Biology subjects

Cullen, B. R.

Publications and source records attributed to Cullen, B. R..

9 recordsLinked to original sources

bioRxiv2022

RNA conformational propensities determine cellular activity

Cellular processes are the product of interactions between biomolecules, which associate to form biologically active complexes 1. These interactions are mediated by intermolecular contacts, which if disrupted, lead to alterations in cell physiology. Nevertheless, the formation of intermolecular contacts nearly universally requires changes in the conformations of the interacting biomolecules. As a result, binding affinity and cellular activity crucially depend not only on the strength of the contacts, but also on the inherent propensities to form binding-competent conformational states2,3. Thus, conformational penalties are ubiquitous in biology and must be known in order to quantitatively model binding energetics for protein and nucleic acid interactions4,5. However, conceptual and technological limitations have hindered our ability to dissect and quantitatively measure how conformational propensities impact cellular activity. Here, we systematically altered and determined the propensities for forming the protein-bound conformation of HIV-1 TAR RNA. These propensities quantitatively predicted the binding affinities of TAR to the RNA-binding region of the Tat protein and predicted the extent of HIV-1 Tat-dependent transactivation in cells. Our results establish the role of ensemble-based conformational propensities in cellular activity and reveal an example of a cellular process driven by an exceptionally rare and short-lived RNA conformational state.

biophysics↗

bioRxiv2021

Mapping of pseudouridine residues on cellular and viral transcripts using a novel antibody-based technique

Pseudouridine ({Psi}) is the most common non-canonical ribonucleoside present on mammalian non-coding RNAs (ncRNAs), including rRNAs, tRNAs and snRNAs, where it contributes [~]7% of the total uridine level. However, {Psi} constitutes only [~]0.1% of the uridines present on mRNAs and its effect on mRNA function remains unclear. {Psi} residues have been shown to inhibit the detection of exogenous RNA transcripts by host innate immune factors, thus raising the possibility that viruses might have subverted the addition of {Psi} residues to mRNAs by host pseudouridine synthase (PUS) enzymes as a way to inhibit antiviral responses in infected cells. Here, we describe and validate a novel antibody-based {Psi} mapping technique called photo-crosslinking assisted {Psi} sequencing (PA-{Psi}-seq) and use it to map {Psi} residues on not only multiple cellular RNAs but also on the mRNAs and genomic RNA encoded by HIV-1. We describe several 293T-derived cell lines in which human PUS enzymes previously reported to add {Psi} residues to human mRNAs, specifically PUS1, PUS7 and TRUB1/PUS4, were inactivated by gene editing. Surprisingly, while this allowed us to assign several sites of {Psi} addition on cellular mRNAs to each of these three PUS enzymes, the {Psi} sites present on HIV-1 transcripts remained unaffected. Moreover, loss of PUS1, PUS7 or TRUB1 function did not significantly reduce the level of {Psi} residues detected on total human mRNA below the [~]0.1% level seen in wild type cells, thus implying that the PUS enzyme(s) that adds the bulk of {Psi} residues to human mRNAs remains to be defined.

molecular biology↗

bioRxiv2021

Epitranscriptomic addition of m6A regulates HIV-1 RNA stability and alternative splicing

Previous work in several laboratories has demonstrated that the epitranscriptomic addition of m6A to viral transcripts promotes the replication and pathogenicity of a wide range of DNA and RNA viruses, yet the underlying mechanisms responsible for this positive effect have remained unclear. It is known that m6A function is largely mediated by cellular m6A binding proteins or readers, yet how m6A readers regulate viral gene expression in general, and HIV-1 gene expression in particular, has been controversial. Here, we confirm that m6A addition indeed regulates HIV-1 RNA expression and demonstrate that this effect is in large part mediated by the the nuclear m6A reader YTHDC1 and the cytoplasmic m6A reader YTHDF2. Both YTHDC1 and YTHDF2 bind to multiple distinct and overlapping sites on the HIV-1 RNA genome, with YTHDC1 recruitment serving to regulate the alternative splicing of HIV-1 RNAs while YTHDF2 binding correlates with increased HIV-1 transcript stability. Author SummaryThis manuscript reports that the expression of mRNAs encoded by the pathogenic human retrovirus HIV-1 is regulated by the methylation of a small number of specific adenosine residues. These in turn recruit a nuclear RNA binding protein, called YTHDC1, which modulates the alternative splicing of HIV-1 transcripts, as well as a cytoplasmic RNA binding protein, called YTHDF2, which stabilizes viral mRNAs. The regulation of HIV-1 gene expression by adenosine methylation is therefore critical for the effective and ordered expression of HIV-1 mRNAs and could represent a novel target for antiviral development.

molecular biology↗

bioRxiv2021

Tax induces the recruitment of NF-kB to unintegrated HIV-1 DNA to rescue viral gene expression and replication

We have previously reported that the normally essential step of integration of the HIV-1 proviral DNA intermediate into the host cell genome becomes dispensable in T cells that express the Human T cell leukemia virus 1 (HTLV-1) Tax protein. The rescue of integrase (IN) deficient HIV-1 replication by Tax results from the strong activation of transcription from the long terminal repeat (LTR) promoter on episomal HIV-1 DNA, an effect that is closely correlated with the recruitment of activating epigenetic marks, such as H3Ac, and depletion of repressive epigenetic marks, such as H3K9me3, from chromatinized unintegrated proviruses. In addition, activation of transcription from unintegrated HIV-1 DNA coincides with the recruitment of NF-kB to the two NF-kB binding sites found in the HIV-1 LTR enhancer. Here we report that the recruitment of NF-kB to unintegrated viral DNA precedes, and is a prerequisite for, Tax-induced changes in epigenetic marks, so that an IN-HIV-1 mutant lacking both LTR NF-kB sites is entirely non-responsive to Tax and fails to undergo the epigenetic changes listed above. We also report that heterologous promoters introduced into IN-HIV-1-based vectors are transcriptionally active even in the absence of Tax. Finally, we failed to reproduce a recent report arguing that heterologous promoters introduced into IN-vectors based on HIV-1 are more active if the HIV-1 promoter and enhancer, located in the LTR U3 region, are deleted, in a so-called self inactivating or SIN lentivector design. ImportanceIntegrase-deficient expression vectors based on HIV-1 are becoming increasingly popular as tools for gene therapy in vivo due to their inability to cause insertional mutagenesis. However, many IN-lentiviral vectors are able to achieve only low levels of gene expression and methods to increase this low level have not been extensively explored. Here we analyze how the HTLV-1 Tax protein is able to rescue the replication of IN-HIV-1 in T cells and describe IN-lentiviral vectors that are able to express a heterologous gene effectively.

molecular biology↗

bioRxiv2020

Revealing the high propensity of RNAs to non-specifically bind drug-like small molecules

Identifying small molecules that selectively bind a single RNA target while discriminating against all other cellular RNAs is an important challenge in RNA-targeted drug discovery. Much effort has been directed toward identifying drug-like small molecules that minimize electrostatic and stacking interactions that lead to non-specific binding of aminoglycosides and intercalators to a variety of RNAs. Many such compounds have been reported to bind RNAs and inhibit their cellular activities, however the ability of such compounds to discriminate against RNA stem-loops commonly found in the transcriptome has not been thoroughly assessed in all cases. Here, we examined the propensities of three drug-like compounds, previously shown to bind and inhibit the cellular activity of three distinct RNAs, to non-specifically bind two HIV-1 stem-loop RNAs: the transactivation response element (TAR) and stem IIB in the rev response element (RREIIB). All three compounds bound to TAR and RREIIB in vitro, and two inhibited TAR-dependent transactivation and RRE-dependent viral export in cell-based assays while also exhibiting substantial off-target interactions consistent with non-specific cellular activity. A survey of X-ray and NMR structures of RNA-small molecule complexes revealed that drug-like molecules form hydrogen bonds with functional groups commonly accessible in canonical stem-loop RNA motifs, much like aminoglycosides, and in contrast to ligands that specifically bind riboswitches. Our results support extending the group of non-selective RNA-binders beyond aminoglycosides and intercalators to encompass drug-like compounds with capacity for non-specific hydrogen-bonding and reinforce the importance of assaying for off-target interactions and RNA selectivity in vitro and in cells when assessing novel RNA-binders.

biochemistry↗

bioRxiv2020

Acetylation of cytidine residues boosts HIV-1 gene expression by increasing viral RNA stability

Covalent modifications added to individual nucleotides on mRNAs, called epitranscriptomic modifications, have recently emerged as key regulators of both cellular and viral mRNA function1,2 and RNA methylation has now been shown to enhance the replication of human immunodeficiency virus 1 (HIV-1) and several other viruses3-11. Recently, acetylation of the N4 position of cytidine (ac4C) was reported to boost cellular mRNA function by increasing mRNA translation and stability12. We therefore hypothesized that ac4C and N-acetyltransferase 10 (NAT10), the cellular enzyme that adds ac4C to RNAs, might also have been subverted by HIV-1 to increase viral gene expression. We now confirm that HIV-1 transcripts are indeed modified by addition of ac4C at multiple discreet sites and demonstrate that silent mutagenesis of a subset of these ac4C addition sites inhibits HIV-1 gene expression in cis. Moreover, reduced expression of NAT10, and the concomitant decrease in the level of ac4C on viral RNAs, inhibits HIV-1 replication by reducing HIV-1 RNA stability. Interestingly Remodelin, a previously reported inhibitor of NAT10 function13,14, also inhibits HIV-1 replication without affecting cell viability, thus raising the possibility that the addition of ac4C to viral mRNAs might emerge as a novel cellular target for antiviral drug development.

microbiology↗

bioRxiv2019

Stabilizing non-native excited states as a therapeutic strategy for targeting RNA

Many regulatory RNAs undergo changes in their structure from the dominant ground-state (GS) toward short-lived low-abundance excited-states (ES) that reorganize local elements of secondary structure. ESs are increasingly observed in vitro and implicated in the folding and biological activities of regulatory RNAs and as targets for developing therapeutics. However, whether these ESs also form with comparable abundance within the complex cellular environment remains unknown. Here, we developed an approach for assessing the relative stability and abundance of RNA ESs within the functional cellular context. The approach uses point substitution mutations to increase the population of an inactive ES relative to the active GS. The cellular activity of such ES-stabilizing mutants then provides an indirect measure of any residual population of the active GS within the functional cellular context. Compensatory rescue mutations that restore the GS are used to control for changes in cellular activity arising due to changes in sequence. The approach is applied to probe ESs in two highly conserved and functionally important regulatory RNAs from HIV-1: the transactivation response element (TAR) and the Rev response element (RRE). For both RNAs, ES-stabilizing mutations inhibited cellular activity to a degree that correlates with the extent to which they stabilize the ES relative to the GS in vitro. These results indicate that the non-native ESs of TAR and RRE likely form in cells with abundances comparable to those measured in vitro and their targeted stabilization provides a new avenue for developing anti-HIV therapeutics.

biochemistry↗

bioRxiv2019

Extensive epitranscriptomic methylation of A and C residues on murine leukemia virus transcripts enhances viral gene expression

While it has been known for several years that viral RNAs are subject to the addition of several distinct covalent modifications to individual nucleotides, collectively referred to as epitranscriptomic modifications, the effect of these editing events on viral gene expression has been controversial. Here, we report the purification of murine leukemia virus (MLV) genomic RNA to homogeneity and show that this viral RNA contains levels of N6-methyladenosine (m6A), 5-methylcytosine (m5C) and 2O-methylated (Nm) ribonucleotides that are an order of magnitude higher than detected on bulk cellular mRNAs. Mapping of m6A and m5C residues on MLV transcripts identified multiple discrete editing sites and allowed the construction of MLV variants bearing silent mutations that removed a subset of these sites. Analysis of the replication potential of these mutants revealed a modest but significant attenuation in viral replication in 3T3 cells in culture. Consistent with a positive role for m6A and m5C in viral replication, we also demonstrate that overexpression of the key m6A reader protein YTHDF2 enhances MLV replication, while downregulation of the m5C writer NSUN2 inhibits MLV replication.\n\nImportanceThe data presented in this manuscript demonstrate that MLV RNAs bear an exceptionally high level of the epitranscriptomic modifications m6A, m5C and Nm, thus suggesting that these each facilitate some aspect of the viral replication cycle. Consistent with this hypothesis, we demonstrate that mutational removal of a subset of these m6A or m5C modifications from MLV transcripts inhibits MLV replication in cis and a similar result was also observed upon manipulation of the level of expression of key cellular epitranscriptomic cofactors in trans. Together, these results argue that the addition of several different epitranscriptomic modifications to viral transcripts stimulates viral gene expression and suggest that MLV has therefore evolved to maximize the level of these modifications that are added to viral RNAs.

microbiology↗

bioRxiv2017

The Epstein-Barr Virus miR-BHRF1 microRNAs Regulate Viral Gene Expression in cis

The Epstein-Barr virus (EBV) miR-BHRF1 microRNA (miRNA) cluster has been shown to facilitate B-cell transformation and promote the rapid growth of the resultant lymphoblastoid cell lines (LCLs). However, we find that expression of physiological levels of the miR-BHRF1 miRNAs in LCLs transformed with a miR-BHRF1 null mutant ({triangleup}123) fails to increase their growth rate. We demonstrate that the pri-miR-BHRF1-2 and 1-3 stem-loops are present in the 3UTR of transcripts encoding EBNA-LP and that excision of pre-miR-BHRF1-2 and 1-3 by Drosha destabilizes these mRNAs and reduces expression of the encoded protein. Therefore, mutational inactivation of primiR-BHRF1-2 and 1-3 in the {triangleup}123 mutant upregulates the expression of not only EBNA-LP but also EBNA-LP-regulated mRNAs and proteins, including LMP1. We hypothesize that this overexpression causes the reduced transformation capacity of the {triangleup}123 EBV mutant. Thus, in addition to regulating cellular mRNAs in trans, miR-BHRF1-2 and 1-3 also regulate EBNA-LP mRNA expression in cis.\n\nHighlightsO_LIThe EBV miR-BHRF1 microRNAs do not up upregulate B cell growth in trans.\nC_LIO_LIEBNA-LP expression is downregulated by pri-miR-BHRF1-2 and 1-3 acting in cis.\nC_LIO_LILoss of miR-BHRF1-2 and 1-3 causes EBNA-LP overexpression and inhibits B cell growth.\nC_LIO_LINovel alternative splicing of EBV Cp/Wp transcripts was identified.\nC_LI

microbiology↗