bims-mitdyn Biomed News
on Mitochondrial dynamics: mechanisms
Issue of 2021‒03‒14
nine papers selected by
Edmond Chan
Queen’s University, School of Medicine

  1. Cell Rep. 2021 Mar 09. pii: S2211-1247(21)00141-8. [Epub ahead of print]34(10): 108827
      Calcium transfer from the endoplasmic reticulum (ER) to mitochondria is a critical contributor to apoptosis. B cell lymphoma 2 (BCL-2) ovarian killer (BOK) localizes to the ER and binds the inositol 1,4,5-trisphosophate receptor (IP3R). Here, we show that BOK is necessary for baseline mitochondrial calcium levels and stimulus-induced calcium transfer from the ER to the mitochondria. Murine embryonic fibroblasts deficient for BOK have decreased proximity of the ER to the mitochondria and altered protein composition of mitochondria-associated membranes (MAMs), which form essential calcium microdomains. Rescue of the ER-mitochondrial juxtaposition with drug-inducible interorganelle linkers reveals a kinetic disruption, which when overcome in Bok-/- cells is still insufficient to rescue thapsigargin-induced calcium transfer and apoptosis. Likewise, a BOK mutant unable to interact with IP3R restores ER-mitochondrial proximity, but not ER-mitochondrial calcium transfer, MAM protein composition, or apoptosis. This work identifies the dynamic coordination of ER-mitochondrial contact by BOK as an important control point for apoptosis.
    Keywords:  BCL-2 family; BOK; IP3R; MAMs; MERCs; apoptosis; calcium; endoplasmic reticulum; mitochondria-ER contact sites; mitochondria-associated membranes
  2. Elife. 2021 Mar 09. pii: e64351. [Epub ahead of print]10
      The hypothalamic orexigenic Agouti-related peptide (AgRP)-expressing neurons are crucial for the regulation of whole-body energy homeostasis. Here, we show that fasting-induced AgRP neuronal activation is associated with dynamin-related peptide 1 (DRP1)-mediated mitochondrial fission and mitochondrial fatty acid utilization in AgRP neurons. In line with this, mice lacking Dnm1l in adult AgRP neurons (Drp1 cKO) show decreased fasting- or ghrelin-induced AgRP neuronal activity and feeding and exhibited a significant decrease in body weight, fat mass, and feeding accompanied by a significant increase in energy expenditure. In support of the role for mitochondrial fission and fatty acids oxidation, Drp1 cKO mice showed attenuated palmitic acid-induced mitochondrial respiration. Altogether, our data revealed that mitochondrial dynamics and fatty acids oxidation in hypothalamic AgRP neurons is a critical mechanism for AgRP neuronal function and body-weight regulation.
    Keywords:  AgRP; feeding; metabolism; mitochondria; mouse; neuroscience
  3. Cancer Res. 2021 Mar 08. pii: canres.1628.2020. [Epub ahead of print]
      Deferoxamine (DFO) represents a widely used iron chelator for the treatment of iron overload. Here we describe the use of mitochondrially targeted deferoxamine (mitoDFO) as a novel approach to preferentially target cancer cells. The agent showed marked cytostatic, cytotoxic, and migrastatic properties in vitro, and it significantly suppressed tumor growth and metastasis in vivo. The underlying molecular mechanisms included (I) impairment of [Fe-S] cluster/heme biogenesis, leading to destabilization and loss of activity of [Fe-S] cluster/heme containing enzymes, (II) inhibition of mitochondrial respiration leading to mitochondrial ROS production, resulting in dysfunctional mitochondria with markedly reduced supercomplexes, and (III) fragmentation of the mitochondrial network and induction of mitophagy. Mitochondrial targeting of DFO represents a way to deprive cancer cells of biologically active iron, which is incompatible with their proliferation and invasion, without disrupting systemic iron metabolism. Our findings highlight the importance of mitochondrial iron metabolism for cancer cells and demonstrate repurposing deferoxamine into an effective anti-cancer drug via mitochondrial targeting.
  4. Curr Biol. 2021 Mar 05. pii: S0960-9822(21)00275-X. [Epub ahead of print]
      The oxidative environment within the mitochondria makes them particularly vulnerable to proteotoxic stress. To maintain a healthy mitochondrial network, eukaryotes have evolved multi-tiered quality control pathways. If the stress cannot be alleviated, defective mitochondria are selectively removed by autophagy via a process termed mitophagy. Despite significant advances in metazoans and yeast, in plants, the molecular underpinnings of mitophagy are largely unknown. Here, using time-lapse imaging, electron tomography, and biochemical assays, we show that uncoupler treatments cause loss of mitochondrial membrane potential and induce autophagy in Arabidopsis. The damaged mitochondria are selectively engulfed by autophagosomes that are labeled by ATG8 proteins in an ATG5-dependent manner. Friendly, a member of the clustered mitochondria protein family, is recruited to the damaged mitochondria to mediate mitophagy. In addition to the stress, mitophagy is also induced during de-etiolation, a major cellular transformation during photomorphogenesis that involves chloroplast biogenesis. De-etiolation-triggered mitophagy is involved in cotyledon greening, pointing toward an inter-organellar crosstalk mechanism. Altogether, our results demonstrate how plants employ mitophagy to recycle damaged mitochondria during stress and development.
    Keywords:  Arabidopsis; autophagy; clustered mitochondria protein; de-etiolation; electron tomography; mitochondria recycling; mitophagy; time-lapse live-cell imaging; uncoupler
  5. J Am Chem Soc. 2021 Mar 12.
      Mitochondria are the site of aerobic respiration, producing ATP via oxidative phosphorylation as protons flow down their electrochemical gradient through ATP synthase. This negative membrane potential across the inner mitochondrial membrane (ΔΨm) represents a fundamental biophysical parameter central to cellular life. Traditional, electrode-based methods for recording membrane potential are impossible to implement on mitochondria within intact cells. Fluorescent ΔΨm indicators based on cationic, lipophilic dyes are a common alternative, but these indicators are complicated by concentration-dependent artifacts and the requirement to maintain dye in the extracellular solution to visualize reversible ΔΨm dynamics. Here, we report the first example of a fluorescent ΔΨm reporter that does not rely on ΔΨm-dependent accumulation. We redirected the localization of a photoinduced electron transfer (PeT)-based indicator, Rhodamine Voltage Reporter (RhoVR), to mitochondria by masking the carboxylate of RhoVR 1 as an acetoxymethyl (AM) ester. Once within mitochondria, esterases remove the AM ester, trapping RhoVR inside of the mitochondrial matrix, where it can incorporate within the inner membrane and reversibly report on changes in ΔΨm. We show that this Small molecule, Permeable, Internally Redistributing for Inner membrane Targeting Rhodamine Voltage Reporter, or SPIRIT RhoVR, localizes to mitochondria across a number of different cell lines and responds reversibly to changes in ΔΨm induced by exceptionally low concentrations of the uncoupler FCCP without the need for exogenous pools of dye (unlike traditional, accumulation-based rhodamine esters). SPIRIT RhoVR is compatible with multi-color imaging, enabling simultaneous, real-time observation of cytosolic Ca2+, plasma membrane potential, and reversible ΔΨm dynamics.
  6. Autophagy. 2021 Mar 08. 1-10
      PINK1 and PRKN, which cause Parkinson disease when mutated, form a quality control mitophagy pathway that is well-characterized in cultured cells. The extent to which the PINK1-PRKN pathway contributes to mitophagy in vivo, however, is controversial. This is due in large part to conflicting results from studies using one of two mitophagy reporters: mt-Keima or mito-QC. Studies using mt-Keima have generally detected PINK1-PRKN mitophagy in vivo, whereas those using mito-QC generally have not. Here, we directly compared the performance of mito-QC and mt-Keima in cell culture and in mice subjected to a PINK1-PRKN activating stress. We found that mito-QC was less sensitive than mt-Keima for mitophagy, and that this difference was more pronounced for PINK1-PRKN mitophagy. These findings suggest that mito-QC's poor sensitivity may account for conflicting reports of PINK1-PRKN mitophagy in vivo and caution against using mito-QC as a reporter for PINK1-PRKN mitophagy.
    Keywords:  Autophagy; PARK2; PARKIN; Parkinson; degradation; disease; mitochondria; neurodegeneration; organelle
  7. Cell Mol Life Sci. 2021 Mar 12.
      Mitochondria supply cellular energy through oxidative phosphorylation and fulfill numerous additional functions that are fundamental to cellular homeostasis and stress responses. Mitochondrial malfunction, arising from inherent defects of the organelle itself, aging, or acute or chronic stress, can cause substantial damage to organismal health. For instance, mitochondrial malfunction contributes to inflammation, neurodegeneration, tumorigenesis and cardiovascular diseases. Therefore, various quality control mechanisms exist that support a functional mitochondrial organelle compartment. The CMLS Forum Reviews introduced here present a collection of articles covering select topics on basic mechanisms and pathophysiological contexts of mitochondrial damage control.
    Keywords:  Apoptosis; Autophagy; Mitochondria; Mitochondrial dynamics; Mitochondrial quality control; Mitochondrial retrograde response; Mitochondrial unfolded protein response; Mitophagy; Stress signaling
  8. Trends Cell Biol. 2021 Mar 04. pii: S0962-8924(21)00028-3. [Epub ahead of print]
      Organelles cooperate with each other to control cellular homeostasis and cell functions by forming close connections through membrane contact sites. Important contacts are present between the endoplasmic reticulum (ER), the main intracellular Ca2+-storage organelle, and the mitochondria, the organelle responsible not only for the majority of cellular ATP production but also for switching on cell death processes. Several Ca2+-transport systems focalize at these contact sites, thereby enabling the efficient transmission of Ca2+ signals from the ER toward mitochondria. This provides tight control of mitochondrial functions at the microdomain level. Here, we discuss how ER-mitochondrial Ca2+ transfers support cell function and how their dysregulation underlies, drives, or contributes to pathogenesis and pathophysiology, with a major focus on cancer and neurodegeneration but also with attention to other diseases such as diabetes and rare genetic diseases.
    Keywords:  Ca(2+) signaling; MAMs; cancer; contact sites; genetic diseases; neurodegeneration
  9. J Cell Biol. 2021 May 03. pii: e202006035. [Epub ahead of print]220(5):
      Of the many crucial functions of the ER, homeostasis of physiological calcium increase is critical for signaling. Plasma membrane (PM) injury causes a pathological calcium influx. Here, we show that the ER helps clear this surge in cytoplasmic calcium through an ER-resident calcium pump, SERCA, and a calcium-activated ion channel, Anoctamin 5 (ANO5). SERCA imports calcium into the ER, and ANO5 supports this by maintaining electroneutrality of the ER lumen through anion import. Preventing either of these transporter activities causes cytosolic calcium overload and disrupts PM repair (PMR). ANO5 deficit in limb girdle muscular dystrophy 2L (LGMD2L) patient cells compromises their cytosolic and ER calcium homeostasis. By generating a mouse model of LGMD2L, we find that PM injury causes cytosolic calcium overload and compromises the ability of ANO5-deficient myofibers to repair. Addressing calcium overload in ANO5-deficient myofibers enables them to repair, supporting the requirement of the ER in calcium homeostasis in injured cells and facilitating PMR.