Protein & Cell

Protein lysine methyltransferases (PKMTs) methylate histone and non-histone proteins to regulate biological outcomes such as development and disease including viral infection. While PKMTs have been extensively studied for modulating the antiviral responses via host gene regulation, their role in methylation of proteins encoded by viruses and its impact on host-pathogen interactions remain poorly understood. In this study, we discovered a distinct nucleo-cytoplasmic form of Euchromatic Histone Methyltransferase1(EHMT1N/C), a PKMT, that phase separates into viral inclusion bodies (IBs) upon cytoplasmic RNA-virus infection (Sendai Virus). EHMT1N/C interacts with cytoplasmic EHMT2 and methylates SeV-Nucleoprotein upon infection. Elevated nucleoprotein methylation during infection correlated with coalescence of small IBs into large mature platforms for efficient replication. Inhibition of EHMT activity by pharmacological inhibitors or genetic depletion of EHMT1N/C reduced the size of IBs with a concomitant reduction in replication. Since IB formation is conserved among all cytoplasmic RNA-viruses, our study will have strong implications in understanding the mechanisms regulating IB formation, discerning RNA viral pathogenesis and designing therapeutic strategies.

The development of the complex nervous system is strictly controlled by diverse isoforms produced from individual genes, but the underlying machinery remains unclear. Our long-read cDNA sequencing identifies more than 700 genes with high isoform diversity in cerebellar granule cell progenitors (GCPs). One such gene, Meis1, produces MEIS1-FL and MEIS1-HdL isoforms, which include and lack the homeodomain, respectively. Our previous study showed that MEIS1-FL localizes to nuclei and promotes ATOH1 protein degradation through transcriptional regulation, thereby promoting GCP differentiation. In contrast, our in vivo electroporation experiment in this study shows that MEIS1-HdL inhibits GCP differentiation. MEIS1-HdL localizes in the cytoplasm and inhibits the degradation of ATOH1 mediated by CUL3, which is a newly identified E3 ligase for ATOH1. MEIS1-HdL enhances the binding of the COP9 signalosome to CUL3, which suppresses ATOH1 polyubiquitination. This study demonstrates that functionally antagonistic isoforms derived from a single gene cleverly control neural progenitor differentiation.

MicroRNAs (miRNAs) are short noncoding RNAs that can regulate gene expression through the binding of their 5-ends to mRNAs. However, the biological effects of miRNA binding via their 3-ends remain unclear. Here, we discover that the exact reverse pairing of the 3-ends of miRNAs or miRNA-like RNAs, collectively termed microsized RNAs (msRNAs), with template RNAs can initiate the production of msRNA-derived RNAs (msdRs), which can subsequently be translated into polypeptides (msdPs). Using 2,632 human msRNAs from miRBase, 11,121 msdRs and 1,239 msdPs were predicted based on a 15-nucleotide pairing threshold, and the presence of representative msdRs and msdPs was confirmed in human cells. Of clinical relevance, msdP0188 is highly expressed in human lung and breast cancers, and its corresponding msRNAs and msdRs represent novel anti-cancer targets. Notably, inhibiting telomerase reverse transcriptase, a putative RNA-dependent RNA polymerase identified by bioinformatic screening, led to reduced levels of msdP0188 in human cancer cells. Our findings reveal a novel "msRNA [->] msdR [->] msdP" axis, expanding the classical central dogma and predicting many previously unrecognized RNAs and polypeptides with potential biological functions in human cells and other systems.

Transposon-associated ribonucleoprotein TnpB is known to be the ancestry endonuclease of diverse Cas12 effector proteins from type-V CRISPR system. Given its small size (409 aa), it is of interest to examine whether engineered TnpB could be used for efficient mammalian genome editing. Here, we showed that the gene editing activity of native TnpB in mouse embryos was already higher than previously identified small-sized Cas12f1. Further stepwise engineering of noncoding RNA ({omega}RNA or reRNA) component of TnpB significantly elevated the nuclease activity of TnpB. Notably, an optimized TnpB-{omega}RNA system could be efficiently delivered in vivo with single adeno-associated virus (AAV) and prevented the disease phenotype in a tyrosinaemia mouse model. Thus, the engineered miniature TnpB system represents a new addition to the current genome editing toolbox, with the unique feature of the smallest effector size that facilitate efficient AAV delivery for editing of cells and tissues.

The CRISPR-Cas13 system is an RNA-guided RNA-targeting system, and has been widely used in transcriptome engineering with potentially important clinical applications. However, it is still controversial whether Cas13 exhibits collateral activity in mammalian cells. Here, we found that knocking down gene expression using RfxCas13d in the adult brain neurons caused death of mice, which was not resulted from the loss of target gene function or off-target effects. Mechanistically, we showed that RfxCas13d exhibited collateral activity in mammalian cells, which is positively correlated with the abundance of target RNA. The collateral activity of RfxCas13d could cleave 28s rRNA into two fragments, leading to translation attenuation and activation of the ZAK-JNK/p38-immediate early gene (IEG) pathway. These results provide new mechanistic insights into the collateral activity of RfxCas13d and warn that the biosafety of CRISPR-Cas13 system needs further evaluation before applying it to clinical treatments.

As important mediators of intercellular communication, exosome have can modulate various cellular functions by transferring a variety of intracellular components to target cells. However, little is known about the role of exosome-mediated communication between distant organs. Hepatopulmonary syndrome (HPS) is a severe lung injury caused by chronic liver disease. A new long noncoding RNA (lncRNA) PICALM-AU1 was found and upregulated in the liver of HPS. It was located in the cholangiocytes of liver and then, secreted as exosome into the serum. PICALM-AU1 carrying serum exosomes induced endothelial-mesenchymal transition (EndMT) of PMVECs and promoted lung injury in vivo and in vitro. Furthermore, overexpression of PICALM-AU1 significantly suppressed miR144-3p and subsequently induced ZEB1 expression. Taken together, our findings identified cholangiocyte-derived exosomal lncRNA PICALM-AU1 plays a critical role in the EndMT of HPS lung. And PICALM-AU1 represents a noninvasive biomarker and potential therapeutic target for HPS.

Under glucose starvation, mammalian cells form lysosome-like structures within the nucleoli to mediate the metabolic shift of carbon sources from glucose to amino acids generated through H2Aub degradation for energy supply, but the mechanism by which lysosome-like structures mediate the degradation of monoubiquitinated histones during glucose starvation remains unclear. Through screening, we discovered that the fibrillarin (FBL) protein in the nucleolus acts as a carrier for mono-ubquitinated histone H2A (H2Aub), regulating the balance between BMI1-mediated mono-ubiquitination of H2A and USP36-mediated deubiquitination of H2Aub. Under glucose starvation, FBL undergoes aggregation, leading to a significant reduction in its interaction with BMI1, accompanied by markedly decreased binding to histone H2A and H2Aub. Concurrently, Midnolin, which serves as a receptor for monoubiquitinated histones, exhibits increased interaction with FBL during glucose starvation. USP36, FBL, Midnolin, and BMI1 collectively form a complex responsible for degrading monoubiquitinated histones. Furthermore, knockdown of FBL, BMI1, or USP36 resulted in cell cycle arrest at S phase and severely compromised cell viability. In summary, we identified a lysosome-like structure in the nucleolus that mediates the degradation of H2Aub. This structure plays a critical role in regulating cell cycle progression and survival, and may offer novel potential therapeutic targets for cancer.

SARS-CoV-2 infection poses a major threat to public health, and understanding the mechanism of viral replication and virion release would help identify therapeutic targets and effective drugs for combating the virus. Herein, we identified E3 ubiquitin-protein ligase Itchy homolog (ITCH) as a central regulator of SARS-CoV-2 at multiple steps and processes. ITCH enhances the ubiquitination of viral envelope and membrane proteins and mutual interactions of structural proteins, thereby aiding in virion assembly. ITCH-mediated ubiquitination also enhances the interaction of viral proteins to the autophagosome receptor p62, promoting their autophagosome-dependent secretion. Additionally, ITCH disrupts the trafficking of the protease furin and the maturation of cathepsin L, thereby suppressing their activities in cleaving and destabilizing the viral spike protein. Furthermore, ITCH exhibits robust activation during the SARS-CoV-2 replication stage, and SARS-CoV-2 replication is significantly decreased by genetic or pharmacological inhibition of ITCH. These findings provide new insights into the mechanisms of the SARS-CoV-2 life cycle and identify a potential target for developing treatments for the virus-related diseases.

Biliary atresia (BA) is a life-threatening neonatal fibro-inflammatory disease characterized by hepatic fibrosis, cirrhosis, and end-stage liver failure. BA is also the most frequent indication of pediatric liver transplantation globally. Despite the devastating condition of BA, the pathogenesis mechanism is unknown. Viral infection has been suggested to be associated with BA, but definitive evidence to support this hypothesis is not available. To elucidate the virus-associated pathogenesis mechanism of BA and to understand the immune ecosystem, we performed single-cell transcriptomic and proteomic profiling of BA livers. We detected human endogenous virus (HERV) in infants with BA and their parents. HERV was mainly found in FOLR2+ resident macrophages, T cells, and NK cells. In addition, HERV activation re-educated the fetal-derived FOLR2+ resident macrophages, and reactive oxygen species scavenging neutrophil recruitment was impaired in patients with BA and HERV+, due to FOLR2+ resident macrophage re-education. Furthermore, we showed depletion of FOLR2+ resident macrophage and N-acetylcysteine treatment could rescue the liver damage in BA. Overall, our study revealed the HERV-associated immunopathology mechanism of BA. These results contribute to potential diagnosis and immunotherapy strategies for BA.

DNA base editors and prime editing technology capable of therapeutic base conversion enable ex vivo gene editing therapy for various genetic diseases. For such therapy, it is critical that the target cells survive well both outside the body and after transplantation. In this regard, chemically derived stem/progenitor cells are attracting attention as the most useful cell sources for clinical trials. Here, we generate chemically derived hepatic progenitors from the hereditary tyrosinemia type1 model mouse (HT1-mCdHs) and successfully correct the disease causing mutation using both adenosine base editors (ABEs) and prime editing tools. After transplantation into HT1 mice, ABE-corrected HT1-mCdHs repopulated the liver with fumarylacetoacetate hydrolase-positive cells and dramatically increased the survival rate of HT1 model mice, suggesting a safe and effective ex vivo gene editing therapy.

Serine/Arginine-Rich Splicing Factor 7 (SRSF7), which is previously recognized as a splicing factor, has been revealed to play oncogenic roles in multiple cancers. However, the mechanisms underlying its oncogenic roles have not been well addressed. Here, based on N6-methyladenosine (m6A) co-methylation network analysis across diverse cell lines, we found SRSF7 positively correlated with glioblastoma cell-specific m6A methylation. We then proved SRSF7 is a novel m6A regulator that specifically facilitates the m6A methylation near its binding sites on the mRNAs involved in cell proliferation and migration through recruiting methyltransferase complex. Moreover, SRSF7 promotes the proliferation and migration of glioblastoma cells largely dependent on the m6A methyltransferase. The two single-nucleotide m6A sites on PBK are regulated by SRSF7 and partially mediate the effects of SRSF7 on glioblastoma cells through recognition by IGF2BP2. Together, our discovery revealed a novel role of SRSF7 in regulating m6A and timely confirmed the existence and functional importance of RNA binding protein (RBP) mediated specific regulation of m6A.

AbstractsNicotinamide adenine dinucleotide (NAD) is a critical metabolite and coenzyme for multiple metabolic pathways and cellular processes (1-4). In this study, we identified Singheart, SGHRT as a nuclear genome-encoded NAD+-binding mitochondrial micropeptide. SGHRT, present in both monomeric and dimeric forms, binds directly to NAD, but not NADH or flavin adenine dinucleotide (FAD). Localized to the inner mitochondrial membrane and mitochondrial matrix, SGHRT interacts with the mitochondrial enzymes Succinate-CoA Ligase and Succinate Dehydrogenase. SGHRT deletion in human embryonic stem cell derived cardiomyocytes disrupted mitochondria morphology, decreased total NAD and ATP abundance, and resulted in defective TCA cycle metabolism, the electron transport chain and in Ox-Phos processes. These results comprise the first report of an NAD+-binding micropeptide, SGHRT, required for mitochondrial function and metabolism.

Double-stranded DNA is recognized as a danger signal by cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS), triggering innate immune responses in mammals. As a DNA sensor, cGAS is generally believed to distribute in the cytoplasm, whereas alternative subcellular localization of cGAS, including cytoplasmic membrane and nucleus, is important to regulate its activity. However, it remains obscure whether cGAS could localized to organelle membrane and the mechanism has yet to be uncovered. Our study reveals that cGAS could localize to the endoplasmic reticulum, Golgi apparatus, and endosomes upon DNA challenge. We identified that the post-translational modification enzymes ZDHHC18 and MARCH8, through their intrinsically disordered regions (IDRs), facilitate the binding of cGAS to the Golgi and endosome, respectively. These IDRs phase separated to recruit cGAS and double-stranded DNA (dsDNA) into biomolecular condensates, suppressing cGAS activity and downstream signaling pathways. These findings highlight the regulatory mechanisms of cGAS activity through the spatial organization, providing new insights into the modulation of innate immune responses.

Human lipoylation pathway deficiencies caused by LIPT2/LIAS/LIPT1 gene mutations lead to inherited metabolic disorders characterized by severe defects of mitochondrial energy and amino acids metabolism. Patients with such mutations suffer from hyperlactic acidemia, encephalopathy, hypotonia and early death. So far there is no effective treatment. Here we introduced the bacteria salvage lipoylation pathway, which is absent in eukaryotes, into lipoylation defective human cells and mouse models. Both E. coli-derived LplA and B. subtilis-derived LplJ restored the phenotypes of LIPT2/LIAS/LIPT1 gene knockout cells to those of wildtype cells with lipoic acid supplementation. Biochemical and isotope tracing analysis demonstrated LplA and LplJ function through direct lipoylation of the H protein of glycine cleavage system and the E2 subunits of 2-oxoacid dehydrogenases, thereby reactivating the short-circuited TCA cycle and amino acids metabolism. This strategy is further extended to treat lipoyl precursor supply defects caused by MECR and FDX1 mutations and its efficacy and safety are validated in Lipt1-/- and LplAOE/+ mouse models. This study provides systematic characterization of lipoylation deficiency and paves the way to gene therapy for treatment of lipoylation-deficient patients.

Dysregulations of lipid metabolism in the liver may trigger steatosis progression leading to potentially severe clinical consequences such as non-alcoholic fatty liver diseases (NAFLD). Molecular mechanisms underlying liver lipogenesis are very complex and fine-tuned by chromatin dynamics and the activity of multiple key transcription factors. Here, we demonstrate that the nuclear factor HMGB1 acts as a strong repressor of liver lipogenesis during metabolic stress in NAFLD. Mice with liver-specific Hmgb1-deficiency display exacerbated liver steatosis and hepatic insulin resistance when subjected to a high-fat diet or after fasting/refeeding. Global transcriptome and functional analysis revealed that the deletion of Hmgb1 gene enhances LXR activity resulting in increased lipogenesis. HMGB1 repression is not mediated through nucleosome landscape re-organization but rather via a preferential DNA occupation in region carrying genes regulated by LXR. Together these findings suggest that hepatocellular HMGB1 protects from liver steatosis development. HMGB1 may constitute a new attractive option to therapeutically target LXR axis during NAFLD.

Dinoflagellate chromosomes are extraordinary, as their organization is independent of architectural nucleosomes unlike typical eukaryotes and shows a cholesteric liquid crystal state. 5-hydroxymethyluridine (5hmU) is present at unusually high levels and its function remains an enigma in dinoflagellates chromosomal DNA. Here, we demonstrate that 5hmU exhibits content variations in different dinoflagellates and is generated at the poly-nucleotide level through hydroxylation of thymidine. Importantly, we identified the enzyme, which is a putative dinoflagellate TET/JBP homologue, catalyzing 5hmU production using either in vivo or in vitro biochemical assay. Based on the near-chromosomal level genome assembly of dinoflagellate Amphidinium carterae, we depicted a comprehensive 5hmU landscape and found that most 5hmU peaks share a conserved TG-rich motif, and are significantly enriched in repeat elements, which mark partially overlapping regions with 5-methylcytosine (5mC) sites. Moreover, inhibition of 5hmU via dioxygenase inhibitor leads to transcriptional activation of 5hmU-marked transposable elements (TEs), implying that 5hmU appears to serve as epigenetic marks for silencing retrotransposon. Together, our results revealed the biogenesis, genome-wide landscape and molecular function of dinoflagellate 5hmU, providing mechanic insight into the function of this enigmatic DNA mark.

De novo lipogenesis (DNL) plays a key role in the excessive fat accumulation present in Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD). Most mechanistic studies and experimental strategies for improving hepatic steatosis in MASLD have focused on the transcriptional regulation of enzymes involved in DNL and triglyceride (TG) synthesis. Here, we provide evidence for a post-translational mechanism that enhances fatty acid (FA) and TG synthesis through the assembly of a multi-protein lipogenic metabolon in liver. Under anabolic conditions, acetyl-CoA carboxylase 1 (ACC1) interacts with additional key enzymes in the DNL and TG synthesis pathway. Immunofluorescence and electron microscopy reveal that this lipogenic metabolon localizes around lipid droplets (LDs) and in proximity to mitochondria and LD interfaces in the anabolic state. The formation of the lipogenic metabolon facilitates the efficient transfer of FA synthesis intermediates to enhance lipogenic flux. These findings uncover a new nutrient-responsive, post-translational regulatory mechanism for hepatic lipogenesis and highlight the lipogenic metabolon as a potential therapeutic target for metabolic liver diseases.

Retroviruses utilize multiple unique RNA elements to control several aspects of RNA processing, such as splicing, subcellular export, and translation. However, it is mostly unclear whether such functional RNA elements are present in endogenous retroviruses (ERVs), many of which were inserted into the host genomes millions of years ago. Previously, in human ERV-derived syncytin-1 gene, we found a cis-acting RNA element named SPRE that enhances its protein expression. In this study, we found a 17-nt common sequence in SPRE of syncytin-1 and another ERV-derived gene, syncytin-2, and the sequence is confirmed to be essential for the expression of the proteins. We detected the sequences of SPRE-like elements in 41 ERV families. Though the SPRE-like elements were not found in currently prevailing (i.e. exogenous) viral sequences, more than thousands of copies of the elements were found in several mammalian genomes, suggesting the ancient integration and propagation of the SPRE-harboring retroviruses in mammalian lineages. Indeed, other mammalian ERV-derived genes: mac-syncytin-3 of macaque, syncytin-Ten1 of tenrec, and syncytin-Car1 of Carnivora contain the SPRE-like elements, and we validated their function for efficient protein expression by in vitro assays. A reporter assay revealed that the enhancement of gene expression by SPRE depended on reporter genes. Moreover, the mutation in SPRE did not affect the gene expression in codon-optimized syncytin-2. However, the same mutation in SPRE impaired the gene expression in wild-type syncytin-2, suggesting that the SPRE dependency of Syncytin-2 expression is due to the negative factors such as inefficient codon frequency or repressive elements within the coding sequence. These results provide new implications that ERVs harbor unique RNA elements involved in the regulation of ERV-derived genes.

The Polycomb group (PcG) protein chromobox 8 (CBX8) is the subunit of Polycomb repressive complex 1 (PRC1) and recognizes the trimethylation of histone H3 on Lysine 27 (H3K27me3), and coordinates with PRC2 complex to function as epigenetic gene silencer. CBX8 plays a key role in cell proliferation, stem cell biology, cell senescence, and cancer development. However, our knowledge of CBX8 post-translational modifications remains elusive. Here, we report that protein kinase D1 (PKD1) interacts and phosphorylates CBX8 at Ser256 and Ser311 in an evolutionarily conserved motif. We found that PKD1 activation triggered by serum stimulation, Nocodazole treatment and oncogene Ras-induced cell senescence (Ras OIS) all promotes CBX8 S256/311 phosphorylation. PKD1-mediated CBX8 S256/311 phosphorylation impairs PRC1 complex integrity by reducing the binding of CBX8 to other PRC1 components BMI1 and RING1B, decreases the monoubiquitination of histone H2AK119, and results in CBX8 dissociation from its target INK4a/ARF locus and the de-repression of p16, and thus ultimately facilitates cellular senescence. CBX8 S256/311 phosphorylation also compromises hepatocellular cancer cells proliferation and migration. Collectively, these results suggest that PKD1-mediated CBX8 S256/311 phosphorylation is a key mechanism governing CBX8 function, including cell senescence and cancer cell proliferation. Financial supportThis work was supported by grants from Ministry of Science and Technology of the Peoples Republic of China (2018YFC2000102), and from National Natural Science Foundation of China (31871382 and 81571369).

Translatable circular RNAs (circRNAs) have emerged as a promising alternative to linear mRNA as new therapeutics due to its improved stability. The translation of circRNAs is mainly driving by internal ribosome entry site (IRES) or IRES-like elements, which is under regulation by various trans-acting RNA binding proteins (RBPs). Here we designed a cell-based system to systematically screen RBPs that enhance translation of circRNAs, and identified a total of 68 proteins as putative activators of noncanonical translation. These translation activators mainly involved in the functions of RNA processing, ribosomal biogenesis and translation initiation. Furthermore, we developed a machine learning algorithm to extract common sequence features of these activators, which predicted more potential RBPs with translation activator activities. The newly identified and predicted activators were subsequently demonstrated to promote the IRES-mediated circRNA translation in a context-dependent fashion. This investigation provides new insights to discover functions for IRES trans-acting factors and to expand the toolbox for engineered RBPs in RNA synthetic biology.