Role of LBPA and Alix in Multivesicular Liposome Formation and Endosome Organization

Comparison of exosome-mimicking liposomes with conventional liposomes for intracellular delivery of siRNA

Exosomes have been extensively explored as delivery vehicles due to low immunogenicity, efficient cargo delivery, and possibly intrinsic homing capacity. However, therapeutic application of exosomes is hampered by structural complexity and lack of efficient techniques for isolation and drug loading. Liposomes represent one of the most successful therapeutic nanocarriers, but are frequently criticized by short blood circulation and inefficient intracellular drug delivery. In this circumstance, a promising strategy is to facilitate a positive feedback between two fields. Herein, exosome-mimicking liposomes were formulated with DOPC/SM/Chol/DOPS/DOPE (21/17.5/30/14/17.5, mol/mol), and harnessed for delivery of VEGF siRNA to A549 and HUVEC cells. Compared with Lipo 2000 and DOTAP liposomes, exosome-mimicking liposomes exhibited less than four-fold cytotoxicity but higher storage stability and anti-serum aggregation effect. Exosome-mimicking liposomes appeared to enter A549 cells through membrane fusion, caveolae-mediated endocytosis, and macropinocytosis, while enter HUVEC through caveolae-mediated endocytosis, which revealed that the uptake pathway was dependent on cell types. Notably, exosome-mimicking liposomes exhibited significantly higher cellular uptake and silencing efficiency than PC-Chol liposomes (>three-fold), suggesting the unique lipid composition did enhance the intracellular delivery efficiency of exosome-mimicking liposomes to a significantly greater extent. However, it still remained far from satisfactory delivery as compared to cationic Lipo 2000 and DOTAP liposomes, which warranted further improvement in future research. This study may encourage further pursuit of more exosome-mimicking delivery vehicles with higher efficiency and biocompatibility.

Extracellular vesicles: lipids as key components of their biogenesis and functions

Intercellular communication has been known for decades to involve either direct contact between cells or to operate via circulating molecules, such as cytokines, growth factors, or lipid mediators. During the last decade, we have begun to appreciate the increasing importance of intercellular communication mediated by extracellular vesicles released by viable cells either from plasma membrane shedding (microvesicles, also named microparticles) or from an intracellular compartment (exosomes). Exosomes and microvesicles circulate in all biological fluids and can trigger biological responses at a distance. Their effects include a large variety of biological processes, such as immune surveillance, modification of tumor microenvironment, or regulation of inflammation. Extracellular vesicles can carry a large array of active molecules, including lipid mediators, such as eicosanoids, proteins, and nucleic acids, able to modify the phenotype of receiving cells. This review will highlight the role of the various lipidic pathways involved in the biogenesis and functions of microvesicles and exosomes.

Rab GTPase Function in Endosome and Lysosome Biogenesis

Eukaryotic cells maintain a highly organized endolysosomal system. This system regulates the protein and lipid content of the plasma membrane, it participates in the intracellular quality control machinery and is needed for the efficient removal of damaged organelles. This complex network comprises an endosomal membrane system that feeds into the lysosomes, yet also allows recycling of membrane proteins, and probably lipids. Moreover, lysosomal degradation provides the cell with macromolecules for further growth. In this review, we focus primarily on the role of the small Rab GTPases Rab5 and Rab7 as organelle markers and interactors of multiple effectors on endosomes and lysosomes and highlight their role in membrane dynamics, particularly fusion along the endolysosomal pathway.

The enigmatic endosome - sorting the ins and outs of endocytic trafficking

The early endosome (EE), also known as the sorting endosome (SE) is a crucial station for the sorting of cargoes, such as receptors and lipids, through the endocytic pathways. The term endosome relates to the receptacle-like nature of this organelle, to which endocytosed cargoes are funneled upon internalization from the plasma membrane. Having been delivered by the fusion of internalized vesicles with the EE or SE, cargo molecules are then sorted to a variety of endocytic pathways, including the endo-lysosomal pathway for degradation, direct or rapid recycling to the plasma membrane, and to a slower recycling pathway that involves a specialized form of endosome known as a recycling endosome (RE), often localized to the perinuclear endocytic recycling compartment (ERC). It is striking that 'the endosome', which plays such essential cellular roles, has managed to avoid a precise description, and its characteristics remain ambiguous and heterogeneous. Moreover, despite the rapid advances in scientific methodologies, including breakthroughs in light microscopy, overall, the endosome remains poorly defined. This Review will attempt to collate key characteristics of the different types of endosomes and provide a platform for discussion of this unique and fascinating collection of organelles. Moreover, under-developed, poorly understood and important open questions will be discussed.

Analysis of novel endosome-to-Golgi retrieval genes reveals a role for PLD3 in regulating endosomal protein sorting and amyloid precursor protein processing

The processing of amyloid precursor protein (APP) to the neurotoxic pro-aggregatory Aβ peptide is controlled by the mechanisms that govern the trafficking and localisation of APP. We hypothesised that genes involved in endosomal protein sorting could play an important role in regulating APP processing and, therefore, analysed ~ 40 novel endosome-to-Golgi retrieval genes previously identified in a genome-wide siRNA screen. We report that phospholipase D3 (PLD3), a type II membrane protein, functions in endosomal protein sorting and plays an important role in regulating APP processing. PLD3 co-localises with APP in endosomes and loss of PLD3 function results in reduced endosomal tubules, impaired trafficking of several membrane proteins and reduced association of sortilin-like 1 with APP.

Separation and identification of phospholipids by hydrophilic interaction liquid chromatography coupled to tandem high resolution mass spectrometry with focus on isomeric phosphatidylglycerol and bis(monoacylglycero)phosphate

Changes in lipid composition of cells or tissue are often linked to various diseases. Studies indicate alterations of bis(monoacylglycero)phosphate (BMP) species in diseases such as cancer. Therefore, an extended phospholipid profiling method based on hydrophilic interaction liquid chromatography (HILIC) coupled to high-resolution mass spectrometry (MS) and data-dependent MS/MS acquisition was developed to separate and unambiguously identify BMP species. Lipid species identification was based on retention time, accurate mass and specific MS/MS fragments. The developed method was applied in a proof of concept study to lipid extracts of a cell culture model of conditional oncogene overexpression in MCF-7/NeuT breast cancer cells. Comparison of control and oncogene-induced MCF-7/NeuT breast cancer cells showed changes in BMP species distribution. Thereby, a shift from long-chain to shorter-chain fatty acid composition in BMP species was detected.

Delta opioid receptors recycle to the membrane after sorting to the degradation path

Soon after internalization delta opioid receptors (DOPrs) are committed to the degradation path by G protein-coupled receptor (GPCR)-associated binding protein. Here we provide evidence that this classical post-endocytic itinerary may be rectified by downstream sorting decisions which allow DOPrs to regain to the membrane after having reached late endosomes (LE). The LE sorting mechanism involved ESCRT accessory protein Alix and the TIP47/Rab9 retrieval complex which supported translocation of the receptor to the TGN, from where it subsequently regained the cell membrane. Preventing DOPrs from completing this itinerary precipitated acute analgesic tolerance to the agonist DPDPE, supporting the relevance of this recycling path in maintaining the analgesic response by this receptor. Taken together, these findings reveal a post-endocytic itinerary where GPCRs that have been sorted for degradation can still recycle to the membrane.

Phospholipase D and phosphatidic acid in the biogenesis and cargo loading of extracellular vesicles

Extracellular vesicles released by viable cells (exosomes and microvesicles) have emerged as important organelles supporting cell-cell communication. Because of their potential therapeutic significance, important efforts are being made toward characterizing the contents of these vesicles and the mechanisms that govern their biogenesis. It has been recently demonstrated that the lipid modifying enzyme, phospholipase D (PLD)2, is involved in exosome production and acts downstream of the small GTPase, ARF6. This review aims to recapitulate our current knowledge of the role of PLD2 and its product, phosphatidic acid, in the biogenesis of exosomes and to propose hypotheses for further investigation of a possible central role of these molecules in the biology of these organelles.

Annexins-Coordinators of Cholesterol Homeostasis in Endocytic Pathways

The spatiotemporal regulation of calcium (Ca2+) storage in late endosomes (LE) and lysosomes (Lys) is increasingly recognized to influence a variety of membrane trafficking events, including endocytosis, exocytosis, and autophagy. Alterations in Ca2+ homeostasis within the LE/Lys compartment are implicated in human diseases, ranging from lysosomal storage diseases (LSDs) to neurodegeneration and cancer, and they correlate with changes in the membrane binding behaviour of Ca2+-binding proteins. This also includes Annexins (AnxA), which is a family of Ca2+-binding proteins participating in membrane traffic and tethering, microdomain organization, cytoskeleton interactions, Ca2+ signalling, and LE/Lys positioning. Although our knowledge regarding the way Annexins contribute to LE/Lys functions is still incomplete, recruitment of Annexins to LE/Lys is greatly influenced by the availability of Annexin bindings sites, including acidic phospholipids, such as phosphatidylserine (PS) and phosphatidic acid (PA), cholesterol, and phosphatidylinositol (4,5)-bisphosphate (PIP2). Moreover, the cytosolic portion of LE/Lys membrane proteins may also, directly or indirectly, determine the recruitment of Annexins to LE. Strikingly, within LE/Lys, AnxA1, A2, A6, and A8 differentially contribute to cholesterol transport along the endocytic route, in particular, cholesterol transfer between LE and other compartments, positioning Annexins at the centre of major pathways mediating cellular cholesterol homeostasis. Underlying mechanisms include the formation of membrane contact sites (MCS) and intraluminal vesicles (ILV), as well as the modulation of LE-cholesterol transporter activity. In this review, we will summarize the current understanding how Annexins contribute to influence LE/Lys membrane transport and associated functions.

Intracellular Trafficking and Endosomal Release of Oligonucleotides: What We Know and What We Don't

Understanding the cellular uptake and intracellular trafficking of oligonucleotides provides an important basic underpinning for the developing field of oligonucleotide-based therapeutics. Whether delivered as "free" oligonucleotides, as ligand-oligonucleotide conjugates, or in association with various nanocarriers, all forms of oligonucleotide enter cells by endocytosis and are initially ensconced within membrane-limited vesicles. Accordingly, the locus and extent of release to the cytosol and nucleus are key determinants of the pharmacological actions of oligonucleotides. A number of recent studies have explored the intracellular trafficking of various forms of oligonucleotides and their release from endomembrane compartments. These studies reveal a surprising convergence on an early-intermediate compartment in the trafficking pathway as the key locus of release for oligonucleotides administered in "free" form as well as those delivered with lipid complexes. Thus, oligonucleotide release from multivesicular bodies or from late endosomes seems to be the crucial endogenous process for attaining pharmacological effects. This intrinsic process of oligonucleotide release may be amplified by delivery agents such as lipid complexes or small molecule enhancers.

Role of Exosomes in Human Retroviral Mediated Disorders

Retroviruses comprise an ancient and varied group of viruses with the unique ability to integrate DNA from an RNA transcript into the genome, a subset of which are able to integrate in humans. The timing of these integrations during human history has dictated whether these viruses have remained exogenous and given rise to various human diseases or have become inseparable from the host genome (endogenous retroviruses). Given the ability of retroviruses to integrate into the host and subsequently co-opt host cellular process for viral propagation, retroviruses have been shown to be closely associated with several cellular processes including exosome formation. Exosomes are 30-150 nm unilamellar extracellular vesicles that originate from intraluminal vesicles (ILVs) that form in the endosomal compartment. Exosomes have been shown to be important in intercellular communication and immune cell function. Almost every cell type studied has been shown to produce these types of vesicles, with the cell type dictating the contents, which include proteins, mRNA, and miRNAs. Importantly, recent evidence has shown that infection by viruses, including retroviruses, alter the contents and subsequent function of produced exosomes. In this review, we will discuss the important retroviruses associated with human health and disease. Furthermore, we will delve into the impact of exosome formation and manipulation by integrated retroviruses on human health, survival, and human retroviral disease pathogenesis.

Role of Extracellular Vesicles in Viral and Bacterial Infections: Pathogenesis, Diagnostics, and Therapeutics

Extracellular vesicles (EVs), or exosomes, are nanovesicles of endocytic origin that carry host and pathogen-derived protein, nucleic acid, and lipid cargos. They are secreted by most cell types and play important roles in normal cell-to-cell communications but can also spread pathogen- and host-derived molecules during infections to alter immune responses and pathophysiological processes. New research is beginning to decipher how EVs influence viral and bacterial pathogenesis. In this review, we will describe how EVs influence viral and bacterial pathogenesis by spreading pathogen-derived factors and how they can promote and inhibit the immune response to these pathogens. We will also discuss the emerging potential of EVs as diagnostic and therapeutic tools.

Role of lipid transfer proteins in loading CD1 antigen-presenting molecules

Research to connect lipids with immunology is growing, but details about the specific roles of lipid transfer proteins (LTPs) in antigen presentation remain unclear. A single class of major histocompatibility class-like molecules, called CD1 molecules, can present lipids and glycolipids to the immune system. These molecules all have a common hydrophobic antigen-binding groove. The loading of this groove with various lipids throughout the life of a CD1 molecule defines the immune recognition of lipids by T cells. At each location of residence, CD1 molecules are exposed to particular physicochemical conditions, particular collections of lipids, and unique combinations of LTPs that will define which lipids bind to CD1 and which do not. The lipid transfer machinery that is used by CD1 molecules is entirely hijacked from the normal synthetic and catalytic pathways of lipids. The precise determinants that regulate the presentation of certain lipids over others with respect to chemistry, solubility, and abundance are still poorly defined and require investigation to allow the use of lipids as regular antigenic targets of immunotherapy and vaccine.

The Multirole of Liposomes in Therapy and Prevention of Infectious Diseases

Liposomes are closed bilayer structures spontaneously formed by hydrated phospholipids that are widely used as efficient delivery systems for drugs or antigens, due to their capability to encapsulate bioactive hydrophilic, amphipathic, and lipophilic molecules into inner water phase or within lipid leaflets. The efficacy of liposomes as drug or antigen carriers has been improved in the last years to ameliorate pharmacokinetics and capacity to release their cargo in selected target organs or cells. Moreover, different formulations and variations in liposome composition have been often proposed to include immunostimulatory molecules, ligands for specific receptors, or stimuli responsive compounds. Intriguingly, independent research has unveiled the capacity of several phospholipids to play critical roles as intracellular messengers in modulating both innate and adaptive immune responses through various mechanisms, including (i) activation of different antimicrobial enzymatic pathways, (ii) driving the fusion–fission events between endosomes with direct consequences to phagosome maturation and/or to antigen presentation pathway, and (iii) modulation of the inflammatory response. These features can be exploited by including selected bioactive phospholipids in the bilayer scaffold of liposomes. This would represent an important step forward since drug or antigen carrying liposomes could be engineered to simultaneously activate different signal transduction pathways and target specific cells or tissues to induce antigen-specific T and/or B cell response. This lipid-based host-directed strategy can provide a focused antimicrobial innate and adaptive immune response against specific pathogens and offer a novel prophylactic or therapeutic option against chronic, recurrent, or drug-resistant infections.

ESCRT-dependent cargo sorting at multivesicular endosomes

The endosomal sorting complex required for transport (ESCRT) machinery is composed of five multi-subunit protein complexes, which act cooperatively at specialized endosomes to facilitate the movement of specific cargoes from the limiting membrane into vesicles that bud into the endosome lumen. Over the past decade, numerous proteins, lipids, and RNAs have been shown to be incorporated into intralumenal vesicles (ILVs), but the mechanisms by which these unique cargoes are captured are only now becoming better understood. Here, we discuss the potential roles that the ESCRT machinery plays during cargo sorting at multivesicular endosomes (MVEs).

Amyloid precursor protein products concentrate in a subset of exosomes specifically endocytosed by neurons

Amyloid beta peptide (Aβ), the main component of senile plaques of Alzheimer's disease brains, is produced by sequential cleavage of amyloid precursor protein (APP) and of its C-terminal fragments (CTFs). An unanswered question is how amyloidogenic peptides spread throughout the brain during the course of the disease. Here, we show that small lipid vesicles called exosomes, secreted in the extracellular milieu by cortical neurons, carry endogenous APP and are strikingly enriched in CTF-α and the newly characterized CTF-η. Exosomes from N2a cells expressing human APP with the autosomal dominant Swedish mutation contain Aβ peptides as well as CTF-α and CTF-η, while those from cells expressing the non-mutated form of APP only contain CTF-α and CTF-η. APP and CTFs are sorted into a subset of exosomes which lack the tetraspanin CD63 and specifically bind to dendrites of neurons, unlike exosomes carrying CD63 which bind to both neurons and glial cells. Thus, neuroblastoma cells secrete distinct populations of exosomes carrying different cargoes and targeting specific cell types. APP-carrying exosomes can be endocytosed by receiving cells, allowing the processing of APP acquired by exosomes to give rise to the APP intracellular domain (AICD). Thus, our results show for the first time that neuronal exosomes may indeed act as vehicles for the intercellular transport of APP and its catabolites.

Biological relevance of fatty acyl heterogeneity to the neural membrane dynamics of rhesus macaques during normative aging

Lipidomic analyses of the frontal cortex of Rhesus macaques across three selected age groups (young, sexually-mature, old) revealed that docosahexaenoic acids (DHAs) displayed notable and unique accretions in sexually-mature macaques for all phospholipid classes examined, which were not observable in all remaining polyunsaturated fatty acids (PUFAs) investigated. On the other hand, arachidonic acid (ARA) exhibited sharp attritions in the membrane lipidomes of sexually-mature macaques, a decline which was attenuated only for cardiolipins (CLs). DHA enrichment in phospholipids was lost in old macaques, with accompanying augmentations in very-long-chain sphingomyelins (VLC-SMs). Age-dependent alterations in membrane lipidomes point to a possibly complex temporal interplay between DHA-enriched membrane microdomains and SM-/cholesterol-rich rafts in neural membranes during normative aging. Lipid co-regulation data revealed an increasingly intense degree of co-regulation between membrane lipid classes with age, and suggest that reduction in CLs during normative brain aging may prompt alternative membrane lipid synthetic pathways driven by a compromised energy availability in the aging brain.

Untargeted lipidomic analysis of primary human epidermal melanocytes acutely and chronically exposed to UV radiation

Ultraviolet (UV) radiation present in sunlight has been related to harmful effects on skin such as premature aging and skin cancer.

Post-translational add-ons mark the path in exosomal protein sorting

Extracellular vesicles (EVs) are released by cells to the extracellular environment to mediate inter-cellular communication. Proteins, lipids, nucleic acids and metabolites shuttled in these vesicles modulate specific functions in recipient cells. The enrichment of selected sets of proteins in EVs compared with global cellular levels suggests the existence of specific sorting mechanisms to specify EV loading. Diverse post-translational modifications (PTMs) of proteins participate in the loading of specific elements into EVs. In this review, we offer a perspective on PTMs found in EVs and discuss the specific role of some PTMs, specifically Ubiquitin and Ubiquitin-like modifiers, in exosomal sorting of protein components. The understanding of these mechanisms will provide new strategies for biomedical applications. Examples include the presence of defined PTM marks on EVs as novel biomarkers for the diagnosis and prognosis of certain diseases, or the specific import of immunogenic components into EVs for vaccine generation.

Role of extracellular vesicles in glioma progression

The role of extracellular vesicles in cancer biology has emerged as a focus of the study of great importance and has been shown to directly influence tumour development in several cancers including brain tumours, such as gliomas. Gliomas are the most aggressive brain tumours, and in the last time, a considerable effort has been made to understand their biology. Studies focus in the signalling pathways involved in the processes of angiogenesis, viability, drug resistance and immune response evasion, as well as gliomas ability to infiltrate healthy tissue, a phenomenon regulated by the migratory and invasive capacity of the cells within a tumour. In this review, we summarize the different types and classifications of extracellular vesicles, their intravesicular content, and their role in the regulation of tumour progression processes in glioma.

Endosomal Redox Signaling in the Antiphospholipid Syndrome

PURPOSE OF REVIEW

It is well established that the antiphospholipid syndrome (APS) is caused by antiphospholipid antibodies (aPL). While several underlying mechanisms have been described in the past, many open questions remain. Here, we will review data on endosomal signaling and, in particular, redox signaling in APS.

RECENT FINDINGS

Endosomal redox signaling has been implicated in several cellular processes including signaling of proinflammatory cytokines. We have shown that certain aPL can activate endosomal NADPH-oxidase (NOX) in several cell types followed by induction of proinflammatory and procoagulant cellular responses in vitro. Involvement of endosomes in aPL signaling has also been reported by others. In wild-type mice but not in NOX-deficient mice, aPL accelerate venous thrombus formation underscoring the relevance of endosomal NOX. Furthermore, hydroxychloroquine (HCQ) inhibits activation of endosomal NOX and prevents thrombus formation in aPL-treated mice. Endosomal redox signaling is an important novel mechanism involved in APS pathogenesis. This makes endosomes a potential target for future treatment approaches of APS.

Role of exosomes in hepatocellular carcinoma cell mobility alteration

Exosomes have gained increased research focus due to their key roles as messengers. The components of exosomes include proteins and RNAs that may be horizontally transferred between adjacent or distant cells. Hepatocellular carcinoma (HCC) is among the most malignant types of cancer worldwide, with exosomes implicated to play a crucial role in its regulation; however, the possible function of exosomes in modulating the motile ability of tumor cells and key molecules in HCC remain largely unknown. To investigate the regulatory effect of exosomes on the motile ability of HCC cells, exosomes from the culture medium of different HCC origins (high metastatic MHCC97-H and low metastatic MHCC97-L cells) were isolated for in vitro migration and invasion assays. The results indicated that the motile ability of MHCC97-L cells was significantly increased by pretreatment with MHCC97-H-derived exosomes when compared with MHCC97-L-exosome pretreatment (P<0.05). To further characterize the function of exosomes at the molecular level, protein profiling of exosomes from different cell origins was performed, which identified 129 proteins. Among these, adenylyl cyclase-associated protein 1, a protein implicated in HCC metastasis, was significantly enriched in exosomes from cells with high motile ability (P<0.05). The results of the present study validated the regulatory effect of exosomes on the motile ability of HCC cells. Furthermore, systematic analysis of the protein profiles of exosomes from different origins identified potential factors correlated with HCC metastasis, which may provide a basis for future functional analysis of exosomes regarding their involvement in cancer metastasis and recurrence.

Structural templating of J-aggregates: Visualizing bis(monoacylglycero)phosphate domains in live cells

Identifying the key structural and dynamical determinants that drive the association of biomolecules, whether in solution, or perhaps more importantly in a membrane environment, has critical implications for our understanding of cellular dynamics, processes, and signaling. With recent advances in high-resolution imaging techniques, from the development of new molecular labels to technical advances in imaging methodologies and platforms, researchers are now reaping the benefits of being able to directly characterize and quantify local dynamics, structures, and conformations in live cells and tissues. These capabilities are providing unique insights into association stoichiometries, interactions, and structures on sub-micron length scales. We previously examined the role of lipid headgroup chemistry and phase state in guiding the formation of pseudoisocyanine (PIC) dye J-aggregates on supported planar bilayers [Langmuir, 25, 10719]. We describe here how these same J-aggregates can report on the in situ formation of organellar membrane domains in live cells. Live cell hyperspectral confocal microscopy using GFP-conjugated GTPase markers of early (Rab5) and late (Rab7) endosomes revealed that the PIC J-aggregates were confined to domains on either the limiting membrane or intralumenal vesicles (ILV) of late endosomes, known to be enriched in the anionic lipid bis(monoacylglycero)phosphate (BMP). Correlated confocal fluorescence - atomic force microscopy performed on endosomal membrane-mimetic supported planar lipid bilayers confirmed BMP-specific templating of the PIC J-aggregates. These data provide strong evidence for the formation of BMP-rich lipid domains during multivesicular body formation and portend the application of structured dye aggregates as markers of cellular membrane domain structure, size, and formation.

Biogenesis and function of ESCRT-dependent extracellular vesicles

From bacteria to humans, cells secrete a large variety of membrane-bound extracellular vesicles. Only relatively recently has it however started to become clear that the exovesicular transport of proteins and RNAs is important for normal physiology and numerous pathological conditions. Extracellular vesicles can be formed through the release of the intralumenal vesicles of multivesicular endosomes as so-called exosomes, or through direct, ectosomal, budding from the cell surface. Through their ability to promote the bending of membranes away from the cytoplasm, the components of the Endosomal Sorting Complex Required for Transport (ESCRT) have been implicated in both exo- and ectosomal biogenesis. Studies of the ESCRT machinery may therefore provide important insights into the formation and function of extracellular vesicles. In the present review, we first describe the cell biological mechanisms through which ESCRT components contribute to the biogenesis of different types of extracellular vesicles. We then discuss how recent functional studies have started to uncover important roles of ESCRT-dependent extracellular vesicles in a wide variety of processes, including the transport of developmental signaling molecules and embryonic morphogenesis, the regulation of social behavior and host-pathogen interactions, as well as the etiology and progression of neurodegenerative pathologies and cancer.

The processing and presentation of lipids and glycolipids to the immune system

The recognition of CD1-lipid complexes by T cells was discovered 20 years ago and has since been an emerging and expanding field of investigation. Unlike protein antigens, which are presented on MHC class I and II molecules, lipids can only be presented by CD1 molecules, a unique family of MHC-like proteins whose singularity is a hydrophobic antigen-binding groove. The processing and loading of lipid antigens inside this groove of CD1 molecules require localization to endosomal and lysosomal subcellular compartments and their acidic pHs. This particular environment provides the necessary glycolytic enzymes and lipases that process lipid and glycolipid antigens, as well as a set of lipid transfer proteins that load the final version of the antigen inside the groove of CD1. The overall sequence of events needed for efficient presentation of lipid antigens is now understood and presented in this review. However, a large number of important details have been elusive. This elusiveness is linked to the inherent technical difficulties of studying lipids and the lipid-protein interface in vitro and in vivo. Here, we will expose some of those limitations and describe new approaches to address them during the characterization of lipids and glycolipids antigen presentation.

The life cycle of phagosomes: formation, maturation, and resolution

Phagocytosis, the regulated uptake of large particles (>0.5 μm in diameter), is essential for tissue homeostasis and is also an early, critical component of the innate immune response. Phagocytosis can be conceptually divided into three stages: phagosome, formation, maturation, and resolution. Each of these involves multiple reactions that require exquisite spatial and temporal orchestration. The molecular events underlying these stages are being unraveled and the current state of knowledge is briefly summarized in this article.

Caspase-8 controls the secretion of inflammatory lysyl-tRNA synthetase in exosomes from cancer cells

Aminoacyl-tRNA synthetases (ARSs), enzymes that normally control protein synthesis, can be secreted and have different activities in the extracellular space, but the mechanism of their secretion is not understood. This study describes the secretion route of the ARS lysyl-tRNA synthetase (KRS) and how this process is regulated by caspase activity, which has been implicated in the unconventional secretion of other proteins. We show that KRS is secreted from colorectal carcinoma cells within the lumen of exosomes that can trigger an inflammatory response. Caspase-8 cleaved the N-terminal of KRS, thus exposing a PDZ-binding motif located in the C terminus of KRS. Syntenin bound to the exposed PDZ-binding motif of KRS and facilitated the exosomic secretion of KRS dissociated from the multi-tRNA synthetase complex. KRS-containing exosomes released by cancer cells induced macrophage migration, and their secretion of TNF-α and cleaved KRS made a significant contribution to these activities, which suggests a novel mechanism by which caspase-8 may promote inflammation.

The HDAC6 Inhibitor Tubacin Induces Release of CD133+ Extracellular Vesicles From Cancer Cells

Tumor-derived extracellular vesicles (EVs) are emerging as an important mode of intercellular communication, capable of transferring biologically active molecules that facilitate the malignant growth and metastatic process. CD133 (Prominin-1), a stem cell marker implicated in tumor initiation, differentiation and resistance to anti-cancer therapy, is reportedly associated with EVs in various types of cancer. However, little is known about the factors that regulate the release of these CD133⁺ EVs. Here, we report that the HDAC6 inhibitor tubacin promoted the extracellular release of CD133⁺ EVs from human FEMX-I metastatic melanoma and Caco-2 colorectal carcinoma cells, with a concomitant downregulation of intracellular CD133. This effect was specific for tubacin, as inhibition of HDAC6 deacetylase activity by another selective HDAC6 inhibitor, ACY-1215 or the pan-HDAC inhibitor trichostatin A (TSA), and knockdown of HDAC6 did not enhance the release of CD133⁺ EVs. The tubacin-induced EV release was associated with changes in cellular lipid composition, loss of clonogenic capacity and decrease in the ability to form multicellular aggregates. These findings indicate a novel potential anti-tumor mechanism for tubacin in CD133-expressing malignancies. J. Cell. Biochem. 118: 4414-4424, 2017. © 2017 Wiley Periodicals, Inc.

Exosomes: From Garbage Bins to Promising Therapeutic Targets

Intercellular communication via cell-released vesicles is a very important process for both normal and tumor cells. Cell communication may involve exosomes, small vesicles of endocytic origin that are released by all types of cells and are found in abundance in body fluids, including blood, saliva, urine, and breast milk. Exosomes have been shown to carry lipids, proteins, mRNAs, non-coding RNAs, and even DNA out of cells. They are more than simply molecular garbage bins, however, in that the molecules they carry can be taken up by other cells. Thus, exosomes transfer biological information to neighboring cells and through this cell-to-cell communication are involved not only in physiological functions such as cell-to-cell communication, but also in the pathogenesis of some diseases, including tumors and neurodegenerative conditions. Our increasing understanding of why cells release exosomes and their role in intercellular communication has revealed the very complex and sophisticated contribution of exosomes to health and disease. The aim of this review is to reveal the emerging roles of exosomes in normal and pathological conditions and describe the controversial biological role of exosomes, as it is now understood, in carcinogenesis. We also summarize what is known about exosome biogenesis, composition, functions, and pathways and discuss the potential clinical applications of exosomes, especially as biomarkers and novel therapeutic agents.

The E3 ubiquitin ligase NEDD4 is an LC3-interactive protein and regulates autophagy

The MAP1LC3/LC3 family plays an essential role in autophagosomal biogenesis and transport. In this report, we show that the HECT family E3 ubiquitin ligase NEDD4 interacts with LC3 and is involved in autophagosomal biogenesis. NEDD4 binds to LC3 through a conserved WXXL LC3-binding motif in a region between the C2 and the WW2 domains. Knockdown of NEDD4 impaired starvation- or rapamycin-induced activation of autophagy and autophagosomal biogenesis and caused aggregates of the LC3 puncta colocalized with endoplasmic reticulum membrane markers. Electron microscopy observed gigantic deformed mitochondria in NEDD4 knockdown cells, suggesting that NEDD4 might function in mitophagy. Furthermore, SQSTM1 is ubiquitinated by NEDD4 while LC3 functions as an activator of NEDD4 ligase activity. Taken together, our studies define an important role of NEDD4 in regulation of autophagy.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

The Multifaceted Functions of Exosomes in Health and Disease: An Overview

Exosomes are extracellular vesicles of 50-150 nm in diameter secreted by basically all cell types. They mediate micro-communication among cells, tissues, and organs under both healthy and disease conditions by virtue of their ability to deliver macromolecules to target cells. Research on exosomes is a rapidly growing field, however many aspects of their biogenesis and functions still await a complete clarification. In our review we summarize most recent findings regarding biogenesis, structure, and functions of exosomes. In addition, an overview regarding the role of exosomes in both infectious and non-infectious diseases is provided. Finally, the use of exosomes as biomarkers and delivery tools for therapeutic molecules is addressed. Considering the body of literature data, exosomes have to be considered key components of the intercellular communication in both health and disease.

Bis(monoacylglycero)phosphate inhibits TLR4-dependent RANTES production in macrophages

Toll-like receptor 4 (TLR4) is the receptor for bacterial lipopolysaccharide (LPS) triggering production of pro-inflammatory cytokines which help eradicate the bacteria but could also be harmful when overproduced. The signaling activity of TLR4 is modulated by cholesterol level in cellular membranes, which in turn is affected by bis(monoacylglycero)phosphate (BMP), a phospholipid enriched in late endosomes. We found that exogenously added BMP isomers become incorporated into the plasma membrane and intracellular vesicles of macrophages and strongly reduced LPS-stimulated production of a chemokine RANTES, which was correlated with inhibition of interferon regulatory factor 3 (IRF3) controlling Rantes expression. To investigate the mechanism underlying the influence of BMP on TLR4 signaling we applied Laurdan and studied the impact of BMP incorporation on lipid packing, a measure for membrane order. Enrichment of model and cellular membranes with BMP significantly reduced their order and the reduction was maintained during stimulation of cells with LPS. This effect of BMP was abolished by enrichment of macrophages with cholesterol. In parallel, the inhibitory effect of BMP exerted on the TLR4-dependent phosphorylation of IRF3 was also reversed. Taken together our results indicate that BMP reduces the order of macrophage membranes which contributes to the inhibition of TLR4-dependent RANTES production.

Scramblase TMEM16F terminates T cell receptor signaling to restrict T cell exhaustion

The scramblase TMEM16F is indispensable for the formation of multivesicular bodies in late endosomes that facilitate TCR degradation and signal termination. Hyperactivation of TMEM16F-deficient T cells in chronic LCMV infection leads to severe T cell exhaustion and uncontrolled virus burden.

In chronic infection, T cells become hyporesponsive to antigenic stimulation to prevent immunopathology. Here, we show that TMEM16F is required to curb excessive T cell responses in chronic infection with virus. TMEM16F-deficient T cells are hyperactivated during the early phase of infection, exhibiting increased proliferation and cytokine production. Interestingly, this overactivation ultimately leads to severe T cell exhaustion and the inability of the host to control viral burden. Mechanistically, we identify TMEM16F as the dominant lipid scramblase in T lymphocytes that transports phospholipids across membranes. TMEM16F is located in late endosomes, where it facilitates the generation of multivesicular bodies for TCR degradation and signal termination. Consequently, TMEM16F deficiency results in sustained signaling and augmented T cell activation. Our results demonstrate that scramblase restricts TCR responses to avoid overactivation, ensuring a well-balanced immune response in chronic infectious disease.

Emerging roles of exosomes in cancer invasion and metastasis

Recent evidence has indicated that nano-sized vesicles called “exosomes” mediate the interaction between cancer cells and their microenvironment and play a critical role in the development of cancers. Exosomes contain cargo consisting of proteins, lipids, mRNAs, and microRNAs that can be delivered to different types of cells in nascent as well as distant locations. Cancer cell-derived exosomes (CCEs) have been identified in body fluids such as urine, plasma, and saliva from patients with cancer. Although their content depends on tumor type and stage, CCEs merit consideration as prognostic and diagnostic markers, as vehicles for drug delivery, and as potential therapeutic targets because they could transport various oncogenic elements. In this review, we summarize recent advances regarding the role of CCEs in cancer invasion and metastasis, as well as its potential clinical applications. [BMB Reports 2016; 49(1): 18-25]

Tetraspan cargo adaptors usher GPI-anchored proteins into multivesicular bodies

Ubiquitinated membrane proteins are sorted into intralumenal endosomal vesicles on their way for degradation in lysosomes. Here we summarize the discovery of the Cos proteins, which work to organize and segregate ubiquitinated cargo prior to its incorporation into intralumenal vesicles of the multivesicular body (MVB). Importantly, cargoes such as GPI-anchored proteins (GPI-APs) that cannot undergo ubiquitination, rely entirely on Cos proteins for sorting into intralumenal vesicles using the same pathway that depends on ESCRTs and ubiquitin ligases that typical polytopic membrane proteins do. Here we show Cos proteins provide functions as not only adaptor proteins for ubiquitin ligases, but also as cargo carriers that can physically usher a variety of other proteins into the MVB pathway. We then discuss the significance of this new sorting model and the broader implications for this cargo adaptor mechanism, whereby yeast Cos proteins, and their likely animal analogs, provide a ubiquitin sorting signal in trans to enable sorting of a membrane protein network into intralumenal vesicles.

ESCRTs and associated proteins in lysosomal fusion with endosomes and autophagosomes

Endolysosomal and autophagosomal degradation pathways are highly connected at various levels, sharing multiple molecular effectors that modulate them individually or simultaneously. These two lysosomal degradative pathways are primarily involved in the disposal of cargo internalized from the cell surface or long-lived proteins or aggregates and aged organelles present in the cytosol. Both of these pathways involve a number of carefully regulated vesicular fusion events that are dependent on ESCRT proteins. The ESCRT proteins especially ESCRT-I and III participate in the regulation of fusion events between autophagosome/amphisome and lysosome. Along with these, a number of functionally diverse ESCRT associated and regulatory proteins such as, endosomal PtdIns (3) P 5-kinase Fab1, ALIX, mahogunin ring finger 1, atrogin 1, syntaxin 17, ATG12-ATG3 complex, and protein kinase CK2α are involved in fusion events in either or both the lysosomal degradative pathways.

Mast Cell Degranulation Is Accompanied by the Release of a Selective Subset of Extracellular Vesicles That Contain Mast Cell-Specific Proteases

Mast cells (MC) are well known for their effector role in allergic disorders; moreover, they are associated with diverse modulatory effects in innate and adaptive immunity. It is largely unclear how MC exert these modulating functions. In this article, we show that IgE-mediated MC degranulation leads to a rapid release of high quantities of extracellular vesicles (EV), comparable to the release of preformed mediators. EV are submicron structures composed of lipid bilayers, proteins, and nucleic acids that are released by cells in a regulated fashion and are involved in intercellular communication. Primary murine mucosal-type MC and connective tissue-type MC released phenotypically different EV populations depending on the stimulus they received. Although unstimulated MC constitutively released CD9⁺ EV, degranulation was accompanied by the release of CD63⁺ EV, which correlated with release of the soluble mediator β-hexosaminidase. This CD63⁺ EV subset was smaller and exhibited a higher buoyant density and distinct phospholipid composition compared with CD9⁺ EV. Marked differences were observed for phosphatidylinositol, phosphatidic acid, and bis(monoacylglycero)phosphate species. Strikingly, proteomic analysis of CD63⁺ EV from connective tissue-type MC unveiled an abundance of MC-specific proteases. With regard to carboxypeptidase A3, it was confirmed that the enzyme was EV associated and biologically active. Our data demonstrate that, depending on their activation status, MC release distinct EV subsets that differ in composition and protease activity and are indicative of differential immunological functions. Concerning the strategic tissue distribution of MC and the presence of degranulated MC in various (allergic) disorders, MC-derived EV should be considered potentially important immune regulators.

Synthesis and transfer of galactolipids in the chloroplast envelope membranes of Arabidopsis thaliana

Galactolipids [monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)] are the hallmark lipids of photosynthetic membranes. The galactolipid synthases MGD1 and DGD1 catalyze consecutive galactosyltransfer reactions but localize to the inner and outer chloroplast envelopes, respectively, necessitating intermembrane lipid transfer. Here we show that the N-terminal sequence of DGD1 (NDGD1) is required for galactolipid transfer between the envelopes. Different diglycosyllipid synthases (DGD1, DGD2, and Chloroflexus glucosyltransferase) were introduced into the dgd1-1 mutant of Arabidopsis in fusion with N-terminal extensions (NDGD1 and NDGD2) targeting to the outer envelope. Reconstruction of DGDG synthesis in the outer envelope membrane was observed only with diglycosyllipid synthase fusion proteins carrying NDGD1, indicating that NDGD1 enables galactolipid translocation between envelopes. NDGD1 binds to phosphatidic acid (PA) in membranes and mediates PA-dependent membrane fusion in vitro. These findings provide a mechanism for the sorting and selective channeling of lipid precursors between the galactolipid pools of the two envelope membranes.

Multifaceted Roles of ALG-2 in Ca(2+)-Regulated Membrane Trafficking

ALG-2 (gene name: PDCD6) is a penta-EF-hand Ca²⁺-binding protein and interacts with a variety of proteins in a Ca²⁺-dependent fashion. ALG-2 recognizes different types of identified motifs in Pro-rich regions by using different hydrophobic pockets, but other unknown modes of binding are also used for non-Pro-rich proteins. Most ALG-2-interacting proteins associate directly or indirectly with the plasma membrane or organelle membranes involving the endosomal sorting complex required for transport (ESCRT) system, coat protein complex II (COPII)-dependent ER-to-Golgi vesicular transport, and signal transduction from membrane receptors to downstream players. Binding of ALG-2 to targets may induce conformational change of the proteins. The ALG-2 dimer may also function as a Ca²⁺-dependent adaptor to bridge different partners and connect the subnetwork of interacting proteins.

Biogenesis and Function of T Cell-Derived Exosomes

Exosomes are a particular type of extracellular vesicle, characterized by their endosomal origin as intraluminal vesicles present in large endosomes with a multivesicular structure. After these endosomes fuse with the plasma membrane, exosomes are secreted into the extracellular space. The ability of exosomes to carry and selectively deliver bioactive molecules (e.g., lipids, proteins, and nucleic acids) confers on them the capacity to modulate the activity of receptor cells, even if these cells are located in distant tissues or organs. Since exosomal cargo depends on cell type, a detailed understanding of the mechanisms that regulate the biochemical composition of exosomes is fundamental to a comprehensive view of exosome function. Here, we review the latest advances concerning exosome function and biogenesis in T cells, with particular focus on the mechanism of protein sorting at multivesicular endosomes. Exosomes secreted by specific T-cell subsets can modulate the activity of immune cells, including other T-cell subsets. Ceramide, tetraspanins and MAL have been revealed to be important in exosome biogenesis by T cells. These molecules, therefore, constitute potential molecular targets for artificially modulating exosome production and, hence, the immune response for therapeutic purposes.

Membrane curvature in cell biology: An integration of molecular mechanisms

Curving biological membranes establishes the complex architecture of the cell and mediates membrane traffic to control flux through subcellular compartments. Common molecular mechanisms for bending membranes are evident in different cell biological contexts across eukaryotic phyla. These mechanisms can be intrinsic to the membrane bilayer (either the lipid or protein components) or can be brought about by extrinsic factors, including the cytoskeleton. Here, we review examples of membrane curvature generation in animals, fungi, and plants. We showcase the molecular mechanisms involved and how they collaborate and go on to highlight contexts of curvature that are exciting areas of future research. Lessons from how membranes are bent in yeast and mammals give hints as to the molecular mechanisms we expect to see used by plants and protists.

Burning cellular bridges: Two pathways to the big breakup

During cytokinetic abscission, the endosomal sorting complex required for transport (ESCRT) proteins are recruited to the midbody and direct the severing of the intercellular bridge. In this issue, Christ et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201507009) demonstrate that two separate but redundant pathways exist to recruit ESCRT-III proteins to the midbody.

Exosomes in Viral Disease

Viruses have evolved many mechanisms by which to evade and subvert the immune system to ensure survival and persistence. However, for each method undertaken by the immune system for pathogen removal, there is a counteracting mechanism utilized by pathogens. The new and emerging role of microvesicles in immune intercellular communication and function is no different. Viruses across many different families have evolved to insert viral components in exosomes, a subtype of microvesicle, with many varying downstream effects. When assessed cumulatively, viral antigens in exosomes increase persistence through cloaking viral genomes, decoying the immune system, and even by increasing viral infection in uninfected cells. Exosomes therefore represent a source of viral antigen that can be used as a biomarker for disease and targeted for therapy in the control and eradication of these disorders. With the rise in the persistence of new and reemerging viruses like Ebola and Zika, exploring the role of exosomes become more important than ever.

Alix-mediated assembly of the actomyosin-tight junction polarity complex preserves epithelial polarity and epithelial barrier

Maintenance of epithelial cell polarity and epithelial barrier relies on the spatial organization of the actin cytoskeleton and proper positioning/assembly of intercellular junctions. However, how these processes are regulated is poorly understood. Here we reveal a key role for the multifunctional protein Alix in both processes. In a knockout mouse model of Alix, we identified overt structural changes in the epithelium of the choroid plexus and in the ependyma, such as asymmetrical cell shape and size, misplacement and abnormal beating of cilia, blebbing of the microvilli. These defects culminate in excessive cell extrusion, enlargement of the lateral ventricles and hydrocephalus. Mechanistically, we find that by interacting with F-actin, the Par complex and ZO-1, Alix ensures the formation and maintenance of the apically restricted actomyosin-tight junction complex. We propose that in this capacity Alix plays a role in the establishment of apical-basal polarity and in the maintenance of the epithelial barrier.