Showing papers by "Laboratory of Molecular Biology published in 2017"

The Human Cell Atlas

Abstract: The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

Cryo-EM structures of tau filaments from Alzheimer’s disease

Abstract: Alzheimer’s disease is the most common neurodegenerative disease, and there are no mechanism-based therapies. The disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. Neurofibrillary lesions comprise paired helical and straight tau filaments, whereas tau filaments with different morphologies characterize other neurodegenerative diseases. No high-resolution structures of tau filaments are available. Here we present cryo-electron microscopy (cryo-EM) maps at 3.4–3.5 A resolution and corresponding atomic models of paired helical and straight filaments from the brain of an individual with Alzheimer’s disease. Filament cores are made of two identical protofilaments comprising residues 306–378 of tau protein, which adopt a combined cross-β/β-helix structure and define the seed for tau aggregation. Paired helical and straight filaments differ in their inter-protofilament packing, showing that they are ultrastructural polymorphs. These findings demonstrate that cryo-EM allows atomic characterization of amyloid filaments from patient-derived material, and pave the way for investigation of a range of neurodegenerative diseases. High-resolution structures of tau filaments shed light on the ultrastructure of neurofibrillary lesions in Alzheimer’s disease. Alzheimer's disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. The lesions are made of paired helical and straight tau filaments (PHFs and SFs, respectively). Different tau filaments characterize other neurodegenerative diseases, suggesting that molecular conformers of aggregated tau underlie human tauopathies. No high-resolution structures of tau filaments are currently available. Here, Sjors Scheres and colleagues present cryo-electron microscopy (cryo-EM) maps at 3.5 A resolution and corresponding atomic models of PHFs and SFs from the brain of an individual with Alzheimer's disease. Their results show that cryo-EM enables atomic characterization of amyloid filaments from patient-derived material and could be used to study a range of neurodegenerative diseases.

3D structures of individual mammalian genomes studied by single-cell Hi-C

Abstract: The folding of genomic DNA from the beads-on-a-string-like structure of nucleosomes into higher-order assemblies is crucially linked to nuclear processes. Here we calculate 3D structures of entire mammalian genomes using data from a new chromosome conformation capture procedure that allows us to first image and then process single cells. The technique enables genome folding to be examined at a scale of less than 100 kb, and chromosome structures to be validated. The structures of individual topological-associated domains and loops vary substantially from cell to cell. By contrast, A and B compartments, lamina-associated domains and active enhancers and promoters are organized in a consistent way on a genome-wide basis in every cell, suggesting that they could drive chromosome and genome folding. By studying genes regulated by pluripotency factor and nucleosome remodelling deacetylase (NuRD), we illustrate how the determination of single-cell genome structure provides a new approach for investigating biological processes.

Cryo-EM structures of tau filaments from Alzheimer's disease

Abstract: Alzheimer’s disease is the most common neurodegenerative disease, and there are no mechanism-based therapies. The disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. Neurofibrillary lesions comprise paired helical and straight tau filaments, whereas tau filaments with different morphologies characterize other neurodegenerative diseases. No high-resolution structures of tau filaments are available. Here we present cryo-electron microscopy (cryo-EM) maps at 3.4–3.5 A resolution and corresponding atomic models of paired helical and straight filaments from the brain of an individual with Alzheimer’s disease. Filament cores are made of two identical protofilaments comprising residues 306–378 of tau protein, which adopt a combined cross-β/β-helix structure and define the seed for tau aggregation. Paired helical and straight filaments differ in their inter-protofilament packing, showing that they are ultrastructural polymorphs. These findings demonstrate that cryo-EM allows atomic characterization of amyloid filaments from patient-derived material, and pave the way for investigation of a range of neurodegenerative diseases. High-resolution structures of tau filaments shed light on the ultrastructure of neurofibrillary lesions in Alzheimer’s disease. Alzheimer's disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. The lesions are made of paired helical and straight tau filaments (PHFs and SFs, respectively). Different tau filaments characterize other neurodegenerative diseases, suggesting that molecular conformers of aggregated tau underlie human tauopathies. No high-resolution structures of tau filaments are currently available. Here, Sjors Scheres and colleagues present cryo-electron microscopy (cryo-EM) maps at 3.5 A resolution and corresponding atomic models of PHFs and SFs from the brain of an individual with Alzheimer's disease. Their results show that cryo-EM enables atomic characterization of amyloid filaments from patient-derived material and could be used to study a range of neurodegenerative diseases.

Mechanisms of Deubiquitinase Specificity and Regulation

Abstract: Protein ubiquitination is one of the most powerful posttranslational modifications of proteins, as it regulates a plethora of cellular processes in distinct manners. Simple monoubiquitination events coexist with more complex forms of polyubiquitination, the latter featuring many different chain architectures. Ubiquitin can be subjected to further posttranslational modifications (e.g., phosphorylation and acetylation) and can also be part of mixed polymers with ubiquitin-like modifiers such as SUMO (small ubiquitin-related modifier) or NEDD8 (neural precursor cell expressed, developmentally downregulated 8). Together, cellular ubiquitination events form a sophisticated and versatile ubiquitin code. Deubiquitinases (DUBs) reverse ubiquitin signals with equally high sophistication. In this review, we conceptualize the many layers of specificity that DUBs encompass to control the ubiquitin code and discuss examples in which DUB specificity has been understood at the molecular level. We further discuss the many mechanisms of DUB regulation with a focus on those that modulate catalytic activity. Our review provides a framework to tackle lingering questions in DUB biology.

Guided self-organization and cortical plate formation in human brain organoids

Abstract: Three-dimensional cell culture models have either relied on the self-organizing properties of mammalian cells or used bioengineered constructs to arrange cells in an organ-like configuration. While self-organizing organoids excel at recapitulating early developmental events, bioengineered constructs reproducibly generate desired tissue architectures. Here, we combine these two approaches to reproducibly generate human forebrain tissue while maintaining its self-organizing capacity. We use poly(lactide-co-glycolide) copolymer (PLGA) fiber microfilaments as a floating scaffold to generate elongated embryoid bodies. Microfilament-engineered cerebral organoids (enCORs) display enhanced neuroectoderm formation and improved cortical development. Furthermore, reconstitution of the basement membrane leads to characteristic cortical tissue architecture, including formation of a polarized cortical plate and radial units. Thus, enCORs model the distinctive radial organization of the cerebral cortex and allow for the study of neuronal migration. Our data demonstrate that combining 3D cell culture with bioengineering can increase reproducibility and improve tissue architecture.

Expanding and reprogramming the genetic code.

Abstract: Nature uses a limited, conservative set of amino acids to synthesize proteins. The ability to genetically encode an expanded set of building blocks with new chemical and physical properties is transforming the study, manipulation and evolution of proteins, and is enabling diverse applications, including approaches to probe, image and control protein function, and to precisely engineer therapeutics. Underpinning this transformation are strategies to engineer and rewire translation. Emerging strategies aim to reprogram the genetic code so that noncanonical biopolymers can be synthesized and evolved, and to test the limits of our ability to engineer the translational machinery and systematically recode genomes.

Propagation of Tau Aggregates and Neurodegeneration.

Abstract: A pathway from the natively unfolded microtubule-associated protein Tau to a highly structured amyloid fibril underlies human Tauopathies. This ordered assembly causes disease and represents the gain of toxic function. In recent years, evidence has accumulated to suggest that Tau inclusions form first in a small number of brain cells, from where they propagate to other regions, resulting in neurodegeneration and disease. Propagation of pathology is often called prion-like, which refers to the capacity of an assembled protein to induce the same abnormal conformation in a protein of the same kind, initiating a self-amplifying cascade. In addition, prion-like encompasses the release of protein aggregates from brain cells and their uptake by neighboring cells. In mice, the intracerebral injection of Tau inclusions induces the ordered assembly of monomeric Tau, followed by its spreading to distant brain regions. Conformational differences between Tau aggregates from transgenic mouse brain and in vitro assembled recombinant protein account for the greater seeding potency of brain aggregates. Short fibrils constitute the major species of seed-competent Tau in the brains of transgenic mice. The existence of multiple human Tauopathies with distinct fibril morphologies has led to the suggestion that different molecular conformers (or strains) of aggregated Tau exist.

The Synucleinopathies: Twenty Years On.

Abstract: In 2017, it is two hundred years since James Parkinson provided the first complete clinical description of the disease named after him, fifty years since the introduction of high-dose D,L-DOPA treatment and twenty years since α-synuclein aggregation came to the fore. In 1998, multiple system atrophy joined Parkinson’s disease and dementia with Lewy bodies as the third major synucleinopathy. Here we review our work, which led to the identification of α-synuclein in Lewy bodies, Lewy neurites and Papp-Lantos bodies, as well as what has happened since. Some of the experiments described were carried out in collaboration with ML Schmidt, VMY Lee and JQ Trojanowski.

Network control principles predict neuron function in the Caenorhabditis elegans connectome

Abstract: Recent studies on the controllability of complex systems offer a powerful mathematical framework to systematically explore the structure-function relationship in biological, social, and technological networks. Despite theoretical advances, we lack direct experimental proof of the validity of these widely used control principles. Here we fill this gap by applying a control framework to the connectome of the nematode Caenorhabditis elegans, allowing us to predict the involvement of each C. elegans neuron in locomotor behaviours. We predict that control of the muscles or motor neurons requires 12 neuronal classes, which include neuronal groups previously implicated in locomotion by laser ablation, as well as one previously uncharacterized neuron, PDB. We validate this prediction experimentally, finding that the ablation of PDB leads to a significant loss of dorsoventral polarity in large body bends. Importantly, control principles also allow us to investigate the involvement of individual neurons within each neuronal class. For example, we predict that, within the class of DD motor neurons, only three (DD04, DD05, or DD06) should affect locomotion when ablated individually. This prediction is also confirmed; single cell ablations of DD04 or DD05 specifically affect posterior body movements, whereas ablations of DD02 or DD03 do not. Our predictions are robust to deletions of weak connections, missing connections, and rewired connections in the current connectome, indicating the potential applicability of this analytical framework to larger and less well-characterized connectomes.

Molecular basis of USP7 inhibition by selective small-molecule inhibitors

Abstract: Ubiquitination controls the stability of most cellular proteins, and its deregulation contributes to human diseases including cancer. Deubiquitinases remove ubiquitin from proteins, and their inhibition can induce the degradation of selected proteins, potentially including otherwise 'undruggable' targets. For example, the inhibition of ubiquitin-specific protease 7 (USP7) results in the degradation of the oncogenic E3 ligase MDM2, and leads to re-activation of the tumour suppressor p53 in various cancers. Here we report that two compounds, FT671 and FT827, inhibit USP7 with high affinity and specificity in vitro and within human cells. Co-crystal structures reveal that both compounds target a dynamic pocket near the catalytic centre of the auto-inhibited apo form of USP7, which differs from other USP deubiquitinases. Consistent with USP7 target engagement in cells, FT671 destabilizes USP7 substrates including MDM2, increases levels of p53, and results in the transcription of p53 target genes, induction of the tumour suppressor p21, and inhibition of tumour growth in mice.

Selectivity determinants of GPCR–G-protein binding

Abstract: The selective coupling of G-protein-coupled receptors (GPCRs) to specific G proteins is critical to trigger the appropriate physiological response. However, the determinants of selective binding have remained elusive. Here we reveal the existence of a selectivity barcode (that is, patterns of amino acids) on each of the 16 human G proteins that is recognized by distinct regions on the approximately 800 human receptors. Although universally conserved positions in the barcode allow the receptors to bind and activate G proteins in a similar manner, different receptors recognize the unique positions of the G-protein barcode through distinct residues, like multiple keys (receptors) opening the same lock (G protein) using non-identical cuts. Considering the evolutionary history of GPCRs allows the identification of these selectivity-determining residues. These findings lay the foundation for understanding the molecular basis of coupling selectivity within individual receptors and G proteins.

Electric Fields and Enzyme Catalysis

Abstract: What happens inside an enzyme's active site to allow slow and difficult chemical reactions to occur so rapidly? This question has occupied biochemists' attention for a long time. Computer models of increasing sophistication have predicted an important role for electrostatic interactions in enzymatic reactions, yet this hypothesis has proved vexingly difficult to test experimentally. Recent experiments utilizing the vibrational Stark effect make it possible to measure the electric field a substrate molecule experiences when bound inside its enzyme's active site. These experiments have provided compelling evidence supporting a major electrostatic contribution to enzymatic catalysis. Here, we review these results and develop a simple model for electrostatic catalysis that enables us to incorporate disparate concepts introduced by many investigators to describe how enzymes work into a more unified framework stressing the importance of electric fields at the active site.

Self‐organized developmental patterning and differentiation in cerebral organoids

Abstract: Cerebral organoids recapitulate human brain development at a considerable level of detail, even in the absence of externally added signaling factors. The patterning events driving this self‐organization are currently unknown. Here, we examine the developmental and differentiative capacity of cerebral organoids. Focusing on forebrain regions, we demonstrate the presence of a variety of discrete ventral and dorsal regions. Clearing and subsequent 3D reconstruction of entire organoids reveal that many of these regions are interconnected, suggesting that the entire range of dorso‐ventral identities can be generated within continuous neuroepithelia. Consistent with this, we demonstrate the presence of forebrain organizing centers that express secreted growth factors, which may be involved in dorso‐ventral patterning within organoids. Furthermore, we demonstrate the timed generation of neurons with mature morphologies, as well as the subsequent generation of astrocytes and oligodendrocytes. Our work provides the methodology and quality criteria for phenotypic analysis of brain organoids and shows that the spatial and temporal patterning events governing human brain development can be recapitulated in vitro .

Guideline of Prevention and Treatment for Chronic Hepatitis B (2015 Update).

Abstract: Department of Infectious Diseases, Nanfang Hospital, Southern Medical University, Guangzhou, China; Department of Infectious Diseases, Center for Liver Diseases, Peking University First Hospital, Beijing, China; The Institute of Translational Hepatology, 302 Hospital of PLA, Peking University, Beijing, China; Center of Liver Diseases, Beijing Ditan Hospital, Capital Medical University, Beijing, China; Institute for Viral Hepatitis, the Key Laboratory of Molecular Biology for Infectious Diseases, the second Affiliated Hospital of Chongqing Medical University, Chongqing, China; Department of Microbiology of Peking University Health Science Center, Beijing, China; State Key Laboratory for Diagnosis and Treatment of Infectious Diseases, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, the First Affiliated Hospital, College of Medicine, Zhejiang University, Hangzhou, China; Serious Illness Medicine Inpatient Area, Beijing Youan Hospital, Capital Medical University, Beijing, China; Department of Pathology, 302 Hospital of PLA, Peking University, Beijing, China; Artificial Liver Center, Beijing Youan Hospital, Capital Medical University, Beijing, China; Liver Research Center, Beijing Friendship Hospital, Capital Medical University, Beijing, China; Center of Infectious Diseases, West China Hospital of Sichuan University, Division of Infectious Diseases, State Key Laboratory of Biotherapy, Sichuan University, Chengdu, China; Department of Infectious Diseases, Shanghai Jiao Tong University School of Medicine Affiliated Ruijin Hospital, Shanghai, China; Hepatology Institute, Peking University People’s Hospital, Beijing, China

Guidelines for morpholino use in zebrafish

Abstract: 1 Department of Developmental Genetics, Max Planck Institute for Heart and Lung Research, Bad Nauheim, Germany, 2 Institute of Cell Biology, ZBME, University of Münster, Münster, Germany, 3 Department of Molecular, Cell, and Cancer Biology, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America, 4 Mayo Clinic, Rochester, Minnesota, United States of America, 5 Department of Molecular Biology, Princeton University, Princeton, New Jersey, United States of America, 6 Institute of Neuroscience, University of Oregon, Eugene, Oregon, United States of America, 7 Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, 8 The Living Systems Institute, University of Exeter, Exeter, United Kingdom, 9 Institute of Cardiovascular Organogenesis and Regeneration, WWU Münster, Faculty of Medicine, Münster, Germany, 10 Division of Biological Sciences, University of California, San Diego, La Jolla, California, United States of America, 11 Division of Developmental Biology, NICHD, NIH, Bethesda, Maryland, United States of America, 12 Department of Cell and Developmental Biology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania, United States of America, 13 Department of Cell and Developmental Biology, University College London, London, United Kingdom, 14 Molecular Immunity Unit, Department of Medicine, University of Cambridge, MRC Laboratory of Molecular Biology, Cambridge, United Kingdom, 15 Departments of Molecular Genetics and Biological Chemistry and Pharmacology, Ohio State University, Columbus, Ohio, United States of America, 16 Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland, 17 School of Life Sciences, Tsinghua University, Beijing, China, 18 National Cerebral and Cardiovascular Center Research Institute, Osaka, Japan, 19 Department of Anatomy and Neuroscience Center, University of Helsinki, Helsinki, Finland, 20 Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America

What is the evidence that tau pathology spreads through prion-like propagation?

Abstract: Emerging experimental evidence suggests that the spread of tau pathology in the brain in Tauopathies reflects the propagation of abnormal tau species along neuroanatomically connected brain areas. This propagation could occur through a “prion-like” mechanism involving transfer of abnormal tau seeds from a “donor cell” to a “recipient cell” and recruitment of normal tau in the latter to generate new tau seeds. This review critically appraises the evidence that the spread of tau pathology occurs via such a “prion-like” mechanism and proposes a number of recommendations for directing future research. Recommendations for definitions of frequently used terms in the tau field are presented in an attempt to clarify and standardize interpretation of research findings. Molecular and cellular factors affecting tau aggregation are briefly reviewed, as are potential contributions of physiological and pathological post-translational modifications of tau. Additionally, the experimental evidence for tau seeding and “prion-like” propagation of tau aggregation that has emerged from cellular assays and in vivo models is discussed. Propagation of tau pathology using “prion-like” mechanisms is expected to incorporate several steps including cellular uptake, templated seeding, secretion and intercellular transfer through synaptic and non-synaptic pathways. The experimental findings supporting each of these steps are reviewed. The clinical validity of these experimental findings is then debated by considering the supportive or contradictory findings from patient samples. Further, the role of physiological tau release in this scenario is examined because emerging data shows that tau is secreted but the physiological function (if any) of this secretion in the context of propagation of pathological tau seeds is unclear. Bona fide prions exhibit specific properties, including transmission from cell to cell, tissue to tissue and organism to organism. The propagation of tau pathology has so far not been shown to exhibit all of these steps and how this influences the debate of whether or not abnormal tau species can propagate in a “prion-like” manner is discussed. The exact nature of tau seeds responsible for propagation of tau pathology in human tauopathies remains controversial; it might be tightly linked to the existence of tau strains stably propagating peculiar patterns of neuropathological lesions, corresponding to the different patterns seen in human tauopathies. That this is a property shared by all seed-competent tau conformers is not yet firmly established. Further investigation is also required to clarify the relationship between propagation of tau aggregates and tau-induced toxicity. Genetic variants identified as risks factors for tauopathies might play a role in propagation of tau pathology, but many more studies are needed to document this. The contribution of selective vulnerability of neuronal populations, as an alternative to prion-like mechanisms to explain spreading of tau pathology needs to be clarified. Learning from the prion field will be helpful to enhance our understanding of propagation of tau pathology. Finally, development of better models is expected to answer some of these key questions and allow for the testing of propagation-centred therapies.

Like prions: the propagation of aggregated tau and α-synuclein in neurodegeneration

Abstract: The abnormal aggregation of a small number of known proteins underlies the most common human neurodegenerative diseases. In tauopathies and synucleinopathies, the normally soluble intracellular proteins tau and α-synuclein become insoluble and filamentous. In recent years, non-cell autonomous mechanisms of aggregate formation have come to the fore, suggesting that nucleation-dependent aggregation may occur in a localized fashion in human tauopathies and synucleinopathies, followed by seed-dependent propagation. There is a long prodromal phase between the formation of protein aggregates and the appearance of the first clinical symptoms, which manifest only after extensive propagation, opening novel therapeutic avenues.

A pipeline approach to single-particle processing in RELION.

Abstract: The formal concept of a workflow to single-particle analysis of cryo-electron microscopy (cryo-EM) images in the RELION program is described. In this approach, the structure-determination process is considered as a graph, where intermediate results in the form of images or metadata are the vertices, and different functionalities of the program are the edges. The new implementation automatically logs all user actions, facilitates file management and disk cleaning, and allows convenient browsing of the history of a project. Moreover, new functionality to iteratively execute consecutive jobs allows on-the-fly image processing, which will lead to more efficient data acquisition by providing faster feedback on data quality. The possibility of exchanging data-processing procedures among users will contribute to the development of standardized image-processing procedures, and hence increase accessibility for new users in this rapidly expanding field.

Ferritin, cellular iron storage and regulation

Abstract: Ferritin is considered the major iron storage protein which maintains a large iron core in its cavity and has ferroxidase activity. There are many types of ferritin particularly in prokaryotes that include the canonical 24-mer FTN molecules, the heme-containing BFR, the smaller 12-mer DPS and the newly recognized EncFtn of encapsulin that forms a very large iron storage compartment. Recent studies show that ferritin function is more dynamic than previous depicted and new mechanisms of ferritin iron recycling are emerging. They participate to the regulation of cellular iron homeostasis as those of ferritin biosynthesis, cooperating also with the iron-dependent mechanism of cellular iron secretion. Some of these basic processes are in common between unicellular and animal cells, and this review aims at discussing the findings on the connections between iron storage, cellular iron regulation and ferritin iron recycling that have been explored in unicellular organisms and in animals. © 2017 IUBMB Life, 69(6):414-422, 2017.

A Complete Electron Microscopy Volume Of The Brain Of Adult Drosophila melanogaster

Abstract: Drosophila melanogaster has a rich repertoire of innate and learned behaviors Its 100,000-neuron brain is a large but tractable target for comprehensive neural circuit mapping Only electron microscopy (EM) enables complete, unbiased mapping of synaptic connectivity; however, the fly brain is too large for conventional EM We developed a custom high-throughput EM platform and imaged the entire brain of an adult female fly We validated the dataset by tracing brain-spanning circuitry involving the mushroom body (MB), intensively studied for its role in learning Here we describe the complete set of olfactory inputs to the MB; find a new cell type providing driving input to Kenyon cells (the intrinsic MB neurons); identify neurons postsynaptic to Kenyon cell dendrites; and find that axonal arbors providing input to the MB calyx are more tightly clustered than previously indicated by light-level data This freely available EM dataset will significantly accelerate Drosophila neuroscience

Mammals divert endogenous genotoxic formaldehyde into one-carbon metabolism

Abstract: The folate-driven one-carbon (1C) cycle is a fundamental metabolic hub in cells that enables the synthesis of nucleotides and amino acids and epigenetic modifications. This cycle might also release formaldehyde, a potent protein and DNA crosslinking agent that organisms produce in substantial quantities. Here we show that supplementation with tetrahydrofolate, the essential cofactor of this cycle, and other oxidation-prone folate derivatives kills human, mouse and chicken cells that cannot detoxify formaldehyde or that lack DNA crosslink repair. Notably, formaldehyde is generated from oxidative decomposition of the folate backbone. Furthermore, we find that formaldehyde detoxification in human cells generates formate, and thereby promotes nucleotide synthesis. This supply of 1C units is sufficient to sustain the growth of cells that are unable to use serine, which is the predominant source of 1C units. These findings identify an unexpected source of formaldehyde and, more generally, indicate that the detoxification of this ubiquitous endogenous genotoxin creates a benign 1C unit that can sustain essential metabolism.