Apoptotic cell death in neural stem cells exposed to toxic stimuli

Abstract

Neural stem cells (NSCs) play an important role in the developing nervous system and adulthood, where mitotic regions of the brain such as the subventricular zone (SVZ) remain active. In spite of the immense interest in NSC research, little is known about the biochemical regulation by which NSCs undergo cell death in the course of normal physiology or in response to neurotoxic insults. Also, before the full potential of NSCs can be appreciated, it is essential to understand the physiological pathways that control their proliferation and differentiation, as well as the influence of extrinsic factors on these processes.

We have studied the general apoptotic machinery in NSCs. As experimental models we used primary culture of adult NSCs (aNSCs) from the SVZ of the adult rat brain, and the neural stem cell line C17.2, initially derived from developing mouse cerebellum. We show that NSCs undergo apoptosis in response to the pankinase inhibitor staurosporine, as well as agents inducing oxidative stress such as 2,3dimethoxy-1,4-naphthoquinone. Exposed cells exhibit apoptotic morphology, phosphatidylserine translocation to outer leaf of the plasma membrane, cytochrome c release, caspase activation and DNA fragmentation. Our findings demonstrate that NSCs are sensitive to cytotoxic stimuli that involve an engagement of the mitochondria, and the importance of the intrinsic apoptotic pathway in NSC-apoptotic cell death. Both aNSCs and C17.2 cells express the Fas receptor, but exposure to agonistic antibodies fails to induce apoptosis.

It is known that Fas not only induces apoptosis, but also can deliver growth stimulatory signals through activation of the extracellular-signal regulated kinase (ERK) pathway. The Fas-induced ERK phosphorylation that we detect in C17.2 cells, suggests that in NSCs Fas may function as a mediator of growth rather than death. We have also investigated the toxic -effects of the environmental organometal MeHg in C17.2 cells and primary embryonic cortical NSCs (cNSCs). Our results show that NSCs are more sensitive than differentiated neurons or glial cells to MeHg. Apoptosis is induced in both models via Bax-activation, cytochrome c release, and activation of caspases and calpains. Remarkably, exposure to MeHg at concentrations lower than observed in cord blood of Swedish pregnant women inhibits spontaneous neuronal differentiation of NSPCC.

In conclusion this study shows that NSPCC are a highly sensitive model system for in vitro developmental neurotoxicity studies and offer new perspectives for evaluating the biological significance of low level exposures to neurotoxicants.