Domain analysis of the tetraspanins: studies of CD9/CD63 chimeric molecules on subcellular localization and upregulation activity for diphtheria toxin binding

Diagnostic and Prognostic Utility of the Extracellular Vesicles Subpopulations Present in Pleural Effusion

Extracellular vesicles (EVs), comprising exosomes, microvesicles, and apoptotic bodies, are released by all cells into the extracellular matrix and body fluids, where they play important roles in intercellular communication and matrix remodeling in various pathological conditions. Malignant pleural mesothelioma (MPM) is a primary tumor of mesothelial origin, predominantly related to asbestos exposure. The detection of MPM at an early stage and distinguishing it from benign conditions and metastatic adenocarcinomas (AD) is sometimes challenging. Pleural effusion is often the first available biological material and an ideal source for characterizing diagnostic and prognostic factors. Specific proteins have previously been identified as diagnostic markers in effusion, but it is not currently known whether these are associated with vesicles or released in soluble form. Here, we study and characterize tumor heterogeneity and extracellular vesicle diversity in pleural effusion as diagnostic or prognostic markers for MPM. We analyzed extracellular vesicles and soluble proteins from 27 pleural effusions, which were collected and processed at the department of pathology and cytology at Karolinska University Hospital, representing three different patient groups, MPM (n = 9), benign (n = 6), and AD (n = 12). The vesicles were fractionated into apoptotic bodies, microvesicles, and exosomes by differential centrifugation and characterized by nanoparticle tracking analysis and Western blotting. Multiplex bead-based flow cytometry analysis showed that exosomal markers were expressed differently on EVs present in different fractions. Further characterization of exosomes by a multiplex immunoassay (Luminex) showed that all soluble proteins studied were also present in exosomes, though the ratio of protein concentration present in supernatant versus exosomes varied. The proportion of Angiopoietin-1 present in exosomes was generally higher in benign compared to malignant samples. The corresponding ratios of Mesothelin, Galectin-1, Osteopontin, and VEGF were higher in MPM effusions compared to those in the benign group. These findings demonstrate that relevant diagnostic markers can be recovered from exosomes.

Extracellular vesicles are the primary source of blood-borne tumour-derived mutant KRAS DNA early in pancreatic cancer

Up to now, the field of liquid biopsies has focused on circulating tumour DNA and cells, though extracellular vesicles (EVs) have been of increasing interest in recent years. Thus, reported sources of tumour-derived nucleic acids include leukocytes, platelets and apoptotic bodies (AB), as well as large (LEV) and small (SEV) EVs. Despite these competing claims, there has yet to be a standardized comparison of the tumour-derived DNA associated with different components of blood. To address this issue, we collected twenty-three blood samples from seventeen patients with pancreatic cancers of known mutant KRAS G12 genotype, and divided them into two groups based on the time of patient survival following sampling. After collecting red and white blood cells, we subjected 1 ml aliquots of platelet rich plasma to differential centrifugation in order to separate the platelets, ABs, LEVs, SEVs and soluble proteins (SP) present. We then confirmed the enrichment of specific blood components in each differential centrifugation fraction using electron microscopy, Western blotting, nanoparticle tracking analysis and bead-based multiplex flow cytometry assays. By targeting wild type and tumour-specific mutant KRAS alleles using digital PCR, we found that the levels of mutant KRAS DNA were highest in association with LEVs and SEVs early, and with SEVs and SP late in disease progression. Importantly, we established that SEVs were the most enriched in tumour-derived DNA throughout disease progression, and verified this association using size exclusion chromatography. This work provides important direction for the rapidly expanding field of liquid biopsies by supporting an increased focus on EVs as a source of tumour-derived DNA.

The roles of tetraspanins in bacterial infections

Tetraspanins, a wide family composed of 33 transmembrane proteins, are associated with different types of proteins through which they arbitrate important cellular processes such as fusion, adhesion, invasion, tissue differentiation and immunological responses. Tetraspanins share a comparable structural design, which consists of four hydrophobic transmembrane domains with cytoplasmic and extracellular loops. They cooperate with different proteins, including other tetraspanins, receptors or signalling proteins to compose functional complexes at the cell surface, designated tetraspanin-enriched microdomains (TEM). Increasing evidences establish that tetraspanins are exploited by numerous intracellular pathogens as a doorway for entering and replicating within human cells. Although previous surveys focused mainly on viruses and parasites, it is now becoming clear that bacteria interact with tetraspanins, using TEM as a "gateway" to infection. In this review, we examine the biological functions of tetraspanins that are relevant to bacterial infective procedures and consider the available data that reveal how different bacteria benefit from host cell tetraspanins in infection and in the pathogenesis of diseases. We will also emphasise the stimulating potentials of targeting tetraspanins for preventing bacterial infectious diseases, using specific neutralising antibodies or anti-adhesion peptide-based therapies. Such innovative therapeutic opportunities may deliver alternatives for fighting difficult-to-manage and drug-resistant bacterial pathogens.

Tetraspanin protein CD9 interacts with metalloprotease CD10 and enhances its release via exosomes

Tetraspanins interact with a wide variety of transmembrane and intracellular proteins called molecular partners, and modulate their function. In this article, we describe a new partner of tetraspanin web, membrane metalloprotease CD10, which is selectively associated with CD9. By constructing chimeras between tetraspanins CD9 and CD82 (the latter does not interact with CD10) or by using site-directed mutagenesis, we determined that a portion of the large extracellular loop from the CCG motif to transmembrane domain 4, as well as the C-terminal tail of CD9, are involved in the interaction with CD10. The stable expression of wild-type CD9 in K562 CD10-positive cells enhanced the level of CD10 released with exosomes five-fold. In contrast, the expression of chimeric CD9, which contained the cytoplasmic C-terminal domain from CD82, had little effect on CD10 release. Short hairpin RNA knockdown of CD9 expression in Nalm-6 pre-B cells resulted in a two-fold reduction in the amount of endogenous CD10 released with microvesicles. The peptidase activity of CD10 measured either on cells or on exosomes correlated with the level of CD10 expression, and was not significantly modulated by CD9 expression as such. Our data suggest that the interaction of CD10 with tetraspanin CD9 can play an important role in the redistribution of peptidase activity from the cell surface to outer microenvironments. In bone marrow, where CD10 presumably contributes to the maturation of pre-B cells and migration of B cells to the blood circulation, release of CD10 peptidase activity with exosomes may effectively regulate extracellular matrix microenvironments.

The C-terminal tail of tetraspanin protein CD9 contributes to its function and molecular organization

Tetraspanin protein CD9 supports sperm-egg fusion, and regulates cell adhesion, motility, metastasis, proliferation and signaling. The large extracellular loop and transmembrane domains of CD9 engage in functionally important interactions with partner proteins. However, neither functional nor biochemical roles have been shown for the CD9 C-terminal tail, despite it being highly conserved throughout vertebrate species. To gain new insight into the CD9 tail, three C-terminal amino acids (Glu-Met-Val) were replaced with residues corresponding to C-terminal amino acids from tetraspanin protein CD82 (Pro-Lys-Tyr). Wild-type and mutant CD9 were then stably expressed in MOLT-4, K562, U937, RD and HT1080 cells. Whereas wild-type CD9 inhibited cell adhesion and spreading on fibronectin, mutant CD9 did not. Wild-type CD9 also promoted homotypic cell-cell aggregation and microvilli formation, whereas mutant CD9 did not. Protein interactions of wild-type and mutant CD9 were compared quantitatively using stable isotope labeling with amino acids in cell culture (SILAC) in conjunction with liquid-chromatography-tandem mass spectrometry (LC-MS/MS) technology. SILAC results showed that, despite wild-type and mutant CD9 having identical expression levels, mutant CD9 and its major transmembrane interacting partners were recovered in substantially reduced amounts from 1% Brij 96 lysates. Immunoprecipitation experiments confirmed that mutant CD9 recovery was decreased in Brij 96, but not in more stringent Triton X-100 detergent. Additionally, compared with wild-type CD9 complexes, mutant CD9 complexes were larger and more oligomerized in Brij 96 detergent, consistent with decreased Brij 96 solubility, perhaps due to more membrane domains packing more tightly together. In conclusion, multiple CD9 functions depend on its C-terminal tail, which affects the molecular organization of CD9 complexes, as manifested by their altered solubilization in Brij 96 and organization on the cell surface.

Tetraspanins as regulators of protein trafficking

Small transmembrane proteins of the tetraspanin superfamily are believed to function as the main structural blocks of specialized membrane microdomains (referred to as tetraspanin-enriched microdomains, TERM or TEM). Through a multitude of homotypic and heterotypic interactions, tetraspanins regulate lateral clustering and, consequently, signalling involving adhesion and growth factor receptors as well as costimulatory proteins. The presence of major histocompatibility complex (MHC) I and MHCII molecules in TERM led to suggestion of tetraspanins' involvement in antigen presentation. In addition, certain tetraspanins function as viral co-receptors and may be important for viral egress from infected cells. It has recently become apparent that in addition to their purely structural function as organizers of TERM, tetraspanins also regulate various aspects of trafficking and biosynthetic processing of associated receptors. Here, we review recent studies, which specifically focus on this issue.

The tetraspanin web modulates immune-signalling complexes

The tetraspanin web represents a new concept of molecular interactions in the immune system. Whereas most surface immune-modulating molecules involve receptor-ligand interactions, tetraspanins associate with partner proteins and facilitate their lateral positioning in the membrane. Moreover, the same tetraspanin molecule can associate with different proteins depending on the cell type. Most importantly, members of this family tend to associate with each other, together with their partners, in membrane microdomains that provide a scaffold for the transmission of external stimuli to intracellular-signalling components.

CD44 variant isoforms associate with tetraspanins and EpCAM

The metastasizing subline of the rat pancreatic adenocarcinoma BSp73 expresses a set of membrane molecules, the combination of which has not been detected on non-metastasizing tumor lines. Hence, it became of interest whether these molecules function independently or may associate and exert specialized functions as membrane complexes. Separation of CD44v4-v7 containing membrane complexes in mild detergent revealed an association with the alpha3 integrin, annexin I, EpCAM, and the tetraspanins D6.1A and CD9. EpCAM and the tetraspanins associate selectively with CD44 variant (CD44v), but not with the CD44 standard (CD44s) isoform. The complexes are found in glycolipid-enriched membrane (GEM) microdomains, which are dissolved by stringent detergents, but the complexes are not destroyed by methyl-beta-cyclodextrin (MbetaCD) treatment, which implies that complex formation does not depend on a lipid-rich microenvironment. However, a complex-associated impact on cell-matrix and cell-cell adhesion as well as on resistance towards apoptosis essentially depended on the location in GEMs. Thus, CD44v-specific functions may well be brought about by complex formation of CD44v with EpCAM, the tetraspanins, and the alpha3 integrin. Because CD44v4-v7-EpCAM complex-specific functions strictly depended on the GEM localization, linker or signal-transducing molecules associating with the complex are likely located in GEMs.

Identification of CD9 extracellular domains important in regulation of CHO cell adhesion to fibronectin and fibronectin pericellular matrix assembly

CD9, a 24-kDa member of the tetraspanin family, influences cellular growth and development, activation, adhesion, and motility. Our investigation focuses on the hypothesis that the CD9 second extracellular loop (EC2) is important in modulating cell adhesive events. Using a Chinese hamster ovary (CHO) cell expression system, we previously reported that CD9 expression inhibited cell adhesion to fibronectin and fibronectin matrix assembly. For the first time, a functional epitope on CD9 EC2 that regulates these processes is described. Binding of mAb7, an EC2-specific anti-CD9 monoclonal antibody, reversed the CD9 inhibitory activity on CHO cell adhesion and fibronectin matrix assembly. This reversal of cell phenotype also was observed in CHO cells expressing CD9 EC2 truncations. Furthermore, our data showed that the EC2 sequence (173)LETFTVKSCPDAIKEVFDNK(192) was largely responsible for the CD9-mediated CHO cell phenotype. Two peptides, (135)K-V(172) (peptide 5b) and (168)P-I(185) (peptide 6a), selectively blocked mAb7 binding to soluble CD9 and to CD9 on intact cells. These active peptides reversed the influence of CD9 expression on CHO cell adhesion to fibronectin. In addition, confocal microscopy revealed that CD9 colocalized with the integrin alpha(5)beta(1) and cytoskeletal F-actin in punctate clusters on the cell surface, particularly at the cell margins. Immunoprecipitation studies confirmed CD9 association with beta(1) integrin. The cellular distribution and colocalization of focal adhesion kinase and alpha-actinin with cytoskeletal actin was also influenced by CD9 expression. Thus, CD9 may exhibit its effect by modulating the composition of adhesive complexes important in facilitating cell adhesion and matrix assembly.

CD9 amino acids critical for upregulation of diphtheria toxin binding

CD9 associates with a diphtheria toxin receptor (DTR) that is identical to the membrane-anchored form of heparin-binding EGF-like growth factor. We determined the region of CD9 important for upregulation activity. Human and monkey CD9 upregulates DT binding activity of DTR, while mouse CD9 has no upregulation activity. Transfection of chimeric constructs comprising monkey and mouse CD9s showed that the human sequence between Ala156 and Asp183 is essential for the upregulation activity. Studies of mutants, replacing a single amino acid within the region between Ala156 and Asp183 of monkey CD9 with the corresponding amino acid residue in mouse CD9, revealed that substitution of Gly158 is critical for the reduction of the upregulation activity and secondly for the substitution of Val159 and Thr175. These three amino acid residues were deduced to be located on the head domain of the second extracellular loop, suggesting that interactions of CD9 with DTR or DT at the domain containing these three amino acids were important for the upregulation of DT binding.

Structure of the tetraspanin main extracellular domain. A partially conserved fold with a structurally variable domain insertion

The tetraspanin family of membrane glycoproteins is involved in the regulation of cellular development, proliferation, activation, and mobility. We have attempted to predict the structural features of the large extracellular domain of tetraspanins (EC2), which is very important in determining their functional specificity. The tetraspanin EC2 is composed of two subdomains: a conserved three-helix subdomain and a variable secondary structure subdomain inserted within the conserved subdomain. The occurrence of key disulphide bridges and other invariant residues leads to a conserved relative topology of both subdomains and also suggests a structural classification of tetraspanins. Using the CD81 EC2 structure as a template, the structures of two other EC2s were predicted by homology modeling and indicate a conserved shape, in which the variable subdomain is located at one side of the structure. The conserved and variable subdomains might contain sites that correspond, respectively, to common and specific interactions of tetraspanins. The tetraspanin EC2 seems to correspond to a new scheme of fold conservation/variability among proteins, namely the insertion of a structurally variable subdomain within an otherwise conserved fold.