Virus induction of human IFN beta gene expression requires the assembly of an enhanceosome

Enhancer cooperativity can compensate for loss of activity over large genomic distances

Enhancers are short DNA sequences that activate their target promoter from a distance; however, increasing the genomic distance between the enhancer and the promoter decreases expression levels. Many genes are controlled by combinations of multiple enhancers, yet the interaction and cooperation of individual enhancer elements are not well understood. Here, we developed a synthetic platform in mouse embryonic stem cells that allows building complex regulatory landscapes from the bottom up. We tested the system by integrating individual enhancers at different distances and confirmed that the strength of an enhancer contributes to how strongly it is affected by increased genomic distance. Furthermore, synergy between two enhancer elements depends on the distance at which the two elements are integrated: introducing a weak enhancer between a strong enhancer and the promoter strongly increases reporter gene expression, allowing enhancers to activate from increased genomic distances.

Enhanced Tumor Immunotherapy by Triple Amplification Effects of Nanomedicine on the STING Signaling Pathway in Dendritic Cells

Insufficient activation of stimulator of interferon genes (STING) signaling pathway in tumor-associated dendritic cells limits the efficiency of tumor immunotherapy. Herein, the "three-in-one" IAHA-LaP/siPTPN6 NPs containing lanthanum ions (La3+), cGAMP, and PTPN6 siRNA are developed for triple amplification of the STING pathway. In vitro results demonstrate that La3+ significantly promotes cGAMP-mediated activation of the STING pathway by enhancing the phosphorylation of STING, TBK1, IRF3, and NF-κB p65. Moreover, the IAHA-LaP/siPTPN6 NPs further significantly enhance the phosphorylation of STING and NF-κB p65 and augment K63-linked ubiquitination of STING protein via siPTPN6-mediated downregulation of SHP-1 protein. Furthermore, NPs improve the secretion of IFNβ (2.4-fold), IL-6 (1.5-fold), and TNF-α (1.4-fold), thereby promoting DCs maturation compared to the mixture of La3+ and cGAMP. In vivo results show that the IAHA-LaP/siPTPN6 NPs remarkably inhibit primary tumor growth by increasing the percentage of mature DCs in tumor-draining lymph nodes, polarizing M2/M1 phenotype in TME, and promoting the infiltration of CD8+T cells into tumors. Moreover, these NPs dramatically prevent the growth of distal tumor by inducing systemic anti-tumor immunity and generating a long-term anti-tumor memory for protection against tumor recurrence in mice bearing bilateral B16F10. These IAHA-LaP/siPTPN6 NPs may offer a promising platform for robust anti-tumor immune responses.

Cellular RNA interacts with MAVS to promote antiviral signaling

Antiviral signaling downstream of RIG-I-like receptors (RLRs) proceeds through a multi-protein complex organized around the adaptor protein mitochondrial antiviral signaling protein (MAVS). Protein complex function can be modulated by RNA molecules that provide allosteric regulation or act as molecular guides or scaffolds. We hypothesized that RNA plays a role in organizing MAVS signaling platforms. We found that MAVS, through its central intrinsically disordered domain, directly interacted with the 3' untranslated regions of cellular messenger RNAs. Elimination of RNA by ribonuclease treatment disrupted the MAVS signalosome, including RNA-modulated MAVS interactors that regulate RLR signaling and viral restriction, and inhibited phosphorylation of transcription factors that induce interferons. This work uncovered a function for cellular RNA in promoting signaling through MAVS and highlights generalizable principles of RNA regulatory control of immune signaling complexes.

Single-molecule states link transcription factor binding to gene expression

The binding of multiple transcription factors (TFs) to genomic enhancers drives gene expression in mammalian cells1. However, the molecular details that link enhancer sequence to TF binding, promoter state and transcription levels remain unclear. Here we applied single-molecule footprinting2,3 to measure the simultaneous occupancy of TFs, nucleosomes and other regulatory proteins on engineered enhancer-promoter constructs with variable numbers of TF binding sites for both a synthetic TF and an endogenous TF involved in the type I interferon response. Although TF binding events on nucleosome-free DNA are independent, activation domains recruit cofactors that destabilize nucleosomes, driving observed TF binding cooperativity. Average TF occupancy linearly determines promoter activity, and we decompose TF strength into separable binding and activation terms. Finally, we develop thermodynamic and kinetic models that quantitatively predict both the enhancer binding microstates and gene expression dynamics. This work provides a template for the quantitative dissection of distinct contributors to gene expression, including TF activation domains, concentration, binding affinity, binding site configuration and recruitment of chromatin regulators.

Functional characteristics and computational model of abundant hyperactive loci in the human genome

Enhancers and promoters are classically considered to be bound by a small set of transcription factors (TFs) in a sequence-specific manner. This assumption has come under increasing skepticism as the datasets of ChIP-seq assays of TFs have expanded. In particular, high-occupancy target (HOT) loci attract hundreds of TFs with often no detectable correlation between ChIP-seq peaks and DNA-binding motif presence. Here, we used a set of 1003 TF ChIP-seq datasets (HepG2, K562, H1) to analyze the patterns of ChIP-seq peak co-occurrence in combination with functional genomics datasets. We identified 43,891 HOT loci forming at the promoter (53%) and enhancer (47%) regions. HOT promoters regulate housekeeping genes, whereas HOT enhancers are involved in tissue-specific process regulation. HOT loci form the foundation of human super-enhancers and evolve under strong negative selection, with some of these loci being located in ultraconserved regions. Sequence-based classification analysis of HOT loci suggested that their formation is driven by the sequence features, and the density of mapped ChIP-seq peaks across TF-bound loci correlates with sequence features and the expression level of flanking genes. Based on the affinities to bind to promoters and enhancers we detected five distinct clusters of TFs that form the core of the HOT loci. We report an abundance of HOT loci in the human genome and a commitment of 51% of all TF ChIP-seq binding events to HOT locus formation thus challenging the classical model of enhancer activity and propose a model of HOT locus formation based on the existence of large transcriptional condensates.

A DNA base-specific sequence interposed between CRX and NRL contributes to RHODOPSIN expression

Gene expression emerges from DNA sequences through the interaction of transcription factors (TFs) with DNA cis-regulatory sequences. In eukaryotes, TFs bind to transcription factor binding sites (TFBSs) with differential affinities, enabling cell-specific gene expression. In this view, DNA enables TF binding along a continuum ranging from low to high affinity depending on its sequence composition; however, it is not known whether evolution has entailed a further level of entanglement between DNA-protein interaction. Here we found that the composition and length (22 bp) of the DNA sequence interposed between the CRX and NRL retinal TFs in the proximal promoter of RHODOPSIN (RHO) largely controls the expression levels of RHO. Mutagenesis of CRX-NRL DNA linking sequences (here termed "DNA-linker") results in uncorrelated gene expression variation. In contrast, mutual exchange of naturally occurring divergent human and mouse Rho cis-regulatory elements conferred similar yet species-specific Rho expression levels. Two orthogonal DNA-binding proteins targeted to the DNA-linker either activate or repress the expression of Rho depending on the DNA-linker orientation relative to the CRX and NRL binding sites. These results argue that, in this instance, DNA itself contributes to CRX and NRL activities through a code based on specific base sequences of a defined length, ultimately determining optimal RHO expression levels.

Dissection of core promoter syntax through single nucleotide resolution modeling of transcription initiation

How the DNA sequence of cis-regulatory elements encode transcription initiation patterns remains poorly understood. Here we introduce CLIPNET, a deep learning model trained on population-scale PRO-cap data that predicts the position and quantity of transcription initiation with single nucleotide resolution from DNA sequence more accurately than existing approaches. Interpretation of CLIPNET revealed a complex regulatory syntax consisting of DNA-protein interactions in five major positions between -200 and +50 bp relative to the transcription start site, as well as more subtle positional preferences among transcriptional activators. Transcriptional activator and core promoter motifs work non-additively to encode distinct aspects of initiation, with the former driving initiation quantity and the latter initiation position. We identified core promoter motifs that explain initiation patterns in the majority of promoters and enhancers, including DPR motifs and AT-rich TBP binding sequences in TATA-less promoters. Our results provide insights into the sequence architecture governing transcription initiation.

Innate and adaptive immune responses that control lymph-borne viruses in the draining lymph node

The interstitial fluids in tissues are constantly drained into the lymph nodes (LNs) as lymph through afferent lymphatic vessels and from LNs into the blood through efferent lymphatics. LNs are strategically positioned and have the appropriate cellular composition to serve as sites of adaptive immune initiation against invading pathogens. However, for lymph-borne viruses, which disseminate from the entry site to other tissues through the lymphatic system, immune cells in the draining LN (dLN) also play critical roles in curbing systemic viral dissemination during primary and secondary infections. Lymph-borne viruses in tissues can be transported to dLNs as free virions in the lymph or within infected cells. Regardless of the entry mechanism, infected myeloid antigen-presenting cells, including various subtypes of dendritic cells, inflammatory monocytes, and macrophages, play a critical role in initiating the innate immune response within the dLN. This innate immune response involves cellular crosstalk between infected and bystander innate immune cells that ultimately produce type I interferons (IFN-Is) and other cytokines and recruit inflammatory monocytes and natural killer (NK) cells. IFN-I and NK cell cytotoxicity can restrict systemic viral spread during primary infections and prevent serious disease. Additionally, the memory CD8+ T-cells that reside or rapidly migrate to the dLN can contribute to disease prevention during secondary viral infections. This review explores the intricate innate immune responses orchestrated within dLNs that contain primary viral infections and the role of memory CD8+ T-cells following secondary infection or CD8+ T-cell vaccination.

Binding to the Other Side: The AT-Hook DNA-Binding Domain Allows Nuclear Factors to Exploit the DNA Minor Groove

The "AT-hook" is a peculiar DNA-binding domain that interacts with DNA in the minor groove in correspondence to AT-rich sequences. This domain has been first described in the HMGA protein family of architectural factors and later in various transcription factors and chromatin proteins, often in association with major groove DNA-binding domains. In this review, using a literature search, we identified about one hundred AT-hook-containing proteins, mainly chromatin proteins and transcription factors. After considering the prototypes of AT-hook-containing proteins, the HMGA family, we review those that have been studied in more detail and that have been involved in various pathologies with a particular focus on cancer. This review shows that the AT-hook is a domain that gives proteins not only the ability to interact with DNA but also with RNA and proteins. This domain can have enzymatic activity and can influence the activity of the major groove DNA-binding domain and chromatin docking modules when present, and its activity can be modulated by post-translational modifications. Future research on the function of AT-hook-containing proteins will allow us to better decipher their function and contribution to the different pathologies and to eventually uncover their mutual influences.

Plant enhancers exhibit both cooperative and additive interactions among their functional elements

Enhancers are cis-regulatory elements that shape gene expression in response to numerous developmental and environmental cues. In animals, several models have been proposed to explain how enhancers integrate the activity of multiple transcription factors. However, it remains largely unclear how plant enhancers integrate transcription factor activity. Here, we use Plant STARR-seq to characterize 3 light-responsive plant enhancers-AB80, Cab-1, and rbcS-E9-derived from genes associated with photosynthesis. Saturation mutagenesis revealed mutations, many of which clustered in short regions, that strongly reduced enhancer activity in the light, in the dark, or in both conditions. When tested in the light, these mutation-sensitive regions did not function on their own; rather, cooperative interactions with other such regions were required for full activity. Epistatic interactions occurred between mutations in adjacent mutation-sensitive regions, and the spacing and order of mutation-sensitive regions in synthetic enhancers affected enhancer activity. In contrast, when tested in the dark, mutation-sensitive regions acted independently and additively in conferring enhancer activity. Taken together, this work demonstrates that plant enhancers show evidence for both cooperative and additive interactions among their functional elements. This knowledge can be harnessed to design strong, condition-specific synthetic enhancers.

NF-κB p65 and TCF-4 interactions are associated with LPS-stimulated IL-6 secretion of macrophages

Proinflammatory cytokine plays a central role in host defense and acute inflammatory responses. Both positive and negative correlations of NF-κB and Wnt/β-catenin pathways have been reported depending on cell types in response to inflammatory stimuli for IL-6 cytokine production. Macrophages are vital to the regulation of immune responses and the development of inflammation, but the crosstalk between two pathways has not been elucidated so far in macrophages. We observed a positive cross-regulation between the NF-κB and Wnt/β-catenin pathways for IL-6 production in human macrophages. To verify the functional validity of this interaction, LY294002 or PNU74654, representative blockers of each pathway, were treated. IL-6 secretion was reduced to the basal level by both inhibitor treatments, even when stimulated by LPS. We also found that NF-κB p65 migrated to the nucleus and interacted with the transcription factor TCF-4 in macrophages upon LPS stimulation.

Two redundant transcription factor binding sites in a single enhancer are essential for mammalian sex determination

Male development in mammals depends on the activity of the two SOX gene: Sry and Sox9, in the embryonic testis. As deletion of Enhancer 13 (Enh13) of the Sox9 gene results in XY male-to-female sex reversal, we explored the critical elements necessary for its function and hence, for testis and male development. Here, we demonstrate that while microdeletions of individual transcription factor binding sites (TFBS) in Enh13 lead to normal testicular development, combined microdeletions of just two SRY/SOX binding motifs can alone fully abolish Enh13 activity leading to XY male-to-female sex reversal. This suggests that for proper male development to occur, these few nucleotides of non-coding DNA must be intact. Interestingly, we show that depending on the nature of these TFBS mutations, dramatically different phenotypic outcomes can occur, providing a molecular explanation for the distinct clinical outcomes observed in patients harboring different variants in the same enhancer.

M. Ahmed S, Alsafar HS, Halwani R. Multiple inborn errors of type I IFN immunity in a 33-year-old male with a fatal case of COVID-19

The host genetic inborn errors of immunity (IEIs) have been shown to contribute to susceptibility to life-threatening coronavirus disease 2019 (COVID-19), as it had been associated previously with other viral infections. Most genetic association studies have described IEIs as a monogenic defect, while there have been no reports of patients with multiple inherited immune deficiencies. This is a complex case of IEIs predisposing to severe viral infections in an unvaccinated 33-year-old male patient. The patient was admitted with no respiratory symptoms, showed a SARS-CoV-2 PCR positive test on the second day of admission, started developing progressive lung consolidation within three days of hospitalization, and was moved from non-invasive to mechanical ventilation within 12 days of hospitalization. Impaired production of type I IFN was detected in patient PBMCs treated with poly(I:C), at both mRNA and protein levels. Whole exome sequencing revealed three mutations across type I IFN production pathway, which were predicted to be loss-of-function (pLOF). The three mutations were predicted to predispose to severe viral infections: monoallelic R488X TLR3, monoallelic His684Arg TLR3, and biallelic Val363Met IRF3. Functional analysis confirmed that all these mutations dysregulated the type I IFN pathway. Evaluation of TLR3 and IRF3 IFN-β1 luciferase reporter activity showed a hypomorphic suppression of function. TOPO TA cloning was used to ascertain the positioning of both TLR3 variants, indicating that both variants were on the same allele. We have described a unique complex IEI patient with multiple mutations, particularly along type I IFN production pathway.

Synergistic innate immune activation and anti-tumor immunity through combined STING and TLR4 stimulation

Previous work has shown that innate immune sensing of tumors involves the host STING pathway, which leads to IFN-β production, dendritic cell (DC) activation, and T cell priming against tumor antigens. This observation has led to the development of STING agonists as a potential cancer therapeutic. However, despite promising results in mouse studies using transplantable tumor models, clinical testing of STING agonists has shown activity in only a minority of patients. Thus, further study of innate immune pathways in anti-tumor immunity is paramount. Innate immune activation in response to a pathogen rarely occurs through stimulation of only one signaling pathway, and activating multiple innate immune pathways similar to a natural infection is one possible strategy to improve the efficacy of STING agonists. To test this, we performed experiments with the STING agonist DMXAA alone or in combination with several TLR agonists. We found that LPS + DMXAA induced significantly greater IFN-β transcription than the sum of either agonist alone. To explain this synergy, we assayed each step of STING pathway signaling. LPS did not increase STING protein aggregation, IRF3 phosphorylation, or IRF3 nuclear translocation beyond what occurred with DMXAA alone. However, since the IFN-β promoter also includes NF-κB binding sites, we additionally examined the NF-κB pathway. In fact, LPS increased the phosphorylation and nuclear translocation of the NF-κB subunit p65, and NF-κB signaling was required for the observed synergy. Intratumoral injection of suboptimal doses of LPS + DMXAA resulted in significantly improved tumor control of B16 melanoma in vivo compared to either agonist alone. Our results suggest that combinatorial signaling through TLR4 and STING results in optimal innate signaling via co-involvement of NF-κB and IRF3, and that combined engagement of these two pathways has therapeutic potential.

Biophysical and functional characterization of N-terminal domain of Human Interferon Regulatory Factor 6

BACKGROUND

Interferon regulatory factor 6 (IRF6) has a key function in palate fusion during palatogenesis during embryonic development, and mutations in IRF6 cause orofacial clefting disorders.

METHODS AND RESULTS

The in silico analysis of IRF6 is done to obtain leads for the domain boundaries and subsequently the sub-cloning of the N-terminal domain of IRF6 into the pGEX-2TK expression vector and successfully optimized the overexpression and purification of recombinant glutathione S-transferase-fused NTD-IRF6 protein under native conditions. After cleavage of the GST tag, NTD-IRF6 was subjected to protein folding studies employing Circular Dichroism and Intrinsic fluorescence spectroscopy at variable pH, temperature, and denaturant. CD studies showed predominantly alpha-helical content and the highest stability of NTD-IRF6 at pH 9.0. A comparison of native and renatured protein depicts loss in the secondary structural content. Intrinsic fluorescence and quenching studies have identified that tryptophan residues are majorly present in the buried areas of the protein and a small fraction was on or near the protein surface. Upon the protein unfolding with a higher concentration of denaturant urea, the peak of fluorescence intensity decreased and red shifted, confirming that tryptophan residues are majorly present in a more polar environment. While regulating IFNβ gene expression during viral infection, the N-terminal domain binds to the promoter region of Virus Response Element-Interferon beta (VRE-IFNβ). Along with the protein folding analysis, this study also aimed to identify the DNA-binding activity and determine the binding affinities of NTD-IRF6 with the VRE-IFNβ promoter region. The protein-DNA interaction is specific as demonstrated by gel retardation assay and the kinetics of molecular interactions as quantified by Biolayer Interferometry showed a strong affinity with an affinity constant (KD) value of 7.96 × 10-10 M.

CONCLUSION

NTD-IRF6 consists of a mix of α-helix and β-sheets that show temperature-dependent cooperative unfolding between 40 °C and 55 °C. Urea-induced unfolding shows moderate tolerance to urea as the mid-transition concentration of urea (Cm) is 3.2 M. The tryptophan residues are majorly buried as depicted by fluorescence quenching studies. NTD-IRF6 has a specific and high affinity toward the promoter region of VRE-IFNβ.

Single-molecule chromatin configurations link transcription factor binding to expression in human cells

The binding of multiple transcription factors (TFs) to genomic enhancers activates gene expression in mammalian cells. However, the molecular details that link enhancer sequence to TF binding, promoter state, and gene expression levels remain opaque. We applied single-molecule footprinting (SMF) to measure the simultaneous occupancy of TFs, nucleosomes, and components of the transcription machinery on engineered enhancer/promoter constructs with variable numbers of TF binding sites for both a synthetic and an endogenous TF. We find that activation domains enhance a TF's capacity to compete with nucleosomes for binding to DNA in a BAF-dependent manner, TF binding on nucleosome-free DNA is consistent with independent binding between TFs, and average TF occupancy linearly contributes to promoter activation rates. We also decompose TF strength into separable binding and activation terms, which can be tuned and perturbed independently. Finally, we develop thermodynamic and kinetic models that quantitatively predict both the binding microstates observed at the enhancer and subsequent time-dependent gene expression. This work provides a template for quantitative dissection of distinct contributors to gene activation, including the activity of chromatin remodelers, TF activation domains, chromatin acetylation, TF concentration, TF binding affinity, and TF binding site configuration.

Mechanisms of type I interferon production by chicken TLR21

The innate immune response relies on the ability of host cells to rapidly detect and respond to microbial nucleic acids. Toll-like receptors (TLRs), a class of pattern recognition receptors (PRRs), play a fundamental role in distinguishing self from non-self at the molecular level. In this study, we focused on TLR21, an avian TLR that recognizes DNA motifs commonly found in bacterial genomic DNA, specifically unmethylated CpG motifs. TLR21 is believed to act as a functional homologue to mammalian TLR9. By analysing TLR21 signalling in chickens, we sought to elucidate avian TLR21 activation outputs in parallel to that of other nucleic acid species. Our analyses revealed that chicken TLR21 (chTLR21) triggers the activation of NF-κB and induces a potent type-I interferon response in chicken macrophages, similar to the signalling cascades observed in mammalian TLR9 activation. Notably, the transcription of interferon beta (IFNB) by chTLR21 was found to be dependent on both NF-κB and IRF7 signalling, but independent of the TBK1 kinase, a distinctive feature of mammalian TLR9 signalling. These findings highlight the conservation of critical signalling components and downstream responses between avian TLR21 and mammalian TLR9, despite their divergent evolutionary origins. These insights into the evolutionarily conserved mechanisms of nucleic acid sensing contribute to the broader understanding of host-pathogen interactions across species.

Affinity-optimizing enhancer variants disrupt development

Enhancers control the location and timing of gene expression and contain the majority of variants associated with disease1-3. The ZRS is arguably the most well-studied vertebrate enhancer and mediates the expression of Shh in the developing limb4. Thirty-one human single-nucleotide variants (SNVs) within the ZRS are associated with polydactyly4-6. However, how this enhancer encodes tissue-specific activity, and the mechanisms by which SNVs alter the number of digits, are poorly understood. Here we show that the ETS sites within the ZRS are low affinity, and identify a functional ETS site, ETS-A, with extremely low affinity. Two human SNVs and a synthetic variant optimize the binding affinity of ETS-A subtly from 15% to around 25% relative to the strongest ETS binding sequence, and cause polydactyly with the same penetrance and severity. A greater increase in affinity results in phenotypes that are more penetrant and more severe. Affinity-optimizing SNVs in other ETS sites in the ZRS, as well as in ETS, interferon regulatory factor (IRF), HOX and activator protein 1 (AP-1) sites within a wide variety of enhancers, cause gain-of-function gene expression. The prevalence of binding sites with suboptimal affinity in enhancers creates a vulnerability in genomes whereby SNVs that optimize affinity, even slightly, can be pathogenic. Searching for affinity-optimizing SNVs in genomes could provide a mechanistic approach to identify causal variants that underlie enhanceropathies.

Epigenetic Control of Innate Immunity: Consequences of Acute Respiratory Virus Infection

Infections caused by acute respiratory viruses induce a systemic innate immune response, which can be measured by the increased levels of expression of inflammatory genes in immune cells. There is growing evidence that these acute viral infections, alongside transient transcriptomic responses, induce epigenetic remodeling as part of the immune response, such as DNA methylation and histone modifications, which might persist after the infection is cleared. In this article, we first review the primary mechanisms of epigenetic remodeling in the context of innate immunity and inflammation, which are crucial for the regulation of the immune response to viral infections. Next, we delve into the existing knowledge concerning the impact of respiratory virus infections on the epigenome, focusing on Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), Influenza A Virus (IAV), and Respiratory Syncytial Virus (RSV). Finally, we offer perspectives on the potential consequences of virus-induced epigenetic remodeling and open questions in the field that are currently under investigation.

Genetic effects of sequence-conserved enhancer-like elements on human complex traits

BACKGROUND

The vast majority of findings from human genome-wide association studies (GWAS) map to non-coding sequences, complicating their mechanistic interpretations and clinical translations. Non-coding sequences that are evolutionarily conserved and biochemically active could offer clues to the mechanisms underpinning GWAS discoveries. However, genetic effects of such sequences have not been systematically examined across a wide range of human tissues and traits, hampering progress to fully understand regulatory causes of human complex traits.

RESULTS

Here we develop a simple yet effective strategy to identify functional elements exhibiting high levels of human-mouse sequence conservation and enhancer-like biochemical activity, which scales well to 313 epigenomic datasets across 106 human tissues and cell types. Combined with 468 GWAS of European (EUR) and East Asian (EAS) ancestries, these elements show tissue-specific enrichments of heritability and causal variants for many traits, which are significantly stronger than enrichments based on enhancers without sequence conservation. These elements also help prioritize candidate genes that are functionally relevant to body mass index (BMI) and schizophrenia but were not reported in previous GWAS with large sample sizes.

CONCLUSIONS

Our findings provide a comprehensive assessment of how sequence-conserved enhancer-like elements affect complex traits in diverse tissues and demonstrate a generalizable strategy of integrating evolutionary and biochemical data to elucidate human disease genetics.

Transcription factor interactions explain the context-dependent activity of CRX binding sites

The effects of transcription factor binding sites (TFBSs) on the activity of a cis-regulatory element (CRE) depend on the local sequence context. In rod photoreceptors, binding sites for the transcription factor (TF) Cone-rod homeobox (CRX) occur in both enhancers and silencers, but the sequence context that determines whether CRX binding sites contribute to activation or repression of transcription is not understood. To investigate the context-dependent activity of CRX sites, we fit neural network-based models to the activities of synthetic CREs composed of photoreceptor TFBSs. The models revealed that CRX binding sites consistently make positive, independent contributions to CRE activity, while negative homotypic interactions between sites cause CREs composed of multiple CRX sites to function as silencers. The effects of negative homotypic interactions can be overcome by the presence of other TFBSs that either interact cooperatively with CRX sites or make independent positive contributions to activity. The context-dependent activity of CRX sites is thus determined by the balance between positive heterotypic interactions, independent contributions of TFBSs, and negative homotypic interactions. Our findings explain observed patterns of activity among genomic CRX-bound enhancers and silencers, and suggest that enhancers may require diverse TFBSs to overcome negative homotypic interactions between TFBSs.

Super-enhancers include classical enhancers and facilitators to fully activate gene expression

Super-enhancers are compound regulatory elements that control expression of key cell identity genes. They recruit high levels of tissue-specific transcription factors and co-activators such as the Mediator complex and contact target gene promoters with high frequency. Most super-enhancers contain multiple constituent regulatory elements, but it is unclear whether these elements have distinct roles in activating target gene expression. Here, by rebuilding the endogenous multipartite α-globin super-enhancer, we show that it contains bioinformatically equivalent but functionally distinct element types: classical enhancers and facilitator elements. Facilitators have no intrinsic enhancer activity, yet in their absence, classical enhancers are unable to fully upregulate their target genes. Without facilitators, classical enhancers exhibit reduced Mediator recruitment, enhancer RNA transcription, and enhancer-promoter interactions. Facilitators are interchangeable but display functional hierarchy based on their position within a multipartite enhancer. Facilitators thus play an important role in potentiating the activity of classical enhancers and ensuring robust activation of target genes.

Inhibition of the lysine demethylase LSD1 modulates the balance between inflammatory and antiviral responses against coronaviruses

Innate immune responses to coronavirus infections are highly cell specific. Tissue-resident macrophages, which are infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in patients but are inconsistently infected in vitro, exert critical but conflicting effects by secreting both antiviral type I interferons (IFNs) and tissue-damaging inflammatory cytokines. Steroids, the only class of host-targeting drugs approved for the treatment of coronavirus disease 2019 (COVID-19), indiscriminately suppress both responses, possibly impairing viral clearance. Here, we established in vitro cell culture systems that enabled us to separately investigate the cell-intrinsic and cell-extrinsic proinflammatory and antiviral activities of mouse macrophages infected with the prototypical murine coronavirus MHV-A59. We showed that the nuclear factor κB-dependent inflammatory response to viral infection was selectively inhibited by loss of the lysine demethylase LSD1, which was previously implicated in innate immune responses to cancer, with negligible effects on the antiviral IFN response. LSD1 ablation also enhanced an IFN-independent antiviral response, blocking viral egress through the lysosomal pathway. The macrophage-intrinsic antiviral and anti-inflammatory activity of Lsd1 inhibition was confirmed in vitro and in a humanized mouse model of SARS-CoV-2 infection. These results suggest that LSD1 controls innate immune responses against coronaviruses at multiple levels and provide a mechanistic rationale for potentially repurposing LSD1 inhibitors for COVID-19 treatment.

Transcription factor condensates and signaling driven transcription

Transcription Factor (TF) condensates are a heterogenous mix of RNA, DNA, and multiple co-factor proteins capable of modulating the transcriptional response of the cell. The dynamic nature and the spatial location of TF-condensates in the 3D nuclear space is believed to provide a fast response, which is on the same pace as the signaling cascade and yet ever-so-specific in the crowded environment of the nucleus. However, the current understanding of how TF-condensates can achieve these feet so quickly and efficiently is still unclear. In this review, we draw parallels with other protein condensates and share our speculations on how the nucleus uses these TF-condensates to achieve high transcriptional specificity and fidelity. We discuss the various constituents of TF-condensates, their properties, and the known and unknown functions of TF-condensates with a particular focus on steroid signaling-induced transcriptional programs.

HMGA2 directly mediates chromatin condensation in association with neuronal fate regulation

Identification of factors that regulate chromatin condensation is important for understanding of gene regulation. High-mobility group AT-hook (HMGA) proteins 1 and 2 are abundant nonhistone chromatin proteins that play a role in many biological processes including tissue stem-progenitor cell regulation, but the nature of their protein function remains unclear. Here we show that HMGA2 mediates direct condensation of polynucleosomes and forms droplets with nucleosomes. Consistently, most endogenous HMGA2 localized to transposase 5- and DNase I-inaccessible chromatin regions, and its binding was mostly associated with gene repression, in mouse embryonic neocortical cells. The AT-hook 1 domain was necessary for chromatin condensation by HMGA2 in vitro and in cellulo, and an HMGA2 mutant lacking this domain was defective in the ability to maintain neuronal progenitors in vivo. Intrinsically disordered regions of other proteins could substitute for the AT-hook 1 domain in promoting this biological function of HMGA2. Taken together, HMGA2 may regulate neural cell fate by its chromatin condensation activity.

Genome-wide enhancer identification by massively parallel reporter assay in Arabidopsis

Enhancers are critical cis-regulatory elements controlling gene expression during cell development and differentiation. However, genome-wide enhancer characterization has been challenging due to the lack of a well-defined relationship between enhancers and genes. Function-based methods are the gold standard for determining the biological function of cis-regulatory elements; however, these methods have not been widely applied to plants. Here, we applied a massively parallel reporter assay on Arabidopsis to measure enhancer activities across the genome. We identified 4327 enhancers with various combinations of epigenetic modifications distinctively different from animal enhancers. Furthermore, we showed that enhancers differ from promoters in their preference for transcription factors. Although some enhancers are not conserved and overlap with transposable elements forming clusters, enhancers are generally conserved across thousand Arabidopsis accessions, suggesting they are selected under evolution pressure and could play critical roles in the regulation of important genes. Moreover, comparison analysis reveals that enhancers identified by different strategies do not overlap, suggesting these methods are complementary in nature. In sum, we systematically investigated the features of enhancers identified by functional assay in A. thaliana, which lays the foundation for further investigation into enhancers' functional mechanisms in plants.

CELLULAR RNA INTERACTS WITH MAVS TO PROMOTE ANTIVIRAL SIGNALING

Immune signaling needs to be well-regulated to promote clearance of pathogens, while preventing aberrant inflammation. Interferons (IFNs) and antiviral genes are activated by the detection of viral RNA by RIG-I-like receptors (RLRs). Signal transduction downstream of RLRs proceeds through a multi-protein complex organized around the central adaptor protein MAVS. Recent work has shown that protein complex function can be modulated by RNA molecules providing allosteric regulation or acting as molecular guides or scaffolds. Thus, we hypothesized that RNA plays a role in organizing MAVS signaling platforms. Here, we show that MAVS, through its central intrinsically disordered domain, directly interacts with the 3' untranslated regions of cellular mRNAs. Importantly, elimination of RNA by RNase treatment disrupts the MAVS signalosome, including newly identified regulators of RLR signaling, and inhibits phosphorylation of the transcription factor IRF3. This supports the hypothesis that RNA molecules scaffold proteins in the MAVS signalosome to induce IFNs. Together, this work uncovers a function for cellular RNA in promoting signaling through MAVS and highlights a generalizable principle of RNA regulatory control of cytoplasmic immune signaling complexes.

Cis-regulatory landscapes in the evolution and development of the mammalian skull

Extensive morphological variation found in mammals reflects the wide spectrum of their ecological adaptations. The highest morphological diversity is present in the craniofacial region, where geometry is mainly dictated by the bony skull. Mammalian craniofacial development represents complex multistep processes governed by numerous conserved genes that require precise spatio-temporal control. A central question in contemporary evolutionary biology is how a defined set of conserved genes can orchestrate formation of fundamentally different structures, and therefore how morphological variability arises. In principle, differential gene expression patterns during development are the source of morphological variation. With the emergence of multicellular organisms, precise regulation of gene expression in time and space is attributed to cis-regulatory elements. These elements contribute to higher-order chromatin structure and together with trans-acting factors control transcriptional landscapes that underlie intricate morphogenetic processes. Consequently, divergence in cis-regulation is believed to rewire existing gene regulatory networks and form the core of morphological evolution. This review outlines the fundamental principles of the genetic code and genomic regulation interplay during development. Recent work that deepened our comprehension of cis-regulatory element origin, divergence and function is presented here to illustrate the state-of-the-art research that uncovered the principles of morphological novelty. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

A variegated model of transcription factor function in the immune system

Specific combinations of transcription factors (TFs) control the gene expression programs that underlie specialized immune responses. Previous models of TF function in immunocytes had restricted each TF to a single functional categorization [e.g., lineage-defining (LDTFs) vs. signal-dependent TFs (SDTFs)] within one cell type. Synthesizing recent results, we instead propose a variegated model of immunological TF function, whereby many TFs have flexible and different roles across distinct cell states, contributing to cell phenotypic diversity. We discuss evidence in support of this variegated model, describe contextual inputs that enable TF diversification, and look to the future to imagine warranted experimental and computational tools to build quantitative and predictive models of immunocyte gene regulatory networks.

The Cytomegalovirus M35 Protein Directly Binds to the Interferon-β Enhancer and Modulates Transcription of Ifnb1 and Other IRF3-Driven Genes

Induction of type I interferon (IFN) gene expression is among the first lines of cellular defense a virus encounters during primary infection. We previously identified the tegument protein M35 of murine cytomegalovirus (MCMV) as an essential antagonist of this antiviral system, showing that M35 interferes with type I IFN induction downstream of pattern-recognition receptor (PRR) activation. Here, we report structural and mechanistic details of M35's function. Determination of M35's crystal structure combined with reverse genetics revealed that homodimerization is a key feature for M35's immunomodulatory activity. In electrophoretic mobility shift assays (EMSAs), purified M35 protein specifically bound to the regulatory DNA element that governs transcription of the first type I IFN gene induced in nonimmune cells, Ifnb1. DNA-binding sites of M35 overlapped with the recognition elements of interferon regulatory factor 3 (IRF3), a key transcription factor activated by PRR signaling. Chromatin immunoprecipitation (ChIP) showed reduced binding of IRF3 to the host Ifnb1 promoter in the presence of M35. We furthermore defined the IRF3-dependent and the type I IFN signaling-responsive genes in murine fibroblasts by RNA sequencing of metabolically labeled transcripts (SLAM-seq) and assessed M35's global effect on gene expression. Stable expression of M35 broadly influenced the transcriptome in untreated cells and specifically downregulated basal expression of IRF3-dependent genes. During MCMV infection, M35 impaired expression of IRF3-responsive genes aside of Ifnb1. Our results suggest that M35-DNA binding directly antagonizes gene induction mediated by IRF3 and impairs the antiviral response more broadly than formerly recognized. IMPORTANCE Replication of the ubiquitous human cytomegalovirus (HCMV) in healthy individuals mostly goes unnoticed but can impair fetal development or cause life-threatening symptoms in immunosuppressed or -deficient patients. Like other herpesviruses, CMV extensively manipulates its hosts and establishes lifelong latent infections. Murine CMV (MCMV) presents an important model system as it allows the study of CMV infection in the host organism. We previously showed that during entry into host cells, MCMV virions release the evolutionary conserved protein M35 protein to immediately dampen the antiviral type I interferon (IFN) response induced by pathogen detection. Here, we show that M35 dimers bind to regulatory DNA elements and interfere with recruitment of interferon regulatory factor 3 (IRF3), a key cellular factor for antiviral gene expression. Thereby, M35 interferes with expression of type I IFNs and other IRF3-dependent genes, reflecting the importance for herpesviruses to avoid IRF3-mediated gene induction.

Viral recognition and the antiviral interferon response

Interferons (IFNs) are antiviral cytokines that play a key role in the innate immune response to viral infections. In response to viral stimuli, cells produce and release interferons, which then act on neighboring cells to induce the transcription of hundreds of genes. Many of these gene products either combat the viral infection directly, e.g., by interfering with viral replication, or help shape the following immune response. Here, we review how viral recognition leads to the production of different types of IFNs and how this production differs in spatial and temporal manners. We then continue to describe how these IFNs play different roles in the ensuing immune response depending on when and where they are produced or act during an infection.

Regulation of IFNβ expression: focusing on the role of its promoter and transcription regulators

IFNβ is a single-copy gene without an intron. Under normal circumstances, it shows low or no expression in cells. It is upregulated only when the body needs it or is stimulated. Stimuli bind to the pattern recognition receptors (PRRs) and pass via various signaling pathways to several basic transcriptional regulators, such as IRFs, NF-кB, and AP-1. Subsequently, the transcriptional regulators enter the nucleus and bind to regulatory elements of the IFNβ promoter. After various modifications, the position of the nucleosome is altered and the complex is assembled to activate the IFNβ expression. However, IFNβ regulation involves a complex network. For the study of immunity and diseases, it is important to understand how transcription factors bind to regulatory elements through specific forms, which elements in cells are involved in regulation, what regulation occurs during the assembly of enhancers and transcription complexes, and the possible regulatory mechanisms after transcription. Thus, this review focuses on the various regulatory mechanisms and elements involved in the activation of IFNβ expression. In addition, we discuss the impact of this regulation in biology.

Control of enhancer and promoter activation in the type I interferon response by the histone demethylase Kdm4d/JMJD2d

Introduction

Transcriptional activation depends on the interplay of chromatin modifiers to establish a permissive epigenetic landscape. While histone 3 lysine 9 (H3K9) methylation has long been associated with gene repression, there is limited evidence to support a role for H3K9 demethylases in gene activation.

Methods

We leveraged knockdown and overexpression of JMJD2d / Kdm4d in mouse embryonic fibroblasts, coupled with extensive epigenomic analysesm to decipher the role of histone 3 lysine 9 demethylases in the innate immune response.

Results

Here we describe the H3K9 demethylase Kdm4d/JMJD2d as a positive regulator of type I interferon responses. In mouse embryonic fibroblasts (MEFs), depletion of JMJD2d attenuates the transcriptional response, conferring increased viral susceptibility, while overexpression of the demethylase results in more robust IFN activation. We find that the underlying mechanism of JMJD2d in type I interferon responses consists of an effect both on the transcription of enhancer RNAs (eRNAs) and on dynamic H3K9me2 at associated promoters. In support of these findings, we establish that JMJD2d is associated with enhancer regions throughout the genome prior to stimulation but is redistributed to inducible promoters in conjunction with transcriptional activation.

Discussion

Taken together, our data reveal JMJD2d as a chromatin modifier that connects enhancer transcription with promoter demethylation to modulate transcriptional responses.

Porcine Respirovirus 1 Suppresses Host Type I Interferon Production and the JAK-STAT Signaling Pathway

Porcine respirovirus 1 (PRV1), first reported in Hong Kong, is currently widely spread in several countries. Our knowledge of the clinical significance and the pathogenicity of this virus is still limited. In this study, we studied the interactions between PRV1 and host innate immune responses. PRV1 exhibited strong inhibitory effects on the production of interferon (IFN), ISG15, and RIG-I induced by SeV infection. Our data generated in vitro suggest that multiple viral proteins can suppress host type I interferon production and signaling, including N, M, and P/C/V/W. The P gene products disrupt both IRF3 and NF-κB dependent type I IFN production and block type I IFN signaling pathway by sequestering STAT1 in the cytoplasm. The V protein disrupts both MDA5 signaling and RIG-I signaling through interaction with TRIM25 and RIG-I, V protein blocks RIG-I polyubiquitination, which is required for RIG-I activation. V protein also binds to MDA5, which may contribute to its inhibitory effect on MDA5 signaling. These findings indicate that PRV1 antagonizes host innate immune responses using various mechanisms, which provides important insights into the pathogenicity of PRV1.

Decoding enhancer complexity with machine learning and high-throughput discovery

Enhancers are genomic DNA elements controlling spatiotemporal gene expression. Their flexible organization and functional redundancies make deciphering their sequence-function relationships challenging. This article provides an overview of the current understanding of enhancer organization and evolution, with an emphasis on factors that influence these relationships. Technological advancements, particularly in machine learning and synthetic biology, are discussed in light of how they provide new ways to understand this complexity. Exciting opportunities lie ahead as we continue to unravel the intricacies of enhancer function.

Multiple and Consecutive Genome Editing Using i-GONAD and Breeding Enrichment Facilitates the Production of Genetically Modified Mice

Genetically modified (GM) mice are essential tools in biomedical research. Traditional methods for generating GM mice are expensive and require specialized personnel and equipment. The use of clustered regularly interspaced short palindromic repeats (CRISPR) coupled with improved-Genome editing via Oviductal Nucleic Acids Delivery (i-GONAD) has highly increased the feasibility of producing GM mice in research laboratories. However, genetic modification in inbred mouse strains of interest such as C57BL/6 (B6) is still challenging because of their low fertility and embryo fragility. We have successfully generated multiple novel GM mouse strains in the B6 background while attempting to optimize i-GONAD. We found that i-GONAD reduced the litter size in superovulated pregnant females but did not impact pregnancy rates. Natural mating or low-hormone dose did not increase the low fertility rate observed in superovulated B6 females. However, diet enrichment had a positive effect on pregnancy success. We also optimized breeding conditions to increase the survival of small litters by co-housing i-GONAD-treated pregnant B6 females with synchronized pregnant FVB/NJ companion mothers. Thus, GM mice generation was increased by an enriched diet and shared pup rearing with highly fertile females such as FVB/NJ. In the present study, we generated 16 GM mice using a CRISPR/Cas system to target individual and multiple loci simultaneously or consecutively. We also compared homology-directed repair efficiency using different methods for LoxP insertion for conditional knockout mouse production. We found that a two-step serial LoxP insertion, in which each LoxP sequence was inserted individually in different i-GONAD procedures, was a low-risk high-efficiency method for generating floxed mice.

Mechanism of SUMOylation-Mediated Regulation of Type I IFN Expression

Type I interferons (IFN) are cytokines that bridge the innate and adaptive immune response, and thus play central roles in human health, including vaccine efficacy, immune response to cancer and pathogen infection, and autoimmune disorders. Post-translational protein modifications by the small ubiquitin-like modifiers (SUMO) have recently emerged as an important regulator of type I IFN expression as shown by studies using murine and cellular models and recent human clinical trials. However, the mechanism regarding how SUMOylation regulates type I IFN expression remains poorly understood. In this study, we show that SUMOylation inhibition does not activate IFNB1 gene promoter that is regulated by known canonical pathways including cytosolic DNA. Instead, we identified a binding site for the chromatin modification enzyme, the SET Domain Bifurcated Histone Lysine Methyltransferase 1 (SETDB1), located between the IFNB1 promoter and a previously identified enhancer. We found that SETDB1 regulates IFNB1 expression and SUMOylation of SETDB1 is required for its binding and enhancing the H3K9me3 heterochromatin signal in this region. Heterochromatin, a tightly packed form of DNA, has been documented to suppress gene expression through suppressing enhancer function. Taken together, our study identified a novel mechanism of regulation of type I IFN expression, at least in part, through SUMOylation of a chromatin modification enzyme.

Strand Asymmetries Across Genomic Processes

Across biological systems, a number of genomic processes, including transcription, replication, DNA repair, and transcription factor binding, display intrinsic directionalities. These directionalities are reflected in the asymmetric distribution of nucleotides, motifs, genes, transposon integration sites, and other functional elements across the two complementary strands. Strand asymmetries, including GC skews and mutational biases, have shaped the nucleotide composition of diverse organisms. The investigation of strand asymmetries often serves as a method to understand underlying biological mechanisms, including protein binding preferences, transcription factor interactions, retrotransposition, DNA damage and repair preferences, transcription-replication collisions, and mutagenesis mechanisms. Research into this subject also enables the identification of functional genomic sites, such as replication origins and transcription start sites. Improvements in our ability to detect and quantify DNA strand asymmetries will provide insights into diverse functionalities of the genome, the contribution of different mutational mechanisms in germline and somatic mutagenesis, and our knowledge of genome instability and evolution, which all have significant clinical implications in human disease, including cancer. In this review, we describe key developments that have been made across the field of genomic strand asymmetries, as well as the discovery of associated mechanisms.

Enhancers display constrained sequence flexibility and context-specific modulation of motif function

The information about when and where each gene is to be expressed is mainly encoded in the DNA sequence of enhancers, sequence elements that comprise binding sites (motifs) for different transcription factors (TFs). Most of the research on enhancer sequences has been focused on TF motif presence, whereas the enhancer syntax, that is, the flexibility of important motif positions and how the sequence context modulates the activity of TF motifs, remains poorly understood. Here, we explore the rules of enhancer syntax by a two-pronged approach in Drosophila melanogaster S2 cells: we (1) replace important TF motifs by all possible 65,536 eight-nucleotide-long sequences and (2) paste eight important TF motif types into 763 positions within 496 enhancers. These complementary strategies reveal that enhancers display constrained sequence flexibility and the context-specific modulation of motif function. Important motifs can be functionally replaced by hundreds of sequences constituting several distinct motif types, but these are only a fraction of all possible sequences and motif types. Moreover, TF motifs contribute with different intrinsic strengths that are strongly modulated by the enhancer sequence context (the flanking sequence, the presence and diversity of other motif types, and the distance between motifs), such that not all motif types can work in all positions. The context-specific modulation of motif function is also a hallmark of human enhancers, as we demonstrate experimentally. Overall, these two general principles of enhancer sequences are important to understand and predict enhancer function during development, evolution, and in disease.

Diverse logics and grammar encode notochord enhancers

The notochord is a defining feature of all chordates. The transcription factors Zic and ETS regulate enhancer activity within the notochord. We conduct high-throughput screens of genomic elements within developing Ciona embryos to understand how Zic and ETS sites encode notochord activity. Our screen discovers an enhancer located near Lama, a gene critical for notochord development. Reversing the orientation of an ETS site within this enhancer abolishes expression, indicating that enhancer grammar is critical for notochord activity. Similarly organized clusters of Zic and ETS sites occur within mouse and human Lama1 introns. Within a Brachyury (Bra) enhancer, FoxA and Bra, in combination with Zic and ETS binding sites, are necessary and sufficient for notochord expression. This binding site logic also occurs within other Ciona and vertebrate Bra enhancers. Collectively, this study uncovers the importance of grammar within notochord enhancers and discovers signatures of enhancer logic and grammar conserved across chordates.

Deciphering the multi-scale, quantitative cis-regulatory code

Uncovering the cis-regulatory code that governs when and how much each gene is transcribed in a given genome and cellular state remains a central goal of biology. Here, we discuss major layers of regulation that influence how transcriptional outputs are encoded by DNA sequence and cellular context. We first discuss how transcription factors bind specific DNA sequences in a dosage-dependent and cooperative manner and then proceed to the cofactors that facilitate transcription factor function and mediate the activity of modular cis-regulatory elements such as enhancers, silencers, and promoters. We then consider the complex and poorly understood interplay of these diverse elements within regulatory landscapes and its relationships with chromatin states and nuclear organization. We propose that a mechanistically informed, quantitative model of transcriptional regulation that integrates these multiple regulatory layers will be the key to ultimately cracking the cis-regulatory code.

Manipulation of innate immune signaling pathways by SARS-CoV-2 non-structural proteins

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the current coronavirus disease 2019 (COVID-19) pandemic, induces an unbalanced immune response in the host. For instance, the production of type I interferon (IFN) and the response to it, which act as a front-line defense against virus invasion, are inhibited during SARS-CoV-2 infection. In addition, tumor necrosis factor alpha (TNF-α), a proinflammatory cytokine, is upregulated in COVID-19 patients with severe symptoms. Studies on the closely related betacoronavirus, SARS-CoV, showed that viral proteins such as Nsp1, Orf6 and nucleocapsid protein inhibit IFN-β production and responses at multiple steps. Given the conservation of these proteins between SARS-CoV and SARS-CoV-2, it is not surprising that SARS-CoV-2 deploys similar immune evasion strategies. Here, we carried out a screen to examine the role of individual SARS-CoV-2 proteins in regulating innate immune signaling, such as the activation of transcription factors IRF3 and NF-κB and the response to type I and type II IFN. In addition to established roles of SARS-CoV-2 proteins, we report that SARS-CoV-2 proteins Nsp6 and Orf8 inhibit the type I IFN response but at different stages. Orf6 blocks the translocation of STAT1 and STAT2 into the nucleus, whereas ORF8 inhibits the pathway in the nucleus after STAT1/2 translocation. SARS-CoV-2 Orf6 also suppresses IRF3 activation and TNF-α-induced NF-κB activation.

Transcriptional enhancers at 40: evolution of a viral DNA element to nuclear architectural structures

Gene regulation by transcriptional enhancers is the dominant mechanism driving cell type- and signal-specific transcriptional diversity in metazoans. However, over four decades since the original discovery, how enhancers operate in the nuclear space remains largely enigmatic. Recent multidisciplinary efforts combining real-time imaging, genome sequencing, and biophysical strategies provide insightful but conflicting models of enhancer-mediated gene control. Here, we review the discovery and progress in enhancer biology, emphasizing the recent findings that acutely activated enhancers assemble regulatory machinery as mesoscale architectural structures with distinct physical properties. These findings help formulate novel models that explain several mysterious features of the assembly of transcriptional enhancers and the mechanisms of spatial control of gene expression.

HMGA2 drives the IGFBP1/AKT pathway to counteract the increase in P27KIP1 protein levels in mtDNA/RNA-less cancer cells

Recent comprehensive analyses of mtDNA and orthogonal RNA-sequencing data revealed that in numerous human cancers, mtDNA copy numbers and mtRNA amounts are significantly reduced, followed by low respiratory gene expression. Under such conditions (called mt-Low), cells encounter severe cell proliferation defects; therefore, they must acquire countermeasures against this fatal disadvantage during malignant transformation. This study elucidated a countermeasure against the mt-Low condition-induced antiproliferative effects in hepatocellular carcinoma (HCC) cells. The mechanism relied on the architectural transcriptional regulator HMGA2, which was preferably expressed in HCC cells of the mt-Low type in vitro and in vivo. Detailed in vitro analyses suggest that HMGA2 regulates insulin-like growth factor binding protein 1 (IGFBP1) expression, leading to AKT activation, which then phosphorylates the cyclin-dependent kinase inhibitor (CKI), P27KIP1, and facilitates its ubiquitin-mediated degradation. Accordingly, intervention in the HMGA2 function by RNAi resulted in an increase in P27KIP1 levels and an induction of senescence-like cell proliferation inhibition in mt-Low-type HCC cells. Conclusively, the HMGA2/IGFBP1/AKT axis has emerged as a countermeasure against P27KIP1 CKI upregulation under mt-Low conditions, thereby circumventing cell proliferation inhibition and supporting the tumorigenic state. Notably, similar to in vitro cell lines, HMGA2 was likely to regulate IGFBP1 expression in HCC in vivo, thereby contributing to poor patient prognosis. Considering the significant number of cases under mt-Low or the threat of CKI upregulation cancer-wide, the axis is noteworthy as a vulnerability of cancer cells or target for tumor-agnostic therapy inducing irreversible cell proliferation inhibition via CKI upregulation in a large population with cancer.

Regulation of cardiac ion channels by transcription factors: Looking for new opportunities of druggable targets for the treatment of arrhythmias

Cardiac electrical activity is governed by different ion channels that generate action potentials. Acquired or inherited abnormalities in the expression and/or function of ion channels usually result in electrophysiological changes that can cause cardiac arrhythmias. Transcription factors (TFs) control gene transcription by binding to specific DNA sequences adjacent to target genes. Linkage analysis, candidate-gene screening within families, and genome-wide association studies have linked rare and common genetic variants in the genes encoding TFs with genetically-determined cardiac arrhythmias. Besides its critical role in cardiac development, recent data demonstrated that they control cardiac electrical activity through the direct regulation of the expression and function of cardiac ion channels in adult hearts. This narrative review summarizes some studies showing functional data on regulation of the main human atrial and ventricular Na⁺, Ca²⁺, and K⁺ channels by cardiac TFs such as Pitx2c, Tbx20, Tbx5, Zfhx3, among others. The results have improved our understanding of the mechanisms regulating cardiac electrical activity and may open new avenues for therapeutic interventions in cardiac acquired or inherited arrhythmias through the identification of TFs as potential drug targets. Even though TFs have for a long time been considered as 'undruggable' targets, advances in structural biology have led to the identification of unique pockets in TFs amenable to be targeted with small-molecule drugs or peptides that are emerging as novel therapeutic drugs.

The chromatin remodeling protein ATRX positively regulates IRF3-dependent type I interferon production and interferon-induced gene expression

The chromatin remodeling protein alpha thalassemia/mental retardation syndrome X-linked (ATRX) is a component of promyelocytic leukemia nuclear bodies (PML-NBs) and thereby mediates intrinsic immunity against several viruses including human cytomegalovirus (HCMV). As a consequence, viruses have evolved different mechanisms to antagonize ATRX, such as displacement from PML-NBs or degradation. Here, we show that depletion of ATRX results in an overall impaired antiviral state by decreasing transcription and subsequent secretion of type I IFNs, which is followed by reduced expression of interferon-stimulated genes (ISGs). ATRX interacts with the transcription factor interferon regulatory factor 3 (IRF3) and associates with the IFN-β promoter to facilitate transcription. Furthermore, whole transcriptome sequencing revealed that ATRX is required for efficient IFN-induced expression of a distinct set of ISGs. Mechanistically, we found that ATRX positively modulates chromatin accessibility specifically upon IFN signaling, thereby affecting promoter regions with recognition motifs for AP-1 family transcription factors. In summary, our study uncovers a novel co-activating function of the chromatin remodeling factor ATRX in innate immunity that regulates chromatin accessibility and subsequent transcription of interferons and ISGs. Consequently, ATRX antagonization by viral proteins and ATRX mutations in tumors represent important strategies to broadly compromise both intrinsic and innate immune responses.

ATRX is a member of a family of chromatin remodeling proteins required for deposition of the histone variant H3.3 at specific genomic regions. This is important to maintain silencing at these sites. Furthermore, ATRX represents a component of PML nuclear bodies (PML-NBs) which are considered as enigmatic nuclear protein accumulations exhibiting a tight link to cell-intrinsic restriction of viral infections. Previous studies demonstrated that many viruses target ATRX by either displacement or degradation. So far, it is believed that this serves to alleviate ATRX-instituted silencing of viral gene expression. Our results reveal a novel and unexpectedly broad function of ATRX as a co-activator of the innate immune response. We show that ATRX is required for both DNA and RNA sensing pathways to activate interferon (IFN) gene expression as well as for upregulation of a distinct set of interferon-stimulated genes. Assessment of chromatin accessibility detected that IFN acts as a switch to regulate the function of ATRX in heterochromatin remodeling. ATRX positively modulates chromatin accessibility specifically upon IFN signaling, thereby affecting promoter regions with recognition motifs for AP-1 family transcription factors. Loss of ATRX due to viral infection or due to tumor mutations may thus broadly compromise cellular innate immunity.

Oncolytic Vaccinia Virus Harboring Aphrocallistes vastus Lectin Inhibits the Growth of Hepatocellular Carcinoma Cells

Oncolytic vaccinia virus has been developed as a novel cancer therapeutic drug in recent years. Our previous studies demonstrated that the antitumor effect of oncolytic vaccina virus harboring Aphrocallistes vastus lectin (oncoVV-AVL) was significantly enhanced in several cancer cells. In the present study, we investigated the underlying mechanisms of AVL that affect virus replication and promote the antitumor efficacy of oncolytic virus in hepatocellular carcinoma (HCC). Our results showed that oncoVV-AVL markedly exhibited antitumor effects in both hepatocellular carcinoma cell lines and a xenograft mouse model. Further investigation illustrated that oncoVV-AVL could activate tumor immunity by upregulating the expression of type I interferons and enhance virus replication by inhibiting ISRE mediated viral defense response. In addition, we inferred that AVL promoted the ability of virus replication by regulating the PI3K/Akt, MAPK/ERK, and Hippo/MST pathways through cross-talk Raf-1, as well as metabolism-related pathways. These findings provide a novel perspective for the exploitation of marine lectins in oncolytic therapy.

HMGA1 chromatin regulators induce transcriptional networks involved in GATA2 and proliferation during MPN progression

Myeloproliferative neoplasms (MPNs) transform to myelofibrosis (MF) and highly lethal acute myeloid leukemia (AML), although the actionable mechanisms driving progression remain elusive. Here, we elucidate the role of the high mobility group A1 (HMGA1) chromatin regulator as a novel driver of MPN progression. HMGA1 is upregulated in MPN, with highest levels after transformation to MF or AML. To define HMGA1 function, we disrupted gene expression via CRISPR/Cas9, short hairpin RNA, or genetic deletion in MPN models. HMGA1 depletion in JAK2V617F AML cell lines disrupts proliferation, clonogenicity, and leukemic engraftment. Surprisingly, loss of just a single Hmga1 allele prevents progression to MF in JAK2V617F mice, decreasing erythrocytosis, thrombocytosis, megakaryocyte hyperplasia, and expansion of stem and progenitors, while preventing splenomegaly and fibrosis within the spleen and BM. RNA-sequencing and chromatin immunoprecipitation sequencing revealed HMGA1 transcriptional networks and chromatin occupancy at genes that govern proliferation (E2F, G2M, mitotic spindle) and cell fate, including the GATA2 master regulatory gene. Silencing GATA2 recapitulates most phenotypes observed with HMGA1 depletion, whereas GATA2 re-expression partially rescues leukemogenesis. HMGA1 transactivates GATA2 through sequences near the developmental enhancer (+9.5), increasing chromatin accessibility and recruiting active histone marks. Further, HMGA1 transcriptional networks, including proliferation pathways and GATA2, are activated in human MF and MPN leukemic transformation. Importantly, HMGA1 depletion enhances responses to the JAK2 inhibitor, ruxolitinib, preventing MF and prolonging survival in murine models of JAK2V617F AML. These findings illuminate HMGA1 as a key epigenetic switch involved in MPN transformation and a promising therapeutic target to treat or prevent disease progression.

TRIM56 overexpression restricts porcine epidemic diarrhoea virus replication in Marc-145 cells by enhancing TLR3-TRAF3-mediated IFN-β antiviral response

Infection with the porcine epidemic diarrhoea virus (PEDV) causes severe enteric disease in suckling piglets, causing massive economic losses in the swine industry worldwide. Tripartite motif-containing 56 (TRIM56) has been shown to augment type I IFN response, but whether it affects PEDV replication remains uncharacterized. Here we investigated the role of TRIM56 in Marc-145 cells during PEDV infection. We found that TRIM56 expression was upregulated in cells infected with PEDV. Overexpression of TRIM56 effectively reduced PEDV replication, while knockdown of TRIM56 resulted in increased viral replication. TRIM56 overexpression significantly increased the phosphorylation of IRF3 and NF-κB P65, and enhanced the IFN-β antiviral response, while silencing TRIM56 did not affect IRF3 activation. TRIM56 overexpression increased the protein level of TRAF3, the component of the TLR3 pathway, thereby significantly activating downstream IRF3 and NF-κB signalling. We demonstrated that TRIM56 overexpression inhibited PEDV replication and upregulated expression of IFN-β, IFN-stimulated genes (ISGs) and chemokines in a dose-dependent manner. Moreover, truncations of the RING domain, N-terminal domain or C-terminal portion on TRIM56 were unable to induce IFN-β expression and failed to restrict PEDV replication. Together, our results suggested that TRIM56 was upregulated in Marc-145 cells in response to PEDV infection. Overexpression of TRIM56 inhibited PEDV replication by positively regulating the TLR3-mediated antiviral signalling pathway. These findings provide evidence that TRIM56 plays a positive role in the innate immune response during PEDV infection.