The MTT-formazan assay: Complementary technical approaches and in vivo validation in Drosophila larvae.

The MTT assay was the first widely accepted method to assess cytotoxicity and cell viability. However, there is controversy on whether this indicator is a useful tool. In this work we intend to expand the interpretability of the MTT study by its combination with widely used cellular biology techniques. We propose complementary approaches to the colorimetric assay, based on the use of measurements in three different settings: confocal microscopy, multi-well plate assay and flow cytometry. Using confocal microscopy, we confirmed that MTT uptake and reduction by cells is a time-dependent process, and that formazan accumulates in round-shaped organelles. Quantitative measurements with a multi-well fluorimeter combined with nuclear staining result in a useful method, yielding a ratio between formazan production and cell number that informs about the average cell metabolic state. We also found that flow cytometry is a suitable technique to measure MTT reduction in large cell populations. When assaying the effect of an oxidizing agent such as paraquat (PQ), this approach allows for the distinction of subpopulations of cells with different reducing power. Finally, we prove that it is feasible to monitor MTT reduction in an in vivo model, the Drosophila larvae, without affecting its survival rate. Formazan accumulates exclusively in the larval fat body, confirming its lipid solubility. The methods explored in this work expand the MTT potential as a useful tool to provide information of the physiological state of cells and organisms.