Protocol for the Differentiation of Human Induced Pluripotent Stem Cells into Mixed Cultures of Neurons and Glia for Neurotoxicity Testing.

Human pluripotent stem cells can differentiate into various cell types that can be applied to human-based in vitro toxicity assays. One major advantage is that the reprogramming of somatic cells to produce human induced pluripotent stem cells (hiPSCs) avoids the ethical and legislative issues related to the use of human embryonic stem cells (hESCs). HiPSCs can be expanded and efficiently differentiated into different types of neuronal and glial cells, serving as test systems for toxicity testing and, in particular, for the assessment of different pathways involved in neurotoxicity. This work describes a protocol for the differentiation of hiPSCs into mixed cultures of neuronal and glial cells. The signaling pathways that are regulated and/or activated by neuronal differentiation are defined. This information is critical to the application of the cell model to the new toxicity testing paradigm, in which chemicals are assessed based on their ability to perturb biological pathways. As a proof of concept, rotenone, an inhibitor of mitochondrial respiratory complex I, was used to assess the activation of the Nrf2 signaling pathway, a key regulator of the antioxidant-response-element-(ARE)-driven cellular defense mechanism against oxidative stress.

Nrf2 pathway activation upon rotenone treatment in human iPSC-derived neural stem cells undergoing differentiation towards neurons and astrocytes.

Activation of Nrf2/ARE signaling pathway occurs ubiquitously in most cell types upon induction of oxidative stress. Rotenone, an inhibitor of mitochondrial complex I, can be used to trigger oxidative stress, stimulate the activation of Nrf2 pathway in neuronal and astrocytic cells and assess neurotoxicity. We have previously demonstrated that an acute treatment with rotenone can induce Nrf2 activation, which leads to astrocyte activation and dopaminergic (DA) neuronal cell death in a mixed neuronal/astrocytic cell model derived from human induced pluripotent stem cells (hiPSCs). In this study, we characterized the effects of a repeated dose treatment with rotenone (14 days) on hiPSC-derived neural stem cells (NSCs) undergoing differentiation, assessing the expression and the activation of the Nrf2 pathway. Our results show that Nrf2 signaling increases during NSC differentiation. Moreover, we observed that rotenone treatment induced a progressive activation of Nrf2 signaling together with a induction of astrocyte reactivity, a reduction of neurite length leading to neuronal cell death, in particular of DA neurons. Altogether these data indicate that hiPSC-NSC models are relevant test systems for the evaluation of Nrf2 pathway activation upon induced oxidative stress, allowing further understanding of the molecular mechanisms underlying exposure to (developmental) neurotoxicants.

Evaluation of the rotenone-induced activation of the Nrf2 pathway in a neuronal model derived from human induced pluripotent stem cells.

Human induced pluripotent stem cells (hiPSCs) are considered as a powerful tool for drug and chemical screening and development of new in vitro testing strategies in the field of toxicology, including neurotoxicity evaluation. These cells are able to expand and efficiently differentiate into different types of neuronal and glial cells as well as peripheral neurons. These human cells-based neuronal models serve as test systems for mechanistic studies on different pathways involved in neurotoxicity. One of the well-known mechanisms that are activated by chemically-induced oxidative stress is the Nrf2 signaling pathway. Therefore, in the current study, we evaluated whether Nrf2 signaling machinery is expressed in human induced pluripotent stem cells (hiPSCs)-derived mixed neuronal/glial culture and if so whether it becomes activated by rotenone-induced oxidative stress mediated by complex I inhibition of mitochondrial respiration. Rotenone was found to induce the activation of Nrf2 signaling particularly at the highest tested concentration (100 nM), as shown by Nrf2 nuclear translocation and the up-regulation of the Nrf2-downstream antioxidant enzymes, NQO1 and SRXN1. Interestingly, exposure to rotenone also increased the number of astroglial cells in which Nrf2 activation may play an important role in neuroprotection. Moreover, rotenone caused cell death of dopaminergic neurons since a decreased percentage of tyrosine hydroxylase (TH+) cells was observed. The obtained results suggest that hiPSC-derived mixed neuronal/glial culture could be a valuable in vitro human model for the establishment of neuronal specific assays in order to link Nrf2 pathway activation (biomarker of oxidative stress) with additional neuronal specific readouts that could be applied to in vitro neurotoxicity evaluation.

Regenerative toxicology: the role of stem cells in the development of chronic toxicities.

INTRODUCTION: Human stem cell lines and their derivatives, as alternatives to the use of animal cells or cancer cell lines, have been widely discussed as cellular models in predictive toxicology. However, the role of stem cells in the development of long-term toxicities and carcinogenesis has not received great attention so far, despite growing evidence indicating the relationship of stem cell damage to adverse effects later in life. However, testing this in vitro is a scientific/technical challenge in particular due to the complex interplay of factors existing under physiological conditions. Current major research programs in stem cell toxicity are not aiming to demonstrate that stem cells can be targeted by toxicants. Therefore, this knowledge gap needs to be addressed in additional research activities developing technical solutions and defining appropriate experimental designs. AREAS COVERED: The current review describes selected examples of the role of stem cells in the development of long-term toxicities in the brain, heart or liver and in the development of cancer. EXPERT OPINION: The presented examples illustrate the need to analyze the contribution of stem cells to chronic toxicity in order to make a final conclusion whether stem cell toxicities are an underestimated risk in mechanism-based safety assessments. This requires the development of predictive in vitro models allowing the assessment of adverse effects to stem cells on chronic toxicity and carcinogenicity.