bims-mecosi 2021-11-14 papers

bims-mecosi

Biomed News

on Membrane contact sites

Issue of 2021–11–14
seven papers selected by
Verena Kohler, Stockholm University

GRP75 mediates Endoplasmic Reticulum-Mitochondria coupling during palmitate-induced pancreatic β-cell apoptosis.
Cal'MAM'ity at the Endoplasmic Reticulum-Mitochondrial Interface: A Potential Therapeutic Target for Neurodegeneration and Human Immunodeficiency Virus-Associated Neurocognitive Disorders.
Endoplasmic Reticulum-Based Calcium Dysfunctions in Synucleinopathies.
Regulation of Endoplasmic Reticulum-Mitochondria Tethering and Ca2+ Fluxes by TDP-43 via GSK3β.
Rab32 uses its effector reticulon 3L to trigger autophagic degradation of mitochondria-associated membrane (MAM) proteins.
Severe neonatal MEGDHEL syndrome with a homozygous truncating mutation in SERAC1.
Regulation of neuronal excitation-transcription coupling by Kv2.1-induced clustering of somatic L-type Ca2+ channels at ER-PM junctions.

J Biol Chem. 2021 Oct 28. pii: S0021-9258(21)01174-1. [Epub ahead of print] 101368

GRP75 mediates Endoplasmic Reticulum-Mitochondria coupling during palmitate-induced pancreatic β-cell apoptosis.

Shweta Tiwary, Arun Nandwani, Rukshar Khan, Malabika Datta.

  The endoplasmic reticulum (ER) and mitochondria are structurally connected with each other at specific sites termed mitochondria-associated membranes (MAM). These physical links are composed of several tethering proteins and are important during varied cellular processes, such as calcium homeostasis, lipid metabolism and transport, membrane biogenesis and organelle remodeling. However, the attributes of specific tethering proteins in these cellular functions remain debatable. Here, we present data to show that one such tether protein, GRP75, is essential in increasing ER-mitochondria contact during palmitate-induced apoptosis in pancreatic insulinoma cells. We demonstrate that palmitate increased GRP75 levels in mouse and rat pancreatic insulinoma cells as well as in mouse primary islet cells. This was associated with increased mitochondrial Ca2+ transfer, impaired mitochondrial membrane potential, increased ROS production, and enhanced physical coupling between the ER and mitochondria. Interestingly, GRP75 inhibition prevented these palmitate-induced cellular aberrations. Additionally, GRP75 overexpression alone was sufficient to impair mitochondrial membrane potential, increase mitochondrial Ca2+ levels and ROS generation, augment ER-mitochondria contact, and induce apoptosis in these cells. In vivo injection of palmitate induced hyperglycemia and hypertriglyceridemia, as well as impaired glucose and insulin tolerance in mice. These animals also exhibited elevated GRP75 levels accompanied by enhanced apoptosis within the pancreatic islets. Our findings suggest that GRP75 is critical in mediating palmitate-induced ER-mitochondrial interaction leading to apoptosis in pancreatic islet cells.

Keywords:  ER-mitochondria contact; GRP75; calcium apoptosis; palmitate

DOI:  https://doi.org/10.1016/j.jbc.2021.101368
Front Neurosci. 2021 ;15 715945

Cal'MAM'ity at the Endoplasmic Reticulum-Mitochondrial Interface: A Potential Therapeutic Target for Neurodegeneration and Human Immunodeficiency Virus-Associated Neurocognitive Disorders.

Jessica Proulx, In-Woo Park, Kathleen Borgmann.

  The endoplasmic reticulum (ER) is a multifunctional organelle and serves as the primary site for intracellular calcium storage, lipid biogenesis, protein synthesis, and quality control. Mitochondria are responsible for producing the majority of cellular energy required for cell survival and function and are integral for many metabolic and signaling processes. Mitochondria-associated ER membranes (MAMs) are direct contact sites between the ER and mitochondria that serve as platforms to coordinate fundamental cellular processes such as mitochondrial dynamics and bioenergetics, calcium and lipid homeostasis, autophagy, apoptosis, inflammation, and intracellular stress responses. Given the importance of MAM-mediated mechanisms in regulating cellular fate and function, MAMs are now known as key molecular and cellular hubs underlying disease pathology. Notably, neurons are uniquely susceptible to mitochondrial dysfunction and intracellular stress, which highlights the importance of MAMs as potential targets to manipulate MAM-associated mechanisms. However, whether altered MAM communication and connectivity are causative agents or compensatory mechanisms in disease development and progression remains elusive. Regardless, exploration is warranted to determine if MAMs are therapeutically targetable to combat neurodegeneration. Here, we review key MAM interactions and proteins both in vitro and in vivo models of Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We further discuss implications of MAMs in HIV-associated neurocognitive disorders (HAND), as MAMs have not yet been explored in this neuropathology. These perspectives specifically focus on mitochondrial dysfunction, calcium dysregulation and ER stress as notable MAM-mediated mechanisms underlying HAND pathology. Finally, we discuss potential targets to manipulate MAM function as a therapeutic intervention against neurodegeneration. Future investigations are warranted to better understand the interplay and therapeutic application of MAMs in glial dysfunction and neurotoxicity.

Keywords:  ER stress; Unfolded protein response; astrocytes; calcium dysregulation; mitochondria-associated ER membranes; mitochondrial dysfunction; neuropathology

DOI:  https://doi.org/10.3389/fnins.2021.715945
Front Neurol. 2021 ;12 742625

Endoplasmic Reticulum-Based Calcium Dysfunctions in Synucleinopathies.

Gergo Kovacs, Lasse Reimer, Poul Henning Jensen.

  Neuronal calcium dyshomeostasis has been associated to Parkinson's disease (PD) development based on epidemiological studies on users of calcium channel antagonists and clinical trials are currently conducted exploring the hypothesis of increased calcium influx into neuronal cytosol as basic premise. We reported in 2018 an opposite hypothesis based on the demonstration that α-synuclein aggregates stimulate the endoplasmic reticulum (ER) calcium pump SERCA and demonstrated in cell models the existence of an α-synuclein-aggregate dependent neuronal state wherein cytosolic calcium is decreased due to an increased pumping of calcium into the ER. Inhibiting the SERCA pump protected both neurons and an α-synuclein transgenic C. elegans model. This models two cellular states that could contribute to development of PD. First the prolonged state with reduced cytosolic calcium that could deregulate multiple signaling pathways. Second the disease ER state with increased calcium concentration. We will discuss our hypothesis in the light of recent papers. First, a mechanistic study describing how variation in the Inositol-1,4,5-triphosphate (IP3) kinase B (ITPKB) may explain GWAS studies identifying the ITPKB gene as a protective factor toward PD. Here it was demonstrated that how increased ITPKB activity reduces influx of ER calcium to mitochondria via contact between IP3-receptors and the mitochondrial calcium uniporter complex in ER-mitochondria contact, known as mitochondria-associated membranes (MAMs). Secondly, it was demonstrated that astrocytes derived from PD patients contain α-synuclein accumulations. A recent study has demonstrated how human astrocytes derived from a few PD patients carrying the LRRK2-2019S mutation express more α-synuclein than control astrocytes, release more calcium from ER upon ryanodine receptor (RyR) stimulation, show changes in ER calcium channels and exhibit a decreased maximal and spare respiration indicating altered mitochondrial function in PD astrocytes. Here, we summarize the previous findings focusing the effect of α-synuclein to SERCA, RyR, IP3R, MCU subunits and other MAM-related channels. We also consider how the SOCE-related events could contribute to the development of PD.

Keywords:  IP3R; Parkinson's disease; RyR; SERCA; calcium; endoplasmic reticulum; α-synuclein

DOI:  https://doi.org/10.3389/fneur.2021.742625
Int J Mol Sci. 2021 Nov 01. pii: 11853. [Epub ahead of print]22(21):

Regulation of Endoplasmic Reticulum-Mitochondria Tethering and Ca2+ Fluxes by TDP-43 via GSK3β.

Caterina Peggion, Maria Lina Massimino, Raphael Severino Bonadio, Federica Lia, Raffaele Lopreiato, Stefano Cagnin, Tito Calì, Alessandro Bertoli.

  Mitochondria-ER contacts (MERCs), tightly regulated by numerous tethering proteins that act as molecular and functional connections between the two organelles, are essential to maintain a variety of cellular functions. Such contacts are often compromised in the early stages of many neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). TDP-43, a nuclear protein mainly involved in RNA metabolism, has been repeatedly associated with ALS pathogenesis and other neurodegenerative diseases. Although TDP-43 neuropathological mechanisms are still unclear, the accumulation of the protein in cytoplasmic inclusions may underlie a protein loss-of-function effect. Accordingly, we investigated the impact of siRNA-mediated TDP-43 silencing on MERCs and the related cellular parameters in HeLa cells using GFP-based probes for MERCs quantification and aequorin-based probes for local Ca2+ measurements, combined with targeted protein and mRNA profiling. Our results demonstrated that TDP-43 down-regulation decreases MERCs density, thereby remarkably reducing mitochondria Ca2+ uptake after ER Ca2+ release. Thorough mRNA and protein analyses did not highlight altered expression of proteins involved in MERCs assembly or Ca2+-mediated ER-mitochondria cross-talk, nor alterations of mitochondrial density and morphology were observed by confocal microscopy. Further mechanistic inspections, however, suggested that the observed cellular alterations are correlated to increased expression/activity of GSK3β, previously associated with MERCs disruption.

Keywords:  ER–mitochondria contacts; GSK3β; SPLICS; TDP-43; amyotrophic lateral sclerosis; calcium homeostasis; neurodegenerative disorders

DOI:  https://doi.org/10.3390/ijms222111853
Biol Direct. 2021 Nov 07. 16(1): 22

Rab32 uses its effector reticulon 3L to trigger autophagic degradation of mitochondria-associated membrane (MAM) proteins.

Maria Sol Herrera-Cruz, Megan C Yap, Nasser Tahbaz, Keelie Phillips, Laurel Thomas, Gary Thomas, Thomas Simmen.

   BACKGROUND: Rab32 is a small GTPase associated with multiple organelles but is particularly enriched at the endoplasmic reticulum (ER). Here, it controls targeting to mitochondria-ER contacts (MERCs), thus influencing composition of the mitochondria-associated membrane (MAM). Moreover, Rab32 regulates mitochondrial membrane dynamics via its effector dynamin-related protein 1 (Drp1). Rab32 has also been reported to induce autophagy, an essential pathway targeting intracellular components for their degradation. However, no autophagy-specific effectors have been identified for Rab32. Similarly, the identity of the intracellular membrane targeted by this small GTPase and the type of autophagy it induces are not known yet.
RESULTS: To investigate the target of autophagic degradation mediated by Rab32, we tested a large panel of organellar proteins. We found that a subset of MERC proteins, including the thioredoxin-related transmembrane protein TMX1, are specifically targeted for degradation in a Rab32-dependent manner. We also identified the long isoform of reticulon-3 (RTN3L), a known ER-phagy receptor, as a Rab32 effector.
CONCLUSIONS: Rab32 promotes degradation of mitochondrial-proximal ER membranes through autophagy with the help of RTN3L. We propose to call this type of selective autophagy "MAM-phagy".

Keywords:  Autophagy; ER-phagy; Mitochondria-associated membrane (MAM); Rab32

DOI:  https://doi.org/10.1186/s13062-021-00311-9
Biochim Biophys Acta Mol Basis Dis. 2021 Oct 28. pii: S0925-4439(21)00231-3. [Epub ahead of print]1868(1): 166298

Severe neonatal MEGDHEL syndrome with a homozygous truncating mutation in SERAC1.

Vineta Fellman, Rishi Banerjee, Kai-Lan Lin, Ilari Pulli, Helen Cooper, Henna Tyynismaa, Jukka Kallijärvi.

  In the diagnostic work-up of a newborn infant with a metabolic crisis, lethal multiorgan failure on day six of life, and increased excretion of 3-methylglutaconic acid, we found using whole genome sequencing a homozygous SERAC1 mutation indicating MEGDHEL syndrome (3-methylglutaconic aciduria with deafness-dystonia, hepatopathy, encephalopathy, and Leigh-like syndrome). The SERAC1 protein is located at the contact site between mitochondria and the endoplasmic reticulum (ER) and is crucial for cholesterol trafficking. Our aim was to investigate the effect of the homozygous truncating mutation on mitochondrial structure and function. In the patient fibroblasts, no SERAC1 protein was detected, the mitochondrial network was severely fragmented, and the cristae morphology was altered. Filipin staining showed uneven localization of unesterified cholesterol. The calcium buffer function between cytoplasm and mitochondria was deficient. In liver mitochondria, complexes I, III, and IV were clearly decreased. In transfected COS-1 cells the mutant protein with the a 45-amino acid C-terminal truncation was distributed throughout the cell, whereas wild-type SERAC1 partially colocalized with the mitochondrial marker MT-CO1. The structural and functional mitochondrial abnormalities, caused by the loss of SERAC1, suggest that the crucial disease mechanism is disrupted interplay between the ER and mitochondria leading to decreased influx of calcium to mitochondria and secondary respiratory chain deficiency.

Keywords:  Cholesterol trafficking; Endoplasmic reticulum; Mitochondrial disease; Newborn infant; Respiratory chain

DOI:  https://doi.org/10.1016/j.bbadis.2021.166298
Proc Natl Acad Sci U S A. 2021 Nov 16. pii: e2110094118. [Epub ahead of print]118(46):

Regulation of neuronal excitation-transcription coupling by Kv2.1-induced clustering of somatic L-type Ca2+ channels at ER-PM junctions.

Nicholas C Vierra, Samantha C O'Dwyer, Collin Matsumoto, L Fernando Santana, James S Trimmer.

  In mammalian brain neurons, membrane depolarization leads to voltage-gated Ca2+ channel-mediated Ca2+ influx that triggers diverse cellular responses, including gene expression, in a process termed excitation-transcription coupling. Neuronal L-type Ca2+ channels, which have prominent populations on the soma and distal dendrites of hippocampal neurons, play a privileged role in excitation-transcription coupling. The voltage-gated K+ channel Kv2.1 organizes signaling complexes containing the L-type Ca2+ channel Cav1.2 at somatic endoplasmic reticulum-plasma membrane junctions. This leads to enhanced clustering of Cav1.2 channels, increasing their activity. However, the downstream consequences of the Kv2.1-mediated regulation of Cav1.2 localization and function on excitation-transcription coupling are not known. Here, we have identified a region between residues 478 to 486 of Kv2.1's C terminus that mediates the Kv2.1-dependent clustering of Cav1.2. By disrupting this Ca2+ channel association domain with either mutations or with a cell-penetrating interfering peptide, we blocked the Kv2.1-mediated clustering of Cav1.2 at endoplasmic reticulum-plasma membrane junctions and the subsequent enhancement of its channel activity and somatic Ca2+ signals without affecting the clustering of Kv2.1. These interventions abolished the depolarization-induced and L-type Ca2+ channel-dependent phosphorylation of the transcription factor CREB and the subsequent expression of c-Fos in hippocampal neurons. Our findings support a model whereby the Kv2.1-Ca2+ channel association domain-mediated clustering of Cav1.2 channels imparts a mechanism to control somatic Ca2+ signals that couple neuronal excitation to gene expression.

Keywords:  calcium signaling; excitation–transcription coupling; membrane contact sites; voltage-gated calcium channels; voltage-gated potassium channels

DOI:  https://doi.org/10.1073/pnas.2110094118