; ADAR1-p150, IFN (interferons) is induced to express and exists in the nucleus and cytoplasm
.

The N-terminal of ADAR1-p150 contains a Zα domain.
In humans, ADAR1 mutations can cause AGS (Aicardi-Goutières syndrome), and the most common ADAR1 mutation (P193A) in AGS patients is located in the Zα domain [4]
.

However, how the different nucleic acid binding domains in ADAR1 select and recruit RNA substrates for subsequent editing and their functions are unclear
.

Therefore, studying the specific contribution of the Zα domain to RNA may provide a wealth of information about the immune function of ADAR1
.

Recently, three studies published in Immunity also reported that mutations in the Zα-RNA binding domain of ADAR1 are sufficient to induce auto-inflammation in mice.
These mutations mimic the human Aicardí-Goutières syndrome and highlight the innate effect of Z-RNA editing.
Important role in immune recognition
.

This article focuses on the research work of Jan Rehwinkel's research group at Oxford University Radcliffe Medical School: Adenosine-to-inosine editing of endogenous Z-form RNA by the deaminase ADAR1 prevents spontaneous MAVS-dependent type I interferon responses.
They constructed Zα- There are two mouse models with missense mutations in the RNA domain.
It was found that these mice spontaneously induce type I IFNs and ISGs in multiple organs and cell types, suggesting that Z-RNA editing is restricting the recognition of endogenous RNA by innate immunity The important role of aspects
.

In order to study the interaction between Z-RNA and the Zα domain of ADAR1-p150 in vivo, the authors constructed a mouse model mZα with two missense mutations in ADAR1: p.
Asn175Ala and p.
Tyr179Ala
.

These residues play an important role in Z-type nucleic acid binding and are homologous to Asn173 and Tyr177 in human ADAR1
.

The authors found that ADAR1-p150 binding to Z-RNA is not necessary for survival at the whole organism level
.

They then tested whether Adar1mZα/mZα mice spontaneously activate type I IFNs
.

By collecting lung, liver, spleen and other tissues, RNA was extracted for qRT-PCR analysis
.

In Adar1mZα/mZα mouse lung RNA samples, the transcription level of Ifnb1 (encoding IFNβ) was significantly increased
.

There are differences in the expression levels of ISG (IFN-stimulated genes) in different types of primary cells, and the expression of ISG is increased in lung fibroblasts with mutations in the Zα domain
.

For embryonic fibroblasts, similar levels of ISG were detected in WT and Adar1mZα/mZα cells, indicating that ISG induction has cell type specificity
.

Among different organs, the lung showed the strongest ISG characteristics, and spontaneous ISG induction was also observed in cultured lung fibroblasts
.

Therefore, the author focused his research on the lungs for follow-up experiments
.

In order to obtain the overall situation of gene expression in Adar1mZα/mZα mice, the authors extracted RNA from the lungs for RNA sequencing analysis
.

Sequencing results showed that the lung tissues of animals with Zα domain mutations showed gene characteristics driven by type I IFNs
.

In order to identify the cell types that show ISG signals in the lungs of Adar1mZα/mZα mice, the authors used MACS to isolate different cells in the lungs
.

Through different types of cells, it was found that in the lungs of Adar1mZα/mZα mice, a variety of cells such as hematopoietic cells and stromal cells would initiate type I IFN responses
.

In order to further analyze the cellular requirements of Adar1mZα/mZα mice for ISG induction, the authors constructed a bone marrow chimeric animal model and found that hematopoietic cells in Adar1mZα/mZα mice were sufficient to induce wild-type mice to produce ISG signals
.

In view of the spontaneous type I IFN response of Adar1mZα/mZα mice, the authors explored whether the Zα domain mutant mice have a protective effect on viral infection
.

The experimental results suggest that Adar1mZα/mZα mice are protected in the early stage of IAV infection
.

Compared with wild-type mice, Adar1mZα/mZα mice have reduced IAV replication and virus-induced inflammation
.

Next, the authors investigated which nucleic acid sensing pathways trigger spontaneous ISG expression in Adar1mZα/mZα mice
.

Because the absence of ADAR1 will cause the activation of the MDA5-MAVS pathway [5], the authors hypothesized that the ISG signal of ADAR1 mZα/mZα mice is driven by MAVS
.

To verify this, they crossed ADAR1 mutant mice with Mavs−/− mice, and observed in Adar1mZα/mZα lungs, liver, and spleen that the absence of MAVS prevented the induction of ISG
.

It shows that the Zα domain of ADAR1-p150 is involved in preventing MAVS-mediated IFN induction
.

Finally, in order to identify natural RNA substrates edited by ADAR1-p150 in a Zα domain-dependent manner, the authors analyzed RNA-seq data from WT and Adar1mZα/mZα lungs
.

In both WT and Adar1mZα/mZα samples, the median editing level of these sites was 10%, indicating that there is no overall defect in RNA editing in Adar1mZα/mZα mice
.

By comparing the editing sites of WT and mutant mice, it is found that about 8% of editing sites require ADAR1-p150 with a complete Zα domain for effective editing
.

In summary, although Z-nucleic acid was discovered 40 years ago, little is known about its biological function
.

In this paper, by introducing mutations into the Zα domain of ADAR1-p150 to prevent it from binding to Z-RNA, it is revealed that type I IFN induction is a biological function of Z-RNA
.

Three studies published in Immunity at the same time revealed the effects of functional mutations in the Zα domain of mouse Adar1 p150 by using three different mutagenesis methods
.

The consistency of these findings highlights the important role of Z-RNA editing in limiting the innate immune recognition of endogenous RNA
.

At the same time, some surprising and different results have led to more new problems
.

Original link: https://doi.
org/10.
1016/j.
immuni.
2021.
08.
011https://doi.
org/10.
1016/j.
immuni.
2021.
08.
022https://doi.
org/10.
1016/j.
immuni.
2021.
07 .
001 Platemaker: Eleven References 1.
Wang AHJ, Quigley GJ, Kolpak FJ, et al.
Molecular structure of a left-handed double helical DNA fragment at atomic resolution[J].
Nature, 1979, 282(5740): 680-686.
2.
Athanasiadis A.
Zalpha-domains: at the intersection between RNA editing and innate immunity[C]//Seminars in cell & developmental biology.
Academic Press, 2012, 23(3): 275-280.
3.
Brown, Bernard A .
, et al.
"The Zα domain of the editing enzyme dsRNA adenosine deaminase binds left-handed Z-RNA as well as Z-DNA.
" Proceedings of the National Academy of Sciences 97.
25 (2000): 13532-13536.
4.
Rice, Gillian I.
, et al.
"Mutations in ADAR1 cause Aicardi-Goutieres syndrome associated with a type I interferon signature.
"Nature genetics 44.
11 (2012): 1243-1248.
5.
Liddicoat, Brian J.
, et al.
"RNA editing by ADAR1 prevents MDA5 sensing of endogenous dsRNA as nonself.
" Science 349.
6252 (2015): 1115-1120.
Reprinting instructions【Original article 】BioArt original articles, personal forwarding and sharing are welcome, reprinting is prohibited without permission, the copyright of all published works is owned by BioArt
.

BioArt reserves all statutory rights and offenders must be investigated
.