Rotenone exerts similar stimulatory effects on H2O2 production by isolated brain mitochondria from young-adult and old rats

Aging-dependent mitochondrial bioenergetic impairment in the skeletal muscle of NNT-deficient mice

Overall health relies on features of skeletal muscle that generally decline with age, partly due to mechanisms associated with mitochondrial redox imbalance and bioenergetic dysfunction. Previously, aged mice genetically devoid of the mitochondrial NAD(P)+ transhydrogenase (NNT, encoded by the nicotinamide nucleotide transhydrogenase gene), an enzyme involved in mitochondrial NADPH supply, were shown to exhibit deficits in locomotor behavior. Here, by using young, middle-aged, and older NNT-deficient (Nnt-/-) mice and age-matched controls (Nnt+/+), we aimed to investigate how muscle bioenergetic function and motor performance are affected by NNT expression and aging. Mice were subjected to the wire-hang test to assess locomotor performance, while mitochondrial bioenergetics was evaluated in fiber bundles from the soleus, vastus lateralis and plantaris muscles. An age-related decrease in the average wire-hang score was observed in middle-aged and older Nnt-/- mice compared to age-matched controls. Although respiratory rates in the soleus, vastus lateralis and plantaris muscles did not significantly differ between the genotypes in young mice, the rates of oxygen consumption did decrease in the soleus and vastus lateralis muscles of middle-aged and older Nnt-/- mice. Notably, the soleus, which exhibited the highest NNT expression level, was the muscle most affected by aging, and NNT loss. Additionally, histology of the soleus fibers revealed increased numbers of centralized nuclei in older Nnt-/- mice, indicating abnormal morphology. In summary, our findings suggest that NNT expression deficiency causes locomotor impairments and muscle dysfunction during aging in mice.

LRRK2 Knockout Confers Resistance in HEK-293 Cells to Rotenone-Induced Oxidative Stress, Mitochondrial Damage, and Apoptosis

Leucine-rich repeat kinase 2 (LRRK2) has been linked to dopaminergic neuronal vulnerability to oxidative stress (OS), mitochondrial impairment, and increased cell death in idiopathic and familial Parkinson's disease (PD). However, how exactly this kinase participates in the OS-mitochondria-apoptosis connection is still unknown. We used clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 LRRK2 knockout (KO) in the human embryonic kidney cell line 293 (HEK-293) to evaluate the cellular response to the mitochondrial inhibitor complex I rotenone (ROT), a well-known OS and cell death inducer. We report successful knockout of the LRRK2 gene in HEK-293 cells using CRISPR editing (ICE, approximately 60%) and flow cytometry (81%) analyses. We found that HEK-293 LRRK2 WT cells exposed to rotenone (ROT, 50 μM) resulted in a significant increase in intracellular reactive oxygen species (ROS, +7400%); oxidized DJ-1-Cys¹⁰⁶-SO₃ (+52%); phosphorylation of LRRK2 (+70%) and c-JUN (+171%); enhanced expression of tumor protein (TP53, +2000%), p53 upregulated modulator of apoptosis (PUMA, +1950%), and Parkin (PRKN, +22%); activation of caspase 3 (CASP3, +8000%), DNA fragmentation (+35%) and decreased mitochondrial membrane potential (ΔΨm, -58%) and PTEN induced putative kinase 1 (PINK1, -49%) when compared to untreated cells. The translocation of the cytoplasmic fission protein dynamin-related Protein 1 (DRP1) to mitochondria was also observed by colocalization with translocase of the outer membrane 20 (TOM20). Outstandingly, HEK-293 LRRK2 KO cells treated with ROT showed unaltered OS and apoptosis markers. We conclude that loss of LRRK2 causes HEK-293 to be resistant to ROT-induced OS, mitochondrial damage, and apoptosis in vitro. Our data support the hypothesis that LRRK2 acts as a proapoptotic kinase by regulating mitochondrial proteins (e.g., PRKN, PINK1, DRP1, and PUMA), transcription factors (e.g., c-JUN and TP53), and CASP3 in cells under stress conditions. Taken together, these observations suggest that LRRK2 is an important kinase in the pathogenesis of PD.

Evaluation of mitochondrial respiratory function in highly glycolytic glioma cells reveals low ADP phosphorylation in relation to oxidative capacity

High-grade gliomas are aggressive and intensely glycolytic tumors. In the present study, we evaluated the mitochondrial respiratory function of glioma cells (T98G and U-87MG) and fresh human glioblastoma (GBM) tissue. To this end, measurements of oxygen consumption rate (OCR) were performed under various experimental conditions. The OCR of T98G and U-87MG cells was well coupled to ADP phosphorylation based on the ratio of ATP produced per oxygen consumed of ~2.5. In agreement, the basal OCR of GBM tissue was also partially associated with ADP phosphorylation. The basal respiration of intact T98G and U-87MG cells was not limited by the supply of endogenous substrates, as indicated by the increased OCR in response to a protonophore. These cells also displayed a high affinity for oxygen, as evidenced by the values of the partial pressure of oxygen when respiration is half maximal (p ₅₀). In permeabilized glioma cells, ADP-stimulated OCR was only approximately 50% of that obtained in the presence of protonophore, revealing a significant limitation in oxidative phosphorylation (OXPHOS) relative to the activity of the electron transport system (ETS). This characteristic was maintained when the cells were grown under low glucose conditions. Flux control coefficient analyses demonstrated that the impaired OXPHOS was associated with the function of both mitochondrial ATP synthase and the adenine nucleotide translocator, but not the phosphate carrier. Altogether, these data indicate that the availability and metabolism of respiratory substrates and mitochondrial ETS are preserved in T98G and U-87MG glioma cells even though these cells possess a relatively restrained OXPHOS capability.

Neuroprotective Effect of the LRRK2 Kinase Inhibitor PF-06447475 in Human Nerve-Like Differentiated Cells Exposed to Oxidative Stress Stimuli: Implications for Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) has been implicated in oxidative stress (OS) and neurodegeneration in Parkinson's disease (PD). However, the pathophysiological mechanism of the LRRK2 kinase in neurons under stress stimuli is not yet understood. We demonstrate that rotenone (ROT), a mitochondria complex I inhibitor frequently used to generate in vitro and in vivo experimental models of PD, induces LRRK2 phosphorylation at serine 935 p-(S935) concomitant with cell death in nerve-like differentiated cells (NLCs). Indeed, ROT (50 µM) at 6 h exposure significantly increased reactive oxygen species (ROS) (~100 %), p-(S935)-LRRK2 kinase [~2 f(old)-(i)ncrease] level, induced nuclei condensation/fragmentation (16 %), increased the expression of NF-κB (5.6 f-i), p53 (5.3 f-i), c-Jun (5.4 f-i) transcription factors, activated caspase-3 (8.0 f-i) and AIF (6.8 f-i) proteins; but significantly decreased mitochondrial membrane potential (∆Ψ_m, ~21 %), indicative of apoptosis -a type of regulated cell death process- compared to untreated cells. Strikingly, the LRRK2 kinase inhibitor PF-06447475 (PF-475, 1 µM) protects NLCs against ROT induced noxious effect. The inhibitor not only blocked the p-(S935)-LRRK2 kinase phosphorylation but also completely abolished ROS, and significantly reversed all ROT-induced apoptosis signaling and OS associated markers to comparable control values. We conclude that wild-type LRRK2 may act as a pro-apoptotic factor under OS stimuli. Our findings suggest an association between OS and LRRK2 phosphorylation in the NLCs death process, as PD model. Therefore, the pharmacological inhibition of LRRK2 might help to understand the OS-mediated kinase activation in PD neurodegenerative disorder.

Underestimation of the Maximal Capacity of the Mitochondrial Electron Transport System in Oligomycin-Treated Cells

The maximal capacity of the mitochondrial electron transport system (ETS) in intact cells is frequently estimated by promoting protonophore-induced maximal oxygen consumption preceded by inhibition of oxidative phosphorylation by oligomycin. In the present study, human glioma (T98G and U-87MG) and prostate cancer (PC-3) cells were titrated with different concentrations of the protonophore CCCP to induce maximal oxygen consumption rate (OCR) within respirometers in a conventional growth medium. The results demonstrate that the presence of oligomycin or its A-isomer leads to underestimation of maximal ETS capacity. In the presence of oligomycin, the spare respiratory capacity (SRC), i.e., the difference between the maximal and basal cellular OCR, was underestimated by 25 to 45%. The inhibitory effect of oligomycin on SRC was more pronounced in T98G cells and was observed in both suspended and attached cells. Underestimation of SRC also occurred when oxidative phosphorylation was fully inhibited by the ATP synthase inhibitor citreoviridin. Further experiments indicated that oligomycin cannot be replaced by the adenine nucleotide translocase inhibitors bongkrekic acid or carboxyatractyloside because, although these compounds have effects in permeabilized cells, they do not inhibit oxidative phosphorylation in intact cells. We replaced CCCP by FCCP, another potent protonophore and similar results were observed. Lower maximal OCR and SRC values were obtained with the weaker protonophore 2,4-dinitrophenol, and these parameters were not affected by the presence of oligomycin. In permeabilized cells or isolated brain mitochondria incubated with respiratory substrates, only a minor inhibitory effect of oligomycin on CCCP-induced maximal OCR was observed. We conclude that unless a previously validated protocol is employed, maximal ETS capacity in intact cells should be estimated without oligomycin. The inhibitory effect of an ATP synthase blocker on potent protonophore-induced maximal OCR may be associated with impaired metabolism of mitochondrial respiratory substrates.