bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021–02–14
fourteen papers selected by
Edmond Chan, Queen’s University, School of Medicine
  1. Zc3h10 regulates adipogenesis by controlling translation and F-actin/mitochondria interaction.
  2. Mitophagy receptor FUNDC1 is regulated by PGC-1α/NRF1 to fine tune mitochondrial homeostasis.
  3. Transient phases of OXPHOS inhibitor resistance reveal underlying metabolic heterogeneity in single cells.
  4. Mitovesicles are a novel population of extracellular vesicles of mitochondrial origin altered in Down syndrome.
  5. Hepatic LDL receptor-related protein-1 deficiency alters mitochondrial dynamics through phosphatidylinositol 4,5-bisphosphate reduction.
  6. Insights into the catalytic properties of the mitochondrial rhomboid protease PARL.
  7. Mitophagy in tumorigenesis and metastasis.
  8. Phosphoproteomics of the developing heart identifies PERM1 - An outer mitochondrial membrane protein.
  9. PINK1 Inhibits Multimeric Aggregation and Signaling of MAVS and MAVS-dependent Lung Pathology.
  10. Enhancing mitochondrial activity in neurons protects against neurodegeneration in a mouse model of multiple sclerosis.
  11. Structural Mechanism of Transport of Mitochondrial Carriers.
  12. Regulation of PRKN-Independent Mitophagy.
  13. Mitochondrial dynamics in cancer stem cells.
  14. Quantifying Mitochondrial Volume Density in Phrenic Motor Neurons.
  1. J Cell Biol. 2021 Mar 01. pii: e202003173. [Epub ahead of print]220(3):
    Zc3h10 regulates adipogenesis by controlling translation and F-actin/mitochondria interaction.
    Matteo Audano, Silvia Pedretti, Simona Ligorio, Francesco Gualdrini, Sara Polletti, Marta Russo, Serena Ghisletti, Camilla Bean, Maurizio Crestani, Donatella Caruso, Emma De Fabiani, Nico Mitro.
      The commitment of mesenchymal stem cells to preadipocytes is stimulated by hormonal induction. Preadipocytes induced to differentiate repress protein synthesis, remodel their cytoskeleton, and increase mitochondrial function to support anabolic pathways. These changes enable differentiation into mature adipocytes. Our understanding of the factors that coordinately regulate the early events of adipocyte differentiation remains incomplete. Here, by using multipronged approaches, we have identified zinc finger CCCH-type containing 10 (Zc3h10) as a critical regulator of the early stages of adipogenesis. Zc3h10 depletion in preadipocytes resulted in increased protein translation and impaired filamentous (F)-actin remodeling, with the latter detrimental effect leading to mitochondrial and metabolic dysfunction. These defects negatively affected differentiation to mature adipocytes. In contrast, Zc3h10 overexpression yielded mature adipocytes with remarkably increased lipid droplet size. Overall, our study establishes Zc3h10 as a fundamental proadipogenic transcription factor that represses protein synthesis and promotes F-actin/mitochondria dynamics to ensure proper energy metabolism and favor lipid accumulation.
    DOI:  https://doi.org/10.1083/jcb.202003173
  2. EMBO Rep. 2021 Feb 08. e50629
    Mitophagy receptor FUNDC1 is regulated by PGC-1α/NRF1 to fine tune mitochondrial homeostasis.
    Lei Liu, Yanjun Li, Jianing Wang, Di Zhang, Hao Wu, Wenhui Li, Huifang Wei, Na Ta, Yuyuan Fan, Yujiao Liu, Xiaohui Wang, Jun Wang, Xin Pan, Xudong Liao, Yushan Zhu, Quan Chen.
      Mitophagy is an essential cellular autophagic process that selectively removes superfluous and damaged mitochondria, and it is coordinated with mitochondrial biogenesis to fine tune the quantity and quality of mitochondria. Coordination between these two opposing processes to maintain the functional mitochondrial network is of paramount importance for normal cellular and organismal metabolism. However, the underlying mechanism is not completely understood. Here we report that PGC-1α and nuclear respiratory factor 1 (NRF1), master regulators of mitochondrial biogenesis and metabolic adaptation, also transcriptionally upregulate the gene encoding FUNDC1, a previously characterized mitophagy receptor, in response to cold stress in brown fat tissue. NRF1 binds to the classic consensus site in the promoter of Fundc1 to upregulate its expression and to enhance mitophagy through its interaction with LC3. Specific knockout of Fundc1 in BAT results in reduced mitochondrial turnover and accumulation of functionally compromised mitochondria, leading to impaired adaptive thermogenesis. Our results demonstrate that FUNDC1-dependent mitophagy is directly coupled with mitochondrial biogenesis through the PGC-1α/NRF1 pathway, which dictates mitochondrial quantity, quality, and turnover and contributes to adaptive thermogenesis.
    Keywords:  adaptive thermogenesis; brown adipose tissue; mitochondrial biogenesis; mitophagy
    DOI:  https://doi.org/10.15252/embr.202050629
  3. Cell Metab. 2021 Feb 04. pii: S1550-4131(21)00014-0. [Epub ahead of print]
    Transient phases of OXPHOS inhibitor resistance reveal underlying metabolic heterogeneity in single cells.
    Nont Kosaisawe, Breanne Sparta, Michael Pargett, Carolyn K Teragawa, John G Albeck.
      Cell-to-cell heterogeneity in metabolism plays an unknown role in physiology and pharmacology. To functionally characterize cellular variability in metabolism, we treated cells with inhibitors of oxidative phosphorylation (OXPHOS) and monitored their responses with live-cell reporters for ATP, ADP/ATP, or activity of the energy-sensing kinase AMPK. Across multiple OXPHOS inhibitors and cell types, we identified a subpopulation of cells resistant to activation of AMPK and reduction of ADP/ATP ratio. This resistant state persists transiently for at least several hours and can be inherited during cell divisions. OXPHOS inhibition suppresses the mTORC1 and ERK growth signaling pathways in sensitive cells, but not in resistant cells. Resistance is linked to a multi-factorial combination of increased glucose uptake, reduced protein biosynthesis, and G0/G1 cell-cycle status. Our results reveal dynamic fluctuations in cellular energetic balance and provide a basis for measuring and predicting the distribution of cellular responses to OXPHOS inhibition.
    Keywords:  AKT; FRET; PI3K; adenosine mono-phosphate-regulated protein kinase; electron transport chain; insulin signaling; mammalian target of rapamycin; metabolic cycle; oligomycin; oscillation; translation regulation
    DOI:  https://doi.org/10.1016/j.cmet.2021.01.014
  4. Sci Adv. 2021 Feb;pii: eabe5085. [Epub ahead of print]7(7):
    Mitovesicles are a novel population of extracellular vesicles of mitochondrial origin altered in Down syndrome.
    Pasquale D'Acunzo, Rocío Pérez-González, Yohan Kim, Tal Hargash, Chelsea Miller, Melissa J Alldred, Hediye Erdjument-Bromage, Sai C Penikalapati, Monika Pawlik, Mitsuo Saito, Mariko Saito, Stephen D Ginsberg, Thomas A Neubert, Chris N Goulbourne, Efrat Levy.
      Mitochondrial dysfunction is an established hallmark of aging and neurodegenerative disorders such as Down syndrome (DS) and Alzheimer's disease (AD). Using a high-resolution density gradient separation of extracellular vesicles (EVs) isolated from murine and human DS and diploid control brains, we identify and characterize a previously unknown population of double-membraned EVs containing multiple mitochondrial proteins distinct from previously described EV subtypes, including microvesicles and exosomes. We term these newly identified mitochondria-derived EVs "mitovesicles." We demonstrate that brain-derived mitovesicles contain a specific subset of mitochondrial constituents and that their levels and cargo are altered during pathophysiological processes where mitochondrial dysfunction occurs, including in DS. The development of a method for the selective isolation of mitovesicles paves the way for the characterization in vivo of biological processes connecting EV biology and mitochondria dynamics and for innovative therapeutic and diagnostic strategies.
    DOI:  https://doi.org/10.1126/sciadv.abe5085
  5. J Biol Chem. 2021 Feb 03. pii: S0021-9258(21)00142-3. [Epub ahead of print] 100370
    Hepatic LDL receptor-related protein-1 deficiency alters mitochondrial dynamics through phosphatidylinositol 4,5-bisphosphate reduction.
    Sivaprakasam Chinnarasu, Fawzi Alogaili, Kevin E Bove, Anja Jaeschke, David Y Hui.
      The LDL receptor-related protein 1 (LRP1) is a multi-functional transmembrane protein with endocytosis and signal transduction functions. Previous studies have shown that hepatic LRP1 deficiency exacerbates diet-induced steatohepatitis and insulin resistance via mechanisms related to increased lysosome and mitochondria permeability and dysfunction. The current study examined the impact of LRP1 deficiency on mitochondrial function in the liver. Hepatocytes isolated from liver-specific LRP1 knockout (hLrp1-/-) mice showed reduced oxygen consumption compared to control mouse hepatocytes. The mitochondria in hLrp1-/- mouse livers have an abnormal morphology and their membranes contain significantly less anionic phospholipids, including lower levels of phosphatidylethanolamine and cardiolipin that increase mitochondrial fission and impair fusion. Additional studies showed that LRP1 complexes with phosphatidylinositol 4-phosphate 5-kinase like protein-1 (PIP5KL1) and phosphatidylinositol 4-phosphate 5-kinase-1β (PIP5K1β). The absence of LRP1 reduces the levels of both PIP5KL1 and PIP5K1β in the plasma membrane, and also lowers phosphatidylinositol(4,5) bisphosphate (PI(4,5)P2) levels in hepatocytes. These data indicate that LRP1 recruits PIP5KL1 and PIP5K1β to the plasma membrane for PI(4,5)P2 biosynthesis. The lack of LRP1 reduces lipid kinase expression, leading to lower PI(4,5)P2 levels thereby decreasing the availability of this lipid metabolite in the cardiolipin biosynthesis pathway to cause cardiolipin reduction and the impairment in mitochondria homeostasis. Taken together, the current study identifies another signaling mechanism by which LRP1 regulates cell functions: Binding and recruitment of PIP5KL1 and PIP5K1β to the membrane for PI(4,5)P2 synthesis. In addition, it highlights the importance of this mechanism for maintaining the integrity and functions of intracellular organelles.
    Keywords:  cardiolipin; inositol phosphate; lipoprotein receptor; lipoprotein receptor-related protein (LRP); liver metabolism; respiration
    DOI:  https://doi.org/10.1016/j.jbc.2021.100370
  6. J Biol Chem. 2021 Feb 05. pii: S0021-9258(21)00155-1. [Epub ahead of print] 100383
    Insights into the catalytic properties of the mitochondrial rhomboid protease PARL.
    Laine Lysyk, Raelynn Brassard, Elena Arutyunova, Verena Siebert, Zhenze Jiang, Emmanuella Takyi, Melissa Morrison, Howard S Young, Marius K Lemberg, Anthony J O'Donoghue, M Joanne Lemieux.
      The rhomboid protease PARL is a critical regulator of mitochondrial homeostasis through its cleavage of substrates such as PINK1, PGAM5, and Smac/Diablo, which have crucial roles in mitochondrial quality control and apoptosis. However, the catalytic properties of PARL, including the effect of lipids on the protease, have never been characterized in vitro. To address this, we isolated human PARL expressed in yeast and used FRET-based kinetic assays to measure proteolytic activity in vitro. We show PARL activity in detergent is enhanced by cardiolipin, a lipid enriched in the mitochondrial inner membrane. Significantly higher turnover rates were observed for PARL reconstituted in proteoliposomes, with Smac/Diablo being cleaved most rapidly at a rate of 1 min-1. In contrast, PGAM5 is cleaved with the highest efficiency (kcat/KM) compared to PINK1 and Smac/Diablo. In proteoliposomes, a truncated β-cleavage form of PARL, a physiological form known to affect mitochondrial fragmentation, is more active than the full-length enzyme for hydrolysis of PINK1, PGAM5 and Smac/Diablo. Multiplex profiling of 228 peptides reveals that PARL prefers substrates with a bulky side chain such as Phe in P1, which is distinct from the preference for small side chain residues typically found with bacterial rhomboid proteases. This study using recombinant PARL provides fundamental insights into its catalytic activity and substrate preferences that enhance our understanding of its role in mitochondrial function and has implications for specific inhibitor design.
    Keywords:  GlpG; PGAM5; PINK1; Smac/Diablo; intramembrane proteolysis; membrane protease; mitochondria; rhomboid protease
    DOI:  https://doi.org/10.1016/j.jbc.2021.100383
  7. Cell Mol Life Sci. 2021 Feb 13.
    Mitophagy in tumorigenesis and metastasis.
    Logan P Poole, Kay F Macleod.
      Cells use mitophagy to remove dysfunctional or excess mitochondria, frequently in response to imposed stresses, such as hypoxia and nutrient deprivation. Mitochondrial cargo receptors (MCR) induced by these stresses target mitochondria to autophagosomes through interaction with members of the LC3/GABARAP family. There are a growing number of these MCRs, including BNIP3, BNIP3L, FUNDC1, Bcl2-L-13, FKBP8, Prohibitin-2, and others, in addition to mitochondrial protein targets of PINK1/Parkin phospho-ubiquitination. There is also an emerging link between mitochondrial lipid signaling and mitophagy where ceramide, sphingosine-1-phosphate, and cardiolipin have all been shown to promote mitophagy. Here, we review the upstream signaling mechanisms that regulate mitophagy, including components of the mitochondrial fission machinery, AMPK, ATF4, FoxOs, Sirtuins, and mtDNA release, and address the significance of these pathways for stress responses in tumorigenesis and metastasis. In particular, we focus on how mitophagy modulators intersect with cell cycle control and survival pathways in cancer, including following ECM detachment and during cell migration and metastasis. Finally, we interrogate how mitophagy affects tissue atrophy during cancer cachexia and therapy responses in the clinic.
    Keywords:  AMPK; ATF4; Autophagy; BCL2-L-13; BNIP3/BNIP3L; Cachexia; DRP1; Electron transport chain; FUNDC1; Fission; FoxOs; LC3/GABARAP; Metabolism; Metastasis; Mitochondria; Mitohormesis; Mitophagy; NAD+; PARP; PINK1/Parkin; ROS; Respiration; Sirtuins; UPRmt
    DOI:  https://doi.org/10.1007/s00018-021-03774-1
  8. J Mol Cell Cardiol. 2021 Feb 05. pii: S0022-2828(21)00025-0. [Epub ahead of print]154 41-59
    Phosphoproteomics of the developing heart identifies PERM1 - An outer mitochondrial membrane protein.
    Sriram Aravamudhan, Clara Türk, Theresa Bock, Lena Keufgens, Hendrik Nolte, Franziska Lang, Ramesh Kumar Krishnan, Tim König, Philipp Hammerschmidt, Natalie Schindler, Susanne Brodesser, Dieu Hien Rozsivalova, Elena Rugarli, Aleksandra Trifunovic, Jens Brüning, Thomas Langer, Thomas Braun, Marcus Krüger.
      Heart development relies on PTMs that control cardiomyocyte proliferation, differentiation and cardiac morphogenesis. We generated a map of phosphorylation sites during the early stages of cardiac postnatal development in mice; we quantified over 10,000 phosphorylation sites and 5000 proteins that were assigned to different pathways. Analysis of mitochondrial proteins led to the identification of PGC-1- and ERR-induced regulator in muscle 1 (PERM1), which is specifically expressed in skeletal muscle and heart tissue and associates with the outer mitochondrial membrane. We demonstrate PERM1 is subject to rapid changes mediated by the UPS through phosphorylation of its PEST motif by casein kinase 2. Ablation of Perm1 in mice results in reduced protein expression of lipin-1 accompanied by accumulation of specific phospholipid species. Isolation of Perm1-deficient mitochondria revealed significant downregulation of mitochondrial transport proteins for amino acids and carnitines, including SLC25A12/13/29/34 and CPT2. Consistently, we observed altered levels of various lipid species, amino acids, and acylcarnitines in Perm1-/- mitochondria. We conclude that the outer mitochondrial membrane protein PERM1 regulates homeostasis of lipid and amino acid metabolites in mitochondria.
    Keywords:  Heart development; Lipid metabolism; Mitochondria; PERM1; Phosphoproteomics; SILAC
    DOI:  https://doi.org/10.1016/j.yjmcc.2021.01.010
  9. Am J Respir Cell Mol Biol. 2021 Feb 12.
    PINK1 Inhibits Multimeric Aggregation and Signaling of MAVS and MAVS-dependent Lung Pathology.
    Sang-Hun Kim, Hyeon Jun Shin, Chang Min Yoon, Sei Won Lee, Lokesh Sharma, Charles S Dela Cruz, Min-Jong Kang.
      Mitochondria have emerged as important signaling organelles where intracellular perturbations are integrated and, consequently, intracellular signaling pathways are modulated to execute appropriate cellular functions. Mitochondrial antiviral signaling (MAVS) protein represents such an example to function as a platform molecule to mediate mitochondrial innate immune signaling. Recently, multimeric aggregation of MAVS has been identified as a key molecular process for its signaling. The underlying mechanisms to regulate this, however, are still incompletely understood. We hypothesized that PTEN-induced kinase 1 (PINK1) plays an important role in the regulation of multimeric MAVS aggregation and its consequent pathobiology. To test whether PINK1 interacts with MAVS, Bimolecular Fluorescence Complementation (BiFC) analysis and immunoprecipitation were performed, respectively. RLH and NLRP3 inflammasomes signaling were evaluated by in vitro assay. In vivo functional significance of PINK1 in the regulation of MAVS signaling was evaluated from both murine modeling of Influenza viral infection and bleomycin-induced experimental pulmonary fibrosis, respectively, wherein MAVS plays important roles. Multimeric MAVS aggregation was induced by mitochondria dysfunction and, during this event, the stabilized PINK1 interacted physically with MAVS and antagonized multimeric MAVS aggregation. Accordingly, the MAVS-mediated antiviral innate immune and NLRP3 inflammasomes signaling were enhanced in PINK1 deficiency. Additionally, in vivo studies revealed that MAVS-mediated pulmonary antiviral innate immune responses and fibrotic responses after bleomycin injury, respectively, were enhanced in PINK1 deficiency. In conclusion, these results establish a new role of PINK1 in the regulation of MAVS signaling and the consequent pulmonary pathobiology.
    Keywords:  MAVS aggregation; Mitochondrial antiviral signaling (MAVS) protein; PTEN‐induced kinase 1 (PINK1); inflammasomes; mitochondrial signaling
    DOI:  https://doi.org/10.1165/rcmb.2020-0490OC
  10. Elife. 2021 Feb 10. pii: e61798. [Epub ahead of print]10
    Enhancing mitochondrial activity in neurons protects against neurodegeneration in a mouse model of multiple sclerosis.
    Sina C Rosenkranz, Artem A Shaposhnykov, Simone Träger, Jan Broder Engler, Maarten E Witte, Vanessa Roth, Vanessa Vieira, Nanne Paauw, Simone Bauer, Celina Schwencke-Westphal, Charlotte Schubert, Lukas Can Bal, Benjamin Schattling, Ole Pless, Jack van Horssen, Marc Freichel, Manuel A Friese.
      While transcripts of neuronal mitochondrial genes are strongly suppressed in central nervous system inflammation, it is unknown whether this results in mitochondrial dysfunction and whether an increase of mitochondrial function can rescue neurodegeneration. Here we show that predominantly genes of the electron transport chain are suppressed in inflamed mouse neurons resulting in impaired mitochondrial complex IV activity. This was associated with posttranslational inactivation of the transcriptional co-regulator PGC-1α. In mice, neuronal overexpression of Ppargc1a, which encodes for PGC-1α, led to increased numbers of mitochondria, complex IV activity and maximum respiratory capacity. Moreover, Ppargc1a overexpressing neurons showed a higher mitochondrial membrane potential that related to an improved calcium buffering capacity. Accordingly, neuronal deletion of Ppargc1a aggravated neurodegeneration during experimental autoimmune encephalomyelitis (EAE), while neuronal overexpression of Ppargc1a ameliorated it. Our study provides systemic insights into mitochondrial dysfunction in neurons during inflammation and commends elevation of mitochondrial activity as a promising neuroprotective strategy.
    Keywords:  immunology; inflammation; mouse; neuroscience
    DOI:  https://doi.org/10.7554/eLife.61798
  11. Annu Rev Biochem. 2021 Feb 08.
    Structural Mechanism of Transport of Mitochondrial Carriers.
    J J Ruprecht, E R S Kunji.
      Members of the mitochondrial carrier family [solute carrier family 25 (SLC25)] transport nucleotides, amino acids, carboxylic acids, fatty acids, inorganic ions, and vitamins across the mitochondrial inner membrane. They are important for many cellular processes, such as oxidative phosphorylation of lipids and sugars, amino acid metabolism, macromolecular synthesis, ion homeostasis, cellular regulation, and differentiation. Here, we describe the functional elements of the transport mechanism of mitochondrial carriers, consisting of one central substrate-binding site and two gates with salt-bridge networks on either side of the carrier. Binding of the substrate during import causes three gate elements to rotate inward, forming the cytoplasmic network and closing access to the substrate-binding site from the intermembrane space. Simultaneously, three core elements rock outward, disrupting the matrix network and opening the substrate-binding site to the matrix side of the membrane. During export, substrate binding triggers conformational changes involving the same elements but operating in reverse. Expected final online publication date for the Annual Review of Biochemistry, Volume 90 is June 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
    DOI:  https://doi.org/10.1146/annurev-biochem-072820-020508
  12. Autophagy. 2021 Feb 11.
    Regulation of PRKN-Independent Mitophagy.
    Petra Terešak, Ana Lapao, Nemanja Subic, Patricia Boya, Zvulun Elazar, Anne Simonsen.
      Mitochondria are dynamic, multifunctional cellular organelles that play a fundamental role in maintaining cellular homeostasis. Keeping the quality of mitochondria in check is of essential importance for functioning and survival of the cells. Selective autophagic clearance of flawed mitochondria, a process termed mitophagy, is one of the most prominent mechanisms through which cells maintain a healthy mitochondrial pool. The best-studied pathway through which mitophagy is exerted is the PINK1-PRKN pathway. However, an increasing number of studies have shown an existence of alternative pathways, where different proteins and lipids are able to recruit autophagic machinery independently of PINK1 and PRKN. The significance of PRKN-independent mitophagy pathways is reflected in various physiological and pathophysiological processes, but many questions regarding the regulation and the interplay between these pathways remain open. Here we review the current knowledge and recent progress made in the field of PRKN-independent mitophagy. Particularly we focus on the regulation of various receptors that participate in targeting impaired mitochondria to autophagosomes independently of PRKN.
    Keywords:  autophagy receptors; mitochondria; mitochondrial dysfunction; mitophagy; selective autophagy
    DOI:  https://doi.org/10.1080/15548627.2021.1888244
  13. Cell Mol Life Sci. 2021 Feb 13.
    Mitochondrial dynamics in cancer stem cells.
    Dane T Sessions, David F Kashatus.
      Many tumors are now understood to be heterogenous cell populations arising from a minority of epithelial-like cancer stem cells (CSCs). CSCs demonstrate distinctive metabolic signatures from the more differentiated surrounding tumor bulk that confer resistance to traditional chemotherapeutic regimens and potential for tumor relapse. Many CSC phenotypes including metabolism, epithelial-to-mesenchymal transition, cellular signaling pathway activity, and others, arise from altered mitochondrial function and turnover, which are regulated by constant cycles of mitochondrial fusion and fission. Further, recycling of mitochondria through mitophagy in CSCs is associated with maintenance of reactive oxygen species levels that dictate gene expression. The protein machinery that drives mitochondrial dynamics is surprisingly simple and may represent attractive new therapeutic avenues to target CSC metabolism and selectively eradicate tumor-generating cells to reduce the risks of metastasis and relapse for a variety of tumor types.
    Keywords:  Cancer stem cells; EMT; Metabolism; Mitochondrial dynamics; Mitochondrial morphology; Signaling; Therapeutic resistance
    DOI:  https://doi.org/10.1007/s00018-021-03773-2
  14. J Neurosci Methods. 2021 Feb 04. pii: S0165-0270(21)00028-5. [Epub ahead of print] 109093
    Quantifying Mitochondrial Volume Density in Phrenic Motor Neurons.
    Matthew J Fogarty, Sabhya Rana, Carlos B Mantilla, Gary C Sieck.
       BACKGROUND: Previous assessments of mitochondrial volume density within motor neurons used electron microscopy (EM) to image mitochondria. However, adequate identification and sampling of motor neurons within a particular motor neuron pool is largely precluded using EM. Here, we present an alternative method for determining mitochondrial volume density in identified motor neurons within the phrenic motor neuron (PhMN) pool, with greatly increased sampling.
    NEW METHOD: This novel method for assessing mitochondrial volume density in PhMNs uses a combination of intrapleural injection of Alexa 488-conjugated cholera toxin B (CTB) to retrogradely label PhMNs, followed by intrathecal application of MitoTracker Red to label mitochondria. This technique was validated by comparison to 3D EM determination of mitochondrial volume density as a "gold standard".
    RESULTS: A mean mitochondrial volume density of ∼11% was observed across PhMNs using the new MitoTracker Red method. This compared favourably with mitochondrial volume density (∼11%) measurements using EM.
    COMPARISON WITH EXISTING METHOD: The range, mean and variance of mitochondrial volume density estimates in PhMNs were not different between EM and fluorescent imaging techniques.
    CONCLUSIONS: Fluorescent imaging may be used to estimate mitochondrial volume density in a large sample of motor neurons, with results similar to EM, although EM did distinguish finer mitochondrion morphology compared to MitoTracker fluorescence. Compared to EM methods, the assessment of a larger sample size and unambiguous identification of motor neurons belonging to a specific motor neuron pool represent major advantages over previous methods.
    Keywords:  confocal microscopy; electron microscopy; mitochondria; motor neuron
    DOI:  https://doi.org/10.1016/j.jneumeth.2021.109093
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