mplex I, complex II and complex IV using isolated mitochondria from DJ-12/2 and +/+ MEFs. After normalization to citrate synthase activity, enzymatic activities of all complexes composing the ETS appear normal in DJ-12/2 MEFs. We then measured the level of ATP in DJ-12/2 and +/+ MEFs using a luciferin/ luciferase assay that provides a direct quantification of ATP concentration. Interestingly, lack of DJ-1 leads to a decrease of ATP concentration in DJ-12/2 MEFs. Thus, loss of DJ-1 does not affect levels of mitochondrial complexes and activities but does cause reduction of ATP concentration. Normal Mitochondrial Calcium Concentration in DJ-12/ 2 MEFs Decreased Mitochondrial Transmembrane Potential in DJ-12/2 MEFs In the absence of enzymatic defects of the ETS complexes but decreased ATP levels, we turned our attention to mitochondrial transmembrane potential, the electrochemical force that modulates the kinetics of proton reentry to the matrix through ATP-synthase. Using two different methods, live cell imaging and flow cytometry, we DJ-1 in ROS Production and mPTP Opening cent dye that binds to free intracellular calcium. Fura-2 is excited at wavelengths 340 nm and 380 nm, and the ratio of the emissions is directly correlated to the amount of intracellular calcium. PF-04447943 increases in Fura-2 signal following FCCP treatment are the same between DJ-12/2 and +/+ MEFs. Thus, loss of DJ-1 does not seem to affect the size of mitochondrial calcium pool. Increased Levels of Oxidative Stress in DJ-12/2 Cells While mitochondrial calcium appears unaffected in the absence of DJ-1, mPTP opening can also be influenced by elevated oxidative stress. Furthermore, mitochondria are the major site where reactive oxygen species are produced in the cell, and several reports showed that DJ-1 can function as oxidative stress sensor and scavenger. We therefore evaluated ROS production in whole cell and in mitochondrial fraction from DJ12/2 and +/+ MEFs. We first evaluated production of oxidative species using live cells loaded with Amplex Red, dihydroethidium or MitoSOX Red. The intensity of Amplex Red fluorescence is modulated by the amount of H2O2 produced in the cell, whereas the fluorescent intensity of DHEt and MitoSOX Red reflects production of intracellular superoxide and intramitochondrial superoxide, respectively. We found that the intensities of all three fluorescent dyes are higher in DJ-12/2 MEFs, indicating increases of ROS production in the absence of DJ-1. Mitotracker Green was used as control, since it is independent of oxidative conditions and membrane potential. We then followed the increase of the fluorescence over time and found that increases of Amplex Red, DHEt and Mitotracker CM-H2XROS fluorescence over time are higher in DJ-12/2 MEFs compared to control cells. Because H2O2 extrusion across the plasma membrane can be limiting, we also measured the rate of H2O2 produced using isolated mitochondria from DJ-12/2 and +/+ MEFs. We found that isolated mitochondria from DJ12/2 MEFs produced more H2O2 than PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22201297 control MEFs. We also performed positive control experiments using different amounts of H2O2 and found that the intensity of Amplex Red fluorescence directly correlated to the concentration of H2O2 in wild-type 
cells. We further used pyocyanin to induce ROS in control cells, and found that the fluorescent intensity of Amplex Red, DHEt and Mitotracker CM-H2XROS is responsive to the induction of ROS production. Since a recent report showed that DJ-1 inf
