NIX compensates lost role of parkin in cd-induced mitophagy in HeLa cells through phosphorylation

Elimination of damaged mitochondria during UVB-induced senescence is orchestrated by NIX-dependent mitophagy

Skin aging is the result of two types of aging, "intrinsic aging" an inevitable consequence of physiologic and genetically determined changes and "extrinsic aging," which is dependent on external factors such as exposure to sunlight, smoking, and dietary habits. UVB causes skin injury through the generation of free radicals and other oxidative byproducts, also contributing to DNA damage. Appearance and accumulation of senescent cells in the skin are considered one of the hallmarks of aging in this tissue. Mitochondria play an important role for the development of cellular senescence, in particular stress-induced senescence of human cells. However, many aspects of mitochondrial physiology relevant to cellular senescence and extrinsic skin aging remain to be unraveled. Here, we demonstrate that mitochondria damaged by UVB irradiation of human dermal fibroblasts (HDF) are eliminated by NIX-dependent mitophagy and that this process is important for cell survival under these conditions. Additionally, UVB-irradiation of human dermal fibroblasts (HDF) induces the shedding of extracellular vesicles (EVs), and this process is significantly enhanced in UVB-irradiated NIX-depleted cells. Our findings establish NIX as the main mitophagy receptor in the process of UVB-induced senescence and suggest the release of EVs as an alternative mechanism of mitochondrial quality control in HDF.

Crosstalk between mitochondrial biogenesis and mitophagy to maintain mitochondrial homeostasis

Mitochondrial mass and quality are tightly regulated by two essential and opposing mechanisms, mitochondrial biogenesis (mitobiogenesis) and mitophagy, in response to cellular energy needs and other cellular and environmental cues. Great strides have been made to uncover key regulators of these complex processes. Emerging evidence has shown that there exists a tight coordination between mitophagy and mitobiogenesis, and their defects may cause many human diseases. In this review, we will first summarize the recent advances made in the discovery of molecular regulations of mitobiogenesis and mitophagy and then focus on the mechanism and signaling pathways involved in the simultaneous regulation of mitobiogenesis and mitophagy in the response of tissue or cultured cells to energy needs, stress, or pathophysiological conditions. Further studies of the crosstalk of these two opposing processes at the molecular level will provide a better understanding of how the cell maintains optimal cellular fitness and function under physiological and pathophysiological conditions, which holds promise for fighting aging and aging-related diseases.

Effect of Electrical Stimulation on Disuse Muscular Atrophy Induced by Immobilization: Correlation With Upregulation of PERK Signal and Parkin-Mediated Mitophagy

OBJECTIVES

The aims of the study are to investigate the effect of electrical stimulation on disuse muscular atrophy induced by immobilization (IM) and to explore the role of PERK signal and Parkin-dependent mitophagy in this process.

DESIGN

In the first subexperiment, 24 rabbits were divided into four groups, which underwent different periods of IM. In the second subexperiment, 24 rabbits were divided into four groups on average in accordance with different kinds of interventions. To test the time-dependent changes of rectus femoris after IM, and to evaluate the effect of electrical stimulation, the wet weights, cross-sectional area and fat deposition of rectus femoris were assessed in this study, along with the protein levels of atrogin-1, p-PERK, Parkin, and COXIV.

RESULTS

The wet weights and cross-sectional area decreased, and the fat deposition increased in rectus femoris after IM, along with the elevated protein levels of atrogin-1, p-PERK, Parkin, and decreased protein levels of COXIV. The above histomorphological and molecular changes can be partially ameliorated by electrical stimulation.

CONCLUSIONS

Immobilization of unilateral lower limb could induce rectus femoris atrophy, which can be partially rectified by electrical stimulation. PERK signal and Parkin-mediated mitophagy may be the mechanisms by which electrical stimulation can play a significant role.

Role of Mitochondria-ER Contact Sites in Mitophagy

Mitochondria are often referred to as the "powerhouse" of the cell. However, this organelle has many more functions than simply satisfying the cells' metabolic needs. Mitochondria are involved in calcium homeostasis and lipid metabolism, and they also regulate apoptotic processes. Many of these functions require contact with the ER, which is mediated by several tether proteins located on the respective organellar surfaces, enabling the formation of mitochondria-ER contact sites (MERCS). Upon damage, mitochondria produce reactive oxygen species (ROS) that can harm the surrounding cell. To circumvent toxicity and to maintain a functional pool of healthy organelles, damaged and excess mitochondria can be targeted for degradation via mitophagy, a form of selective autophagy. Defects in mitochondria-ER tethers and the accumulation of damaged mitochondria are found in several neurodegenerative diseases, including Parkinson's disease and amyotrophic lateral sclerosis, which argues that the interplay between the two organelles is vital for neuronal health. This review provides an overview of the different mechanisms of mitochondrial quality control that are implicated with the different mitochondria-ER tether proteins, and also provides a novel perspective on how MERCS are involved in mediating mitophagy upon mitochondrial damage.

To eat or not to eat mitochondria? How do host cells cope with mitophagy upon bacterial infection?

Mitochondria fulfil a plethora of cellular functions ranging from energy production to regulation of inflammation and cell death control. The fundamental role of mitochondria makes them a target of choice for invading pathogens, with either an intracellular or extracellular lifestyle. Indeed, the modulation of mitochondrial functions by several bacterial pathogens has been shown to be beneficial for bacterial survival inside their host. However, so far, relatively little is known about the importance of mitochondrial recycling and degradation pathways through mitophagy in the outcome (success or failure) of bacterial infection. On the one hand, mitophagy could be considered as a defensive response triggered by the host upon infection to maintain mitochondrial homeostasis. However, on the other hand, the pathogen itself may initiate the host mitophagy to escape from mitochondrial-mediated inflammation or antibacterial oxidative stress. In this review, we will discuss the diversity of various mechanisms of mitophagy in a general context, as well as what is currently known about the different bacterial pathogens that have developed strategies to manipulate the host mitophagy.

Mitophagy and reactive oxygen species interplay in Parkinson's disease

Mitophagy impairment and oxidative stress are cardinal pathological hallmarks in Parkinson's disease (PD), a common age-related neurodegenerative condition. The specific interactions between mitophagy and reactive oxygen species (ROS) have attracted considerable attention even though their exact interplay in PD has not been fully elucidated. We highlight the interactions between ROS and mitophagy, with a focus on the signalling pathways downstream to ROS that triggers mitophagy and draw attention to potential therapeutic compounds that target these pathways in both experimental and clinical models. Identifying a combination of ROS inhibitors and mitophagy activators to provide a physiologic balance in this complex signalling pathways may lead to a more optimal outcome. Deciphering the exact temporal relationship between mitophagy and oxidative stress and their triggers early in the course of neurodegeneration can unravel mechanistic clues that potentially lead to the development of compounds for clinical drug trials focusing on prodromic PD or at-risk individuals.

Senegenin alleviates Aβ1-42 induced cell damage through triggering mitophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Senegenin (SEN), an active compound extracted from the traditional Chinese herb Polygala tenuifolia Willd. (a species in the genus Polygala, family Polygalaceae), could nourish neurons and resist neuronal damage in mouse models of Alzheimer's disease (AD). Amyloid-β (Aβ) depositions in neuronal cells may cause pathological changes such as oxidative stress which one return could cause severe damage to mitochondria in AD patients or animal models. Mitophagy is an important mechanism to selectively remove damaged mitochondria. In neurons, this process is mainly mediated by PTEN-induced putative kinase 1 (PINK1)/Parkin pathway. Previous studies have shown that SEN could reduce mitochondrial damage and inhibit apoptosis in neurons. Therefore, this study speculated that SEN might activate mitophagy to clear damaged mitochondria, thereby mitigating Aβ-induced cell damage in neuronal cells.

AIM OF THE STUDY

This study aimed to determine the effects of SEN on Aβ-induced cell damage, and further to explore whether SEN could induce mitophagy. Moreover, the regulatory role of mitophagy in the neuroptrotective effect of SEN would be elucidated.

MATERIALS AND METHODS

This study established an in vitro cell damage model using Aβ_1-42 to treat mouse hippocampal neuron HT22 cells. The effects of SEN on cell damage were determined by MTT assay and lactate dehydrogenase (LDH) release assay. Reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) were detected by Cytation™5 cell imaging microplate detection system. The apoptotic rate was analyzed by flow cytometry. The effects of SEN on mitophagy were detected by transmission electron microscope, immunofluorescence and immunoblotting.

RESULTS

Firstly, HT22 cells were treated with 30 μM Aβ_1-42 for 24 h to establish the damage model. It was found that 30 μM Aβ_1-42 caused neuronal damages as evidenced by reduced cell viability, increased LDH release and ROS, collapsed MMP and elevated apoptosis. Secondly, Aβ_1-42-incubated cells were treated with 10, 20, 40 and 60 μM SEN for 24 h. SEN significantly reduced the damage of Aβ_1-42-incubated cells as shown by recovered cell viability and MMP, reduced apoptosis and ROS. Notably, SEN induced the formation of mitophagosomes and mitolysosomes, and elevated the conversion of LC3 I to LC3 II. Moreover, SEN down-regulated the expression of p62, promoted the accumulation of full-length PINK1 and the translocation of Parkin to mitochondria, decreased the expression of mitochondrial matrix protein HSP60, thus activating the PINK1/Parkin-mediated mitophagy. However, when cells were pretreated with 5 μM CsA (Cyclosporine A, a mitophagy inhibitor) for 2 h and then co-treated with 20 and 40 μM SEN for 24 h, the protective effects of SEN were compromised.

CONCLUSIONS

The present study demonstrated that SEN could alleviate Aβ_1-42-induced cell damage through PINK1/Parkin-mediated mitophagy. Our findings justify the traditional use of P. tenuifolia in China with anti-aging or anti-neurodegenerative effects.

Long non-coding RNA Opa interacting protein 5-antisense RNA 1 promotes mitochondrial autophagy and protects SH-SY5Y cells from 1-methyl-4-phenylpyridine-induced damage by binding to microRNA-137 and upregulating NIX

Parkinson's disease (PD) is a leading cause of disability. Long noncoding RNA (LncRNA) OIP5-AS1 alleviates the accumulation and toxicity of 1-methyl-4-phenylpyridine (MPP⁺ )/-induced α-synuclein in human neuroblastoma SH-SY5Y cells, which may be involved in the pathological process of PD. This study explored the neuroprotective effect of lncRNA OIP5-AS1 on MPP⁺ /-induced SH-SY5Y cell model of PD, so as to provide a theoretical basis for PD treatment. The PD cell model was established (MPP⁺ group). The overexpression vector oe-OIP5-AS1 was constructed and transfected into MPP+/-induced SH-SY5Y cells, which were further transfected with miR-137 mimic or si-NIX plasmids. The localization of OIP5-AS1 and its binding sites with miR-137 were predicted by subcellular isolation and fluorescence in situ hybridization analysis. The targeting relationships between OIP5-AS1 and miR-137, and miR-137 and NIX were detected by dual-luciferase reporter assays. The mitochondrial membrane potential (Δψm) and total reactive oxygen species (ROS) levels, and expressions of α-synuclein, inflammatory cytokines, and microglia-activated chemokines, cell activity, and apoptosis were assessed. OIP5-AS1 was downregulated in MPP⁺ cells. After OIP5-AS1 overexpression, miR-137 was downregulated and NIX was upregulated in MPP⁺ cells, inflammatory factors and chemokines were downregulated. There were target relationships between OIP5-AS1 and miR-137, and miR-137 and NIX. After OIP5-AS1 overexpression, miR-137 overexpression or NIX downregulation inhibited mitochondrial autophagy and ROS levels and aggravated mitochondrial vacuolation; and partially reversed the effect of OIP5-AS1 overexpression on promoting mitochondrial autophagy and protection on MPP+ cells. Collectively, lncRNA OIP5-AS1 promoted NIX expression through competitively binding to miR-137, and promoted mitochondrial autophagy, thus protecting neurons from degeneration which might be seen in patients with PD.

Puerarin alleviates cadmium-induced mitochondrial mass decrease by inhibiting PINK1-Parkin and Nix-mediated mitophagy in rat cortical neurons

Cadmium (Cd) has well-known central nervous system toxicity, and mitochondria are direct targets of Cd-induced neuronal toxicity. However, how Cd induces mitochondrial mass decrease in terms of its neurotoxic effects remains unknown. Puerarin, an isoflavone extracted from kudzu root, can cross the blood-brain barrier and exert protective effects in nervous system disease. The purpose of the study was to determine the mechanism of Cd-induced mitochondrial mass decrease and the protective role of puerarin in rat cortical neurons. The results indicated that Cd induced mitochondrial mass decrease by activating mitophagy mediated by the PTEN-induced putative kinase protein 1 (PINK1)-E3 ubiquitin ligase (Parkin) and Nip3-like protein X (Nix) pathways in rat cortical neurons. Puerarin improved the Cd-induced decrease in mitochondrial membrane potential (MMP) in vitro, and blocked PINK1-Parkin and Nix-mediated mitophagy, inhibiting Cd-induced mitochondrial mass decrease in rat cortical neurons in vitro and in vivo. In summary, our data clearly indicated that puerarin protects rat cortical neurons against Cd-induced neurotoxicity by ameliorating mitochondrial damage, inhibiting mitophagy-mediated mitochondrial mass decrease. Puerarin appears to have great potential as a neuroprotective agent.

Molecular Mechanisms and Regulation of Mammalian Mitophagy

Mitochondria in the cell are the center for energy production, essential biomolecule synthesis, and cell fate determination. Moreover, the mitochondrial functional versatility enables cells to adapt to the changes in cellular environment and various stresses. In the process of discharging its cellular duties, mitochondria face multiple types of challenges, such as oxidative stress, protein-related challenges (import, folding, and degradation) and mitochondrial DNA damage. They mitigate all these challenges with robust quality control mechanisms which include antioxidant defenses, proteostasis systems (chaperones and proteases) and mitochondrial biogenesis. Failure of these quality control mechanisms leaves mitochondria as terminally damaged, which then have to be promptly cleared from the cells before they become a threat to cell survival. Such damaged mitochondria are degraded by a selective form of autophagy called mitophagy. Rigorous research in the field has identified multiple types of mitophagy processes based on targeting signals on damaged or superfluous mitochondria. In this review, we provide an in-depth overview of mammalian mitophagy and its importance in human health and diseases. We also attempted to highlight the future area of investigation in the field of mitophagy.

BNIP3L/NIX-mediated mitophagy: molecular mechanisms and implications for human disease

Mitophagy is a highly conserved cellular process that maintains the mitochondrial quantity by eliminating dysfunctional or superfluous mitochondria through autophagy machinery. The mitochondrial outer membrane protein BNIP3L/Nix serves as a mitophagy receptor by recognizing autophagosomes. BNIP3L is initially known to clear the mitochondria during the development of reticulocytes. Recent studies indicated it also engages in a variety of physiological and pathological processes. In this review, we provide an overview of how BNIP3L induces mitophagy and discuss the biological functions of BNIP3L and its regulation at the molecular level. We further discuss current evidence indicating the involvement of BNIP3L-mediated mitophagy in human disease, particularly in cancer and neurological disorders.

Dual role of cadmium in rat liver: Inducing liver injury and inhibiting the progression of early liver cancer

The heavy metal cadmium (Cd) can induce damage in liver and liver cancer cells; however, the mechanism underlying its toxicity needs to be further verified in vivo. We daily administered CdCl₂ to adult male rats at different dosages via gavage for 12 weeks and established rat liver injury model and liver cancer model to study the dual role of Cd in rat liver. Increased exposure to Cd resulted in abnormal liver function indicators, pathological degeneration, rat liver cell necrosis, and proliferation of collagen fibres. Using immunohistochemistry, we found that the area of GST-P-positive precancerous liver lesions decreased in a dose-dependent manner. Real-time quantitative polymerase chain reaction, western blot, immunohistochemistry, and transmission electron microscopy revealed that Cd induced mitophagy, as well as mitophagy blockade, as evidenced by the downregulation of TOMM20 and upregulation of LC3II and P62 with increasing Cd dose. Next, the expression of PINK1/Parkin, a classic signalling pathway protein that regulates mitophagy, was examined. Cd was found to promote PINK1/Parkin expression, which was proportional to the Cd dose. In conclusion, Cd activates PINK1/Parkin-mediated mitophagy in a dose-dependent manner. Mitophagy blockade likely aggravates Cd toxicity, leading to the dual role of inducing liver injury and inhibiting the progression of early liver cancer.

Low-dose cadmium exposure facilitates cell proliferation by promoter hypermethylation of RASSF1A and DAPK1 genes

Cadmium (Cd) at low concentrations has a potential to promote cell proliferation. However, the molecular mechanisms of Cd-induced proliferation are not well understood. Here, we reported that Cd (0-500 nM) significantly promoted the proliferation of HepG2 cells as demonstrated by elevated cell viability, more EdU-positive cells and increased gene expression of KI-67 and COX-2. Meanwhile, the gene expression of DNA methyltransferases was found to be elevated while that of tumor suppressor genes DAPK1 and RASSF1A were decreased under Cd exposure. Correspondingly, the methylation level of promoters in DAPK1 and RASSF1A were increased. Specifically, the CpG sites at -461 (Chr3:50, 374, 481) of RASSF1A promoter, and that at -260 (Chr9:90, 113, 207), -239 (Chr9:90, 113, 228), and -68 (Chr9:90, 113, 399) of DAPK1 promoter, were significantly hypermethylated. Moreover, 5-azacytidine (an inhibitor of DNA methyltransferase) partly impaired Cd-induced promoter hypermethylation of RASSF1A and DAPK1 genes, increased their expressions and slowed down Cd-induced cell proliferation, suggesting that DNA methylation play an essential part in Cd-boosted proliferation. The study showed that Cd caused promoter hypermethylation of RASSF1A and DAPK1, decreasing their expression and leading to higher level of cell proliferation. Furthermore, Cd at low concentrations could influence DNA methylation, which may serve as the proliferative mechanism of Cd.

The role of mitophagy in pulmonary sepsis

Sepsis is a systemic inflammatory disease with an unacceptably high mortality rate caused by an infection or trauma that involves both innate and adaptive immune systems. Inflammatory events activate different downstream pathways leading to tissue damage and ultimately multi-organ failure. Mitochondria are responsible for cellular energy, thermoregulation, metabolite biosynthesis, intracellular calcium regulation, and cell death. Damaged mitochondria induce the high Ca²⁺ influx through mitochondrial calcium uniporter (MCU). It also generates excessive Reactive oxygen species (ROS) and releases mtDNA into the cytoplasm, which causes induction of NLRP3 inflammasome and apoptosis. Mitophagy (Autophagy of damaged mitochondria) controls mitochondrial dynamics and function. It also maintains cellular homeostasis. This review is about how pulmonary sepsis affects the body. What is the aftermath of sepsis, and how mitophagy affects Acute Lung Injury and macrophage polarisation to overcome the damages.

Parkin: A targetable linchpin in human malignancies

Parkin, an E3 ubiquitin ligase has been found to be deregulated in a variety of human cancers. Our current understanding is endowed with strong evidences that Parkin plays crucial role in the pathogenesis of cancer by controlling/interfering with major hallmarks of cancer delineated till today. Consistent with the idea of mitophagy, the existing studies imitates the tumor suppressive potential of Parkin, resolved by its capacity to regulate cell proliferation, cell migration, angiogenesis, apoptosis and overall cellular survival. Dysfunction of Parkin has resulted in the loss of ubiquitination of cell cycle components followed by their accumulation leading to genomic instability, perturbed cell cycle and eventually tumor progression. In this review, we provide an overview of current knowledge about the critical role of Parkin in cancer development and progression and have focussed on its therapeutic implications highlighting the diagnostic and prognostic value of Parkin as a biomarker. We earnestly hope that an in-depth knowledge of Parkin will provide a linchpin to target in various cancers that will open a new door of clinical applications and therapeutics.

Targeting Mitochondrial Impairment in Parkinson's Disease: Challenges and Opportunities

The underlying pathophysiology of Parkinson's disease is complex, but mitochondrial dysfunction has an established and prominent role. This is supported by an already large and rapidly growing body of evidence showing that the role of mitochondrial (dys)function is central and multifaceted. However, there are clear gaps in knowledge, including the dilemma of explaining why inherited mitochondriopathies do not usually present with parkinsonian symptoms. Many aspects of mitochondrial function are potential therapeutic targets, including reactive oxygen species production, mitophagy, mitochondrial biogenesis, mitochondrial dynamics and trafficking, mitochondrial metal ion homeostasis, sirtuins, and endoplasmic reticulum links with mitochondria. Potential therapeutic strategies may also incorporate exercise, microRNAs, mitochondrial transplantation, stem cell therapies, and photobiomodulation. Despite multiple studies adopting numerous treatment strategies, clinical trials to date have generally failed to show benefit. To overcome this hurdle, more accurate biomarkers of mitochondrial dysfunction are required to detect subtle beneficial effects. Furthermore, selecting study participants early in the disease course, studying them for suitable durations, and stratifying them according to genetic and neuroimaging findings may increase the likelihood of successful clinical trials. Moreover, treatments involving combined approaches will likely better address the complexity of mitochondrial dysfunction in Parkinson's disease. Therefore, selecting the right patients, at the right time, and using targeted combination treatments, may offer the best chance for development of an effective novel therapy targeting mitochondrial dysfunction in Parkinson's disease.

Mitophagy: A New Player in Stem Cell Biology

The fundamental importance of functional mitochondria in the survival of most eukaryotic cells, through regulation of bioenergetics, cell death, calcium dynamics and reactive oxygen species (ROS) generation, is undisputed. However, with new avenues of research in stem cell biology these organelles have now emerged as signaling entities, actively involved in many aspects of stem cell functions, including self-renewal, commitment and differentiation. With this recent knowledge, it becomes evident that regulatory pathways that would ensure the maintenance of mitochondria with state-specific characteristics and the selective removal of organelles with sub-optimal functions must play a pivotal role in stem cells. As such, mitophagy, as an essential mitochondrial quality control mechanism, is beginning to gain appreciation within the stem cell field. Here we review and discuss recent advances in our knowledge pertaining to the roles of mitophagy in stem cell functions and the potential contributions of this specific quality control process on to the progression of aging and diseases.