Image analysis of Ca2+ signals as a basis for neurotoxicity assays: Promises and challenges

Free intracellular calcium ([Ca2+]i) controls a wide range of cellular functions such as contraction, neurotransmitter and hormone release, metabolism, cell division and differentiation. Cytosolic Ca2+ levels are abnormal in cells exposed to toxicants and understanding how these levels become altered may improve our ability to design high-throughput methods for the sensitive detection of cellular responses to a toxic exposure. Because Ca2+ is involved in multiple aspects of cellular function, its role in signaling is complex. It is therefore necessary to identify the individual pathways targeted during toxicant exposure in order to use them as a tool for predictive measurements of toxicity and as targets for prevention or reversal of injury. This review illustrates several methods available for analysis of Ca2+ responses in vitro and their applicability for understanding mechanisms of toxicity at the molecular and cellular levels. The review will also consider the usefulness of Ca2+ imaging for predicting a unique signature for classes of toxicants. Towards this end, two methodological approaches for assessment of Ca2+ responses to toxicants are examined: steady state measurements and complex spatial and/or temporal measurements. Each of the methods described and appropriately used results in reliable and reproducible measurements which may be applied in a high-throughput fashion to individualize in vitro assessment of cellular responses caused by toxicants.