bims-mecosi Biomed News
on Membrane contact sites
Issue of 2022–10–30
seven papers selected by
Verena Kohler, University of Graz



  1. Front Mol Biosci. 2022 ;9 959844
      Skeletal muscle has a critical role in the regulation of the energy balance of the organism, particularly as the principal tissue responsible for insulin-stimulated glucose disposal and as the major site of peripheral insulin resistance (IR), which has been related to accumulation of lipid intermediates, reduced oxidative capacity of mitochondria and endoplasmic reticulum (ER) stress. These organelles form contact sites, known as mitochondria-associated ER membranes (MAMs). This interconnection seems to be involved in various cellular processes, including Ca2+ transport and energy metabolism; therefore, MAMs could play an important role in maintaining cellular homeostasis. Evidence suggests that alterations in MAMs may contribute to IR. However, the evidence does not refer to a specific subcellular location, which is of interest due to the fact that skeletal muscle is constituted by oxidative and glycolytic fibers as well as different mitochondrial populations that appear to respond differently to stimuli and pathological conditions. In this review, we show the available evidence of possible differential responses in the formation of MAMs in skeletal muscle as well as its role in insulin signaling and the beneficial effect it could have in the regulation of energetic metabolism and muscular contraction.
    Keywords:  insulin resistance; mitochondria-associated ER membranes (MAMs); mitochondrial dysfunction; mitochondrial skeletal muscle; mitochondrial subpopulations; obesity
    DOI:  https://doi.org/10.3389/fmolb.2022.959844
  2. J Cell Biol. 2022 Dec 05. pii: e202207022. [Epub ahead of print]221(12):
      Lipid transport proteins at membrane contacts, where organelles are closely apposed, are critical in redistributing lipids from the endoplasmic reticulum (ER), where they are made, to other cellular membranes. Such protein-mediated transfer is especially important for maintaining organelles disconnected from secretory pathways, like mitochondria. We identify mitoguardin-2, a mitochondrial protein at contacts with the ER and/or lipid droplets (LDs), as a lipid transporter. An x-ray structure shows that the C-terminal domain of mitoguardin-2 has a hydrophobic cavity that binds lipids. Mass spectrometry analysis reveals that both glycerophospholipids and free-fatty acids co-purify with mitoguardin-2 from cells, and that each mitoguardin-2 can accommodate up to two lipids. Mitoguardin-2 transfers glycerophospholipids between membranes in vitro, and this transport ability is required for roles both in mitochondrial and LD biology. While it is not established that protein-mediated transfer at contacts plays a role in LD metabolism, our findings raise the possibility that mitoguardin-2 functions in transporting fatty acids and glycerophospholipids at mitochondria-LD contacts.
    DOI:  https://doi.org/10.1083/jcb.202207022
  3. PLoS Biol. 2022 Oct;20(10): e3001854
      Centrioles are non-membrane-bound organelles that participate in fundamental cellular processes through their ability to form physical contacts with other structures. During interphase, two mature centrioles can associate to form a single centrosome-a phenomenon known as centrosome cohesion. Centrosome cohesion is important for processes such as cell migration, and yet how it is maintained is unclear. Current models indicate that pericentriolar fibres termed rootlets, also known as the centrosome linker, entangle to maintain centriole proximity. Here, I uncover a centriole-centriole contact site and mechanism of centrosome cohesion based on coalescence of the proximal centriole component cNap1. Using live-cell imaging of endogenously tagged cNap1, I show that proximal centrioles form dynamic contacts in response to physical force from the cytoskeleton. Expansion microscopy reveals that cNap1 bridges between these contact sites, physically linking proximal centrioles on the nanoscale. Fluorescence correlation spectroscopy (FCS)-calibrated imaging shows that cNap1 accumulates at nearly micromolar concentrations on proximal centrioles, corresponding to a few hundred protein copy numbers. When ectopically tethered to organelles such as lysosomes, cNap1 forms viscous and cohesive assemblies that promote organelle spatial proximity. These results suggest a mechanism of centrosome cohesion by cNap1 at the proximal centriole and illustrate how a non-membrane-bound organelle forms organelle contact sites.
    DOI:  https://doi.org/10.1371/journal.pbio.3001854
  4. Biochem Soc Trans. 2022 Oct 28. pii: BST20220519. [Epub ahead of print]
      Advances in public health have nearly doubled life expectancy over the last century, but this demographic shift has also changed the landscape of human illness. Today, chronic and age-dependent diseases dominate the leading causes of morbidity and mortality worldwide. Targeting the underlying molecular, genetic and cell biological drivers of the aging process itself appears to be an increasingly viable strategy for developing therapeutics against these diseases of aging. Towards this end, one of the most exciting developments in cell biology over the last decade is the explosion of research into organelle contact sites and related mechanisms of inter-organelle communication. Identification of the molecular mediators of inter-organelle tethering and signaling is now allowing the field to investigate the consequences of aberrant organelle interactions, which frequently seem to correlate with age-onset pathophysiology. This review introduces the major cellular roles for inter-organelle interactions, including the regulation of organelle morphology, the transfer of ions, lipids and other metabolites, and the formation of hubs for nutrient and stress signaling. We explore how these interactions are disrupted in aging and present findings that modulation of inter-organelle communication is a promising avenue for promoting longevity. Through this review, we propose that the maintenance of inter-organelle interactions is a pillar of healthy aging. Learning how to target the cellular mechanisms for sensing and controlling inter-organelle communication is a key next hurdle for geroscience.
    Keywords:  aging; endoplasmic reticulum; inter-organelle; longevity; mitochondria
    DOI:  https://doi.org/10.1042/BST20220519
  5. Life Sci. 2022 Oct 22. pii: S0024-3205(22)00812-8. [Epub ahead of print] 121112
       AIM: Mitochondrial fission-fusion events, distribution, and Ca2+-buffering abilities are relevant for several diseases, yet are poorly understood events. TRPV4 channels are a group of thermosensitive ion channel which regulate cellular and mitochondrial Ca2+-level. The underlying mechanisms of the change in mitochondrial dynamics upon modulation of TRPV4 channel are ill explored.
    MAIN METHODS: We have used TRPV4 expressing stable cell line CHO-K1-V4 and compared with CHO-K1-MOCK as a control cell. We have also used mouse bone marrow derived mesenchymal stem cells and purified mitochondria from mouse brain for the interaction study.
    KEY FINDINGS: Now we demonstrate that expression and/or pharmacological modulation of TRPV4 regulates mitochondrial morphologies and Ca2+-level. TRPV4 interacts with MFN1/MFN2, the mitochondrial regulatory factors. TRPV4 regulates ER-mito contact points. We used different cellular conditions where cytosolic or ER Ca2+-levels were pharmacologically altered. Analysis of ~55,000 mitochondrial particles, ~125,000 ER-mito contact points from ~900 cells in 10 different cellular conditions suggest that ER-mito contact points are inversely regulated with mitochondrial Ca2+-levels where TRPV4 always elevates mitochondrial Ca2+-levels. These findings link TRPV4 with MFN2-mediated diseases and suggest that different TRPV4-induced channelopathies are likely due to mitochondrial abnormalities.
    Keywords:  CMT disease; Ca(2+)-chelation; Channelopathy; Mitochondria-associated ER membrane; Mitochondrial fission-fusion; Thermosensitive ion channel
    DOI:  https://doi.org/10.1016/j.lfs.2022.121112
  6. Antioxid Redox Signal. 2022 Oct 27.
       SIGNIFICANCE: Cells depend on well-functioning mitochondria for essential processes such as energy production, redox signaling, coordination of metabolic pathways, and cofactor biosynthesis. Mitochondrial dysfunction, metabolic decline and protein stress have been implicated in the etiology of multiple late-onset diseases, including various ataxias, diabetes, sarcopenia, neuromuscular disorders, and neurodegenerative diseases such as parkinsonism, amyotrophic lateral sclerosis and glaucoma.
    RECENT ADVANCES: New evidence supports increased energy metabolism protects neuron function during aging. Key energy metabolic enzymes, however, are susceptible to oxidative damage making it imperative that the mitochondrial proteome is protected. Over 40 different enzymes have been identified as important factors for guarding mitochondrial health and maintaining a dynamic pool of mitochondria.
    CRITICAL ISSUES: Understanding shared mechanisms of age-related disorders of neurodegenerative diseases such as glaucoma, Alzheimer's Disease, and Parkinson's Disease (PD) is important for developing new therapies. Functional mitochondrial shape and dynamics rely on complex interactions between mitochondrial proteases and membrane proteins. Identifying the sequence of molecular events that lead to mitochondrial dysfunction and metabolic stress is a major challenge.
    FUTURE DIRECTIONS: A critical need exists for new strategies that reduce mitochondrial protein stress and promote mitochondrial dynamics in age-related neurological disorders. Discovering how mitochondria-associated degradation is related to proteostatic mechanisms in mitochondrial compartments may reveal new opportunities for therapeutic interventions. Also, little is known about how protein and membrane contacts in the inner and outer mitochondrial membrane are regulated, even though they are pivotal for mitochondrial architecture. Future work will need to delineate the molecular details of these processes.
    DOI:  https://doi.org/10.1089/ars.2022.0124
  7. Open Biol. 2022 Oct;12(10): 220155
      Lysosomal storage diseases (LSDs) comprise a group of inherited monogenic disorders characterized by lysosomal dysfunctions due to undegraded substrate accumulation. They are caused by a deficiency in specific lysosomal hydrolases involved in cellular catabolism, or non-enzymatic proteins essential for normal lysosomal functions. In LSDs, the lack of degradation of the accumulated substrate and its lysosomal storage impairs lysosome functions resulting in the perturbation of cellular homeostasis and, in turn, the damage of multiple organ systems. A substantial number of studies on the pathogenesis of LSDs has highlighted how the accumulation of lysosomal substrates is only the first event of a cascade of processes including the accumulation of secondary metabolites and the impairment of cellular trafficking, cell signalling, autophagic flux, mitochondria functionality and calcium homeostasis, that significantly contribute to the onset and progression of these diseases. Emerging studies on lysosomal biology have described the fundamental roles of these organelles in a variety of physiological functions and pathological conditions beyond their canonical activity in cellular waste clearance. Here, we discuss recent advances in the knowledge of cellular and molecular mechanisms linking lysosomal positioning and trafficking to LSDs.
    Keywords:  lysosomal storage diseases; lysosome; membrane contact sites; microtubule tracks; positioning; trafficking
    DOI:  https://doi.org/10.1098/rsob.220155