Characterization of mitochondrial dysfunction due to laser damage by 2-photon FLIM microscopy

Mitochondria are the central organelles in cellular bio-energetics with key roles to play in energy metabolism and cell fate decisions. Fluorescence Lifetime Imaging microscopy (FLIM) is used to track metabolic changes by following the intrinsic co-enzymes NAD(P)H and FAD, present in metabolic pathways. FLIM records-lifetimes and the relative fractions of free (unbound) and bound states of NAD(P)H and FAD are achieved by multiphoton excitation of a pulsed femto-second infra-red laser. Optimization of multiphoton laser power levels is critical to achieve sufficient photon counts for correct lifetime fitting while avoiding phototoxic effects. We have characterized two photon (2p) laser induced changes at the intra-cellular level, specifically in the mitochondria, where damage was assessed at rising 2p laser average power excitation. Our results show that NAD(P)H-a2%-the lifetime-based enzyme bound fraction, an indicator of mitochondrial OXPHOS activity is increased by rising average power, while inducing changes in the mitochondria at higher power levels, quantified by different probes. Treatment response tracked by means of NAD(P)H-a2% can be confounded by laser-induced damage producing the same effect. Our study demonstrates that 2p-laser power optimization is critical by characterizing changes in the mitochondria at increasing laser average power.