Substrate induction of osteogenesis from marrow-derived mesenchymal precursors

Therapeutic modalities aimed at bone regeneration are increasingly employing extracellular matrix (ECM) constituents to control bone marrow progenitor cell (BMPC) commitment, growth, and differentiation. However, the precise role these ECM elements play during stem cell differentiation remains unclear. (See also Salaszynk et al., Stem Cells Dev 14(6):608-620, 2005; and Schwartz et al., Stem Cells Dev. 14(6), 643-655, 2005, both in this issue.) Because bone formation ultimately begins with the recruitment and commitment of BMPCs into the osteogenic lineage, factors that enhance this process are clearly therapeutic targets. We hypothesized that BMPC attachment, proliferation, and osteogenic differentiation would be potentiated when cultured on ECM proteins normally found in the bone niche. To examine this, we cultured murine BMPCs on laminin-1, fibronectin, and collagen type-1 substrates for up to 14 days and assessed their homogeneity, attachment, proliferation, and expression of the specific bone lineage markers RUNX2, collagen-1, alkaline phosphatase, and osteocalcin. We found that freshly harvested mBMPCs contain a mixed population of progenitor cells and that the mesenchymal pool can be enriched by adherent culture in the presence of leucine methyl ester. Furthermore, mBMPCs attached to laminin, fibronectin, and collagen-1 with varying affinity up to 3 h (fibronectin>or=collagen>laminin), after which time no difference could be detected. Despite this, growth was unaffected; cells thereafter proliferated equally well on all substrates up to confluence (7 days). Notably, commitment to the osteoblast lineage (RUNX2) increased up to 14 days for cells cultured on the various substrates, yet no difference was observed at day 14 in the expression of collagen-1, alkaline phosphatase, or osteocalcin. We conclude that mBMPC differentiation down the osteoblastic lineage is time-dependent in osteogenic culture and that attachment to ECM matrices potentiates lineage commitment rather than growth.

Role of growth hormone receptor signaling in osteogenesis from murine bone marrow progenitor cells

Growth hormone (GH) regulates many of the factors responsible for controlling the development of bone marrow progenitor cells (BMPCs). The aim of this study was to elucidate the role of GH in osteogenic differentiation of BMPCs using GH receptor null mice (GHRKO). BMPCs from GHRKO and their wild-type (WT) littermates were quantified by flow cytometry and their osteogenic differentiation in vitro was determined by cell morphology, real-time RT-PCR, and biochemical analyses. We found that freshly harvested GHRKO marrow contains 3% CD34 (hematopoietic lineage), 43.5% CD45 (monocyte/macrophage lineage), and 2.5% CD106 positive (CFU-F/BMPC) cells compared to 11.2%, 45%, and 3.4% positive cells for (WT) marrow cells, respectively. When cultured for 14 days under conditions suitable for CFU-F expansion, GHRKO marrow cells lost CD34 positivity, and were markedly reduced for CD45, but 3- to 4-fold higher for CD106. While WT marrow cells also lost CD34 expression, they maintained CD45 and increased CD106 levels by 16-fold. When BMPCs from GHRKO mice were cultured under osteogenic conditions, they failed to elongate, in contrast to WT cells. Furthermore, GHRKO cultures expressed less alkaline phosphatase, contained less mineralized calcium, and displayed lower osteocalcin expression than WT cells. However, GHRKO cells displayed similar or higher expression of cbfa-1, collagen I, and osteopontin mRNA compared to WT. In conclusion, we show that GH has an effect on the proportions of hematopoietic and mesenchymal progenitor cells in the bone marrow, and that GH is essential for both the induction and later progression of osteogenesis.

Alternative testing systems for evaluating noncarcinogenic, hematologic toxicity

Hematopoietic tissues are the targets of numerous xenobiotics. Clinical hematotoxicity is either a decrease or an increase in peripheral blood cell counts in one or more cell lineages--a cytopenia or a cytosis, respectively--that carries a risk of an adverse clinical event. The purpose of in vitro hematotoxicology is the prediction of these adverse hematologic effects from the effects of the toxicants on human hematopoietic targets under controlled experimental conditions in the laboratory. Building on its important foundations in experimental hematology and the wealth of hematotoxicology data found in experimental oncology, this field of alternative toxicology has developed rapidly during the past decade. Although the colony-forming unit-granulocyte/monocyte neutrophil progenitor is most frequently evaluated, other defined progenitors and stem cells as well as cell types found in the marrow stroma can be evaluated in vitro. End points have been proposed for predicting toxicant exposure levels at the maximum tolerated dose and the no observable adverse effect level for the neutrophil lineage, and several clinical prediction models for neutropenia have developed to the point that they are ready for prospective evaluation and validation in both preclinical species and humans. Known predictive end points are the key to successful comparisons across species or across chemical structures when in vitro dose-response curves are nonparallel. Analytical chemistry support is critical for accurate interpretation of in vitro data and for relating the in vitro pharmacodynamics to the in vivo pharmacokinetics. In contrast to acute neutropenia, anemia and acute thrombocytopenia, as well as adverse effects from chronic toxicant exposure, are much more difficult to predict from in vitro data. Pharmacologic principles critical for clinical predictions from in vitro data very likely will apply to toxicities to other proliferative tissues, such as mucositis.

Mathematical models of marrow cell kinetics: differential effects of protracted irradiations on stromal and stem cells in mice

It is known that hematopoiesis is supported by bone-marrow stem cells, but those cells must seed and grow on a stromal microenvironment. Typically, studies have shown that a surviving fraction of about 30 hematopoietic stem cells (HSCs) (i.e., about 0.04%) correspond to the LD50, although other studies have shown that marrow can repopulate from a single viable cell under strong regiments of antibiotics and infusions of irradiated blood elements.

PURPOSE

This paper describes comparisons between our results (from maximum-likelihood estimation techniques for cellular damage, repair, and compensatory repopulation) and published experimental data on marrow stromal cells.

METHODS AND MATERIALS

After biophysical consideration of the rate constants that were derived by maximizing the likelihood function (a consideration necessary to extend the model to cell populations not indicated by the model as "critical" for recovery), the rate constants for cellular damage to stem cells are fitted to experimental data. Rate constants for repair and proliferation of stem cells are assigned based on published data on repair/proliferation half-times, and these assignments affect the evaluation of the rate constants for cellular damage. From the two models, that is one for "critical" cells (having radiosensitive and repopulation characteristics similar to stromal cells) and another for stem cells, effects on two cell populations of different radiosensitivities and repopulation rates can be demonstrated for complex schedules of protracted irradiations which could reduce either cell population below a critical need for marrow repopulation.

RESULTS

Our analysis of animal mortality data has indicated that recovery of an animal from potentially lethal irradiation is dominantly regulated by cells with survival and repopulation characteristics similar to those of stroma cells.

CONCLUSION

In contrast to the surviving fraction of hematopoietic stem cells, it appears that the probability of an animal's recovery is high if the "critical" population of cells is above 1% (our "best" maximum likelihood estimate, from mouse data, with the corresponding lower confidence bound at about 0.2%). Of course, a few stem cells--perhaps only one--must maintain a potential for repopulation of blood and marrow.

Interactions in recovery and in residual injury from sequential treatments of mouse haemopoietic and stromal marrow cell populations, using X-rays, cyclophosphamide and busulphan

The acute recovery of populations of day 11 CFU-S, iv-CFC and CFU-F in mouse bone marrow, following a test dose of X-rays, cyclophosphamide (CP) or busulphan (BUS) given to mice previously treated with repeated priming doses of X-rays or CP, was in general predictable from the amount of residual injury after the priming doses. A marked exception was iv-CFC after X-rays, which although amplified to near normal levels during the residual injury phase, recovered after the test irradiation from low levels of CFU-S. The amount of residual injury after sequential treatments of different agents was in general less than expected on the basis of the product of the effects of the individual agents. This was most marked for CP priming treatments, where the long-term recovery of day 11 CFU-S after the test dose remained persistently above control levels. Also, some correlation was found between improved stromal recovery (CFU-F) and the CFU-S content following the sequential treatment protocols.

Cumulative bone marrow stromal damage caused by X-irradiation and cytosine-arabinoside in leukemic mice

A study of treated murine acute myeloid leukemia (AML) with an emphasis on the bone marrow stromal function is reported. Leukemia was induced in C57Bl mice through intraperitoneal (i.p.) inoculation of C-1498 myelogenous leukemic cells. The leukemic mice were administered: (1) total body lethal X-irradiation (t.b.i.); (2) two i.p. cytosine-arabinoside (Ara-C) injections followed by X-irradiation. Control mice received similar regimens. Bone marrow of experimental and control mice was processed for stromal cell cultures (SCC) and in vitro engraftment of hematopoietic cells onto the cultures. The results of this study indicate that the bone marrow stromal deficiency which occurs in leukemia is aggravated by Ara-C and irradiation treatments. Moreover, SCC of treated leukemic mice sustain in vitro hematopoiesis only to a limited degree. Stromal deficiency, as possible cause for graft failure in bone marrow transplanted leukemic patients, is discussed.

Residual haemopoietic damage in the mouse after fractionated gamma-irradiation, down to 0.1 Gy per fraction

Residual damage in haemopoietic progenitor cell populations, spleen and granulocyte-macrophage colony-forming cells (CFU-S and GM-CFC) was detected in mice after 15 daily fractions where the dose per fraction was as low as 0.1 Gy. The injury was dose-dependent and after higher total fractionated doses of 7.5-10 Gy the CFU-S population recovered to about 50% of control between 2 and 12 months after irradiation. Residual damage was also detected in the stroma, in the form of reduced numbers of fibroblastoid colony-forming cells and of CFU-S in ossicles under the kidney capsule. The response to a second course of 15 fractions, given 3 weeks after the end of the first course, was similar and additive to the response to the first course in the short term. However, in the long term, recovery levels were similar after either one or two courses.

Long-term haemopoietic injury in mice after repeated irradiation: precursor-cell cycling and its regulation

The cycling rate of haemopoietic stem cells (day 9 CFU-S) and granulocyte-macrophage colony forming cells (GM-CFC) in mouse femora was, in response to reduced numbers, elevated at all times of sampling between 3 weeks and 10 months after 4 repeated doses of 4.5 Gy X-rays (3 wk between doses). The level of a stimulator of CFU-S cycling was also elevated, and this was observed in both axial and marginal regions of the marrow inside the shaft. However, the rate of production of the stimulator was low; lower than previously reported in marrow regenerating after a single dose of 4.5 Gy, indicating damage to the regulatory stromal cells. The distribution of CFU-S across the axial and marginal zones of femoral marrow was changed from that in normal mice, where higher concentrations were found near the bone surface, to a more uniform distribution.

Effect of irradiation on thermal sensitivity of bone marrow progenitors

The heat sensitivity of murine CFU-GM and CFU-E following 2.5 Gy of total body irradiation (TBI) was studied. C3H f/Sed female mice were treated with 2.5 Gy TBI and femoral bone marrow was heated in vitro at 43 degrees C. CFU-GM show heat radiosensitization when bone marrow was heated immediately following irradiation. There was a brief decline in heat and radiation interaction when cells were heated 3 hours following 2.5 Gy of TBI, but heat radiosensitization returned to its maximum from 1 to 2 days following irradiation and remained significantly different from the control on days 5 and 7 following irradiation. The heat and radiation interaction disappeared by 30 days. CFU-E shows significant heat radiosensitization only on day 2 following 2.5 Gy of TBI. Total nucleated cells per femur showed a decrease by 70 per cent in days 1 to 2 following TBI, recovered to control values by day 5, and did not correlate with the changes in heat radiosensitization. Cell cycle analysis of CFU-GM using hydroxyurea showed no significant changes in cell cycle parameters on days 1 and 2 following 2.5 Gy, when maximum heat sensitization was observed. It is concluded that bone marrow progenitors may respond in a different way from other normal tissues to heat and irradiation sequencing, and that these differences must be considered when designing clinical trials.

The radiation sensitivity of the haemopoietic microenvironment--effect of dose rate on ectopic ossicle formation

The haemopoietic microenvironment (HM) consists of a complex mixture of cellular types and extra-cellular matrix. It is essential for prolonged haemopoiesis in both the normal situation and after bone marrow transplantation. The competence of the HM can be assessed by ectopic grafting of femoral marrow. A complete haemopoietic organ develops at the site of implantation. Stem cells (CFU-S) which inhabit the ossicle formed after ectopic implantation can be measured, to assess the function of the engrafted HM to support haemopoiesis. Using this functional endpoint we have examined the radiation sensitivity of the HM at both high and low dose rates, and conclude that high doses of gamma-irradiation delivered at 4 Gy/min or 0.016 Gy/min have widely different effects on the HM, the former proving much more damaging than the latter.

The cellular basis of long-term marrow injury after irradiation

Haemopoietic recovery from radiation injury can appear complete when measured by blood cell counts, but this can hide deficiencies in the precursor cell populations because of compensatory mechanisms of increased numbers of divisions in the maturing cell populations and increased cycling of the stem cells. These mechanisms can operate for quite long but finite periods, before they fail which then leads to hypoplasia. Also, while these mechanisms are operating, small further injuries could precipitate marrow failure. Persistent injury in the stem cell population can be induced by quite small doses, and in mice the threshold total dose is probably in the region of 1.5 Gy using fractionated whole-body irradiations. The sensitivity of the environment varies enormously, depending largely on the proliferative stress applied to the cell populations involved in the particular assay technique used. When similar tests of reproductive integrity are applied, stromal progenitor cells are more radioresistant than haemopoietic stem cells. The contribution of environmental injuries to haemopoietic defects is uncertain and difficult to assess.

A cellular analysis of residual hemopoietic deficiencies in mice after 4 repeated doses of 4.5 Gray X rays

A sub-optimal plateau in numbers of femoral stem-cells (CFU-S) in mice after 4 doses of 4.5 Gray X rays (each separated by 21 days), was shown to persist at 20-30% of control up to 1 year after the last dose, when about 50% of the mice had survived. The concentration of white cells in the blood was maintained persistently at about 70% of control, whereas the concentration of red cells was normal up to 4 months and then it declined to about 75% of control at 10 months after irradiation. Concentrations of some committed progenitor cells in the marrow (GM-CFC and ERC), which are capable of amplification cell divisions, were intermediate between the concentrations of marrow stem cells and mature blood cells in both the granuloid and the erythroid cell lineages, respectively. Hence increased amplification was a mechanism operating for a prolonged period in the production of numbers of mature cells. The numbers were subnormal, however, and this corresponded to only 1 extra amplification division on average. There was a slow decline after 6 months in the numbers of CFU-S, BFU-E and GM-CFC, and in the hematocrit, with reference to age-matched controls. The decline was due partly to a prevention of the natural increase in cell numbers in the marrow with the age of the mice, which was also seen with the femoral content of a stromal progenitor cell (CFU-F). A defect in the repeatedly-irradiated CFU-S population was detected as a persistent inability to produce colonies containing the same number of daughter CFU-S as contained in colonies derived from unirradiated marrow and assayed at the same time.