(R)evolutionary considerations in hematopoietic development

Therapy resistance mediated by cancer stem cells

Cancer stem cells (CSC) possess abilities generally associated with embryonic or adult stem cells, especially self-renewal and differentiation. The CSC model assumes that this subpopulation of cells sustains malignant growth, which suggests a hierarchical organization of tumors in which CSCs are on top and responsible for the generation of intratumoral heterogeneity. Effective tumor therapy requires the eradication of CSC as they can support regrowth of the tumor resulting in recurrence. However, eradication of CSC is difficult because they frequently are therapy resistant. Therapy resistance is mediated by the acquisition of dormancy, increased DNA repair and drug efflux capacity, decreased apoptosis as well as the interaction between CSC and their supporting microenvironment, the CSC niche. This review highlights the role of CSC in chemo- and radiotherapy resistance as well as possible ways to overcome CSC mediated therapy resistance.

Intestinal crypt properties fit a model that incorporates replicative ageing and deep and proximate stem cells

A model of intestinal crypt organization is suggested based on the assumption that stem cells have a finite replicative life span. The model assumes the existence in a crypt of a quiescent ('deep') stem cell and a few more actively cycling ('proximate') stem cells. Monte Carlo computer simulation of published intestinal crypt mutagenesis data is used to test the model. The results of the simulation indicate that stabilization of the crypt mutant phenotype following treatment with external mutagen is consistent with a stem cell replicative life span of about 40 divisions for mouse colon and 90-100 divisions for mouse small intestine, corresponding to a deep stem cell cycle time of about 3.9 and 8.5 weeks for colon and small intestine, respectively. Simulation of the data obtained for human colorectal crypts suggests that the proximate stem cell cycle time is about 80 h, assuming a replicative life span of 50-150 divisions, and that the deep stem cell divides approximately every 30 weeks.

The spleen is a major site of megakaryopoiesis following transplantation of murine hematopoietic stem cells

The stem cell pool can be fractionated by using the mitochondrial dye, rhodamine-123, into Rho(low) hematopoietic stem cells and Rho(high) progenitors. Rho(low) stem cells permanently engraft all lineages, whereas Rho(high) progenitors transiently produce erythrocytes, without substantial platelet or granulocyte production. We hypothesized that the inability of the Rho(high) cells to produce platelets in vivo was due to the fact that these cells preferentially engraft in the spleen and lack marrow engraftment. Initially, we demonstrated that Rho(high) progenitors produced more megakaryocytes in vitro than Rho(low) stem cells did. To study the activity of the Rho(low) and Rho(high) subsets in vivo, we used mice allelic at the hemoglobin and glucose phosphate isomerase loci to track donor-derived erythropoiesis and thrombopoiesis. Rho(low) stem cells contributed to robust and long-term erythroid and platelet engraftment, whereas Rho(high) progenitors contributed only to transient erythroid engraftment and produced very low numbers of platelets in vivo. Donor-derived megakaryopoiesis occurred at higher densities in the spleen than in the bone marrow in animals receiving Rho(low) stem cells and peaked around day 28. Blockade of splenic engraftment using pertussis toxin did not affect the peak of splenic megakaryopoiesis, supporting the hypothesis that these megakaryocytes were derived from progenitors that originated in the bone marrow. These data emphasize that in vitro behavior of hematopoietic progenitor cell subsets does not always predict their behavior following transplantation. This study supports a major role for the spleen in thrombopoiesis following engraftment of transplanted stem cells in irradiated mice.

The ABCG2 transporter is an efficient Hoechst 33342 efflux pump and is preferentially expressed by immature human hematopoietic progenitors

A promising and increasingly exploited property of hematopoietic stem cells is their ability to efflux the fluorescent dye Hoechst 33342. The Hoechst-negative cells are isolated by fluorescence-activated cell sorting as a so-called side "population" (SP) of bone marrow. This SP from bone marrow, as well as other tissues, is reported to contain immature stem cells with considerable plasticity. Some cell lines also efflux Hoechst and generate SP profiles. Reverse transcription-polymerase chain reaction (RT-PCR) and efflux inhibition studies with the lung carcinoma cell line, A549, implicated the ABCG2 transporter as a Hoechst efflux pump. Furthermore, it is shown that transient expression of ABCG2 generates a robust SP phenotype in human embryonic kidney (HEK293) cells. The results allow the conclusion that ABCG2 is a potent Hoechst efflux pump. Semiquantitative RT-PCR was used to characterize the developmental pattern of expression of ABCG2 in hematopoiesis. It is expressed at relatively high levels in putative hematopoietic stem cells (isolated as SP, 34+/38- or 34+/KDR+ populations) and drops sharply in committed progenitors (34+/38+, 34+/33+, or 34+/10+). Expression remains low in most maturing populations, but rises again in natural killer cells and erythroblasts. Comparison of messenger RNA (mRNA) levels for the 3 major multidrug-resistant efflux pumps, MDR1, MRP1, and ABCG2, in bone marrow SP cells reveals that ABCG2 is the predominant form in these cells. These data suggest that ABCG2 contributes significantly to the generation of the SP phenotype in hematopoietic stem cells. Furthermore, the sharp down-regulation of ABCG2 at the stage of lineage commitment suggests that this gene may play an important role in the unique physiology of the pluripotent stem cell.

The fleet feet of haematopoietic stem cells: rapid motility, interaction and proteopodia

Haematopoietic stem cells (HSCs) have been extensively characterized regarding in vivo engraftment, surface epitopes and genetic regulation. However, little is known about the homing of these rare cells, and their intrinsic motility and membrane deformation capacity. We used high-speed optical-sectioning microscopy and inverted fluorescent videomicroscopy to study highly purified murine lineage-negative, rhodamine-low, Hoechst-low HSCs over time under various in vitro conditions. We discovered extremely rapid motility, directed migration to stromal cells and marked membrane modulation. High resolution images with three-dimensional reconstruction showed the general presence of microspikes. Further, pseudopodia (proteopodia) were observed that were induced by stromal-derived factor-1 and steel factor. Proteopodia were directed towards and were quenched by stromal cells, at times bridged HSCs, and could rapidly retract or detach from cells. Proteopodia were also observed in vivo with homed HSCs in frozen sections of murine spleen, lung and heart. This is the first demonstration that HSCs are both fast and highly malleable in phenotype.

Hematopoietic stem cells can be CD34+ or CD34-

Until recently, it was thought that the most primitive HSC have a fixed phenotype within a hierarchical differentiation system, and that changes in engraftment and renewal potential occur in a stepwise fashion linked with differentiation. In this review, we summarize the data from several different species and different animal models of hematopoietic stem cell function. Taking into account all of the published data, it becomes clear that the hematopoietic stem cell compartment contains more than one phenotypically identifiable population capable of self-renewal and long term pluripotent engraftment. It is clear that some stem cells express CD34, and others do not. The exact phenotypic progression between these cells needs to be further defined, because different in vivo and ex vivo manipulations may shift the stem cells from one phenotype to another, and this can complicate interpretation of experimental transplant data.

Mobilization of hematopoietic stem cells

Hematopoietic stem cell transplantation has been extensively exploited as a therapeutic and research modality and has revolutionized current patient care. At present, more and more medical centers use peripheral blood progenitor cells for transplantation by mobilizing hematopoietic stem cells from bone marrow to peripheral blood because of potential advantages of peripheral blood stem cell transplantation over bone-marrow transplantation. Different effective mobilization regimens have been developed recently with chemotherapeutic agents, hematopoietic growth factors or their combination. This article reviews current developments related to hematopoietic stem cell mobilization including the biology of hematopoietic stem cells, strategies for mobilization, management for mobilization failure, mechanisms of mobilization, and side effects during mobilization. Finally, the Initiation-Amplification-Emigration-Adaptation Model is proposed to help aid understanding of the mechanisms of hematopoietic stem cell mobilization and to stimulate development of novel and optimal mobilization strategies for patient care.

Life cycle of Trypanosoma cruzi (Y strain) in mice

Since 1958, we have studied experimental Chagas' disease (CD) by subcutaneous inoculation of 1,000 blood forms of Trypanosoma cruzi (Y strain) in Balb/C. mice. Evolution of parasitemia remained constant, beginning on the 5th and 6th day of the disease, increasing progressively, achieving a maximum on about the 30th day. After another month, only a few forms were present, and they disappeared from the circulation after the third month, as determined from direct examination of slides and the use of a Neubauer Counting Chamber. These events coincided with the appearance of amastigote nests in the tissues (especially the cardiac ones), starting the first week, and following the Gauss parasitemia curve, but they were not in parallel until the chronic stage. In 1997, we began to note the following changes: Parasites appeared in the circulation during the first week and disappeared starting on the 7th day, and there was a coincident absence of the amastigote nests in the tissues. A careful study verified that young forms in the evolutionary cycle of T. cruzi (epi + amastigotes) began to appear alongside the trypomastigotes in the circulation on the 5th and 7th post-inoculation day. At the same time, rounded, oval, and spindle shapes were seen circulating through the capillaries and sinusoids of the tissues, principally of the hematopoietic organs. Stasis occurs because the diameter of the circulating parasites is greater than the vessels, and this makes them more visible. Examination of the sternal bone marrow revealed young cells with elongated forms and others truncated in the shape of a "C" occupying the internal surface of the blood cells that had empty central portions (erythrocytes?). We hypothesize that there could be a loss of virulence or mutation of the Y strain of Trypanosoma cruzi.