Cyrille F. Dunant

Hematopoietic Stem Cells Reversibly Switch from Dormancy to Self-Renewal during Homeostasis and Repair

Bone marrow hematopoietic stem cells (HSCs) are crucial to maintain lifelong production of all blood cells. Although HSCs divide infrequently, it is thought that the entire HSC pool turns over every few weeks, suggesting that HSCs regularly enter and exit cell cycle. Here, we combine flow cytometry with label-retaining assays (BrdU and histone H2B-GFP) to identify a population of dormant mouse HSCs (d-HSCs) within the lin(-)Sca1+cKit+CD150+CD48(-)CD34(-) population. Computational modeling suggests that d-HSCs divide about every 145 days, or five times per lifetime. d-HSCs harbor the vast majority of multilineage long-term self-renewal activity. While they form a silent reservoir of the most potent HSCs during homeostasis, they are efficiently activated to self-renew in response to bone marrow injury or G-CSF stimulation. After re-establishment of homeostasis, activated HSCs return to dormancy, suggesting that HSCs are not stochastically entering the cell cycle but reversibly switch from dormancy to self-renewal under conditions of hematopoietic stress.

Distinct routes of lineage development reshape the human blood hierarchy across ontogeny

Adjusting hematopoietic hierarchy In adults, more than 300 billion blood cells are replenished daily. This output arises from a cellular hierarchy where stem cells differentiate into a series of multilineage progenitors, culminating in unilineage progenitors that generate over 10 different mature blood cell types. Notta et al. mapped the lineage potential of nearly 3000 single cells from 33 different cell populations of stem and progenitor cells from fetal liver, cord blood, and adult bone marrow (see the Perspective by Cabezas-Wallscheid and Trumpp). Prenatally, stem cell and progenitor populations were multilineage with few unilineage progenitors. In adults, multilineage cell potential was only seen in stem cell populations. Science, this issue p. 10.1126/science.aab2116; see also p. 126 As humans age, progenitor cells take over from stem cells the task of producing a steady supply of blood cells. [Also see Perspective by Cabezas-Wallscheid and Trumpp] INTRODUCTION The hematopoietic road map is a compilation of the various lineage differentiation routes that a stem cell takes to make blood. This program produces greater than 10 blood cell fates and is responsible for generating more than 300 billion cells daily. On several occasions over the past six decades, the murine road map has been reconceived due to new information overturning dogma. However, the human road map has changed little. In the human model, blood differentiation initiates at the level of multipotent stem cells and passes through a series of increasingly lineage-restricted oligopotent and, finally, unipotent progenitor intermediates. One critical oligopotent intermediate is the common myeloid progenitor (CMP), believed to be the origin of all myeloid (My), erythroid (Er), and megakaryocyte (Mk) cells. Although murine studies challenge the existence of oligopotent progenitors, a comprehensive analysis of human My-Er-Mk differentiation is lacking. Moreover, whether the pool of oligopotent intermediates is fixed across human development (fetal to adult) is unknown. RATIONALE The differentiation road map taken by human hematopoietic stem cells (HSCs) is fundamental to our understanding of blood homeostasis, hematopoietic malignancies, and regenerative medicine. RESULTS We mapped the cellular origins of My, Er, and Mk lineages across three time points in human blood development: fetal liver (FL), neonatal cord blood (CB), and adult bone marrow (BM). Using a cell-sorting scheme based on markers linked to Er and Mk lineage specification (CD71 and CD110), we found that previously described populations of multipotent progenitors (MPPs), CMPs, and megakaryocyte-erythroid progenitors (MEPs) were heterogeneous and could be further purified. Nearly 3000 single cells from 11 cellular subsets from the CD34+ compartment of FL, CB, and BM (33 subsets in total) were evaluated for their My, Er, and Mk lineage potential using an optimized single-cell assay. In FL, the ratio of cells with multilineage versus unilineage potential remained constant in both the stem cell (CD34+CD38–) and progenitor cell (CD34+CD38+) enriched compartments. By contrast, in BM, nearly all multipotent cells were restricted to the stem cell compartment, whereas unilineage progenitors dominated the progenitor cell compartment. Oligopotent progenitors were only a negligible component of the human blood hierarchy in BM, leading to the inference that multipotent cells differentiate into unipotent cells directly by adulthood. Mk/Er activity predominantly originated from the stem cell compartment at all developmental time points. In CB and BM, most Mks emerged as part of mixed clones from HSCs/MPPs, indicating that Mks directly branch from a multipotent cell and not from oligopotent progenitors like CMP. In FL, an almost pure Mk/Er progenitor was identified in the stem cell compartment, although less potent Mk/Er progenitors were also present in the progenitor compartment. In a hematological condition of HSC loss (aplastic anemia), Mk/Er but not My progenitors were more severely depleted, pinpointing a close physiological connection between HSC and the Mk/Er lineage. CONCLUSION Our data indicate that there are distinct road maps of blood differentiation across human development. Prenatally, Mk/Er lineage branching occurs throughout the cellular hierarchy. By adulthood, both Mk/Er activity and multipotency are restricted to the stem cell compartment, whereas the progenitor compartment is composed of unilineage progenitors forming a “two-tier” system, with few intervening oligopotent intermediates. Roadmaps of human blood stem cell differentiation. The classical model envisions that oligopotent progenitors such as CMP are an essential intermediate stage from which My/Er/Mk differentiation originates. The redefined model proposes a developmental shift in the progenitor cell architecture from the fetus, where many stem and progenitor cell types are multipotent, to the adult, where the stem cell compartment is multipotent but the progenitors are unipotent. The grayed planes represent theoretical tiers of differentiation. In a classical view of hematopoiesis, the various blood cell lineages arise via a hierarchical scheme starting with multipotent stem cells that become increasingly restricted in their differentiation potential through oligopotent and then unipotent progenitors. We developed a cell-sorting scheme to resolve myeloid (My), erythroid (Er), and megakaryocytic (Mk) fates from single CD34+ cells and then mapped the progenitor hierarchy across human development. Fetal liver contained large numbers of distinct oligopotent progenitors with intermingled My, Er, and Mk fates. However, few oligopotent progenitor intermediates were present in the adult bone marrow. Instead, only two progenitor classes predominate, multipotent and unipotent, with Er-Mk lineages emerging from multipotent cells. The developmental shift to an adult “two-tier” hierarchy challenges current dogma and provides a revised framework to understand normal and disease states of human hematopoiesis.

CDK6 Levels Regulate Quiescence Exit in Human Hematopoietic Stem Cells

Summary Regulated blood production is achieved through the hierarchical organization of dormant hematopoietic stem cell (HSC) subsets that differ in self-renewal potential and division frequency, with long-term (LT)-HSCs dividing the least. The molecular mechanisms underlying this variability in HSC division kinetics are unknown. We report here that quiescence exit kinetics are differentially regulated within human HSC subsets through the expression level of CDK6. LT-HSCs lack CDK6 protein. Short-term (ST)-HSCs are also quiescent but contain high CDK6 protein levels that permit rapid cell cycle entry upon mitogenic stimulation. Enforced CDK6 expression in LT-HSCs shortens quiescence exit and confers competitive advantage without impacting function. Computational modeling suggests that this independent control of quiescence exit kinetics inherently limits LT-HSC divisions and preserves the HSC pool to ensure lifelong hematopoiesis. Thus, differential expression of CDK6 underlies heterogeneity in stem cell quiescence states that functionally regulates this highly regenerative system.

A critical comparison of several numerical methods for computing effective properties of highly heterogeneous materials

Modelling transport and long-term creep in concrete materials is a difficult problem when the complexity of the microstructure is taken into account, because it is hard to predict instantaneous elastic responses. In this work, several numerical methods are compared to assess their properties and suitability to model concrete-like microstructures with large phase properties contrast. The methods are classical finite elements, a novel extended finite element method (@m-xfem), an unconstrained heuristic meshing technique (amie), and a locally homogenising preprocessor in combination with various solvers (benhur). The benchmark itself consists of a number of simple and complex microstructures, which are tested with a range of phase contrasts designed to cover the needs of creep and transport modelling in concrete. The calculations are performed assuming linear elasticity and thermal conduction. The methods are compared in term of precision, ease of implementation and appropriateness to the problem type. We find that xfem is the most suitable when the mesh if coarse, and methods based on Cartesian grids are best when a very fine mesh can be used. Finite element methods are good compromises with high flexibility.

Architecture tradeoffs of integrating a mesh generator to partition of unity enriched object-oriented finite element software

We explore the tradeoffs of using an internal mesher in a XFEM code. We show that it allows an efficient enrichement detection scheme, while retaining the ability to have welladapted meshes. We provide benchmarks highlighting the considerable gains which can be expected from a well designed architecture. The efficiency of the proposed algorithm is shown by solving fracture mechanics problems of densely micro-cracked bodies including adaptive mesh refinement.

Computing Creep-Damage Interactions in Irradiated Concrete

AbstractAmong various degradation mechanisms possibly affecting the long-term operation of nuclear power plants, the effects of induced expansion and internal degradation occurring in concrete expo...

Molecular landscapes of human hematopoietic stem cells in health and leukemia.

Blood cells are organized as a hierarchy with hematopoietic stem cells (HSCs) at the root. The advent of genomic technologies has opened the way for global characterization of the molecular landscape of HSCs and their progeny, both in mouse and human models, at the genetic, transcriptomic, epigenetic, and proteomics levels. Here, we outline our current understanding of the molecular programs that govern human HSCs and how dynamic changes occurring during HSC differentiation are necessary for well‐regulated blood formation under homeostasis and upon injury. A large body of evidence is accumulating on how the programs of normal hematopoiesis are modified in acute myeloid leukemia, an aggressive adult malignancy driven by leukemic stem cells. We summarize these findings and their clinical implications.

Numerical benchmark campaign of COST Action TU1404 – microstructural modelling

This paper presents the results of the numerical benchmark campaign on modelling of hydration and microstructure development of cementitious materials. This numerical benchmark was performed in the scope of COST Action TU1404 “Towards the next generation of standards for service life of cement-based materials and structures”. Seven modelling groups took part in the campaign applying different models for prediction of mechanical properties (elastic moduli or compressive strength) in cement pastes and mortars. The simulations were based on published experimental data. The experimental data (both input and results used for validation) were open to the participants. The purpose of the benchmark campaign was to identify the needs of different models in terms of input experimental data, verify predictive potential of the models and finally to provide reference cases for new models in the future. The results of the benchmark show that a relatively high scatter in the predictions can arise between different models, in particular at early ages (e.g. elastic Young’s modulus predicted at 1 d in the range 6-20 GPa), while it reduces at later age, providing relatively good agreement with experimental data. Even though the input data was based on a single experimental dataset, the large differences between the results of the different models were found to be caused by distinct assumed properties for the individual phases at the microstructural level, mainly because of the scatter in the nanoindentation-derived properties of the C-S-H phase.

Fully-Coupled Creep-Damage Models for Concrete

© ASCE. The long term analysis of structures and structural elements depends on accounting both for the creep stress relaxation and the development of structural cracks. The development of such cracks may take place over long periods and therefore be difficult to distinguish from the viscous effects. Modelling structures under such slow-loading conditions requires the development of appropriate numerical models for material behaviour, and novel strategies for computing the link between creep and damage. In this paper, we present a new empirical model combining both aspects. The creep component is represented here using a Maxwell chain, and the material laws are based on the modified compression field theory (MCFT) and the Mazars model. The effects of speed-of-loading are investigated, and the effect of creep on the post-peak behaviour is assessed. A particular attention is given to the implementation which integrates models developed with widely differing aims.

Alkali Silica Reaction Mitigating Properties of Ternary Blended Cement with Calcined Clay and Limestone

Supplementary cementitious materials (SCMs) are widely used in concrete either in blended cements or added separately in the concrete mixer. SCMs such as calcined clays, slags or fly ashes are widely used to partially substitute plain Portland cement (PC). A particurlarly promising blend is a blend with a high level of substitution by widely available SCMs such low grade calcined clay and limestone. The use of such materials, where no additional clinkering process is involved leads to a significant reduction in CO2 emissions per ton of material. Further, blended systems have numerous well established benefits in terms of durability. ASR is the most important such issue not related to reinforcing steel. Prevention of this phenomenon is critical as sources of non-reactive aggregates are increasingly scarce. The most economical path to ASR resistant concrete is through ternary blends. Since the reaction occurs between alkalis in pore solution and reactive silica, most mitigation methods rely on lowering the alkalinity of the solution through Supplementary Cementitious Materials (SCMs). The effectiveness of SCMs in mitigating ASR is attributable to pore refinement, alkali binding by secondary hydration due to replacing part of the Portland cement, but mainly to the inhibition of silica dissolution when Al ions provided by the SCM are present in the solution. Due to yet uncommon usage in the field, the performances and mechanisms which underlie the properties of such blends are still not wholly understood. In this study, we demonstrate the performance of blends with high level of replacement (reaching 50 %) of cement with limestone and calcined clay. The use of these two SCMs at such high level of replacement promise improvement of the resistance to expansion compared to PC in environmentally friendly blends.