New method to assess mitophagy flux by flow cytometry

Quantitative imaging and semiotic phenotyping of mitochondrial network morphology in live human cells

The importance of mitochondria in tissue homeostasis, stress responses and human diseases, combined to their ability to transition between various structural and functional states, makes them excellent organelles for monitoring cell health. There is therefore a need for technologies to accurately analyze and quantify changes in mitochondrial organization in a variety of cells and cellular contexts. Here we present an innovative computerized method that enables accurate, multiscale, fast and cost-effective analysis of mitochondrial shape and network architecture from confocal fluorescence images by providing more than thirty features. In order to facilitate interpretation of the quantitative results, we introduced two innovations: the use of Kiviat-graphs (herein named MitoSpider plots) to present highly multidimensional data and visualization of the various mito-cellular configurations in the form of morphospace diagrams (called MitoSigils). We tested our fully automated image analysis tool on rich datasets gathered from live normal human skin cells cultured under basal conditions or exposed to specific stress including UVB irradiation and pesticide exposure. We demonstrated the ability of our proprietary software (named MitoTouch) to sensitively discriminate between control and stressed dermal fibroblasts, and between normal fibroblasts and other cell types (including cancer tissue-derived fibroblasts and primary keratinocytes), showing that our automated analysis captures subtle differences in morphology. Based on this novel algorithm, we report the identification of a protective natural ingredient that mitigates the deleterious impact of hydrogen peroxide (H2O2) on mitochondrial organization. Hence we conceived a novel wet-plus-dry pipeline combining cell cultures, quantitative imaging and semiotic analysis for exhaustive analysis of mitochondrial morphology in living adherent cells. Our tool has potential for broader applications in other research areas such as cell biology and medicine, high-throughput drug screening as well as predictive and environmental toxicology.

Melatonin-Loaded Nanoparticles Augment Mitophagy to Retard Parkinson's Disease

The molecular pathways that melatonin follows as a Parkinson's disease (PD) antagonist remain poorly elucidated, despite it being a safe and a potential neurotherapeutic drug with a few limitations such as less bioavailability, premature oxidation, brain delivery, etc. Here, we used a biocompatible protein (HSA) nanocarrier for the delivery of melatonin to the brain. This nanomelatonin showed better antioxidative and neuroprotective properties, and it not only improves mitophagy to remove unhealthy mitochondria but also improves mitochondrial biogenesis to counteract rotenone-induced toxicity in an in vitro PD model. We also showed BMI1, a member of the PRC1 complex that regulates mitophagy, whose protein expression was enhanced after nanomelatonin dosage. These effects were translated to a rodent model, where nanomelatonin improves the TH+ve neuron population in SNPC and protects against rotenone-mediated toxicity. Our findings highlight the significantly better in vitro and in vivo neuroprotective effect of nanomelatonin as well as the molecular/cellular dynamics it influences to regulate mitophagy as a measure of the potential therapeutic candidate for PD.

Fusion-fission-mitophagy cycling and metabolic reprogramming coordinate nerve growth factor (NGF)-dependent neuronal differentiation

Neuronal differentiation is regulated by nerve growth factor (NGF) and other neurotrophins. We explored the impact of NGF on mitochondrial dynamics and metabolism through time-lapse imaging, metabolomics profiling, and computer modeling studies. We show that NGF may direct differentiation by stimulating fission, thereby causing selective mitochondrial network fragmentation and mitophagy, ultimately leading to increased mitochondrial quality and respiration. Then, we reconstructed the dynamic fusion-fission-mitophagy cycling of mitochondria in a computer model, integrating these processes into a single network mechanism. Both the computational model and the simulations are able to reproduce the proposed mechanism in terms of mitochondrial dynamics, levels of reactive oxygen species (ROS), mitophagy, and mitochondrial quality, thus providing a computational tool for the interpretation of the experimental data and for future studies aiming to detail further the action of NGF on mitochondrial processes. We also show that changes in these mitochondrial processes are intertwined with a metabolic function of NGF in differentiation: NGF directs a profound metabolic rearrangement involving glycolysis, TCA cycle, and the pentose phosphate pathway, altering the redox balance. This metabolic rewiring may ensure: (a) supply of both energy and building blocks for the anabolic processes needed for morphological reorganization, as well as (b) redox homeostasis.

Impaired Removal of the Damaged Mitochondria in the Metabolic Memory Phenomenon Associated with Continued Progression of Diabetic Retinopathy

Retinopathy fails to halt even after diabetic patients in poor glycemic control try to institute tight glycemic control, suggesting a "metabolic memory" phenomenon, and the experimental models have demonstrated that mitochondria continue to be damaged/dysfunctional, fueling into the vicious cycle of free radicals. Our aim was to investigate the role of removal of the damaged mitochondria in the metabolic memory. Using human retinal endothelial cells (HRECs), incubated in 20 mM D-glucose for 4 days, followed by 5 mM D-glucose for 4 additional days, mitochondrial turnover, formation of mitophagosome, and mitophagy flux were evaluated. Mitophagy was confirmed in a rat model of metabolic memory where the rats were kept in poor glycemic control (blood glucose ~ 400 mg/dl) for 3 months soon after induction of streptozotocin-induced diabetes, followed by 3 additional months of good control (BG < 150 mg/dl). Reversal of high glucose by normal glucose had no effect on mitochondrial turnover and mitophagosome formation, and mitophagy flux remained compromised. Similarly, 3 months of good glycemic control in rats, which had followed 3 months of poor glycemic control, had no effect on mitophagy flux. Thus, poor turnover/removal of the damaged mitochondria, initiated during poor glycemic control, does not benefit from the termination of hyperglycemic insult, and the damaged mitochondria continue to produce free radicals, suggesting the importance of mitophagy in the metabolic memory phenomenon associated with the continued progression of diabetic retinopathy.

Cell death induction and intracellular vesicle formation in human colorectal cancer cells treated with Δ9-Tetrahydrocannabinol

BACKGROUND

Δ9-Tetrahydrocannabinol (Δ9-THC) is a principal psychoactive extract of Cannabis sativa and has been traditionally used as palliative medicine for neuropathic pain. Cannabidiol (CBD), an extract of hemp species, has recently attracted increased attention as a cancer treatment, but Δ9-THC is also requiring explored pharmacological application.

OBJECTIVE

This study evaluated the pharmacological effects of Δ9-THC in two human colorectal cancer cell lines. We investigated whether Δ9-THC treatment induces cell death in human colorectal cancer cells.

METHODS

We performed an MTT assay to determine the pharmacological concentration of Δ9-THC. Annxein V and Western blot analysis confirmed that Δ9-THC induced apoptosis in colorectal cancer cells. Metabolic activity was evaluated using MitoTracker staining and ATP determination. We investigated vesicle formation by Δ9-THC treatment using GW9662, known as a PPARγ inhibitor.

RESULTS

The MTT assay showed that treatment with 40 μM Δ9-THC and above inhibited the proliferation of colorectal cancer cells. Multiple intracytoplasmic vesicles were detected upon microscopic observation, and fluorescence-activated cell sorting analysis showed cell death via G1 arrest. Δ9-THC treatment increased the expression of cell death marker proteins, including p53, cleaved PARP-1, RIP1, and RIP3, suggesting that Δ9-THC induced the death of colorectal cancer cells. Δ9-THC treatment also reduced ATP production via changes in Bax and Bcl-2. Δ9-THC regulated intracytoplasmic vesicle formation by modulating the expression of PPARγ and clathrin, adding that antiproliferative activity of Δ9-THC was also affected.

CONCLUSION

In conclusion, Δ9-THC regulated two functional mechanisms, intracellular vesicle formation and cell death. These findings can help to determine how cannabinoids can be used most effectively to improve the efficacy of cancer treatment.

Mitophagy and mitochondrion-related expression profiles in response to physiological and pathological hypoxia in the corneal epithelium

To study the mitochondrial and cellular responses to physiological and pathological hypoxia, corneal epithelial cells were preconditioned under 21% O2, 8% O2 or 1% O2. The cell survival rate, mitochondrial fluorescence and mitophagy flux were quantified using flow cytometry. After RNA sequencing, gene set enrichment analysis (GSEA) was performed. When the oxygen level decreased from 21% to 8%, mitochondrial fluorescence decreased by 45% (p < 0.001), accompanied by an 80% increase in mitophagy flux (p < 0.001). When the oxygen level dropped to 1%, the cell survival rate and mitochondrial fluorescence decreased, while mitophagy flux further increased (each p < 0.001). Comparison of 1% O2 vs. 21% O2 revealed enrichment of the HYPOXIA hallmark. Most of the significantly enriched mitochondrion-related gene sets were involved in apoptosis. The corresponding foremost leading edge genes belonged to the BCL-2 family. Corneal epithelial cell fate decisions under hypoxia may involve noncanonical pathways of mitophagy.

Targeting mitochondrial degradation by chimeric autophagy-tethering compounds

The ability to regulate mitophagy in a living system with small molecules remains a great challenge. We hypothesize that adding fragments specific to the key autophagosome protein LC3 to mitochondria will mimic receptor-mediated mitophagy, thus engaging the autophagy-lysosome pathway to induce mitochondrial degradation. Herein, we develop a general biochemical approach to modulate mitophagy, dubbed mito-ATTECs, which employ chimera molecules composed of LC3-binding moieties linked to mitochondria-targeting ligands. Mito-ATTECs trigger mitophagy via targeting mitochondria to autophagosomes through direct interaction between mito-ATTECs and LC3 on mitochondrial membranes. Subsequently, autophagosomes containing mitochondria rapidly fuse with lysosomes to facilitate the degradation of mitochondria. Therefore, mito-ATTECs circumvent the detrimental effects related to disruption of mitochondrial membrane integrity by inducers routinely used to manipulate mitophagy, and provide a versatile biochemical approach to investigate the physiological roles of mitophagy. Furthermore, we found that sustained mitophagy lead to mitochondrial depletion and autophagic cell death in several malignant cell lines (lethal mitophagy). Among them, apoptosis-resistant malignant melanoma cell lines are particularly sensitive to lethal mitophagy. The therapeutic efficacy of mito-ATTECs has been further evaluated by using subcutaneous and pulmonary metastatic melanoma models. Together, the mitochondrial depletion achieved by mito-ATTECs may demonstrate the general concept of inducing cancer cell lethality through excessive mitochondrial clearance, establishing a promising therapeutic paradigm for apoptosis-resistant tumors.

Complementary Approaches to Interrogate Mitophagy Flux in Pancreatic β-Cells

Mitophagy is a quality control mechanism necessary to maintain optimal mitochondrial function. Dysfunctional β-cell mitophagy results in insufficient insulin release. Advanced quantitative assessments of mitophagy often require the use of genetic reporters. The mt-Keima mouse model, which expresses a mitochondria-targeted pH-sensitive dual-excitation ratiometric probe for quantifying mitophagy via flow cytometry, has been optimized in β-cells. The ratio of acidic-to-neutral mt-Keima wavelength emissions can be used to robustly quantify mitophagy. However, using genetic mitophagy reporters can be challenging when working with complex genetic mouse models or difficult-to-transfect cells, such as primary human islets. This protocol describes a novel complementary dye-based method to quantify β-cell mitophagy in primary islets using MtPhagy. MtPhagy is a pH-sensitive, cell-permeable dye that accumulates in the mitochondria and increases its fluorescence intensity when mitochondria are in low pH environments, such as lysosomes during mitophagy. By combining the MtPhagy dye with Fluozin-3-AM, a Zn2+ indicator that selects for β-cells, and Tetramethylrhodamine, ethyl ester (TMRE) to assess mitochondrial membrane potential, mitophagy flux can be quantified specifically in β-cells via flow cytometry. These two approaches are highly complementary, allowing for flexibility and precision in assessing mitochondrial quality control in numerous β-cell models.

Mitophagy in the retina: Viewing mitochondrial homeostasis through a new lens

Mitochondrial function is key to support metabolism and homeostasis in the retina, an organ that has one of the highest metabolic rates body-wide and is constantly exposed to photooxidative damage and external stressors. Mitophagy is the selective autophagic degradation of mitochondria within lysosomes, and can be triggered by distinct stimuli such as mitochondrial damage or hypoxia. Here, we review the importance of mitophagy in retinal physiology and pathology. In the developing retina, mitophagy is essential for metabolic reprogramming and differentiation of retina ganglion cells (RGCs). In basal conditions, mitophagy acts as a quality control mechanism, maintaining a healthy mitochondrial pool to meet cellular demands. We summarize the different autophagy- and mitophagy-deficient mouse models described in the literature, and discuss the potential role of mitophagy dysregulation in retinal diseases such as glaucoma, diabetic retinopathy, retinitis pigmentosa, and age-related macular degeneration. Finally, we provide an overview of methods used to monitor mitophagy in vitro, ex vivo, and in vivo. This review highlights the important role of mitophagy in sustaining visual function, and its potential as a putative therapeutic target for retinal and other diseases.

Regulatory roles of Osteopontin in lung epithelial inflammation and epithelial-telocyte interaction

BACKGROUND

Lung epithelial cells play important roles in lung inflammation and injury, although mechanisms remain unclear. Osteopontin (OPN) has essential roles in epithelial damage and repair and in lung cancer biological behaviours. Telocyte (TC) is a type of interstitial cell that interacts with epithelial cells to alleviate acute inflammation and lung injury. The present studies aim at exploring potential mechanisms by which OPN regulates the epithelial origin lung inflammation and the interaction of epithelial cells with TCs in acute and chronic lung injury.

METHODS

The lung disease specificity of OPN and epithelial inflammation were defined by bioinformatics. We evaluated the regulatory roles of OPN in OPN-knockdown or over-expressed bronchial epithelia (HBEs) challenged with cigarette smoke extracts (CSE) or in animals with genome OPN knockout (gKO) or lung conditional OPN knockout (cKO). Acute lung injury and chronic obstructive pulmonary disease (COPD) were induced by smoking or lipopolysaccharide (LPS). Effects of OPN on PI3K subunits and ERK were assessed using the inhibitors. Spatialization and distribution of OPN, OPN-positive epithelial subtypes, and TCs were defined by spatial transcriptomics. The interaction between HBEs and TCs was assayed by the co-culture system.

RESULTS

Levels of OPN expression increased in smokers, smokers with COPD, and smokers with COPD and lung cancer, as compared with healthy nonsmokers. LPS and/or CSE induced over-production of cytokines from HBEs, dependent upon the dysfunction of OPN. The severity of lung inflammation and injury was significantly lower in OPN-gKO or OPN-cKO mice. HBEs transferred with OPN enhanced the expression of phosphoinositide 3-kinase (PI3K)CA/p110α, PIK3CB/p110β, PIK3CD/p110δ, PIK3CG/p110γ, PIK3R1, PIK3R2 or PIK3R3. Spatial locations of OPN and OPN-positive epithelial subtypes showed the tight contact of airway epithelia and TCs. Epithelial OPN regulated the epithelial communication with TCs, and the down-regulation of OPN induced more alterations in transcriptomic profiles than the up-regulation.

CONCLUSION

Our data evidenced that OPN regulated lung epithelial inflammation, injury, and cell communication between epithelium and TCs in acute and chronic lung injury. The conditional control of lung epithelial OPN may be an alternative for preventing and treating epithelial-origin lung inflammation and injury.

Morin exhibits a neuroprotective effect in MPTP-induced Parkinson's disease model via TFEB/AMPK-mediated mitophagy

BACKGROUND

Parkinson's disease (PD) is one of the most common neurodegenerative diseases in the world. Mitophagy has been implicated in PD etiology for decades and its pharmacological activation is recognized as a promising treatment strategy for PD. For mitophagy initiation, low mitochondrial membrane potential (ΔΨm) is essential. We identified a natural compound morin that could induce mitophagy without affecting ΔΨm. Morin is a flavonoid that can be isolated from fruits like mulberry.

PURPOSE

To reveal the effect of morin on the PD mice model and their potential underlying molecular mechanism.

METHODS

Mitophagy process induced by morin in N2a cells meditation were measured using flow cytometry and immunofluorescence. JC-1 fluorescence dye used to detect the mitochondrial membrane potential (ΔΨm). The TFEB nuclear translocation were examined by immunofluorescence staining and western blot assay. The PD mice model was induced by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) intraperitoneal administration.

RESULTS

We found that morin also promoted nuclear translocation of the mitophagy regulator TFEB and activated the AMPK-ULK1 pathway. In MPTP-induced PD in vivo models, morin protected DA neurons from MPTP neurotoxicity and ameliorated behavioral deficit.

CONCLUSION

Although morin was previously reported to be neuroprotective in PD, the detailed molecular mechanisms remain to be elucidated. For the first time, we report morin served as a novel and safe mitophagy enhancer underlying AMPK-ULK1 pathway and exhibited anti-Parkinsonian effects indicating its potential as a clinical drug for PD treatment.

mTOR signaling inhibition decreases lysosome migration and impairs the success of Trypanosoma cruzi infection and replication in cardiomyocytes

Chagas disease is caused by the parasite Trypanosoma cruzi (T. cruzi) and, among all the chronic manifestations of the disease, Chronic Chagas Cardiomyopathy (CCC) is the most severe outcome. Despite high burden and public health importance in Latin America, there is a gap in understanding the molecular mechanisms that results in CCC development. Previous studies showed that T. cruzi uses the host machinery for infection and replication, including the repurposing of the responses to intracellular infection such as mitochondrial activity, vacuolar membrane, and lysosomal activation in benefit of parasite infection and replication. One common signaling upstream to many responses to parasite infection is mTOR pathway, previous associated to several downstream cellular mechanisms including autophagy, mitophagy and lysosomal activation. Here, using human iPSC derived cardiomyocytes (hiPSCCM), we show the mTOR pathway is activated in hiPSCCM after T. cruzi infection, and the inhibition of mTOR with rapamycin reduced number of T. cruzi 48 h post infection (hpi). Rapamycin treatment also reduced lysosome migration from nuclei region to cell periphery resulting in less T. cruzi inside the parasitophorous vacuole (PV) in the first hour of infection. In addition, the number of parasites leaving the PV to the cytoplasm to replicate in later times of infection was also lower after rapamycin treatment. Altogether, our data suggest that host's mTOR activation concomitant with parasite infection modulates lysosome migration and that T. cruzi uses this mechanism to achieve infection and replication. Modulating this mechanism with rapamycin impaired the success of T. cruzi life cycle independent of mitophagy.

Inhibition of ACSF2 protects against renal ischemia/reperfusion injury via mediating mitophagy in proximal tubular cells

Acute kidney injury (AKI) is a prevalent clinical condition caused by sepsis and ischemia reperfusion (IR) injury. The principal driver of IR-induced AKI involves renal tubular structural changes triggered by the impairment of function in renal tubular cells. The target gene, Acyl-CoA Synthetase Family Member 2 (ACSF2), was retrieved from the GEO database based on high specific expression in renal tubular cells and location in mitochondria. Here, we substantiate that ACSF2 is specifically localized in the mitochondria of the renal tubular epithelium. Functionally silencing ACSF2 in HK2 cells enhanced hypoxia-reoxygenation (HR)-induced mitophagy, restored mitochondrial function and decreased the production of mitochondrial superoxide. Our study demonstrated that these effects were reversed by silencing Bcl-2 19-kDa interacting protein 3 (BNIP3), a receptor regulating mitophagy. In vivo, ACSF2 knockdown significantly enhanced IR-induced mitophagy and improved renal function in mice with IR injury. Conversely, BNIP3 knockdown inhibited mitophagy and exacerbated renal damage in ACSF2-knockdown mice with IR injury. In conclusion, our study demonstrated that inhibition of ACSF2 enhances mitophagy, restoring mitochondrial function and protects against IR-induced AKI, providing a new target and potential strategy for therapy.

Carbon Monoxide Stimulates Both Mitophagy And Mitochondrial Biogenesis to Mediate Protection Against Oxidative Stress in Astrocytes

Astrocytes are key glial cells for the metabolic and functional support of the brain. Mitochondrial quality control (MQC), in particular the balance between mitophagy and mitochondrial biogenesis, is a major event for the maintenance of cellular homeostasis. Carbon monoxide (CO) is an endogenous gasotransmitter that inhibits cell death and inflammation by targeting mitochondria. It is well established that CO promotes cytoprotection by increasing mitochondrial population and metabolism (oxidative phosphorylation). Thus, it is hypothesized that CO-induced cytoprotection may also be mediated by the balance between mitophagy and mitochondrial biogenesis. Herein, the carbon monoxide releasing molecule-A1 (CORM-A1) was used in primary cultures of astrocytes to assess CO role on mitochondrial turnover. PINK1/Parkin-dependent mitophagy was stimulated by CORM-A1 following 1 h of treatment. While at 24 h after treatment, CORM-A1 increased mitochondrial population, which may indicate mitochondrial biogenesis. In fact, mitochondrial biogenesis was confirmed by the enhancement of PGC-1α expression that upregulates several mitochondrial transcription factors. Furthermore, inhibition of mitophagy by knocking down PINK1 expression reverted CO-induced mitochondrial biogenesis, indicating that mitochondrial turnover is dependent on modulation of mitophagy. Finally, CORM-A1 prevented astrocytic cell death induced by oxidative stress in a mitophagy-dependent manner. In fact, whenever PINK1 was knocked down, CORM-A1-induced cytoprotection was lost. In summary, CORM-A1 stimulates mitochondrial turnover, which in turn prevents astrocytic cell death. CO cytoprotection depends on increasing mitochondrial population and on eliminating dysfunctional mitochondria.

BNIP3L/NIX regulates both mitophagy and pexophagy

Mitochondria and peroxisomes are closely related metabolic organelles, both in terms of origin and in terms of function. Mitochondria and peroxisomes can also be turned over by autophagy, in processes termed mitophagy and pexophagy, respectively. However, despite their close relationship, it is not known if both organelles are turned over under similar conditions, and if so, how this might be coordinated molecularly. Here, we find that multiple selective autophagy pathways are activated upon iron chelation and show that mitophagy and pexophagy occur in a BNIP3L/NIX-dependent manner. We reveal that the outer mitochondrial membrane-anchored NIX protein, previously described as a mitophagy receptor, also independently localises to peroxisomes and drives pexophagy. We show this process happens in vivo, with mouse tissue that lacks NIX having a higher peroxisomal content. We further show that pexophagy is stimulated under the same physiological conditions that activate mitophagy, including cardiomyocyte and erythrocyte differentiation. Taken together, our work uncovers a dual role for NIX, not only in mitophagy but also in pexophagy, thus illustrating the interconnection between selective autophagy pathways.

Characterization of immune features and immunotherapy response in subtypes of hepatocellular carcinoma based on mitophagy

Mitophagy is suggested to be involved in tumor initiation and development; however, mitophagy heterogeneity in hepatocellular carcinoma (HCC) and its association with immune status and prognosis remain unclear. Differentially expressed genes (DEGs) were identified using expression profiles acquired from The Cancer Genome Atlas (TCGA). Mitophagy-related subtypes were identified using the ConsensusClusterPlus software. The differences in prognosis, clinical characteristics, and immune status, including immune cell infiltration, immune function, immune-checkpoint gene expression, and response to immunotherapy, were compared between subtypes. A mitophagy-related gene signature was constructed by applying least absolute shrinkage and selection operator regression to the TCGA cohort. The International Cancer Genome Consortium cohort and the cohort from Peking Union Medical College Hospital were utilized for validation. Carbonyl cyanide m-chlorophenylhydrazone was used to induce mitophagy in HCC cell lines to obtain our own mitophagy signature. Real-time polymerase chain reaction was used for the experimental validation of the expression of model genes. Two mitophagy-related subtypes with distinct prognoses, clinical characteristics, immune states, and biological function patterns were identified based on the mitophagy-related DEGs. The subtype that showed higher mitophagy-related DEG expression had worse survival outcomes, suppressed immune function, higher immune-checkpoint gene expression, and a better response to immunotherapy, indicating that this subpopulation in HCC may benefit from immune-checkpoint blockade therapy and other immunotherapies. A risk model consisting of nine mitophagy-related genes was constructed and its performance was confirmed in two validation cohorts. The risk score was an independent risk factor even when age, sex, and tumor stage were considered. Our study identified two distinct mitophagy subtypes and built a mitophagy signature, uncovering mitophagy heterogeneity in HCC and its association with immune status and prognosis. These findings shed light on the treatment of HCC, especially with immunotherapy.

Mitophagy: A novel perspective for insighting into cancer and cancer treatment

BACKGROUND

Mitophagy refers to the selective self-elimination of mitochondria under damaged or certain developmental conditions. As an important regulatory mechanism to remove damaged mitochondria and maintain the internal and external cellular balance, mitophagy plays pivotal roles in carcinogenesis and progression as well as treatment.

MATERIALS AND METHODS

Here, we combined data from recent years to comprehensively describe the regulatory mechanisms of mitophagy and its multifaceted significance in cancer, and discusse the potential of targeted mitophagy as a cancer treatment strategy.

RESULTS

The molecular mechanisms regulating mitophagy are complex, diverse, and cross-talk. Inducing or blocking mitophagy has the same or completely different effects in different cancer contexts. Mitophagy plays an indispensable role in regulating cancer metabolic reprogramming, cell stemness, and chemotherapy resistance for better adaptation to tumor microenvironment. In cancer cell biology, mitophagy is considered to be a double-edged sword. And to fully understand the role of mitophagy in cancer development can provide new targets for cancer treatment in clinical practice.

CONCLUSIONS

This review synthesizes a large body of data to comprehensively describe the molecular mechanisms of mitophagy and its multidimensional significance in cancer and cancer treatment, which will undoubtedly deepen the understanding of mitophagy.

Glial cell derived neurotrophic factor prevents western diet and palmitate-induced hepatocyte oxidative damage and death through SIRT3

Nonalcoholic fatty liver disease (NAFLD) is associated with increased oxidative stress that leads to hepatocyte and mitochondrial damage. In this study we investigated the mechanisms involved in the induction of oxidative stress and impairment of mitochondrial quality control and mitophagy in hepatocytes by the saturated fatty acid palmitate and Western diet feeding in mice and if their harmful effects could be reversed by the neurotrophic factor glial cell derived neurotrophic factor (GDNF). Western diet (WD)-feeding increased hepatic lipid peroxidation in control mice and, in vitro palmitate induced oxidative stress and impaired the mitophagic clearance of damaged mitochondria in hepatocytes. This was accompanied by reductions in hepatocyte sirtuin 3 (SIRT3) deacetylase activity, gene expression and protein levels as well as in superoxide dismutase enzyme activity. These reductions were reversed in the liver of Western diet fed GDNF transgenic mice and in hepatocytes exposed to palmitate in the presence of GDNF. We demonstrate an important role for Western diet and palmitate in inducing oxidative stress and impairing mitophagy in hepatocytes and an ability of GDNF to prevent this. These findings suggest that GDNF or its agonists may be a potential therapy for the prevention or treatment of NAFLD.

Validation of a new multiparametric protocol to assess viability, acrosome integrity and mitochondrial activity in cooled and frozen thawed boar spermatozoa

BACKGROUND

Motility, morphology, membrane integrity and DNA fragmentation are sperm characteristics routinely used to assess quality of boar spermatozoa. However, the evaluation of individual parameters has intrinsic restrictions in the estimation of potential fertility. Therefore, we aimed to validate a new multiparametric protocol to assess fertility potential through the evaluation of viability, acrosome integrity and mitochondrial activity within the same sperm population for cooled and frozen-thawed boar spermatozoa.

METHOD

Three multicolor protocols to assess viability, acrosome integrity and/or mitochondrial activity were compared for agreement containing two dyes (HM-panel; Hoechst 33342, MitoTracker™ Deep Red), three dyes (3-panel; SYBR®14, propidium iodide and lectin PNA-Alexa™ 647) or four dyes (4-panel; Hoechst 33342, lectin PNA-Alexa™ 488, propidium iodide and MitoTracker™ Deep Red). Cooled (n = 132) and frozen-thawed (n = 254) samples of boar spermatozoa were assessed by flow cytometry.

RESULTS

4-Panel enabled the detection of several sperm subpopulations based on plasma membrane integrity, acrosome status and mitochondrial activity in cooled and frozen-thawed spermatozoa. No significant differences were observed between 3-panel and 4-panel for the percentage of live, live-acrosome intact, and dead-acrosome reacted spermatozoa. However, the percentage of acrosome-intact spermatozoa was significantly higher in cooled samples when stained by 3-panel than 4-panel. Percentages of sperm parameters between protocols were strongly correlated, and agreement analysis demonstrated that both assays resulted in similar values for both sperm sample type.

CONCLUSION

Our results indicate that a four-color protocol is a practical, simple and reliable procedure to simultaneously evaluate boar sperm viability, acrosome integrity and mitochondrial activity under clinical conditions.

Hallmarks and Molecular Tools for the Study of Mitophagy in Parkinson’s Disease

The best-known hallmarks of Parkinson’s disease (PD) are the motor deficits that result from the degeneration of dopaminergic neurons in the substantia nigra. Dopaminergic neurons are thought to be particularly susceptible to mitochondrial dysfunction. As such, for their survival, they rely on the elaborate quality control mechanisms that have evolved in mammalian cells to monitor mitochondrial function and eliminate dysfunctional mitochondria. Mitophagy is a specialized type of autophagy that mediates the selective removal of damaged mitochondria from cells, with the net effect of dampening the toxicity arising from these dysfunctional organelles. Despite an increasing understanding of the molecular mechanisms that regulate the removal of damaged mitochondria, the detailed molecular link to PD pathophysiology is still not entirely clear. Herein, we review the fundamental molecular pathways involved in PINK1/Parkin-mediated and receptor-mediated mitophagy, the evidence for the dysfunction of these pathways in PD, and recently-developed state-of-the art assays for measuring mitophagy in vitro and in vivo.

Assessment of sperm mitochondrial activity by flow cytometry and fluorescent microscopy: a comparative study of mitochondrial fluorescent probes in bovine spermatozoa

CONTEXT

While conventional semen analysis is a simple, time-saving, and economical means to evaluate sperm quality, it leaves biochemical and metabolic characteristics of spermatozoa aside. To address this issue, the use of fluorescent probes assessing functional sperm parameters, such as JC-1, DiOC6 (3) and MitoTracker, has increased over the last decades. Apparently contradictory observations have nevertheless fostered an ongoing debate on their sensitivity and ability to evaluate the mitochondrial membrane potential (MMP) of sperm cells, thus warranting a re-examination of these probes.

AIMS

The present study aims to elucidate the suitability and sensitivity of each probe to evaluate the MMP of bovine spermatozoa by flow cytometry.

METHODS

Cryopreserved spermatozoa from ten bulls were thawed, stained with JC-1/SYTOXRed, DiOC6 (3)/propidium iodide (PI) or MitoTracker Deep Red (MTDR)/PI, and evaluated with flow cytometry and fluorescence microscopy.

KEY RESULTS

DiOC6 (3), JC-1 and MTDR can be simultaneously co-stained with a viability marker. The results of the present study support the ability of DiOC6 (3)/PI and JC-1/SYTOXRed, but not that of MTDR/PI, to monitor the MMP of spermatozoa.

CONCLUSIONS

JC-1/SYTOXRed assessed by flow cytometry was found to be the most sensitive and robust fluorescent probe to assess MMP. Moreover, DiOC6 (3)/PI could be a suitable alternative when the flow cytometer is not equipped with a red laser and/or an adequate optical filter.

IMPLICATIONS

Both DiOC6 (3) and JC-1, but not MTDR, could be used as probes to assess the mitochondrial membrane potential of bovine spermatozoa.

Tools and Biomarkers for the Study of Retinal Ganglion Cell Degeneration

The retina is part of the central nervous system, its analysis may provide an idea of the health and functionality, not only of the retina, but also of the entire central nervous system, as has been shown in Alzheimer’s or Parkinson’s diseases. Within the retina, the ganglion cells (RGC) are the neurons in charge of processing and sending light information to higher brain centers. Diverse insults and pathological states cause degeneration of RGC, leading to irreversible blindness or impaired vision. RGCs are the measurable endpoints in current research into experimental therapies and diagnosis in multiple ocular pathologies, like glaucoma. RGC subtype classifications are based on morphological, functional, genetical, and immunohistochemical aspects. Although great efforts are being made, there is still no classification accepted by consensus. Moreover, it has been observed that each RGC subtype has a different susceptibility to injury. Characterizing these subtypes together with cell death pathway identification will help to understand the degenerative process in the different injury and pathological models, and therefore prevent it. Here we review the known RGC subtypes, as well as the diagnostic techniques, probes, and biomarkers for programmed and unprogrammed cell death in RGC.

p38 MAPK priming boosts VSMC proliferation and arteriogenesis by promoting PGC1α-dependent mitochondrial dynamics

Vascular smooth muscle cell (VSMC) proliferation is essential for arteriogenesis to restore blood flow after artery occlusion, but the mechanisms underlying this response remain unclear. Based on our previous findings showing increased VSMC proliferation in the neonatal aorta of mice lacking the protease MT4-MMP, we aimed at discovering new players in this process. We demonstrate that MT4-MMP absence boosted VSMC proliferation in vitro in response to PDGF-BB in a cell-autonomous manner through enhanced p38 MAPK activity. Increased phospho-p38 in basal MT4-MMP-null VSMCs augmented the rate of mitochondrial degradation by promoting mitochondrial morphological changes through the co-activator PGC1α as demonstrated in PGC1α^−/− VSMCs. We tested the in vivo implications of this pathway in a novel conditional mouse line for selective MT4-MMP deletion in VSMCs and in mice pre-treated with the p38 MAPK activator anisomycin. Priming of p38 MAPK activity in vivo by the absence of the protease MT4-MMP or by anisomycin treatment led to enhanced arteriogenesis and improved flow recovery after femoral artery occlusion. These findings may open new therapeutic opportunities for peripheral vascular diseases.

Induction and Detection of Mitophagy

Mitophagy, a process of selective elimination of mitochondria by autophagy, is a mechanism of mitochondrial quality control that maintains mitochondrial network functionality. The elimination of damaged mitochondria through autophagy requires two steps: induction of general autophagy and priming of damaged mitochondria for selective autophagic recognition. Mitophagy impairment is linked to various pathologies; thus, removal of malfunctioning or even harmful mitochondria is vital to cellular physiology. Here, we describe methods that can be applied to the investigation of mitophagy.

Quantifying Regulated Mitochondrial Fission in Macrophages

Mitochondria have co-evolved with eukaryotic cells for more than a billion years, becoming an important cog in their machinery. They are best known for being tasked with energy generation through the production of adenosine triphosphate, but they also have roles in several other cellular processes, for example, immune and inflammatory responses. Mitochondria have important functions in macrophages, key innate immune cells that detect pathogens and drive inflammation. Mitochondrial activity is influenced by the highly dynamic nature of the mitochondrial network, which alternates between interconnected tubular and fragmented forms. The dynamic balance between this interconnected fused network and fission-mediated mitochondrial fragmentation modulates inflammatory responses such as production of cytokines and mitochondrial reactive oxygen species. Here we describe methods to differentiate mouse bone marrow cells into macrophages and the use of light microscopy, electron microscopy, flow cytometry, and Western blotting to quantify regulated mitochondrial dynamics in these differentiated macrophages.

Phenotypic Assay Leads to Discovery of Mitophagy Inducers with Therapeutic Potential for Parkinson's Disease

Mitophagy, the selective degradation of mitochondria by autophagy, involved in important physiological processes and defects in pathways has been reported in pathological conditions, such as neurodegeneration. Thus, mitophagy is an interesting target for drug discovery programs. In this investigation, we used robust phenotypic assay to screen a set of 50 small heterocyclic compounds to identify inducers of mitophagy. We identified two compounds, VP07 and JAR1.39, that induce Parkin-dependent mitophagy. Based on structure-activity relationship studies, we proposed the ability of the compounds to act as light chain 3 (LC3) interactors, similar to cardiolipin or ceramide, triggering mitophagy via Pink1/Parkin. Finally, we show promising therapeutic applicability in a cellular model of Parkinson's disease.

Decreased mitochondrial DNA copy number in nerve cells and the hippocampus during nicotine exposure is mediated by autophagy

Cigarette smoke is a harmful air pollutant and nicotine dependence is the essential cause of the tobacco epidemic. Since mitochondrial abnormalities are associated with substance addiction, in this work we used mitochondrial DNA (mtDNA) copy number as an indicator of mitochondrial function to investigate whether nicotine addicts also exhibit mitochondrial abnormalities. We found significantly lower mtDNA copy number in the peripheral blood of healthy nicotine addicts than in non-smokers, indicating that long-term nicotine exposure through smoking has detrimental effects on mitochondria. We also examined the effects of nicotine on mtDNA levels in a rat conditioned place preference (CPP) model of addiction and in cultured neuron cells, which revealed that the mtDNA copy number was significantly reduced in the hippocampus of CPP rats, in human neuroblastoma SH-SY5Y cells, and in rat pheochromocytoma PC12 cells, suggesting that significantly reduced mtDNA copy number is a potential biomarker of nicotine addiction. In SH-SY5Y cells, nicotine treatment induced several mitochondrial defects, such as increased mtDNA damage, increased reactive oxygen species (ROS) levels, decreased mitochondrial membrane potential (△Ψm), and stimulation of autophagic flux via transcriptional up-regulation of several autophagy-related genes and elevated marker protein accumulation, although genes controlling mtDNA replication were unaffected. In addition, pretreatment with the autophagy inhibitor Bafilomycin A1 led to accumulation of microtubule-associated protein 1 light chain 3b-II (LC3B-II) and counteracted the nicotine-induced decrease in mtDNA copy number. These results were recapitulated in PC12 cells, which also showed significant down-regulation of the marker SQSTM1/P62, suggesting that the decrease in mtDNA copy number is mediated by autophagy. This study shows that prolonged nicotine exposure, such as that in nicotine addicts, leads to a decrease of mtDNA copy number in neurons due to enhanced induction of autophagy. CAPSULE: It was found that smoking or nicotine exposure decreased mtDNA copy number based on population, animal, and cell models, and these effects appear to be mediated by autophagy.

AMPK protects against alcohol-induced liver injury through UQCRC2 to up-regulate mitophagy

Recent reports indicated that mitophagy protects against alcohol-induced liver injury, which helps remove damaged mitochondria to reduce the accumulation of reactive oxygen species (ROS). AMP-activated protein kinase (AMPK) has been recently used in ALD (alcoholic liver disease) and mitochondrial dysfunction research. However, the inner mechanism, whether AMPK can regulate mitophagy in ALD, remains unknown. Here we found that AMPK can significantly reduce alcohol-induced liver injury and enhances hepatocytes' mitophagy level. Next, we identified that AMPK rescued alcohol-induced low expression of UQCRC2 (ubiquinol-cytochrome c reductase core protein 2). Interestingly, UQCRC2 knockdown (KD) treatment causes impaired mitophagy, whereas UQCRC2 overexpression (OE) can significantly increase mitophagy to attenuate liver injury. Also, we identified that AMPK indirectly upregulates UQCRC2 protein level, and RNA-seq, chromatin immunoprecipitation (ChIP) assay, bioinformatics, and luciferase assays helped us understand that AMPK enhanced UQCRC2 gene transcription through activating NFE2L2/NRF2 (nuclear factor, erythroid 2 like 2). Our results demonstrate that AMPK regulating UQCRC2 is a significant mitochondrial event in mitophagy. It identifies a new signaling axis, AMPK-NFE2L2-UQCRC2, in the regulation of mitophagy levels in the liver, suggesting a possible therapeutic strategy to treat ALD.Abbreviations: AAV: AENO-associated virus; ALD: alcoholic liver disease; AMPK: AMP-activated protein kinase; BUN: blood urea nitrogen; H&E: hematoxylin and eosin; CCCP: carbonyl cyanide 3-chlorophenylhydrazone; ChIP: chromatin immunoprecipitation assay; CO-IP: co-immunoprecipitation; COPD: chronic obstructive pulmonary disease; EM: electron microscope; GOT1/AST: glutamic-oxaloacetic transaminase 1; GPT/ALT: glutamic-pyruvic transaminase; IF: immunofluorescence; IHC: immunohistochemistry; KD: knockdown; MAP1LC3/LC3: microtubule associated protein 1 light chain protein 3; MTDR: MitoTracker Deep Red; NFE2L2/NRF2: nuclear factor, erythroid 2 like 2; mtDNA: mitochondrial DNA; MTRC: MitoTracker Red CMXRos; OCR: Oxygen consumption rate; OE: overexpress; PINK1: PTEN induced kinase 1; qRT-PCR: quantitative real-time PCR; ROS: reactive oxygen species; SD: standard deviation; SOD2: superoxide dismutase 2; UQCRC2: ubiquinol-cytochrome c reductase core protein 2; WB: western blot; ΔΨ: mitochondrial membrane potential.

MAPK1/3 kinase-dependent ULK1 degradation attenuates mitophagy and promotes breast cancer bone metastasis

The function of mitophagy in cancer is controversial. ULK1 is critical for induction of macroautophagy/autophagy and has a more specific role in mitophagy in response to hypoxia. Here, we show that ULK1 deficiency induces an invasive phenotype of breast cancer cells under hypoxia and increases osteolytic bone metastasis. Mechanistically, ULK1 depletion attenuates mitophagy ability during hypoxia. As a result, the accumulation of damaged, ROS-generating mitochondria leads to activation of the NLRP3 inflammasome, which induces abnormal soluble cytokines secretion, then promotes the differentiation and maturation of osteoclasts, and ultimately results in bone metastasis. Notably, phosphorylation of ULK1 by MAPK1/ERK2-MAPK3/ERK1 kinase triggers its interaction with BTRC and subsequent K48-linked ubiquitination and proteasome degradation. Also, a clearly negative correlation between the expression levels of ULK1 and p-MAPK1/3 was observed in human breast cancer tissues. The MAP2K/MEK inhibitor trametinib is sufficient to restore mitophagy function via upregulation of ULK1, leading to inhibition of NLRP3 inflammasome activation, thereby reduces bone metastasis. These results indicate that ULK1 knockout-mediated mitophagy defect promotes breast cancer bone metastasis and provide evidence to explore MAP2K/MEK- MAPK1/3 pathway inhibitors for therapy, especially in cancers displaying low levels of ULK1.Abbreviations: ATG: autophagy-related; Baf A1: bafilomycin A₁; BTRC/β-TrCP: beta-transducin repeat containing E3 ubiquitin protein ligase; CHX: cycloheximide; CM: conditioned media; FBXW7/FBW7: F-box and WD repeat domain containing 7; MAPK1: mitogen-activated protein kinase 1; MTDR: MitoTracker Deep Red; mtROS: mitochondrial reactive oxygen species; microCT: micro-computed tomography; mtROS: mitochondrial reactive oxygen species; OCR: oxygen consumption rate; SQSTM1: sequestosome 1; ACP5/TRAP: acid phosphatase, tartrate resistant; ULK1: unc-51 like autophagy activating kinase 1.

Decreased PGC1β expression results in disrupted human erythroid differentiation, impaired hemoglobinization and cell cycle exit

Production of red blood cells relies on proper mitochondrial function, both for their increased energy demands during differentiation and for proper heme and iron homeostasis. Mutations in genes regulating mitochondrial function have been reported in patients with anemia, yet their pathophysiological role often remains unclear. PGC1β is a critical coactivator of mitochondrial biogenesis, with increased expression during terminal erythroid differentiation. The role of PGC1β has however mainly been studied in skeletal muscle, adipose and hepatic tissues, and its function in erythropoiesis remains largely unknown. Here we show that perturbed PGC1β expression in human hematopoietic stem/progenitor cells from both bone marrow and cord blood results in impaired formation of early erythroid progenitors and delayed terminal erythroid differentiation in vitro, with accumulations of polychromatic erythroblasts, similar to MDS-related refractory anemia. Reduced levels of PGC1β resulted in deregulated expression of iron, heme and globin related genes in polychromatic erythroblasts, and reduced hemoglobin content in the more mature bone marrow derived reticulocytes. Furthermore, PGC1β knock-down resulted in disturbed cell cycle exit with accumulation of erythroblasts in S-phase and enhanced expression of G1-S regulating genes, with smaller reticulocytes as a result. Taken together, we demonstrate that PGC1β is directly involved in production of hemoglobin and regulation of G1-S transition and is ultimately required for proper terminal erythroid differentiation.

Mitochondrial dysfunction associated with autophagy and mitophagy in cerebrospinal fluid cells of patients with delayed cerebral ischemia following subarachnoid hemorrhage

Decreased mitochondrial membrane potential in cerebrospinal fluid (CSF) was observed in patients with subarachnoid hemorrhage (SAH) accompanied by delayed cerebral ischemia (DCI). However, whether abnormal mechanisms of mitochondria are associated with the development of DCI has not been reported yet. Under cerebral ischemia, mitochondria can transfer into the extracellular space. Mitochondrial dysfunction can aggravate neurologic complications. The objective of this study was to evaluate whether mitochondrial dysfunction might be associated with autophagy and mitophagy in CSF cells to provide possible insight into DCI pathogenesis. CSF samples were collected from 56 SAH patients (DCI, n = 21; and non-DCI, n = 35). We analyzed CSF cells using autophagy and mitophagy markers (DAPK1, BNIP3L, BAX, PINK1, ULK1, and NDP52) via qRT-PCR and western blotting of proteins (BECN1, LC3, and p62). Confocal microscopy and immunogold staining were performed to demonstrate the differentially expression of markers within dysfunctional mitochondria. Significant induction of autophagic flux with accumulation of autophagic vacuoles, increased expression of BECN1, LC3-II, and p62 degradation were observed during DCI. Compared to non-DCI patients, DCI patients showed significantly increased mRNA expression levels (2^-ΔCt) of DAPK1, BNIP3L, and PINK1, but not BAX, ULK1, or NDP52. Multivariable logistic regression analysis revealed that Hunt and Hess grade ≥ IV (p = 0.023), DAPK1 (p = 0.003), and BNIP3L (p = 0.039) were related to DCI. Increased mitochondrial dysfunction associated with autophagy and mitophagy could play an important role in DCI pathogenesis.

Mitochondrial-derived vesicles compensate for loss of LC3-mediated mitophagy

Mitochondria are critical metabolic and signaling hubs, and dysregulated mitochondrial homeostasis is implicated in many diseases. Degradation of damaged mitochondria by selective GABARAP/LC3-dependent macro-autophagy (mitophagy) is critical for maintaining mitochondrial homeostasis. To identify alternate forms of mitochondrial quality control that functionally compensate if mitophagy is inactive, we selected for autophagy-dependent cancer cells that survived loss of LC3-dependent autophagosome formation caused by inactivation of ATG7 or RB1CC1/FIP200. We discovered rare surviving autophagy-deficient clones that adapted to maintain mitochondrial homeostasis after gene inactivation and identified two enhanced mechanisms affecting mitochondria including mitochondrial dynamics and mitochondrial-derived vesicles (MDVs). To further understand these mechanisms, we quantified MDVs via flow cytometry and confirmed an SNX9-mediated mechanism necessary for flux of MDVs to lysosomes. We show that the autophagy-dependent cells acquire unique dependencies on these processes, indicating that these alternate forms of mitochondrial homeostasis compensate for loss of autophagy to maintain mitochondrial health.

Urolithin A improves muscle function by inducing mitophagy in muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy, and despite advances in genetic and pharmacological disease-modifying treatments, its management remains a major challenge. Mitochondrial dysfunction contributes to DMD, yet the mechanisms by which this occurs remain elusive. Our data in experimental models and patients with DMD show that reduced expression of genes involved in mitochondrial autophagy, or mitophagy, contributes to mitochondrial dysfunction. Mitophagy markers were reduced in skeletal muscle and in muscle stem cells (MuSCs) of a mouse model of DMD. Administration of the mitophagy activator urolithin A (UA) rescued mitophagy in DMD worms and mice and in primary myoblasts from patients with DMD, increased skeletal muscle respiratory capacity, and improved MuSCs' regenerative ability, resulting in the recovery of muscle function and increased survival in DMD mouse models. These data indicate that restoration of mitophagy alleviates symptoms of DMD and suggest that UA may have potential therapeutic applications for muscular dystrophies.

Delineating the Role of Mitophagy Inducers for Alzheimer Disease Patients

Alzheimer’s disease (AD) is the most common cause of dementia in elderly that serves to be a formidable socio-economic and healthcare challenge in the 21^st century. Mitochondrial dysfunction and impairment of mitochondrial-specific autophagy, namely mitophagy, have emerged as important components of the cellular processes contributing to the development of AD pathologies, namely amyloid-β plaques (Aβ) and neurofibrillary tangles (NFT). Here, we highlight the recent advances in the association between impaired mitophagy and AD, as well as delineate the potential underlying mechanisms. Furthermore, we conduct a systematic review the current status of mitophagy modulators and analyzed their relevant mechanisms, evaluating on their advantages, limitations and current applications in clinical trials for AD patients. Finally, we describe how deep learning may be a promising method to rapid and efficient discovery of mitophagy inducers as well as general guidance for the workflow of the process.

The parkinsonian LRRK2 R1441G mutation shows macroautophagy-mitophagy dysregulation concomitant with endoplasmic reticulum stress

Autophagy is a mechanism responsible for the degradation of cellular components to maintain their homeostasis. However, autophagy is commonly altered and compromised in several diseases, including neurodegenerative disorders. Parkinson's disease (PD) can be considered a multifactorial disease because environmental factors, genetic factors, and aging are involved. Several genes are involved in PD pathology, among which the LRRK2 gene and its mutations, inherited in an autosomal dominant manner, are responsible for most genetic PD cases. The R1441G LRRK2 mutation is, after G2019S, the most important in PD pathogenesis. Our results demonstrate a relationship between the R1441G LRRK2 mutation and a mechanistic dysregulation of autophagy that compromises cell viability. This altered autophagy mechanism is associated with organellar stress including mitochondrial (which induces mitophagy) and endoplasmic reticulum (ER) stress, consistent with the fact that patients with this mutation are more vulnerable to toxins related to PD, such as MPP⁺.

Mitophagy: An Emerging Target in Ocular Pathology

Mitochondrial function is essential for the viability of aerobic eukaryotic cells, as mitochondria provide energy through the generation of adenosine triphosphate (ATP), regulate cellular metabolism, provide redox balancing, participate in immune signaling, and can initiate apoptosis. Mitochondria are dynamic organelles that participate in a cyclical and ongoing process of regeneration and autophagy (clearance), termed mitophagy specifically for mitochondrial (macro)autophagy. An imbalance in mitochondrial function toward mitochondrial dysfunction can be catastrophic for cells and has been characterized in several common ophthalmic diseases. In this article, we review mitochondrial homeostasis in detail, focusing on the balance of mitochondrial dynamics including the processes of fission and fusion, and provide a description of the mechanisms involved in mitophagy. Furthermore, this article reviews investigations of ocular diseases with impaired mitophagy, including Fuchs endothelial corneal dystrophy, primary open-angle glaucoma, diabetic retinopathy, and age-related macular degeneration, as well as several primary mitochondrial diseases with ocular phenotypes that display impaired mitophagy, including mitochondrial encephalopathy lactic acidosis stroke, Leber hereditary optic neuropathy, and chronic progressive external ophthalmoplegia. The results of various studies using cell culture, animal, and human tissue models are presented and reflect a growing awareness of mitophagy impairment as an important feature of ophthalmic disease pathology. As this review indicates, it is imperative that mitophagy be investigated as a targetable mechanism in developing therapies for ocular diseases characterized by oxidative stress and mitochondrial dysfunction.

Notoginsenoside R1 alleviates TEGDMA-induced mitochondrial apoptosis in preodontoblasts through activation of Akt/Nrf2 pathway-dependent mitophagy

Incomplete polymerization or biodegradation of dental resin materials results in the release of resin monomers such as triethylene glycol dimethacrylate (TEGDMA), causing severe injury of dental pulp cells. To date, there has been no efficient treatment option for this complication, in part due to the lack of understanding of the mechanism underlying these phenomena. Here, for the first time, we found that notoginsenoside R1 (NR1), a bioactive ingredient extracted from Panax notoginseng, exerted an obvious protective effect on TEGDMA-induced mitochondrial apoptosis in the preodontoblast mDPC6T cell line. In terms of the mechanism of action, NR1 enhanced the level of phosphorylated Akt (protein kinase B), resulting in the activation of a transcriptional factor, nuclear factor erythroid 2-related factor 2 (Nrf2), and eventually upregulating cellular ability to resist TEGDMA-related toxicity. Inhibiting the Akt/Nrf2 pathway by pharmaceutical inhibitors significantly decreased NR1-mediated cellular antioxidant properties and aggravated mitochondrial oxidative damage in TEGDMA-treated cells. Interestingly, NR1 also promoted mitophagy, which was identified as the potential downstream of the Akt/Nrf2 pathway. Blocking the Akt/Nrf2 pathway inhibited mitophagy and abolished the protection of NR1 on cells exposed to TEGDMA. In conclusion, these findings reveal that the activation of Akt/Nrf2 pathway-mediated mitophagy by NR1 might be a promising approach for preventing resin monomer-induced dental pulp injury.

Mutant HTT (huntingtin) impairs mitophagy in a cellular model of Huntington disease

The precise degradation of dysfunctional mitochondria by mitophagy is essential for maintaining neuronal homeostasis. HTT (huntingtin) can interact with numerous other proteins and thereby perform multiple biological functions within the cell. In this study, we investigated the role of HTT during mitophagy and analyzed the impact of the expansion of its polyglutamine (polyQ) tract. HTT is involved in different mitophagy steps, promoting the physical proximity of different protein complexes during the initiation of mitophagy and recruiting mitophagy receptors essential for promoting the interaction between damaged mitochondria and the nascent autophagosome. The presence of the polyQ tract in mutant HTT affects the formation of these protein complexes and determines the negative consequences of mutant HTT on mitophagy, leading to the accumulation of damaged mitochondria and an increase in oxidative stress. These outcomes contribute to general mitochondrial dysfunction and neurodegeneration in Huntington disease.Abbreviations: AMPK: AMP-activated protein kinase; ATG13: autophagy related 13; BECN1: beclin 1, autophagy related; BNIP3: BCL2/adenovirus E1B interacting protein 3; BNIP3L/Nix: BCL2/adenovirus E1B interacting protein 3-like; CCCP: carbonyl cyanide 3-chlorophenyl hydrazone; DMEM: Dulbecco's modified eagle medium; EDTA: ethylene-diamine-tetra-acetic acid; EGFP: enhanced green fluorescent protein; EGTA: ethylene glycol bis(2-aminoethyl ether)tetraacetic acid; FUNDC1: FUN14 domain containing 1; HD: Huntington disease; HRP: horseradish peroxidase; HTT: huntingtin; LC3-II: lipidated form of MAP1LC3/LC3; mtDNA: mitochondrial deoxyribonucleic acid; MTDR: MitoTracker Deep Red; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin kinase complex 1; NBR1: NBR1, autophagy cargo receptor; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; OCR: oxygen consumption rate; OPTN: optineurin; OXPHOS: oxidative phosphorylation; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PIK3R4/VPS15: phosphoinositide-3-kinase regulatory subunit 4; PINK1: PTEN induced putative kinase 1; PLA: proximity ligation assay; PMSF: phenylmethylsulfonyl fluoride; polyQ: polyglutamine; PtdIns3K: phosphatidylinositol 3-kinase; ROS: reactive oxygen species; Rot: rotenone; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SEM: standard error of the mean; SQSTM1/p62: sequestosome 1; TMRM: tetramethylrhodamine methyl ester; UB: ubiquitin; ULK1: unc-51 like kinase 1.

Towards Tissue-Specific Stem Cell Therapy for the Intervertebral Disc: PPARδ Agonist Increases the Yield of Human Nucleus Pulposus Progenitor Cells in Expansion

(1) Background: Low back pain (LBP) is often associated with intervertebral disc degeneration (IVDD). Autochthonous progenitor cells isolated from the center, i.e., the nucleus pulposus, of the IVD (so-called nucleus pulposus progenitor cells (NPPCs)) could be a future cell source for therapy. The NPPCs were also identified to be positive for the angiopoietin-1 receptor (Tie2). Similar to hematopoietic stem cells, Tie2 might be involved in peroxisome proliferator-activated receptor delta (PPARδ) agonist-induced self-renewal regulation. The purpose of this study was to investigate whether a PPARδ agonist (GW501516) increases the Tie2+ NPPCs’ yield within the heterogeneous nucleus pulposus cell (NPC) population. (2) Methods: Primary NPCs were treated with 10 µM of GW501516 for eight days. Mitochondrial mass was determined by microscopy, using mitotracker red dye, and the relative gene expression was quantified by qPCR, using extracellular matrix and mitophagy-related genes. (3) The NPC’s group treated with the PPARδ agonist showed a significant increase of the Tie2+ NPCs yield from ~7% in passage 1 to ~50% in passage two, compared to the NPCs vehicle-treated group. Furthermore, no significant differences were found among treatment and control, using qPCR and mitotracker deep red. (4) Conclusion: PPARδ agonist could help to increase the Tie2+ NPCs yield during NPC expansion.

The Potent Antioxidant MitoQ Protects Against Preeclampsia During Late Gestation but Increases the Risk of Preeclampsia When Administered in Early Pregnancy

Aims: Although preeclampsia (PE) has been attributed to excessive oxidative stress (OS) in the placenta, mild antioxidants failed to prevent PE in clinical trials. As mitochondria are a major source of OS, this study assessed the potential of a potent mitochondria-targeting antioxidant MitoQ in the prevention of PE. Results: Placentas from women with PE and from reduced uterine perfusion pressure (RUPP) mice demonstrated significantly higher OS, along with increased mitochondrial damage and compromised glutathione peroxidase (GPx) activities. MitoQ administration during late gestation alleviated RUPP-induced PE; whereas early-pregnancy MitoQ treatment not only exacerbated blood pressure, fetal growth restriction, and proteinuria but also reduced the labyrinth/spongiotrophoblast ratio and blood sinuses in the labyrinth. Invasion (Matrigel transwell) and migration (wound healing assay) of trophoblasts were greatly improved by 1 μM hydrogen peroxide (H₂O₂), but this improvement was abolished by MitoQ or MitoTempo. Mild OS enhanced the expression of miR-29b-3p, which regulates five genes involved in viability and mobility, in HTR8-S/Vneo cells. Innovation and Conclusions: Although the potent mitochondrial-targeting antioxidant MitoQ protects against hypertension and kidney damage induced by RUPP in mice when administered in late gestation, it exacerbates the PE-like phenotype when given in early gestation by interfering with placenta formation because mild OS is required to stimulate trophoblast proliferation, invasion, and migration. Eliminating trophoblastic OS during early pregnancy may lead to compromised placentation and a risk of diseases of placental origin. Therefore, antioxidant therapy for pregnant women should be carefully considered.

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)(1)

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field.

Aberrant mitochondrial morphology and function associated with impaired mitophagy and DNM1L-MAPK/ERK signaling are found in aged mutant Parkinsonian LRRK2R1441G mice

Mitochondrial dysfunction causes energy deficiency and nigrostriatal neurodegeneration which is integral to the pathogenesis of Parkinson disease (PD). Clearance of defective mitochondria involves fission and ubiquitin-dependent degradation via mitophagy to maintain energy homeostasis. We hypothesize that LRRK2 (leucine-rich repeat kinase 2) mutation disrupts mitochondrial turnover causing accumulation of defective mitochondria in aging brain. We found more ubiquitinated mitochondria with aberrant morphology associated with impaired function in aged (but not young) LRRK2^R1441G knockin mutant mouse striatum compared to wild-type (WT) controls. LRRK2^R1441G mutant mouse embryonic fibroblasts (MEFs) exhibited reduced MAP1LC3/LC3 activation indicating impaired macroautophagy/autophagy. Mutant MEFs under FCCP-induced (mitochondrial uncoupler) stress showed increased LC3-aggregates demonstrating impaired mitophagy. Using a novel flow cytometry assay to quantify mitophagic rates in MEFs expressing photoactivatable mito-PAmCherry, we found significantly slower mitochondria clearance in mutant cells. Specific LRRK2 kinase inhibition using GNE-7915 did not alleviate impaired mitochondrial clearance suggesting a lack of direct relationship to increased kinase activity alone. DNM1L/Drp1 knockdown in MEFs slowed mitochondrial clearance indicating that DNM1L is a prerequisite for mitophagy. DNM1L knockdown in slowing mitochondrial clearance was less pronounced in mutant MEFs, indicating preexisting impaired DNM1L activation. DNM1L knockdown disrupted mitochondrial network which was more evident in mutant MEFs. DNM1L-Ser616 and MAPK/ERK phosphorylation which mediate mitochondrial fission and downstream mitophagic processes was apparent in WT using FCCP-induced stress but not mutant MEFs, despite similar total MAPK/ERK and DNM1L levels. In conclusion, aberrant mitochondria morphology and dysfunction associated with impaired mitophagy and DNM1L-MAPK/ERK signaling are found in mutant LRRK2 MEFs and mouse brain.

Abbreviations: ATP: adenosine triphosphate; BAX: BCL2-associated X protein; CDK1: cyclin-dependent kinase 1; CDK5: cyclin-dependent kinase 5; CQ: chloroquine; CSF: cerebrospinal fluid; DNM1L/DRP1: dynamin 1-like; ELISA: enzyme-linked immunosorbent assay; FACS: fluorescence-activated cell sorting; FCCP: carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; LAMP2A: lysosomal-associated membrane protein 2A; LRRK2: leucine-rich repeat kinase 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAPK1/ERK2: mitogen-activated protein kinase 1; MEF: mouse embryonic fibroblast; MFN1: mitofusin 1; MMP: mitochondrial membrane potential; PAmCherry: photoactivatable-mCherry; PD: Parkinson disease; PINK1: PTEN induced putative kinase 1; PRKN/PARKIN: parkin RBR E3 ubiquitin protein ligase; RAB10: RAB10, member RAS oncogene family; RAF: v-raf-leukemia oncogene; SNCA: synuclein, alpha; TEM: transmission electron microscopy; VDAC: voltage-dependent anion channel; WT: wild type; SQSTM1/p62: sequestosome 1.

MAPK1/3 kinase-dependent ULK1 degradation attenuates mitophagy and promotes breast cancer bone metastasis

The function of mitophagy in cancer is controversial. ULK1 is critical for induction of macroautophagy/autophagy and has a more specific role in mitophagy in response to hypoxia. Here, we show that ULK1 deficiency induces an invasive phenotype of breast cancer cells under hypoxia and increases osteolytic bone metastasis. Mechanistically, ULK1 depletion attenuates mitophagy ability during hypoxia. As a result, the accumulation of damaged, ROS-generating mitochondria leads to activation of the NLRP3 inflammasome, which induces abnormal soluble cytokines secretion, then promotes the differentiation and maturation of osteoclasts, and ultimately results in bone metastasis. Notably, phosphorylation of ULK1 by MAPK1/ERK2-MAPK3/ERK1 kinase triggers its interaction with BTRC and subsequent K48-linked ubiquitination and proteasome degradation. Also, a clearly negative correlation between the expression levels of ULK1 and p-MAPK1/3 was observed in human breast cancer tissues. The MAP2K/MEK inhibitor trametinib is sufficient to restore mitophagy function via upregulation of ULK1, leading to inhibition of NLRP3 inflammasome activation, thereby reduces bone metastasis. These results indicate that ULK1 knockout-mediated mitophagy defect promotes breast cancer bone metastasis and provide evidence to explore MAP2K/MEK- MAPK1/3 pathway inhibitors for therapy, especially in cancers displaying low levels of ULK1.Abbreviations: ATG: autophagy-related; Baf A1: bafilomycin A₁; BTRC/β-TrCP: beta-transducin repeat containing E3 ubiquitin protein ligase; CHX: cycloheximide; CM: conditioned media; FBXW7/FBW7: F-box and WD repeat domain containing 7; MAPK1: mitogen-activated protein kinase 1; MTDR: MitoTracker Deep Red; mtROS: mitochondrial reactive oxygen species; microCT: micro-computed tomography; mtROS: mitochondrial reactive oxygen species; OCR: oxygen consumption rate; SQSTM1: sequestosome 1; ACP5/TRAP: acid phosphatase, tartrate resistant; ULK1: unc-51 like autophagy activating kinase 1.

The fluorescence toolbox for visualizing autophagy

Autophagy is an adaptive catabolic process functioning to promote cell survival in the event of inappropriate living conditions such as nutrient shortage and to cope with diverse cytotoxic insults. It is regarded as one of the key survival mechanisms of living organisms. Cells undergo autophagy to accomplish the lysosomal digestion of intracellular materials including damaged proteins, organelles, and foreign bodies, in a bulk, non-selective or a cargo-specific manner. Studies in the past decades have shed light on the association of autophagy pathways with various diseases and also highlighted the therapeutic value of autophagy modulation. Hence, it is crucial to develop effective approaches for monitoring intracellular autophagy dynamics, as a comprehensive account of methodology establishment is far from complete. In this review, we aim to provide an overview of the major current fluorescence-based techniques utilized for visualizing, sensing or measuring autophagic activities in cells or tissues, which are categorized firstly by targets detected and further by the types of fluorescence tools. We will mainly focus on the working mechanisms of these techniques, put emphasis on the insight into their roles in biomedical science and provide perspectives on the challenges and future opportunities in this field.

Correlative Light and Transmission Electron Microscopy Showed Details of Mitophagy by Mitochondria Quality Control in Propionic Acid Treated SH-SY5Y Cell

Propionic acid is a metabolite of the microbiome and can be transported to the brain. Previous data show that propionic acid changes mitochondrial biogenesis in SH-SY5Y cells and induces abnormal autophagy in primary hippocampal neurons. Maintaining mitochondrial function is key to homeostasis in neuronal cells, and mitophagy is the selective autophagy involved in regulating mitochondrial quality. Monitoring mitophagy though light microscopy or conventional transmission electron microscopy separately is insufficient because phases of mitophagy, including autophagosome and autolysosome in nano-resolution, are critical for studies of function. Therefore, we used correlative light and electron microscopy to investigate mitochondrial quality in SH-SY5Y cells after propionic acid treatment to use the advantages of both techniques. We showed, with this approach, that propionic acid induces mitophagy associated with mitochondrial quality.

Mitophagy impairment is involved in sevoflurane-induced cognitive dysfunction in aged rats

Postoperative cognitive dysfunction (POCD) is frequently observed in elderly patients following anesthesia, but its pathophysiological mechanisms have not been fully elucidated. Sevoflurane was reported to repress autophagy in aged rat neurons; however, the role of mitophagy, which is crucial for the control of mitochondrial quality and neuronal health, in sevoflurane-induced POCD in aged rats remains undetermined. Therefore, this study investigated whether mitophagy impairment is involved in sevoflurane-induced cognitive dysfunction. We found sevoflurane treatment inhibited mitochondrial respiration and mitophagic flux, changes in mitochondria morphology, impaired lysosomal acidification, and increased Tomm20 and deceased LAMP1 accumulation were observed in H4 cell and aged rat models. Rapamycin counteracted ROS induced by sevoflurane, restored mitophagy and improved mitochondrial function. Furthermore, rapamycin ameliorated the cognitive deficits observed in aged rats given sevoflurane anesthesia as determined by the Morris water maze test; this improvement was associated with an increased number of dendritic spines and pyramidal neurons. Overexpression of PARK2, but not mutant PARK2 lacking enzyme activity, in H4 cells decreased ROS and Tomm20 accumulation and reversed mitophagy dysfunction after sevoflurane treatment. These findings suggest that mitophagy dysfunction could be a mechanism underlying sevoflurane-induced POCD and that activating mitophagy may provide a new strategy to rescue cognitive deficits.

HIF-1α-BNIP3-mediated mitophagy in tubular cells protects against renal ischemia/reperfusion injury

In the present study, we hypothesized that hypoxia-inducible factor 1α (HIF-1α)-mediated mitophagy plays a protective role in ischemia/reperfusion (I/R)-induced acute kidney injury (AKI). Mitophagy was evaluated by measuring the changes of mitophagy flux, mitochondria DNA copy number, and the changes of mitophagy-related proteins including translocase of outer mitochondrial membrane 20 (TOMM20), cytochrome c oxidase IV (COX IV), microtubule-associated protein 1 light chain 3B (LC3B), and mitochondria adaptor nucleoporin p62 in HK2 cells, a human tubular cell line. Results show that HIF-1α knockout significantly attenuated hypoxia/reoxygenation (H/R)-induced mitophagy, aggravated H/R-induced apoptosis, and increased the production of reactive oxygen species (ROS). Similarly, H/R induced significantly increase in Bcl-2 19-kDa interacting protein 3 (BNIP3), a downstream regulator of HIF-1α. Notably, BNIP3 overexpression reversed the inhibitory effect of HIF-1α knockout on H/R-induced mitophagy, and prevented the enhancing effect of HIF-1α knockout on H/R-induced apoptosis and ROS production. For in vivo study, we established HIF-1αflox/flox; cadherin-16-cre mice in which tubular HIF-1α was specifically knockout. It was found that tubular HIF-1α knockout significantly inhibited I/R-induced mitophagy, and aggravated I/R-induced tubular apoptosis and kidney damage. In contrast, adenovirus-mediated BNIP3 overexpression significantly reversed the decreased mitophagy, and prevented enhanced kidney damage in tubular HIF-1α knockout mice with I/R injury. In summary, our study demonstrated that HIF-1α-BNIP3-mediated mitophagy in tubular cells plays a protective role through inhibition of apoptosis and ROS production in acute kidney damage.