Osteogenic Differentiation of Human Embryonic Stem Cell-Derived Mesenchymal Progenitor Cells as a Model for Assessing Developmental Bone ToxicityIn Vitro

Comparison of the Anabolic Effects of Reported Osteogenic Compounds on Human Mesenchymal Progenitor-derived Osteoblasts

There is variability in the reported effects of compounds on osteoblasts arising from differences in experimental design and choice of cell type/origin. This makes it difficult to discern a compound's action outside its original study and compare efficacy between compounds. Here, we investigated five compounds frequently reported as anabolic for osteoblasts (17β-estradiol (oestrogen), icariin, lactoferrin, lithium chloride, and menaquinone-4 (MK-4)) on human mesenchymal progenitors to assess their potential for bone tissue engineering with the aim of identifying a potential alternative to expensive recombinant growth factors such as bone morphogenetic protein 2 (BMP-2). Experiments were performed using the same culture conditions to allow direct comparison. The concentrations of compounds spanned two orders of magnitude to encompass the reported efficacious range and were applied continuously for 22 days. The effects on the proliferation (resazurin reduction and DNA quantification), osteogenic differentiation (alkaline phosphatase (ALP) activity), and mineralised matrix deposition (calcium and collagen quantification) were assessed. Of these compounds, only 10 µM MK-4 stimulated a significant anabolic response with 50% greater calcium deposition. Oestrogen and icariin had no significant effects, with the exception of 1 µM icariin, which increased the metabolic activity on days 8 and 22. 1000 µg/mL of lactoferrin and 10 mM lithium chloride both significantly reduced the mineralised matrix deposition in comparison to the vehicle control, despite the ALP activity being higher in lithium chloride-treated cells at day 15. This demonstrates that MK-4 is the most powerful stimulant of bone formation in hES-MPs of the compounds investigated, highlighting its potential in bone tissue engineering as a method of promoting bone formation, as well as its prospective use as an osteoporosis treatment.

An engineered mouse embryonic stem cell model with survivin as a molecular marker and EGFP as the reporter for high throughput screening of embryotoxic chemicals in vitro

Embryonic stem cell test (EST) is the only generally accepted in vitro method for assessing embryotoxicity without animal sacrifice. However, the implementation and application of EST for regulatory embryotoxicity screening are impeded by its technical complexity, long testing period, and limited endpoint data. In this study, a high throughput embryotoxicity screening based on mouse embryonic stem cells (mESCs) expressing enhanced green fluorescent protein (EGFP) driven by a human survivin promoter and a human cytomegalovirus promoter, respectively, was developed. These EGFP expressing mESCs were cultured in three-dimensional (3D) fibrous scaffolds in microbioreactors on a multiwell plate with EGFP fluorescence signals as cell responses to chemicals monitored noninvasively in a high throughput manner. Nine chemicals with known developmental toxicity were used to validate the survivin-based embryotoxicity assay, which showed that strongly embryotoxic compounds such as 5-fluorouracil, retinoic acid, and methotrexate downregulated survivin expression by more than 50% in 3 days, while weakly embryotoxic compounds such as boric acid, methoxyacetic acid, and tetracyclin showed modest downregulation effect and nonembryotoxic saccharin, penicillin G, and acrylamide had negligible downregulation effect on survivin expression, confirming that survivin can be used as a molecular endpoint for high throughput screening of embryotoxicants. The potential developmental toxicity of three Chinese herbal medicines were also evaluated using this assay, demonstrating its application in in vitro developmental toxicity test for drug safety assessment.

Human Pluripotent Stem Cells to Assess Developmental Toxicity in the Osteogenic Lineage

Musculoskeletal birth defects are frequent, yet their causes remain insufficiently investigated. Aside from genetic factors, exposure to environmental toxicants is suspected to contribute to the etiology of skeletal malformations. However, most chemicals in the environment are insufficiently characterized for their potential to cause harm to the differentiation of osteoblasts, the bone-forming cells and thereby the development of the skeleton.This lack of information primarily stems from animal testing being prohibitively expensive and time-consuming, which has prompted the development of predictive in vitro alternative methods. With the advent of mouse embryonic stem cells, which represent cells with the potential to become any of the 200 cell types in the body, among them osteoblasts, the past 15 years have borne suitable opportunities to assess chemicals in vitro. However, with an increasing understanding of the differences between mouse and human embryonic development, a need for human-specific developmental toxicity testing has risen. This chapter provides a detailed protocol on how to differentiate human embryonic stem cells into the osteogenic lineage, how to assess differentiation inhibition and how to evaluate such findings in relation to the mitochondrial activity of human embryonic stem cells and human fibroblasts, while exposed to a potential toxicant. Together, these endpoints allow for a human-specific screening of developmental toxicity specifically related to the osteogenic lineage.

Transgenic Mouse Models Transferred into the Test Tube: New Perspectives for Developmental Toxicity Testing In Vitro?

Despite our increasing understanding of molecular mechanisms controlling embryogenesis, the identification and characterization of teratogenic substances still heavily relies on animal testing. Embryonic development depends on cell-autonomous and non-autonomous processes including spatiotemporally regulated extracellular signaling activities. These have been elucidated in transgenic mouse models harboring easily detectable reporter genes under the control of evolutionarily conserved signaling cascades. We propose combining these transgenic mouse models and cells derived thereof with existing alternative toxicological testing strategies. This would enable the plausibility of in vitro data to be verified in light of in vivo data and, ultimately, facilitate regulatory acceptance of in vitro test methods.