Complexes of tetraspanins with integrins: more than meets the eye

KAI1/CD82, a tumor metastasis suppressor

Tetraspanin KAI1/CD82 is a wide-spectrum tumor metastasis suppressor. KAI1/CD82 suppresses tumor metastasis by primarily inhibiting cancer cell motility and invasiveness. In tetraspanin-enriched microdomain, KAI1/CD82 associates with the proteins important for cell migration such as cell adhesion molecule, growth factor receptor, and signaling molecule. Likely, KAI1/CD82 down-regulates the functions of these motility-related proteins to inhibit cell migration. The loss of KAI1/CD82 expression in invasive and metastatic cancers is due to a complex, epigenetic mechanism that probably involves transcription factors such as NFkappaB, p53, and beta-catenin.

MHC class II/CD38/CD9: a lipid-raft–dependent signaling complex in human monocytes

Despite a lack of signaling motifs in their cytoplasmic domain, major histocompatibility complex (MHC) class II molecules trigger a variety of intracellular signals that regulate antigen-presenting cell function. They thus may use associated effector molecules as demonstrated on B cells and dendritic cells. The starting point of this study comes from our previous work, which demonstrated that the ecto-enzyme CD38 is functionally linked to MHC class II molecules. We report that CD38 and human leukocyte antigen-DR (HLA-DR) are functionally and physically associated in lipid rafts microdomains of cellsurface monocytes and that the integrity of these domains is necessary for the HLA-DR and CD38 signaling events. Moreover, we identified the tetraspanin CD9 molecule as a partner of the CD38/HLA-DR complex and demonstrated that HLA-DR, CD38, and CD9 share a common pathway of tyrosine kinase activation in human monocytes. The analysis of conjugate formation between monocytes presenting superantigen and T cells shows the active participation of CD9 and HLA-DR on the monocyte surface. Together, these observations demonstrate the presence of a CD38 and HLA-DR signaling complex within tetraspanin-containing lipid rafts and the functional impact of their molecular partner CD9 in antigen presentation.

Regulation of EGF receptor signaling by the MARVEL domain-containing protein CKLFSF8

It is known that chemokine-like factor superfamily 8 (CKLFSF8), a member of the CKLF superfamily, has four putative transmembrane regions and a MARVEL domain. Its structure is similar to TM4SF11 (plasmolipin) and widely distributed in normal tissue. However, its function is not yet known. We show here that CKLFSF8 is associated with the epidermal growth factor receptor (EGFR) and that ectopic expression of CKLFSF8 in several cell lines suppresses EGF-induced cell proliferation, whereas knockdown of CKLFSF8 by siRNA promotes cell proliferation. In cells overexpressing CKLFSF8, the initial activation of EGFR was not affected, but subsequent desensitization of EGF-induced signaling occurred rapidly. This attenuation was correlated with an increased rate of receptor endocytosis. In contrast, knockdown of CKLFSF8 by siCKLFSF8 delayed EGFR endocytosis. These results identify CKLFSF8 as a novel regulator of EGF-induced signaling and indicate that the association of EGFR with four transmembrane proteins is critical for EGFR desensitization.

Galectin-1 knocking down in human U87 glioblastoma cells alters their gene expression pattern

We have previously reported that (i) progression of malignancy in patients bearing astrocytic tumors correlates with increased tumor levels of galectin-1; (ii) in vitro addition of purified galectin-1 to U87 human glioblastoma cells enhances tumor cell motility; and (iii) conversely, knocking down galectin-1 expression in this cell line by stable transfection with antisense galectin-1 mRNA impairs motility and delays mortality after their intracranial grafting to nude mice. We here used cDNA microarray analysis to compare the effect on gene expression of stable transfection with antisense galectin-1 vector to mock-transfected and wild-type cells. Among the 631 spots probing genes potentially involved in cancer that were valid for analysis on all the arrays the expression of 86 genes was increased at least 2-fold. Confirmation of increased protein levels was provided by immunocytochemistry for p21waf/cip1, cullin-2, p53, ADAM-15, and MAP-2. Major differences in the expression patterns of ADAM-15 and the actin stress fiber organization were also observed. U87 cells stably deficient for galectin-1 expression were significantly less motile than control. We conclude that the stable inhibition of galectin-1 expression alters the expression of a number of genes that either directly or indirectly influence adhesion, motility and invasion of human glioblastoma cells.

Colocalization of the tetraspanins, CO-029 and CD151, with integrins in human pancreatic adenocarcinoma: impact on cell motility

PURPOSE

Patients with pancreatic adenocarcinoma have a poor prognosis due to the extraordinary high invasive capacity of this tumor. Altered integrin and tetraspanin expression is suggested to be an important factor. We recently reported that after protein kinase C activation, colocalization of alpha6beta4 with the tetraspanin CO-029 strongly supports migration of a rat pancreatic adenocarcinoma. The finding led us to explore whether and which integrin-tetraspanin complexes influence the motility of human pancreatic tumors.

EXPERIMENTAL DESIGN

Integrin and tetraspanin expression of pancreatic and colorectal adenocarcinoma was evaluated with emphasis on colocalization and the impact of integrin-tetraspanin associations on tumor cell motility.

RESULTS

The majority of pancreatic and colorectal tumors expressed the alpha2, alpha3, alpha6, beta1, and beta4 integrins and the tetraspanins CD9, CD63, CD81, CD151, and CO-029. Expression of alpha6beta4 and CO-029 was restricted to tumor cells, whereas alpha1, alpha2, alpha3, alpha6, beta1, and CD9, CD81, CD151 were also expressed by the surrounding stroma. CD63, CD81, and beta1 expression was observed at comparably high levels in healthy pancreatic tissue. alpha3beta1 frequently colocalized and coimmunoprecipitated with CD9, CD81, and CD151, whereas alpha6beta4 colocalized and coimmunoprecipitated mostly with CD151 and CO-029. Notably, protein kinase C activation strengthened only the colocalization of CD151 and CO-029 with beta4 and was accompanied by internalization of the integrin-tetraspanin complex, decreased laminin 5 adhesion, and increased cell migration.

CONCLUSION

alpha6beta4 is selectively up-regulated in pancreatic and colorectal cancer. The association of alpha6beta4 with CD151 and CO-029 correlates with increased tumor cell motility.

Aberrant expression of novel and previously described cell membrane markers in human breast cancer cell lines and tumors

PURPOSE

In a previous gene expression array study, we identified some 300 genes that were differentially expressed in human epidermal growth factor receptor tyrosine kinase 2 (HER2)-positive versus HER2-negative breast cancer cells. We have now done validation experiments on a group of three cell membrane components that had previously not been implicated in breast cancer. We also studied the expression of three other cell membrane proteins known to play a role in mammary neoplasia.

EXPERIMENTAL DESIGN

By immunohistochemistry, we examined up to 130 archival breast carcinomas for Celsr2, E-cadherin, Kai1, and CD9 expression. The expression levels of NET-6 and TROP-2 were determined by quantitative reverse transcription-PCR in a subset of frozen tumors. We also studied fresh pellets and paraffin-embedded cell buttons of nine human breast cell lines. The relationship between the expression of all six membrane proteins and a variety of pathologic and biological variables, including estrogen receptor, HER2, and epidermal growth factor receptor status, was also examined. The NET-6 gene was transfected into a low-expressing cell line, and the effect on cellular morphology, growth, and invasion in vitro was recorded.

RESULTS

Celsr2 was down-regulated in one cell line and in 7% of breast cancers. E-cadherin, Kai1, and CD9 were down-regulated in 35%, 76%, and 79% of tumors, respectively, confirming the important role of these markers in human mammary neoplasia. In breast cancer cell lines and tissues, TROP-2 was generally expressed at low levels, although a few specimens showed relative overexpression. NET-6 levels were lower in HER2-negative breast carcinoma cells. In addition, NET-6 was markedly down-regulated in estrogen receptor-negative breast cancers, and expression was lowest in "basal-like" tumors. Ectopic expression of NET-6 in low-expressing MDA-MB-231 cells altered cellular morphology, inhibited growth in vitro, and decreased invasion in a Boyden chamber assay.

CONCLUSIONS

We have confirmed the expression of three new membrane markers that had previously not been implicated in human breast cancer, and one of them (NET-6) was correlated with HER2 and estrogen receptor status. NET-6 levels were decreased in estrogen receptor-negative and high-grade tumors, and ectopic expression of this gene had an inhibitory effect on proliferation and invasion. Thus, NET-6 may represent a novel breast cancer suppressor gene.

Cyclic FEE peptide increases human gamete fusion and potentiates its RGD-induced inhibition

BACKGROUND

Alpha6beta1 integrin has been proposed to act as a sperm receptor on the mouse oocyte by interacting with spermatozoon fertilin beta. We investigated, in humans, whether oocyte integrins could act similarly in gamete fusion, using a cyclic peptide containing the putative disintegrin-binding domain of human fertilin beta [cyclic FEE (cFEE)] and RGD peptide.

METHODS

Zona-free eggs were inseminated in the absence or presence of peptides. To maintain the membrane protein pattern, the zona pellucida was removed by microdissection. Immunofluorescence and confocal microscopy were used to detect integrin subunits on the oocyte.

RESULTS

Unexpectedly, cFEE alone increased human gamete fusion by 94% instead of inhibiting fertilization. Furthermore, cFEE together with RGD potentiated the RGD-induced inhibition of fertilization in a dose-dependent manner. The data suggested the hypothesis of integrin cross-talk, further supported by the co-localization of alpha6beta1 and alphavbeta3 integrins, the putative receptors of cFEE and RGD peptides, respectively.

CONCLUSIONS

RGD-sensitive and -insensitive integrins may be associated in a multimolecular complex working as a sperm receptor on the human oocyte membrane. Supplementation of human IVF culture medium with cFEE peptide might improve fertilization rates in ART.

Hypersialylation of beta1 integrins, observed in colon adenocarcinoma, may contribute to cancer progression by up-regulating cell motility

Colon adenocarcinomas are known to express elevated levels of alpha2-6 sialylation and increased activity of ST6Gal-I, the Golgi glycosyltransferase that creates alpha2-6 linkages. Elevated ST6Gal-I positively correlates with metastasis and poor survival, and therefore ST6Gal-I-mediated hypersialylation likely plays a role in colorectal tumor invasion. Previously we found that oncogenic ras (present in roughly 50% of colon adenocarcinomas) up-regulates ST6Gal-I and, in turn, increases sialylation of beta1 integrin adhesion receptors in colon epithelial cells. However, we wanted to know if this pattern held true in vivo and, if so, how beta1 hypersialylation might contribute to colon tumor progression. In the present study, we find that beta1 integrins from colon adenocarcinomas consistently carry higher levels of alpha2-6 sialic acid. To explore the effects of increased alpha2-6 sialylation on beta1-integrin function, we stably expressed ST6Gal-I in a colon epithelial cell line lacking endogenous ST6Gal-I. ST6Gal-I expressors (with alpha2-6 sialylated beta1 integrins) exhibited up-regulated attachment to collagen I and laminin and increased haptotactic migration toward collagen I, relative to parental cells (with completely unsialylated beta1 integrins). Blockade of ST6Gal-I expression with short interfering RNA reversed collagen binding back to the level of ST6Gal-I nonexpressors, confirming that alpha2-6 sialylation regulates beta1 integrin function. Finally, we show that beta1 integrins from ST6Gal-I expressors have increased association with talin, a marker for integrin activation. Collectively, these findings suggest that beta1 hypersialylation may augment colon tumor progression by altering cell preference for certain extracellular matrix milieus, as well as by stimulating cell migration.

Embryonic implantation and leukocyte transendothelial migration: different processes with similar players?

A clear parallelism between the different steps in human embryo-endometrial apposition/adhesion/invasion and leukocyte-endothelium rolling/adhesion/extravasation can be established. During human implantation and leukocyte trafficking, a first wave of soluble mediators regulates the expression and functional activity of adhesion molecules such as L-selectin and integrins, which mediate both processes. Apical surfaces of human endometrial epithelium and endothelium are key elements for the initiation of molecular interactions to capture the blastocyst or leukocyte, respectively. Subsequently, the blastocyst and the leukocyte migrate through the epithelium and endothelium toward their final destination, the endometrial stroma, to initiate placentation or the inflammatory foci as part of the immune response. Similarities between the intermediate molecular mechanisms of these two physiologically unrelated processes are discussed.

Structural organization and interactions of transmembrane domains in tetraspanin proteins

Background

Proteins of the tetraspanin family contain four transmembrane domains (TM1-4) linked by two extracellular loops and a short intracellular loop, and have short intracellular N- and C-termini. While structure and function analysis of the larger extracellular loop has been performed, the organization and role of transmembrane domains have not been systematically assessed.

Results

Among 28 human tetraspanin proteins, the TM1-3 sequences display a distinct heptad repeat motif (abcdefg)_n. In TM1, position a is occupied by structurally conserved bulky residues and position d contains highly conserved Asn and Gly residues. In TM2, position a is occupied by conserved small residues (Gly/Ala/Thr), and position d has a conserved Gly and two bulky aliphatic residues. In TM3, three a positions of the heptad repeat are filled by two leucines and a glutamate/glutamine residue, and two d positions are occupied by either Phe/Tyr or Val/Ile/Leu residues. No heptad motif is apparent in TM4 sequences. Mutations of conserved glycines in human CD9 (Gly25 and Gly32 in TM1; Gly67 and Gly74 in TM2) caused aggregation of mutant proteins inside the cell. Modeling of the TM1-TM2 interface in CD9, using a novel algorithm, predicts tight packing of conserved bulky residues against conserved Gly residues along the two helices. The homodimeric interface of CD9 was mapped, by disulfide cross-linking of single-cysteine mutants, to the vicinity of residues Leu14 and Phe17 in TM1 (positions g and c) and Gly77, Gly80 and Ala81 in TM2 (positions d, g and a, respectively). Mutations of a and d residues in both TM1 and TM2 (Gly25, Gly32, Gly67 and Gly74), involved in intramolecular TM1-TM2 interaction, also strongly diminished intermolecular interaction, as assessed by cross-linking of Cys80.

Conclusion

Our results suggest that tetraspanin intra- and intermolecular interactions are mediated by conserved residues in adjacent, but distinct regions of TM1 and TM2. A key structural element that defines TM1-TM2 interaction in tetraspanins is the specific packing of bulky residues against small residues.

Aberrant expression of tetraspanin molecules in B-cell chronic lymphoproliferative disorders and its correlation with normal B-cell maturation

Tetraspanin proteins form signaling complexes between them and with other membrane proteins and modulate cell adhesion and migration properties. The surface expression of several tetraspanin antigens (CD9, CD37, CD53, CD63, and CD81), and their interacting proteins (CD19, CD21, and HLA-DR) were analyzed during normal B-cell maturation and compared to a group of 67 B-cell neoplasias. Three patterns of tetraspanin expression were identified in normal B cells. The first corresponded to bone marrow CD10(+) B-cell precursors (BCP) which showed high expression of CD81 and CD9, low reactivity for CD53 and negativity for CD37. CD10(-) B-lymphocytes showed downregulation of CD9/CD81 and upregulation of CD53/CD37. Plasma cells showed re-expressed CD9 and downregulated CD37. Hierarchical clustering analysis of flow cytometry immunophenotypic data showed a good correlation between the tumor differentiation stage and the pattern of tetraspanin expression, with all analyzed individual samples classified into three major groups, independently of their normal or neoplastic origin. Despite this, neoplastic B-cells frequently showed aberrantly high/low expression of the different markers analyzed. Interestingly, in B-cell chronic lymphocytic leukemia, abnormal expression of CD53 and CD9 were associated with different patterns of disease infiltration, which would support the role of these molecules on modulating adhesion and migration of neoplastic B cells.

Enamel matrix protein interactions

The recognized structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. While a large volume of data exists showing that amelogenin self-assembles into multimeric units referred to as nanospheres, other reports of enamel matrix protein-protein interactions are scant. We believe that each of these enamel matrix proteins must interact with other organic components of ameloblasts and the enamel matrix. Likely protein partners would include integral membrane proteins and additional secreted proteins.

INTRODUCTION

The purpose of this study was to identify and catalog additional proteins that play a significant role in enamel formation.

MATERIALS AND METHODS

We used the yeast two-hybrid assay to identify protein partners for amelogenin, ameloblastin, and enamelin. Once identified, RT-PCR was used to assess gene transcription of these newly identified and potential "enamel" proteins in ameloblast-like LS8 cells.

RESULTS

In the context of this yeast assay, we identified a number of secreted proteins and integral membrane proteins that interact with amelogenin, ameloblastin, and enamelin. Additionally, proteins whose functions range from the inhibition of soft tissue mineralization, calcium ion transport, and phosphorylation events have been identified as protein partners to these enamel matrix proteins. For each protein identified using this screening strategy, future studies are planned to confirm this physiological relationship to biomineralization in vivo.

CONCLUSION

Identifying integral membrane proteins of the secretory surface of ameloblast cells (Tomes' processes) and additional enamel matrix proteins, based on their abilities to interact with the most abundant enamel matrix proteins, will better define the molecular mechanisms of enamel formation at its most rudimentary level.

Down-regulation of CD9 in human ovarian carcinoma cell might contribute to peritoneal dissemination: morphologic alteration and reduced expression of beta1 integrin subsets

Peritoneal dissemination is one of the main causes of death in cancer patients. Pathophysiology of metastasis has been well investigated, but the mechanism of diffuse spread of tumor colonies in the peritoneal cavity is not fully understood. CD9 is a member of tetraspanin and its down-regulation is known to be involved in poor prognosis. To investigate the significance of the down-regulation of CD9, HTOA, an ovarian carcinoma cell line that highly expressed CD9, was transiently transfected with small interfering RNA (siRNA) against CD9, and CD9-negative cells (HTOA(CD9-)) were purified. HTOA(CD9-) showed altered adhesion patterns on Matrigel, collagen, fibronectin, and laminin compared with those of control siRNA-transfected HTOA (control-HTOA). Flow cytometry and fluorescence cytostainings revealed that the expression levels of integrins beta1, alpha2, alpha3beta1, alpha5, and alpha6 were lower in HTOA(CD9-) than those of control-HTOA. HTOA(CD9-) showed altered expression of junctional and cytoskeletal molecules. By time-lapse video microscopy, control-HTOA showed solid adhesion to extracellular matrix and formed cobblestone pattern, whereas HTOA(CD9-) showed weaker adhesion and were distributed as diffuse spots. To examine whether the expression level of CD9 change during tumor dissemination, HTOA-P, a highly disseminative subclone of HTOA, was established. HTOA-P showed distinctive down-regulation of CD9 at mRNA and protein levels, and showed similar morphologic alteration as HTOA(CD9-) did. These findings indicate that the down-regulation of CD9 may be an acquired event in the process of tumor dissemination. Down-regulated CD9 may attenuate the expression of several integrins and rearrange junctional and cytoskeletal molecules that might contribute to dissemination of ovarian carcinomas.

Iterative microarray and RNA interference-based interrogation of the SRC-induced invasive phenotype

Src kinase has long been recognized as a factor in the progression of colorectal cancer and seems to play a specific role in the development of the metastatic phenotype. In spite of numerous studies conducted to elucidate the exact role of Src in cancer progression, downstream targets of Src remain poorly understood. Gene expression profiling has permitted the identification of large sets of genes that may be functionally interrelated but it is often unclear as to which molecular pathways they belong. Here we have developed an iterative approach to experimentally reconstruct a network of gene activity regulated by Src and contributing to the invasive phenotype. Our strategy uses a combination of phenotypic anchoring of gene expression profiles and loss-of-function screening by way of RNA-mediated interference. Using a panel of human colon cancer cell lines exhibiting differential Src-specific activity and invasivity, we identify the first two levels of gene transcription responsible for the invasive phenotype, where first-tier genes are controlled by Src activity and the second-tier genes are under the influence of the first tier. Specifically, perturbation of first-tier gene activity by either pharmacologic inhibition of Src or RNA-mediated interference-directed knockdown leads to a loss of invasivity and decline of second-tier gene activity. The targeting of first-tier genes may be bypassed altogether because knockdown of second-tier genes led to a similar loss of invasive potential. In this manner, numerous members of a "transcriptional cascade" pathway for metastatic activity have been identified and functionally validated.

Tetraspanin-5 (Tm4sf9) mRNA expression parallels neuronal maturation in the cerebellum of normal and L7En-2 transgenic mice

Tetraspanin-5 (Tspan-5) mRNA was recently shown to be strongly expressed within the central nervous system. In order to address Tspan-5 function during nervous system development, we performed a detailed expression analysis in the postnatal FVB/N mouse cerebellum using in situ hybridizations. Tspan-5 mRNA was expressed within cerebellar Purkinje cells (PCs) throughout postnatal development. The expression level, however, changed significantly with ongoing development. At the day of birth (P0), Tspan-5 mRNA was expressed at very low levels in PCs. At this time, PCs of the FVB/N strain are postmitotic and bear axons, but no dendrites. At P7, Tspan-5 mRNA expression was visible in all PCs, but was more prominent in those of the posterior lobules as compared to those of the anterior lobules. After P7, high levels of Tspan-5 mRNA were seen in all PCs, which is when PCs elaborate and maintain their typical dendritic tree. This demonstrates that the level of Tspan-5 mRNA is related to the developmental status of PCs. Consistently, expression of Tspan-5 mRNA was specifically reduced in PCs of L7En-2 animals, which display a delay in PC maturation during postnatal cerebellar development. In addition, whereas no Tspan-5 mRNA signal could be detected in the proliferating granule cell layer, low levels could be found in postmitotic, premigratory granule cells and high levels in settled and differentiated granule cells. Thus, the level of Tspan-5 mRNA expression correlates very well with the differentiation status of particular neurons. The level of Tspan-5 expression might therefore be important for distinct phases of neuronal maturation.

Increased expression of Drosophila tetraspanin, Tsp68C, suppresses the abnormal proliferation of ytr-deficient and Ras/Raf-activated hemocytes

Tetraspanins are evolutionary conserved transmembrane proteins thought to facilitate cell proliferation, movement or fusion by acting as organizers of different signaling events. Despite their prevalence and conservation, their specific role and functions remain largely elusive, as their redundancy in various organisms has hindered loss of function studies. Here, we take a gain of function approach to study Drosophila tetraspanin Tsp68C and its effect on larval hemocytes. We recently characterized a lethal mutation in ytr, a conserved gene that encodes a nuclear arginine-rich protein of unknown function, which is accompanied by abnormal differentiation and proliferation of the larval hematopoietic tissue in flies. A hemolectin (hml)-Gal4 construct carried by hml-Gal4 transgenic flies was sufficient by itself to abrogate the hematopoietic defects in ytr mutant larvae. This rescue correlated with the overexpression of tsp68C, a tetraspanin gene nested in the hml promoter. The suppression of abnormal proliferation by the hml-Gal4 construct was not restricted to ytr-deficient hemocytes, but was also observed in hemocytes expressing the oncogenic forms of Raf or Ras proteins. However, it had no effect on overproliferation mediated by a constitutively active form of Jak. New hml-Gal4 lines, in which the tsp68C gene was silenced or deleted from the promoter, no longer rescued the hematopoietic defect in ytr mutants nor suppressed the activated Raf-induced overproliferation. Therefore, change in tetraspanin Tsp68C expression has a strong suppressor effect on abnormal proliferation and differentiation of hemocytes in the context of specific lesions, such as overactivation of the Ras/Raf/MAPK pathway.

Evolution of metazoan cell junction proteins: the scaffold protein MAGI and the transmembrane receptor tetraspanin in the demosponge Suberites domuncula

Until recently the positioning of the sponges (phylum Porifera) within the metazoan systematics was hampered by the lack of molecular evidence for the existence of junctional structures in the surface cell layers. In this study two genes related to the tight junctions are characterized from the demosponge Suberites domuncula: tetraspanin (SDTM4SF), a cell surface receptor, and MAGI (SDMAGI), a MAGUK (membrane-associated guanylate kinase homologue) protein. Especially the MAGI protein is known in other metazoan animal phyla to exist exclusively in tight junctions. The characteristic domains of MAGI proteins (six PDZ domains, two WW domains, and a truncated guanylate kinase motif) are conserved in the sponge protein. The functional analysis of SDMAGI done by in situ hybridization shows its expression in the surface epithelial layers (exopinacoderm and endopinacoderm). Northern blot studies reveal that expression of SDMAGI and SDTM4SF increases after formation of the pinacoderm layer in the animals as well as in primmorphs. These results support earlier notions that sponges contain junctional structures. We conclude that sponges contain epithelia whose cells are organized by cell junctions.

Integrin-dependent interaction of lipid rafts with the actin cytoskeleton in activated human platelets

Dynamic connections between actin filaments and the plasma membrane are crucial for the regulation of blood platelet functions. Protein complexes associated with αIIbβ3 integrin-based cytoskeleton structures are known to play a role in these processes. However, mechanisms involving lateral organizations of the plasma membrane remain to be investigated. Here, we demonstrate that a large fraction of platelet lipid rafts specifically associates with the actin cytoskeleton upon activation. This association was inhibited by antagonists of fibrinogen-αIIbβ3 binding and did not occur in type I Glanzman's thrombasthenic platelets. The raft-cytoskeleton interaction is a reversible process correlating with the intensity and stability of platelet aggregation. Although only a minor fraction of αIIbβ3 was recovered in rafts upon activation, this integrin specifically upregulated the level of PtdIns(4,5)P2 in membrane microdomains and induced the recruitment of several actin-modulating proteins known to directly or indirectly interact with this lipid. Controlled disruption of rafts did not affect αIIbβ3-mediated platelet aggregation in response to high concentrations of thrombin but significantly inhibited fibrin clot retraction. We propose that rafts participate in the organization of membrane-cytoskeleton interactions where αIIbβ3-mediated tension forces apply during the late phase of platelet activation.

Roles of uroplakins in plaque formation, umbrella cell enlargement, and urinary tract diseases

The apical surface of mouse urothelium is covered by two-dimensional crystals (plaques) of uroplakin (UP) particles. To study uroplakin function, we ablated the mouse UPII gene. A comparison of the phenotypes of UPII- and UPIII-deficient mice yielded new insights into the mechanism of plaque formation and some fundamental features of urothelial differentiation. Although UPIII knockout yielded small plaques, UPII knockout abolished plaque formation, indicating that both uroplakin heterodimers (UPIa/II and UPIb/III or IIIb) are required for plaque assembly. Both knockouts had elevated UPIb gene expression, suggesting that this is a general response to defective plaque assembly. Both knockouts also had small superficial cells, suggesting that continued fusion of uroplakin-delivering vesicles with the apical surface may contribute to umbrella cell enlargement. Both knockouts experienced vesicoureteral reflux, hydronephrosis, renal dysfunction, and, in the offspring of some breeding pairs, renal failure and neonatal death. These results highlight the functional importance of uroplakins and establish uroplakin defects as a possible cause of major urinary tract anomalies and death.

Palmitoylation supports assembly and function of integrin-tetraspanin complexes

As observed previously, tetraspanin palmitoylation promotes tetraspanin microdomain assembly. Here, we show that palmitoylated integrins (α3, α6, and β4 subunits) and tetraspanins (CD9, CD81, and CD63) coexist in substantially overlapping complexes. Removal of β4 palmitoylation sites markedly impaired cell spreading and signaling through p130Cas on laminin substrate. Also in palmitoylation-deficient β4, secondary associations with tetraspanins (CD9, CD81, and CD63) were diminished and cell surface CD9 clustering was decreased, whereas core α6β4–CD151 complex formation was unaltered. There is also a functional connection between CD9 and β4 integrins, as evidenced by anti-CD9 antibody effects on β4-dependent cell spreading. Notably, β4 palmitoylation neither increased localization into “light membrane” fractions of sucrose gradients nor decreased solubility in nonionic detergents—hence it does not promote lipid raft association. Instead, palmitoylation of β4 (and of the closely associated tetraspanin CD151) promotes CD151–α6β4 incorporation into a network of secondary tetraspanin interactions (with CD9, CD81, CD63, etc.), which provides a novel framework for functional regulation.

CD9-mediated activation of the p46 Shc isoform leads to apoptosis in cancer cells

CD9, a member of the tetraspanin family, has been shown to be involved in a range of cellular activities, including migration, proliferation and adhesion, but the molecular mechanisms by which it mediates such events is unclear. Here, we found that anti-CD9 monoclonal antibody ALB6 inhibited cell proliferation, reduced cell viability and induced not only morphological changes specific to apoptosis but also molecular changes, as evidenced by TUNEL and annexin-V staining. For the possible mechanism of ALB6-induced apoptosis, ALB6 activated the c-Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 mitogen-activated-protein kinase (MAPK) within 5-15 minutes, as well as caspase-3 within 24-48 hours. It is noteworthy that ALB6 induced tyrosine phosphorylation of the p46 Shc isoform specifically and that the overexpression of its dominant-negative form completely suppressed the ALB6-induced activation of JNK/SAPK, p38 MAPK and caspase-3, resulting in the inhibition of apoptotic cell death. These results suggest that CD9 might regulate apoptosis through the specialized signals in human cancer cell lines.

Inhibición de la motilidad de células de melanoma con ARN de interferencia de CD9

INTRODUCTION

Although 65 % of the deaths caused by skin cancer are due to melanomas, the possible causes that may explain the aggressiveness of this type of tumor are still unknown. Different approaches have been used to try to find an effective treatment, but they have been unsuccessful. Interference RNA (iRNA) is an essential technique for this research. With this technique, we can selectively "knock down" or silence protein expression. The tetraspanins CD9 and CD151 are molecules involved in cell motility, including melanoma cells.

MATERIAL AND METHODS

A375 melanoma cell line, anti-CD9 and anti-CD151 monoclonal antibodies, and iRNA against CD9 and CD151. Immunofluorescence techniques, flow cytometry, cell transfection, cell selection with magnetic beads and evaluation of cell migration in a wound-healing model.

RESULTS

The A375 cells express CD9 and CD151. By using iRNA against CD9 and CD151, we managed to inhibit the expression of these proteins. The cells transfected with iRNA against CD9 showed significant inhibition of their motility.

DISCUSSION

We managed to silence the expression of CD9 and CD151 using iRNA techniques in the A375 melanoma cell line. The reduction in CD9 caused the inhibition of cell motility, while the interference with CD151 expression had a more moderate effect. This data indicates that the knockdown of tetraspanins may be a future target for the treatment of melanoma.

Identification of FGF10 Targets in the Embryonic Lung Epithelium during Bud Morphogenesis

Genetic studies implicate Fgf10-Fgfr2 signaling as a critical regulator of bud morphogenesis in the embryo. However, little is known about the transcriptional targets of Fgf10 during this process. Here we identified global changes in gene expression in lung epithelial explants undergoing FGF10-mediated budding in the absence of other growth factors and mesenchyme. Targets were confirmed by their localization at sites where endogenous Fgf10 signaling is active in embryonic lungs and by demonstrating their induction in intact lungs in response to local application of FGF10 protein. We show that the initial stages of budding are characterized by marked up-regulation of genes associated with cell rearrangement and cell migration, inflammatory process, and lipid metabolism but not cell proliferation. We also found that some genes implicated in tumor invasion and metastatic behavior are epithelial targets of Fgf10 in the lung and other developing organs that depend on Fgf10-Fgfr2 signaling to properly form. Our approach identifies Fgf10 targets that are common to multiple biological processes and provides insights into potential mechanisms by which Fgf signaling regulates epithelial cell behavior.

Tetraspanin CD82 Attenuates Cellular Morphogenesis through Down-regulating Integrin α6-Mediated Cell Adhesion

Tetraspanin CD82 has been implicated in integrin-mediated functions such as cell motility and invasiveness. Although tetraspanins associate with integrins, it is unknown if and how CD82 regulates the functionality of integrins. In this study, we found that Du145 prostate cancer cells underwent morphogenesis on the reconstituted basement membrane Matrigel to form an anastomosing network of multicellular structures. This process entirely depends on integrin alpha6, a receptor for laminin. After CD82 is expressed in Du145 cells, this cellular morphogenesis was abolished, indicating a functional cross-talk between CD82 and alpha6 integrins. Interestingly, antibodies against other tetraspanins expressed in Du145 cells such as CD9, CD81, and CD151 did not block this integrin alpha6-dependent morphogenesis. We further found that CD82 significantly inhibited cell adhesion on laminin 1. Notably, the level of alpha6 integrins on the cell surface was down-regulated upon CD82 expression, although total cellular alpha6 protein levels remained unchanged in CD82-expressing cells. This down-regulation indicates that the diminished cell adhesiveness of CD82-expressing Du145 cells on laminin likely resulted from less cell surface expression of alpha6 integrins. As expected, CD82 physically associated with the integrin alpha6 in Du145-CD82 transfectant cells, suggesting that the formation of the CD82-integrin alpha6 complex reduces alpha6 integrin cell surface expression. Finally, the internalization of cell surface integrin alpha6 is significantly enhanced upon CD82 expression. In conclusion, our results indicate that 1) CD82 attenuates integrin alpha6 signaling during a cellular morphogenic process; 2) the decreased surface expression of alpha6 integrins in CD82-expressing cells is likely responsible for the diminished adhesiveness on laminin and, subsequently, results in the attenuation of alpha6 integrin-mediated cellular morphogenesis; and 3) the accelerated internalization of integrin alpha6 upon CD82 expression correlates with the down-regulation of cell surface integrin alpha6.

Anti-CD63 antibodies suppress IgE-dependent allergic reactions in vitro and in vivo

High-affinity IgE receptor (FcɛRI) cross-linking on mast cells (MCs) induces secretion of preformed allergy mediators (degranulation) and synthesis of lipid mediators and cytokines. Degranulation produces many symptoms of immediate-type allergic reactions and is modulated by adhesion to surfaces coated with specific extracellular matrix (ECM) proteins. The signals involved in this modulation are mostly unknown and their contribution to allergic reactions in vivo is unclear. Here we report the generation of monoclonal antibodies that potently suppress FcɛRI-induced degranulation, but not leukotriene synthesis. We identified the antibody target as the tetraspanin CD63. Tetraspanins are membrane molecules that form multimolecular complexes with a broad array of molecules including ECM protein-binding β integrins. We found that anti-CD63 inhibits MC adhesion to fibronectin and vitronectin. Furthermore, anti-CD63 inhibits FcɛRI-mediated degranulation in cells adherent to those ECM proteins but not in nonadherent cells. Thus the inhibition of degranulation by anti-CD63 correlates with its effect on adhesion. In support of a mechanistic linkage between the two types of inhibition, anti-CD63 had no effect on FcɛRI-induced global tyrosine phosphorylation and calcium mobilization but impaired the Gab2–PI3K pathway that is known to be essential for both degranulation and adhesion. Finally, we showed that these antibodies inhibited FcɛRI-mediated allergic reactions in vivo. These properties raise the possibility that anti-CD63 could be used as therapeutic agents in MC-dependent diseases.

Regulation of urokinase receptor proteolytic function by the tetraspanin CD82

The high affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored cellular receptor (uPAR) promotes plasminogen activation and the efficient generation of pericellular proteolytic activity. We demonstrate here that expression of the tetraspanin CD82/KAI1 (a tumor metastasis suppressor) leads to a profound effect on uPAR function. Pericellular plasminogen activation was reduced by approximately 50-fold in the presence of CD82, although levels of components of the plasminogen activation system were unchanged. uPAR was present on the cell surface and molecularly intact, but radioligand binding analysis with uPA and anti-uPAR antibodies revealed that it was in a previously undetected cryptic form unable to bind uPA. This was not due to direct interactions between uPAR and CD82, as they neither co-localized on the cell surface nor could be co-immunoprecipitated. However, expression of CD82 led to a redistribution of uPAR to focal adhesions, where it was shown by double immunofluorescence labeling to co-localize with the integrin alpha(5)beta(1), which was also redistributed in the presence of CD82. Co-immunoprecipitation experiments showed that, in the presence of CD82, uPAR preferentially formed stable associations with alpha(5)beta(1), but not with a variety of other integrins, including alpha(3)beta(1). These data suggest that CD82 inhibits the proteolytic function of uPAR indirectly, directing uPAR and alpha(5)beta(1) to focal adhesions and promoting their association with a resultant loss of uPA binding. This represents a novel mechanism whereby tetraspanins, integrins, and uPAR, systems involved in cell adhesion and migration, cooperate to regulate pericellular proteolytic activity and may suggest a mechanism for the tumor-suppressive effects of CD82/KAI1.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

A novel tetraspanin fusion protein, peripherin-2, requires a region upstream of the fusion domain for activity

Peripherin-2 (also known as peripherin/rds), a photoreceptor specific tetraspanin protein, is required to maintain normal cell structure through its role in renewal processes requiring membrane fusion. It is the first tetraspanin fusogen and has been shown to directly mediate fusion between disk membranes and opposing membranes to maintain the highly ordered structure of rod outer segments. Localized to the C terminus of human, bovine, and murine peripherin-2 is an amphiphilic fusion peptide domain (residues 312-326) and a highly conserved region upstream of this domain that we hypothesize is essential for fusogenic function. Our previous studies indicated that substitution of a threonine for a proline at position 296 within this highly conserved region enhanced fusion activity. In this study we wanted to determine whether this proline is essential with the introduction of three additional substitutions of proline with alanine, leucine, and glutamic acid. Wild type, P296T, P296A, P296L, and P296E mutants of peripherin-2 were expressed as His6-tagged full-length proteins in Madin-Darby canine kidney (MDCK) cells. All of the proteins were localized to intracellular membranes and detected as 42-kDa monomers by Western blot analysis. The wild type, P296A, and P296L assembled into core tetramers; in contrast the P296T and P296E formed higher order oligomers. Fusogenic activity of full-length protein expressed in MDCK membranes and purified protein reconstituted in model membrane liposomes was determined using fluorescence quenching techniques. Fusion activity was decreased in the P296L, P296A, and P296E mutants both in endogenous MDCK membranes and in model liposomes. Collectively, these results suggest that the proline at position 296 is necessary for optimal function.

GATA-6 regulates genes promoting synthetic functions in vascular smooth muscle cells

OBJECTIVE

Previous studies suggested the zinc-finger transcription factor GATA-6 inhibits vascular smooth muscle cell (VSMC) proliferation and promotes the contractile VSMC phenotype. The objective of this study was to identify bona fide target genes regulated by GATA-6 in VSMCs.

METHODS AND RESULTS

Microarray analyses were performed comparing mRNA from rat aortic smooth muscle cells (SMCs) infected with either adenovirus encoding a dominant-negative GATA-6/engrailed fusion protein or with control adenovirus. These studies identified 122 genes differentially expressed by at least 2-fold, including multiple genes involved in cell-cell signaling and cell-matrix interactions. Among these, endothelin-1 and the angiotensin type(1a) (AT(1a)) receptor are known to be induced in VSMCs in response to inflammatory stimuli and to be expressed in a GATA-dependent manner in cardiac myocytes in response to hemodynamic stress. Consistent with these findings, the endothelin-1 and AT(1a) receptor promoters were activated by forced expression of GATA-6 and repressed by forced expression of GATA-6/engrailed. Surprisingly, genes encoding SMC contractile proteins were not altered, and myocardin-induced SMC differentiation was not impaired in GATA-6(-/-) embryonic stem cells.

CONCLUSIONS

These data demonstrate that in VSMCs, GATA-6 regulates a set of genes associated with synthetic SMC functions and suggest that this transcriptional pathway may be independent from myocardin-induced SMC differentiation. An unbiased microarray screen of genes regulated by GATA-6 in VSMCs identified multiple genes involved in cell-cell signaling and cell-matrix interactions. The endothelin-1 and the AT1a receptor genes were shown to be direct GATA-6 target genes. These data suggest that GATA-6 plays a role in promoting synthetic functions in VSMCs.

Endothelial tetraspanin microdomains regulate leukocyte firm adhesion during extravasation

Tetraspanins associate with several transmembrane proteins forming microdomains involved in intercellular adhesion and migration. Here, we show that endothelial tetraspanins relocalize to the contact site with transmigrating leukocytes and associate laterally with both intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). Alteration of endothelial tetraspanin microdomains by CD9-large extracellular loop (LEL)-glutathione S-transferase (GST) peptides or CD9/CD151 siRNA oligonucleotides interfered with ICAM-1 and VCAM-1 function, preventing lymphocyte transendothelial migration and increasing lymphocyte detachment under shear flow. Heterotypic intercellular adhesion mediated by VCAM-1 or ICAM-1 was augmented when expressed exogenously in the appropriate tetraspanin environment. Therefore, tetraspanin microdomains have a crucial role in the proper adhesive function of ICAM-1 and VCAM-1 during leukocyte adhesion and transendothelial migration.

The palmitoylation of metastasis suppressor KAI1/CD82 is important for its motility- and invasiveness-inhibitory activity

The cancer metastasis suppressor protein KAI1/CD82 is a member of the tetraspanin superfamily. Recent studies have demonstrated that tetraspanins are palmitoylated and that palmitoylation contributes to the organization of tetraspanin webs or tetraspanin-enriched microdomains. However, the effect of palmitoylation on tetraspanin-mediated cellular functions remains obscure. In this study, we found that tetraspanin KAI1/CD82 was palmitoylated when expressed in PC3 metastatic prostate cancer cells and that palmitoylation involved all of the cytoplasmic cysteine residues proximal to the plasma membrane. Notably, the palmitoylation-deficient KAI1/CD82 mutant largely reversed the wild-type KAI1/CD82's inhibitory effects on migration and invasion of PC3 cells. Also, palmitoylation regulates the subcellular distribution of KAI1/CD82 and its association with other tetraspanins, suggesting that the localized interaction of KAI1/CD82 with tetraspanin webs or tetraspanin-enriched microdomains is important for KAI1/CD82's motility-inhibitory activity. Moreover, we found that KAI1/CD82 palmitoylation affected motility-related subcellular events such as lamellipodia formation and actin cytoskeleton organization and that the alteration of these processes likely contributes to KAI1/CD82's inhibition of motility. Finally, the reversal of cell motility seen in the palmitoylation-deficient KAI1/CD82 mutant correlates with regaining of p130(CAS)-CrkII coupling, a signaling step important for KAI1/CD82's activity. Taken together, our results indicate that palmitoylation is crucial for the functional integrity of tetraspanin KAI1/CD82 during the suppression of cancer cell migration and invasion.

Disruption of WT1 gene expression and exon 5 splicing following cytotoxic drug treatment: Antisense down-regulation of exon 5 alters target gene expression and inhibits cell survival

Deregulated expression of the Wilms' tumor gene (WT1) has been implicated in the maintenance of a malignant phenotype in leukemias and a wide range of solid tumors through interference with normal signaling in differentiation and apoptotic pathways. Expression of high levels of WT1 is associated with poor prognosis in leukemias and breast cancer. Using real-time (Taqman) reverse transcription-PCR and RNase protection assay, we have shown up-regulation of WT1 expression following cytotoxic treatment of cells exhibiting drug resistance, a phenomenon not seen in sensitive cells. WT1 is subject to alternative splicing involving exon 5 and three amino acids (KTS) at the end of exon 9, producing four major isoforms. Exon 5 splicing was disrupted in all cell lines studied following a cytotoxic insult probably due to increased exon 5 skipping. Disruption of exon 5 splicing may be a proapoptotic signal because specific targeting of WT1 exon 5–containing transcripts using a nuclease-resistant antisense oligonucleotide (ASO) killed HL60 leukemia cells, which were resistant to an ASO targeting all four alternatively spliced transcripts simultaneously. K562 cells were sensitive to both target-specific ASOs. Gene expression profiling following treatment with WT1 exon 5–targeted antisense showed up-regulation of the known WT1 target gene, thrombospondin 1, in HL60 cells, which correlated with cell death. In addition, novel potential WT1 target genes were identified in each cell line. These studies highlight a new layer of complexity in the regulation and function of the WT1 gene product and suggest that antisense directed to WT1 exon 5 might have therapeutic potential.

The tetraspanin superfamily member CD151 regulates outside-in integrin αIIbβ3 signaling and platelet function

The tetraspanin family member CD151 forms complexes with integrins and regulates cell adhesion and migration. While CD151 is highly expressed in megakaryocytes and to a lesser extent in platelets, its physiologic role in platelets is unclear. In this study, we investigate the physical and functional importance of CD151 in murine platelets. Immunoprecipitation/Western blot studies reveal a constitutive physical association of CD151 with integrin αIIbβ3 complex under strong detergent conditions. Using CD151-deficient mice, we show that the platelets have impaired “outside-in” integrin αIIbβ3 signaling with defective platelet aggregation responses to protease-activated receptor 4 (PAR-4) agonist peptide, collagen, and adenosine diphosphate (ADP); impaired platelet spreading on fibrinogen; and delayed kinetics of clot retraction in vitro. This functional integrin αIIbβ3 defect could not be attributed to altered expression of integrin αIIbβ3. CD151–/– platelets displayed normal platelet alpha granule secretion, dense granule secretion, and static platelet adhesion. In addition, CD151–/– platelets displayed normal “inside-out” integrin αIIbβ3 signaling properties as demonstrated by normal agonist-induced binding of soluble fluorescein isothiocyanate (FITC)–fibrinogen, JON/A antibody binding, and increases in cytosolic-free calcium and inositol 1,4,5 triphosphate (IP3) levels. This study provides the first direct evidence that CD151 is essential for normal platelet function and that disruption of CD151 induced a moderate outside-in integrin αIIbβ3 signaling defect.

Tetraspanin protein (TSP-15) is required for epidermal integrity inCaenorhabditis elegans

Epidermal integrity is essential for animal development and survival. Here, we demonstrate that TSP-15, a member of the tetraspanin protein family, is required for epithelial membrane integrity in Caenorhabditis elegans. Reduction of tsp-15 function by mutation or by RNA interference elicits abnormalities of the hypodermis, including dissociation of the cuticle and degeneration of the hypodermis. Lethality during molting often results. Examination of GFP transgenic animals, genetic mosaic analysis and rescue assays revealed that TSP-15 functions in hyp7, a large syncytium that composes most of the hypodermis. Assays with a membrane-impermeable dye or leakage analysis of a hypodermal-specific marker indicate that the barrier function of the hypodermal membrane is impaired owing to the loss or reduction of TSP-15. These results indicate that TSP-15 functions in the maintenance of epithelial cell integrity.

Developmental expression of OAP-1/Tspan-3, a member of the tetraspanin superfamily

OSP/claudin-11-associated protein (OAP-1/Tspan-3), originally isolated by yeast two-hybrid screening using OSP/claudin-11 (oligodendrocyte-specific protein) as bait, is a member of the tetraspanin superfamily and OAP-1/Tspan-3, OSP/claudin-11, and beta1 integrin form a protein complex that seems to be involved in oligodendrocyte proliferation and migration. This study investigated the temporal and regional expression, glycosylation status, and tissue distribution of OAP-1/Tspan-3. OAP-1/Tspan-3 mRNA was expressed as a single transcript throughout brain development, with high levels of expression in the germinal zones. OAP-1/Tspan-3 protein contains N-terminal glycosylation sites in extracellular loop 2 and deglycosylation studies indicated a decrease in apparent molecular weight of OAP-1/Tspan-3, consistent with removal of N-glycans. Similar to OSP/claudin-11, OAP-1/Tspan-3 is expressed in all stages of oligodendrocyte development and in the myelin sheath. Unlike OSP/claudin-11, however, it is expressed in all cell types tested in the central nervous system (CNS), including neurons and astrocytes. The association of OAP-1/Tspan-3 with OSP/claudin-11 and beta1 integrin, its subcellular distribution as a cell surface, membrane-spanning glycoprotein, and its widespread distribution supports its potential role in cell migration, proliferation, and interactions between cells and extracellular matrix.

Sertoli-Sertoli and Sertoli-germ cell interactions and their significance in germ cell movement in the seminiferous epithelium during spermatogenesis

Spermatogenesis is the process by which a single spermatogonium develops into 256 spermatozoa, one of which will fertilize the ovum. Since the 1950s when the stages of the epithelial cycle were first described, reproductive biologists have been in pursuit of one question: How can a spermatogonium traverse the epithelium, while at the same time differentiating into elongate spermatids that remain attached to the Sertoli cell throughout their development? Although it was generally agreed upon that junction restructuring was involved, at that time the types of junctions present in the testis were not even discerned. Today, it is known that tight, anchoring, and gap junctions are found in the testis. The testis also has two unique anchoring junction types, the ectoplasmic specialization and tubulobulbar complex. However, attention has recently shifted on identifying the regulatory molecules that "open" and "close" junctions, because this information will be useful in elucidating the mechanism of germ cell movement. For instance, cytokines have been shown to induce Sertoli cell tight junction disassembly by shutting down the production of tight junction proteins. Other factors such as proteases, protease inhibitors, GTPases, kinases, and phosphatases also come into play. In this review, we focus on this cellular phenomenon, recapping recent developments in the field.

Tetraspanin CD82 controls the association of cholesterol-dependent microdomains with the actin cytoskeleton in T lymphocytes: relevance to co-stimulation

T-cell activation is initiated by the concerted engagement of the T-cell receptor and different co-stimulatory molecules, and requires cytoskeleton-dependent membrane dynamics. Here, we have studied the relationships between tetraspanins, cytoskeleton and raft microdomains, and their relevance in T-cell signaling. Localization studies and density-gradient flotation experiments indicate that part of tetraspanins localizes in raft microdomains linked to the actin cytoskeleton. First, partial coalescence of lipid raft is triggered by tetraspanin cross-linking and results in large caps in which F-actin also concentrates. Second, the amount of tetraspanins, which are recovered in the cholesterol-dependent insoluble fractions of low and intermediate density, and which appears to be membrane vesicles by electron microscopy, is under cytoskeletal influence. Disruption of actin filaments enhances the amount of tetraspanins recovered in typical raft fractions, whereas F-actin-stabilizing agents induce the opposite effect. Our data also reveal that CD82 constitutes a link between raft domains and the actin cytoskeleton, which is functionally relevant. First, tetraspanin signaling induces a selective translocation of CD82 from detergent-resistant membrane fractions to the cytoskeleton-associated pellet. Second, all functional effects linked to CD82 engagement, such as adhesion to culture plates, formation of actin bundles and early events of tyrosine phosphorylation, are abolished, or strongly reduced, by cholesterol depletion. We also show that dynamic relocalization of CD82 and F-actin at the periphery of the immune synapse is induced upon contact of T cells with antigen-presenting cells. This suggests that the tetraspanin web might participate in the membrane dynamics required for proper T-cell signaling. More generally, the interaction of tetraspanins with raft domains and with the actin cytoskeleton might relate with their role in many cellular functions as membrane organizers.

Partitioning of the Serotonin Transporter into Lipid Microdomains Modulates Transport of Serotonin

The serotonin transporter (SERT) is an integral membrane protein responsible for the clearance of serotonin from the synaptic cleft following the release of the neurotransmitter. SERT plays a prominent role in the regulation of serotoninergic neurotransmission and is a molecular target for multiple antidepressants as well as substances of abuse. Here we show that SERT associates with lipid rafts in both heterologous expression systems and rat brain and that the inclusion of the transporter into lipid microdomains is critical for serotonin uptake activity. SERT is present in a subpopulation of lipid rafts, which is soluble in Triton X-100 but insoluble in other non-ionic detergents such as Brij 58. Disaggregation of lipid rafts upon depletion of cellular cholesterol results in a decrease of serotonin transport capacity (V(max)), due to the reduction of turnover number of serotonin transport. Our data suggest that the association of SERT with lipid rafts may represent a mechanism for regulating the transporter activity and, consequently, serotoninergic signaling in the central nervous system, through the modulation of the cholesterol content in the cell membrane. Furthermore, SERT-containing rafts are detected in both intracellular and cell surface fractions, suggesting that raft association may be important for trafficking and targeting of SERT.

Characterization of mice lacking the tetraspanin superfamily member CD151

The tetraspanin membrane protein CD151 is a broadly expressed molecule noted for its strong molecular associations with integrins, especially alpha3beta1, alpha6beta1, alpha7beta1, and alpha6beta4. In vitro functional studies have pointed to a role for CD151 in cell-cell adhesion, cell migration, platelet aggregation, and angiogenesis. It has also been implicated in epithelial tumor progression and metastasis. Here we describe the generation and initial characterization of CD151-null mice. The mice are viable, healthy, and fertile and show normal Mendelian inheritance. They have essentially normal blood and bone marrow cell counts and grossly normal tissue morphology, including hemidesmosomes in skin, and expression of alpha3 and alpha6 integrins. However, the CD151-null mice do show phenotypes in several different tissue types. An absence of CD151 leads to a minor abnormality in hemostasis, with CD151-null mice showing longer average bleeding times, greater average blood loss, and an increased incidence of rebleeding occurrences. CD151-null keratinocytes migrate poorly in skin explant cultures. Finally, CD151-null T lymphocytes are hyperproliferative in response to in vitro mitogenic stimulation.

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.

Differential modulation of human melanoma cell metalloproteinase expression by alpha2beta1 integrin and CD44 triple-helical ligands derived from type IV collagen

Tumor cell binding to components of the basement membrane is well known to trigger intracellular signaling pathways. Signaling ultimately results in the modulation of gene expression, facilitating metastasis. Type IV collagen is the major structural component of the basement membrane and is known to be a polyvalent ligand, possessing sequences bound by the alpha1beta1, alpha2beta1, and alpha3beta1 integrins, as well as cell surface proteoglycan receptors, such as CD44/chondroitin sulfate proteoglycan (CSPG). The role of alpha2beta1 integrin and CD44/CSPG receptor binding on human melanoma cell activation has been evaluated herein using triple-helical peptide ligands incorporating the alpha1(IV)382-393 and alpha1(IV)1263-1277 sequences, respectively. Gene expression and protein production of matrix metalloproteinases-1 (MMP-1), -2, -3, -13, and -14 were modulated with the alpha2beta1-specific sequence, whereas the CD44-specific sequence yielded significant stimulation of MMP-8 and lower levels of modulation of MMP-1, -2, -13, and -14. Analysis of enzyme activity confirmed different melanoma cell proteolytic potentials based on engagement of either the alpha2beta1 integrin or CD44/CSPG. These results are indicative of specific activation events that tumor cells undergo upon binding to select regions of basement membrane collagen. Based on the present study, triple-helical peptide ligands provide a general approach for monitoring the regulation of proteolysis in cellular systems.

Control of Immune Responses by Trafficking Cell Surface Proteins, Vesicles and Lipid Rafts to and from the Immunological Synapse

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra- and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory as required lysosomes. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.

CD151, the first member of the tetraspanin (TM4) superfamily detected on erythrocytes, is essential for the correct assembly of human basement membranes in kidney and skin

Tetraspanins are thought to facilitate the formation of multiprotein complexes at cell surfaces, but evidence illuminating the biologic importance of this role is sparse. Tetraspanin CD151 forms very stable laminin-binding complexes with integrins alpha3beta1 and alpha6beta1 in kidney and alpha3beta1 and alpha6beta4 in skin. It is encoded by a gene at the same position on chromosome 11p15.5 as the MER2 blood group gene. We show that CD151 expresses the MER2 blood group antigen and is located on erythrocytes. We examined CD151 in 3 MER2-negative patients (2 are sibs) of Indian Jewish origin with end-stage kidney disease. In addition to hereditary nephritis the sibs have sensorineural deafness, pretibial epidermolysis bullosa, and beta-thalassemia minor. The 3 patients are homozygous for a single nucleotide insertion (G383) in exon 5 of CD151, causing a frameshift and premature stop signal at codon 140. The resultant truncated protein would lack its integrin-binding domain. We conclude that CD151 is essential for the proper assembly of the glomerular and tubular basement membrane in kidney, has functional significance in the skin, is probably a component of the inner ear, and could play a role in erythropoiesis.

Localization of CD9 in pig oocytes and its effects on sperm-egg interaction

CD9 is a cell surface protein that participates in many cellular processes, such as cell adhesion. Fertilization involves sperm and oocyte interactions including sperm binding to oocytes and sperm-oocyte fusion. Thus CD9 may play an essential role during fertilization in mammals. The present study was conducted to examine whether CD9 is present in porcine gametes and whether it participates in the regulation of sperm-oocyte interactions. The presence of CD9 in ovarian tissues, oocytes and spermatozoa was examined by immunohistochemistry, immunofluorescence and immunoblotting. Sperm binding and penetration of oocytes treated with CD9 antibody were examined by in vitro fertilization. The results showed that CD9 was present on the plasma membrane of oocytes at different developmental stages. A 24 kDa protein was found in oocytes during in vitro maturation by immunoblotting and its quantity was significantly (P < 0.001) increased as oocytes underwent maturation and reached the highest level after the oocytes had been cultured for 44 h. No positive CD9 staining was found in the spermatozoa. Both sperm binding to ooplasma and sperm penetration into oocytes were significantly (P < 0.01) reduced in anti-CD9 antibody-treated oocytes (1.2 +/- 0.2 per oocyte and 16.6% respectively) as compared with oocytes in the controls (2.5 +/- 0.4 per oocyte and 70.3% respectively). These results indicated that CD9 is expressed in pig oocytes during early growth and meiotic maturation and that it participates in sperm-oocyte interactions during fertilization.

Transcriptomic changes in human breast cancer progression as determined by serial analysis of gene expression

Introduction

Genomic and transcriptomic alterations affecting key cellular processes such us cell proliferation, differentiation and genomic stability are considered crucial for the development and progression of cancer. Most invasive breast carcinomas are known to derive from precursor in situ lesions. It is proposed that major global expression abnormalities occur in the transition from normal to premalignant stages and further progression to invasive stages. Serial analysis of gene expression (SAGE) was employed to generate a comprehensive global gene expression profile of the major changes occurring during breast cancer malignant evolution.

Methods

In the present study we combined various normal and tumor SAGE libraries available in the public domain with sets of breast cancer SAGE libraries recently generated and sequenced in our laboratory. A recently developed modified t test was used to detect the genes differentially expressed.

Results

We accumulated a total of approximately 1.7 million breast tissue-specific SAGE tags and monitored the behavior of more than 25,157 genes during early breast carcinogenesis. We detected 52 transcripts commonly deregulated across the board when comparing normal tissue with ductal carcinoma in situ, and 149 transcripts when comparing ductal carcinoma in situ with invasive ductal carcinoma (P < 0.01).

Conclusion

A major novelty of our study was the use of a statistical method that correctly accounts for the intra-SAGE and inter-SAGE library sources of variation. The most useful result of applying this modified t statistics beta binomial test is the identification of genes and gene families commonly deregulated across samples within each specific stage in the transition from normal to preinvasive and invasive stages of breast cancer development. Most of the gene expression abnormalities detected at the in situ stage were related to specific genes in charge of regulating the proper homeostasis between cell death and cell proliferation. The comparison of in situ lesions with fully invasive lesions, a much more heterogeneous group, clearly identified as the most importantly deregulated group of transcripts those encoding for various families of proteins in charge of extracellular matrix remodeling, invasion and cell motility functions.

A regulatory role for CD37 in T cell proliferation

CD37 is a leukocyte-specific protein belonging to the tetraspanin superfamily. Previously thought to be predominantly a B cell molecule, CD37 is shown in this study to regulate T cell proliferation. CD37-deficient (CD37(-/-)) T cells were notably hyperproliferative in MLR, in response to Con A, or CD3-TCR engagement particularly in the absence of CD28 costimulation. Hyperproliferation was not due to differences in memory to naive T cell ratios in CD37(-/-) mice, apoptosis, or TCR down-modulation. Division cycle analyses revealed CD37(-/-) T cells to enter first division earlier than wild-type T cells. Importantly, proliferation of CD37(-/-) T cells was preceded by enhanced early IL-2 production. We hypothesized CD37 to be involved in TCR signaling and this was supported by the observation that CD4/CD8-associated p56(Lck) kinase activity was increased in CD37(-/-) T cells. Remarkably, CD37 cross-linking on human T cells transduced signals that led to complete inhibition of CD3-induced proliferation. In the presence of CD28 costimulation, CD37 engagement still significantly reduced proliferation. Taken together, these results demonstrate a regulatory role for CD37 in T cell proliferation by influencing early events of TCR signaling.

Translation-state analysis of gene expression in mouse brain after focal ischemia

Confounding any genome-scale analysis of gene expression after cerebral ischemia is massive suppression of protein synthesis. This inefficient translation questions the utility of examining profiles of total transcripts. Our approach to such postischemic gene profiling in the mouse by microarray analysis was to concentrate on those mRNAs bound to polyribosomes. In our proof-of-principle study, polysomally bound and unbound mRNAs were subjected to microarray analysis: of the 1,161 transcripts that we found to increase after ischemia, only 36% were bound to polyribosomes. In addition to the expected increases in heat-shock proteins and metallothioneins, increases in several other bound transcripts involved in the promotion of cell survival or antiinflammatory behavior were noted, such as CD63 (Lamp3), Lcn2 (lipocalin-2), Msn (moesin), and UCP2 (uncoupling protein 2), all of which showed increases in cognate protein by Western blotting. The list of heretofore nonfunctionally annotated transcripts (RIKEN clones/ESTs) that increased appeared to be novel. How some transcripts are selected in ischemic brain for translation into protein, while others are rejected, is not clear. The length of the 5'-UTR in the ischemically induced transcripts that occur in the NCBI RefSeq database did not indicate any general tendency to be more than 200 nt, nor to be longer than the 5'-UTRs of the unbound transcripts. Thus, the presence of a complex 5'-UTR region with internal ribosome entry sites (IRES) or polypyrimidine tracts (TOP) does not appear to be the basis of selection for translation in ischemic brain.

Variant glycosylation: an underappreciated regulatory mechanism for β1 integrins

Although it has been known for many years that beta1 integrins undergo differential glycosylation in accordance with changes in cell phenotype, the potential role of N-glycosylation as a modulator of integrin function has received little attention. One reason for the relatively limited interest in this topic likely relates to the fact that much of the prior research was correlative in nature. However, new results now bolster the hypothesis that there is a causal relationship between variant glycosylation and altered integrin activity. In this review, the evidence for variant glycosylation as a regulatory mechanism for beta1 integrins are summarized, with particular emphasis on: (1). outlining the instances in which cell phenotypic variation is associated with differential beta1 glycosylation, (2). describing the specific alterations in glycan structure that accompany phenotypic changes and (3). presenting potential mechanisms by which variant glycosylation might regulate integrin function.