Terminal Deoxynucleotidyl Transferase Mediated Production of Labeled Probes for Single-molecule FISH or RNA Capture

m6A Reader YTHDC1 Impairs Respiratory Syncytial Virus Infection by Downregulating Membrane CX3CR1 Expression

Respiratory syncytial virus (RSV) is the most prevalent cause of acute lower respiratory infection in young children. Currently, the first RSV vaccines are approved by the FDA. Recently, N6-methyladenosine (m6A) RNA methylation has been implicated in the regulation of the viral life cycle and replication of many viruses, including RSV. m6A methylation of RSV RNA has been demonstrated to promote replication and prevent anti-viral immune responses by the host. Whether m6A is also involved in viral entry and whether m6A can also affect RSV infection via different mechanisms than methylation of viral RNA is poorly understood. Here, we identify m6A reader YTH domain-containing protein 1 (YTHDC1) as a novel negative regulator of RSV infection. We demonstrate that YTHDC1 abrogates RSV infection by reducing the expression of RSV entry receptor CX3C motif chemokine receptor 1 (CX3CR1) on the cell surface of lung epithelial cells. Altogether, these data reveal a novel role for m6A methylation and YTHDC1 in the viral entry of RSV. These findings may contribute to the development of novel treatment options to control RSV infection.

Distinct roles of LARP1 and 4EBP1/2 in regulating translation and stability of 5'TOP mRNAs

A central mechanism of mTOR complex 1 (mTORC1) signaling is the coordinated translation of ribosomal protein and translation factor mRNAs mediated by the 5'-terminal oligopyrimidine motif (5'TOP). Recently, La-related protein 1 (LARP1) was proposed to be the specific regulator of 5'TOP mRNA translation downstream of mTORC1, while eIF4E-binding proteins (4EBP1/2) were suggested to have a general role in translational repression of all transcripts. Here, we use single-molecule translation site imaging of 5'TOP and canonical mRNAs to study the translation of single mRNAs in living cells. Our data reveal that 4EBP1/2 has a dominant role in repression of translation of both 5'TOP and canonical mRNAs during pharmacological inhibition of mTOR. In contrast, we find that LARP1 selectively protects 5'TOP mRNAs from degradation in a transcriptome-wide analysis of mRNA half-lives. Our results clarify the roles of 4EBP1/2 and LARP1 in regulating 5'TOP mRNAs and provide a framework to further study how these factors control cell growth during development and disease.

Antiviral responses are shaped by heterogeneity in viral replication dynamics

Antiviral signalling, which can be activated in host cells upon virus infection, restricts virus replication and communicates infection status to neighbouring cells. The antiviral response is heterogeneous, both quantitatively (efficiency of response activation) and qualitatively (transcribed antiviral gene set). To investigate the basis of this heterogeneity, we combined Virus Infection Real-time IMaging (VIRIM), a live-cell single-molecule imaging method, with real-time readouts of the dsRNA sensing pathway to analyse the response of human cells to encephalomyocarditis virus (EMCV) infection. We find that cell-to-cell heterogeneity in viral replication rates early in infection affect the efficiency of antiviral response activation, with lower replication rates leading to more antiviral response activation. Furthermore, we show that qualitatively distinct antiviral responses can be linked to the strength of the antiviral signalling pathway. Our analyses identify variation in early viral replication rates as an important parameter contributing to heterogeneity in antiviral response activation.

Functional and molecular dissection of HCMV long non-coding RNAs

Small, compact genomes confer a selective advantage to viruses, yet human cytomegalovirus (HCMV) expresses the long non-coding RNAs (lncRNAs); RNA1.2, RNA2.7, RNA4.9, and RNA5.0. Little is known about the function of these lncRNAs in the virus life cycle. Here, we dissected the functional and molecular landscape of HCMV lncRNAs. We found that HCMV lncRNAs occupy ~ 30% and 50-60% of total and poly(A)+viral transcriptome, respectively, throughout virus life cycle. RNA1.2, RNA2.7, and RNA4.9, the three abundantly expressed lncRNAs, appear to be essential in all infection states. Among these three lncRNAs, depletion of RNA2.7 and RNA4.9 results in the greatest defect in maintaining latent reservoir and promoting lytic replication, respectively. Moreover, we delineated the global post-transcriptional nature of HCMV lncRNAs by nanopore direct RNA sequencing and interactome analysis. We revealed that the lncRNAs are modified with N⁶-methyladenosine (m⁶A) and interact with m⁶A readers in all infection states. In-depth analysis demonstrated that m⁶A machineries stabilize HCMV lncRNAs, which could account for the overwhelming abundance of viral lncRNAs. Our study lays the groundwork for understanding the viral lncRNA-mediated regulation of host-virus interaction throughout the HCMV life cycle.

Translation and Replication Dynamics of Single RNA Viruses

RNA viruses are among the most prevalent pathogens and are a major burden on society. Although RNA viruses have been studied extensively, little is known about the processes that occur during the first several hours of infection because of a lack of sensitive assays. Here we develop a single-molecule imaging assay, virus infection real-time imaging (VIRIM), to study translation and replication of individual RNA viruses in live cells. VIRIM uncovered a striking heterogeneity in replication dynamics between cells and revealed extensive coordination between translation and replication of single viral RNAs. Furthermore, using VIRIM, we identify the replication step of the incoming viral RNA as a major bottleneck of successful infection and identify host genes that are responsible for inhibition of early virus replication. Single-molecule imaging of virus infection is a powerful tool to study virus replication and virus-host interactions that may be broadly applicable to RNA viruses.

Single-Molecule Imaging Reveals Translation of mRNAs Localized to Stress Granules

Cellular stress leads to reprogramming of mRNA translation and formation of stress granules (SGs), membraneless organelles consisting of mRNA and RNA-binding proteins. Although the function of SGs remains largely unknown, it is widely assumed they contain exclusively non-translating mRNA. Here, we re-examine this hypothesis using single-molecule imaging of mRNA translation in living cells. Although we observe non-translating mRNAs are preferentially recruited to SGs, we find unequivocal evidence that mRNAs localized to SGs can undergo translation. Our data indicate that SG-associated translation is not rare, and the entire translation cycle (initiation, elongation, and termination) can occur on SG-localized transcripts. Furthermore, translating mRNAs can be observed transitioning between the cytosol and SGs without changing their translational status. Together, these results demonstrate that mRNA localization to SGs is compatible with translation and argue against a direct role for SGs in inhibition of protein synthesis.

Probing the Conformational State of mRNPs Using smFISH and SIM

mRNAs and lncRNAs assemble with RNA-binding proteins (RBPs) to form ribonucleoprotein complexes (RNPs ). The assembly of RNPs initiates co-transcriptionally, and their composition and organization is thought to change during the different steps of an RNP life cycle. Modulation of RNP structural organization has been implicated in the regulation of different aspects of RNA metabolism, including establishing interactions between the 5' and 3' ends in regulating mRNA translation and turnover. In this chapter, we describe a single-molecule microscopy approach that combines fluorescent RNA in situ hybridization (smFISH) and structured illumination microscopy (SIM ) and allows to measure different aspects of RNP organization in cells, including distances between different regions within individual mRNAs, as well as the overall compaction state of RNAs in different subcellular compartments and environmental conditions. Moreover, we describe a detailed workflow required for image registration and analysis that allows determining distances at sub-diffraction resolution.

Transcript specific mRNP capture from Drosophila egg-chambers for proteomic analysis

mRNA binding proteins (RBPs) play a major role in post-transcriptional control of gene expression. To understand the complex regulatory processes regulating a specific mRNA during its life-time, a comprehensive view of the bound RBPs is essential. Here, we describe a method for transcript-specific isolation of endogenous ribonucleoprotein complexes (RNPs) from Drosophila egg-chambers. The method, which is based on in-solution hybridization of short biotinylated antisense DNA oligonucleotide probes to multiple segments of a transcript of interest allows unbiased identification of associated proteins by quantitative proteomics.

Staufen2-mediated RNA recognition and localization requires combinatorial action of multiple domains

Throughout metazoans, Staufen (Stau) proteins are core factors of mRNA localization particles. They consist of three to four double-stranded RNA binding domains (dsRBDs) and a C-terminal dsRBD-like domain. Mouse Staufen2 (mStau2)-like Drosophila Stau (dmStau) contains four dsRBDs. Existing data suggest that only dsRBDs 3-4 are necessary and sufficient for mRNA binding. Here, we show that dsRBDs 1 and 2 of mStau2 bind RNA with similar affinities and kinetics as dsRBDs 3 and 4. While RNA binding by these tandem domains is transient, all four dsRBDs recognize their target RNAs with high stability. Rescue experiments in Drosophila oocytes demonstrate that mStau2 partially rescues dmStau-dependent mRNA localization. In contrast, a rescue with mStau2 bearing RNA-binding mutations in dsRBD1-2 fails, confirming the physiological relevance of our findings. In summary, our data show that the dsRBDs 1-2 play essential roles in the mRNA recognition and function of Stau-family proteins of different species.

Single-molecule Fluorescence in situ Hybridization (smFISH) for RNA Detection in Adherent Animal Cells

Transcription and RNA decay play critical roles in the process of gene expression and the ability to accurately measure cellular mRNA levels is essential for understanding this regulation. Here, we describe a single-molecule fluorescent in situ hybridization (smFISH) method (as performed in Haimovich et al., 2017 ) that detects single RNA molecules in individual cells. This technique employs multiple single-stranded, fluorescent labeled, short DNA probes that hybridize to target RNAs in fixed cells, allowing for both the quantification and localization of cytoplasmic and nuclear RNAs at the single-cell level and single-molecule resolution. Analyzing smFISH data provides absolute quantitative data of the number of cytoplasmic ("mature") mRNAs, the number of nascent RNA molecules at distinct transcription sites, and the spatial localization of these RNAs in the cytoplasm and/or nucleoplasm.

Transposon silencing in the Drosophila female germline is essential for genome stability in progeny embryos

The Piwi-interacting RNA pathway functions in transposon control in the germline of metazoans. The conserved RNA helicase Vasa is an essential Piwi-interacting RNA pathway component, but has additional important developmental functions. Here, we address the importance of Vasa-dependent transposon control in the Drosophila female germline and early embryos. We find that transient loss of vasa expression during early oogenesis leads to transposon up-regulation in supporting nurse cells of the fly egg-chamber. We show that elevated transposon levels have dramatic consequences, as de-repressed transposons accumulate in the oocyte where they cause DNA damage. We find that suppression of Chk2-mediated DNA damage signaling in vasa mutant females restores oogenesis and egg production. Damaged DNA and up-regulated transposons are transmitted from the mother to the embryos, which sustain severe nuclear defects and arrest development. Our findings reveal that the Vasa-dependent protection against selfish genetic elements in the nuage of nurse cell is essential to prevent DNA damage-induced arrest of embryonic development.