Stem cell biology for the transfusionist

Telomere biology in aging and cancer: early history and perspectives

The ends of eukaryotic linear chromosomes are protected from undesired enzymatic activities by a nucleoprotein complex called the telomere. Expanding evidence indicates that telomeres have central functions in human aging and tumorigenesis. While it is undoubtedly important to follow current advances in telomere biology, it is also fruitful to be well informed in seminal historical studies for a comprehensive understanding of telomere biology, and for the anticipation of future directions. With this in mind, I here summarize the early history of telomere biology and current advances in the field, mostly focusing on mammalian studies relevant to aging and cancer.

Cell division patterns in acute myeloid leukemia stem-like cells determine clinical course: a model to predict patient survival

Acute myeloid leukemia (AML) is a heterogeneous disease in which a variety of distinct genetic alterations might occur. Recent attempts to identify the leukemia stem-like cells (LSC) have also indicated heterogeneity of these cells. On the basis of mathematical modeling and computer simulations, we have provided evidence that proliferation and self-renewal rates of the LSC population have greater impact on the course of disease than proliferation and self-renewal rates of leukemia blast populations, that is, leukemia progenitor cells. The modeling approach has enabled us to estimate the LSC properties of 31 individuals with relapsed AML and to link them to patient survival. On the basis of the estimated LSC properties, the patients can be divided into two prognostic groups that differ significantly with respect to overall survival after first relapse. The results suggest that high LSC self-renewal and proliferation rates are indicators of poor prognosis. Nevertheless, high LSC self-renewal rate may partially compensate for slow LSC proliferation and vice versa. Thus, model-based interpretation of clinical data allows estimation of prognostic factors that cannot be measured directly. This may have clinical implications for designing treatment strategies.

Mathematical Modelling as a Tool to Understand Cell Self-renewal and Differentiation

Mathematical modeling is a powerful technique to address key questions and paradigms in a variety of complex biological systems and can provide quantitative insights into cell kinetics, fate determination and development of cell populations. The chapter is devoted to a review of modeling of the dynamics of stem cell-initiated systems using mathematical methods of ordinary differential equations. Some basic concepts and tools for cell population dynamics are summarized and presented as a gentle introduction to non-mathematicians. The models take into account different plausible mechanisms regulating homeostasis. Two mathematical frameworks are proposed reflecting, respectively, a discrete (punctuated by division events) and a continuous character of transitions between differentiation stages. Advantages and constraints of the mathematical approaches are presented on examples of models of blood systems and compared to patients data on healthy hematopoiesis.

MicroRNA-125 family members exert a similar role in the regulation of murine hematopoiesis

MicroRNAs (miRNAs) are crucial for proper functioning of hematopoietic stem and progenitor cells (HSPCs). Members of the miRNA-125 family (consisting of miR-125a, miR-125b1, and miR-125b2) are known to confer a proliferative advantage on cells upon overexpression, to decrease the rate of apoptosis by targeting proapoptotic genes, and to promote differentiation toward the myeloid lineage in mice. However, many distinct biological effects of the three miR-125 species have been reported as well. In the current study, we set out to assess whether the three miRNA-125s that carry identical seed sequences could be functionally different. Our data show that overexpression of each of the three miR-125 family members preserves HSPCs in a primitive state in vitro, results in a competitive advantage upon serial transplantation, and promotes skewing toward the myeloid lineage. All miR-125 family members decreased the pool of phenotypically defined Lin(-)Sca(+)Kit(+)CD48(-)CD150(+) long-term hematopoietic stem cells, simultaneously increasing the self-renewal activity upon secondary transplantation. The downregulation of miR-125s in hematopoietic stem cells abolishes these effects and impairs long-term contribution to blood cell production. The introduction of a point mutation within the miRNA-125 seed sequence abolishes all abovementioned effects and leads to the restoration of normal hematopoiesis. Our results show that all miR-125 family members are similar in function, they likely operate in a seed-sequence-dependent manner, and they induce a highly comparable hematopoietic phenotype.

Clonal selection and therapy resistance in acute leukaemias: mathematical modelling explains different proliferation patterns at diagnosis and relapse

Recent experimental evidence suggests that acute myeloid leukaemias may originate from multiple clones of malignant cells. Nevertheless, it is not known how the observed clones may differ with respect to cell properties, such as proliferation and self-renewal. There are scarcely any data on how these cell properties change due to chemotherapy and relapse. We propose a new mathematical model to investigate the impact of cell properties on the multi-clonal composition of leukaemias. Model results imply that enhanced self-renewal may be a key mechanism in the clonal selection process. Simulations suggest that fast proliferating and highly self-renewing cells dominate at primary diagnosis, while relapse following therapy-induced remission is triggered mostly by highly self-renewing but slowly proliferating cells. Comparison of simulation results to patient data demonstrates that the proposed model is consistent with clinically observed dynamics based on a clonal selection process.

Assessing hematopoietic (stem-) cell behavior during regenerative pressure

Hematopoiesis is a complex and strongly regulated process. In case of regenerative pressure, efficient recovery of blood cell counts is crucial for survival of an individual. We propose a quantitative mathematical model of white blood cell formation based on the following cell parameters: (1) proliferation rate, (2) self-renewal, and (3) cell death. Simulating this model we assess the change of these parameters under regenerative pressure. The proposed model allows to quantitatively describe the impact of these cell parameters on engraftment time after stem cell transplantation. Results indicate that enhanced self-renewal during the posttransplant period is crucial for efficient regeneration of blood cell counts while constant or reduced self-renewal leads to delayed recovery or graft failure. Increased cell death in the posttransplant period has a similar impact. In contrast, reduced proliferation or pre-homing cell death causes only mild delays in blood cell recovery which can be compensated sufficiently by increasing the dose of transplanted cells.

Prospective identification and skeletal localization of cells capable of multilineage differentiation in vivo

A prospective in vivo assay was used to identify cells with potential for multiple lineage differentiation. With this assay, it was first determined that the 5-fluorouracil resistant cells capable of osseous tissue formation in vivo also migrated toward stromal derived factor-1 (SDF-1) in vitro. In parallel, an isolation method based on fluorescence-activated cell sorting was employed to identify a very small cell embryonic-like Lin-/Sca-1+CD45- cell that with as few as 500 cells was capable of forming bone-like structures in vivo. Differential marrow fractionation studies determined that the majority of the Lin-Sca-1+CD45- cells reside in the subendosteal regions of marrow. To determine whether these cells were capable of differentiating into multiple lineages, stromal cells harvested from Col2.3 Delta TK mice were implanted with a gelatin sponge into SCID mice to generate thymidine kinase sensitive ossicles. At 1.5 months, 2,000 green fluorescent protein (GFP)+ Lin-Sca-1+CD45- cells were injected into the ossicles. At harvest, colocalization of GFP-expressing cells with antibodies to the osteoblast-specific marker Runx-2 and the adipocyte marker PPAP gamma were observed. Based on the ability of the noncultured cells to differentiate into multiple mesenchymal lineages in vivo and the ability to generate osseous tissues at low density, we propose that this population fulfills many of the characteristics of mesenchymal stem cells.

Modeling of replicative senescence in hematopoietic development

Hematopoietic stem cells (HSC) give rise to an enormous number of blood cells throughout our life. In contrast their number of cell divisions preceding senescence is limited underin vitro culture conditions. Here we consider the question whether HSC can rejuvenate indefinitely or if the number of cell divisions is restricted. We have developed a multi-compartmental model for hematopoietic differentiation based on ordinary differential equations. The model is based on the hypothesis that in each step of maturation, the percentage of self-renewal versus differentiation is regulated by a single external feedback mechanism. We simulate the model under the assumption that hematopoietic differentiation precedes the six steps of maturation and the cells ultimately cease to proliferate after 50 divisions. Our results demonstrate that it is conceivable to maintain hematopoiesis over a life-time if HSC have a slow division rate and a high self-renewal rate. With age, the feedback signal increases and this enhances self-renewal, which results in the increase of the number of stem and progenitor cells. This study demonstrates that replicative senescence is compatible with life-long hematopoiesis and that model predictions are in line with experimental observations. Thus, HSC might not divide indefinitely with potentially important clinical implications.

Does the reservoir for self-renewal stem from the ends?

Stem cell research is a burgeoning field with an alluring potential for therapeutic intervention, and thus begs a critical understanding of the long-term consequences of stem cell replacement. Operationally, a stem cell may be defined as a rarely dividing cell with the capacity for self-renewal throughout the lifetime of the organism, and an ability to reconstitute its appropriate lineages via proliferation and differentiation. In many differentiated normal and cancer cell types, the maintenance of telomeres plays a pivotal role in their continued division potential. Taken together with the presence of the enzymatic activity responsible for telomere addition, telomerase, in several progenitor cell lineages, it is presumed that telomere maintenance will be critical for the replenishment of stem cells or their successors. The purpose of this review is to discuss the role of telomere length maintenance in self-renewal, and the consequent challenges and potential pitfalls to the manipulation of normal and cancer-derived stem cells.

Telomere length in subpopulations of human hematopoietic cells

In order to test the hypothesis that the telomere length in human hematopoietic cells correlates with their proliferative potential, we analyzed the telomere length in highly purified subpopulations of bone marrow cells. Cells were sorted on the basis of CD34 and CD38 cell surface markers, and two samples were additionally sorted on the basis of Hoechst 33342 dye efflux allowing isolation of side population (SP) cells. The telomere length in limiting numbers of sorted cells was analyzed using a newly developed fluorescence in situ hybridization (flow-FISH) method in which hybridization of telomere probe in cells of interest is measured relative to control cells in the same tube. In all seven bone marrow samples analyzed, the telomere length in CD34(+)CD38(-) cells was longer than in CD34(+)CD38(+) cells from the same donor (p < 0.02). Results with sorted SP cells were less clear: the telomere fluorescence in these cells was very heterogeneous, and a reproducible difference in telomere length relative to CD34(+)CD38(-) cells could not be observed. We conclude that the telomere length in subpopulations of hematopoietic cells does appear to be correlated with the known proliferative potential of such cells and that further characterization of cells on the basis of telomere length is warranted for enrichment of very rare precursors of hematopoietic and other tissues.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

Hematopoietic cells and replicative senescence

Replicative senescence describes the finite cell replicative capacity in response to chronic proliferative stimulation. A key element in this process is the shortening of the telomeres, which to a major extent is caused by the lack of expression of telomerase. Whereas this situation has been well documented for a variety of somatic cell types, the question of whether stem cells "senesce" in the course of enforced chronic sequential divisions is as yet unresolved. This article examines several distinct features of hematopoietic cells (HC) in light of their similarity to certain aspects of memory T cells. It appears that although the capacity of HC for replication is not exhausted under normal physiological conditions in vivo, under certain experimental conditions and in specific in clinical situations HC do show signs of telomere shortening. This limited potential should be taken into account both with respect to aging in vivo, and also in terms of attempts to expand these cells ex vivo for therapeutic use.

Peripheral blood stem cell versus bone marrow allotransplantation: does the source of hematopoietic stem cells matter?

Hematopoietic stem cells from 4 different sources have been or are being used for the reconstitution of lymphohematopoietic function after myeloablative, near-myeloablative, or nonmyeloablative treatment. Bone marrow (BM)-derived stem cells, introduced by E. D. Thomas in 1963, are considered the classical stem cell source. Fetal liver stem cell transplantation has been performed on a limited number of patients with aplastic anemia or acute leukemia, but only transient engraftment has been demonstrated. Peripheral blood as a stem cell source was introduced in 1981, and cord blood was introduced as a source in 1988. The various stem cell sources differ in their reconstitutive and immunogenic characteristics, which are based on the proportion of early pluripotent and self-renewing stem cells to lineage-committed late progenitor cells and on the number and characteristics of accompanying "accessory cells" contained in stem cell allografts.