GPATCH4 regulates rRNA and snRNA 2'-O-methylation in both DHX15-dependent and DHX15-independent manners

Regulation of RNA helicase activity, often accomplished by protein cofactors, is essential to ensure target specificity within the complex cellular environment. The largest family of RNA helicase cofactors are the G-patch proteins, but the cognate RNA helicases and cellular functions of numerous human G-patch proteins remain elusive. Here, we discover that GPATCH4 is a stimulatory cofactor of DHX15 that interacts with the DEAH box helicase in the nucleolus via residues in its G-patch domain. We reveal that GPATCH4 associates with pre-ribosomal particles, and crosslinks to the transcribed ribosomal DNA locus and precursor ribosomal RNAs as well as binding to small nucleolar- and small Cajal body-associated RNAs that guide rRNA and snRNA modifications. Loss of GPATCH4 impairs 2'-O-methylation at various rRNA and snRNA sites leading to decreased protein synthesis and cell growth. We demonstrate that the regulation of 2'-O-methylation by GPATCH4 is both dependent on, and independent of, its interaction with DHX15. Intriguingly, the ATPase activity of DHX15 is necessary for efficient methylation of DHX15-dependent sites, suggesting a function of DHX15 in regulating snoRNA-guided 2'-O-methylation of rRNA that requires activation by GPATCH4. Overall, our findings extend knowledge on RNA helicase regulation by G-patch proteins and also provide important new insights into the mechanisms regulating installation of rRNA and snRNA modifications, which are essential for ribosome function and pre-mRNA splicing.

Remodeling of the endothelial cell transcriptional program via paracrine and DNA-binding activities of MPO

Myeloperoxidase (MPO) is an enzyme that functions in host defense. MPO is released into the vascular lumen by neutrophils during inflammation and may adhere and subsequently penetrate endothelial cells (ECs) coating vascular walls. We show that MPO enters the nucleus of ECs and binds chromatin independently of its enzymatic activity. MPO drives chromatin decondensation at its binding sites and enhances condensation at neighboring regions. It binds loci relevant for endothelial-to-mesenchymal transition (EndMT) and affects the migratory potential of ECs. Finally, MPO interacts with the RNA-binding factor ILF3 thereby affecting its relative abundance between cytoplasm and nucleus. This interaction leads to change in stability of ILF3-bound transcripts. MPO-knockout mice exhibit reduced number of ECs at scar sites following myocardial infarction, indicating reduced neovascularization. In summary, we describe a non-enzymatic role for MPO in coordinating EndMT and controlling the fate of endothelial cells through direct chromatin binding and association with co-factors.

K6-linked ubiquitylation marks formaldehyde-induced RNA-protein crosslinks for resolution

Reactive aldehydes are produced by normal cellular metabolism or after alcohol consumption, and they accumulate in human tissues if aldehyde clearance mechanisms are impaired. Their toxicity has been attributed to the damage they cause to genomic DNA and the subsequent inhibition of transcription and replication. However, whether interference with other cellular processes contributes to aldehyde toxicity has not been investigated. We demonstrate that formaldehyde induces RNA-protein crosslinks (RPCs) that stall the ribosome and inhibit translation in human cells. RPCs in the messenger RNA (mRNA) are recognized by the translating ribosomes, marked by atypical K6-linked ubiquitylation catalyzed by the RING-in-between-RING (RBR) E3 ligase RNF14, and subsequently resolved by the ubiquitin- and ATP-dependent unfoldase VCP. Our findings uncover an evolutionary conserved formaldehyde-induced stress response pathway that protects cells against RPC accumulation in the cytoplasm, and they suggest that RPCs contribute to the cellular and tissue toxicity of reactive aldehydes.

A fluorophore-conjugated reagent enabling rapid detection, isolation and live tracking of senescent cells

Cellular senescence is a stress-response mechanism implicated in various physiological processes, diseases, and aging. Current detection approaches have partially addressed the issue of senescent cell identification in clinical specimens. Effective methodologies enabling precise isolation or live tracking of senescent cells are still lacking. In-depth analysis of truly senescent cells is, therefore, an extremely challenging task. We report (1) the synthesis and validation of a fluorophore-conjugated, Sudan Black-B analog (GLF16), suitable for in vivo and in vitro analysis of senescence by fluorescence microscopy and flow cytometry and (2) the development and application of a GLF16-carrying micelle vector facilitating GLF16 uptake by living senescent cells in vivo and in vitro. The compound and the applied methodology render isolation of senescent cells an easy, rapid, and precise process. Straightforward nanocarrier-mediated GLF16 delivery in live senescent cells comprises a unique tool for characterization of senescence at an unprecedented depth.

Ageing-associated changes in transcriptional elongation influence longevity

Physiological homeostasis becomes compromised during ageing, as a result of impairment of cellular processes, including transcription and RNA splicing1-4. However, the molecular mechanisms leading to the loss of transcriptional fidelity are so far elusive, as are ways of preventing it. Here we profiled and analysed genome-wide, ageing-related changes in transcriptional processes across different organisms: nematodes, fruitflies, mice, rats and humans. The average transcriptional elongation speed (RNA polymerase II speed) increased with age in all five species. Along with these changes in elongation speed, we observed changes in splicing, including a reduction of unspliced transcripts and the formation of more circular RNAs. Two lifespan-extending interventions, dietary restriction and lowered insulin-IGF signalling, both reversed most of these ageing-related changes. Genetic variants in RNA polymerase II that reduced its speed in worms5 and flies6 increased their lifespan. Similarly, reducing the speed of RNA polymerase II by overexpressing histone components, to counter age-associated changes in nucleosome positioning, also extended lifespan in flies and the division potential of human cells. Our findings uncover fundamental molecular mechanisms underlying animal ageing and lifespan-extending interventions, and point to possible preventive measures.

Microgravity-induced stress mechanisms in human stem cell-derived cardiomyocytes

Exposure to outer space microgravity poses a risk for the development of various pathologies including cardiovascular disease. To study this, we derived cardiomyocytes (CMs) from human-induced pluripotent stem cells and exposed them to simulated microgravity (SMG). We combined different “omics” and chromosome conformation capture technologies with live-cell imaging of various transgenic lines to discover that SMG impacts on the contractile velocity and function of CMs via the induction of senescence processes. This is linked to SMG-induced changes of reactive oxygen species (ROS) generation and energy metabolism by mitochondria. Taken together, we uncover a microgravity-controlled axis causing contractile dysfunctions to CMs. Our findings can contribute to the design of preventive and therapeutic strategies against senescence-associated disease.

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Simulated microgravity (SMG) causes ROS production in human cardiomyocytes (CMs)

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SMG inhibits mitochondria function and energy metabolism and induces senescence of CMs

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SMG attenuates contractile velocity, beating frequency and Ca²⁺ influx in CMs

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SMG induces chromosomal changes and modifies the chromosomal architecture in CMs

A recurrent chromosomal inversion suffices for driving escape from oncogene-induced senescence via subTAD reorganization

Oncogene-induced senescence (OIS) is an inherent and important tumor suppressor mechanism. However, if not removed timely via immune surveillance, senescent cells also have detrimental effects. Although this has mostly been attributed to the senescence-associated secretory phenotype (SASP) of these cells, we recently proposed that "escape" from the senescent state is another unfavorable outcome. The mechanism underlying this phenomenon remains elusive. Here, we exploit genomic and functional data from a prototypical human epithelial cell model carrying an inducible CDC6 oncogene to identify an early-acquired recurrent chromosomal inversion that harbors a locus encoding the circadian transcription factor BHLHE40. This inversion alone suffices for BHLHE40 activation upon CDC6 induction and driving cell cycle re-entry of senescent cells, and malignant transformation. Ectopic overexpression of BHLHE40 prevented induction of CDC6-triggered senescence. We provide strong evidence in support of replication stress-induced genomic instability being a causative factor underlying "escape" from oncogene-induced senescence.

TNF-α-producing macrophages determine subtype identity and prognosis via AP1 enhancer reprogramming in pancreatic cancer

Large-scale genomic profiling of pancreatic cancer (PDAC) has revealed two distinct subtypes: 'classical' and 'basal-like'. Their variable coexistence within the stromal immune microenvironment is linked to differential prognosis; however, the extent to which these neoplastic subtypes shape the stromal immune landscape and impact clinical outcome remains unclear. By combining preclinical models, patient-derived xenografts, as well as FACS-sorted PDAC patient biopsies, we show that the basal-like neoplastic state is sustained via BRD4-mediated cJUN/AP1 expression, which induces CCL2 to recruit tumor necrosis factor (TNF)-α-secreting macrophages. TNF-α+ macrophages force classical neoplastic cells into an aggressive phenotypic state via lineage reprogramming. Integration of ATAC-, ChIP- and RNA-seq data revealed distinct JUNB/AP1 (classical) and cJUN/AP1 (basal-like)-driven regulation of PDAC subtype identity. Pharmacological inhibition of BRD4 led to suppression of the BRD4-cJUN-CCL2-TNF-α axis, restoration of classical subtype identity and a favorable prognosis. Hence, patient-tailored therapy for a cJUNhigh/TNF-αhigh subtype is paramount in overcoming highly inflamed and aggressive PDAC states.

HMGB1 coordinates SASP-related chromatin folding and RNA homeostasis on the path to senescence

Spatial organization and gene expression of mammalian chromosomes are maintained and regulated in conjunction with cell cycle progression. This is perturbed once cells enter senescence and the highly abundant HMGB1 protein is depleted from nuclei to act as an extracellular proinflammatory stimulus. Despite its physiological importance, we know little about the positioning of HMGB1 on chromatin and its nuclear roles. To address this, we mapped HMGB1 binding genome‐wide in two primary cell lines. We integrated ChIP‐seq and Hi‐C with graph theory to uncover clustering of HMGB1‐marked topological domains that harbor genes involved in paracrine senescence. Using simplified Cross‐Linking and Immuno‐Precipitation and functional tests, we show that HMGB1 is also a bona fide RNA‐binding protein (RBP) binding hundreds of mRNAs. It presents an interactome rich in RBPs implicated in senescence regulation. The mRNAs of many of these RBPs are directly bound by HMGB1 and regulate availability of SASP‐relevant transcripts. Our findings reveal a broader than hitherto assumed role for HMGB1 in coordinating chromatin folding and RNA homeostasis as part of a regulatory loop controlling cell‐autonomous and paracrine senescence.

Overarching control of autophagy and DNA damage response by CHD6 revealed by modeling a rare human pathology

Members of the chromodomain-helicase-DNA binding (CHD) protein family are chromatin remodelers implicated in human pathologies, with CHD6 being one of its least studied members. We discovered a de novo CHD6 missense mutation in a patient clinically presenting the rare Hallermann-Streiff syndrome (HSS). We used genome editing to generate isogenic iPSC lines and model HSS in relevant cell types. By combining genomics with functional in vivo and in vitro assays, we show that CHD6 binds a cohort of autophagy and stress response genes across cell types. The HSS mutation affects CHD6 protein folding and impairs its ability to recruit co-remodelers in response to DNA damage or autophagy stimulation. This leads to accumulation of DNA damage burden and senescence-like phenotypes. We therefore uncovered a molecular mechanism explaining HSS onset via chromatin control of autophagic flux and genotoxic stress surveillance.

Genome folding and refolding in differentiation and cellular senescence

The spatial conformation of chromatin within the confines of eukaryotic cell nuclei is now acknowledged as a decisive epigenetic mechanism for the modulation of such cellular functions as gene expression regulation, DNA replication or DNA damage repair. Of course, these processes are tightly regulated during organismal development and markedly affected by cellular ageing. Thus, the question that arises is to what extent does folding or refolding of the genome in three-dimensional space underlie the progression of development or ageing? Herein, we discuss recent experimental and modelling evidence to address this question and revisit how these seemingly different processed might represent two sides of the same coin.

Distinct IL-1α-responsive enhancers promote acute and coordinated changes in chromatin topology in a hierarchical manner

How cytokine-driven changes in chromatin topology are converted into gene regulatory circuits during inflammation still remains unclear. Here, we show that interleukin (IL)-1α induces acute and widespread changes in chromatin accessibility via the TAK1 kinase and NF-κB at regions that are highly enriched for inflammatory disease-relevant SNPs. Two enhancers in the extended chemokine locus on human chromosome 4 regulate the IL-1α-inducible IL8 and CXCL1-3 genes. Both enhancers engage in dynamic spatial interactions with gene promoters in an IL-1α/TAK1-inducible manner. Microdeletions of p65-binding sites in either of the two enhancers impair NF-κB recruitment, suppress activation and biallelic transcription of the IL8/CXCL2 genes, and reshuffle higher-order chromatin interactions as judged by i4C interactome profiles. Notably, these findings support a dominant role of the IL8 "master" enhancer in the regulation of sustained IL-1α signaling, as well as for IL-8 and IL-6 secretion. CRISPR-guided transactivation of the IL8 locus or cross-TAD regulation by TNFα-responsive enhancers in a different model locus supports the existence of complex enhancer hierarchies in response to cytokine stimulation that prime and orchestrate proinflammatory chromatin responses downstream of NF-κB.

GemC1 governs multiciliogenesis through direct interaction with and transcriptional regulation of p73

A distinct combination of transcription factors elicits the acquisition of a specific fate and the initiation of a differentiation program. Multiciliated cells (MCCs) are a specialized type of epithelial cells that possess dozens of motile cilia on their apical surface. Defects in cilia function have been associated with ciliopathies that affect many organs, including brain and airway epithelium. Here we show that the geminin coiled-coil domain-containing protein 1 GemC1 (also known as Lynkeas) regulates the transcriptional activation of p73, a transcription factor central to multiciliogenesis. Moreover, we show that GemC1 acts in a trimeric complex with transcription factor E2F5 and tumor protein p73 (officially known as TP73), and that this complex is important for the activation of the p73 promoter. We also provide in vivo evidence that GemC1 is necessary for p73 expression in different multiciliated epithelia. We further show that GemC1 regulates multiciliogenesis through the control of chromatin organization, and the epigenetic marks/tags of p73 and Foxj 1. Our results highlight novel signaling cues involved in the commitment program of MCCs across species and tissues.This article has an associated First Person interview with the first author of the paper.

Anaerobic Glycolysis Maintains the Glomerular Filtration Barrier Independent of Mitochondrial Metabolism and Dynamics

The cellular responses induced by mitochondrial dysfunction remain elusive. Intrigued by the lack of almost any glomerular phenotype in patients with profound renal ischemia, we comprehensively investigated the primary sources of energy of glomerular podocytes. Combining functional measurements of oxygen consumption rates, glomerular metabolite analysis, and determination of mitochondrial density of podocytes in vivo, we demonstrate that anaerobic glycolysis and fermentation of glucose to lactate represent the key energy source of podocytes. Under physiological conditions, we could detect neither a developmental nor late-onset pathological phenotype in podocytes with impaired mitochondrial biogenesis machinery, defective mitochondrial fusion-fission apparatus, or reduced mtDNA stability and transcription caused by podocyte-specific deletion of Pgc-1α, Drp1, or Tfam, respectively. Anaerobic glycolysis represents the predominant metabolic pathway of podocytes. These findings offer a strategy to therapeutically interfere with the enhanced podocyte metabolism in various progressive kidney diseases, such as diabetic nephropathy or focal segmental glomerulosclerosis (FSGS).

Nucleotide Content Gradients in Maternally and Paternally Inherited Mitochondrial Genomes of the Mussel Mytilus

Several studies have shown that in vertebrate mtDNAs the nucleotide content at fourfold degenerate sites is well correlated with the site's time of exposure to the single-strand state, as predicted from the asymmetrical model of mtDNA replication. Here we examine whether the same explanation may hold for the regional variation in nucleotide content in the maternal and paternal mtDNAs of the mussel Mytilus galloprovincialis. The origin of replication of the heavy strand (O(H)) of these genomes has been previously established. A systematic search of the two genomes for sequences that are likely to act as the origin of replication of the light strand (O(L)) suggested that the most probable site lies within the ND3 gene. By adopting this O(L) position we calculated times of exposure for 0(FD) (nondegenerate), 2(FD) (twofold degenerate), and 4(FD) (fourfold degenerate) sites of the protein-coding part of the genome and for the rRNA, tRNA and noncoding parts. The presence of thymine and absence of guanine at 4(FD) sites was highly correlated with the presumed time of exposure. Such an effect was not found for the 2(FD) sites, the rRNA, the tRNA, or the noncoding parts. There was a trend for a small increase in cytosine at 0(FD) sites with exposure time, which is explicable as the result of biased usage of 4(FD) codons. The same analysis was applied to a recently sequenced mitochondrial genome of Mytilus trossulus and produced similar results. These results are consistent with the asymmetrical model of replication and suggest that guanine oxidation due to single-strand exposure is the main cause of regional variation of nucleotide content in Mytilus mitochondrial genomes.

Multiple events are responsible for an insertion in a paternally inherited mitochondrial genome of the mussel Mytilus galloprovincialis

In a sperm-transmitted mtDNA of Mytilus galloprovincialis we found an insertion that is not present in the typical genome and whose origin can be explained by a sequence of three events: a tandem duplication, a nonhomologous recombination, and a deletion. Unless such events are extremely rare in this species, the identical gene arrangement of the two gender-specific genomes should imply strong selection for same gene order and size.

The Complete Maternal and Paternal Mitochondrial Genomes of the Mediterranean Mussel Mytilus galloprovincialis: Implications for the Doubly Uniparental Inheritance Mode of mtDNA

The maternal (F) and paternal (M) mitochondrial genomes of the mussel Mytilus galloprovincialis have diverged by about 20% in nucleotide sequence but retained identical gene content and gene arrangement and similar nucleotide composition and codon usage bias. Both lack the ATPase8 subunit gene, have two tRNAs for methionine and a longer open-reading frame for cox3 than seen in other mollusks. Between the F and M genomes, tRNAs are most conserved followed by rRNAs and protein-coding genes, even though the degree of divergence varies considerably among the latter. Divergence at nad3 is exceptionally low most likely because this gene includes the origin of transcription of the lagging strand (O(L)). Noncoding regions are the least conserved with the notable exception of the central domain of the main control region and a segment of another noncoding region immediately following nad3. The amino acid divergence (14%) of the two genomes is smaller than in two other pairs of conspecific genomes that are available in GenBank, that of the clam Venerupis philippinarum (34%) and of the fresh water mussel Inversidens japanensis (50%), suggesting that doubly uniparental inheritance of mtDNA emerged at different times in the three species or that there has been a relatively recent replacement of the male genome by the female in the Mytilus line. The latter hypothesis is supported from phylogenetic and population studies of Mytilidae. That the M genome contains a full complement of genes with no premature termination codons argues against it being a selfish element that rides with the sperm. It is shorter than the F by 118 bp, which apparently cannot account for the postulated replicative advantage of this genome over the F in male gonads. The high similarity of the two genomes explains why the F genome may assume the role of the M genome, but it does not exclude the possibility that for this to happen some M-specific sequences must be transferred on to the F genome by means of recombination. If such sequences exist they would most likely be located in noncoding regions.