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.

Mitochondrial acyltransferases and glycerophospholipid metabolism

Our understanding of the synthesis and remodeling of mitochondrial phospholipids remains incomplete. Two isoforms of glycerol-3-phosphate acyltransferase (GPAT1 and 2) and two isoforms of acylglycerol-3-phosphate acyltransferase (AGPAT4 and 5) are located on the outer mitochondrial membrane, suggesting that both lysophosphatidic acid and phosphatidic acid are synthesized in situ for de novo glycerolipid biosynthesis. However, it is believed that the phosphatidic acid substrate for cardiolipin and phosphatidylethanolamine biosynthesis is produced at the endoplasmic reticulum whereas the phosphatidic acid synthesized in the mitochondria must be transferred to the endoplasmic reticulum before it undergoes additional steps to form the mature phospholipids that are trafficked back to the mitochondria. It is unclear whether mitochondrial phospholipids are remodeled by mitochondrial acyltransferases or whether lysophospholipids must return to the endoplasmic reticulum or to the mitochondrial associated membrane for reesterification. In this review we will focus on the few glycerolipid acyltransferases that are known to be mitochondrial. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Mitochondrial lipid transport and biosynthesis: A complex balance

Little is known about how mitochondrial lipids reach inner membrane-localized metabolic enzymes for phosphatidylethanolamine synthesis. Aaltonen et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201602007) and Miyata et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201601082) now report roles for two mitochondrial complexes, Ups2-Mdm35 and mitochondrial contact site and cristae organizing system, in the biosynthesis and transport of mitochondrial lipids.

Effects of lipids on mitochondrial functions

Mitochondria contain two membranes: the outer and inner membrane. Whereas the outer membrane is particularly enriched in phospholipids, the inner membrane has an unusual high protein content and forms large invaginations termed cristae. The proper phospholipid composition of the membranes is crucial for mitochondrial functions. Phospholipids affect activity, biogenesis and stability of protein complexes including protein translocases and respiratory chain supercomplexes. Negatively charged phospholipids such as cardiolipin are important for the architecture of the membranes and recruit soluble factors to the membranes to support mitochondrial dynamics. Thus, phospholipids not only form the hydrophobic core of biological membranes that surround mitochondria, but also create a specific environment to promote functions of various protein machineries. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Endoplasmic Reticulum: The Favorite Intracellular Niche for Viral Replication and Assembly

The endoplasmic reticulum (ER) is the largest intracellular organelle. It forms a complex network of continuous sheets and tubules, extending from the nuclear envelope (NE) to the plasma membrane. This network is frequently perturbed by positive-strand RNA viruses utilizing the ER to create membranous replication factories (RFs), where amplification of their genomes occurs. In addition, many enveloped viruses assemble progeny virions in association with ER membranes, and viruses replicating in the nucleus need to overcome the NE barrier, requiring transient changes of the NE morphology. This review first summarizes some key aspects of ER morphology and then focuses on the exploitation of the ER by viruses for the sake of promoting the different steps of their replication cycles.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Detection of atherosclerotic plaques in ApoE-deficient mice using (99m)Tc-duramycin

Apoptosis of macrophages and smooth muscle cells is linked to atherosclerotic plaque destabilization. The apoptotic cascade leads to exposure of phosphatidylethanolamine (PE) on the outer leaflet of the cell membrane, thereby making apoptosis detectable using probes targeting PE. The objective of this study was to exploit capabilities of a PE-specific imaging probe, (99m)Tc-duramycin, in localizing atherosclerotic plaque and assessing plaque evolution in apolipoprotein-E knockout (ApoE(-/-)) mice.

METHODS

Atherosclerosis was induced in ApoE(-/-) mice by feeding an atherogenic diet. (99m)Tc-duramycin images were acquired using a small-animal SPECT imager. Six ApoE(-/-) mice at 20weeks of age (Group I) were imaged and then sacrificed for ex vivo analyses. Six additional ApoE(-/-) mice (Group II) were imaged at 20 and 40weeks of age before sacrifice. Six ApoE wild-type (ApoE(+/+)) mice (Group III) were imaged at 40weeks as controls. Five additional ApoE(-/-) mice (40weeks of age) (Group IV) were imaged with a (99m)Tc-labeled inactive peptide, (99m)Tc-LinDUR, to assess (99m)Tc-duramycin targeting specificity.

RESULTS

Focal (99m)Tc-duramycin uptake in the ascending aorta and aortic arch was detected at 20 and 40weeks in the ApoE(-/-) mice but not in ApoE(+/+) mice. (99m)Tc-duramycin uptake in the aortic lesions increased 2.2-fold on quantitative imaging in the ApoE(-/-) mice between 20 and 40weeks. Autoradiographic and histological data indicated significantly increased (99m)Tc-duramycin uptake in the ascending aorta and aortic arch associated with advanced plaques. Quantitative autoradiography showed that the ratio of activity in the aortic arch to descending thoracic aorta, which had no plaques or radioactive uptake, was 2.1 times higher at 40weeks than at 20weeks (6.62±0.89 vs. 3.18±0.29, P<0.01). There was barely detectable focal uptake of (99m)Tc-duramycin in the aortic arch of ApoE(+/+) mice. No detectable (99m)Tc-LinDUR uptake was observed in the aortas of ApoE(-/-) mice.

CONCLUSIONS

PE-targeting properties of (99m)Tc-duramycin in the atherosclerotic mouse aortas were noninvasively characterized. (99m)Tc-duramycin is promising in localizing advanced atherosclerotic plaques.

Is Spontaneous Translocation of Polar Lipids Between Cellular Organelles Negligible?

In most reviews addressing intracellular lipid trafficking, spontaneous diffusion of lipid monomers between the cellular organelles is considered biologically irrelevant because it is thought to be far too slow to significantly contribute to organelle biogenesis. This view is based on intervesicle transfer experiments carried out in vitro with few lipids as well as on the view that lipids are highly hydrophobic and thus cannot undergo spontaneous intermembrane diffusion at a significant rate. However, besides that single-chain lipids can translocate between vesicles in seconds, it has been demonstrated that the rate of spontaneous transfer of two-chain polar lipids can vary even 1000-fold, depending on the number of carbons and double bonds in the acyl chains. In addition, the rate of spontaneous lipid transfer can strongly depend on the experimental conditions such as vesicle composition and concentration. This review examines the studies suggesting that spontaneous lipid transfer is probably more relevant to intracellular trafficking of amphipathic lipids than commonly thought.

iPLA2β deficiency attenuates obesity and hepatic steatosis in ob/ob mice through hepatic fatty-acyl phospholipid remodeling

PLA2G6 or GVIA calcium-independent PLA2 (iPLA2β) is identified as one of the NAFLD modifier genes in humans, and thought to be a target for NAFLD therapy. iPLA2β is known to play a house-keeping role in phospholipid metabolism and remodeling. However, its role in NAFLD pathogenesis has not been supported by results obtained from high-fat feeding of iPLA2β-null (PKO) mice. Unlike livers of human NAFLD and genetically obese rodents, fatty liver induced by high-fat diet is not associated with depletion of hepatic phospholipids. We therefore tested whether iPLA2β could regulate obesity and hepatic steatosis in leptin-deficient mice by cross-breeding PKO with ob/ob mice to generate ob/ob-PKO mice. Here we observed an improvement in ob/ob-PKO mice with significant reduction in serum enzymes, lipids, glucose, insulin as well as improved glucose tolerance, and reduction in islet hyperplasia. The improvement in hepatic steatosis measured by liver triglycerides, fatty acids and cholesterol esters was associated with decreased expression of PPARγ and de novo lipogenesis genes, and the reversal of β-oxidation gene expression. Notably, ob/ob livers contained depleted levels of lysophospholipids and phospholipids, and iPLA2β deficiency in ob/ob-PKO livers lowers the former, but replenished the latter particularly phosphatidylethanolamine (PE) and phosphatidylcholine (PC) that contained arachidonic (AA) and docosahexaenoic (DHA) acids. Compared with WT livers, PKO livers also contained increased PE and PC containing AA and DHA. Thus, iPLA2β deficiency protected against obesity and ob/ob fatty liver which was associated with hepatic fatty-acyl phospholipid remodeling. Our results support the deleterious role of iPLA2β in severe obesity associated NAFLD.

The Ebola Virus matrix protein, VP40, requires phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) for extensive oligomerization at the plasma membrane and viral egress

VP40 is one of eight proteins encoded by the Ebola Virus (EBOV) and serves as the primary matrix protein, forming virus like particles (VLPs) from mammalian cells without the need for other EBOV proteins. While VP40 is required for viral assembly and budding from host cells during infection, the mechanisms that target VP40 to the plasma membrane are not well understood. Phosphatidylserine is required for VP40 plasma membrane binding, VP40 hexamer formation, and VLP egress, However, PS also becomes exposed on the outer membrane leaflet at sites of VP40 budding, raising the question of how VP40 maintains an interaction with the plasma membrane inner leaflet when PS is flipped to the opposite side. To address this question, cellular and in vitro assays were employed to determine if phosphoinositides are important for efficient VP40 localization to the plasma membrane. Cellular studies demonstrated that PI(4,5)P₂ was an important component of VP40 assembly at the plasma membrane and subsequent virus like particle formation. Additionally, PI(4,5)P₂ was required for formation of extensive oligomers of VP40, suggesting PS and PI(4,5)P₂ have different roles in VP40 assembly where PS regulates formation of hexamers from VP40 dimers and PI(4,5)P₂ stabilizes and/or induces extensive VP40 oligomerization at the plasma membrane.

Human rhinovirus-induced inflammatory responses are inhibited by phosphatidylserine containing liposomes

Human rhinovirus (HRV) infections are major contributors to the healthcare burden associated with acute exacerbations of chronic airway disease, such as chronic obstructive pulmonary disease and asthma. Cellular responses to HRV are mediated through pattern recognition receptors that may in part signal from membrane microdomains. We previously found Toll-like receptor signaling is reduced, by targeting membrane microdomains with a specific liposomal phosphatidylserine species, 1-stearoyl-2-arachidonoyl-sn-glycero-3-phospho-L-serine (SAPS). Here we explored the ability of this approach to target a clinically important pathogen. We determined the biochemical and biophysical properties and stability of SAPS liposomes and studied their ability to modulate rhinovirus-induced inflammation, measured by cytokine production, and rhinovirus replication in both immortalized and normal primary bronchial epithelial cells. SAPS liposomes rapidly partitioned throughout the plasma membrane and internal cellular membranes of epithelial cells. Uptake of liposomes did not cause cell death, but was associated with markedly reduced inflammatory responses to rhinovirus, at the expense of only modest non-significant increases in viral replication, and without impairment of interferon receptor signaling. Thus using liposomes of phosphatidylserine to target membrane microdomains is a feasible mechanism for modulating rhinovirus-induced signaling, and potentially a prototypic new therapy for viral-mediated inflammation.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

Membrane-bound selenoproteins

SIGNIFICANCE

Selenoproteins employ selenium to supplement the chemistry available through the common 20 amino acids. These powerful enzymes are affiliated with redox biology, often in connection with the detection, management, and signaling of oxidative stress. Among them, membrane-bound selenoproteins play prominent roles in signaling pathways, Ca(2+) regulation, membrane complexes integrity, and biosynthesis of lipophilic molecules.

RECENT ADVANCES

The number of selenoproteins whose physiological roles, protein partners, expression, evolution, and biosynthesis are characterized is steadily increasing, thus offering a more nuanced view of this specialized family. This review focuses on human membrane selenoproteins, particularly the five least characterized ones: selenoproteins I, K, N, S, and T.

CRITICAL ISSUES

Membrane-bound selenoproteins are the least understood, as it is challenging to provide the membrane-like environment required for their biochemical and biophysical characterization. Hence, their studies rely mostly on biological rather than structural and biochemical assays. Another aspect that has not received much attention is the particular role that their membrane association plays in their physiological function.

FUTURE DIRECTIONS

Findings cited in this review show that it is possible to infer the structure and the membrane-binding mode of these lesser-studied selenoproteins and design experiments to examine the role of the rare amino acid selenocysteine.

Structural and mechanistic insights into phospholipid transfer by Ups1-Mdm35 in mitochondria

Eukaryotic cells are compartmentalized into membrane-bounded organelles whose functions rely on lipid trafficking to achieve membrane-specific compositions of lipids. Here we focused on the Ups1–Mdm35 system, which mediates phosphatidic acid (PA) transfer between the outer and inner mitochondrial membranes, and determined the X-ray structures of Mdm35 and Ups1–Mdm35 with and without PA. The Ups1–Mdm35 complex constitutes a single domain that has a deep pocket and flexible Ω-loop lid. Structure-based mutational analyses revealed that a basic residue at the pocket bottom and the Ω-loop lid are important for PA extraction from the membrane following Ups1 binding. Ups1 binding to the membrane is enhanced by the dissociation of Mdm35. We also show that basic residues around the pocket entrance are important for Ups1 binding to the membrane and PA extraction. These results provide a structural basis for understanding the mechanism of PA transfer between mitochondrial membranes.

Phospholipid trafficking between membranes is essential to maintain the structural integrity and function of membrane-bound cellular compartments. Here the authors establish the structural basis for transport of phosphatidic acid between the outer and inner membranes of the mitochondria by the Ups1–Mdm35 lipid-transport complex.

Synthesis of Fluorophores that Target Small Molecules to the Endoplasmic Reticulum of Living Mammalian Cells

The endoplasmic reticulum (ER) plays critical roles in the processing of secreted and transmembrane proteins. To deliver small molecules to this organelle, we synthesized fluorinated hydrophobic analogues of the fluorophore rhodol. These cell-permeable fluorophores are exceptionally bright, with quantum yields of around 0.8, and they were found to specifically accumulate in the ER of living HeLa cells, as imaged by confocal laser scanning microscopy. To target a biological pathway controlled by the ER, we linked a fluorinated hydrophobic rhodol to 5-nitrofuran-2-acrylaldehyde. In contrast to an untargeted nitrofuran warhead, delivery of this electrophilic nitrofuran to the ER by the rhodol resulted in cytotoxicity comparable to the ER-targeted cytotoxin eeyarestatin I, and specifically inhibited protein processing by the ubiquitin-proteasome system. Fluorinated hydrophobic rhodols are outstanding fluorophores that enable the delivery of small molecules for targeting ER-associated proteins and pathways.

A new family of StART domain proteins at membrane contact sites has a role in ER-PM sterol transport

Sterol traffic between the endoplasmic reticulum (ER) and plasma membrane (PM) is a fundamental cellular process that occurs by a poorly understood non-vesicular mechanism. We identified a novel, evolutionarily diverse family of ER membrane proteins with StART-like lipid transfer domains and studied them in yeast. StART-like domains from Ysp2p and its paralog Lam4p specifically bind sterols, and Ysp2p, Lam4p and their homologs Ysp1p and Sip3p target punctate ER-PM contact sites distinct from those occupied by known ER-PM tethers. The activity of Ysp2p, reflected in amphotericin-sensitivity assays, requires its second StART-like domain to be positioned so that it can reach across ER-PM contacts. Absence of Ysp2p, Ysp1p or Sip3p reduces the rate at which exogenously supplied sterols traffic from the PM to the ER. Our data suggest that these StART-like proteins act in trans to mediate a step in sterol exchange between the PM and ER.

DOI:http://dx.doi.org/10.7554/eLife.07253.001

Membranes are crucial structures for cells that are made primarily of fat molecules. The most important membrane is the external one that surrounds cells and keeps the outside world out and cellular contents in. The single most common fat component in the external membrane is cholesterol, which makes the membrane rigid and better able to withstand the outside world. So even though excess cholesterol contributes to diseases such as heart disease, stroke and Alzheimer's, the external membrane of every cell needs about a billion cholesterol molecules for its normal function. But how do cells manage the traffic of these molecules to their destination?

It is known that when external membranes are short of cholesterol they make it at a different cellular location. There is an internal network—called the endoplasmic reticulum—that spreads just about everywhere throughout the cell. This network is where fats like cholesterol are made when the cell has not got enough, and where they are converted into an inert form when the cell has too much. What is not known is how cholesterol moves to and fro between this network and the external membrane.

One theory is that cholesterol and other fats move only where the internal network comes into close contact with the external membrane, without quite touching. This theory comes in part from the finding that many of the proteins found in the narrow gaps between the internal network and the external membrane are capable of transferring fats across the gap. However, one of the missing supports for this theory is that no protein that transfers cholesterol across this gap has been found.

Gatta, Wong, Sere et al. used computational tools to scan the database of known proteins for those that might be able to transfer cholesterol, and found a new family of fat transfer proteins. Further experiments showed that these proteins only bind to cholesterol out of all the fats. Next, Gatta, Wong, Sere et al. studied what the proteins do in cells, but instead of looking at the proteins in human cells they studied the related proteins in yeast. This is because the details of both the traffic of cholesterol and contacts between the internal network and the external membrane are in many respects understood better in yeast than in human cells.

Gatta, Wong, Sere et al. found the cholesterol transfer proteins were embedded in regions where the internal network was in close contact with the external membrane. Also, in cells that lacked these proteins, cholesterol added to the external membrane had difficulty transferring to the internal network.

These results together suggest that the newly identified lipid transfer proteins exchange lipids between the plasma membrane and endoplasmic reticulum at membrane contact sites. Further research is required to understand in detail how these proteins work.

DOI:http://dx.doi.org/10.7554/eLife.07253.002

Calcium signaling at ER membrane contact sites

Communication between organelles is a necessary consequence of intracellular compartmentalization. Membrane contact sites (MCSs) are regions where the membranes of two organelles come into close apposition allowing exchange of small molecules and ions including Ca²⁺. The ER, the cell's major Ca²⁺ store, forms an extensive and dynamic network of contacts with multiple organelles. Here we review established and emerging roles of ER contacts as platforms for Ca²⁺ exchange and further consider a potential role for Ca²⁺ in the regulation of MCS formation. We additionally discuss the challenges associated with the study of MCS biology and highlight advances in microscopy-based solutions. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.