ER-mitochondria Contact Research Articles

AKT-mTOR signaling modulates the dynamics of IRE1 RNAse activity by regulating ER-mitochondria contacts

Inositol Requiring Enzyme-1 (IRE1) is the most conserved transducer of the Unfolded Protein Response (UPR), a surveillance mechanism that ensures homeostasis of the endoplasmic reticulum (ER) in eukaryotes. IRE1 activation orchestrates adaptive responses, including lipid anabolism, metabolic reprogramming, increases in protein folding competency, and ER expansion/remodeling. However, we still know surprisingly little regarding the principles by which this ER transducer is deactivated upon ER stress clearance. Here we show that Protein Kinase B-mechanistic Target of Rapamycin (PKB/AKT-mTOR) signaling controls the dynamics of IRE1 deactivation by regulating ER-mitochondria physical contacts and the autophosphorylation state of IRE1. AKT-mTOR-mediated attenuation of IRE1 activity is important for ER remodelling dynamics and cell survival in the face of recursive, transient ER stress. Our observations suggest that IRE1 attenuation is an integral component of anabolic programmes regulated by AKT-mTOR. We suggest that AKT-mTOR activity is part of a ‘timing mechanism’ to deactivate IRE1 immediately following engagement of the UPR, in order to limit prolonged IRE1 RNAse activity that could lead to damaging inflammation or apoptosis.

INF2-mediated actin polymerization at the ER stimulates mitochondrial calcium uptake, inner membrane constriction, and division.

Mitochondrial division requires division of both the inner and outer mitochondrial membranes (IMM and OMM, respectively). Interaction with endoplasmic reticulum (ER) promotes OMM division by recruitment of the dynamin Drp1, but effects on IMM division are not well characterized. We previously showed that actin polymerization through ER-bound inverted formin 2 (INF2) stimulates Drp1 recruitment in mammalian cells. Here, we show that INF2-mediated actin polymerization stimulates a second mitochondrial response independent of Drp1: a rise in mitochondrial matrix calcium through the mitochondrial calcium uniporter. ER stores supply the increased mitochondrial calcium, and the role of actin is to increase ER-mitochondria contact. Myosin IIA is also required for this mitochondrial calcium increase. Elevated mitochondrial calcium in turn activates IMM constriction in a Drp1-independent manner. IMM constriction requires electron transport chain activity. IMM division precedes OMM division. These results demonstrate that actin polymerization independently stimulates the dynamics of both membranes during mitochondrial division: IMM through increased matrix calcium, and OMM through Drp1 recruitment.

ER-mitochondria tethering by PDZD8 regulates Ca2+ dynamics in mammalian neurons.

Interfaces between organelles are emerging as critical platforms for many biological responses in eukaryotic cells. In yeast, the ERMES complex is an endoplasmic reticulum (ER)-mitochondria tether composed of four proteins, three of which contain a SMP (synaptotagmin-like mitochondrial-lipid binding protein) domain. No functional ortholog for any ERMES protein has been identified in metazoans. Here, we identified PDZD8 as an ER protein present at ER-mitochondria contacts. The SMP domain of PDZD8 is functionally orthologous to the SMP domain found in yeast Mmm1. PDZD8 was necessary for the formation of ER-mitochondria contacts in mammalian cells. In neurons, PDZD8 was required for calcium ion (Ca2+) uptake by mitochondria after synaptically induced Ca2+-release from ER and thereby regulated cytoplasmic Ca2+ dynamics. Thus, PDZD8 represents a critical ER-mitochondria tethering protein in metazoans. We suggest that ER-mitochondria coupling is involved in the regulation of dendritic Ca2+ dynamics in mammalian neurons.

Making the right contacts

Cell Biology Contacts between the endoplasmic reticulum (ER) and mitochondria mediate key physiological processes such as Ca2+ exchange and lipid biogenesis. In yeast, ER and mitochondria are tethered by a complex of four proteins called ERMES. However, no functional orthologs of these ERMES complex proteins have been identified in metazoans. Hirabayashi et al. identified PDZD8 as a structural and functional ortholog of the yeast ERMES protein MMM1 (see the Perspective by Lombardi and Elrod). PDZD8 was found at ER-mitochondria contact sites and was required for ER-mitochondria tethering in mammalian cells. In neuronal dendrites, PDZD8 regulated synaptically evoked Ca2+ dynamics, which underscores the importance of interorganelle membrane contacts in cell physiology. Science , this issue p. [623][1]; see also p. [591][2] [1]: /lookup/doi/10.1126/science.aan6009 [2]: /lookup/doi/10.1126/science.aaq0141

Crystal structures of Mmm1 and Mdm12-Mmm1 reveal mechanistic insight into phospholipid trafficking at ER-mitochondria contact sites.

The endoplasmic reticulum (ER)-mitochondria encounter structure (ERMES) comprises mitochondrial distribution and morphology 12 (Mdm12), maintenance of mitochondrial morphology 1 (Mmm1), Mdm34, and Mdm10 and mediates physical membrane contact sites and nonvesicular lipid trafficking between the ER and mitochondria in yeast. Herein, we report two crystal structures of the synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain of Mmm1 and the Mdm12-Mmm1 complex at 2.8 Å and 3.8 Å resolution, respectively. Mmm1 adopts a dimeric SMP structure augmented with two extra structural elements at the N and C termini that are involved in tight self-association and phospholipid coordination. Mmm1 binds two phospholipids inside the hydrophobic cavity, and the phosphate ion of the distal phospholipid is specifically recognized through extensive H-bonds. A positively charged concave surface on the SMP domain not only mediates ER membrane docking but also results in preferential binding to glycerophospholipids such as phosphatidylcholine (PC), phosphatidic acid (PA), phosphatidylglycerol (PG), and phosphatidylserine (PS), some of which are substrates for lipid-modifying enzymes in mitochondria. The Mdm12-Mmm1 structure reveals two Mdm12s binding to the SMP domains of the Mmm1 dimer in a pairwise head-to-tail manner. Direct association of Mmm1 and Mdm12 generates a 210-Å-long continuous hydrophobic tunnel that facilitates phospholipid transport. The Mdm12-Mmm1 complex binds all glycerophospholipids except for phosphatidylethanolamine (PE) in vitro.

Fishing for key players in ER–mitochondrial contacts

Endoplasmic reticulum-mitochondrial contacts (EMCs) regulate multiple critical cellular activities, dysregulation of which correlates with various human maladies such as neurodegenerative diseases. A new study makes use of the ascorbate peroxidase proximity-labeling proteomics approach to scrutinize the components of EMCs in live cells, leading to the identification of reticulon 1A as a novel promoter of EMCs.

Identification of the N-terminal transmembrane domain of StarD7 and its importance for mitochondrial outer membrane localization and phosphatidylcholine transfer

StarD7 facilitates phosphatidylcholine (PC) transfer to mitochondria, and is essential for mitochondrial homeostasis. However, the molecular mechanism for PC transfer by protein remains poorly understood. Herein, we describe a putative novel transmembrane (TM) domain C-terminal to the mitochondria-targeting signal (MTS) sequence at the N-terminus of StarD7. The mature form of StarD7 is integrated and/or associated onto the outer leaflet of the outer mitochondrial membrane (OMM) in HEPA-1 and HepG2 cells. A truncated form of StarD7 lacking the TM domain is distributed in the inner space of the mitochondria, and cannot reverse mitochondrial abnormalities, such as complex formation and PC content, when re-expressed in StarD7-KO HEPA-1 cells. Re-expression of wild StarD7 can compensate these mitochondrial functions of StarD7-KO HEPA-1 cells. The precursor form of StarD7 is cleaved between Met76 and Ala77, and Ala77 and Ala78 in the TM domain to produce the mature form. These results suggest that StarD7 is anchored onto the OMM through its N-terminal TM domain, and the C-terminal START domain may extend into the cytoplasm and shuttle PC between the ER and OMM at the ER-mitochondria contact sites.

Mitochondria-Associated Membranes As Networking Platforms and Regulators of Cancer Cell Fate.

The tight cross talk between two essential organelles of the cell, the endoplasmic reticulum (ER) and mitochondria, is spatially and functionally regulated by specific microdomains known as the mitochondria-associated membranes (MAMs). MAMs are hot spots of Ca2+ transfer between the ER and mitochondria, and emerging data indicate their vital role in the regulation of fundamental physiological processes, chief among them mitochondria bioenergetics, proteostasis, cell death, and autophagy. Moreover, and perhaps not surprisingly, it has become clear that signaling events regulated at the ER–mitochondria intersection regulate key processes in oncogenesis and in the response of cancer cells to therapeutics. ER–mitochondria appositions have been shown to dynamically recruit oncogenes and tumor suppressors, modulating their activity and protein complex formation, adapt the bioenergetic demand of cancer cells and to regulate cell death pathways and redox signaling in cancer cells. In this review, we discuss some emerging players of the ER–mitochondria contact sites in mammalian cells, the key processes they regulate and recent evidence highlighting the role of MAMs in shaping cell-autonomous and non-autonomous signals that regulate cancer growth.

The “sweet” side of ER-mitochondria contact sites

ABSTRACTThe regions at which the ER and mitochondria come into close proximity, known as ER-mitochondria contact sites provide essential platforms for the exchange of molecules between the two organelles and the coordination of various fundamental cellular processes. In addition to the well-established role of ER-mitochondria interfaces in calcium and lipid crosstalk, emerging evidence supports that a proper communication between ER and mitochondria is critical for the regulation of mitochondrial morphology and the initiation of autophagy. Within this context, our recent data indicate that glycogen is targeted to ER-mitochondria contacts through the Stbd1 protein, a proposed autophagy receptor for glycogen. Glycogen-bound Stbd1 influences ER-mitochondria tethering and the morphology of the mitochondrial network. We here suggest possible roles of glycogen recruitment to ER-mitochondria contact sites. Stbd1-mediated targeting of glycogen to ER-mitochondria junctions could represent a mechanism through which glycogen is sequestered into autophagosomes for lysosomal degradation, a process described as glycogen autophagy or glycophagy. Additionally, we discuss a possible mechanism which links the observed effects of Stbd1 on mitochondrial morphology with the previously reported impact of nutrient availability on mitochondrial dynamics. In this model we propose that glycogen-bound Stbd1 signals nutrient status to ER-mitochondria junctions resulting in adaptations in the morphology of the mitochondrial network.

Combustion-Derived Nanoparticles in Key Brain Target Cells and Organelles in Young Urbanites: Culprit Hidden in Plain Sight in Alzheimer's Disease Development.

Millions of children and young adults are exposed to fine particulate matter (PM2.5) and ozone, associated with Alzheimer's disease (AD) risk. Mexico City (MC) children exhibit systemic and brain inflammation, low cerebrospinal fluid (CSF) Aβ1-42, breakdown of nasal, olfactory, alveolar-capillary, duodenal, and blood-brain barriers, volumetric and metabolic brain changes, attention and short-term memory deficits, and hallmarks of AD and Parkinson's disease. Airborne iron-rich strongly magnetic combustion-derived nanoparticles (CDNPs) are present in young urbanites' brains. Using transmission electron microscopy, we documented CDNPs in neurons, glia, choroid plexus, and neurovascular units of young MC residents versus matched clean air controls. CDNPs are associated with pathology in mitochondria, endoplasmic reticulum (ER), mitochondria-ER contacts (MERCs), axons,and dendrites. There is a significant difference in size and numbers between spherical CDNPs (>85%) and the angular, euhedral endogenous NPs (<15%). Spherical CDNPs (dogs 21.2±7.1 nm in diameter versus humans 29.1±11.2 nm, p = 0.002) are present in neurons, glia, choroid plexus, endothelium, nasal and olfactory epithelium, and in CSF at significantly higher in numbers in MC residents (p < 0.0001). Degenerated MERCs, abnormal mitochondria, and dilated ER are widespread, and CDNPs in close contact with neurofilaments, glial fibers, and chromatin are a potential source for altered microtubule dynamics, mitochondrial dysfunction, accumulation and aggregation of unfolded proteins, abnormal endosomal systems, altered insulin signaling, calcium homeostasis, apoptotic signaling, autophagy, and epigenetic changes. Highly oxidative, ubiquitous CDNPs constitute a novel path into AD pathogenesis. Exposed children and young adults need early neuroprotection and multidisciplinary prevention efforts to modify the course of AD at early stages.

THE VAPB-PTPIP51 ER-MITOCHONDRIA TETHERS IN FRONTOTEMPORAL DEMENTIA AND RELATED AMYOTROPHIC LATERAL SCLEROSIS

Signaling between the ER and mitochondria regulates many of the neurophysiological functions that are damaged in FTD/ALS. This signaling involves close physical contacts between the two organelles. We showed that the integral ER protein VAPB binds to the outer mitochondrial membrane protein PTPIP51 to mediate these contacts. VAPB and PTPIP51 thus act as molecular scaffolds to tether the two organelles. We have investigated how FTD/ALS insults affect ER-mitochondria contacts and signaling, and the VAPB-PTPIP51 tethers. ER-mitochondria contacts and the VAPB-PTPIP51 interaction were quantified in cell and transgenic mouse models of FTD/ALS. Contacts were quantified by EM and super-resolution microscopy. The VAPB-PTPIP51 interaction was monitored by proximity ligation, immunoprecipitation and dimerization-dependent EGFP fusion assays. Ca2+ imaging was used to determine ER-mitochondria Ca2+ exchange. Signal transduction mechanisms linking FTD/ALS insults with damage to the VAPB-PTPIP51 tethers involved mass spectrometry. FTD/ALS associated TDP-43 and FUS both disrupt ER-mitochondria contacts, Ca2+ exchange and linked mitochondrial ATP production. This disruption involved activation of GSK3beta and breaking of the VAPB-PTPIP51 tethers. Potential GSK3beta phosphorylation sites were identified in both VAPB and PTPIP51. The Sigma-1 receptor was identified as a regulator of the VAPB-PTPIP51 interaction that may be targeted for damage by TDP-43 and FUS. The identification of VAPB and PTPIP51 as tethering proteins enabled the construction of novel drug screens. Damage to ER-mitochondria signaling and the VAPB-PTPIP51 ER-mitochondria tethering proteins is a feature of FTD/ALS. Together these findings identify a new pathogenic process in FTD/ALS that can explain many disease features and which represents a novel therapeutic target.

THE EFFECT OF AMYLOID β-PEPTIDE ON ER-MITOCHONDRIA CONTACT IN ALZHEIMER’S DISEASE

Cellular organelles are in close proximity at specialized contact sites where important cellular processes take place. For example, Ca2+-homeostasis, lipid synthesis, apoptosis and autophagy are regulated at the ER-mitochondria axis and interestingly all these processes are altered in Alzheimer's disease (AD). We have previously shown that modulation of ER-mitochondria contacts affect the assembly and maturation of the γ-secretase complex as well as generation of amyloid β-peptide (Aβ) (Leal et al 2016). Here we have investigated the effect of Aβ on the structure and dynamics of ER-mitochondria contacts in human tissue and AD mouse models. Human brain biopsies were collected from patients undergoing neurosurgery due to idiopathic normal pressure hydrocephalus (iNPH). The biopsies were stained for amyloid plaques (Aβ) and neurofibriallary tangles (tau). Mouse brain tissue was obtained from wt and tgAPPswe/lon mice of different ages. ER-mitochondria contact length and number were analyzed by structural electron microscopy. In addition, ER-mitochondria contact is being currently analyzed by proximity ligation assay (PLA) in primary neurons derived from a novel APP-knockin mouse model, APPNL-F. From brain biopsies we were able to characterise the structure of ER-mitochondria contacts in human brain. EM analysis of mouse brain material shows that ER-mitochondria apposition is affected by Aβ accumulation. Increased length of ER-mitochondria contacts were detected in tgAPPswe/lon mice at 6 months of age. Using PLA we can also visualize and quantify contact sites in APPNL-F mice. On-going analysis will reveal if Aβ accumulation affects ER-mitochondria apposition at an early stage before plaque pathology. The mechanisms underlying synapse loss and neurodegeneration in AD are not yet clarified. Here we show a close relationship between Aβ accumulation and regulation of ER-mitochondria contact and discuss its role in AD pathogenesis.

BAX inhibitor-1 regulates autophagy by controlling the IRE1α branch of the unfolded protein response.

Corrigendum1 June 2017free access Source Data BAX inhibitor-1 regulates autophagy by controlling the IRE1α branch of the unfolded protein response Karen Castillo Karen Castillo Search for more papers by this author Diego Rojas-Rivera Diego Rojas-Rivera orcid.org/0000-0002-8773-9830 Search for more papers by this author Fernanda Lisbona Fernanda Lisbona Search for more papers by this author Benjamín Caballero Benjamín Caballero Search for more papers by this author Melissa Nassif Melissa Nassif Search for more papers by this author Felipe A Court Felipe A Court Search for more papers by this author Sebastian Schuck Sebastian Schuck Search for more papers by this author Consuelo Ibar Consuelo Ibar Search for more papers by this author Peter Walter Peter Walter Search for more papers by this author Jimena Sierralta Jimena Sierralta Search for more papers by this author Alvaro Glavic Alvaro Glavic Search for more papers by this author Claudio Hetz Claudio Hetz Search for more papers by this author Karen Castillo Karen Castillo Search for more papers by this author Diego Rojas-Rivera Diego Rojas-Rivera orcid.org/0000-0002-8773-9830 Search for more papers by this author Fernanda Lisbona Fernanda Lisbona Search for more papers by this author Benjamín Caballero Benjamín Caballero Search for more papers by this author Melissa Nassif Melissa Nassif Search for more papers by this author Felipe A Court Felipe A Court Search for more papers by this author Sebastian Schuck Sebastian Schuck Search for more papers by this author Consuelo Ibar Consuelo Ibar Search for more papers by this author Peter Walter Peter Walter Search for more papers by this author Jimena Sierralta Jimena Sierralta Search for more papers by this author Alvaro Glavic Alvaro Glavic Search for more papers by this author Claudio Hetz Claudio Hetz Search for more papers by this author Author Information Karen Castillo, Diego Rojas-Rivera, Fernanda Lisbona, Benjamín Caballero, Melissa Nassif, Felipe A Court, Sebastian Schuck, Consuelo Ibar, Peter Walter, Jimena Sierralta, Alvaro Glavic and Claudio Hetz The EMBO Journal (2017)36:1640-1640https://doi.org/10.15252/embj.201797020 This article corrects the following: BAX inhibitor-1 regulates autophagy by controlling the IRE1α branch of the unfolded protein response16 September 2011 ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Figure 6D was assembled by directly pasting cropped sections of the original images, after adjustment of individual channel levels. A zoom of the dBI-1EY03662 NT panel was placed in the Control NT panel, duplicating the image by mistake. It is worth mentioning that the lysotracker signal was never significant in the non-treated conditions. We have revised the figure to correct the Control NT image and adjusted the contrast settings to make them comparable for all six images. Source data for Fig 6D are available online. Figure 6D.Revised figure. Source data are available online for this figure. Source Data for Figure 6D [embj201797020-sup-0001-SDataFig6.zip] Download figure Download PowerPoint The authors apologise for this oversight. Supporting Information Previous ArticleNext Article Read MoreAbout the coverClose modalView large imageVolume 36,Issue 11,01 June 2017Cover: NSU251 human glioma stem‐like cells expressing mitochondrial targeted YFP and OMMER, an artificial ER‐mitochondria tether. From Esen Yonca Bassoy, Atsuko Kasahara, Valentina Chiusolo, Denis Martinvalet and colleagues: ER–mitochondria contacts control surface glycan expression and sensitivity to killer lymphocytes in glioma stem‐like cells. For details, see the Article on p 1493, also highlighted by Lakshminarayanan and Scorrano on p 1465. Scientific image by Valentina Chiusolo and Denis Martinvalet, Department of Cell Physiology and Metabolism, University of Geneva, Geneva, Switzerland. Volume 36Issue 111 June 2017In this issue FiguresRelatedDetailsLoading ...

Comparative Proteomic Analysis of the Mitochondria-associated ER Membrane (MAM) in a Long-term Type 2 Diabetic Rodent Model

The mitochondria-associated ER membrane (MAM) plays a critical role in cellular energetics and calcium homeostasis; however, how MAM is affected under diabetic condition remains elusive. This study presented a comprehensive proteome profiling of isolated brain MAM from long-term type 2 diabetic mice vs. non-diabetic controls. MAM protein was extracted efficiently by a surfactant-aided precipitation/on-pellet digestion (SOD) method, and MAM proteome was quantified by an ion-current-based MS1 method combined with nanoLC-MS/MS. A total of 1,313 non-redundant proteins of MAM were identified, among which 144 proteins were found significantly altered by diabetes. In-depth IPA analysis identified multiple disease-relevant signaling pathways associated with the MAM proteome changes in diabetes, most significantly the unfolded protein response (UPR), p53, hypoxia-related transcription factors, and methyl CpG binding protein 2. Using immunofluorescence labeling we confirmed the activation of three UPR branches and increased ERp29 and calreticulin in diabetic retinas. Moreover, we found GRP75, a key MAM tethering protein, was drastically reduced by long-term diabetes. In vitro, acute high glucose treatment reduces ER-mitochondrial contact in retinal endothelial cells. This study provides first insight into the significant alterations in MAM proteome associated with activation of the UPR in diabetes, which may serve as novel benchmarks for the future studies of diabetic complications.

Proteomic mapping of cytosol-facing outer mitochondrial and ER membranes in living human cells by proximity biotinylation.

The cytosol-facing membranes of cellular organelles contain proteins that enable signal transduction, regulation of morphology and trafficking, protein import and export, and other specialized processes. Discovery of these proteins by traditional biochemical fractionation can be plagued with contaminants and loss of key components. Using peroxidase-mediated proximity biotinylation, we captured and identified endogenous proteins on the outer mitochondrial membrane (OMM) and endoplasmic reticulum membrane (ERM) of living human fibroblasts. The proteomes of 137 and 634 proteins, respectively, are highly specific and highlight 94 potentially novel mitochondrial or ER proteins. Dataset intersection identified protein candidates potentially localized to mitochondria-ER contact sites. We found that one candidate, the tail-anchored, PDZ-domain-containing OMM protein SYNJ2BP, dramatically increases mitochondrial contacts with rough ER when overexpressed. Immunoprecipitation-mass spectrometry identified ribosome-binding protein 1 (RRBP1) as SYNJ2BP's ERM binding partner. Our results highlight the power of proximity biotinylation to yield insights into the molecular composition and function of intracellular membranes.

A novel CISD2 mutation associated with a classical Wolfram syndrome phenotype alters Ca2+ homeostasis and ER-mitochondria interactions

Wolfram syndrome (WS) is a progressive neurodegenerative disease characterized by early-onset optic atrophy and diabetes mellitus, which can be associated with more extensive central nervous system and endocrine complications. The majority of patients harbour pathogenic WFS1 mutations, but recessive mutations in a second gene, CISD2, have been described in a small number of families with Wolfram syndrome type 2 (WFS2). The defining diagnostic criteria for WFS2 also consist of optic atrophy and diabetes mellitus, but unlike WFS1, this phenotypic subgroup has been associated with peptic ulcer disease and an increased bleeding tendency. Here, we report on a novel homozygous CISD2 mutation (c.215A > G; p.Asn72Ser) in a Moroccan patient with an overlapping phenotype suggesting that Wolfram syndrome type 1 and type 2 form a continuous clinical spectrum with genetic heterogeneity. The present study provides strong evidence that this particular CISD2 mutation disturbs cellular Ca2+ homeostasis with enhanced Ca2+ flux from the ER to mitochondria and cytosolic Ca2+ abnormalities in patient-derived fibroblasts. This Ca2+ dysregulation was associated with increased ER-mitochondria contact, a swollen ER lumen and a hyperfused mitochondrial network in the absence of overt ER stress. Although there was no marked alteration in mitochondrial bioenergetics under basal conditions, culture of patient-derived fibroblasts in glucose-free galactose medium revealed a respiratory chain defect in complexes I and II, and a trend towards decreased ATP levels. Our results provide important novel insight into the potential disease mechanisms underlying the neurodegenerative consequences of CISD2 mutations and the subsequent development of multisystemic disease.

From dysfunctional endoplasmic reticulum-mitochondria coupling to neurodegeneration

Over the last years, contact sites between the endoplasmic reticulum (ER) and mitochondria have attracted great attention in the study of cell homeostasis and dysfunction, especially in the context of neurodegenerative disorders. This is largely due to the critical involvement of this subcellular compartment in a plethora of vital cellular functions: Ca2+ homeostasis, mitochondrial dynamics, transport, bioenergetics and turnover, ER stress, apoptotic signaling and inflammation. An increasing number of disease-associated proteins have been reported to physically associate with the ER-mitochondria interface, and cause structural and/or functional perturbations of this compartment. In the present review, we summarize current knowledge about the architecture and functions of the ER-mitochondria contact sites, and the consequences of their alteration in different neurodegenerative disorders. Special emphasis is placed on the caveats and difficulties in defining the nature and origin of the highlighted defects in ER-mitochondria communication, and their exact contribution to the neurodegenerative process.

ER-mitochondria contacts control surface glycan expression and sensitivity to killer lymphocytes in glioma stem-like cells.

Glioblastoma is a highly heterogeneous aggressive primary brain tumor, with the glioma stem-like cells (GSC) being more sensitive to cytotoxic lymphocyte-mediated killing than glioma differentiated cells (GDC). However, the mechanism behind this higher sensitivity is unclear. Here, we found that the mitochondrial morphology of GSCs modulates the ER-mitochondria contacts that regulate the surface expression of sialylated glycans and their recognition by cytotoxic T lymphocytes and natural killer cells. GSCs displayed diminished ER-mitochondria contacts compared to GDCs. Forced ER-mitochondria contacts in GSCs increased their cell surface expression of sialylated glycans and reduced their susceptibility to cytotoxic lymphocytes. Therefore, mitochondrial morphology and dynamism dictate the ER-mitochondria contacts in order to regulate the surface expression of certain glycans and thus play a role in GSC recognition and elimination by immune effector cells. Targeting the mitochondrial morphology, dynamism, and contacts with the ER could be an innovative strategy to deplete the cancer stem cell compartment to successfully treat glioblastoma.

Phosphatidylserine synthesis at membrane contact sites promotes its transport out of the ER

Close contacts between organelles, often called membrane contact sites (MCSs), are regions where lipids are exchanged between organelles. Here, we identify a novel mechanism by which cells promote phospholipid exchange at MCSs. Previous studies have shown that phosphatidylserine (PS) synthase activity is highly enriched in portions of the endoplasmic reticulum (ER) in contact with mitochondria. The objective of this study was to determine whether this enrichment promotes PS transport out of the ER. We found that PS transport to mitochondria was more efficient when PS synthase was fused to a protein in the ER at ER-mitochondria contacts than when it was fused to a protein in all portions of the ER. Inefficient PS transport to mitochondria was corrected by increasing tethering between these organelles. PS transport to endosomes was similarly enhanced by PS production in regions of the ER in contact with endosomes. Together, these findings indicate that PS production at MCSs promotes PS transport out of the ER and suggest that phospholipid production at MCSs may be a general mechanism of channeling lipids to specific cellular compartments.

PINK1 and BECN1 relocalize at mitochondria-associated membranes during mitophagy and promote ER-mitochondria tethering and autophagosome formation

ABSTRACTMitophagy is a highly specialized process to remove dysfunctional or superfluous mitochondria through the macroautophagy/autophagy pathway, aimed at protecting cells from the damage of disordered mitochondrial metabolism and apoptosis induction. PINK1, a neuroprotective protein mutated in autosomal recessive Parkinson disease, has been implicated in the activation of mitophagy by selectively accumulating on depolarized mitochondria, and promoting PARK2/Parkin translocation to them. While these steps have been characterized in depth, less is known about the process and site of autophagosome formation upon mitophagic stimuli. A previous study reported that, in starvation-induced autophagy, the proautophagic protein BECN1/Beclin1 (which we previously showed to interact with PINK1) relocalizes at specific regions of contact between the endoplasmic reticulum (ER) and mitochondria called mitochondria-associated membranes (MAM), from which the autophagosome originates. Here we show that, following mitophagic stimuli, autophagosomes also form at MAM; moreover, endogenous PINK1 and BECN1 were both found to relocalize at MAM, where they promoted the enhancement of ER-mitochondria contact sites and the formation of omegasomes, that represent autophagosome precursors. PARK2 was also enhanced at MAM following mitophagy induction. However, PINK1 silencing impaired BECN1 enrichment at MAM independently of PARK2, suggesting a novel role for PINK1 in regulating mitophagy. MAM have been recently implicated in many key cellular events. In this light, the observed prevalent localization of PINK1 at MAM may well explain other neuroprotective activities of this protein, such as modulation of mitochondrial calcium levels, mitochondrial dynamics, and apoptosis.