Mitochondrial Tethers and Their Impact on Lifespan in Budding Yeast

Tethers that link mitochondria to other organelles are critical for lipid and calcium transport as well as mitochondrial genome replication and fission of the organelle. Here, we review recent advances in the characterization of interorganellar mitochondrial tethers in the budding yeast, Saccharomyces cerevisiae. We specifically focus on evidence for a role for mitochondrial tethers that anchor mitochondria to specific regions within yeast cells. These tethering events contribute to two processes that are critical for normal replicative lifespan: inheritance of fitter mitochondria by daughter cells, and retention of a small pool of higher-functioning mitochondria in mother cells. Since asymmetric inheritance of mitochondria also occurs in human mammary stem-like cells, it is possible that mechanisms underlying mitochondrial segregation in yeast also operate in other cell types.

PARL partitions the lipid transfer protein STARD7 between the cytosol and mitochondria

Intramembrane-cleaving peptidases of the rhomboid family regulate diverse cellular processes that are critical for development and cell survival. The function of the rhomboid protease PARL in the mitochondrial inner membrane has been linked to mitophagy and apoptosis, but other regulatory functions are likely to exist. Here, we identify the START domain-containing protein STARD7 as an intramitochondrial lipid transfer protein for phosphatidylcholine. We demonstrate that PARL-mediated cleavage during mitochondrial import partitions STARD7 to the cytosol and the mitochondrial intermembrane space. Negatively charged amino acids in STARD7 serve as a sorting signal allowing mitochondrial release of mature STARD7 upon cleavage by PARL On the other hand, membrane insertion of STARD7 mediated by the TIM23 complex promotes mitochondrial localization of mature STARD7. Mitochondrial STARD7 is necessary and sufficient for the accumulation of phosphatidylcholine in the inner membrane and for the maintenance of respiration and cristae morphogenesis. Thus, PARL preserves mitochondrial membrane homeostasis via STARD7 processing and is emerging as a critical regulator of protein localization between mitochondria and the cytosol.

Arabidopsis Serine Decarboxylase 1 (SDC1) in Phospholipid and Amino Acid Metabolism

Arabidopsis thaliana serine decarboxylase 1 (SDC1) catalyzes conversion of serine to ethanolamine, the first reaction step of phosphatidylcholine and phosphatidylethanolamine biosynthesis. However, an involvement of SDC1 in amino acid metabolism remains elusive despite that serine is the substrate of SDC1. Here, we showed that SDC1 localizes in mitochondria although phosphatidylcholine and phosphatidylethanolamine are known to be produced in the endoplasmic reticulum (ER). Moreover, we found that overexpression of SDC1 decreased levels of amino acid compounds derived from mitochondrial tricarboxylic acid cycle. These results suggest that mitochondria-localized SDC1 plays an important role in both phospholipid and amino acid metabolism in A. thaliana.

High-resolution lipidomics reveals dysregulation of lipid metabolism in respiratory syncytial virus pneumonia mice

Respiratory syncytial virus (RSV) is a leading viral pathogen responsible for lower respiratory tract infections, particularly in children under five years worldwide, often resulting in hospitalization. At present, the molecular-level interactions between RSV and its host and the underlying mechanisms of RSV-induced inflammation are poorly understood. Herein, we describe an untargeted high-resolution lipidomics platform based on UHPLC-Q-Exactive-MS to assess the lipid alterations of lung tissues and plasma from a mouse model of RSV pneumonia. Untargeted lipidomics using LC-MS with multivariate analysis was applied to describe the lipidomic profiling of the lung tissues and plasma in RSV pneumonia mice. Lipid identification was conducted via an in silico MS/MS LipidBlast library using the MS-DIAL software. We observed distinct compartmental lipid signatures in the mice lung tissues and plasma and significant lipid profile changes between the systematic and localized host responses to RSV. A total of 87 and 68 differential lipids were captured in the mice lung tissue and plasma, respectively, including phospholipids, sphingolipids, acylcarnitine, and fatty acids. Some of these lipids belong to pulmonary surfactants, illustrating that RSV pneumonia-induced aberrations of the pulmonary surfactant system may play a vital role in the etiology of respiratory inflammation. Our findings reveal that the host responses to RSV and various lipid metabolic pathways were linked to disease pathology. Furthermore, our findings could provide mechanistic insights into RSV pneumonia.

Respiratory syncytial virus (RSV) is a leading viral pathogen responsible for lower respiratory tract infections, particularly in children under five years worldwide, often resulting in hospitalization.

Lipid Droplet Fusion in Mammary Epithelial Cells is Regulated by Phosphatidylethanolamine Metabolism

Mammary epithelial cells (MEC) secrete fat in the form of milk fat globules (MFG) which are found in milk in diverse sizes. MFG originate from intracellular lipid droplets, and the mechanism underlying their size regulation is still elusive. Two main mechanisms have been suggested to control lipid droplet size. The first is a well-documented pathway, which involves regulation of cellular triglyceride content. The second is the fusion pathway, which is less-documented, especially in mammalian cells, and its importance in the regulation of droplet size is still unclear. Using biochemical and molecular inhibitors, we provide evidence that in MEC, lipid droplet size is determined by fusion, independent of cellular triglyceride content. The extent of fusion is determined by the cell membrane's phospholipid composition. In particular, increasing phosphatidylethanolamine (PE) content enhances fusion between lipid droplets and hence increases lipid droplet size. We further identified the underlying biochemical mechanism that controls this content as the mitochondrial enzyme phosphatidylserine decarboxylase; siRNA knockdown of this enzyme reduced the number of large lipid droplets threefold. Further, inhibition of phosphatidylserine transfer to the mitochondria, where its conversion to PE occurs, diminished the large lipid droplet phenotype in these cells. These results reveal, for the first time to our knowledge in mammalian cells and specifically in mammary epithelium, the missing biochemical link between the metabolism of cellular complex lipids and lipid-droplet fusion, which ultimately defines lipid droplet size.

Structural insights into the committed step of bacterial phospholipid biosynthesis

The membrane-integral glycerol 3-phosphate (G3P) acyltransferase PlsY catalyses the committed and essential step in bacterial phospholipid biosynthesis by acylation of G3P, forming lysophosphatidic acid. It contains no known acyltransferase motifs, lacks eukaryotic homologs, and uses the unusual acyl-phosphate as acyl donor, as opposed to acyl-CoA or acyl-carrier protein for other acyltransferases. Previous studies have identified several PlsY inhibitors as potential antimicrobials. Here we determine the crystal structure of PlsY at 1.48 Å resolution, revealing a seven-transmembrane helix fold. Four additional substrate- and product-bound structures uncover the atomic details of its relatively inflexible active site. Structure and mutagenesis suggest a different acylation mechanism of ‘substrate-assisted catalysis’ that, unlike other acyltransferases, does not require a proteinaceous catalytic base to complete. The structure data and a high-throughput enzymatic assay developed in this work should prove useful for virtual and experimental screening of inhibitors against this vital bacterial enzyme.

The first step in bacterial phospholipid biosynthesis is the acylation of glycerol 3-phosphate to form lysophosphatidic acid. Here, the authors present the high resolution crystal structure of the glycerol 3-phosphate acyltransferase PlsY, a membrane protein and give insights into its catalytical mechanism.

Phospholipids of Animal and Marine Origin: Structure, Function, and Anti-Inflammatory Properties

In this review paper, the latest literature on the functional properties of phospholipids in relation to inflammation and inflammation-related disorders has been critically appraised and evaluated. The paper is divided into three sections: Section 1 presents an overview of the relationship between structures and biological activities (pro-inflammatory or anti-inflammatory) of several phospholipids with respect to inflammation. Section 2 and Section 3 are dedicated to the structures, functions, compositions and anti-inflammatory properties of dietary phospholipids from animal and marine sources. Most of the dietary phospholipids of animal origin come from meat, egg and dairy products. To date, there is very limited work published on meat phospholipids, undoubtedly due to the negative perception that meat consumption is an unhealthy option because of its putative associations with several chronic diseases. These assumptions are addressed with respect to the phospholipid composition of meat products. Recent research trends indicate that dairy phospholipids possess anti-inflammatory properties, which has led to an increased interest into their molecular structures and reputed health benefits. Finally, the structural composition of phospholipids of marine origin is discussed. Extensive research has been published in relation to ω-3 polyunsaturated fatty acids (PUFAs) and inflammation, however this research has recently come under scrutiny and has proved to be unreliable and controversial in terms of the therapeutic effects of ω-3 PUFA, which are generally in the form of triglycerides and esters. Therefore, this review focuses on recent publications concerning marine phospholipids and their structural composition and related health benefits. Finally, the strong nutritional value of dietary phospholipids are highlighted with respect to marine and animal origin and avenues for future research are discussed.

Phospholipid flippase ATP11C is endocytosed and downregulated following Ca2+-mediated protein kinase C activation

We and others showed that ATP11A and ATP11C, members of the P4-ATPase family, translocate phosphatidylserine (PS) and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflets at the plasma membrane. PS exposure on the outer leaflet of the plasma membrane in activated platelets, erythrocytes, and apoptotic cells was proposed to require the inhibition of PS-flippases, as well as activation of scramblases. Although ATP11A and ATP11C are cleaved by caspases in apoptotic cells, it remains unclear how PS-flippase activity is regulated in non-apoptotic cells. Here we report that the PS-flippase ATP11C, but not ATP11A, is sequestered from the plasma membrane via clathrin-mediated endocytosis upon Ca²⁺-mediated PKC activation. Importantly, we show that a characteristic di-leucine motif (SVRPLL) in the C-terminal cytoplasmic region of ATP11C becomes functional upon PKC activation. Moreover endocytosis of ATP11C is induced by Ca²⁺-signaling via Gq-coupled receptors. Our data provide the first evidence for signal-dependent regulation of mammalian P4-ATPase.

ATP11C is a flippase that uses ATP hydrolysis to translocate phospholipids at the plasma membrane. Here, the authors show that the activation of Ca2+-dependent protein kinase C increases ATP11C endocytosis thus downregulating phospholipid translocation.

Interleukin-1β affects the phospholipid biosynthesis of fibroblast-like synoviocytes from human osteoarthritic knee joints

OBJECTIVE

Phospholipids (PLs), together with hyaluronan and lubricin, are involved in boundary lubrication within human articular joints. Levels of lubricants in synovial fluid (SF) have been found to be associated with the health status of the joint. However, the biosynthesis and release of PLs within human joints remains poorly understood. This study contributes to our understanding of the effects of cytokines on the biosynthesis of PLs using cultured fibroblast-like synoviocytes (FLS) from human osteoarthritic knee joints.

METHODS

Cultured FLS were stimulated with IL-1β, TNFα, IL-6, or inhibitors of cell signaling pathways such as QNZ, SB203580 and SP600125 in the presence of stable isotope-labeled precursors of PLs. Lipids were extracted and quantified using electrospray ionization tandem mass spectrometry (ESI-MS/MS).

RESULTS

Our analyses provide for the first time a detailed overview of PL species being synthesized by FLS. IL-1β increased the biosynthesis of both phosphatidylethanolamine (PE) and PE-based plasmalogens. We show here that the NF-κB, p38 MAPK and JNK signaling pathways are all involved in IL-1β-induced PL biosynthesis. IL-6 had no impact on PLs, whereas TNFα increased the biosynthesis of all PL classes.

CONCLUSION

The biosynthesis of various PLs is controlled by IL-1β and TNFα. Our detailed PL species analysis revealed that FLS can partly contribute to the elevated PL levels found in human osteoarthritis (OA) SF. IL-1β in particular stimulates PE and PE-based plasmalogens which can act as cell-protective antioxidants. These results suggest that during OA progression, FLS undergo alterations in their PL composition to adapt to the new diseased environment.

Structure, function, and regulation of mitofusin-2 in health and disease

Mitochondria are highly dynamic organelles that constantly migrate, fuse, and divide to regulate their shape, size, number, and bioenergetic function. Mitofusins (Mfn1/2), optic atrophy 1 (OPA1), and dynamin-related protein 1 (Drp1), are key regulators of mitochondrial fusion and fission. Mutations in these molecules are associated with severe neurodegenerative and non-neurological diseases pointing to the importance of functional mitochondrial dynamics in normal cell physiology. In recent years, significant progress has been made in our understanding of mitochondrial dynamics, which has raised interest in defining the physiological roles of key regulators of fusion and fission and led to the identification of additional functions of Mfn2 in mitochondrial metabolism, cell signalling, and apoptosis. In this review, we summarize the current knowledge of the structural and functional properties of Mfn2 as well as its regulation in different tissues, and also discuss the consequences of aberrant Mfn2 expression.

Clickable Substrate Mimics Enable Imaging of Phospholipase D Activity

Chemical imaging techniques have played instrumental roles in dissecting the spatiotemporal regulation of signal transduction pathways. Phospholipase D (PLD) enzymes affect cell signaling by producing the pleiotropic lipid second messenger phosphatidic acid via hydrolysis of phosphatidylcholine. It remains a mystery how this one lipid signal can cause such diverse physiological and pathological signaling outcomes, due in large part to a lack of suitable tools for visualizing the spatial and temporal dynamics of its production within cells. Here, we report a chemical method for imaging phosphatidic acid synthesis by PLD enzymes in live cells. Our approach capitalizes upon the enzymatic promiscuity of PLDs, which we show can accept azidoalcohols as reporters in a transphosphatidylation reaction. The resultant azidolipids are then fluorescently tagged using the strain-promoted azide-alkyne cycloaddition, enabling visualization of cellular membranes bearing active PLD enzymes. Our method, termed IMPACT (Imaging Phospholipase D Activity with Clickable Alcohols via Transphosphatidylation), reveals pools of basal and stimulated PLD activities in expected and unexpected locations. As well, we reveal a striking heterogeneity in PLD activities at both the cellular and subcellular levels. Collectively, our studies highlight the importance of using chemical tools to directly visualize, with high spatial and temporal resolution, the subset of signaling enzymes that are active.

Natural phospholipids: Occurrence, biosynthesis, separation, identification, and beneficial health aspects

During the last years, phospholipids (PLs) have attracted great attention because of their crucial roles in providing nutritional values, technological and medical applications. There are considerable proofs that PLs have unique nutritional benefits on human health, such as reducing cholesterol absorption, improving liver functions, and decreasing the risk of cardiovascular diseases. PLs are the main structural lipid components of cell and organelle membranes in all living organisms, and therefore, they occur in all organisms and the derived food products. PLs are distinguished by the presence of a hydrophilic head and a hydrophobic tail, consequently they possess amphiphilic features. Due to their unique characteristics, the extraction, separation, and identification of PLs are critical issues to be concerned. This review is focused on the content of PLs classes in several sources (including milk, vegetable oils, egg yolk, and mitochondria). As well, it highlights PLs biosynthesis, and the methodologies applied for PLs extraction and separation, such as solvent extraction and solid-phase extraction. In addition, the determination and quantification of PLs classes by using thin layer chromatography, high-performance liquid chromatography coupled with different detectors, and nuclear magnetic resonance spectroscopy techniques.

Robust Regression Analysis of GCMS Data Reveals Differential Rewiring of Metabolic Networks in Hepatitis B and C Patients

About one in 15 of the world’s population is chronically infected with either hepatitis virus B (HBV) or C (HCV), with enormous public health consequences. The metabolic alterations caused by these infections have never been directly compared and contrasted. We investigated groups of HBV-positive, HCV-positive, and uninfected healthy controls using gas chromatography-mass spectrometry analyses of their plasma and urine. A robust regression analysis of the metabolite data was conducted to reveal correlations between metabolite pairs. Ten metabolite correlations appeared for HBV plasma and urine, with 18 for HCV plasma and urine, none of which were present in the controls. Metabolic perturbation networks were constructed, which permitted a differential view of the HBV- and HCV-infected liver. HBV hepatitis was consistent with enhanced glucose uptake, glycolysis, and pentose phosphate pathway metabolism, the latter using xylitol and producing threonic acid, which may also be imported by glucose transporters. HCV hepatitis was consistent with impaired glucose uptake, glycolysis, and pentose phosphate pathway metabolism, with the tricarboxylic acid pathway fueled by branched-chain amino acids feeding gluconeogenesis and the hepatocellular loss of glucose, which most probably contributed to hyperglycemia. It is concluded that robust regression analyses can uncover metabolic rewiring in disease states.

Phosphatidic Acid and Cardiolipin Coordinate Mitochondrial Dynamics

Membrane organelles comprise both proteins and lipids. Remodeling of these membrane structures is controlled by interactions between specific proteins and lipids. Mitochondrial structure and function depend on regulated fusion and the division of both the outer and inner membranes. Here we discuss recent advances in the regulation of mitochondrial dynamics by two critical phospholipids, phosphatidic acid (PA) and cardiolipin (CL). These two lipids interact with the core components of mitochondrial fusion and division (Opa1, mitofusin, and Drp1) to activate and inhibit these dynamin-related GTPases. Moreover, lipid-modifying enzymes such as phospholipases and lipid phosphatases may organize local lipid composition to spatially and temporarily coordinate a balance between fusion and division to establish mitochondrial morphology.

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.

Plasmalogen homeostasis - regulation of plasmalogen biosynthesis and its physiological consequence in mammals

Plasmalogens, mostly ethanolamine-containing alkenyl ether phospholipids, are a major subclass of glycerophospholipids. Plasmalogen synthesis is initiated in peroxisomes and completed in the endoplasmic reticulum. The absence of plasmalogens in several organs of peroxisome biogenesis-defective patients suggests that the de novo synthesis of plasmalogens plays a pivotal role in its homeostasis in tissues. Plasmalogen synthesis is regulated by modulating the stability of fatty acyl-CoA reductase 1 on peroxisomal membranes, a rate-limiting enzyme in plasmalogen synthesis, by sensing plasmalogens in the inner leaflet of plasma membranes. Dysregulation of plasmalogen homeostasis impairs cholesterol biosynthesis by altering the stability of squalene monooxygenase, a key enzyme in cholesterol biosynthesis, implying physiological consequences of plasmalogen homeostasis with respect to cholesterol metabolism in cells, as well as in organs such as the liver.

Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins

Axons contain a smooth tubular endoplasmic reticulum (ER) network that is thought to be continuous with ER throughout the neuron; the mechanisms that form this axonal network are unknown. Mutations affecting reticulon or REEP proteins, with intramembrane hairpin domains that model ER membranes, cause an axon degenerative disease, hereditary spastic paraplegia (HSP). We show that Drosophila axons have a dynamic axonal ER network, which these proteins help to model. Loss of HSP hairpin proteins causes ER sheet expansion, partial loss of ER from distal motor axons, and occasional discontinuities in axonal ER. Ultrastructural analysis reveals an extensive ER network in axons, which shows larger and fewer tubules in larvae that lack reticulon and REEP proteins, consistent with loss of membrane curvature. Therefore HSP hairpin-containing proteins are required for shaping and continuity of axonal ER, thus suggesting roles for ER modeling in axon maintenance and function.

DOI:http://dx.doi.org/10.7554/eLife.23882.001

The way we move – from simple motions like reaching out to grab something, to playing the piano or dancing – is coordinated in our brain. These processes involve many regions and steps, in which nerve cells transport signals along projections known as axons. Axons rely on sophisticated ‘engineering’ to work properly over long distances and are vulnerable to diseases that disrupt their engineering. For example, in genetic diseases called ‘hereditary spastic paraplegias’, damages to the ‘distal’ end of axons – the end furthest from the nerve cell body – cause paralysis of the lower body.

Axons have several internal structures that make sure everything works properly. One of these structures is the endoplasmic reticulum, which is a network of tubular membranes that runs lengthwise along the axon. It is known that spastic paraplegias are sometimes caused by mutations affecting proteins that help to build and shape the endoplasmic reticulum, for example, the proteins of the reticulon and REEP families. However, until now it was not known how the ER forms its network in the axons and if this is influenced by these proteins.

To see whether reticulons and REEPs affect the shape of the endoplasmic reticulum, Yalçιn et al. used healthy fruit fly larvae, and genetically modified ones that lacked the proteins. The results show that in healthy flies, the tubular network runs continuously along the axons. When either reticulon or REEP proteins were removed, the distal axons contained less endoplasmic reticulum. In mutant fly larvae that lacked both protein families, the endoplasmic reticulum was more interrupted and contained more gaps than in normal larvae. Using high-magnification electron microscopy confirmed these findings, and showed that the tubules of the endoplasmic reticulum in mutant axons were larger, but fewer.

A next step will be to test whether these mutations also affect how the axons work and communicate over long distances. A better knowledge of the role of the endoplasmic reticulum in axons will help us to understand how damages to it could affect hereditary spastic paraplegias and other degenerative conditions.

DOI:http://dx.doi.org/10.7554/eLife.23882.002

The expanding role of sphingolipids in lipid droplet biogenesis

Sphingolipids are a diverse class of lipids that have regulatory, structural, and metabolic functions. Although chemically distinct from the neutral lipids and the glycerophospholipids, which are the main lipid components of the lipid droplets, sphingolipids have nonetheless been shown to influence lipid droplet formation. The goal of this article is to review the available information and provide a cohesive picture of the role sphingolipids play in lipid droplet biogenesis. The following topics are discussed: (i) the abundance of sphingolipids in lipid droplets and their functional significance; (ii) cross-talk between the synthetic pathways of sphingolipids, glycerophospholipids, and neutral lipids; (iii) the impact of bioactive sphingolipids on TAG synthesis and degradation; (iv) interactions between sphingolipids and other lipid droplet components, like cholesterol esters and proteins; (v) inhibition/genetic deletion of specific sphingolipid metabolic enzymes and the resulting effects on lipid droplet formation in mouse models. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Directing lipid transport at membrane contact sites

Contact sites between the endoplasmic reticulum and the plasma membrane mediate receptor signalling. How this function is controlled physically and functionally is poorly understood. Extended synaptotagmins are now shown to shuttle the lipid metabolite diacylglycerol from the plasma membrane to the endoplasmic reticulum in receptor-stimulated cells.

Ethanolamine and Phosphatidylethanolamine: Partners in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant phospholipid in mammalian cells. PE comprises about 15–25% of the total lipid in mammalian cells; it is enriched in the inner leaflet of membranes, and it is especially abundant in the inner mitochondrial membrane. PE has quite remarkable activities: it is a lipid chaperone that assists in the folding of certain membrane proteins, it is required for the activity of several of the respiratory complexes, and it plays a key role in the initiation of autophagy. In this review, we focus on PE's roles in lipid-induced stress in the endoplasmic reticulum (ER), Parkinson's disease (PD), ferroptosis, and cancer.

α-Synuclein binds to the ER-mitochondria tethering protein VAPB to disrupt Ca2+ homeostasis and mitochondrial ATP production

α-Synuclein is strongly linked to Parkinson’s disease but the molecular targets for its toxicity are not fully clear. However, many neuronal functions damaged in Parkinson’s disease are regulated by signalling between the endoplasmic reticulum (ER) and mitochondria. This signalling involves close physical associations between the two organelles that are mediated by binding of the integral ER protein vesicle-associated membrane protein-associated protein B (VAPB) to the outer mitochondrial membrane protein, protein tyrosine phosphatase-interacting protein 51 (PTPIP51). VAPB and PTPIP51 thus act as a scaffold to tether the two organelles. Here we show that α-synuclein binds to VAPB and that overexpression of wild-type and familial Parkinson’s disease mutant α-synuclein disrupt the VAPB-PTPIP51 tethers to loosen ER–mitochondria associations. This disruption to the VAPB-PTPIP51 tethers is also seen in neurons derived from induced pluripotent stem cells from familial Parkinson’s disease patients harbouring pathogenic triplication of the α-synuclein gene. We also show that the α-synuclein induced loosening of ER–mitochondria contacts is accompanied by disruption to Ca²⁺ exchange between the two organelles and mitochondrial ATP production. Such disruptions are likely to be particularly damaging to neurons that are heavily dependent on correct Ca²⁺ signaling and ATP.

Lipids and Their Trafficking: An Integral Part of Cellular Organization

An evolutionarily conserved feature of cellular organelles is the distinct phospholipid composition of their bounding membranes, which is essential to their identity and function. Within eukaryotic cells, two major lipid territories can be discerned, one centered on the endoplasmic reticulum and characterized by membranes with lipid packing defects, the other comprising plasma-membrane-derived organelles and characterized by membrane charge. We discuss how this cellular lipid organization is maintained, how lipid flux is regulated, and how perturbations in cellular lipid homeostasis can lead to disease.

Crystal structure of Mdm12 and combinatorial reconstitution of Mdm12/Mmm1 ERMES complexes for structural studies

Membrane contact sites between organelles serve as molecular hubs for the exchange of metabolites and signals. In yeast, the Endoplasmic Reticulum - Mitochondrion Encounter Structure (ERMES) tethers these two organelles likely to facilitate the non-vesicular exchange of essential phospholipids. Present in Fungi and Amoebas but not in Metazoans, ERMES is composed of five distinct subunits; among those, Mdm12, Mmm1 and Mdm34 each contain an SMP domain functioning as a lipid transfer module. We previously showed that the SMP domains of Mdm12 and Mmm1 form a hetero-tetramer. Here we describe our strategy to diversify the number of Mdm12/Mmm1 complexes suited for structural studies. We use sequence analysis of orthologues combined to protein engineering of disordered regions to guide the design of protein constructs and expand the repertoire of Mdm12/Mmm1 complexes more likely to crystallize. Using this combinatorial approach we report crystals of Mdm12/Mmm1 ERMES complexes currently diffracting to 4.5 Å resolution and a new structure of Mdm12 solved at 4.1 Å resolution. Our structure reveals a monomeric form of Mdm12 with a conformationally dynamic N-terminal β-strand; it differs from a previously reported homodimeric structure where the N-terminal β strands where swapped to promote dimerization. Based on our electron microscopy data, we propose a refined pseudo-atomic model of the Mdm12/Mmm1 complex that agrees with our crystallographic and small-angle X-ray scattering (SAXS) solution data.

Blue light regenerates functional visual pigments in mammals through a retinyl-phospholipid intermediate

The light absorbing chromophore in opsin visual pigments is the protonated Schiff base of 11-cis-retinaldehyde (11cRAL). Absorption of a photon isomerizes 11cRAL to all-trans-retinaldehyde (atRAL), briefly activating the pigment before it dissociates. Light sensitivity is restored when apo-opsin combines with another 11cRAL to form a new visual pigment. Conversion of atRAL to 11cRAL is carried out by enzyme pathways in neighboring cells. Here we show that blue (450-nm) light converts atRAL specifically to 11cRAL through a retinyl-phospholipid intermediate in photoreceptor membranes. The quantum efficiency of this photoconversion is similar to rhodopsin. Photoreceptor membranes synthesize 11cRAL chromophore faster under blue light than in darkness. Live mice regenerate rhodopsin more rapidly in blue light. Finally, whole retinas and isolated cone cells show increased photosensitivity following exposure to blue light. These results indicate that light contributes to visual-pigment renewal in mammalian rods and cones through a non-enzymatic process involving retinyl-phospholipids.

It is currently thought that visual pigments in vertebrate photoreceptors are regenerated exclusively through enzymatic cycles. Here the authors show that mammalian photoreceptors also regenerate opsin pigments in light through photoisomerization of N-ret-PE (N-retinylidene-phosphatidylethanolamine.

Lipidomic Evaluation of Aryl Hydrocarbon Receptor-Mediated Hepatic Steatosis in Male and Female Mice Elicited by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces hepatic steatosis mediated by the aryl hydrocarbon receptor. To further characterize TCDD-elicited hepatic lipid accumulation, mice were gavaged with TCDD every 4 days for 28 days. Liver samples were examined using untargeted lipidomics with structural confirmation of lipid species by targeted high-resolution MS/MS, and data were integrated with complementary RNA-Seq analyses. Approximately 936 unique spectral features were detected, of which 379 were confirmed as unique lipid species. Both male and female samples exhibited similar qualitative changes (lipid species) but differed in quantitative changes. A shift to higher mass lipid species was observed, indicative of increased free fatty acid (FFA) packaging. For example, of the 13 lipid classes examined, triglycerides increased from 46 to 48% of total lipids to 68-83% in TCDD treated animals. Hepatic cholesterol esters increased 11.3-fold in male mice with moieties consisting largely of dietary fatty acids (FAs) (i.e., linolenate, palmitate, and oleate). Phosphatidylserines, phosphatidylethanolamines, phosphatidic acids, and cardiolipins decreased 4.1-, 5.0-, 5.4- and 7.4-fold, respectively, while ceramides increased 6.6-fold. Accordingly, the integration of lipidomic data with differential gene expression associated with lipid metabolism suggests that in addition to the repression of de novo fatty acid synthesis and β-oxidation, TCDD also increased hepatic uptake and packaging of lipids, while inhibiting VLDL secretion, consistent with hepatic fat accumulation and the progression to steatohepatitis with fibrosis.

Endoplasmic Reticulum Stress and Homeostasis in Reproductive Physiology and Pathology

The endoplasmic reticulum (ER), comprises 60% of the total cell membrane and interacts directly or indirectly with several cell organelles i.e., Golgi bodies, mitochondria and proteasomes. The ER is usually associated with large numbers of attached ribosomes. During evolution, ER developed as the specific cellular site of synthesis, folding, modification and trafficking of secretory and cell-surface proteins. The ER is also the major intracellular calcium storage compartment that maintains cellular calcium homeostasis. During the production of functionally effective proteins, several ER-specific molecular steps sense quantity and quality of synthesized proteins as well as proper folding into their native structures. During this process, excess accumulation of unfolded/misfolded proteins in the ER lumen results in ER stress, the homeostatic coping mechanism that activates an ER-specific adaptation program, (the unfolded protein response; UPR) to increase ER-associated degradation of structurally and/or functionally defective proteins, thus sustaining ER homeostasis. Impaired ER homeostasis results in aberrant cellular responses, contributing to the pathogenesis of various diseases. Both female and male reproductive tissues undergo highly dynamic cellular, molecular and genetic changes such as oogenesis and spermatogenesis starting in prenatal life, mainly controlled by sex-steroids but also cytokines and growth factors throughout reproductive life. These reproductive changes require ER to provide extensive protein synthesis, folding, maturation and then their trafficking to appropriate cellular location as well as destroying unfolded/misfolded proteins via activating ER-associated degradation mediated proteasomes. Many studies have now shown roles for ER stress/UPR signaling cascades in the endometrial menstrual cycle, ovarian folliculogenesis and oocyte maturation, spermatogenesis, fertilization, pre-implantation embryo development and pregnancy and parturition. Conversely, the contribution of impaired ER homeostasis by severe/prolong ER stress-mediated UPR signaling pathways to several reproductive tissue pathologies including endometriosis, cancers, recurrent pregnancy loss and pregnancy complications associated with pre-term birth have been reported. This review focuses on ER stress and UPR signaling mechanisms, and their potential roles in female and male reproductive physiopathology involving in menstrual cycle changes, gametogenesis, preimplantation embryo development, implantation and placentation, labor, endometriosis, pregnancy complications and preterm birth as well as reproductive system tumorigenesis.

Sarcoplasmic reticulum-mitochondria communication in cardiovascular pathophysiology

Repetitive, calcium-mediated contractile activity renders cardiomyocytes critically dependent on a sustained energy supply and adequate calcium buffering, both of which are provided by mitochondria. Moreover, in vascular smooth muscle cells, mitochondrial metabolism modulates cell growth and proliferation, whereas cytosolic calcium levels regulate the arterial vascular tone. Physical and functional communication between mitochondria and sarco/endoplasmic reticulum and balanced mitochondrial dynamics seem to have a critical role for optimal calcium transfer to mitochondria, which is crucial in calcium homeostasis and mitochondrial metabolism in both types of muscle cells. Moreover, mitochondrial dysfunction has been associated with myocardial damage and dysregulation of vascular smooth muscle proliferation. Therefore, sarco/endoplasmic reticulum-mitochondria coupling and mitochondrial dynamics are now viewed as relevant factors in the pathogenesis of cardiac and vascular diseases, including coronary artery disease, heart failure, and pulmonary arterial hypertension. In this Review, we summarize the evidence related to the role of sarco/endoplasmic reticulum-mitochondria communication in cardiac and vascular muscle physiology, with a focus on how perturbations contribute to the pathogenesis of cardiovascular disorders.

Viral rewiring of cellular lipid metabolism to create membranous replication compartments

Positive-strand RNA (+RNA) viruses (e.g. poliovirus, hepatitis C virus, dengue virus, SARS-coronavirus) remodel cellular membranes to form so-called viral replication compartments (VRCs), which are the sites where viral RNA genome replication takes place. To induce VRC formation, these viruses extensively rewire lipid metabolism. Disparate viruses have many commonalities as well as disparities in their interactions with the host lipidome and accumulate specific sets of lipids (sterols, glycerophospholipids, sphingolipids) at their VRCs. Recent years have seen an upsurge in studies investigating the role of lipids in +RNA virus replication, in particular of sterols, and uncovered that membrane contact sites and lipid transfer proteins are hijacked by viruses and play pivotal roles in VRC formation.

Of yeast, mice and men: MAMs come in two flavors

The past decade has seen dramatic progress in our understanding of membrane contact sites (MCS). Important examples of these are endoplasmic reticulum (ER)-mitochondria contact sites. ER-mitochondria contacts have originally been discovered in mammalian tissue, where they have been designated as mitochondria-associated membranes (MAMs). It is also in this model system, where the first critical MAM proteins have been identified, including MAM tethering regulators such as phospho-furin acidic cluster sorting protein 2 (PACS-2) and mitofusin-2. However, the past decade has seen the discovery of the MAM also in the powerful yeast model system Saccharomyces cerevisiae. This has led to the discovery of novel MAM tethers such as the yeast ER-mitochondria encounter structure (ERMES), absent in the mammalian system, but whose regulators Gem1 and Lam6 are conserved. While MAMs, sometimes referred to as mitochondria-ER contacts (MERCs), regulate lipid metabolism, Ca²⁺ signaling, bioenergetics, inflammation, autophagy and apoptosis, not all of these functions exist in both systems or operate differently. This biological difference has led to puzzling discrepancies on findings obtained in yeast or mammalian cells at the moment. Our review aims to shed some light onto mechanistic differences between yeast and mammalian MAM and their underlying causes.

Reviewers: This article was reviewed by Paola Pizzo (nominated by Luca Pellegrini), Maya Schuldiner and György Szabadkai (nominated by Luca Pellegrini).

Elimination of the unnecessary: Intra- and extracellular signaling by anionic phospholipids

High fidelity of biological systems is frequently achieved by duplication of the essential intracellular machineries or, removal of the entire cell, which becomes unnecessary or even harmful in altered physiological environments. Carefully controlled removal of these cells, without damaging normal cells, requires precise signaling, and is critical to maintaining homeostasis. This review describes how two anionic phospholipids - phosphatidylserine (PS) and cardiolipin (CL) - residing in distinct compartments of the cell, signal removal of "the unnecessary" using several uniform principles. One of these principles is realized by collapse of inherent transmembrane asymmetry and the externalization of the signal on the outer membrane surface - mitochondria for CL and the plasma membrane for PS - to trigger mitophagy and phagocytosis, respectively. Release from damaged cells of intracellular structures with externalized CL or externalized PS triggers their elimination by phagocytosis. Another of these principles is realized by oxidation of polyunsaturated species of CL and PS. Highly specific oxidation of CL by cytochrome c serves as a signal for mitochondria-dependent apoptosis, while oxidation of externalized PS improves its effectiveness to trigger phagocytosis of effete cells.

Endoplasmic reticulum-mitochondria Ca2+ crosstalk in the control of the tumor cell fate

Mitochondria-associated membranes are juxtaposed between the endoplasmic reticulum and mitochondria and have been identified as a critical hub in the regulation of apoptosis and tumor growth. One key function of mitochondria-associated membranes is to provide asylum to a number of proteins with tumor suppressor and oncogenic properties. In this review, we discuss how Ca²⁺ flux manipulation represents the primary mechanism underlying the action of several oncogenes and tumor-suppressor genes and how these networks might be manipulated to provide novel therapies for cancer. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Domain Interaction Studies of Herpes Simplex Virus 1 Tegument Protein UL16 Reveal Its Interaction with Mitochondria

The UL16 tegument protein of herpes simplex virus 1 (HSV-1) is conserved among all herpesviruses and plays many roles during replication. This protein has an N-terminal domain (NTD) that has been shown to bind to several viral proteins, including UL11, VP22, and glycoprotein E, and these interactions are negatively regulated by a C-terminal domain (CTD). Thus, in pairwise transfections, UL16 binding is enabled only when the CTD is absent or altered. Based on these results, we hypothesized that direct interactions occur between the NTD and the CTD. Here we report that the separated and coexpressed functional domains of UL16 are mutually responsive to each other in transfected cells and form complexes that are stable enough to be captured in coimmunoprecipitation assays. Moreover, we found that the CTD can associate with itself. To our surprise, the CTD was also found to contain a novel and intrinsic ability to localize to specific spots on mitochondria in transfected cells. Subsequent analyses of HSV-infected cells by immunogold electron microscopy and live-cell confocal imaging revealed a population of UL16 that does not merely accumulate on mitochondria but in fact makes dynamic contacts with these organelles in a time-dependent manner. These findings suggest that the domain interactions of UL16 serve to regulate not just the interaction of this tegument protein with its viral binding partners but also its interactions with mitochondria. The purpose of this novel interaction remains to be determined.

IMPORTANCE

The HSV-1-encoded tegument protein UL16 is involved in multiple events of the virus replication cycle, ranging from virus assembly to cell-cell spread of the virus, and hence it can serve as an important drug target. Unfortunately, a lack of both structural and functional information limits our understanding of this protein. The discovery of domain interactions within UL16 and the novel ability of UL16 to interact with mitochondria in HSV-infected cells lays a foundational framework for future investigations aimed at deciphering the structure and function of not just UL16 of HSV-1 but also its homologs in other herpesviruses.

Isolation of Mitochondria-Associated Membranes (MAM) from Mouse Brain Tissue

During the last decades, increasing evidence indicated that subcellular organelles do not exist as autarkic units but instead communicate constantly and extensively with each other in various ways. Some communication, for example, the exchange of small molecules, requires the marked convergence of two distinct organelles for a certain period of time. The cross talk between endoplasmic reticulum (ER) and mitochondria, two subcellular organelles of utmost importance for cellular bioenergetics and protein homeostasis, has been increasingly investigated under the last years. This development was significantly driven by the establishment of optimized subcellular fractionation techniques. In this chapter, we will describe and critically discuss the currently used protocol for the isolation of the membrane fraction containing mitochondria-associated membranes (MAM).

Ceramide Transport from the Endoplasmic Reticulum to the Trans Golgi Region at Organelle Membrane Contact Sites

Lipids are the major constituents of all cell membranes and play dynamic roles in organelle structure and function. Although the spontaneous transfer of lipids between different membranes rarely occurs, lipids are appropriately transported between different organelles for their metabolism and to exert their functions in living cells. Proteins that have the biochemical capability to catalyze the intermembrane transfer of lipids are called lipid transfer proteins (LTPs). All organisms possess many types of LTPs. Recent studies revealed that LTPs are key players in the interorganelle transport of lipids at organelle membrane contact sites (MCSs). This chapter depicts how LTPs rationally operate at MCSs by using the ceramide transport protein CERT as a typical model for the LTP-mediated interorganelle transport of lipids.

Over Six Decades of Discovery and Characterization of the Architecture at Mitochondria-Associated Membranes (MAMs)

The discovery of proteins regulating ER-mitochondria tethering including phosphofurin acidic cluster sorting protein 2 (PACS-2) and mitofusin-2 has pushed contact sites between the endoplasmic reticulum (ER) and mitochondria into the spotlight of cell biology. While the field is developing rapidly and controversies have come and gone multiple times during its history, it is sometimes overlooked that significant research has been done decades ago with the original discovery of these structures in the 1950s and the first characterization of their function (and coining of the term mitochondria-associated membrane, MAM) in 1990. Today, an ever-increasing array of proteins localize to the MAM fraction of the endoplasmic reticulum (ER) to regulate the interaction of this organelle with mitochondria. These mitochondria-ER contacts, sometimes referred to as MERCs, regulate a multitude of biological functions, including lipid metabolism, Ca²⁺ signaling, bioenergetics, inflammation, autophagy, mitochondrial structure, and apoptosis.

Hydrophobic resorufamine derivatives: potent and selective red fluorescent probes of the endoplasmic reticulum of mammalian cells

The endoplasmic reticulum (ER) of eukaryotic cells plays critical roles in the processing of secreted and transmembrane proteins. Defects in these functions are associated with a wide range of pathologies. To image this organelle, cells are often treated with fluorescent ER-Tracker dyes. Although these compounds are selective, existing red fluorescent probes of the ER are costly glibenclamide derivatives that inhibit ER-associated sulphonylurea receptors. To provide simpler and more cost-effective red fluorescent probes of the ER, we synthesized amino analogues of the fluorophore resorufin. By varying the polarity of linked substituents, we identified hexyl resorufamine (HRA) as a novel hydrophobic (cLogD (pH 7.4) = 3.8) red fluorescent (Ex. 565 nm; Em. 614 nm in ethanol) molecular probe. HRA is exceptionally bright in organic solvents (quantum yield = 0.70), it exclusively localizes to the ER of living HeLa cells as imaged by confocal microscopy, it is effective at concentrations as low as 100 nM, and it is non-toxic under these conditions. To examine its utility, we used HRA to facilitate visualization of small molecule-mediated release of a GFP-GPI fusion protein from the ER into the secretory pathway. HRA represents a potent, selective, and cost-effective probe for imaging and labeling the ER.

Disturbed phospholipid homeostasis in endoplasmic reticulum initiates tri-o-cresyl phosphate-induced delayed neurotoxicity

Tri-o-cresyl phosphate (TOCP) is a widely used organophosphorus compound, which can cause a neurodegenerative disorder, i.e., organophosphate-induced delayed neurotoxicity (OPIDN). The biochemical events in the initiation of OPIDN were not fully understood except for the essential inhibition of neuropathy target esterase (NTE). NTE, located in endoplasmic reticulum (ER), catalyzes the deacylation of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) to glycerophosphocholine (GPC). The present study aims to study the changes of ER phospholipids profile as well as levels of important intermediates of phospholipid synthesis such as diacylglycerol (DAG) and phosphatidic acid (PA) at the initiation stage of OPIDN. Hens are the most commonly used animal models of OPIDN. The spinal cord phospholipidomic profiles of hens treated by TOCP were studied by using HPLC-MS-MS. The results revealed that TOCP induced an increase of PC, LPC, and sphingomyelin (SM) levels and a decrease of GPC, phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), phosphatidylglycerol (PG), and phosphatidylinositol (PI) levels., Levels of DAG and PA were also decreased. Pretreatment with phenylmethylsulfonyl fluoride (PMSF) 24 h before TOCP administration prevented OPIDN and restored the TOCP-induced changes of phospholipids except GPC. Thus, the disruption of ER phospholipid homeostasis may contribute to the initiation of organophosphate-induced delayed neurotoxicity.

Lethal H1N1 influenza A virus infection alters the murine alveolar type II cell surfactant lipidome

Alveolar type II (ATII) epithelial cells are the primary site of influenza virus replication in the distal lung. Development of acute respiratory distress syndrome in influenza-infected mice correlates with significant alterations in ATII cell function. However, the impact of infection on ATII cell surfactant lipid metabolism has not been explored. C57BL/6 mice were inoculated intranasally with influenza A/WSN/33 (H1N1) virus (10,000 plaque-forming units/mouse) or mock-infected with virus diluent. ATII cells were isolated by a standard lung digestion protocol at 2 and 6 days postinfection. Levels of 77 surfactant lipid-related compounds of known identity in each ATII cell sample were measured by ultra-high-performance liquid chromatography-mass spectrometry. In other mice, bronchoalveolar lavage fluid was collected to measure lipid and protein content using commercial assay kits. Relative to mock-infected animals, ATII cells from influenza-infected mice contained reduced levels of major surfactant phospholipids (phosphatidylcholine, phosphatidylglycerol, and phosphatidylethanolamine) but increased levels of minor phospholipids (phosphatidylserine, phosphatidylinositol, and sphingomyelin), cholesterol, and diacylglycerol. These changes were accompanied by reductions in cytidine 5'-diphosphocholine and 5'-diphosphoethanolamine (liponucleotide precursors for ATII cell phosphatidylcholine and phosphatidylethanolamine synthesis, respectively). ATII cell lamellar bodies were ultrastructurally abnormal after infection. Changes in ATII cell phospholipids were reflected in the composition of bronchoalveolar lavage fluid, which contained reduced amounts of phosphatidylcholine and phosphatidylglycerol but increased amounts of sphingomyelin, cholesterol, and protein. Influenza infection significantly alters ATII cell surfactant lipid metabolism, which may contribute to surfactant dysfunction and development of acute respiratory distress syndrome in influenza-infected mice.

Piecing Together the Patchwork of Contact Sites

Contact sites are places where two organelles join together to carry out a shared activity requiring nonvesicular communication. A large number of contact sites have been discovered, and almost any two organelles can contact each other. General rules about contacts include constraints on bridging proteins, with only a minority of bridges physically creating contacts by acting as 'tethers'. The downstream effects of contacts include changing the physical behaviour of organelles, and also forming biochemically heterogeneous subdomains. However, some functions typically localized to contact sites, such as lipid transfer, have no absolute requirement to be situated there. Therefore, the key aspect of contacts is the directness of communication, which allows metabolic channelling and collective regulation.

Suppression of StarD7 promotes endoplasmic reticulum stress and induces ROS production

StarD7 is an intracellular lipid transport protein identified as up-regulated in the choriocarcinoma JEG-3 cell line. StarD7 facilitates the delivery of phosphatidylcholine (PC) to the mitochondria, and StarD7 knockdown causes a reduction in phospholipid synthesis. Since inhibition of PC synthesis may lead to endoplasmic reticulum (ER) stress we hypothesized that StarD7 may be involved in maintaining cell homeostasis. Here, we examined the effect of StarD7 silencing on ER stress response and on the levels of reactive oxygen species (ROS) in the human hepatoma cell line HepG2. StarD7 knockdown induced alterations in mitochondria and ER morphology. These changes were accompanied with an ER stress response as determined by increased expression of inositol-requiring enzyme 1α (IRE1α), calnexin, glucose regulated protein 78/immunoglobulin heavy chain-binding protein (Grp78/BiP), protein kinase-like ER kinase (PERK) as well as the phosphorylated eukaryotic translation initiation factor 2, subunit 1α (p-eIF2α). Additionally, a downregulation of the tumor suppressor p53 by a degradation mechanism was observed in StarD7 siRNA cells. Furthermore, StarD7 silencing induced ROS generation and reduced cell viability after H₂O₂ exposure. Decreased expression of StarD7 was associated to increased levels of the heme oxygenase-1 (HO-1) and catalase enzymes as well as in catalase enzymatic activity. Finally, no changes in levels of autophagy and apoptosis markers were observed in StarD7 siRNA treated cells respect to control cells. Taken together, these results indicate that StarD7 contributes to modulate cellular redox homeostasis.

StarD7 Protein Deficiency Adversely Affects the Phosphatidylcholine Composition, Respiratory Activity, and Cristae Structure of Mitochondria

Phosphatidylcholine (PC) is a major phospholipid of mitochondria, comprising 40-50% of both the outer and the inner membranes. However, PC must be imported from its production organelles because mitochondria lack the enzymes essential for PC biosynthesis. In a previous study, we found that StarD7 mediates the intracellular transfer of PC to mitochondria. Therefore, in this study, we analyzed the contribution of StarD7 to the maintenance of mitochondrial phospholipid content and function using siRNA-mediated knockdown and knock-out (KO) of the StarD7 gene in HEPA-1 cells. Real time analysis of respiratory activity demonstrated that the oxygen consumption rate and activity of mitochondrial complexes were impaired in StarD7-KD cells. To confirm these results, we established StarD7-KO HEPA-1 cells by double nicking using CRISPR/Cas9n. As expected, StarD7-KD and -KO cells showed a significant reduction in mitochondrial PC content. The ATP level and growth rate of KO cells were notably lower compared with wild-type cells when cultured in glucose-free galactose-containing medium to force cells to rely on mitochondrial ATP production. In KO cells, the level of the MTCO1 protein, a primary subunit of complex IV, was reduced without a concomitant decrease in its mRNA, but the level was restored when StarD7-I was overexpressed. StarD7-KO cells showed impaired formation of the mitochondrial supercomplexes and exhibited a disorganized cristae structure, with no changes in optic atrophy 1 protein. These findings indicate that StarD7 plays important roles in maintaining the proper composition of mitochondrial phospholipids as well as mitochondrial function and morphogenesis.

Endoplasmic reticulum stress in obesity and obesity-related disorders: An expanded view

The endoplasmic reticulum (ER) is most notable for its central roles in calcium ion storage, lipid biosynthesis, and protein sorting and processing. By virtue of its extensive membrane contact sites that connect the ER to most other organelles and to the plasma membrane, the ER can also regulate diverse cellular processes including inflammatory and insulin signaling, nutrient metabolism, and cell proliferation and death via a signaling pathway called the unfolded protein response (UPR). Chronic UPR activation has been observed in liver and/or adipose tissue of dietary and genetic murine models of obesity, and in human obesity and non-alcoholic fatty liver disease (NAFLD). Activation of the UPR in obesity and obesity-related disorders likely has two origins. One linked to classic ER stress involving the ER lumen and one linked to alterations to the ER membrane environment. This review discusses both of these origins and also considers the role of post-translational protein modifications, such as acetylation and palmitoylation, and ER-mitochondrial interactions to obesity-mediated impairments in the ER and activation of the UPR.

Targeting mitochondria: how intravacuolar bacterial pathogens manipulate mitochondria

Manipulation of host cell function by bacterial pathogens is paramount for successful invasion and creation of a niche conducive to bacterial replication. Mitochondria play a role in many important cellular processes including energy production, cellular calcium homeostasis, lipid metabolism, haeme biosynthesis, immune signalling and apoptosis. The sophisticated integration of host cell processes by the mitochondrion have seen it emerge as a key target during bacterial infection of human host cells. This review highlights the targeting and interaction of this dynamic organelle by intravacuolar bacterial pathogens and the way that the modulation of mitochondrial function might contribute to pathogenesis.

Variation in human cancer cell external phosphatidylserine is regulated by flippase activity and intracellular calcium

Viable cancer cells expose elevated levels of phosphatidylserine (PS) on the exoplasmic face of the plasma membrane. However, the mechanisms leading to elevated PS exposure in viable cancer cells have not been defined. We previously showed that externalized PS may be used to monitor, target and kill tumor cells. In addition, PS on tumor cells is recognized by macrophages and has implications in antitumor immunity. Therefore, it is important to understand the molecular details of PS exposure on cancer cells in order to improve therapeutic targeting. Here we explored the mechanisms regulating the surface PS exposure in human cancer cells and found that differential flippase activity and intracellular calcium are the major regulators of surface PS exposure in viable human cancer cells. In general, cancer cell lines with high surface PS exhibited low flippase activity and high intracellular calcium, whereas cancer cells with low surface PS exhibited high flippase activity and low intracellular calcium. High surface PS cancer cells also had higher total cellular PS than low surface PS cells. Together, our results indicate that the amount of external PS in cancer cells is regulated by calcium dependent flippase activity and may also be influenced by total cellular PS.

Mitochondrial contact sites as platforms for phospholipid exchange

Mitochondria are unique organelles that contain their own - although strongly reduced - genome, and are surrounded by two membranes. While most cellular phospholipid biosynthesis takes place in the ER, mitochondria harbor the whole spectrum of glycerophospholipids common to biological membranes. Mitochondria also contribute to overall phospholipid biosynthesis in cells by producing phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin. Considering these features, it is not surprising that mitochondria maintain highly active exchange of phospholipids with other cellular compartments. In this contribution we describe the transport of phospholipids between mitochondria and other organelles, and discuss recent developments in our understanding of the molecular functions of the protein complexes that mediate these processes. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.