Membrane microdomains and proteomics: lessons from tetraspanin microdomains and comparison with lipid rafts

Biophysical aspects of migrasome organelle formation and their diverse cellular functions

The transient cellular organelles known as migrasomes, which form during cell migration along retraction fibers, have emerged as a crutial factor in various fundamental cellular processes and pathologies. These membrane vesicles originate from local membrane swellings, encapsulate specific cytoplasmic content, and are eventually released to the extracellular environment or taken up by recipient cells. Migrasome biogenesis entails a sequential membrane remodeling process involving a complex interplay between various molecular factors such as tetraspanin proteins, and mechanical properties like membrane tension and bending rigidity. In this review, we summarize recent studies exploring the mechanism of migrasome formation. We emphasize how physical forces, together with molecular factors, shape migrasome biogenesis, and detail the involvement of migrasomes in various cellular processes and pathologies. A comprehensive understanding of the exact mechanism underlying migrasome formation and the identification of key molecules involved hold promise for advancing their therapeutic and diagnostic applications.

Tetraspanin proteins in membrane remodeling processes

Membrane remodeling is a fundamental cellular process that is crucial for physiological functions such as signaling, membrane fusion and cell migration. Tetraspanins (TSPANs) are transmembrane proteins of central importance to membrane remodeling events. During these events, TSPANs are known to interact with themselves and other proteins and lipids; however, their mechanism of action in controlling membrane dynamics is not fully understood. Since these proteins span the membrane, membrane properties such as rigidity, curvature and tension can influence their behavior. In this Review, we summarize recent studies that explore the roles of TSPANs in membrane remodeling processes and highlight the unique structural features of TSPANs that mediate their interactions and localization. Further, we emphasize the influence of membrane curvature on TSPAN distribution and membrane domain formation and describe how these behaviors affect cellular functions. This Review provides a comprehensive perspective on the multifaceted function of TSPANs in membrane remodeling processes and can help readers to understand the intricate molecular mechanisms that govern cellular membrane dynamics.

Extracellular domain 2 of TSPAN4 governs its functions

Tetraspanin 4, a protein with four transmembrane helices and three connecting loops, senses membrane curvature and localizes to membrane tubes. This enrichment in tubular membranes enhances its diverse interactions. While the transmembrane part of the protein likely contributes to curvature sensitivity, the possible roles of the ectodomains in curvature sensitivity of tetraspanin 4 are still unknown. Here, using micropipette aspiration combined with confocal microscopy and optical tweezers, we show that the extracellular loop 2 contributes to the curvature sensitivity and curvature-induced interactions of tetraspanin 4. To this end, we created truncated tetraspanin 4 mutants by deleting each of the connecting loops. Subsequently, we pulled membrane tubes from giant plasma membrane vesicles containing tetraspanin 4-GFP or its mutants while maintaining controllable membrane tension and curvature. Among the mutations tested, the removal of the extracellular loop 2 had the most significant impact on both the curvature sensitivity and interactions of tetraspanin 4. Based on the results, we suggest that the extracellular loop 2 regulates the affinity of tetraspanin 4 towards curved membranes and affects its lateral interactions.

Proximity-Based Labeling Identifies MHC Class II and CD37 as B Cell Receptor-Proximal Proteins with Immunological Functions

The BCR allows for Ag-driven B cell activation and subsequent Ag endocytosis, processing, and presentation to recruit T cell help. Core drivers of BCR signaling and endocytosis are motifs within the receptor's cytoplasmic tail (primarily CD79). However, BCR function can be tuned by other proximal cellular elements, such as CD20 and membrane lipid microdomains. To identify additional proteins that could modulate BCR function, we used a proximity-based biotinylation technique paired with mass spectrometry to identify molecular neighbors of the murine IgM BCR. Those neighbors include MHC class II molecules, integrins, various transporters, and membrane microdomain proteins. Class II molecules, some of which are invariant chain-associated nascent class II, are a readily detected BCR neighbor. This finding is consistent with reports of BCR-class II association within intracellular compartments. The BCR is also in close proximity to multiple proteins involved in the formation of membrane microdomains, including CD37, raftlin, and Ig superfamily member 8. Known defects in T cell-dependent humoral immunity in CD37 knockout mice suggest a role for CD37 in BCR function. In line with this notion, CRISPR-based knockout of CD37 expression in a B cell line heightens BCR signaling, slows BCR endocytosis, and tempers formation of peptide-class II complexes. These results indicate that BCR molecular neighbors can impact membrane-mediated BCR functions. Overall, a proximity-based labeling technique allowed for identification of multiple previously unknown BCR molecular neighbors, including the tetraspanin protein CD37, which can modulate BCR function.

Migrasome, a novel organelle, differs from exosomes

Migrasomes, a newly discovered organelle produced by migrating cells, are vesicles with membranous structure that form on the tips and intersections of retraction fibers (RFs). These structures are released into the extracellular environment or taken up by surrounding cells, mediating the release of cytoplasmic contents and intercellular communication. Retractosomes, a new type of small extracellular vesicles generated from broken-off RFs, are closely related to migrasomes in their physical location and origin, but were defined later. Despite their widespread existence in cells and biological organisms, little is known about the regulatory mechanisms underlying their formation and potential function. In this review, we provide an overview of the discovery, biogenesis, distribution, and functions of migrasomes and retractosomes, as well as their differences from exosomes.

The molecular mechanism of CD81 antibody inhibition of metastasis

Metastases are reduced in CD81KO mice. In addition, a unique anti-CD81 antibody, 5A6, inhibits metastasis in vivo and invasion and migration in vitro. Here, we probed the structural components of CD81 required for the antimetastatic activity induced by 5A6. We found that the removal of either cholesterol or the intracellular domains of CD81 did not affect inhibition by the antibody. We show that the uniqueness of 5A6 is due not to increased affinity but rather to its recognition of a specific epitope on the large extracellular loop of CD81. Finally, we present a number of CD81 membrane-associated partners that may play a role in mediating the 5A6 antimetastatic attributes, including integrins and transferrin receptors.

Tetraspanin 4 stabilizes membrane swellings and facilitates their maturation into migrasomes

Migrasomes are newly discovered cell organelles forming by local swelling of retraction fibers. The migrasome formation critically depends on tetraspanin proteins present in the retraction fiber membranes and is modulated by the membrane tension and bending rigidity. It remained unknown how and in which time sequence these factors are involved in migrasome nucleation, growth, and stabilization, and what are the possible intermediate stages of migrasome biogenesis. Here using live cell imaging and a biomimetic system for migrasomes and retraction fibers, we reveal that migrasome formation is a two-stage process. At the first stage, which in biomimetic system is mediated by membrane tension, local swellings largely devoid of tetraspanin 4 form on the retraction fibers. At the second stage, tetraspanin 4 molecules migrate toward and onto these swellings, which grow up to several microns in size and transform into migrasomes. This tetraspanin 4 recruitment to the swellings is essential for migrasome growth and stabilization. Based on these findings we propose that the major role of tetraspanin proteins is in stabilizing the migrasome structure, while the migrasome nucleation and initial growth stages can be driven by membrane mechanical stresses.

Migrasome biogenesis and functions

The migrasome is a newly discovered organelle produced by migrating cells. As cells migrate, long and thin retraction fibers are left in their wake. On these fibers, we discovered the production of a pomegranate-like structure, which we named migrasomes. The production of migrasomes is highly correlated with the migration of cells. Currently, it has been demonstrated the migrasomes exhibit three modes of action: release of signaling molecules through rupturing or leaking, carriers of damaged mitochondria, and lateral transfer of mRNA or proteins. In this review, we would like to discuss, in detail, the functions, mechanisms, and potential applications of this newly discovered cell organelle.

Transmembrane proteins tetraspanin 4 and CD9 sense membrane curvature

Multiple membrane-shaping and remodeling processes are associated with tetraspanin proteins by yet unknown mechanisms. Tetraspanins constitute a family of proteins with four transmembrane domains present in every cell type. Prominent examples are tetraspanin4 and CD9, which are required for the fundamental cellular processes of migrasome formation and fertilization, respectively. These proteins are enriched in curved membrane structures, such as cellular retraction fibers and oocyte microvilli. The factors driving this enrichment are, however, unknown. Here, we revealed that tetraspanin4 and CD9 are curvature sensors with a preference for positive membrane curvature. To this end, we used a biomimetic system emulating membranes of cell retraction fibers and oocyte microvilli by membrane tubes pulled out of giant plasma membrane vesicles with controllable membrane tension and curvature. We developed a simple thermodynamic model for the partitioning of curvature sensors between flat and tubular membranes, which allowed us to estimate the individual intrinsic curvatures of the two proteins. Overall, our findings illuminate the process of migrasome formation and oocyte microvilli shaping and provide insight into the role of tetraspanin proteins in membrane remodeling processes.

Machine learning-assisted elucidation of CD81-CD44 interactions in promoting cancer stemness and extracellular vesicle integrity

Tumor-initiating cells with reprogramming plasticity or stem-progenitor cell properties (stemness) are thought to be essential for cancer development and metastatic regeneration in many cancers; however, elucidation of the underlying molecular network and pathways remains demanding. Combining machine learning and experimental investigation, here we report CD81, a tetraspanin transmembrane protein known to be enriched in extracellular vesicles (EVs), as a newly identified driver of breast cancer stemness and metastasis. Using protein structure modeling and interface prediction-guided mutagenesis, we demonstrate that membrane CD81 interacts with CD44 through their extracellular regions in promoting tumor cell cluster formation and lung metastasis of triple negative breast cancer (TNBC) in human and mouse models. In-depth global and phosphoproteomic analyses of tumor cells deficient with CD81 or CD44 unveils endocytosis-related pathway alterations, leading to further identification of a quality-keeping role of CD44 and CD81 in EV secretion as well as in EV-associated stemness-promoting function. CD81 is coexpressed along with CD44 in human circulating tumor cells (CTCs) and enriched in clustered CTCs that promote cancer stemness and metastasis, supporting the clinical significance of CD81 in association with patient outcomes. Our study highlights machine learning as a powerful tool in facilitating the molecular understanding of new molecular targets in regulating stemness and metastasis of TNBC.

Assembly of Tetraspanin-enriched macrodomains contains membrane damage to facilitate repair

Various mechanisms contribute to membrane repair^1-8 but the machinery that mediates the repair of large wounds on the plasma membrane is less clear. We found that shortly after membrane damage, Tetraspanin-enriched macrodomains are assembled around the damage site. Tetraspanin-enriched macrodomains are in the liquid-ordered phase and form a rigid ring around the damaged site. This restricts the spread of the damage and prevents membrane disintegration, thus facilitating membrane repair by other mechanisms. Functionally, Tetraspanin 4 helps cells mitigate damage caused by laser, detergent, pyroptosis and natural killer cells. We propose that assembly of Tetraspanin-enriched macrodomains creates a physical barrier to contain membrane damage.

Tspan15 Is a New Stemness-Related Marker in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second cause of cancer-related deaths worldwide. A clearer understanding of the molecular mechanisms underlying tumor growth and invasiveness remains crucial for developing new therapies. Here, the expression of tetraspanins, a family of plasma membrane organizers involved in tumor progression, has been addressed. Integrative approaches combining transcriptomics and bioinformatics allow demonstrating the induced and heterogeneous expression of Tspan15 in HCC. Tspan15 positive tumors exhibit signatures related to hepatic progenitor cells as well as recurrence of cancer. Immunohistochemistry experiments confirm Tspan15 expression in the subset of HCC expressing stemness-related markers such as EpCAM and Cytokeratin-19. Functional networks reveal that most of these genes expressed in correlation to Tspan15 support cell proliferation. Furthermore, Tspan15 overexpression in the hepatoma cell line HepG2 significantly increases cell proliferation. A quantitative proteomic analysis of the secretome reveals a higher abundance of the protein connective tissue growth factor (CTGF), a pleiotropic matricellular signaling protein. Proteomic profiling of Tspan15 complexes allows identifying numerous membrane proteins including several growth factor receptors. Finally, Tspan15 increases ERK1/2 phosphorylation that directly controls CTGF expression and secretion. In conclusion, Tspan15 is a new stemness-related marker in HCC which exhibits high potential of tumor growth and recurrence.

Migrasome formation is mediated by assembly of micron-scale tetraspanin macrodomains

Migrasomes are recently discovered cellular organelles that form as large vesicle-like structures on retraction fibres of migrating cells. While the process of migrasome formation has been described before, the molecular mechanism underlying migrasome biogenesis remains unclear. Here, we propose that the mechanism of migrasome formation consists of the assembly of tetraspanin- and cholesterol-enriched membrane microdomains into micron-scale macrodomains, which swell into migrasomes. The major finding underlying the mechanism is that tetraspanins and cholesterol are necessary and sufficient for migrasome formation. We demonstrate the necessity of tetraspanins and cholesterol via live-cell experiments, and their sufficiency by generating migrasome-like structures in reconstituted membrane systems. We substantiate the mechanism by a theoretical model proposing that the key factor driving migrasome formation is the elevated membrane stiffness of the tetraspanin- and cholesterol-enriched macrodomains. Finally, the theoretical model was quantitatively validated by experimental demonstration of the membrane-stiffening effect of tetraspanin 4 and cholesterol.

Tetraspanins: Architects of Viral Entry and Exit Platforms

Host factors render cells susceptible to viral infection. One family of susceptibility factors, the tetraspanin proteins, facilitate enveloped virus entry by promoting virus-cell membrane fusion.

Host factors render cells susceptible to viral infection. One family of susceptibility factors, the tetraspanin proteins, facilitate enveloped virus entry by promoting virus-cell membrane fusion. They also facilitate viral egress from infected cells. In this Gem, we discuss recent insights into how tetraspanins assemble viral entry and exit platforms on cell membranes, and we speculate that tetraspanins contribute to nonviral membrane fusions by similar mechanisms.

Uroplakins play conserved roles in egg fertilization and acquired additional urothelial functions during mammalian divergence

Uroplakin (UP) tetraspanins and their associated proteins are major mammalian urothelial differentiation products that form unique two-dimensional crystals of 16-nm particles (“urothelial plaques”) covering the apical urothelial surface. Although uroplakins are highly expressed only in mammalian urothelium and are often referred to as being urothelium specific, they are also expressed in several mouse nonurothelial cell types in stomach, kidney, prostate, epididymis, testis/sperms, and ovary/oocytes. In oocytes, uroplakins colocalize with CD9 on cell-surface and multivesicular body-derived exosomes, and the cytoplasmic tail of UPIIIa undergoes a conserved fertilization-dependent, Fyn-mediated tyrosine phosphorylation that also occurs in Xenopus laevis eggs. Uroplakin knockout and antibody blocking reduce mouse eggs’ fertilization rate in in vitro fertilization assays, and UPII/IIIa double-knockout mice have a smaller litter size. Phylogenetic analyses showed that uroplakin sequences underwent significant mammal-specific changes. These results suggest that, by mediating signal transduction and modulating membrane stability that do not require two-dimensional-crystal formation, uroplakins can perform conserved and more ancestral fertilization functions in mouse and frog eggs. Uroplakins acquired the ability to form two-dimensional-crystalline plaques during mammalian divergence, enabling them to perform additional functions, including umbrella cell enlargement and the formation of permeability and mechanical barriers, to protect/modify the apical surface of the modern-day mammalian urothelium.

Tetraspanin CD9: A Key Regulator of Cell Adhesion in the Immune System

The tetraspanin CD9 is expressed by all the major subsets of leukocytes (B cells, CD4⁺ T cells, CD8⁺ T cells, natural killer cells, granulocytes, monocytes and macrophages, and immature and mature dendritic cells) and also at a high level by endothelial cells. As a typical member of the tetraspanin superfamily, a prominent feature of CD9 is its propensity to engage in a multitude of interactions with other tetraspanins as well as with different transmembrane and intracellular proteins within the context of defined membranal domains termed tetraspanin-enriched microdomains (TEMs). Through these associations, CD9 influences many cellular activities in the different subtypes of leukocytes and in endothelial cells, including intracellular signaling, proliferation, activation, survival, migration, invasion, adhesion, and diapedesis. Several excellent reviews have already covered the topic of how tetraspanins, including CD9, regulate these cellular processes in the different cells of the immune system. In this mini-review, however, we will focus particularly on describing and discussing the regulatory effects exerted by CD9 on different adhesion molecules that play pivotal roles in the physiology of leukocytes and endothelial cells, with a particular emphasis in the regulation of adhesion molecules of the integrin and immunoglobulin superfamilies.

Glycosylation as a Main Regulator of Growth and Death Factor Receptors Signaling

Glycosylation is a very frequent and functionally important post-translational protein modification that undergoes profound changes in cancer. Growth and death factor receptors and plasma membrane glycoproteins, which upon activation by extracellular ligands trigger a signal transduction cascade, are targets of several molecular anti-cancer drugs. In this review, we provide a thorough picture of the mechanisms bywhich glycosylation affects the activity of growth and death factor receptors in normal and pathological conditions. Glycosylation affects receptor activity through three non-mutually exclusive basic mechanisms: (1) by directly regulating intracellular transport, ligand binding, oligomerization and signaling of receptors; (2) through the binding of receptor carbohydrate structures to galectins, forming a lattice thatregulates receptor turnover on the plasma membrane; and (3) by receptor interaction with gangliosides inside membrane microdomains. Some carbohydrate chains, for example core fucose and β1,6-branching, exert a stimulatory effect on all receptors, while other structures exert opposite effects on different receptors or in different cellular contexts. In light of the crucial role played by glycosylation in the regulation of receptor activity, the development of next-generation drugs targeting glyco-epitopes of growth factor receptors should be considered a therapeutically interesting goal.

Port-to-port delivery: Mobilization of toxic sphingolipids via extracellular vesicles

The discovery that most cells produce extracellular vesicles (EVs) and release them in the extracellular milieu has spurred the idea that these membranous cargoes spread pathogenic mechanisms. In the brain, EVs may have multifold and important physiological functions, from deregulating synaptic activity to promoting demyelination to changes in microglial activity. The finding that small EVs (exosomes) contain α-synuclein and β-amyloid, among other pathogenic proteins, is an example of this notion, underscoring their potential role in the brains of patients with Parkinson's and Alzheimer's diseases. Given that they are membranous vesicles, we speculate that EVs also have an intrinsic capacity to incorporate sphingolipids. In conditions under which these lipids are elevated to toxic levels, such as in Krabbe's disease and metachromatic leukodystrophy, EVs may contribute to spread disease from sick to healthy cells. In this essay, we discuss a working hypothesis that brain cells in sphingolipidoses clear some of the accumulated lipid material to attempt restoring cell homeostasis via EV secretion. We hypothesize that secreted sphingolipid-loaded EVs shuttle pathogenic lipids to cells that are not intrinsically affected, contributing to establishing non-cell-autonomous defects. © 2016 Wiley Periodicals, Inc.

Lipidation increases antiviral activities of coronavirus fusion-inhibiting peptides

Coronaviruses (CoVs) can cause life-threatening respiratory diseases. Their infectious entry requires viral spike (S) proteins, which attach to cell receptors, undergo proteolytic cleavage, and then refold in a process that catalyzes virus-cell membrane fusion. Fusion-inhibiting peptides bind to S proteins, interfere with refolding, and prevent infection. Here we conjugated fusion-inhibiting peptides to various lipids, expecting this to secure peptides onto cell membranes and thereby increase antiviral potencies. Cholesterol or palmitate adducts increased antiviral potencies up to 1000-fold. Antiviral effects were evident after S proteolytic cleavage, implying that lipid conjugates affixed the peptides at sites of protease-triggered fusion activation. Unlike lipid-free peptides, the lipopeptides suppressed CoV S protein-directed virus entry taking place within endosomes. Cell imaging revealed intracellular peptide aggregates, consistent with their endocytosis into compartments where CoV entry takes place. These findings suggest that lipidations localize antiviral peptides to protease-rich sites of CoV fusion, thereby protecting cells from diverse CoVs.

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Lipidation increases antiviral activities of CoV fusion-inhibiting peptides.

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Fusion-inhibiting peptides target proteolytically-triggered CoV spike proteins.

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Lipidated peptides suppress CoVs that are occluded within endosomes before cytosolic entry.

Exosomal tetraspanins mediate cancer metastasis by altering host microenvironment

The metastases of malignant tumors develop through a cascade of events. The establishment of a pre-metastatic micro-environment is initiated by communication between tumors and host. Exosomes come into focus as the most potent intercellular communicators playing a pivotal role in this process. Cancer cells release exosomes into the extracellular environment prior to metastasis. Tetraspanin is a type of 4 times transmembrane proteins. It may be involved in cell motility, adhesion, morphogenesis, as well as cell and vesicular membrane fusion. The exosomal tetraspanin network is a molecular scaffold connecting various proteins for signaling transduction. The complex of tetraspanin-integrin determines the recruiting cancer exosomes to pre-metastatic sites. Tetraspanin is a key element for the target cell selection of exosomes uptake that may lead to the reprogramming of target cells. Reprogrammed target cells assist pre-metastatic niche formation. Previous reviews have described the biogenesis, secretion and intercellular interaction of exosomes in various tumors. However, there is a lack of reviews on the topic of exosomal tetraspanin in the context of cancer. In this review, we will describe the main characteristics of exosomal tetraspanin in cancer cells. We will also discuss how the cancer exosomal tetraspanin alters extracellular environment and regulates cancer metastasis.

CD63 Regulates Epstein-Barr Virus LMP1 Exosomal Packaging, Enhancement of Vesicle Production, and Noncanonical NF-κB Signaling

Latent membrane protein 1 (LMP1) is an Epstein-Barr virus (EBV)-encoded oncoprotein that is packaged into small extracellular vesicles (EVs) called exosomes. Trafficking of LMP1 into multivesicular bodies (MVBs) alters the content and function of exosomes. LMP1-modified exosomes enhance the growth, migration, and invasion of malignant cells, demonstrating the capacity to manipulate the tumor microenvironment and enhance the progression of EBV-associated cancers. Despite the growing evidence surrounding the significance of LMP1-modified exosomes in cancer, very little is understood about the mechanisms that orchestrate LMP1 incorporation into these vesicles. Recently, LMP1 was shown to be copurified with CD63, a conserved tetraspanin protein enriched in late endosomal and lysosomal compartments. Here, we demonstrate the importance of CD63 presence for exosomal packaging of LMP1. Nanoparticle tracking analysis and gradient purification revealed an increase in extracellular vesicle secretion and exosomal proteins following LMP1 expression. Immunoisolation of CD63-positive exosomes exhibited accumulation of LMP1 in this vesicle population. Functionally, CRISPR/Cas9 knockout of CD63 resulted in a reduction of LMP1-induced particle secretion. Furthermore, LMP1 packaging was severely impaired in CD63 knockout cells, concomitant with a disruption in the perinuclear localization of LMP1. Importantly, LMP1 trafficking to lipid rafts and activation of NF-κB and PI3K/Akt pathways remained intact following CD63 knockout, while mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and noncanonical NF-κB activation were observed to be increased. These results suggest that CD63 is a critical player in LMP1 exosomal trafficking and LMP1-mediated enhancement of exosome production and may play further roles in limiting downstream LMP1 signaling.IMPORTANCE EBV is a ubiquitous gamma herpesvirus linked to malignancies such as nasopharyngeal carcinoma, Burkitt's lymphoma, and Hodgkin's lymphoma. In the context of cancer, EBV hijacks the exosomal pathway to modulate cell-to-cell signaling by secreting viral components such as an oncoprotein, LMP1, into host cell membrane-bound EVs. Trafficking of LMP1 into exosomes is associated with increased oncogenicity of these secreted vesicles. However, we have only a limited understanding of the mechanisms surrounding exosomal cargo packaging, including viral proteins. Here, we describe a role of LMP1 in EV production that requires CD63 and provide an extensive demonstration of CD63-mediated exosomal LMP1 release that is distinct from lipid raft trafficking. Finally, we present further evidence of the role of CD63 in limiting LMP1-induced noncanonical NF-κB and ERK activation. Our findings have implications for future investigations of physiological and pathological mechanisms of exosome biogenesis, protein trafficking, and signal transduction, especially in viral-associated tumorigenesis.

Exosomes in Cancer Disease

Cancer diagnosis and therapy is steadily improving. Still, diagnosis is frequently late and diagnosis and follow-up procedures mostly are time-consuming and expensive. Searching for tumor-derived exosomes (TEX) in body fluids may provide an alternative, minimally invasive, yet highly reliable diagnostic tool. Beyond this, there is strong evidence that TEX could become a potent therapeutics. Exosomes, small vesicles delivered by many cells of the organism, are found in all body fluids. Exosomes are characterized by lipid composition, common and donor cell specific proteins, mRNA, small non-coding RNA including miRNA and DNA. Particularly the protein and miRNA markers received much attention as they may allow for highly specific diagnosis and can provide hints toward tumor aggressiveness and progression, where exosome-based diagnosis and follow-up is greatly facilitated by the recovery of exosomes in body fluids, particularly the peripheral blood. Beyond this, exosomes are the most important intercellular communicators that modulate, instruct, and reprogram their surrounding as well as distant organs. In concern about TEX this includes message transfer from tumor cells toward the tumor stroma, the premetastatic niche, the hematopoietic system and, last but not least, the instruction of non-cancer stem cells by cancer-initiating cells (CIC). Taking this into account, it becomes obvious that "tailored" exosomes offer themselves as potent therapeutic delivery system. In brief, during the last 4-5 years there is an ever-increasing, overwhelming interest in exosome research. This boom appears fully justified provided the content of the exosomes becomes most thoroughly analyzed and their mode of intercellular interaction can be unraveled in detail as this knowledge will open new doors toward cancer diagnosis and therapy including immunotherapy and CIC reprogramming.

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.

Pancreatic cancer stem cell markers and exosomes - the incentive push

Pancreatic cancer (PaCa) has the highest death rate and incidence is increasing. Poor prognosis is due to late diagnosis and early metastatic spread, which is ascribed to a minor population of so called cancer stem cells (CSC) within the mass of the primary tumor. CSC are defined by biological features, which they share with adult stem cells like longevity, rare cell division, the capacity for self renewal, differentiation, drug resistance and the requirement for a niche. CSC can also be identified by sets of markers, which for pancreatic CSC (Pa-CSC) include CD44v6, c-Met, Tspan8, alpha6beta4, CXCR4, CD133, EpCAM and claudin7. The functional relevance of CSC markers is still disputed. We hypothesize that Pa-CSC markers play a decisive role in tumor progression. This is fostered by the location in glycolipid-enriched membrane domains, which function as signaling platform and support connectivity of the individual Pa-CSC markers. Outside-in signaling supports apoptosis resistance, stem cell gene expression and tumor suppressor gene repression as well as miRNA transcription and silencing. Pa-CSC markers also contribute to motility and invasiveness. By ligand binding host cells are triggered towards creating a milieu supporting Pa-CSC maintenance. Furthermore, CSC markers contribute to the generation, loading and delivery of exosomes, whereby CSC gain the capacity for a cell-cell contact independent crosstalk with the host and neighboring non-CSC. This allows Pa-CSC exosomes (TEX) to reprogram neighboring non-CSC towards epithelial mesenchymal transition and to stimulate host cells towards preparing a niche for metastasizing tumor cells. Finally, TEX communicate with the matrix to support tumor cell motility, invasion and homing. We will discuss the possibility that CSC markers are the initial trigger for these processes and what is the special contribution of CSC-TEX.

Similarity of a 16.5kDa tegumental protein of the human liver fluke Opisthorchis viverrini to nematode cytoplasmic motility protein

Opisthorchis viverrini is the causative agent of human opisthorchiasis in Thailand and long lasting infection with the parasite has been correlated with the development of cholangiocarcinoma. In this work we have molecularly characterized the first member of a protein family carrying two DM9 repeats in this parasite (OvDM9-1). InterPro and other protein family databases describe the DM9 repeat as a protein domain of unknown function that has been first noted in Drosophila melanogaster. Two paralogous proteins have been partially characterized in the genus Fasciola, Fasciola hepatica TP16.5, a novel tegumental antigen in human fascioliasis and, recently F. gigantica DM9-1, a parenchymal protein with structural similarity to nematode cytoplasmic motility protein (MFP2). In this study, we show further evidence that this family of trematode proteins is related to MFP2 in sequence and structure. Soluble recombinant OvDM9-1 was used for structural analyses and for production of specific antisera. The native protein was detected in soluble and insoluble crude worm extracts and in seemingly various oligomeric forms in the latter. The potential for oligomerization was supported by cross-linking experiments of recombinant OvDM9-1. Structure prediction suggested a β-rich secondary structure of the protein and this was supported by a circular dichroism analysis. Molecular modeling in Phyre2 identified both MFP2 domains as distant homologs of OvDM9-1. The protein was located in tegumental type tissue and the cecal epithelium in the mature parasite. Recombinant OvDM9-1 was used as target in indirect ELISA but sera from infected hamsters showed only marginal reactivity towards it. It is proposed that OvDM9-1 and other members of this protein family have a role in cellular transport through functions on the cytoskeleton.

The tetraspanin web revisited by super-resolution microscopy

The spatial organization of membrane proteins in the plasma membrane is critical for signal transduction, cell communication and membrane trafficking. Tetraspanins organize functional higher-order protein complexes called ‘tetraspanin-enriched microdomains (TEMs)’ via interactions with partner molecules and other tetraspanins. Still, the nanoscale organization of TEMs in native plasma membranes has not been resolved. Here, we elucidated the size, density and distribution of TEMs in the plasma membrane of human B cells and dendritic cells using dual color stimulated emission depletion (STED) microscopy. We demonstrate that tetraspanins form individual nanoclusters smaller than 120 nm and quantified that a single tetraspanin CD53 cluster contains less than ten CD53 molecules. CD53 and CD37 domains were adjacent to and displayed only minor overlap with clusters containing tetraspanins CD81 or CD82. Moreover, CD53 and CD81 were found in closer proximity to their partners MHC class II and CD19 than to other tetraspanins. Although these results indicate that tetraspanin domains are adjacently positioned in the plasma membrane, they challenge the current view of the tetraspanin web of multiple tetraspanin species organized into a single domain. This study increases the molecular understanding of TEMs at the nanoscale level which is essential for comprehending tetraspanin function in cell biology.

Charming neighborhoods on the cell surface: plasma membrane microdomains regulate receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTK) are an important family of growth factor and hormone receptors that regulate many aspects of cellular physiology. Ligand binding by RTKs at the plasma membrane elicits activation of many signaling intermediates. The spatial and temporal regulation of RTK signaling within cells is an important determinant of receptor signaling outcome. In particular, the compartmentalization of the plasma membrane into a number of microdomains allows context-specific control of RTK signaling. Indeed various RTKs are recruited to and enriched within specific plasma membrane microdomains under various conditions, including lipid-ordered domains such as caveolae and lipid rafts, clathrin-coated structures, tetraspanin-enriched microdomains, and actin-dependent protrusive membrane microdomains such as dorsal ruffles and invadosomes. We examine the evidence for control of RTK signaling by each of these plasma membrane microdomains, as well as molecular mechanisms for how this spatial organization controls receptor signaling.

Proteomics of extracellular vesicles: Exosomes and ectosomes

Almost all bacteria, archaea, and eukaryotic cells shed extracellular vesicles either constitutively or in a regulated manner. These nanosized membrane vesicles are spherical, bilayered proteolipids that harbor specific subsets of proteins, DNAs, RNAs, and lipids. Recent research has facilitated conceptual advancements in this emerging field that indicate that extracellular vesicles act as intercellular communicasomes by transferring signals to their target cell via surface ligands and delivering receptors and functional molecules. Recent progress in mass spectrometry-based proteomic analyses of mammalian extracellular vesicles derived from diverse cell types and body fluids has resulted in the identification of several thousand vesicular proteins that provide us with essential clues to the molecular mechanisms involved in vesicle cargo sorting and biogenesis. Furthermore, cell-type- or disease-specific vesicular proteins help us to understand the pathophysiological functions of extracellular vesicles and contribute to the discovery of diagnostic and therapeutic target proteins. This review focuses on the high-throughput mass spectrometry-based proteomic analyses of mammalian extracellular vesicles (i.e., exosomes and ectosomes), EVpedia (a free web-based integrated database of high-throughput data for systematic analyses of extracellular vesicles; http://evpedia.info), and the intravesicular protein-protein interaction network analyses of mammalian extracellular vesicles. The goal of this article is to encourage further studies to construct a comprehensive proteome database for extracellular vesicles that will help us to not only decode the biogenesis and cargo-sorting mechanisms during vesicle formation but also elucidate the pathophysiological roles of these complex extracellular organelles.

Different states of integrin LFA-1 aggregation are controlled through its association with tetraspanin CD9

The tetraspanin CD9 has been shown to interact with different members of the β1 and β3 subfamilies of integrins, regulating through these interactions cell adhesion, migration and signaling. Based on confocal microscopy co-localization and on co-immunoprecipitation results, we report here that CD9 associates with the β2 integrin LFA-1 in different types of leukocytes including T, B and monocytic cells. This association is resistant to stringent solubilization conditions which, together with data from chemical crosslinking, in situ Proximity Ligation Assays and pull-down experiments, suggest a primary/direct type of interaction mediated by the Large Extracellular Loop of the tetraspanin. CD9 exerts inhibitory effects on the adhesive function of LFA-1 and on LFA-1-dependent leukocyte cytotoxic activity. The mechanism responsible for this negative regulation exerted by CD9 on LFA-1 adhesion does not involve changes in the affinity state of this integrin but seems to be related to alterations in its state of aggregation.

Coronavirus and influenza virus proteolytic priming takes place in tetraspanin-enriched membrane microdomains

Coronaviruses (CoVs) and low-pathogenicity influenza A viruses (LP IAVs) depend on target cell proteases to cleave their viral glycoproteins and prime them for virus-cell membrane fusion. Several proteases cluster into tetraspanin-enriched microdomains (TEMs), suggesting that TEMs are preferred virus entry portals. Here we found that several CoV receptors and virus-priming proteases were indeed present in TEMs. Isolated TEMs, when mixed with CoV and LP IAV pseudoparticles, cleaved viral fusion proteins to fusion-primed fragments and potentiated viral transductions. That entering viruses utilize TEMs as a protease source was further confirmed using tetraspanin antibodies and tetraspanin short hairpin RNAs (shRNAs). Tetraspanin antibodies inhibited CoV and LP IAV infections, but their virus-blocking activities were overcome by expressing excess TEM-associated proteases. Similarly, cells with reduced levels of the tetraspanin CD9 resisted CoV pseudoparticle transductions but were made susceptible by overproducing TEM-associated proteases. These findings indicated that antibodies and CD9 depletions interfere with viral proteolytic priming in ways that are overcome by surplus proteases. TEMs appear to be exploited by some CoVs and LP IAVs for appropriate coengagement with cell receptors and proteases.

IMPORTANCE

Enveloped viruses use their surface glycoproteins to catalyze membrane fusion, an essential cell entry step. Host cell components prime these viral surface glycoproteins to catalyze membrane fusion at specific times and places during virus cell entry. Among these priming components are proteases, which cleave viral surface glycoproteins, unleashing them to refold in ways that catalyze virus-cell membrane fusions. For some enveloped viruses, these proteases are known to reside on target cell surfaces. This research focuses on coronavirus and influenza A virus cell entry and identifies TEMs as sites of viral proteolysis, thereby defining subcellular locations of virus priming with greater precision. Implications of these findings extend to the use of virus entry antagonists, such as protease inhibitors, which might be most effective when localized to these microdomains.

Tetraspanins as therapeutic targets in hematological malignancy: a concise review

Tetraspanins belong to a family of transmembrane proteins which play a major role in the organization of the plasma membrane. While all immune cells express tetraspanins, most of these are present in a variety of other cell types. There are a select few, such as CD37 and CD53, which are restricted to hematopoietic lineages. Tetraspanins associate with numerous partners involved in a diverse set of biological processes, including cell activation, survival, proliferation, adhesion, and migration. The historical view has assigned them a scaffolding role, but recent discoveries suggest some tetraspanins can directly participate in signaling through interactions with cytoplasmic proteins. Given their potential roles in supporting tumor survival and immune evasion, an improved understanding of tetraspanin activity could prove clinically valuable. This review will focus on emerging data in the study of tetraspanins, advances in the clinical development of anti-CD37 therapeutics, and the future prospects of targeting tetraspanins in hematological malignancy.

Distinct Transcriptomic Features are Associated with Transitional and Mature B-Cell Populations in the Mouse Spleen

Splenic transitional B-cells (T1 and T2) are selected to avoid self-reactivity and to safeguard against autoimmunity, then differentiate into mature follicular (FO-I and FO-II) and marginal zone (MZ) subsets. Transcriptomic analysis by RNA-seq of the five B-cell subsets revealed T1 cell signature genes included RAG suggesting a potential for receptor revision. T1 to T2 B-cell differentiation was marked by a switch from Myb to Myc, increased expression of the PI3K adapter DAP10 and MHC class II. FO-II may be an intermediate in FO-I differentiation and may also become MZ B-cells as suggested by principle component analysis. MZ B-cells possessed the most distinct transcriptome including down-regulation of CD45 phosphatase-associated protein (CD45-AP/PTPRC-AP), as well as upregulation of IL-9R and innate molecules TLR3, TLR7, and bactericidal Perforin-2 (MPEG1). Among the endosomal TLRs, stimulation via TLR3 further enhanced Perforin-2 expression exclusively in MZ B-cells. Using gene-deleted and overexpressing transgenic mice we show that IL-9/IL-9R interaction resulted in rapid activation of STAT1, 3, and 5, primarily in MZ B-cells. Importantly, CD45-AP mutant mice had reduced transitional and increased mature MZ and FO B-cells, suggesting that it prevents premature entry of transitional B-cells to the mature B-cell pool or their survival and proliferation. Together, these findings suggest, developmental plasticity among splenic B-cell subsets, potential for receptor revision in peripheral tolerance whereas enhanced metabolism coincides with T2 to mature B-cell differentiation. Further, unique core transcriptional signatures in MZ B-cells may control their innate features.

Distinct regions of the large extracellular domain of tetraspanin CD9 are involved in the control of human multinucleated giant cell formation

Multinucleated giant cells, formed by the fusion of monocytes/macrophages, are features of chronic granulomatous inflammation associated with infections or the persistent presence of foreign material. The tetraspanins CD9 and CD81 regulate multinucleated giant cell formation: soluble recombinant proteins corresponding to the large extracellular domain (EC2) of human but not mouse CD9 can inhibit multinucleated giant cell formation, whereas human CD81 EC2 can antagonise this effect. Tetraspanin EC2 are all likely to have a conserved three helix sub-domain and a much less well-conserved or hypervariable sub-domain formed by short helices and interconnecting loops stabilised by two or more disulfide bridges. Using CD9/CD81 EC2 chimeras and point mutants we have mapped the specific regions of the CD9 EC2 involved in multinucleated giant cell formation. These were primarily located in two helices, one in each sub-domain. The cysteine residues involved in the formation of the disulfide bridges in CD9 EC2 were all essential for inhibitory activity but a conserved glycine residue in the tetraspanin-defining 'CCG' motif was not. A tyrosine residue in one of the active regions that is not conserved between human and mouse CD9 EC2, predicted to be solvent-exposed, was found to be only peripherally involved in this activity. We have defined two spatially-distinct sites on the CD9 EC2 that are required for inhibitory activity. Agents that target these sites could have therapeutic applications in diseases in which multinucleated giant cells play a pathogenic role.

Sperm-egg fusion: a molecular enigma of mammalian reproduction

The mechanism of gamete fusion remains largely unknown on a molecular level despite its indisputable significance. Only a few of the molecules required for membrane interaction are known, among them IZUMO1, which is present on sperm, tetraspanin CD9, which is present on the egg, and the newly found oolema protein named Juno. A concept of a large multiprotein complex on both membranes forming fusion machinery has recently emerged. The Juno and IZUMO1, up to present, is the only known extracellular receptor pair in the process of fertilization, thus, facilitating the essential binding of gametes. However, neither IZUMO1 nor Juno appears to be the fusogenic protein. At the same time, the tetraspanin is expected to play a role in organizing the egg membrane order and to interact laterally with other factors. This review summarizes, to present, the known molecules involved in the process of sperm-egg fusion. The complexity and expected redundancy of the involved factors makes the process an intricate and still poorly understood mechanism, which is difficult to comprehend in its full distinction.

Outsmart tumor exosomes to steal the cancer initiating cell its niche

Exosomes are small vesicles that derive from endosomes and are delivered by many cells, including tumor cells that are a particular rich source of exosomes. Exosomes are suggested to be the most potent intercellular communicators. Being recovered in all body fluids, they can communicate with neighboring as well as distant cells. The latter was first described for dendritic cell exosomes that can initiate T cell activation. However, tumor exosomes (TEX) may impede this crosstalk. Besides with hematopoietic cells, TEX communicate with the tumor cell itself, but also with host stroma cells and endothelial cells. This crosstalk received much attention as there is strong evidence that TEX account for angiogenesis and premetastatic niche formation, which may proceed directly via binding and uptake of TEX by cells in the premetastatic organ or indirectly via TEX being taken up by hematopoietic progenitors in the bone marrow (BM), which mature toward lineages with immunosuppressive features or are forced toward premature release from the BM and homing into premetastatic organs. Knowing these deleterious activities of TEX, it becomes demanding to search for modes of therapeutic interference. I here introduce our hypothesis that metastasis formation may be hampered by tailored exosomes that outsmart TEX. The essential prerequisites are an in depth knowledge on TEX binding, uptake, binding-initiated signal transduction and uptake-promoted target cell reprogramming.

CD81-receptor associations--impact for hepatitis C virus entry and antiviral therapies

Tetraspanins are integral transmembrane proteins organized in microdomains displaying specific and direct interactions with other tetraspanins and molecular partners. Among them, CD81 has been implicated in a variety of physiological and pathological processes. CD81 also plays a crucial role in pathogen entry into host cells, including hepatitis C virus (HCV) entry into hepatocytes. HCV is a major cause of liver cirrhosis and hepatocellular carcinoma. HCV entry into hepatocytes is a complex process that requires the coordinated interaction of viral and host factors for the initiation of infection, including CD81, scavenger receptor BI, claudin-1, occludin, membrane-bound host cell kinases, Niemann-Pick C1 Like 1, Harvey rat sarcoma viral oncogene homolog (HRas), CD63 and transferrin receptor 1. Furthermore, recent data in HCV model systems have demonstrated that targeting critical components of tetraspanins and associated cell membrane proteins open new avenues to prevent and treat viral infection.

Recent advances in HCV entry

ABSTRACT: 

The elucidation of the mechanisms by which HCV infects hepatocytes and replicates has been paramount for identifying therapeutic targets and developing the highly efficacious antiviral drugs from which we benefit today. The earliest stage of HCV infection is viral entry, a process in which a complex interplay is thought to occur between host molecules (including glycosaminoglycans, low-density lipoprotein receptor, CD81, SR-B1, CLDN1, OCLN, EGF receptor, ephrin type A receptor 2 and transferrin receptor 1) and envelope viral glycoproteins E1 and E2. The wealth of experimental data produced in the field of HCV entry is summarized in a proposed mechanism, updated to include the most recently published data on the topic. Compounds with putative entry-blocking and/or entry-inhibiting activity in vitro and in vivo are also briefly reviewed.

Palmitoylated transmembrane adaptor proteins in leukocyte signaling

Transmembrane adaptor proteins (TRAPs) are structurally related proteins that have no enzymatic function, but enable inducible recruitment of effector molecules to the plasma membrane, usually in a phosphorylation dependent manner. Numerous surface receptors employ TRAPs for either propagation or negative regulation of the signal transduction. Several TRAPs (LAT, NTAL, PAG, LIME, PRR7, SCIMP, LST1/A, and putatively GAPT) are known to be palmitoylated that could facilitate their localization in lipid rafts or tetraspanin enriched microdomains. This review summarizes expression patterns, binding partners, signaling pathways, and biological functions of particular palmitoylated TRAPs with an emphasis on the three most recently discovered members, PRR7, SCIMP, and LST1/A. Moreover, we discuss in silico methodology used for discovery of new family members, nature of their binding partners, and microdomain localization.

Toward atomic force microscopy and mass spectrometry to visualize and identify lipid rafts in plasmodesmata

Plant cell-to-cell communication is mediated by nanopores called plasmodesmata (PDs) which are complex structures comprising plasma membrane (PM), highly packed endoplasmic reticulum and numerous membrane proteins. Although recent advances on proteomics have led to insights into mechanisms of transport, there is still an inadequate characterization of the lipidic composition of the PM where membrane proteins are inserted. It has been postulated that PDs could be formed by lipid rafts, however no structural evidence has shown to visualize and analyse their lipid components. In this perspective article, we discuss proposed experiments to characterize lipid rafts and proteins in the PDs. By using atomic force microscopy (AFM) and mass spectrometry (MS) of purified PD vesicles it is possible to determine the presence of lipid rafts, specific bound proteins and the lipidomic profile of the PD under physiological conditions and after changing transport permeability. In addition, MS can determine the stoichiometry of intact membrane proteins inserted in lipid rafts. This will give novel insights into the role of membrane proteins and lipid rafts on the PD structure.

Characterization of hydrophobic peptides in the presence of detergent by photoionization mass spectrometry

The characterization of membrane proteins is still challenging. The major issue is the high hydrophobicity of membrane proteins that necessitates the use of detergents for their extraction and solubilization. The very poor compatibility of mass spectrometry with detergents remains a tremendous obstacle in studies of membrane proteins. Here, we investigated the potential of atmospheric pressure photoionization (APPI) for mass spectrometry study of membrane proteins. This work was focused on the tetraspanin CD9 and the multidrug transporter BmrA. A set of peptides from CD9, exhibiting a broad range of hydropathicity, was investigated using APPI as compared to electrospray ionization (ESI). Mass spectrometry experiments revealed that the most hydrophobic peptides were hardly ionized by ESI whereas all peptides, including the highly hydrophobic one that corresponds to the full sequence of the first transmembrane domain of CD9, were easily ionized by APPI. The native protein BmrA purified in the presence of the non-ionic detergent beta-D-dodecyl maltoside (DDM) was digested in-solution using trypsin. The resulting peptides were investigated by flow injection analysis of the mixture followed by mass spectrometry. Upon ESI, only detergent ions were detected and the ionic signals from the peptides were totally suppressed. In contrast, APPI allowed many peptides distributed along the sequence of the protein to be detected. Furthermore, the parent ion corresponding to the first transmembrane domain of the protein BmrA was detected under APPI conditions. Careful examination of the APPI mass spectrum revealed a-, b-, c- and y- fragment ions generated by in-source fragmentation. Those fragment ions allowed unambiguous structural characterization of the transmembrane domain. In conclusion, APPI-MS appears as a versatile method allowing the ionization and fragmentation of hydrophobic peptides in the presence of detergent.

Microdomains in the membrane landscape shape antigen-presenting cell function

The plasma membrane of immune cells is a highly organized cell structure that is key to the initiation and regulation of innate and adaptive immune responses. It is well-established that immunoreceptors embedded in the plasma membrane have a nonrandom spatial distribution that is important for coupling to components of intracellular signaling cascades. In the last two decades, specialized membrane microdomains, including lipid rafts and TEMs, have been identified. These domains are preformed structures ("physical entities") that compartmentalize proteins, lipids, and signaling molecules into multimolecular assemblies. In APCs, different microdomains containing immunoreceptors (MHC proteins, PRRs, integrins, among others) have been reported that are imperative for efficient pathogen recognition, the formation of the immunological synapse, and subsequent T cell activation. In addition, recent work has demonstrated that tetraspanin microdomains and lipid rafts are involved in BCR signaling and B cell activation. Research into the molecular mechanisms underlying membrane domain formation is fundamental to a comprehensive understanding of membrane-proximal signaling and APC function. This review will also discuss the advances in the microscopy field for the visualization of the plasma membrane, as well as the recent progress in targeting microdomains as novel, therapeutic approach for infectious and malignant diseases.

HRas signal transduction promotes hepatitis C virus cell entry by triggering assembly of the host tetraspanin receptor complex

Hepatitis C virus (HCV) entry is dependent on coreceptor complex formation between the tetraspanin superfamily member CD81 and the tight junction protein claudin-1 (CLDN1) on the host cell membrane. The receptor tyrosine kinase EGFR acts as a cofactor for HCV entry by promoting CD81-CLDN1 complex formation via unknown mechanisms. We identify the GTPase HRas, activated downstream of EGFR signaling, as a key host signal transducer for EGFR-mediated HCV entry. Proteomic analysis revealed that HRas associates with tetraspanin CD81, CLDN1, and the previously unrecognized HCV entry cofactors integrin β1 and Ras-related protein Rap2B in hepatocyte membranes. HRas signaling is required for lateral membrane diffusion of CD81, which enables tetraspanin receptor complex assembly. HRas was also found to be relevant for entry of other viruses, including influenza. Our data demonstrate that viruses exploit HRas signaling for cellular entry by compartmentalization of entry factors and receptor trafficking.

Colorectal cancer defeating? Challenge accepted!

Colorectal tumours are actually considered as aberrant organs, within it is possible to notice a different stage of cell growth and differentiation. Their origin is reported to arise from a subpopulation of tumour cells endowed with, just like the healthy stem cells, self-renewal and aberrant multi-lineage differentiation capacity likely to be called colorectal cancer stem cells (CCSCs). Cancer stem cells (CSCs) fate, since their origin, reflects the influences from their microenvironment (or niche) both in the maintenance of stemness, in promoting their differentiation, and in inducing epithelial-mesenchymal transition, responsible of CSCs dissemination and subsequent formation of metastatic lesions. The tumour cells heterogeneity and their immuno-response resistance nowadays probably responsible of the failure of the conventional therapies, make this research field an open issue. Even more importantly, our increasing understanding of the cellular and molecular mechanisms that regulate CSC quiescence and cell cycle regulation, self-renewal, chemotaxis and resistance to cytotoxic agents, is expected to eventually result in tailor-made therapies with a significant impact on the morbidity and overall survival of colorectal cancer patients.

Substrate-attached materials are enriched with tetraspanins and are analogous to the structures associated with rear-end retraction in migrating cells

Substrate-attached materials (SAMs) are cellular feet that remain on substrates after the treatment of adherent cells with EGTA. SAMs are thought to contain cell adhesion machineries, but their biochemical properties have not been addressed in detail. To gain insight into the molecular mechanisms operating in cell adhesions, we comprehensively identified the protein components of SAMs by liquid chromatography coupled with tandem mass spectrometry, followed by immunoblot analysis. We found that the tetraspanins CD9, CD81, and CD151 were enriched in SAMs along with other transmembrane proteins that are known to associate with tetraspanins. Notably, integrins were detected in SAMs, but the components of focal adhesions were scarcely detected. These observations are reminiscent of the “footprints” that remain on substrates when the retraction fibers at the rear of migrating cells are released, because such footprints have been reported to contain tetraspanins and integrins but not focal adhesion proteins. In support of this hypothesis, the formation of SAMs was attenuated by inhibitors of ROCK, myosin II and dynamin, all of which are known to participate in rear-end retraction in migrating cells. Furthermore, SAMs left on collagen-coated substrates were found by electron microscopy to be fewer and thinner than those on laminin-coated substrates, reflecting the thin and fragile retraction fibers of cells migrating on collagen. Collectively, these results indicate that SAMs closely resemble the footprints and retraction fibers of migrating cells in their protein components, and that they are yielded by similar mechanisms.

EpCAM: structure and function in health and disease

Injection of tumor cells in mice more than 30 years ago resulted in the discovery of an epithelial antigen, later defined as a cell adhesion molecule (EpCAM). Although EpCAM has since evoked significant interest as a target in cancer therapy, mechanistic insights on the functions of this glycoprotein have been emerging only very recently. This may have been caused by the multitude of functions attributed to the glycoprotein, its localization at different subcellular sites and complex posttranslational modifications. Here, we review how EpCAM modifies cell-cell contact adhesion strength and tissue plasticity, and how it regulates cell proliferation and differentiation. Major knowledge derived from human diseases will be highlighted: Mutant EpCAM that is absent from the cell surface leads to fatal intestinal abnormalities (congenital tufting enteropathy). EpCAM-mediated cell proliferation in cancer may result from signaling (i) via regulated intramembrane proteolysis and/or (ii) the localization and association with binding partners in specialized membrane microdomains. New insight in EpCAM signaling will help to develop optimized cancer therapies and open new avenues in the field of regenerative medicine.

EpCAM proteolysis: new fragments with distinct functions?

EpCAM [epithelial cell adhesion molecule; CD326 (cluster of differentiation 326)] is highly expressed on epithelium-derived tumours and can play a role in cell proliferation. Recently, RIP (regulated intramembrane proteolysis) has been implicated as the trigger for EpCAM-mediated proliferative signalling. However, RIP does not explain all EpCAM-derived protein fragments. To shed light on how proteolytic cleavage is involved in EpCAM signalling, we characterized the protein biochemically using antibodies binding to three different EpCAM domains. Using a newly generated anti-EpCAM antibody, we find that EpCAM can be cleaved at multiple positions within its ectodomain in addition to described peptides, revealing that EpCAM is processed via distinct proteolytic pathways. Here, we report on four new peptides, but also discuss the previously described cleavage products to provide a comprehensive picture of EpCAM cleavage at multiple positions. The complex regulation of EpCAM might not only result in the absence of full-length EpCAM, but the newly formed EpCAM-derived proteins may have their own signalling properties.

The tetraspanin network modulates MT1-MMP cell surface trafficking

The membrane-type 1 matrix metalloproteinase (MT1-MMP) drives fundamental physiological and pathophysiological processes. Among other substrates, MT1-MMP cleaves components of the extracellular matrix and activates other matrix-cleaving proteases such as MMP-2. Trafficking is a highly effective means to modulate MT1-MMP cell surface expression, and hence regulate its function. Here, we describe the complex interaction of MT1-MMP with tetraspanins, their effects on MT1-MMP intracellular trafficking and proteolytic function. Tetraspanins are credited as membrane organizers that form a network within the membrane to regulate the trafficking of associated proteins. In short, we found MT1-MMP to interact with the tetraspanin-associated EWI-2 protein by a yeast two-hybrid screen. Immunoprecipitation analysis confirmed this interaction and further revealed that MT1-MMP also stably interacts with distinct tetraspanins (CD9, CD37, CD53, CD63, CD81, and CD82) and the tetraspanin-like MAL protein. By using different MT1-MMP truncation constructs and mutants, we observed that all tetraspanins and MAL associated with the hemopexin domain of MT1-MMP. Moreover, this interaction was independent of O-glycosylation of MT1-MMP and exclusively occurred in the endoplasmic reticulum. Here, the respective subcellular compartment was identified by fitting the MT1-MMP interaction pattern to a model for post-translational processing of MT1-MMP. In addition, tetraspanins differentially affected the cell surface localization of MT1-MMP, its capacity to activate pro-MMP-2, and the collagen invasion capacity. Interestingly, the degree of tetraspanin-MT1-MMP association did not correlate with its impact on MT1-MMP function. Tetraspanins thus distinctly affect MT1-MMP subcellular localization and function, and may constitute an effective mechanism to control MT1-MMP-dependent proteolysis at the cell surface.

EpCAM-associated claudin-7 supports lymphatic spread and drug resistance in rat pancreatic cancer

Pancreatic cancer has a dismal prognosis because of early metastatic spread, a suggested feature of cancer-initiating cells (CIC). To control for a functional contribution of the pancreatic CIC-marker EpCAM, we explored metastasis formation by a stable EpCAM-knockdown (ASML-EpC(kd)) of the rat pancreatic adenocarcinoma line BSp73ASML (ASML(wt)). As EpCAM associates with claudin-7, an ASML-claudin-7-knockdown (ASML-cld7(kd)) was included to differentiate between EpC- and EpC-cld7-mediated effects. The metastatic capacity of ASML-EpC(kd) and more pronounced ASML-cld7(kd) cells is strikingly reduced. EpC-associated cld7 interferes with EpC-mediated cell-cell adhesion and supports migration. This requires cld7 phosphorylation and formation of an EpC-cld7-tetraspanin-alpha6beta4 complex in glycolipid-enriched membrane domains (GEM), where cld7 associates via the tetraspanin-alpha6beta4 complex with phosphorylated ezrin. The association of cld7 with alpha6beta4 and cytoskeleton strongly stimulates tumor cell migration. However, EpC does not actively contribute. Instead, GEM-located cld7 associates with presenilin-2, which facilitates EpC cleavage and thereby tumor cell proliferation. Finally, the EpC-cld7 complex promotes drug resistance. Both EpC and cld7 support MAPK and JNK activation, such that in ASML-EpC(kd) and ASML-cld7(kd) cells an undue expansion of proapoptotic molecules is observed. Only cld7 promotes activation of the PI3K/Akt pathway by a strong downregulation of Pten. Accordingly, cisplatin treatment prolongs the survival time of ASML-cld7(kd)-bearing rats. Taken together, cld7 supports tumorigenic features of EpC by provoking EpC cleavage and thereby its cotranscription factor activity. On the other hand, only cld7 is directly engaged in motility and apoptosis resistance. Thus, at least in concern of migrating CIC, it is cld7 that acts as a CIC biomarker.

The intracellular interactome of tetraspanin-enriched microdomains reveals their function as sorting machineries toward exosomes

Extracellular vesicles are emerging as a potent mechanism of intercellular communication because they can systemically exchange genetic and protein material between cells. Tetraspanin molecules are commonly used as protein markers of extracellular vesicles, although their role in the unexplored mechanisms of cargo selection into exosomes has not been addressed. For that purpose, we have characterized the intracellular tetraspanin-enriched microdomain (TEM) interactome by high throughput mass spectrometry, in both human lymphoblasts and their derived exosomes, revealing a clear pattern of interaction networks. Proteins interacting with TEM receptors cytoplasmic regions presented a considerable degree of overlap, although some highly specific CD81 tetraspanin ligands, such as Rac GTPase, were detected. Quantitative proteomics showed that TEM ligands account for a great proportion of the exosome proteome and that a selective repertoire of CD81-associated molecules, including Rac, is not correctly routed to exosomes in cells from CD81-deficient animals. Our data provide evidence that insertion into TEM may be necessary for protein inclusion into the exosome structure.