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The Babraham Institute Publications database contains details of all publications resulting from our research groups and scientific facilities. Pre-prints by Institute authors can be viewed on the Institute's bioRxiv channel. We believe that free and open access to the outputs of publicly‐funded research offers significant social and economic benefits, as well as aiding the development of new research. We are working to provide Open Access to as many publications as possible and these can be identified below by the padlock icon. Where this hasn't been possible, subscriptions may be required to view the full text.
 

Alissafi T, Kalafati L, Lazari M, Filia A, Kloukina I, Manifava M, Lim JH, Alexaki VI, Ktistakis NT, Doskas T, Garinis GA, Chavakis T, Boumpas DT, Verginis P Signalling

Regulatory T cells (Tregs) are vital for the maintenance of immune homeostasis, while their dysfunction constitutes a cardinal feature of autoimmunity. Under steady-state conditions, mitochondrial metabolism is critical for Treg function; however, the metabolic adaptations of Tregs during autoimmunity are ill-defined. Herein, we report that elevated mitochondrial oxidative stress and a robust DNA damage response (DDR) associated with cell death occur in Tregs in individuals with autoimmunity. In an experimental autoimmune encephalitis (EAE) mouse model of autoimmunity, we found a Treg dysfunction recapitulating the features of autoimmune Tregs with a prominent mtROS signature. Scavenging of mtROS in Tregs of EAE mice reversed the DDR and prevented Treg death, while attenuating the Th1 and Th17 autoimmune responses. These findings highlight an unrecognized role of mitochondrial oxidative stress in defining Treg fate during autoimmunity, which may facilitate the design of novel immunotherapies for diseases with disturbed immune tolerance.

+view abstract Cell metabolism , PMID: 32738205

Wauters E, Thevissen K, Wouters C, Bosisio FM, De Smet F, Gunst J, Humblet-Baron S, Lambrechts D, Liston A, Matthys P, Neyts J, Proost P, Weynand B, Wauters J, Tejpar S, Garg AD Immunology

No abstract available

+view abstract Frontiers in immunology , PMID: 32719686

D'Anna F, Van Dyck L, Xiong J, Zhao H, Berrens RV, Qian J, Bieniasz-Krzywiec P, Chandra V, Schoonjans L, Matthews J, De Smedt J, Minnoye L, Amorim R, Khorasanizadeh S, Yu Q, Zhao L, De Borre M, Savvides SN, Simon MC, Carmeliet P, Reik W, Rastinejad F, Mazzone M, Thienpont B, Lambrechts D Epigenetics

Hypoxia is pervasive in cancer and other diseases. Cells sense and adapt to hypoxia by activating hypoxia-inducible transcription factors (HIFs), but it is still an outstanding question why cell types differ in their transcriptional response to hypoxia.

+view abstract Genome Biology , PMID: 32718321

Pasciuto E, Burton OT, Roca CP, Lagou V, Rajan WD, Theys T, Mancuso R, Tito RY, Kouser L, Callaerts-Vegh Z, de la Fuente AG, Prezzemolo T, Mascali LG, Brajic A, Whyte CE, Yshii L, Martinez-Muriana A, Naughton M, Young A, Moudra A, Lemaitre P, Poovathingal S, Raes J, De Strooper B, Fitzgerald DC, Dooley J, Liston A Immunology

The brain is a site of relative immune privilege. Although CD4 T cells have been reported in the central nervous system, their presence in the healthy brain remains controversial, and their function remains largely unknown. We used a combination of imaging, single cell, and surgical approaches to identify a CD69 CD4 T cell population in both the mouse and human brain, distinct from circulating CD4 T cells. The brain-resident population was derived through in situ differentiation from activated circulatory cells and was shaped by self-antigen and the peripheral microbiome. Single-cell sequencing revealed that in the absence of murine CD4 T cells, resident microglia remained suspended between the fetal and adult states. This maturation defect resulted in excess immature neuronal synapses and behavioral abnormalities. These results illuminate a role for CD4 T cells in brain development and a potential interconnected dynamic between the evolution of the immunological and neurological systems. VIDEO ABSTRACT.

+view abstract Cell , PMID: 32702313

McGough IJ, Vecchia L, Bishop B, Malinauskas T, Beckett K, Joshi D, O'Reilly N, Siebold C, Jones EY, Vincent JP Signalling

A relatively small number of proteins have been suggested to act as morphogens-signalling molecules that spread within tissues to organize tissue repair and the specification of cell fate during development. Among them are Wnt proteins, which carry a palmitoleate moiety that is essential for signalling activity. How a hydrophobic lipoprotein can spread in the aqueous extracellular space is unknown. Several mechanisms, such as those involving lipoprotein particles, exosomes or a specific chaperone, have been proposed to overcome this so-called Wnt solubility problem. Here we provide evidence against these models and show that the Wnt lipid is shielded by the core domain of a subclass of glypicans defined by the Dally-like protein (Dlp). Structural analysis shows that, in the presence of palmitoleoylated peptides, these glypicans change conformation to create a hydrophobic space. Thus, glypicans of the Dlp family protect the lipid of Wnt proteins from the aqueous environment and serve as a reservoir from which Wnt proteins can be handed over to signalling receptors.

+view abstract Nature , PMID: 32699409

Patani H, Rushton MD, Higham J, Teijeiro SA, Oxley D, Cutillas P, Sproul D, Ficz G Epigenetics , Mass Spectrometry

Epigenetic reprogramming is a cancer hallmark, but how it unfolds during early neoplastic events and its role in carcinogenesis and cancer progression is not fully understood. Here we show that resetting from primed to naïve human pluripotency results in acquisition of a DNA methylation landscape mirroring the cancer DNA methylome, with gradual hypermethylation of bivalent developmental genes. We identify a dichotomy between bivalent genes that do and do not become hypermethylated, which is also mirrored in cancer. We find that loss of H3K4me3 at bivalent regions is associated with gain of methylation. Additionally, we observe that promoter CpG island hypermethylation is not restricted solely to emerging naïve cells, suggesting that it is a feature of a heterogeneous intermediate population during resetting. These results indicate that transition to naïve pluripotency and oncogenic transformation share common epigenetic trajectories, which implicates reprogramming and the pluripotency network as a central hub in cancer formation.

+view abstract Nature communications , PMID: 32699299

Thiecke MJ, Wutz G, Muhar M, Tang W, Bevan S, Malysheva V, Stocsits R, Neumann T, Zuber J, Fraser P, Schoenfelder S, Peters JM, Spivakov M Epigenetics

It is currently assumed that 3D chromosomal organization plays a central role in transcriptional control. However, depletion of cohesin and CTCF affects the steady-state levels of only a minority of transcripts. Here, we use high-resolution Capture Hi-C to interrogate the dynamics of chromosomal contacts of all annotated human gene promoters upon degradation of cohesin and CTCF. We show that a majority of promoter-anchored contacts are lost in these conditions, but many contacts with distinct properties are maintained, and some new ones are gained. The rewiring of contacts between promoters and active enhancers upon cohesin degradation associates with rapid changes in target gene transcription as detected by SLAM sequencing (SLAM-seq). These results provide a mechanistic explanation for the limited, but consistent, effects of cohesin and CTCF depletion on steady-state transcription and suggest the existence of both cohesin-dependent and -independent mechanisms of enhancer-promoter pairing.

+view abstract Cell reports , PMID: 32698000

Morf J, Wingett SW Bioinformatics

RNA localization is an important regulatory layer of gene expression and cell functioning. The protocol guides through the Proximity RNA-seq method, in which RNA molecules are sequenced in their spatial, cellular context to derive RNA co-localization and transcriptome organization. Transcripts in individual subcellular particles from chemically crosslinked cells are tagged with the same, unique DNA barcode in water-in-oil emulsion droplets. First, single DNA barcodes are PCR amplified and immobilized on single, small magnetic beads in droplets. Subsequently, 3' ends of bead-bound barcode copies are tailed with random pentadecamers. Then beads are encapsulated again into droplets together with crosslinked subcellular particles containing RNA. Reverse transcription using random pentadecamers as primers is performed in droplets, which optimally contain one bead and one particle, in order to tag RNAs co-localized to the same particle. Sequencing such cDNA molecules identifies the RNA molecule and the barcode. Subsequent analysis of transcripts that share the same barcode, i.e., co-barcoding, reveals RNA co-localization and interactions. The technique is not restricted to pairs of RNAs but can as well detect groups of transcripts and estimates local RNA density or connectivity for individual transcripts. We provide here a detailed protocol to perform and analyze Proximity RNA-seq on cell nuclei to study spatial, nuclear RNA organization.

+view abstract Methods in molecular biology , PMID: 32681513

Dooley J, Lagou V, Goveia J, Ulrich A, Rohlenova K, Heirman N, Karakach T, Lampi Y, Khan S, Wang J, Dresselaers T, Himmelreich U, Gunter MJ, Prokopenko I, Carmeliet P, Liston A Immunology

Pancreatic cancer is a rare but fatal form of cancer, the fourth highest in absolute mortality. Known risk factors include obesity, diet, and type 2 diabetes; however, the low incidence rate and interconnection of these factors confound the isolation of individual effects. Here, we use epidemiological analysis of prospective human cohorts and parallel tracking of pancreatic cancer in mice to dissect the effects of obesity, diet, and diabetes on pancreatic cancer. Through longitudinal monitoring and multi-omics analysis in mice, we found distinct effects of protein, sugar, and fat dietary components, with dietary sugars increasing Mad2l1 expression and tumor proliferation. Using epidemiological approaches in humans, we find that dietary sugars give a MAD2L1 genotype-dependent increased susceptibility to pancreatic cancer. The translation of these results to a clinical setting could aid in the identification of the at-risk population for screening and potentially harness dietary modification as a therapeutic measure.

+view abstract Cell reports , PMID: 32668252

Deniz Ö, Ahmed M, Todd CD, Rio-Machin A, Dawson MA, Branco MR Epigenetics

Acute myeloid leukemia (AML) is characterised by a series of genetic and epigenetic alterations that result in deregulation of transcriptional networks. One understudied source of transcriptional regulators are transposable elements (TEs), whose aberrant usage could contribute to oncogenic transcriptional circuits. However, the regulatory influence of TEs and their links to AML pathogenesis remain unexplored. Here we identify six endogenous retrovirus (ERV) families with AML-associated enhancer chromatin signatures that are enriched in binding of key regulators of hematopoiesis and AML pathogenesis. Using both locus-specific genetic editing and simultaneous epigenetic silencing of multiple ERVs, we demonstrate that ERV deregulation directly alters the expression of adjacent genes in AML. Strikingly, deletion or epigenetic silencing of an ERV-derived enhancer suppresses cell growth by inducing apoptosis in leukemia cell lines. This work reveals that ERVs are a previously unappreciated source of AML enhancers that may be exploited by cancer cells to help drive tumour heterogeneity and evolution.

+view abstract Nature communications , PMID: 32665538

Gallotta I, Sandhu A, Peters M, Haslbeck M, Jung R, Agilkaya S, Blersch JL, Rödelsperger C, Röseler W, Huang C, Sommer RJ, David DC Signalling

In metazoans, the secreted proteome participates in intercellular signalling and innate immunity, and builds the extracellular matrix scaffold around cells. Compared with the relatively constant intracellular environment, conditions for proteins in the extracellular space are harsher, and low concentrations of ATP prevent the activity of intracellular components of the protein quality-control machinery. Until now, only a few bona fide extracellular chaperones and proteases have been shown to limit the aggregation of extracellular proteins. Here we performed a systematic analysis of the extracellular proteostasis network in Caenorhabditis elegans with an RNA interference screen that targets genes that encode the secreted proteome. We discovered 57 regulators of extracellular protein aggregation, including several proteins related to innate immunity. Because intracellular proteostasis is upregulated in response to pathogens, we investigated whether pathogens also stimulate extracellular proteostasis. Using a pore-forming toxin to mimic a pathogenic attack, we found that C. elegans responded by increasing the expression of components of extracellular proteostasis and by limiting aggregation of extracellular proteins. The activation of extracellular proteostasis was dependent on stress-activated MAP kinase signalling. Notably, the overexpression of components of extracellular proteostasis delayed ageing and rendered worms resistant to intoxication. We propose that enhanced extracellular proteostasis contributes to systemic host defence by maintaining a functional secreted proteome and avoiding proteotoxicity.

+view abstract Nature , PMID: 32641833

Alda-Catalinas C, Bredikhin D, Hernando-Herraez I, Santos F, Kubinyecz O, Eckersley-Maslin MA, Stegle O, Reik W Epigenetics

Zygotic genome activation (ZGA) is an essential transcriptional event in embryonic development that coincides with extensive epigenetic reprogramming. Complex manipulation techniques and maternal stores of proteins preclude large-scale functional screens for ZGA regulators within early embryos. Here, we combined pooled CRISPR activation (CRISPRa) with single-cell transcriptomics to identify regulators of ZGA-like transcription in mouse embryonic stem cells, which serve as a tractable, in vitro proxy of early mouse embryos. Using multi-omics factor analysis (MOFA+) applied to ∼200,000 single-cell transcriptomes comprising 230 CRISPRa perturbations, we characterized molecular signatures of ZGA and uncovered 24 factors that promote a ZGA-like response. Follow-up assays validated top screen hits, including the DNA-binding protein Dppa2, the chromatin remodeler Smarca5, and the transcription factor Patz1, and functional experiments revealed that Smarca5's regulation of ZGA-like transcription is dependent on Dppa2. Together, our single-cell transcriptomic profiling of CRISPRa-perturbed cells provides both system-level and molecular insights into the mechanisms that orchestrate ZGA.

+view abstract Cell systems , PMID: 32634384

Zhang F, Smith LP, Blinov ML, Faeder J, Hlavacek WS, Juan Tapia J, Keating SM, Rodriguez N, Dräger A, Harris LA, Finney A, Hu B, Hucka M, Meier-Schellersheim M

Rule-based modeling is an approach that permits constructing reaction networks based on the specification of rules for molecular interactions and transformations. These rules can encompass details such as the interacting sub-molecular domains and the states and binding status of the involved components. Conceptually, fine-grained spatial information such as locations can also be provided. Through "wildcards" representing component states, entire families of molecule complexes sharing certain properties can be specified as patterns. This can significantly simplify the definition of models involving species with multiple components, multiple states, and multiple compartments. The systems biology markup language (SBML) Level 3 Multi Package Version 1 extends the SBML Level 3 Version 1 core with the "type" concept in the Species and Compartment classes. Therefore, reaction rules may contain species that can be patterns and exist in multiple locations. Multiple software tools such as Simmune and BioNetGen support this standard that thus also becomes a medium for exchanging rule-based models. This document provides the specification for Release 2 of Version 1 of the SBML Level 3 Multi package. No design changes have been made to the description of models between Release 1 and Release 2; changes are restricted to the correction of errata and the addition of clarifications.

+view abstract Journal of integrative bioinformatics , PMID: 32628633

Klobučar T, Kreibich E, Krueger F, Arez M, Pólvora-Brandão D, von Meyenn F, da Rocha ST, Eckersley-Maslin M Epigenetics , Bioinformatics

Genomic imprinting is an epigenetic phenomenon leading to parental allele-specific expression. Dosage of imprinted genes is crucial for normal development and its dysregulation accounts for several human disorders. This unusual expression pattern is mostly dictated by differences in DNA methylation between parental alleles at specific regulatory elements known as imprinting control regions (ICRs). Although several approaches can be used for methylation inspection, we lack an easy and cost-effective method to simultaneously measure DNA methylation at multiple imprinted regions. Here, we present IMPLICON, a high-throughput method measuring DNA methylation levels at imprinted regions with base-pair resolution and over 1000-fold coverage. We adapted amplicon bisulfite-sequencing protocols to design IMPLICON for ICRs in adult tissues of inbred mice, validating it in hybrid mice from reciprocal crosses for which we could discriminate methylation profiles in the two parental alleles. Lastly, we developed a human version of IMPLICON and detected imprinting errors in embryonic and induced pluripotent stem cells. We also provide rules and guidelines to adapt this method for investigating the DNA methylation landscape of any set of genomic regions. In summary, IMPLICON is a rapid, cost-effective and scalable method, which could become the gold standard in both imprinting research and diagnostics.

+view abstract Nucleic acids research , PMID: 32621604

Prada-Luengo I, Møller HD, Henriksen RA, Gao Q, Larsen CE, Alizadeh S, Maretty L, Houseley J, Regenberg B Epigenetics

Circular DNA can arise from all parts of eukaryotic chromosomes. In yeast, circular ribosomal DNA (rDNA) accumulates dramatically as cells age, however little is known about the accumulation of other chromosome-derived circles or the contribution of such circles to genetic variation in aged cells. We profiled circular DNA in Saccharomyces cerevisiae populations sampled when young and after extensive aging. Young cells possessed highly diverse circular DNA populations but 94% of the circular DNA were lost after ∼15 divisions, whereas rDNA circles underwent massive accumulation to >95% of circular DNA. Circles present in both young and old cells were characterized by replication origins including circles from unique regions of the genome and repetitive regions: rDNA and telomeric Y' regions. We further observed that circles can have flexible inheritance patterns: [HXT6/7circle] normally segregates to mother cells but in low glucose is present in up to 50% of cells, the majority of which must have inherited this circle from their mother. Interestingly, [HXT6/7circle] cells are eventually replaced by cells carrying stable chromosomal HXT6 HXT6/7 HXT7 amplifications, suggesting circular DNAs are intermediates in chromosomal amplifications. In conclusion, the heterogeneity of circular DNA offers flexibility in adaptation, but this heterogeneity is remarkably diminished with age.

+view abstract Nucleic acids research , PMID: 32609810

Ohashi Y, Tremel S, Masson GR, McGinney L, Boulanger J, Rostislavleva K, Johnson CM, Niewczas I, Clark J, Williams RL Biological Chemistry

The lipid kinase VPS34 orchestrates diverse processes, including autophagy, endocytic sorting, phagocytosis, anabolic responses and cell division. VPS34 forms various complexes that help adapt it to specific pathways, with complexes I and II being the most prominent ones. We found that physicochemical properties of membranes strongly modulate VPS34 activity. Greater unsaturation of both substrate and non-substrate lipids, negative charge and curvature activate VPS34 complexes, adapting them to their cellular compartments. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) of complexes I and II on membranes elucidated structural determinants that enable them to bind membranes. Among these are the Barkor/ATG14L autophagosome targeting sequence (BATS), which makes autophagy-specific complex I more active than the endocytic complex II, and the Beclin1 BARA domain. Interestingly, even though Beclin1 BARA is common to both complexes, its membrane-interacting loops are critical for complex II, but have only a minor role for complex I.

+view abstract eLife , PMID: 32602837

Dodsworth BT, Hatje K, Rostovskaya M, Flynn R, Meyer CA, Cowley SA Epigenetics

Naïve human pluripotent stem cells (hPSC) resemble the embryonic epiblast at an earlier time-point in development than conventional, 'primed' hPSC. We present a comprehensive miRNA profiling of naïve-to-primed transition in hPSC, a process recapitulating aspects of early in vivo embryogenesis. We identify miR-143-3p and miR-22-3p as markers of the naïve state and miR-363-5p, several members of the miR-17 family, miR-302 family as primed markers. We uncover that miR-371-373 are highly expressed in naïve hPSC. MiR-371-373 are the human homologs of the mouse miR-290 family, which are the most highly expressed miRNAs in naïve mouse PSC. This aligns with the consensus that naïve hPSC resemble mouse naive PSC, showing that the absence of miR-371-373 in conventional hPSC is due to cell state rather than a species difference.

+view abstract Scientific reports , PMID: 32601281

Ivanova E, Canovas S, Garcia-Martínez S, Romar R, Lopes JS, Rizos D, Sanchez-Calabuig MJ, Krueger F, Andrews S, Perez-Sanz F, Kelsey G, Coy P Epigenetics , Bioinformatics

An amendment to this paper has been published and can be accessed via the original article.

+view abstract Clinical epigenetics , PMID: 32600441

Hay IM, Fearnley GW, Rios P, Köhn M, Sharpe HJ, Deane JE Signalling

The receptor-linked protein tyrosine phosphatases (RPTPs) are key regulators of cell-cell communication through the control of cellular phosphotyrosine levels. Most human RPTPs possess an extracellular receptor domain and tandem intracellular phosphatase domains: comprising an active membrane proximal (D1) domain and an inactive distal (D2) pseudophosphatase domain. Here we demonstrate that PTPRU is unique amongst the RPTPs in possessing two pseudophosphatase domains. The PTPRU-D1 displays no detectable catalytic activity against a range of phosphorylated substrates and we show that this is due to multiple structural rearrangements that destabilise the active site pocket and block the catalytic cysteine. Upon oxidation, this cysteine forms an intramolecular disulphide bond with a vicinal "backdoor" cysteine, a process thought to reversibly inactivate related phosphatases. Importantly, despite the absence of catalytic activity, PTPRU binds substrates of related phosphatases strongly suggesting that this pseudophosphatase functions in tyrosine phosphorylation by competing with active phosphatases for the binding of substrates.

+view abstract Nature communications , PMID: 32591542

Glont M, Arankalle C, Tiwari K, Nguyen TVN, Hermjakob H, Malik Sheriff RS Bioinformatics

One of the major bottlenecks in building systems biology models is identification and estimation of model parameters for model calibration. Searching for model parameters from published literature and models is an essential, yet laborious task.

+view abstract Bioinformatics , PMID: 32573648

Eckersley-Maslin MA, Parry A, Blotenburg M, Krueger C, Ito Y, Franklin VNR, Narita M, D'Santos CS, Reik W Epigenetics , Bioinformatics

How the epigenetic landscape is established in development is still being elucidated. Here, we uncover developmental pluripotency associated 2 and 4 (DPPA2/4) as epigenetic priming factors that establish a permissive epigenetic landscape at a subset of developmentally important bivalent promoters characterized by low expression and poised RNA-polymerase. Differentiation assays reveal that Dppa2/4 double knockout mouse embryonic stem cells fail to exit pluripotency and differentiate efficiently. DPPA2/4 bind both H3K4me3-marked and bivalent gene promoters and associate with COMPASS- and Polycomb-bound chromatin. Comparing knockout and inducible knockdown systems, we find that acute depletion of DPPA2/4 results in rapid loss of H3K4me3 from key bivalent genes, while H3K27me3 is initially more stable but lost following extended culture. Consequently, upon DPPA2/4 depletion, these promoters gain DNA methylation and are unable to be activated upon differentiation. Our findings uncover a novel epigenetic priming mechanism at developmental promoters, poising them for future lineage-specific activation.

+view abstract Nature structural & molecular biology , PMID: 32572255

Furmanik M, Chatrou M, van Gorp R, Akbulut A, Willems B, Schmidt H, van Eys G, Bochaton-Piallat ML, Proudfoot D, Biessen E, Hedin U, Perisic L, Mees B, Shanahan C, Reutelingsperger C, Schurgers L

Vascular calcification, the formation of calcium phosphate crystals in the vessel wall, is mediated by vascular smooth muscle cells (VSMCs). However, the underlying molecular mechanisms remain elusive, precluding mechanism-based therapies.

+view abstract Circulation research , PMID: 32564697

Nieuwenhuis B, Barber AC, Evans RS, Pearson CS, Fuchs J, MacQueen AR, van Erp S, Haenzi B, Hulshof LA, Osborne A, Conceicao R, Khatib TZ, Deshpande SS, Cave J, Ffrench-Constant C, Smith PD, Okkenhaug K, Eickholt BJ, Martin KR, Fawcett JW, Eva R

Peripheral nervous system (PNS) neurons support axon regeneration into adulthood, whereas central nervous system (CNS) neurons lose regenerative ability after development. To better understand this decline whilst aiming to improve regeneration, we focused on phosphoinositide 3-kinase (PI3K) and its product phosphatidylinositol (3,4,5)-trisphosphate (PIP ). We demonstrate that adult PNS neurons utilise two catalytic subunits of PI3K for axon regeneration: p110α and p110δ. However, in the CNS, axonal PIP decreases with development at the time when axon transport declines and regenerative competence is lost. Overexpressing p110α in CNS neurons had no effect; however, expression of p110δ restored axonal PIP and increased regenerative axon transport. p110δ expression enhanced CNS regeneration in both rat and human neurons and in transgenic mice, functioning in the same way as the hyperactivating H1047R mutation of p110α. Furthermore, viral delivery of p110δ promoted robust regeneration after optic nerve injury. These findings establish a deficit of axonal PIP as a key reason for intrinsic regeneration failure and demonstrate that native p110δ facilitates axon regeneration by functioning in a hyperactive fashion.

+view abstract EMBO molecular medicine , PMID: 32558386

Stammers M, Niewczas IS, Segonds-Pichon A, Clark J Biological Chemistry

Collagen I is a major tendon protein whose polypeptide chains are linked by covalent cross-links. It is unknown how the cross-linking contributes to the mechanical properties of tendon or whether cross-linking changes in response to stretching or relaxation. Since their discovery, imine bonds within collagen have been recognized as being important in both cross-link formation and collagen structure. They are often described as acidic or thermally labile, but no evidence is available from direct measurements of cross-link levels whether these bonds contribute to the mechanical properties of collagen. Here, we used MS to analyze these imine bonds after reduction with sodium borohydride while under tension and found that their levels are altered in stretched tendon. We studied the changes in cross-link bonding in tail tendon from 11-week-old C57Bl/6 mice at 4% physical strain, at 10% strain, and at breaking point. The cross-links hydroxy-lysino-norleucine (HLNL), dihydroxy-lysino-norleucine (DHLNL), and lysino-norleucine (LNL) increased or decreased depending on the specific cross-link and amount of mechanical strain. We also noted a decrease in glycated lysine residues in collagen, indicating that the imine formed between circulating glucose and lysine is also stress-labile. We also carried out mechanical testing, including cyclic testing at 4% strain, stress relaxation tests, and stress-strain profiles taken at breaking point, both with and without sodium borohydride reduction. The results from both the MS studies and mechanical testing provide insights into the chemical changes during tendon stretching and directly link these chemical changes to functional collagen properties.

+view abstract The Journal of biological chemistry , PMID: 32546479