Newly made phosphatidylserine and phosphatidylethanolamine are preferentially translocated between rat liver mitochondria and endoplasmic reticulum

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

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

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).

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.

Phospholipids in mitochondrial dysfunction during hemorrhagic shock

Energy deficiency plays a key role in the development of irreversible shock conditions. Therefore, identifying mitochondrial functional disturbances during hemorrhagic shock should be considered a prospective direction for studying its pathogenesis. Phospholipid (PL)-dependent mechanisms of mitochondrial dysfunction in the brain (i.e., in the frontal lobes of the cerebral hemispheres and medulla oblongata) and liver, which, when damaged, leads to an encephalopathy, are examined in this review. These mechanisms show strong regional specificity. Analyzing the data presented in this review suggests that the basis for mitochondrial functional disturbances is cholinergic hyperactivation, accompanied by a choline deficiency and membrane phosphatidylcholine (PC) depletion. Stabilization of the PL composition in mitochondrial membranes using "empty" PC liposomes could be one of the most important methods for eliminating energy deficiency during massive blood loss.

Intermediate Filaments as Organizers of Cellular Space: How They Affect Mitochondrial Structure and Function

Intermediate filaments together with actin filaments and microtubules form the cytoskeleton, which is a complex and highly dynamic 3D network. Intermediate filaments are the major mechanical stress protectors but also affect cell growth, differentiation, signal transduction, and migration. Using intermediate filament-mitochondrial crosstalk as a prominent example, this review emphasizes the importance of intermediate filaments as crucial organizers of cytoplasmic space to support these functions. We summarize observations in different mammalian cell types which demonstrate how intermediate filaments influence mitochondrial morphology, subcellular localization, and function through direct and indirect interactions and how perturbations of these interactions may lead to human diseases.

The coming of age of the mitochondria-ER contact: a matter of thickness

The sites of near-contact between the mitochondrion and the endoplasmic reticulum (ER) have earned a lot of attention due to their key role in the maintenance of lipid and calcium (Ca(2+)) homeostasis, in the initiation of autophagy and mitochondrial division, and in sensing metabolic shifts. At these sites, typically called MAMs (mitochondria-associated ER membranes) or MERCs (mitochondria-ER contacts), the organelles juxtapose at a distance that can range from ~10 to ~50 nm. The multifunctional role of this subcellular compartment is puzzling; further, recent studies have shown that mitochondria-ER contacts are highly plastic structures that remodel upon metabolic transitions and that their activity in controlling lipid homeostasis could be involved in Alzheimer's disease pathogenesis. This review aims at integrating the functions of this subcellular compartment to its most characterizing and unexplored structural parameter, their 'thickness': that is, the width of the cleft that separates the cytosolic face of the outer mitochondrial membrane from that of the ER. We describe and discuss the reasons why the thickness of a MERC should be considered a regulated structural parameter of the cell that defines and controls its function. Further, we propose a MERC classification that will help organize the expanding field of MERCs biology and of their role in cell physiology and human disease.

OSBP-Related Protein Family: Mediators of Lipid Transport and Signaling at Membrane Contact Sites

Oxysterol-binding protein (OSBP) and its related protein homologs, ORPs, constitute a conserved family of lipid-binding/transfer proteins (LTPs) expressed ubiquitously in eukaryotes. The ligand-binding domain of ORPs accommodates cholesterol and oxysterols, but also glycerophospholipids, particularly phosphatidylinositol-4-phosphate (PI4P). ORPs have been implicated as intracellular lipid sensors or transporters. Most ORPs carry targeting determinants for the endoplasmic reticulum (ER) and non-ER organelle membrane. ORPs are located and function at membrane contact sites (MCSs), at which ER is closely apposed with other organelle limiting membranes. Such sites have roles in lipid transport and metabolism, control of Ca(2+) fluxes, and signaling events. ORPs are postulated either to transport lipids over MCSs to maintain the distinct lipid compositions of organelle membranes, or to control the activity of enzymes/protein complexes with functions in signaling and lipid metabolism. ORPs may transfer PI4P and another lipid class bidirectionally. Transport of PI4P followed by its hydrolysis would in this model provide the energy for transfer of the other lipid against its concentration gradient. Control of organelle lipid compositions by OSBP/ORPs is important for the life cycles of several pathogenic viruses. Targeting ORPs with small-molecular antagonists is proposed as a new strategy to combat viral infections. Several ORPs are reported to modulate vesicle transport along the secretory or endocytic pathways. Moreover, antagonists of certain ORPs inhibit cancer cell proliferation. Thus, ORPs are LTPs, which mediate interorganelle lipid transport and coordinate lipid signals with a variety of cellular regimes.

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.

The different facets of organelle interplay-an overview of organelle interactions

Membrane-bound organelles such as mitochondria, peroxisomes, or the endoplasmic reticulum (ER) create distinct environments to promote specific cellular tasks such as ATP production, lipid breakdown, or protein export. During recent years, it has become evident that organelles are integrated into cellular networks regulating metabolism, intracellular signaling, cellular maintenance, cell fate decision, and pathogen defence. In order to facilitate such signaling events, specialized membrane regions between apposing organelles bear distinct sets of proteins to enable tethering and exchange of metabolites and signaling molecules. Such membrane associations between the mitochondria and a specialized site of the ER, the mitochondria associated-membrane (MAM), as well as between the ER and the plasma membrane (PAM) have been partially characterized at the molecular level. However, historical and recent observations imply that other organelles like peroxisomes, lysosomes, and lipid droplets might also be involved in the formation of such apposing membrane contact sites. Alternatively, reports on so-called mitochondria derived-vesicles (MDV) suggest alternative mechanisms of organelle interaction. Moreover, maintenance of cellular homeostasis requires the precise removal of aged organelles by autophagy—a process which involves the detection of ubiquitinated organelle proteins by the autophagosome membrane, representing another site of membrane associated-signaling. This review will summarize the available data on the existence and composition of organelle contact sites and the molecular specializations each site uses in order to provide a timely overview on the potential functions of organelle interaction.

Phosphatidylserine and the human brain

OBJECTIVE

The aim of this study was to assess the roles and importance of phosphatidylserine (PS), an endogenous phospholipid and dietary nutrient, in human brain biochemistry, physiology, and function.

METHODS

A scientific literature search was conducted on MEDLINE for relevant articles regarding PS and the human brain published before June 2014. Additional publications were identified from references provided in original papers; 127 articles were selected for inclusion in this review.

RESULTS

A large body of scientific evidence describes the interactions among PS, cognitive activity, cognitive aging, and retention of cognitive functioning ability.

CONCLUSION

Phosphatidylserine is required for healthy nerve cell membranes and myelin. Aging of the human brain is associated with biochemical alterations and structural deterioration that impair neurotransmission. Exogenous PS (300-800 mg/d) is absorbed efficiently in humans, crosses the blood-brain barrier, and safely slows, halts, or reverses biochemical alterations and structural deterioration in nerve cells. It supports human cognitive functions, including the formation of short-term memory, the consolidation of long-term memory, the ability to create new memories, the ability to retrieve memories, the ability to learn and recall information, the ability to focus attention and concentrate, the ability to reason and solve problems, language skills, and the ability to communicate. It also supports locomotor functions, especially rapid reactions and reflexes.

Lipid transport in the lactating mammary gland

Mammalian cells depend on phospholipid (PL) and fatty acid (FA) transport to maintain membrane structure and organization, and to fuel and regulate cellular functions. In mammary glands of lactating animals, copious milk secretion, including large quantities of lipid in some species, requires adaptation and integration of PL and FA synthesis and transport processes to meet secretion demands. At present few details exist about how these processes are regulated within the mammary gland. However, recent advances in our understanding of the structural and molecular biology of membrane systems and cellular lipid trafficking provide insights into the mechanisms underlying the regulation and integration of PL and FA transport processes the lactating mammary gland. This review discusses the PL and FA transport processes required to maintain the structural integrity and organization of the mammary gland and support its secretory functions within the context of current molecular and cellular models of their regulation.

MAM (mitochondria-associated membranes) in mammalian cells: lipids and beyond

One mechanism by which communication between the endoplasmic reticulum (ER) and mitochondria is achieved is by close juxtaposition between these organelles via mitochondria-associated membranes (MAM). The MAM consist of a region of the ER that is enriched in several lipid biosynthetic enzyme activities and becomes reversibly tethered to mitochondria. Specific proteins are localized, sometimes transiently, in the MAM. Several of these proteins have been implicated in tethering the MAM to mitochondria. In mammalian cells, formation of these contact sites between MAM and mitochondria appears to be required for key cellular events including the transport of calcium from the ER to mitochondria, the import of phosphatidylserine into mitochondria from the ER for decarboxylation to phosphatidylethanolamine, the formation of autophagosomes, regulation of the morphology, dynamics and functions of mitochondria, and cell survival. This review focuses on the functions proposed for MAM in mediating these events in mammalian cells. In light of the apparent involvement of MAM in multiple fundamental cellular processes, recent studies indicate that impaired contact between MAM and mitochondria might underlie the pathology of several human neurodegenerative diseases, including Alzheimer's disease. Moreover, MAM has been implicated in modulating glucose homeostasis and insulin resistance, as well as in some viral infections.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Phosphatidylethanolamine deficiency in Mammalian mitochondria impairs oxidative phosphorylation and alters mitochondrial morphology

Mitochondrial dysfunction is implicated in neurodegenerative, cardiovascular, and metabolic disorders, but the role of phospholipids, particularly the nonbilayer-forming lipid phosphatidylethanolamine (PE), in mitochondrial function is poorly understood. Elimination of mitochondrial PE (mtPE) synthesis via phosphatidylserine decarboxylase in mice profoundly alters mitochondrial morphology and is embryonic lethal (Steenbergen, R., Nanowski, T. S., Beigneux, A., Kulinski, A., Young, S. G., and Vance, J. E. (2005) J. Biol. Chem. 280, 40032-40040). We now report that moderate <30% depletion of mtPE alters mitochondrial morphology and function and impairs cell growth. Acute reduction of mtPE by RNAi silencing of phosphatidylserine decarboxylase and chronic reduction of mtPE in PSB-2 cells that have only 5% of normal phosphatidylserine synthesis decreased respiratory capacity, ATP production, and activities of electron transport chain complexes (C) I and CIV but not CV. Blue native-PAGE analysis revealed defects in the organization of CI and CIV into supercomplexes in PE-deficient mitochondria, correlated with reduced amounts of CI and CIV proteins. Thus, mtPE deficiency impairs formation and/or membrane integration of respiratory supercomplexes. Despite normal or increased levels of mitochondrial fusion proteins in mtPE-deficient cells, and no reduction in mitochondrial membrane potential, mitochondria were extensively fragmented, and mitochondrial ultrastructure was grossly aberrant. In general, chronic reduction of mtPE caused more pronounced mitochondrial defects than did acute mtPE depletion. The functional and morphological changes in PSB-2 cells were largely reversed by normalization of mtPE content by supplementation with lyso-PE, a mtPE precursor. These studies demonstrate that even a modest reduction of mtPE in mammalian cells profoundly alters mitochondrial functions.

Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are metabolically related membrane aminophospholipids. In mammalian cells, PS is required for targeting and function of several intracellular signaling proteins. Moreover, PS is asymmetrically distributed in the plasma membrane. Although PS is highly enriched in the cytoplasmic leaflet of plasma membranes, PS exposure on the cell surface initiates blood clotting and removal of apoptotic cells. PS is synthesized in mammalian cells by two distinct PS synthases that exchange serine for choline or ethanolamine in phosphatidylcholine (PC) or PE, respectively. Targeted disruption of each PS synthase individually in mice demonstrated that neither enzyme is required for viability whereas elimination of both synthases was embryonic lethal. Thus, mammalian cells require a threshold amount of PS. PE is synthesized in mammalian cells by four different pathways, the quantitatively most important of which are the CDP-ethanolamine pathway that produces PE in the ER, and PS decarboxylation that occurs in mitochondria. PS is made in ER membranes and is imported into mitochondria for decarboxylation to PE via a domain of the ER [mitochondria-associated membranes (MAM)] that transiently associates with mitochondria. Elimination of PS decarboxylase in mice caused mitochondrial defects and embryonic lethality. Global elimination of the CDP-ethanolamine pathway was also incompatible with mouse survival. Thus, PE made by each of these pathways has independent and necessary functions. In mammals PE is a substrate for methylation to PC in the liver, a substrate for anandamide synthesis, and supplies ethanolamine for glycosylphosphatidylinositol anchors of cell-surface signaling proteins. Thus, PS and PE participate in many previously unanticipated facets of mammalian cell biology. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Forms, crosstalks, and the role of phospholipid biosynthesis in autophagy

Autophagy is a highly conserved cellular process occurring during periods of stress to ensure a cell's survival by recycling cytosolic constituents and making products that can be used in energy generation and other essential processes. Three major forms of autophagy exist according to the specific mechanism through which cytoplasmic material is transported to a lysosome. Chaperone-mediated autophagy is a highly selective form of autophagy that delivers specific proteins for lysosomal degradation. Microautophagy is a less selective form of autophagy that occurs through lysosomal membrane invaginations, forming tubes and directly engulfing cytoplasm. Finally, macroautophagy involves formation of new membrane bilayers (autophagosomes) that engulf cytosolic material and deliver it to lysosomes. This review provides new insights on the crosstalks between different forms of autophagy and the significance of bilayer-forming phospholipid synthesis in autophagosomal membrane formation.

Integration of non-vesicular and vesicular transport processes at the Golgi complex by the PKD-CERT network

Non-vesicular transport of ceramide from endoplasmic reticulum to Golgi membranes is essential for cellular lipid homeostasis. Protein kinase D (PKD) is a serine-threonine kinase that controls vesicle fission at Golgi membranes. Here we highlight the intimate connections between non-vesicular and vesicular transport at the level of the Golgi complex, and suggest that PKD and its substrate CERT, the ceramide transfer protein, play central roles in coordinating these processes by fine-tuning the local membrane lipid composition to maintain Golgi secretory function. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

De novo synthesis of phospholipids is coupled with autophagosome formation

Autophagy, the process involved in the breakdown of intracellular proteins and organelles, has become an area of great importance in both cell survival and cell death. Despite the abundance of information on this topic, persisting issues remain about the origin and mechanism of formation of the autophagosomal membrane. The endoplasmic reticulum (ER) plays a critical role in the initiation of autophagy, especially in the formation of early lipid particles, termed the phagophores or the isolation membranes. The bulk, if not all of the lipid biosynthetic pathways cease at the level of the ER where the main synthesizing enzymes are resident proteins. We postulate that if the initial isolation membrane is formed from the locally synthesized lipids at the level of the ER, than an increase in the biosynthesis of the bilayer-forming phospholipids (phosphatidylcholine-PC, phosphatidylethanolamine-PE, and phosphatidylserine-PS) would occur simultaneously with induction of autophagy. As part of the isolation membranes, PE conjugates the cytosolic microtubule-associated protein 1 light chain 3 (LC3-I), to form LC3-II, the selective autophagosomal protein. Phosphatidylinositol-3-kinase (PI3K) action on the ER phosphatidylinositol occurs with phospholipid biogenesis, and together they act to contribute to the elongation and assembly of the autophagosomal particle.

The targeting of plasmalemmal ceramide to mitochondria during apoptosis

Ceramide is a key lipid mediator of cellular processes such as differentiation, proliferation, growth arrest and apoptosis. During apoptosis, ceramide is produced within the plasma membrane. Although recent data suggest that the generation of intracellular ceramide increases mitochondrial permeability, the source of mitochondrial ceramide remains unknown. Here, we determine whether a stress-mediated plasmalemmal pool of ceramide might become available to the mitochondria of apoptotic cells. We have previously established annexin A1—a member of a family of Ca²⁺ and membrane-binding proteins—to be a marker of ceramide platforms. Using fluorescently tagged annexin A1, we show that, upon its generation within the plasma membrane, ceramide self-associates into platforms that subsequently invaginate and fuse with mitochondria. An accumulation of ceramide within the mitochondria of apoptotic cells was also confirmed using a ceramide-specific antibody. Electron microscopic tomography confirmed that upon the formation of ceramide platforms, the invaginated regions of the plasma membrane extend deep into the cytoplasm forming direct physical contacts with mitochondrial outer membranes. Ceramide might thus be directly transferred from the plasma membrane to the mitochondrial outer membrane. It is conceivable that this “kiss-of-death” increases the permeability of the mitochondrial outer membrane thereby triggering apoptosis.

Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

The endoplasmic reticulum (ER) is the biggest organelle in most cell types, but its characterization as an organelle with a continuous membrane belies the fact that the ER is actually an assembly of several, distinct membrane domains that execute diverse functions. Almost 20 years ago, an essay by Sitia and Meldolesi first listed what was known at the time about domain formation within the ER. In the time that has passed since, additional ER domains have been discovered and characterized. These include the mitochondria-associated membrane (MAM), the ER quality control compartment (ERQC), where ER-associated degradation (ERAD) occurs, and the plasma membrane-associated membrane (PAM). Insight has been gained into the separation of nuclear envelope proteins from the remainder of the ER. Research has also shown that the biogenesis of peroxisomes and lipid droplets occurs on specialized membranes of the ER. Several studies have shown the existence of specific marker proteins found on all these domains and how they are targeted there. Moreover, a first set of cytosolic ER-associated sorting proteins, including phosphofurin acidic cluster sorting protein 2 (PACS-2) and Rab32 have been identified. Intra-ER targeting mechanisms appear to be superimposed onto ER retention mechanisms and rely on transmembrane and cytosolic sequences. The crucial roles of ER domain formation for cell physiology are highlighted with the specific targeting of the tumor metastasis regulator gp78 to ERAD-mediating membranes or of the promyelocytic leukemia protein to the MAM.

Ca2+ dysfunction in neurodegenerative disorders: Alzheimer's disease

More than one century ago "a peculiar disorder of the cerebral cortex" was noticed in a middle-aged patient who had been affected by dementia in the last years of his life. The postmortem hallmarks of his brain were protein plaques, neurofibrillary tangles, and atherosclerotic changes: the neuropathologist who found these alterations and gave his name to the disease that underlied them was Alois Alzheimer (Alzheimer et al., Clin Anat 1995;8:429-431). Following its discovery, the disease has been studied with a vigor that went parallel to the increase of its social importance. The amount of information amassed in the literature is impressive, but knowledge on the mechanism underlying its onset and its progression is still very limited. Numerous hypotheses on the molecular pathogenesis of the Alzheimer's disease (AD) have been proposed and two have gradually gained wide consensus: (i) the amyloid cascade hypothesis, first proposed on the basis of the toxicity evoked by the deposition of amyloid β (Aβ) aggregates; (ii) the Ca(2+) hypothesis, which focuses on the correlation between the dysfunction of Ca(2+) homeostasis and the neurodegeneration process. This succinct review will discuss the essential aspects of the role of Ca(2+) homeostasis dysregulation in the onset and development of AD.

Making heads or tails of phospholipids in mitochondria

Mitochondria are dynamic organelles whose functional integrity requires a coordinated supply of proteins and phospholipids. Defined functions of specific phospholipids, like the mitochondrial signature lipid cardiolipin, are emerging in diverse processes, ranging from protein biogenesis and energy production to membrane fusion and apoptosis. The accumulation of phospholipids within mitochondria depends on interorganellar lipid transport between the endoplasmic reticulum (ER) and mitochondria as well as intramitochondrial lipid trafficking. The discovery of proteins that regulate mitochondrial membrane lipid composition and of a multiprotein complex tethering ER to mitochondrial membranes has unveiled novel mechanisms of mitochondrial membrane biogenesis.

A mitochondrial phosphatase required for cardiolipin biosynthesis: the PGP phosphatase Gep4

Cardiolipin (CL), a unique dimeric phosphoglycerolipid predominantly present in mitochondrial membranes, has pivotal functions for the cellular energy metabolism, mitochondrial dynamics and the initiation of apoptotic pathways. Perturbations in the mitochondrial CL metabolism cause cardiomyopathy in Barth syndrome. Here, we identify a novel phosphatase in the mitochondrial matrix space, Gep4, and demonstrate that it dephosphorylates phosphatidylglycerolphosphate to generate phosphatidylglycerol, an essential step during CL biosynthesis. Expression of a mitochondrially targeted variant of Escherichia coli phosphatase PgpA restores CL levels in Gep4-deficient cells, indicating functional conservation. A genetic epistasis analysis combined with the identification of intermediates of CL biosynthesis allowed us to integrate Gep4 in the CL-biosynthetic pathway and assign an essential function during early steps of CL synthesis to Tam41, which has previously been shown to be essential for the maintenance of normal CL levels. Our experiments provide the framework for the further dissection of mechanisms that are required for accumulation and maintenance of CL levels in mitochondria.

Scaling properties of cell and organelle size

How size is controlled is a fundamental question in biology. In this review, we discuss the use of scaling relationships-for example, power-laws of the form y∝x(α)-to provide a framework for comparison and interpretation of size measurements. Such analysis can illustrate the biological and physical principles underlying observed trends, as has been proposed for the allometric dependence of metabolic rate or limb structure on organism mass. Techniques for measuring size at smaller length-scales continue to improve, leading to more data on the control of size in cells and organelles. Size scaling of these structures is expected to influence growth patterns, functional capacity and intracellular transport. Furthermore, organelles such as the nucleus, mitochondria and endoplasmic reticulum show widely varying morphologies that affect their scaling properties. We provide brief summaries of these issues for individual organelles, and conclude with a discussion on how to apply this concept to better understand the mechanisms of size control in the cellular environment.

Intracellular trafficking of ceramide by ceramide transfer protein

The transport and sorting of lipids are fundamental to membrane biogenesis. In the synthesis of sphingolipids in mammalian cells, ceramide is newly produced at the endoplasmic reticulum (ER), and transported from the ER to the trans Golgi regions, where it is converted to sphingomyelin. Ceramide transfer protein (CERT) mediates the ER-to-Golgi trafficking of ceramide. It has been suggested that CERT extracts ceramide from the ER and carries it to the Golgi apparatus in a non-vesicular manner and that efficient CERT-mediated trafficking of ceramide occurs at membrane contact sites between the ER and the Golgi apparatus.

An ER-mitochondria tethering complex revealed by a synthetic biology screen

Communication between organelles is an important feature of all eukaryotic cells. To uncover components involved in mitochondria/endoplasmic reticulum (ER) junctions, we screened for mutants that could be complemented by a synthetic protein designed to artificially tether the two organelles. We identified the Mmm1/Mdm10/Mdm12/Mdm34 complex as a molecular tether between ER and mitochondria. The tethering complex was composed of proteins resident of both ER and mitochondria. With the use of genome-wide mapping of genetic interactions, we showed that the components of the tethering complex were functionally connected to phospholipid biosynthesis and calcium-signaling genes. In mutant cells, phospholipid biosynthesis was impaired. The tethering complex localized to discrete foci, suggesting that discrete sites of close apposition between ER and mitochondria facilitate interorganelle calcium and phospholipid exchange.

Mitochondrial degeneration and not apoptosis is the primary cause of embryonic lethality in ceramide transfer protein mutant mice

Ceramide transfer protein (CERT) functions in the transfer of ceramide from the endoplasmic reticulum (ER) to the Golgi. In this study, we show that CERT is an essential gene for mouse development and embryonic survival and, quite strikingly, is critical for mitochondrial integrity. CERT mutant embryos accumulate ceramide in the ER but also mislocalize ceramide to the mitochondria, compromising their function. Cells in mutant embryos show abnormal dilation of the ER and degenerating mitochondria. These subcellular changes manifest as heart defects and cause severely compromised cardiac function and embryonic death around embryonic day 11.5. In spite of ceramide accumulation, CERT mutant mice do not die as a result of enhanced apoptosis. Instead, cell proliferation is impaired, and expression levels of cell cycle–associated proteins are altered. Individual cells survive, perhaps because cell survival mechanisms are activated. Thus, global compromise of ER and mitochondrial integrity caused by ceramide accumulation in CERT mutant mice primarily affects organogenesis rather than causing cell death via apoptotic pathways.

CERT-mediated trafficking of ceramide

The transport and sorting of lipids from the sites of their synthesis to their appropriate destinations are fundamental for membrane biogenesis. In the synthesis of sphingolipids in mammalian cells, ceramide is newly produced at the endoplasmic reticulum (ER), and transported from the ER to the trans Golgi regions, where it is converted to sphingomyelin. CERT mediates the ER-to-Golgi trafficking of ceramide. CERT contains several functional domains and motifs including i) a START domain capable of catalyzing inter-membrane transfer of ceramide, ii) a pleckstrin homology domain, which serves to target the Golgi apparatus, iii) a FFAT motif which interacts with the ER-resident membrane protein VAP, and iv) a serine-repeat motif, of which hyperphosphorylation down-regulates CERT activity. It has been suggested that CERT extracts ceramide from the ER and carries it to the Golgi apparatus in a non-vesicular manner and that efficient CERT-mediated trafficking of ceramide occurs at membrane contact sites between the ER and the Golgi apparatus.

Lipid-transfer proteins in biosynthetic pathways

Compartmentalization is a defining feature of eukaryotic cells that allows the spatial segregation of different functions, such as protein and lipid synthesis, and ensures their fidelity and efficiency. This imposes the need for an intense flux of metabolic intermediates between segregated enzymatic activities, as seen for the sequential transport of neosynthesized proteins through the segments of the secretory pathway during their post-translational modification. For lipid synthesis, the identification of proteins that transfer lipids between membranes has revealed an additional mechanism for this intercompartment exchange. The intense interest elicited by the lipid-transfer proteins over the last few years has led to the definition of their central role in key processes, such as lipid metabolism, membrane trafficking, and signaling.

Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

Ceramide synthesis in the endoplasmic reticulum can permeabilize mitochondria to proapoptotic proteins

Increased mitochondrial ceramide levels are associated with the initiation of apoptosis. There is evidence that ceramide is causal. Thus, the conversion of the precursor, dihydroceramide, to ceramide by the enzyme dihydroceramide desaturase may be important in preparing the cell for apoptosis. Ceramide can initiate apoptosis by permeabilizing the mitochondrial outer membrane to apoptosis-inducing proteins. However, the mitochondrion's ability to produce ceramide may be limited by its proteome. Here, we show that ceramide synthesized in isolated mammalian endoplasmic reticulum (ER) vesicles from either C8-dihydroceramide or sphingosine to produce long-chain ceramide can transfer to isolated mitochondria. The rate of transfer is consistent with a simple collision model. The transfer of the long-chain ceramide is faster than expected for an uncatalyzed process. Sufficient ceramide is transferred to permeabilize the outer membrane to cytochrome c and adenylate kinase. The mitochondria-associated membranes, ER-like membranes that are tightly associated with isolated mitochondria, can produce enough ceramide to permeabilize the outer membrane transiently. Thus, this ceramide exchange obviates the need for a complete ceramide de novo pathway in mitochondria to increase ceramide levels to the critical value required for functional changes, such as ceramide channel self-assembly followed by protein release.

Metabolic and molecular aspects of ethanolamine phospholipid biosynthesis: the role of CTP:phosphoethanolamine cytidylyltransferase (Pcyt2)

The CDP-ethanolamine branch of the Kennedy pathway is the major route for the formation of ethanolamine-derived phospholipids, including diacyl phosphatidylethanolamine and alkenylacyl phosphatidylethanolamine derivatives, known as plasmalogens. Ethanolamine phospholipids are essential structural components of the cell membranes and play regulatory roles in cell division, cell signaling, activation, autophagy, and phagocytosis. The physiological importance of plasmalogens has not been not fully elucidated, although they are known for their antioxidant properties and deficiencies in a number of inherited peroxisomal disorders. This review highlights important aspects of ethanolamine phospholipid metabolism and reports current molecular information on 1 of the regulatory enzymes in their synthesis, CTP:phosphoethanolamine cytidylyltransferase (Pcyt2). Pcyt2 is encoded by a single, nonredundant gene in animal species that could be alternatively spliced into 2 potential protein products. We describe properties of the mouse and human Pcyt2 genes and their regulatory promoters and provide molecular evidence for the existence of 2 distinct Pcyt2 proteins. The goal is to obtain more insight into Pcyt2 catalytic function and regulation to facilitate a better understanding of the production of ethanolamine phospholipids via the CDP-ethanolamine branch of the Kennedy pathway.

Toll-like receptor 2 senses beta-cell death and contributes to the initiation of autoimmune diabetes

Although it is established that defective clearance and, hence, increased accumulation of apoptotic cells can lead to autoimmunity, the mechanism by which this occurs remains elusive. Here, we observed that apoptotic cells undergoing secondary necrosis but not intact apoptotic cells provoked substantial immune responses, which were mediated through the toll-like receptor 2 (TLR2) pathway. The development of autoimmune diabetes was markedly inhibited in Tlr2(-/-) mice but not in Tlr4(-/-) mice, showing that TLR2 plays an important role in the initiation of the disease. Apoptotic beta-cell injury could stimulate the priming of diabetogenic T cells through a TLR2-dependent, but TLR4-independent, activation of antigen-presenting cells. These findings suggest that beta-cell death and its sensing via TLR2 may be an initial event for the stimulation of antigen-presenting cells and development of autoimmune diabetes.

The CDP-ethanolamine pathway and phosphatidylserine decarboxylation generate different phosphatidylethanolamine molecular species

In mammalian cells, phosphatidylethanolamine (PtdEtn) is mainly synthesized via the CDP-ethanolamine (Kennedy) pathway and by decarboxylation of phosphatidylserine (PtdSer). However, the extent to which these two pathways contribute to overall PtdEtn synthesis both quantitatively and qualitatively is still not clear. To assess their contributions, PtdEtn species synthesized by the two routes were labeled with pathway-specific stable isotope precursors, d(3)-serine and d(4)-ethanolamine, and analyzed by high performance liquid chromatography-mass spectrometry. The major conclusions from this study are that (i) in both McA-RH7777 and Chinese hamster ovary K1 cells, the CDP-ethanolamine pathway was favored over PtdSer decarboxylation, and (ii) both pathways for PtdEtn synthesis are able to produce all diacyl-PtdEtn species, but most of these species were preferentially made by one pathway. For example, the CDP-ethanolamine pathway preferentially synthesized phospholipids with mono- or di-unsaturated fatty acids on the sn-2 position (e.g. (16:0-18:2)PtdEtn and (18:1-18:2)PtdEtn), whereas PtdSer decarboxylation generated species with mainly polyunsaturated fatty acids on the sn-2 position (e.g. (18:0-20:4)PtdEtn and (18:0-20:5)PtdEtn in McArdle and (18: 0-20:4)PtdEtn and (18:0-22:6)PtdEtn in Chinese hamster ovary K1 cells). (iii) The main PtdEtn species newly synthesized from the Kennedy pathway in the microsomal fraction appeared to equilibrate rapidly between the endoplasmic reticulum and mitochondria. (iv) Newly synthesized PtdEtn species preferably formed in the mitochondria, which is at least in part due to the substrate specificity of the phosphatidylserine decarboxylase, seemed to be retained in this organelle. Our data suggest a potentially essential role of the PtdSer decarboxylation pathway in mitochondrial functioning.

Membrane contact sites: physical attachment between chloroplasts and endoplasmic reticulum revealed by optical manipulation

Chloroplasts and their surrounding cell are highly interdependent. One example is lipid metabolism, where the cell depends on its chloroplasts to provide fatty acids for lipid synthesis in the endoplasmic reticulum (ER) and in turn, chloroplasts rely on import of lipid precursors from the ER. Despite its fundamental importance, the route for lipid trafficking into and out of chloroplasts remains unknown. Biochemical studies of plant membrane lipid metabolism have suggested the possibility of lipid transport at membrane contact sites (MCSs) between the ER and chloroplasts. With the aid of optical manipulation, we recently could present physical evidence for this association. Leaf protoplasts isolated from Arabidopsis thaliana expressing green fluorescent protein (GFP) in the ER lumen were observed by confocal microscopy. A laser scalpel was used to rupture the protoplasts. ER fragments associated with the released chloroplasts could be stretched out by optical tweezers but remained attached to the chloroplast surface, even when a stretching force of 400 pN was applied. We thus provided the first physical evidence for MCSs between two membranes and we propose for the ER-chloroplast pair, that such tight associations are involved in bidirectional lipid trafficking between the two compartments.

Optical manipulation reveals strong attracting forces at membrane contact sites between endoplasmic reticulum and chloroplasts

Eukaryote cells depend on membrane lipid trafficking from biogenic membranes, like the endoplasmic reticulum (ER), to other membranes in the cell. Two major routes for membrane lipid transport are recognized: vesicular trafficking and lipid transfer at zones of close contact between membranes. Specific ER regions involved in such membrane contact sites (MCSs) have been isolated, and lipid transfer at MCSs as well as protein-protein interactions between the partaking membranes have been demonstrated (reviewed by Holthuis, J. C. M., and Levine, T. P. (2005) Nat. Rev. 6, 209-220). Here we present the first demonstration of the physical association between membranes involved in MCSs: by using optical imaging and manipulation, strong attracting forces between ER and chloroplasts are revealed. We used Arabidopsis thaliana expressing green fluorescent protein in the ER lumen and observed leaf protoplasts by confocal microscopy. The ER network was evident, with ER branch end points apparently localized at chloroplast surfaces. After rupture of a protoplast using a laser scalpel, the cell content was released. ER fragments remained attached to the released chloroplasts and could be stretched out by optical tweezers. The applied force, 400 pN, could not drag a chloroplast free from its attached ER, which could reflect protein-protein interactions at the ER-chloroplast MCSs. As chloroplasts rely on import of ER-synthesized lipids, we propose that lipid transfer occurs at these MCSs. We suggest that lipid transfer at the MCSs also occurs in the opposite direction, for example to channel plastid-synthesized acyl groups to supply substrates for ER-localized synthesis of membrane and storage lipids.

Glycerolipid transfer for the building of membranes in plant cells

Membranes of plant organelles have specific glycerolipid compositions. Selective distribution of lipids at the levels of subcellular organelles, membrane leaflets and membrane domains reflects a complex and finely tuned lipid homeostasis. Glycerolipid neosynthesis occurs mainly in plastid envelope and endoplasmic reticulum membranes. Since most lipids are not only present in the membranes where they are synthesized, one cannot explain membrane specific lipid distribution by metabolic processes confined in each membrane compartment. In this review, we present our current understanding of glycerolipid trafficking in plant cells. We examine the potential mechanisms involved in lipid transport inside bilayers and from one membrane to another. We survey lipid transfers going through vesicular membrane flow and those dependent on lipid transfer proteins at membrane contact sites. By introducing recently described membrane lipid reorganization during phosphate deprivation and recent developments issued from mutant analyses, we detail the specific lipid transfers towards or outwards the chloroplast envelope.

Phospholipid biosynthesis in mammalian cells

Identification of the genes and gene products involved in the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine has lagged behind that in many other fields because of difficulties encountered in purifying the respective proteins. Nevertheless, most of these genes have now been identified. In this review article, we have highlighted important new findings on the individual enzymes and the corresponding genes of phosphatidylcholine synthesis via its two major biosynthetic pathways: the CDP-choline pathway and the methylation pathway. We also review recent studies on phosphatidylethanolamine biosynthesis by two pathways: the CDP-ethanolamine pathway, which is active in the endoplasmic reticulum, and the phosphatidylserine decarboxylase pathway, which operates in mitochondria. Finally, the two base-exchange enzymes, phosphatidylserine synthase-1 and phosphatidylserine synthase-2, that synthesize phosphatidylserine in mammalian cells are also discussed.

Contribution of different biosynthetic pathways to species selectivity of aminoglycerophospholipids assembled into mitochondrial membranes of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, three pathways lead to the formation of cellular phosphatidylethanolamine (PtdEtn), namely the mitochondrial conversion of phosphatidylserine (PtdSer) to PtdEtn catalyzed by phosphatidylserine decarboxylase 1 (Psd1p), the equivalent reaction catalyzed by phosphatidylserine decarboxylase 2 (Psd2p) in the Golgi, and the CDP-ethanolamine branch of the so-called Kennedy pathway which is located to the microsomal fraction. To investigate the contributions of these three pathways to the cellular pattern of PtdEtn species (fatty acid composition) we subjected lipids of wild-type and yeast mutant strains with distinct defects in the respective pathways to mass spectrometric analysis. We also analyzed species of PtdSer and phosphatidylcholine (PtdCho) of these strains because formation of the three aminoglycerophospholipids is linked through their biosynthetic route. We demonstrate that all three pathways involved in PtdEtn synthesis exhibit a preference for the formation of C34:2 and C32:2 species resulting in a high degree of unsaturation in total cellular PtdEtn. In PtdSer, the ratio of unsaturated to saturated fatty acids is much lower than in PtdEtn, suggesting a high species selectivity of PtdSer decarboxylases. Finally, PtdCho is characterized by its higher ratio of C16 to C18 fatty acids compared to PtdSer and PtdEtn. In contrast to biosynthetic steps, import of all three aminoglycerophospholipids into mitochondria of wild-type and mutant cells is not highly specific with respect to species transported. Thus, the species pattern of aminoglycerophospholipids in mitochondria is mainly the result of enzyme specificities, but not of translocation processes involved. Our results support a model that suggests equilibrium transport of aminoglycerophospholipids between mitochondria and microsomes based on membrane contact between the two compartments.

Characterization of a non-mitochondrial type I phosphatidylserine decarboxylase in Plasmodium falciparum

In search of key enzymes in Plasmodium phospholipid metabolism, we demonstrate the presence of a parasite-encoded phosphatidylserine decarboxylase (PSD) in the membrane fraction of Plasmodium falciparum-infected erythrocytes. PSD cDNA, encoding phosphatidylserine decarboxylase (PfPSD), was cloned by screening a directional cDNA library derived from the trophozoite erythrocytic stage. The corresponding PfPSD gene is located on chromosome 9 of P. falciparum, contains one intron of 938 nucleotides and is transcribed into a 3.7 kb mRNA. PfPSD cDNA encodes a putative protein of 362 amino acids, with a predicted molecular mass of 42.6 kDa, which clearly belongs to the type I PSD family. Only a 35 kDa polypeptide was detected in the parasite using a specific rabbit antiserum. PfPSD has a 314VGSS317 sequence near its carboxyl-terminus that is related to the Escherichia coli, yeast and human LGST motif, which is the site of proenzyme processing. PSD enzyme was expressed in E. coli with a KM of 63 +/- 19 microM and a VMAX of 680 +/- 49 nmol of phosphatidylethanolamine formed h-1 mg-1 protein. Site-directed mutagenesis of the VGSS active site demonstrated that the PfPSD proenzyme was processed into two non-identical subunits (alpha and beta) and revealed the crucial role played by each residue in enzyme processing and activity. Using indirect immunofluorescence, PfPSD labelling was co-localized with an endoplasmic reticulum marker, but not with a mitochondrial vital dye. This P. falciparum PSD is the first type I PSD identified in the endoplasmic reticulum compartment.