<> "The repository administrator has not yet configured an RDF license."^^ . <> . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model."^^ . "Ageing is traditionally viewed as the deterministic functional decline of organisms. However,\r\nfrom an evolutionary perspective, aged organisms have surpassed their reproductive phase and\r\nare therefore less subjected to selective pressures, leading to the accumulation of random errors\r\nacross various biological tiers of their being. These stochastic errors are crucial in understanding\r\nwhy ageing manifests differently across individuals.\r\nThis thesis generally aimed to explore the stochastic aspects of ageing, using the hematopoietic\r\nsystem as a model. In the first part, I focused on examining the endogenous mechanisms\r\nfacilitating the gradual acquisition of random somatic mutations in hematopoietic stem cells\r\n(HSC) during normal ageing, as these cells are responsible for the lifelong system regeneration.\r\nSomatic mutagenesis is universal and constant over time; however, the mutational rates vary\r\namong different cell types. Here, I used whole genome sequencing (WGS) and mouse models to\r\ninspect single HSC genomes in depth and quantify their mutational burdens, by interrogating\r\ntheir clonal progeny. Assuming that this acquisition takes place in a linear fashion, as previously\r\nsuggested, I estimated that murine HSCs accumulate 44 single nucleotide variants (SNV) per\r\ngenome per year, a rate almost three times faster than that observed in human counterparts.\r\nMoreover, the SNV landscapes formed in murine HSCs appeared analogous to those in humans,\r\nshaped by processes reflecting self-renewal activity, metabolic dependencies and cellular state.\r\nHowever, no further alterations were detected regarding other mutational types.\r\nApoptosis has been regarded as the guardian of the HSC genome under stress, providing\r\ntumor-suppressive function against cells harboring driver mutations. To understand its impact\r\nduring normal ageing, I blocked the intrinsic apoptotic pathway in HSCs using a conditional\r\nBak -/- Baxδ/δ mouse model. Strikingly, the perturbation did not alter the mutational burdens in\r\nHSCs, challenging the established views about this mechanism. Shifting the focus of analysis on\r\nthe states of HSCs, I examined dormant and actively cycling cells, using a label-retention mouse\r\nmodel that allows the easy separation of these two fractions by flow cytometry. Despite\r\noriginating from the same donors, dormant HSCs exhibited significantly fewer mutations than\r\ntheir active counterparts. Moreover, mutational signature analysis attributed this difference\r\nmainly to the decreased divisional activity of dormant cells; otherwise, their mutational\r\nlandscapes appeared similar. This observation highlighted that dormancy, rather than apoptosis,\r\nis the primary mechanism maintaining genomic integrity within the HSC pool during\r\nphysiological ageing, while HSC activation allows SNV accumulation over time.\r\nIn the second part of my thesis, I aimed to explore the intrinsic origins of the inter-individual\r\nheterogeneity observed in hematological ageing. To address this, I first compiled a\r\ncomprehensive murine dataset characterizing extensively the hematological profiles of young, middle-aged and old individuals using flow cytometry, multi-omics and bone histology. With this\r\ndataset in hand, I first explored potential associations among classical indicators of hematological\r\nageing, such as HSC expansion, myeloid cell production, and HSC functionality, as assessed\r\nthrough competitive transplantation assays. Interestingly, in young mice, some of the metrics\r\nwere interconnected, suggesting a finely regulated system under homeostasis. However, in old\r\nmice, a unique variability of phenotypes emerged, alongside a lack of correlations among the\r\ndifferent metrics.\r\nTransplantations using bone marrow from donors of the three different age groups into\r\nyoung myeloablated recipients revealed some degree of overlap in hematologic phenotypes\r\nregardless of donor age. This observation pinpointed that the aged phenotype is also regulated\r\nby the niche. Analysis of single-cell RNA-sequencing data from both hematopoietic and niche\r\ncells revealed a remarkable bone marrow remodeling in old mice, including the emergence of a\r\nnovel and heterogeneous mesenchymal stromal progenitor population -Adipo-CAR3-, observed\r\nin this study for the first time. This population served as the epicenter of an inflammatory\r\nnetwork involving neutrophils and HSCs. However, the niche phenotype did not correlate with\r\nother hematopoietic metrics in a manner that could explain the overall system behavior,\r\nsuggesting that the lack of regulation that characterizes old age may expose each organism to\r\nstochastic events able to determine its phenotype.\r\nIn conclusion, this thesis highlighted the often overlooked role of stochasticity as a\r\nfundamental aspect of ageing. The results from the first part challenged conventional beliefs\r\nabout how HSCs manage DNA damage and mutagenesis under physiological conditions. Unlike\r\nstudies relying on strong stressors to explore genotoxic responses, my findings shed light on\r\nsources of endogenous routine damage that accumulates steadily over time. Furthermore,\r\nthrough a murine epidemiological study included in the second part of this work, I observed that\r\nas the hematopoietic system ages, it gradually loses its ability to regulate its phenotype.\r\nUltimately, it is likely that this leads to a transition towards a highly stochastic state, where\r\nrandom errors can alter its characteristics causing phenotypic variability."^^ . "2025" . . . . . . . "Foteini"^^ . "Fotopoulou"^^ . "Foteini Fotopoulou"^^ . . . . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (PDF)"^^ . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (Other)"^^ . . . . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (Other)"^^ . . . . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (Other)"^^ . . . . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (Other)"^^ . . . . . . "Exploring the stochastic nature of ageing: Insights from the hematopoietic system as a model. (Other)"^^ . . . . . "HTML Summary of #35410 \n\nExploring the stochastic nature of ageing: Insights from the hematopoietic system as a model.\n\n" . "text/html" . . . "570 Biowissenschaften, Biologie"@de . "570 Life sciences"@en . .