Re-evaluating microglia expression profiles using RiboTag and cell isolation strategies.

Transcriptome profiling is widely used to infer functional states of specific cell types, as well as their responses to stimuli, to define contributions to physiology and pathophysiology. Focusing on microglia, the brain's macrophages, we report here a side-by-side comparison of classical cell-sorting-based transcriptome sequencing and the 'RiboTag' method, which avoids cell retrieval from tissue context and yields translatome sequencing information. Conventional whole-cell microglial transcriptomes were found to be significantly tainted by artifacts introduced by tissue dissociation, cargo contamination and transcripts sequestered from ribosomes. Conversely, our data highlight the added value of RiboTag profiling for assessing the lineage accuracy of Cre recombinase expression in transgenic mice. Collectively, this study indicates method-based biases, reveals observer effects and establishes RiboTag-based translatome profiling as a valuable complement to standard sorting-based profiling strategies.

Gene Architecture and Sequence Composition Underpin Selective Dependency of Nuclear Export of Long RNAs on NXF1 and the TREX Complex.

The core components of the nuclear RNA export pathway are thought to be required for export of virtually all polyadenylated RNAs. Here, we depleted different proteins that act in nuclear export in human cells and quantified the transcriptome-wide consequences on RNA localization. Different genes exhibited substantially variable sensitivities, with depletion of NXF1 and TREX components causing some transcripts to become strongly retained in the nucleus while others were not affected. Specifically, NXF1 is preferentially required for export of single- or few-exon transcripts with long exons or high A/U content, whereas depletion of TREX complex components preferentially affects spliced and G/C-rich transcripts. Using massively parallel reporter assays, we identified short sequence elements that render transcripts dependent on NXF1 for their export and identified synergistic effects of splicing and NXF1. These results revise the current model of how nuclear export shapes the distribution of RNA within human cells.

Transcription Dynamics Regulate Poly(A) Tails and Expression of the RNA Degradation Machinery to Balance mRNA Levels.

Gene expression is regulated by the rates of synthesis and degradation of mRNAs, but how these processes are coordinated is poorly understood. Here, we show that reduced transcription dynamics of specific genes leads to enhanced m6A deposition, preferential activity of the CCR4-Not complex, shortened poly(A) tails, and reduced stability of the respective mRNAs. These effects are also exerted by internal ribosome entry site (IRES) elements, which we found to be transcriptional pause sites. However, when transcription dynamics, and subsequently poly(A) tails, are globally altered, cells buffer mRNA levels by adjusting the expression of mRNA degradation machinery. Stress-provoked global impediment of transcription elongation leads to a dramatic inhibition of the mRNA degradation machinery and massive mRNA stabilization. Accordingly, globally enhanced transcription, such as following B cell activation or glucose stimulation, has the opposite effects. This study uncovers two molecular pathways that maintain balanced gene expression in mammalian cells by linking transcription to mRNA stability.

SARS-CoV-2 uses a multipronged strategy to impede host protein synthesis.

The coronavirus SARS-CoV-2 is the cause of the ongoing pandemic of COVID-191. Coronaviruses have developed a variety of mechanisms to repress host mRNA translation to allow the translation of viral mRNA, and concomitantly block the cellular innate immune response2,3. Although several different proteins of SARS-CoV-2 have previously been implicated in shutting off host expression4-7, a comprehensive picture of the effects of SARS-CoV-2 infection on cellular gene expression is lacking. Here we combine RNA sequencing, ribosome profiling and metabolic labelling of newly synthesized RNA to comprehensively define the mechanisms that are used by SARS-CoV-2 to shut off cellular protein synthesis. We show that infection leads to a global reduction in translation, but that viral transcripts are not preferentially translated. Instead, we find that infection leads to the accelerated degradation of cytosolic cellular mRNAs, which facilitates viral takeover of the mRNA pool in infected cells. We reveal that the translation of transcripts that are induced in response to infection (including innate immune genes) is impaired. We demonstrate this impairment is probably mediated by inhibition of nuclear mRNA export, which prevents newly transcribed cellular mRNA from accessing ribosomes. Overall, our results uncover a multipronged strategy that is used by SARS-CoV-2 to take over the translation machinery and to suppress host defences.

High-resolution mapping of function and protein binding in an RNA nuclear enrichment sequence.

The functions of long RNAs, including mRNAs and long noncoding RNAs (lncRNAs), critically depend on their subcellular localization. The identity of the sequences that dictate subcellular localization and their high-resolution anatomy remain largely unknown. We used a suite of massively parallel RNA assays and libraries containing thousands of sequence variants to pinpoint the functional features within the SIRLOIN element, which dictates nuclear enrichment through hnRNPK recruitment. In addition, we profiled the endogenous SIRLOIN RNA-nucleoprotein complex and identified the nuclear RNA-binding proteins SLTM and SNRNP70 as novel SIRLOIN binders. Taken together, using massively parallel assays, we identified the features that dictate binding of hnRNPK, SLTM, and SNRNP70 to SIRLOIN and found that these factors are jointly required for SIRLOIN activity. Our study thus provides a roadmap for high-throughput dissection of functional sequence elements in long RNAs.

Cap-independent translation and a precisely located RNA sequence enable SARS-CoV-2 to control host translation and escape anti-viral response.

Translation of SARS-CoV-2-encoded mRNAs by the host ribosomes is essential for its propagation. Following infection, the early expressed viral protein NSP1 binds the ribosome, represses translation, and induces mRNA degradation, while the host elicits an anti-viral response. The mechanisms enabling viral mRNAs to escape this multifaceted repression remain obscure. Here we show that expression of NSP1 leads to destabilization of multi-exon cellular mRNAs, while intron-less transcripts, such as viral mRNAs and anti-viral interferon genes, remain relatively stable. We identified a conserved and precisely located cap-proximal RNA element devoid of guanosines that confers resistance to NSP1-mediated translation inhibition. Importantly, the primary sequence rather than the secondary structure is critical for protection. We further show that the genomic 5'UTR of SARS-CoV-2 drives cap-independent translation and promotes expression of NSP1 in an eIF4E-independent and Torin1-resistant manner. Upon expression, NSP1 further enhances cap-independent translation. However, the sub-genomic 5'UTRs are highly sensitive to eIF4E availability, rendering viral propagation partially sensitive to Torin1. We conclude that the combined NSP1-mediated degradation of spliced mRNAs and translation inhibition of single-exon genes, along with the unique features present in the viral 5'UTRs, ensure robust expression of viral mRNAs. These features can be exploited as potential therapeutic targets.

Predictive models of subcellular localization of long RNAs.

Export to the cytoplasm is a key regulatory junction for both protein-coding mRNAs and long noncoding RNAs (lncRNAs), and cytoplasmic enrichment varies dramatically both within and between those groups. We used a new computational approach and RNA-seq data from human and mouse cells to quantify the genome-wide association between cytoplasmic/nuclear ratios of both gene groups and various factors, including expression levels, splicing efficiency, gene architecture, chromatin marks, and sequence elements. Splicing efficiency emerged as the main predictive factor, explaining up to a third of the variability in localization. Combination with other features allowed predictive models that could explain up to 45% of the variance for protein-coding genes and up to 34% for lncRNAs. Factors associated with localization were similar between lncRNAs and mRNAs with some important differences. Readily accessible features can thus be used to predict RNA localization.

Regulation of Neuronal Oxygen Responses in C. elegans Is Mediated through Interactions between Globin 5 and the H-NOX Domains of Soluble Guanylate Cyclases.

Soluble guanylate cyclases (sGCs) are gas-binding proteins that control diverse physiological processes such as vasodilation, platelet aggregation, and synaptic plasticity. In the nematode Caenorhabditis elegans, a complex of sGCs, GCY-35 and GCY-36, functions in oxygen (O2) sensing. Previous studies suggested that the neuroglobin GLB-5 genetically interacts with GCY-35, and that the inhibitory effect of GLB-5 on GCY-35 function is necessary for fast recovery from prolonged hypoxia. In this study, we identified mutations in gcy-35 and gcy-36 that impact fast recovery and other phenotypes associated with GLB-5, without undermining sGC activity. These mutations, heb1 and heb3, change conserved amino acid residues in the regulatory H-NOX domains of GCY-35 and GCY-36, respectively, and appear to suppress GLB-5 activity by different mechanisms. Moreover, we observed that short exposure to 35% O2 desensitized the neurons responsible for ambient O2 sensing and that this phenomenon does not occur in heb1 animals. These observations may implicate sGCs in neuronal desensitization mechanisms far beyond the specific case of O2 sensing in nematodes. The conservation of functionally important regions of sGCs is supported by examining site-directed mutants of GCY-35, which suggested that similar regions in the H-NOX domains of O2 and NO-sensing sGCs are important for heme/gas interactions. Overall, our studies provide novel insights into sGC activity and regulation, and implicate similar structural determinants in the control of both O2 and NO sensors. Significance statement: Soluble guanylate cyclases (sGCs) control essential and diverse physiological processes, including memory processing. We used Caenorhabditis elegans to explore how a neuroglobin inhibits a complex of oxygen-sensing sGCs, identifying sGC mutants that resist inhibition. Resistance appears to arise by two different mechanisms: increased basal sGC activity or disruption of an interaction with neuroglobin. Our findings demonstrate that the inhibition of sGCs by neuroglobin is essential for rapid adaptation to either low or high oxygen levels, and that similar structural regions are key for regulating both oxygen and nitric oxide sensors. Based on our structural and functional analyses, we present the hypothesis that neuroglobin-sGC interactions may be generally important for adaptation processes, including those in organisms with more complex neurological functions.

Comparative analysis between single-cell RNA-seq and single-molecule RNA FISH indicates that the pyrimidine nucleobase idoxuridine (IdU) globally amplifies transcriptional noise.

Stochastic fluctuations (noise) in transcription generate substantial cell-to-cell variability, but the physiological roles of noise have remained difficult to determine in the absence of generalized noise-modulation approaches. Previous single-cell RNA-sequencing (scRNA-seq) suggested that the pyrimidine-base analog (5'-iodo-2'-deoxyuridine, IdU) could generally amplify noise without substantially altering mean-expression levels but scRNA-seq technical drawbacks potentially obscured the penetrance of IdU-induced transcriptional noise amplification. Here we quantify global-vs.-partial penetrance of IdU-induced noise amplification by assessing scRNA-seq data using numerous normalization algorithms and directly quantifying noise using single-molecule RNA FISH (smFISH) for a panel of genes from across the transcriptome. Alternate scRNA-seq analyses indicate IdU-induced noise amplification for ~90% of genes, and smFISH data verified noise amplification for ~90% of tested genes. Collectively, this analysis indicates which scRNA-seq algorithms are appropriate for quantifying noise and argues that IdU is a globally penetrant noise-enhancer molecule that could enable investigations of the physiological impacts of transcriptional noise.

Expression of thioredoxin-1 in the ASJ neuron corresponds with and enhances intrinsic regenerative capacity under lesion conditioning in C. elegans.

A conditioning lesion of the peripheral sensory axon triggers robust central axon regeneration in mammals. We trigger conditioned regeneration in the Caenorhabditis elegans ASJ neuron by laser surgery or genetic disruption of sensory pathways. Conditioning upregulates thioredoxin-1 (trx-1) expression, as indicated by trx-1 promoter-driven expression of green fluorescent protein and fluorescence in situ hybridization (FISH), suggesting trx-1 levels and associated fluorescence indicate regenerative capacity. The redox activity of trx-1 functionally enhances conditioned regeneration, but both redox-dependent and -independent activity inhibit non-conditioned regeneration. Six strains isolated in a forward genetic screen for reduced fluorescence, which suggests diminished regenerative potential, also show reduced axon outgrowth. We demonstrate an association between trx-1 expression and the conditioned state that we leverage to rapidly assess regenerative capacity.

Nucleoporin-93 reveals a common feature of aggressive breast cancers: robust nucleocytoplasmic transport of transcription factors.

By establishing multi-omics pipelines, we uncover overexpression and gene copy-number alterations of nucleoporin-93 (NUP93), a nuclear pore component, in aggressive human mammary tumors. NUP93 overexpression enhances transendothelial migration and matrix invasion in vitro, along with tumor growth and metastasis in animal models. These findings are supported by analyses of two sets of naturally occurring mutations: rare oncogenic mutations and inactivating familial nephrotic syndrome mutations. Mechanistically, NUP93 binds with importins, boosts nuclear transport of importins' cargoes, such as ß-catenin, and activates MYC. Likewise, NUP93 overexpression enhances the ultimate nuclear transport step shared by additional signaling pathways, including TGF-ß/SMAD and EGF/ERK. The emerging addiction to nuclear transport exposes vulnerabilities of NUP93-overexpressing tumors. Congruently, myristoylated peptides corresponding to the nuclear translocation signals of SMAD and ERK can inhibit tumor growth and metastasis. Our study sheds light on an emerging hallmark of advanced tumors, which derive benefit from robust nucleocytoplasmic transport.

Ferritin is regulated by a neuro-intestinal axis in the nematode Caenorhabditis elegans.

Iron is vital for the life of most organisms. However, when dysregulated, iron can catalyze the formation of oxygen (O2) radicals that can destroy any biological molecule and thus lead to oxidative injury and death. Therefore, iron metabolism must be tightly regulated at all times, as well as coordinated with the metabolism of O2. However, how is this achieved at the whole animal level is not well understood. Here, we explore this question using the nematode Caenorhabditis elegans. Exposure of worms to O2 starvation conditions (i.e. hypoxia) induces a major upregulation in levels of the conserved iron-cage protein ferritin 1 (ftn-1) in the intestine, while exposure to 21% O2 decreases ftn-1 level. This O2-dependent inhibition is mediated by O2-sensing neurons that communicate with the intestine through neurotransmitter and neuropeptide signalling, and requires the activity of hydroxylated HIF-1. By contrast, the induction of ftn-1 in hypoxia appears to be HIF-1-independent. This upregulation provides protection against Pseudomonas aeruginosa bacteria and oxidative injury. Taken together, our studies uncover a neuro-intestine axis that coordinates O2 and iron responses at the whole animal level.

Characterization of gene expression associated with the adaptation of the nematode C. elegans to hypoxia and reoxygenation stress reveals an unexpected function of the neuroglobin GLB-5 in innate immunity.

Oxygen (O2) is a double-edged sword to cells, for while it is vital for energy production in all aerobic animals and insufficient O2 (hypoxia) can lead to cell death, the reoxygenation of hypoxic tissues may trigger the generation of reactive oxygen species (ROS) that can destroy any biological molecule. Indeed, both hypoxia and hypoxia-reoxygenation (H/R) stress are harmful, and may play a critical role in the pathophysiology of many human diseases, such as myocardial ischemia and stroke. Therefore, understanding how animals adapt to hypoxia and H/R stress is critical for developing better treatments for these diseases. Previous studies showed that the neuroglobin GLB-5(Haw) is essential for the fast recovery of the nematode Caenorhabditis elegans (C. elegans) from H/R stress. Here, we characterize the changes in neuronal gene expression during the adaptation of worms to hypoxia and recovery from H/R stress. Our analysis shows that innate immunity genes are differentially expressed during both adaptation to hypoxia and recovery from H/R stress. Moreover, we reveal that the prolyl hydroxylase EGL-9, a known regulator of both adaptation to hypoxia and the innate immune response, inhibits the fast recovery from H/R stress through its activity in the O2-sensing neurons AQR, PQR, and URX. Finally, we show that GLB-5(Haw) acts in AQR, PQR, and URX to increase the tolerance of worms to Pseudomonas aeruginosa pathogenesis. Together, our studies suggest that innate immunity and recovery from H/R stress are regulated by overlapping signaling pathways.