Epitope mapping of anti-rat CD53 monoclonal antibodies. Implications for the membrane orientation of the Transmembrane 4 Superfamily

Morphologic, cytometric, quantitative transcriptomic and functional characterisation provide insights into the haemocyte immune responses of Pacific abalone (Haliotis discus hannai)

In recent years, the abalone aquaculture industry has been threatened by the bacterial pathogens. The immune responses mechanisms underlying the phagocytosis of haemocytes remain unclear in Haliotis discus hannai. It is necessary to investigate the immune mechanism in response to these bacterial pathogens challenges. In this study, the phagocytic activities of haemocytes in H. discus hannai were examined by flow cytometry combined with electron microscopy and transcriptomic analyses. The results of Vibrio parahaemolyticus, Vibrio alginolyticus and Staphylococcus aureu challenge using electron microscopy showed a process during phagosome formation in haemocytes. The phagocytic rate (PP) of S. aureus was higher than the other five foreign particles, which was about 63%. The PP of Vibrio harveyi was about 43%, the PP peak of V. alginolyticus in haemocyte was 63.7% at 1.5 h. After V. parahaemolyticus and V. alginolyticus challenge, acid phosphatase, alkaline phosphatase, total superoxide dismutase, lysozyme, total antioxidant capacity, catalase, nitric oxide synthase and glutathione peroxidase activities in haemocytes were measured at different times, differentially expressed genes (DEGs) were identified by quantitative transcriptomic analysis. The identified DEGs after V. parahaemolyticus challenge included haemagglutinin/amebocyte aggregation factor-like, supervillin-like isoform X4, calmodulin-like and kyphoscoliosis peptidase-like; the identified DEGs after V. alginolyticus challenge included interleukin-6 receptor subunit beta-like, protein turtle homolog B-like, rho GTPase-activating protein 6-like isoform X2, leukocyte surface antigen CD53-like, calponin-1-like, calmodulin-like, troponin C, troponin I-like isoform X4, troponin T-like isoform X18, tumor necrosis factor ligand superfamily member 10-like, rho-related protein racA-like and haemagglutinin/amebocyte aggregation factor-like. Some immune-related KEGG pathways were significantly up-regulated or down-regulated after challenge, including thyroid hormone synthesis, Th17 cell differentiation signalling pathway, focal adhesion, melanogenesis, leukocyte transendothelial migration, inflammatory mediator regulation of TRP channels, ras signalling pathway, rap1 signalling pathway. This study is the first step towards understanding the H. discus hannai immune system by adapting several tools to gastropods and providing a first detailed morpho-functional study of their haemocytes.

The tetraspanin Tspan15 is an essential subunit of an ADAM10 scissor complex

A disintegrin and metalloprotease 10 (ADAM10) is a transmembrane protein essential for embryonic development, and its dysregulation underlies disorders such as cancer, Alzheimer's disease, and inflammation. ADAM10 is a "molecular scissor" that proteolytically cleaves the extracellular region from >100 substrates, including Notch, amyloid precursor protein, cadherins, growth factors, and chemokines. ADAM10 has been recently proposed to function as six distinct scissors with different substrates, depending on its association with one of six regulatory tetraspanins, termed TspanC8s. However, it remains unclear to what degree ADAM10 function critically depends on a TspanC8 partner, and a lack of monoclonal antibodies specific for most TspanC8s has hindered investigation of this question. To address this knowledge gap, here we designed an immunogen to generate the first monoclonal antibodies targeting Tspan15, a model TspanC8. The immunogen was created in an ADAM10-knockout mouse cell line stably overexpressing human Tspan15, because we hypothesized that expression in this cell line would expose epitopes that are normally blocked by ADAM10. Following immunization of mice, this immunogen strategy generated four Tspan15 antibodies. Using these antibodies, we show that endogenous Tspan15 and ADAM10 co-localize on the cell surface, that ADAM10 is the principal Tspan15-interacting protein, that endogenous Tspan15 expression requires ADAM10 in cell lines and primary cells, and that a synthetic ADAM10/Tspan15 fusion protein is a functional scissor. Furthermore, two of the four antibodies impaired ADAM10/Tspan15 activity. These findings suggest that Tspan15 directly interacts with ADAM10 in a functional scissor complex.

Leukocyte Tetraspanin CD53 Restrains α3 Integrin Mobilization and Facilitates Cytoskeletal Remodeling and Transmigration in Mice

The importance of tetraspanin proteins in regulating migration has been demonstrated in many diverse cellular systems. However, the function of the leukocyte-restricted tetraspanin CD53 remains obscure. We therefore hypothesized that CD53 plays a role in regulating leukocyte recruitment and tested this hypothesis by examining responses of CD53-deficient mice to a range of inflammatory stimuli. Deletion of CD53 significantly reduced neutrophil recruitment to the acutely inflamed peritoneal cavity. Intravital microscopy revealed that in response to several inflammatory and chemotactic stimuli, absence of CD53 had only minor effects on leukocyte rolling and adhesion in postcapillary venules. In contrast, Cd53^-/- mice showed a defect in leukocyte transmigration induced by TNF, CXCL1 and CCL2, and a reduced capacity for leukocyte retention on the endothelial surface under shear flow. Comparison of adhesion molecule expression in wild-type and Cd53^-/- neutrophils revealed no alteration in expression of β₂ integrins, whereas L-selectin was almost completely absent from Cd53^-/- neutrophils. In addition, Cd53^-/- neutrophils showed defects in activation-induced cytoskeletal remodeling and translocation to the cell periphery, responses necessary for efficient transendothelial migration, as well as increased α₃ integrin expression. These alterations were associated with effects on inflammation, so that in Cd53^-/- mice, the onset of neutrophil-dependent serum-induced arthritis was delayed. Together, these findings demonstrate a role for tetraspanin CD53 in promotion of neutrophil transendothelial migration and inflammation, associated with CD53-mediated regulation of L-selectin expression, attachment to the endothelial surface, integrin expression and trafficking, and cytoskeletal function.

Tetraspanins Function as Regulators of Cellular Signaling

Tetraspanins are molecular scaffolds that distribute proteins into highly organized microdomains consisting of adhesion, signaling, and adaptor proteins. Many reports have identified interactions between tetraspanins and signaling molecules, finding unique downstream cellular consequences. In this review, we will explore these interactions as well as the specific cellular responses to signal activation, focusing on tetraspanin regulation of adhesion-mediated (integrins/FAK), receptor-mediated (EGFR, TNF-α, c-Met, c-Kit), and intracellular signaling (PKC, PI4K, β-catenin). Additionally, we will summarize our current understanding for how tetraspanin post-translational modifications (palmitoylation, N-linked glycosylation, and ubiquitination) can regulate signal propagation. Many of the studies outlined in this review suggest that tetraspanins offer a potential therapeutic target to modulate aberrant signal transduction pathways that directly impact a host of cellular behaviors and disease states.

Islet inflammation and fibrosis in a spontaneous model of type 2 diabetes, the GK rat

The molecular pathways leading to islet fibrosis in diabetes are unknown. Therefore, we studied gene expression in islets of 4-month-old Goto-Kakizaki (GK) and Wistar control rats. Of 71 genes found to be overexpressed in GK islets, 24% belong to extracellular matrix (ECM)/cell adhesion and 34% to inflammatory/immune response families. Based on gene data, we selected several antibodies to study fibrosis development during progression of hyperglycemia by immunohistochemistry. One-month-old GK and Wistar islets appeared to be similar. Two-month-old GK islets were strongly heterogenous in terms of ECM accumulation compared with Wistar islets. GK islet vascularization, labeled by von Willebrand factor, was altered after 1 month of mild hyperglycemia. Numerous macrophages (major histocompatibility complex class II(+) and CD68(+)) and granulocytes were found in/around GK islets. These data demonstrate that marked inflammatory reaction accompanies GK islet fibrosis and suggest that islet alterations in this nonobese model of type 2 diabetes develop in a way reminiscent of microangiopathy.

Characterization and cloning of T24, a Taenia solium antigen diagnostic for cysticercosis

The third and final diagnostic antigen of the lentil lectin purified glycoproteins (LLGP) extracted from the larval stage of Taenia solium has been characterized, cloned, and expressed. T24 is an integral membrane protein that belongs to the tetraspanin superfamily. It migrates at a position corresponding to 24-kDa and as a homodimer at 42-kDa. Antibodies from cysticercosis patients recognize secondary structure epitopes that are dependent upon correctly formed disulfide bonds. A portion of T24, the large, extracellular loop domain, was expressed in an immunologically reactive form in insect cells. When tested in a Western blot assay with a large battery of serum samples, this protein, T24H, has a sensitivity of 94% (101/107), for detecting cases of cysticercosis with two or more viable cysts, and a specificity of 98% (284/290). The identification and expression of T24H sets the stage for the development of an ELISA suitable for testing single samples and for large-scale serosurveys that is not dependent upon the isolation and purification of antigens from parasite materials.

Contrasting effects of EWI proteins, integrins, and protein palmitoylation on cell surface CD9 organization

CD9, a tetraspanin protein, makes crucial contributions to sperm egg fusion, other cellular fusions, epidermal growth factor receptor signaling, cell motility, and tumor suppression. Here we characterize a low affinity anti-CD9 antibody, C9BB, which binds preferentially to homoclustered CD9. Using mAb C9BB as a tool, we show that cell surface CD9 homoclustering is promoted by expression of alpha3beta1 and alpha6beta4 integrins and by palmitoylation of the CD9 and beta4 proteins. Conversely, CD9 is shifted toward heteroclusters upon expression of CD9 partner proteins (EWI-2 and EWI-F) or other tetraspanins, or upon ablation of CD9 palmitoylation. Furthermore, unpalmitoylated CD9 showed enhanced EWI-2 association, thereby demonstrating a previously unappreciated role for tetraspanin palmitoylation, and underscoring how depalmitoylation and EWI-2 association may collaborate to shift CD9 from homo- to heteroclusters. In conclusion, we have used a novel molecular probe (mAb C9BB) to demonstrate the existence of multiple types of CD9 complex on the cell surface. A shift from homo- to heteroclustered CD9 may be functionally significant because the latter was especially obvious on malignant epithelial tumor cells. Hence, because of its specialized properties, C9BB may be more useful than other anti-CD9 antibodies for monitoring CD9 during tumor progression.

Expression of tetraspanins in peripheral blood leukocytes: a comparison between normal and infectious conditions

The role of tetraspanins is undefined, despite their detection in diverse cell types and functions. This study addresses the characterization of tetraspanin expression levels in normal peripheral blood leukocytes (PBL) and in patients with bacterial infection. Membranal and cytoplasmic expression of CD9, CD53, CD63, CD81, CD82 and CD151 in polymorphonuclears (PMN), monocytes, B and T lymphocytes was assessed using flow cytometry. Results suggested that for normal PBL, PMN are distinguished by dominant cytoplasmic CD63; monocytes and B cells prevailingly express CD53; CD82 is primarily expressed on T-cell membranes. However, a major trend of downregulation was demonstrated for the examined tetraspanins, except CD63, in all patients' PBL subtypes. Therefore, tetraspanin modulation in infections may be attributed to elevated leukocyte motility in immune reactions and this is compatible with the previous publications of tetraspanins as metastasis suppressors. This work represents the first comprehensive baseline of tetraspanin expression in normal PBL and in infectious disorders.

Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain

This review summarizes key aspects of tetraspanin proteins, with a focus on the functional relevance and structural features of these proteins and how they are organized into a novel type of membrane microdomain. Despite the size of the tetraspanin family and their abundance and wide distribution over many cell types, most have not been studied. However, from studies of prototype tetraspanins, information regarding functions, cell biology, and structural organization has begun to emerge. Genetic evidence points to critical roles for tetraspanins on oocytes during fertilization, in fungi during leaf invasion, in Drosophila embryos during neuromuscular synapse formation, during T and B lymphocyte activation, in brain function, and in retinal degeneration. From structure and mutagenesis studies, we are beginning to understand functional subregions within tetraspanins, as well as the levels of connections among tetraspanins and their many associated proteins. Tetraspanin-enriched microdomains (TEMs) are emerging as entities physically and functionally distinct from lipid rafts. These microdomains now provide a context in which to evaluate tetraspanins in the regulation of growth factor signaling and in the modulation of integrin-mediated post-cell adhesion events. Finally, the enrichment of tetraspanins within secreted vesicles called exosomes, coupled with hints that tetraspanins may regulate vesicle fusion and/or fission, suggests exciting new directions for future research.

Characterization of cell types during rat liver development

Hepatic stem cells have been identified in adult liver. Recently, the origin of hepatic progenitors and hepatocytes from bone marrow was demonstrated. Hematopoietic and hepatic stem cells share the markers CD 34, c-kit, and Thy1. Little is known about liver stem cells during liver development. In this study, we investigated the potential stem cell marker Thy1 and hepatocytic marker CK-18 during liver development to identify putative fetal liver stem cell candidates. Livers were harvested from embryonic and fetal day (ED) 16, ED 18, ED 20, and neonatal ED 22 stage rat fetuses from Sprague-Dawley rats. Fetal livers were digested by collagenase-DNAse solution and purified by percoll centrifugation. Magnetic cell sorting (MACS) depletion of fetal liver cells was performed using OX43 and OX44 antibodies. Cells were characterized by immunocytochemistry for Thy1, CK-18, and proliferating cell antigen Ki-67 and double labeling for Thy1 and CK-18. Thy1 expression was found at all stages of liver development before and after MACS in immunocytochemistry. Thy1 positive cells were enriched after MACS only in early developmental stages. An enrichment of CK-18 positive cells was found after MACS at all developmental stages. Cells coexpressing Thy1 and CK-18 were identified by double labeling of fetal liver cell isolates. In conclusion, hepatic progenitor cells (CK-18 positive) in fetal rat liver express Thy1. Other progenitors express only CK-18. This indicates the coexistence of different hepatic cell compartments. Isolation and further characterization of such cells is needed to demonstrate their biologic properties.

Molecular characterisation of mouse and human TSSC6: evidence that TSSC6 is a genuine member of the tetraspanin superfamily and is expressed specifically in haematopoietic organs

Previous analyses of the murine and human TSSC6 (also known as Phemx) proteins were not carried out using the full length sequence. Using 5'-RACE and cDNA library screening, we identified an additional 5' sequence for both the murine Tssc6 cDNA and its human homologue TSSC6. This novel sequence encodes a 5' exon encoding an in frame, upstream start codon, an N-terminal cytoplasmic domain and a transmembrane domain. The deduced, and now full length, murine and human TSSC6 proteins contained four hydrophobic regions together with other features characteristic of the tetraspanin superfamily. Computational analyses of the full length sequences show that TSSC6 is a genuine, albeit relatively divergent member of this superfamily. Using RNA from a number of mouse tissues, we identified seven splice variants of Tssc6. Splice variants of the human gene were also detected. Tssc6 expression was detected early in embryogenesis in primitive blood cells and was confined to haematopoietic organs in the adult mouse. Tssc6 expression was detected in many haematopoietic cell lines and was highest in cell lines of the erythroid lineage.

The molecular characterisation of mouse tspan-3

The Tetraspanin/Transmembrane-4 Superfamily of cell surface molecules is defined by their four highly conserved transmembrane domains and is found in a wide variety of species and cell types. A common function for these molecules has yet to be discovered, however their broad expression patterns and conservation over evolution suggests that they will have an important general function relevant to many cell lineages. Here we describe the cloning and characterisation of the murine homologue of a recently described member of this superfamily, tspan-3. Murine tspan-3 was remarkably similar to the human molecule showing 88% identity at the nucleic acid level and 98% homology on the amino acid level. Northern blot analyses of mouse tspan-3 show a very broad pattern of expression, with expression readily detected in most organs including neural and bone marrow derived tissues.

The L6 membrane proteins--a new four-transmembrane superfamily

L6, IL-TMP, and TM4SF5 are cell surface proteins predicted to have four transmembrane domains. Previous sequence analysis led to their assignment as members of the tetraspanin superfamily. In this paper, we identify a new sequence (L6D) that is strikingly similar to L6, IL-TMP, and TM4SF5. Analyses of these four sequences indicate that they are not significantly related to genuine tetraspanins, but instead constitute their own L6 superfamily.

CD9 expression enhances the susceptibility of myeloma cell lines to cell-mediated cytolysis

Myeloma tumor cells, both freshly excised and cultured, are extremely resistant to cell-mediated cytolysis. As evidence suggests that B-cell susceptibility to lysis is dependent upon its state of differentiation and activation, we tested the ability of a variety of B-cell proliferation and differentiation agents, including pokeweed mitogen (PWM), to enhance the sensitivity of myeloma cells to cell-mediated lysis. PWM was found to significantly enhance the susceptibility of myeloma cell lines and freshly isolated myeloma cells to interleukin-2 (IL-2)–activated cell-mediated cytolysis. This effect was seen with the use of both IL-2–stimulated natural killer (NK) cells and T cells as effectors. The enhanced sensitivity of myeloma cells to cytolysis correlated with an increase in their cell surface expression of CD9, a pre-B cell marker and member of the transmembrane 4 superfamily. Incubation of PWM-stimulated myeloma cells with either monoclonal antibodies or antisense oligonucleotides directed against CD9 abrogated the effect of PWM. In order to determine whether there was a direct relationship between the expression of CD9 and enhanced sensitivity to cytolysis, myeloma cell lines that lacked CD9 expression were transfected with the CD9 gene. The level of cell surface CD9 expression correlates with enhanced susceptibility to lysis. Therefore, CD9 appears to be an important component in enhancing the sensitivity of myeloma cells to lysis mediated by IL-2–activated T cells and NK cells.

CD9 expression enhances the susceptibility of myeloma cell lines to cell-mediated cytolysis

Myeloma tumor cells, both freshly excised and cultured, are extremely resistant to cell-mediated cytolysis. As evidence suggests that B-cell susceptibility to lysis is dependent upon its state of differentiation and activation, we tested the ability of a variety of B-cell proliferation and differentiation agents, including pokeweed mitogen (PWM), to enhance the sensitivity of myeloma cells to cell-mediated lysis. PWM was found to significantly enhance the susceptibility of myeloma cell lines and freshly isolated myeloma cells to interleukin-2 (IL-2)–activated cell-mediated cytolysis. This effect was seen with the use of both IL-2–stimulated natural killer (NK) cells and T cells as effectors. The enhanced sensitivity of myeloma cells to cytolysis correlated with an increase in their cell surface expression of CD9, a pre-B cell marker and member of the transmembrane 4 superfamily. Incubation of PWM-stimulated myeloma cells with either monoclonal antibodies or antisense oligonucleotides directed against CD9 abrogated the effect of PWM. In order to determine whether there was a direct relationship between the expression of CD9 and enhanced sensitivity to cytolysis, myeloma cell lines that lacked CD9 expression were transfected with the CD9 gene. The level of cell surface CD9 expression correlates with enhanced susceptibility to lysis. Therefore, CD9 appears to be an important component in enhancing the sensitivity of myeloma cells to lysis mediated by IL-2–activated T cells and NK cells.

Direct extracellular contact between integrin alpha(3)beta(1) and TM4SF protein CD151

Previously we established that the alpha(3)beta(1) integrin shows stable, specific, and stoichiometric association with the TM4SF (tetraspannin) protein CD151. Here we used a membrane impermeable cross-linking agent to show a direct association between extracellular domains of alpha(3)beta(1) and CD151. The alpha(3)beta(1)-CD151 association site was then mapped using chimeric alpha(6)/alpha(3) integrins and CD151/NAG2 TM4SF proteins. Complex formation required an extracellular alpha(3) site (amino acids (aa) 570-705) not previously known to be involved in specific integrin contacts with other proteins and a region (aa 186-217) within the large extracellular loop of CD151. Notably, the anti-CD151 monoclonal antibody TS151r binding epitope, previously implicated in alpha(3) integrin association, was mapped to the same region of CD151 (aa 186-217). Finally, we demonstrated that both NH(2)- and COOH-terminal domains of CD151 are located on the inside of the plasma membrane, thus confirming a long suspected model of TM4SF protein topology.

Apoptosis in human autoimmune diseases

The description of apoptosis or programmed cell death nearly thirty years ago did not initially stimulate a great deal of interest. However, the ways cells die is clearly an essential part of biological homeostasis and well worth of study in its own right as the enormous literature on the subject in the past 15 years confirms. In the past decade new avenues of apoptosis research have opened up as the relationship between this form of cell death and autoimmune disease has come under increasing scrutiny. Although most research to date has been in animal study models, there are a variety of studies which have begun to explore links between apoptosis and a wider range of human autoimmune conditions. In this review we analyse briefly the background to what is known about apoptosis and focus on the increasing likelihood that abnormalities in apoptosis are contributory factors in the development of human autoimmunity.

A general approach to the generation of monoclonal antibodies against members of the tetraspanin superfamily using recombinant GST fusion proteins

Tetraspanins belong to a rapidly growing family of proteins characterized by the presence of four conserved transmembrane segments and are involved in such diverse functions as cellular activation, adhesion, migration and differentiation. In an effort to develop reagents against newly discovered tetraspanins, we have devised a simple method for the screening of monoclonal antibodies (mAbs) using recombinant GST fusion proteins. GST fusion proteins containing the second extracellular domain of different tetraspanins (CD9, CD63, CD53, CD81, A15 or CO-029) were produced separately. Mice were immunized with cells having a high expression of the chosen tetraspanin and the constructs were used to screen hybridomas in a solid phase ELISA. Several clones binding the fusion protein were identified for each construct tested: four anti-CD9 hybridoma clones, four anti-CD63, two anti-CD53, two anti-CD81, three anti-A15 and one anti-CO-029. All the newly developed mAbs recognized the native proteins by flow cytometry, immunofluorescence staining of cells and immunoprecipitation and bound to the denatured proteins on immunoblotting. Use of GST fusion protein constructs in a simple ELISA can facilitate screening for mAbs to members of the tetraspanin family, especially in cases where information is limited to the nucleotide sequence. The mAbs obtained by this strategy should prove to be valuable tools for functional studies of newly discovered tetraspanins.

CD81 (TAPA-1): a molecule involved in signal transduction and cell adhesion in the immune system

CD81 (TAPA-1) is a widely expressed cell-surface protein involved in an astonishing variety of biologic responses. It has been cloned independently several times for different functional effects and is reported to influence adhesion, morphology, activation, proliferation, and differentiation of B, T, and other cells. On B cells CD81 is part of a complex with CD21, CD19, and Leu13. This complex reduces the threshold for B cell activation via the B cell receptor by bridging Ag specific recognition and CD21-mediated complement recognition. Similarly on T cells CD81 associates with CD4 and CD8 and provides a costimulatory signal with CD3. In fetal thymic organ culture, mAb to CD81 block maturation of CD4-CD8- thymocytes, and expression of CD81 on CHO cells endows those cells with the ability to support T cell maturation. However, CD81-deficient mice express normal numbers and subsets of T cells. These mice do exhibit diminished antibody responses to protein antigens. CD81 is also physically and functionally associated with several integrins. Anti-CD81 can activate integrin alpha 4 beta 1 (VLA-4) on B cells, facilitating their adhesion to tonsilar interfollicular stroma. Similarly, anti-CD81 can activate alpha L beta 2 (LFA-1) on human thymocytes. CD81 can also affect cognate B-T cell interactions because anti-CD81 increases IL-4 synthesis by T cells responding to antigen presented by B cells but not by monocytes. The tetraspanin superfamily (or TM4SF) includes CD81, CD9, CD37, CD53, CD63, CD82, CD151, and an increasing number of additional proteins. Like CD81, several tetraspanins are involved in cell adhesion, motility, and metastasis, as well as cell activation and signal transduction.

Anti-CD53 monoclonal antibody induced LFA-1/ICAM-1-dependent and -independent lymphocyte homotypic cell aggregation

CD53 is a pan-leukocyte glycoprotein and belongs to a member of the tetraspan family of cell membrane proteins. The predicted structure and functional characteristics of CD53 suggest that it may play important roles in transmembrane signaling, but its roles in cell adhesion have not been clarified. The present study shows that anti-CD53 monoclonal antibody (mAb), HI29 induced homotypic cell aggregation of lymphoid cell lines including a B cell line from a patient with leukocyte adhesion deficiency syndrome (LAD). The homotypic cell aggregation was blocked by another anti-CD53 mAb, MEM53, in all the examined cell lines and by anti-LFA-1 (CD11a/CD18) or anti-ICAM-1 (CD54) mAbs in the cell lines except for the LAD line, but it was not blocked by anti-CD44 or anti-CD49d mAb. The induced homotypic cell aggregation was energy-dependent. These findings suggest that CD53 relates to LFA-1/ICAM-1-dependent and -independent pathways of homotypic cell aggregation of lymphocytes and that it plays an important role in lymphocyte activation and cell adhesion.

Characterisation of mouse CD37: cDNA and genomic cloning

The leukocyte surface antigen CD37 is a member of the transmembrane 4 superfamily (TM4SF) of glycoproteins which are predicted to span the lipid bilayer four times. The protein sequence and gene structure of mouse CD37 (Cd37) have been deduced through the isolation of cDNA and genomic clones. The Cd37 gene produces a major mRNA transcript of 1.2 kb that is restricted to cells of lymphoid and myeloid origin. Mouse CD37 is a glycoprotein of 281 amino acids in length, encoded by eight exons that span approximately 5.2 kb. CD37 is highly conserved between species, the mouse sequence sharing amino acid identities of 98% and 79% with rat and human, respectively. Cd37 shows a striking similarity in genomic organisation to other members of the TM4SF, which is consistent with the theory that this superfamily has evolved by gene duplication and divergence from a common ancestral gene.

Characterization of mouse CD53: epitope mapping, cellular distribution and induction by T cell receptor engagement during repertoire selection

The pan-leukocyte antigen CD53 is a member of the poorly understood transmembrane 4 superfamily (TM4SF) of cell membrane glycoproteins. CD53 is proposed to play a role in thymopoiesis, since rat CD53 is expressed on immature CD4-8-thymocytes and the functionally mature single-positive subset, but is largely absent from the intermediate CD4+8+ cells. We have characterized CD53 in the mouse through the production of two new monoclonal antibodies, MRC OX-79 and OX-80, which were raised against the RAW 264 cell line and screened on recombinant CD53 fusion proteins. The epitopes recognized by both antibodies are dependent on disulfide bonding and map to the major extracellular region of CD53, requiring the presence of a single threonine residue at position 154. Mouse CD53 has a molecular mass of 35-45 kDa and is expressed on virtually all peripheral leukocytes, but not on cells outside the lymphoid or myeloid lineages. CD53 expression distinguishes subpopulations of thymocytes in the mouse and resembles the expression pattern of rat CD53. Amongst the immature CD4-8-thymocytes, mouse CD53 is clearly detectable on the earliest CD44high25- subset, but down-regulated on the later CD44high25+, CD44low25+ and CD44low25- stages. Also, the subsequent transient TcR-/low CD4-8+ cells and most CD4+8+ thymocytes express little or no CD53. This is consistent with the idea that cells which are committed to enter the selectable CD4+8+ compartment switch off CD53. The effect of T cell receptor (TcR) engagement on the re-expression of CD53 on CD4+8+ thymocytes was studied both ex vivo and in vitro using F5 mice, transgenic for the H-2b/influenza nucleoprotein-peptide-specific TcR, back-crossed onto an H-2q or H-2b background of RAG-2-deficient mice. CD4+8+ thymocytes from non-selecting H-2q F5 mice are CD53 negative, but in vitro stimulation through the TcR dramatically induces CD53 expression. In contrast, a fraction of CD4+8+ thymocytes from positively selecting H-2b F5 transgenic mice express CD53. Therefore TcR engagement by selecting major histocompatibility complex peptide complexes, or surrogate ligands, induces CD53 expression on otherwise CD53-negative, non-selected CD4+8+ thymocytes. Whether CD53 itself participates as a signaling molecule in further stages of thymic selection is still a matter of speculation.

Localization of multiple functional domains on human PECAM-1 (CD31) by monoclonal antibody epitope mapping

PECAM-1, a cell adhesion molecule of the immunoglobulin gene (Ig) superfamily, has been implicated in white cell transmigration, integrin activation on lymphocytes, and cell-cell adhesion. The purpose of this investigation was to identify specific regions of the PECAM-1 extracellular domain mediating these functions by identifying the location of epitopes of bioactive anti-PECAM-1 monoclonal antibodies. The binding regions of mAbs important in PECAM-1-mediated leukocyte transmigration (Hec 7.2 and 3D2) were mapped to N-terminal Ig-like domains. The epitopes of monoclonal antibodies that activated integrin function on lymphocytes were dispersed over the entire extracellular region, but those that had the strongest activating effect were preferentially localized to the N-terminus of the molecule. The binding regions of mAbs that blocked PECAM-1-mediated heterophilic L-cell aggregation were located either in Ig-like domain 2 (NIH31.4) or Ig-like domain 6 (4G6 and 1.2). Site-directed mutagenesis further pinpointed the epitope of the 4G6 mAb to a hexapeptide, CAVNEG, within Ig-like domain 6. These results demonstrate that PECAM-1 contains multiple functional domains. Regions within N-terminal Ig-like domains appear to be required for transmigration. In contrast, two distinct regions were implicated in L-cell mediated heterophilic aggregation.

Expression of cell interaction molecules by immature rat thymocytes during passage through the CD4+8+ compartment: developmental regulation and induction by T cell receptor engagement of CD2, CD5, CD28, CD11a, CD44 and CD53

Rat thymocytes of the T cell receptorlow (TcRlow) CD4+8+ subset which is the target of repertoire selection are heterogeneous with respect to expression of the cell interaction (CI) molecules CD2, CD5, CD11a/CD18 (LFA-1), CD28 and CD44. We show that this heterogeneity is due to the developmental regulation of these CI molecules during passage through the CD4+8+ compartment, and to up-regulation by TcR engagement. Thus, cohorts of CD4+8+ cells differentiating synchronously in vitro from their direct precursors, the immature CD4-8+ cells, were homogeneous with regard to CI molecule expression. Upon entry into the CD4+8+ compartment, they expressed relatively high levels of CD2 and CD44, and moderate levels of CD5, CD28 and CD11a. CD2, CD28 and CD44 were slightly down-regulated during the following 2 days, whereas CD5 slightly increased and CD11a remained constant. TcR stimulation using immobilized monoclonal antibodies resulted in rapid and dramatic up-regulation of CD2, CD5 and CD28 and, to a lesser extent, of CD11a and CD44. Finally CD53, a triggering structure absent from unstimulated CD4+8+ thymocytes was also rapidly induced by TcR stimulation. Inclusion of interleukin (IL)-2, IL-4, or IL-7 in this in vitro differentiation system did not affect the levels of CI molecules studied. Since the high levels of CI molecules induced by TcR-stimulation correspond to those found in vivo on TcRintermediate thymocytes known to be undergoing repertoire selection, these results suggest that upregulation of CI molecules by TcR engagement provides a mechanism by which thymocytes that have entered the selection process gain preferential access to further interactions with stromal and lymphoid cells in the thymus.

The ins and outs of the transmembrane 4 superfamily

The recently discovered transmembrane 4 superfamily comprises a group of cell-surface proteins that are characterized by the presence of four hydrophobic domains, which are presumed to be membrane spanning. At least seven of these molecules are expressed on leukocytes, and it seems likely that they mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility.

Uroplakins Ia and Ib, two major differentiation products of bladder epithelium, belong to a family of four transmembrane domain (4TM) proteins

The mammalian bladder epithelium elaborates, as a terminal differentiation product, a specialized plasma membrane called asymmetric unit membrane (AUM) which is believed to play a role in strengthening and stabilizing the urothelial apical surface through its interactions with an underlying cytoskeleton. Previous studies indicate that the outer leaflet of AUM is composed of crystalline patches of 12-nm protein particles, and that bovine AUMs contain three major proteins: the 27- to 28-kD uroplakin I, the 15-kD uroplakin II and the 47-kD uroplakin III. As a step towards elucidating the AUM structure and function, we have cloned the cDNAs of bovine uroplakin I (UPI). Our results established the existence of two isoforms of bovine uroplakin I: a 27-kD uroplakin Ia and a 28-kD uroplakin Ib. These two glycoproteins are closely related with 39% identity in their amino acid sequences. Hydropathy plot revealed that both have four potential transmembrane domains (TMDs) with connecting loops of similar length. Proteolytic digestion of UPIa inserted in vitro into microsomal vesicles suggested that its two main hydrophilic loops are exposed to the luminal space, possibly involved in interacting with the luminal domains of other uroplakins to form the 12-nm protein particles. The larger loop connecting TMD3 and TMD4 of both UPIa and UPIb contains six highly conserved cysteine residues; at least one centrally located cysteine doublet in UPIa is involved in forming intramolecular disulfide bridges. The sequences of UPIa and UPIb (the latter is almost identical to a hypothetical, TGF beta-inducible, TI-1 protein of mink lung epithelial cells) are homologous to members of a recently described family all possessing four transmembrane domains (the "4TM family"); members of this family include many important leukocyte differentiation markers such as CD9, CD37, CD53, and CD63. The tissue-specific and differentiation-dependent expression as well as the naturally occurring crystalline state of uroplakin I molecules make them uniquely suitable, as prototype members of the 4TM family, for studying the structure and function of these integral membrane proteins.

Induction of nitric oxide release by MRC OX-44 (anti-CD53) through a protein kinase C-dependent pathway in rat macrophages

Many membrane proteins are implicated in the control of cell function by triggering specific signaling pathways. There is a new family of membrane proteins, defined by its structural motifs, which includes several lymphoid antigens, but lacks a function. To study its biological role, we determined which signaling pathways are affected by the CD53 antigen, a prototypic member of this family, in rat macrophages. Activation of CD53 by cross-linking results in an increase in inositol phosphates and diacylglycerol and in Ca2+ mobilization, which are insensitive to pertussis or cholera toxins. There is a translocation of protein kinase C to the membrane accompanied by nitric oxide (NO) release in macrophages. This effect is the result of the expression of the inducible nitric oxide synthase (iNOS), which is dependent on protein kinase C and protein synthesis. These results have linked a new receptor with a specific pathway of NO induction and thus have opened up a novel aspect of NO regulation in cell biology.