Metabolic mapping of MCF10A human breast cells via multiphoton fluorescence lifetime imaging of the coenzyme NADH

Biochemical estimation of NADH concentration is a useful method for monitoring cellular metabolism, because the NADH/NAD+ reduction-oxidation pair is crucial for electron transfer in the mitochondrial electron chain. In this article, we present a novel method for deriving functional maps of intracellular reduction-oxidation ratio in vivo via measurement of the fluorescence lifetimes and the ratio of free and protein-bound NADH using two-photon fluorescence lifetime imaging (FLIM). Through systematic analysis of FLIM data from the control cells, it was observed that there is a statistically significant decrease in the fluorescence lifetime of both free and protein-bound NADH and the contribution of protein-bound NADH as cells progress from an early to logarithmic to confluent phase. Potassium cyanide (KCN) treatment and serum starvation of cells yielded similar changes. There was a statistically significant decrease in the fluorescence lifetime of protein-bound and free NADH at the early and logarithmic phase of the growth curve and a statistically significant decrease in the contribution of protein-bound NADH relative to that observed in the control cells at all three phases of the growth curve. The imposed perturbations (confluence, serum starvation, and KCN treatment) are all expected to result in an increase in the ratio of NADH/NAD+. Our studies suggest that the fluorescence lifetime of both the free and the protein-bound components of NADH and the ratio of free to protein-bound NADH is related to changes in the NADH/NAD+ ratio.