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

Derivation of embryonic germ cells and male gametes from embryonic stem cells

Egg and sperm cells (gametes) of the mouse are derived from a founder population of primordial germ cells that are set aside early in embryogenesis. Primordial germ cells arise from the proximal epiblast, a region of the early mouse embryo that also contributes to the first blood lineages of the embryonic yolk sac. Embryonic stem cells differentiate in vitro into cystic structures called embryoid bodies consisting of tissue lineages typical of the early mouse embryo. Because embryoid bodies sustain blood development, we reasoned that they might also support primordial germ cell formation. Here we isolate primordial germ cells from embryoid bodies, and derive continuously growing lines of embryonic germ cells. Embryonic germ cells show erasure of the methylation markers (imprints) of the Igf2r and H19 genes, a property characteristic of the germ lineage. We show that embryoid bodies support maturation of the primordial germ cells into haploid male gametes, which when injected into oocytes restore the somatic diploid chromosome complement and develop into blastocysts. Our ability to derive germ cells from embryonic stem cells provides an accessible in vitro model system for studies of germline epigenetic modification and mammalian gametogenesis.

In vitro development of mouse embryonic stem cells lacking JNK/stress-activated protein kinase-associated protein 1 (JSAP1) scaffold protein revealed its requirement during early embryonic neurogenesis

The Jsap1 gene encodes a scaffold protein for c-Jun N-terminal kinase cascades. We established c-Jun N-terminal kinase (JNK)/stress-activated protein kinase-associated protein 1 (JSAP1)-null mouse embryonic stem cell lines by homologous recombination. The JSAP1-null embryonic stem cells were viable, however, exhibited hyperplasia of the ectoderm during embryoid body formation, and spontaneously differentiated into neurons more efficiently than did wild type. The expression of components of c-Jun N-terminal kinase cascades and a subset of marker mRNAs during early embryogenesis was altered in the JSAP1-null mutants. Retinoic acid dramatically increased the expression of JSAP1 and JNK3, which were co-precipitated with anti-JNK3 in the neuroectoderm of wild type but not JSAP1-null embryoid bodies. In the neurons differentiated from the wild type embryoid bodies, JSAP1 was localized in the soma, neurites, and growth cone-like structure of the neurites, and neurite outgrowth from the JSAP1-null embryoid bodies was apparently less efficient than from wild type. JSAP1 and c-Jun N-terminal kinase 3 were coexpressed in the embryonic ectoderm of E7.5 mouse embryo, whereas Wnt1 and Pax2 were coexpressed with JSAP1 at the midbrain-hindbrain junction in E12.5 mouse embryo, thus suggesting that JSAP1 is required for early embryonic neurogenesis.

The appearance of truncated cyclin A2 correlates with differentiation of mouse embryonic stem cells

The presence of a form of cyclin A2 with an N-terminal truncation has recently been reported in various murine cell lines and tissues. The truncated cyclin A2 binds to and activates the cyclin-dependent kinase 2 (CDK2). However, CDK2 bound by the truncated cyclin A2 is located in the cytoplasm in contrast to CDK2 bound to full-length cyclin A2, which is in the nucleus. Here, we show that proliferating mouse embryonic stem cells (ES cells) contain very little truncated cyclin A2 but as the cells are induced to differentiate the amount of truncated cyclin A2 increases. The expression pattern of truncated cyclin A2 was the same in p27(Kip1) -/- differentiating ES cells as in the differentiating wild-type cells. We conclude that p27(Kip1) is not necessary for the proteolytic cleavage that gives rise to the truncated form of cyclin A2 in differentiating ES cells and that this post-translational modification is not a function of the cell density but is correlated with differentiation.

Generation of confluent cardiomyocyte monolayers derived from embryonic stem cells in suspension: a cell source for new therapies and screening strategies

BACKGROUND

Cellular cardiomyoplasty is evolving as a new strategy to treat cardiac diseases. A prerequisite is a reliable source of pure cardiomyocytes, which could also help in the exploitation of recent advances in genomics and drug screening. Our goal was to establish a robust lab-scale process for the generation of embryonic stem (ES)-cell-derived cardiomyocytes in suspension.

METHODS

A 71 ES cell clone carrying a construct consisting of the alpha-cardiac myosin heavy chain (alphaMHC) promoter driving the neomycin resistance gene was used for antibiotic-driven cardiomyocyte enrichment. Rotating suspension culture was established to initiate embryoid body (EB) formation. To track growth and differentiation kinetics, cell count and flow cytometry for SSEA-I, E-cadherin (stem-cell marker)and sarcomeric myosin (cardiomyocytes marker) was performed. Oct4 expression was measured via real time (RT)-PCR.

RESULTS

Cultures comprising 2.5-8 x 10(6) differentiating FS cells/mL were obtained after 9 days in rotating suspension. Upon G418 addition,vigorous contracting spheres, termed cardiac bodies (CB), developed. These cultures consisted of about 2.1 x 10(5) enriched cardiomyocytes/mL after 6- 10 days of selection. Suspensions comprising 90- 95%viable single cells were generated using an improved dissociation method. Seeding of cardiomyocytes with 7 x 10(4) cell/cm(2) resulted in a homogeneous monolayer of synchronously contracting cells. Myocyte specific immunohistochemistry indicated purity of > 99%.

DISCUSSION

We have established a reliable lab-scale protocol to generate cultures of highly enriched cardiomyocytes in suspension. This will facilitate development of larger-scale processes for stem-cell based cardiomyocyte supply. An improved method is provided to derive vital suspensions of cardiomyocytes, which could be utilized for transplantation as well as for drug screening purposes.

Common and distinct features of cytokine effects on hematopoietic stem and progenitor cells revealed by dose-response surface analysis

Recent studies have identified thrombopoietin (TPO), flt-3 ligand (FL), Steel factor (SF), and interleukin-11 (IL-11) as cytokines able to stimulate amplification of the most primitive murine hematopoietic cells in vitro. However, dose-response and interaction parameters that predict how to optimize mixtures of these cytokines have not been previously defined. To obtain this information, Sca-1(+)lin(-) and c-kit(+)Sca-1(+)lin(-) adult mouse bone marrow cells were cultured for 10 and 14 days, respectively, in serum-free medium with varying concentrations of these cytokines. Quantitative assays were performed to determine the influences of the cytokine combinations tested on changes in long-term repopulating hematopoietic stem cells (HSCs), in vitro colony-forming cells (CFCs), and total cell numbers. A two-level factorial design was first used to screen the effects of TPO, SF, FL, and IL-11 as well as two different incubation temperatures. IL-11 and SF were found to be the most significant stimulators of murine HSC expansion. More detailed analyses of the effects on c-kit(+)Sca-1(+)lin(-) cells of IL-11, SF, and FL concentrations and their interactions using response surface methodology showed IL-11 to have a maximal stimulatory effect on HSC expansion at 20 ng/mL with higher concentrations being inhibitory. In contrast, not even high concentration saturation of the effects of either SF or FL was observed as the stimulatory effect of both SF and FL increased beyond 300 ng/mL. A negative interaction between SF and FL on HSCs was discovered. Interestingly, a generally similar pattern of cytokine effects was found to influence the 14-day output of CFCs and total cells from the same c-kit(+)Sca-1(+)lin(-) starting cell population. However, compared with HSCs, the cytokine requirements for maximizing the generation of CFCs and total cells were at much lower cytokine doses. From the information provided by the factorial analysis, mathematical models based on Monod kinetics for inhibitory substrates were developed that allow total cell, CFC, and HSC expansion to be predicted as a function of the IL-11, SF, and FL concentrations in terms of more widely recognized parameters. Overall, these methods should also serve as a guide for the future design and testing of other ex vivo stem cell expansion 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.

LIF-induced STAT3 signaling in murine versus human embryonal carcinoma (EC) cells

Self-renewal and the maintenance of pluripotency of mouse embryonal stem (ES) cells in vitro requires exogenous leukemia inhibitory factor (LIF). Mouse ES cells can be cultured and kept undifferentiated in the absence of embryonal feeder-cell layers when exogenous LIF concentrations are maintained above a threshold concentration. An important downstream target of LIF signal transduction in mouse ES cells is the transcription factor signal transducer and activator of transcription 3 (STAT3). In contrast to mouse ES cells, human ES cells are unresponsive to LIF and depend on feeder cells for undifferentiated growth. Here, we investigated the activation patterns of LIF-downstream effectors in mouse and human embryonal carcinoma (EC) cells. We report that LIF induces both ERK-1 as well as STAT3 activation in mouse P19 EC cells. LIF enhances the proliferation rate of P19 EC cells, which depends on ERK activity but does not require activation of STAT3. In contrast, LIF does not activate STAT3, ERK, or the gp130 receptor in human N tera-2/D1 EC cells, although all receptor components are expressed. The negative feedback protein suppressor of cytokine signaling 1 (SOCS-1) is constitutively expressed in N tera-2/D1 EC cells, suggesting that LIF signal transduction is inhibited by elevated levels of SOCS-1 expression.