Innate immunity against microbial pathogens relies on the pattern recognition of cell wall components on invading microbes. Recent evidence has shown that a mammalian Toll-like receptor (TLR) is activated by bacterial lipopolysaccharides (LPS). The innate immunity in invertebrates is also triggered by LPS, as seen in the hemolymph coagulation in horseshoe crab. We report the cloning of a TLR from the Japanese horseshoe crab Tachypleus tridentatus. A cDNA coding for Tachypleus Toll was isolated from a hemocyte cDNA library and the open reading frame codes for a proprotein including a signal sequence. Like Drosophila Toll, Tachypleus Toll is a type I transmembrane protein with an extracellular domain consisting of two leucine-rich repeats flanked by two cystein-rich clusters and a cytoplasmic domain exhibiting striking similarity with the cytoplasmic domain of interleukin-1 receptor. Tachypleus Toll is most similar to Drosophila Toll in the domain architecture and the overall length.
A 14-kDa lectin, named tachylectin-3, was newly identified from hemocytes of the Japanese horseshoe crab, Tachypleus tridentatus. This lectin exhibited hemagglutinating activity against human A-type erythrocytes, but not against the B- and O-types of erythrocytes and animal erythrocytes, including those of sheep, rabbit, horse, and bovine. The hemagglutinating activity of tachylectin-3 was equivalent to that of a previously identified lectin, named tachylectin-2, with affinity forN-acetyl-d-glucosamine orN-acetyl-d-galactosamine. However, the activity of tachylectin-3 was not inhibited by these twoN-acetylhexosamines at 100 mm but was inhibited by a blood group A-pentasaccharide at a minimum inhibitory concentration of 0.16 mm. Furthermore, the hemagglutinating activity was strongly inhibited by bacterial S-type lipopolysaccharides (LPSs) from Gram-negative bacteria but not by R-type LPSs lacking O-antigens. One of the most effective S-type LPSs was fromEscherichia coli O111:B4, with a minimum inhibitory concentration of 6 ng/ml. These data suggest that tachylectin-3 specifically recognizes Gram-negative bacteria through the unique structural units of O-antigens. Ultracentrifugation analysis revealed that tachylectin-3 is present in dimer in solution. A cDNA coding for tachylectin-3 was isolated from a hemocyte cDNA library. Tachylectin-3 consisted of two repeating sequences, each with a partial sequence similarity to rinderpest virus neuraminidase. Tachylectin-3 and three previously isolated types of tachylectins were all predominantly expressed in hemocytes and released from hemocytes in response to external stimuli. These lectins present at injured sites suggest that they probably serve synergistically to accomplish an effective host defense against invading microbes.
For successful human placentation, invasion of trophoblast cells into the uterus and its associated vasculature is essential, and the regulation of this process is controlled by many factors at the fetal-maternal interface. N-acetylglucosaminyltransferase V (GnT-V) is a key enzyme that catalyzes β1, 6-N-acetylglucosamine (β1-6GlcNAc) branching on asparagine-linked oligosaccharides of cell proteins. GnT-V and its product, β1-6GlcNAc, are known to regulate cellular transformation and correlate with the metastatic potential of various cancer cells. The aim of the present study was to determine whether extravillous trophoblast (EVT) expressed this molecule and examine the role of GnT-V in the regulation of human trophoblast invasion. Immunohistochemistry showed that GnT-V was strongly expressed within the cytoplasm of EVT in the anchoring villi; this expression was down-regulated in EVTs invading the decidua. Suppression of β1-6GlcNAc glycosylation by swainsonine enhanced the migratory potential and invasive capability of both primary EVTs and the EVT cell line, HTR-8/SVneo. Down-regulation of GnT-V expression by small interfering RNA in the choriocarcinoma cell line Jar consistently enhanced the migration and invasive capacity of these cells and elevated cellular adhesion to extracellular matrix proteins, such as fibronectin and collagen type I/IV. The extent of β1-6 branching of α5β1 integrin was significantly reduced in small interfering GnT-V-transfected Jar cells compared with mock transfectants, although the expression of α1, α5, α6, and β1 integrin on the cell surface was not changed. These results suggest that GnT-V is expressed in human EVT and is involved in regulating trophoblast invasion through modifications of the oligosaccharide chains of α5β1 integrin. N-acetylglucosaminyltransferase V (GnT-V) is expressed in human extravillous trophoblast and is involved in regulating trophoblast invasion through modifications of the oligosaccharide chains of α5β1 integrin.
Bacterial lipopolysaccharide (LPS)-induced exocytosis of granular hemocytes is a key component of the horseshoe crab's innate immunity to infectious microorganisms; stimulation by LPS induces the secretion of various defense molecules from the granular hemocytes. Using a previously uncharacterized assay for exocytosis, we clearly show that hemocytes respond only to LPS and not to other pathogen-associated molecular patterns, such as β-1,3-glucans and peptidoglycans. Furthermore, we show that a granular protein called factor C, an LPS-recognizing serine protease zymogen that initiates the hemolymph coagulation cascade, also exists on the hemocyte surface as a biosensor for LPS. Our data demonstrate that the proteolytic activity of factor C is both necessary and sufficient to trigger exocytosis through a heterotrimeric GTP-binding protein-mediating signaling pathway. Exocytosis of hemocytes was not induced by thrombin, but it was induced by hexapeptides corresponding to the tethered ligands of protease-activated G protein-coupled receptors (PARs). This finding suggested the presence of a PAR-like receptor on the hemocyte surface. We conclude that the serine protease zymogen on the hemocyte surface functions as a pattern-recognition protein for LPS.
Dystroglycan is a cell membrane receptor that organizes the basement membrane by binding ligands in the extracellular matrix. Proper glycosylation of the α-dystroglycan (α-DG) subunit is essential for these activities, and lack thereof results in neuromuscular disease. Currently, neither the glycan synthesis pathway nor the roles of many known or putative glycosyltransferases that are essential for this process are well understood. Here we show that FKRP, FKTN, TMEM5 and B4GAT1 (formerly known as B3GNT1) localize to the Golgi and contribute to the O-mannosyl post-phosphorylation modification of α-DG. Moreover, we assigned B4GAT1 a function as a xylose β1,4-glucuronyltransferase. Nuclear magnetic resonance studies confirmed that a glucuronic acid β1,4-xylose disaccharide synthesized by B4GAT1 acts as an acceptor primer that can be elongated by LARGE with the ligand-binding heteropolysaccharide. Our findings greatly broaden the understanding of α-DG glycosylation and provide mechanistic insight into why mutations in B4GAT1 disrupt dystroglycan function and cause disease.
