bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021‒03‒28
fifteen papers selected by
Edmond Chan
Queen’s University, School of Medicine

  1. Nat Commun. 2021 03 22. 12(1): 1807
      Mitochondria-lysosome contacts are recently identified sites for mediating crosstalk between both organelles, but their role in normal and diseased human neurons remains unknown. In this study, we demonstrate that mitochondria-lysosome contacts can dynamically form in the soma, axons, and dendrites of human neurons, allowing for their bidirectional crosstalk. Parkinson's disease patient derived neurons harboring mutant GBA1 exhibited prolonged mitochondria-lysosome contacts due to defective modulation of the untethering protein TBC1D15, which mediates Rab7 GTP hydrolysis for contact untethering. This dysregulation was due to decreased GBA1 (β-glucocerebrosidase (GCase)) lysosomal enzyme activity in patient derived neurons, and could be rescued by increasing enzyme activity with a GCase modulator. These defects resulted in disrupted mitochondrial distribution and function, and could be further rescued by TBC1D15 in Parkinson's patient derived GBA1-linked neurons. Together, our work demonstrates a potential role of mitochondria-lysosome contacts as an upstream regulator of mitochondrial function and dynamics in midbrain dopaminergic neurons in GBA1-linked Parkinson's disease.
  2. Nat Commun. 2021 03 22. 12(1): 1812
      Human hexokinase 2 is an essential regulator of glycolysis that couples metabolic and proliferative activities in cancer cells. The binding of hexokinase 2 to the outer membrane of mitochondria is critical for its oncogenic activity. However, the regulation of hexokinase 2 binding to mitochondria remains unclear. Here, we report that SUMOylation regulates the binding of hexokinase 2 to mitochondria. We find that hexokinase 2 can be SUMOylated at K315 and K492. SUMO-specific protease SENP1 mediates the de-SUMOylation of hexokinase 2. SUMO-defective hexokinase 2 preferably binds to mitochondria and enhances both glucose consumption and lactate production and decreases mitochondrial respiration in parallel. This metabolic reprogramming supports prostate cancer cell proliferation and protects cells from chemotherapy-induced cell apoptosis. Moreover, we demonstrate an inverse relationship between SENP1-hexokinase 2 axis and chemotherapy response in prostate cancer samples. Our data provide evidence for a previously uncovered posttranslational modification of hexokinase 2 in cancer cells, suggesting a potentially actionable strategy for preventing chemotherapy resistance in prostate cancer.
  3. Cell Metab. 2021 Mar 23. pii: S1550-4131(21)00110-8. [Epub ahead of print]
      Mitochondria have an independent genome (mtDNA) and protein synthesis machinery that coordinately activate for mitochondrial generation. Here, we report that the Krebs cycle intermediate fumarate links metabolism to mitobiogenesis through binding to malic enzyme 2 (ME2). Mechanistically, fumarate binds ME2 with two complementary consequences. First, promoting the formation of ME2 dimers, which activate deoxyuridine 5'-triphosphate nucleotidohydrolase (DUT). DUT fosters thymidine generation and an increase of mtDNA. Second, fumarate-induced ME2 dimers abrogate ME2 monomer binding to mitochondrial ribosome protein L45, freeing it for mitoribosome assembly and mtDNA-encoded protein production. Methylation of the ME2-fumarate binding site by protein arginine methyltransferase-1 inhibits fumarate signaling to constrain mitobiogenesis. Notably, acute myeloid leukemia is highly dependent on mitochondrial function and is sensitive to targeting of the fumarate-ME2 axis. Therefore, mitobiogenesis can be manipulated in normal and malignant cells through ME2, an unanticipated governor of mitochondrial biomass production that senses nutrient availability through fumarate.
    Keywords:  acute myeloid leukemia; arginine methylation; deoxyuridine 5′-triphosphate nucleotidohydrolase; fumarate; malic enzyme 2; mitobiogenesis; mitochondrial ribosome; mitochondrial ribosome protein L45; protein arginine methyltransferase 1
  4. Nat Metab. 2021 Mar;3(3): 428-441
      Obesity reduces adipocyte mitochondrial function, and expanding adipocyte oxidative capacity is an emerging strategy to improve systemic metabolism. Here, we report that serine/threonine-protein kinase 3 (STK3) and STK4 are key physiological suppressors of mitochondrial capacity in brown, beige and white adipose tissues. Levels of STK3 and STK4, kinases in the Hippo signalling pathway, are greater in white than brown adipose tissues, and levels in brown adipose tissue are suppressed by cold exposure and greatly elevated by surgical denervation. Genetic inactivation of Stk3 and Stk4 increases mitochondrial mass and function, stabilizes uncoupling protein 1 in beige adipose tissue and confers resistance to metabolic dysfunction induced by high-fat diet feeding. Mechanistically, STK3 and STK4 increase adipocyte mitophagy in part by regulating the phosphorylation and dimerization status of the mitophagy receptor BNIP3. STK3 and STK4 expression levels are elevated in human obesity, and pharmacological inhibition improves metabolic profiles in a mouse model of obesity, suggesting STK3 and STK4 as potential targets for treating obesity-related diseases.
  5. Proc Natl Acad Sci U S A. 2021 Mar 30. pii: e2021385118. [Epub ahead of print]118(13):
      Conventional T cell fate and function are determined by coordination between cellular signaling and mitochondrial metabolism. Invariant natural killer T (iNKT) cells are an important subset of "innate-like" T cells that exist in a preactivated effector state, and their dependence on mitochondrial metabolism has not been previously defined genetically or in vivo. Here, we show that mature iNKT cells have reduced mitochondrial respiratory reserve and iNKT cell development was highly sensitive to perturbation of mitochondrial function. Mice with T cell-specific ablation of Rieske iron-sulfur protein (RISP; T-Uqcrfs1 -/- ), an essential subunit of mitochondrial complex III, had a dramatic reduction of iNKT cells in the thymus and periphery, but no significant perturbation on the development of conventional T cells. The impaired development observed in T-Uqcrfs1 -/- mice stems from a cell-autonomous defect in iNKT cells, resulting in a differentiation block at the early stages of iNKT cell development. Residual iNKT cells in T-Uqcrfs1 -/- mice displayed increased apoptosis but retained the ability to proliferate in vivo, suggesting that their bioenergetic and biosynthetic demands were not compromised. However, they exhibited reduced expression of activation markers, decreased T cell receptor (TCR) signaling and impaired responses to TCR and interleukin-15 stimulation. Furthermore, knocking down RISP in mature iNKT cells diminished their cytokine production, correlating with reduced NFATc2 activity. Collectively, our data provide evidence for a critical role of mitochondrial metabolism in iNKT cell development and activation outside of its traditional role in supporting cellular bioenergetic demands.
    Keywords:  CD1; NKT cells; T cell development; knockout mice; mitochondrial metabolism
  6. Proc Natl Acad Sci U S A. 2021 Mar 30. pii: e2100558118. [Epub ahead of print]118(13):
      Human mitochondrial ATP synthase is a molecular machine with a rotary action bound in the inner organellar membranes. Turning of the rotor, driven by a proton motive force, provides energy to make ATP from ADP and phosphate. Among the 29 component proteins of 18 kinds, ATP6 and ATP8 are mitochondrial gene products, and the rest are nuclear gene products that are imported into the organelle. The ATP synthase is assembled from them via intermediate modules representing the main structural elements of the enzyme. One such module is the c8-ring, which provides the membrane sector of the enzyme's rotor, and its assembly is influenced by another transmembrane (TMEM) protein, TMEM70. We have shown that subunit c interacts with TMEM70 and another hitherto unidentified mitochondrial transmembrane protein, TMEM242. Deletion of TMEM242, similar to deletion of TMEM70, affects but does not completely eliminate the assembly of ATP synthase, and to a lesser degree the assembly of respiratory enzyme complexes I, III, and IV. Deletion of TMEM70 and TMEM242 together prevents assembly of ATP synthase and the impact on complex I is enhanced. Removal of TMEM242, but not of TMEM70, also affects the introduction of subunits ATP6, ATP8, j, and k into the enzyme. TMEM70 and TMEM242 interact with the mitochondrial complex I assembly (the MCIA) complex that supports assembly of the membrane arm of complex I. The interactions of TMEM70 and TMEM242 with MCIA could be part of either the assembly of ATP synthase and complex I or the regulation of their levels.
    Keywords:  ATP synthase; TMEM242; TMEM70; assembly; human mitochondria
  7. Cell Rep. 2021 Mar 23. pii: S2211-1247(21)00189-3. [Epub ahead of print]34(12): 108875
      The maintenance of mitochondrial homeostasis requires PTEN-induced kinase 1 (PINK1)-dependent mitophagy, and mutations in PINK1 are associated with Parkinson's disease (PD). PINK1 is also downregulated in tumor cells with PTEN mutations. However, there is limited information concerning the role of PINK1 in tissue growth and tumorigenesis. Here, we show that the loss of pink1 caused multiple growth defects independent of its pathological target, Parkin. Moreover, knocking down pink1 in muscle cells induced hyperglycemia and limited systemic organismal growth by the induction of Imaginal morphogenesis protein-Late 2 (ImpL2). Similarly, disrupting PTEN activity in multiple tissues impaired systemic growth by reducing pink1 expression, resembling wasting-like syndrome in cancer patients. Furthermore, the re-expression of PINK1 fully rescued defects in carbohydrate metabolism and systemic growth induced by the tissue-specific pten mutations. Our data suggest a function for PINK1 in regulating systemic growth in Drosophila and shed light on its role in wasting in the context of PTEN mutations.
    Keywords:  ImpL2; PINK1; PTEN; Parkin; mitochondria
  8. Elife. 2021 Mar 24. pii: e59687. [Epub ahead of print]10
      Endoplasmic reticulum (ER) and mitochondria form close physical associations to facilitate calcium transfer, thereby regulating mitochondrial function. Neurons with high metabolic demands, such as sensory hair cells, are especially dependent on precisely regulated ER-mitochondria associations. We previously showed that the secreted metalloprotease Pregnancy associated plasma protein-aa (Pappaa) regulates mitochondrial function in zebrafish lateral line hair cells (Alassaf et al., 2019). Here, we show that pappaa mutant hair cells exhibit excessive and abnormally close ER-mitochondria associations, suggesting increased ER-mitochondria calcium transfer. pappaa mutant hair cells are more vulnerable to pharmacological induction of ER-calcium transfer. Additionally, pappaa mutant hair cells display ER stress and dysfunctional downstream processes of the ER-mitochondria axis including altered mitochondrial morphology and reduced autophagy. We further show that Pappaa influences ER-calcium transfer and autophagy via its ability to stimulate insulin-like growth factor-1 bioavailability. Together our results identify Pappaa as a novel regulator of the ER-mitochondria axis.
    Keywords:  cell biology; neuroscience; zebrafish
  9. FASEB J. 2021 Apr;35(4): e21426
      Mitochondrial remodeling through fusion and fission is crucial for progenitor cell differentiation but its role in myogenesis is poorly understood. Here, we characterized the function of mitofusin 2 (Mfn2), a mitochondrial outer membrane protein critical for mitochondrial fusion, in muscle progenitor cells (myoblasts). Mfn2 expression is upregulated during myoblast differentiation in vitro and muscle regeneration in vivo. Targeted deletion of Mfn2 gene in myoblasts (Mfn2MKO ) increases oxygen-consumption rates (OCR) associated with the maximal respiration and spare respiratory capacity, and increased levels of reactive oxygen species (ROS). Skeletal muscles of Mfn2MKO mice exhibit robust mitochondrial swelling with normal mitochondrial DNA content. Additionally, mitochondria isolated from Mfn2MKO muscles have reduced OCR at basal state and for complex I respiration, associated with decreased levels of complex I proteins NDUFB8 (NADH ubiquinone oxidoreductase subunit B8) and NDUFS3 (NADH ubiquinone oxidoreductase subunit S3). However, Mfn2MKO has no obvious effects on myoblast differentiation, muscle development and function, and muscle regeneration. These results demonstrate a novel role of Mfn2 in regulating mitochondrial complex I protein abundance and respiratory functions in myogenic progenitors and myofibers.
    Keywords:  Mfn2; mitochondrion; myogenesis; myogenic progenitor cells; oxidative respiration
  10. BMC Biol. 2021 Mar 24. 19(1): 57
      BACKGROUND: Mitochondrial dysfunction is a common feature of aging, neurodegeneration, and metabolic diseases. Hence, mitotherapeutics may be valuable disease modifiers for a large number of conditions. In this study, we have set up a large-scale screening platform for mitochondrial-based modulators with promising therapeutic potential.RESULTS: Using differentiated human neuroblastoma cells, we screened 1200 FDA-approved compounds and identified 61 molecules that significantly increased cellular ATP without any cytotoxic effect. Following dose response curve-dependent selection, we identified the flavonoid luteolin as a primary hit. Further validation in neuronal models indicated that luteolin increased mitochondrial respiration in primary neurons, despite not affecting mitochondrial mass, structure, or mitochondria-derived reactive oxygen species. However, we found that luteolin increased contacts between mitochondria and endoplasmic reticulum (ER), contributing to increased mitochondrial calcium (Ca2+) and Ca2+-dependent pyruvate dehydrogenase activity. This signaling pathway likely contributed to the observed effect of luteolin on enhanced mitochondrial complexes I and II activities. Importantly, we observed that increased mitochondrial functions were dependent on the activity of ER Ca2+-releasing channels inositol 1,4,5-trisphosphate receptors (IP3Rs) both in neurons and in isolated synaptosomes. Additionally, luteolin treatment improved mitochondrial and locomotory activities in primary neurons and Caenorhabditis elegans expressing an expanded polyglutamine tract of the huntingtin protein.
    CONCLUSION: We provide a new screening platform for drug discovery validated in vitro and ex vivo. In addition, we describe a novel mechanism through which luteolin modulates mitochondrial activity in neuronal models with potential therapeutic validity for treatment of a variety of human diseases.
    Keywords:  High-throughput screen; Luteolin; Mitochondria; Mitochondria-ER contacts; Mitochondrial calcium
  11. Hum Mol Genet. 2021 Mar 22. pii: ddab078. [Epub ahead of print]
      Mutations in CHCHD10, coding for a mitochondrial intermembrane space protein, are a rare cause of autosomal dominant amyotrophic lateral sclerosis (ALS). Mutation-specific toxic gain of function or haploinsufficiency models have been proposed to explain pathogenicity. To decipher the metabolic dysfunction associated with the haploinsufficient p.R15L variant we integrated transcriptomic, metabolomic and proteomic data sets in patient cells subjected to an energetic stress that forces the cells to rely on oxidative phosphorylation for ATP production. Patient cells had a complex I deficiency that resulted in an increased NADH/NAD+ ratio, diminished TCA cycle activity, a reorganization of one carbon metabolism, and an increased AMP/ATP ratio leading to phosphorylation of AMPK and inhibition of mTORC1. These metabolic changes activated the unfolded protein response (UPR) in the ER through the IRE1/XBP1 pathway, upregulating downstream targets including ATF3, ATF4, CHOP and EGLN3, and two cytokine markers of mitochondrial disease, GDF15 and FGF21. Activation of the mitochondrial UPR was mediated through an upregulation of the transcription factors ATF4 and ATF5, leading to increased expression of mitochondrial proteases and heat shock proteins. There was a striking transcriptional up regulation of at least seven dual specific phosphatases, associated with an almost complete dephosphorylation of JNK isoforms, suggesting a concerted deactivation of MAP kinase pathways. This study demonstrates that loss of CHCHD10 function elicits an energy deficit that activates unique responses to nutrient stress in both the mitochondria and ER, which may contribute to the selective vulnerability of motor neurons.
  12. Biochem Soc Trans. 2021 Mar 26. pii: BST20190236. [Epub ahead of print]
      Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects around 2% of individuals over 60 years old. It is characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta of the midbrain, which is thought to account for the major clinical symptoms such as tremor, slowness of movement and muscle stiffness. Its aetiology is poorly understood as the physiological and molecular mechanisms leading to this neuronal loss are currently unclear. However, mitochondrial and lysosomal dysfunction seem to play a central role in this disease. In recent years, defective mitochondrial elimination through autophagy, termed mitophagy, has emerged as a potential contributing factor to disease pathology. PINK1 and Parkin, two proteins mutated in familial PD, were found to eliminate mitochondria under distinct mitochondrial depolarisation-induced stress. However, PINK1 and Parkin are not essential for all types of mitophagy and such pathways occur in most cell types and tissues in vivo, even in the absence of overt mitochondrial stress - so-called basal mitophagy. The most common mutation in PD, that of glycine at position 2019 to serine in the protein kinase LRRK2, results in increased activity and this was recently shown to disrupt basal mitophagy in vivo. Thus, different modalities of mitophagy are affected by distinct proteins implicated in PD, suggesting impaired mitophagy may be a common denominator for the disease. In this short review, we discuss the current knowledge about the link between PD pathogenic mutations and mitophagy, with a particular focus on LRRK2.
    Keywords:  Parkinsons disease; autophagy; leucine-rich repeat kinase; mitophagy
  13. Nat Commun. 2021 Mar 26. 12(1): 1924
      Mutations in coiled-coil-helix-coiled-coil-helix domain containing 10 (CHCHD10) can cause amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). However, the underlying mechanisms are unclear. Here, we generate CHCH10S59L-mutant Drosophila melanogaster and HeLa cell lines to model CHCHD10-associated ALS-FTD. The CHCHD10S59L mutation results in cell toxicity in several tissues and mitochondrial defects. CHCHD10S59L independently affects the TDP-43 and PINK1 pathways. CHCHD10S59L expression increases TDP-43 insolubility and mitochondrial translocation. Blocking TDP-43 mitochondrial translocation with a peptide inhibitor reduced CHCHD10S59L-mediated toxicity. While genetic and pharmacological modulation of PINK1 expression and activity of its substrates rescues and mitigates the CHCHD10S59L-induced phenotypes and mitochondrial defects, respectively, in both Drosophila and HeLa cells. Our findings suggest that CHCHD10S59L-induced TDP-43 mitochondrial translocation and chronic activation of PINK1-mediated pathways result in dominant toxicity, providing a mechanistic insight into the CHCHD10 mutations associated with ALS-FTD.
  14. FASEB J. 2021 Apr;35(4): e21278
      Mitochondria share attributes of vesicular transport with their bacterial ancestors given their ability to form mitochondrial-derived vesicles (MDVs). MDVs are involved in mitochondrial quality control and their formation is enhanced with stress and may, therefore, play a potential role in mitochondrial-cellular communication. However, MDV proteomic cargo has remained mostly undefined. In this study, we strategically used an in vitro MDV budding/reconstitution assay on cardiac mitochondria, followed by graded oxidative stress, to identify and characterize the MDV proteome. Our results confirmed previously identified cardiac MDV markers, while also revealing a complete map of the MDV proteome, paving the way to a better understanding of the role of MDVs. The oxidative stress vulnerability of proteins directed the cargo loading of MDVs, which was enhanced by antimycin A (Ant-A). Among OXPHOS complexes, complexes III and V were found to be Ant-A-sensitive. Proteins from metabolic pathways such as the TCA cycle and fatty acid metabolism, along with Fe-S cluster, antioxidant response proteins, and autophagy were also found to be Ant-A sensitive. Intriguingly, proteins containing hyper-reactive cysteine residues, metabolic redox switches, including professional redox enzymes and those that mediate iron metabolism, were found to be components of MDV cargo with Ant-A sensitivity. Last, we revealed a possible contribution of MDVs to the formation of extracellular vesicles, which may indicate mitochondrial stress. In conclusion, our study provides an MDV proteomics signature that delineates MDV cargo selectivity and hints at the potential for MDVs and their novel protein cargo to serve as vital biomarkers during mitochondrial stress and related pathologies.
    Keywords:  hyper-reactive cysteine residues; mitochondrial iron transport; mitochondrial quality control; mitochondrial stress; mitochondrial-derived vesicle proteome
  15. Autophagy. 2021 Mar 23.
      Mitochondria are the main cellular energy powerhouses and supply most of the energy in the form of ATP to fuel essential neuronal functions through oxidative phosphorylation (OXPHOS). In Alzheimer disease (AD), metabolic and mitochondrial disruptions are an early feature preceding any histopathological and clinical manifestations. Mitochondrial malfunction is also linked to synaptic defects in early AD. Mitophagy serves as a key cellular quality control mechanism involving sequestration of damaged mitochondria within autophagosomes and their subsequent degradation in lysosomes. However, it remains largely unknown whether mitophagy is involved in the regulation of energy metabolism in neurons, and if so, whether metabolic deficiency in AD is attributed to mitophagy dysfunction. Here we reveal that mitophagy is broadly activated in metabolically enhanced neurons upon OXPHOS stimulation, which sustains high energetic activity by increasing mitochondrial turnover and hence facilitating mitochondrial maintenance. Unexpectedly, in AD-related mutant HsAPP Tg mouse brains, early stimulation of OXPHOS activity fails to correct energy deficits but exacerbates synapse loss as a consequence of mitophagy failure. Excitingly, lysosomal enhancement in AD neurons restores impaired metabolic function by promoting elimination of damaged mitochondria, protecting against synaptic damage in AD mouse brains. Taken together, we propose a new mechanism by which mitophagy controls bioenergetic status in neurons, furthering our understanding of the direct impact of mitophagy defects on AD-linked metabolic deficits and shedding light on the development of novel therapeutic strategies to treat AD by the early stimulation of mitochondrial metabolism combined with elevation of lysosomal proteolytic activity.
    Keywords:  Alzheimer; bioenergetics; energy metabolism; lysosomal proteolysis; metabolic deficiency; mitochondrial stress; mitophagosome; neuronal mitophagy; retrograde transport; synapse loss