Accumulation of Nuclear ADAR2 Regulates Adenosine-To-Inosine

RESEARCH ARTICLE Accumulation of nuclear ADAR2 regulates adenosine-to-inosine RNA editing during neuronal development Mikaela Behm, Helene Wahlstedt*, Albin Widmark, Maria Eriksson‡ and Marie Öhman§

ABSTRACT Owing to structural similarities, inosine is recognized as Adenosine to inosine (A-to-I) RNA editing is important for a functional guanosine (G) by the cellular machineries. Hence, A-to-I editing brain, and most known sites that are subject to selective RNA editing has the capacity to diversify neuronal gene expression through have been found to result in diversified protein isoforms that are alteration of protein sequences, splicing patterns and base-pairing involved in neurotransmission. In the absence of the active editing properties. Editing within coding sequences mostly occurs in genes enzymes ADAR1 or ADAR2 (also known as ADAR and ADARB1, involved in neurotransmission, such as in the transcript coding for respectively), mice fail to survive until adulthood. Nuclear A-to-I the GluA2 subunit of the AMPA glutamate receptor, where editing editing of neuronal transcripts is regulated during brain development, is essential for survival (Brusa et al., 1995; Higuchi et al., 2000). with low levels of editing in the embryo and a dramatic increase after Another example of a recoding editing event occurs in the Gabra-3 α birth. Yet, little is known about the mechanisms that regulate editing transcript, encoding the 3 subunit of the inhibitory GABA during development. Here, we demonstrate lower levels of ADAR2 in receptor, where editing leads to an isoleucine to methionine (I/M) the nucleus of immature neurons than in mature neurons. We show that change in the translated protein (Ohlson et al., 2007). Here, the importin-α4 (encoded by Kpna3), which increases during neuronal edited isoform is necessary for proper trafficking and recycling α maturation, interacts with ADAR2 and contributes to the editing properties of the 3 subunit during brain development (Daniel et al., efficiency by bringing it into the nucleus. Moreover, we detect an 2011). Most sites edited within coding sequences are inefficiently increased number of interactions between ADAR2 and the nuclear edited in the embryonic brain, while they are edited to much greater isomerase Pin1 as neurons mature, which contribute to ADAR2 protein extents in the adult brain (Dillman et al., 2013; Hwang et al., 2016; stability. Together, these findings explain how the nuclear editing of Lomeli et al., 1994; Wahlstedt et al., 2009). substrates that are important for neuronal function can increase as the Just like edited sites within coding sequences, editing within brain develops. repetitive elements in introns and untranslated regions (UTRs) is lower in embryonic tissue than in adult human tissue, including the KEY WORDS: A-to-I RNA editing, ADAR2, Importin-α4, Pin1 brain (Shtrichman et al., 2012). Although most A-to-I editing sites within these repetitive elements have not been assigned specific INTRODUCTION functions, there are examples where editing leads to alternative Diversification of the neuronal transcriptome through co- and splicing and exonization of introns (Lev-Maor et al., 2007). The post-transcriptional events is crucial for proper function of the non-coding microRNAs (miRNAs) are also edited, particularly in mammalian brain. One of the most common modifications of the targeting seed sequence (Alon et al., 2012; Ekdahl et al., 2012; neuronal transcripts is deamination of adenosine to inosine (A-to-I) in Kawahara et al., 2007; Vesely et al., 2014). The ADAR enzymes double-stranded RNA (dsRNA), which is catalyzed by one of two target the nuclear miRNA precursors and can affect their maturation active adenosine deaminases that act on RNA (ADARs), ADAR1 or mRNA-targeting capacity. The frequency of miRNA editing is and ADAR2 (also known as ADAR and ADARB1, respectively) also a regulated event during brain development (Ekdahl et al., (Bass, 2002; Bass et al., 1997; Nishikura, 2010; Paul and Bass, 2012). 1998). Most ADAR substrates depend on intronic sequences to To date, mechanisms for how A-to-I editing is regulated have been form the dsRNA structure required for editing (reviewed in sparse. We have previously seen that the protein levels of ADAR1 and Wahlstedt and Öhman, 2011). Thus, editing of many substrates has ADAR2 are constant during mouse brain development, not following to occur in the nucleus before splicing. Both ADAR1 and ADAR2 thegradual increase ofA-to-I editing (Wahlstedt et al.,2009). A number have been shown to accumulate in the nucleolus, where the enzymes ofediting enhancerandrepressorcandidates have been identified by the bind but do not edit ribosomal RNA (Desterro et al., 2003; Sansam Jantsch laboratory (Garncarz et al., 2013; Tariq et al., 2013), and work et al., 2003). However, the nucleolar association is highly dynamic from the O’Connell laboratory has revealed that ADAR2 is post- and the ADAR enzymes shuttle between the nucleolus and translationally regulated by the phosphorylation-dependent peptidyl- nucleoplasm. prolyl isomerase Pin1 (Marcucci et al., 2011). Pin1 recognizes a conserved phosphorylated Ser/Thr-Pro motif in the N-terminal region

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm of ADAR2, and this interaction is required for nuclear localization of University, Svante Arrheniusväg 20C, Stockholm, 106 91, Sweden. ADAR2. Nevertheless, there are no indications that these factors *Present address: Department of Oncology-Pathology, Cancer centrum Karolinska, regulate editing specifically during brain development. Karolinska Institutet, Stockholm 171 76, Sweden. ‡Present address: BioArctic AB Warfvinges väg 35, Stockholm 112 51, Sweden. Here, we show that developmentally regulated RNA editing in neurons can be explained by an increasing presence of the ADAR2 §Author for correspondence ([email protected]) enzyme in the nucleus. We show that ADAR2 is imported into the α M.O., 0000-0002-6636-5841 nucleus by importin- 4 and is stabilized by a nuclear interaction with Pin1. These factors increase the accessibility of ADAR2 to the

Received 23 November 2016; Accepted 30 December 2016 editing substrates during neuronal maturation. Journal of Cell Science

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RESULTS DIV, the amounts of transcripts edited resembled those detected in the RNA editing increases in cultured primary cortical neurons adult mouse brain. Gabra-3 and pri-miR-376b are substrates for both as they mature ADAR1 and ADAR2 (Fig. S1B; Ohlson et al., 2007). Editing at the We have previously shown that A-to-I RNA editing increases in both +4 site in the ADAR1-specific non-coding substrate pri-miR-381 coding and non-coding substrates in the developing mouse brain also increased as the neurons matured (Fig. 1B; Fig. S1B). (Ekdahl et al., 2012; Wahlstedt et al., 2009). To study the mechanisms Interestingly, editing at the lysine-to-glutamate (K/E) site in the that regulate A-to-I editing during neuronal maturation, we isolated cytoplasmic FMR1 interacting protein 2 (Cyfip2), appeared later in cortical neural progenitor cells from mouse cortices at embryonic day the developing neurons than editing at the other sites – only after 6 16 and cultured them for up to 14 days in vitro (DIV). During this DIV a clear G peak was visible in the chromatogram (Fig. 1B). At 12 timeframe, neural progenitors matured and formed an extensive DIV, 50% of the Cyfip transcripts were edited at the K/E site, which neurite network (Fig. 1A; Fig. S1A). To determine if editing has been shown to be exclusively edited by ADAR2 (Nishimoto increased in a similar way in developing cultured neurons as in the et al., 2008; Rueter et al., 1999). To determine if this is a specific brain, coding as well as non-coding RNA substrates with both pattern for ADAR2-mediated editing, we analyzed auto-editing at the specific and overlapping specificity for ADAR1 and ADAR2 editing −1 site in the ADAR2 pre-mRNA. Editing at the −1sitegaveriseto were selected for analyses after 1–12 days in culture (Ekdahl et al., alternative splicing and a subsequent inclusion of 47 nucleotides in 2012; Nishimoto et al., 2008; Ohlson et al., 2007). Similar to that in exon 5 (Fig. 1C) (Rueter et al., 1999). Editing-induced alternative the embryonic brain, low levels of editing were observed at the I/M splicing was measured by performing semi-quantitative reverse- site in the coding region of the Gabra-3 transcript and at the +6 site in transcriptase (RT)-PCR, using exonic primers that amplify both the miRNA precursor transcript pri-miR-376b after 1 DIV (Fig. 1B). normal and alternatively spliced ADAR2 transcripts. Using this As the neurons matured, editing levels gradually increased, and at 12 approach, we could confirm that editing mediated by ADAR2

Fig. 1. Editing levels increase in developing cortical neurons. (A) Neurites were visualized by βIII-tubulin immunoreactivity (green) in primary mouse cortical neurons cultured for up to 14 days in vitro (DIV). DAPI staining (blue) was used to visualize the nuclei. Scale bar: 50 μm. (B) Sanger sequencing chromatograms showing editing levels as a dual A and G peak after RT-PCR analysis of the I/M site in the Gabra-3 transcript, the +6 site in pri-miR-376b, the +4 site in pri-miR-381 and the lysine-to-glutamate (K/E) site in Cyfip2 in neurons cultured for 1, 3, 6 and 12 DIV. Arrows indicate the edited sites. (C) Schematic illustration of alternative ADAR2 pre-mRNA processing of exon 4 and 5 due to editing at the −1 site in intron 4. Editing at the −1 site creates an alternative 3′ splice site, which results in the inclusion of 47 nucleotides. The illustration is not drawn to scale. (D) Semi- quantitative RT-PCR of alternative ADAR2 pre- mRNA processing of exon 4 and 5 using exonic primers in neurons cultured for 1, 3, 6 and 12 DIV. The upper band corresponds to the alternatively spliced product as a result of editing, while the lower band represents canonical exon 4–5 splicing. Primers against 18S RNA were used as loading control. Journal of Cell Science

746 RESEARCH ARTICLE Journal of Cell Science (2017) 130, 745-753 doi:10.1242/jcs.200055 gradually increases during neuronal maturation (Fig. 1D). Similar 12 DIV compared to that at 2 DIV (Fig. 2B,C). Moreover, the results have been shown previously in primary rat cortical neurons ADAR2 total protein expression level increases after 3 DIV and during maturation (Hang et al., 2008). Taken together, these findings remains high until 12 DIV (Fig. S2). Taken together, these results show that the editing of several ADAR targets increases during indicate a dual effect of both increased nuclear localization of maturation of cultured neurons in a similar manner to that which we ADAR2, as well as an increased expression as neurons mature. have previously shown in the developing mouse brain. ADAR2 harbors two nuclear localization signals in its ADAR2 accumulates in the nucleus during neuronal N-terminal region development Selective import of proteins into the nucleus is mediated by specific Nuclear RNA editing substrates are edited co-transcriptionally, before amino acid sequences known as nuclear localization signals (NLSs). splicing, since intronic sequences often are needed to form the A long stretch of N-terminally positioned residues (residues 4–72) in double-stranded structure required for editing. The low efficiency of ADAR2 has been shown, by using deletion mutants, to be crucial for editing during early brain development may therefore be explained by nuclear localization (Desterro et al., 2003; Wong et al., 2003). To the ADAR enzymes not being located in the same subcellular refine the sequence that is required for import of ADAR2, we used the compartment as the substrate – i.e. not in the nucleus – and/or low first 72 amino acid residues of rat ADAR2 as the input into several expression of the ADAR enzymes in embryos. The ADAR2 NLS in silico prediction programs (Fig. S4A,B) (Kosugi et al., 2009; subcellular localization has been described as being exclusively Nakai and Horton, 1999; Nguyen Ba et al., 2009). The region nuclear, with an accumulation in the nucleolus, in adult rat neurons spanning amino acid residues 49–72 contains two basic clusters and cell lines, such as HeLa and Cos-7 (Desterro et al., 2003; Sansam spaced by 14 residues, which resemble the motif for either two et al., 2003). Co-immunostaining of immature cortical neurons at 1 monopartite or one bipartite NLS (Dingwall and Laskey, 1991; DIV showed that ADAR2 localized to the soma and nucleus, with no Kalderon et al., 1984; Robbins et al., 1991). Moreover, multiple apparent overlap with the nucleolar fibrillarin protein (Fig. 2A). After sequence alignment analysis of the region containing the predicted 6 DIV, ADAR2 localized predominantly to the nucleus, and NLSs showed that the first cluster (52–53) and the basic amino acids pronounced colocalization between fibrillarin and ADAR2 in of the second cluster (68–72) are conserved between 11 selected nucleoli could be detected. After 12 DIV, ADAR2 was almost mammalian species (Fig. S4C). We tested the function of the exclusively localized to the nucleus with an accumulation in the predicted NLS clusters by deleting sequences encoding amino acids nucleoli in most cells (Fig. 2A). The subcellular localization of 48–72 and 4–47 in the context of N-terminally FLAG-tagged ADAR1 was less clear because the two isoforms of the enzyme, p110 ADAR2 (Fig. 3A,B). Subcellular localization was determined and p150, are both cytoplasmic and nuclear in immature as well as in by performing immunofluorescent microscopy after transient mature neurons. Nevertheless, we observed a clear increase in nuclear expression of NLS mutant constructs in human embryonic kidney localization of ADAR1 in neurons at 12 DIV compared to that in 293 (HEK293) cells. Expression of full-length fusion proteins was immature neurons at 1 DIV, whereas ADAR1 expression levels confirmed by western blotting (data not shown). Nuclear localization remained constant (Figs S2 and S3). To examine the possibility that depended on residues 48–72 but not on residues 4–47 (Fig. 3B). nuclear levels of ADAR enzymes underlie the regulation of editing Fusion of rat ADAR2 residues 48–72 to EGFP demonstrated that this during neuronal development, we focused our studies on ADAR2. sequence was sufficient to target the protein to the nucleus (Fig. 3C). Western blot analysis of nuclear and cytoplasmic extracts prepared Separately introducing alanine mutations in the first cluster, 52KR53 from neurons that had been cultured for 2 and 12 DIV independently (ΔNLS1), or in the second cluster, 68KKRRK72 (ΔNLS2), had only confirmed a 2.5-fold increase of ADAR2 protein in the nucleus at small effects on preventing nuclear localization, whereas combining

Fig. 2. Nuclear ADAR2 increases during neuronal maturation. (A) Immunostaining of ADAR2 (red) and fibrillarin (green) in cortical neurons cultured for 1, 6 and 12 DIV. DAPI staining was used to visualize the nucleus (blue). Scale bar: 10 µm. (B) Western blot analysis of ADAR2 expression in nuclear and cytoplasmic fractions from cortical neurons cultured for 2 or 12 DIV. Enrichment of expression of a nuclear protein, U1-70K, and of a cytoplasmic protein, βIII-tubulin, was used as a fractionation control. (C) Quantitative densitometry of ADAR2 protein expression in nuclear fractions from 2 and 12 DIV. Results are presented as the fold change (mean±s.e.m.) of three independent experiments. *P=0.0136 (Student’s t-test). Journal of Cell Science

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Fig. 3. ADAR2 nuclear localization is dependent on two N-terminal nuclear localization signals. (A) Schematic diagram of the N-terminal FLAG-tagged ADAR2 NLS deletion and alanine substitution mutants, as well as of the protein domains – the dsRNA-binding domains (dsRBD) and deaminase domain. The illustration is not drawn to scale. (B) Subcellular localization of ADAR2 NLS mutants that were transiently expressed in HEK293 cells using anti-FLAG (red) immunostaining and fluorescence microscopy. DAPI staining was used to visualize the nucleus (blue). (C) Subcellular localization of EGFP–ADAR2 fusion proteins transiently expressed in HEK293 cells. Scale bars: 10 µm.

the mutations (ΔNLS1+2) fully prevented nuclear localization Using antibodies against ADAR2 and importin-α4, specific (Fig. 3C). Similarly, the ΔNLS1+2 mutations prevented nuclear interactions could be visualized as dots by immunofluorescence accumulation of FLAG-tagged ADAR2 protein (Fig. 3B). We (Fig. 4D). At 2 DIV, only a few interactions were detected, mostly in conclude that ADAR2 uses the two highly conserved basic regions in the cytoplasm, whereas after 6–9 DIV, an increasing number of its N-terminal region for its nuclear targeting. interactions were detected in the nucleus. In summary, this result shows that importin-α4 interacts more frequently with ADAR2 as ADAR2 nuclear localization is mediated by an importin-α4- the neuron matures, plausibly bringing it into the nucleus. dependent sequence Import of proteins into the nucleus depends on interactions between Importin-α4 positively regulates ADAR2-specific RNA editing NLSs and importin-α adaptor proteins. To evaluate which importin- To determine if the expression level of importin-α4 influences the α isoform interacts with ADAR2 in neurons, we focused on two level of RNA editing, we transiently co-expressed an editing importin-α isoforms that have been previously shown to have reporter and an ADAR2 expression vector with and without an affinity towards human ADAR2 in a yeast two-hybrid assay (Maas importin-α4 expression vector in SH-SY5Y neuroblastoma cells. and Gommans, 2009). By performing co-immunoprecipitation (co- The editing reporter was a rat Adar2 minigene expressing exon 4, IP) experiments using transiently expressed FLAG-tagged ADAR2 intron 4 and exon 5 (A2-Ex4-5), in which editing leads to alternative in HEK293 cells, we found that ADAR2 specifically interacts with splicing as described above (Rueter et al., 1999). Editing was importin-α4 but not with importin-α5 (Fig. 4A). Importin-α1was detected by performing RT-PCR using primers amplifying both the used as a negative control as it has previously been shown to lack normal (unedited) and the alternatively spliced (edited) variants binding affinity to ADAR2 (Maas and Gommans, 2009). We further (Fig. 5A). Overexpression of importin-α4 resulted in a 2.2-fold show that the ADAR2–importin-α4 interaction was lost in the increase in editing of the reporter, indicating that the expression nuclear-import-incompetent mutant, Δ48–72, while there was a level of importin-α4 affects nuclear editing (Fig. 5B,C). Protein stronger interaction of importin-α4 with the nuclear-import- expression of transiently expressed ADAR2 and importin-α4was competent Δ4–47 mutant (Fig. 4B), possibly due to an altered assayed using western blot (Fig. 5B). Subcellular localization of the nuclear import–export rate, increasing the accessibility of the NLS transiently expressed ADAR2 was as expected – localized to the in the cytoplasm. A previous study has demonstrated that importin- nucleus and the nucleolus (data not shown). Thus, an elevated level α4 mRNA is developmentally upregulated in mouse brain of importin-α4 resulted in increased ADAR2-specific editing in (Hosokawa et al., 2008). To determine if importin-α4 protein neuronal cells, indicating that the level of importin-α4 regulates expression is regulated during neuronal maturation as described for editing efficiency during neuronal maturation. other importin-α isoforms (α1, α3 and α5) (Yasuhara et al., 2007), we performed western blot analysis on total protein extracts during Pin1–ADAR2 nuclear interactions at later stages of neuronal neuronal maturation. We observed a 60% increase in importin-α4 maturation protein expression over 12 days of neuronal maturation in vitro Interaction between the proline isomerase Pin1 and phosphorylated (Fig. 4C). To confirm an interaction between ADAR2 and importin- ADAR2 stabilizes ADAR2 in the nucleus and has been shown to be α4 in the developing neurons, we used an in situ proximity ligation required for efficient editing of endogenously and transiently assay (PLA). In this assay, only proteins within close proximity are expressed GluA2 transcripts in cell lines (Marcucci et al., 2011). detected using rolling circle amplification (Söderberg et al., 2006). We wanted to investigate the role of Pin1 for the function of ADAR2 Journal of Cell Science

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Fig. 4. ADAR2 interacts with importin-α4 during neuronal maturation. (A) Wild-type ADAR2 was transiently expressed in HEK293 cells and co- immunoprecipitated using an antibody against FLAG. Western blots (WB) were probed for importin-α4, importin-α5 and importin-α1 or FLAG. Rabbit IgG antibodies were used as background control in the immunoprecipitation (IP). (B) Co- immunoprecipitation of ADAR2 wild type (WT) and the deletion mutants Δ48–72 and Δ4–47 with importin-α4. (C) Western blot analysis of importin- α4 expression in primary mouse cortical neurons cultured for 1–12 DIV. The graph shows the intensities of importin-α4 bands normalized to the total amount of protein loaded using Coomassie staining. The data are presented as the fold-change (mean±s.e.m.) relative to expression at 1 DIV. Statistical analysis was performed on paired raw data using one-way ANOVA (n=3), P=0.0339, and Dunnett’s multiple comparison test, *P=0.0227 (1 DIV vs 12 DIV). (D) In situ immunofluorescence detection of endogenous ADAR2–importin-α4 protein complexes (red) using PLA in neurons at 2, 6 and 9 DIV. As negative controls for the PLA, an antibody against only ADAR2 or against only importin-α4 antibody was used, or both were excluded (no primary antibodies). DAPI staining was used to visualize the nucleus (blue). Scale bars: 10 µm.

in developing neurons and therefore asked if Pin1 expression is altered and ADAR2 are constant during mouse brain development, not during maturation. Indeed, western blot analysis showed that Pin1 was following the gradual increase of A-to-I editing (Wahlstedt et al., expressed at low levels at 1–3 DIV and that the expression increased 2009). In this study, we have addressed the mechanism behind during maturation, up to 13 DIV (Fig. 6A). Importantly, this was developmentally regulated A-to-I RNA editing. We found low also observed during mouse brain development (Fig. 6B). levels of total and nuclear ADAR2 in immature cortical neurons that Furthermore, we determined the subcellular localization of Pin1 increase during maturation in accordance with increased nuclear in relation to that of ADAR2 during maturation of neurons by editing. To date, regulation of the ADAR proteins has mainly been immunocytological staining. At 1 DIV, Pin1 was mostly found to studied in non-neuronal cell lines. Although a substantial amount of localize to the cytoplasm with only a diffuse staining pattern in the information can be obtained from analysis in cell lines, it may not nucleus (Fig. 6C). At this stage, ADAR2 co-localized with Pin1 at reflect dynamic processes within a developing neuron. We therefore defined spots in the soma. As the neurons matured, Pin1 intensely analyzed editing and regulation of the ADAR2 protein during stained the cell nucleus but was also present in the cytoplasm. In maturation of primary neurons and showed that editing of specific the mature neurons, Pin1 and ADAR2 colocalization was targets increases in vitro in a similar way to that in the developing exclusively seen in the nucleus. Specific interactions between brain. Many ADAR substrates must be edited in their premature Pin1 and ADAR2 in the developing mouse neurons were analyzed nuclear transcript state as the editing reaction requires the presence using PLA. At 1 DIV, only a few interactions between ADAR2 and of intronic sequences to form the necessary double-stranded Pin1 were detected, mostly in the cytoplasm (Fig. 6D). At 6 DIV, structures. Thus, cytoplasmic mature mRNAs or miRNAs are no several interactions were detected in the nucleus, but some longer substrates for A-to-I editing, and the concentration of nuclear complexes were also found in the soma. However, at 13 DIV, a ADAR enzymes is crucial for editing efficiency. In a recent report, it dramatic increase in the number of interactions was observed, has been shown that sites where editing increases during brain mostly in the nucleus. Taken together, the data indicate that development are more sensitive to fluctuations in ADAR expression nuclear interactions between Pin1 and ADAR2 increase during than other sites (Hwang et al., 2016). In neurons, we saw a less neuronal maturation. frequent presence of ADAR2 in the nucleus of immature neurons compared to mature neurons. A reasonable explanation for the low DISCUSSION level of editing during early maturation is therefore the low Several previous analyses have revealed that editing in general concentration of nuclear ADAR2 protein. Moreover, even though increases during neuronal maturation, which has profound effects we have chosen to analyze ADAR2, ADAR1-specific editing also on both development and functionality of the mammalian brain increases as neurons mature and, accordingly, a nuclear (Ekdahl et al., 2012; Shtrichman et al., 2012; Wahlstedt et al., accumulation of ADAR1 was seen during maturation although the

2009). We have previously seen that the protein levels of ADAR1 total protein levels remained constant (Figs S2 and S3). This result Journal of Cell Science

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Fig. 5. Increased expression of importin-α4 increases A-to-I RNA editing. (A) Schematic illustration of the ADAR2 transcript exon4, intron 4, exon 5 editing-splicing reporter, A2-Ex4-5. In the absence of editing, the transcript expressed from the reporter is normally spliced (left), whereas editing of the AA dinucleotide into AI creates an alternative 3′ splice acceptor site and an inclusion of 47 nucleotides (right). ADAR2- induced alternative splicing of the A2-Ex4-5 reporter was analyzed by RT-PCR using primers (arrows) that amplify both the normal and alternatively spliced transcripts. The illustration is not drawn to scale. (B) RT-PCR analysis of SH- SY5Y cells that transiently expressed equal amounts of the A2-Ex4-5 editing-splicing reporter and the indicated protein: ADAR2 and/or importin- α4. Expression of ADAR2 and importin-α4was analyzed by western blotting (WB). β-Actin was used as a loading control. (C) The fold-change in the amount of alternative splicing of the A2-Ex4-5 reporter in cells that overexpressed importin-α4 compared to levels of endogenous expression (EV, empty vector). The fold-change was calculated by using densitometry to measure the amount of alternatively spliced product compared to that of the normally spliced product after RT- PCR. Results shown are representative of four independent experiments. *P=0.0177 (Student’s t-test).

indicates that the subcellular localization of ADAR1 plays an Intriguingly, ADAR2-specific editing is post-translationally important role in the regulation of nuclear editing. regulated by the phosphorylation-dependent isomerase Pin1 A previous study suggests that ADAR2 has a non-canonical NLS (Marcucci et al., 2011). We show that Pin1 expression is within its first 64 amino acids, but this is not enough for nucleolar developmentally regulated in cultured primary neurons, which is localization, which requires more than the first 139 amino acids in line with previous results that have shown a strong induction of (Desterro et al., 2003). In our analysis, we detected two short Pin1 during neuronal differentiation (Hamdane et al., 2006). conserved NLS sequences in ADAR2 between amino acids 48 and Accordingly, we detected an increasing number of interactions 72. These NLS sequences are functionally redundant since only one between ADAR2 and Pin1 during neuronal maturation. The of them was enough for nuclear localization of GFP fused to amino interactionbetweenPin1andADAR2dependson acids 48–72. Strikingly, these results explain why a previous deletion phosphorylation of a Ser/Thr-Pro motif in the N-terminal region of amino acids 64 to 75 in a study by Desterro et al. (2003) retained of ADAR2 (Marcucci et al., 2011). The kinase responsible for this ADAR2 in the nucleus. This indicates that there are two nuclear action is currently unknown and possibly, a developmentally localization signals in the N-terminus of ADAR2, but a nucleolar timed phosphorylation event induces the conformational change in localization signal elsewhere in the protein. Most likely, the dsRNA- ADAR2 that triggers the stabilization of ADAR2 in the nucleus. In binding domains of ADAR2 interact with ribosomal RNA and retain all, this will contribute to the high level of nuclear ADAR2 in the protein in the nucleolus as ADAR2 is excluded from the nucleolus mature neurons. Recently, ADAR-related hypo-editing has been upon overexpression of dsRNA or through disruption of its RNA- foundinAlzheimer’s disease (AD) brains with no corresponding binding capacity (Desterro et al., 2003; Sansam et al., 2003). decrease in ADAR mRNA levels (Gaisler-Salomon et al., 2014; In a previous yeast two-hybrid screen, ADAR2 was shown to Khermesh et al., 2016). Also, decreased nuclear levels of Pin1 interact with both importin-α4 and importin-α5 (Maas and have been described in brain regions that are known to be prone to Gommans, 2009). In agreement with this, we show that importin- neurodegeneration during AD (Liou et al., 2003; Lu et al., 1999). α4 interacts with ADAR2, an interaction that is dependent on the This may be a reflection of dysfunctional Pin1 post-translational region necessary for nuclear import. However, we could not confirm control of nuclear ADAR2 concentration in affected neurons in any binding affinity towards importin-α5, which might be due to AD. Taken together, our findings explain the restraint on A-to-I differences in the experimental techniques. Furthermore, we saw an editing during early brain development, which occurs as a result of increase in interactions between ADAR2 and importin-α4in the limited nuclear concentration of the ADAR2 editing enzyme developing neurons, as well as an increase in the protein that is due to lack of interactions with importin-α4 and Pin1 expression of importin-α4. This is in accordance with increased (Fig. 7). During development the nuclear import of ADAR2 is ADAR2-specific editing after elevating importin-α4 expression in mediated by the gradual increase in importin-α4 expression and neuroblastoma cells. Our result therefore explains how editing can stabilized by an increased expression of Pin1. As a result, the increase during development through increased import of ADAR2, concentration of ADAR2 in the nucleus is elevated and editing elevating the nuclear concentration. increases. Journal of Cell Science

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Fig. 6. ADAR2–Pin1 complexes accumulate in the nucleus during neuronal maturation. (A) Western blot analysis of Pin1 expression in developing neurons at 1–13 DIV, as indicated. βIII-tubulin was used as a loading control. (B) Western blot analysis of Pin1 expression in total mouse brain homogenates from embryonic day 19 (E19), postnatal day 2, 7 and 21 (P2, P7, P21). βIII-tubulin was used as a loading control. (C) Immunofluorescent staining of Pin1 (green) and ADAR2 (red) in cortical neurons cultured for 1 and 13 DIV. Colocalization between Pin1 and ADAR2 is indicated by arrows. DAPI staining was used to visualize the nucleus (blue). (D) In situ PLA of ADAR2– Pin1 protein complexes (red) in cortical neurons at 1, 6 and 13 DIV. As negative controls for the PLA, only antibodies against ADAR2 or Pin1 were used, or both were excluded (no primary antibodies). DAPI staining was used to visualize the nucleus (blue). Scale bars: 10 µm.

Our findings suggest a mechanism of ADAR editing regulation streptomycin (Invitrogen). Cells were transfected using Lipofectamine that can rapidly change the editing status of multiple substrates 2000 (Invitrogen) according to the manufacturer’s instructions. simultaneously. This may be important not only for neuronal development but also for neuronal plasticity. Indeed, acute and RNA isolation, RT-PCR and editing analysis Total RNA was isolated from cell cultures using the mammalian total RNA chronic neuronal activation of both hippocampal and cortical neurons kit (Sigma) or Trizol (Invitrogen). After DNase I (Sigma) treatment, the alters the editing of several substrates (Balik et al., 2013; Sanjana RNA was reverse-transcribed using SuperScript II and random hexamers et al., 2012). However, differences in nuclear concentration of (Invitrogen). For editing analysis of endogenous transcripts from neuronal ADAR2 upon neuronal activation remain to be investigated. cells, RT-PCR was performed using gene-specific primers and Phusion DNA polymerase (ThermoScientific). Editing frequencies were determined MATERIALS AND METHODS by Sanger sequencing (Eurofins MWG Operon) of gel purified RT-PCR Cell culture amplicons. Analysis of editing in the editing-splicing reporter A2-Ex4-5, Primary neurons were prepared from cortices dissected from embryonic- cDNA synthesis, DNase I treatment and RT-PCR using Phusion DNA day-16 pregnant NMRI mice (Charles River). After trituration and filtration polymerase (ThermoScientific) was performed as above. The fold-change in through a 40 μm membrane, cells were plated on poly-D-lysine- (Sigma) the amount of editing-induced alternative over that of the normally spliced coated vessels and grown in Neurobasal® medium supplemented with 2% transcript was assayed by measuring the intensities of the PCR products B27, 0.5 mM L-glutamine, 20 U/ml penicillin-streptomycin (Invitrogen) using ImageLab (Bio-Rad). and 12.5 μM L-glutamate (Sigma). All animal procedures were approved by the Animal Ethics Committee of the North Stockholm region. HEK293, SH- Plasmids and site-directed mutagenesis SY5Y and N2a cells were cultured in Dulbecco’s modified Eagle’s medium Pri-miR-381 and pri-miR-376b constructs were constructed from PCR-

(DMEM) supplemented with 10% FBS and 100 U/ml penicillin- amplified mouse genomic DNA using primers approximately 200 nucleotides Journal of Cell Science

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Lysis-M reagent (Roche) supplemented with protease and phosphatase inhibitor cocktails (Roche). Total protein concentration was determined with Bradford assay, and samples were separated on 4–15% SDS-polyacrylamide gels (BioRad) and blotted onto PVDF membranes. The blots were probed with the following primary antibodies overnight at 4°C: rabbit polyclonal anti- ADAR2 (1:200, Sigma), rabbit polyclonal anti-ADAR1 (1:500, Sigma), mouse monoclonal anti-Pin1 (1:1000: G-8, Santa Cruz Biotechnology), mouse monoclonal anti-βIII-Tubulin (1:2000, SDL.3D10, Sigma), mouse monoclonal anti-U1-70K (1:1000, H111, Synaptic Systems), mouse monoclonal anti-MAP2 (1:250, HM-2, Sigma), mouse monoclonal anti- KPNA1 (1:200, 187.1, Santa Cruz Biotechnology), mouse monoclonal anti- KPNA2 (1:200, B-9, Santa Cruz Biotechnology), mouse monoclonal anti- KPNA3 (1:200, B-1, Santa Cruz Biotechnology), mouse monoclonal anti- GAPDH (1:10,000, 6C5, Abcam), mouse monoclonal anti-nestin (1:500, rat- 401, Millipore), guinea-pig polyclonal anti-NeuN (1:500, Millipore), mouse monoclonal anti-PSD-95 (1:500, 7E3-1B8, Millipore), rat monoclonal anti- GFAP (1:500, 2.2B10, Invitrogen), mouse monoclonal β-actin (1:10,000, AC- 15 Sigma). Horseradish peroxidase (HRP)-conjugated secondary antibodies (DAKO) were used at a 1:2000 dilution. Chemiluminescent detection of protein was performed using the WesternBright Sirius detection system (Advansta), and band intensities were quantified using ImageLab (BioRad). For total protein measurements, a sister polyacrylamide gel was stained with Coomassie Blue (R-250) and destained according to standard procedures. To Fig. 7. Elevated ADAR2 expression in neuronal nuclei increases A-to-I quantify nuclear expression or total protein content, intensities of bands RNA editing during neuronal maturation. As neurons mature, A-to-I RNA or lanes, were measured using Bio-Rad Image Analysis. For co- editing increases gradually in the transcripts of several substrates. immunoprecipitation, cells were resuspended in lysis buffer [125 mM NaCl, Simultaneously, the expression of importin-α4 increases, bringing ADAR2 into 50 mM Tris-HCl pH 7.5, 1% NP-40, 1 mM EGTA and 10% glycerol, and the nucleus where it is stabilized by Pin1. As a result, an elevated amount of supplemented with Complete protease and phosphatase inhibitor cocktail nuclear ADAR2 leads to increased A-to-I RNA editing in mature neurons (right) (Roche)]. After removal of insoluble material, the supernatants were incubated α α compared to in immature neurons (left). A, adenosine; I, inosine; 4, importin- 4. with rabbit anti-FLAG (1:2000, Sigma) followed by addition of Sepharose-G- breads (Invitrogen). upstream and downstream of the annotated pre-miRNA. Amplicons were cloned into pcDNA3. All point mutations and deletion constructs were Statistics generated through QuickChange site-directed mutagenesis (Stratagene) All numerical data are presented as means±s.e.m., calculated using Prism 7 according to the manufacturer’s instructions using the rat pcDNA3-FLAG- software (GraphPad). Statistical significance was assessed using paired two- ADAR2 as template (Bratt and Öhman, 2003). To construct GFP-rADAR2- tailed Student’s t-test or one-way ANOVA, indicated in the respective figure NLS, the rat ADAR2 residues 48–72weresubclonedintotheEcoRI and SalI legends. P-values less than 0.05 were considered statistically significant. sites of the enhanced GFP (pEGFP-C2, CLONTECH) vector. The editing- splicing reporter pRC-CMV-A2 exon 4–5 (A2-Ex4-5) plasmid has been Acknowledgements described previously (Rueter et al., 1999). The pCMVTNT-T7-KPNA3 We thank Mary O’Connell for invaluable scientific discussions, Anna-Stina Höglund plasmid was obtained from Addgene (plasmid # 26679) and is originally for microscopy technical support, Roger Karlsson (Stockholm University, described in Kelley et al. (2010). The pcDNA3-Pin1-HA plasmid was a gift Stockholm, Sweden) for kindly donating the pEGFP plasmid and Claes Andréasson from Mary O’Connell (Masaryk University, Brno, Czech Republic). for critically reading the manuscript. We acknowledge the Imaging Facility at Stockholm University (IFSU) for support with microscopy.

Immunofluorescence, proximity ligation assay and image analysis Competing interests Cells grown on glass slides for the indicated times were fixed in 4% PFA and The authors declare no competing or financial interests. permeabilized with 0.05% SDS. After blocking with 3% BSA, slides were incubated with the following primary antibodies overnight at 4°C: mouse Author contributions ̈ monoclonal anti-βIII-tubulin (1:1000, SDL.3D10, Sigma), rabbit M.B., H.W. and M.O. designed the experiments. M.B., H.W. and A.W. performed the polyclonal anti-ADAR2 (1:200, Sigma), mouse monoclonal anti-ADAR1 experiments. M.E. set up the primary cortical culture system and performed initial experiments. M.B. and M.Ö. wrote the manuscript. (1:500, 15.8.6, Santa Cruz Biotechnology), mouse monoclonal anti- fibrillarin (1:200, 38F3, GeneTex), mouse monoclonal anti-Pin1 (1:200: Funding G-8, Santa Cruz Biotechnology), mouse monoclonal anti-KPNA3 (1:200, This work was supported by the Swedish Research Council (Vetenskapsrådet) B-1, Santa Cruz Biotechnology), rabbit anti-FLAG (1:3000, Sigma). For (grant K2013-66X-20702-06-4 to M.Ö.). immunofluorescence, cells were incubated with anti-mouse-IgG conjugated to Alexa-Fluor-488 and/or anti-rabbit-IgG conjugated to Alexa-Fluor-555 Supplementary information (Invitrogen) and mounted using Prolong Gold antifade reagent containing Supplementary information available online at DAPI (Invitrogen). In situ Proximity Ligation Assay (Duolink®)was http://jcs.biologists.org/lookup/doi/10.1242/jcs.200055.supplemental conducted following the manufacturer’s instructions (Sigma). Images were acquired using an Axiovert 200M inverted fluorescence microscope (Carl References Zeiss) equipped with a Plan-Apochromate 63×1.4 oil objective lens. Alon, S., Mor, E., Vigneault, F., Church, G. M., Locatelli, F., Galeano, F., Gallo, A., Shomron, N. and Eisenberg, E. (2012). Systematic identification of edited Subcellular fractionation, co-immunoprecipitation and western microRNAs in the human brain. Genome Res. 22, 1533-1540. Balik, A., Penn, A. C., Nemoda, Z. and Greger, I. H. (2013). Activity-regulated RNA blots editing in select neuronal subfields in hippocampus. Nucleic Acids Res. 41, Nuclear and cytoplasmic extracts were prepared using NE-PER nuclear 1124-1134. and cytoplasmic extraction kit (Thermo Scientific) according to the Bass, B. L. (2002). RNA editing by adenosine deaminases that act on RNA. Annu. manufacturer’s instructions. For total protein extracts, cells were lysed using Rev. Biochem. 71, 817-846. Journal of Cell Science

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Bass, B. L., Nishikura, K., Keller, W., Seeburg, P. H., Emeson, R. B., O’Connell, Lomeli, H., Mosbacher, J., Melcher, T., Hoger, T., Geiger, J. R., Kuner, T., M. A., Samuel, C. E. and Herbert, A. (1997). A standardized nomenclature for Monyer, H., Higuchi, M., Bach, A. and Seeburg, P. H. (1994). Control of kinetic adenosine deaminases that act on RNA. RNA 3, 947-949. properties of AMPA receptor channels by nuclear RNA editing. Science 266, Bratt, E. and Öhman, M. (2003). Coordination of editing and splicing of glutamate 1709-1713. receptor pre-mRNA. RNA 9, 309-318. Lu, P. J., Wulf, G., Zhou, X. Z., Davies, P. and Lu, K. P. (1999). The prolyl Brusa, R., Zimmermann, F., Koh, D.-S., Feldmeyer, D., Gass, P., Seeburg, P. H. isomerase Pin1 restores the function of Alzheimer-associated phosphorylated tau and Sprengel, R. (1995). Early-onset epilepsy and postnatal lethality associated protein. Nature 399, 784-788. with an editing-deficient GluR-B allele in mice. Science 270, 1677-1680. Maas, S. and Gommans, W. M. (2009). Identification of a selective nuclear import Daniel, C., Wahlstedt, H., Ohlson, J., Björk, P. and Öhman, M. (2011). signal in adenosine deaminases acting on RNA. Nucleic Acids Res. 37, Adenosine-to-inosine RNA editing affects trafficking of the gamma-aminobutyric 5822-5829. acid type A (GABA(A)) receptor. J. Biol. Chem. 286, 2031-2040. Marcucci, R., Brindle, J., Paro, S., Casadio, A., Hempel, S., Morrice, N., Bisso, Desterro, J. M., Keegan, L. P., Lafarga, M., Berciano, M. T., O’Connell, M. and A., Keegan, L. P., Del Sal, G. and O’Connell, M. A. (2011). Pin1 and WWP2 Carmo-Fonseca, M. (2003). Dynamic association of RNA-editing enzymes with regulate GluR2 Q/R site RNA editing by ADAR2 with opposing effects. EMBO J. the nucleolus. J. Cell Sci. 116, 1805-1818. 30, 4211-4222. Dillman, A. A., Hauser, D. N., Gibbs, J. R., Nalls, M. A., McCoy, M. K., Rudenko, Nakai, K. and Horton, P. (1999). PSORT: a program for detecting sorting signals in I. N., Galter, D. and Cookson, M. R. (2013). mRNA expression, splicing and proteins and predicting their subcellular localization. Trends Biochem. Sci. 24, editing in the embryonic and adult mouse cerebral cortex. Nat. Neurosci. 16, 34-36. 499-506. Nguyen Ba, A. N., Pogoutse, A., Provart, N. and Moses, A. M. (2009). Dingwall, C. and Laskey, R. A. (1991). Nuclear targeting sequences–a NLStradamus: a simple Hidden Markov Model for nuclear localization signal consensus? Trends Biochem. Sci. 16, 478-481. prediction. BMC Bioinf. 10, 202. Ekdahl, Y., Farahani, H. S., Behm, M., Lagergren, J. and Öhman, M. (2012). A-to- Nishikura, K. (2010). Functions and regulation of RNA editing by ADAR I editing of microRNAs in the mammalian brain increases during development. deaminases. Annu. Rev. Biochem. 79, 321-349. Genome Res. 22, 1477-1487. Nishimoto, Y., Yamashita, T., Hideyama, T., Tsuji, S., Suzuki, N. and Kwak, S. Gaisler-Salomon, I., Kravitz, E., Feiler, Y., Safran, M., Biegon, A., Amariglio, N. (2008). Determination of editors at the novel A-to-I editing positions. Neurosci. and Rechavi, G. (2014). Hippocampus-specific deficiency in RNA editing of Res. 61, 201-206. GluA2 in Alzheimer’s disease. Neurobiol. Aging 35, 1785-1791. Ohlson, J., Pedersen, J. S., Haussler, D. and Öhman, M. (2007). Editing modifies Garncarz, W., Tariq, A., Handl, C., Pusch, O. and Jantsch, M. F. (2013). A high- the GABA(A) receptor subunit alpha3. RNA 13, 698-703. throughput screen to identify enhancers of ADAR-mediated RNA-editing. RNA Paul, M. S. and Bass, B. L. (1998). Inosine exists in mRNA at tissue-specific levels Biology 10, 192-204. and is most abundant in brain mRNA. EMBO J. 17, 1120-1127. Hamdane, M., Dourlen, P., Bretteville, A., Sambo, A.-V., Ferreira, S., Ando, K., Robbins, J., Dilworth, S. M., Laskey, R. A. and Dingwall, C. (1991). Two Kerdraon, O., Bégard, S., Geay, L., Lippens, G. et al. (2006). Pin1 allows for interdependent basic domains in nucleoplasmin nuclear targeting sequence: differential Tau dephosphorylation in neuronal cells. Mol. Cell. Neurosci. 32, identification of a class of bipartite nuclear targeting sequence. Cell 64, 615-623. 155-160. Rueter, S. M., Dawson, T. R. and Emeson, R. B. (1999). Regulation of alternative Hang, P. N. T., Tohda, M. and Matsumoto, K. (2008). Developmental changes in splicing by RNA editing. Nature 399, 75-80. expression and self-editing of adenosine deaminase type 2 pre-mRNA and mRNA Sanjana, N. E., Levanon, E. Y., Hueske, E. A., Ambrose, J. M. and Li, J. B. (2012). in rat brain and cultured cortical neurons. Neurosci. Res. 61, 398-403. Activity-dependent A-to-I RNA editing in rat cortical neurons. Genetics 192, Higuchi, M., Maas, S., Single, F. N., Hartner, J., Rozov, A., Burnashev, N., 281-287. Feldmeyer, D., Sprengel, R. and Seeburg, P. H. (2000). Point mutation in an Sansam, C. L., Wells, K. S. and Emeson, R. B. (2003). Modulation of RNA editing AMPA receptor gene rescues lethality in mice deficient in the RNA-editing enzyme by functional nucleolar sequestration of ADAR2. Proc. Natl. Acad. Sci. USA 100, ADAR2. Nature 406, 78-81. 14018-14023. Hosokawa, K., Nishi, M., Sakamoto, H., Tanaka, Y. and Kawata, M. (2008). Shtrichman, R., Germanguz, I., Mandel, R., Ziskind, A., Nahor, I., Safran, M., Regional distribution of importin subtype mRNA expression in the nervous Osenberg, S., Sherf, O., Rechavi, G. and Itskovitz-Eldor, J. (2012). Altered A- system: study of early postnatal and adult mouse. Neuroscience 157, 864-877. to-I RNA editing in human embryogenesis. PLoS ONE 7, e41576. Hwang, T., Park, C.-K., Leung, A. K. L., Gao, Y., Hyde, T. M., Kleinman, J. E., Söderberg, O., Gullberg, M., Jarvius, M., Ridderstråle, K., Leuchowius, K.-J., Rajpurohit, A., Tao, R., Shin, J. H. and Weinberger, D. R. (2016). Dynamic Jarvius, J., Wester, K., Hydbring, P., Bahram, F., Larsson, L.-G. et al. (2006). regulation of RNA editing in human brain development and disease. Nat. Direct observation of individual endogenous protein complexes in situ by proximity Neurosci. 19, 1093-1099. ligation. Nat. Methods 3, 995-1000. Kalderon, D., Richardson, W. D., Markham, A. F. and Smith, A. E. (1984). Tariq, A., Garncarz, W., Handl, C., Balik, A., Pusch, O. and Jantsch, M. F. (2013). Sequence requirements for nuclear location of simian virus 40 large-T antigen. RNA-interacting proteins act as site-specific repressors of ADAR2-mediated RNA Nature 311, 33-38. editing and fluctuate upon neuronal stimulation. Nucleic Acids Res. 41, Kawahara, Y., Zinshteyn, B., Sethupathy, P., Iizasa, H., Hatzigeorgiou, A. G. and 2581-2593. Nishikura, K. (2007). Redirection of silencing targets by adenosine-to-inosine Vesely, C., Tauber, S., Sedlazeck, F. J., Tajaddod, M., Haeseler, A. V. and editing of miRNAs. Science 315, 1137-1140. Jantsch, M. F. (2014). ADAR2 induces reproducible changes in sequence and Kelley, J. B., Talley, A. M., Spencer, A., Gioeli, D. and Paschal, B. M. (2010). abundance of mature microRNAs in the mouse brain. Nucleic Acids Res. 42, Karyopherin alpha7 (KPNA7), a divergent member of the importin alpha family of 12155-12168. nuclear import receptors. BMC Cell. Biol. 11, 63. Wahlstedt, H. and Öhman, M. (2011). Site-selective versus promiscuous A-to-I Khermesh, K., D’Erchia, A. M., Barak, M., Annese, A., Wachtel, C., Levanon, editing. Wiley Interdiscip. Rev. RNA 2, 761-771. E. Y., Picardi, E. and Eisenberg, E. (2016). Reduced levels of protein recoding Wahlstedt, H., Daniel, C., Ensterö, M. and Öhman, M. (2009). Large-scale mRNA by A-to-I RNA editing in Alzheimer’s disease. RNA 22, 290-302. sequencing determines global regulation of RNA editing during brain Kosugi, S., Hasebe, M., Tomita, M. and Yanagawa, H. (2009). Systematic development. Genome Res. 19, 978-986. identification of cell cycle-dependent yeast nucleocytoplasmic shuttling proteins Wong, S. K., Sato, S. and Lazinski, D. W. (2003). Elevated activity of the large form by prediction of composite motifs. Proc. Natl. Acad. Sci. USA 106, 10171-10176. of ADAR1 in vivo: very efficient RNA editing occurs in the cytoplasm. RNA 9, Lev-Maor, G., Sorek, R., Levanon, E. Y., Paz, N., Eisenberg, E. and Ast, G. 586-598. (2007). RNA-editing-mediated exon evolution. Genome Biol. 8, R29. Yasuhara, N., Shibazaki, N., Tanaka, S., Nagai, M., Kamikawa, Y., Oe, S., Asally, Liou, Y.-C., Sun, A., Ryo, A., Zhou, X. Z., Yu, Z.-X., Huang, H.-K., Uchida, T., M., Kamachi, Y., Kondoh, H. and Yoneda, Y. (2007). Triggering neural Bronson, R., Bing, G., Li, X. et al. (2003). Role of the prolyl isomerase Pin1 in differentiation of ES cells by subtype switching of importin-alpha. Nat. Cell Biol. protecting against age-dependent neurodegeneration. Nature 424, 556-561. 9, 72-79. 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