Lysophosphatidic acid-induced platelet shape change proceeds via Rho/Rho kinase-mediated myosin light-chain and moesin phosphorylation.

Platelet activation plays an important role in arterial thrombotic disorders. Here we show that the serum-borne phospholipid lysophosphatidic acid (LPA) activates the GTPase Rho and its target Rho-kinase to induce myosin light-chain (MLC) and moesin phosphorylation, leading to platelet shape change. MLC phosphorylation, moesin phosphorylation, and shape change were blocked by preincubating platelets with C3 transferase from Clostridium botulinum and Y-27632-specific inhibitors of Rho and Rho kinase, respectively. LPA did not increase the cytosolic Ca(2+) concentration during shape change. Our results suggest that LPA via Rho-Rho kinase induces MLC and moesin phosphorylation leading to shape change in the absence of an increase in the cytosolic Ca(2+) concentration. Rho/Rho kinase inhibition could be a therapeutic strategy to prevent pathologic platelet activation during arterial thrombotic disorders.

ADP-induced platelet shape change: an investigation of the signalling pathways involved and their dependence on the method of platelet preparation.

Shape change is an important early event in platelet activation. In this study we show that the Ca2+ chelator BAPTA and the Rho-kinase inhibitor Y-27632 inhibit ADP-induced myosin light chain (MLC) phosphorylation and platelet shape change through distinct pathways and with distinct kinetics. Ca2+ is largely responsible for the initial onset of shape change, whilst Rho-kinase plays a major role in the maintenance of the response. The relative contribution of these two pathways to each stage of the response was dependent on the method of platelet preparation, but in all cases shape change was shown to be downstream of the P2Y1 receptor. Similar observations were made in murine platelets. The shape change response was modulated via changes in cAMP levels, possibly via the P2TAC receptor, but not by tyrosine phosphorylation. We conclude that ADP-induced shape change occurs via the P2Y1 receptor, which can be differentially coupled to Rho-kinase and Ca2+-linked pathways dependent on the method of platelet preparation.

Mildly oxidised low density lipoprotein induces platelet shape change via Rho-kinase-dependent phosphorylation of myosin light chain and moesin.

Oxidised low density lipoprotein (LDL) plays an important role in the pathogenesis of atherosclerosis. Here we demonstrate that mildly oxidised (mox) LDL engages the GTPase Rho and its effector molecule p160 Rho-kinase to induce phosphorylation of myosin light chain and of moesin leading to platelet shape change. Pretreatment of platelets with the selective Rho inhibitor C3-transferase from Clostridium botulinum or with the Rho-kinase inhibitor Y-27632 blocked mox-LDL-induced myosin light chain phosphorylation, moesin phosphorylation and shape change. Mox-LDL did not induce an increase in cytosolic Ca(2+) during shape change. We propose that Rho/Rho-kinase inhibition could be a strategy for prevention of the pathologic platelet activation during atherogenesis.

Mildly oxidized low density lipoprotein induces contraction of human endothelial cells through activation of Rho/Rho kinase and inhibition of myosin light chain phosphatase.

Mildly oxidized low density lipoprotein (mox-LDL) is critically involved in the early atherogenic responses of the endothelium and increases endothelial permeability through an unknown signal pathway. Here we show that (i) exposure of confluent human endothelial cells (HUVEC) to mox-LDL but not to native LDL induces the formation of actin stress fibers and intercellular gaps within minutes, leading to an increase in endothelial permeability; (ii) mox-LDL induces a transient decrease in myosin light chain (MLC) phosphatase that is paralleled by an increase in MLC phosphorylation; (iii) phosphorylated MLC stimulated by mox-LDL is incorporated into stress fibers; (iv) cytoskeletal rearrangements and MLC phosphorylation are inhibited by C3 transferase from Clostridium botulinum, a specific Rho inhibitor, and Y-27632, an inhibitor of Rho kinase; and (v) mox-LDL does not increase intracellular Ca(2+) concentration. Our data indicate that mox-LDL induces endothelial cell contraction through activation of Rho and its effector Rho kinase which inhibits MLC phosphatase and phosphorylates MLC. We suggest that inhibition of this novel cell signaling pathway of mox-LDL could be relevant for the prevention of atherosclerosis.

Dichotomous regulation of myosin phosphorylation and shape change by Rho-kinase and calcium in intact human platelets.

Both Rho-kinase and the Ca(2+)/calmodulin-dependent myosin light chain (MLC) kinase increase the phosphorylation of MLC. We show that upon thrombin receptor stimulation by low-dose thrombin or the peptide ligand YFLLRNP, or upon thromboxane receptor activation by U46619, shape change and MLC phosphorylation in human platelets proceed through a pathway that does not involve an increase in cytosolic Ca(2+). Under these conditions, Y-27632, a specific Rho-kinase inhibitor, prevented shape change and reduced the stimulation of MLC-phosphorylation. In contrast, Y-27632 barely affected shape change and MLC-phosphorylation by adenosine diphosphate (ADP), collagen-related peptide, and ionomycin that were associated with an increase in cytosolic Ca(2+) and inhibited by BAPTA-AM/EGTA treatment. Furthermore, C3 exoenzyme, which inactivates Rho, inhibited preferentially the shape change induced by YFLLRNP compared with ADP and ionomycin. The results indicate that the Rho/Rho-kinase pathway is pivotal in mediating the MLC phosphorylation and platelet shape change by low concentrations of certain G protein-coupled platelet receptors, independent of an increase in cytosolic Ca(2+). Our study defines 2 alternate pathways, Rho/Rho-kinase and Ca(2+)/calmodulin-regulated MLC-kinase, that lead independently of each other through stimulation of MLC-phosphorylation to the same physiological response in human platelets (ie, shape change).

Identification of sequences in human transferrin that bind to the bacterial receptor protein, transferrin-binding protein B.

Alignment of amino-acid sequences from the N-terminal and C-terminal halves of transferrin-binding protein B revealed an underlying bilobed nature with several regions of identity. Based on this analysis, purified recombinant fusion proteins of maltose-binding protein (Mbp) with intact TbpB, its N-terminal half or C-terminal half from the human pathogens Neisseria meningitidis and Moraxella catarrhalis were produced. Solid-phase binding assays and affinity isolation assays demonstrated that the N-terminal and C-terminal halves of TbpB could bind independently to human transferrin (hTf). A solid-phase overlapping synthetic peptide library representing the amino-acid sequence of hTf was probed with soluble, labelled Mbp-TbpB fusions to localize TbpB-binding regions on hTf. An essentially identical series of peptides from domains within both lobes of hTf was recognized by intact TbpB from both organisms, demonstrating a conserved TbpB-hTf interaction. Both halves of TbpB from N. meningitidis bound the same series of peptides, which included peptides from equivalent regions on the two hTf lobes, indicating that TbpB interacts with each lobe of hTf in a similar manner. Mapping of the peptide-binding regions on a molecular model of hTf revealed a series of nearly adjacent surface regions that nearly encircled each lobe. Binding studies with chimeric hTf/bTf transferrins demonstrated that regions in the C-lobe of hTf were preferentially recognized by the N-terminal half of TbpB. Collectively, these results provide evidence that TbpB consists of two lobes, each with distinct yet homologous Tf-binding regions.

Purified meningococcal transferrin-binding protein B interacts with a secondary, strain-specific, binding site in the N-terminal lobe of human transferrin.

Neisseria meningitidis, grown in iron-limited conditions, produces two transferrin-binding proteins (TbpA and TbpB) that independently and specifically bind human serum transferrin (hTF) but not bovine serum transferrin (bTF). We have used surface plasmon resonance to characterize the interaction between individual TbpA and TbpB and a series of full-length human-bovine chimaeric transferrins (hbTFs) under conditions of variable saturation with iron. A comparative analysis of hTF and hbTF chimaera-binding data confirmed that the major features involved in Tbp binding are located in the C-terminal lobe of hTF and that isolated TbpA can recognize distinct sites present in, or conformationally influenced by, residues 598-679. Binding by TbpB was maintained at a significant but decreased level after replacement of the entire hTF C-terminal lobe by the equivalent bovine sequence. The extent of this binding difference was dependent on the meningococcal strain and on the presence of hTF residues 255-350. This indicated that TbpB from strain SD has a secondary, strain-specific, binding site located within this region, whereas TbpB from strain B16B6 does not share this recognition site. Binding of TbpA was influenced primarily by sequence substitutions in the hTF C-terminal lobe, and co-purified TbpA and TbpB (TbpA+B) was functionally distinct from either of its components. The limited divergence between hTF and bTF has been related to observed differences in binding by Tbps and has been used to delineate those regions of hTF that are important for such interactions.

Discrimination between apo and iron-loaded forms of transferrin by transferrin binding protein B and its N-terminal subfragment.

Many pathogens of the Pasteurellaceae and Neisseriaceae possess a surface receptor that binds transferrin (Tf) as an initial step in an iron acquisition process. This receptor is comprised of two proteins, transferrin binding protein A (TbpA) and transferrin binding protein B (TbpB). Since the ability to recognize the iron-loaded form of Tf preferentially would be a useful attribute of these receptors, we examined this property in a number of bacterial species. In solid-phase binding assays with isolated membranes, only the receptor from Moraxella catarrhalis was capable of preferentially binding iron-loaded Tf. In a competitive affinity isolation assay which enabled us to resolve TbpA and TbpB, TbpA from all tested species was shown to bind both apo and iron-loaded Tf. Under these assay conditions TbpB from M. catarrhalis, Haemophilus somnus and Pasteurella haemolytica discriminated between apo and holo Tf, whereas TbpB from Neisseria meningitidis showed no discrimination. The ability of TbpB from N. meningitidis to bind iron-saturated hTf preferentially became evident in a TbpA- background or by using recombinant TbpB. In binding assays with recombinant fusion proteins, both intact TbpB and the N-terminal half of TbpB from all the tested species preferentially bound Fe-loaded Tf, indicating that this may be a conserved mechanism by which these organisms optimize their ability to acquire iron.

Bacterial lactoferrin receptors.

Lactoferrin is thought to play a pivotal role in prevention of infection in the host and its ability to sequester iron from potential pathogens has been considered an important component of its antimicrobial function. A number of bacterial species in the Neisseriaceae have developed a mechanism for acquiring iron directly from this host glycoprotein which involves surface receptors capable of specifically binding lactoferrin. Initial attempts at identifying the receptor proteins in Neisseria and Moraxella species using affinity isolation with immobilized lactoferrin under high stringency conditions presumptively identified a single 100 kDa receptor protein, LbpA (formerly Lbp1). Under modified affinity isolation conditions a second 84 kDa lactoferrin binding protein was isolated and had been presumptively identified as LbpB. This protein was not isolated from a CopB-ve isogenic mutant of Moraxella catarrhalis, indicating that it was in fact CopB. However, another lactoferrin binding protein isolated under high stringency conditions, that comigrated with LbpA in most, but not all, M. catarrhalis strains, was identified by convalescent antisera. Its biochemical properties suggested that it indeed was LbpB. The identity of these proteins was confirmed by preparing isogenic mutants with the lbpA and lbpB genes. Growth studies with isogenic mutants deficient in LbpB, LbpA, CopB or FbpA were performed to evaluate their role in iron acquisition from lactoferrin. LbpA and FbpA were essential for this process, supporting prior models of the iron acquisition pathway. LbpB was not essential which is remniscent of studies with the bacterial transferrin receptors. The isogenic CopB-ve isogenic mutants were deficient in iron acquisition from both transferrin and lactoferrin, suggesting that it is a key component in both pathways. A model providing an alternate explanation of the data is presented. The role and surface accessibility of the lactoferrin receptor proteins suggests that they might be useful vaccine antigens and the preferentially reactivity of convalescent antisera with LbpB suggests that it may be the prime candidate.

Production and characterization of chimeric transferrins for the determination of the binding domains for bacterial transferrin receptors.

Pathogenic bacteria in the Neisseriaceae and Pasteurellaceae possess outer membrane proteins that specifically bind transferrin from the host as the first step in the iron acquisition process. As a logical progression from prior studies of the ligand-receptor interaction using biochemical approaches, we have initiated an approach involving the production of recombinant chimeric transferrins to further identify the regions of transferrin involved in receptor binding. In order to prepare bovine/human hybrids, the bovine transferrin gene was cloned, sequenced, and compared with the existing human transferrin gene sequence. After identification of potential splice sites, hybrid transferrin genes were constructed using the polymerase chain reaction-based approach of splicing by overlap extension. Five hybrid genes containing sequences from both bovine and human transferrin were constructed. Recombinant transferrins were produced in a baculovirus expression vector system and affinity-purified using concanavalin A-Sepharose. The recombinant proteins were analyzed for reactivity against polyclonal and monoclonal antibodies and assessed for binding to Neisseria meningitidis transferrin receptor proteins in solid-phase binding assays and affinity isolation experiments. These experiments enabled us to localize the regions of human transferrin predominantly involved in binding to the N. meningitidis receptor to amino acid residues 346-588. The construction of these chimeras provides unique tools for the investigation of transferrin binding to receptors from both human and bovine bacterial pathogens.