Signal transduction during exocytosis in Limulus polyphemus granulocytes

One recombinant C-type lectin (LvLec) from white shrimp Litopenaeus vannamei affected the haemocyte immune response in vitro

C-type lectin has received widespread attention in animal immunomodulation functions since it was discovered, but it is still limited in crustaceans. The present study is to explore effects of one recombinant C-type lectin (LvLec protein) on haemocyte immune response in Litopenaeus vannamei (L. vannamei). The methods of keeping haemocyte immune activity were optimised by the Key Laboratory of Mariculture. The experiment was divided into four groups: control group, recombinant protein group (LvLec protein, 1.0 mg mL^-1), Lipopolysaccharide group (LPS, 1.0 mg mL^-1), and LPS combine with LvLec protein group (LPS + LvLec protein, 1.0 mg mL^-1 + 1.0 mg mL^-1), while each group processes 0, 3, 6, 9, 12, and 24 h respectively. The results showed that the haemocyte count reduced, while the exocytosis PO activity, hemagglutinating activity and phagocytic activity promoted, and the concentration of cGMP and PKA increased after LvLec protein treatment. However, the levels of antibacterial activity and bacteriolytic activity as well as the concentrations of cAMP and PKG did not change significantly after treating with LvLec protein, LPS or LPS + LvLec protein. Therefore, these results suggest that LvLec protein can stimulate the exocytosis PO activity through cGMP-PKA pathway to affect the phagocytic activity and hemagglutinating activity of L. vannamei haemocytes in vitro.

Fine-tuned ATP signals are acute mediators in osteocyte mechanotransduction

Osteocytes are considered the primary mechanosensors of bone, but the signaling pathways they apply in mechanotransduction are still incompletely investigated and characterized. A growing body of data strongly indicates that P2 receptor signaling among osteoblasts and osteoclasts has regulatory effects on bone remodeling. Therefore, we hypothesized that ATP signaling is also applied by osteocytes in mechanotransduction. We applied a short fluid pulse on MLO-Y4 osteocyte-like cells during real-time detection of ATP and demonstrated that mechanical stimulation activates the acute release of ATP and that these acute ATP signals are fine-tuned according to the magnitude of loading. ATP release was then challenged by pharmacological inhibitors, which indicated a vesicular release pathway for acute ATP signals. Finally, we showed that osteocytes express functional P2X2 and P2X7 receptors and respond to even low concentrations of nucleotides by increasing intracellular calcium concentration. These results indicate that in osteocytes, vesicular ATP release is an acute mediator of mechanical signals and the magnitude of loading. These and previous results, therefore, implicate purinergic signaling as an early signaling pathway in osteocyte mechanotransduction.

Development of a monoclonal antibody specific to granulocytes and its application for variation of granulocytes in scallop Chlamys farreri after acute viral necrobiotic virus (AVNV) infection

A monoclonal antibody (MAb 6H7) specific to granulocytes of scallop Chlamys farreri was produced by immunising mice with separated granulocytes as an antigen. Characterised using a flow cytometric immunofluorescence assay, MAb 6H7 reacted to granulocytes by 87.1% of total positive haemocytes. At the ultrastructural level, MAb 6H7 demonstrated epitope in cytoplasmic granules of granulocytes. Western blotting analysis indicated that a peptide of 155 kDa was recognised by MAb 6H7. It was therefore used to investigate granulocyte variation in C. farreri after acute viral necrobiotic virus (AVNV) infection using an enzyme-linked immunosorbent assay. The result illustrated that granulocytes varied greatly by AVNV infection, and their amount significantly increased on day 1 post-injection, then decreased on days 2, 3 and 4, thereafter, rebounded and approached to a second peak on day 6, finally went down gradually to the control level on day 8.

Identification of an in vitro interaction between an insect immune suppressor protein (CrV2) and G alpha proteins

The protein CrV2 is encoded by a polydnavirus integrated into the genome of the endoparasitoid Cotesia rubecula (Hymenoptera:Braconidae:Microgastrinae) and is expressed in host larvae with other gene products of the polydnavirus to allow successful development of the parasitoid. CrV2 expression has previously been associated with immune suppression, although the molecular basis for this was not known. Here, we have used time-resolved Förster resonance energy transfer (TR-FRET) to demonstrate high affinity binding of CrV2 to Gα subunits (but not the Gβγ dimer) of heterotrimeric G-proteins. Signals up to 5-fold above background were generated, and an apparent dissociation constant of 6.2 nm was calculated. Protease treatment abolished the TR-FRET signal, and the presence of unlabeled CrV2 or Gα proteins also reduced the TR-FRET signal. The activation state of the Gα subunit was altered with aluminum fluoride, and this decreased the affinity of the interaction with CrV2. It was also demonstrated that CrV2 preferentially bound to Drosophila Gα(o) compared with rat Gα(i1). In addition, three CrV2 homologs were detected in sequences derived from polydnaviruses from Cotesia plutellae and Cotesia congregata (including the immune-related early expressed transcript, EP2). These data suggest a potential mode-of-action of immune suppressors not previously reported, which in addition to furthering our understanding of insect immunity may have practical benefits such as facilitating development of novel controls for pest insect species.

Blood collection from the American horseshoe crab, Limulus polyphemus

The horseshoe crab has the best-characterized immune system of any long-lived invertebrate. The study of immunity in horseshoe crabs has been facilitated by the ease in collecting large volumes of blood and from the simplicity of the blood. Horseshoe crabs show only a single cell type in the general circulation, the granular amebocyte. The plasma has the salt content of sea water and only three abundant proteins, hemocyanin, the respiratory protein, the C-reactive proteins, which function in the cytolytic destruction of foreign cells, including bacterial cells, and α2-macroglobulin, which inhibits the proteases of invading pathogens. Blood is collected by direct cardiac puncture under conditions that minimize contamination by lipopolysaccharide (a.k.a., endotoxin, LPS), a product of the Gram-negative bacteria. A large animal can yield 200 - 400 mL of blood. For the study of the plasma, blood cells are immediately removed from the plasma by centrifugation and the plasma can then be fractionated into its constituent proteins. The blood cells are conveniently studied microscopically by collecting small volumes of blood into LPS-free isotonic saline (0.5 M NaCl) under conditions that permit direct microscopic examination by placing one of more LPS-free coverglasses on the culture dish surface, then mounting those coverglasses in simple observation chambers following cell attachment. A second preparation for direct observation is to collect 3 - 5 mL of blood in a LPS-free embryo dish and then explanting fragments of aggregated amebocytes to a chamber that sandwiches the tissue between a slide and a coverglass. In this preparation, the motile amebocytes migrate onto the coverglass surface, where they can readily be observed. The blood clotting system involves aggregation of amebocytes and the formation of an extracellular clot of a protein, coagulin, which is released from the secretory granules of the blood cells. Biochemical analysis of washed blood cells requires that aggregation and degranulation does not occur, which can be accomplished by collecting blood into 0.1 volumes of 2% Tween-20, 0.5 M LPS-free NaCl, followed by centrifugation of the cells and washing with 0.5 M NaCl.

Recognition of pathogens and activation of immune responses in Drosophila and horseshoe crab innate immunity

In innate immunity, pattern recognition receptors discriminate between self- and infectious non-self-matter. Mammalian homologs of the Drosophila Toll protein, which are collectively referred to as Toll-like receptors (TLRs), recognize pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS) and lipoproteins, whereas the Drosophila Toll protein does not act as a PAMP receptor, but rather binds to Spätzle, an endogenous peptide. In Drosophila, innate immune surveillance is mediated by members of the peptidoglycan recognition protein (PGRP) family, which recognize diverse bacteria-derived peptidoglycans and initiate appropriate immune reactions including the release of antimicrobial peptides and the activation of the prophenoloxidase cascade, the latter effecting localized wound healing, melanization, and microbial phagocytosis. In the horseshoe crab, LPS induces hemocyte exocytotic degranulation, resulting in the secretion of various defense molecules, such as coagulation factors, antimicrobial peptides, and lectins. Recent studies have demonstrated that the zymogen form of the serine protease factor C, a major granular component of hemocyte, also exists on the hemocyte surface and functions as a biosensor for LPS. The proteolytic activity of activated factor C initiates hemocyte exocytosis via a G protein mediated signal transduction pathway. Furthermore, it has become clear that an endogenous mechanism for the feedback amplification of the innate immune response exists and is dependent upon a granular component of the horseshoe crab hemocyte.

Spatial and temporal coordination of expression of immune response genes during Pseudomonas infection of horseshoe crab, Carcinoscorpius rotundicauda

Knowledge on how genes are turned on/off during infection and immunity is lacking. Here, we report the co-regulation of diverse clusters of functionally related immune response genes in a horseshoe crab, Carcinoscorpius rotundicauda. Expressed sequence tag (EST) clusters for frontline immune defense, cell signalling, apoptosis and stress response genes were expressed or repressed spatio-temporally during the acute phase of Pseudomonas infection. An infection time course monitored by virtual Northern evaluation indicates upregulation of genes in blood cells (amebocytes) at 3-h postinfection, whereas most of the hepatopancreas genes remained down regulated over 72 h of infection. Thus, the two tissues orchestrate a coordinated and timely response to infection. The hepatopancreas probably immuno-modulates the expression of other genes and serves as a reservoir for later response, if/when chronic infection ensues. On the other hand, being the first to encounter pathogens, we reasoned that amebocytes would respond acutely to infection. Besides acute transactivation of the immune genes, the amebocytes maintained morphological integrity, indicating their ability to synthesise and store/secrete the immune proteins and effectors to sustain the frontline innate immune defense, while simultaneously elicit complement-mediated phagocytosis of the invading pathogen. Our results show that the immune response against Pseudomonas infection is spatially and temporally coordinated.

Signal transduction of the prophenoloxidase activating system of prawn haemocytes triggered by CpG oligodeoxynucleotides

Intracellular phenoloxidase (PO) activity in haemocyte lysate supernatant (HLS) of giant freshwater prawn (Macrobrachium rosenbergii) was shown to be enhanced by CpG oligodeoxynucleotide (ODN) 2006, but not by so-ODN13. When haemocytes were treated in vitro with 50 microg/ml of ODN2006 for 30 min, the increases in both intra- and extracellular stimulated PO activity (POS) and extracellular total PO activity (POT) and the reduction of POT suggest that the PO activity of haemocytes is enhanced by ODN2006 stimulation, but new prophenoloxidase (proPO) is not synthesised. In an attempt to determine which signal transduction pathway is involved in the activation of the proPO system, haemocytes were separately treated with activators or inhibitors of specific signalling components. The results show that there was an increase in both intra- and extracellular POT of haemocytes treated with sodium fluoride (a G-protein activator); the addition of phosphokinase A (PKA)-activating 8-bromo-cAMP to haemocytes only increased intracellular POT, and the addition of either phorbol-12-myristate-13-acetate (PMA; a phosphokinase C (PKC) activator) or caffeine (a phosphodiesterase inhibitor) only increased extracellular POT. When PMA-stimulated haemocytes were treated with chelerythrine (a PKC inhibitor), the induced extracellular POT was significantly reduced. Furthermore, the study of ODN2006-stimulated haemocytes treated with chelerythrine or palmitoyl-DL-carnitine (a PKC inhibitor) showed that the enhancement effects of ODN2006 on the intra- and extracellular POS and extracellular POT were significantly decreased. ODN-stimulated haemocytes treated with genistein (an inhibitor of protein tyrosine kinase) showed a further increase in extracellular POT, but the other PO activities remained the same as those of the ODN-stimulated group. These results suggest that the activation of the proPO system of prawn haemocytes, including degranulation and PO activity, is induced by ODN2006 via a PKC-activating signalling pathway, but negatively regulated via the tyrosine kinase pathway.

Microarray analyses identify molecular biomarkers of Atlantic salmon macrophage and hematopoietic kidney response to Piscirickettsia salmonis infection

Piscirickettsia salmonis is the intracellular bacterium that causes salmonid rickettsial septicemia, an infectious disease that kills millions of farmed fish each year. The mechanisms used by P. salmonis to survive and replicate within host cells are not known. Piscirickettsiosis causes severe necrosis of hematopoietic kidney. Microarray-based experiments with QPCR validation were used to identify Atlantic salmon macrophage and hematopoietic kidney genes differentially transcribed in response to P. salmonis infection. Infections were confirmed by microscopy and RT-PCR with pathogen-specific primers. In infected salmon macrophages, 71 different transcripts were upregulated and 31 different transcripts were downregulated. In infected hematopoietic kidney, 30 different transcripts were upregulated and 39 different transcripts were downregulated. Ten antioxidant genes, including glutathione S-transferase, glutathione reductase, glutathione peroxidase, and cytochrome b558 alpha- and beta-subunits, were upregulated in infected macrophages but not in infected hematopoietic kidney. Changes in redox status of infected macrophages may allow these cells to tolerate P. salmonis infection, raising the possibility that treatment with antioxidants may reduce hematopoietic tissue damage caused by this rickettsial infection. The downregulation of transcripts involved in adaptive immune responses (e.g., T cell receptor alpha-chain and C-C chemokine receptor 7) in infected hematopoietic kidney but not in infected macrophages may contribute to infection-induced kidney tissue damage. Molecular biomarkers of P. salmonis infection, characterized by immune-relevant functional annotations and high fold differences in expression between infected and noninfected samples, may aid in the development of anti-piscirickettsial vaccines and therapeutics.

Exocytosis of a complement component C3-like protein by tunicate hemocytes

This study investigates the exocytic responses of invertebrate hemocytes to pathogen-associated antigens. It demonstrates that a homologue of complement component C3, a key defensive protein of the innate immune system, is expressed by phagocytic hemocytes (non-refractile vacuolated cells) of the tunicate, Styela plicata. C3-like molecules are localized in sub-cellular vesicles and are rapidly exocytosed after stimulation with bacterial, fungal or algal cell surface molecules. Signal transduction analysis indicated that the induced secretion of C3-like molecules is mediated by a G-protein dependent signaling pathway, which modulates tubulin microtubules. All of this evidence indicates that hemocytes can contribute to host defense responses by rapidly exocytosing C3-like proteins at sites of infection.

A serine protease zymogen functions as a pattern-recognition receptor for lipopolysaccharides

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 beta-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.

An 18.5 kDa protein from the amebocyte of Limulus polyphemus, homologous to the previously described amebocyte aggregation factor, expresses alternative phospholipase A2 activity

A protein expressing phospholipase A2 activity was purified from the granular amebocyte of the horseshoe crab, Limulus polyphemus by cation-exchange, size-exclusion chromatography and semi-preparative reverse-phase-high pressure liquid chromatography (RP-HPLC). The protein had an apparent mass of 17.7 kDa by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), but a more accurate estimate of 18.5 kDa was assigned by electrospray ionization-mass spectrometry (ESI-MS). A partial sequence of this protein demonstrated total sequence homology with an 18.5 kDa protein with cell aggregating properties from Limulus reported by Fujii et al. [J. Biol. Chem. 267:22452.]. In these studies, the Limulus protein demonstrated a positive cross-reaction to polyclonal anti-human recombinant phospholipase A2 (group II, 14 kDa). The protein did not display a significant loss of biological activity after boiling, but all enzymatic activity was lost after boiling in the presence of the reducing agent betamercaptoethanol (beta-mercaptoethanol). The Limulus protein was inhibited by manoalide, a covalent irreversible phospholipase A2 inhibitor, in a dose-dependent fashion with 50% inhibition occurring at a concentration of 0.48 microM. The Limulus protein displayed no activity in a triglyceride lipase assay. These studies characterize an alternative phospholipase A2 activity for the previously described 18.5 kDa protein from the L. polyphemus amebocyte.

Localization of a putative inositol 1,4,5-triphosphate receptor in the Limulus granulocyte

The horseshoe crab (Limulus polyphemus) granulocyte (GR) degranulates upon contact with bacteria and release factors that mediate an immune response. Stimulated cells produce IP3, which binds to receptors (IP3R, M.W.240-300 kD) that function to release stored Ca2+ into the cytoplasm that mediates degranulation. This mechanism is believed to mediate exocytosis in the Limulus GR but IP3R in the GR has not been shown. The present study utilized monoclonal antibody 4C11 and a commercially available anti-IP3R antibody, both of which label amino acids of the N-terminal of all known isoforms. Electron microscopy, immunohistochemistry, SDS-PAGE, and Western blot analysis, which employed the use of the two antibodies, demonstrates that a putative IP3R exists in the: plasma membrane, smooth surfaced vesicles, nucleus and nuclear membrane. We hypothesize that this putative IP3R is involved in mediating the immune response of the Limulus GR.