LIF-mediated control of embryonic stem cell self-renewal emerges due to an autoregulatory loop

Stem cells convert graded stimuli into all-or-nothing cell-fate responses. We investigated how embryonic stem cells (ESCs) convert leukemia inhibitory factor (LIF) concentration into an all-or-nothing cell-fate decision (self-renewal). Using a combined experimental/computational approach we demonstrate unexpected switch-like (on/off) signaling in response to LIF. This behavior emerges over time due to a positive feedback loop controlling transcriptional expression of LIF signaling pathway components. The autoregulatory loop maintains robust pathway responsiveness ("on") at sufficient concentrations of exogenous LIF, while autocrine signaling and low concentrations of exogenous LIF cause ESCs to adopt the weakly responsive ("off") state of differentiated cells. We demonstrate that loss of ligand responsiveness is reversible and precedes loss of the ESC transcription factors Oct4 and Nanog, suggesting an early step in the hierarchical control of differentiation. While endogenously produced ligands were insufficient to sustain the "on" state, they buffer it, influencing the timing of differentiation. These results demonstrate a novel switch-like behavior, which establishes the LIF threshold for ESC self-renewal.

Understanding cellular networks to improve hematopoietic stem cell expansion cultures

Efforts to develop culture technologies capable of eliciting robust human blood stem cell growth have met with limited success. Considering that adult stem cell cultures are complex systems, comprising multiple cell types with dynamically changing intracellular signalling environments and cellular compositions, this is not surprising. Typically treated as single-input single-output systems, adult stem cell cultures are better described as complex, non-linear, multiple-input multiple-output systems wherein the proliferation of subpopulations of cells leads to the formation of intercellular endogenously secreted protein interaction networks. Genomic and proteomic tools need to be applied to generate high-throughput (and ideally high-content) biological measurements of stem cell culture evolution. Datasets describing cellular interaction networks need to be integrated into predictive models of in vitro stem cell development. Ultimately, such models will serve as a starting point for the rational design of blood stem cell expansion bioprocesses utilizing dynamic system perturbations to achieve the preferential expansion of target cell populations.

Spatial organization of embryonic stem cell responsiveness to autocrine gp130 ligands reveals an autoregulatory stem cell niche

Highly ordered aggregates of cells, or niches, regulate stem cell fate. Specific tissue location need not be an obligatory requirement for a stem cell niche, particularly during embryogenesis, where cells exist in a dynamic environment. We investigated autoregulatory fixed-location-independent processes controlling cell fate by analyzing the spatial organization of embryonic stem cells (ESCs) using quantitative single-cell immunocytochemistry and a computational approach involving Delaunay triangulation. ESC colonies demonstrated radial organization of phosphorylated signal transducer and activator of transcription 3, Nanog, and Oct4 (among others) in the presence and absence of exogenous leukemia inhibitory factor (LIF). Endogenous self-renewal signaling resulted from autocrine non-LIF gp130 ligands, which buffered cells against differentiation upon exogenous LIF deprivation. Together with a radial organization of differential responsiveness to gp130 ligands within colonies, autocrine signaling produced a radial organization of self-renewal, generating a fixed-location-independent autoregulatory niche. These findings reveal fundamental properties of niches and elucidate mechanisms colonies of cells use to transition between fates during morphogenesis.

Culture development for human embryonic stem cell propagation: molecular aspects and challenges

Basic fibroblast growth factor and members of the transforming growth factor-beta superfamily are important regulators of human embryonic stem cell (hESC) self-renewal. Extensive cross-talk between the intracellular signaling pathways activated by these factors contributes to maintenance of the undifferentiated hESC state. Understanding the molecular regulation of hESC self-renewal will facilitate the design of improved systems for hESC propagation and provide a foundation for strategies to direct the differentiation of hESCs to clinically relevant cell types.

Dynamic changes in cellular and microenvironmental composition can be controlled to elicit in vitro human hematopoietic stem cell expansion

OBJECTIVE

The absence of effective strategies for the ex vivo expansion of human hematopoietic stem cells (HSCs) limits the development of many cell-based therapies. Prior attempts to stimulate HSC expansion have focused on media supplementation using cytokines and growth factors. In these cultures, cellular and microenvironmental compositions change with time. In this study, the impact of controlling these dynamic changes on HSC output is determined.

MATERIALS AND METHODS

Cord blood-derived lin(-) cells were cultured for 8 days in serum-free medium supplemented with stem cell factor, Flt3 ligand, and thrombopoietin. Functional, phenotypic, and molecular (gene and protein) analyses were used to characterize dynamic changes in cellular and microenvironmental composition. The effects of these changes and the mechanism behind their effects on HSC expansion were assessed using a selection/media exchange-based global culture manipulation (GCM) technique.

RESULTS

We show that the direct secretion of negative regulators by culture-generated lin(+) cells, and the indirect stimulation of cells to secrete negative regulators by culture-conditioned media, limits in vitro HSC generation. The GCM strategy was able to abrogate these effects to produce elevated numbers of LTC-ICs (14.6-fold relative to input), migrating rapid NOD/SCID repopulating cells (12.1-fold), and long-term NOD/SCID repopulating cells (5.2-fold).

CONCLUSIONS

Cellular and microenvironmental changes that occur during all in vitro HSC cultures can significantly affect HSC output through the direct or indirect secretion of negative regulators. This study provides insight into the mechanisms regulating HSC fate in vitro and describes a novel methodology to regulate overall in vitro microenvironmental dynamics to enable the generation of clinically relevant numbers of HSCs.

Polyurethane films seeded with embryonic stem cell-derived cardiomyocytes for use in cardiac tissue engineering applications

Cardiomyocytes are terminally differentiated cells and therefore unable to regenerate heart tissue after infarction. The successful engraftment of various cell types resulting in improved cardiac function has been reported, however methods for improving the delivery of donor cells to the infarct site still need to be developed. The use of bioengineered cardiac grafts has been suggested to replace infarcted myocardium and enhance cardiac function. In this study, we cultured embryonic stem (ES) cell-derived cardiomyocytes on thin polyurethane (PU) films. The films were coated with gelatin, laminin or collagen IV in order to encourage cell adhesion. Constructs were examined for 30 days after seeding. Cells cultured on laminin and collagen IV, exhibited preferential attachment, as assessed by cellular counts, and viability assays. These surfaces also resulted in a greater number of contracting films compared to controls. A degradable elastomer seeded with embryonic stem cell-derived cardiomyocytes may hold potential for the repair of damaged heart tissue.

Controlled, scalable embryonic stem cell differentiation culture

Embryonic stem (ES) cells are of significant interest as a renewable source of therapeutically useful cells. ES cell aggregation is important for both human and mouse embryoid body (EB) formation and the subsequent generation of ES cell derivatives. Aggregation between EBs (agglomeration), however, inhibits cell growth and differentiation in stirred or high-cell-density static cultures. We demonstrate that the agglomeration of two EBs is initiated by E-cadherin-mediated cell attachment and followed by active cell migration. We report the development of a technology capable of controlling cell-cell interactions in scalable culture by the mass encapsulation of ES cells in size-specified agarose capsules. When placed in stirred-suspension bioreactors, encapsulated ES cells can be used to produce scalable quantities of hematopoietic progenitor cells in a controlled environment.

Clonal evolution of stem and differentiated cells can be predicted by integrating cell-intrinsic and -extrinsic parameters

Stem cells and their derivatives represent a renewable source of cells for therapeutic applications. However, the inability to quantitatively integrate and exploit the effects of multiple parameters on the fate of stem cells limits their use in clinical applications. To address this, we developed a computational model that combines probabilistic, individual-cell and deterministic cell-population parameters to simultaneously calculate the specific effects of exogenous and endogenous factors on the overall population-dynamics behaviour. The model tracks the progeny trajectory of individual cells over several generations as a threshold function of ligand-receptor signalling interactions. Simulations in silico were validated against an Oct 4-promoter-driven green-fluorescent-protein-expressing murine embryonic stem cell line, and used to understand the effects of key parameters on the clonal evolution of stem versus differentiated cells in this system. Our approach demonstrated the ability to distinguish between individual-cell and population-averaged parameters with respect to their effects on governing dynamic behaviour. Moreover, we could discriminate between digital versus graded regulation of the Oct 4 transcription factor in accounting for experimental observations. Finally, we showed that our approach could be generalized to other stem-cell systems, in particular the previously characterized intestinal crypt system, in elucidating relative contributions of stem and progenitor cells to population output. On the basis of all these results, we believe that our iterative experimental and computational approach has been found to be useful for the study of various stem-cell systems.

Development of a perfusion fed bioreactor for embryonic stem cell-derived cardiomyocyte generation: oxygen-mediated enhancement of cardiomyocyte output

Cell transplantation is emerging as a promising new approach to replace scarred, nonfunctional myocardium in a diseased heart. At present, however, generating the numbers of donor cardiomyocytes required to develop and test animal models is a major limitation. Embryonic stem (ES) cells may be a promising source for therapeutic applications, potentially providing sufficient numbers of functionally relevant cells for transplantation into a variety of organs. We developed a single-step bioprocess for ES cell-derived cardiomyocyte production that enables both medium perfusion and direct monitoring and control of dissolved oxygen. Implementation of the bioprocess required combining methods to prevent ES cell aggregation (hydrogel encapsulation) and to purify for cardiomyocytes from the heterogeneous cell populations (genetic selection), with medium perfusion in a controlled bioreactor environment. We used this bioprocess to investigate the effects of oxygen on cardiomyocyte generation. Parallel vessels (250 mL culture volume) were run under normoxic (20% oxygen tension) or hypoxic (4% oxygen tension) conditions. After 14 days of differentiation (including 5 days of selection), the cardiomyocyte yield per input ES cell achieved in hypoxic vessels was 3.77 +/- 0.13, higher than has previously been reported. We have developed a bioprocess that improves the efficiency of ES cell-derived cardiomyocyte production, and allows the investigation of bioprocess parameters on ES cell-derived cardiomyogenesis. Using this system we have demonstrated that medium oxygen tension is a culture parameter that can be manipulated to improve cardiomyocyte yield.

Shear-controlled single-step mouse embryonic stem cell expansion and embryoid body-based differentiation

To facilitate the exploitation of embryonic stem cells (ESCs) and ESC-derived cells, scale-up of cell production and optimization of culture conditions are necessary. Conventional ESC culture methods are impractical for large-scale cell production and lack robust microenvironmental control. We developed two stirred-suspension culture systems for the propagation of undifferentiated ESCs--microcarrier and aggregate cultures--and compared them with tissue-culture flask and Petri dish controls. ESCs cultured on glass microcarriers had population doubling times (approximately 14-17 hours) comparable to tissue-culture flask controls. ESC growth could be elicited in shear-controlled stirred-suspension culture, with population doubling times ranging between 24 and 39 hours at 100 rpm impeller speed. Upon removal of leukemia inhibitory factor, the size-controlled ESC aggregates developed into embryoid bodies (EBs) capable of multilineage differentiation. A comprehensive analysis of ESC developmental potential, including flow cytometry for Oct-4, SSEA-1, and E-cadherinprotein expression, reverse transcription-polymerase chain reaction for Flk-1, HNF3-beta, MHC, and Sox-1 gene expression, and EB differentiation analysis, demonstrated that the suspension-cultured ESCs retained the developmental potential of the starting cell population. Analysis of E-cadherin-/- and E-cadherin+/- cells using both systems provided insight into the mechanisms behind the role of cell aggregation control, which is fundamental to these observations. These cell-culture tools should prove useful for both the production of ESCs and ESC-derived cells and for investigations into adhesion, survival, and differentiation phenomena during ESC propagation and differentiation.

Soluble factor cross-talk between human bone marrow-derived hematopoietic and mesenchymal cells enhances in vitro CFU-F and CFU-O growth and reveals heterogeneity in the mesenchymal progenitor cell compartment

The homeostatic adult bone marrow (BM) is a complex tissue wherein physical and biochemical interactions serve to maintain a balance between the hematopoietic and nonhematopoietic compartments. To focus on soluble factor interactions occurring between mesenchymal and hematopoietic cells, a serum-free adhesion-independent culture system was developed that allows manipulation of the growth of both mesenchymal and hematopoietic human BM-derived progenitors and the balance between these compartments. Factorial experiments demonstrated a role for stem cell factor (SCF) and interleukin 3 (IL-3) in the concomitant growth of hematopoietic (CD45+) and nonhematopoietic (CD45-) cells, as well as their derivatives. Kinetic tracking of IL-3alpha receptor (CD123) and SCF receptor (CD117) expression on a sorted CD45- cell population revealed the emergence of CD45-CD123+ cells capable of osteogenesis. Of the total fibroblast colony-forming units (CFU-Fs) and osteoblast colony-forming units (CFU-O), approximately 24% of CFU-Fs and about 22% of CFU-Os were recovered from this population. Cell-sorting experiments demonstrated that the CD45+ cell population secreted soluble factors that positively affect the survival and proliferation of CFU-Fs and CFU-Os generated from the CD45- cells. Together, our results provide insight into the intercellular cytokine network between hematopoietic and mesenchymal cells and provide a strategy to mutually culture both mesenchymal and hematopoietic cells in a defined scalable bioprocess.

Scalable production of embryonic stem cell-derived cells

Embryonic stem (ES) cells have the ability to self-renew as well as differentiate into any cell type in the body. These traits make ES cells an attractive "raw material" for a variety of cell-based technologies. However, uncontrolled cell aggregation in ES cell differentiation culture inhibits cell proliferation and differentiation and thwarts the use of stirred suspension bioreactors. Encapsulation of ES cells in agarose microdrops prevents physical interaction between developing embryoid bodies (EBs) that, in turn, prevents EB agglomeration. This enables use of stirred suspension bioreactors that can generate large numbers of ES-derived cells under controlled conditions.

Quantitative screening of embryonic stem cell differentiation: Endoderm formation as a model

Embryonic stem (ES) cells have attracted much attention as a possible source of functional cells for regenerative medicine. Therapeutic use of ES cells requires control over the types and frequencies of cells generated during their in vitro differentiation. Due to the complexity of factors that impact upon ES cell differentiation, novel approaches for the optimization of tissue-specific development are required. This motivates our use of factorial and composite design methods to make empirical investigations more efficient, and to reveal unexpected interactions missed by conventional dose-response analysis. Factorial experiments would benefit from the high content evaluation of a large number of test conditions, necessitating the development of a quantitative screening technology (QST) capable of reporting the absolute number and frequency of target cells. We have developed and validated such a technology for ES cell differentiation analysis using automated fluorescence microscopy, employing endoderm differentiation as a model system. To test this platform, a two-level factorial experiment was carried out to identify major and interactive effects of glucose, insulin, retinoic acid (RA), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) on endoderm formation. RA was found to have inhibitory effects on endoderm formation, while low glucose proved beneficial. QST was demonstrated to be a powerful tool to study factors impacting endoderm-specific ES cell differentiation, and should be applicable to the analysis of a range of ES cell-derived tissues.

Supplementation-dependent differences in the rates of embryonic stem cell self-renewal, differentiation, and apoptosis

Although it is known that leukemia inhibitory factor (LIF) supports the derivation and expansion of murine embryonic stem (ES) cells, it is unclear whether this is due to inhibitory effects of LIF on ES cell differentiation or stimulatory effects on ES cell survival and proliferation. Using an ES cell line transgenic for green fluorescent protein (GFP) expression under control of the Oct4 promoter, we were able to simultaneously track the responses of live Oct4-GFP-positive (ES) and -negative (differentiated) fractions to LIF, serum, and other growth factors. Our findings show that, in addition to inhibiting differentiation of undifferentiated cells, the administration of LIF resulted in a distinct dose-dependent survival and proliferation advantage, thus enabling the long-term propagation of undifferentiated cells. Competitive responses from the differentiated cell fraction could only be elicited upon addition of serum, fibroblast growth factor-4 (FGF-4), or insulin-like growth factor-1 (IGF-1). The growth factors did not induce additional differentiation of ES cells, but rather they significantly improved the proliferation of already differentiated cells. Our analyses show that, by adjusting culture conditions, including the type and amount of growth factors or cytokines present, the frequency of media exchange, and the presence or absence of serum, we could selectively and specifically alter the survival, proliferation, and differentiation dynamics of the two subpopulations, and thus effectively control population outputs. Our findings therefore have important applications in engineering stem cell culture systems to predictably generate desired stem cells or their derivatives for various regenerative therapies.

Scalable production of embryonic stem cell-derived cardiomyocytes

Cardiomyocyte transplantation could offer a new approach to replace scarred, nonfunctional myocardium in a diseased heart. Clinical application of this approach would require the ability to generate large numbers of donor cells. The purpose of this study was to develop a scalable, robust, and reproducible process to derive purified cardiomyocytes from genetically engineered embryonic stem (ES) cells. ES cells transfected with a fusion gene consisting of the alpha-cardiac myosin heavy chain (MHC) promoter driving the aminoglycoside phosphotransferase (neomycin resistance) gene were used for cardiomyocyte enrichment. The transfected cells were aggregated into embyroid bodies (EBs), inoculated into stirred suspension cultures, and differentiated for 9 days before selection of cardiomyocytes by the addition of G418 with or without retinoic acid (RA). Throughout the culture period, EB and viable cell numbers were measured. In addition, flow cytometric analysis was performed to monitor sarcomeric myosin (a marker for cardiomyocytes) and Oct-4 (a marker for undifferentiated ES cells) expression. Enrichment of cardiomyocytes was achieved in cultures treated with either G418 and retinoic acid (RA) or with G418 alone. Eighteen days after differentiation, G418-selected flasks treated with RA contained approximately twice as many cells as the nontreated flasks, as well as undetectable levels of Oct-4 expression, suggesting that RA may promote cardiac differentiation and/or survival. Immunohistological and electron microscopic analysis showed that the harvested cardiomyocytes displayed many features characteristic of native cardiomyocytes. Our results demonstrate the feasibility of large-scale production of viable, ES cell-derived cardiomyocytes for tissue engineering and/or implantation, an approach that should be transferable to other ES cell derived lineages, as well as to adult stem cells with in vitro cardiomyogenic activity.

Signal processing underlying extrinsic control of stem cell fate

PURPOSE OF REVIEW

Strategies to manipulate stem cells for therapeutic applications are limited by our inability to control or predict stem cell fate decisions in response to exogenous stimuli. This review focuses on the mechanisms by which exogenous stimuli influence cell fate.

RECENT FINDINGS

Limitations in our ability to control cell fate arises from our primarily qualitative understanding of stem cell regulation, which proposes straightforward cue-fate relationships that appear to be the exception rather than the rule. Alternatively, consideration of the underlying quantitative, temporal, and spatial mechanisms governing extrinsic regulation of stem cell fate may enable novel approaches to control stem cell output predictively.

SUMMARY

The authors review advances in the understanding of these underlying mechanisms, and discuss experimental and analytic tools necessary to investigate and use these mechanisms to control stem cell fate.

Sustained In Vitro Expansion of Bone Progenitors Is Cell Density Dependent

Osteogenic cells are an integral part of the dynamic tissue-remodeling process in bone and are potential tools for tissue engineering and cell-based therapies. We examined the role of glucocorticoids and cell density in the expansion of primary rat calvaria cell populations and osteoprogenitor subpopulations in adherent cell culture. Osteoprogenitor response to dexamethasone (dex, a synthetic glucocorticoid known to stimulate bone formation in vitro) supplementation and long-term osteoprogenitor cell proliferation and differentiation were quantified using functional (colony forming unit-osteoblast [CFU-O]) and phenotypic analyses. Although osteoprogenitor self-renewal occurred at both standard and high initiating cell densities, progenitor cell expansion (measured by changes in CFU-O number relative to input) was sustained and dramatically increased at high initiating cell densities (30-fold CFU-O expansion for standard-density cultures compared with a greater than 10,000-fold CFU-O expansion in high-density cultures). Cell density was also found to impact upon the potential of dex to recruit additional progenitors towards bone development. These multifaceted effects appeared to be independent of cell proliferation rates or population phenotypic expression. Together, our results emphasize a roll for cell-cell interactions and/or community effects in the control and maintenance of progenitor cells during in vitro culture.

Fluorescence activated cell sorting reveals heterogeneous and cell non-autonomous osteoprogenitor differentiation in fetal rat calvaria cell populations

Identification of osteoblast progenitors, with defined developmental capacity, would facilitate studies on a variety of parameters of bone development. We used expression of alkaline phosphatase (ALP) and the parathyroid hormone/parathyroid hormone-related protein receptor (PTH1R) as osteoblast markers in dual-color fluorescence activated cell sorting (FACS) to fractionate rat calvaria (RC) cells into ALP(-)PTH1R(-), ALP(+)PTH1R(-), ALP(-)PTH1R(+), and ALP(+)PTH1R(+) populations. These fractionated populations were seeded clonally (n = 96) or over a range of cell densities ( approximately 150-8,500 cell/cm(2); n = 3). Our results indicate that colony forming unit-osteoblast (CFU-O)/bone nodule-forming cells are found in all fractions, but the frequency of CFU-O and total mineralized area is different across fractions. Analysis of these differences suggests that ALP(-)PTH1R(-), ALP(-)PTH1R(+), ALP(+)PTH1R(-), and ALP(+)PTH1R(+) cell populations are separated in order of increasing bone formation capacity. Dexamethasone (dex) differentially increased the CFU-O number in the four fractions, with the largest stimulation in the ALP(-) cell populations. However, there was no significant difference in the number or size distribution of CFU-F (fibroblast) colonies that formed in vehicle versus dex. Finally, both cell autonomous and cell non-autonomous (i.e., inhibitory/stimulatory effects of cell neighbors) differentiation of osteoprogenitors was seen. Only the ALP(-)PTH1R(-) population was capable of forming nodules at the clonal level, at approximately 3- or 12-times the predicted frequency of unfractionated populations in dex or vehicle, respectively. These data suggest that osteoprogenitors can be significantly enriched by fractionation of RC populations, that assay conditions modify the osteoprogenitor frequencies observed and that fractionation of osteogenic populations is useful for interrogation of their developmental status and osteogenic capacity.

Adult human bone marrow-derived mesenchymal progenitor cells are capable of adhesion-independent survival and expansion

OVERVIEW

We show the existence of adult human mesenchymal progenitor cells (hMPCs) that can proliferate, in a cytokine-dependent manner, as individual cells in stirred suspension cultures (SSC) while maintaining their ability to form functional differentiated mesenchymal cell types.

MATERIALS AND METHODS

Ficolled human bone marrow (BM)-derived cells were grown in SSC (and adherent controls) in the presence and absence of exogenously added cytokines. Phenotypic, gene expression, and functional assays for hematopoietic and nonhematopoietic cell populations were used to kinetically track cell production. Limiting-dilution analysis was used to relate culture-produced cells to input cell populations.

RESULTS

Cytokine cocktail influenced total and progenitor cell expansion, as well as the types of cells generated upon plating. Flow cytometric analysis of CD117, CD123, and CD45 expression showed that cytokine supplementation influenced SSC output. The concomitant growth of CD45(+) and CD45(-) cells in the cultures that exhibited the greatest hMPC expansions suggests that the growth of these cells may benefit from interactions with hematopoietic cells. Functional assays demonstrated that the SSC-derived cells (input CFU-O number: 1990+/-377) grown in the presence of SCF+IL-3 resulted, after 21 days, in the generation of a significantly greater number (p<0.05) of bone progenitors (33,700+/-8763 CFU-O) than similarly initiated adherent cultures (214+/-75 CFU-O). RT-PCR analysis confirmed that the SSC-derived cells grown in osteogenic conditions express bone-specific genes (Cbfa1/Runx2, bone sialoprotein, and osteocalcin).

CONCLUSIONS

Our approach not only provides an alternative strategy to expand adult BM-derived nonhematopoietic progenitor cell numbers in a scalable and controllable bioprocess, but also questions established biological paradigms concerning the properties of connective-tissue stem and progenitor cells.

Two-color image analysis discriminates between mineralized and unmineralized bone nodules in vitro

Functional assays of progenitor cell capacity for colony formation in vitro typically depend on the investigator's expertise with quantification. The ability to enumerate and analyze colony types with standardized criteria with no bias would be a useful tool for research and drug development. We report the development of a two-color automated analysis system for colony-forming unit-osteoblasts that is capable of reporting progenitor frequency and bone nodule number size, and type (mineralized or unmineralized). Our image analysis system was validated using the rat calvaria cell model to measure in vitro bone nodule development. With computer-aided image analysis, data on nodules can be rapidly generated with a minimum of user bias and fatigue. This novel tool will distinguish mineralized and unmineralized bone nodules, facilitates quantification, enable large-scale experimental design, allow for long-term data storage and tracking, and lead to the identification of new parameters that impact bone development.

Modeling stem cell development by retrospective analysis of gene expression profiles in single progenitor-derived colonies

The process of development of various cell types is often based on a linear or deterministic paradigm. This is true, for example, for osteoblast development, a process that occurs through the differentiation of a subset of primitive fibroblast progenitors called colony-forming unit-osteoblasts (CFU-Os). CFU-O differentiation has been subdivided into three stages: proliferation, extracellular matrix development and maturation, and mineralization, with characteristic changes in gene expression at each stage. Few analyses have asked whether CFU-O differentiation, or indeed stem cell differentiation in general, may follow more complex and nondeterministic paths, a possibility that may underlie the substantial number of discrepancies in published reports of progenitor cell developmental sequences. We analyzed 99 single colonies of osteoblast stem/primitive progenitor cells cultured under identical conditions. The colonies were analyzed by global amplification poly(A) polymerase chain reaction to determine which of nine genes had been expressed. We used the expression profiles to develop a statistically rigorous map of the cell fate decisions that occur during osteoprogenitor differentiation and show that different developmental routes can be taken to achieve the same end point phenotype. These routes appear to involve both developmental "dead ends" (leading to the expression of genes not correlated with osteoblast-associated genes or the mature osteoblast phenotype) and developmental flexibility (the existence of multiple gene expression routes to the same developmental end point). Our results provide new insight into the biology of primitive progenitor cell differentiation and introduce a powerful new quantitative method for stem cell lineage analysis that should be applicable to a wide variety of stem cell systems.

Ligand/receptor signaling threshold (LIST) model accounts for gp130-mediated embryonic stem cell self-renewal responses to LIF and HIL-6

We previously demonstrated that embryonic stem (ES) cell self-renewal required sustained signaling by leukemia inhibitory factor (LIF) in a concentration-dependent manner, allowing us to hypothesize that thresholds in ligand-receptor signaling modulate stem cell differentiation control. To test this hypothesis, we have experimentally and computationally compared the abilities of two gp130-signaling cytokines (LIF and Hyper-interleukin-6 [HIL-6]) to sustain ES cell self-renewal. Quantitative measurements of ES cell phenotypic markers (stage-specific embryonic antigen-1 and E-cadherin), functional assays (alkaline phosphatase activity and embryoid body formation efficiency), and transcription factor (Oct-4) expression over a range of LIF and HIL-6 concentrations demonstrated a superior ability of LIF to maintain ES cell pluripotentiality at higher concentrations (> or =500 pM). Additionally, we observed distinct qualitative differences in the ES cell self-renewal dose response profiles between the two cytokines. A computational model permitted calculation of the number of signaling complexes as a function of receptor expression, ligand concentration, and ligand/receptor-binding properties, generating predictions for the degree of self-renewal as a function of cytokine concentration by comparison of these calculated complex numbers to experimentally determined threshold cytokine concentrations. Model predictions, consistent with experimental data, indicated that differences in the potencies of these two cytokines were based primarily on differences in receptor-binding stoichiometries and properties. These results support a ligand/receptor signaling threshold model of ES cell fate modulation through appropriate types and levels of cytokine stimulation. Insights from these results may be more generally applicable to tissue-specific stem cells and could aid in the development of stem cell-based technologies.

Efficiency of embryoid body formation and hematopoietic development from embryonic stem cells in different culture systems

Embryonic stem (ES) cells have tremendous potential as a cell source for cell-based therapies. Realization of that potential will depend on our ability to understand and manipulate the factors that influence cell fate decisions and to develop scalable methods of cell production. We compared four standard ES cell differentiation culture systems by measuring aspects of embryoid body (EB) formation efficiency and cell proliferation, and by tracking development of a specific differentiated tissue type-blood-using functional (colony-forming cell) and phenotypic (Flk-1 and CD34 expression) assays. We report that individual murine ES cells form EBs with an efficiency of 42 +/- 9%, but this value is rarely obtained because of EB aggregation-a process whereby two or more individual ES cells or EBs fuse to form a single, larger cell aggregate. Regardless of whether EBs were generated from a single ES cell in methylcellulose or liquid suspension culture, or aggregates of ES cells in hanging drop culture, they grew to a similar maximum cell number of 28,000 +/- 9,000 cells per EB. Among the three methods for EB generation in suspension culture there were no differences in the kinetics or frequency of hematopoietic development. Thus, initiating EBs with a single ES cell and preventing EB aggregation should allow for maximum yield of differentiated cells in the EB system. EB differentiation cultures were also compared to attached differentiation culture using the same outputs. Attached colonies were not similarly limited in cell number; however, hematopoietic development in attached culture was impaired. The percentage of early Flk-1 and CD34 expressing cells was dramatically lower than in EBs cultured in suspension, whereas hematopoietic colony formation was almost completely inhibited. These results provide a foundation for development of efficient, scalable bioprocesses for ES cell differentiation, and inform novel methods for the production of hematopoietic tissues.

Stem cell bioengineering

Tissue engineering and cellular therapies, either on their own or in combination with therapeutic gene delivery, have the potential to significantly impact medicine. Implementation of technologies based on these approaches requires a readily available source of cells for the generation of cells and tissues outside a living body. Because of their unique capacity to regenerate functional tissue for the lifetime of an organism, stem cells are an attractive "raw material" for multiple biotechnological applications. By definition they are self-renewing because on cell division they can generate daughter stem cells. They are also multipotent because they can differentiate into numerous specialized, functional cells. Recent findings have shown that stem cells exist in most, if not all, tissues, and that stem cell tissue specificity may be more flexible than originally thought. Although the potential for producing novel cell-based products from stem cells is large, currently there are no effective technologically relevant methodologies for culturing stem cells outside the body, or for reproducibly stimulating them to differentiate into functional cells. A mechanistic understanding of the parameters important in the control of stem cell self-renewal and lineage commitment is thus necessary to guide the development of bioprocesses for the ex vivo culture of stem cells and their derivates.

Controlling culture dynamics for the expansion of hematopoietic stem cells

The ex vivo expansion of hematopoietic stem cells (HSCs) is the subject of intense commercial and academic interest due to the potential of HSCs to be a renewable source of material for cellular therapeutics. Unfortunately, because methodologies have not yet been developed to grow clinically relevant numbers of HSCs (or their derivatives) consistently, the potential of this technology is limited. Manipulation of the in vitro culture microenvironment, primarily through cytokine supplementation, has been the predominant approach in studies attempting to expand primary human HSC numbers in vitro. While promising results have been obtained, it is becoming clear that novel methods must be developed before cellular therapies using these stem cells can become routine. Ideally, bioprocesses must be designed to target specifically the growth of stem cell populations while incorporating positive and negative feedback from potentially dynamic mature and maturing cell populations. The product of these culture systems should consist of not only HSCs, but also of cells that allow the engraftment of HSCs and, ideally, cells responsible for the immediate or accelerated functional support of patients. Development of such "designer transplants" will require combining optimal culture conditions capable of amplifying HSC numbers with novel approaches for finely controlling the number, functional capabilities, and characteristics of potentially therapeutic cells in these very complex cell culture systems.

Leukemia inhibitory factor (LIF) concentration modulates embryonic stem cell self-renewal and differentiation independently of proliferation

A major limitation of the widespread use of stem cells in a variety of biotechnological applications is the relatively low level of knowledge about how to maintain these cells in vitro without losing the long-term multilineage growth properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of stem cell stimulation by exogenous cytokines would thus be useful. An emerging theme from recent hematopoietic stem cell (HSC)-expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by cytokine presentation on cell surfaces. According to such a model, when the relevant ligand-receptor interaction falls below this threshold level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Taking advantage of the ability of the cytokine leukemia inhibitory factor (LIF) to maintain embryonic stem (ES) cell pluripotentiality at high concentrations, we are testing this model by investigating critical parameters in the control of ES cell responses. We have developed quantitative assays of ES cell differentiation by measuring cell-surface alkaline phosphatase activity, cell-surface stage specific embryonic antigen (SSEA)-1 expression, and the ability of ES cells to form embryoid bodies. Examination of ES cell responses over a range of LIF concentrations shows that LIF supplementation has little effect on ES cell-growth rate but significantly alters the probability of a cell undergoing a self-renewal vs. a differentiation division. In vitro culture parameters such as inoculum cell density, medium exchange, as well as cell-intrinsic processes such as autocrine secretion are shown to affect this decision. In addition to yielding new information on stem cell regulation by exogenous factors, these studies provide important clues about culture of these cells and should stimulate further investigations into the mechanistic basis of stem cell differentiation control.

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events. (Blood. 2000;96:1215-1222)

Concentration-dependent internalization of a cytokine/cytokine receptor complex in human hematopoietic cells

Soluble steel factor (SF) is a potent stimulator of hematopoietic progenitor cell proliferation in vitro, and cytokine combinations that include SF can support extensive expansions of hematopoietic cells. Recently, we showed that very primitive progenitor cells from normal human bone marrow require exposure to very high concentrations of cytokines to maintain their primitive status while proliferating. These cells also display higher cell-specific cytokine uptake rates than more differentiated types of hematopoietic cells. As a first step toward identifying the mechanisms involved in mediating such cytokine dose-dependent effects, we have now investigated the kinetics of SF receptor (c-kit) internalization by human Mo7e cells exposed to different extracellular concentrations of soluble SF. Transfer of Mo7e cells to a higher concentration of SF caused an initially rapid downregulation of cell surface c-kit which was accompanied by a rapid depletion of extracellular SF. Confocal microscopy showed a concomitant increase in the number and intensity of intracellular c-kit aggregates. After the first 30 min, the cells continued to deplete SF from the medium but at a much slower rate. During this period, there was a gradual recovery of expression of c-kit on the cell surface. A mathematical analysis of bulk medium to cell-surface SF-mass transport indicated that the cytokine-depletion rates measured were not likely to have significantly depleted the SF concentration in the microenvironment of the cells. Taken together, these results underscore the importance of monitoring and appropriately regulating cytokine concentrations in hematopoietic cell expansion cultures. They may also help to explain the different biological responses exhibited by primitive hematopoietic cells exposed to different types and concentrations of cytokines for periods of days.

Ontogeny-associated changes in the cytokine responses of primitive human haemopoietic cells

Time course studies revealed that the combination of Flt-3 ligand (FL), Steel factor (SF) and interleukin-3 (IL-3) did not elicit as large an amplification of the long-term culture-initiating cell (LTC-IC) population in serum-free cultures of CD34+ CD38- cord blood (CB) cells as was obtained in similar cultures of adult human CD34+ CD38- bone marrow (BM) cells (4- v 90-fold maximum increases), even though both total and colony-forming cell (CFC) numbers initially increased more rapidly in CB cultures. Multifactorial analysis of the short-term (10 d) effects of different cytokines identified FL and IL-6 in combination with the soluble IL-6 receptor (sIL-6R) as most important for expanding the CB LTC-IC population. In contrast, their counterparts in adult BM were most effectively stimulated by FL, SF and IL-3. For rapid generation of increased numbers of CFC, SF with either FL or IL-6/sIL-6R were found to be the most important contributors in cultures of CD34+ CD38- CB cells, whereas, in analogous BM cultures, IL-6/sIL-6R and TPO (in addition to FL, SF and IL-3) were required. These findings reinforce the principle of altered cytokine responsiveness as a hallmark of early haemopoietic cell differentiation and demonstrate how cytokine requirements may change during human ontogeny. Identification of conditions for optimizing the expansion of different subsets of primitive CB cells has additional important implications for clinical transplantation and gene transfer.

Advances in hematopoietic stem cell culture

Recent advances in our understanding of the earliest stages of hematopoietic cell differentiation, and how these may be manipulated under defined conditions in vitro, have set the stage for the development of robust bioprocess technology applicable to hematopoietic cells. Sensitive and specific assays now exist for measuring the frequency of hematopoietic stem cells with long-term in vivo repopulating activity from human as well as murine sources. The production of natural or engineered ligands through recombinant DNA and/or combinatorial chemistry strategies is providing new reagents for enhancing the productivity of hematopoietic cell cultures. Multifactorial and dose-response analyses have yielded new insight into the different types and concentrations of factors required to optimize the rate and the extent of amplification of specific subpopulations of primitive hematopoietic cells. In addition, the rate of cytokine depletion from the medium has also been found to be dependent on the types of cell present. The discovery of these cell-type-specific parameters affecting cytokine concentrations and responses has introduced a new level of complexity into the design of optimized hematopoietic bioprocess systems.

Cytokine manipulation of primitive human hematopoietic cell self-renewal

Previous studies have shown that primitive human hematopoietic cells detectable as long-term culture-initiating cells (LTC-ICs) and colony-forming cells (CFCs) can be amplified when CD34(+) CD38(-) marrow cells are cultured for 10 days in serum-free medium containing flt3 ligand (FL), Steel factor (SF), interleukin (IL)-3, IL-6, and granulocyte colony-stimulating factor. We now show that the generation of these two cell types in such cultures is differentially affected at the single cell level by changes in the concentrations of these cytokines. Thus, maximal expansion of LTC-ICs (60-fold) was obtained in the presence of 30 times more FL, SF, IL-3, IL-6, and granulocyte colony-stimulating factor than could concomitantly stimulate the near-maximal (280-fold) amplification of CFCs. Furthermore, the reduced ability of suboptimal cytokine concentrations to support the production of LTC-ICs could be ascribed to a differential response of the stimulated cells since this was not accompanied by a change in the number of input CD34(+) CD38(-) cells that proliferated. Reduced LTC-IC amplification in the absence of a significant effect on CFC generation also occurred when the concentrations of FL and SF were decreased but the concentration of IL-3 was high (as compared with cultures containing high levels of all three cytokines). To our knowledge, these findings provide the first evidence suggesting that extrinsically acting cytokines can alter the self-renewal behavior of primary human hematopoietic stem cells independent of effects on their viability or proliferation.

Differential cytokine effects on primitive (CD34+CD38-) human hematopoietic cells: novel responses to Flt3-ligand and thrombopoietin

A high proportion of the CD34+CD38- cells in normal human marrow are defined as long-term culture-initiating cells (LTC-IC) because they can proliferate and differentiate when co-cultured with cytokine-producing stromal feeder layers. In contrast, very few CD34+CD38- cells will divide in cytokine-containing methylcellulose and thus are not classifiable as direct colony-forming cells (CFC), although most can proliferate in serum-free liquid cultures containing certain soluble cytokines. Analysis of the effects of 16 cytokines on CD34+CD38- cells in the latter type of culture showed that Flt3-ligand (FL), Steel factor (SF), and interleukin (IL)-3 were both necessary and sufficient to obtain an approximately 30-fold amplification of the input LTC-IC population within 10 d. As single factors, only FL and thrombopoietin (TPO) stimulated a net increase in LTC-IC within 10 d. Interestingly, a significantly increased proportion of the CFC produced from the TPO-amplified LTC-IC were erythroid. Increases in the number of directly detectable CFC of > 500-fold were also obtainable within 10 d in serum-free cultures of CD34+CD38- cells. However, this required the presence of IL-6 and/or granulocyte/colony-stimulating factor and/or nerve growth factor beta in addition to FL, SF, and IL-3. Also, for this response, the most potent single-acting factor tested was IL-3, not FL. Identification of cytokine combinations that differentially stimulate primitive human hematopoietic cell self-renewal and lineage determination should facilitate analysis of the intracellular pathways that regulate these decisions as well as the development of improved ex vivo expansion and gene transfer protocols.

Expansion of hematopoietic progenitor cell populations in stirred suspension bioreactors of normal human bone marrow cells

We have investigated the potential of stirred suspension cultures to support hematopoiesis from starting innocula of normal human bone marrow cells. Initial studies showed that the short-term maintenance of both colony-forming cell (CFC) numbers and their precursors, detected as long-term culture-initiating cells (LTC-IC), could be achieved as well in stirred suspension cultures as in static cultures. Neither of these progenitor cell populations was affected in either type of culture when porous microcarriers were added to provide an increased surface for adherent cell attachment. Supplementation of the medium with 10 ng/ml of Steel factor (SF) and 2 ng/ml of interleukin-3 (IL-3) resulted in a significant expansion of LTC-IC, CFC and total cell numbers in stirred cultures. Both the duration and ultimate magnitude of these expansions were correlated with the initial cell density and after 4 weeks the number of LTC-IC and CFC present in stirred cultures initiated with the highest starting cell concentration tested reflected average increases of 7- and 22-fold, respectively, above input values. Stirred suspension cultures offer the combined advantages of homogeneity and lack of dependence on the formation and maintenance of an adherent cell layer. Our results suggest their applicability to the development of scaled-up bioreactor systems for clinical procedures requiring the production of primitive hematopoietic cell populations. In addition, stirred suspension cultures may offer a new tool for the analysis of hematopoietic regulatory mechanisms.