Search

Variants of the RNA-editing enzyme ADAR1 cause Aicardi-Goutières syndrome (AGS), in which severe inflammation occurs in the brain due to innate immune activation. Here, we analyze the RNA-editing status and innate immune activation in an AGS mouse model that carries the Adar P195A mutation in the N terminus of the ADAR1 p150 isoform, the equivalent of the P193A human Zα variant causal for disease. This mutation alone can cause interferon-stimulated gene (ISG) expression in the brain, especially in the periventricular areas, reflecting the pathologic feature of AGS. However, in these mice, ISG expression does not correlate with an overall decrease in RNA editing. Rather, the enhanced ISG expression in the brain due to the P195A mutant is dose dependent. Our findings indicate that ADAR1 can regulate innate immune responses through Z-RNA binding without changing overall RNA editing., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The RNA-sensing signaling pathway has been well studied as an essential antiviral mechanism of innate immunity. However, its role in non-infected cells is yet to be thoroughly characterized. Here, we demonstrated that the RNA sensing signaling pathway also reacts to the endogenous cellular RNAs in endothelial cells (ECs), and this reaction is regulated by the RNA-editing enzyme ADAR1. Cellular RNA sequencing analysis showed that EC RNAs endure extensive RNA editing, especially in the RNA transcripts of short interspersed nuclear elements. The EC-specific deletion of ADAR1 dramatically reduced the editing level on short interspersed nuclear element RNAs, resulting in newborn death in mice with damage evident in multiple organs. Genome-wide gene expression analysis revealed a prominent innate immune activation with a dramatically elevated expression of interferon-stimulated genes. However, blocking the RNA sensing signaling pathway by deletion of the cellular RNA receptor MDA-5 prevented interferon-stimulated gene expression and rescued the newborn mice from death. This evidence demonstrated that the RNA-editing/RNA-sensing signaling pathway dramatically modulates EC function, representing a novel molecular mechanism for the regulation of EC functions., (© 2021 Guo et al.)

In mammalian cells two active enzymes, ADAR1 and ADAR2, carry out A-to-I RNA editing. These two editases share many common features in their protein structures, catalytic activities, and substrate requirements. However, the phenotypes of the knockout animals are remarkably different, which indicate the distinct functions they play. The most striking effect of ADAR1 knockout is cell death and interruption of embryonic development that are not observed in ADAR2 knockout. Evidences have shown that ADAR1 plays critical roles in the differentiating cells in embryo and adult tissues to support the cell's survival and permit their further differentiation and maturation. However, our knowledge in understanding of the mechanism by which ADAR1 exerts its unique effects is very limited. Many efforts had been made trying to understand why ADAR1 is so important that it is indispensible for animal survival, including studies that identify the RNA editing substrates and studies on non-editing mechanisms. The interest of this review is focused on the question why ADAR1 and not ADAR2 is required for cell survival. Therefore, only the data, published and unpublished, potentially connecting ADAR1 to its cell death effect is selectively cited and discussed here. The features of cell death caused by ADAR1 deletion are summarized. Potential involvement of interferon and protein kinase RNA-activated (PKR) pathways is proposed, but obviously more experimental evaluations are needed.

Adenosine Deaminase Acting on RNA 1 (ADAR1) is an RNA-editing enzyme that converts adenosine to inosine, following RNA transcription. ADAR1's essential role in embryonic development, especially within the hematopoietic lineage, has been demonstrated in knock-out mice. However, a specific role for ADAR1 in adult hematopoietic progenitor cells (HPCs) remains elusive. In this report, we show that ADAR1 is required for survival of differentiating HPCs as opposed to more primitive cells in adult mice by multiple strategies targeting floxed ADAR1 for deletion by Cre recombinase. As a consequence, ADAR1-deficient hematopoietic stem cells (HSCs) were incapable of reconstituting irradiated recipients although being phenotypically present in the recipient bone marrow. While an effect on HSCs cannot be completely ruled out, the preferential effect of ADAR1 absence on HPCs over more primitive hematopoietic cells was consistent with the increased expression of ADAR1 within HPCs, as well as the inability of ADAR1-deficient HPCs to form differentiated colonies and increased apoptotic fraction during ex vivo culture. Moreover, we have obtained direct evidence that ADAR1 functions in HPCs via an RNA-editing dependent mechanism. Therefore, ADAR1 plays an essential role in adult hematopoiesis through its RNA editing activity in HPCs.

Adenosine deaminases acting on RNA (ADARs) are involved in editing of adenosine residues to inosine in double-stranded RNA (dsRNA). Although this editing recodes and alters functions of several mammalian genes, its most common targets are noncoding repeat sequences, indicating the involvement of this editing system in currently unknown functions other than recoding of protein sequences. Here we show that specific adenosine residues of certain microRNA (miRNA) precursors are edited by ADAR1 and ADAR2. Editing of pri-miR-142, the precursor of miRNA-142, expressed in hematopoietic tissues, resulted in suppression of its processing by Drosha. The edited pri-miR-142 was degraded by Tudor-SN, a component of RISC and also a ribonuclease specific to inosine-containing dsRNAs. Consequently, mature miRNA-142 expression levels increased substantially in ADAR1 null or ADAR2 null mice. Our results demonstrate a new function of RNA editing in the control of miRNA biogenesis.

Members of the ADAR (adenosine deaminases acting on RNA) gene family are involved in one type of RNA editing that converts adenosine residues to inosine. The A-to-I editing of serotonin receptor subtype 2C (5-HT(2C)R) mRNA leads to replacement of three amino acid residues located within the intracellular loop II domain, resulting in dramatic alterations in G-protein coupling functions of the receptor. It has been speculated that RNA editing may play a role in several pharmacological and behavioral processes where the serotonergic plasticity is mediated through 5-HT(2C)R. Interferon-alpha (IFN-alpha) often causes severe depression in patients treated for chronic viral hepatitis and certain malignancies. In this study, we examined the effects of IFN-alpha on RNA editing in human glioblastoma cell lines, which express 5-HT(2C)R mRNAs. ADAR1 expression and the pattern of the 5-HT(2C)R mRNA editing rapidly changed in response to IFN-alpha, leading to the dominant expression of the 5-HT(2C)R-VSI isoform predicted to have reduced G-protein coupling functions. Our results support the hypothesis that 5-HT(2C)R mRNA editing has causative relevance in the pathophysiology of depression associated with cytokine therapy.

One type of RNA editing involves the conversion of adenosine residues into inosine in double-stranded RNA through the action of adenosine deaminases acting on RNA (ADAR). A-to-I RNA editing of the coding sequence could result in synthesis of proteins not directly encoded in the genome. ADAR edits also non-coding sequences of target RNAs, such as introns and 3'-untranslated regions, which may affect splicing, translation, and mRNA stability. Three mammalian ADAR gene family members (ADAR1-3) have been identified. Here we investigated phenotypes of mice homozygous for ADAR1 null mutation. Although live ADAR1-/- embryos with normal gross appearance could be recovered up to E11.5, widespread apoptosis was detected in many tissues. Fibroblasts derived from ADAR1-/- embryos were also prone to apoptosis induced by serum deprivation. Our results demonstrate an essential requirement for ADAR1 in embryogenesis and suggest that it functions to promote survival of numerous tissues by editing one or more double-stranded RNAs required for protection against stress-induced apoptosis.

Patients with chronic myelogenous leukemia (CML) respond well to tyrosine kinase inhibitors (TKIs) of the Bcr-Abl oncoprotein. However, intolerance and resistance to these agents remains a challenge, and TKIs are unable to eradicate rare leukemia-initiating cells. Leukemia treatment would benefit from a better understanding of molecular signals that are necessary for the survival of leukemia-initiating cells but dispensable for normal hematopoietic stem cells. Leukemia-initiating cells in CML can arise from myeloid progenitor cells, a population that we have reported in normal hematopoiesis to depend on the RNA-editing enzyme adenosine deaminase acting on RNA-1 (ADAR1). We now report that Bcr-Abl transformed leukemic cells were ADAR1-dependent in a conditional ADAR1 knockout mouse model. ADAR1 deletion reversed leukocytosis and splenomegaly, and preferentially depleted primitive Lin-Sca+Kit+ (LSK) leukemic cells but not LSK cells lacking the leukemic oncoprotein. ADAR1 deletion ultimately normalized the peripheral white blood count, eliminating leukemic cells as assessed by PCR. These results uncover a novel requirement for ADAR1 in myeloid leukemic cells and indicate that ADAR1 may comprise a new molecular target for CML-directed therapeutics. [ABSTRACT FROM AUTHOR]

In mammalian cells two active enzymes, ADAR1 and ADAR2, carry out A-to-I RNA editing. These two editases share many common features in their protein structures, catalytic activities, and substrate requirements. However, the phenotypes of the knockout animals are remarkably different, which indicate the distinct functions they play. The most striking effect of ADAR1 knockout is cell death and interruption of embryonic development that are not observed in ADAR2 knockout. Evidences have shown that ADAR1 plays critical roles in the differentiating cells in embryo and adult tissues to support the cell's survival and permit their further differentiation and maturation. However, our knowledge in understanding of the mechanism by which ADAR1 exerts its unique effects is very limited. Many efforts had been made trying to understand why ADAR1 is so important that it is indispensible for animal survival, including studies that identify the RNA editing substrates and studies on non-editing mechanisms. The interest of this review is focused on the question why ADAR1 and not ADAR2 is required for cell survival. Therefore, only the data, published and unpublished, potentially connecting ADAR1 to its cell death effect is selectively cited and discussed here. The features of cell death caused by ADAR1 deletion are summarized. Potential involvement of interferon and protein kinase RNA-activated (PKR) pathways is proposed, but obviously more experimental evaluations are needed. [ABSTRACT FROM AUTHOR]

Members of the ADAR (adenosine deaminases acting on RNA) gene family are involved in one type of RNA editing that converts adenosine residues to inosine. The A-to-I editing of serotonin receptor subtype 2C (5-HT2CR) mRNA leads to replacement of three amino acid residues located within the intracellular loop II domain, resulting in dramatic alterations in G-protein coupling functions of the receptor. It has been speculated that RNA editing may play a role in several pharmacological and behavioral processes where the serotonergic plasticity is mediated through 5-HT2CR. Interferon-α (IFN-α) often causes severe depression in patients treated for chronic viral hepatitis and certain malignancies. In this study, we examined the effects of IFN-α on RNA editing in human glioblastoma cell lines, which express 5-HT2CR mRNAs. ADAR1 expression and the pattern of the 5-HT2CR mRNA editing rapidly changed in response to IFN-α, leading to the dominant expression of the 5-HT2CR-VSI isoform predicted to have reduced G-protein coupling functions. Our results support the hypothesis that 5-HT2CR mRNA editing has causative relevance in the pathophysiology of depression associated with cytokine therapy. [Copyright &y& Elsevier]

Abstract: Bone mass is controlled through a delicate balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. We show here that RNA editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) is critical for proper control of bone mass. Postnatal conditional knockout of Adar1 (the gene encoding ADAR1) resulted in a severe osteopenic phenotype. Ablation of the Adar1 gene significantly suppressed osteoblast differentiation without affecting osteoclast differentiation in bone. In vitro deletion of the Adar1 gene decreased expression of osteoblast-specific osteocalcin and bone sialoprotein genes, alkaline phosphatase activity, and mineralization, suggesting a direct intrinsic role of ADAR1 in osteoblasts. ADAR1 regulates osteoblast differentiation by, at least in part, modulation of osterix expression, which is essential for bone formation. Further, ablation of the Adar1 gene decreased the proliferation and survival of bone marrow stromal cells and inhibited the differentiation of mesenchymal stem cells towards osteoblast lineage. Finally, shRNA knockdown of the Adar1 gene in MC-4 pre-osteoblasts reduced cyclin D1 and cyclin A1 expression and cell growth. Our results identify ADAR1 as a new key regulator of bone mass and suggest that ADAR1 functions in this process mainly through modulation of the intrinsic properties of osteoblasts (i.e., proliferation, survival and differentiation). [Copyright &y& Elsevier]