Absence of the Fragile X Mental Retardation Protein results in defects of RNA editing of neuronal mRNAs in mouse

Abstract

The fragile X syndrome (FXS), the most common form of inherited intellectual disability, is due to the absence of FMRP, a protein regulating RNA metabolism. Recently, an unexpected function of FMRP in modulating the activity of Adenosine Deaminase Acting on RNA (ADAR) enzymes has been reported both in Drosophila and Zebrafish. ADARs are RNA-binding proteins that increase transcriptional complexity through a post-transcriptional mechanism called RNA editing.To evaluate the ADAR2-FMRP interaction in mammals we analyzed several RNA editing re-coding sites in the fmr1 knockout (KO) mice. Ex vivo and in vitro analysis revealed that absence of FMRP leads to an increase in the editing levels of brain specific mRNAs, indicating that FMRP might act as an inhibitor of editing activity. Proximity Ligation Assay (PLA) in mouse primary cortical neurons and in non-neuronal cells revealed that ADAR2 and FMRP co-localize in the nucleus. The ADAR2-FMRP co-localization was further observed by double-immunogold Electron Microscopy (EM) in the hippocampus. Moreover, ADAR2-FMRP interaction appeared to be RNA independent.Because changes in the editing pattern are associated with neuropsychiatric and neurodevelopmental disorders, we propose that the increased editing observed in the fmr1-KO mice might contribute to the FXS molecular phenotypes.

Multifarious Functions of the Fragile X Mental Retardation Protein

“…Downstream of DNA interactions, but prior to canonical translational regulation roles, FMRP is proposed to act at multiple RNA life stages: in mRNA editing, pre-mRNA splicing, and in the microRNA pathway (Table 1). Recent work from zebrafish and mouse FXS models shows FMRP alters RNA-editing via interaction with the adenosine deaminase ADAR , supporting earlier Drosophila studies (Table 1) [6,7]. In the mouse FXS model, FMRP also works with RNA-binding protein 14 (RBM14) in pre-mRNA alternative splicing (Table 1) [8].…”

“…While most recent studies continue to identify new FMRP mRNA targets (Table 2), other work reveals new FMRP protein partners in an ever-broadening arena of biology (Table 3). Although some interactions support canonical roles in translation control, others have expanded FMRP biology to a dizzying scale spanning from chromatin-binding to channel-binding (Table 1) [3,4,6,7,8]. Considerable work has shown the importance of channel-binding interactions in FXS, especially for disease state hyperexcitability.…”

Differential Enzymatic Activity of Rat ADAR2 Splicing Variants Is Due to Altered Capability to Interact with RNA in the Deaminase Domain

“…Furthermore, the extent of editing varies within different tissues during human embryogenesis and, again, does not always correlate with ADAR expression [ 44 ]. To address this feature, several studies focused on ADAR post-translational modifications/cofactors interaction [ 5 , 29 , 45 , 46 , 47 , 48 , 49 , 50 ] and on splicing isoforms activity [ 19 , 21 ]. In particular, the activity of ADAR2 has been shown to be regulated by pre-mRNA processing events including self-editing and several alternative splicing events [ 19 , 21 , 22 ].…”

Single Cell Transcriptomics Reveals Dysregulated Cellular and Molecular Networks in a Fragile X Syndrome model

“…More importantly, focusing on FMRP's translational function in dendritic synapses overlooks the fact that the great majority of this protein is located in the cell soma 9 . Indeed, a wide range of research has associated FMRP to multiple steps of the mRNA life cycle, including pre-mRNA splicing 10 , mRNA editing 11,12 , miRNA activity 13,14 , and mRNA stability 15,16 . Additionally, FMRP may function outside the RNA-binding scope, by chromatin binding and regulating genome stability 17,18 , as well as directly binding to and regulating ion channels 19,20 .…”