Simultaneous quantification of protein-DNA interactions and transcriptomes in single cells with scDam&T-seq.

Protein-DNA interactions are essential for establishing cell type-specific chromatin architecture and gene expression. We recently developed scDam&T-seq, a multi-omics method that can simultaneously quantify protein-DNA interactions and the transcriptome in single cells. The method effectively combines two existing methods: DNA adenine methyltransferase identification (DamID) and CEL-Seq2. DamID works through the tethering of a protein of interest (POI) to the Escherichia coli DNA adenine methyltransferase (Dam). Upon expression of this fusion protein, DNA in proximity to the POI is methylated by Dam and can be selectively digested and amplified. CEL-Seq2, in contrast, makes use of poly-dT primers to reverse transcribe mRNA, followed by linear amplification through in vitro transcription. scDam&T-seq is the first technique capable of providing a combined readout of protein-DNA contact and transcription from single-cell samples. Once suitable cell lines have been established, the protocol can be completed in 5 d, with a throughput of hundreds to thousands of cells. The processing of raw sequencing data takes an additional 1-2 d. Our method can be used to understand the transcriptional changes a cell undergoes upon the DNA binding of a POI. It can be performed in any laboratory with access to FACS, robotic and high-throughput-sequencing facilities.

Alcohol-derived DNA crosslinks are repaired by two distinct mechanisms.

Acetaldehyde is a highly reactive, DNA-damaging metabolite that is produced upon alcohol consumption1. Impaired detoxification of acetaldehyde is common in the Asian population, and is associated with alcohol-related cancers1,2. Cells are protected against acetaldehyde-induced damage by DNA crosslink repair, which when impaired causes Fanconi anaemia (FA), a disease resulting in failure to produce blood cells and a predisposition to cancer3,4. The combined inactivation of acetaldehyde detoxification and the FA pathway induces mutation, accelerates malignancies and causes the rapid attrition of blood stem cells5-7. However, the nature of the DNA damage induced by acetaldehyde and how this is repaired remains a key question. Here we generate acetaldehyde-induced DNA interstrand crosslinks and determine their repair mechanism in Xenopus egg extracts. We find that two replication-coupled pathways repair these lesions. The first is the FA pathway, which operates using excision-analogous to the mechanism used to repair the interstrand crosslinks caused by the chemotherapeutic agent cisplatin. However, the repair of acetaldehyde-induced crosslinks results in increased mutation frequency and an altered mutational spectrum compared with the repair of cisplatin-induced crosslinks. The second repair mechanism requires replication fork convergence, but does not involve DNA incisions-instead the acetaldehyde crosslink itself is broken. The Y-family DNA polymerase REV1 completes repair of the crosslink, culminating in a distinct mutational spectrum. These results define the repair pathways of DNA interstrand crosslinks caused by an endogenous and alcohol-derived metabolite, and identify an excision-independent mechanism.

Simultaneous quantification of protein-DNA contacts and transcriptomes in single cells.

Protein-DNA interactions are critical to the regulation of gene expression, but it remains challenging to define how cell-to-cell heterogeneity in protein-DNA binding influences gene expression variability. Here we report a method for the simultaneous quantification of protein-DNA contacts by combining single-cell DNA adenine methyltransferase identification (DamID) with messenger RNA sequencing of the same cell (scDam&T-seq). We apply scDam&T-seq to reveal how genome-lamina contacts or chromatin accessibility correlate with gene expression in individual cells. Furthermore, we provide single-cell genome-wide interaction data on a polycomb-group protein, RING1B, and the associated transcriptome. Our results show that scDam&T-seq is sensitive enough to distinguish mouse embryonic stem cells cultured under different conditions and their different chromatin landscapes. Our method will enable the analysis of protein-mediated mechanisms that regulate cell-type-specific transcriptional programs in heterogeneous tissues.

TGFß1-induced leucine limitation uncovered by differential ribosome codon reading.

Cancer cells modulate their metabolic networks to support cell proliferation and a higher demand of building blocks. These changes may restrict the availability of certain amino acids for protein synthesis, which can be utilized for cancer therapy. However, little is known about the amino acid demand changes occurring during aggressive and invasive stages of cancer. Recently, we developed diricore, an approach based on ribosome profiling that can uncover amino acid limitations. Here, we applied diricore to a cellular model in which epithelial breast cells respond rapidly to TGFß1, a cytokine essential for cancer progression and metastasis, and uncovered shortage of leucine. Further analyses indicated that TGFß1 treatment of human breast epithelial cells reduces the expression of SLC3A2, a subunit of the leucine transporter, which diminishes leucine uptake and inhibits cell proliferation. Thus, we identified a specific amino acid limitation associated with the TGFß1 response, a vulnerability that might be associated with aggressiveness in cancer.

Using mitoribosomal profiling to investigate human mitochondrial translation.

Background: Gene expression in human mitochondria has various idiosyncratic features. One of these was recently revealed as the unprecedented recruitment of a mitochondrially-encoded tRNA as a structural component of the large mitoribosomal subunit. In porcine particles this is mt-tRNA Phe whilst in humans it is mt-tRNA Val. We have previously shown that when a mutation in mt-tRNA Val causes very low steady state levels, there is preferential recruitment of mt-tRNA Phe. We have investigated whether this altered mitoribosome affects intra-organellar protein synthesis. Methods: By using mitoribosomal profiling we have revealed aspects of mitoribosome behaviour with its template mt-mRNA under both normal conditions as well as those where the mitoribosome has incorporated mt-tRNA Phe. Results: Analysis of the mitoribosome residency on transcripts under control conditions reveals that although mitochondria employ only 22 mt-tRNAs for protein synthesis, the use of non-canonical wobble base pairs at codon position 3 does not cause any measurable difference in mitoribosome occupancy irrespective of the codon. Comparison of the profile of aberrant mt-tRNA Phe containing mitoribosomes with those of controls that integrate mt-tRNA Val revealed that the impaired translation seen in the latter was not due to stalling on triplets encoding either of these amino acids. The alterations in mitoribosome interactions with start codons was not directly attributable to the either the use of non-cognate initiation codons or the presence or absence of 5' leader sequences, except in the two bicistronic RNA units, RNA7 and RNA14 where the initiation sites are internal. Conclusions: These data report the power of mitoribosomal profiling in helping to understand the subtleties of mammalian mitochondrial protein synthesis. Analysis of profiles from the mutant mt-tRNA Val cell line suggest that despite mt-tRNA Phe being preferred in the porcine mitoribosome, its integration into the human counterpart results in a suboptimal structure that modifies its interaction with mt-mRNAs.

Tumour-specific proline vulnerability uncovered by differential ribosome codon reading.

Tumour growth and metabolic adaptation may restrict the availability of certain amino acids for protein synthesis. It has recently been shown that certain types of cancer cells depend on glycine, glutamine, leucine and serine metabolism to proliferate and survive. In addition, successful therapies using L-asparaginase-induced asparagine deprivation have been developed for acute lymphoblastic leukaemia. However, a tailored detection system for measuring restrictive amino acids in each tumour is currently not available. Here we harness ribosome profiling for sensing restrictive amino acids, and develop diricore, a procedure for differential ribosome measurements of codon reading. We first demonstrate the functionality and constraints of diricore using metabolic inhibitors and nutrient deprivation assays. Notably, treatment with L-asparaginase elicited both specific diricore signals at asparagine codons and high levels of asparagine synthetase (ASNS). We then applied diricore to kidney cancer and discover signals indicating restrictive proline. As for asparagine, this observation was linked to high levels of PYCR1, a key enzyme in proline production, suggesting a compensatory mechanism allowing tumour expansion. Indeed, PYCR1 is induced by shortage of proline precursors, and its suppression attenuated kidney cancer cell proliferation when proline was limiting. High PYCR1 is frequently observed in invasive breast carcinoma. In an in vivo model system of this tumour, we also uncover signals indicating restrictive proline. We further show that CRISPR-mediated knockout of PYCR1 impedes tumorigenic growth in this system. Thus, diricore has the potential to reveal unknown amino acid deficiencies, vulnerabilities that can be used to target key metabolic pathways for cancer treatment.

Human bone marrow- and adipose-mesenchymal stem cells secrete exosomes enriched in distinctive miRNA and tRNA species.

INTRODUCTION: Administration of mesenchymal stem cells (MSCs) represents a promising treatment option for patients suffering from immunological and degenerative disorders. Accumulating evidence indicates that the healing effects of MSCs are mainly related to unique paracrine properties, opening opportunities for secretome-based therapies. Apart from soluble factors, MSCs release functional small RNAs via extracellular vesicles (EVs) that seem to convey essential features of MSCs. Here we set out to characterize the full small RNAome of MSC-produced exosomes. METHODS: We set up a protocol for isolating exosomes released by early passage adipose- (ASC) and bone marrow-MSCs (BMSC) and characterized them via electron microscopy, protein analysis and small RNA-sequencing. We developed a bioinformatics pipeline to define the exosome-enclosed RNA species and performed the first complete small RNA characterization of BMSCs and ASCs and their corresponding exosomes in biological replicates. RESULTS: Our analysis revealed that primary ASCs and BMSCs have highly similar small RNA expression profiles dominated by miRNAs and snoRNAs (together 64-71 %), of which 150-200 miRNAs are present at physiological levels. In contrast, the miRNA pool in MSC exosomes is only 2-5 % of the total small RNAome and is dominated by a minor subset of miRNAs. Nevertheless, the miRNAs in exosomes do not merely reflect the cellular content and a defined set of miRNAs are overrepresented in exosomes compared to the cell of origin. Moreover, multiple highly expressed miRNAs are precluded from exosomal sorting, consistent with the notion that these miRNAs are involved in functional repression of RNA targets. While ASC and BMSC exosomes are similar in RNA class distribution and composition, we observed striking differences in the sorting of evolutionary conserved tRNA species that seems associated with the differentiation status of MSCs, as defined by Sox2, POU5F1A/B and Nanog expression. CONCLUSIONS: We demonstrate that primary MSCs release small RNAs via exosomes, which are increasingly implicated in intercellular communications. tRNAs species, and in particular tRNA halves, are preferentially released and their specific sorting into exosomes is related to MSC tissue origin and stemness. These findings may help to understand how MSCs impact neighboring or distant cells with possible consequences for their therapeutic usage.

Parasite-induced ER stress response in hepatocytes facilitates Plasmodium liver stage infection.

Upon infection of a mammalian host, Plasmodium parasites first replicate inside hepatocytes, generating thousands of new parasites. Although Plasmodium intra-hepatic development represents a substantial metabolic challenge to the host hepatocyte, how infected cells respond to and integrate this stress remains poorly understood. Here, we present proteomic and transcriptomic analyses, revealing that the endoplasmic reticulum (ER)-resident unfolded protein response (UPR) is activated in host hepatocytes upon Plasmodium berghei infection. The expression of XBP1s--the active form of the UPR mediator XBP1--and the liver-specific UPR mediator CREBH is induced by P. berghei infection in vivo. Furthermore, this UPR induction increases parasite liver burden. Altogether, our data suggest that ER stress is a central feature of P. berghei intra-hepatic development, contributing to the success of infection.

Exposure to arginine analog canavanine induces aberrant mitochondrial translation products, mitoribosome stalling, and instability of the mitochondrial proteome.

Impairment of mitochondrial protein homeostasis disrupts mitochondrial function and causes human diseases and aging, but the molecular mechanisms of protein synthesis and quality control in mammalian mitochondria are not fully understood. Here we demonstrate in human cells that misincorporation of an arginine analog, canavanine, during mitochondrial protein synthesis, induced aberrant translation products and destabilized the mtDNA-encoded proteome, leading to loss of mitochondrial respiratory chain complexes. Furthermore, in the presence of a high concentration of canavanine, mitoribosome stalling could be demonstrated. The stalling did not, however, occur at arginine codons, but downstream of those codons. In particular, two adjacent arginines induced the most prominent downstream stalling effect, with the distance between the arginine codons and the stalling peak corresponding roughly to the length of the ribosomal exit tunnel. These results suggest that misincorporated canavanine disrupted the proper folding of the hydrophobic nascent polypeptides within the exit tunnel or while being inserted into the inner mitochondrial membrane. The canavanine treatment provides a model system for studying the consequences of mitoribosome stalling and the responses to misfolded proteins exiting the mitochondrial ribosome.

Genome-wide profiling of p53-regulated enhancer RNAs uncovers a subset of enhancers controlled by a lncRNA.

p53 binds enhancers to regulate key target genes. Here, we globally mapped p53-regulated enhancers by looking at enhancer RNA (eRNA) production. Intriguingly, while many p53-induced enhancers contained p53-binding sites, most did not. As long non-coding RNAs (lncRNAs) are prominent regulators of chromatin dynamics, we hypothesized that p53-induced lncRNAs contribute to the activation of enhancers by p53. Among p53-induced lncRNAs, we identified LED and demonstrate that its suppression attenuates p53 function. Chromatin-binding and eRNA expression analyses show that LED associates with and activates strong enhancers. One prominent target of LED was located at an enhancer region within CDKN1A gene, a potent p53-responsive cell cycle inhibitor. LED knockdown reduces CDKN1A enhancer induction and activity, and cell cycle arrest following p53 activation. Finally, promoter-associated hypermethylation analysis shows silencing of LED in human tumours. Thus, our study identifies a new layer of complexity in the p53 pathway and suggests its dysregulation in cancer.

Ribosome profiling reveals features of normal and disease-associated mitochondrial translation.

Mitochondria are essential cellular organelles for generation of energy and their dysfunction may cause diabetes, Parkinson's disease and multi-systemic failure marked by failure to thrive, gastrointestinal problems, lactic acidosis and early lethality. Disease-associated mitochondrial mutations often affect components of the mitochondrial translation machinery. Here we perform ribosome profiling to measure mitochondrial translation at nucleotide resolution. Using a protocol optimized for the retrieval of mitochondrial ribosome protected fragments (RPFs) we show that the size distribution of wild-type mitochondrial RPFs follows a bimodal distribution peaking at 27 and 33 nucleotides, which is distinct from the 30-nucleotide peak of nuclear RPFs. Their cross-correlation suggests generation of mitochondrial RPFs during ribosome progression. In contrast, RPFs from patient-derived mitochondria mutated in tRNA-Tryptophan are centered on tryptophan codons and reduced downstream, indicating ribosome stalling. Intriguingly, long RPFs are enriched in mutated mitochondria, suggesting they characterize stalled ribosomes. Our findings provide the first model for translation in wild-type and disease-triggering mitochondria.

p53 induces transcriptional and translational programs to suppress cell proliferation and growth.

BACKGROUND: Cell growth and proliferation are tightly connected to ensure that appropriately sized daughter cells are generated following mitosis. Energy stress blocks cell growth and proliferation, a critical response for survival under extreme conditions. Excessive oncogenic stress leads to p53 activation and the induction of senescence, an irreversible state of cell-cycle arrest and a critical component in the suppression of tumorigenesis. Nutrient-sensing and mitogenic cues converge on a major signaling node, which regulates the activity of the mTOR kinase. Although transcriptional responses to energy and oncogenic stresses have been examined by many gene-expression experiments, a global exploration of the modulation of mRNA translation in response to these conditions is lacking. RESULTS: We combine RNA sequencing and ribosomal profiling analyses to systematically delineate modes of transcriptional and translational regulation induced in response to conditions of limited energy, oncogenic stress and cellular transformation. We detect a key role for mTOR and p53 in these distinct physiological states, and provide the first genome-wide demonstration that p53 activation results in mTOR inhibition and a consequent global repression of protein translation. We confirm the role of the direct p53 target genes Sestrin1 and Sestrin2 in this response, as part of the broad modulation of gene expression induced by p53 activation. CONCLUSIONS: We delineate a bimodal tumor-suppressive regulatory program activated by p53, in which cell-cycle arrest is imposed mainly at the transcriptional level, whereas cell growth inhibition is enforced by global repression of the translation machinery.

Alternative cleavage and polyadenylation during colorectal cancer development.

PURPOSE: Alternative cleavage and polyadenylation (APA) of mRNAs is a phenomenon that alters 3'-untranslated region length leading to altered posttranscriptional regulation of gene expression. Changing APA patterns have been shown to result in misregulation of genes involved in carcinogenesis; therefore, we hypothesized that altered APA contributes to progression of colorectal cancer, and that measurement of APA may lead to discovery of novel biomarkers. EXPERIMENTAL DESIGN: We used next-generation sequencing to directly measure global patterns of APA changes during colorectal carcinoma progression in 15 human patient samples. Results were validated in a larger cohort of 50 patients, including 5 normal/carcinoma pairs from individuals. RESULTS: We discovered numerous genes presenting progressive changes in APA. Genes undergoing untranslated region (3'UTR) shortening were enriched for functional groups such as cell-cycle and nucleic acid-binding and processing factors, and those undergoing 3'UTR lengthening or alternative 3'UTR usage were enriched for categories such as cell-cell adhesion and extracellular matrix. We found indications that APA changes result from differential processing of transcripts because of increased expression of cleavage and polyadenylation factors. Quantitative PCR analysis in a larger series of human patient samples, including matched pairs, confirmed APA changes in DMKN, PDXK, and PPIE genes. CONCLUSIONS: Our results suggest that genes undergoing altered APA during human cancer progression may be useful novel biomarkers and potentially targeted for disease prevention and treatment. We propose that the strategy presented here may be broadly useful in discovery of novel biomarkers for other types of cancer and human disease.

The poly(A)-binding protein nuclear 1 suppresses alternative cleavage and polyadenylation sites.

Alternative cleavage and polyadenylation (APA) is emerging as an important layer of gene regulation. Factors controlling APA are largely unknown. We developed a reporter-based RNAi screen for APA and identified PABPN1 as a regulator of this process. Genome-wide analysis of APA in human cells showed that loss of PABPN1 resulted in extensive 3' untranslated region shortening. Messenger RNA transcription, stability analyses, and in vitro cleavage assays indicated enhanced usage of proximal cleavage sites (CSs) as the underlying mechanism. Using Cyclin D1 as a test case, we demonstrated that enhanced usage of proximal CSs compromises microRNA-mediated repression. Triplet-repeat expansion in PABPN1 (trePABPN1) causes autosomal-dominant oculopharyngeal muscular dystrophy (OPMD). The expression of trePABPN1 in both a mouse model of OPMD and human cells elicited broad induction of proximal CS usage, linked to binding to endogenous PABPN1 and its sequestration in nuclear aggregates. Our results elucidate a novel function for PABPN1 as a suppressor of APA.

An iterative workflow for mining the human intestinal metaproteome.

BACKGROUND: Peptide spectrum matching (PSM) is the standard method in shotgun proteomics data analysis. It relies on the availability of an accurate and complete sample proteome that is used to make interpretation of the spectra feasible. Although this procedure has proven to be effective in many proteomics studies, the approach has limitations when applied on complex samples of microbial communities, such as those found in the human intestinal tract. Metagenome studies have indicated that the human intestinal microbiome contains over 100 times more genes than the human genome and it has been estimated that this ecosystem contains over 5000 bacterial species. The genomes of the vast majority of these species have not yet been sequenced and hence their proteomes remain unknown. To enable data analysis of shotgun proteomics data using PSM, and circumvent the lack of a defined matched metaproteome, an iterative workflow was developed that is based on a synthetic metaproteome and the developing metagenomic databases that are both representative for but not necessarily originating from the sample of interest. RESULTS: Two human fecal samples for which metagenomic data had been collected, were analyzed for their metaproteome using liquid chromatography-mass spectrometry and used to benchmark the developed iterative workflow to other methods. The results show that the developed method is able to detect over 3,000 peptides per fecal sample from the spectral data by circumventing the lack of a defined proteome without naive translation of matched metagenomes and cross-species peptide identification. CONCLUSIONS: The developed iterative workflow achieved an approximate two-fold increase in the amount of identified spectra at a false discovery rate of 1% and can be applied in metaproteomic studies of the human intestinal tract or other complex ecosystems.