Development of an in vitro blood–brain barrier model—cytotoxicity of mercury and aluminum

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In this study, in vitro blood–brain barrier (BBB) models composed of two different cell types were compared. The aim of our study was to find an alternative human cell line that could be used in BBB models. Inorganic and organic mercury and aluminum were studied as model chemicals in the testing of the system. BBB models were composed of endothelial RBE4 cell line or retinal pigment epithelial (RPE) cell line ARPE-19 and neuronal SH-SY5Y cells as target cells. Glial U-373 MG cells were included in part of the tests to induce the formation of a tighter barrier. Millicell CM filter inserts were coated with rat-tail collagen, and RBE4 or ARPE-19 cells were placed on the filters at the density of 3.5–4 × 105 cells/filter. During culture, the state of confluency was microscopically observed and confirmed by the measurement of electrical resistance caused by the developing cell layer. The target cells, SH-SY5Y neuroblastoma cells, were plated on the bottom of cell culture wells at the density of 100 000 cells/cm2. In part of the studies, glial U-373 MG cells were placed on the under side of the membrane filter. When confluent filters with ARPE-19 or RBE4 cells were placed on top of the SH-SY5Y cells, different concentrations of mercuric chloride, methyl mercury chloride, and aluminum chloride were added into the filter cups along with a fluorescent tracer. Exposure time was 24 h, after which the cytotoxicity in the SH-SY5Y cell layer, as well as in the ARPE-19 or RBE4 cell layer, was evaluated by the luminescent measurement of total ATP. The leakage of the fluorescent tracer was also monitored. The results showed that both barrier cell types were induced by glial cells. Inorganic and organic mercury caused a leakage of the dye and cytotoxicity in SH-SY5Y cells. Especially, methyl mercury chloride could exert an effect on target cells before any profound cytotoxicity in barrier cells could be seen. Aluminum did not cause any leakage in the barrier cell layer, and even the highest concentration (1 mM) of aluminum did not cause any cytotoxicity in the SH-SY5Y cells. In conclusion, BBB models composed of RBE4 and ARPE-19 cells were able to distinguish between different toxicities, and ARPE-19 cells are thus promising candidates for studies of drug penetration through the blood–brain barrier.

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