bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2023‒06‒11
eighteen papers selected by
Edmond Chan
Queen’s University, School of Medicine
  1. A cytosolic surveillance mechanism activates the mitochondrial UPR.
  2. Delivery of low-density lipoprotein from endocytic carriers to mitochondria supports steroidogenesis.
  3. LDLs take a shortcut to mitochondria.
  4. ROS signaling-induced mitochondrial Sgk1 expression regulates epithelial cell renewal.
  5. PLIN5 interacts with FATP4 at membrane contact sites to promote lipid droplet-to-mitochondria fatty acid transport.
  6. Reduced mitochondrial calcium uptake in macrophages is a major driver of inflammaging.
  7. Endogenous PP2A inhibitor CIP2A degradation by chaperone-mediated autophagy contributes to the antitumor effect of mitochondrial complex I inhibition.
  8. Human mitochondrial ADP/ATP carrier SLC25A4 operates with a ping-pong kinetic mechanism.
  9. A molecular rotor FLIM probe reveals dynamic coupling between mitochondrial inner membrane fluidity and cellular respiration.
  10. The multiple links between actin and mitochondria.
  11. Eating your mitochondria-when too much of a good thing turns bad.
  12. Pancreatic islet protection at the expense of secretory function involves serine-linked mitochondrial one-carbon metabolism.
  13. Mitochondrial metabolism and dynamics in pancreatic beta cell glucose sensing.
  14. Loss of the mitochondrial protein Abcb10 results in altered arginine metabolism in MEL and K562 cells and nutrient stress signaling through ATF4.
  15. ATF4 regulates mitochondrial content, morphology, and function in differentiating skeletal muscle myotubes.
  16. DNA Damage Response and Mitochondrial Translation Mediate ROS Response.
  17. Defective mitochondrial import as a challenge for cellular protein homeostasis.
  18. Mitochondrial networks through the lens of mathematics.
  1. Nature. 2023 Jun 07.
    A cytosolic surveillance mechanism activates the mitochondrial UPR.
    F X Reymond Sutandy, Ines Gößner, Georg Tascher, Christian Münch.
      The mitochondrial unfolded protein response (UPRmt) is essential to safeguard mitochondria from proteotoxic damage by activating a dedicated transcriptional response in the nucleus to restore proteostasis1,2. Yet, it remains unclear how the information on mitochondria misfolding stress (MMS) is signalled to the nucleus as part of the human UPRmt (refs. 3,4). Here, we show that UPRmt signalling is driven by the release of two individual signals in the cytosol-mitochondrial reactive oxygen species (mtROS) and accumulation of mitochondrial protein precursors in the cytosol (c-mtProt). Combining proteomics and genetic approaches, we identified that MMS causes the release of mtROS into the cytosol. In parallel, MMS leads to mitochondrial protein import defects causing c-mtProt accumulation. Both signals integrate to activate the UPRmt; released mtROS oxidize the cytosolic HSP40 protein DNAJA1, which leads to enhanced recruitment of cytosolic HSP70 to c-mtProt. Consequently, HSP70 releases HSF1, which translocates to the nucleus and activates transcription of UPRmt genes. Together, we identify a highly controlled cytosolic surveillance mechanism that integrates independent mitochondrial stress signals to initiate the UPRmt. These observations reveal a link between mitochondrial and cytosolic proteostasis and provide molecular insight into UPRmt signalling in human cells.
    DOI:  https://doi.org/10.1038/s41586-023-06142-0
  2. Nat Cell Biol. 2023 Jun 05.
    Delivery of low-density lipoprotein from endocytic carriers to mitochondria supports steroidogenesis.
    Yu-Xia Zhou, Jian Wei, Gang Deng, Ao Hu, Pu-Yu Sun, Xiaolu Zhao, Bao-Liang Song, Jie Luo.
      The low-density lipoprotein (LDL) is a major cholesterol carrier in circulation and is internalized into cells through LDL receptor (LDLR)-mediated endocytosis. The LDLR protein is highly expressed in the steroidogenic organs and LDL cholesterol is an important source for steroidogenesis. Cholesterol must be transported into the mitochondria, where steroid hormone biosynthesis initiates. However, how LDL cholesterol is conveyed to the mitochondria is poorly defined. Here, through genome-wide small hairpin RNA screening, we find that the outer mitochondrial membrane protein phospholipase D6 (PLD6), which hydrolyses cardiolipin to phosphatidic acid, accelerates LDLR degradation. PLD6 promotes the entrance of LDL and LDLR into the mitochondria, where LDLR is degraded by mitochondrial proteases and LDL-carried cholesterol is used for steroid hormone biosynthesis. Mechanistically, the outer mitochondrial membrane protein CISD2 binds to the cytosolic tail of LDLR and tethers LDLR+ vesicles to the mitochondria. The fusogenic lipid phosphatidic acid generated by PLD6 facilitates the membrane fusion of LDLR+ vesicles with the mitochondria. This intracellular transport pathway of LDL-LDLR bypasses the lysosomes and delivers cholesterol to the mitochondria for steroidogenesis.
    DOI:  https://doi.org/10.1038/s41556-023-01160-6
  3. Nat Cell Biol. 2023 Jun 05.
    LDLs take a shortcut to mitochondria.
    Satoko Shinjo, Luca Scorrano.
      
    DOI:  https://doi.org/10.1038/s41556-023-01166-0
  4. Proc Natl Acad Sci U S A. 2023 Jun 13. 120(24): e2216310120
    ROS signaling-induced mitochondrial Sgk1 expression regulates epithelial cell renewal.
    Yingxiang Li, Chengdong Liu, Luke Rolling, Veronica Sikora, Zhimin Chen, Jack Gurwin, Caroline Barabell, Jiandie Lin, Cunming Duan.
      Many types of differentiated cells can reenter the cell cycle upon injury or stress. The underlying mechanisms are still poorly understood. Here, we investigated how quiescent cells are reactivated using a zebrafish model, in which a population of differentiated epithelial cells are reactivated under a physiological context. A robust and sustained increase in mitochondrial membrane potential was observed in the reactivated cells. Genetic and pharmacological perturbations show that elevated mitochondrial metabolism and ATP synthesis are critical for cell reactivation. Further analyses showed that elevated mitochondrial metabolism increases mitochondrial ROS levels, which induces Sgk1 expression in the mitochondria. Genetic deletion and inhibition of Sgk1 in zebrafish abolished epithelial cell reactivation. Similarly, ROS-dependent mitochondrial expression of SGK1 promotes S phase entry in human breast cancer cells. Mechanistically, SGK1 coordinates mitochondrial activity with ATP synthesis by phosphorylating F1Fo-ATP synthase. These findings suggest a conserved intramitochondrial signaling loop regulating epithelial cell renewal.
    Keywords:  F1Fo-ATP synthase; IGF/insulin signaling; mitochondrial membrane potential; reactive oxygen species; serum- and glucocorticoid-regulated kinase 1
    DOI:  https://doi.org/10.1073/pnas.2216310120
  5. Dev Cell. 2023 Jun 02. pii: S1534-5807(23)00239-3. [Epub ahead of print]
    PLIN5 interacts with FATP4 at membrane contact sites to promote lipid droplet-to-mitochondria fatty acid transport.
    Gregory E Miner, Christina M So, Whitney Edwards, Joey V Ragusa, Jonathan T Wine, Daniel Wong Gutierrez, Michael V Airola, Laura E Herring, Rosalind A Coleman, Eric L Klett, Sarah Cohen.
      Cells adjust their metabolism by remodeling membrane contact sites that channel metabolites to different fates. Lipid droplet (LD)-mitochondria contacts change in response to fasting, cold exposure, and exercise. However, their function and mechanism of formation have remained controversial. We focused on perilipin 5 (PLIN5), an LD protein that tethers mitochondria, to probe the function and regulation of LD-mitochondria contacts. We demonstrate that efficient LD-to-mitochondria fatty acid (FA) trafficking and ß-oxidation during starvation of myoblasts are promoted by phosphorylation of PLIN5 and require an intact PLIN5 mitochondrial tethering domain. Using human and murine cells, we further identified the acyl-CoA synthetase, FATP4 (ACSVL4), as a mitochondrial interactor of PLIN5. The C-terminal domains of PLIN5 and FATP4 constitute a minimal protein interaction capable of inducing organelle contacts. Our work suggests that starvation leads to phosphorylation of PLIN5, lipolysis, and subsequent channeling of FAs from LDs to FATP4 on mitochondria for conversion to fatty-acyl-CoAs and subsequent oxidation.
    Keywords:  FATP4; PLIN5; acyl-CoA; fatty acids; lipid droplets; membrane contact sites; metabolism; mitochondria; organelles
    DOI:  https://doi.org/10.1016/j.devcel.2023.05.006
  6. Nat Aging. 2023 Jun 05.
    Reduced mitochondrial calcium uptake in macrophages is a major driver of inflammaging.
    Philip V Seegren, Logan R Harper, Taylor K Downs, Xiao-Yu Zhao, Shivapriya B Viswanathan, Marta E Stremska, Rachel J Olson, Joel Kennedy, Sarah E Ewald, Pankaj Kumar, Bimal N Desai.
      Mitochondrial dysfunction is linked to age-associated inflammation or inflammaging, but underlying mechanisms are not understood. Analyses of 700 human blood transcriptomes revealed clear signs of age-associated low-grade inflammation. Among changes in mitochondrial components, we found that the expression of mitochondrial calcium uniporter (MCU) and its regulatory subunit MICU1, genes central to mitochondrial Ca2+ (mCa2+) signaling, correlated inversely with age. Indeed, mCa2+ uptake capacity of mouse macrophages decreased significantly with age. We show that in both human and mouse macrophages, reduced mCa2+ uptake amplifies cytosolic Ca2+ oscillations and potentiates downstream nuclear factor kappa B activation, which is central to inflammation. Our findings pinpoint the mitochondrial calcium uniporter complex as a keystone molecular apparatus that links age-related changes in mitochondrial physiology to systemic macrophage-mediated age-associated inflammation. The findings raise the exciting possibility that restoring mCa2+ uptake capacity in tissue-resident macrophages may decrease inflammaging of specific organs and alleviate age-associated conditions such as neurodegenerative and cardiometabolic diseases.
    DOI:  https://doi.org/10.1038/s43587-023-00436-8
  7. Cell Rep. 2023 Jun 07. pii: S2211-1247(23)00627-7. [Epub ahead of print]42(6): 112616
    Endogenous PP2A inhibitor CIP2A degradation by chaperone-mediated autophagy contributes to the antitumor effect of mitochondrial complex I inhibition.
    Riccardo Cazzoli, Francesco Romeo, Isabella Pallavicini, Sebastiano Peri, Mauro Romanenghi, Juan Alberto Pérez-Valencia, Eman Hagag, Filippo Ferrucci, Mohamed Elgendy, Orazio Vittorio, Salvatore Pece, Marco Foiani, Jukka Westermarck, Saverio Minucci.
      Combined inhibition of oxidative phosphorylation (OXPHOS) and glycolysis has been shown to activate a PP2A-dependent signaling pathway, leading to tumor cell death. Here, we analyze highly selective mitochondrial complex I or III inhibitors in vitro and in vivo to elucidate the molecular mechanisms leading to cell death following OXPHOS inhibition. We show that IACS-010759 treatment (complex I inhibitor) induces a reactive oxygen species (ROS)-dependent dissociation of CIP2A from PP2A, leading to its destabilization and degradation through chaperone-mediated autophagy. Mitochondrial complex III inhibition has analogous effects. We establish that activation of the PP2A holoenzyme containing B56δ regulatory subunit selectively mediates tumor cell death, while the arrest in proliferation that is observed upon IACS-010759 treatment does not depend on the PP2A-B56δ complex. These studies provide a molecular characterization of the events subsequent to the alteration of critical bioenergetic pathways and help to refine clinical studies aimed to exploit metabolic vulnerabilities of tumor cells.
    Keywords:  CIP2A; CP: Cancer; CP: Molecular biology; OXPHOS; PP2A; cancer; chaperone-mediated autophagy; fasting; glycolysis; metabolism
    DOI:  https://doi.org/10.1016/j.celrep.2023.112616
  8. EMBO Rep. 2023 Jun 06. e57127
    Human mitochondrial ADP/ATP carrier SLC25A4 operates with a ping-pong kinetic mechanism.
    Camila Cimadamore-Werthein, Stephany Jaiquel Baron, Martin S King, Roger Springett, Edmund Rs Kunji.
      The mitochondrial ADP/ATP carrier (SLC25A4), also called the adenine nucleotide translocase, imports ADP into the mitochondrial matrix and exports ATP, which are key steps in oxidative phosphorylation. Historically, the carrier was thought to form a homodimer and to operate by a sequential kinetic mechanism, which involves the formation of a ternary complex with the two exchanged substrates bound simultaneously. However, recent structural and functional data have demonstrated that the mitochondrial ADP/ATP carrier works as a monomer and has a single substrate binding site, which cannot be reconciled with a sequential kinetic mechanism. Here, we study the kinetic properties of the human mitochondrial ADP/ATP carrier by using proteoliposomes and transport robotics. We show that the Km/Vmax ratio is constant for all of the measured internal concentrations. Thus, in contrast to earlier claims, we conclude that the carrier operates with a ping-pong kinetic mechanism in which substrate exchange across the membrane occurs consecutively rather than simultaneously. These data unite the kinetic and structural models, showing that the carrier operates with an alternating access mechanism.
    Keywords:  ADP/ATP translocase; SLC25; adenine nucleotide translocator; bioenergetics; mitochondrial carrier family
    DOI:  https://doi.org/10.15252/embr.202357127
  9. Proc Natl Acad Sci U S A. 2023 Jun 13. 120(24): e2213241120
    A molecular rotor FLIM probe reveals dynamic coupling between mitochondrial inner membrane fluidity and cellular respiration.
    Gaurav Singh, Geen George, Sufi O Raja, Ponnuvel Kandaswamy, Manoj Kumar, Shashi Thutupalli, Sunil Laxman, Akash Gulyani.
      The inner mitochondrial membrane (IMM), housing components of the electron transport chain (ETC), is the site for respiration. The ETC relies on mobile carriers; therefore, it has long been argued that the fluidity of the densely packed IMM can potentially influence ETC flux and cell physiology. However, it is unclear if cells temporally modulate IMM fluidity upon metabolic or other stimulation. Using a photostable, red-shifted, cell-permeable molecular-rotor, Mitorotor-1, we present a multiplexed approach for quantitatively mapping IMM fluidity in living cells. This reveals IMM fluidity to be linked to cellular-respiration and responsive to stimuli. Multiple approaches combining in vitro experiments and live-cell fluorescence (FLIM) lifetime imaging microscopy (FLIM) show Mitorotor-1 to robustly report IMM 'microviscosity'/fluidity through changes in molecular free volume. Interestingly, external osmotic stimuli cause controlled swelling/compaction of mitochondria, thereby revealing a graded Mitorotor-1 response to IMM microviscosity. Lateral diffusion measurements of IMM correlate with microviscosity reported via Mitorotor-1 FLIM-lifetime, showing convergence of independent approaches for measuring IMM local-order. Mitorotor-1 FLIM reveals mitochondrial heterogeneity in IMM fluidity; between-and-within cells and across single mitochondrion. Multiplexed FLIM lifetime imaging of Mitorotor-1 and NADH autofluorescence reveals that IMM fluidity positively correlates with respiration, across individual cells. Remarkably, we find that stimulating respiration, through nutrient deprivation or chemically, also leads to increase in IMM fluidity. These data suggest that modulating IMM fluidity supports enhanced respiratory flux. Our study presents a robust method for measuring IMM fluidity and suggests a dynamic regulatory paradigm of modulating IMM local order on changing metabolic demand.
    Keywords:  fluidity; fluorescence lifetime; fluorescent probe; metabolism; mitochondria
    DOI:  https://doi.org/10.1073/pnas.2213241120
  10. Nat Rev Mol Cell Biol. 2023 Jun 05.
    The multiple links between actin and mitochondria.
    Tak Shun Fung, Rajarshi Chakrabarti, Henry N Higgs.
      Actin plays many well-known roles in cells, and understanding any specific role is often confounded by the overlap of multiple actin-based structures in space and time. Here, we review our rapidly expanding understanding of actin in mitochondrial biology, where actin plays multiple distinct roles, exemplifying the versatility of actin and its functions in cell biology. One well-studied role of actin in mitochondrial biology is its role in mitochondrial fission, where actin polymerization from the endoplasmic reticulum through the formin INF2 has been shown to stimulate two distinct steps. However, roles for actin during other types of mitochondrial fission, dependent on the Arp2/3 complex, have also been described. In addition, actin performs functions independent of mitochondrial fission. During mitochondrial dysfunction, two distinct phases of Arp2/3 complex-mediated actin polymerization can be triggered. First, within 5 min of dysfunction, rapid actin assembly around mitochondria serves to suppress mitochondrial shape changes and to stimulate glycolysis. At a later time point, at more than 1 h post-dysfunction, a second round of actin polymerization prepares mitochondria for mitophagy. Finally, actin can both stimulate and inhibit mitochondrial motility depending on the context. These motility effects can either be through the polymerization of actin itself or through myosin-based processes, with myosin 19 being an important mitochondrially attached myosin. Overall, distinct actin structures assemble in response to diverse stimuli to affect specific changes to mitochondria.
    DOI:  https://doi.org/10.1038/s41580-023-00613-y
  11. EMBO J. 2023 Jun 05. e114542
    Eating your mitochondria-when too much of a good thing turns bad.
    Léa P Wilhelm, Ian G Ganley.
      How mitophagy is turned on to remove damaged or excess mitochondria from cells has been well-studied, but less is known about how the pathway is turned off to avoid "over-eating" of mitochondria under basal conditions. Three new studies now reveal the disease-associated FBXL4 protein as an important negative regulator of constitutive mitophagy, controlling the stability of mitophagy receptors BNIP3 and NIX.
    DOI:  https://doi.org/10.15252/embj.2023114542
  12. Cell Rep. 2023 Jun 07. pii: S2211-1247(23)00626-5. [Epub ahead of print]42(6): 112615
    Pancreatic islet protection at the expense of secretory function involves serine-linked mitochondrial one-carbon metabolism.
    Angela Pelligra, Jessica Mrugala, Kerstin Griess, Philip Kirschner, Oliver Nortmann, Barbara Bartosinska, Andrea Köster, Natalia I Krupenko, Dominik Gebel, Philipp Westhoff, Bodo Steckel, Daniel Eberhard, Diran Herebian, Bengt-Frederik Belgardt, Jürgen Schrader, Andreas P M Weber, Sergey A Krupenko, Eckhard Lammert.
      Type 2 diabetes is characterized by insulin hypersecretion followed by reduced glucose-stimulated insulin secretion (GSIS). Here we show that acute stimulation of pancreatic islets with the insulin secretagogue dextrorphan (DXO) or glibenclamide enhances GSIS, whereas chronic treatment with high concentrations of these drugs reduces GSIS but protect islets from cell death. Bulk RNA sequencing of islets shows increased expression of genes for serine-linked mitochondrial one-carbon metabolism (OCM) after chronic, but not acute, stimulation. In chronically stimulated islets, more glucose is metabolized to serine than to citrate, and the mitochondrial ATP/ADP ratio decreases, whereas the NAPDH/NADP+ ratio increases. Activating transcription factor-4 (Atf4) is required and sufficient to activate serine-linked mitochondrial OCM genes in islets, with gain- and loss-of-function experiments showing that Atf4 reduces GSIS and is required, but not sufficient, for full DXO-mediated islet protection. In sum, we identify a reversible metabolic pathway that provides islet protection at the expense of secretory function.
    Keywords:  Activating transcription factor-4 (Atf4); CP: Metabolism; K(ATP) channel; beta cell exhaustion; beta cell survival; de novo serine synthesis; diabetes; mitochondria; one-carbon metabolism; pancreatic beta cell; pancreatic islets
    DOI:  https://doi.org/10.1016/j.celrep.2023.112615
  13. Biochem J. 2023 Jun 15. 480(11): 773-789
    Mitochondrial metabolism and dynamics in pancreatic beta cell glucose sensing.
    Guy A Rutter, Vaibhav Sidarala, Brett A Kaufman, Scott A Soleimanpour.
      Glucose-regulated insulin secretion becomes defective in all forms of diabetes. The signaling mechanisms through which the sugar acts on the ensemble of beta cells within the islet remain a vigorous area of research after more than 60 years. Here, we focus firstly on the role that the privileged oxidative metabolism of glucose plays in glucose detection, discussing the importance of 'disallowing' in the beta cell the expression of genes including Lactate dehydrogenase (Ldha) and the lactate transporter Mct1/Slc16a1 to restrict other metabolic fates for glucose. We next explore the regulation of mitochondrial metabolism by Ca2+ and its possible role in sustaining glucose signaling towards insulin secretion. Finally, we discuss in depth the importance of mitochondrial structure and dynamics in the beta cell, and their potential for therapeutic targeting by incretin hormones or direct regulators of mitochondrial fusion. This review, and the 2023 Sir Philip Randle Lecture which GAR will give at the Islet Study Group meeting in Vancouver, Canada in June 2023, honor the foundational, and sometimes under-appreciated, contributions made by Professor Randle and his colleagues towards our understanding of the regulation of insulin secretion.
    Keywords:  diabetes; glucose homeostasis; hormone secretion; insulin; mitochondria; pancreatic beta cell
    DOI:  https://doi.org/10.1042/BCJ20230167
  14. J Biol Chem. 2023 Jun 01. pii: S0021-9258(23)01905-1. [Epub ahead of print] 104877
    Loss of the mitochondrial protein Abcb10 results in altered arginine metabolism in MEL and K562 cells and nutrient stress signaling through ATF4.
    Marisa Miljkovic, Alexandra Seguin, Xuan Jia, James E Cox, Jonathan Leon Catrow, Hector Bergonia, John D Phillips, W Zac Stephens, Diane M Ward.
      Abcb10 is a mitochondrial membrane protein involved in hemoglobinization of red cells. Abcb10 topology and ATPase domain localization suggest it exports a substrate, likely biliverdin, out of mitochondria that is necessary for hemoglobinization. In this study we generated Abcb10 deletion cell lines in both mouse murine erythroleukemia (MEL) and human erythroid precursor human myelogenous leukemia (K562) cells to better understand the consequences of Abcb10 loss. Loss of Abcb10 resulted in an inability to hemoglobinize upon differentiation in both K562 and MEL cells with reduced heme and intermediate porphyrins and decreased levels of aminolevulinic acid synthase 2 activity. Metabolomic and transcriptional analyses revealed that Abcb10 loss gave rise to decreased cellular arginine levels, increased transcripts for cationic and neutral amino acid transporters with reduced levels of the citrulline to arginine converting enzymes argininosuccinate synthetase and argininosuccinate lyase. The reduced arginine levels in Abcb10 null cells gave rise to decreased proliferative capacity. Arginine supplementation improved both Abcb10 null proliferation and hemoglobinization upon differentiation. Abcb10 null cells showed increased phosphorylation of Eukaryotic Translation Initiation Factor 2 Subunit Alpha (eIF2A), increased expression of nutrient sensing transcription factor ATF4 and downstream targets DNA damage inducible transcript 3 (Chop), ChaC glutathione specific gamma-glutamylcyclotransferase 1 (Chac1) and arginyl-tRNA synthetase 1 (Rars). These results suggest that when the Abcb10 substrate is trapped in the mitochondria, the nutrient sensing machinery is turned on remodeling transcription to block protein synthesis necessary for proliferation and hemoglobin biosynthesis in erythroid models.
    Keywords:  Arginine; differentiation; erythroid; metabolism; nutrient; transporter
    DOI:  https://doi.org/10.1016/j.jbc.2023.104877
  15. Am J Physiol Cell Physiol. 2023 Jun 05.
    ATF4 regulates mitochondrial content, morphology, and function in differentiating skeletal muscle myotubes.
    Jonathan M Memme, Victoria C Sanfrancesco, David A Hood.
      Mitochondrial function is widely recognized as a major determinant of health, emphasizing the importance of understanding the mechanisms promoting mitochondrial quality in various tissues. Recently, the mitochondrial unfolded protein response (UPRmt) has come into focus as a modulator of mitochondrial homeostasis, particularly in stress conditions. In muscle, the necessity for ATF4 and its role in regulating mitochondrial quality control (MQC) has yet to be determined. We overexpressed (OE) and knocked down ATF4 in C2C12 myoblasts, differentiated them to myotubes for 5 days, and subjected them to acute (ACA) or chronic (CCA) contractile activity. ATF4 mediated myotube formation through the regulated expression of myogenic factors, mainly Myc and MyoD, and supressed mitochondrial biogenesis basally through PGC-1a. However, our data also show that ATF4 expression levels are directly related to mitochondrial fusion and dynamics, UPRmt activation, as well as lysosomal biogenesis and autophagy. Thus, ATF4 promoted enhanced mitochondrial networking, protein handling, and capacity for clearance of dysfunctional organelles under stress conditions, despite lower levels of mitophagy flux with OE. Indeed, we found that ATF4 promoted the formation of a smaller pool of high functioning mitochondria that are more responsive to contractile activity, have higher oxygen consumption rates and lower reactive oxygen species levels. These data provide evidence that ATF4 is both necessary and sufficient for mitochondrial quality control and adaptation during both differentiation and contractile activity, thus advancing the current understanding of ATF4 beyond its canonical functions, to include the regulation of mitochondrial morphology, lysosomal biogenesis and mitophagy in muscle cells.
    Keywords:  ATF4; mitochondrial quality control; mitochondrial unfolded protein response; mitophagy and lysosomal biogenesis; skeletal muscle C2C12
    DOI:  https://doi.org/10.1152/ajpcell.00080.2023
  16. Cancer Discov. 2023 Jun 09. OF1
    DNA Damage Response and Mitochondrial Translation Mediate ROS Response.
      The cellular response to ROS depends on coordination of activities in the nucleus and mitochondria.
    DOI:  https://doi.org/10.1158/2159-8290.CD-RW2023-085
  17. FEBS Lett. 2023 Jun 05.
    Defective mitochondrial import as a challenge for cellular protein homeostasis.
    Klaudia K Maruszczak, Selvaraj Ayyamperumal, Agnieszka Chacinska.
      Mitochondria are organelles indispensable for the correct functioning of eukaryotic cells. Their significance for cellular homeostasis is manifested by the existence of complex quality control pathways that monitor organellar fitness. Mitochondrial biogenesis relies on the efficient import of mitochondrial precursor proteins, a large majority of which are encoded by nuclear DNA and synthesized in the cytosol. This creates a demand for highly specialized import routes that comprise cytosolic factors and organellar translocases. The passage of newly encoded mitochondrial precursor proteins through the cytosol to the translocase of the outer mitochondrial membrane (TOM) is under tight surveillance. As a result of mitochondrial import defects, mitochondrial precursor proteins accumulate in the cytosol or clog the TOM complex, which in turn stimulates cellular stress responses to minimize the consequences of these challenges. These responses are critical for maintaining protein homeostasis under conditions of mitochondrial stress. The present review summarizes recent advances in the field of mitochondrial protein import quality control and discusses the role of this quality control within the network of cellular mechanisms that maintain the cellular homeostasis of proteins.
    Keywords:  cellular stress responses; mitochondria; mitochondrial dysfunction; mitochondrial quality control; protein aggregates; protein homeostasis
    DOI:  https://doi.org/10.1002/1873-3468.14677
  18. Phys Biol. 2023 Jun 08.
    Mitochondrial networks through the lens of mathematics.
    Greyson Lewis, Wallace F Marshall.
      Mitochondria serve a wide range of functions within cells, most notably via their production of ATP. Although their morphology is commonly described as bean-like, mitochondria often form interconnected networks within cells that exhibit dynamic restructuring through a variety of physical changes. Further, though relationships between form and function in biology are well established, the extant toolkit for understanding mitochondrial morphology is limited. Here, we emphasize new and established methods for quantitatively describing mitochondrial networks, ranging from unweighted graph-theoretic representations to multi-scale approaches from applied topology, in particular persistent homology. We also show fundamental relationships between mitochondrial networks, mathematics, and physics, using ideas of graph planarity and statistical mechanics to better understand the full possible morphological
space of mitochondrial network structures. Lastly, we provide suggestions for how examination of mitochondrial network form through the language of mathematics can inform biological understanding, and vice versa.
    Keywords:  graph theory; mitochondrial networks; persistent homology; planar graphs; scaling
    DOI:  https://doi.org/10.1088/1478-3975/acdcdb
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