Early mitochondrial fragmentation is a potential in vitro biomarker of environmental stress

AutoMitoNetwork: Software for analyzing mitochondrial networks in autofluorescence images to enable label-free cell classification

High-resolution mitochondria imaging in combination with image analysis tools have significantly advanced our understanding of cellular function in health and disease. However, most image analysis tools for mitochondrial studies have been designed to work with fluorescently labeled images only. Additionally, efforts to integrate features describing mitochondrial networks with machine learning techniques for the differentiation of cell types have been limited. Herein, we present AutoMitoNetwork software for image-based assessment of mitochondrial networks in label-free autofluorescence images using a range of interpretable morphological, intensity, and textural features. To demonstrate its utility, we characterized unstained mitochondrial networks in healthy retinal cells and in retinal cells exposed to two types of treatments: rotenone, which directly inhibited mitochondrial respiration and ATP production, and iodoacetic acid, which had a milder impact on mitochondrial networks via the inhibition of anaerobic glycolysis. For both cases, our multi-dimensional feature analysis combined with a support vector machine classifier distinguished between healthy cells and those treated with rotenone or iodoacetic acid. Subtle changes in morphological features were measured including increased fragmentation in the treated retinal cells, pointing to an association with metabolic mechanisms. AutoMitoNetwork opens new options for image-based machine learning in label-free imaging, diagnostics, and mitochondrial disease drug development.

A time window for rescuing dying retinal ganglion cells

BACKGROUND

Retinal ganglion cell (RGC) degeneration and death cause vision loss in patients with glaucoma. Regulated cell death, once initiated, is generally considered to be an irreversible process. Recently, we showed that, by timely removing the cell death stimulus, stressed neuronal PC12 cells can recover from phosphatidylserine (PS) exposure, nuclear shrinkage, DNA damage, mitochondrial fragmentation, mitochondrial membrane potential loss, and retraction of neurites, all hallmarks of an activated cell death program. Whether the cell death process can be reversed in neurons of the central nervous system, like RGCs, is still unknown. Here, we studied reversibility of the activated cell death program in primary rat RGCs (prRGCs).

METHODS

prRGCs were exposed to ethanol (5%, vol/vol) to induce cell death. At different stages of the cell death process, ethanol was removed by washing and injured prRGCs were further cultured in fresh medium to see whether they recovered. The dynamics of single cells were monitored by high-resolution live-cell spinning disk microscopy. PS exposure, mitochondrial structure, membrane potential, and intracellular Ca2+ were revealed by annexin A5-FITC, Mito-tracker, TMRM, and Fluo 8-AM staining, respectively. The distribution of cytochrome c was investigated by immunofluorescence. The ultrastructure of mitochondria was studied by electron microscopy.

RESULTS

Analysis of temporal relationships between mitochondrial changes and PS exposure showed that fragmentation of the mitochondrial network and loss of mitochondrial membrane potential occurred before PS exposure. Mitochondrial changes proceeded caspase-independently, while PS exposure was caspase dependent. Interestingly, prRGCs recovered quickly from these mitochondrial changes but not from PS exposure at the plasma membrane. Correlative light and electron microscopy showed that stress-induced decrease in mitochondrial area, length and cristae number was reversible. Intracellular Ca2+ was elevated during this stage of reversible mitochondrial injury, but there was no sign of mitochondrial cytochrome c release.

CONCLUSIONS

Our study demonstrates that RGCs with impaired mitochondrial structure and function can fully recover if there is no mitochondrial cytochrome c release yet, and no PS is exposed at the plasma membrane. This finding indicates that there is a time window for rescuing dying or injured RGCs, by simply removing the cell death stimulus. Video Abstract.

Quantifying mitochondrial heteroplasmy diversity: A computational approach

Biodiversity plays a pivotal role in sustaining ecosystem processes, encompassing diverse biological species, genetic types and the intricacies of ecosystem composition. However, the precise definition of biodiversity at the individual level remains a challenging endeavour. Hill numbers, derived from Rényi's entropy, have emerged as a popular measure of diversity, with a recent unified framework extending their application across various levels, from genetics to ecosystems. In this study, we employ a computational approach to exploring the diversity of mitochondrial heteroplasmy using real-world data. By adopting Hill numbers with q = 2, we demonstrate the feasibility of quantifying mitochondrial heteroplasmy diversity within and between individuals and populations. Furthermore, we investigate the alpha diversity of mitochondrial heteroplasmy among different species, revealing heterogeneity at multiple levels, including mitogenome components and protein-coding genes (PCGs). Our analysis explores large-scale mitochondrial heteroplasmy data in humans, examining the relationship between alpha diversity at the mitogenome components and PCGs level. Notably, we do not find a significant correlation between these two levels. Additionally, we observe significant correlations in alpha diversity between mothers and children in blood samples, exceeding the reported R2 value for allele frequency correlations. Moreover, our investigation of beta diversity and local overlay similarity demonstrates that heteroplasmy variant distributions in different tissues of children more closely resemble those of their mothers. Through systematic quantification and analysis of mitochondrial heteroplasmy diversity, this study enhances our understanding of heterogeneity at multiple levels, from individuals to populations, providing new insights into this fundamental dimension of biodiversity.

Quantitative assessment of mitochondrial morphology relevant for studies on cellular health and environmental toxicity

Mitochondria are essential organelles that play crucial roles in cellular energy metabolism, calcium signaling and apoptosis. Their importance in tissue homeostasis and stress responses, combined to their ability to transition between various structural and functional states, make them excellent organelles for monitoring cellular health. Quantitative assessment of mitochondrial morphology can therefore provide valuable insights into environmentally-induced cell damage. High-content screening (HCS) provides a powerful tool for analyzing organelles and cellular substructures. We developed a fully automated and miniaturized HCS wet-plus-dry pipeline (MITOMATICS) exploiting mitochondrial morphology as a marker for monitoring cellular health or damage. MITOMATICS uses an in-house, proprietary software (MitoRadar) to enable fast, exhaustive and cost-effective analysis of mitochondrial morphology and its inherent diversity in live cells. We applied our pipeline and big data analytics software to assess the mitotoxicity of selected chemicals, using the mitochondrial uncoupler CCCP as an internal control. Six different pesticides (inhibiting complexes I, II and III of the mitochondrial respiratory chain) were tested as individual compounds and five other pesticides present locally in Occitanie (Southern France) were assessed in combination to determine acute mitotoxicity. Our results show that the assayed pesticides exhibit specific signatures when used as single compounds or chemical mixtures and that they function synergistically to impact mitochondrial architecture. Study of environment-induced mitochondrial damage has the potential to open new fields in mechanistic toxicology, currently underexplored by regulatory toxicology and exposome research. Such exploration could inform health policy guidelines and foster pharmacological intervention, water, air and soil pollution control and food safety.

Restoration of mitochondrial structure and function within Helicobacter pylori VacA intoxicated cells

The Helicobacter pylori vacuolating cytotoxin (VacA) is an intracellular, mitochondrial-targeting exotoxin that rapidly causes mitochondrial dysfunction and fragmentation. Although VacA targeting of mitochondria has been reported to alter overall cellular metabolism, there is little known about the consequences of extended exposure to the toxin. Here, we describe studies to address this gap in knowledge, which have revealed that mitochondrial dysfunction and fragmentation are followed by a time-dependent recovery of mitochondrial structure, mitochondrial transmembrane potential, and cellular ATP levels. Cells exposed to VacA also initially demonstrated a reduction in oxidative phosphorylation, as well as increase in compensatory aerobic glycolysis. These metabolic alterations were reversed in cells with limited toxin exposure, congruent with the recovery of mitochondrial transmembrane potential and the absence of cytochrome c release from the mitochondria. Taken together, these results are consistent with a model that mitochondrial structure and function are restored in VacA-intoxicated cells.

Palmitic Acid Induced a Long-Lasting Lipotoxic Insult in Human Retinal Pigment Epithelial Cells, which Is Partially Counteracted by TRAIL

Hyperglycaemia and increased circulating saturated fatty acids are key metabolic features of type 2 diabetes mellitus (T2DM) that contribute to diabetic retinopathy pathogenesis. Contrarily, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) has been shown to improve or prevent T2DM. This study aimed at investigating the effect of TRAIL in an in vitro model of human retinal pigment epithelium: the ARPE-19 cell line, treated with palmitic acid (PA) in the presence of high glucose concentration. PA caused a drop in cellular metabolic activity and cell viability as well as an increase in apoptosis rates, which were paralleled by an upregulation of reactive oxygen species (ROS) generation as well as mitochondrial fragmentation. Despite ARPE-19 cells expressing TRAIL-R2 at the cell surface, TRAIL failed to counteract the cytotoxic effects of PA. However, when TRAIL was used alongside PA and then removed or used alone following PA challenge, it partially attenuated PA-induced lipotoxicity. This effect of TRAIL appeared to rely upon the modulation of inflammation and ROS production. Thus, TRAIL exerted a trophic effect on ARPE-19 cells, which became evident only when the lipotoxic insult was removed. Nevertheless, whether recombinant TRAIL might have a therapeutic potential for the treatment of diabetic retinopathy requires further investigation.

Chlorinated organophosphorus flame retardants-induced mitochondrial abnormalities and the correlation with progesterone production in mLTC-1 cells

Environmental monitoring data have indicated that three chlorinated organophosphorus flame retardants (Cl-OPFRs), including tris(2-chloroethyl)-phosphate (TCEP), tris(2-chloropropyl)-phosphate (TCPP), and tris(1,3-dichloro-2-propyl)-phosphate (TDCPP) are the predominant chemicals in various environmental matrices and exhibit reproductive endocrine disrupting activities. Currently, mitochondrial abnormality is a new paradigm for evaluating chemical-mediated cell dysfunction. However, a comprehensive correlation between these two aspects of Cl-OPFRs remains unclear. In this research, the effects of TCEP, TCPP, and TDCPP on progesterone production and mitochondrial impairment were investigated by using mouse Leydig tumor cells (mLTC-1). The half maximal inhibitory concentration (IC₅₀) values at 48 h exposure indicated that the rank order of anti-androgenic activity was TDCPP > TCPP. Whereas, TCEP exhibited elevation of progesterone production. At concentrations close to IC₅₀ of progesterone production by TCPP and TDCPP, the elevation of intracellular reactive oxygen species (ROS), depletion of mitochondrial membrane potential (MMP), reduction of cellular adenosine triphosphate (ATP) content, and alteration of mitochondrial structures was observed. In addition, the expression of main genes related to progesterone synthesis was dramatically down-regulated by TCPP and TDCPP treatments. These results imply that the inhibition effect of TCPP and TDCPP on progesterone production might be related to mitochondrial damage and down-regulated steroidogenic genes.

Battery wastewater induces nephrotoxicity via disordering the mitochondrial dynamics

The rapid development of new energy battery enterprises manifolds the obsolete and scrapped batteries which are considered serious concern for the environment and ecology. Increasing trend of recycling batteries waste is public hazard throughout the world. The batteries wastes affect the various body systems but exact toxicological mechanism of battery wastewater is still unexplored. The present study was designed to observe the toxicological effects of batteries wastes on kidney functional dynamics. In this experiment, a total of 20 male mice were randomly divided into two groups including control and treatment (battery wastewater) group. The control group was provided the normal saline while the battery wastewater group were provided battery waste-water for a period of 21 days. The isolated kidneys were processed for histopathological analysis, biochemical assays, mRNA and protein estimation. The results showed that battery wastewater provision increased the mitochondrial division-related genes and proteins (Drp1, MFF, Fis1) and decreased the expression level of fusion-related nuclear proteins (MFN1, MFN2, OPA1) in kidneys. Moreover, the battery wastewater exposure significantly up-regulated the autophagy (PINK, Parkin, mTOR, ATG5, LC3-b, p62) and apoptosis (Bax, Cytc, APAF1, P53, Caspase3, Caspase8) related mRNA and proteins levels in kidneys. However, down-regulation of mRNA and proteins levels of Bcl2 and Beclin1 were also observed in kidneys after batteries wastes exposure. In conclusion, it is evident that the battery wastewater leads to renal apoptosis and autophagy by disrupting the mitochondrial dynamics in mice kidneys.

Resveratrol inhibits HeLa cell proliferation by regulating mitochondrial function

The beneficial roles of resveratrol (RES) in affecting proliferation of multiple cancer cells have attracted intensive attention. However, the underlying mechanism remains unclear. This study aims to bridge the knowledge gap by investigating RES-induced growth inhibition of HeLa cells. Our work focuses on the metergasis of mitochondria in the RES-exposed cells. Therefore, HeLa cells were treated with different concentrations of RES for 30 min and 24 h, respectively. As a result, concentration-dependent increases in cell growth inhibition, ROS (reactive oxygen species) triggering, and LC3-II (light chain 3-II) expression were detected in the HeLa cells exposed to RES for 24 h. Interestingly, a specific concentration-dependent effect was observed in the HeLa cells exposed to RES for 30 min, that is, low concentration RES (≤ 25 μmol/L) reduced ROS levels, inhibited transcription and expression levels of LC3-II, and stimulated mitochondrial respiratory capacities. In contrast, high concentration RES (50 and 100 μmol/L) induced ROS over-production and autophagy in the cells, resulting in decreased levels of mitochondrial membrane potential, mitochondrial DNA copy numbers, and mitochondrial respiratory capacities. Together, our data concluded that RES inhibited HeLa cell proliferation through perturbation of mitochondrial structure and function, and ROS-induced autophagy also played a critical role in the process.

Mammalian AKT, the Emerging Roles on Mitochondrial Function in Diseases

Mitochondrial dysfunction may play a crucial role in various diseases due to its roles in the regulation of energy production and cellular metabolism. Serine/threonine kinase (AKT) is a highly recognized antioxidant, immunomodulatory, anti-proliferation, and endocrine modulatory molecule. Interestingly, increasing studies have revealed that AKT can modulate mitochondria-mediated apoptosis, redox states, dynamic balance, autophagy, and metabolism. AKT thus plays multifaceted roles in mitochondrial function and is involved in the modulation of mitochondria-related diseases. This paper reviews the protective effects of AKT and its potential mechanisms of action in relation to mitochondrial function in various diseases.

Pb induced mitochondrial fission of fibroblast cells via ATM activation

Previous study showed that lead (Pb) could induce ATM-dependent mitophagy. However, whether Pb has any impact on mitochondrial fusion and fission, the upstream events of mitophagy, and how ATM connects to these processes remain unclear. In this study, we found that Pb can disrupt mitochondrial network morphology as indicated by increased percentage of shortened mitochondria and by decreased mitochondrial footprints. Correspondingly, the expression of fission protein Drp1 and its association with mitochondrial marker Hsp60 were significantly increased, while those of fusion proteins Mfn2 and Opa1 and their co-localization with Hsp60 were drastically attenuated. Notably, the expression of p-Drp1 (Ser616) and its translocation to mitochondria were dramatically elevated. Moreover, a small amount of ATM could be detected in the cytoplasm around mitochondria in response to Pb, and the co-localization of p-ATM (Ser1981) with Drp1 and p-Drp1 (Ser616) was obviously increased while its co-localization with Mfn2 and Opa1 was dramatically decreased. Furthermore, siRNA silencing of ATM evidently promoted greater fission in response to Pb stress, indicating that ATM is involved in mitochondrial fragmentation. Our results suggest that cytoplasmic ATM is an important regulator of Pb-induced mitochondrial fission.

Comprehensive analysis of organophosphorus flame retardant-induced mitochondrial abnormalities: Potential role in lipid accumulation

Organophosphorus flame retardants (OPFRs), a group of new emerging endocrine disruption chemicals, have been reported to cause metabolic disturbance. Currently, mitochondrial abnormality is a new paradigm for evaluating chemical-mediated metabolic disruption. However, a comprehensive correlation between these two aspects of OPFR remains elusive. In the work reported here, 3 markers for morphological abnormality, and 7 markers of mitochondrial dysfunction were detected after treatment with two aryl-OPFRs (TCP and TPhP) and three chlorinated-OPFRs (TDCPP, TCPP, and TCEP) on hepatocyte. The two aryl-OPFRs and TDCPP can cause intracellular lipid accumulation at non-cytotoxic concentrations (<10 μM), while the other two chlorinated-OPFRs only caused lipid deposition at 10 μM. Furthermore, at the tested concentrations, all of them reduced mitochondrial (mito)-network numbers, enlarged mito-area/cells, and skewed mitoATP/glycoATP. Excluding TCEP, the other four chemicals induced mito-ROS and depleted mitochondrial membrane potential (MMP). Notably, only TCP, TPhP and TDCPP impeded mitoATP generation rate and mito-respiratory rate. Based on potency estimates, the capacity for lipid accumulation was significantly correlated with mito-network numbers (R² = 0.6481, p < 0.01), mitoATP/glycoATP (R² = 0.5197, p < 0.01), mitoROS (R² = 0.7197, p < 0.01), and MMP (R² = 0.7715, p < 0.01). Remarkably, the mito-respiratory rate (R² = 0.8753, p < 0.01) exhibited the highest correlation. Thus, the more potent lipid inducers TPhP, TCP and TDCPP could be identified. The results of this study demonstrate that aryl-OPFRs are more potent in metabolic disruption than other esters examined. Metabolic disruption should be examined further for chemicals that have the capacity to counteract the aforementioned functions of mitochondrial.

Polydatin attenuates cadmium-induced oxidative stress via stimulating SOD activity and regulating mitochondrial function in Musca domestica larvae

Cadmium (Cd) is a widespread environment contaminant due to the development of electroplating and metallurgical industry. Cd can be enriched by organisms via food chain, causing the enlarged environmental problems and posing threats to the health of humans. Polydatin (PD), a natural stilbenoid compound derived from Polygonum cuspidatum, shows pronouncedly curative effect on oxidative damage. In this work, the protective effects of PD on oxidative damage induced by Cd in Musca domestica (housefly) larvae were evaluated. The larvae were exposed to Cd and/or PD, subsequently, the oxidative stress status, mitochondria activity, oxidative phosphorylation efficiency, and survival rate were assessed. Cd exposure generated significant increases of malondialdehyde (MDA), reactive oxygen species (ROS) and 8-hydroxy-2-deoxyguanosine (8-oxoG) in the housefly larvae, causing mitochondrial dysfunction and survival rate decline. Interestingly, pretreatment with PD exhibited obviously mitochondrial protective effects in the Cd-exposed larvae, as evidenced by reduced MDA, ROS and 8-oxoG levels, and increased activities of superoxide dismutase (SOD), mitochondrial electron transfer chain, and mitochondrial membrane potential, as well as respiratory control ratio. These results suggested that PD could attenuate Cd-induced damage via maintaining redox balance, stimulating SOD activity, and regulating mitochondria activity in housefly larvae. As a natural polyphenolic chemical, PD can act as a potential candidate compounds to relieve Cd injury.

Application of the Key Characteristics of Carcinogens to Per and Polyfluoroalkyl Substances

Per- and polyfluoroalkyl substances (PFAS) constitute a large class of environmentally persistent chemicals used in industrial and consumer products. Human exposure to PFAS is extensive, and PFAS contamination has been reported in drinking water and food supplies as well as in the serum of nearly all people. The most well-studied member of the PFAS class, perfluorooctanoic acid (PFOA), induces tumors in animal bioassays and has been associated with elevated risk of cancer in human populations. GenX, one of the PFOA replacement chemicals, induces tumors in animal bioassays as well. Using the Key Characteristics of Carcinogens framework for cancer hazard identification, we considered the existing epidemiological, toxicological and mechanistic data for 26 different PFAS. We found strong evidence that multiple PFAS induce oxidative stress, are immunosuppressive, and modulate receptor-mediated effects. We also found suggestive evidence indicating that some PFAS can induce epigenetic alterations and influence cell proliferation. Experimental data indicate that PFAS are not genotoxic and generally do not undergo metabolic activation. Data are currently insufficient to assess whether any PFAS promote chronic inflammation, cellular immortalization or alter DNA repair. While more research is needed to address data gaps, evidence exists that several PFAS exhibit one or more of the key characteristics of carcinogens.