bims-nenemi 2021-10-10 papers

bims-nenemi

Biomed News

on Neuroinflammation, neurodegeneration and mitochondria

Issue of 2021–10–10
twenty papers selected by
Marco Tigano, Thomas Jefferson University

Mild mitochondrial impairment enhances inn3ate immunity and longevity through ATFS-1 and p38 signaling.
TFAM knockdown-triggered mtDNA-nucleoid aggregation and a decrease in mtDNA copy number induce the reorganization of nucleoid populations and mitochondria-associated ER-membrane contacts.
Labeling and Measuring Stressed Mitochondria Using a PINK1-Based Ratiometric Fluorescent Sensor.
Remdesivir triphosphate blocks DNA synthesis and increases exonucleolysis by the replicative mitochondrial DNA polymerase, Pol γ.
Modulation of SF3B1 in the pre-mRNA spliceosome induces a RIG-I-dependent type I IFN response.
A Benzenesulfonamide-based Mitochondrial Uncoupler Induces Endoplasmic Reticulum Stress and Immunogenic Cell Death in Epithelial Ovarian Cancer.
A replication-linked mutational gradient drives somatic mutation accumulation and influences germline polymorphisms and genome composition in mitochondrial DNA.
Nucleus-mitochondria positive feedback loop formed by ERK5 S496 phosphorylation-mediated poly (ADP-ribose) polymerase activation provokes persistent pro-inflammatory senescent phenotype and accelerates coronary atherosclerosis after chemo-radiation.
Intercellular transfer of mitochondrial DNA carrying metastasis-enhancing pathogenic mutations from high- to low-metastatic tumor cells and stromal cells via extracellular vesicles.
Mitofusin 2: the missing link between mtDNA maintenance defects and neurotransmitter disorders.
Stress response proteins NRP1 and NRP2 are pro-survival factors that inhibit cell death during ER stress.
Visualizing formation of the active site in the mitochondrial ribosome.
PINK1 drives production of mtDNA-containing extracellular vesicles to promote invasiveness.
Protective mitochondrial fission induced by stress-responsive protein GJA1-20k.
ERRFI1 induces apoptosis of hepatocellular carcinoma cells in response to tryptophan deficiency.
STING protects breast cancer cells from intrinsic and genotoxic-induced DNA instability via a non-canonical, cell-autonomous pathway.
A Mild Phenotype of Mitochondrial DNA Depletion Syndrome Type 13 with a Novel FBXL4 Variant.
The REDD1/TXNIP Complex Accelerates Oxidative Stress-Induced Apoptosis of Nucleus Pulposus Cells through the Mitochondrial Pathway.
Seeking protective mitochondria.
Higher-order phosphatase-substrate contacts terminate the integrated stress response.

EMBO Rep. 2021 Oct 07. e52964

Mild mitochondrial impairment enhances inn3ate immunity and longevity through ATFS-1 and p38 signaling.

Juliane C Campos, Ziyun Wu, Paige D Rudich, Sonja K Soo, Meeta Mistry, Julio Cb Ferreira, T Keith Blackwell, Jeremy M Van Raamsdonk.

  While mitochondrial function is essential for life in all multicellular organisms, a mild impairment of mitochondrial function can extend longevity in model organisms. By understanding the molecular mechanisms involved, these pathways might be targeted to promote healthy aging. In studying two long-lived mitochondrial mutants in C. elegans, we found that disrupting subunits of the mitochondrial electron transport chain results in upregulation of genes involved in innate immunity, which is driven by the mitochondrial unfolded protein response (mitoUPR) but also dependent on the canonical p38-mediated innate immune signaling pathway. Both of these pathways are required for the increased resistance to bacterial pathogens and extended longevity of the long-lived mitochondrial mutants, as is the FOXO transcription factor DAF-16. This work demonstrates that both the p38-mediated innate immune signaling pathway and the mitoUPR act in concert on the same innate immunity genes to promote pathogen resistance and longevity and that input from the mitochondria can extend longevity by signaling through these pathways. This indicates that multiple evolutionarily conserved genetic pathways controlling innate immunity also function to modulate lifespan.

Keywords:   C. elegans ; aging; innate immunity; mitochondria; mitochondrial unfolded protein response

DOI:  https://doi.org/10.15252/embr.202152964
Biochem Biophys Rep. 2021 Dec;28 101142

TFAM knockdown-triggered mtDNA-nucleoid aggregation and a decrease in mtDNA copy number induce the reorganization of nucleoid populations and mitochondria-associated ER-membrane contacts.

Koit Aasumets, Yuliya Basikhina, Jaakko L Pohjoismäki, Steffi Goffart, Joachim Gerhold.

  The correct organization of mitochondrial DNA (mtDNA) in nucleoids and the contacts of mitochondria with the ER play an important role in maintaining the mitochondrial genome distribution within the cell. Mitochondria-associated ER membranes (MAMs) consist of interacting proteins and lipids located in the outer mitochondrial membrane and ER membrane, forming a platform for the mitochondrial inner membrane-associated genome replication factory as well as connecting the nucleoids with the mitochondrial division machinery. We show here that knockdown of a core component of mitochondrial nucleoids, TFAM, causes changes in the mitochondrial nucleoid populations, which subsequently impact ER-mitochondria membrane contacts. Knockdown of TFAM causes a significant decrease in the copy number of mtDNA as well as aggregation of mtDNA nucleoids. At the same time, it causes significant upregulation of the replicative TWNK helicase in the membrane-associated nucleoid fraction. This is accompanied by a transient elevation of MAM proteins, indicating a rearrangement of the linkage between ER and mitochondria triggered by changes in mitochondrial nucleoids. Reciprocal knockdown of the mitochondrial replicative helicase TWNK causes a decrease in mtDNA copy number and modifies mtDNA membrane association, however, it does not cause nucleoid aggregation and considerable alterations of MAM proteins in the membrane-associated fraction. Our explanation is that the aggregation of mitochondrial nucleoids resulting from TFAM knockdown triggers a compensatory mechanism involving the reorganization of both mitochondrial nucleoids and MAM. These results could provide an important insight into pathological conditions associated with impaired nucleoid organization or defects of mtDNA distribution.

Keywords:  Mitochondrial DNA; Mitochondrial transcription factor A (TFAM); Nucleoids; Organellar membranes; TWNK helicase

DOI:  https://doi.org/10.1016/j.bbrep.2021.101142
J Biol Chem. 2021 Oct 05. pii: S0021-9258(21)01082-6. [Epub ahead of print] 101279

Labeling and Measuring Stressed Mitochondria Using a PINK1-Based Ratiometric Fluorescent Sensor.

Rie Uesugi, Shunsuke Ishii, Akira Matsuura, Eisuke Itakura.

  Mitochondria are essential organelles that carry out a number of pivotal metabolic processes and maintain cellular homeostasis. Mitochondrial dysfunction caused by various stresses is associated with many diseases such as type 2 diabetes, obesity, cancer, heart failure, neurodegenerative disorders, and aging. Therefore, it is important to understand the stimuli that induce mitochondrial stress. However, broad analysis of mitochondrial stress has not been carried out to date. Here, we present a set of fluorescent tools, called mito-Pain (mitochondrial PINK1 accumulation index), which enables the labeling of stressed mitochondria. Mito-Pain utilizes PINK1 stabilization on mitochondria and quantifies mitochondrial stress levels by comparison with PINK1-GFP, which is stabilized under mitochondrial stress, and RFP-Omp25, which is constitutively localized on mitochondria. To identify compounds that induce mitochondrial stress, we screened a library of 3374 compounds using mito-Pain and identified 57 compounds as mitochondrial stress inducers. Furthermore, we classified each compound into several categories based on mitochondrial response: depolarization, mitochondrial morphology, or Parkin recruitment. Parkin recruitment to mitochondria was often associated with mitochondrial depolarization and aggregation, suggesting that Parkin is recruited to heavily damaged mitochondria. In addition, many of the compounds led to various mitochondrial morphological changes, including fragmentation, aggregation, elongation, and swelling, with or without Parkin recruitment or mitochondrial depolarization. We also found that several compounds induced an ectopic response of Parkin, leading to the formation of cytosolic puncta dependent on PINK1. Thus, mito-Pain enables the detection of stressed mitochondria under a wide variety of conditions and provide insights into mitochondrial quality control systems.

Keywords:  PTEN‐induced putative kinase 1 (PINK1); Parkin; mitochondria; mitochondrial membrane potential; mitochondrial sensor; mitochondrial stress

DOI:  https://doi.org/10.1016/j.jbc.2021.101279
Mitochondrion. 2021 Oct 04. pii: S1567-7249(21)00136-7. [Epub ahead of print]

Remdesivir triphosphate blocks DNA synthesis and increases exonucleolysis by the replicative mitochondrial DNA polymerase, Pol γ.

Elena J Ciesielska, Shalom Kim, Hyacintha-Ghislaine M Bisimwa, Cody Grier, Md Mostafijur Rahman, Carolyn K J Young, Matthew J Young, Marcos T Oliveira, Grzegorz L Ciesielski.

  The COVID-19 pandemic prompted the FDA to authorize a new nucleoside analogue, remdesivir, for emergency use in affected individuals. We examined the effects of its active metabolite, remdesivir triphosphate (RTP), on the activity of the replicative mitochondrial DNA polymerase, Pol γ. We found that while RTP is not incorporated by Pol γ into a nascent DNA strand, it remains associated with the enzyme impeding its synthetic activity and stimulating exonucleolysis. In spite of that, we found no evidence for deleterious effects of remdesivir treatment on the integrity of the mitochondrial genome in human cells in culture.

Keywords:  Antiviral nucleoside analogues; COVID-19; DNA polymerase gamma; Mitochondrial DNA; Remdesivir

DOI:  https://doi.org/10.1016/j.mito.2021.09.010
J Biol Chem. 2021 Oct 04. pii: S0021-9258(21)01080-2. [Epub ahead of print] 101277

Modulation of SF3B1 in the pre-mRNA spliceosome induces a RIG-I-dependent type I IFN response.

Aaron Y Chang, Yu Jerry Zhou, Sharanya Iyengar, Piotr W Pobiarzyn, Pavel Tishchenko, Kesha M Shah, Heather Wheeler, Yue-Ming Wang, Paula M Loria, Frank Loganzo, Seng-Ryong Woo.

  Nucleic acid-sensing pathways play critical roles in innate immune activation through the production of type I interferon (IFN-I) and proinflammatory cytokines. These factors are required for effective anti-tumor immune responses. Pharmacological modulators of the pre-mRNA spliceosome splicing factor 3b subunit 1 (SF3B1) are under clinical investigation as cancer cytotoxic agents. However, potential roles of these agents in aberrant RNA generation and subsequent RNA-sensing pathway activation have not been studied. In this study, we observed that SF3B1 pharmacological modulation using pladienolide B (Plad B) induces production of aberrant RNA species and robust IFN-I responses via engagement of the dsRNA sensor retinoic acid-inducible gene I (RIG-I) and downstream interferon regulatory factor 3 (IRF3). We found that Plad B synergized with canonical RIG-I agonism to induce the IFN-I response. In addition, Plad B induced NF-κB responses and secretion of proinflammatory cytokines and chemokines. Finally, we showed that cancer cells bearing the hotspot SF3B1K700E mutation, which leads to global aberrant splicing, had enhanced IFN-I response to canonical RIG-I agonism. Together, these results demonstrate that pharmacological modulation of SF3B1 in cancer cells can induce an enhanced IFN-I response dependent on RIG-I expression. The study suggests that spliceosome modulation may not only induce direct cancer cell cytotoxicity but also initiate an innate immune response via activation of RNA sensing pathways.

Keywords:  Alternative splicing; Anticancer drug; Cytokine; Innate immunity; Interferon; Interferon regulatory factor (IRF); RIG-I-like receptor (RLR); RNA splicing; Spliceosome; mRNA

DOI:  https://doi.org/10.1016/j.jbc.2021.101277
Mol Cancer Ther. 2021 Oct 08. pii: molcanther.MCT-21-0396-A.2021. [Epub ahead of print]

A Benzenesulfonamide-based Mitochondrial Uncoupler Induces Endoplasmic Reticulum Stress and Immunogenic Cell Death in Epithelial Ovarian Cancer.

Fangfang Bi, Ziyan Jiang, Wonmin Park, Tobias Mp Hartwich, Zhiping Ge, Kay Yi Chong, Kevin Yang, Madeline J Morrisson, Dongin Kim, Jaeyeon Kim, Wen Zhang, Liliia M Kril, David S Watt, Chunming Liu, Yang Yang-Hartwich.

Epithelial ovarian cancer (EOC) is a leading cause of death from gynecologic malignancies and requires new therapeutic strategies to improve clinical outcomes. EOCs metastasize in the abdominal cavity through dissemination in the peritoneal fluid and ascites, efficiently adapt to the nutrient-deprived microenvironment, and resist current chemotherapeutic agents. Accumulating evidence suggests that mitochondrial oxidative phosphorylation is critical for the adaptation of EOC cells to this otherwise hostile microenvironment. Although chemical mitochondrial uncouplers can impair mitochondrial functions and thereby target multiple, essential pathways for cancer cell proliferation, traditional mitochondria uncouplers often cause toxicity that precludes their clinical application. In this study, we demonstrated that a mitochondrial uncoupler, specifically 2,5-dichloro-N-(4-nitronaphthalen-1-yl)benzenesulfonamide, hereinafter named Y3, was an antineoplastic agent in ovarian cancer models. Y3 treatment activated AMP-activated protein kinase and resulted in the activation of endoplasmic reticulum stress sensors as well as growth inhibition and apoptosis in ovarian cancer cells in vitro. Y3 was well tolerated in vivo and effectively suppressed tumor progression in three mouse models of EOC, and Y3 also induced immunogenic cell death of cancer cells that involved the release of damage-associated molecular patterns and the activation of antitumor adaptive immune responses. These findings suggest that mitochondrial uncouplers hold promise in developing new anticancer therapies that delay tumor progression and protect ovarian cancer patients against relapse.

DOI: https://doi.org/10.1158/1535-7163.MCT-21-0396
Nucleic Acids Res. 2021 Oct 06. pii: gkab901. [Epub ahead of print]

A replication-linked mutational gradient drives somatic mutation accumulation and influences germline polymorphisms and genome composition in mitochondrial DNA.

Monica Sanchez-Contreras, Mariya T Sweetwyne, Brendan F Kohrn, Kristine A Tsantilas, Michael J Hipp, Elizabeth K Schmidt, Jeanne Fredrickson, Jeremy A Whitson, Matthew D Campbell, Peter S Rabinovitch, David J Marcinek, Scott R Kennedy.

Mutations in mitochondrial DNA (mtDNA) cause maternally inherited diseases, while somatic mutations are linked to common diseases of aging. Although mtDNA mutations impact health, the processes that give rise to them are under considerable debate. To investigate the mechanism by which de novo mutations arise, we analyzed the distribution of naturally occurring somatic mutations across the mouse and human mtDNA obtained by Duplex Sequencing. We observe distinct mutational gradients in G→A and T→C transitions delimited by the light-strand origin and the mitochondrial Control Region (mCR). The gradient increases unequally across the mtDNA with age and is lost in the absence of DNA polymerase γ proofreading activity. In addition, high-resolution analysis of the mCR shows that important regulatory elements exhibit considerable variability in mutation frequency, consistent with them being mutational 'hot-spots' or 'cold-spots'. Collectively, these patterns support genome replication via a deamination prone asymmetric strand-displacement mechanism as the fundamental driver of mutagenesis in mammalian DNA. Moreover, the distribution of mtDNA single nucleotide polymorphisms in humans and the distribution of bases in the mtDNA across vertebrate species mirror this gradient, indicating that replication-linked mutations are likely the primary source of inherited polymorphisms that, over evolutionary timescales, influences genome composition during speciation.

DOI: https://doi.org/10.1093/nar/gkab901
Redox Biol. 2021 Sep 20. pii: S2213-2317(21)00291-3. [Epub ahead of print]47 102132

Nucleus-mitochondria positive feedback loop formed by ERK5 S496 phosphorylation-mediated poly (ADP-ribose) polymerase activation provokes persistent pro-inflammatory senescent phenotype and accelerates coronary atherosclerosis after chemo-radiation.

Sivareddy Kotla, Aijun Zhang, Masaki Imanishi, Kyung Ae Ko, Steven H Lin, Young Jin Gi, Margie Moczygemba, Sevinj Isgandarova, Keri L Schadler, Caroline Chung, Sarah A Milgrom, Jose Banchs, Syed Wamique Yusuf, Diana N Amaya, Huifang Guo, Tamlyn N Thomas, Ying H Shen, Anita Deswal, Joerg Herrmann, Eugenie S Kleinerman, Mark L Entman, John P Cooke, Giovanni Schifitto, Sanjay B Maggirwar, Elena McBeath, Anisha A Gupte, Sunil Krishnan, Zarana S Patel, Yisang Yoon, Jared K Burks, Keigi Fujiwara, Paul S Brookes, Nhat-Tu Le, Dale J Hamilton, Jun-Ichi Abe.

  The incidence of cardiovascular disease (CVD) is higher in cancer survivors than in the general population. Several cancer treatments are recognized as risk factors for CVD, but specific therapies are unavailable. Many cancer treatments activate shared signaling events, which reprogram myeloid cells (MCs) towards persistent senescence-associated secretory phenotype (SASP) and consequently CVD, but the exact mechanisms remain unclear. This study aimed to provide mechanistic insights and potential treatments by investigating how chemo-radiation can induce persistent SASP. We generated ERK5 S496A knock-in mice and determined SASP in myeloid cells (MCs) by evaluating their efferocytotic ability, antioxidation-related molecule expression, telomere length, and inflammatory gene expression. Candidate SASP inducers were identified by high-throughput screening, using the ERK5 transcriptional activity reporter cell system. Various chemotherapy agents and ionizing radiation (IR) up-regulated p90RSK-mediated ERK5 S496 phosphorylation. Doxorubicin and IR caused metabolic changes with nicotinamide adenine dinucleotide depletion and ensuing mitochondrial stunning (reversible mitochondria dysfunction without showing any cell death under ATP depletion) via p90RSK-ERK5 modulation and poly (ADP-ribose) polymerase (PARP) activation, which formed a nucleus-mitochondria positive feedback loop. This feedback loop reprogramed MCs to induce a sustained SASP state, and ultimately primed MCs to be more sensitive to reactive oxygen species. This priming was also detected in circulating monocytes from cancer patients after IR. When PARP activity was transiently inhibited at the time of IR, mitochondrial stunning, priming, macrophage infiltration, and coronary atherosclerosis were all eradicated. The p90RSK-ERK5 module plays a crucial role in SASP-mediated mitochondrial stunning via regulating PARP activation. Our data show for the first time that the nucleus-mitochondria positive feedback loop formed by p90RSK-ERK5 S496 phosphorylation-mediated PARP activation plays a crucial role of persistent SASP state, and also provide preclinical evidence supporting that transient inhibition of PARP activation only at the time of radiation therapy can prevent future CVD in cancer survivors.

Keywords:  Antioxidants; Atherosclerosis; ERK5; Efferocytosis; Ionizing radiation; Mitochondrial stunning; Poly (ADP-Ribose) polymerase; Senescence-associated secretory phenotype (SASP); Telomere length; p90RSK

DOI:  https://doi.org/10.1016/j.redox.2021.102132
BMC Mol Cell Biol. 2021 Oct 07. 22(1): 52

Intercellular transfer of mitochondrial DNA carrying metastasis-enhancing pathogenic mutations from high- to low-metastatic tumor cells and stromal cells via extracellular vesicles.

Keizo Takenaga, Nobuko Koshikawa, Hiroki Nagase.

   BACKGROUND: Mitochondrial DNA (mtDNA) carrying certain pathogenic mutations or single nucleotide variants (SNVs) enhances the invasion and metastasis of tumor cells, and some of these mutations are homoplasmic in tumor cells and even in tumor tissues. On the other hand, intercellular transfer of mitochondria and cellular components via extracellular vesicles (EVs) and tunneling nanotubes (TNTs) has recently attracted intense attention in terms of cell-to-cell communication in the tumor microenvironment. It remains unclear whether metastasis-enhancing pathogenic mutant mtDNA in tumor cells is intercellularly transferred between tumor cells and stromal cells. In this study, we investigated whether mtDNA with the NADH dehydrogenase subunit 6 (ND6) G13997A pathogenic mutation in highly metastatic cells can be horizontally transferred to low-metastatic cells and stromal cells in the tumor microenvironment.
RESULTS: When MitoTracker Deep Red-labeled high-metastatic Lewis lung carcinoma A11 cells carrying the ND6 G13997A mtDNA mutation were cocultured with CellLight mitochondria-GFP-labeled low-metastatic P29 cells harboring wild-type mtDNA, bidirectional transfer of red- and green-colored vesicles, probably mitochondria-related EVs, was observed in a time-dependent manner. Similarly, intercellular transfer of mitochondria-related EVs occurred between A11 cells and α-smooth muscle actin (α-SMA)-positive cancer-associated fibroblasts (CAFs, WA-mFib), macrophages (RAW264.7) and cytotoxic T cells (CTLL-2). Intercellular transfer was suppressed by inhibitors of EV release. The large and small EV fractions (L-EV and S-EV, respectively) prepared from the conditioned medium by differential ultracentrifugation both were found to contain mtDNA, although only S-EVs were efficiently incorporated into the cells. Several subpopulations had evidence of LC3-II and contained degenerated mitochondrial components in the S-EV fraction, signaling to the existence of autophagy-related S-EVs. Interestingly, the S-EV fraction contained a MitoTracker-positive subpopulation, which was inhibited by the respiration inhibitor antimycin A, indicating the presence of mitochondria with membrane potential. It was also demonstrated that mtDNA was transferred into mtDNA-less ρ0 cells after coculture with the S-EV fraction. In syngeneic mouse subcutaneous tumors formed by a mixture of A11 and P29 cells, the mitochondria-related EVs released from A11 cells reached distantly positioned P29 cells and CAFs.
CONCLUSIONS: These results suggest that metastasis-enhancing pathogenic mtDNA derived from metastatic tumor cells is transferred to low-metastatic tumor cells and stromal cells via S-EVs in vitro and in the tumor microenvironment, inferring a novel mechanism of enhancement of metastatic potential during tumor progression.

Keywords:  Extracellular vesicles; Intercellular transfer; Lung cancer; Metastasis; Mitochondria; Mitochondrial DNA; Mutation; Tumor microenvironment

DOI:  https://doi.org/10.1186/s12860-021-00391-5
Mitochondrion. 2021 Sep 29. pii: S1567-7249(21)00137-9. [Epub ahead of print]

Mitofusin 2: the missing link between mtDNA maintenance defects and neurotransmitter disorders.

Giuditta Pellino, Raffaella Faggioli, Anna Galuppi, Alberta Leon, Carlo Fusco, Valeria Tugnoli, Agnese Suppiej.

  Mitofusin (MFN) 2 belongs to the large family of mitochondrial transmembrane GTPases and has a role in dynamic mitochondrial remodeling process governed by fusion and fission. MFN2 pathogenic variants classically cause Charcot-Marie-Tooth disease type 2A (CMT2A), the most common axonal form of CMT, but patients with complex and unusual phenotypes involving the central and peripheral nervous system have been described, with mitochondrial dysfunction proposed as the underlying pathogenic mechanism. Here, we report the first description of a neurochemical pattern of secondary alterations in the metabolism of biogenic amines linked to the de novo presence of the hotspot MFN2 pathogenic variant p.Arg104Trp. The infant presented a very early onset choreic movement disorder associated with severe axial hypotonia and fluctuating dystonia of limbs. The relationship between mitochondrial DNA (mtDNA) maintenance defects and dopaminergic neurotransmitter disorders, governed by MFN2, is discussed.

Keywords:  CSF biogenic amine disorder; Charcot-Marie-Tooth disease; MFN2; mtDNA depletion syndrome

DOI:  https://doi.org/10.1016/j.mito.2021.09.011
Plant Physiol. 2021 Jul 20. pii: kiab335. [Epub ahead of print]

Stress response proteins NRP1 and NRP2 are pro-survival factors that inhibit cell death during ER stress.

Yuhua Yang, Xu Liu, Wenbin Zhang, Qian Qian, Limeng Zhou, Shu Liu, Yuge Li, Xingliang Hou.

Environmental stresses cause an increased number of unfolded or misfolded proteins to accumulate in the endoplasmic reticulum (ER), resulting in ER stress. To restore ER homeostasis and survive, plants initiate an orchestrated signaling pathway known as the unfolded protein response (UPR). Asparagine-rich protein (NRP) 1 and NRP2, two homologous proteins harboring a Development and Cell Death domain, are associated with various stress responses in Arabidopsis (Arabidopsis thaliana), but the relevant molecular mechanism remains obscure. Here, we show that NRP1 and NRP2 act as key pro-survival factors during the ER stress response and that they inhibit cell death. Loss-of-function of NRP1 and NRP2 results in decreased tolerance to the ER stress inducer tunicamycin (TM), accelerating cell death. NRP2 is constitutively expressed while NRP1 is induced in plants under ER stress. In Arabidopsis, basic leucine zipper protein (bZIP) 28 and bZIP60 are important transcription factors in the UPR that activates the expression of many ER stress-related genes. Notably, under ER stress, bZIP60 activates NRP1 by directly binding to the UPRE-I element in the NRP1 promoter. These findings reveal a pro-survival strategy in plants wherein the bZIP60-NRPs cascade suppresses cell death signal transmission, improving survival under adverse conditions.

DOI: https://doi.org/10.1093/plphys/kiab335
Elife. 2021 10 05. pii: e68806. [Epub ahead of print]10

Visualizing formation of the active site in the mitochondrial ribosome.

Viswanathan Chandrasekaran, Nirupa Desai, Nicholas O Burton, Hanting Yang, Jon Price, Eric A Miska, V Ramakrishnan.

  Ribosome assembly is an essential and conserved process that is regulated at each step by specific factors. Using cryo-electron microscopy (cryo-EM), we visualize the formation of the conserved peptidyl transferase center (PTC) of the human mitochondrial ribosome. The conserved GTPase GTPBP7 regulates the correct folding of 16S ribosomal RNA (rRNA) helices and ensures 2'-O-methylation of the PTC base U3039. GTPBP7 binds the RNA methyltransferase NSUN4 and MTERF4, which sequester H68-71 of the 16S rRNA and allow biogenesis factors to access the maturing PTC. Mutations that disrupt binding of their Caenorhabditis elegans orthologs to the large subunit potently activate mitochondrial stress and cause viability, development, and sterility defects. Next-generation RNA sequencing reveals widespread gene expression changes in these mutant animals that are indicative of mitochondrial stress response activation. We also answer the long-standing question of why NSUN4, but not its enzymatic activity, is indispensable for mitochondrial protein synthesis.

Keywords:  C. elegans; RNA modifications; biochemistry; chemical biology; cryo-EM; human; mitochondrial ribosome; molecular biophysics; peptidyl transferase center; structural biology

DOI:  https://doi.org/10.7554/eLife.68806
J Cell Biol. 2021 Dec 06. pii: e202006049. [Epub ahead of print]220(12):

PINK1 drives production of mtDNA-containing extracellular vesicles to promote invasiveness.

Nicolas Rabas, Sarah Palmer, Louise Mitchell, Shehab Ismail, Andrea Gohlke, Joel S Riley, Stephen W G Tait, Payam Gammage, Leandro Lemgruber Soares, Iain R Macpherson, Jim C Norman.

The cystine-glutamate antiporter, xCT, supports a glutathione synthesis program enabling cancer cells to cope with metabolically stressful microenvironments. Up-regulated xCT, in combination with glutaminolysis, leads to increased extracellular glutamate, which promotes invasive behavior by activating metabotropic glutamate receptor 3 (mGluR3). Here we show that activation of mGluR3 in breast cancer cells activates Rab27-dependent release of extracellular vesicles (EVs), which can transfer invasive characteristics to "recipient" tumor cells. These EVs contain mitochondrial DNA (mtDNA), which is packaged via a PINK1-dependent mechanism. We highlight mtDNA as a key EV cargo necessary and sufficient for intercellular transfer of invasive behavior by activating Toll-like receptor 9 in recipient cells, and this involves increased endosomal trafficking of pro-invasive receptors. We propose that an EV-mediated mechanism, through which altered cellular metabolism in one cell influences endosomal trafficking in other cells, is key to generation and dissemination of pro-invasive microenvironments during mammary carcinoma progression.

DOI: https://doi.org/10.1083/jcb.202006049
Elife. 2021 10 05. pii: e69207. [Epub ahead of print]10

Protective mitochondrial fission induced by stress-responsive protein GJA1-20k.

Daisuke Shimura, Esther Nuebel, Rachel Baum, Steven E Valdez, Shaohua Xiao, Junco S Warren, Joseph A Palatinus, TingTing Hong, Jared Rutter, Robin M Shaw.

  The Connexin43 gap junction gene GJA1 has one coding exon, but its mRNA undergoes internal translation to generate N-terminal truncated isoforms of Connexin43 with the predominant isoform being only 20 kDa in size (GJA1-20k). Endogenous GJA1-20k protein is not membrane bound and has been found to increase in response to ischemic stress, localize to mitochondria, and mimic ischemic preconditioning protection in the heart. However, it is not known how GJA1-20k benefits mitochondria to provide this protection. Here, using human cells and mice, we identify that GJA1-20k polymerizes actin around mitochondria which induces focal constriction sites. Mitochondrial fission events occur within about 45 s of GJA1-20k recruitment of actin. Interestingly, GJA1-20k mediated fission is independent of canonical Dynamin-Related Protein 1 (DRP1). We find that GJA1-20k-induced smaller mitochondria have decreased reactive oxygen species (ROS) generation and, in hearts, provide potent protection against ischemia-reperfusion injury. The results indicate that stress responsive internally translated GJA1-20k stabilizes polymerized actin filaments to stimulate non-canonical mitochondrial fission which limits ischemic-reperfusion induced myocardial infarction.

Keywords:  GJA1-20k; actin dynamics; cell biology; human; ischemia/reperfusion; mitochondria; mitochondria dynamics; mouse; organ protection

DOI:  https://doi.org/10.7554/eLife.69207
Cell Death Discov. 2021 Oct 04. 7(1): 274

ERRFI1 induces apoptosis of hepatocellular carcinoma cells in response to tryptophan deficiency.

Mingqing Cui, Dan Liu, Wujun Xiong, Yugang Wang, Jun Mi.

Tryptophan metabolism is an essential regulator of tumor immune evasion. However, the effect of tryptophan metabolism on cancer cells remains largely unknown. Here, we find that tumor cells have distinct responses to tryptophan deficiency in terms of cell growth, no matter hepatocellular carcinoma (HCC) cells, lung cancer cells, or breast cancer cells. Further study shows that ERRFI1 is upregulated in sensitive HCC cells, but not in resistant HCC cells, in response to tryptophan deficiency, and ERRFI1 expression level positively correlates with HCC patient overall survival. ERRFI1 knockdown recovers tryptophan deficiency-suppressed cell growth of sensitive HCC cells. In contrast, ERRFI1 overexpression sensitizes resistant HCC cells to tryptophan deficiency. Moreover, ERRFI1 induces apoptosis by binding PDCD2 in HCC cells, PDCD2 knockdown decreases the ERRFI1-induced apoptosis in HCC cells. Thus, we conclude that ERRFI1-induced apoptosis increases the sensitivity of HCC cells to tryptophan deficiency and ERRFI1 interacts with PDCD2 to induce apoptosis in HCC cells.

DOI: https://doi.org/10.1038/s41420-021-00666-y
Oncogene. 2021 Oct 08.

STING protects breast cancer cells from intrinsic and genotoxic-induced DNA instability via a non-canonical, cell-autonomous pathway.

Laura Cheradame, Ida Chiara Guerrera, Julie Gaston, Alain Schmitt, Vincent Jung, Nicolas Goudin, Marion Pouillard, Nina Radosevic-Robin, Mauro Modesti, Jean-Gabriel Judde, Stefano Cairo, Vincent Goffin.

STING (Stimulator of Interferon Genes) is an endoplasmic reticulum-anchored adaptor of the innate immunity best known to trigger pro-inflammatory cytokine expression in response to pathogen infection. In cancer, this canonical pathway can be activated by intrinsic or drug-induced genomic instability, potentiating antitumor immune responses. Here we report that STING downregulation decreases cell survival and increases sensitivity to genotoxic treatment in a panel of breast cancer cell lines in a cell-autonomous manner. STING silencing impaired DNA Damage Response (53BP1) foci formation and increased DNA break accumulation. These newly identified properties were found to be independent of STING partner cGAS and of its canonical pro-inflammatory pathway. STING was shown to partially localize at the inner nuclear membrane in a variety of breast cancer cell models and clinical tumor samples. Interactomics analysis of nuclear STING identified several proteins of the DNA Damage Response, including the three proteins of the DNA-PK complex, further supporting a role of STING in the regulation of genomic stability. In breast and ovarian cancer patients that received adjuvant chemotherapy, high STING expression is associated with increased risk of relapse. In summary, this study highlights an alternative, non-canonical tumor-promoting role of STING that opposes its well-documented function in tumor immunosurveillance.

DOI: https://doi.org/10.1038/s41388-021-02037-4
Mol Syndromol. 2021 Aug;12(5): 294-299

A Mild Phenotype of Mitochondrial DNA Depletion Syndrome Type 13 with a Novel FBXL4 Variant.

Ummuhan Oncul, Engin Kose, Fatma Tuba Eminoglu.

  Mitochondrial DNA depletion syndromes (MDDS) are a group of rare genetic disorders caused by defects in multiple genes involved in mitochondrial DNA maintenance. Among these, FBXL4 gene variants result in encephalomyopathic mtDNA depletion syndrome 13 (MTDPS13), which commonly presents as a combination of failure to thrive, neurodevelopmental delays, encephalopathy, hypotonia, a pattern of mild facial dysmorphisms, and persistent lactic acidosis. To date, 53 pathogenic FBXL4 variants and 100 cases have been described in the literature. In the present case report, we report on a 4.5-year-old boy with MTDPS13 and a novel variant. The patient had a history of antenatal hydrocephalus, severe developmental delay and mental motor retardation with psychomotor delay, severe hypotonia, mild left ventricular hypertrophic cardiomyopathy, mild facial dysmorphism, and elevated lactate levels. Symptoms suggested mitochondrial myopathy; subsequently, whole-exome sequencing was performed and a novel homozygous variant FBXL4 (NM_012160.4): c.486T>G (p.Tyr162Ter) was identified. While most of the patients with FBLX4 gene mutation have severe clinical manifestation and die at a very young age, clinical progress of our case was milder than previously reported. MDDS are very rare and can present with many different clinical signs and symptoms. In this report, we identified a novel pathogenic variant in the FBXL4 gene. This report shows that patients with FBLX4 gene mutations may present with a milder clinical phenotype than previously reported.

Keywords:  Encephalopathy; FBXL4; Mitochondrial DNA depletion; Myopathy, MTDPS13; mtDNA depletion syndrome

DOI:  https://doi.org/10.1159/000515928
Oxid Med Cell Longev. 2021 ;2021 7397516

The REDD1/TXNIP Complex Accelerates Oxidative Stress-Induced Apoptosis of Nucleus Pulposus Cells through the Mitochondrial Pathway.

Huipeng Yin, Kun Wang, Abhirup Das, Gaocai Li, Yu Song, Rongjin Luo, Jason Pui Yin Cheung, Teng Zhang, Shuai Li, Cao Yang.

The death of nucleus pulposus (NP) cells is an important cause of intervertebral disc (IVD) degeneration. Redox disturbance caused by dysfunctional mitochondria has been considered as a vital risk for NP cell survival. It is valuable to identify key proteins maintaining mitochondrial function in NP cells. A previous study found that regulated in development and DNA damage response 1 (REDD1) are upregulated during intervertebral disc degeneration and that REDD1 can cause NP cell apoptosis. Thus, the present study further explores the effect of REDD1 on IVD degeneration. Our results showed that REDD1 promotes NP cell apoptosis via the mitochondrial pathway. Importantly, REDD1 formed a complex with TXNIP to strengthen its own action, and the combination was consolidated under H2O2-induced oxidative stress. The combined inhibition of the REDD1/TXNIP complex was better than that of REDD1 or TXNIP alone in restoring cell proliferation and accelerating apoptosis. Moreover, p53 acts as the transcription factor of REDD1 to regulate the REDD1/TXNIP complex under oxidative stress. Altogether, our results demonstrated that the REDD1/TXNIP complex mediated H2O2-induced human NP cell apoptosis and IVD degeneration through the mitochondrial pathway. Interferences on these sites to achieve mitochondrial redox homeostasis may be a novel therapeutic strategy for oxidative stress-associated IVD degeneration.

DOI: https://doi.org/10.1155/2021/7397516
Nat Cell Biol. 2021 Oct 06.

Seeking protective mitochondria.

Melina Casadio.

DOI: https://doi.org/10.1038/s41556-021-00771-1
Nat Struct Mol Biol. 2021 Oct;28(10): 835-846

Higher-order phosphatase-substrate contacts terminate the integrated stress response.

Yahui Yan, Heather P Harding, David Ron.

Many regulatory PPP1R subunits join few catalytic PP1c subunits to mediate phosphoserine and phosphothreonine dephosphorylation in metazoans. Regulatory subunits engage the surface of PP1c, locally affecting flexible access of the phosphopeptide to the active site. However, catalytic efficiency of holophosphatases towards their phosphoprotein substrates remains unexplained. Here we present a cryo-EM structure of the tripartite PP1c-PPP1R15A-G-actin holophosphatase that terminates signaling in the mammalian integrated stress response (ISR) in the pre-dephosphorylation complex with its substrate, translation initiation factor 2α (eIF2α). G-actin, whose essential role in eIF2α dephosphorylation is supported crystallographically, biochemically and genetically, aligns the catalytic and regulatory subunits, creating a composite surface that engages the N-terminal domain of eIF2α to position the distant phosphoserine-51 at the active site. Substrate residues that mediate affinity for the holophosphatase also make critical contacts with eIF2α kinases. Thus, a convergent process of higher-order substrate recognition specifies functionally antagonistic phosphorylation and dephosphorylation in the ISR.

DOI: https://doi.org/10.1038/s41594-021-00666-7