Acetylation of Cytidine in mRNA Promotes Translation Efficiency.

Generation of the "epitranscriptome" through post-transcriptional ribonucleoside modification embeds a layer of regulatory complexity into RNA structure and function. Here, we describe N4-acetylcytidine (ac4C) as an mRNA modification that is catalyzed by the acetyltransferase NAT10. Transcriptome-wide mapping of ac4C revealed discretely acetylated regions that were enriched within coding sequences. Ablation of NAT10 reduced ac4C detection at the mapped mRNA sites and was globally associated with target mRNA downregulation. Analysis of mRNA half-lives revealed a NAT10-dependent increase in stability in the cohort of acetylated mRNAs. mRNA acetylation was further demonstrated to enhance substrate translation in vitro and in vivo. Codon content analysis within ac4C peaks uncovered a biased representation of cytidine within wobble sites that was empirically determined to influence mRNA decoding efficiency. These findings expand the repertoire of mRNA modifications to include an acetylated residue and establish a role for ac4C in the regulation of mRNA translation.

Detection of ac4C in human mRNA is preserved upon data reassessment.

We recently reported the distribution of N4-acetylcytidine (ac4C) in HeLa mRNA at base resolution through chemical reduction and the induction of C:T mismatches in sequencing (RedaC:T-seq). Our results contradicted an earlier report from Schwartz and colleagues utilizing a similar method termed ac4C-seq. Here, we revisit both datasets and reaffirm our findings. Through RedaC:T-seq reanalysis, we establish a low basal error rate at unmodified nucleotides that is not skewed to any specific mismatch type and a prominent increase in C:T substitutions as the dominant mismatch type in both treated wild-type replicates, with a high degree of reproducibility across replicates. In contrast, through ac4C-seq reanalysis, we uncover significant data quality issues including insufficient depth, with one wild-type replicate yielding 2.7 million reads, inconsistencies in reduction efficiencies between replicates, and an overall increase in mismatches involving thymine that could obscure ac4C detection. These analyses bolster the detection of ac4C in HeLa mRNA through RedaC:T-seq.

Direct epitranscriptomic regulation of mammalian translation initiation through N4-acetylcytidine.

mRNA function is influenced by modifications that modulate canonical nucleobase behavior. We show that a single modification mediates distinct impacts on mRNA translation in a position-dependent manner. Although cytidine acetylation (ac4C) within protein-coding sequences stimulates translation, ac4C within 5' UTRs impacts protein synthesis at the level of initiation. 5' UTR acetylation promotes initiation at upstream sequences, competitively inhibiting annotated start codons. Acetylation further directly impedes initiation at optimal AUG contexts: ac4C within AUG-flanking Kozak sequences reduced initiation in base-resolved transcriptome-wide HeLa results and in vitro utilizing substrates with site-specific ac4C incorporation. Cryo-EM of mammalian 80S initiation complexes revealed that ac4C in the -1 position adjacent to an AUG start codon disrupts an interaction between C and hypermodified t6A at nucleotide 37 of the initiator tRNA. These findings demonstrate the impact of RNA modifications on nucleobase function at a molecular level and introduce mRNA acetylation as a factor regulating translation in a location-specific manner.

ac4C: a fragile modification with stabilizing functions in RNA metabolism.

In recent years, concerted efforts to map and understand epitranscriptomic modifications in mRNA have unveiled new complexities in the regulation of gene expression. These studies cumulatively point to diverse functions in mRNA metabolism, spanning pre-mRNA processing, mRNA degradation, and translation. However, this emerging landscape is not without its intricacies and sources of discrepancies. Disparities in detection methodologies, divergent interpretations of functional outcomes, and the complex nature of biological systems across different cell types pose significant challenges. With a focus of N4-acetylcytidine (ac4C), this review endeavors to unravel conflicting narratives by examining the technological, biological, and methodological factors that have contributed to discrepancies and thwarted research progress. Our goal is to mitigate detection inconsistencies and establish a unified model to elucidate the contribution of ac4C to mRNA metabolism and cellular equilibrium.

Enhanced ac4C detection in RNA via chemical reduction and cDNA synthesis with modified dNTPs.

The functional analysis of epitranscriptomic modifications in RNA is constrained by a lack of methods that accurately capture their locations and levels. We previously demonstrated that the RNA modification N4-acetylcytidine (ac4C) can be mapped at base resolution through sodium borohydride reduction to tetrahydroacetylcytidine (tetrahydro-ac4C), followed by cDNA synthesis to misincorporate adenosine opposite reduced ac4C sites, culminating in C:T mismatches at acetylated cytidines (RedaC:T). However, this process is relatively inefficient, resulting in <20% C:T mismatches at a fully modified ac4C site in 18S rRNA. Considering that ac4C locations in other substrates including mRNA are unlikely to reach full penetrance, this method is not ideal for comprehensive mapping. Here, we introduce "RetraC:T" (reduction to tetrahydro-ac4C and reverse transcription with amino-dATP to induce C:T mismatches) as a method with enhanced ability to detect ac4C in cellular RNA. In brief, RNA is reduced through NaBH4 or the closely related reagent sodium cyanoborohydride (NaCNBH3) followed by cDNA synthesis in the presence of a modified DNA nucleotide, 2-amino-dATP, that preferentially binds to tetrahydro-ac4C. Incorporation of the modified dNTP substantially improved C:T mismatch rates, reaching stoichiometric detection of ac4C in 18S rRNA. Importantly, 2-amino-dATP did not result in truncated cDNA products nor increase mismatches at other locations. Thus, modified dNTPs are introduced as a new addition to the toolbox for detecting ac4C at base resolution.

TET-catalyzed oxidation of intragenic 5-methylcytosine regulates CTCF-dependent alternative splicing.

Intragenic 5-methylcytosine and CTCF mediate opposing effects on pre-mRNA splicing: CTCF promotes inclusion of weak upstream exons through RNA polymerase II pausing, whereas 5-methylcytosine evicts CTCF, leading to exon exclusion. However, the mechanisms governing dynamic DNA methylation at CTCF-binding sites were unclear. Here, we reveal the methylcytosine dioxygenases TET1 and TET2 as active regulators of CTCF-mediated alternative splicing through conversion of 5-methylcytosine to its oxidation derivatives. 5-hydroxymethylcytosine and 5-carboxylcytosine are enriched at an intragenic CTCF-binding sites in the CD45 model gene and are associated with alternative exon inclusion. Reduced TET levels culminate in increased 5-methylcytosine, resulting in CTCF eviction and exon exclusion. In vitro analyses establish the oxidation derivatives are not sufficient to stimulate splicing, but efficiently promote CTCF association. We further show genomewide that reciprocal exchange of 5-hydroxymethylcytosine and 5-methylcytosine at downstream CTCF-binding sites is a general feature of alternative splicing in naïve and activated CD4(+) T cells. These findings significantly expand our current concept of the pre-mRNA "splicing code" to include dynamic intragenic DNA methylation catalyzed by the TET proteins.

Independence between pre-mRNA splicing and DNA methylation in an isogenic minigene resource.

Actively transcribed genes adopt a unique chromatin environment with characteristic patterns of enrichment. Within gene bodies, H3K36me3 and cytosine DNA methylation are elevated at exons of spliced genes and have been implicated in the regulation of pre-mRNA splicing. H3K36me3 is further responsive to splicing, wherein splicing inhibition led to a redistribution and general reduction over gene bodies. In contrast, little is known of the mechanisms supporting elevated DNA methylation at actively spliced genic locations. Recent evidence associating the de novo DNA methyltransferase Dnmt3b with H3K36me3-rich chromatin raises the possibility that genic DNA methylation is influenced by splicing-associated H3K36me3. Here, we report the generation of an isogenic resource to test the direct impact of splicing on chromatin. A panel of minigenes of varying splicing potential were integrated into a single FRT site for inducible expression. Profiling of H3K36me3 confirmed the established relationship to splicing, wherein levels were directly correlated with splicing efficiency. In contrast, DNA methylation was equivalently detected across the minigene panel, irrespective of splicing and H3K36me3 status. In addition to revealing a degree of independence between genic H3K36me3 and DNA methylation, these findings highlight the generated minigene panel as a flexible platform for the query of splicing-dependent chromatin modifications.

A Chemical Signature for Cytidine Acetylation in RNA.

N4-acetylcytidine (ac4C) is a highly conserved modified RNA nucleobase whose formation is catalyzed by the disease-associated N-acetyltransferase 10 (NAT10). Here we report a sensitive chemical method to localize ac4C in RNA. Specifically, we characterize the susceptibility of ac4C to borohydride-based reduction and show this reaction can cause introduction of noncognate base pairs during reverse transcription (RT). Combining borohydride-dependent misincorporation with ac4C's known base-sensitivity provides a unique chemical signature for this modified nucleobase. We show this unique reactivity can be used to quantitatively analyze cellular RNA acetylation, study adapters responsible for ac4C targeting, and probe the timing of RNA acetylation during ribosome biogenesis. Overall, our studies provide a chemical foundation for defining an expanding landscape of cytidine acetyltransferase activity and its impact on biology and disease.

CTCF-promoted RNA polymerase II pausing links DNA methylation to splicing.

Alternative splicing of pre-messenger RNA is a key feature of transcriptome expansion in eukaryotic cells, yet its regulation is poorly understood. Spliceosome assembly occurs co-transcriptionally, raising the possibility that DNA structure may directly influence alternative splicing. Supporting such an association, recent reports have identified distinct histone methylation patterns, elevated nucleosome occupancy and enriched DNA methylation at exons relative to introns. Moreover, the rate of transcription elongation has been linked to alternative splicing. Here we provide the first evidence that a DNA-binding protein, CCCTC-binding factor (CTCF), can promote inclusion of weak upstream exons by mediating local RNA polymerase II pausing both in a mammalian model system for alternative splicing, CD45, and genome-wide. We further show that CTCF binding to CD45 exon 5 is inhibited by DNA methylation, leading to reciprocal effects on exon 5 inclusion. These findings provide a mechanistic basis for developmental regulation of splicing outcome through heritable epigenetic marks.

Heterogeneous nuclear ribonucleoprotein L-like (hnRNPLL) and elongation factor, RNA polymerase II, 2 (ELL2) are regulators of mRNA processing in plasma cells.

B cells and plasma cells possess distinct RNA processing environments that respectively promote the expression of membrane-associated Ig by B cells versus the secretion of Ig by plasma cells. Through a combination of transcriptional profiling and screening using a lentiviral short-hairpin RNA interference library, we show that both the splicing factor hnRNPLL and the transcription elongation factor ELL2 modulate the ratio of secreted versus membrane-encoding Ighg2b transcripts in MPC11 plasmacytoma cell lines. hnRNPLL and ELL2 are both highly expressed in primary plasma cells relative to B cells, but hnRNPLL binds Ighg2b mRNA transcripts and promotes an increase in levels of the membrane-encoding Ighg2b isoform at the expense of the secreted Ighg2b isoform, whereas ELL2 counteracts this effect and drives Ig secretion by increasing the frequency of the secreted Ighg2b isoform. As in T cells, hnRNPLL also alters the splicing pattern of mRNA encoding the adhesion receptor CD44, promoting exon inclusion, and decreasing the overall level of CD44 expression. Further characterization of ELL2-dependent transcription by RNA-Seq revealed that ∼12% of transcripts expressed by plasma cells were differentially processed because of the activities of ELL2, including B-cell maturation antigen BCMA, a receptor with a defined role in plasma cell survival. Taken together, our data identify hnRNPLL and ELL2 as regulators of pre-mRNA processing in plasma cells.

Dephosphorylation of the nuclear factor of activated T cells (NFAT) transcription factor is regulated by an RNA-protein scaffold complex.

Nuclear factor of activated T cells (NFAT) proteins are Ca(2+)-regulated transcription factors that control gene expression in many cell types. NFAT proteins are heavily phosphorylated and reside in the cytoplasm of resting cells; when cells are stimulated by a rise in intracellular Ca(2+), NFAT proteins are dephosphorylated by the Ca(2+)/calmodulin-dependent phosphatase calcineurin and translocate to the nucleus to activate target gene expression. Here we show that phosphorylated NFAT1 is present in a large cytoplasmic RNA-protein scaffold complex that contains a long intergenic noncoding RNA (lincRNA), NRON [noncoding (RNA) repressor of NFAT]; a scaffold protein, IQ motif containing GTPase activating protein (IQGAP); and three NFAT kinases, casein kinase 1, glycogen synthase kinase 3, and dual specificity tyrosine phosphorylation regulated kinase. Combined knockdown of NRON and IQGAP1 increased NFAT dephosphorylation and nuclear import exclusively after stimulation, without affecting the rate of NFAT rephosphorylation and nuclear export; and both NRON-depleted T cells and T cells from IQGAP1-deficient mice showed increased production of NFAT-dependent cytokines. Our results provide evidence that a complex of lincRNA and protein forms a scaffold for a latent transcription factor and its regulatory kinases, and support an emerging consensus that lincRNAs that bind transcriptional regulators have a similar scaffold function.

NAT10 and cytidine acetylation in mRNA: intersecting paths in development and disease.

N4-acetylcytidine (ac4C) is an RNA modification that is catalyzed by the enzyme NAT10. Constitutively found in tRNA and rRNA, ac4C displays a dynamic presence in mRNA that is shaped by developmental and induced shifts in NAT10 levels. However, deciphering ac4C functions in mRNA has been hampered by its context-dependent influences in translation and the complexity of isolating effects on specific mRNAs from other NAT10 activities. Recent advances have begun to overcome these obstacles by leveraging natural variations in mRNA acetylation in cancer, developmental transitions, and immune responses. Here, we synthesize the current literature with a focus on nuances that may fuel the perception of cellular discrepancies toward the development of a cohesive model of ac4C function in mRNA.

Voices: Challenges and opportunities in RNA biology.

In the first of many thematic issues marking the 30th anniversary of Cell Chemical Biology, we highlight the contribution of chemical biology to RNA biology in a special issue on RNA modulation. We asked several leaders in the field to share their opinions on the current challenges and opportunities in RNA biology.

Co-transcriptional regulation of alternative pre-mRNA splicing.

While studies of alternative pre-mRNA splicing regulation have typically focused on RNA-binding proteins and their target sequences within nascent message, it is becoming increasingly evident that mRNA splicing, RNA polymerase II (pol II) elongation and chromatin structure are intricately intertwined. The majority of introns in higher eukaryotes are excised prior to transcript release in a manner that is dependent on transcription through pol II. As a result of co-transcriptional splicing, variations in pol II elongation influence alternative splicing patterns, wherein a slower elongation rate is associated with increased inclusion of alternative exons within mature mRNA. Physiological barriers to pol II elongation, such as repressive chromatin structure, can thereby similarly impact splicing decisions. Surprisingly, pre-mRNA splicing can reciprocally influence pol II elongation and chromatin structure. Here, we highlight recent advances in co-transcriptional splicing that reveal an extensive network of coupling between splicing, transcription and chromatin remodeling complexes. This article is part of a Special Issue entitled: Chromatin in time and space.

Protocol for base resolution mapping of ac4C using RedaC:T-seq.

N4-acetylcytidine (ac4C) is an mRNA modification catalyzed by the enzyme N-acetyltransferase 10 (NAT10), with position-dependent effects on mRNA translation. This protocol details a procedure to map ac4C at base resolution using NaBH4-induced reduction of ac4C and conversion to thymidine followed by sequencing (RedaC:T-seq). Total RNA is ribodepleted and then treated with NaBH4 to reduce ac4C to tetrahydro-ac4C, which specifically alters base pairing during cDNA synthesis, allowing the detection of ac4C at positions called as thymidine following Illumina sequencing. For complete details on the use and execution of this protocol, please refer to Arango et al. (2022).1.

Immunoprecipitation and Sequencing of Acetylated RNA.

Generation of the epitranscriptome through chemical modifications of protein-coding messenger RNAs (mRNAs) has emerged as a new mechanism of post-transcriptional gene regulation. While most mRNA modifications are methylation events, a single acetylated ribonucleoside has been described in eukaryotes, occurring at the N4-position of cytidine (N4-acetylcytidine or ac4C). Using a combination of antibody-based enrichment of acetylated regions and deep sequencing, we recently reported ac4C as a novel mRNA modification that is catalyzed by the N-acetyltransferase enzyme NAT10. In this protocol, we describe in detail the procedures to identify acetylated mRNA regions transcriptome-wide using acetylated RNA immunoprecipitation and sequencing (acRIP-seq).

TET-Catalyzed 5-Carboxylcytosine Promotes CTCF Binding to Suboptimal Sequences Genome-wide.

The mechanisms supporting dynamic regulation of CTCF-binding sites remain poorly understood. Here we describe the TET-catalyzed 5-methylcytosine derivative, 5-carboxylcytosine (5caC), as a factor driving new CTCF binding within genomic DNA. Through a combination of in vivo and in vitro approaches, we reveal that 5caC generally strengthens CTCF association with DNA and facilitates binding to suboptimal sequences. Dramatically, profiling of CTCF binding in a cellular model that accumulates genomic 5caC identified ~13,000 new CTCF sites. The new sites were enriched for overlapping 5caC and were marked by an overall reduction in CTCF motif strength. As CTCF has multiple roles in gene expression, these findings have wide-reaching implications and point to induced 5caC as a potential mechanism to achieve differential CTCF binding in cells.

Rbfox1 up-regulation impairs BDNF-dependent hippocampal LTP by dysregulating TrkB isoform expression levels.

Brain-derived neurotrophic factor (BDNF) is a potent modulator of brain synaptic plasticity. Signaling defects caused by dysregulation of its Ntrk2 (TrkB) kinase (TrkB.FL) and truncated receptors (TrkB.T1) have been linked to the pathophysiology of several neurological and neurodegenerative disorders. We found that upregulation of Rbfox1, an RNA binding protein associated with intellectual disability, epilepsy and autism, increases selectively hippocampal TrkB.T1 isoform expression. Physiologically, increased Rbfox1 impairs BDNF-dependent LTP which can be rescued by genetically restoring TrkB.T1 levels. RNA-seq analysis of hippocampi with upregulation of Rbfox1 in conjunction with the specific increase of TrkB.T1 isoform expression also shows that the genes affected by Rbfox1 gain of function are surprisingly different from those influenced by Rbfox1 deletion. These findings not only identify TrkB as a major target of Rbfox1 pathophysiology but also suggest that gain or loss of function of Rbfox1 regulate different genetic landscapes.

Profiling Cytidine Acetylation with Specific Affinity and Reactivity.

The human acetyltransferase NAT10 has recently been shown to catalyze formation of N4-acetylcytidine (ac4C), a minor nucleobase known to alter RNA structure and function. In order to better understand the role of RNA acetyltransferases in biology and disease, here we report the development and application of chemical methods to study ac4C. First, we demonstrate that ac4C can be conjugated to carrier proteins using optimized protocols. Next, we describe methods to access ac4C-containing RNAs, enabling the screening of anti-ac4C antibodies. Finally, we validate the specificity of an optimized ac4C affinity reagent in the context of cellular RNA by demonstrating its ability to accurately report on chemical deacetylation of ac4C. Overall, these studies provide a powerful new tool for studying ac4C in biological contexts, as well as new insights into the stability and half-life of this highly conserved RNA modification. More broadly, they demonstrate how chemical reactivity may be exploited to aid the development and validation of nucleobase-targeting affinity reagents designed to target the emerging epitranscriptome.