Xinyang tablet alleviated cardiac dysfunction in a cardiac pressure overload model by regulating the receptor-interacting serum/three-protein kinase 3/FUN14 domain containing 1-mediated mitochondrial unfolded protein response and mitophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Xinyang tablet (XYT) has been used for heart failure (HF) for over twenty years in clinical practice, but the underlying molecular mechanism remains poorly understood.

AIMS OF THE STUDY

In the present study, we aimed to explore the protective effects of XYT in HF in vivo and in vitro.

MATERIALS AND METHODS

Transverse aortic constriction was performed in vivo to establish a mouse model of cardiac pressure overload. Echocardiography, tissue staining, and real-time quantitative PCR (qPCR) were examined to evaluate the protective effects of XYT on cardiac function and structure. Adenosine 5'-triphosphate production, reactive oxygen species staining, and measurement of malondialdehyde and superoxide dismutase was used to detect mitochondrial damage. Mitochondrial ultrastructure was observed by transmission electron microscope. Immunofluorescence staining, qPCR, and Western blotting were performed to evaluate the effect of XYT on the mitochondrial unfolded protein response and mitophagy, and to identify its potential pharmacological mechanism. In vitro, HL-1 cells and neonatal mouse cardiomyocytes were stimulated with Angiotensin II to establish the cell model. Western blotting, qPCR, immunofluorescence staining, and flow cytometry were utilized to determine the effects of XYT on cardiomyocytes. HL-1 cells overexpressing receptor-interacting serum/three-protein kinase 3 (RIPK3) were generated by transfection of RIPK3-overexpressing lentiviral vectors. Cells were then co-treated with XYT to determine the molecular mechanisms.

RESULTS

In the present study, XYT was found to exerta protective effect on cardiac function and structure in the pressure overload mice. And it was also found XYT reduced mitochondrial damage by enhancing mitochondrial unfolded protein response and restoring mitophagy. Further studies showed that XYT achieved its cardioprotective role through regulating the RIPK3/FUN14 domain containing 1 (FUNDC1) signaling. Moreover, the overexpression of RIPK3 successfully reversed the XYT-induced protective effects and significantly attenuated the positive effects on the mitochondrial unfolded protein response and mitophagy.

CONCLUSIONS

Our findings indicated that XYT prevented pressure overload-induced HF through regulating the RIPK3/FUNDC1-mediated mitochondrial unfolded protein response and mitophagy. The information gained from this study provides a potential strategy for attenuating mitochondrial damage in the context of pressure overload-induced heart failure using XYT.

ANKZF1 knockdown inhibits glioblastoma progression by promoting intramitochondrial protein aggregation through mitoRQC

Protein homeostasis is fundamental to the development of tumors. Ribosome-associated quality-control (RQC) is able to add alanine and threonine to the stagnant polypeptide chain C-terminal (CAT-tail) when protein translation is hindered, while Ankyrin repeat and zinc-finger domain-containing-protein 1 (ANKZF1) can counteract the formation of the CAT-tail, preventing the aggregation of polypeptide chains. In particular, ANKZF1 plays an important role in maintaining mitochondrial protein homeostasis by mitochondrial RQC (mitoRQC) after translation stagnation of precursor proteins targeting mitochondria. However, the role of ANKZF1 in glioblastoma is unclear. Therefore, the current study was aimed to investigate the effects of ANKZF1 in glioblastoma cells and a nude mouse glioblastoma xenograft model. Here, we reported that knockdown of ANKZF1 in glioblastoma cells resulted in the accumulation of CAT-tail in mitochondria, leading to the activated mitochondrial unfolded protein response (UPRmt) and inhibits glioblastoma malignant progression. Excessive CAT-tail sequestered mitochondrial chaperones HSP60, mtHSP70 and proteases LONP1 as well as mitochondrial respiratory chain subunits ND1, Cytb, mtCO2 and ATP6, leading to mitochondrial oxidative phosphorylation dysfunction, membrane potential impairment, and mitochondrial apoptotic pathway activation. Our study highlights ANKZF1 as a valuable target for glioblastoma intervention and provides an innovative insight for the treatment of glioblastoma through the regulating of mitochondrial protein homeostasis.

Mitochondrial perturbation in immune cells enhances cell-mediated innate immunity in Drosophila

BACKGROUND

Mitochondria participate in various cellular processes including energy metabolism, apoptosis, autophagy, production of reactive oxygen species, stress responses, inflammation and immunity. However, the role of mitochondrial metabolism in immune cells and tissues shaping the innate immune responses are not yet fully understood. We investigated the effects of tissue-specific mitochondrial perturbation on the immune responses at the organismal level. Genes for oxidative phosphorylation (OXPHOS) complexes cI-cV were knocked down in the fruit fly Drosophila melanogaster, targeting the two main immune tissues, the fat body and the immune cells (hemocytes).

RESULTS

While OXPHOS perturbation in the fat body was detrimental, hemocyte-specific perturbation led to an enhanced immunocompetence. This was accompanied by the formation of melanized hemocyte aggregates (melanotic nodules), a sign of activation of cell-mediated innate immunity. Furthermore, the hemocyte-specific OXPHOS perturbation induced immune activation of hemocytes, resulting in an infection-like hemocyte profile and an enhanced immune response against parasitoid wasp infection. In addition, OXPHOS perturbation in hemocytes resulted in mitochondrial membrane depolarization and upregulation of genes associated with the mitochondrial unfolded protein response.

CONCLUSIONS

Overall, we show that while the effects of mitochondrial perturbation on immune responses are highly tissue-specific, mild mitochondrial dysfunction can be beneficial in immune-challenged individuals and contributes to variation in infection outcomes among individuals.

The mitochondrial UPR induced by ATF5 attenuates intervertebral disc degeneration via cooperating with mitophagy

Intervertebral disc degeneration (IVDD) is an aging disease that results in a low quality of life and heavy socioeconomic burden. The mitochondrial unfolded protein response (UPRmt) take part in various aging-related diseases. Our research intents to explore the role and underlying mechanism of UPRmt in IVDD. Nucleus pulposus (NP) cells were exposed to IL-1β and nicotinamide riboside (NR) served as UPRmt inducer to treat NP cells. Detection of ATP, NAD + and NADH were used to determine the function of mitochondria. MRI, Safranin O-fast green and Immunohistochemical examination were used to determine the degree of IVDD in vivo. In this study, we discovered that UPRmt was increased markedly in the NP cells of human IVDD tissues than in healthy controls. In vitro, UPRmt and mitophagy levels were promoted in NP cells treated with IL-1β. Upregulation of UPRmt by NR and Atf5 overexpression inhibited NP cell apoptosis and further improved mitophagy. Silencing of Pink1 reversed the protective effects of NR and inhibited mitophagy induced by the UPRmt. In vivo, NR might attenuate the degree of IDD by activating the UPRmt in rats. In summary, the UPRmt was involved in IVDD by regulating Pink1-induced mitophagy. Mitophagy induced by the UPRmt might be a latent treated target for IVDD.

ULP-2 SUMO protease regulates UPRmt and mitochondrial homeostasis in Caenorhabditis elegans

Mitochondria are the powerhouses of cells, responsible for energy production and regulation of cellular homeostasis. When mitochondrial function is impaired, a stress response termed mitochondrial unfolded protein response (UPRmt) is initiated to restore mitochondrial function. Since mitochondria and UPRmt are implicated in many diseases, it is important to understand UPRmt regulation. In this study, we show that the SUMO protease ULP-2 has a key role in regulating mitochondrial function and UPRmt. Specifically, down-regulation of ulp-2 suppresses UPRmt and reduces mitochondrial membrane potential without significantly affecting cellular ROS. Mitochondrial networks are expanded in ulp-2 null mutants with larger mitochondrial area and increased branching. Moreover, the amount of mitochondrial DNA is increased in ulp-2 mutants. Downregulation of ULP-2 also leads to alterations in expression levels of mitochondrial genes involved in protein import and mtDNA replication, however, mitophagy remains unaltered. In summary, this study demonstrates that ULP-2 is required for mitochondrial homeostasis and the UPRmt.

ATF5-regulated Mitochondrial Unfolded Protein Response Attenuates Neuronal Damage in Epileptic Rat by Reducing Endoplasmic Reticulum Stress Through Mitochondrial ROS

Endoplasmic reticulum (ER) dysfunction caused by excessive ER stress is a crucial mechanism underlying seizures-induced neuronal injury. Studies have shown that mitochondrial reactive oxygen species (ROS) are closely related to ER stress, and our previous study showed that activating transcription factor 5 (ATF5)-regulated mitochondrial unfolded protein response (mtUPR) modulated mitochondrial ROS generation in a hippocampal neuronal culture model of seizures. However, the effects of ATF5-regulated mtUPR on ER stress and the underlying mechanisms remain uncertain in epilepsy. In this study, ATF5 upregulation by lentivirus infection attenuated seizures-induced neuronal damage and apoptosis in a rat model of pilocarpine-induced epilepsy, whereas ATF5 downregulation by lentivirus infection had the opposite effects. ATF5 upregulation potentiated mtUPR by increasing the expression of mitochondrial chaperone heat shock protein 60 (HSP60) and caseinolytic protease proteolytic subunit (ClpP) and reducing mitochondrial ROS generation in pilocarpine-induced seizures in rats. Additionally, upregulation of ATF5 reduced the expression of glucose-regulated protein 78 (GRP78), protein kinase RNA-like endoplasmic reticulum kinase (PERK), activating transcription factor 4 (ATF4), and C/EBP homologous protein (CHOP), suggesting suppression of ER stress; Moreover, ATF5 upregulation attenuated apoptosis-related proteins such as B-cell lymphoma-2 (BCL2) downregulation, BCL2-associated X (BAX) and cleaved-caspase-3 upregulation. However, ATF5 downregulation exerted the opposite effects. Furthermore, pretreatment with the mitochondria-targeted antioxidant mito-TEMPO attenuated the harmful effects of ATF5 downregulation on ER stress and neuronal apoptosis by reducing mitochondrial ROS generation. Overall, our study suggested that ATF5-regulated mtUPR exerted neuroprotective effects against pilocarpine-induced seizures in rats and the underlying mechanisms might involve mitochondrial ROS-mediated ER stress.

Mitochondrial defects triggered by amg-1 mutation elicit UPRmt and phagocytic clearance during spermatogenesis in C. elegans

Mitochondria are the powerhouses of many biological processes. During spermatogenesis, post-transcriptional regulation of mitochondrial gene expression is mediated by nuclear-encoded mitochondrial RNA-binding proteins (mtRBPs). We identified AMG-1 as an mtRBP required for reproductive success in Caenorhabditis elegans. amg-1 mutation led to defects in mitochondrial structure and sperm budding, resulting in mitochondria being discarded into residual bodies, which ultimately delayed spermatogenesis in the proximal gonad. In addition, mitochondrial defects triggered the gonadal mitochondrial unfolded protein response and phagocytic clearance to ensure spermatogenesis but ultimately failed to rescue hermaphroditic fertility. These findings reveal a previously undiscovered role for AMG-1 in regulating C. elegans spermatogenesis, in which mitochondrial-damaged sperm prevented the transmission of defective mitochondria to mature sperm by budding and phagocytic clearance, a process which may also exist in the reproductive systems of higher organisms.

Stress response mechanisms in protein misfolding diseases: Profiling a cellular model of Huntington's disease

Stress response pathways like the integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt) and the heat shock response (HSR) have emerged as part of the pathophysiology of neurodegenerative diseases, including Huntington's disease (HD) - a currently incurable disease caused by the production of mutant huntingtin (mut-Htt). Previous data from HD patients suggest that ISR is activated while UPRmt and HSR are impaired in HD. The study of these stress response pathways as potential therapeutic targets in HD requires cellular models that mimic the activation status found in HD patients of such pathways. PC12 cells with inducible expression of the N-terminal fragment of mut-Htt are among the most used cell lines to model HD, however the activation of stress responses remains unclear in this model. The goal of this study is to characterize the activation of ISR, UPRmt and HSR in this HD cell model and evaluate if it mimics the activation status found in HD patients. We show that PC12 HD cell model presents reduced levels of Hsp90 and mitochondrial chaperones, suggesting an impaired activation or function of HSR and UPRmt. This HD model also presents increased levels of phosphorylated eIF2α, the master regulator of the ISR, but overall similar levels of ATF4 and decreased levels of CHOP - transcription factors downstream to eIF2α - in comparison to control, suggesting an initial activation of ISR. These results show that this model mimics the ISR activation and the impaired UPRmt and HSR found in HD patients. This work suggests that the PC12 N-terminal HD model is suitable for studying the role of stress response pathways in the pathophysiology of HD and for exploratory studies investigating the therapeutic potential of drugs targeting stress responses.

Activating Transcription Factor 4-mediated Mitochondrial Unfolded Protein Response Alleviates Hippocampal Neuronal Damage in an In Vitro Model of Epileptiform Discharges

The mitochondrial unfolded protein response (mtUPR) has been shown to restore protein homeostasis and cell function under stress, and recent studies have confirmed that the activating transcription factor 4 (ATF4) regulates mtUPR. However, the role of ATF4-mediated mtUPR in a hippocampal neuronal culture model of seizures remains unclear. Our results showed that the expression of mtUPR-related proteins (HSP60 and CLpP) increased in primary hippocampal neurons with seizures induced by a magnesium-free solution, suggesting mtUPR activation. Furthermore, ATF4 overexpression by lentiviral vector transfection enhanced the expression of HSP60 and CLpP, whereas ATF4 low expression by lentiviral vector transfection weakened the expression of HSP60 and CLpP. In addition, ATF4 overexpression increased neuronal viability and reduced seizure-induced apoptosis. ATF4 overexpression reduced reactive oxygen species (ROS) production and improved mitochondrial membrane potential damage during seizures. Moreover, ATF4 overexpression reduced the BCL2-associated X protein (Bax) expression and increased the expression of B-cell lymphoma 2 (BCL2). In contrast, ATF4 expression showed the opposite trend. In conclusion, our results showed that ATF4-mediated mtUPR may delay the cascade activation of apoptotic pathways by reducing ROS-mediated oxidative stress, thereby attenuating seizure-induced stress injury.

Melatonin activates mitochondrial unfolded protein response to preserve osteogenic potential of senescent BMSCs via upregulating PDI-6

Bone marrow stromal cells (BMSCs) possess the capability to differentiate into osteogenic or adipogenic lineages. With aging, BMSCs suffer from mitochondrial dysfunction and undergo senescence, favoring adipogenesis at the expense of osteoblastogenesis. It leads to decreased bone formation and contributes to senile osteoporosis (SOP). In the current study, RNA-seq analysis unveiled that senescent BMSCs from mice exhibited a significant suppression in the expression of the protein disulfide isomerase PDI-6, an important regulator of mitochondrial unfolded protein response (UPR^mt) as well as maintenance of mitochondrial homeostasis. Overexpression of PDI-6 in senescent BMSCs partially rescued mitochondrial function and enhanced osteogenic differentiation. In contrast, osteoblastogenesis of BMSCs remarkably deteriorated under the condition of PDI-6 silencing. Furthermore, melatonin, an endocrine hormone, effectively enhanced PDI-6 expression and repaired injured mitochondria, and the effect of melatonin on PDI-6 expression was melatonin receptor dependent. We further identified that PDI-6 was a downstream effector of Wnt/β-catenin pathway, as the inhibitor of Wnt3A/TCF signaling, Wnt-C59, inhibited PDI-6 expression. Potential β-catenin-TCF/LEF binding sites on the promoter of PDI-6 gene were also validated by chromatin immunoprecipitation (ChIP) assay. Thus, our study suggests that PDI-6 is a pharmacological target of melatonin for the intervention of age-related osteoporosis via mitigating mitochondrial dysfunction in senescent BMSCs.

ATF5 Attenuates Apoptosis in Hippocampal Neurons with Seizures Evoked by Mg2+-Free Medium via Regulating Mitochondrial Unfolded Protein Response

The mitochondrial unfolded protein response (mtUPR)-a stress response pathway for maintaining protein homeostasis-is critical in seizures-induced neuronal injury. The activating transcription factor 5 (ATF5) regulates mtUPR; however, whether ATF5-regulated mtUPR has a role in neuronal injury in epilepsy remains uncertain. Here, we investigated the effects of ATF5-regulated mtUPR on neuronal injury in hippocampal neurons with seizures evoked by Mg²⁺-free medium. HSP60 and ClpP, key proteins of mtUPR, were upregulated, indicating mtUPR activation. ATF5 overexpression by lentiviral vector infection potentiated mtUPR, whereas ATF5 downregulation by lentiviral vector infection attenuated this response. Moreover, ATF5 overexpression elevated mitochondrial membrane potential and reduced reactive oxygen species (ROS) generation, suggesting that ATF5 overexpression protected mitochondrial homeostasis, while ATF5 downregulation had the opposite effect. ATF5 overexpression also reversed Bcl2 downregulation and Bax upregulation and attenuated seizures-induced neuronal apoptosis, while ATF5 downregulation aggravated the injury. Our study demonstrates that ATF5 attenuates seizures-induced neuronal injury, possibly by regulating mtUPR pathways, to prevent mitochondrial dysfunction.

Mitochondrial unfolded protein response in ischemia-reperfusion injury

Mitochondrial unfolded protein response (UPRmt) is a mitochondrial stress response that activates the transcriptional program of mitochondrial chaperone proteins and proteases to keep protein homeostasis in mitochondria. Ischemia-reperfusion injury results in multiple severe clinical issues linked to high morbidity and mortality in various disorders. The pathophysiology and pathogenesis of ischemia-reperfusion injury are complex and multifactorial. Emerging evidence showed the roles of UPRmt signaling in ischemia-reperfusion injury. Herein, we discuss the regulatory mechanisms underlying UPRmt signaling in C. elegans and mammals. Furthermore, we review the recent studies into the roles and mechanisms of UPRmt signaling in ischemia-reperfusion injury of the heart, brain, kidney, and liver. Further research of UPRmt signaling will potentially develop novel therapeutic strategies against ischemia-reperfusion injury.

Puerarin alleviates cadmium-induced rat neurocyte injury by alleviating Nrf2-mediated oxidative stress and inhibiting mitochondrial unfolded protein response

Cadmium (Cd) is a highly neurotoxic environmental pollutant. Puerarin (Pur) is a natural antioxidant isolated from Kudzu root that exhibits a powerful neuroprotective effect. Herein, we illustrated the mechanism underlying the protective effect of Pur on Cd-induced rat neurocyte injury in an in vivo rat model as well as in vitro using PC12 cells and primary rat cerebral cortical neurons. First, the results showed that Pur alleviated Cd-induced cerebral cortical pathological damage and decreased the viability of neurocytes. Furthermore, Cd activated the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway, which plays a negative role in Cd-induced rat neurocyte injury. In addition, Pur alleviated Cd-induced oxidative stress by enhancing antioxidant defense, reducing reactive oxygen species (ROS) accumulation and lipid peroxidation, and inhibiting activation of the Nrf2 signaling pathway in rat neurocytes. Moreover, Pur inhibited the Cd-induced mitochondrial unfolded protein response (UPRmt) in rat neurocytes. Overall, Pur alleviated Cd-induced rat neurocyte injury by alleviating Nrf2-mediated oxidative stress and inhibiting UPRmt.

The mitochondrial unfolded protein response: A multitasking giant in the fight against human diseases

Mitochondria, which serve as the energy factories of cells, are involved in cell differentiation, calcium homeostasis, amino acid and fatty acid metabolism and apoptosis. In response to environmental stresses, mitochondrial homeostasis is regulated at both the organelle and molecular levels to effectively maintain the number and function of mitochondria. The mitochondrial unfolded protein response (UPR^mt) is an adaptive intracellular stress mechanism that responds to stress signals by promoting the transcription of genes encoding mitochondrial chaperones and proteases. The mechanism of the UPR^mt in Caenorhabditis elegans (C. elegans) has been clarified over time, and the main regulatory factors include ATFS-1, UBL-5 and DVE-1. In mammals, the activation of the UPR^mt involves eIF2α phosphorylation and the uORF-regulated expression of CHOP, ATF4 and ATF5. Several additional factors, such as SIRT3 and HSF1, are also involved in regulating the UPR^mt. A deep and comprehensive exploration of the UPR^mt can provide new directions and strategies for the treatment of human diseases, including aging, neurodegenerative diseases, cardiovascular diseases and diabetes. In this review, we mainly discuss the function of UPR^mt, describe the regulatory mechanisms of UPR^mt in C. elegans and mammals, and summarize the relationship between UPR^mt and various human diseases.

Mitochondrial quality control in cardiac ischemia/reperfusion injury: new insights into mechanisms and implications

The current effective method for the treatment of myocardial infarction is timely restoration of the blood supply to the ischemic area of the heart. Although reperfusion is essential for reestablishing oxygen and nutrient supplies, it often leads to additional myocardial damage, creating an important clinical dilemma. Reports from long-term studies have confirmed that mitochondrial damage is the critical mechanism in cardiac ischemia/reperfusion (I/R) injury. Mitochondria are dynamic and possess a quality control system that targets mitochondrial quantity and quality by modifying mitochondrial fusion, fission, mitophagy, and biogenesis and protein homeostasis to maintain a healthy mitochondrial network. The system of mitochondrial quality control involves complex molecular machinery that is highly interconnected and associated with pathological changes such as oxidative stress, calcium overload, and endoplasmic reticulum (ER) stress. Because of the critical role of the mitochondrial quality control systems, many reports have suggested that defects in this system are among the molecular mechanisms underlying myocardial reperfusion injury. In this review, we briefly summarize the important role of the mitochondrial quality control in cardiomyocyte function and focus on the current understanding of the regulatory mechanisms and molecular pathways involved in mitochondrial quality control in cardiac I/R damage.

The mitochondrial unfolded protein response (UPRmt): shielding against toxicity to mitochondria in cancer

Mitochondria are essential for tumor growth and progression. However, the heavy demand for mitochondrial activity in cancer leads to increased production of mitochondrial reactive oxygen species (mtROS), accumulation of mutations in mitochondrial DNA, and development of mitochondrial dysfunction. If left unchecked, excessive mtROS can damage and unfold proteins in the mitochondria to an extent that becomes lethal to the tumor. Cellular systems have evolved to combat mtROS and alleviate mitochondrial stress through a quality control mechanism called the mitochondrial unfolded protein response (UPR^mt). The UPR^mt system is composed of chaperones and proteases, which promote protein folding or eliminate mitochondrial proteins damaged by mtROS, respectively. UPR^mt is conserved and activated in cancer in response to mitochondrial stress to maintain mitochondrial integrity and support tumor growth. In this review, we discuss how mitochondria become dysfunctional in cancer and highlight the tumor-promoting functions of key components of the UPR^mt.

Itaconate prolongs the healthy lifespan by activating UPRmt in Caenorhabditis elegans

Itaconate (ItA), a byproduct of the Krebs cycle, has recently emerged as an anti-inflammatory metabolite for inhibiting the overactive immune response. In addition to its immunomodulatory and antimicrobial effects, ItA may have other therapeutic avenues. Herein, the effect of ItA on aging was explored in order to better establish the therapeutic potential of this promising metabolite. ItA extended the lifespan and enhanced the stress resistance of Caenorhabditis elegans (C. elegans), even at the doses of 0.01 and 0.1 μM. Moreover, the lifespan extension effect of ItA was pronounced even for the aged worms (days 7 and 9 post adult stage). Furthermore, ItA was found to extend the healthy longevity of C. elegans in a mitochondria-dependent manner. ItA protected the mitochondrial integrity, increased ATP content, and decreased the reactive oxygen species (ROS) in C. elegans. Mechanistic investigations showed that ItA specifically activated the mitochondrial unfolded protein response (UPR^mt) in worms and significantly increased the expression of activating transcription factor associated with stress-1 (ATFS-1) that senses mitochondrial stress and communicates with the nucleus during the UPR^mt. ItA extended the lifespan of C. elegans in an ATFS-1-dependent manner. In summary, this study elucidates the molecular mechanism by which ItA extends the healthy lifespan and highlights the importance of mitochondrial integrity in the intervention of aging.

Insight into the mitochondrial unfolded protein response and cancer: opportunities and challenges

The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved protective transcriptional response that maintains mitochondrial proteostasis by inducing the expression of mitochondrial chaperones and proteases in response to various stresses. The UPR^mt-mediated transcriptional program requires the participation of various upstream signaling pathways and molecules. The factors regulating the UPR^mt in Caenorhabditis elegans (C. elegans) and mammals are both similar and different. Cancer cells, as malignant cells with uncontrolled proliferation, are exposed to various challenges from endogenous and exogenous stresses. Therefore, in cancer cells, the UPR^mt is hijacked and exploited for the repair of mitochondria and the promotion of tumor growth, invasion and metastasis. In this review, we systematically introduce the inducers of UPR^mt, the biological processes in which UPR^mt participates, the mechanisms regulating the UPR^mt in C. elegans and mammals, cross-tissue signal transduction of the UPR^mt and the roles of the UPR^mt in promoting cancer initiation and progression. Disrupting proteostasis in cancer cells by targeting UPR^mt constitutes a novel anticancer therapeutic strategy.

Lycopene mitigates DEHP-induced hepatic mitochondrial quality control disorder via regulating SIRT1/PINK1/mitophagy axis and mitochondrial unfolded protein response

Di (2-ethylhexyl) phthalate (DEHP) is a hazardous chemical which is used as a plasticizer in the plastic products. Lycopene (LYC) is a carotenoid that has protective roles against cellular damage in different organs. The present study sought to explore the role of the interaction between mitophagy and mitochondrial unfolded protein response (UPR^mt) in the LYC mitigating DEHP-induced hepatic mitochondrial quality control disorder. The mice were treated with LYC (5 mg/kg) and/or DEHP (500 or 1000 mg/kg). In our findings, LYC prevented DEHP-induced histopathological alterations including steatosis and fibrosis, and ultrastructural injuries including decreased mitochondrial membrane potential (ΔΨm) and mitochondria volume density. Furthermore, LYC alleviated DEHP-induced mitochondrial biogenesis disorder by suppressing SIRT1-PGC-1α axis, PINK1-mediated mitophagy and the activation of mitochondrial unfolded protein response (UPR^mt). This research suggested that LYC could prevent DEHP-induced hepatic mitochondrial quality control disorder via regulating SIRT1/PINK1/mitophagy axis and UPR^mt. The present study provided a current understanding about the potential implication of the SIRT1/PINK1/mitophagy axis and UPR^mt in LYC preventing DEHP-induced hepatic mitochondrial quality control disorder.

Inorganic Polyphosphate, Mitochondria, and Neurodegeneration

With an aging population, the presence of aging-associated pathologies is expected to increase within the next decades. Regrettably, we still do not have any valid pharmacological or non-pharmacological tools to prevent, revert, or cure these pathologies. The absence of therapeutical approaches against aging-associated pathologies can be at least partially explained by the relatively lack of knowledge that we still have regarding the molecular mechanisms underlying them, as well as by the complexity of their etiopathology. In fact, a complex number of changes in the physiological function of the cell has been described in all these aging-associated pathologies, including neurodegenerative disorders. Based on multiple scientific manuscripts produced by us and others, it seems clear that mitochondria are dysfunctional in many of these aging-associated pathologies. For example, mitochondrial dysfunction is an early event in the etiopathology of all the main neurodegenerative disorders, and it could be a trigger of many of the other deleterious changes which are present at the cellular level in these pathologies. While mitochondria are complex organelles and their regulation is still not yet entirely understood, inorganic polyphosphate (polyP) could play a crucial role in the regulation of some mitochondrial processes, which are dysfunctional in neurodegeneration. PolyP is a well-preserved biopolymer; it has been identified in every organism that has been studied. It is constituted by a series of orthophosphates connected by highly energetic phosphoanhydride bonds, comparable to those found in ATP. The literature suggests that the role of polyP in maintaining mitochondrial physiology might be related, at least partially, to its effects as a key regulator of cellular bioenergetics. However, further research needs to be conducted to fully elucidate the molecular mechanisms underlying the effects of polyP in the regulation of mitochondrial physiology in aging-associated pathologies, including neurodegenerative disorders. With a significant lack of therapeutic options for the prevention and/or treatment of neurodegeneration, the search for new pharmacological tools against these conditions has been continuous in past decades, even though very few therapeutic approaches have shown potential in treating these pathologies. Therefore, increasing our knowledge about the molecular mechanisms underlying the effects of polyP in mitochondrial physiology as well as its metabolism could place this polymer as a promising and innovative pharmacological target not only in neurodegeneration, but also in a wide range of aging-associated pathologies and conditions where mitochondrial dysfunction has been described as a crucial component of its etiopathology, such as diabetes, musculoskeletal disorders, and cardiovascular disorders.

BI-1 ameliorates myocardial injury by activating the mitochondrial unfolded protein response and FUNDC1-related mitophagy in cardiorenal syndrome type 3

It has been suggested that mitochondrial dysfunction underlies the myocardial injury seen following cardiorenal syndrome type 3 (CRS-3). Both mitophagy and the mitochondrial unfolded protein response (UPR^mt) are protective programs that preserve mitochondrial homeostasis. Here, we explored whether Bax inhibitor-1 (BI-1) overexpression attenuates CRS-3-related myocardial injury through activation of mitophagy and the UPR^mt in cardiomyocytes. Following CRS-3 induction via renal ischemia-reperfusion injury, BI-1 transgenic (BI1^TG) mice showed greater preservation of myocardial integrity and relaxation function and less cardiomyocyte apoptosis than wild-type (WT) mice. Moreover, BI-1 overexpression attenuated CRS-3-mediated myocardial inflammation, as indicated by decreased MCP-1 and IL-6 expression and normalized ATP production in cardiomyocytes. After CRS-3 induction, mitophagy was inhibited in cardiomyocytes from WT mice, as indicated by both decreased Fundc1 transcription and mt-Keima fluorescence, and modest activation of the UPR^mt, denoted by a slight increase in Atf6 mRNA levels. By contrast, activation of mitophagy and marked UPR^mt upregulation were observed in cardiac tissue from BI1^TG mice. shRNA-mediated silencing of Fundc1 or Atf6 greatly impaired mitochondrial metabolism and survival in cultured cardiomyocytes overexpressing BI-1. Thus, upregulation of BI-1 expression aimed at activating mitophagy and the UPR^mt may represent a useful therapeutic approach for the treatment of CRS-3.

Copper exposure induces mitochondrial dynamic disorder and oxidative stress via mitochondrial unfolded protein response in pig fundic gland

Cu is a metallic element that widely spread over in the environment, which have raised wide concerns about the potential toxic effects and public health threat. The objective of this study aimed to investigate the impression of copper (Cu)-triggered toxicity on mitochondrial dynamic, oxidative stress, and unfolded protein response (UPRmt) in fundic gland of pigs. Weaned pigs were randomly distributed into three groups, fed with different Cu of 10 mg/kg (control group), 125 mg/kg (group I), and 250 mg/kg (group Ⅱ). The trial persisted for 80 days and the fundic gland tissues were collected for further researches. Moreover, the markers participated to mitochondrial dynamic, UPRmt，and oxidative stress in fundic gland were determined. Results revealed that vacuolar degeneration were observed in the treated groups contrast with control group, and the Cu level was boosted with the increasing intake of Cu. Besides that, the levels of CAT, TRX, H2O2, and G6PDH were reduced in group Ⅰ and group Ⅱ, the mRNA levels of NRF2, HO-1, SOD-1, CAT, SOD-2, GSR, GPX1, GPX4, and TRX in the treated groups were promoted contrast to control group. Furthermore, the protein expression of KEAP1 was dramatically decreased, and the protein expression of NRF2, TRX and HO-1 were markedly enhanced in group Ⅰ and Ⅱ at 80 days. Moreover, the mRNA and protein expression levels of MFN1, MFN2, and OPA1 down-regulated and protein level of DRP1 was increased with the adding levels of Cu. Nevertheless, the UPRmt-related mRNA levels of CLPP, HTRA-2, CHOP, HSP10, and HSP60 were enhanced dramatically in Cu treatment group compared with control group. In general, our current study demonstrated that excessive absorption of Cu in fundic gland were related with stimulating UPRmt, oxidative stress, and the NRF2 interceded antioxidant defense. These results could afford an updated evidence on molecular theory of Cu-invited toxicity.

SIRT3-mediated mitochondrial unfolded protein response weakens breast cancer sensitivity to cisplatin

BACKGROUND

Mitochondrial unfolded protein response plays an important role in the occurrence and development of breast cancer. However, the role of mitochondrial unfolded protein response (UPR^mt) in the sensitivity of breast cancer to cisplatin chemotherapy has not yet been cleared.

OBJECTIVES

The purpose of this study is to explore the role of mitochondrial unfolded protein response in breast cancer sensitivity to cisplatin.

METHODS

In this study, qRT-PCR, Western blotting, Immunofluorescence, CCK-8, Colony formation, Transwell assay and TUNEL staining assay were used to confirm the role of UPR^mt in breast cancer cells treated with cisplatin.

RESULTS

Cisplatin increased the levels of UPR^mt including CLPP, HSP60, LONP1 in MCF7 and MDA-MB-231 cells. UPR^mt inducer Nicotinamide ribose (NR) could promote the proliferation and invasion of breast cancer cells treated with cisplatin. Importantly, SIRT3 was discovered to increase UPR^mt in breast cancer cells and silencing of SIRT3 could inhibit the effect of NR in breast cancer.

CONCLUSIONS

UPR^mt regulated by SIRT3 could protect breast cancer cell from cisplatin. Controlling SIRT3-induced UPR may be a potential therapeutic target to increase the sensitivity of breast cancer chemotherapy.

Roles of mitochondrial unfolded protein response in mammalian stem cells

The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved adaptive mechanism for improving cell survival under mitochondrial stress. Under physiological and pathological conditions, the UPR^mt is the key to maintaining intracellular homeostasis and proteostasis. Important roles of the UPR^mt have been demonstrated in a variety of cell types and in cell development, metabolism, and immune processes. UPR^mt dysfunction leads to a variety of pathologies, including cancer, inflammation, neurodegenerative disease, metabolic disease, and immune disease. Stem cells have a special ability to self-renew and differentiate into a variety of somatic cells and have been shown to exist in a variety of tissues. These cells are involved in development, tissue renewal, and some disease processes. Although the roles and regulatory mechanisms of the UPR^mt in somatic cells have been widely reported, the roles of the UPR^mt in stem cells are not fully understood. The roles and functions of the UPR^mt depend on stem cell type. Therefore, this paper summarizes the potential significance of the UPR^mt in embryonic stem cells, tissue stem cells, tumor stem cells, and induced pluripotent stem cells. The purpose of this review is to provide new insights into stem cell differentiation and tumor pathogenesis.

Mitophagy coordinates the mitochondrial unfolded protein response to attenuate inflammation-mediated myocardial injury

Mitochondrial dysfunction is a fundamental challenge in septic cardiomyopathy. Mitophagy and the mitochondrial unfolded protein response (UPR^mt) are the predominant stress-responsive and protective mechanisms involved in repairing damaged mitochondria. Although mitochondrial homeostasis requires the coordinated actions of mitophagy and UPR^mt, their molecular basis and interactive actions are poorly understood in sepsis-induced myocardial injury. Our investigations showed that lipopolysaccharide (LPS)-induced sepsis contributed to cardiac dysfunction and mitochondrial damage. Although both mitophagy and UPR^mt were slightly activated by LPS in cardiomyocytes, their endogenous activation failed to prevent sepsis-mediated myocardial injury. However, administration of urolithin A, an inducer of mitophagy, obviously reduced sepsis-mediated cardiac depression by normalizing mitochondrial function. Interestingly, this beneficial action was undetectable in cardiomyocyte-specific FUNDC1 knockout (FUNDC1^CKO) mice. Notably, supplementation with a mitophagy inducer had no impact on UPR^mt, whereas genetic ablation of FUNDC1 significantly upregulated the expression of genes related to UPR^mt in LPS-treated hearts. In contrast, enhancement of endogenous UPR^mt through oligomycin administration reduced sepsis-mediated mitochondrial injury and myocardial dysfunction; this cardioprotective effect was imperceptible in FUNDC1^CKO mice. Lastly, once UPR^mt was inhibited, mitophagy-mediated protection of mitochondria and cardiomyocytes was partly blunted. Taken together, it is plausible that endogenous UPR^mt and mitophagy are slightly activated by myocardial stress and they work together to sustain mitochondrial performance and cardiac function. Endogenous UPR^mt, a downstream signal of mitophagy, played a compensatory role in maintaining mitochondrial homeostasis in the case of mitophagy inhibition. Although UPR^mt activation had no negative impact on mitophagy, UPR^mt inhibition compromised the partial cardioprotective actions of mitophagy. This study shows how mitophagy modulates UPR^mt to attenuate inflammation-related myocardial injury and suggests the potential application of mitophagy and UPR^mt targeting in the treatment of myocardial stress.

•

Mitochondrial dysfunction is a fundamental challenge in septic cardiomyopathy.

•

LPS-induced sepsis contributes to cardiac dysfunction and mitochondrial damage.

•

Endogenous UPR^mt and mitophagy could be slightly activated by myocardial stress.

•

Mitophagy modulates UPR^mt to attenuate inflammation-related myocardial injury.

•

Mitophagy and UPR^mt targeting can be applied in treatment of myocardial stress.

Mitochondrial function in development and disease

Mitochondria are organelles with vital functions in almost all eukaryotic cells. Often described as the cellular ‘powerhouses’ due to their essential role in aerobic oxidative phosphorylation, mitochondria perform many other essential functions beyond energy production. As signaling organelles, mitochondria communicate with the nucleus and other organelles to help maintain cellular homeostasis, allow cellular adaptation to diverse stresses, and help steer cell fate decisions during development. Mitochondria have taken center stage in the research of normal and pathological processes, including normal tissue homeostasis and metabolism, neurodegeneration, immunity and infectious diseases. The central role that mitochondria assume within cells is evidenced by the broad impact of mitochondrial diseases, caused by defects in either mitochondrial or nuclear genes encoding for mitochondrial proteins, on different organ systems. In this Review, we will provide the reader with a foundation of the mitochondrial ‘hardware’, the mitochondrion itself, with its specific dynamics, quality control mechanisms and cross-organelle communication, including its roles as a driver of an innate immune response, all with a focus on development, disease and aging. We will further discuss how mitochondrial DNA is inherited, how its mutation affects cell and organismal fitness, and current therapeutic approaches for mitochondrial diseases in both model organisms and humans.

Summary: Mitochondria have a plethora of functions beyond metabolism. This Review discusses the emerging and multifaceted roles of mitochondria in different model organisms and human disease biology.

CMLS forum reviews: mitochondrial damage control

Mitochondria supply cellular energy through oxidative phosphorylation and fulfill numerous additional functions that are fundamental to cellular homeostasis and stress responses. Mitochondrial malfunction, arising from inherent defects of the organelle itself, aging, or acute or chronic stress, can cause substantial damage to organismal health. For instance, mitochondrial malfunction contributes to inflammation, neurodegeneration, tumorigenesis and cardiovascular diseases. Therefore, various quality control mechanisms exist that support a functional mitochondrial organelle compartment. The CMLS Forum Reviews introduced here present a collection of articles covering select topics on basic mechanisms and pathophysiological contexts of mitochondrial damage control.

Depletion of TMEM65 leads to oxidative stress, apoptosis, induction of mitochondrial unfolded protein response, and upregulation of mitochondrial protein import receptor TOMM22

Mutation in the transmembrane protein 65 gene (TMEM65) results in mitochondrial dysfunction and a severe mitochondrial encephalomyopathy phenotype. However, neither the function of TMEM65 nor the cellular responses to its depletion have been fully elucidated. Hence, we knocked down TMEM65 in human cultured cells and analyzed the resulting cellular responses. Depletion of TMEM65 led to a mild increase in ROS generation and upregulation of the mRNA levels of oxidative stress suppressors, such as NFE2L2 and SESN3, indicating that TMEM65 knockdown induced an oxidative stress response. A mild induction of apoptosis was also observed upon depletion of TMEM65. Depletion of TMEM65 upregulated protein levels of the mitochondrial chaperone HSPD1 and mitochondrial protease LONP1, indicating that mitochondrial unfolded protein response (UPR^mt) was induced in response to TMEM65 depletion. Additionally, we found that the mitochondrial protein import receptor TOMM22 and HSPA9 (mitochondrial Hsp70), were also upregulated in TMEM65-depleted cells. Notably, the depletion of TMEM65 did not lead to upregulation of TOMM22 in an ATF5-dependent manner, although upregulation of LONP1 reportedly occurs in an ATF5-dependent manner. Taken together, our findings suggest that depletion of TMEM65 causes mild oxidative stress and apoptosis, induces UPR^mt, and upregulates protein expression of mitochondrial protein import receptor TOMM22 in an ATF5-independent manner.

Protection against renal ischemia and reperfusion injury by short-term time-restricted feeding involves the mitochondrial unfolded protein response

Food restriction improves metabolic health and increases resistance to stress in experimental animals. However, most studies have focused on long-term dietary restriction protocols consisting of several weeks or months of limited food ingestion. Here it was investigated the impact of 2-h time-restricted feeding (TRF) for one week on stress resistance in a rat model of kidney injury induced by ischemia and reperfusion (IR). At baseline, TRF reduced blood glucose, increased β-hydroxybutyrate and improved body composition in male Wistar rats. Importantly, implementing the one-week TRF schedule before ischemia significantly improved renal function, suppressed tubular injury, prevented the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and inhibited the development of interstitial fibrosis. These benefits were related to increased antioxidant protection, reduction in dynamin-related protein 1 (DRP1)-mediated mitochondrial fragmentation and modulation of the mitochondrial unfolded protein response (UPRmt). Specifically, preoperative TRF boosted the activation of the UPRmt in the acute phase after renal IR while promoted its resolution at the stage of fibrosis. Our study indicates that dietary preconditioning by short-term TRF improves the outcome of renal IR injury, and suggests that an optimal intervention that promotes kidney protection may not necessarily require adherence to restrictive diets for prolonged periods of time.

The mitohormetic response as part of the cytoprotection mechanism of berberine : Berberine induces mitohormesis and mechanisms

It was well-known that Berberine, a major bioactive compound extracted from natural plants Coptis chinensis, has anti-diabetic effects for decades in china. Other types of pharmacological activities, such as anti-inflammatory, antimicrobial, hypolipidemic, and anti-cancer effects, have also been examined. At cellular level, these pharmacological activities were mostly an inhibitory effect. However, the cytoprotective effect of berberine was also observed in various types of cells, such as neurons, endothelial cells, fibroblasts, and β-cells. The paradoxical result may be closely associated with characteristics and distribution of berberine within cells, and they can be explained mechanically by mitohormesis, one particular form of hormesis. Here, we reviewed the mitohormetic response and assessed the berberine-induced effects and the possible signaling pathway involved. These findings may contribute to better clinical applications of berberine and indicate that some mitochondria-targeted conventional drugs should be considered carefully in clinical application.

Impaired Mitochondrial Stress Response due to CLPP Deletion Is Associated with Altered Mitochondrial Dynamics and Increased Apoptosis in Cumulus Cells

Caseinolytic peptidase P (CLPP) plays a central role in mitochondrial unfolded protein response (mtUPR) and is required for maintaining protein homeostasis in the mitochondria. Global germline Clpp deletion causes female infertility and accelerated follicular depletion. In the current study, we aimed to characterize the role of CLPP in cumulus cell function, gene expression, and mitochondrial ultrastructure. We found that mitochondria in Clpp-deficient cumulus cells have a smaller aspect ratio (length/width) and have a larger coverage area (mitochondrial area/cytoplasmic area) under electron microscopy. These ultrastructural changes were accompanied with diminished expression of mitochondrial dynamics genes. RNA sequencing analysis revealed a significant change in genes related to cellular metabolism in Clpp-deficient cumulus cells compared to wild type. In addition, apoptosis and phagosome pathways were significantly affected. Immunofluorescence assessment confirmed increased apoptotic activity and decreased cell proliferation in cumulus oophorus complexes (COCs) of Clpp-deficient mice. Our findings demonstrate that deletion of CLPP results in significant structural and functional changes in cumulus cells and suggests that mtUPR is required for cumulus cell function.

The intersection of exercise and aging on mitochondrial protein quality control

Skeletal muscle quality and quantity are negatively impacted with age. Part of this decline in function can be attributed to alterations in mitochondrial turnover, and in the mechanisms that regulate mitochondrial homeostasis. Protein quality control within the mitochondria relies on a number of interconnected processes, namely the mitochondrial unfolded protein response (UPR^mt), protein import and mitophagy. In particular, the post-transcriptional regulation of protein import into the organelle has generated considerable recent interest in view of its dynamic versatility. The capacity for import can be increased by chronic exercise, and diminished by muscle disuse, and defects in the import pathway can be rescued by exercise. Within mitochondria, the unfolded protein response (UPR) is activated if protein import is altered, or if protein misfolding takes place. This UPR generates retrograde signaling to the nucleus to activate compensatory gene expression and protein synthesis. Mitophagy is also elevated with age, contributing to the lower mitochondrial content in aging muscle. However, mitophagy is amenable to exercise adaptations, as it is activated with each exercise bout, presumably to mediate mitochondrial quality control. However, this response is attenuated in older subjects. Although not yet completely elucidated, numerous molecular processes involved in mitochondrial biogenesis and turnover are affected with age. The contrasting and often opposite consequences of exercise and age suggest that exercise can serve as non-pharmacological "mitochondrial medicine" for aging muscle to ameliorate mitochondrial content and function, via pathways that implicate organelle protein quality control mechanisms.

Does perturbation in the mitochondrial protein folding pave the way for neurodegeneration diseases?

Mitochondria, which are cell compartments that are widely present in eukaryotic cells, have been shown to be involved in a variety of synthetic, metabolic, and signaling processes, thereby playing a vital role in cells. The mitochondrial unfolded protein response (mtUPR) is a response in which mitochondria reverse the signal to the nucleus and maintain mitochondrial protein homeostasis when unfolded and misfolded proteins continue to accumulate. Multiple neurodegeneration diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and familial amyotrophic lateral sclerosis (fALS), are public health challenges. Every year, countless efforts are expended trying to clarify the pathogenesis and treatment of neurological disorders, which are associated with mitochondrial dysfunction to some extent. Numerous studies have shown that mtUPR is involved in and plays an important role in the pathogenesis of neurological disorders, but the exact mechanism of the disorders is still unclear. Further study of the process of mtUPR in neurological disorders can help us more accurately understand their pathogenesis in order to provide new therapeutic targets. In this paper, we briefly review mtUPR signaling in Caenorhabditis elegans (C. elegans) and mammals and summarize the role of mtUPR in neurodegeneration diseases, including AD, PD and fALS.

Oxidative stress induces different tissue dependent effects on Mutyh-deficient mice

8-oxoguanine (8-oxoG) is one of the most prevalent genotoxic lesions, and it is generated in DNA attacked by reactive oxygen species (ROS). Adenine misincorporated opposite to 8-oxoG during replication is excised by MutY homolog (MUTYH), an important protein of the base excision repair (BER) system. Mutyh plays an important role in the maintenance of genomic integrity, but the functional consequences of Mutyh deficiency are not fully understood. In the current study, we investigated the histological and functional changes of five tissues (hippocampus, heart, liver, kidney and lung) and their molecular basis in Mutyh^-/- and wild-type mice exposed to D-galactose (D-gal). Our data indicated that Mutyh deficiency hindered the weight gain of experimental mice and induced substantial alterations of 8-oxoG content and superoxide dismutase (SOD) activity, but no significant histological and functional impairment appeared in the investigated tissues of Mutyh- deficient mice without D-gal exposure. Under low-dose D-gal exposure, Mutyh deficiency altered expression of genes involved in mitochondrial unfolded protein response (UPR^mt) in the heart, liver and lung, and caused an enhanced expression of mitochondrial dynamics proteins (MDPs) in hippocampus and liver. The stress responses could maintain mitochondrial proteostasis and function. However, such responses were not noted when experiencing excessive damage burden induced by high-dose D-gal exposure, in which Mutyh deficiency increased accumulation of 8-oxoG and aggravated mitonuclear protein imbalance, as well as histological lesions in heart, liver and kidney. A higher sensitivity to ROS-induced cardiotoxicity with high-dose D-gal exposure was noticed in Mutyh^-/- mice. However, no differences in learning and memory impairments were observed between Mutyh^-/- and wild-type mice with high-dose D-gal exposure. In conclusion, our data demonstrated that Mutyh deficiency has different impacts on various tissues based on the degree of oxidative stress.

Cadmium exposure triggers mitochondrial dysfunction and oxidative stress in chicken (Gallus gallus) kidney via mitochondrial UPR inhibition and Nrf2-mediated antioxidant defense activation

Cadmium (Cd) is a widespread environmental pollutant that accumulates in living systems and represents a significant global health hazard. Cd poses a toxicity threat to both human and animal health, including that of birds. Further knowledge of Cd toxicology pathways will allow for a better understanding of Cd-induced nephrotoxicity. To evaluate Cd-induced nephrotoxicity through potential oxidative damage, male chickens were treated with 0 mg/kg, 35 mg/kg or 70 mg/kg CdCl₂ in diet for 90 days. Markedly, histopathology indicated renal tubular epithelial cell swelling, renal function CREA content abnormalities, biochemical and morphologic indices indicative of Cd-induced kidney injury. Cd toxicity induced the up-regulation of Nrf2 and downstream target genes that relieve oxidative stress. Meanwhile, Cd disrupted the homeostasis of trace elements and promoted oxidative damage. Cd interfered with mitochondrial unfolded protein response (UPR^mt)-related factors (SIRT1, SIRT3, PGC-1α, TFAM, Nrf1, and HTRA2) and disrupted the homeostasis of mitochondrial dynamics (OPA1, MFN1, MFN2, Fis1 and MFF), thereby exacerbating mitochondrial structural damage and mitochondrial dysfunction. In conclusion, our study demonstrated that the nephrotoxicity of Cd exposure results in oxidative stress and mitochondrial dysfunction by activating the Nrf2 signaling pathway and inhibiting UPR^mt in the kidneys.

NAD+ repletion inhibits the endothelial-to-mesenchymal transition induced by TGF-β in endothelial cells through improving mitochondrial unfolded protein response

Endothelial-to-mesenchymal transition (EndMT) plays an important role in the progression of cardiac fibrosis but its mechanism and treatment need to be further understood. Herein, we have found that mitochondrial unfolded protein response (mtUPR) played a critical role in transforming growth factor beta 1 (TGF-β1)-induced EndMT in endothelial cells (ECs). MtUPR was repressed in endothelial cells after exposure to TGF-β1. NAD + precursor nicotinamide riboside (NR) could attenuate TGF-β1-induced EndMT and improve the levels of mtUPR. Significantly, prohibitin proteins (PHB and PHB2) was also regulated by nicotinamide riboside. Moreover, we found that inhibition of prohibitin proteins could prevent the protective effect of nicotinamide riboside on mtUPR and TGF-β1-induced EndMT. Overexpression of prohibitin proteins could alleviate mitochondrial function and TGF-β1-induced EndMT through improving mtUPR. In vivo, The EndMT of ECs induced by Transverse aortic constriction (TAC) in mouse was inhibited by NR. In conclusion, our results indicate that nicotinamide riboside improved the expression of prohibitin proteins to ameliorate EndMT via promotion of mtUPR. Nicotinamide riboside is a potential therapeutic target for cardiac fibrosis.

Mitochondrial Alkbh1 localizes to mtRNA granules and its knockdown induces the mitochondrial UPR in humans and C. elegans

The Fe(II) and 2-oxoglutarate-dependent oxygenase Alkb homologue 1 (Alkbh1) has been shown to act on a wide range of substrates, like DNA, tRNA and histones. Thereby different enzymatic activities have been identified including, among others, demethylation of N ³-methylcytosine (m³C) in RNA- and single-stranded DNA oligonucleotides, demethylation of N ¹-methyladenosine (m¹A) in tRNA or formation of 5-formyl cytosine (f⁵C) in tRNA. In accordance with the different substrates, Alkbh1 has also been proposed to reside in distinct cellular compartments in human and mouse cells, including the nucleus, cytoplasm and mitochondria. Here, we describe further evidence for a role of human Alkbh1 in regulation of mitochondrial protein biogenesis, including visualizing localization of Alkbh1 into mitochondrial RNA granules with super-resolution 3D SIM microscopy. Electron microscopy and high-resolution respirometry analyses revealed an impact of Alkbh1 level on mitochondrial respiration, but not on mitochondrial structure. Downregulation of Alkbh1 impacts cell growth in HeLa cells and delays development in Caenorhabditis elegans, where the mitochondrial role of Alkbh1 seems to be conserved. Alkbh1 knockdown, but not Alkbh7 knockdown, triggers the mitochondrial unfolded protein response (UPR^mt) in C. elegans.

Mitochondrial unfolded protein response gene CLPP changes mitochondrial dynamics and affects mitochondrial function

Mitochondrial dynamics is associated with mitochondrial function, which is associated with diabetes. Although an important indicator of the mitochondrial unfolded protein response, to the best of our knowledge, CLPP and its effects on mitochondrial dynamics in islet cells have not been studied to date. We analyzed the effects of CLPP on mitochondrial dynamics and mitochondrial function in the mice islet β-cell line Min6 under high glucose and high fat conditions. Min6 cells were assigned to: Normal, HG, HG+NC, HG+siCLPP, HF, HF+NC and HF+ siCLPP groups. High glucose and high fat can promote the mRNA and protein expression of CLPP in mitochondria. The increase of mitochondrial fission, the decrese of mitochondrial fusion, and the damage of mintocondrial ultrastructure were significant in the siCLPP cell groups as compared to no-siCLPP treated groups. Meanwhile, mitochondrial functions of MIN6 cells treated with siCLPP were impaired, such as ATP decreased, ROS increased, mitochondrial membrane potential decreased. In addition, cell insulin secretion decreased and cell apoptosis rate increased in siCLPP groups. These results revealed that mitochondrial unfolded protein response geneCLPP alleviated high glucose and high fat-induced mitochondrial dynamics imbalance and mitochondrial dysfunction.

Mutant huntingtin inhibits the mitochondrial unfolded protein response by impairing ABCB10 mRNA stability

Numerous studies have shown that mitochondrial dysfunction contributes to consequential phenotypes of Huntington's disease (HD), a fatal and inherited neurodegenerative disease caused by the expanded CAG repeats in the N-terminus of the huntingtin (Htt) gene. To maintain proper function, mitochondria develop a dedicated protein quality control mechanism by activating a stress response termed the mitochondrial unfolded protein response (UPR^mt). Defects in the UPR^mt have been linked to aging and are also associated with neurodegenerative diseases. However, little is known about the role of the UPR^mt in HD. In this study, we find that ABCB10, a mitochondrial transporter involved in the UPR^mt pathway, is downregulated in HD mouse striatal cells, HD patient fibroblasts, and HD R6/2 mice. Deletion of ABCB10 causes increased mitochondrial reactive oxygen species (ROS) production and cell death, whereas overexpression of ABCB10 reduces these aberrant events. Moreover, the mitochondrial chaperone HSP60 and mitochondrial protease Clpp, two well-established markers of the UPR^mt, are reduced in the in vitro ABCB10-deficienct HD models. CHOP, a key transcription factor of HSP60 and Clpp, is regulated by ABCB10 in HD mouse striatal cells. Furthermore, we find that mutant huntingtin (mtHtt) inhibits the mtUPR by impairing ABCB10 mRNA stability. These findings demonstrate a suppression of the UPR^mt by mtHtt, suggesting that disturbance of mitochondrial protein quality control may contribute to the pathogenesis of HD.

Mitochondrial unfolded protein response transcription factor ATFS-1 promotes longevity in a long-lived mitochondrial mutant through activation of stress response pathways

Background

The mitochondrial unfolded protein response (mitoUPR) is a stress response pathway activated by disruption of proteostasis in the mitochondria. This pathway has been proposed to influence lifespan, with studies suggesting that mitoUPR activation has complex effects on longevity.

Results

Here, we examined the contribution of the mitoUPR to the survival and lifespan of three long-lived mitochondrial mutants in Caenorhabditis elegans by modulating the levels of ATFS-1, the central transcription factor that mediates the mitoUPR. We found that clk-1, isp-1, and nuo-6 worms all exhibit an ATFS-1-dependent activation of the mitoUPR. While loss of atfs-1 during adulthood does not affect lifespan in any of these strains, absence of atfs-1 during development prevents clk-1 and isp-1 worms from reaching adulthood and reduces the lifespan of nuo-6 mutants. Examining the mechanism by which deletion of atfs-1 reverts nuo-6 lifespan to wild-type, we find that many of the transcriptional changes present in nuo-6 worms are mediated by ATFS-1. Genes exhibiting an ATFS-1-dependent upregulation in nuo-6 worms are enriched for transcripts that function in stress response and metabolism. Consistent, with this finding, loss of atfs-1 abolishes the enhanced stress resistance observed in nuo-6 mutants and prevents upregulation of multiple stress response pathways including the HIF-1-mediated hypoxia response, SKN-1-mediated oxidative stress response and DAF-16-mediated stress response.

Conclusions

Our results suggest that in the long-lived mitochondrial mutant nuo-6 activation of the mitoUPR causes atfs-1-dependent changes in the expression of genes involved in stress response and metabolism, which contributes to the extended longevity observed in this mutant. This work demonstrates that the mitoUPR can modulate multiple stress response pathways and suggests that it is crucial for the development and lifespan of long-lived mitochondrial mutants.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0615-3) contains supplementary material, which is available to authorized users.

Disentangling the effect of dietary restriction on mitochondrial function using recombinant inbred mice

Dietary restriction (DR) extends lifespan and healthspan in many species, but precisely how it elicits its beneficial effects is unclear. We investigated the impact of DR on mitochondrial function within liver and skeletal muscle of female ILSXISS mice that exhibit strain-specific variation in lifespan under 40% DR. Strains TejJ89 (lifespan increased under DR), TejJ48 (lifespan unaffected by DR) and TejJ114 (lifespan decreased under DR) were studied following 10 months of 40% DR (13 months of age). Oxygen consumption rates (OCR) within isolated liver mitochondria were unaffected by DR in TejJ89 and TejJ48, but decreased by DR in TejJ114. DR had no effect on hepatic protein levels of PGC-1a, TFAM, and OXPHOS complexes IV. Mitonuclear protein imbalance (nDNA:mtDNA ratio) was unaffected by DR, but HSP90 protein levels were reduced in TejJ114 under DR. Surprisingly hepatic mitochondrial hydrogen peroxide (H₂O₂) production was elevated by DR in TejJ89, with total superoxide dismutase activity and protein carbonyls increased by DR in both TejJ89 and TejJ114. In skeletal muscle, DR had no effect on mitochondrial OCR, OXPHOS complexes or mitonuclear protein imbalance, but H₂O₂ production was decreased in TejJ114 and nuclear PGC-1a increased in TejJ89 under DR. Our findings indicate that hepatic mitochondrial dysfunction associated with reduced lifespan of TejJ114 mice under 40% DR, but similar dysfunction was not apparent in skeletal muscle mitochondria. We highlight tissue-specific differences in the mitochondrial response in ILSXISS mice to DR, and underline the importance and challenges of exploiting genetic heterogeneity to help understand mechanisms of ageing.

Molecular hydrogen modulates gene expression via histone modification and induces the mitochondrial unfolded protein response

Molecular hydrogen (H₂) is a biologically active gas that is used medically to ameliorate various systemic pathological conditions. H₂ also regulates gene expression involved in intracellular signaling and metabolic pathways. However, it is unclear whether H₂ affects gene expression directly or through indirect effects as a consequence of health improvement. Therefore, we attempted to identify genes that exhibit similar changes in expression in response to H₂ by employing DNA microarrays and gene set enrichment analysis to analyze RNA from liver and lung of rats and mice with or without dietary stress. We found that H₂ activated the expression of sets of genes regulated by histone H3K27 methylation status. H₂ also modified the expression of many genes regulated by a wide variety of signaling pathways. RT-qPCR showed that H₂ up-regulated expression of Kcnc3, a H3K27-regulated gene, in organs such as liver, lung, kidney and brain. Furthermore, using immunohistochemistry and immunoblot analysis, we observed changes in H3K27 methylation status in the liver of mice and rats administered H₂. Moreover, we showed that H₂ simultaneously induced the H3K27 demethylase, Jmjd3, and mitochondrial unfolded protein response (mtUPR)-related genes. Recently, alteration of mitochondrial function was shown to cause induction of H3K27 demethylase or chromatin restructuring, followed by mtUPR activation through the alteration of H3K27 or H3K9 methylation states. Taken together, our study suggests that H₂ can induce beneficial effects through mtUPR activation via epigenetic histone modification and by modification of gene expression.

A fish oil diet induces mitochondrial uncoupling and mitochondrial unfolded protein response in epididymal white adipose tissue of mice

White adipose tissue (WAT) mitochondrial dysfunction is linked to the pathogenesis of obesity driven insulin resistance. Dietary conditions that alter fat mass are known to affect white adipocyte mitochondrial function, however, the impact of high calorie diets on white adipocyte mitochondria is not fully understood. The aim of this study is to assess the effect of a diet rich in saturated or polyunsaturated fat on mitochondrial unfolded protein response (UPR^mt), a retrograde signaling response that maintains mitochondrial homeostasis, in epididymal WAT (eWAT). Mice were fed a low fat diet (LFD), saturated fat diet (SFD) or fish oil (unsaturated fat diet, UFD) and assessed changes in eWAT mitochondria. Compared to mice fed a LFD, SFD-fed mice have reduced mitochondrial biogenesis markers, mitochondrial fatty acid oxidation enzymes and TCA cycle enzymes, suggesting an impaired mitochondrial function that could contribute to increased fat mass. In contrast, isocaloric UFD-fed mice have increased expression of mitochondrial uncoupling protein 1 (UCP1) and peroxisomal fatty acid oxidation enzymes suggesting that elevated mitochondrial uncoupling and peroxisomal fatty acid oxidation could contribute to the reduction in fat mass. Interestingly, expression of UPR^mt-associated proteins caseinolytic peptidase (ClpP) and heat shock protein 60 (Hsp60) are induced by UFD, whereas SFD reduced the expression of ClpP. Based on our data, we propose that induction of UPR^mt helps to preserve a functional mitochondria and efficient utilization of fat by UFD whereas a dampened UPR^mt response might impair mitochondrial function and promote fat accumulation by SFD. Thus, our findings suggest a potential role of UPR^mt in mediating the beneficial effects of fish oil.

Integrating the UPRmt into the mitochondrial maintenance network

Mitochondrial function is central to many different processes in the cell, from oxidative phosphorylation to the synthesis of iron-sulfur clusters. Therefore, mitochondrial dysfunction underlies a diverse array of diseases, from neurodegenerative diseases to cancer. Stress can be communicated to the cytosol and nucleus from the mitochondria through many different signals, and in response the cell can effect everything from transcriptional to post-transcriptional responses to protect the mitochondrial network. How these responses are coordinated have only recently begun to be understood. In this review, we explore how the cell maintains mitochondrial function, focusing on the mitochondrial unfolded protein response (UPR^mt), a transcriptional response that can activate a wide array of programs to repair and restore mitochondrial function.

Mitochondrial remodeling: Rearranging, recycling, and reprogramming

Mitochondria are highly dynamic and responsive organelles that respond to environmental cues with fission and fusion. They undergo mitophagy and biogenesis, and are subject to extensive post-translational modifications. Calcium plays an important role in regulating mitochondrial functions. Mitochondria play a central role in metabolism of glucose, fatty acids, and amino acids, and generate ATP with effects on redox poise, oxidative stress, pH, and other metabolites including acetyl-CoA and NAD(+) which in turn have effects on chromatin remodeling. The complex interplay of mitochondria, cytosolic factors, and the nucleus ensure a well-coordinated response to environmental stresses.

Identification and characterization of a mitochondrial unfolded protein response transcription factor ATFS-1 in Litopenaeus vannamei

A mitochondrial specific stress response termed mitochondrial unfolded protein response (UPR(mt)) is activated in responding to disturbance of protein homeostasis in mitochondria. The activating transcription factor associated with stress-1 (designated as ATFS-1) is the key regulator of UPR(mt). To investigating the roles of ATFS-1 (LvATFS-1) in Litopenaeus vannamei mitochondrial stress remission and immunity, it's full length cDNA was cloned. The open reading frame of LvATFS-1 was 1, 557 bp in length, deducing to a 268 amino acids protein. LvATFS-1 was highly expressed in muscle, hemocytes and eyestalk. Subcellular location assays showed that N-terminal of LvATFS-1 contained a mitochondrial targeting sequence, which could directed the fused EGFP located to mitochondria. And the C-terminal of LvATFS-1, which had a nuclear localization signal, expressed in nucleus. The in vitro experiments verified that LvATFS-1 could reduced the level of intracellular reactive oxygen species (ROS). And results of real-time RT-PCR indicated that LvATFS-1 might scavenge excess ROS via ROS-eliminating genes regulation. Reporter gene assays showed that LvATFS-1 could upregulated the expression of the antimicrobial peptide genes in Drosophila Schneider 2 cells. Results of real-time RT-PCR showed that Vibrio alginolyticus or white spot syndrome virus (WSSV) infection induced the expression of LvATFS-1. And knocked-down LvATFS-1 by RNAi resulted in a higher cumulative mortality of L. vannamei upon V. alginolyticus or WSSV infection. These results suggested that LvATFS-1 not only rolled in mitochondrial specific stress responding, but also important for L. vannamei immunologic defence.

A quantitative genome-wide RNAi screen in C. elegans for antifungal innate immunity genes

BACKGROUND

Caenorhabditis elegans has emerged over the last decade as a useful model for the study of innate immunity. Its infection with the pathogenic fungus Drechmeria coniospora leads to the rapid up-regulation in the epidermis of genes encoding antimicrobial peptides. The molecular basis of antimicrobial peptide gene regulation has been previously characterized through forward genetic screens. Reverse genetics, based on RNAi, provide a complementary approach to dissect the worm's immune defenses.

RESULTS

We report here the full results of a quantitative whole-genome RNAi screen in C. elegans for genes involved in regulating antimicrobial peptide gene expression. The results will be a valuable resource for those contemplating similar RNAi-based screens and also reveal the limitations of such an approach. We present several strategies, including a comprehensive class clustering method, to overcome these limitations and which allowed us to characterize the different steps of the interaction between C. elegans and the fungus D. coniospora, leading to a complete description of the MAPK pathway central to innate immunity in C. elegans. The results further revealed a cross-tissue signaling, triggered by mitochondrial dysfunction in the intestine, that suppresses antimicrobial peptide gene expression in the nematode epidermis.

CONCLUSIONS

Overall, our results provide an unprecedented system's level insight into the regulation of C. elegans innate immunity. They represent a significant contribution to our understanding of host defenses and will lead to a better comprehension of the function and evolution of animal innate immunity.

Down-regulation of the mitochondrial matrix peptidase ClpP in muscle cells causes mitochondrial dysfunction and decreases cell proliferation

The caseinolytic peptidase P (ClpP) is the endopeptidase component of the mitochondrial matrix ATP-dependent ClpXP protease. ClpP degrades unfolded proteins to maintain mitochondrial protein homeostasis and is involved in the initiation of the mitochondrial unfolded protein response (UPR(mt)). Outside of an integral role in the UPR(mt), the cellular function of ClpP is not well characterized in mammalian cells. To investigate the role of ClpP in mitochondrial function, we generated C2C12 muscle cells that are deficient in ClpP using siRNA or stable knockdown using lentiviral transduction. Reduction of ClpP levels by ~70% in C2C12 muscle cells resulted in a number of mitochondrial alterations including reduced mitochondrial respiration and reduced oxygen consumption rate in response to electron transport chain (ETC) complex I and II substrates. The reduction in ClpP altered mitochondrial morphology, changed the expression level of mitochondrial fission protein Drp1 and blunted UPR(mt) induction. In addition, ClpP deficient cells showed increased generation of reactive oxygen species (ROS) and decreased membrane potential. At the cellular level, reduction of ClpP impaired myoblast differentiation, cell proliferation and elevated phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) suggesting an inhibition of translation. Our study is the first to define the effects of ClpP deficiency on mitochondrial function in muscle cells in vitro. In addition, we have uncovered novel effects of ClpP on mitochondrial morphology, cell proliferation and protein translation pathways in muscle cells.

UPR(mt)-mediated cytoprotection and organismal aging

Time- or age-dependent accumulation of mitochondrial damage and dysfunction is strongly associated with aging [1]. Thus, a major biomedical goal is to identify and therapeutically manipulate those inherent programs that protect against mitochondrial dysfunction to promote cell survival and organismal health. The mitochondrial unfolded protein response (UPR(mt)) is such a protective transcriptional response mediated by mitochondrial-to-nuclear signaling that includes mitochondrial proteostasis genes to stabilize mitochondrial function, metabolic adaptations, as well as an innate immunity program. Here, we review the UPR(mt) and its role during a variety of forms of mitochondrial dysfunction including those caused by mutations in respiratory chain genes as well as upon exposure to pathogens that produce mitochondrial toxins. We also review recent data in support of and against the emerging role of the UPR(mt) during aging and longevity. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.