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N6-methyladenosine (m 6 A), the most abundant modification of mRNA, is essential for normal development and dysregulation promotes cancer. m 6 A is highly enriched in the 3’ untranslated region (UTR) of a large subset of mRNAs to influence mRNA stability and/or translation. However, the mechanism responsible for the observed m 6 A distribution remains enigmatic. Here we find the exon junction complex shapes the m 6 A landscape by blocking METTL3-mediated m 6 A modification close to exon junctions within coding sequence (CDS). Depletion of EIF4A3, a core component of the EJC, causes increased METTL3 binding and m 6 A modification of short internal exons, and sites close to exon-exon junctions within mRNA. Reporter gene experiments further support the role of splicing and EIF4A3 deposition in controlling m 6 A modification via the local steric blockade of METTL3. Our results explain how characteristic patterns of m 6 A mRNA modification are established and uncover a role of the EJC in shaping the m 6 A epitranscriptome. Here the authors show the exon junction complex (EJC) component, EIF4A3, locally restricts METTL3- mediated mRNA methylation at exon junctions to explain the observed widespread enrichment of m6A modification in 3’ untranslated regions.

This study shows that Dis3l2 is the 3′–5′ exonuclease that mediates the degradation of uridylated precursor let-7 microRNA; this is the first physiological RNA substrate identified for this new exonuclease, which causes the Perlman syndrome of fetal overgrowth and Wilms’ tumour susceptibility when mutated. Function of Perlman syndrome exonuclease The Lin28-let-7 signalling cascade has been linked to stem cell function, cancers and various aspects of cellular metabolism. The pluripotency factor Lin28 recruits the 3′ terminal uridylyl transferases that add an oligouridine tail to let-7 precursor RNA. Here Richard Gregory and colleagues demonstrate that Dis3l2, a protein mutated in Perlman syndrome of fetal overgrowth and predisposition to Wilms' tumour, is the exonuclease that mediates the degradation of uridylated pre-let-7 in mouse embryonic stem cells. The identification of a decay pathway for uridylated RNAs raises the possibility that this type of post-transcriptional regulation might occur more widely. The pluripotency factor Lin28 blocks the expression of let-7 microRNAs in undifferentiated cells during development, and functions as an oncogene in a subset of cancers 1 . Lin28 binds to let-7 precursor (pre-let-7) RNAs and recruits 3′ terminal uridylyl transferases to selectively inhibit let-7 biogenesis 2 , 3 , 4 . Uridylated pre-let-7 is refractory to processing by Dicer, and is rapidly degraded by an unknown RNase 5 . Here we identify Dis3l2 as the 3′–5′ exonuclease responsible for the decay of uridylated pre-let-7 in mouse embryonic stem cells. Biochemical reconstitution assays show that 3′ oligouridylation stimulates Dis3l2 activity in vitro , and knockdown of Dis3l2 in mouse embryonic stem cells leads to the stabilization of pre-let-7. Our study establishes 3′ oligouridylation as an RNA decay signal for Dis3l2, and identifies the first physiological RNA substrate of this new exonuclease, which is mutated in the Perlman syndrome of fetal overgrowth and causes a predisposition to Wilms’ tumour development 6 .

Background Intravascular leukocyte recruitment in most vertebrate tissues is restricted to postcapillary and collecting venules, whereas capillaries and arterioles usually support little or no leukocyte adhesion. This segmental restriction is thought to be mediated by endothelial, rather than hemodynamic, differences. The underlying mechanisms are largely unknown, in part because effective tools to distinguish, isolate, and analyze venular endothelial cells (V-ECs) and non-venular endothelial cells (NV-ECs) have been unavailable. We hypothesized that the atypical chemokine receptor DARC (Duffy Antigen Receptor for Chemokines, a.k.a. ACKR1 or CD234) may distinguish V-ECs versus NV-ECs in mice. Methods We generated a rat-anti-mouse monoclonal antibody (MAb) that specifically recognizes the erythroid and endothelial forms of native, surface-expressed DARC. Using this reagent, we characterized DARC expression and distribution in the microvasculature of murine tissues. Results DARC was exquisitely restricted to post-capillary and small collecting venules and completely absent from arteries, arterioles, capillaries, veins, and most lymphatics in every tissue analyzed. Accordingly, intravital microscopy showed that adhesive leukocyte-endothelial interactions were restricted to DARC + venules. DARC was detectable over the entire circumference of V-ECs, but was more concentrated at cell-cell junctions. Analysis of single-cell suspensions suggested that the frequency of V-ECs among the total microvascular EC pool varies considerably between different tissues. Conclusions Immunostaining of endothelial DARC allows the identification and isolation of intact V-ECs from multiple murine tissues. This strategy may be useful to dissect the mechanisms underlying segmental microvascular specialization in healthy and diseased tissues and to characterize the role of EC subsets in tissue-homeostasis, immune surveillance, infection, inflammation, and malignancies.

Patient-derived induced pluripotent stem cells (iPSCs) hold great promise for autologous cell replacement. However, for many inherited diseases, treatment will likely require genetic repair pre-transplantation. Genome editing technologies are useful for this application. The purpose of this study was to develop CRISPR-Cas9-mediated genome editing strategies to target and correct the three most common types of disease-causing variants in patient-derived iPSCs: (1) exonic, (2) deep intronic, and (3) dominant gain of function. We developed a homology-directed repair strategy targeting a homozygous Alu insertion in exon 9 of male germ cell-associated kinase (MAK) and demonstrated restoration of the retinal transcript and protein in patient cells. We generated a CRISPR-Cas9-mediated non-homologous end joining (NHEJ) approach to excise a major contributor to Leber congenital amaurosis, the IVS26 cryptic-splice mutation in CEP290, and demonstrated correction of the transcript and protein in patient iPSCs. Lastly, we designed allele-specific CRISPR guides that selectively target the mutant Pro23His rhodopsin (RHO) allele, which, following delivery to both patient iPSCs in vitro and pig retina in vivo, created a frameshift and premature stop that would prevent transcription of the disease-causing variant. The strategies developed in this study will prove useful for correcting a wide range of genetic variants in genes that cause inherited retinal degeneration. Using CRISPR-Cas9 to generate gene-corrected autologous iPSCs for the treatment of inherited retinal degeneration, induced pluripotent stem cells, and genome editing hold great promise for autologous cell replacement therapies for inherited diseases. Here, Burnight and colleagues developed CRISPR-Cas9-mediated genome editing strategies to target and repair the three most common types of retinal disease-causing variants in patient-derived iPSCs: exonic, deep intronic, and dominant gain of function.

MicroRNAs (miRNAs) are single-stranded noncoding RNAs of 19 to 25 nucleotides that function as gene regulators and as a host cell defense against both RNA and DNA viruses. We provide evidence for a physiological role of the miRNA-silencing machinery in controlling HIV-1 replication. Type III RNAses Dicer and Drosha, responsible for miRNA processing, inhibited virus replication both in peripheral blood mononuclear cells from HIV-1-infected donors and in latently infected cells. In turn, HIV-1 actively suppressed the expression of the polycistronic miRNA cluster miR-17/92. This suppression was found to be required for efficient viral replication and was dependent on the histone acetyltransferase Tat cofactor PCAF. Our results highlight the involvement of the miRNA-silencing pathway in HIV-1 replication and latency.

The rate of HIV-1 gene expression is a key step that determines the kinetics of virus spread and AIDS progression. Viral entry and gene expression were described to be the key determinants for cell permissiveness to HIV. Recent reports highlighted the involvement of miRNA in regulating HIV-1 replication post-transcriptionally. In this study we explored the role of cellular factors required for miRNA-mediated mRNA translational inhibition in regulating HIV-1 gene expression. Here we show that HIV-1 mRNAs associate and co-localize with components of the RNA Induced Silencing Complex (RISC), and we characterize some of the proteins required for miRNA-mediated silencing (miRNA effectors). RCK/p54, GW182, LSm-1 and XRN1 negatively regulate HIV-1 gene expression by preventing viral mRNA association with polysomes. Interestingly, knockdown of RCK/p54 or DGCR8 resulted in virus reactivation in PBMCs isolated from HIV infected patients treated with suppressive HAART.

Exportin-5 is involved in microRNA precursor nuclear export. Dicer mRNA is now found to compete with pre-miRNAs for exportin-5 binding, suggesting that cytoplasmic levels of Dicer and its pre-miRNA substrate engage in a feedback loop. Analysis of adenoviral RNA indicates that it can also compete for exportin-5, suggesting that this might be a mechanism by which adenovirus can interfere with host miRNA processing. MicroRNAs (miRNAs) are a class of small, noncoding RNAs that function by regulating gene expression post-transcriptionally. Alterations in miRNA expression can strongly influence cellular physiology. Here we demonstrated cross-regulation between two components of the RNA interference (RNAi) machinery in human cells. Inhibition of exportin-5, the karyopherin responsible for pre-miRNA export, downregulated expression of Dicer, the RNase III required for pre-miRNA maturation. This effect was post-transcriptional and resulted from an increased nuclear localization of Dicer mRNA. In vitro assays and cellular RNA immunoprecipitation experiments showed that exportin-5 interacted directly with Dicer mRNA. Titration of exportin-5 by overexpression of either pre-miRNA or the adenoviral VA1 RNA resulted in loss of Dicer mRNA–exportin-5 interaction and reduction of Dicer level. This saturation also occurred during adenoviral infection and enhanced viral replication. Our study reveals an important cross-regulatory mechanism between pre-miRNA or viral small RNAs and Dicer through exportin-5.

The p300–CBP-associated factor (PCAF) is a histone acetyltransferase (HAT) involved in the reversible acetylation of various transcriptional regulators 1 , including the tumour suppressor p53. It is implicated in many cellular processes, such as transcription, differentiation, proliferation and apoptosis. We observed that knockdown of PCAF expression in HeLa or U2OS cell lines induces stabilization of the oncoprotein Hdm2, a RING finger E3 ligase primarily known for its role in controlling p53 stability 2 , 3 . To investigate the molecular basis of this effect, we examined whether PCAF is involved in Hdm2 ubiquitination. Here, we show that PCAF, in addition to its acetyltransferase activity, possesses an intrinsic ubiquitination activity that is critical for controlling Hdm2 expression levels, and thus p53 functions. Our data highlight a regulatory crosstalk between PCAF and Hdm2 activities, which is likely to have a central role in the subtle control of p53 activity after DNA damage.

RNAi-based gene therapy using miRNA-adapted short hairpin RNAs (shRNAmiR) is a powerful approach to modulate gene expression. However, we have observed low viral titers with shRNAmiR-containing recombinant vectors and hypothesized that this could be due to cleavage of viral genomic RNA by the endogenous microprocessor complex during virus assembly. To test this hypothesis, we targeted DROSHA, the core component of the microprocessor complex, and successfully generated monoallelic and biallelic DROSHA knockout (KO) HEK293T cells for vector production. DROSHA KO was verified by polymerase chain reaction (PCR) and western blot analysis. We produced lentiviral vectors containing Venus with or without shRNA hairpins and generated virus supernatants using DROSHA KO packaging cells. We observed an increase in the fluorescence intensity of hairpin-containing Venus transcripts in DROSHA KO producer cells consistent with reduced microprocessor cleavage of encoded mRNA transcripts, and recovery in the viral titer of hairpin-containing vectors compared with non-hairpin-containing constructs. We confirmed the absence of significant shRNAmiR processing by northern blot analysis and showed that this correlated with an increase in the amount of full-length vector genomic RNA. These findings may have important implications in future production of viral shRNAmiR-containing vectors for RNAi-based therapy.

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